Design And Synthesis Of Metal Nanocatalyst Based On Cerium Dioxide And Their Application In The Photocatalytic Hydrogen Generation From Formic Acid

As a clean energy,hydrogen has attracted much attention because of its high combustion calorific value and the combustion products without pollution.However,there are some problems in the storage,transport and release of hydrogen.Formic acid（HCOOH）with 4.4 wt%of hydrogen content,which is liquid at room temperature,is a promising chemical hydrogen storage material.Therefore,in order to achieve the efficient hydrogen generation from formic acid under mild conditions,a series of supported Au Pd Ni photocatalysts are designed in the thesis.By adjusting the semiconducting properties of CeO₂ support in the catalysts to enhance the electron density of metal surface,the catalytic hydrogen generation performance of Au Pd Ni nanoparticles（NPs）is improved.The main results are as follows:（1）Based on the close relationship between the oxygen vacancy defect and the semiconducting characteristics of metal oxides,the CeO₂ is treated by plasma to control the oxygen vacancy content.Then a series of Au Pd Ni NPs supported by defective CeO₂ with different oxygen vacancy contents are synthesized and their photocatalytic hydrogen generation performance is studied.The results show that compared with the Au Pd Ni NPs is supported on original CeO₂,the catalytic activity of all the defective catalysts is improved and the optimized turnover frequency（TOF）value reaches 603 h^-1.The reason is attributed to the increase of oxygen vacancy concentration,the decrease of bandgaps and the improvement of photoinduced carrier separation efficiency,which leads to the increase of electron density on metal surface and resulting enhanced catalytic performance.（2）From the viewpoint of broadening the absorption range of semiconducting supports in the catalysts,a series of Au Pd Ni NPs are synthesized by using CeO₂modified by carbon quantum dots（CQDs）as supports,and their photocatalytic hydrogen generation performance is studied.The results show that the catalytic performance of nanocatalysts with CQDs modified CeO₂ as support is improved.The highest performance of catalyst with 5 wt%of the amount of CQDs was obtained and the corresponding TOF value reaches 693 h^-1.The enhanced catalytic performance is attributed to that CQDs could promote the catalytic reaction.The reason is that the addition of CQDs broadens the optical absorption range and the CQDs with conjugatedπbond are benefical for the enhancement of conductivity of catalysts,which promotes the separation of carriers and then more photoelectrons are transferred to the active metal surface.At the same time,the CQDs have a special up-conversion characteristics,which can convert the long wavelength light into short wavelength light,which can be reused by CeO₂ in the photocatalytic process.（3）On the basis of the principle that the formation of heterojunction among composite semiconductors can benefit the change the structure of energy band,the CeO₂ and C₃N₄ are combined firstly,and then the bandgap of CeO₂ is adjusted by adjusting the oxygen vacancy concentration of CeO₂.A series of supported Au Pd Ni photocatalysts are prepared and their photocatalytic performance is studied.The results show that compared with the metal catalysts with CeO₂ or C₃N₄ as individual support,the catalytic performance of metal catalysts with composite supports has been improved.With the increase of oxygen vacancy content in CeO₂,the catalytic performance of metal catalysts is gradually enhanced,and the highest TOF value is725 h^-1.The reason for the improvement of the performance is attributed to the formation of type II heterojunction in the composite semiconductor,which changes the energy band structure and enhances the electron transfer ability of semiconductor,which leads to the increased electron density on the metal surface and the catalytic performance.At the same time,the change of oxygen vacancy concentration can further benefit the tunable bandgap of the composite supports,and the oxygen vacancy may also participate in the adsorption and activation of HCOOH molecule in the photocatalytic process.