Study on the Design,synthesis, Characterization and Catalytic Performance of Fe-based Catalysts Supported on Beta Coated by Oxide Thin Films for Nh3-SCR Reaction

Author:Liu Ji Sheng

Supervisor:liu jian zhao zhen


Degree Year:2018





Nitrogen oxides(NOx),originated from diesel exhaust,have given rise to serious environmental problems.Restrictions that confine the NOx emission from diesel exhaust are becoming more and more stringent due to the enhancing of people’s environmental awareness as well as the growing of stringent environmental protection laws and regulations.Ammonia selective catalytic reduction(NH3-SCR)is considered as an economic and effictive method for NOx abatement.The commonly used NH3-SCR catalyst is V2O5-WO3(MoO3)/TiO2.However,some inevitable drawbacks still remain in this system,such as the narrow temperature window(300400°C),the low N2selectivity and the high activity for SO2 oxidation to SO3 as well as the toxicity of vanadium species,which greatly limit its use in industrial applications.Therefore,there is a starvation for developing superior and environmentally-benign vanadium-free SCR catalysts.Recently,attracted by the competitive activity,Fe-based zeolite catalysts have been extensively studied over the past decades and considered as a promising substitute for vanadium-based catalyst for NH3-SCR of NOx.However,the low-temperature SCR performance,the high-temperature hydrothermal stability,the H2O-,SO2-and hydrocarbons-tolerance of Fe-based zeolite catalysts is still undesirable.Therefore,herein series of core-shell structural deNOx catalysts using small-grain Beta supporting FeOx nanoparticles(NPs)as the core and oxide thin films(TiO2,SBA-15,CeO2,)as sheaths were designed and controllably synthesized.Their structure and physico-chemical properties were characterized by XRD,TEM,N2 adsor-desorption,SEM,XPS,NH3-TPD,XANES,ICR-AES,H2-TPR and In-situ DRIFTS,and their catalytic performances were tested for NH3-SCR of NOx.The main conclusions are as follows:(1)Fe/Beta@TiO2 core-shell structural catalyst with TiO2 thin film as shell(5nm)was successfully synthesized by a self-assembly method.It was found that TiO2shells not only decrease the acidity of the catalyst,which could be beneficial to inhibiting the adsorption of acrolein species from covering active sites,but also promote the formation of chemisorbed oxygen species(O2-,O-),and then improve the ability of NO oxidation to NO2 during NH3-SCR.Moreover,TiO2 shells can serve as an effective barrier to suppress the active metal oxide nanoparticles from aggregating at high temperature,and thereafter leading to an improved thermal stability.The NOx conversion still keeps above 87%at 450°C over Fe/Beta@TiO2 in the presence of 500ppm propene.(2)A series of Fe/Beta@SBA-15 core-shell structural catalysts were designed and controllablly constructed by an ultra dilute liquid phase coating method.It was found that their acidity and redox properties were strongly dependent on SBA-15 shells.Fe/Beta@SBA-15-1 catalyst with10 nm SBA-15 thin film thickness exhibits the best NH3-SCR performance.NOx conversion reaches above 80%in the wide temperature range of 325600°C in the presence of 5%water vapor and 500 ppm propene under GHSV=125,000 h-1.The kinetics results indicate that the mesoporous structure of SBA-15 shell lowers the diffusion limitation and promotes the reactants accessing to active sites.Moreover,TGA and in-situ DRIFT results reveal that SBA-15 shell can not only prevent the generation and deposition of coke and nitrate species from blocking active sites but also serve as an effective“obstacle”to inhibit active FeOx nanoparticles from agglomerating at high temperature,leading to the higher propene resistance than Fe/Beta.(3)A series of Fe/Beta@CeO2 core-shell structural deNOx catalysts with small-grain Beta supporting FeOx NPs as the core and tunable CeO2 thin film as sheaths,were designed and controllably synthesized by a self-assembly method.The results reveal that CeO2 shell thickness plays an important role in influencing the acidity and redox properties of the catalysts,and thereafter affects their catalytic performances.Fe/Beta@CeO2-1 catalyst with proper CeO2 thickness(10 nm)can not only enhance the redox property,but also promote the formation of NO2 and cis-N2O2-species,and thereby exhibit excellent resistance to H2O and SO2 and high NOx conversion(above90%)in the wide temperature range(225565°C).But too thick CeO2 shell(20 nm)can not only result in the dramatically reduction of acidity but also induce the formation of inactive nitrate species,and thereby lead to a remarkable decrease of high-temperature activity of Fe/Beta@CeO2-2 catalyst.Moreover,the kinetics result indicates that the coating of CeO2 shell significantly increases the pore diffusion resistance of Fe/Beta@CeO2 catalysts.(4)MoFe/Beta@CeO2 core-shell structural catalyst with CeO2 thin film as shell(10 nm)was successfully prepared by a self-assembly method.CeO2 shells can not only serve as an effective barrier to inhibit the active metal oxides nanoparticles from aggregating at high temperature but also restrain the generation of iron sulfate,leading to a higher SO2 and H2O-tolerance.Thus,it can be a promising substitute for vanadium-based catalyst for NH3-SCR of NOx and open up a new way for designing highly effective NH3-SCR catalysts.Therefore,series of NH3-SCR core-shell catalyst with excellent propene-,H2O-and SO2-tolerance were obtained using the interface effect between oxide thin film and zeolite.