Design Of LDHs Functional Materials And The Performances Of Heavy Metal Ions Removaland Photocatalysis

Layered double hydroxides（LDHs）are a kind of unique two-dimensional layered structures with many structural advantages,including highly dispersed metal components,adjustable composition,interlayer anions exchangeability,high specific surface area,low cost,etc.Therefore,LDHs show great application prospects in heavy metal removal and photocatalysis.In this dissertation,on the one hand,based on the unique bonding mode and abundant multiple forces of LDHs,it was proposed to use metal oxides as a mineralizing agent to mineralize trivalent heavy metal ions(e.g.Cr³⁺)into the lattice of the laminate of LDHs to form Cr-based LDHs with minimal solubility product constant(K_sp),and thus,the heavy metal ions were super-stable mineralized.The mineralization mechanism of heavy metal ions was investigated by advanced spectroscopy characterizations and ex-situ techniques.Furthermore,from the perspective of resource recovery,the photocatalytic and adsorption performances of mineralized products were investigated.On the other hand,based on the characteristics of the adjustable metal composition of LDHs laminates and highly dispersed active metals,catalytic active metals such as Zn,Ni,and Ti were introduced into LDHs laminates.Furthermore,combined with the regulation of band structure and defect structure,the selective oxidation of photocatalytic small molecules（such as benzene and ethane）reaction was realized.Further,the deep understanding of the photocatalytic reaction mechanism was got through the combination of in situ DRIFTS,in situ XAFS characterization,and DFT theoretical calculation.It provides a new idea and method for the design of LDHs photocatalysts with high activity and selectivity.The specific research content of the thesis is as follows:1.Given the removal and resource recovery of Cr³⁺heavy metal ions from tanning leather wastewater,we used Cu O NPs as a mineralizing agent to remove Cr³⁺ions and form a CuCr-LDH structure through super-stable mineralization.The results showed that the adsorption capacity for Cr³⁺ions can reach 207.6 mg/g.Moreover,the actual tannery wastewater can be treated to meet the standard（<1.5 mg/L）.Furthermore,the mechanism of super-stable mineralization of Cr³⁺ion was studied by HRTEM,in situ and quasi-in situ XAFS spectroscopy.It was found that the process mainly involves two reaction pathways:"dissolution-reprecipitation"and"topological transformation".On the one hand,partial hydrolysis of Cu O occurred in weakly acidic chromium solutions,resulting in the release of Cu²⁺.Cu²⁺was then co-precipitated with Cr³⁺,Cl^-,and H₂O in solution to form CuCr-LDH products.On the other hand,Cr³⁺ions adsorbed on the Cu O surface can also form CuCr-LDH through the“topological transformation”process.More importantly,the mineralized CuCr-LDH can be utilized as a resource,showing excellent performance in the removal of azo dyes（Congo red and Evans blue）and photocatalytic CO₂reduction.The saturated adsorption capacities of Evans blue and Congo red was 248.0 mg·g^-1and240.1 mg·g^-1,respectively.In addition,under visible light（λ>400 nm）,CuCr-LDH can photocatalytic reduction of CO₂to obtain syngas（H₂/CO=2.66）,and the CO yield was as high as 52.06μmol?g^-1h^-1.This work provides a green chemical strategy for the super-stable mineralization and reuse of heavy metal ions in wastewater.2.To achieve selective hydroxylation of benzene to obtain phenol under normal conditions,green,cheap,and easily available air and water were used as the oxidant.Then,a series of Zn_xTi-LDHs（x=1,2,3,4）with various ratios of Zn²⁺/Ti⁴⁺were prepared by taking advantage of the adjustable ratio of LDHs laminate elements and controllable band structure and defect structure.Among them,Zn₂Ti-LDH showed the best phenol selectivity of 87.18%.Compared with conventional commercial photocatalysts P25,Ti O₂（anatase）and Zn O,Zn₂Ti-LDH also displayed the highest phenol selectivity.The results showed that the suitable band structure gave the appropriate oxidation capacity of LDHs thermodynamically,and the construction of defect structure improved the efficiency of carrier’s separation and migration kinetically,thus improving the performance of the photocatalytic selective oxidation of benzene.In addition,electron paramagnetic resonance and free radical quenching experiments displayed that the reactive oxygen species·O₂^-formed on the Zn Ti-LDH surface played a key role in the reaction.This work provides a new way for green fine chemical synthesis by combining band structure and defect engineering.3.Benefiting from the adjustable metal elements and highly dispersed active metal sites of LDHs,a series of MTi-LDHs（M=Zn,Ni,Cu）photocatalysts were prepared.The catalytic performance of the photocatalytic oxidation of ethane under full spectrum（IR-UV-vis）light assisted by CO₂was investigated.Ni Ti-LDH showed excellent n-butane selectivity（84.57%）and production rate（44.5μmol/g/h）.When the reaction gas（C₂H₆:CO₂）ratio was adjusted to 8:2,the selectivity and production rate of n-butane can be increased to 92.35%and 62.06μmol/g/h,respectively,and a large amount of syngas（CO and H₂）can be produced.XAFS,ESR,and photoelectric characterizations results showed that Ni Ti-LDH had the appropriate concentration of oxygen and metal defects,which can promote the separation and migration of photogenerated electron-holes,to improve the photocatalytic performance.In addition,DFT calculations displayed that C₂H₅·intermediates were more likely to be coupled to form n-C₄H₁₀products in Ni Ti-LDH catalysts,rather than over-dehydrogenation to produce other products such as C₂H₄and CH₄.