3Ti Alloys Research Articles

CO adsorption on (111) and (100) surfaces of the Pt 3Ti alloy: Evidence for parallel binding and strong activation of CO

An atom superposition and electron delocalization molecular orbital (ASED-MO) study has been made of CO adsorption on a 40-atom cluster model of the (111) surface and a 36-atom cluster model of the (100) surface of the Pt 3Ti alloy. Parallel binding to high-coordinate sites associated with Ti and low CO bond scission barriers are predicted for both surfaces. The preference for parallel adsorption is a consequence of the nature of the CO π-to-surface donation interactions. On Ti sites the π orbitals donate to the nearly empty Ti 3 d band and the antibonding counterpart orbitals are empty. Thus the π donation makes substantial contributions to the adsorption bond order that are in addition to the contributions from 5σ donation and metal backbonding to the π ∗ orbitals. Altogether these bonding interactions favor the lying down orientation. On Pt sites, on the other hand, the π donation antibonding counterpart orbitals are occupied so that the net interaction with Pt is a closed-shell repulsion. CO bonds upright in order to minimize the π interaction and, concomitantly, the closed-shell repulsion, while maintaining 5σ donation and π ∗ backbonding stabilizations. Comparisons are made with the results for a 40-atom cluster model of the unalloyed Pt(111) surface. It is shown that the extended Hückel parameterization is inappropriate for studying CO adsorption to Pt with the ASED-MO theory because it incorrectly favors adsorption bonding through the oxygen end.