Investigation On Direct Decomposition Of NO Over Cu-ZSM-5 Zeolite Catalysts

A large amount of NOx emissions will cause many environmental problems such as acid rain,photochemical smog and greenhouse effect.It is imperative to eliminate NOx.Direct decomposition of NO to N₂ and O₂ is an ideal route for NO removal due to the simple reacting network with no additional reductant and no secondary pollution.In recent years,Cu-ZSM-5 catalysts have received much attention due to the high activity compared with perovskite-type oxides and noble metal catalysts.In this work,the optimum preparation conditions of Cu-ZSM-5 catalyst are studied,and different promoters are introduced to modify the catalyst by ion exchange method to improve the NO decomposition activity of a Cu-ZSM-5 catalyst.In chapter 2,the Cu-ZSM-5 catalyst is prepared by ion exchange method.The catalytic decomposition activity of NO is tested for the all samples.The selectivity of NO conversion to N₂ is used as the criterion to evaluate the performance of the catalysts.The effect of four molecular sieves?ZSM-5,SAPO-34,TS-1 and SSZ-13?with different pore structures on NO decomposition activity is investigated.ZSM-5molecular sieve shows the highest NO decomposition activity in the temperature range investigated.Then the effects of ion exchange time,ion exchange temperature and ion exchange solution concentration on the preparation of Cu-ZSM-5 catalyst are investigated.The results show that the best activity is obtained at 40 ^oC,solution with Cu content of 0.01 mol/L for 24 h.In addition,the effect of the space velocity on the catalytic decomposition activity of NO is investigated.The results show that the decomposition activity decreases with the increase of space velocity,and so the final test space velocity is set at 1200 h^-1.In chapter 3,four rare earth elements?La,Ce,Y,Pr?,five transition metal elements?Zn,Fe,Ni,Mn,Co?and alkali earth metal element?Sr?are introduced to modify Cu-ZSM-5 by ion exchange method.The catalyst with the best catalytic activity for NO direct decomposition is La-Ni-Cu-ZSM-5 catalyst.The promotion effect of La,Ni on the Cu-ZSM-5 catalyst in NO decomposition reaction is discussed.M-Cu-ZSM-5?M=La,Ni,La-Ni?is characterized using XRD,ICP,BET,XPS and H₂-TPR.The co-doping with Ni and La into a Cu-ZSM-5 catalyst enhances the NO decomposition activity in a temperature range of 350-550 ^oC.The characterization results indicate that La effectively improves the dispersion of the Cu species,forming more Cu²⁺ions in the sample,while Ni is likely to facilitate the implantation of Cu²⁺to form(Cu²⁺-O^2–-Cu²⁺)²⁺dimers.The synergistic effect of La and Ni contributes to the increased(Cu²⁺-O^2–-Cu²⁺)²⁺content in the co-doped sample,thus enhancing the NO decomposition activity.