THE CHEMICAL NATURE OF ATOMIC OXYGEN ADSORBED ON RH(111) AND PT(111) - A DENSITY-FUNCTIONAL STUDY

The bonding of atomic oxygen on Pt(111) and Rh(111) was examined using density functional theory in order to understand their different chem ical properties. The oxygen-surface interactions were modeled by bondi ng atomic oxygen to 10-atom clusters of Pt and Ph designed to model th e (111) surface. Density functional theory was applied using the local density and generalized gradient approximations; results were obtaine d for both double-zeta and triple-zeta basis sets. Optimized geometrie s and binding energies were computed and favorably compared to availab le experimental values. Interestingly, the ionic bonding in the two ca ses is nearly the same, based on the similarities in the charge on oxy gen. The Hirshfeld charges on oxygen were -0.225 and -0.207 for Rh-10- O and Pt-10-O, respectively, using the double-zeta basis set. A more d etailed analysis of the covalent bonding using crystal orbital overlap populations indicated that the 2p orbitals of oxygen interact in a gr eater bonding fashion with both the sp and d orbitals of Ph than with those of Pt. Additional calculations with adsorbed hydroxyl on these m etal clusters show differences in covalent bonding similar to that of oxygen. In this case, however, differences in ionic bonding play a rol e; oxygen in hydroxyl has a greater charge on Pt than Ph. This leads t o smaller differences in the interaction energies of hydroxyl on Ph an d Pt compared with oxygen, resulting in differences in chemical reacti vity between the two metals, especially with respect to reactions invo lving hydrogen transfer.