Selective Catalytic Reduction (SCR) systems will be widely used to meet the Heavy Duty Diesel (HDD) Euro IV emissions legislation. Reports on a number of demonstrations of such systems have already been published, but the long-term durability of such systems is still to be proven. The potential catalyst deactivation induced by oil-derived species and thermal processes have, up to now, received very little attention, despite the fact that these HDD emission control systems will need to be durable for distances of the order of 500,000 km or more.
This paper describes the development and performance of a new family of SCR catalyst with very high thermal durability and poison resistance. The thermal durability of the catalyst was initially demonstrated within long-term, high temperature engine bench ageing studies. Following the demonstration of the excellent thermal durability of the new catalyst, its resistance to poisoning by oil-derived species was studied using a mixed driving, engine bench ageing cycle, with substantial periods of high temperature exposure. Very little catalyst deactivation was observed, even after treatments corresponding to over 1 million km of real-world driving (based on oil consumption).
The catalyst has been incorporated into an SCRT® system, a four-way emission control system comprising a CRT® particulate filter system and an SCR system. This system has accumulated over 125,000 km of on-road driving in a mid-west to west coast long-haul US application. The performance of the system has been checked periodically. Average on-road NOx conversions of up to 82% were observed, when using an injection strategy in which the ammonia-to-NOx ratio was only 0.85 (ie giving a maximum possible NOx conversion of 85%). These high conversions were averaged over an on-road drive cycle which covered 850 km. During the course of the 125,000 km accumulated on the road, no deterioration in the NOx conversion of the system was observed, attesting to the excellent field durability of the SCR catalysts within this SCRT system.