Theoretical Study of the TiO2 and MgO Surface Acidity and the Adsorption of Acids and Bases |
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Authors: | Markovits Alexis Ahdjoudj Jamila Minot Christian |
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Institution: | (1) Laboratoire de Chimie Théorique, UPR 9070 CNRS, Tour 23-22, Boîte 137, Université P. et M. Curie, 75252 Paris Cedex05, France |
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Abstract: | We present ab-initio periodic Hartree–Fock calculations (crystal program) of small molecules on TiO2 and MgO. The adsorption of the molecules may be molecular or dissociative. This depends on their acid and basic properties in the gas phase. For the molecular adsorption, the molecules are adsorbed as bases on Ti(+IV) sites, the adsorption energies correlate with the proton affinities. The dissociations on the surface correlate with the gas phase cleavages: thus, the dissociation of MeOH leads to a preferential basic cleavage (the fragment HO– is adsorbed on a Ti+4 ion and the fragment Me+ is adsorbed on a O2– ion of the oxide). The opposite result is obtained with MeSH. Another important factor is the adsorbate–adsorbate interaction: favorable cases are a sequence of H-bonds for the hydroxyl groups resulting from the water dissociation and the mode of adsorption for the ammonium ions. Lateral interactions also force the adsorbed CO2 molecules to bend over the surface so that their mutual orientation resembles the geometry of the CO2 dimer. With respect to water adsorption, MgO appears to be a basic oxide. As experimentally observed, NH3 adsorbs preferentially on TiO2 and CO2 on MgO. However, this difference of reactivity should not be expressed in terms of acid vs. basic behaviour but in terms of hard and soft acidity. The MgO surface is a 'soft' acidic surface that reacts preferentially with the soft base, CO2. |
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Keywords: | Periodic Hartree– Fock calculations metal oxide surface acidity absorption hard or soft acid or base |
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