A Novel Photocatalytic Plasma Reactor for Reforming of Waste Gases

Capp, S. C. (2021) A Novel Photocatalytic Plasma Reactor for Reforming of Waste Gases. Doctoral thesis (PhD), Manchester Metropolitan University.

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Abstract

Non-thermal plasma (NTP) is an area of research that is receiving ever more attention. The unique properties of NTP make it useful in many applications. One such application is combining NTP with catalysts known as plasma catalysis. Plasma catalysis can allow chemical reactions to occur at temperatures otherwise insufficient when using catalysts alone. A specific type of catalyst was employed termed photocatalysts. This is due to their unique property by which they can be activated by ultraviolet light or even visible light, both of which are naturally produced by plasma. This has the ability to further enhance the synergistic effect of NTP and catalyst. Therefore, investigation into the effects of combining photocatalysts and NTP plasma was studied. The research focused on using a physical deposition technique, magnetron sputtering, to introduce the photocatalysts in a novel way, into the NTP reactor for plasma catalysis. Specifically, the photocatalytic coatings were produced on dielectric particulates which are used within the NTP reactor. Titanium dioxide based photocatalytic coatings, produced by magnetron sputtering onto particulates, were characterised by various characterisation techniques such as Raman spectroscopy, scanning electron microscopy and X-ray photoelectron microscopy and subsequently tested for photocatalytic activity through methylene blue degradation, which is a common method to determine photocatalytic activity, and showed an increase in degradation of methylene blue by up to 12.5 times. In order to understand the mechanisms involved between photocatalytic coatings and NTP, their effect on the reforming of oxygen and nitrogen gas mixtures and the dry reforming of methane was investigated. The presence of titanium dioxide was observed to affect the plasma chemistry in the NTP by acting as a sink for atomic oxygen, through photocatalytic formation of superoxide anion radical (O2-), and alteration of the dielectric constant of the dielectric particulates. The titanium dioxide coatings had a significant effect on the oxygen and nitrogen plasma chemistry, it was measured that the coatings enhanced ozone production by up to 80 times for a residence time of 0.011 s and up to 10 times for 0.0037 s compared to without any coatings and significantly decreased the formation of harmful nitrogen dioxide (NO2) and nitrous oxide (N2O) at a residence time of 0.011 s. In the dry reforming of methane experiments, it was found that the photocatalytic coatings enhanced the conversion of methane by 1.6 times and enhanced the conversion of CO2 by 1.35 times. However, it was also observed that some photocatalysts, such as the tungsten doped titanium dioxide coatings can reduce the performance by effecting the plasma discharge. The finding of this study shows that with careful understanding of the plasma-photocatalyst surface interaction, chemical processes could be tailored towards improving the production of desired product and limiting the formation of undesired by-products. Moreover, the photocatalysts produced by magnetron sputtering can enhance the surface interaction between plasma and photocatalysts with minimal disturbance to gas flow.