Adsorbate-adsorbent and adsorbate-adsorbate interactions in CO overlayers adsorbed on α-Cr 20 3 as investigated by infrared spectroscopy

Abstract

CO is adsorbed at 298 K only on the (0001) faces of polycrystaltine αCr 20 3 with the appearance of a single i.r. peak whose frequency shifts from 2185 (θ ≈ 0) to 2170 cm -1 (θ max). This shift is due to the building up of adsorbate-adsorbate interactions (dynamic and static). The dipole--dipole (dynamic) interaction has been measured by means of the diluted isotopic method and can be explained in terms of CO oscillators adsorbed on identical sites and with their axes parallel. This suggests that the (0001) face is fiat and that the Cr 3+-CO bond, although mainly σ in character, has some participation of the d-π * orbitals, The adsorption of CO causes distinct perturbations in the i.r, spectrum of the α-Cr 2O 3 (intensity erosions in the multiphonon and fundamental regions): this is ascribed to changes in the anharmonieity and force constants ofsurface vibrations. Attempts to detect the Me-CO stretching and bending vibrations in the low frequency range of the spectrum (700--200 cm -1) have been unsuccessful; the reasons for this negative result are discussed.