Abstract
A bench-mounted naturally-aspirated direct injection (NA/DI) Hatz single-cylinder diesel engine fitted with an exhaust aftertreatment system, consisting of an upstream DOC, a Fe-zeolite SCR catalyst and a downstream DPF, is used to perform accelerated thermal aging of the SCR catalyst. The rapid aging utilizes the high temperature exhaust gases obtained during the DPF regeneration cycles where supplemental fuel is periodically injected upstream of the DOC. Cyclic aging is performed at SCR inlet temperatures of 650, 750 and 850°C for up to 50 DPF regeneration cycles. Additionally, three field-aged SCR catalysts of similar formulation obtained from a high-mileage bus fleet are cut into three different sections: front, middle, and rear. To assess the validity of the implemented accelerated thermal aging protocol, cored samples of 2.2 cm diameter and 76 cm long from front and rear sections of accelerated engine-aged and field-aged SCR catalysts are evaluated for NOx performance in a bench-flow reactor.
Evidence of thermal aging is apparent on all of the engine-aged catalysts. However, while the rear section of the engine-aged SCR catalysts maintains relatively high NOx performance, the front section exhibits a drastic reduction in the NOx performance. This severely degraded performance of the front section is also observed in the field-aged samples. At an aging temperature of 750°C both the rear and front sections of the engine-aged SCR catalyst show a strong similarity to those of field-aged catalyst, which indicates that the implemented accelerated engine aging protocol at 750°C can replicate the field-aged conditions.
The degradation in the NOx performance in the front section of the engine-aged and field-aged catalysts appears to be due to not only the collapse of the zeolite structure—severe cracking and delamination of the washcoat—but also to possible platinum contamination from upstream DOC.
Keywords: Fe-zeolite SCR catalyst; Accelerated engine aging; Field-aged SCR catalysts; Bench-flow reactor; SEM.