Heat treatment-induced structural changes in SiC-derived carbons and their impact on gas storage potential

We investigate the effect of heat treatment on the structure of carbide-derived carbons (CDC) prepared by chlorination from nanosized beta SiC particles and on their methane as well as hydrogen storage and delivery performance. Pore size and pore wall thickness distributions of the CDCs are obtained from interpretation of argon adsorption data using the finite wall thickness (FWT) model. The adequacy of the FWT model for adsorption modeling in the SiC-CDC samples is demonstrated by satisfactory prediction of subatmospheric and high pressure adsorption isotherms of CO(2) and CH(4) at 313 and 333 K. From the characterization results, it is observed that the SiC-CDC particles are predominantly amorphous with slight graphitization of the external surface. The degree of graphitization is more pronounced in the sample prepared at 1000 degrees C and increases slowly with heat treatment time. During this time the accessibility of methane molecules is found to increase, as a result of short-range ordering and opening up of pore entrances. Nevertheless, methane storage capacity is unsatisfactory, despite the high surface area and porosity, clue to accessibility problems. On the other hand improvement in high pressure H(2) uptake (4.61 wt % at 77 K) is obtained for SiC-CDC chlorinated at 800 degrees C and heat treated for one day. The recently predicted optimal delivery temperature of 115 K for hydrogen storage is found to be appropriate for this material. It is demonstrated that accessibility is an important issue to be addressed for methane storage in carbons, but which has hitherto not received attention for this application

Kinetic restriction of simple gases in porous carbons: Transition-state theory study

The separation of simple gases such as N-2, Ar, CO2, and CH4 is an industrially important problem, particularly for the mitigation of greenhouse emissions. Furthermore, these gases are widely accepted as standard probing gases for the characterization of the microstructure of porous solids. However, a consistent set of microstructural parameters of a microporous solid deter-mined from the use of adsorption measurements of these different gases is not always achieved because of differences in their pore accessibility. This is a long-standing and poorly understood problem. Here, we present the calculated results of the crossing time of N-2, Ar, CO2, and CH4 between two neighboring cages through a constricted window in a realistic structural model of saccharose char, generated from hybrid reverse Monte Carlo (HRMC) simulation (Nguyen, T. X.; Bhatia, S. K.; Jain, S. K.; Gubbins, K. E. Mol. Simul. 2006,32,567-577) using transition state theory (TST), as described in our recent work (Nguyen, T. X.; Bhatia, S. K. J. Phys. Chem. 2007, 111, 2212-2222). The striking feature in these results is that whereas very fast diffusion of carbon dioxide within the temperature range of 273-343 K, with crossing time on the molecular dynamics scale (10(-4)-10(-6) S), leads to instantaneous equilibrium and no hysteresis on the experimental time scale, slower diffusion of Ar and N-2 at the low temperature of analysis indicates an accessibility problem. These results rationalize the experimental results of hysteresis for N-2 at 77 K and At at 87 K but not for CO2 at 273 K in Takeda 3 angstrom carbon molecular sieves. Furthermore, it is shown that CH4 diffusion through narrow pore mouths can be hindered even at ambient temperature. Finally, we show that the use of pore size and wall thickness distributions extracted from the adsorption of At at 87 K using the finite wall thickness (FWT) model (Nguyen, T. X.; Bhatia, S. K. Langmuir 2004,20,3532-3535 and Nguyen, T. X.; Bhatia, S. K. J. Phys. Chem. B 2004, 108, 14032-14042) provides the correct prediction of experimental CO2 adsorption in BPL and PCB carbons whereas that from N-2 at 77 K gives a significant underprediction for both CO2 and CH4 in the BPL carbon. These trends are in excellent agreement with those predicted using the calculated crossing times