Preparation Of High Surface Area Ce-Zr Rare Earth TWC Material And Catalytic Performance Optimization

With the rapid development of motor vehicle economy,vehicle emission becomes the largest pollution source for nature environment.Domestic and overseas both work to improve emission control standard according to current serious situation.Rare earth mixed oxide catalyst developed to the 3^rd generation since year 2000,this concept is produced base on Ce Zr solid solution but doped with La family.The main composition Ce started to be applied in 2^nd generation of rare earth mixed oxide catalyst,La family dopants started to doped with Ce Zr includes La,Pr,Nd and Y.According to the new regulation of vehicle emission control modified by the European commission,current EU6 regulation will be promoted to be more strict which means vehicle emission control standard will transit to EU7 in near future.China government announced CN6a and CN6b for the moment and the former standard is implemented,and CN6a and b fit with EU6 and 7 respectively.Rare earth mixed oxide catalyst meets serious challenge that the automotive market is asking higher aged thermal stability product.This research works base on Ce Zr La 30/65/5 composition to optimize process parameter control and recipe improvement to dig the potential of producing higher aged thermal stability mixed oxide catalyst for market requirement and vehicle emission regulations.（1）Prepare Ce Zr solid solution with co-precipitation method doped with La³⁺formulates Ce O₂（30%）-Zr O₂（70%-X%）-La₂O₃（X%）,（X=2%-8%）to analyze the difference on product structure,aging performance and other properties.The results show that higher La₂O₃leads no change on crystallite structure but lower crystallite size,pore volume and pore size increases with higher La₂O₃content but tap density maintains and particle size distribution are at the same level.The very promising direction obtained in term of aged thermal stability that higher La₂O₃content leads better aged thermal stability,1000°C/4h SA increases to 53.4m2/g at La₂O₃8%comparing with46m2/g at La₂O₃2%and 1100°C/4h SA increases to 27.5m2/g from 26m2/g.（2）Prepare Ce Zr solid solution with co-precipitation method with fixed dopant ratio of La₂O₃at 5%as Ce O₂（30%）-Zr O₂（65%）-La₂O₃（5%）composition,pilot trails made with process parameter metal concentration X=40-80g/L to study the difference on product structure,aging performance and other properties.The general trend show that lower concentration from 80g/L maximum to 40g/L minimum leads better aged thermal stability.Regarding crystallization and precipitation theory,metal concentration impacts the particle dispersion and homogeneity when precipitation happened in solution,lower metal concentration supports the crystal nucleation to grow up when it is generated during precipitation which makes particle structure more stable and to avoid collapsing at high temperature which maintains the porous structure and high aged thermal stability,the catalyst working efficiency will maintain longer cycle time to support oxidation and reduction reactions for vehicle emission control.The best result is obtained at metal concentration at 40g/L which achieves 53.8m2/g at1000°C/4h and 28.2m2/g at 1100°C/4h.（3）Prepare Ce Zr solid solution with co-precipitation method and fixed dopant ratio of La₂O₃at 5%as Ce O₂（30%）-Zr O₂（65%）-La₂O₃（5%）composition,pilot trails with process parameter agitation speed X=20%-95%to study the difference on product structure,aging performance and other properties.The general trend show high speed leads better aged thermal stability at 1000°C/4h and 1100°C/4h.Generally speaking,stirring provides consistent energy for crystal generation,nucleation and growing up.From a micro perspective,crystal generation phenomenon is that liquid moves from atom,molecule or other crystal unit to crystal surface.From macro perspective,it’s a transportation between heat and quantity for crystal generation.To better understand crystal growth is to understand the inner structure of crystal,defect,growth conditions and the correlation between crystal morphology.To control crystal growth conditions to restrain the inner defects generation to improve crystal quality and performance which will enhance the rare earth mixed oxide catalyst and increase its aged thermal stability.Generally,there are 3 phases of crystal growth,1^st phase is the media achieves supersaturating and supercoiling,2^nd phase is crystal nucleation which is crystal generation phase,3^rd phase is the growth of crystal.Crystal-liquid surface will be formed once crystal nucleation is generated and crystal will start to grow which means the atom or molecule who makes crystal will get together according to the structure.Stirring will support consistent energy for crystal generation and nucleation,and supports crystal nucleation,growth smoothly with well dispersion from the agitation,the aggregation will be restrained in parallel.From this parameter research,95%agitation speed show the best aged thermal stability that 1000°C/4h SA is 52m2/g and1100°C/4h SA is 27.4m2/g.