Recent advances in metal atom catalysts for photo- and electro-catalytic nitrate-to-ammonia conversion.

The catalytic conversion of nitrate (NO3-) to ammonia (NH3) offers a promising route to simultaneously address water pollution and sustainable nitrogen utilization. This review critically examines recent progress in metal atom catalysts, including single-atom, dual-atom, and sub-nanocluster systems, for photocatalytic, electrocatalytic, and photoelectrochemical nitrate-to-ammonia conversion. We show that the principal advantage of metal atom catalysts extends beyond maximizing atom utilization, and their greatest contribution lies in enabling atomic-level control of adsorption energetics, proton-coupled electron-transfer pathways, and competing side reactions. Recent studies reveal that synergetic electronic interactions, dynamic active-site reconstruction, defect engineering, and metal-support coupling can partially overcome conventional activity-selectivity trade-offs and break linear scaling relationships that limit traditional catalysts. Comparative analysis further indicates that electrocatalytic systems currently provide superior activity and scalability, whereas photocatalytic systems offer a more sustainable energy input. Photoelectrocatalytic platforms emerge as a potentially optimal compromise by integrating efficient charge separation with reduced external energy demand. Despite remarkable laboratory performance, catalyst instability, mass-transfer limitations at low nitrate concentrations, and insufficient reactor-level integration remain major barriers to industrial implementation. The central finding of this review is that future progress will depend more on coupling atomically precise catalyst design with operando mechanistic understanding, intelligent material discovery, and integrated reactor-separation engineering. Such convergence is essential for translating nitrate-to-ammonia conversion from a promising laboratory reaction into a practical technology for sustainable wastewater remediation and green ammonia production.