Dependence of stretching frequency on surface coverage and adsorbate–adsorbate interactions: a density-functional theory approach of CO on Pd (111)

Abstract

Total energy and stretching frequency calculations ν(C–O) are reported for the chemisorption of CO on a Pd (111) surface as a function of coverage with a periodic density-functional theory approach including generalized gradient approximation. At low [0.33 monolayer (ML)] and high (0.75 ML) coverages, the most stable structures always present CO in threefold fcc and hcp hollow sites, mixed with top sites for the high coverage structure, in agreement with the interpretation of the RAIRS or HREELS spectra. The calculated anharmonic frequencies for these sites (1828–1830 cm −1 at 0.33 ML) and (1893 and 2085 cm −1 at 0.75 ML) are coherent with the observed IR peaks. At medium coverage, 0.5 ML, the most stable model is the one with both fcc and hcp hollow sites. The observed blue shift of the frequency band between 0.33 ML and 0.5 ML is usually interpreted as a change of the chemisorption site from a hollow to a bridge adsorption. Although a direct comparison of the IR peak with the calculated anharmonic values at 0.5 ML for the structures with two hollow sites (1906 cm −1) or with two bridge sites (1937 cm −1) is this time more delicate, the calculations show that the observed frequency shift is fully compatible with the hollow site model and can be attributed to increased coverage and adsorbate–adsorbate interactions. The results show that two main phenomena occur when the coverage increases. First the shift caused by static interactions between adsorbates is ruled by the coverage-dependent backdonation from the surface metallic atoms towards the antibonding 2π molecular orbitals of the chemisorbed CO molecules. The second shift is due to dynamic interactions between adsorbates. The stability of a vibrational configuration taken from a particular mode is ruled by the electrostatic energy cost due to the competitive charge transfer between the adsorbates and the metallic surface.