CO ADSORPTION AND CO OXIDATION ON RH(100)

The adsorption of CO on a Rh(100) surface and the interaction of CO wi th p(2 x 2) and (2 x 2)p4g oxygen layers on Rh(100) at different tempe ratures have been studied by means of Thermal Programmed Desorption (T PD), X-Ray Photoelectron Spectroscopy (XPS) and Low Energy Electron Di ffraction (LEED). As a measure of the reaction rate the changes in the partial pressure of CO2 (mass 44) during titration of the oxygen laye rs with CO were used. The composition of the surface layer and the sur face structure were monitored at various stages of the reaction. The C O O 1s XPS spectra showed that CO adsorption occurs in two bonding con figurations characterized by O 1s binding energies of 532.4 and 531.5 eV, assigned to occupation of on-top and bridge sites, respectively. T he on-top sites are occupied first yielding an ordered c(2 x 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 root x root 2)R45 degre es and 'split' (2 x 1) structures occurs. Three CO desorption states w ere observed in the CO TPD curves which correlate well with the develo pment 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 dur ing titration of p(2 x 2) and (2 x 2)p4g layers was studied at tempera tures ranging from 290 to 670 K, The variation of the CO2 production r evealed dependence on the temperature, reactant surface structure and coverages. It has been found that the following factors are of importa nce for the reaction rate: (i) the mobility of CO which at the same re action 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.