Impact of redox conditions on thermal deactivation of NOx traps for diesel

Performance of NOx traps after high-temperature treatments in different redox environments was studied. Two types of treatments were considered: aging and pretreatment. Lean and rich agings were examined for a model NOx trap, Pt–Ba/Al2O3. These were done at 950 °C for 3 h, in air and in 1% H2/N2, respectively. Lean aging had a severe impact on NOx trap performance, including HC and CO oxidation, and NH3 and N2O formation. Rich aging had minimal impact on performance, compared to fresh/degreened performance. Deactivation from lean aging was essentially irreversible due to Pt sintering, but Pt remained dispersed with the rich aging. Pretreatments were examined for a commercially feasible fully formulated NOx trap and two model NOx traps, Pt–Ba/Al2O3 and Pt–Ba–Ce/Al2O3. Pretreatments were done at 600 °C for 10 min, and used feed gas that simulated diesel exhaust under several conditions. Lean pretreatment severely suppressed NOx, HC, CO, NH3 and N2O activities for the ceria-containing NOx traps, but had no impact on Pt–Ba/Al2O3. Subsequently, a relatively mild rich pretreatment reversed this deactivation, which appears to be due to a form of Pt–ceria interaction, an effect that is well known from early work on three-way catalysts. Practical applications of results of this work are discussed with respect to NOx traps for light-duty diesel vehicles.