Origin of Poisoning in Methanol Photoreforming on TiO2(110): The Importance of Thermal Back-Reaction Steps in Photocatalysis

Alcohol photoreforming on titania represents a perfect model system for elucidating fundamental processes in the heterogeneous photocatalysis of semiconductors. One important but open question is the origin of poisoning during the photoreaction of primary alcohols on a bare, reduced rutile TiO2(110) crystal under ultrahigh vacuum conditions. By comparing the photocatalytic properties of methanol and 2-methyl-2-pentanol, it is demonstrated that the fading activity in methanol photoreforming does not originate from the often-assigned increase of trap states for photon-generated charge carriers. Instead, we attribute the apparent catalyst poisoning to an increased rate of thermal back reactions, particularly to that of the photochemical oxidation step. While overall back reactions are generally considered in photocatalysis, back reactions of individual steps are largely neglected so far. Our work shows that their inclusion in the reaction scheme is inevitable for the comprehensive modeling of photocatalytic processes.