Study Of NO Catalytic Oxidation On High-silica ZSM-5Modified And Its Monolithic Catalyst Prepared In-situ Hydrothermal Method

Nowdays, the most popular technologies used to reduce NOx emission are selective catalytic reduction(SCR), absorption and adsorption. The removal efficiency of NOx in these technologies is beneficial to maintain a high ratio of NO2/NOx in the feed. So it is important to oxidize NO to NO2. The results in early researches indicated that high catalytic activity in NO oxidation reaction can be obtained for ZSM-5under room temperature, but no activity under higher temperature condition. To overcome this disadvantage, we prepared Mn-Ce/ZSM-5catalyst with impregnation method and evaluated its activity. In order to lower the pressure drop and improve the efficiency of mass transfer in the NO oxidation fixed-bed reactor, high silica ZSM-5/cordierite honeycomb ceramic monolith catalysts were prepared with in-situ synthesized hydrothermal method. Further, Mn-Ce/ZSM-5/cordierite monolith catalysts were prepared with impregnation method. The catalytic activity for NO oxidation was tested. Conclusions were listed as followed: Mn-Ce/ZSM-5catalysts exhibited thecatalytic activity for NO oxidation when the temperature ranged widely from10℃to280℃. The NO conversion decreased from79%to10%with the reaction temperature increased from10℃to100℃, but increased from10%to79%when the reaction temperature increased from100℃to280℃.NOx species on Mn-Ce/ZSM-5after NO adsorption and oxidation were studied by means of TPD, H2-TPR and In-situ DRIFTS to obtain the reaction mechanism. Two different mechanisms were proposed for NO oxidation at room and higher temperature respectively:a. at low temperature: NO(?)NO(ad)or(NO)2(ad)â†'2NO2(g)(?)N2O4(?)NO(ad)++NO3-(â… )b. at higher temperature:1/2O2+v(?)O(ad)(â…¡) NO+v(?)NO(ad)o (â…¢) NO(ad)+O(ad)(?)NO2(ad)(â…£) NO2(ad)(?)NO2(g)+v (â…¤) which could explain the difference of NO oxidation over Mn-Ce/ZSM-5catalysts well at different temperature window zone.ZSM-5/cordierite were prepared on the cordierite support with in-situ synthesized hydrothermal method. The results showed that a coating of ZSM-5grew preferably on the cordierite support.The optimal preparation conditions were confirmed as follows:the cordierite support was pretreated by37%hydrochloric acid solution; the optimization ratio of n(SiO2):n(H2O) was1:45; the optimization ratio of n(SiO2):n(TPAOH) was1:0.2;Crystallization proceeded under the facts of175℃with3days. the crystallization silicon sol was prepared the molar composition of n(SiO2):n(TPAOH):n(H2O)=1:0.2:45.Finally, the activity tests of ZSM-5/cordierite catalyst showed that ZSM-5/cordierite monolith catalyst gave high catalytic activity for NO oxidation at low temperature and exhibited resistance to moisture. Mn-Ce/ZSM-5/cordierite catalyst exhibited catalytic activity for NO oxidation at a broader activity temperature window zone.