High oxygen storage capacity and enhanced catalytic performance of NiO/NixCe1-xO2-δ nanorods: synergy between Ni-doping and 1D morphology

Article abstract of DOI:10.1039/c5ra04259c

A series of Ni-doped ceria nanorods with low Ni content was synthesized via a hydrothermal approach and characterized by DRX, SEM, HRTEM, Raman and TPR techniques. Simultaneous tuning of morphology and composition, i.e. exposure of reactive (100) and (110) facets together with Ni substitution, provide great improvements in their physical chemical properties. Segregated NiO as a secondary phase was observed as Ni content increases affecting the growth of the nanorods. The system is described as NiO dispersed on Ni_xCe_1-xO_2-δ nanorods. The materials showed a remarkably high OSC at 400°C and good activity for CO oxidation. Results corroborate the synergic effects between Ni-doping and one-dimensional morphology on ceria structure.

Full text of DOI:10.1039/c5ra04259c

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High oxygen storage capacity and enhanced
catalytic performance of NiO/Ni Ce O
nanorods: synergy between Ni-doping and 1D
x
1ꢀx 2ꢀd
Cite this: RSC Adv., 2015, 5, 54571
morphology
Araceli Romero-Nunez and Gabriela Dıaz*
´
˜
´
A series of Ni-doped ceria nanorods with low Ni content was synthesized via a hydrothermal approach and
characterized by DRX, SEM, HRTEM, Raman and TPR techniques. Simultaneous tuning of morphology and
composition, i.e. exposure of reactive (100) and (110) facets together with Ni substitution, provide great
improvements in their physical chemical properties. Segregated NiO as a secondary phase was observed
Received 10th March 2015
Accepted 11th June 2015
as Ni content increases affecting the growth of the nanorods. The system is described as NiO dispersed
ꢁ
x
on Ni Ce1ꢀxO
2ꢀd nanorods. The materials showed a remarkably high OSC at 400 C and good activity
DOI: 10.1039/c5ra04259c
for CO oxidation. Results corroborate the synergic effects between Ni-doping and one-dimensional
www.rsc.org/advances
morphology on ceria structure.
to obtain anisotropic growth and the consequent 1D structure
have been well documented; hydrothermal synthesis using high
1
. Introduction
Ceria (CeO
2
) is widely used in heterogeneous catalysis because concentrations of NaOH is one of the most efficient procedures
15
of its unique redox properties. Through a reversible redox cycle to obtain these morphologies.
4
+
3+
between Ce and Ce ions the material can intrinsically
On the other hand, not only morphology, but also compo-
17
generate oxygen vacancies and form a non-stoichiometric sition can improve the catalytic performance of materials.
CeO2 . The reversible redox reaction allows ceria to either Incorporation of an additional component, for example a metal,
release or take oxygen depending on the surface demand; this either by impregnation or doping allows the materials to
property is called oxygen storage capacity (OSC). DFT studies enhance their catalytic properties. Ceria host structure has been
have already shown that the stability of the three low-index doped with a wide variety of cations from which divalent tran-
1
ꢀd
2,3
18–20
lattice planes of ceria surface follows the sequence: (111) > sition metals are specially interesting.
110) > (100); while the activity follows the opposite order.
Through the forma-
4
(
tion of a substitutional solid solution, M Ce(1ꢀx)O2ꢀd, with the
x
Accordingly, vacancy formation is energetically favored at the divalent cation, an extrinsic oxygen vacancy is created in order
1,17
most reactive ceria surfaces {110} and {100} and as a conse- to fulll charge neutrality.
quence, the preferential exposure of these facets is desirable for
Recently, the Ni–O–Ce system has attracted great interest
5
catalytic reactions, mainly oxidations. Morphology of a mate- due to their high activity and stability and low cost for catalytic
rial controls the crystallographic orientation and exposes crystal applications; also, it has been described as an alternative for
1
8,21,22
surfaces that strongly inuence the surface atom densities, noble metal catalysts for CO oxidation.
6
electronic structure, and chemical reactivity. Ceria 1D
morphologies such as nanorods, nanotubes and nanowires are Ni
An oxygen-decient Ni-doped ceria solid solution,
Ce1ꢀxO2ꢀd, is formed when Ni substitutes a Ce ion into
2
+
4+
x
structures with high aspect ratio which can expose dened the ceria host structure. Once a limit of solubility is reached, the
facets. When anisotropic growth of cubic uorite-like struc- excess of Ni is segregated and NiO is formed; this second phase
tured ceria is driven at the [011] and [211] directions, 1D could be highly dispersed at the surface of the principal phase
structures with preferentially exposed {110} and {100} reactive or forming larger particles. Even though some studies have
7
–9
facets are developed.
An enhanced redox-behavior and reported the limit of solubility for the Ni–O–Ce system;
improvements in catalytic performance of 1D ceria materials complementary studies show that it depends on the preparation
7,10–16
23
have been proved several times.
Diverse routes of synthesis method and post-treatments. The distribution of Ni species is
the most critical factor for catalytic properties, thus, the system
has been described as a hetero-structure NiO/Ni Ce1ꢀxO
2ꢀd
x
Instituto de F ´ı sica, Universidad Nacional Aut ´o noma de M ´e xico, Circuito de la where Ni species have different interactions with the CeO
host
2
Investigaci ´o n Cient ´ı ca s/n, Cd. Universitaria, M ´e xico, D.F. 04510, Mexico. E-mail:
diaz@sica.unam.mx; Fax: +52 (55) 56225008; Tel: +52 (55) 56225097
24–26
2+
structure.
The strongest interaction is that of the Ni which
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is incorporated inside the ceria structure forming a solid solu- min; total cation concentration was maintained at 0.15 M. The
tion. According to the dispersion degree and particle size of the resulting solution was added drop wise to a 9 M NaOH solution
ꢁ
outside NiO, this phase and the solid solution could have previously heated at 70 C and aged for 1 h under vigorous
interactions which vary from strong to weak. Structural char- stirring. Subsequently, the formed slurry was transferred into a
acterization of the Ni–O–Ce system and other doped hetero- 150 mL Teon line stainless steel reactor for hydrothermal
ꢁ
structures requires the use of several techniques. In general, treatment at 140 C for 24 h under autogenous pressure.
identication of this and other solid solutions is achieved by a Aerwards, the reactor was quenched with cold water; the
combination of XRD with another M–O bond sensitive tech- obtained products were centrifuged and washed with deionized
1
7,27
nique as Raman, XPS or H
2
-TPR.
water several times until neutral pH was reached. Finally,
ꢁ
ꢁ
Although 1D morphology and metal-doping have been samples were dried at 70 C for 12 h and calcined at 400 C
widely separately studied, only a small number of materials that under air ow (30 mL min ) for 3 h. Ni Ce
ꢀ1
O
samples
x
1ꢀx 2ꢀd
28–32
combine both modications have been synthesized.
Theo- with Ni-doping atomic percentages of 0, 3, 7, 9 were prepared
, 1D-CeNi3, 1D-CeNi7 and 1D-CeNi9.
been done by Nolan. Ni is predicted to adopt a four- The Ni-doping atomic percentage was calculated as:
retical studies of (111) and (110) Ni-doped ceria surfaces have and identied as 1D-CeO
2
33
ꢀ
ꢁ
coordinated square planar environment near the surface; in
order to maintain this conguration, active oxygen vacancies
will be easily created by removing oxygen coordinated with two
Ce ions. These active vacancies are especially active in oxidation
reactions. In the basis of this evidence, the results found by
Ni
at:% ¼
ꢂ 100
Ce þ Ni
In addition, a 1D Ni-doped sample with high Ni loading (25
Nolan could be seen as a prediction of the utility of 1D Ni-doped at.% Ni) was prepared (1D-CeNi25) and thermally treated at 400
ꢁ
ceria materials for catalysis, in particular for oxidation
C as the other 1D samples. The corresponding weight
reactions.
percentages of Ni and Ni/Ce atomic ratio in each sample are
As control over morphology and composition are the main shown in Table 1.
approach to promote the catalytic performance of ceria based
Reference NiO supported on 1D-CeO
nanomaterials, this work concerns the synthesis and charac- sized by incipient wetness impregnation (IWI)
terization of a series of 1D ceria nanorods doped with low Ni Ni(NO $6H O (Aldrich, 99.99%) as Ni precursor. The sample
content.
was calcined at 400 C for 3 h and identied as X-IWI-NiO/
At present time, no 1D Ni-doped ceria materials have been CeO
2
sample was synthe-
34
36
using
2
)
3
2
ꢁ
1D, where X ¼ 1 corresponds to the Ni loading (1 wt% Ni).
2
reported. Synthesis involves a controlled precipitation step fol- Bulk NiO reference sample was prepared following the hydro-
lowed by a hydrothermal treatment and a nal calcination. The thermal procedure used for the 1D Ni–Ce–O system. This
ꢁ
synergic effects of simultaneously Ni-doping and 1D material was calcined also at 400 C.
morphology of ceria were conrmed using OSC and CO oxida-
tion probes.
2.2. Structure characterization
Structural characterization of the materials was achieved by
electron microscopy techniques in a JEM 2010 FasTem analyt-
ical microscope equipped with a Z-contrast annular detector
2
. Experimental
2.1. Synthesis of materials
Synthesis of 1D undoped-ceria and novel 1D Ni–Ce–O system (HAADF). The samples were ultrasonically dispersed in ethanol
was achieved by hydrothermal approach. The method proposed for 20 s and a single drop of the dispersion was deposited on
35
by Mai et al. for 1D-CeO synthesis was modied in order to get commercial lacey-carbon coated Cu grids (300 mesh) for
2
simultaneously Ni into the ceria host structure and anisotropic observation. SEM analysis was performed in a JEOL 7800
morphology. In particular, precipitation and hydrothermal Scanning Electron Microscopy equipped with eld emission
temperatures as well as type of precursors were modied. gun and X-Max® SDD X-ray Detector. X-ray diffraction (XRD)
Appropriated amounts of NiCl
2
(Aldrich, 98%) and CeCl
3
analysis was carried out in a Bruker D8 Advance diffractometer
(l ¼ 0.154 nm) radiation equipped with a Lynx Eye
(Aldrich, 99.99%) were mixed in 50 mL of deionized water for 10 with Cu K
a
Table 1 Ni content of the 1D Ni–O–Ce system
Nominal Ni-doping
Sample
Ni (at.%)
x in Ni
x
Ce1ꢀxO
2ꢀd
Ni/Ce at. ratio
Ni (wt%)
EDS Ni (wt%)
1
1
1
1
1
D-CeO
2
—
3
7
9
25
0
0
—
—
D-CeNi3
D-CeNi7
D-CeNi9
D-CeNi25
0.03
0.07
0.09
0.25
0.031
0.075
0.099
0.333
1.04
2.48
3.23
9.93
1.0 (3)
2.4 (1)
3.0 (3)
9.8 (4)
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detector. Rietveld renement of XRD data using the whole
37
diffraction pattern was performed with the Bruker TOPAS 3
code. The nature of M–O bond was analyzed with Raman
spectroscopy using a Thermo Scientic DRX Raman Microscope
with an excitation wavelength of 532 nm and a resolution of 2
ꢀ
1
cm . Two spectra were obtained for each sample. All Raman
spectra were recorded at ambient conditions. BET specic
surface areas were obtained from analysis in a Quantachrome
2
Autosorb MP-1 using N as adsorbate.
2.3. Physical chemical properties and CO oxidation
Reduction properties of the 1D samples and bulk NiO were
studied by hydrogen temperature programmed reduction (H2-
TPR) in a RIG multitask unit equipped with a thermal
conductivity detector (TCD). Reduction proles were recorded
using 0.050 g of each sample under a 5 vol.% H
2
ꢀ1
/N
2
ow of 30
ꢀ1
ꢁ
mL min
and a heating rate of 5 C min
from room
ꢁ
temperature to 650 C. The H
2
O produced by the reduction
process was trapped before the TCD.
Oxygen storage capacity (OSC) was determined at 400
following the procedure described by Mai et al. using pulses of
ꢁ
C
35
Fig. 1 Typical electron microscopy images of the 1D ceria-based
structures (a) SEM image of 1D-CeNi7 sample, (b) SEM image of 1D-
CeNi25 sample, (c) and (d) HRTEM images of 1D-CeNi3 sample
showing [110] and [211] growth directions.
CO under oxygen free atmosphere to achieve reaction (1)
xCO + CeO
2
/ CeO2ꢀx + xCO
2
(1)
A xed bed ow reactor in line with a HIDEN mass spec-
trometer was used for this purpose. Firstly, 0.020 g of catalyst identied: the classical [110] and also the [211]. The [110]
ꢁ
was pretreated in situ at 300 C for 30 min under 20 vol.% O /He growth direction, which was found in all samples, exhibits the
2

ow; then the system was purged with argon for 10 min at the (111), (200) and (220) planes with interplanar spacing of 0.31
same temperature. Aerwards, pulses of 1 mL of 5 vol.% CO/Ar nm, 0.27 nm and 0.19 nm respectively, Fig. 1(c). This growth
were injected and the CO and CO
concentrations were followed direction is the most commonly reported by other works on
by MS. A 5 min lapse between pulses was established. OSC ceria nanorods. On its side, the [211] growth direction exhibits
values were determined by the amount of CO
produced aer 20 {220} facets, with interplanar spacing of 0.27 nm, Fig. 1(d). This
pulses as no modication in the emerging CO pulses was conguration suggests an oriented-attached growth mecha-
2
2
38–40
observed. Each experiment was carried out by duplicate and the nism as previous works describe.
By combining [110] and
results averaged to obtain the nal OSC value.
[211] growth directions, is expected a preferential exposure of
Activity tests for CO oxidation were carried out in a xed bed {110} and {110} reactive surfaces in combination with a low

ow reactor charged with 0.050 g of catalyst. The feed gas proportion of {111} less reactive surfaces.
composition was 1 vol.% CO, 1 vol.% O
2
and He balance with a
The Ni content as determined by SEM–EDS analysis was in
total ow of 60 mL min . Reaction was followed from 50 C to good agreement with the nominal one, Table 1. In order to
00 C and the products were analyzed by gas chromatography examine the Ni distribution at the large scale in 1D Ni-doped
ꢀ
1
ꢁ
ꢁ
4
using a Carboxen 1000 Supelco column and a TCD.
samples, SEM–EDS mapping of cerium and nickel was recor-
ded, Fig. 2(a). Results reveal that both nickel and cerium are
uniformly distributed. Complementarily, analysis of few nano-
rods was also performed through HAADF–EDS elemental anal-
ysis, Fig. 2(b). As observed, also at this scale, both Ni and Ce are
3. Results and discussion
3.1. Electron microscopy and elemental analyses
Synthetized undoped ceria and Ni-doped ceria materials show a also well distributed.
one-dimensional (1D) rod-like morphology as can be seen in
On the other hand, few nanoparticle-like structures attached
typical SEM images presented in Fig. 1(a) and (b). The length to the nanorods of both undoped and Ni-doped samples were
value of the 1D structures is wide ranging from 100–1000 nm observed by TEM. In order to establish the nature of these
with diameters around 20 nm for low nickel doping (#9 at.% particle-like structures, EDS analysis was performed. Fig. 3(a)
substitution), Fig. 1(a), but these parameters changed signi- shows a typical image of 1D-CeNi3 sample along with the EDS
cantly when the Ni doping was higher (25 at.% substitution), spectrum of the region enclosed in a square. As the analysis
Fig. 1(b); 1D structures became clearly wider (30–150 nm) and showed, no conclusive evidence of Ni was obtained with
shorter (60–500 nm). Lattice fringe spacing determined by predominance of Ce in the EDS spectrum; however at higher Ni
HRTEM in 1D Ni-doped samples is consistent with those of the loadings the evident Ni peak conrms that some of these
ceria structure, Fig. 1(c) and (d). Two direction growths were structures are segregated NiO, Fig. 3(b) and (c). Some other
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Fig. 2 Elemental mapping (Ce, Ni) by EDS in 1D-CeNi3 sample (a) SEM
and (b) HAADF images.
structures showed only Ce atoms. The presence of CeO struc-
2
tures attached to the nanorods may be explained by the growth
mechanism which consists of self-assembling of adjacent
nanoparticles sharing common crystallographic orientation
developing by this an anisotropic structure. The CeO
may come from a second growth process via Ostwald ripening.
2
structures
39
On the other hand, as the preparation method of these 1D
systems is focused not only on the assembling of the 1D
morphology but also on the formation of the solid solution,
further research on the synthesis parameters and growth
mechanism in these 1D Ni-doped ceria nanostructures to
control segregation of NiO needs to be further investigated.
3.2. X-ray diffraction
Once the 1D morphology was conrmed, the next step is to
corroborate the formation of a Ni–ceria solid solution. XRD,
Fig. 4(a), revealed a uorite-like ceria structure (JCPDS-34-0394)
for all samples together with signals corresponding to NiO
cubic phase (JCPDS-89-7130). The ratio between the two phases
is dependent on the Ni content. In the case of 1D-CeNi3 sample,
a signal whose intensity is in the signal/noise limit gives some Fig. 3 EDS elemental analysis performed on particle-like structures at
ꢁ
the surface of synthetized 1D Ni-doped samples. Spectra correspond
to the enclosed area in the TEM image of (a) 1D-CeNi3, (b) 1D-CeNi9
and (c) 1D-CeNi25 samples.
indication of NiO at 2-theta value around 43 . For higher Ni
doping diffraction peaks corresponding to NiO phase are
recognizable as small peaks. XRD pattern of sample 1D-CeNi25
showed in addition, an effect of Ni loading on the crystal growth
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of the solid solution results to be very close to that of the
23,45
undoped ceria.
This effect is observed in the present study as
listed in Table 2. Then, according to the Rietveld analysis two Ni
species can be identied in these samples: Ni incorporated in
the ceria structure leading to a solid solution and dispersed NiO
phase. In the particular case of 1D-CeNi3, either all Ni is
incorporated in the ceria structure or some amount of Ni
coexists as well dispersed or amorphous NiO with the
18,46–49
former.
As Ni loading increases, contribution of NiO
becomes important and the trend in the lattice parameter of
ceria suggests that the amount of Ni in the ceria framework is
also increasing, Table 2. According with empirical parameters
of the formation of solid solutions, only low Ni loadings are
expected to be incorporated in the ceria structure due to
Fig. 4 (a) XRD patterns of (a) 1D-CeO
2
, (b) 1D-CeNi3, (c) 1D-CeNi7, (d)
ꢁ
1
D-CeNi9, (e) 1D-CeNi25 samples after calcination at 400 C. (b) Shift
to higher 2-theta values of (111) reflection of ceria as a function of Ni
loading. (c) Region where the most intense NiO diffraction peaks differences in electronegativity, valence and crystal structure
appear.
2
between NiO and CeO . Further discussion on the solid solution
formation will be provided by the Raman analysis.
Finally, average crystallite size for 1D-Ni doped samples is
of ceria as evidenced by the lower intensity of the diffraction around 20 nm for low Ni content while it is lower for 1D-CeO
peaks of ceria, Fig. 4(c). As the SEM analysis showed the 1D and 1D-CeNi25.
structures in this sample were shorter and wider, Fig. 1(b).
2
Regarding the ceria phase, a shi to higher 2-theta values is
observed for Ni-doped samples, Fig. 4(b).
3.3. Raman spectroscopy
Rietveld structure renement taking the entire diffraction
pattern was done in order to analyze these features, Table 2. The
quantitative phase analysis evidence that only a fraction of the
total loaded Ni in the samples is forming the NiO phase; sug-
gesting that Ni can be incorporated in the ceria host structure.
In addition, Rietveld analysis of the ceria-like phase revealed a
contraction of the lattice of 1D Ni-doped samples when
Raman spectra of 1D-CeO and 1D Ni-doped sample are shown
2
in Fig. 5. Raman spectrum of 1D-CeO
active modes of uorite crystal structure. The F2g signal, which
is ascribed to the symmetrical stretching mode of the [Ce–O
2
shows the typical Raman
8
]
ꢀ1
vibrational unit, appears at 466 cm . Also, bands can be seen at
10 cm and 260 cm , which are assigned to the so-called
ꢀ
1
ꢀ1
6
Frenkel defect-induced mode (D mode) and to the transverse
compared with the undoped 1D-CeO . A clear trend in this
50,51
2
second-order acoustic vibrational mode (2TA), respectively.
feature is observed as Ni loading increases. This behavior is
related to the formation of a substitutional solid solution in
Spectra of doped samples resemble the one of the 1D-CeO , but
2
a slight shi to lower frequency is observed for the F2g and 2TA
signals. Also, the intensity of 2TA and D peaks increased with
Ni-doping concentration. The down shi in F2g signal is
17,41–43
2+
˚
agreement with previous reports.
Since Ni (69 A) has a
4
+
˚
much smaller ionic size compared to Ce (97 A), Ni ion can
replace a Ce ion in the uorite lattice producing shrinkage of
the lattice. Although the dopant could also occupy an interstitial
site, solid solutions involving substitutional Ni ions are pre-
dicted to become thermodynamically more stable than those
commonly interpreted as lattice distortions due to formation of
18,47
the solid solution.
This assumption is further conrmed by
ꢀ
1
ꢀ1
the increase of the 260 cm and 610 cm signals which are
related to the formation of lattice defects, mostly oxygen
44
containing interstitial defects. The formation of the substitu-
18,24,52
vacancies.
In Fig. 6 the 2TA/F2g intensity ratios of Ni-doped
2
+
4+
tional solid solution, in which a Ni replace a Ce , produces an
samples suggest that the content of oxygen vacancies increased
with Ni-doping. According to this trend, the oxygen vacancy
17,41
oxygen vacancy in order to maintain charge neutrality.
The
contraction originated by the introduction of the smaller size
dopant is compensated by the expansion caused by the forma-
tion of the oxygen vacancy, and therefore the lattice parameter
content follows the order 1D-CeO
D-CeNi9 < 1D-CeNi25. Only for 1D-CeNi25 signals of NiO
phase seem to be observed at the reported frequency for such
2
ꢃ 1D-CeNi3 < 1D-CeNi7 <
1
ꢁ
Table 2 Rietveld analysis of 1D samples after calcination at 400 C
a
Sample
Nominal Ni (wt%)
Ni (wt%) as NiO
Cell parameter ( A˚ )
Mean crystal size ( A˚ )
Rwp
1
1
1
1
1
D-CeO
2
—
—
n.d.
1.10
1.17
7.81
5.4145 (3)
5.4130 (2)
5.4123 (2)
5.4117 (2)
5.4109 (3)
135 (2)
193 (1)
197 (2)
186 (3)
160 (1)
9.29
9.99
8.96
8.61
8.46
b
D-CeNi3
D-CeNi7
D-CeNi9
D-CeNi25
1.04
2.48
3.23
9.93
a
b
Calculated based on the total NiO wt% obtained by Rietveld renement. Not available due to very weak NiO signals in XRD pattern.
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systems. As reduction of bulk CeO
2
takes place at higher
ꢁ
1
temperatures (ꢄ700–800 C), this feature is not studied in the
present work. For 1D-CeO sample, a wide signal centered at
2
ꢁ
5
00 C appears which is ascribed to surface oxygen reduction of
the material; the shape of the signal also resembles the already
2
reported for ceria nanorods. In the case of 1D Ni-doped system
a well-dened hydrogen consumption peak which changed as a
function of the material composition characterizes the TPR
proles. Three main hydrogen uptake zones can be identied:
ꢁ
ꢁ
ꢁ
below 305 C (A); below 380 C (B) and below 460 C (C).
D-CeNi3 sample shows a single peak which dominates the
1
C zone and low hydrogen uptake at lower temperature (A zone).
By increasing Ni concentration, peaks at B zone develop along
with increased uptakes at the A zone. On the ceria side, the
2
Fig. 5 Raman spectra of (a) 1D-CeO , (b) 1D-CeNi3, (c) 1D-CeNi7, (d)
1
D-CeNi9 and (e) 1D-CeNi25 doped materials.
ꢁ
reduction peak around 500 C is no longer observed.
Hydrogen uptakes at low temperature have been attributed
to the reduction of reactive oxygen species like peroxides
adsorbed at surface oxygen vacancies of Ni-ceria solid solu-
18,22,24,26,47,53
tion.
Reduction temperatures of the Ni–CeO system
2
will depend on the interaction between the Ni and the ceria host
structure; thus, stronger interactions are characterized by the
23,46,48,54
higher reduction temperatures.
On the other hand, reduction temperatures of bulk-like or
2
Fig. 6 2TA/F2g intensity ratio of 1D-CeO , 1D-CeNi3, 1D-CeNi7, 1D-
CeNi9 and 1D-CeNi25 samples.
free NiO phase reported in the literature are quite diverse
23,25,53,55
depending on the particle size.
The reduction peaks at
higher temperatures are ascribed to reduction of larger NiO
particles and these temperatures are further modied if inter-
action with the support takes place.
According to this and results in Fig. 7 various Ni species are
being reduced in these 1D-Ni-doped samples. The peak in the
4
1
ꢀ1
material (lit., 524 cm ). In the other Ni-doped samples the
apparent absence of NiO signals could be a sign of their very low
concentration when compared with the principal phase. This is
in line with the NiO concentration obtained from Rietveld
quantitative phase analysis.
1D-CeNi3 sample is attributed to the reduction of a dispersed
NiO phase interacting strongly with the 1D-Ni 2ꢀd solid
x
Ce1ꢀxO
solution. As Ni content increases, larger NiO particles are
ꢁ
formed whose reduction temperature in the range 380–306 C
3
.4. Thermo-programmed reduction (H -TPR)
2
suggests bulk-like behavior with lower interaction with the
support. The reduction at 355 C of bulk NiO powder taken as
reference, Fig. 7(f), corroborates this. It is worth to notice that B
reduction zone is dominated by the reduction of NiO species in
In order to evaluate the reducibility of 1D-NiCe materials and
the strength of the interaction between the NiO secondary
ꢁ
phase and the solid solution, H -TPR characterization was
2
ꢁ
performed in the temperature range 100–650 C, Fig. 7. Proles
low interaction with the 1D-Ni Ce1ꢀxO2ꢀd solid solution, while
x
with hydrogen consumptions in this range are well documented
C reduction zone includes the reduction of both NiO in weak
and strong interaction with the solid solution support.
The quantitative analysis of the hydrogen consumption in
to provide information of the surface reduction of metal-CeO
2
ꢁ
the range 150–500 C is presented in Table 3. As observed, the
experimental value is higher than the required for the reduction
of all the Ni detected by EDS assuming the reaction: NiO + H
2
/
NiO + H O. This conrms that the surface reduction of ceria
2
support is taking place at the same time as that of nickel oxide
ꢁ
explaining the absence of the reduction peak at 500 C. The
reducibility of ceria improves in the presence of metals due to
56
spillover effects. Decomposition of the reduction peaks
allowed to identity the hydrogen uptake associated to each
reduction zone. The uptake in A zone increases as oxygen
vacancies increases following the results of Raman character-
ization. The hydrogen uptake of B zone follows the increase of
Ni concentration, which is in line with XRD quantitative phase
analysis. The trend observed in the hydrogen uptake indicates
that the reduction of larger NiO particles identied by TEM in
Fig. 7
2 2
H -TPR profiles of (a) 1D-CeO , (b) 1D-CeNi3, (c) 1D-CeNi7, (d)
1
D-CeNi9, (e) 1D-CeNi25 and (f) NiO samples.
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Table
nanorods
3
Quantitative analysis of
H
2
uptake of Ni-doped ceria host structure leads to a lower formation energy, DE
, required
v
17
to make an oxygen vacancy near the location of the dopant.
Indeed, DE is lower for the (110) surface than for the (111)
surface for Ni-doped ceria solid solutions.
v
Experimental Theoretical
H
2
uptake (mmol)
B
33
a
H
2
uptake
2
H uptake
Sample
(mmol)
(mmol)
A
C
On the other hand, the volcano behavior of the OSC observed
with Ni-doping is believed to be related to the contribution of
NiO second phase. At rst, the well-dispersed NiO provides
additional active sites for the catalytic reaction, as seen for 1D-
CeNi3 and 1D-CeNi7 samples. Shortly aer, an excess of NiO
could cause the formation of larger aggregates with lower
dispersion and lower oxygen mobility, as 1D-CeNi9 sample
seems to indicate. Also, the poor OSC value of the 1-IWI-NiO/
1
1
1
1
D-CeNi3
D-CeNi7
D-CeNi9
0.0447
0.0589
0.0669
0.0089
0.0211
0.0275
0.0827
0.0091 0.0063 0.0293
0.0102 0.0344 0.0142
0.0105 0.0370 0.0194
0.0119 0.0892 0.0201
D-CeNi25 0.1212
a
Calculated by assuming total reduction of Ni detected by EDS.
2
CeO 1D sample corroborates the premise that lower Ni substi-
samples with a Ni loading >3 at.% dominates the process. The
contribution of ceria surface reduction assumed to take place at
the same time that the one of NiO is difficult to establish only
with the current technique.
tution, if any in this sample, provides less OSC. Additionally, as-
prepared 1D Ni-doped materials exhibit exceptionally high OSC
values when compared with some previously reported Ni-ceria
4
1
ꢀ1
solid solutions (lit., 500 mmol [O] g ). The mutual promo-
tion effect of 1D morphology as well as Ni-doping is believed to
synergistically contribute for the higher OSC of the prepared
In summary, the present characterization results (TEM, XRD,
TPR and Raman results), as other reports in the literature about
2
+
the Ni–CeO system, identify three Ni species; Ni present into
2
33
samples corroborating the theoretical prediction.
the uorite lattice forming the 1D-Ni Ce
O
solid solution,
x
1ꢀx 2ꢀd
and NiO in low and high interaction with the surface of the
former allowing to describe the materials as NiO dispersed on 3.6. Catalytic properties for CO oxidation
Ni
x
Ce1ꢀxO2ꢀd nanorods were Ni content determines the inter-
CO is not only an important probe molecule for catalytic activity
of heterogeneous catalysts, but also its oxidation has clear
action degree of NiO second phase.
57
signicance in real application on environmental issues. The
catalytic performance for CO oxidation displayed by 1D-CeO2
and and 1D Ni-doped material with Ni doping #9 at.%, is shown in
3.5. Oxygen storage capacity (OSC)
Fig. 8 displays the OSC of the 1D nanostructures (1D-CeO
2
1
D-CeNi samples with Ni doping #9 at.%) and the 1-IWI-NiO/ Fig. 9. As expected, the CO oxidation activity follows the OSC
CeO 1D reference sample. The 1D-CeO
shows an OSC value of trend shown in Fig. 8. The improvement in the activity is related
25 mmol [O] g ; other reports for ceria nanorods show a lower to the high number of oxygen vacancies which can be achieved
2
2
ꢀ1
6
35
ꢀ1
value (lit., 554 mmol [O] g ). A great improvement in the OSC in the 1D Ni-doped structure and to the NiO dispersed over its
of ceria nanorods is achieved with Ni-doping. The OSC of all 1D surface which acts as a second active phase. However, a balance
doped samples exceed by more than twice the value of 1D-CeO
between these two features is needed as the 1D-CeNi9 sample
sample; OSC values of 1615, 1785 and 1472 mmol [O] g were seems to indicate; despite the high number of defects in this
2
ꢀ1
obtained for 1D-CeNi3, 1D-CeNi7 and 1D-CeNi9, respectively. A sample as shown by Raman spectroscopy, Fig. 6, activity is not
reference 1-IWI-NiO/CeO
[
2
1D sample shows an OSC of 548 mmol enhanced in comparison to 1D-CeNi7 sample.
O] g , which is lower than the OSC of the 1D-CeO support.
In order to compare the catalytic activity showed by different
ꢀ
1
2
The origin of the enhancement of the OSC for 1D Ni-doped reported Ni-doped ceria systems and our 1D Ni-doped samples,
samples is supposed to be related to their unique structure Table 4 tabulates their respective half-conversion temperature,
and morphology. In addition to the oxygen vacancy stabilization
T50, which is the temperature at which the 50% of CO conver-
previously mentioned, the incorporation of Ni into the ceria sion is achieved. It must be noticed that the oxygen concen-
tration in some reported studies exceeds signicantly the one
Fig. 9 Catalytic performance for CO oxidation of 1D-CeO
, 1D-CeNi3, 1D-CeNi7, 1D-CeNi9 and CeNi3, 1D-CeNi7 and 1D-CeNi9 materials. Feed: 1% CO, 1% O and He
2
1D reference sample. balance, of 0.050 g of catalyst.
2
, 1D-
Fig. 8 OSC values of 1D-CeO
-IWI-NiO/CeO
2
2
1
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Table 4 Catalytic performance for CO oxidation of Ni–O–Ce systems
2
ꢀ1
ꢁ
Catalyst
Ni wt%
S
BET (m g
)
T
50 ( C)
Conditions
CO : O
2
Ref.
4
6
Ni5–CeO
2
2.12
2.12
3.8
1.75
0
1.04
2.48
3.23
58
ꢄ410
290
175
175
351
229
176
216
1% CO, 1% O
4% CO, 2.05% O
1% CO, 16% O
2
/He
/He
/He
2 2
1.6% CO, 20.8% O /N
1 : 1
2 : 1
1 : 16
1 : 13
1 : 1
1 : 1
1 : 1
1 : 1
Y. Park
Yakimova
5
8
5
NiO/CeO
Ni–CeO 3.8%
meso-Ni0.05Ce
2
220
122
111
50
51
54
2
34
2
2
T. Li
C. Tang
2
2
1
1
1
1
D-CeO
2
1% CO, 1% O
1% CO, 1% O
1% CO, 1% O
1% CO, 1% O
2
2
2
2
/He
/He
/He
/He
This work
This work
This work
This work
D-CeNi3
D-CeNi7
D-CeNi9
55
used in this paper along with larger SBET areas and Ni loading.
Despite this, the as-prepared 1D catalysts with less Ni content
show lower T50 for CO oxidation compared to samples with no-
anisotropic morphologies. These results indicate that the use of
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. Conclusions
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A series of Ni-doped ceria nanorods with low Ni contents (3, 7 and
at.%) were synthesized using a hydrothermal approach.
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Anisotropic growth in [110] and [211] directions was identied
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surfaces in combination with the less reactive (111) one. Different
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Acknowledgements
8 S. Mahammadunnisa, P. Manoj Kumar Reddy, N. Lingaiah
Authors want to thank CONACyT-176509 and PAPIIT-IN107512
and C. Subrahmanyam, Catal. Sci. Technol., 2013, 3, 730–736.
for funding. Antonio G ´o mez-Cort ´e s for technical support, LCM- 19 A. B. Kehoe, D. O. Scanlon and G. W. Watson, Chem. Mater.,
IF (Roberto Hern ´a ndez, Orlando Hern ´a ndez, Mario Monroy and
2011, 23, 4464–4468.
Carlos Maga n˜ a) for TEM and SEM images; Cristina Zorrilla for 20 Y. Zuo, L. Li, X. Huang and G. Li, Catal. Sci. Technol., 2014, 4,
Raman spectra and LAREC-IF (Manuel Aguilar and Antonio 3368–3378.
Morales) for XRD assistance. ARN also thanks CONACyT Becas 21 J. Zhang, J. Guo, W. Liu, S. Wang, A. Xie, X. Liu, J. Wang and
Nacionales for the 332648 scholarship.
Y. Yang, Eur. J. Inorg. Chem., 2015, 2015, 969–976.
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