Thermodynamics of CO2 adsorption on functionalized SBA-15 silica. NLDFT analysis of surface energetic heterogeneity

Siliceous SBA-15 mesoporous molecular sieves were functionalized with different amounts of 3-aminopropyl-trimethoxysilane. To obtain a more detailed insight into the material properties of the prepared samples, their textural parameters were combined with results of thermal analysis. Adsorption isotherms of carbon dioxide on parent and functionalized SBA-15 were measured in the temperature range from 273 to 333 K. From the temperature dependence of CO2 isotherms the isosteric adsorption heats of CO2 were determined and discussed. Information about the surface energetic heterogeneity caused by tethered 3-aminopropyl groups were obtained from CO2 adsorption energy distributions calculated using the theoretical CO2 adsorption isotherms derived from the non-local density functional theory. The values of isosteric heats and the energy distributions of CO2 adsorption detect highly energetic sites and enabled quantification of their concentrations.