The preferential oxidation of CO in excess hydrogen: A study of the influence of KOH/K2CO3 on CuO-CeO2-x catalysts

Article abstract of DOI:10.1016/j.molcata.2006.04.003

The preferential oxidation of CO in rich hydrogen has been studied for CuO-CeO_2-x catalysts. The catalysts were prepared by co-precipitation with K₂CO₃ + KOH as precipitators and characterized using N₂ physisorption isotherms, high resolution transmission electron microscopy (TEM), and X-ray diffraction (XRD). The results indicate that nano-structured CuO-CeO_2-x catalysts are obtained, and that the ratio of KOH/K₂CO₃ has a beneficial influence on the properties of CuO-CeO_2-x catalysts, i.e., with the addition of KOH, the particle sizes grow smaller, CuO-CeO_2-x catalysts have larger surface areas, CuO species in CuO-CeO_2-x are more easily reduced and achieve higher catalytic performance in the preferential oxidation of CO in rich hydrogen. CO conversion higher than 99% with O₂ selectivity of 100% is obtained for CuO-CeO_2-x catalysts with KOH/K₂CO₃ equal to 4:0 at a temperature of 90-110 °C and a space velocity of 30,000-120,000 ml g^-1 h^-1.

Full text of DOI:10.1016/j.molcata.2006.04.003

Journal of Molecular Catalysis A: Chemical 255 (2006) 103–108
The preferential oxidation of CO in excess hydrogen: A study of the
influence of KOH/K2CO3 on CuO–CeO2−x catalysts
Zhigang Liu, Renxian Zhou∗, Xiaoming Zheng
Institute of catalysis, Zhejiang University, Hangzhou 310028, PR China
Received 24 November 2005; received in revised form 1 April 2006; accepted 4 April 2006
Available online 8 May 2006
Abstract
The preferential oxidation of CO in rich hydrogen has been studied for CuO–CeO2−x catalysts. The catalysts were prepared by co-precipitation
with K2CO3 + KOH as precipitators and characterized using N2 physisorption isotherms, high resolution transmission electron microscopy (TEM),
and X-ray diffraction (XRD). The results indicate that nano-structured CuO–CeO2−x catalysts are obtained, and that the ratio of KOH/K2CO3 has a
beneficial influence on the properties of CuO–CeO2−x catalysts, i.e., with the addition of KOH, the particle sizes grow smaller, CuO–CeO2−x catalysts
have larger surface areas, CuO species in CuO–CeO2−x are more easily reduced and achieve higher catalytic performance in the preferential oxidation
of CO in rich hydrogen. CO conversion higher than 99% with O2 selectivity of 100% is obtained for CuO–CeO2−x catalysts with KOH/K2CO3
equal to 4:0 at a temperature of 90–110 ◦C and a space velocity of 30,000–120,000 ml g−1 h−1
© 2006 Elsevier B.V. All rights reserved.
.
Keywords: Preferential oxidation; CO; Excess hydrogen; CuO–CeO2−x; Precipitator
1. Introduction
ied by Avgouropoulos et al. [13] and Kim and Cha [14]. The
catalysts are prepared with Na2CO3 or NaOH solution as pre-
cipitators and show superior performance in the preferential
oxidation of CO in excess hydrogen at the temperature below
170 ◦C for a feed of 1% CO, 1% or 1.25% O2, 50% H2 in the
presence of H2O and CO2.
In this work, we prepared the nano-structured CuO–CeO2−x
catalysts by co-precipitation with different ratios of
KOH/K2CO3 as precipitators, and the preferential oxida-
tion properties of CO in rich hydrogen over the catalysts were
studied.
Ceria has been widely used in catalytic systems for the elimi-
nationofthepollutantscontainedintheexhaustgasesofautomo-
biles [1]. This material contains a high concentration of highly
mobile oxygen vacancies, which act as local sources or sinks for
oxygen involved in reactions taking place on its surface [2].
Recently, CeO2−x supported CuO catalyst has been attracting
attentions because of its activity higher than that of conventional
copper-based catalysts and comparable or superior to platinum
catalysts for the preferential oxidation of CO in excess hydrogen
[3–5]. It is believed that the reaction is catalyzed by the inter-
facial copper oxide–ceria centers in which ceria presents a high
number of oxygen vacancies that permits a high mobility of lat-
tice oxygen [6,7], and the properties of CuO–CeO2−x catalysts
have a great dependence on the preparation methods.
2. Experimental
2.1. Catalyst preparation
For CuO–CeO2−x catalysts, a number of preparation meth-
ods have been described, including co-precipitation [8,9], urea-
nitrate combustion method [10,11], sol–gel peroxo route [12],
etc. CuO–CeO2−x prepared by co-precipitation had been stud-
The CuO–CeO2−x catalysts were prepared by co-
precipitating aqueous salt solution of the metal with different
ratios of KOH/K2CO3 (4:0, 3:1, 2:2, 1:3 and 0:4). The obtained
precipitate was washed with de-ionized water to remove
residual potassium, and then washed with ethanol before dried
at 105 ◦C for 1 h. The dried samples were thermally treated at
500 ◦C for about 3 h. Thus, treated catalysts were crushed and
sieved to 60–80 meshes. The Cu content in the catalysts was
∗
Corresponding author. Tel.: +86 571 88273272; fax: +86 571 88273283.
E-mail address: zhourenxian@zju.edu.cn (R. Zhou).
1381-1169/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2006.04.003

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Z. Liu et al. / Journal of Molecular Catalysis A: Chemical 255 (2006) 103–108
expressed as the Cu/(Cu + CeO2) % weight ratio and all data
below were obtained with the 5.0 wt.% Cu catalysts.
2.2. Catalyst characterization
The specific areas of the catalysts were obtained at −196 ◦C
using a Coulter Omnisorp 100CX. Prior to the measurement, the
catalysts were pretreated at 250 ◦C for 2 h. TEM profiles were
conducted using a JEOL JEM-2010 (HR) microscope operat-
ing at 100 kV. X-ray powder diffraction (XRD) patterns were
recorded on a Rigaku D/Max 2550PC powder diffractometer
using nickel-filtered Cu K␣ radiation. The mean particle diam-
eter was calculated from the X-ray line broadening, according
to Scherrer’s equation. H2 temperature-programmed reduction
(TPR) was carried out using a conventional reactor equipped
with a TCD.
2.3. Catalytic oxidation performance tests
The catalytic performance measurement was carried out in a
fixed micro-reactor (quartz glass, 4 mm i.d., 6 mm o.d., length
250 mm) at atmospheric pressure. The reactor temperature was
measured by a K-type thermocouple located at the top of the
packed catalyst bed and controlled by a temperature controller.
The mass of catalyst used in the experiments ranged from 50 to
100 mg, and the catalyst was diluted with inert alumina particles
(60–80 meshes) with a dilution ratio of 1:1 by mass. A desired
mixture of gases H2, O2, CO, CO2, and Ar was prepared by
adjusting the ratio of flows with mass flow controller (Scientific
Alicat). The water vapor was introduced with reacting gases
bubbling through a heated water bath.
The reactor inlet and outlet streams were measured using an
on-line gas chromatograph equipped with a thermal conductivity
detector (TCD) and a flame ionization detector (FID). H2, O2,
CO and CO2 were separated using a carbon molecular sieve
(TDX-01) column. CO and CO2 were converted to methane by
a methanation reactor and analyzed by FID. The detection limit
of CO is less than 5 ppm.
Taking into consideration of the existence of CO2 in feed-
stock, the CO conversion was calculated based on the CO
decrease as follows:
Fig. 1. TEM images of the CuO–CeO2−x catalyst prepared with different of
KOH/K2CO3: (a) 0:4, (b) 4:0.
addition of KOH results in a decrease in particle sizes from 8.3 to
5.4 nm and increase in pH values from 10.5 to 13.0 and surface
area from 40 to 138 m2 g−1. In addition, when the KOH/K2CO3
ratio, λ, increased from 0:4 to 4:0, the oxidation performance
of CuO–CeO2−x catalysts is improved greatly; i.e., the reac-
tion temperatures with 50% conversion of CO are lowered from
182.6 to 82.5 ◦C.
TEMimagesofCuO–CeO2−x catalystspreparedwithλof0:4
and 4:0, respectively, are presented in Fig. 1. As shown in Fig. 1,
the statistical analysis of the dimension of the CuO–CeO2−x
particles results in a narrow distribution with a mean diameter
around dTEM = 6.5 and 8.5 nm, respectively. In comparison with
the data in Table 1, the trend of size variation for both catalysts
is similar, though the Scherrer’s equation yields slightly lower
values for the crystal size than TEM analysis [15].
[CO]in − [CO]out
% of conversion of CO =
× 100
[CO]in
The selectivity is defined as the oxygen consumed by CO
oxidation, namely:
0.5([CO]in − [CO]out
[O2]in − [O2]out
)
% of selectivity =
× 100
3. Results and discussion
3.1. Structural studies of CuO–CeO2−x catalysts
The pH values, the surface area, the particle sizes and the tem-
perature required for 50% conversion of CO of CuO–CeO2−x
catalysts are listed in Table 1. From Table 1, it can be seen that the
The XRD patterns of all CuO–CeO2−x catalysts are given
in Fig. 2. The distinct fluorite-type oxide structure of CeO2

Z. Liu et al. / Journal of Molecular Catalysis A: Chemical 255 (2006) 103–108
105
Table 1
Characteristics of CuO–CeO2−x catalysts and comparison of their activity in CO oxidation
KOH/K2CO3
(molar ratio)
pH
D(1 1 1) (nm)a
SBET (m2 g−1
)
rCO at 100 ◦Cb
T50 (◦C)c
−1
−1
−1
−1
)
Cu
mmolCO (s
g
)
mmolCO (s
g
␮molCO (s−1 m2−1
)
cat
0:4
1:3
2:2
3:1
4:0
10.5
11.4
12.2
12.7
13.0
8.3
7.9
6.9
6.2
5.4
40
72
97
115
138
–
–
4.2
14.3
23.2
28.2
–
5.2
7.4
10.1
10.2
182.6
113.0
101.0
93.0
0.2
0.7
1.2
1.4
82.5
a
From line broadening of CeO2 (1 1 1) peak.
According to literature [7].
b
c
W/F = 0.03 g s cm−3
.
is present in all catalysts. It can be seen that with decreas-
ing λ value, the peaks of the catalysts become sharper and
more intense, corresponding to an increase in size of the crys-
talline particles. Obviously, the addition of KOH into precipi-
tator inhibits the growth of CeO2 particles. No observable shift
in the diffraction lines of CeO2−x can be found in these cata-
seen that the curve of CuO–CeO2−x catalyst prepared with λ of
4:0 shows two reduction peaks at 178 and 196 ◦C, respectively.
The peak at 178 ◦C (called peak ␣) is ascribed to the copper ions
strongly interacting with ceria, and the peak at 196 ◦C (called
peak ␤) is assigned to the CuO clusters non-associated with ceria
[16]. By addition of K2CO3 into the precipitator, the hydrogen
consumption peaks shift to higher temperatures and the areas of
peak ␣ (namely the H2 consumption) sharply diminish from 22
to 0 ␮mol/gcat. When λ = 1:3, it can be seen that peaks ␣ and ␤
coalesce into overlap to one peak and a new peak, called peak
␥, appears at 242 ◦C. According to the results of XRD measure-
ments above, peak ␥ is assigned to bulk CuO. When λ = 0:4, the
peak ␣ vanishes and the H2-TPR profile of the CuO–CeO2−x
catalyst only shows a main peak of hydrogen consumption at
245 ◦C (peak ␥) and a shoulder peak at 227 ◦C (peak ␤) in the
range of 100–350 ◦C. It is suggested that the addition of KOH
into the precipitator is beneficial to the formation of dispersed
CuOx clusters; however, the addition of K2CO3 inhibits this pro-
cess and yield bulk CuO with a large diameter.
´
lysts, although Marban and Fuertes [15] reported that the XRD
peaks of CeO2−x are rather broad and shifts of small magnitude
might be undetectable. Therefore, it can be concluded that this
is dispersed among the ceria crystallites and not forming a true
solid solution. CuOx must be in the form of clusters of XRD
amorphous material on the surface of ceria crystals, at least for
λ > 0:4, since, as observed in Fig. 3, at λ = 0:4 small but con-
spicuous diffraction peaks of CuO are detected, meaning that
crystallization of CuO starts to occur. It is evident that the addi-
tion of K2CO3 into precipitator causes the growth of CuO and
CeO2−x particles.
3.2. Redox properties of CuO–CeO2−x catalysts
In addition, we find the observed trend in activity with vari-
ation of λ values (seen in Table 2) correlates well with that of
H2 consumption of ␣ peak in H2-TPR of CuO–CeO2−x cat-
alysts. This is in agreement with the proposition [7,17] that
H2-TPR profiles of CuO–CeO2−x catalysts are shown in
Fig. 3 and values of the H2 consumption and peak temperature
maxima are listed in Table 2. From Fig. 3 and Table 2, it can be
Fig. 2. XRD images of the CuO–CeO2−x catalyst prepared with different ratios
of KOH/K2CO3: (a) 4:0, (b) 3:1, (c) 2:2, (d) 1:3, (e) 0:4.
Fig. 3. TPR images of the CuO–CeO2−x catalysts with different ratios of
KOH/K2CO3: (a) 4:0, (b) 3:1, (c) 2:2, (d) 1:3, (e) 0:4.

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Z. Liu et al. / Journal of Molecular Catalysis A: Chemical 255 (2006) 103–108
Table 2
H2 consumption and temperature of TPR peaks for catalysts with different λ values
Catalysts
␣ Peak
␤ Peak
␥ Peak
(λ values)
H2 consumption
Peak temperature
(◦C)
H2 consumption
Peak temperature
(◦C)
H2 consumption
(␮mol/gcat)
Peak temperature
(◦C)
(␮mol/gcat
)
(␮mol/gcat
)
4:0
3:1
2:2
1:3
0:4
22
21
13
10
–
176
180
184
197
–
40
45
46
3
196
207
208
214
227
–
–
–
42
39
–
–
–
242
245
16
the high activity of CuO–CeO2−x catalysts is due to the pres-
ence of highly dispersed CuO clusters in strongly interacting
with CeO2−x, which has a superior reducibility; i.e., the higher
reducibility of CuO–CeO2−x catalysts, the higher the catalytic
activity. Therefore, the reduction peak ␣ appears at the lowest
temperature and possesses the largest H2 consumption for the
catalyst with λ = 4:0 among all catalysts, and the catalyst is most
active.
with the increase of reaction temperatures, H2-oxidation reac-
tion emerges and the selectivity of O2 decreases with a further
increase in the reaction temperature. Arias et al. [18] proposed
a reaction scheme, in which Ce4+–O2−–Cu2+ pairs are rapidly
reduced by CO to produce CO2. According to these authors
CO (and also H2) molecules are adsorbed on the copper–ceria
interfacial region of the catalyst and react with lattice oxygen
to produce CO2 (and H2O), reducing the copper ion Cu2+ to
Cu+. The ceria lattice then supplies the lost oxygen anion to the
copper–ceria interface and finally the so formed vacancies are
refilled by gaseous oxygen. It is evident that in the above pro-
cess, the existence on the catalytic surface of relatively higher
amounts of easily reducible well dispersed copper oxide species
strongly interacting with ceria is the key to the achievement of
the superior catalytic performance. This is consistent with the
above TPR results.
As for the influence of WGSR (water–gas shift reaction) or
reverse WGSR, the catalytic performance of CuO–CeO2−x cat-
alyst prepared with λ of 4:0 in the feed gas of 1% O2, 8%
CO2, 50% H2, and argon in balance is also investigated, and
the results is illustrated in Fig. 5. From Fig. 5, it is found that
the CuO–CeO2−x catalyst is inactive for H2 oxidation at tem-
peratures up to 150 ◦C, and the concentration of CO is less than
10 ppm at 165 ◦C. Liu and Flytzani-Stephanopoulos [7] have
also found that the thermodynamic equilibrium constant of the
WGSR is at least 25 orders of magnitude lower than the thermo-
dynamic equilibrium constant of CO and H2 oxidation reactions
3.3. Oxidation performance of catalysts
The catalytic behavior toward preferential oxidation of CO
in rich hydrogen over CuO–CeO2−x catalysts are compared in
Fig. 4. The precipitators have an important effect on the cat-
alytic performance of CuO–CeO2−x catalyst. From Fig. 4, it
can be seen that the CuO–CeO2−x catalysts prepared with λ
of 4:0 shows the highest catalytic activity. Even at 110 ◦C,
CO conversion higher than 99% is observed at a space veloc-
ity of 120,000 ml g−1 h−1 in the absence of CO2 and H2O.
CuO–CeO2−x catalysts prepared with λ of 1:3, 2:2, and 3:1
achieve the similar conversion of CO at the temperature of 130,
145, 175 ◦C, respectively, while the CuO–CeO2−x catalyst pre-
pared with λ of 0:4 even cannot achieve the same conversion of
CO in the similar temperature range of 70–190 ◦C. The selec-
tivity of O2 for CuO–CeO2−x catalysts shows a contrary trend.
At low temperature range (lower than 120 ◦C), the selectivity of
O2 reaches 100%, due to no H2-oxidation reaction. However,
Fig. 4. The activity and selectivity of CuO–CeO2−x catalysts prepared with different ratios of KOH/K2CO3, at a space velocity of 120,000 ml g−1 h−1 in a feed of
1% O2, 1% CO, 50% H2, Ar in balance, in the absence of CO2 and H2O: (ꢀ) 0:4, (᭹) 1:3, (ꢁ) 2:2, (ꢂ) 3:1, (ꢃ) 4:0.

Z. Liu et al. / Journal of Molecular Catalysis A: Chemical 255 (2006) 103–108
107
in the temperature range of performed kinetic experiments, and
that no reverse WGS reaction is observed over the CuO–CeO2−x
catalyst at temperatures below 155 ◦C. The results show that
the reverse water gas shift reaction has a slight influence on
the selectivity and activity of the CuO–CeO2−x catalyst at the
temperatures lower than 160 ◦C. Therefore, the O2 selectivity
of 100% can be obtained over the CuO–CeO2−x catalyst when
reaction temperatures are lower than 120 ◦C, due to no compet-
itive oxidation of H2 and side reactions.
The influence of O2/CO is depicted in Fig. 6. As shown in
Fig. 6 (left), at a ratio of O2/CO up to 0.75, the CO conversion
shows a very weak dependence of the O2/CO ratio. However,
if the stoichiometric ratio of oxygen (O2/CO = 0.5) is fed to the
reactorinthepresenceofhydrogen, theCOreachesitsmaximum
value (about 98%) at a temperature of 100 ◦C. At higher temper-
atures, the CO conversion curve lowers again. Correspondingly,
the selectivity of O2, seen in Fig. 6 (right), decreases as the
increase in the O2/CO ratio. Because more and more water is
formed when O2/CO ratio increases, oxygen more than the sto-
ichiometric ratio is available for the H2 oxidation reaction.
We also investigated the influence of space velocity, CO2 and
H2O (seen in Fig. 7). According to literatures, both increase of
Fig. 5. The reverse water gas-shift reaction over the CuO–CeO2−x catalyst with
λ = 4:0. Reaction conditions: 50 mg of catalyst; feed gas of 1% O2, 8% CO2,
50% H2 and Ar in balance; 120,000 ml g−1 h−1 of GHSV.
Fig. 6. The activity and selectivity of CuO–CeO2−x catalysts prepared with the ratio of KOH/K2CO3 equal to 4:0, at a space velocity of 120,000 ml g−1 h−1, in the
absence of CO2 and H2O, O2/CO = 0.5 (ꢀ), 0.75 (᭹), 1.0 (ꢁ), 1.25 (ꢂ).
Fig. 7. The conversion of CO and selectivity of O2 over CuO–CeO2−x at a space velocity of 30,000 ml g−1 h−1 in the absence of CO2 and H2O (♦), at a space
velocity of 120,000 ml g−1 h−1 in the absence of CO2 and H2O (ꢃ), in the presence of CO2 (ꢁ), and in the presence of CO2 and H2O (ꢂ).

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Z. Liu et al. / Journal of Molecular Catalysis A: Chemical 255 (2006) 103–108
tators. The BET, XRD and TPR results indicate that the addition
of KOH as precipitators inhibits the growth of CuO–CeO2−x par-
ticles and yields nano-structured catalysts (dXRD = 5.4–8.3 nm),
increases the surface areas of CuO–CeO2−x catalysts from 39.9
to 138.0 m2 g−1. It is also find that the intensity of ␣ peak
in H2-TPR of CuO–CeO2−x catalysts is in a good agreement
with the oxidation performance of the CuO–CeO2−x catalysts.
Namely, the catalyst prepared with λ of 4:0, maintaining the
largest area of ␣ peak, achieves the superior activity for the
preferential oxidation of CO in rich hydrogen. CO conversion
higher than 99% with O2 selectivity of 100% can be obtained
for this CuO–CeO2−x catalyst at 90–110 ◦C and a space velocity
of 30,000–120,000 ml g−1 h−1 in the absence of CO2 and H2O
or at 150 ◦C and a space velocity of 120,000 ml g−1 h−1 in the
presence of CO2 and H2O.
Fig. 8. Time on stream over the CuO–CeO2−x catalyst with λ = 4:0 at 120 ◦C.
Reaction conditions: 50 mg of catalyst; feed gas of 1% O2, 8% CO2, 50% H2
and Ar in balance; 120,000 ml g−1 h−1 of GHSV.
Acknowledgment
The authors gratefully acknowledge the Ministry of Science
and Technology of China (No. 2004 CB 719504).
space velocity and addition of CO2 and H2O have a negative
effect on the performance of CuO–CeO2−x catalysts. In the case
of the space velocity of 120,000 ml g−1 h−1, the curves of CO
conversion and selectivity of O2 over CuO–CeO2−x catalyst, in
comparison with 30,000 ml g−1 h−1, are slightly lower, as seen
from both left and right images of Fig. 7. Similar trends can
be seen for the addition of CO2 and H2O. The temperature with
CO conversion of 99% in the presence of CO2 or in the presence
of CO2 and H2O is 120 and 150 ◦C, respectively. It is evident
that the addition of CO2 and H2O depresses the performance of
CuO–CeO2−x catalysts.
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