Laboratory Postmortem Analysis of 120k mi Engine Aged Urea SCR Catalyst

Selective Catalytic Reduction (SCR) of NOx with aqueous urea and a Catalyzed Diesel Particulate Filter (CDPF) has been considered as one of the emission control systems for diesel vehicles required to meet Federal Tier 2 and California LEVII emission standards. At Ford Motor Company, a DOC-SCR-CDPF system containing a copper / zeolite SCR catalyst was aged to 120k mi on the engine dynamometer using an aging cycle that mimicked both city and highway driving modes. A total of 643 CDPF regenerations occurred during the aging that raised the SCR catalyst to a temperature of up to 650°C on a regular basis. A series of lab analyses including activity tests, ammonia thermal desorption, BET surface area, XRF, XRD, and EPMA was conducted on cores taken from the 120k mi engine aged SCR catalyst brick. The lab post-mortem characterizations revealed the changes of catalyst properties, and the deterioration profile of the SCR catalyst brick after undergoing real aging conditions. The lab activity data showed that deterioration of the brick was not uniform, and the inlet section of catalyst brick was deactivated the most. The analysis results suggested that the aging mechanism of engine aged SCR catalyst was “dynamic”, and the aging process was different along the flow and brick radial directions due to non-uniform exposure to exotherms, urea, ash, and poisoning components, and the engine operation conditions during the aging. This system probably had the first available zeolite-based SCR in the world that was engine-aged in a system with a downstream CDPF and provided very valuable information on understanding the deactivation process and mechanisms of urea SCR for light-duty diesel vehicle applications.