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

Abstract

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.

Light-induced water splitting by titanium-tetrahydroxide: a computational study

“…This energy is determined by the energies of the products Ti(OH) 3 6 and Ti 3 O 4 (OH) 4 that are obtained by the hydrolysis of the Ti 2 O 2 and Ti 3 O 6 species, respectively. 30 The main intermediates of the reactions (1)-(7) are compared for the corresponding reactants. The products P are obtained by attaching a hydrogen atom to a terminal a or a bridging oxygen.…”

“…By a subsequent Ti-O bond breaking, the O radical binds to the adsorbed hydroxyl radical leading to the formation of a Ti-O-O-Ti peroxide species. [29][30][31][32][33] In a recent study, Fang and Dixon investigated the ground and excited state H 2 and O 2 production reactions initiated, in particular, by Ti(OH) 4 -the hydrolysis product of TiO 2 . At the surface the water decomposition is viewed as an electrocatalytic process, driven by the electrochemical potential in which the system is considered to be in the ground state.…”

“…In an aqueous environment one can envisage that Ti(OH) 4 will react with an additional water molecule. The Ti(OH) 4 molecule is readily obtained in the hydrolysis reaction of TiO 2 , 30 representing thereby a minimal model for the hydroxylated titanium oxide system. The Ti(OH) 4 molecule is readily obtained in the hydrolysis reaction of TiO 2 , 30 representing thereby a minimal model for the hydroxylated titanium oxide system.…”

Dissociative Water Adsorption on Gas-Phase Titanium Dioxide Cluster Anions Probed with Infrared Photodissociation Spectroscopy

“…Several theoretical groups have reported calculations of molecular and dissociative water adsorption and water splitting on small (TiO 2 ) n clusters [50][51][52][53][54][55][56]; others have studied these interactions on larger TiO 2 nanostructures [57][58][59][60]. The most thorough body of work comes from calculations by Dixon and coworkers [61][62][63] regarding water adsorption, hydrolysis, and H 2 /O 2 evolution on small neutral titanium dioxide clusters. Most recently, these authors used a hybrid genetic algorithm to rigorously identify the lowest-energy structures for the neutral [(TiO 2 ) n (H 2 O) m ] (n ≤ 4; m ≤ 2n) clusters [63].…”

ZnO and TiO2 clusters as catalyst in the addition and abstraction reaction of acrylic acid with the OH radical

“…36,37] and TiO 2 [Refs. [38][39][40][41] bare metal oxide clusters. Hence, the optimized structures of ZnO and TiO 2 clusters are given in Figure S1 (abstraction) and Figure S2 (…”