Ethylene adsorption on the Pt-Cu bimetallic catalysts. Density functional theory cluster study

The adsorption of ethylene on Cu12Pt2 clusters has been studied within the density functional theory (DFT) approach to understand the high ethylene selectivity of Cu-rich Pt-Cu catalyst particles in the reaction of hydrogen assisted 1,2-dichloroethane dechlorination. The structural parameters for Cu12Pt2 clusters with D-4h, D-2d, and C-3v symmetry have been calculated. The relative stability of the isomeric Cu12Pt2 clusters follows the order: C-3v > D-2d > D-4h. Each isomer has an active site for ethylene adsorption that consists of a single Pt atom surrounded by Cu atoms. The interaction of ethylene with the active site yields a pi-C2H4 adsorption complex. The strongest pi-C2H4 complex forms with the cluster Of C-3v symmetry; the bonding energy, Delta Epsilon pi(C2H4), is - 15.6 kcal mo(-1). The bonding energies for the pi-C2H4 complex with Cu-14 and Pt-14 clusters are -6.5 and -18.8 kcal mo1(-1), respectively. The addition of Pt to Cu modifies the valence spd-band of the cluster as compared to a Cu-14 cluster. The DOS near the Fermi level increases when C2H4 adsorbs on the Cu12Pt2 cluster. As well, the center of the d-band shifts toward lower binding energies. Ethylene adsorption also induces a number of states below the d-band. These states correspond to those of gas-phase C2H4. The vibrational frequencies of C2H4 adsorbed on the clusters of D-4h and C-3v, symmetry have been calculated. The phonon vibrations occur below 250 cm(-1). The intense bands around 200 cm(-1) are attributed to stretching vibrations of the Pt-Cu bonds normal to the cluster surface. The stretching vibrations of the Pt-C bonds depend on the local structure of the active site: nu(s)(Pt-C) = 268 cm(-1) and nu(as)(Pt-C) = 357 cm(-1) for the cluster of the D-4h symmetry; nu(s)(Pt-C) = 335 cm(-1) and v(as)(Pt-C) = 397 cm(-1) for the cluster of the C-3v symmetry. Bands in the range of 800-3100 cm(-1) are attributed to vibrations of the adsorbed C2H4 molecule. The signature frequencies of the pi-C2H4 adsorption complex are the delta(s)(CH2) deformation vibration at similar to 1200 cm(-1) and the nu(C-C) stretching vibration at similar to 1500 cm(-1). These vibration are absent for di-sigma-C2H4 adsorption complexes. (c) 2005 Elsevier B.V. All rights reserved.