Comparison of O2 and NO2 impact on PtOx and PdOx formation in diesel oxidation catalysts and their reduction by CO and C3H6 pulses

NO oxidation on diesel oxidation catalyst (DOC) increases NO2/NO ratio in the exhaust gas, which is beneficial for both NOx reduction and soot oxidation processes in the catalysts and filters located downstream, such as SCR, LNT and DPF. The impact of O-2 and NO2 on the PtOx and PdOx formation and subsequent changes in NO oxidation activity was studied on Pt/gamma-Al2O3 and Pd/gamma-Al2O3 catalysts. Repeated heat-up and cool-down temperature ramps in the range of 80-450 degrees C and isothermal deactivation/reactivation experiments at 150, 175 and 200 degrees C with NO oxidation as a probe reaction were performed. Inverse hysteresis of NO2 yield during temperature ramps was observed with both catalysts. Though NO2 is stronger oxidizing agent than O-2, PtOx and PdOx formation was induced to a similar extent by both O-2 and NO2 when present at their typical concentration levels (8% O-2, 250 ppm NO2). The NO oxidation activity of both Pt and Pd sites was restored by CO or C3H6 pulses while keeping overall lean conditions (excess of oxygen). The maximum efficiency of CO pulses was achieved at 200 degrees C for PtOx reduction, and at 150 degrees C for PdOx reduction.