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Use of Gd2O3-CeO2 mixed oxides, with specific nanoformations, as supports of mono- and bimetallic Ir-Ni Catalysts on the Dry Reforming of Biogas

Chatziprodromou Vasiliki

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Summary

The aim of this diploma thesis is to study the catalytic behavior of mono - and bi - metallic Iridium-Nickel (Ir-Ni) catalysts, which were supported on a mixed oxide carrier prepared by two methods, the hydrothermal method and the precipitation method, during the process of dry reforming of biogas reaction. The dry reforming of biogas process refers to the reaction of methane (CH4) and carbon dioxide (CO2), producing hydrogen (H2) and carbon monoxide (CO). Carbon dioxide and methane are two key gases associated with the greenhouse effect, which is a major topic of debate in modern society, as its consequences can be serious and harmful in the years to come. At the same time, CH4 and CO2 are part of the main components of natural gas and biogas, which are relatively inexpensive raw materials for the production of synthesis gas (syngas), which can be used to synthesize Oxygenated Chemicals and hydrocarbons through the Fischer-Tropsch process. Synthesis gas (H2/CO) is also used in the chemical industry to produce main chemicals and as a fuel in many applications. H2 has been identified as the fuel of the future for electricity generation, making this process very important for further investigation.This reaction, however, addresses a major problem, the gradual inactivation of catalysts. This is often due to carbon deposition and in this case catalysts are needed that oppose this phenomenon. The nickel catalyst, while having a fairly low cost, is found in abundance and is quite active, is not characterized by good stability, because coke formation and particle aggregation takes place. On the contrary, while Iridium, like other noble metals, resists such phenomena better, its high cost prevents it from being a frequent choice. For this reason, the use of bimetallic catalysts for this reaction is preferred and they are studied more extensively, as the co-operative action of the two metals reduces the tendency to inactivation.More specifically, an isomolecular composition of gas reactants was selected (CH4:CO2=50%: 50%) and monometallic catalysts, Ir/GDC-Pr, Ni/GDC-Pr, Ir/GDC-NR and Ni/GDC-NR, with a recommendation of 2% w.t Ir and 10%w.t Ni, and the bimetallic Ir-Ni/GDC-Pr and Ir/Ni-GDC-NR with a recommendation of 2%w.t Ir-10%w.t Ni. The catalysts were prepared using the liquid impregnation method and characterized for their physicochemical properties and crystallinity using the BET method and X-ray diffraction (XRD). The GDC-Pr mixed oxide carrier was prepared by the condensing method, while the GDC-NR mixed oxide carrier was prepared by the hydrothermal method, in which nanorods are formed, to prevent particle formation. As has been shown, vectors play an important role in enhancing catalytic behaviour, as well as reducing the chances of carbon deposition during dry biogas reformulation processes. First, they started the activity experiments, ranging from 350oC-750oC followed by the stability experiments, at a constant temperature of 750oC, for 30 hours to study the performance of the catalysts.The results confirm the importance of using bimetallic catalysts in this process, as they noted the highest conversion rates of methane and carbon dioxide reactants, as well as the efficiency of hydrogen and carbon dioxide during warming. More specifically, the bimetallic catalyst Ir-Ni/GDC-NR proved to be the most efficient, as the temperature increased to 750°C and its activity improved and always held the highest position in conversion rates. Then, in the stability experiments, although the nickel catalyst had higher gas conversion rates compared to the noble metal iridium catalyst, the iridium catalyst showed a minimal increase in the conversion of the reactants CH4 and CO2, while the efficiency of the nickel catalyst decreased, which indicates the occurrence of the phenomenon of carbon deposition. As observed, both IR-Ni/GDC-Pr and IR/Ni/GDC-NR bimetallic catalysts had higher conversion and efficiency rates than the monometallic catalysts examined, but the bimetallic catalyst supported on a carrier prepared by hydrothermal method proved to be the most active, stable and efficient of the other catalysts examined, which raises interest for further research and study for application to other reactions as well.

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