Electrocatalytic Reduction of Nitrate to Ammonia at Oxidized Vanadium Surfaces with V(3+) and V(4+) Oxidation States

The electrochemical reduction of nitrate to ammonia is of interest as an energy/environmentally friendly source of ammonia for agriculture and energy applications and as a route toward groundwater purification. We report in situ photoemission data, electrochemical results, and density functional theory calculations that demonstrate vanadium oxide – prepared by ambient exposure of V metal, with a distribution of surface V3+ and V4+ oxidation states – specifically adsorbs and reduces nitrate to ammonia at pH 3.2 at cathodic potentials. Negligible cathodic activity in the absence of NO3- indicates high selectivity with respect to non-nitrate reduction processes. In situ photoemission data indicate that nitrate adsorption and reduction to adsorbed NO2 is a key step in the reduction process. NO3RR activity is also observed at pH 7, albeit at a much slower rate. The results indicate that intermediate (non-d0) oxidation states are important for both molecular nitrogen and nitrate reduction to ammonia.