The Promotion Mechanism Of G-C₃N₄ Modified By Alkali In CO₂ Photocatalytic Reduction And Its Expanded Application In Constructing 3-component Catalyst

As an ideal approach to alleviate global warming and achieve a sustainable use of energy,photocatalytic reduction of CO₂ aims to convert CO₂ into fuels such as carbon monoxide,methane and methanol using photocatalysts,solar energy and water.However,the efficiency and cost of currently developed photocatalysts are still far from meeting the demands for practical application.Among all the semiconductor photocatalysts,non-metal semiconductor graphitic carbon nitride?g-C₃N₄?is low in cost,easy to obtain and with highly tunable properties.However,limited attention has been paid on its application in CO₂ photocatalytic reduction.To explore efficient g-C₃N₄ based photocatalyts for CO₂ reduction,in this thesis,following work has been conducted.First,the properties of g-C₃N₄ synthesized from four different precursors,i.e.urea,dicyandiamide,melamine and thiourea,as well as their performance in CO₂ photocatalytic reduction under the irradiation of light with different ranges of wavelength???420 nm,?? 320 nm,?? 200 nm?were investigated.It was found that CO₂ conversion rate on g-C₃N₄ was influenced by its surface area,basic-NHx groups,light absorption range and recombination rate of the photo-generated electron and holes.And the factors show varied importance under the different irradiation conditions.Then,surface alkali decoration was applied to improve the performance of bulk g-C₃N₄ and the role of the alkali was explored in detail.It was found that the decoration of KOH on the surface of g-C₃N₄ exert little influence on the intrinsic properties of g-C₃N₄.But the CO₂ adsorption was enhanced for the existence of KOH.An optimal amount of KOH could improve the CO₂ reduction activity of g-C₃N₄ effectively.The promoting effect of the surface alkali was mainly contributed by the roles of OH while the cation of the alkali also showed some influence.To further improve the CO₂ photocatalytic reduction performance of g-C₃N₄,the KOH chemical activation method for carbon was applied on g-C₃N₄,which is supposed to not only increase the amount of-NHx groups on g-C₃N₄ but also introduce K+ into the g-C₃N₄ layers,so that the CO₂ activation and electron transfer on g-C₃N₄ could be both enhanced.The resultant samples were characterized to show enhanced ability in CO₂ adsorption and electron transfer as expected.Under the irradiation of simulated sunlight,the optimal 5-h productions of CO and CH4 achieved on the KOH activated g-C₃N₄ were more-than-5 folds higher than the productions on pristine g-C₃N₄.Bases on the mechanism that the CO₂ reduction efficiency could be promoted by simultaneously enhancing CO₂ adsorption and electron transfer in a photocatalyst system,a 3-component structure with photocatalyst?TiO₂?,electron transfer promoter?AuCu?and CO₂ adsorbent?ZIF-8?was constructed.While the optimal form of composite was selected,the mechanism how the enhanced CO₂ adsorption and electron transfer synergistically contributed to the promotion of CO₂ conversion was elucidated.The studies in this thesis not only lay a solid research foundation for the development of g-C₃N₄ based photocatalysts for CO₂ reduction,but also provide new ideas for the modification of photocatalysts especially for CO₂ photocatalytic reduction.They are of important value as theoretical reference for the exploration of practically applicable photocatalysts with a low cost and a high efficiency for converting CO₂ into fuels.