Correlation of structural characteristics with catalytic performance of CuO/CexZr1-xO2 catalysts for NO reduction by CO

Article abstract of DOI:10.1016/j.jcat.2010.07.024

NO reduction by CO reaction was studied over a series of CuO/Ce _xZr_1-xO₂ catalysts with different copper loadings and Ce/Zr molar ratios to evaluate the correlation of their structural characteristics with catalytic performance. These catalysts were investigated in detail by means of thermogravimetric analysis (TGA/DSC), X-ray diffraction (XRD), Raman spectroscopy, high-resolution transmission electron microscopy (HR-TEM), electron paramagnetic resonance (EPR), UV-vis spectroscopy, X-ray photoelectron spectroscopy (XPS) and H₂-temperature-programmed reduction (H₂-TPR) and in situ Fourier transform infrared spectroscopy (FTIR). The results demonstrated that the ceria-rich (pseudocubic t″) phase could disperse and stabilize the copper species more effectively and resulted in stronger interaction with copper than the zirconia-rich (t) phase. Furthermore, compared with the zirconia-rich phase, the synergistic interaction of copper with ceria-rich phase easily promoted the reduction of copper species and support surface oxygen, as well as the activation of adsorbed NO species. Therefore, CuO/Ce_0.8Zr_0.2O₂ catalyst exhibited the higher activity for NO reduction than CuO/Ce _0.5Zr_0.5O₂ and CuO/Ce_0.2Zr _0.8O₂. A surface model was proposed to discuss these catalytic properties. The copper species at the interfacial area did not maintain an epitaxial relationship with Ce_xZr_{1- x}O₂, while could penetrate into the Ce_xZr _1-xO₂ surface lattice by occupying the vacant site on the exposed (1 1 1) plane. The type and coordination environment of copper species were different in ceria-rich and zirconia-rich phases surface, and their stabilities were related to the lattice strains.

Full text of DOI:10.1016/j.jcat.2010.07.024

Journal of Catalysis 275 (2010) 45–60
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Correlation of structural characteristics with catalytic performance
of CuO/CexZr1ꢀxO2 catalysts for NO reduction by CO
Lianjun Liu a, Zhijian Yao a, Bin Liu a, Lin Dong a,b,
*
a Key Laboratory of Mesoscopic Chemistry of MOE, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, PR China
b Modern Analysis Center, Nanjing University, Nanjing 210093, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
NO reduction by CO reaction was studied over a series of CuO/CexZr1ꢀxO2 catalysts with different copper
loadings and Ce/Zr molar ratios to evaluate the correlation of their structural characteristics with cata-
lytic performance. These catalysts were investigated in detail by means of thermogravimetric analysis
(TGA/DSC), X-ray diffraction (XRD), Raman spectroscopy, high-resolution transmission electron micros-
copy (HR-TEM), electron paramagnetic resonance (EPR), UV–vis spectroscopy, X-ray photoelectron spec-
troscopy (XPS) and H2-temperature-programmed reduction (H2-TPR) and in situ Fourier transform
infrared spectroscopy (FTIR). The results demonstrated that the ceria-rich (pseudocubic t00) phase could
disperse and stabilize the copper species more effectively and resulted in stronger interaction with cop-
per than the zirconia-rich (t) phase. Furthermore, compared with the zirconia-rich phase, the synergistic
interaction of copper with ceria-rich phase easily promoted the reduction of copper species and support
surface oxygen, as well as the activation of adsorbed NO species. Therefore, CuO/Ce0.8Zr0.2O2 catalyst
exhibited the higher activity for NO reduction than CuO/Ce0.5Zr0.5O2 and CuO/Ce0.2Zr0.8O2. A surface
model was proposed to discuss these catalytic properties. The copper species at the interfacial area did
not maintain an epitaxial relationship with CexZr1ꢀxO2, while could penetrate into the CexZr1ꢀxO2 surface
lattice by occupying the vacant site on the exposed (1 1 1) plane. The type and coordination environment
of copper species were different in ceria-rich and zirconia-rich phases surface, and their stabilities were
related to the lattice strains.
Received 28 April 2010
Revised 12 July 2010
Accepted 13 July 2010
Keywords:
CexZr1ꢀxO2
Structure
Interaction
Adsorption
NO reduction
Ó 2010 Elsevier Inc. All rights reserved.
1. Introduction
size) [12,13], oxygen storage capacity [14–17], redox and adsorp-
tion behaviors (CH3OH, CO, NO, NO2) [18–21], and dynamics of
Substitutional ceria–zirconia solid solutions have attracted
much interest in recent years because of their extensive use in a
number of different fields, for example, as active supports or ‘‘oxy-
gen buffers” in three-way catalysis elimination of CO, NOx and
hydrocarbons. Their crystal structures and local order are therefore
widely investigated by either theoretical calculation or XRD,
Raman, neutron spectroscopy, extended X-ray absorption fine
structure (EXAFS) and HR-TEM [1–8]. Until now, five different
phases are identified depending on the cerium content: one mono-
clinic phase (m, < 5%, P21/c), three tetragonal phases (t, t0, t00,
15% < x < 90%, P42/nmc) and one cubic phase (c, > 90%, Fm3m) [5].
The physicochemical properties of these solids are also intensively
studied. Along with the influence of preparation method and com-
plex composition (chemical homogeneity) [9–11], the structural
and textural properties (thermal stability, surface area, crystallite
oxygen storage [22], as well as catalytic activity [23,24], are
checked. The focus has been on examining the possible correlation
of structural features with these properties. Results seem to indi-
cate that CexZr1ꢀxO2 (0.5 < x < 0.8) solid solution has a superior per-
formance in terms of overall reduction and total oxygen storage.
This behavior is consistent with a structural modification of the
fluorite cell induced by the introduction of zirconia, which brings
out a higher lability in surface and bulk oxygen ions, thus enhanc-
ing their diffusion rate [22].
Other than this, in addition to noble metals (Rh, Pt, Pd, Au)
supported on ceria–zirconia-related catalysts [25–30], consider-
able efforts have been devoted to develop the promising
copper-based catalyst [31–41] for preferential oxidation of CO
and selective reduction of NO. Typically, these reports pay close
attention to the influence of particle size, metal–support interac-
tion and active species on the catalytic activity, in order to improve
the intrinsic performance and understand the reaction mechanism.
It has been agreed that the well-known enhancement of the total
oxidation activity of CuO–CeO2 is attributed to the ‘‘synergistic”
effect. Skårman et al. [35a] have attributed this to an epitaxial
* Corresponding author at: Key Laboratory of Mesoscopic Chemistry of MOE,
School of Chemistry and Chemical Engineering, Nanjing University, Nanjing
210093, PR China. Fax: +86 25 83317761.
E-mail address: donglin@nju.edu.cn (L. Dong).
0021-9517/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2010.07.024

46
L. Liu et al. / Journal of Catalysis 275 (2010) 45–60
copper phase on top of the fluorite type CeO2. Although a large
amount of structural and electronic studies on the CuO–CeZrO2
system are carried out, the interaction of copper species with the
predominately exposed plane of ceria–zirconia has been frequently
dismissed. It is still important to emphasize that the analysis of the
dispersion behaviors and structural characteristics of copper spe-
cies on the surface of metastable t (zirconia-rich), t0 and t00 (ceria-
rich) phases cannot be ruled out, especially the influence on the
catalytic properties for NO reduction. These issues remain ambigu-
ous that needs a better understanding at the atomic level.
Previously, we studied the coordination states and reduc-
tion properties of CuO/WO3/CeO2, CuO/Fe2O3 and CuO/t–ZrO2
catalysts [42,43]. On this basis, the present work fundamentally
approached the structural characteristic, dispersion, reduction,
adsorption/desorption behaviors and activity of the representative
CuO/CexZr1ꢀxO2 catalysts with varying the copper loadings and
Ce/Zr molar ratios, which were characterized by TGA/DSC, XRD,
Raman, HR-TEM, UV–vis, EPR, XPS, H2-TPR and in situ FTIR. The
good NO + CO model reaction was also performed. The aim was
to establish the composition–structure–property relationships,
which would be beneficial for the design and rationalization of
the practical catalysts.
kept in an ultrasonic bath for 15 min, then deposited on a
carbon-covered copper grid for measurement.
UV–vis DRS spectra were recorded in the range of 200–900 nm
by a UV–vis–NIR 5000 spectrophotometer. BaSO4 was used as the
reference for baseline emendation.
EPR measurements were performed on a Bruker EMX-10/12
spectrometer operating at X-band frequency (
100-kHz filed modulation. The spectra of these catalysts were
m
ꢂ 9.4 GHz) and
recorded at 77 K.
XPS analysis was performed on a Thermo ESCALAB 250 high
performance electron spectrometer, using Al
Ka radiation
(1486.6 eV) operating at an accelerating power of 150 W. All bind-
ing energies (BE) were referenced to the adventitious C 1s at
284.6 eV. This reference gave BE values with an accuracy at
0.1 eV. Charge effects were corrected by adjusting the Ce 3d3/2
peak (u000) to a position of 917.0 eV.
H2-TPR was carried out in a quartz U-tube reactor connected to
a thermal conduction detector with H2–Ar mixture (7.0% H2 by vol-
ume) as reductant. Before switched to the H2–Ar stream, the sam-
ple (50 mg) was pretreated in a N2 stream at 100 °C for 1 h. After
that, TPR started from room temperature to 650 °C at a rate of
10 °C minꢀ1
.
In situ FTIR spectra were collected from 400 to 4000 cmꢀ1 at a
resolution 4 cmꢀ1 (number of scans, ꢃ32) on a Nicolet 5700FTIR
spectrometer. The catalysts (about 25 mg) were mounted in an IR
cell and pretreated for 1 h at 100 °C in the flowing N2 atmosphere.
After cooling to room temperature, the self-supporting wafer were
exposed to a controlled stream of CO–Ar (10% CO) or/and NO–Ar
(5% NO) at a rate of 5.0 ml minꢀ1 for 30 min. In situ FTIR spectra
(temperature-programmed reduction and reaction) were recorded
at various target temperatures (at every 25 °C) by subtraction of
the background reference (collected from the gas data at each
temperature without the samples).
2. Experimental
2.1. Catalyst preparation
CexZr1ꢀxO2 (x = 0.2, 0.5, 0.8) oxides were prepared by the
co-precipitation method [41]. In brief, a requisite amount of
(NH4)2Ce(NO3)6ꢁ6H2O and Zr(NO3)4ꢁ5H2O were dissolved in dis-
tilled water, and the excess ammonia (25%) was slowly dropped
to the mixture solution with vigorously stirring until pH = 10.
The resulting solution was kept in stirring for another 3 h, aged
overnight and then filtered, washed with distilled water until no
pH change. The obtained solids were first dried at 110 °C overnight,
and then calcined at 500 °C in the flowing air for 4 h. The CuO/
CexZr1ꢀxO2 catalysts were prepared by incipient wetness impreg-
nation method with the solution containing Cu(NO3)2. The CuO
loading amount was 0.3, 0.6, 0.9, 1.2, 1.5 mmol/100 m2 CexZr1ꢀxO2.
The mixture was kept stirring for 3 h and was evaporated at 100 °C.
The resultant materials were calcined at 450 °C in the flowing air
for 4 h. For simplification, these catalysts were denoted as yCu/
C2Z8, C5Z5, C8Z2, where y was the copper loading amount.
2.3. Catalytic activity tests
The activities of the catalysts were determined under light-off
procedure, involving a feed steam with a fixed composition, 5%
NO, 10% CO and 85% He by volume as diluents. The catalysts
(50 mg) were pretreated in N2 stream at 100 °C for 1 h and then
cooled to room temperature, after that, the mixed gases were
switched on. The reactions were carried out at different tempera-
tures with a space velocity of 12,000 mL gꢀ1 hꢀ1. Two gas chroma-
tographs equipped with thermal conduction detections were used
for analyzing the production. Column A with Paropak Q for sepa-
rating N2O and CO2 and column B packed with 5A and 13X mole-
cule sieve (40–60 M) for separating N2, NO and CO. Kinetic
measurements were performed under differential reaction condi-
tions, with ꢃ15 mg catalyst to limit the conversion between 5%
and 40%. The turnover frequency (TOF, sꢀ1) of NO over per copper
atom was calculated by this equation, i.e., TOF = P ꢁ Vs ꢁ C ꢁ VNO% ꢄ
10ꢀ6/R ꢁ T ꢁ nCu (Vs = space velocity, C = NO conversion, VNO% = NO
concentration, nCu = molar ratio of copper species). The content of
surface copper species was determined by XPS.
2.2. Catalyst characterization
The specific surface areas of these supports were determined
via nitrogen adsorption at 77 K (Micromeritics ASAP-2020) with
the Brunauer–Emmett–Teller (BET) method.
The change in the weight of the uncalcined supports was mon-
itored by thermogravimetric analysis (TGA) in the flowing air from
room temperature to 900 C. A commercial Netzsch TGA/DSC ana-
lyzer (ATA 449C-Thernak Star 300) was employed for this purpose.
XRD patterns were recorded on a Philips X’pert Pro diffractom-
eter using Ni-filtered Cu K
tube was operated at 40 kV and 40 mA. The mean grain sizes (Db)
from the (1 1 1) plane and strain ( ) of these samples were calcu-
a radiation (k = 0.15418 nm). The X-ray
3. Results and discussion
e
lated by the Scherrer equation (Db = Kk/bcos h) and
e
= b/4tgh,
3.1. Structural characterization (TGA, XRD, Raman, HR-TEM)
respectively.
Raman spectra were collected on a Jobin–Yvon (France–Japan)
T64000 type Laser Raman spectroscopy using Ar+ laser beam, oper-
ating with an excitation wavelength of 516 nm and the laser power
of 300 mW.
HR-TEM images of these samples were obtained by a JEM-
2100CX system. The six samples were dispersed in ethanol and
3.1.1. TGA and XRD analysis
The TGA/DSC analysis of these supports before calcination
provided some information on the interaction between cerium
and zirconium hydrate with different molar ratios. As shown in
Fig. 1, these endothermic peaks of the three samples in the range
70–300 °C were primarily due to the loss of nondissociative

L. Liu et al. / Journal of Catalysis 275 (2010) 45–60
47
399oC
(a) 100
(a)
0.2
C2Z8
0.1
95
90
85
80
ZrO2
C2Z8
0.0
-0.1
-0.2
-0.3
C5Z5
18.43%
C8Z2
CeO2
100oC
10
20
30
40
50
60
70
80
150
300
450
600
750
900
2θ/o
Temperature/oC
0.0
(b)
•
CuO
(b)
C5Z5
336oC
100
-0.1
-0.2
-0.3
-0.4
12Cu/C2Z8
•
95
90
85
•
12Cu/C5Z5
12Cu/C8Z2
100oC
13.79%
06Cu/C2Z8
06Cu/C5Z5
06Cu/C8Z2
10
20
30
40
50
60
70
80
2θ/o
150
300
450
600
750
900
Temperature/oC
5.40
5.35
5.30
5.25
5.20
5.15
5.10
(c)
0.1
xZ1-x-100
03Cu/C
(c)
C8Z2
100
xZ1-x-450
xZ1-x-100
xZ1-x-450
03Cu/C
06Cu/C
06Cu/C
0.0
-0.1
-0.2
-0.3
-0.4
-0.5
98oC
95
90
11.88%
C2Z8
C5Z5
C8Z2
150
300
450
600
750
900
Fig. 2. XRD patterns of these supports and CuO/CexZr1ꢀxO2 catalysts.
Temperature/oC
Fig. 1. TG–DSC curves of these ceria–zirconia supports (a) C2Z8, (b) C5Z5, and (c)
C8Z2.
over, the weight loss of the samples was from ambient up to
500 °C. Apparently, the amorphous to crystalline transition of
CexZr1ꢀxO2 was affected by the sample compositions [11], since
the transition temperatures and weight loss increased as the cer-
ium content decreased.
adsorbed water and the hydroxyl groups [6]. These second exo-
thermic peaks at higher temperatures (C2Z8: 399 °C, C5Z5: 336 °C)
were related to the heat effect on the phase transformation from
amorphous to tetragonal crystallites, and the broad peak in C8Z2
should be associated with the crystallization of cubic phase. More-
When compared with cubic ceria and tetragonal zirconia pat-
terns (Fig. 2a), the diffraction peaks of the mixed oxides were

48
L. Liu et al. / Journal of Catalysis 275 (2010) 45–60
Table 1
broadened due to the small crystalline size [3] and shifted to high
angles as a function of zirconium content, indicating the phase
transition from the cubic fluorite structure to the tetragonal struc-
ture [5]. The symmetrical peaks of CexZr1ꢀxO2 suggested the forma-
tion of single ceria–zirconia solid solution (no peak splitting for the
presence of two phases) [12a]. However, it was difficult to explore
the intermediate tetragonal (t0, t00) phase at high zirconium content
due to the limitation of XRD. As shown in Fig. 2b, after the impreg-
nation of copper species with low loading, very weak peaks for
crystalline CuO appeared in 06Cu/C2Z8 and 06Cu/C5Z5 but were ab-
sent in 06Cu/C8Z2. When at high loading, the diffraction intensity
of crystalline CuO increased distinguishingly in 12Cu/C2Z8 and
The surface area, grain size, lattice parameter, strain and space group of CexZr1ꢀxO2
and 06Cu/CexZr1ꢀxO2 catalysts.
Samples
BET
Grain
size (nm)
Lattice
parameter (Å)
Lattice
strain (%)
Space
group
(m2/g)
ZrO2
CeO2
C2Z8
C5Z5
56.9
27.0
63.1
56.6
48.8
–
16.4
14.4
7.7
5.3
5.7
9.9
6.8
6.5
5.09
5.40
5.18
5.27
5.37
5.12
5.21
5.32
0.46
0.55
0.99
1.44
1.42
0.76
1.13
1.21
t, P42/nmc
c, Fm3m
t, P42/nmc
t00, P42/nmc
t00, P42/nmc
t, P42/nmc
t00, P42/nmc
t00, P42/nmc
C8Z2
06Cu/C2Z8
06Cu/C5Z5
06Cu/C8Z2
–
–
459
(a)
(b)
474
t-ZrO2
CeO2
299
624
293
306
612
06Cu/C8Z2
440
450
635
247
314
457
C8Z2
C5Z5
C2Z8
06Cu/C5Z5
06Cu/C2Z8
625
600
300
600
750
900
300
450
750
900
Raman shift (cm-1)
Raman shift (cm-1)
Fig. 3. Raman spectra of these supports and 06Cu/CexZr1ꢀxO2 catalysts.
(a)
(b)
(c)
5 nm
5 nm
5 nm
Fig. 4. HR-TEM and selected area electron diffraction (SAED) images of these 06Cu/CexZr1ꢀxO2 catalysts (a and d) 06Cu/C2Z8, (b and e) 06Cu/C5Z5, and (c and f) 06Cu/C8Z2.

L. Liu et al. / Journal of Catalysis 275 (2010) 45–60
49
12Cu/C5Z5, while was very weak in 12Cu/C8Z2. These results
indicated the difference in their dispersion capacity (details seen
in Fig. S1), and the ceria-rich support seemed to disperse and
anchor the copper species more effectively than the zirconia-rich
one.
displayed the low level. As reported previously [2,17], strain was
a measurement of the lattice stress existing in the materials be-
cause of surface effects and/or the crystal imperfections. Accord-
ingly, the above difference in strains should be related to the
large density of oxygen defects in C8Z2 and small in C2Z8, which
might be caused by the defective pseudocubic (t00) structure of
C8Z2 and nondefective tetragonal structure of C2Z8 [17]. In addi-
tion, the impregnation of copper species slightly reduced the lat-
tice strains of C8Z2, C5Z5 and C2Z8. The strains of the resultant
catalysts were in the order of 06Cu/C8Z2 > 06Cu/C5Z5 > 06Cu/C2Z8.
Therefore, these stabilized copper species could interact with the
underlying supports, and the interaction between them was
06Cu/C8Z2 > 06Cu/C5Z5 > 06Cu/C2Z8 in sequence.
The grain size, lattice parameter and strain of the mixed oxides
before and after copper impregnation are summarized in Table 1.
The data shows that the grain size of C2Z8 is bigger than that of
C8Z2 and C5Z5, and all of them slightly increase after the introduc-
tion of copper species. Table 1 also shows that the lattice parame-
ters of cubic CeO2 and tetragonal ZrO2 are 5.40, 5.09 Å,
respectively. When x = 0.8, 0.5, 0.2, the lattice parameters of
CexZr1ꢀxO2 decrease from 5.37, 5.27 to 5.18 Å, respectively. This
observation is due to the displacement of Ce4+ ions (r4þ = 0.97 Å)
Ce
4þ
by the smaller zirconium ions (r = 0.84 Å) [9–12]. Interestingly,
3.1.2. Raman spectra analysis
Zr
Fig. 2c showed that the lattice parameter of ceria–zirconia did
not change after the addition of copper species without calcination.
However, calcination at 450 °C led to the lattice contraction, prob-
The XRD results were well complemented by Raman spectra. As
shown in Fig. 3a, C2Z8 maintained the tetragonal structure (identi-
cal to t-ZrO2) [3,5]. C5Z5 showed one weak band at ca. 474 cmꢀ1
,
2þ
ably due to the incorporation of monolayer copper (r = 0.73 Å)
which was due to the characteristic F2g Raman active mode of
the fluorite type lattice [9,12]. Two weak bands at ca. 306 and
635 cmꢀ1 were attributed to the tetragonal substitution of oxygen
atoms from their ideal fluorite lattice position and the nondegener-
ate Longitudinal Optical (LO) mode of ceria (linked to oxygen
vacancies in the ceria lattice), respectively [6,9,12]. Compared with
pure CeO2, C8Z2 displayed the broad F2g band, which was related to
either the small size or the formation of oxygen vacancy [6,30].
Cu
into ceria–zirconia surface/subsurface layers. Nevertheless, the
copper did not insert into the bulk lattice to form a new CuCeZrOx
solid solution, because ceria–zirconia still maintained their original
crystal structures with no additional Bragg-position shift (Fig. S2).
Apart from this, the introduction of Zr species led to extra force
that increased the lattice strain of CeO2. The ceria-rich support
exhibited the relatively high lattice strain, while the zirconia-rich
(a)
(b)
Amorphous
I
(111)
0.296 nm
0.289nm
0.248 nm
C2Z8
(111), ZrO2
(111)
Crystalline CuO
0.305 nm
(c)
(d)
Amorphous
(111)
0.299 nm (111)
0.255 nm
(111)
(111)
0.297 nm
0.299 nm
(111)
(e)
(f)
0.309 nm
(111)
I
0.309 nm
(111)
0.306 nm
(220)
0.308 nm
0.185 nm
(111)
(200)
(111)
II
Amorphous
Fig. 5. HR-TEM images of these 06Cu/CexZr1ꢀxO2 catalysts dried at 100 °C and calcined at 450 °C, respectively, (a and b) 06Cu/C2Z8, (c and d) 06Cu/C5Z5, and (e and f) 06Cu/
C8Z2.

50
L. Liu et al. / Journal of Catalysis 275 (2010) 45–60
Additionally, Raman bands at 306 and 635 cmꢀ1 red shifted to 293
and 612 cmꢀ1 with changing C5Z5 to C8Z2. This shift is associated
with more Zr ions in C5Z5 and accompanied by t phase-like lattice
distortions [5]. The above observations also predicted the possibil-
ity of the existence of metastable t00 phase or/and the association
with t phase-like lattice distortion in the C5Z5 and C8Z2 systems
[5,17]. A comparison of the data from TGA/DSC, XRD and Raman
showed that the tetragonal (t) phase was predominant in C2Z8;
pseoducubic (t00) phase was predominant in C8Z2 and C5Z5. In fact,
the t00 phase differed from the c phase only in that some oxygen
ions were displaced from their ideal fluorite positions to new
tetragonal positions, while all cations remained in their fcc posi-
tions [5]. Although C8Z2 and C5Z5 were in the same pseoducubic
ðt00Þ phase, C8Z2 preferentially exhibited the cubic characteristic,
while C5Z5 mainly showed the tetragonality. This was in agreement
with the report that the tetragonality of three tetragonal phases
ðt; t00; t00Þ increased with zirconia content [14]. As shown in
Fig. 3b, further evidence of copper incorporation into the surface/
subsurface position of CexZr1ꢀxO2 was given by the red shift (from
474 to 459 and 457 to 440 cmꢀ1) and width observed in the F2g
peaks. Since the grain size of CuO/CexZr1ꢀxO2 increased, these
observations certainly presented the influences of the interaction
between copper species and ceria–zirconia and their inhomoge-
neous strains.
3.1.3. HR-TEM analysis
HR-TEM was performed to ascertain the crystallite growth and
morphology of the three representative catalysts. For 06Cu/C2Z8 at
high magnifications (Fig. 4a), it contained the smoothly interfaced
particles (ꢃ6–10 nm) with the irregular shapes. Only one kind of
1.6
(a)
(b)
341
06Cu/C2Z8
271
504
271
06Cu/C5Z5
06Cu/C8Z2
1.2
0.8
0.4
1.2
527
220
216
0.8
342
483
0.4
744
C8Z2
0.0
C5Z5
672
C2Z8
200
400
600
800
200
400
600
800
Wavelength (nm)
Wavelength (nm)
Fig. 6. UV–vis spectra of (a) these supports and (b) 06Cu/CexZr1ꢀxO2 catalysts.
(a)
(b)
100 oC
×5
06Cu/C8Z2
100
C2Z8
C5Z5
06Cu/C5Z5
06Cu/C2Z8
450 oC
×5
06Cu/C8Z2
06Cu/C5Z5
2.020
C8Z2
06Cu/C2Z8
1.943
1.961(Ce3+)
2.047
2.031
2400
2800
3200
3600
2500
2750
3000
3250
3500
3750
Magnetic Field (G)
Magnetic Field (G)
Fig. 7. EPR spectra of (a) CexZr1ꢀxO2 and (b) 06Cu/CexZr1ꢀxO2 catalysts dried at 100 °C and calcined at 450 °C, respectively.

L. Liu et al. / Journal of Catalysis 275 (2010) 45–60
51
periodicity of lattice fringes (ꢃ0.292 nm) can be observed, which
was compatible with the distance expected between the (1 1 1)
reticular plane of C2Z8. The results indicated that the most fre-
quently exposed crystal plane of C2Z8 should be (1 1 1) plane. Judg-
ing from the continuous rings in the selected area diffraction (SAD)
pattern (Fig. 4d), there was an evidence for the presence of the
tetragonal phase, viewed in the (1 1 2) facet with the spacing of
0.211 nm [6]. For 06Cu/C5Z5 sample (Fig. 4b), the high presence
of Moiré fringes was taken as an interaction of aggregates com-
posed of epitaxially interfaced nanocrystallites, and it was also en-
Table 2
EPR spin Hamiltonian parameters.
Samples
Temperature (°C)
Assignment
g
g\
A (Cu) (G)
k
k
C2Z8
C5Z5
C8Z2
500
Ce3+
1.961
2.031
g = 2.047
1.943
2.020
–
–
–
Ce4+–Oꢀ2 ꢁ
Ce3+ and oxygen vacancy
06Cu/C2Z8
06Cu/C5Z5
06Cu/C8Z2
100
450
Isolated surface Cu2+
2.318
2.128
2.329
2.122
2.055
2.128
2.044
2.122
179
Not resolved
140
Interacting Cu2+
Isolated surface Cu2+ in the tetragonal vacant site
Interacting Cu2+
Not resolved
100
450
Isolated surface Cu2+
Interacting Cu2+
2.302
2.114
2.329
2.106
2.051
2.114
2.039
2.106
199
Not resolved
140
Isolated surface Cu2+
Not resolved
100
450
Isolated surface Cu2+
2.330
2.113
2.335
2.110
2.045
2.113
2.038
2.110
125
Not resolved
132
Interacting Cu2+
Isolated surface Cu2+ in the cubic vacant site
Interacting Cu2+
Not resolved
Zr 3d
v'''
Ce 3d
u'''
uo,u
vo,v
(c)
(b)
u''
u'
(c)
v'
v''
(b)
(a)
(a)
Ce 3d(3/2)
Ce 3d(5/2)
920
910
900
890
880
188
186
184
182
180
178
Binding Energy/eV
932.7eV
930.5
O'
Cu 2p
O 1s
(c)
2p1/2
shake up
(c)
O''
(b)
(b)
(a)
(a)
960 955 950 945 940 935 930
534
532
530
528
526
Binding Energy/eV
Fig. 8. XPS spectra (Ce 3d, Zr 3d, Cu 2p, O 1s) of these 06Cu/CexZr1ꢀxO2 catalysts.

52
L. Liu et al. / Journal of Catalysis 275 (2010) 45–60
closed by the (1 1 1) plane with the interplanar spacing of
0.296 nm. The patterns of the majority of nanocrystal imaged in
the SAD (Fig. 4e) could again be indexed to the fluorite structure
(metastable pseudocubic and tetragonal without monoclinic
trace). For 06Cu/C8Z2 sample (Fig. 4c and f), the image (number
of block-shaped particles, ꢃ5–8 nm) clearly displayed the (1 1 1)
lattice fringes with interplanar spacing of 0.308 nm, and some ex-
tended defects such as dislocations, grain boundaries and staking
faults observed inside particles. From the SAD pattern, the pseu-
docubic structure could be identified [7,11]. Therefore, the
above-mentioned analysis reasonably revealed that CexZr1ꢀxO2
oxides were shown to predominantly expose the (1 1 1) plane.
The crystal phases and space groups of these solid oxides were
summarized in Table 1.
HR-TEM was further performed on the fresh and calcined 06Cu/
CexZr1ꢀxO2 samples to obtain fine structural details of the nano-
sized particles when dispersed onto supports. Fig. 5a shows a
HR-TEM image of 06Cu/C2Z8 before calcination, which contains
the aggregated particles with the average size ꢃ7 nm. The spacing
of these lattice fringes is around 0.289 nm, which is ascribed to the
(1 1 1) crystallographic planes of C2Z8 support. However, no crystal
planes of copper can be seen in the lattice-fringe image. After cal-
cined at 450 °C (Fig. 5b), a few CuO crystallites appear with the lat-
tice spacing 0.248 nm, but not frequently enough to account for all
impregnated copper and without any obvious epitaxial relation-
ship to C2Z8 support. Crystallographic planes at 0.296 nm corre-
sponding to (1 1 1) planes of C2Z8 support are not parallel to
planes at 0.248 nm due to (1 1 1) plane of CuO [35b]. In addition,
apparently amorphous surface CuOx clusters are found evenly
distributed on top of or at the edge of zirconia crystal. Similar
phenomena could also be observed in 06Cu/C5Z5 sample (Fig. 5c
and d). In comparison with 06Cu/C8Z2 before and after calcination
(Fig. 5e and f), HR-TEM images provide obvious evidence that the
original spherical-shaped particles (5–7 nm) disappear, while
block-shaped particles exhibit sharp edges and distinct flat surface
presenting a higher crystallinity factors. Interestingly, the C8Z2 par-
ticle size distribution has not been broadened with respect to the
fresh one. The frequently exposed plane is also the (1 1 1) facet
with the lattice spacing of 0.308 nm. Both these electron diffraction
results and lack of detection of distinct copper crystals in the HR-
TEM images suggest a high dispersion of copper phase. Moreover,
in the zone of I and II, it is seen that the amorphous copper seem to
be embedded in the lattice of C8Z2 support. These results reveal
that (1) copper oxide and ceria–zirconia unlikely maintain an epi-
taxial relationship due to the mismatch between crystal lattices of
CuO and the supports. Skårman et al. [35a] also proposed that ten-
orite oxides, where all Cu was in the +2 state, imposed a strong
monoclinic distortion of the unit cell and would break loose from
the CeO2 film support. (2) Compared with the zirconia-rich phase,
a stronger interaction between copper species and ceria-rich phase
induces the change of its morphology. (3) Copper in small amounts
(lower than the dispersion capacity) has a strong tendency to asso-
ciate with ceria-rich phase, and excess copper (higher than the dis-
persion capacity) forms bulk CuO particles on the zirconia-rich
surface. This supports the XRD results.
3.2. Chemical and electronic states of CuO/CexZr1ꢀxO2 catalysts (UV,
EPR, XPS)
3.2.1. UV–vis and EPR analysis
The information on the surface electronic states can be obtained
from UV–vis diffuse reflectance measurements. As shown in
Table 3
XPS elementary surface concentration of 06Cu/CexZr1ꢀxO2 catalysts.
Samples
Atomic concentration
Atomic ratios
Ce (at.%)
Zr (at.%)
Cu (at.%)
O (at.%)
Ce/Zr
Cu/(Ce + Zr)
O/(Ce + Zr + Cu)
06Cu/C2Z8
06Cu/C5Z5
06Cu/C8Z2
5.3
12.0
20.4
16.7
10.1
4.1
2.9
3.7
4.0
60.0
55.5
52.1
0.32 (0.25)
1.18 (1.0)
4.97 (4.0)
0.13
0.16
0.16
2.41
2.15
1.83
(a)
(b)
0.32
0.20
0.28
0.24
0.20
0.16
0.15
0.10
0.05
0.00
0.46
0.73
0.0
0.3
0.6
0.9
1.2
1.5
0.3
0.6
0.9
1.2
1.5
Copper loading (mmol/100m2C2Z8)
Copper loading (mmol/100m2C8Z2)
Fig. 9. XPS peak intensity ratio ICu/I(Ce+Zr) versus the content of copper oxide in Cu/C2Z8 and Cu/C8Z2 samples.

L. Liu et al. / Journal of Catalysis 275 (2010) 45–60
53
Fig. 6a, two bands for C5Z5 and C8Z2 supports at ca. 271 and 342 nm
can be attributed to O2ꢀ ? Ce4+ charge transfer and interband tran-
sitions [12a], whereas only a broad one at 271 nm appeared in C2Z8
sample. The absorption band at 255 nm corresponding to the
O2ꢀ ? Ce3+ charge transfer transition [17] was not clearly ob-
served. Eight-coordinated tetravalent Zr species (like those of cubic
and tetragonal zirconia) were responsible for the absorption in the
range 200–220 nm [44]. On the other hand, the position of absorp-
tion edges was located in 504, 527 and 483 nm for C8Z2, C5Z5 and
C2Z8 in sequence and did not keep any direct linear relationship
with the composition. This may be due to the similar particle sizes
of these oxides (shown in XRD, TEM results). The CuO-based cata-
lysts (Fig. 6b) showed a broad d–d band in the visible region [36].
This band shifted from 672 to 744 nm with a change in the sup-
ports (C2Z8, C5Z5, and C8Z2, respectively), indicating the coordina-
tion environment of copper species was influenced by support
structures. Ratnasamy et al. [36] also observed the shift from 780
(Cu2+ in sixfold coordination) to 680 nm (Cu2+ in fourfold coordina-
tion) with changing the support from ZrO2 to CeO2–ZrO2 to CeO2.
The EPR spectra of CexZr1ꢀxO2 are presented in Fig. 7a. The C8Z2
spectra show a line at g = 1.961 and g\ = 1.943, which has been as-
k
signed previously to Ce3+ ions [36]. It also exhibits a strong reso-
nance line at g = 2.031 and g\ = 2.020, which is attributed to Oꢀꢁ
k
2
species bound to Ce4+ ions [29b]. The additional signal corresponds
to g = 2.047. Since Zr4+ cations do not contribute to EPR, this line is
probably due to the presence of Ce3+ ions and oxygen vacancies.
Such vacancies may trap electrons that give rise to paramagnetic
signal, in contrast to vacancies originated from mere relocation
of O2ꢀ ions into the octahedral interstitial sites [14a]. The C5Z5
and C2Z8 samples display the similar resonance line, the fine struc-
ture of which is a little poorly resolved. After the impregnation of
copper species, at least two types of paramagnetic copper can be
identified in the EPR spectrum (Fig. 7b). No trace of either Ce3+ or
superoxide ions (Oꢀ) is detected except fresh 06Cu/C8Z2. Above
2
all, the resonance line intensities of the three fresh 06Cu/
CexZr1ꢀxO2 samples correspondingly decrease after calcination.
This result indicates that heat treatment reduces the concentration
of Cu2+ ions in the system, probably due to the electron interaction
Ce 3d(5/2)
Ce 3d
Zr 3d
(B)
Ce 3d(3/2)
181.7
184.2
(d)
(c)
(d)
(c)
(b)
(a)
(b)
(a)
930
920
910
900
890
880
870
192
188
184
180
176
Binding Energy (eV)
Binding Energy (eV)
529.2
Cu 2p
(d)
O'
O 1s
932.3
930.3
shake up
2p1/2
529.6
O''
531.4
933.6
932.9
(d)
(c)
(c)
(b)
532.1
531.5
934.6
(a)
529.7
(b)
(a)
531.9
965
960
955
950
945
940
935
930
925
540
537
534
531
528
525
Binding Energy (eV)
Binding Energy (eV)
Fig. 10. XPS spectra (Ce 3d, Zr 3d, Cu 2p, O 1s) of these 03Cu/C2Z8 (a and b) and 03Cu/C8Z2 (c and d) catalysts dried at 100 °C and calcined at 450 °C, respectively.

54
L. Liu et al. / Journal of Catalysis 275 (2010) 45–60
between Cu2+ and Ce3+ ions (Cu2+ + Ce3+ ? Cu+ + Ce4+). A compari-
son of spin Hamiltonian parameters of 06Cu/CexZr1ꢀxO2 (Table 2)
original copper species, which are distributed on the top of surface,
now possibly diffuse and insert into the surface lattice by occupy-
ing the vacant sites on the (1 1 1) surface of supports. On the other
hand, the isolated Cu2+ in the aged 06Cu/C2Z8 and 06Cu/C8Z2 are
different in their g , g\ and A (Cu) (former: g = 2.329, g\ = 2.044,
shows that these g , g\ and A (Cu) values are different before and
k
k
after calcination. The EPR parameter values of the calcined samples
are also characteristic of isolated Cu2+ ions located in the tetrago-
nally distorted octahedral sites [31,37]. This accounts for that the
k
k
k
and A (Cu) = 140 G; latter: g = 2.335, g\ = 2.038, and A (Cu) =
k
k
k
132 G). The species characterized by a broad isotropic signal are
attributed to dipolar, interacting Cu2+ ions, which are centered at
average g = 2.122, 2.106, 2.110 in 06Cu/C2Z8, 06Cu/C5Z5 and
06Cu/C8Z2, respectively. Therefore, the types and coordination
environments are influenced by the crystal structure of ceria–
zirconia [36].
(a)
0.4
(a)
06Cu/C2Z8
5
C8Z2
Cu+-CO
1098
300oC
250oC
C5Z5
C2Z8
200oC
150oC
100
200
300
400
500
600
1
100oC
25oC
Temperature/ oC
1546 1387
2100 1600 1400
2107
(b)
2400
1200
1000
(I)
(II)
Wavenumber (cm-1)
(b)
06Cu/C5Z5
5
06Cu/C8Z2
Cu+-CO
300oC
250oC
200oC
06Cu/C5Z5
150oC
06Cu/C2Z8
400 500
Temperature/ oC
100oC
25oC
2400
1363
1475
100
200
300
1557
2100 1600
2106
1400
1200
1000
Wavenumber (cm-1)
(c)
1
(II)
(I)
(c)
06Cu/C8Z2
5
12Cu/C8Z2
1065
Cu+-CO
300oC
250oC
12Cu/C5Z5
12Cu/C2Z8
200oC
150oC
100oC
25oC
1360
1496
1576
2108
100
200
300
400
500
2400
2100 1600 1400 1200 1000
Wavenumber (cm-1)
Temperature/ oC
Fig. 11. H2-TPR profiles of (a) CexZr1ꢀxO2, (b) 06Cu/CexZr1ꢀxO2, and (c) 12Cu/
CexZr1ꢀxO2 catalysts.
Fig. 12. In situ FTIR spectra of CO interaction with 06Cu/CexZr1ꢀxO2 catalysts from
25 to 325 °C (a) 06Cu/C2Z8, (b) 06Cu/C5Z5, and (c) 06Cu/C8Z2.

L. Liu et al. / Journal of Catalysis 275 (2010) 45–60
55
3.2.2. XPS analysis
In order to substantiate XRD results and further understand the
difference of copper dispersion, XPS is performed on yCu/C2Z8 and
yCu/C8Z2 catalysts with a series of amounts (seen in Fig. S4). The
XPS peak intensity ratio ICu/I(Ce+Zr) is linear fitted as a function of
copper loading amount. As represented in Fig. 9a and b, the disper-
sion capacity of copper oxide on zirconia-rich and ceria-rich phases
is 0.46 and 0.73 mmol/100 m2 CexZr1ꢀxO2, respectively. Remark-
ably, ceria-rich phase can disperse copper species more effectively
than zirconia-rich phase. This is consistent with the XRD quantita-
tive results.
XPS was further performed on the fresh and calcined 03Cu/C2Z8
and 03Cu/C8Z2, which provided information about the surface
interaction and the chemical state of elements in the near-surface
region. As present in Fig. 10A and B, in comparison with zirconia-
rich phase catalyst before and after calcination (a and b), the
valence of cerium was predominant in a 4+, and no obvious
chemical shift took place on Ce 3d. However, the intensity of v0
and u0 peaks for Ce3+ increased, and the Zr 3d3/2 slightly shifted
to low binding energy. For ceria-rich phase catalyst (c and d), cal-
cination at 450 °C resulted in the chemical shift of Ce 3d and Zr 3d
peaks, and the decrease in peak intensity. These differences indi-
cated that the metals’ environments were different near the cerium
or zirconium sites, which may be due to the electronic state inter-
action between ceria–zirconia and the introduced species in the
catalysts [12b].
The catalyst systems were analyzed by XPS to verify surface
composition and elementary oxidation states. The complex spec-
trum of Ce 3d was decomposed into eight components with the
assignment defined in Fig. 8. The chemical valence of cerium on
the surface of CexZr1ꢀxO2 was mainly in a +4 oxidation state, and
a small quantity of Ce3+ v0, u0) co-existed [10,13,17]. Nagai et al.
(
[8] concluded the enhancement of homogeneity of the Ce and Zr
atoms could ease the valence change of the Ce (Ce4+ ? Ce3+).
Therefore, the presence of Ce3+ was partly attributed to the relative
homogeneous CexZr1ꢀxO2 (seen in Fig. S3, XRD patterns and Raman
spectra of the three samples calcined at 750 °C for 5 h did not ex-
hibit extra peaks, indicating no phase segregation occurred
[11,12a]). Another reason was probably the substitution of Ce4+
by Zr4+ ions. The spontaneous transformation of Ce4+ (r
=
4þ
Ce
0.97 Å) ion to the larger Ce3+ (r3þ = 1.10 Å) can compensate for this
Ce
lattice contraction. Nevertheless, the part photoreduction of ceria
cannot be ignored. The relative intensities of v0 and u0 to the other
six bands increased from 16.6% to 17.2% to 22.4% with the increase
in zirconium content. This revealed that the surface ratio of Ce3+ to
Ce4+ on the three samples was enhanced by the incorporation of
zirconia. The Zr 3d3/2 (183.7 eV) and Zr 3d5/2 (181.5 eV) peak areas
and amplitudes decreased as a function of reducing zirconium con-
tent. Interestingly, the BE of the latter was higher than that in Zr
metal (180.0 eV), lower than that in ZrO2 (182.9 eV), but similar
to that in ZrOx (0 < x < 2, 181.4 eV) [44]. Hence, the XPS study also
proved the displacement of Zr4+ ions into the ceria lattice. Further-
more, all the cerium to zirconium atomic ratios were a little higher
than the nominal ratios calculated from the bulk composition, as
indicated by the values in parentheses (Table 3), which suggested
that a spot of cerium was enriched on the surface [14].
Concerning the Cu 2p spectra, it was apparent that most Cu ini-
tially present in these samples was in a +2 oxidation state and
small amount in a +1 oxidation state, located at 933.2 and
930.8 eV respectively [34]. The generation of Cu+ together with
observation of Ce3+ species was indicative of the redox equilibrium
(Cu2+ + Ce3+ M Cu+ + Ce4+) shifting to right in these samples, which
was claimed to be the source of a synergistic effect on catalytic
properties [45]. This confirmed the earlier deduction from EPR.
Moreover, the main peak of Cu 2p3/2 over 06Cu/C8Z2 sample was
broad, whereas over 06Cu/C2Z8 was relatively sharp. This resulted
from the stronger interaction between copper species and ceria-
rich support than zirconia-rich support. Table 3 also showed that
the surface Cu/Ce + Zr atomic ratios over the three catalysts
appeared to be in the same level, and nearly three times the value
of the nominal ratio (0.05). In this case, copper species with low
loading appeared to segregate on the surface of ceria–zirconia,
and the formation of a CuCeZrOx bulk solid solution did not take
place.
The high-resolution spectrum for the O 1s ionization features
was numerically fitted with the Gaussian features representing
the primary O 1s ionization feature and chemically shifted O 1s
features from chemisorbed surface species. The strong band O0
(529.9 eV) was attributed to characteristic oxygen of the metal oxi-
des [32], while the shoulder O00 with higher binding energy
(532.1 eV) was the result of chemisorbed oxygen [12]. Although
no chemical shift was found in the O 1s feature with changing
the oxide composition, the O/(Cu + Ce + Zr) atomic ratios (Table 3)
exceptionally increased with the increase in zirconium. This should
be due to the presence of Ce3+ over ceria-rich sample that favored
the formation of oxygen vacancy in the oxide surface. In addition,
the O/(Cu + Ce + Zr) ratios over 06Cu/C2Z8 and 06Cu/C5Z5 were
higher than the nominal ratio (1.95) of the fully oxidized surface,
but over 06Cu/C8Z2 were lower. The excess surface oxygen was
attributed to the high concentration of surface oxygen as an ad-
sorbed layer of CO2, CO or water [13].
(a)
2Z8
5Z5
8Z2
06Cu/C
06Cu/C
06Cu/C
50
40
30
20
10
0
0
50
100
150
200
250
300
350
Temperature/ oC
(b)
900
600
300
0
06Cu/C2Z8
06Cu/C5Z5
06Cu/C8Z2
0
50
100
150
200
250
300
350
Temperature/ oC
Fig. 13. Integrated intensity of IR spectra for (a) copper carbonyls and (b)
carbonates over 06Cu/CexZr1ꢀxO2 catalysts as a function of temperature.

56
L. Liu et al. / Journal of Catalysis 275 (2010) 45–60
As shown in Fig. 10C, calcination led to not only the chemical
Regarding the O 1s spectra (Fig. 10D), the O00 at higher energies,
attributed to adsorbed hydroxyl, moved from 531.9 to 531.4 and
from 532.1 to 531.5 eV on 03Cu/C2Z8 and 03Cu/C8Z2, respectively.
However, the shift of O0 at 529–530 eV, due to the lattice oxygen
bound to metal ions, just happened on ceria-rich catalyst. Interest-
ingly, the surface O/(Cu + Ce + Zr) atomic ratios of 03Cu/C2Z8 and
03Cu/C8Z2 after calcination decreased from initial 2.06 to 1.99
and 2.55 to 2.29. Other than this, the proportion of O00/(O00 + O0)
correspondingly decreased from 32.9% to 29.1% and from 34.0%
to 27.5%. The increase in the concentration of lattice oxygen replied
the incorporation of copper into the lattice with the capping
oxygen.
shift of Cu 2p3/2 and 2p1/2 signals to low binding energy but also
the decrease in shake-up peak due to the characteristic Cu2+. More-
over, a shoulder at 930.3 eV appeared in ceria-rich catalyst, which
was assigned to the reduced state of copper species [12b]. The sur-
face Cu/(Cu + Ce + Zr) atomic ratios decreased from 0.08 to 0.06
and 0.12 to 0.09 for zirconia-rich and ceria-rich catalysts, respec-
tively. Generally, the coordination geometry of Cu2+ ions can be
studied by determining the intensity ratios of the satellite to the
corresponding principal peak (Isat/Ipp). The ratio of Isat/Ipp decreases
as the number of oxygen ions coordinated with Cu2+ ions increases
[[42b] and therein]. When compared with the Isat/Ipp values of fresh
03Cu/C2Z8 and 03Cu/C8Z2, after calcination they increase from 0.35
to 0.37 and from 0.51 to 0.59, respectively. Therefore, the local
electron property and the coordination environment of dispersed
copper species are greatly modified after calcination, probably
due to an interfacial interaction between copper and the underly-
ing supports, which resulted in the insertion of copper from the
surface into lattice. In addition, the Isat/Ipp ratios were distinguish-
able in zirconia-rich (0.37) and ceria-rich (0.59) phases, which
were all different from the characteristic value for pure Cu2+
(0.55) [12b]. This indicated that the coordination environment of
cupric ions on ceria-rich phase was close to octahedral symmetry,
whereas on zirconia-rich phase was distorted. This coincided with
EPR analysis.
3.3. Reduction properties of CuO/CexZr1ꢀxO2 catalysts (H2-TPR, CO-IR)
Fig. 11 compared the H2-TPR results of these samples. The pure
ceria–zirconia supports started the reduction of surface oxygen
[15,16] from 150 °C and showed the maximum at 560 °C
(Fig. 11a). For the catalysts with low copper loading (Fig. 11b), at
least two reduction peaks were observed in low-temperature
range. These intensive peaks were associated with the concurrent
reduction of different types of copper species and CexZr1ꢀxO2 sur-
face oxygen [32]. The H2 consumption, which was higher than that
expected for complete reduction of Cu2+ to Cu0 (Fig. S5), could
prove this viewpoint. The reduction of CexZr1ꢀxO2 supports was
(a)
(b)
06Cu/C2Z8
06Cu/C5Z5
06Cu/C8Z2
C2Z8
06Cu/C2Z8
100
100
80
60
40
20
0
06Cu/C5Z5
06Cu/C8Z2
80
60
40
20
0
C2Z8
C5Z5
C8Z2
C5Z5
C8Z2
100
150
200
250
300
350
400
100
150
200
250
300
350
400
Temperature/ oC
Temperature/ oC
(c)
(d)
100
80
60
40
20
0
100
80
60
40
20
0
12Cu/C2Z8
12Cu/C5Z5
12Cu/C8Z2
12Cu/C2Z8
12Cu/C5Z5
12Cu/C8Z2
100
150
200
250
300
350
400
100
150
200
250
300
350
400
Temperature/ oC
Temperature/ oC
Fig. 14. NO conversion (%) and N2 selectivity (%) of supports and CuO/CexZr1ꢀxO2 catalysts as a function of temperature.

L. Liu et al. / Journal of Catalysis 275 (2010) 45–60
57
due to the promotion by hydrogen spill from the reduced copper
entity to its surface [33]. In the zone (I), two peaks were proposed
as the reduction of the dispersed copper species strongly interact-
ing with ceria, whereas in the zone (II) as copper oxide in proxim-
ity to zirconia and the less dispersed one. Obviously, an effect of
the support composition on the TPR profiles can be discerned. A
moderate shift of the reduction peaks to low temperatures took
place with the increase in cerium content, and no reduction peak
was found in the zone (II) of 06Cu/C8Z2, probably because it dis-
perses well. Similar information was also obtained from the sam-
ples with high copper loading (Fig. 11c). These reduction peaks
in the zone (II) were considered partially from the contribution
of crystalline CuO [38]. These assignments were in accordance with
the above XRD, Raman, UV–vis and XPS results. From the H2-TPR
profiles, it was reasonably imagined that copper species seemed
to preferentially interact with ceria, and the pseudocubic (t00) struc-
ture could promote the reduction of the resultant catalyst more
easily than the tetragonal (t) did. The stronger interaction between
copper species and ceria-rich support could account for this
behavior.
carbonates (1065–1098, 1360–1390, 1470–1499 cmꢀ1 and 1540–
1580 cmꢀ1 for polydentate and bidentate, respectively
[18,29,30,45]) were clearly seen over 06Cu/C2Z8, 06Cu/C5Z5 and
06Cu/C8Z2, which were very stable even after heat treatment up
to 300 °C. Important information was the evolution of the copper
carbonyl intensity with temperature during the CO-TPR runs. As
shown in Fig. 13a, Cu+–CO on 06Cu/C2Z8 showed the maximum
intensity at 150 °C and completely disappeared at 300 °C. The
increase and disappearance of copper carbonyl suggested that
the progressive reduction of Cu2+ to Cu+ was favored up to that
temperature, while reduction of Cu+ to metallic copper would hap-
pen at higher temperature. In turn, reduction of Cu2+ to Cu+ on
06Cu/C5Z5 would be produced up to 125 °C and subsequent reduc-
tion to metallic copper would occur above this temperature. The
above behaviors on 06Cu/C8Z2 were favored at 100 °C. This analysis
was in agreement with the reports about CuCeTb catalysts [46].
Correspondingly, the intensity of carbonates (Fig. 13b) increased
in parallel from 25 to 200 °C and kept consistent at high tempera-
tures. These differences accounted for that the stability of Cu+-car-
bonyl was affected by the support composition. The ceria-rich
phase had the greater ability to assist the copper oxide in changing
the chemical valence (Cu2+ to Cu+/Cu0) and support in supplying
surface oxygen (the formation of oxygen vacancies) at low temper-
atures, but difficult for the zirconia-rich phase.
To further gain insight into the reduction property of the
catalysts, in situ FTIR of CO as a probe molecule interacting with
06Cu/CexZr1ꢀxO2 was measured in a TPR procedure. As shown in
Fig. 12, exposure of 06Cu/CexZr1ꢀxO2 to CO at room temperature
gave IR bands at about 2107 cmꢀ1 due to the adsorption of CO on
Cu+ ions [45]. Similar IR spectra assignable to chemisorbed
3.4. Relationship of structure with catalytic activity
Fig. 14 showed the NO conversion and N2 selectivity as a func-
tion of temperature for NO + CO reaction over pure CexZr1ꢀxO2 and
Cu/CexZr1ꢀxO2 catalysts. At low temperatures (ꢅ300 °C), the NO
conversion and N2 selectivity over the supports (Fig. 14a and b)
were identical (close to 0%). While at high temperatures
(ꢆ350 °C), the values were dependent on the compositions. C8Z2
showed the higher activity and N2 selectivity than C5Z5 and C2Z8
did, although its surface area was lowest among them. This was
possibly due to the formation of more oxygen vacancies in C8Z2
[39,47]. After the impregnation of copper species with low loading
(Fig. 14a and b), however, these catalysts exhibited the different
activity and selectivity blow 300 °C following the order of 06Cu/
C8Z2 > 06Cu/C5Z5 > 06Cu/C2Z8, but no difference could be found
over 06Cu/C8Z2 and 06Cu/C5Z5 above 300 °C. For the samples with
high loading (Fig. 14c and d), the order was ranked as 12Cu/
C8Z2 > 12Cu/C5Z5 ꢂ 12Cu/C2Z8 below 350 °C. We also compared
the TOF of NO over per copper atom, as shown in Fig. 15. In the
temperature range 100–250 °C, 06Cu/C8Z2 obviously showed the
highest value, followed by 06Cu/C5Z5 and 06Cu/C2Z8 in sequence.
On the other hand, 12Cu/C8Z2 showed a TOF of 0.34 sꢀ1 at 180 °C,
which was lower than that on 06Cu/C8Z2 (0.81 sꢀ1 at 180 °C).
Therefore, the crystalline copper oxide appeared not to contribute
06Cu/C8Z2
0.75
06Cu/C5Z5
06Cu/C2Z8
12Cu/C8Z2
0.60
12Cu/C5Z5
12Cu/C2Z8
0.45
0.30
0.15
0.00
100 120 140 160 180 200 220 240 260
Temperature (oC)
Fig. 15. Turnover frequencies (TOF, sꢀ1) of NO over per copper atom in these CuO/
CexZr1ꢀxO2 catalysts as a function of temperature.
(a)
(b)
(c)
I
II
II
I
I
II
lattice oxygen vacant site
Cu2+
Cu+
Zr4+
Ce4+
capping oxygen
Fig. 16. The diagram of Cu2+ and Cu+ incorporated in the vacant sites on the (1 1 1) plane of the CexZr1ꢀxO2 supports (a) Ce0.2Zr0.8O2, (b) Ce0.5Zr0.5O2, and (c) Ce0.8Zr0.2O2.

58
L. Liu et al. / Journal of Catalysis 275 (2010) 45–60
to NO conversion. Since all these catalysts have the similar particle
size in the nanoscale, the nanosize effect can be ruled out. Sec-
ondly, the surface area of supports and the loading amounts of cop-
per species were not the main factors influencing the activity.
Consequently, these unusual results at low temperatures were
originated from the differently synergistic interaction between
copper and CexZr1ꢀxO2 supports. When trying to understand and
interpret the measured catalytic data from the viewpoint of inter-
action, two central questions evolved:
interaction between the incorporated copper species and the cer-
ia-rich surface could bring down the reduction temperature of
Cu2+ ions and support surface oxygen. The facility for achieving
partial reduction of copper oxide entities at the interface zone,
which was favored by an increase in dispersion of copper compo-
nent, may be most significant in accounting for the different activ-
ities. In the second place, the quick change of copper valence in
ceria-rich catalyst and strong electron state interaction also greatly
contributed to the higher activity. The EPR, XPS, H2-TPR and CO-IR
results could support these ideas. Hence, the reaction pathway of
NO transformation into N2 and N2O essentially depended on the
I. What caused the observed differences in the synergetic
interaction between copper species and CexZr1ꢀxO2 phases?
II. Why was copper oxide more active on ceria-rich than on
zirconia-rich phase?
(a)
06Cu/C2Z8
5
When addressing the first question, we imagined that the sur-
face structure of CexZr1ꢀxO2 supports would be an essential reason.
It is generally accepted that tetragonal ZrO2 represents a slightly
distorted CaF2 type structure [43], while CeO2 possesses a typical
fluorite structure. Alternatively, it can be described as an array of
cations forming the face-centered cubic lattice with oxygen ions
occupying the tetrahedral interstitial sites [14b]. The octahedral
sites in a perfect fluorite structure are empty, which are considered
to locate in the exposed (1 1 1) surface. The crystal structure of
tetragonal and pseudocubic CexZr1ꢀxO2 are similar to pure t-ZrO2
and CeO2, thus being proposed to have the similar vacant sites
on the (1 1 1) plane. Noticeably, their ionic radius, lattice parame-
ters and compositions (Ce/Zr ratios) determined the different atom
arrangement on this surface. According to our previous incorpora-
tion model [42,43], it was presumed that tetragonal vacant sites
existed on the C2Z8 (1 1 1) surface, cubic vacant sites on the C8Z2,
and the mixed on the C5Z5. In this case, how were the copper spe-
cies stabilized? As discussed in Sections 3.1 and 3.2 about struc-
tural and surface characteristics, when at low loadings, copper
ions could be incorporated into the surface lattice by occupying
the vacant site on the (1 1 1) plane. The capping oxygen was
accompanied for charge compensation, as shown in Fig. 16. As a re-
sult, the incorporation led to the generation of synergetic interac-
tion between copper and ceria–zirconia (1 1 1) surface. Liu et al.
[49] also proposed that the copper ions at the interfacial region
of the finely dispersed CuO clusters and CeO2 could penetrate into
the cerium oxide lattice by occupying the vacant sites of the cer-
ium ions on the (0 0 1) plane. For zirconia-rich sample, Cu2+ ions
were probably located as the elongated and compressed coordina-
tion with the C3v or trigonal bipyramid symmetry. For ceria-rich
sample, Cu2+ ions at the interfacial area can penetrate into the cu-
bic vacant sites to form an unstable five-coordination structure and
can associate with the adjacent cerium ions to generate the elec-
tronic interaction. The sites geometry of copper species was conse-
quently dissimilar. As reported elsewhere [[2] and therein], lattice
strain can reflect surface effects (differences in local symmetry and
distances with respect to the bulk). Hence, we imagined that there
should be some connection between the structure stability and lat-
tice strain of these studied catalysts. It was proposed that the
above coordination structure of ceria-rich catalyst with the high
strain would be less stable, while that of zirconia-rich catalyst with
the low strain would be stable. Therefore, this information ex-
plained why there was a stronger synergetic interaction between
copper- and ceria-rich phase than zirconia-rich phase.
1410
2108
300oC
1011
250oC
200oC
150oC
100oC
25oC
1248
1287
1622
1525
2400
2100 1600 1400 1200 1000
Wavenumber (cm-1)
(b)
06Cu/C5Z5
5
1360
1536
2113
300oC
250oC
200oC
150oC
100oC
25oC
1019
1235
1278
1620
2400
2100 1600 1400 1200 1000
Wavenumber (cm-1)
(c)
06Cu/C8Z2
5
1082
1360
1550
2108
300oC
250oC
1019
o
2239
200 C
150oC
100oC
25oC
According to the reports [25,48], these easily reduced Pt/CeZrO2
catalyst showed the higher activity for NO + CO reaction, which
was pronounced as a structure-sensitive reaction. Moreover, the
redox-type mechanism between Cu2+ and Cu0 was promising for
the simple NO + CO reaction over the supported copper species
[50]. Along this line, addressing the second question, we suggested
that when compared with the zirconia-rich surface, the stronger
1228
1288
1618
1489
2400
2100 1600 1400 1200 1000
Wavenumber (cm-1)
Fig. 17. In situ FTIR spectra of CO and NO co-interaction with 06Cu/CexZr1ꢀxO2
catalysts at different temperatures (a) 06Cu/C2Z8, (b) 06Cu/C5Z5, and (c) 06Cu/C8Z2.

L. Liu et al. / Journal of Catalysis 275 (2010) 45–60
59
225
(a)
(b)
300
250
200
150
100
present
abesent
1248
1242
1236
1230
chelating nitro
Cu+-CO
200
-1
1010-1020 cm
1220-1250 cm
1278-1288 cm
175
150
125
-1
-1
C5Z5
C2Z8
C2Z8
C5Z5
C8Z2
C8Z2
Fig. 18. (a) The temperature for the presence and absence of Cu+–CO over these three catalysts; (b) the reaction temperature of adsorbed NO and wave number of chelating
nitro over these three catalysts.
characteristics of the synergistic interaction between copper spe-
cies and ceria–zirconia supports.
that the ceria-rich phase had the greater ability to accelerate the
CO desorption or/and rapid change in valence states of copper than
zirconia-rich did during the reaction process. According to the re-
port [48], Cu2+ ions acted as NO adsorption sites, while Cu+ ions
acted as recipients of the O atoms produced through the dissocia-
tion. Therefore, it ensured an inhibition of the reduction of NO by
increased CO partial pressures, due to the adsorption of CO on
these sites. It was presumed that the easy desorption of CO was
beneficial for the enhancement of activity. So, our above suggestion
could explain why the higher activity was obtained over the ceria-
rich phase sample in the present reaction system (CO:NO = 2:1).
This information further confirmed our above consideration (Sec-
tion 3.4). The rapid change in valence state of copper species was
a crucial part of this reaction mechanism. Apart from the above
discussion, it was found that the adsorbed bridge, monodentate
nitrates and chelating nitro over these samples, positioned at
1010–1020, 1278–1288 and 1220–1250 cmꢀ1, respectively, com-
pletely disappeared with the decrease in temperature as C2Z8,
C5Z5, C8Z2 in sequence (Fig. 18b). Furthermore, the wave number
of chelating nitro also red shifted from 1248 to 1228 cmꢀ1 in turn,
suggesting the interaction of NO with Cu/C8Z2 was strengthened,
thus the N–O bond was weakened. These findings demonstrated
that the adsorbed NO species over zirconia-rich (t) catalyst were
stable, whereas over ceria-rich (t00) catalyst were less stable due
to its superior redox activity.
3.5. NO and CO co-interaction with CuO/CexZr1ꢀxO2 catalysts
In situ FTIR for NO and CO co-interaction with CuO/CexZr1ꢀxO2
was performed to further probe the surface species and the corre-
lation of structure–activity over these catalysts. As shown in
Fig. 17, NO molecules appeared to preferentially interact with
these catalysts at room temperature, thus very intensive bands
for four differently coordinated NOx species in the region 1000–
1700 cmꢀ1 now dominated the spectra. An increase in temperature
up to 200 °C led to a decrease and disappearance, respectively, of
chelating nitro (1220–1250 cmꢀ1
) [40], monodentate (1275–
1290, 1470–1510 cmꢀ1) [20,21] and bridge nitrates (1010–1020/
1610–1620 cmꢀ1) [23,41], and to the progressive accumulation of
bidentate nitrates and carbonates (1360, 1480–1570 cmꢀ1, formed
by the adsorption of CO2) [27,28]. Further increasing the tempera-
ture to 300 °C resulted in the decrease in the residual bands (1360,
1480–1540 cmꢀ1). Concerning the spectral features in the region of
2000–2500 cmꢀ1, one can distinguish bands at ca. 2116, 2220–
2240 and 2360 cmꢀ1 for Cu+–CO, N2O [28] and CO2, respectively.
These results indicated that reaction of the adsorbed NO species
with CO had readily taken place rather than their rearrangement
or desorption (seen in Fig. S6, the adsorbed NO species were stable
up to 300 °C during the TPD process). The formation of N2O should
not be originated from the decomposition of adsorbed nitrates, be-
cause N2O was not detected during the IR-TPD process (data not
shown). In addition, Cu+–CO and N2O over these samples totally
vanished when reaching 300 °C, which might be due to either no
production of N2O or its reaction with CO at high temperature. This
was in accordance with our activity test results (low N2 selectivity
at low temperature and high at above 250 °C). This was related to
its reaction mechanism over copper-based catalysts. The NO reduc-
tion by CO was believed to proceed in two steps, first partial reduc-
tion of NO to N2O and then subsequent reduction of N2O to N2 [[48]
and therein].
4. Conclusions
This work studied the correlation of structural characteristics
with catalytic performance of CuO/CexZr1ꢀxO2 (x = 0.2, 0.5, 0.8) cat-
alysts for NO reduction by CO. Combined with XRD, Raman, HR-
TEM, UV–vis, EPR and XPS results, it was demonstrated that (1)
the dispersion capacity of CuO on the CexZr1ꢀxO2 was influenced
by the crystal structures of supports, and the ceria-rich (pseudoc-
ubic t00) phase could anchor and stabilize the copper species more
effectively than the zirconia-rich (t) phase. (2) The copper species
could penetrate into the surface lattice by occupying the vacant
sites on the predominantly exposed (1 1 1) surface of ceria–zirco-
nia, and their interaction with ceria-rich phase was much stronger
than that with zirconia-rich phase. (3) The differences in the coor-
dination environments and lattice strains of CuO/CexZr1ꢀxO2 were
More important to this system was the results obtained upon
the comparison of the stability of copper carbonyl and adsorbed
NO species. In Fig. 18a, the temperatures for the presence/absence
of Cu+–CO were in the order of C2Z8 > C5Z5 > C8Z2, which suggested

60
L. Liu et al. / Journal of Catalysis 275 (2010) 45–60
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The financial supports of the National Natural Science Founda-
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