The Preparation And Measurement Of SCR Catalysts For The Purification Of Diesel Exhaust

The diesel engine has been widely used in the broad areas of marine power, generationpower, irrigation, vehicle dynamics and so on, especially in the vehicle power, the trend ofglobal automotive power of "diesel" has been increasingly formed. The combustion of dieselengine determines that the fuel economy is superior to gasoline. But as China’s auto possessionincrease, the diesel exhausts emissions brings the environment pollution is more and moreserious. Harmful components in diesel exhaust are: carbon monoxide （CO）, hydrocarbons （HC）,nitrogen oxides （NOX）, sulfur dioxide （SO₂）, odor, other harmful gases and a variety of solidparticles （PM）. According to statistics,65percent of the total emissions of NOx of China’smobile come from diesel vehicles. In order to control the increasingly serious trend ofenvironmental pollution, diesel exhaust emissions standards in China will become increasinglyharsh. In the area of diesel engine exhaust purification, there are two major relatively maturetechnologies routes in the international. One is the U.S. engine companies on behalf of the EGR（Exhaust Gas Recirculation）+DPF route, and the other is the European engine companies onbehalf of optimization of the combustion+SCR route. Since in most regions of China the salesof the sulfur content of diesel are higher, and it is difficult to decrease the sulfur content of dieselwhich can meet the DPF requirements. Therefore in the choice of technology route, we need totake into account of China’s specific national conditions. So take use of the route of optimizedcombustion+SCR, first the technology route can reduce engine fuel consumption by5%to7%,second it is more easily to gain the urea solution in China. In the SCR system, as a key part ofthe study is to select the appropriate and efficient SCR catalysts.Based on the current state of development of the SCR catalyst and future developmentrequirements, the V₂O₅–WO₃/TiO₂catalysts was synthesized by the combine of sol–gel methodand impregnation method and used for selective catalytic reduction （SCR） of NOXwithammonia. The physical and chemical properties of the catalyst samples were characterized byXRD and BET test, and using the laboratory-made exhaust gas simulation system to test theactivity of the catalyst samples. The experimental results showed that when the V₂O₅content is 3%, the catalyst showed higher catalytic activity and a wide active window. When the O₂concentration is higher than2%, O₂concentration has less impact on the catalytic activity. At thelow temperature, high air velocity causes the catalyst for NOx removal rate with greaterreduction, but at the high temperature conditions, the air velocity has little effect on the NOxremoval rate. Take the use of the above-mentioned catalyst sample preparation methods, and it ismore conducive to the catalyst coating on the metal carrier. In order to better simulate the actualsituation, we made the small metal SCR catalytic reactor by the V₂O₅-WO₃/TiO₂catalystcoating on a metal carrier. Through the homemade SCR reaction system, we test catalyticactivity of the sample. The test results show that the metal SCR catalytic reactor has achievedexcellent result of NO purification.Because the V₂O₅-WO₃/TiO₂traditional catalysts have a good catalytic effect on NOx inthe temperature range of350-450℃, but when the temperature range of100-300℃, Its catalyticactivity is not ideal, therefore we prepared the tungsten modified MnOx/TiO₂as a superiorcatalyst for selective catalytic reduction with NH₃at low temperature by the method of wetimpregnation. By XRD, SEM, and NH₃-TPD characterization methods as well as the activitytests and get the following conclusions: Tungsten modified MnOx/TiO₂catalysts showed highactivity for the low temperature SCR of NO with NH₃. More than99%of NO conversion wasobtained on the Mn（0.2）W（0.1）Ti at temperature range from140to280°C. SEM showedtungsten could provide thermal stability of catalyst and inhibit the occurrence sinteringphenomenon. Furthermore, NH₃-TPD results indicated that tungsten could increase the numberof activity sites and enhance the strength of acidity.