NMR studies of adsorption processes upon activated carbon

Abstract

This thesis is concerned with the adsorption behaviour of adsorbate molecules upon activated carbon. Limitations in bulk gravimetric measurements mean that the mechanism of adsorption for adsorbates upon carbonaceous materials is poorly characterised. The information obtained from NMR experiments performed upon the adsorbate molecules is applied to provide additional understanding of the adsorption process. A systematic study of a phosphate/phosphonate - adsorbent system is performed, and a theoretical basis for the results observed is discussed in terms of molecular mobility and the penetration of molecules into pores of differing sizes. Calculated pore volumes obtained from NMR data are discussed. It is demonstrated that phosphate and phosphonate molecules adsorb initially within larger pores at the surface of the adsorbent and subsequently progress deeper within the porous volume. The effects of magic-angle spinning and proton decoupling upon the adsorbate spectra obtained are also addressed. Relaxation measurements are used to study the motional freedom of adsorbed molecules and the measurements provide further corroboration to the interpretation of spectra expounded. The study is then extended to analyse the adsorption of adsorbate molecules upon carbon adsorbents with other adsorbate molecules pre-adsorbed. Analysis of spectra provides information upon the movement of both the primarily adsorbed material and secondary adsorbate within the pore volume. Preferential adsorption of phosphate molecules over water molecules is demonstrated. Finally, dynamic NMR techniques are used to characterise the exchange of adsorbate within the porous volume with excess adsorbate upon the exterior of the carbon adsorbent.