| Soot and NOx are the main pollutants in the emission of diesel engines.DPF,SCR and NSR are mostly adopted in diesel engine after-treatment technology for reducing the soot and NOx emissions.The catalytic oxidation method,which is employed for reducing light-off temperature of the soot and partly oxidizing NO,is supposed to be beneficial to DPF,SCR and NSR.In this paper,Mn-Ce/?-Al2O3 catalysts are selected as the main research object,through which thermo-gravimetric analysis method was taken to study its influence on light-off temperature and oxidation rate peak temperature of the soot.The NO oxidation activity tests were also carried out to explore the impact of temperature,different manganese cerium ratios and NO concentration on the NO oxidation activity in low temperature,which provided the basis data for the application of Mn-Ce/?-Al2O3 catalysts.xMnyCe/?-Al2O3 catalysts?x:y=mole ratio,x=4,5,6,7,8,9;y=10?were synthesized by a sol-gel method.A combination of characterization techniques,namely XRD,SEM,TEM,XPS and H2-TPR,were adopted to analysis its degree of crystallization,surface morphology,dispersion of different constituents regarding on support and active components,surface element valence and reducibility,respectively.The characterization results illustrate that CeO2 existed in cubic fluorite structure and manganese found in the catalysts mainly presented in the form of MnO2 and Mn2O3,and manganese cerium solid solution generated,by which part of Mnn + entered into the lattice of CeO2.The activated particles of catalysts dispersed evenly on the surface of the carrier,whose diameter range was from 100 nm to 200 nm.Furthermore,the existence of chemisorbed oxygen O-?O??and lattice oxygen O2-?O??were also found in the catalyst surface,and O? prevailed even more better mobility over O?,which was believed that the larger quantity of O?,the better catalytic activity of catalysts could possess.Among all of catalysts,6Mn10Ce/?-Al2O3 catalyst gained the highest value of O?/?O?+O??,and its reduction temperature obtained in the H2-TPR was the lowest.The results of TG experiments show that the Ti and Tm of the pure soot,of which corresponded to 469.4 ? and 623.8 ?,reduced around 110 ? and 54 ? by 8Mn10Ce/?-Al2O3 whose Ti and Tm were 360 ? and 570 ?,respectively.Nevertheless,soot oxidation activity of 4Mn10Ce/?-Al2O3 and 9Mn10Ce/?-Al2O3 catalyst was considerably inferior to others.It was observed that the Ti and Tm decreased about 55 ? and 30 ? in the comparison of pour soot,separately.Therefore,the conclusion could be made that Mn-Ce/?-Al2O3 catalysts?x:y=mole ratio,x=4,5,6,7,8,9;y=10?reduced the light-off temperature and oxidation rate peak temperature of the soot.The results of NO oxidation activity tests show that the NO conversion rate of catalysts increased gradually within the temperature window of 250 ?350 ?,and it reached its optimal value when the temperature was at 350 ?.However,the NO conversion rate would decreased moderately if the temperature rising continually.The NO oxidation activity increased in order of 9Mn10Ce/?-Al2O3,8Mn10Ce/?-Al2O3,4Mn10Ce/?-Al2O3,5Mn10Ce/?-Al2O3,7Mn10Ce/?-Al2O3,6Mn10Ce/?-Al2O3.Especially,the NO conversion rate of 6Mn10Ce/?-Al2O3 catalysts acquired 22.04% at 250 ? and 60% at 350 ?.Moreover,NO oxidation activity tests under different NO concentrations were carried out over 6Mn10Ce/?-Al2O3 catalyst.It shows that the effect of NO concentration on NO conversion rate was the greatest at the temperature of 250 ?.Compared with 500×10-6 v/v NO concentration,the NO conversion rate decreased about 2% and 6.4% under the condition of NO concentration was 750×10-6 v/v and 1000×10-6 v/v,separately.However,when the temperature is higher than 350 ?,the influence of NO concentration on NO oxidation activity was not obvious over 6Mn10Ce/?-Al2O3 catalyst. |
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