The Study Of Preparation Of Metal Doped LaCoO₃ Perovskite Catalysts For Simultaneous Removal NO And PM

In diesel exhaust pollutants,there exists a relative high proportion of NO_x and PM.However the catalysts for the diesel exhaust purification are insufficient for the simultaneous removal of NO_x and PM at low temperature.Thus,the development of high-performance catalyst to separate simultaneously the NO_x and PM is an important trend.It is well known that LaCoO₃ perovskite catalyst has excellent catalytic activity,so it is a promising catalyst on diesel exhaust purification.For the development of high activity catalyst on the simultaneous removal of NO_x and PM,the LaCoO₃ perovskite catalyst was investigated by different metal modifications in this work.explorationThis work are divided five parts,as follow: 1)We investigated that both the metals doped and doped amounts in A site of LaCoO₃ perovskite catalysts influence on its performance,and that the complexing agent amounts and calcination conditions effects on the activity of catalyst with metal A site doped for simultaneous removal NO and PM.2)Comparing with first part,the doping of B site in LaCoO₃ was also studied under different calcination conditions.3)In this part,the catalysts were simultaneously doped in A and B sites using metal.Then,the relationship between the catalyst activity and metal doped were established.4)We preliminarily studied on the sulfuration tolerance ability of LaCoO₃ catalyst and discussed the effect of metal doped on the sulfuration tolerance ability.5)We explored that the correlations of the surface active species of the catalyst with the different dopings?A site,B site,or A and B site?and the effect of sulfuration on the surface active species.Based on this,we built the relationship between surface active species and the activity of catalysts.The experimental results in detail are as follows:?1?The first part of experiments study the effect of alkaline earth metal?Mg,Ca,Sr,Ba?doped in A site on the property of LaCoO₃ perovskite catalyst.The results showed that the doping of Sr can effectively improve the activity of catalyst at low temperature.The experimental results for different doping amounts of Sr showed that Sr was difficult to be inset into the lattice of catalyst,and with the increasing of Sr doping amount,part of Sr precipitated in the form of strontium carbonate and further reunited on the surface of catalyst.Thus,the La_0.6Sr_0.4CoO₃ showed the highest activity among the La_1-xSr_xCoO₃ catalysts,and the maximum conversion rate of NO was 70%;the study on the additive content of citric acid complexing agent indicated that the ratio of 1:1.5 and 1:2 were more suitable for catalyst preparation.Too much complexi ng agent would increase the carbonate content in the catalyst,which is not conducive to the doped Sr ions into the lattice of catalyst;when the appropriate ratio of citric acid and EDTA is 1:0.2,the catalyst showed highly activity.The study of calcination condition showed that the optimal calcination temperature and time are 700?C and 5h,respectively.If the calcined temperature was too low or the calcined time was too short,there existed the impurities in the catalyst.However,too high temperature or the too long time would cause the collapsed structure,leading the decreasing of the activity,?2?The study on transition metals?Mn,Fe,Ni?doped in B sites of LaCoO₃ catalyst showed that the doping of Fe can promoted the activity of LaCoO₃.When the doping amount of Fe was 0.1,the LaCo_0.9Fe_0.1O₃ possessed high NO catalytic activity at low temperature.However,the activity of LaCoO₃ with the ratio of Fe doping amount of 0.2 was lower for NO reaction than the former,,despite it showed excellent combustion activity of PM at low temperature.Furthermore,we found that the optimal calcination condition and the suitable preparation time were 700°C and 5 hours.?3?The study on the simultaneous doping of metal in both the A and B sites showed that this doping can enhance the activity and the redox ability of LaCoO₃ at low temperature.The redox ability of catalysts after the doping of Sr and Fe was higher than the catalysts with only doping of Fe.The La_0.6Sr_0.4Co_0.8Fe_0.2O₃ showed the optimal activity at low temperature after the doping of two metals,the maximum conversion rate of NO reach 68%;for PM combustion activity,the initial combustion temperature?Ti?was 240 °C,and the maximum combustion rate temperature?Tma x?was 380 °C.?4?The experiments on SO₂ tolerance ability of catalyst showed that the sulfuration has a significant effect on the catalytic activity of NO and PM,and found that the doping of metal can improve the SO₂ tolerance ability of the catalyst.The doping with 40% Sr can improve the SO₂ tolerance ability of the catalyst.Moreover,it can be found that,when the A and B sites were respectively doped the 40% Sr and 20% Fe,the SO₂ tolerance ability can be obviously improved.?5?XPS Characterization showed that the doping of Sr in A site or B site of LaCoO₃ can increase the percentages of Co³⁺ and Oa on the surface of catalyst,leading to the increasing of activity trend.That is to say,the surface active species of catalyst in the reaction were Co³⁺ and Oa.Furthermore,we found that,although the doping of Sr can effectively improve the percentage of Co³⁺ and Oa,Fe doped can only increase the percentage of lattice oxygen and the total ratio of O atom on the surface of catalyst.However,the increase of lattice oxygen reduced the mobility of oxygen species,leading to a decrease of activity.So that,the activity of LaCoO₃ with Sr was better enhanced than that of Fe.Finally,La_0.6Sr_0.4Co_0.8Fe_0.2O₃ was found to possess a high level of content of Co³⁺ and Oa on the surface,which led to a high activity on removal of NO and PM.The investigation about the SO₂ tolerance ability indicated that the Co³⁺ and Oa were covered by SO₂.It is difficult to expose the Co³⁺ and Oa on the surface and react with the NO and PM,so the activity of the catalyst decreased obviously at low temperature.