Oxygen-vacancy-mediated photocatalytic activity of antimony molybdenum oxide toward green ammonia synthesis

Abstract

Oxygen vacancies in metal oxide photocatalysts are essential to activate photocatalytic activity toward nitrogen fixation, although their roles and underlying mechanisms remain poorly understood. Unfortunately, oxygen vacancies are prone to disappear during photocatalysis processes. Herein, antimony molybdate (Sb₂MoO₆) is proposed as a photocatalyst for green ammonia synthesis, which achieves ammonia generation rate of 6.39 μM h⁻¹⋅g⁻¹. Oxygen vacancies modulate the MoO₆ octahedra, not the tetrahedral, SbO₄ bilayers in the triclinic structure of Sb₂MoO₆ and result in partial reduction of Mo⁶⁺ to Mo⁵⁺. Instead of oxygen vacancies themselves, Mo⁵⁺ serves as active sites, favoring surface adoption of N₂ and formation of reaction intermediates. Oxygen vacancies are predominantly refilled over extended photocatalysis, leading to conversion of Mo⁵⁺ back to Mo⁶⁺ and consequent photocatalyst deactivation. Therefore, cobalt dopant is introduced to stabilize the oxygen vacancies effectively. The scientific insights obtained will guide the development of stable photocatalysts for green ammonia synthesis.