Simulation study on the relationship between a high resolution αs-plot and the pore size distribution for activated carbon Original Research Article

The effects of the micropore structure of activated carbons on the high resolution αs-plot for nitrogen adsorption isotherms were examined with the grand canonical Monte Carlo simulation. Adsorption isotherms of nitrogen were simulated in graphitic slit pores at 77 K as a function of the slit width (w). As no pore effect was observed below P/P0=0.6 for w=3.5 nm, αs-plots for the simulated adsorption isotherms were constructed using the standard isotherm simulated for w=3.5 nm. The simulated αs-plots had filling and cooperative swings which were experimentally shown in the previous works, and the shape of the simulated αs-plot varied with the micropore structure. As the subtracting pore effect (SPE) method for the specific surface area (SSA) determination using the αs-plot was proposed in the previous experimental works, the theoretical ground for the SPE method was discussed. The best evaluation method of SSA using the αs-plot was shown, almost agreeing with the SPE method. This simulation study showed clearly that the SPE method is available for pore systems of w≥0.7 nm, whereas even the SPE method underestimates the SSA of the pores of w≤0.6 nm. The observed swings of the αs-plot were simulated using the different micropore size distribution. The bimodal micropore size distribution lead to both of filling and cooperative swings, while a single pore size distribution <0.9 nm gave only the filling swing. Thus, four representative types of the αs-plot for activated carbons were proposed and it was shown how to understand the micropore size distribution through the αs-plot.