| Summary: | Automotive three-way catalytic converters employ well-established technology for the control of NO<SUB>x</SUB>, CO, and hydrocarbon emissions, which are known to contribute to air pollution. This technology exploits the catalytic properties of platinum, palladium, and rhodium metals, of which rhodium is the crucial component in relation to the reduction of NO<SUB>x. </SUB> In order to minimise costs and meet increasingly stringent emission legislation, vehicle manufacturers have an interest in improving the performance of current catalyst systems, as well as developing catalyst compatible with lean-burn (oxygen rich) engines, which offer substantial improvements in fuel economy. The first part of this research, "Idealised system", has focused on studying the promotional effect of alkali metals in order to (i) enhance the catalytic properties of supported platinum-group metals, and (ii) promote the reduction of NO<SUB>x </SUB> in the presence of excess oxygen. Atmospheric microreactor measurements were used to demonstrate that, in a similar manner to platinum and palladium, the addition of sodium to rhodium catalysts enhances the activity and nitrogen selectivity for the reduction of NO in the absence of oxygen. This promotional effect was also apparent for sodium-promoted platinum and palladium catalysts operated under simulated exhaust conditions, with oxygen present in the feed stream. Significantly, however, under identical conditions sodium addition to rhodium has a detrimental effect as manifested by severe poisoning and decreased selectivity. A single mechanism has been established to account for the catalytic behaviour of the metals in the presence of sodium, based on the electronic influence of sodium on the coverages of the reactants over the metal surface. In the second part, "Practical systems", the relationship between ageing conditions and catalytic performance has been investigated for commercial three-way catalysts. Variations in oxygen storage and activity are observed depending on the ageing conditions experienced, and the original position of the catalyst in the exhaust system. Extensive catalyst characterisation was performed, including techniques used for the first time with these types of samples, in an attempt to explain the differences in performance.
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