Studying The Dispersion Behaviors Of Metal And Metal Oxide On Composite Oxide Supports And The Performance Of NO_x-SCR By C₃H₆

NO_x is one of the major air pollution sources,which can do great harm to human health and environment.Therefore,it is of great significance to investigate the elimination of NO_x pollution.At present,selective catalytic reduction（SCR）is the most widely used technology,and NH₃-SCR（NH₃ as reductant）and HC-SCR（hydrocarbon as reductant）are the two representative ones.Noble metals such as Pt,Rh and Pd are generally used in the processes because of their high catalytic efficiency.However,the limited resources and high price are driving people to seek for base metal oxide or metal replacement materials with low cost but with still excellent performance.With this objective,a series of effective catalysts were prepared by supporting metal or metal oxides onto composite oxide supports have been prepared and used for NO_x-SCR by C₃H₆.In addition,cold plasma treatment method is employed to further improve their performance.By using XRD,XPS,H₂-TPR,NH₃-TPD,In situ DRIFT,Raman,O₂-TPD,EPR and some other characterization techniques,the relationship between the surface physical-chemical properties and the catalytic performance of the catalysts has been explored.The mjor conclusions are summarized here:Part one:A series of SnO₂/ZSM-5 catalysts with different SnO₂loadings were prepared by deposistion-precipitation method,and used for C₃H₆-SCR reaction.Moreover,cold plasma treatment method is used to further improve the performance.According to the monolayer dispersion theory,SnO₂ can spontaneously disperse on H-ZSM-5 surface with a certain capacity.By using XRD extrapolation method,it is discovered that the monolayer dispersion capacity of SnO₂ on H-ZSM-5 is 0.271mmol/100 m²,being equivalent to 11.7 wt.%SnO₂.Furthermore,the dispersion degree of SnO₂ can be obviously improved by low temperature plasma treatment,which was increased to 0.380 mmol/100 m²,being equivalent to 15.5 wt.%SnO₂.Interestingly,both the non-treated or plasma-treated catalysts show evident threshold effect.In both cases,the catalysts with SnO₂ loadings around the monolayer dispersion capacity（10%SnO₂/ZSM-5 or15%SnO₂/ZSM-5-P）demonstrate the best reaction performance.It is worth noting that the number of acid sites on the surface of H-ZSM-5 was increased obviously after loading SnO₂,which can facilitate the adsorption and activation of the reactants,and is favorable to NO_x-SCR.By cold plasma treatment,the number of surface acid sites can be further increased,indicating the improved SnO₂ dispersion can increase the number of Sn⁴⁺cations.In addition,the surface active oxygenO₂^-species has also certain influence on the reaction performance of the catalysts.As testified by FTIR,the reaction follows a Langmuir-Hinshelwood mechanism,and the main reaction intermediates are acetate,monodentate and bridging nitrates.Part two:A series of Ag/Y₂Sn₂O₇ catalysts with different SnO₂loadings were prepared by impregnation method and used for C₃H₆-SCR reaction.Raman,UV-vis and XPS results have confirmed that Ag species mainly exists as metallic form.By using XRD extrapolation method,it is quantified that the dispersion threshold of Ag on Y₂Sn₂O₇ support is 0.516 mmol/100 m²,equaling to 4.06 wt.%.When Ag loading is not above this threshold,Ag is in a highly dispersed state.When it is above this threshold,Ag micro-crystallites are formed.The reaction results have proved that 4%Ag/Y₂Sn₂O₇ catalyst has the optimal reaction activity and N₂ selectivity.As evidenced by NH₃-TPD results,4%Ag/Y₂Sn₂O₇ possesses the largest number of surface acidic sites.Furthermore,EPR and O₂-TPD results substantiate that the active oxygen species on the catalyst surface is superoxide O₂^-anions.The addition of Ag can facilaite the generation of O₂^-species,thus being favorable to the reaction.