Adsorption of Methanol on TiO₂(110): A First-Principles Investigation

We have performed first-principles static and dynamic calculations based on density functional theory and the pseudopotential method to investigate the adsorption and deprotonation of methanol on the stoichiometric (110) surface of TiO2. Static calculations, employing full relaxation of adsorbate and substrate atom positions, are performed. In the high-coverage limit (θ = 1), we find that there are several structures of approximately equal stability. In two of these, the methanol molecule is dissociated, resulting from scission of the O−H or C−O bonds. In the third, methanol is molecularly adsorbed. Other structures of approximately equivalent energy contain 1:1 mixtures of these conformations. At lower coverage (θ = 1/2), we find that the two dissociative modes of adsorption found at θ = 1 are favored over molecular adsorption by 19 kJ/mol (O−H scission) and 7 kJ/mol (C−O scission). The adsorption energy of the most stable θ = 1/2 conformation changes by approximately ±5% as the coverage is reduced to θ = 1/3 and θ = 1/4. Intermolecular attractions and repulsions are found to play a crucial role in determining the stability of different conformations at different coverages. Conversion of the metastable θ = 1/2 molecularly adsorbed complex via O−H scission to a dissociated complex 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 bond and also that C−O bond breaking is likely to be an activated process. Further dynamical simulations indicate that the probability of finding conformations other than that obtained after O−H bond rupture is small.