NO ADSORPTION ON RH(110)

NO adsorption on Rh(110) has been investigated using a molecular beam system and TPD. NO appears to adsorb initially dissociatively at 300 K, followed by molecular adsorption at higher exposures. The initial sticking coefficient is high (0.67) at all temperatures, declines only slowly with increasing coverage and the saturation coverage at 300 K is 1.1 (+/- 0.1) monolayers. This seems to be composed of a mix of atomic adsorption in the trough sites and molecular adsorption on on top sites. If the adsorption measurements are performed above 380 K, N2 is seen to evolve during the experiments, with kinetics which depend on the binding strength of the N atoms to the surface. TPD shows that is dramatically weakened above a total atomic coverage of approximately 0.5 monolayers, due to the presence of coadsorbed oxygen atoms. Since the saturation atomic coverage under these circumstances can be as high as 1.5 monolayers it appears that this destabilisation is due to a structural rearrangement of the surface which displaces nitrogen atoms to low coordination sites (perhaps on top) while the oxygen remains in the more favourable sites.