First principles study of adsorption and dissociation of H$_2$, O$_2$, and CO on Pt$_4$ and Pt$_3$Co clusters*

Pt and Pt based alloy nanoparticles have received much attention recently in designing improved catalysts for the oxygen reduction reaction in fuel cell electrodes. In particular, Pt-Co alloy systems have attracted significant interest in proton exchange membrane fuel cells. To gain physical insight into the catalytic properties of Pt and Pt-Co alloys we have performed fundamental studies on Pt$_4$ and Pt$_3$Co alloy clusters using first principles density functional calculations. The structural and physicochemical properties of the pure metal and the alloy have been analyzed, and for Pt$_4$, the tetrahedral geometry is found to be more stable than the square planar geometry. On the optimized tetrahedral geometries the interactions of H$_2$, O$_2$, and CO with different orientations have been studied. The calculated energies of adsorption indicate H$_2$ and CO prefers to adsorb on Pt atop while O$_2$ prefers Co atop. The adsorption energy of CO is found to be the highest of all the three adsorbing species in both pure Pt$_4$ and Pt$_3$Co alloy clusters.