Origin of site preference of CO and NO adsorption on Pd(111) at different coverages: A density functional theory study

First-principles calculations based on density functional theory have been performed to investigate the adsorption of CO and NO on Pd(111) surface. The average and differential adsorption energies of CO and NO at the high-symmetry adsorption sites on Pd(111) at different coverages, namely 0.11, 0.25, 0.50, and 0.75ML are calculated. The results show that at low coverages of 0.11 and 0.25ML, both CO and NO are preferentially adsorbed at the threefold hollow sites. At the medium coverage of 0.50ML, CO molecules occupy both hollow and bridge sites with almost degenerate adsorption energies, while NO molecules prefer the hollow sites to the bridge sites, which can be attributed to the more corrugated potential energy surface of NO on Pd(111) surface. At high coverage of 0.75ML, both CO and NO form the p(3×3)- 3CO structures with the adsorbates at the hollow and atop sites. The calculated stretching vibrational frequencies are in good agreement with the experimental values. The electronic structure analysis shows that the adsorption of CO and NO on Pd(111) surface follows a donation and back-donation mechanism, and the high-coordinated sites promote the donation and back-donation process. The energy decomposition scheme indicates that the site preference at higher coverage is a compromise between the adsorbate–substrate interaction and adsorbate–adsorbate interaction. The relationship between the adsorption energies and the surface coverage has been well established, and linear relations are identified for both CO and NO. We suggest that the coverage-dependent adsorption energies of CO and NO on Pd(111) are useful parameters for the kinetic studies on the relevant catalytic reaction systems.