Photoelectrochemical H₂ Production Coupled With Oxidation Coupling Of Aromatic Amine Compounds

Solar-driven photoelectric water hydrolysis technology is an important method to realize the green and efficient preparation of hydrogen energy.However,in the traditional process of hydrogen production from photoelectric water,the kinetics of oxygen evolution reaction at the anode is slow,which greatly restricts the efficiency of hydrogen production at the cathode,and the added value of oxygen is not high.If the anode oxygen evolution reaction is replaced by an important organic oxidation reaction,it can not only improve the rate of hydrogen production,but also realize the preparation of high value-added chemicals in the anode,providing a new strategy for the realization of photoelectric water hydrolysis"improving efficiency and reducing cost".Among many oxidation reactions,the preparation of aromatic azo/imine compounds by oxidative coupling of aromatic amine compounds is an important reaction,and its oxidation products are widely used in dyes,pharmaceutical synthesis,food additives and other fields.Traditional oxidation coupling methods of aromatic amine compounds are mainly homogeneous or heterogeneous thermal catalytic methods.Although aromatic azo/imine compounds can be obtained with high selectivity,they are faced with problems such as difficult recycling of catalysts,the need to use organic solvents,and high temperature and pressure.In recent years,photocatalytic methods driven by renewable energy sources have been successfully used for the preparation of aromatic azo/imine compounds.However,the efficiency of photocatalytic reaction system is still low,and additional oxidants and organic solvents are required.Therefore,it is of great significance to develop more green and efficient methods for the preparation of aromatic azo/imine compounds.In this paper,we focus on the oxidative coupling of aromatic amine compounds,and successfully realize the highly selective preparation of a series of aromatic azo/imine compounds at the anode by photoelectrochemical（PEC）method.The PEC reaction system does not require additional oxidants and organic solvents,while achieving efficient hydrogen production at the cathode.In situ electron paramagnetic resonance spectroscopy,free radical trapping experiment,in situ adsorption infrared and other tests were carried out to explore the reaction mechanism of the oxidative coupling of aromatic amines of PEC.The following is the detailed research content and conclusions.（1）Firstly,we used the oxidation coupling of aniline to prepare azobenzene as a model reaction.By preparing different photoanode materials,we compared and screened out bismuth vanadate（Bi VO₄）photoanode with better performance for the oxidation coupling of aniline.In order to improve the reaction performance of Bi VO₄photoanode,we further designed and fabricated a porous bismuth vanadate nanoarray（Bi VO₄-NA）photoanode with higher specific surface area.The experimental results show that the porous nanoarray structure can promote the exposure of the active site,and can significantly enhance the absorption of visible light and promote the separation of photogenerated charge carriers,thus improving the reaction performance of the oxidation coupling of aniline to azobenes(H₂production rate reached 26.7μmol cm^-2h^-1,azobenes selectivity>99%,faraday efficiency reached 87.4%).In addition,through in situ infrared and free radical trapping experiments,we proved that the reaction followed the photohole-induced free radical oxidation coupling reaction mechanism,and Bi VO₄-NA had a moderate adsorption strength for aniline,which could ensure the desorption of aniline oxidation intermediates on the catalyst surface and avoid excessive oxidation.Finally,we demonstrate the universality of the PEC strategy for the synthesis of a series of aromatic azo compounds.（2）We further explored the feasibility of the PEC strategy in the preparation of imine by oxidative coupling of benzylamine.Firstly,two photoanodes,Ti O₂and Bi VO₄of nanosheet array,were used for benzylamine oxidation coupling,and it was found that Ti O₂photoanodes had higher benzylamine oxidation performance.In order to improve the reaction performance,we loaded Au nanoparticles on Ti O₂by photodeposition method,and prepared a series of Au-x/Ti O₂（x=3,5,10）composite photoanode materials with different Au loads by adjusting the photodeposition time.The comparison of reaction performance shows that the Au-5/Ti O₂composite photoanode obtained by photodeposition of Au for 5 minutes has the best performance for benzylamine oxidation coupling(H₂production rate reaches 80.0μmol cm^-2h^-1,imine product selectivity reaches 94.2%,and Faraday efficiency is 78%).The results of UV-visible spectroscopy and photoluminescence show that the plasma resonance effect of Au nanoparticles can effectively enhance the light absorption and promote the separation of photogenerated charge carriers,and improve the reaction performance of benzylamine oxidation coupling to imine.Finally,we demonstrate the universality of the PEC strategy for the synthesis of a series of aromatic imine compounds.