Mechanisms of electrochemical reduction and oxidation of nitric oxide

A summary is given of recent work on the reactivity of nitric oxide on various metal electrodes. The significant differences between the reactivity of adsorbed NO and NO in solution are pointed out, both for the reduction and the oxidation reaction(s). Whereas adsorbed NO can be reduced only to hydroxylamine and/or ammonia, it takes NO in solution to produce N2O and N2. From the reduction of NO on a series on stepped single-crystal Pt electrodes, it is concluded that NOads reduction is not a structure sensitive process. The protonation of the adsorbed NO is rate-determining; neither the NO adsorption strength nor the NO bond breaking play a significant role in its reduction rate. Whereas adsorbed NO on polycrystalline Pt can only be oxidized to nitrate, in the presence of NO in solution nitrous acid HNO2 may also be formed, in a potential region where adsorbed NO is otherwise stable. Interestingly, on Pt(1 1 1) and Pt(5 5 4) NOads may be oxidized to HNO2 in a surface-bonded redox couple. Whereas surface oxides appear to catalyze the oxidation of solution NO to HNO2, the further oxidation to nitrate seems to be inhibited by the presence of surface oxides. Both the reduction and oxidation of solution NO appear to be not very metal-dependent reactions, as they take place with approximately equal rate on all electrode metals studied, including gold. This suggests the involvement of weakly adsorbed intermediates, and the relatively unimportant role of surface-bonded NO in the bulk NO reduction and oxidation activity.