Study On Preparation And Performance Of SCR Denitration Catalysts For Diesel Vehicle Exhaust

Nitrogen Oxides(NOx)is one of the major atmospheric pollutants nowadays.NOx emissions from diesel engine accounted for about 70%in total emissions from motor vehicles in China.Effective control of NOx emissions in diesel exhaust is a main focus at the present stage.The selective catalytic reduction(SCR)of NOx with NH3 in the presence of excess oxygen has proven to be an efficient process for NOx removal in diesel exhaust.Based on typical NOx sources Diesel exhaust,this paper is committed to the investigation of the catalytic performance of SCR catalysts as well as its preparation and modification.Hence,this study aims at looking for simple and direct synthesis and its modification methods to prepare catalyst with excellent performance.Firstly,based on the Precious Metals Ag/Al catalysts,we found that a H2 or CO pretreatment could have a notably enhancement for NH3-SCR.The NOx conversion could increased from 22%to 93%at 270 ? by a pretreatment in 1 v%H2+N2 mixed gas at 300 ?for 3 h.The Ag+ ion was converted to low valence agglomeration Ag clusters such as Ag-?n and Agm clusters.These clusters could be the active center for SCR reaction and promoted the formation of surface nitrates,thereby the activity of the SCR reaction was enhanced.However,by the viewing from the diesel SCR applications perspective,the activity temperature window of the catalyst was still relatively narrow.What more,part of the activity were reduced after a period of time,which need further research.For this reason,hydrogen pretreatment was processed on the amorphous Ce-Ti catalyst which also enhanced its NH3-SCR performance.We concluded that 3-hour and 400 ? were the most suitable and efficient pretreatment conditions,whereas the NOx conversion rate was above 95%from 210 ? to 360 ? for such pretreated catalyst.Similar to F doping,the enhancement was attributed to the effects generated by the pretreatment for instance,Ce4+reduced to active Ce3+,the increase of the chemisorbed oxygen and the acid sites,therefore advanced its SCR performance.The partially substitution by F in lattice had caused the electron transfer from Ti to Ce,which strengthened the interaction between Ce and Ti,so that more Ce3+ was produced.The pret-eatment might have affected the Ce-O-Ti structure,and more low valence Ce site was released and the oxygen vacancies generated.Compared to Ag/Al-H,Ce-Ti/H has more excellent NOx conversion rate and active temperature window.It also demonstrated good activity stability,which attracted interests in further study on low-temperature NH3-SCR.Based on the commercial V/Ti catalyst,CuO-V2O5/TiO2 catalyst was prepared by a hydrothermal method for anatase titania preparation,followed by impregnation for Cu and then V.The catalyst component had both good low temperature performance of CuO,as well as good mid-range performance and sulfur resistance of V2O5.The results indicated that copper species had a good interaction with TiO2,vanadium species interacted with TiO2 by copper species and thereby generating more V4+.This has led to the increase of chemisorb oxygen,better reduction property,more surface acid sites and improvement of NH3 and NO adsorption capacity,all these resulting in better NH3-SCR performance of catalysts.However,the hydrothermal stability of TiO2 support was poor.The catalytic activity decreased and irreversible phase transferred after a 2-hour hydrothermal treatment at 700 ? with water vapor.Further research is needed to enhance its hydrothermal stabilityThereafter,in view of the demand for the hydrothermal stability of the catalyst,and with the introduction of solvent-free method,copper-contained SAPO-34 small pore zeolite was synthesized with Cu-TEPA added in the precursor.The direct synthesis with the absence of solvent has effectively reduced the waste production,increased the yield and accelerated the crystallization period as well.The so-prepared sample exhibited high degree of crystallinity,ideal crystallized cube-like structure and excellent BET surface area and pore volume which are positive to adsorption and SCR.It has a high NOx conversion and N2 selectivity in the temperature range of 210 ? to 450 ?.Moreover,it also retained the excellent hydrothermal stability of chabazite structure.The activity could be fully maintained after a 12-hour hydrothermal treatment at 700 ? with water vapor.The Cu-SAPO-34 catalyst prepared by solvent-free method will have a big potentiality and deserves further study.Finally,the Cu-SAPO-34 catalyst was prepared into slurry and coated on cordierite honeycomb ceramic carrier to prepare monolithic catalysts.The monolithic catalysts were evaluated in laboratory simulated exhaust gas and 60kW diesel generator actual exhaust conditions respectively.In the simulated exhaust gas test,the evaluation results of the 8-hole monolithic catalyst are close to the powder catalyst,which had about 95%NOx conversion from 250 ? to 450 ?.In the actual exhaust gas test,the denitration activity of the 3 5-hole catalyst was relatively lower,the best NOx conversion was 85.4%.The experience acquired in the preparation and evaluation of catalysts have also laid a foundation for the development of better performance and stability of the monolithic Cu-SAPO-34 cordierite catalysts.