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Development of new catalysts for methane oxidation in dual-fuel HGV engines

Kamieniak-Rodziewicz, Joanna (2017) Development of new catalysts for methane oxidation in dual-fuel HGV engines. Doctoral thesis (PhD), Manchester Metropolitan University.

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Available under License Creative Commons Attribution Non-commercial No Derivatives.

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Abstract

1.4 billion tonnes of cargo is transported in the UK every year, using 1,445,000 heavy goods vehicles (HGVs) over a collective distance of 19 billion km and these figures are likely to increase in the future. For example, the number of trucks is expected to rise by 75% by 2040, and the demand for transport fuels will also grow rapidly. In view of that, natural gas has become subject to big investments for new businesses lines, such as dual-fuel engines. This type of engine typically utilises diesel as a primary fuel with the substitutions of natural gas in order to reduce running costs, as well as for environmental benefits. However, the main downside of the utilisation of natural gas is that it has a higher combustion enthalpy per unit mass, when compared to other conventional fuels. Also, it is not fully burned in the engine, thus results in increased methane emissions in the exhaust. The aim of this project is, therefore, to develop new catalysts to manage emissions of methane to meet the requirements established by the Euro VI regulations. This thesis reports the synthesis and full characterisation of hydroxyapatite (HAP) as the support for a range of catalysts, using several methods and templates to improve its porosity. Moreover, carbon nanorods were employed for the first time as a hard-template in the synthesis of HAP, obtaining high BET surface areas that corresponded to 242.2± 2.3 m2g-1. Then, the thermal, chemical and mechanical stability of HAP was investigated by reproducing possible environmental conditions, which the catalyst would be exposed to in real exhausts from HGV engines. The main findings were that mesoporous HAP is fully stable to any change of pH and any mechanical disturbance, and only started to dehydroxylate at temperatures above 650ᵒC, which is, nonetheless, higher than the engine operating temperature. In consequence, HAP was confirmed as an extremely powerful catalyst support. Additionally, new methods for doping HAP with Pd and Ni metals were explored in order to improve the metal distribution on the support and, hence its catalytic activity. Ultrasound was utilised to assist conventional ion exchange (IE) and incipient wetness impregnation (IW) methodologies. The results for IW revealed that the ultrasound breaks down metal clusters and subsequently improves their distribution, when compared to the standard IW protocol, and in the case of IE, even though the distribution remains stable, the utilisation of ultrasound significantly accelerated the process from 3 days to 3 hours. Furthermore, pretreatment of HAP with different pH before doping with Pd using the IW protocol considerably enhanced the metal distribution when compared to the conventional IW procedure, and remained the high metal distribution when IE took place in a different pH buffer solution instead of neutral water. All synthesised and characterised samples were tested towards dry reforming of methane (DRM) and oxidation of methane, reproducing the oxygen lean and rich conditions found in an exhaust of a HGV, respectively. Products of the reactions were analysed using an in-house built catalysis rig equipped with GC-TCD. For DRM, the most active catalyst impressively exceeded the commercially available catalyst tested under same conditions; converting 100% at a temperature of 250ᵒC, and still achieving 80% conversion after 88 hours continuous reaction. On the other hand, it was found that the oxidation of methane in the presence of oxygen species proceeds through a redox cycle between reduced metal and metal oxide. Based upon the catalytic profiles of previously synthesised catalysts, the metal oxide was more active and revealed more stable conversions when compared to the reduced metal. The results obtained, therefore, suggest that the adsorbed lattice oxygen plays a key role in the catalysis reaction. Lastly, coking process was also studied via TGA as a preliminary deactivation process of the catalysts. It was found that all Pd and Ni based catalysts were resistant to the formation of carbon on their surface.

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