Transition metal bonding functions and their applications in catalytic adsorptions and reactions: Part II. Micromechanism of CO chemisorption and a new concept of σ-π coordination

Abstract

A micromechanism of CO adsorption and a new concept of σ-π coordination on transition metal are proposed in this article. Based on experimental facts, we assume CO 5σ- and/ or CO 1 π interacts with the representative M.O.s of the metal valence band, ψ(Mi, Vs) and ψ(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 orbitais (denoted as M-CO σ-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 5σ, CO 1 π and CO 4σ. The calculated data indicate that the peak positions of adsorbed CO 5σ, CO 1 π and CO 4σ 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 (Δ E) from adsorbed CO 5σ, CO 1 π and CO 4σ 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 orbitais. The hybridization of CO 2π with dπ- orbitais in the Vs, Vd bands and dπ orbitais of the antibonding M.O. group of M-CO bondings results in the formation of unoccupied M.O.s with CO 2π-M dπ character. These M.O.s plus those unoccupied M.O.s without CO 2π-M dπ 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 5σ, dσ, Vd) and S(CO 2π, dπ, Vd) to characterize the relative competitive abilities for hybridization of CO 5σ and CO 2π with d orbitais. The calculated results show that CO 5σ possesses a stronger ability to hybridize d orbitals in the Vd band than does CO 2π-, thus the peaks of adsorbate-induced empty levels are shifted farther from the d band when the competitive hybridizing factor [CHF=S(CO 5σ, dσ, Vd)/S(CO 2π, dπ, 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 σ-bonding stimulates d electrons to transfer upward from the d band to the Vs band, where much more CO 2π-M dπ 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 2π-M dπ character in the Vs and Vd bands, which weakens the π 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 σ-π coordination provide a unified interpretation of various CO adsorption electronic spectra from below to above the E F, i.e. from occupied orbitals to empty orbitals; and 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 2π-derived resonances.