Photocatalytic Materials With The Synergy Of Metal Cocatalyst And Surface Oxygen Vacancies For Organic Transformation

With the increasing importance of sustainable development in the world,the green chemical reactions with the merits of resource saving,atomic economy and eco-friendly have received more and more attention.The employment of photocatalytic technology can utilize sunlight to drive redox reaction for producing valuable chemical products.By designing and optimizing the photocatalytic materials,target products can be produced with high selectivity,thus providing a green and energy-saving strategy for chemical transformations.In order to achieve this goal,the preparation of high efficiency photocatalytic materials has become an important research content in the field of photocatalysis.Metal-oxide semiconductors have attracted extensive attention because most of them have the distinct advantages,such as appropriate energy levels of valence band（VB）and conduction band（CB）,good stability and easy regulation of surface structure.However,the conventional single metal-oxide photocatalytic materials have the disadvantages of low utilization of sunlight,high recombination of photoexcited charge carriers,insufficient surface active sites,and thus exhibit the low photocatalytic efficiency.Therefore,from multiple perspectives,we proposed to engineer the surface structure of oxide semiconductors along with loading metal cocatalyst and therefore improve the photocatalytic performance by the synergy of metal sites and surface oxygen vacancies（V_Os）.In this thesis,taking two typical metal oxide semiconductors as examples,the oxygen vacancies were constructed and a small amount of metal cocatalyst were loaded on their surface,to achieve the goals of enhancing light absorption,promoting charge carrier separation and fabricating the catalytic active sites,and thus significantly enhance the photocatalytic performance and achieve the high conversion and selectivity of photocatalytic selective oxidation of organics.The main research contents of this thesis are as follows:1.The BiOBr photocatalyst with flower-like nanostructures were prepared via a solvothermal process.The Pd nanoparticles（NPs）were anchored on the surface of BiOBr nanostructures through the LED-light induced deposition and abundant surface V_Os were createded by in situ induction simultaneously.The Pd/BiOBr photocatalyst prepared by this method exhibited the superior photocatalytic performance in the photocatalytic selective oxidation of toluene to benzaldehyde,compared with the other related photocatalysts.Experimental measurements and theoretical calculations indicated that the interfacial interaction between Pd and BiOBr was not only beneficial to the high dispersion of Pd NPs,but also stabilized the abundant surface V_O.The Pd NPs and V_Oco-existing on the surface of BiOBr had stronger adsorption capacity for oxygen and toluene molecules,respectively.The charge separation induced by the interface facilitated the activation of oxygen molecules and C（sp³）-H bonds of toluene molecules,respectively.A series of controlled experiments and in situ ESR measurements（ESR）confirmed the existence of the active species and intermediates in the photocatalytic reactions.The research results confirmed that Pd sites and V_Ocan synergically improve the conversion rate and selectivity of photocatalytic selective oxidation of toluene to benzaldehyde.This study provided insight into the interfacial synergy of metal sites and V_O-rich oxide semiconductors for facilitating photocatalytic selective oxidation reactions,and was expected to direct the structural design and modulation of photocatalysts in the future.2.The HNb₃O₈ photocatalytic material with porous structure was synthesized via the solvothermal and protonation processes.Then through the process of impregnation adsorption and hydrogen thermal reduction,Pd cocatalysts were loaded on the surface of HNb₃O₈and oxygen vacancies were constructed by in-situ thermal reduction.Pd/HNb₃O₈-H₂photocatalyst with the best photocatalytic performance was obtained by optimizing the experimental conditions.The theoretical calculations indicated that the presence of Pd reduces the formation energy of V_Oon the surface of HNb₃O₈.Moreover,the hydrogen spillover effect caused by Pd cocatalyst during the hydrogen thermal reduction can promote the formation of V_Oon the surface of HNb₃O₈.The theoretical calculations further indicated that the presence of Pd and V_Ofacilitated the adsorption and activation of H₂O and benzyl alcohol on the surface of photocatalyst.Therefore,the optimized Pd/HNb₃O₈-H₂photocatalyst showed excellent performance in the selective oxidation of benzyl alcohol coupled with water splitting.The production rates of H₂and benzaldehyde reached 3.17 mmol·g^-1_cat·h^-1and 2.76 mmol·g^-1_cat·h^-1,respectively.In situ ESR tests further determined the active radicals and reaction intermediates produced in the reaction system.This research provided an effective method for the design and synthesis of photocatalysts with the synergy of metal cocatalyst and also provided an effective strategy for the utilization of photocatalytic technology to efficiently produce clean energy and high value-added organics.