Fabrication Of Reactive Site For Bismuth-based Photocatalyst And The Mechanism Of Enhanced Activity

Although the heterogeneous catalysis has highlighted its unique advantages in the field of clean energy regeneration,it still suffers from the drawbacks of poor catalytic activity and low selectivity of products and so on.To solve these problems,it is of prime importance to select and design photocatalyst based on the catalytic reaction mechanism and process.In this thesis,based on the basic principles of photocatalytic N₂ and CO₂ reduction reactions,we aim to mediate the active sites of bismuth-based semiconductor photocatalysts to improve their photocatalytic activity and the selectivity of final products by employing surface vacancy engineering,noble metal loading and metal modification.By analyzing the effects of active site on the intrinsic structure and photochemical properties of bismuth-based semiconductor materials,the adsorption and activation of the substrate molecule,as well as the catalytic reaction path,the structure-activity relationship between active site and photocatalytic activity and product selectivity was discussed.The main works are summarized as follows:1.Bi₂MO₆(M=W,Mo)nanomaterials with oxygen vacancy were successfully prepared by controlling the hydrothermal reaction time,and their photocatalytic N₂ reduction activity of the samples were evaluated.The NH₄⁺determination by Nessler's reagent method showed that the photocatalytic N₂ reduction activity of Bi₂WO₆ and Bi₂MoO₆ samples increased with the increase of oxygen vacancy amount of the samples.However,there was no NH₄⁺generation was detected by ion chromatography in all of the photocatalytic systems.Further study showed that many uncertainties resulted the inaccurate results of NH₄⁺determination in the photocatalytic N₂ reduction process.The residual alcohols,amines,and other N contained surfactant in the reaction system could significantly influence the determination of NH₄⁺.As many uncertainties existed in the determination of NH₄⁺,it is quite essential to select an appropriate NH₄⁺determination method of and design fully reasonable comparison experiments to obtain accurate data of photocatalytic N₂reduction.2.The controllable construction of oxygen vacancy on the surface of Bi₂MO₆(M=W,Mo)and BiOX(X=Cl,Br)samples were realized via a one-step solvothermal treatment by controlling the solvothermal reaction times in polyols solvent.It was found that the photocatalytic CO₂ reduction activity of Bi₂WO₆ and Bi₂MoO₆ samples was poor,while BiOCl and BiOBr samples had excellent photocatalytic CO₂ reduction performance.Furthermore,the photocatalytic CO₂ reduction activity of BiOCl and BiOBr samples was increased and then decreased with the increase of the amount of surface oxygen vacancy.The CO₂ temperature programmed desorption(CO₂-TPD)and photoelectrochemical property determination results revealed that an appropriate amount of surface oxygen vacancy could enhance the CO₂ adsorption capacity,improve the reduction ability and migration rate of the photo-generated electrons and promote the separation of photo-generated electrons and holes.3.Noble metal(M=Au,Ag,Pt and Pd)supported BiOCl samples were prepared via a sodium borohydride reduction method,and their photocatalytic CO₂ reduction activities were evaluated.The results showed that only a little CO was generated over different pure BiOCl samples.However,the photocatalytic CO₂ reduction activities of noble metal supported BiOCl samples were significant improved.Moreover,the products of CH₄,C₂H₆and C₂H₄ were also detected in the reaction system.The supported BiOCl sample with 1%amount of noble metal possessed highest photocatalytic CO₂ reduction activity.Moreover,noble metal supported BiOCl samples displayed different product selectivity in photocatalytic CO₂ reduction reaction.The highest yield of C₂H₆(39.31?mol g^-1)was detected in the products of photocatalytic CO₂ reduction over 1%Au-BOC-1 sample,which accounted for 70.94%in the final products.While the highest yield(15.46?mol g^-1)and highest percentage(83.54%)of CH₄was generated in the photocatalytic CO₂ reduction over1%Pt-BOC-1 sample.The CO-TPD and photocatalytic CO reduction experiment result showed that the loaded noble metal improved the CO adsorption ability of samples,which was beneficial to the further hydrogenation and reduction of the intermediate CO*,thus leading to the generation of CH₄,C₂H₄ and C₂H₆ products.It was also found that the selectivity of products was related to the CO adsorption and hydrogen evolution ability of the supported noble metals.The stronger CO adsorption abilities of the loaded noble metal,the lower selectivity of CO product.The stronger hydrogen evolution ability of the loaded noble metal was more beneficial for the generation of the·H,thus facilitating the generation of CH₄ in photocatalytic CO₂ reduction.The selectivity of C₂H₆ was enhanced with the decrease of the hydrogen evolution ability of the surface loaded noble metal.Moreover,the similar phenomena and results were also found in the photocatalytic CO₂ reduction system over noble metal(M=Au,Ag,Pt,Pd)supported P25 and g-C₃N₄ samples.However,different with noble metal supported BiOCl sample,the stronger the hydrogen evolution ability of the noble metals on P25 and g-C₃N₄ samples was favorable for the generation of by-product H₂.The yield and selectivity of C₂H₆ over noble metal supported P25 and g-C₃N₄(M=Au,Ag,Pt,Pd)sample in photocatalytic CO₂ reduction was much lower than that of noble metal supported BiOCl samples,suggesting that the BiOCl substrate could effectively inhibit the hydrogen-evolution reactions and promote the generation of C₂H₆ product.4.Cu modified BiOCl samples were successfully prepared via a sodium borohydride reduction method,and their photocatalytic CO₂ reduction activity of the samples were evaluated.The X-ray photoelectron spectroscopy(XPS)and Electron paramagnetic resonance(EPR)data revealed that Cu²⁺species existed on the catalyst surface.The photocatalytic CO₂ reduction experiment results showed that Cu modified BiOCl samples exhibited enhanced photocatalytic CO₂ reduction activity.Besides CO,the products of CH₄,C₂H₄ and C₂H₆ was also detected in photocatalytic CO₂ reduction reaction over Cu modified BiOCl samples.It was found that Cu⁺was generated from the photoreduction of Cu²⁺by photo-generated,and Cu⁺was proposed to be more effective for CO₂ photoreduction.The Cu²⁺/Cu⁺on BiOCl samples can effectively improve photogenerated electron-hole separation efficiency.In addition,the enhanced CO₂ adsorption ability and relatively fast charge migration rate of Cu modified BiOCl sample was synergistically contributed to the CO₂ photoreduction.Meanwhile,enhanced CO adsorption capacity was beneficial to the generation of the CH₄,C₂H₄ and C₂H₆ products.