Determination of the coverage dependent isosteric heat of adsorption of COon Rh(111) by high resolution core level photoemission

High resolution core level photoemission is used to measure the equilibrium coverages of CO on Rh(l 1 1) at different sample temperatures and CO backg round pressures. Use of the chemical shifts of the C Is binding energy allo ws us to divide the total CO coverage into separate coverages for the diffe rent adsorption sites of CO on Rh(I 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 adsorpt ion E-ads for this adsorption system. The present results are found to agre e well with a recent calculation of the adsorption energy for the Rh(l 1 1) -(2 x 2)-1CO structure at 0.25 ML (monolayer, ML). Furthermore, the present determination of E-ads is consistent with a recent measurement of the zero coverage limit E-ads(theta = 0 ML) but is inconsistent with some previous determinations of this quantity. The coverage variation of the isosteric he at of adsorption is briefly discussed and put in relation to the relative o ccupations of different sites by the CO molecules. (C) 2001 Elsevier Scienc e B.V. All rights reserved.