(498b) Mn-Fe/ZSM5 as a Low Temperature SCR Catalyst for Diesel Exhaust Aftertreatment

              When diesel engine with high fuel efficiency is operated under lean condition, large amounts of NOx, a source of acid rain and urban photochemical smog, are emitted to atmosphere [1].  Since engine modification is hardly ready yet to meet the recent air pollution regulation including EURO V and SULEV, diesel exhaust aftertreatment may be unavoidable for removing the air pollutants including NOx.  Urea-SCR is the most effective and promising technology to remove NOx without any fuel penalty efficiency [2].

               A low temperature Urea-SCR catalyst may be required to meet the future emission standards for diesel engine operated under urban driving condition.  Particularly, the temperature of the catalyst during the NEDC test (The New European Driving Cycle) remains below 150 ^oC in 70% of the cycle [3].  CuZSM5 and V₂O₅/TiO₂ catalysts have been commonly regarded as a commercial catalyst for the Urea-SCR system.  However, these catalysts may not be well qualified for their commercial application, mainly due to their weak low temperature activity and the use of Cu and V classified as a heavy metal for preparing the catalysts [4-5].  Mn-Fe/ZSM5 as an eco-friendly low temperature Urea-SCR catalyst has been developed to resolve the current catalyst issues raised for its application to diesel engine. 

              MnOx-based catalytic system has been commonly reported as a low temperature SCR catalyst [6-8].  Most of them were active over the ranges of the reactor space velocity from 8,000 to 30,000 hr^-1 and the reaction temperature from 80 to 250 ^oC under the feed gas condition without H₂O, mainly due to their application to the stationary source of NOx.  Mobile diesel engine, however requires wider operating temperature window (150~350 ^oC) and higher reactor space velocity due to its transient engine operation under excess lean air condition [3,9].  The exhaust gas temperature from diesel engine dynamically varies with respect to the engine size, light (150~250 ^oC) and heavy duties (200~350 ^oC ).  Also, water seriously reducing the deNOx activity of the catalyst is always included in the feed gas stream containing NOx to be removed [8].  The purpose of the present study is to develop an effective low temperature SCR catalyst, mainly Mn-Fe/ZSM5 catalyst for its diesel application under the realistic reactor operating condition including high reactor space velocity (100,000 hr^-1) and wide operating temperature window (150 ^oC~500 ^oC) under the feed condition containing H₂O.

              The catalytic systems screened in the present study including Mn-Fe/ZSM5, Mn/ZSM5, Mn-Fe/TiO₂, and Mn-Fe/Al₂O₃, were prepared by incipient wetness method for their comparative study [10].  CuZSM5 recognized as a standard and target SCR catalyst was prepared by ion-exchange method [11].  The deNOx activity of the catalysts has been examined over a packed-bed flow reactor system under the feed gas containing 500ppm NO, 500ppm NH₃, 5% O₂, 10% H₂O and N₂ balance.  As shown in Fig. 1, the Mn-Fe/ZSM5 catalyst reveals the highest NO conversion of 80% at 160 ^oC, while CuZSM5 catalyst shows 50%.  The deNOx performance of the Mn/ZSM5 catalyst has been further enhanced by adding the 2^nd metal (Fe) onto the catalyst, probably due to the interaction of Mn with Fe improving the dispersion of Mn on the catalyst surface [10].  Mn-Fe/ZSM5 shows the highest deNOx performance including N₂ selectivity among the catalysts prepared in the temperature range upto 350 ^oC.  However, the deNOx activity of Mn-Fe/ZSM5 immediately decreases by further increase of the reaction temperature, mainly due to the competitive NH₃ oxidation reaction.  In addition, Mn-Fe based Al₂O₃ and TiO₂ catalysts reveal inferior deNOx activity and N₂ selectivity to the Mn-Fe/ZSM5 catalyst containing the identical Mn and Fe contents.  Large amount of N₂O has been formed during the course of SCR reaction over Mn-Fe based Al₂O₃ and TiO₂ by the decomposition of NH₄NO₃ (NH₄NO₃à N₂O + 2H₂O) and the oxidation of NH₃ (4NH₃ + 2O₂à N₂O + 3H₂O) at low and high reaction temperatures, respectively [12].  Although the catalytic performance of the Mn-Fe/ZSM5 is relatively low in the temperature region higher than 350 ^oC compared to that of the CuZSM5 catalyst, the catalyst still holds an excellent low temperature activity, proper operating temperature window and high N₂ selectivity for reducing NOx from diesel engine.  It should be noted that the highest temperature of the exhaust stream from heavy duty diesel engine is typically 350 ^oC.

              The formation of MnO₂ recognized as an active reaction site for SCR reaction over the Mn based catalysts prepared in the present study [6-8], has been identified by XPS and XAFS.  The surface atomic concentration of Mn on Mn/ZSM5 increased from 2.6 to 5.1 by adding Fe onto the catalyst.  The concentrations of Mn on Mn-Fe/TiO₂ and /Al₂O₃ catalysts, however were 3.5 and 2.8, respectively lower than that of Mn-Fe/ZSM5.  It is well known that the surface concentration of Mn over the Mn-based catalysts is critical for their low temperature SCR activity [6].  The high surface concentration of Mn on the surface of Mn-Fe/ZSM5 may be responsible for the high deNOx performance of the catalyst.  In addition, the improvement of the Mn dispersion on the surface of Mn-Fe/ZSM5 has been determined by EXAFS study identifying the alteration of the coordination number of the Mn-Mn bond by the addition of Fe to Mn/ZSM5 catalyst [13].  In addition, the Mn based catalyst prepared in the present study has been characterized by XRD, H₂-TPR, TPD, in-situ FT-IR to elucidate their high deNOx performance.

Figure 1. DeNOx activity of Mn based catalysts.  Feed condition: 500ppm NO, 500ppm NH₃, 5% O₂, 10% H₂O and N₂ balance; Reactor SV: 100,000h^-1

References

1. W.S. Epling, L.E. Campbell, A. Yezerets, N.W. Currier, E.P. James, Catal. Rev. 46 (2004) 163.

2. K.-I. Shimizu, A. Satsuma, Appl. Catal. B: Enviorn., 77 (2007) 202.

3. F. Klingstedt, K. Arve, K. Ernen, D.Y. Murzin, Acc. Chem. Res., 39 (2006) 273.

4. Kim, B-E., Nevitt, T., and Thiele, D.J., Nat. Chem. Bio., 4 (2008) 176.

5. Li, Y., Cheng, H., Li, D., Qin, Y., and Wang, S., Chem. Comm., 12 (2008) 1470.

6. P. G. Smirniotis, D. A. Pena, B. S. Uphade, Angew. Chem. Int. Ed., 40 (2001) 2479.

7. P. R. Ettireddy, N. Ettireddy, S. Mamedov, P. Boolchand, P. G. Smirniotis, Appl. Catal. B:Environ., 76 (2007) 123.

8. G. Qi, R. T. Yang, Appl. Catal. B:Environ., 44 (2003) 217.

9. M. Koebel, M. Elsener, M. Kleemann, Catal. Today, 59 (2000) 335.

10. G. Qi, R. T. Yang, R. Chang, Catal. Lett., 87 (2003) 67.

11. J. H. Park, H. J .Park, J. H. Baik, I.-S. Nam, C. H. Shin, J.-H. Lee, B. K. Cho, S. H. Oh, J. Catal., 240 (2006) 47.

12. G. Li, C.A. Jones, V.H. Grassian, S.C. Larsen, J. Catal., 234 (2005) 401.

13. R. Radhakrishnan, S.T. Oyama, J.G. Chen, K. Asakura, J. Phys. Chem. B, 105 (2001) 4245.