Development of a Ni–Pd/CeZrO2/Al2O3 catalyst for the effective conversion of methane into hydrogen-containing gas

Article abstract of DOI:10.1134/S002315841705010X

The effects of the Pd content (0–1 wt %) and the synthesis method (joint impregnation with Ni + Pd and Pd/Ni or Ni/Pd sequential impregnation) on the physicochemical and catalytic properties of Ni–Pd/CeZrO₂/Al₂O₃ were studied in order to develop an efficient catalyst for the conversion of methane into hydrogen-containing gas. It was shown that variation in the palladium content and a change in the method used for the introduction of an active constituent into the support matrix make it possible to regulate the redox properties of nickel cations but do not affect the size of NiO particles (14.0 ± 0.5 nm) and the phase composition of the catalyst ((γ + δ)-Al₂O₃, CeZrO₂ solid solution, and NiO). It was established that the activity of Ni–Pd catalysts in the reaction of autothermal methane reforming depends on the method of synthesis and increases in the following order: Ni + Pd < Ni/Pd < Pd/Ni. It was found that, as the Pd content of the Ni–Pd/CeZrO₂/Al₂O₃ catalyst was decreased from 1 to 0.05 wt %, the ability for self-activation, high activity, and operational stability of the catalyst under the conditions of autothermal methane reforming remained unaffected: at 850°C, the yield of hydrogen was ~70% at a methane conversion of ~100% during a 24-h reaction.

Full text of DOI:10.1134/S002315841705010X

ISSN 0023-1584, Kinetics and Catalysis, 2017, Vol. 58, No. 5, pp. 601–609. © Pleiades Publishing, Ltd., 2017.
Original Russian Text © M.A. Kerzhentsev, E.V. Matus, I.A. Rundau, V.V. Kuznetsov, I.Z. Ismagilov, V.A. Ushakov, S.A. Yashnik, Z.R. Ismagilov, 2017, published in Kinetika
i Kataliz, 2017, Vol. 58, No. 5, pp. 614–622.
Development of a Ni–Pd/CeZrO2/Al2O3 Catalyst for the Effective
Conversion of Methane into Hydrogen-Containing Gas
M. A. Kerzhentseva, E. V. Matusa, *, I. A. Rundaua, b, V. V. Kuznetsova, I. Z. Ismagilova, V. A. Ushakova,
S. A. Yashnika, 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 Chemical Materials Science, Siberian Branch, Russian Academy of Sciences,
Kemerovo, 650000 Russia
*e-mail: matus@catalysis.ru
Received November 16, 2016
Abstract—The effects of the Pd content (0–1 wt %) and the synthesis method (joint impregnation with Ni +
Pd and Pd/Ni or Ni/Pd sequential impregnation) on the physicochemical and catalytic properties of Ni–
Pd/CeZrO2/Al2O3 were studied in order to develop an efficient catalyst for the conversion of methane into
hydrogen-containing gas. It was shown that variation in the palladium content and a change in the method
used for the introduction of an active constituent into the support matrix make it possible to regulate the redox
properties of nickel cations but do not affect the size of NiO particles (14.0 0.5 nm) and the phase compo-
sition of the catalyst ((γ + δ)-Al2O3, CeZrO2 solid solution, and NiO). It was established that the activity of
Ni–Pd catalysts in the reaction of autothermal methane reforming depends on the method of synthesis and
increases in the following order: Ni + Pd < Ni/Pd < Pd/Ni. It was found that, as the Pd content of the Ni–
Pd/CeZrO2/Al2O3 catalyst was decreased from 1 to 0.05 wt %, the ability for self-activation, high activity, and
operational stability of the catalyst under the conditions of autothermal methane reforming remained unaf-
fected: at 850°C, the yield of hydrogen was ~70% at a methane conversion of ~100% during a 24-h reaction.
Keywords: Ni–Pd catalysts, aluminum oxide, autothermal reforming, methane, hydrogen
DOI: 10.1134/S002315841705010X
INTRODUCTION
gas conversion (II) followed by the removal of impuri-
ties from hydrogen [7, 8]:
Hydrogen is an important reagent for the chemical,
petroleum, and food industries. The current global
market of hydrogen is estimated at 55 million tons per
annum, and its increase by 30% is expected in the near
five years [1–3]. The general tendency toward deeper
petroleum refining and the development of the pro-
duction of ammonia, methanol, and synthetic liquid
fuel are responsible for a steady increase in the con-
sumption of hydrogen. Its active use within the frame-
work of hydrogen-oriented power engineering along
with traditional areas of application is predicted
because hydrogen is considered as the most promising
energy carrier due to its high fuel rating and ecological
compatibility [4–6]. The main currently available
technologies for the production of hydrogen on an
industrial scale include the steam reforming of natural
gas, the reforming of petrochemical waste gases, the
gasification of coal, and the electrolysis of water [1].
The steam hydrocarbon reforming technology affords
to 85% of the hydrogen produced and includes the fol-
lowing stages of synthesis gas generation (I) and water
CH4 + H2O ↔ CO + 3H2,
(I)
ΔH 2098 K = +206 kJ/mol,
CO + H2O ↔ CO2 + H2,
(II)
ΔH2098 K = – 41 kJ/mol.
Because the steam reforming of methane is an endo-
thermic reaction, heat supply from without is required.
A more advantageous energy balance is reached in auto-
thermal reforming (ATR), which additionally includes
the following exothermic reactions of methane oxida-
tion [9]:
CH4 + 1 2O2 ↔ CO + 2H2,
(III)
ΔH2098K = −44 kJ/mol,
CH4 + 2O2 ↔ CO2 + 2H2O,
(IV)
ΔH2098 K = –802 kJ/mol.
601

602
KERZHENTSEV et al.
Bimetallic catalysts on different supports (Al2O3, lished [28] that the structure of bimetallic particles in
the 0.9Ni–0.1Pd/γ-Al2O3 catalyst depends on the
preparation method: the enrichment of the surface of
bimetallic particles in palladium, which ensured the
stability of Ni–Pd particles to oxidation, was observed
upon the sequential impregnation (in contrast to the
joint impregnation). On the contrary, in the case of
the 10.6Ni–0.2Pd/α-Al2O3 catalyst [24], the joint
La2O3, ZrO2, MgAl2O4, and Ce1 – xZrxO2) are effective
catalytic systems for hydrocarbon reforming processes
[10–22]. As a rule, nickel serves as the main constitu-
ent of an active component, and noble metals (0.1–
2.0 wt % of Rh, Pd, Pt, Ir, and Ru) serve as promoting
additives. As compared with the monometallic cata-
lysts based on noble metals, bimetallic catalysts are
characterized by smaller noble metal contents, which impregnation led to a higher surface concentration of
make their use more profitable. As a rule, a joint [18, palladium in the bimetallic particles and to improved
20, 23, 24] or sequential [25–27] impregnation process characteristics in the oxidative steam reform-
method is used in the preparation of the Ni-contain- ing of methane.
ing bimetallic catalysts.
Thus, the type, concentration, and method of the
From an analysis of published data, it follows that
the presence of a noble metal (1) stabilizes an active
component in a reduced state to decrease the proba-
bility of catalyst deactivation due to the oxidation of
the active component [15, 16], (2) decreases the rate of
formation of carbonaceous deposits [17], and (3)
increases the resistance of the catalyst to sulfur-con-
taining compounds [18]. In addition to a change in the
redox properties, the introduction of the second metal
can exert an effect on the electronic structure [17] and
dispersity [19] of the active component. Thus, for
instance, Yoshida et al. [20] noted that the average
particle size of Ni in the Ni–M/Al2O3 catalysts
increased with the metal content for M = Pd, Rh, or
Ru or, on the contrary, decreased in the case of M =
Pt, Ir, and Au. In a comparative study of the Ni–
M/Al2O3 bimetallic catalysts (M = Pd, Pt, Au, Ir, Rh,
or Ru), Yoshida et al. [20] found that Pd is the most
effective modifier, which increases the dispersity of Ni
particles and the capability of Ni2+ to be reduced and
suppresses the oxidation of Ni0 in the course of the
oxidative steam reforming of methane. Based on an
EXAFS study, Yoshida et al. [20] concluded that an
improvement in the characteristics of the catalyst
upon the introduction of palladium was caused by the
formation of Pd clusters, which interacted with the
particles of Ni. In the case of a Ni–M/CeZrO2 catalyst
(M = Pt, Pd, Fe, and Ag), an additive of Ag was rec-
ommended as an optimum promoter [21]. According
to Dantas et al. [21], the higher activity and opera-
tional stability of the Ni–Ag/CeZrO2 sample in a CH4
ATR reaction is related to its redox properties. It was
found [22] that the activity of the Ni–M/La2O3 cata-
lysts (M = Pt, Sn, Mo, Re, and Pd) in the CH4 ATR
reaction increases in the order Pt < Sn < Mo < Re <
Pd, which correlates with an increase of the reducibil-
ity of Nin+ cations in LaNi1–xMxO3.
introduction of a promoting additive vary over a wide
range, and their optimum values depend on both the
catalyst composition and the conditions of a catalytic
reaction. This work is a continuation of studies on
the development of an efficient catalyst for the con-
version of methane into hydrogen-containing gas
[22, 29–31], and it is dedicated to a systematic study of
the physicochemical and catalytic properties of the Ni–
Pd/CeZrO2/Al2O3 catalysts for CH4 ATR depending on
the Pd content and method used for the introduction of
an active component into the support matrix. The pro-
moting additive and support composition were chosen
based on the previously obtained data [22, 29–31]
according to which Pd is the most effective additive
(among Pt, Sn, Mo, Re, and Pd) [22] and Al2O3 modi-
fied with 10 wt % Ce0.5Zr0.5O2 is an optimum support
material among La2O3, CexGd1–xOy, CexZr1–xOy,
Ce0.5Zr0.5O2/Al2O3, and La2O3/Ce0.5Zr0.5O2/Al2O3
[29–31]. In comparison with the previous works, we
studied the Ni–Pd catalytic systems, which are char-
acterized by low Pd content, and examined for the
first time the effect of the composition of an active
component and the method of synthesis on the capabil-
ity of the Ni–Pd/CeZrO2/Al2O3 catalysts to undergo
self-activation.
EXPERIMENTAL
Catalyst Synthesis Procedure
The Ni–Pd/CeZrO2/Al2O3 catalysts were obtained
by the joint and sequential incipient wetness impreg-
nation of the 10CeZrO2/Al2O3 support, whose charac-
teristics were reported preciously [29]. On the joint
impregnation, the support was impregnated with an
aqueous solution of a mixture of metal salts (nickel
nitrate Ni(NO3)2 ⋅ 6H2O and palladium nitrate
Pd(NO3)2 with specified concentrations); the sam-
ples were dried under an IR lamp at 80–85°C and
calcined in a muffle furnace at 500°C for 4 h. In the
Pd/Ni sequential impregnation, the support was
impregnated with an aqueous solution of nickel
nitrate, dried, and then calcined at 500°C for 4 h. The
resulting Ni/CeZrO2/Al2O3 sample was impregnated
The degree of interaction between metals in the
bimetallic catalysts is different depending on the
chemical composition of the catalyst, its preparation
method, and the conditions of its activation and oper-
ation [10, 11]. In the case of weak interaction between
nickel and a promoting additive, monometallic parti-
cles were formed, whereas a surface or bulk alloy was
formed in the case of strong interaction. It was estab- with a solution of palladium nitrate. In the Ni/Pd
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DEVELOPMENT OF A Ni–Pd/CeZrO2/Al2O3 CATALYST
603
sequential impregnation, the order of supporting Germany) in a temperature range of 25–900°C with a
metal salts was changed: first, the support was impreg- heating rate of 10 K/min in an atmosphere of air.
nated with a solution of palladium nitrate; then, after
intermediate heat treatment at 500°C for 4 h, the
Procedure for Studying the Activity of Catalysts
Pd/CeZrO2/Al2O3 sample was impregnated with a
solution of nickel nitrate. The final stage of the heat
treatment of the catalysts obtained by the above method
was the same as that for the catalysts synthesized by the
joint impregnation method. In the designations of the
samples, the numbers before Pd indicate its weight con-
tents in the catalysts.
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 [30, 31] at atmospheric pressure, a tempera-
ture of 700–900°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. Before the beginning of an
experiment, the catalyst was reduced in the flow of a
30% H2/He mixture at 800°C and a gas flow rate of
100 mLN/min for 2 h. The long-term tests of the sam-
ples in the CH4 ATR reaction were conducted at a
reaction temperature of 850°C without preliminary
catalyst activation in hydrogen. The reaction mixture
was analyzed with the aid of a QMS 300 mass-spectro-
metric analyzer (Stanford Research Systems, the
United States).
Physicochemical Methods of Catalyst Characterization
The concentrations of metals in the test catalysts were
determined by X-ray spectral 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 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.
RESULTS AND DISCUSSION
Physicochemical Properties of the Catalysts
According to the X-ray spectral fluorescence
analysis data, the metal contents of all of the Ni–
Pd/CeZrO2/Al2O3 catalysts obtained were consistent
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 with the calculated values: the Ni content was 10
0.5 wt %, and the Pd contents were 0.05, 0.1, 0.2, 0.5,
and 1.0 wt % depending on the sample.
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 nickel oxide were calculated
from the 2.0.0. diffraction peak broadening of a NiO
phase; the CSR of Ni was found from the 2.0.0. peak
of a nickel phase.
The high-resolution transmission electron micros-
copy (HR TEM) images were obtained on a JEM-
2010 microscope (JEOL, Japan) with a lattice resolu-
tion of 0.14 nm at an accelerating voltage of 200 kV.
The local elemental microanalysis of the samples was
performed by energy-dispersive X-ray (EDX) spec-
trometry on an EDAX spectrometer (EDAX Inc., the
United States) equipped with a Si(Li) detector with an
energy resolution of no less than 130 eV. The samples
for HR TEM were applied to perforated carbon sub-
strates fixed on copper gauzes.
The temperature-programmed reduction with
hydrogen (TPR-H2) was accomplished in a flow reac-
tor (inside diameter, 5 mm) in accordance with a pre-
viously published procedure [29].
The thermal analysis (differential thermal analysis
(DTA), thermogravimetric analysis (TGA), and dif-
ferential thermogravimetric analysis (DTG)) of the
samples was carried out on a NETZSCH STA 449 C
thermal analyzer (NETZSCH Geratebau GmbH,
From the results of investigating the samples by the
low-temperature adsorption of nitrogen, it follows that
the composition of catalysts and the method of their
synthesis do not exert a considerable effect on the tex-
tural characteristics of the samples. The specific sur-
face area of the catalysts calcined at 500°C was 75
10 m2/g, and the pore volume was ~0.3 cm3/g.
An analysis of the diffraction patterns of Ni–
Pd/CeZrO2/Al2O3 showed that the formal cell param-
eter of aluminum oxide remained almost unchanged
(a = 7.90 Å) upon the introduction of an active con-
stituent into the support. The phases of (γ + δ)-Al2O3,
a solid solution based on CeO2 (JSPDS 43-1002) and
ZrO2 (JSPDS 42-1164) related to the support, and the
NiO phase (JSPDS 47-1049) of the active constituent
were present in the composition of the samples. Fur-
thermore, a highly dispersed PdO phase (JSPDS 43-
1024) was observed in the samples with a Pd high con-
tent (1 wt %). The average particle size of nickel oxide
in the Ni–Pd/CeZrO2/Al2O3 test catalysts estimated
according to the Scherrer equation showed that the
CSR size of NiO was 14.0 0.5 nm regardless of their
composition and preparation method. The high dis-
persity of active constituent particles was also retained
after the reduction of Ni–Pd/CeZrO2/Al2O3 in 30%
H2/He at 800°C. In this case, the formation of a Ni
KINETICS AND CATALYSIS Vol. 58 No. 5 2017

604
KERZHENTSEV et al.
0.2157
(a)
(b)
(c)
NiO
1
Ce–Zr–O
5
3
2
0.1487
4
Ce–Zr–O
0.2072
NiO
0.4451
0.1975
0.3519
1
0.2983
1
2
0.1230
0.2459
NiO
Ce–Zr
10 nm
10 nm
10 nm
Fig. 1. HR TEM micrographs of the Ni–Pd/CeZrO /Al O catalysts prepared by the Pd/Ni sequential impregnation method.
2
2 3
Pd contents: (a) 0.05, (b) 0.2, and (c) 0.5 wt %.
metal phase (at a Pd content of <1 wt %) or a Ni–Pd for which the reduction of nickel oxide is predomi-
alloy (at a Pd content of 1 wt %) with a particle size of nantly observed at T > 500°C.
10.0 1.0 nm. The evaluation of the presence of alloy
at a low Pd content is difficult to perform because the
determination of the cell parameter of Ni with a high
accuracy is impossible due to the superposition of the
lines of other phases.
The introduction of palladium changed the profile
of the TPR-H2 curve: a peak at T1 ~ 50–150°C due to
the reduction of palladium cations [1, 33] and a peak
at T2 ~ 400°C related to the reduction of nickel cations
appeared. An increase in the palladium content led to
an increase in the absorption of hydrogen in a tem-
perature range of the second peak with a maximum at
T2 ~ 380–400°C and a decrease in the absorption of
hydrogen in a high-temperature region with a maxi-
mum at T3 ~ 550–570°C (Fig. 2a). The ratio of the
peak areas with maximums at T3 and T2 decreased
from 4.1 to 0.4 as the palladium content of the catalyst
was increased from 0 to 0.5 wt %. This fact is indicative
of a decrease in the fraction of difficult-to-reduce Ni2+
cations in the presence of Pd, apparently, due to the
effect of spillover [11, 36]. The sample with a high
content palladium (Ni–1.0Pd/CeZrO2/Al2O3) in a
temperature range of 300–800°C exhibited one peak
of the absorption of hydrogen with a maximum at T2 =
400°C.
Changes in the hydrogen absorption curve profile
with increasing the Pd content of the sample were
somewhat different upon varying the method of intro-
ducing metal precursors into the support matrix. In
particular, two peaks of the absorption of hydrogen
with maximums at T2 ~ 400°C and T3 ~ 550°C were
detected in a region of 300–800°C for the catalysts
obtained by the Pd/Ni sequential impregnation
method at Pd concentrations of 0.2 wt % (Fig. 2b) and
0.5 wt % [30]. At the same time, in the samples pre-
pared by the joint Pd + Ni and Ni/Pd sequential
impregnation, one peak at T2 ~ 400°C was detected,
and the absorption of hydrogen in a high-temperature
region increased insignificantly. As noted above, the
curve of H2 absorption by Ni–Pd/CeZrO2/Al2O3 syn-
According to the HR TEM data for the Ni–
0.5Pd/CeZrO2/Al2O3 catalysts obtained by both joint
and sequential impregnation methods, the particles of
(Ce,Zr)O2 with a size of 2–10 nm were present on the
surface of aluminum oxide [30]. The Ni-containing
phase is formed by the particles of NiO and NiAl2O4
10–15 nm in diameter. The Pd-containing phase pre-
dominantly occurs in a highly dispersed state and also
in the form of PdO particles of size 2–3 nm. As can be
seen in the micrographs of the Ni–Pd/CeZrO2/Al2O3
samples (Fig. 1), the variation of the concentration of
Pd does not lead to a change in the structure of the cat-
alysts.
For studying the effects of the composition of the
Ni–Pd/CeZrO2/Al2O3 catalyst and its synthesis
method on the reducibility of Nin+ cations, the sam-
ples obtained were investigated by TPR-H2. Figure 2a
shows the TPR-H2 curves for the Ni and Ni–Pd sam-
ples obtained by the Pd/Ni sequential impregnation
method with different concentrations of palladium.
The Ni/CeZrO2/Al2O3 catalyst exhibited a wide peak
with a maximum at 525°C due to the reduction of Ni2+
[11, 32, 33]. According to published data [33–35], the
TPR-H2 curves for the bulk NiO exhibited a peak of
the absorption of hydrogen (T = 350 20°C) or sev-
eral peaks in the case of supported nickel oxide. Their
positions depend on the composition of the support. It
is well known [32] that the reduction of large NiO par-
ticles, which are characterized by weak metal–support
interaction, occurs in a low-temperature region (T < thesized by the Pd/Ni sequential impregnation method
500°C), whereas the reduction of highly dispersed NiO exhibited only one peak at a Pd content of 1.0 wt %.
particles, which are characterized by strong metal–sup- Consequently, at a Pd concentration of <1.0 wt %, the
port interaction, occurs in a high-temperature region
(T > 500°C). Consequently, the strong metal–support
interaction occurs in the Ni/CeZrO2/Al2O3 catalyst,
fraction of difficult-to-reduce Ni2+ cations was higher
in the samples obtained by the Pd/Ni sequential
impregnation method than that in the samples synthe-
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DEVELOPMENT OF A Ni–Pd/CeZrO2/Al2O3 CATALYST
(a)
605
(a)
400°С
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
100
80
60
40
20
0
Pd content,
wt %:
525°С
570°С
0
0.05
0.1
0.2
0.5
1.0
380°С
65°С
80°С
110°С
550°С
810°С
0
200
400
600
800
1000
700
750
800
850
900
950
Temperature, °C
(b)
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
(b)
370°С
Synthesis method:
100
80
60
40
20
0
Pd+Ni
Pd/Ni
Ni/Pd
550°С
65°С
810°С
90°С
110°С
0
200
400
600
800
1000
Temperature, °С
700
750
800
850
900
950
Temperature, °C
Fig. 2. TPR-H curves of the Ni–Pd/CeZrO /Al O samples:
2
2
2 3
Fig. 3. The temperature dependence of (a) the conversion of
effects of (a) the Pd content of the Ni–Pd/CeZrO / Al O cat-
2
2 3
methane and (b) the yield of hydrogen in the presence of the
alyst obtained by the Pd/Ni sequential impregnation method
and (b) the procedure of the preparation of the Ni–
0.2Pd/CeZrO /Al O catalyst on the absorption of hydrogen.
monometallic catalysts of the CH ATR reaction. Sample
4
compositions (j) CeZrO /Al O ; (d) Ni/CeZrO /Al O ;
2
2
3
2
2 3
2 3
(e) 0.05Pd/CeZrO /Al O ; (q) 1Pd/CeZrO /Al O .
2
2 3
2
2
3
2
sequential impregnation) ensured the almost complete
absence of the difficult-to-reduce Ni2+ cations at a lower
Pd content of the catalyst.
sized by the joint or Ni/Pd sequential impregnation
methods.
Thus, based on the study of the Ni–Pd/CeZrO2/
Al2O3 catalysts by a set of physicochemical methods
(N2 adsorption, XRD analysis, HR TEM, and TPR-
H2), we can conclude that the fraction of nickel cat-
ions, which are reduced in a low-temperature region
(350–450°C), increases with the Pd content of the
samples. In this case, the textural characteristics of the
samples and their phase composition did not change.
The exception was provided by the catalysts with high
Pd content (1 wt %), in which the phases of highly dis-
persed PdO were formed. The variation of the method
Activity of the Catalysts in the CH4 ATR Reaction
Figure 3 illustrates the results of investigating the
activity of monometallic catalysts in the CH4 ATR
reaction. In the case of the CeZrO2/Al2O3 support, the
conversion of CH4 weakly depended on reaction tem-
perature (700–950°C), and it was ~40%; the yield of
H2 was no higher than 5% (Fig. 3b), and the main
reaction products were CO2 and H2O. The introduc-
of introducing an active constituent into the support tion of nickel led to a considerable increase in the con-
matrix also makes it possible to regulate the reducibil- version of CH4 and the yield of H2 (Fig. 3). These indi-
ity of the Ni2+ cations. The joint and Ni/Pd sequential ces insignificantly increased with reaction tempera-
impregnation methods (in comparison with the Pd/Ni ture, and they were ~100 and 55%, respectively, at
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KERZHENTSEV et al.
Table 1. Activity of the Ni–Pd/CeZrO2/Al2O3 catalysts in the CH4 ATR reaction at 850°C
Conversion of CH4, %
Yield of H2, %
Н2/СО
Pd content, wt %
Yield of CO, %
Ni + Pd joint impregnation
0.06
0.11
0.20
0.52
1.00
100
97
99
100
100
47
58
57
69
69
56
56
57
60
53
2.5
3.1
2.9
3.3
3.8
Pd/Ni sequential impregnation
0.03
0.08
0.18
0.54
1.00
100
100
100
100
100
55
59
63
75
57
61
61
57
49
2.8
2.8
3.1
3.8
3.9
66
Ni/Pd sequential impregnation
0.05
0.11
0.20
0.53
0.99
100
97
100
100
100
47
50
54
62
64
62
52
62
59
59
2.2
2.8
2.5
2.9
3.1
850°C. The main reaction products were H2, CO, lyst afforded better process characteristics for the ATR
of CH4. The activity of Ni/CeZrO2/Al2O3 at 700–
850°C was higher than or comparable with the activity
of Pd/CeZrO2/Al2O3 at 850–950°C.
CO2, and H2O. In the presence of the Pd/CeZrO2/
Al2O3 catalysts, the temperature dependence of the
conversion of CH4 and the yield of H2 was more pro-
nounced: the process characteristics increased as the
For the bimetallic Ni–Pd catalysts, the tempera-
ATR process characteris-
reaction temperature was increased (Fig. 3). As the ture dependence of the CH4
palladium content of the Pd/CeZrO2/Al2O3 catalyst tics is analogous to that observed for the Ni catalyst:
the conversion of CH4 and the yield of H2 insignifi-
cantly increased with the reaction temperature. In a
wide temperature range (800–900°C), the Ni–
Pd/CeZrO2/Al2O3 catalysts ensured the yield of H2 of
no lower than 50% at a CH4 conversion of >90%. An
increase in the Pd content of the catalyst led to a cer-
tain increase in the yield of H2 (see Table 1); with con-
sideration for the TPR-H2 data, this can be related to
an improvement in the reducibility of nickel cations
and, as a consequence, an increase in the number of Ni0
active centers. This effect was maximally expressed in
the samples obtained by the Pd/Ni sequential impreg-
nation method. In particular, in the yield of H2 at
850°C increased from 55 to 75% as the Pd content was
increased from 0.05 to 0.5 wt % (see the table). The yield
of H2 also depended on the catalyst synthesis method. At
the same chemical composition of the catalysts, the sam-
ples prepared by the Pd/Ni sequential impregnation
method exhibited higher process characteristics (see
Table 1 and Fig. 4).
was increased from 0.05 to 1.0 wt %, the catalyst activ-
ity increased. It should be noted that, as compared
with Pd/CeZrO2/Al2O3, the Ni/CeZrO2/Al2O3 cata-
45
40
35
30
Synthesis method:
Pd + Ni
Pd/Ni
Ni/Pd
25
0.05
0.10
0.20
0.50
1.00
Pd content, wt %
Fig. 4. Effect of the method of the synthesis of the Ni–
We investigated the operational stability of the Ni–
Pd/CeZrO2/Al2O3 catalysts in the CH4 ATR reaction
depending on the composition and method used for
Pd/CeZrO /Al O catalysts on the concentration of
2
2 3
hydrogen in the reaction mixture in the course of the CH
ATR reaction. T = 850°C.
4
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DEVELOPMENT OF A Ni–Pd/CeZrO2/Al2O3 CATALYST
the introduction of an active constituent. Because the
607
(а)
CH4 ATR process characteristics in the presence of the
Pd, Ni (Fig. 3), and Ni–Pd samples reached maxi-
mum values at a reaction temperature of 850°C and
then remained unchanged with a further increase in
the temperature, the long-term tests of the samples
were conducted at T = 850°C. As noted above, in this
case, the catalysts were tested without preliminary
activation in hydrogen. From Fig. 5, it follows that the
catalytic properties of the samples depend on the com-
position of an active constituent. In the case of the
Ni/CeZrO2/Al2O3 catalyst not modified with palla-
dium, the conversion of CH4 was only ~40%, and the
yield of H2 was lower than 3%. The products of the
complete oxidation of methane—CO2 and H2O—were
mainly formed. This fact is indicative of the absence of
NiO reduction under the action of a reaction mixture
under the CH4 ATR reaction conditions. Indeed,
according to the XRD analysis data, the Ni-containing
phase in the Ni/CeZrO2/Al2O3 sample after a 24-h reac-
tion of the ATR of CH4 was a solid solution phase with
the structure of spinel (NiO–Al2O3, a = 7.960 Å). The
preliminary treatment of the Ni/CeZrO2/Al2O3 cata-
lyst in hydrogen (30% H2/He; 800°C; 2 h), which
100
80
60
40
20
Sample compositions:
Ni/CeZrO2/Al2O3
Pd/CeZrO2/Al2O3
Ni1.0Pd/CeZrO2/Al2O3
0
5
10
15
20
Reaction time, h
(b)
100
80
60
40
20
ensured the formation of a Ni0 phase, led to its activa-
tion: the conversion of CH4 increased to 100%, and
the yield of hydrogen increased to 65%. However, the
activity of this sample decreased in the course of time.
In particular, the yield of H2 was 65 or 50% after a 2- or
24-h reaction, respectively. Among the main reasons
for the deactivation of Ni-containing catalysts in the
processes of reforming are the oxidation and sintering
of an active constituent, changes in the phase compo-
sition of the catalyst (for example, the formation of an
inactive phase of nickel aluminate), and the formation
of carbon deposits [11, 14].
Sample compositions:
Ni/CeZrO2/Al2O3
Pd/CeZrO2/Al2O3
Ni1.0Pd/CeZrO2/Al2O3
0
5
10
15
20
Reaction time, h
(c)
100
80
60
40
20
In contrast to the Ni/CeZrO2/Al2O3 sample, the
1.0Pd/CeZrO2/Al2O3 and Ni–1.0Pd/CeZrO2/Al2O3
catalysts ensured the following stable process charac-
teristics: the conversion of CH4 was as high as 100%,
and the yields of hydrogen were 55 and 70%, respec-
tively (Figs. 5a, 5b). Note that preliminary treatment
in hydrogen is not required for these samples. As can be
seen in Fig. 5, at the initial stage of reaction (1–2 h), an
increase in the process characteristics was observed. It
is likely that the formation of a Ni metal or Ni–Pd
alloy phase occurred during this period because of the
reduction of Ni2+ cations under the action of a reac-
tion mixture under the CH4 ATR reaction conditions.
As the Pd content of the Ni–Pd/CeZrO2/Al2O3 cata-
lyst was decreased from 1 to 0.05 wt %, both the cata-
lyst capability for self-activation and the high yields of
hydrogen (~70%) were retained (Fig. 5c). A study of
the phase composition of the Ni–Pd/CeZrO2/Al2O3
samples after a 24-h reaction showed that they con-
tained a Ni–Pd alloy phase whose CSR size was 17
Pd content, wt %
0.05
0.1
0.2
0.5
0
5
10
15
20
Reaction time, h
Fig. 5. Effect of the catalyst composition on the character-
istics of the CH ATR reaction. Preparation method:
4
Pd/Ni sequential impregnation. T = 850°C.
decrease in the fraction of difficult-to-reduce Ni2+
cations upon the introduction of Pd into the composi-
3 nm regardless of the quantity of Pd. Accordingly, a tion of the catalysts, which was found using the TPR-
KINETICS AND CATALYSIS Vol. 58 No. 5 2017

608
KERZHENTSEV et al.
and the formation of carbon deposits, was more pro-
100
80
60
40
20
Conversion of СH4
nounced. In this connection, it is possible to assume that
differences in the activity and operational stability of the
Ni–Pd/CeZrO2/Al2O3 catalysts obtained by different
methods in the ATR of CH4 were caused by the different
structures of bimetallic particles.
The process characteristics reached (see the table
and Fig. 5c) are comparable with or higher than those
described in the literature [11, 21, 37–39]. This fact
allowed us to consider the Ni–0.05Pd/CeZrO2/Al2O3
catalyst as a promising catalytic system for the effective
conversion of methane into hydrogen-containing gas.
Yield of H2
Synthesis method:
,
,
,
Ni + Pd
Pd/Ni
Ni/Pd
0
5
10
15
20
CONCLUSIONS
Reaction time, h
Based on a systematic study of the physicochemical
and catalytic properties of the Ni–Pd/CeZrO2/Al2O3
catalysts depending on the Pd content (0–1 wt %) and
the method of introducing an active constituent into
the support matrix (Ni + Pd joint impregnation,
Pd/Ni sequential impregnation, and Ni/Pd sequential
impregnation), an optimum catalyst composition and
a method of its preparation were determined. The Ni–
0.05Pd/CeZrO2/Al2O3 sample prepared by the Pd/Ni
sequential impregnation method stably operated
under CH4 ATR reaction conditions and afforded a
high yield of hydrogen (~70%) at a methane conver-
sion of 100% and a reaction temperature of 850°C.
Upon the introduction of Pd into the composition of
the Ni/CeZrO2/Al2O3 catalyst, the reducibility of Ni2+
cations was improved; this ensured the ability of Ni–
Pd systems to be activated under the action of a reac-
tion mixture in the CH4 ATR reaction conditions.
Fig. 6. Effect of the method of the preparation of the Ni–
0.5Pd/CeZrO /Al O catalysts on the characteristics of
2
2 3
the CH ATR reaction. T = 850°C.
4
H2 method, facilitated the formation of active centers
immediately under the conditions of the CH4 ATR
reaction; this makes it possible to exclude the stage of
their preliminary treatment in a reducing atmosphere.
The activity of the Ni–Pd/CeZrO2/Al2O3 catalysts
depended on the method of introducing an of active
constituent and increased in the following order: Ni +
Pd < Ni/Pd < Pd/Ni (Fig. 6). The catalysts obtained
by the sequential impregnation method stably oper-
ated under the CH4 ATR reaction conditions. In their
presence, the CH4 ATR process characteristics—the
conversion of CH4, the concentration of H2, and the
yield of H2—were close to equilibrium values: 100, 35–
38, and 58–68%, respectively. In the case of the Ni–
Pd/CeZrO2/Al2O3 catalyst obtained by the joint
impregnation method, the yield of hydrogen
decreased from 60 to 40% for 24 h of reaction.
Changes in the activity of the samples of Ni–
0.5Pd/CeZrO2/Al2O3 prepared by the joint impregna-
tion method and Ni/CeZrO2/Al2O3 with time were
similar. Note that, according to the XRD data, the
Ni–0.5Pd/CeZrO2/Al2O3 catalysts synthesized by dif-
ferent methods were characterized by identical phase
compositions after a 24-h reaction of the ATR of CH4.
According to published data [11], the joint impregna-
tion method ensures the mixing of active constituent
precursors on the molecular level already at the stage
of support impregnation. This led to the formation of
bimetallic particles with a uniform distribution of pal-
ladium and, as a result, caused a lowered surface con-
centration of Pd. In the sequential impregnation
method, bimetallic particles with a high surface con-
centration of palladium (palladium clusters on the
surface of Ni particles) were formed. At a higher sur-
face concentration of Pd, the effect of promotion,
namely, the stability of an active constituent to oxidation
ACKNOWLEDGMENTS
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 character-
ization 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