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

Abstract

Water oxidation by Ti(OH)₄ in the ground and excited states was investigated using density functional (ΔSCF, TDDFT) methods gauged by the coupled cluster (CCSD, CCSD(T)) calculations. O₂ and H₂ are generated in a reaction sequence that starts with Ti(OH)₄ reacting with H₂O. This reaction can proceed by either nucleophilic attack by H₂O or by H-atom abstraction from H₂O. The nucleophilic attack has high energy barriers (40–120 kcal mol⁻¹) in both the ground and excited states. On the other hand, H abstraction is effected by Ti(OH)₄ in the excited state with a low energy barrier (4–8 kcal mol⁻¹), generating OH˙. This is the rate-limiting barrier in the chain of O₂ formation reactions proposed in this work. The production of free OH˙ radicals is not energetically feasible in the ground state. By absorbing two photons, two hydroxyl radicals are produced, which then form H₂O₂. By a stepwise H-abstraction from H₂O₂ and OOH˙, O₂ is generated by absorbing two more photons. In each H-abstraction reaction a Ti(OH)₄ is consumed and a Ti(OH)₃H₂O is produced. H₂ production can proceed thermally from the latter in a very exothermic (68–105 kcal mol⁻¹) bimolecular reaction. The solvent effects, modelled by explicit water molecules, have a limited influence on the reactivity.