The influence of adsorbate interactions on kinetics and equilibrium for CO on Ru(001). II. Desorption kinetics and equilibrium

Abstract

A variety of methods [temperature programmed desorption via pressure rise and via work function changes (Δφ); isothermal desorption via Δφ: quasiequilibrium measurements via isobars monitored by Δφ, in combination with sticking coefficients] has been used to obtain detailed data on the coverage dependence of the adsorption equilibrium and desorption kinetics for CO on the basal Ru(001) face. While the deviation from reversibility varies strongly over these methods, no significant influence of the degree of irreversibility on the results has been found. Desorption energies and isosteric heats are constant at 160 kJ/mol for 0<Θ<0.2, then rise slowly up to 175 kJ/mol at Θ=0.33, where they fall abruptly to 120 kJ/mol and more gradually at higher coverage. The ‘‘first order’’ frequency factor (Arrhenius preexponential normalized by the coverage) is 1016 s−1 at Θ=0, rises precipitously, especially in the range 0.2<Θ<0.33, to 1019 s−1 at Θ≊0.33, where it drops abruptly to ≊1014 s−1. The main conclusions drawn are: (1) The dependence on coverage of the desorption energies and preexponentials can be understood in terms of the equilibrium statistical mechanics of the chemisorption layer, governed by lateral adsorbate–adsorbate interactions. In particular, the high preexponentials and their strong increase close to Θ=0.33 are due to strong localization in the adlayer. (2) No strong influence of precursor kinetics exists in desorption. (3) Possible dynamic effects have constant influence throughout the range of measurements and can be described by the behavior of the sticking coefficient. These conclusions are discussed in connection with other recent findings.