THE COADSORPTION OF AMMONIA AND OXYGEN ON NI(110) - A RAIRS STUDY

The adsorption of NH3 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 ND3]. A second band, which is fully resolved for ND3, Where the bands are narrower, grows in as saturation is approached, and is assigned to adsorption on bridge sites. The NH3/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 x 1) and p(2 x 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 x 1) surface, but due to a higher Ni-O chain density, at less stable bridge sites on the p(2 x 1) surface. We propose that dissociation of ammonia occurs only on the low coverage oxygen phase, theta(O) < 0.3 ML, and that dissociation depends upon an optimum orientation rather than simply upon the strength of the interaction of NH3 With the surface.