HARTREE-FOCK-SLATER CALCULATIONS ON CATION-INDUCED CHANGES IN THE ADSORPTION OF CO ON IR-4 CLUSTERS

The adsorption of CO on various sites of a tetrahedral Ir4 cluster has been studied by using the Hartree-Fock-Slater method. The changes in the adsorption when a small metal cluster is deposited in a pore of a zeolite are modeled with a Mg2+ ion at the opposite side of the Ir4 cluster. The formation of strong bonds of metal d and CO 2π* orbitals favors the 2- and 3-fold adsorption geometry. In the presence of a Mg2+ ion the distance between CO and the metal cluster increases, and the frequency of the CO stretch vibration is shifted upwards. The changes can be ascribed to polarization of the Ir4 clusters in the electrostatic field generated by the Mg2+ ion. As a result the repulsive interaction between the CO 5σ orbital and the Ir4 cluster is reduced, and back-donation into the unoccupied CO 2π* orbital decreases. Implications of our results to catalysis are discussed. © 1990 American Chemical Society.