Adsorption analysis of carbon dioxide, hydrocarbons and moisture on carbon

Abstract

This thesis presents a fundamental investigation on the role of the structure of microporous silicon carbide-derived carbon (SiCDC) and its functionalisation in the adsorption equilibria and transport of gases. The SiCDCs with different particle size distributions were synthesized in our laboratory and characterized using a combination of techniques including scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), helium pycnometry, thermogravimetric analyses (TGA), nuclear magnetic resonance (NMR), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FTIR), Raman spectroscopy, and gas adsorption. Based on the characterisation results, a bidisperse pore structure model is proposed for the synthesized SiCDC to explain the kinetics. In the mathematical modelling of adsorption kinetics, the internal structure of SiCDC is assumed to have ultra-microporous grains with larger particle scale micropores forming the intergrain pathways. It is shown that CH4 adsorption kinetics is governed by two distinct diffusional resistances, arising from slow grain scale diffusion in ultra-micropores and faster particle scale diffusion in large micropores. For CO2, it is found that the adsorption kinetics is strongly influenced by a barrier resistance at the grain surface where entry into the ultra-microporosity occurs. The uptake of CO2 in the bidisperse pore structure of CDCs occurs through rapid diffusion in the large particle-scale micropores, in which a Henry law isotherm holds, and a combination of barrier resistance at the grain surface and diffusional resistance in the grain interior with a Langmuirian isotherm. It is shown that the grain scale activation energies are comparable with values for carbon molecular sieves, and consistent with values expected for the size range of the ultra-micropores, while the activation energies for transport in the larger particle scale micropores are comparable to those for conventional activated carbons. The experimental uptake-based data are compared with self-diffusivities obtained through equilibrium molecular dynamics (EMD) simulations using a realistic model of SiCDC developed by the hybrid reverse Monte Carlo (HRMC) method. It is observed that MD-based CO2 diffusivities are as much as two orders of magnitude larger than the CO2 particle scale diffusion coefficients, however such discrepancy is not found for CH4. The quasi elastic neutron scattering (QENS) measurements for CH4 suggests a smaller activation energy barrier compared to the results obtained from MD simulations and experimental kinetic uptake. The difference between the activation energies obtained from the experimental uptake measurements and MD simulations suggests that there are some internal barriers and structural constrictions which are not captured by QENS measurement and MD simulation. This thesis also presents an investigation on the effect of fluorine doping of microporous CDC on its structural as well as hydrophobic/hydrophilic character. The morphology and structure of samples fluorinated to three different F/C ratios are characterized by several analysis techniques and gas adsorption. It is shown that stronger C-F bonds are formed at high levels of fluorination and increasing fluorination level leads to a decrease of specific surface area and total pore volume. It is demonstrated that fluorination has little effect on the ultra-microporosity at low levels of fluorination, but leads to significant decrease at high levels of fluorination. The comparison of the CO2 uptake-time curves for the fluorinated and non-fluorinated samples shows slightly slower uptake with increasing fluorination level, largely due to decrease in pore volume and surface area. By the application of different semi-empirical models to the experimental water adsorption isotherms of fluorinated and non-fluorinated CDCs, the effects of fluorine-doping on the adsorption mechanism is also analysed. The comparison of different model parameters with characterization results of the samples is used as the methodology to understand the water adsorption mechanism in virgin and fluorinated CDCs. It is demonstrated that with increasing the level of fluorination the hydrophobic character of the low and medium fluorinated CDCs remains almost constant while high level of fluorination decreases the hydrophobicity. It is shown that fluorine doping causes blockage of the carbon pores for the nonpolar argon molecules while allowing polar H2O molecules to grow into clusters and migrate into the internal volume of the micropores. The findings of this thesis should aid better understanding of the gas adsorption and diffusion mechanism in microporous CDCs.