METHANE ADSORPTION ON MICROPOROUS CARBONS - A COMPARISON OF EXPERIMENT, THEORY, AND SIMULATION

The adsorption of methane and nitrogen on a KOH-activated carbon, AX21, has been studied and the resulting data analysed using both conventional and molecular techniques. Molecular simulation clearly shows that the large methane-adsorption capacity of AX21 is due to an enhanced heat of adsorption brought about by the overlap of the wall forces from opposing pore walls in the microstructure. It is demonstrated that the methane-adsorption characteristics can only be rationalised using a nitrogen pore-size distribution derived using SNAP, an analysis method based on molecular methods. Conventional methods either severely underestimate (volume filling) or overestimate (capillary condensation-Kelvin equation) the mean pore size and also fail to reveal the bi-modal nature of the pore distribution. The molecular simulation approach has also demonstrated that the effective density of the adsorbed nitrogen deviates substantially from that of liquid nitrogen, particularly in small pores. A closer approximation to the true pore volume for microporous carbons may be obtained using a value of 0.93 g/ml rather than 0.808 g/ml for the density of liquid nitrogen.