Photocatalytic Ammonia Synthesis: Mechanistic Insights into N₂ Activation at Oxygen Vacancies under Visible Light Excitation

Abstract

Oxide semiconductors like bismuth-based oxide or layered-double-hydroxide accompanied by many surface oxygen vacancies (OVs) are emerging as highly promising photocatalysts for artificial N2 fixation. However, their band edge reduction potentials actually do not meet nitrogen fixation requirements at all. The mechanism that triggers the photocatalytic NH3 synthesis reaction still remains unclear. Herein, taking BiOBr as a prototypical photocatalyst, we reveal a photoexcitation-assisted N2 activation mechanism, which can perfectly address the abovementioned problem. Specifically, the OV defect states serve as a springboard that offers the photogenerated electrons the reduction potential that is much higher than conduction band edge under visible light. The physically adsorbed *N2 can trap the electron to form the *N2•– transient state and collapse into the *N2 vibrational excited state. This process deposits a high amount of energy into *N2 and sharply lowers the π* orbital of *N2 below the band edge, thereby allowing *N2 to capture photogenerated electrons at band edge and trigger the following NH3 synthesis. This study advances the fundamental understanding of photocatalytic N2 fixation and may provide an alternative way for the design of efficient ammonia photocatalysts.