IR study of CO adsorption on α-Cr2O3

CO is adsorbed at 298 K on the (0001) faces of polycrystalline α-Cr 2 O 3 with the appearance of a single IR 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 pure dipole-dipole (dynamic) interaction as measured by means of the diluted isotopic method, can be explained in terms of CO oscillators adsorbed on identical sites and with their axes parallel. This suggest that the (0001) face is flat 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 of the IR spectrum of the solid (intensity erosions in the multiphonon and fundamental regions); these are ascribed to changes in the anharmonicity and force constants of surface vibrations. Attempts to detect the MeCO stretching and bending modes in the low-frequency range of the spectrum (700−200 cm −1 ) have been unsuccessful; the reasons of this negative result are discussed.