Investigation on NOx Storage Characteristics and Aging Influence of Lean NOx Trap Model Catalyst BaO-Pt/Al2O3 2010-01-0364
Barium oxide (BaO) and platinum (Pt) are the key components in lean NOx 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 (NO₂). To better understand the roles of both BaO and Pt, a series of BaO/Al₂O₃ and BaO-Pt/Al₂O₃ were prepared and used to investigate the release characteristics of stored NO₂. Two typical BaO-Pt/Al₂O₃ catalysts having 75% and 44% of dispersed BaO, respectively, were aged and used for both the static NO₂ storage and the dynamic NOx elimination by lean-rich cyclic reactions. Results showed that bulk-like BaO plays a lesser role in storing NO₂ than does dispersed BaO. Pt inhibits the formation of nitrates and thus decreases NO₂ storage capacity (NSC) as Pt can catalyze the release, particularly for NO₂ 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 NO₂ stored on BaO/Al₂O₃, indicating that the Pt inhibiting role weakens on the formation of nitrate over aged BaO-Pt/Al₂O₃. Lean-rich cyclic reactions have shown that the BaO-Pt/Al₂O₃ catalyst that was aged at 950°C totally lost the ability to eliminate NOx 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 NO₂ and the rich cycle's reduction of NO, suggesting that the sintered Pt particles significantly decreased the spillover ability of NOx from Pt to BaO species.