A comparative study of CO chemisorption on Al2O3 and Ti2O3 nonpolar surfaces

Density functional molecular cluster calculations have been used to investigate the interaction of CO with the M2O3(10 (1) over bar2) (M = Al and Ti) nonpolar surface. The electronic structure of the clean surface, the adsorbate geometry, vibrational parameters, and chemisorption enthalpies are computed and discussed. Theoretical results pertaining to the clean surface agree quite well with experimental measurements and other theoretical investigations. As far as the adsorbate-substrate interaction is concerned, our data indicate that the CO-M2O3(10 (1) over bar2) bonding is charaterized, in both Al2O3 and Ti2O3, by a two-way electron flow involving both donation from CO based sigma levels into virtual orbitals of the unsaturated surface Lewis acid site and back-donation from surface states into the CO pi* virtual levels. However, the nature of surface orbitals involved in back-donation and the concomitant effects on the adsorbate structure are very different in the two cases. CO is only slightly affected upon chemisorption on Al2O3(10 (1) over bar2), while perturbations induced into the CO electronic and molecular structure by the interaction with the Ti2O3(10 (1) over bar2) surface are very intense and, consistently with experimental data, the C-O bond becomes strongly weakened.