(216c) Steam Effect on NoX Reduction over Pt-Bao/Al2O3 Catalyst

Pt-BaO/Al₂O₃ is a lean NO_x trap (LNT) model catalyst that provides a very effective method for NO_x removal in lean-burn gasoline engine exhaust. For a typical cyclic operation, NO is oxidized to NO₂ on the Pt sites, and the NO₂ is then being trapped by the BaO to form barium nitrate during the lean cycle. During the rich cycle, the nitrate decomposes and the released NO_x is reduced on the Pt sites into N₂. Many studies have been carried out on catalytic reduction of NO by H₂, CO, and by a mixture of H₂/CO over supported Pt group metal catalysts. With H₂ as reductant, NH₃ or HCN is the major reaction product. Steam effects in three-way catalysts have also been widely investigated, and recently the formation of NH₃ was reported for the reduction of NO by CO in the presence of water vapor on Al₂O₃-supported Pt catalyst. Although NH₃ is currently not an emission component regulated by EPA, it is still interesting to study in more detail the catalytic formation of NH₃ and its influence on the NO_x reduction for automotive applications, especially in view of potential use of H₂ in future H₂ internal combustion engines (ICE).

In this work, we investigated the decomposition characteristics of Pt-Ba(NO₃)₂/Al₂O₃ by TGA-IR, and the lean/rich (L/R) cycle reaction characteristics of a Pt-BaO/Al₂O₃ LNT model catalyst, focusing on the formation of NH₃ and exploring ways to minimize the NH₃ formation when CO or hydrocarbons are used as reductants. TGA-IR results showed that the original main decomposition products NO₂ and NO obtained under N₂ atmosphere transfer to NH₃ and H₂O in presence of H₂ atmosphere. The influences of BaO loading, the presence of Pt, and the TGA heating rate on the decomposition temperature and product distribution are discussed. L/R cycle experiments showed that under rich condition the major reduction product of NO_x in the presence of steam is NH₃. The formation of NH₃ is attributed the reaction between NO_x and H₂, with H₂ stemming from water gas shift (CO as reductant) or steam reforming (HC as reductant). Catalysts operated in L-R cycle mode are more effective to reduce NO_x into NH₃ than catalysts operated only under rich conditions. Additional experiments showed that the combination of Pt-BaO/Al₂O₃ and Co²⁺ exchanged zeolite could effectively inhibit the NH₃ formation in L-R cycle (L/R=60/20 sec). However, if operation under rich condition is prolonged to 60 seconds (L/R=60/60), the major product under rich condition is still NH₃.