Adsorption and Reaction of CO₂ on Metal Surfaces. Detection of an Intrinsic Precursor to Dissociation

Abstract

In the late seventies much progress was made in understanding the mecha nism of the CO oxidation reaction using the spectroscopic machinery of surface science /1–2/. ERTL and his group /3–5/ unambiguously showed by molecular beam and other experiments that CO oxidation on transition metal surfaces proceeds via a Langmuir-Hinshelwood mechanism rather than an Eley-Rideal mechanism. This implies that both reactants, CO and oxygen are adsorbed on the surface when CO₂ is formed. The latter is readily desorbed at the temperatures used in the above-mentioned studies. Therefore, adsorbed CO₂ has not been observed in those studies but only detected after desorption in the gas-phase /3–5/. In light of the fact that in addition to the importance of CO oxidation, CO₂ dissociation /6–9/, the reverse reaction, is of considerable - even technical /10/ - interest, several groups have started to investigate the interaction and reactivity of CO₂ with and on metal surfaces /7–9, 12–16/. In particular, from molecular beam experiments on CO₂ reaction dynamics performed on different surfaces a picture arises that can schematically be represented in a simplified manner by the twodimensional potential energy diagram shown in Fig. 1 /17–18/. CO₂ approaches the surface along the entrance channel and may, after passing through some kind of physisorbed (van der Waals) state, be trapped into an intermediate state which then dissociates along the exit channel into adsorbed CO and adsorbed oxygen.