(233d) Lean NOX Trap Pt-Ba/Al2O3 Model Catalyst: Stability and Reactivity of Barium Species under Different Purging Conditions

The usage of lean-burn engines is a promising strategy for improving the fuel economy of vehicles and reducing the emission of greenhouse gas CO₂. However, gasoline engines operating at a high air-to-fuel ratio prevent the use of the conventional three-way catalyst for NO_x reduction, as the three-way activity (simultaneous oxidation of carbon monoxide and hydrocarbons, and reduction of NO_x) is restricted to stoichiometric combustion. In fact, it is this limitation of the three-way catalyst that presently hinders the use of more fuel-efficient gasoline engines. Lean NO_x trap (LNT) catalysts have been developed as a promising alternative to meet the upcoming regulations. These catalysts operate in a cyclic manner where during the lean period of operation, the catalyst stores or "traps" NO_x as nitrate species. A periodic and short rich pulse is introduced so that the trapped NO_x is released and reduced to N₂ and the catalyst is regenerated. Typically, a LNT catalyst consists of precious metal (generally Pt) and trap components including alkali or alkaline earth metal oxides (mainly BaO) on a high-surface-area support such as alumina. For a typical model LNT containing Pt and Ba on alumina, NO is oxidized to NO₂ on the Pt sites, the NO₂ is then being trapped by the Ba to form barium nitrate under lean cycles. During rich cycles, the nitrate decomposes and the released NO_x is reduced on the Pt sites into N₂.

In recent years, many efforts have been made to understand at a fundamental level the mechanism of storage of NO_x, and the effects of different storage loadings. However, it is equally important to elucidate the release and reduction of NO_x within the rich cycle, where the stored NOx needs to be converted into N₂.  Very recently, studies showed that the release of stored NO_x depends on the gas composition and the presence of Pt in the LNT catalyst.

Previous work in our laboratories has shown that highly dispersed barium phases behave quite differently than bulk-like phases. The thermal decomposition of highly dispersed Ba(NO₃)₂ occurs at significantly lower temperatures than that of bulk Ba(NO₃)₂. The release of NO₂ stored over these two sites of Ba species mimics the decomposition characteristics of impregnated Ba(NO₃)₂ on Al₂O₃. Further study showed that the dispersed and bulk-like Ba species play different role for NO₂ storage during lean cycle. In fact, the dispersed phase was found to be primarily responsible for NO₂ storage.

In this paper, we compare the decomposition of alumina-supported barium nitrate species and the release of stored NOx under different reactive atmospheres, including nitrogen, carbon monoxide, and propylene.  Our major focus is on the influence of platinum and the different atmospheres on the stability and reactivity of dispersed and bulk-like barium species in LNT Pt-Ba/Al₂O₃ model catalysts.