Methane Decomposition on the Surface of Molybdenum Nanoparticles at Room Temperature

Article abstract of DOI:10.1134/S0023158420020068

Abstract: The decomposition of methane on molybdenum nanoparticles was studied experimentally at room temperature. The molybdenum nanoparticles were synthesized in the gas phase using UV laser photolysis of Mo(CO)₆ vapor in a flow reactor. The working part of the flow reactor was equipped with quartz windows for introducing the radiation from a pulsed Nd:YaG laser operating at the fourth harmonic (266 nm) at a frequency of 10 Hz. Methane was used as a carrier gas. As a result of irradiation of a mixture of methane with Mo(CO)₆ vapors in the gas phase at room temperature, nanoparticles with sizes of 2–50 nm were synthesized. The phase composition of the nanoparticles included pure molybdenum, molybdenum carbide Mo₂C, and molybdenum oxide MoO₃. During the reaction, the hydrogen yield was measured with a VG-7 highly sensitive hydrogen analyzer based on a semiconductor metal–dielectric sensor. The measured H₂ concentration varied from 5 to 25 ppm depending on the concentration of Mo(CO)₆. The possibility of methane decomposition on molybdenum nanoparticles at room temperature was discussed based on the obtained data.

Full text of DOI:10.1134/S0023158420020068

ISSN 0023-1584, Kinetics and Catalysis, 2020, Vol. 61, No. 2, pp. 224–231. © Pleiades Publishing, Ltd., 2020.
Russian Text © The Author(s), 2020, published in Kinetika i Kataliz, 2020, Vol. 61, No. 2, pp. 225–234.
Methane Decomposition on the Surface of Molybdenum
Nanoparticles at Room Temperature
E. V. Gurentsova, *, A. V. Eremina, R. N. Kolotushkina, b, D. N. Khmeleninc, and Yu. V. Grigorievc
aJoint Institute for High Temperatures, Russian Academy of Sciences, Moscow, 125412 Russia
bBauman Moscow State Technical University, Moscow,105005 Russia
cShubnikov Institute of Crystallography, Federal Scientific Research Center Crystallography and Photonics,
Russian Academy of Sciences, Leninsky pr. 59, Moscow, 119333 Russia
*e-mail: gurentsov@ihed.ras.ru
Received June 10, 2019; revised September 24, 2019; accepted October 22, 2019
Abstract—The decomposition of methane on molybdenum nanoparticles was studied experimentally at room
temperature. The molybdenum nanoparticles were synthesized in the gas phase using UV laser photolysis of
Mo(CO)6 vapor in a flow reactor. The working part of the flow reactor was equipped with quartz windows for
introducing the radiation from a pulsed Nd:YaG laser operating at the fourth harmonic (266 nm) at a fre-
quency of 10 Hz. Methane was used as a carrier gas. As a result of irradiation of a mixture of methane with
Mo(CO)6 vapors in the gas phase at room temperature, nanoparticles with sizes of 2–50 nm were synthesized.
The phase composition of the nanoparticles included pure molybdenum, molybdenum carbide Mo2C, and
molybdenum oxide MoO3. During the reaction, the hydrogen yield was measured with a VG-7 highly sensi-
tive hydrogen analyzer based on a semiconductor metal–dielectric sensor. The measured H2 concentration
varied from 5 to 25 ppm depending on the concentration of Mo(CO)6. The possibility of methane decompo-
sition on molybdenum nanoparticles at room temperature was discussed based on the obtained data.
Keywords: photodissociation of Mo(CO)6, molybdenum nanoparticles, decomposition of hydrocarbons,
hydrogen gas, molybdenum carbide
DOI: 10.1134/S0023158420020068
INTRODUCTION
single UV laser pulse at a wavelength of 266 nm. On
the other hand, UV laser irradiation of a mixture of
toluene, 1-butanol, and methane with Fe(CO)5
formed only iron nanoparticles without a carbon shell.
The use of metal nanoparticles in various fields of
science and technology has aroused interest in
improvement of methods for their synthesis. To pre-
vent the oxidation and exposure to aggressive media
and ensure compatibility with biological materials,
metal nanoparticles are proposed to be coated with a
carbon shell [1]. There are many procedures for the
synthesis of metal–carbon nanoparticles [2–9].
Among these, the electric arc method [2–4] and laser
pyrolysis of gaseous precursors [6–8] are the most
common techniques. One of the gas-phase methods
for producing nanoparticles is UV photodissociation
of metal carbonyl vapors followed by formation of
metal nanoparticles in the gas phase [10]. This method
was used to synthesize iron and molybdenum
nanoparticles encapsulated in a carbon shell as a result
of photodissociation of metal carbonyl vapors with
hydrocarbons [11, 12]. Iron nanoparticles in a carbon
shell were obtained by irradiating Fe(CO)5 + C2H2 and
Fe(CO)5 + C6H6 mixtures diluted with argon with a
Molybdenum nanoparticles enclosed in a carbon
shell were synthesized under the conditions of UV
laser photodissociation of Mo(CO)6 vapors in mix-
tures with methane and with acetylene, toluene,
1-butanol, and benzene vapors diluted with argon, as
reported in [12]. The nanoparticle samples had
molybdenum nuclei with molybdenum oxide and car-
bide impurities surrounded by the amorphous carbon
material. Amorphous carbon was detected in all the
samples regardless of the type of the hydrocarbon used
and its mole fraction in the mixture. Thus, it was
found that a single UV laser pulse with an energy of up
to 100 mJ was sufficient for dissociation of Mo(CO)6
molecules, formation of molybdenum nanoparticles
with sizes of 2–4 nm (270–2000 atoms), and subse-
quent decomposition of hydrocarbons on the surface
of these nanoparticles.
Note that metal–carbon nanoparticles were syn-
thesized at room temperature, as reported in [11, 12].
However, there is no information in the literature on
Abbreviations: EDS, energy dispersive X-ray spectroscopy;
HAADF-STEM, high-angle annular dark-field scanning trans-
mission electron microscopy.
224

METHANE DECOMPOSITION ON THE SURFACE
225
Energy
meter
Protective
glass
Mo(CO)6
Reactor
Damper
container
Flow meter
Filter
Nd:YaG laser
(266 nm)
Hydrogen
analyzer
CH4
To the atmosphere
Fig. 1. Diagram of a flow reactor.
the catalytic decomposition of hydrocarbons at room reactor. The gas was fed in the reactor for 1 min during
temperature with formation of solid carbon [13–15]. the experiments. The mixture was irradiated with
As is known, UV laser radiation is effectively absorbed 600 laser pulses at a frequency of 10 Hz with an average
by Fe(CO)5 or Mo(CO)6 molecules [10]. The absorption
of hydrocarbon molecules at a wavelength of 266 nm,
however, is rather weak and cannot lead to the forma-
tion of solid carbon from the gas phase [16].
The goal of this study was more detailed investiga-
tion of the catalytic decomposition of methane on the
surface of molybdenum nanoparticles using new data
on the yield of gaseous hydrogen.
pulse energy of 30 mJ during this time. The interval
between two laser pulses was such that the irradiated
part of the gas shifted to a distance longer than the
diameter of the laser beam to prevent the heating of the
condensed molybdenum nanoparticles.
It was assumed that metal–carbon nanoparticles
were synthesized as a result of the experiments, as in
[12]. The hydrogen yield in this process was measured
using a VG-7 hydrogen analyzer (National Research
Nuclear University MEPhI). This analyzer has high
sensitivity to hydrogen concentration (up to 0.1 ppm),
but high inertia. To determine the hydrogen concen-
tration, a 5 L damper container was used, which was
initially filled with air at a pressure of 1 atm because
the hydrogen analyzer was manufactured and cali-
brated for measurements in hydrogen–air mixtures.
EXPERIMENTAL
The experiments were performed in a stainless steel
flow reactor with a gas mixture flow rate of 3 L/min.
The diagram of the experimental unit is shown in
Fig. 1. The gas mixture consisted of a methane carrier
gas (CH4 ≥ 99.9%) and molybdenum carbonyl vapors,
which saturated methane that passed through a con- When a gas flow was passed through the reactor, the
tainer with a Mo(CO)6 powder. As the pressure of the hydrogen content increased to a maximum value
within ~1 h (Fig. 2). The relative hydrogen concentra-
tion during reactor operation for 1 min without laser
irradiation of a methane mixture with Mo(CO)6
vapors and hence in the absence of molybdenum
nanoparticles was found to be 2.3 ppm. At a standard
pressure of 1000 mbar and a room temperature of
23°С, this value corresponds to the total hydrogen
concentration of 5.5 × 1013 cm–3. After laser irradia-
tion of the same mixture for 1 min, the hydrogen yield
minus its concentration level in the initial mixture was
saturated Mo(CO)6 vapors under the experimental
conditions was extremely low; it was determined by
measuring the room temperature from the data of [17].
At a room temperature of 23°C, the Mo(CO)6 vapor
pressure was 156 μbar. For photodissociation of
Mo(CO)6 vapors, an LQ-129 pulsed Nd:YaG laser
(Solar laser systems, Belarus) working at a wavelength
of 266 nm (pulse duration 15 ns) was used. Laser irra-
diation of the gas and energy measurement in a laser
pulse were performed through a pair of parallel quartz
windows installed in the working chamber of a flow 5.9 ppm (1.4 × 1014 cm–3). When the reactor zone was
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226
GURENTSOV et al.
10
8
6
4
2
0
10
20
30
40
50
60
70
Time, min
100 nm
Fig. 2. Readings of the VG-7 sensor during the measure-
ment of a mixture of Mo(CO) and methane in a damper
6
tank after UV irradiation in a flow reactor at 23°C.
Fig. 3. Micrographs of the nanomaterials synthesized in
the mixture of methane with Mo(CO) vapor using
6
600 laser pulses with a repetition rate of 10 Hz.
heated to 40°С, it increased to 25 ppm, which is most
likely associated with increased Mo(CO)6 vapor pressure
and, accordingly, with a higher yield of molybdenum
nanoparticles after laser irradiation of the mixture.
To clarify the composition of the synthesized
nanomaterials, the microdiffraction patterns of
nanoparticles in the samples were analyzed (Fig. 4). It
was found that almost all large nuclei of nanoparticles
were crystalline, as evidenced by the ring reflections in
the electron diffraction pattern. An analysis of the
electron diffraction patterns shows the presence of the
following interplanar spacings (Å): 2.54 – MoO3,
2.36 – Mo2C, 2.20 – Mo, 1.73 – MoO3, 1.56 – Mo,
1.48 – Mo2C, 1.31 – Mo2C, 1.27 – Mo, and 1.1 – Mo.
According to these data, the nuclei of the synthesized
materials consist of pure molybdenum, molybdenum
carbide, and molybdenum oxide. Mo2C is the prevail-
ing molybdenum carbide phase, which is stable at low
temperatures and is characterized by a close-packed
hexagonal crystalline structure with carbon atoms
located in the octahedral voids.
To analyze the composition of the synthesized
nanomaterials, transmission electron microscopy
(TEM) was used. The samples were collected on spe-
cial copper grids coated with a perforated carbon layer
for electron microscope. The grids were installed on a
special filter element perpendicularly to the gas flow
after the working section of the reactor. The nanopar-
ticle samples were analyzed using an Osiris transmis-
sion electron microscope (FEI Company, United
States) with an accelerating voltage of 200 kV at the
Federal Research Center for Crystallography and
Photonics, Russian Academy of Sciences. The micro-
graphs of nanoparticles with different resolutions were
used to determine the nanoparticle size and to esti-
mate the volume ratio of molybdenum and carbon in
the samples. The data on the phase and elemental
compositions of the nanomaterials were obtained by
electron microdiffraction and energy dispersive X-ray
spectroscopy (EDS), respectively, using the same
electron microscope. In addition, the latter was also
used to visualize the spatial distribution of elements in
the sample by high-angle annular dark-field scanning
transmission electron microscopy (HAADF-STEM).
In addition, the nanoparticle samples were ana-
lyzed by EDS and HAADF-STEM. The EDS spec-
trum (not given in the figures) showed the peaks of the
Mo, C, O, Cu, and Si elements. The Mo, C, and O
atoms are the components of the synthesized
nanoparticles. Copper and silicon are the materials
used in the manufacture of grids for electron micros-
copy. The distribution maps of the element Mo, C,
and O atoms in the synthesized nanoparticles (four
images of the same site in the sample) are presented in
Fig. 5. Figure 5a shows the image of a sample of
molybdenum–carbon nanoparticles obtained by the
HAADF-STEM method. The distribution of Mo
atoms (Fig. 5b) reflects the arrangement of nuclei in
RESULTS AND DISCUSSION
Figure 3 shows the micrograph of the nanomateri-
als synthesized in a flow reactor during the irradiation
of the CH4 + Mo(CO)6 mixture using 600 UV laser
pulses. The micrograph shows chains of dark
nanoparticles with an average size of ~10–50 nm sur- the aggregates of nanoparticles. The localization of
rounded and interconnected by the lighter material. carbon atoms (Fig. 5c) coincides with the position of
Along with large particles of 10–50 nm in diameter, nanoparticle aggregates shown in the HAADF-STEM
smaller particles with sizes of 1.5–2.5 nm were found. image and indicates that the molybdenum nucleus is
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METHANE DECOMPOSITION ON THE SURFACE
227
It should be emphasized that the decomposition of
CH4 into solid carbon and hydrogen as a result of the
reaction
(I)
CH4 → C(s) + 2H2(gas).
at room temperature is impossible from the thermody-
namic viewpoint [18] (the Gibbs energy of this reac-
tion, calculated as the difference between the Gibbs
energies of the reaction products and the reactants, is
ΔG2f98
positive at room temperature and equals
=
50.83 kJ/mol). A significant increase in temperature is
necessary for reaction (I) to proceed, which can occur
when the nanoparticles are heated with repeated laser
pulses. Under the conditions of a flow reactor, how-
ever, there was no secondary heating of the formed
molybdenum nanoparticles because of their displace-
ment downstream in the interval between two laser
pulses. In addition, any effect of laser radiation on the
methane molecule is also unlikely, which is confirmed
by the absence of absorption bands at 266 nm [16].
Nevertheless, in order to completely eliminate the
heating of molybdenum nanoparticles under the con-
ditions of the present experiments by repeated laser
pulses in a flow reactor, we performed experiments
with methane introduced significantly lower down-
stream, outside the working part, where Mo(CO)6 was
photodissociated. In this case, Mo(CO)6 vapors were
transferred with argon at a flow rate of 1.5 L/min. The
total consumption of argon and methane remained
equal to 3 L/min. The results indicate that the hydro-
gen yield was the same under these conditions as when
using methane as a carrier gas in the working part of
the reactor.
10 nm–1
Fig. 4. Electron diffraction pattern of the nanoparticle
samples synthesized by UV irradiation of the mixture of
methane with Mo(CO) vapor.
6
surrounded by the amorphous carbon material. The
distribution of oxygen atoms (Fig. 5d) covers the
region of localization of molybdenum atoms, but does
not exceed the area of distribution of carbon atoms. It
can be assumed that oxygen is both a component of
the nanoparticle nuclei, partially consisting of molyb-
denum oxides, and is adsorbed in a molecular form in
the carbon material. The presence of metal oxides in
the nanoparticles can be explained by their partial oxi-
dation during prolonged exposure to air, despite the
presence of a carbon shell [11, 12].
It follows from the electron microscopy data that
amorphous carbon is present on the surface of
nanoparticles, and that some part of carbon is found in
the nuclei of nanoparticles in the form of molybdenum
carbide Mo2C. The hydrogen gas released during the
experiment can be quantified.
The direct catalytic decomposition of methane into
hydrogen and carbon material proceeds quite effi-
ciently only at temperatures of 500–1000°С [19–23].
The main catalysts are Fe, Ni, and Co nanoparticles
and their alloys deposited on ceramic supports
[19, 21–23], as well as carbon black [20]. In the case of
the metal catalysts, the carbide cycle mechanism of
growth of the carbon material is generally accepted
[24]. The hydrocarbons are adsorbed and decompose
on a certain crystallographic surface of the metal par-
ticle microcrystal via the intermediate formation of
unstable carbide-like compounds. The evolving car-
bon atoms dissolve in the metal particle and are car-
ried out to another face by diffusion transfer through
the “body” of the metal particle. Carbon is released
into an independent graphite phase on it. The struc-
ture of this phase forms epitaxially under the action of
some features of this face.
The formation of a relatively large amount of
amorphous carbon material in the synthesized sam-
ples is explained by the influence of the electron beam
during the TEM analysis. It was found that the molyb-
denum nanoparticles were quickly coated with carbon
directly under the action of the electron beam; the
longer the exposure time and the larger the accelerat-
ing voltage of electrons, the higher the rate of coating.
The layer sprayed onto the microscope grid was the
source of the carbon material. Unfortunately, the grids
with other (e.g., silicon) coatings proved unsuitable for
analysis on the instrument used in this work. Figure 6
shows the micrographs of the same molybdenum
Thus, instead of the thermodynamically forbidden
nanoparticles coated with the carbon material to vary- reaction (I), we can assume that methane interacts
ing degrees after treatment with the electron beam. In with molybdenum, forming molybdenum carbide,
the left micrograph, the carbon layer around the which should also lead to a release of the hydrogen gas:
nanoparticles is narrower than that on the right micro-
(II)
CH + 2Mo → Mo C s + 2H gas .
( )
(
)
graph as it was made 10 s earlier.
4
2
2
KINETICS AND CATALYSIS Vol. 61 No. 2 2020

228
GURENTSOV et al.
(b)
(a)
Mo
200 nm
200 nm
(c)
(d)
O
C
200 nm
200 nm
Fig. 5. (a) HAADF-STEM image and the corresponding distribution maps of (b) Mo, (c) C, and (d) O atoms in the nanomaterial
samples synthesized by UV irradiation of the mixture of methane with Mo(CO) vapor.
6
100 nm
100 nm
Fig. 6. Buildup of carbon on the nanoparticles synthesized by UV irradiation of the mixture of methane with Mo(CO) vapor
6
under the action of an electron beam. The time difference between the left and right shots is 10 s.
KINETICS AND CATALYSIS
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METHANE DECOMPOSITION ON THE SURFACE
229
According to various sources, the Gibbs energy of hydrogen coincides in the order of magnitude with the
formation of molybdenum carbide Mo2C by reaction value calculated for reaction (II). Exact coincidence is
possible if it is assumed that 80% of condensed molyb-
denum is converted into carbide in its interaction with
methane.
(II) at room temperature is –52.2 [25], –51.4 [26], and
–65.8 kJ/mol [27], which makes this reaction thermo-
dynamically possible. Overcoming the energy barrier
of the reaction can be explained by the high activity of
free (without substrate) molybdenum nanoparticles of
~2 nm in the gas phase. The number of atoms in the
particles of these sizes is ~270. Thus, the majority of
atoms lie on the surface, and a nanoparticle of this
type has a high surface energy, which allows it to over-
come the barrier of methane decomposition and
molybdenum carbide formation. The overall low level
of conversion of hydrocarbons into hydrogen in our
experiments can be explained by the small number of
molybdenum nanoparticles relative to the amount of
methane and room temperature conditions. As is
known, low temperature conditions lower the rate of
chemical reactions according to the Arrhenius law.
If the formation of the recorded amount of hydro-
gen is really associated with methane decomposition
on the surface of molybdenum nanoparticles, then the
hydrogen yield should increase with their amount.
This assumption was confirmed experimentally: an
increase in the temperature in the reactor zone to 40°C
led to an increase in the vapor pressure of Mo(CO)6
and, accordingly, in the amount of condensed molyb-
denum. In this case, the hydrogen yield increased to
25 ppm.
The probability of formation of higher hydrocar-
bons (ethane, pentane, propane, butane) with hydro-
gen evolution from methane on a molybdenum cata-
lyst was considered in addition to reactions (I) and
(II). All of these reactions proved thermodynamically
impossible at room temperature. The only thermody-
namically allowed reaction was the decomposition of
CO molecules on the molybdenum nanoparticles that
formed during the photodissociation of Mo(CO)6:
Note that, unlike molybdenum carbide, the forma-
tion of iron, nickel, or cobalt carbides, often consid-
ered as catalysts for direct decomposition of methane,
is thermodynamically impossible at room temperature
[28, 29].
Given the number of laser pulses, measured laser
(III)
3CO + 7Mo → 3Mo2C(s) + MoO3(s).
energy, and absorption cross section of the Mo(CO)6
The Gibbs energy of reaction (III) is approximately
–413 kJ/mol at room temperature. However, this
reaction does not explain the hydrogen yield observed
in the experiment.
Another possible reason for hydrogen release might
be the presence of water vapor in the gases used. The
mass concentration of water vapor in methane is
0.03 g/m2; this corresponds to the volume fraction of
water vapor of 0.0039% (39 ppm), which is higher than
the observed hydrogen yield. The catalytic dissocia-
tion of water molecules by the reaction
molecule at a wavelength of 266 nm, 3 × 10–17 cm2, we
can determine the amount of condensed molybdenum
that formed during the experiment [30]. As indicated
above, the concentration of Mo(CO)6 molecules was
determined based on room temperature measure-
ments from the data on the saturated vapor pressure
[17]. According to the Lambert–Beer law, the ratio of
the laser energy transmitted through the absorbing
medium and incident energy can be calculated by the
equation
(1)
E E0 = exp(–Сσl) ,
(IV)
2Н О gas → 2Н gas + О gas
(
)
(
)
(
)
2
2
2
where E and E0 are the transmitted and incident
energy in the laser pulse, respectively; C is the concen-
tration of Mo(CO)6; σ is the absorption cross section
of Mo(CO)6; and l is the optical path of the laser beam
in the reactor (1.7 cm). Taking into account the exper-
imental average laser pulse energy (30 mJ) and the
Mo(CO)6 concentration (2.9 × 1015 cm–3) calculated
from the temperature dependence of vapor pressure
[17], the absorbed laser radiation energy in the pulse
was 3 mJ. As the release of one Mo atom requires three
photons with an energy of 7.5 × 10–19 J [31], the num-
ber of Mo atoms formed after 600 laser pulses was
1.1 × 1018. This corresponds to 1.8 × 10–6 mol of con-
densed molybdenum.
at room temperature is impossible because the Gibbs
energy of this reaction at Тroom is positive and equals
457 kJ/mol.
However, hydrogen can be released by the reaction
(V)
2Н О gas + Mo s → МоО s + 2Н gas .
(
)
( )
2 ( )
(
)
2
2
f
The Gibbs energy of reaction (V) is
= –76 kJ/mol,
which makes it possible at room temperature from the
ΔG298
thermodynamic viewpoint.
The formation of other molybdenum oxides is
impossible because of the positive Gibbs energies. For
example, for the formation of MoO3
(VI)
3Н О gas + Мо s → МоО s + 3Н gas ,
( ) 3 ( )
(
)
(
)
In accordance with reaction (II), 100% conversion
of molybdenum into carbide will lead to the same
molar yield of hydrogen gas collected in the damper
container. In the experiment, the hydrogen yield was
6 ppm (in a volume of 5 L), which corresponds to
2
2
f
is 18 kJ/mol. The same relates to other molyb-
ΔG298
denum oxides, Мо2О3 and Мо2О5.
Another variant of the mechanism of hydrogen for-
1.46 × 10–6 mol. Thus, the experimental yield of mation with the yield observed in the experiment
KINETICS AND CATALYSIS Vol. 61 No. 2 2020

230
GURENTSOV et al.
might be dissociative adsorption of water on the sur- with molybdenum nanoparticles with formation of
face of molybdenum nanoparticles:
MoO3 is impossible at room temperature.
(VII)
Н О g → Н О ad ,
( )
(
)
The dissociative adsorption of methane on the sur-
face of molybdenum nanoparticles with liberation of
hydrogen gas may be another route of hydrogen for-
mation.
2
2
(VIII)
Н О ad → ОН ad + Н(ad)
(
)
(
)
2
followed by recombination of hydrogen atoms
(IX)
Н ad + Н ad → Н ad
(
)
(
)
(
)
(XII)
(XII)
(XIV)
СН4 ad → CH ad + H ad ,
(
)
(
)
(
)
2
3
and their desorption from the surface
H ad + H ad → H ad ,
(
)
(
)
(
)
2
(X)
Н2 ad → Н g .
(
)
2 ( )
Н2 ad → Н g .
2 ( )
(
)
The overall reaction of the above mechanism is
For the first reaction to be thermodynamically pos-
sible at room temperature, the Gibbs energy of forma-
tion of adsorbed CH3 should exceed 180 kJ/mol in
magnitude (while the energy of formation of the free
CH3 radical at room temperature is –19.6 kJ/mol)
[33]. This is unlikely, taking into account that the
Gibbs energy of formation of a nonadsorbed stable
methane molecule is –50.82 at Troom. Indeed, accord-
ing to the data of [34], the probability of dissociative
adsorption of methane on the platinum surface at
400 K is ~10–8 and, accordingly, it is at least an order
of magnitude lower at 300 K. This is associated with
the relatively high activation energy of this process,
which is ~60 kJ/mol. Similar data were obtained more
recently in a study of the dissociative adsorption of
methane on crystalline nickel [35].
(XI)
Н О g → 2ОН ad + Н g .
( ) 2 ( )
(
)
2
For reaction (XI) to be thermodynamically possi-
ble at room temperature, the Gibbs energy of forma-
tion of the adsorbed OH radical should be lower than
for the water molecule (–223 kJ/mol) in the gas phase.
We think this unlikely because, according to published
data, the Gibbs energy of the adsorbed OH radical on
platinum is of the order of –58 kJ/mol [32].
Nevertheless, an additional series of experiments
were performed to analyze the possible effect of water
vapor on the yield of hydrogen during the interaction
with molybdenum nanoparticles. Nitrogen of 99.6%
purity with 0.009 vol % (90 ppm, according to the cer-
tificate) water vapor was used as a carrier gas. To intro-
duce additional water, nitrogen was bubbled through a
layer of distilled water poured into a separate sealed
container. The experiments were carried out at a tem-
perature of 21.4°C, which corresponds to the saturated
water vapor pressure of 2.5 kPa (0.025% in nitrogen or
250 ppm). Nitrogen saturated with the water vapor
passed through the container with Mo(CO)6 vapors
and entered the chamber with quartz windows for
introducing laser UV radiation. The flow mode in the
flow reactor was the same as in the experiments with
pure methane: 3 L/min for 1 min. At first, the hydro-
gen yield was measured with the sensor located in the
buffer container without exposure to laser radiation.
The yield was 0.3 ppm and corresponded to the hydro-
gen impurity concentration in the gas. In the experi-
ments conducted under similar conditions but with
exposure to 600 UV laser pulses with an average pulse
energy of 30 mJ for 1 min and, accordingly, with the
formation of molybdenum nanoparticles and water
vapor, a similar hydrogen yield of 0.3 ppm was
obtained. Thus, the release of hydrogen during the
interaction of water vapor with molybdenum nanopar-
ticles was not recorded in our experiments. The
absence of microdiffraction rings corresponding to
molybdenum oxide MoO2 on the electron diffraction
patterns of the samples was indirect evidence that
there was no interaction of water vapor with molybde-
num nanoparticles at room temperature. At the same
time, the data obtained indicated the existence of
MoO3. As shown above, the interaction of water vapor
Thus, having considered all of the above mecha-
nisms, we can state that the interaction of methane
with molybdenum nanoparticles with formation of
molybdenum carbide Mo2C by the carbide cycle
mechanism is the most probable explanation of the
yield of hydrogen under the given conditions [24].
CONCLUSIONS
Nanoparticles consisting of molybdenum, molyb-
denum carbide, and molybdenum oxide were synthe-
sized in a flow reactor by laser photodissociation of
gaseous precursors. Mo(CO)6 vapor was the source of
molybdenum atoms, and methane gas was the source
of carbon. It was found that the measured concentra-
tion of molecular hydrogen generated during the
decomposition of methane on the surface of molybde-
num nanoparticles was 5–25 ppm depending on the
temperature in the reactor zone (23–40°С). The main
result of the present study is experimental evidence for
the decomposition of methane on molybdenum
nanoparticles with formation of molybdenum carbide
and hydrogen at room temperature. The data obtained
show that it is necessary to take into account the yield
of hydrogen when hydrocarbons contact with metal
aerosols, which can affect, e.g., the combustion kinet-
ics of explosive mixtures.
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METHANE DECOMPOSITION ON THE SURFACE
231
FUNDING
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the Federal Research Center “Crystallography and Photon-
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KINETICS AND CATALYSIS Vol. 61 No. 2 2020