| With the increasing of aromatic hydrocarbon in water environment,the security of aquatic ecosystem is seriously threatened,which also affects the safety and health of human beings.Photocatalytic wastewater treatment technology has attracted much attention because it can directly utilize solar energy to degrade and mineralize organic pollutants without secondary pollution.Graphitic carbon nitride?g-C3N4?,with its unique electronic structure,excellent visible light absorption ability and super stability,has become the most promising material for many TiO2 alternative semiconductor catalysts.In addition,the supported heteropoly acid can effectively improve the specific surface area of heteropoly acid,benefit for separation and recovery,while retaining the original strong oxidation property of heteropoly acid.Therefore,in this paper,new composite photocatalysts with large specific surface area and high photogenerated charge separation efficiency were fabricated by combined graphitic carbon nitride with SiO2?or TiO2?supported heteropoly acid,SiO2/PW12/g-C3N4 and TiO2/PW12/g-C3N4.X?ray powder diffraction?XRD?,X?ray photoelectron spectroscopy?XPS?,ultraviolet visible diffuse reflection?UV?vis/DRS?,N2adsorption/desorption porosity analysis,and transmission electron microscopy?TEM?were used to characterize the crystal phase structure,light absorption properties,surface physicochemical properties and morphology of the catalyst.The degradation,mineralization and stability properties of SiO2/PW12/g-C3N4 and TiO2/PW12/g-C3N4were studied by the photocatalytic degradation of aqueous toluene and m-xylene under simulated sunlight.In addition,photocatalytic degradation mechanisms of SiO2/PW12/g-C3N4 and TiO2/PW12/g-C3N4 were studied by photochemical experiments and free radical detection.The specific research contents of this paper were summarized as follows:1.Preparation,characterization and photocatalytic performance of SiO2/PW12/g-C3N4 composite photocatalyst.SiO2/PW12 sol with different heteropoly acid loadings was successfully prepared via sol-gel technology by using P123 as template and ethyl orthosilicate as silicon source.By the combination of SiO2/PW12 sol with ultrathin g-C3N4 nanosheets prepared by thermal condensation-HCl hydrothermal treatment,SiO2/PW12/g-C3N4composite photocatalysts with the heteropoly acid loading of 2.2%?4.5%and 6.2%were obtained.In the composite photocatalysts,the crystal phase structure of g-C3N4and the Keggin structure of H3PW12O40 remained intact.Due to the interaction of template,SiO2/PW12/g-C3N4 possessed a larger specific surface area than g-C3N4.The band gap of SiO2/PW12/g-C3N4 was between 2.76?2.83 eV.Toluene and m-xylene were selected as the target pollutants to investigate the photocatalytic performance of the catalyst under the simulated sunlight irradiation?320nm?27???27?680 nm?.The results showed that the photocatalytic activity of SiO2/PW12/g-C3N4 towards the degradation of two target pollutants was better than that of pure g-C3N4.When the loading of H3PW12O40 was 4.5%,SiO2/PW12/g-C3N4-4.5 showed the best photocatalytic degradation activity towards aqueous toluene and m-xylene,and the removal efficiency of toluene and m-xylene were 99%and 97%,respectively,after simulated sunlight illumination for 120 min.SiO2/PW12/g-C3N4 also exhibited good mineralization capacity and catalytic stability after 5 consecutive photocatalytic degradation of toluene in water.According to the results of photochemical experiments and free radical and hole scavenging experiments,the mechanism of photocatalytic degradation of aqueous aromatic hydrocarbon organic pollutants by SiO2/PW12/g-C3N4 was proposed,and the reason for its enhanced photocatalytic activity was explained.First,the introduction of SiO2 makes the composite photocatalyst have good mesoporous structure and increased specific surface area,which provides more adequate active sites for the degradation of pollutants.Secondly,H3PW12O40 is closely connected with g-C3N4 by acid-base reaction,which effectively promotes the separation and migration of photogenerated charge carriers and prolongs the lifetime of photogenerated charge carriers.Finally,the migration path of photogenerated charge carrier in SiO2/PW12/g-C3N4 composites follows the extraordinary Z-scheme-dictated charge carrier transfer mechanism,producing holes(h+VB)and hydroxyl radicals?·OH?with strong oxidation besides superoxide radicals?·O2??,which are more conducive to the rapid removal of pollutants in water.2.Preparation,characterization and photocatalytic performance of TiO2/PW12/g-C3N4 composite photocatalystOn the basis of ultrathin g-C3N4 nanosheets prepared by thermal condensation-HCl hydrothermal treatment,a series of TiO2/PW12/g-C3N4 composite photocatalysts were successfully prepared via hydrothermal method?HT?or evaporation-induced self-assembly method?EISA?by using P123 as template agent and titanium tetraisopropanolate as titanium source.In the TiO2/PW12/g-C3N4 photocatalysts obtained by the above two methods,the crystal phase structure of g-C3N4 and the Keggin structure of H3PW12O40 remained intact.And the composite photocatalysts presented the morphology of nano-particle accumulation.As for the composite photocatalysts prepared by EISA,TiO2 in s-TiO2/PW12/g-C3N4 exhibited anatase crystal phase structure and the specific surface area of s-TiO2/PW12/g-C3N4 was between 204?252 m2 g-1.The band gap?Eg?of s-TiO2/PW12/g-C3N4 was between 2.71?2.84 eV.Compared with EISA,anatase TiO2 in the catalyst prepared by HT method shows much better crystallinity,slightly smaller specific surface area(165 m2 g?1),and a smaller band gap of 2.67 eV.Toluene and m-xylene were selected as the target pollutants to investigate the photocatalytic performance of the catalyst under the simulated sunlight?320 nm?27???27?680 nm?irradiation.The results showed that the photocatalytic activity of TiO2/PW12/g-C3N4 was better than that of TiO2/PW12 and pure TiO2 or g-C3N4.The loading of H3PW12O40 has a significant effect on the photocatalytic activity of the composite photocatalyst.h-TiO2/PW12/g-C3N4-15.5 prepared by HT method showed the highest photocatalytic degradation activity towards aqueous toluene and m-xylene.Under the simulated sunlight irradiation,the complete degradation of toluene and m-xylene required only 90 and 60 min,respectively.At the same time,h-TiO2/PW12/g-C3N4-15.5possessed good mineralization capacity and catalytic stability after 5 consecutive photocatalytic degradation of toluene in water.Based on the electrochemical experiments and characterization results,the reasons for the enhanced photocatalytic degradation activity of h-TiO2/PW12/g-C3N4 were attributed to good anatase crystal structure,proper heteropoly acid loading,large specific surface area and fast separation ability of photogenerated charge carriers.Free radical and hole scavenging experiments illustrated that·O2?,·OH and h+VB were all the main active species in the photocatalytic degradation of toluene and m-xylene,and the photocatalytic degradation mechanism of h-TiO2/PW12/g-C3N4 was proposed accordingly. |
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