Mechanism of CO oxidation in excess H2 over CuO/CeO2 catalysts: ESR and TPD studies

Article abstract of DOI:10.1134/S002315840604015X

The oxidation of CO with oxygen(0.25-6.4)% CuO/CeO₂ catalysts in excess H₂ is studied. CO conversion increases and the temperature range of the reaction decreases by 100 K as the CuO content is raised. The maximal CO conversion, 98.5%, is achieved on 6.4% CuO/CeO ₂ at 150°C. At T > 150°C, the CO conversion decreases as a result of the deactivation of part of the active sites because of the dissociative adsorption of hydrogen. CO is efficiently adsorbed on the oxidized catalyst to form CO-Cu⁺ carbonyls on Cu₂O clusters and is oxidized by the oxygen of these clusters, whereas it is neither adsorbed nor oxidized on Cu⁰ of the reduced catalysts. The activity of the catalysts is recovered after the dissociative adsorption of O₂ on Cu⁰ at T ～ 150°C. The activation energies of CO, CO ₂, and H₂O desorption are estimated, and the activation energy of CO adsorption yielding CO-Cu⁺ carbonyls is calculated in the framework of the Langmuir-Hinshelwood model. MAIK Nauka/Interperiodica , 2006.

Full text of DOI:10.1134/S002315840604015X

ISSN 0023-1584, Kinetics and Catalysis, 2006, Vol. 47, No. 4, pp. 585–592. © MAIK “Nauka /Interperiodica” (Russia), 2006.
Original Russian Text © A.N. Il’ichev, A.A. Firsova, V.N. Korchak, 2006, published in Kinetika i Kataliz, 2006, Vol. 47, No. 4, pp. 602–609.
Mechanism of CO Oxidation in Excess H2
over CuO/CeO2 Catalysts: ESR and TPD Studies
A. N. Il’ichev, A. A. Firsova, and V. N. Korchak
Semenov Institute of Chemical Physics, Russian Academy of Sciences, Moscow, 117977 Russia
Received December 6, 2004
Abstract—The oxidation of CO with oxygen over (0.25–6.4)% CuO/CeO2 catalysts in excess H2 is studied.
CO conversion increases and the temperature range of the reaction decreases by 100 K as the CuO content is
raised. The maximal CO conversion, 98.5%, is achieved on 6.4% CuO/CeO2 at 150°C. At T > 150°C, the CO
conversion decreases as a result of the deactivation of part of the active sites because of the dissociative adsorp-
tion of hydrogen. CO is efficiently adsorbed on the oxidized catalyst to form CO–Cu+ carbonyls on Cu2O clus-
ters and is oxidized by the oxygen of these clusters, whereas it is neither adsorbed nor oxidized on Cu0 of the
reduced catalysts. The activity of the catalysts is recovered after the dissociative adsorption of O2 on Cu0 at T ~
150°C. The activation energies of CO, CO2, and H2O desorption are estimated, and the activation energy of CO
adsorption yielding CO–Cu+ carbonyls is calculated in the framework of the Langmuir–Hinshelwood model.
DOI: 10.1134/S002315840604015X
INTRODUCTION
reduced (0.25–6.4)% CuO/CeO2 catalysts was studied
by ESR and TPD in the temperature range 20–200°C.
The low-temperature heterogeneous oxidation of
CO with oxygen in excess hydrogen is a promising
reaction for efficient fuel cells [1]. A survey of catalysts
for CO oxidation shows that the oxide system CuO–
CeO2 is one order of magnitude more active than plati-
num group metals supported onAl2O3 or zeolites [2–4].
The mechanism of this reaction on CuO–CeO2 and
CuO–CeO2–ZrO2 systems is being actively debated
[5, 6], and new catalysts are being sought for this reac-
tion [7–11].
EXPERIMENTAL
Ceria with a specific surface area of Ssp = 73 m2/g
was prepared by the decomposition of Ce(NO3)4 · 6H2O
in air at 500°C for 2 h. The specific surface area was
determined by the BET method from low-temperature
argon adsorption data.
CuO/CeO2 samples containing 0.25, 0.5, 1.0, 2.1,
and 6.4 wt % CuO were prepared by impregnating
CeO2 with copper nitrate solutions with various salt
concentrations followed by drying at 100°C and calci-
nation in air at 500°C for 2 h. The specific surface area
of the samples decreases from 73 to 63 m2/g as the CuO
content is raised from 0.25 to 6.4%. The ratio of the
number of copper ions to the number of surface cerium
ions was varied from 0.1 to 2.1 under the assumption
that copper ions are uniformly distributed on the CeO2
surface.
The high activity of CuO–CeO2 catalysts in low-
temperature CO oxidation is due to the synergistic
effect arising from the Cu–Ce interaction, which facili-
tates the reduction and oxidation of both components.
The formation of the radical anion O–2 during the
adsorption of CO + O2 at room temperature is believed
to be evidence of this synergism. However, the
O–2 (CO + O2) species has recently been found on pure
oxidized CeO2 [12, 13], in the absence of a synergistic
effect. In this connection, it is of interest to obtain fur-
ther evidence of the synergistic effect in CuO–CeO2 by
investigating the formation of O–2 during the adsorption
of CO + O2 on CuO/CeO2 samples with various CuO
contents.
The catalytic activity of the samples in CO oxida-
tion with oxygen in excess hydrogen was studied in a
flow reactor. A catalyst sample (20 mg) was charged
into a quartz reactor (tube 3 mm in diameter), and the
reactor was purged with the reaction mixture
(98 vol % H2 + 1 vol % CO + 1 vol % O2) at a flow rate
of 40 ml/min. The reaction products were analyzed
using two columns packed with the molecular sieve
NaX (13 Å) and Porapak QS, respectively. Thermal
conductivity detectors were used.
Here, we report the catalytic activity of CeO2 and
(0.25–6.4)% CuO/CeO2 as a function of CuO content
for CO oxidation with oxygen in excess hydrogen in the
temperature range 100–350°C. In order to elucidate the
nature of the active sites and intermediates, the adsorp-
tion of CO, O2, H2, and their mixtures on oxidized and
585

586
IL’ICHEV et al.
CO conversion, %
100
were studied by vacuum TPD. A 50-mg sample was
5
preliminarily oxidized or reduced at 500°C, and then a
gas was adsorbed at 20°C. After adsorption, the gas was
pumped out and the sample was heated at a rate of
10 K/min under continuous pumping. TPD spectra as
the change in the pressure of desorbed gas (W) as a
function of temperature (T) were obtained using a
Pirani manometer [14]. The composition of desorbed
gas was monitored with an MX-7304 mass-spectrome-
ter. The amount of desorbed gas was estimated in a TPD
run conducted in a sealed reactor.
80
60
4
2
40
1
3
20
0
100
200
300
400
T, °C
The gases CO, O2, and H2 were obtained according
to standard procedures [15]. They were admitted to the
sample through a liquid nitrogen trap to remove the
uncontrolled CO2 and H2O impurities.
Fig. 1. Catalytic activity of CuO/CeO samples in CO oxi-
dation with oxygen in excess hydrogen. CuO content: (1) 0,
(2) 0.5, (3) 1.0, (4) 2.1 and (5) 6.4%.
2
A
RESULTS
A2 = 180 G
2
g|| = 2.34
||
Catalytic Activity of CuO/CeO2 in CO Oxidation
with Oxygen in Excess Hydrogen
× 3.3
Figure 1 plots CO conversion versus temperature on
samples with various CuO contents that were prelimi-
narily heated in air for 1 h. As can be seen in Fig. 1, the
CO conversion on CeO2 is not high, being at most 30%
at 350°C. Supporting a small amount of CuO (0.5%
CuO/CeO2 sample) raises the conversion from 5 to 85%
in the temperature range from 100 to 200°C. The con-
version is maximal at Tmax = 200°C and decreases to
75% as the temperature is further increased to 220°C.
An extremum in the temperature dependence of CO
conversion is also observed for the other CuO-contain-
ing samples. As the CuO content is increased from 0.5
to 6.4%, the maximal CO conversion changes from 85
to 98.5% and Tmax decreases from 200 to 150°C. The
decrease in CO conversion at T > Tmax cannot be due to
an irreversible poisoning of the catalyst since the initial
value of conversion is restored as the reaction tempera-
ture is decreased to Tmax. The maximal CO conversion
is 98.5% and is observed for 6.4% CuO/CeO2.
1
A|| = 180 G
g1|| = 2.27
100 G
H
g⊥1 = 2.023
A⊥1 = 27 G
2+
Fig. 2. ESR spectrum of Cu in the 0.25% CuO/CeO (O ,
500°C) in a vacuum at –196°C.
2
2
The state of copper ions in the oxidized and reduced
samples (0.25–6.4)% CuO/CeO2 was studied by ESR.
A 50-mg sample was placed into an ESR ampoule,
pumped to 10–4 Pa, heated in vacuo at 500°C for 1 h,
and then oxidized with O2 (P = 2 × 102 Pa) or reduced
ESR Data
Isolated Cu2+ cations in (0.25–6.4)% CuO/CeO2.
with H2 (P = 1 × 103 Pa) at this temperature for 20 min.
After oxidation, the sample was cooled to 20°C in O2
and pumped. In the case of reduction, the sample was
pumped at 500°C for 15 min and then cooled to room
temperature. ESR spectra were recorded at –196 and
20°C in the X-band region on an ESR-V spectrometer
with a Diapazon temperature attachment. The amount
of paramagnetic species was determined from the ESR
spectra after double integration and comparison with
the reference sample, CuSO4 · H2O. The accuracy of
these measurements was 20%.
Figure 2 presents the ESR spectrum of Cu2+ cations in
the oxidized 0.25% CuO/CeO2 (O2, 500°C) sample
recorded at –196°C. This spectrum is typical of isolated
Cu2+ cations in both zeolites [16, 17] and CeO2 [18].
The spectrum consists of axially symmetric signals
with g⊥ > g|| > ge. The HFS lines are due to the interac-
tion between unpaired electrons with the nuclear spin
I = 3/2. Two signals can be distinguished here. Signal 1
is characterized by g1⊥ = 2.023, A⊥1 = 27 G, g|1| = 2.27,
and A1|| = 180 G; signal 2, by g2|| = 2.34, and A|2| ~
The CO adsorption species and their interaction 180 G. The lines of signal 2 likely overlap with the lines
of signal 1 in the high-field region. This allows one to
with O2 and H2 on CeO2 and (0.25–6.4)% CuO/CeO2
KINETICS AND CATALYSIS Vol. 47 No. 4 2006

MECHANISM OF CO OXIDATION IN EXCESS H2 OVER CuO/CeO2 CATALYSTS
587
assume that g1⊥ ~ g⊥2 and A|1| ~ A|2| . The main contribu-
Jrel
1.2
tion to the ESR spectrum is from signal 1. The total
concentration of paramagnetic Cu2+ cations in 0.25%
CuO/CeO2 is 1.3 × 1017 m–2, ~30% of the concentration
of supported copper ions.
The ESR spectra of Cu2+ in the (0.5–6.4)%
CuO/CeO2 samples (O2, 500°C) are similar to the spec-
trum presented in Fig. 2. Figure 3 plots the relative
intensity (Jrel = J/Jmax) of line A in the ESR spectrum of
1.0
0.8
0.6
0.4
0.2
0
2
4
6
8
CuO content, %
Cu2+ versus the CuO content of CeO2. The intensity of
this line decreases by a factor of 15 as the CuO content
of CeO2 increases from 0.25 to 6.4%. The other lines of
the spectrum change in a similar way. This is evidence
of a decrease in the proportion of isolated Cu2+ cations
in CeO2.
2+
Fig. 3. Intensity of line A in the ESR spectrum of Cu (see
Fig. 2) versus the CuO content of CeO .
2
1
Thus, isolated Cu2+ ions are present in (0.5–6.4)%
CuO/CeO2 (O2, 500°C), and their concentration
decreases with an increase in the copper content of
CeO2. Isolated Cu2+ cations in CeO2 likely form oxo
complexes with two different coordination spheres. The
g-tensors of these signals are in agreement with those of
Cu2+ in various oxide systems, in which the coordina-
tion sphere of copper cations varies from distorted octa-
hedral to pyramidal and square-planar [18]. In this con-
nection, note that the parameters of the ESR spectrum
g⊥ = 2.023
A⊥ = 27 G
2
3
2.034
2.047
of Cu2+ in CuO/CeO2 (g1⊥ = g2⊥ = 2.023, A⊥1 = A2⊥ =
27 G, g2|| = 2.34, A|2| ~180 G, g1|| = 2.27, and A|1| =
180 G) are close to those for Cu2+ cations in CuZSM-5,
which have a square-pyramidal or square-planar coor-
dination sphere [16]. Further investigation is required
for a more detailed consideration of this point.
2.009
2+
Formation of O–2 during adsorption of CO + O2
and O2 on (0.25–6.4)% CuO/CeO2. Upon the admis-
sion of CO or O2 (P = 2 × 102 Pa, 20°C) to the oxidized
(0.25–6.4)% CuO/CeO2 (O2, 500°C) samples, the ESR
Fig. 4. (1) Cu ESR spectrum in the g region for 0.25%
CuO/CeO (O , 500°C) in a vacuum at 20°C, (2) the same
spectrum after the adsorption of CO + O , and (3) the dif-
⊥
2
2
2
ference spectrum.
signal of Cu2+ does not change in the case of CO and
broadens in the case of O2. However, after CO adsorp-
characteristic of the O2– radical anion. This species
tion for 1–2 min followed by removal of the gas and results from the reaction CO + O2 on oxidized CeO2
admission of O2, a new signal appears in the ESR spec-
trum. Its intensity decreases by 1/4 upon pumping the
sample at 20°C for 1 h. The signal disappears upon
holding the sample at 100°C in vacuo for 10 min. Only
a signal from isolated Cu2+ cations remains in the ESR
spectrum. Its intensity is equal to the intensity of the
signal from Cu2+ in the initial sample. Figure 4 presents
the Cu2+ ESR spectrum in the region of g⊥ values for the
sample 0.25% CuO/CeO2 (O2, 500°C) in vacuo at 20°C
(spectrum 1), the ESR spectrum recorded after the
adsorption of CO + O2 on the same catalyst (O2, 500°C)
(spectrum 2), and their difference (spectrum 3). Spec-
trum 3 is complicated and shows an anisotropic signal
with g1 = 2.047, g2 = 2.034, and g3 = 2.009. The shape,
[13] or from O2 adsorption on CeO2 preliminarily
reduced in vacuo at T = 400°C [13, 19] and is stabilized
in the coordination sphere of Ce4+ cations in various
anionic vacancies. Based on our earlier study of the
temperature dependence of the ESR spectrum of O2–
[13], we believe that spectrum 3 consists of at least two
anisotropic signals, whose lines are separated in the gzz
range from 2.055 to 2.016 and are greatly overlapped at
gxx and gyy of 2.016 to 2.003 at 20°C. The line with
g1zz = 2.047 is evidence of the stabilization of O–2 on
associated anionic vacancies, whereas the line with
g2zz = 2.034 points to the stabilization of O–2 in isolated
thermal behavior, and the g-tensor of the spectrum are anionic vacancies. The concentration of radical anions
KINETICS AND CATALYSIS Vol. 47 No. 4 2006

588
IL’ICHEV et al.
For the radical anion O–2 , gxx ~ 2.009 is higher than
Jrel
ge = 2.0023, indicating the formation of a covalent bond
due to the overlap of the occupied πy-orbital of oxygen
1.0
0.8
0.6
0.4
0.2
with the unoccupied 4f orbital of the Ce4+ cation [12].
2
1
The covalent bond in this complex is less strong than
the ionic bond in O–2 Zr4+ [20]. The presence of several
kinds of O–2 species and their different thermal stability
0
2
4
6
8
are likely due to their stabilization on cerium cations
with different coordination spheres. The possibility of
CuO content, %
O–2 radical anions (gzz = 2.030) being stabilized on
Fig. 5. Relative intensity of the O–2 signal versus CuO con-
tent for (1) CuO/CeO (O , 500°C) samples after the
CeO2 anions was mentioned in an earlier work [12].
2
2
adsorption of CO + O and (2) CuO/CeO (H , 500°C)
2
2
2
samples after O adsorption.
2
TPD Data
CO and CO2 adsorption species on (0.25–6.4)%
CuO/CeO2. As can be seen in Fig. 6, the TPD spectra
of reduced CeO2 and (0.5, 6.4)% CuO/CeO2 (H2,
500°C) contain a peak at Tmax ≈ 300°C after adsorption
W, arb. units
7
6
5
4
3
2
1
1
2
3
of CO (P = 2 × 102 Pa, T = 20°C, t = 10 min). The inten-
sity of the peak is maximal for CeO2. The amount of
desorbed CO per unit surface area is 1.0 × 1017 m–2. As
the copper content of the sample is increased from
0.5 to 6.4%, the amount of desorbed CO is halved
(spectra 1, 3). The presence of a CO peak at 300°C in
the TPD spectra of CeO2 and (0.5–6.4)% CuO/CeO2 is
evidence that CO desorbs from the surface of the CeO2
support of the copper-containing samples and does not
adsorb on reduced copper, which is likely to be in the
Cu0 state.
0
100
200
300
400
500
600
T, °C
Fig. 6. TPD spectra for CuO/CeO (H , 500°C) samples
2
2
after CO adsorption at 20°C. CuO content: (1) 0, (2) 0.5,
and (3) 6.4%.
It is also evident from Fig. 6 that a small amount of
CO desorbs from CuO/CeO2 at ~100°C. This is likely
due to the decomposition of the surface complexes
formed by adsorbed CO and copper cations.
on the 0.25% CuO/CeO2 (O2, 500°C) surface is 5.0 ×
1016 m–2 and is close to their concentration on pure
CeO2.
After CO adsorption (P = 2 × 102 Pa, T = 20°C, t =
The dependence of the relative intensity (Jrel) of the
10 min) on oxidized CeO2 and (0.25–6.4)% CuO/CeO2
ESR line of O–2 (ëé + é2) on the CuO content for the (O2, 500°C), gas desorption from CeO2 is negligible as
compared to desorption from the copper-containing
samples. Several H2 desorption peaks are observed in
the TPD spectra of the copper-containing samples. As
can be seen in Fig. 7, the peaks at 100 and 160°C in the
low-temperature range of the spectrum grow with an
increase in the CuO content from 0.25 to 6.4%. At
higher temperatures, the maximum of the broad peak
shifts from 450 to 270°C. CO is desorbed in the low-
temperature range of the spectrum, whereas CO2 is des-
orbed at higher temperatures. In the case of 6.4%
CuO/CeO2 (O2, 500°C), the CO and CO2 desorption
peaks are overlapped to a considerable extent, but they
can still be separated from one another. Figure 8 pre-
sents the TPD spectra recorded after CO adsorption on
6.4% CuO/CeO2 (O2, 500°C). Spectrum 1 is the super-
position of the CO and CO2 desorption spectra. Spec-
trum 2 corresponds to the desorption of CO alone. This
spectrum was recorded upon CO2 freezing in a liquid
nitrogen trap placed at the outlet of the reactor before
oxidized samples (Fig. 5, curve 1) shows that the
amount of O–2 decreases monotonously to zero as the
CuO content is increased from 0.25 to 6.4%. Hence, the
copper ion suppresses the formation of O–2 (ëé + é2)
on the oxidized CeO2 surface.
The Cu2+ ESR signal from the reduced (0.25–6.4)%
CuO/CeO2 (H2, 500°C) samples is 20–30 times weaker
than the same signal from the oxidized samples. How-
ever, the radical anion O–2 (g1 = 2.047, g2 = 2.034, g3 =
2.009) results from the adsorption of oxygen (P = 2 ×
102 Pa, 20°C) on the reduced samples, and their con-
centration, (1.5–3.0) × 1017 m–2, is higher than the
O–2 (ëé + é2) concentration on the oxidized samples.
As can be seen in Fig. 5 (curve 2), the amount of O–2 in
this case decreases only by a factor of 2 as the CuO con-
tent increases from 0.25 to 6.4%.
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MECHANISM OF CO OXIDATION IN EXCESS H2 OVER CuO/CeO2 CATALYSTS
589
the Pirani manometer. Spectrum 3 is the difference
W, arb. units
between spectra 1 and 2 and characterizes CO2 desorp-
tion. Spectrum 2 has a maximum at Tmax = 100°C and a
shoulder at ~150°C. In this temperature range, CO des-
orption is due to the decomposition of CO–Cu+ carbo-
nyls of two types, in which copper cations have differ-
ent coordination spheres [5]. Their concentration is 8 ×
1017 m–2. Spectrum 3 is complicated, with the CO2 des-
orption rate peaking at 150 and 250°C. The desorption
peak at 150°C is likely due to the oxidation of adsorbed
CO with surface oxygen, since the maximal CO conver-
sion on this sample is observed at this temperature (see
Fig. 1). The broad peak at 250°C is possibly due to the
slow decomposition of surface carbonates [5]. The
concentrations of desorbed CO2 calculated from spec-
trum 3 and from the amount of CO2 frozen in the liquid
nitrogen trap in the TPD run are similar, being 1.2 ×
1018 and 1.7 × 1018 m–2, respectively.
35
30
25
20
15
10
5
1
2
3
4
5
0
100
200
300
400
500
T, °C
Fig. 7. TPD spectra for CuO/CeO (O , 500°C) samples
after CO adsorption. CuO content: (1) 0.25, (2) 0.5, (3) 1.0,
(4) 2.1, and (5) 6.4%.
2
2
W, arb. units
18
1
16
14
12
In a TPD run carried out after CO adsorption on the
6.4% CuO/CeO2 sample not oxidized with oxygen, the
amounts of desorbed CO and CO2 were half as large.
After two successive runs, these amounts were smaller
by a factor of 5. Heating the sample in oxygen at T ≥
150°ë for 10 min resulted in the complete recovery of
catalytic activity.
10
8
3
6
2
4
2
0
100
200
300
400
500
T, °C
Thus, as the CuO content of CeO2 in the oxidized
samples increases, both CO adsorption and the amount
of CO oxidized with surface oxygen increase and the
CO2 desorption temperature decreases. The surface
oxygen consumed in oxidation is recovered upon heat-
ing the sample in O2 at 150°C.
Fig. 8. TPD spectra of the sample 6.4% CuO/CeO (O ,
2
2
500°C) after CO adsorption: (1) CO + CO , (2) CO, and
2
(3) CO .
2
W, arb. units
18
16
14
12
10
8
Effect of oxygen adsorption on CO2 desorption.
Figure 9 presents the TPD spectra for 6.4% CuO/CeO2
1
(O2, 500°C) after CO adsorption (P = 2 × 102 Pa, T =
20°C, t = 10 min) (spectrum 1) and after oxygen
adsorption (P = 2 × 102 Pa, T = 20°C, t = 5 min) on a
pumped sample containing preliminarily adsorbed CO
(spectrum 2). The broad CO2 desorption peak in the
200–450°C range in spectrum 1 is stronger than the
same peak in spectrum 2. In the former case, the con-
centration of desorbed molecules is 2.5 × 1018 m–2; in
the latter case, this concentration is 2.1 × 1018 m–2. The
difference is 4.0 × 1017 m–2. It was found that, as O2 is
adsorbed, CO2 desorbs from the surface. The amount of
2
6
4
2
0
100
200
300
400
500
T, °C
Fig. 9. TPD spectra of the sample 6.4% CuO/CeO (O ,
2
2
500°C) after the adsorption of (1) CO and (2) CO + O .
2
desorbed CO2 frozen in the liquid nitrogen trap sug- tion on CO adsorption. This point was investigated in
gests a value of 4.3 × 1017 m–2, which is equal to the
greater detail.
above difference. Lengthening the O2 adsorption time
from 5 min to 1 h increases the amount of desorbed CO2
Effect of adsorbed H2 on CO adsorption on 6.4%
CuO/CeO2. The effect of adsorbed H2 on CO adsorp-
tion was studied as follows. Initially, H2 was adsorbed
on 6.4% CuO/CeO2 (O2, 500°C) (P = 4 × 102 Pa, t =
only to 5.0 × 1017 m–2. Hence, part of the strongly
adsorbed CO2 is replaced from the surface by oxygen
at 20°C.
10 min) at a chosen temperature TH . After H2 adsorp-
2
tion, the sample was pumped and cooled to room tem-
perature, CO was adsorbed (P = 2 × 102 Pa, t = 10 min),
and a TPD run was performed. TPD spectra for various
As shown above, the decrease in the CO conversion
at T > Tmax is not caused by an irreversible poisoning of
the catalyst and can be due to the effect of H2 adsorp-
KINETICS AND CATALYSIS Vol. 47 No. 4 2006

590
IL’ICHEV et al.
broad H2O desorption peak appears at 400°C (spec-
trum 3).
The TPD spectrum of the 6.4% CuO/CeO2 (O2,
W, arb. units
18
16
14
12
10
8
1
500°C) sample on which H2 was adsorbed at 160°C and
O2 was then adsorbed at 160°C for 5 min coincides with
spectrum 2. After CO adsorption at 20°C on 6.4%
CuO/CeO2 (O2, 500°C) containing H2 and O2 adsorbed
in sequence at 160°C, the TPD spectrum shows an H2O
desorption peaks at 270°C, which is equal in intensity
to the H2O peak in spectrum 2, and CO and CO2 des-
orption peaks at 100 and 150°C (spectrum 4). Their
intensities are comparable with the intensities of the
CO and CO2 peaks in the TPD spectrum of the oxidized
sample after CO adsorption.
Hence, H2 adsorption at 100–200°C deactivates the
CO adsorption and oxidation sites. It is likely that
hydrogen is adsorbed dissociatively, is stabilized on
oxygen ions, and desorbs from the surface as H2O at
270–300°C. The active sites are regenerated upon heat
treatment in oxygen at 160°C, which does not affect the
hydrogen coverage of the surface.
3
2
6
4
4
2
0
100
200
300
400
500 600
T, °C
Fig. 10. TPD spectra recorded after CO adsorption on
(1) 6.4% CuO/CeO (O , 500°C) and (2–4) 6.4%
2
2
CuO/CeO (O , 500°C) containing H adsorbed at (2) 100,
2
2
2
(3) 160 and (4) 200°C.
W, arb. units
14
12
10
8
2
4
3
6
4
2
1
DISCUSSION
As the CuO content of CeO2 is changed from 0 to
6.4%, CO conversion increases from 30 to 98.5%,
accompanied by a decrease in Tmax from 350 to 150°C
(Fig. 1), indicating that the reaction occurs on sup-
ported CuO.
0
100
200
300
400 500 600
T, °C
Fig. 11. H O TPD spectra of the sample 6.4% CuO/CeO
2
2
(O , 500°C) after H adsorption at (1) 100, (2) 160, and
2
2
Depending on treatment conditions, copper in the
catalysts is in the Cu2+, Cu+, or Cu0 state, and these ions
participate differently in the reaction. CO is not
adsorbed on Cu0 in the reduced catalysts and is not oxi-
dized (Fig. 6). The amounts of adsorbed and oxidized
CO on the oxidized catalysts increase with an increase
in the CuO content of CeO2 (Figs. 7, 8). Hence, the
(3) 200°C. (4) TPD spectrum of the sample 6.4%
CuO/CeO (O , 500°C) recorded after the adsorption of H
2
2
2
and, thereafter, O at 160°C and CO at 20°C.
2
TH values are presented in Fig. 10. For TH = 100°ë,
2
2
the intensities of the CO and CO2 desorption peaks
around 100 and 150°C are 25% lower than the intensity
of the TPD peaks measured after CO adsorption on the
sample not treated with hydrogen (spectra 1, 2). For
TH = 160°C, the peak at 100°C is 4 times weaker and
the peak at 160°C is not observed (spectrum 3). For
TH = 200°C, neither peak is observed (spectrum 4). At
the same time, the intensity of the TPD peak at Tmax
270°C increases as TH is raised from 100 to 160°C
(spectra 2, 3) and then drops at T = 200°C. The
active sites can include Cu2+ and Cu+ cations.
According to ESR data, the Cu2+ cations form oxo
complexes, likely with a square-pyramidal or square-
planar coordination sphere, on the oxidized surface
(Fig. 2). It is improbable that they participate in CO
oxidation, since CO is not adsorbed on these cations at
room temperature. Furthermore, the concentration of
Cu2+ cations decreases with an increase in the CuO con-
tent of CeO2, whereas the CO conversion increases (see
Figs. 1 and 3). This inference was confirmed by quanti-
tative data. The concentration of Cu2+ oxo complexes in
the most active catalyst 6.4% CuO/CeO2 (~1.0 ×
2
2
~
2
H2
changes in the intensity of the desorption peak near
1016 m–2) is two orders of magnitude lower than the
270°C are due to H2O desorption after hydrogen
adsorption. Indeed, as can be seen in Fig. 11, after H2 concentration of desorbed CO (1.0 × 1018 m–2) or CO2
(1.5 × 1018 m–2).
adsorption on 6.4% CuO/CeO2 (O2, 500°C) at 100,
160, and 200°C for 5 min, only the H2O desorption
The decrease in the amount of isolated Cu2+ oxo
peak at 270–300°C is observed in the TPD spectra. Its complexes caused by an increase in the CuO content of
CeO2 is apparently due to the formation of associated
Cu2+ oxo complexes on the surface. The increasing
intensity increases by a factor of ~10 as the hydrogen
adsorption temperature is raised from 100 to 160°C
(spectra 1, 2). At 200°C, its intensity is halved and a CuO content causes an increase in the amount of CO
KINETICS AND CATALYSIS Vol. 47 No. 4 2006

MECHANISM OF CO OXIDATION IN EXCESS H2 OVER CuO/CeO2 CATALYSTS
591
adsorbed on the Cu+ cations and in the amount of CO
oxidized with surface oxygen (Fig. 7), indicating that
the active sites are in these associates (Fig. 8) [5]. The
temperature range of the deactivation of the active sites
in H2 (100–200°C, Fig. 10) coincides with the temper-
ature range of the reduction of finely dispersed copper
clusters on CeO2 [21]. The structure of these clusters on
nanostructured CuOx/CeO2 catalysts has been deter-
mined [9]: the Cu+ cations form either a linear structure
similar to that of the Cu2O compound, in which they
have two coordination bonds with oxygen ligands, or a
planar triangle, whereas the coordination polyhedron of
a Cu2+ cation is either a CuO4 tetrahedron or a heavily
distorted CuO6 octahedron. The Cu+/Cu2+ ratio depends
on catalyst activation conditions and varies between
0.35 and 0.05. Assuming that the concentration of Cu+
is equal to the concentration of CO–Cu+ carbonyls, one
can conclude that the Cu+/Cu2+ ratio in 6.4%
CuO/CeO2 is ~0.3.
is insufficient for the regeneration of the adsorption and
CO oxidation sites, whose concentrations are 0.8 × 1018
and 1.7 × 1018 m–2, respectively. Note that these sites
are not regenerated with oxygen at 20°C, but, when the
reoxidation temperature is increased to 160°C, their
concentration corresponds to the number of active sites
in the oxidized catalyst. Apparently, catalyst reoxida-
tion occurs through the dissociative adsorption of O2 on
Cu0 species, contrary to the opinion that it occurs
through O–2 transformation to ionic species [5, 6, 22].
Hydrogen on the surface of 6.4% CuO/CeO2 at 100–
200°C is stabilized as the hydroxyl species H+–O2–, as
is indicated by the fact that it desorbs as H2O at 270–
300°C (Fig. 11). Hydrogen adsorption decreases the
number of CO adsorption and oxidation sites (Fig. 10),
because of the reduction of Cu+ cations to Cu0 and the
formation of hydroxyl groups. Since the temperature
range of hydrogen adsorption (100–200°C) overlaps
partially with the range in which CO conversion on
6.4% CuO/CeO2 decreases (Tmax > 150°C), the
Thus, the coincidence of the temperature ranges in
which the active sites are deactivated and the copper decrease in catalytic activity is due to the additional
oxide clusters are reduced with hydrogen, as well as the deactivation of part of the active sites by adsorbed
similarity between the Cu+/Cu2+ ratios in 6.4%
CuO/CeO2 and nanostructured CuOx/CeO2, suggests
that CO likely adsorbs on the Cu2O clusters to form
hydrogen. Note that, after hydrogen adsorption on the
surface, the active sites are regenerated by oxygen at
150°C (Fig. 11).
CO–Cu+ carbonyl complexes.
The adsorption of a CO + O2 mixture on the oxi-
dized CuO/CeO2catalysts yields O–2 radical anions on
The amount of oxidized CO increases and the CO2
desorption temperature decreases with an increase in
the CuO coverage of the CeO2 surface (Fig. 7). There-
fore, adsorbed CO is oxidized with the oxygen of the
Cu2O clusters, possibly the bridging or terminal oxygen
coordinatively unsaturated Ce4+ cations. These species
do not participate in CO oxidation on CuO/CeO2, since
the rate of O–2 (ëé + é2) generation drops with increas-
ing CuO content (see Fig. 5), whereas CO conversion
increases (Fig. 1). The finding that the O–2 (ëé + é2)
of the Cu2+–O2––Cu2+ or ëÂ4+–O2––Cu2+ species.
The O–2 radical anions are stabilized in the coordi- concentration increases monotonically with increasing
CuO content of CeO2 indicates that O–2 (ëé + é2) for-
mation on CeO2 is inhibited by copper oxide and there
is no additional channel of O–2 (ëé + é2) generation
nation sphere of the coordinatively unsaturated Ce4+
cations. Their amount is proportional to the surface area
of the support that is not occupied by supported copper
species. As can be seen in Fig. 5, the copper coverage
of the surface in the reduced (2.1–6.4)% CuO/CeO2
catalysts is at most ~50% (curve 2), whereas it is close
to 100% in the oxidized 6.4% CuO/CeO2 catalyst. This
fact is in agreement with the high degree of dispersion
of copper oxide on the CeO2 surface. The increase in
the free surface area of the support upon the reduction
of the catalyst is due to the aggregation of Cu0 atoms
into three-dimensional species. The oxidation of copper
likely yields two-dimensional association species of
copper oxo complexes dispersed on the surface. These
structural changes in supported copper occur through
the migration of ions and atoms. It was assumed that the
copper atoms migrate to anionic vacancies [9]. On the
other hand, oxygen adsorbed on anionic vacancies is
due to the synergistic effect in this reaction. The inhib-
iting effect is due to the fact that the free surface area of
CeO2 decreases with increasing CuO coverage. In this
case, a decrease in the rate of O–2 (ëé + é2) formation
can be due to either a decrease in the number of O–2 sta-
bilization sites or a decrease in the number of electron
donor complexes (active carboxylates and carbonates)
on the CeO2 surface [12, 13].
As shown above, oxygen is necessary for maintain-
ing the catalyst in the oxidized state active in CO con-
version. Furthermore, oxygen adsorption at 20°C stim-
ulates the desorption of part of the CO2 strongly bound
to the surface, which occurs at T ≥ 160°ë in the absence
of oxygen (Fig. 8). CO conversion at 20°C [5] is likely
due to this replacement effect.
stabilized as O–2 . This oxygen species was considered
in some studies [5, 6, 22] to be an intermediate in the
catalyst reoxidation. However, on the reduced catalyst
The TPD data presented in Figs. 8 and 11 enable one
to estimate the activation energies of CO, CO2, and
H2O desorption from the equation Edes = 25RTmax [23]
6.4% CuO/CeO2, the O–2 concentration (1.5 × 1017 m–2)
KINETICS AND CATALYSIS Vol. 47 No. 4 2006

592
IL’ICHEV et al.
5. Matinez-Arias, A., Fernandez-Garcia, M., Galvez, O.,
Coronado, J.M., Anderson, J.A., Conesa, J.C., Soria, J.,
and Munuera, G., J. Catal., 2000, vol. 195, p. 207.
(where R is the universal gas constant and Tmax is the
desorption peak temperature (K)). For CO desorption,
Edes = 77 and 87 kJ/mol for Tmax = 373 and 423 K,
respectively; for CO2, Edes = 87 and 107 kJ/mol for
Tmax = 423 and 523 K, respectively (Fig. 8); for H2O,
Edes = 117 kJ/mol for Tmax = 573 K (Fig. 11).
6. Sedmak, G., Hocevar, S., and Levec, J., J. Catal., 2003,
vol. 213, p. 135.
7. Dow, W.-P., Wang, Y.-P., and Huang, T.-G., Appl. Catal.,
A, 2000, vol. 244 (1), p. 155.
It was suggested that the low-temperature CO oxi-
dation on CuO–CeO2 occurs through the Langmuir–
Hinshelwood mechanism [3, 6]. The kinetics of this
reaction were studied, and it was found that the activa-
tion energy of CO oxidation is 57 kJ/mol and the heat
of CO adsorption is 27 kJ/mol. The heat of adsorption
and activation energy of CO desorption allow one to
estimate the activation energy of CO adsorption from
the equation Eads = Edes – Q. The formation of the two
carbonyl species is characterized by Eads = 50 and
60 kJ/mol, respectively.
8. Avgoropoulos, G. and Ioannides, T., Appl. Catal., A,
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Thus, when CO is oxidized with oxygen in excess
hydrogen on CuO/CeO2, carbon monoxide adsorbs on
Cu2O clusters to form two types of CO–Cu2+ com-
plexes. Part of these complexes decomposes at 100–
160°C, and the other part is oxidized with the bridging
oxygen of Cu2O clusters. The resulting CO2 desorbs
into the gas phase at 160 and 250°C. When CO is oxi-
dized, Cu+ is reduced to Cu0. The Cu+ ions are regener-
ated upon the dissociative adsorption of O2 on Cu0.
Hydrogen also adsorbs dissociatively on Cu2O to form
hydroxyl groups and desorbs as H2O at 300°C. The
decrease in CO conversion at T > 150°C is mainly due
to the increase in the number of deactivated CO oxida-
tion sites during H2 adsorption.
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KINETICS AND CATALYSIS Vol. 47 No. 4 2006