The reactivity of copper clusters supported on carbon studied by XPS

The electronic structure and reactivity of metal clusters at solid surfaces is the subject of much interest, not least because of their importance in modeling industrial heterogeneous catalysts. In this article we investigate the reactivity of copper clusters deposited on a carbon substrate. In contrast with oxygen chemisorption at bulk copper surfaces at 298 K, two oxygen species are observed after exposure to dioxygen, reflecting the unusual properties of copper clusters supported on carbon. One of the oxygen species is atomic and adsorbed on the copper metal, whereas the second species is molecular and associated with the carbon substrate. The latter species is formed via spillover from the metal and can only be formed on the carbon surface in the absence of copper under much more extreme conditions of pressure and temperature. There are marked differences in the behaviour of the two oxygen states: the atomic species is unstable on heating to 540 K and is reactive towards ammonia at 298 K, being desorbed as water via an oxydehydrogenation reaction, whereas the molecular species is unaffected in both cases. The coverage of adsorbed imide, NH(a), formed from the oxydehydrogenation of ammonia is strongly dependent on reaction conditions, dynamic coadsorption from dioxygen ammonia mixtures leading to much higher surface concentrations than observed during the sequential adsorption of oxygen followed by ammonia. In the former case, the instantaneous concentration of adsorbed oxygen atoms remains low throughout much of the reaction due to their continuous removal by reaction with ammonia molecules. Preventing the development of oxygen islands, the interior atoms of which are relatively unreactive, is the key to maintaining a high reactivity. (C) 1999 Elsevier Science B.V. All rights reserved.