Study On Preparation And Photocatalytic Performance Of Polyoxometalate Nanocrystalline/Carbon Nitride Composites

Nowadays,the semiconductor photocatalysis technology provides an efficient way for humans with applying sunlight to deal with the increasingly serious environmental pollution problem.Photocatalysis technology can use sunlight directly as the driving force of the reaction.It has the advantages of mild reaction conditions,high photocatalytic degradation efficiency,no secondary pollution and so on,which has been recognized as an ideal method for solving environmental pollution problems.Titanium dioxide（TiO₂）has been widely studied and used because of its low cost,non-toxicity,high photocatalytic activity and stability.However,TiO₂ has a large band gap（about 3.0-3.2 eV）,which can only utilize the ultraviolet light（5%）in the solar spectrum,and the spectral efficiency is low.At the same time,the photogenerated carriers in TiO₂ are easy to recombine,which leads to the low quantum efficiency.The above factors hinder the practical application of TiO₂.In this paper,we employ TiO2 as the carrier material to prepare POMs/TiO2 composite by the electrospinning-high temperature calcination technology.And the Pt/PMo₁₂/TiO₂and g-C₃N₄/PW₁₂/TiO₂ composite photocatalyst systems were constructed by loading precious metal Pt nanoparticles or mixing with urea by calcination,the photocatalytic degradation behavior and mechanism for environmental pollutants by different catalyst systems were studied in detail.The details are as follows:1.A series of PMo₁₂/TiO₂ nanofiber composites loaded with Pt nanoparticles were constructed through electrospinning/calcining technology and photoreduction method.And the effect of Pt amount loaded on photocatalytic activity of the composite catalyst was investigated.The visible light absorption of Pt/PMo₁₂/TiO₂ composite was significantly enhanced due to the narrow band gap（2.4 eV）of PMo₁₂ and the SPR absorption of Pt nanoparticles.At the same time,the strong local electromagnetic field generated by Pt nanoparticles of SPR improved the separation efficiency of photogenerated charge carriers in the composite.The results of photocatalysis experiments showed that Pt/PMo₁₂/TiO₂ had excellent and stable visible light（λ>420 nm）catalytic activity for degrading methyl orange,tetracycline,bisphenol A and reducing Cr（VI）.2.A series of g-C₃N₄/PW₁₂/TiO₂ composites were prepared by electrospinning/calcining technology and mixed calcination methods.And the effect of g-C3N4 content on the photocatalytic performance of the composite was studied.In the composite,PW₁₂2 as an electron transport medium,can accelerate the transfer of photogenerated electrons from conduction band of g-C₃N₄ to conduction band of TiO₂,and effectively promote the separation of electron-hole pairs in g-C₃N₄,thus improving the photocatalytic activity of the composite.The results of photocatalysis tests showed that g-C3N4/PW12/TiO2 had high and stable catalytic activity for visible light degradation of tetracycline,bisphenol A and reduced Cr（VI）.The successful preparation of the above two series of composite catalyst systems not only effectively expanded the spectral absorption range of TiO₂ materials,improved the separation efficiency of photogenerated carrier,but also solved the problem of recovery and utilization of POMs as homogeneous catalyst.At the same time,it addressed the disadvantage that the POMs/TiO₂ material system can only use ultraviolet light.This work provides new ideas for the design and preparation of other new high-efficiency composite catalyst systems.