A modulated molecular beam study of the extent of H2O dissociation on TiO2(110)

The adsorption-desorption kinetics of H2O have been studied on three well-defined surface configurations of TiO3(110) using temperature programmed desorption (TPD) and modulated molecular beam scattering in order to measure the extent of dissociative adsorption. The condensation coefficient for H2O is unity below 150 K and falls to zero near 200 K. The sticking coefficient is unity for all coverages and temperatures investigated. The adsorption-desorption kinetics are the same on the 1 x 1, 1 x 2 and argon-sputtered surfaces at the high coverages probed in TPD measurements. Small differences are observed at coverages between 10(-2) and 10(-4) monolayers probed in the beam studies. Separate monolayer and multilayer desorption peaks are observed with TPD, but it is not possible to distinguish between dissociative and non-dissociative adsorption based on a peak shape analysis. Based on the temperature dependence of the scattered H2O waveforms and the linearity of the kinetics as revealed by a Fourier analysis of the waveforms, several models for the adsorption-desorption kinetics can be ruled out. These include first- and second-order desorption from a uniform surface, and defect-mediated first-order desorption in the limit of rapid surface diffusion prior to desorption. The data are best fit by a model which assumes first-order desorption from an immobile adsorbate on a heterogeneous surface for air three surface phases. The degree of heterogeneity deduced from the modulated beam measurements is consistent with the TPD data. A very small degree of non-linearity in the desorption kinetics is the only evidence of H2O dissociation for surface temperatures as high as 500 K. We conclude that very few of the water molecules incident on the surface undergo dissociative adsorption, even in the limit of zero coverage. (C) 1998 Elsevier Science B.V.