Theoretical Study On The Construction Of Layered Double Hydroxides As Photocatalytic And Soil Remediation Materials And The Related Mechanisms

Energy shortage and environmental pollution seriously restrict social and economic development.Layered double hydroxides（LDHs）have shown a bright application prospect in the field of photocatalysis and soil remediation because of their rich adjustability of host-guest structure,band structure and large specific surface area,which has attracted the extensive attention from both of industry and academia.The design and construction of materials are the basis of its further application.Therefore,in this dissertation,density functional theory（DFT）and molecular dynamics（MD）simulation methods are used to study the construction of LDHs materials for photocatalytic CO₂ reduction and soil remediation of super-stable mineralized LDHs materials,and reveal the related mechanism.The main research contents and conclusions of this dissertation are as follows:（1）The electronic properties of M₂^IIM^III/^IV-NO₃-LDHs(M^II=Mg²⁺,Co²⁺,Ni²⁺,Zn²⁺;M^III=Al³⁺,In³⁺,Cr³⁺,Fe³⁺;M^IV=Ti⁴⁺),such as band structure,electronic density of states（DOS）and band edge placement,as well as the thermodynamic mechanism of photocatalytic CO₂ reduction（CO₂PR）over these LDHs are calculated by DFT method.By analyzing of band structure and density of states,it is found that all LDHs except Mg₂Al-and Mg₂In-LDHs are responsive to visible light.The band edge placement indicates that all the LDHs except Ni₂Al-and Ni₂Fe-LDHs have sufficient driving force to undergo CO₂PR to products.According to the minimum-free-energy reaction pathway,for Co₂Fe-LDH,CO₂PR is more inclined to generate HCOOH,and for Mg₂Al-,Mg₂In-,Co₂Al-,Ni₂Ti-,Zn₂Al-,Zn₂Cr-,Zn₂Ti-NO₃-LDHs,CH₄ is the favorable final product via HCOOH,HCHO or CH₃OH intermediates.It is found that CO products can be obtained from Mg₂Al-,Mg₂In-,Co₂Al-,Zn₂Al-,Zn₂Ti-,Zn₂Cr-and Co₂Fe-NO₃-LDHs.The effective driving force（ΔΔG_b）is defined as the difference between the driving force of photocatalytic reduction of CO₂ and the potential barrier of the potential-determining step on the minimum-free-energy reaction pathway,it is found that Mg₂In-LDH has the largest effective driving force for photocatalytic reduction of CO₂ to CH₄,which is tend to the formation of CH₄ by CO₂photocatalytic reduction.Mg₂Al-LDH has the largest effective driving force photocatalytic reduction of CO₂ to CO,which is conducive to the formation of CO by CO₂ photocatalytic reduction.（2）For the mechanism of super-stable mineralization of heavy metal cations by LDHs,the solubility product,lattice energy and interaction force of M₂^IIM^III-A-LDHs(M^II=Mg²⁺,Cu²⁺,Cd²⁺,Ca²⁺,Fe²⁺,Ni²⁺,Zn²⁺,Co²⁺;M^III=Al³⁺or Fe³⁺;A=CO₃^2-,Cl^-)are calculated.Compared with the corresponding hydroxides and carbonates,this indicates that the ultra-low solubility product of LDHs is the driving force for its super-stable mineralization.The three removal mechanisms of Ca Al-LDH on five common heavy metal cations(Cu²⁺,Ni²⁺,Zn²⁺,Co²⁺,Cd²⁺),chemical binding mechanism,electrostatic binding mechanism and isomorphic substitution mechanism,are calculated.The effects of the introduction of H vacancy,OH vacancy and Ca vacancy on the action mechanism are also investigated.The results show that the mechanism of LDHs removing heavy metal cations tended to electrostatic binding mechanism regardless of the introduction of vacancy.Compared with the chemical binding mechanism and electrostatic binding mechanism,the energy barrier of isomorphic substitution of heavy metal cations with Ca vacancy is reduced the most,indicating that the presence of Ca vacancy is conducive to the occurrence of isomorphic substitution reaction.（3）Aiming at the problem of anion exchange,the binding energy between anions and laminates in 159 different anions intercalated M_R^IIAl-A-LDHs(M^II=Mg²⁺,Ni²⁺,Zn²⁺;R=1.4-8,A=OH^-,Cl^-,Br^-,NO₃^-,HCOO^-,C₆H₅SO₃^-,CO₃^2-,SO₄^2-,PO₄^3-)are calculated.The study indicates that for the same laminate ratio,the higher the charge of the anion,the stronger the binding ability with the laminate,and the easier it is to introduce into the LDHs interlayer.The calculation of anion exchange mechanism shows that the anion exchange order of anions in LDHs(PO₄^3->CO₃^2->SO₄^2->OH^->Cl^->Br^->HCOO^->NO₃^->C₆H₅SO₃^-)is not only consistent with the order of relative binding energy,but also closely related to the strength of hydrogen bond between host and guest,charge transfer and electronegativity of interlayer anions.By calculating the diffusion kinetics of anions between the layers of LDHs,the results show that the anion exchange reaction mainly occurs in the ab direction of LDHs cell,but not along the c direction.From the perspective of kinetics and thermodynamics.It is found that when the cell parameter c<24（?）,the anion exchange reaction is mainly affected by thermodynamic factors.When the cell parameter c>24（?）,both the thermodynamic and dynamic factors jointly affect the anion exchange reaction.These conclusions of the above work are consistent with the experimental results well.The work of this dissertation provides valuable theoretical information and helpful academic direction for the design and construction of LDHs based photocatalytic reduction of CO₂ and soil remediation materials.