Selective Catalytic Reduction Of NO_x With NH₃on Manganese-based Catalysts For Diesel Emission Control

Diesel exhaust emissions seriously affect the urban atmospheric environment.NH₃-SCR is an effective aftertreatment technology for removal of NO_xin exhaustgas.The most important thing in NH₃-SCR technology is the development of efficientSCR catalyst. So this paper mainly researches on one of SCR catalysts, manganeseoxides, by doping SO₄^2-or mixing cerium oxide to improve the performance of thecatalysts. The specific contents are as follows:（1）The SO₄^2--MnO_x/TiO₂catalysts with different SO₄^2-loadings weresynthesized via the sol–gel method and the NH₃-SCR reaction was investigated withthe catalysts above. The SCR results prove that, compared with MnO_x/TiO₂,SO₄^2--MnO_x/TiO₂catalysts exhibit much better catalytic performances for the NO_xreduction at200⁴00°C. Moreover, the catalyst with the mole ratio ofS:Mn:Ti=1:4:10exhibits the best catalytic performance and the NO_xconversionexceeds90%at150³50°C. The catalysts’ properties were characterized with XRD,BET, NH₃-TPD and H₂-TPR. The results indicate that the main reason for the changeof catalytic performances after incorporating sulfate to MnO_x/TiO₂is the increase ofMnO_x/TiO₂acidity and the decrease of its oxidability. However, when the temperatureis below300°C, air velocity impacts of NO_xconversion rate and the air velocityincrease will lead to significantly NO_xconversion rate reducing. When thetemperature increases, this effect decreases continuously.（2） To develope a pre-oxidation catalyst for cooperating with V2O5/WO3/TiO₂catalyst in NH₃-SCR system, this paper prepared MnO_x-CeO₂composite oxidescatalyst by citric acid sol-gel method and discussed the Mn, Ce ratio and calcinationtemperature effect on oxidation activity of the catalyst.First，at the same calcining temperature （400°C）, compared with single MnO_x,or CeO₂, MnO_x-CeO₂composite catalysts exhibit much better oxidation activity in thelow temperature. When the reaction temperature is below300°C, MnO_x-CeO₂catalyst oxidation activity increases with Mn content increasing: Mn3Ce3> Mn2Ce3>Mn1Ce3.What’s more, the Mn2Ce3catalyst’s NO oxidation rate is much closer to50%, so I selected Mn2Ce3as the best ratio pre-oxidation catalyst. The result of XRDshows, in the different proportion of MnO_x-CeO₂, MnO_xmainly exists in the amorphous state and CeO_xexists as cubic fluorite CeO₂.Second, the optimal ratio catalyst（Mn2Ce3） is calcined at400°C,500°C,600°C,700°C, to compare the oxidation activity of the catalyst.To some extent, thecalcination temperature have some effect on the catalytic oxidation activity. Theoptimum calcination temperature was500°C, followed by400°C. When thetemperature is400°C, the oxidation rate of NO is cloest to50%. So we think that thereasonable calcination temperature is400°C. Depend on the result of XRD, we knowthat the CeO₂inhibits the crystallization of MnO_x. MnO_xexist in amorphous state athigher calcination temperature.