Mechanism Of SO₂ Resistance And N₂O Formation In Low-temperature Selective Catalytic Reduction Of NO Over CeOx-based Catalysts

Nitrogen oxides(NOx)emitted from industrial production and automobile exhaust are one of the major air pollutants.Ammonia selective catalytic reduction(NH3-SCR)has been considered as one of the most effective technology for NOx abatement.Re-cently,low-temperature SCR has been extensively studied due to its energy efficiency and potential application.However,the low activity,poor selectivity and SO2 deactiva-tion of the catalysts have limited the industrial application of low-temperature SCR.A detailed knowledge about mechanisms of the NOx reduction,the SO2 poisoning/anti-poisoning and N2O formation over SCR catalysts is indispensable for the development of effective catalysts with high durability and selectivity.CeO2 exhibits unique redox performance and potential SO2 resistance in low-temperature SCR.Here in this thesis,we reported our studies about the mechanism for SO2 induced deactivation and regenra-tion of CeO2 catalyst,the reaction pathways for NOx reduction and the possible N2O formation rotues over CeO2-based catalysts.The details are listed below.1.CeO2 nanorods and nanopolyhedrals were prepared by hydrothermal method and applied as SCR catalysts.CeO2 nanorods exhibited a higher de-NOx performance than nanopolyhedrals due to the higher surface Ce3+and adsorbed oxygen which pro-moted the adsorption and activation of NO and NH3.The NH3-SCR reaction route was affected by temperature,"fast-SCR"was the main route for NO removal at lower tem-perature.While for higher temperature,"standard-SCR"became the main route.2.Mechanism of SO2 resistance over ceria-based catalyst was revealed.SO2 tol-erance test up to 1000 hours for CeO2 nanorods declared its outstanding SO2 resistance.For the first time,atomic-scale dynamic evolution of CeO2 after interaction with NO,O2,SO2,and NH3 has been directly observed in in-situ TEM,and it was found that NH3 can promote the decomposition of Ce2(SO4)3 at low temperature.The establishment of dynamical equilibrium between the formation and decomposition of sulfates was ac-counted for the great SO2 resistance of CeO2.For the first time,MnOx/CeO2 catalysts applied in NH3-SCR de-NOx achieved a 1000 hours SO2 resistance at 200 ? and 250 ? through such a dynamical equilibrium.A feasible solution to the regeneration of the SO2 deactivated catalysts in other catalytic reactions was proposed based on the above mentioned mechanism.3.NO reduction and N2O formation routes over Pd/CeO2 catalyst were proposed in the case where O2 involved or not in the reactive gases.Under oxygen-free condition:NO2 formation was disrupted and"standard-SCR"was the only route for NO removal.Pd/CeO2 catalyst can achieve a 100%NO conversion and N2 selectivity at a temperature range of 175-400 ? due to the enhanced adsorption and activation of NH3 by Pd.N2O formation depends on the reaction temperature and reactive gas.At lower temperature,HON dissociation is the only pathway for N2O yield.At higher temperature,N2O was generated through the dissociation of NO when NH3 was insufficient.For oxygen in-volved condition:the preferential occupation of oxygen on Pd competes with the ad-sorption and activation of NH3.At lower temperature,the NH3 activation was inhibited and NO was removed through the "fast-SCR" pathways,which inhibited the N2O gen-eration.At higher temperature,"standard-SCR"was the dominated pathways for NO reduction.The high activity of oxygen species resulted in the deep dehydrogenation of NH3,accompanied with the formation of a large amount of N2O.The deep dehydro-genation of NH3 was the rate-determining step for N2O yield.Our results indicated that for noble metal based SCR catalysts,the presence of O2 is not only detrimental to the NO removal,but also causes excessive yield of N2O at high temperature.4.The effect of SO2 on the catalytic performance of Pd/CeO2 was described and the the results achieved from in-situ TEM was evidenced by ex-situ experimental results.NO adsorption was prevented on the sulfated Pd/CeO2 catalyst and E-R is the only reaction route for NO reduction.The sulfated catalyst possessed high oxidation ability both for NH3 and NO.The deep dehydrogenation of NH3 cause the N2O formation un-der O2 free condition.While with O2 in presence,the existence of NO2 faciliate the NO reduction through"fast-SCR"and N2O formation was cut off at low temperature.The ex-.situ TEM observation of CeO2 nanoploycrystals and the FTIR results of sulfated Pd/CeO2 catalyst after treatment in NO+NH3 indicated that NH4HSO4 and Ce2(SO4)3 can decompose at low temperature,which is in line with the in-situ experimental results.5.The invalidity of addition of base metals to inhibit the passivation of O2 to the de-NOx performance over noble metal based catalyst was demonstrated.In-situ TEM observation proved the preferential oxidation of Ni can prevent the formation of bulk PtOx,but the oxidation of Pt in the surface layer still occurs.Therefore,the de-NOx performance of PtNi/CeO2 will still be passivated by oxygen.