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 series of metals (Pd, Rh, Ru, Ir, and Au) has been studied, both for the reduction 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 at low potentials, whereas N2 is formed at intermediate potentials (although gold forms mainly N2O, and very little NH3). The behavior of the transition metals is very similar to that of platinum, suggesting that the reaction schemes are essentially the same (especially the potential windows in which the products are formed are similar). The mechanism that leads to N2O is believed to involve the formation of a weakly adsorbed NO dimer intermediate, similar to recent suggestions made for the gas-phase reduction of NO. The reduction of adsorbed NO leads only to formation of NH3 and not to N2O or N2. The electrochemical measurements suggest that NH3 formation involves a combined electron–proton transfer in equilibrium, followed by a nonelectrochemical rate-determining step. The formation of N2, produced at potentials between the formation of N2O and NH3, most likely takes place by the reduction of previously formed N2O.