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Chemical vapour impregnation as a synthesis technique for propane total oxidation catalysts

Bailey, Liam 2021. Chemical vapour impregnation as a synthesis technique for propane total oxidation catalysts. PhD Thesis, Cardiff University.
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Abstract

Volatile organic compounds (VOCs) are a group of chemicals particularly damaging to the environment and human health. Improvements to current removal techniques are needed in order to meet increasingly stringent legislation on VOC emissions. Chemical vapour impregnation (CVI) is a novel catalyst synthesis technique that has only seen application in a small number of reactions. This thesis aims to investigate CVI as a preparation method for total propane oxidation catalysts. CVI prepared catalysts were first compared against the more traditional synthesis technique of wet impregnation (WI) for the deep oxidation of propane. Three Pd/Al2O3 catalysts were synthesised by both techniques with a weight loading of 1 %, 2.5 %, and 5 %. In all cases the CVI catalysts were more active in terms of propane conversion than the analogous WI catalyst. XRD and XPS analysis was performed, identifying that both techniques prepared palladium in its highly active PdO form. TEM showed large differences in the palladium structure with CVI producing discrete nanoparticles 3-5 nm in diameter while WI produced large palladium rich areas but no discernible nanoparticles. CO chemisorption showed CVI produced significantly more active sites than WI catalysts with this being proposed as a major reason for increased activity. TOF calculations, suggested active sites on WI catalysts were more active per site than CVI sites. A range of 1% PdPt/Al2O3 bimetallic catalysts were synthesised by CVI, where the Pd:Pt ratios were 3:1, 1:1, and 1:3. The 3:1 Pd:Pt ratio was found to be the most active. XPS found that the palladium species on the bimetallic catalysts existed as a mixture of Pd0 and PdO. A strong correlation was found between increasing the proportion of metallic palladium to PdO and propane conversion. XRD suggested these bimetallics produced larger platinum particles, which was confirmed by SEM. Microscopy coupled with EDX identified segregation of the platinum and palladium with the platinum existing III as large particles while the palladium was disperse nanoparticles, too small for identification on the microscope used. PdFe, PdNi, and PdCo on alumina catalysts with a Pd:X of 3:1 were prepared by CVI and tested for propane oxidation. The PdFe catalyst was found to be the most active with a T50 of 298 °C, making it more active than the PdPt reference. XPS suggests that these catalysts also follow relationship between palladium oxidation state ratio and propane conversion; however, the relationship was not as strong. Heat treatment has been shown to influence activity, with catalysts reduced then calcined being more active than those calcined only. TEM imaging suggests that very small disperse nanoparticles have been synthesised. XPS shows catalysts that are almost exclusively in the oxide form with the literature suggesting a highly active PdO shell has formed around an alloy core. Overall, it was demonstrated that CVI produces highly active monometallic and bimetallic catalysts for the deep oxidation of propane, outperforming more traditional preparation techniques and should be considered for further investigation for use in making oxidation catalysts.

Item Type: Thesis (PhD)
Date Type: Completion
Status: Unpublished
Schools: Chemistry
Date of First Compliant Deposit: 11 May 2022
Last Modified: 11 May 2022 15:17
URI: https://orca.cardiff.ac.uk/id/eprint/149700

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