Effect of Preparation Methods on the Physicochemical and Functional Properties of Ni/CeO2 Catalysts

Article abstract of DOI:10.1134/S002315841902006X

Abstract: The effect of preparation procedures (a polymer ester precursor method and incipient wetness impregnation) on the physicochemical and functional properties of Ni/CeO₂ catalysts with different nickel contents (0–15 wt %) was studied in

Full text of DOI:10.1134/S002315841902006X

ISSN 0023-1584, Kinetics and Catalysis, 2019, Vol. 60, No. 2, pp. 221–230. © Pleiades Publishing, Ltd., 2019.
Russian Text © E.V. Matus, A.S. Shlyakhtina, O.B. Sukhova, I.Z. Ismagilov, V.A. Ushakov, S.A. Yashnik, A.P. Nikitin, P. Bharali, M.A. Kerzhentsev, Z.R. Ismagilov, 2019,
published in Kinetika i Kataliz, 2019, Vol. 60, No. 2, pp. 245–255.
Effect of Preparation Methods on the Physicochemical
and Functional Properties of Ni/CeO2 Catalysts
E. V. Matusa, b, *, A. S. Shlyakhtinaa, b, O. B. Sukhovaa, I. Z. Ismagilova, V. A. Ushakova, S. A. Yashnika,
A. P. Nikitinc, P. Bharalid, M. A. Kerzhentseva, and Z. R. Ismagilova, c
aBoreskov Institute of Catalysis, Siberian Branch, Russian Academy of Sciences, Novosibirsk, 630090 Russia
bNovosibirsk State Technical University, Novosibirsk, 630073 Russia
cInstitute of Coal Chemistry and Material Science FRS CCC, Siberian Branch,
Russian Academy of Sciences, Kemerovo, 650000 Russia
dTezpur University, Napaam, Tezpur, 784 028 Assam, India
*e-mail: matus@catalysis.ru
Received November 29, 2018; revised November 29, 2018; accepted November 29, 2018
Abstract—The effect of preparation procedures (a polymer ester precursor method and incipient wetness
impregnation) on the physicochemical and functional properties of Ni/CeO2 catalysts with different nickel
contents (0–15 wt %) was studied in order to develop highly active and carbonization-resistant catalysts for
hydrocarbon reforming. Based on the results of studying the samples by low-temperature nitrogen adsorp-
tion, X-ray phase analysis, Raman spectroscopy, transmission electron microscopy and temperature-pro-
grammed reduction with hydrogen, it was found that the textural, structural, and redox properties of the
materials depend on the method of their synthesis. As compared with the samples prepared by impregnation,
the Ni/CeO2 catalysts obtained by the polymer ester precursor method were characterized by different active
component stabilization forms (a Ce1 – xNixOy solid solution phase and NiO particles <5 nm in size vs. a NiO
phase with a particle size of 5–50 nm), a smaller average size of CeO2 crystallites (5.5 vs. 11 nm), a high spe-
cific surface area (105 vs. 75 m2/g), a defect structure, and a decreased reducibility. It was found that the sam-
ples of both series provided comparable yields of hydrogen (to 50% at 600°C) in an autothermal ethanol
reforming reaction, but the Ni/CeO2 catalysts synthesized by the polymer ester precursor method were more
resistant to the formation of carbonaceous deposits.
Keywords: Ni catalysts, cerium dioxide, preparation method, ethanol, reforming
DOI: 10.1134/S002315841902006X
INTRODUCTION
It was established [12–16] that the efficiency of
supported metal nanoparticles in catalytic reactions
depends on the morphology of CeO2 support particles.
In particular [12], the activity of RhPd/CeO2 in the
ethanol steam reforming (ESR) reaction increased in
the following order: polycrystalline-structure CeO2 <
rod-shaped CeO2 crystallites (CeO2 rods) < cubic
CeO2 crystallites (CeO2 cubes). According to Soykal
et al. [15], the catalytic properties of Co/CeO2 cube
samples in ESR can be improved, as compared to
those of Co/CeO2 rod samples, due to both an
increase in the dispersion and reducibility of the active
component and a higher oxygen mobility in the CeO2
crystal lattice. A similar dependence was observed for
Ni catalysts in a decomposition reaction of N2O,
which was explained by a higher concentration of the
Сe–O–Ni centers active in the reaction [16]. Sohn
et al. [17] found that a difference in the reaction mech-
anisms of ESR, namely, the formation of formate or
acetate intermediates in the presence of Co catalysts
Cerium dioxide is widely used in catalysis as an
active component, support, or modifying additive [1–
5]. The stability of the fluorite-type cubic structure of
nonstoichiometric cerium dioxide and the presence of
the Ce4+/Ce3+ redox pair are responsible for the
advantages of using systems based on it. The properties
of this material are unique due to the high oxygen
capacity and the mobility of crystal lattice oxygen, the
ability to form Ce1 – xMxOy solid solutions of various
compositions (M = Zr, La, Pr, Gd, Y, Mg, etc.), and
the possibility of strong interactions with supported
transition metals [6–11]. With the use of CeO2 as a
carrier of oxide and metal active component nanopar-
ticles, the key factors determining the functional prop-
erties of the catalyst are the morphology and textural
and structural properties of cerium dioxide, the cata-
lyst composition, and the method and conditions of
catalyst synthesis [11–15].
221

222
MATUS et al.
based on micro- and nanoparticles of cerium dioxide, This work continues a cycle of studies directed to the
correlated with different concentrations of oxygen development of approaches to the control of the func-
vacancies in the test samples.
tional properties of catalysts through the regulation of
their physicochemical characteristics [10, 20, 32–35].
It was found that an increase in the degree of
metal–support interaction has a positive effect on the
catalytic properties of materials based on CeO2 [3, 9,
18–21]. For example, the activity of X–CeO2 catalysts
(X = Ru, Pd, and Pt) in the oxidation reactions of CO
and СH4 increased in the metal order Ru < Pd < Pt
due to an increase in the metal–cerium dioxide inter-
EXPERIMENTAL
Catalyst Synthesis Procedures
The Ni/CeO2 catalysts were prepared by the poly-
mer ester precursor method (P series) and incipient
action observed in this order. It was noted [18, 19] wetness impregnation (I series). For the synthesis of
samples by the polymer ester precursor method, citric
acid was dissolved in ethylene glycol at a temperature
that, in the case of a strong interaction between sup-
ported metal nanoparticles (clusters) and cerium
dioxide, the transport of reactive oxygen species of 70°C. An aqueous solution with a given concentra-
tion of cerium nitrate and nickel nitrate (Сe + Ni) and
ethylenediamine were sequentially added to the solu-
improved the resistance of the catalysts to coking. The
doping of cerium dioxide [10, 20] and the varying of
active component introduction methods [22–24] or tion prepared. The molar ratio (Сe + Ni) : citric acid :
ethylene glycol : ethylenediamine was 1 : 4 : 12 : 4. The
resulting gel was stirred and then dried under an IR
lamp for 72 h to remove excess solvent. The resulting
resinous substance was calcined in a muffle furnace at
a temperature of 500°C for 4 h.
To obtain catalyst samples by incipient wetness
impregnation, the CeO2 support, which was synthe-
sized according to a published procedure [36], was
impregnated with an aqueous solution of nickel nitrate
with a given concentration; the samples were dried
under an IR lamp and calcined in a muffle furnace at
500°C for 4 h.
In the designation of the samples, the numbers
before Ni show its mass concentration in the catalyst
(2, 5, 10, or 15 wt %). Before the reaction, the catalysts
were activated in a flow of 30 vol % H2/He at 600°C
for one hour.
catalyst heat treatment conditions [8, 25–27] were
used in order to control the degree of metal–CeO2
interactions. It was shown that the modification of
cerium dioxide with lanthanum improved the disper-
sion of the Ni active component and the stability of
catalyst operation in ethanol autothermal reforming of
(ATR) [10]. In contrast to impregnation and solvo-
thermal decomposition, the use of an inverse emul-
sion method for the preparation of Co/CeO2 resulted
in higher ESR process characteristics due to an
increase in the dispersion of the active component and
the formation of a developed metal–support interface.
The ultimate variant of the strong interaction between
the supported active component and the carrier is the
formation of joint surface or bulk phases, for example,
solid solutions with fluorite or perovskite structures
and mixed oxides with a spinel structure. With the use
of these materials as catalyst precursors, in some cases,
their activation in a reducing or reaction atmosphere is
required, which leads to the destruction of the initial joint
phase and the formation of highly dispersed supported
active component particles on the surface of the oxide
carrier [28–31].
Physicochemical Methods for Catalyst Characterization
The metal content of the test catalysts was deter-
mined by X-ray fluorescence analysis on an ARL
ADVANT’X analyzer (ThermoTechno Scientific,
Switzerland) with an Rh anode of the X-ray tube.
The textural characteristics of the catalysts (spe-
cific surface area (SBET), pore volume (Vpore), and
average pore diameter (Dpore)) were determined on an
ASAP 2400 automated volumetric instrument
(Micromeritics, the United States) by measuring and
treating the isotherms of low-temperature nitrogen
adsorption at 77 K.
Thus, cerium dioxide–based catalysts are promis-
ing materials, which can be improved by the target-
oriented regulation of the degree of metal–support
interactions. In this work, in order to develop highly
active and carbonization-resistant catalysts for hydro-
carbon reforming, we synthesized and comprehen-
sively studied (by low-temperature nitrogen adsorp-
tion, X-ray phase analysis, Raman spectroscopy,
transmission electron microscopy, temperature-pro-
grammed reduction with hydrogen, and testing in
The X-ray diffraction (XRD) analysis of the sam-
autothermal ethanol reforming) Ni/CeO2 catalysts ples was performed on an HZG-4C diffractometer
(Freiberger Prazisionmechanik, Germany) in mono-
chromatic CoKα radiation (λ = 1.79021 Å). The phase
composition was determined from diffraction patterns
obtained by scanning the angle region 2θ = 10°–80°
with a step of 0.1 deg and an accumulation time of 6–
15 s. The dimensions of coherent scattering regions
(CSRs) of Ce1 – xNixOy (x = 0–0.4) were calculated
with different nickel contents (0–15 wt %). To opti-
mize the form of stabilization, dispersion, and redox
properties of the active component, the catalyst prepa-
ration methods (polymer ester precursor method and
incipient wetness impregnation) were varied to pro-
vide different degrees of interaction between the Ni-
containing active component and cerium dioxide.
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EFFECT OF PREPARATION METHODS
223
from the 1.1.1. diffraction peak broadening of detected has a significant effect on the textural characteristics
phases having a fluorite-type cubic structure; the of the catalysts (Table 1). The specific surface area of
CSRs of NiO and Ni were determined based on the samples from the P series was higher than that of sam-
2.0.0. peak of a NiO phase and the 2.0.0. peak of a Ni0
phase, respectively.
ples from the I series. This effect increased with the
fraction of nickel in the catalyst (Table 1). Thus, at a
2 wt % Ni content of Ni/CeO2 (P) and Ni/CeO2 (I)
The Raman spectra were measured in a spectral
frequency shift range of 120–900 cm–1 on a Renishaw
Invia Raman spectrometer (Renishaw, the United
Kingdom) using an argon laser with a wavelength of
514.5 nm as an excitation source, a 1800 l/mm diffrac-
tion grating, and an L50x lens. The laser radiation
power on the sample did not exceed 2 mW, and the
useful signal accumulation time was 30 s.
The high-resolution transmission electron micros-
copy (HR TEM) images were taken on a JEM–2010
electron microscope (JEOL, Japan) with a lattice res-
olution of 0.14 nm at an accelerating voltage of 200 kV.
The local elemental analysis of the samples was carried
out by energy-dispersive X-ray (EDX) spectrometry
on an EDAX spectrometer (EDAX, the United States)
equipped with a Si(Li) detector with an energy resolu-
tion of no less than 130 eV. The samples for HR TEM
were supported onto perforated carbon substrates
fixed on copper gauze.
samples, SBET = 105 and 85 m2/g, respectively, or
SBET = 115 and 65 m2/g, respectively, as the nickel
concentration was increased to 15 wt %. Note that the
SBET of P-series catalysts varied only slightly with the
nickel content and reached 110 5 m2/g, which is higher
than that of similar systems obtained by a hydrothermal
method [38]. On the contrary, for I-series catalysts, the
specific surface area decreased from 85 to 65 m2/g with
an increase in the fraction of Ni; this can be due to the
partial blocking of support pores with active compo-
nent crystallites [37]. Regardless of the method of syn-
thesis, the Ni/CeO2 catalysts were mesoporous mate-
rials, as indicated by their type IV adsorption iso-
therms with an H3 hysteresis loop.
An analysis of Ni/CeO2 diffraction patterns
showed that the phase composition of the samples
depended on their preparation methods (Fig. 1).
According to the XRD analysis data, the P-series sam-
ples were single-phase, and they consisted of a cerium
dioxide phase. The absence of peaks related to Ni-
Temperature-programmed reduction with hydro-
gen (TPR-H2) was performed in a flow reactor (inner
diameter, 5 mm) according to a procedure described containing phases can indicate the stabilization of
elsewhere [37].
nickel cations as a constituent of either solid solu-
tion based on cerium dioxide or finely dispersed
nickel oxide particles (CSR < 4 nm). The formation
of a Ce1 – xNixOy solid solution could be indicated by a
change in the CeO2 unit cell parameter. As a rule, the
The thermal analysis (differential thermal analysis
(DTA), thermogravimetric analysis (TGA), and dif-
ferential thermogravimetric analysis (DTG)) of the
samples was performed on a NETZSCH STA 449 C
thermal analyzer (NETZSCH-Geratebau GmbH,
Germany) in a temperature range of 25–900°C at a
heating rate of 10 K/min in an atmosphere of air.
replacement of cerium cations (
= 0.097 nm) by
rCe4+
cations with a smaller radius (in particular,
=
rNi2+
0.081 nm) is accompanied by a decrease in the cell
parameter and a shift of diffraction peaks to larger
angles [36, 38]. However, regardless of the catalyst syn-
thesis method and the nickel content of the catalyst, the
Methods for Studying the Activity of Catalysts
The activity of catalysts in the ATR reaction of eth-
anol was studied in a quartz flow reactor (inner diam-
eter, 11 mm) according to a procedure described pre-
viously [10] at atmospheric pressure, a temperature of
200–600°C, a gas flow rate of 320 mL/min (NTP),
and the reactant molar ratio C2H5OH : H2O : O2 :
He = 1 : 3 : 0.4 : 0.7.
cerium dioxide cell parameter (5.413
0.002 Å)
remained unchanged upon the introduction of an
active component into the carrier matrix. The I-series
samples were two-phase: a NiO active component
phase was present in addition to the support phase.
The estimation of the average size of crystalline
domains (the size of a coherent scattering region)
using the Scherrer equation showed that the CSR size
(CeO2) decreases from 11.0 to 5.5 nm as the nickel
content of the P-series samples was increased from 0
to 15 wt %, whereas it remained unchanged (11.0 nm)
RESULTS AND DISCUSSION
Physicochemical Properties
Table 1 shows the textural and structural properties in the case of the I series. The observed decrease in the
of the synthesized Ni/CeO2 catalysts and the results of CSR (CeO2) in Ni/CeO2 (P) samples, which were
their study using the TPR-H2 method. According to obtained by the polymer ester precursor method, was
similar to the effect observed upon the doping of
cerium dioxide with La, Gd, or Mg cations [36]. In
particular, Kerzhentsev et al. [40] found that an
increase in the mole fraction of a doping cation from
0.1 to 0.9 led to a decrease in the average size of CeO2
the X-ray fluorescence analysis data, the concentra-
tions of metals in all of the samples were consistent
with the calculated values. From the results of the
study of the samples by the low-temperature adsorp-
tion of nitrogen, it follows that the method of synthesis
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MATUS et al.
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EFFECT OF PREPARATION METHODS
225
crystallites from 12 to 3 nm. A decrease in the size of
CeO2
NiO
CSRs upon doping is a widely known phenomenon for
the CeO2 system caused by the inhibition of crystallite
growth in the presence of a doping cation [41]. Note
that the cell parameter increased from 5.41 to 5.54 Å
upon a decrease in the particle size of unmodified
CeO2 from 70 to 2.8 nm [42]. In this regard, the
absence of an effect of the introduced nickel cation on
the CeO2 cell parameter in the P-series samples can be
due to a decrease in the size of CeO2 crystallites.
Accordingly, it can be assumed that, during the syn-
thesis of Ni/CeO2 (P), a portion of nickel was included
into the solid solution based on cerium dioxide; this
led to an observed decrease in crystallite sizes and to
an increase in the specific surface area of samples, as
compared to those of unmodified cerium dioxide.
This indicates an increase in the sintering resistance of
the structure of P-series samples in comparison with
those of the structures of the initial CeO2 support and
the I-series samples.
9
8
7
6
5
4
3
2
1
20
30
40
2θ, deg
50
60
70
Fig. 1. Effect of the preparation method on the phase com-
position of the Ni/CeO catalysts: (1) CeO , (2)
2
2
2Ni/CeO (P), (3) 2Ni/CeO (I), (4) 5Ni/CeO (P), (5)
2
2
2
2
5Ni/CeO (I), (6) 10Ni/CeO (P), (7) 10Ni/CeO (I), (8)
2
2
It is well known that the nonisovalent replacement
of cerium cations with Ni2+ cations leads to the disrup-
tion of oxygen stoichiometry and, consequently, an
increase in the defect structure of the material [43–45].
Indeed, an analysis of the Raman spectroscopy data
(Fig. 2, Table 1) showed that the structure imperfec-
tion of materials related to the presence of oxygen
vacancies increased in the following order: CeO2 <
Ni/CeO2 (I) < Ni/CeO2 (P). This was confirmed by
(1) the presence of Raman scattering bands in a region
of 550–650 cm–1, which appeared upon doping
cerium dioxide with metal cations of a different radius
and valence; (2) the values of a I570/I466 band intensity
ratio, which is used to evaluate the defect structure of
CeO2; and (3) an increase in the Raman scattering band
15Ni/CeO (P), and (9) 15Ni/CeO (I).
2
2
466
452
3
2
5
4
1
230
570
intensity at 220–230 cm–1 due to transverse acoustic
vibrations [43–45]. In particular, it can be seen (Table 1)
that the parameter I570/I466 for the P-series samples (0.4–
1.4) was significantly higher than that for the I-series
samples (0.1–0.2). In addition, in contrast to
Ni/CeO2 (I), the value of I570/I466 for Ni/CeO2 (P)
increased from 0.4 to 1.4 as the Ni content of the cat-
alyst was increased from 2 to 15 wt %, and the maxi-
mum position of the F2g mode of CeO2 (466 cm–1)
100 200 300 400 500 600 700 800 900
Wavenumber, cm–1
Fig. 2. Raman spectra of (1) the support and the Ni/CeO
2
catalysts: (2) 2Ni/CeO (P), (3) 15Ni/CeO (P), (4)
2
2
2Ni/CeO (I), and (5) 15Ni/CeO (I).
2
2
shifted toward lower frequencies by 14 cm–1. The
observed effects can be due to the partial replacement
of cerium cations by nickel cations and the formation
of the Ce1 – xNixOy solid solution [43–45].
(I) or from 3 to 30 nm for 10Ni/CeO2 (I) (Fig. 3b). The
Ni/CeO2 (P) samples were formed by crystallites with
characteristic sizes to 8 nm; their structure was highly
defective, and nickel oxide particles were almost not
observed with the exception of single particles smaller
than 5 nm in the samples with a high nickel content
(10–15 wt %) (Fig. 3a).
The results of the TEM analysis of the samples
(Fig. 3) are in good agreement with the XRD analysis
and Raman spectroscopy data. It was established that
cerium dioxide particles were crystallites ~10 nm in
size, which formed polycrystalline agglomerates
mainly in the form of plates. In the I-series samples,
nickel oxide particles were observed on the support
surface; the range of sizes of these particles depended
Because catalysts were activated by reduction
before the ethanol ATR reaction for the formation of
the active component, namely, Ni0, the phase compo-
sition of the samples after activation at 600°C in 30%
on the nickel content: from 3 to 10 nm for 2Ni/CeO2 H2/He was studied and the characteristics of their
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MATUS et al.
tv49
NiO
CeO2
2
0.3100 nm
NiO
0.2071 nm
1
10 nm
10 nm
(а)
(b)
Fig. 3. HR TEM micrographs of (a) 10Ni/CeO (P) and (b) 10Ni/CeO (I) catalysts.
2
2
reduction under TPR-H2 conditions were determined tallites. Lamonier et al. [47] proposed the following
reduction scheme, in which nickel metal (initially formed
(Table 1). Table 1 indicates that, despite the high-tem-
perature treatment in a reducing atmosphere, the P- from Ni2+ cations) promotes the reduction of surface
Ce4+ cations to Ce3+ to become reoxidized to Ni2+:
series catalysts, unlike the I-series samples, retained
the high dispersion of both the support (14–19 nm
versus 25 nm) and the active component (6–8 nm
against 20–50 nm).
Ni2+ + O2– + H2 → Ni0 + H2O + ꢀ,
2Ce4+ + Ni0 → 2Ce3+ + Ni2+,
Figure 4 shows TPR-H2 curves for the Ni/CeO2
(P) and Ni/CeO2 (I) samples with different nickel
contents. In the TPR-H2 curves of unmodified cerium
dioxide, two hydrogen consumption regions can be
distinguished in temperature ranges of 100–600°C
(a peak maximum at T = 510°C) and 600–1000°C (a
peak maximum at T = 880°C) due to the reduction of
Ce4+ cations localized on the surface and in the bulk of
the particles, respectively [46]. The introduction of
nickel into the support matrix changed the profile of
TPR-H2 curves mainly in a low-temperature region
(T < 600°C). In the case of the P-series catalysts, in
addition to a peak with a maximum at 880°C, there
were two peaks with maximums at ~160 and 230 5°C
and a broad peak at 250–450°C without a pronounced
maximum (Fig. 4a). In the case of the I-series sam-
ples, the presence of Ni in their composition led to the
appearance of two peaks with maximums at 235 10
and 340 20°C (Fig. 4b). Regardless of the prepara-
tion method, the position and intensity of peaks at T <
250°C remained unchanged as the nickel content was
increased from 2 to 15 wt %, whereas the consumption
intensity at 250–450°C increased (Fig. 4). According
to an analysis of the literature data [47–49], the fol-
lowing three regions of hydrogen consumption can be
distinguished for the Ni–Ce–O materials: low-tem-
perature (T < 250°C), medium (250°C < T <550°C),
and high-temperature (T > 550°C) regions. The first
region of H2 consumption was due to the presence of
[Ni–O–Ce] structures localized on the surface of a
□
where is an oxygen vacancy.
The consumption of H2 at 250 < T < 550°C is
related to the reduction of nickel cations as the constit-
uents of particles, nickel oxide clusters, and solid solu-
tion. An increase in the Ni2+ reduction temperature
can be caused by (1) an increase in the degree of
metal–support interaction and (2) the formation of
large nickel oxide particles [47–49]. Lamonier et al.
[47] noted that, on the formation of a Ce1 – xMxOy solid
solution (M = Cu or Ni), the reducibility of Ce4+ cat-
ions increased and that of Mn+ decreased, as compared
with the corresponding single-component oxides. The
third region of hydrogen consumption was related to
the reduction of Ce4+ cations localized in the bulk of
the particles. In accordance with the above and based
on the experimental TPR-H2 data for the samples of P
and I series (Fig. 4), we can conclude that the contact
area between the active component and the support was
more developed and the range of stabilized nickel cation
species was wider when the polymer ester precursor
method was used. Moreover, lower values of both total
hydrogen consumption and H2/Ni ratios for the P-series
samples, as compared with those of the I-series samples,
are indicative of a lower reducibility of the systems
obtained by the polymer ester precursor method; this
can be due to the formation of the Ce1 – xNixOy solid
solution.
To establish a synthesis method–structure–prop-
solid solution or at the points of contact of highly dis- erties relationship, we tested the catalysts obtained in
persed nickel oxide particles with cerium dioxide crys- the autothermal reforming reaction of ethanol.
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EFFECT OF PREPARATION METHODS
227
1000
1000
Activity of the Catalysts in the Autothermal
(а)
8
7
6
5
4
3
2
1
Reforming of Ethanol
Table 2 summarizes the results of a study of the
activity of the support and Ni/CeO2 catalysts in the
ATR reaction of ethanol. In addition to hydrogen, a
wide range of C-containing products was formed: car-
bon oxides, methane, ethylene, acetaldehyde, and
acetone. Regardless of the catalyst composition and
preparation method, an increase in the conversion of
ethanol and the yield of hydrogen with reaction tem-
perature was observed; selectivity for the formation of
carbon monoxide increased, and selectivity for the
formation of C2–C3 by-products decreased. This is
consistent with previously obtained data and reflects
the features of the mechanism of this reaction [50].
370
5
4
270
225–235
160
880
1
2
3
0
200
400
600
800
Temperature, °С
As follows from the data of Table 2, the yield of
hydrogen was no higher than 20% in the presence of
CeO2. The introduction of nickel led to an improve-
ment in the process performance characteristics: the
conversion of ethanol and the yield of hydrogen
increased, while the yield of C2–C3 by-products
decreased (Table 2). As the nickel content of the P-
series was increased from 2 to 15 wt %, the yield of
hydrogen increased from 35 to 55%, whereas it
remained almost unchanged (40–45%) for the I
series. For all of the Ni/CeO2 test samples, the process
parameters remained almost unchanged in the course
of reaction for 6 h. However, in the presence of CeO2,
the conversion of ethanol decreased from 100 to 70%.
To assess the degree of carbonization of catalysts in
the course of the autothermal reforming of ethanol,
the catalysts were studied 6 h after the ATR reaction of
ethanol (reaction temperature, 600°C) by thermal
analysis in an atmosphere of air. It was established
(Table 2) that the amount of carbon deposits depends
on the composition and preparation method of
Ni/CeO2. In the case of the P-series samples, the frac-
tion of carbon deposits was 3–4 wt %, and it changed
only slightly upon varying the sample composition.
For the I-series samples, the amount of carbon depos-
its increased significantly from ~3 to 30% as the nickel
content was increased from 2 to 15 wt %. Therefore,
the catalysts obtained by the polymer ester precursor dioxide and the formation of the Ce1 – x
(b)
8
7
6
5
4
3
2
1
0
360
5
320
4
3
245
225
880
1
2 510
0
200
400
600
800
Temperature, °С
Fig. 4. Effect of the preparation method on the TPR-H of
2
Ni/CeO catalysts: (a) P-series and (b) I series. Ni con-
2
tents, wt %: (1) 0, (2) 2, (3) 5, (4) 10, and (5) 15.
crystallite size (5–8 against 11 nm), a higher specific
surface area (105–115 against 65–85 m2/g), and a
defect structure but lower reducibility. It is likely that the
observed differences resulted from the partial incorpora-
tion of nickel cations into the crystal lattice of cerium
NixOy solid solu-
tion. It is important to note that, as a result of reduc-
tive activation, the highly dispersed Ni0 active phase
particles were formed in Ni/CeO2 (P) samples, unlike
the I-series catalysts: these particles were characterized
method were characterized by high resistance to the
formation of carbon deposits in contrast to the sam-
ples prepared by impregnation.
It is reasonable that differences in the functional
properties of the samples are due to their different by a high dispersion of the active component (6–8 nm
physicochemical characteristics. As shown above, the against 20–50 nm). It is well known that the rate of
method of preparation is responsible for the textural
and structural properties of these materials and for
their reducibility. As compared with the I-series sam-
ples, the P-series Ni/CeO2 catalysts were character-
ized by different forms of stabilization of the active
component (a Ce1 – xNixOy solid solution phase and
NiO particles of size <5 nm against a NiO phase with
a particle size of 5–50 nm), a smaller average CeO2
formation of carbon deposits decreases with decreas-
ing Ni0 particle size [51], and the stable growth of
fibrous carbon occurs on particles of sizes 20–60 nm
[52]. The high dispersion of the active component also
causes a developed metal–support contact surface,
which facilitates the efficient transport of reactive oxy-
gen species from the support to C-containing interme-
diates formed at the Ni centers. A higher concentra-
KINETICS AND CATALYSIS Vol. 60 No. 2 2019

228
MATUS et al.
Table 2. Activity* of the support and catalysts in the autothermal reforming reaction of ethanol and the degree of their car-
bonization
S, %
XC H OH
,
2
5
Sample
CeO2
T, °C
%
С**, %
1.7
YH ,
2
%
CO2
CH4
C2H4
C2H4O C3H6O
CO
300
400
500
600
300
400
500
600
300
400
500
600
300
400
500
600
300
400
500
600
42
72
5
4
6
36
31
30
41
52
45
41
46
46
40
41
63
56
63
63
61
38
63
63
59
1
3
19
21
14
11
4
18
8
22
31
38
8
9
15
22
5
88
6
6
6
99
16
3
20
1
4
2Ni/CeO2 (P)
2Ni/CeO2 (I)
15Ni/CeO2 (P)
15Ni/CeO2 (I)
26
19
33
26
9
21
11
9
3.6
2.9
54
58
12
20
35
15
24
30
42
20
25
40
54
17
21
32
40
6
1
4
17
30
10
11
18
23
1
6
1
97
11
4
1
2
56
78
6
5
29
32
26
1
4
5
8
88
5
5
5
99
11
42
34
27
12
25
31
26
18
0
2
99
1
0
0
3.1
100
100
100
83
3
0
0
0
10
27
18
5
0
0
0
0
0
0
2
13
1
4
33.8
99
0
0
99
9
0
1
1
100
23
0
0
0
* Ethanol conversion (
), yield of hydrogen (
), and selectivity for product formation (S).
Y
H2
XC H OH
2
5
** The amount of carbon deposits in the catalysts after 6 h of the ATR reaction of ethanol at 600°C according to the results of thermal
analysis.
tion of oxygen vacancies in the P-series samples affects this method, the Ni active component was stabilized
the oxygen capacity of the material and the mobility of in a highly dispersed state, which ensured its resistance
oxygen and, hence, the efficiency of the removal of to the formation of carbon deposits in the course of the
carbon deposits [45, 53]. Thus, we can conclude that autothermal reforming of ethanol.
the polymer ester precursor method ensures a strong
metal–support interaction, which leads to the resis-
ACKNOWLEDGMENTS
tance of the active component against agglomeration
and its stabilization in a highly dispersed state. As a
consequence, the P-series catalysts exhibit high resis-
tance to carbonization during the catalytic process.
We are grateful to I.L. Kraevskaya, T.Ya. Efi-
menko, G.S. Litvak, and Cand. Sci. (Chem.)
E.Yu. Gerasimov for their assistance in the study of
the samples by physicochemical methods.
This work was supported by the Russian Foundation
CONCLUSIONS
for Basic Research (project no. 18-53-45012 IND_a).
We systematically studied the physicochemical and
catalytic properties of Ni/CeO2 (2, 5, 10, and 15 wt %)
depending on the method of their synthesis (the poly-
mer ester precursor method or incipient wetness
impregnation). We established that the best way to
prepare Ni/CeO2 is the polymer ester precursor
method. In contrast to impregnation, with the use of
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