Dynamics of CO2 molecules confined in the micropores of solids as studied by 13C NMR

The pressure and temperature dependence of 13C NMR of CO2 adsorbed in several porous materials was measured. For CO2 in activated carbon fiber (ACF), the spectrum observed in the pressure range from 0 to 10 MPa consisted of two lines. A very sharp peak at δ = 126 ppm was attributed to free CO2 gas and a broad peak at δ = 123 ppm was attributed to confined CO2 molecules in the micropores of ACF, although CO2 in microporous materials such as zeolites and mesoporous silica, gave only a single peak attributed to free CO2 gas. In the low-pressure region, the peak at δ = 123 ppm shifted to 118 ppm and a very broad peak with a line width of about 200 ppm appeared. This indicates that there are two kinds of CO2 molecules confined in ACF with different rates of molecular motion: one is undergoing isotropic rotation and the other is undergoing anisotropic motion, which rotates around an axis tilted by 30° from the molecular axis. This implies that small pockets with a characteristic diameter exist on the surface of the ACF micropore.