Conversion of ethanol over supported cobalt oxide catalysts

Article abstract of DOI:10.1016/j.cattod.2011.06.015

Conversion of ethanol was investigated on supported (ceria, zirconia and ceria-zirconia) cobalt oxide catalysts. The catalysts were prepared by support impregnation with cobalt nitrate-citric acid solution and they were explored by comparing results from different characterization techniques: X-ray fluorescence, X-ray diffraction, Raman spectroscopy and nitrogen adsorption techniques. Their catalytic properties at 693 K were characterized in a fixed-bed reactor. The CoO_x/CeO₂ catalyst displayed the highest catalytic activity. The conversion of ethanol decreased with the increase of the ZrO₂/CeO₂ ratio in the support of catalyst. All catalysts exhibited high selectivity of ethanol conversion to hydrogen and acetone. The coking of catalysts under reaction conditions was also characterized by gravimetric method. The results indicated that the increase of the ZrO₂/CeO₂ ratio in the support exerts significant influence on the coke formation. The amount of carbon deposited on CoO _x/ZrO₂ at 693 K was higher than on any other catalyst. Raman studies of used catalysts proved that their surface was almost completely covered with carbonaceous deposit, which was probably the main reason of deactivation of catalysts under reaction conditions.

Full text of DOI:10.1016/j.cattod.2011.06.015

Catalysis Today 176 (2011) 14–20
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Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
Conversion of ethanol over supported cobalt oxide catalysts
P. Rybak∗, B. Tomaszewska, A. Machocki, W. Grzegorczyk, A. Denis
The Maria Curie-Sklodowska University, Faculty of Chemistry, Department of Chemical Technology, 3 Maria Curie-Sklodowska Square, 20-031 Lublin, Poland
a r t i c l e i n f o
a b s t r a c t
Article history:
Conversion of ethanol was investigated on supported (ceria, zirconia and ceria–zirconia) cobalt oxide cat-
alysts. The catalysts were prepared by support impregnation with cobalt nitrate–citric acid solution and
they were explored by comparing results from different characterization techniques: X-ray fluorescence,
X-ray diffraction, Raman spectroscopy and nitrogen adsorption techniques. Their catalytic properties at
693 K were characterized in a fixed-bed reactor. The CoOx/CeO2 catalyst displayed the highest catalytic
activity. The conversion of ethanol decreased with the increase of the ZrO2/CeO2 ratio in the support
of catalyst. All catalysts exhibited high selectivity of ethanol conversion to hydrogen and acetone. The
coking of catalysts under reaction conditions was also characterized by gravimetric method. The results
indicated that the increase of the ZrO2/CeO2 ratio in the support exerts significant influence on the coke
formation. The amount of carbon deposited on CoOx/ZrO2 at 693 K was higher than on any other catalyst.
Raman studies of used catalysts proved that their surface was almost completely covered with carbona-
ceous deposit, which was probably the main reason of deactivation of catalysts under reaction conditions.
Received 28 September 2010
Received in revised form 28 February 2011
Accepted 16 June 2011
Available online 14 July 2011
Keywords:
Ethanol
Conversion
Cobalt oxide-based catalysts
Support nature
Deactivation
Crown Copyright © 2011 Published by Elsevier B.V. All rights reserved.
1. Introduction
expensive alternative. Several oxides such as Al2O3, MgO, SiO2,
ZrO2, and ZnO have been used as a cobalt support in steam reform-
The issues of environmental protection and the growing
demand for energy forced people to search for new renewable
energy sources. Hydrogen produced from bioethanol in the steam
reforming process may become such future fuel. Currently, more
than 90% of hydrogen is being produced from natural gas [1–3]
or light oil fraction [4]. The use of these unrenewable resources
is associated with the emission of greenhouse gases, mainly CO2,
whereas hydrogen produced from ethanol would not only be envi-
ronmentally friendly but also renewable. Furthermore, ethanol is
becoming increasingly available and is free from catalyst poisons
such as sulphur compounds [5–9].
A catalyst plays an important role in achieving complete and
selective ethanol conversion and is essential for the process to
be economically acceptable. In scientific literature there is no
strong tendency to use a specific composition of the catalyst that
would be most profitable. Various catalytic systems are taken into
consideration: transition metals (Ni and Co), noble-metals and
bimetallic-based as well as various oxides [5,10–12]. Noble met-
als (Rh, Ru, Pt, and Pd) supported catalysts are very active in steam
reforming of ethanol, but the high cost of these materials limits
their application. Cobalt-based catalysts proof to be a much less
ing of ethanol [13–17]. Furthermore, the use of cerium oxide as
the support has also been reported. The main advantage of ceria
is its oxygen storage capacity and high oxygen mobility associated
with oxygen vacancies [18–27]. The insertion of ZrO2 into Co/CeO2
structure has been found to enhance the redox properties as well as
thermal stability and good anti-carbon deposition ability of these
catalysts [28].
Due to the high effectiveness of ceria oxide used as a support
and the low cost of cobalt, these materials can form a highly active
and selective catalytic system [16,29–37]. However, this system is
associated with a major problem which is the deactivation of the
catalysts during steam reforming of ethanol due to carbon deposits
[31,32,37–40]. In particular Co/CeO2 was investigated for this rea-
son. Wang et al. [32] studied the deposition of carbon during steam
reforming of ethanol over Co/CeO2. They observed that cobalt par-
ticles are completely covered with coke and suggested that this
phenomenon was caused by ethylene which played a role of a
carbon deposition precursor. The mechanism of Co/ceria catalyst
deactivation was studied by de Lima et al. [30]. They suggested that
at high temperature acetaldehyde is converted to acetate via sup-
port bound –OH. Next, steam promotes decomposition of acetate to
carbonate and CH4, the latter of which decomposes to carbon and
H2. Moreover, in the absence of steam the dehydration to ethylene
is favored.
∗
Corresponding author at: Uniwersytet Marii Curie-Skłodowskiej, Wydział
Jilei et al. [41] prepared the catalyst with the use of citric com-
plexes of cerium, zirconium and nickel. They studied the activity of
NiO/Ce0.5Zr0.5O2 during the steam reforming process as a function
Chemii, Zakład Technologii Chemicznej, Plac Marii Curie-Skłodowskiej 3, 20-031
Lublin, Poland. Tel.: +48 815375514/815375596; fax: +48 815375565.
E-mail address: rybak-piotr@o2.pl (P. Rybak).
0920-5861/$ – see front matter. Crown Copyright © 2011 Published by Elsevier B.V. All rights reserved.
doi:10.1016/j.cattod.2011.06.015

P. Rybak et al. / Catalysis Today 176 (2011) 14–20
15
of reaction temperature. They reported that the catalyst prepared
by citrate method was more active and selective for hydrogen pro-
duction at low temperature compared to catalysts prepared by
co-precipitation method. In addition, that catalyst demonstrates
satisfactory stability, good carbon resistance and anti-sintering
properties.
The main results and discussion in the papers listed above were
obtained with reduced catalysts, however there are no results
concerning unreduced catalysts. The goal of this article was to
determine the influence of the nature of the support (ceria, zir-
conia and ceria–zirconia) and the cerium modifier on the activity
and selectivity of unreduced cobalt oxide-based catalysts and on
their stability in the ethanol–water conversion.
products of the reaction were analysed with two on-line gas chro-
matographs. One of them, Varian CP-3800, was equipped with two
detectors – a FID and a TCD – as well as two capillary columns:
Poropak-Q (for all organics, carbon dioxide and water vapour) and
Carbosieve (for methane and carbon monoxide analysis). Helium
was used as carrier gas. Hydrogen was analysed by the second gas
chromatograph, Chromatron GCHF-18.3, equipped with a charcoal
packed column, nitrogen as a carrier gas and a TCD detector.
The total conversion of ethanol XEtOH, conversion of water XH
O
and conversions of ethanol into particular carbon-containing pro2d-
ucts, XCP, were calculated on the basis of their concentrations
before and after the reaction, with a correction introduced for
the change in gas volume during the reaction, from the below
equations:
CHin O − CHoutOK
in
out
C
− C
C
K
2. Experimental
EtOH
EtOH
2
2
XEtOH
=
=
× 100%; XH
=
× 100%;
O
2
in
CHin
O
2
EtOH
The
CoOx/CeO2,
CoOx/ZrO2,
CoOx/Ce0.75Zr0.25O2,
CCoPutK
CoOx/Ce0.6Zr0.4O2 and CoOxCeO2/ZrO2 catalysts were prepared by
the impregnation method. Prior to the impregnation all supports
(Aldrich) were dried at 393 K for 3 h. The solutions of cobalt nitrate
and citric acid CA (the relative molar concentrations of Co and
CA were 1/1) were used for impregnation. In the case of the
CoOxCeO2/ZrO2 catalyst the support was impregnated with cobalt
and cerium nitrates and citric acid aqueous solution (the Co/Ce and
Co + Ce/CA molar ratios were 1/1). After impregnation, the catalyst
precursors were dried at 393 K for 12 h, then calcined at 673 K
with heating rate of 2.2 K min−1 up to the calcination set point and
maintained for 1 h at this temperature.
The X-ray fluorescence spectroscopy (XRF) technique was used
to determine the bulk contents of cobalt in the catalysts. The spectra
of the samples were obtained with the XRF spectrometer (Can-
berra 1510) equipped with a liquid nitrogen-cooled Si(Li) detector.
The AXIL software package for spectra deconvolution and for the
calculation of cobalt contents was used.
X-ray diffraction patterns of the catalysts were taken by a Zeiss
HZG-4 diffractometer using Mn-filtered CuK␣ radiation. The sam-
ples were scanned by the step-by-step technique, at 2ꢀ intervals of
0.05◦ and recording time of 10 s for each step. The diffraction data
were collected at 20–85◦. The Joint Committee on Power Diffraction
Standards (JCPDS) database was used for the phase identification.
The Raman spectra were recorded with a resolution of 2 cm1
in the inVia Raman microscope Renishaw with Raman dispersion
system, using a semiconducting laser 785 nm (3 mW), working in
a back scattered confocal arrangement. The spectra of all samples
were recorded at room temperature.
The Brunauer–Emmett–Teller (BET) surface area of the catalysts
was determined by low-temperature (77 K) nitrogen adsorp-
tion in the ASAP 2405N v1.0 analyser (Micromeritics). The
pore volume and their diameter were evaluated applying the
Barrett–Joyner–Halenda (BJH) method.
XCP
× 100%
in
(n/2)C
EtOH
in
where C
and CHin represent the molar concentrations of
O
2
EtOH
out
EtOH
ethanol and water in the reaction mixture, mol%; C
and
CHout are the molar concentrations of ethanol and water in the
O
2
out
CP
post-reaction mixture, mol%; C
is the molar concentration of
carbon-containing products in the post-reaction mixture, mol%; n
is the number of carbon atoms in the carbon-containing molecule
of the reaction product; K is the volume contraction factor (K =
Ccin/Ccout where: Ccin and Ccout are the molar concentrations of car-
bon in ethanol fed to the reaction and in all carbon-containing
compounds which were present in post-reaction gases, respec-
tively). The selectivity of ethanol conversion into individual
carbon-containing products was expressed as (XCP/XEtOH) × 100.
The carbon mass balances, based on the carbon selectivities
at each of the reaction temperatures, were close to 100 3%.
The selectivity of hydrogen formation was determined from the
equation:
CHout
+ 2CCout +22Cout CHO + 3Cout
H2-selectivity =
out
4
CHin + 2C
CH
H
CH
3
(CH3) CO
2
2
4
2
× 100%
where Cout refers to the molar concentrations of the hydrogen-
containing reaction products, mol%.
The gravimetric method was used to determine the catalyst cok-
ing under the ethanol steam reforming conditions. The experiments
were conducted in the TG121 microbalance system (Cahn) in a
quartz reactor with a continuous flow of ethanol–water vapours.
The molar ratio of ethanol and water vapours was 1:4.
Temperature-programmed reduction (TPR) experiments were
carried out with the Altamira Instruments apparatus equipped with
a TCD detector. The reduction profiles were obtained by passing
6% H2/Ar flow at the rate of 30 mL min−1 through 0.05 g of cat-
alyst (0.1–0.2 mm). The temperature was increased from 290 to
800 K at the rate of 10 K min−1. The water vapour that formed dur-
ing reduction was removed in a cold trap (immersed in a liquid
nitrogen–methanol slush) placed before the thermal conductivity
detector (TCD).
The reaction of ethanol conversion was carried out at 693 K
under atmospheric pressure, in a fixed-bed continuous-flow quartz
reactor, using 0.1 g (0.15–0.3 mm) of catalyst diluted (1/10, w/w)
with 0.15–0.3 mm grains of quartz. The vapours of ethanol and
water were supplied to the reactor with nitrogen as a carrier
gas. Molar ratio of EtOH/H2O/N2 was equalled to 5/20/75. The
3. Results and discussions
Table 1 shows that all catalysts used in the conversion of ethanol
had a high surface area 40–60 m2/g; only CoOx/Ce0.75Zr0.25O2 was
characterized by 120 m2/g. The catalyst CoOx/Ce0.75Zr0.25O2 exhib-
ited the highest porosity. The pore diameter of that catalyst was
7.7 nm and the pores volume – 0.284 cm3/g. In the case of the
CoOx/CeO2 catalyst the pore diameter was similar and amounted to
8.8 nm. It can be noticed that the higher content of zirconia in the
support, the larger pore diameter could be expected. In the case
of CoOx/Ce0.6Zr0.4O2 the pore size was 10.7 nm and it increased
to 19.1 nm for CoOx/ZrO2. The cobalt content was in the range of
8–10 wt.%.
Fig. 1A shows XRD patterns of cobalt and cobalt–cerium
supported catalysts. Ceria and ceria–zirconia (with high cerium

16
P. Rybak et al. / Catalysis Today 176 (2011) 14–20
Table 1
Results of characterization of cobalt oxide-based catalysts.
Catalysts
BET surface area (m2/g)
Average diameter of pores (nm)
Volume of pores (cm3/g)
Cobalt (wt.%)
CoOx/CeO2
66.9
118.2
56.0
40.5
63.5
8.8
7.7
10.7
19.1
9.4
0.1907
0.2840
0.1923
0.2411
0.1748
7.89 0.32
9.61 0.38
8.59 0.34
8.19 0.33
9.07 0.36
CoOx/Ce0.75Zr0.25O2
CoOx/Ce0.6Zr0.4O2
CoOx/ZrO2
CeOxCoOx/ZrO2
content) supports enabled us to obtain a very high dispersion of
the cobalt oxide phase. Similar effect was achieved when cobalt
was deposited simultaneously with cerium from the same solution
on the zirconia support (the case of the CoOxCeO2/ZrO2 catalyst).
Neither Co3O4 nor Co2O3 or CoO peaks could be seen in the XRD pat-
terns for ceria-contained supported cobalt catalysts. The absence of
the peaks of cobalt oxides proves their very small crystallite size.
Only in the case of CoOx/ZrO2, it was possible to determine the
cobalt oxide crystallite size, which was 9.2 nm. The XRD study also
revealed the presence of a new phase, which we have not identified
yet.
Fig. 1B displays the Raman spectra of the catalysts. They reveal
five bands in the range of 100–1200 cm−1. The signals located at
approximately 196, 484, 522, 622, and 693 cm−1 correspond to the
F2g, Eg, F2g, F2g, and A1g modes, respectively, of the crystalline Co3O4
phase [42–44]. It is known that the Raman spectrum for pure ceria
displays only one peak at 465 cm−1, as expected on the basis of the
factor group analysis and in agreement with the literature [45–55].
This peak corresponds to the triply degenerate F2g mode of fluorite
CeO2. Furthermore, the pure zirconium oxide exhibits peaks at 182
(Ag), 333 (Bg), 377 (Bg), 475 (Ag), 559 (Ag), and 634 cm−1 (Ag) char-
acteristic for monoclinic ZrO2 [56–58] and six Raman bands at 145
(B1g), 265 (Eg), 313 (B1g), 460 (Eg), 600 (B1g) and 645 cm−1 (Eg) pre-
dicted theoretically for tetragonal ZrO2 [48,51–55,59,60,57,61]. It
can be observed in the Raman spectra of our catalysts that, except
for the ceria supported CoOx/CeO2 catalyst, there are no signals
derived from any of the support. Only a small peak of ceria is visible
at 465 cm−1. Additionally, there occurs no masking effect associ-
ated with cobalt oxide. The presence of only very strong Raman
signals of Co3O4 suggests that the surface of the catalysts is com-
pletely covered with crystallites of Co3O4.
The temperature-programmed reduction profiles (Fig. 2) for all
catalysts were complex and difficult to interpret unambiguously.
They show two groups of peaks: the low-temperature group (in
the range of 423–573 K) attributed to the reduction of Co3O4 to
CoO and then to metallic cobalt and the second group in the range
of 573–673 K [28,33]. The reduction of some amounts of cobalt
oxides in all catalysts was very hard. Those more difficult reducible
cobalt oxide forms are most probably strongly interacting with the
support (e.g., as a solid solution). Liu et al. [62] studied the soot
catalytic combustion over the nanometric CeO2-supported cobalt
oxide catalysts prepared by means of ultrasonic-assisted incipient-
wetness impregnation. They reported that the structures of the
catalyst were altered with the loading amount of cobalt. At low Co
loading, cobalt oxide existed as CoO or Co–Ce–O solid solution, and
the medium or high Co loading resulted in the presence of Co3O4
spinel. The existence of a strong interaction between cobalt oxide
species and the mesoporous support CeO2 was also concluded by
Dongsheng et al. [63]. They studied CO and CH4 catalytic oxidation
over CeO2–MOx (M = Cu, Mn, Fe, Co, and Ni) prepared by a cit-
ric acid complexation–combustion method. Their XRD and Raman
results confirmed presence of the solid solutions between CeO2 and
Co3O4. As reported in the literature, ceria is characterized by a low-
temperature peak and high-temperature peak of reduction with
hydrogen [64,65]. In the case of our ceria-containing catalysts the
reduction of ceria is seen only in the case of CoOx/CeO2 above 973 K.
de la Piscina et al. [66] studied the steam reforming of ethanol
over cerium oxide. They carried out the process between 573 and
723 K, at atmospheric pressure, using a 1:13:70 C2H5OH:H2O:Ar
stream (molar ratio). The conversion of ethanol increased with
temperature and at 723 K, it reached the value of 24%. The main
products obtained after 20 h of reaction were hydrogen, ethylene,
Fig. 1. (A) XRD patterns of ceria-, ceria–zirconia- and zirconia-supported cobalt oxide and cobalt oxide–cerium oxide catalysts: ꢀ, CeO2; ♦, Ce0.75Zr0.25O2; ꢀ, Ce0.6Zr0.4O2;
ꢁ, ꢁ ZrO2; ᭹, Co3O4; ?, unknown phase; (B) Raman spectra of ceria-, ceria–zirconia- and zirconia-supported cobalt oxide and cobalt oxide–cerium oxide catalysts.

P. Rybak et al. / Catalysis Today 176 (2011) 14–20
17
the case of CoOx/Ce0.75Zr0.25O2 and CoOx/Ce0.6Zr0.4O2 catalysts this
selectivity was very similar. The lowest selectivity, below 20%, was
for CoOx/ZrO2. Acetone, acetaldehyde and small amounts of other
organic by-products, as well as traces of carbon monoxide were
also formed in side reactions. In particular, the selectivity towards
acetone appeared exceptionally high, initially even above 60%. The
presence of cerium oxide in the support favored high selectivity of
acetone formation. That direction of ethanol conversion was less
significant for the CoOx/ZrO2 catalyst.
The presence of CeO2 in the cobalt oxide phase can be consid-
ered a valuable modification of this phase in the zirconia supported
catalyst. It resulted in the CoOxCeO2/ZrO2 catalyst being character-
ized by smaller cobalt oxide crystallite size (no signal in the X-ray
diffraction pattern) and by higher BET surface area (63.5 m2/g) than
in the case of the CoOx/ZrO2. The pore diameter was 9.4 nm and it
was lower in comparison with CoOx/ZrO2 (Table 1). Those positive
facts also caused a change in the activity of the catalyst (Fig. 2). The
selectivity of the ethanol conversion to hydrogen and carbon diox-
ide increased by 10%, parallel to the decrease of selectivity towards
acetaldehyde, ethylene and methane with respect to CoOx/ZrO2
(Fig. 3).
The stability of activity and selectivity is an important character-
istic of any catalyst since it determines its efficient use in a reaction.
A decrease in stability of catalysts may be attributed to the depo-
sition of coke and sintering of the active phase. Ethylene is known
as a strong coke precursor and carbon formation appeared primar-
ily due to its presence in the product stream. It should be noted
that the selectivity of ethylene formation is inversely proportional
to the activity of catalysts and the quickness of their deactivation
which can be observed. This relation is most probably connected
with carbonaceous deposits formation [30–32]. The weight of coke
or carbon deposited on catalysts during ethanol conversion was
obtained from TG results (Fig. 4A). At first, the catalysts were heated
to constant weight at 623 K. Then the catalysts were treated in the
stream of ethanol and water vapours (molar ratio H2O/EtOH = 4:1)
and heated to 693 K. It can be observed that TG profiles are a
product of two phenomena: the reduction of catalysts and car-
bon deposition at 693 K. The rate and amount of carbon deposited
on CoOx/ZrO2 were significantly higher than on any other catalyst.
Moreover, initial conversion of ethanol was the lowest on this cat-
alyst (Fig. 3). Relatively large amounts of coke were also deposited
over CoOx/Ce0.6Zr0.4O2, but with much lower rate than in the case of
the CoOx/ZrO2 catalyst. Ethanol conversion decreased very quickly
on that catalyst (Fig. 3). The formation of coke deposits was also
confirmed by the Raman spectroscopy of used catalysts after the
Fig. 2. TPR profiles of ceria-, ceria–zirconia- and zirconia-supported cobalt oxide
and cobalt oxide–cerium oxide catalysts.
carbon dioxide and acetone. Also de Lima et al. [67] studied the
steam reforming of ethanol over CeZrO2 (Ce/Zr ratio = 3.0) at 773 K
(residence time = 0.02 g s/mL). They concluded that the ethanol
conversion was nearly 70% on CeZrO2 and it was quite stable during
6 h (H2O/ethanol molar ratio 5.0). The product distributions exhib-
ited mainly formation of ethylene and hydrogen. Trace amounts
of carbon dioxide, methane and acetaldehyde were also detected,
while the formation of CO was not observed. These researches
suggested that cerium oxide shows small activity in the steam
reforming of ethanol. The coupling of cerium oxide and zirco-
nium oxide forms an active catalytic system but this system mainly
exhibits the formation of ethylene (a strong coke precursor).
The catalytic behaviour of our catalysts in the ethanol steam
reforming conditions is presented in Fig. 3 in terms of ethanol
and water conversions and selectivities versus reaction time.
The initial activity of catalysts formed the following order:
CoOx/CeO ≈ CoOx/Ce0.75Zr0.25O > CoOx/Ce0.6Zr0.4O > CoOxCeO /ZrO >reCaocOtio/nZroOf e. thanol conversion. Fig. 4B shows the Raman spectra
x
2
2
2
2
2
2
The content of zirconium oxide in the catalyst support had a signif-
icant effect on the conversion of ethanol and water. The conversion
of ethanol decreased parallel to the increase of the ZrO2/CeO2
ratio. In terms of stability of catalytic properties, all catalysts
deactivated with time on stream under reaction conditions and
the ethanol conversion decreased considerably within a few hours.
Among the tested catalysts, CoOx/CeO2 and CoOx/Ce0.75Zr0.25O2
displayed the highest stability. Over the CoOx/CeO2 catalyst, the
initial conversion of ethanol was 70% and it decreased after 20 h of
reaction to 60% at 693 K. In the case of the most unstable catalyst,
CoOx/Ce0.6Zr0.4O2, the conversion of ethanol decreased from 60%
to 20% after 20 h, most probably due to intensive coking and/or
sintering of the catalyst. Over oxide form of cobalt catalysts, the
steam reforming of ethanol occurs only to the limited extent,
which is suggested by low water conversion.
taken over CoOx/CeO2, CoOx/ZrO2 and CoOxCeO2/ZrO2 after 25 h
on the steam reforming at 693 K. Two bands are observed around
1312 cm−1 and 1599 cm−1 and these signals are characteristic of
disordered carbonaceous and ordered graphitic species [68,23]. No
bands of cobalt oxide or support oxides were seen. Those results
revealed that the surface of catalysts was completely covered by
coke. We may conclude that the coke formation and encapsulation
of catalyst particles are justified reasons of deactivation of catalysts
in the ethanol conversion reaction.
Song and Ozkan [23] have conducted research on Co/ZrO2 cata-
lysts in the steam reforming of ethanol. They obtained a complete
conversion of ethanol at 400 ◦C (EtOH:H2O = 1:10 (molar ratio)),
while that catalyst deactivated rapidly due to carbon deposition
on the surface. Addition of 10% CeO2 to the support maintained its
stable activity and also imparted higher reforming activity to the
catalyst. The main products obtained after 25 h of reaction were
hydrogen and carbon dioxide on Co/10%CeO2–ZrO2. Also Zhang
et al. [33] studied the steam reforming of ethanol over Co/CeO2.
They concluded that the ethanol conversion was complete on this
catalyst at 400 ◦C (EtOH/H2O/He = 2/18/80 vol%) and it was quite
The influence of the nature of the support had no significant
impact on the selectivity of ethanol conversion to the most desir-
able products – hydrogen and carbon dioxide (Fig. 3). They were in
the range of 56–64% and 18–25%, respectively. The CoOx/CeO2 dis-
played the highest selectivity of ethanol conversion to CO2 (25%). In

18
P. Rybak et al. / Catalysis Today 176 (2011) 14–20
Fig. 3. Conversion of ethanol and water and selectivity to the reaction products over supported cobalt oxide-based catalysts (693 K, EtOH/H2O/N2 = 5/20/75 mol/mol/mol):
ꢁ, CoOx/CeO2; ꢁ, CoOx/Ce0.75Zr0.25O2; ꢀ, CoOx/Ce0.6Zr0.4O2; ♦, CoOx/ZrO2; ×, CoOxCeO2/ZrO2.
stable. The product distribution exhibited mainly the formation
of hydrogen and carbon dioxide, which was constant during the
reaction. Slight amounts of methane and acetaldehyde were also
detected. The main results and discussions in the above mentioned
papers were obtained over reduced catalysts and for this reason
they are not easily comparable with our results. Our studies of
unreduced cobalt oxide-based catalysts showed that there is a
possibility to obtain not only hydrogen but also acetone. We also

P. Rybak et al. / Catalysis Today 176 (2011) 14–20
19
Fig. 4. (A) Thermogravimetric profiles of ꢁ, CoOx/CeO2; ꢁ, CoOx/Ce0.75Zr0.25O2; ꢀ, CoOx/Ce0.6Zr0.4O2; ♦, CoOx/ZrO2; ×, CoOxCeO2/ZrO2 under reaction conditions (the molar
ratio of ethanol and water vapours was 1:4); (B) Raman spectra of ceria- and zirconia-supported cobalt oxide and cobalt oxide–cerium oxide catalysts after the ethanol–water
reaction.
obtained the improvement of stability of zirconia supported cobalt
catalysts by addition of cerium into the cobalt phase. Moreover, the
clear relation between the amount of ceria in the support and the
stability of catalysts could be observed. The more ceria oxide was
in the support, the higher stability had the catalyst.
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4. Conclusions
It was shown that there is a possibility to obtain not only hydro-
gen but also acetone over unreduced cobalt oxide-based catalysts
from bio-ethanol. The conversion of ethanol and its selectivity are
strongly influenced by the nature of the support. The CoOx/CeO2
and CoOx/Ce0.75Zr0.25O2 catalysts enabled us to achieve the high-
est and most stable activity measured in the conversion of ethanol,
whereas the presence of higher amounts of zirconium oxide in the
support resulted in a decrease in conversion and the CoOx/ZrO2
catalyst was the least active. An improvement of the catalytic prop-
erties of the latter catalyst is possible by modifying its active phase
with cerium. However, all catalysts demonstrated some instabil-
ity of their activity, which was directly proportional to the amount
of zirconia in the catalyst support. The most significant reason for
the diminishing stability was most probably the coke deposition.
Its formation seems to be connected with the polymerization of
ethylene on the catalyst surface.
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Acknowledgements
These results have been achieved within the framework of
the 1st call on Applied Catalysis carried out by ACENET ERA-NET
(project ACE.07.009), with funding from the Ministry of Science
and Higher Education of Poland.
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