RelayCascade Catalysis on Ru–Co Dual SitesEnables Selective Nitrate-to-Ammonia Conversion in Wastewater

Electrocatalytic nitrate (NO3-) reduction reaction (NO3RR) enables simultaneous nitrate removal and ammonia (NH3) recovery from wastewater, yet kinetic mismatch among sequential hydrogenation steps limits NH3 selectivity. Here, we report an atomic-scale relay cascade catalyst composed of adjacent Ru-Co dual sites anchored on a CeOx support (Co3Ru4-CeOx) that spatially coordinates these elementary steps. Ru sites promote nitrate adsorption and deoxygenation, while neighboring Co sites accelerate nitrite hydrogenation, as supported by in situ spectroscopy and theoretical calculations. This establishes a kinetically favorable regime (R2 > R1), where downstream conversion outpaces intermediate formation, thereby suppressing nitrite accumulation and enabling selective NH3 synthesis with a Faradaic efficiency (FE) of 97.1%. Integrated into an electrocatalytic membrane reactor, the catalyst mitigates mass-transfer limitations via convective transport and sustains high selectivity in low-concentration nitrate wastewater (1-10 mM). Techno-economic analysis indicates an estimated production cost of $2.08 per kg NH3, highlighting a scalable approach for coupling wastewater treatment with nitrogen recovery.