Effect of interactions between components in nickel-silica catalysts on the yield of carbon in methane decomposition

Article abstract of DOI:10.1023/A:1023364714847

A number of 90% Ni-10% SiO₂ catalysts for methane decomposition were studied at different stages of preparation and operation in the reaction using X-ray diffraction analysis, differential dissolution, temperature-programmed reduction, IR spectroscopy, and high-resolution electron microscopy. It was found that an increase in the interaction between components in the catalytic system decreased the ability of nickel to accumulate carbon in the decomposition of methane.

Full text of DOI:10.1023/A:1023364714847

Kinetics and Catalysis, Vol. 44, No. 2, 2003, pp. 268–276. Translated from Kinetika i Kataliz, Vol. 44, No. 2, 2003, pp. 290–299.
Original Russian Text Copyright © 2003 by Ermakova, Ermakov.
Effect of Interactions between Components in Nickel–Silica
Catalysts on the Yield of Carbon in Methane Decomposition
M. A. Ermakova and D. Yu. Ermakov
Boreskov Institute of Catalysis, Siberian Division, Russian Academy of Sciences, Novosibirsk, 630090 Russia
Received January 28, 2002
Abstract—A number of 90% Ni–10% SiO2 catalysts for methane decomposition were studied at different
stages of preparation and operation in the reaction using X-ray diffraction analysis, differential dissolution,
temperature-programmed reduction, IR spectroscopy, and high-resolution electron microscopy. It was found
that an increase in the interaction between components in the catalytic system decreased the ability of nickel to
accumulate carbon in the decomposition of methane.
INTRODUCTION
should be used as oxide additives that are difficult to
reduce to the active component. Previously [8, 9], it was
reported that carbon yields as high as 300–386 (g C)/(g Ni)
were obtained with the use of nickel–silica catalysts
with extremely high nickel contents in the reaction of
methane decomposition; the catalysts were prepared
using heterophase sol–gel synthesis. With the use of
catalysts in which nickel was combined with other
widespread supports such as alumina, zirconia, magne-
sia, and titania, the yield of carbon was much lower
than that in the case of a Ni–SiO2 sample, even though
the nickel concentrations were equal and the average
sizes of nickel crystallites were similar in these samples
[9]. We assumed that the interaction between catalyst
components (nickel–refractory oxide) affects catalytic
properties in hydrocarbon decomposition reactions.
Methane is the most widespread gaseous hydrocar-
bon and the main component of natural gas; therefore,
reactions with the participation of methane are of great
practical importance. Methane decomposition is one of
these reactions. In the presence of catalysts (iron group
metals), methane decomposes into molecular hydrogen
and filamentous carbon. Hydrogen obtained in this
manner is free of carbon monoxide. This is important
when the presence of CO is undesirable or even inad-
missible, for example, in the use of hydrogen as fuel for
electrocatalytic cells, because the presence of CO
results in catalyst poisoning [1, 2]. If hydrogen is not
separated from methane, the mixture of these gases is a
more efficient fuel for internal-combustion engines and
gas-turbine power plants than natural gas or oil gas.
Previously, Baker and Chludzinski [10] studied the
effects of various additives to Ni–Fe powders on the
growth of carbon nanofibers in the decomposition of
acetylene. They hypothesized that some materials (for
example, Al2O3) can produce a physical barrier to
hydrocarbon adsorption; other materials (for example,
MoO3) can decrease the solubility of carbon in the
metal, but they have no effect on its diffusion; and
materials of the third kind (SiO2) can decrease both the
solubility of carbon and its diffusion through a catalyst
particle. However, to explain differences in the behav-
ior of catalysts in the accumulation of filamentous car-
bon, it is most likely that not only the impurity material
but also the preparation procedure, which is responsible
for the types of interactions between components of the
catalytic system, should be taken into consideration.
The second reaction product, carbon that consists of
nanofibers, is of interest as a highly pure graphitized
carbon material that can potentially be used in different
branches of industry. Carbon fibers grow because car-
bon atoms, which are formed in the dissociative chemi-
sorption of hydrocarbons on particular faces of a cata-
lytic metal particle, diffuse to opposite faces and crys-
tallize on them as continuous graphite-like structures
[3, 4]. The rate of carbon deposition is controlled by the
isothermal diffusion of carbon through a metal particle
[5, 6]. Catalyst deactivation results from the blocking of
the active surface by coke; however, the cause of this
remains unclear.
Nickel is the most active catalyst for the decomposi-
tion of hydrocarbons, including methane. However,
nickel crystallites of size greater than 100–150 nm can-
not produce nanofibers; they become encrusted with
Thus, an understanding of the interactions between
carbon, which isolates them from the reaction medium. catalytic system components provides an opportunity
For this reason, nickel blacks and powders, which are to reasonably select an optimum catalyst for the process
prone to agglomeration in a hydrocarbon medium, do of methane decomposition. In this work, we continue a
not produce carbon fibers and exhibit zero conversion study of the structure and properties of previously pro-
of methane [7]. Although methane decomposition does posed promising Ni–SiO2 catalysts [8, 11] from the
not require high dispersity of a catalyst, promoters standpoint of the effect of the interaction between
0023-1584/03/4402-0268$25.00 © 2003 MAIK “Nauka /Interperiodica”

EFFECT OF INTERACTIONS BETWEEN COMPONENTS IN NICKEL–SILICA CATALYSTS
269
nickel and silica on the time of catalyst deactivation in
The IR spectra were measured on a BOMEM MB-102
the reaction and hence on the yield of carbon in meth- instrument (21 scans) with a resolution of 4 cm–1 at
ane decomposition.
EXPERIMENTAL
room temperature. The samples were ground with KBr
in the proportion of 1.5 mg to 500 mg of KBr.
The electron-microscopic studies of the samples
were performed with the use of a JEM-2010 transmis-
sion electron microscope.
Highly dispersed nickel hydroxide was prepared by
hydrolysis of the ammonia complex obtained by dis-
The texture characteristics of the samples of nickel
solving nickel nitrate in an aqueous ammonia solution. oxide were determined by a precision static method
Next, the resulting precipitate was filtered off and from the full isotherms of low-temperature (at 77 K)
washed several times with distilled water to remove nitrogen adsorption with the use of an ASAP-2400
ammonium nitrate. After drying in air at 110°ë, the automated analyzer (Micrometrics). The calculation
specific surface area of nickel hydroxide was 460 m2/g. was performed by the BET method.
Hydrated nickel oxide was prepared by the calcination
The differential dissolution method [12] was used to
of the hydroxide in a flow of air at 250°ë. The specific
study the phase composition of the samples and the
surface area of NiO · H2O was 400 m2/g, and the coher-
ent-scattering region was no greater than 3–4 nm in
interactions between components. The method is based
on the regularities of stoichiography and the dynamic
accordance with X-ray diffraction data. The texture of
mode of dissolution. In this method, the criterion for the
nickel oxide (specific surface area and pore volume)
separation of individual phases is the time profile of
was changed by calcination for 1 h at a certain temper-
stoichiograms—the molar ratio between elements that
ature within the range 350–800°C.
pass into solution as a function of time. The sample
weight was 3–10 mg. In the dynamic mode of dissolu-
tion, the solvent composition was varied from H2O
(pH 7) to H2O : HCl (pH 2) or to H2O : acid mixture
(10 : 1, pH 1.2) at the component ratio H2SO4 : H3PO4 :
HNO3 : HF = 4 : 3 : 1 : 2. The process of dissolution was
performed with a linear increase in the temperature at a
rate of 1 K/min up to 80°ë. A PST inductively coupled
plasma atomic emission spectrometer was used as a
detector.
To prepare nickel–silica catalysts (in this case, with
the composition 90%Ni–10%SiO2), oxide precursors of
nickel with different textures characteristics were
impregnated with an alcoholic solution of tetraethox-
ysilane as a source of silica, which was prehydrolyzed
in an acidic medium in the presence of a substoichio-
metric amount of water. The SiO2 content of the alcosol
was 0.147 g in 1 ml of solution. The hydrated nickel
oxide contained 63.3 wt % Ni, which corresponded to
the composition NiO · H2O. The other nickel oxide
samples calcined at higher temperatures (350°ë or
higher) were stoichiometric; that is, the nickel content
was ~79%. Taking into account the total pore volume of
nickel oxide (6–0.7 cm3/g depending on the calcination
temperature), the hydrolyzate was diluted with ethanol
so that the same amount of SiO2 (10 wt %) was intro-
duced in all cases. The mixture was brought to the con-
sistency of homogeneous paste and dried in a flow of air
at room temperature; thereafter, the drying temperature
was increased up to 150°ë.
The catalyst samples were tested in the reaction of
methane decomposition using a laboratory setup with a
vibratory fluidized-bed flow quartz reactor at a temper-
ature of 550°ë. The catalysts were prereduced in
hydrogen at 550°ë. Next, the reactor was cooled, and
hydrogen was replaced with methane. The methane
used was of 99.99% purity. The volume of the working
area of the reactor was 10 cm3. The weight of nickel in
a sample was 10 mg. The flow rate of methane was
adjusted to 20 cm3/min. The concentration of hydrogen
at the reactor outlet was determined by chromatography
with the use of a column packed with zeolite NaX. The
activity of catalysts was determined from the concen-
tration of hydrogen in a mixture at the reactor outlet; in
the case of a fresh catalyst, this value was 28–30%. The
reaction was stopped when the concentration of hydro-
gen decreased to 5%. The conversion of methane was
determined using the equation
The temperature-programmed reduction (TPR) of
nickel–silica systems was performed in a U-shaped
quartz tube. An argon–hydrogen mixture with a hydro-
gen content of 10% additionally dried with KOH was
used for the reduction. The initial weight of a loaded
sample was 6 mg. The flow rate of the gas mixture was
30 cm3/min. The rate of heating was 5 K/min.
1 – c
1 + c
X-ray diffraction (XRD) analysis was used to deter-
mine the phase composition and dispersity of samples.
The average particle sizes of nickel oxide and nickel
metal were calculated from the Scherrer equation using
the half-widths of (111) and (200) diffraction lines. The
X-ray diffraction patterns were taken on a URD-63 dif-
-----------
X =
× 100%,
where c is the methane concentration in volume (mole)
fractions at the reactor outlet.
The amount of carbon deposited on the catalyst dur-
fractometer (CuK radiation, λ = 0.15418 nm) with a ing the reaction time was determined by weighing the
graphite monochromator using silicon as an internal unloaded samples. The yield of carbon (G, (g C)/(g Ni))
standard.
was calculated as follows: the amount of deposited car-
KINETICS AND CATALYSIS Vol. 44 No. 2 2003

270
ERMAKOVA, ERMAKOV
depending on the calcination temperature. Starting with
350°ë, the finest flat flakes of the parent oxide were
agglomerated to form bulk particles with a face-cen-
tered cubic lattice and an average size of 8 nm, whereas
the average size of nickel crystallites increased up to
12 nm at 400°ë (Fig. 1b). As the calcination temperature
was further increased, the particle size of nickel oxide
further increased because of coalescence (Table 1).
Table 2 summarizes the characteristics of a number
of 90%Ni–10%SiO2 catalysts, which were prepared by
treating the corresponding NiO samples with an alcosol
and were reduced with hydrogen at 550°ë. It can be
seen that the yield of carbon strongly depends on the
dispersity of nickel oxide used as the catalyst precursor
rather than on the average size of nickel particles in the
reduced samples. Thus, at almost the same average par-
ticle size of nickel in catalysts 1 and 2, the yield of car-
bon on sample 2 was many times higher. This consider-
able increase in the yield of carbon is difficult to explain
by the small increase in the average size of metal parti-
cles. Indeed, the average size of nickel particles in sam-
ples 2–5 increased from 10 to 60 nm, whereas the yield
of carbon increased by only 22%. Because the nickel
content of all the catalyst samples was the same (90 wt %),
the difference in the yields of carbon can be explained
only by taking into account the presence or absence of
unreduced silicates in the catalyst. These silicates can
be formed at the stage of mixing nickel oxide with the
hydrolyzate. In the test series, the oxide precursor of
sample 1 exhibited a maximum surface area of
400 m2/g; it was hydrated and hence most reactive in
terms of the formation of silicate compounds through
its interaction with polysiloxane molecules from the
alcosol. At a calcination temperature of 350°ë or
higher, the particles of nickel oxide coarsened; in this
case, the amount of hydroxyl groups at their surface
dramatically decreased; as a consequence, the reactiv-
ity of the surface decreased. Evidently, a tendency to
form silicate compounds upon treatment with the alco-
sol is much weaker in the case of nickel oxide samples
2–5 than that in sample 1.
50 nm
(‡)
40 nm
(b)
Fig. 1. Electron micrographs of nickel oxides prepared by
the calcination of α-Ni(OH) at (a) 250 and (b) 400°C.
2
bon was divided by 0.01 g, where 0.01 g is the weight
of nickel in the catalyst sample.
To test this hypothesis, samples 1 and 5 (henceforth,
the sample numbers are given in accordance with Table 2)
were studied in detail at different stages of preparation and
in the reaction of methane decomposition, because these
samples are characterized by extreme yields of carbon in
the test series of catalysts (40 and 384 (g C)/(g Ni), respec-
tively). At the same nickel content, the time it takes for
the complete deactivation of these catalysts in the reac-
tion differed by a factor of ~10; this difference also
resulted in different yields of carbon (Fig. 2). Note that
the curves of methane conversion for catalysts 2–4 are
similar to the curve obtained with catalyst 5 and almost
coincide with it. Differences were observed only in the
last 400–500 min of the reaction.
RESULTS AND DISCUSSION
Figure 1a demonstrates the micrograph of highly
dispersed hydrated nickel oxide prepared by the calci-
nation of the hydroxide at 250°ë. The formation of a
weakly crystallized oxide phase at this temperature was
supported by X-ray diffraction analysis, and this is con-
sistent with published data [13]. Judging from the
structure, this oxide is a pseudomorphosis on the initial
substance, α-nickel hydroxide. The hydrated oxide was
divided into six portions. One of them remained
unchanged, and the other five portions were calcined
Nickel oxides with surface areas of 400 and 7 m2/g,
for 1 h at different temperatures. Table 1 summarizes which were prepared by calcination at 250 and 700°ë,
the texture characteristics of nickel oxide samples respectively, were used as the precursors of samples 1
KINETICS AND CATALYSIS Vol. 44 No. 2 2003

EFFECT OF INTERACTIONS BETWEEN COMPONENTS IN NICKEL–SILICA CATALYSTS
271
Table 1. Texture and structure characteristics of NiO samples depending on calcination temperature
Calcination
temperature, °C
Volume of 1- to
Volume of mi- Average pore Average particle size
Sample
Ssp, m2/g
100-nm pores, cm3/g cropores, cm3/g
size, nm
of NiO, nm (XRD)
1
2
3
4
5
6
250
350
400
450
700
800
400
144
90
56
7
1.09
0.57
0.40
0.28
0.03
0.013
0.0246
0.0102
0.0013
0.0002
–
17
18
20
19
21
20
3–4
8
12
19
~90
>100
3
–
Table 2. Average size of nickel particles in reduced catalysts (90% Ni–10% SiO2) and the yields of carbon on them
Average particle size of NiO, nm
Average particle size
of NiO*, nm (XRD)
Yield of carbon,
(g C)/(g Ni)
Sample
(XRD)
(EM)
6.8
3–4
1
2
3
4
5
6
7
10
40
300
375
375
384
2
8
12
9
12
16
62
–
12
19
17
~90
>100
60
>100
Note: * For samples used as catalyst precursors before the treatment with tetraethoxysilane.
and 5, respectively. Figure 3 demonstrates the TPR pro- tion of silica to nickel oxide significantly increased the
files of catalysts 1 and 5. The TPR profiles of parent temperature of sample reduction and broadened the
oxides are given for comparison. It can be seen that two second peak of hydrogen absorption. In this case,
peaks of hydrogen absorption with maximums at 236 according to XRD data, the average size of nickel crys-
and 246°ë were observed in the reduction of highly tallites decreased to 7 nm. This was also supported by
dispersed silica-free nickel oxide. Note that the former electron-microscopic data (Fig. 4a).
peak was observed only in the reduction of NiO that
The oxide precursor of sample 5 (NiO with Ssp =
was prepared at temperatures lower than 400°ë.
According to Mile et al. [14], this effect is associated
with the conversion of “Ni2O3” to NiO. The presence of
nonstoichiometric nickel oxide species in nickel(II)
oxide was also mentioned by other researchers [15].
The conversion of Ni2+ into Ni0 began at 240°ë and
occurred over a very narrow range of temperatures to
complete at 255°ë. In this case, even though the reduc-
tion temperature was low, the average size of nickel
crystallites (according to XRD data) increased as com-
pared to that of the parent oxide particles (3–4 nm) by
an order of magnitude and was as large as 44 nm.
7 m2/g) was reduced starting at a temperature of 300°ë.
The rate of hydrogen absorption reached a maximum at
332°ë, and the reduction was complete at 380°ë
(Fig. 3). This reduction behavior is characteristic of sto-
ichiometric nickel oxide [16]. After the modification of
this oxide with silica, the reduction started at the spec-
ified temperature and stopped at 450°ë, which is higher
than the reduction temperature of unmodified nickel
oxide by 70°ë. The peak was split into two new peaks
with maximums at 335 and 370°ë. It is likely that the
first maximum can be ascribed to NiO unbound to sil-
ica, whereas the second peak characterizes the reduc-
An analysis of the TPR profile obtained by the tion of bound oxide species. In contrast to sample 1, the
reduction of sample 1 (Fig. 3) indicated that the addi- peak that characterizes the reduction of sample 5 exhib-
KINETICS AND CATALYSIS Vol. 44 No. 2 2003

272
ERMAKOVA, ERMAKOV
Methane conversion, %
18
16
14
2
12
1
2
10
8
6
4
4
1
2
0
1000
2000
3000
Reaction time, min
Fig. 2. Methane conversion at 550°ë on catalysts (1) 1 and
(2) 5 as a function of reaction time.
3
ited no tailing; this fact indicates that the interaction
between components in the test system was weak.
Figure 4b demonstrates the micrograph of reduced sam-
ple 5. It can be seen that, in contrast to sample 1
(Fig. 4a), sample 5 consisted of coarse nickel particles.
The system was fairly uniform, although relatively
small nickel crystallites were present (<25 nm). A
translucent silica film, which prevents particles from
agglomeration, can be seen at the edges of metal parti-
cles.
We used IR spectroscopy to reveal differences in the
interaction of phases in samples 1 and 5 at different
stages of preparation. We compared the IR spectra of
samples (Fig. 5) only within a frequency region from
200 to 1300 cm–1, because changes in the absorption
profile characterizing nickel–silica interactions were
observed in this region. At frequencies higher than
1300 cm–1, all the IR spectra of the test samples exhib-
ited absorption bands due to impurity anions (at 1305
and 1380 cm–1) and absorption bands that correspond to
deformation (at 1630 cm–1) and stretching vibrations of
water (a broad absorption band with a maximum at
3440 cm–1).
It can be seen in Fig. 5 that bands with maximums
at 583 (≡Si–OH deformation vibrations), 950 (≡Si–OH
stretching vibrations), and 793 cm–1 (symmetric
≡Si−O–Si≡ stretching vibrations, which are typical of a
silica skeleton) disappeared from the spectrum of sam-
ple 1, whereas these bands were present in the IR spec-
trum of the pure xerogel (the alcosol was dried and cal-
cined at 150°ë) [17]. All these data, as well as the
strong shift of the most intense absorption band of silica
at 1078 cm–1 (asymmetric ≡Si–O–Si≡ stretching vibrations
200
300
400
500
T, °C
Fig. 3. TPR profiles obtained in the reduction of catalysts
(1) 1 (NiO (250°C) + 10% SiO ) and (2) 5 (NiO (700°C) +
10% SiO ) and the corresponding parent nickel oxides (3)
NiO (250°C) and (4) NiO (700°C) (oxide calcination tem-
peratures are given in parentheses); the heating rate was
5 K/min; the sample weight was 6 mg; and the flow rate of
2
2
3
the gas mixture was 30 cm /min.
silica is sensitive to the formation of silicate com-
pounds. A comparison with the samples of SiO2 and
NiO (250°C) indicates that the bands at 450 and
630 cm–1 in the spectrum of sample 1 remained
unchanged and belonged to the deformation vibrations
of the silica skeleton and the O–H group in hydrated
nickel oxide, respectively. Thus, we can state that silica
introduced into a porous matrix of hydrated nickel
oxide is chemically bound with the formation of silicate
compounds because of the high surface area (400 m2/g)
and the specific layered structure of the hydrated nickel
oxide. Dzis’ko et al. [20] related the appearance of
bands at 665 and 1050 cm–1 in consecutively precipi-
tated and coprecipitated nickel–silica samples to strong
chemical interactions and the formation of a new chem-
ical compound. The shift of a band at 1100 cm–1 to
1050 cm–1 was attributed in [21, 22] to the formation of
poorly crystallized 1 : 1 phyllosilicate of the nepouite
type (Ni3Si2O5(OH)4).
An analysis of the IR spectrum of compound 1 after
of the skeleton) toward lower frequencies (to 1050 cm–1), reduction indicated that silicate compounds formed on
are indicative of the presence of the ≡Si–O–Ni≡ bond. mixing the components (nickel oxide and the alcosol)
As noted previously [18, 19], the most intense band of did not disappear, although the reduction temperature
KINETICS AND CATALYSIS Vol. 44 No. 2 2003

EFFECT OF INTERACTIONS BETWEEN COMPONENTS IN NICKEL–SILICA CATALYSTS
273
460
4
1070
1160
1080
970
785
50 nm
450
450
785
3
2
(‡)
640
1050
1030
450
460
1
670
50 nm
8
7
406
1040
(b)
632
Fig. 4. Electron micrographs of catalysts after reduction in
2
H at 550°C: (a) catalyst 1 and (b) catalyst 5.
1067
1094
838
was high (550°ë). As judged from the shift of the main
band of silica to 1030 cm–1, as well as from the appear-
ance of a weak band at 670 cm–1, we can conclude that
instable silicate compounds of the nepouite type were
converted into more stable talcite-like compounds [22].
462
455
1078
1162
1200
6
5
802
Let us analyze in the same manner the IR spectra of
sample 5 at different stages of preparation. The IR spec-
trum of this sample (oxide form) seems to be a super-
position of the spectra of SiO2 (150°ë) and NiO
(700°ë). In this spectrum, adsorption bands at 455 and
460 cm–1, which are characteristic of the deformation
vibrations of SiO2 and the stretching vibrations of NiO,
respectively, are superimposed on each other to merge
into a single more intense band. In the oxide sample of
catalyst 5, a band at 1078 cm–1, which corresponds to the
asymmetric stretching vibrations of SiO2, was somewhat
shifted to the low-frequency region (to 1070 cm–1); this
950
793
800
583
600
ν, cm–1
1000
Fig. 5. IR spectra of catalysts (1, 2) 1 and (3, 4) 5 (2, 4)
before reduction and (1, 3) after reduction in H at 550°ë;
2
SiO samples heated at (5) 150 and (6) 600°ë; and NiO
2
samples used for the preparation of catalysts and calcined at
(7) 250 and (8) 700°ë.
KINETICS AND CATALYSIS Vol. 44 No. 2 2003

274
ERMAKOVA, ERMAKOV
Concentration, arb. units
(a)
2
1
(b)
1
2
0
10
20
30
40
50
Time, min
Fig. 6. Differential dissolution profiles of reduced samples (a) 1 and (b) 5: (1) nickel and (2) silica.
suggests the appearance of weak interactions at the intense peak of silica dissolution. This fact indicates
NiO–polysiloxane film interface. Upon the reduction of that a very small portion of nickel in sample 5 can also
sample 5 at 550°ë, its IR spectrum became similar to be chemically bound to silica; however, in this case, it
the spectrum of SiO2 (600°ë); this fact is indicative of is difficult to speak of stoichiometry. Because the dif-
a minimum interaction between the components of the ference in the peak intensities is very great, we can only
binary system under consideration.
Figure 6 demonstrates the differential dissolution
profiles of reduced samples 1 and 5. This method sup-
state that the amount of bound nickel in sample 5 is
much lower than that in sample 1.
The behavior of catalysts for methane decomposi-
ported the presence of silicate compounds in sample 1. tion over the first minutes after being placed in a reac-
The dissolution curves of nickel and silica exhibited tion medium is of paramount importance because it is
synchronous peaks after approximately 45 min; these responsible for the course of the subsequent reaction
peaks are indicative of the dissolution of a chemical over many hours. Figures 7a and 7c demonstrate the
compound, which can be characterized as nepouite micrographs of catalysts 1 and 5, respectively, after the
because the Ni/Si ratio is 1.5 and corresponds to the sto- first 30 min of reaction. It is well known that, when a
ichiometry of nepouite. The concentration of nepouite catalyst is placed in methane, free metal particles
was no higher than 2% of the reduced sample weight. acquire an increased fluidity under the action of dis-
As for sample 5, the dissolution curve of nickel also solved carbon, and they can coalesce to very large sizes,
exhibited a small peak after 40 min; the maximum of even though a support is present in the catalyst [23].
this peak coincides with the maximum of a much more This explains why initially disperse catalyst 1 (the aver-
KINETICS AND CATALYSIS Vol. 44 No. 2 2003

EFFECT OF INTERACTIONS BETWEEN COMPONENTS IN NICKEL–SILICA CATALYSTS
275
age particle size of Ni dNi is ~7 nm) produced both thin
and thick carbon fibers with nickel crystallites as great
as 120 nm in size at the ends of the fibers (Fig. 7a). The
uncoalesced small particles can be subdivided into
those at which fibers grew and those covered with car-
bon; the latter are much greater in number. It is evident
that this behavior of the catalyst is a consequence of the
strong interaction of nickel with silica, as supported by
TPR, IR spectroscopy, and differential dissolution.
Nickel silicates that remained unreduced decreased the
rate of carbon diffusion through nickel nanoparticles;
because of this, they became covered with carbon
formed in methane decomposition and deactivated.
Because these particles constitute the major portion of
catalyst 1, it is most likely that this is responsible for a
low yield of carbon on this catalyst.
200 nm
In contrast to catalyst 1, catalyst 5 produced carbon
fibers of approximately equal diameters within a range
of 80–118 nm (Fig. 7c), which is much the same as the
particle size of Ni in the reduced catalyst (Fig. 4b). The
nickel particles at the ends of fibers are well-faceted
crystals, which acquired this shape under exposure to a
reaction medium [24]. Nickel particles encapsulated in
carbon are absent from the micrograph. Thus, the
behaviors of catalysts 1 and 5 over the first hour of reac-
tion supported the conclusion that the strong interaction
between nickel and silica has an extremely adverse
effect on the yield of carbon.
(‡)
Data for sample 6 are also given in Table 2. The
yield of carbon on this catalyst was no higher than
2 (g C)/(g Ni), although it is evident that the interaction
between components in this sample was even weaker
than that in sample 5. However, this fact can be easily
explained. Nickel oxide used for the preparation of cat-
alyst 6 was calcined at 800°ë, and its surface area was
no higher than 3 m2/g. Based on this surface area, the
average size of oxide particles was equal to 270–
300 nm. The stabilization of this coarsely dispersed
oxide system by the addition of silica did not allow us
to prepare a catalyst with optimum structure character-
istics because the size of nickel crystallites did not
decrease after reduction (dispersion did not occur);
consequently, this size was greater than the critical size
(150–200 nm) for the production of fibers.
50 nm
(b)
Thus, we found that the interaction between compo-
nents of the catalytic system depends on the specific
surface area of nickel oxide used as a precursor. The
most disperse nickel oxide with a specific surface area
of 400 m2/g exhibited the greatest tendency to form sil-
icates on treatment with a solution of hydrolyzed tetra-
ethoxysilane followed by reduction. The calcination of
this oxide at higher temperatures (350–700°ë)
decreased the reactivity of its surface toward silica and
hence facilitated a decrease in the amount of residual
silicates in the reduced catalyst.
400 nm
(c)
Fig. 7. Electron micrographs of catalysts after exposure to
methane for 30 min: (a) carbonized catalyst 1, (b) finest par-
ticles of catalyst 1 encapsulated in carbon, and (c) carbon-
ized catalyst 5.
The yield of carbon depends on the interaction
between nickel and silica. The presence of ~2% silicates
decreased the yield of carbon down to 40 (g C)/(g Ni).
KINETICS AND CATALYSIS Vol. 44 No. 2 2003

276
ERMAKOVA, ERMAKOV
10. Baker, R.T.K. and Chludzinski, J.J., J. Catal., 1980,
The catalyst in which silicates were not detected
after reduction was characterized by a carbon yield
of 384 (g C)/(g Ni).
vol. 64, p. 464.
11. Ermakova, M.A., Ermakov, D.Yu., and Kuvshinov, G.G.,
Appl. Catal., A, 2000, vol. 201, p. 61.
12. Malakhov, V.V. and Vlasov, A.A., Kinet. Katal., 1995,
ACKNOWLEDGMENTS
vol. 36, no. 4, p. 503.
We are grateful to Cand. Sci. (Chem.) A.L. Chuvilin
for the provision of micrographs, to Cand. Sci. (Chem.)
G.N. Kustov for the measurement of IR spectra, and to
Dr. Sci. (Chem.) V.V. Malakhov and L.S. Dovlitova for
the provision of data on the differentiated dissolution of
samples.
13. Bihan, S.Le. and Figlarz, M., Thermochim. Acta, 1973,
vol. 6, p. 319.
14. Mile, B., Stirling, D., Zammit, M.A., Lovell, A., and
Webb, M., J. Catal., 1988, vol. 114, p. 217.
15. Chemistry of the Solid State, Garner, W.E., Ed., London:
University of Bristol, 1955.
16. Robertson, S.D., McNicol, B.D., De Baas, J.H.,
Kloet, S.C., and Jenkins, J.W., J. Catal., 1975, vol. 37,
p. 424.
REFERENCES
1. Kuvshinov, G.G., Mogilnykh, Yu.I., Kuvshinov, D.G.,
Parmon, V.N., and Zavarukhin, S.G., Proc. 11th World
Hydrogen Energy Conference, Stuttgart, 1996, p. 2.
17. Ro, J. and Chung, I.J., J. Non-Cryst. Solids, 1991,
vol. 130, p. 8.
18. Lazarev, A.N., Kolebatel’nye spektry i stroenie silikatov
(Vibrational Spectra and the Structyure of Silicates),
Leningrad: Nauka, 1968.
2. Zhang, T. and Amiridis, M.D., Appl. Catal., A, 1998,
vol. 167, p. 161.
3. Baker, R.T.K., Barber, M.A., Harris, P.S., Feates, F.S.,
19. Sviderskii, V.A., Voronkov, M.G., Klimenko, V.S., and
and Waite, R.J., J. Catal., 1972, vol. 26, p. 51.
Klimenko, S.V., Zh. Prikl. Khim., 1997, vol. 70, p. 1698.
4. Fenelonov, B., Likholobov, V.A., Derevyankin, A.Yu.,
and Mel’gunov, M.S., Catal. Today, 1998, vol. 42,
p. 341.
5. Rodriguez, N.M., J. Mater. Res., 1993, vol. 8, p. 3233.
6. Holstein, W.L., J. Catal., 1995, vol. 152, p. 42.
20. Dzis’ko, V.A., Noskova S.P., Karakchiev L.G., Bori-
sova M.S., Bolgova V.D., and Tyulikova T.Ya., Kinet.
Katal., 1972, vol. 13, no. 2, p. 366.
21. Clause, O., Kermarec, M., Bonneviot, L., Villain, F., and
Che, M., J. Am. Chem. Soc., 1992, vol. 114, p. 4709.
7. Kim, M.S., Rodriguez, N.M., and Baker, R.T.K.,
22. Kermarec, M., Carriat, J.Y., Burattin, P., Che, M., and
J. Catal., 1991, vol. 131, p. 60.
Decarreau, A., J. Phys. Chem., 1994, vol. 98, p. 12008.
8. Ermakova, M.A., Ermakov, D.Yu., Kuvshinov, G.G., and
23. Ermakova, M.A., Ermakov, D.Yu., Plyasova, L.M., and
Plyasova, L.M., J. Catal., 1999, vol. 187, p. 77.
Kuvshinov, G.G., Catal. Lett., 1999, vol. 62, p. 93.
9. Ermakova, M.A., Ermakov, D.Yu., Kuvshinov, G.G., and
Plyasova, L.M., Kinet. Katal., 1998, vol. 39, no. 5, 24. Chesnokov, V.V. and Buyanov, R.A., Rus. Chem. Rev.,
p. 791.
vol. 69, p. 623.
KINETICS AND CATALYSIS Vol. 44 No. 2 2003