Study On The Mechanochemical Synthesis And Photocatalytic Performance Of LDH-based Nano Composite Mineral Materials

Layered double hydroxide(LDH),also known as layered anionic clay minerals,has a small amount of reserves in nature.It is currently mainly obtained by artificial synthesis and is widely used in the field of resources and environment.According to two important characteristics of the slab cation exchangeability and the interlayer anion exchangeability of the LDH minerals,the types of LDHs such as zinc,titanium and manganese having photocatalytic ability can be prepared.This type of LDH has excellent adsorption capacity and considerable photodegradation of organic pollutants,and can be used as a photocatalytic material or a photocatalytic carrier material.However,at present,LDH synthesis methods are mainly liquid phase method and mechanochemistry method.The liquid phase method is relatively mature,but it is difficult to simultaneously load heterogeneous materials onto the LDH mineral matrix because of the liquid phase between the reactions.The difference in pH of the precipitation is too large and there is interference between them.Mechanochemical method can effectively overcome the interaction between hydrotalcite and other materials,and realize the simultaneous synthesis of hydrotalcite and supported heterogeneous catalytic materials.Firstly,the MgAl-LDH precursor was used as the research object.The mechanochemical process was used to overcome the problem of excessive preparation of various components in the liquid phase synthesis of multi-component composite mineral materials.The visible light response CdS was synthesized by ball milling and loaded on the MgAl-LDH matix.Al(OH)3,dry grinding activated Mg(OH)2,anhydrous CdCl2 and anhydrous Na2 S were mixed and dry-milled according to the reaction ratio to obtain the powdered CdS/MgAl-LDH precursor.Then the precursor samples were stirred with water to remove the impurities and crystallize the LDH at the same time.This solid phase reactions were not affected by the difference in coprecipitation pH.After the CdS nanocrystals are highly dispersed on the MgAl-LDH matrix while the degradation rate of the organic dye methylene blue in 90 minutes was increased by 40% compared with the bulk CdS.Secondly,taking ZnAl-LDH mineral with semiconductor properties as the research object,the mechanochemical method was used to treat the zinc carbonate as the unified zinc source.The large gap between the formation condition of zinc sulfide and zinc-aluminum LDH during liquid precipitation,resulting in the difficult in silmultaneously perparation and combination,was overcomed by directly loading UV-responsive ZnS nanoparticles onto the zinc-aluminum LDH.While fully uniformly dispersed,ZnS forms a band matched heterojunction structure with zinc-aluminum LDH,which promotes the separation and transfer efficiency of photogenerated electrons and holes.After 180 minutes under UV irradiation,2ZnS/LDH can degrade 94% of Rhodamine B dye(250 mL,4 mg/L).On this basis,the ZnAl-LDH minerals were continuously studied.The ZnxCd1-xS solid solution with visible light response capability and Bi2S3,a typical lanthanide photocatalytic material,were loaded onto LDH by mechanochemical method.Simultaneous realization of synthesis,loading and composition of heterojunctions was occured.The obstacles that the liquid phase synthesis reaction of the LDH and the transition metal sulfide cannot be simultaneously performed in the same reaction field were overcome.The synthesis process of the heterojunction structure catalytic material was greatly shortened,and the operation complexity was reduced.At the same time,the formation of the heterojunction structure effectively promoted the utilization of visible light in the composite material.The band structure of the sulfide and hydrotalcite can effectively separate the photogenerated electrons and holes and inhibit their recombination.The elemental noble metal was used as raw material.The noble metal element was directly loaded on the ZnAl-LDH mineral by mechanochemical method to form a composite photocatalytic mineral material with surface plasmon resonance characteristics.The mechanochemical process overcomes the disadvantages during the noble metals doping in liquid phase,the rare kinds and high price of noble metal soluble salts,and the need of organic reducing agents.The noble metal can be directly loaded onto the LDH semiconductor to form a heterojunction with SPR characteristic which could enhance the migration ability of the photo-generated carriers and expand the visible light utilization range of LDH.Within 180 minutes,the photodegradation rate of the composite to methyl orange(200 mL,100 mg/L)was close to 100%.Based on the above synthetic studies,the synthesized CdS/MgAl-LDH,ZnS/ZnAl-LDH,ZnxCd1-x S/ZnAl-LDH,Bi2S3/ZnAl-LDH and Ag/ZnAl-LDH samples were subjected to experiments for photodegradation of organic dyes.The results show that the photocatalytic ability and efficiency of each composite mineral material are greatly improved compared with the pure phase sulfide and hydrotalcite synthesized by the same mechanical force chemistry,indicating that the mechanical chemical process is highly efficient in dispersing the catalyst and The composition of the heterojunction can effectively promote the photoelectrochemical properties of the mineral materials.At the same time,the photocatalytic mechanism of each composite mineral material was systematically analyzed and studied,which established the theoretical and experimental basis for the application of LDH mineral material as photocatalyst and photocatalyst carrier.