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Catalytic exhaust gas reforming of hydrocarbon fuels

Dixon, Timothy 2018. Catalytic exhaust gas reforming of hydrocarbon fuels. PhD Thesis, Cardiff University.
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Exhaust gas reforming is a technology that has the potential to serve as an intermediate technology between the current fossil fuel vehicle fleet, and future hydrogen-fueled and battery-powered systems, analogous to the role played by hybrid electric vehicles. The technology involves reacting a portion of the primary fuel with the exhaust gases of an internal combustion engine, in the presence of a catalyst, yielding a reformed exhaust gas containing hydrogen that can be recirculated back into the engine of the vehicle. If operation is optimized, such a system can operate as an energy recover system, therefore improving miles to the gallon, while also improving engine emissions. The barrier to entry for this technology is the availability of attractively priced, active and durable catalytic materials. The common catalytic candidates for exhaust gas reforming are precious metal based formulations, but their cost is often prohibitive if the loading of the precious metal is high. Base metal formulations have more competitive pricing, however can present issues with activity and durability. This work was designed as a compare and contrast study of precious metal and base metal catalysts in the exhaust gas reforming of gaseous and liquid hydrocarbon fuels, under conditions typical of those found in the exhaust of a spark ignition internal combustion engine. The key aim was to determine whether base metal catalysts could show suitable catalytic activity and durability during exhaust gas reforming, both in absolute terms, but also relative to a commercial precious metal reforming catalyst, and hence whether such a formulation had the potential for use in a real vehicle. A 10% Ni/CeO2-ZrO2 catalyst formulation was prepared via wet impregnation, as the first phase base metal formulation. Its performance was compared to a commercial precious metal based reforming catalyst, and a 1% Rh/CeO2-ZrO2 formulation, under a range of realistic exhaust conditions expected from spark ignition internal combustion engines. The testing was carried out using a reactor set up that required modification in order to be able to test with liquid hydrocarbon fuels. The surrogate fuels for gasoline used in this work were propane and iso-octane. The key factors for consideration were activity, in terms of quantity of hydrogen in the reformed gas, and resistance to deactivation. Catalysts were characterized pre and post-testing using TPR, BET, TGA and XRD techniques. The results presented here show that the activity, in terms of hydrogen production, of the nickel based catalyst was comparable to that of the precious metal catalysts with both gaseous and liquid fuels, and produced hydrogen in quantities sufficient to be of benefit when fed back into an internal combustion engine. However, significant issues remained with the nickel catalyst with respect to its stability and durability. The rate of coking with the heavier feedstock was a significant issue. Additionally, the nickel catalyst showed sensitivity to the atmosphere it was exposed to during the post-operation cooling phase of testing. Exposing the hot catalyst to an oxidizing atmosphere during this phase was found to be severely deactivating. This is important because this would be the situation expected in a real vehicle once the engine has been switched off and the exhaust is exposed to air while still hot. This was assigned to a thermal sintering effect, initiated by the oxidation of surface carbon deposited under reaction conditions, hence highlighting that the thermal stability of the catalyst was also inadequate. The rate of coking with the heavier feedstock was found to be of lesser problem with the precious metal catalysts, and the sensitivity to the post-operation cooling conditions was not observed. Three avenues of further work were therefore suggested, with particular focus on optimizing the nickel catalyst in an attempt to overcome the aforementioned durability issues, since as this work makes clear, these are the key roadblocks that need to be overcome for an exhaust gas reforming system to be competitive for use in real vehicles.

Item Type: Thesis (PhD)
Date Type: Submission
Status: Unpublished
Schools: Chemistry
Subjects: Q Science > QD Chemistry
Date of First Compliant Deposit: 30 May 2019
Last Modified: 04 Aug 2022 02:00

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