Study On The Design, Synthesis Of High-performance Bismuth-based Catalysts And Their Photocatalytic CO₂ Reduction Mechanism

In order to achieve the goal of Carbon Peak,Carbon Neutrality,it is imperative to recycle carbon dioxide（greenhouse gas）.As a sustainable green technology,photocatalysis technology can convert carbon dioxide（CO₂）into compounds with higher calorific value,which is regarded as one of the most promising measures to solve the energy and environmental issues in one stroke.However,the development of photocatalytic carbon dioxide reduction technology is still restricted by low light utilization efficiency,low product yield,poor selectivity and poor stability.Therefore,the design and preparation of photocatalytic carbon dioxide reduction catalyst materials with high efficiency and high stability is the key to enhance the practical application potential of this technology.In this study,a series of bismuth-containing composite catalysts（Bi/TiO₂,Bi₂O₃/Bi₂WO₆@g-C₃N₄,g-C₃N₄/Bi₂MoO₆ and Au/Bi₂MoO₆）were successfully synthesized based on bismuth related materials.In order to improve the visible light absorption of catalyst materials,promote carrier separation and utilization efficiency,the localized surface plasmon resonance effect and heterojunction effect were mainly used to realize the efficient photocatalytic conversion of carbon dioxide.In addition,the relationship among the physicochemical properties,electronic structures and catalytic performance of the catalyst was further studied by in-situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy（DRIFTS）combined with Density Functional Theory Calculation（DFT）.The specific research and innovative conclusions are as follows:（1）The metallic bismuth modified TiO₂ microsphere（namely BTO）catalysts were synthesized by one pot solvothermal method.The introduced metallic Bi nanoparticles contribute to the visible light absorption by LSPR（Localized Surface Plasmon Resonance）effect,and donate hot electrons to the valence band of TiO₂ for inhibiting the recombination of light generated electron-hole pairs.BTO catalysts perform the visible light response activity,and the optimized BTO-2 catalyst exhibited the CH₄production amount of 49.12μmol g^-1（4 h）as well as the CH₄ selectivity of 85.48%.The analysis of in-situ DRIFTS demonstrates that the visible light driven Bi-LSPR effect can boost the adsorption,activation and conversion of moist CO₂.And the result of DFT calculation shown the superior process of CO₂*-to-HCO*on BTO catalyst contributes to the selective generation of CH₄.It inspires the design of efficient photocatalysts for CO₂ conversion.（2）The dual-core@shell structured Bi₂O₃/Bi₂WO₆@g-C₃N₄（BB@CN）photocatalysts were fabricated by the co-grind and calcination method.The separation/transfer efficiencies of photogenerated electron-hole pairs could be improved via the EXAFS（Extended X-ray Absorption Fine Structure）proved strong interaction of core/shell interface.The BB@CN catalysts with a strong interaction interface exhibit high activity for CO₂-to-CO,and the BB@10CN（10 g melamine is used）catalyst has the largest formation rate(263.70μmol g^-1 after 9 h visible light irradiation)and selectivity（97.00%）of CO product.Based on the results of in-situ DRIFTS and DFT calculation,the photoconversion mechanism of CO₂-to-CO has been proposed:the COOH*formation is the rate-determined step of CO₂ conversion,and the selectivity of CO product is dependent on its suitable desorption energy.It inspires the fabrication strategy of efficient hetero photocatalysts with a strong interaction interface for selective photocatalytic CO₂ conversion.（3）The catalysts of ultrathin g-C₃N₄ surface-modified hollow spherical Bi₂MoO₆（g-C₃N₄/Bi₂MoO₆,abbreviated as CN/BMO）were fabricated by the co-solvothermal method.The variable valence Mo⁵⁺/Mo⁶⁺ionic bridge in CN/BMO catalysts can boost the rapid transfer of photogenerated electrons from Bi₂MoO₆ to g-C₃N₄.And the synergy effect of g-C₃N₄ and Bi₂MoO₆ components remarkably enhance CO₂adsorption capability.CN/BMO-2 catalyst has the best performances for visible light-driven CO₂ reduction compared with single Bi₂MoO₆ and g-C₃N₄,i.e.,its amount and selectivity of CO product are 139.50μmol g^-1 and 96.88%for 9 h,respectively.Based on the results of characterizations and DFT calculation,the photocatalytic mechanism for CO₂ reduction is proposed:The high-efficient separation efficiency of photogenerated electron-hole pairs,induced by variable valence Mo⁵⁺/Mo⁶⁺ionic bridge,can boost the rate-limiting steps（COOH*-to-CO*and CO*desorption）of selective visible light-driven CO₂ conversion into CO.It inspires the establishment of efficient photocatalysts for CO₂ conversion.（4）The nanocatalysts of Au nanoparticles（NPs）supported on hollow microsphere structured Bi₂MoO₆（BMO）were successfully synthesized by template solvent thermal and in-situ ammonium hydroxide-assisted deposition methods.The hollow microsphere morphology of Bi₂MoO₆ support contributes to light absorption and possesses high surface area.Supported Au NPs with surface plasmon resonance effect are favorable for enhancing visible light absorption and high-energy electron enrichment.The Au1.5/BMO catalyst shows the highest conversion efficiency（96.60%）of CO₂,the corresponding CH₄ product amount is 338.40μmol g^-1 and CH₄ product selectivity is94.20%after 9 h visible light irradiation.The apparent quantum efficiency of CH₄formation can reach to 1.71%.Based on the results of in-situ DRIFTS and DFT calculation,the photocatalytic mechanism of Au/BMO catalyst for CO₂ reduction is researched.The possible hydrogenation path of CO₂ methanation is proposed:CO₂*→COOH*→HCO*→CH₃O*→CH₄.It provides a novel strategy for designing efficient photocatalysts for CO₂ reduction.