48 commercial three-way catalytic converters (TWC) aged on-road were investigated. The results show that there was no significant impact of aging on light off temperatures for CO, HC, and NO up to 120 000 km. Above 120 000 km, the light off temperatures in general increased. The steady state conversions for CO and HC are not significantly affected by aging. However, the steady state conversion of NO is decreased with increased mileage at the conditions studied. The impact of aging on oxygen storage capacity (OSC) of the converters was studied by propene pulse experiments. A trend of decreased pulse conversion was noted above 120 000 km. OSC and NOconversion showed considerable difference between individuals with similar mileage. A number of high-mileage units perform well while a few low-mileage converters appear to be next to destroyed. The surface characterization shows that the main cause of deactivation was loss of specific surface, both of washcoat and precious metals. This is most likely due to exposure to high temperatures. Poisoning elements, typically phosphorus, lead, sulphur and calcium, are found on the catalysts surface.

SUMMARYCatalytic converters are crucial for the reduction of car emissions. The performance of aging catalysts in use has not been well known. Data on catalysts from scrapped cars of different ages has been taken in well controlled laboratory conditions. The statistical analysis of these data compares and uses different models of the regression type and also generalized regression for ordered categorical data. A significant, though not very dramatic, deterioration with age is shown for the catalytic converters. The overall impression is, however, that converters like those examined in this study do function reasonably well at least up to about 200000 km driving distances.