METAL-METAL BONDING ON SURFACES - MOLECULAR-ORBITAL STUDY OF PD/TI(001) AND PD/RU(001)

The interaction of a Pd atom with Ti(001), Ru(001) and Pd(111) surfaces has been studied using semiempirical MO-SCF calculations (INDO/1) and cluster models. In addition, the electronic properties of the diatomic PdTi, PdRu and Pd, molecules have been examined using ab initio SCF calculations. The results of the calculations indicate that the charge transfer in the Pd-Ti and Pd-Ru bonds is small. For supported Pd monolayers, the Pd-substrate bonds can be described as mainly metallic, with a small degree of ionic character. Adsorption of Pd on an early transition metal induces a reduction in the electron population of the Pd(4d) orbitals by: (1) charge transfer from Pd to the metal substrate, and (2) rehybridization of the Pd(4d,5s,5p) levels. The magnitude of both phenomena increases when the fraction of empty orbitals in the valence band of the metal substrate rises. The Pd(4d) --> Pd(5s,5p) electron transfer plays an important role in the strength of the bimetallic bonds: the larger this rehybridization, the stronger the Pd-substrate bond. Electronic perturbations induced by Ti or Ru on Pd reduce the CO-chemisorption ability of Pd by weakening simultaneously the Pd(4d)-CO(2pi*) and Pd(5s,5p)-CO(5sigma) bonding interactions. For adsorption of CO on supported Pd, the pi back-donation and strength of the Pd-CO bond decrease when the fraction of empty states in the valence band of the support increases.