CO adsorption and CO oxidation on Rh(100)

Abstract

The adsorption of CO on a Rh(100) surface and the interaction of CO with p(2 × 2) and (2 × 2)p4g oxygen layers on Rh(100) at different temperatures have been studied by means of Thermal Programmed Desorption (TPD), X-Ray Photoelectron Spectroscopy (XPS) and Low Energy Electron Diffraction (LEED). As a measure of the reaction rate the changes in the partial pressure of CO 2 (mass 44) during titration of the oxygen layers with CO were used. The composition of the surface layer and the surface structure were monitored at various stages of the reaction. The CO O 1s XPS spectra showed that CO adsorption occurs in two bonding configurations characterized by O 1s binding energies of 532.4 and 531.5 eV, assigned to occupation of on-top and bridge sites, respectively. The on-top sites are occupied first yielding an ordered c(2 × 2) layer at coverage of 0.5 ML. At higher coverages a fraction of CO adsorbs in bridge sites and conversion to compressed p(4√2 × √2) and ‘split’ (2 × 1) structures occurs. Three CO desorption states were observed in the CO TPD curves which correlate well with the development of the three ordered structures with increasing CO coverage. The desorption parameters of each ordered structure and their variations with CO coverage were evaluated. The rate of CO oxidation reaction during titration of p(2 × 2) and (2 × 2)p4g layers was studied at temperatures ranging from 290 to 670 K. The variation of the CO 2 production revealed dependence on the temperature, reactant surface structure and coverages. It has been found that the following factors are of importance for the reaction rate: (i) the mobility of CO which at the same reaction temperature is affected by the actual oxygen surface structure; (ii) the ordering of the reactants and the structural changes in the course of the titration reaction.