Reaction Kinetics of Water Molecules with Oxygen Vacancies on Rutile TiO2(110)

The formation of bridging hydroxyls (OHb) via reactions of water molecules with oxygen vacancies (V-O) on reduced TiO2(110) surfaces is studied using polarized infrared reflection-absorption spectroscopy (IRAS), electron-stimulated desorption (ESD), and photon-stimulated desorption (PSD). Narrow IR peaks at 2737 and 3711 cm(-1) are observed for the stretching vibrations of ODb and OHb, respectively. The IRAS spectra indicate that the bridging hydroxyls are oriented normal to the TiO2(110) surface. Using IRAS, we have studied the kinetics of water reacting with the vacancies by monitoring the formation of bridging hydroxyls as a function of the annealing temperature on the TiO2(110). Separate experiments have also monitored the loss of water molecules (using water ESD) and vacancies (using the CO photooxidation reaction) due to the reactions of water molecules with the vacancies. All three techniques show that the reaction rate becomes appreciable for T> 150 K and that the reactions are largely complete for T> 250 K. The temperature-dependent water-V-O reaction kinetics are consistent with a Gaussian distribution of activation energies with E-a = 0.545 eV, Delta E-a(fwhm) = 0.125 eV, and a normal prefactor, v = 10(12) s(-1). In contrast, a single activation energy with a physically reasonable prefactor does not fit the data well. Our experimental activation energy is close to theoretical estimates for the diffusion of water molecules along the Ti-5c, rows on the reduced TiO2(110) surface, which suggests that the diffusion of water controls the water-V-O reaction rate.