Publisher: Cardiff University

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Publication date(s): 2019 (print) 2019 (online)

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Journal: Doctoral Thesis

Link: http://orca.cf.ac.uk/129474/

URL: http://orca.cf.ac.uk/129474/

The preparation method of a heterogeneous catalyst is one of the most fundamental aspects that can determine its morphology, surface area, phases present, elemental mixing and of course ultimately its catalytic activity. Currently there are a large number of different ways of preparing metal oxide catalysts such as co-precipitation or sol gel but over the last 20-30 years there has been a large number of solvent systems that have been used to develop alternative synthesis techniques such as supercritical solvents, ionic liquids, deep eutectic solvents and switchable solvents. These systems contain interesting properties that are not found in conventional solvent systems which could be utilised to synthesise metal oxide or metal oxide precursors that have unique properties that gives them an advantage over the metal oxide catalysts prepared by conventional methods. The aim of this thesis was to investigate the potential for these novel systems and adapt them for the application of metal oxide catalyst preparation and to see how these techniques compare with more established methods. In order to assess these systems three catalytic reactions were chosen to use as a model to see how metal oxide catalyst prepared by these methods compare with methods such as co-precipitation or supercritical anti-solvent. 1) the use of Co3O4, Mn2O3 and Fe2O3 for the total oxidation of propane to CO2. 2) the use of copper-manganese oxide (hopcalite) for low temperature carbon monoxide oxidation and 3) Cu/ZnO catalyst for methanol synthesis from CO2 and H2. Cobalt oxalate, manganese oxalate and iron oxalate were prepared using choline chloride-oxalic acid based deep eutectic solvent with a water or water-alcohol anti-solvent. It was found the for cobalt and iron precursors a rod shape morphology could be achieved and this morphology was retained after calcination although the precipitated manganese did not form rods. Varying the anti-solvent mixture changed morphology and surface area of the cobalt oxide and iron oxide catalysts. Mixed cobalt manganese oxide and ii iron manganese oxide prepared using deep eutectic solvents were also shown to form rod like morphologies similar to the single cobalt oxide and iron oxide catalysts. These catalysts that were tested for propane total oxidation method and did show some variations in activity between the different preparation methods but there was no significant improvement over the reference catalysts. The use of hydrothermal synthesis to make a crednerite phase CuMnO2 as a precursor to the spinel phase copper-manganese oxide was found to produce spinel copper-manganese oxide catalysts with properties that differed from co-precipitated equivalents. These catalysts demonstrated lower deactivation during the first 30 minutes of CO oxidation despite having generally having lower a surface area, although these catalysts showed deactivation after temperature ramp to 50 °C. Characterisation on the crednerite derived spinel showed that they differed from the regular co-precipitated hopcalite with XPS showing a higher Cu+:Cu2+ at lower temperature heat treatment which may indicate greater Cu-Mn integration. The use of a switchable solvent system was demonstrated for the preparation of carbonate precursors to copper manganese oxides CO oxidation catalysts which were shown to have high surface areas and excellent CO conversion comparable to copper-manganese oxide catalysts prepared by supercritical anti-solvent methods, presenting a less energy intensive method of making metal oxide catalysts to supercritical anti-solvent precipitation. The use of choline chloride-urea deep eutectic solvents to prepare copper-zinc oxide methanol synthesis catalysts was shown to be an ineffective method, with the MP-AES showing loss of zinc at higher copper loadings and XPS showing large amounts of surface chlorine present after calcination resulting in inactive catalysts. An initial study using switchable solvents to prepare Cu/ZnO catalysts was shown to produce catalysts that were active for methanol synthesis and presents a promising potential for future development.

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