Origin of the synergistic interaction between MoO3 and iron molybdate for the selective oxidation of methanol to formaldehyde

The origin of the enhanced catalytic performance of bulk iron molybdate catalysts with excess crystalline MoO3 for methanol oxidation to formaldehyde was investigated with MoO3, Fe2O3, Fe-2(MoO4)(3), MoO3/Fe-2(MoO4)(3) and model supported MoO3/Fe2O3 catalysts. Low-energy ion scattering (LEIS) analysis of the outermost surface layer revealed that the molybdate catalysts possess a monolayer of surface MoOx species. Temperature programmed CH3OH-IR spectroscopy revealed that both intact surface CH3OH center dot and surface CH3O center dot species are present on the catalysts with both yielding HCHO for the redox molybdate catalysts. The addition of excess crystalline MoO3 to the crystalline Fe-2(MoO4)(3) phase significantly increases the overall steady-state catalytic performance toward HCHO formation. The enhanced catalytic performance of bulk iron molybdate catalysts in the presence of excess MoO3 is related to the formation of a surface MoOx monolayer on the bulk Fe-2(MoO4)(3) phase. Thus, the catalytic active phase for bulk iron molybdate catalysts is the surface MoOx monolayer on the bulk crystalline Fe-2(MoO4)(3) phase and the only role of the excess crystalline MoO3 is to replenish the surface MoOx lost by volatilization during methanol oxidation. (C) 2010 Elsevier Inc. All rights reserved.