It is well known for decades that supported vanadium oxide species are active catalysts for selective oxidation reactions, for example for the oxidation of alcohols to the corresponding aldehydes. The activity was suggested to be influenced by the size of the supported vanadia species (monomer, dimer, oligomer, nanoparticle), as well as the nature of the oxide support (e.g. SiO2, Al2O3, ZrO2...). In this contribution, we show that isolated VO4 species on silica are also active for the photocatalytic selective oxidation of methanol to formaldehyde at room temperature. The selectivity appears to be ~100%. Different from the thermal heterogeneous catalytic reaction, the rate-determining step in the photocatalytic reaction is formaldehyde desorption. We can clearly demonstrate that V2O5 nanoparticles are instead not active at all in the photocatalytic reaction. This is tentatively attributed to the different position of occupied and unoccupied energy levels on the electrochemical energy scale caused by the quantum size effect. Apart from this size dependence, the reactivity also depends on the nature of the support. When an alumina support is used instead of silica, formaldehyde is not formed. Instead, surface species are obtained that contain an OCO functional group (e.g. formate, carbonate, acetate).

Teaching an old dog new tricks: Size-dependent photocatalytic activity of supported vanadium oxide species

255th National Meeting (2018)

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