On Study Of Ag-based Catalysts For The Combustion Of Soot From Diesel Engine Emission

The emission of diesel motor vehicle exhaust contains CO,NO_x,unbumt organic compounds?HC?and soot particulate matter?PM?and so on.The HC and soot particles as a main source of urban atmospheric particulate matters?PM 10 and PM2.5,particle size<10 and 2.5 ? m respectively?have caused acute human health and environmental problem.At present,there are mature processing techniques for CO,NO_x and HC,but there is no satisfactory solution to the soot particles.The best catalytic materials for soot are generally platinum,rhodium,palladium and other noble metal catalysts,especially in low temperature conditions,platinum based catalyst can use the procedure of converting NO to NO₂ to catalyze soot combustion.However,the catalytic activity is very low in the absence of NO_x conditions.With the increasingly stringent laws and regulations,the requirements of NO_x emissions are becoming lower and lower,and the existing soot combustion catalyst system will face a great challenge.Therefore,it is important to study the catalytic soot combustion catalyst with high activity under non NO_x conditions.According to the reported literature,the Ag based catalyst has a high catalytic activity for soot combustion in the absence of NO_x compared to platinum,rhodium,palladium and other precious metal based catalysts.In this work,a series of catalysts(Ce_0.75Zr_0.25O₂?K-Agn/Ce1-xZrxO₂?N-Agn/Ce_0.75Zr_0.25O₂ and KN-Ag22%/Ce_0.75Zr_0.25O₂-n)were prepared using coprecipitation method.Active component distribution in the catalyst was controlled by using different precipitants.The catalytic activities of the catalysts for model soot combustion were evaluated by the temperature-programmed oxidation reaction.The catalysts were characterized by means of TG,BET,XRD,TEM,XPS,HS-LEISS,H₂-TPR,O₂-TPD and in situ-XRD.Based on the results of characterizations,the relationships between distribution of Ag state in the catalyst and catalytic soot combustion was studied in detail.The main research results are as follows:?1?Ceria-zirconia solid solution catalysts were prepared by respectively using KOH,NH₃·H₂O,KOH and NH₃·H₂O mixture as precipitating agent,denoted as follows:K-Ce_0.75Zr_0.25O₂,N-Ce_0.75Zr_0.25O₂,KN-Ce_0.75Zr_0.25O₂.The activity of catalysts can be significantly improved as zirconium doped into CeO₂ crystal structure.KN-Ce_0.75Zr_0.25O₂ catalyst prepared by using KOH and NH₃·H₂O mixture as precipitating agent has the highest activity for soot combustion,which has ignition temperature of soot combustion?T10?at 367.6?.?2?Utilizing the property that KOH and NH₃·H₂O have different effects on Ag+precipitation,the distribution state of Ag in catalyst can be controlled.Ag nanoparticles uniform distribution in the K-Ag22%/Ce_0.75Zr_0.25O₂ catalyst and gradually increase distribution from inside to outside in the N-Ag22%/Ce_0.75Zr_0.25O₂ catalyst,while the distribution of Ag nanoparticles in the KN-Ag22%/Ce_0.75Zr_0.25O₂-5 catalyst is between K-Ag22%/Ce_0.75Zr_0.25O₂ and N-Ag22%/Ce_0.75Zr_0.25O₂ catalysts.?3?By using different precipitating agents and their ratios,the Ag/Ce_0.75Zr_0.25O₂ catalyst with different reduction properties can be controlled.The initial reduction temperature of the N-Ag/Ce_0.75Zr_0.25O₂ catalyst is about 150 ?,while that for K-Ag/Ce_0.75Zr_0.25O₂ catalyst is about 100 ?.In particular,the KN-Ag/Ce_0.75Zr_0.25O₂ catalyst prepared by KOH and H₂O NH₃ mixed solution as the precipitating agent,the reduction temperature was significantly decreased,and the reduction signal was detected at 50 ?.It also confirmed that the distribution of more Ag on surface enhance the interaction beween Ag and Ce support.?4?The interaction between Ag and Ce is also beneficial to improve the adsorption capacity of Ag/Ce_0.75Zr_0.25O₂ catalyst for oxygen species,at the same time,the surface of the catalyst has a high Ag+/Ag0 ratio,which is beneficial to the improvement of surface adsorption of oxygen species.Combining with the activity results,it is also further explained that the interaction between Ag and Ag-Ce can significantly improve the oxidation ability of the catalyst.Among them,the KN-Ag22%/Ce_0.75Zr_0.25O₂-5 catalyst had the best catalytic activity on soot combustion and the trigger temperature of soot T10 was 254.1 ?.The second comes to N-Ag22%/Ce_0.75Zr_0.25O₂ catalys with T10=277.0 ?,and the last is the K-Ag22%/Ce_0.75Zr_0.25O₂ with T10=306.3 ?.?5?Based on the results of various characterization and catalytic activity data,the intrinsic mechanism of Ag/Ce_0.75Zr_0.25O₂ catalyst for catalytic oxidation of soot is preliminarily explored.It can be inferred that the active oxygen species on the surface of the catalyst at low temperature are derived from two aspects.One is the reactive oxygen species generated by the adsorption and activation of the Ag nanoparticles on the molecular oxygen species O₂ + Ag?Ag-O₂-.Based on the results of in situ XRD and HS-LEISS,we can speculate that more Ag outside the catalysts is more conducive to Ag particles for oxygen activation;CeZr activation or stored oxygen species overflow to the Ag surface will further promote the activation of Ag particles for oxygen species.Another aspect,the stronger interaction between the metal?Ag?-and?Ce?-carrier is caused by an increase in the number of reactive oxygen species,Ce3++ Ag+ ? Ce4+ + Ag + Vo,Vo + O₂ ? O-.Combined with the results of H₂-TPR experiment,the interaction of Ag-Ce and the special distribution of Ce and Ag significantly decreased the reduction temperature of the catalyst,especially that of the surface oxygen of CeO₂.This indicates that more Ag outside is more conducive to optimize the interaction between Ag and Ce.Moreover,the results of O₂-TPD and XPS also shown that the interaction between Ag and Ag-Ce can significantly improve the oxidation ability of the catalyst.?6?The Ag/Ce_0.75Zr_0.25O₂ catalyst prepared by NH₃ and/or NH₃ · H₂O as the precipitation agent has good stability,no matter in the water bearing or water reaction atmosphere,the catalyst activity of the catalyst decreased no more than 10 ? after 5 cycles of T10.The reduction of the catalyst activity was related to the occurrence of Ag.