Experimental and theoretical studies of H₂O oxidation by neutral Ti₂O_4,5 clusters under visible light irradiation

Abstract

A new photo excitation fast flow reactor system is constructed and used to investigate reactions of neutral TimOn 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. TimOn 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 TimOn 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.

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

“…Clusters exhibit varied geometries and stoichiometries, with under-coordinated atoms and dangling moieties that mimic reactive point defects on a bulk surface [35,36]. Size-selected clusters can be complexed with a well-defined number of substrate molecules, allowing for the systematic characterization of how cluster size, stoichiometry, and local structure affect reactivity [37][38][39][40]. These systems are also tractable for simulation with high level quantum chemistry calculations, allowing for analysis of geometries and reaction mechanisms with clarity not available in bulk studies [41].…”

“…Early work from the Castleman group reported on the mass spectrum of the water-solvated TiO + cation [47]. Yin and Bernstein [38] [49], and ascribe both sets of measurements to structures involving dissociatively adsorbed water molecules.…”

Titanium dioxide defect structures as catalytic sites

“…В работе [27] рассматривалась суперъячейка 2 × 2 × 1, содержащая 16 атомов Ti и 32 атома O. Для сравнения некоторые вычисления проводились на вдвое увеличенной ячейке 2 × 2 × 2. Такого размера ячейки и кластеры обычно применяются для исследования примесей и дефектов в диоксиде титана [28], где с использованием кластеров Ti 16 O 32 и Ti 32 O 64 изучалось поведение кислородных вакансий и атомов фтора и хлора в рутиле и анатазе.…”

Excess charge driven dissociative hydrogen adsorption on Ti2O4−

“…The gas phase reactivity of titanium oxide clusters is less well studied experimentally and their characterization has been limited to reactions with molecular oxygen 33 and water. 34 Computationally, the geometries of neutral and charged (TiO 2 ) n clusters have been extensively investigated at both DFT and CCSD(T) levels of theory. 28,31,[35][36][37][38][39][40][41][42] Reactions of titanium oxide clusters with hydrogen and water have also been examined.…”

“…28,31,[35][36][37][38][39][40][41][42] Reactions of titanium oxide clusters with hydrogen and water have also been examined. 34,[43][44][45][46][47] As the number of TiO 2 units n in (TiO 2 ) n +/0/À clusters is increased, the number of isomers grows dramatically. This complicates the identification of low-energy structures and advanced algorithms for performing global minimum searches are required.…”