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A comparison of two different types of NOx reducing catalysts will be worked out. The potential of two De-NOx catalysts using engine out hydrocarbon emissions for NOx conversion will be shown by variation of different engine parameters. An analysis of the hydrocarbon species upstream and downstream catalyst will demonstrate, which components are responsible for the NOx reduction in the exhaust gas of a lean burn engine. By variation of different parameters during adsorbtion and regeneration phases of the adsorber catalyst the efficiency in NOx reduction will be optimized. An assessment of the suitability for lean burn engines will consider the emission reduction efficiency as well as the influence on engine fuel consumption.

Most automobile manufacturers are looking for methods to increase fuel economy. It is possible to achieve this by operating under lean conditions. The problem remains, however, that while hydrocarbon and carbon monoxide oxidation is very efficient on a 3-way catalyst in an oxidising atmosphere, reduction of oxides of nitrogen (NOx) is poor. One method to improve the efficiency for NOx reduction under lean conditions is to use a 3-way catalyst in conjunction with a NOx trap. The NOx trap stores NOx under lean conditions which is then released and reduced under rich conditions. Bench evaluations using a lean tune engine have been carried out on a number of 3-way catalysts containing a range of different NOx adsorbent components. Measurements of adsorption capacity, temperature window for lean NOx conversion and thermal durability have been made and a comparison of different adsorption materials determined. The effect of Pt loading on these parameters has also been ascertained.

A test programme has been conducted to study any potential long term effects of gasoline sulphur on catalyst performance, using a newly developed transient engine-bed ageing cycle. The ageing cycle, which was based on repeated European Extra Urban Drive Cycles, was chosen to ensure that the catalyst experienced a realistically wide range of temperatures and space velocities, together with transients, idle and periods of overrun. Two nominally identical platinum/rhodium catalysts (manufactured from the same batch) with matched lambda sensors, were aged for a period of 80,000 km each, one being aged using a gasoline containing 50 mg/kg (ppm wt) sulphur, the other being aged on the same fuel doped to 450 ppm wt S. The emissions performance of both catalysts was measured after 6,000, 40,000 and 80,000 km ageing, by fitting the catalysts to a test vehicle, and performing emissions tests over the European test cycle at both sulphur levels.

New advanced Pd only, Pd:Rh and Pt:Pd:Rh catalysts are compared with a current platinum rhodium catalyst after poisoning and thermal ageing. The results indicate that at equivalent precious metal cost (at 1994 prices) the advanced palladium based catalysts achieve significantly improved performance compared with current Pt, Rh and Pd technology. The new Pd:Rh formulation is recommended for close coupled locations and the Pt:Pd:Rh formulation recommended for underfloor locations where residual fuel lead may be present. The formation of H2S is shown to be low with the palladium based catalysts. Finally, it is shown that the new catalysts with balanced oxidation and reduction capability perform better in multi-brick systems than addition of a highly loaded palladium only front brick.

The durability of platinum-rhodium and palladium-rhodium three way catalysts has been investigated after ageing at elevated temperatures. Catalysts were subjected to inlet temperatures of between 780 and 960 °C for periods up to 100 hours on an engine bench. It may be concluded that platinum-rhodium catalysts have a better resistance to ageing at catalyst inlet temperatures above 840 °C than palladium-rhodium catalysts at the same loading. A new formulation palladium-rhodium catalyst loaded at 60 g/ft3 and a ratio of 9:1 showed substantially improved high temperature durability.