Computational Study of the Hydrolysis Reactions of the Ground and First Excited Triplet States of Small TiO2 Nanoclusters

Density functional theory and coupled cluster theory are used to study the hydrolysis reactions of (TiO2)(n) (n = 1-4) nanoclusters to provide insight into H2O activation on TiO2. The singlet-triplet energy gaps of (TiO2)(n) are predicted to lie between 30 and 65 kcal/mol, depending on the cluster size and structure, consistent with our previous studies. The excitation energies for the various hydroxides, TinO2n-m(OH)(2m) (n = 1-4, 1 <= m <= n) are predicted to be, in general, higher than those for (TiO2)(n). The partial charge on Ti increases as the Ti=O bonds are replaced with the Ti-OH bonds. The Ti=O and Ti-O frequencies in the triplet state of (TiO2)(n) and TinO2n-m(OH)(2m) are, in general, lower than those in the singlet state. The first H2O adsorption (physisorption) energies for these TiO2 nanoclusters are predicted to be -10 to -35 kcal/mol for the singlet states and -10 to -50 kcal/mol for the triplet states. These physisorption energies depend on the cluster size and the site of adsorption, consistent with existing experimental studies. In general, H2O prefers to physisorb on the Ti site with one Ti=O bond and two Ti-O bonds and at the Ti site with no Ti=O bond and three Ti-O bonds. The first hydrolysis (dissociative chemisorption) reaction energies of the TiO2 nanoclusters are predicted to be -20 to -70 kcal/mol for the singlet states and -15 to -80 kcal/mol for the triplet states. Both singlet and triplet potential energy surfaces for the hydrolysis are calculated. Our calculations show that H2O readily reacts with both the singlet and the triplet states of the TiO2 nanoclusters to form the hydroxides with reaction barriers of 5-16 kcal/mol for the singlet states and 5-26 kcal/mol for the triplet states for the first hydrolysis steps, which are, in general, less than the H2O complexation energies. Because H2O splitting to form H-2 and O-2 is a strongly endothermic process by similar to 116 kcal/mol, photocatalytic processes are necessary only in the subsequent steps.