Coadsorption of ammonia and electrolyte anions on a Pt(111) electrode

Abstract

The adsorption of NH 3 and the coadsorption of NH 3 and an electrolyte anion on a Pt(111) electrode have been studied by voltammetry and infra-red reflection absorption spectroscopy (IRAS). In IRA spectra of NH 3 adsorbed on a Pt(111) electrode, an absorption band appeared around 1300 cm −1 in a 0.01 M NH 3+0.1 M HF solution. The band was considerably broad (FWHM is 50 cm −1) and showed a frequency shift with an electrode potential change (45 cm −1 V −1). The absorption band was assignable to a symmetric deformation mode of NH 3 adsorbed on Pt(111) via a nitrogen lone pair orbital. The formation of hydrogen bonds between NH 3 and H 2O molecules, which was recognized by the band frequency shift and broadening, was confirmed by the coadsorption study of NH 3 and H 2O using IRAS under UHV conditions (UHV modelling). The coadsorption of NH 3 and HSO − 4 was observed by IRAS in the 0.1–0.01 M NH 3+0.1 M H 2SO 4 solution. The band shape of NH 3 coadsorbed with HSO − 4 was extremely sharp. The appearance of the sharp absorption band of NH 3 symmetric deformation mode can be explained by the existence of well-ordered hydrogen bonds. The SO 3 symmetric stretching mode of HSO − 4 coadsorbed with NH 3 exhibited substantially lower frequency shifts compared with that of HSO − 4 alone over the entire potential range. This IRAS result indicates that adsorption strength of the HSO − 4 on Pt(111) is enhanced in the course of the coadsorption with NH 3.