ESTIMATION OF CO HEATS OF ADSORPTION ON METAL-SURFACES FROM VIBRATIONAL-SPECTRA

An empirical relationship between the bond dissociation energy at 0 K, D-0, and the force constant, k, was obtained for a series of heteronu clear carbon-containing and homonuclear metal diatomic species, sugges ting that the potential wells for these species have similar curvature . This D-0-k relationship was then used as part of a simple mathematic al formalism to calculate the metal-carbon and carbon-oxygen bond stre ngths of CO adsorbed on metal surfaces directly from experimental valu es of A(1) vibrational modes. By assuming a rigid metal lattice, whose bonds remain unperturbed as a result of CO adsorption, it was thus po ssible to directly calculate the heat of adsorption of CO, Q(ad), from the calculated bond strengths. Although calculated values of Q(ad) fo r CO on 3d and 4d transition metals were in reasonable agreement with experimental values reported in the literature, agreement was not sati sfactory for the 5d transition metals. Further analysis indicates that the discrepancy is likely due to the assumption of a rigid metal latt ice and that CO adsorption on some metal surfaces, particularly those of platinum and iridium, induces some bond relaxation on the metal sur face. It is thus suggested that metal surfaces which have both a large curvature of the cohesive function and adsorb CO primarily via 5 sigm a donation to the surface, i.e., little metal back-bonding, are strong ly susceptible to bond relaxation and possible reconstruction.