Hartree-Fock-Slater-LCAO studies of the acetylene-transition metal interaction: I. Chemisorption on Ni surfaces; cluster models

Abstract

The interaction of C 2H 2 with Ni surfaces has been studied by the Hartree-Fock-Slater-LCAO method (with core pseudopotentials). Different adsorption sites (π, di-σ, μ 2, μ 3) at the Ni(111) surface have been modelled by clusters of 1 to 4 Ni atoms; the structure of C 2H 2 and the Ni-C distance have been varied (3 structures, 2 distances). The acetylene-metal bonding can be interpreted in terms of π to metal donation and, especially, metal to π ∗ back donation effects which considerably weaken the C-C bond. These effects become increasingly important when more metal atoms are directly involved in the adsorption bonding: π < di-σ < μ 2 < μ 3. The calculated shifts in the ionization energies are in fair agreement with the experimentally observed shifts (by UPS) for C 2H 2 adsorbed on Ni(111) (and other Ni surfaces); these shifts do not depend very sensitively on the bonding situation, however, so that we could not assign the structure of adsorbed C 2H 2 solely on this basis. From the comparison between the measured C-C stretch frequency (by ELS) and the calculated C-C overlap populations, using a relation calibrated on Ni-acetylene complexes, we find that μ 3 bonding of C 2H 2 with a Ni-C distance of about 1.9 Å is most probable on the Ni(111) surface; the CCH angle is estimated to be somewhat smaller than 150°. We have suggested an explanation for the surface specific dissociation of C 2H 2: C 2 fragments (C-H bond breaking) have been observed on stepped Ni surfaces (at low temperature), CH fragments (C-C bond breaking) have been found on ideal surfaces (at higher temperatures).