An infrared reflection-absorption study of CO chemisorbed on clean and sulfided Ni(111) — Evidence for local surface interactions

Abstract

Infrared Reflection-Absorption Spectroscopy (IRAS) has been used to characterize CO chemisorption on a clean and sulfur-modified Ni(111) surface under both ultrahigh vacuum and high CO pressure conditions. On clean Ni(111) correlations between the observed LEED patterns and the infrared spectra show that for the c(4 × 2) structure with a CO coverage of θ co = 0.50, all CO molecules occupy two-fold bridge sites as indicated by a single infrared band and 1910 cm −1. For θ co = 0.57, a ( 7 2 × 7 2 )R19.1° structure forms with infrared bands at 2058 cm −1 corresponding to terminally bound CO and at 1925 cm −1 for the bridge-bonded CO. The change in the overlayer structures is accompanied by an abrupt change in the coverage dependence of the bridge-bonded CO frequency for adsorption at 80 K. In studies involving known sulfur coverages on Ni(111). a new sulfur-induced CO state is observed with an infrared band which shifts from 2109 cm −1 to 2103 cm −1 corresponding to increasing sulfur coverage. The sulfur-induced CO state is only populated at 300 K when a high ambient CO pressure is present. The infrared data clearly indicate that the CO and S interact on the Ni surface through a local short-range mechanism: this short range COS interaction model is supported by kinetic studies of CO desorption from the sulfided Ni(111) surface.