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Abstract

Owing to an increasing awareness over the last few decades, of the importance of vehicle exhaust emissions to the growing problem of air pollution, a number of methods for reducing such emissions have been developed. Of these, the catalytic converter is probably the most important. The function of a catalytic converter is to oxidise completely carbon monoxide and hydrocarbons and also to reduce oxides of nitrogen without the formation of ammonia. However, the presence of various lead compounds resulting from the combustion of lead tetra-alkyl antiknock additives has been found to deactivate or poison the catalysts used in such converters. The purpose of this work has been to investigate the mechanism of deactivation of catalysts by lead compounds.

In the Introduction, an account is given of the general problem of pollution with particular reference to the role of catalysis in the control of automotive air pollution. In addition, the main theories of heterogeneous catalysis, including catalytic poisoning, are summarised together with a description of the main experimental techniques used. Finally a review is given of the current status of heterogeneous catalytic oxidation, as applied to the control of automotive air pollution, with particular emphasis on catalyst deactivation.

The Experimental Section describes the compounds, apparatus and procedures used in this work. The catalysts selected for study were mainly transition metal oxides (both supported and unsupported), the most important of which were nickel (II) oxide ‘and cobalt (II,III,III) oxide. In addition to these, an alumina supported platinum catalyst was investigated. The lead compounds chosen were lead (II) bromide and lead (II) chloride. Catalysts were deactivated by use of a specially designed apparatus, which allowed a regulated stream of lead halide vapour to come into contact with the catalyst. The effect of lead halide on catalytic activity with respect to carbon monoxide oxidation was studied using a pulsed-flow micro-reactor. In conjunction with this, a number of physicochemical techniques were employed to examine further the interaction of catalysts with the lead halides.

In the Results Section, the lead halides are shown to deactivate all the catalysts examined, with the supported catalysts showing a significantly greater resistance to deactivation than the unsupported catalysts. Of the unsupported catalysts, cobalt (II,III,III) oxide was clearly the most resistant to deactivation. Furthermore lead (II) chloride was found to be far more effective than lead (II) bromide in deactivating both supported and unsupported cobalt (II,III,III) oxide. Nitrogen adsorption studies have revealed that there is little correlation between the deactivation by lead halides of unsupported catalysts, on the one hand, and the surface properties of these catalysts on the other. However, it has been shown that both surface area and porosity decrease during the deactivation of supported catalysts. Various physicochemical techniques have shown that bulk chemical reaction between the lead halides and the catalysts does not take place.

Finally, in the Discussion, the significance and implications of the experimental results are considered in detail. Adsorption of the lead halides appears to be the principal mode of deactivation both with supported and unsupported catalysts. However, the support provides a large surface area for adsorption of the halides, and this increases the tolerance of the catalyst to deactivation. In addition, with supported catalysts, lead halides appear to be concentrated primarily on the outer surface of the catalyst, without deep penetration into the porous structure, thus causing pore blocking. | The mechanism of pore blocking was not found, however, to be important.

Publication Type:	Thesis (Doctoral)
Subjects:	Q Science > QD Chemistry
Departments:	School of Science & Technology > School of Science & Technology Doctoral Theses Doctoral Theses

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