Study Of Denitration Based On NO Decomposition Over Metal-loaded Zeolite Catalysts

Ocean transportation assumes 90%of global trade transportation,which results in serious emission of NO_xfrom ship exhaust gases,bringing serious and severe environmental issues.The International Maritime Organization（IMO）makes a regulation on NO_x emissions from ships need to meet Tier III standards in some areas from January 1,2016,but currently,there is no maturity technology for the deterination of ship exhaust gas.Due to NO as the major one（90%）in NO_x emission from ship exhaust gases,this thesis proposes to decompose NO directly into N₂ and O₂ as denitration method for ship exhaust gas,which has the advantages such as simple process,no reducing agent,no secondary pollution,and thus it is considered to be an ideal way for denitration of ship exhaust gas.Catalyst is a key factor in catalyzing NO decomposition,therefore this thesis focuses on preparation of metal loaded zeolite catalyst and evaluation activity of prepared catalysts in NO decomposition under stimulated exhaust gas with high concentration of O₂ and SO₂（NO 1000 ppm,SO₂ 500 ppm,10 vol.%O₂,N₂ as balance gas,gas flow=60 L/min）,the results and conclusions achieved in this thesis as following:1.HZSM-5 used as the catalyst carrier,a series of metal-HZSM-5 catalysts were prepared through ion-exchange method and their performance were evaluated.It was found that,（1）transition metal（Fe,Co,Ni,Cu）significantly affects the catalytic activity of metal-HZSM-5,and Fe-HZSM-5 shows better catalytic activity towards NO removal.（2）modification by promoter,such as alkali metal（Li,Na,K,Cs）,alkaline earth metal（Mg,Ca,Ba）and lanthanide（La,Ce,Sm）,can improve the catalytic activity of Fe-HZSM-5,and Na,Ba,Cs,K are better ones.（3）ion exchange step by step is a better way than co-exchange for preparation of M/Fe-HZSM-5.（4）the best Cs/Fe ratio for Cs-Fe-HZSM-5 is 1:4.（5）the results of characterization analysis and reaction performance reveals that,isolate Fe（III）in Fe-HZSM-5 catalyst is favorable to decompose NO,and Cs modification improves the concentration of isolate Fe（III）species and reduces SO₂adsorption on surface of catalyst,which is the main reason for the high activity of Cs-Fe-HZSM-5 catalyst obtained in NO decomposition.（6）Cs-Fe-Hz SM-5 catalyst exhibited higher stability and activity with 45-50%NO removal in a 10 h continuous experiment at 350℃,but NO removal over Fe-HZSM-5 was only 20-25%at the same conditions.2.Series of Fe-HZSM-5 catalysts were prepared by excess impregnation and evaluated in NO decomposition.It was found that,（1）the loading of Fe（0.3-12.5 wt%）significantly affects the catalytic activity of Fe-HZSM-5,and the optimum Fe loading is 7.5 wt%.（2）the phase state of Fe species greatly affects the catalytic activity of Fe-HZSM-5,and the activity decrease in the order of iron nitride>iron oxide>metal iron.（3）the results of characterization analysis and reaction performance reveals that isolate Fe（III）species is the major active site towards NO decomposition.（4）78%NO removal rate was achieved over 7.5 wt%Fe N-HZSM-5 at150℃,but the stability of this catalyst is very poor.3.Effects of zeolite acidity and type on the activity of Cs-Fe based catalyst was tested in NO decomposition.It was found that,（1）acidity of HZSM-5（Si/Al ratio=25,30,80,150）can significantly affects the activity of Cs-Fe-HZSM-5,and the lower Si/Al ratio is better Cs_0.01-Fe_0.04-HZSM-5 with Si/Al ratio of 25 showed best catalytic activity of 52%NO removal at350℃.The results of characterization analysis and reaction performance reveals that,lower Si/Al ratio favors to increase the metal loading over HZSM-5,which is reason for the highest activity of Cs_0.01-Fe_0.04-HZSM-5 with Si/Al ratio of 25.（2）The type of zeolite（HZSM-5,13X,Beta,SSZ-13,MOR）significantly affects the activity of catalyst as well.Besides acidity,the pore size of zeolite might be another factor for tuning catalyst activity in NO decomposition.