Utilization of high specific surface area CuO-CeO2 catalysts for high temperature processes of hydrogen production: steam re-forming of ethanol and methane dry re-forming

Article abstract of DOI:10.1021/jp907345u

CuO-CeO₂ mixed oxide catalysts with 10, 15, and 20 mol % CuO content were prepared by the hard template method using KIT-6 silica as a template. The applied synthesis method yields solids with BET surface area in excess of 147 m²/g, highly porous nanocrystalline CeO₂ morphology and dispersion of CuO phase between 28 and 40%, corresponding to CuO particle size between 1.3 and 1.9 nm. Increasing the CuO content caused a decrease in dispersion of this phase and a further decrease of surface acid site abundance, determined by NH₃ chemisorption/TPD method, but improved the reducibility extent of CeO₂ (14.5, 16.1 and 24.5% for CuCe10, CuCe15, and CuCe20 catalyst, respectively) and oxygen mobility of prepared powders. It was discovered during ethanol steam re-forming experiments that increasing CuO content is favorable in terms of ethanol conversion but also causes quicker catalyst deactivation, primarily as a result of sintering and loss of CuO dispersion. Reaction temperatures in excess of 550 °C strongly promoted ethanol dehydratation reaction, leading to a rise in methane production and extensive coking of the catalyst surface. Coking was slower in the case of CuO-CeO₂ catalysts with a higher CuO content as a result of lower acid site abundance and more pronounced oxygen mobility. Temperatures in excess of 450 °C are required for any noticeable CO₂ and CH₄ conversion in methane dry re-forming reaction over CuO-CeO₂ materials. The examined materials displayed steady performance during stability tests at a reaction temperature of 650 °C, with catalysts containing 15 and 20 mol % CuO exhibiting the highest activity. Additionally, very low amounts of carbon were deposited on spent catalyst samples.

Full text of DOI:10.1021/jp907345u

J. Phys. Chem. A 2010, 114, 3939–3949
3939
Utilization of High Specific Surface Area CuO-CeO2 Catalysts for High Temperature
Processes of Hydrogen Production: Steam Re-forming of Ethanol and Methane Dry
Re-forming†
‡
‡
‡
,‡,§
´
Petar Djinovic´, Jurka Batista, Benis Cehic´, and Albin Pintar*
Laboratory for Catalysis and Chemical Reaction Engineering, National Institute of Chemistry, P.O. Box 660,
SI-1001 Ljubljana, SloVenia, and Chair of Chemical, Biochemical and EnVironmental Engineering, Faculty of
Chemistry and Chemical Technology, UniVersity of Ljubljana, P.O. Box 537, SI-1001 Ljubljana, SloVenia
ReceiVed: July 31, 2009; ReVised Manuscript ReceiVed: October 08, 2009
CuO-CeO2 mixed oxide catalysts with 10, 15, and 20 mol % CuO content were prepared by the hard template
method using KIT-6 silica as a template. The applied synthesis method yields solids with BET surface area
in excess of 147 m2/g, highly porous nanocrystalline CeO2 morphology and dispersion of CuO phase between
28 and 40%, corresponding to CuO particle size between 1.3 and 1.9 nm. Increasing the CuO content caused
a decrease in dispersion of this phase and a further decrease of surface acid site abundance, determined by
NH3 chemisorption/TPD method, but improved the reducibility extent of CeO2 (14.5, 16.1 and 24.5% for
CuCe10, CuCe15, and CuCe20 catalyst, respectively) and oxygen mobility of prepared powders. It was
discovered during ethanol steam re-forming experiments that increasing CuO content is favorable in terms of
ethanol conversion but also causes quicker catalyst deactivation, primarily as a result of sintering and loss of
CuO dispersion. Reaction temperatures in excess of 550 °C strongly promoted ethanol dehydratation reaction,
leading to a rise in methane production and extensive coking of the catalyst surface. Coking was slower in
the case of CuO-CeO2 catalysts with a higher CuO content as a result of lower acid site abundance and
more pronounced oxygen mobility. Temperatures in excess of 450 °C are required for any noticeable CO2
and CH4 conversion in methane dry re-forming reaction over CuO-CeO2 materials. The examined materials
displayed steady performance during stability tests at a reaction temperature of 650 °C, with catalysts containing
15 and 20 mol % CuO exhibiting the highest activity. Additionally, very low amounts of carbon were deposited
on spent catalyst samples.
∆H0298
174 kJ/mol (1)
)
1. Introduction
C2H5OH + 3H2O f 6H2 + 2CO2
Hydrogen is an important feedstock for the production of
methanol,1 ammonia,2 various hydrogenations of organic mol-
ecules and naphtha fractions,3 synthesis of value added fine
chemicals, and recently also an alternative fuel.4 As a result of
rising prices of fossil fuels and with the environmental regula-
tions becoming more stringent, there is a growing interest in
the use of hydrogen as an alternative fuel produced from
renewable resources such as biomass (e.g., sugar cane, cellulose,
corn, etc.).
Compared to methanol steam re-forming, reaction pathways of
ethanol steam re-forming are more complicated due to the
presence of a C-C bond, which requires higher re-forming
temperature, usually in the range from 400 to 700 °C.6 Ethanol
steam re-forming is an endothermic reaction and produces only
hydrogen and CO2 if ethanol reacts in a most desirable way.
However, byproduct such as CO, CH4, acetaldehyde, ethylene,
and ethyl acetate are also formed.5 Steam re-forming of ethanol
to produce hydrogen is favored at high temperatures, because
high H2 selectivity can be thermodynamically achieved at these
temperatures. On the other hand, C2 byproduct formation is
rather dominant at low temperatures.7 It should be pointed out
that high temperature re-forming, however, favors coproduction
of CO, resulting in a loss of potential H2 yield (due to
unfavorable water gas shift equilibrium).
Catalysts tested in this reaction are mainly Ni, Cu, Co, and
supported noble metals, such as Rh, Ru, Re, Pd, Ir, and Pt, which
have been reviewed by Haryanto et al.6 and Vaidya et al.8
Although supported noble metal catalysts have been demon-
strated to have significant activity in the 500-600 °C range
and at high space velocities,8,9 the high cost of these metals
limits their application. As a less expensive alternative, cobalt-
based catalysts have been reported to have superior ethanol
steam re-forming performance due to their high activity for C-C
bond cleavage at temperatures as low as 350-400 °C.5 In most
Ethanol is obtained by fermentation of biomass as an aqueous
solution, which can be used for steam re-forming without
extensive elimination of water. The use of downstream condi-
tioning processes, such as distillation is not necessary, which
reduces the overall cost of hydrogen produced from renewable
feedstocks. From an environmental point of view, the use of
ethanol is favorable, because its production is CO2 neutral
(renewable feedstock); further, it is substantially less toxic
compared to methanol.5 Moreover, a high yield of hydrogen
can be obtained if ethanol reacts, as depicted in reaction 1, via
the steam re-forming pathway:
† Part of the special issue “Green Chemistry in Energy Production
Symposium”.
* Corresponding author. Tel.: +386-1-47-60-237Fax: +386-1-47-60-300.
E-mail address: albin.pintar@ki.si.
‡ National Institute of Chemistry.
§ University of Ljubljana.
10.1021/jp907345u  2010 American Chemical Society
Published on Web 11/02/2009

3940 J. Phys. Chem. A, Vol. 114, No. 11, 2010
Djinovic´ et al.
of these studies, metal sintering as a result of high reaction
temperature and deposition of carbonaceous species such as
amorphous carbon, carbon filaments, and carbonate species have
been recognized as the main causes of catalyst deactivation.10
To retard deactivation by coking, higher than stoichiometric
water-to-ethanol ratios (up to 10) can be employed.11 Addition-
ally, catalyst supports with pronounced surface acidity, such as
γ-Al2O3 should be omitted, since acidic sites are the main reason
for accelerating ethanol dehydratation reaction, which leads to
ethylene production, that readily polymerizes to carbon and leads
to premature activity drop. Basic supports such as MgO exhibit
improved coking resistance as a result of surface hydrophilicity
and abundant hydroxyl species chemisorbed on the surface but
cause no improvement of catalyst activity.12 Redox catalyst
supports such as CeO2 and ZrO2 exhibit enhanced resistance to
coking as a result of high oxygen mobility in the support and
consequently provide abundance of active oxygen species that
are involved in scavenging and oxidizing formed surface carbon
at the CuO-CeO2 interface. Further, copper possesses high
activity in C-C bond cleavage.10
KIT-6 silica as a template. The KIT-6 template was prepared
by adjusting the molar ratio of SiO2/P123 close to 60 and the
HCl concentration close to 0.75 M. Four grams of Pluronic P123
(EO20PO70EO20 poly(alkylene oxide) based triblock copolymer,
MW ) 5800, from Aldrich) was added to 144 g of distilled
water and 7.5 g of concentrated HCl (37%, from Merck).
Afterward, 4 g of butanol (absolute, p.a., from Merck) was added
under stirring and left for 1 h at 35 °C. Finally, 8.6 g of TEOS
(Si(OC2H5)4, 99.0% purity, from Fluka) was added and stirred
for additional 24 h at the same temperature.
The obtained gel was transferred into autoclaves and aged at
100 °C for 24 h under static conditions. Aged slurry was filtered
and dried overnight at 100 °C. The dried product was first mixed
with 500 mL of ethanol (absolute, p.a., from Riedel-de Hae¨n)
and 30 mL of concentrated HCl and stirred at room temperature
for 1 h. It was then filtered and washed with 250 mL of distilled
water and 150 mL of ethanol. Finally, the product was dried at
60 °C overnight and calcined in air at 550 °C for 5 h to remove
the polymer template.18-20
Appropriate amounts of Cu(NO3)2 ·3H2O (99.5% purity,
Merck) and Ce(NO3)3 ·6H2O (99% purity, Aldrich) were dis-
solved in 25 mL of ethanol (absolute, p.a., Riedel-de Hae¨n).
The amounts of added metal salts were calculated to yield a
total metal ion concentration of 0.7 M. Into 15 mL of this
solution, 1 g of KIT-6 silica was added and stirred at room
temperature for 1 h. The impregnated template was dried
overnight at 60 °C. The obtained CuCe15 catalyst precursor
was heated in an oven at 400 °C for 3 h to completely
decompose nitrate species. CuCe10 and CuCe20 precursors were
calcined at 550 and 450 °C, respectively. The impregnation step
was repeated with 10 mL of the remaining ethanol-metal salt
mixture. After overnight drying at 60 °C, CuCe15 precursor
was again calcined at 400 °C (CuCe10 at 550 °C and CuCe20
at 450 °C) for 3 h. Optimal calcination temperatures for catalysts
with different CuO loadings, which were determined in our
previous work, were employed in this study. Utilization of these
temperatures produced catalysts with the highest WGS activity
as a result of optimal morphological properties of these materials
(high BET specific surface area, abundance of surface acidic
sites, extensive oxygen mobility in the ceria structure, and lowest
possible sintering) for a certain CuO loading and investigated
range of calcination temperatures from 400 to 850 °C.21
By using the preparation procedure described above, pure
CeO2 was synthesized by impregnation of KIT-6 template and
calcined at 550 °C for 3 h. KIT-6 silica template was removed
from the resulting solids by leaching twice with 2 M NaOH
(Merck) at 50 °C. Traces of NaOH were removed by continu-
ously washing samples with distilled water and centrifugation
until pH value of the slurry reached 7. Finally, the mesoporous
CuO-CeO2 mixed oxide samples were dried overnight at 50
°C. Actual CuO content values, determined with the ICP-AES
(inductively coupled plasma-atomic emission spectroscopy)
method were 9.2% for CuCe10, 14% for CuCe15, and 18% for
CuCe20, respectively.
Catalytic re-forming of methane with carbon dioxide, also
known as dry re-forming of methane (reaction 2), has recently
attracted considerable attention because of simultaneous con-
sumption of two greenhouse gases: CO2, a side product
generated in vast quantities by numerous chemical processes
and CH4, abundant fossil fuel with high H2 content.
CH4 + CO2 f 2H2 + 2CO
∆H0298 ) 247 kJ/mol
(2)
Dry re-forming reaction may be particularly useful for further
applications requiring synthesis gas with a low H2/CO ratio. It
produces synthesis gas with a H2/CO ratio of unity, which is a
suitable feedstock for synthesis of oxygenates as well as
methanol synthesis, a globally widespread large scale industrial
catalytic process.13
Performance of noble metals (Rh, Ru, Pt, and Pd) and
transition metals, such as Ni, supported on Al2O3, SiO2, ZrO2,
and La2O3, was investigated previously in methane dry re-
forming reaction.14 Li et al.15 compared supported group VIII
metals with the aim of finding the optimal catalyst for this
reaction. Among tested materials, Rh exhibited the highest
activity and chemical stability. Moreover, the use of supports
with low concentration of Lewis acid sites and/or presence of
basic sites results in enhancement of catalyst activity, lower coke
deposition and therefore more stable catalysts.16 Coke deposition
is the major cause of catalyst deactivation during methane dry
re-forming,15 as it leads to the coverage of active sites and a
decrease in catalytic performance. This being said, knowledge
about the reaction mechanism and kinetics is still a matter of
debate.17 Although supported noble metal catalysts exhibit
considerable catalytic activity at high temperatures (above 750
°C)15 and are less sensitive to carbon deposition, non-noble metal
catalysts are more practical in the view of availability and their
lower price.14
C-analysis was carried out by means of a Rosemount/
Dohrmann DC-190 TOC analyzer, where a carefully weighed
mass of carbon covered catalyst is oxidized by purified air to
produce CO2. The quantity of CO2 is measured by a TCD
detector and quantified when compared to oxidation of a known
standard; in this study, anhydrous Na2CO3 was used.
In this work, activity, stability, and selectivity of CuO-CeO2
catalysts in stoichiometric reactions of ethanol steam re-forming
and dry re-forming of methane were tested and correlated with
different morphological/chemical properties of tested solids.
Powdered catalysts containing 10, 15, and 20 mol % CuO were
prepared by hard template method.
XRD diffractograms of CuO-CeO2 catalyst samples were
recorded on a PANalytical X’pert PRO diffractometer using Cu
KR radiation (λ ) 0.154 06 nm). Samples were scanned in the
2θ ranges between 0.5-5° and 10-85° with 0.017° and 0.034°
2. Experimental Methods
CuO-CeO2 catalysts with a nominal 10, 15, and 20 mol %
CuO content were synthesized by hard template method using

High Temperature Processes of Hydrogen Production
J. Phys. Chem. A, Vol. 114, No. 11, 2010 3941
TABLE 1: BET Specific Surface Area, Total Pore Volume, Average CeO2 Crystallite Size, CuO Dispersion, and Average CuO
Particle Size of Fresh CeO2 and CuCe Catalyst Samples
specific BET surface area, m2/g
annealed at
600 °C for 24 h
annealed at
700 °C for 24 h
total pore
average CeO2
crystallite size, nm
CuO
dispersion, %
average CuO
particle size, nm
sample
fresh
volume, cm3/g
CeO2
134
147
166
161
124
90
116
88
106
14
29
0.30
0.29
0.31
0.33
8.2
7.8
6.5
6.6
CuCe10
CuCe15
CuCe20
40.4
28.7
32.2
1.3
1.9
1.7
24
increments, respectively, and recording time of 100 s at each
increment. N2 adsorption/desorption measurements were per-
formed at T ) -196 °C using a Micromeritics ASAP 2020
MP/C apparatus. Prior to characterization, the samples were
degassed in vacuum (4 µm Hg) at 300 °C.
AutoChem II 2920 apparatus. During activity probing tests
(increasing the reaction temperature from 200 to 800 °C with a
5 °C/min ramp), 55 mg of catalyst was used, while during
stability tests reaction was monitored for a time period of 72 h
at constant temperature of either 600 or 650 °C and 150 mg of
catalyst was placed on loosely packed quartz wool at the bottom
of U-shaped quartz test tube, which was inserted into an electric
furnace of AutoChem II 2920 apparatus. Prior to reaction,
catalyst samples were reduced in situ in a 50 mL/min flow of
either 5% CO/He or 5% H2/Ar by heating from room temper-
ature to 400 °C with a rate of 5 °C/min. During all dry re-
forming experiments, a reactant gas mixture comprising CH4
(9.1%) and CO2 (9.1%) diluted with He was fed into the fixed
bed reactor with a total volumetric flow rate of 51 mL/min
(STP). Reaction products as well as remaining reactants during
activity probing tests were simultaneously analyzed by mass
spectrometer (Pfeiffer Vacuum, model ThermoStar) and Rose-
mount BINOS 1001 IR detector. During stability tests, catalyst
performance was additionally monitored by consecutively
analyzing the effluent gas stream by means of a PerkinElmer
Spectrum 100 FT-IR spectrometer, equipped with a gas cell.
H2-TPR and H2-TPD measurements were performed using a
Micromeritics AutoChem II 2920 apparatus on 250 mg of
catalyst samples. TPR analysis was performed in a 50 mL/min
stream of 5% H2/Ar as a reducing agent. The samples were
heated from -20 to +400 °C with a 5 °C/min ramp during
analysis. A liquid nitrogen-isopropyl alcohol cold trap was
mounted on exhaust to condense water vapor and remove it
from the effluent gas mixture, before entering the TCD detector.
Samples subjected to TPD analysis were investigated in the
temperature range -20 to +700 °C with a 5 °C/min heating
ramp in a flow of Ar with a 50 mL/min flow rate. The quantity
of H2, used to reduce individual catalyst samples and the H2
amount desorbed from these samples, was evaluated by integra-
tion of peaks with a Micromeritics’ Peak Editor software.
For a detailed description of the utilized protocol, for
determination of CuO dispersion with selective pulse N2O
decomposition and investigation of surface acidic site abundance
with NH3 chemisorption, refer to our previous study.21
3. Results and Discussion
Ethanol steam re-forming (ESR) experiments were carried
out using a fixed-bed reactor system manufactured by PID
Eng&Tech (model Microactivity Reference) with integrated gas
flow, pressure, and temperature control modules. 300 mg of a
calcined catalyst was positioned inside the 9 mm i.d. Hastelloy
C reactor between two quartz wool beads. Prior to reaction, the
catalyst was reduced in situ in a 20% CO/He gas mixture at
400 °C for 1 h. Reaction temperature (measured with a
thermocouple located in the middle of the catalyst bed) was
increased in 25 °C increments from 400 to 600 °C. Activity of
each catalyst sample was monitored for a total of 75 h time on
stream. Reactant gas mixture composed of a stoichiometric (3:
1) molar ratio of water/ethanol solution, which was dosed by
means of a Gilson 307 HPLC pump (0.1 mL/min) and
evaporated in a heat box, operating at 180 °C and integrated in
the reactor system. N2 (50 mL/min) was used as an internal
standard in the gas mixture to inspect GC consistency, and He
(50 mL/min) as ballast gas. The total reactant flow was 180
mL/min (STP), which corresponds to GHSV of 21 580 h-1. The
operating pressure was in all runs equal to 3.0 bar, due to
pressure drop caused by the catalyst bed and protective stainless-
steel frits inside the reactor system. Reaction products and
remaining reactants were efficiently separated by means of HP
Plot Q and Molesieve capillary columns, and qualitatively and
quantitatively analyzed with an Agilent 7890 gas chromatograph
equipped with TCD and FID detectors. The transfer line between
the reactor system and gas chromatograph was heated to 200
°C using a heating tape to prevent water vapor condensation
and to ensure acquisition of reliable data.
3.1. Characterization. As determined by N2 adsorption/
desorption experiments, synthesized powders exhibited BET
surface areas (multipoint analysis) of 134, 147, 166, and 161
m2/g for CeO2, CuCe10, CuCe15, and CuCe20 samples,
respectively. The trend of decreasing surface area of CuO-CeO2
mixed oxides is consistent with increasing calcination temper-
ature of these solids (see Table 1). Addition of CuO enhanced
the surface area in comparison to pure CeO2 powder (134 vs
147 m2/g for CeO2 and CuCe10, respectively), indicating a very
small size of CuO entities and negligible pore blocking of the
CeO2 structure as a result of CuO presence (total pore volume
of 0.30 and 0.29 cm3/g for CeO2 and CuCe10, respectively).
XRD examination of fresh CuO-CeO2 materials revealed the
presence of FCC CeO2 crystalline structure only (Figure 1), as
evaluated from the comparison of measured diffractograms with
the PDF data file (03-065-5923). No presence of any copper
species containing ordered phase could be confirmed, as a result
of their high dispersion and consequently small size. By applying
the Scherrer equation to the characteristic (111) CeO2 peak, the
average crystallite size was calculated.22 Their size was in the
6.5-7.8 nm range (Table 1) and increased with rising calcination
temperature. With the intention of monitoring structural changes
at high temperatures, fresh CuO-CeO2 materials were annealed
for 24 h at either 600 or 700 °C in argon; the results of measured
BET surface areas are presented in Table 1. After the exposure
to 600 °C, BET surface area is found in the range between 88
and 116 m2/g, and slight alterations in total pore volume,
compared to fresh materials are noted (0.21, 0.25, and 0.20
cm3/g for CuCe10, CuCe15, and CuCe20 samples, respectively).
Additionally, the exposure of solids to 600 °C caused growth
of CeO2 crystallites, determined from the narrowing of char-
Catalytic experiments of CH4 re-forming with CO2 were
performed at atmospheric pressure using a Micromeritics

3942 J. Phys. Chem. A, Vol. 114, No. 11, 2010
Djinovic´ et al.
Figure 1. XRD diffractograms of fresh and annealed CuO-CeO2 catalyst powders.
acteristic XRD peaks, while the presence of ordered CuO phase
still could not be confirmed. Annealing at 700 °C resulted in a
drastic BET surface area drop compared to fresh materials (14,
29, and 25 m2/g) and severe reduction of total pore volume (0.05,
0.08, and 0.07 cm3/g for CuCe10, CuCe15, and CuCe20
samples, respectively). After the exposure to 700 °C, narrow
and intense peaks attributed to monoclinic CuO tenorite (PDF
data file 00-048-1548), were also observed (Figure 1, marked
with an asterisk), indicating growth and agglomeration of CuO
nanoparticles.
Pure CeO2 displayed considerably lower alterations of its
morphology when exposed to high temperatures in comparison
with mixed oxide analogs, disclosing a negative influence of
CuO addition on thermal stability of investigated CuO-CeO2
mixed oxide powders.
With the use of low-angle XRD experiments (results not
shown), we confirmed the presence of cubic symmetry also in
the CuO-CeO2 structure after template removal, indicating
resemblance with the KIT-6 template pore network and some
extent of ordered Ia3d cubic symmetry in the structure of
obtained CuCe powdered materials.
Since XRD experiments did not reveal the presence of any
copper containing ordered phase in fresh CuO-CeO2 materials,
CuO dispersion was evaluated with the use of pulse N2O
decomposition experiments.23 Values of 40.4, 28.7, and 32.2%
were obtained (Table 1) that correspond to the average CuO
particle size of 1.3, 1.9, and 1.7 nm for CuCe10, CuCe15, and
CuCe20, respectively, under the assumption they are spherical.
Calculation of CuO nanoparticle size using the results of
selective N2O decomposition experiments and assumptions
adopted from ref 24 confirmed that their size is indeed too small
and could therefore not be identified during XRD examination
of fresh CuO-CeO2 powders.
positively influences the reducibility extent of CeO2, which in
turn increases oxygen mobility of tested materials through the
formation of oxygen vacancies in CeO2 structure. It is interesting
to note that temperature interval and the overall shape of
recorded TPR curves of fresh CuO-CeO2 materials did not
differ much when CuO content was increased from 10 to 20
mol % (Figure 2). This reveals the presence of finely dispersed
CuO on CeO2 with strong electronic interactions between both
metal oxide phases in all tested samples. Low surface area CeO2
with identical CuO loadings investigated in our previous work,21
proved to be unable to accommodate finely dispersed CuO at
loadings in excess of 15%, resulting in the formation of bulk
CuO. Consequently, H2-TPR curves of these materials, synthe-
sized by means of coprecipitation, sol-gel peroxide route and
citric acid-assisted synthesis methods28 were found to be very
similar to the ones presented in Figure 2 for annealed and
sintered materials at 700 °C.
Extent of partial CeO2 reduction in examined solids was
evaluated by subtracting the contribution for complete CuO to
Cu reduction (calculated for actual CuO content in prepared
materials) and H2 adsorbed on the catalyst surface, which does
not contribute to CuO-CeO2 reduction (evaluated by TPD
experiments and listed in Table 2) from the total amount of
consumed H2. It was determined that under employed H2-TPR
conditions, 14.4, 16.1, and 24.5% of total CeO2 is capable of
releasing an oxygen atom and transforming into Ce2O3 state
for CuCe10, CuCe15, and CuCe20 solids, respectively (Table
2). This provides a beneficial effect of increasing CuO content
on the reducibility extent of CeO2.
Annealed CuO-CeO2 materials, according to results pre-
sented in Figure 2, exhibited lower H2 consumption ability
leading to a lower extent of CeO2 phase reducibility. Sintering
of CuO-CeO2 materials at elevated temperatures results in a
loss of CuO dispersion and consequently lower abundance of
CuO-CeO2 juncture points, where CeO2 reduction is initiated.27
The shape of obtained TPR curves, consistent with the results
presented in our previous study,21 is attributed to the simulta-
neous reduction of bulk CuO and CeO2 at the CuO-CeO2
interface, characteristic for heavily sintered low surface area
CuO-CeO2 materials.
Oxygen mobility within the CeO2 structure is greatly en-
hanced when CeO2 is coupled with CuO.25,26 This characteristic
of CuO-CeO2 materials is well documented as an important
parameter influencing both catalyst activity and stability during
numerous catalytic reactions, such as preferential oxidation of
CO (PROX) and water-gas shift (WGS).21,27 It was discovered
during H2-TPR/TPD experiments that increasing CuO content

High Temperature Processes of Hydrogen Production
J. Phys. Chem. A, Vol. 114, No. 11, 2010 3943
Figure 2. H2-TPR profiles of pure CeO2 and fresh and annealed CuO-CeO2 materials.
TABLE 2: H2-TPR/TPD, Partial CeO2 Reduction and
Amounts of Desorbed NH3 and CO2 from CeO2 and CuCe
Catalyst Samples
TPR, TPD,
mL/g mL/g partial CeO2
sample (STP) (STP) reduction, %
desorbed NH3, desorbed CO2,
µmol/m2
µmol/m2
(STP)
(STP)
CeO2
CuCe10 30.8
CuCe15 41.5 10.7
CuCe20 49.8 9.5
22.4
3.5
9.3
28.9
14.4
16.1
24.5
1.39
1.24
0.89
0.92
0.99
1.78
2.22
2.41
It was reported earlier that acidic centers on the catalyst surface
originating from finely dispersed and coordinatively unsaturated
metal entities/cations, influence the activity and stability of these
materials in heterogeneous catalytic reactions in two possible ways:
(i) act as particularly active centers on the catalyst surface for
splitting the C-C and C-H bonds and (ii) act as active centers
where coking is initiated.29,30 NH3, the probe molecule used for
surface acidity characterization of investigated CuO-CeO2 materi-
als, can react with both Lewis and Brønsted acid sites as it can
bond to the exposed surface metal cations through the free electron
pair of the nitrogen atom, or to Brønsted acid sites by forming a
hydrogen bond with the surface hydroxyl groups. Consequently,
NH3 consumption accounts for titration of all acid sites, capable
of reacting with NH3. Results presented in Table 2 revealed a
positive correlation between the abundance of surface acidic sites
and CuO dispersion (Table 1). Consequently, the measured acidity
is very likely a consequence of Lewis acidity, connected to finely
dispersed metallic Cu nanoparticles on the surface of investigated
CuO-CeO2 materials. Similar conclusions were drawn in ref 29,
who attributed enhanced surface acidity of Nb/Cu materials to the
presence of Lewis acid sites, originating from surface Cu cations.
Pure CeO2, according to acidity probing examinations,
possesses the highest quantity of surface sites capable of reacting
with NH3 probe molecule. It could be proposed that oxygen
vacancies formed on the partly reduced CeO2 material (prior to
NH3 chemisorption, the catalyst sample was reduced in 5% CO/
He for 1 h at 400 °C), due to their electrophilic character exhibit
a strong attraction for the free electron pair in the NH3 molecule,
thus contributing to a high surface acidity of pure and nanoc-
rystalline CeO2 material.
Figure 3. (a) Ethanol and water conversion as a function of reaction
temperature and time on stream, obtained over CeO2 catalyst. (b)
Reactor outlet composition as a function of reaction temperature and
time on stream for CeO2 catalyst.
3.2. Ethanol Steam Re-forming Activity. In Figures 3-6,
results of ethanol steam re-forming reaction over CeO2 and
CuO-CeO2 catalysts are presented. By increasing reaction
temperature from 400 to 600 °C, ethanol conversion calculated
as XEtOH ) (1 - (nEtOHout)/(nEtOHin))·100%, increased from about

3944 J. Phys. Chem. A, Vol. 114, No. 11, 2010
Djinovic´ et al.
Figure 4. (a) Ethanol and water conversion as a function of reaction
temperature and time on stream, obtained over CuCe10 catalyst. (b)
Reactor outlet composition as a function of reaction temperature and
time on stream for CuCe10 catalyst.
Figure 5. (a) Ethanol and water conversion as a function of reaction
temperature and time on stream, obtained over CuCe15 catalyst. (b)
Reactor outlet composition as a function of reaction temperature and
time on stream for CuCe15 catalyst.
20 to over 90%. At reaction temperatures below 500 °C,
performance of CeO2 and CuCe10 samples was stable during
tested time intervals, while CuCe15 and CuCe20 catalysts
exhibited slight deactivation caused by sintering, and more
importantly, agglomeration of CuO phase under hydrothermal
operating conditions, since the deactivation rate was more
pronounced for samples with a higher CuO content. Increasing
CuO content of tested materials positively influenced the
conversion of ethanol in investigated reaction (CuCe20 >
CuCe15 > CuCe10), as can be concluded from the comparison
of data illustrated in Figures 3a-6a.
When applying reaction temperatures of 500 °C and above,
we observed a noticeable drop in ethanol conversion for all
CuO-CeO2 samples within tested time intervals (see Figures
4-6). However, the drop was negligible for CeO2. These
findings correspond well to the results of thermal stability tests
of these materials, reported in the previous section, where pure
and nanocrystalline CeO2 proved substantially more stable
compared to mixed oxide powders; this reveals sintering as a
source of catalysts deactivation. Changes in water conversion
were much less pronounced during time on stream at each
temperature increment regardless of the catalyst used, indicating
the occurrence of ethanol dehydratation reaction that results in
the production of C2H4 and water. Furthermore, ethanol dehy-
drogenation followed by decomposition reactions, which also
do not require water, could lead to an unchangeable water
conversion. This explains why noticeably lower water conver-
sion, compared to ethanol, was achieved despite the stoichio-
metric ratio of both reactants in the feed gas stream. The
remaining high water content in the effluent gas stream could
also be attributed to the unfavorable WGS reaction equilibrium
at elevated temperatures. Coupled with a continuous decrease
of ethanol conversion at reaction temperatures of 500 °C and
above, pressure in the reactor unit started to gradually increase;
consequently, all catalysts exhibited a slight increase in CH4
production with time on stream. The pressure rise in the reactor
was caused by the deposition of carbon, which blocked catalyst
pores and voids within the catalyst bed. Catalyst coking was
slower over CuCe15 and CuCe20 samples, allowing us to reach
the final temperature of 600 °C, whereas for CuCe10 and CeO2
samples, extensive pressure rise in the reactor prevented us from
reaching the final reaction temperature. Reasons for higher
coking resistance and consequently delayed onset of pressure
rise in the reactor probably lie in the enhanced surface oxygen
mobility and facile creation of oxygen vacancies in the structure
of catalyst samples with a higher CuO content (CuCe20 >
CuCe15 > CuCe10). Oxygen vacancies in the CeO2 structure
play an important role in adsorption and splitting of water
molecules, which represent the source of oxygen atoms during
the ethanol steam re-forming reaction, accordingly to Song et
al.10 Mobile oxygen species on the catalyst surface are believed
to play a significant role in scavenging and oxidizing carbon
deposits on the catalyst surface, therefore greatly retarding
fouling processes. Catalysts with a higher CuO content exhibit
also a lower abundance of surface acidic sites (see Table 2),
which are generally considered as active centers, where coking
is initiated.12
When results from different characterization techniques are
combined, the emerging picture suggests that the availability
of oxygen plays a key role in determining the stability of

High Temperature Processes of Hydrogen Production
J. Phys. Chem. A, Vol. 114, No. 11, 2010 3945
stream at 400 °C from initial ∼1.5% to about 0.3% and remained
at this value at all examined reaction temperatures. Production
of C2H6 passed through a maximum of ∼1 vol % at a reaction
temperature of 500 °C and decreased to the initial value of
∼0.1% at 575 °C. Generation of C2H4 was very low (∼0.2 vol
% for CuO-CeO2 materials and ∼0.5% for pure CeO2)
throughout the whole investigated temperature range. The
absence of copper in pure and nanocrystalline CeO2, which
according to Song et al.10 possesses high activity for C-C bond
cleavage, could account for slower ethylene decomposition
reaction (eq 4) and the remaining higher concentration of this
byproduct in the effluent gas stream, compared to CuO-CeO2
catalysts. It should be pointed out that during performed SRE
experiments the calculated carbon balance on the basis of
identified reaction products was 100 ( 5%.
At reaction temperatures of 575 and 600 °C, a decrease of
EtOH conversion with time on stream, observed at lower
reaction temperatures, stopped and EtOH conversion either
remained constant (CeO2, CuCe15) or started to increase with
time on stream (CuCe10 and CuCe20). Simultaneously with
the increase of ethanol conversion (Figures 3b-6b), concentra-
tion of CH4 in the reactor outlet increased to 6 vol % with time
on stream. These observations are consistent with the occurrence
of enhanced ethanol dehydratation reaction, where water and
C2H4 are produced in the first step (reaction 3). Ethylene then
further decomposes to carbon and methane (reaction 4).
CH3CH2OH f H2O + C2H4
∆H0298 ) 45 kJ/mol
(3)
Figure 6. (a) Ethanol and water conversion as a function of reaction
temperature and time on stream, obtained over CuCe20 catalyst. (b)
Reactor outlet composition as a function of reaction temperature and
time on stream for CuCe20 catalyst.
C2H4 f CH4 + C
(4)
Reactions 3 and 4 seem to be self-accelerating over the ever
increasing amount of carbon formed in the catalytic bed, as
carbon buildup positively influenced ethanol conversion. The
concentration of C2H4, a primary product of ethanol dehydra-
tation reaction, did not exhibit a substantial rise with the onset
of accelerated coking, which indicates the reaction of C2H4
decomposition and polymerization (eq 4) is very fast and is very
likely the major contributor to carbon formation on the catalyst
surface. This further implies that under conditions employed in
this study C2H4 is an unstable intermediate product. On the other
hand, the Boudouard disproportionation reaction as an additional
carbon formation source cannot be excluded.
All CuO-CeO2 samples as well as pure CeO2 were addition-
ally tested for long-term stability at T ) 475 °C, i.e., the highest
temperature that should, according to the results presented in
Figures 3a-6a, enable stable catalyst performance. In Figure
7a results of stability tests for CeO2 and CuCe materials are
presented, while Figure 7b shows reactor outlet composition
for CuCe20 catalyst sample as a function of time on stream. It
can be seen that the initial activity of all CuCe samples dropped
very quickly during the first few hours of time on stream and
continued to decline slowly until stabilizing at considerably
lower values (ethanol conversions are equal to 39, 32, and 39%
for CuCe10, CuCe15, and CuCe20 solids, respectively), com-
pared to the ones measured during activity tests (57, 58, and
63% for CuCe10, CuCe15, and CuCe20 samples, respectively),
thus indicating gradual stabilization of catalyst morphology at
reaction temperatures below 475 °C. The extent of catalyst
deactivation, presented in Figure 7a, coincides with the CuO
content of investigated materials (20, 39, and 49% for CuCe10,
CuCe15, and CuCe20 solids, respectively), while ethanol
CuO-CeO2 catalysts. Ethanol steam re-forming can be con-
sidered as a redox reaction, where ethanol is oxidized by oxygen
species originating from water. If there is sufficient oxygen
available/accessible, ethanol can be fully converted to CO2 and
H2 via the steam re-forming pathway (eq 1). However, if
replenishment of oxygen on the surface is slower compared to
its depletion, other byproducts with intermediate oxidation states
will be formed, including CH4, C2H4, C2H6, carbon, and CO.
Hydrogen yield will be maximized when carbon in ethanol is
oxidized all the way to CO2. Therefore, effective delivery of
oxygen to oxidation sites plays a key role in determining the
hydrogen yield and maintaining high catalyst selectivity. Similar
findings were made by Song et al.,10 who report that higher
oxygen mobility determines the oxygen transfer process across
catalyst surface, resulting in favorable ethanol oxidation and
maximization of hydrogen production.
By increasing the reaction temperature above 500 °C, the rate
of oxygen consumption via the steam re-forming pathway
overtook the rate of water adsorption/splitting and oxygen
replenishment, which resulted in a loss of catalyst selectivity,
enhanced formation of CH4, C2H6, and CO side products and
initiation of catalyst coking (Figures 3-6). The amount of
carbon on the surface of spent catalysts, determined by means
of C-analysis, was 22.8, 20.9, and 20.8% for CuCe10, CuCe15,
and CuCe20 samples, respectively.
Besides the presence of H2O, ethanol, H2, CO2, CO, and
methane in the effluent gas stream (Figures 3b-6b), minute
amounts of C2H6, C2H4, and CH3CHO were also detected (not
shown). CH3CHO production was very similar for all
CuO-CeO2 materials, exhibiting a fast decrease with time on

3946 J. Phys. Chem. A, Vol. 114, No. 11, 2010
Djinovic´ et al.
noticeable CH4 and CO2 conversion. CuCe15 and CuCe20
catalysts exhibited very similar activities (55 and 53% CO2
conversion at 800 °C for CuCe15 and CuCe20, respectively).
Lower activity was measured in the case of CuCe10 sample
(48% CO2 conversion at 800 °C). More active CuCe15 and
CuCe20 samples were additionally tested after reduction in 5%
CO/He to determine the influence of various reduction atmo-
spheres (either 5% H2/Ar or 5% CO/He) on activity in methane
dry re-forming reaction. It can be seen in Figure 8 that only
minor difference as a result of catalyst pretreatment in 5% CO/
He can be observed (52 and 56% CO2 conversion at 800 °C
for CuCe15 and CuCe20 samples, respectively), compared to
the pretreatment in 5% H2/Ar. Consequently, it can be concluded
that the methane dry re-forming reaction is unaffected by the
presence of different carbonates formed on the CuO-CeO2
surface, during either the reduction process or reaction itself,
since the temperature window where tested CuO-CeO2 materi-
als are active causes their thermal decomposition. This assump-
tion was confirmed by in situ DRIFTS experiments, where no
surface carbonates could be identified on CO-reduced catalysts,
after running the reaction for 24 h at 600 °C (results not shown).
On the other hand, one should note that the presence of stable
carbonates on the surface of CuO-CeO2 catalysts strongly
affects the performance of these solids in the WGS reaction
conducted at much lower temperatures.31
Catalytic performance of nanocrystalline CeO2 was also tested
in the methane dry re-forming reaction. Of all tested materials,
it achieved the lowest CO2 conversion (27% at 800 °C, Figure
8), which leads us to the conclusion that the activity of
investigated catalysts can be correlated to the amount of CuO
on the catalyst surface.
It can be seen in Figures 8 and 9 that further increasing CuO
content above 15 mol % does not improve the activity but
contributes only to a more pronounced deactivation (described
below). These findings are consistent with the ones reported
previously by Bradford et al.,17 who state that CH4 activation
on the metal surface is the rate-limiting step during dry
re-forming reaction, and that the catalytic activity depends on
the percentage of d-character metal exposed.
Figure 7. (a) Results of stability tests for CeO2, CuCe10, CuCe15,
and CuCe20 catalyst samples. (b) Reactor outlet composition during
stability test of CuCe20 catalyst.
Spent catalyst samples reduced in 5% H2/He (Figure 9a) were
analyzed for carbon deposition and the following values were
obtained: 0.13 wt % (CeO2), 0.21 wt % (CuCe10), 0.25 wt %
(CuCe15), and 0.52 wt % (CuCe20). The amount of carbon on
the surface of spent CuCe15 and CuCe20 catalysts reduced in
5% CO/He (Figure 9b) was equal to 0.57 and 0.58 wt %, which
is somewhat higher than in the case when CuO-CeO2 powders
were reduced in H2. The presence of carbonaceous deposits on
the surface of spent catalysts was confirmed also by means of
DRIFTS analysis, which reveals a broadband around 1500
Figure 8. CO2 conversion as a function of reaction temperature for
CeO2, CuCe10, CuCe15, and CuCe20 catalyst samples during methane
dry re-forming reaction. Inset: CO concentration as a function of
reaction temperature. Symbols: full, reduction in 5% H2/Ar; empty,
reduction in 5% CO/He.
cm-1 32
. According to activity trends presented in Figures 8 and
9, more active catalysts also exhibited more pronounced coking,
but the amount of deposited carbon is low, which exhibits
negligible influence on measured activity. This can be confirmed
with the fact that CuCe15 and CuCe20 catalysts performed very
similarly in spite of a different degree of surface covered with
carbonaceous deposits. Since the measured amounts of carbon
formed are consistent with increasing CuO content of investi-
gated catalysts, one can conclude that carbon is formed
preferentially on the CuO surface, where it cannot be oxidized
by active oxygen species, whose mobility is somewhat limited
to the CuO-CeO2 interface area.10
conversion increased by 11% over the nanocrystalline CeO2.
Consequently, changes of CuO morphology and loss of disper-
sion can be considered as the primary reasons for the observed
activity decline. Nevertheless, the investigated CuO-CeO2
catalysts maintain more than 50% of initial activity in the
process of ethanol steam re-forming.
3.3. Methane Dry Re-forming Reaction. In Figure 8, results
of CuCe10, CuCe15, CuCe20 activity tests obtained during dry
re-forming of methane are depicted. It can be seen that
temperatures in excess of 450 °C are required to obtain any
Besides CO and H2, water was also identified in the product
gas mixture (Figure 9). H2O production cannot be omitted with
the use of CuO-CeO2 catalysts at high temperatures, since they

High Temperature Processes of Hydrogen Production
J. Phys. Chem. A, Vol. 114, No. 11, 2010 3947
Figure 9. (a) Results of CuCe10, CuCe15, and CuCe20 activity tests obtained during dry re-forming of methane after reduction in 5% H2/Ar. (b)
Results of CuCe15 and CuCe20 activity tests obtained during dry re-forming of methane after reduction in 5% CO/He.
are also very active in the WGS reaction.31 Unfavorable WGSR
equilibrium at temperatures above 450 °C greatly enhances
production of H2O and CO, resulting in a loss of H2 yield and
reaction selectivity. Besides water, no other side product could
be identified.
To examine the performance of synthesized CuO-CeO2
catalysts on a longer time scale, stability tests were carried out
at reaction temperatures of 600 and 650 °C; the obtained results
are depicted in Figures 10-12. Despite the utilization of high
reaction temperature, stable CH4 and CO2 conversion was
observed during the whole time period of 72 h for CuCe10 and
CuCe15 catalysts. On the other hand, the CuCe20 sample
exhibited a slight activity drop during the initial 15 h time on
stream at a reaction temperature of 650 °C, whereas at 600 °C
Figure 10. Long-term stability of CuCe10 catalyst sample at T ) 650
°C.
no noticeable deactivation of this solid could be observed.
Nevertheless, the investigated CuO-CeO2 catalysts maintain
more than 95% of initial activity in the process of methane dry
re-forming. It is interesting to point out that the examined
CuO-CeO2 catalysts exhibit considerable activity for methane
dry re-forming in a very similar temperature window (Figure
9) as catalysts containing precious metals.15

3948 J. Phys. Chem. A, Vol. 114, No. 11, 2010
Djinovic´ et al.
ability of these catalysts to be used in high temperature
applications.
4. Conclusions
Preparation of CuO-CeO2 mixed oxide catalysts with the
hard template method using KIT-6 silica as the template results
in nanocrystalline powders which exhibit BET surface area in
excess of 145 m2/g, highly dispersed CuO nanoparticles, and
facile reducibility of CeO2 support, indicating strong interactions
between both metal oxide phases.
During the stoichiometric ethanol steam re-forming reaction
pure CeO2 displayed stable performance throughout the whole
temperature range and the highest activity at temperatures above
500 °C. Among CuO-CeO2 solids, catalysts with a higher CuO
content exhibited higher activity, demonstrating that higher
content of exposed copper on the catalyst surface and enhanced
oxygen mobility positively influence the performance of inves-
tigated materials. Deactivation observed under utilized reaction
conditions was more pronounced for materials with a higher
CuO content and was primarily caused by sintering and loss of
CuO dispersion, resulting from the exposure to hydrothermal
operating conditions. Nevertheless, stable operation of CuCe10
and CuCe15 samples was achieved at reaction temperatures
below 500 °C. At 575 °C and above, the ethanol dehydratation
reaction is strongly accelerated, resulting in extensive formation
of coke and methane. Ethanol dehydratation reaction is self-
accelerating over the ever increasing amount of carbon. The
onset of this side reaction can be delayed by using catalysts
with higher CuO loadings, as more abundant mobile oxygen
species favor the ethanol steam re-forming pathway.
Figure 11. Long-term stability of CuCe15 catalyst sample at T ) 650
°C.
Methane dry re-forming reaction occurred over CuO-CeO2
materials at temperatures in excess of 450 °C. It was discovered
that carbonate species on the catalyst surface, as a result of
exposure to CO had a negligible influence on the catalyst
performance, as they are decomposed at the applied reaction
temperatures. Deposition of carbon on spent CuO-CeO2
catalysts occurred to a very low extent. Catalysts with a higher
CuO content generally exhibited better activity but were more
prone to deactivation by sintering. Contrary to ethanol steam
re-forming, pure CeO2 exhibited noticeably lower activity
compared to CuO-CeO2 mixed oxides. Unfavorable WGSR
equilibrium at utilized reaction temperatures favors the produc-
tion of H2O and CO, resulting in a noticeable loss of hydrogen
yield. This calls for carrying out the process in a multifunctional
reaction/separation system.
Acknowledgment. We gratefully acknowledge the financial
support of the Ministry of Education, Science, and Technology
of the Republic of Slovenia through Research program No. P2-
0152.
Figure 12. Long-term stability of CuCe20 catalyst sample at (a) T )
600 °C and (b) T ) 650 °C.
The major cause for deactivation can in our opinion be
attributed to the agglomeration of CuO and sintering of catalysts.
BET surface area of spent materials tested at 650 °C was found
to be equal to 48 m2/g (CuCe10), 46 m2/g (CuCe15), 43 m2/g
(CuCe20), and 56 m2/g (CuCe20, 600 °C). The extent of
deactivation increased with rising CuO content (see Figures
10-12) and also follows the trend of decreasing BET surface
area of spent catalysts. Consistently with data presented in Figure
9a, CuCe15 and CuCe20 samples exhibited higher conversion
of CH4 and CO2, thus indicating superior activity compared to
the activity of CuCe10 catalyst examined at the same reaction
conditions. To conclude, the above facts confirm that the
examined CuO-CeO2 catalysts maintain a considerable fraction
of initially available specific surface area, demonstrating the
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