Effect of the support composition on the physicochemical properties of Ni/Ce1–xLaxOy catalysts and their activity in an autothermal methane reforming reaction

Article abstract of DOI:10.1134/S0023158417050160

The effect of the Ce_1–xLa_xO_y (x = 0–1, 1.5 ≤ y ≤ 2.0) support composition on the physicochemical properties of supported Ni catalysts and their activity in autothermal methane reforming was studied. The textural and struct

Full text of DOI:10.1134/S0023158417050160

ISSN 0023-1584, Kinetics and Catalysis, 2017, Vol. 58, No. 5, pp. 610–621. © Pleiades Publishing, Ltd., 2017.
Original Russian Text © E.V. Matus, D.V. Nefedova, V.V. Kuznetsov, V.A. Ushakov, O.A. Stonkus, I.Z. Ismagilov, M.A. Kerzhentsev, Z.R. Ismagilov, 2017, published in Kinetika
i Kataliz, 2017, Vol. 58, No. 5, pp. 623–633.
Effect of the Support Composition on the Physicochemical Properties
of Ni/Ce1–xLaxOy Catalysts and Their Activity in an Autothermal
Methane Reforming Reaction
E. V. Matusa, *, D. V. Nefedovaa, b, V. V. Kuznetsova, V. A. Ushakova, O. A. Stonkusa, c, I. Z. Ismagilova,
M. A. Kerzhentseva, and Z. R. Ismagilova, d
a Boreskov Institute of Catalysis, Siberian Branch, Russian Academy of Sciences, Novosibirsk, 630090 Russia
b Novosibirsk State Technical University, Novosibirsk, 630073 Russia
c National Research Novosibirsk State University, Novosibirsk, 630090 Russia
d Institute of Coal Chemistry and Chemical Materials Science, Siberian Branch, Russian Academy of Sciences,
Kemerovo, 650000 Russia
*e-mail: matus@catalysis.ru
Received November 16, 2016
Abstract—The effect of the Ce1 – xLaxOy (x = 0–1, 1.5 ≤ y ≤ 2.0) support composition on the physicochemical
properties of supported Ni catalysts and their activity in autothermal methane reforming was studied. The tex-
tural and structural characteristics of Ce1–xLaxOy and Ni/Ce1–xLaxOy samples and the process of their
reduction in an atmosphere of hydrogen were examined using a set of techniques (low-temperature nitrogen
adsorption, X-ray diffraction analysis, transmission electron microscopy, and thermal analysis). It was estab-
lished that the Ce1–xLaxOy supports (x = 0–0.9) are mesoporous materials containing a fluorite-like solid
solution based on cerium dioxide, in which the unit cell parameter increases and the average crystallite size
decreases with the mole fraction of La. It was shown that the average size and composition of Ni-containing
particles in the Ni/Ce1–xLaxOy catalysts depends on the composition of the support: at x = 0–0.8, a phase of
NiO was formed, whereas a phase of LaNiO3 was formed at x = 0.9–1. The dispersity of the active constituent
and its stability to agglomeration increased as the mole fraction of La in the Ce1–xLaxOy support was
increased from 0 to 0.8, whereas the reduction of Ni-containing oxide particles shifted to the higher tempera-
ture region. The Ni/Ce1–xLaxOy catalysts provided high methane conversion (96–100%) and the yield of H2
(35–55%). The yield of hydrogen increased with decreasing the mole fraction of La in the Ce1–xLaxOy sup-
port composition; this can be caused by a decrease in the fraction of difficult-to-reduce Nin+ cations due to
the weakening of metal–support interactions.
Keywords: Ni catalysts, cerium dioxide, autothermal reforming, methane, synthesis gas
DOI: 10.1134/S0023158417050160
INTRODUCTION
CH4 + 1 2O2 ꢀ CO + 2H2,
ΔH2098K = −44 kJ/mol.
(III)
The oxidative conversion of methane into synthesis
gas (a mixture of CO and H2) is the base process of the
chemical conversion of natural gas, and it includes
three main reactions [1–5] of steam methane reform-
ing (SMR) (I), carbon dioxide methane reforming
(CDMR) (II), and the partial oxidation of methane
(POM) (III):
The combination of an exothermic POM reaction
with endothermic SMR and/or CDMR reactions is
the basis of an autothermal reforming (ATR) method.
The combined reforming ensures a decrease in energy
consumption and the more flexible regulation of syn-
thesis gas composition [5]. As a rule use, catalytic sys-
tems containing noble metals, Ni, or Co as active
components are used to perform processes (I)–(III)
[1, 6–10]. Different oxides—individual (Al2O3, ZrO2,
CeO2, and La2O3), binary (CeO2–ZrO2, MgO–Al2O3,
Y2O3–ZrO2, and La2O3–Al2O3), and more complex
systems (СeO2–La2O3–Al2O3, CeO2–ZrO2–La2O3,
CH 4 + H 2O ꢀ 3H 2 + CO,
(I)
0
ΔH 298 K = + 206 kJ/m ol,
CH4 + СO2 ꢀ 2H2 + 2CO,
(II)
ΔH2098 K = +261 kJ/mol,
610

EFFECT OF THE SUPPORT COMPOSITION
611
MgO–SiO2–Al2O3–Fe2O3)—serve as catalyst sup- the reduction temperature of the material and an
increase in its activity in the reactions of CO and soot
oxidation [17].
ports [1, 6–10]. Cerium-containing mixed oxides
(Сe1 – xMxOy, M = Gd, Zr, La, Y, and Pr) belong to
the most effective supports, which are characterized
by the high mobility of crystal lattice oxygen, high
oxygen capacity, and structural stability in a compara-
tively wide range of temperatures at different compo-
sitions of reaction atmospheres [11–16].
With the use of the Сe1–xMxOy materials as sup-
ports, the high dispersity and improved catalytic prop-
erties of the supported metal particles because of spe-
cific metal–support interactions and the good resis-
tance of catalysts to the formation carbon deposits were
noted [20, 24–31]. In addition to a positive influence on
It was established [17] that the introduction of
Zr4+, La3+, Eu3+, or Sm3+ cations (20 mol %) into the the properties of the active constituent, Сe1 – xMxOy can
lattice of cerium dioxide leads to an increase in its spe-
cific surface area, a decrease in the size of crystallites,
and an increase in the resistance of the material to
agglomeration. It was noted [18] that the doping of
directly participate in a catalytic process [17, 18, 21,
32, 33]. It is obvious that the optimum composition of
a mixed Ce-containing depends on its area of applica-
tion. Thus, for instance, Zhang et al. [20] found that
CeO2 with the cations Zr4+, La3+, and Y3+ (10 mol %) the yield of hydrogen in the ATR reaction of С2H5OH
also improves the thermal stability of the material, but in the presence of Rh/Сe1–xLaxOy catalysts increased
it does almost not influence its textural and morpho-
logical properties. A change in the physicochemical
properties of Сe1 – xMxOy materials with increasing the
as the mole fraction of La in the support was increased
from 0 to 0.3. According to Zhang et al. [20], this was
due to an improvement in the dispersity of the active
doping cation content is nonlinear. Thus, for instance, constituent (63 against 93%) and by an increase in
as the mole fraction of Zr, La, or Gd was increased the oxygen capacity of the material (131 against
from 0 to 0.33 [19], from 0 to 0.2–0.3 [18, 20], or from 256 (μmol O)/(g Cat)). For a methane tri-reforming
0 to 0.1 [19], respectively, the specific surface area of process, which combines all of the three methods of
the materials increased, but it decreased with a further oxidative conversion of CH4 (I)–(III), the catalyst
increase in the mole fraction. It was found [20, 21] that
the maximum reducibility of Сe1 – xLaxOy was reached
at x = 0.3 [20] or 0.4 [21]. Upon the modification of
cerium dioxide with trivalent cations, in accordance
with the electroneutrality of a lattice, every two cations
that replace a tetravalent cation of cerium form one
oxygen vacancy:
composition Сe0.7La0.2Ni0.1Oy, which is character-
ized by a minimum average size and a maximum
reducibility (H2/Ni) of Ni-containing particles, was
found as optimum.
Thus, by varying the composition of cerium-con-
taining mixed oxides and the methods and conditions
of their synthesis, it is possible to purposefully regulate
their textural, structural, and redox properties and
hence to affect the degree of metal–support interac-
tion for the stabilization of the specific forms of an
active constituent in the support matrix [20, 24–29,
34–36]. In turn, this provides an opportunity to con-
trol the physicochemical and functional properties of
the catalyst, and, correspondingly, the catalytic reac-
tion characteristics.
2CeO2 + 2M → 2MCe + 3OOx + VÖ + 1 2O2,
'
(IV)
where M = La3+, Eu3+, or Sm3+;
is M3+ that
'
MCe
x
replaces Ce4+;
gen vacancy.
is lattice oxygen, and VÖ is an oxy-
OO
Furthermore, an increase in the concentration of
oxygen vacancies in the materials based on cerium
dioxide doped with M3+ cations is also caused by a
decrease in the enthalpy of their formation with
decreasing the size of Сe1–xMxOy crystallites [22].
This process is accompanied by a decrease in the
effective degree of oxidation of cerium, and it can be
represented as follows:
In this work, we performed a comparative study of
the textural, structural, and reducing properties of Ni
catalysts depending on the composition of the
Сe1 ‒ xLaxOy (x = 0–1, 1.5 ≤ y ≤ 2.0) support in order
to develop methods for the target-oriented regulation
of the physicochemical properties and activity of cat-
alysts for methane conversion. We revealed a rela-
tionship between the physicochemical characteristics
of Ni/Се1 – xLaxOy materials and their activity in an
autothermal methane reforming reaction.
СеО2 → СеО2 – х + x/2O2, where 0 < x < 0.5, (V)
4Ce4+ + 2O2−
(VI)
→ 2Ce4+ + O2− + 2Ce3+ + V + 0.5O .
ꢀꢀ
O
2
According to Ismagilov et al. [23], the Се3+/(Се3+
+
Се4+) molar ratio increases with decreasing the size of
particles to approach 1 at a crystallite size of ~3 nm.
The structure disordering and an increase in the con-
centration of oxygen vacancies in the Сe1 – xMxOy
mixed oxides, as compared with CeO2, increase the
EXPERIMENTAL
Procedures of Support and Catalyst Synthesis
The Се1 – xLaxOy supports (x = 0, 0.1, 0.2, 0.5, 0.8,
0.9, and 1) were synthesized by a polymer ester precur-
mobility of oxygen. In turn, this leads to a decrease in sor method described previously [19, 24].
KINETICS AND CATALYSIS Vol. 58 No. 5 2017

612
MATUS et al.
The Ni/Се1 – xLaxOy catalysts were prepared by the
Procedure for Studying the Activity of Catalysts
incipient wetness impregnation of the support with an
aqueous solution of nickel nitrate with a specified con-
centration. The samples obtained were dried under an
IR lamp and then calcined in a muffle furnace at
500°C for 4 h. The Ni content of the resulting samples
was 10 0.5 wt %.
The activity of catalysts in an ATR reaction of CH4
was studied in a quartz flow reactor (inside diameter,
14 mm) in accordance with a procedure described pre-
viously [37, 38] at atmospheric pressure, a tempera-
ture of 300–900°C, a gas flow rate of 200 mLN/min,
and the molar ratio between reagents CH4 : H2O : O2 :
He = 1 : 1 : 0.75 : 2.5.
Physicochemical Methods of Support
and Catalyst Characterization
RESULTS AND DISCUSSION
Physicochemical Properties of the Supports
The concentrations of metals in the test materials
were determined by X-ray spectral fluorescence analysis
on an ARL ADVANT’X analyzer (ThermoTechno Sci-
entific, Switzerland) with an Rh anode of the X-ray tube.
Table 1 summarizes data on the chemical composi-
tion and the textural and structural characteristics of
Се1–xLaxOy supports. For all of the test samples, the
experimentally determined metal contents were con-
sistent with the calculated values.
According to the data on the low-temperature
adsorption of N2, the Се1– xLaxOy materials exhibited
the type IV adsorption isotherm with an H3 hysteresis
loop. Hysteresis at the partial pressure P/P0 = 0.5–0.9
indicates the presence of both primary (pores inside
the primary particles) and textural (pores between the
primary particles) mesoporosity [39]. After heat treat-
ment at 500°C, the specific surface area of Се1 – xLaxOy
The textural characteristics of the materials (spe-
cific surface area (SBET), pore volume (Vpore), and
average pore diameter (Dpore)) were studied on an
ASAP 2400 automated volumetric instrument
(Micromeritics, the United States) by the measure-
ment and treatment of low-temperature nitrogen
adsorption isotherms at 77 K.
The X-ray diffraction (XRD) analysis of the sam-
ples was carried out on an HZG-4C diffractometer
(Freiberger Prazisionmechanik, Germany) in mono-
chromatic CoKα radiation (λ = 1.79021 Å). Phase
composition was characterized based on the diffrac-
tion patterns obtained by scanning a region of the
angles 2θ = 10°–80° with a step of 0.1 deg and an accu-
mulation time of 6–15 s. The sizes of coherent scatter-
ing regions (CSRs) in Се1–xLaxOy (x = 0–0.9) were
calculated from the 1.1.1. diffraction peak broadening
of detected phases, which have a cubic structure of the
fluorite type; the CSRs of NiO and Ni were found
from the 2.0.0. peaks of NiO and Ni0 phases, respec-
tively.
was 17–95 m2/g, the pore volume was 0.11–
0.19 cm3/g, and the mean pore diameter was 8.5–
26.1 nm. As the temperature of the heat treatment of
Се1–xLaxOy was increased from 500 to 800°C, the spe-
cific surface area of the materials decreased by a factor
of 1.5–5 due to the intensification of sintering pro-
cesses [40]. From a ratio between the specific surface
areas of the samples calcined at 500 and 800°C, it fol-
lows that the resistance to sintering increases with the
La content of the material.
Figure 1a shows typical X-ray diffraction patterns
of the Се1–xLaxOy supports (x = 0–1) calcined at
500°C. The analysis of experimental data showed that
the support samples with x = 0–0.9 calcined at 300–
700°C were the single-phase crystalline systems of
cubic type fluorite-like solid solutions based on
cerium dioxide (JCPDS 34-394). According to the
XRD analysis data, the absence of additional phases,
for example, lanthanum hydroxycarbonate, oxide, or
hydroxide, does not exclude their presence in a highly
dispersed or amorphous state in the samples. The
sample of La2O3 calcined at 500–600°C was also a sin-
gle-phase sample of lanthanum dioxo monocarbonate
(JCPDS 23-322). As the support calcination tempera-
ture was increased to 800°C, the phase composition of
the Се1–xLaxOy (x = 0–0.8) materials remained
unchanged, whereas the composition of Се1 – xLaxOy
The high-resolution transmission electron micros-
copy (HR TEM) images were obtained on JEM-2010
and JEM-2200FS microscopes (JEOL, Japan) oper-
ating at an accelerating voltage of 200 kV. The spatial
lattice resolutions of the instruments were 1.4 and 1 Å,
respectively. The electron microscopes were equipped
with analytical attachments for local elemental micro-
analysis by energy-dispersive X-ray spectrometry
(EDX spectroscopy). The JEM-2200FS microscope
operates in both TEM and scanning (STEM) modes.
In this instrument, the EDX technique is compatible
with both TEM and STEM modes; this allowed us to
carry out EDX mapping with a locality of less than 1 nm.
The thermal analysis (differential thermal analysis
(DTA), thermogravimetric analysis (TGA), and dif-
ferential thermogravimetric analysis (DTG)) of the (x = 0.9–1) underwent some changes (Table 1). In
samples was carried out on a NETZSCH STA 449 C particular, in the structure of the sample of
thermal analyzer (NETZSCH Geratebau GmbH, Ger- Ce0.1La0.9O1.55, the traces of a lanthanum hydroxide
many) in a temperature range of 25–800°C with a heat-
phase (JCPDS 36-1481) were detected in addition to a
phase based on cerium dioxide; this is consistent with
ing rate of 10 K/min in an atmosphere of 5 vol % H2/He.
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EFFECT OF THE SUPPORT COMPOSITION
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614
MATUS et al.
the phase diagram of the Ce–La–O system [41]. In
(a)
the case of the sample of La2O3, the phase of lantha-
num dioxo monocarbonate decomposed because of
high-temperature heat treatment with the formation
of a lanthanum oxide phase (JCPDS 05-602).
Mole fraction
of La
CeO2
An analysis of the EDX maps of distribution
obtained in a STEM microscope showed that La was
evenly distributed in the samples of Се1–xLaxOy with
x = 0.1–0.5 [36]. As can be seen in Fig. 2b, regions
with an increased concentration of lanthanum were
absent from the images.
(111)
1.0
(200)
(222)
(311)
(220)
0.9
0.8
0.5
0.2
0.1
The diffraction patterns of the samples of Се1–xLaxOy
(x = 0.1–0.9) are characterized by wide symmetrical
peaks. The diffraction maximums shifted to the region
of smaller angles with an increase in the mole fraction
of lanthanum (Fig. 1a); this fact confirms the entry of
lanthanum cations into the lattice of cerium dioxide,
and it is related to differences in the ionic radii of the
cations of Ce (0.97 Å) and La (1.16 Å) [42, 43]. The
replacement of cerium cations by lanthanum cations is
evident from an increase in the lattice parameter a of
the solid solution Се1 – xLaxOy with the mole fraction
of La in its composition. The volume of a unit cell in
the samples of Се1 – xLaxOy (x = 0–0.9) changed pro-
portionally to an increase in the molar concentration
of La (Fig. 1b) to indicate a gradual increase in its con-
centration in the lattice of CeO2 and the formation of
a homogeneous solid solution.
20
30
40
50
60
70
2θ, deg
(b)
200
15
12
9
190
180
170
160
150
6
3
The average size of CSRs for the Се1–xLaxOy mate-
rials (x = 0–0.9) calcined at 500°C was 2.8–12.5 nm.
This parameter decreased with the mole fraction of La
in the material (Fig. 1b). A decrease in the size of
CSRs in Сe1–xMxOy, as compared with that of CSRs
in CeO2, is a widely known phenomenon caused by
the inhibition of crystallite growth in the presence of a
doping cation [21, 22, 44]. As the heat treatment tem-
perature was increased to 800°C, the size of crystallites
increased to 20–50 nm (Table 1). As in the samples
0
0
0.2
0.4
0.6
0.8
1.0
Mole fraction of La in Ce1 – xLaxOy
Fig. 1. Dependence of (a) the phase composition and (b)
the average volume of a unit cell and the CSR size of the
Ce
La O supports on the mole fraction of lanthanum
x y
1 – x
(x). Calcination temperature, 500°C.
(а)
(b)
(c)
CeL
LaL
100 nm
DF
100 nm
100 nm
Fig. 2. HAADF-STEM images of (a) a section of the Сe La
O
sample and (b, c) the EDX mapping that shows the distri-
0.5 0.5 1.75
bution of elements. Calcination temperature, 500°C.
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EFFECT OF THE SUPPORT COMPOSITION
Table 2. Physicochemical properties of the catalysts*
Texture characteristics
615
XRD data
unit cell
parameter
of CeO2 (а), Å
Catalyst
SBET
m2/g
,
Vpore,
cm3/g
Dpore
nm
,
CSR
size, nm
phase composition
NiO
СеО2
–
5.413
–
5.445
–
5.473
–
5.557
–
25.0
15.0
14.5
9.5
13.5
8
–
6.5
–
Ni/CeO2
50
0.13
10.8
10.5
10.8
18.1
20.6
NiO
СеО2**
NiO
Ni/Сe0.9La0.1O1.95
Ni/Сe0.8La0.2O1.9
Ni/Сe0.5La0.5O1.75
Ni/Сe0.2La0.8O1.6
73
0.19
0.19
0.18
69
СеО2**
Traces of a highly dispersed NiO phase
СеО2**
40
Traces of a highly dispersed NiO phase
СеО2**
LaNiO3
СеО2**
12
0.06
5.638
–
4.0
–
Ni/Сe0.1La0.9O1.55
8
0.05
23.0
–
–
La2О2СО3
LaNiO3
La2О2СО3
–
–
–
–
–
–
Ni/La2O3
10
0.06
23.6
* The catalyst calcination temperature was 500°C.
** The diffraction pattern of a phase similar to CeO .
2
Dashes indicate that the cell parameters and CSR sizes are given for a phase based on CeO .
2
treated at 500°C, a high mole fraction of La in the can be seen in Table 2, these samples are characterized
by SBET ≈ 10–75 m2/g and Vpore ≈ 0.1–0.2 cm3/g. The
higher values of specific surface areas were observed in
the catalysts based on the supports with a low La con-
tent (x = 0.1–0.2); this completely correlates with the
dependence of the textural characteristics of the sup-
port on its composition.
composition of the material ensured a smaller size of
its crystallites (Table 1). According to HR TEM data
[36], the crystallites formed polycrystalline agglomer-
ates predominantly as plates.
Thus, all of the Се1 – xLaxOy supports obtained (x =
0–0.9) were mesoporous materials containing a fluo-
rite-like solid solution based on CeO2, in which the
From a comparison between data given in Tables 1 and
unit cell parameter increased and the average size of 2, it follows that the phase composition of the Се1 – xLaxOy
crystallites decreased with the mole fraction of La.
Unlike a citrate sol–gel method [21] and coprecipita-
tion [45], the use of the polymer ester precursor
method for the synthesis of the Се1 – xLaxOy materials
makes it possible to obtain solid solutions based on
CeO2 in a wider range of La concentrations. Although
(x = 0–0.8) supports remained unchanged after the
introduction of a nickel oxide precursor and the sub-
sequent heat treatment at 500°C. The sample of
Ni/Сe0.1La0.9O1.55 with a high La content (x = 0.9) was
the exception; in the composition of this sample, lan-
thanum dioxo monocarbonate traces were observed
the resistance of the Се1 – xLaxOy material to sintering together with a solid solution phase based on CeO2
increases with the La content, a developed specific (Table 2).
surface area together with a constant phase composi-
tion in a wide range of temperatures was reached at a
La mole fraction of 0.1–0.5. The synthesized Се1 – xLaxOy
mixed oxides were used as supports for the Ni-con-
taining particles of active constituents in the catalysts
for the ATR of CH4.
The Ni/Се1 – xLaxOy (x = 0–0.2) catalysts exhibited
peaks due to the formation of a phase of NiO against
the background of a diffraction pattern of the support.
The dispersity of the active component increased with
the lanthanum content of the support. Thus, as the
fraction of La changed from 0 to 0.2, the average CSR
size of NiO decreased from 25 to 13.5 nm. With a fur-
ther increase in the quantity of La to x = 0.5–0.8, the
Ni-containing phase occurred as the traces of highly
dispersed NiO. It should be noted that the dependence
of the particle size of NiO on the support composition
Physicochemical Properties of the Catalysts
Table 2 summarizes the textural characteristics and
phase composition of the Ni/Се1 – xLaxOy catalysts. As
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616
MATUS et al.
(b)
(a)
NiO
NiO
50 nm 20 nm
(c)
(d)
(e)
Cel
NiK
100 nm
(f)
200 nm
(h)
(g)
Cel
NiK
100 nm
Fig. 3. Electron-microscopic images of the (a) Ni/CeO and (b) Ni/Ce La
O
samples; HAADF-STEM images of the sec-
2
0.8 0.2 1.9
tions of the (c) Ni/CeO and (d) Ni/Ce La
O
samples; and EDX mapping (e–h) that shows the distribution of elements.
2
0.5 0.5 1.8
does not correlate with their specific surface areas. increased from 0 to 0.5. Thus, the results of EDX map-
Consequently, the interaction of Ni2+ cations with the
ping for the Ni/Ce0.5La0.5O1.8 catalyst indicated the
support strengthened with the fraction of La in the presence of only small particles (smaller than 8 nm) of
composition of Се1 – xLaxOy; this led to the formation nickel oxide or nickel in an atomically dispersed state
(Fig. 3f).
of a highly dispersed Ni-containing oxide phase.
Another behavior was observed in the samples
obtained on the basis of the Се1 – xLaxOy supports with
high lanthanum content (x = 0.9–1). In this case, the
formation of the oxide LaNiO3 with the structure of
perovskite was observed.
The reduction of a Ni-containing phase depending
on the support composition was studied by the thermal
analysis of the Ni/Се1 – xLaxOy catalysts in a reducing
atmosphere. On the interaction of hydrogen with the
catalyst at elevated temperatures, the occurrence of
several processes is expected: the desorption of physi-
cally adsorbed water, the reduction of cerium dioxide,
and the reduction of nickel oxide. As a typical exam-
ple, Fig. 4 shows thermal analysis data for the samples
of CeO2 and Ni/CeO2 in 5% H2/He. CeO2 exhibited
endothermic effects in a low-temperature region (T <
200°C) accompanied by a sample weight loss caused
by the desorption of water. Clearly pronounced endo
effects were absent from the high-temperature
The XRD analysis data (Table 2) are consistent with
the results obtained by HR TEM, HAADF-STEM, and
EDX mapping (Fig. 3), according to which single and
aggregated nickel oxide particles were observed on the
surface of the polycrystalline Се1 – xLaxOy (x = 0–0.5)
support (Figs. 3a, 3b). The particle size depends on the
support composition and varies in a range from 8 to 50 nm.
Furthermore, according to the EDX mapping of cata-
lyst sections, a portion of the active constituent
occurred in a highly dispersed state. The fraction of
region, but sample weight losses occurred at TDTG
=
the active constituent stabilized in the highly dispersed 275°C (–∆m/m = 0.05 wt %) and 580°C (–∆m/m =
state increased as the mole fraction of La was 0.4 wt %). According to published data [43], the TPR-
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EFFECT OF THE SUPPORT COMPOSITION
(a)
617
100
98
96
94
92
0.6
0.5
0.4
0.3
0.2
0.1
0
0.1
650°С
–0.1%
–0.4%
–0.8%
–0.05%
TG
0
275°С
DTG
580°С
75°С
–0.1
–0.2
–0.3
120°С
DTA
–0.1
–0.2
60°С
200
400
600
800
Temperature, °С
(b)
–0.4%
–0.6%
100
98
96
94
92
0.6
0.5
0.4
0.3
0.2
0.1
0
0.1
–0.2%
–0.6%
DTA
TG
695°С
0
–2.2%
DTG
295°С
255°С
580°С
–0.3%
–0.1
–0.2
–0.3
600°С
70°С
–0.3%
405°С
470°С
255°С
125°С
0
200
400
600
800
Temperature, °С
(c)
0.1
255°С
0
–0.1
–0.2
–0.3
–0.4
–0.5
650°С
280°С
400°С
470°С
100°С
545°С
Composition of the support:
CeO2
745oC
Ce0.8La0.2O1.9
Ce0.5La0.5O1.75
La2O3
0
200
400
600
800
Temperature, °С
Fig. 4. Thermal analysis of the (a) CeO , (b) Ni/CeO , and (c) Ni/Ce
La O samples in a reducing atmosphere (H /He).
x y 2
2
2
1 – x
H2 curves of cerium dioxide exhibited two regions of curve (Fig. 4c): the Ni-containing oxide particles were
reduced in a higher temperature region as the La con-
tent of the samples was increased. This is indicative of an
increase in the fraction of difficult-to-reduce Ni2+ cat-
ions, which can be related to the strengthening of active
constituent interaction with the support, as confirmed by
the XRD analysis and electron-microscopic data.
hydrogen absorption at 100–600 and 600–900°C due
to the reduction of Ce4+ cations localized on the sur-
face and in the bulk of particles, respectively. It is
believed that the weight loss observed in the region of
T > 200°C was caused by the reduction of Ce4+ cations
localized on the surface of the particles.
Thus, the composition, dispersity, and reducibility
of Ni-containing particles in the Ni/Се1 – xLaxOy catalysts
depend on the composition of the support. The active con-
stituent interaction with the support matrix was strength-
ened with the mole fraction of La in Се1 – xLaxOy; this ini-
tially led to an increase in the dispersity of the NiO
The main difference in the profile of TG/DTG
curves for the Ni/CeO2 sample is that a considerable
weight loss was observed in a temperature range of
350–550°C (TDTG = 405 and 470°C; –∆m/m = 3.0 wt %).
This effect may be caused by the reduction of nickel
oxide [46]. A change in the support composition led to
the displacement of minimum positions in the DTG phase (at x = 0–0.8) and then to the formation of the
KINETICS AND CATALYSIS Vol. 58 No. 5 2017

618
MATUS et al.
LaNiO3 phase at x = 0.9–1 and an increase in the frac-
tion of difficult-to-reduce nickel cations. To reveal a
relationship between the physicochemical and func-
tional properties of the Ni/Се1 – xLaxOy catalysts, we
studied their activity in the CH4 ATR reaction.
(а)
100
80
60
40
20
0
Conversion of CH4
Yield of H2
Yield of СО
Yield of СО2
Activity of the Catalysts in the CH4 ATR Reaction
Figure 5 illustrates the temperature dependence of
the conversion of CH4 and the yield of products in the
CH4 ATR reaction in the presence of Ni catalysts
based on the individual oxide supports. From Fig. 5a,
it is evident that, in the presence of the Ni/CeO2 cata-
lyst, the conversion of CH4 increased from 5 to 100%
as Treaction was increased from 400 to 800°C, and it
remained unchanged upon further heating. The main
reaction products are H2, CO, and CO2. The forma-
tion of hydrogen begins at 550°C; as the temperature
was increased to 800°C, the yield of hydrogen
increased to 55% and than remained unchanged.
Analogous temperature dependence was observed for
the yield of CO. The temperature of the onset of CO2
formation (400°C) coincides with the temperature of
the onset of CH4 conversion. The yield of CO2
increased as Treaction was increased from 400 to 450°C;
at 450–700°C, it changed only slightly (30–35%) and
further decreased. A somewhat different behavior was
characteristic of the Ni/La2O3 catalyst (Fig. 5b). In
this case, the conversion of CH4 increased from 10 to
35% as Treaction was increased from 450 to 500°C, and it
remained at this level in a temperature range of 500–
750°C. On further heating, the conversion of methane
increased to reach 100% at 900°C. The formation of
hydrogen and CO began at Treaction = 750°C; then,
their yields increased with temperature to 40 and 45%,
respectively, at 900°C. The yield of CO2 increased
with temperature to 40–45% at 500°C and then
decreased. The temperature dependence of the com-
position of reaction products indicates that the com-
plete oxidation of methane is the main reaction in the
presence of the Ni/CeO2 catalyst at 400–500°C. At
550°C, the conversion of CH4 and the yields of CO
and H2 increased due to the occurrence of methane
reforming reactions. In the presence of the Ni/La2O3
catalyst, the temperatures of the onset of CH4 conver-
sion and the appearance of H2 in the reaction products
were shifted to the high-temperature region. The com-
plete oxidation of methane predominantly occurred in
a wider temperature range of 500–750°C. The conver-
sion of CH4 and the yields of CO and H2 began to
increase due to the occurrence of methane reforming
reactions only at 750°C.
300 400 500 600 700 800 900
Temperature, °С
(b)
100
80
60
40
20
0
Conversion of CH4
Yield of H2
Yield of СО
Yield of СО2
300 400 500 600 700 800 900
Temperature, °С
(c)
60
50
40
30
20
10
0
Composition of the support:
CeO2
Ce0.8La0.2O1.9
Ce0.5La0.5O1.75
Ce0.2La0.8O1.6
La2O3
400 500 600 700 800 900
Reaction temperature, °C
Fig. 5. The temperature dependence of (a, b) the conver-
sion of methane and (a–c) the yields of products in the
CH ATR reaction on the (a) Ni/CeO , (b) Ni/La O ,
4
2
2 3
and (c) Ni/Ce
La O catalysts.
x y
1 – x
As follows from the data shown in Fig. 5c, the curve
of the temperature dependence of the yield of hydrogen
shifted to the high-temperature region as the mole frac-
tion of lanthanum in the composition of the Ce1– xLaxOy
onset of hydrogen formation (yield, 5–10%) in the
CH4 ATR reaction were 700 and 550°C for the
Ni/Се1 – xLaxOy catalysts with x = 0.8 and 0.2, respec-
support was increased. Thus, the temperatures of the tively.
KINETICS AND CATALYSIS
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No. 5
2017

EFFECT OF THE SUPPORT COMPOSITION
619
Table 3. Characteristics of the activity of Ni/Ce1–xLaxOy catalysts in the CH4 ATR reaction at 850°C
Conversion of СН4, %
Yield of H2, %
H2/CO
Catalyst
Yield of CO, %
Ni/СеО2
100
100
100
100
100
100
96
56
56
55
43
48
38
34
46
48
45
45
43
44
41
3.7
3.5
3.7
2.8
3.1
2.5
3.4
Ni/Сe0.9La0.1O1.95
Ni/Сe0.8La0.2O1.9
Ni/Сe0.5La0.5O1.75
Ni/Сe0.2La0.8O1.7
Ni/Сe0.1La0.9O1.55
Ni/La2O3
Table 3 summarizes data on the effect of the sup- [28]. Because Ce-containing oxides are the effective
port composition upon the catalytic properties of promoters of this reaction [45], its contribution to the
Ni/Се1 – xLaxOy in the CH4 ATR reaction at 850°C. All overall process varies depending on the catalyst com-
position. Note that, at a high concentration of surface
defects, the Ce1–xMxOy materials are characterized by
a sufficient amount of H2O/O2 activation centers, and
they are a source of active oxygen, which participates
in the oxidation of carbon at the metal–support inter-
face [13, 46]. The efficiency of the participation of a
support in the catalytic process depends on the ratio
and dispersity of active constituent and support
phases, their reducibility, and the oxygen capacity of
the material and the mobility of oxygen in the support
matrix. With increasing the fraction of La in the sup-
port composition, the oxidation of carbon formed on
the active constituent surface can occur according to
the reaction La2O2CO3 + C–Ni → La2O3 + CO + Ni
[26]. Consequently, it is likely that the Ni/Ce1 – xLaxOy
test catalysts are resistant to the formation of carbon
deposits. Because the resistance of both the support
and the active constituent to sintering increases with
the La content of the support, it is believed that the
Ni/Ce1 – xLaxOy catalysts (x = 0.1–0.2) will ensure sta-
ble operation under the CH4 ATR reaction conditions
along with high activity. This is the subject matter of
our further studies.
of the test catalysts ensured the high conversion of
methane (96–100%), and the yield of hydrogen
decreased from 56 to 34% as the mole fraction of La in
the support composition was increased from 0 to 1.
According to the results of the studies of samples by
physicochemical methods, the interaction of an active
constituent with the support strengthened with the La
content of Ce1–xLaxOy; this manifested itself as a
change in the form of its stabilization (NiO particles,
highly dispersed forms of NiO, and LaNiO3), and the
fraction of Nin+ cations, which are reduced in a high-
temperature region, increased. It is well known that an
increase in the reducibility of Nin+ and an increase in
the dispersity of Ni0 ensure an improvement in the cat-
alytic properties of Ni catalysts in the reactions of CH4
conversion into synthesis gas [47–49]. It was shown
that the reducibility of Ni cations determines the sta-
bility of the active constituent to oxidation, and the
particle size of Ni affects the rate of catalyst carboniza-
tion under CH4 ATR reaction conditions. It was estab-
lished [50] that the Ni-containing particles, which are
characterized by weak metal–support interaction,
exhibited high activity in the POM reaction, and the
dispersed forms of nickel, which are characterized by
strong metal–support interaction, exhibited high resis-
tance to deactivation. Accordingly, the different activity
of the Ni/Ce1–xLaxOy catalysts can be caused by differ-
ences in their redox properties. The reducibility of the
cations of nickel and the resistance of the active constitu-
ent to oxidation increased with decreasing the mole frac-
tion of La; this led to an increase in the concentration of
an active phase of Ni0, and, as a result, to the higher pro-
cess characteristics of the ATR of CH4.
CONCLUSIONS
We synthesized and studied a series of the
Ni/Ce1 – xLaxOy catalysts for the process of autother-
mal methane reforming, We found that the form of
active constituent stabilization in the support matrix
depends on the composition of the support. The compo-
sition and dispersity of the Ni-containing phase changed
as the mole fraction of La was increased from 0 to 1
(a phase of NiO (13–25 nm) was observed at x = 0–
0.2; a highly dispersed phase of NiO was observed at
x = 0.5–0.8, and a phase of LaNiO3 was formed at x =
0.9–1), and the temperature of formation of a phase of
Ni0 increased upon catalyst reduction; this fact is
As follows from the data of Table 3, the H2/CO
molar ratio was 2.5–3.7. The relatively high values of
H2/CO > 3 can be indicative of the occurrence of a
water gas shift reaction (СО + H O
СО2 + H2)
ꢀ
2
KINETICS AND CATALYSIS Vol. 58 No. 5 2017

620
MATUS et al.
tions, and Synthesis Methods), Novosibirsk: Sib. Otd.
Ross. Akad. Nauk, 2010.
indicative of an increase in the degree of interaction of
the active constituent with the support. An increase in
the La content leads to a shift of the temperature
dependence of the yield of hydrogen in the CH4 ATR
reaction to the high-temperature region. In the presence
15. Kuznetsova, T.G. and Sadykov, V.A., Kinet. Catal.,
2008, vol. 49, no. 6, p. 840.
16. Ivanov, V.K., Polezhaeva, O.S., and Tret’yakov, Yu.D.,
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of the Ni/Се1–xLaxOy catalysts (x = 0–0.2) at Treaction
=
850°C, the yield of hydrogen was ~55%, which is close
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ACKNOWLEDGMENTS
19. Kaneko, H., Taku, S., and Tamaura, Y., Solar Energy,
We are grateful to I.L. Kraevskaya, T.Ya. Efimenko,
and G.S. Litvak for their assistance in the characteri-
zation of the samples by physicochemical methods.
This work was performed within the framework of
a state contract at the Boreskov Institute of Catalysis,
Siberian Branch, Russian Academy of Sciences (proj-
ect no. 0303-2016-0004).
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Translated by V. Makhlyarchuk
KINETICS AND CATALYSIS Vol. 58 No. 5 2017