The Mechanism Study Of Cu-Mn Bimetal Doped Zeolites Catalysts To Remove NO_x From Diesel Engine Exhaust

Nitrogen oxides(NO_x)emitted from diesel engine exhaust is the major air pollutant in our country,and it leads to acid rain and photochemical pollution.Selective catalytic reduction(SCR)technology is currently the main technology to remove NO_x from diesel engine exhaust.NH3-SCR catalysts based on zeolites have received extensively interests due to the unique structure,better adsorption ability and high stability.Although Cu-Mn/ZSM-5 and Cu-Mn/SAPO-34 catalysts show high catalytic activity in NH3-SCR at low temperature,the reaction mechanism on the two catalysts and the difference in stability have not been reported.In this study,in situ diffuse reflectance infrared Fourier transform spectroscopy(in situ DRIFTS)was used to investigate the adsorption of the reactant and their transformation on the surface of catalysts.Furthermore,the key intermediates that influence the activity of the catalysts and the SCR reaction mechanism were also analyzed.From the in situ DRIFTS analysis results,both the Lamgmuir-Hinshelwood(L-H)and the Eley-Rideal(E-R)reaction pathways are involved simultaneously on Cu-Mn/ZSM-5 and Cu-Mn/SAPO-34 in the NH3-SCR reaction at low temperature.The L-H reaction pathway is mainly the reaction between adsorbed NO2 and NH4+,and the E-R reaction pathway is between the gaseous NO_x and coordinated NH3.There is no difference in the catalytic activity between Cu-Mn/ZSM-5 and Cu-Mn/SAPO-34 through the E-R reaction pathway,and the main difference is due to the L-H reaction pathway.On Cu-Mn/SAPO-34,bidentate nitrate can betransformed to monodentate nitrate,and further to adsorbed NO2.Since the amount of nitrates formed on Cu-Mn/ZSM-5 is very low,there is not sufficient adsorbed NO2 generated from nitrates and participates in the L-H reaction pathway.Therefore,the catalytic activity of Cu-Mn/ZSM-5 was lower than Cu-Mn/SAPO-34.Base on the SCR reaction mechanism,the difference in hydrothermal stability and resistance to HC between the two catalysts was further invesigated.After hydrothermal aging treatment,the amount of adsorbed NO2 reduced significantly and the NH4+ on Bronsted acid sites was not generated on Cu-Mn/ZSM-5,which leads to the inhibition of the L-H reaction.On Cu-Mn/SAPO-34,the NO_x species,including adsorbed NO2 and nitrates,were still formed with high amount after hydrothermal aging treatment,and the amount of NH4+ on Bronsted acid sites even increased.Thus,much more NO_x can be removed through L-H reaction way on Cu-Mn/SAPO-34 than on Cu-Mn/ZSM-5,leading to higher hydrothermal stability of Cu-Mn/SAPO-34.From the in situ DRIFTS results of C3H6 adsorption,high amount of C3H6 could be adsorbed on Cu-Mn/ZSM-5,which occupied the active sites,thus NO_x was difficult to adsorb on Cu-Mn/ZSM-5,and NH4+ cannot formed on Br(?)nsted acid sites either.It leads to the inhibition of the reaction through L-H way.However,the adsorption of C3H6 on Cu-Mn/SAPO-34 is only one tenth of Cu-Mn/ZSM-5.After C3H6 adsorption,adsorbed NO2 and bidentate nitrate can be still fomed,and only part of monodentate nitrate formation is restrain.Furthermore,NH3 species can beeasily formed with large amount.Therefore,there are enough active intermediates to participate in SCR reaction,which means Cu-Mn/SAPO-34 has higher resistance to HC performance than Cu-Mn/ZSM-5.