Preparation Of C₃N₄ Composite Materials For Photocatalytic Conversion Of Small Molecules And Their Catalytic Mechanism

Carbon nitride?C₃N₄?,as an attractive non-metallic photocatalyst,has attracted worldwide interest in the field of solar energy conversion due to its easy synthesis,abundant elements,physical and chemical stability and visible light response.Surface defects of C₃N₄ can be used as activation sites for small molecules such as N₂,O₂ and CO₂ in the air,which provides a possibility for further reaction.However,since its wide band gap,easy recombination of photogenerated carriers and limited surface-active sites,the reaction activity is low,so it can not be put into industrial application.As a nonpolar molecule,the dissociation energy of N?N bond is as high as 941 KJ/mol,and the nucleophilicity of proton is weak,so it is very difficult to be reduced to ammonia.Nitrogen vacancies on the surface of C₃N₄ can be used as activation sites of N₂,which can reduce the dissociation energy of N?N triple bond and can be used in photocatalytic nitrogen fixation reaction.Because the surface defects of C₃N₄ are limited and the activity of oxidizing water is low,it cannot provide the protons for nitrogen fixation reaction,and organic molecules such as methanol or ethanol are often added as sacrificial agents.In this paper,the photocatalytic activity of C₃N₄-based materials for nitrogen fixation in pure water was investigated.Starting from the steps of coupling transfer of electron protons and adsorption activation of nitrogen gas in the reaction process,silicotungstic acid(SiW₁₂)with strong oxidation ability were selected as co-catalysts to study photocatalytic nitrogen fixation reaction.In addition,based on the activation of oxygen in air by C₃N₄,the selective oxidation of benzyl alcohol in water medium was studied.In this paper,the conversion of benzyl alcohol and the selectivity of benzyl alcohol were improved by using W₁₈O₄₉ as co-catalyst in aqueous medium,which provided a new idea for improving the photocatalytic reaction efficiency of C₃N₄and had great significance for its application in the field of photocatalysis.The main contents of this paper include:?1?This chapter focuses on improving the adsorption and activation of N₂ on C₃N₄and the oxidation ability of water to provide more protons for nitrogen fixation reaction.Firstly,K-doped C₃N₄?K-C₃N₄?was prepared by introducing KOH into the synthesis process of C₃N₄.More nitrogen vacancies were introduced into the structure of carbon nitride to enhance the adsorption and activation of N₂.Then,phosphoric acid was used as electron proton transport bridge,and silicotungstic acid(SiW₁₂)was loaded onto the surface of K-C₃N₄ by impregnation method to enhance the material's water oxidation activity and provide more protons for nitrogen fixation reactions.The protons released by oxidized water and photogenerated electrons are transferred and stored in SiW₁₂.In N₂ atmosphere,the electron-coupled protons are transferred to the adsorbed and activated N₂ on the surface of the composite material,and NH₃ is formed.Electron-coupled proton transfer accelerates the synthesis rate of NH₃.Phosphate as an electron proton bridge also accelerates the carrier separation efficiency,thereby improving the photocatalytic nitrogen fixation performance of the composite.?2?This chapter focuses on enhancing the adsorption and activation of oxygen by C₃N₄ materials,as well as the adsorption of reactants and the desorption of products.The bulk phase C₃N₄ was prepared by pyrolysis of melamine.Ultrathin porous C₃N₄nanosheets?HU-CNS?were obtained by further heat treatment,which accelerated the carrier migration rate on the surface of HU-CNS.The W₁₈O₄₉ nanowires were grown on the surface of HU-CNS by hydrothermal method,and W₁₈O₄₉/HU-CNS composites were prepared to accelerate the carrier migration rate at the interface.The introduction of W₁₈O₄₉ nanowires with abundant oxygen vacancies not only improves the adsorption and activation of O₂,but also changes the adsorption properties of reactants and products on the surface of catalysts.The W₁₈O₄₉/HU-CNS composite has a strong adsorption for benzyl alcohol,which accelerates the reaction of benzyl alcohol on the catalyst surface,and has a weak adsorption for benzaldehyde,thus inhibiting the further oxidation of the product to CO₂.Therefore,the selective oxidation of benzyl alcohol to benzyl alcohol over W₁₈O₄₉/HU-CNS composite materials has achieved high selectivity and conversion.