Metal-Organic Frameworks And ZnCdS Based Heterogeneous Photo-Fenton Pollutants Degradation As Influenced By Textural Properties,Lewis Acidity And Charge Separation

Heterogeneous photocatalytic technology promises a solution to the many challenges associated with water pollution and energy crises.Metal-organic frameworks（MOFs）and metal sulfides have received significant attention as promising photocatalysts due to their uniform but tunable porosity,tailorable chemistry,well-dispersed metallic components,suitable electronic and optical properties.However,small pores in microporous MOFs limit mass diffusion and access of bigger molecules to MOF cavities,thus preventing their applications in photocatalysis.Metals in the majority of MOFs are bounded by organic linkers,which reduces the Lewis acid and active metal sites.Moreover,the slow charge separation and redox reaction in metal sulfides are hindrance their photocatalytic activity.Improving the mass diffusion efficiency,structural properties,Lewis acidity and charge separation can ultimately enhance the pollutants degradation efficiency of a photocatalyst.Therefore,we tried to tackle those issues by the construction of a hierarchical mesocellular structure,creating unsaturated metal centers with the higher Lewis acidity and incorporation of metal sites.The main research works and results are as follows.We successfully synthesized stable M.MIL-100（Fe）（M;used for mesoporous hereafter）with mesocellular structure,open-pore channel and high specific surface area by in situ selfassembly method using unstable metal-organic assemblies（MOAs）.The prepared catalyst was then loaded with a certain amount of ZnO nanosphere（NS）to reduce the possible charge recombination rate during catalytic reactions.N2-sorption isotherm and pore size distribution（PSD）of M.MIL-100（Fe）and M.MIL-100（Fe）/ZnO NS showed the hierarchical mesocellular structure in the range of 3.31 to 5.5 nm with the BET specific surface area of 650 to 770 m2 g1.M.MIL-100（Fe）/ZnO NS showed enhanced photo-Fenton degradation efficiency of phenol（95%）,bisphenol-A（97%）and atrazine（82%）in the presence of 10 mmol H₂O₂ concentration at lower pH value.This degradation efficiency was much better than the degradation efficiency of pure MIL-100（Fe）catalyst（less than 45%for all three pollutants）.Similarly,M.MIL-100（Fe）/ZnO NS showed better TOC removal of phenol（48%）,bisphenol-A（43%）and atrazine（49%）than pure MIL-100（Fe）catalyst（less than 30%for all three pollutants）.M.MIL-100（Fe）/ZnO NS also showed good stability and reusability after four times the recycling experiment.The maximum leached Fe was 1.17 mg L-1 in M.MIL-100（Fe）/ZnO NS,which was lower than the environmental standard（2 mg L-1）.EPR analysis and using TBA as·OH scavenger confirmed that ·OH radical is the main reactive species produced during the photo-Fenton reaction.Such higher catalytic activity is attributed to the mesoporous structure and rich nanoscale cavities that let the reactants quickly diffuse to interior media and allow external and internal surfaces for entirely contact with the reactants.We tuned the Lewis acid sites by creating coordinatively unsaturated centers（CUCs）in MIL-88B-Fe with mix-valence（Fe2+/Fe3+）on ultrathin Ti₃C₂ nanosheet using thermal activation under vacuum condition.CUCs-MIL-88B-Fe showed larger and stronger sorption sites than MIL-88B-Fe,suggesting its higher Lewis acid sites for NH3 adsorption,as confirmed from TPD-NH3 analysis.The IR-pyridine study showed that CUCs-MIL-88BFe observed three peaks positioned at 1435,1470 and 1541 cm-1 suggesting its abundant Lewis,Br?nsted+Lewis and Br?nsted acid sites,respectively,CUCs-MIL-88B-Fe/Ti3C2 showed comparatively higher photo-Fenton degradation activity in terms of TOC removal（71%）,degradation of sulfamethoxazole（99%）and phenol（94%）than pristine MIL-88B-Fe（less than 50%）at pH 3.0 and 10 mmol H₂O₂ concentration.Very low phenol degradation（11%）and sulfamethoxazole（3%）were observed when we used H₂O₂ alone.Furthermore,CUCsMIL-88B-Fe/Ti3C2 exhibited no obvious declined in photo-Fenton degradation efficiency after five consecutive series of experiments,indicating its good stability.DMPO-trapped EPR spectra showed that-OH radical is the main reactive oxidizing species for the degradation of phenol and SMX.After creating unsaturated metal centers in MIL-88B-Fe,its Lewis acid sites increased,which ultimately enhanced H₂O₂ activation and thus catalytic degradation efficiency.The separation of photogenerated charge carriers is enhanced by the coupling of CUCs-MIL88B-Fe with 2D Ti3C2 nanosheet due to its high charge mobility.We proposed an active photocatalyst comprising of Pt-CrOx sites loaded on ZnCdS nanocrystals（ZnCdS/Pt-CrOx）that implying in situ process for H₂O₂ generation,efficient charge separation and fast redox reaction.The prepared catalyst showed excellent photoFenton phenol degradation（87%）,TOC removal（45%）with in situ H₂O₂ generation（1731.10μmol L-1）.The generated H₂O₂ could be utilized by applied Fe2+ to produce ·OH radicals during the photo-Fenton reaction.The catalyst also showed efficient photocatalytic hydrogen evolution reaction（HER）activity（21.1 mmol g-1 h-1）by operating the redox couple transport mechanism at higher pH conditions.Pt-CrOx sites provide efficient charge separation and appropriate reaction center for the reduction of O2 to H₂O₂ and proton to H2 during photoFenton and hydrogen evolution reaction,respectively.Pt acts as an electron sink and offers optimum hydrogen binding energy while CrOx act as a surface modifier that avoid back unwanted reaction and increase free charge carrier around Pt metal.Spectroscopic analysis of 2-hydroxyterephthalic acid（H2BDC-OH）showed that significantly more ·OH radicals are produced by ZnCdS/Pt-CrOx compared to pure ZnCdS.Pt-CrOx also enhances water dissociation kinetics as confirmed from the increasing spectra of adsorbed OH and water molecules of near-ambient pressure XPS（NAP-XPS）analysis.No obvious declined in photoFenton degradation and HER was observed after five consecutive series of experiments,demonstrating its high stability.