Identification of the Active Species in Photochemical Hole Scavenging Reactions of Methanol on TiO2

Molecular and dissociative forms of adsorbed methanol were prepared on the rutile TiO2(110) surface to study their relative photocatalytic activity for hole-mediated oxidation. Molecular methanol is the dominant surface species on the vacuum-annealed TiO2(110) surface in ultrahigh vacuum (UHV). Coadsorption of methanol with oxygen results in ∼20% of the adsorbed methanol decomposing to methoxy and OH. Subsequent heating of the surface to ∼350 K desorbs unreacted methanol and OH (as water), leaving a surface with only adsorbed methoxy groups. Using temperature-programmed desorption (TPD), we show that adsorbed methoxy is at least an order of magnitude more reactive than molecularly adsorbed methanol for hole-mediated photooxidation. Methoxy photodecomposes through cleavage of a C–H bond forming adsorbed formaldehyde and a surface OH group. These results suggest that methoxy, and not molecular methanol, is the effective hole scavenger in photochemical reactions of methanol on TiO2.