Selective electroreduction of nitrate wastewater to ammonia fertilizer via highly dispersed ruthenium nanocluster catalyst

Abstract

Electrochemical reduction of nitrate (NO3-) wastewater to ammonia (NH3) offers a promising but challenging approach for wastewater treatment of nitrate pollutants and resource recovery of ammonia fertilizer. However, the low catalytic activities and selective efficiency have limited their development and application. Herein, we report a facile chemical reduction method for the successful preparation of Ru nanoclusters (NCs) on ultrathin MoO2 nanosheet substrates (Ru NCs/MoO2 NSs). The Ru NCs/MoO2 NSs electrocatalyst exhibits superior activity and selectivity for NO3- electroreduction to NH3, with an NH3 yield rate of 50.9 mg·h−1·g−1cat. at − 0.4 V and a faradaic efficiency of 83.5 % at − 0.3 V versus reversible hydrogen electrode. Operando electrochemical impedance spectroscopy further indicates the faster reaction kinetics of the Ru SCs/MoO2 NSs at the electrode-electrolyte interfaces compared with MoO2 NSs. In situ spectroscopic studies and on-line differential electrochemical mass spectrometry reveal the proposed reaction pathway for the conversion of NO3- to NH3, namely, deoxygenation reactions *NO3-→*NO2-→*NO and hydrogenation steps *NO→*NOH→*NH2OH→*NH3. Density functional theory calculations further verify the effective N-end NOH pathway with the conversion of *NO to *NOH as the rate-determining step requiring a low energy barrier of 0.44 eV, which is lower than that of the O-end ONH pathway (1.17 eV).