(233c) Mechanistic Investigation of Nsr (Nitrogen Oxide Storage and Reduction) Catalysts

As the most promising strategy so far for NOx abatement of lean engines, Nitrogen oxide Storage and Reduction (NSR) catalysts can greatly reduce gas consumption as well as the consequent CO2 emission. Extensive studies have been made recently by many groups; some aspects, however, are still under discussion.

A comprehensive investigation of the NSR mechanism has been conducted over different catalysts. The formation of both nitrite and nitrate species was observed on a model 1 wt.% Pt/20 wt.% BaO/Al2O3 NOx Storage Reduction (NSR) catalyst during the NOx storage phase. These species were subsequently reduced under ?rich? conditions. In comparison, a Pd/BaO/Al2O3 catalyst of similar composition exhibits higher overall NOx reduction activity at temperatures between 250 and 350 °C. This difference is mainly ascribed to the higher amount of nitrites formed and the higher activity of Pd for propylene activation. Reactivity studies show a higher reactivity of nitrites than nitrates towards reduction by propylene, suggesting nitrite is the key intermediate for the NOx storage/reduction. The effect of the support (i.e., Al2O3, TiO2 and SiO2) and storage component (i.e., K, Ca, Sr, and Ba) was also examined. The results suggest a correlation with acid/basic properties in both cases, which can facilitate NOx storage (basicity) and propylene activation (acidity).

Based on the results, the use of hydrotalcites was attempted due to the unique properties of these materials. Indeed, hydrotalcite-derived Pd-Ni catalysts exhibit improved reactivity thanks to the high amounts of the nitrites formed under the ?lean? conditions and the efficient C3H6 activation under ?rich? pulses.