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ORIGINAL PAPER
Hydrogen Production from Glycerol Over Nickel CatalystsSupported on Al2O3 Modified by Mg, Zr, Ce or La
A. Iriondo Æ V. L. Barrio Æ J. F. Cambra Æ P. L. Arias ÆM. B. Guemez Æ R. M. Navarro Æ M. C. Sanchez-Sanchez ÆJ. L. G. Fierro
Published online: 23 April 2008
� Springer Science+Business Media, LLC 2008
Abstract Hydrogen production from glycerol reforming
in liquid (aqueous phase reforming, APR) and vapor (steam
reforming SR) phase over alumina-supported nickel cata-
lysts modified with Ce, Mg, Zr and La was studied.
Characterization of catalysts by temperature programmed
reduction and XPS analyses revealed important structural
effects: (i) the intercalation of Mg between nickel and
alumina that inhibited the alumina incorporation to nickel
phases, (ii) the close contact between Ni and Zr phases and,
(iii) the close surface interaction of La and Ce ions with
NiO phases. The catalytic activity of the samples studied in
this work clearly indicated the different catalyst function-
alities necessary to carry out aqueous-phase and vapor-
phase steam reforming of glycerol. For aqueous phase
reforming of glycerol, the addition of Ce, La and Zr to Ni/
Al2O3 improves the initial glycerol conversions obtained
over the Ni/Al2O3 supported catalyst. It is suggested that
the differences in catalytic activities are related with geo-
metric effects caused by the decoration of Ni phases by Ce
and La or by the close interaction between Ni and Zr. In
spite that nickel catalysts showed high APR activities at
initial times on stream, all samples showed, independently
of support, important deactivation rates that deactivate the
catalysts after few hours under operation. Catalysts char-
acterization after APR showed the oxidation of the active
metallic Ni during reaction as the main cause of the
observed deactivation. In the case of the glycerol steam
reforming in vapor phase, the use of Ce, La, Mg and Zr as
promoters of Ni based catalysts increases the hydrogen
selectivity. Differences in activity were explained in terms
of enhancement in: surface nickel concentration (Mg),
capacity to activate steam (Zr) and stability of nickel
phases under reaction conditions (Ce and La).
Keywords Glycerol � Nickel catalyst � Steam reforming �Aqueous phase reforming � Ceria � Magnesia � Lanthana �Zirconia
1 Introduction
The diminution of fossil fuel reserves and the pollution
caused by the continuous energy demands make hydrogen
an attractive alternative energy vector, especially if it is
produced from renewable resources. Nowadays biodiesel
has become one of the more promising carbon neutral bio-
fuels. However the cost of biodiesel is the main barrier for
this product commercialization. The recovery of biodiesel
co-products, like the glycerol, is one of the main options to
be considered to lower the overall cost of the biodiesel
production [1]. Thus, in recent years, many studies have
focused in the recovery of high quality glycerol by-product.
In this context, the glycerol may represent a potential
source for hydrogen production. Glycerol can be efficiently
converted in hydrogen by means of its catalytic reaction
with steam according to the following reaction:
C3H8O3 þ 3H2O! 3CO2 þ 7H2 ð1Þ
The steam reforming of hydrocarbons (SR) is a catalytic
process that typically takes place in vapour phase at
A. Iriondo � V. L. Barrio � J. F. Cambra �P. L. Arias � M. B. Guemez
Department of Chemical and Environmental Engineering,
University of the Basque Country (School of Engineering),
C/Alameda Urquijo s/n, 48013 Bilbao, Spain
R. M. Navarro (&) � M. C. Sanchez-Sanchez � J. L. G. Fierro
Instituto de Catalisis y Petroleoquımica, CSIC, C/Marie Curie 2
Cantoblanco, 28049 Madrid, Spain
e-mail: [email protected]
123
Top Catal (2008) 49:46–58
DOI 10.1007/s11244-008-9060-9
atmospheric pressure and temperatures around 1073 K.
Nevertheless it has been recently reported [2] the possibility
to obtain hydrogen from oxygenated hydrocarbons having a
C:O stoichiometry of 1:1 at low temperature (500 K) and
high pressure (2–5 MPa) by aqueous-phase reforming
(APR).
Catalysts for steam reforming of hydrocarbons are
mainly based on nickel as active component supported on
oxides with high thermal stability [3]. Although noble
metals (Ru, Rh, Pt) are more effective for the steam
reforming of hydrocarbons than Ni and less susceptible to
carbon formation, such catalysts are not common in
industrial applications because of their cost [3]. Effective
catalyst for production of hydrogen by aqueous phase
reforming of oxygenated hydrocarbons must break C–C,
O–H and C–H bonds in the oxygenated hydrocarbon
reactant and facilitate the water–gas shift reaction to
remove adsorbed CO from the surface. Studies on aqueous-
phase reforming of oxygenated hydrocarbons over various
supported metals [4, 5] indicate that Pt and Pd catalysts are,
especially in the case of Pt, active and selective for the
production of hydrogen. Nevertheless, the high cost and
limited availability of noble metals make it of particular
interest to develop less expensive catalyst for aqueous-
phase reforming. From the above reasons, in the present
work nickel was selected as active phase to be included in
catalyst formulations applied to glycerol reforming.
The structural characteristics and performance of sup-
ported nickel catalysts are strongly influenced by the nature
of support where the metallic crystallites are deposited.
The use of supports with high thermal stability that stabi-
lizes nickel particles against sintering and that promotes the
carbon gasification are necessary to develop catalysts with
high activity and stability in the steam reforming of glyc-
erol. To provide these operational advantages different
promoters are usually included in the reforming catalyst
formulations. Magnesium is widely used as promoter in Ni/
Al2O3 catalysts applied to steam reforming of hydrocar-
bons since it enhances both the steam adsorption capability
and the stability of nickel against sintering [6, 7]. The
addition of ZrO2 to reforming catalysts is also claimed in
literature [8] as an element able to improve the stability of
nickel catalysts in the steam reforming of hydrocarbons.
Ceria [9] and lanthana [10] are also found in several for-
mulations of Ni steam reforming catalysts since they are
known promoters of carbon removal from metallic sur-
faces. For aqueous-phase reforming catalysts, there are no
studies reporting the effect of the addition of Mg, Zr, Ce
and La on the reforming chemistry over nickel catalysts
being therefore necessary to be explored.
With this background, the aim of the present work was
to study the role and effect of La, Mg, Zr and Ce on
the behaviour of alumina-supported Ni catalysts in the
production of hydrogen by both aqueous-phase reforming
and vapour phase steam reforming of glycerol. Careful
investigations of the structure of the catalysts were per-
formed in an attempt to understand the relationship
between activity and their structural and surface
characteristics.
2 Experimental
2.1 Supports and Catalysts Preparation
MxOy–Al2O3 supports (M=Zr, Ce, La or Mg) were pre-
pared by impregnation of a commercial c-Al2O3 (Alfa
Aesar, SBET = 212 m2/g) with aqueous solutions of metal
nitrates. MxOy loading in the different supports were
selected in order to achieve 0.3 theoretical monolayers on
alumina carrier. The impregnated solids were dried under
air at 393 K for 3 h and subsequently calcined in air at
923 K for 6 h (except for ZrO2–Al2O3 support which was
calcined at 823 K). The supports were designated as A–M
where M refers to the oxide (C=CeO2, M=MgO, Z=ZrO2
and L=La2O3) added to Al2O3 carrier.
Supported Ni catalysts were prepared by impregnation
under stirring at 333 K of each of the above supports using
aqueous solutions of Ni(NO3)2. Ni loading in the different
catalysts was selected in order to achieve 0.75 theoretical
monolayers over supports. After Ni loading, the samples
were dried at 393 K for 2 h and subsequently calcined
in air at 773 K for 4 h. The catalysts were designated as
Ni/A–M.
2.2 Catalyst Characterization
The chemical composition of the catalysts was determined
by inductively coupled plasma atomic emission spectros-
copy (ICP-AES), using a Perkin-Elmer Optima 3300DV
apparatus. The samples were first dissolved in acid solu-
tions (a mixture of HF, HCl and HNO3), microwaved for
15 min, and diluted to concentrations within the detection
range of the instrument.
N2 adsorption–desorption isotherms were obtained at
77 K over the whole range of relative pressures using a
Micromeritics ASAP 2100 automatic device on samples
previously outgassed at 423 K for 12 h. BET specific areas
were calculated from these isotherms using the BET
method and taking a value of 0.162 nm2 for the cross-
section of the physically adsorbed N2 molecule.
Temperature-programmed reduction experiments were
carried out with a semiautomatic Micromeritics TPD/TPR
2900 apparatus equipped with a TC detector. Prior to
reduction experiments, the samples, about 30 mg, were
treated thermally under an air stream at 573 K to remove
Top Catal (2008) 49:46–58 47
123
water and other contaminants. TPR profiles were obtained
by heating the samples under a 10% H2/Ar flow (50 mL/
min) from 298 to 1173 K at a linearly programmed rate of
10 K/min. The effluent gas was passed through a cold trap
to remove water before measuring the amount of hydrogen
consumed during reduction by the TC detector.
X-ray powder diffractograms were recorded following
the step-scanning procedure (step size 0.02�, 2h scanning
from 20 to 80�) using a computerised Seifert XRD 3000P
diffractometer (Cu Ka radiation, k = 0.15418 nm) equip-
ped with a PW Bragg-Brentano h/2h goniometer and a bent
graphite monochromator and automatic slit. Volume-
averaged crystallite sizes were determined by applying the
Debye-Scherrer equation.
X-ray photoelectron spectroscopy (XPS) was used to
study the chemical composition and oxidation state of the
catalyst surfaces. Photoelectron spectra were recorded with
a VG Escalab 200R electron spectrometer equipped with a
Mg Ka X-ray source (hm = 1253.6 eV) and a hemispher-
ical electron analyser operating at constant transmission
energy (50 eV). The X-ray source was operated at low
X-ray fluxes in order to minimize X-ray-induced reduction
of Ce species. The C 1s, Al 2p, Zr 3d, Mg 2p, La 3d, Ce 3d
and Ni 2p core-level spectra were recorded and the corre-
sponding binding energies were referenced to the C 1s line
at 284.6 eV (accuracy within ± 0.1 eV). Due to the partial
overlapping of La 3d5/2 and Ni 2p3/2 signals, the chemical
state of nickel element was elucidated after carefully sub-
traction of La 3d5/2 in the Ni 2p3/2 peak. The reduction
treatment was carried out ex situ at 923 K in H2/N2 (1/9
vol.) flow for 90 min followed by reduction in situ at
773 K for 30 min. After outgassing at 10-5 mbar, the
calcined or reduced samples were transferred to the ion-
pumped analysis chamber, whose residual pressure was
kept below 7 9 10-9 mbar during data acquisition. The C
1s peak at 284.6 eV was used as an internal standard for
peak position measurement. The areas of the peaks were
estimated by calculating the integral of each peak after
subtracting a Shirley background and fitting the experi-
mental peak to a combination of Lorentzian/Gaussian lines
of variable proportions.
2.3 Activity Tests
The catalytic tests for the steam reforming (SR) or aque-
ous-phase reforming (APR) of glycerol were carried out in
a bench-scale unit equipped with a stainless steel fixed-bed
catalytic reactor. In order to avoid preferential gas flow
paths and hot spots, 200 mg of catalyst were diluted with
SiC (1:9 w/w), both in the 0.42–0.5-mm particle size
range. Each catalyst was activated in situ by reduction at
atmospheric pressure under H2 flow (75 mL/min) at 973 K
for 2 h. After catalyst activation, the activity measurements
were performed. The feed was 1 wt% of glycerol in water.
Aqueous phase reforming measurements were performed at
3 MPa of total pressure, at 498 K and with a WHSV equal
to 1.25 h-1. Steam reforming measurements were con-
ducted at atmospheric pressure, at 873 K and with a
WHSV equal to 2.5 h-1. The reaction was kept running at
this space velocity until steady-state was reached. Liquid
products was collected and analyzed by a GC equipped
with a MS detector and CHN-analyzer. The gas product
was analyzed on-line by a GC equipped with a FID and a
TCD detectors.
3 Results
3.1 Chemical Composition and Textural Properties
Table 1 shows the chemical compositions obtained from
ICP analyses, expressed as weight percentages, of the
calcined catalysts. The textural properties of all catalysts
and supports were evaluated from nitrogen adsorption–
desorption isotherms. The specific surface areas of the
catalysts and supports together with the pore volume and
average pore diameter are summarized in Table 2. For the
sake of a valid comparison, the N2 adsorption–desorption
data were normalized to unit weight of Al2O3 carrier.
Textural data in Table 2 show that the surface area of
Al2O3 was almost constant after the incorporation of Ce
and Mg and increases for lanthanum and zirconium con-
taining supports. The increase in surface area observed in
the case of A–L and A–Z supports may be related to the
known capacity of La [11] and Zr [12] to stabilize the
surface area of c-Al2O3 against the thermal sintering
induced by the treatments done in the calcination step.
The specific area and pore volume data for the Ni-loa-
ded samples indicate that the surface area of the bare
MxOy–Al2O3 supports does not change significantly upon
incorporating Ni to the supports. The almost constant sur-
face area after Ni loading is probably indicating a slight
increase in the external area associated to NiO phases
which is not counterbalanced by the simultaneous filling of
the pores by nickel species as indicated by the slight
decrease in pore volume.
Table 1 Chemical composition (wt%) of calcined catalysts
Sample NiO ZrO2 CeO2 La2O3 MgO Al2O3
Ni/A 16.0 – – – – Balance
Ni/A–Z 17.0 7.0 – – – Balance
Ni/A–C 16.0 – 8.4 – – Balance
Ni/A–M 16.2 – – – 2.5 Balance
Ni/A–L 17.0 – – 5.0 – Balance
48 Top Catal (2008) 49:46–58
123
3.2 X-Ray Diffraction (XRD)
Figure 1 shows the X-ray diffraction patterns of the fresh
calcined supports. The characteristic peaks of poorly
crystalline c-Al2O3 (at 46.8� and 66.7 JCPDS 86-1410)
were observed in the bare alumina support (Fig. 1). The
XRD pattern of the La, Zr and Mg supports (A–L, A–Z and
A–M in Fig. 1) exhibited lines only attributable to the bare
c-Al2O3 support. The Ce-containing support was the
only modified alumina that showed diffraction peaks
characteristics of bulk crystalline CeO2 besides the Al2O3
phases of bare alumina.
XRD patterns of calcined and reduced catalysts depos-
ited on MxOy–Al2O3 supports are shown in Fig. 2. As
Fig. 2a shows, calcined catalysts displayed the reflections
already detected in Ni-free supports and new reflections
associated to nickel phases. All calcined catalysts exhibited
diffraction lines at 37.2� and 43.3� associated to crystalline
phases of NiO (JCPDS 44-1159). Calculated crystallite size
of the NiO phase applying the Scherrer equation are
summarized in Table 3. The size of the NiO crystallites
varies with the support and decreases following the trend:
Ni/A–Z & Ni/A & Ni/A–C [ Ni/A–M [ Ni/A–L.
The X-ray diffractograms of reduced catalysts supported
on MxOy–Al2O3 supports (after treatment under H2/N2 (1/
9 vol, 100 mL/min at 923 K for 2 h)) are displayed in
Fig. 2b. XRD patterns of reduced catalysts in Fig. 2b
showed the diffraction line corresponding to the (200)
reflection of Ni0 phase (at 2H angle of 51.8� JCPD
04-850). No diffraction lines ascribed to NiO (at 2H angles
Table 2 Textural properties of calcined supports and catalysts as
measured by N2 adsorption desorption isotherms at 77 K
SBET (m2/gAl2O3) Vpore (cm3/gAl2O3) Dpore (nm)
A 212 0.81 15.4
A–Z 229 0.81 14.2
A–C 217 0.80 14.7
A–M 214 0.80 14.9
A–L 225 0.81 14.3
Ni/A 207 0.77 14.8
Ni/A–Z 229 0.81 14.0
NiA–C 215 0.81 14.3
Ni/A–M 218 0.82 14.1
Ni/A–L 225 0.81 14.3
20 40 60 80
Inte
nsi
ty (
au)
2 θ°
A-M
A-L
A-Z
A
A-C
Fig. 1 XRD patterns of calcined supports (j Al2O3, d CeO2)
20 40 60 80
a
Ni/A
Ni/A-L
Ni/A-M
Ni/A-Z
Ni/A-C
o
o
Inte
nsi
ty (
a.u
.)
o
o
o
oo
o
o
o
o
o
Ni/A
Inte
nsi
ty (
a.u
.)
Ni/A-L
2 θ°
20 40 60 802 θ°
Ni/A-M
Ni/A-Z
Ni/A-C
b
Fig. 2 XRD patterns of Ni catalysts after: (a) calcination in air at
773 K and (b) reduction under H2/N2 flow at 923 K (¤ NiO, o Ni0, d
CeO2 , j Al2O3)
Top Catal (2008) 49:46–58 49
123
of 37.2 and 43.3) were found in these reduced samples.
Quantitative estimation of crystallite sizes of Ni0 phase by
applying the Scherrer equation (Table 3) indicates the
larger crystal size for Ni/A–Z than that of Ni/A, Ni/A–C
and Ni/A–M counterparts. It is interesting to note the
absence of any diffraction peak associated to reduced
phases of nickel for the catalysts supported on lanthana-
loaded alumina (Ni/A–L).
3.3 Temperature Programmed Reduction (TPR)
The TPR profiles corresponding to Ni catalysts deposited
on MxOy–Al2O3 supports are depicted in Fig. 3. Quantifi-
cation of TPR data are summarized in Table 4. TPR data
showed differences in the relative proportion of nickel
species depending on the support used to disperse the
nickel entities. As it is observed in Fig. 3, the catalyst
supported on pure Al2O3 exhibits a broad reduction peak in
the 650 and 1,150 K range which could be deconvoluted
into three components at reduction temperatures of 818,
942, and 1041 K. According to literature studies, the
reduction peak at 818 K is attributed to reduction of NiO
species with weak interaction with alumina support [13]
while the reduction peaks appearing at higher temperatures
were related to the reduction of highly dispersed non-
stoichiometric amorphous nickel aluminate spinels and to a
diluted NiAl2O4-like phase respectively [14]. The incor-
poration of Mg to Al2O3 (Ni/A–M in Fig. 3) provokes a
slight increase in the intensity of the reduction peak cor-
responding to the NiO species with weak interaction with
support (peak at 822 K). From data in Table 4, the pres-
ence of Mg also increases the proportion of highly
dispersed non-stoichiometric amorphous nickel aluminate
(peak at 943 K) at the expense of the diluted NiAl2O4
phase (peak at 1068 K). Similar reduction behaviour was
observed for nickel supported on La-containing Al2O3 (Ni/
A–L in Fig. 3). The Zr-containing catalyst (Ni/A–Z in
Fig. 3) showed hydrogen uptakes at 829, 935 and 1028 K.
The peak at 829 K, ascribed to NiO particles with low
interaction with support, showed a similar shape and
intensity than those corresponding to Mg- and La-loaded
catalysts. However, with the addition of Zr to Al2O3, the
reduction peaks in the high-temperature range increases
considerably in intensity (Table 4). Similar reduction
behaviour was observed for the sample including Ce in the
composition. For this sample, the low intensity of the peak
at 822 K (Table 4) indicates the low concentration of
nickel oxide species weakly interacting with aluminium
ions in this sample. The reduction peak at 958 K, that
includes the reduction of non stoichiometric nickel spinel
species and the bulk reduction of CeO2, showed the highest
intensity among the samples studied.
3.4 X-Ray Photoelectron Spectroscopy (XPS)
The binding energies of core-electrons and the surface
atomic ratios of the calcined supports are summarized in
Table 5. Zr containing support (A–Z in Table 5) shows a
single component in the Zr 3d5/2 level at 181.8 eV char-
acteristic of Zr4+ ions in an oxygen environment [15].
Surface Zr/Al ratio (Table 5) was higher than nominal Zr/
Al ratio indicating that most of the Zr has been supported
on the surface of Al2O3. Mg added support presents a
single component photoelectron Mg 2p peak at 50.3 eV.
This binding energy is close to that reported in literature for
MgO (50.2 eV) or MgAl2O4 (50.2 eV) [16]. Surface XPS
Mg/Al ratio was higher than the corresponding nominal
Table 3 Average size of NiO
and Ni0 crystalline particles
from XRD data of calcined and
reduced Ni catalysts
Calcined
NiO (nm)
Reduced
Ni0 (nm)
Ni/A 10 7
Ni/A–Z 10 9
Ni/A–C 10 6
Ni/A–M 7 7
Ni/A–L 5 n.d.
200 400 600 800 1000 1200 1400
H2
con
sum
pti
on
(a.
u.)
Ni/A
Ni/A-L
Temperature (K)
Ni/A-C
Ni/A-Z
Ni/A-M
Fig. 3 Temperature-programmed reduction profiles of Ni catalysts
(10% vol H2/Ar, heating rate 10 K/min)
50 Top Catal (2008) 49:46–58
123
one indicating, as occurred previously for Zr, the disper-
sion, without diffusion, of Mg across the Al2O3 surface.
For La-containing alumina, the La 3d5/2 peak envelopes are
centred at a binding energy of 835.7 eV. This BE is higher
than the characteristic value for La2O3 (834.3 eV) and
LaAlO3 (833.8) [17, 18] and can be assigned to well-dis-
persed lanthanum species on alumina [19]. The La/Al
atomic ratio obtained from XPS (Table 5) was similar to
the values corresponding to the bulk composition. This fact
may be indicative of diffusion at surface level of lanthanum
ions into the alumina framework [19]. The calculated XPS
Ce/Al atomic ratio for the A–C support (Table 5) was close
to the expected value derived from its chemical composi-
tion (0.033). This fact may indicate some diffusion of Ce
on Al2O3 but as previously commented, the low tempera-
ture of calcination used in the preparation and the presence
of CeO2 entities detected by XRD discards this possibility.
The presence of this agglomeration of CeO2 particles may
be the reason of the low Ce/Al ratio since Ce atoms in the
bulk particles will not contribute to the Ce signal.
The characteristic Ni 2p3/2 levels for the calcined cata-
lysts are displayed in Fig. 4a. The binding energies of Ni
2p3/2 peaks and surface atomic concentrations are sum-
marized in Table 6. As it is seen in Fig. 4a, all calcined
catalysts show the main line of Ni 2p3/2 level at 856.4 eV.
This binding energy was slightly higher than the value
reported for pure NiO species (854.4 eV) and close to
nickel aluminate (855.6 eV). Calculated XPS Ni/Al and M/
Al (M=Zr, Ce, Mg and La) surface atomic ratios in Table 6
indicate differences in the relative metal exposition
depending on the support used. The relation between Ni/Al
surface ratio decreases according the sequence: Ni/A–L [Ni/A–M [ Ni/A [ Ni/A–Z [ Ni/A–C. Regarding the
support additives Zr and Mg, their XPS M/Al surface ratio
decreased after Ni deposition respect to the values
observed on the bare support. The loss of surface exposure
of additives on supports after nickel deposition was much
lower in the case of cerium and negligible in the case of
lanthanum.
Chemical changes in the catalysts after reduction in H2
at 923 K were also investigated by XPS and the results are
summarized in Table 7. Figure 4b shows the Ni 2p3/2 core
level spectra of catalysts after reduction. Reduced catalysts
showed binding energies characteristic of Ni0 (852.6 eV)
and Ni2+ ions in nickel aluminate entities (856.2 eV). As it
can be observed in Table 7, it was clear that none of the
samples were completely reduced after the reduction pro-
tocol applied. This fact confirms the difficulties observed in
TPR for the reduction of nickel ions with interactions with
ions from the supports. After reduction, all the catalysts
showed a strong decrease in the intensity of the Ni 2p3/2
core level that provokes a decrease in the surface exposi-
tion of metallic Ni as it was derived from the Ni/Al surface
ratios presented in Table 7. The loss in Ni XPS signal after
reduction observed for all the analyzed catalysts may be
indicative of sintering phenomena of nickel particles dur-
ing the reduction process and/or ‘decoration’ of metallic
particles by dispersed species from support. Surface
exposition of metallic nickel decreases in the order: Ni/A–C
[ Ni/A–M [ NiA–Z & Ni/A. In the case of Ni/A–L
catalysts the Ni 2p XPS level signal was very weak after
reduction and hence not quantification was done.
3.5 Aqueous Phase Reforming (APR) and Steam
Reforming (SR) Activity Tests
The conversion of glycerol in the aqueous phase reforming
over nickel based catalysts is presented in Fig. 5. Table 8
shows gaseous and liquid product compositions corre-
sponding to the 2.5 hour on stream as representative values
Table 4 Quantitative TPR data of calcined catalysts
H2/Ni Free NiO NiO–Al Surface NiAl2O4 NiAl2O4
Ni/A 0.79 – 809 K––62% 940 K––24% 1,043 K––13%
Ni/A–Z 1.14a – 829 K––50% 935 K––24%b 1,028 K––25%b
Ni/A–C 0.86b 539 K––2.3% 822 K––33% 958 K––48%a 1,062 K––15%
Ni/A–M 0.88 – 822 K––64% 943 K––29% 1,069 K––7%
Ni/A–L 0.92 – 808 K––61% 929 K––30% 1,054 K––9%
a Includes partial surface zirconia reductionb Includes ceria reduction
Table 5 Binding energies (eV) of core electrons and surface atomic
ratios of calcined supports
Al
2p
La
3d5/2
Mg
2p
Zr
3d5/2
Ce 3d5/2
u0 0 0 (% in
Ce 3d)
M/Al at
A–Z 74.5 181.9 0.090 (0.035)a
A–C 74.5 883.0 (12.1%) 0.033 (0.033)a
A–M 74.5 50.3 0.065 (0.039)a
A–L 74.5 835.7 0.021 (0.020)a
a In parenthesis the value corresponding to bulk M/Al ratio
Top Catal (2008) 49:46–58 51
123
of the first hours in the aqueous phase reforming of glyc-
erol over the different tested catalysts. In spite of the low
stability under reaction conditions showed by the catalysts,
differences in glycerol conversion at the initial activity
measurements were observed for the different nickel cat-
alysts. As Fig. 5 shows, initial glycerol conversions were
found to decrease following the sequence: Ni/A–L [ Ni/
A–C & Ni/A–Z [ Ni/A & Ni/A–M. However, all the
catalysts exhibit severe deactivation phenomena. For all
catalysts the glycerol conversion falls below 3% after 30 h
on-stream. When comparing the initial gas product com-
position of the various catalysts shown in Table 8, the
845 850 855 860 865 870 875 845 850 855 860 865
BE (eV)
Ni/A
Ni/A-M
Ni/A-C
Ni/A-Z
BE (eV)
Ni/A-L
a b
cou
nts
per
sec
on
d (
a.u
.)
Ni 2p3/2 Ni 2p
3/2
Ni/A
Ni/A-M
Ni/A-C
Ni/A-Z
cou
nts
per
sec
on
d (
a.u
.)
Fig. 4 Ni 2p3/2 photoelectron
spectra of Ni catalysts: (a) fresh
calcined in air at 773 K and (b)
after reduction in H2/N2 flow at
923 K
Table 6 Binding energy (eV) of Ni 2p3/2 core electrons and surface
atomic ratios of calcined catalysts
Ni 2p3/2 (eV) Ni/Al M/Al (M=Zr, Ce, Mg, La)
XPS Nominal XPS Nominal
Ni/A 856.4 0.110 0.130
Ni/A–Z 856.4 0.123 0.153 0.026 (0.090)a 0.035
Ni/A–C 856.4 0.115 0.148 0.029 (0.033)a 0.033
Ni/A–M 856.4 0.149 0.136 0.053 (0.065)a 0.039
Ni/A–L 856.2 0.172 0.147 0.020 (0.021)a 0.020
a In parentheses values corresponding to bare supports
Table 7 Binding energy (eV) of Ni 2p3/2 core electrons and surface
atomic ratios of reduced catalysts
Ni 2p3/2 (eV) Ni/Al M/Al (M=Zr, Ce,
Mg, La)
XPS Nominal XPS Nominal
Ni/A 852.7 (30.6%) 0.026 0.130
856.4 (69.4%)
Ni/A–Z 852.0 (50.1%) 0.016 0.153 0.019 0.035
856.5 (49.9%)
Ni/A–C 852.4 (47.2%) 0.02 0.148 0.026 0.033
856.4 (52.8%)
Ni/A–M 852.3 (31.5%) 0.028 0.136 0.038 0.039
856.3 (66.5%)
Ni/A–L n.d. – – 0.022 0.020
0
20
40
60
0 5 10 15 20 25 30
Time on stream (h)
Gly
cero
l co
nve
rsio
n (
mo
l %)
Fig. 5 Glycerol conversion as a function of time-on-stream (TOS
(h)) for aqueous phase reforming of glycerol over nickel catalysts: (¤)
Ni/A–Z, (j) Ni/A–C, (D) Ni/A–M, (d) Ni/A–L, (9) Ni/A.
(T = 498 K, P = 3 MPa, WHSV = 1.25 h-1)
52 Top Catal (2008) 49:46–58
123
catalysts supported on Ce- and La–Al2O3 exhibit relatively
higher rates of H2 production. The catalyst supported on
Mg–Al2O3 presents a higher hydrogen molar composition
although its glycerol conversion is rather low. For all the
catalysts the gas product is saturated by water vapour and
contains no significant CO amounts. Analyses of the
effluent liquid from the aqueous-phase reforming of glyc-
erol showed, for all the catalysts tested, the presence of
propylenglycol and ethylenglycol as the main products.
In a second set of experiments, the activity of these
nickel catalysts was also evaluated in the steam reforming
of glycerol performed, as indicated in the experimental
section, at atmospheric pressure and 873 K. Figure 6 pre-
sents the product composition of the glycerol steam
reforming over the different nickel based catalysts. For all
catalysts, glycerol conversion was 100 % and hydrogen
was the major component in the gas phase. From the
comparison of the molar fractions towards the various
reaction products (Fig. 6) it can be seen that the Ni/A–Z
sample was the most selective catalyst with a gas
composition after steam reforming similar to the values
predicted by equilibrium. On the contrary, the catalyst
supported on bare alumina showed the lower hydrogen
selectivity among the tested samples. Compared to the Ni/
A–Z catalyst, lower hydrogen selectivities were found for
the catalysts supported on alumina modified with Mg (Ni/
A–M), Ce (Ni/A–C) and La (Ni/A–L). Among the results
of these last samples, slight differences in hydrogen
selectivity were observed, showing the Ni/A–M and Ni/
A–C samples the higher and lower H2 selectivities,
respectively.
Long-term activity test was performed for the Zr-sup-
ported catalysts in order to study its stability under
reforming conditions. The evolution of gaseous products as
a function of reaction time is presented in Fig. 7. It can be
see that in spite that total glycerol conversion maintains
with reaction time on Ni/A–Z catalyst, the product distri-
bution changes with time-on-stream. The reaction starts
with high conversion and selectivity to H2 and CO2 but the
product distribution changes in the first hours on stream
with a decrease in the hydrogen formation and concomitant
CO production. Nevertheless, after several hours under
reaction stream the reverse was observed with increasing
CO2 formation and decreasing of CO levels. After this
change, the gas composition maintains stable for the last
25 h of the test: high H2 selectivity and low CO contents.
3.6 Characterization of Used Catalysts
XPS analyses of Ni/A–Z catalyst used in the glycerol
reforming in both aqueous and vapour phase were also per-
formed in order to know the evolution of active phases after
reaction. Binding energy for Ni 2p3/2 core of Ni/A–Z catalyst
used in the aqueous phase reforming of glycerol appears at
Table 8 Glycerol conversion and product compositions in the
aqueous phase reforming of glycerol over nickel catalysts after 2.5 h
on stream (T = 498 K, P = 3 MPa, WHSV = 1.25 h-1)
Ni/A Ni/A–Z Ni/A–C Ni/A–M Ni/A–L
Glycerol conversion
(%)
25 30 36 15 37
Gas product (%)
H2 32 32 48 32
CO2 31 42 40 40
CO – – – –
CH4 37 26 12 28
C2+ – – – –
Liquid products (%)
Propylenglycol 66 75 57 57
Ethylenglycol 24 15 33 34
Acetol 3 1 7 6
Ethanol 7 8 3 3
0%
20%
40%
60%
80%
100%
Ni/A-M Ni/A-L Ni/A-C Ni/A-Z Ni/A Equilibrium
Pro
du
ct d
istr
ibu
tio
n (
mo
lar
%)
H2 CO2
CO CH4
Fig. 6 Product distribution for steam reforming of glycerol over Ni
catalysts (T = 873 K, P = 0.1 MPa, WHSV = 2.5 h-1)
0%
20%
40%
60%
80%
100%
0 10 20 30 40 50 60
Time on stream (h)
Co
nve
rsio
n, p
rod
uct
dis
trib
uti
on
(m
ol %
)
Fig. 7 Glycerol conversion and product distribution as a function of
time-on stream (TOS (h)) for steam reforming glycerol over the Ni/
A–Z catalyst ((X) glycerol conversion, (j) H2, (¤) CO2, (D) CH4,
(d) CO, T = 873 K, P = 0.1 MPa, WHSV = 2.5 h-1)
Top Catal (2008) 49:46–58 53
123
856.4 eV (Fig. 8). This binding energy, as it was previously
indicated, corresponds to Ni2+ ions in species close to nickel
aluminate structures. Calculated XPS Ni/Al ratio for the
sample used in the liquid reforming revealed an increase in
Ni surface exposition after reaction (Table 9). The observed
changes in binding energy and surface Ni exposition points
to the formation, under reaction conditions, of the nickel
aluminate entities over Ni particles that leads to a surface
situation very close to that observed for the calcined non
activated sample. In the case of the sample used in the gas-
phase reforming of glycerol, the Ni 2p3/2 profile did not
change in comparison with the binding energies and per-
centages of Ni species detected on the fresh reduced sample.
The main difference observed for the used sample is related
to the quantity of carbon present on the surface of the sam-
ples, being much higher in the used sample.
4 Discussion
4.1 Structure of Supports
Physichochemical characterization of supports revealed the
different interaction degree of Zr-, Ce-, La- and Mg-pro-
moters with alumina carrier that modifies the properties of
the supports at different level. For lanthana–alumina sup-
port, the absence of XRD peaks characteristics of discrete
La phases (A–L in Fig. 1) together with the XPS results
showing a La 3d5/2 binding energy and a La/Al intensity
ratio (Table 5) similar to the value corresponding to bulk
composition suggest that lanthanum is highly dispersed
over alumina support. The characterization of the ceria–
alumina support by XRD and XPS showed the coexistence
of large CeO2 particles (9 nm) and highly dispersed ceria
entities in close contact with the Al2O3 surface. For
magnesia–alumina support, both the absence of XRD dif-
fraction lines corresponding to Mg phases together with the
high Mg/Al ratio point out to a high dispersion of Mg ions
over the surface of alumina. From XPS analysis was not
possible to identify any specific magnesium phase because
the BEs values of Mg 2p in MgAl2O4 and MgO are indis-
tinguishable. Nevertheless magnesium surface spinels
might well be the dominant magnesium phase on the sur-
face taking into account that, at low Mg/Al ratios, the
reaction between Mg and Al2O3 is very favourable to form
surface spinels [20]. For zirconia–alumina support, the
measured XPS Zr/Al ratio indicates that ZrO2 appears well
dispersed on Al2O3 surface. The good dispersion of zirconia
on the alumina surface was also corroborated by the
absence, in spite of the relatively high ZrO2 loading, of
XRD peaks and by the absence of loss in specific surface
area indicative of pore plugging of alumina by ZrO2
crystallites.
4.2 Structure of Nickel Catalysts
The modifications of alumina support due to the addition of
Zr, Ce, La and Mg directly affect the structure and mor-
phology of Ni particles in both oxidized and reduced forms.
Although the Scherrer’ equation should not be used for
absolute determination of particle size distribution, the
sizes it provides allows to compare average crystalline
sizes. XRD profiles of calcined catalysts (Fig. 2a) showed
NiO crystallites with size increasing following the trend:
Ni/A & Ni/A–Z & Ni/A–C [ Ni/A–M [ Ni/A–L. This
result agrees with several studies in literature that indicates
the ability of lanthanum [21, 22] and Mg [23] to disperse
nickel crystallites. TPR data of calcined Ni catalysts pro-
vided additional information about Ni oxidized entities not
detected by XRD. According to literature [24, 25] a num-
ber of factors can influence on the reduction of NiO
850 860 870
Co
un
ts p
er s
eco
nd
(a.
u.)
(c)
(b)
Binding energy (eV)
(a)
x 0.25
Ni 2p3/2
Fig. 8 Ni 2p3/2 core-level spectra of Ni/A–Z catalyst after: (a)
reduction, (b) glycerol reforming reaction in gas phase and (c)
aqueous phase glycerol reforming
Table 9 Binding energy (eV) of Ni 2p3/2 core electrons and surface
atomic ratios of used Ni/A–Z catalyst
Ni2p3/2 (eV) Ni/Al C/Al
After aqueous phase reforming 856.5 (100%) 0.093 0.429
After steam reforming 851.9 (29%) 0.017 1.513
856.5 (71%)
54 Top Catal (2008) 49:46–58
123
including particle size and/or the level of interactions of
these oxides with the support. Stronger interactions of
nickel oxide entities with supports provoke higher diffi-
culty in their reducibility. The origin of the interaction
between NiO particles and support remains a matter of
controversy, and not unambiguous theoretical explanation
of this phenomenon has yet been offered. Two different
explanations about the origin of this interaction appear in
the studies published in literature: (i), nickel ocupying
different sites in the alumina, octahedral or tetrahedral, this
latter more difficult to reduce [26, 27]; or (ii), the incor-
poration of Al3+ [13], Ce3+ [28] or La3+ [29] ions on the
surface of NiO particles during impregnation. The latter
possibility is in accordance with XPS characterization of
calcined catalysts (Table 6) since the absence of Ni 2p3/2
signal corresponding to NiO species together with the low
Ni/Al surface XPS ratios seem to indicate that in all cata-
lysts the surface of NiO particles is covered by dispersed
support species. XPS characterization of Ni/A–L and Ni/
A–C catalysts showing low Ni/Al ratios and the similarity
in La/Ce surface exposure before and after nickel deposi-
tion supports the notion that a part of the NiO particles
could be covered by surface nickel bound to aluminum and
lanthanum/cerium ions. The catalyst containing Zr shows
intense reduction peaks in the high temperature range
(Table 4). This fact may be indicative, as indicated previ-
ously for Ce- and La-added catalysts, of some surface
interaction between Ni Al and Zr ions that retarded the
reduction of nickel entities. Nevertheless XPS analysis of
Ni/A–Z does not support the above assumption. On the
contrary, the decreasing of Zr/Al XPS ratio after Ni
deposition respect to the values observed on the bare
support (Table 6) was indicative of the partial coverage
and the close contact of Zr with the deposited nickel oxide
entities. In the case of Ni/A–M catalyst, the presence of Mg
in the support affects to the proportion of Ni strongly
interacting with alumina as it is indicated by the lower
intensity of reduction peak at 1069 K respect to that cor-
responding to bare alumina (Table 4). One possible reason
for this reduction behavior could be related with the pro-
pensity of Mg to stabilize on Al2O3 lattice, as it was
previously suggested from the characterization data of A–M
support, that reduces the strong Ni–Al interaction
inhibiting the incorporation of Ni into the alumina lattice
[30].
The above changes in reducibility and interaction of
NiO particles with supports imply variations in exposition
of metallic Ni after reduction. XRD data in Table 3 showed
differences in particle size of Ni crystallites after reduction.
Crystalline sizes of Ni0 phase decreased following the
sequence: Ni/A–Z [ Ni/A = Ni/A–C = Ni/A–M [ Ni/
A–L. The differences on crystalline sizes were in good
agreement with Ni/Al ratios from XPS measurements
(Table 5). However, taking into account the differences in
nominal Ni/Al compositions, the sequence of surface Ni/Al
concentration was the reverse to that previously determined
from XRD: Ni/A = Ni/A–M [ Ni/A–Z. For Ni/A–C and
Ni/A–L catalysts the XPS surface Ni concentrations did not
also fit with XRD sequence, showing a lower Ni/Al ratio
than that expected from their Ni crystalline size. This lower
Ni surface concentration may be related, as it was previ-
ously shown, to the fact that calcined Ni/A–C and Ni/A–L
catalysts presented NiO phases partially covered by entities
with aluminum, cerium and lanthanum ions that after
reduction make difficult or even impossible the access to
metallic function.
4.3 Catalytic Structure and Activity Correlations
4.3.1 Aqueous Phase Reforming (APR)
The activity of the catalysts studied in this work clearly
indicated the different catalyst functionality necessary to
carry out aqueous-phase and vapor-phase steam reforming
reactions of glycerol. According to the mechanism
described for aqueous reforming of oxygenated hydrocar-
bons [31], the organic molecule adsorbs on metallic phases
that activate the desired C–C cleavage to form adsorbed
CO and H2 or the undesired C–O cleavage to produce
propyleneglycol, ethylenglycol, ethanol,…. It is apparent
from Fig. 6, despite the strong instability of the catalytic
systems under reaction, that with the exception of Mg, the
addition of La-, Ce- and Zr-oxides to Al2O3 improves the
initial glycerol conversions with respect the ones obtained
on bare Al2O3 supported catalyst. Results in literature [32]
report the important effect of the support on activity of
platinum based catalysts for hydrogen production by APR
of oxygenated hydrocarbons. In these studies evidence was
found that supports promoting dehydration reactions lead
to lower hydrogen productions. In our case, the composi-
tion of the liquid products collected in Table 8 does not
show significant differences on the selectivity toward the
formation of dehydration products with the type of the
support, indicating therefore that other parameters than
dehydration capacity of the support affect to catalytic
behavior of nickel based catalysts. Effects of the support on
nickel dispersion may be discarded because metallic Ni
surface concentrations from XPS and XRD analyses of the
different catalysts do not correlate with the initial glycerol
conversion values derived from Fig. 5. Therefore, the
observed differences in catalytic activities may be related
with geometric effects caused by the presence of promoters
on Ni surfaces as proposed by Shabaker et al. [33], who
found a promotional effect of Sn in the production of
hydrogen by APR of oxygenated compounds over Ni/
Al2O3 catalyst that they assign to the selective poison by
Top Catal (2008) 49:46–58 55
123
Sn of Ni defect sites responsible for methanation of CO or
CO2. Our characterization studies of Ni/A–C and Ni/A–L
catalysts suggest that cerium and lanthanum atoms partic-
ipate in nickel metal particles coverage after reduction.
Therefore in a similar way than that proposed by Shabaker
et al [33], the improvement in activity observed for the Ni/
A–C and Ni/A–L catalyst might be related to geometric
effects caused by the decoration of metallic Ni phases by
Ce and La atoms. In the case of Ni/A–Z and Ni/A–M
catalysts, however, no surface modification of Ni particles
by Zr or Mg atoms was observed. As characterization data
indicate, the main difference of Ni/A–M and Ni/A–Z cat-
alysts respect to Ni/A one are related to the propensity of
Mg to stabilize on Al2O3 that reduces the proportion of
nickel strongly interacting with aluminum and the close
interaction between Ni and ZrO2, respectively. Therefore,
the differences in APR activity between Ni/A and Ni/A–M
and Ni/A–Z samples strongly suggest that, apart of metallic
nickel, the interaction between Ni and ZrO2 and the pro-
portion of nickel strongly interacting with aluminum take
part in the APR reaction. However, the question as to how
these interactions enhance the activity was not evidenced
from the data presented here requiring therefore a further
investigation.
In spite that nickel catalysts showed differences on their
initial APR activities depending on the support used, the
stability under reaction conditions were very low for all the
catalysts prepared. Severe deactivation phenomena of Ni
catalysts supported on Al2O3, ZrO2 and SiO2 were previ-
ously reported in high-pressure aqueous environments [33].
According to these studies, the sintering of Ni particles has
been suggested as the cause of the deactivation observed
for Ni catalysts after exposure to reaction conditions.
Contrary to that, the increase of Ni/Al XPS signal after
reaction observed in our case over the representative Ni/
A–Z catalyst (Table 9) was not indicative of some sintering
phenomena. The changes in the Ni 2p XPS spectra col-
lected in Fig. 8 indicates that the main reason for the
observed deactivation phenomena on supported nickel
catalyst after APR reaction was ascribed to the gradually
change of nickel under reaction conditions from metallic to
oxidized state with formation of surface nickel aluminate
spinels. This result is in accordance with results in litera-
ture [34] that reported similar formation of nickel
aluminate spinels as the main cause of deactivation of
supported nickel catalyst subjected to high temperatures in
presence of steam.
4.3.2 Steam Reforming
For glycerol steam reforming in vapor phase, the hydrogen
selectivity over the different catalysts was higher when
nickel was supported on modified alumina compared to the
reference catalyst supported on bare alumina (Fig. 6). This
improvement in performance of Ni catalysts supported on
modified alumina was shown as higher H2 and CO2 yields
and lower CO yield. For steam reforming of hydrocarbons
over nickel catalysts a bifunctional mechanism [35, 36] has
been proposed in which Ni and support participates in the
activation of hydrocarbon and water molecules. Nickel
metal activates the organic molecule, by means of O–H,
–CH2–, C–C and CH2 bond breaking [37] and promotes the
reaction between the organic fragments with the OH
groups from the water reagent. Bearing this fact in mind,
the increase in activity observed for catalysts supported on
modified alumina may be caused by an increase in the
exposed metal surface area and/or in the capacity to acti-
vate steam. As stated above, XPS and XRD analyses of the
different Ni catalysts show changes in the metallic nickel
surface concentrations with the type of support used to
disperse Ni (Tables 3 and 7). Thus the increase in the
hydrogen production observed for the sample Ni/A–M may
be related to the better surface nickel concentration
achieved on this sample catalyst respect to that corre-
sponding to the Ni/A reference. However, the absence of
correlation between activity and surface nickel concentra-
tion in the steam reforming of glycerol for the Ni/A–C, Ni/
A–L and Ni/A–Z samples suggests that additional proper-
ties to the metallic phase exposition play a role in driving
the catalytic activity over these samples. In spite of the low
Ni dispersion and taking into account that ZrO2 by itself is
not active for steam reforming [38], the high activity
observed for the Ni/A–Z catalyst suggests that ZrO2,
modifying the Al2O3 and interacting with Ni active phase,
is able to enhance the H2O activation. A similar result was
reported by Takanabe et al. for steam reforming of acetic
acid over ZrO2-based supported Pt catalysts [38]. These
authors suggest that the reaction occurs at the periphery of
the Pt particles in the proximity of ZrO2. As our results
suggest that a part of the Ni atoms in Ni/A–Z catalyst have
an enhanced activity, we propose in line with the above
explanation, that steam reforming of glycerol occurs
probably at those Ni sites which are in close proximity of
the ZrO2 entities. In the case of Ce- and La-containing
catalysts, there were not clear evidences of the ability of
ceria and lanthana to promote the activation of water. In
literature the positive effect of lanthanum and cerium in the
activity of nickel catalysts applied to steam reforming of
hydrocarbons is generally ascribed to the good contact
between lanthana/ceria and Ni phases that inhibits the
effects, coking and/or sintering, that reduce the loss of
metal surface area under reaction [39, 40]. In our case,
characterization of Ni/A–C and Ni/A–L catalysts showed
the presence of ceria and lantana entities on top of metallic
nickel crystallites that may explain, as suggested previ-
ously, their better hydrogen production respect to the Ni/A
56 Top Catal (2008) 49:46–58
123
reference by an enhancement in the stability of the cata-
lysts that means an increase of the amount of active sites
available during the reforming reaction.
5 Conclusions
Aqueous phase reforming and steam reforming of glycerol
were investigated over nickel catalysts supported on
MxOy–Al2O3 (M=Ce, La, Mg and Zr). Characterization of
Ni catalysts showed that the modifications of alumina
support induced by the addition of Zr, Ce, La and Mg
directly affect the structure and morphology of Ni particles
in both oxidized and reduced forms. The presence of Mg
modifies the interaction degree of Ni with alumina by
intercalation of the promoter between nickel and alumina
that inhibited the incorporation of nickel to Al2O3 phase.
The addition of Zr to the Al2O3 support implies a decrease
in the dispersion of Ni phases respect to that achieved on
bare Al2O3 and the development of strong Ni–ZrO2 inter-
actions. The use of Ce- and La-added Al2O3 supports leads
to nickel phases partially covered by ceria and lanthana
entities. The activity of the catalysts studied in this work
clearly indicated the different catalyst functionality nec-
essary to carry out aqueous-phase and vapor-phase steam
reforming reaction of glycerol. For aqueous phase
reforming of glycerol (APR), the addition of Ce, La and Zr
to Al2O3 improves the initial glycerol conversions obtained
over the bare Al2O3 supported catalyst. For Ce- and La-
promoted catalysts, the improvement in catalytic perfor-
mance is suggested to be related with geometric effects
caused by the presence of cerium and lanthanum on Ni
surfaces. In this respect, the decoration of Ni phases by Ce
and La may be in the origin of the improvement in activity
observed for these samples. The differences in APR
activity between Ni/A and Ni/A–Z and Ni/A–M catalysts
strongly suggest that, apart of metallic nickel, the interac-
tion between Ni and ZrO2 and the proportion of nickel
strongly interacting with alumina take part in the APR
reaction. In spite that nickel catalysts showed high APR
activities at initial times of reaction they deactivate quickly
under reaction conditions. Characterization of catalysts
after APR indicates that the main reason for the observed
deactivation phenomena is ascribed to the gradually change
of nickel under reaction conditions from metallic to oxi-
dized state with formation of surface NiAl2O4. For glycerol
steam reforming in vapour phase, the use of Mg, Zr, Ce and
La as promoter of Ni based catalysts increases the hydro-
gen selectivity compared to the reference catalyst
supported on bare alumina. Differences in activity were
explained in terms of surface nickel concentration (Mg),
capacity to activate steam (Zr) and stability of nickel
phases under reaction conditions (Ce and La).
Acknowledgments The authors thank the financial support to
Ministerio de Educacion y Ciencia of Spain (Projects MAT2003-
08348-C04-01 and ENE2007-6753-C02-01) and the University of
the Basque Country. R.M.N also acknowledges the Ministerio de
Educacion y Ciencia for a Ramon y Cajal research program.
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