The Effects of Thermal Degradation on the Performance of a NO _X Storage/Reduction Catalyst

The performance characteristics of a commercial lean-NO_X trap catalyst were evaluated between 200 and 500°C, using H₂, CO, and a mixture of both H₂ and CO as reductants before and after different high-temperature aging steps, from 600 to 750°C. Tests included NO_X reduction efficiency during cycling, NO_X storage capacity (NSC), oxygen storage capacity (OSC), and water-gas-shift (WGS) and NO oxidation reaction extents. The WGS reaction extent at 200 and 300°C was negatively affected by thermal degradation, but at 400 and 500°C no significant change was observed. Changes in the extent of NO oxidation did not show a consistent trend as a function of thermal degradation. The total NSC was tested at 200, 350 and 500°C. Little change was observed at 500°C with thermal degradation but a steady decrease was observed at 350°C as the thermal degradation temperature was increased. At 200°C, there was also a steady decrease in NO_X storage capacity, except after aging at 700°C for 2 hours, where the capacity actually increased. The OSC decreased at test temperatures between 200 and 500°C after each increase in thermal degradation temperature, except again when the sample was degraded at 700°C for 2 hours, where an increase was observed. In the cycling experiments, a gradual drop in NO_X conversion was observed after each thermal degradation temperature, but when the catalyst was aged at 700°C for up to 2 to 5 hours, an increase in conversion was observed. These data suggest that there was redispersion or activation of a trapping material component during initial 700°C thermal degradation treatments while the OSC data indicate redispersion or activation of oxygen storage components. We propose that the oxygen storage components become “activated” as trapping materials when an initial 700°C thermal degradation is imposed.