Design Of Cobalt-based Heterogeneous Catalysts With Highly Efficient Performance For Activation Of Peroxymonosulfate In Organic Pollutants Degradation

Sulfate radical(SO₄^–·)based advanced oxidation process(SR-AOP)is one of the most effective and promising technologies for organic wastewater treatment due to its outstanding advantages such as high degradation efficiency,mild conditions,wide p H range,and easy operation.Activation of peroxymonosufalte(PMS)is an important way to produce SO₄^–·,and various activation pathways have received extensive attention.Numerous studies have shown that cobalt-based catalysts show excellent potential for PMS activation,which not only overcome the limitation of physical activation requiring specific instrumentation but also have superior electron transfer and regeneration properties compared with other transition metals.However,homogeneous cobalt-based catalysts are difficult to be separated and recovered and are prone to secondary contamination and high cost,which seriously hinder their practical application.In this context,cobalt-based heterogeneous catalysts(especially Co₃O₄)have rapidly become a research hotspot in this field owing to their advantages including recyclability and effective avoidance of secondary pollution.However,compared with homogeneous cobalt-based catalysts,heterogeneous cobalt sites often exhibit significantly lower catalytic activity due to material constraints,resulting in less satisfactory treatment of many difficult-to-degrade organic pollutants.Therefore,the rational design of new cobalt-based heterogeneous catalysts to significantly improve the catalytic activity is one of the key research topics for SR-AOPs.The modulation of chemical composition can usually directly affect the electronic structure of active sites,modulate the redox properties and occupy a key position in improving the catalytic activity.Special morphological structures(such as spherical,floral spherical,eggshell-yolk structures,etc.)have significant advantages in fully exposing the sites,enhancing mass transfer,and improving the host-object contact rate.Based on composition engineering strategies,this thesis has successfully explored a variety of simple and controllable material preparation methods,developed a series of new cobalt-based heterogeneous catalysts combining inert metals(Al,Ti),nonmetal(phosphorus),and cobalt,carved out the physical and chemical structure information in detail,realized the efficient and long-lasting degradation of various organic pollutants,characterized the electronic structure,redox properties,and interaction behavior with PMS,elaborated the intrinsic activation mechanism of the catalytic materials with the help of modern characterization techniques.The details of the study are as follows.(1)Enhance activation performance of Co₃O₄ toward PMS for rapid degradation of organic pollutants via surface chemical composition modulation.First,postsynthetic incorporation of catalytically inert Al into Co₃O₄ boost catalytic performance for peroxymonosulfate activation.The Al-doped Co₃O₄ nanomaterials(ACO)were obtained by an ion-exchange-calcination process with commercial cobalt trioxide as the parent material.The characterization results showed that the introduction of Al did not change the microsphere morphology and crystal structure of the parent material.In degradation experiments with use of metronidazole(MNZ)as the target pollutant,ACO-2 showed excellent catalytic activity and could completely degrade MNZ at a concentration of 10mg L^–1 within 40 min at p H=7,t=25^oC and[PMS]=1.5 m M.The removal rate of total organic carbon(TOC)reached 51%.DFT calculations showed that the introduction of Al significantly promoted the adsorption of Coonto PMS,enhanced the electron transfer of Coto PMS and improved the activation of peroxide bonds.Besides,the ACO-2 catalyst maintained high catalytic activity after six cycles with cobalt leaching below 0.079 mg L^–1,implying excellent cycling stability.Moreover,enhance PMS activation by modulating the electronic structure of the cobalt site using P.Partially phosphorus-substituted Co₃O₄catalyst materials(P-Co₃O₄)were prepared by high-temperature calcination of commercial Co₃O₄ in an environment containing NaH₂PO₂.After a series of characterization tests,the phosphorus-doped material maintained the microsphere morphology of the parent intact,and significantly increased the oxygen vacancy concentration and hydrophilicity.More importantly,the P doping not only induce Cosites more exposed,and makes it easier to adsorb PMS,but also allows the electronic restructuring of the Cosites,which in turn increases the number of electrons transferred to PMS and enhances the activation of O–O bond.In the experiments of catalytic degradation of organic pollutants,P-Co₃O₄ showed excellent catalytic activity and good cycling stability.Combined with EPR tests and a series of control experiments,it was confirmed that SO₄^–·and singlet oxygen(¹O₂)were the dominant oxidation active species,and the mechanism of free radical generation was systematically discussed.The possible degradation pathways of rhodamine B were given based on the results of degradation intermediate product analysis tests.(2)Constructing new Co₂TiO₄ spinel catalyts with highly exposed sites and easy mass transfer for the efficient activation of PMS.The Co₂TiO₄ spinel material was prepared in a simple and controlled manner by the"one-step hydrothermal"method.The material exhibits monodisperse,highly open three-dimensional spherical microscopic morphology with a high specific surface area of 113.6 m² g^–1.EPR,H₂-TPR,and DFT calculations show that the Cosites exhibit electron-rich properties due to the presence of Tielements,thereby induce excellent electron-giving ability for the rapid activation of PMS.Under the conditions of pH=7,t=25^oC,and[PMS]=1 m M,the ofloxacin(OFX)at a concentration of 25 mg L^–1 could be completely degraded within 20 min.The fitted apparent reaction rate constant k(1.394 min^–1)was 32 times higher than that of the benchmark catalyst Co₃O₄(0.044 min^–1)under the same conditions and was superior to most reported Co-based heterogeneous catalysts.In addition,the material exhibited excellent catalytic activity for a wide range of organic pollutants.(3)Large scale of fabrication of CoTiO₃/TiO₂ three-dimensional flower-like microspheres and"yolk-shell"structured CoTiO₃@Co₃O₄ nanoreactor with rich"Co–O–Ti"units toward efficient activation of PMS for degradation of organic pollutants.A novel CoTiO₃/TiO₂ composite composed of CoTiO₃ overlayer on nanosheets-assembled hierarchical TiO₂ nanospheres was synthesized involved a hydrothermal–calcination process with the use of amorphous TiO₂ nanosphere as a template precursor and CoCl₂·6H₂O as cobalt source,respectively.(100 g could be obtained in a single synthesis).The characterization results showed that the material exhibited a three-dimensional flower-like morphology with a specific surface area of 114.8 m² g^–1.Theoretical calculations indicated that the low electronegativity Titransferred electrons to the Cosites to obtain low oxidation state Co,which enhanced the adsorption ability to PMS,increased the number of transferred electrons,and facilitated the peroxide bond breaking.Based on its unique advantages in chemical composition and texture,CoTiO₃/TiO₂ can rapidly degrade a variety of organic pollutants such as ofloxacin,norfloxacin,methyl orange,and rhodamine B in a short period of time.Its catalytic activity was much higher than that of CoTiO₃ alone and the benchmark catalyst Co₃O₄ under the same conditions.In addition,the catalyst showed an excellent catalytic effect in real water samples and exhibited satisfying cycling stability.Secondly,CoTiO₃@Co₃O₄ nanoreactors with unique"yolk-shell"structure were prepared via hydrothermal etching-calcination using amorphous TiO₂ and CoCl₂·6H₂O as titanium and cobalt sources and employed to simulated organic wastewater degradation experiments with rhodamine B.The characterization and experimental data show that the Co₃O₄ shell of CoTiO₃@Co₃O₄ is permeable,allowing the rapid passage of guest molecules such as PMS and RhB,and the nano-scale cavity provides a sui Table site for the catalytic reaction,which is conducive to improving the mutual contact between the host and the guest,and enabling the catalytic generation of free radicals to oxidize organic pollutants nearby.Comparative experimental results show that the catalytic activity of this nano-reactor is significantly better than that of any single component under the same conditions.Meanwhile,the"yolk-shell"structure and high catalytic activity were retained after seven cycles,indicating that the material has good structural rigidity and catalytic stability.