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.