ADSORPTION OF CO ON RH(100) STUDIED BY AB-INITIO LOCAL-DENSITY FUNCTIONAL CALCULATIONS

Ab initio local-density functional studies of the adsorption of CO on the (100) surface of Rh have been performed. We show that although ads orption in the bridge site is always energetically more favorable than adsorption in either the on-top or the hollow sites, two different me chanisms can lead to a relatively high occupation of the on-top sites: (i) At higher coverage the interactions between the adsorbates stabil ize a pseudohexagonal coincidence lattice with the experimentally obse rved p(4 (root)2 X (root)2) structure with a bridge/on-top ratio of 2: 1 (all adsorbates being slightly shifted from their high-symmetry posi tions). (ii) At lower coverages there seems to be a contradiction betw een the energetic preference for bridge-site adsorption and the mixed top/bridge adsorption reported in the experiments. This could simply b e dismissed as a failure of density-functional theory. However, we spe culate about a possible way to reconcile the calculated potential ener gy surface and the experimental observations: At distances larger than 1.75 Angstrom from the surface (this is smaller than the equilibrium height for on-top adsorption), the interaction-energy with the on-top site is always larger by similar to 0.5 eV than with the bridge site. The on-top adsorbed molecule is metastable, because there is a barrier of 0.06 eV for migration to the bridge site and the reaction channel is very narrow. This leads to the observed mixed occupation of top and bridge sites at lower coverage (Theta less than or equal to 0.5). (C) 1998 American Institute of Physics. [S0021-9606(98)70137-0]