Research Of Preparation Of Carbon Nitride Composite Photocatalyst And Its Catalytic Performance

With the development of economy and society,the problem of water pollution is becoming more and more serious,and people's production and life are inseparable from water.The quality of water is related to people's quality of life,so water pollution control is extremely important.Dye wastewater is one of the common water pollutants.Therefore,the development of efficient,stable,and low-cost wastewater purification materials is of great practical value.Semiconductor photocatalytic technology is one of the important ways to solve this problem.Carbon nitride?g-C₃N₄?is a widely-studied visible-light semiconductor catalyst,and it possesses the characteristics of stable physical and chemical properties,direct utilization of visible light,low cost,and large specific surface area.However,the problems of easy recombination of photogenerated carriers and low visible-light catalytic activity limit its actual applications.Ag₃PO₄ is a new type of photocatalyst with high visible light catalytic activity,but it has the problem of photocorrosion.To solve the above issues,the following work has been done in this dissertation:?1?In order to enhance the visible light catalytic activity of g-C₃N₄,g-C₃N₄ was firstly prepared by calcination method using urea as a precursor,and then Ag₃PO₄ was supported on g-C₃N₄ by precipitation method,and finally,a small amount of Ag₃PO₄was replaced with Ag₂S by liquid phase ion exchange method to obtain the Ag₂S/Ag₃PO₄/g-C₃N₄ composite photocatalyst.The photocatalytic performance of the composite was evaluated by visible-light degradation of Rhodamine B?Rh B?as a model pollutant.The results show that,the formation of a small amount of silver sulfide can significantly improve the visible light absorption and visible light catalytic activity of the composite catalyst.When When the molar raio?n?Na₂S?/n?Ag₃PO₄??is in the range of 0?1.5%,the reaction rate constant increases with the molar ratio,and further increasing the amount of Na₂S,the rate constant will decrease.When the molar ratio exceeds 5%,the activity of the composite photocatalyst shows an obvious decrease.The possible reasons are that the generation of too much Ag₂S will cause a large amount of Ag₃PO₄ to be replaced,and the visible-light catalytic activity of Ag₂S is not high,so resulting in a decrease in the activity of the composite photocatalyst.The mass fraction of g-C₃N₄ has a significant effect on the performance of the composite photocatalyst.As the mass of g-C₃N₄ is increased,the photocatalytic reaction rate constant of the composite photocatalyst first increases and then decreases.And when the mass ratio of the two is 3:7,the best catalytic performance is achieved.Excessive addition of g-C₃N₄will reduce the activity of the composite catalyst.This is because Ag₃PO₄ is thickly wrapped with the excessive g-C₃N₄,which is not beneficial to the absorption of light,resulting in a decline in photocatalytic activity.The results of active species experiments show that·OH,·O^2-and h⁺are the main active species in the photocatalytic process.The order of effects of three active species are as follows:h⁺>·O^2->·OH.Also,the catalytic mechanism of degradation of rhodamine B over this the composite photocatalyst is proposed.?2?In order to improve the separation efficiency of photo-generated electron-hole of g-C₃N₄,urea was used as the precursor to prepare g-C₃N₄ by calcination,and medical cotton was calcined and carbonized to prepare multi-dimensional?1D/2D/3D?carbon materials,and then C/g-C₃N₄ composite photocatalyst was obtained by sonitation.The photocatalytic performance of the composite was evaluated by visible-light degradation of rhodamine B as a model pollutant.The results show that:The photocatalytic activity of the sample first increases and then decreases as the carbon content increases.When the mass ratio of C to g-C₃N₄ is 1:100?CCN1.0?,the sample shows the best photocatalytic activity and stability.Among the three dimensions?1D/2D/3D?of carbon materials,2D carbon has the greatest effect on improving photocatalytic activity.Because 2D carbon has a 2D structure similar to g-C₃N₄,the largest contact interface is formed between 2D carbon and g-C₃N₄,and 2D planar network structure conducive to the rapid electron transfer,which can effectively improve the separation efficiency of photogenerated carriers and the absorption of visible light.Compared with the common activated carbon?AC?supported catalyst?AC1.0/g-C₃N₄?,CCN1.0 shows higher visible-light catalytic activity.The first-order rate constant?k?value of CCN1.0 is about5.5 times and 3.4 times of pure g-C₃N₄ and AC1.0/g-C₃N₄,respectively.The CCN1.0catalyst also shows excellent photocatalytic stability in the cycling experiment.The results of impedance spectroscopy and photocurrent experiments show that a small amount of carbon doping can significantly improve the charge transfer rate of the sample and reduce the resistance.And·O^2-and h⁺are the main active species in the photocatalytic process,and the role of·O^2-is greater than h⁺.On this basis,the catalytic reaction mechanism is proposed.During to the advantages of low cost,simple preparation method,high photocatalytic efficiency,and excellent catalytic stability and so on,the composite catalyst has a potential application prospects in the field of degrading organic pollutants.