Determination of the coverage dependent isosteric heat of adsorption of CO on Rh(1 1 1) by high resolution core level photoemission

High resolution core level photoemission is used to measure the equilibrium coverages of CO on Rh(1 1 1) at different sample temperatures and CO background pressures. Use of the chemical shifts of the C 1s binding energy allows us to divide the total CO coverage into separate coverages for the different adsorption sites of CO on Rh(1 1 1). A Clausius–Clapeyron analysis of isosteres constructed from the measured isotherms for the total CO coverage is used to derive the coverage dependence of the isosteric heat of adsorption Eads for this adsorption system. The present results are found to agree well with a recent calculation of the adsorption energy for the Rh(1 1 1)-(2×2)-1CO structure at 0.25 ML (monolayer, ML). Furthermore, the present determination of Eads is consistent with a recent measurement of the zero coverage limit Eads(θ=0 ML) but is inconsistent with some previous determinations of this quantity. The coverage variation of the isosteric heat of adsorption is briefly discussed and put in relation to the relative occupations of different sites by the CO molecules.