LNT+SCR Catalyst Systems Optimized for NO _x Conversion on Diesel Applications

A laboratory study was performed to assess the effectiveness of LNT+SCR systems for NO_x control in lean exhaust. The effects of the catalyst system length and the spatial configuration of the LNT & SCR catalysts were evaluated for their effects on the NO_x conversion, NH₃ yield, N₂O yield, and HC conversion. It was found that multi-zone catalyst architectures with four or eight alternating LNT and SCR catalyst zones had equivalent gross NO_x conversion, lower NH₃ and N₂O yield, and significantly higher net conversion of NO_x to N₂ than an all-LNT design or a standard LNT+SCR configuration, where all of the SCR volume is placed downstream of the LNT. The lower NH₃ emissions of the two multi-zone designs relative to the standard LNT+SCR design were attributed to the improved balance of NO_x and NH₃ in the SCR zones. The lower N₂O emissions with the multi-zone designs were attributed to the fact that a significant portion of the NO_x conversion occurred in the SCR zones in the front of the catalyst system, as the copper/zeolite SCR catalyst is less prone for generating N₂O than the platinum-containing LNT. The two multi-zone designs also had improved HC conversion relative to the standard LNT+SCR design. This was due to the adsorption of some of the HC on the zeolite-containing SCR zones during a rich purge; some of this adsorbed HC was released during the subsequent lean period and oxidized by the downstream LNT. For the same overall length, a four-zone LNT+SCR design was nearly as beneficial as an eight-zone design. This has important implications for manufacturability, as a four-zone design would be considerably simpler to produce than an eight-zone design.