Bircumshaw, Lucy Deborah; (2000) alpha-Alumina supported catalysts for pollution control. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The exhaust gas from an automobile contains hydrocarbons, carbon monoxide and nitrogen oxides. These pollutants are removed using a catalytic converter consisting of a monolith support, washcoated with γ-Al2O3 and impregnated with catalyst. Current converter designs provide good warmed-up performance, however during cold-start a significant amount of pollutants pass unconverted through the converter. Placing the converter nearer the exhaust manifold, in a close-coupled position, reduces cold-start emissions, however it also results in thermal deactivation caused, in part, by sintering of the γ-Al2O3 washcoat. This work examined the use of α-Al2O3 as a thermally stable catalyst support. Firstly, issues relating to catalyst preparation using α-Al2O3 as a support were investigated. The influence of drying severity on catalyst crystallite size and activity for CO oxidation on Pd/α-Al2O3 was examined. Experimental results, which demonstrated that a slow drying rate was critical in attaining a small crystallite size and good catalyst activity, were rationalised using a four-stage model of the drying process. NMR imaging was used to follow the drying of an α-Al2O3 pellet. The control of catalyst axial redistribution during preparation of α-Al2O3 and γ-Al2O3 monolith supported catalysts was investigated, and samples were prepared with non-uniform distributions. The effect of catalyst axial distribution on cold-start behaviour was examined using a one-dimensional model. Results demonstrated the importance of incorporating changing inlet species concentrations in simulations. The relationship between catalyst loading of Pd/α-Al2O3 on catalyst crystallite size and activity for CO oxidation was experimentally established. Results indicated that the origin of the catalyst precursor can affect catalyst characteristics. Finally, experimental data fitted to an empirical rate expression enabled quantitative comparisons to be made regarding the effect of atmosphere and temperature on the sintering behaviour of α-Al2O3 compared to γ-Al2O3 supported catalysts. It was concluded that α-Al2O3 based close-coupled converters are a promising competitor to commercial converters.

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