Mechanism Analysis Of Metal Oxide Crystal Surface Effect Regulating Photocatalytic Oxidation Reactivity

The use of fossil energy promotes the rapid development of society,but the massive burning of fossil fuel causes serious environmental pollution and directly threatens human life and health.As a kind of green sustainable clean energy,solar energy can replace the traditional fossil energy,has attracted wide attention.Photocatalysis technology,which can convert solar energy into chemical energy by using photocatalyst,has become a hot spot and frontier in many fields such as chemistry,energy and environmental engineering.However,the photocatalyst has problems such as low utilization rate of light and easy recombination of electron holes,which lead to low photocatalytic efficiency.Based on this,a variety of new photocatalysts have been synthesized to improve the photocatalytic efficiency.Different crystal faces of nanomaterials have different band structure and atomic arrangement.The photocatalytic performance can be effectively improved by adjusting the crystal face type and ratio of the photocatalytic materials.Therefore,the crystal surface effect is considered to be a good method to regulate photocatalytic activity.Metal oxide is a kind of common photocatalyst.The photocatalytic activity can be regulated by using metal oxide with different crystal planes.However,the mechanism of regulating the photocatalytic activity by the effect of metal oxide crystal faces is still unclear.Most studies analyze the mechanism of the crystal plane effect based on the average catalytic activity measured at the global level.However,such a global test method ignores the uneven distribution of active sites caused by local differences in the structure of photocatalysts caused by defects,heterojunctions,doping,etc.,and conceals the heterogeneity between different particles and different parts of the same particle.Therefore,in-depth analysis of the mechanism of regulating photocatalytic activity by metal oxide crystal surface effect at the level of single particle is of scientific research value.In this thesis,the photocatalytic C-C bond oxidative fracture and methylene blue oxidative degradation of lignin were used as model reactions.CeO₂and TiO₂were used as photocatalysts respectively to analyze the mechanism of metal oxide crystal surface effect regulating the catalytic activity of lignin C-C bond oxidative fracture and methylene blue oxidative degradation.The specific research contents are as follows:（1）Rod-shaped CeO₂（CeO₂-R）and octahedral CeO₂（CeO₂-O）photocatalysts were synthesized,and the main exposed crystal planes were（110）and（111）crystal planes,respectively.The effect of CeO₂photocatalysts with different crystal surfaces on the oxidative fracture activity of C-C bond of lignin model compound 1,2-diphenyl ethanol was analyzed.The results showed that CeO₂-R exhibited excellent activity of lignin C-C bond oxidative fracture.Raman and XPS results show that CeO₂-R contains more oxygen vacancy content.The photocurrent response,electrochemical impedance,photoluminescence and UV-Vis techniques demonstrate that CeO₂-R has a wider light absorption range and better electron hole separation ability.The adsorption of 1,2-diphenyl ethanol andO₂on CeO₂-R surface was proved by DFT simulation.The results of reactive oxygen capture experiment and EPR showed that·O₂^-was the main reactive oxygen species that promoted the oxidative fracture of C-C bond in lignin.In this chapter,by analyzing the mechanism of CeO₂crystal face effect regulating lignin’s C-C bond oxidative fracture reactivity,it was revealed that different CeO₂crystal faces could affect the production rate of·O₂^-and the adsorption of reactants,so as to realize the regulation of lignin’s C-C bond oxidative fracture reactivity and provide the basis for the study of real lignin C-C bond fracture with high efficiency photocatalysis.（2）TiO₂photocatalysts with different（101）crystal plane proportions were synthesized,and the effects of TiO₂photocatalysts with different（101）crystal plane proportions on the oxidative degradation of methylene blue were explored.The results showed that TiO₂with higher ratio of（101）crystal planes had higher methylene blue oxidation and degradation activity.The single molecule fluorescence imaging technology was used to further explore the regulatory mechanism of TiO₂crystal surface effect in the degradation of methylene blue oxidation reaction.Hydroxyphenyl fluorescein（HPF）can specifically label the main active oxygen·OH of methylene blue oxidative degradation.By collecting,processing and analyzing the single molecular fluorescence signals of single TiO₂particle surface·OH with different（101）crystal plane proportions,high-resolution positioning maps and generation rates of single TiO₂particle surface·OH with different（101）crystal plane proportions were obtained.In situ real-time monitoring of the spatial distribution of the surface·OH of single TiO₂particles with different（101）crystal plane proportions can be achieved.The results of high-resolution positioning map showed that·OH was not uniformly distributed on the surface of single TiO₂particles,indicating that single TiO₂particles had activity heterogeneity,and the rate of·OH was faster on the surface of single TiO₂photocatalyst with a high ratio of（101）crystal planes.This chapter reveals that TiO₂with different（101）crystal plane proportions can regulate the oxidative degradation activity of methylene blue by influencing the formation rate of·OH at the single particle level,which is crucial for the design and development of highly active photocatalysts for the degradation of methylene blue.In summary,this thesis analyzed the photocatalytic oxidation reactivity at the whole and single particle levels respectively,revealing the mechanism of metal oxide crystal surface effect regulating the photocatalytic oxidation reactivity,which is expected to provide theoretical guidance and research basis for the precise design of excellent photocatalytic reactivity catalysts.