Adsorption of Water and Methanol on Silica Hydroxyls: Ab Initio Energy and Frequency Calculations

Two hypothetical mechanisms proposed in the literature for the coordination of H 2 O and CH 3 OH on the isolated silica OH group are reexamined by computations on the related molecular models at SCF, MP2, and DFT levels. The computed energies suggest that these molecules stabilize on the isolated OH group by means of two hydrogen bonds, acting as a proton acceptor toward the silica OH group and as a proton donor toward a nearby SiOSi bridge. The alternative surface structures, with H 2 O and CH 3 OH acting exclusively as a proton acceptor toward the isolated OH group, are found to be less stable by ∼8 kJ/mol. The check of these mechanisms against experimental IR data using the computed frequency shifts of CH 3 OH upon the adsorption supports these results. The ab initio calculations suggest no significant difference in mechanism between the adsorptions on the isolated and geminal OH groups of silica. When stabilizing on a geminal OH group, H 2 O and CH 3 OH can form the extra hydrogen bond not only with the nearby SiOSi bridge but also with the complementary geminal OH group.