The Study On Ce_x/HBEA Catalyst For Selective Catalytic Reduction Of NO_x And Its Resistance To SO₂ And C₃H₆

Nitrogen oxides?NOx?are major air pollutants.Diesel engine is a major source of NOx emission.In order to improve the air quality,NOx emissions from diesel engine should be controlled.NH3 selective catalytic reduction?NH3-SCR?of NOx is recognized as an effective technology to control NOx emissions from diesel engines.Based on the features of diesel engine exhaust,Cex/HBEA catalysts with high activity,excellent propene poisoning resistance and good sulfur tolerance for NOx reduction were developed.These results are crucial to develop active SCR catalysts for diesel engines.Cex/HBEA catalysts doped with different amount of CeO2 were prepared by impregnation methods.According to the results of activity test,the N2 selectivity was above 98%between 175? and 500? for Cex/HBEA catalysts.Cei/HBEA showed the best NH3-SCR performance with gas mixture contianing NH3,NOx,O2 and N2.The NOx conversion of Cei/HBEA was above 90%between 250? and 500?.The temperature window of Cex/HBEA was wider than that of Cex/SAPO-34.In addition,the NH3-SCR activity of Cei/HBEA was inhibited by H2O,SO2 and C3H6.The inhibitory effects of C3H6 on the activity of Cei/HBEA were the most obvious.Although Cei/HBEA had excellent NH3-SCR performance,it was easily deactivated by C3H6.The effects of CeO2 doping amount on the propene poisoning resistance of Cex/HBEA catalysts were investigated.The propene resistance of Cex/HBEA increased with the increase of CeO2 doping amount,and Ce48/HBEA showed the best propene poisoning resistance.The NOx conversion of Ce48/HBEA was above 90%between 300? and 450?.Based on the results of catalyst characterization and activity test,the promotional role of ceria on the propene resistance was shown in three ways:1)A HBEA-CeO2 core-shell structure was formed in Ce48/HBEA catalyst,2)Ceria inhibited the adsorption of C3H6,3)Ceria promoted C3H6 oxidation.Due to the presencee of SO2 in the exhaust of diesel engines,the effects of SO2 on the NH3-SCR activity of Cex/HBEA were investigated.The results show that CeO2 reacted with SO2 to form Ce2?SO4?3 which could partially cover part of the acid sites over Cex/HBEA catalysts,inhibiting the NH3-SCR activity of Cex/HBEA catalysts.The sulfur tolerance of Cex/HBEA was improved by doping CeO2.Ce48/HBEA catalyst showed the best sulfur tolerance.In addition,sulfates could be removed from sulfur-poisoned CeO2 containing catalysts under high temperature thermal treatment in air.After regenerated at 800?,NOx conversion of sulfur-poisoned Ce48/HBEA catalyst was recovered.The NH3-SCR reaction mechanism was investigated by in situ DRIFT.NH4+adsorbed on BrOnsted acid site and NO3-adsorbed on Ceo2 were the intermediates during NH3-SCR reaction over Cex/HBEA catalysts.Both the L-H mechanism and the E-R mechanism existed over the Ce48/HBEA catalyst.C3H6 competed with NH3 to adsorb on the Bronsted acid sites over HBEA,inhibiting the NH3-SCR activity at low temperatures.However,C3H6 was oxidized to carbonyl and carboxylate by CeO2 over Ce48/HBEA.The carbonyl and carboxylate were further oxidized to CO and CO2.Thus,Ce48/HBEA catalyst had excellent propene poisoning resistance.In addition,after CeO2 sulfated at 350? for 12 h,So42-/Ceo2 solid super acid was formed,promoting the NH3-SCR activity.However,non-active Ce2?So4?3 was formed over catalyst by over-sulfating.On the one hand,Ce2?So4?3 covered the acid sites,inhibiting the adsorption of NH3;on the other hand,the formation of Ce2?So4?3 inhibited the redox cycle between Ce4+ and Ce3+.Thus,the formation of Ce2?SO4?3 inhibited the NH3-SCR activity of Cex/HBEA catalysts.Steady-state kinetic experiments were investigated on Ce4s/HBEA.At 225?,the reaction order of NH3 and O2 was 0,and the reaction order of NO was 0.75,indicating that NH3-SCR reaction was only affected by NO concentration.The reaction rate constant was 17.1 L g^-1 min^-1 at 200 OC and 22.7 L g^-1 min^-1 at 225?,indicating that the higher the reaction temperature,the faster the reaction rate.The reaction activation energy of Ce48/HBEA was 22.19 kJ mol^-1 which was lower than that of comercial vanadium-based catalysts(70-90 kJ mol^-1).