ADSORPTION OF METHANOL ON TIO2(110) - A FIRST-PRINCIPLES INVESTIGATION

We have performed first-principles static and dynamic calculations bas ed on density functional theory and the pseudopotential method to inve stigate the adsorption sind deprotonation of methanol on the stoichiom etric (110) surface of TiO2. Static calculations, employing full relax ation of adsorbate and substrate atom positions, are performed. In the high-coverage limit (theta = 1), we find that there are several struc tures of approximately equal stability. In two of these, the methanol molecule is dissociated, resulting from scission of the O-H or C-O bon ds. In the third, methanol is molecularly adsorbed. Other structures o f approximately equivalent energy contain 1:1 mixtures of these confor mations. At lower coverage (theta = 1/2), we find that the two dissoci ative modes of adsorption found at theta = 1 are favored over molecula r adsorption by 19 kJ/mol (O-H scission) and 7 kJ/mol (C-O scission). The adsorption energy of the most stable theta = 1/2 conformation chan ges by approximately +/-5% as the coverage is reduced to theta = 1/3 a nd theta = 1/4. Intermolecular attractions and repulsions are found to play a crucial role in determining the stability of different conform ations at different coverages. Conversion of the metastable theta = 1/ 2 molecularly adsorbed complex via O-H scission to a dissociated compl ex is predicted to be barrierless. First-principles molecular dynamics calculations on this system in which the methanol molecule approaches the surface predict spontaneous dissociation by rupture of the O-H bo nd and also that C-O bond breaking is likely to be an activated proces s. Further dynamical simulations indicate that the probability of find ing conformations other than that obtained after O-H bond rupture is s mall.