Temperature- and Coverage-Dependent Kinetics of Photocatalytic Reaction of Methanol on TiO2 (110)-(1 x 1) Surface

We systematically investigated the photocatalytic reaction of methanol on the TiO2 (110)-(1 x 1) surface under irradiation with ultraviolet (UV) light performed at various conditions, using scanning tunneling microscopy (STM) jointed with temperature-programmed desorption (TPD) techniques. Our STM and TPD results show that the photocatalytic reaction is indeed initiated from the molecular methanol at the S-fold coordinated Ti sites, as commonly ascribed to the methanol oxidation by the photogenerated holes, reflecting the highly photoactive nature of methanol. The formaldehyde yield from the TPD results is much smaller by a factor of 2/3 than the amount of dissociated methanol from the STM results at 80 K. This observation can be assigned to the reverse reaction during the TPD measurement, and may explain the lower yield of formaldehyde using molecular methanol than using methoxy. From the fractal-like reaction kinetics of methanol, we can associate the coverage-dependence of the spectral dimensions with the change for the diffusion of holes across the surface from a one-dimensional to a two-dimensional behavior because of the increased scattering species at higher coverages. Our results here provide a clear picture for the photocatalytic reaction of molecular methanol and may rationalize the different observations performed at various conditions.