On the mechanism of sulphur poisoning and regeneration of a commercial gasoline NOx-storage catalyst

A commercial NOx-storage catalyst for gasoline applications containing platinum, palladium and rhodium together with Ba/CeO2/Al2O3 has been sulfated on the engine bench at 390 and 510 °C with nominal exposures of 0.3 and 1.3 g sulphur/l catalyst. Exposures of 0.3 g/l are too low to have a notable impact on the catalytic performance. At 390 °C, the sulphur is quantitatively adsorbed as opposed to 510 °C. Sulphur is mainly adsorbed at the inlet side of the catalyst thereby shielding the downstream region. Desulfation on synthetic gas bench at 700 °C leads to a removal of 50% of the sulphur. The residual sulphur is more evenly distributed along the length of the catalyst compared to the sulphur profile in the sulfated catalyst. This leads to an improvement of the NOx-activity at the inlet side and a drop in NOx-performance at the outlet side after desulfation. Results from SEM-EDX, TPD, TPR and IR suggest that the barium component is selectively poisoned by the sulphur while alumina and ceria are not affected significantly. The insufficient desulfation of the catalyst is almost entirely due to the incomplete sulphur removal from the barium sites. In the desulfated state the IR and TPD-data suggest the presence of barium carbonate species of low thermal stability together with bulk carbonate. After sulfation the IR data indicate the presence of different sulphate species, both bulk and surface phases. Thermal ageing and sulphur poisoning affect different parts of the operational temperature window of the NOx-storage catalyst. While thermal stress alone mainly affects low temperature NOx-performance, sulphur poisoning mainly reduces high temperature activity. The results of this work help to identify strategies to improve both the sulfur resistance of the catalyst and the operational conditions for a better management of the NOx-trap system.