Comparison of the properties of some molybdates with physicochemical characteristics of their cations

The investigation concerned the molybdates of elements belonging to various periods and groups of the Periodic system. The interaction between ions of a metal and molybdenum leads to a re-distribution of the electron density as expressed by the variation of the chemical bond value (valence), of their lability and symmetry of structure. In various molybdates the infrared spectra differ by a specific combination of frequencies corresponding to maxima of absorption. This is related both to the degree of distortion of structures and to various strength of the metal-oxygen bonds. An element exerts its influence through a bond with oxygen. In molybdates of the first and the second group of scheelite structure, the absorption end ν3(F2) is shifted to the low-frequency range as compared to that of MoO3. The molybdates of elements of the second group feature a relatively small distortion of molybdate-ion tetrahedrons and have clear-cut absorption bands in the ν3(F2) range. In molybdates of the transitional elements the distortion of the symmetry of molybdate-ions results in a substantial splitting of the absorption bands ν3(F2). The greater the electronegativeness of a metal, the greater the shift of the characteristic absorption bands toward the low-frequency range, thus evidencing of a greater lability of said bonds. The increase in the ionization potential is parallelled by that of the acceptor capacity of said cations, of the strength of adsorption of propylene and, accordingly, of the CO2 yield. A major part is that of the strength of the metal-oxygen bonds which govern the reactivity of oxygen. A general conclusion is made that the nature of the cation has a decisive effect upon the strength of adsorption of the oxidized olefins and the reactivity of the metal-oxygen bonds, which, in the final analysis, controls the variation of the catalytic properties in the molybdate series