Studies On NH₃-SCR Catalysts Formulations And Coating Process Of Honeycomb

Nitrogen oxides （NO_x） is one of the major atmosphericpollutants.With the increase in the amount of urban vehicles in recentyears, in particular the increase in the number of diesel vehicles, the NO_xpollution problems of city automobile emission have become increasinglyprominent. So many countries have developed stringent emissionsregulations to control NO_xemissions of vehicles. NH₃-SCR is one of themost development potential technology in the control of diesel vehicleNO_xemissions.Two aspects of SCR：powder catalysts formulations and honeycombcoating process of the monolith catalyst were investigated in this paper.（1）Studies on SCR powder catalyst formulationThe most widely used commercial vanadium-based NH₃-SCRcatalysts have been prohibited in Japan and the United States, due to thebiological toxicity of the active component V2O5. Therefore studies onnon-vanadium-based catalyst formulations becomes a hot topic.Taking into account of the different exhaust temperature characteristics of passenger and commercial diesel vehicles， two series ofnon-vanadium-based SCR catalyst powder formulations were investigatedin this study.On one hand，the temperature of passenger diesel vehicles exhaust isrelatively low， so a serious novel modified catalyst powder wasinvestigated by doping a transition metal on low temperature manganesecerium-based catalyst in this study.Firstly, manganese cerium-basedcatalysts （MnCe₃O_x） and a series of its reforming catalysts doped withdifferent transition metal M （MnCe₃MO_x, M=W, Fe, Cu） was preparedwith method of coprecipitation. Secondly the activity and N₂selectivity ofeach catalyst in DeNO reaction was evaluated. The experimental resultsshowed that the high temperature activity window of manganesecerium-based catalysts was greatly broadened by doping tungsten andtungsten also improved its N₂selectivity obviously. The results of SO2resistance test of optimal MnCe₃WO_xcatalyst showed it has excellentresistance ability in high temperature, but its resistance needs to beimproved in low temperature; Finally the results of the characterization ofthe catalysts by BET, XRD, SEM and in situ explained how W modifiesthe manganese cerium-based catalysts.On the other hand，the temperature of commercial diesel vehicleexhaust is relatively high，the Ce₃WO_xcatalysts were prepared withmethod of coprecipitation.Then the activity and N₂selectivity of each catalyst in DeNO reaction was evaluated，and the experimental resultsshowed that Ce₃WO_xcatalysts have excellent activity and N2selectivity inmid and high temperature.The results of SO2resistance test of Ce₃WO_xcatalyst showed it has excellent SO2resistance ability in250℃and400℃.Finally, the characterization of the catalysts by XRD, BET, SEM and insitu DRIFTS showed it has ideal microscopic physical structure.（2）Studies on honeycomb coating process of the monolith catalystIn real diesel engine SCR system，the catalysts is typically made intoslurry and then coated on the honeycomb. However, the the traditionalslurry coating process is very complex, and it have other drawbacks suchas coating nonuniformity.The method of self-propagating combustionsynthesis （SHS） is innovatively introduced to synthesis and coat SCRcatalysts on honeycomb by one single step. Then the activity and N2selectivity of honeycomb catalysts and commercial catalysts wasevaluated，and the experimental results showed that the catalysts havebetter performance than some commercial catalysts. Analyzed by thequality measurement, characterizations of BET and SEM， monolithcatalyst has low coating amount of catalyst but its washcoat is uniform,while exhibiting excellent performance.