Oxygen Vacancy and Alkali Ion-Assisted Nitrogen Fixation with Efficient Solar Ammonia Production over NiTiO3 Based Photocatalysts

Abstract

Among the artificial nitrogen fixation routes, photocatalytic N2 fixation offers a sustainable and environmentally benign approach using abundant solar energy and water at ambient conditions. Despite significant advancements in photocatalytic material synthesis, major bottlenecks such as low solubility of N2 in aqueous medium, limited diffusion, and insufficient adsorption sites contribute to the sluggish reaction kinetics and need to be addressed for achieving high N2 fixation efficiency. In this work, we demonstrate that strategic oxygen vacancy nanoarchitectonics in a perovskite oxide photocatalyst, NiTiO3, significantly amplifies its efficiency towards photocatalytic NH3 production. As a result, the optimized NiTiO3 sample with ample oxygen vacancies delivered a significantly higher NH3 production rate of 100.51 μmol g–1 h–1. Furthermore, the addition of alkali metal ions in the reaction medium, ranging from Li+ to Cs+, improved the NH3 production, reflecting their strong electronic and interfacial interactions. The most pronounced increase was observed in the case of K+ ions, resulting in about a two-fold increase (197.56 μmol g–1 h–1) in comparison to the pure water reaction medium, consistent with its established role as an electron promoter in the case of the Haber–Bosch process. The addition of alkali metal ions, supported by oxygen vacancies, tunes the electronic structure and generates an electric field that further accelerates reactant transport to interfacial active sites by weakening the N≡N bond, thereby accelerating solar NH3 production. The density functional theory calculations revealed that oxygen vacancies act as dual-function sites that facilitate N2 chemisorption and activation and provide atomic channels for hydrogenation to NH3, with experimental results preferentially supporting the associative distal pathway for NH3 generation. These findings establish a clear and definite strategy to achieve high-efficiency solar-driven NH3 production with the synergy of defect-modulation in perovskite oxides and the presence of alkali ions in the reaction mixture.