Kinetics of germanium tetrachloride reduction with hydrogen in the presence of pyrolytic tungsten

Article abstract of DOI:10.1134/S002016851609017X

Pyrolytic tungsten coatings have been produced on the surface of ash microspheres under steady-state conditions using tungsten hexacarbonyl as a precursor. The nanostructured composites thus obtained were characterized by X-ray diffraction and scanning electron microscopy. We have studied the kinetics of the catalytic reduction of germanium tetrachloride with hydrogen in the temperature range 423–973 K in the presence of the composites as catalysts and determined the reaction order and activation energy for the catalytic reduction of germanium tetrachloride with hydrogen.

Full text of DOI:10.1134/S002016851609017X

ISSN 0020-1685, Inorganic Materials, 2016, Vol. 52, No. 9, pp. 919–924. © Pleiades Publishing, Ltd., 2016.
Original Russian Text © A.V. Vorotyntsev, V.M. Vorotyntsev, A.N. Petukhov, A.V. Kadomtseva, I.Yu. Kopersak, M.M. Trubyanov, A.M. Ob’’edkov, I.V. Pikulin, V.S. Drozhzhin,
A.A. Aushev, 2016, published in Neorganicheskie Materialy, 2016, Vol. 52, No. 9, pp. 985–990.
Kinetics of Germanium Tetrachloride Reduction with Hydrogen
in the Presence of Pyrolytic Tungsten
a
a
a
a
a
A. V. Vorotyntsev , V. M. Vorotyntsev , A. N. Petukhov , A. V. Kadomtseva , I. Yu. Kopersak ,
a
b
c
c
c
M. M. Trubyanov , A. M. Ob’’edkov , I. V. Pikulin , V. S. Drozhzhin , and A. A. Aushev
Nizhny Novgorod State Technical University n.a. R.E. Alekseev, ul. Minina 24, Nizhny Novgorod, 603950 Russia
a
b
Razuvaev Institute of Organometallic Chemistry, Russian Academy of Sciences,
ul. Tropinina 49, Nizhny Novgorod, 603950 Russia
c
All-Russia Research Institute of Experimental Physics, Russian Federal Nuclear Center,
pr. Mira 37, Sarov, Nizhny Novgorod oblast, 607188 Russia
e-mail: an.vorotyntsev@gmail.com
Received September 15, 2015; in final form, January 19, 2016
Abstract—Pyrolytic tungsten coatings have been produced on the surface of ash microspheres under steady-
state conditions using tungsten hexacarbonyl as a precursor. The nanostructured composites thus obtained
were characterized by X-ray diffraction and scanning electron microscopy. We have studied the kinetics of the
catalytic reduction of germanium tetrachloride with hydrogen in the temperature range 423–973 K in the
presence of the composites as catalysts and determined the reaction order and activation energy for the cata-
lytic reduction of germanium tetrachloride with hydrogen.
Keywords: germanium tetrachloride, germanium, pyrolytic tungsten, ash microspheres, heterogeneous catal-
ysis, kinetics, reduction
DOI: 10.1134/S002016851609017X
INTRODUCTION
tetrachloride with hydrogen on a heated germanium
substrate:
Elemental germanium is widely used in the fabrica-
tion of fast-recovery, parametric, and tunnel diodes;
transistors; and microwave converters and in IR engi-
neering for the fabrication of optical elements, such as
lenses, reflecting mirrors, windows, and lasers [1].
High-purity germanium doped with specialty impuri-
ties continues to play a key role as one of the most
promising materials for the fabrication of highly sensi-
tive IR detectors. On a commercial scale, high-purity
germanium is produced via a “chloride” route [2]. The
main problem with the chloride route is the low ger-
manium yield (no higher than 70%), the high loss with
hydrochloric acid discharges, and the contamination
of the final product in the germanium tetrachloride
hydrolysis step for the preparation of germanium diox-
ide [3–6].
GeCl + 2H = Ge + 4HCl.
(1)
4
2
According to thermodynamic calculations, equi-
librium in reaction (1) can be displaced to the right at
a temperature as low as 973 K [10]. Germanium is,
however, formed only at temperatures in the range
1
173–1473 K in the presence of a considerable excess
of hydrogen (H /GeCl = 15–20 [10]). The process is
2
4
assumed to comprise two sequential steps [6–10]:
GeCl + H = GeCl + 2HCl,
(2)
(3)
4
2
2
GeCl + H = Ge + 2HCl.
2
2
If there is an insufficient hydrogen excess
H /GeCl < 10 at 900 K), equilibrium in reaction (2)
(
2
4
is displaced to the left and the formation of dichloro-
germylene (GeCl ) and chlorinated polygermanes is
2
In several publications [4–6], an electrochemical
process was proposed for monogermane synthesis.
The germanium-containing reactant in the reactions
involved is germanium dioxide. The monogermane
prepared by this process requires further purification
to remove a number of impurities [7–9].
possible [9].
Thus, the objectives of this work were to produce
new, highly selective catalysts for the hydrogen reduc-
tion of germanium tetrachloride and determine the
principal kinetic and technological process parame-
ters ensuring stable operation of the catalysts under
In the context of the above difficulties, there is aggressive conditions. It is also of interest to find new
considerable interest in the preparation of germanium germanium synthesis methods capable of lowering the
tetrachloride by the direct reduction of germanium process temperature.
919

9
20
VOROTYNTSEV et al.
One such method is catalytic hydrogen reduction in a tungsten coating is determined to a significant
of germanium tetrachloride, which allows the reaction degree by the substrate temperature [16].
temperature to be lowered.
Drozhzhin et al. [16] investigated the morphology
A key issue in the preparation of germanium by cat- and structure of tungsten coatings produced at various
alytic reduction is a search for new, highly selective deposition temperatures. Their results demonstrate
catalysts capable of operating in atmospheres of that, at a tungsten hexacarbonyl evaporator tempera-
aggressive substances, such as germanium tetrachlo- ture of 323 K and a substrate temperature of 673 K, the
ride and hydrogen chloride.
tungsten coatings have a horizontal layer structure. At
substrate temperatures from 773 to 973 K, the coatings
have a vertical columnar structure. At substrate tem-
peratures above 973 K, the coatings have a microcrys-
talline structure.
One potentially attractive catalyst support from the
viewpoints of the feasibility of wide application, avail-
ability of raw materials, and low cost is ash micro-
spheres (AMs). They have a spherical shape, low loose
bulk density, high mechanical strength, good thermal
In this study, pyrolytic tungsten coatings were
stability, and sufficient chemical inertness. AMs are grown at a reactor temperature of 673–693 K over a
promising raw materials for the preparation of catalysts period of 2–3 h, using tungsten hexacarbonyl as a pre-
and adsorbents capable of functioning when exposed to cursor. The volume of the microspheres loaded into
3
aggressive media and high temperatures [11].
It is of interest to produce catalysts via additional
deposition of metals onto AMs.
As shown earlier [12], silicon can be obtained with
a conversion yield of 85% based on silicon tetrachlo-
ride at a temperature as low as 523 K.
the reactor was 100 cm and the weight of the tungsten
hexacarbonyl was 1.7–2.0 g. The reactor rotation rate
was 15–20 rpm. The samples were preheated for 1.5 h,
and the tungsten coating growth time proper was 2 h.
The thickness of the tungsten layer grown on the sur-
face of the microspheres in one deposition cycle did
not exceed 130 nm.
In a previous study [13], metals were shown to be
capable of causing dissociative chemisorption of
molecular hydrogen. It is worth giving special mention
to the fact that tungsten ensures significantly better
results as to increasing the H–H bond length in an
optimized state (1.79718 Å for tungsten against, for
example, 0.17765 Å for titanium), which is ten times
greater in comparison with the other transition metals.
This suggests that tungsten has considerable potential
as a metal for depositing onto AMs in order to obtain a
highly selective catalyst [14].
Figure 1 shows micrographs of the surface of a
tungsten coating on AMs. The micrographs were
obtained on a JEOL JSM-733 scanning electron
microscope. It is seen that the pyrolytic tungsten forms
a uniform coating on the surface of the AMs, without
peeling off from the surface of the AMs.
The phase composition of the pyrolytic tungsten-
coated AM-based catalysts thus prepared was deter-
mined by X-ray diffraction on a DRON-3M X-ray dif-
fractometer (CuK radiation, step scan mode, angular
α
range 2θ = 10°–80°, scan step of 0.05°, counting time
of 5 s per data point, diffracted beam graphite mono-
chromator).
Thus, AM-based composite materials coated with
pyrolytic tungsten can be used as catalysts for the
hydrogen reduction of germanium tetrachloride.
Figure 2 shows an X-ray diffraction pattern of AM
samples coated with pyrolytic tungsten. It is seen from
the X-ray diffraction data in Fig. 2 that the starting
AMs are a two-phase system composed of a crystalline
Al SiO phase and an amorphous tungsten phase evi-
EXPERIMENTAL
Catalyst synthesis. The main reaction in the ther-
mal decomposition of tungsten hexacarbonyl can be
represented by the scheme
2
5
denced by a broad peak in the range 2θ = 36° to 40°.
The pyrolytic tungsten-coated AM samples
obtained were used for hydrogen reduction of germa-
nium tetrachloride.
W(CO) → W + 6CO.
(4)
6
According to thermodynamic calculations, the
main reaction in tungsten hexacarbonyl decomposi-
Experimental setup. To experimentally determine
tion can be accompanied by reactions that lead to the the key kinetic parameters of the catalytic hydrogen
formation of free carbon, tungsten carbides, and tung- reduction of germanium tetrachloride, we used an
sten oxides of various compositions [15], which can be experimental setup schematized in Fig. 3. It was made
obtained under various conditions of the thermal of 316SS stainless steel and included various Swageloc
decomposition process.
globe valves with Dow Corning 111 valve lubricant and
a Teflon sealant, which were nonreactive with germa-
nium chloride and hydrogen chloride.
The growth rate, composition, and surface mor-
phology of the resulting tungsten coatings are influ-
enced primarily by the thermal conditions and pres-
The reactor temperature was varied in the tempera-
sure in the reactor and pumping rate. The thermody- ture range from 423 to 1123 K. Germanium tetrachlo-
namic feasibility of the formation of impurities in the ride was fed into the reactor by hydrogen bubbling
form of free carbon and tungsten carbides and oxides through germanium tetrachloride in the ratio H /GeCl =
2 4
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KINETICS OF GERMANIUM TETRACHLORIDE REDUCTION
921
20 μm
200 μm
(a)
(b)
Fig. 1. Micrographs of the surface of a pyrolytic tungsten coating on ash microspheres.
2
.3 at a bubbler temperature of 293 K and in the ratio
It is known that, in the absence of a catalyst, reac-
H /GeCl = 4.2 at a bubbler temperature of 273 K. The tion (5) is quantitative at a temperature of 1273 K. The
2
4
hydrogen flow rates were 5, 15, and 30 mL/min. We hydrogen reduction of germanium tetrachloride in the
used 99.999%-pure germanium tetrachloride and
presence of an AM-based catalyst showed (Fig. 4) that
the reaction proceeded with 96% germanium tetra-
chloride conversion at a temperature of 973 K.
9
9.9999% hydrogen. The experimental setup included
two reactors, which could operate both in sequence
and in parallel. The inputs and outputs of the reactors
had pressure and temperature sensors. The bubbler
Further raising the temperature of reaction (5) was
vessel had a siphon input and output for the gas phase unreasonable because it changed the composition of
and a bypass line for reactor blowdown, which pre-
cluded bubbling through germanium tetrachloride.
the catalyst. The result was the formation of dark violet
tungsten(VI) chloride crystals:
The substances forming in our experiments were
quantitatively analyzed by gas chromatography using a
thermal conductivity detector on a Tswett-800 gas
chromatograph equipped with a vacuum sample inlet
system. We used a column packed with N-AW-HMDS
chromaton (0.16–0.20 mm), which supported E-301 liq-
uid phase (15%). The temperature was 373 K (6 min),
and 99.9999% hydrogen was used as a carrier gas.
The major reaction products were identified using
a Shimadzu GCMS-QP2010Plus gas chromato-
graph/mass spectrometer system equipped with a
heated vacuum sample inlet through a Valco Instruments
automatic injection valve and a sampling system. We
used an Agilent capillary column with a trifluoropropyl-
methylpolysiloxane-based stationary phase. The tem-
perature was 323 K (10 min), and 99.99999% helium
was used as a carrier gas. The reaction products were
identified using the NIST-11 Mass Spectral Library
and GCMS Real Time Analysis software.
W + 2GeCl + H → WCl + 2Ge + 2HCl. (6)
4
2
6
Al SiO5
2
α-SiO2
2
W
1
3
10
16 22 28 34 40 46 52 58 64 70
RESULTS AND DISCUSSION
2θ, deg
The results obtained in this study demonstrate that
the process in question involves the following reaction:
Fig. 2. X-ray diffraction data for AM samples (size fraction
100–160 μm) coated with pyrolytic tungsten: (1) X-ray dif-
fraction pattern of W (Fm3m), (2) X-ray diffraction pattern
cub
GeCl + 2H → Ge + 4HCl, W AМcatalyst. (5)
of uncoated AMs, (3) X-ray diffraction pattern of Al SiO .
4
2
2
5
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9
22
VOROTYNTSEV et al.
5
GC–TCD
Gas phase to analysis
Abgas
5
GC–MS
A
9
6
7
4
4
A
A
11
11
TCD
8
A
A
10
2
1
Three-way valve
Two-way valve
Pressure control
H2
3
A
T
GeCl4
Temperature control
Working mode
Blowdown
GC/MS analysis line
GC analysis line
Fig. 3. Schematic of the experimental setup for the catalytic reduction of germanium tetrachloride: (1) hydrogen generator,
(
2) gas flow division and rate controller, (3) GeCl -filled bubbler, (4) direct flow reactor, (5) cartridge filter, (6) gas chromato-
4
graph/mass spectrometer, (7) six-way stopcock, (8) gas chromatograph (with TCD), (9) vacuum pump, (10) computer, (11) elec-
tric heater.
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KINETICS OF GERMANIUM TETRACHLORIDE REDUCTION
923
mol % GeCl4
.08
lnk
12
0
1
2
3
0
.06
.04
I
8
0
II
0.02
4
0
.8
1.0
1.2
1.4
1.6
1
03/T, K
–1
0
4
70
620
770
920
Fig. 5. Arrhenius plot of the reaction rate: the points rep-
resent the experimental data and the line represents a lin-
ear fit.
Temperature, K
Fig. 4. Germanium tetrachloride concentration as a func-
tion of temperature at different flow rates: H /GeCl = (I)
2
4
2
.3 and (II) 4.2; flow rates of (1) 5.2, (2) 15.3, and (3)
cesses on catalysts based on multiwalled carbon nano-
tubes modified with copper and nickel chloride
nanoparticles was found to be lower than that for the
hydrogen reduction of germanium tetrachloride with
no catalyst, which was 48.49 kJ/mol. Thus, the addi-
tion of a W/AM catalyst to the reaction zone also
decreases the activation energy by 16.23 kJ/mol.
30 mL/min.
The obtained experimental data were used to con-
struct a log–log plot of the reaction rate against con-
centration, which can be represented by the equation
lnv = (0.401 ± 0.012)lnC – (6.897 ± 0.201), (7)
where v (mol/(L s)) is the rate of the chemical reac-
tion and C (mol/L) is the germanium tetrachloride
concentration.
CONCLUSIONS
Pyrolytic tungsten coatings have been produced on
the surface of AMs under steady-state conditions
using tungsten hexacarbonyl as a precursor. The tung-
sten coating thickness per deposition cycle does not
exceed 130 nm. The nanostructured composites thus
produced have been characterized by X-ray diffraction
and scanning electron microscopy. According to X-ray
diffraction data, the pyrolytic tungsten coating grown
by this process at a temperature of 673–693 K con-
sisted of an amorphous tungsten phase.
The slope of the plot indicates that the catalytic
reduction of germanium tetrachloride with hydrogen
is a zero-order reaction (0.401). Note that there is a
large amount of reactant adsorption on the catalyst, so
the reaction rate is essentially independent of the reac-
tant concentration because the rate of reactant diffu-
sion to an active catalyst center exceeds the rate at
which the active centers become empty after chemical
interaction. Moreover, a change in the rate at which
the mixture of germanium tetrachloride and hydrogen
was fed to the reactor, with an equivalent change in
The experimental data were used to construct a
catalyst layer thickness (from 0.05 to 0.20 m), did not log–log plot of the reaction rate against concentration
change the gas mixture composition, which also sug- for the catalytic reduction of germanium tetrachloride
gests that the process was not diffusion-limited.
Using the present experimental data, we plotted the
logarithm of the rate constant against inverse tempera-
ture. The Arrhenius plot was found to be linear (Fig. 5).
with hydrogen. The reaction was shown to be well rep-
resented by a zero-order equation. Using the present
experimental data, we plotted the logarithm of the rate
constant against inverse temperature. From the slope
of the plot, we evaluated the apparent activation
From least squares fitting of the experimental data,
we obtained an equation for determining the rate con-
stant of the chemical reaction:
energy: E = 32.36 kJ/mol.
a
Comparison of the apparent activation energies
obtained with different catalysts for the catalytic
reduction of germanium tetrachloride with hydrogen
demonstrates that the activation energy is lower than
that for the hydrogen reduction of germanium tetra-
3
lnk = (3.051 ± 0.091) – (3.881 ± 0.116) × 10 /T. (8)
From the slope of the straight line in Fig. 5, we evalu-
ated the apparent activation energy: E = 32.36 kJ/mol.
a
Kadomtseva et al. [17] and Mochalov et al. [18] chloride with no catalyst, which was 48.49 kJ/mol.
considered the reduction of germanium tetrachloride The use of a pyrolytic tungsten-coated AM-based cat-
on various catalysts. The activation energy for the pro- alyst (W/AM) was shown to reduce the activation
INORGANIC MATERIALS Vol. 52 No. 9 2016

9
24
VOROTYNTSEV et al.
energy for the catalytic reduction of germanium tetra-
chloride with hydrogen by 16.23 kJ/mol.
high-purity monogermane, J. Anal. Chem., 2010,
vol. 65, no. 6, pp. 634–639.
It has also been shown that, at a temperature of 973 K, 10. Hyun Jae Song, Seok Min Yoon, and Hyun Joon Shin,
Growth of germanium nanowires using liquid GeCl as
the catalytic reduction of germanium tetrachloride
with hydrogen on the catalyst proposed here proceeds
with 96% germanium tetrachloride conversion.
4
a precursor: the critical role of Si impurities, Chem.
Commun., 2009, pp. 5124–5126.
1
1. Polye mikrosfery v zolakh unosa elektrostantsii: sbornik
nauchnykh statei (Hollow Microspheres in Power Plant Fly
Ash: A Collection of Research Papers), Drozhzhin, V.S.,
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ACKNOWLEDGMENTS
The study of the kinetic characteristics was sup-
ported by the Russian Federation Ministry of Educa-
tion and Science (state research target no. 2014/133,
project no. 2897, basic stage). The syntheses of the
tungsten-based catalytic systems were performed with
support from the Russian Foundation for Basic
Research (project no. 16-38-00876 mol_a).
1
2. Vorotyntsev, A.V., Mochalov, G.M., and Vorotyn-
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clusters, Russ. Chem. Bull., 2015, vol. 64, no. 4,
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Translated by O. Tsarev
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