NOx Reduction Over Perovskite Based Lean NOx Trap

Abstract

Conventional Lean NO_x Trap (LNT) catalysts contain high loadings of Pt in order to oxidize NO to NO₂, a critical reaction in LNT application, and NO₂ is stored as nitrates/nitrites on NO_x trap materials (such as BaO) during lean periods and reduced to nitrogen/ammonia during shorter rich periods.  Hence it is highly desirable to develop non-Pt catalysts with high NO oxidation activities.  We have recently reported that perovskites are active catalysts for NO oxidation.  This paper presents our investigation on using perovskites to substitute Pt in LNT catalysts.  LNT catalysts based on LaMO₃ and Sr-LaMO₃ (M = Mn, Fe and Co) perovskites were evaluated and their thermal durability under reducing conditions, required for desulfation was probed.  Temperature Programmed Reduction (TPR) and X-ray Diffraction (XRD) results show that Co-perovskite is not stable in reducing environment at temperatures above 500^oC, whereas both Fe and Mn-perovskites could be stable at 700^oC under the same condition. Considering both NO oxidation to NO₂ efficiency and thermal durability under rich condition, Mn-perovskite is likely the best choice for LNT applications.  Similar NO_x reduction performance was achieved over a Mn-perovskite based LNT when compared with a Pt based LNT, and its sulfation and desulfation characteristics were evaluated.