Diffusion and Photon-Stimulated Desorption of CO on TiO₂(110)

Abstract

Thermal diffusion of CO adsorbed on rutile TiO₂(110) was studied in the 20–110 K range using photon-stimulated desorption (PSD), temperature-programmed desorption (TPD), and scanning tunneling microscopy. During UV irradiation, CO desorbs from certain photoactive sites (e.g., oxygen vacancies). This phenomenon was exploited to study CO thermal diffusion in three steps: first, empty these sites during a first irradiation cycle, then replenish them with CO during annealing, and finally probe the active site occupancy in the second PSD cycle. The PSD and TPD experiments show that the CO diffusion rate correlates with the CO adsorption energystronger binding corresponds to slower diffusion. Increasing the CO coverage from 0.06 to 0.44 monolayer (ML) or hydroxylation of the surface decreases the CO binding and increases the CO diffusion rate. Relative to the reduced surface, the CO adsorption energy increases and the diffusion decreases on the oxidized surface. The CO diffusion kinetics can be modeled satisfactorily as an Arrhenius process with a “normal” prefactor (i.e., ν = 10¹² s^–1) and a Gaussian distribution of activation energies where the peak of the distribution is ∼0.26 eV and the full width at half-maximum (fwhm) is ∼0.1 eV at the lowest coverage. The observations are consistent with a significant electrostatic component of the CO binding energy on the TiO₂(110) surface which is affected by changes in the surface dipole and dipole–dipole interactions