Pore structure characterisation: The challenge to understand heterogeneous catalysts and fuel cells

Abstract

The work in this thesis is focussed on improving the accuracy, precision and range of descriptors that can be obtained from the gas sorption and cryoporometry characterisation techniques.

The conversion of gas adsorption isotherms into pore size distributions generally relies upon the assumption of independent pores. Hence, co-operative effects between pores, which might result in a significantly skewed pore size distribution, are neglected. In this work, co-operative adsorption effects (advanced adsorption) in water adsorption on a mesoporous silica material have been studied using MRI experiments. Evidence for advanced adsorption has been seen directly using transverse relaxation time weighted MR images. The spatial distribution of filled pores has been found to be highly non-random. Moreover, pixels containing the largest pores present in the material have been observed to fill in conjunction with pixels containing much smaller pores.

The theories of adsorption-desorption hysteresis of mesoporous solids have been rigorously tested using the integrated gas-mercury-gas experiment. The experiment has been used to deconvolve the capillary condensation and evaporation processes within a specific subset of pores contained within mesoporous materials. The size of these pores has been obtained independently of gas sorption using mercury porosimetry. This has enabled the meniscus geometry for the capillary condensation process to be determined in these pores. In addition, the Kelvin-Cohan equations and DFT theories of capillary condensation and evaporation have been directly tested. This has been done by comparing the relationship between the adsorption and desorption relative pressures for the pores which entrap mercury. It was found that the Kelvin-Cohan equations and DFT do not correctly predict the width of adsorption-desorption hysteresis in independent pores.

Gas sorption scanning loop experiments on a mesoporous material have been studied. It was observed that hysteresis in scanning loop experiments can be virtually purged which indicates a reversible adsorption and desorption mechanism. Additionally, scanning curve experiments have been studied as part of the integrated gas-mercury-gas experiment. The data for pores which entrapped mercury were deconvolved from that for all other pores and the scanning curves for these pores were determined. It was found that the scanning curves for the pores which entrapped mercury crossed between the boundary curves, which is anticipated for independent pores.

The theories of freeze-thaw hysteresis in cryoporometry of mesoporous solids have been rigorously tested using analogous scanning loop and curve experiments together with concurrently obtained (PFG) NMR diffusometry and relaxometry data. PFG NMR and relaxometry have revealed that the spatial disposition of frozen and molten phases, at a particular point in a loop, depends upon the prior thermal history. The data has also shown that there is an advanced melting process similar to advanced adsorption, and that it must be accounted for in order to adequately interpret cryoporometry data.

An experimental method has been developed that determines the amount of platinum incorporated inside the micropores of a ZSM-5 zeolite support. The method studies a chemisorption experiment before and after a nonane pre-adsorption experiment. The nonane becomes entrapped only in the micropores so the amount of platinum in these pores has been determined.

One of the main findings throughout the work has been that co-operative effects between pores (advanced adsorption and advanced melting) are neglected in conventional gas adsorption and cryoporometry measurements. Hence, pore size distributions derived are highly inaccurate. The work in this thesis illustrates the importance of understanding the underlying physics of these techniques before they are applied to characterise porous materials.

Date of Award	1 May 2011
Original language	English
Awarding Institution	University of Bath
Supervisor	Davide Mattia (Supervisor)