The structure of adsorbed CO2 in carbon nanopores: a neutron diffraction study

The behaviour of CO2 adsorbed on a microporous carbon at a temperature slightly above the critical point, was studied by adsorption in conjunction with in situ neutron powder diffraction. The carbon sample was produced by carbonisation and activation of a polymer precursor and the experiment was carried out by means of a specially designed adsorption apparatus, directly mounted on the neutron diffractometer. The CO2 diffraction patterns provide evidence that the adsorbed phase is—at pressures well below the critical one—in a state comparable to high-pressure dense supercritical fluid or even bulk liquid. Furthermore, the features of the diffraction patterns are consistent with the presence of orientational correlations between adsorbed molecules. Even though, the scattering data cover a limited Q space and thus the overall resolution is rather poor, the radial distribution functions deduced at different equilibrium pressures, reveal differences, which could be attributed to changes in the arrangement of CO2 molecules along the 308 K isothermal scan. The results are discussed and compared with permeability measurements as well as Monte Carlo calculations.