Preparation And Photocatalytic Properties Of Metal Sulfide Supported KTA_0.75Nb_0.25O₃ And C/KNb₃O₈ Composite Catalysts

Energy shortage and environmental pollution are two major problems facing human society,which drives people to use hydrogen and other new energy to replace fossil energy.Ammonia（NH₃）and hydrogen are both important carriers of hydrogen energy,and their economical and effective synthesis is very important for the practical application of hydrogen energy.Moreover,ammonia is one of the basic components of amino acids,nucleotides and other compounds in all living organisms,and it is also an indispensable chemical in agricultural fertilizers.With the development of human society,the demand for ammonia（NH₃）and hydrogen is increasing day by day.For this reason,many scientists have done a lot of research work on catalytic synthesis of NH₃ and hydrogen evolution,in which the application of photocatalysis and piezoelectric catalysis technology to synthesize NH₃ and hydrogen evolution shows a bright prospect.In traditional industrial production,Haber-Bosch is usually used to synthesize NH₃,which not only consumes high energy,but also produces a lot of CO₂greenhouse gases,which leads to huge energy consumption and environmental pollution.In industrial production,hydrogen production mainly uses two methods:hydrogen production by electrolysis of water and hydrogen production from fossil fuels,both of which consume a lot of energy,and the latter will cause a series of environmental problems due to the combustion of fossil fuels.In comparison,using solar energy and ultrasonic pressure energy to catalyze the synthesis of NH₃ and production of H₂ has the characteristics of green and low cost,so it is an ideal way for nitrogen fixation and hydrogen evolution.In this paper,on the basis of two excellent photocatalysts,KTa_0.75Nb_0.25O₃（KTN）and KNb₃O₈,three new types of catalysts were prepared by metal sulfide and non-metallic carbon,and their photocatalytic activity and photocatalytic mechanism were studied:1.Using sodium sulfide hexahydrate as sulfur source and copper chloride dihydrate as copper source,a high efficiency CuS/KTN composite catalyst with both photocatalytic and pressure catalytic nitrogen fixation efficiency was prepared by simple hydrothermal method.The catalytic nitrogen fixation rates of the optimized CuS/KTN reached 167.4μmol·g^-1·h^-1 and 36.2μmol·g^-1·h^-1 under simulated sunlight and 60 W ultrasound,which were 4.2 and 9.8 times higher than those of the pure phase KTN,respectively.Further combined light and pressure catalytic nitrogen fixation experiments revealed that the nitrogen fixation rate was further enhanced compared to the pure photocatalytic experiments.The optimum CuS/KTN combined photocatalysis and pressure nitrogen fixation rate reached 212.2μmol·g^-1·h^-1.The nitrogen fixation rates of the synthesized catalysts were determined by X-ray diffraction（XRD）,X-ray photoelectron spectroscopy（XPS）,scanning electron microscopy（SEM）,transmission electron microscopy（TEM）,Raman spectroscopy（Raman）and Mott-Schottky N₂ adsorption and desorption,UV-vis diffuse reflectance spectroscopy（DRS）,transient photocurrent response（PC）,impedance spectroscopy（EIS）and linear scanning voltammetry（LSV）characterization indicated that the enhanced catalytic performance of the CuS/KTN composite catalyst could be attributed to the loading of CuS to enhance the carrier separation efficiency,thus enhancing the catalytic performance.2.Bi₂S₃/KTN photocatalyst for nitrogen fixation was synthesized by a simple hydrothermal method at 120°C,using sodium sulfide as the sulfur source and bismuth nitrate pentahydrate as the bismuth source（the molar ratio of sulfur to bismuth is 4:1）.Under simulated sunlight and 60 W ultrasound,the best Bi₂S₃/KTN samples exhibited photo-and pressure-catalyzed nitrogen fixation rates of 561.6μmol·g^-1·h^-1 and 14.9μmol·g^-1·h^-1,respectively,reaching 4.7 and 4 times that of pure-phase KTN,respectively（Note:The catalytic performance of the Bi₂S₃/KTN group catalyst was tested by replacing the xenon lamp light source.The measured photocatalytic rate values were not comparable to those of the CuS/KTN group catalysts）.The results of the catalytic experiments showed that the addition of Bi₂S₃had a significant contribution to the KTN.Through the characterization of the composite catalyst,it was found that Bi₂S₃ could enhance the separation efficiency of electron-hole pairs by trapping photoelectrons and piezoelectrons.Further,six photocatalytic nitrogen fixation cycles were performed with the best Bi₂S₃/KTN,and the measured six photocatalytic nitrogen fixation rates were 561.6μmol·g^-1·h^-1,589.0μmol·g^-1·h^-1,571.8μmol·g^-1·h^-1,563.8μmol·g^-1·h^-1,560.0μmol·g^-1·h^-1,574.4μmol·g^-1·h^-1.After several cycles of testing,the nitrogen fixation rate of Bi₂S₃/KTN increased rather than decreased.This study indicates that Bi₂S₃/KTN has great potential for application in photocatalytic/piezoelectric catalytic nitrogen fixation.3.Pure phase KNb₃O₈ was synthesized by two-step hydrothermal method,and then C/KNb₃O₈ nanocomposite catalyst was synthesized by hydrothermal method with glucose as carbon source.The synthesized catalyst showed excellent performance in photocatalytic hydrogen evolution.The best C/KNb₃O₈ sample produced hydrogen at a rate of 1148μmol·g^-1·h^-1 under simulated solar illumination,which was 1.9 times higher than that of pure phase KNb₃O₈.The photocatalytic mechanism was investigated using N₂ adsorption,XRD,XPS,Raman,DRS,transient photocurrent,EIS,LSV and other characterization tools.The results showed that the introduction of carbon had little effect on the specific surface area of the material,but significantly improved the light absorption performance and charge separation efficiency of the material,which played a key role in improving the photocatalytic reaction.The cycling experiments further demonstrated the high photocatalytic stability of C/KNb₃O₈,indicating its good potential for photocatalytic hydrogen precipitation.