Investigation on NO_x Storage Characteristics and Aging Influence of Lean NO_x Trap Model Catalyst BaO-Pt/Al_2O_3

摘要

Barium oxide (BaO) and platinum (Pt) are the key components in lean NO_x trap (LNT) catalysts. Previous work has demonstrated that highly dispersed and bulk-like BaO species have low-temperature (LT) and high-temperature (HT) release features for stored nitrogen dioxide (NO_2). To better understand the roles of both BaO and Pt, a series of BaO/Al_2O_3 and BaO-Pt/Al_2O_3 were prepared and used to investigate the release characteristics of stored NO_2. Two typical BaO-Pt/Al_2O_3 catalysts having 75% and 44% of dispersed BaO, respectively, were aged and used for both the static NO_2 storage and the dynamic NO_x elimination by lean-rich cyclic reactions. Results showed that bulk-like BaO plays a lesser role in storing NO_2 than does dispersed BaO. Pt inhibits the formation of nitrates and thus decreases NO_2 storage capacity (NSC) as Pt can catalyze the release, particularly for NO_2 species stored on bulk-like BaO. Aging in the range of 750-950°C resulted in a decrease of NSC but the HT release peak was restored with release features similar to those for NO_2 stored on BaO/Al_2O_3, indicating that the Pt inhibiting role weakens on the formation of nitrate over aged BaO-Pt/Al_2O_3. Lean-rich cyclic reactions have shown that the BaO-Pt/Al_2O_3 catalyst that was aged at 950°C totally lost the ability to eliminate NO_x in the range of 250-600°C in the lean cycles due to Pt sintering. These Pt crystallites were still able to catalyze the lean cycle's oxidation of NO to NO_2 and the rich cycle's reduction of NO, suggesting that the sintered Pt particles significantly decreased the spillover ability of NO_x from Pt to BaO species.