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Applied Catalysis B: Environmental 102 (2011) 291–301

Contents lists available at ScienceDirect

Applied Catalysis B: Environmental

journa l homepage: www.e lsev ier .com/ locate /apcatb

urface reactivity of LaCoO3 and Ru/LaCoO3 towards CO, CO2 and C3H8:ffect of H2 and O2 pretreatments

. Consuelo Álvarez-Galvána, Domna A. Constantinoub, Rufino M. Navarroa, José A. Villoriaa,osé Luis G. Fierroa, Angelos M. Efstathioub,∗

Instituto de Catálisis y Petroleoquímica, CSIC, C/Marie Curie 2, Cantoblanco, 28049, Madrid, SpainDepartment of Chemistry, Heterogeneous Catalysis Laboratory, University of Cyprus, P.O. Box 20537, CY 1678 Nicosia, Cyprus

r t i c l e i n f o

rticle history:eceived 17 September 2010eceived in revised form 5 December 2010ccepted 8 December 2010vailable online 16 December 2010

eywords:xidative steam reforming of propane2 productionaCoO3

uerovskiteO-TPDO2-TPDropane TPSR

a b s t r a c t

The differences in surface reactivity of LaCoO3 and Ru/LaCoO3 solids after pre-treatment in a hydrogen oroxygen gas atmosphere towards oxidative steam reforming (OSR) of propane was probed by performingtemperature-programmed desorption (TPD) of CO and CO2, temperature-programmed surface reaction(TPSR) of C3H8, transient isothermal oxidation of “carbon” formed after TPSR of C3H8, X-ray diffraction(XRD), and X-ray photoelectron spectroscopy (XPS) studies. The TPD of CO and CO2 studies revealed agreater adsorption of these molecular product species of the OSR of propane on the surface of LaCoO3

than Ru/LaCoO3 solid, while XPS studies performed after reduction in hydrogen revealed a higher con-centration of Co2+ in the LaCoO3 than Ru/LaCoO3 solid catalyst composition. The Ru/LaCoO3 solid wasfound to exhibit after an induction period enhanced reactivity towards H2 production for the OSR ofpropane reaction compared to LaCoO3 after hydrogen reduction at 750 ◦C. These results led to the con-clusion that the introduction of a small amount of Ru (0.8 wt%) on lanthanum cobaltite surface leads to ahigher concentration of metallic cobalt (Co0) after hydrogen reduction is performed, thus favouring theenhancement of necessary active catalytic sites on the Co, LaCoO3 and La2O3 surfaces formed. TPSR ofpropane used as model compound of diesel revealed the participation of surface lattice oxygen of both

LaCoO3 and Ru/LaCoO3 pre-oxidized solids as well as the formation of hydrogen by reaction of propaneon the Co0 (metallic state) surface once the latter is formed. A greater reactivity of propane during TPSRwas observed over the Ru/LaCoO3 solid surface for either oxidative or reductive pre-treatments. Also,a lower amount (2.13 mmol/g) of “carbon” deposition was found on Ru/LaCoO3 compared to LaCoO3

(3.65 mmol/g) at the end of the TPSR of propane experiment. These results provide important fundamen-tal information on the role of Ru in the Ru/LaCoO3 system towards catalytic oxidative steam reforming

of propane.

. Introduction

Nowadays, the use of hydrogen as an energy carrier in fuelells is receiving considerable attention due to environmental con-erns and to the higher efficiency of these systems compared tonternal combustion engines [1]. Widely available logistic fuelse.g., diesel) with a high volumetric energy density can be readilyeformed into a hydrogen-rich gas and directly be applied in fuelells [2]. The production of molecular hydrogen using diesel hydro-

arbon compounds can be achieved by different catalytic processesuch as partial oxidation (POX), steam reforming (SR) or oxidativeteam reforming (OSR) [3]. The latter process is more efficient dueo the in situ integration of the reaction heat of the exothermic

∗ Corresponding author. Tel.: +35722 892776; fax: +357 22 892801.E-mail address: efstath@ucy.ac.cy (A.M. Efstathiou).

926-3373/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.apcatb.2010.12.015

© 2010 Elsevier B.V. All rights reserved.

partial oxidation into the steam reforming endothermic reaction[1,4,5].

The oxidative steam reforming reaction can be performed athigh reaction temperatures (800–900 ◦C) in the presence of anactive and stable catalyst having a good resistance to coking andsulphur poisoning [6]. The dispersion of a metal throughout thestructure of a stable metal oxide could maximize the active phaseexposition and reduce the possibility of large metal clusters forma-tion that are potential sites for sulphur and carbon accumulation,and as a result of this, catalysts less susceptible to deactivation havebeen developed [7,8]. Mixed metal oxides including perovskites(ABO3) are becoming increasingly attractive as catalyst precursors

for oxidative steam reforming reactions due to their ability to gen-erate a good dispersion of the active phase B in metallic state (B0)over a matrix of the oxide A and be stable in the highly reductivegas atmosphere at the reaction conditions used [9,10]. In addi-tion, it has been reported [11,12] that the use of a transition metal

2 alysis

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ioaaso

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2

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92 M.C. Álvarez-Galván et al. / Applied Cat

uch as La in the A position of a perovskite (ABO3) structure cane a good metal candidate since it leads to the formation of lan-hanum oxycarbonate under reaction conditions which was foundo participate in the reduction of carbon deposition via gasifica-ion. Moreover, the high interaction between both metallic cobaltnd lanthana phases derived by reduction of the perovskite struc-ure produces a synergy which promotes the formation of syngas10]. The incorporation of small amounts (ca. less than 1 wt%) ofransition metals, like ruthenium (Ru) has been found to resultn the increase of catalytic activity and stability for the oxidativeteam reforming of heavy hydrocarbons towards hydrogen produc-ion not only by the higher intrinsic activity of this metal, but alsoecause small proportion of ruthenium promotes the reduction ofhe system, thus increasing the surface proportion of active phases10]. Liu and Krumpelt [13] found that the incorporation of Runto a perovskite type structure (LaCr0.95Ru0.05O3) led to a catalyt-cally active system for the autothermal reforming of n-dodecanento hydrogen having good resistance to sulphur deposition. Also,avarro et al. [10] have reported an increased efficiency in hydro-en production during oxidative steam reforming of diesel overRu/LaCoO3 catalyst (0.02:1:1 molar, Ru/Co/La) compared to Ce

nd/or La alumina-doped supported Pt or Ni catalysts.Different reactions are involved in the oxidative steam reform-

ng of diesel such as partial oxidation, steam reforming, totalxidation and water–gas shift (WGS), all of them resulting to H2, COnd CO2 reaction products [14,15]. Thus, understanding the inter-ctions of H2, CO and CO2 with the surface of ABO3 and Ru/ABO3olids mentioned above is essential for establishing relationshipsf their catalytic performance with physicochemical properties.

According to the above mentioned, the present study pro-ides for the first time fundamental information on the CO andO2 chemisorptive behaviour of LaCoO3 and Ru/LaCoO3 solids viaemperature-programmed desorption experiments (CO-TPD andO2-TPD) for different hydrogen or oxygen gas pre-treatment con-itions applied on the solids. X-ray diffraction (XRD) and X-rayhotoelectron spectroscopy (XPS) studies have also been con-ucted for additional surface characterization of the catalyticystems investigated (e.g., crystal phases, primary crystal size, sur-ace atom% composition, extent of cobalt reduction following H2re-treatment of the solids). Propane is derived via the homolyticracking of hydrocarbon molecules present in diesel [16]. In a blankxperiment performed in the present work in the absence of aatalyst, it was found that after passing vaporized diesel in theeactor propane was the hydrocarbon formed with the highest con-entration in the product stream for the reaction conditions (T,HSV) used in this study. Therefore, propane has been used as aodel hydrocarbon present in a catalytic reactor during oxidative

team reforming of diesel. For this, temperature-programmed sur-ace reaction (TPSR) studies using propane were performed afterifferent reductive or oxidative pre-treatments of the LaCoO3 andu/LaCoO3 solids were made in order to probe the participation of

attice oxygen during propane oxidative steam reforming and toharacterize the “carbon” formed.

. Experimental

.1. Synthesis of catalysts

LaCoO3 perovskite was prepared by simultaneous precipitationf cobalt and lanthanum from a solution of nitrate salts of cobalt

nd lanthanum after using K2CO3 as a precipitating agent. Morerecisely, the required amount of La(NO3)3·6H2O (99.9%, Johnsonatthey) or Co(NO3)2·6H2O (97.7% minimum, Johnson Matthey)

alt was dissolved in distilled water in order to obtain a 1 M solution.oth solutions were then mixed under vigorous stirring. Subse-

B: Environmental 102 (2011) 291–301

quently, an excess (10 vol%) of the required aqueous solution ofK2CO3 (>99.0%, Johnson Matthey) was rapidly added. Under theprevailed basic conditions (pH > 9), water was partially evaporatedby heating the solution to 70 ◦C. Before filtering, the precipitatewas washed with ice-cooled distilled water until the pH of thefiltrate became neutral. The solid precursors thus obtained werethen dried at 110 ◦C for 4 h and calcined in air (static conditions)at 750 ◦C for 4 h (heating rate used was 3 ◦C/min from 110 to750 ◦C). The Ru/LaCoO3 solid was prepared by the wet impreg-nation method using an appropriate amount of RuCl3 (rutheniumchloride, 48.91% Ru, Premion, Johnson Matthey) in order to achievea Ru:La:Co = 0.02:1:1 molar ratio. Impregnation was carried out ina rotary evaporator at 80 ◦C for 2 h. Finally, the samples were driedat 110 ◦C for 4 h and then calcined in air (static conditions) at 500 ◦Cfor 3 h.

2.2. Catalysts characterization studies

2.2.1. X-ray diffraction (XRD)X-ray diffraction (XRD) patterns were recorded using a Seifert

3000P vertical diffractometer and nickel-filtered CuK� radiation(� = 0.1538 nm). The mean particle size of LaCoO3 in the calcined(static air, 750 ◦C, 4 h) samples, and of the Co0, La2O3 and La(OH)3obtained after hydrogen reduction of the samples (10 vol% H2/N2 gasmixture, 750 ◦C, 2 h) was estimated using the Scherrer equation[17]; the width of XRD peak was considered as the full-width athalf maximum intensity of the most intense and least overlappedpeak (2� = 47.7◦ for LaCoO3; 44.3◦ for Co0; 30.0◦ for La2O3; 48.6◦

for La(OH)3)). The samples for analysis were reduced ex-situ andtransferred in the XRD instrument in a vial containing iso-octaneto avoid oxidation. Hydrogen-containing gas was passed throughthe reactor when the sample was transferred in the vial.

2.2.2. X-ray photoelectron spectroscopy (XPS)X-ray photoelectron spectroscopy measurements were per-

formed using an Escalab 200R spectrometer equipped with ahemispherical electron analyser and an Al K� (h� = 1486.6, 120 W)X-ray source. The area of a given peak was estimated by calculat-ing the integral of it after smoothing and subtracting an S-shapedbackground, while fitting of the thus derived experimental spec-trum was made using a mixture of Lorentzian and Gaussian linesof variable proportions. All binding energies (BE) were referencedto the C 1s signal at 284.6 eV based on carbon contamination ofthe sample. Quantification of the surface atomic composition wasobtained after integration of the peaks and using appropriate cor-rections for sensitivity factors [18]. XPS analyses on the LaCoO3 andRu/LaCoO3 solids were performed following in situ reduction of thesolid in the pre-treatment chamber of the XPS apparatus.

2.2.3. Temperature-programmed desorption (TPD)All TPD runs (H2-TPD, CO-TPD and CO2-TPD) were conducted

in a specially designed gas-flow system described previously [19]and after using a 0.5 g catalyst sample, 30 NmL min−1 He flow, and30 ◦C min−1 heating rate. Quantitative analysis of the effluent gasstream from the micro-reactor was done with an on line quadrupolemass spectrometer (MS) (Omnistar, Balzers) equipped with a fastresponse inlet capillary/leak valve (SVI050, Balzers) and data acqui-sition systems [19]. The mass numbers (m/z) 2, 28 and 44 used forH2, CO and CO2, respectively, were continuously recorded. Cali-

bration of H2, CO and CO2 signals of the mass spectrometer wasmade based on certified 1 vol% H2, 1 vol% CO and 1.0 vol% CO2 in Hediluent gas mixtures. For the quantitative analysis of a gas mixturecontaining both CO and CO2, the contribution of CO2 to the m/z = 28MS signal was considered.

alysis

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dpwahtmiw

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3.1.1. XRD analysesThe XRD patterns of the pre-calcined LaCoO3 and Ru/LaCoO3

samples shown in Fig. 1 were found to be similar to those previ-ously reported [10], having strong reflections at 32.9◦ (1 1 0) and33.3◦ (1 0 4) which correspond to stoichiometric perovskite with a

Ru/LaCoO3

I/Io

(a.

u.)

LaCoO3

R. Rhombohedral

R

R R

R

R

R

R R

R

R

R

R

R

R

R R

M.C. Álvarez-Galván et al. / Applied Cat

.2.4. Metal dispersion measurements (H2-TPD)The dispersion of cobalt and ruthenium/cobalt metals in the

aCoO3 and Ru/LaCoO3 catalysts, respectively, was determined byelective H2 chemisorption at 25 ◦C followed by TPD in He flowccording to the following procedure. The fresh catalyst sample (asrepared, Section 2.1) was first heated in He flow to 750 ◦C and theneduced in a 10 vol% H2/He gas mixture at 750 ◦C for 2 h. Followingeduction, the feed was changed to He and the temperature wasecreased to 100 ◦C in He flow. The feed was then changed to avol% H2/He gas mixture for 30 min. After this adsorption step theatalyst was cooled in 1 vol% H2/He to room temperature and leftor 15 min. A switch to He flow was then made for 15 min until no

2 signal could be observed in the mass spectrometer, and the tem-erature of the catalyst was then increased to 700 ◦C at the rate of0 ◦C/min in order to carry out a TPD run. Integration of the H2-TPDesponse curve obtained and considering the amount of cobalt orhe ruthenium/cobalt in the respective catalyst sample, the metalispersion was estimated.

.2.5. CO Temperature-programmed desorption (CO-TPD)CO-TPD experiments were performed following chemisorption

rom a 2 vol% CO/He gas mixture at room temperature for 30 min.efore CO chemisorption the fresh sample of LaCoO3 or Ru/LaCoO3as prepared, Section 2.1) was reduced in a 10% H2/He gas mix-ure at 750 ◦C for 2 h and then brought quickly in He flow to roomemperature. Following CO chemisorption the catalyst sample wasurged in He flow at room temperature until no CO evolution wasbserved. CO-TPD runs were conducted in the 25–700 ◦C range.

.2.6. CO2 Temperature-programmed desorption (CO2-TPD)CO2-TPD experiments were performed following chemisorption

f CO2 at 25 ◦C from a 3 vol% CO2/He gas mixture for 30 min. Theresh catalyst sample was first reduced in 10 vol% H2/He at 300,00 or 750 ◦C for 2 h. This different hydrogen reduction treatmentas performed in order to investigate the effect of oxygen vacant

ite concentration formed on the surface of LaCoO3 and Ru/LaCoO3olids by hydrogen reduction on the chemisorptive behaviour ofO2. The temperature of the catalyst was then increased from roomemperature to 700 ◦C to carry out a TPD run.

.3. Temperature-programmed surface reaction (TPSR) of C3H8

TPSR of C3H8 experiments were conducted in a speciallyesigned gas-flow system previously described [19]. The reactantropane (C3H8) and the H2, CH4, H2O, CO and CO2 gas productsere monitored by on line quadrupole mass spectrometer (MS)

t the m/z = 43, 2, 16, 18, 28, and 44, respectively. It is mentionedere that propane leads to m/z = 28, 43 and 44 signals in the ioniza-ion chamber of the mass spectrometer. In addition, the signal at/z = 28 also arises from CO and CO2 (formation of CO+ in the ion-

zation chamber of MS). Therefore, the signal recorded at m/z = 43as derived exclusively from the propane molecule.

In order to investigate the influence of the extent of reductionr oxidation of LaCoO3 perovskite structure on the TPSR of propaneraces, three different gas pre-treatment conditions were used: (a)ydrogen reduction at 750 ◦C for 2 h in a 10 vol% H2/He gas mixture,b) oxidation at 400 ◦C for 2 h in a 10 vol% O2/He gas mixture, and (c)xidation at 600 ◦C for 2 h in a 10 vol% O2/He gas mixture. After the

bove described solid pre-treatment, the solid catalyst (0.5 g) wasooled to room temperature in He flow. The feed was subsequentlywitched to a 2 vol% C3H8/He gas mixture and at the same timehe temperature of the solid was increased to 700 ◦C at the rate of0 ◦C/min.

B: Environmental 102 (2011) 291–301 293

2.4. Estimation of carbon deposition under TPSR in C3H8/He flow

The amount of carbonaceous species accumulated on the surfaceof the investigated solid catalysts following TPSR of C3H8 (Section2.3), where the solids were first oxidized at 600 ◦C for 2 h (Section2.3) was measured as follows. At the end of TPSR run (700 ◦C) thefeed was changed to He until all MS signals, and in particular theCO (m/z = 28) and CO2 (m/z = 44) reached their respective baselinevalue. Then, the catalyst sample was cooled to 600 ◦C in He flow. Thegas-flow was then switched to 2 vol% O2/He gas mixture at 600 ◦C,and the signals of CO and CO2 were continuously monitored byon line mass spectrometer. The amount of surface carbon deposits(g C/g catalyst) was calculated based on the CO and CO2 responsecurves calibrated against standard 1 vol% CO and 1 vol% CO2 gasmixtures in He diluent.

2.5. Catalytic oxidative steam reforming of propane

Catalytic activity performance tests of the investigated cata-lyst samples (Section 2.1) towards oxidative steam reforming ofpropane for hydrogen production were carried out in a fixed-bedcontinuous flow stainless steel micro-reactor. The LaCoO3-basedsolid catalyst samples were reduced in 10 vol% H2N2 gas mixture at750 ◦C for 2 h before activity tests. The flow rates of the propane andliquid water feed streams were controlled by mass flow controllerand liquid pump, respectively. The liquid feed was pre-heated to200 ◦C in an evaporator before entering the catalyst bed. Nitrogengas (25 NmL/min) was also fed into the evaporator in order to facili-tate evaporation of water and gas mixing. The resulting gas mixturewas mixed with an appropriate oxygen flow before entering thecatalytic reactor at a molar ratio of H2O/O2/C = 3/0.5/1, whereasthe total pressure was close to 1 atm and the total gas flow ratewas 75 NmL/min (GHSV = 20,000 h−1). Catalyst activity in terms ofH2-yield (%) was measured at 750 ◦C. The reaction products wereanalysed by on line gas chromatograph (HP 5890 Series II) equippedwith a thermal conductivity detector.

3. Results and discussion

3.1. Catalysts characterization

80706050403020102θ (º)

Fig. 1. XRD patterns of pre-calcined LaCoO3 and Ru/LaCoO3 perovskites (see Section2.2.1).

294 M.C. Álvarez-Galván et al. / Applied Catalysis B: Environmental 102 (2011) 291–301

Table 1Average primary crystal size (nm) of LaCoO3, Co0, La2O3 and La(OH)3 solids for the fresh (as prepared, Section 2.1), calcined, and reduced LaCoO3 and Ru/LaCoO3 solidscalculated using the Scherrer equation.

Fresh samples

LaCoO3 Ru/LaCoO3

O3 LaCoO3

42.13 La(OH)3 Co0 La2O3 La(OH)3

13.1 28.6 30.6 11.7

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810800790780770

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LaCoO3

Cou

nts

per

seco

nd (

a.u.

)

Binding Energy (eV)

Co 2p a

Co 2p

Ru/LaCoO3

CalcinedT = 750 ◦C/4 h

LaCo44.2

ReducedT = 750 ◦C/2 h

Co0 La2O35.0 31.8

hombohedral distortion (JCPDS 48-123) of the ideal cubic struc-ure of perovskite. No segregated phases corresponding to La oro oxides were detected. Estimation of the mean primary LaCoO3rystallite size using the Scherrer equation [17] led to a slightlymaller crystallite size for the Ru/LaCoO3 compared to LaCoO3 solidTable 1). The XRD patterns of the reduced at 750 ◦C/2 h LaCoO3nd Ru/LaCoO3 samples are presented in Fig. 2. After comparinghe diffractograms recorded for the calcined samples with thosebtained after reduction (Figs. 1 and 2), it is concluded that theiffraction peaks corresponding to LaCoO3 were replaced by 2�-iffraction peaks attributed to La2O3 (JCPDS 74-2430), La(OH)3JCPDS 006-585) and Co0 (JCPDS 15-806). The mean primary crys-al sizes of Co0, La2O3 and La(OH)3 solids derived from the reducedu/LaCoO3 solid formulation were found to be smaller than thosebtained for the reduced LaCoO3 solid as reported in Table 1.

.1.2. XPS studiesX-ray photoelectron spectroscopy was used to determine the

xidation state of the surface metal atoms of LaCoO3 and Ru/LaCoO3olids and to estimate their surface atom ratios in their reducedtate (10 vol% H2/N2, 750 ◦C, 2 h). Fig. 3a and b show XP spectra forhe photoemission level of Co 2p measured for the reduced LaCoO3nd Ru/LaCoO3 solids, respectively.

The XPS spectra presented in Fig. 3a and b for Co 2p core levelppear to be practically the same for both solids investigated, hav-ng a multiple splitting at 778.5 and 794.1 eV which correspondso 2p3/2 and 2p1/2 level, respectively. Shake-up peaks were alsoecorded around 786 and 802 eV, which are characteristic of Co2+

20]. This species was formed after hydrogen reduction of theu/LaCoO3 solid (Section 2.2.2) indicating that a fraction of cobaltemains on its surface as Co2+ ions. In addition, the relative inten-

ity of Co 2p shake-up peaks is larger in the case of LaCoO3 (Fig. 3a)han Ru/LaCoO3 solid (Fig. 3b). It is important to note that Co2+ ionsre hardly reduced in the presence of water vapour, the latter gen-rated either in the reduction process or during decomposition ofanthanum oxo-hydroxide (LaO(OH)) [21].

8070605040302010

1

2θ (º)

21

1

1 1

1121

31

31

2

1

1

I/Io

(a.

u.)

2

1. La2O

3

2. La(OH)3

3. Co1 1

LaCoO3

Ru/LaCoO3

ig. 2. XRD patterns of catalysts derived from LaCoO3 and Ru/LaCoO3 precursorsfter hydrogen reduction in a 10 vol% H2/N2 gas mixture at 750 ◦C for 2 h.

Fig. 3. X-ray photoelectron spectra of Co 2p obtained on the (a) pre-reduced LaCoO3

solid (10 vol% H2/N2 gas mixture, 750 ◦C, 2 h), and (b) pre-reduced Ru/LaCoO3 solid(10 vol% H2/N2 gas mixture, 750 ◦C, 2 h).

Table 2 presents values for the Co/La and (Ru + Co)/La surfaceatom ratio, and the adsorbed “C”/La surface ratio for the freshreduced LaCoO3 and Ru/LaCoO3 solids. It is seen that LaCoO3exhibits a higher metal/La ratio and a greater proportion ofadsorbed “C” to surface La species compared to the Ru/LaCoO3 solid.These results will be further discussed in Section 3.1.5.

3.1.3. Metal dispersion measurements

The amount of H2 desorbed (Section 2.2.4) from the surface of

LaCoO3 and Ru/LaCoO3 solids was found to be 19.6 and 33.2 �mol/g,respectively. This indicates that Ru/LaCoO3 solid exposed a higheramount of surface metallic phase (Ru, Co) per gram of solid thanLaCoO3 after H2 reduction at 750 ◦C. On the other hand, in both

Table 2Surface atomic ratios calculated by XPS in LaCoO3 and Ru/LaCoO3 solids followingH2 reduction at 750 ◦C.

Catalyst precursora Co/La or (Ru + Co)/La Adsorbed “C”/La

LaCoO3 1.10 0.59Ru/LaCoO3 0.85 0.41

a Before XPS studies the solid catalyst was reduced in 10 vol% H2N2 at 750 ◦C for2 h (Section 2.2.2).

M.C. Álvarez-Galván et al. / Applied Catalysis

700500300100

0

50

100

150

200

LaCoO3a

isothermalRu/LaCoO3

CO

Con

cent

rati

on (

ppm

)

Temperature (ºC)

LaCoO3

700500300100

0

100

200

300

400

500

600

Ru/LaCoO3

isothermal

b

CO

2 C

once

ntra

tion

(pp

m)

Temperature (ºC)

LaCoO3

FpC

cStuo2cf

3

tcrgfas

TAl

7

ig. 4. (a) CO and (b) CO2-TPD response curves obtained over LaCoO3 and Ru/LaCoO3

re-reduced (10 vol% H2/He, 750 ◦C, 2 h) catalysts following adsorption of CO (2 vol%O/He gas mixture, 30 min, 30 ◦C).

ases metal dispersion values were very small (lower than 1%).pecifically, metal dispersion was 0.4% and 0.8%, respectively forhe LaCoO3 and Ru/LaCoO3 solids. These very low dispersion val-es are largely due to the presence of a large surface concentrationf Co2+ and Run+ in both samples (no hydrogen chemisorption at5 ◦C takes place), in agreement with the XPS measurements in thease of Co2+ (Fig. 3) and the low BET values observed [10]; 3.3 m2/gor LaCoO3 and 7.2 m2/g for Ru/LaCoO3.

.1.4. CO chemisorption at 25 ◦C followed by TPDFig. 4a and b present CO and CO2-TPD response curves, respec-

ively, obtained over the LaCoO3 and Ru/LaCoO3 solids followinghemisorption of CO at 25 ◦C for 30 min (Section 2.2.5). Table 3eports the quantities of CO and CO2 (�mol/g) estimated after inte-

rating the respective TPD response curves and accounting alsoor the complete CO and CO2 desorption occurred isothermallyt 700 ◦C in He flow. The obtained results clearly indicate that aignificantly larger amount of CO2 was desorbed compared to CO

able 3mounts (�mol/g) of CO and CO2 obtained over LaCoO3 and Ru/LaCoO3 solids fol-

owing CO-TPD studies (Section 2.2.5).

Catalyst precursora CO (�mol/g) CO2 (�mol/g)

LaCoO3 0.9 14.6Ru/LaCoO3 0.8 5.4

a Before CO chemisorption the solid catalyst was reduced in 10 vol% H2/He at50 ◦C for 2 h (Section 2.2.5).

B: Environmental 102 (2011) 291–301 295

(Table 3) for both solids. In the case of LaCoO3 the amount of des-orbed CO was 0.9 �mol/g compared to 14.6 �mol/g of CO2. Thisresult is attributed to the formation of CO2 through the Boudouardreaction [22,23]:

2CO(g) + s → CO2(g) + C-s (1)

The fact that reaction (1) takes place during CO-TPD has beenreported for partially reduced LaCoO3 [23] as well as for �-alumina[24] and silica-supported Ru solids [25]. Carbon dioxide can also beproduced from the reaction between adsorbed carbon monoxidewith surface hydroxyl groups of the metal oxide support locatedat the periphery of the metal-support interface, leading to the for-mation of CO2 and molecular hydrogen according to reaction (2)[26,27], and by surface reaction of adsorbed CO with surface labileoxygen (OL) species (reactions (3) and (4)) [21,28]:

COads + OHL → CO2(g) + 1/2H2(g) (2)

CO-s + OL2e− → CO2-s2e− (3)

CO2-s2e− ↔ CO2(g) + (�s/2e−) (4)

where �s/2e− is an oxygen vacant site. In the present work, verysmall H2 concentrations were detected (<10 ppm), thus reaction (2)is minor.

In the case of LaCoO3 two sharp and well-resolved CO desorp-tion peaks were observed (Fig. 4a) with peak maxima at 160 and700 ◦C (isothermal desorption). On the contrary, a very weak andbroad desorption peak was detected for the Ru/LaCoO3 catalyticsurface (Fig. 4a). The amount (�mol/g) of CO desorbed from thesurface of both catalysts was approximately the same (Table 3). Asreported by González Tejuca et al. [21], the peak position of COdesorption (binding strength) is related to the oxidation state ofcobalt and to the adsorption type of CO molecule (e.g., linearly orbridged CO). Following FTIR spectroscopic studies, González Tejucaet al. [21] suggested that the first low-temperature desorption peakobtained after interaction of CO or CO2 with the LaCoO3 surfacecould be assigned to monodentate carbonate, while the secondpeak to bridged carbonate. Similar results for the CO-TPD behaviouron LaCoO3 were reported by Rhee and Lee [28] during CO-TPD stud-ies under vacuum conditions. The latter work reported also onelow-temperature desorption peak (165 ◦C) and a high-temperatureone (360 ◦C) with a tail out to 630 ◦C. The authors suggested thatthe weak adsorption state of CO represents the bonding of CO withCo3+ via electron back-donation. The strong adsorption state of COwas assigned to a bridge-type CO bond with two lattice oxygens.

According to what mentioned in the previous paragraph, thepeak detected at 160 ◦C for the pre-reduced LaCoO3 solid (Fig. 4a)could be attributed to linearly adsorbed CO on Co2+, while the sec-ond peak at 700 ◦C to bridge-type carbonate as evidenced by theCO2 desorption peak shown in Fig. 4b. However, it is also likelythat some of adsorbed CO produced a small quantity of carbon viathe Boudouard reaction (1). The latter may had reacted with lat-tice oxygen to form gaseous CO at high temperatures (ca. 700 ◦C).The broad desorption trace of CO obtained for the catalyst derivedafter reduction of the Ru/LaCoO3 solid (Fig. 4a) clearly implies theinfluence of the presence of Ru on the adsorption states of CO bothon Ru and Co sites. Dulaurent et al. [29] reported that the modifi-cation of the Ru particles in the course of CO adsorption leads tonew adsorbed states with heats of adsorption higher than that ofthe adsorbed species formed initially at 25 ◦C. This important find-ing could explain the broadening of CO desorption trace in the caseof Ru/LaCoO3 solid, where the interaction of Ru particles with sup-

port Co sites might have also influenced the heat of adsorption ofCO associated with the latter sites.

The CO2-TPD response curve obtained with the LaCoO3 solid(Fig. 4b) consists of an intense and broad peak centred at 600 ◦Cwith a broad shoulder on its falling part. In the case of Ru/LaCoO3

2 alysis B: Environmental 102 (2011) 291–301

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Table 4Amount (�mol/g) of CO2 adsorbed on LaCoO3 and Ru/LaCoO3 surfaces after hydro-gen pre-treatment at 300, 500 and 750 ◦C for 2 h, estimated after CO2-TPD studies.

Reduction temperature (◦C) CO2 (�mol/g)

300 24.2a

4.8b

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96 M.C. Álvarez-Galván et al. / Applied Cat

largely different CO2-TPD curve was obtained. More precisely,very weak CO2 desorption peak centred at 35 ◦C and a larger

nd broad one centred at 425 ◦C with a shoulder on its rising partere detected (Fig. 4b). The amount of CO2 desorbed from the sur-

ace of LaCoO3 (14.6 �mol/g) was found to be nearly three timesarger than the amount of CO2 desorbed from the Ru/LaCoO3 sur-ace (5.4 �mol/g, Table 3). As stated in the previous paragraph, theormation of CO2 is the result of decomposition of bridge-type car-onate species formed upon CO chemisorption (reactions (3) and4)). The latter is confirmed by the CO2-TPDs to be presented in theollowing section. It appears from the CO-TPD results of Fig. 4 thathe presence of Ru on the reduced LaCoO3 state retards significantlyhe rate of carbon deposition via the Boudouard reaction and likelyhat of reaction of lattice oxygen of the oxidic phases present (e.g.,a2O3) with adsorbed CO.

.1.5. CO2 chemisorption at 25 ◦C followed by TPDCO2 chemisorption at 25 ◦C followed by TPD (Section 2.2.6) was

onducted following catalyst reduction at 300, 500 or 750 ◦C forh in order to create different concentration of oxygen vacancies

n both solids, LaCoO3 and Ru/LaCoO3. It is important to mentionhat for both solids and for all reduction temperatures studiedo desorption of CO was observed. Fig. 5a–c present CO2-TPDesponse curves obtained following catalysts reduction at 300, 500nd 750 ◦C, respectively. For both solids, all CO2-TPD profiles consistf two main desorption peaks; a small one in the low-temperatureange of 25–240 ◦C, and a larger one in the high-temperature rangef 200–700 ◦C with a shoulder at the falling part of it. In particular,ig. 5a shows a weak CO2 desorption peak centred at 68 ◦C and aroad one at 400 ◦C in the case of Ru/LaCoO3, while a sharp peakentred at 85 ◦C and a broad one at 500 ◦C with a shoulder on theising and falling part of it were observed in the case of pre-reducedaCoO3 catalyst. After increasing the hydrogen reduction tempera-ure of Ru/LaCoO3 solid to 500 ◦C (Fig. 5b), the CO2 desorption peaksere shifted to lower desorption temperatures compared to thoseetected when reduction of the solid occurred at 300 ◦C (Fig. 5a).herefore, the increase in reduction temperature from 300 to 500 ◦Ced to the decrease in the strength of surface basic sites, possiblyue to the formation of surface and/or metal-support interface oxy-en vacancies [30,31]. The opposite behaviour was observed for theaCoO3 solid indicating desorption peaks with maxima at 92 and12 ◦C (Fig. 5b).

After increasing the reduction temperature to 750 ◦C (Fig. 5c),ll CO2 desorption peaks related to LaCoO3 were shifted to higheremperatures when compared to those observed following H2eduction at 500 ◦C (Fig. 5b). The opposite behaviour was seen forhe Ru/LaCoO3 which revealed peaks at 59 and 450 ◦C close to theeaks observed after catalyst reduction at 300 ◦C (Fig. 5a). It is con-luded that the increase in H2 reduction temperature results in thencrease of the strength of surface active basic sites in the case ofaCoO3 in agreement with previous studies [30,31]. On the otherand, in the case of Ru/LaCoO3 the strength of surface basic sitesith increasing reduction temperature in the 300-750 ◦C rangeasses through a minimum.

The amount of desorbed CO2 (�mol/g) from the surface of eachatalyst at the three different reduction temperatures applied iseported in Table 4. A slight increase (∼8%) of the surface basicityith increasing hydrogen reduction temperature in the 300–750 ◦C

ange was observed in the case of LaCoO3 catalyst. It is interest-ng to state that reduction of the Ru/LaCoO3 catalyst at 300 and50 ◦C resulted in the formation of a similar concentration of oxy-

en surface/interface basic sites. Moreover, the amount of desorbedO2 from the surface of Ru/LaCoO3 solid was found to be five toix times lower than that obtained for the LaCoO3 solid; a maxi-um amount of 4.8 �mol CO2/g was obtained for the Ru/LaCoO3

ollowing hydrogen reduction at 300 or 750 ◦C.

3.8

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a LaCoO3.b Ru/LaCoO3.

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M.C. Álvarez-Galván et al. / Applied Cat

As stated earlier in Section 3.1.4, the first CO2 desorption peakn the case of LaCoO3 is associated with monodentate carbonatedsorbed on Co2+ sites, whereas the second peak to bridged carbon-te associated with surface Co2+ and La3+ cation sites. The shoulderbserved in the high-T peak should also be associated with bridgedarbonates known for their high thermal stability. According to thebove mentioned, the higher concentration of CO2 accommodatedn the surface of LaCoO3 compared to Ru/LaCoO3 indicates a greaterurface concentration for Co2+ and/or La3+ derived from the reducedaCoO3 compared to Ru/LaCoO3 solid. These results are in goodgreement with other studies reported in the literature [32]. More-ver, the higher CO2 desorption temperatures observed over there-reduced LaCoO3 compared to Ru/LaCoO3 (Fig. 5) may have aole in the catalyst stability since adsorption of CO2 on lanthanaeads to the formation of La2O2CO3 and formate species, which

ere reported to be involved in the gasification of carbon speciesormed during reaction [11,33].

The CO2-TPD results obtained indicate that the LaCoO3 surfaceas less reduced compared to that of Ru/LaCoO3 as also revealed

rom the CO-TPD experiments (Fig. 4). The above observations are ingreement to those of less contribution of the shake-up peaks cor-esponding to Co2+ in the Ru/LaCoO3 than LaCoO3 solid as revealedn the XPS studies (Fig. 3). This is related to the higher reducibilityf Ru/LaCoO3 solid structure compared to that of perovskite alonen agreement with results of previous work [10]. The higher surfaceroportion of Co2+ in the solid derived from the reduction of LaCoO3

s in agreement with the greater “C”/La atomic ratio found by XPSn the reduced samples, associated with the remaining carbonatesormed on its surface after the reduction treatment (Table 2).

All the above results strongly support the view that thencreased amount of CO2 adsorbed, thus of the increased concentra-ion of active basic sites formed on the surface of LaCoO3 comparedo Ru/LaCoO3 following hydrogen pre-treatment is related to thencreased concentration of surface Co2+ in the former solid due tohe different degree of reduction achieved in the two solids.

CO2-TPD studies on clean LaCoO3 under vacuum conditionsevealed only a single desorption peak (TM = 220 ◦C) arising fromhe monodentate carbonate state [28]. In addition, a new adsorp-ion state of CO2 was found to be formed during oxidation of COn the LaCoO3 surface. These results could explain the differencesn the position and shape of CO2-TPD traces in the CO-TPD (Fig. 4)nd CO2-TPD (Fig. 5) experiments.

.2. Catalytic performance of LaCoO3 and Ru/LaCoO3 towardsxidative steam reforming of propane

The activity of pre-reduced in H2 at 750 ◦C (Section 2.5) LaCoO3nd Ru/LaCoO3 catalysts towards the oxidative steam reformingOSR) of propane was measured in terms of hydrogen yield (%)t 750 ◦C (Fig. 6). Hydrogen yield is defined as the percentage ofydrogen composition formed experimentally with respect to theaximum theoretical hydrogen composition expected under the

resent feed gas composition used. It is seen (Fig. 6) that after 6 h ontream H2-yields of 60% and 65% for LaCoO3 and Ru/LaCoO3 solids,espectively were obtained. The activity of both catalysts changesignificantly within the first 4 h of reaction, and this is most likelyue to the progressive increase in the extent of cobalt reduction.slightly higher hydrogen production was found for the catalyst

erived from LaCoO3 at the beginning of reaction, while a bettererformance was observed as reaction proceeded further for theatalyst derived from Ru/LaCoO3. The latter should be partly related

o the extent of reduction of Co2+ to Co0 in both catalyst composi-ions without considering the intrinsic activity of ruthenium phase.ccording to the present CO and CO2-TPD studies, a greater pro-ortion of unreduced cobalt was found in LaCoO3 than Ru/LaCoO3atalytic system. It should be noted the slightly larger relative inten-

Fig. 6. Hydrogen yield (%) versus reaction time obtained under oxidativesteam reforming of propane performed at 750 ◦C (GHSV = 20,000 h−1, H2O/C = 3,O2/C = 0.5 vol%) over the LaCoO3 and Ru/LaCoO3 solids pre-reduced in hydrogen(10 vol% H2/N2 gas mixture, 750 ◦C, 2 h).

sity of the satellite peaks of Co 2p shown in Fig. 3 which are relatedto the presence of Co2+ on the surface. Furthermore, the higheractivity of catalyst derived from LaCoO3 should be influenced bythe larger amount of cobalt per gram of catalyst in the metallicstate [10,34]. Even though the Ru + Co/La atomic ratio is lower thanthat of Co/La corresponding to the reduced system derived fromLaCoO3 (Table 2), the catalyst containing Ru has a greater propor-tion of cobalt in the reduced state as indicated by the lower intensityof the satellite line associated with the principal peaks of the Co 2pdoublet (Fig. 3b).

The present results seem to indicate that the applied hydro-gen reduction pre-treatment of the present LaCoO3 and Ru/LaCoO3solids at 750 ◦C does not ensure complete reduction of cobalt to Co0.The presence of Co2+ contributes to re-adsorption of the CO and CO2reaction products of OSR of propane reaction on the catalyst surface(Figs. 4 and 5), which is likely to have a suppressing effect (block-age of active sites) in the production of hydrogen during the OSR ofpropane reaction. It is well documented in the literature that waterdissociation requires a pair site of Mx+–Oy− on metal oxide surfaces,thus occupation of these sites by carbonates would reduce the rateof steam reforming reactions in general.

3.3. C3H8 TPSR studies

Propane TPSR experiments were conducted in order to inves-tigate the interaction of the latter molecular species with thesurface oxygen of the catalytic surfaces derived from LaCoO3 andRu/LaCoO3 following different hydrogen or oxygen pre-treatments.Fig. 7 shows the evolution of mass spectrometer (MS) signals (m/z)recorded during propane TPSR over LaCoO3, the latter pre-treatedunder three different conditions. More precisely, Fig. 7a and bpresent TPSR traces of m/z = 2 (H2), 16 (CH4), 28 (CO), 43 (C3H8)and 44 (CO2) obtained over the pre-oxidized LaCoO3 at 600 and400 ◦C, respectively, while Fig. 7c shows the H2-TPSR trace (m/z = 2)obtained over the pre-reduced LaCoO3 solid at 500 ◦C. For the lat-ter case it should be noted that all other m/z traces (16, 28, 43 and44) obtained in the case of pre-oxidized solid were absent in thepresent case of pre-reduced solid, within the accuracy of the massspectrometry analysis performed (see also Section 2.3). Fig. 8a–c

show similar C3H8-TPSR traces obtained over the Ru/LaCoO3 cat-alytic system.

Reaction of propane over the catalytic surfaces derived fromLaCoO3 and Ru/LaCoO3 following oxidation or reduction occurswith the surface oxygen ions (On−) present. Thus, the observed

298 M.C. Álvarez-Galván et al. / Applied Catalysis B: Environmental 102 (2011) 291–301

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ig. 7. Temperature-programmed surface reaction (TPSR) of C3H8 performed overaCoO3 following oxidation at 600 ◦C (a), 400 ◦C (b), and hydrogen reduction at50 ◦C (c). Mass numbers (m/z): 2 (H2); 16 (CH4); 28 (CO); 43 (C3H8); 44 (CO2).

ntensities of MS signals associated with a given reaction productust depend on the catalyst’s BET surface area. It is evident from

igs. 7 and 8 that almost complete consumption of C3H8 takes placerom the beginning of TPSR (∼30 ◦C). On the other hand, formationf H2, CH4, CO and CO2 gas products starts at T > 400 ◦C. In particu-ar, H2 formation starts between 400 and 450 ◦C and increases witheaction temperature (400 → 700 ◦C) and with time on stream at00 ◦C (isothermal reaction) in the case of pre-oxidized catalystsFigs. 7a and b and 8a and b). On the other hand, a pseudo-steadytate rate of H2 production is observed at lower temperatures overhe pre-reduced than pre-oxidized solids but of similar magni-ude (compare Fig. 7a and c). Furthermore, the general features ofhe TPSR trace for hydrogen formation are largely different in thease of pre-oxidized than pre-reduced solid. In particular, in thease of Ru/LaCoO3 the oxidative pre-treatment (Fig. 8a and b) ledo a significantly higher initial hydrogen formation rate at 700 ◦Cisothermal conditions) compared to the reductive pre-treatmentFig. 8c). However, as time on C3H8/He stream at 700 ◦C increases,here is a larger drop in H2 formation rate for the pre-oxidized600 ◦C) than pre-reduced Ru/LaCoO3 solid (Figs. 8a and c).

In the case of catalysts pre-treated in an oxidative atmosphere,sudden increase in the H2 formation rate occurred after the cat-

lyst was kept at 700 ◦C under the C3H8/He feed stream for nearly0 min (Figs. 7a and b and 8a and b). In this case, the formation of2 is accompanied by a significant increase of m/z = 28 (CO) signalut not of m/z = 44 (CO2) signal, the latter showing a decline withime on stream. These results indicate the formation of synthesis

as (CO/H2) via partial oxidation of propane by the aid of catalysturface lattice oxygen. The formation of synthesis gas very likelyartly reduces the perovskite structure of LaCoO3 to metallic cobaltormed on the surface of lanthana (Co/La2O3), in agreement with

Fig. 8. Temperature-programmed surface reaction (TPSR) of C3H8 performed overRu/LaCoO3 following oxidation at 600 ◦C (a), 400 ◦C (b), and hydrogen reduction at750 ◦C (c). Mass numbers (m/z): 2 (H2); 16 (CH4); 28 (CO); 43 (C3H8); 44 (CO2).

the XRD studies (Section 3.1.1). This reductive process favours fur-ther conversion of propane to CO/H2, as evidenced by the evolutionof the transient isothermal (T = 700 ◦C) response curves of H2 andCO. This process of cobalt reduction that significantly enhances syn-gas formation was found to be largely influenced by the presenceof Ru according to the results presented in Figs. 7a and 8a.

3.3.1. Mechanistic aspects of C3H8-TPSRAs proposed in the literature [35] the propane-TPSR behaviour

in perovskite-type materials (e.g., LaCoO3) for which an oxidativepre-treatment is performed could be explained by a two-stage pro-cess. In the first stage, propane and lower hydrocarbons formedfollowing propane cracking (e.g., CH4, C2H6, C2H4) react with sur-face lattice oxygen of the solid producing carbon monoxide, carbondioxide and water according to reaction (5) in the case of propane.During this stage process, Co(III) is reduced to Co(II) and then toCo0. In a following stage, non reacted propane could further reactwith metallic cobalt (Co0) formed producing hydrogen and propy-lene according to reaction (6), and other hydrocarbons, e.g., CH4and C2H4 according to reaction (7):

C3H8 + 9OL(lattice) → CO + 2CO2 + 4H2O + 9(�s/e−)

(perovskite surface) (5)

C3H8 → H2 + C3H6 (metal cobalt surface, Co0) (6)

3 8 4 2 4

Steam reforming of non reacted propane and of other hydro-carbons formed could also take place on the perovskite and cobaltsurfaces, according to reaction (8) in the case of propane, where the

alysis B: Environmental 102 (2011) 291–301 299

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ater is provided via the combustion reaction (5).

3H8 + 3H2O → 3CO + 7H2

(perovskite and metal cobalt surface, Co0) (8)

The abrupt production of H2 at 700 ◦C with concomitant produc-ion of CO (Figs. 7a and 8a) can now be explained according to thebove provided reaction scheme at the point where Co(III) reduc-ion to Co0 was significant, followed by selective reforming of CxHy

nto CO/H2. It should be noted here that in the case of Ru/LaCoO3,he Ru metal surface also participates in reactions described by Eqs.6)–(8).

The MS signals recorded at m/z = 16 and 44 for both catalystsfter oxidative pre-treatment (Figs. 7a and b and 8a and b) consistf two peaks with maxima between 400 and 700 ◦C due to CH4 andO2 formation. In the case of m/z = 44, since no propane (m/z = 43)

s present at the exit of the reactor, this signal is due only to carbonioxide. The first peak of carbon dioxide should be related to theombustion of a fraction of produced methane and of other C2-ydrocarbons derived from propane cracking by the aid of surface

attice oxygen of perovskite, according to reaction (5), leading toartial reduction of cobalt (Co3+ to Co2+) as previously discussed.he second peak is most likely to be the result of combustion ofropane with surface oxygen of the partially reduced perovskite,esulting in the reduction of Co2+ to metallic cobalt. Therefore, pre-xidation of LaCoO3 and Ru/LaCoO3 seems to create new catalyticites influencing metallic cobalt formation during OSR of propane,hus to higher hydrogen production. The presence of Ru seems tolay a positive role on this process.

In the case of pre-reduction of LaCoO3 and Ru/LaCoO3 solids at00 ◦C before C3H8 TPSR, neither CO, CO2 nor CH4 is formed underPSR conditions or even under isothermal (T = 700 ◦C) reactionf C3H8 with the catalyst surface (Figs. 7c and 8c). This impor-ant result strongly suggests that all “carbon-containing” speciesormed upon propane decomposition are strongly held on theatalyst surface. The latter was probed by performing isothermalxygen titration experiments to be described next. Based on theO2-TPDs described earlier (Fig. 5), it seems that the nature ofhese “carbon-containing” species is largely not adsorbed CO2 (e.g.,arbonate species) but rather CxHy hydrocarbon fragments.

It is to be noted that the differences observed in the variousPSR traces for LaCoO3 and Ru/LaCoO3 catalytic surfaces may notully justify the difference (∼10%) in H2-yield observed in Fig. 6 forhe OSR of propane reaction performed over the same catalysts.owever, this might be expected since in the present C3H8-TPSRxperiments no H2O and O2 were present in the feed.

.4. Characterization of carbonaceous species formed afterropane TPSR

Isothermal oxygen titration (T = 600 ◦C) studies following C3H8-PSR were contacted on the pre-oxidized solids at 600 ◦C (Figs. 7and 8a) in order to determine the amount of “carbonaceous” speciesccumulated on the surface of LaCoO3 and Ru/LaCoO3 due to crack-ng of propane itself and of other lower hydrocarbons (e.g., CH4)ormed by cracking or de-hydrogenation of propane (e.g., C3H6nd CH4; see reactions (6) and (7)). Fig. 9a and b show transientesponse curves of CO2 and CO, respectively, obtained under theow of a 2 vol% O2/He gas mixture at 600 ◦C over LaCoO3. Simi-

ar transient response curves are shown in Fig. 10a and b for theu/LaCoO3 solid. Three distinct peaks of CO2 formation after 3,

3 and 43 min in O2/He gas flow were observed in the case ofaCoO3 (Fig. 9a). On the other hand, in the case of CO formationFig. 9b) no defined peaks were obtained, where the CO produc-ion rate was practically constant for 30 min on stream. After thatime, the rate started decreasing and became zero after 35 min on

Fig. 9. CO2 (a) and CO (b) transient response curves obtained during isothermal(T = 600 ◦C) oxygen titration of the “carbon” formed on the LaCoO3 (oxidative pre-treatment at 600 ◦C) after TPSR of propane.

stream.A different transient response curve of CO production was found

for the Ru/LaCoO3 solid with prolonged period of time formation(50 min, Fig. 10b) and of decreasing rate. For the same catalyst,a large CO2 peak was observed with a maximum appearing after40 min in O2/He gas flow and with a broad shoulder in the low-reaction time side (Fig. 10a). It is interesting to note that the reactionrate of oxygen with “carbonaceous” species to form CO was slowerin the case of Ru/LaCoO3 (Fig. 10b) compared to LaCoO3 (Fig. 9b).Furthermore, large differences are observed in the kinetic rateof oxidation of the “carbonaceous” species to form CO2 betweenLaCoO3 (Fig. 9a) and Ru/LaCoO3 (Fig. 10a) solids. The latter likelyreflects both the different chemical composition of “carbonaceous”species formed after C3H8-TPSR as evidenced by the three distinctCO2 peaks in the case of LaCoO3 (Fig. 9a), and the different kineticrate constant (k) associated with the oxidation process in the twocatalysts. Kinetic modelling of the isothermal transient oxidationexperiments of Figs. 9 and 10 could lead to detailed characteriza-tion of the “carbonaceous” species formed as shown in previousstudies [36,37] but this was out of the scope of the present work.

The quantity (�mol/g) of “carbonaceous” deposits formed dur-ing the propane TPSR was obtained by integrating the respective

transient CO and CO2 response curves (Figs. 9 and 10). This quantitycould also be referred as percentage of the “total carbon” fed dur-ing the propane TPSR experiment and which was reactive in oxygenup to 600 ◦C. The latter quantities of “carbonaceous” deposits werefound to be 3.65 mmol/g or 38% of the total carbon fed for the

300 M.C. Álvarez-Galván et al. / Applied Catalysis

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of X-ray Photoelectron Spectroscopy, Perkin-Elmer Corporation, Eden Prairie,

ig. 10. CO2 (a) and CO (b) transient response curves obtained during isothermalT = 600 ◦C) oxygen titration of the “carbon” formed on the Ru/LaCoO3 (oxidativere-treatment at 600 ◦C) after TPSR of propane.

aCoO3 solid, and 2.13 mmol/g or 17% for the Ru/LaCoO3 solid. Thebove results are in harmony with the lower “carbon” depositionbserved in the Ru/LaCoO3 catalyst compared to LaCoO3 during thexidative steam reforming of diesel [10]. Thus, it is evident fromhese studies that the role of Ru is to suppress significantly theate of formation of “carbonaceous” deposits during interaction of3H8 or higher hydrocarbons (e.g., those present in diesel) withurface lattice oxygen of Ru/LaCoO3 catalyst. It was reported [38]hat Ru/TiO2 promotes the direct partial oxidation of CH4 to CO/H2no formation of CO2 and H2O) as opposed to Ru/�-Al2O3 whichromotes the indirect route of partial oxidation (total oxidation ofH4 followed by steam and CO2 reforming of unconverted CH4).his result was interpreted based on DRIFTS studies which showedhat the support influenced in a detrimental manner the oxidationtate of Ru; oxidized Ru sites promote the direct partial oxidationf CH4 to CO/H2. In the present Ru/LaCoO3 (pre-oxidized catalyst)nd Ru/La2O3–Co/La2O3 (pre-reduced Ru/LaCoO3 solid) systems, its likely that the effect of support on the oxidation state of Ru influ-nces the oxidation rate of CxHy adsorbed species (formed uponropane chemisorption) towards H2 and COx (x = 1, 2).

. Conclusions

The following conclusions can be derived from the results of theresent work:

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B: Environmental 102 (2011) 291–301

(a) CO and CO2-TPD along with XPS studies conducted over pre-reduced (10% H2/N2 gas mixture at 750 ◦C for 2 h) LaCoO3 andRu/LaCoO3 solids led to the conclusion that cobalt cannot becompletely reduced to Co0.

(b) A greater fraction of unreduced Co2+ species was obtained forthe solid derived from LaCoO3 after pre-reduction at 750 ◦C for2 h in 10% H2/N2 gas mixture. This had a detrimental effect inthe production of hydrogen in the oxidative steam reforming ofpropane since unreduced cobalt is not active towards the latterprocess. In addition, the presence of an increased surface con-centration of Co2+ favours re-adsorption of the main reactionproducts, CO and CO2 which could eventually reduce catalystactivity by blocking active catalytic sites for water dissociationat least.

(c) TPSR of propane indicated that surface lattice oxygen presentin pre-oxidized LaCoO3 and Ru/LaCoO3 solids participate in theoxidation of propane which results in the formation of CO, CO2,H2 and CH4 and “carbonaceous” deposits on the catalyst sur-face. It was found that the presence of Ru in the Ru/LaCoO3solid composition largely reduces the rate of formation of “car-bonaceous” deposits. Also, the kinetics of H2 formation wasfound to be largely influenced by the catalyst pre-treatmentperformed (oxidation versus reduction) and largely promotedby the presence of Ru and of reduced cobalt phase.

(d) Enhanced reactivity of propane over the Ru/LaCoO3 thanLaCoO3 surface for either oxidative or reductive pre-treatmentswas observed during TPSR. It appears that the presence of Ruretards “carbonaceous” deposits and promotes syngas (CO/H2)formation.

Acknowledgements

The Research Committee of the University of Cyprus is acknowl-edged for financial support. Also, Dr. M. Consuelo Álvarez-Galvángratefully acknowledges financial support from MCYT within theframework of the Ramon y Cajal research programme. We alsothank Noelia Mota for performing the OSR of propane catalyticexperiments.

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