A comparison of the effects of Cu and Au on the surface reactivity of Ru(0001)

Abstract

A programme of work is described directed towards an understanding of the influence of deposits of copper and gold on the surface reactivity of Ru(0001). A combination of Auger electron spectroscopy, static secondary-ion mass spectroscopy, work-function measurements and thermal desorption studies have been used to devise surface preparation procedures which allow the concentration and dispersion of Cu or Au at the Ru surface to be controlled. Then using static secondary-ion mass spectroscopy low-energy electron diffraction, thermal desorption studies and electron energy loss spectroscopy the adsorption of CO and C₂H₄ have been studied on a range of surfaces having differing coverages and dispersions of Cu or Au. In CO adsorption at 150 and 300 K the main function of Au is to reduce coverage by physically blocking the adsorption sites. As would be expected, the highly dispersed Au is far more effective in this respect than Au clusters. Cu on the other hand, provides new adsorption sites at low temperature. Even at 300 K, although CO coverage is reduced by the Cu atoms CO is moved from the linear bonding invariably found on Ru into higher coordination sites some of which seem to involve Cu. At 130 K surface Cu and Au result in significant quantities of π-bonded ethene as compared to the di-σ-bonded ethene normally observed on Ru(0001). Whilst it is evident that Cu and Au influence adsorption on the Ru, Ru also influences adsorption on Cu and Au. π-Adsorption of C₂H₄ is observed on 1 monolayer Au on Ru. When the layer is thicker ethene will not adsorb on Au at 130 K. Where di-σ-adsorbed ethene is present, annealing the surface to 230 K produces adsorbed ethylidyne, whereas π-adsorbed ethene just desorbs. The ‘promoter’ action of Cu and Au is discussed in terms of the relative influence of ensemble and ligand effects.