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Abstract The electrochemical reduction of nitrates (NO₃⁻) enables a pathway for the carbon neutral synthesis of ammonia (NH₃), via the nitrate reduction reaction (NO₃RR), which has been demonstrated at high selectivity. However, to make NH₃synthesis cost‐competitive with current technologies, high NH₃partial current densities (j_NH3) must be achieved to reduce the levelized cost of NH₃. Here, the high NO₃RR activity of Fe‐based materials is leveraged to synthesize a novel active particle‐active support system with Fe₂O₃nanoparticles supported on atomically dispersed Fe–N–C. The optimized 3×Fe₂O₃/Fe–N–C catalyst demonstrates an ultrahigh NO₃RR activity, reaching a maximum j_NH3of 1.95 A cm⁻²at a Faradaic efficiency (FE) for NH₃of 100% and an NH₃yield rate over 9 mmol hr⁻¹cm⁻². Operando XANES and post‐mortem XPS reveal the importance of a pre‐reduction activation step, reducing the surface Fe₂O₃(Fe³⁺) to highly active Fe⁰sites, which are maintained during electrolysis. Durability studies demonstrate the robustness of both the Fe₂O₃particles and Fe–N_xsites at highly cathodic potentials, maintaining a current of −1.3 A cm⁻²over 24 hours. This work exhibits an effective and durable active particle‐active support system enhancing the performance of the NO₃RR, enabling industrially relevant current densities and near 100% selectivity. 

Abstract Single-atom catalysts (SACs) offer efficient metal utilization and distinct reactivity compared to supported metal nanoparticles. Structure-function relationships for SACs often assume that active sites have uniform coordination environments at particular binding sites on support surfaces. Here, we investigate the distribution of coordination environments of Pt SAs dispersed on shape-controlled anatase TiO₂supports specifically exposing (001) and (101) surfaces. Pt SAs on (101) are found on the surface, consistent with existing structural models, whereas those on (001) are beneath the surface after calcination. Pt SAs under (001) surfaces exhibit lower reactivity for CO oxidation than those on (101) surfaces due to their limited accessibility to gas phase species. Pt SAs deposited on commercial-TiO₂are found both at the surface and in the bulk, posing challenges to structure-function relationship development. This study highlights heterogeneity in SA coordination environments on oxide supports, emphasizing a previously overlooked consideration in the design of SACs.