Adsorption and dissociation of CO2 on a potassium-promoted Rh(111) surface

Abstract

Adsorption of CO₂ on a clean Rh(111) surface has been found to be weak and non-dissociative. However preadsorbed potassium dramatically affected the behaviour of CO₂ on Rh(111) and caused significant changes in the electron energy-loss spectrum of adsorbed CO₂. (i) It increased the rate of adsorption of CO₂; (ii) it induced the formation of strongly bonded CO₂ species and (iii) it initiated the dissociation of CO₂. While CO₂ desorbs from a clean Rh(111) surface at 170–244K, in the presence of preadsorbed potassium ( < θ_K= 0.4) new adsorption states developed at 343 and 500 K. From a potassium coverage θ_K= 0.4 another high-temperature state appears with T_p= 714 K. At the same time potassium was stabilized by adsorbed CO₂ on the surface; it desorbed in a narrow peak with T≈ 720 K. The dissociation of CO₂ was observed even at a low potassium coverage, θ_K= 0.03. The amount of CO formed and the peak temperature for CO desorption increased with increasing θ_K up to θ_K≈ 0.25. The adsorption of CO₂ on potassium-dosed Rh led to a significant work-function increase. This indicates a substantial charge transfer from the potassium-dosed metal to an empty CO₂π-orbital; as a result the bonding and structure of adsorbed CO₂ are changed. The mutual stabilization of CO₂ and K observed at higher K coverages is attributed to a direct interaction between CO₂ and K and to the formation of carbonate-like species.