Chemisorption of CO on Pd(100): An lcgto‐lsd cluster study

Abstract

AbstractLCGTO‐LSD model potential calculations have been performed for CO interacting with two‐, four‐, and eight‐atom clusters of Pd, chosen to model the bridge site of the (100) surface. The geometry and vibrational frequencies are not very sensitive to the cluster size. For Pd8 + CO we obtain dC—O = 1.18 Å (1.13 ± 0.1 exp.), dPd—C = 1.87 Å (1.93 ± 0.07 exp.), and (uncoupled) estimates for ωC—O = 1828 cm−1 (1895 exp.) and ωPd—CO = 454 cm−1 (339 exp.) Binding energies of 4.8, 3.8, and 2.6 eV are calculated, respectively, for Pd2 + CO, Pd4 + CO, and Pd8 + CO which may be compared with the experimental initial heat of adsorption of 1.6 eV. Ionization potentials for CO‐derived levels are in excellent agreement with experiment (relative to ϵF: 4σ (‐11.0 eV, ‐11.2 exp.); 5σ (‐8.0, −8.2 exp.); 1π [−7.5 (b1), −7.3 (b2), −7.5 exp.]). The main negative ion states of 2π* character are calculated at 2.8 eV (b1) and 2.7 eV (b2) above EF. Other states with appreciable 2π* character are found near 5 eV. These may be compared with inverse photoemission results which show a broad peak centered at 4.8 eV. Interactions of the 4σ, 5σ, 1π, and 2π* orbitals of CO with the metal are discussed. The 4σ and 5σ levels are highly mixed, each receiving appreciable contributions from the 4σ and 5σ orbitals of isolated CO. This is discussed in relation to the dispersion of the 4σ and 5σ levels observed in UPS and to the photon‐energy dependent intensities of the 4σ and 5σ resonances. The 2π* component of the backbonding comes through several levels in the upper part of the d band which contain small 2π* contributions in bonding combination with Pd d orbitals. The main 2π* orbitals (contaminated by small antibonding contributions from the metal) are empty (see above).