Heterosturcture NiO/Ce1-xNixO2: Synthesis and synergistic effect of simultaneous surface modification and internal doping for superior catalytic performance

Article abstract of DOI:10.1039/c3ra44186e

In this work, a two-step method that involves a hydrothermal reaction and a subsequent calcination was initiated to prepare a novel heterostructure (1 - x)CeO₂·xNiO. All samples were systematically characterized by X-ray diffraction, Raman spectra, Transmission electron microscopy, In situ DRIFT spectra, Brunauer-Emmett-Teller technique, H₂-temperature- programmed-reduction, and O₂-temperature-programmed-desorption. Distinct from the conventional sol-gel or co-precipitation methods, the present methodology allows Ni²⁺ to be simultaneously doped in the CeO ₂ lattice and dispersed onto the surfaces of CeO₂ nanoparticles by simply varying the calcination temperature from 400 to 700 °C. When calcined at an optimized temperature of 500°C, the heterostructure at x = 0.1 showed a mean grain size of 18 nm, specific surface area of 69.3 m² g^-1, the smallest lattice parameter, and the largest amount of surface adsorbed and desorped oxygen species, which has led to an excellent catalytic performance towards CO and CH₄ oxidation. Such a superior performance benefits from the unique synergistic effects of the finely dispersed NiO and Ni-O-Ce solid solution species detected in the heterostructure.

Full text of DOI:10.1039/c3ra44186e

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PAPER
Heterosturcture NiO/Ce Ni O : synthesis and
1
ꢀx x 2
synergistic effect of simultaneous surface
modification and internal doping for superior
catalytic performance†
Cite this: RSC Adv., 2014, 4, 6397
ab
a
a
a
a
Xin Liu, Ying Zuo, Liping Li, Xinsong Huang and Guangshe Li*
In this work, a two-step method that involves a hydrothermal reaction and a subsequent calcination was
initiated to prepare a novel heterostructure (1 ꢀ x)CeO $xNiO. All samples were systematically
characterized by X-ray diffraction, Raman spectra, Transmission electron microscopy, In situ DRIFT
spectra, Brunauer–Emmett–Teller technique, -temperature-programmed-reduction, and
temperature-programmed-desorption. Distinct from the conventional sol–gel or co-precipitation
2
H
2
2
O -
2+
methods, the present methodology allows Ni to be simultaneously doped in the CeO
2
lattice and
dispersed onto the surfaces of CeO
2
nanoparticles by simply varying the calcination temperature from
ꢁ
ꢁ
4
00 to 700 C. When calcined at an optimized temperature of 500 C, the heterostructure at x ¼ 0.1
2
ꢀ1
, the smallest lattice parameter,
showed a mean grain size of 18 nm, specific surface area of 69.3 m
g
Received 6th August 2013
Accepted 20th December 2013
and the largest amount of surface adsorbed and desorped oxygen species, which has led to an excellent
catalytic performance towards CO and CH oxidation. Such a superior performance benefits from the
4
DOI: 10.1039/c3ra44186e
unique synergistic effects of the finely dispersed NiO and Ni–O–Ce solid solution species detected in
www.rsc.org/advances
the heterostructure.
transition oxide enables the promotion of oxygen storage
capacity and redox properties of CeO . As a consequence, there
2
1
. Introduction
Catalytic oxidation reactions have drawn great attention in is a strong demand for development of composite CeO
recent decades due to the energy utilization and environmental catalysts with low cost and relatively high activity.
–MO
X
2
1–3
4
requirements. Many types of catalysts such as pure oxides,
Catalytic performance enhancement of CeO
mixed oxides, and supported systerms have been studied for is not only due to the interactions between MO
catalytic oxidations. CeO
has a relatively high thermal stability can further modulate the intrinsic surface properties of sup-
and redox behavior, and therefore is widely used as an impor- ported phases, but also due to the internal doping of transition
2
–MO
X
catalysts
5
6
X
and CeO
2
that
2
7
11–16
tant support in CO oxidation reactions. It is well established metal species for creating more defects (oxygen vacancies).
that noble metals supported on CeO usually exhibit excellent Besides these, CeO shows the merits in preventing the sinter-
catalytic performance, while these supported noble metals are ing of transition metal ions, as indicated for transition-metal
expensive. It is hence necessary to develop cheap substitution oxide supported systems (e.g., CeO –Fe , CeO –NiO, CeO
when applying in catalytic oxidation reactions. Among
oxides are promising alternatives because of the low price. all transition metal oxide supported systems, CeO –NiO mate-
Recent reports have shown that MO
(M ¼ transition metal rials could be a highly promising catalyst for various reactions.
supported on CeO exhibits Many methodologies have been reported in literature to prepare
2
2
O
2
–
2
2
3
2
17–19
catalysts for catalytic oxidation reactions. Transition metal CuO)
2
X
3
+
2+
2+
2+ 8–10
ions, like Co /Co , Cu , Ni )
excellent activity of catalytic oxidation since addition of CeO
which include co-precipitation, citrate acid method, or
2
2
–NiO mixed oxide catalysts with different Ni contents,
20
21
22,23
impregnation method.
However, there still exists certain
a
State Key laboratory of Structural Chemistry, Fujian Institute of Research on the
controversies in these studies. For instance, it is reported that
Ni species are doped in CeO2 lattice, which favors to form
Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, P. R. China.
E-mail: guangshe@irsm.ac.cn; Fax: +86-591-83702122
24,25
oxygen vacancies and improves reducibility of CeO
others thought that NiO particles are highly dispersed on CeO
surfaces, providing strong metal-support interactions between
2
.
While,
b
Key Laboratory for Photonic and Electronic Bandgap Materials, Ministry of Education,
School of Physics and Electronic Engineering, Harbin Normal University, Harbin
2
1
50025, P. R. China
2
6,27
†
Electronic supplementary information (ESI) available: Experimental details, NiO particles and CeO
.
One may wonder if it is possible to
$xNiO that simultaneously
2
XRD, SEM, Raman, CO oxidation activity and pseudocapacitance performance.
achieve heterostructure (1 ꢀ x)CeO
2
See DOI: 10.1039/c3ra44186e
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Paper
ꢁ
ꢀ1
involves the doped Ni species and surface modication of NiO 0.15418 nm). The scanning rate was set at 0.3 min , and the
ꢁ
nanocrystals, since such heterostructure expects to show the step size was 0.02 . Aluminum powders were added into the
synergistic effects in producing excellent catalytic performance samples (amounting 10–15 wt%) to serve as an internal stan-
essential for applications.
dard for peak positions calibration. Raman spectra of the
In this work, we adopted a two-step method that involves a samples were obtained on a Renishaw, UV-vis raman system
hydrothermal reaction and a subsequent calcination to prepare 1000 with an excitation line of 785 nm. Morphologies of the
a novel heterostructure (1 ꢀ x)CeO
is based on the following considerations: (i) hydrothermal (Phenom G
method is effective to synthesize the precusors of CeO and (JEM-2010). Sample was dispersed in anhydrous ethanol and
beta-Ni(OH)2, and (ii) both NiO and CeO have the similar then treated under ultrasound for 30 minutes to get a well-
2
$xNiO. The sample synthesis samples were observed by scanning electron microscopy
2
), and transmission electron microscopy (TEM)
2
28
2
2
+
4+
cubic structures, while Ni is far smaller than Ce . When dispersed solution before TEM measurements. The molar ratios
calcined at high temperatures, part of Ni ions could be doped in of Ce to Ni and weight proportions for the samples were
CeO
onto CeO
With the successful sample synthesis, the catalytic oxidation
behaviors of the heterostructure were investigated, which have adsorption data measured at liquid nitrogen temperature using
shown an excellent activity towards oxidation of CO and CH Brunauer–Emmett–Teller (BET) technique on a Micromeritics
2
lattice, while part of Ni species would be highly dispersed measured by Inductively Coupled Plasma OES spectroscopy
grains to form heterostructure (1 ꢀ x)CeO $xNiO. (Ultima2).
Specic surface areas were determined from nitrogen
2
2
4
.
Further, the impacts of calcination temperature on surface ASAP 2000 Surface Area and Porosity Analyzer. The samples were
ꢁ
structure, internal defect structure, and furthermore the pre-treated in N at 120 C before N absorption analysis. In situ
2
2
catalytic activities were discussed.
diffuse reectance infrared Fourier transform spectroscopy
DRIFTS) was performed on a Nicolet 6700 apparatus equipped
(
with diffuse reectance accessory (Harrick CHC-CHA-3). All
2
. Experimental section
ꢁ
sample powders were pre-treated in situ at 400 C in pure He
ꢀ1
2.1. Sample syntheses
(99.99%) with a ow rate of 60 mL min to eliminate water
ꢁ
traces. Then, all powders were cooled to 160 C in owing He.
(
1 ꢀ x)CeO $xNiO (x ¼ 0.1). All samples were prepared by a
2
Subsequently, 1% CO and 99% He at a total ow rate of 40 mL
hydrothermal method followed with calcination treatment. The
synthetic procedure can be briey described as follows. A mixed
solution that contains 0.225
Ni(NO $6H
ꢀ
1
min were introduced into the in situ chamber, and the spectra
were recorded in 10 min.
M
Ce(NO
3
)
3
$6H
2
O
and
H
2
temperature-programmed-reduction (TPR) for all
3
)
2
2
O at a molar ratio of Ce : Ni ¼ 9 : 1 was sufficiently
samples were measured. Prior to H
2
-TPR experiments, 50 mg
stirred, into which 5 M NaOH solution was added till pH ¼ 13.
ꢁ
samples were treated at 200 C for 2 h in Ar. Aer cooling down
The mixed solution was then transferred to 100 mL Teon-lined
ꢁ
ꢁ
ꢁ
ꢀ1
ꢁ
to 30 C, samples were heated to 900 C at a rate of 5 min in
stainless steel autoclave, which was allowed to react at 220 C
for 24 h. Aer naturally cooling down to room temperature, the
ꢀ1
1
0 vol%H
of the samples was calculated by comparing to CuO reference.
To examine the mobility of lattice oxygen, O temperature-
2 2
in Ar at a ow rate of 30 mL min . H consumption
precipitates were ltered, washed with distilled water till pH ¼
ꢁ
2
7, and dried at 80 C in air. Eventually, the dried precipitates
ꢁ
programmed-desorption (TPD) for all samples were measured
using a conventional chemisorption analyzer (Finesorb 3010C).
The measurements were performed under a ow of pure He (30
were calcined at 400, 500, 600, and 700 C, respectively, in air
for 4 h.
(
1 ꢀ x)CeO
2
$xNiO (0.05#x # 0.7). Using the procedures for
$xNiO with different nickel
ꢀ
1
ꢁ
ꢀ1
ml min ) over 100 mg catalyst at a heating rate of 10 C min
x ¼ 0.1, the samples (1 ꢀ x)CeO
2
ꢁ
from room temperature to 900 C. All samples were pre-treated
in He at 120 C for 40 min, and then in pure O
contents were prepared, when the calcination temperature was
ꢁ
ꢁ
ꢁ
2
at 120 C for 60

xed at 600 C.
min before TPD test.
Pure-phases of the end compounds (1 ꢀ x)CeO $xNiO (x ¼
2
0
and 1) were also prepared for comparison.
x ¼ 0. The synthetic procedure for CeO
2
was similar to that
$6H O was involved during
2
.3. Catalytic activity test
for x ¼ 0.1, while no Ni(NO
3
)
2
2
CO catalytic oxidation. The catalytic test was carried out on a
hydrothermal reactions. The precipitates by hydrothermal
method were also calcined at 400, 500, 600, and 700 C in air for
h, respectively.
ꢁ
xed-bed quartz microreactor equipped with an online gas
chromatograph (Shimadzu GC-2014) and a quadrupole mass
spectrometer (Pfeiffer Prisma Plus QMG 220 M2). All measure-
ments were done at atmospheric pressure with 50 mg catalyst
4
x ¼ 1. The synthetic procedure for NiO was similar to that for
x ¼ 0.1, while no Ce(NO ) $6H O was involved during the
3
3
2
(50–70 mesh) without any pre-treatment. The reagent gas con-
hydrothermal reactions. The precipitate thus obtained was
ꢁ
sisted of 1.0% CO, 20% O
min . The specic velocity was set at 24 000 mL g
2
, and 79% Ar with a ow rate of 20 mL
calcined at 500 and 600 C in air for 4 h.
ꢀ
1
ꢀ1 ꢀ1
h . The
conversion of CO was calculated from the difference in CO
concentration between the inlet and outlet gases.
2
.2. Sample characterization
All samples were characterized by X-ray diffraction (XRD) using
a Rigaku Miniex apparatus equipped with Cu Ka radiation (l ¼ on the same reactor for CO catalytic oxidation, but with a
4 4
CH catalytic oxidation. CH oxidation test was performed
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4 2
catalytst mass of 100 mg. 1% CH and 4% O balanced with Ar
ꢀ
1
at a ow rate of 50 mL min were set as the reagent gas, and the
ꢀ
1
ꢀ1
specic velocity was set at 30 000 mL g
h .
3. Results and discussion
3.1. CO catalytic oxidation activity of the samples
ꢁ
All samples (1 ꢀ x)CeO $xNiO (0.05 # x # 0.7) calcined at 600 C
2
showed a pronounced catalytic performance towards CO
oxidation. As indicated in ESI (Fig. S1†), NiO content showed a
signicant inuence on the catalytic performance. The catalyst
(
1 ꢀ x)CeO
2
$xNiO at x ¼ 0.05 has a relatively poor activity and
ꢁ
the complete temperature of CO conversion (T100%) was 200 C.
Fig. 2 CO oxidation activities of the samples (1 ꢀ x)CeO
2
$xNiO at x ¼
As NiO content increased to x ¼ 0.1, CO conversion rate
ꢁ
0
.1 prepared after calcination at (a) 500, (b) 600, (c) 400, and (d) 700 C.
ꢁ
increased obviously, showing a decrease in T
to 180 C.
1
00%
However, the activity of catalysts were not remarkably improved
when varying the NiO content in the range of 0.1 # x # 0.5.
Further increasing NiO content results in a decrease in catalytic
activity. Especially for x ¼ 0.6 and 0.7, T100% started to increase
ꢁ
needed a higher temperature of 210 C. Therefore, the opti-
mized calcination temperature for x ¼ 0.1 that shows the
ꢁ
excellent catalytic activity was determined to be 500 C.
ꢁ
to 190 and 200 C. In the following, we will address the sample
ꢁ
Reproducibility of the sample at x ¼ 0.1 calcined at 500 C
at x ¼ 0.1 due to its greatly improved oxidation activity.
were investigated. As shown in Fig. 3, the catalytic performance
Fig. 1 compares the CO catalytic oxidation activity of the
ꢁ
for x ¼ 0.1 aer calcination at 500 C was reproducible for four
sample (1 ꢀ x)CeO $xNiO at x ¼ 0.1 with those for the end
2
runs, no matter what the run sequence was. In addition, the
stability of the catalytic performance was further examined at a
conversion rate of 93–94% for x ¼ 0.1 and x ¼ 1. As indicated in
inset of Fig. 3, for x ¼ 0.1, CO conversion showed no sign of
compositions (x ¼ 0, 1). All these samples were prepared aer
ꢁ
calcination at 600 C. It is seen that the sample at x ¼ 0.1 performs
more excellent catalytic performance when comparing to the end
compounds at x ¼ 0 and 1. For the latter cases, the sample at x ¼
ꢁ
deactivation at 155 C, even when the catalytic reaction periods
0
<
was least active, almost showing no conversion at temperatures
were prolonged beyond 100 h. CO conversion for x ¼ 1 slightly
ꢁ
200 C. Meanwhile, the sample at x ¼ 1 gave an intermediate

uctuate from 25 h to 40 h. Therefore, synergistic effect of NiO
ꢁ
catalytic activity with 100% conversion around 210 C.
and CeO could improve the stability of the catalyst compared
2
In order to further improve the catalyst properties, calcina-
tion temperature for x ¼ 0.1 was optimized. As shown in Fig. 2,
with pure oxide.
To get insights into the optimum catalytic activity for x ¼ 0.1,
ꢁ
when the calcination temperature was set at 400 C, the sample
we systematically characterized our samples by XRD, Raman
ꢁ
at x ¼ 0.1 showed a complete CO conversion around 190 C.
2 2
spectra, TEM, BET, H -TPR, and O -TPD as below. All relevant
results were compared with those reported in literature for
similar systems.
ꢁ
Increasing calcination temperature to 500 C led to an optimum
catalytic performance by reducing the complete conversion
ꢁ
temperature to 160 C. Further increasing calcination temper-
ꢁ
ature beyond 500 C, catalytic activity decreased readily. For
ꢁ
instance, aer calcination at 700 C, the complete conversion
Fig. 3 CO oxidation reproducibility of the samples (1 ꢀ x)CeO
2
$xNiO
ꢁ
at x ¼ 0.1 prepared after calcination at 500 C. All catalytic tests were
run for four times. Inset shows the long-term catalytic durability at a
Fig. 1 CO oxidation activities of the samples (1 ꢀ x)CeO
2
$xNiO at (a) x 93–94% conversion for a long period of 100 h for the samples at x ¼
ꢁ ꢁ
¼
0.1, (b) x ¼ 1, and (c) x ¼ 0 prepared after calcination at 600 C.
0.1 (black dot) and 1 (red dot) after calcinations at 500 C.
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.2. XRD
Paper
3
strongly inuenced by calcination. As indicated in Fig. 4, with
increasing the calcination temperature, diffraction intensities
for components CeO and NiO gradually increased, while the
relevant diffraction width became narrowed, which suggests an
increased crystalline degree of the component phases.
Table 1 shows the mean crystallite size, lattice constant, and
lattice strain for samples (x ¼ 0, 0.1) prepared aer calcinations
at different temperatures. It is well documented that either
particle size reduction or lattice strain effect could contribute to
the peak broadening of XRD patterns. To gure out what is the
main reason for diffraction broadening, we employed Wil-
Fig. 4 shows XRD patterns of the samples (1 ꢀ x)CeO $xNiO (x ¼
2
2
0.1) prepared aer calcinations at different temperatures. To
ꢀ
assign the peaks correctly, Al powder (space group Fm3m,
JCPDS, no. 04-0787) was used as an internal standard. For
comparison, XRD data for the end compounds (x ¼ 0, 1) were
also given. From Fig. 4, it is seen that both the end compounds
showed the diffractions that can be well tted in a cubic uorite
ꢀ
phase of CeO (space group Fm3m, JCPDS, no. 65-5923) at x ¼
2
ꢀ
0
and NiO (space group Fm3m, JCPDS, no. 47-1049) at x ¼ 1.
Comparatively, for x ¼ 0.1, majority of the intensive diffraction
30
31,32
liams and Scherrer relation
and calculated the mean crys-
peaks can be assigned to CeO , while several weak diffraction
2
tallite size and lattice strain for the dominant cubic uorite
phases of x ¼ 0 and 0.1 prepared aer calcinations at different
peaks for NiO were also observed, as indicated by the enlarged
diffraction patterns in ESI (Fig. S2†). One may thus conclude
that traces of NiO co-existed with the dominant phase CeO
33
temperatures using the following equation,
2
.
This observation is completely different from a previous report
b cos q 0:89 h sin q
¼
þ
29
by Wrobel et al., who concluded that Ni tends to dope in CeO2
lattice to form a solid solution at Ni/Ce < 0.5. For both
component phases in x ¼ 0.1, the crystalline degrees were
l
D
l
where b is the full width at half maximum (FWHM), q is the
diffraction angle, l is the X-ray wavelength, D is the mean
particle size, and h is the lattice strain. Four dominant diffrac-
2
tion peaks (111), (200), (220), and (311) of CeO uorite struc-
tures were used for all calculations. Since crystallite sizes and
dopants can have great impacts on lattice dimension of uorite
structures, the lattice parameters for x ¼ 0 and 0.1 were calcu-
lated using a prole tting by a least-squares method employing
34
the computer program GSAS implemented with EXPGUI.
From Table 1, it is seen that the crystallite sizes for a doping
2
+
of 10% Ni strongly depend on calcination temperatures.
ꢁ
When calcination temperature was lower than 700 C, mean
crystallite size for x ¼ 0 and 0.1 were roughly the same, around
1
7
8 nm. However, increasing the calcination temperature to
00 C, there occurred an obvious growth of the crystallite size.
ꢁ
For example, the crystallite size for x ¼ 0 grew up to approxi-
mately 26 nm. Similar growth of crystallite size was observed for
x ¼ 0.1. Such observations could be attributed to the kinetics
factors of the nanosized CeO . It is well established that crys-
2
tallite growth of CeO
is lower than 600 C. Above this temperature, there appears an
exponential increase in crystallite size.
2
is very slow when calcination temperature
ꢁ
35,36
As indicated in
Table 1, the molar ratios of Ce to Ni for x ¼ 0.1 measured
using ICP were closer to the initial ones, the stoichiometric
atomic concentration was retained when varying calcination
temperature.
Fig. 4 XRD patterns of the samples (1 ꢀ x)CeO
2
$xNiO at x ¼ 0 after
ꢁ
calcination at 600 C (a), x ¼ 0.1 after calcinations at (b) 400, (c) 500,
ꢁ
ꢁ
(
d) 600, (e) 700 C, and (f) x ¼ 1 after calcination at 600 C. Al was used
as an internal standard for peak positions determination.
Table 1 Mean particle size (D), lattice parameter (nm), and lattice strain (h) of (1 ꢀ x) CeO
2
$xNiO at x ¼ 0 and 0.1 after calcinations at different
temperatures, and molar ratios of Ce to Ni for x ¼ 0.1 after calcinations at different temperatures
Samples (1 ꢀ x)CeO $xNiO
2
Molar ratios
of Ce to Ni
Mean particle size (nm)
Lattice parameters (nm)
Lattice strain (h)
x ¼ 0
Calcination
temperature ( C)
ꢁ
x ¼ 0
x ¼ 0.1
x ¼ 0
x ¼ 0.1
x ¼ 0.1
x ¼ 0.1
4
5
6
7
00
00
00
00
17.6
18.5
18.5
26.3
17.1
17.7
17.9
25.8
0.5414
0.5414
0.5413
0.5414
0.5410
0.5405
0.5412
0.5413
0.18%
0.26%
0.32%
0.25%
0.68%
0.56%
0.51%
0.34%
0.92 : 0.11
0.91 : 0.11
0.91 : 0.11
0.92 : 0.12
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The lattice parameters for x ¼ 0 and 0.1 were comparatively
2
+
studied to comprehend the doping behavior of Ni in uorite
CeO lattice. As indicated in Table 1, with increasing calcination
2
temperature, the lattice parameters for x ¼ 0 did not show
apparent changes, but retaining around 0.5414 nm, consistent
with the bulk nature for big crystallite size. Alternatively, for x ¼
0
0
.1, the lattice parameters slightly decreased from 0.5410 to
.5405 nm, as the calcination temperature increased from 400
ꢁ
to 500 C. Further increasing calcination temperature led to an
increase in lattice parameter. It means that the sample at x ¼ 0.1
ꢁ
calcined at 500 C might have more Ni ions doped in CeO
2
2
+
lattice. This is because Ni with an ionic radius of 0.072 nm is
4
+
2+
smaller than that of 0.101 nm for Ce . When Ni ions are
incorporated in ceria lattice to form a solid solution Ce1ꢀxNi
one may expect a decrease in lattice parameter, as reported
elsewhere.
To further conrm the incorporation of part Ni in CeO
x 2
O ,
37
2
+
2
to
form solid solutions aer calcinations, we compared the
diffraction peak (111) for uorite structure of all samples
(
Fig. S3†). When xing the intensive diffraction line of internal
ꢁ
standard (Al) at two theta of 38.47 for the standard diffraction
data for Al, one can clearly see that the (111) diffraction peak for
x ¼ 0.1 shied towards higher diffraction angles, relative to x ¼
Fig. 5 Raman spectra of the samples (1 ꢀ x) CeO
2
$xNiO at x ¼ 0.1 after
25,38
0
. This observation is similar to those previously reported.
ꢁ
calcination at (a) 400, (b) 500, (c) 700 C, and (d) x ¼ 0 after calcina-
ꢁ
For x ¼ 0.1, the calcination at 500 C had led to a largest shis
towards higher diffraction angles, which conrms that more Ni
species were incorporated in uorite lattice and formed a solid
solution.
ꢁ
tions at 600 C.
4
2,43
Raman bands is different from a previous report.
For the
ꢀ1
The lattice strains for all samples were also highly dependent latter cases, no Raman signal at 260 cm is observed, but only
on the calcination temperatures. As indicated in Table 1, the one band at 225 cm for CeO –NiO systems. It is noted that the
ꢀ
1
2
lattice strain for x ¼ 0 was all much smaller, being distributed most intensive Raman signals for x ¼ 0.1 aer calcination at 400
ꢁ
around h ¼ 0.2–0.3% when varying the calcination temperature and 500 C are much broadened when comparing to that aer
ꢁ
ꢁ
from 400 to 700 C. However, the lattice strain for x ¼ 0.1 aer calcination at 700 C. Such a Raman signal broadening could be
ꢁ
calcination at 400 C was 0.7%, at least double that for x ¼ 0, explained by the inhomogeneous strain effects (Table 1) asso-
2
+
which is a good proof of the dislocation in CeO structure as a ciated with dispersion in particle size and Ni incorporation.
2
33,39
2+
result of nickel ion incorporation.
As calcination tempera-
2
Doping of smaller ions (like Ni in CeO ) can give a lattice
ture increased, lattice strain for x ¼ 0.1 decreased continuously. constriction and therefore a decrease in lattice spacing and
ꢁ
Further increasing the calcination temperature up to 700 C, the bond length, leading to a red shi in Raman mode. To examine
lattice strain for x ¼ 0.1 droped down to 0.3%, which is probably the impacts of the calcination on the doping behavior, the most
ꢀ
1
due to the signicantly increased crystallite size.
intensive Raman signal at 461 cm was carefully studied, as
2
+
It appears that doping part of low-valence Ni in CeO
2
aer indicated in ESI (Fig. S4†). It is seen that when calcined at lower
calcination at temperatures <600 C has produced a smaller temperatures of 400 and 500 C, the Raman signal at 461 cm
ꢁ
ꢁ
ꢀ1
lattice parameter (Table 1). In the following, Raman spectra for x ¼ 0.1 shied towards lower frequencies, comparing to that
were used to study the impacts of calcination temperature on for x ¼ 0. This phenomenon could be attributed to the forma-
2
+
the lattice strain and oxygen vacancies.
tion of Ce Ni O solid solution with Ni ions replacing for
1ꢀx x 2
4
+
Ce ions in CeO lattice. This assumption is further conrmed
2
by the appearance of well-dened weak Raman signals at 225
and 550 cm , since these weak signals are closely linked to the
3.3. Raman spectra
ꢀ1
Fig. 5 shows Raman spectra of the samples (1ꢀx)CeO
2
$xNiO lattice defects originated from the oxygen vacancies as reported
43
(
x ¼ 0.1) prepared aer calcination at given temperatures. For elesewhere. Therefore, the solid solutions Ce1–xNi
x
O
2
were
ꢁ
comparison, the Raman data for x ¼ 0 were also given. For x ¼ formed when calcination temperature was lower than 600 C.
ꢁ
2+
0
.1 prepared aer calcination at 400 C, a most intensive signal Another important feature for Ni doping is the modication of
ꢀ1
42
at 461 cm
6
4
and four weak signals at 225, 260, 530, and oxygen sublattice, which is demonstrated by the weak Raman
30 cm were observed. The most intensive Raman signal at signal observed at 630 cm in Fig. 5(a) and (b). When the
61 cm is characteristic of the intrinsic vibration mode F of calcination temperature was further increased to 700 C, the
ceria, while the weaker one at 260 cm is associated with the intensities of Raman signals at 225, 550, and 630 cm
ꢀ
1
ꢀ1
ꢀ1
ꢁ
2
g
40
ꢀ1
ꢀ1
ꢀ1
41
second-order transverse acoustic mode of ceria. This set of decreased, while that of the most intensive one at 461 cm
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increased, which indicates a sharply decrease in the concen- SEM images of the sample (1 ꢀ x)CeO
2
$xNiO at x ¼ 0.1 aer
ꢁ
tration of lattice defects when calcined at 700 C. One may thus calcination at different temperatures in ESI (Fig. S6†). Fig. 6(c
2
+
imagine that dopant Ni and the relevant defects might be and d) show the HRTEM images of the sample (1 ꢀ x)CeO $x-
2
ꢁ
expelled from the bulk lattice to the surface aer high-temper- NiO at x ¼ 0.1 aer calcination at 500 C. It is seen that most of
ature calcination.
the particles exhibited a subspheroidal morphology, which is
completely different from the shape of nanoclusters when NiO/
CeO
2
catalyst was prepared by solution combustion synthesis
3
.4. TEM
As stated above, syntheses of the samples (1 ꢀ x)CeO
involved two key processes: The rst step is hydrothermal reac-
6
using citric acid as the oxidant. Despite of very low concen-
tration of Ni , the sample aer calcination was still observed to
be composed of two types of particles: roundish CeO particles
with surfaces attached by small NiO particles. CeO particles is
2
$xNiO
2
+
2
tions between Ce(NO
3 3 2 3 2 2
) $6H O and Ni(NO ) $6H O aqueous
ꢁ
2
solution at 220 C, and the second one is the calcination at
different temperatures. As indicated in Fig. 6a, morphologies for
x ¼ 0.1 before calcination were primarily nanocuboids and
nanorods. Fig. 6b shows the inter-planar spacings of nanorods,
which were about 0.31 nm, in good agreement with that for (111)
facet of CeO
contained a small amount of b-Ni(OH)
difficult to detect b-Ni(OH) nanoparticles in HRTEM, since its
low concentration and poor crystalline degree. To certicicate the
uniformity of Ni element in the sample, EDX mapping was also
taken in Fig. 6c, which revealed that Ni element is uniformly
distributed in the sample before calcination.
proved by calculation of the lattice spacing of 0.27 nm for facet
of CeO
estimated to about 13 nm. NiO particles (6 nm) attached on
CeO surface is evidenced by TEM and SAED results (insert
2 2
(200). As shown in Fig. 6(e), the CeO nanoparticle was
2
of Fig. 6e). According to the fast Fourier transform (FFT)
analysis of the lattice fringes, SAED pattern can be indexed to be
the [12 1] zone axis of cubic phase NiO, and these pattern spots
are consistent with XRD results. The angles between the facets
of (111) and (101), (210) were about 90.32 and 139.59 ,
respectively, in good agreement with the theoretical values of
2
. Besides, the samples obtained before calcination
2
(shown in Fig. S5†). It is
ꢀ
2
ꢁ
ꢁ
ꢀ
ꢁ
ꢁ
90.82 and 140.80 .
Aer high-temperature calcinations, the phase compositions
and the relevant morphologies were changed, as indicated by
3.5. BET
We compared the CO conversion at a reaction temperature of
ꢁ
1
50 C and BET surface area for x ¼ 0.1 and 0 as a function of
calcination temperature. CO oxidation curve of (1 ꢀ x)CeO
NiO catalysts at x ¼ 0 calcined at different temperatures was
shown in Fig. S8.† As indicated in Fig. 7 and S8,† for pure CeO
2
$x-
2
(
x ¼ 0) calcined at different temperatures, there was no activity
ꢁ
when reaction temperature was increased to 150 C, while the
sample calcined at 600 C showed the best activity among the
ꢁ
samples for x ¼ 0. However, for x ¼ 0.1, the maximum activity is
ꢁ
obtained aer calcination at 500 C. This indicates that
ꢁ
Fig. 6 (a) TEM, (b) HRTEM, and (c) TEM-EDX mapping for x ¼ 0.1 Fig. 7 Relationships among CO conversion at 150 C, specific surface
before calcinations, (d) TEM and (e) HRTEM images of x ¼ 0.1 after area, and calciniation temperature for samples (1 ꢀ x)CeO
2
$xNiO at x ¼
ꢁ
calcination at 500 C. Insert shows the SAED pattern of NiO 0.1 and 0 after calcination at different temperatures from 400 to
ꢁ
nanoparticle.
700 C.
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2
addition of Ni has a great effect on CeO activity and structure,
and that the catalytic activity miximun for x ¼ 0.1 calcined at
ꢁ
5
00 C is not simply caused by the state of CeO but probably
2
the distribution of Ni species. Furthermore, when calcined at
ꢁ
temperature <600 C, the BET surface area for x ¼ 0.1 was larger
than that for x ¼ 0, though the mean particle size for both
samples did not show apparent changes. One may deduce that
the increased BET surface area may be related to the change of
catalyst structure. At lower calcination temperature, part of Ni
ions were doped in CeO lattice, while other parts of Ni ions
2
were highly dispersed as segregation at surfaces for (1 ꢀ x)
CeO $xNiO at x ¼ 0.1. Alternatively, higher calcination
2
ꢁ
temperatures >700 C may distruct the structure of solid solu-
tion and cause sintering and growth of the particles, resulting in
the gradual loss of surface area.
ꢁ
As indicated in Table 1, when calcined at 500 C, the lattice
parameters for the dominant uorite phase were smallest. It
2
+
means that more Ni ions could be doped in CeO2 lattice,
which expects to give more oxygen vacancies essential for high
activity. In the following, H -TPR spectra were used to study the
2
reduction behaviour of catalyst surface introduced by Ni species
doped into CeO
2
lattice and surface NiO species for the samples
at x ¼ 0.1 calcined at different temperature.
3
2
.6. H -TPR
Fig. 8
H
2
-TPR data of the samples (1 ꢀ x)CeO
2
$xNiO at x ¼ 0.1 after
ꢁ
calcinations at (a) 400, (b) 500, (c) 600, (d) 700 C, and those at (e) x ¼ 1
Generally, large surface area enables more surface active centers
exposed to the reactants, and therefore is helpful to enhance the
ꢁ
and (f) x ¼ 0 after calcinations at 600 C.
catalytic activity. The BET surface area for x ¼ 0.1 aer calci-
ꢁ
ꢁ
nation at 500 C is not the largest, which however gave a rela- x ¼ 0.1 calcined at 500 C, H consumption for peak a was
2
2
tively high catalytic activity among the samples at x ¼ 0.1. To highest, and that for b peak was the lowest among the samples
2
+
uncover the structural nature behind the abnormal catalytic for x ¼ 0.1. This phenomenon further certies that more Ni
performance, H
2
-TPR datas of the samples (1 ꢀ x)CeO
2
$xNiO at ions were doped in CeO lattice to form solid solution aer
2
ꢁ
x ¼ 0, 0.1, and 1 were comparatively studied. For x ¼ 0 (Fig. 8f), 500 C calcination, which led to a drop in b peak intensity
there appeared two reduction peaks. Both peaks centered at caused by the reduction of dispersed NiO species. When calci-
ꢁ
ꢁ
ꢁ
4
00 C and 750 C were assigned to the reduction of surface and nation temperature was higher than 500 C, the intensities for
44
bulk oxygen species of CeO
2
, respectively. For x ¼ 1, there were peaks a and a became weak, while that for b peak became
1
2
ꢁ
two reduction peaks at 368 and 480 C, which correspond to the strong, which implies that high temperature calcinations above
ꢁ
stepwise reduction of NiO, respectively, as represented by the 500 C led to the destruction of solid solution structure and the
20
symbols “b” and “g” (Fig. 8e).
2
increase of NiO amount distributed onto CeO surface.
As shown in Fig. 8a–d, the samples at x ¼ 0.1 calcined at
different temperature showed four hydrogen consumption
3
.7. O -TPD
2
1 2
peaks, a , a , b, and d. The rst two peaks could be ascribed to
the reduction of adsorbed oxygen species, which is easily acces-
2
O -TPD was used to examine the mobility of the lattice oxygen.
45
2+
sible by H
2
at low temperatures. When Ni was incorporated in Fig. 9 shows the O
2
-TPD of (1 ꢀ x)CeO
2
$xNiO samples at x ¼ 0,
ꢁ
CeO lattice to form solid solution, oxygen vacancies were formed x ¼ 0.1 and x ¼ 1. It is seen that calcination of x ¼ 0 at 600 C
2
ꢁ
which favor to surface oxygen adsorption. The b and d peaks are (Fig. 9e) gave three obvious oxygen desorption signals at 112 C,
ꢁ
ꢁ
ascribed to NiO reduction and bulk oxygen reduction of crystal- 250 C and 500 C, which are related to the weakly adsorbed
line CeO , respectively. The lower reduction temperature of b molecular oxygen, the chemically adsorbed oxygen on surface,
peaks for x ¼ 0.1 when compared to that of x ¼ 1 indicates that and the chemically adsorbed oxygen adjacent to the oxygen
2
46
NiO nanoparticles have a high dispersion degree on surfaces of vacancies. The sample at x ¼ 1 gave two weak desorptions at
ꢁ
ꢁ
catalysts and simultaneously proved the interactions existing about 180 and 400 C and one strong signal at 800 C. (Fig. 9f),
between NiO and support CeO which are attributed to the desorption of non-stoichiometric
As indicated in Table 2, the calculated H2 consumption oxygen (O and O ) and lattice oxygen (O ) of NiO, respectively.
2
.
ꢀ
2ꢀ
47
2
ratios of the b peak to a2 peak associated with Ni species
For all samples at x ¼ 0.1, the desorption peaks were related
distributed just followed such a sequence: sample at x ¼ to surface oxygen species desorption, since bulk oxygen is
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
0
.1 aer calcinations at 500 C < 600 C < 400 C < 700 C. For difficult to be desorbed. The sample at x ¼ 0.1 calcined at 400 C
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Table 2 Reduction peak positions, H
temperatures from 400 to 700 C
2
consumption, desorption peak positions, and desorption areas for x ¼ 0.1 after calcinations at different
ꢁ
2
(1 ꢀ x)CeO $xNiO (x ¼ 0.1)
Desorptionpeak
position ( C)
ꢁ
ꢀ1
ꢁ
Peak position ( C)
H
2
consumption(mmolg
)
Desorption area
Calcination
temperature( C)
n
n
1
+
+ n
3
ꢁ
T
1
(a
1
)
T
2
(a
2
)
T
3
(b)
T
4
(g)
n
2
(a
2
)
n
3
(b)
n
4
(g)
n
3
/n
2
b/a
2
T
1
T
2
T
3
n
1
n
2
n
3
2
400
500
600
700
157
156
156
166
222
212
206
293
298
297
251
733
740
746
715
0.089
0.104
0.048
0.027
0.477
0.135
0.157
0.317
0.357
0.337
0.288
0.320
5.4
1.3
3.3
120 271 591 535 104 295
934
147 (a peak)
112 (a peak)
101 337
3034
2285
212 186
3034
2285
398
11.7
ꢁ
ꢁ
showed three obvious desorption signals at 120 , 271 , and 591 desorption peaks for different samples follows such a sequence:
ꢁ
C, the shape of which is quite similar to that of pure CeO
2
. It is the sample (1 ꢀ x)CeO $xNiO catalysts at x ¼ 0.1 calcined at
2
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
thus indicated that incorporation of Ni ions in CeO has little 500 C > 600 C > 400 C > 700 C. Sample calcined at 500 C has
2
effect on the desorption of surface oxygen species aer calci- a larger surface oxygen desorption amount, which would give
ꢁ
nation at 400 C. With increasing the calcination temperature to rise to
a better activity. When calcination temperature
ꢁ
5
00 C, O
2
-TPD prole for x ¼ 0.1 (Fig. 9b) showed a strong and increases, the desorption ability for surface oxygen species
ꢁ
broad a peak in a wider temperature range (80–430 C), which is drops, which accounts for the decrease in CO oxidation activity.
attributed to the desorption of nonstoichiometric oxygen
ꢀ
species (O
2
), indicating an enhanced surface oxygen mobility.
3
.8. CH oxidation
4
ꢁ
As the calcination temperature increases to 600 and 700 C, a
peak became slightly smaller and divided in two signals at 101
and 337 C.
Desorption area of each peak was also calculated and inte-
grated (shown in Table 2). It is observed that area summation of
The above experiments have demonstrated that the sample at
ꢁ
x ¼ 0.1 aer calcination at 500 C showed the best catalytic
ꢁ
activity towards CO oxidation. In the following, this sample was
further tested for oxidation of CH
4
. For comparison, pure CeO
2
ꢁ
(
x ¼ 0) calcined at 500 C was also tested under the similar
conditions. As monitored by mass spectra, CO
2 2
and H O were
the only detectable products during complete CH
4
oxidation. As
ꢁ
shown in Fig. 10, for x ¼ 0.1 aer calcination at 500 C, there
ꢁ
occurred a CH conversion of 50% at 478 C, which appears to
4
be superior to that of 12% for x ¼ 0 at the same temperature.
Samples at x ¼ 0.1 calcined at different temperatures were
tested for CH
4
oxidation and the results are shown in ESI
ꢁ
(Fig. S9†). It is seen that the samples calcined at 400 and 500 C
4
showed a respectively high CH oxidation activity, among which
Fig. 9
O
2
-TPD of the samples (1 ꢀ x)CeO
2
$xNiO at x ¼ 0.1 after
ꢁ
calcination at (a) 400, (b) 500, (c) 600, (d) 700 C, and those at (e) x ¼ Fig. 10 CH
4
oxidation activities of the samples (1 ꢀ x)CeO
2
$xNiO at (a)
ꢁ
ꢁ
0
and (f) x ¼ 1 after calcinations at 600 C.
x ¼ 0.1 and (b) x ¼ 0 prepared after calcinations at 500 C.
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there were no obviously difference in CH
4
oxidation activity.
8 J.-Y. Luo, M. Meng, J.-S. Yao, X.-G. Li, Y.-Q. Zha, X. Wang and
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ꢁ
7
00 C, the activity was dramatically suppressed, with a
ꢁ
complete CH conversion of 50% at 570 and 750 C, respec-
4
4
tively. The activity variation order for CO and CH oxidation are
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ꢁ
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4
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ꢁ
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+
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C
yielded the optimum catalytic activities towards CO oxidation
and CH
ration of more Ni in CeO
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