The Research On Mixed Metal Oxide Catalysts Applied For Eliminating Nitrogen Oxides In Diesel Engine Exhaust With A Selective Catalytic Reduction Technique

The major efforts have been paid to improve the activity of two mixed metaloxide catalysts with monitoring the preparation conditions.A V2O5-WO3/TiO2catalyst was prepared with an impregnation method. Theeffects of the preparation conditions, such as the dipping procedure, calciningtemperature, calcining time and the value of n(oxalic acid)/n(WO3) on the selectivecatalytic reduction of NOxwith ammonia were investigated. Besides, the influence ofthe space velocity on the catalytic activity was examined.The experimental results showed the optimum preparation conditions for theV2O5-WO3/TiO2catalyst was set for the step impregnation procedure, the water bathtemperature80oC, the n(oxalic acid)/n(WO3)=2:1, the calcination temperature500oC, and the calcining time3h. The optimum catalyst showed a good performancewith82.3%NO conversion at200oC, and100%NO conversion between250oC-400oC, still91.8%NO conversion at450oC at a space velocity of50000h-1. If thespace velocity is too high, the NO conversion is lowered.In order to find the new catalysts which have a high catalytic activity at lowtemperatures, a Cu-Mn-Ce catalyst was investigated. A number of samples withdifferent copper contents were prepared with a co-precipitation technique. X-raydiffraction(XRD), Temperature programmed reduction(TPR), Temperatureprogrammed desorption(TPD) techniques were employed for characterizing thematerials. The effects of the copper doping amount and the calcination temperature onthe catalytic activity were examined.The order of the catalytic activity was found as: Cu (0.05)> Cu (0.01)> Cu(0.1)> Cu (0)> Cu (0.5)> Cu (2). The Cu (0.05) catalyst gave the best lowtemperature performance with90.2%NO conversion at100oC and nearly100%conversion between125and250oC at a space velocity of50000h-1.The XRD profiles showed all the samples consisted of characteristic peaksattributed to CeO2with a cubic fluorite structure. The NH3-TPD results showed that asmall amount of copper doping enhanced the NH3desorption amount significantly.The NO2-TPD curves proved that the catalyst showed higher NO conversion desorbsmore NO2. More NO2desorption means more nitrates forms on the catalyst whichresults in more effective SCR catalyst, especially in low temperature. The effect of the calcining temperature on the catalytic activity was alsoinvestigated. The results showed that: at low temperature (<150oC), the order of thecatalytic activity is: Cu (0.05)-400> Cu (0.05)-500> Cu (0.05)-600. At hightemperature (>250oC), the order of the catalytic activity is: Cu (0.05)-600> Cu(0.05)-500> Cu (0.05)-400.The intensity of the XRD peaks increased with the increase of the calcinationtemperatures, suggesting that a high temperature calcination leads to the sintering ofthe catalyst which is the main reason for the activity loss at low reaction temperature.However, this effect also weakens the excess oxidation ability of NH3, resulting in abetter performance of the catalyst at high temperatures as a consequence.