The role of the suprastoichiometric molybdenum during methanol to formaldehyde oxidation over Mo–Fe mixed oxides

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Iron molybdates are common industrial catalysts for methanol oxidation. The role of Mo excess, usually present in the catalyst formulation still doubtful. Some authors claim a stability effect whereas others claim that surface MoO3, segregated from bulk, is the active phase.In order to clarify the role of Mo excess on the catalytic performances, catalysts were prepared by Mo surface loading of stoichiometric Fe2(MoO4)3. No crystalline MoO3 was detected even for the sample prepared with five monolayers (maximum number) of MoO3. XPS data reveals a Mo surface enrichment even for the stoichiometric mixed oxide sample. All the samples presented Mo/Fe atomic ratio higher than the stoichiometric value (1.5). Adding of MoO3 monolayers seems to promote surface aggregation of Mo species.Catalytic data showed that Mo surface enrichment of the stoichiometric phase leads to an increase of formaldehyde yield for loadings higher than 0.5 monolayers. The observed behavior seems to result from a compromise between the higher catalytic activity of the Fe2(MoO4)3 phase and the lower activity but higher selectivity of the MoO3 phase. A cooperative effect between the MoO3 and the molybdate phase seems to dictate the catalytic performances.