Theoretical Study of the Chemisorption of CO on Al2O3(0001)

Density functional molecular cluster calculations have been used to study the adsorption of CO on the α-Al2O3(0001) surface. Substrate and adsorbate geometry modifications, adsorption enthalpies, and adsorbate vibrations are computed. Despite the rather small size of the employed cluster, relaxation phenomena evaluated for the clean surface agree well with experimental measurements and periodic slab calculations and mainly consist of an inward relaxation of the Lewis acid site (Las ). Different adsorbate arrangements, perpendicular and parallel to the surface, have been considered. Among them, the most stable CO chemisorption geometry (ΔHads ≈ −13 kcal/mol) is that corresponding to the adsorbate perpendicular to the surface, atop and C-down oriented. The C−O stretching frequency (νC-O) computed for such an arrangement is 2158 cm-1, i.e., blue shifted by 44 cm-1 with respect to the free adsorbate. The lack of experimental evidence pertaining to CO interacting with a well-defined α-Al2O3(0001) surface prevents the possibility of a direct check of the computed quantities. Nevertheless, low-temperature IR data for CO on alumina powders (Zecchina, A.; Escalona Platero, E.; Otero Areán, C. J. Catal. 1987, 107, 244) indicate for the chemisorbed species a Δν = 12 cm-1. The adsorbate−substrate interaction relieves some of the Las relaxation, even if the Las electronic structure is only slightly affected upon chemisorption.