Influence Of Alkali And Alkaline-earth Metals In Ferrierite On C₂H₂-SCR

The elimination of NO_x with high selectivity in the temperature range of 300～400℃from lean-burn and diesel engines exhaust is a much improtant subject in the world.The aim of this work is to investigate the catalyst with high activity for the selective catalytic reduction using acetylene as reductant(C₂H₂-SCR)under oxygen-rich conditions,in order to provide the basis for the elimination of NO_x.To improve the activity of catalysts at low temperature in the selective catalytic reduction of NO_x,the performance of the catalysts prepared by incorporating different metals into H-ferrierite(HFER)zeolite was investigated.It was found that higher NO conversion was achieved over the resulting materials obtained by part of protons in the zeolite exchanged with metals(e.g.Na,Ba or Mg)compared to that over HFER.It is the frist time to propose that the appropriate amount of alkali or alkaline-earth metals presented in FER zeolite is favorable for C₂H₂-SCR.The influence of sodium in FER zeolite on selective catalytic reduction of NO by acetylene(C₂H₂-SCR)was investigated in detail in the temperature range of 250-450℃. Sodium incorporation into the zeolite(with an exchange level of protons less than 11.8%) significantly promoted the activity of the zeolite for C₂H₂-SCR,and large amount of sodium (more than 31.5%)depressed the title reaction.Based on the characterizion of NO_x-TPD and FT-IR,it is proposed that active nitric species(NO⁺,bridging and bidentate nitrate)formation on FER zeolite is a crucial step for C₂H₂-SCR.The active nitrate species(bridging and bidentate nitrate)formation capacity of the zeolite was improved by the small amount of sodium incorporation into HFER.However,the exchange of proton by sodium with the higher level resulted in the formation of inactive nitrate species on the zeolite rather than the active nitrates.Furthermore,the over-exchange of proton by sodium led to the activity of the zeolite for NO oxidation diminishing and thus made the rate of active nitric species formation decrease remarkably.Therefore,the zeolite containing large amount of sodium obtained by exchange behaved rather worse catalytic performance in C₂H₂-SCR.In addition,we found that NO⁺ was also the active nitric species for C₂H₂-SCR,which mainly produced on HFER zeolite.The formation of NO⁺ in the zeolite was considerably depressed by the over-exchange of proton by sodium,which is also a reason leading to the lower NO conversion to N₂ over the zeolite.Small sodium incorporated in FER zeolite not only promoted the title reaction but also suppressed the combustion of C₂H₂,and therefore 91%NO conversion over Na(1)HFER zeolite(with the exchange level of 11.8%)was achieved,which is 9%higher than that obtained over HFER in the same conditions.