Study Of Ag-Mn_xCe_1-xO_y Catalysts For The Oxidation Of Soot Particles From Diesel Engine Emissions

Diesel engines are widely used in heavy loading trucks and ships.Traditionally diesel engines are less flexible and portable than gasoline counterparts,and the power of per liter fuel for diesel engines is not as good as that of gasoline engines(low speed).Besides,the noise and vibration caused by diesel engines are more severe,and the emissions of particulate matter(PM)and soot from diesel engines are also more than those from gasoline engines.Therefore,cars made of diesel engines are rarely seen.The exhaust from diesel engines usually contains a large proportion of CO,NOx,unburned organic hydrocarbons(HC),and PM,which have very negative impact on the quality of atmosphere and human health.Because of its high ignition temperature,soot particles can only be fully oxidized at the temperature above 600℃,which is hard to achieve for a catalytic process.The traditional Pt,Rh and Pd based catalysts are usually poor in activity for this reaction and expensive in cost,which limits their wide applications.The Ag-based catalysts,however,possess high activity and are relatively low in price,thus making them the promising candidates for replacing precious metal-based catalysts in the elimination of pollutants(especially soot particles)from diesel engines.In this paper,the activity and production cost of Ag-CeO2 based catalysts were studied and then optimized.Currently,the silver loading in the Ag-based catalysts still remains high(>20 wt.%),and the simple reduction of silver content in the catalyst would lead to a dramatically decrease in activity.Thus,other methods would be needed.In this work,we proposed to dope certain amount of manganese in the catalyst to maintain its high activity while significantly reducing the amount of silver loading.In the experiment,NOx-assisted temperature programmed soot oxidation was employed to evaluate the activity of the prepared catalysts.The physicochemical properties of the synthesized catalysts were also characterized by BET,SEM,EDS,TEM,SEAD,XRF,XRD,H2-O2 pulse reaction,H2-TPR,O2-TPD,EPR,XPS and the etching experiments.The1.Ag-MnxCei-xOy catalysts were prepared by the reverse co-precipitation method,with Mn(NO3)2,Ce(NO3)3 mixed solution and silver ammonia solution reacting with KOH,the precipitant.The activity evaluation indicated that the doping of manganese greatly increased the catalytic activity of Ag-CeO2 catalyst for the oxidation of soot particles.And the catalysts could effectively catalyze the oxidation of soot particles even at the temperature below 400 ℃.2.The activity of catalysts followed a regular change pattern when different molar ratios of manganese were doped,that is,it increases first and then decreases with a maximum activity existing.For Ag-Mn0.iCe0.9Oy catalyst,its Tm was 321 ℃;for Ag-CeO2 catalyst,its Tm was 452 ℃,while for Ag-Mn0.5Ce0.5Oy,its Tm was 384℃.3.The H2-TPR experiment showed that the Ag-Mn0.iCe0.9Oy catalyst had the lowest reduction temperature for the support,which implied that the interaction between Ag and Mn0.1Ce0.9Oy favors the generation of active oxygen.In O2-TPD experiment,Ag-Mn0.1Ce0.9Oy had also the lowest oxygen desorption temperature for the MnCeOx support and the lowest oxygen desorption peak at the interface of silver oxides.Ag-CeO2 catalyst also possesses the oxygen desorption peak,but its oxygen desorption temperature for the CeO2 support was about 445 ℃,thus limiting its activity at low temperature.XPS,and its etching experiments showed that in the samples with high manganese content,the silver species are easy to combine with manganese oxides to form Ag-Mn complex oxides,resulting in the appearance of Ag2+ and the disappearance of elemental silver,thus leading to the decrease of activity due to insufficient activation of oxygen species.4.Based on the results of various characterizations and discussions above,we found that doping certain amount of manganese could promote the mobility of lattice oxygen and activation of oxygen species,which is probably caused by the modification of manganese to the crystal phase of ceria and its regulation to the proportion of silver species.5.3DOM Ag-AlCe catalysts were also successfully synthesized and characterized preliminarily.Besides,the Ag-CeO2 catalysts with specific morphology were also studied.