Bismuth-based Composite Materials For Photocatalytic Oxidation Of Cyclohexane

Selective catalytic oxidation of alkanes to produce oxygenated organic compounds with high added value is one of the most important challenges in the chemical and pharmaceutical industries.Among them,the catalytic oxidation of cyclohexane to cyclohexanol and cyclohexanone(KA oil)has been a research hotspot in the field of saturated C(sp3)-H bond oxidation.However,it is difficult to oxidize the C-H bonds to the C-OH and/or the C=O bonds because of their high bond energy.Meanwhile,the activation energy barrier of KA oil is lower than that of cyclohexane,and it is easy to be deeply oxidized,so it is difficult to balance the high conversion rate of cyclohexane and the selectivity to KA oil.Although new catalysts and oxidation processes have been developed and improved,the industrial process of cyclohexane oxidation still faces enormous economic and environmental pressure.As a new advanced oxidation technology,photocatalysis can directly convert green and sustainable solar energy into high-value-added chemical energy,which shows great potential in solving energy shortages and environmental pollution.The stable form((3 phase)of bismuth-based(Bi-based)catalysts has a layered structure formed by interlacing[Bi2O2]and anion or/and anion groups.Moreover,the Bi 6s orbital in Bi(Ⅲ)can be hybridized with the O 2p orbital to form a wider hybrid state,which can not only narrow the band gap and enhance optical absorption but also form a highly dispersible band structure to promote the rapid transfer of charge.So,they have a good application prospect in the fields of photohydrolysis of water,photocatalytic CO-,reduction,photocatalytic nitrogen fixation,and photo catalytic organic synthesis.However,the main reasons for the limiting photocatalytic activity are the low efficiency of photogenerated carrier separation,weak oxidative capacity,and low solar energy utilization.Therefore,in this dissertation,a series of new semiconductiver catalysts such as Bi2WO6 and BiOCl were developed by the strategies of building heterojunction,controlling morphology,and defect engineering.The active species and mechanism of cyclohexane oxidation of photocatalytic reaction were systematically described,which provided theoretical guidance for improving the photocatalytic activity of Bi-based catalysts.The main research contents of this work are as follows:(1)The Bi/Bi2WO6 heterojunction with adjustable metal Bi and oxygen vacancy(OV-Bi/Bi2WO6)was prepared by a simple one-step solvothermal method,and it is first used as a photocatalyst for the selective oxidation of cyclohexane to cyclohexanone.The effect of solvothermal temperature on the morphology,phase structure,and composition of the catalyst was systematically studied.The ratio of Bi:Bi2WO6 in OV-Bi/Bi2WO6 heterojunction can be adjusted from 0.08 to 8.43,and the introduction of DMF greatly increases the metal Bi content.The OV-Bi/Bi2WO6-160 heterojunction obtained at 160℃ solvothermal temperature showed the best photocatalytic activity(128.8 μmol with a selectivity of 93.6%)in cyclohexane to cyclohexanone,which was 4.4 and 8.8 times of pure Bi2WO6 and metal Bi,respectively.The results show that the enhanced activity of the photocatalyst is mainly attributed to the unique double active site in OV-Bi/Bi2WO6,oxygen vacancies can enhance the adsorption and activation capacity of Bi2WO6 for O2,while metal Bi can enhance the adsorption and activation capacity of Bi2WO6 for cyclohexane.At the same time,the photocatalyst also showed excellent durability due to the strong interaction between metal Bi and Bi2WO6.The active species in the photocatalytic process were determined according to the active species capture experiment and electron paramagnetic resonance(EPR)analysis,and the photocatalytic reaction mechanism was proposed based on the synergistic effect of oxygen vacancy and SPR of metal Bi nanoparticle.(2)Based on the results of the first part,the ultrathin Bi2WO6/BiOCl heterojunction nanosheet modified by oxygen vacancy(OV-Bi2WO6/BiOCl)was synthesized via the solvothermal method by changing the amount of potassium chloride(KCl).The microstructure,chemical composition,structure,and photochemical properties of the photocatalyst have been systematically analyzed by various characterization techniques.The photocatalytic activity(KA oil,484.4μmol)was significantly improved over OV-Bi2WO6/BiOCl with 1.8 and 9.8 times compared with that of the pure BiOCl and Bi2WO6 catalysts,respectively.Meanwhile,up to 92.5%selectivity to KA oil and 5 cycles without photocatalytic activity loss were achieved.The photogenerated holes(h+)and ·OH radicals are verified to be the predominant reactive species by trapping experiments and EPR tests,and a photocatalytic reaction mechanism was proposed based on heterojunction photogenerated carrier transfer mechanism and oxygen vacancy synergism.(3)Based on the unique open crystal structure of BiOCl,a PVP-coated stacked superstructure with exposed {001} face(OV-BiOCl-P)was synthesized by the solvothermal method.The effects of NH4Cl and PVP on the crystallinity,morphology and electronic structure of BiOCl were systematically investigated.The KA oil yield over OV-BiOCl-3@1.5PVP is 466.7μmol(340.8μmol for cyclohexanone and 125.9 μmol for cyclohexanol)with a selectivity of 93.9%.The photocatalytic process is accompanied by the in-situ formation of oxygen vacancies.After 5 cycles of experiments,the yield of KA oil can reach 92%,and the selectivity is unchanged.The active species in the photocatalytic process were determined according to the active species capture experiment and EPR analysis,and the photocatalytic mechanism was proposed.(4)Based on the results of the third part,the 2D nanosheet self-assembled 3D hierarchical structure BiOCl1-xBrx solid solution nanoflower with oxygen vacancy(OV-BiOCl1-xBrx)was synthesized by one-step PVP/EG assisted solvothermal method.The effects of the Cl/Br ratio on the composition,texture,morphology,and optical and electrochemical properties of OV-BiOCl1-xBrx were systematically investigated by various characterization techniques.The introduction of Broptimized the band structure and microstructure,thus broadening the light absorption performance of BiOCl.The OV-BiOCl0.67Br0.33 showed the best photocatalytic activity and the KA oil yield of 708.7 μmol(94.6%selectivity).It is 4.5 and 1.5 times that of the pure OV-BiOBr and OV-BiOCl,respectively.After 5 cycles,the production and selectivity of KA oil remained stable.Based on the active species capture experiment and EPR analysis,the active species in the photocatalytic process were determined,and the photocatalytic mechanism optimized by solid solution was proposed.