THEORETICAL-STUDY OF THE ADSORPTION OF ACIDS AND BASES ON TIO2 AND MGO SURFACES

The adsorption of small molecules on TiO2 and MgO is investigated by n b initio periodic Hartree-Fock calculations. The adsorption may be mol ecular or dissociative. This depends on their acidic and basic propert ies in gas phase. For the molecular adsorption, the molecules are adso rbed as bases on the metal sites and the adsorption energies correlate with the proton affinities. This also true on MgO, even if CO2 is ord inary considered as an acidic probe molecule. HSBA theory allows to co mpare the relative adsorption of molecules on TiO2 and MgO. The dissoc iations on the surface correlate with the gas phase cleavages: basic c leavage for MeOH and acidic cleavage for MeSH. Another important facto r is the adsorbate-adsorbate interaction: at high coverage, stabilizat ion through H-bonds favors the vicinity of adsorbates on the surface ( islands of adsorbates and coadsorption effects). Electrostatic interac tions explain the change of orientation (CO2) with the coverage.