Combined steam and carbon dioxide reforming of methane over porous nickel based catalysts

Article abstract of DOI:10.1039/c4cy01614a

The phase composition and texture of nickel catalysts supported on a porous nickel ribbon with a MgO underlayer were studied. In the supported reduced catalysts (900 °C, H₂), nickel crystallites were epitaxially bound to MgO. The reaction of combined steam and carbon dioxide reforming of methane to synthesis gas (750 °C, CH₄/CO₂/H₂O/N₂ = 35/23/39/3, GHSV = 62.5 L g^-1 h^-1) was performed in a flow reactor in the presence of the developed catalysts. The catalysts with the MgO underlayer completely covering the nickel ribbon showed stable activity throughout the test period (18 h); after the reaction, no carbon deposits were found in them. The resistance of these catalysts to carbonization was attributed to both the formation of nickel crystallites epitaxially bound to MgO and the formation of the MgO underlayer that covers the nickel ribbon and prevents its contact with the reaction medium. Based on the supported nickel catalysts, a monolithic catalyst was prepared and tested in the developed flow reactor. According to the test data, methane conversion over the monolithic catalyst at a contact time of 0.29 s was not lower than 94% and the synthesis gas yield was 90%; at this contact time, experimental values were close to the calculated equilibrium values.

Full text of DOI:10.1039/c4cy01614a

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Page 1 of 18
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DOI: 10.1039/C4CY01614A
COMBINED STEAM AND CARBON DIOXIDE REFORMING OF
METHANE OVER POROUS NICKEL BASED CATALYSTS
M.M. Danilova1, Z.A. Fedorova1,*, V.A. Kuzmin1, V.I. Zaikovskii1, A.V. Porsin1, T.A. Krieger1,2
Abstract
The phase composition and texture of nickel catalysts supported on porous nickel ribbon with a MgO
underlayer were studied. In the supported reduced catalysts (900ºC, H2), nickel crystallites were epitaxially bound
with MgO. The reaction of combined steam and carbon dioxide reforming of methane to synthesis gas (750ºС,
СН4/СО2/Н2О/N2 = 35/23/39/3, GHSV = 62.5 L/g h) was performed in flow reactor in the presence of the developed
catalysts. The catalysts with the MgO underlayer completely covering the nickel ribbon showed stable activity
throughout the test period (18h); after the reaction no carbon deposits were found in them. The resistance of these
catalysts to carbonization was attributed both to the formation of nickel crystallites epitaxially bound with MgO and
to the formation of the MgO underlayer that covers the nickel ribbon and prevents its contact with the reaction
medium. Based on the supported nickel catalyst the monolithic catalyst was prepared and tested in the developed
flow reactor. According to the test data, methane conversion on the monolithic catalyst at a contact time 0.29 s was
not lower than 94 % and the synthesis gas yield was 90%; at this contact time experimental values are close to the
calculated equilibrium values.
1. Introduction
The combined steam and carbon dioxide reforming (SCDR) of methane to synthesis gas attracts much
attention in the recent years. This process allows to convert two major greenhouse gases in synthesis gas.
In comparison with dry reforming of methane, the addition of steam in the feed for CO2ꢀreforming
reduces the formation of carbon.1ꢀ3 By changing the H2O/CO2 ratio in the reaction feed, the H2/CO
product ratio can be controlled,1ꢀ3 and this makes it possible to produce synthesis gas with the H2/CO ratio
of about 2, which is suitable for the oxoꢀ and FisherꢀTropsch synthesis processes.4,5 The SCDR of
methane enables the production of synthesis gas from such renewable energy sources as biogas, whose
main components are methane, carbon dioxide and water.
1Boreskov Institute of Catalysis SB RAS
2Novosibirsk State University
*Corresponding author. Fax: +738333269402
Eꢀmail address: sabirova@catalysis.ru (Fedorova Z.A.)

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According to the literature available the main direction of development of catalysts for SCDR of
methane is associated with supported nickel catalysts. Nickel catalysts are active and selective in this
process and are less expensive than catalysts containing platinum group metals. The characteristic
drawback of nickel catalysts is their carbonization, which leads to a decrease in their activity.1,3,6ꢀ12
The carbonization of nickel catalysts can be diminished by the addition of promoters, such as calcium
oxide,1,7,8,10 magnesium oxide,3,6,8,11 oxides of rare earth elements,8 noble metals12 and by optimization of
synthesis methods.13,14
The implementation of the endothermic reactions of SCDR of methane requires an intensive heat
transfer from an external source into the reaction zone and the equalization of the temperature inside the
catalyst bed. It means that the catalysts for these reactions must have high thermal conductivity, which
can be imparted by the use of metallic supports.15,16 A number of catalysts on such supports were tested in
the steam reforming of methane: nickel composite catalyst reinforced with a stainless steel gauze,17,18
a
nickel catalyst supported onto an oxide underlayer deposited on porous nickel,19 a composite nickel
catalyst obtained by the capsulation of lanthanum nickelate particles in metalꢀceramics,20 a nickel catalyst
supported on a porous nickel plate with an underlayer of MgO.21
Analysis of published data shows that the use of MgO as an underlayer for the nickel catalysts is very
promising: it has high thermal stability, reduces carbonization because of basic properties, and it easily
forms solid solutions with NiO owing to the similarity of the structural parameters, which facilitates the
formation of dispersed crystallites of reduced nickel.5,22 A number of supported nickel catalysts with a
MgO underlayer have been studied: nickel catalysts supported on metal foams,23 porous nickel plate,21 αꢀ
Al2O3,6 and Al2O3ꢀSiO2.8,9
The aim of the current investigation was to elucidate the catalytic properties of nickel catalysts
supported on porous nickel ribbon with a MgO underlayer including monolithic catalyst in the SCDR of
methane to synthesis gas.
2. Experimental
2.1. Catalyst preparation
The supported nickel catalysts were prepared on a 0.1 mm thick porous nickel ribbon (pNirb)
obtained by rolling of a nickel powder followed by calcination at 1200°С in a Н2 flow.24 The ribbon
nickel support had a specific surface area of 0.15 m2/g and the total pore volume of ca. 0.1 cm3/g. The
MgO underlayer was prepared by impregnating the nickel ribbon support with a Mg(NO3)2 solution
followed by drying and then by calcination at 550°С in flowing H2; the support I contained 6.0 wt.%
MgO, the support II – 10.3 wt.% MgO. The decomposition of Mg(NO3)2 in air is a usual method for the
preparation of MgO support,6,9,23,25 but in the case of a nickel support, it can lead to loosening of the
nickel surface.26,27 Therefore, to reduce the loosening of the nickel ribbon surface, decomposition of

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Mg(NO3) was carried out in a H2 flow. Catalysts I and II were prepared by supporting of nickel on
supports I and II via their impregnation with a mixture of Ni(NO3)2 and Mg(NO3)2 solutions. After
impregnation, all the catalysts were dried and then calcined at 450°С in a N2 flow. The prepared catalysts
were reduced in flowing Н2 at 900°С for 1 h. The content of supported nickel in the reduced catalysts was
1ꢀ4 wt. %; the content of MgO in catalysts was 7ꢀ12 wt.%.
2.2. Characterization
The specific surface area and the pore radius distribution were determined by lowꢀtemperature
nitrogen adsorption at 77 K using an АSАР–2400 (Micromeritics, USA) precision instrument. The error
of the evaluation of the specific surface area did not exceed 5 rel.%. For some samples, the total pore
volume and the pore radius distribution were determined using mercury porosimetry. Particle morphology
was examined by scanning electron microscopy (SEM) and by transition electron microscopy (TEМ) with
the respective use of JSMꢀ6460LV and JEMꢀ2010 (JEOL Co., Japan) instruments with a lattice resolution
of 0.14 nm. The energy dispersive Xꢀray microanalysis (EDX) of some samples was performed using an
EDAX energy dispersive spectrometer with a Si(Li) detector with an energy resolution of 130 eV. Crystal
faces were indexed by high resolution TEM (HRTEM) of lattice fringe images. Phase analysis was
carried out using Xꢀray diffraction patterns taken with an ARLX´TRACT (Thermo, Switzerland)
instrument (monochromatized CuKα radiation).
2.3. Catalytic activity
Catalyst activity in the SCDR of methane was evaluated by the flow method under atmospheric
pressure at 750°С using a conventional continuous flow quartz reactor with an inner diameter of 14 mm.
The initial reaction mixture comprised of 35 vol.% of СН4, 23 vol.% of CO2, 39 vol.% of H2O, and 3
vol.% of N2. According to,1ꢀ3 such feed composition allows a production of synthesis gas with the H2/CO
ratio of about 2. The feed rate of the initial mixture was 25.0 L/h. For the catalytic activity measurements,
ribbon catalysts were cut to pieces (1.5mm×1.5mm×0.1mm). Catalyst samples weighing 0.40 g were used
in the tests. A commercial catalyst for steam reforming of methane NIAPꢀ18 (10.5 wt.%Ni/Al2O3)28 was
used for the comparison. The catalyst sample weighing 0.40 g was tested in the form of a 0.25–0.50 mm
fraction. Before the catalytic activity measurements, the samples were reduced in situ at 750°С in flowing
Н2 for 1 h. Then hydrogen was substituted for the reaction mixture and measurements were taken after
keeping the catalysts in the reaction mixture for 1 h. The catalyst bed temperature was controlled with a
thermocouple mounted in the reactor at the level with the catalyst bed. The temperature evaluation error
was ± 2°C. The composition of reaction mixtures was analyzed chromatographically: separation of
oxygen, nitrogen and methane was performed using a column with NaX at room temperature, a heat
conductivity detector, helium as a carrier gas; the concentrations of methane, carbon oxide, carbon

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dioxide and hydrogen were measured on a chromatograph equipped with a heat conductivity detector and
a temperatureꢀprogrammed column (in a temperature range of 50ꢀ200°C) filled with activated carbon,
argon served as a carrier gas; hydrocarbons were separated on a chromatograph equipped with a flameꢀ
ionization detector and a temperatureꢀprogrammed column (in a temperature range of 50ꢀ200°C) filled
with alumina, hydrogen as a carrier gas. The error of the chromatographic measurements was less than
10%.
Catalytic activity measurements in the empty reactor showed that the methane conversion in this
case was less than 1%.
The supported nickel catalyst 4.0% Ni/(rpNi + 10.4% MgO) (I) was used to prepare a monolithic
catalyst in the form of a rectangular block with the crossꢀsection of 100 × 6 mm2 and a height of 70 mm.
The monolithic catalyst consisted of alternating corrugated (corrugation height of 1.8 mm, the base of the
triangular channel of 4.0 mm) and smooth ribbons with the supported nickel catalyst.
The SCDR of methane to synthesis gas was performed in the developed flow reactor (Fig.1),
which consisted of an evaporator 1 (crossꢀsection of 20×90 mm2, height of 70 mm), a reaction zone with
the monolithic catalyst 2, and a gas burner 3 mounted successively in one housing. The monolithic
catalyst was mounted in the center of the reaction zone. The fuel mixture (natural gas and air) was
supplied to the burner (d = 100 mm) through a tube 4 (d = 20 mm) with several rows of holes (d = 2 mm).
An additional flow of air was fed tangentially through the tube 5 (d = 8 mm), which facilitated the
combustion. Ignition of the gas mixture was provided with an electroꢀspark between the tube 4 and
electrode 6. Combustion products were directed to heat the reaction zone. The temperatures of the heating
gases and the monolithic catalyst were measured using chromelꢀalumel thermocouples 7,8,9. The
temperature of the heating gases measured before and after the reactor was 1070ꢀ1125°C and 860ꢀ930°C,
respectively. The evaporator 1 contained plates (shelves) mounted at 10ꢀmm intervals. Water entered the
evaporator through a capillary 10 (d = 1 mm). The capillary was mounted inside a tube 11 (d = 5 mm)
through which carbon dioxide was supplied. This design protected the capillary from the hot gases exiting
the burner and prevented water from boiling, which could lead to a nonꢀuniformity of the flow. After the
evaporator, the mixture of water and carbon dioxide (12), with admixed methane 13, was fed into the
reactor through a tube 14 (d = 5 mm). The tube contained holes (d = 1 mm) located at the opposite sides
of the catalyst unit, which provided a better distribution of the reaction mixture over the reactor crossꢀ
section. The flows of gases and water were measured and adjusted with flow mass meters. The
composition of reaction products was analyzed with a gas analyzer.
3. Results and Discussion
3.1. Phase composition and texture of initial catalysts

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Xꢀray diffraction data showed that the metallic support contains only the nickel phase. Supports I
and II contain phases of nickel and MgO with a = 4.219 Å (Table 1). One can assume that the obtained
lattice parameter of MgO was higher than that given in29 because of a lower degree of crystallinity and a
higher degree of defects, which resulted from the formation of MgO in a hydrogen flow. In the reduced
catalysts with supported nickel, the diffraction peaks correspond to only one oxide phase and nickel.
Diffraction peaks of the oxide phases are very weak compared to the diffraction peaks of nickel and have
an interjacent position between the peaks of pure oxides and are shifted to the diffraction peaks of MgO
(Fig.2). According to,31 this diffraction pattern indicates the formation of a solid solution of NiO in MgO.
The formation of NiO in MgO solid solution at low calcination temperature, 500°C32 and 400°C33 was
shown by Xꢀray diffraction in32,33. It is believed that the heating of supports I and II impregnated with a
mixture of Ni(NO3)2 and Mg(NO3)2 solutions in N2 flow at 450°C yields the dispersed solid solution of
NiO in MgO. The presence of a phase of the solid solution after reduction treatment (900ºC, H2) can be
explained by hindered reduction of Ni2+ cations embedded into the lattice of hardly reducible MgO (the
heat of formation of MgO is higher than that of NiO34). An increase in the temperature of NiO reduction
after the formation of its solid solution in MgO is shown in.9,22,35
Table 1 Unit cell parameters of oxide phases in the samples.
1
Catalyst
aMgO (Å) aNi
(Å)
a
NiO (Å)
ꢀ
X Mg1− X
O
pNirb + 6.0% MgO (I)
4.219
ꢀ
pNirb + 10.3% MgO (II)
1.0% Ni/(pNirb + 6.7% MgO) (I)
2.7% Ni/(pNirb + 8.6% MgO) (I)
4.0% Ni/(pNirb + 10.4% MgO) (I)
2.3% Ni/(pNirb + 12.2% MgO) (II)
MgO
4.219
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
4.210
4.208
4.208
4.209
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
4.2112
ꢀ
ꢀ
NiO
ꢀ
4.1773
1NiXMg1ꢀXO – assumed solid solution of NiO in MgO. 2 According to Ref.29
3 According to Ref.30
The ribbon nickel support has a macroporous structure with a low total pore volume: according to
the mercury porosimetry data, most pores have the radius from 5 to 60 µm. The macroporous nature of
the support is confirmed by SEM data: the support has a corpuscular structure formed by rounded nickel
particles with a size of 3ꢀ30 µm fused at the contact points.

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According to SEM data, the MgO underlayer forms a loose porous layer with a thickness of 2ꢀ3
µm. According to the lowꢀtemperature nitrogen adsorption data, the specific surface area of support I is
7.5 m2/g and of support II is 11 m2/g (Table 2). The MgO underlayer in supports I and II has fine pores
with an average diameter of 4ꢀ6 nm.
Table 2 Texture parameters of the catalysts.
Catalyst
Ssq, m2/g
7.5
DNi, nm2
ꢀ
pNirb + 6.0% MgO (I)1
pNirb + 10.3% MgO (II)1
11
1.7
ꢀ
1.0% Ni/(pNirb + 6.7% MgO) (I)
2.7% Ni/(pNirb+ 8.6% MgO) (I)
3ꢀ5
3ꢀ5
3ꢀ7
3ꢀ5
3ꢀ5
1.6
4.0% Ni/(pNirb + 10.4% MgO) (I) 1.9
1.2% Ni/(pNirb + 11.3% MgO) (II) 3.0
2.3% Ni/(pNirb + 12.2% MgO) (II)
ꢀ
1The samples were reduced at 750°C, H2
2Dominant nickel particle size on the MgO surface according to
TEM data.
As follows from the TEM images obtained, the MgO underlayer in supports I and II consists of
aggregates 50ꢀ300 nm in size formed by primary 2ꢀ10ꢀnm particles. The MgO underlayer does not
contain separate nickel particles except for the sparse large (100 nm) particles probably detached from the
nickel support during the sample preparation. Nevertheless, the EDXꢀspectra of MgO in supports I and II
show Ni in the form of weak spectral lines corresponding to the Ni content not exceeding 2 at.% relative
to Mg. The intensities of the nickel lines in the EDX spectra from different areas of the MgO surface are
different, which indicates that during calcination, MgO nonꢀuniformly interacts with the NiO film.
Probably, the adjoined particles of MgO and NiO (Fig. 3) react to form a dispersed mixed oxide, whereas
the particles of MgO that are not in contact with the surface of nickel ribbon do not contain dissolved
nickel. The possibility of interaction of NiO with MgO during heating in hydrogen at temperatures of
250ꢀ400°C is shown in,37 where the authors assumed that the diffusion of Ni2+ cations into the surface
layer of MgO proceeds in parallel with their reduction. The same assumption can be made on the basis of
the data in.38 In addition, the calcination of the impregnated nickel ribbon leads to the decomposition of
Mg(NO3)2 with the formation of nitrogen oxides, which are able to oxidize some parts of the nickel
ribbon surface.

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Impregnation of the nickel ribbon with a weak acid solution of Mg(NO3)2 (pH = 5.8), with the
subsequent drying and calcination, may be accompanied by corrosion of nickel with the transition of Ni2+
cations into solution. These cations can be incorporated into the bulk of MgO particles.
TEM data indicate that in reduced catalysts, crystallites of supported nickel are dispersal (3ꢀ7 nm)
(Table 2) and epitaxially bound with MgO36,39 (Fig. 4): the image of nickel particles exhibits a moire
fringe pattern in the form of parallel fringes with a period of about 1 nm as a result of lattice spacing d111
of Ni and MgO superposition.40 In catalysts, epitaxially unbound crystallites of supported nickel are very
rare; these crystallites are oxidized and their surface contains a 1ꢀ2ꢀnm layer of NiO. In catalysts, the
epitaxially bound highly dispersed nickel crystallites less than 7 nm do not have the oxide layer after
exposure to air.21,36,39
It is believed that in the studied catalysts, epitaxial binding of nickel crystallites with MgO is due to
the formation of nickel crystallites from a solid solution of NiXMg1ꢀXО and due to their high dispersion.
3.2. Catalytic activity
Figure 5 shows the activity of the prepared catalysts in the SCDR of methane. The conversion of
methane over the initial nickel ribbon was 7% and decreased to 0.5% during the reaction, apparently
because of carbonization. The supporting of the MgO underlayer on the initial nickel ribbon increases the
conversion of methane to 56% (support I). The increase in conversion can be attributed to the interaction
of NiO and MgO with the formation of the dispersed mixed oxide, from which dispersed nickel
crystallites are formed under the influence of the reaction medium.21The increase in activity is also
facilitated by loosening of the nickel ribbon surface during the heating in the presence of nitrogen oxides,
which are formed by the decomposition of Mg(NO3)2, and subsequent reduction in hydrogen.26,27 During
the reaction, the conversion on support I decreases; this can be caused by sintering of the loosened nickel
ribbon and by its carbonization. As shown in Figure 3, in support I some parts of the nickel ribbon
surface do not contact with the particles of MgO. It is known41 that sintering of the particles by the
surface diffusion mechanism starts at a temperature (the soꢀcalled Tamman temperature) which is much
lower than the conventional melting point of a bulk material; this is caused by defectness of the nearꢀ
surface layers of a solid. It is expected that under the influence of the reaction medium at 750°C, the parts
of nickel ribbon in support I that are not covered with magnesium oxide undergo sintering by the surface
diffusion mechanism.
Carbonization of nickel catalysts supported on αꢀAl2O3 and Al2O3ꢀSiO2 with a magnesium oxide
underlayer was observed in SCDR of methane.6,8,9 This suggests that the action of reaction medium on the
regions of nickel ribbon in support I that are not covered with MgO leads to their carbonization.
The supporting of Ni (1.0 and 2.7 wt.%) onto the support I decreases its catalytic activity (Fig. 5);
this can be explained as follow: upon impregnation of the support I with the aqueous solutions of nitrates

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a substantial part of MgO is rehydrated;39,40 earlier we have shown21 that thermal treatment of nickel
ribbon impregnated with a Mg(NO3)2 solution in N2 flow at 550°C yields the MgO with a smaller specific
surface area compared to its annealing in hydrogen; in the supported catalysts (1.0% Ni/(pNirb + 6.7%
MgO) (I) and 2.7% Ni/(pNirb + 8.6% MgO) (I)) the supported nickel particles have the larger
predominant size and the smaller surface area of metallic nickel than those in the support I.
In the supported catalysts with the increase in the content of supported nickel, the catalyst activity
increases, which may result from an increase in the surface area of metallic nickel per unit weight of the
catalyst (Fig. 5).
Figure 5 shows the effect of test duration on the conversion of methane. The decrease in the
activity of catalyst I containing 1.0 % Ni/pNirb + 6.7 % MgO was about 20%. For comparison, Fig. 5
depicts the activity of the latter catalyst but reduced at 750ºC. The decrease in the activity of the catalysts
reduced at 750 and 900ºC was comparable. The activity of catalysts I with high content of MgO (8.6 and
10.4 wt.%) and catalysts II remained stable throughout the test period.
The H2/CO ratio in the reaction products over supported catalysts I with low content of MgO (6.7
вес.%) was 2.7ꢀ2.8, whereas for catalysts I with high content of MgO and catalysts II, this ratio was 2.1ꢀ
2.2 and almost did not change during the reaction. An increased H2/CO ratio (2.7ꢀ2.8) in the reaction
products in the case of catalyst I with a low content of MgO (1.0% Ni/(pNirb + 6.7% MgO) (I)) may be
caused by carbonization of some parts of the nickel ribbon surface which can be free from the MgO
underlayer and a decrease in CO content in the reaction products.
For comparison, Figure 5 presents the effect of test duration on the conversion of methane over a
commercial catalyst NIAPꢀ18. These results show that supported catalysts I with high content of MgO
and catalysts II on the highly thermally conductive support are comparable in activity with the catalyst
NIAPꢀ18 and surpass it in stability under conditions of SCDR of methane.
Table 3 lists the test data for the monolithic catalyst in SCDR of methane to synthesis gas
performed in the developed flow reactor.
Table 3 Parameters of SCDR of methane to synthesis gas for the monolithic catalyst.
Flow rate, L/min
CH4 H2O CO2
τ, s
Product composition, %
CH4 CO2 CO H2
CH4 conversion, %
T cat., °С
1
2
3
Т7
T8
T9

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3.0 3.53 1.95 0.29 657 697 806
1.4 7.9 32 59
94
experimental
0.27 6.95 31.5 61.3
98.9
equilibrium4
experimental
experimental
3.0 3.70 1.95
4.50 2.90 5.20
0.29
0.20
672 715
696 742
816
809
1.3 7.7
3.7 11
32
60
95
86
29 57
0.28 6.80 31.7 61.2
98.8
equilibrium4
1
Thermocouple is mounted in the monolithic catalyst at a distance of 5 mm from the reaction
2
mixture inlet. Thermocouple is mounted in the monolithic catalyst at a distance of 15 mm from the
reaction mixture inlet.3 Thermocouple is mounted in the monolithic catalyst at a distance of 5 mm from
the reaction mixture outlet.4 The equilibrium values of methane conversion and synthesis gas yield
correspond to the temperature of monolithic catalyst at the reactor outlet. Designations of thermocouples
in Table 3 are in accord with Figure 1.
According to the test data, methane conversion on the monolithic catalyst at a contact time 0.29 s
was not lower than 94 % and the synthesis gas yield was 90%; at this contact time experimental values
are close to the calculated equilibrium values. The H2/CO ratio in the reaction products over monolithic
catalyst was close to 1,9. These results indicate that the monolithic catalyst based on 4.0% Ni/(pNirb +
10.4% MgO) (I) is an effective catalyst for the SCDR of methane to synthesis gas.
In studies44ꢀ47 the structured catalysts based on metallic supports with oxide undelayer were used
for the synthesis gas production by methane and liquefied fuels conversion. Noble metals,44ꢀ47 nickel45 and
nickel doped with noble metals44,45 were used as the active component. The active component containing
mixed oxides of La, Ce, Zr and nickel doped with noble metals is considered as the most promising one
for the development of monolithic catalysts for reforming of methane and liquefied fuels to synthesis
gas.44,48
The test data in the reaction of partial oxidation of methane to synthesis gas for the monolithic
catalyst based on 3.0% Ni/(rpNi + 5.0% MgO), whose composition is close to the catalysts studied in our
paper, are presented in ref.39. The obtained values of methane conversion and synthesis gas yield are
comparable with those reported for the same range of contact time and composition of the reaction
mixture for the Ptꢀpromoted Niꢀcontaining active component supported on Fecralloy monolithic
substrate.49

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3.3. Texture of catalysts after tests in SCDR of methane
According to TEM data after the tests in catalysts I and II a small number of enlarged nickel
crystallites (20ꢀ40 nm) appear, but the most of the supported nickel crystallites remain in the stable state
of dispersed crystallites epitaxially bound with MgO. Some of these crystallites do not have the oxide
film on their surface. Besides, there appear highly dispersed nickel crystallites (2ꢀ3 nm) epitaxially bound
with MgO. In catalysts I with the high content of MgO, on large nickel particles (~ 100 nm) detached
from the nickel support, there appear nickel crystallites of 5ꢀ10 nm in size epitaxially bound with MgO
probably containing NiO (Fig. 6). In initial catalysts I with high content of MgO these crystallites were
not found. These results indicate that in catalysts I with high content of MgO under the influence of the
reaction medium, there proceeds a reduction of nickel oxide from the hardly reducible solid solution NiOꢀ
MgO, which is formed on the surface of the nickel ribbon.
In the supported catalysts I and II the formation of dispersed solid solution and mixed oxide is
facilitated both by the supporting of nickel from a mixture of Ni(NO3)2 and Mg(NO2)2 solutions and by
treatment in hydrogen at 900°C, which increases the diffusion mobility of cations and intensifies the
incorporation of Ni2+ cations into the lattice of MgO and Mg2+ cations into the lattice of NiO; the
reduction of NiO from the solid solution and mixed oxide leads to the formation of dispersed nickel
crystallites; unreduced part of the solid solution and mixed oxide may disperse the nickel particles.
The treatment under reaction conditions of catalyst I with low content of MgO (1.0 wt.%
Ni/pNirb+6.7% MgO) does not result in the formation of carbon on dispersed nickel crystallites
epitaxially bound with MgO. However, the islet formation of graphiteꢀlike carbon was observed on the
large particles of nickel (~ 100 nm) detached from the nickel support. It is known22,31,50 that large nickel
particles are more prone to carbonization than small ones. As shown in Figure 3, in support I some parts
of the nickel ribbon surface do not contact the particles of MgO. Because the content of MgO in catalyst I
(1.0 wt.% Ni/pNirb+6.7% MgO) was increased insignificantly compared with the support I, one can
suppose that some parts of the nickel support surface can be free from the MgO underlayer. When
exposed to the reaction medium, these parts can be carbonized. Thus, the observed decrease in the activity
of catalyst I with low content of MgO can be attributed to the carbonization of large crystallites of the
nickel ribbon that are not covered with MgO.
Catalysts I with high content of MgO and catalysts II exhibited stable activity and contained no
carbon deposits after reaction. Their resistance to carbonization is possibly a result of the formation of
dispersed nickel crystallites epitaxially bound with MgO, as well as result of the formation of the MgO
underlayer, which covers completely the nickel ribbon and prevents the contact of the reaction medium
with large nickel crystallites and thus prevents their carbonization. It is known1,3,6ꢀ10 that during the SCDR
of methane, nickel catalysts are inclined to carbonization. TEM investigation of our catalysts showed that
no carbon is deposited on nickel crystallites epitaxially bound with MgO. In studied catalysts, a

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considerable part of supported nickel crystallites is formed during the reduction of the solid solution of
NiO in MgO (Table 1). It can be supposed that there is the interaction of these crystallites with MgO. The
interaction of dispersed nickel crystallites with MgO can be indicated by their epitaxial binding and by
the absence of the oxide layer on their surface. In this case, the catalytic properties of the catalyst are no
longer determined only by supported metal, but refer to a unified system “metalꢀsupport” and depend on
the nature of the support and the nature of its interaction with the metal.51,52 It is believed that the epitaxial
binding of nickel with MgO leads to a change in the catalyst properties and suppresses its activity in the
reactions leading to the formation of carbon.53
An increased resistance to carbonization of the catalysts in the case when the precursor of the
active component is a solid solution of NiO in MgO is shown in.5,31,35,54 It is supposed that the carbon
deposition is suppressed due to the formation of highly dispersed crystallites of nickel that are in
interaction with MgO.5,28 According to,55 the formation of the partially reducible solid solution of NiO in
MgO increases the stability of Ni–Ni bonds on the reduced surfaces; this higher surface stability inhibits
carbon diffusion into the nickel lattice, and thus prevents the formation of carbon. Resistance to
carbonization may also increase because of the basic properties of the surface of MgO: owing to the
presence of labile hydroxyl groups56ꢀ58 and activated molecules of CO2,59 which oxidize hydrocarbon
fragments.
4. Conclusion
In the nickel catalysts supported on a porous nickel ribbon with the MgO underlayer, nickel
crystallites are epitaxially bound with MgO and some of the dispersed nickel crystallites do not have an
oxide film on their surface. It is believed that the epitaxial binding of nickel with MgO leads to a change
the catalyst properties and suppresses its activity in the reactions leading to the formation of carbon.
When tested in the SCDR of methane, the catalysts with the MgO underlayer completely covering the
nickel ribbon exhibit a stable activity; no carbon deposits are found in them.
The formation of dispersed nickel crystallites epitaxially bound with MgO and the formation of
the MgO underlayer, which covers the nickel ribbon, prevents its contact with the reaction medium and
thus prevents its carbonization, can determine the stability of these catalysts.
Acknowledgements
The authors thank V.A.Kirillov, N.A.Kuzin, Yu.I.Amosov, A.B.Shigarov, N.F.Saputina for help in this
work. This work was supported by the Russian Foundation of Basic Research (grant 15ꢀ03ꢀ04001).

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Fig.1 Scheme of reactor for the monolithic nickel catalyst testing in SCDR of methane to synthesis gas: 1
– evaporator, 2 – monolithic catalyst, 3 – gas burner, 4 – fuel mixture inlet, 5 – additional flow air inlet, 6
– electrode, 7,8,9 – thermocouples mounted in monolithic catalyst, 10 – water inlet, 11 – carbon dioxide
inlet, 12 – steam and carbon dioxide mixture inlet, 13 – methane inlet, 14 – reaction mixture inlet, 15 –
reaction product outlet.
Ni
(111)
(220)
MgO
I
NiO
*
Ni
(200)
1
2
3
4
Ni
(220)
61,5 62,0 62,5 63,0 63,5
*
I
*
I
*
I
1
2
3
4
30
40
50
60
70
80
2 Theta, grad
Fig.2. XRD pattern of samples: 1 ꢀ 2.3% Ni/(pNirb + 12.2% MgO) (II), 900ºC, H2; 2 – 4.0% Ni/(pNirb +
10.4% MgO) (I), 900ºC, H2; 3 – 2.7%Ni/ (pNirb + 8.6% MgO) (I), 900ºC, H2; 4 – pNirb + 6.0% MgO (I),
550ºC, H2. The insert – fragment of XRD patterns of samples. Denoted as (|) and (*) location of
diffraction peaks of MgO and NiO, respectively.

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Fig. 3. HRTEM image of the nickel ribbon with MgO underlayer (support I), 550°С, Н2.36
Fig.4. HRTEM image of the catalyst 2.7 % Ni/(pNirb+8.6% MgO) (I), 900°С, Н2 (1 ꢀ dispersed nickel
crystallites epitaxially bound with MgO).

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Fig.5. Methane conversion versus timeꢀonꢀstream in the steamꢀCO2 reforming of methane to
synthesis gas over the catalysts reduced in H2 (T = 750°С, р = 1 аtm, СН4/СО2/Н2О/N2 = 35/23/39/3,
weight of catalyst: 0,40 g).

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Fig.6. HRTEM image of the catalyst 4.0% Ni/(pNirb + 10.4% MgO) (I) tested in the steamꢀCO2
reforming of methane (1 ꢀ dispersed nickel crystallites epitaxially bound with MgO).