Mechanistic Study Of Frustrated Lewis Acid-base Pair(FLP) In NO_x Selective Catalytic Reduction

Environmental problems have become a focus of close attention in today’s society,not only affecting our economy,culture and society,but also threatening our survival and development.Protecting the environment and realizing"sustainable development"have become the trend of the times in today’s society.Nitrogen oxides（NO_x）produced by the combustion of fossil energy are one of the main components of atmospheric pollution and the main cause of acid rain,photochemical smog,PM_2.5 and O₃.NH₃ selective catalytic reduction of NO_x（NH₃-SCR）technology is one of the most important tools used for NO_x elimination,and the core of this technology is the catalyst.The activation of N-H/N-O bonds on the catalyst surface involved in the reaction process is usually considered to be the rate-determining step of the reaction,which determines the NO_x removal efficiency.Frustrated Lewis pairs（FLPs）developed from the field of homogeneous catalysis have been widely explored and investigated in heterogeneous catalysis in recent years,and a series of heterogeneous catalysts represented by Ce-based catalysts have emerged to show significant advantages in catalytic activation of H₂ and catalytic reduction of CO₂.The essence of the enhanced catalytic performance is attributed to the effective activation of H-H and C=O bonds by FLP.In this thesis,the application of FLP was introduced into the NH₃-SCR reaction,the mechanism of N-H/N-O bond activation by FLP structure was clarified,and the structure-activity relationship between FLP-containing catalysts in the catalytic reduction of NO_x was investigated,and a high-performance de NO_x catalyst was designed and synthesized accordingly,which provides an important guideline for designing high-efficiency catalysts based on the basic principles of catalysis.In this thesis,Ce-dopedα-Fe₂O₃ high-efficiency metal oxide catalysts were firstly synthesised,which exhibited excellent de NO_x performance and water and sulphur resistance,and the effects of Ce doping on the acidity,redox properties and reaction mechanism of the catalysts were preliminarily investigated.Further,In order to reveal the structure-activity relationship of Ce-modified catalysts in the catalytic reduction of NO_x at the geometrical and electronic structure levels,single-atom Ce-dopedα-Fe₂O₃（001）catalysts with a defined morphology exposing specific crystalline facet were prepared.In the simulation of the reaction process of NH₃+NO+O₂ on the catalyst surface by density functional theory（DFT）calculations,it was found that the formation of-NH₂NO₂ intermediates in the L-H pathway induced the elongation and rupture of Ce-O bonds and the formation of FLP sites with the Ce site as the Lewis acid and O_lattice as the Lewis base,which facilitated the N-O bond activation in the rate-determining step.The FLP site appears during the reaction and disappears again after catalyst recovery.Based on such formation and action characteristics,the concept of transient frustrated Lewis pair（TFLP）was creatively proposed,which well explains the origin of the excellent catalytic performance exhibited by Ce-modified Fe₂O₃ catalysts.The generalisability of the mechanism of solid FLP in the NH₃-SCR reaction was further investigated in this thesis.CeO₂ nanorod catalysts with FLP sites for efficient conversion of NO_xwere designed by oxygen vacancy engineering,and it was found that the number of FLP sites in the catalysts could be further increased by prolonging the H₂ treatment time,and the ability to convert NO_x was also enhanced,thus the correlation between the appearance of FLP sites and the improvement of catalytic performance was clarified.Density functional theory（DFT）calculations clearly revealed that the FLP sites facilitated the catalytic reaction by achieving low-energy barrier activation of N-H/N-O bonds.Furthermore,it was confirmed that loading acidic metal oxides on the surface of CeO₂ nanorod catalysts with FLP（FR-CeO₂）could enhance the acidity of the catalysts,and the MoO₃/FR-CeO₂ catalysts were designed through the synergistic effect of acidic sites and FLP sites.Finally,it was found that the Lewis acid sites of FLP could also be directionally modulated by doping with other elements（e.g.Zr）,and the acidity enhancement promoted the adsorption behaviour of reactant molecules and lowered the activation energy barrier of the N-H bond,which resulted in a significant improvement of the catalytic NO_x elimination.