Yu, Haiyue; (2021) Development of a novel ultra-high vacuum diffusion apparatus for investigating Knudsen diffusion in complex pore channels. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Disordered porous materials with rough surfaces are widely used in several industrial applications, such as fuel cells, heterogeneous catalysis, molecular separation, and oil and gas recovery. Many of these processes are diffusion-controlled, due to narrow pore size. In porous catalysts, molecules diffuse through the pore network and react on the active sites of the pore walls. This indicates that the shape and surface morphology of the pores may affect catalytic performance. Knudsen diffusion plays a crucial role in catalysts where gaseous molecular transport is dominated by molecule–wall collisions. Consequently, the pore geometry of disordered porous materials, including the pore shapes and pore wall roughness, has attracted significant research interest. However, the high cost and limited availability of techniques for building a system that can simulate the Knudsen regime in complex pores with rough inner surface representative of porous catalysts have hindered the practical investigation of the effect of the pore morphology on Knudsen diffusion. In this study, to address these limitations and accurately measure the Knudsen diffusivity in complex pore channels, a unique vacuum diffusion apparatus was designed and employed. The inherent characteristics of the novel apparatus enabled the system to rapidly simulate the Knudsen regime, emulating the complexity of actual porous media, and to accurately measure the relevant data for calculations. In addition, this high-vacuum diffusion apparatus was used to investigate Knudsen diffusion in complex pore channels with various lengths, pore shapes, and surface roughness. The measurement of the Knudsen diffusivity in the channels with various lengths and shapes was guided by computer simulations and theoretical calculations to validate the functioning and the accuracy of the apparatus. Three fractal surface channels (with N= 1, 2, and 3 generations) were fabricated using a three-dimensional printer via selective laser sintering, and the effect of the surface roughness of the fabricated channels on the Knudsen diffusion can be analysed in the unique apparatus. The developed ultra-high vacuum diffusion apparatus enables us to experimentally probe Knudsen diffusion in complex geometries more directly than has ever been achieved.

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