Preparation Of Gold And Platinum Nanoparticles Deposited ZnWO₄ For Photocatalytic CO₂ Reduction

The increase of fossil energy consumption has resulted in the release of large amount of carbon dioxide(CO2)into the atmosphere,which causes serious environmental problems.Photocatalytic reduction of CO2 into carbonaceous compounds driven by solar energy is an effective way to reduce the concentration of CO2 in the atmosphere.Semiconductor photocatalysts play an important role in CO2 reduction.In recent years,zinc tungstate semiconductor has attracted wide attention in the field of photocatalysis due to its stable chemical properties.However,the low efficiency of photogenerated carrier separation and lack of surface active sites seriously limit their application in photocatalysis.In order to solve these problems,we synthesized Au/ZnWO4 and Pt/ZnWO4 composites,which were expect to introduce Schottky heterojunction and surface reaction sites simultaneously through cocatalyst loading.As the same time,the cocatalyst loading can improve the separation and transport efficiency of photogeneratd carriers,which can enhance the activity of photocatalytic CO2 reduction.The main results are as follows:1.A series of Au/ZnWO4 composites were prepared by the deposition-precipitation method.The effect of Au particle size was investigated for photocatalytic reduction of CO2 by controlling the calcination temperatures.As the calcination temperature increased from 150 to 450℃ the size of Au nanoparticles increased from 0.7 nm to 4.5 nm.The results show that the photocatalytic activity of Au/ZnWO4 composite is significantly higher than that of pure ZnWO4.The photocatalytic activity of Au/ZnWO4 first increases and then decreases with the increase of Au particle size.After 3 h irradiation,the yields of CO,CH4 and H2 of the optimal sample were 1.25 μmol·g-1,7.43 μmol·g-1and 30.59 μmol·g-1,respectively.The enhanced photocatalytic activity is mainly attributed to the formation of Schottky heterojunction between Au and ZnWO4,which improves the separation efficiency of photogenerated carriers.In addition,the selectivity of CH4 increases with the increase of Au particle size.The reason may be explained by the fact that the change of Au particle size makes the proportion of surface atoms and atomic coordination number change,which affects the stability of reaction intermediates and regulates the selectivity of products.2.A series of ZnWO4 nanorods were synthesized by hydrothermal method by adjusting the pH value.The effect of the preparation conditions on the photocatalytic reduction of CO2 was discussed.The results show that ZnWO4 synthesized at pH 5.5,6.0 and 6.5 exhibits high CH4 selectivity,while ZnWO4 synthesized at pH 7.0,8.0 and 9.0 exhibits high CO selectivity.The ZnWO4 sample synthesized at pH 6.5 has the highest photocatalytic activity for CO2 reduction,and the selectivity for CH4 was 82.8%.ICP and FTIR characterization confirmed that the surface of ZnWO4 synthesized under acidic conditions was rich in tungsten,and the surface hydroxyl content increased after illumination,which promoted the formation of CH4.Then,Pt nanoparticles were deposited on ZnWO4 nanorods by oil bath method,and the effect of Pt loading on the photocatalytic reduction of CO2 was studied.The results show that the photocatalytic activity of Pt/ZnWO4 composite is significantly higher than that of pure ZnWO4.After 3 h irradiation,the yields of CO,CH4 and H2 of the optimal sample were 2.14 μmol·g-1、16.11 μmol·g-1 and 23.89 μmol·g-1,respectively.The enhanced photocatalytic activity of the composite is mainly attributed to the formation of Schottky heterojunction between Pt and ZnWO4,which improves the separation efficiency of photogenerated carriers and increases the activity of photocatalytic reduction of CO2.