Mechanistic study on the electrocatalytic reduction of nitric oxide on transition-metal electrodes

The mechanism of the electrochemical reduction of nitric oxide (NO) on a se ries of metals (Pd, Rh, Ru, Ir, and An) has been studied, both for the redu ction of adsorbed NO and for the continuous NO reduction. All metals show a high selectivity to N2O at high potentials and a high selectivity to NH3 a t low potentials, whereas N-2 is formed at intermediate potentials (althoug h gold forms mainly N2O, and very little NH3). The behavior of the transiti on metals is very similar to that of platinum, suggesting that the reaction schemes are essentially the same (especially the potential windows in whic h the products are formed are similar). The mechanism that leads to N2O is believed to involve the formation of a weakly adsorbed NO dimer intermediat e, similar to recent suggestions made for the gas-phase reduction of NO. Th e reduction of adsorbed NO leads only to formation of NH3 and not to N2O or N-2. The electrochemical measurements suggest that NH3 formation involves a combined electron-proton transfer in equilibrium, followed by a nonelectr ochemical rate-determining step. The formation of N-2, produced at potentia ls between the formation of N2O and NH3, Most likely takes place by the red uction of previously formed N2O. (C) 2001 Academic Press.