MORPHOLOGY OF MOLYBDENA SUPPORTED ON VARIOUS OXIDES AND ITS ACTIVITY FOR METHANOL OXIDATION

This paper describes a method of benzaldehyde-ammonia titration (BAT) that can distinguish surfaces of supports and loaded oxides. Such a method has been found to be effective in examining the morphology of molybdena when held on a variety of supports; the results achieved are corroborated by spectroscopic and electron microscopic studies. A molybdena monolayer of tetrahedral or octahedral coordination was loaded at a surface concentration of <7 nm-2, with ≲80% coverage, irrespective of the kind of support. On further loading, however, the MoO3 was not piled on the surface molybdate to form a multilayer but collected in the macropores of the support. This way of loading for MoO3 was completely different from that on supported V2O5 where the multilayer was formed depending on the electronegativity of the support. Methanol oxidation to formaldehyde and methyl formate was then investigated with use of these catalysts, and the turnover frequency (TOF) was given on the basis of the discriminated surface area of the loaded oxide. The TOF increased in proportion to the loaded molybdena in the monolayer region, which indicated that the active sites were the surface molybdate obtained at high concentrations of molybdenum, possibly the monolayer with the octahedrally coordinated MoO6. Strong dependence of the TOF upon the support was observed. © 1990 American Chemical Society.