Production of a highly dispersed carbon material and hydrogen from natural gas in a microwave reactor with metallic catalysts

Article abstract of DOI:10.1134/S1070427211060176

Effect of a microwave field on metallic catalysts was studied in the reaction of decomposition of methane into hydrogen and highly dispersed carbon. The dependence of the conversion of methane, yield of carbon, and its composition on the chemical nature of a catalysts and reaction conditions was examined.

Full text of DOI:10.1134/S1070427211060176

ISSN 1070-4272, Russian Journal of Applied Chemistry, 2011, Vol. 84, No. 6, pp. 997–1002. © Pleiades Publishing, Ltd., 2011.
Original Russian Text © S.I. Galanov, A.G. Zherlitsyn, Yu.V. Medvedev, O.I. Sidorova, V.P. Shiyan, 2011, published in Zhurnal Prikladnoi Khimii, 2011,
Vol. 84, No. 6, pp. 973–978.
ORGANIC SYNTHESIS AND INDUSTRIAL
ORGANIC CHEMISTRY
Production of a Highly Dispersed Carbon Material
and Hydrogen from Natural Gas in a Microwave Reactor
with Metallic Catalysts
S. I. Galanovb, A. G. Zherlitsyna, Yu. V. Medvedeva,
O. I. Sidorovab, and V. P. Shiyana
a Research Institute of Nuclear Physics, Tomsk Polytechnic University, State Educational Institution for Higher Professional
Education, Tomsk, Russia
b Tomsk State University, State Educational Institution for Higher Professional Education, Tomsk, Russia
Received June 17, 2010
Abstract—Effect of a microwave field on metallic catalysts was studied in the reaction of decomposition of
methane into hydrogen and highly dispersed carbon. The dependence of the conversion of methane, yield of
carbon, and its composition on the chemical nature of a catalysts and reaction conditions was examined.
DOI: 10.1134/S1070427211060176
СН4(g) = С(s) + 2Н2(g), ΔН1300 K = 92.11 kJ;
(1)
One of the most important problems of rational use
of hydrocarbon raw materials consists in utilization of
low-pressure natural gas and petroleum gases. It is
commonly believed that the most promising, in
addition to burning in flares, are the gas-to-liquid
conversion (GTL) and(or) burning to produce electric
power [1].
2СН4(g) = С2Н4(g) + Н2(g), ΔН1300 K = 108.86 kJ; (2)
2СН4(g) = С2Н2(g) + 2Н2(g), ΔН1300 K = 406.12 kJ. (3)
In addition to the target reaction (1) of hydrogen
formation, also possible are reactions (2) and (3). In
this case, it is rather promising to employ combined
processes of physical treatment and catalysis, i.e.,
catalysis combined with exposure of the catalyst and
methane to a microwave field [3].
Generation of electric power at production sites
seems to be rational, but requires capital investment
with nearly zero payoff because the amount of
generated power markedly exceeds the needs of oil
fields and a problem arises of utilizing the excess
electric power. In this case, it is promising to construct,
together with power plants, power-consuming installa-
tions for processing of a gas into easily transportable
products. At present, new processes are required for
conversion of methane into valuable products in order
to use in due way resources of natural and petroleum
gases. This is particularly topical because of the
development of the hydrogen power engineering, in
which natural gases are regarded as the main raw
materials for production of hydrogen.
EXPERIMENTAL
Experiments were performed in the installation
shown schematically in Fig. 1. Natural gas was con-
verted in a flow-through quartz reactor of a waveguide
type with an inner diameter of 18.5 mm in the presence
of a microwave discharge at a frequency of 2450 MHz.
The discharge was maintained and the catalyst was
heated (300–650°C) with a microwave generator
operating in the continuous mode with an output power
finely adjusted in the range 700–1500 W. The
temperature was measured with a Kel’vin KB-Dipol’
noncontact IR thermometer with a measurement range
200–2000°C. As catalysts served bulky granulated
metals (Ni, Fe, Mo) and intermetallic compounds
The thermal conversion of methane may occur by
the following endothermic (energy-consuming) reac-
tions [2]:
997

998
GALANOV et al.
Discharge
Fig. 1. Schematic of laboratory installation. (1) Power source of the microwave generator, (2) microwave generator, (3) circulator,
(4) waveguide, (5) reactor, (6) gas flow control valve, (7) meter of reflected microwave power, (8) meter of transmitted microwave
power, (9) carbon collector, (10) gas-mixture-sampling unit, and (11) pyrometer.
(TiNI and AlNi) with a grain size of 0.1–2 mm. As raw
materials was used natural gas containing 94 vol %
CH4 and C2-hydrocarbons the rest.
The catalysts used in the study belong to “bulk
metals.” When a catalyst is exposed to a microwave
field, two processes occur simultaneously. First,
catalyst grains are heated under the action of the
microwave energy because of the dissipative loss. In
the experiments, the samples are heated to tempera-
tures of 300–650°C. These values fall within the
temperature range characteristic of catalytic methods
for synthesis of nanosize carbon (nanofibers, nano-
tubes) from methane [4, 5]. Second, “partial” electric
discharges (microdischarges) between grains are
caused by induced electromagnetic fields when a
catalyst is exposed to a microwave field. This
promotes decomposition of methane to unsaturated
hydrocarbons by reactions (2) and (3). For example, a
similar effect of the microwave field on methane to
give acetylene was observed in [3], with metallic
catalysts or grids used. It is also probable that nanotube
precursors are formed on the surface of metallic
catalysts by reaction (1). All the reactions mentioned
above are endothermic. That is why, when nitrogen, in
whose flow the catalyst is heated, is replaced in gas
A chromatographic analysis of gas mixtures was
made on a Khromatek-Kristall 5000.1 chromatograph
with DTP and PID detectors and the following
columns: HP-PLOT capillary column (50 m) with
supported Al2O3/KCl, packed column (2 m) with
Carbosieve S-II sorbent (60–80 mesh), and packed
column (3 m) with NaX sorbent (60–80 mesh). The
thermostat temperature was 80°C, as carrier gases
served helium and argon. The concentrations of the gas
mixture components were calculated by the absolute-
calibration method with Khromatek Analitik 2.5 software.
The specific surface area of the carbon product was
measured by the multiple-point BET method with a
Sorbi 4.1 instrument. Surface micrographs of carbon
samples were obtained with a Philips SEW 515
scanning electron microscope. X-ray diffraction
analyses of the samples were made on a Shimadzu
XRD 6000 X-ray diffractometer.
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PRODUCTION OF A HIGHLY DISPERSED CARBON MATERIAL
999
Table 1. Parameters of conversion of natural gas, composition and properties of the carbon product
Content, %
Yield of carbon
H2
Content of C2-
Catalyst
Methane
Specific surface
area, m2 g–1
per passed
concentration, hydrocarbons,
composition conversion, %
amorphous
carbon
methane, %
vol %
vol %
nanotubes
TiNi
AlNi
Ni
42.4
15.8
79.5
54.5
77.6
11.2
23.5
62.9
46.7
35.9
45.00
55.00
21.2
46.1
33.1
56.5
78.2
24.9
20.9
18.7
42.9
69.7
19.5
52.3
28.3
84.9
69.4
84.6
8.8
2.2
4.0
2.2
3.1
Fe
4.04
Mo
64.9
Table 2. Types and SCR-based sizes of nanotubes being formed
Nanotubes
Catalyst
composition
multi-walled
single-walled
onion-like
amount, % SCR-based size, nm amount, % SCR-based size, nm amount, % SCR-based size, nm
TiNi
AlNi
Ni
100
100
9.1
10.6
14.9
12.4
26.3
–
–
–
7.8
7.3
37.2
–
–
–
7.1
3.7
37.2
–
–
19.4
45.0
38.8
32.9
35.6
33.0
20.1
Fe
28.2
47.0
Mo
flows with methane, the temperature of the catalyst
bed decreases by 20–80°C relative to the initial value,
with this decrease dependent on the amount of
methane fed into the reactor (residence time).
their electrical properties, and, accordingly, the effect
of a microwave field on the bulk of the catalyst bed
and the nature of “microdischarges” may change. The
chemical composition of catalysts can affect the
structure of precursors of dispersed carbon, formed in
the process. It was shown in [4, 5] that the chemical
composition of methane decomposition catalysts
affects both the conversion of methane and the
structure of a carbon product being formed. According
to the data in Table 1, the conversion of methane is
low with AlNi and TiNi intermetallic compounds as
catalysts, with only multi-walled nanotubes found in
the carbon product (Table 2).
Reactions (1)–(3) result in that hydrogen and
unsaturated hydrocarbons accumulate in reaction
gases, as also do particles (precursors) of carbon.
Accordingly, ingress of products formed in decom-
position of methane into a gradient microwave field
favors formation of a plasma jet fed by the microwave
energy at the exit from the catalyst bed. Under gas
discharge conditions, there occur fast chemical
transformations of natural gas (methane) into carbon,
hydrogen, and unsaturated hydrocarbons [6]. About
80% of the microwave power is consumed for main-
taining the plasma jet, and 10–15%, for maintaining
the temperature in the catalysts bed. Therefore, it can
be assumed that the main conversion of methane
occurs in the plasma being formed.
When Ni and Fe catalysts used for decomposition
of methane and formation of carbon [4, 5], the
conversion of methane increases and the content of
nanotubes becomes larger than that in amorphous
carbon (Table 1), with three types of nanotubes
observed (predominantly single-walled and onion-like)
(Table 2). According to [7], metallic molybdenum
supported by a high-silica zeolite is an effective
catalyst for activation of methane to give ethane and
ethylene, but carbon formed on the surface of
The conversion of methane and the amount and size
of nanotubes in the carbon product are determined by
the chemical nature of a catalyst (Tables 1, 2). The
varied chemical nature of metallic catalysts can affect
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 84 No. 6 2011

1000
GALANOV et al.
1
2
3
v, cm s–1
τ, s
Fig. 3. Dependence of (1) methane conversion K(CH4) and
concentrations of (2) hydrogen [c(H2)] and (3) C2
hydrocarbons (ethylene, ethane) [c(C2)] on the linear flow
velocity ν of methane.
Fig. 2. Effect of the contact duration τ on the concentra-
tions c of (1) hydrogen, (2) methane, (3) acetylene, and
(4) ethylene and (5) on the conversion K of methane.
molybdenum particles has a graphite-like structure.
According to the data in Table 1, a high conversion of
methane is observed with Mo, and the yield of the
carbon product and nanotubes is low, compared with
other samples of metallic catalysts.
determined from the coherent scattering region (CSR),
which vary with the range 9.1–26.3, 7.3–37.2, and 3.7–
37.2 nm for multi-walled, single-walled, and onion-
like nanotubes, respectively. The specific surface area
of the carbon product varies from 18.7 to 69.7 m2,
depending on the catalyst used.
Combination of catalytic and plasmochemical
processes leads to complex dependences in the endo-
thermic reactions of methane decomposition. Making
shorter the time of contact with the catalyst at a
constant linear flow velocity of methane diminishes
the conversion of methane and the concentration of
hydrogen, the concentration of C2-hydrocarbons being
constant (Fig. 2). Raising the linear flow velocity of
methane markedly diminishes the conversion of
methane and the yield of hydrogen and raises the
concentration of C2-hydrocarbons in reaction gases
(Fig. 3).
With the supply rate of raw materials and catalyst
composition varied, it is possible to obtain carbon
materials with different relative amounts of nanotubes
and amorphous dispersed carbon. The most frequently
encountered problem of the catalytic synthesis of
carbon is occlusion of catalyst particles by the carbon
product. Occluded metal particles are not dissolved in
acids and contaminate nanotubes and nanofibers [8]. In
a microwave reactor, precursors of nanotubes and
nanofibers, formed on the catalyst surface, are carried
by the hydrogen plasma into the discharge zone and
contain no metal, which makes unnecessary the stage
of washing of the carbon product to remove the metal
or metal oxides.
Table 1 lists concentrations of hydrogen and by-
products (ethylene, acetylene) obtained in a single pass
of the reaction mixture. The highest conversion of
methane and concentration of hydrogen in reaction
gases are observed on nickel and molybdenum cata-
lysts, and the highest yield of carbon nanotubes is
characteristic of Ni and Fe.
Figure 4 shows micrographs of carbon, from which
it is seen that the product obtained can be conditionally
divided into two kinds: the first structure is formed by
associates with “cotton wool” structure and sizes of
10–20 μm (Figs. 4a–4c), constituted by finer structures
200–250 nm in size (Fig. 4); the smallest among the
observed particles, 30–40 nm in size, form disordered
associates (Fig. 4c). Probably, this is amorphous
carbon, whose content in the final product may reach a
value of 64.8% (Table 1) in the case of a Mo-system.
The second structure of carbon being formed are nano-
The carbon product is predominantly composed of
amorphous carbon and carbon nanotubes, with a minor
amount of graphite, orta carbon, and fullerenes. Orta
carbon is formed only on Ni-containing catalysts (Ni,
AlNi, and TiNi). The chemical composition of the
catalyst (Table 2) strongly affects the type of nano-
tubes being formed, their relative amounts, and sizes
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 84 No. 6 2011

PRODUCTION OF A HIGHLY DISPERSED CARBON MATERIAL
(a) (d)
1001
(b)
10 μm
(e)
10 μm
1 μm
1 μm
(c)
(f)
0.1 μm
100 nm
Fig. 4. Micrographs of samples of the carbon product obtained. Associates of “cotton wool” texture with sizes (μm): (a) 10–20,
(b) 200–250, (c) 30–40; (d) fibrous carbon; (e) fibrous carbon distributed over the surface of “cotton wool” associates; and
(f) carbon fiber with a diameter of 100 nm.
fibers that may be chaotically interweaved (Fig. 4d) or
redistributed over the surface of “cotton-wool”
associates (Fig. 4e). The diameter of carbon fibers is,
on average, 100 nm (Fig. 4f). Unfortunately, the instru-
ment gives no way of studying finer forms of the car-
bon, which are apparently present in the carbon product.
oxidizing agent. Wide introduction of PEMFCs
encounters a number of difficulties, one of which is the
necessity for effective reduction of the concentration
of CO (“poison” for a fuel cell) to 10–100 ppm in the
hydrogen-containing gas. Because the presently
employed techniques for production of hydrogen from
natural gases are based on selective oxidation of
methane to synthesis gas, conversion with steam, or a
combination of these, the resulting hydrogen is always
contaminated with CO, which requires an additional
stage of catalytic purification [9]. In the case of carbon
synthesis in a microwave reactor, the resulting
In recent years, studies have been performed in
order to develop and commercialize power instal-
lations based on fuel cells with a proton-exchange
membrane (PEMFCs). Hydrogen serves as a fuel for
PEMFCs, with atmospheric oxygen serving as the
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 84 No. 6 2011

1002
GALANOV et al.
hydrogen contains no CO and residual unsaturated C2-
hydrocarbons are rather easily separated from
hydrogen, which will make it possible to obtain high-
purity hydrogen.
REFERENCES
1. Novikov, A.A. and Chukhareva, N.V., Neft’, Gaz
Energet., 2007, no. 6. p. 28.
2. Petrenko, I.G. and Filippova, V.I., Termodinamika
reaktsii
prevrashcheniya
uglevodorodov
C1–C5
CONCLUSIONS
(Thermodynamics of Conversion Reactions of C1–C5
Hydrocarbons), Moscow: Khimiya, 1972.
(1) The process described is based on combined
effects of the microwave radiation. These are heating
of the catalyst and initiation of microdischarges in the
catalyst bed, which excite gas molecules to give
methane decomposition products. The microwave
radiation also maintains the plasma jet, in which
natural gas is fully decomposed and conversion pro-
ducts are removed from the reactor space.
3. Fedoseev, V.I., Aristov, Yu.I., Tanashev, Yu.Yu., and
Parmon, V.N., Kinet. Kataliz, 1996, vol. 37, no. 6,
p. 869.
4. Reshetenko, T.V., Development of Bimetallic Catalysts
Based on Metals of the Subgroup of Iron for Synthesis
of Mesoporous Carbon Materials from Methane, Cand.
Sci. Dissertation, Novosibirsk, 2003.
5. RF Patent 2146648.
(2) The capacity in terms of highly dispersed
carbon at an average methane conversion of 70% is
0.3–0.5 g h–1 from a cubic centimeter of the reactor.
6. Slovetskii, D.I., Khim. Vysok. Energ., 2006, vol. 40,
no. 2, p. 112.
7. Vosmerikov, Conversion of Gaseous Hydrocarbons to
Aromatic Compounds on Bifunctional Zeolite-
containing Catalysts, Doctoral Dissertation, Tomsk,
2009.
ACKNOWLEDGMENTS
8. Rakov, E.G., Khim. Tekhnol., 2003, no. 10, p. 2.
The study was financially supported by the Federal
target program “Scientific and scientific-pedagogical
personnel of the innovative Russia” for the years of
2009–2013.
9. Snytnikov, P.V., Catalytic Oxidation of CO in
Hydrogen-containing Gas Mixtures, Cand. Sci.
Dissertation, Novosibirsk, 2004.
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