Non-thermodynamical CO_2 adsorption behavior for amine-modified nanoporous silica is clarified by evaluating the mobility of organic functional group inside mesopores by using pulsed NMR technique. CO_2 adsorption behavior of nanoporous silica modified with amino-propyl silane (AP) changes significantly depending on the amount of AP loaded. A low AP loaded sample shows normal adsorption behavior; the amount of CO_2 adsorbed decreases with increasing temperature. In contrast, a high AP loaded sample possesses non-thermodynamic CO_2 adsorption behavior in which the amount of CO_2 adsorbed increases with increasing temperature in within a certain temperature range. To address the mechanism, a pulsed NMR technique was employed to clarify the mobility of AP molecules, and it was found that the mobility of mobile components in a high AP loaded sample increased drastically with increasing temperature while the mobility in a low AP loaded sample remained unchanged. It is understood that the enhancement of the diffusion of CO_2 inside nanopores leads to the non-thermodynamic adsorption behavior.
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