Characterization Of The BET Surface Area, Pore Size Distribution And Intraparticle Diffusivity Of Porous Materials

The specific surface area, pore size distribution (PSD) and intraparticle diffusivity are important properties of porous materials. The accurate and facile characterization of these properties is important for the evaluation of porous materials for potential applications, e.g. as solid catalysts. Single wall carbon nanotubes (SWNT) have many potential applications and the characterization of the purity, diameter distribution and open-ended fraction of a SWNT sample by an easily available method, e.g. gas adsorption, is also important.An adsorption manometry apparatus was developed that can overcome some limitations of currently available apparatuses based on the use of a new method to measure the amount of gas dosed and the optimization of the apparatus. The apparatus is capable of high resolution measurement of adsorption isotherms. It is also capable of extremely low pressure adsorption isotherm measurements. The apparatus improves the measurement precision by using a differential pressure method and high precision differential pressure sensors, and using extremely small dosing rates by using a leak valve. It can be used for measuring adsorption isotherms by both intermittent dosing and continuous dosing. Experimental and theoretical error analyses were performed that indicate that the new apparatus is suitable for the accurate characterization of microporous materials.Density functional theory (DFT) was used to simulate N2 adsorption in SWNTs, which used the Lennard-Jones potential for the interactions between N2 molecules and between N2 and carbon atoms. The DFT calculation of the excess Helmholtz free energy used the modified fundamental measurement theory of Yu and Wu and the Weeks-Chandler-Anderson potential model. The calculations studied the interaction energy between N2 and the SWNT wall, density distribution of N2 in the SWNT and the characteristics of the adsorption isotherms of SWNT with different diameters.DFT simulation of N2 adsorption in SWNTs and adsorption isotherm measurements were suggested for the characterization of the purity, diameter distribution and open-ended fraction of SWNTs. The effect of acid and heat treatments on SWNT samples was discussed. It was found that the PSD calculated by DFT simulation was not unique, that is, many different PSDs can give the same adsorption isotherm.A temporal-analysis-of-products (TAP) reactor was investigated for making porous material diffusivity measurement. The mathematical model of the TAP reactor, the measurement of the diffusivities of packed powder beds and intraparticle diffusivities, and the sensitivity analysis of intraparticle diffusivity measurements were discussed. Some problems in the experiments and simulations used for intraparticle diffusivities measurement were discussed, including the non-agreement of the temperature dependence of the bed diffusivities with that predicted by Knudsen flow and the inaccuracy arising from the existence of a wide particle size distribution caused by the grinding of solid particles. At last, the differences between the differential and integral TAP mathematical model were also analyzed and discussed.