Impact of dopants on the sulfation, desulfation and NO_x reduction performance of Ba-based NO_x storage-reduction catalysts

The performance of a NO_x storage-reduction (NSR) catalyst is strongly dependent on the relative stabilities of nitrates and sulfates on the catalyst surface. This effort studies the effects of introducing 5 mol% La, Ca, and K-dopants into a BaO phase of a model Pt/Ba/Al₂O₃ NSR catalyst. The dopants were chosen with a range of properties to affect the BaO lattice spacing and/or the number of oxygen vacancies. The resulting changes in the storage material, in turn, impact the stability of stored nitrates and sulfates, as measured by NO_x conversions and desulfation temperatures. The Ca- and La-doped material shows equivalent or better NO _x reduction performance between 200 and 400 °C, while the K-doped NSR catalyst showed significant decreases in performance at 200 °C but maintained the high performance at 300 and 400 °C. Following the performance measurements, the samples are then sulfated to 5.5 mg S/gcat and desulfated to 1000 °C. All NSR catalysts generally show similar desulfation behavior, but in determining the temperature of 20% sulfur removal (T_20%), it is shown that the Ca + Ba sample has a 25 °C lower T_20% than both the Ba-only and La + Ba samples, while K + Ba is 70 °C higher. Additional Ca-based samples were than prepared at the 10, 20 and 100 mol% levels. The higher Ca-doped materials show similar NO_x reduction performance, but the 5% Ca + Ba sample maintains the lowest T_20%. Interestingly, the Ca-only sample has a T_20% that is 60 °C higher than the Ba-only NSR catalyst, which indicates that the introduction of 5% Ca into the Ba-lattice has a synergistic effect in lowering the desulfation temperature.