Elucidation of Surface Species through in Situ FTIR Spectroscopy of Carbon Dioxide Adsorption on Amine-Grafted SBA-15

The nature of the surface species formed through the adsorption of CO₂on amine-grafted mesoporous silica is investigated through in situ FTIR spectroscopy with the aid of¹⁵N dynamic nuclear polarization (DNP) and¹³C NMR spectroscopy. Primary, secondary, and tertiary amines are functionalized onto a mesoporous SBA-15 silica. Both isotopically labeled¹³CO₂and natural-abundance CO₂are used for accurate FTIR peak assignments, which are compared with assignments reported previously. The results support the formation of monomeric and dimeric carbamic acid species on secondary amines that are stabilized differently to the monocarbamic acid species on primary amines. Furthermore, the results from isotopically labelled¹³CO₂experiments suggest the existence of two carbamate species on primary amines, whereas only one species is observed predominantly on secondary amines. The analysis of the IR peak intensities and frequencies indicate that the second carbamate species on primary amines is probably more asymmetric in nature and forms in a relatively smaller amount. Only the formation of bicarbonate ions at a low concentration is observed on tertiary amines; therefore, physisorbed water on the surface plays a role in the hydrolysis of CO₂even if water is not added intentionally and dry gases are used. This suggests that a small amount of bicarbonate ions could be expected to form on primary and secondary amines, which are more hydrophilic than tertiary amines, and these low concentration species are difficult to observe on such samples.