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Article 
Effect of Ni doping on the catalytic properties of nanostructured
peony‐like CeO2
XIAN Cunni a, WANG Shaofei a, SUN Chunwen a, LI Hong a,*, CHAN Suiwai b, CHEN Liquan a
a Key Laboratory for Renewable Energy, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
b Department of Applied Physics and Applied Mathematics, Columbia University, New York, USA
A R T I C L E I N F O
 
A B S T R A C T
Article history:
Received 13 August 2012
Accepted 12 October 2012
Published 20 February 2013
Nanostructured ceria materials have attracted wide attention as catalysts, and the doping of these
materials with rare earth elements to modify their catalytic activity has been comprehensively in‐
vestigated. A novel type of Ni‐doped hierarchical nanostructured peony‐like ceria (PCO) has been
prepared and its catalytic activity is investigated and compared with that of Ni‐loaded samples. The
prepared Ni‐doped ceria have nanoscale grain sizes and open mesopores. This unique morphology
endows it with superior catalytic activity for the oxidation of CO and the partial oxidation of me‐
thane. It is found that extra oxygen vacancies are generated in the ceria, and the reducibility of the
ceria is highly enhanced after Ni‐doping. The catalytic activity for CO oxidation is improved after
Ni‐doping, compared with that of pure ceria and Ni‐loaded ceria. In the reaction for the partial oxi‐
dation of methane, the 3.8 atm% Ni‐loaded PCO sample realizes a higher CH4 conversion than the
Ni‐doped ceria. However, it is found that the onset temperature for CH4 conversion decreases from
400 °C for the pure PCO and 3.8 atm% Ni‐loaded PCO sample, to 340 °C for the 5.7 atm% Ni‐doped
PCO sample.
© 2013, Dalian Institute of Chemical Physics, Chinese Academy of Sciences.
Published by Elsevier B.V. All rights reserved.
Keywords:
Nanostructured ceria
Nickel
Carbon monoxide
Oxidation
Methane
Partial oxidation
 
1. Introduction
CeO2‐based materials have attracted much attention in re‐
cent years, due to their high catalytic activity for CO oxidation
[1–3], water‐gas shift reactions [4–6], reformation of hydro‐
carbons [7,8], and three‐way catalysis for the elimination of
toxic auto‐exhaust [6,9]. The excellent catalytic activity of
CeO2‐based materials is related to the redox reaction of
CeIII/CeIV and the presence of intrinsic oxygen vacancies
[10,11]. The catalytic activity can be significantly enhanced by
changing the morphology of the catalysts [12,13]. Recently,
hierarchical nanostructured ceria materials have been pre‐
pared using a hydrothermal method in our group [14,15]. They
have a similar morphology to the Chinese peony, so we named
these materials as peony‐like CeO2 (PCO). The PCO‐based ma‐
terials have an open mesoporous microstructure. The thickness
of the petals is in the range of 20–50 nm, and the grain size is
approximately 6–8 nm. Their surface area is above 100 m2/g.
Compared with commercial ceria, they show higher thermal
stability [16] and much higher activity for CO oxidation [14,15],
ethanol reformation [17], and as active supporters in the anode
active layer and cathode layer in solid oxide fuel cells (SOFCs)
[18,19].
The catalytic activity of CeO2 can also be modified signifi‐
cantly by metal loading. It has been found that CeO2 interacts
with the loaded metal, and the interfacial interaction gives the
composite oxide unusual features compared with pure ceria
[20]. One of the most studied catalysts is Ni supported by


* Corresponding author. Tel.: +86‐10‐82648067; Fax: +86‐10‐82649046; E‐mail: hli@iphy.ac.cn
This work was supported by the National Natural Science Foundation of China (511172275) and the National Basic Research Program of China (973
Program, 2012CB215402).
DOI: 10.1016/S1872‐2067(11)60466‐X | http://www.sciencedirect.com/science/journal/18722067 | Chin. J. Catal., Vol. 34, No. 2, February 2013
306 XIAN Cunni et al. / Chinese Journal of Catalysis 34 (2013) 305–312
CeO2‐based materials; this catalyst is effective in exploiting the
synergetic effects of Ni and CeO2. Nickel shows high activity for
the reformation of hydrocarbons, similar to precious metals,
but with a much lower cost [21,22]. Nickel is also used as a
catalyst for carbon nanotube growth because of its high cata‐
lytic activity for the cleavage of the C–C bond in hydrocarbons
[23–25]. This also gives rise to the deactivation of the Ni‐based
catalyst in the reformation of the hydrocarbons, due to the
formation of carbon. Fortunately, the coking problem can be
alleviated by modifying the properties of the support [24,26–
28]. As an oxygen ion conductor, CeO2 has been demonstrated
to be effective in impeding the formation of carbon; it achieves
this by transporting oxygen vacancies from the bulk to the sur‐
face, to promote the oxidation of carbon species [10,11]. Ac‐
cordingly, Ni‐loaded CeO2 catalysts are widely used in wa‐
ter‐gas shift reactions [4,29], the partial oxidation of methane
(POM) [30–32], and the reformation of ethanol [17,33]. The
most important component in this composite catalyst is be‐
lieved to be the metallic Ni, while the ceria support also plays a
noticeable role. It has been confirmed that catalysts with CeO2
support have higher activity and lower deactivation rates than
those with inert supports, such as Al2O3 or SiO2 [34–36]. The
former benefits from the superior oxygen storage capacity and
transport activity in the ceria.
The catalytic activity of ceria can be also modified by doping
with Ga, La, Zr, Sn, Pr, or Cu [10,12,37]. The Cu‐doped ceria
material shows unusual structural and chemical properties that
favor the formation of oxygen vacancies, and can be reversibly
reduced [37]. Taking this approach further, we expect that
Ni‐doped ceria materials would show some novel properties.
However, it is difficult to dope Ni into the CeO2 lattice with a
large molar ratio, due to the large discrepancy in ionic radius
between Ce and Ni. This could be one of the reasons for the
relative lack of reports in the literature on Ni‐doped ceria.
Thurber et al. [38] studied the ferromagnetism of Ni doped into
CeO2. It was claimed that the CeO2 retained its cubic fluorite
structure even when the Ni doping concentration reached as
high as 10 atm% of the nominal composition, and that the dop‐
ing could decrease the grain size and generate additional strain
in the lattice. Jalowiecki‐Duhamel et al. [39] found that CeNixOy
mixed oxides are large catalytic hydrogen reservoirs with
marked diffusion properties for hydrogen species. Yisup et al.
[40] reported that Ce‐Ni‐O mixed oxides were highly active for
the catalytic oxidation of methane. Kaneko et al. [41] claimed
that the reactivity in a two‐step water‐splitting reaction for
solar H2 production was improved by Ni‐doping in ceria. How‐
ever, none of these studies clarified this basic fact for Ni‐doped
ceria; did the nickel really enter the lattice of the ceria?
The activities of Ni‐loaded and Ni‐doped ceria catalysts have
been extensively studied [17,21,22,39–41], but the difference in
the catalytic activity between them is rarely reported. In this
work, Ni‐doped peony‐like ceria catalysts were successfully
prepared, and the effects of Ni doping on the structure and
catalytic activity of CeO2 were studied. For comparison,
Ni‐loaded PCO samples were also prepared at the same time.
The catalytic activities for CO oxidation and POM reactions
were also investigated.
2. Experimental
2.1. Catalyst preparation
2.1.1. Synthesis of Ni‐doped peony‐like ceria
All of the doped peony‐like ceria materials were synthesized
using the hydrothermal method described in our previous re‐
port [14]. In brief, hydrate cerium (III) nitrate (AR, 99%), hy‐
drate nickel nitrate (AR, ≥ 98%), glucose, acrylamide, and am‐
monia aqueous solutions were purchased from Beijing Chemi‐
cal Reagents Company. The hexahydrated nickel nitrate was
dissolved in de‐ionized water to obtain a solution with a Ni2+
concentration of 0.2 mol/L. Then, hydrate cerium nitrate, the
Ni2+ solution (the total cation concentration was 0.005 mol,
with molar ratios of dopant cation Ni2+ to Ce3+ of 0.05:0.95,
0.1:0.9, 0.15:0.85, and 0.2:0.8), 0.01 mol glucose, 0.015 mol
acrylamide, and 80 ml of de‐ionized water were mixed in a
Teflon autoclave with a capacity of 100 ml. The pH value of the
solution was then adjusted to approximately 10 via dropwise
addition of an aqueous ammonia solution. After continuous
stirring at room temperature for 3 h, the autoclave was sealed
and kept in an oven at 180 C for 24 h. The precipitate was col‐
lected from the autoclave after the suspension was cooled to
room temperature. After being washed with de‐ionized water
and alcohol and dried at 80 C, the Ni‐doped peony‐like ceria
materials were obtained via calcining, using a previously re‐
ported two‐step procedure [14].
2.1.2. Synthesis of Ni‐loaded peony‐like ceria
The peony‐like CeO2 materials were synthesized using the
same process as that described in a previous paper [14]. The
Ni‐loading was achieved using the nitrate impregnation meth‐
od. Peony‐like CeO2 (0.5 g) was added to 40 ml of de‐ionized
water, and after stirring at room temperature for 3 h, 855 μl
(5.7 atm% Ni content) of a 0.2 mol/L Ni2+ solution was added; 8
h of additional, vigorous stirring was performed, and the sam‐
ple was dried at 80 C. The Ni‐loaded material was obtained
after the sample was dried at 110 C for 2 h and calcined at 450
C for 4 h in air.
2.2. Characterization
X‐ray diffraction (XRD) measurements were carried out us‐
ing a Holland X’Pert Pro MPD X‐ray diffractometer equipped
with a monochromatized Cu Kα radiation source (λ = 0.15405
nm) operated at 40 kV and 40 mA using a step rate of 0.017.
The morphology of the samples was observed using a scanning
electron microscope (SEM, XL30s‐FEG, 10 kV). To determine
the true amount of Ni doped into the CeO2, induced coupled
plasma (ICP, Thermo Electron Corporation) analysis was used
to test the chemical composition. The Raman spectra of the
Ni‐containing materials were recorded on a JY HR800 spec‐
trometer at ambient temperature using a laser source of 514
nm and resolution of 1 cm–1. The X‐ray photoelectron spec‐
troscopy (XPS) analysis was performed on a PHI Quantera SXM
instrument, using an Al X‐ray source, and the binding energies
were calibrated using the binding energy of the C 1s (284.8 eV)
XIAN Cunni et al. / Chinese Journal of Catalysis 34 (2013) 305–312 307
peak as a reference. Temperature‐programmed reduction
(TPR) was performed to determine the reduction behavior of
the 5.7 atm% Ni‐doped and 3.8 atm% Ni‐loaded catalysts, using
10% H2‐90% Ar as the reducing gas on a Micromeritics Chem
2920 instrument equipped with a thermal conductivity detec‐
tor. The samples were heated from room temperature to 900
C at a rate of 10 C/min, and the gas flow rate was 50 ml/min.
Typically, 20 mg of fresh catalyst was used in each experiment.
A sample pretreatment was performed by flowing Ar over the
sample at 300 C for 0.5 h, to remove the adsorbed species on
the surface. The H2 consumption was determined from the
integrated peak area of the reduction profiles.
2.3. Catalytic testing
All of the Ni‐doped and Ni‐loaded samples were examined
in the catalysis tests. Activity tests were performed under at‐
mospheric pressure, in a continuous down‐flow quartz
fixed‐bed reactor. The catalyst (50 mg) was loaded in the reac‐
tor, using quartz wool. A feed with a constant CO:O2:N2 volume
ratio of 2:3:95 (for CO oxidation), and CH4:O2:N2 volume ratio of
2:1:4 (for POM) was used. The total gas flow rate was 50
ml/min for CO oxidation, and 49 ml/min for POM. The effluent
gas was analyzed using an online gas chromatograph (Agilent
7890a) equipped with Porapak Q, ShinCarbon ST, and a ther‐
mal conductivity detector (TCD). The conversions of CO and
CH4 were calculated as:
XCO = mol CO2out/(mol COout + mol CO2out) × 100%
XCH4 = (1 – mol CH4out/mol CH4in) × 100%
3. Results and discussion
3.1. Characterization results of the catalysts
All of the Ni‐doped ceria catalysts present a peony‐like
nanostructure morphology, as shown in Fig. 1. Some small par‐
ticles can be clearly observed on the surface of the Ni‐loaded
ceria sample. These are suggested to be residual Ni salt. As
listed in Table 1, the actual Ni contents in the samples with
nominal compositions of 5, 10, 15, and 20 atm% Ni doping
were determined as 1.1, 3.2, 5.7, and 10.7 atm%, respectively,
using ICP analysis. Some of the Ni was lost during the washing
(a) (b)
(c) (d)
 
Fig. 1. SEM images of the Ni‐doped peony‐like CeO2 (PCO) and 3.8 atm% Ni‐loaded PCO catalysts. (a) 1.1 atm% Ni doping; (b) 3.2 atm% Ni doping; (c)
5.7 atm% Ni doping; (d) 3.8 atm% Ni loading.
Table 1
Characterization results for the Ni‐doped/loaded peony‐like ceria catalysts.
Ni content (atm%) NiO phase b Lattice parameter c (nm)
CeO2 grain
size d (nm)
Surface element e (atm%) Surface Ce/O
ratio (%)
ΔH2
Expected a ICP result O Ni P1 P2 P3
0 — no 0.54156 6.2 77.27 — 29.41 0 0.329 0.606
5D 1.1 no 0.54169 6.6 79.18 0 26.30 — — —
10D 3.2 no 0.54171 6.8 79.18 0 26.30 — — —
15D 5.7 no 0.54172 6.3 71.13 5.40 38.40 0.00387 0.471 0.585
20D 10.7 yes 0.54157 6.5 — — — — — —
5.7L 3.8 no 0.54122 10.9 75.89 13.99 31.77 0.116 0.0678 0.571
a The superscript D refers to doping, and L refers to loading. b From Fig. 2. c Calculated from the XRD data shown in Fig. 2 using the Jade program.
d Calculated from the (111) XRD data for the ceria, as shown in Fig. 2. e From XPS. 
308 XIAN Cunni et al. / Chinese Journal of Catalysis 34 (2013) 305–312
with water and alcohol. Figure 1(d) shows the Ni‐loaded PCO
catalyst with a nominal composition of 5.7 atm%. The actual Ni
content was 3.8 atm%.
The XRD patterns of the samples are shown in Fig. 2. The
NiO phase is not found in the 3.8 atm% Ni‐loaded PCO catalyst,
which could be due to the detection limit of the XRD measure‐
ments. It is not clear from these results whether some of the Ni
was doped into the CeO2 lattice, or if it was all dispersed on the
surface. This will be clarified in a later section. All of the doped
catalysts show a cubic fluorite structure. The NiO phase pre‐
sents only in the XRD pattern of the 10.7 atm% Ni‐doped PCO.
The lattice parameters of CeO2 increase with increasing of
Ni‐doping amounts, as listed in Table 1. The NiII radius (0.069
nm) is much smaller than that of CeIV (0.092 nm), and the lat‐
tice should shrink when Ni enters the CeO2 lattice. In fact, the
conversion of CeIV (0.092 nm) to CeIII (0.103 nm) will happen
after the Ni‐doping. The increase in the amount of CeIII will ex‐
pand the CeO2 lattice [38], which will counteract the shrinking
effects caused by the Ni‐doping. The more Ni doping that oc‐
curs, the more CeIII is generated. Therefore, the CeO2 lattice
expands until the shrinking lattice effects of the NiII doping
begin to dominate. As shown in Table 1, the lattice parameters
of CeO2 decrease to 0.54157 nm under 10.7 atm% of Ni doping.
These values are much smaller than those of the other
Ni‐doped samples. Although it is difficult to determine the real
Ni‐doping content (due to the appearance of NiO impurities), it
seems that the shrinking effects of the Ni‐doping overcome the
expanding effects of the increasing CeIII amounts after 10.7
atm% of Ni‐doping. It should be noted that the lattice parame‐
ters for the 3.8 atm% Ni‐loaded PCO is much smaller than those
of pure PCO, as listed in Table 1. After the impregnation pro‐
cess, the 3.8 atm% Ni‐loaded PCO sample was calcined at 450
C in air for 4 h, to make the nickel nitrate decompose. Due to
extra sintering procedure in air, the amount of CeIII in the bulk
material of ceria decreases compared to pure ceria and hence
the lattice parameter of ceria decreases in this case [42].
The grain sizes in the prepared catalysts are calculated us‐
ing the Scherrer equation. The grain size is approximately 6 nm
in all of the Ni‐doped samples, but is 10.9 nm in the 3.8 atm%
Ni loaded PCO, as shown in Table 1. After the impregnation
process, the 3.8 atm% Ni‐loaded PCO sample was calcined at
450 C for 4 h in air to make the nickel nitrate decompose. This
extra sintering also made the PCO grains grow bigger com‐
pared with those in the doped samples, which did not occur
extra sintering.
The Raman spectra are shown in Fig. 3. The peak at 462.0
cm–1, the typical F2g Raman active modes of a fluorite struc‐
tured ceria [43], shift to 455.2, 455.0, and 459.8 cm–1 for the
1.1, 3.2, and 5.7 atm% Ni‐doped samples, respectively. It does
not shift for the Ni‐loaded sample. For clarity, an enlargement
of the spectra is shown in the inset. The red‐shift suggests a
significant modification of the M–O bonding symmetry, which
probably results from the presence of Ni in the fluorite lattice
[40,44]. As mentioned above, the doping of Ni into the ceria
lattice results in extra strain and increases the concentration of
CeIII in the bulk [38], which also influences the Raman bands.
The shoulder peaks around 230 and 590 cm–1 are related to the
addition of Ni. The relative intensity of the band from 540 to
636 cm–1 increases with increases in the Ni‐doping amount.
This is ascribed to the formation of oxygen vacancies in the
Ni‐doped ceria [45].
The XPS spectra for the binding energies of Ce 3d and Ni 2p
core levels are given in Fig. 4. The signals from CeIV are marked
as u, u, and u for 3d3/2, while those for 3d5/2 are marked as v,
v, and v. The four peaks labeled as u0, u, v0, and v were as‐
signed to CeIII, as reported in the literature [46]. Although the
absolute binding energies are referenced against the C 1s pho‐
toelectron peak at 284.8 eV, some scattering of the binding
energies in the Ce 3d and Ni 2p regions could occur. This is due
to the low conductivity of the catalysts, which caused charging
effects. Therefore, the positions of the peaks may not have re‐
flected the exact binding energies. However, it is clear from Fig.
4(a) that the relative intensities of the v0/v, v, u0/u, and u
peaks increase with increasing amounts of doped Ni. This be‐
havior was not observed in the Ni‐loaded catalyst. The u0/u and
v0/v peaks are too close to be distinguished, but from the in‐
20 30 40 50 60 70 80
Int
en
si
ty
(6)
(5)
(4)
(3)
(1)
(2)
2/( o )
NiO
Fig. 2. XRD patterns of different catalyst samples. (1) Pure PCO; (2) 1.1
atm% Ni‐doped PCO; (3) 3.2 atm% Ni‐doped PCO; (4) 5.7 atm%
Ni‐doped PCO; (5) 10.7 atm% Ni‐doped PCO; (6) 3.8 atm% Ni‐loaded
PCO. 
150 300 450 600 750 900 1050 1200 1350 1500
 1.2
(5)
(4)
(3)
 2
 4
 1/10
(2)
Raman shift (cm1)
In
te
ns
ity
(1)
440 460 480
(5)
(4)
(3)
(2)


(1)
Fig. 3. Raman spectra of different catalyst samples. (1) Pure PCO; (2)
3.8 atm%Ni‐loaded PCO; (3) 1.1 atm% Ni‐doped PCO; (4) 3.2
atm%Ni‐doped PCO; (5) 5.7 atm%Ni‐doped PCO. The small graph inset
shows an enlargement of the peak at 450 cm–1. 
XIAN Cunni et al. / Chinese Journal of Catalysis 34 (2013) 305–312 309
crease of the relative intensity of the v and u peaks, it is clear
that the CeIII concentration increases with increasing Ni‐doping
amounts. A similar phenomenon has been reported for Cu and
Zr‐doped ceria materials [47,48].
Compared with pure ceria, the v and u peak intensities for
the 3.8 atm% Ni‐loaded PCO show insignificant change com‐
pared with those of the 5.7 atm% Ni‐doped catalyst, which is in
accordance with the Raman spectra results. The Ni contents on
the catalyst surface (as detected by XPS) are listed in Table 1.
There is no obvious Ni signal from the surfaces of the 1.1 atm%
Ni‐doped or 3.2 atm% Ni‐doped catalysts, and the surface nick‐
el content is 5.4 atm% for the 5.7 atm% Ni‐doped sample,
which is similar to the bulk nickel content. In contrast, the sur‐
face nickel content is 13.44 atm% for the Ni‐loaded catalyst,
which is three times higher than the actual composition of 3.8
atm%. This is consistent with the fact that the Ni particles are
mostly dispersed on the surface of the 3.8 atm% Ni‐loaded PCO
sample. In addition, it is found that the peak at approximately
919.4 eV in the Ce 3d core level grows larger with increases in
the doped Ni amount. This is clearly related to the nick‐
el‐doping behavior, and the same peak is not observed in the
spectra of the 3.8 atm% Ni‐loaded PCO. The Ni 2p spectra of the
5.7 atm% Ni‐doped and 3.8 atm% Ni‐loaded PCOs are com‐
pared in Fig. 4(b). In the Ni‐loaded catalyst, there are two dis‐
tinct peaks at ~855.3 and ~861 eV, which were assigned to NiII.
The peak at approximately 861 eV does not present, and the
peak at approximately 855.3 eV broadens. This indicates the
different chemical environments surrounding the Ni in the
loaded and doped catalysts.
The O 1s spectra of the samples are shown in Fig. 4(c). There
are two peaks in the pure PCO and Ni‐doped PCO samples and
three peaks in the Ni‐loaded PCO sample. The O 1s peaks at
529.25 and 531.75 eV in PCO, which are assigned to the ce‐
ria‐related lattice oxygen, and the adsorbed oxygen species on
the surface, respectively [49,50], shifted to 529.20 and 531.30
eV after 1.1 atm% Ni and 3.2 atm% Ni doping, and to 528.65
and 530.95 eV after 5.7 atm% Ni doping. The O 1s peaks at
528.10, 529.25, and 530.60 eV in the 3.8 atm% Ni‐loaded sam‐
ples and the new peak at 528.10 eV that was related to O2–
were also found in LaNiO3 [51]. The surface O contents are
listed in Table 1. They slightly increase from 77.27% to 79.18%
after 1.1 atm% Ni or 3.2 atm% Ni doping, but rapidly reduce to
71.13% and 67.99% after 5.7 atm% Ni doping and 3.8 atm% Ni
loading, respectively. The surface O content is closely related to
the reduction of surface ceria [49]. According to the changes in
the ratio of Ce to O on the surface, as listed in Table 1, it is
clearly that the Ni loading and doping reduce the surface oxy‐
gen content. That is, they enhance the oxygen vacancy content
on the surface. However, it is found that the Ce/O ratio in the
1.1 atm% Ni doping and 3.2 atm% Ni doping samples is a little
lower than that in PCO. In these two samples, the doped nickel
was not be detected using XPS. However, it is likely that the
nickel combines with the oxygen on the surface, which contrib‐
utes to the slightly lower Ce/O ratio of 26.30% in the 1.1 atm%
Ni doping and 3.2 atm% Ni doping samples, compared with of
the value of 29.41% found for PCO. It is found that there are
more oxygen vacancies on the surface of the 5.7 atm% Ni‐
doped sample compared with the 3.8 atm% Ni‐loaded sample.
According to the XRD, Raman spectroscopy, and XPS results
given here, the lattice shrinking of CeO2 is induced after Ni
doping. To suppress the structure deformation, more CeIII is
generated, since its ionic radius is larger than that of CeIV. This
can explain the increases in the CeIII concentration after Ni
doping. According to the above results, the defect chemistry
could be written in Kröger‐Vink notation as follows:
2NiO + CeO2 = NiCeIV + 2VO·· + CeCeIV + NiCeIII + 2O2
3.2. TPR results
The 5.7 atm% Ni‐doped PCO sample and the 3.8 atm%
Ni‐loaded PCO sample were selected to compare the influence
of Ni doping and loading on the reduction properties of PCO.
The TPR spectra are shown in Fig. 5 and the results are sum‐
marized in Table 1. Two peaks are identified. The peak at 380
C (marked as P2) is attributed to the surface reduction of the
ceria, and the second peak at 655 C (marked as P3) is related
to the bulk reduction of the ceria. The reduction temperatures
are much lower than the reported temperatures of ~500 and
880 890 900 910 920 930 850 855 860 865 870 875 526 528 530 532 534
(2)
(4)
(3)
(1)
In
te
ns
ity
Binding energy (eV)
v0,v
v
v
v
u0,u
u
u
?u (5)
Ce 3d(a)
Ni-doped PCO
Ni-loaded PCO
5.7 atm%
(b)
Inte
ns
ity
Binding energy (eV)
3.8 atm%
Ni 2p3/2
shoulder peak
(3)
In
te
ns
ity
Binding energy (eV)
(5)
(4)
(c) (2)
(1)
O 1s
Fig. 4. Ce 3d (a), Ni 2p (b), and O 1s (c) XPS spectra of the samples. (1) Pure PCO; (2) 1.1 atm% Ni‐doped PCO; (3) 3.2 atm% Ni‐doped PCO; (4) 5.7
atm% Ni‐doped PCO; (5) 3.8 atm% Ni‐loaded PCO. CeIV peaks are marked as u, u, and u for 3d3/2 and v, v, and v for 3d5/2. The four peaks labeled
as u0, u, v0, and v were assigned to CeIII. 
310 XIAN Cunni et al. / Chinese Journal of Catalysis 34 (2013) 305–312
~800 C [9,48]. When Ni is introduced, regardless of the doping
or loading, the P2 and P3 shift to lower temperature. This
shows that the presence of Ni promotes the reduction of ceria.
In addition, the P1 below 300 C in the spectra of the
Ni‐containing samples was assigned to the reduction of Ni [52].
For the 5.7 atm% Ni‐doped PCO, the P1 may be related to the
reduction of Ni in the ceria close to the surface. The P2 for the
Ni‐doped sample is at 315 C, much lower than that for PCO,
which is at 380 C. The P2 peak consumes much more H2 for
reduction on the Ni doped PCO catalyst than that on PCO. This
indicates that the Ni doping promotes the reduction of the ceria
surface. The P2 for the 3.8 atm% Ni‐loaded PCO decreases to
323 C, indicating that interactions occur between the loaded
Ni and the surface of the ceria. However, the H2 consumption of
the 3.8 atm% Ni‐loaded PCO is lower than that of PCO. This is
probably because the dispersion of Ni particles on the surface
partly blocks the reduction of the ceria. The P3 associated with
the reduction of the bulk ceria does not shift significantly for
either the 5.7 atm% Ni‐doped sample or the 3.8 atm%
Ni‐loaded sample, and the P3 peak area for the three samples is
similar, which shows that the Ni loading and doping have no
significant influence on the reduction of bulk ceria. Comparing
the relative intensity of P1 to that of P2 and P3, it is quite clear
that the amount of reducible Ni is higher in the 3.8 atm%
Ni‐loaded PCO sample than in the 5.7 atm% Ni‐doped PCO. This
is consistent with the XPS results showing that Ni is distributed
uniformly within the lattice of the Ni‐doped sample.
3.3. Catalytic activity
The CO oxidation results are shown in Fig. 6. All of the
Ni‐containing catalysts show higher activities compared to the
pristine PCO. It is clear that this improvement effect is not re‐
lated to changes in the specific surface area of the catalyst since
the specific surface area of the pure PCO is above 100 m2/g
[14,15], and is not significantly changed after small amounts of
Ni loading or doping. The catalytic activity of the Ni‐doped cat‐
alyst increases with the increasing of the Ni doping amount.
The activity of the 3.8 atm% Ni‐loaded PCO is lower than that of
3.2 atm% Ni‐doped PCO. To further clarify why the effects of
the Ni doping is superior to those of the Ni loading, a sample of
13.4 atm% Ni‐loaded PCO (actual composition) was also pre‐
pared using the impregnation method, and was tested for CO
oxidation. The activity of the 13.4 atm% Ni‐loaded PCO is also
lower than the activity of the 3.2 atm% Ni‐doped PCO and the
5.7 atm% Ni‐doped PCO. Therefore, the Ni doping is more ef‐
fective in enhancing the activity than the Ni‐loading. It has been
reported that the releasable lattice oxygen on the ceria surface
could be the key reactant for the oxidation of the adsorbed CO
[12,53–56]. As shown in the TPR profiles in Fig. 5, the 3.8 atm%
Ni‐loaded PCO has more reducible Ni, but less reducible surface
ceria than the 5.7 atm% Ni‐doped PCO sample. This result
supports the findings from previous investigations that the
surface reducibility of CeO2 is an essential factor for CO oxida‐
tion, at least for ceria‐based catalysts [12,53–56].
Data for the partial oxidation of methane is shown in Fig. 7.
The CH4 conversion ratio of the Ni‐doped catalysts increases
with the increasing of the doped Ni amount. The activity of PCO

0 200 400 600 800
655
645
638
380
323
315
220
215
P3
P2
P1
(3)
(2)
H 2
c
on
su
m
pt
io
n
Temperature (oC)
(1)
Fig. 5. TPR profiles for the selected samples. (1) Pure PCO; (2) 5.7
atm% Ni‐doped PCO; (3) 3.8 atm% Ni‐loaded PCO. 
60 90 120 150 180 210 240 270 300 330
0
20
40
60
80
100
Temperature (oC)
PCO
1.1 atm% Ni doped
3.8 atm% Ni loaded
3.2 atm% Ni doped
5.7 atm% Ni doped
13.4 atm% Ni loadedC
O
c
on
ve
rs
io
n
(%
)
Fig. 6. CO conversion for all Ni‐doped and Ni‐loaded PCO catalysts. The
catalyst (50 mg) was loaded in the reactor using quartz wool. A feed
with a constant CO:O2:N2 volume ratio of 2:3:95 was used. The total gas
flow rate was 50 ml/min. 
300 330 360 390 420 450 480 510 540 570 600
0
5
10
15
20
25
30
35
PCO
1.1 atm% Ni doped
3.2 atm% Ni doped
5.7 atm% Ni doped
3.8 atm% Ni loaded

Temperature (oC)
C
H
4 c
on
ve
rs
io
n
(%
)
Fig. 7. Partial oxidation of CH4 on catalysts. The catalyst (50 mg) was
loaded in the reactor, using quartz wool. A feed with a constant
CH4:O2:N2 volume ratio of 2:1:4 was used. The total gas flow rate was 49
ml/min. 
XIAN Cunni et al. / Chinese Journal of Catalysis 34 (2013) 305–312 311
is similar to that of the 3.2 atm% Ni‐doped PCO for the temper‐
ature range tested, and is slightly higher than the activity
shown by the 1.1 atm% Ni‐doped PCO sample. The 5.7 atm%
Ni‐doped PCO shows a 20% conversion ratio at 350 C. Com‐
pared with the Ni‐doped PCO catalysts, the 3.8 atm% Ni‐loaded
PCO sample shows a much higher conversion ratio of 28% at
540 C, which is also higher than the ratios shown by all of the
doped and un‐doped PCO samples. Ni metal sites are known for
methane activation [10,57], and our results indicate that the
reducible Ni plays an important role in the POM reaction. The
methane conversion ratio begins to decrease at 480 C and 540
C on 5.7 atm% Ni‐doped PCO and 3.8 atm% Ni‐loaded PCO
catalyst respectively. This is attributed to the degradation of
the catalysts after the reaction, which likely occurs because of
carbon formation and the coarsening of the PCO grain growth.
This was confirmed by the XRD measurements performed after
the POM tests (data not shown).
It is interesting to note that for the PCO‐based catalysts, the
onset temperature for CH4 conversion is strongly influenced by
the Ni doping. The reforming reaction starts at 400 C for the
PCO and the Ni‐loaded PCO samples. It decreases to 360, 360,
and 340 C for the 1.1, 3.2, and 5.7 atm% Ni‐doped PCO sam‐
ples, respectively. These onset temperatures are much lower
than those reported previously for Ni‐loaded ceria catalysts, or
catalysts with other supports [9,57–60]. The decreasing trend
for the onset temperature in the POM reaction could be related
to the oxygen vacancy generation, or the enhancement in the
reduction of surface ceria, caused by the Ni doping. Due to the
complicated reaction mechanism of POM, a clear understanding
of this process will require further investigations.
4. Conclusions
Hierarchically nanostructured Ni‐doped and Ni‐loaded pe‐
ony‐like ceria materials have been successfully prepared, and
show high activity for CO oxidation and the partial oxidation of
methane. Ni doping is favorable for the formation of CeIII in
ceria, and enhances the reduction of surface ceria. The
Ni‐doped samples therefore show much higher activity for CO
oxidation than the Ni‐loaded samples. It is believed that the
surface oxygen vacancy induced by Ni doping is essential for CO
oxidation. The onset temperature for CH4 conversion is highly
dependent on the Ni doping amount. It decreases from 400 C
for the pure PCO and the 3.8 atm% Ni‐loaded PCO sample, to
340 C for the 5.7 atm% Ni‐doped PCO sample. The 3.8 atm%
Ni‐loaded PCO sample realizes the highest CH4 conversion ratio
of 28% at 540 C. It is thought that the exposed reducible Ni is
more important for the partial oxidation of methane than the
ceria surface or the doped Ni. The Ni‐doped PCO samples show
interesting effects for the oxidation of CO and the partial oxida‐
tion of methane, which should be useful for the design of cata‐
lysts and the understanding of catalytic mechanisms.
Acknowledgments
Dr. Xia Lu is thanked for the helpful discussions. Dr. K. Y.
Cheung’s assistance on editing the manuscript is appreciated.
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Ni 掺杂对纳米结构牡丹花状 CeO2 材料催化特性的影响
仙存妮 a, 王少飞 a, 孙春文 a, 李 泓 a,*, 陈晓惠 b, 陈立泉 a
a中国科学院物理研究所清洁能源重点实验室, 北京 100190
b哥伦比亚大学应用物理与应用数学学院, 美国纽约

摘要: 制备了一种新型 Ni 掺杂多层纳米结构牡丹花状 CeO2 材料, 研究了其催化性能, 同时与 Ni 负载牡丹花状 CeO2 样品进行了比
较. 结果表明, Ni 掺杂 CeO2 样品具有纳米晶粒和开放的介孔结构, 特殊的形貌使其在 CO 氧化和甲烷部分氧化反应中具有独特的
催化特性. Ni 掺杂后, CeO2 中产生了多余氧空位, 同时其氧化还原活性也增强, 其在 CO 氧化反应中的催化活性明显高于纯 CeO2 和
Ni 负载 CeO2 样品; 在甲烷部分氧化反应中, 牡丹花状 CeO2 负载 3 atm% Ni 催化剂样品上甲烷转化率高于所有 Ni 掺杂的催化剂样
品. 但是在 Ni 负载型催化剂和花状 CeO2 催化剂上, 甲烷的起始转化温度为 400 oC, 而 5.7 atm%Ni 的掺杂使其降至 340 oC.
关键词: 纳米结构氧化铈; 镍; 一氧化碳; 氧化; 甲烷; 部分氧化
收稿日期: 2012-08-13. 接受日期: 2012-10-12. 出版日期: 2013-02-20.
*通讯联系人. 电话: (010)82649047; 传真: (010)82649046; 电子信箱: hli@iphy.ac.cn
基金来源: 国家自然科学基金 (511172275); 国家重点基础研究发展计划 (973 计划, 2012CB215402).
本文的英文电子版由Elsevier出版社在ScienceDirect上出版(http://www.sciencedirect.com/science/journal/18722067).

 
Graphical Abstract
Chin. J. Catal., 2013, 34: 305–312 doi: 10.1016/S1872‐2067(11)60466‐X
Effect of Ni doping on the catalytic properties of nanostructured peony‐like CeO2
XIAN Cunni, WANG Shaofei, SUN Chunwen, LI Hong*, CHAN Suiwai, CHEN Liquan
Institute of Physics, Chinese Academy of Sciences, China; Columbia University, USA
Ni-doped peony-like CeO2 (PCO)
Ce Ni O
CO CO2
C
Ni-loaded PCO
Ni-doped PCO
Temperature (oC)
PCO


60 90 120 150 180 210 240 270 300
0
20
40
60
80
100


C
O
c
on
ve
rs
io
n
(%
)

Oxygen vacancies are generated in bulk ceria after Ni doping, which promotes the reducibility of peony‐like CeO2, and hence enhances the
catalytic activity for CO oxidation.