| Nitrogen oxide is one of the major air pollutants,which is extremely harmful to the natural environment and health of mankind.It is significant to study the removal of NOx.Selective catalytic reduction is currently the most widely used de-NOx technology,among which NH3-SCR is the most mature method.Noble metals such as Pt,Pd and Rh have been widely studied because of their excellent catalytic performance.However,high price and scarce resources restrict their industrial applications.It is imperative to find non-noble metal catalytic materials with wide sources and excellent performance.In this paper,based on the monolayer dispersion theory,ZSM-5 supports are used to load single component or multi-metal oxide as active components to prepare catalysts with excellent performance.The monolayer dispersion threshold of Co3O4 over ZSM-5 was determined with XRD and XPS extrapolation methods.The relationship between the surface physical and chemical properties,and the catalytic performance of NH3-SCR was investigated by XRD,N2-sorption,NH3-TPD,NO-TPD,O2-TPD,EPR and Py-IR.The mjor conclusions are summarized here:PartⅠ:A series of Co3O4/ZSM-5 catalysts were prepared by impregnation method for NH3-SCR.According to the monolayer dispersion theory,Co3O4 can spontaneously migrate and disperse on the ZSM-5 support to form sub-monolayers or monolayers.As determined by XRD extrapolation,the monolayer dispersion threshold of Co3O4 on the support is 0.061 mmol 100 m-2,which is equivalent to 4.5 wt.%Co3O4.As also determined by XPS extrapolation,which is 0.058 mmol 100 m-2 ZSM-5.In addition,the Co3O4/ZSM-5 catalyst showed a significant threshold effect,for which the catalyst near the threshold loading showed the best NOx removal efficiency.The results of H2-TPR and NO-TPD indicated that the addition of Co3O4 improved the redox ability and NO adsorption capacity of Co3O4/ZSM-5 catalyst.The results of NH3-TPD and Py-IR indicated that the catalyst near the threshold loading has the largest number of surface acidic sites,which is beneficial to the adsorption and activation of NH3 and played an important role in NH3-SCR reaction.The active oxygen species（O2-）on the catalyst surface also played a certain role in the reaction.The results of O2-TPD and XPS O1s showed that the catalyst near the threshold loading owns the largest number of the surface-active oxygen.In situ DRIFT results indicated that the reaction follows a Langmuir-Hinshelwood mechanism.PartⅡ:The first part of work has proved that 4%Co3O4/ZSM-5 prepared by the impregnation method shows good performance for NOx-SCR with NH3,but its reaction temperature range is narrow,and further research and improvement are needed.Both surface active oxygen and acidic sites are the key factors affecting the reaction.In order to investigate the regulation influence of adding metal oxides with different properties on the active oxygen species and acidic sites on Co3O4/ZSM-5 catalyst,a series of M-Co/ZSM-5（M=Cr,Pr,Sb,W）catalysts based on 4%Co3O4/ZSM-5 were synthesized in this part by adding the secondary metal oxides with redox or acidic properties.The effect on the activity of NH3-SCR was investigated,and the catalysts were characterized by XRD,NH3-TPD and EPR methods.XRD results demonstrated that the supported metal oxides were highly dispersed on the surface of the ZSM-5 and in an amorphous state.The reaction results showed that Cr-Co/ZSM-5 and Pr-Co/ZSM-5catalysts had better activity at low temperatures,while Sb-Co/ZSM-5 and W-Co/ZSM-5 catalysts had better activity at high temperatures.The results of NH3-TPD proved that Cr-Co/ZSM-5 had the most abundant surface acidic sites,which is the major reason accounting for its excellent low-temperature activity.The EPR results showed that the surface-active oxygen species of the catalysts was surface superoxide（O2-）species.In particular,the addition of W and Sb could promote the generation of O2-species,which was beneficial to the high temperature activity. |
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