The chemisorption of carbon monoxide on palladium single crystal surfaces: IR spectroscopic evidence for localised site adsorption

Abstract

Combined IR reflection-absorption and LEED studies of the room temperature adsorption of carbon monoxide on the (100), (111) and (210) surfaces of palladium are reported. The CO stretch frequency on the (100) surface varies from 1895 cm −1 at low coverages to 1949 cm −1 at θ = 0.5. A further strong shift to higher frequency is observed during the compression stage for θ > 0.5. On the (111) surface the CO stretch frequency is found at 1823 cm −1 at very low coverages, rising to 1936 cm −1 at θ = 0.5. In the compression stage above θ = 0.5 a second peak is observed above 2000 cm −1. On the much more open (210) surface, where surface atoms with the next nearest neighbour distance of 2.73 Å do not exist, the behaviour of the IR band resembles surprisingly that of the (100) surface. The infrared reflection studies provide detailed information on the CO/Pd system which other surface techniques have not as yet revealed, namely, evidence for surface specificity and localised site adsorption at low CO coverage. The frequency shifts observed in the spectra are discussed in terms of the three important intermolecular interaction mechanisms, namely, dipole-dipole coupling, direct intermolecular repulsion and indirect effects via the metal atoms.