The coadsorption of ammonia and oxygen on Ni{110}: a RAIRS study

Abstract

The adsorption of NH 3 on clean Ni{110} and Ni{110} preexposed with oxygen has been investigated in the temperature range 150 to 300 K using high resolution infrared spectroscopy (RAIRS). Prior to saturation on the clean surface at 150 K, ammonia adsorbs into atop sites, giving a symmetric deformation (umbrella) mode frequency of 1122 cm −1 (0.55%), FWHM = 17 cm −1 [867 cm −1 (0.25%), FWHM = 11 cm −1 for ND 3]. A second band, which is fully resolved for ND 3, where the bands are narrowe in as saturation is approached, and is assigned to adsorption on bridge sites. The NH 3/O system exhibits a more complex behaviour, with at least four vibrationally distinct surface ammonia species in the oxygen coverage range 0–0.5 ML. Surface structures as seen by STM in the literature are used to aid assignment of the infrared bands. On the regular p(3 × 1) and p(2 × 1) surfaces ammonia adsorbs onto strongly perturbed atop sites on Ni-O chains. On both these surfaces ammonia is also adsorbed on second layer sites between the chains, at atop sites on the p(3 × 1) surface, but due to a higher Ni-O chain density, at less stable bridge sites on the p(2 × 1) surface. We propose that dissociation of ammonia occurs only on the low coverage oxygen phase, θ o < 0.3 ML, and that dissociation depends upon an optimum orientation rather than simply upon the strength of the interaction of NH 3 with the surface.