Theoretical study of the chemisorption of CO on Al2O3(0001)

Density functional molecular cluster calculations have been used to study t he adsorption of CO on the alpha -Al2O3-(0001) surface. Substrate and adsor bate geometry modifications, adsorption enthalpies, and adsorbate vibration s are computed. Despite the rather small size of the employed cluster, rela xation phenomena evaluated for the clean surface agree well with experiment al measurements and periodic slab calculations and mainly consist of an inw ard relaxation of the Lewis acid site (L-s(a)). Different adsorbate arrange ments, perpendicular and parallel to the surface, have been considered. Amo ng them, the most stable CO chemisorption geometry (Delta H (ads) approxima te to -13 kcal/ mel) is that corresponding to the adsorbate perpendicular t o the surface, atop L-s(a) and C-down oriented. The C-O stretching frequenc y' (nuc-o) computed for such an arrangement is 2158 cm(-1), i.e., blue shif ted by 44 cm(-1) with respect to the free adsorbate. The lack of experiment al evidence pertaining to CO interacting with a well-defined alpha -Al2O3(0 001) surface prevents the possibility of a direct check of the computed qua ntities. Nevertheless, low-temperature IR data for CO on alumina powders (Z ecchina, A.; Escalona Platero, E.; Otero Arean, C. J. Catnl. 1987, 107, 244 ) indicate for the chemisorbed species a Delta nu = 12 cm(-1). The adsorbat e-substrate interaction relieves some of the L-s(a) relaxation, even if the L-s(a) electronic structure is only slightly affected upon chemisorption.