| Recent years,with domestic attentions to nitrogen oxide pollution and the promulgation and implementation to relevant laws and regulations,our country have been more and more stricter with NOx emissions standard.NH3-SCR?the selective catalytic reduction of NOx with NH3?is one of the most mature and effective technology in the abatement of NOx from diesel exhaust.The most widely used commercial catalyst system is V2O5–WO3?MoO3?/TiO2 which has the high operation temperature,the relatively narrow operation temperature window?350-400 °C?,the easy SO2 oxidation to SO3,the low N2 selectivity and the biotoxicity of V2O5.Such a catalyst system is inadequacy industrial application.Therefore,we need to be devoted to the development of catalyst which has a high SCR activity and resistance to SO2 at low temperature.However,Mn based catalysts show an excellent low-temperature SCR activity and Fe based catalysts possess a good resistance to SO2.Based on this,Mn as the main active component,Fe as a promoter and the HZSM-5 as support were added respectively to prepare Mn-Fe/ZSM-5 catalysts with high reactivity for NH3-SCR of NO.By optimizing the preparation conditions,we got the best catalyst.Taking a series of characterization analysis,we explored the effects of different methods for the SCR activity and resistance to SO2 of Mn-Fe/ZSM-5 catalysts,which was aimed at seeking a high activity and resistance to SO2 at low temperature for the diesel engine exhaust purification.First of all,Mn-Fe/ZSM-5?P-CVD?catalysts were synthesized through precipitation-chemical vapor deposition method.By optimizing the exchange time of FeCl3 vapor,flow rate of N2,addition of FeCl3 raw material,loading amounts of Mn component and supporting order of Mn and Fe,we examined effects of these factors on Mn-Fe/ZSM-5?P-CVD?.The results showed that Mn-Fe/ZSM-5?P-CVD?catalyst exhibited an excellent SCR activity when the exchange time was 2 h,the N2 flow rate was 20 mL/min,the FeCl3 addition was 6 g,the supporting order was firstly Fe and then Mn,and the loading amounts was 10 wt%.NO conversion of this best Mn-Fe/ZSM-5?P-CVD?catalyst reached over 90% at 180-450 °C.Then,we chosen the highest SCR activity of Mn-Fe/ZSM-5?P-CVD?catalyst which compared with Mn-Fe/ZSM-5?CP?and Mn-Fe/ZSM-5?IM?catalysts synthesized respectively through ammonia co-precipitation and impregnation method on their catalytic activity and relative characterization.The test and characterization results demonstrated that Mn-Fe/ZSM-5?CP?catalyst showed an excellent and low temperature activity.NO conversion of that catalyst reached to over 90% at 120-390 °C temperature window but achieving the conversion of 90% over Mn-Fe/ZSM-5?IM?and Mn-Fe/ZSM-5?P-CVD?catalysts were respectively at 150-360 °C and 180-450 °C.Mn-Fe/ZSM-5?CP?catalyst exhibited highest activity due to better dispersion of active species on the catalyst surface,higher specific surface area,more amounts of NO adsorption,stronger NH3 adsorption and appropriate ratio of Mn4+/Mn3+.The mechanism indicated that the NH3-SCR reaction over Mn-Fe/ZSM-5 catalyst was mainly controlled by the L-H mechanism at low temperature range?< 150 °C?and E-R mechanism at high temperature range?> 150 °C?.Therefore,the enhanced simultaneously NO and NH3 adsorption should be the main reason for the higher SCR activity of Mn-Fe/ZSM-5 catalyst in the lower temperature range?< 150 °C?.However,the increase of adsorbed NH3 could facilitate the promotion of SCR activity in the higher temperature?> 150 °C?.Last,introducing SO2 to the mixture gas,we contrasted the sulfur resistance of these three Mn-Fe/ZSM-5 catalysts and analyzed the reason of the difference of sulfur resistance through a variety of technical characterization.The results indicated that the NO conversion of Mn-Fe/ZSM-5?P-CVD?catalyst was up to over 90% at rang temperature 240-390 °C and maintained 85% in the stability of 40 h at 240 °C.The activity of Mn-Fe/ZSM-5?P-CVD?catalyst was recovered when cut off the SO2 gas.However,NO conversion of Mn-Fe/ZSM-5?CP?and Mn-Fe/ZSM-5?IM?catalysts attained 90% only when the reaction temperature was at 390 °C.From the characterization results to analyze the reason of sulfur resistance Mn-Fe/ZSM-5?P-CVD?catalyst,we found that it had the least reduction extent of special surface area,the weakest decrease extent of NO adsorption,the strongest ability of NH3 adsorption,the least ratio of S6+/S4+,the lowest amounts of ammonium sulfate deposited and the suitable proportion Mn4+/Mn3+.Mn-Fe/ZSM-5?P-CVD?catalyst showed a high activity at 240-450 °C?> 150 °C?in presence of SO2.We speculated that this catalyst was abided by E-R mechanism at 240-450 °C and the adsorption ability of NH3 became the major factor for NH3-SCR.The results of NH3-TPD showed that Mn-Fe/ZSM-5?P-CVD?catalyst possessed a stronger NH3 adsorption ability so it owned high resistance to SO2. |
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