Adsorption energetics of NO and CO on Pt(111)

The adsorption energetics of NO and CO on Pt(111) are studied using an ab i nitio embedding theory. The Pt(111) surface is modeled as a three-layer, 28 -atom cluster with the Pt atoms fixed at bulk lattice sites. Molecular NO i s adsorbed at high symmetry sites on Pt(111), with the fee threefold site e nergetically more favorable than the hcp threefold and bridge sites. The ca lculated adsorption energy at the fcc threefold site is 1.90 eV, with an N- surface distance of 1.23 A. The NO molecular axis is perpendicular to the P t(111) surface. Tilting the O atom away from the surface normal destablizes adsorbed NO at all adsorption sites considered. On-top Pt adsorption has b een ruled out. The Pt(111) potential surface is very flat for CO adsorption , and the diffusion barriers from hcp to fee sites are 0.03 eV and less tha n 0.06 eV across the bridge and the atop sites, respectively. Calculated ad sorption energies are 1.67, 1.54, 1.51, and 1.60 eV at the fcc threefold, h cp threefold, bridge, and atop sites, respectively. Calculated C-surface di stances are 1.24 Angstrom at the fcc threefold site and 1.83 Angstrom at th e atop site. It is concluded that NO and CO adsorption energetics and geome tries are different on Pt(111).