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

Electrocatalytic nitrate reduction reaction (NORR) to harmless nitrogen (N) presents a viable approach for purifying NO-contaminated wastewater, yet most current electrocatalysts predominantly produce ammonium/ammonia (NH/NH) due to challenges in facilitating N-N coupling. This study focuses on identifying metal catalysts that preferentially generate N 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 N selectivity (79.3, 70.0, and 57.0%, respectively, under conditions of 6 h electrolysis, a current density of 10 mA/cm, and an initial NO-N concentration of 100 mg/L), while others largely favored NH production. Comprehensive experimental and theoretical analyses indicate that NH-selective catalysts (e.g., Co) exhibited high water activity that enhances H coverage, thereby promoting the hydrogenation of NO to NH through the hydrogen atom transfer mechanism. In contrast, N-selective catalysts, with their lower water activity, promoted the formation of N-containing intermediates, which likely undergo dimerization to form N via the proton-coupled electron transfer mechanism. Enhancing NO adsorption was beneficial to improve N selectivity by competitively reducing H coverage. Our findings highlight the crucial role of water activity in NORR performance and offer a rational design of electrocatalysts with enhanced N selectivity.