Fabrication Of CeO₂-Based Visible Light Photocatalysts And Their Photocatalytic Performances Toward Organic Reactions

To diminish the detrimentally environmental impact in chemical industries,environment friend and energy sustainable alternative for organic transformations is an important development direct in future.Sunlight,being an abundantly and easily available energy resource,possesses a great potential in driving organic transformations through an environmentally benign way.The illuminated light will not only shift apparently the thermodynamic equilibrium of redox reactions to favour chemical synthesis at room temperature,but also can avoid thermally induced side reactions.Photochemical reactions have been already used as key steps in the synthesis of complicated molecules.However,ultraviolet light with short wavelength is usually required to drive these reactions and can only be realized in special reaction vessels.In addition,the selectivity in photocatalytic organic transformations is still needed to be highly improved.Therefore,it is of great significance to prepare photocatalysts with high photocatalytic activity and selectivity for organic transformations under visible light.As an important rare earth oxide,CeO2 has attracted more and more attention in the field of photocatalysis due to its advantages of Ce3+/Ce4+ redox cycle,abundant oxygen defect sites,high oxygen storage and release capacity and chemical/photochemical stability.Therefore,in this thesis,we synthesized various visible light photocatalysts based on CeO2 nano-catalyst through reasonably tailoring their composites and structures.Meanwhile,the activity,and selectivity of the CeO2-based photocatalysts were evaluated by taking the photocatalytic organic reaction as model reactions.Furthermore,the enhance mechanism in photocatalysis was revealed in depth.This thesis comprises four sections depended on the different CeO2-based photocatalysts and their applications:(1)We developed a strategy to tuning the chemiadsorped oxygen in CeO2 nano-catalysts to further improve its photocatalytic activity under visible light illumination.CeO2 nanostructures with different concentrations of chemisorbed oxygen were prepared by hydrolyzing Ce(NO3)3 in Ar,Air and O2 saturated atmospheres.The rich chemical adsorbed oxygen formed during the preparation can produce the Ce peroxo structure in CeO2,which can enhance the visible light response of CeO2.Meanwhile,Ce peroxo structure promotes the formation of superoxygen species and reduces the recombination probability of photogenerated electron-hole pairs.Thus,the photocatalytic performances for the oxidation of benzyl alcohol under visible light were greatly improved.The presence of abundant Ce peroxo structure in CeO2-O2 photocatalyst makes them exhibit an activity and selectivity as high as almost 100%in theoxidation of benzyl alcohol.(2)The photocatalytic activity of CeO2-based catalysts under visiblie light is enhanced by tailoring their composite and morphological architecture.We employed a spontaneous chemical redox approach at ambient temperature to self-assembly synthesize Au@CeO2 core-shell composites with different Au loadings.At room temperature,the Au@CeO2 nanostructures,composited with small size Au nanoparticles as the core and CeO2 particles as the shell,can be rapidly prepared without the introduction of reducing agent and surfactant.The results demonstrate that the as-synthesized Au@CeO2 composites exhibit spherical-shape core-shell structures with porous characteristics.As the core,Au nanoparticles with the plasmonic feature,effectively harvest the utilization of visible light.Moreover,the shell constituted CeO2 nanostructures not only prevent agglomeration of Au cores,but also exhibit abundant defect sites.These features are conducive to the photocatalytic performance of the Au@CeO2 composites.The high photocatalytic activity,selectivity,stability and versatility on the selective photocatalytic oxidation of alcohols to aldehydes or ketones were demonstrated by using Au@CeO2-4.0wt.%as photocatalysts.(3)The optimization in reaction system and the control of defect in photocatalysts were explored to investigate the photo-and thermal-catalytic properties of CeO2 catalyststs under visible light.We employed an easy pathway to quickly preparing a flower-like CeO2 nanostructure.The Au nanoparticles were syhthesized by a plasma reduction technique and uniformly supported on the CeO2 nanoflowers to form a structurally stable Au/CeO2 composite catalyst.The plasma reduction technology was chosen for its high reaction rate to reduce Au ions,and the high efficiency in the production of the oxygen defect sites in CeO2.For evaluation their catalytic performances,thermal catalysis and thermal-assisted photocatalytic oxidation of benzyl alcohol were investigated.We found that the oxidation of benzyl alcohol to benzaldehyde under photocatalytic process exhibited satisfactory selectivity,while thermal heat will have an adverse effect on the selectivity of benzaldehyde during thermal catalytic procedure.In addition,the catalytic efficiency in heat-assisted photocatalytic oxidation performance is higher than that of the thermocatalysis or photocatalysis.It is well-known that the photoelectrons are more likely to migrate under heat-assisted conditions,and CeO2 nanostructures can release active oxygen.Therefore,both the migration of the photoelectrons and concentration of the active oxygen are promoted in Au/CeO2 under visible light illumination in photocatalytic reaction,leading to a higher catalytic activitiy and selectivity.(4)The visible-light photocatalytic performances of CeO2-based catalysts toward the reduction of nitro compounds were investigated via a hybriding methodology.CdS nanoparticles with a narrow bandgap were introduced on CeO2 nanorods to form CdS/CeO2 photocatalysts using a solvothermal method.In the presence of CdS/CeO2 nanostructures,the produced H2 through photolysis of water can reduce nitro compounds to the corresponding amines under visible light conditions.The reduction reaction is also versatile toward the nitro compounds.Furthermore,we also investigated the enhanced mechanism in detail when active hydrogen as the reduction agent in the photocatalysis of CdS/C eO2 nanostructures.