Preparation Of Molybdenum-based Nanoparticles Anchored PVDF Composite Mebmrane And Its Catalytic Reduction For Hexavalent Chromium

Aiming at the technical problems of persistent toxic heavy metal pollutants such as strong toxicity and difficult to repair,this paper uses polyvinylidene fluoride（PVDF）as a carrier to prepare nonprecious bimetallic（Mo,Fe）-N/C nanostructures loaded on PVDF membrane and zinc-doped molybdenum sulfide nanocatalysts anchored in porous membrane.The heavy metal hexavalent chromium was used as the target pollutant to construct a catalytic membrane-coupled formic acid catalytic reduction reaction system.In addition,the catalytic reaction system was deeply explored from multiple perspectives,such as reaction kinetics,influencing factors,and recycling.The mechanism of the chemical reduction of chromium is explained by the coupling structure of the membrane material,micro-composite state and metal nanoparticles.Through modern analytical testing techniques（XPS,XRD,AFM,TEM,SEM）,the catalyst composition,crystal,surface structure,and assembly rules are analyzed to explore the relationship among the catalyst composition,the structure and the performance.The main research contents and results are as follows:（1）With PVDF as the carrier,oxalic acid dihydrate（C₂H₂O₄·2H₂O）as the carbon source,ferrous sulfate heptahydrate（Fe SO₄·7H₂O）as the iron source,ammonium molybdate tetrahydrate(（NH4）₆Mo₇O₂₄·4H₂O)as the molybdenum source,and melamine（C₃H₆N₆）as a nitrogen source,a Mo_xFe_y-NC@PVDF composite catalytic membrane was synthesized by a two-step method（where x and y represent the mass ratio of Mo and Fe in the obtained composite material）.A series of characterization results show that the composite membrane material has a porous structure and Mo_xFe_y-NC nanoparticles are highly dispersed in the PVDF matrix.The composite membrane’s high surface roughness,asymmetric structure,and high porosity provide more landing points for the reaction.Therefore,the reaction efficiency is improved,the metal ions are effectively protected from air oxidation,and the technical problems of powder loss and difficult recovery are solved.The heavy metal hexavalent chromium was selected as the pollutant and formic acid was used as the induced reducing agent.The effects of a series of influencing factors on the percentage of catalytic reduction were investigated.The results show that the Mo_0.27Fe_0.8-NC@PVDF/FA system has the best catalytic performance,and the reaction conforms to the kinetic model.Repeatable experiments show that the catalytic membrane has good catalytic performance stability and structural stability.Based on the relationship among catalyst composition,structure,and performance,the mechanism of synergistic catalysis among composite components,micro-composite state,and catalytic performance was studied.The mechanism of Mo_xFe_y@NC/PVDF catalyst activating formic acid to remove hexavalent chromium was explained to provide relevant theoretical and technical support for the toxic heavy metal ion hexavalent chromium in highly efficient mineralized water.（2）Using zinc chloride（ZnCl₂）as the zinc source,thiourea（CH₄N₂S）as the sulfur source,and ammonium molybdate tetrahydrate(（NH4）₆Mo₇O₂₄·4H₂O)as the molybdenum source,a Zn-MoS₂ nanoparticle catalyst was prepared by high-temperature pyrolysis.Then,the Zn-MoS₂@PVDF catalytic membrane material was prepared by the dry-wet phase conversion method,and was successfully applied to the reaction system for the catalytic reduction of hexavalent chromium with formic acid.Comparison of different metal doping experiments shows that the catalytic efficiency shows a trend of Zn-MoS₂@PVDF>Cu-MoS₂@PVDF>Ni-MoS₂@PVDF>Co-MoS₂@PVDF.Taking Zn-MoS₂@PVDF as the object of investigation,the effects of different influence parameters such as reaction temperature,formic acid concentration,p H value,and hexavalent chromium concentration on the percentage of catalytic reduction were systematically studied.In addition,cyclic experiments prove that the composite catalytic membrane has excellent stability.The results of a series of characterization experiments show that the Zn-MoS₂ powder catalyst is uniformly doped in the PVDF membrane base and in the membrane pores,achieving high dispersion and no aggregation,reducing mass transfer resistance,and the powder catalyst can be used as a pore-forming agent to improve membrane porosity and hydrophilicity.In addition,the mechanism study shows that the strong electronic interaction between the tight interfaces between Zn S and MoS₂ nanosheets effectively improves the conductivity of Zn-MoS₂ and its electronic structure.The coexistence and synergy of Zn and Mo make it have high density defect sites.Non-metal nitrogen-carbon co-doping increases the active site,reduces the mass transfer energy barrier,promotes electron transfer,and significantly improves the catalytic reduction efficiency.