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Kinetics of Germanium Tetrachloride Reduction with Hydrogen in the Presence of Pyrolytic Tungsten A

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Kinetics of Germanium Tetrachloride Reduction with Hydrogen in the Presence of Pyrolytic Tungsten A 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. 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 “chloride” 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) hydrochloric acid 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 germanium dioxide. 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 hydrogen chloride. 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.
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