Influence of nitrate concentration on its electrochemical reduction on tin cathode: Identification of reaction intermediates

The influence of the concentration of nitrate in the range between 100 and 62,000 mg L⁻¹ NaNO₃ in NaCl solutions was studied under constant potential electrolysis at −2.8 V vs. Ag/AgCl. The rate of the reduction follows Langmuir-Hinshelwood kinetics, according to which zero order kinetics is followed at concentrations higher than 0.3 M whereas first order at lower concentrations.The selectivity to nitrogen increases from 70 to 83% as the concentration of nitrate increases from 100 to 1500 mg L⁻¹ and it remains almost constant for higher nitrate concentrations, whereas that of ammonia exhibits the opposite trend decreasing from 25 to 11%. The % Faradaic Efficiency (%FE) increased with the increase of the concentration of nitrate from 25% at 0.1 M to 78% at 1 M when 95% of nitrate was reduced in both cases. At high concentrations of nitrate, hyponitrite and hydroxylamine were detected as intermediates of the reduction and a reaction scheme which is in agreement with the experimental results has been proposed.The hydrogen evolution in our conditions probably takes place through the discharge of the cation of the supporting electrolyte instead of the Volmer-Tafel mechanism and the reduction of nitrate proceeds through electrochemical hydrogenation.