Insights into Electrochemical Nitrate Reduction to Nitrogen on Metal Catalysts for Wastewater Treatment

Electrocatalytic nitrate reduction reaction (NO3RR) to harmless nitrogen (N2) presents a viable approach for purifying NO3–-contaminated wastewater, yet most current electrocatalysts predominantly produce ammonium/ammonia (NH4+/NH3) due to challenges in facilitating N–N coupling. This study focuses on identifying metal catalysts that preferentially generate N2 and elucidating the mechanistic origins of their high selectivity. Our evaluation of 16 commercially available metals reveals that only Pb, Sn, and In demonstrated substantial N2 selectivity (79.3, 70.0, and 57.0%, respectively, under conditions of 6 h electrolysis, a current density of 10 mA/cm2, and an initial NO3–-N concentration of 100 mg/L), while others largely favored NH4+ production. Comprehensive experimental and theoretical analyses indicate that NH4+-selective catalysts (e.g., Co) exhibited high water activity that enhances •H coverage, thereby promoting the hydrogenation of NO3– to NH4+ through the hydrogen atom transfer mechanism. In contrast, N2-selective catalysts, with their lower water activity, promoted the formation of N-containing intermediates, which likely undergo dimerization to form N2 via the proton-coupled electron transfer mechanism. Enhancing NO3– adsorption was beneficial to improve N2 selectivity by competitively reducing •H coverage. Our findings highlight the crucial role of water activity in NO3RR performance and offer a rational design of electrocatalysts with enhanced N2 selectivity.