Photocatalytic Nitrogen Fixation via the Oxidation Pathway: A Comprehensive Review
Industrial nitric acid production relies on the Haber-Bosch process for ammonia synthesis followed by the Ostwald process for oxidation, consuming vast fossil energy and emitting substantial greenhouse gases. Direct photocatalytic conversion of dinitrogen (N2), oxygen (O2), and water (H2O) into nitric acid (HNO3) under ambient conditions offers a sustainable alternative. This review critically examines recent advances in photocatalytic nitrogen oxidation (NOF), focusing on catalyst design, mechanistic insights into N2 activation, and performance metrics. The NOF pathway requires only 4 electrons versus a minimum of 6 for nitrogen reduction (NRF), lowering reaction energy barriers and potentially enhancing efficiency. Key challenges include the extreme inertness of the N≡N triple bond (bond dissociation energy 940.95 kJ mol−1), competing oxygen evolution reaction (OER), and the need for precise control over product selectivity. We analyze state-of-the-art photocatalysts, including tungsten oxide, bismuth oxychloride, and titania-based systems, and discuss strategies such as vacancy engineering, heterojunction construction, and co-catalyst loading. Performance benchmarks from recent studies reveal nitrate yields ranging from micromolar to millimolar levels, with apparent quantum efficiencies (AQE) often below 1% under visible light. The review identifies critical gaps in mechanistic understanding, particularly regarding the role of oxygen vacancies and reactive oxygen species, and proposes standardized testing protocols to enable meaningful comparison. We conclude that while NOF holds promise for decentralized fertilizer production, substantial improvements in catalyst stability, selectivity, and solar-to-chemical conversion efficiency are required for practical implementation.