Conversion of methanol on rutile TiO2(110) and tungsten oxide clusters: 2. The role of defects and electron transfer in bifunctional oxidic photocatalysts

Abstract

The photochemical conversion of organic compounds on tailored transition metal oxide surfaces by UV irradiation has found wide applications ranging from the production of chemicals to the degradation of organic pollutants e.g. in waste water treatment. Here, we present a systematic surface science-based study of the UV photoconversion of methanol on a rutile TiO2(110) surface. Under the used conditions, the dominant photoreaction is the photo-oxidation forming formaldehyde, that is drastically boosted by the presence of adsorbed oxygen as well as (sub-)surface defects such as oxygen vacancies and Ti3+ interstitials. Moreover, a photostimulated and Ti3+ mediated C-C coupling was observed leading to the production of ethene. We have further deposited tungsten oxide clusters on the rutile surface and examined the impact on the methanol photochemistry. In this case, the C-C coupling can be suppressed. Surprisingly, especially for high Ti3+ contents the population of the photochemical pathway is quenched in favor of the population of the thermal reaction yielding more methane from the deoxygenation reaction. So, the common concept that long time charge separation is efficient by combining two photocatalysts with similar band gaps, but different work functions in order to enhance photochemical yields is apparently too naive for certain systems. We attribute the loss of photoproducts with tungsten oxide coadsorption to the "pinning"of Ti3+ centers and a related enhancement of electron density near the oxide clusters which makes a concomitant recombination of the photochemical relevant holes with the excess surface electrons more likely.