Experimental and theoretical studies of H2O oxidation by neutral Ti2O4,5 clusters under visible light irradiation

Abstract

A new photo excitation fast flow reactor system is constructed and used to investigate reactions of neutral Ti_mO_n clusters with H₂O under visible (532 nm) light irradiation. Single photon ionization at 118 nm (10.5 eV) is used to detect neutral cluster distributions through time of flight mass spectrometry. Ti_mO_n clusters are generated through laser ablation of a titanium target in the presence of 4% O₂/He carrier gas. Association products Ti₂O₄(H₂O) and Ti₂O₅(H₂O) are observed for reactions of H₂O and Ti_mO_n clusters without irradiation. Under 532 nm visible light irradiation of the fast flow reactor, only the Ti₂O₅(H₂O) feature disappears. This light activated reaction suggests that visible radiation can induce chemistry for Ti₂O₅(H₂O), but not for Ti₂O₄(H₂O). Density functional theory (DFT) and time-dependent (TD) DFT calculations are performed to explore the ground and first excited state potential energy surfaces (PES) for the reaction Ti₂O₅ + H₂O → Ti₂O₄ + H₂O₂. A high barrier (1.33 eV) and a thermodynamically unfavorable (1.14 eV) pathway are obtained on the ground state PES for the Ti₂O₅ + H₂O reaction; the reaction is also thermodynamically unfavorable (1.54 eV) on the first singlet excited state PES. The reaction is proposed to occur on the ground state PES through a conical intersection ((S₁/S₀)_CI), and to generate products Ti₂O₄ and H₂O₂ on the ground state PES. This mechanism is substantiated by a multi-reference ab initio calculation at the complete active space self-consistent field (CASSCF) level. The S₀–S₁ vertical excitation energy of Ti₂O₄ (3.66 eV) is much higher than the 532 nm photon energy (2.33 eV), suggesting this visible light driven reaction is unfavorable for the Ti₂O₄ cluster. The TDDFT calculated optical absorption spectra of Ti₂O₄ and Ti₂O₅ further indicate that Ti₂O₅ like structures on a titanium oxide surface are the active catalytic sites for visible light photo-catalytic oxidation of water.