Molybdenum Carbides: Synthesis and Application in Catalysis
- PDF / 463,320 Bytes
- 8 Pages / 612 x 792 pts (letter) Page_size
- 70 Downloads / 344 Views
bdenum Carbides: Synthesis and Application in Catalysis A. V. Vasilevicha,*, O. N. Baklanovaa,**, and A. V. Lavrenova,*** a
Center of New Chemical Technologies, Boreskov Institute of Catalysis, Siberian Branch, Russian Academy of Sciences, Omsk, 644040 Russia *e-mail: [email protected] **e-mail: [email protected] ***e-mail: [email protected] Received February 14, 2020; revised May 18, 2020; accepted July 15, 2020
Abstract—The applications of catalysts based on molybdenum carbide are considered. Attention is focused on the effects of molybdenum carbide preparation methods on the crystal structure and catalytic activity of molybdenum carbide in the redox reactions of organic compounds. Keywords: molybdenum carbide, catalysis, hydrogenation, hydrodeoxygenation, hydrodesulfurization, methane reforming DOI: 10.3103/S0361521920060130
In recent years, there has been a growing interest in transition metal carbides, namely, molybdenum carbide. This paper summarizes data on currently available methods for the synthesis of molybdenum carbide and its practical use in catalysis. STRUCTURE OF TRANSITION METAL CARBIDES Transition metal carbides have unique physicochemical properties: high mechanical hardness, thermal stability, high melting point, resistance to mechanical and corrosive wear, and good thermal and electrical conductivity [1]. At the same time, catalysis on carbides, including the reactions of ammonia synthesis and decomposition, oxidation of hydrocarbons, hydrogenation, and hydrogenolysis, is of no less interest. Levy and Boudart [2] were the first to use carbides as catalysts; they found the formation of water from a hydrogen–oxygen mixture at room temperature in the presence of tungsten carbide. Metal carbides, most often molybdenum carbide, are considered as an alternative to supported metal and sulfide catalysts traditionally used in oil refining and petrochemical processes. Molybdenum carbides are compounds whose electronic structure is similar to the electronic structure of Group IX and X noble metals [3–5]. The crystal structures of transition metal compounds with carbon can be described as the closest packing of metal atoms with smaller carbon atoms inserted in the interstices. According to the Hegg rule,
the structures of transition metal carbides are determined by the ratio of radii R = Rx/RM, where Rx and RM are the atomic radii of the inserted element and the transition metal, respectively. If R < 0.59, the transition metal atoms in these phases are arranged according to the type of one of the simple crystal lattices, cubic or hexagonal, and the atoms of the inserted element occupy voids. If R > 0.59, compounds with more complex crystal lattices are formed. Molybdenum carbide is characterized by a hexagonal close-packed crystal structure. Molybdenum oxycarbide has a facecentered cubic lattice. MoC has a hexagonal structure with carbon atoms in trigonal prismatic voids [6, 7]. The crystal structure depends on geometrical and electronic factors. The introduction of carbon into the lattice of a transiti
Data Loading...
