Quantitative Investigation of MgO Bronsted Basicity: DFT, IR, and Calorimetry Study of Methanol Adsorption
Résumé
The adsorption geometries, energies, and vibrational frequencies of methanol on MgO defective surfaces have been calcd. by periodic DFT simulations. The results are very comparable with those obtained with water and are also in very good accordance with microcalorimetry and IR expts. At low coverage, the dissocn. is obsd. on all defects involving ions in low coordinations. Over and above the coordination no. of surface ions, the adsorption energy is strongly governed by the surface topol.: dissocn. on confined sites gives rise to methoxy groups highly stabilized by bridging two or even three cations. The occurrence of such very strong sites on MgO powder is confirmed by microcalorimetry. The dissocn. ability depends on the methanol coverage because it modifies the surface relaxation and the network of H bonds, resulting, for a given defect, in similar adsorption energies for mol. and dissocd. species at high coverage. This explains why there are more strong sites (quantified by microcalorimetry) than dissocg. sites (quantified by IR). [on SciFinder(R)]