Study On The Preparation Of Mesoporous Alumina Basic Rare-earth Composite Oxides And Their Structure-performance Relationship

Increasingly stringent vehicle emission regulations require vehicles to achieve ultra low emission or zero emission,and the key to solve this problem is exhaust gas purification technology in the cold start phase.The most promising solution is to design new materials with special components and special pore structure to improve its catalytic activity and selective adsorption performance at low temperature by supporting precious metals on it.In order to design and develop new materials with rich porous structure,good thermal stability and a large oxygen storage capacity,the interactive nature of CeO2 and La2O3 with γ-Al2O3 was studied and mesoporous Al2O3-CeO2-ZrO2-Ti02(AlCeZrTiOx)mixed oxides and mesoporous Al2O3-La2O3-ZrO2-TiO2(AlLaZrTiOx)mixed oxides were synthesized in this paper.Also,application research of new mesoporous materials was carried out by evaluating of sample,CNG(compressed natural gas)engine bench testing and gasoline vehicle emission experiment.The main works are as follows:1.Study on the interaction of CeO2/γ-Al2O3 system prepared by impregnation method.The interaction between CeO2 and γ-Al2O3 in CeO2/γ-Al2O3 system was characterized by XRD,HR-TEM,N2 physisorption apparatus,H2-TPR technology and H2-TG thermogravimetric analysis.The monolayer dispersion threshold of CeO2 in the surface of γ-Al2O3 was estimated at different dispersion temperature,and was about 2g CeO2/100g·γ-Al2O3.With the dispersion temperature increasing,the monolayer dispersion threshold decreased slowly.The surface pore structure and redox properties of CeO2/γ-Al2O3 system have a threshold effect before and after the dispersion threshold.2.Study on the interaction of CeO2/γ-Al2O3 system prepared by co-precipitation method.The interaction between CeO2 and γ-Al2O3 in CeO2/γ-Al2O3 system was studied by XRD,HR-TEM,N2 physisorption apparatus,H2-TPR technology and H2-TG thermogravimetric analysis.Due to the fact that the mixed oxides system cannot be explained by the monolayer dispersion theory,the concept of bulk dispersion,bulk dispersion threshold and threshold effect is proposed.The threshold of Ce02 in CeO2/γ-Al2O3 system is estimated to be between 6g·CeO2/100·γ-Al2O3 and 9g·CeO2/100g·γ-Al2O3,and found that the bulk dispersion threshold decreases with the increase of calcination temperature.The crystal structure and redox properties of CeO2/γ-Al2O3 system have a threshold effect before and after the bulk dispersion threshold.3.Study on the interaction of La2O3/γ-Al2O3 system.The interaction between La2O3 andγ-Al2O3 3 in La2O3/γ-Al2O3 system prepared by mechanical mixing method,impregnation method and coprecipitation method was characterized by XRD,HR-TEM and N2 physisorption apparatus.The results showed that the La2O3/γ-Al2O3 system prepared by coprecipitation method has the best thermal stability and dispersibility.Because the reaction between La2O3 and γ-Al2O3 occurs at less than 550℃ and obtained LaAlO3 product,which reduces the content of γ-Al2O3 in system.While γ-Al2O3 is the decision components which embodies the specific surface area and pore structure characteristics of La2O3/γ-Al2O3 system.Therefore,the content of La has a great impact on the thermal stability of the La2O3/γ-Al2O3system,and the system with high La content is very poor in surface area and pore volume.At the same time,the experimental results also showed that La2O3/γ-Al2O3 system prepared by the impregnation method has good thermal stability and dispersibility when the La content is less than 20g·La2O3/100l·γ-Al2O3 for the surface reaction of La2O3 in γ-Al2O3 surface,and the system prepared by the coprecipitation method has good thermal stability and dispersibility when the La content is less than 30g·La2O3/100g·γ-Al2O3 due to the bulk reaction between La2O3 andγ-Al2O3.4.New mesoporous AlLaZrTiOx composite oxides and mesoporous AlCeZrTiOx composite oxides was prepared by sol-gel method,and their surface area,pore structure and redox ability were characterized by N2 physisorption apparatus,small angle X-ray diffraction,X-ray diffraction,HR-TEM,H2-TPR and H2-TG technology.The results showed that the AlLaZrTiOx and AlCeZrTiOx composite oxides are mesoporous materials,the most probable distribution of pore size is between 8～10nm,the hole shape is worm-like,optimum calcination temperature is 800℃,and their thermal stability are good.The mesoporous AlCeZrTiOx composite oxides has high oxygen storage capacity,and its oxygen storage capacity is 487 umol/g even been calcined at 1000℃ for 4 hour.At the same time the synthetic process of mesoporous rare-earth composite oxides were explained by cooperative formation mechanism,and put forward the definition of"particles gap template" method.This definition is more accurate to illustrate the formation process of mesoporous rare-earth composite oxides than soft template method or colloidal crystal template method.5.The Pd-Rh/AL(Pd/AlLaZrTiOx+Rh/AlLaZrTiOx)catalysts,Pd-Rh/AC(Pd/AlCeZrTiOx+Rh/AlCeZrTiOx)catalysts and Pd-Rh/ALC(Pd/AlLaZrTiOx+Rh/AlCeZrTiOx)catalysts were prepared by supporting Pd and Rh on these mesoporous materials with different oxygen storage capacity.Catalysts were tested by sample evaluation method,CNG engine bench testing and gasoline vehicle emission experiment.The results showed as follows:For the Pd-Rh/AL catalysts with poor oxygen storage capacity,it is beneficial to the catalytic reduction of NOx under lean burn condition,good for catalytic combustion of C3H8 and CH4 near the theory air-fuel ratio and favorable to the complete oxidation of CO and C3H6 under the condition of rich oxygen.For the Pd-Rh/AC catalysts with good oxygen storage capacity,it is favorable to the catalytic reduction of NOx under the condition of poor oxygen,good for complete oxidation of C3H6 under the condition of rich oxygen,and beneficial to catalytic combustion of CH4,CO and C3H8 near the theory air-fuel ratio.For the Pd-Rh/ALC catalyst with moderate oxygen storage capacity,it is beneficial to the catalytic reduction of NOx under lean burn condition,good for catalytic reaction of NOx,C3H8 and CH4 near the theory air-fuel ratio and favorable to the complete oxidation of CO and C3H6 under the condition of rich oxygen.The Pd-Rh/ALC catalyst prepared with mesoporous rare-earth composite oxides can be successfully applied to exhaust purification of CNG vehicle.The conversion efficiency of CH4 is as high as 97.8%.After thermal aging,the conversion efficiency of CH4,CO and NMHC(no-methane hydrocarbons)still reach 94.7%,82.4%and 80.0%.Although the treatment effect of NOx is not very good,the highest conversion efficiency can reach about 75%.Also,the Pd-Rh/ALC catalyst is suitable for the purification of gasoline vehicle exhaust gas.Fresh Pd-Rh/ALC catalyst play excellent catalytic performance for treating CH4,NMHC,CO and NOx,after aging,it exercises certain negative influence to the catalytic activity on CH4 and CO.