IR spectroscopy of adsorbed dinitrogen: a sensitive probe of defect sites on Pt(111)

Fourier transform infrared reflection absorption spectroscopy (FT-IRAS) has been used to probe the non-dissociative adsorption of N₂ on an atomically clean Pt(111) single crystal. In contradiction to a previous IRAS study of nitrogen adsorption on a Pt(111) foil at 120 K, no nitrogen infrared (IR) band was observed on a fully annealed Pt(111) surface at 90 K. Following Ar⁺ ion bombardment, adsorption of nitrogen at 90 K produces an intense IR band at 2222 cm⁻¹ attributed to the N---N stretching mode of molecular nitrogen adsorbed on defect sites produced by ion bombardment. Annealing the Ar⁺ ion sputtered surface to a temperature above 750 K completely suppresses the adsorption of nitrogen at 90 K. Based on these and other results, we postulate that nitrogen adsorbs at 90 K mainly on monovacancies on platinum. We suggest that this specific adsorption occurs by sigma donation from nitrogen to the base of monovacancy sites which possess a low d-electron density compared to surface Pt atoms.