Interaction of ethene and ethyne with bare and hydrogenated Ir₄ clusters. A density functional study

The paper reports a computational study of species that can be formed during ethene hydrogenation on iridium clusters. The simulated concentrations of the complexes (C₂H_m)Ir₄H_n (m = 2-5, 0 ≤ n ≤ 14 - m) based on calculated Gibbs free energies suggest at low temperature and high hydrogen pressure π-bonded ethene to be the dominant species at the Ir₄ cluster covered by hydrides. At higher temperature and lower H₂ pressure, this model predicts ethylidyne and, subsequently, di-σ-coordinated ethyne with a minor or zero amount of hydride ligands on the metal cluster. Ethyl, vinyl, and vinylidene species were calculated to be less stable over the range of the hydrogen coverage studied. Ethane desorption from the most stable complex was calculated to be thermodynamically favorable for systems in which at least three hydride ligands will remain on the metal cluster after desorption. Adsorption of one of these organic ligands and/or hydrogen results in an oxidation of the metal moiety; this effect is more pronounced in complexes with ethylidyne, vinyl, and vinylidene. The calculated vibrational spectra of ethylidyne on Ir ₄H_n clusters agree well with available experimental data for this species on iridium surfaces and supported metal particles. The spectra of the various organic species in the region of C-H stretching modes (3300-2700 cm^-1) were calculated to overlap, in particular in the presence of hydride ligands on the metal moiety.