Interaction, Noncrystalline Structure and Catalysis Property on the Interface of Five Transition Metal Binary Oxides (II) DSC, Semiconductor Gas-sensitivity, Catalysis Activity and Meta-monolayer Dispersion Model

Continuing last paper, DSC results show that the exothermic and endothemic peaks could be attributed the formation of solid solution, lattice distortion or disintegration, melting, sintering and interface chemical reactions. Conductibility measurements prove these binary oxides are N-type semiconductors, their resistance decrease in o-dimethylbenzene atmosphere, i. e., they are sensitive to this vapor, and all the sensitivities depend linearly on the vapor concentration during the initial stage of chemical adsorption. Catalysis tests show V2O5-MoO3 and WO3-MoO3 are all catalytically active in the selective oxidation of o-dimethylbenzene to phthalic anhydride. Conversion and selectivity measurements indicate that non-crystalline dispersed MoO3 and V2O5 are obviously advantageous to the catalytic reaction, and selectivity is at its best when catalyst composition is close to the dispersed threshold value. In order to explain satisfactorily the relation of great dispersed threshold values with small specific area, the meta-monolayer dispersion model was suggested: each component oxide should be dispersed on other's surface with the unit of spherical octahedron MeO6 close-packed with each other by sharing O atoms to form a two-dimensional monolayer, and limited numbers of such monolayers are stacked up to form the meta-monolayer dispersed interfacial transition layer of a binary oxide. Seven parameters of the meta-monolayer model were calculated and listed:average mono- layer number (nL=1.6-8.5 layers), thickness per monolayer (tL=0.678-0.718nm), stacking-up thickness of each component oxide (tp=1.10-5.81nm), total thickness of interfacial transition layer (tI=5.43-8.78nm), octahedron radius (r0=0.199-0.219nm), close packed-monolayer capacity (C = 0.088-0.264g/100m2) and dispersed threshold value (Dt=0.33-1.43g/100m2). By discussing the relation of meta-monolayer dispersion with non-crystalline phase, the mechanism of crystalline phase loss, the non-crystalline structure and the thermal meta-stability of interfacial transition layer of binary oxide catalyst were described, respectively.