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In order to achieve ultra low emission levels with three-way catalysts, an early accurate air/fuel ratio control is essential. Positioning the oxygen sensor in the first part of the substrate helps to protect the oxygen sensor from being splashed by water during cold start, so that early heating and activation becomes a less limiting factor. For emission control purpose, a position of a rear sensor in the warm part of the catalyst gives improved possibilities for oxygen buffer control during catalyst warming up conditions. This enhances balancing HC and NOx in an early phase. In addition, for OBD reasons it is possible to locate the sensor in any axial position in the catalyst, which improves design possibilities for cold start detection, even for single brick catalyst systems. The paper describes the construction of the catalyst with an integrated oxygen sensor.

Catalysts aged under different on-road conditions were analysed with respect to their conversion of CO and HC at step changes of the synthetic exhaust gas composition. Time resolved diode laser spectroscopy and fast response FID analysis were used to characterise the catalyst response to transient changes of CO and hydrocarbons in the exhaust gas. The oxygen storage capacity was monitored at various conditions; flow rate, catalyst temperature, previous exposure to oxidizing or reducing atmosphere and amplitude of the perturbation. The technique appeared to provide a sensitive probe for analysis of the dynamic oxygen storage capacity of new and aged catalysts at exhaust like conditions. The results correlate well with the transient emission performance during vehicle tests. Further, surface characterization using SEM/EDS and XPS techniques indicated that phosphate formation was the most probable cause of deactivation.

Methane and nitric oxide conversion was studied over a Pd-based catalyst at steady state conditions. The gas mixture contained methane (0.125 %), Nitric oxide (0.125 %), carbon monoxide (0.7 %), oxygen and argon as carrier gas. The experiments were performed in a well-stirred reactor (Berty reactor) which provided constant gas composition over the catalyst. Lambda scans from λ=1.01 to 0.99 and back performed by varying the oxygen content, revealed a hysteresis in both the methane conversion and the nitric oxide conversion. The temperature and presence of nitric oxide affected the hysteresis. Complementary experiments in a synthetic exhaust gas rig revealed a more pronounced hysteresis in the presence of carbon dioxide and water. An attempt to model the hysteresis effect as a function of the palladium and palladium-oxide transformations was made.