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 Tetrachloride Reduction with Hydrogen in the Presence of Pyrolytic Tungsten A. V. Vorotyntseva, V. M. Vorotyntseva, A. N. Petukhova, A. V. Kadomtsevaa, I. Yu. Kopersaka, M. M. Trubyanova, A. M. Ob’’edkovb, I. V. Pikulinc, V. S. Drozhzhinc, and A. A. Aushevc aNizhny Novgorod State Technical University n.a. R.E. Alekseev, ul. Minina 24, Nizhny Novgorod, 603950 Russia bRazuvaev Institute of Organometallic Chemistry, Russian Academy of Sciences, ul. Tropinina 49, Nizhny Novgorod, 603950 Russia cAll-Russia Research Institute of Experimental Physics, Russian Federal Nuclear Center, pr. Mira 37, Sarov, Nizhny Novgorod oblast, 607188 Russia e-mail: [email protected] 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; GeCl4 + 2H2 = Ge + 4HCl. (1) transistors; and microwave converters and in IR engi- According to thermodynamic calculations, equi- neering for the fabrication of optical elements, such as librium in reaction (1) can be displaced to the right at lenses, reflecting mirrors, windows, and lasers [1]. a temperature as low as 973 K [10]. Germanium is, High-purity germanium doped with specialty impuri- however, formed only at temperatures in the range ties continues to play a key role as one of the most 1173–1473 K in the presence of a considerable excess promising materials for the fabrication of highly sensi- of hydrogen (H2/GeCl4 = 15–20 [10]). The process is tive IR detectors. On a commercial scale, high-purity assumed to comprise two sequential steps [6–10]: germanium is produced via a “” route [2]. The main problem with the chloride route is the low ger- GeCl4 + H2 = GeCl2 + 2HCl, (2) manium yield (no higher than 70%), the high loss with GeCl2 + H2 = Ge + 2HCl. (3) discharges, and the contamination of the final product in the germanium tetrachloride If there is an insufficient hydrogen excess hydrolysis step for the preparation of germanium diox- (H2/GeCl4 < 10 at 900 K), equilibrium in reaction (2) ide [3–6]. is displaced to the left and the formation of dichloro- germylene (GeCl2) and chlorinated polygermanes is In several publications [4–6], an electrochemical possible [9]. process was proposed for monogermane synthesis. Thus, the objectives of this work were to produce The germanium-containing reactant in the reactions new, highly selective catalysts for the hydrogen reduc- involved is . The monogermane tion of germanium tetrachloride and determine the prepared by this process requires further purification principal kinetic and technological process parame- to remove a number of impurities [7–9]. 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.

919 920 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 . tungsten coatings have a horizontal layer structure. At One potentially attractive catalyst support from the substrate temperatures from 773 to 973 K, the coatings viewpoints of the feasibility of wide application, avail- have a vertical columnar structure. At substrate tem- ability of raw materials, and low cost is ash micro- peratures above 973 K, the coatings have a microcrys- spheres (AMs). They have a spherical shape, low loose talline structure. 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 aggressive media and high temperatures [11]. the reactor was 100 cm3 and the weight of the tungsten It is of interest to produce catalysts via additional hexacarbonyl was 1.7–2.0 g. The reactor rotation rate deposition of metals onto AMs. was 15–20 rpm. The samples were preheated for 1.5 h, and the tungsten coating growth time proper was 2 h. As shown earlier [12], silicon can be obtained with The thickness of the tungsten layer grown on the sur- a conversion yield of 85% based on silicon tetrachlo- face of the microspheres in one deposition cycle did ride at a temperature as low as 523 K. not exceed 130 nm. In a previous study [13], metals were shown to be Figure 1 shows micrographs of the surface of a capable of causing dissociative chemisorption of tungsten coating on AMs. The micrographs were molecular hydrogen. It is worth giving special mention obtained on a JEOL JSM-733 scanning electron to the fact that tungsten ensures significantly better microscope. It is seen that the pyrolytic tungsten forms results as to increasing the H–H bond length in an a uniform coating on the surface of the AMs, without optimized state (1.79718 Å for tungsten against, for peeling off from the surface of the AMs. example, 0.17765 Å for titanium), which is ten times greater in comparison with the other transition metals. The phase composition of the pyrolytic tungsten- This suggests that tungsten has considerable potential coated AM-based catalysts thus prepared was deter- as a metal for depositing onto AMs in order to obtain a mined by X-ray diffraction on a DRON-3M X-ray dif- highly selective catalyst [14]. fractometer (CuKα radiation, step scan mode, angular range 2θ = 10°–80°, scan step of 0.05°, counting time Thus, AM-based composite materials coated with of 5 s per data point, diffracted beam graphite mono- pyrolytic tungsten can be used as catalysts for the chromator). hydrogen reduction of germanium tetrachloride. Figure 2 shows an X-ray diffraction pattern of AM samples coated with pyrolytic tungsten. It is seen from EXPERIMENTAL the X-ray diffraction data in Fig. 2 that the starting AMs are a two-phase system composed of a crystalline Catalyst synthesis. The main reaction in the ther- Al2SiO5 phase and an amorphous tungsten phase evi- mal decomposition of tungsten hexacarbonyl can be ° ° represented by the scheme denced by a broad peak in the range 2θ = 36 to 40 . The pyrolytic tungsten-coated AM samples → W(CO)6 W + 6CO. (4) obtained were used for hydrogen reduction of germa- According to thermodynamic calculations, the nium tetrachloride. 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 The growth rate, composition, and surface mor- a Teflon sealant, which were nonreactive with germa- phology of the resulting tungsten coatings are influ- nium chloride and hydrogen chloride. 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 H2/GeCl4 =

INORGANIC MATERIALS Vol. 52 No. 9 2016 KINETICS OF GERMANIUM TETRACHLORIDE REDUCTION 921

μ (a) 20 m (b) 200 μm

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- H2/GeCl4 = 4.2 at a bubbler temperature of 273 K. The tion (5) is quantitative at a temperature of 1273 K. The 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 99.9999% hydrogen. The experimental setup included the reaction proceeded with 96% germanium tetra- two reactors, which could operate both in sequence chloride conversion at a temperature of 973 K. 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- the catalyst. The result was the formation of dark violet cluded bubbling through germanium tetrachloride. tungsten(VI) chloride crystals: The substances forming in our experiments were quantitatively analyzed by gas chromatography using a W2GeClH++→++42 WCl2Ge2HCl. 6 (6) 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. Al2SiO5 α-SiO2 The major reaction products were identified using 2 W 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- Intensity perature was 323 K (10 min), and 99.99999% helium was used as a carrier gas. The reaction products were 1 3 identified using the NIST-11 Mass Spectral Library and GCMS Real Time Analysis software. 10 16 22 28 34 40 46 52 58 64 70 RESULTS AND DISCUSSION 2θ, deg

The results obtained in this study demonstrate that Fig. 2. X-ray diffraction data for AM samples (size fraction the process in question involves the following reaction: 100–160 μm) coated with pyrolytic tungsten: (1) X-ray dif- fraction pattern of W (Fm3m), (2) X-ray diffraction pattern +→+ cub GeCl42 2H Ge 4HCl, W AМcatalyst. (5) of uncoated AMs, (3) X-ray diffraction pattern of Al2SiO5.

INORGANIC MATERIALS Vol. 52 No. 9 2016 922 VOROTYNTSEV et al.

5 GC–TCD Abgas Gas phase to analysis 5 Abgas GC–MS

A 9 6

7

44 A A 11 11 TCD T T 8 Catalyst Catalyst

A A

10

2 1

Three-way valve 3 H2 Two-way valve

Pressure control A GeCl4

Temperature control T 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) GeCl4-filled bubbler, (4) direct flow reactor, (5) cartridge filter, (6) gas chromato- graph/mass spectrometer, (7) six-way stopcock, (8) gas chromatograph (with TCD), (9) vacuum pump, (10) computer, (11) elec- tric heater.

INORGANIC MATERIALS Vol. 52 No. 9 2016 KINETICS OF GERMANIUM TETRACHLORIDE REDUCTION 923

mol % GeCl4 ln k 0.08 12

1 0.06 I 2 3 8 0.04

0.02 II 4 0.8 1.0 1.2 1.4 1.6 103/T, K–1 0 470 620 770 920 Fig. 5. Arrhenius plot of the reaction rate: the points rep- Temperature, K resent the experimental data and the line represents a lin- ear fit. Fig. 4. Germanium tetrachloride concentration as a func- tion of temperature at different flow rates: H2/GeCl4 = (I) 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- 30 mL/min. tubes modified with copper and nickel chloride nanoparticles was found to be lower than that for the hydrogen reduction of germanium tetrachloride with The obtained experimental data were used to con- no catalyst, which was 48.49 kJ/mol. Thus, the addi- struct a log–log plot of the reaction rate against con- tion of a W/AM catalyst to the reaction zone also centration, which can be represented by the equation decreases the activation energy by 16.23 kJ/mol. ln(v =±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 CONCLUSIONS concentration. Pyrolytic tungsten coatings have been produced on The slope of the plot indicates that the catalytic the surface of AMs under steady-state conditions reduction of germanium tetrachloride with hydrogen using tungsten hexacarbonyl as a precursor. The tung- is a zero-order reaction (0.401). Note that there is a sten coating thickness per deposition cycle does not large amount of reactant adsorption on the catalyst, so exceed 130 nm. The nanostructured composites thus the reaction rate is essentially independent of the reac- produced have been characterized by X-ray diffraction tant concentration because the rate of reactant diffu- and scanning electron microscopy. According to X-ray sion to an active catalyst center exceeds the rate at diffraction data, the pyrolytic tungsten coating grown which the active centers become empty after chemical by this process at a temperature of 673–693 K con- interaction. Moreover, a change in the rate at which sisted of an amorphous tungsten phase. 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. with hydrogen. The reaction was shown to be well rep- Using the present experimental data, we plotted the resented by a zero-order equation. Using the present logarithm of the rate constant against inverse tempera- experimental data, we plotted the logarithm of the rate ture. The Arrhenius plot was found to be linear (Fig. 5). constant against inverse temperature. From the slope of the plot, we evaluated the apparent activation From least squares fitting of the experimental data, energy: E = 32.36 kJ/mol. we obtained an equation for determining the rate con- a stant of the chemical reaction: Comparison of the apparent activation energies lnk = (3.051 ± 0.091) – (3.881 ± 0.116) × 103/T. (8) obtained with different catalysts for the catalytic reduction of germanium tetrachloride with hydrogen From the slope of the straight line in Fig. 5, we evalu- demonstrates that the activation energy is lower than ated the apparent activation energy: Ea = 32.36 kJ/mol. that for the hydrogen reduction of germanium tetra- 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 924 VOROTYNTSEV et al. energy for the catalytic reduction of germanium tetra- high-purity monogermane, J. Anal. Chem., 2010, chloride with hydrogen by 16.23 kJ/mol. 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, the catalytic reduction of germanium tetrachloride Growth of germanium nanowires using liquid GeCl4 as with hydrogen on the catalyst proposed here proceeds a precursor: the critical role of Si impurities, Chem. with 96% germanium tetrachloride conversion. Commun., 2009, pp. 5124–5126. 11. Polye mikrosfery v zolakh unosa elektrostantsii: sbornik nauchnykh statei (Hollow Microspheres in Power Plant Fly ACKNOWLEDGMENTS Ash: A Collection of Research Papers), Drozhzhin, V.S., The study of the kinetic characteristics was sup- Ed., Sarov: FGUP RFYaTs–VNIIEF, 2009. ported by the Russian Federation Ministry of Educa- 12. Vorotyntsev, A.V., Mochalov, G.M., and Vorotyn- tion and Science (state research target no. 2014/133, tsev, V.M., Kinetics of catalytic hydrogen reduction of project no. 2897, basic stage). The syntheses of the SiCl4 in the presence of nickel chloride, Inorg. Mater., tungsten-based catalytic systems were performed with 2013, vol. 49, no. 1, pp. 1–5. support from the Russian Foundation for Basic 13. Vorotyntsev, A.V., Zelentsov, S.V., Vorotyntsev, V.M., Research (project no. 16-38-00876 mol_a). et al., Quantum chemical simulation of dissociative hydrogen chemisorption on metallic surfaces of nano- clusters, Russ. Chem. Bull., 2015, vol. 64, no. 4, REFERENCES pp. 759–765. 1. Devyatykh, G.G., Gusev, A.V., and Vorotyntsev, V.M., 14. Bochmann, S., Untersuchungen zur Reaktivität im Preparation of high-purity germanium, Vysokochist. stofflichen System Ge–Metall–Cl–H, Doctoral Veshchestva, 1988, no. 1, pp. 5–16. (Chem.) Dissertation, Freiberg: Technischen Univ. Ber- 2. Vorotyntsev, V.M., RF Patent 2 230 830, 2004. gakademie Freiberg, 2008. 3. Vorotyntsev, A.V., Petukhov, A.N., Vorotyntsev, I.V., 15. Caussat, B. and Vahlas, C., CVD and powders: a great Sazanova, T.S., Trubyanov, M.M., Kopersak, I.Y., potential to create new materials, Chem. Vapor Deposi- Razov, E.N., Vorotyntsev, V.M., Low-temperature cat- tion, 2007, vol. 13, no. 9, pp. 443–445. alytic hydrogenation of silicon and germanium tetra- 16. Drozhzhin, V.S., Danilin, L.D., Pikulin, I.V., et al., on the modified nickel chloride. Appl. Cat. B: Functional materials based on fly ash microspheres, in Env., 2016, vol. 198, pp. 334–346. Polye mikrosfery v zolakh unosa elektrostantsii (Hollow 4. EEC Patent 1 654 400, 2006. Microspheres in Power Plant Fly Ash), Sarov: FGUP 5. WIPO Patent Application 2005005673, 2005. RFYaTs–VNIIEF, 2009, pp. 92–104. 6. Vorotyntsev, A.V., Zelentsov, S.V., and Vorotyntsev, V.M., 17. Kadomtseva, A.V., Vorotyntsev, A.V., Vorotyntsev, V.M., Quantum chemical simulation of et al., Effect of the catalytic system based on multi- hydrogenation, Russ. Chem. Bull., 2011, vol. 60, walled carbon nanotubes modified with copper pp. 1531–1536. nanoparticles on the kinetics of catalytic reduction of 7. Vorotyntsev, V.M., Drozdov, P.N., Vorotyntsev, I.V., germanium tetrachloride by hydrogen, Russ. J. Appl. and Smirnov, K.Y., Germane high purification by Chem. 2015, vol. 88, no. 4, pp. 595–602. membrane gas separation, Desalination, 2006, vol. 200, 18. Mochalov, L.A., Kornev, R.A., Nezhdanov, A.V., no. 1, pp. 232–233. Mashin, A.I., Lobanov, A.S., Kostrov, A.V., Vorotyn- 8. Vorotyntsev, V.M., Drozdov, P.N., and Vorotyntsev, I.V., tsev, V.M., and Vorotyntsev, A.V., Preparation of sili- High purification of substances by a gas separation con thin films of different phase composition from method, Desalination, 2009, vol. 240, no. 1, pp. 301– monochlorosilane as a precursor by rf capacitive 305. plasma discharge, Plasma Chem. Plasma Process., 2016, 9. Vorotyntsev, V.M., Mochalov, G.M., Shishkin, A.O., vol. 36, issue 3, pp. 849–856. doi 10.1007/s11090-016- and Suvorov, S.S., Gas-chromatographic determina- 9703-8 tion of the impurity composition of permanent gases, methane, carbon monoxide, and carbon dioxide in Translated by O. Tsarev

INORGANIC MATERIALS Vol. 52 No. 9 2016