Adsorption equilibrium of methane on layered graphene sheets and activated carbon

In order to develop a new kind of adsorbent for the storage of natural gas by adsorption, the activated carbon SAC-01 and the layered graphene GS(3D), which have a specific surface area about 2062 m²/g and 1507 m²/g, respectively, were comparatively studied as per adsorption equilibrium data measured at the temperature of 283.15-303.15 K and the pressure of 0-10 MPa. The pore size distribution (PSD) and BET specific surface area of the GS(3D) and the activated carbon were firstly determined by analyzing adsorption isotherms of nitrogen at 77.15 K through Horvath-Kawazoe equation calculation. The Henry law constant was used to calculate the limit isosteric heat of methane adsorption in correspondence with the low surface coverage, the interaction potentials between methane molecule and the surface of two adsorbents were then plotted by employing Virial equation and 10-4-3 potential function. The adsorption data, which were volumetrically measured under high pressures, were correlatively fitted by Langmuirian equations through nonlinear regression. Toth equation with the highest accuracy in predicting adsorption data was then used to calculate the absolute adsorption amount, which was finally employed to calculate the isosteric heat of adsorption via Clausius-Clapeyron equation. The result shows that the limit isosteric heat of methane adsorption on the GS(3D) and the activated carbon is about 23.07 and 20.67 kJ/mol, respectively, and the corresponding interaction potential ε_sf/k between methane molecule and the GS(3D) or the activated carbon is about 67.19 and 64.23 K at temperature 283.15 K, respectively, which are similar to 64.60 K determined by Lorenz-Berthelot mixing law. The accumulated relative error of the Toth equation for predicting the adsorption equilibrium of methane on the activated carbon and GS(3D) is 0.25% and 2.29%, respectively, and the mean isosteric heat of methane adsorption on the GS(3D) and the activated carbon is about 18.3 and 16.8 kJ/mol, respectively. It suggests that the GS (3D) with a larger specific surface area and micro-pore volume takes more advantages in methane adsorption in comparing with the activated carbon.