Efficient electrochemical reduction of nitrate to nitrogen on tin cathode at very high cathodic potentials

The electrochemical reduction of nitrate on tin cathode at very high cathodic potentials was studied in 0.1 M K2SO4, 0.05 M KNO3 electrolyte. A high rate of nitrate reduction (0.206 mmol min−1 cm−2) and a high selectivity (%S) of nitrogen (92%) was obtained at −2.9 V versus Ag/AgCl. The main by-products were ammonia (8%) and nitrite (<0.02%). Small amounts of N2O and traces of NO were also detected. As the cathodic potential increases, the %S of nitrogen increases, while that of ammonia displays a maximum at −2.2 V. The %S of nitrite decreases from 65% at −1.8 V to <0.02% at −2.4 V. The kinetic analysis indicated that the formation of nitrogen and ammonia proceeds through the intermediate nitrite. The reduction follows first order kinetics for both nitrate and nitrite at more cathodic potentials than −2.4 V, while at less negative potentials the kinetics is more complicated. The %Faradaic efficiency (%FE) of the reduction at −2.9 V was about 60% initially and decreased to 22% at 40 min. A cathodic corrosion of tin was observed, which was more intensive in the absence of nitrate. At potentials more negative than −2.4 V, small amounts of tin hydride were detected.