Carbon monoxide adsorbed on Ru(101̄0)

Abstract

The adsorption of CO on the Ru(101̄0) surface has been studied by means of XPS, LEED and TPD. The CO O1s spectra, including the satellites, were used to obtain information about the CO bonding configuration and the population of different adsorption sites as a function of the CO coverage and adlayer ordered structures. The on-top bonded CO, which is the only species up to one monolayer, is characterized by an O1s binding energy of 531.7 eV at coverages less than 0.6 ML. The slight shift of the on-top CO O1s binding energy to 531.9 eV at higher coverages has been ascribed to the compression and displacement of CO from the equilibrium on-top position. Beyond 1 ML the adsorption of extra 0.25 ML CO in a higher coordinated site results in a second O1s peak at 530.9 eV. On the basis of the XPS and LEED data a structural model for the arrangement of the CO molecules in the compressed (4−112) saturated layer has been suggested. The coverage dependence of desorption energy, Ed, and pre-exponential factor, v, evaluated by using two methods for fitting the TPD data, have shown almost constant Ed and v values of ∼ 140 kJ mol−1 and 1014 s−1 up to ∼ 0.33 ML, a linear decline to ∼ 80 kJ mol−1 and 109 s−1 at 0.8–1.0 ML and a sharp drop to ∼ 50 kJ mol−1 and 106 s−1 at saturation (∼ 1.25 ML). This coverage dependence has been explained in terms of CO-CO repulsive interactions and reduced CO-substrate coupling in the higher coordinated site.