The Effect Of Crystal Size On Selective Catalytic Reduction Of NO_x By Acetylene Over Zeolites

The lean burn engine has many advantages such as enhancing the using rate of fuel,adjusting fuel's economical efficiency, decreasing emition of CO, CO₂ and hydrocarbon. Atthe lean burn condition, NO_x selective catalytic reduction by hydrocarbon becomes the bestway. The high selectivity becomes a serious problem of global significance in the NO_xselective catalytic reduction by hydrocarbon (SCR) in the temperature of exhaust at the exessoxygen conditions. The aim of this work is to investigate the influence of particle size ofHZSM-5 zeolite on selective catalytic reduction of NO by acetylene. High selective catalystfor the SCR using acetylene as reductant under oxygen-rich conditions for providing the basisfor the application of elimination of NO_x from lean-burn and diesel engines exhaust.CH₄ and C₃H₆ (in the same concentration of carbon as that of C₂H₂) as reductant weretested individually over the nano-particle HZSM-5(25) catalyst for SCR of NO. At 300℃,only 1.5ï¼…and 10.7ï¼…NO conversion to N₂ were obtained individually over thenano-particle HZSM-5(25) catalyst, but 74ï¼…was observed at the same temprature. Theseresults further indicate the promising advantage of C₂H₂ compared with CH₄ and C₃H₆ asreductant for SCR of NO_x in the exsess of O₂The role of Na⁺ ions in the C₂H₂-SCR over the micro-particle Na-HZSM-5 (25) catalystwas discussed. As more Na⁺ loading on HZSM-5, the conversion of NO to N₂ decreased. Andover Na-ZSM-5 (25), no NO conversion to N₂ was observed both in NO_x selective catalyticreduction and NO oxidation reaction. This result indicated that protons in the zeolite play animportant role in the two reactions. And we noticed that the rate of NO₂ adsorption wasaffected by the content of Na⁺ which loaded on HZSM-5. Then we found that the slower therate of NO₂ adsorption, the poorer the activity of catalyst of selective catalytic reduction.The zeolite with nano-particle behaved considerable higher activity than themicro-particle one for the reaction of selective catalytic reduction. And we got the same resultin the reaction of NO oxidation. In this paper, we discuss the reason of the different activitybetween micro-paticle and nano-paticle HZSM-5 detailedly, and use a kind of new theory toexplain the reason. Firstly, the characterization of NH₃-TPD and FTIR implyed that, theHZSM-5 zeolite with micro-particle has more population of acid sites both with weak and strong acidity. It implies that the reactions were retarded by the intracrystalline diffusion ofthe reactants when HZSM-5 zeolite was used as catalyst for the reactions. But the result ofreductant adsorption was that they had the same adsorption rate. So it was revealed that thelarge difference in the activity for the C₂H₂-SCR of NO arising from the particle size ofzeolite was not caused by limited intracrystalline diffusion of the reductant, but that of NO₂,which was strongly supported by the adsorption results obtained over the zeolites. As a result,the strong adsorption of NO₂ in the channels of zeolite limited the intracrystalline diffusion ofreactant molecules, and it was well interpreted that the HZSM-5 zeolite with nano-particlebehaved higher activity than the one with micro-particle for NO oxidation to NO₂ and for theC₂H₂-SCR of NO.In the thesis, Hβzeolites with micro-paticle and with nano-paticle were also compared interms of activity for the C₂H₂-SCR of NO. Similar to those of HZSM-5, the nano-particle onebehaved considerable higher activity than the meso-particle one, especially in lowtemperature range of 300～400℃.