Influence of composition, structure and symmetry of polyhedrons in solids on catalytical activity

A study of catalysis is made based on the participation of polyhedra of a definite composition, structure and symmetry in the elementary act for the case of metal oxide catalysts, used, as a rule, in reactions of hydrocarbon compounds. The influence of the type of polyhedra arrangement in the lattice, as well as their association by edges, vertices or faces is considered. A greater number of ''insular'' than that of ''condensed'' polyhedra in the lattice structure results in the maximum activity of a catalyst. It is shown that the orientation of polyhedra in the surface layer (with their vertices, edges or faces oriented outward) strongly affects the catalytic activity. The tetrahedron in one of these orientations will be the most active in adsorption and catalysis. Under the effect of the reaction mixture, temperature or other factors a re-orientation can take place in the bulk, as well as in the surface layer. This leads, as a rule to a decrease in adsorptive and catalytic activities of metal oxide catalysts. The application of catalysts is discussed of the type InMgInO4 having the structure of inverse spinels which enhances the catalytic effect. The influence on the catalytic activity of the coordination number and the nature of a neighbour polyhedron is considered for molybdates. In propylene oxidation Mg2Mo3O8 is more active han Zn2Mo3O8. Molybdates CuMoO4, Fe2(MoO4)3, V2MoO8, BaMoO4, Cr(MoO4)3, BaMoO4, CdMoO4, SrMoO4, ZnMoO4, MgMoO4 contain the following polyhedra: [MoO4]-, [CuO8]-, [ZnO6]-, [SrO8]-, [CdO8]-, [BaO8]-, [MgO6]-, [VO6], [CrO6], [MnO6], [FeO6]. The activity of these compounds in the oxidation reaction C3H6 can be presented by a series V > Fe >= Cu > Ni > Ti > Mn >= Zn > Cr > Co. These data indicate that both [MoO4]- and [MoO6]-polyhedra are active in the oxidation reaction but their activity depends substantially on the structure, composition and state of other polyhedra associated with them