Aging of Cu-exchanged Zeolite SCR Catalysts Under Lean, Stoichiometric, and Rich Gas Compositions Relevant to Gasoline Applications
Calvin Thomas, Oak Ridge National Laboratory
Cu-exchanged zeolites are very effective for selective catalytic reduction (SCR) of NOx by ammonia in diesel engine exhaust. Some Cu-exchanged zeolites (notably SSZ-13) have also proven to be quite durable under the continuously fuel-lean conditions in diesel exhaust. These materials are under consideration for applications in gasoline engine exhaust, including passive SCR for NOx control during fuel-lean operation as well as ammonia control for stoichiometric applications. In gasoline applications, high temperature excursions will likely occur under stoichiometric or fuel-rich exhaust conditions. The shift in exhaust composition could significantly alter the mechanisms of high temperature degradation, impacting the durability of the Cu-exchanged zeolites. While there has been some previous work investigating the effect of exhaust composition on the aging of Cu-exchanged zeolites, the stability of commercially formulated SCR catalysts and the degradation mechanisms under stoichiometric and rich conditions have not been fully established.
In this work, we have used a synthetic exhaust flow reactor to evaluate three different commercially formulated Cu-exchanged zeolite SCR catalysts before and after aging under fuel-lean, fuel-rich, and stoichiometric conditions. The evaluations included steady state SCR activity, NOx conversion during passive SCR cycles, ammonia storage, and ammonia temperature programmed desorption profiles. Each of the catalyst samples was degreened under fuel-lean conditions at 500°C and evaluated to determine the baseline activity. Each sample was then sequentially aged in 4 h increments at 600, 700, and 800 °C under either fuel-lean, stoichiometric, or fuel-rich exhaust conditions, with evaluations conducted after each aging step. This presentation will discuss trends in catalyst deactivation for three different Cu-exchanged zeolite formulations as a function of aging temperature and gas composition. The findings will help to define an appropriate operating envelope for Cu-exchanged zeolites in gasoline engine exhaust applications.