Evolution of adsorbed species during C2H2 adsorption on Ni(111) in relation to their vibrational spectra

Abstract

Acetylene adsorption at room temperature on Ni (111) has been studied by low energy electron diffraction (l.e.e.d.), thermal desorption, work function changes and high resolution electron energy loss spectroscopy (h.r.e.l.s.). The existence of two adsorbed states has been demonstrated depending upon exposure.

At low exposure, C₂H₂ is associatively adsorbed but a large rehybridization occurs, characterized by a carbon–carbon bond order close to 1.15. According to its hybridization state C₂H₂ should be adsorbed on a triangular site through two σ bonds and one π bond with nickel atoms.

With increasing exposure, large modifications occur. In addition to self-hydrogenated species, benzene should be formed as shown by comparison of the vibrational spectra of adsorbed C₂H₂ and adsorbed C₆H₆. This conclusion is in agreement with the benzene formation as detected after contact between C₂H₂ and Ni/SiO₂ catalysts with (111) exposed faces.