Characterization of amorphous porous carbons based upon structural enhancement of sorption potentials

Analysis of gas sorption isotherms may yield information about structural heterogeneity that can be used to control selectivity of separations processes, catalytic reactions, and gas storage. Disordered porous materials may be analyzed via simulation of sorption isotherms using methods that rely on simple geometric structures with a single source of heterogeneity to render the problem more tractable. Molecular simulations (MS) and density functional theory (DFT) are considered exact for a given simple pore structure and chemical makeup. DFT model isotherms have been used to analyze experimental isotherms for several amorphous porous carbons at 77K for both argon and nitrogen. The pore size distributions (PSD) of the two gases generally agreed within 8% for both the total volume and mode of the distribution. Although both probe gases yielded similar results the argon is recommended for use as the probe gas of choice, as it is smaller than nitrogen, is a monatomic molecule, and has no permanent multipole moment. Elemental analysis of the carbons indicates that the two probe gases are in better agreement when the carbons do not contain functional groups.; Sorption analysis is very dependant upon the model parameters used to describe the potential functions. The potential parameters may be determined from bulk physical characteristics of a molecule such as the polarizability and the magnetic susceptibility, or may be fitted to isotherm data. The monolayer filling pressure of carbon pores larger than 10 molecular diameters approaches the maximum therefore choosing the parameters to match the monolayer filling pressure of a large slit pore with that of a nonporous carbon is an excellent way to fit the potential parameters. An alternate means to accomplish the same goal is to fit an isotherm to a known distribution of pore sizes. Transmission electron microscope pictures may be taken of single-walled carbon nanotubes to determine their PSD, which can be used to determine the potential parameters. The technique has be verified through comparison PSD of isotherms generated with the use of nonporous carbon fitted parameters with the TEM data resulting in an error of less than 3%.; Similar to the DFT and MS methods the Horvath-Kawazoe (HK) method relies on the superposition of the adsorbent-adsorbate potential to enhance adsorption in micropores, and may be used to determine isotherms for a pore of a given size and structure. The HK method with a generalized potential function requires less than one hundredth of the time required to generate a series of isotherms using either the DFT or MS methods, while remaining remarkably accurate for its simplicity. Both MS and DFT predict pore wall wetting prior to condensation in the pore, whereas the original HK method predicted that the pores are either completely empty or completely full. The HK method was modified to approximate pore wall wetting at pressures lower than the filling pressure, yielding closer agreement of predicted pore size as compared to DFF. The addition of the monolayer filling in the HK method has the advantage of rendering it possible to fit the potential parameters of the system to a nonporous reference, in addition to improved accuracy for the mesopore range.