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Hydrothermally produced mono- and bi- metallic Ir-Ni/CeO2-NR catalysts in the dry reforming of methane reaction (DRM)

Lyra Markella-Zizel

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Summary

The subject of the present diploma thesis examines the catalytic behavior of the mono- and bi- metallic Iridium-Nickel (Ir-Ni) catalysts supported on the Ceria (CeO2) carrier during the dry reforming of methane reaction. This process involves the conversion of carbon dioxide (CO2) and methane (CH4) into synthesis gas, a mixture of hydrogen (H2) and carbon monoxide (CO). Indeed, the reaction has gained much attention in the environmental and industrial sector as it exploits two main greenhouse gases (CH4 and CO2) which have a significant impact on global warming and climate change. Moreover, both greenhouse gases can be used as feedstock to produce synthesis gas (H2/CO) and its subsequent use for the synthesis of oxygenated chemicals and hydrocarbons by means of Fischer–Tropsch process. Also, CH4 and CO2 constitute the main components of biogas, hence a route for its direct utilization can be provided. A major drawback of the reaction is the gradual deactivation of the catalysts due to carbon deposition. For this reason, it is important to develop catalysts that resist this phenomenon. Nickel catalysts are the most studied ones as they are abundant and highly active. However, these catalysts suffer from poor stability due to coke formation and sintering. Precious metals such as Iridium are less prone to these phenomena, but their high cost is an obstacle for becoming an attractive option. Therefore, in order to exploit the advantages of precious, as well as non-precious metals, bimetallic catalysts have been extensively explored for DRM, because of their ability to mitigate deactivation due to the synergistic effects of the two active metal phases. The examined monometallic catalysts Ir/CeO2-NR and Ni/CeO2-NR have a composition of 2% w.t Ir and 10% w.t Ni respectively, whereas the bimetallic one has 2% w.t Ir-10% w.t Ni/CeO2-NR. These catalysts are produced by the wet impregnation method, while the CeO2 carrier is produced by the hydrothermal method, which favors the form of nanorods. In general, the role of the carriers or supports, especially during the dry reforming of methane process is critical since they enhance the catalytic activity and mitigate carbon deposition on the surface of the catalysts. More specifically, the catalytic performance is examined first through activity experiments in the temperature range of 350-750℃ and afterwards through 30 hours time-on-stream stability experiments under a constant temperature of 750℃. The present experimental procedure operates under specific reaction conditions in which the reactants simulate an equimolar constitution of biogas (CO2:CH4=50%:50%). From the results obtained, it is found that the examined catalysts present high rates of CH4, CO2 conversions and H2, CO yields of the process as the temperature increases. In particular, the Ir-Ni bimetallic catalyst is the most efficient one as it is activated at lower temperatures and its activity improves as the temperature approaches to 750℃. In the following stability experiments, the performance of the monometallic Ni catalyst gradually decreases, implying the tendency of the catalyst to form carbon. In contrast, the noble metal Ir catalyst shows a slight increase of CH4, CO2 conversions over time, showing no sensitivity to carbon deposition. However, the Ni catalyst shows higher conversion values compared to the Ir catalyst. As for the Ir-Ni catalyst, it exhibits excellent activity throughout the continuous operation of the reaction with particularly high rates of conversions and yields of the aforementioned gases. Therefore, it shows greater stability than the monometallic catalysts, qualifying it as a promising option in terms of activity and resistance to carbon formation, especially when dispersed on Ceria (CeO2) containing substrates.

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