Comparative study of different methods of preparing CuO-CeO2 catalysts for preferential oxidation of CO in excess hydrogen

Article abstract of DOI:10.1016/j.molcata.2006.11.036

Influence of preparation methods (i.e. co-precipitation, chelating, citric acid and critical phase) on the preferential oxidation of CO in excess hydrogen over CuO-CeO₂ catalysts has been investigated. CuO-CeO₂ catalysts are characterized by using BET, X-ray powder diffraction (XRD), UV Raman and temperature-programmed reduction (TPR) techniques. The catalyst prepared by chelating is most active for the preferential oxidation of CO. In addition, XRD, UV Raman and TPR show that the chelating method enhances the formation of defects of ceria and produces a synergic effect between the cycle of Cu¹⁺/Cu²⁺ and that of Ce³⁺/Ce⁴⁺, which is beneficial to the improvement of the performance of CuO-CeO₂ catalysts.

Full text of DOI:10.1016/j.molcata.2006.11.036

Journal of Molecular Catalysis A: Chemical 267 (2007) 137–142
Comparative study of different methods of preparing CuO-CeO2 catalysts
for preferential oxidation of CO in excess hydrogen
Zhigang Liu, Renxian Zhou∗, Xiaoming Zheng
Institute of catalysis, Zhejiang University, Hangzhou 310028, PR China
Received 26 May 2006; received in revised form 11 November 2006; accepted 19 November 2006
Available online 25 November 2006
Abstract
Influence of preparation methods (i.e. co-precipitation, chelating, citric acid and critical phase) on the preferential oxidation of CO in excess
hydrogen over CuO-CeO2 catalysts has been investigated. CuO-CeO2 catalysts are characterized by using BET, X-ray powder diffraction (XRD),
UV Raman and temperature-programmed reduction (TPR) techniques. The catalyst prepared by chelating is most active for the preferential oxidation
of CO. In addition, XRD, UV Raman and TPR show that the chelating method enhances the formation of defects of ceria and produces a synergic
effect between the cycle of Cu1+/Cu2+ and that of Ce3+/Ce4+, which is beneficial to the improvement of the performance of CuO-CeO2 catalysts.
© 2006 Elsevier B.V. All rights reserved.
Keywords: CuO-CeO2; Preferential oxidation; CO; Fuel cell; Preparation method
1. Introduction
CuO-CeO2 catalysts synthesized by Kim and Cha [3] using
co-precipitation methods, with a BET specific surface area of
91 m2/g, showed superior performance in the preferential oxi-
dation of CO in excess hydrogen; i.e., these catalysts reduced
the CO content to less than 100 ppm 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. Obviously, the preparation methods
had a marked effect on the performance of CuO-CeO2 catalysts
in the preferential oxidation of CO.
In this work, influence of the preparation methods (i.e. co-
precipitation, chelating, citric acid and critical phase) on the
preferential oxidation of CO in excess hydrogen over CuO-CeO2
catalysts has been investigated, and the catalysts have been char-
acterized by BET, X-ray powder diffraction (XRD), UV Raman
and temperature-programmed reduction (TPR) techniques. The
catalyst prepared by chelating gave a catalytic performance in
the preferential oxidation of CO superior to the catalysts pre-
pared by the other preparation methods.
Recently, the CuO-CeO2 catalyst has been attracting atten-
tions because of its activity higher than that of conventional
copper-based catalysts and comparable or superior to that of
platinum catalysts for the preferential oxidation of CO in excess
hydrogen [1–5], which has been used for PEM fuel cells (PEM-
FCS). Itisbelievedthatthereactioniscatalyzedbytheinterfacial
copper oxide-ceria centers in which ceria presents a high num-
ber of oxygen vacancies that permits a high mobility of lattice
oxygen [6,7].
For CuO-CeO2 catalysts, preparation methods have a critical
influence on preferential oxidation of CO in excess hydro-
gen. Preparation methods reported are co-precipitation [8–10],
urea-nitrate combustion method [11,12], sol–gel peroxo route
[13,14], etc. Avgouropoulos et al. [15] investigated the dif-
ference among four CuO-CeO2 catalysts prepared by using
the co-precipitation, the citrate-hydrothermal, the urea-nitrates
combustion and the impregnation methods. They found that
the combustion-prepared sample exhibited the best catalytic
performance, followed by the citrate-hydrothermally-prepared
sample. The impregnated one was least active. However,
2. Experimental
2.1. Catalyst preparation
∗
Sample 1 (designated as 5CuC-CP) was synthesized by co-
precipitationfromthemixture(250 ml)of0.055 mol/lCe(NO3)2
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.2006.11.036

138
Z. Liu et al. / Journal of Molecular Catalysis A: Chemical 267 (2007) 137–142
and 0.008 mol/l Cu(NO3)2. 0.362 mol/l KOH was added drop-
wise with vigorous stirring, and then the mixture was aged for
about 20 min. The pH value of the mixture was 12.5 and the
obtained precipitate was washed with distilled water until pH
was decreased to 7.0, followed by washing with 200 ml ethanol
before being dried at 105 ◦C for about 3 h. The dried sample was
thermally treated at 500 ◦C for about 2 h.
Sample 2 (designated as 5CuC-CE) was synthesized by dry-
ing under a critical phase of ethanol. The other steps were similar
to those for the co-precipitation-prepared sample.
Sample 3 (designated as 5CuC-CH) was synthesized by the
chelating method. The solution of cetytrimethyammonium bro-
mide (C19H42BrN) was added dropwise into the mixture of
0.055 mol/l Ce(NO3)2 and 0.008 mol/l Cu(NO3)2 solutions with
vigorous stirring, and the obtained sol–gel was aged for 30 min
at ambient temperatures. It should be noted that all the solvent
in the experiment is not water but ethanol. Then, the sol–gel was
dried at 100 ◦C for about 5 h and then thermally treated at 500 ◦C
for 2 h.
catalyst was diluted with inert alumina particles (60–80 meshes)
with a mass ratio of 1:1. A desired mixture of gases (i.e. 50% H2,
1.0% O2, 1.0% CO and Ar in balance) was prepared by adjust-
ing the ratio of flows with the mass flow controllers (Scientific
Alicat). The steam was introduced with reacting gases bubbling
through a thermostat 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 and O2
were separated by a carbon molecular sieve (TDX-01) column
and detected with TCD. CO and CO2 were separated by a carbon
molecular sieve (TDX-01) column, then converted to methane
by a methanation reactor and analyzed by FID. The detection
limit of FID for CO is less than 3 ppm.
Taking into consideration of the existence of CO2 in the
feedstock, the CO conversion was calculated based on the CO
decrease as follows:
[CO]in − [CO]out
% of conversion of CO =
× 100
[CO]in
Sample 4 (designated as 5CuC-CA) was prepared by the cit-
ric acid method. Aqueous solution of 0.060 mol/l citric acid was
added dropwise into the mixture of equal amount of 0.055 mol/l
Ce(NO3)2 and 0.008 mol/l Cu(NO3)2 under continuous stirring
and then the mixture was kept stirring for 6 h. The mixed solu-
tion was dried at 120 ◦C for a few hours to obtain the powder,
followed by thermal treatment at 500 ◦C for 2 h.
All treated catalysts were crushed and sieved to 60–80
meshes. The Cu content over all the catalysts was expressed as
the Cu/(Cu + CeO2) wt% ratio and all data below were obtained
with 5.0 wt% Cu catalysts.
The selectivity was defined as the oxygen consumed by CO
oxidation, namely:
0.5([CO]in − [CO]out
[O2]in − [O2]out
)
% of selectivity =
× 100
2.3. Characterization of catalysts
The specific area of the sample was obtained at −196 ◦C
using a Coulter Omnisorp 100CX. Prior to the measurement, the
sample was pretreated at 250 ◦C for 2 h. X-ray powder diffrac-
tion patterns were recorded on a Rigaku D/Max 2550PC powder
diffractometer using nickel-filtered Cu K␣ radiation. UV Raman
spectra were recorded on a UV-HR Raman spectrograph. The
laser power measured at the samples was below 4.0 mW for
325 nm radiation. H2 temperature-programmed reduction was
carried out using a conventional reactor equipped with TCD.
Fifty milligrams catalyst and 40 ml/min flowing velocity of 5%
H2/Ar were applied in this work. The temperature rate was
10 ◦C/min from 30 ◦C to 570 ◦C.
2.2. Measurement of catalytic performance
The activity measurement was carried out in a fixed-
bed micro-reactor (quartz glass, i.d. 4 mm, o.d. 6 mm, length
250 mm) at atmospheric pressure. The reactor temperature was
measured with 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 was 50 mg, and the
Fig. 1. The activity and selectivity of CuO-CeO2 catalysts prepared with different preparation methods, at a space velocity of 120,000 ml g−1 h−1, in a feed of 1.0%
CO, 1.0% O2, 50% H2, balance Ar.

Z. Liu et al. / Journal of Molecular Catalysis A: Chemical 267 (2007) 137–142
139
3. Results and discussion
3.1. The catalytic performance of CuO-CeO2 catalysts
The preferential oxidation of CO in excess hydrogen over
the CuO-CeO2 catalysts prepared by different methods is
illustrated in Fig. 1. From Fig. 1, it can be seen that the prepa-
ration methods have a significant influence on the catalytic
performance of CuO-CeO2 catalysts. 5CuC-CH exhibits the
highest catalytic activity in the preferential oxidation of CO
and 5CuC-CA shows the lowest catalytic activity. T60 (i.e. the
temperature required for 60% conversion of CO) for 5CuC-
CP, 5CuC-CH, 5CuC-CA and 5CuC-CE are 106 ◦C, 81 ◦C,
109 ◦C and 95 ◦C, respectively. However, in the temperature
range from 60 ◦C to 160 ◦C, the highest conversions of CO
appear in the light-off curves of the catalysts, and the cor-
responding temperatures for 5CuC-CP, 5CuC-CH, 5CuC-CA
and 5CuC-CE are 160 ◦C (98.0% of CO conversion), 120 ◦C
(99.6% of CO conversion), 160 ◦C (99.2% of CO conver-
sion) and 145 ◦C (97.3% of CO conversion), respectively. In
addition, at a space velocity of 120,000 ml g−1 h−1, 5CuC-CP
and 5CuC-CE cannot reach CO conversion higher than 99.0%
(namely the CO concentration less than 100 ppm) in con-
trast with 5CuC-CH and 5CuC-CA attaining CO conversion
higher than 99.0%. It is evident that 5CuC-CH shows out-
standing catalytic activity in the preferential oxidation of CO
in excess hydrogen compared with 5CuC-CP, 5CuC-CA and
5CuC-CE.
Fig. 2. The influence of the space velocity on the catalytic performance of
CuO-CeO2 catalyst prepared with the chelating method, at a space velocity of
30,000–120,000 ml g−1 h−1, in a feed of 1.0% CO, 1.0% O2, 50% H2, balance
Ar.
The selectivity of O2 shows a contrary trend with the increase
in reaction temperatures, due to the fact that displacement of
adsorbed hydrogen by carbon monoxide and blocking of hydro-
gen adsorption by preadsorbed carbon monoxide occur during
the competing adsorption of hydrogen and carbon monoxide
[16]. For example, in case of 5CuC-CH, the selectivity declines
from 100.0% at 100 ◦C to about 48.0% at 145 ◦C. In addition, for
5CuC-CAand5CuC-CE, 100%selectivityofO2 isstillobserved
at 110 ◦C. Consequently, 5CuC-CH has a lower selectivity of O2
than 5CuC-CA and 5CuC-CE.
Fig. 3. Time on stream over the CuO-CeO2 catalyst prepared with the chelating
method at 120 ◦C.
In order to investigate the stability of 5CuC-CH and elimi-
nate the influence of initial activity of CuO-CeO2 catalysts on
the measurement of activity, the duration test is run for 12 h at
120 ◦C and 120,000 ml g−1 h−1 of GHSV in a feed of 1% CO,
1% O2, 50% H2 and Ar in balance. As shown in Fig. 3, the CO
conversions is kept at the values higher than 99% during the
run. Selectivity of O2 exhibits a slight increase, about 1% in the
end of the test. It is evident that 5CuC-CH possesses a desirable
stability, which is consistent with some literature [2,3]. Thus,
Fig. 2 illustrates the influence of the space velocity. As seen
from Fig. 2, increase in the space velocity has a negative effect
on the performance of CuO-CeO2 catalysts prepared with the
chelating method. At the space velocity of 120,000 ml g−1 h−1
,
the CO conversion and selectivity of O2 over CuO-CeO2 catalyst
are slightly lower than at 30,000 ml g−1 h−1
.
Table 1
Characteristics of CuO-CeO2 catalysts and comparison of their activity in CO oxidation
Catalysts
SBET (m2/g)
PV (ml/g)a
APD (nm)b
rCO at 100 ◦Cc
T60 (◦C)d
−1
−1
␮molCO (s gcat
)
␮molCO (s gCu
)
␮molCO (s m2)−1
5CuC-CP
5CuC-CH
5CuC-CA
5CuC-CE
111
99
31
0.22
0.18
0.16
0.16
7.9
7.3
20.6
7.2
5.2
14.4
4.4
104
288
88
0.047
0.145
1.419
0.126
106
81
109
95
89
11.2
224
a
PV, pore volume.
b
c
d
APD, average pore diameter (APD = 4 × pore volume/BET surface area).
According to literature [7].
W/F = 0.03 g s cm−3
.

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Z. Liu et al. / Journal of Molecular Catalysis A: Chemical 267 (2007) 137–142
Fig. 4. XRD patterns of CuO-CeO2 catalysts prepared by different methods.
the influence of initial activity of CuO-CeO2 catalysts on the
measurement of activity is eliminated.
Fig. 5. UV Raman spectra of CuO-CeO2 catalysts prepared by different meth-
ods.
3.2. The structural studies of CuO-CeO2 catalysts
can be linked to oxygen vacancies in the CeO2 lattice and
attributed to the CeO2 defects [18,19]. It can also be seen that
the intensity of 584 cm−1 and 1176 cm−1 bands of CuO-CeO2
catalysts builds up with the increase in the turn of 5CuC-CP,
5CuC-CE, 5CuC-CA and 5CuC-CH, which indicates that the
amount of defects in CuO-CeO2 catalysts is increased. Defects
have a beneficial influence on the catalytic performance of
CuO-based catalysts in preferential oxidation of CO in excess
hydrogen [20]. Consequently, the amount of defects correlates
well with the activity of CuO-CeO2 catalysts.
The H2-TPR profiles of CuO-CeO2 catalysts prepared by dif-
ferent methods are illustrated in Fig. 6. From Fig. 6, it can be
seen that the overlapping reduction peaks of CuOx species, for
5CuC-CP, 5CuC-CE and 5CuC-CA, consist of a low-intensity,
low-temperature peak at about 161 ◦C (α peak) and a higher-
Table 1 lists the pore volume, average pore diameter and
surface areas and catalytic activity of the CuO-CeO2 catalysts
prepared by different methods. From Table 1, it can be seen that
5CuC-CP has the largest BET area (111 m2 g−1) and the lowest
TOF (0.047 ␮molCO (s m2)−1), while 5CuC-CA has a BET
area of only about one third of that of 5CuC-CP and the highest
TOF (1.419 ␮molCO (s m2)−1). 5CuC-CH with a surface area of
99 m2 g−1 achieves the highest activity in preferential oxidation
of CO. Namely T60 of 5CuC-CH is as low as 81 ◦C, compared
with 109 ◦C over 5CuC-CA and 106 ◦C over 5CuC-CP.
Fig. 4 shows the XRD patterns of CuO-CeO2 catalysts pre-
pared by different preparation methods. It can be seen that the
distinct fluorite-type oxide structure of CeO2 is observed in all
samples [17], and that the peaks of 5CuC-CH are broader than
those of other catalysts, which indicates that 5CuC-CH has a
smaller particle size than the other catalysts. In addition, four
peaks can be found at 36◦, 38.6◦, 42◦ and 43◦. Peaks at 36◦ and
38.6◦ are assigned to the peak of crystal CuO. The peaks of CuO
appear in the XRD patterns of 5CuC-CP, 5CuC-CA and 5CuC-
CE. However, no peaks of CuO can be seen in that of 5CuC-CH,
which means that CuO is well dispersed in 5CuC-CH. Two peaks
at 42◦ and 43◦ are assigned to the formation of cerium and cop-
per solid solution. The peaks of cerium and copper solid solution
appear in the XRD patterns of 5CuC-CP, 5CuC-CA and 5CuC-
CE. No peaks of cerium and copper solid solution can be seen in
that of 5CuC-CH. However, it should also be noted that the XRD
peaks of CeO2 are rather broad and shifts of small magnitude
cannot be detectable. Therefore, the possibility of existence of
trace copper and cerium solid solution cannot be excluded.
Fig. 5 shows the UV Raman spectra recorded by using a
laser at 325 nm as the excitation source. A broad band with rel-
atively high intensity at 462 cm−1 is assigned to cubic CeO2 in
CuO-CeO2 catalysts together with two bands at 584 cm−1 and
1176 cm−1 [12]. The bands at about 584 cm−1 and 1176 cm−1
Fig. 6. TPR profiles of CuO-CeO2 catalysts prepared by different methods.

Z. Liu et al. / Journal of Molecular Catalysis A: Chemical 267 (2007) 137–142
141
Table 2
H2 consumption and temperature of TPR peaks for catalysts prepared by different methods
Catalysts
α peak
β peak
βꢀ or γ peak
H2 consumption
Peak
temperature (◦C)
H2 consumption
Peak temperature
(◦C)
H2 consumption
Peak temperature
(◦C)
(␮mol/gcat
)
(␮mol/gcat
)
(␮mol/gcat
)
5CuC-CP
5CuC-CH
5CuC-CA
5CuC-CE
325
295
199
312
162.6
161.7
161.8
160.5
479
568
522
551
183.1
199.5
189.8
178.9
256
157
274.6
203.7
intensity peak at about 180 ◦C (β peak). In addition, a shoulder
peak is found at 203.7 ◦C in the case of 5CuC-CA, and assigned
to βꢀ peak. Generally, there are several types of copper-oxygen
entities [20,21]: (a) isolated Cu2+ ions can strongly interact with
thesupport;(b)weakmagneticassociatesconsistofseveralCu2+
ions and these Cu2+ ions have close contact with each other; (c)
small two- and three-dimensional clusters have so loose struc-
tures that they have no specific and regular lattice arrangement;
(d) large three-dimensional clusters and bulk CuO phase have
characters and properties identical to those of pure CuO powder.
Therefore, in this work, α peak may be attributed to a type or b
type, β peak may be assigned to c type and γ or βꢀ peak may be
assigned to d type.
enough to change its oxide ion transport properties and enable
bulk reduction at 400 ◦C [9].
In addition, γ peaks in TPR patterns of 5CuC-CH cannot be
assigned to the reduction of bulk CuO. If γ peak were assigned
to the reduction of bulk CuO, signal of bulk CuO should have
been strong enough to be seen in the XRD pattern, due to the
existence of large amount of bulk CuO. However, in the case
of 5CuC-CH, bulk CuO or solid solution of copper and cerium
cannot be found in the XRD patterns, which indicates that some
other species are reduced at the position of γ peak.
In general, pure CuO and Cu2O catalysts exhibit H2 reduction
peaksatapproximately180 ◦Cand300 ◦C[22]. Therefore, αand
γ peaks may be assigned to the reduction of CuO and Cu2O,
respectively. Moreover, Fig. 8 indicates the influence of redox
cycles on reducibility of 5CuC-CH. With the increase in redox
cycles, α peak shifts to higher temperature and β peaks appear
between α and γ peaks, while the position of γ peaks slightly
changes. Thus, β peak is assigned to the reduction of CuO with
larger particle sizes.
However, for 5CuC-CH, not only α peak at 161.7 ◦C but also
γ peak at 274.6 ◦C is found. There is an overlapping reduc-
tion peak between α peak and γ peak. H2 consumptions of α,
β and γ peaks are (multi-peaks fitted according to the Lorentz
method and shown in Table 2) 295 ␮mol/gcat, 568 ␮mol/gcat and
256 ␮mol/gcat, respectively. The amount of H2 consumption for
α, β and γ peaks are 1119 ␮mol/gcat and larger than the theoreti-
cal amount of H2 consumption in 5CuC-CH (i.e. 787 ␮mol/gcat).
Furthermore, in Fig. 7, CeO2-CH (prepared by the chelating
method) has two peaks in the TPR profile, one at 414 ◦C and the
other at 513 ◦C. When copper oxide and cerium oxide is pre-
pared by the chelating method, 5CuC-CH exhibits H2 reduction
peaks well below 450 ◦C. Obviously, the reduction of surface
or bulk oxygen species of ceria is involved, which indicates
that ceria has an amount of copper oxide dissolved in it. The
amount of copper oxide, even though perhaps small, is large
According to above discussion, the reduction of 5CuC-CH
includes several types: (a) reduction of CuO with small particle
sizes;(b)reductionofCuOwithlargeparticlesizes;(c)reduction
of Cu2O; (d) reduction of surface or bulk oxygen species of
ceria.During the reduction of 5CuC-CH the equilibrium of the
above reduction types can be expressed as follow [23,24]:
Ce4+ + Cu1+ ↔ Ce3+ + Cu2+
This equilibrium can stabilize the cationic copper species in the
structure even in highly reductive atmosphere [9]. If so, the high
Fig. 7. TPR profiles of 5CuC-CH and CeO2-CH.
Fig. 8. TPR profiles of 5CuC-CH reoxidized with different times.

142
Z. Liu et al. / Journal of Molecular Catalysis A: Chemical 267 (2007) 137–142
activity of CO oxidation in excess hydrogen over 5CuC-CH can
evolve as a synergic effect between the cycle of Cu1+/Cu2+ and
that of Ce3+/Ce4+.
[3] D.H. Kim, J.E. Cha, Catal. Lett. 86 (2003) 107.
[4] P. Chin, X.L. Sun, G.W. Roberts, J.J. Spivey, Appl. Catal. A: Gen. 302
(2006) 22.
[5] K. Tanaka, Y. Moro-oka, K. Ishigure, et al., Catal. Lett. 92 (2004)
115.
4. Conclusions
[6] J.B. Wang, W.H. Shih, T.J. Huang, Appl. Catal. A: Gen. 203 (2000)
191.
[7] W. Liu, M. Flytzani-Stephanopoulos, J. Catal. 153 (1995) 304.
[8] G. Avgouropoulos, T. Ioannides, Appl. Catal. A 244 (2003) 155.
[9] G. Sedmak, S. Hocevar, J. Levec, J. Catal. 213 (2003) 135.
[10] D.H. Kim, M.S. Lim, Appl. Catal. A 224 (2002) 27.
[11] G. Avgouropoulos, Th. Ioannides, Appl. Catal. A: Gen. 244 (2003)
155.
[12] W. Shan, Z. Feng, Z. Li, J. Zhang, W. Shen, C. Li, J. Catal. 228 (2004)
206.
[13] G. Avgouropoulos, T. Ioannides, Ch. Papadopoulou, J. Batista, S. Hocevar,
H.K. Matralis, Catal. Today 75 (2002) 157.
[14] Y. Liu, Q. Fu, M. Flytzani-Stephanopoulos, Catal. Today 93–95 (2000)
241.
[15] G. Avgouropoulos, T. Loannides, H.K. Matralis, Appl. Catal. B: Environ.
56 (2005) 87.
[16] B.E. Nieuwenhuys, Surf. Sci. 126 (1983) 307.
[17] V. Oerrichon, A. Laachir, S. Abouarnadasse, O. Touret, G. Blanchard, Appl.
Catal. A: Gen. 129 (1995) 69.
[18] J.E. Spanier, R.D. Robinson, F. Zhang, S.W. Chan, I.P. Herman, Phys. Rev.
B 64 (2001) 245407.
In this work, the influence of preparation methods (i.e.
co-precipitation, chelating, citric acid and critical phase) on
the preferential oxidation of CO in excess hydrogen over
CuO-CeO2 catalysts has been investigated and CuO-CeO2
catalysts are characterized by using BET, XRD, UV Raman
and TPR techniques. The catalyst prepared by chelating is most
active in preferential oxidation of CO. CO conversion over
5CuC-CH is 99.6% at the temperature of 120 ◦C, while CO
conversions over 5CuC-CE and 5CuC-CP cannot reach 99%
in all reaction temperature range and that over 5CuC-CA is
99.2% at the temperature of 160 ◦C. In addition, the chelating
method enhances the formation of defects of ceria and produces
a synergic effect between the cycle of Cu1+/Cu2+ and that of
Ce3+/Ce4+, which are beneficial to the improvement of the
performance of CuO-CeO2 catalysts.
[19] J.R. McBride, K.C. Hass, B.D. Poindexter, W.H. Weber, J. Appl. Phys. 76
(1994) 2435.
Acknowledgment
[20] W.P. Dow, Y.P. Wang, T.J. Huang, J. Catal. 160 (1996) 171.
[21] W.P. Dow, Y.P. Wang, T.J. Huang, J. Catal. 160 (1996) 155.
[22] G. Fierro, M. Lo Jacono, M. Inversi, P. Porta, R. Lavecchia, F. Cioci, J.
Catal. 148 (1994) 709.
The authors gratefully acknowledge the Ministry of Science
and Technology of China (No: 2004 CB 719504).
[23] S. Hocevar, J. Batista, J. Levec, J. Catal. 184 (1999) 39.
[24] C. Lamonier, A. Ponchel, A. D’Huysser, L. Jalowiecki-Duhamel, Catal.
Today 50 (1999) 247.
References
[1] W. Liu, M. Flytzani-Stephanopoulos, Chem. Eng. J. 64 (1996) 283.
[2] G. Avgouropoulos, T. Loannides, H.K. Matralis, Catal. Lett. 73 (2001) 33.