Proof of Equivalent Catalytic Functionality upon Photon‐Induced and Thermal Activation of Supported Isolated Vanadia Species in Methanol Oxidation

In this study, evidence is provided that isolated surface vanadia (VO4) species on SiO2 can similarly act as a thermal heterogeneous catalyst and as a heterogeneous photocatalyst. Structurally identical surface VO4 species catalyze the selective oxidation of methanol both by thermal activation and by UV‐light induction. Selectivity to formaldehyde appears to be unity. For the photocatalytic reaction at room temperature, formaldehyde desorption is rate limiting. With larger agglomerates or V2O5 nanoparticles, on the contrary, only the thermal reaction is feasible. This is tentatively attributed to the different positions of electronic states (HOMO/LUMO, valence/conduction band) on the electrochemical energy scale owing to the quantum size effect. Besides providing new fundamental insight into the mode of action of nanosized photocatalysts, our results demonstrate that tuning the photocatalytic reactivity of supported transition‐metal oxides by adjusting the degree of agglomeration is feasible. Isolation preferred: Supported isolated surface vanadia species on silica can catalyze the selective oxidation of methanol to formaldehyde similarly with thermal activation and in a photon‐induced process. Larger agglomerates and V2O5 nanoparticles are only active in thermal catalysis. As the catalytic properties are similar for all exposed vanadium sites, the differing reactivity must be related to changes in the photoabsorption properties as a function of size.