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, Ti n O2n–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 Ti n O2n–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 H2 and O2 is a strongly endothermic process by ∼116 kcal/mol, photocatalytic processes are necessary only in the subsequent steps.

Density functional theory (DFT) has been used to study the hydrolysis reaction of (MO2) n (M = Zr, Hf, n = 1–4) nanoclusters in the ground singlet and first triplet states. The reactions for singlet n = 1 were benchmarked at the CCSD(T) level of theory. The reactions of H2O with the metal site having an MO bond and/or M–O bonds as well as H transfer to both terminal O atoms and bridge −O atoms have been studied. The partial charge on M increases as the MO bonds are replaced with M–OH bonds. The first H2O adsorption (physisorption) energies for these MO2 nanoclusters are calculated to be −20 to −30 kcal/mol for the singlet state and −15 to −48 kcal/mol for the triplet state. These physisorption energies depend on the cluster size and the adsorption site, consistent with existing experimental and computational studies. The first hydrolysis (dissociative chemisorption) reaction energies of the MO2 nanoclusters are calculated to have a much broader range, −30 to −80 kcal/mol for the singlet states and −30 to −100 kcal/mol for the triplet states. Steric effects play an important role in determining the physisorption and chemisorption energies, especially for the trimers and tetramers. The potential energy surfaces for hydrolysis in both the singlet and triplet states are calculated. The calculated Lewis acidities (fluoride affinities) correlate with the hydrolysis properties of the nanoclusters. Our calculations show that H2O readily reacts with both the singlet and triplet states of the MO2 nanoclusters to form the hydroxides. The reaction barriers are generally less than 10 kcal/mol for the singlet states, and because the H2O physisorption energies are large, the barriers occur below the (MO2) n asymptote.