STRUCTURAL INFLUENCES ON ADSORBATE BINDING ENERGY .I. CARBON MONOXIDE ON (100) PALLADIUM

The structural arrangement and binding energy of CO adsorbed on a (100) surface of palladium have been determined and related to each other over the submonolayer coverage range. There are four distinct equilibrium structural arrangements of CO: (1) At low coverages (0.4 monolayers) a random lattice gas configuration is observed in which each CO molecule is localized on a specific adsorption site. (2) Just below 0.5 monolayers a liquidlike short-range order arrangement is found. (3) At exactly 0.5 monolayers a c (4X2) -45° overlayer structure forms which is in complete registry with the substrate. (4) For coverages greater than 0.5 monolayers, the c(4X2)-45° structure is uniaxially compressed to form an overlayer which is out of registry with the palladium substrate. The observed linear decrease in binding energy with coverage up to 0.5 monolayers suggests that the only important CO-CO interaction occurs between nearest neighbors. Near 0.5 monolayers an almost discontinuous decrease in the binding energy of 0.35 eV is attributed to loss of registry between the overlayer and substrate. As the coverage increased above 0.6 monolayers the binding energy again decreased sharply as a consequence of neighboring CO orbital overlap.