THEORETICAL INTERPRETATION OF THE REACTIVITY OF HIGHLY DISPERSED RH CATALYSTS TOWARD CO ADSORPTION

The interaction of CO with precrystalline Rh-4 and Rh-13 structures is analyzed by means of intermediate neglect of differential overlap (IN DO) calculations. The strength of the interaction, as well as the rela tive contribution of the sigma and pi bonds to the stabilization of CO on the different sites, is studied as a function of the electronic ch aracteristics of the cluster surface. Larger binding energies (BE) are found on the electron-deficient structures and are associated with la rger sigma interactions. This brings about a greater charge transfer t o the metal moiety and a subsequent geometric distortion due to the we akness of the Rh-Rh bonds in comparison to the Rh-C bonds that are for med. The larger activation energy for diffusion between the sites defi ned on the smaller structures which has been experimentally derived fr om C-13 NMR determinations is associated with the large BE for adsorpt ion on electron-deficient sites. These sites are present in higher con centration in the smaller structures.