Role of Lewis Acidic Sites on Anatase Titanium Dioxide in Photocatalytic Hydrogen Production from Aqueous Ammonia

Development of energy society using NH3 as a hydrogen carrier requires technology for on-site generation of H2 gas from aqueous NH3. Photocatalytic decomposition of NH3 into H2 and N2 using titanium dioxide (TiO2) powders is a promising method. Here, we report that the crystal structure of TiO2 strongly affects activity. Anatase or anatase/rutile-mixed TiO2 quantitatively generates H2 and N2, while rutile TiO2 is inactive. Photoelectrochemical analysis and density functional theory calculations revealed that the oxidation of NH3 by the photogenerated valence band holes is the critical step to promote the reaction. The Lewis acidic four-coordinated Ti (Ti4c) sites, which are specifically present on the anatase(110) surface, behave as the sites for NH3 oxidation. The Ti4c sites facilitate preferential adsorption of NH3 even in water and promote NH3 oxidation, whereas other Ti sites are inactive owing to strong adsorption of water. In situ spectroscopic analysis revealed that the holes photogenerated on the lattice O atoms adjacent to the Ti4c atoms trigger the deprotonation of adsorbed NH3. This promotes N–N coupling and subsequent oxidation of the formed intermediates to produce N2.