Study On The Efficiency And Mechanism Of Cobalt-chromium Bimetallic Sulfide And Its Photocatalytic Reduction Of Cr(Ⅵ)

Heavy metal Cr（VI）ions are common pollutants in the wastewater from metallurgy,metal processing and electroplating,dyeing,leather tanning,textile and other industries.Due to its high water solubility,strong mobility and the carcinogenic,mutagenic and teratogenic effects on humans,developing efficient,energy-saving and environmental friendly technology for the removal of Cr（VI）pollutant has become a research hotspot in environmental field.Visible light photocatalytic technology has the characteristics of high efficiency,good economy,safety and environmental protection in the treatment of heavy metal pollutants in water.It is therefore considered to be a promising technology in the water pollution treatment.Cadmium sulfide is a common sulfide semiconductor photocatalyst.It is rich in resources,low in synthesis price,and shows excellent photochemical properties and quantum efficiency in the visible region.Therefore,it has been widely studied in the field of photocatalysis.However,the catalyst is limited by the high recombination rate of photogenerated carriers and serious photo corrosion.Cobalt sulfide is widely used as cocatalyst in the field of photocatalysis because of its narrow band gap and high flat band potential,and the bonding or coord ination of surface sulfur atoms with other metals can improve the separation and transfer of photogenerated carriers.Considering that cadmium sulfide and cobalt sulfide semiconductor photocatalysts have suitable energy band structure,in this paper,aiming at the efficient reduction of heavy metal Cr（VI）pollutants in wastewater,a series of cobalt cadmium bimetallic sulfide composite photocatalysts with high catalytic activity and stable performance were prepared by means of morphology regulation,ion doping and defect construction.The structure-activity relationship between its physicochemical properties and photocatalytic activity was explored by means of various characterization methods and theoretical calculation,and the transfer pathway of photogenerated carriers under visible light irradiation and the photocatalytic reaction mechanism of Cr（VI）pollutants are also revealed.Firstly,visible-light driven Co S_x/Cd S composite photocatalyst was synthesized by the combination of sacrificial template and solvothermal method.Through the optimization of the proportion of active components,it was found that the optimal molar ratio of Co S_x to Cd S was 1/16 with the Cr（VI）photoreduction efficiency of100%within 30 min.After five cycle experiments,the composite photocatalyst can still maintain high photocatalytic activity and good photostability.Besides,various characterization techniques and theoretical calculation results show that the hollow polyhedrons structure of Co S_x in the hybrids photocatalyst can greatly improve the light capture ability and expose more catalytic active sites.More importantly,the close combination of Co S_x and Cd S component can lead to the formation of direct Z-scheme heterojunction,which can not only improve the separation and transfer of photogenerated electron-hole pairs,but also can retain the strong reduction ability,so as to further enhance the Cr（VI）reduction efficiency for the developed Co S_x/Cd S heterojunction photocatalyst.To improve the photocatalytic performance of cobalt cadmium sulfide composites,N-Co S_x/Cd S composite photocatalysts were prepared by plasma assisted nitrogen doping method.It is found that the plasma treatment with nitrogen as gas source time is 5 min and the molar ratio of N-Co S_x to Cd S is 1/8,the photocatalytic reduction efficiency of Cr（VI）for the so-obtained photocatalyst is100%within 25 min.In addition,the mechanism study shows that the plasma treatment can not only covalently dope N elements into Co Sx by forming Co-N bond,but also can enhance the photocatalytic reduction of Cr（VI）by adjusting its surface wettability,conductivity and Fermi energy level.Moreover,a variety of characterization and experimental results confirmed that the N-Co S_x/Cd S composite photocatalyst has a direct Z-scheme heterojunction structure,and the e^-and^·O₂^–radicals play major roles in the photocatalytic reduction of Cr（VI）.To further improve the adsorption performance and photocatalytic activity of cobalt cadmium sulfide composites,3D/1D Co_1-xS/Cd S composite photocatalyst was prepared by surfactant assisted solvothermal method.The research shows that the so-obtained composite is a direct Z-scheme heterojunction photocatalyst,in which the nanowire structure of Cd S is conducive to the rapid and long-distance transmission of photogenerated carriers,and the nano cubic structure of Co _1-xS has strong"light capture"ability,which is helpful to improve the light utilization.Due to its unique heterostructure and good 3D/1D interface coupling,the so-obtained composite photocatalyst could exhibit a wide light absorption edge and good photo-generated carrier separation efficiency,which enables the optimized composite photocatalyst to completely remove Cr（VI）within 20 min,in which e^-and^·O₂^–radicals are the main active species during the Cr（VI）photocatalytic reduction.Cyclic experiments show that the composite photocatalyst has strong photocatalytic stability and corrosion resistance.In addition,the so-developed composite photocatalyst is applied to the study of photocatalytic performance of Cr（VI）pollutants in simulated wastewater,which shows good photocatalytic reduction efficiency and has certain practical application potential.