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Citation: Pakharukova, V.P.;
Potemkin, D.I.; Rogozhnikov, V.N.;
Stonkus, O.A.; Gorlova, A.M.;
Nikitina, N.A.; Suprun, E.A.; Brayko,
A.S.; Rogov, V.A.; Snytnikov, P.V.
Effect of Ce/Zr Composition on
Structure and Properties of
Ce1−xZrxO2Oxides and Related
Ni/Ce1−xZrxO2Catalysts for CO2
Methanation. Nanomaterials 2022,12,
3207. https://doi.org/10.3390/
nano12183207
Academic Editor: Giorgio Vilardi
Received: 25 August 2022
Accepted: 10 September 2022
Published: 15 September 2022
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nanomaterials
Article
Effect of Ce/Zr Composition on Structure and Properties of
Ce1−xZrxO2Oxides and Related Ni/Ce1−xZrxO2Catalysts for
CO2Methanation
Vera P. Pakharukova 1,* , Dmitriy I. Potemkin 1, Vladimir N. Rogozhnikov 1, Olga A. Stonkus 1,
Anna M. Gorlova 1,2 , Nadezhda A. Nikitina 1,3, Evgeniy A. Suprun 1, Andrey S. Brayko 1, Vladimir A. Rogov 1
and Pavel V. Snytnikov 1
1Boreskov Institute of Catalysis SB RAS, Pr. Lavrentieva 5, 630090 Novosibirsk, Russia
2
Department of Natural Sciences, Novosibirsk State University, Pirogova Street 2, 630090 Novosibirsk, Russia
3Department of Chemistry, Moscow State University, Leninskie Gory St., 1, 119991 Moscow, Russia
*Correspondence: verapakh@catalysis.ru; Tel.: +7-383-326-9597; Fax: +7-383-330-8056
Abstract:
Ce
1−x
Zr
x
O
2
oxides (x = 0.1, 0.25, 0.5) prepared via the Pechini route were investigated
using XRD analysis, N
2
physisorption, TEM, and TPR in combination with density functional theory
calculations. The Ni/Ce
1−x
Zr
x
O
2
catalysts were characterized via XRD analysis, SEM-EDX, TEM-
EDX, and CO chemisorption and tested in carbon dioxide methanation. The obtained Ce
1−x
Zr
x
O
2
materials were single-phase solid solutions. The increase in Zr content intensified crystal structure
strains and favored the reducibility of the Ce
1−x
Zr
x
O
2
oxides but strongly affected their microstruc-
ture. The catalytic activity of the Ni/Ce
1−x
Zr
x
O
2
catalysts was found to depend on the composition
of the Ce
1−x
Zr
x
O
2
supports. The detected negative effect of Zr content on the catalytic activity was
attributed to the decrease in the dispersion of the Ni
0
nanoparticles and the length of metal–support
contacts due to the worsening microstructure of Ce
1−x
Zr
x
O
2
oxides. The improvement of the redox
properties of the Ce
1−x
Zr
x
O
2
oxide supports through cation modification can be negated by changes
in their microstructure and textural characteristics.
Keywords:
Ni/Ce
1−x
Zr
x
O
2
catalysts; mixed oxides; structure; microstructure; methanation;
carbon dioxide
1. Introduction
The CO
2
methanation process has received great interest during the last few years as
a promising method of the production of synthetic natural gas as a hydrogen storage ap-
proach [
1
–
3
]. Nickel-based catalysts exhibit high activity and selectivity in the methanation
of carbon oxides and are less expensive than systems containing noble metals [
3
–
5
]. Because
of this, nickel catalysts are widely investigated. The impact of a support on the dispersion
of nickel particles and catalytic performance of nickel catalysts is significant [
6
–
8
]. CeO
2
was shown to be one of the most effective support materials [
6
–
16
]. Ceria works as both
a support for nickel particles and a reaction promoter. The promoting effect is related to
the easiness and reversibility of Ce4+–Ce3+ transition associated with appearing or healing
oxygen vacancies [
17
]. Oxygen vacancies on the support surface were reported to participate
in the activation of CO2molecules [14,18–22].
The modification of ceria by doping with foreign cations is a well-known strategy to
improve its redox properties [
17
]. Nickel catalysts supported on mixed Ce
1−x
Zr
x
O
2
oxides
are promising systems in terms of catalytic activity and stability [
12
,
23
–
26
]. Zirconium is
recognized as a promoter of the reducibility and oxygen storage capacity of ceria [
27
–
29
].
Our previous study [
26
] showed that the formation of oxygen vacancies on the support
surface is essential for activity of Ni/Ce
1−x
Zr
x
O
2
catalysts in the methanation of carbon
oxides. The reducibility of Ce
1−x
Zr
x
O
2
oxides was experimentally shown to increase with
Nanomaterials 2022,12, 3207. https://doi.org/10.3390/nano12183207 https://www.mdpi.com/journal/nanomaterials
Nanomaterials 2022,12, 3207 2 of 16
the Zr content [
29
,
30
]. The regulation of the support redox properties through the variation
of Zr content is of great interest. However, there have been few studies on the effect of the
Ce/Zr ratio on the performance of Ni/Ce
1−x
Zr
x
O
2
catalysts in CO
2
methanation. Ocampo
et al. [
23
] studied CO
2
methanation over 5 wt.% Ni/Ce
1−x
Zr
x
O
2
catalysts differing in
Zr content (x = 0.28, 0.5, 0.86). The 5 wt.% Ni/Ce
0.5
Zr
0.5
O
2
catalyst showed the highest
activity. Nie et al. [
31
] studied NiO-CeO
2
-ZrO
2
mixed oxides containing 40 wt.% Ni.
The catalyst with a Ce/Zr molar ratio of 9:1 (Ce
0.9
Zr
0.1
O
2
) exhibited the best catalytic
properties. Atzori et al. [
32
] tested NiO-CeO
2
-ZrO
2
catalysts (30 wt.% Ni—Ce
1−x
Zr
x
O
2
) in
CO
2
and CO co-methanation. The activity was the same for the catalysts with Zr content in
x = 0–0.5 and decreased at a higher content. To summarize, a correlation between the
activity of Ni/Ce
1−x
Zr
x
O
2
catalysts and the Ce/Zr ratio has not been fully understood.
All the reported studies indicated that nickel catalysts based on highly doped Ce
1−x
Zr
x
O
2
oxides (x > 0.5) are less effective. The data concerning systems based on Ce
1−x
Zr
x
O
2
oxides
with a lower Zr content (x ≤0.5) are somewhat controversial.
The present work aims to provide further insight into the relation between the com-
position of the Ce
1−x
Zr
x
O
2
support materials and the performance of Ni/Ce
1−x
Zr
x
O
2
catalysts in CO
2
methanation. A series of Ce
1−x
Zr
x
O
2
supports with different Zr con-
tents (x = 0.1, 0.25, 0.5) were prepared via the Pechini method. The supported catalysts
Ni/Ce
1−x
Zr
x
O
2
containing 10 wt.% Ni were synthesized via the impregnation method.
DFT calculations and TPR studies were employed to evaluate the impact of Zr content on
the reducibility of the Ce
1−x
Zr
x
O
2
oxides. A wide range of physical methods was used
to reveal the structure features of the Ce
1−x
Zr
x
O
2
support materials and Ni/Ce
1−x
Zr
x
O
2
catalysts. The catalytic properties of the Ni/Ce
1−x
Zr
x
O
2
samples were analyzed with
regard to the structural features of both the support materials and catalysts.
2. Experimental Section
2.1. Computational Methods and Details
The DFT+U calculations were performed using the Vienna Ab initio Simulation Pack-
age (VASP) program [
33
]. The generalized gradient approximation (GGA) PBE96 functional
was employed [
34
]. The core electrons were described by projector augmented-wave (PAW)
potentials [
35
,
36
], and the valence electrons were described by a plane-wave basis set. The
DFT+U method (U
eff
= 5 eV) [
37
] was used to correct the strong Coulomb repulsion of
cerium and DFT-D3 [
38
] to take into account dispersion corrections. The cutoff energy was
400 eV. A 2
×
2
×
1 Monkhorst–Pack k-point set was used for the Brillouin-zone integration.
The face-centered cubic unit cell of a fluorite-type structure (space group: Fm
3
m) was
used as the initial geometry in the calculations [
39
,
40
]. Ce
1−x
Zr
x
O
2
supercells (x = 0.25, 0.50,
0.75) were built by replacing Ce atoms with Zr ones (Supplementary Materials, Figure S1).
Ce
1−x
Zr
x
O
2
(100) and (111) surfaces were modeled as p (2
×
2) and p (3
×
3) slab cells.
The (100) and (111) Ce
1−x
Zr
x
O
2
supercells contained four and three layers, respectively. A
vacuum space of 15
◦
Å was set between neighboring slabs to keep the spurious interaction.
The energy of oxygen vacancy formation (Ef) was computed as:
Ef= E(Ce1−xZrxO2−δ) + 1/2E(O2)−E(Ce1−xZrxO2),
where E(Ce
1−x
Zr
x
O
2−δ
) and E(Ce
1−x
Zr
x
O
2
) are the energies for surfaces with and without
oxygen vacancy, and E(O2) is the energy of gas-phase O2.
2.2. Samples Preparation
2.2.1. Synthesis of Ce1−xZrxO2Mixed Oxides
A series of Ce
1−x
Zr
x
O
2
(x = 0.1, 0.25, 0.5) mixed oxides was prepared via the Pechini
method [
41
]. The Ce(NO
3
)
3
*6H
2
O (99.4%) and ZrO(NO
3
)
2
*8H
2
O (99.98%) salts were used
as precursors. Aqueous solutions of the salts with required Ce:Zr molar ratios (9:1; 3:1;
1:1) were prepared. Citric acid (CA) was added to the aqueous solutions at 80
◦
C and
vigorously stirred for 30 min. The CA:metal molar ratio was 1:1. Subsequently, ethylene
Nanomaterials 2022,12, 3207 3 of 16
glycol C
2
H
4
(OH)
2
(EG) was added. The CA:EG molar ratio was 3:2. Next, the solution was
heated at 100
◦
C to promote the polyesterification reaction and water evaporation with the
formation of a polymeric resin. The obtained solids were mechanically milled to powders
and calcined at 450 ◦C for 8 h.
2.2.2. Synthesis of Ni/Ce1−xZrxO2Catalysts
The catalysts containing 10 wt.% Ni were prepared by the impregnation of the obtained
Ce
1−x
Zr
x
O
2
oxide materials. Metal precursor Ni(CH
3
COO)
2·
4H
2
O (99.0%) and ethylene
glycol (99.5%) were dissolved in the distilled water under stirring at 70
◦
C for 20 min. Then,
the support material was added into the solution. The EG:Ni molar ratio was set to 5.3. The
suspension of the support and impregnating solution was stirred for 2 h at 70
◦
C and dried
at 120
◦
C for 12 h in air. The dried samples were heated at a rate of 2
◦
C/min and calcined
in air at 400
◦
C for 2 h. The catalysts were designated as Ni/Ce
1−x
Zr
x
O
2
, with x indicating
Zr content.
2.3. Samples Characterization
2.3.1. XRF Analysis
Elemental compositions of the catalysts were determined via X-ray fluorescent spec-
troscopy (XRF) using an ARL-Advant’x device (Thermo Fisher, Vienna, Austria). Mea-
surements were carried out in a helium atmosphere using the Rh X-Ray tube. UniQuant
software was used to calculate the element percentages.
2.3.2. BET Surface Area Analysis
The BET specific surface areas (S
BET
, m
2
/g) of the Ce
1−x
Zr
x
O
2
oxides were determined
by N
2
-physisorption at
−
196
◦
C. The experiments were carried out using an ASAP 2400
instrument (Micrometrics, Norcross, GA, USA).
2.3.3. XRD Analysis
X-ray diffraction (XRD) measurements were carried out using a D8 Advance diffrac-
tometer (Bruker, Germany) equipped with a Lynxeye linear detector. The measurements
were carried out using the Cu K
α
radiation (
λ
= 1.5418 Å) in the 2
θ
range of 10–83
◦
with
a step of 0.05
◦
. XRD phase analysis was performed using the ICDD PDF-4+ database.
The evaluation of substructure parameters of the Ce1−xZrxO2oxides was performed. The
separation of the crystallite size (D
XRD
) and microstrain (
∆
d/d) effects to the line broad-
ening was performed by means of Williamson–Hall plots [
42
]. From the D
XRD
values,
the crystallite surface area (S
XRD
) values were calculated assuming the crystallites were
quasi-spherical:
SXRD =
6000
ρDXRD
where ρis the theoretical density of the material (g/cm3).
To estimate the degree of agglomeration of the Ce
1−x
Zr
x
O
2
crystallites, an agglomera-
tion coefficient (ξ) was calculated on the basis of the ratio of SBET and SXRD values:
ξ=1−SBET
SXRD
The average crystallite size of nickel-containing phases was estimated by the line
broadening analysis according to the Scherrer equation [
43
]. The Rietveld refinement was
carried out using the software package Topas v.4.2 (Bruker-AXS, Karlsruhe, Germany).
2.3.4. CO Pulse Chemisorption
The average sizes of metallic Ni
0
nanoparticles in catalysts after catalytic experiments
were determined using the pulse chemisorption of CO on the assumption that each sur-
face Ni atom adsorbed one CO molecule. The measurements were carried out using a
Nanomaterials 2022,12, 3207 4 of 16
Chemosorb analyzer (Modern Laboratory Equipment, Novosibirsk, Russia). An amount of
50 mg of each sample was placed inside a U-shape quartz reactor and reduced at 350
◦
C in
H
2
flow (100 mL/min) for 30 min. The treated sample was subsequently cooled down to
room temperature, followed by Ar purge. After that, pulses of CO were fed to the reactor
(100
µ
L) until the amount of CO in the outlet stopped changing according to the thermal
conductivity detector. The amount of chemisorbed CO was estimated. CO adsorption over
the pure supports was negligible.
2.3.5. TEM-EDX
Transmission electron microscopy (TEM) studies were carried out using a JEM-2200FS
(JEOL, Tokyo, Japan) and a Themis Z electron microscope (Thermo Fisher Scientific, Eind-
hoven, The Netherlands) operated at 200 kV. Images in Scanning-TEM (STEM) mode were
acquired using a high-angle annular dark field (HAADF) detector. The local elemental
composition of the samples was studied using a Thermo Fisher Scientific Super-X EDX
spectrometer. The samples were ground, suspended in ethanol, and placed on a copper
grid coated with a holey carbon film.
2.3.6. SEM-EDX
Scanning electron microscopy–energy-dispersive X-ray analysis (SEM-EDX) studies
were carried out using a dual-beam scanning electron microscope, Tescan Solaris FE/SEM
(Tescan, Brno, Czech Republic). The experiments were performed in secondary electron
mode at an accelerating voltage of 20 kV. The microscope is equipped with an AztecLive
EDX spectrometer (Oxford Instruments, High Wycombe, UK) with a Silicon Drift Detector
and energy resolution of 128 eV. The cross-sections of catalyst granules in epoxy resin were
prepared for the examination. The cross-sections with a given flatness of 0.25
µ
m were
covered with a conductive carbon layer of 10–20 nm thickness.
2.3.7. H2-TPR
The temperature-programmed reduction (TPR) via hydrogen was performed with
40–60 mg of sample in a quartz reactor using a flow setup with a thermal conductivity
detector. The gas mixture containing 10 vol.% of H
2
in Ar was fed at 40 mL/min. The
rate of heating from 25 to 800
◦
C was 10
◦
C/min. The TPR curves were normalized per
sample mass.
2.4. Tests of CO2Methanation Activity
The catalytic tests were performed in a U-shaped tubular continuous-flow reactor
(i.d. = 3 mm) at ambient pressure and the temperature range from 200 to 400
◦
C. The
temperature was controlled using a K-type thermocouple placed in the middle of the
catalyst bed. The feed gas contained 4 vol.% CO
2
, 16 vol.% H
2
, and Ar as balance. The
catalyst load was 150 mg, 0.2–0.5 mm fraction, and the flow rate—75 mL/min. Prior to
the experiment, each catalyst was reduced in 10 vol.% H
2
in Ar flow (50 mL/min) at
400
◦
C for 1 h. The compositions of the inlet and outlet gas mixtures were determined
using a gas chromatograph, KHROMOS-1000 (Khromos, Dzerzhinsk, Russia), equipped
with a thermal conductivity detector (CaA molecular sieves column) and flame ionization
detector (Porapak Q column) with a methanator characterized by sensitivity to CO, CH
4
,
and CO
2
of ~1 ppm. The separation of CO, CH
4
, and CO
2
on the column followed by the
methanation of carbon oxides allowed the flame ionization detector to be used to analyze
their concentration. The equilibrium compositions were calculated using equilibrium
software HSC 7.0. It was assumed that equilibrium mixtures only contained gaseous
substances (CH
4
, CO, CO
2
, H
2
, and H
2
O), i.e., no carbon deposition processes were taken
into account. The catalytic properties of Ni/Ce
1−x
Zr
x
O
2
catalysts were also compared with
those of industrial catalyst NIAP-07-05 for the methanation of carbon oxides, which contains
38 wt.% NiO, 12 wt.% Cr
2
O
3
, and 50 wt.% Al
2
O
3
(further denoted as 38Ni
−
Cr
−
Al). The
catalyst has a Ni0-specific surface area of 3.3 m2/g in a reduced state [26].
Nanomaterials 2022,12, 3207 5 of 16
3. Results and Discussion
3.1. Calculation Results
According to DFT+U calculations, the parameter of the Ce
1−x
Zr
x
O
2
lattice decreases
with an increase in Zr content (Table 1) due to a smaller radius of the Zr
4+
cation (0.84 Å)
compared to the Ce
4+
cation (0.97 Å). Doping CeO
2
with Zr leads to a slight decrease in
Ce–O distances. The calculated Ce–O and Zr–O distances are in the range of 2.314–2.375 Å
and 2.205–2.258 Å, respectively.
Table 1.
Calculated geometric characteristics, energies of oxygen vacancy formation (E
f
), and surface
energies (Esurf) for Ce1−xZrxO2.
Ce1−xZrxO2
Composition Lattice Parameter (Å) rCe-O (Å) rZr-O (Å) Esurf (J/m2)Ef(eV)
(100) (111) (100) (111)
CeO25.46 2.375 - 1.76 0.72 2.03 2.71
Ce0.75Zr0.25 O25.38 2.347 2.258 1.79 0.74 1.45 2.23
Ce0.5Zr0.5 O25.26 2.326 2.242 1.81 0.75 1.22 1.95
Ce0.25Zr0.75 O25.17 2.314 2.220 1.85 0.79 1.97 2.54
ZrO25.08 - 2.205 1.94 0.89 4.26 5.08
The calculated values of the surface energy are presented in Table 1. The Ce
1−x
Zr
x
O
2
(111) surface is more stable than the (100) surface. The CeO
2
(111) surface is known to
be the most stable surface among the low-index surfaces (100), (110), and (111) [
44
,
45
].
Zirconium incorporation slightly increases the surface energy at 0 < x < 0.5, while a sharp
increase in the energy is observed at x > 0.5.
The calculated energies of oxygen vacancy formation in Ce
1−x
Zr
x
O
2
are also listed
in Table 1. The E
f
values decrease considerably in the range of compositions 0 < x < 0.5,
while at x > 0.5, the E
f
values increase. The Ce
0.5
Zr
0.5
O
2
surfaces are characterized by the
lowest energies of oxygen formation. It was found that Zr incorporation facilitates the
generation of oxygen vacancy. The obtained result agrees with data of interatomic potential
simulations reported by G. Balducci et al. [
46
,
47
], which showed that Ce
4+
/Ce
3+
reduction
energy is reduced even by small amounts of zirconium incorporated into ceria. Based
on DFT+U calculations, Yang et al. [
48
,
49
] also found that zirconium addition leads to
lowering energy of the oxygen vacancy formation. The structure distortion induced by Zr
cations can be responsible for the decrease in the reduction energy. It was assumed that the
smaller Zr
4+
cations counterbalance steric strains arising at formation Ce
3+
cations, which
are larger than Ce4+ cations [46,49].
3.2. Experimental Results
3.2.1. Ce1−xZrxO2Support Materials
The powder XRD patterns of the Ce
1−x
Zr
x
O
2
samples are shown in Figure 1. Only
Bragg peaks related to oxide with a fluorite-type cubic crystal structure (S.G. Fm
3
m)
are observed.
Table 2lists data on the structural parameters obtained by Rietveld refinement. The lattice
parameters of all the oxides are lower than the value characteristic of CeO
2
(a = 5.411 Å, PDF#
00-028-0753). The lattice shrinkage indicates the formation of substitutional solid solution
Ce
1−x
Zr
x
O
2
. The gradual decrease in the lattice parameter with the increase in Zr content
is observed. The composition of the Ce
1−x
Zr
x
O
2
solid solutions was evaluated with use of
Vegard’s rule. The linear dependence of the lattice parameter on the Zr content is provided
elsewhere [
26
]. Estimated values of zirconium content coincide with those set at the synthesis
(Table 2). This implies that obtained materials are single-phase Ce
1−x
Zr
x
O
2
solid solutions. As
can be seen from Table 2, the isotropic temperature factors of atoms increase with Zr content.
This suggests the increase in crystal lattice distortion resulted from Zr incorporation.
Nanomaterials 2022,12, 3207 6 of 16
Figure 1.
XRD patterns fitted using the Rietveld refinement method for Ce
0.9
Zr
0.1
O
2
(
a
), Ce
0.75
Zr
0.25
O
2
(b), and Ce0.5Zr0.5O2(c) mixed oxides.
Table 2. Structural parameters of Ce1−xZrxO2oxides determined from XRD data.
Sample
Ce1−xZrxO2Lattice Parameter (Å) Estimated Zirconium
Content xBMe (Å−1) * BO(Å−1) * Rwp ** χ2**
Ce0.9Zr0.1 O25.388 (1) 0.10 0.06 0.17 3.62 0.97
Ce0.75Zr0.25 O25.353 (1) 0.25 0.13 0.29 4.07 1.22
Ce0.5Zr0.5 O25.290 (1) 0.51 0.45 1.20 3.92 1.58
* Isotropic temperature factors were refined on an assumption of Ce
1−x
Zr
x
O
2
composition set at the synthesis.
The factors of Ce and Zr atoms were constrained to be equal (BMe). ** Rietveld analysis agreement indices.
Nanomaterials 2022,12, 3207 7 of 16
Microstrain analysis also indicates the intensification of structure deformation with
an increasing Zr concentration in Ce
1−x
Zr
x
O
2
oxides (Table 3). The microstrain value
for the Ce
0.5
Zr
0.5
O
2
sample is roughly two times higher than that for the Ce
0.9
Zr
0.1
O
2
sample. A difference in the radii of Ce
4+
and Zr
4+
cations is the main reason for the crystal
lattice distortion. As mentioned above, the structure strains induced by Zr doping can be
responsible for the improved reducibility of the Ce1−xZrxO2mixed oxides.
Table 3.
Average crystallite sizes and microstrain values according to XRD data, specific surface
areas calculated from XRD-derived crystallite sizes and determined by BET method, agglomeration
coefficients, and average crystallite sizes according to HRTEM data.
Sample ∆d/d DXRD (nm) SXRD (m2/g) SBET (m2/g) Agglomeration
Coefficient ξdHRTEM (nm)
Ce0.9Zr0.1 O2
(2.6
±
0.2)
×
10
−37.0 120 71 0.41 6.4
Ce0.75Zr0.25 O2
(4.5
±
0.2)
×
10
−35.5 157 83 0.47 5.5
Ce0.5Zr0.5 O2
(6.9
±
0.2)
×
10
−35.0 180 53 0.70 4.1
All the oxides are highly dispersed; the D
XRD
are in the range of 5–7 nm. However,
quite low S
BET
values were obtained. Calculated S
XRD
values significantly exceed S
BET
ones (Table 3). Such a difference between S
XRD
and S
BET
values can be explained by the
agglomeration of primary crystallites. The analysis of estimated agglomeration coefficients
(Table 3) showed that an increase in Zr content in the Ce
1−x
Zr
x
O
2
oxides is accompanied
with an increase in the agglomeration of crystallites. TEM data confirmed these results. The
TEM images shown in Figure 2a–c demonstrate the loose agglomeration of crystallites in the
low-doped Ce
0.9
Zr
0.1
O
2
sample and the significantly denser agglomeration of crystallites
in the high-doped Ce
0.5
Zr
0.5
O
2
sample. In the HRTEM images (Figure 2d–f), the individual
Ce
1−x
Zr
x
O
2
crystallites are distinguishable. The mean size of Ce
1−x
Zr
x
O
2
crystallites
(Table 3, d
HRTEM
) was determined from particle size distribution histograms shown in
insets in Figure 2d–f. There is a clear tendency for the Ce
1−x
Zr
x
O
2
crystallite size to
decrease as the Zr content increases in accordance with the XRD results.
Figure 2.
TEM data for Ce
0.9
Zr
0.1
O
2
(
a
,
d
), Ce
0.75
Zr
0.25
O
2
(
b
,
e
), and Ce
0.5
Zr
0.5
O
2
(
c
,
f
) catalysts:
(
a
–
c
) TEM images and SAED patterns in insets with indication of fluorite phase rings; (
d
–
f
) HRTEM
images and particle size distribution histograms.
Nanomaterials 2022,12, 3207 8 of 16
The reduction behavior of the Ce
1−x
Zr
x
O
2
oxides was studied. The H
2
-TPR profiles
are reported in Figure 3. The H
2
-TPR profile of the low-doped Ce
0.9
Zr
0.1
O
2
oxide exhibits
two broad peaks at 500
◦
C and 820
◦
C. Such a two-peak pattern is characteristic of CeO
2
oxide and reflects the stepwise reduction in the surface and bulk [
50
,
51
]. The TPR profiles
of the Ce
0.75
Zr
0.25
O
2
and Ce
0.5
Zr
0.5
O
2
oxides exhibit asymmetric peaks with significantly
increased intensities. The second high-temperature reduction peak is not observed. These data
indicate that surface and bulk reduction processes occur almost simultaneously. The observed
intensive asymmetric peak reflects the co-reduction in surface and bulk. This suggests the
improvement of the reducibility of the mixed Ce
1−x
Zr
x
O
2
oxides with an increase in Zr
content in full agreement with previous TPR studies of Ce1−xZrxO2oxides [28–30].
Figure 3. The H2-TPR profiles of the Ce1−xZrxO2samples.
3.2.2. Ni/Ce1−xZrxO2Catalysts
Figure 4shows XRD patterns of the Ni/Ce
1−x
Zr
x
O
2
catalysts. The broad peaks
from the oxide NiO phase (PDF#00-047-1049) are detected in the XRD patterns of the
as-prepared catalysts, while the narrow peaks from the metallic Ni
0
phase (PDF #00-004-
0085) are observed in the XRD patterns of the catalysts aged under reductive conditions of
CO2methanation.
Table 4summarizes the average size characteristics of nickel species in the as-prepared
and used Ni/Ce
1−x
Zr
x
O
2
catalysts according to the XRD and CO chemisorption data. The
Ni/Ce
0.9
Zr
0.1
O
2
catalyst is characterized by a higher dispersion of initial NiO crystallites as
well as Ni0crystallites formed upon reduction. Reaction conditions provoke the sintering
of nickel species. The average size of Ni
0
crystallites in the aged catalysts is larger than
the size of NiO crystallites in the as-prepared catalysts. The Ni/Ce
0.9
Zr
0.1
O
2
catalyst is
characterized by a higher resistance of Ni
0
crystallites to sintering. The average size of
Ni
0
crystallites in the Ni/Ce
0.9
Zr
0.1
O
2
catalyst is about two times smaller than in other
catalysts. CO chemisorption results (Table 4) confirmed that the Ni/Ce
0.9
Zr
0.1
O
2
catalyst
contains Ni
0
particles of the highest dispersion. The determined sizes of Ni
0
nanoparticles
in the aged catalysts are in the increasing order of Ni/Ce
0.9
Zr
0.1
O
2
< Ni/Ce
0.75
Zr
0.25
O
2
<
Ni/Ce
0.5
Zr
0.5
O
2
. The chemisorption method is considerably more sensitive to ultrafine
particles compared to XRD analysis. The observed differences in sizes measured via the
chemisorption and XRD techniques suggest that highly dispersed particles of metallic Ni
0
,
undetectable via XRD, are present in the catalysts.
The comparison of values of Ni
0
content determined from the Rietveld refinement of
XRD data and XRF analysis (Supplementary material, Table S1) confirmed that a part of
the loaded nickel in the catalysts is not detected via XRD analysis. The fraction of XRD-
undetectable nickel species in the catalysts decreases in the sequence: Ni/Ce
0.9
Zr
0.1
O
2
>
Ni/Ce0.75Zr0.25O2> Ni/Ce0.5Zr0.5O2.
Nanomaterials 2022,12, 3207 9 of 16
Figure 4.
Fragments of XRD patterns fitted using the Rietveld refinement method for Ni/Ce
0.9
Zr
0.1
O
2
(
a
,
d
), Ni/Ce
0.75
Zr
0.25
O
2
(
b
,
e
), and Ni/Ce
0.5
Zr
0.5
O
2
(
c
,
f
) catalysts before (
a
–
c
) and after (
d
–
f
) the
catalytic reaction.
Table 4.
Average size characteristics of nickel species in the as-prepared and used Ni/Ce
1−x
Zr
x
O
2
catalysts according to the XRD and CO chemisorption data.
Sample As-Prepared Catalysts Aged Catalysts
dNiOXRD (nm) dNi XRD (nm) dNichem (nm) SNichem (m2/gcat )
Ni/Ce0.9Zr0.1 O28.5(5) 20.0(5) 11.2 6
Ni/Ce0.75Zr0.25 O212.0(5) 54.0(5) 23.2 2.9
Ni/Ce0.5Zr0.5 O211.0(5) 53.0(5) 33.7 2
The HAADF STEM images and EDX mapping patterns are presented in Figure 5. The
analysis of EDX data revealed that the spatial distribution of Ce and Zr in the catalysts is
homogeneous. No Ce-rich or Zr-rich areas are observed. This confirms that Ce
1−x
Zr
x
O
2
oxides used as supports are single-phase substitutional solid solutions. The analysis of
Ni distribution via EDX allows us to see Ni-rich nanoparticles of 5–10 nm in size in the
catalysts. These particles in all the studied samples were shown to be NiO particles
(Supplementary material Figure S2). Individual NiO nanoparticles in contact with support
particles and ones assembled into large aggregates are observed. The Ni/Ce
1−x
Zr
x
O
2
catalysts were found to differ in the amount of non-agglomerated NiO nanoparticles
and the possibility of their fixation as single particles. Thus, a large number of single
NiO particles being in contact with support particles are observed in the images of the
Ni/Ce
0.9
Zr
0.1
O
2
catalyst (Figure 5b). More developed contacts between NiO particles and
Ce
0.9
Zr
0.1
O
2
support are likely responsible for the higher resistance of nickel particles
to sintering under conditions of CO
2
methanation. In the case of the Ni/Ce
0.5
Zr
0.5
O
2
Nanomaterials 2022,12, 3207 10 of 16
catalyst, agglomerated NiO nanoparticles with slight contact with the support are observed
(Figure 5h). The Ni particles undergo reduction and sintering during the reaction, as
revealed by XRD analysis. However, the highly dispersed particles fixed on the support
are retained (Supplementary material Figure S3).
Figure 5.
TEM data for Ni/Ce
0.9
Zr
0.1
O
2
(
a
,
b
), Ni/Ce
0.75
Zr
0.25
O
2
(
c
–
e
), and Ni/Ce
0.5
Zr
0.5
O
2
(
f
–
h
)
catalysts: TEM (
c
) and HAADF STEM (
a
,
d
,
f
,
g
) images and corresponding EDX mapping patterns
(
b
,
e
,
h
) showing distribution of Ni (red), Zr (blue), and Ce (green). The maps are presented in
background-corrected intensities.
The observed differences in the dispersion of nickel species in the catalysts seem to
be related to effect of the microstructure of the Ce
1−x
Zr
x
O
2
oxides on the formation of
supported nanoparticles. As noted above, the Ce
1−x
Zr
x
O
2
support materials differed in
particle organization and the agglomeration of crystallites. The microstructure features
of the catalysts were additionally studied via SEM coupled with EDX analysis. SEM anal-
ysis showed quite different internal organization and porous structure in grains of the
Ni/Ce
0.9
Zr
0.1
O
2
and Ni/Ce
0.5
Zr
0.5
O
2
catalysts. In the Ni/Ce
0.5
Zr
0.5
O
2
catalyst, coarse elon-
gated support particles are aggregated with the formation of large, unevenly distributed
voids (Figure 6a,b). Large NiO particles are visualized in the big voids, while the main
part of the support material is weakly covered with nickel compounds according to EDX
mapping (Figure 6c,f).
Nanomaterials 2022,12, 3207 11 of 16
Figure 6.
SEM images (
a
,
b
) and EDX mapping patterns (
c
–
f
) for nickel (red), zirconium (orange), and
cerium (blue) of cross-section area of the Ni/Ce0.5Zr0.5O2catalyst.
In the Ni/Ce
0.9
Zr
0.1
O
2
catalyst, aggregates of smaller and thinner support particles
have a more uniform distribution of narrow interparticle voids (Figure 7a,b). EDX map-
ping showed the more homogeneous distribution of nickel over the Ce
0.9
Zr
0.1
O
2
support
(Figure 7c,f) than over the Ce
0.5
Zr
0.5
O
2
support (Figure 6c,f). It appears that a network of
narrow channels provided a more uniform distribution of the nickel compounds over the
support surface in the impregnation step. As a result, the support is effectively covered
with nickel compounds and there is no pronounced gradient in the size of formed NiO
particles. Thus, the SEM-EDX results allowed one to explain the observed differences in
Ni/Ce
1−x
Zr
x
O
2
catalysts in the dispersion of nickel species. The microstructure features of
the Ce
1−x
Zr
x
O
2
oxides affected the dispersion of nickel compounds as well as their spatial
distribution over the supports. The microstructure of the Ce
0.9
Zr
0.1
O
2
support is more
beneficial for the uniform distribution of the nickel compounds without their segregation.
The formation of larger particles in the Ce
0.5
Zr
0.5
O
2
oxide seems to be caused by the
considerable aggregation of the constituent crystallites (Table 3, Figure 2). On the one
hand, greater aggregation is likely related to the higher dispersion of crystallites. Ultrafine
crystallites assemble to lower the surface energy [
52
,
53
]. On the other hand, differences in
the microstructure can be related with specifics of the formation of Ce
1−x
Zr
x
O
2
oxides at
their preparation via the Pechini route. Thus, the intensity of combustion processes when
burning the organic polymer matrix depends on the relative contents of Ce and Zr atoms.
During the preparation of Ce
0.9
Zr
0.1
O
2
oxide, a higher content of easily oxidized Ce cations
intensifies combustion processes. The explosive formation of gaseous products favors the
formation of loose particles with the low agglomeration of crystallites.
Nanomaterials 2022,12, 3207 12 of 16
Figure 7.
SEM images (
a
,
b
) and EDX mapping patterns (
c
–
f
) for nickel (red), zirconium (green), and
cerium (blue) of cross-section area of the Ni/Ce0.9Zr0.1O2catalyst.
The catalytic performance of the Ni/Ce
1−x
Zr
x
O
2
catalysts in CO
2
methanation was
studied. Figure 8shows light-off curves for the CO
2
methanation. For all the catalysts, the
CH
4
concentration increases with temperature, reaches a maximum, and then decreases
coinciding with the equilibrium values. All the Ni/Ce
1−x
Zr
x
O
2
catalysts had comparable
or higher activity in comparison with the industrial 38Ni-Cr-Al catalyst containing a
significantly higher amount of nickel (38 wt.% vs. 10 wt.%). In a previous work, we
reported that the synergism of redox properties of the Ni/Ce
1−x
Zr
x
O
2
system enhances
the catalytic performance in the methanation of carbon oxides [26].
Figure 8.
Temperature dependences of the outlet concentrations of CH
4
(
a
) and CO (
b
) during CO
2
methanation over the Ni/Ce
1−x
Zr
x
O
2
catalysts and 38Ni-Cr-Al catalyst. The dashed lines correspond
to the calculated equilibrium concentrations.
Nanomaterials 2022,12, 3207 13 of 16
The Ni/Ce
0.9
Zr
0.1
O
2
catalyst exhibited the highest catalytic activity. The CO
2
half-
conversion temperature (T
50
) was 244, 266, and 280
◦
C for Ni/Ce
0.9
Zr
0.1
O
2
, Ni/Ce
0.75
Zr
0.25
O
2
,
and Ni/Ce
0.5
Zr
0.5
O
2
, respectively. The observed decrease in activity in the sequence
Ni/Ce
0.9
Zr
0.1
O
2
>> Ni/Ce
0.75
Zr
0.25
O
2
> Ni/Ce
0.5
Zr
0.5
O
2
correlates with a decrease in Ni
0
-
specific surface area: 6 >> 2.9 > 2 m
Ni2
/g
cat
. The revealed higher dispersion of Ni
0
particles
and more developed contacts between them and support surface can explain the higher
catalytic activity of the Ni/Ce
0.9
Zr
0.1
O
2
catalyst. As was shown above, the Ni/Ce
0.9
Zr
0.1
O
2
catalyst contained Ni
0
nanoparticles with d
Nichem
sizes of 11.2 nm, and the least active
Ni/Ce
0.5
Zr
0.5
O
2
catalyst contained Ni
0
nanoparticles with d
Nichem
sizes of 33.7 nm (Table 4).
It was also revealed that the most active Ni/Ce
0.9
Zr
0.1
O
2
catalyst is characterized by more
developed contacts between nickel particles and the support (Figure 5b). It is well accepted
that high Ni
0
dispersion and a developed metal–support interface area are highly important
for the catalytic activity for CO2methanation [6,8,23,54,55].
It was recently suggested that zirconium addition impacts the dispersion of nickel
nanoparticles in Ni/Ce
1−x
Zr
x
O
2
catalysts as well as the degree of metal–support interac-
tion [
23
,
31
]. The results of this study imply that the cation composition of the Ce
1−x
Zr
x
O
2
oxides can affect their microstructure and texture features and, consequently, the dispersion
of supported nickel nanoparticles. This influence is likely related to the synthesis technique
used. The Ce
1−x
Zr
x
O
2
oxides under study were prepared via the Pechini method. A similar
effect was observed by Iglesias et al. [
24
] in the study of Ni/Ce
1−x
Zr
x
O
2
catalysts with
supports prepared using the coprecipitation method. It was shown that an increase in Zr
content diminishes the specific surface area of the Ce
1−x
Zr
x
O
2
supports with a decrease in
the dispersion of supported Ni0nanoparticles. It was also reported that the increase in Zr
content reduces the specific surface area of the Ce
1−x
Zr
x
O
2
oxides prepared via the sol–gel
technique [56] and the citrate complexation route [57].
4. Conclusions
In this study, Ce
1−x
Zr
x
O
2
mixed oxides with different compositions were prepared
using the Pechini method and used as the supports for Ni/Ce
1−x
Zr
x
O
2
catalysts. It was
demonstrated that an increase in Zr content enhances the distortion of the crystal structure
of Ce
1−x
Zr
x
O
2
oxides and leads to improvements in their redox properties. It was also
found that microstructure features of Ce
1−x
Zr
x
O
2
oxides change with cation composition.
The effect of the Ce
1−x
Zr
x
O
2
composition on the structure and catalytic activity of
the Ni/Ce
1−x
Zr
x
O
2
catalysts in CO
2
methanation was investigated. The activity of the
Ni/Ce
1−x
Zr
x
O
2
catalysts decreased in the order: Ni/Ce
0.9
Zr
0.1
O
2
>> Ni/Ce
0.75
Zr
0.25
O
2
>
Ni/Ce
0.5
Zr
0.5
O
2
. The drop in the activity correlated with the decrease in the dispersion
of metallic Ni
0
nanoparticles. It was revealed that differences in the microstructural char-
acteristics of the Ce
1−x
Zr
x
O
2
supports are responsible for differences in the dispersion of
supported Ni0nanoparticles and the length of the metal–support interface.
It was shown that improving the redox properties of Ce
1−x
Zr
x
O
2
oxides, which are
important for catalysis, through cation modification can be counterbalanced by wors-
ening their microstructure characteristics, which determine the dispersion of supported
Ni0nanoparticles.
Supplementary Materials:
The following supporting information can be downloaded at: https://
www.mdpi.com/article/10.3390/nano12183207/s1, Figure S1: The models of fluorite Ce
1−x
Zr
x
O
2
unit
cells: (a) x = 0, (b) x = 0.25, (c) x = 0.5, (d) x = 0.75, (e) x = 1; Figure S2: TEM image of as-prepared
Ni/Ce
0.9
Zr
0.1
O
2
catalyst (a), electron diffraction pattern (b); Figure S3: HAADF-STEM images and
corresponding EDX-mapping patterns for aged Ni/Ce
0.9
Zr
0.1
O
2
(a,b) Ni/Ce
0.5
Zr
0.5
O
2
(c,d) catalysts.
Table S1: Quantities of nickel compounds in the used Ni/Ce
1−x
Zr
x
O
2
catalysts according to XRD phase
analysis and XRF analysis.
Author Contributions:
V.P.P.: investigation, writing—original draft, project administration, writing—
review and editing, funding acquisition. D.I.P.: investigation, visualization, writing—review and
editing. V.N.R.: methodology, investigation. O.A.S.: investigation, visualization. A.M.G.: investiga-
Nanomaterials 2022,12, 3207 14 of 16
tion. N.A.N.: methodology, investigation. E.A.S.: investigation. A.S.B.: methodology, investigation.
V.A.R.: investigation. P.V.S.: investigation, writing—review and editing. All authors have read and
agreed to the published version of the manuscript.
Funding: This research was funded by the Russian Science Foundation (project 21-73-20075).
Data Availability Statement: Not applicable.
Acknowledgments:
The XRF, HRTEM, and SEM studies were carried out using facilities of the shared
research center “National center of investigation of catalysts” at Boreskov Institute of Catalysis. The
authors also acknowledge resource center “VTAN” (Novosibirsk State University) for the access to
TEM equipment. The quantum–chemical calculations were carried out using computing resources of
the federal collective usage center Complex for Simulation and Data Processing for Mega-science
Facilities at NRC “Kurchatov Institute”.
Conflicts of Interest:
The authors declare that they have no known competing financial interests or
personal relationships that could have appeared to influence the work reported in this paper.
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