Development of heterogeneous catalysts for clean hydrogen production from
biomass resources
Laura Pastor Pérez
Departamento de Química Inorgánica – Instituto Universitario de Materiales
Facultad de Ciencias
“Development of heterogeneous catalysts
for clean hydrogen production from
biomass resources”
Laura Pastor Pérez
Programa de Doctorado en Ciencia de Materiales
Tesis presentada para aspirar al grado de
DOCTORA POR LA UNIVERSIDAD DE ALICANTE
MENCIÓN DE DOCTORA INTERNACIONAL
Dirigida por:
Antonio Sepúlveda Escribano
Catedrático de Química Inorgánica de la Universidad de Alicante
Julio 2016
LIST OF CONTENTS
1 Abstract / Resumen……………………………………………………….……………….
CAPÍTULO I: Introducción general y objetivos
1. LOS COMBUSTIBLES FÓSILES Y SU PROBLEMÁTICA…………………….. 7
2. EL HIDRÓGENO: ALTERNATIVA ENERGÉTICA RENOVABLE Y NO
CONTAMINANTE………………………………………………………………………… 9
2.1 Pilas de combustible………………………………………………………………. 10
2.2 Producción de hidrógeno………………………………………………………….. 11
3. BIOMASA………………………………………………………………………………... 13
3.1 Métodos de producción de H2 y otros combustibles a partir de biomasa….. 14
3.2 Biocombustibles……………………………………………………………………. 17
3.2.1 Bioetanol………………………………………………………………………. 17
3.2.2 Biodiésel……………………………………………………………………….. 18
3.3 Valorización del glicerol…………………………………………………………... 19
4. REFORMADO CATALÍTICO DE GLICEROL…………………………………….. 21
5. REACCIÓN DE DESPLAZMIENTO DEL GAS DE AGUA (WGS).……………. 25
7. OBJETIVOS………………………………………………………………………..…… 28
8. REFERENCIAS………………………………………………………………………… 29
CHAPTER II: Low temperature glycerol steam reforming on bimetallic PtSn/C
catalysts: on the effect of the Sn content
Abstract……………………………………………………………………………………... 35
1. INTRODUCTION…………………………………………………………………….... 36
2. EXPERIMENTAL SECTION………………………………………………………… 38
2.1 Catalysts preparation…………………………………………………………….. 38
2.2 Catalysts characterization……………………………………………………….. 38
2.3 Catalytic test..……………………………………………………………………… 39
3. RESULTS AND DISCUSSIONS…………………………………………………….. 40
3.1 Catalysts characterization……………………………………………………….. 40
3.1.1 Chemical analysis……………………………………………………………. 40
3.1.2 X-ray photoelectron spectra……….……………………………………….. 41
3.1.3 Transmission electron microscopy………..…………………..…………… 42
3.1.4 H2-Temperature programmed reduction……………….………………... 43
3.2 Catalytic behaviour………………………..……………………………………… 44
4. CONCLUSIONS………………………………………………………………………... 49
5. REFERENCES……………………………………………………………………......... 49
CHAPTER III: Multicomponent NiSnCeO2/C catalysts for the low-temperature
glycerol steam reforming
Abstract……………………………………………………………………………………... 55
1. INTRODUCTION……………………………………………………………………..... 56
2. EXPERIMENTAL SECTION………………………………………………………… 58
2.1 Catalysts preparation…………………………………………………………….. 58
2.2 Catalysts characterization……………………………………………………….. 59
2.3 Catalytic tests……………………………………………………………………… 60
3. RESULTS AND DISCUSSIONS…………………………………………………….. 61
3.1 Catalysts characterization……………………………………………………….. 61
3.1.1 Textural properties …………………………………………………….…… 61
3.1.2 H2-temperature programmed reduction………………………………… 62
3.1.3 XPS characterization of reduced catalysts………………………….…… 63
3.2 Catalytic behaviour……………………………………………………………….. 65
3.3 XRD post-reaction characterization…………………………………………….. 69
4. CONCLUSIONS………………………………………………………………………... 71
5. REFERENCES……………………………………………………………………......... 72
CHAPTER IV: Aqueous phase reforming of glycerol for hydrogen production
over Pt, Ni andPt-Ni catalysts supported on CeO2
Abstract……………………………………………………………………………………... 77
1. INTRODUCTION……………………………………………………………………..... 78
2. EXPERIMENTAL SECTION………………………………………………………… 80
2.1 Catalysts preparation…………………………………………………………….. 80
2.2 Catalysts characterization……………………………………………………….. 81
2.3 Catalytic activity measurements……...………………………………………… 82
3. RESULTS AND DISCUSSIONS…………………………………………………….. 83
3.1 Catalysts characterization……………….…...………………………………….. 83
3.1.1 Textural and chemical characterization………………………………….. 83
3.1.2 X-ray diffraction……………………………………………………………… 84
3.1.3 H2-temperature programmed reduction.…………………………………. 84
3.1.4 X-ray photoelectron spectra……….……………………………………….. 86
3.1.5 Transmission electron microscopy……….………………………………... 88
3.2 Catalytic test..……………………………………………………………………… 88
3.2.1 XPS post-reaction characterization……………………………………….. 93
3.2.2 TEM post-reaction characterization………………………………………. 95
3.2.3 “In situ“ ATR-IR spectroscopy…………….………..……………………… 95
4. CONCLUSIONS………………………………………………………………………... 100
5. REFERENCES……………………………………………………………………......... 101
CHAPTER V: CeO2-promoted Ni/activated carbon catalysts for the water-gas
shift (WGS) reaction
Abstract……………………………………………………………………………………... 107
1. INTRODUCTION………………………………………………………………………. 108
2. EXPERIMENTAL SECTION………………………………………………………… 110
2.1 Catalysts preparation…………………………………………………………….. 110
2.2 Catalysts characterization……………………………………………………..… 111
2.3 Catalytic tests……………………………………………………………………… 112
3. RESULTS AND DISCUSSIONS…..………………………………………………… 113
3.1 Catalysts characterization……..………………………………………………… 113
3.1.1 Textural and chemical characterization………………………………….. 113
3.1.2 X-ray diffraction……………………………………………………………… 115
3.1.3 H2-temperature programmed reduction………………………………….. 116
3.1.4 XPS characterization of reduced catalysts………………………………. 118
3.1.5 Transmission electron microscopy………………………………………… 119
3.2 Catalytic behaviour……………………………………………………………….. 120
4. CONCLUSIONS……………………………………………………………………….. 126
5. REFERENCES…………………………………………………………………………. 127
CHAPTER VI: Ni-CeO2 catalysts whit enhance OSC for the WGS reaction
Abstract……………………………………………………………………………………... 133
1. INTRODUCTION……………………………………………………………….……… 134
2. EXPERIMENTAL SECTION………………………………………………………… 136
2.1 Catalysts preparation…………………………………………………………….. 136
2.2 Catalysts characterization……………………………………………………….. 136
2.3 WGS Catalytic tests………………………………………………………………. 138
3. RESULTS AND DISCUSSIONS…………………………………………………….. 138
3.1 Catalysts characterization……………………..………………………………… 138
3.1.1 Textural and chemical characterization………………………………….. 138
3.1.2 OSCC and OSC………………………………………………………………. 139
3.2 WGS Behaviour……………………………………………………………………. 141
4. CONCLUSIONS………………………………………………………………………... 145
5. REFERENCES…………………………………………………………………………. 146
General conclusions / Conclusiones generales………………………………………… 149
List of Publications………………………………………………………………………... 165
1
ABSTRACT
Chapter I is focused in the current energy crisis and provides a brief introduction
to the use of hydrogen as an energy carrier, mentioning the different methods that
can be used for production/purification of hydrogen from renewable resources. This
chapter also includes a description of the role that biomass can play as an alternative
to fossil fuels, and its conversion into biofuels and value-added chemicals.
Catalytic reforming of glycerol for hydrogen-rich streams or synthesis gas
production is presented as a potential, promising alternative route that has attracted
attention in recent years. This reaction is often carried out in metal-based catalysts
supported on stable materials.
Chapter II explores Sn addition effect on the properties and stability of Pt catalysts
supported on carbon in glycerol steam reforming reaction. To this end, a series of
samples with different Pt/Sn atomic ratios were prepared and characterized.
The high price of noble metals motivates the search and use of cheaper and more
abundant metals that also provide a good catalytic behaviour in this reaction.
Therefore, in Chapter III Ni-based catalyst promoted by ceria for glycerol steam
reforming were used. Furthermore, it is necessary to optimize the use of the CeO2 due
to its limited availability and extensive applications. Thus, in this Chapter CeO2 was
dispersed on activated carbon of high surface area, obtaining great cerium oxide
surface exposed with a remarkable reduction on the ceria loading. Also the effect of
tin was studied in these catalysts.
Several advantages are obtained when glycerol reforming is carried out in liquid
phase. For instance, the produced gas obtained is rich in H2 and poor in CO. This is
due to the moderate temperatures and high pressures employed, favouring the water
gas shift reaction. Furthermore, the overall energy consumption is reduced by the fact
that glycerol is converted into hydrogen in an aqueous liquid phase rather than in the
gas phase, eliminating the need to vaporize the high-boiling biomass derived
oxygenated. Moreover, glycerol upgrading at low temperature prevents the
undesirable thermal decomposition reaction of glycerol when high temperatures are
used.
2
In this context, in Chapter IV, a comparative study of the catalytic properties of
three samples, Pt/CeO2, Ni/CeO2 and Pt-Ni/CeO2 is tested in glycerol aqueous phase
reforming reaction. In addition, in-situ attenuated total reflectance spectroscopy was
employed to obtain relevant information about reaction intermediates and the
evolution of the catalysts during the reaction. This study allowed to propose the more
likely reaction pathways.
For clean energy production, pure hydrogen is required as a feed gas for electricity
generation in low temperature fuel cells. To obtain clean hydrogen, the reforming
stream generated must be processed in several steps, including CO elimination via
water-gas shift reaction.
In Chapter V the same series of Ni catalysts promoted by CeO2 supported in carbon
were tested in the low temperature water gas shift. Two different feed gas mixtures
were used in this study: an idealized one (only CO and H2O) and, a post-reforming
surrogate stream (CO, CO2, H2 and H2O).
To conclude, in Chapter VI, the catalyst which presented the best catalytic
behaviour in the previous chapter was studied in greater depth, linking its properties
with catalytic activity. Finally this sample was submitted to several stability tests
under more demanding reaction conditions aiming to check its potential application in
an integrated fuel processor.
3
RESUMEN
El Capítulo I trata la actual crisis energética y hace una breve introducción sobre el
uso del hidrógeno como vector energético, mencionando los diferentes métodos que
pueden utilizarse para la producción/purificación de hidrógeno a partir de recursos
renovables. También incluye una breve descripción del papel que puede jugar la
biomasa como alternativa a los combustibles fósiles, y su conversión a biocombustibles
y productos químicos de valor añadido.
El reformado catalítico de glicerol para la producción de gas de síntesis o corrientes
ricas en hidrógeno se presenta como una ruta potencial, alternativa y prometedora
que ha llamado la atención en los últimos años. Esta reacción se suele llevar a cabo
sobre catalizadores basados en metales soportados en materiales estables.
En el Capítulo II se estudia el efecto de la adición de Sn sobre las propiedades y la
estabilidad de catalizadores de Pt soportado en carbón en la reacción de reformado de
glicerol en fase gas. Para ello, se preparó y caracterizó una serie de catalizadores con
diferentes relaciones atómicas Pt/Sn.
El alto precio de los metales nobles motiva la búsqueda y empleo de metales más
baratos y abundantes que también tengan un buen comportamiento catalítico en esta
reacción. Por ello, en el Capítulo III se emplearon catalizadores basados en Ni
promovidos por óxido de cerio para el reformado de glicerol. Por otro lado, se hace
necesario optimizar el uso del CeO2 debido a su disponibilidad limitada y sus extensas
aplicaciones. Así, en este trabajo se dispersó CeO2 sobre carbón activado de alta área
superficial, obteniendo una gran superficie de óxido de cerio expuesta al mismo
tiempo que se redujo su consumo. También se estudió el efecto de la presencia de
estaño en estos catalizadores.
Se consiguen varias ventajas al realizar el reformado de glicerol en fase líquida.
Así, se obtienen corrientes más ricas en H2 con menor cantidad de CO. Esto se debe al
empleo de temperaturas moderadas y presiones altas, que favorecen la reacción de
desplazamiento del gas de agua. También se suprime la necesidad de evaporar la
disolución acuosa de glicerol, por lo que el requerimiento energético es menor y se
evitan reacciones indeseadas de descomposición térmica.
4
De este modo, en el Capítulo IV se hizo un estudio comparativo sobre las
propiedades catalíticas de tres muestras, Pt/CeO2, Ni/CeO2 y Pt-Ni/CeO2, en la
reacción de reformado de glicerol en fase líquida. Además, se empleó espectroscopía de
reflectancia total atenuada in situ para obtener información relevante sobre los
intermedios de reacción y la evolución de los catalizadores durante la reacción,
permitiendo así proponer los caminos de reacción más probables.
Para obtener corrientes de hidrógeno suficientemente puro para su uso es las pilas
de combustible, la corriente obtenida después del reformado debe ser procesada en
varias etapas, entre las que se incluyen la eliminación del CO por medio de la reacción
de desplazamiento del gas de agua (water-gas shift, WGS).
En el Capítulo V se estudió la serie de catalizadores de Ni promovidos por CeO2
soportados en carbón en la reacción de desplazamiento del gas de agua a bajas
temperaturas. Para este estudio se emplearon diferentes corrientes de entrada, tanto
ideales (sólo CO y H2O) como reales (CO, CO2, H2 y H2O). Por último, en el Capítulo
VI, el catalizador que presentó mejor comportamiento catalítico en el apartado
anterior fue estudiado en mayor profundidad, relacionando sus propiedades con la
actividad catalítica, sometiéndolo finalmente a ensayos de estabilidad en condiciones
más exigentes.
CAPÍTULO I
Introducción general
y objetivos
Capítulo I
7
1. LOS COMBUSTIBLES FÓSILES Y SU PROBLEMÁTICA
Los combustibles fósiles (petróleo, carbón y gas natural) constituyen en la
actualidad la principal fuente de energía a nivel mundial. Según datos recientes de
consumo energético el petróleo, el carbón y el gas natural suministran alrededor del
90 % de la energía total consumida [1]. El consumo a gran escala de estos recursos
naturales ha permitido a nuestra sociedad alcanzar cotas de desarrollo nunca antes
vistas aunque, al mismo tiempo, también se ha originado una alta dependencia hacia
los mismos. Uno de los sectores que mejor representa esta dependencia es el
transporte, pues depende casi en exclusiva del suministro de petróleo para su
funcionamiento [2]. De entre todos los sectores de la sociedad también es el transporte
el que ha experimentado un crecimiento más fuerte en los últimos años, y en la
actualidad es responsable de casi un tercio de la energía total consumida en el mundo.
Por otro lado, el consumo masivo de combustibles fósiles lleva asociado importantes
problemas económicos, sociales y medioambientales. La primera preocupación está
relaciona con su disponibilidad. Las reservas de combustibles fósiles son finitas, y el
actual ritmo de consumo aumenta cada año para satisfacer la creciente demanda de
países industrializados y el rápido desarrollo de economías emergentes como China e
India. Las proyecciones de consumo energético auguran un aumento del 35 % en el
consumo mundial de energía en los próximos 20 años [3]. Por tanto, teniendo en
cuenta la velocidad actual de consumo y los niveles de las reservas de combustibles
fósiles, se ha estimado que el petróleo, el gas natural y el carbón se agotarán dentro de
los próximos 40, 60 y 120 años, respectivamente [4].
Figura 1. Consumo de energía por fuente [1].
1980 1990 2000 2010 2020 2030 20400
20
40
60
80
100
1202015 Proyecciones
Petróleo y otros líquidos
Cu
atri
mill
ón
Btu
Carbón
Nuclear
Biocumbustibles
Renovables
Gas natural
Otros combustibles
Introducción general y objetivos
8
Otra desventaja de los combustibles fósiles es que su combustión para la
producción de energía genera emisiones netas de gases tales como CO2, CO y otros
gases que han contribuido y aún contribuyen a generar y potenciar el efecto
invernadero, la lluvia ácida y la contaminación del aire, suelo y agua. Estudios
recientes demuestran que la quema de combustibles fósiles para la producción de
energía es responsable de un 70 % del calentamiento global [4].
El tercer gran problema del uso de los combustibles fósiles es su distribución
geográfica desigual. Los países de Oriente Próximo controlan más de un 60 % de las
reservas mundiales de petróleo y el 40 % del gas natural disponible en el mundo.
Además, sólo tres países (China, Rusia y Estados Unidos) monopolizan dos tercios de
los yacimientos de carbón existentes en la actualidad [3]. Esta situación origina
multitud de problemas y tensiones internacionales, y obliga a transportar el
combustible fósil largas distancias para suministrar a los países no productores.
Figura 2. Producción mundial de petróleo (2015) [3].
Por estas y otras razones, es necesario buscar nuevas alternativas energéticas, y
esto se ha convertido en uno de los temas más investigados en los últimos años, y aún
está pendiente de desarrollar.
Con este objetivo los gobiernos, mediante ambiciosas directivas, están estimulando
el uso de fuentes de energías renovables que puedan sustituir a los combustibles
fósiles en el actual sistema energético [5]. Estas fuentes renovables de energía
deberían, además, estar bien distribuidas por el mundo y no contribuir a la
acumulación de gases de efecto invernadero en la atmósfera. Recursos naturales como
el sol, el viento, los recursos geotérmicos, el agua acumulada en pantanos y la biomasa
cumplen con estos requisitos, y permiten el desarrollo de tecnologías limpias con
> 10
7 - 10
4 - 7
2 - 4
1 - 2
< 1
Millones de barriles por día
Capítulo I
9
potencial para sustituir las actuales basadas en los combustibles fósiles. Esta
sustitución será progresiva en el tiempo y también selectiva, pues no todas las formas
renovables de energía pueden sustituir a los combustibles fósiles. De este modo,
estudios recientes indican que la energía solar, la eólica, la geotérmica y la
hidroeléctrica están naturalmente diseñadas para generar calor y electricidad en
instalaciones estacionarias, lo que permite una eventual sustitución del carbón y el
gas natural [6]. Por otro lado, la biomasa es la única fuente renovable de carbono
orgánico disponible en la Tierra y, por tanto, este recurso natural está considerado
como el sustituto ideal del petróleo para la producción de combustibles y productos
químicos [7]. Los biocombustibles, o combustibles generados a partir de biomasa,
generan significativamente menos emisiones de gases causantes de efecto invernadero
que los combustibles de origen fósil, y pueden incluso llegar a ser neutros en el
balance de carbono si se desarrollan métodos eficientes de producción [8,9]
2. EL HIDRÓGENO: ALTERNATIVA ENERGÉTICA RENOVABLE Y NO
CONTAMINANTE
Son muchas las posibilidades que ofrece el hidrógeno para cubrir las demandas
energéticas futuras, y por ello es uno de los grandes candidatos a suplir estas
necesidades. Esto viene impulsado por a su gran abundancia como elemento y su
distribución en infinidad de compuestos. Se considera una alternativa frente al
acuciante problema del cambio climático, ya que su uso evita la producción de gases
de efecto invernadero y el empleo de los combustibles fósiles. Actualmente, incluso se
plantea una futura economía basada en el hidrógeno que reemplace en la mayor
proporción posible a la existente basada en los combustibles fósiles.
Sin embargo, conviene aclarar que el hidrógeno no es un recurso energético, ya que
no se encuentra libre en la naturaleza y, por tanto es necesario producirlo a través de
diferentes métodos y haciendo uso de otras fuentes de energía primarias, tanto
renovables como no renovables. En este sentido, el hidrógeno es considerado como un
portador de energía o, más comúnmente llamado, un vector energético [10].
También cabe destacar que el hidrógeno presenta varias ventajas en comparación
con otros portadores de energía. Por ejemplo, no es tóxico, se quema rápidamente con
poca radiación de calor y, debido a su baja densidad en comparación con el aire, se
Introducción general y objetivos
10
diluye rápidamente en espacios abiertos. Además, el aprovechamiento de la energía
química del hidrógeno no se ve limitada por el ciclo de Carnot, debido a que puede ser
convertida directamente en energía eléctrica utilizando las pilas de combustible,
suprimiendo así el paso intermedio del acondicionamiento térmico de un ciclo de
potencia. Por otro lado, el hidrógeno contiene casi el triple de energía por unidad de
masa que los demás combustibles. Por ejemplo H2 gas (300 bar, 25 °C) contiene una
energía de 33,3 kWh/kg, mientras que el gas natural (1 bar, 25 °C) o la gasolina
presentan 12,7 y 12,3 kWh/kg respectivamente. Sin embargo, al ser tan poco denso, su
contenido energético por unidad de volumen es muy bajo, lo que supone ciertos
problemas de almacenamiento y transporte [11].
2.1 Pilas de combustible
Las pilas de combustible son una de las tecnologías más prometedoras para el
futuro de una industria energética limpia [12,13]. Se trata de un dispositivo
electroquímico que convierte la energía química almacenada en un combustible, como
el hidrógeno, en energía eléctrica con un abastecimiento continuo de reactivos. Tienen
aplicación en un amplio rango de dispositivos, que van desde sistemas estacionarios
para producir energía eléctrica a gran escala, hasta dispositivos móviles para
alimentar equipos microelectrónicos o unidades de energía auxiliares [14,15].
Existe una gran variedad de pilas de combustible, algunas ya comercializadas y
otras en proceso de desarrollo e investigación, con diferencias en los reactivos y en el
electrolito utilizado, la temperatura de trabajo y su eficiencia. Hoy en día, se emplea
hidrógeno como combustible en las llamadas pilas de membrana de intercambio
protónico (PEMFC, de la terminología inglesa Proton Exchange Membrane Fuel Cell
(Figura 3)).
Capítulo I
11
Figura 3. Esquema general de una PEMFC.
La combustión de hidrógeno en la pila de combustible se considera una alternativa
energética "verde", ya que el único subproducto es el agua, con la ventaja adicional,
como se ha mencionado anteriormente, de que el hidrógeno como combustible posee
alrededor de tres veces más energía neta por unidad de masa, que los combustibles
fósiles y derivados [16].
2.2 Producción de hidrógeno
Como se ha mencionado anteriormente, el hidrógeno es un portador energético que
tiene que ser producido a partir de compuestos que lo contengan, tales como
hidrocarburos, amoniaco, hidruros, biomasa, etc. [17].
La opción más empleada a corto plazo es la producción de hidrógeno mediante el
reformado de hidrocarburos. No obstante, el producto de este proceso es una mezcla de
hidrógeno y óxidos de carbono como componentes principales. El CO2 puede ser
eliminado fácilmente, ya que éste no afecta a la pila de combustible. En cambio, la
presencia de CO en la corriente de reformado obtenida [18] es un gran inconveniente
para las PEMFC ya que su ánodo, comúnmente fabricado en Pt o Pt-Ru, se envenena
por la fuerte adsorción irreversible de moléculas de CO en su superficie.
Por este motivo, la cantidad de CO presente en la corriente de hidrógeno que
alimenta a la pila de combustible debe ser lo más baja posible. Se necesitan menos de
50 ppm en el caso de ánodos de Pt-Ru, y menos de 10 ppm para los ánodos de Pt para
Hidrógeno Oxígeno
Cat
aliz
ado
r
Cat
aliz
ado
r
Exceso de H2
(para reutilizar)Agua
Energía eléctrica
Ele
ctro
lito
Introducción general y objetivos
12
envenenar el electrocatalizador [19], de forma que la vida útil del ánodo de PEMFC
se reduce drásticamente. Para evitar este envenenamiento y obtener corrientes de
hidrógeno suficientemente puras, la corriente obtenida después del reformado debe
ser procesada en varias etapas, entre las que se incluyen la eliminación del CO por
medio de la reacción de desplazamiento del gas de agua (water-gas shift, WGS), la
oxidación preferencial de CO y/o la reacción de metanación [20,21].
De esta forma, las distintas etapas de procesado de combustible convierten los
hidrocarburos líquidos o gaseosos (el contenido de energía por unidad de volumen es
mucho mayor que en el hidrógeno en las mismas condiciones) en una corriente rica en
H2 y casi libre de CO, según la secuencia esquematizada en la Figura 4.
Figura 4. Etapas de procesado de combustible para la producción de hidrogeno limpio.
Por ende, para que el empleo de hidrógeno como alternativa a los combustibles
fósiles sea viable, es necesario desarrollar catalizadores eficientes para cada uno de
estos procesos. En concreto, esta memoria se centra en el desarrollo y optimización de
catalizadores para los dos primeros procesos de procesado: reformado y reacción de
desplazamiento del gas de agua (WGS).
COMBUSTIBLECH4 + H2O CO + 3H2
CalorCalor
CO + H2O ↔ CO2 + H2
ReformadoWGS
CO + ½ O CO2
PROX
Metanación
CO + 3H2 H2O + CH4
Calor
CalorH2
O2
H2O
2H2 + O2 2H2O
Capítulo I
13
3. BIOMASA
La biomasa, en términos energéticos, se define como la materia orgánica originada
en un proceso biológico, espontáneo o provocado, utilizable como fuente de energía
[22]. Puede ser de origen vegetal, derivada de todo tipo de plantas, o de origen animal,
incluyendo los abonos y el estiércol. La biomasa está compuesta por materia orgánica
en la que la energía solar se encuentra almacenada en forma de enlaces químicos. Su
composición elemental está formada por C, H, O y N, con cantidades pequeñas de
azufre y otros elementos. Los carbohidratos constituyen la parte más importante de la
biomasa energética de origen vegetal, que contiene cantidades variables de celulosa
(80-40 % del peso), hemicelulosa (30-15 % del peso) y lignina (25-10 % del peso)
dependiendo de su procedencia (Figura 5) [23,24,25,26].
Figura 5. Estructura de diferentes fracciones de la biomasa vegetal (lignocelulosa, celulosa,
lignina y hemicelulosa).
En la actualidad es la cuarta fuente de energía más importante del mundo, y es el
recurso renovable más versátil. Representa alrededor del 15 % del consumo de energía
primaria y el 38 % del consumo energético en los países en desarrollo [27]
Introducción general y objetivos
14
Figura 6. Ciclo de la biomasa.
Además del gran potencial energético que tiene la biomasa, la reducción del CO2
neto que es liberado a la atmósfera hace que su uso sea muy atractivo y amigable con
el medio ambiente, reduciendo el impacto ambiental en comparación con el uso de
combustibles fósiles. Este CO2 liberado al gasificar, pirolizar o reformar la biomasa, es
nuevamente fijado por las plantas mediante la fotosíntesis (Figura 6) [28,29].
3.1 Métodos de producción de H2 y otros combustibles a partir de biomasa
Aunque el hidrógeno es una alternativa viable, en la actualidad no existe una sola
fuente energética que pueda reemplazar por completo al petróleo. Se hace necesaria la
búsqueda de alternativas paralelas para dar solución al problema energético, y dentro
de éstas, la biomasa es una de las fuentes energéticas renovables y sostenibles con
mayor aceptación.
A pesar de ser el compuesto principal para alimentar las pilas de combustible de
membrana (PEMFC), el hidrógeno también puede ser utilizado como intermedio para
la producción de biocombustibles, de igual modo que lo es en la producción de gasolina
y diésel. Por tanto, la producción de hidrógeno y los procesos que lleva involucrados
pueden ser parte integral de las futuras biorefinerías, de igual modo que ya lo son
para las refinerías de petróleo actuales. Según los expertos, todos los beneficios de una
economía basada en el hidrógeno sólo se alcanzan cuando el hidrógeno se obtiene a
partir de recursos renovables como la biomasa [30]. A ello se suma que la biomasa,
además de poder ser utilizada como fuente de producción de hidrógeno, es también
uno de los precursores más atractivos para la síntesis de combustibles y productos
químicos.
Fotosíntesis
CO2
CO2
Bosque
Biomasa
Transformación de la biomasa
Combustión
Capítulo I
15
Por otra parte un sistema energético basado en la utilización de biomasa como
combustible puede ayudar a igualar la economía, ayudando a mejorar la de países que
no tienen reservas de petróleo y hacer además un sistema menos dependiente de las
fluctuaciones de su precio. También hay que tener en cuenta que la compatibilidad
con las infraestructuras existentes para el uso de combustibles convencionales ha sido
un factor clave que ha permitido la rápida expansión de los biocombustibles [8].
Uno de los primeros inconvenientes que se planteó en relación con la producción a
gran escala de biocombustibles fue el problema que podía surgir por el uso de tierras
dedicadas al cultivo de alimentos. Esto impulsó la creación de los biocombustibles de
segunda generación, que se obtienen a partir de biomasa no comestible y, más
específicamente, de material lignocelulósico. Teniendo en cuenta que, además, la
biomasa lignocelulósica tiene un crecimiento más acelerado, menor coste (comparada
con la biomasa alimentaria) y mayor disponibilidad (su producción no está limitada a
pequeñas áreas) este tipo de materia constituye un recurso atractivo y viable para
sustituir al petróleo [31,32,33].
Figura 7. Ejemplos de biocombustibles de primera y segunda generación.
La producción de biocombustibles a partir de material lignocelulósico involucra la
eliminación de oxígeno y la formación de enlaces C-C, controlando el peso final de los
hidrocarburos formados e intentando emplear la menor cantidad de hidrógeno
procedente de una fuente externa. Esto se consigue mediante dos etapas principales,
primero la conversión del material lignocelulósico en productos químicos plataforma
(líquidos o gaseosos), lo que implica la eliminación parcial de oxígeno, y en segundo
BIOCOMBUSTIBLES DE PRIMERA GENERACIÓN
BIOCOMBUSTIBLES DE SEGUNDA GENERACIÓN
Cereal
Almidón
Azúcares
Cultivos de aceite
Algas
Residuos
Paja, Bagazo, Cáscaras
Deshechos de uva y posos
Estiércol animal
Brea celulósica y glicerina cruda
Introducción general y objetivos
16
lugar la formación de los hidrocarburos junto con la eliminación final de grupos
oxigenados mediante procesos catalíticos.
Debido a la complejidad química y estructural de la biomasa lignocelulósica, se han
desarrollado diferentes métodos para convertir este material en productos químicos y
combustibles. Las tecnologías actuales implican tres rutas principales, como se
muestra en la Figura 8, que incluyen la producción de gas de síntesis (CO + H2) por
gasificación, la producción de bioaceites por pirólisis o licuefacción, o la hidrólisis de la
biomasa para producir unidades de monómeros de azúcar. El gas de síntesis se puede
utilizar para producir hidrocarburos (diésel o gasolina), metanol y otros combustibles.
Los bioaceites son posteriormente mejorados para que puedan ser utilizados como
combustibles en el transporte. También, a partir de los azúcares y otros productos
intermedios se puede producir etanol, gasolina y gasóleo para su uso en este sector.
Otro método para la producción de biocombustibles es hacer crecer cultivos
energéticos con estructuras de alta densidad energética, que pueden ser convertidas
fácilmente en combustibles líquidos como aceites vegetales o hidrocarburos [34].
Figura 8. Estrategias para la producción de combustibles a partir de biomasa
lignocelulósica [32].
La biomasa y los biocombustibles parecen ser la clave para el abastecimiento de las
necesidades energéticas básicas de nuestra sociedad, por medio de la producción
BIOMASA
Celulósica (desechos forestales, agrícolas, cultivos energéticos, plantas acuáticas)
Gasificación Pirólisis/Licuef. Hidrólisis
Gas de síntesis Bio-aceites Azúcares líquidos Lignina
Fish
er-T
rop
sch
Met
an
ol
WG
S
Des
hid
roxi
gen
.
Zeo
lita
s
Ferm
enta
ció
n
Des
hid
rata
ció
n
Pro
cesa
do
F.A
c
Va
lori
zaci
ón
Alcanos Metanol H2 Combustibles líquidos EtanolHidroc.
aromáticosAlcanos liq./ H2
Gasolina esterificada
Capítulo I
17
sostenible de combustibles líquidos para el transporte y productos químicos, sin
comprometer las necesidades de futuras generaciones. Un objetivo importante del
siglo XXI para la academia, la industria y el gobierno debe ser conseguir una
utilización eficiente y económica de los recursos de la biomasa.
3. 2 Biocombustibles
El bioetanol y el biodiésel son actualmente los biocombustibles líquidos más
utilizados para el transporte. Se utilizan como aditivos o sustitutivos de combustibles
convencionales derivados del petróleo como la gasolina y el diésel, respectivamente.
Esta compatibilidad con las infraestructuras existentes para los combustibles
convencionales es el factor clave que ha permitido una rápida expansión de los
biocombustibles. De esta forma, la producción mundial de bioetanol y biodiésel ha
crecido exponencialmente en los últimos años, alcanzando en la actualidad un 2 % del
total de los combustibles consumidos en el mundo [8]. Otro factor que explica el rápido
desarrollo de estos biocombustibles es la simpleza de las tecnologías para su
producción, lo que ha permitido la construcción de un gran número de biorefinerías en
los últimos años.
3.2.1 Bioetanol
El bioetanol es el biocombustible más producido en la actualidad. Representa más
del 80 % de la producción total de biocarburantes y su producción continúa
aumentando. Los dos principales productores de bioetanol son EE.UU y Brasil, que
acaparan cerca de un 90 % de la producción mundial de este biocombustible. El
bioetanol se produce por fermentación anaeróbica de disoluciones acuosas diluidas de
azúcares derivados de la biomasa a temperaturas suaves (30-50 °C). El bioetanol
producido se purifica por destilación para eliminar el agua, y una vez purificado ya es
apropiado para su uso directo como combustible o aditivo. El bioetanol se suele añadir
a la gasolina en bajas concentraciones (5-10 % en volumen, E5-E10) ya que para la
utilización de mezclas ricas en etanol (E-85 y superior) es necesaria la modificación de
los vehículos. Como se ha comentado más arriba, la utilización de biomasa comestible
para la producción de etanol es un problema añadido, pues produce competición con
tierras dedicadas al cultivo de alimentos y, por ello, la investigación se centra
Introducción general y objetivos
18
actualmente en la producción de bioetanol a partir de fuentes de biomasa no
comestibles como la lignocelulósica [35]. Finalmente, es necesario indicar que su alta
miscibilidad con agua y su baja densidad energética en comparación con la gasolina
también son importantes limitaciones para la implementación del bioetanol como
combustible para el transporte [36].
3.2.2 Biodiésel
El biodiésel es el segundo biocombustible más empleado a nivel mundial y el
primero en Europa, representando el 80 % del mercado de los biocombustibles
actuales en Europa. La producción mundial de biodiésel ha crecido exponencialmente,
y se espera que alcance los 45 billones de litros en 2020 [37]. El biodiésel es una
mezcla de ésteres metílicos de cadena larga derivados de aceites vegetales como
girasol, soja, palma o colza. El proceso industrial de producción del biodiésel involucra
la reacción entre los triglicéridos de estos aceites vegetales y metanol, en presencia de
un catalizador básico (normalmente NaOH o KOH) a temperaturas suaves, en un
proceso denominado transesterificación (Figura 9).
Figura 9. Esquema del proceso de transesterificación para la producción de biodiésel.
En este proceso se producen grandes cantidades de glicerol (1,2,3-propanotriol)
junto con el biodiésel. Éste se separa por decantación del glicerol (mucho más denso) y
se purifica para su uso directo como combustible. Al igual que el bioetanol, el biodiésel
se usa en forma de mezclas diluidas (B-5, B-20) con diésel convencional, ya que el
biodiésel puro (B-100) no es compatible con los motores diésel actuales. Además, en la
actualidad también se está investigando la utilización de triglicéridos no comestibles,
como aceites usados y algas, para la producción de biodiésel [36].
Capítulo I
19
3.3 Valorización de glicerol
El glicerol, como se ha mencionado anteriormente, es el principal producto de
desecho de la creciente industria del biodiésel. Como consecuencia del proceso de
transesterificación de triglicéridos con metanol (Figura 9), se generan 100 Kg de
glicerol (en forma de disoluciones acuosas concentradas) por cada tonelada de
biodiésel producido.
Figura 10. Aplicaciones industriales del glicerol [38].
A pesar del gran número de aplicaciones actuales que posee el glicerol (Figura 10)
la sobreproducción generada por la industria del biodiésel excede con creces su
demanda actual, generando un remanente importante de glicerol crudo en la
industria. Además, el glicerol también se puede obtener como subproducto de la
conversión de lignocelulosa en etanol [39], que se espera sea una de las industrias más
importantes para la producción de biocombustibles en los próximos años. Por tanto,
un reto importante para la industria de los biocombustibles consiste en la búsqueda
de vías para la valorización de este glicerol residual, mejorando de esta forma la
economía de los procesos de producción de biocarburantes. En este sentido, se estima
que la venta del glicerol residual permitiría reducir los costes de producción del
biodiesel en un 6 % [40], y que la coproducción de glicerol y bioetanol disminuya los
costes de producción e incremente los márgenes de beneficio en la producción de
bioetanol lignocelulósico [41].
Una de los procesos que permitiría el consumo a gran escala de glicerol sería su uso
como combustible en el sector del transporte. Sin embargo, a diferencia del bioetanol,
el glicerol no se puede añadir directamente a los combustibles convencionales debido a
su baja solubilidad en hidrocarburos. Además, es un compuesto viscoso e inestable a
Medicamentos /Drogas18%
Resinas8%
Triacetín10%
Explosivos2%
2%Celofán2%
Detergentes
Tabaco6%
Otros 11%
Alimentos 11%
Plásticos14%
Cuidado personal16%
Glicerol
OH
OHHO
Introducción general y objetivos
20
altas temperaturas, por lo que se desaconseja su uso en motores de combustión. Por
tanto, una ruta interesante para la utilización del glicerol a gran escala implicaría
ajustar sus propiedades a la de los hidrocarburos. Por ello, la conversión del glicerol
en gas de síntesis (mezcla de CO e H2) o en una corriente rica en H2 mediante procesos
de reformado, y su posterior conversión en alcanos a través de tecnologías bien
conocidas como la síntesis de Fischer-Tropsch (F-T) o la posterior purificación de la
corriente de H2 mediante las reacciones de WGS y PROX, son vías con un gran
potencial [42].
La estructura química del glicerol, con tres grupos hidroxilo adyacentes, determina
las propiedades físicas y químicas de este compuesto. Es una sustancia
completamente soluble en agua, con un elevado punto de ebullición (290 °C) y con una
alta y variada reactividad química que puede ser aprovechada para la producción de
un gran número de compuestos. Algunas de las rutas catalíticas más importantes
para la transformación de glicerol en combustibles y productos químicos de alto valor
añadido se detallan en la Figura 11.
Figura 11. Principales rutas catalíticas para la valorización de glicerol a combustibles y
productos químicos.
El glicerol es un componente básico para la síntesis de numerosos productos
químicos, como ácidos glicéridos, propilenglicol, 1,3-propanodiol, poliésteres
ramificados y polioles. La oxidación selectiva, la hidrogenólisis y la deshidratación son
Capítulo I
21
las rutas más utilizadas para generar estos productos [43,44,45,46]. Muchos de estos
procesos son conocidos desde hace tiempo, pero no fueron investigados en profundidad
por el alto precio del glicerol. El desarrollo de la industria del biodiésel y el
consiguiente abaratamiento del glicerol ha relanzado la investigación en este campo
[47]. El desafío, en la mayoría de estos procesos, es el control de la alta reactividad del
glicerol que, en muchas ocasiones, impide la obtención de buenas selectividades a los
productos deseados. Para ello es imprescindible la preparación de catalizadores
adecuados y la determinación de condiciones de reacción óptimas.
4. REFORMADO CATALÍTICO DE GLICEROL
Como se ha indicado anteriormente, una de las rutas más interesantes para la
valorización de glicerol es el reformado catalítico. Esta reacción permite la
transformación de disoluciones acuosas de glicerol en una corriente gaseosa de CO,
CO2 e H2 sobre catalizadores metálicos soportados. Esta ruta se ha desarrollado con
rapidez en los últimos años como consecuencia de su aplicación en la producción
catalítica de H2 a partir de bioetanol utilizando catalizadores basados en Ni, Co y
metales nobles (Pt, Ru, Rh) a altas temperaturas (600-800 °C) [48]. El reformado
catalítico de glicerol ofrece una serie de ventajas con respecto al mismo proceso con
etanol. En primer lugar, la mayor reactividad química del glicerol en comparación con
el etanol permite realizar el reformado a temperaturas más bajas (250-400 °C). En
estas condiciones las reacciones no deseadas que conducen a la formación de coque
están cinéticamente controladas, facilitando un control óptimo sobre la composición de
la fase gas y alargando la vida media del catalizador. Además, el proceso de reformado
de glicerol es más versátil que el del etanol. En el reformado catalítico de glicerol las
condiciones de reacción y los catalizadores pueden ser seleccionados para producir gas
de síntesis para procesos de F-T o síntesis de metanol [42] o, alternativamente,
corrientes ricas en H2 mediante el acoplamiento del reformado con la reacción de
desplazamiento del gas de agua (Figura 11) [49]. Por otro lado, el reformado de
glicerol da lugar a un mayor número de productos secundarios en comparación con el
del etanol, que es una molécula más sencilla y menos reactiva.
El glicerol, por tanto, puede actuar como fuente renovable de gas de síntesis o de
hidrógeno mediante procesos de reformado catalítico. En la actualidad, las principales
Introducción general y objetivos
22
rutas para la producción de gas de síntesis e/o hidrógeno a partir de biomasa se basan
en su gasificación en atmósfera parcialmente oxidante a temperaturas entre 900-1100
°C. Mediante esta tecnología se obtiene una corriente de gas de síntesis diluida, con
cierto grado de impurezas procedentes de la biomasa (que hay que eliminar para
posteriores procesos de F-T) y con poco control de la relación H2/CO debido a las altas
temperaturas empleadas. El reformado catalítico de glicerol ofrece una interesante
alternativa al proceso de gasificación, ya que se obtiene gas de síntesis sin diluir y
libre de impurezas a temperaturas considerablemente más bajas (250-400 °C). El uso
de temperaturas bajas durante el reformado es muy interesante, pues permite el
acoplamiento directo (en un sólo reactor) con reacciones de F-T [50] o WGS [49]
(reacciones exotérmicas y por tanto, favorecidas a bajas temperaturas) reduciendo así
la complejidad del proceso. De esta forma, el calor necesario para llevar a cabo el
reformado endotérmico podría ser teóricamente suministrado por las reacciones
exotérmicas de F-T o WGS, mejorando así la eficiencia térmica del proceso.
Comúnmente, el reformado catalítico de glicerol se lleva a cabo sobre catalizadores
metálicos soportados. Industrialmente, la mayoría de los procesos de reformado
emplean catalizadores de Ni soportado sobre Al2O3. El Ni es un metal muy atractivo
para este tipo de procesos, ya que tiene una relación actividad/precio excelente [51], lo
que permite incorporar altas cargas de metal al catalizador, alcanzándose actividades
catalíticas similares a las obtenidas con los metales nobles pero a costes más
reducidos. Sin embargo, el Ni se desactiva por la sinterización de las partículas y por
la formación de depósitos de coque sobre su superficie [52,53,54,55]. Las principales
reacciones que pueden tener lugar durante el reformado de glicerol sobre
catalizadores metálicos se detallan en la Figura 12.
Capítulo I
23
Figura 12. Esquema de reacción para el reformado catalítico de disoluciones acuosas de
glicerol sobre catalizadores metálicos. (*) indica unión con la superficie catalítica [30].
El primer paso en el reformado es la adsorción del glicerol sobre la superficie
metálica. Este proceso tiene lugar a través de reacciones de deshidrogenación. A
continuación, los intermedios adsorbidos pueden sufrir reacciones de rotura de enlaces
C-C o C-O que darán lugar a los distintos productos de reacción. De esta forma, la
rotura de enlaces C-C da lugar a la formación de CO e H2. El CO en presencia de agua
puede reaccionar mediante la reacción de WGS para producir CO2 y más H2. Por
tanto, si se pretende generar una corriente rica en H2 en el reformado de glicerol, se
deben seleccionar condiciones de reacción y catalizadores que favorezcan reacciones de
rotura de C-C junto con la WGS. En caso contrario, si lo que se pretende es producir
una corriente rica en gas de síntesis debemos usar catalizadores y condiciones que
favorezcan reacciones de rotura C-C pero no así el proceso de WGS. De esta forma,
operando a bajas presiones y altas concentraciones de glicerol (o bajas concentraciones
de agua) se disminuye la presión parcial de agua en el reactor y se consigue desplazar
el equilibrio WGS hacia el CO. Alternativamente, se puede inhibir la reacción de WGS
mediante el uso de soportes inertes que no sean capaces de activar la molécula de
agua.
Como se indica en la Figura 12, si las condiciones de reacción no son
cuidadosamente controladas, tanto el CO como el CO2 pueden reaccionar con el H2
formado para producir metano (a través de procesos de metanación sobre el metal),
Introducción general y objetivos
24
mientras que el CO y el H2 pueden generar alcanos mediante procesos de F-T. Estos
procesos conllevan consumo de H2 y, por tanto, se deben evitar en el reactor de
reformado.
Al contrario de los procesos de escisión de enlaces C-C, la rotura de enlaces C-O
conlleva la formación de alcanos (productos indeseados) a través de la formación
intermedia de alcoholes. Esta rotura de enlaces C-O también se puede producir
mediante procesos de deshidratación (en sitios ácidos del soporte, por ejemplo) seguido
de hidrogenación del intermedio insaturado sobre el metal (Figura 12). Los procesos
de deshidrogenación sobre el metal y posterior reordenamiento generan ácidos
orgánicos. Estos intermedios de reacción, que se encuentran con frecuencia en la fase
acuosa obtenida tras el proceso de reformado, pueden reaccionar sobre el metal y
producir alcanos y CO2 a través de procesos de descarboxilación.
Por otro lado, cabe destacar que el reformado catalítico de glicerol en disolución
acuosa puede llevarse a cabo en fase vapor (si se opera a temperaturas elevadas y
presiones bajas) o, alternativamente, en fase líquida, si se trabaja a temperaturas
moderadas y presiones elevadas (por encima de la presión de vapor del agua a la
temperatura de reacción). Ambas estrategias tienen sus ventajas e inconvenientes.
Por ejemplo, trabajar en fase líquida permite ahorrar la energía necesaria para
vaporizar disoluciones acuosas de glicerol pero, por el contrario, obliga a operar a
elevadas presiones. Estas elevadas presiones favorecen la presión parcial del agua,
favoreciendo la reacción WGS, por lo que el reformado en fase líquida está
especialmente indicado para la producción de corrientes ricas en H2 con bajos niveles
de CO. El reformado en fase vapor permite mayor flexibilidad en la producción de H2 o
gas de síntesis (seleccionando, por ejemplo, el catalizador adecuado) y, además, opera
a mayores temperaturas, lo que incrementa la velocidad de reacción. Sin embargo, el
reformado en fase vapor necesita de catalizadores estables a altas temperaturas que,
por otro lado, favorecen las reacciones de formación de coque y la consiguiente
desactivación del catalizador de reformado.
De forma general, el reformado de glicerol es un proceso endotérmico que se puede
definir como la combinación entre la pirólisis y la reacción de desplazamiento del gas
de agua. Viene determinado por la siguiente reacción química:
Capítulo I
25
donde x indica el grado de participación de la reacción de desplazamiento del gas de
agua. X puede tener valores desde 0, en cuyo caso se trataría de una pirólisis, hasta 3,
que sería la reacción de reformado de glicerol con vapor de agua.
Termodinámicamente, el proceso está favorecido a altas temperaturas y presiones
bajas, mientras que la reacción de WGS se inhibe a altas temperaturas y no está
afectada por la presión.
Por tanto, la relación molar teórica H2/CO en el gas de síntesis obtenido a partir de
glicerol es de 1,3. Al introducir agua en el reactor esta relación puede aumentar a
consecuencia de las reacciones de WGS produciéndose, además, un cambio notable en
la relación molar CO/CO2 de la corriente gaseosa de salida. La reacción de WGS juega,
por tanto, un papel importante a la hora de regular la relación molar H2/CO del gas de
salida producido.
5. REACCIÓN DE DESPLAZAMIENTO DEL GAS DE AGUA (WGS)
La reacción de desplazamiento de gas de agua o WGS (de sus siglas en inglés
“water-gas shift”) es una reacción exotérmica y reversible, cuya constante de
equilibrio disminuye al aumentar la temperatura [13,56].
Pese a ser un proceso aparentemente sencillo, el mecanismo de reacción ha sido y
continua siendo un tema de debate, habiéndose propuesto dos tipos de mecanismos
sobre la base de resultados cinéticos y evidencias espectroscópicas [57] Además, el tipo
de mecanismo puede variar en función del catalizador y de las condiciones de reacción
empleadas. Uno de ellos es el llamado mecanismo redox o mecanismo regenerativo
tipo Eley-Rideal, en el cual el agua oxida la superficie del catalizador y el CO vuelve a
reducir la superficie oxidada. Este proceso ha sido descrito frecuentemente como un
Introducción general y objetivos
26
proceso bi-funcional, en el que el CO adsorbido sobre el metal es oxidado por el soporte
(que participa activamente en la reacción) y el agua suple la vacante de oxígeno
formada en la superficie del soporte [58,59].
Figura 13. Esquema del mecanismo redox para la reacción de WGS.
Por otro lado, el segundo mecanismo propuesto es el mecanismo asociativo o tipo
Langmuir-Hinshelwood, que transcurre a través de la formación de grupos
formiato/carboxilato superficiales. En este caso,el agua adsorbida o disociada forma
grupos hidroxilos reactivos que se unen al CO, formando un formiato/carboxilo
superficial que posteriormente se descompone en CO2 y H2 [60].
Figura 14. Esquema del mecanismo asociativo para la reacción de WGS.
La reacción de WGS es una reacción clásica, y a la vez muy relevante y con una
amplia trayectoria histórica en la industria química. Desde principios del año 1940, la
CO
Soporte
C
O
MO
CO2
H2O
Soporte
CO2
M
Soporte
MO
H
H
Soporte
H
MOH
Soporte
H2
MO
H2
CO
CO2
H2O
Soporte
M
H2
Soporte
M
C
O
Soporte
M
C
O
O
H
H
Soporte
M
C
O
O H H
Soporte
M
C
O
O H H
Capítulo I
27
reacción de WGS ha representado un paso importante en el proceso industrial de
producción de hidrógeno. Fue con el desarrollo del proceso Haber-Bosch para la
producción de amoníaco, cuando comenzó su aplicación práctica [57]. En este proceso,
la reacción de WGS se combina con el reformado de gas natural o con la gasificación
de carbón para producir una mezcla adecuada H2/CO para la síntesis de amoníaco. A
partir de este momento, la reacción de WGS ganó importancia y se convirtió en parte
inseparable de diversos procesos industriales, tales como la síntesis de metanol, la
producción de hidrocarburos alternativos como combustible a través de la reacción de
Fisher-Tropsch y la conversión de hidrocarburos mediante reacciones de reformado,
entre otras muchas aplicaciones.
Como se ha señalado antes, el hecho de que la reacción sea exotérmica hace que
esté favorecida termodinámicamente a bajas temperaturas y cinéticamente a altas
temperaturas. Por tanto, se puede lograr un rendimiento significativamente mejor
llevando a cabo primero la reacción a temperaturas relativamente altas (HTWGS:
high-temperature WGS), aprovechando una alta velocidad de reacción, cuando la
composición del gas está lejos del equilibrio. Acto seguido, para seguir convirtiendo el
CO restante, se baja la temperatura de reacción (LTWGS: low-temperature WGS),
cuando la termodinámica comienza a limitar la conversión de CO [61]. Por tanto, la
menor temperatura a la cual se puede alcanzar una máxima concentración de CO está
determinada por la actividad del catalizador y por la temperatura de rocío del gas
[62].
Los primeros catalizadores empleados para la reacción de WGS a alta temperatura
fueron mezclas de óxidos de Fe-Cr. Presentan la ventaja de su bajo coste y una
resistencia razonable a los venenos azufrados, pero solo son activos en un intervalo de
temperatura entre 400-500 °C [61]. En un intento de mejorar la actividad de estos
catalizadores se han realizado numerosos estudios en los que adicionan pequeñas
cantidades de metales nobles tales como Pt, Pd, Rh [63,64]. Más tarde se utilizaron
catalizadores basados en óxidos de Cu-Zn para la reacción de WGS a baja
temperatura en una serie de reactores adiabáticos para el suministro de hidrógeno en
condiciones industriales. El problema que presentan estos catalizadores es su carácter
pirofórico por encima de los 300 °C, lo que los hace indeseables por cuestiones de
seguridad, limitando sus aplicaciones. Algunos estudios sobre catalizadores de cobre
soportados en óxido de cerio demuestran que se obtiene una estabilidad térmica
Introducción general y objetivos
28
mayor que con los catalizadores convencionales Cu-Zn [61]. Adicionalmente, estos
catalizadores comerciales solo resultan activos a bajas velocidades espaciales, lo que
implica grandes volúmenes de reactor y limita su campo de aplicación a sistemas
estacionarios.
Recientemente, la reacción de WGS ha cobrado un renovado interés debido a su
papel crucial en la producción de hidrógeno puro para aplicaciones en pilas de
combustible [21,65]. Independientemente de los compuestos empleados para el
reformado, ya sean hidrocarburos o moléculas procedentes de la biomasa, la reacción
de WGS suele estar acoplada a ésta para modular la relación H2/CO que se obtiene a
la salida. Esta reacción es el proceso que recibe la mayor cantidad de CO (entre 1-13
% de CO en función del gas de reformado [57]. Esta concentración debe ser reducida
hasta aproximadamente el 1 % (v/v) antes de alimentar la siguiente unidad de
procesado. Por otro lado, la reciente aparición de esta tecnología basada en pilas de
combustible, y su empleo en equipos electrónicos móviles, hace necesario redefinir los
requisitos que debe tener un catalizador para ser empleado en estas unidades. Por lo
tanto, se hace indispensable el desarrollo de nuevos catalizadores no pirofóricos,
activos a bajas temperaturas (y que por tanto permitan conversiones elevadas de
acuerdo al equilibrio) y capaces de funcionar a velocidades espaciales elevadas. Estas
propiedades permitirían reducir considerablemente el volumen de la unidad de WGS
para su aplicación en los procesos integrados de generación de hidrógeno puro in situ
que alimente dispositivos móviles.
6. OBJETIVOS
En los últimos años, el interés y las regulaciones ambientales se han incrementado
en el ámbito público, político y económico, puesto que la calidad de vida está
directamente relacionada con un medioambiente limpio. Con este fin aparece el
concepto de “Química Verde” (también llamada química sostenible), que consiste en
una filosofía del uso de la Química dirigida hacia el diseño de productos y procesos
químicos que impliquen la reducción o eliminación de sustancias químicas peligrosas.
En este marco, la Catálisis juega un papel fundamental en el logro y mejora de estas
metas ambientales y económicas. La necesidad de desarrollar nuevos catalizadores,
más eficientes y selectivos, está directamente relacionada con la compatibilidad de los
Capítulo I
29
productos y los procesos químicos, con el medioambiente. La Catálisis representa una
parte muy importante en el desarrollo de nuevos procesos químicos sostenibles a
través de la Química Verde.
Teniendo en cuenta lo anteriormente expuesto, es necesario buscar alternativas
para los procesos de producción y/o limpieza de corrientes de hidrogeno, y el uso de
productos derivados de la biomasa. Se trataría de diferentes procesos catalíticos en los
cuales es indispensable el desarrollo de catalizadores eficientes para cada uno de ellos,
y sin los cuales no sería posible garantizar el éxito de la tecnología basada en el H2 y
las pilas de combustibles.
Dada la diferente naturaleza de estos procesos catalíticos y la multitud de
condiciones de reacción en que pueden transcurrir, cada uno de ellos requiere
catalizadores con características específicas. En la presente Tesis Doctoral, las
reacciones estudiadas han sido el reformado catalítico de glicerol (tanto en fase gas
como en fase líquida) y la reacción de desplazamiento del gas de agua (WGS). Para
ello, en este estudio se han desarrollado catalizadores de muy variada naturaleza
orientados a la optimización de cada uno de estos procesos. El conjunto de
catalizadores que se irán presentado abarcan desde metales nobles como el Pt, hasta
metales de transiciónmás económicos como el Ni. Adicionalmente, algunos parámetros
fundamentales en Catálisis como, por ejemplo, el método de preparación, la carga
metálica y su dispersión, así como el efecto del soporte, han sido evaluados de distinta
manera según la reacción considerada.
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CHAPTER II
Low temperature glycerol steam
reforming on bimetallic PtSn/C catalysts:
on the effect of the Sn content
GGrraapphhiiccaall aabbssttrraacctt
Chapter II
35
Abstract
Steam reforming of glycerol to produce syngas or H2-enriched streams is a
promising route that has caught the attention of many researchers around the world.
This reaction is typically carried out over metal-based catalysts supported on stable
materials. Herein we report a study on the effect of Sn on the catalytic properties of
Pt/C in the aforementioned reaction. A series of Pt-Sn/C catalysts with different Pt:Sn
ratios were prepared and characterized using ICP, H2-TPR, TEM and XPS techniques,
and they were tested in the glycerol steam reforming reaction at 350 ºC and
atmospheric pressure. The best performance was observed for the catalysts with low
tin contents. It was found that Sn promoted oxidation reactions and inhibited
methanation. Furthermore, the presence of Sn improved the stability of the catalysts
when operating at harsher conditions of temperature and glycerol concentration. A
promoter effect of Sn hindering platinum sintering and the formation of coke
precursors is proposed as the origin of the observed behaviour.
Keywords: Bimetallic catalysts; PtSn/C; Glycerol steam reforming; Deactivation.
Low temperature glycerol steam reforming on bimetallic PtSn/C catalysts: on the effect of the Sn content
36
1. INTRODUCTION
Fossil fuels are the world’s main energy source, but supplies are diminishing while
the consumers’ demand is progressively growing. In this scenario, there is a need to
develop clean and suitable energy alternatives to satisfy the global requirements.
Herein, next-generation biofuels, such as biodiesel, have become the most considered
substitutes for conventional fossil fuels due to technical, economic, and environmental
sustainability.
Biodiesel is comprised of mono-alkyl esters of long-chain fatty acids, which are
derived from vegetable oils or animal fats [1-3]. In the biodiesel synthesis process an
important amount of glycerol, ca. 10 wt%, is obtained as by-product. Although glycerol
is used as raw material in chemical industry, for instance in food, pharmaceutical,
cosmetic, and tobacco industries, the supply of glycerol is larger than the demand due
to the increasing biodiesel production. This over stock of glycerol obligates scientific
researchers to develop processes which transform it into valuable products and, in
this way, to improve the economic viability of the biodiesel production [4,5].
With this perspective, the conversion of glycerol to hydrogen via steam reforming
(GSR: glycerol steam reforming) is an interesting alternative [6]. This reaction is very
attractive since glycerol reforming can be performed using well-known technologies,
conducted at atmospheric pressure, at relatively low temperatures due to the
reactivity of the alcohol, and using conventional fixed-bed reactors [7]. The use of low
temperatures during reforming is very interesting because it allows direct coupling
(in one reactor) with Fischer-Tropsch (FT) reactions [8] or water-gas shift (WGS) [9].
However, glycerol steam reforming involves complex reactions affecting the hydrogen
selectivity and producing high carbon formation rates [10,11].
The glycerol steam reforming reaction can be represented as follows:
C3H8O3 (g) + 3H2O (g) 3CO2 (g) + 7H2 (g)
Thus, it is the combination of two reactions, glycerol decomposition (C3H8O3 (g) →
3CO (g) + 4H2) and water-gas shift (CO (g) + H2O (g) ↔ CO2 (g) + H2 (g)).
Chapter II
37
Many metal catalysts have been reported for the GSR, among which Ru, Rh, Ir, Pd,
Pt, Co and mainly Ni are the most representative ones, being the Pt-based catalysts
among the most effective for its efficient C-C, O-H and C-H bond cleavages with high
activity and selectivity levels [12-14]. The use of bimetallic formulations has been
considered as a strategy to enhance the catalytic performance of Pt based catalysts. It
is well-established that the second metal may influence the first one through
electronic interactions and/or by modifying the morphology of the active sites. Among
the metallic promoters, Sn is one of the most preferable options. Sn addition as
promoter of catalytic activity and selectivity has been extensively studied in reactions
such as hydrogenations in the field of fine chemistry, alkane dehydrogenation and
methane reforming. In all the cases it has been observed that Sn addition improves
significantly the catalyst activity, selectivity and/or stability [15, 16].
The nature of the support influences the catalytic performance of the catalysts in
steam reforming reactions. Catalysts with Pt supported on Al2O3, ZrO2, CeO2/ZrO2,
MgO/ZrO2 and carbon have been studied, and it was found that the oxide-supported
systems are deactivated during the reaction, while carbon-supported catalysts showed
a stable conversion of glycerol to synthesis gas for at least 30 hours [6]. High levels of
unsaturated hydrocarbons such as ethylene were found in the gas stream of the
catalyst suffering a more rapid deactivation. These unsaturated compounds, formed
by dehydration reactions on the substrate, are precursors in the formation of coke,
and may explain the observed deactivation [17]. Due to its relatively inert chemical
nature, carbon does not catalyse such dehydration reactions and, therefore, it exhibits
an excellent performance in the reforming of aqueous glycerol solutions. Furthermore,
this material has excellent stability under hydrothermal conditions (moderate
temperatures and high concentrations of water) [6,18].
In this paper, a series of Pt-Sn/C catalysts with different Sn contents were
evaluated in the glycerol steam reforming reaction, in order to determine the effect of
tin content on the activity, selectivity and stability of carbon-supported Pt catalysts.
Low temperature glycerol steam reforming on bimetallic PtSn/C catalysts: on the effect of the Sn content
38
2. EXPERIMENTAL SECTION
2.1 Catalysts preparation
A commercial activated carbon (RGC-30) with a high BET surface area (1598 m2/g)
was used as support. The monometallic catalyst, Pt/C, was prepared by impregnating
the dry support (110 ºC, overnight) with an acetone solution (10mL per gram of
support) of H2PtCl6·6H2O (Sigma-Aldrich 99.5 %) with the appropriate concentration
to obtain a Pt loading of 5 wt%. After stirring for 12 h, the excess of solvent was
slowly evaporated at 40 ºC under vacuum in a rotary evaporator. Then, the sample
was dried at 110 ºC until complete removal of the solvent.
Bimetallic PtSn catalysts were prepared by sequential impregnation. Tin addition
to the dried Pt/C sample was carried out using the proper amount of SnCl2 (99 %,
Sigma-Aldrich) dissolved in acetone (10 mL of solution per gram of solid) to obtain
Pt:Sn atomic ratios of 50:1, 10:1, 1:1, 1:5. After stirring for 12 h, the solvent was
removed under vacuum at 40 ºC. In this way five catalysts were prepared, which were
labelled as Pt/C, Pt-Sn/C (50:1), Pt-Sn/C (10:1), Pt-Sn/C (1:1) and Pt-Sn/C (5:1).
2.2 Catalysts characterization
The Pt content of Pt/C and Pt-Sn/C catalysts was determined by burning off the
catalysts in air at 800 ºC and analysing the residue (dissolved in aqua regia) by ICP-
OES (Perkin Elmer, Optima 4300 DV).
Temperature-programmed reduction (TPR) with H2 measurements were carried
out on the fresh (dried) catalysts in a U-shaped quartz cell using a 5% H2/He gas flow
of 50 mL/min, with a heating rate of 10 ºC/min. Before the TPR run, the catalyst were
pre-treated with flowing He (50 mL/min) at 150 ºC for 1 h. Hydrogen consumption was
followed by on-line mass spectrometry (Pfeiffer, OmniStar GSD 301).
X-Ray photoelectron spectroscopy (XPS) analyses were performed with a VG-
Microtech Multilab 3000 spectrometer equipped with a hemispherical electron
analyser and a Mg-Kα (h = 1253.6 eV; 1 eV = 1.6302·10-19 J) 300 W X-ray source. The
powder samples were pressed into small Inox cylinders. Before recording the spectra,
Chapter II
39
the samples were maintained in the analysis chamber until a residual pressure of ca.
5 × 10−7 N/m2 was reached. The spectra were collected at pass energy of 50 eV. The
intensities were estimated by calculating the integral of each peak, after subtracting
the S-shaped background, and by fitting the experimental curve to a combination of
Lorentzian (30 %) and Gaussian (70 %) lines. The binding energy (BE) of the C l s
peak of the support at 284.6 eV was taken as an internal standard. The accuracy of
the BE values is ± 0.2 eV. Samples were reduced “ex-situ” in flowing H2 for 2 h at 250
ºC and conserved in octane before the analysis.
Conventional TEM analysis was carried out with a JOEL model JEM-210 electron
microscope working at 200 kV and equipped with a INCA Energy TEM 100 analytical
system and a SIS MegaView II camera. Samples for analysis were suspended in
methanol and placed on copper grids with a holey-carbon film support. Catalysts were
analysed before (samples reduced at 250 ºC, 2h) and after being used in the glycerol
steam reforming reaction.
2.3 Catalytic tests
The catalytic behaviour of the prepared catalysts in the glycerol steam reforming
reaction was evaluated under mild and harder reaction conditions in a fixed bed
reactor (Microactivity Refererence). Prior to the activity test, catalysts were in-situ
reduced under 50 mL/min of H2 at 250 ºC during 2 h. Then, the H2 stream was
changed to He, and the temperature was risen up to that of the reaction test, 350 or
400 ºC. The reaction was carried out at atmospheric pressure, with a feeding (0.05
mL/min) containing 10 or 30 % w/w glycerol in water. This 10 % w/w glycerol feed
composition is similar to that of the glycerol residue obtained from the biodiesel
production process after alcohol removal and acid neutralization of the glycerol
fraction. Activity tests were performed using 0.200 g of catalyst diluted with SiC, to
avoid thermal effects. The composition of the gas stream exiting the reactor was
determined by gas chromatography (Agilent Technologies), with two columns
(Carboxen-1000 and Porapak-Q) and two detectors (FID and TCD).
The catalytic performance was evaluated in terms of conversion into gaseous
products (based on a carbon balance between the inlet and the outlet of the reactor),
Low temperature glycerol steam reforming on bimetallic PtSn/C catalysts: on the effect of the Sn content
40
selectivity to main reaction products (where “i” is CO2, CO and CH4) and also
hydrogen yield, which were defined as:
3. RESULTS AND DISCUSSION
3.1 Catalysts characterization
3.1.1 Chemical analysis
Table 1 presents the results of ICP analysis of the Pt content of the different
catalysts. It can be seen that the actual metal loading is very close to the nominal one
in all cases, this evidencing the efficiency of the impregnation method. Unfortunately,
the analysis method used did not allow for the determination of the Sn content due to
the volatility of the SnCl4 formed during the digestion process. Thus, these values were
estimated by XPS analysis.
% Conversion =C in the gas products
C fed into reactor· 100
% “i” selectivity =“i” produced experimentally
C atoms in the gas products· 100
% H2 Yield =H2 produced experimentally
H2 calculated according to Eq (1)· 100
Chapter II
41
Table 1. Atomic ratios and binding energies of the Pt 4f7/2 and Sn 3d5/2 levels in catalysts
reduced at 250 ºC, and Pt loading.
* Determined by ICP analysis.
3.1.2 X-ray photoelectron spectra (XPS)
The binding energies of the Pt 4f7/2 and Sn 3d5/2 levels for the catalysts reduced in
flowing hydrogen at 250 ºC for 2 h are reported in Table 1. XPS results show that Pt is
completely reduced in all the catalysts, as the spectra show only one peak centred at
71.3 eV, which is assigned to metallic Pt. It is noteworthy to mention that a small
shift to lower binding energies can be observed in the PtSn/C catalysts compared to
the monometallic Pt/C catalyst. This shift may indicate the formation of Pt-Sn alloy
phases, as the differences in electronegativity of Pt and Sn could lead to charge
transfer from the less-electronegative Sn to the more-electronegative Pt [19,20]. In
fact, the analysis of the Sn 3d5/2 level indicates the presence of two contributions
(Table 1). The first one, centred at a binding energy around 486.2-486.4 eV, is
assigned to oxidized tin species, Sn(II, IV), and the second one, at around 485.2 eV,
corresponds to metallic tin (Sn(0)) [21]. Thus, the presence of metallic tin in the
catalysts reduced at 250 ºC opens up the possibility for the existence of Pt-Sn alloy
phases after this treatment, although this cannot be readily assessed by XPS [22,23].
It has also to be taken into account that the use of a relatively inert support such as
carbon decreases the possibility of a strong interaction between the tin precursor and
the support, this facilitating the Pt-Sn interaction and the formation of Pt-Sn alloy
phases [24]. Results obtained for the Pt/Sn atomic ratios are also presented in Table 1.
They confirm that the intended amount of tin has been deposited.
Catalyst Binding energies (eV)
Sn(0)/[Sn(II,IV)+Sn(0)] Pt/Sn (at/at) *Pt (wt%)
Pt 4f7/2 Sn 3d5/2
Pt/C 71.4 -- -- 4.8
Pt-Sn/C (50:1) 71.3 485.2 - 486.4 0.048 46.3 4.9
Pt-Sn/C (10:1) 71.3 485.2 - 486.3 0.107 11.4 4.8
Pt-Sn/C (1:1) 71.3 485.2 - 486.2 0.086 0.87 4.6
Pt-Sn/C (1:5) 71.2 485.2 - 486.3 0.074 0.28 4.7
Low temperature glycerol steam reforming on bimetallic PtSn/C catalysts: on the effect of the Sn content
42
3.1.3 Transmission electron microscopy (TEM)
Figure 1 shows the TEM images of all samples, before and after being used in the
glycerol steam reforming reaction (350 ºC, 1 atm, 10 % w/w glycerol). It can be seen
that before the reaction, (left column) platinum and tin particles are well dispersed on
the carbon support, with a homogeneous distribution of the active phase over the
catalyst’s surface and no agglomerations being observed. It is important to point out
that it was not possible to distinguish between Pt and Sn, as both species appear in
the images as dark dots.
Figure 1. TEM images before (left column) and after glycerol steam reforming (right
column) for a), b) Pt/C; c), d) Pt-Sn/C (50:1); e), f) Pt-Sn/C (10:1); g), h) Pt-Sn/C (1:1) and i), j)
Pt-Sn/C (5:1) catalysts. (Scale 5 nm).
Chapter II
43
TEM micrographs of all catalyst were also taken after reaction (right column) and
some agglomerations can be observed in this case. It can be seen that a considerable
sintering of the metal particles has been produced to different extents depending on
the sample. In this way, it is noteworthy to note that agglomerations in the tin-
containing catalysts are less evident than in the monometallic Pt/C catalyst, and a
lesser amount of agglomerates are formed when the amount of tin in the catalyst is
increased. This indicates that Sn provides a stabilizing effect on the Pt particles
avoiding their sintering, as it has also been observed in other works. [15,25]. Under
these conditions, the presence of coke is not observed. It is well known that Sn
inhibits the reactions leading to the formation of coke precursors, and improves in
this way the stability of the catalysts under reaction conditions [24].
3.1.4 Temperature programmed reduction
Figure 2 shows the TPR profiles of the carbon-supported Pt-Sn catalysts after the
drying process. The TPR profile of the carbon support only shows one H2 consumption
peak at high temperature (530-830 ºC). This peak appears in all the catalysts, and it
has been attributed to the reaction of H2 with reactive surface sites created by
decomposition of surface functional groups (mainly those that evolve as CO) on the
support [26]. For the monometallic Pt/C catalyst three H2 consumption peaks can be
clearly observed. The first one (150-250 ºC) is assigned to the reduction of the
impregnated metal chloride complex to form metallic Pt particles; the second one, at
intermediate temperatures (250-350 ºC), can be attributed to H2 consumption by
oxygen surface functionalities present on the support. Hydrogen which is previously
chemisorbed and dissociated on Pt particles is transported to the carbon surface by
spillover, where it reacts with the oxygen groups located at the metal-carbon interface
[27]. The third peak at higher temperatures (500-700 ºC) is attributed to hydrogen
consumption by the reduction of oxygen superficial groups on the carbon support that
are not located near the metal particles.
Low temperature glycerol steam reforming on bimetallic PtSn/C catalysts: on the effect of the Sn content
44
Figure 2. H2-TPR profiles for the fresh dried catalysts.
On the other hand, the H2-TPR profiles for the bimetallic catalysts differ as the
amount of tin is increased. The peak at low temperatures becomes more pronounced
with the increase of the amount of tin. The reduction profile of Pt-Sn/C (50:1) is very
similar to that of the Pt/C catalyst. The increase of the amount of Sn produces a more
intense peak due to the reduction of both platinum and tin precursors, this revealing
the close proximity and strong interaction between both species. Thus, tin is reduced
at lower temperature due to the presence of platinum in the catalysts [28,29]. The
observed reduction process at intermediate temperatures can be attributed to the
reduction of oxidised Sn species (Sn(II) and/or Sn(IV)) that are not in intimate contact
with Pt. Interestingly, no high temperature reduction peaks are observed in these
profiles, what can be due to the partial blockage/decomposition of the oxygen surface
groups during the impregnation with the tin precursor and drying.
Based on the reduction profiles shown, and considering that the reduction
treatment applied previously to the reforming reaction is carried out at 250 ºC, we can
assume that the bimetallic catalysts are, before reaction, composed of metallic Pt, a
fraction of oxidized tin species (Sn (II)/Sn (IV)) and metallic Sn, likely forming alloy
phases with Pt. This conclusion is also supported by XPS data (Table 1).
3.2 Catalytic behaviour
The catalytic behaviour of the prepared catalysts in terms of gas phase conversion
as a function of time on stream, after being reduced at 250 ºC, is reported in Figure 3.
0 100 200 300 400 500 600 700 800 900 1000
Inte
nsity
(a.
u.)
Pt/C
Temperature (ºC)
Pt-Sn/C (50:1)
Pt-Sn/C (10:1)
Pt-Sn/C (1:1)
Pt-Sn/C (1:5)
Carbon
Chapter II
45
The steam reforming of glycerol was performed at atmospheric pressure, 350 ºC and
0.05 mL/min of a 10 % w/w of aqueous glycerol solution. The gas phase analysis was
carried out every 30 min by on line chromatography. A blank run without catalyst
was also performed, and negligible glycerol conversion was obtained.
Figure 3. Gas phase conversion vs. time on stream of the different catalysts in glycerol
steam reforming at 350 ºC.
The gas phase conversions, after an initial increase during the first 2-3 h on
stream, were stable during the period of time studied (7.5 h). For the monometallic
catalyst, Pt/C, and the bimetallic catalysts with low tin content (Pt-Sn/C (50:1) and
Pt-Sn/C (10:1)) the conversion was complete, with all the glycerol solution being
transformed to gaseous products. A decrease of the gas phase conversion was observed
with the increase of the tin content in the samples. For Pt-Sn/C (1:1) the conversion
was about 78 %, and for the catalyst with the highest Sn content (Pt-Sn/C (1:5)) the
conversion was very low, about 10 %. This low conversion may be due to excessive
blocking of the active sites of Pt by the large amount of Sn present. In fact, a certain
free adjacent active sites of Pt are needed to adsorb and activate the glycerol molecule
(cleavage of C-C bond) so that it can be reformed [30].
The composition of the gas phase product stream is reported in Table 2. These
results were obtained when the conversion reached a stable value, after 5 h on
stream. It can be observed in Table 2 that the H2/CO molar ratio is close to 1 for the
monometallic catalyst and for the bimetallic catalysts with low Sn contents. For the
samples with higher Sn content an increase of this ratio was observed, reaching a
0 1 2 3 4 5 6 7 8
0
20
40
60
80
100
Gas
pha
se c
onve
rsio
n (%
)
Time (h)
Pt/C
Pt-Sn/C (50:1)
Pt-Sn/C (10:1)
Pt-Sn/C (1:1)
Pt-Sn/C (1:5)
Low temperature glycerol steam reforming on bimetallic PtSn/C catalysts: on the effect of the Sn content
46
value of 3.56 for the Pt-Sn/C (1:5) catalyst. Furthermore, the CO/CO2 molar ratio
tends to decrease with increasing the amount of tin in the catalysts.
Table 2. Phase gas composition (molar ratios) and conversion in glycerol steam reforming at
350 ºC for all samples.
Catalyst H2/CO CO/CO2 CH4/H2 Gas phase conv. (%)
Pt/C 0.97 3.46 0.09 100
Pt-Sn/C (50:1) 1.00 3.10 0.08 98
Pt-Sn/C (10:1) 0.98 2.60 0.12 100
Pt-Sn/C (1:1) 1.28 2.78 0.16 78
Pt-Sn/C (1:5) 3.56 0.13 0.01 9
These results point to an enhancement of the water-gas shift reaction, which would
be favoured by tin species. There are few reports in the literature on the role of Sn in
the water-gas shift reaction. Recently, Gupta and Hegde reported the catalytic
behaviour of a Ce0.78Sn0.2Pt0.02O2-δ catalyst, which was able to convert over 99.5 % CO
to H2 at 300 ºC [31]. In this case, the role of tin species was claimed to be the
stabilization of the catalyst surface against the formation of deactivating carbonate
species. On the other hand, Azzam et al. studied the role of the support in Pt-based
catalysts for water gas shift [32], including a titania-supported bimetallic 0.5%Pt-
0.3%Sn catalyst. Although it was less active than other catalysts studied, such as Pt-
Re/TiO2, it showed a high catalytic stability at 300 ºC.
Values for the CH4/H2 molar ratio in Table 2 show that methane formation is very
low for all catalysts. In fact, nearly no methane was produced by the catalyst with the
highest amount of tin, Pt-Sn/C (1:5). The low methane formation is indicative of the
role of tin in inhibiting hydrogenation reactions between CO/CO2 and the H2 formed
[15].
The H2, CO, CO2 and CH4 selectivities (after 5 hours of reaction, time when
catalytic activity is stable) are presented in Figure 4. The effect of tin is clearly
observed. The CO2 selectivity slightly increases with the amount of Sn while the CO
selectivity decreases. This change is more pronounced with the Pt-Sn/C (1:5) catalyst,
so the Sn addition in large amounts promotes the CO oxidation producing H2-rich gas
streams and not streams for syngas. Furthermore the H2 selectivity increases with Sn
Chapter II
47
addition except for the catalyst with the highest amount of Sn, but this could be
explained as due to the low conversion to gas phase and the possible H2 consumption
in the formation of liquid products.
Figure 4. Selectivity for the gaseous products in the GSR at 350 ºC.
Finally, considering the above results a stability test (not shown) was carried out
with the Pt/C and Pt-Sn/C (1:1) catalysts, in order to determining the effect of tin
during longer reaction (24 hours) in the same temperature conditions (350 ºC). In both
cases, the conversion to gas phase products remained stable during all the
experiment.
In view of these results, it was decided to test these catalysts under harder reaction
conditions: higher temperature (400 ºC) and a more concentrated of glycerol feed
solution (30 % w/w glycerol in water). The results of these experiments are shown in
Figure 5.
Under these conditions the monometallic Pt/C was nearly inactive, producing only
about 5 % conversion in the first hour on stream. In contrast, for the bimetallic
catalyst Pt-Sn/C (1:1) the conversion decreased with respect to reaction under milder
reaction conditions (78 % versus 50 %), but an excellent performance stability, for
almost 10 hours on stream, was obtained under these harder reaction conditions.
0
10
20
30
40
50
60
70
CO2 CO CH4 H2 H2 Yield
Se
lectivity,
%
Pt/C
Pt-Sn/C (50:1)
Pt-Sn/C (10:1)
Pt-Sn/C (1:1)
Pt-Sn/C (1:5)
CO2 CO CH4 H2 H2 Yield
Se
lectivity (
%)
Low temperature glycerol steam reforming on bimetallic PtSn/C catalysts: on the effect of the Sn content
48
Figure 5. Gas phase conversion vs time of reaction for Pt/C and Pt-Sn/C (1:1) catalysts at
400 ºC and feed of 30 % w/w of glycerol.
The formation of coke is favoured at high reaction temperature and with high
glycerol concentration in the feed stream. This fact can explain the fast deactivation of
the Pt/C catalyst, which showed a good activity and stability under milder reaction
conditions. It is well known that Sn is able to inhibit coke formation reactions in
processes such as the dehydrogenation of hydrocarbons [24]. Thus, by electronic
and/or geometric effects Sn is able to modify the catalytic properties of Pt, inhibiting
reactions that form coke precursors and, thereby, improving the stability of the
catalyst. The effect of Sn may be similar in the reforming reaction of glycerol.
Probably a strong Pt-Sn interaction (as indicated by the results of TPR, Figure 2)
inhibits reactions that produce olefins (coke precursors) during glycerol reforming.
Furthermore, Sn has also a textural promoter effect, inhibiting the sintering of Pt
particles (as shown by TEM photographs) at these higher reaction temperature
conditions. Nevertheless, it has to be taken into account that the beneficial effects of
the presence of Sn become detrimental if a too high amount of this promoter is
present, as the amount of active Pt surface sites decrease and, thus, the catalytic
activity.
0 2 4 6 8 10
0
10
20
30
40
50
60
Time (h)
Pt-Sn/C (1:1)
Gas
pha
se c
onve
rsio
n (%
)
Pt/C
Chapter II
49
4. CONCLUSIONS
The effect of Sn addition to Pt/C catalyst with different Pt:Sn ratios was
investigated in the glycerol steam reforming reaction. XPS and TPR-H2 data revealed
the close proximity and strong interaction between Pt and Sn. TEM images showed
that metal agglomerations in the tin-containing catalysts were less evident than in
the monometallic Pt/C catalyst, and a lesser amount of agglomerates were formed
when the amount of tin in the catalyst is increased.
Regarding catalytic tests, a good behaviour in terms of activity and stability was
obtained with bimetallic PtSn/C catalysts with low Sn/Pt ratios, both under mild
reaction conditions (10 wt% glycerol and 350 ºC) and under more severe conditions (30
wt% glycerol and 400 ºC). It was found that Sn promotes the CO oxidation reaction
producing H2-rich gas streams. Furthermore, the H2 selectivity increased with low
Sn/Pt ratios. Sn is also able to inhibit coke formation reactions and hinder Pt
sintering, thereby improving the stability of the catalyst.
7. REFERENCES
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[9]. E.L. Kunkes, R.R. Soares, D.A. Simonetti, J.A. Dumesic, Appl. Catal. B 90
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[10]. P.R. Piscina, N. Homs, Chem. Soc. Rev. 37 (2008) 2459-2467.
[11]. A. Iriondo, V.L. Barrio, J.F. Cambra, P.L Arias, M.B. Güemez, R.M. Navarro RM
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[12]. S. Adhikari, S. Fernando, A. Haryanto, Catal. Today 129 (2007) 355-364.
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[18]. B. Kaya, S. Irmak, A. Hasanoglu, O. Erbatur, Int. J. Hydrogen Energ. 39 (2014)
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82 (2008) 89-102.
[20]. G.J. Siri, J.M. Ramallo-López, M.L. Casell, J.L.G. José Fierro, F.G. Requejo,
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CHAPTER III
Multicomponent NiSnCeO2/C catalysts
for the low-temperature glycerol steam
reforming
GGrraapphhiiccaall aabbssttrraacctt
Chapter III
55
Abstract
In this work, the low-temperature hydrogen production via glycerol steam
reforming over activated carbon-supported Ni and Ni-Sn catalysts promoted by ceria
was studied. A combination of N2 adsorption, powder X-ray diffraction, temperature-
programmed reduction with H2, X-ray photoelectron spectroscopy and TEM analysis
were used to characterize the Ni-Sn-CeO2 interactions and the CeO2 dispersion over
the activated carbon support. The catalytic activity results show that the presence of
ceria enhances the water-gas shift reaction, thus promoting the selectivity towards
hydrogen. The inclusion of Sn stresses the influence of ceria in the displayed
performance. Moreover the formation of a Ni-Sn alloy seems to be an efficient way to
mitigate Ni sintering and therefore to improve the overall catalyst’s stability.
Keywords: H2 production; Nickel-tin; Glycerol; Ceria; Activated carbon.
Multicomponent NiSnCeO2/C catalysts for the low-temperature glycerol steam reforming
56
1. INTRODUCTION
Developing ways to utilize more effectively the abundant and renewable biomass
resources available should be imperative nowadays. This provides new sources of
energy and chemical intermediates, diminishing both the petroleum society
dependence and its consequent global warming effect [1]. The use of biomass as raw
material to produce hydrogen or synthesis gas is an interesting alternative route
because it is renewable and its CO2 emissions are almost neutral [2].
Among the multiple sources of biomass, the low-value glycerol that is produced as a
waste in biodiesel synthesis from the transesterification of vegetable oils and animal
fats is an interesting feedstock to be upgraded [3]. The production levels of biodiesel
are increasing constantly; therefore, finding a profitable outcome to waste glycerol
solutions can lower the manufacture costs of biodiesel [4].
Typically, glycerol is converted to more valuable products through reforming
reactions [5]. In this sense, steam reforming of glycerol is one of the most common
route. However, this process is rather endothermic and normally requires elevated
temperatures in order to obtain high performances. Alternatively, glycerol steam
reforming to produce H2 rich streams or H2/CO mixtures can be performed at
relatively low temperatures over metal-based catalysts in the so called low-
temperature steam reforming. This approach improves the economic viability of the
process, allowing the efficient combination of the endothermic conversions of glycerol
with the exothermic following steps in a hypothetical integrated process (water-gas
shift, Fischer-Tropsch or methanol synthesis). At the same time, the low temperature
steam reforming imposes some challenges in the catalyst design that have to be
covered.
On the other hand, the catalytic conversion of polyols to H2, CO and CO2 involves
the preferential cleavage of C-C bonds as opposed to C-O bonds [1], being Pt-based
catalysts the most effective system for this process. Indeed, we have recently
demonstrated the suitability of monometallic and bimetallic systems based on Pt and
Pt-Sn supported on carbon for glycerol upgrading [6]. In particular, the presence of Sn
benefits the catalyst’s stability allowing higher glycerol conversions during long term
periods. Furthermore, carbon supports seem to be a good choice since they are not
Chapter III
57
able to catalyse dehydration reactions, due to their relatively inert chemical nature
under reaction conditions, while exhibiting an excellent performance in the reforming
of aqueous glycerol solutions when combined with a metallic active phase [7, 8].
Although Pt-based catalysts are more active in terms of specific activity, we
propose in this paper Ni as a potential candidate to replace Pt from the bimetallic
metal-Sn ensemble, since this transition metal represents a rather cheaper
alternative with good skills for reforming processes [9]. Furthermore, addition of Sn is
reported to avoid alkane formation by the methanation reaction over metallic Ni.
However, the catalytic activity of Ni-based catalysts is not modified by adding Sn
[10,11]. On the other hand, the formation of nickel carbide has been proposed as the
initial step of coke formation. The similar electronic structure of carbon and elements
of groups IV and V (Ge, Sn, Pb, etc.) may favour the interaction of these metals (free p
electrons) with Ni 3d electrons, thereby reducing the chance of nickel carbide
formation [12, 13].
And not least, the presence of an active support/promoter as ceria provides several
interesting advantages for a reforming catalyst. Its well-known activity in the WGS
reaction enriches hydrogen concentration in the reformate gas and reduces carbon
monoxide formation [14]. In addition, the excellent redox properties of this rare earth
oxide improve the redox reversibility of the metallic phase [15], and the basic
character of ceria mitigates coke formation, which is usually due to dehydration
reaction taking place in the catalyst´s acid sites [16]. Moreover, ceria dispersion on a
high surface carrier as activated carbon potentiates its catalytic skills, allowing to
obtain smaller oxide nanoparticles and higher oxygen mobility while reducing the
metal oxide spending [17].
In this way, a series of multicomponent NiSn/CeO2/C catalysts for hydrogen
production via glycerol reforming is proposed. One of the aims of this work is to
prepare optimized Ni-CeO2 catalysts, reducing the amount of ceria, in which the
interaction between the metal and the oxide is enhanced, thus improving the catalytic
activity. The reaction is carried out at relatively low temperatures and the catalytic
performance is compared to that of Sn-free NiCeO2/C systems. The effect of Sn on the
activity, stability and physicochemical properties of the catalysts is also a subject of
this study.
Multicomponent NiSnCeO2/C catalysts for the low-temperature glycerol steam reforming
58
2. EXPERIMENTAL SECTION
2.1 Catalysts preparation
The procedure used for the synthesis of the catalysts was similar to that reported
in our previous work [18]. The support was an industrial activated carbon (RGC30,
from Westvaco). This carbon was grinded and meshed (300-500 μm). The
corresponding amount of Ce(NO3)3·6H2O (99.99 %, Sigma–Aldrich) to obtain 20, 30
and 40 wt.% of CeO2 was dissolved in acetone. Dried carbon was added to the solution,
in a proportion of 10 mL/gsupport, with stirring. After 12 h, the excess of solvent was
slowly removed under vacuum at 40 ºC, and the solid was then dried in the oven
overnight. Finally, the dried solid was heat treated during 4 h at 350 ºC under flowing
He (50 mL/min), with a heating rate of 1 ºC/min, in order to slowly decompose the
cerium nitrate to form CeO2, and trying to avoid the modification of the carbon
surface by the evolved nitrogen oxides [19].Three CeO2/C samples were prepared with
different nominal CeO2 loadings: 20, 30 and 40 wt.%.
Nickel addition to the CeO2/C solid was carried out using the proper amount of
Ni(NO3)2·6H2O (99.9 %, Sigma-Aldrich) in acetone to obtain 15 wt.% Ni, using 10 mL
of solution per gram of solid. After stirring for 12 h, the solvent was removed under
vacuum at 40 ºC. Finally the solid was treated at 350 ºC for 4 h under flowing He (50
mL/min). For the sake of comparison, Ni/C and Ni/CeO2 catalysts were also
synthesized. The ceria support was prepared by homogeneous precipitation from an
aqueous solution of Ce(NO3)3·6H2O (99.99 %, Sigma-Aldrich) containing an excess of
urea. The solution was heated at 80 ºC and kept at this temperature, with slow
stirring, during 12 h. The solid formed was filtered and calcined at 350 ºC for 4 h. The
CeO2 support prepared in this way was impregnated with the Ni precursor as
described for the carbon supported catalysts. In this way, five samples were prepared,
which were labelled as Ni/C, Ni20CeO2/C, Ni30CeO2/C, Ni40CeO2/C and Ni/CeO2.
In addition, Ni-Sn catalysts were prepared by sequential impregnation. Tin
addition to the dried NiCeO2/C samples was carried out using the proper amount of
SnCl2 (99 %, Sigma-Aldrich) dissolved in acetone (10 mL of solution per gram of solid)
to obtain catalysts with a Ni:Sn atomic ratio of 10:1. After stirring for 12 h, the
solvent was removed under vacuum at 40 ºC. Five tin-containing catalysts were
Chapter III
59
prepared, which were labelled as NiSn/C, NiSn20CeO2/C, NiSn30CeO2/C,
NiSn40CeO2/C and NiSn/CeO2.
2.2 Catalysts characterization
The textural properties of the supports were characterized by nitrogen
adsorption measurements at -196 °C. Gas adsorption experiments were performed in
a home-made fully automated manometric equipment. Prior to the adsorption
experiments, samples were out-gassed under vacuum (10-4 Pa) at 150 °C for 4 h. The
specific surface area was estimated after application of the BET equation.
X-Ray powder diffraction patterns were recorded on a Bruker D8-Advance with
a Göebel mirror and a Kristalloflex K 760-80 F X-Ray generation system, fitted with a
Cu cathode and a Ni filter. Spectra were registered between 20 and 80° (2θ) with a
step of 0.05° and a time per step of 3 seconds.
Temperature-programmed reduction (TPR) with H2 measurements were
carried out with the calcined catalysts in a U-shaped quartz cell using a 5 % H2/He
gas flow of 50 mL/min, with a heating rate of 10 °C/min. Hydrogen consumption was
followed by on-line mass spectrometry (Pfeiffer, OmniStar GSD 301).
X-Ray photoelectron spectroscopy (XPS) analyses were performed with a VG
Microtech Multilab 3000 spectrometer equipped with a hemispherical electron
analyzer and a Mg-Kα (h = 1253.6 eV; 1 eV = 1.6302·10-19 J) 300-W X-ray source. The
powder samples were pressed into small Inox cylinders. Before recording the spectra,
the samples were maintained in the analysis chamber until a residual pressure of ca.
5·10−7 N·m-2 was reached. The spectra were collected at pass energy of 50 eV. The
intensities were estimated by calculating the integral of each peak, after subtracting
the S-shaped background, and by fitting the experimental curve to a combination of
Lorentzian (30 %) and Gaussian (70 %) lines. The binding energy (BE) of the C l s
peak of the support at 284.9 eV was taken as an internal standard. The accuracy of
the BE values is ± 0.2 eV.
Multicomponent NiSnCeO2/C catalysts for the low-temperature glycerol steam reforming
60
2.3 Catalytic tests
The catalytic behaviour of the prepared catalysts in the glycerol steam reforming
reaction was evaluated under mild reaction conditions in a fixed bed reactor
(Microactivity Refererence). Prior to the activity tests, catalysts were in-situ reduced
under 50mL/min of H2 at 350 ºC during 2 h. The reaction was carried out during 30 h,
at atmospheric pressure and 350 ºC, with a feeding (0.05 mL/min) containing 10 w/w
glycerol in water. Activity tests were performed using 0.200 g of catalyst diluted with
SiC, to avoid thermal effects. The composition of the gas stream exiting the reactor
was determined by gas chromatography (Agilent Technologies), with two columns
(Carboxen-1000 and Porapak-Q) and two detectors (FID and TCD).
The catalytic performance was evaluated in terms of conversion into gaseous
products (based on a carbon balance between the inlet and the outlet of the reactor),
selectivity to main reaction products (where ―i‖ is CO2, CO and CH4) and also
hydrogen yield, which were defined as:
% Conversion =C in the gas products
C fed into reactor· 100
% “i” selectivity =“i” produced experimentally
C atoms in the gas products· 100
% H2 Yield =H2 produced experimentally
H2 calculated according to Eq (1)· 100
Chapter III
61
3. RESULTS AND DISCUSSION
3.1 Catalysts characterization
3.1.1 Textural properties
Table 1 shows the specific surface area (N2, -196 °C, BET), the micropore volume
(Vmicro, N2, -196 °C, D-R) and the volume of mesopores (Vmeso) for the prepared samples
and the parent carbon support. The textural parameters of bulk ceria are also
included for sake of comparison.
Table 1.Textural properties of supports and catalysts.
Samples SBET(m2/g) Vmicro(cm3/g) Vmeso(cm3/g)
C 1487 0.52 0.62
CeO2 101 0.04 0.07
Ni/C 1238 0.45 0.52
Ni20CeO2/C 807 0.28 0.34
Ni30CeO2/C 677 0.23 0.28
Ni40CeO2/C 512 0.19 0.21
Ni/CeO2 70 0.03 0.04
NiSn/C 1228 0.41 0.52
NiSn20CeO2/C 800 0.25 0.35
NiSn30CeO2/C 676 0.23 0.29
NiSn40CeO2/C 502 0.17 0.20
NiSn/CeO2 69 0.02 0.03
The N2 adsorption isotherms at -196 °C for all the carbon-based materials (not
shown) correspond to a combination of Type I and Type IV isotherms, which are
characteristic of materials containing both micro- and mesopores. In fact, the
mesoporous volume is always slightly larger than the microporous one. Data in Table
1 show a continuous decrease of the BET surface area for the Ni-ceria-loaded carbons
as the amount of ceria increases, and the same trend is observed for both the
micropore and the mesopore volumes. This effect has been previously reported [19]
and it is attributed to the blockage of porosity by Ni and ceria crystallites to a certain
extent. It has also to be taken into consideration that ceria addition decreases the
Multicomponent NiSnCeO2/C catalysts for the low-temperature glycerol steam reforming
62
textural parameters of these materials, as compared with those of the parent Ni-
carbon, as an effect of the increase of mass and the much lower porosity of ceria as
compared with carbon.
It has to be noted the much lower surface area of the ceria support and of the ceria-
supported nickel catalyst as compared to that their carbon-supported counterparts.
The impregnation stage with the tin precursor also produces a smooth decrease in
the BET surface area of the catalysts, which is also attributed to presence of Sn in low
concentration.
3.1.2 H2-temperature-programmed reduction
Temperature programmed reduction studies were conducted in order to assess the
redox properties of the catalysts and to analyze the interaction between the different
metallic species and the support. Figure 1 shows the H2-TRP profiles of the prepared
materials. The profile obtained with the Ni/C catalyst presents two peaks at low and
medium temperatures, respectively. The first peak, centred at 240 ºC, is ascribed to
the reduction of small and well dispersed nickel oxide particles, while the peak at
higher temperature accounts for the reduction of medium size NiOx species, as
described elsewhere [18]. For the Ni/CeO2 catalyst two reduction zones were observed.
The low temperature one, between 150 and 400 ºC, is related to the reduction of NiOx
particles in contact with ceria and also to the surface reduction of ceria, while the
peak at high temperatures (700-900 ºC) is due to the bulk ceria reduction normally
observed in this temperature range [20]. The ternary NiCeO2/C systems show
intermediate redox behaviour with both NiOx and CeO2 contributions. More
specifically, three reduction processes were observed in the H2-TPR profiles for
NixCeO2/C. The first low temperature peak is attributed to the reduction of highly
dispersed NiO on the support, and the intermediate ones correspond to the surface
reduction of ceria with different degree of interaction with nickel particles. Several
types of Ni species coexist depending of the Ni/Ce ratios. In addition, for solids with
higher ceria loadings, Ni-Ce solid solution and well dispersed NiO phase may coexist,
as has been reported before [21,22]
Chapter III
63
Figure 1. H2-TPR profiles for NiCeO2/C (—) and NiSnCeO2/C (---) catalysts.
The addition of Sn provokes notable changes in the TPR profiles. In principle, one
should expect an enhanced reducibility due to the electronic Ni-Sn interaction.
Indeed, Ni is a more electronegative element than Sn (1.9 vs. 1.5 in Pauling scale) and
thus, the electronic transfer from Sn to Ni is plausible and should facilitate Ni
reducibility. However the situation is much more complex due to the heterogeneity of
the studied catalysts. For instance, the profile of Ni/C shows two peaks as commented
above due to two types of Ni particles with different size. Sn addition makes the
reduction process more homogeneous, merging both reduction peaks in a broader
reduction zone centred at 350 ºC. This effect is somewhat repeated along the whole
series of catalysts. Basically all the Sn-promoted catalysts show only one reduction
zone involving the reduction of CeO2, NiO and SnOx together. This fact may indicate
the role of Sn as an electronic bridge between Ni and Ce, favouring the electronic
transfer between both species, this resulting in a more homogenous reduction profiles.
3.1.3 XPS characterization of reduced catalysts
Further insights on the chemical modifications of the catalysts after reduction in
H2 at 350 ºC were investigated by XPS. The samples were reduced ex-situ and
conserved in octane until the analysis. The binding energies (B.E.) of the main peaks
of the Ni 2p3/2 and Sn 3d5/2 energy levels are summarized in Table 2. In the absence of
NiSn/CeO2
Ni20CeO2C
NiSn20CeO2/C
NiSn30CeO2/C
NiSn40CeO2/C
0 100 200 300 400 500 600 700 800 900 1000
Inte
nsity
(a.
u.)
Ni/C
Ni20CeO2/C
Ni30CeO2/C
Ni40CeO2/C
Temperature (ºC)
Ni/CeO2
Ni/C
NiSn/C
Ni/C
NiSn/C
Ni20CeO2/C
NiSn20CeO2/C
Ni30CeO2/C
NiSn30CeO2/C
Ni40CeO2/C
NiSn40CeO2/C
Ni/CeO2
NiSn/CeO2
Multicomponent NiSnCeO2/C catalysts for the low-temperature glycerol steam reforming
64
tin, the reduced catalysts exhibited complex Ni 2p3/2 spectra, pointing to the
presence of different nickel species. For instance, the band at ca. 852.6 eV is assigned
to metallic Ni0 species, while the bands at about 854.7 and 856 eV are ascribed to NiO
and Ni(OH)2 species, respectively [23,24]. The presence of ceria shifts the Ni2+ bands
towards higher binding energies, what indicates a strong Ni-CeO2 interaction in good
agreement with the TPR data. This interaction may modify the structure and
electronic properties of Ni, this enhancing the catalytic behaviour of these materials
[25]. Furthermore, such a strong interaction depends also on the ceria particle size
and dispersion, as deduced from the different position of the bands in the carbon-
supported catalysts as compared to those of Ni supported on bulk ceria. In any case, it
should be mentioned that Ni was not completely reduced in any of the samples after
the reduction treatment.
Table 2. Binding energies of the Ni 2p3/2 and Sn 3d5/2 levels in catalysts reduced at 350 ºC.
1 values in parenthesis indicate the percentage of metallic Ni and Sn calculated from XPS
areas
The situation is more complex for the tin containing samples. Table 2 summarizes
the positions (binding energies) of the Sn 3d5/2 XPS bands observed for the Sn-doped
samples. This band is constituted by two contributions. The peak centred at ca. 485
Catalysts Ni 2p3/2 (eV) Sn 3d5/2 (eV)
Ni2+
Ni0 Sn
2+,4+ Sn
0
Ni/C 854.1 - 856.3 852.3 (38)1 -- --
NiSn/C 854.1 - 856.2 852.8 (38) 486.4 484.9 (27)1
Ni20CeO2/C 854.8 - 856.9 852.8 (33) -- --
NiSn20CeO2/C 854.8 - 856.6 852.9 (49) 486.4 485.0 (49)
Ni30CeO2/C 854.2 - 856.5 852.6 (35) -- --
NiSn30CeO2/C 854.7 - 856.7 852.7 (55) 486.4 485.0 (33)
Ni40CeO2/C 854.7 - 856.8 852.7 (35) -- --
NiSn40CeO2/C 855.2 - 857.2 853.0 (50) 486.7 485.6 (41)
Ni/CeO2 855.4 - 857.6 853.6 (35) -- --
NiSn/CeO2 854.7 - 857.6 852.6 (51) 486.4 484.9 (16.5)
Chapter III
65
eV is attributed to Sn0 while the peak centred at 486.4 eV is related to oxidized tin
species [11]. It is worth mentioning that the presence of metallic Sn can be associated
to the formation of a Ni-Sn alloy, as pure tin oxide is known to reduce only at above
527 ºC [26]. Indeed, Shabaker et al. demonstrated, by using Mossbauer spectroscopy,
the presence of Ni3Sn after a reduction treatment similar to that employed this work
[27]. In fact, and according to our Ni/Sn atomic ratio, Ni3Sn is the possible alloy when
the Ni-Sn phase diagram is considered [28].
Furthermore, tin addition also modifies the Ni XPS features. In particular, the
presence of Sn promotes Ni reducibility leading to a higher concentration of metallic
Ni species (calculated values shown in Table 2). This observation agrees with the
redox features discussed in the TPR section, and underlines the intimate Ni-Sn
contact. The presence of reduced Ni can be linked to the above mentioned formation of
a Ni-Sn alloy. However, reduced Ni particles and Ni3Sn species are indistinguishable
by photoelectron spectroscopy. In any case, the Ni-Sn interaction is evident, and
correlates well with the results obtained in the TPR experiments. In fact, this Ni-Sn
contact is also influenced by the presence of ceria and its loading. In this sense, it
must be underlined that 30 wt.% of CeO2 seems to be the optimal loading to provide
an adequate environment facilitating the chemical interactions among the active
phase and the promoters. As shown in Table 2, NiSn30CeO2/C is the catalyst with the
highest concentration of reduced Ni and with a richer proportion of Sn0.
3.2 Catalytic behaviour
The catalytic behaviour of the prepared multicomponent catalysts in terms of gas
phase conversion as a function of time on stream, after being reduced at 350 ºC, is
reported in Figure 2. The steam reforming of glycerol was performed at atmospheric
pressure, 350 ºC and 0.05 mL/min of a 10% w/w of aqueous glycerol solution. The
reaction was carried out for 30 h in order to assess the deactivation process and to
determine the effect of tin during these long reaction runs. The study of the catalytic
stability is, from the industrial application point of view, as relevant as the catalytic
activity. The gas phase analysis was carried out every 30 min by on line
chromatography. A blank run with only the carbon support was also performed, and
negligible glycerol conversion was obtained.
Multicomponent NiSnCeO2/C catalysts for the low-temperature glycerol steam reforming
66
Figure 2. Gas phase conversion vs. time on stream for (A) NiCeO2/C catalysts and (B)
NiSnCeO2/C catalysts in glycerol steam reforming at 350 ºC.
It can be seen in Figure 2 that there is a clear effect of the ceria loading on the
catalytic activity, being the sample containing 30 wt.% CeO2 the most active one
(Figure 2A). Furthermore, in all cases, catalysts with CeO2 were the most active
materials. On the contrary, Ni/C shows a lower activity, this assessing the
determinant role of ceria in this reaction due to its well-known redox properties, and
also its ability to improve the stability and the catalytic performance of these
catalysts by the synergistic effect between the Ni and CeO2.
On the other hand, two different activity windows can be distinguished. Firstly, at
early reaction stages (less than 3 h), all the studied samples displayed good catalytic
activity reaching high glycerol conversions. In this range most of the catalysts exhibit
comparable conversions, with the NiSn30CeO2/C sample outstanding among the
others. This catalyst resulted to be the best one, reaching full alcohol conversion in
the first 5 h on stream. From then on glycerol conversion progressively dropped for all
the studied samples. Due to the complex nature of our catalysts, several factors might
be responsible of the observed deactivation. In the simplest system, Ni/C, deactivation
is mainly due to the sintering of metal particles. The absence of any other component
(apart from the mere support) to stabilize and prevent Ni agglomeration leads to
rapid Ni sintering, what explicates the activity depletion. In addition, C deposition is
favoured in these agglomerated Ni particles, this also contributing to the deactivation.
Figure 3 shows TEM micrographs of the Ni/C catalyst after reaction. It can be seen
that a considerable sintering of the metal particles has taken place (Figure 3A).
Chapter III
67
Furthermore, the possibility of having coke covering the metal particles, although
difficult to assess due to the carbonaceous nature of the carbon support, has also to be
taken into account (Figure 3B). These processes also occurs in all the other catalysts
(see below), but the presence of ceria and tin slows down the sintering and coke
poisoning. Even so, the ceria-supported Ni catalyst also experienced a relatively fast
deactivation. A similarly quick deactivation was found by Dumesic et al. when basic
supports were applied in the low-temperature glycerol reforming [7]. Apparently, the
activity drop is caused by glycerol dehydration on the oxide catalyst support, which
leads to the formation of carbonaceous blocking species that are accumulated on the
surface [7, 29]. Very interestingly, the tin promoted materials seem to be less prone to
deactivation. Despite the fact that the activity falls for all the tin-doped samples after
5 h on stream, this drop is smoother compared to that of the tin-free samples. It seems
that tin allows greater glycerol conversion even when the catalyst is operating under
deactivation conditions. The addition of Sn improves catalytic stability, avoiding Ni
sintering, what is maybe due to the formation of Ni-Sn alloys.
Figure 3. TEM micrographs of the Ni /C catalyst after reaction at 350 °C during 30 h.
The H2/CO, CO/CO2, CH2/H2 molar ratios for the product streams at 25 h on stream
(when the steady state is reached) with NixCeO2/C and NiSnCeO2/C catalysts are
summarized in Table 3. The stoichiometric H2/CO ratio, if all glycerol were converted
into CO and H2 (C3O3H8→3CO + 4H2), is 1.3. In contrast, the H2/CO ratios obtained
over all samples were higher, this indicating a remarkable contribution from the
water-gas shift reaction (CO + H2O H2 + CO2). This behaviour is more clearly
evidenced in the CO/CO2 molar ratio, which is higher for Ni/C and NiSn/C (ceria-free
samples). Therefore, it can be partially concluded that ceria benefits the hydrogen
Multicomponent NiSnCeO2/C catalysts for the low-temperature glycerol steam reforming
68
yield and influences the products distribution since its presence is linked to the WGS
reaction [30].
Table 3. Gas phase composition (molar ratios) and conversion in glycerol steam reforming
at 350 ºC for all samples.
Catalyst H2/CO CO/CO2 CH4/H2 Gas phase conv. (%)
Ni/C 11.6 0.249 0.008 15
Ni20CeO2/C 43.1 0.062 0.012 34
Ni30CeO2/C 34.9 0.054 0.010 45
Ni40CeO2/C 32.1 0.072 0.008 35
Ni/CeO2 32.5 0.074 0.003 38
NiSn/C 5.6 0.849 0.006 18
NiSn20CeO2/C 11.6 0.302 0.009 40
NiSn30CeO2/C 9.4 0.479 0.016 52
NiSn40CeO2/C 7.9 0.294 0.012 41
NiSn/CeO2 12.9 0.173 0 26
The maximum H2/CO ratio was obtained for samples containing 20 wt.% ceria,
suggesting a promoted WGS activity when ceria is dispersed over a high surface area
carbon material, in good agreement with previous results [17]. For the Sn-containing
samples a decrease of this ratio was obtained, which can be due to the presence of tin
species blocking CeO2 active sites. Furthermore, the CO/CO2 molar ratio tends to
increase with the presence of tin in the catalysts, this indicating that Sn slows down
the WGS reaction in the multicomponent catalysts. On the other hand, the
methanation reaction seems to have a minor influence in our process. Values for the
CH4/H2 molar ratio in Table 3 show that methane formation remains low for all
catalysts.
The H2 yield and CO, CO2, CH4, CHx selectivities are presented in Figure 4. For all
the ceria-containing catalysts (Figure 4A) the H2 yield and selectivities to the
different product are rather similar, independently of the ceria content. However,
some pronounced differences were obtained upon tin addition. This change is more
notorious for the NiSn30CeO2/C catalyst, which showed the best catalytic behaviour
in terms of conversion and stability, and also reached a maximum H2 yield of about 48
Chapter III
69
% (Figure 4B). In this respect, the beneficial effect we have observed by adding Sn to
the Ni30CeO2/C sample on the selectivity for H2 production may be caused by the
presence of Sn at Ni-defect sites, and by the formation of Ni-Sn surface alloy species,
such a Ni3Sn alloy, which maintain the high rates of C-C cleavage needed for H2
production [11]. Furthermore, the best catalytic behaviour obtained with the
NiSn30CeO2/C catalyst is in fair agreement with the characterization data discussed
above. This sample is the one with a higher concentration of reduced Ni and with a
richer proportion of Sn0, both metals synergistically interacting and thus favouring
the catalytic performance. For the remaining NiSnxCeO2/C samples a tendency was
observed: CO2 selectivity slightly increased with the amount of ceria while CO
selectivity decreased, an effect that backs up the WGS contribution ascribed to ceria.
Finally, tin addition affected to different extents the catalytic activity of these
samples, blocking part of ceria active sites. Therefore, a larger amount of CeO2 to
increase the water-gas shift contribution is needed.
Figure 4. H2 yield and selectivity for gas products in the GSR at 350 ºC for NiCeO2/C (A)
and NiSnCeO2/C (B) catalysts.
3.3 XRD post-reaction characterization
Post-reaction characterization provides some clues to back up the catalytic activity
discussion. Thus, XRD diffraction patterns of the spent catalysts are presented in
Figure 5. Some remarkable differences can be established between the studied
catalysts. As a general trend, cerium oxide presents smaller particle sizes and higher
amorphous character when supported on carbon. The typical fluorite cell reflexions
are sharper and more intense for the ceria bulk materials compared to ceria
(B) (A)
Multicomponent NiSnCeO2/C catalysts for the low-temperature glycerol steam reforming
70
supported on carbon. The later points to a greater ceria dispersion, which is beneficial
for the reforming reaction and underlines the fact that better catalytic skills can be
achieved with less amount of ceria if this oxide is dispersed in a high surface carrier
as carbon. Similar results were obtained in recent works using ceria-based catalysts
for WGS, with the oxide dispersed on an inert support [18,31]. On the other hand, the
XRD analysis confirms Ni sintering during the reforming reaction, especially for the
NixCeO2/C catalysts (Figure 5A). All these samples present the typical metallic Ni
reflections accounting for the Ni reduction and agglomeration during the process. Ni
particle size is around 19 nm for all the samples presented in Figure 5A, except for
the Ni/C solid, whose Ni particle size is somewhat larger than ca. 23 nm (values
estimated by the Scherrer equation). This is an interesting observation since,
apparently, the ceria-free systems underwent stronger sintering. In other words, it
seems that ceria mitigates to some extent metallic Ni agglomeration. It is well known
that the oxygen vacancies present on ceria are electron rich sites where metallic
particles preferentially nucleate [32]. This preferential nucleation of Ni on ceria’s
oxygen vacancies could explain the strong Ni-Ce interaction, which is evidenced also
in the TPR profile, and accounts for the higher resistance of the ceria-containing
materials to metal sintering.
Figure 5. XRD post-reaction characterization for NiCeO2/C (A) and NiSnCeO2/C (B)
catalysts.
20 25 30 35 40 45 50 55 60 65 70 75 80
*
º
º
+
+
*
*º
NiSn/C
*
+ Ni3Sn
Ni metallic
NiSn20CeO2/C
NiO
CeO2
Inte
nsity
(a.
u.)
NiSn30CeO2/C
NiSn40CeO2/C
2 degree)
NiSn/CeO2
20 25 30 35 40 45 50 55 60 65 70 75 80
CeO2
NiOº
ºº
*
**
Ni/C
* Ni metallic
º
Ni20CeO2/C
Inte
nsity
(a.
u.)
Ni30CeO2/C
Ni40CeO2/C
2 degree)
Ni/CeO2
Chapter III
71
The situation becomes more interesting when the NiSnXCeO2/C series of catalysts
are considered (Figure 5B). Some peaks ascribed to Ni-Sn alloy were identified for the
NiSn/C sample, in good agreement with the XPS data [33,34] In addition, some
reflexions related to metallic nickel and to nickel oxide were found for this sample.
Contrary to the NixCeO2/C series (Figure 5A), no peaks related to metallic Ni were
observed for the multicomponent NiSnxCeO2/C catalysts after reaction. Also, the
peaks of the alloy phase are broader and smoother for these samples. This is a crucial
difference between both series of catalysts, which reveals the absence of sintering of
metallic particles when the Ni-Sn-Ce ensemble is considered. Shabaker et al.
suggested that Sn migrates into the Ni particles to form Ni-Sn alloys during high-
temperature reduction [27]. Indeed, diffusion of Sn into Ni occurs rapidly at elevated
temperature, especially above the melting point of Sn (323 ºC) [35]. Therefore the use
of Sn in the metallic phase has a strong effect in the catalytic properties of NiCeO2/C
solids. The Ni-Sn alloy formed is better dispersed on the ceria-carbon support and,
what is more important, it presents a higher tolerance to sintering in comparison with
the monometallic NiCeO2/C systems, this enhancing the catalytic behaviour in terms
of larger hydrogen yields and stability with time on reaction.
4. CONCLUSIONS
We have developed a series of active ceria-promoted Ni and Ni-Sn catalysts
supported on carbon for the low-temperature glycerol steam reforming. Our most
active samples reached full glycerol conversion at early reaction stages with
reasonably good hydrogen yields. In fact, hydrogen production is boosted via WGS due
to the ability of ceria to carry out the shift process in the studied temperature window.
Additionally, the strong influence of the WGS is reflected in the CO/CO2 and the
H2/CO2 ratios, in such a way that the obtained gas stream is rich in hydrogen and
relatively poor in CO, an added value for further downstream processing.
The catalytic trends observed are a consequence of the complex nature of these
multicomponent systems. The presence of Sn modulates the redox properties, acting
as an electronic bridge between Ni and Ce and favouring the electronic transfer
between both species, this resulting in more homogenous reducibility profiles. Also,
our data indicate the formation of Ni-Sn alloy that plays a crucial role in the catalytic
Multicomponent NiSnCeO2/C catalysts for the low-temperature glycerol steam reforming
72
activity. The bimetallic Ni-Sn catalysts are more active and more stable than the
monometallic ones based exclusively on Ni. Apparently, the Ni-Sn alloy palliates the
metallic sintering thus maintaining a good metallic dispersion of the active phase
during the reaction. This effect is high-lightened in the presence of ceria. The Ni-Sn
alloy is better dispersed in the CeO2/C support than in the pure C one, this helping to
avoid the sintering process. Consequently, the presence of all the components Ni, Sn,
CeO2 and the carbon support is mandatory for an optimum performance. In the case
of CeO2, its dispersion on carbon leads to smaller ceria particles that are more active
in the shift process, this favouring the hydrogen production with a low ceria loading.
Overall, the results presented in this paper demonstrate that a successful glycerol
upgrading in mild conditions can be achieved without a noble metal based catalyst.
Herein, we propose a multicomponent system in which every component is carefully
selected with the aim to potentiate the catalytic performance, gaining stability and
selectivity towards a rich hydrogen stream with relatively low CO concentration.
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128-134.
CHAPTER IV
Aqueous phase reforming of glycerol for
hydrogen production over Pt, Ni andPt-
Ni catalysts supported on CeO2
GGrraapphhiiccaall aabbssttrraacctt
Chapter IV
77
Abstract
Herein, efficient Pt/CeO2, Ni/CeO2 and Pt-Ni/CeO2 catalysts have been developed
for hydrogen production via glycerol aqueous-phase reforming. Fundamental
understanding of the catalytic behaviour was obtained due to broad range of
physicochemical techniques employed for the catalysts characterization. The
superiority of Pt over Ni for this reaction has been evidenced. Pt containing systems
exhibit higher glycerol conversions and greater liquid yields being the bimetallic Pt-Ni
catalyst the most active material presented in this study. Besides, noteworthy
catalytic activity toward gas phase products with the non-reduced samples was
obtained. Moreover, rather relevant information about the reaction intermediates
and the catalysts evolution during the reaction was obtained using in situ ATR
spectroscopy allowing an approach towards the most likely reaction pathways.
Keywords: H2 production; APR; Glycerol; Ni-Pt; Ceria; “In situ“ ATR-IR.
Aqueous phase reforming of glycerol for hydrogen production over Pt, Ni and Pt-Ni catalysts supported on CeO2
78
1. INTRODUCTION
Hydrogen is considered as the most efficient and environmentally benign energy
fuel [1]. It allows additional flexibility in the selection of energy resources and could
play a major role mitigating effects of climate change. The depletion of fossil fuels and
the environmental problems associate to their use as energy sources have encouraged
the research towards this renewable and sustainable energetic vector [2, 3]. Although,
nowadays, hydrogen is produced from fossil fuels, considerable research has focused
on the production of hydrogen from renewable sources [4]. Dumesic and co-workers
pioneered a more suitable production of H2 from biomass-derived oxygenates through
catalytic processes, i.e., aqueous phase reforming (APR). This reaction requires
supported metal catalysts under mild conditions (200-250 ºC, 20-50 bar) to produce a
H2-rich stream with low levels of CO, which is a poison for the anode’s catalysts in
fuel cell applications [5, 6]. This process presents important advantages in comparison
to conventional steam reforming, which include diminished energy requirements by
the fact that oxygenates are converted into hydrogen in an aqueous liquid phase
rather than in the gas phase, thus eliminating the need to vaporize the high-boiling
biomass derived oxygenated. It also prevents the undesirable thermal decomposition
reaction of these oxygenates when high temperatures are used. Another particular
advantage is that the produced gas is rich in H2 and poor in CO due to water gas shift
reaction (WGS), which is thermodynamically favoured at the temperatures and
pressures that are employed in the APR process.
Among the different biomass-derived oxygenates that can be used for H2
production, glycerol has been targeted as one of the most adequate. Glycerol is
obtained in large quantities as a by-product in the production of bio-diesel, and its
carbon-oxygen ratio 1:1 is optimal for its use of glycerol in APR.
The APR process is in fact a combination of two processes, glycerol reforming and
water-gas shift. However, undesired reactions, such as carbon monoxide
hydrogenation, partial breaking of C-C, C-H, C-O bonds of glycerol and intermediates,
dehydrogenation and dehydration/hydrogenation of glycerol to diols might occur
(Figure 1). These unwanted reaction pathways are highly dependent on the catalyst
nature [7-11]. In this sense, the development of a catalyst active for C-C, C-H bond
scissions and also for water-gas shift reaction is a must. Additionally, this catalyst
Chapter IV
79
should not have acidity, as acid sites promote the formation of diols, mono-alcohols
and alkanes via C-O cleavage.
Figure 1. Reaction pathways for H2 production by reactions of oxygenated hydrocarbons
with water. Reproduced from ref [5].
Group VIII metals, particularly Pt, Pd and Ni, are especially effective for the APR
of glycerol. More specifically, Pt and Pd are highly selective to hydrogen, whereas Ni
shows a higher activity for alkane formation [12]. Pt is one of the preferred choices
due to its activity for C-C/C-H bond cleavage and WGS reaction, but the high cost of
this noble metal has stimulated the use of catalysts based on non-precious metals. Ni
is widely used due to its low cost, high availability and also good performance for C-C
scission. Ni catalytic activity can be improved by addition of Pt. Indeed, the positive
Pt-Ni synergetic effect has been reported elsewhere [13] Furthermore the use of Ni
catalysts will be considered as a cheaper alternative that has shown initial APR
activity comparable to that of Pt, but was subject to significant deactivation [14]. Thus
efforts have been made to improve catalytic behaviour of Ni combining it with other
metallic element.
Aqueous phase reforming of glycerol for hydrogen production over Pt, Ni and Pt-Ni catalysts supported on CeO2
80
The catalytic activity of APR catalysts can be improved by additional metals. In
particular, the Pt-Ni system has been thoroughly studied in a range of applications
because of its synergic catalytic effect [7]. Crisafulli et al. reported that adding noble
metal to Ni catalyst for dry methane reforming reduces coke deposition and enhance
the stability [15]. Ko et al. showed that Pt-Ni bimetallic catalysts had more active
sites than monometallic ones, under the same pretreatment conditions. Huber et al.
found that the catalytic activity for Pt(3% wt)-base catalysts for APR increased after
alloying Pt with Ni, which decreased the strength with which CO and H2 interact with
the surface, thereby increasing the amount on catalytic sites available to react with
ethylene glycol [10].
The nature of the support moreover influences the efficiency of glycerol APR, being
the use of basic supports generally favourable for its high hydrogen selectivity.
Specifically, CeO2 improves initial glycerol conversion and hydrogen selectivity for
APR of glycerol promoting WGS reaction [16]. Additionally, ceria inhibits
dehydration, avoiding formation of unsaturated hydrocarbons that are coke
precursors, increasing catalytic stability by promoting carbon removal from the
metallic surface [17].
In this context, the aim of this work is to study Pt or/and Ni catalysts supported on
ceria, in aqueous-phase reforming reaction of glycerol for hydrogen production. The
goal is to reach high glycerol conversion towards gaseous products, especially H2, but
the presence of both gas and liquid products were also studied. The effect of the Pt
addition to Ni catalysts and the interactions of these metals with the support on the
catalytic activity for APR reaction were investigated. In addition, in-situ ATR
experiments are employed to gain some clues regarding the reaction intermediates
and possible pathways depending on the involved metal.
2. EXPERIMENTAL SECTION
2.1 Catalysts preparation
The CeO2 support was prepared by homogeneous precipitation from an aqueous
solution of Ce(NO3)3·6H2O (99.99 %, Sigma-Aldrich) containing an excess of urea. The
Chapter IV
81
solution was heated at 80 °C and kept at this temperature, with slow stirring, during
12 h. The solid formed was filtered and calcined at 350 °C for 4 h.
Platinum and nickel were incorporated to the CeO2 support by impregnation with
acetone solutions containing the adequate amount of H2PtCl6·6H2O (99.95 %, Alfa
Aesar) and/or Ni(NO3)2·6H2O (99.9 %, Sigma-Aldrich) to obtain 1 wt.% Pt or 5 wt.%
Ni, using 10 mL of solution per gram of solid. After stirring for 12 h, the solvent was
removed under vacuum at 40 °C in a rotary evaporator. Finally, the dried solids were
calcined at 350 °C for 4 h. In this way, three ceria-supported samples were prepared
and labelled as Pt/CeO2, Ni/CeO2 and PtNi/CeO2.
2.2 Catalysts characterization
The textural properties of the supports were determined by nitrogen adsorption
measurements at -196 °C. Gas adsorption experiments were performed in a home-
made fully automated manometric equipment. Prior to the adsorption experiments
samples were out-gassed under vacuum (10-4 Pa) at 250 °C for 4 h. The specific
surface area was estimated after application of the BET equation.
The actual metal loading was determined by inductively coupled plasma optical
emission spectroscopy (ICP-OES) in a Perkin–Elmer device (Optimal 3000). The
metals were extracted from the catalysts by digestion in aqua regia for 30 min, in a
microwave oven, at 200 °C.
X-Ray powder diffraction patterns were recorded on a Bruker D8-Advance with a
Göebel mirror and a Kristalloflex K 760-80 F X-Ray generation system, fitted with a
Cu cathode and a Ni filter. Spectra were registered between 20 and 80º (2θ) with a
step of 0.05º and a time per step of 3 seconds.
Transmission electron microgrphs were acquired on a JEOL electron microscope
(model JEM-2010) working at 200 kV, which It was equipped with an INCA Energy
TEM 100 analytical system and a SIS MegaView II camera. Samples for analysis
were suspended in ethanol and placed on copper grids with a holey-carbon film
support.
Aqueous phase reforming of glycerol for hydrogen production over Pt, Ni and Pt-Ni catalysts supported on CeO2
82
Temperature-programmed reduction (TPR) with H2 experiments were performed
on the calcined catalysts in a U-shaped quartz cell, using a 5 % H2/He gas flow of 50
mL/min, with a heating rate of 10 °C/min. Samples were treated with flowing He at
150 °C for 1 h before the TPR run. Hydrogen consumption was followed by on-line
mass spectrometry (Pfeiffer, OmniStar GSD 301).
X-Ray photoelectron spectroscopy was performed with a K-ALPHA spectrometer
(Thermo Scientific). All spectra were collected using Al-Kα radiation (1486.6 eV),
monochromatized by a twin crystal monochromator, yielding a focused X-ray spot
with a diameter of 400_m, at 3 mA × 12 kV. The alpha hemispherical analyser was
operated at the constant energy mode, with survey scan pass energies of 200 eV to
measure the whole energy band and 50 eV in a narrow scan to selectively measure the
particular elements. Charge compensation was achieved with the system flood gun
that provides low energy electrons and low energy argon ions from a single source.
The powder samples were pressed and mounted on the sample holder and placed in
the vacuum chamber. Before recording the spectrum, the samples were maintained in
the analysis chamber until a residual pressure of ca. 5×10−7 N/m2 was reached. The
quantitative analysis were estimated by calculating the integral of each peak, after
subtracting the S-shaped background and fitting the experimental curve to a
combination of Lorentzian (30 %) and Gaussian (70 %) lines. Reduced samples (ex
situ) were conserved in octane solution (inert atmosphere). Suspensions were
evaporated in the XPS system under vacuum conditions.
2.3 Catalytic activity measurements
Aqueous-phase reforming reactions of glycerol were performed under semi-batch
conditions in a 40 mL stainless steel bench top Parr reactor with a back-pressure
regulator. The reactor was charged with 0.2 g of catalyst and 10 mL of a 10 % aqueous
solution of glycerol, a feed composition similar to that of the solutions obtained from
biodiesel production after alcohol removal and acid neutralization of the glycerol
fraction [18]. Reactions were carried out at 225 °C for 5 h with a constant pressure of
29 bars (pressurized with He). The gas phase reactions products (H2, CO, CO2, CH4)
were analyzed by online dual channel micro-GC with COX column and a TCD (Varian
CP4900). The liquid phase was analyzed with a Shimadzu HPLC equipped with an
Aminex HPX-87H column.
Chapter IV
83
In situ ATR-IR analysis of reactions were conducted in a 40 mL Parr stainless steel
autoclave equipped with a MT ReactIR 45 m ATR-IR sentinel with a diamond probe.
The experiments were performed in the same conditions described above. ATR-IR
spectra were recorded every 5 min during 5 h of reaction.
3. RESULTS AND DISCUSSION
3.1 Catalysts characterization
3.1.1 Textural and chemical characterization
The actual metal content in the catalysts is reported in Table 1. As it can be seen,
the values are very close to the nominal ones, this confirming the success of the
impregnation process.
Table 1 also shows the textural parameters of the ceria support and the three
catalysts, namely the specific surface area (N2, -196 °C, BET), the micropore volume
(Vmicro, N2, -196 °C, D-R) and the volume of mesopores (Vmeso). The N2 adsorption
isotherms at -196 ºC for all materials (not shown) correspond to the Type II according
to the IUPAC classification, which is characteristic of non-porous materials. All the
isotherms show a hysteresis cycle that corresponds to N2 condensation in mesopores
formed by particle agglomerations. Table 1 shows that the surface area of the ceria
support is hardly modified after the metal loading, the smaller values being mainly
due to the weight increase as metals are incorporated.
Table 1. Textural characteristics and metal loading of catalysts and crystallite size of the
CeO2 support.
S
BET(m
2/g) V
micro(cm
3/g) V
meso(cm
3/g) *dCeO2 (nm) **Pt (wt.%) **Ni (wt.%)
CeO2 101 0.04 0.07 12.1 -- --
Pt/CeO 105 0.05 0.09 12.7 1.17 --
PtNi/CeO2 80 0.04 0.06 11.6 1.20 5.01
Ni/CeO2 95 0.05 0.06 11.9 -- 4.80
*Calculated by the Scherrer equation using the (111) plane of CeO2.
**Determined by ICP analysis.
Aqueous phase reforming of glycerol for hydrogen production over Pt, Ni and Pt-Ni catalysts supported on CeO2
84
3.1.2 X-ray diffraction
Figure 2 shows the XRD patterns of the support and the catalysts after calcination
at 350 ºC, where the characteristic CeO2 peaks (JCPDS 34-0394) at 28.6, 33.4, 47.8
and 56.7 º, corresponding to the reflections in the (111), (200), (220) and (331)
crystalline planes, respectively, of the cubic fluorite type phase, can be clearly
observed. By application of the Scherrer equation to the (111) diffraction peak, the
mean crystal sizes of CeO2 in the different catalysts have been determined and are
reported in Table 1. It can be seen that the XRD profile of the CeO2 support shows
very narrow diffraction peaks indicating high crystallinity and large crystal size. On
the other hand, diffraction peaks of Pt and Ni species were not observed, which is
attributed to a high dispersion of the supported metals.
Figure 2. XRD patterns of the support and catalysts calcined at 350 ºC.
3.1.3 H2-temperature-programmed reduction
H2-TPR measurements were employed in order to assess the type of reducible
species formed after the calcination treatment and to evaluate the interaction
between Pt-Ni and Pt or Ni with the support. The TPR profiles of the ceria support
and those of the ceria-supported Pt, PtNi, and Ni catalysts are presented in Figure 3.
The TPR profile of the CeO2 support shows two reduction peaks at around 450 and
20 30 40 50 60 70 80
CeO2
Pt/CeO2
Inte
nsity (
a.u
.)
PtNi/CeO2
2
Ni/CeO2
Chapter IV
85
800 °C, which are associated to the reduction of surface and bulk ceria, respectively
[19,20].
Figure 3. H2-TPR profiles of the support and catalysts calcined at 350 ºC.
The TPR profile obtained with Pt/CeO2 is also similar to those found in the
literature [21, 22. 23]. The intense peak centred at 300 °C is assigned to the surface
reduction of ceria in close contact with the metal, as well as the platinum reduction
(breakdown of Pt-O-CeO2 species created upon calcination). It has to be noted that
this peak appears at a lower temperature than for surface reduction in pure CeO2,
this being related to the easier hydrogen availability by spillover from the metal
surface. Finally, hydrogen consumption at 830 °C corresponds to the reduction of bulk
CeO2.
The H2-TPR profile of Ni/CeO2 reveals several peaks of hydrogen consumption
corresponding to the reduction of nickel species and ceria in different environments.
The first low temperature peak at 220 ºC can be attributed to the reduction of highly
dispersed NiO on the support [24], and the intermediates ones (at 250 and 340 ºC)
correspond to the surface reduction of ceria which interacts with nickel particles to
different extent, together with the reduction of NiO particles strongly interacting with
the ceria support, respectively [25,26,27]. It has been shown in the literature that
different Ni species can coexist in this system depending on the Ni/Ce ratio and the
0 100 200 300 400 500 600 700 800 900 1000
Pt/CeO2
Inte
nsid
ad
(u
.a.)
PtNi/CeO2
Ni/CeO2
Temperatura oC
CeO2
Temperature (ºC)
Inte
nsity (
a.u
.)
Aqueous phase reforming of glycerol for hydrogen production over Pt, Ni and Pt-Ni catalysts supported on CeO2
86
calcination temperature, and that the presence of nickel can decrease the temperature
at which the surface of ceria is reduced [28,29]. Also, a small peak at high
temperatures (around 800 ºC) can also be observed, which is assigned to the reduction
of bulk ceria in the Ni/CeO2 catalyst [30].
The high temperature peak also appears in the TPR profile of PtNi/CeO2, as it
could be expected. But the low temperature region for this catalyst is very different
than those of its counterparts. On one hand, a small reduction peak appears centred
at around 75 ºC. The very low temperature allows to assign this peak to the reduction
of PtOx species that do not interact with the ceria support, maybe because they are
located on NiOx particles and they are not in contact with the support. The reduction
profile reveals two more broad and asymmetric bands at larger temperatures, which
seem to be composed of different contributions, this indicating the complex
composition of this catalyst and the different reduction processes that take place, such
as the reduction of NiOx species interacting to different extents with Pt and the
support and the surface reduction of ceria interacting with Pt and Ni species.
Furthermore, these processes are all of them favoured by the spillover of hydrogen
atoms from the platinum nanoparticles to the catalyst’s surface, this decreasing the
temperature at which reduction would proceed in the absence of platinum [31].
3.1.4 X-ray photoelectron spectra (XPS)
The chemical composition of the catalyst surface, and the chemical state of the
different components was assessed by XPS. The calcined samples were reduced ex-
situ at 350 ºC in flowing H2 (50 mL/min) and conserved in octane until the analysis, to
avoid re-oxidation. The binding energies (B.E.) of the main peaks, with the
corresponding deconvolutions of the Pt 4f7/2 and Ni 2p3/2 energy levels are summarized
in Table 2.
It can be seen from data in Table 2 that both Pt and Ni are in form of oxidized
species in the fresh calcined catalysts. After the reduction treatment at 350 ºC only a
part of Pt (around 60 %) and Ni (12 – 14 %) were in the metallic state.
Chapter IV
87
Table 2. XPS binding energies and surface atomic ratios in fresh (calcined and reduced) and
spent (calcined and reduced) catalysts in APR of glycerol.
*Atomic composition of carbonaceous species only calculated with the peak at ≈286.4 eV (C 1s)
After the reduction treatment at 350 ºC, only a part of Pt and Ni were in the
metallic state (Table 2), which is in agreement with the H2-TPR results. The Pt 4f7/2
binding energy (B.E.) in the reduced PtNi/CeO2 catalyst (71.4 eV) was slightly lower
than in Pt/CeO2 (71.8 eV). The platinum electronic environment is probably altered by
nickel, this making platinum electronically richer and therefore shifting the XPS Pt
peaks toward lower B.E. in the bimetallic sample compared to the monometallic one.
In fact, Pt presents higher electronegativity than Ni (according to Pauling scale) and
therefore, it can be expected that the interaction between Pt and Ni results in an
electronic transfer from the later to the former. If such effect occurs, the superficial
platinum particles will tend to maintain some negative density while Ni will have
positive density [32-33]. Furthermore, these results are in agreement with the H2-
TPR profiles, which showed overlap between Pt and Ni reduction peaks. For the
calcined Pt/CeO2 and PtNi/CeO2samples, two different contributions at around 73.2
and 74.8 eV appear, which are ascribed to Pt2+ and Pt4+, respectively. This can be
explained by the reduction of a certain amount of the starting Pt4+to Pt2+ upon the
decomposition of the platinum precursor during support impregnation [34].
Similarly, the Ni B.E. of the 2p3/2 level in the reduced Ni/CeO2 catalyst (852.3 eV) is
shifted towards lower values compared to the bimetallic sample (853.0 eV).This
indicates that the Pt-Ni interaction changes the Ni electronic environment by electron
transfer. The deconvoluted spectra (not shown) indicate the presence of Ni in different
chemical environments. For the reduced samples, a certain amount of Ni0 was
observed. This low amount of reduced nickel can be explained through the high
Sample
Pt 4f7/2 (eV) Ni 2p3/2 (eV) Pt0/Ptotal Ni0/Nitotal Pt/Ce Ni/Ce C/Ce
*Cspecie
s (%) Pt0 Pt2+ Pt4+ Ni0 Ni2+
Pt/CeO2 Calc.
Calc. spent
--
70.7
72.7
71.9
74.5
--
--
--
--
--
--
0.710
--
--
0.015
0.045
--
--
0.906
1.136
4.0
15.6
Red.
Red. spent
71.5
71.0
72.7
72.1
--
--
--
--
--
--
0.555
0.583
--
--
0.007
0.031
--
--
0.440
1.051
3.9
18.0
PtNi/CeO2 Calc.
Calc. spent
--
71.0
72.9
71.9
74.4
--
--
--
853.4-855.3
853.5-855.8
--
0.639
--
--
0.019
0.035
0.189
0.166
1.394
2.213
2.6
13.2
Red.
Red. spent
71.1
71.0
72.1
72.1
--
--
852.7
852.5
855.1-857.0
855.6
0.590
0.739
0.140
0.297
0.008
0.033
0.127
0.093
0.690
4.197
3.2
14.1
Ni/CeO2 Calc.
Calc. spent
--
--
--
--
--
--
--
--
853.5-855.5
854.0-855.9
--
--
--
--
--
--
0.205
0.118
1.343
1.610
3.8
9.7
Red.
Red. spent
--
--
--
--
--
--
852.0
852.9
854.1-856.2
855.4
--
--
0.122
0.304
--
--
0.129
0.099
1.088
1.670
6.1
12.2
Aqueous phase reforming of glycerol for hydrogen production over Pt, Ni and Pt-Ni catalysts supported on CeO2
88
interaction Ni-Ce, in agreement with the H2-TPR results. This strong interaction
leads into a lower Ni reducibility [35,31]. Moreover, for both calcined and reduced Ni-
containing catalysts, different Ni2+ species appear, what is attributed to Ni with
different chemical environment (the two bands between 853.8 and 856.5 eV) [36,37].
Binding energies of these two Ni2+ species in the ceria-containing catalysts are
slightly larger than the values reported for pure compounds, which is indicative of
their interaction with ceria. The existence of a strong metal-support interaction
between Ni and CeO2 can modify the structure and electronic properties of Ni, which
could improve the catalytic behaviour of this system [38].
3.1.5 Transmission electron microscopy (TEM)
Figure 4 shows some representative TEM images of reduced samples. It can be
seen that the active phases are homogeneously dispersed on the support as no large
agglomerations are observed. It is important to point out that it was not possible to
distinguish Ni and Pt because of their similar electron density and high dispersion.
However, EDX experiments (not shown) revealed the presence of these metallic
species in direct contact.
Figure 4. TEM micrographs of catalysts reduced at 350 ºC. A) Pt/CeO2 (scale bar: 2 nm), B)
PtNi/CeO2 and C) Ni/CeO2 (scale bars: 5 nm).
3.2 Catalytic test
Glycerol APR experiments were run in a semi batch reactor (at 225 ºC and 29 bar)
during 5 hours, with a back pressure regulator to keep the system pressure constant
Chapter IV
89
at just above the vapour pressure of water at the reaction temperature. Some authors
have previously shown that a batch reaction system is a convenient and reliable tool
for initial screening studies of catalytic behaviour in the APR of oxygenates [10, 39].
The performance of the calcined and reduced catalysts in the APR of glycerol (10 wt.%
aqueous solution) is summarized in Figure 5, in which conversion of glycerol is
separated in conversion into gas products and conversion into liquids. In the gas
phase, CO, CO2, H2 and CH4 were detected. Alkanes larger than methane were only
detected in trace amounts and therefore no quantified. Furthermore, gas phase
products distribution for all catalysts is given in Figure 5.
Figure 5. Glycerol conversion into gas products (Gas phase products distribution in colour)
and into liquids (grey), obtained by APR of 10 wt.% glycerol with calcined and reduced
catalysts at 225 ºC and 29 bar.
For Pt/CeO2, the best glycerol conversion (86 %) was obtained with the calcined
sample, with also a higher conversion to gas phase products than the reduced sample.
In spite of the fact that no metallic platinum was detected in the fresh calcined
sample, a certain degree of reduction takes place upon reaction (Table 2). The surface
Pt/Ce atomic ratio in Table 2 indicates a relative higher amount of surface platinum
in the calcined sample than in the reduced catalyst. Then, it can be inferred that the
reduction process taking place during the APR reaction generates more dispersed
Aqueous phase reforming of glycerol for hydrogen production over Pt, Ni and Pt-Ni catalysts supported on CeO2
90
metallic platinum sites than the conventional reduction treatment with hydrogen,
which could produce a certain degree of sintering and/or agglomeration. Thus, the
higher activity of the calcined catalyst as compared to its reduced counterpart can be
explained by a higher amount of surface metallic platinum sites. In fact, the activity
and H2 selectivity of supported metal APR catalysts has been correlated by other
authors with the metal dispersion, concluding that the rate of H2 formation decreased
with increasing the Pt particle size [40,32].
Despite the higher metallic loading in the Ni/CeO2 catalyst, a poorer glycerol
conversion was obtained. This underlines the superior performance of noble metals for
this catalytic process. Even though, good activity and a very high H2 selectivity were
obtained with the calcined Ni/CeO2 sample (Figure 5). Most of the glycerol was
transformed into gaseous products. In spite of the well-known methanation activity of
the nickel based catalysts, no methane was obtained. To be more specific, only the
reduced Ni/CeO2 sample and the Pt-containing catalysts produced low amounts of
methane. This indicates that only Ni metallic particles are able to carry out the
methanation side reaction, as metallic Ni was not detected by XPS neither in the
fresh calcined nor in the spent calcined catalyst (Table 2) [41,42].
The use of CeO2 as a support promotes the water gas-shift reaction converting
adsorbed CO into CO2. However, samples containing reduced Ni produce a small
amount of CO. It can be considered that the deactivation of metallic Ni species, due to
their high susceptibility to oxidation by water excess under APR conditions, results in
a loss of the activity for C-C/C-H bond scission. In this way, an increase of
intermediate products could be produced, whose decomposition to CO and CH4 can
explain the observed results [31].
Both the calcined and the reduced PtNi/CeO2 catalysts show a high glycerol
conversion degree. It has to be noted that the reduced PtNi/CeO2 sample is the best
catalyst in terms of total glycerol conversion (liquid + gaseous products). However the
improved catalytic activity could be due to the cooperative effect of Ni and Pt but also
to an additive effect of the total metallic loading. The Pt-Ni synergy is clearly
established in view of the physicochemical properties discussed above, namely XPS
and H2-TPR results. On the one hand, the addition of Pt improves the dispersion of Ni
species and facilitates the reduction of NiO to metallic Ni through a hydrogen
Chapter IV
91
spillover mechanism resulting in improved redox chemistry of the bimetallic system,
as it has been previously shown [43]. On the other hand, as it has been demonstrated
by the XPS results, the Ni-Pt interaction results in an electronic density transfer from
Ni to Pt, making the Pt-Ni interface an electric rich site to anchor and activate
molecules.
The additive effect was roughly evaluated by normalising glycerol conversion per
mole of active metal (Pt, Ni or Pt + Ni). The results of these calculations are shown in
Table 3. As shown in the table, the bimetallic catalysts presented slightly superior
activity than the monimetallic Ni system. However, the Pt/CeO2 catalyst resulted to
be the most efficient in terms of conversion per unit of active metal. Therefore,
although the Ni-Pt synergy may result in advanced catalytic properties like improved
redox and electronic features, in the studied reaction conditions a monometallic
system based on Pt seems to be the best option. Also when comparing PtNi/CeO2 vs
Ni/CeO2 the overall effect in the catalytic activity is not really remarkable thus
making the monometallic Ni a more sensible choice.
Table 3. Normalising glycerol conversion per mole of active metal for calcined and reduced
samples.
Sample gly. conv. (mol)/ M (mol)
Pt/CeO2 Calc.
Red.
915
782
PtNi/CeO2 Calc.
Red.
48
53
Ni/CeO2 Calc.
Red.
43
44
Glycerol conversion into liquid phase compounds has been also analysed. Figure 6
shows the distribution of liquid phase products distribution, grouped in their carbon
numbers, for all catalysts. It can be seen that the reduced catalysts generated a larger
amount of liquid products than their calcined counterparts. This can be explained by
the smaller number of available active sites because of sintering after reduction
(Table 2), this decreasing the capacity of cleavage the OHC intermediates into
gaseous products.
Aqueous phase reforming of glycerol for hydrogen production over Pt, Ni and Pt-Ni catalysts supported on CeO2
92
On the other hand, calcined Pt/CeO2 catalyst promoted glycerol conversions with
higher gas phase formation and H2 selectivity than the calcined PtNi/CeO2 sample.
The better H2 selectivity obtained by Pt is due to high activity of this metal for
dehydrogenation reactions (C-H). The presence of Ni favours selectivity to OHC
intermediates, due to its high activity for the hydrogenation of CO/CO2 and for C-C/C-
O cleavage from oxygenated compounds via consecutive and/or parallel reactions
[44,45]. In this sense PtNi/CeO2 catalysts have an intermediate behaviour with a
moderate H2 selectivity and OHC intermediates.
Figure 6. Distribution of liquid phase products for calcined and reduced catalysts.
Figure 7 shows the possible reaction pathways during the APR of glycerol. The
main products obtained in the liquid phase after APR process were identified as lactic
acid (C3), acetic acid (C2), ethylene glycol (C2), acetaldehyde (C2), ethanol (C2) and
methanol (C1). Although the same products were formed over all catalysts, major
differences in product distribution were observed (Figure 6). Lactic acid (C3) is one of
the major products on Pt/CeO2 catalysts, and its formation is believed to involve
pyruvaldehyde as an intermediate, a conversion that is in turn base-catalyzed via
dehydration/hydrogenation [46,47]. The decrease of C3 and the increase of C2-C1
compounds such an EG, acetaldehyde, ethanol and methanol was a consequence of the
enhanced competition between the C-C/C-H against C-O bonds cleavage over the
PtNi/CeO2 and Ni/CeO2 catalysts.
Chapter IV
93
Figure 7. Possible reaction pathways during APR of glycerol.
3.2.1 XPS post-reaction characterization
The spent catalysts were analysed by XPS. This analysis provides useful
information about the surface state in the samples tested. Table 2 summarizes the
surface composition of all samples and the binding energy of the Pt 4f7/2 and Ni 2p3/2
core level in the spent catalysts. It also includes the surface Pt/Ce, Ni/Ce, and C/Ce
atomic ratios, as well as the relative content of carbonaceous species obtained by XPS
for fresh (calcined and reduced) and spent (calcined and reduced) catalysts.
The surface Pt/Ce and Ni/Ce atomic ratios in the reduced catalysts decreased as
compared to the calcined counterparts, this suggesting some sintering of Pt and Ni
particles after the reduction treatment. For Pt/CeO2 catalyst the slight sintering of
the particles produces consequent decreased of glycerol conversion. This sintering also
happens with PtNi/CeO2 and Ni/CeO2 catalysts but the beneficial effects obtained by
the reduction of the samples and metals interactions are more notorious than the
sintering effect, increasing slightly glycerol conversion in reduced samples.
Aqueous phase reforming of glycerol for hydrogen production over Pt, Ni and Pt-Ni catalysts supported on CeO2
94
Besides, the Ni/Ce surface atomic ratio in the spent catalysts decreased as
compared to the fresh ones, this suggesting some sintering of Ni particles during the
APR of glycerol. However for platinum containing samples, the opposite trend is
observed. Although Pt sintering after reaction also occurs, as it will be discussed
below, the Pt/Ce atomic ratio is higher. A possible explanation of Pt/Ce increment is
the higher carbonaceous species covering ceria surface in relation to Ni/CeO2 sample.
In order to clarify this point the surface C/Ce atomic ratios and the content of
carbonaceous species were calculated (Table 2). Three carbon peaks were obtained in
the XPS spectra of the C 1s region. Surface carbon contamination appears as a peak
at 285.8 eV, the adsorbed CO species appear at 286.5 eV, and a high binding energy
peak (between 288-290 eV) is present when surface carbonates like species appeared
[48,11] For the estimation of the surface C/Ce atomic ratios and carbonaceous species
content only this last peak was employed.
It can be seen that the surface C/Ce atomic ratios and the surface %C increase
after aqueous phase reforming reaction for all samples. Carbonaceous species have
been deposited on the catalysts surface upon reaction, partially covering the ceria
support [11]. These carbonate-like species deposits are more notorious in the Pt-
containing catalysts due to the better activity of this metal both in C-C and C-H
scissions and water-gas shift reaction.
On the other hand, Table 2 also shows the binding energies of the Pt 4f7/2 and Ni
2p3/2 core levels for the spent catalysts. After glycerol aqueous phase reforming, Pt-
containing samples, both reduced and calcined, contents Pt0. This confirms that
oxidized platinum species in the fresh catalysts are reduced under reaction conditions,
and explains the good performance of the calcined samples. Also, platinum is
electronically richer, as the B.E. of the XPS Pt 4f7/2 core level is shifted towards lower
values as compared with the fresh counterparts.
XPS results indicate that Ni is partially re oxidized in the Ni-containing spent
catalysts, both calcined and reduced. Nevertheless, the presence of reduced Ni species
cannot be discarded since they could be formed during the reaction. Indeed, a certain
amount of metallic nickel remains in the Ni/CeO2 reduced spent sample [49].
Chapter IV
95
3.2.2 TEM post-reaction characterization
Figure 8 shows the TEM micrographs of the reduced catalysts after glycerol
aqueous phase reforming reaction. It can be seen that sintering has taken place in all
samples, but more significantly in the Ni/CeO2catalyst. This is due to the higher Ni
mobility under hydrothermal conditions. Furthermore, the formation of superficial
carbonaceous species is evidenced in the micrograph of PtNi/CeO2, in good agreement
with the XPS data.
Figure 8. TEM micrographs of spent reduced catalysts (APR 10% glycerol, 225 ºC, 29 bar.
A) Pt/CeO2 (scale bar: 10 nm), B) PtNi/CeO2 (scale bar: 10 nm), and C) Ni/CeO2 (scale bar: 20
nm).
3.2.3 “In situ“ ATR-IR spectroscopy
Finally the ATR-IR spectra of the surface species formed during the glycerol
reforming on the three calcined catalyst at 225 °C and a pressure of 29 bars are shown
in Figure 10. Only spectra of calcined catalysts were studied, due to the good
performance of these samples without any previous reduction treatment. For a better
assignment of the characteristic bands to the reactant, intermediate products and
surface species formed, the water background was subtracted.
The multiple peaks between 1350-900 cm-1 observed for the three samples can be
assigned to the deformation vibration of COH and to twisting and rocking vibrations
of CH2 groups, which are present in glycerol as well as in the intermediates formed
during the reaction. [50,51]. Two of these bands can be clearly assigned in the three
Aqueous phase reforming of glycerol for hydrogen production over Pt, Ni and Pt-Ni catalysts supported on CeO2
96
samples: 1180 cm-1 ( (COH)) and 966 cm-1 ( (CH2)) [52]. Glycerol was identified by
the C-O stretching vibration bands in α (1065 cm-1) and β positions (1104 cm-1).
A)
B)
Chapter IV
97
Figure 9. Operando ATR-IR spectra of A) Pt/CeO2, B)PtNi/CeO2 and C) Ni/CeO2 calcined
catalysts during glycerol aqueous phase reforming (10 % wt. glycerol, 225 ºC, 29 bar).
For the Pt containing catalyst, three carbonyl bands ( (C=O)), centred at 1745,
1720 and 1717 cm-1 appear, and they can be assigned to glyceraldehyde, pyrualdehyde
and lactic acid respectively, indicating the formation and increase of these
intermediates during the reaction in agreement with the results obtained for the
liquid products [53] (Figure 8). This indicates that part of the hydrogen obtained can
be produced via dehydrogenation (C-H cleavage) to form glyceraldehydes and it is
further converted to lactic acid via cannizzaro type reactions, catalyzed by support
basic sites [54,55]. Moreover C2 and C1 are also obtained for these calcined samples,
therefore a subsequent C-C cleavage step takes place to form small intermediate
oxygenated compounds, ethanol and methanol, as well as the water gas shift reaction.
The presence of this C2 and C1 oxygenated intermediates is confirmed by the
appearance of some overlapped bands with high complexity and difficult assignment
in 1400-1610 cm-1 region. They can be assigned to asymmetric and symmetric
vibrations of carboxylate species (H)COx formed after adsorption of ethanol, methanol
and other small HCOs on ceria [56]. It has been reported that bands at 1390-1450 cm-
1 and 1590-1610 cm-1 are usually attributed to hydrogen carbonates together with
bands at 3610 cm-1 (attributed to the presence of OH species on Ce3+/Ce4+, at 3600-
3700 cm-1 [57]). Besides it is well reported that strong bands appeared ca. 1580 cm-1,
C)
Aqueous phase reforming of glycerol for hydrogen production over Pt, Ni and Pt-Ni catalysts supported on CeO2
98
and they are considered to have two components, one of them due to formates and the
other one due to “bidentate” carbonates. Also, bands in the region 1434-1510 cm-1 are
assigned by different groups to “monodentate”, “polydentate”, or polymeric carbonate
or carboxylate species coordinated via the carbon atom to a surface Ce cation [58]. As
this (H)COx intermediate species also must precede the formation of H2, CO and CO2,
the higher concentration of the mentioned species during the reaction over the
Pt/CeO2 and PtNi/CeO2 catalyst denotes the slightly better activity of Pt-based
samples in comparison with the Ni/CeO2 sample. Indeed, the better Pt activity for
water dissociation and C-C/C-H bond cleavage than Ni, increased the presence of
these carbonate species in the platinum-containing samples, correlated with the XPS
analysis discussed above [59].
Additionally, a notorious negative broad band around 1400 cm-1 is formed. One
possible contribution to this band is a δOH mode that is attributed to the removal of
hydroxyl groups from the Pt surface [60].
In contrast, the contribution of carbonate species (1400-1650 cm-1) for the Ni/CeO2
catalysts decreased significantly as the reaction progressed corroborating the less
liquid phase obtained with this sample [61]. Moreover the bands of carbonyl groups
(C=O stretching) of glyceraldehydes, pyruvaldehyde and lactic acid at 1745, 1728,
1717 cm-1 respectively can also be distinguish. Furthermore, the band ca 1456 cm-1
can be attributed to carboxylic (COO) stretching vibrations of lactic acid.
As mentioned above very high H2 selectivity was obtained with the calcined
Ni/CeO2 sample. Most of the glycerol was transformed into gaseous products and the
low amount of liquid phase was composed by C2 and C3 products (mainly ethanol and
lactic acid). This H2 can be produced to glycerol dehydrogenation to form
glyceraldehyde and the consecutive lactic acid formation (in agreement to the bands
assignment). Also the decarboxylation of lactic acid to ethanol can occur, however this
reaction does not consume hydrogen.
The great contribution of broad bands appearing in the next region is related to
CO-Metal bonds. On one hand, peaks at ca. 2050 and 1990 cm-1 are evident in Pt-
containing samples, due to the stretching mode of linear bonded CO on Pt. This band
is expected in this region of the spectra when the Pt-CO bond is present in an aqueous
Chapter IV
99
environment [62]. Moreover, bridged bonded CO on Pt can also be evidenced at 1940
cm-1 [53].
On the other hand, a strong contribution of bands related to CO on Ni is present
between 1900-2200 cm-1 in the Ni-containing catalysts. The complexity of these peaks
makes it difficult their attribution, although they are generally associated to terminal
and bridged CO on Ni particles. Molecular structures like Nix(CO)y (with x>1) also
produce these bands, due to coupled liner CO oscillators in these kind of complexes
[63]. In addition, the presence of oxidized nickel species is identified by bands at 2174
cm−1, associated with the presence of Ni2+-CO bonds, and at 2128 and 2090 cm−1,
which could be due to the doublet assigned to Ni+(CO)2 species corroboration the
oxidation of Ni during the reaction. The sharp features at 2057, 2039 and 2032 cm−1
could be due to species like physisorbed Ni(CO)4 and Ni(CO)3, which are more stable
at low temperatures (210 ºC) [11,64]. The strong adsorption of CO on Ni particles may
decrease the activity and enhance deactivation by Ni sintering through the formation
of carbonyl clusters and by the formation of stable polycarbonates, which block the
active sites. This fact can explain the small amount of CO2 in the gas phase products
obtained with the calcined catalyst due to the limitation to the water gas shift
reaction for the strong adsorption of CO [65].
Continuing with larger wavelengths, a band at 2280 cm-1 in the Pt/CeO2 spectra
and a broad band at ca. 2450 cm-1 in the Ni-containing catalysts are observed, which
can be assigned to CO2 adsorbed on basic sites. Besides, small negative peaks at 2960,
2930 and 2857 cm-1, assigned to C-H stretching vibrations of methylene groups, are
resolved. This indicates the evolution of these carbonaceous species acting as reaction
intermediates and disappearing from the catalysts surface at a certain point of the
process [50]. Finally, we can observe some peaks related to the presence of OH on
Ce3+/Ce4+ at 3600-3700 cm-1 [57]. The evolution of the OH linked to ceria indirectly
points the participation of the support on the WGS process promoting the hydrogen
production, as discussed before [66].
Overall, the in-situ ATR data underlines the different reaction intermediates
formed depending on the considered metal. It seems that carbonates like surface
species, together with glyceraldehyde and lactic acid, are the main reaction
intermediates. The formation of these species and their further decomposition to the
Aqueous phase reforming of glycerol for hydrogen production over Pt, Ni and Pt-Ni catalysts supported on CeO2
100
desired products is favoured in the Pt containing samples compared to the Ni/CeO2
catalyst, this indicating the suitability of Pt for this reaction.
4. CONCLUSIONS
An interesting catalytic screening using mono and bimetallic Ni-Pt catalysts for
hydrogen production via glycerol APR has been carried out in this paper.
Our data reveal that monometallic platinum is the most active catalysts within the
studied series. In addition it seems that the samples activated “in situ” are more
active than those pre-reduced before the reaction. Apparently, the catalyst activation
in the reaction mixture leads to a higher stability towards metallic sintering.
Within the calcined samples the comparison Pt vs Ni is also valuable. Although Pt
is more efficient in terms of total glycerol conversion, the monometallic Ni/CeO2
provides a good balance between catalytic activity and economic viability. Indeed,
Ni/CeO2 is highly selective towards gas phase conversion yielding to hydrogen rich
streams (without CH4 and CO).
The bimetallic PtNi/CeO2 presented advanced physicochemical properties as for
example, improved redox and electronic properties arising from the intimate Pt-Ni
contact. However under the studied conditions the bimetallic catalysts are not the
best choice.
Finally, our study provides deeper insights in the complicated APR reaction
pathway using operando ATR as a tool to identify the main reaction intermediates.
This analysis reveals notable differences between Pt and Ni based catalysts. The
higher Pt activity is associated to the capacity of Pt to stabilised the intermediates
which further decompose to the desired towards products. The poorer activity of Ni
could be due the strong adsorption of CO on Ni particles may result in Ni sintering
through the formation of carbonyl clusters and stable polycarbonates, which block the
active sites.
Chapter IV
101
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CHAPTER V
CeO2-promoted Ni/activated carbon
catalysts for the water-gas shift (WGS)
reaction
GGrraapphhiiccaall aabbssttrraacctt
Chapter V
107
Abstract
The low temperature water-gas shift (WGS) reaction has been studied over carbon-
supported nickel catalysts promoted by ceria. To this end, cerium oxide has been
dispersed (at different loadings: 10, 20, 30 and 40 wt.%) on the activated carbon
surface with the aim of obtaining small ceria particles and a highly available surface
area. Furthermore, carbon- and ceria-supported nickel catalysts have also been
studied as references. A combination of N2 adsorption analysis, powder X-ray
diffraction, temperature-programmed reduction with H2, X-ray photoelectron
spectroscopy and TEM analysis were used to characterize the Ni-CeO2 interactions
and the CeO2 dispersion over the activated carbon support. Catalysts were tested in
the low temperature WGS reaction with two different feed gas mixtures: the idealized
one (with only CO and H2O) and a slightly harder one (with CO, CO2, H2, and H2O).
The obtained results show that there is a clear effect of the ceria loading on the
catalytic activity. In both cases, catalysts with 20 and 10 wt.% CeO2 were the most
active materials at low temperature. On the other hand, Ni/C shows a lower activity,
this assessing the determinant role of ceria in this reaction. Methane, a product of
side reactions, was observed in very low amounts, when CO2 and H2 were included in
the WGS feed. Nevertheless, our data indicate that the methanation process is mainly
due to CO2, and no CO consumption via methanation takes place at the relevant WGS
temperatures. Finally, a stability test was carried out, obtaining CO conversions
greater than 40 % after 150 h of reaction.
Keywords: Ni; CeO2; Activated carbon; WGS; Low temperature.
CeO2-promoted Ni/activated carbon for the water-gas shift reaction
108
1. INTRODUCTION
Hydrogen is considered as the most promising energy carrier in order to take
advantage of sustainable and renewable energy sources. It allows additional
flexibility in the selection of energy resources and could play a major role in
mitigating the effects of climate change by avoiding or limiting the emission of
greenhouse gases to the atmosphere. The depletion of fossil fuels and the
environmental problems associated to their use as energy sources, have encouraged
the research towards this renewable and sustainable energetic vector [1,2]. For clean
energy production, pure hydrogen is required as a feed gas for electricity generation in
low temperature fuel cells. Hydrogen is generally produced by steam reforming of
hydrocarbons, and reformate includes some other gases, in most cases as co-products,
such as CO, CO2, H2O, CH4 and other hydrocarbons. Among these impurities, CO is
especially damaging as it poisons the Pt catalyst in the polymer electrolyte membrane
(PEM) fuel cell anode, that is deactivated by low levels of CO, and its concentration
must be reduced before entering the fuel cell system [3,4]. Latest technologies are
tolerant up to 100 ppm CO in the feed gas, but the typical reformate exiting a fuel
processor contains higher quantities of carbon monoxide [5]. In this sense, some
processes have been considered in order to minimize the reformate CO concentration
for PEM fuel cell applications as, for example water-gas shift (WGS), CO-methanation
and selective oxidation in the presence of oxygen and/or water. Among them, WGS is
conventionally the first clean-up unit, and it is the reaction that removes the most
important amount of CO in the H2 stream.
In this way, the water-gas shift reaction has been used for long time coupled to the
steam reforming of hydrocarbons to modulate the CO/H2 ratio of the produced gas
stream, as in the case of Fischer-Tropsch synthesis, or even for nearly completely
remove the poisonous CO, as in the case of ammonia synthesis [6]. The WGS reaction
is described as follows:
This reaction is moderately exothermic, being thermodynamically favoured at low
temperatures but kinetically favoured at high temperatures; in this way, the
equilibrium CO conversion decreases with increasing the reaction temperature.
Chapter V
109
Because of this, the reaction is normally carried out in two stages: the high-
temperature shift (350-370 ºC) with iron based catalysts, and the low-temperature
shift (200-220 ºC) with copper-based catalysts [7]. The two stage converter systems
easily and consistently reduce the CO level to 3000-4000 ppm compared to the single
stage converters, which cannot reduce the CO amount to much less than 1 %. In this
sense, a catalyst is needed to keep a good conversion rate at low temperature, as the
copper-based catalysts do not fulfil the security and transient behaviour requirements
which are needed for the new applications.
Due to its high oxygen storage capacity and reducibility (via the Ce4+ ↔ Ce3+ redox
process), cerium oxide is widely used as catalyst support and promoter. One of the
proposed mechanisms for WGS involves the chemisorption of CO on the metal
particles and its subsequent oxidation by the support, through the interaction with
active OH groups on the ceria surface, yielding surface formates. Then, water
decomposes the surface formates to give H2 and CO2, re-oxidizing the ceria surface [8].
In this sense, it seems clear that the role of the ceria surface is determinant for this
reaction, both in terms of extension (large surface areas are needed) but also in terms
of reducibility.
On the other hand, many studies have shown that ceria-supported noble metals are
very efficient systems for the WGS reaction [9-11]. Ceria reducibility is also strongly
enhanced in the presence of these metals, and its reducibility can be correlated with
the WGS performance [12]. Despite the high activity of ceria-supported noble metals
such Pt, Rh, Au, Pd, etc., in the WGS reaction, their high prizes motivate the search
for cheaper and more abundant metals which can be useful in the shift reaction [13].
Several studies have reported good activities of transition metals such a Ni, Cu, Co,
Fe, Mn on many supports for WGS reaction [14-16]. Among these metals, Ni has
received quite attention and has been employed as a promoter of many reactions. Ni-
based catalysts are widely employed in many industrial processes such as reforming
of alcohols, hydrogenation reactions, hydrocracking or oxidation processes [17].
Previous studies have addressed the behaviour of NiO/CeO2 or Ni/CeO2 systems as
catalysts for the title reaction [17-20]. Thus, the Ni-CeO2 system has proven to be
quiet active especially when compared to catalysts containing other metals, both noble
and basic ones. One disadvantage of these Ni-based systems is the possibility of
catalyst deactivation due to the carbon deposits generated by the proved activity of Ni
CeO2-promoted Ni/activated carbon for the water-gas shift reaction
110
in the methanation reaction and in methane decomposition. Several studies have
shown the importance of the amount of deposited Ni in decreasing the extent of the
methanation reaction. In this way, it has been shown that catalysts containing
limited amounts of highly dispersed nickel on ceria apparently do not produce
methane [17,21,22].
In this work, ceria-promoted nickel catalysts supported on activated carbon have
been prepared, characterized and studied in the low-temperature WGS reaction. The
limited supply and extensive applications of CeO2 make it necessary to optimize its
use. With this background, the aim of this work has been to prepare optimized Ni-
CeO2 catalysts in which the interaction between the metal and the oxide is enhanced,
and a large ceria surface is available while reducing the ceria amount in the catalyst.
To this end, CeO2 has been dispersed on a high surface area activated carbon. It has
been previously shown in other works from our group that large ceria dispersions (low
ceria crystallite sizes) can be achieved by this approach, and that this method can be
used to enhance the catalytic behaviour in different reactions [23-25].
2. EXPERIMENTAL SECTION
2.1 Catalysts preparation
The support was an industrial activated carbon (RGC30, from Westvaco). This
carbon was grinded and meshed (300-500 mm). The corresponding amount of
Ce(NO3)3·6H2O (99.99 %, Sigma-Aldrich) was dissolved in acetone. Acetone was used
as solvent to improve the accessibility of the metal precursor to the carbon surface,
taking into account its hydrophobicity and the lower polarity of acetone as compared
with water. Dried carbon was added to the solution, in a proportion of 10 mL/g of
support, with stirring. After 12 h, the excess of solvent was slowly removed under
vacuum at 40 ºC and the solid was then dried in the oven overnight at 80 ºC. Finally,
the dried solid was heat treated during 4 h at 350 ºC under flowing He (50 mL/min),
with a heating rate of 1 ºC/min, in order to slowly decompose the cerium nitrate to
form CeO2, trying to avoid the modification of the carbon surface by the evolved
nitrogen oxides [25]. Four CeO2/C samples were prepared with different nominal CeO2
loadings: 10, 20, 30 and 40 wt.%.
Chapter V
111
Nickel addition to the CeO2/C samples was carried out using the proper amount of
Ni(NO3)2·6H2O (99.9 %, Sigma-Aldrich) in acetone to obtain 15 wt.% Ni, using 10 mL
of solution per gram of solid. After stirring for 12 h, the solvent was removed under
vacuum at 40 ºC. Finally the catalysts were treated at 350 ºC for 4 h under flowing He
(50 mL/min). In this way, four ceria-containing catalysts were prepared, which were
labeled as Ni10CeO2/C, Ni20CeO2/C, Ni30CeO2/C and Ni40CeO2/C. For the sake of
comparison, Ni/C and Ni/CeO2 catalysts were also synthesized. The Ni/C was
prepared similarly to the ceria containing samples, as described above.
For the Ni/CeO2 catalysts, the ceria support was prepared by homogeneous
precipitation from an aqueous solution of Ce(NO3)3·6H2O (99.99 %, Sigma-Aldrich)
containing an excess of urea. The solution was heated at 80 ºC and kept at this
temperature, with slow stirring, during 12 h. The solid formed was filtered and
calcined at 350 ºC for 4 h. The CeO2 support prepared in this way was impregnated
with the Ni precursor as described for the carbon supported catalysts.
2.2 Catalysts characterization
The textural properties of the supports were characterized by nitrogen adsorption
measurements at -196 ºC. Gas adsorption experiments were performed in home-made
fully automated manometric equipment. Prior to the adsorption experiments, samples
were out-gassed under vacuum (10-4 Pa) at 250 ºC for 4 h. The specific surface area
was estimated after application of the BET equation.
The actual metal loading of the different catalysts was determined by ICP-OES in
a Perkine Elmer device (Optimal 3000). To this end, the metal was extracted from the
catalysts by digestion in HNO3/H2O2 (4:1) for 30 min, in a microwave oven at 200 ºC.
X-Ray powder diffraction patterns were recorded on a Bruker D8-Advance with a
Göebel mirror and a Kristalloflex K 760-80 F X-Ray generation system, fitted with a
Cu cathode and a Ni filter. Spectra were registered between 20 and 80° (2θ) with a
step of 0.05° and a time per step of 3 s.
TEM images were taken with a JEOL electron microscope (model JEM-2010)
working at 200 kV. It was equipped with an INCA Energy TEM 100 analytical system
CeO2-promoted Ni/activated carbon for the water-gas shift reaction
112
and a SIS MegaView II camera. Samples for analysis were suspended in ethanol and
placed on copper grids with a holey-carbon film support.
Temperature-programmed reduction (TPR) with H2 experiments were carried out
on the calcined catalysts in a U-shaped quartz cell using a 5% H2/He gas flow of 50
mL/min, with a heating rate of 10 ºC/min. Samples were treated with flowing He at
150 ºC for 1 h before the TPR run. Hydrogen consumption was followed by on-line
mass spectrometry (Pfeiffer, OmniStar GSD 301).
X-Ray photoelectron spectroscopy (XPS) analyses were performed with a VG-
Microtech Multilab 3000 spectrometer equipped with a hemispherical electron
analyzer and a Mg-Kα (h = 1253.6 eV; 1 eV = 1.6302·10-19 J) 300-W X-ray source. The
powder samples were pressed into small Inox cylinders. Before recording the spectra,
the samples were maintained in the analysis chamber until a residual pressure of ca.
5·10-7 N/m2 was reached. The spectra were collected at pass energy of 50 eV. The
intensities were estimated by calculating the integral of each peak, after subtracting
the S-shaped background, and by fitting the experimental curve to a combination of
Lorentzian (30 %) and Gaussian (70 %) lines. The binding energy (BE) of the C l s
peak of the support at 284.9 eV was taken as an internal standard. The accuracy of
the BE values is ±0.2 eV.
2.3 Catalytic tests
The catalytic behaviour of the prepared samples in the low temperature water-gas
shift reaction was evaluated in a fixed bed flow reactor under atmospheric pressure in
the range of temperatures from 140 ºC to 360 ºC. Two different feed gas mixtures were
used, both of them with a total flow of 100 mL/min. The idealized feed gas mixture
contained 1.75 mol% CO and 35.92 mol% H2O in helium; on the other hand, trying to
simulate a closer to actual outgas mixture from a reformer, experiments with a feed
gas composition of 1.75 mol% CO, 35.92 mol% H2O, 34.45 mol% H2, and 1.12 mol%
CO2 in helium were carried out. Activity tests were performed using 0.150 g of
catalyst diluted with SiC, to avoid thermal effects. The corresponding contact time
was 0.09 g s/mL. Prior to reaction, the catalysts were reduced under flowing H2 (50
mL/min) for 2 h at 350 ºC. The composition of the gas stream exiting the reactor was
determined by mass spectrometry (Pfeiffer, OmniStar GSD 301), and the catalytic
Chapter V
113
activity will be expressed by degree of CO conversion as a function of the reaction
temperature. The stabilization time for each temperature was 1 h and the CO
conversion percentage was calculated by this equation:
% CO conversion = (xCO/xCOinitial) · 100
where xCO is the concentration of CO in the outlet of the reactor and xCOinitial is
the CO concentration in the initial gas mixture. The carbon balance was checked
taking into account all the carbon containing products.
3. RESULTS AND DISCUSSION
3.1 Catalysts characterization
3.1.1 Textural and chemical characterization
Table 1 shows the specific surface area (N2, -196 ºC, BET), the micropore volume
(Vmicro, N2, -196 ºC, D-R) and the volume of mesopores (Vmeso) for the parent carbon,
the four ceria-loaded carbons, the four nickel catalysts prepared with the ceria-loaded
carbons as support, the bulk CeO2 and the Ni/CeO2 catalyst. Table 1 also reports the
chemical composition (ICP analysis) of the different catalysts.
The N2 adsorption isotherms at -196 ºC for all the carbon-based materials (not
shown) correspond to a combination of Type I and Type IV isotherms, which are
characteristic of materials containing both micro and mesopores. In fact, the
mesoporous volume is always slightly larger than the microporous volume. Data in
Table 1 show a continuous decrease of the BET surface area for the ceria-loaded
carbons as the amount of ceria increases, and the same trend is observed for both the
microporous and the mesoporous volumes. This effect has been previously reported
[25] and it is attributed to the blockage of porosity by ceria crystallites to a certain
extent. It has also to be taken into consideration that ceria addition decreases the
textural parameters of these materials as compared with those of the parent carbon,
as an effect of the increase of mass and the much lower porosity of ceria as compared
with carbon. The percentage of decrease of these textural parameters was calculated
CeO2-promoted Ni/activated carbon for the water-gas shift reaction
114
and it was found that it is directly proportional to the mass increase due to the CeO2
loaded. In any case, the fact that both the micro- and mesoporous volumes decrease in
the same way is indicative of a good dispersion of ceria on the carbon surface.
Table 1. Textural properties of supports and catalysts, and chemical composition of
catalysts.
SBET (m2/g) Vmicro (cm3/g) Vmeso (cm3/g) Ni (wt.%)* CeO2 (wt.%)*
C 1487 0.52 0.62 -- --
10CeO2/C 1299 0.47 0.59 -- 11.3
20CeO2/C 1083 0.37 0.47 -- 22.2
30CeO2/C 922 0.33 0.40 -- 31.5
40CeO2/C 758 0.26 0.32 -- 40.3
CeO2 101 0.04 0.07 -- --
Ni/C 1238 0.45 0.52 16.3 --
Ni10CeO2/C 1037 0.39 0.42 13.8 --
Ni20CeO2/C 807 0.28 0.34 12.7 --
Ni30CeO2/C 677 0.23 0.28 14.3 --
Ni40CeO2/C 512 0.19 0.21 12.5 --
Ni/CeO2 70 0.03 0.04 14.1 --
* Determined by ICP analysis.
The impregnation stage with the nickel precursor, followed by the thermal
treatment under helium, also produces a decrease in the BET surface area of the
catalysts, which is also attributed to presence of Ni in a high load (15 wt.%).
It has to be noted the much lower surface area of the ceria support and the ceria-
supported nickel catalyst as compared to that their carbon-supported counterparts.
Finally, the actual Ni and CeO2 content of the catalysts were determined by ICP
measurements, and they are reported in Table 1. The obtained values are very close to
the nominal ones.
Chapter V
115
3.1.2 X-ray diffraction
Figure 1 shows the XRD patterns of all the prepared supports after calcination at
350 ºC, where the characteristic CeO2 peaks (JCPDS 34-0394) at 28.6, 33.4, 47.8 and
56.7°, corresponding to the reflections in the (111), (200), (220) and (331) crystalline
planes of the cubic fluorite type phase respectively, can be clearly observed. By
application of the Scherrer equation to the (111) diffraction peak, the mean crystal
sizes of CeO2 in the different catalysts have been determined, and they are presented
in the Table inserted in Figure 1. Whereas the CeO2 XRD profile shows very narrow
diffraction peaks, this indicating a high crystallinity and large crystal size, the
diffraction peaks obtained with the carbon-supported catalysts are much wider, this
indicating a small crystal size and a high ceria dispersion. Thus, a large exposed
surface is expected in these materials, in spite of the fact that they contain a lower
amount of CeO2 than the bulk support [23].
Figure 1. X-ray diffraction patterns and CeO2 crystal sizes both in the bulk and dispersed
on the carbon support.
On the other hand, Figure 2 shows the XRD patterns of the different catalysts after
the heat treatment in He at 350 ºC. Diffraction peaks at 37.2, 43.3, and 62.9°
correspond to the (111), (200) and (220) planes of the NiO fcc phase, respectively, and
are in agreement with JCPDS no. 04-0835 and with others DRX profiles found in the
literature [24-26]. The mean crystal sizes of NiO in the different catalysts are
presented in the Table inserted in Figure 2. The Ni/CeO2 catalyst shows very narrow
20 30 40 50 60 70 80
10CeO2C
20CeO2/C
30CeO2/C
40CeO2/C
Inte
nsity
(a.
u.)
2 (degree)
CeO2
Supports DCeO2 (nm)
10CeO2/C 3.9
20CeO2/C 5.4
30CeO2/C 5.2
40CeO2/C 6.0
CeO2 15.5
CeO2-promoted Ni/activated carbon for the water-gas shift reaction
116
20 30 40 50 60 70 80
Ni/C
Ni10CeO2/C
Ni20CeO2/C
Ni30CeO2/C
Ni40CeO2/C
Inte
nsity
(a.u
.)
2 (degree)
Ni/CeO2
Catalysts DNiO (nm)
Ni/C 2.2
Ni10CeO2/C 2.9
Ni20CeO2/C 4.2
Ni30CeO2/C 5.0
Ni40CeO2/C 4.9
Ni/CeO2 19.6
diffraction peaks, thus indicating a high crystallinity and large NiO crystal size; on
the other hand, results are different for Ni/C, with a very small size of NiO
crystallites and thus, a high dispersion.
Figure 2. X-ray diffraction patterns of catalysts.
3.1.3. H2-temperature-programmed reduction
H2-TPR measurements were employed in order to assess the type of reducible
species formed after the heat treatment of the catalysts under He flow. The TPR
profiles are presented in Figure 3. The TPR profiles of the CeO2/C supports (Figure
3a) showed two close reduction peaks at around 500 and 600 ºC. This profile is
different to those obtained from massive CeO2. Thus, the typical TPR profile for
unsupported CeO2 includes two distinct reductions peaks, one at low temperature,
around 500 ºC, and a second one at higher temperatures, around 850 ºC, which are
associated to the reduction of surface and bulk ceria, respectively [23,25,27-31]. In the
case of CeO2/C, the high temperature peak shifts to lower temperatures and it now
appears overlapped with the low temperature one. This indicates a high dispersion of
cerium oxide on the carbon support, as pointed out in our previous results. It can be
assumed that ceria is present on the carbon surface as small size particles, as
evidenced by XRD; this leads to a larger proportion of surface ceria that is reduced at
low temperature, and to an easier reduction of the remaining bulk ceria as compared
to the bulk oxide. Furthermore, the TPR profile of the activated carbon is also shown.
Chapter V
117
One peak appears around 630 ºC due to the reduction of the low amount of oxygen
surface functional groups it contains.
Figure 3. H2-TPR profiles for supports (a) and catalysts (b).
On the other hand, three reduction peaks can be observed in the H2-TPR profiles
for NixCeO2/C (Figure 3b). The first low temperature peak is attributed to the
reduction of highly dispersed NiO on the support, and the intermediate ones
correspond to the surface reduction of ceria which interacts with the nickel particles
to different extents. Several types of Ni species coexist depending of the Ni/Ce ratios.
In the high nickel content samples, crystallized nickel oxide particles, Ni-Ce solid
solution and highly dispersed NiO phase can coexist [32,33]. For the Ni/C catalyst two
peaks can be observed. The first one at low temperature corresponds to the reduction
of highly dispersed NiO on the support, and the second one at intermediates
temperatures is ascribed to the reduction of medium sized particles. Previous results
in the literature show that the presence of Ni decreases the reduction temperature of
ceria, what can be associated to the spillover of hydrogen atoms from the nickel
particles to the ceria support [34]. This can be seen in the Ni/CeO2 reduction profile,
where there is an intermediate temperature peak slightly shifted to lower
temperatures due to the Ni-Ce interaction that decreases the ceria reduction
temperature. Also, two small peaks can be observed at lower temperatures that
correspond to the reduction of highly dispersed NiO on the ceria. On the other hand, a
small peak at high temperatures (around 830 ºC) can also be observed, which is
assigned to the reduction of bulk CeO2 in the Ni/CeO2 catalyst [35,36].
0 100 200 300 400 500 600 700 800 900 1000
Temperature (ºC)
Carbon
Inte
nsity
(a.
u.)
10CeO2/C
20CeO2/C
30CeO2/C
40CeO2/C
CeO2
0 100 200 300 400 500 600 700 800 900 1000
Inte
nsi
ty (
a.u
.)
Ni/CeO2
Ni40CeO2/C
Ni30CeO2/C
Ni20CeO2/C
Ni/C
Temperature (ºC)
Ni10CeO2/C
a) b)
(a)
CeO2-promoted Ni/activated carbon for the water-gas shift reaction
118
3.1.4 XPS characterization of reduced catalysts
Chemical changes in the catalysts after reduction in H2 at 350 ºC were investigated
by XPS. The samples were reduced ex situ and conserved in octane until the analysis.
The binding energies (B.E.) of the main peaks of the Ni 2p3/2 and Ce 3d5/2 energy levels
are summarized in Table 2. Figure 4 shows the XPS spectra obtained for the Ni 2p3/2
region.
Table 2. Binding energies of the Ni 2p3/2 and Ce 3d5/2 levels in the catalysts reduced at
350 ºC.
Catalysts Ni 2p3/2 (eV) Ce 3d5/2 (eV)
Ni2+ Ni0
Ni/C 854.0-855.8 852.6 --
Ni10CeO2/C 854.1-856.4 852.7 881.6
Ni20CeO2/C 854.8-856.9 852.8 882.7
Ni30CeO2/C 854.2-856.5 852.6 882.4
Ni40CeO2/C 854.7-856.8 852.7 882.4
Ni/CeO2 855.4-857.6 853.5 883.3
The reduced catalysts showed complex Ni 2p3/2 spectra, this indicating the presence
of different nickel species. In this way, the band at 852.6 eV is assigned to Ni0; the
following two bands, at around 854.7 and 856 eV are assigned to NiO and Ni(OH)2
species, respectively [21,27,37]. The binding energies of these two Ni2+ species in the
ceria-containing catalysts are slightly larger than the values reported for the pure
compounds and also than these species on the carbon supported Ni/C catalyst, which
is indicative of their interaction with ceria. The existence of a strong metal-support
interaction between Ni and CeO2 can modify the structure and electronic properties of
Ni which could improve the catalytic behavior of this system [38]. It was clear that
none of the samples were completely reduced after the reduction treatment.
Furthermore, the band corresponding to metallic Ni in Ni/CeO2 is strongly shifted to a
higher binding energy, 853.5 eV, as compared to the carbon-supported catalyst. These
results confirm the existence of Ni-CeO2 interactions, as also observed in H2-TPR.
Chapter V
119
Figure 4. Core level XPS of the Ni 2p3/2 region for catalysts reduced at 350 ºC.
The main line of Ce 3d5/2 appears at 882.4 eV (not shown), which is typically
ascribed to CeO2 species. The bulk catalyst presents B.E. values slightly higher than
for carbon supported catalysts due to the interactions of bulk CeO2 with nickel [26,36-
38].
3.1.5 Transmission electron microscopy
Figure 5 shows TEM images of the fresh calcined Ni/CeO2, Ni10CeO2/C,
Ni20CeO2/C, Ni30CeO2/C, Ni40CeO2/C and Ni/C materials. It can be seen that the
active phases are homogeneously dispersed on the carbon support, with no large
agglomerations being observed. It is important to point out that it was not possible to
distinguish between CeO2 and Ni in the carbon-supported catalysts, as both species
appear in the images as black dots due to their similar electron density. However,
EDX experiments (not shown) revealed the presence of both oxides in direct contact.
The Ni/CeO2 catalyst shows large crystal sizes of both CeO2 and NiO, thus confirming
the XRD results.
850 852 854 856 858 860 862 864 866 868
Inte
nsi
ty (
a.u
.)
B.E. (eV)
Ni/CeO2
Ni40CeO2/C
Ni30CeO2/C
Ni20CeO2/C
Ni10CeO2/C
Ni/C
CeO2-promoted Ni/activated carbon for the water-gas shift reaction
120
f))
a)
)
c)
)
b)
)
d)
)
e)
)
Figure 5. TEM micrographs of a) Ni/C, b) Ni10CeO2/C, c) Ni20CeO2/C, d) Ni30CeO2/C, e)
Ni40CeO2/C, f) Ni/CeO2 (scale bar: 10 nm).
TEM micrographs also show that even with the large ceria and nickel loadings
used, the surface of the carbon support is not completely covered. This indicates that
the nickel particles may be located both on ceria and on the bare carbon surface, with
different degrees of Ni-Ce interactions which are clearly evidenced by the complex
TPR profiles obtained.
3.2 Catalytic behaviour
The catalytic performance of the reduced catalysts (flowing H2, 350 ºC), in terms of
CO conversion as a function of temperature during the low-temperature WGS
reaction with an ideal mixture (1.75 % CO, 35.92 % H2O, and He balance), is reported
in Figure 6. The catalytic activity of the CeO2/C supports was also checked, and
conversions lower than 2 % were obtained underlining the role of Ni as an active
phase in the WGS.
It can be seen that the Ni/C catalyst is less active than the ceria-containing
samples. This result reflects the essential role of ceria in this reaction due not only to
its well-known redox properties, but also to its ability to improve the stability and the
catalytic performance of these catalysts by the synergistic effect between the Ni and
Chapter V
121
CeO2 [38-42]. For the NixCeO2/C catalysts there is a clear effect of the ceria loading on
the catalytic activity.
Figure 6. CO conversion vs. reaction temperature in idealized feed (1.75% CO, 35.92% H2O,
and He balance), for catalysts reduced at 350 ºC.
Figure 7 shows the CO conversion at 200 ºC for all the catalysts checked as a bars
graph. It can be seen that Ni20CeO2/C is the most active sample at this low
temperature. Only at temperatures above 230 ºC all the catalysts show the same
activity, close to 97 % of CO conversion (equilibrium conversion).
Figure 7. CO conversion at 200 ºC for the NixCeO2/C catalysts reduced at 350 ºC (ideal gas
stream feed).
140 160 180 200 220 240 260 2800
10
20
30
40
50
60
70
80
90
100
CO
co
nve
rsio
n (
%)
Temperature (ºC)
Ni/C
Ni10CeO2/C
Ni20CeO2/C
Ni30CeO2/C
Ni40CeO2/C
Ni/CeO2
equilibrium curve
CeO2-promoted Ni/activated carbon for the water-gas shift reaction
122
The synergism between the metal and the oxide support is clearly established
[20,43]. There are two general proposed mechanisms for this reaction, the redox
mechanism and the associative mechanism. In both cases the interaction between the
metal and the oxide support is of great importance [8]. As it has been shown by H2-
TPR (Figure 3b), the relative intensity of the medium temperature peak attributed to
the surface reduction CeO2 interacting with Ni particles is higher for the samples
containing 10-20 wt.% ceria. In principle, the strongest Ni-CeO2 interaction is
expected for these solids. As proposed by Rodriguez and co-workers, most of the WGS
reaction steps take place in the metal-support interface [42]. Therefore, the best WGS
performance is expected when the population of Ni-Ce interface sites is maximum.
For this reason, it is important to obtain a high surface area of the ceria promoter
that can interact with the nickel particles providing high concentration of active
reaction sites [44].
Undesirable products from side reactions, such as CH4 and other hydrocarbons,
were not detected in these experiments carried out with an idealized feed composition.
Even at 97 % CO conversion, the only products were CO2 and H2, this indicating the
high selectivity of these catalysts toward H2 production. These findings are also
consistent with the literature, as either suppression or no detection of methanation
activity is reported over Ni-modified ceria catalysts [17,20].
The effect of H2 and CO2 addition into the feed gas stream on the CO conversion
over all catalysts was also studied; these gases are present in many practical gas
streams exiting a reformer, and it is reported that these compounds usually inhibit
the WGS reaction rate [21]. Thus, it has been reported that CO2 can produce
deactivation due to the formation of carbonates on the ceria support [45,46], and an
excess of hydrogen may produce irreversible over-reduction of ceria [47].
Figure 8 shows the catalytic activity results in terms of CO conversion as a
function of temperature of the prepared catalysis, reduced in situ at 350 ºC, during
the low-temperature WGS reaction with a feed mixture containing the main
components of a post-reforming stream (1.75 % CO, 35.92 % H2O, 34.45 % H2, 1.12 %
CO2 and He balance).
Chapter V
123
Figure 8. CO conversion vs. reaction temperature in harder feed (1.75 % CO, 35.92 % H2O,
34.45 % H2, 1.12 % CO2 and He balance) for catalysts reduced at 350 ºC.
It can be seen that the H2 and CO2 addition into the feed decreases the activity of
all the catalysts, although to different extents. Thus, the most affected one is Ni/C,
which was able to achieve a nearly complete CO conversion at 240 ºC with the ideal
gas mixture, but it now needs to work at around 330 ºC to reach a somewhat lower CO
conversion. The effect of the feed stream composition is less dramatic for the ceria
containing catalysts. Thus, Ni/CeO2 reached a 50 % CO conversion at 220 ºC with the
ideal gas mixture and at 230 ºC with the harder gas mixture, and similar differences
are obtained for the NixCe/C catalysts. Interestingly, all the ceria-promoted carbon-
supported catalysts show a better behaviour than Ni/CeO2 also under this feed, this
indicating the importance of a good ceria dispersion (small particle size) and a good
interaction with the active metal [48]. Thus, an optimum load of 10 wt.% CeO2 was
enough to obtain an efficient performance under reaction with the feed containing all
the reformate products. Actually, this sample is the one that presents the smallest
ceria particle size (Figure 1) and, therefore, the highest population of Ni-CeO2
reaction sites are expected.
It is broadly reported that bulk Ni species promote the formation of CH4 under
these WGS reaction conditions since metallic nickel is a well-known catalyst for
methanation [16]. On the other hand, Ni-based catalysts are among the most effective
and affordable metal catalysts for WGS [49]. Another important issue that has been
extensively studied is the nature of catalytically active species in the methanation
0
10
20
30
40
50
60
70
80
90
100
140 160 180 200 220 240 260 280 300 320 340
Ni/C
Ni10CeO2/C
Ni20CeO2/C
Ni30CeO2/C
Ni40CeO2/C
Ni/CeO2
CO
con
vers
ion
(%)
Temperature (ºC)
equilibrium curve
CeO2-promoted Ni/activated carbon for the water-gas shift reaction
124
process. It is well known that the hydrogenation process occurs only at metallic Ni0
sites and that other Ni species, as oxides, hydroxides or carbides are not active in the
methanation reaction. In this context, the methanation issue must be addressed to
validate the excellent performance of our Ni-based catalysts in the WGS reaction.
Figure 9 shows the evolution of CO, CO2 and CH4 concentrations at the reactor exit
for the Ni/C catalyst, as a function of temperature, using a CO2-free feed stream (1.75
% CO, 35.92 % H2O, 34.45 % H2, and He balance). The plot clearly demonstrates that
methane formation was exclusively due to CO2 hydrogenation, since no methane
appears at high CO concentrations (temperatures below 270 ºC). Indeed methane
concentration increases inversely to that of CO2, this indicating that CO2 is being
consumed to produce methane. Furthermore and very importantly, methanation
starts when all CO has been converted to CO2 via WGS. At this point, the CO2
concentration is maximum, favouring its transformation to methane. The CO
conversion observed in the WGS test is only attributable to the shift process. This
result reinforces the successful application of our Ni-based catalysts in the WGS
reaction.
Figure 9. CO, CO2 and CH4 concentrations at the exit of the reactor for Ni/C, reduced at 350
ºC, as a function of reaction temperature.
Finally, a stability test has been carried out on the most active catalyst,
Ni10CeO2/C, in harder feed at 220 ºC. Figure 10 shows the CO conversion as a
function of time. In a first stage, the gas stream composition was stabilized before
being put in contact with the catalyst. It can be seen that a high CO conversion of
about 55 % is achieved during the first 5 h on stream; then, a marked deactivation
140 160 180 200 220 240 260 280 300 320 340
0.0
0.5
1.0
1.5
2.0
2.5
3.0
Gas
con
cent
ratio
n (%
)
Temperature (ºC)
CO
CO2
CH4
Chapter V
125
takes place during next hours, but after 150 h of reaction the CO conversion is still
larger than 40 %.
Figure 10. Stability test for Ni10CeO2/C at 220 ºC.
The observed deactivation trend can be attributed to a decrease of the WGS
catalytic activity when Ni particles become larger by sintering [42]. In this sense,
TEM micrographs of Ni10CeO2/C catalyst were taken before and after the stability
test (Figure 11a) and b)), and it can be seen that some agglomerations were observed
in the used catalyst. It has been reported that sintering of nickel particles cannot be
avoided under conditions of high temperature and high water content [44,49] as water
favours the growing of Ni particles. This reduces the catalytic surface area and results
in a decreased activity [48]. It has been recently demonstrated that adsorbate-metal
complexes can be formed with low formation energies and/or low diffusion barriers
which may induce coarsening phenomena. This led to the assumption that Ni2-OH
complexes are the major transport species at the surface of nickel particles in H2O/H2
atmospheres [50-55].
Figure 11. TEM micrographs of the Ni10CeO2/C catalyst before (a) and after (b) the
stability reaction test at 220 ºC (scale 10 nm).
0 20 40 60 80 100 120 140
0
10
20
30
40
50
60
CO
con
vers
ion
(%)
Time (h)
CeO2-promoted Ni/activated carbon for the water-gas shift reaction
126
The slight modification of the size of nickel particles due to sintering results in a
slow decrease of activity. Suppression of the metal particle growing over the ceria-
based supports has been reported, and it was found that the strong metal-support
interaction plays an important role in controlling the sintering and it is also known
that ceria-based supports improve the thermal stability of the metal particles [49].
4. CONCLUSIONS
We have developed highly active activated carbon-supported Ni-ceria catalysts
(NixCeO2/C) for the low temperature water-gas shift reaction. Cerium oxide has been
dispersed over an activated carbon support at different loadings, obtaining small ceria
particles and a highly available surface area, in spite of the fact that they contain a
lower amount of CeO2 than the bulk support. XRD and TEM studies have confirmed
the small crystal size (4-6 nm) and high dispersion of CeO2 in comparison with
massive CeO2 catalysts (15 nm). This leads to an easier reduction of ceria, which
interacts with the nickel particles to different extents (H2-TPR). After a reduction
treatment at 350 ºC, XPS analysis showed the presence of metallic nickel and
different oxidized nickel species, and a shift of the binding energy to larger values,
which is attributed to the Ni-CeO2 interaction. Catalytic activity using an ideal
mixture feed shows the essential role of ceria in this reaction. In addition, a clear
effect of the ceria loading and dispersion on the catalytic behaviour was observed. We
found that the optimum load was 20 wt.% CeO2. Side reactions such as methanation
were not detected in these conditions.
The effect of the addition of H2 and CO2 to the feed gas mixture on the CO
conversion was also studied. The catalytic activity was slightly diminished for the
ceria-carbon based catalysts, but a strong deactivation was observed in Ni/C. Under
these more realistic conditions, the catalyst with 10 wt.% CeO2 exhibited the best
performance. A point to be highlighted is the absence of CO methanation. All the CO
conversion observed was achieved via WGS. Nevertheless, methanation reaction was
detected at high temperatures, but it resulted to be completely selective towards CO2
hydrogenation.
Chapter V
127
Finally, a CO conversion greater than 40 % was obtained in a stability test after
150 h of reaction at 220 ºC with Ni10CeO2/C catalyst. The results obtained in this
study permit to conclude that this kind of catalysts are promising for the low-
temperature water-gas shift reaction, allowing to decrease the amount of ceria needed
to obtain high CO conversions by increasing its available surface area.
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CHAPTER VI
Ni-CeO2/C catalysts with enhanced OSC
for the WGS reaction reaction
GGrraapphhiiccaall aabbssttrraacctt
Chapter VI
133
Abstract
In this work, the WGS performance of a conventional Ni/CeO2 bulk catalyst is
compared to that of a carbon-supported Ni-CeO2 catalyst. The carbon-supported
sample resulted to be much more active than the bulk one. The higher activity of the
Ni-CeO2/C catalyst is associated to its oxygen storage capacity, a parameter that
strongly influences the WGS behavior. The stability of the carbon-supported catalyst
under realistic operation conditions is also a subject of this paper. In summary, our
study represents an approach towards a new generation of Ni-ceria based catalyst for
the pure hydrogen production via WGS. The dispersion of ceria nanoparticles on an
activated carbon support drives to improved catalytic skills with a considerable
reduction of the amount of ceria in the catalyst formulation.
Keywords: WGS; cerium oxide; carbon support; OSC; Ni based catalysts
Ni-CeO2 catalysts whit enhance OSC for the WGS reaction
134
1. INTRODUCTION
Nowadays, synthesis gas (a mixture of hydrogen and carbon monoxide) is generally
produced by steam reforming of hydrocarbons, especially if the principal objective is
the generation of gas streams with high H2/CO ratios. For clean energy production,
pure hydrogen is required as a feed gas for electricity generation in low temperature
fuel cells [1]. In this sense, some processes have been considered in order to minimize
the reformate CO concentration for PEM fuel cell applications as for example water-
gas shift (WGS). Conventionally, in an integrated fuel processor, the WGS reactor is
the first clean-up unit, and it is the reaction that removes the most important amount
of CO in the H2 stream [2].
Due to its high oxygen storage capacity and reducibility (via the Ce4+↔Ce3+ redox
process), cerium oxide is widely used as catalyst support and promoter in the water-
gas shift reaction [3]. The role of the ceria surface is determinant for this reaction,
both in terms of reducibility but also in terms of extension (large exposed surface
areas are needed). Furthermore ceria reducibility is favored by the presence of metals
[4,5].
One of the key points that convert ceria in a highly suitable support for the shift
reaction relies on its ability to provide oxygen to the process. Indeed the WGS is a
redox reaction, thus the oxygen mobility of the catalyst is regarded as a main issue in
the catalytic design [6]. The later makes obligatory an accurate study of the redox
features exhibited by the WGS systems, namely, the oxygen storage complete capacity
(OSCC) and oxygen storage capacity (OSC). Recently, a great increase in both the
OSC and the OSCC has been proposed as one of the main reasons to explain the
activity promotion when ceria is dispersed on a high surface alumina [7]. In this way,
promoting redox properties of the catalysts, namely surface and bulk oxygen mobility
should benefit the WGS performance. Therefore, oxygen supply from the support is a
highly desired feature for an effective WGS catalyst. Dispersing ceria over a high
surface support such as carbon provides higher surface/bulk ratios improving oxygen
mobility. In addition, the limited supply and extensive applications of CeO2 makes
desirable to optimize its use [8].
Chapter VI
135
On the other hand, ceria-supported noble metals are mostly used due to the high
activity in the WGS reaction. However, their elevated prices motivate the search for
cheaper and more abundant metals which can be useful in the shift reaction. Ni-based
catalysts are widely employed in many industrial processes such as reforming of
alcohols, hydrogenation reactions and hydrocracking or oxidation processes [9].
Previous studies have addressed the behaviour of NiO/CeO2 or Ni/CeO2 systems as
catalysts for the title reaction. Despite good activities were reported, Ni containing
catalysts suffer for deactivation mainly due to the well-known metallic particles
sintering process together with the accumulation of carbon deposits on the catalysts
surface [10,11].
Another point to consider is the adaptation of the new catalysts formulation to the
budding market demands. The emerging fuel cell technology for portable electronic
devices is changing the requisites for the WGS catalysts. Apart from developing a
solid with the properties mentioned above, an industrial catalyst for WGS reaction
requires in addition high long term stability and good resistance toward start/stop
situations [12].
In a previous work [13] NixCeO2/C catalysts with different CeO2 contents (x = 10,
20, 30, 40 %wt.) were tested in the WGS reaction. Detailed characterization was
performed, deepening in both physical and chemical characteristics of these materials.
The results showed that there was a clear effect of the ceria loading on the catalytic
activity. It could be seen that the Ni-CeO2-carbon system yielded catalysts which
showed better performance than bulk Ni/CeO2.
In this scenario, and taking into account our previous results [13] the objective of
this work is to correlate the OSC of these catalysts with their high activity. A study of
the enhancement of the oxygen storage capacity when ceria is dispersed on carbon is
carried out. For this study, a sample with a 20 wt.% CeO2 loading was chosen due to
the interesting results obtained before. Additionally, and differently to our previous
study, water-gas shift reactions employing more demanding WGS conditions were
carried out. Further, stability tests and start/stop cycles were developed in order to
evaluate the viability of this catalyst in real applications.
Ni-CeO2 catalysts whit enhance OSC for the WGS reaction
136
2. EXPERIMENTAL SECTION
2.1 Catalysts preparation
The procedure used for the catalyst synthesis was the same than in our previous
work [13]. The support was an industrial activated carbon (RGC-30, from Westvaco,
New York, NY, USA). This carbon was grinded and meshed (300–500 μm). The
corresponding amount of Ce(NO3)3·6H2O (99.99 %, Sigma–Aldrich, St. Louis, MO,
USA) to obtain 20 wt.% of CeO2 was dissolved in acetone. Dried carbon was added to
the solution, in a proportion of 10 mL/g of support, with stirring. After 12 h, the excess
of solvent was slowly removed under vacuum at 40 °C and the solid was then dried in
the oven overnight. Finally, the dried solid was heat treated during 4 h at 350 °C
under flowing He (50 mL/min), with a heating rate of 1 °C/min, in order to slowly
decompose the cerium nitrate to form CeO2, trying to avoid the modification of the
carbon surface by the evolved nitrogen oxides [25].
Nickel addition to the CeO2/C solid was carried out using the proper amount of
Ni(NO3)2·6H2O (99.9 %, Sigma-Aldrich, St. Louis, MO, USA) in acetone to obtain 15
wt.% Ni, using 10 mL of solution per gram of solid. After stirring for 12 h, the solvent
was removed under vacuum at 40 °C. Finally, the solid was treated at 350 °C for 4 h
under flowing He (50 mL/min). The catalyst prepared was labeled Ni-CeO2/C. For the
sake of comparison, a Ni/CeO2 catalyst was also synthesized. The ceria support was
prepared by homogeneous precipitation from an aqueous solution of Ce(NO3)3·6H2O
(99.99 %, Sigma-Aldrich, St. Louis, MO, USA) containing an excess of urea. The
solution was heated at 80 °C and kept at this temperature, with slow stirring, during
12 h. The solid formed was filtered and calcined at 350 °C for 4 h. The CeO2 support
prepared in this way was impregnated with the Ni precursor as described for the
carbon supported catalysts.
2.2 Catalysts characterization
The textural properties of the supports were characterized by nitrogen adsorption
measurements at −196 °C. Gas adsorption experiments were performed in home-made
fully automated manometric equipment. Prior to the adsorption experiments, samples
Chapter VI
137
were out-gassed under vacuum (10−4 Pa) at 250 °C for 4 h. The specific surface area
was estimated after application of the BET equation.
The actual metal loading of the different catalysts was determined by ICP-OES in
a Perkin Elmer device (Optimal 3000). To this end, the metal was extracted from the
catalysts by digestion in HNO3/H2O2 (4:1) for 30 min, in a microwave oven at 200 °C.
For the Oxygen Storage Complete Capacity (OSCC) 100 mg of catalyst was loaded
into a U-shaped quartz reactor and the temperature was raised in a He flow (50
mL/min) until 350 °C. Then, the system was cooled and set to the desired temperature
(150, 250 and 350 °C). For each temperature 10 O2 pulses of 1 mL were injected every
2 min. After that, the sample was submitted to 10 CO pulses of 1 mL each (every 2
min). The OSCC was calculated from the sum of the CO2 formed after each CO pulse.
The sample was then degassed during 10 min in a He flow to perform the OSC
measurements. Four alternating series of pulses (CO-O2-CO-O2-CO-O2-CO-O2) were
applied for each temperature. The OSC was determined by the average amount of
CO2 per pulse formed after the first CO pulse of the alternated ones. This method is
based on the one proposed by Duprez et al. [26]. The gas composition at the exit of the
reactor was analyzed by a mass spectrometer PFEIFFER Vacuum PrismaPlus
controlled by Quadera® software (version 4.0, INFICON, LI-9496 Balzers,
Furstentum Liechtenstein).
Several assumptions were contemplated for the OSC calculations. Concretely, it
was considered that (i) only oxygen atoms bonded to the cerium participate in the
oxygen storage process; (ii) the surface is assumed homogeneous and (iii) only one of
the four oxygen atoms is involved in the storage (2CeO2→Ce2O3 + “O”)
TEM images were taken with a JEOL electron microscope (model JEM-2010)
working at 200 kV. It was equipped with an INCA Energy TEM 100 analytical system
and a SIS MegaView II camera. Copper grids with a holey-carbon film support were
used for the microscopy measurements. The samples were suspended in ethanol and
placed on the grids for the analysis.
Ni-CeO2 catalysts whit enhance OSC for the WGS reaction
138
2.3 WGS Catalytic tests
The catalytic behaviour of the prepared samples in the low temperature water-gas
shift reaction was evaluated in a fixed bed flow reactor under atmospheric pressure in
the range of temperatures from 160 °C to 380 °C. For the stability tests the
temperature employed was 250 °C. Trying to simulate a more close to real outgas
mixture from a reformer, experiments with a feed gas composition of 7 mol% CO, 30
mol% H2O, 50 mol% H2, and 9 mol% CO2 balanced to 100 mL/min with helium was
tested. Activity tests were performed using 0.150 g of catalyst diluted with SiC, to
avoid thermal effects. The corresponding contact time was 0.09 g·s/mL Prior to
reaction, the catalysts were reduced under flowing H2 (50 mL/min) during 2 h at 350
°C. The composition of the gas stream exiting the reactor was determined by mass
spectrometry (Pfeiffer, OmniStar GSD 301). The stabilization time for each
temperature was 1 h and the CO conversion percentage was calculated by this
equation:
% CO conversion = 100 − (xCO/xCOinitial)·100
where xCO is the molar concentration of CO in the outlet of the reactor and
xCOinitial is the CO concentration in the initial gas mixture. The carbon balance was
checked taking into account all the carbon containing products.
3. RESULTS AND DISCUSSION
3.1 Catalysts characterization
3.1.1 Textural and chemical characterization
The chemical composition and the main textural properties of the prepared
catalysts are presented in Table 1. The enhanced textural properties of the carbon
materials are evidenced, all of them exhibiting larger surface area compared to the
bulk solids. As previously reported, carbon is playing a role of textural promoter in
these type of catalysts [13]. Data in Table 1 show a decrease of the BET surface area
for ceria-carbon based samples, attributed to the blockage of porosity by ceria
Chapter VI
139
crystallites and/or an effect of mass increment and the much lower porosity of ceria as
compared with carbon. The addition of nickel also produces a decrease in the BET
surface area of the catalysts, which is also attributed to presence of high amounts of
Ni (15 wt.%).
Regarding the elemental analysis, the actual Ni and CeO2 content of the catalysts
were determined by ICP (Inductively Coupled Plasma) measurements, and they are
reported in Table 1. The obtained values are very close to the targeted ones, this
validating the preparation method used.
Table 1. Catalysts composition and textural properties of the prepared samples.
SBET (m2/g) Vmicro (cm3/g) Vmeso (cm3/g) Ni (wt.%)* CeO2(wt.%)*
C 1487 0.52 0.62 -- --
20CeO2/C 1083 0.37 0.47 -- 22.2
CeO2 101 0.04 0.07 -- --
Ni20CeO2/C 807 0.28 0.34 12.7 21.6
Ni/CeO2 70 0.03 0.04 14.1 --
* Determined by ICP analysis.
3.1.2 OSCC and OSC
As mentioned above, redox properties constitute one of the main issues to be
optimized for the successful design of an efficient WGS catalyst. An accurate
understanding of such skills is obtained by the oxygen storage capacity
measurements. In particular, OSCC measurements provide information about the
maximum reducibility of the samples, while OSC informs about the most reactive and
most available oxygen atoms that are involved in the redox process [14]. This study
was carried out at two different temperatures (150 °C and 250 °C) both of them low
enough to avoid any possible carbon combustion due to the oxygen pulses. Actually
our TGA data (not shown for sake of briefness) point that carbon combustion in our
catalysts under air atmosphere starts at 320 °C. The selected temperatures are
relevant points in the catalytic study. Therefore, the analysis of the redox behavior of
both catalysts at these temperatures provides valuable information to understand
their WGS performance. Figure 1 shows the OSCC results of the prepared solids. As
expected, the OSCC increases with the temperature for both supports and catalysts,
indicating higher degree of ceria reduction at 250 °C. In the case of the supports
Ni-CeO2 catalysts whit enhance OSC for the WGS reaction
140
(Figure 1A) it must be underlined the superior OSCC exhibited for the CeO2/C system
at the studied temperatures. These data indicate that the dispersion of ceria
nanoparticles on a high surface carrier as activated carbon results in a notorious
improvement of its reducibility. In other words, enhanced redox properties are
achieved for the CeO2/C solid in comparison to the bulk CeO2. Even though carbon is
not playing a chemical role in the oxygen storage process, it acts as an ideal media to
support ceria nanoparticles providing high surface area (as shown in Table 1). Similar
results were obtained when CeO2 is dispersed over gamma alumina [15]. In this
sense, dispersing ceria over a high-surface carrier opens up the possibility of having
higher surface/bulk ratios thus improving the oxygen mobility no matter the used
support. However it is worth noting that our ceria/carbon based samples present
higher OSCC values than those observed for ceria/alumina systems measured under
the same conditions [15]. The latter indicates the suitability of carbon as a textural
promoter allowing better dispersion of ceria nanoparticles.
Nickel addition (Figure 1B) enhances the OSCC for both, bulk CeO2 and CeO2/C
support. This result agrees with H2-TPR data recently published for these solids
evidencing a remarkable increase of the support reducibility due to the strong Ni-
CeO2 interaction [13]. Again, as observed for the bare supports, much higher OSCC
values are obtained when ceria and nickel phases are dispersed on the activated
carbon.
Figure 1.OSCC of the prepared solids. A) supports B) Ni based catalysts.
The OSC study is summarized in Table 2. At 150 °C the supports presented low
oxygen mobility; however, the OSC improves when the temperature is raised. In a
similar way to the OSCC, the OSC of the CeO2/C sample is always higher than that of
100 150 200 250 3000
500
1000
1500
2000
2500
3000
3500
m
ol
CO
2/
g s
am
ple
Temperature (oC)
CeO2
CeO2/C
A OSCC
100 150 200 250 3000
500
1000
1500
2000
2500
3000
3500
B
m
ol
CO
2/
g s
am
ple
Temperature (oC)
Ni/CeO2
Ni-CeO2/C
OSCC
Chapter VI
141
the bulk CeO2 support. As stated above, Ni influences CeO2 reducibility due to the
intimate Ni-CeO2 contact [16]. This effect is highlighted at low temperature (150 °C),
where the metallic particles broadly boost the OSC of the parent supports. In any
case, for all the studied temperatures, the Ni-CeO2/C catalyst shows the best oxygen
mobility. The promoted redox features of this catalyst are related with two inherent
aspects of its composition: (i) the dispersion of ceria nanoparticles on the activated
carbon, which increases the surface/bulk ratio, thus potentiating the oxygen mobility
in the ceria lattice and, (ii) the facilitated CeO2 reduction due to the presence of Ni,
arising from a strong metal-support interaction.
Table 2. OSC in μmol CO2/g sample for the studied samples at different temperatures.
3.2 WGS Behaviour
The catalysts were evaluated under a surrogate post reforming stream (7 mol%
CO, 30 mol% H2O, 50 mol% H2, and 9 mol% CO2 balanced with He) with the aim to
test their viability for a real application, for instance the use of a WGS reactor in an
integrated fuel processor for pure hydrogen production. The catalytic activity of the
prepared Ni-ceria based catalysts in the shift reaction is presented in Figure 2. It
should be mentioned that the supports were also tested under the same conditions,
showing an almost nil activity in the studied temperature range. This underlines the
importance of the metallic phase to achieve good performance in the WGS when ceria-
based solids are considered. As intended from the plot, the samples are not very
effective in the low temperature range (<200 °C). At these conditions the solids
present limited oxygen mobility (low OSC values) that may contribute to the observed
low CO conversion. Nevertheless, the catalytic activity rapidly increases with the
temperature. The Ni-CeO2/C sample evidences superior WGS activity compared to the
Ni/CeO2 one in the whole temperature range. Taking into account that the amount of
metallic phase (Ni) is comparable in both materials, as obtained by ICP, the higher
Sample OSC150ºC OSC250ºC
CeO2 10 180
CeO2/C 25 255
Ni/CeO2 410 726
Ni-CeO2/C 620 850
Ni-CeO2 catalysts whit enhance OSC for the WGS reaction
142
activity in WGS correlates with the boosted OSC and OSCC demonstrated for this
solid. In addition to the excellent redox skills, the Ni-Ce/C sample presents greater
metallic dispersion and superior specific surface area contributing to activity
enhancement. This catalyst quadruplicates the activity of the Ni/CeO2 solid at 240 °C,
and reaches equilibrium conversion at 260 °C. Compared to others Ni-ceria catalysts
previously reported, our carbon-supported sample performs better even under harder
WGS conditions [17,18]. This is a very promising result since this material seems to
be as efficient as some well-known noble metal based catalysts for this reaction [19].
However, contrary to the noble metal based solids, Ni-ceria solids also catalyze the
methanation reaction. As can be seen in Figure 2, both catalysts exceed the
equilibrium conversion at high temperatures. The additional CO consumption is due
to the methanation process. The high concentrations of CO used in this stream favor
the methanation reaction [20]. The good point is that the methanation reaction for
both catalysts starts at temperatures higher than 260 °C and thus, this does not affect
our WGS operating window.
Figure 2. CO conversion vs. reaction temperature for catalysts reduced at 350 °C. Gas
mixture: 7 mol% CO, 30 mol% H2O, 50 mol% H2, and 9 mol% CO2 balanced in He.
This result is very interesting from the point of view of catalyst design since,
according to our data, a much better catalytic activity is obtained when CeO2 is
dispersed on activated carbon. This approach permits to develop very active WGS
catalysts reducing significantly the amount of ceria on the catalyst formulation. This
160 180 200 220 240 260 280 300 320 340 360 380
0
20
40
60
80
100
equilibrium
CO
con
vers
ion
(%)
Temperature (oC)
Ni-CeO2/C
Ni/CeO2
Chapter VI
143
outstanding behavior is linked to a strong enhancement of the redox properties when
ceria nanoparticles are dispersed on a high surface carrier.
From the industrial perspective, the catalytic stability is as relevant as the
activity. In this sense, a complete stability study of the selected sample was carried
out. The stability test was developed at 250 °C, a reaction temperature at which the
methanation reaction was not observed and good WGS activity was achieved. Figure 3
shows the long term stability test of the Ni-CeO2/C catalyst. For the aim of
comparison the same test was carried out with the Ni/CeO2 solid; however, this
catalyst exhibited fast deactivation and very poor CO conversion. Therefore it will not
be longer considered. Regarding our Ni-CeO2/C material, at the first stages of the
reaction the system exhibits a good catalytic behavior. However, the CO conversion
notably drops after 8 h of continuous reaction and remains stable after more than 120
h. Afterwards, several start/stop cycles were introduced simulating likely situations in
real application. It was observed that the catalyst recovers the conversion after the
start/stop operation. Apparently, once the initial deactivation occurs (at the early
reaction stages) the start-up/shutdown actions do not influence the activity.
Figure 3. Long term stability test of the prepared catalysts at 250 °C. 1 and 2 start/stop
cycles. Gas mixture: 7 mol% CO, 30 mol% H2O, 50 mol% H2, and 9 mol% CO2 balanced in He.
For a deeper understanding of this result, a second stability test was carried out.
At this time, the catalyst was directly submitted to a series of start/stop cycles to
check its actual tolerance under these conditions. Indeed, this type of stability
0 20 40 60 80 100 120 1400
20
40
60
80
100
Ni-CeO2/C
21
CO
co
nvers
ion
(%
)
Time (h)
2
Ni/CeO2
Ni-CeO2 catalysts whit enhance OSC for the WGS reaction
144
experiment is considered as the most exigent test for a WGS catalyst, since during the
stop stages the system is cooled down at room temperature with the reaction flow
passing through the catalytic bed [21].The later involves liquid water may condense
on the pores of the catalyst damaging the system. Figure 4 reveals that the Ni-CeO2/C
catalyst withstands 4 start/stop cycles within 6 h of reaction while preserving the
initial activity. However, the activity drops after the fourth cycle, and it reaches
practically the same value attributed to the steady state in the long term stability
test. This result is very interesting from the catalyst deactivation point of view. Our
data reveal that in a long term stability test high conversions are preserved during at
least 10 hours of reaction, but start-up/shutdowns cycles strongly affect the catalysts
stability, in such a way that the activity drop takes place earlier (after 6 h of
reaction).
Figure 4. Stability test: start/stop cycles of the Ni-CeO2/C catalyst at 250 °C. Gas mixture: 7
mol% CO, 30 mol% H2O, 50 mol% H2, and 9 mol% CO2 balanced in He.
In both cases, long term and start-up/shut-down, the deactivation is related to the
well-known phenomenon of Ni particles sintering. Actually, as previously proposed,
the presence of water potentiates Ni particles agglomeration especially at high
temperatures [22,23]. TEM images presented in Figure 5, corresponding to the Ni-
CeO2/C catalyst, evidences the Ni sintering process.
0 1 2 3 4 5 6 7 8
0
20
40
60
80
100
20 21 22 23 24 25 26 27 28
CO
con
vers
ion
(%)
Time (h)
Chapter VI
145
Figure 5. TEM micrographs of the Ni-CeO2/C catalyst before (scale 10 nm) (a) and after
(scale 200 nm) (b) the stability reaction test at 250 °C.
Our stability study indicates that water favours Ni agglomeration. This reduces
the catalytic surface area and results in a decreased activity [24]. According to our
data, the sintering process occurs earlier when liquid water (during the start/stop
cycles) enters in contact with the catalyst.
4. CONCLUSIONS
The impact of the oxygen storage capacity on the WGS behaviour of a Ni/CeO2
based catalysts is underlined in this paper. The dispersion of ceria nanoparticles on a
high surface activated carbon drives to a strong enhancement of the catalyst’s oxygen
mobility. The excellent redox skills together with the better metallic dispersion of the
Ni-CeO2/C catalyst due to the carbon textural promotion, result in a remarkable
improvement of the WGS activity.
The stability of the Ni-CeO2/C catalysts under continuous operation, as well as
interrupting cycles, was tested. Although high activity was found under these
conditions at early stages of the reaction, a notable activity loss was observed and
attributed to the sintering of Ni particles. Despite the issue of the stability should be
improved, the Ni-CeO2/C catalyst could be considered as an interesting alternative for
the low temperature WGS reaction and merits further investigations.
As final remark, it should not be disregarded that the high WGS activity observed
for the developed sample was achieved with a relatively low amount of cerium oxide
Ni-CeO2 catalysts whit enhance OSC for the WGS reaction
146
on its composition. This fact indicates that our preparation procedure could constitute
an alternative approach towards a new generation of Ni/CeO2 based catalysts.
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General conclusions
Conclusiones Generales
151
GENERAL CONCLUSIONS
The findings presented in this Thesis represent a step forward towards the
development of highly efficient catalytic systems for production/purification of
hydrogen from renewable resources.
An overview of the current energy status and a brief description of the role that
biomass could play as an alternative to fossil fuels is presented in Chapter I. In
addition, the main routes for biomass conversion into biofuels and value-added
chemicals are summarized in this chapter.
After this introduction, extensive studies in the different fuel processing steps to
obtain clean hydrogen are presented in this memory. The main goal in this respect
has been to find out the optimum catalytic design for every studied reaction. Overall,
a number of conclusions can be extracted from the different chapters:
Chapter II: Low temperature glycerol steam reforming on bimetallic PtSn/C
catalysts: on the effect of the Sn content.
Carbon supported Pt-Sn catalysts are rather efficient catalysts for the low
temperature glycerol steam reforming. Furthermore, our results state that Pt/Sn ratio
is an important parameter which influences the catalytic behaviour. More specifically,
low Sn/Pt ratios seemed to be the best option under mild reaction conditions (10 wt%
glycerol and 350 ºC) and also under more severe conditions (30 wt% glycerol and 400
ºC)
Chemical characterization techniques such as XPS and TPR-H2, revealed the
close proximity and strong interaction between Pt and Sn which alters the electronic
properties of the designed catalysts and thus affecting the catalytic trends.
In addition, TEM images showed that the bimetallic combinations are less
prone to sintering than the monometallic Pt/C catalyst after glycerol steam reforming
reaction. Again, Pt:Sn ratio plays a major role with enhanced sintering suppression
upon increasing tin loading.
152
As for the catalytic test, it could be concluded that Sn promotes the CO
oxidation reaction producing H2-rich gas streams. Hydrogen production is favoured at
low Sn/Pt ratios. Sn is also able to inhibit coke formation reactions and hinder Pt
sintering, thereby improving the overall stability of the catalyst.
Chapter III: Multicomponent NiSnCeO2/C catalysts for the low-temperature
glycerol steam reforming.
Following the interesting results from Chapter II regarding the beneficial effect of
Sn for the low temperature reforming process, Chapter III aimed to develop
economically viable catalysts by replacing Pt with Ni and including CeO2 to promote
the catalytic activity and the H2 selectivity. The following conclusions can be drawn:
The results presented in this study demonstrated that a successful glycerol
reforming in mild conditions can be achieved without a noble metal based catalyst.
Herein, it was demonstrated that both tin and ceria boost the catalytic performance,
gaining stability and selectivity towards a rich hydrogen stream. In this sense,
NiXCeO2/C and NiSnXCeO2/C catalysts (X=20, 30, 40 %) were prepared, characterised
and tested in this reaction.
XRD data indicated the formation of Ni-Sn alloy in the samples. This played a
crucial role in the catalytic stability. Furthermore, the Ni-Sn alloy was better
dispersed in the CeO2/C support than in the pure C one, this helping to avoid the
sintering process.
The most active sample, NiSn30CeO2/C reached full glycerol conversion at
early reaction stages with reasonably good hydrogen yields.
Hydrogen production was boosted via WGS due to the ability of ceria to carry
out the shift process in the studied temperature window. Additionally, its dispersion
on carbon led to smaller ceria particles that are more active in the shift process which
has a strong influence in the reaction as reflected in the CO/CO2 and H2/CO2 ratios.
Overall, the bimetallic Ni-Sn catalysts were more active and more stable than
the monometallic ones based exclusively on Ni.
153
Chapter IV: Aqueous phase reforming of glycerol for hydrogen production over
Pt, Ni and Pt-Ni catalysts supported on CeO2.
As discussed in this chapter several advantages could be obtained when glycerol
reforming is carried out in liquid phase.
In order to evaluate the possible benefits of this reaction, a comparative study
of the catalytic properties of three samples, Pt/CeO2, Ni/CeO2 and Pt-Ni/CeO2 was
carried out.
It was obtained that the samples activated “in situ” are more active than those
pre-reduced before the reaction, being monometallic platinum the most active
catalysts within the studied series.
Although Pt was more efficient in terms of total glycerol conversion, the
monometallic Ni/CeO2 provided a good balance between catalytic activity and
economic viability. Indeed, Ni/CeO2 was highly selective towards gas phase conversion
yielding to hydrogen rich streams.
The bimetallic PtNi/CeO2 presented advanced physicochemical properties.
However under the studied conditions the bimetallic catalyst was not the best choice.
Finally, operando ATR analysis revealed notable differences between Pt and Ni
based catalysts in the APR reaction pathway. The higher Pt activity was associated to
the capacity of Pt to stabilise the intermediates which further decompose to the
desired products. The poorer activity of Ni could be due the strong adsorption of CO
on Ni particles which block the active sites.
Chapter V: CeO2-promoted Ni/activated carbon catalysts for the water-gas
shift reaction.
For clean energy production, pure hydrogen is required as a feed gas for electricity
generation in low temperature fuel cells. Hydrogen purification after the reforming
unit involves several fuel processing steps, including CO elimination via water-gas
shift reaction.
154
In this sense, the same series of Ni catalysts promoted by CeO2 supported in
carbon employed in Chapter III were tested in the low temperature water gas shift.
Some relevant outputs could be extracted from this study:
The designed carbon-supported Ni-ceria catalysts with different ceria loadings
(NixCeO2/C with x=10, 20, 30, 40 wt.% CeO2) are highly efficient systems the low
temperature water-gas shift reaction.
Activated carbon is an ideal carrier for ceria nanoparticles dispersion. Indeed,
XRD and TEM studies confirmed the small crystal size (4-6 nm) and high dispersion
of CeO2 in comparison with a massive CeO2 catalyst (15 nm).
A direct correlation between the cerium oxide loading in the catalyst and the
catalytic activity could be established. It was found that the optimum load was 20
wt.% CeO2 in an ideal mixture feed reaction. Side reactions such as methanation were
not detected in an idealised WGS mixture (including only CO and H2O).
Under more realistic conditions (post reforming gas mixtures, containing CO2
and H2 ), the catalyst with 10 wt.% CeO2 exhibited the best performance. Very
interestingly this material if completely selective towards the WGS reaction and no
CO conversion via methanation was detected. The latter is an excellent result given
the well-known activity of Ni for the methanation reaction. Indeed, some means of
methanation were detected at high temperatures, but it resulted to be completely
selective towards CO2 hydrogenation.
Not at least place, the developed Ni-CeO2/C catalysts were able to maintain
stable conversion (greater than 40 %) during 150 h of reaction at 220 ºC a rather
promising result for realistic applications in an integrated fuel processor.
Chapter VI: Ni-CeO2/C catalysts with enhanced OSC for the WGS reaction.
The catalyst which presented better catalytic behaviour in the previous chapter
was studied in greater depth. Bearing in mind that, redox properties constitute one of
155
the key parameters tuning the WGS performance.. In this chapter, the impact of the
oxygen storage capacity on the WGS behaviour of the developed Ni based catalysts
was analysed. The following conclusions could be extracted:
It was clearly stated that the dispersion of ceria nanoparticles on a high
surface activated carbon drives to a strong enhancement of the catalyst’s oxygen
mobility.
A remarkable improvement of the WGS activity resulted from both an
excellent redox skills and a better metallic dispersion of the Ni-CeO2/C catalyst due to
the carbon textural promotion. A high WGS conversion was obtained with a relatively
low amount of cerium oxide on its composition.
The stability of the Ni-CeO2/C catalysts under continuous operation, as well as
interrupting cycles, was tested. Although high activity was found under these
conditions at early stages of the reaction, a notable activity loss was observed and
attributed to the sintering of Ni particles.
Despite the issue of the stability should be improved, the Ni-CeO2/C catalyst
could be considered as an interesting alternative for the low temperature WGS
reaction.
157
CONCLUSIONES GENERALES
Los resultados mostrados en esta tesis doctoral representan un paso hacia adelante
en el desarrollo de sistemas catalíticos altamente eficientes para la
producción/purificación de hidrógeno a partir de recursos renovables.
En el Capítulo I se presentó una visión general de la actual situación energética
global junto con una breve descripción del papel que podría desempeñar la biomasa
como alternativa a los combustibles fósiles. Además en este capítulo se resumieron las
principales rutas existentes para la conversión de biomasa en biocombustibles y
productos químicos de alto valor añadido.
Tras la introducción, en esta memoria se desarrollaron extensos estudios sobre las
diferentes etapas de procesado de combustible para obtener hidrógeno limpio. Por
tanto, el objetivo principal de esta tesis ha sido diseñar y desarrollar sistemas
catalíticos óptimos para cada reacción objeto estudio. De manera general, las
conclusiones que podemos extraer de cada uno de los capítulos son las siguientes:
Capítulo II: Reformado de glicerol con vapor de agua a baja temperatura sobre
catalizadores bimetálicos PtSn/C: estudio del contenido en Sn.
Se desarrolló una serie de catalizadores de Pt-Sn soportados en carbón con
diferentes relaciones atómicas Pt/Sn. Dichos sistemas fueron caracterizados y
probados en la reacción de reformado de glicerol a baja temperatura.
La caracterización físico-química de los sólidos mediante XPS y RTP-H2,
revelaron la proximidad y fuerte interacción entre el Pt y el Sn, lo que modifica las
propiedades electrónicas de los sistemas bimetálicos
Los catalizadores mostraron un buen comportamiento catalítico, en términos
de actividad y estabilidad. Además se observó un marcado efecto de la relación Pt/Sn
en los resultados catalíticos siendo los sistemas bimetálicos PtSn/C con bajas
relaciones Sn/Pt los más eficientes, tanto en condiciones suaves (10 %peso glicerol y
350 ºC) como severas (30 %eso glicerol y 400 ºC).
158
Además, en las micrografías TEM se vio como los catalizadores bimetálicos son
menos propensos a la sinterización en comparación con la muestra monometálica
Pt/C, después de haberlos sometido al reformado de glicerol. Es más, la supresión de
la sinterización se ve mejorada con la cantidad de estaño en el catalizador.
Referente a los resultados de actividad catalítica, se pudo concluir que el Sn
promovió la reacción de oxidación de CO produciendo corrientes con alta relación
H2/CO. Las bajas relaciones Sn/Pt favorecieron la producción de hidrógeno. La
presencia de Sn también inhibió las reacciones de formación de coque e impidió las
sinterización del Pt, mejorando así la estabilidad el catalizador.
Capítulo III: Catalizador multicomponente NiSnCeO2/C para la reacción de
reformado de glicerol con vapor de agua a baja temperatura.
Después de los interesantes resultados obtenidos en el Capítulo II referentes al
efecto favorable del Sn en el proceso de reformado a baja temperatura, el Capítulo III
tuvo como objetivo desarrollar catalizadores económicamente más viables
sustituyendo el Pt por el Ni. Además se incluyó CeO2 en la composición de las
muestras para promover la actividad catalítica y selectividad hacia H2. Pudieron
extraerse las siguientes conclusiones:
Se prepararon y caracterizaron catalizadores de NixCeO2/C y NiSnxCeO2/C (x=
20, 30, 40 % peso). Dichos sistemas se probaron en la reacción de reformado de glicerol
con vapor de agua. Los resultados obtenidos demostraron que se pueden alcanzar
buena conversión de glicerol sin un catalizador basado en un metal noble.
Los datos de DRX evidenciaron la formación de la aleación Ni-Sn en las
muestras, hecho que jugó un papel crucial en la estabilidad catalítica. Además las
muestras con óxido de cerio, mejoraron la dispersión de la aleación formada, evitando
la sinterización de las partículas de Ni.
NiSn30CeO2/C fue la muestra más activa, alcanzando una conversión completa
de glicerol en las primeras horas de reacción, y con un buen rendimiento a hidrógeno.
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La capacidad del CeO2 para llevar a cabo la reacción de desplazamiento del gas
de agua en la temperatura estudiada, incrementó la producción de hidrógeno en estas
muestras. Además la dispersión del CeO2 sobre carbón, condujo a la formación de
pequeñas partículas que son más activas en esta reacción. También, la presencia de
esta reacción fue reflejada en las relaciones de CO/CO2 y H2/CO2 obtenidas.
Los catalizadores bimetálicos Ni-Sn fueron más activos y estables que los
catalizadores monometálicos.
Capítulo IV: Reformado de glicerol en fase acuosa para producción de
hidrógeno sobre catalizadores de Pt, Ni y Pt-Ni soportados en CeO2.
Como se discutió en el Capítulo IV, se pueden obtener una serie de ventajas cuando
el reformado de glicerol se lleva a cabo en fase acuosa.
Se realizó un estudio comparativo de las propiedades catalíticas de tres
muestras, Pt/CeO2, Ni/CeO2 y Pt-Ni/CeO2 en esta reacción.
Las muestras activadas “in situ” fueron más activas que las reducidas antes de
reacción, siendo la muestra Pt/CeO2 la que presentó mejor comportamiento catalítico.
Aunque las muestras con Pt fueron más eficientes en términos de conversión
total de glicerol, el catalizador de Ni/CeO2 presentó un buen compromiso entre
resultados de actividad catalítica y viabilidad económica. De hecho, Ni/CeO2 fue
altamente selectivo hacia la conversión a fase gas, produciendo una corriente rica en
H2.
El catalizador bimetálico PtNi/CeO2 destacó por sus avanzadas propiedades
fisicoquímicas. Sin embargo, bajo las condiciones estudiadas de reacción, esta muestra
no fue la mejor opción.
Por último, el análisis “in situ” mediante espectroscopia de Reflectancia Total
Atenuada de las muestras sin reducir, reveló notables diferencias en las diferentes
vías de reacción entre los catalizadores de Pt y Ni. La mayor actividad obtenida en las
muestras que contienen Pt fue asociada a la capacidad del Pt a estabilizar los
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intermedios de reacción que descompusieron en los productos gaseosos deseados. La
menor actividad del Ni puede ser debida a la fuerte adsorción del CO en las partículas
de Ni, bloqueando los sitios activos.
Capítulo V: Catalizadores de Ni/carbón activado promovidos por CeO2 para la
reacción de desplazamiento del gas de agua.
Para alimentar una pila de combustible tipo PEM con hidrógeno, éste debe ser de
alta pureza. Para la obtención de H2 limpio, la corriente de reformado generada debe
ser procesada en varias etapas, incluyendo la eliminación de CO a través de la
reacción de desplazamiento del gas de agua.
Con esta finalidad, la misma serie de catalizadores de Ni promovidos por CeO2
soportados en carbón empleados en el Capítulo III, fueron probados en la reacción de
desplazamiento del gas de agua. Esto nos permitió concluir que:
Los catalizadores Ni-CeO2 soportados en carbón activado (NixCeO2/C con x=10,
20, 30, 40 % en peso de CeO2) fueron altamente eficientes de, en la reacción de
desplazamiento del gas de agua.
Al dispersar distintas cantidades de óxido de cerio sobre carbón activado, se
obtuvieron pequeñas partículas de óxido con alta área superficial disponible. Estudios
de DRX y TEM confirmaron el pequeño tamaño de partícula (4-6 nm) y alta dispersión
para los sitemas CeO2/C en comparación con el catalizador de CeO2 másico.
Se observó una correlación directa entre la cantidad de CeO2 presente en los
catalizadores y la actividad catalítica obtenida para cada uno de ellos. La cantidad
óptima encontrada en condiciones de reacción ideales (CO y H2O) fue de un 20% en
peso de CeO2. En estas condiciones no se obtuvieron reacciones secundarias como la
metanación.
Bajo condiciones más reales de reacción (mezcla de gases similar a la de salida
en un reformador), el catalizador con un 10 % en peso de CeO2 obtuvo el mejor
comportamiento catalítico. Destacó que este material fue completamente selectivo a la
reacción deseada y no a la conversión de CO vía metanación. Esto resultó ser un
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excelente resultado dada la conocida actividad del níquel en la reacción de
metanación. De hecho, cierta cantidad de CH4 fue detectada a altas temperaturas,
pero resultó proceder exclusivamente de la hidrogenación de CO2.
Una última prueba catalítica corroboró la estabilidad del catalizador de Ni-
CeO2/C. Este mantuvo una conversión de CO estable durante 150 h de reacción a 220
ºC, siendo un resultado prometedor para una posible aplicación real.
Capítulo VI: Catalizador de Ni-CeO2/C con elevada capacidad de
almacenamiento de oxígeno para la reacción de desplazamiento del gas de agua.
En este capítulo, el catalizador que presentó mejor comportamiento catalítico en el
estudio anterior, fue analizado en mayor profundidad. En concreto se estudiaron
detalladamente las propiedades redox del mismo ya que éstas constituyen uno de los
parámetros clave que favorecen la excelente actividad de los catalizadores Ni/CeO2/C.
Por tanto, se investigó el efecto que tiene la capacidad de almacenamiento de oxígeno
de la muestra en la actividad catalítica obtenida para la reacción de desplazamiento
del gas de agua. Esto nos condujo a las siguientes conclusiones:
La dispersión de las partículas de óxido de cerio en un carbón activado de alta
área superficial generó una elevada movilidad de oxígeno en la muestra.
El efecto de las excelentes propiedades redox del catalizador, junto con su alta
dispersión debido a las propiedades texturales del carbón se vio reflejado en una
notable mejora en su actividad catalítica. Se obtuvo una alta conversión con
relativamente poca cantidad de CeO2 en su composición.
Fue probada la estabilidad del catalizador tanto en condiciones continuas de
reacción como en ciclos de encendido y apagado. Se obtuvo un buen comportamiento
catalítico durante las primeras horas del test de estabilidad, seguido de una notable
perdida de actividad debido a la sinterización de las partículas de níquel.
A pesar de que la estabilidad del catalizador puede ser mejorada, esta muestra
Ni-CeO2/C, puede ser considerada una interesante alternativa para su uso en la
reacción de desplazamiento de gas de agua a baja temperatura.
List of Publications
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LIST OF PUBLICATIONS
1. Alonso, F.; Moglie, Y.; Pastor-Pérez, L.; Sepúlveda-Escribano, A. Solvent- and
ligand-free diboration of alkynes and alkenes catalyzed by platinum nanoparticles on
titania. ChemCatChem. 6 (2014) 857- 865.
2. Pastor-Pérez, L.; Buitrago-Sierra, R.; Sepúlveda-Escribano, A.CeO2-promoted
Ni/activated carbon catalysts for the water-gas shift (WGS) reaction. International
Journal of Hydrogen Energy. 39 (2014) 17589-17599.
3. Dongil, A.B.; Rivera-Cárcamo, C.; Pastor-Pérez, L.; Sepúlveda-Escribano, A.;
Reyes, P.Ir supported over carbon materials for the selective hydrogenation of
chloronitrobenzenes. Catalysis Today. 249 (2015) 72-79.
4. Pastor-Pérez, L., Reina, T. R., Ivanova, S., Centeno, M.A., Odriozola, J.A.,
Sepúlveda-Escribano, A. Ni-CeO2/C catalysts with enhanced OSC for the WGS
reaction. Catalysts 5(1) (2015) 298-309.
5. Pastor-Pérez, L., Merlo, A., Buitrago Sierra, R., Casella, M. Sepúlveda-
Escribano, A. Bimetallic PtSn/C catalysts obtained via SOMC/M for glycerol steam
reforming, Journal of Colloid and Interface Science 459 (2015) 160-166.
6. Dongil, A. B., Pastor-Pérez, L., Sepúlveda-Escribano, A., Reyes, P. Promoter
effect of sodium in graphene-supported Ni and Ni-CeO2 catalyst for the low
temperature WGS reaction, Applied Catalysis A 505 (2015) 98-104.
7. Dongil, A. B., Pastor-Pérez, L., Sepúlveda-Escribano A., García, R.
Escalona, N., Hydrodeoxygenation of guaiacol: Tuning the selectivity to
cyclohexene by introducing Ni nanoparticles inside carbon nanotubes, Fuel 172
(2016) 65–69.
8. Dongil, A. B., Pastor-Pérez, L., Escalona, N. Sepúlveda-Escribano A., Carbon
nanotube-supported Ni-CeO2 catalysts. Effect of the support on the catalytic
performance in the lowtemperature WGS reaction, Carbon 101 (2016) 296-304.
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9. Dongil, A. B., Pastor-Pérez, L., Fierro, J.L.G., Escalona, N., Sepúlveda-
Escribano A., Synthesis of palladium nanoparticles over graphite oxide and carbon
nanotubes by reduction in ethylene glycol and their catalytic performance on the
chemoselective hydrogenation of para-chloronitrobenzene, Applied Catalysis A
General 513 (2016) 89–97.
10. Dongil, A. B., Pastor-Pérez, L., Fierro, J.L.G., Escalona, N., Sepúlveda-
Escribano A., Synthesis of palladium nanoparticles on carbon nanotubes and
graphene for the chemoselective hydrogenation of para-chloronitrobenzene, Catalysis
Communications 75 (2016) 55–59.
11. Dongil, A. B., Pastor-Pérez, L., Fierro, J.L.G., Escalona, N., Sepúlveda-
Escribano A., Effect of the surface oxidation of carbon nanotubes on the selective
cyclization of citronellal, Applied Catalysis A General 524 (2016) 25-31.