| Severe environmental problems and energy problems are still the major contradictions that must be resolved in today’s social development.Volatile organic compounds(VOCs)are a generic term for a class of air pollutants mainly including various alcohols,aromatic hydrocarbons(benzene,toluene),aldehydes(acetaldehyde,formaldehyde)and volatile halogenated hydrocarbons.Benzene series(BTEX)mainly come from the release of household products and chemical industry production.The generated volatile organic compounds released into the atmosphere will not only seriously harm the air environment,but also cause pollution to the soil and water environment.When the human body is exposed to or breathed in air containing a certain concentration of volatile organic compounds for a long time,human health will be endangered seriously.Toluene is a common volatile organic compound,which will cause irreversible damage to the human respiratory system and central nervous system when exposure to toluene for long-term.Therefore,residual toluene in the environment will seriously endanger human health and cause serious environmental problems.Thus,it is particularly important to use clean and efficient environmental treatment technologies to degrade volatile organic compounds in the atmosphere environment.Sunlight is a kind of renewable energy in a certain sense,and it is widely used in various fields,such as solar power generation,solar cells and so on.Photocatalytic technology is also a light-driven catalytic technology that converts light energy into chemical energy,such as photocatalytic synthesis for ammonia,photocatalytic organic synthesis,and photocatalytic degradation for pollutants.Due to taking light energy to drive the reaction,photocatalytic technology shows the advantages of cleanliness,high efficiency,and no secondary pollution,which has been attracted wide attention from worldwide experts and scholars.For benzene series treatment,the applications of wide-band gap metal oxides are the most widely used because of stronger oxidation ability due to their positive valence band position.Variable structure means that it can be easily to control crystal structure.However,the narrow photo-response range,the high cost of preparation and synthesis limited the application for photocatalysis.Because benzene series reaction mechanism is particularly complex,the reaction mechanism of wide band gap metal oxides in the photocatalytic benzene series degradation need to understand further,especially the relationship between the material surface/interface structure and the catalytic performance.Through experimental design and theoretical simulation,the relationship between the identification/modification of active sites on wide-band gap metal oxides surface and the intermediates conversion in photocatalytic benzene series degradation can be revealed.Therefore,the main points of this paper are summarized as follows:(1)Revealing intermediates conversion and the opening benzene-ring mechanism during the process of photocatalytic toluene mineralization in antimony-based wide band gap metal oxides Ca2Sb2O7:Compared with commercial titanium dioxide(P25),Ca2Sb2O7 exhibit stronger electron transmission with surface oxygen molecules and water molecules,toluene molecules and intermediate products,which promotes the generation of active free radicals and the activation of pollutant molecules,thereby inducing selective conversion of intermediate products to benzoic acid.At the same time,the theoretical calculation results show that the energy barrier from toluene to benzoic acid on Ca2Sb2O7 surface is lower,which means that can promote the selectivity of the intermediate product benzoic acid,and improve the efficiency of toluene ring-opening and degradation.Wide band gap metal oxide Ca2Sb2O7 as a model,the method of closely combining experiments and theoretical simulations are be used to explore the nature effect in photocatalysis.The mechanism of performance improvement in toluene degradation,the mutual conversion of intermediate products and the benzene-ring opening in the process of toluene mineralization are revealed.That has laid a certain theoretical foundation for the subsequent work.(2)Explore the influence of the surface lattice oxygen in antimony-based wide-band gap metal oxide Sr2Sb2O7 during the photocatalytic toluene mineralization:Due to the characteristics of the Sb atom in the center of the SbO6 structure,the Sb-O bond is elongated and the surface oxygen atoms gather more electrons,which promotes the activation of surface lattice oxygen on Sr2Sb2O7.The activated lattice oxygen promotes the generation of superoxide radicals and hydroxyl radicals.The adsorption and activation of toluene molecules can be enhanced,which induces the selective conversion of intermediate products to benzoic acid.In addition,the final product CO2 can be effectively desorbed from the catalyst surface.Wide-band gap metal oxide Sr2Sb2O7 shows excellent performance during the photocatalytic toluene mineralization process.Through theoretical simulation and experimental design,the activation mechanism of the lattice oxygen,and the effect of the activated lattice oxygen for the active free radicals generation,the conversion of intermediate products and the desorption of final products are revealed on Sr2Sb2O7 surface.(3)Explain the influence of surface electronic structure adjustment in antimony-based wide-band gap metal oxide BaSb2O6 on photocatalytic toluene mineralization:Transition metal element Ga is decorated on ternary oxide BaSb2O6 surface to adjust the surface electronic structure,which cause a greatly improving for photocatalysis toluene mineralization performance in comparison with bulk-BaSb2O6.The analysis results show that the surface modification by Ga makes BaSb2O6 surface form an electron delocalization center.The center reduces the surface potential of BaSb2O6,and promotes the separation of photogenerated carriers and the transport of photogenerated electrons on BaSb2O6 surface.The impurity levels forming in the band structure of BaSb2O6 by Ga modification broaden the light absorption range of the black catalyst and promote the separation of photogenerated carriers.Ga/BaSb2O6 promotes the generation of radicals(superoxide radicals and hydroxyl radicals)and the activation of toluene molecules,which is conducive to the conversion and ring opening of toluene.Through the combination of experimental characterization and theoretical calculation,the electronic structure regulation mechanism of wide-band gap metal oxide doped by transition metal element and the improvement mechanism of toluene degradation performance are explained.(4)To clarify the effect of hydroxyl activation on the surface/interface of tin-based wide-bandgap double metal hydroxide during photocatalytic toluene mineralization:SnO2-modified ZnSn(OH)6 heterostructure was successfully synthesized by in-situ growth hydrothermal method.The built-in electric field at the SnO2/ZnSn(OH)6 interface increases the dipole moment of the O-H bond in the interface hydroxyl group.At the same time,the O-H bond can be elongated,indicating that the interface hydroxyl group show a significant polarization.The polarized surface hydroxyl groups promote the adsorption and activation of oxygen molecules,water molecules,toluene and the oxidation intermediates.The formation of the built-in electric field at the SnO2/ZnSn(OH)6 interface also promotes the separation efficiency of photogenerated carriers.Thereby,the photocatalytic toluene mineralization performance in SnO2/ZnSn(OH)6 can be greatly enhanced.The combination of material design,performance testing,in-situ DRIFTS investigation and the first-principles simulation calculations,we revealed the reaction mechanism and reaction path during the photocatalytic toluene degradation on the surface/interface of wide band gap metal oxides,laying a certain theoretical foundation for subsequent research.Then the identification/modification of the active sites,the source and their role of the active sites in the photocatalytic toluene degradation are clarified.Finally,the mechanism of the surface/interface activation on wide band gap material and the improvement of the photocatalytic toluene degradation performance is explained through modified wide-band gap metal oxide/double hydroxide.This work reveals the role of surface/interface activation on wide band gap metal oxide during the photocatalytic benzene series degradation,and provides new insights to understand the reaction mechanism better on wide band gap semiconductor materials in the photocatalytic degradation of volatile organic pollutants. |
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