TRANSITION-METAL BONDING FUNCTIONS AND THEIR APPLICATIONS IN CATALYTIC ADSORPTIONS AND REACTIONS 2 MICROMECHANISM OF CO CHEMISORPTION AND A NEW CONCEPT OF SIGMA-PI COORDINATION
- 化学化工－已发表论文 
A micromechanism of CO adsorption and a new concept of sigma-pi coordination on transition metal are proposed in this article. Based on experimental facts, we assume CO 5-sigma and/or CO 1-pi interacts with the representative M.O.s of the metal valence band, PSI-(Mi, Vs) and PSI-(Mi, Vd), to form the bonding M.O. group and antibonding M.O. group. The bonding group is located below the Fermi level (E(F)), in which some M.O.s are much more characteristic of metal orbitals (denoted as M-CO sigma-bondings) while some M.O.s exhibit slight metal orbital characteristics, which belong to the excited valence M.O.s of adsorbed CO, conventionally assigned as adsorbed CO 5-sigma, CO 1-pi and CO 4-sigma. The calculated data indicate that the peak positions of adsorbed CO 5-sigma, CO 1-pi and CO 4-sigma are significantly higher than their corresponding M.O.s in the gaseous CO molecule, i.e. adsorbed CO is in an excited (or activated) state. The total energy generated (DELTA-E) from adsorbed CO 5-sigma, CO 1-pi and CO 4-sigma can be used as a qualitative parameter for characterizing the ability for CO dissociation. On the other hand, the antibonding empty M.O. group of M-CO is located above the E(F), which exhibits some characteristics of metal d orbitals. The hybridization of CO 2-pi with d-pi orbitals in the Vs, Vd bands and d-pi orbitals of the antibonding M.O. group of M-CO bondings results in the formation of unoccupied M.O.s with CO 2-pi-M d-pi character. These M.O.s plus those unoccupied M.O.s without CO 2-pi-M d-pi character contribute the adsorbate-derived resonances, located 3-5 eV above E(F) and observed by Inverse Photo-Emission (IPE) difference spectra. We have used orbital overlap integrals of S(CO 5-sigma, d-sigma, Vd) and S(CO 2-pi, Vd) to characterize the relative competitive abilities for hybridization of CO 5-sigma and CO 2-sigma with d orbitals. The calculated results show that CO 5-sigma possesses a stronger ability to hybridize d orbitals in the Vd band than does CO 2-pi, thus the peaks of adsorbate-induced empty levels are shifted farther from the d band when the competitive hybridizing factor [CHF = S(CO 5-sigma, d-sigma, Vd)/S(CO 2-pi, d-pi, Vd)] is increased. The calculated data demonstrate that the peak positions of CO adsorbate-derived resonances of Cu, Ni, Pd and Pt metals, observed by IPE difference spectra, are in good parallel with their CHF values. Moreover, the values of CHE also demonstrate that CO sigma-bonding stimulates d electrons to transfer upward from the d band to the Vs band, where much more CO 2-pi-M d-pi character exists. We propose here a new concept of d back-donation, i.e. d electrons transfer from the occupied d band to the unoccupied M.O.s exhibiting CO 2-pi-M d-pi character in the Vs and Vd bands, which weakens the pi bond of C-O and simultaneously strengthens the M-C bond; these phenomena have been confirmed by IR spectroscopy and EELS. The d back-donation is represented by the B bonding function. The calculations of A, B and AB bonding functions indicate that the AB bonding function of CO adsorption on Cu is significantly smaller than that on Ni, Pd and Pt, so that CO adsorbtion is weak on Cu and is strong on Ni, Pd and Pt. Our micromechanism and our new concept of sigma-pi coordination provide a unified interpretation of the adsorbate vibration spectra measured by EELS and IR spectroscopy. The advantages of our new concept have been discussed and compared with the conventional concepts of Blyholder and CO 2-pi-derived resonances.