Optimization of LNT/SCR Aftertreatment Catalysts: Investigation of NH3 Storage/Release Dynamics Over Fe-Loaded Zeolites

Lean burn engines like spark ignition direct injection engines (SIDI) offer significant fuel economy benefits and reduced CO₂ emissions. A major challenge is the difficulty of reducing effectively lean NO_x as the conventional three-way catalyst does not work for NO_x reduction under lean conditions. An aftertreatment system containing lean NO_x trap (LNT) catalysts provides one of the promising options to eliminate NO_x when the engine works under so-called lean–rich repeatable operation cycles. LNT catalyst components store NO_x during the lean cycle, and then the stored NO_x is released and catalytically reduced when the engine operates under rich condition. Therefore, the role of LNT catalyst in the both rich and lean cycles is equally critical in terms of the ultimate NO_x removal efficiency of aftertreatment systems.

During fuel combustion in the engine, steam is generated and represents one of the major components in automotive exhaust. While steam effects have been widely investigated for TWC catalysts, less is known about how steam effects NO_x reduction over LNT catalysts. In previous studies, we found that during the rich cycle the primary NO_x reduction product over Pt–BaO/Al₂O₃ was NH₃. Based on results from combined temperature programmed reduction of stored NO_x by H₂ and from measurements of the NO_x reduction efficiency and NH₃ selectivity versus reaction temperature over Pt–BaO/Al₂O₃, we suggested that steam causes NH₃ formation via the reduction of NO_x by H₂ that is generated by water gas shift when CO used as reductant or by steam reforming when hydrocarbon (C₃H₈) used as reductant. Thus, a method was established using a dual-bed reactor system containing a zeolite catalyst down-stream of the LNT bed to capture NH₃ generated during the rich cycle. The zeolite serves then as NH₃-SCR catalyst to eliminate lean NO_x by using desorbed NH₃ as reductant when the rich cycle is switched to the lean cycle.

Zeolites, particularly Fe- and Cu-containing ZSM-5 and Beta, have showed remarkable lean NO_x reduction ability for NH₃ used as reductant (NH₃-SCR). So far, extensive NH₃-SCR studies have been carried out over these types of zeolite catalysts. However, many fundamental issues are still poorly understood regarding the use of NH₃ generated during the LNT rich cycle to further reduce NO_x in the lean cycle over zeolite catalysts. In order to optimize LNT/SCR aftertreatment systems for SIDI application, in this work, NH₃ storage/release will be investigated over zeolites and compared with Fe-loaded zeolites that prepared using different Fe loadings. The effects of preparation method, aging and operation conditions on NH₃ storage/release dynamics will also be evaluated. The role and status of Fe species in zeolite will be discussed in light of the performance of NH₃ release or conversion.