Ammonia Oxidation for Energy Conversion and Environmental Remediation: Mechanism-Guided Catalyst Design for Electrochemical, Photocatalytic, and Photoelectrochemical Systems.

Ammonia oxidation reaction (AOR) has emerged as a promising technology for sustainable energy conversion and the treatment of ammonia-containing wastewater. This review summarizes recent advances in electrochemical (EC), photocatalytic (PC), and photoelectrochemical (PEC) AOR systems, emphasizing how mechanistic understanding guides rational catalyst design. Because catalyst requirements depend on the intended application, EC-, PC-, and PEC-AOR are comparatively discussed from both energy-conversion and environmental-remediation perspectives. Structure-mechanism-performance relationships are established by correlating catalyst architecture, electronic structure, charge-transfer behavior, oxidative intermediates, and product selectivity with the underlying AOR pathways. In addition, various catalyst-engineering strategies are critically compared in terms of their ability to regulate reaction intermediates and interfacial reaction kinetics. In particular, the Gerischer-Mauerer and Oswin-Salomon pathways, together with reactive oxygen/chlorine species-mediated oxidation mechanisms, are discussed to clarify how reaction mechanisms influence catalytic activity, selectivity, and stability. Finally, the remaining challenges in mechanistic ambiguity, photoconversion efficiency, device integration, and practical scalability are highlighted to provide future directions for sustainable AOR technologies.