Photocatalytic fixation of nitrogen to ammonia with a Ce/S co-doped TiO2 catalyst: synergistic tuning of heterovalent metal states and oxygen vacancy defects

Abstract

A novel Ce/S co-doped TiO₂ sulfur-oxide catalyst (labeled TiCeOS) with heterovalent metal states and oxygen-vacancy defects was synthesized for effective photocatalytic nitrogen fixation into ammonia under visible light. The hydrazine-driven TiCeOS-3 with an appropriate heterovalent Ce³⁺/Ce⁴⁺ state ratio and oxygen vacancies exhibited excellent photocatalytic nitrogen fixation activity of 382.4 μmol g⁻¹ h⁻¹ without a sacrificial agent and maintained stability. An apparent quantum efficiency (AQE) of 3.32% at 420 nm and a solar-to-ammonia (STA) conversion efficiency of 0.058% in pure water under AM1.5 G light irradiation were achieved. Synergistic transition metal Ce^3+/4+-cation and S²⁻-anion co-doped TiO₂ adjusted the energy bandgap to improve its utilization of visible light, where the ratio of Ce³⁺/Ce⁴⁺ was regulated via hydrazine to introduce oxygen vacancies to balance the charge and maintain the lattice electroneutrality. The oxygen vacancies act as active sites for trapping water molecules and activating the water molecules to generate protons for N₂ reduction. The surface Ce³⁺ acts as active sites for adsorbed and activated N₂, and the photoproduced electrons facilitate efficient electron transfer through electron hopping between Ce³⁺ and Ce⁴⁺ for N₂ reduction. This study presents a novel approach to designing efficient catalysts for sustainable nitrogen fixation through a photocatalytic process.