Electrocatalytic reduction of nitrate at low concentration on coinage and transition-metal electrodes in acid solutions

A comparative study was performed to determine the reactivity of nitrate ions at 0.1 M on eight different polycrystalline electrodes (platinum, palladium, rhodium, ruthenium, iridium, copper, silver and gold) in acidic solution using cyclic voltammetry (CV), chronoamperometry and differential electrochemical mass spectroscopy (DEMS). Cyclic voltammetry shows that the current densities for nitrate reduction depend strongly on the nature of the electrode. The activities decrease in the order Rh>Ru>Ir>Pd and Pt for the transition-metal electrodes and in the order Cu>Ag>Au for the coinage metals. The rate-determining step on Ru, Rh, Ir, Pt, Cu, and Ag is concluded to be the reduction of nitrate to nitrite, as is evident from the Tafel slope, the kinetic reaction order in nitrate, and the anion effect. Transfer experiments with Pt suggest that chemisorbed nitric oxide is the key surface intermediate in the nitrate reduction. Since on-line mass spectrometry (DEMS) measurements on Pt and Rh show no formation of gaseous products such as nitric oxide (NO), nitrous oxide (N2O) or nitrogen (N2), it is suggested that ammonia and hydroxylamine are the main products on transition-metal electrodes. This is in agreement with the known mechanism for NO reduction, which forms N2O or N2 only if NO is in solution. On Cu, DEMS measurements show the production of gaseous NO, which is explained by the weaker binding of NO to Cu as compared to the transition metals.