Redox-transitions in NO/NH3 adlayers on a Pt(111) electrode in an acidic solution

Abstract

The process of electroreduction of nitrate anions is of considerable interest for researchers both as a fundamental (investigation of a multistage reaction, properties of intermediate products) and practical (water treatment) problem. A key intermediate product of nitrate anion electroreduction is nitric oxide NO that occupies two different types of sites on the Pt(111) surface: atop and 3-fold hollow. This paper studies redox transitions of N-containing species in the adsorption layer on Pt(111) in a wide range of potentials in a nitrate solution and in a preadsorbed saturated NO layer after its partial reduction in a negatively going potential sweep. Such partial reduction results in the appearance of a stable reversible pair of peaks in voltammograms at 0.6 V RHE. Analysis of charge densities of partial NO reduction and of this redox process shows that this pair of peaks corresponds to a one–electron redox transition between NH3(ads) and NH2(ads). We suggest a scheme based on the obtained results and literature data that explains the voltammetric signals observed in a wide range of potentials on an adsorption layer on Pt(111) formed by partial reduction of full NO adlayer. In this scheme, NH3(ads) molecules are formed in reduction of atop-bound NO and do not desorb from the surface owing to formation of coadsorption complexes with unreduced hollow-bound NO molecules. Such complexes are quite stable up to the potentials of hollow-bound NO reduction. In the potential range around 0.6 V, NH3(ads) molecules in such complexes are reversibly oxidized to NH2(ads) species that do not desorb either and remain within the coadsorption lattice. Moreover, the obtained NO/NH2(ads) is so stable that it suppresses oxidation of unreduced hollow-bound NO molecules in this complex to nitrite species.