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 TiO 2 (110) surface to study their relative photocatalytic activity for hole-mediated oxidation. Molecular methanol is the dominant surface species on the vacuum-annealed TiO 2 (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 TiO 2 .