The Impact of Sulfur Poisoning on NOx-Storage Catalysts in Gasoline Applications

A commercial NOx-storage catalyst for gasoline applications containing Ba/CeO₂/Al₂O₃, platinum, palladium and rhodium has been sulfated on the engine bench at 390 and 510°C with a nominal exposure of 1.3 g sulfur/liter catalyst. Lower exposures proved too low to have a notable impact on the catalytic performance. At 390°C the sulfur is completely adsorbed while at 510°C only partial adsorption is being observed. Sulfur is mainly deposited at the catalyst inlet thereby shielding the downstream region. Desulfation on synthetic gas bench at 700°C leads to a partial removal of the sulfur. The residual sulfur is more evenly distributed along the length of the catalyst compared to the sulfur profile in the sulfated catalyst. This causes an improvement of the NOx-activity at the inlet side while the NOx-performance at the outlet side decreases after desulfation. Characterisation with SEM-EDX, TPD, TPR and IR suggests that the barium component is selectively poisoned by the sulfur while alumina and ceria are not affected significantly. 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 aging alone mainly affects low temperature NOx-performance, while sulfur poisoning mainly reduces high temperature activity.