AXIAL CHARACTERIZATION OF OXYGEN STORAGE CAPACITY IN CLOSE-COUPLED LIGHTOFF AND UNDERFLOOR CATALYTIC-CONVERTERS AND IMPACT OF SULFUR

The oxygen storage capacity of a 56,000 mile aged warmup and underfloo r converter system was characterized as a function of axial location a long the converters and compared with fresh samples having the same fo rmulation. Measurements of oxygen storage were made using a titration technique and at conditions expected to be commonly encountered during OBD-II diagnosis of catalyst performance. Vehicle aging resulted in a dramatic loss of oxygen storage in the warmup converter presumably du e to the severe thermal sintering, but the significant amount of phosp horus (P) and zinc (Zn) poison accumulation on this converter was foun d to impact oxygen storage minimally. This is in contrast to the measu red impact of P and Zn deposition on warmed-up hydrocarbon conversion, which was found to be significant relative to the impact of thermal s intering. The underfloor converter was found to have retained nearly a ll of its original oxygen storage after vehicle aging, consistent with operation of this converter at moderate temperatures which do no resu lt in severe thermal sintering of the noble metals and the ceria. The impact of sulfur on the oxygen storage of both warmup and underfloor c onverter sections was dramatic. Sections in the forward part of the wa rmup converter and in the front brick of the underfloor converter had relatively modest oxygen storage capacity which was almost completely blocked as the sulfur concentration reached 75-150 ppm (equivalent in gasoline). Other sections such as the rear of the warmup converter and the rear monolith of the underfloor converter had more oxygen storage capacity, which was significantly decreased as the sulfur concentrati on reached 150 ppm equivalent in fuel, and was approached complete los s near 500 ppm sulfur equivalent in fuel.