Novel approach to the characterization of the pore structure and surface chemistry of porous carbon with Ar, N2, H2O and CH3OH adsorption

Abstract

Characterization of porous carbon for its pore size distribution is traditionally carried out with simple pore models of well-defined geometry (slit or cylinder) with both ends opened to the gas surroundings. These idealised models do not adequately describe the inherently complex structure of porous carbons. An improved model should be sufficiently simple, not only for characterization, but also for the design of separation and purification processes. The following factors should be included: (1) energetic variation along the pore axis due to constrictions or strong energy sites, (2) connectivity between adjacent pores and (3) the presence of functional groups. In this work, we report results from computer simulations, using argon and nitrogen as non–polar or weakly polar adsorbates and water and methanol as examples of associating fluids. The new model is able to produce all hysteresis loops classified by the IUPAC as well as loop types reported earlier by de Boer [1]. Experimental isotherms are commonly measured at 77 K and 87 K (nitrogen and argon boiling points, respectively), but in recent years cryostats have become increasingly available in characterization laboratories and our simulations show that by measuring isotherms at different temperatures new insights about the porous structure can be gained from the variation of the hysteresis loop with respect to temperature which may change, for example, from a single-loop to a double-loop via a fused-loop as the temperature is varied.