Construction Of TiO₂-based Heterojunction Catalysts And The Photocatalytic Selective Oxidation Of Cyclohexane Under Visible Light

Semiconductor photocatalytic technology has great potential in solving the energy crisis.As an inorganic semiconductor material,titanium dioxide（TiO₂）has been widely used in the field of photocatalysis.However,the large band gap and the rapid recombination of photogenerated carriers severely limit the activity and application of TiO₂in visible light.In this study,a variety of TiO₂-based semiconductor photocatalysts were prepared through chemical structure regulation and the construction of surface heterojunction,aiming at improving the visible light absorption and expanding the lifetime of photon-generated electrons and holes,and the relationship between the structure and photocatalytic activity was systematically discussed.The main contents are as follows:（1）Anatase/rutile TiO₂mixed phase heterojunction was successfully prepared by in-situ hydrothermal method.Then,nitrogen doped anatase/rutile TiO₂（N-TiO₂）photocatalyst was obtained by elemental doping selecting urea as nitrogen source.The N-TiO₂mixed phase heterojunction exhibited higher catalytic activity than single nitrogen doped anatase or nitrogen doped rutile.Through optical and electrochemical characterization methods,we further explored the transfer direction of photoproduction electronic between anatase and rutile phase interface.Free radical control experiments showed that photogenerated holes and hydroxyl radicals are the main free radical species in photocatalytic oxidation of cyclohexane,which were crucial to the activation of C-H bond in cyclohexane.（2）NiS_1.03@C/N-TiO₂P-N heterojunction photocatalyst was constructed by introducing p-type NiS@C with n-type TiO₂semiconductor.Compared with pure TiO₂and N-TiO₂,P-N heterojunction showed better cyclohexane oxidation performance under visible light,among them NiS_1.03@C/N-TiO₂-3 showed the best conversion and selectivity,reaching 1.86%and 90%,respectively.It was mainly attributed to the construction of N-doping and P-N heterojunction interface,both of which could effectively enhance the absorption of visible light and promote the separation of photogenerated carriers.The presence of carbon as a structural buffer would inhibit the agglomeration of NiS_1.03and N-TiO₂particles during the cycling process,promoting the adsorption of cyclohexane and the desorption of oxidation products,and further improve the cyclohexane selective oxidation activity and stability of the photocatalyst.（3）Mixed metal oxides were prepared by high temperature calcination with hydrotalcite as precursor.The band structure of the mixed metal oxides was precisely regulated by using different metal cation intercalation,and the regulation of appropriate valence band range of the catalyst for selective oxidation of cyclohexane under visible light was investigated.Then Cr-Mg Al O was further combined with TiO₂to construct heterojunction,the Cr-Mg Al O/TiO₂heterojunction still maintained the lamellar structure of the original mixed metal oxides,and its cyclohexane selective oxidation performance and cyclic stability were further improved.When the content of Cr-Mg Al O was 50%,the heterojunction catalyst showed the best photocatalytic activity,the cyclohexane conversion reached 17.32%.The optical and electrochemical characterization was further used to explore the transfer direction of photogenerated carriers at the heterojunction,and the mechanism of heterojunction catalyst was studied.（4）TiO₂nanoparticles were grown on MIL-101-Fe substrate by hydrolysis at room temperature to prepare TiO₂@MIL-101-Fe heterojunction catalysts.Transmission electron microscopy showed that TiO₂nanoparticles not only adhered on the surface,but also entered into the pores of MIL-101-Fe,and dispersed evenly in the whole frame structure.When the content of TiO₂was 50%,the heterojunction catalyst demonstrated the highest photocatalytic activity,and the cyclohexane conversion reached 14.11%.After 5 cycles,the conversion rate can still reach 98.6%of the initial conversion.In summary,all the prepared TiO₂-based heterojunction photocatalytic materials significantly improved the selective oxidation activity of cyclohexane under visible light.The oxidation mechanism of cyclohexane was also studied by means of optical,electrochemistry,free radical control experiment and electron spin resonance.It provides theoretical guidance for the construction of heterojunction catalysts with high activity under visible light.