The role of impregnation medium on the activity of ceria-supported cobalt catalysts for ethanol steam reforming

Article abstract of DOI:10.1016/j.molcata.2009.11.003

The effect of impregnation medium on the activity of Co/CeO₂ catalysts in ethanol steam reforming was investigated using various characterization techniques including temperature programmed calcination, temperature programmed reduction, X-ray d

Full text of DOI:10.1016/j.molcata.2009.11.003

Journal of Molecular Catalysis A: Chemical 318 (2010) 21–29
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
The role of impregnation medium on the activity of ceria-supported cobalt
catalysts for ethanol steam reforming
Hua Song, Umit S. Ozkan∗
Department of Chemical & Biomolecular Engineering, The Ohio State University, Columbus, OH 43210, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 2 May 2009
Received in revised form 22 October 2009
Accepted 4 November 2009
Available online 12 November 2009
The effect of impregnation medium on the activity of Co/CeO2 catalysts in ethanol steam reforming was
investigated using various characterization techniques including temperature programmed calcination,
temperature programmed reduction, X-ray diffraction, laser Raman spectroscopy, X-ray photoelectron
spectroscopy, and diffuse reflectance infrared fourier transform spectroscopy. The steady-state reac-
tion experiments showed that catalysts that were prepared in an organic medium (e.g., ethanol) during
impregnation gave higher H2 yields than those prepared in aqueous media. Characterization results
showed the presence of oxygenated carbonaceous species left on the surface from the impregnation step.
These species, which were stable through oxidation and reduction pre-treatment steps, may possibly con-
tribute to the activity, selectivity and stability of the catalysts by keeping the Co particles segregated and
by blocking the sites for side reactions.
Keywords:
Cobalt catalyst
Ceria
Ethanol steam reforming
Impregnation medium
DRIFTS
© 2009 Elsevier B.V. All rights reserved.
Raman
XPS
1. Introduction
ating cost of these metals limits their wide applications. As a less
expensive alternative, cobalt-based catalysts have been reported
Hydrogen continues to hold promise as an important energy car-
rier for the future, with its high gravimetric energy density, its low
impact on the environment, and its potential as a fuel for more effi-
cient energy conversion devices such as fuel cells. With increasing
demand for hydrogen, producing it from renewable sources, such
as bio-derived liquids, is also gaining increased importance. Bio-
ethanol offers a promising starting material for hydrogen due to its
low toxicity, its ease of handling and its availability from many
different renewable sources, ranging from sugar cane to algae.
Hydrogen produced from a bio-source has the potential to recycle
the CO2 produced during its production through photosynthesis
during plant growth. Finally, producing hydrogen through steam
reforming of bio-derived liquids such as bio-ethanol lends itself
well to a distributed production strategy.
Recent years have seen a significant increase in the number of
studies on ethanol steam reforming (ESR). Catalysts utilized are
mainly Ni, Cu, Co and noble metals, such as Rh, Ru, Re, Pd, Ir and
Pt [1–4], which are reviewed by Haryanto et al. [5] and Vaidya and
Rodrigues [6]. Although supported noble metal catalysts have been
shown to have excellent performance during ESR in 500–600 ◦C
range and high space velocities [7–9], the high and widely fluctu-
to have superior ethanol steam reforming performance due to
their high activity for C–C bond cleavage at temperatures as low
as 350–400 ◦C [10–12]. At these temperatures, researchers have
reported good selectivity to CO2 and H2 with CH4 being the only
by-product.
Various metal oxides, most of which have been reported by
Llorca et al. [12], have been used as support materials for Co in
order to provide a high surface area as well as good thermal stabil-
ity leading to better cobalt dispersion even at higher temperatures.
Although cobalt catalysts supported on these oxides showed com-
parable activity, the product distribution was significantly different
indicating that these selected supports actively participated in the
ESR reaction. Among the various supports, ceria offers increased
resistance to coking due to its high oxygen storage capacity (OSC)
and oxygen mobility, as shown in many other reactions that involve
an oxidation step such as CO oxidation, water–gas shift, and steam
reforming of methane [13–15]. Increased oxygen mobility pro-
motes gasification and oxidation of carbon deposits on the surface,
hence improving stability, which was demonstrated in our previous
studies [16].
In addition to the support effects mentioned above, several other
promotional effects have been extensively investigated aiming to
enhance the catalytic activity as well as stability during ESR. For
instance, the addition of alkaline metals (e.g., Li, Na, and K) has been
shown to improve the catalytic performance [17–19]. In addition,
∗
Corresponding author. Tel.: +1 614 292 6623; fax: +1 614 292 3769.
E-mail address: Ozkan.1@osu.edu (U.S. Ozkan).
1381-1169/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2009.11.003

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H. Song, U.S. Ozkan / Journal of Molecular Catalysis A: Chemical 318 (2010) 21–29
use of different cobalt precursors [20–22], different preparation
techniques, and synthesis/pre-treatment parameters [23–25] have
been shown to have a significant impact on the catalytic perfor-
mance. One conclusion emerging from various studies is that the
catalytic activity is directly related to cobalt dispersion [26,27].
There have been reports in the literature, showing an effect of
the impregnation medium used in the preparation of supported
Co catalysts for Fischer-Tropsch reaction [28]. In this paper, as a
continuation of our previous work [16,23,29], we examine the role
of the impregnation medium on the activity of Co-based catalysts
for ethanol steam reforming. The supported catalysts were pre-
pared by incipient wetness impregnation (IWI) and characterized
through temperature programmed calcination (TPC), temperature
programmed reduction (TPR), in situ X-ray diffraction (XRD), in
situ laser Raman spectroscopy (LRS), X-ray photoelectron spec-
troscopy (XPS), and diffuse reflectance infrared fourier transform
spectroscopy (DRIFTS). The results from characterization studies
were used in explaining the activity differences observed for cata-
lysts prepared in different media.
H2 TPR experiments were performed using a laboratory flow
system equipped with a thermal conductivity detector (TCD). Sam-
ples of 50 mg were loaded into a U-tube and then subjected to
an oxidative cleaning step at the samples’ calcination tempera-
ture in air, followed by cooling to room temperature under helium.
TPR experiments were subsequently performed under 5%H2/N2
(30 ml/min) with a heating rate of 10 ◦C/min. The exit gas from the
reactor was sent to a column filled with silica gel for trapping water
before being sent to the TCD for analysis.
XRD profiles were collected from 20◦ to 90◦ at a step width
of 0.0144◦ using Bruker D8 advance X-ray diffractometer. In situ
XRD was also performed during calcination process under air
(30 ml/min) and reduction process under 5%H2/N2 (30 ml/min)
using a linear heating rate of 0.3 ◦C/min and holding at different
preset temperatures for a given time for stabilizing and data collec-
tion. The XRD patterns used for particle size estimation at various
stages of the catalyst life-time were collected from 20◦ to 90◦ at a
step width of 0.002◦ using Rigaku Ultima III X-ray diffractometer
equipped with a CuK␣ source (ꢀ = 1.5406 Å).
Raman spectra were taken with a LabRAM HR-800 spectro-
metric analyzer integrated with OLYMPUS BX41 microscope (50×
magnification) and CCD detector. The in situ calcination experiment
was performed under air (30 ml/min) using an “operando cell”.
The sample was first heated to various temperatures and cooled
down to room temperature where the spectra were taken using an
argon ion laser (514.5 nm, operated at 3 mW). A similar procedure
was followed while performing in situ reduction experiment under
5%H2/He (30 ml/min).
XPS analysis was performed using an AXIS His, 165 Spectrome-
ter manufactured by Kratos Analytical with a monochromatized Al
X-ray source. 2.3 V voltage was chosen to make the charge balance.
Thesample was pressedinto astainless steelcup beforeloading into
the instrument. The survey scan was performed to identify all the
elements within the sample, followed by regional scans for Co 2p,
C 1s, O 1s, Ce 3d orbitals in order to achieve the high resolution for
these elements of interest. A controlled-atmosphere transfer cham-
ber was used for transferring the samples to the XPS instrument in
order to prevent the reduced samples from being re-oxidized again.
For quantification of the surface and near surface concentrations
of elements, the instrument-dependent atomic sensitivity factors
were used.
2. Experimental
2.1. Catalysts preparation
Supported cobalt catalysts with 10% weight loading were pre-
pared in air by IWI technique. Cerium (IV) oxide (nanopowder,
<25 nm, Aldrich) was calcined at 550 ◦C for 4 h, resulting in a sup-
port with a surface area and pore volume of 71 m2/g and 0.34 cm3/g,
respectively. Cobalt (II) nitrate hexahydrate (Aldrich 99.999%),
which was used as the cobalt precursor, was dissolved in either
ethanol or deionized water and then impregnated on the previously
calcined support. After repeated impregnation and drying steps
(overnight at approximately 95 ◦C) as many times as required by
the pore volume of the CeO2 support, the as-prepared samples were
calcined at 450 ◦C under air for 3 h and stored for later use. In the
nomenclature used in this paper, the letter in parenthesis following
the catalyst formulation represents the impregnation medium, i.e.,
A for aqueous (DI water) and E for ethanol.
2.2. Catalysts characterization
DRIFTS was performed with a Thermo NICOLET 6700 FTIR spec-
trometer equipped with a liquid-nitrogen-cooled MCT detector
and a KBr beam splitter. The experiments were performed using
a Smart collector DRIFT environmental chamber with ZnSe win-
dows. For room-temperature spectra, following a cleaning step
under He at 450 ◦C for 30 min to remove impurities from sample
surface that might have adsorbed during storage, the chamber was
cooled down to room temperature under He and sample spectra
were then collected. As background, the spectrum taken over the
sample impregnated in aqueous media was used.
Cobalt dispersion was determined through a H2 chemisorption
technique using a Micromeritics ASAP 2010 Chemisorption system.
Prior to adsorption measurements, calcined samples were pre-
treated at 450 ◦C and then reduced in situ under 5%H2/He at 400 ◦C
for 2 h followed by evacuation to 1.33–0.67 KPa and cooling down
to 50 ◦C to maximize activated chemisorption while minimizing H2
spillover [30]. The adsorption isotherms were measured at equi-
librium pressures between 6.67 and 66.7 kPa. The first adsorption
isotherm was established by measuring the amount of H2 adsorbed
as a function of pressure. After completing the first adsorption
isotherm, the system was evacuated for 1 h at 1.33–0.67 kPa. Then
a second adsorption isotherm was obtained. The amount of probe
molecule chemisorbed was calculated by taking the difference
between the two isothermal adsorption amounts. Metallic sur-
face area was determined by assuming a one-to-one stoichiometry
between Co and atomic hydrogen and a cross-sectional area of
0.0662 nm2 for a Co atom.
The calcination process was investigated through TPC experi-
ment. The sample was placed between two layers of quartz wool
in a stainless steel reactor which was placed in the center of a
temperature programmable furnace. A constant air flow rate was
maintained over the sample throughout the process and the reactor
outlet stream was monitored by a Cirrus Mass Spectrometer (MKS
Instruments, 1–300 amu).
2.3. Activity tests
The catalytic activity tests were performed in
a tubular
quartz reactor (4 mm internal diameter). The catalyst was placed
between two layers of quartz wool. The aqueous ethanol solution
(EtOH:H2O = 1:10 at molar ratio) was delivered into the evaporator
by an isocratic pump (Eldex MicroPro). The feed vapors gener-
ated were carried by helium and sent into the reactor after being
combined with nitrogen, which was used as internal standard. The
delivery lines were heated with heat tapes to prevent condensation.
All catalysts were first pretreated at 450 ◦C for 30 min under He
and then reduced in situ at 400 ◦C for 2 h under 5%H2/He. Subse-
quently the catalytic performances were tested in the temperature
range of 300–500 ◦C, in 50 ◦C increments. The catalyst was held at

H. Song, U.S. Ozkan / Journal of Molecular Catalysis A: Chemical 318 (2010) 21–29
23
Table 1
Product distribution during steady-state reaction over 10%Co/CeO2(A) and 10%Co/CeO2(E) at various temperatures: H2O:EtOH = 10:1 (molar ratio), WHSV = 0.48 gEtOH/g Cat/h,
GHSV = ∼20,000 h−1, and CEtOH = 2%.
Productsa
300 ◦
C
350 ◦
(A)
C
400 ◦
(A)
C
450 ◦
(A)
C
500 ◦
(A)
C
(A)
(E)
(E)
(E)
(E)
(E)
% yield
H2
10.9
42.4
32.9
83.3
64.7
88.9
71.4
89.4
74.9
91.4
% selectivity
CO2
CO
CH4
C2H4
5.6
4.1
3.5
0
55.1
2.3
3.2
0
17.4
7.5
5.2
0.0
0.0
37.0
29.5
3.4
85.6
2.9
8.4
0.0
0.0
3.1
0.0
0.0
29.9
4.4
2.9
0.0
0.0
62.8
0.0
0.0
91.2
2.8
6.0
0.0
0.0
0.0
0.0
0.0
35.0
5.5
2.7
1.2
0.0
55.6
0.0
0.0
90.4
4.6
4.9
0.0
0.0
0.0
0.0
0.0
59.4
5.8
10.9
2.7
0.7
20.6
0.0
88.5
6.6
4.9
0.0
0.0
0.0
0.0
0.0
C2H6
0
0
CH3COCH3
CH3CHO
(C2H5)2O
6.0
80.7
0
3.5
35.9
0
0.0
a
The values reported in the table are defined in Section 2.3.
each temperature for at least 2 h. At the end of the catalytic test, the
flow of EtOH + H2O was stopped and the catalyst was cooled under
a He stream.
organic medium, give higher H2 yields and CO2 selectivity while
selectivities to CO and CH4 are lower. Another important aspect
of the product distribution comparison is that Co/CeO2(E) catalyst
does not have any liquid products left in the product stream above
350 ◦C, with CH4 and CO being the only other products besides CO2
and H2. Catalyst prepared in an aqueous media, on the other hand,
continues to have significant levels of acetone in the product stream
even at higher temperatures. In addition, although their amount
is relatively small, other undesirable by-products such as diethyl
ether, ethylene, and ethane are also observed over Co/CeO2(A)
at various temperatures. Formation of C2 hydrocarbons suggests
hydration steps that might lead to coke formation.
When TOFs for the (E) and (A) catalysts are compared, it is seen
that ethanol conversion TOFs are comparable for the two catalysts
(0.006 and 0.007 s−1 at 300 ◦C, respectively). TOFs for H2 forma-
tion, however, show a much higher activity for Co/CeO2(E), with a
value more than twice that of the (A) catalysts (0.011 and 0.024 s−1
at 300 ◦C, respectively), indicating a higher ability for C–C bond
cleavage and for complete oxidation of C in the ethanol molecule.
The ethanol-impregnated catalyst was kept on stream for 40 h
to examine its stability. Fig. 1 shows the ethanol conversion and
product yields during the run at 350 ◦C. For the duration of the
experiment, the catalyst has shown no sign of deactivation, main-
taining an ethanol conversion of 85%, a H2 yield of ∼82% and
a CO2 yield of ∼70%. The yield of all other products remained
low, below 8%. When a similar time-on-stream experiment was
The analysis of the reactants and all the reaction products
was carried out online by gas chromatography (Shimadzu Scien-
tific 2010). Analysis was done using two different detectors and
the separation was achieved using two different sample injec-
tion loops/valves and two analysis lines: the first line consisted
of a Carboxen column and a 5-Å molecular sieve column con-
nected in series in a column isolation scheme. This combined
column arrangement was used in conjunction with a pulse dis-
charge helium ionization detector (PDHID). The second line was
a separate 30 m-long Q-Plot column used with a methanizer and a
flame ionization detector (FID) to allow detection of CO as low as
10 ppm. Helium was used as the carrier gas for both of the anal-
ysis lines. All the carbon-containing products could be separated
by the Q-Plot column and detected by FID. PDHID can detect all the
products in the stream, including CO, CO2, and H2. Response factors
for all products were obtained and the system was calibrated with
appropriate standards before each catalytic test.
The H2 yield, selectivity and yield of carbon-containing prod-
ucts, and ethanol conversion were defined as follows:
moles of H2 produced
6 × (moles of ethanol fed)
H2 yield % =
× 100
Yield of C-containing product, i %
#C × (moles of i produced)
=
× 100
2 × (moles of ethanol fed)
Selectivity of C-containing product i
#C × (moles of i produced)
6 =
2 × (moles of ethanol converted)
moles of ethanol converted
EtOH Conv.% =
× 100
moles of ethanol fed
The turnover frequency (TOF) reported in the paper is calculated
based on the ethanol conversion rate and hydrogen produc-
tion, respectively, divided by the total metallic cobalt active sites
exposed on the surface of the catalysts in the reactor, which was
determined from H2 chemisorption experiment described above.
3. Results and discussion
3.1. Catalytic performance comparison
Fig. 1. Time-on-stream behavior of 10 Co/CeO2(E) catalyst at 350 ◦C. Reac-
tion conditions: H2O:EtOH = 10:1 (molar ratio), WHSV = 0.48 g EtOH/g Cat/h,
GHSV = ∼20,000 h−1, and CEtOH = 2%.
Table 1 shows the H2 yields and the selectivities for C-containing
products. At every temperature tested, the catalysts prepared in an

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H. Song, U.S. Ozkan / Journal of Molecular Catalysis A: Chemical 318 (2010) 21–29
Fig. 2. Elution of gas phase species over Co/CeO2(E) during the calcination process.
performed over the Co/CeO2(A) catalyst, rapid deactivation was
observed starting within 15 h (data not shown). Once activity loss
started, the main products were hydrocarbons, signaling dehydra-
tion reaction becoming important. Poor carbon balance during this
period suggested coking on the surface, which was verified by post-
deactivation examination of the spent catalyst.
3.2. Characterization
Catalysts prepared in the two different impregnation media
were characterized using various techniques, in an effort to under-
stand the possible surface and structural characteristics that might
contribute to the differences in performance.
Fig. 4. In situ Raman spectra collected over/CeO2(E) during the calcination process.
Fig. 2 shows the gas phase species eluting from the Co/CeO2(E)
sample during the calcination process. Water is seen to form at two
distinct temperatures. The low-temperature feature indicates the
release of the crystal water present in the cobalt precursor. The
larger peak at higher temperature might be derived from the OH
groups in the sample formed from the ethanol impregnation during
sample preparation. The combination of 30 and 46 ion fragments
Fig. 3. In situ XRD patterns of Co/CeO2(E) taken during the calcination process.

H. Song, U.S. Ozkan / Journal of Molecular Catalysis A: Chemical 318 (2010) 21–29
25
Fig. 5. H2 TPR profiles of (a) Co/CeO2(A) and (b) Co/CeO2(E).
reveals the production of NOx from the cobalt nitrate oxidative
decomposition, which is accompanied by the corresponding oxy-
gen consumption. No carbon-containing gas products are detected.
The gas phase species eluting from the sample prepared in the
aqueous media exhibited similar features of H2O and NOx species
(profiles not shown).
The calcination process of the catalyst impregnated in an
ethanol medium was further examined using the XRD technique.
Although there is a broad feature from 35◦ to 45◦ due to the flu-
orescence of Co, the signal can still be discriminated from the
background. As shown in Fig. 3, the cubic CeO2 support is visible at
each temperature. However, the evolution of the cobalt oxide can
seen be with increasing calcination temperature. When the tem-
perature is above 300 ◦C, the diffraction peaks corresponding to
cubic Co3O4 begin to appear. The gradual increase of the Co3O4 peak
along with increasing calcination temperature implies the growth
of Co3O4 particles at higher temperatures.
The calcination process over the Co/CeO2(E) sample was also
examined by laser Raman spectroscopy. Fig. 4 shows the in situ
Raman spectra collected during calcination. The low wavenum-
ber region (100–800 cm−1) shows the formation of Co3O4 and
higher region (4500–5500 cm−1) shows the decomposition of
cobalt nitrate, left behind from the impregnation step. Although
cobalt nitrate has a major Raman band centered around 1050 cm−1
,
when impregnated onto CeO2, its signal is masked due to the
highly Raman active nature of the support. Therefore, the evolu-
tion of the nitrate phase was followed in the high wavenumber
region. As seen in Fig. 4, CeO2 peak is observed throughout the
process without changing, Co3O4 begins to appear when the tem-
perature is above 300 ◦C. The gradual increase of the Co3O4 peak
along with increasing calcination temperature implies the growth
of Co3O4 particles at higher temperature. The doublet shown in
high wavenumber region shows the cobalt nitrate decomposition
which is completed when the temperature is above 300 ◦C which
coincides with the formation of Co3O4. The catalyst precursor for
the Co/CeO2(A) showed a similar evolution through the calcination
process (data not shown).
Fig. 6. In situ XRD patterns taken during reduction process over (a) Co/CeO2(E) and
(b) Co3O4.
The reduction behaviors of the Co/CeO2(E) and Co/CeO2(A) are
studied using the TPR technique and the results are shown in Fig. 5.
Two peaks are observed and identified as Co3O4–CoO and CoO–Co
reduction features, using CuO to calibrate H2 consumption and
Co3O4 as a standard reference. When the reduction profiles for
the two catalysts were compared, the sample synthesized in the
organic media showed noticeably better reducibility, which corre-
lates well with ESR activity [23]. Neither surface nor bulk reduction
of CeO2 occurs within the reduction temperature range investi-
gated.
The evolution of the crystalline phases during the reduction pro-
cess over the calcined Co/CeO2(E) is shown in the XRD patterns

26
H. Song, U.S. Ozkan / Journal of Molecular Catalysis A: Chemical 318 (2010) 21–29
Table 2
Co element. The presence of shake-up lines in the Co 2p XPS spectra
of the sample reduced at 400 ◦C for 2 h points to the reduction of
Co3O4 to CoO since the satellite peaks are characteristic of the Co2+
oxidation state [32–34]. The shoulder at 777.8 eV near the main
Cobalt/cobalt oxide crystal size estimation from XRD (nm).
Catalyst
Fresha
Reducedb
Spentc
Co/CeO2(A)
Co/CeO2(E)
15
14
12
8
25
8
a
Calculations are based on (3 1 1) diffraction line for Co3O4.
b
Samples were reduced at 400 ◦C for 2 h. Calculations are based on (1 1 1) diffrac-
tion line for Co.
c
XRD patterns were collected after time-on-stream for 40 h over 10%Co/CeO2(E)
and 10 h over 10%Co/CeO2(A). Calculations are based on (1 1 1) diffraction line for
Co.
Table 3
Surface elemental analysis using XPS technique over 10%Co/CeO2(A) and
10%Co/CeO2(E) after calcination and reduction.
After calcination
Co/CeO2(A)
After reduction
Co/CeO2(A)
Co/CeO2(E)
Co/CeO2(E)
%Co
Co/Ce
7.3
0.27
5.9
0.43
10.0
0.25
7.8
0.34
acquired in situ (Fig. 6a). In situ XRD patterns obtained over bulk
Co3O4 during a similar reduction process are also included (Fig. 6b)
for comparison. The disappearance of the Co3O4 phase and the
appearance of the CoO phase coincide at a reduction temperature
of 350 ◦C. The diffraction line that corresponds to CoO phase disap-
pears above 400 ◦C and a metallic Co phase appears at 450 ◦C, which
is consistent with the TPR profile shown in Fig. 5. Compared to the
standard Co3O4, the phase transformation takes place at a higher
temperature, possibly due to the metal–support interaction. The
crystal phase transformations during the reduction of Co/CeO2(A)
are also shifted to higher temperatures (data not shown) com-
pared to the Co/CeO2(E), sample, implying that this sample is more
difficult to reduce, consistent with the TPR profiles shown earlier
(Fig. 5).
Crystallite sizes of Co3O4 and metallic Co were determined using
X-ray diffraction data. Table 2 summarizes the crystallite sizes for
the two catalysts at different life stages calculated using Scher-
rer equation. The diffraction lines used for these calculations are
(3 1 1) and (1 1 1) for the Co3O4 and Co phases, respectively. The
fact that both of the samples have similar particle sizes for Co3O4
after calcination implies that the preparation in organic media has
no effect on controlling the Co3O4 crystallite size. However, the
crystallite sizes for the reduced and spent sample show a major
difference, suggesting a strong effect of the impregnation medium
in suppressing particle growth during reduction and reaction.
The reduction characteristics of the two samples prepared in
organic versus aqueous media are compared through in situ LRS
technique in Fig. 7a and b, respectively. The spectra corresponding
to pure CeO2, Co3O4 and CoO are also shown as reference. As the
temperature is raised step-wise in a 5% H2 stream, no significant
phase transformation takes place below 400 ◦C. When the tempera-
ture is at 400 ◦C, Co/CeO2(A) gives signal from both CO3O4 and CoO.
When the temperature is raised up to 600 ◦C, cobalt oxide species
are still visible in this sample. For the sample prepared in organic
media, however, the reduction to metal takes place much more
readily. At 450 ◦C, the bands that correspond to Co–O vibrations
are no longer visible.
XPS surface analysis was performed over both samples at three
different stages: (a) following calcination, (b) following reduction
at 400 ◦C, and (c) following reduction at 600 ◦C. The spectra for
Ce 3d and Co 2p regions are presented in Fig. 8 for Co/CeO2(E).
The oxidation state of Ce remains at +4 even in the sample that
has been reduced at 600 ◦C, indicating that surface CeO2 is not
reduced within the tested reduction temperature range [31]. How-
ever, changes have been observed in the oxidation state of surface
Fig. 7. In situ Raman spectra collected over (a) Co/CeO2(E) and (b) Co/CeO2(A) during
reduction process.

H. Song, U.S. Ozkan / Journal of Molecular Catalysis A: Chemical 318 (2010) 21–29
27
Fig. 8. The Ce (3d) and Co (2p) XPS spectra collected over Co/CeO2(E) after (a) cal-
cination at 450 ◦C for 3 h, (b) reduction at 400 ◦C for 2 h, and (c) reduction at 600 ◦
for 2 h.
C
Fig. 9. The C (1s) and O (1s) XPS spectra collected over Co/CeO2(E) after (a) calcina-
tion at 450 ◦C for 3 h, (b) reduction at 400 ◦C for 2 h, (c) reduction at 600 ◦C for 2 h,
and (d) over Co/CeO2(A) after calcination at 450 ◦C for 3 h.
peak at 780.2 eV shows the existence of metallic Co on the sur-
face [35], implying that after the reduction treatment at 400 ◦C for
2 h, the Co3O4 particles have been reduced to a mixture of CoO
and metallic Co. The spectrum taken following reduction at 600 ◦C
shows a higher intensity for metallic Co signal, but Co2+ signal is still
present. Although at 600 ◦C, reduction of the cobalt oxide phase
to metallic Co is expected to be complete, presence of Co-oxide
species can be due to partial reoxidation of the surface by lattice
oxygen from the support. The oxygen mobility in CeO2 has been
demonstrated before in several different systems [16,36,37].
In order to evaluate the surface concentrations of the Co and
Ce elements before and after reduction and compare this ratio
between the two catalysts evaluated, the Co/Ce atomic ratios deter-
mined by XPS results have been quantified and tabulated in Table 3.
As can be seen, the Co/CeO2(E) has significantly higher Co/Ce ratio
than its counterpart both before and after reduction, indicating bet-
ter cobalt dispersion over Co/CeO2(E), which is consistent with H2
chemisorption results, which showed 9.4% and 15.1% Co dispersion
for Co/CeO2(A) and Co/CeO2(E), respectively. In addition, it is worth
noting that although Co concentration on the surface decreases for
both of the catalysts following reduction, the % decrease is larger for
the Co/CeO2(A) sample, suggesting a sintering effect during reduc-
tion over this catalyst. The reduction temperature did not affect
much the Co concentration on the surface of the Co/CeO2(E) when
it was reduced at 600 ◦C. There was no additional decrease in the
Co surface concentration over this catalyst compared to what was
obtained following the lower temperature reduction process.
The XPS spectra taken over the Co/CeO2(A) (data not shown)
exhibited features similar to those of Co/CeO2(E). When the C 1s
and O 1s regions of the spectra were compared, however, the two
catalysts showed major differences (Fig. 9). C 1s spectrum showed
a feature at a binding energy of 287.8 eV over the Co/CeO2(E) cata-
lyst. This peak was observed in all three spectra, the one taken over
the fully oxidized sample as well as the ones taken following reduc-
tion at 400 and 600 ◦C. Although a weak peak also appeared around
287 eV over the Co/CeO2(A) sample, it was much weaker and close
to 1 eV apart. When O 1s spectra were compared, the Co/CeO2(E)
catalyst showed a peak at 531.1 eV, which remained even after
reduction at 600 ◦C. This peak was absent in the O 1s spectrum
taken over the sample prepared in aqueous media. Appearance of

28
H. Song, U.S. Ozkan / Journal of Molecular Catalysis A: Chemical 318 (2010) 21–29
and aldol condensation-type reactions, which would lead to coking
and acetone formation, respectively. A related possibility is that the
presence of acetate species on the surface of the catalysts prior to
any reactions may make it easier for ethanol to be converted to sur-
face acetate species, by following an “imprint” left on the surface
during the impregnation step. The concept of molecular imprint-
ing on metal complexes has been previously reported in different
studies [38–42] and many of these studies have been summarized
in a review article by Tada and Iwasawa [43]. In these studies, usu-
ally ligands are used as template molecules, which, when removed,
leave behind cavities near the metal site. One of the possible mech-
anisms through which such imprinting can help is by providing
“shape-selective reaction space on the active metal center” [38].
It is conceivable that what is observed in our study stems from
such a phenomenon where the acetate-type surface species formed
during the impregnation step may provide necessary surface geom-
etry for the selective reaction pathway in ethanol steam reforming.
However, the current results do not provide sufficient evidence to
prove or to refute any of these aforementioned possibilities.
4. Conclusions
Fig. 10. DRIFTS spectra of Co/CeO2(E) at various life stages.
The Co–CeO2 catalysts prepared in ethanol medium showed sig-
nificant improvement in catalytic performance (higher H2 yield,
higher stability and fewer side reactions) compared to the ones
prepared in aqueous media. Characterization results showed the
presence of oxygenated carbon species, possibly metal-coordinated
acetates, on the surface. These species are likely to play a role in the
improved performance. Although the nature of this role is not clear
at this point, possibilities include a segregation effect that prevents
sintering, a site-blocking effect that suppresses the side reactions,
or an “imprinting” effect that makes it easier for the acetate inter-
mediates to form on the surface. Additional studies are needed to
elucidate the exact role played by these surface species resulting
from the impregnation medium.
these additional peaks in the O 1s and C 1s spectra suggests the
presence of some oxygenated carbon species on the surface of the
catalyst prepared in organic media [35]. It should be noted that the
presence of a shoulder at 533.5 eV in O 1s spectrum over the fully
oxidized sample and the noticeable change in the relative intensi-
ties of the C 1s peaks suggest that these species go through some
transformation during the reduction steps, but are stable enough
to remain on the surface even when exposed to a high-temperature
reducing atmosphere.
The presence of oxygenated carbonaceous species was further
verified by the DRIFTS experiments performed over the samples
prepared in different media. Fig. 10 shows the difference spectra
plotted by subtracting the spectrum collected over the sample pre-
pared in aqueous media from the ones taken over the Co/CeO2(E)
sample, following three different pre-treatment steps. The band
located at 1787 cm−1 suggests the presence of a carbonyl group
(ꢁ(C O)), possibly in a carboxylate species. However, this band is
Acknowledgement
We gratefully acknowledge the funding from the U.S. Depart-
ment of Energy through the grant DE-FG36-05GO15033.
much weaker compared to the ones located at 1552 and 1450 cm−1
.
References
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880 cm−1, suggests an acetate species, possibly coordinated to a
metal center. Formation of acetate species has been identified as the
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Although there are slight shifts in the band position when the
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media. One possibility is that the presence of these organic ligands
may keep the Co particles segregated, ensuring a high level of dis-
persion throughout their life history. The other possibility is that
these species may be blocking the sites that lead to the dehydration
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