Abstract
The origin of the enhanced catalytic performance of bulk iron molybdate catalysts with excess crystalline MoO 3 for methanol oxidation to formaldehyde was investigated with MoO 3, Fe 2O 3, Fe 2(MoO 4) 3, MoO 3/Fe 2(MoO 4) 3 and model supported MoO 3/Fe 2O 3 catalysts. Low-energy ion scattering (LEIS) analysis of the outermost surface layer revealed that the molybdate catalysts possess a monolayer of surface MoO x species. Temperature programmed CH 3OH-IR spectroscopy revealed that both intact surface CH 3OH * and surface CH 3O * species are present on the catalysts with both yielding HCHO for the redox molybdate catalysts. The addition of excess crystalline MoO 3 to the crystalline Fe 2(MoO 4) 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 MoO 3 is related to the formation of a surface MoO x monolayer on the bulk Fe 2(MoO 4) 3 phase. Thus, the catalytic active phase for bulk iron molybdate catalysts is the surface MoO x monolayer on the bulk crystalline Fe 2(MoO 4) 3 phase and the only role of the excess crystalline MoO 3 is to replenish the surface MoO x lost by volatilization during methanol oxidation.
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