Experimental Study On Adsorption And Desorption Of NO_x Emission From Diesel Engine By LNT Catalysts

NO_x?NO and NO₂?emissions from diesel engines are one of the major sources of air pollution and can cause environmental damage,such as acid rain,photochemical smog,greenhouse effect,and ozone depletion.And also cause damage to human health.Diesel engine exhaust is oxygen enriched environment,therefore the traditional three-way catalytic converter?TWC?can not effectively remove NO_x.With the emission regulations becoming increasingly stringent,based on the in-engine purification measures,coordinating with corresponding aftertreatment technologies is the focus of diesel engine emissions control.Currently,the most promising ways to reduce NO_x under lean conditions are selective catalytic reduction?SCR?and lean NO_x trap?LNT?.SCR is the most effective technology to reduce NO_x emission from fixed sources and diesel engines which employing HC,NH3 and H2 as reductants.LNT technology,also known as NO_x storage and reduction?NSR?,operates under periodic lean and rich environments while needing no additional reductants and is of great prospect in the application of light-duty diesel vehicles.In this paper,LNT catalysts were prepared by modified sol-gel method and impregnation method.The physicochemical properties of the catalysts were studied by characterization technologies.A catalyst simulating gas test platform was designed and built.Based on this platform,the performance of catalyst adsorption and desorption of NO_x was studied.The main work and conclusions are as follows:?1?A series of 1%Pt/xBa-?30-x?Ce/?-Al2O3 catalysts with different Ba/Ce weight ratio were prepared by modified sol-gel method and impregnation method?x=0,5,10,15,20,25,30?.The specific surface area,crystalline phase structure,microstructure,dispersion of active components,valence state of surface elements,reduction performance and functional groups of the catalysts were characterized by Brunauer-Emmett-Teller?BET?,X-ray diffraction?XRD?,Scanning Electron Microscope/Energy Dispersive Spectrometer?SEM/EDS?,Transmission Electron Microscope?TEM?,X-ray Photoelectron Spectroscopy?XPS?,H2 Temperature Programmed Reduction?H2-TPR?and Fourier Transform Infrared Spectrometer?FTIR?.Results show that 1%Pt/15Ba-15Ce/?-Al2O3 catalyst has a maximum specific surface area of 91.803 m2/g;CeO₂ is mainly present in cubic fluorite structures and CeO₂?111?grain size is between 5 nm and 8 nm,1%Pt/15Ba-15Ce/?-Al2O3 catalyst has the smallest grain size,which is 5.7 nm.On the surface of the catalysts exited chemical adsorption oxygen and lattice oxygen,and the ratio of O?/?O?+O??are 0.356,0.506 and 0.561 for 1%Pt/30Ba/?-Al2O3,1%Pt/30Ce/?-Al2O3 and 1%Pt/15Ba-15Ce/?-Al2O3 catalysts,indicating that 1%Pt/15Ba-15Ce/?-Al2O3 catalyst obtains better catalytic activity.?2?A catalyst simulating gas test platform was designed and built.Based on this platform,the effects of temperature,O₂,NO and NO₂ on the adsorption of NO_x by the catalyst were investigated.Through NO_x adsorption and desorption cycling experiments,the effects of lean/rich cycle timing on the NO_x conversion were considered.What's more,the thermal stability of adsorbed NO_x during the lean phase by the catalyst was studied by NO-TPD.Results showed that at temperatures between 250°C and 450°C,the adsorption capacity of the catalyst increases and then decreases with the increase of temperature.The maximum adsorption capacity was obtained at 350°C,up to 641.14 ?mol/gcat,and the adsorption efficiency was 51.56% when NO_x adsorption was saturated.In addition,the NO_x adsorption efficiency decreased with time,of which was higher than 80% at the first 30 min of the lean phase.When the concentration of NO in the reaction gas is in the range of 500×10-6-1000×10-6,the penetration time and the adsorption saturation time decrease with the increase of NO concentration leading to the result that the NO_x adsorption efficiency decreased with the increase of NO concentration.On the contrary,NO_x adsorption efficiency increased with the increase of NO concentration.It was found that NO₂ was more readily to be adsorbed on the surface of the catalyst than NO.In the NO_x adsorption and desorption cycle experiments,maintaining the lean period 240 s,while extending the rich period from 30 s to 60 s at 350°C,resulted in an increase in NO_x adsorption efficiency from 90% to 94.7%.And NO_x removal efficiency also increased with the increase of rich period time,from 76.3% to 83.9%.When the lean period is 120 s,while extending the rich period from 30 s to 60 s,resulted in an increase in NO_x adsorption efficiency from 91.9% to 95.1% and the NO_x removal efficiency increased from 82.4% to 89.6%.It can be seen that by controlling the lean / rich time,increasing the rich period time and shortening the lean period time can increase the removal efficiency of NO_x.