An XPS study of desorption and dissociation kinetics of CO on W(110)

The purpose of this paper is to demonstrate the usefulness of XPS as a quantitative in-situ technique for species-resolving studies of adsorbate reactions and for the investigation of surface kinetics. The desorption and conversion (dissociation) processes in the well-known adsorption system CO/W(110) are chosen for this test. Some details of experimental procedures and necessary precautions are given. CO layers on W(110) prepared at 100 K are molecular (v-CO). Desorption from saturated layers occurs in three main steps. In the first, the compressed CO overlayer is depleted between 225 and 300 K by desorption with low energies and preexponentials. The main desorption from the molecular layer takes place in a second step between 300 and 375 K with E_d = 86kJ/mol and k₀ ≅ 10¹¹ s^-1. The competing dissociation process is hindered by site blocking by the v-CO species. For lower coverages where site blocking becomes negligible, dissociation without desorption occurs at lower temperatures (between 200 and 300 K). The activation energy (≈21 kJ/mol) and the ("first order") preexponential (≈ 2 × 10² s^-1) for dissociation are very small, in agreement with observations made for room temperature adsorption. Finally, the dissociated layer (β-CO) desorbs between 800 and 1200 K. The desorption and dissociation results of CO/W(110) are discussed, and plausibility arguments are given for the observed kinetic parameters.