Design,Synthesis And Properties Of Rare Earth Composited LDH Based Heterogeneous Catalysts

In the fields of catalytic science,developing catalysts with high activity,selectivity and stability is the eternal theme.Precious metals（e.g.,Pt,Pd,Au,Ir and their alloys）are always employed to drive most of catalytic processes because they can exhibit inimitable performance beyond many other catalytic materials in the heterogeneous catalysis.However,precious metal catalysts all suffer from the disadvantages of limited reserve and high cost,determining that they are not viable for largescale industrial applications.In view of this problem,searching for easy-to-operate solutions has become a hot spot in the current field of heterogeneous catalysis.At present,there are two main types of design ideas:one is to optimize the catalyst support or introduce a promoter.Reducing the amount of precious metals under the premise that the performance is not affected（or even enhanced）.The second is developing transition metal catalysts by using nano-engineering design and other strategies to replace the traditional precious metals,which achieve the same or even exceed catalytic performance.Therefore,the design of new catalyst support,promoter and catalyst synthesis methods have important theoretical significance and application value for the development of heterogeneous catalysis.Layered Double Hydroxides,（LDHs）are a class of anionic two-dimensional layered compounds.LDHs are built up by the periodic stacking of positively charged(M²⁺,M³⁺)（OH）₆ octahedral layers and negatively charged interlayer galleries consisting of anions and water.In the field of heterogeneous catalysis,LDHs offer a number of advantages:（I）cation-tunability of the main layers and anionic exchangeability gives LDHs flexible versatility in composition;（II）the high adsorption capacity of LDHs make them effective supports for the immobilization of catalytically active species on the surface;（III）the uniform dispersion of M²⁺and M³⁺cations in the layers,as well as preferred orientation of anions in the interlayer,provide a prerequisite for the LDHs to be highly stable dispersed heterogeneous catalysts;（IV）combining LDHs with other materials to form nanocomposites with hierarchical structure enhances the catalytic activity and stability by the benefits of synergistic effects between the two materials.Consequently,the rational design and controllable preparation of LDHs based catalyst with enhanced activity and stability have valuable application prospects.Our group is dedicated to the research work of rare earth functional nanocomposites.Rare earth elements known as"industrial vitamins"have 4f layer electronic structure,coordination number,high oxygen storage capacity and other unique properties,which determine its significant application in catalysis field.In this thesis,we used LDHs as nanomaterial matrix and controlled the structure by introducing rare earth elements.Then we discussed the material assembly strategy in detail and studied the application as heterogeneous catalysts in the fields of organic pollutant degradation and electrocatalytic oxygen evolution reaction（OER）.This research provides controllable synthesis,performance control and catalytic mechanism of heterogeneous catalysts from three dimensions:designing new catalyst supports,new promoters and new catalyst synthesis method,which provides a novel idea for broadening the application of rare earth composites in the field of catalysis.The dissertation includes following five chapters:Chapter 1:a brief review of investigation progress of rare earth related and LDHs-based heterostructure were summarized.Chapter 2:yttrium doped magnetic LDHs loaded with nano-gold toward organic pollutant degradation.In this part of the work,a rare earth Y-doped magnetic core-shell structure was prepared as a catalyst support by a simple co-precipitation method.After loading the Au nanoparticles,it exhibited excellent catalytic reduction of 4-nitrophenol.The magnetic Fe₃O₄ core allows the catalyst to be easily separated from the reaction system by a magnet.The LDHs shell can increase the loading efficiency of Au while effectively avoiding the agglomeration of nanoparticles.More importantly,doping of the rare earth Y effectively can adjust the electronic structure of the catalytically active component,thereby improving the catalytic efficiency.Chapter 3:in situ growth of ceria on cerium–nitrogen–carbon as promoter for oxygen evolution reaction.In this chapter,we successfully designed and synthesized CeO₂ nanoparticles uniformly distributed on CeNC structure with graphene oxide as precursor.It combines with exfoliated NiFe-LDH to exhibit excellent electrocatalytic OER activity and stability.In the 1 M KOH solution,when the current density is 10 mA cm^-2,the overpotential of the catalyst is only 235 mV,which is far superior to the commercial20%Ir/C and other Ni-based OER catalysts reported in papers.As a promoter,CeO₂@CeNC plays a key role in the electrocatalytic process.This part of the work provides a new idea for the development of rare earth doped high performance electrocatalysts.Chapter 4:core-shell heterostructural,ZIF-67 derived layer double hydroxides with encapsulated CeO₂ for boosting oxygen evolution reaction.We constructed a novel CeO₂@NC/LDH core-shell catalyst using a self-success template synthesis strategy.The catalyst exhibits excellent OER catalytic activity and stability in an alkaline electrolyte.This high performance mainly benefits from the introduction of rare earth CeO₂,which effectively regulates the electronic state of the catalyst and forms more defect structures.In addition,this surface in situ growth strategy can also prevent LDH sheets agglomeration and increase the number of active sites.More importantly,the synthetic strategy of this heterogeneous core-shell structure can be further extended and applied to other catalytic fields.Chapter 5:well defined Ni_xFe_3-xO₄ Nanoparticles as catalytic anodes to oxide 5-hydroxymethylfurfural（HMF）.We explored the activity and selectivity of different sizes of Ni NPs as HMF electrochemical oxidation catalysts,demonstrating the size effect of the catalyst.In view of the poor stability of Ni NPs and the lack of agglomeration during the catalytic process,we synthesized Ni_xFe_3-x-x O₄ NPs and stabilized the active component Ni with Fe_3-xO₄ matrix.When the ratio of Ni/Fe is 1/1,the catalyst has the best HMF conversion rate and FDCA faradic efficiency,which is similar to the activity of pure Ni NPs.In view of the formation of bimetallic oxides after hydrotalcite（LDHs）calcination,we suspect that LDHs and rare earth composited LDHs may have potential applications in the field of HMF catalytic oxidation.