Investigating The Electronic Structure Modulation And Toluene Degradation Mechanism Of Bismuth Carbonate Photocatalytic Materials

Photocatalysis has become a promising technology for air pollution remediation,with green,mild,and efficient advantages.For volatile organic pollutants causing severe environmental problems,especially toluene,the critical challenges to achieving effective photocatalytic purification are developing inexpensive,environmentally friendly,efficient,and stable photocatalysts and revealing the degradation mechanism.Bi₂O₂CO₃photocatalyst in wide band gap semiconductors meets the cost requirements for practical application,and its valence band position gives the potential to show strong oxidation ability.In addition,the anion-cation layered crossover composition of bismuth-based semiconductors makes the electronic structure of Bi₂O₂CO₃ easy to regulate.However,Bi₂O₂CO₃ is yet to exhibit satisfactory photocatalytic performance in continuous flow toluene degradation systems.In this dissertation,we aimed to optimize the photocatalytic toluene degradation activity and stability of Bi₂O₂CO₃.By modulating the morphology and electronic structure,the intrinsic light absorption and reactant adsorption activation are enhanced.The mechanism of the surface/interface structure facilitating the generation of reactive oxygen species and ring-opening of benzene-ring-containing reactants was systematically elucidated.The main findings are as follows.（1）By changing the catalyst preparation conditions,the Bi₂O₂CO₃ nanosheets were transformed from a closely packed state to a multi-directional stacked state.As a result,more nanosheet surfaces can receive photon energy to enhance light utilization during the reaction process.The boosted photogenerated carrier generation and separation efficiency can promote the production of superoxide radicals（·O₂^-）.The dynamic evolution of reaction species on the photocatalyst surface during photocatalytic toluene degradation was investigated.It was revealed that the improved generation of·O₂^-facilitates the conversion of toluene to benzene-ring-containing intermediates.The photocatalytic toluene degradation efficiency was increased from 16.2%to 52.8%.（2）To improve the stability of photocatalysts during toluene degradation,alkali metal ions were introduced as active sites on the surface of Bi₂O₂CO₃.Theoretical calculations proved that the alkali metal ions existed on the Bi₂O₂CO₃ surface in a substitution doping manner,breaking the uniform surface charge distribution.The formation of new active sites enhances the adsorption of water molecules and promotes the generation of hydroxyl radicals（·OH）.Thus,the ring-opening degradation of benzene-ring-containing reactants was facilitated,reducing the accumulation of intermediates and enabling the photocatalyst to maintain great toluene degradation efficiency during the long-time reaction.（3）Based on the critical roles of·O₂^-and·OH during photocatalytic toluene degradation,spatially separated electron and hole active sites that can promote the adsorption and activation of O₂ and H₂O molecules,respectively,were constructed.Combining the results of in situ characterization and theoretical calculations,it was demonstrated that the electron and hole active sites formed on the surface of Bi₂O₂CO₃have adsorption specificity and more vital activation ability for O₂ and H₂O,respectively.The simultaneously improved generation of·O₂^-and·OH accelerates the conversion and ring-opening degradation of toluene and its intermediates.Thus,the photocatalytic toluene degradation efficiency was further improved to 68.1%,and the degradation stability could be maintained for the long term.（4）For the stable conjugated benzene ring structure of benzene-ring-containing reactants,the delocalized electrons on the surface of Bi₂O₂CO₃ were constructed to enhance the activation of critical ring-opening sites.Theoretical calculations demonstrated that the presence of delocalized electrons would promote the interaction of benzene-ring-containing reactants with the Bi₂O₂CO₃ surface,destabilizing the benzene-ring conjugated electronic structure.Thus,the ring-opening energy barrier（ΔG）of toluene,benzyl alcohol,benzaldehyde,and benzoic acid were significantly reduced from1.95,0.52,2.15,and 1.34 e V,respectively,to 0.26,0.25,0.52,and-0.12 e V.The crucial intermediate(1-Hexene,C₇H₁₄)in the toluene degradation pathway was detected using GC-MS,which indicates that the benzene-ring structure of toluene could be directly ring-opened when it was activated.Thus,the dual degradation pathways that methyl oxidation to benzoic acid and direct ring opening resulted in toluene degradation activity and mineralization rate of more than 72.0%and exhibited superior environmental humidity compatibility and stability.The research in this dissertation realizes the efficient and stable toluene degradation by Bi₂O₂CO₃ in a continuous flow reaction system.The formation mechanism of active sites on the Bi₂O₂CO₃surface and the influence of the surface/interface activation process on the photocatalytic toluene degradation pathway were revealed by combining in situ characterization and theoretical calculation.It is of great significance to promote the practical application of photocatalytic toluene degradation technology and provides a new research idea to enhance active species generation and reactant activation.