The Role of Defects in the Photocatalytic Conversion of Alcohols on Rutile TiO₂(110)

The role of defects is controversially discussed in photocatalysis. Commonly, they are seen as trap states for photon-generated charge carriers, which either promote charge separation and enhance the activity of the photocatalyst or act as recombination centers and lower the photocatalytic performance of the material. The present work illustrates the crucial role of defects as potential reaction centers in photocatalysis. We show that the photocatalytic activity of a rutile TiO₂(110) single crystal toward alcohols in the gas phase can be varied by its degree of reduction. Specific heat treatments of the semiconductor lead to the formation of different concentrations of defect states, in particular bridge-bonded oxygen vacancies (BBO_V), to which we assign the role of photoactive sites. Here, the dissociative adsorption of the alcohol and its subsequent photoconversion occurs. Our evaluation of the catalytic methanol photooxidation under different reaction conditions further reveals the importance of alcohol surface diffusion, whose influence on the catalytic activity is rationalized based on a capture zone around each BBO_V. If a methanol molecule hits the TiO₂ surface in this zone, it can be photoconverted even when it does not land directly on the photoactive site. Finally, an analysis of the return to the dark state of reactant and products upon interrupting the illumination enables us to determine methoxy coverages on the photocatalyst prior to illumination, which also scale with the concentration of BBO_Vs. We find that methoxies abound on Pt-loaded titania in the dark, which explains the increased product formation in the first seconds of photocatalysis.