CHARACTERIZATION OF MICROPOROUS CARBONS: FROM MATHEMATICAL MODELING TO ATOMISTIC CONSTRUCTION

摘要

Microporous carbon is one of most important adsorbents for gaseous mixture separation and energy storage due to its extremely high surface area, as well as ease in fabrication and modification. The microstructure of the carbon is normally semi-amorphous with short and medium range order induced by complexity of its pore structure. Indeed, local structural images of carbon taken by transmission electron microscopy (TEM) normally show fragmented carbon sheets with various degrees of curvature intercalated by voids or pore spaces. As a result, the slit-pore model, whereby the actual microstructure of the carbon is approximated as an ensemble of disconnected graphitic slit-pores, has been widely used to characterize the microstructure of the carbon. Despite such crude approximation, the slit-pore model with its latest improved version~1, the Finite Wall Thickness (FWT) model, provides correct prediction of adsorption isotherms in carbons, free of pore connectivity problems over a wide range of temperatures and pressures.~2Unfortunately, the pore connectivity in microporous carbons such as molecular sieve carbons (MSC) and coals is too important to be disregarded due to its serious impact on capability of the adsorbent in gaseous separation and storage.~3 Furthermore, the slit-pore model is unable to correctly predict isosteric heat of adsorption at low coverage~4 as well as to directly determine transport coefficient. This underscores the need for a realistic characterization method.