Study On The Simultaneous Removal Of PM And NO_x From Diesel Engines With 3DOM Ce-based Catalysts

Soot particulates（PM）and nitrogen oxides（NOx）emitted from diesel engines poses even greater threat to human and environment.Many researches have been focused on the simultaneous PM and NOx removal,but the catalytic activity was not satisfied.The traditional technology for PM and NOx removal is the combination DPF and SCR technology,which increases the overall system volume,mass and cost of diesel exhaust gas treatment systems.Here,we present a new catalytic technology,named SCRPF（Selective Catalytic Reduction and Particulate Filter）that combines the both steps in one material.High efficiency catalyst is the core for this technology.In this work,three-dimensionally ordered macroporous（3DOM）catalysts were designed to enhance the contact between catalyst and PM.The oxygen levels in diesel exhaust are much higher precluding direct NO_x reduction.Therefore,a reductant has to be added to the after-treatment system.To achieve the high elimination efficient,the catalysts were modified by adding the transition metal into Ce-based catalyst.W/3DOM Ce_0.8Zr_0.2O₂,Ce_0.9-xFe_xTi_0.1O₂,Ce-Fe-ZrO₂ catalysts were designed and various characterization methods were used to analyze its physical and chemical properties and further explore the mechanism of simultaneous elimination of reaction.The main research contents and results are as follows:（1）x%W/3DOM Ce_0.8Zr_0.2O₂（x=0.5,0.8,1,3）catalysts were prepared and employed for the simultaneous removal of PM and NOx from diesel engine exhausts.The contact between solid PM and catalyst active site has been strengthened due to the special 3DOM structure.0.8%W/3DOM Ce_0.8Zr_0.2O₂ possessed superior catalytic activity with a maximum concentration of CO₂ at 408℃ and nearly a completed NO conversion at 378-492℃,which also presented highly catalytic activity even under a space velocity of 50000 h^-1.Furthermore,the stability of the catalyst was also excellent even after being aged 900℃ for 5 h.（2）3DOM Ce_0.8M_0.1Zr_0.1O₂（M=Mn,Co,Ni）were synthesized by CCT method and used for the simultaneous removal.This series of catalysts owned a well-arranged three-dimensional ordered macropore structure and periodic pore channels.The effective contact area of the catalyst and PM was obviously enhanced,and the catalytic activity was much improved.3DOM Ce_0.8Mn_0.1Zr_0.1O₂ catalyst gave a maximum concentration of CO₂ at 402℃ for PM combustion,and showed excellent NH₃-SCR performance in the temperature range of 374-512℃.The specific 3DOM architecture,high Ce³⁺/Ce⁴⁺ratio and chemisorbed oxygen species amounts,good low-temperature reducibility as well as the abundant of acid sites are responsible for the excellent catalytic efficiency of Ce_0.8Mn_0.1Zr_0.1O₂ sample for the simultaneous removal of PM and NOx from diesel engines.（3）The hierarchically ordered mixed Fe-Ce-Zr-oxides were prepared for the simultaneous capture and oxidation of soot and reduction of NOx by ammonia in a single step.The optimized material can effectively trap the model soot particles in its open macroporous structure and oxidize the soot below 400℃,while completely removing NO in the 285℃-420℃ range.The characterization results and DFT calculations emphasize the defective nature of Fe-doped ceria.The isolated Fe ions and associated oxygen vacancies catalyze facile NO reduction to N₂.A mechanism for the reduction of NO with NH₃ on Fe-doped ceria is proposed involving adsorbed O₂.Such adsorbed O₂ species will also contribute to the oxidation of soot.（4）Fe modified Ce-Ti catalysts(3DOM Ce_0.9-xFe_xTi_0.1O₂)were prepared by CCT method.The catalytic purification taking place over the material with double efficacy is cost-efficient.The contact between solid PM and catalyst active site has been process intensified by 3DOM unique structure.3DOM Ce_0.7Fe_0.2Ti_0.1O₂ catalyst possess a high SCR activity and an excellent selectivity to N₂,giving a maximum concentration of CO₂at 385℃ for PM combustion and completed NO conversion in the temperature range of 281-425℃(Fe³⁺+Ce³⁺?Fe²⁺+Ce⁴⁺,Fe³⁺+Ti³⁺?Fe²⁺+Ti⁴⁺),the dual redox cycles and the excellent reducibility and sufficient acid sites of catalysts play key roles for the highly catalytic performance.