Model bimetallic catalysts: The preparation and characterisation of Au/Ru(0001) surfaces and the adsorption of carbon monoxide

Abstract

We have studied the growth and structure of gold films deposited onto a Ru(0001) single-crystal substrate in UHV using LEED, Auger electron spectroscopy (AES) and thermal desorption mass spectroscopy (TDS). The gold concentrations ( θ Au) varied from fractions of a monolayer up to several monolayers with the Ru substrate held at either 540 K (low-temperature series) or at 1110 K (high-temperature series) during deposition. A correlation between the AES signal heights of Ru and Au in conjunction with TDS peak areas revealed a layer-by-layer growth of Au, particularly for the high- T series. TDS revealed three different Au binding states which are subsequently populated, viz., a β 3 state around 1320 K desorbing in a first-order kinetics and associated with the first Au monolayer, followed by a β 2 state ( T des around 1180–1280 K, fractional-order desorption) reflecting the second monolayer and a β 1 state ( T des around 1160–1260 K, zero-order desorption) corresponding to the third and all following Au layers. The activation energy of desorption. ΔE des, increases with increasing coverage in the submonolayer regime; it rises from 330 J mol −1 at θ Au = 0.1 to about 400 J mol −1 at θ = 0.6 ML. As a general tendency, the lower substrate temperatures favour an island growth of the Au whereas higher Ru temperatures lead to a more pronounced spreading and dispersion of the Au atoms. Epitaxial growth of Au can be observed at higher coverages ( θ Au ≲ 3 ML) as indicated by double-scattering LEED features. The CO adsorption data support the conclusions about the lateral dispersion of the Au on the Ru substrate in that the high- T Au films suppress the CO uptake more effectively than the low- T films.