Development of Emission Transfer Functions for Predicting the Deterioration of a Cu-Zeolite SCR Catalyst 2009-01-1282

Urea selective catalytic reduction (SCR) catalysts have the capability to deliver the high NOx conversion efficiencies required for future emission standards. However, the potential for the occasional over-temperature can lead to the irreversible deactivation of the SCR catalyst. On-board diagnostics (OBD) compliance requires monitoring of the SCR function to make sure it is operating properly. Initially, SCR catalyst performance metrics such as NOx conversion, NH₃ oxidation, NH₃ storage capacity, and BET surface area are within normal limits. However, these features degrade with high temperature aging. In this work, a laboratory flow reactor was utilized to determine the impact on these performance metrics as a function of aging condition. Upon the completion of a full time-at-temperature durability study, four performance criteria were established to help determine a likely SCR failure. The following four performance criteria were established: (1) < 80% NOx conversion (between 200°C–400°C), (2) < 100% NH₃ oxidation (at T > 500°C), (3) < 600mg/L NH₃ storage capacity, and (4) < 70% normalized BET surface area. Implementation of the performance criteria resulted in the identification of the following five “Borderline” failure aging conditions: (1) 1hr/1000°C, (2) 2hr/950°C, (3) 8hr/900°C, (4) 64hr/850°C, and (5) 256hr/800°C. Based on these SCR performance time and temperature limits, two simple mathematical equations were developed that related the threshold temperature to the threshold duration time. In addition, two transfer functions were developed that characterized the NH₃ storage capacity and the BET surface area results (via N₂ adsorption) as functions of both aging time and aging temperature. From these two functions, a NOx conversion transfer function was also developed that predicted the NOx conversion vs. the measured NOx conversion within 10% margin of error.