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Title: The application of superacidic materials for the oxidation of methane
Author: Brown, Adrian St Clair
ISNI:       0000 0001 3483 7990
Awarding Body: Nottingham Trent University
Current Institution: Nottingham Trent University
Date of Award: 2000
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In recent years so called "superacidic" metal oxides have attracted a lot of attention, primarily because of their ability to catalyse hydrocarbon isomerisation reactions at low temperatures. In this study, two particular classes of "superacidic" metal oxides - sulfated haematite and molybdated zirconia - have been examined for their efficacy in methane oxidation. To date, there have been few studies, which have aimed at investigating the potential of this class of oxides for hydrocarbon oxidation reactions, which is surprising in view of their well known ability to activate paraffins. Sulfation has been employed for iron oxide catalysts prepared from a number of oxyhydroxide and hydroxide precursors. Although, as has been shown, iron oxide catalysts prepared by different routes exhibit different reaction pathways during methane oxidation, sulfation has been found to produce a number of general effects such as increased surface area, suppression of low temperature total oxidation and the enhancement of oxidation activity at higher temperatures producing low levels of selective partial oxidation products. TEM examination demonstrates that the morphology of the different iron oxide samples are related to that of their precursors and is little effected by sulfation, except in the case of iron oxide samples prepared from a goethite precursor where the influence of sulfation on porosity is evidenced. Powder x-ray diffraction indicates that all the materials are in the haematite polymorph following calcination and that the sulfation again produces little effect, except for modifying the particle size and crystallinity in some cases. However, in the case of haematite produced from goethite, sulfation causes an anisotropic effect on the width of powder diffraction reflections. Further investigation has demonstrated that this is highly calcination temperature dependent, and whilst Mossbauer spectroscopy indicates that cationic disorder contributes to this effect HRTEM has shown it to be primarily due to the presence of mesoporous dimension voids running axially along the acicular Fe₂O₃ crystallites. It is proposed that sulfation causes this effect by hindering the Fe³⁺ cation migration during the transformation of goethite to haematite and this is associated with bridging C₂v complexation of SO₄²⁻ as evidenced in the sulfated goethite precursor. The catalytic activity of a series of zirconia-supported molybdenum catalysts with different loadings in the range 2, 5 and 10wt% were evaluated for both the partial and complete oxidation of methane. The catalytic performance of these materials has been shown to depend markedly on molybdenum content and it has tentatively been suggested that the formation of a monolayer species over the zirconia support is of importance in the reaction. Molybdation increases surface area and quantitative powder X-ray diffraction has indicated that this is associated with an increasing proportion of the metastable tetragonal phase of zirconia with increasing M0 O3 content. Hydrogen TPR studies indicate that molybdate undergoes a two stage reduction process and that Mo species on monoclinic zirconia are more susceptible to reduction. Laser Raman spectroscopy studies have indicated that the nature of the molybdenum oxo species is a function of loading, with polymolybdate species being replaced by XRD invisible molybdenum trioxide crystallites as MoO₃ content is increased. No direct relationship between MoO₃ formation or ZrO₂ polymorph on catalytie performance could be discerned and it is proposed that at reaction temperatures, which are significantly above the Tamman temperature of MoO₃ molybdenum oxo species are highly mobile forming monolayer type entities. Consistent with this, a catalyst with close monolayer MoO₃ content is found to possess a much higher efficacy than catalysts with sub- and super- monolayer contents. Finally, "superacidic" metal oxides have been screened for efficacy in methane combustion and the results determined under differential reaction conditions have been compared with those of materials reported to have high efficacy in the literature. It is found that, consistent with their potential enhancement of methane activation, modification of catalysts by literature methods so as to introduce "superacidity" generally promotes methane oxidation performance. However, supporting low levels of platinum on "Superacidic" oxides generally suppresses their performance, which is the opposite effect to that observed on their base oxides, which may imply a direct interaction of platinum with the aetive sites of reaction.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Metal oxides; Catalysts