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Robust on-board diagnosis of emission catalyst performance requires the development of artificially damaged "threshold" catalysts that accurately mimic the performance of damaged catalysts in customer use. The threshold catalysts are used by emissions calibrators to determine fore-aft exhaust oxygen sensor responses that indicate catalyst failure. Rather than rely on traditional trial-and-error processes to generate threshold catalysts, we have used a DFSS (Design For Six-Sigma) approach that explores, at a research level, the relationship between oxygen storage capacity (OSC) of the catalyst (i.e., the fundamental property dictating the response of the aft oxygen sensor) and key process input variables: high-temperature exposure, phosphorus poisoning, and catalyst "recovery."

This paper summarizes some of the research which has been carried out at Ford Motor Co. in the area of NOx traps. Results from a large body of experimental work are reviewed and used to provide insight into the fundamental processes which govern NOx trap performance. In particular, the key parameters which control thermal durability and sulfur poisoning of the NOx trap are discussed in detail. In addition, a theoretical model of the NOx trap is described and used in the analysis and interpretation of the experimental results.