First-principle analysis of ethylene chemisorption on copper chloride clusters

Ethylene chemisorption on model copper chloride [Cu_xCl_y, Cu_xCl_y(OH)_z] and supported copper chloride [Cu_xCl_y(OH)_z/Al_r(OH)_s] clusters was examined using spin-polarized gradient corrected density functional theory. Both the mode and the energy of ethylene chemisorption are affected by the oxidation state, the coordination number, and the ligand field at the Cu center. In addition, the specific location of the vacant site (atop vs in-plane), adsorbate orientation, and support interactions were also found to be important in dictating the strength of the bond between ethylene and Cu. Ethylene weakly physisorbs atop a central Cu atom, in an axial ligand position, with an energy of less than 2 kcal/mol on the fully saturated Cu²⁺ adsorption site. The more favorable adsorption state is one which contains a defect site within the Cu_xCl_y plane. Ethylene strongly chemisorbs at this site, with its C double bond C bond oriented perpendicular to the Cu_xCl_y plane, with an energy of -18.7 kcal/mol. Two intermediate states also exist whereby ethylene chemisorbs less strongly, at -9.3 and -10.9 kcal/mol. These energies are consistent with the ethylene physisorption and chemisorption states reported experimentally at -2.3, -10.3, and -17.5 (to -19.8) kcal/mol. The interaction between ethylene and copper follows the classic Dewer-Chatt donation/back-donation model. Hydroxyl ligands at the Cu center act as trans-directing agents for ethylene chemisorption. They increase the binding energy of ethylene if they sit trans to ethylene; otherwise, they weaken the interaction between Cu and ethylene. These interactions are, in general, on the order of 5 kcal/mol. The interaction of the Cu_xCl_y complex with the alumina support strongly depends on the nature of the ligands that anchor the cluster to the support. Bridging oxygen atoms form a much stronger bond between the Cu_xCl_y complex and the support than the bonds formed by Cl or OH bridges. The alumina support increases the negative charge on the bridging oxygen species, which in turn increases the positive charge on the Cu center. This leads to an increase in the binding energies of the supported Cu centers over the unsupported centers of 3-5 kcal/mol.