Study On Ceria-based Oxide Catalysts For NO_x Storage And Reduction

Lean-burn engines are becoming more popular than traditional engines,mainly because of their higher fuel efficiency and lower carbon oxide emissions.However,lots of NO_xis contained in the exhaust gas when a large air-fuel ratio occurs.The traditional three-way catalyst（TWC）cannot achieve a high NO_xreduction efficiency and the existing Pt/Ba O/Al₂O₃catalyst only has a good NO_xremoval efficiency at high temperatures.So,it is necessary to develop an NSR catalyst with higher NO_xremoval efficiency and wider operating temperature window.In this paper,on the basis of CeO₂and/or Mg-Al mixed oxides,a series of cerium-based catalysts were synthesized through the modification with precious metal Pt and/or alkaline earth metal Ba.The physicochemical properties of all samples were characterized by XRD,SEM,TEM,N₂adsorption/desorption,H₂–TPR and XPS.The NO_xadsorption/desorption experiments were used to test the NO_xstorage performance and the thermal stability of the stored species.Moreover,the NO_xstorage mechanism of the catalysts was explored by in situ DRIFTS and the NSR performance was investigated through the lean-rich fuel cycle test.At the end,we have investigated the hydrothermal aging resistance of the catalysts.The specific contents are as follows:Three CeO₂ catalysts with different morphology,including nanosphere,nanorod and nanoparticle,were synthesized by different methods.The as–prepared CeO₂nanosphere possessed excellent NO oxidation capacity,smaller mesopores,better reducibility,more surface Ce³⁺content.The exposed（100）and（110）facets on CeO₂nanosphere facilitated the formation of more oxygen vacancies,thereby promoting the NO_xstorage.Compared to CeO₂with nanorod and nanoparticle morphology,CeO₂nanosphere showed better intrinsic low temperature NO_xtrapping performance,with a wide operating temperature window（150–300℃）,high NO_xadsorption capacity（NAC,640–745μmol/g）and high NO_xstorage capacity（NSC,250–350μmol/g）.Two reaction pathways were speculated for NO_xadsorption on CeO₂nanosphere,including“nitrate route”and“nitrite route”.The higher the temperature,the more the NO_xstorage shifted to the"nitrate route".The thermally unstable surface nitrites formed on the CeO₂nanosphere allowed ceria to release more NO_xduring the desorption process.The good medium-low temperature NO_xstorage performance of CeO₂nanosphere makes it to be a potential excellent NO_xstorage material.In order to broaden the operating temperature and improve the NSR performance of the catalyst,a new NSR catalyst Pt/MgAlO+CeO₂was prepared by combining CeO₂nanospheres with Pt/MgAlO through a mechanical mixing method.According to the research results,the catalyst Pt/MgAlO+CeO₂had a larger surface area and pore volume,stronger reduction performance,a large number of surface oxygen species and more oxygen vacancies.Pt particles were highly dispersed and captured on the CeO₂surface by migration,thereby reducing the loss of precious metals caused by high temperature and enhancing the reduction performance of the catalyst.Compared with CeO₂and Pt/MgAlO,the Pt/MgAlO+CeO₂sample exhibited better NO_xstorage capacity and higher NO_xremoval efficiency,which was due to its excellent medium and low temperature（<300℃）NO oxidation ability and higher surface oxygen content.At the same time,the NO_xspecies stored on Pt/MgAlO+CeO₂ had lower thermal stability and were easy to desorb from the catalyst,which was beneficial to promote the regeneration of active sites under rich fuel condition.According to the in situ DRIFTS results,there were two routes of"nitrate route"and"nitrite route"on Pt/MgAlO+CeO₂to store NO_x,and the storage sites included Ce site and Mg site.The presence of two storage sites allowed the catalyst to have a wider operating temperature window（100-400℃）and excellent NSR performance（NAC:640-745μmol/g,NO_xconversion rate:78-85%）.At the same time,the catalyst was less affected by adsorption temperature and had higher stability.In order to further improve the NSR performance of the catalyst Pt/MgAlO+CeO₂,the different content Ba was loaded on CeO₂to form a new type of Pt/MgAlO+（n）Ba/CeO₂catalyst.After loading Ba,the sample showed spherical particle morphology with slight agglomeration,its surface area and pore volume decreased,and the pore size increased.Ba was highly dispersed on the catalyst with the form of Ba CO₃,and a stable Mg Al₂O₄spinel structure was formed on the catalyst at the same time,which was conducive to the enhancement of the catalyst NSR performance.The precious metal Pt was captured on the CeO₂surface by migration,and was highly dispersed on the catalyst surface with the form of Pt particles and oxides,which could enhance the reduction performance of the catalyst.The H₂-TPR results showed that the higher the Ba loading,the stronger the reduction performance of the catalyst.From the results of NO oxidation,it was found that Pt/MgAlO+15BC possessed the strongest NO oxidation ability.The loading of Ba could significantly improve the NO_xstorage capacity of the catalyst.When the loading of Ba was 15wt%,the catalyst had the largest NO_xadsorption capacity（NAC>1000μmol/g）and a higher NO_xconversion rate（78-95%）.Therefore,the catalyst Pt/MgAlO+15BC showed excellent medium and high temperature（300-400℃）storage performance and NSR performance.According to the in situ DRIFTS results,the NO_xwas stored through the"nitrate route"and the"nitrite route"simultaneously on Pt/MgAlO+15BC.The storage sites included Ce site,Mg site and Ba site.The existence of multiple storage sites allowed catalyst to possess a wider operating temperature window（100-400℃）.In addition,when H₂was used as the reducing gas,the NSR performance of the Pt/MgAlO+15BC catalyst was the best.In order to explore the hydrothermal stability and sulfur resistance of the catalysts,CeO₂,Pt/MgAlO+CeO₂and Pt/MgAlO+15BC were selected for hydrothermal aging tests.The process of hydrothermal aging would lead to sample sintering,catalyst structure collapse and blockage,resulting in significantly reduced specific surface area and increased pore size.Pt/MgAlO+15BC had the smallest change in surface area and pore size,and stronger reducibility,indicating that Pt/MgAlO+15BC possessed higher hydrothermal stability.After hydrothermal aging,the NO_xstorage performance and NO_xconversion rate of the all samples decreased.Compared with the other two samples,the NO_xadsorption capacity and NO_xconversion rate of Pt/MgAlO+15BC were better,indicating its higher hydrothermal stability.In addition,cerium oxide had strong sulfur resistance,and Ba-containing catalysts were greatly affected by sulfides,and their sulfur resistance was weaker.