High activity of Ce1-xNixO2-y for H 2 production through ethanol steam reforming: Tuning catalytic performance through metal-oxide interactions

Article abstract of DOI:10.1002/anie.201004966

The importance of the oxide: Ce_0.8Ni_0.2O _2-y is an excellent catalyst for ethanol steam reforming (see graph). Metal-oxide interactions perturb the electronic properties of the small particles of metallic nickel present in

Full text of DOI:10.1002/anie.201004966

Communications
DOI: 10.1002/anie.201004966
Heterogeneous Catalysis
High Activity of Ce1ꢀxNixO2ꢀy for H2 Production through Ethanol
Steam Reforming: Tuning Catalytic Performance through Metal–Oxide
Interactions**
Gong Zhou, Laura Barrio, Stefano Agnoli, Sanjaya D. Senanayake, Jaime Evans,
Anna Kubacka, Michael Estrella, Jonathan C. Hanson, Arturo Martꢀnez-Arias,
Marcos Fernꢁndez-Garcꢀa, and Josꢂ A. Rodriguez*
Hydrogen production by the steam reforming of ethanol
(C2H5OH + 3H2O!6H2 + 2CO2) is receiving significant
attention because ethanol can be obtained readily from
renewable-energy sources and is environmentally friendly.[1–3]
There is also a strong interest in the use of ethanol as a fuel for
mobile-fuel-cell applications.[4,5] The ethanol-reforming pro-
cess occurs by a complex mechanism involving multiple
reactions.[2,6] Noble-metal supported catalysts, in particular
Rh, have been shown to have significant activity;[3,6–10]
however, the high cost of these materials limits their
Recently, the structural and electronic properties of
Ce1ꢀxNixO2ꢀy nanosystems prepared by a reverse-microemul-
sion method were characterized by synchrotron-based X-ray
diffraction (XRD), X-ray absorption fine structure (XAFS),
Raman spectroscopy, and density-functional calculations.[19]
XRD showed that the solubility limit of a Ce–Ni exchange in
ceria is in the range of 10–12%. In a Ce0.9Ni0.1O2ꢀy system,
most of the Ni forms a solid solution, whereas in a sample with
higher Ni content (20%), there is also a segregated phase of
NiO present.[19] Results of in situ time-resolved XRD indicate
that strong Ce–O–Ni interactions in the mixed-metal oxide
delay the reduction of the nickel cations by hydrogen up to
temperatures above 4008C. Hydrogen reduction of the
Ce1ꢀxNixO2ꢀy mixed-metal oxides involves the reduction of
CeIV species to CeIII, as well as Ni0 formation.[19]
applications. As
a less expensive alternative, highly
active[3,11–15] nickel-based catalysts have been studied for the
production of H2 through ethanol steam reforming.[16–18]
These catalysts readily undergo deactivation as a result of
coke deposition.[17,18] Herein, we describe
a successful
approach to the production of highly efficient, stable, and
inexpensive nickel-based catalysts for ethanol steam reform-
ing. Within the Ce1ꢀxNixO2ꢀy system, metal–oxide interactions
alter the chemical properties of nickel and ceria to produce
During the steam reforming of ethanol on a Ce0.8Ni0.2O2ꢀy
catalyst, substantial evolution of H2 and CO2 was observed at
temperatures above 3008C (Figure 1). The variations in the
H2 and CO2 signals track each other, as expected for a
catalytic process. Ni is a well-known methanation catalyst.[20]
In the experiments with Ce0.8Ni0.2O2ꢀy, the amount of methane
formed was either very small (300–4008C) or negligible (400–
ꢀ
surface sites which are very active for the cleavage of C C
ꢀ
and C H bonds and for the subsequent formation of CO2 and
H2.
[*] Dr. G. Zhou, Dr. L. Barrio, Dr. S. Agnoli, Dr. S. D. Senanayake,
M. Estrella, Dr. J. C. Hanson, Dr. J. A. Rodriguez
Chemistry Department, Brookhaven National Laboratory
Upton, NY 11973 (USA)
Fax: (+1)631-344-5815
E-mail: rodrigez@bnl.gov
Prof. J. Evans
Facultad de Ciencias, Universidad Central de Venezuela
Caracas 1020 A (Venezuela)
Dr. A. Kubacka, Dr. A. Martꢀnez-Arias, Dr. M. Fernꢁndez-Garcꢀa
Instituto de Catꢁlisis y Petroleoquꢀmica, CSIC
Campus Cantoblanco, 28049 Madrid (Spain)
[**] The research at BNL was financed by the US Department of Energy
(DOE), Chemical Sciences Division, Office of Basic Energy Science
(DE-AC02-98CH10086). The National Synchrotron Light Source is
supported by the Divisions of Materials and Chemical Sciences of
the US DOE. L.B. acknowledges funding by the FP7 People program
under the project Marie Curie IOF-219674. J.E. thanks INTEVEP and
IDB for research grants that made possible part of this research at
the Universidad Central de Venezuela. Research at the ICP-CSIC was
financed by the Comunidad de Madrid (DIVERCEL S2009/ENE-
1745) and the MICINN (CTQ2006-15600/BQU and CTQ2009-
14527/BQU), to whom we are grateful.
Figure 1. Production of H2, CO2, and CH4 during the steam reforming
of ethanol over a Ce0.8Ni0.2O2ꢀy catalyst. The sample was held under
isothermal conditions at 250, 300, 350, 400, 450, and 5008C for
periods of 1 h. The amount of H2, CO2, and CH4 was measured with a
mass spectrometer located at the exit of the microreactor.[19] The
measured values are reported without correction for the relative
sensitivities of H2, CO2, and CH4 in the mass spectrometer.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201004966.
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Chemie
5008C). This behavior is very different from that observed for
the steam reforming of ethanol on other NiO/CeO2 cata-
lysts,[16–18] with which a significant amount of methane was
formed, with eventual deactivation owing to coke deposition.
We tested the catalytic activity of Ce0.8Ni0.2O2ꢀy for long
periods (10–40 h) at 400–5008C and always observed the
stable production of H2 and CO2 (see Figure S1 in the
Supporting Information). Metal–oxide interactions give the
nickel present in Ce0.8Ni0.2O2ꢀy special chemical properties.
We examined the production of hydrogen by the steam
reforming of ethanol in a temperature range between 250 and
5008C over CeO2, Rh/CeO2, Ce1ꢀxNixO2ꢀy, and Ni/CeO2
catalysts (Figure 2). The pure ceria support was not active
for the ethanol-steam-reforming reaction and produced a
negligible amount of hydrogen even at 5008C. With both
Ce0.9Ni0.1O2ꢀy and Ce0.8Ni0.2O2ꢀy, a small amount of H2 was
produced as soon as the temperature was raised to 3008C. At
temperatures below 4008C, the reforming activities of both
mixed oxides were nearly identical. However, for
Ce0.8Ni0.2O2ꢀy, substantial catalytic activity was observed at
4008C; this activity was very stable over time (see also
Figure S1 in the Supporting Information). Hydrogen produc-
tion with Ce0.8Ni0.2O2ꢀy was almost twice that observed for
Ce0.9Ni0.1O2ꢀy. Between 400 and 5008C, the reforming activ-
ities of both mixed oxides were relatively stable and only
increased slightly. The estimated ethanol consumption of
Ni0.2Ce0.8O2ꢀy at 4008C was 100%, and the selectivity for H2
formation was 67%. The production of hydrogen over a
typical Rh/CeO2 catalyst is described in Figure 2a. At low
temperatures (< 3008C), the activity of Rh/CeO2 was higher
than that of the mixed-metal oxides, but at high temperatures
(> 4008C), the performance of the Ce1ꢀxNixO2ꢀy systems was
far superior. Clearly, Ce0.8Ni0.2O2ꢀy is a nonexpensive highly
efficient catalyst for ethanol steam reforming.
The data in Figure 2b for the nickel-impregnated samples
Ni10/CeO2 and Ni20/CeO2 (10 and 20 mol% Ni deposited on
the ceria support) show that hydrogen production started at a
temperature of 3508C. Ni10/CeO2 showed the highest activity
at 4508C; its hydrogen production decreased slightly as the
temperature was raised to 5008C. On the other hand,
Ni20/CeO2 became catalytically unstable at temperatures
above 4008C, with rapid deactivation. When the temperature
was raised to 450 and 5008C, Ni20/CeO2 regained some
activity, but its hydrogen production was unstable. The
deactivation of Ni20/CeO2 is probably due to metal-particle
sintering and coke deposition.[16–18] The two best catalysts
studied are Ce0.8Ni0.2O2ꢀy and Ni10/CeO2, whereby the mixed-
metal oxide displays the highest activity for H2 production.
The catalysts described in Figure 2 were characterized by
XAFS spectroscopy and XRD (see Figure S2 and S3 in the
Supporting Information). An analysis of the Ni K-edge XAFS
region showed that nickel in the as-prepared catalysts was in a
+ 2 oxidation state. In their XRD data, all samples showed a
major contribution from a CeO2 fluorite-type structure.[21]
The lattice parameters of the fresh catalysts were nearly
identical at 5.417 ꢀ, and very close to the reported value for
bulk ceria of 5.411 ꢀ.[21] The Ce0.9Ni0.1O2ꢀy sample appeared
to be a single-phase solid solution and did not exhibit any
clear peaks associated with the NiO phase.[19] The diffraction
patterns of Ce0.8Ni0.2O2ꢀy (Figure 3), Ni10/CeO2, and Ni20/CeO2
samples exhibited weak NiO peaks that indicated the
presence of an NiO phase in these systems. The XRD
Rietveld refinements gave NiO mole fractions for
Ce0.8Ni0.2O2ꢀy, Ni10/CeO2, and Ni20/CeO2 of 0.11, 0.1, and
0.18, respectively. The XRD results suggest that for
Ce0.8Ni0.2O2ꢀy, only about half of the Ni atoms present in the
sample form a periodic structure of NiO. The rest of the Ni
atoms form a solid solution in the ceria lattice.[19]
Figure 3 shows a series of XRD patterns for Ce0.8Ni0.2O2ꢀy
collected during the steam reforming of ethanol. The
diffraction signal is governed by ceria contributions, with
weak features for NiO. Rietveld refinement revealed that
under the reaction conditions, NiO survives up to about
4008C, at which temperature a NiO!Ni transformation takes
place. The appearance of the metallic Ni phase correlated
with a substantial increase in the production of H2 (Figure 1).
Thus, the best catalyst in Figure 2 contains a small amount of
Ni (particles less than 3 nm in diameter) dispersed on a
nickel-doped ceria support. In the case of Ce0.9Ni0.1O2ꢀy, which
Figure 2. a) Hydrogen-production plots for ethanol steam reforming
over Ni0.1Ce0.9O2ꢀy, Ni0.2Ce0.8O2ꢀy, and 1 wt% Rh/CeO2. b) Hydrogen-
production plots for ethanol steam reforming over Ni10/CeO2, Ni20/
CeO2, and CeO2. The samples were held under isothermal conditions
at 250, 300, 350, 400, 450, and 5008C for periods of 1 h.
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Figure 4. Sequential Rietveld refinement analysis of the ceria lattice
parameter for Ce0.8Ni0.2O2ꢀy and Ni10/CeO2 catalysts under ethanol
steam reforming at different temperatures.
The data for Ni10/CeO2 and Ni20/CeO2 in Figure 2 indicate
that the Ni coverage has a drastic effect on the catalytic
activity. Figure 5 shows valence photoemission spectra
recorded after Ni had been deposited at submonolayer
coverage on a CeO2(111) film grown on Ru(0001) according
to a previously described procedure.[26,27] Clean CeO2(111) is
characterized by a large band gap, with the O 2p states
appearing at binding energies between 3 and 8 eV.[27] The
addition of small amounts of nickel (< 0.4 monolayer)
produces new features at binding energies between 0 and
2 eV, in the region in which the Ni 3d states are expected.[28,29]
A detailed comparison of the valence spectra for Ni/CeO2 and
bulk Ni[28,29] shows that metal–oxide interactions significantly
Figure 3. Time-resolved X-ray diffraction patterns for Ce0.8Ni0.2O2ꢀy
collected during the ethanol-steam-reforming reaction. The graph at
the top is an expanded view of the bottom graph in the 2q range
6.5–9.58; the dotted lines show the positions of the NiO and Ni
peaks.
was not as active as Ce0.8Ni0.2O2ꢀy, no significant amounts of
Ni were detected by XRD under the reaction conditions (see
Figure S4 in the Supporting Information).
A comparison of the XRD data for Ce0.8Ni0.2O2ꢀy and
Ni10/CeO2 illustrates the important role played by the oxide
support in the catalytic process. Initially, the mole fractions of
NiO in these systems were very close (ca. 0.1), and high
catalytic activity was observed following an NiO!Ni trans-
formation at temperatures above 4008C. Thus, differences in
the catalytic activity of these systems (Figure 2) reflect
variations in the properties of the oxide component that
may directly affect the reaction process or metal–oxide
interactions. Rietveld refinement of the in situ XRD patterns
showed that for both catalysts there was a large expansion of
the oxide lattice cell at temperatures between 200 and 2508C
(Figure 4). Ni10/CeO2 exhibited a lattice expansion very close
to that found in similar experiments with pure CeO2. Upon
thermal treatment, an expansion of the lattice is expected;
however, sudden changes in the slope can be related to
chemical processes and particularly to the creation of O
vacancies and CeIII sites, which have larger atomic radii than
CeIV sites.[22,23] It is known that ceria surfaces undergo
significant reduction upon reaction with ethanol.[24,25] The
cell expansion in Ce0.8Ni0.2O2ꢀy is 1.5 times larger than in
Ni10/CeO2, which indicates that the number of O vacancies is
substantially larger in the mixed-metal oxide. The doping of
ceria with Ni induces strain in the oxide lattice and favors the
formation of O vacancies.[19] A large concentration of O
vacancies, and related defects, should enhance the dispersion
Figure 5. Valence photoemission spectra (He II radiation; 40.8 eV) for
the deposition of small amounts of Ni (<0.4 monolayer) on a
CeO2(111) surface.
of reduced Ni on the oxide surface[26] and facilitate the
[22–25]
ꢀ
cleavage of the O H bonds in water and ethanol.
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decrease the density of occupied Ni 3d states near the Fermi
level.
ethanol and water. These studies show the importance of both
the metal and the oxide phase in catalysts for ethanol steam
reforming. Both phases must be taken into consideration
when trying to improve catalyst performance.
The smaller the Ni coverage, the larger the electronic
perturbations of Ni: at the smallest Ni coverage in Figure 5,
Ni 2p X-ray photoelectron spectra (see Figure S5 in the
Supporting Information) showed the dominant presence of
Ni2+ on the oxide surface. An electronically perturbed Ni0 Experimental Section
The Ni0.1Ce0.9O2ꢀy and Ni0.2Ce0.8O2ꢀy catalysts were prepared by the
species becomes the dominant species for the other nickel
coverages. The electronic perturbations of nickel modify its
chemical properties and thus decrease its activity for the
generation of methane. Figure 6 shows the CO-methanation
use of reverse microemulsions.[19] Ni K-edge XAFS spectra were
collected in air at room temperature on beamline X18A of the
National Synchrotron Light Source (NSLS) at Brookhaven National
Laboratory.[19] In situ time-resolved XRD experiments were carried
out on beamline X7B of the NSLS (l = 0.3184 ꢀ). The sample
(ca. 5 mg) was loaded into a glass capillary cell, which was attached to
a flow system.[19,22] A small resistance heater was wrapped around the
capillary, and the temperature was monitored with
a 1.0 mm
Chromel–Alumel thermocouple, which was placed directly in the
capillary near the sample. Two-dimensional powder patterns were
collected with an Mar345 image-plate detector, and the powder rings
were integrated by using the FIT2D code.[31] The short wavelength
enabled powder-profile refinement of data to q = 10 ꢀ-1 Lattice
constants were determined by Rietveld analysis with the General
Structure Analysis System (GSAS) program.[32,33] Diffraction patterns
were collected over the catalysts during ethanol steam reforming. The
reaction was carried out isothermally at several temperatures (250,
300, 350, 400, 450, and 5008C) with He as the carrier gas. The He flow
rate to the reactor was maintained at 10 mLminꢀ1. Avapor mixture of
water and ethanol (6:1) was injected into the gas stream by using a
syringe pump at a vapor flow rate of 0.532 mLminꢀ1. The reaction
products were identified with a quadrupole mass spectrometer.
The experiments with Ni/CeO2(111) surfaces were undertaken in
two different ultrahigh-vacuum (UHV) chambers.[34,35] One of the
chambers was used to collect the XPS (MgKa) and valence photo-
emission spectra (He II) data. To avoid problems with charging in the
photoemission experiments, films of CeO2(111) were grown in situ on
Ru(0001).[26,27] Studies of CO methanation and ethanol steam
reforming on Ni/CeO2(111) were conducted in a second UHV
chamber with a batch reactor attached.[34,35] The kinetic tests were
carried out by using a CeO2(111) single crystal cleaned by standard
procedures.[35] After preparation and characterization of the
Ni/CeO2(111) surfaces by vapor-deposition of Ni on the oxide
substrate, the sample was transferred to the reactor. The reported
rates are for steady-state conditions. The amount of hydrogen
produced was normalized according to the exposed active area of
the sample face that contained nickel.[35]
Figure 6. CO-methanation activity of model Ni/CeO2(111) catalysts as
a function of admetal coverage. Each Ni/CeO2(111) surface was
exposed to a mixture of CO (24 torr) and H2 (96 torr) at approximately
3508C. The reported values correspond to the number of CH4
molecules produced per square centimeter of the catalyst surface
during a reaction time of 5 min under steady-state conditions.
activity of Ni/CeO2(111) surfaces as a function of nickel
coverage. This reaction is a major source of CH4 in the
ethanol-steam-reforming process.[6] At large coverages of Ni,
the Ni/CeO2(111) systems exhibit comparable methanation
activity to that of bulk-nickel surfaces.[20,30] In contrast, at low
coverages (< 0.4 monolayer) of Ni, the amount of methane
produced is very small or negligible. These Ni/CeO2(111)
systems with a low nickel content were transformed into
Ni/CeO2ꢀx(111) during the reaction. They displayed a high
activity/selectivity for ethanol steam reforming and did not
undergo deactivation with time. In this respect, their behavior
is similar to that found for the powder Ni10/CeO2 catalyst,
which did not show signs of deactivation after a reaction time
of 40 h.
Received: August 9, 2010
Published online: November 9, 2010
Keywords: ceria · ethanol · hydrogen production · nickel ·
.
steam reforming
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