Characterization of accessible and inaccessible pores in microporous carbons by a combination of adsorption and small angle neutron scattering

We determine the pore size distribution for five activated carbons (comprising carbide derived as well as commercial activated carbon samples) by the interpretation of experimental small angle neutron scattering (SANS) intensity profiles, based on the primary assumption of an infinitely dilute solution of hollow spherical particles. The interpretation yields the pore size distribution of the carbon samples that have predominantly micropore populations (size <20 Å), but not for carbons which have significant mesopore populations of sizes up to 48 Å and high mass fractal degrees. The pore size distribution (PSD) results based on SANS data reveal significant populations of micropores of size <6.1 Å, and mesopores of size >20 Å, which are not present in the PSD results based on adsorption isotherms of either Ar at 87 K or CO2 273 K. This inaccessible porosity becomes accessible to CO2 and Ar on heat treatment, leading to increase in the adsorption based pore volume. However, the surface area does not commensurately increase, indicating the inaccessible microporosity to predominantly comprise surface defects and roughness that are removed on heat treatment or activation. This finding sheds the light onto the evolution of porosity of activated carbons during gasification or post synthesis-treatment.