Study Of Structural Regulation Of Iron Based Catalysts And Reaction Mechanism For The Heterogeneous Fenton Reaction

Iron oxides have been extensively used in heterogeneous Fenton reactions.Among them,Fe₂O₃ is one of the most promising catalysts for industrial organic wastewater treatment in the future due to its advantages of low cost,environmental benign,fairly strong chemical reactivity.Unfortunately,the large particle size and easy agglomeration,low specific surface area,poor redox capacity and stability lead to tardy Fenton reaction kinetics and iron release,and then affect the catalyst circulation and regeneration.These drawbacks have been hampered its industrial application.In addition,the co mplexity of the heterogeneous Fenton reaction process and catalytic system hinders the study of the reaction principle,and therefore it is very difficult to explore the real active center,reactive species and degradative path.Herein,Fe₂O₃ was taken as the research object,and the Fenton reaction activity of the iron-based catalyst was improved by adjusting its morphology,improving the dispersion of the active components,and designing efficient bimetal synergistic active sites.Furthermore,redox property and stability of the catalytic material was enhanced by constructing Fe-N-C coordination active sites and form abundant oxygen vacancies on the surface,thereby increasing its application value.Finally,the structure-activity relationship and Fenton reaction mechanism of the heterogeneous catalyst were studied in depth.The specific research contents were obtained as follows:（1）The structure and properties of Fe-based（Fe₂O₃）and non-Fe-based（vanadium phosphate,VPO）materials with excellent redox properties were compared in order to select suitable heterogeneous Fenton catalyst.Nanoporous VPO and Fe₂O₃ catalysts were prepared,the results of Fenton oxidation degradation of MB dye showed that reaction performance of Fe₂O₃（210 min,67%degradation）was superior to VPO and reached 5 times the commercial Fe₂O₃ due to its smaller grain size,larger specific surface area,suitable Fe^Ⅱ/Fe^Ⅲratio and stronger oxidation-reduction property.In addition,Fe₂O₃ catalyst had higher stability than VPO catalyst,the reason was that the VPO catalyst was rapidly lost,while ICP test indicated that the reaction liquid contained no iron ions,thus Fe₂O₃ was used as heterogeneous Fenton catalyst for next research.（2）In order to improve the Fenton reaction activity of the iron-base catalyst,iron-based bimetallic oxide with different morphologies,structures and compositions were constructed and the structure-activity relationship of catalysts and the synergistic catalytic effect formed by metal doping was explored.Specifically,Fe₂O₃-Ti O₂ composite oxide nanofibers with given morphology and structure were synthesized by electrospinning.The grains in the composite catalyst were very evenly dispersed,and a more efficient triple heterojunction as new active site(Ti O₂/Fe₃Ti₃O₁₀/Fe₂O₃)was formed.The nanofiber structure was able to decrease grain size of Fe₂O₃,enhance specific surface area and pore volume,which were more beneficial to expose the active site on the surface and transfer reactants.It was found that when the mass fraction of Fe₂O₃ in the composite fiber is 40%,the Fenton degradation activity of MB was the best,the MB degradation rate reached 80%after 120min reaction.For further optimizing the structure of iron-based catalyst,and exploring the synergistic effect of Fe₂O₃ with other transition metal oxides,CuO with strong redox cycle ability was used as dopants to prepare Fe₂O₃-CuO composite nanoparticle by simple and green low-temperature pyrolysis method.Compared with Fe₂O₃-Ti O₂ composite nanofiber,Fe₂O₃-CuO composite nanoparticle possessed smaller grain size of Fe₂O₃,larger specific surface area,and CuO promote the electron transfer on the surface of Fe₂O₃,resulting in the stronger synergistic effect of two metal oxides.Therefore,the activity of Fe₂O₃-CuO catalyst（120 min,93%degradation）was obviously higher than Fe₂O₃-Ti O₂,（3）In order to further enhance the application value of iron-based catalysts,on the basis of the Fe₂O₃-CuO composite oxide catalyst,polyvinylpyrrolidone（PVP）was added to the synthesis system as a source of C and N,and PVP was carbonized to form pyrrolidine to bond with Fe atoms in Fe₂O₃ and simultaneously the Fe-O bond broke to create oxygen vacancies.It was found that the catalyst（FeCu@CN-Py-250-3）possessed multistage pore and honeycomb structure and rich hydroxyl groups on the surface,which was more benefical to the adsorption of reactants and H₂O₂.Moreover,it also had strong electron transfer ability and high oxidation reducibility,and the incorporation of pyrrole nitrogen can stabilize the structure of Fe₂O₃ so as to no iron dissolution after the reaction,and the carbon layer has a good dispersion effect on the active site.The above significantly enhanced the activity（120 min,98%degradation）and cycle stability（the degradation rate of MB was not found to decrease distinctly after the reaction was repeated for 5 times）of FeCu@CN-Py-250-3.Further,the high magnetic property of the catalyst made it very convenient to separate and recovered from the testing solution.It is worth pointing out that the cyclic stability of the catalyst was higher than that of most reported catalysts,and a variety of anions were added to the reaction solution,the degradation rate of MB did not decrease significantly,and FeCu@CN-Py-250-3 was used for catalytic oxidation of some common organic pollutants,the good degradation results were obtained.The above provided a new guidance for rational design of iron-based catalysts.（4）To deeply investigate the mechanism of MB degradation by heterogeneous iron-based catalyst,FeCu@CN-Py-250-3 with the best comprehensive performance in this paper was studied as an example.The active substance trapping agent experiment and EPR characterization were first carried out.The results showed that·OH and ¹O₂ were major active species in the degradation of MB.In addition,the intermediate products of MB Fenton degradation were analyzed by LC-MS,and a relatively complete reaction mechanism was proposed.The active species preferentially attacked the C—S⁺=C and C—N=C bonds of MB,which caused its phenothiazine ring to break and the benzene ring to rupture further.Eventually,C₄-C₈ products such as N-formyl-N-phenylformamide,6-formyl-2-hexenoic acid,4-nitro-2-butenic acid and benzenesulfonic acid were formed.The reaction process includes hydroxylation,demethylation（decolorization）,ring cracking,oxidation,hydrolysis,deamination,denitrosylation,desulfation and other complex reactions.In this paper,reasonable proposition and detailed verification are carried out.Furthermore,the results of DFT calculation showed that the decomposition energy barrier of H₂O₂ on the surface of catalyst with different structures was distinguishing（Fe₂O₃-CuO（FeCu-Py-250-1）and FeCu@CN-Py-250-3 were 0.36 e V and 0.32 e V,respectively）.Therefore,H₂O₂is more likely to break into·OH on the surface of FeCu@CN-Py-250-3.Based on the above experimental results and theoretical calculations,the reaction mechanism of FeCu@CN-Py-250-3 degradation of MB was obtained,which made up for the deficiency of exploration of MB degradation path by current heterogeneous iron-based catalysts to a certain extent.