Study of the Interaction between Silica Surfaces and the Carbon Dioxide Molecule

Three sphere packing (SP) and three particle packing (PP) silica xerogels, two silica aerogels (AERs), one all-silica mesoporous molecular sieve (MMS), one all-silica molecular sieve (MS), two high Si/Al relation molecular sieves, and an extremely high Si/Al relation molecular sieve were carefully studied with SEM, TGA, SAXS, DRIFTS, and adsorption of nitrogen at 77 K and carbon dioxide at 273 K. The study of different amorphous, ordered, and crystalline silicas, applying numerous powerful and complementary experimental methods and a proper theoretical methodology, is, by itself, an original approach that allows the recognition of the similar features exhibited by the surface of these materials. With the help of SEM, TGA, SAXS, and DRIFTS, it was found that the SP, PP, and AER samples present a complicated pore size distribution composed of micropores and mesopores formed by the agglomeration of primary nanoparticles. The specific surface areas of the synthesized PP silica and one of the AER are extremely high, a fact very significant for the application of these materials as catalyst supports and adsorbents. The specific surface area of the amorphous materials was confirmed with a novel methodology, applying the TGA data. The carbon dioxide adsorption data allowed the measurement of the isosteric heat of adsorption with a single isotherm, applying an original methodology. The contribution of the dispersive interactions to the adsorption energy was theoretically calculated to analyze the separate contribution of the dispersive and the quadrupole interactions. Additionally, the careful analysis of the DRIFT spectra of carbon dioxide adsorbed on silica at 298 K generated information about the carbon dioxide−silica interactions, which allowed us to conclude that, in this interaction, besides the dispersive and quadrupole interactions, there is present a weakly bonded adduct, that is, Hδ+···δ−O═C═Oδ−. This is a new finding as far as we know.