Preparation Of High-Efficiency Photocatalysts Based On Two-Dimensional Materials Regulation And Study Of Their Selective Organic Synthesis Performance

The continuous consumption of fossil fuels leads to the increasingly serious problems such as energy shortage and environmental pollution.The development and utilization of solar energy has always been the direction explored by the scientific community.As a new technology that can convert solar energy into chemical energy,photocatalysis can realize reactions such as water splitting to produce H2,CO2 reduction and pollutant degradation under the driving of sunlight.It is regarded as a promising way to solve energy and environmental problems in the future.Through constant exploration and practice,photocatalysis has made important progress in environmental restoration and energy production.With the deepening of research,photocatalysis has also shown great application potential in the field of organic synthesis in recent years.Compared with traditional industrial production,the selective synthesis of organic compounds by photocatalysis can make full use of solar energy,reducing the consumption of fossil energy,and can also effectively reduce the reaction barrier,making the reaction proceed under milder conditions.Therefore,under the goals of "carbon peaking and carbon neutrality" in our country,the selective synthesis of organic compounds using photocatalysis will have positive practical significance and huge development space.However,the current photocatalytic organic synthesis generally suffers from the low conversion efficiency and poor product selectivity,and there is still a huge gap from practical application.In order to improve the activity and selectivity of photocatalytic organic synthesis,researchers have carried out extensive work and explored a series of control strategies,such as:doping,loading,defect and heterojunction.However,these strategies mainly focus on improving catalytic activity by tuning the band structure and electronic properties of the semiconductor itself.Whereas,the conversion efficiency and product selectivity are not only affected by the light absorption capacity and the carrier separation efficiency of the semiconductor,but also closely related to the adsorption and activation process between the catalyst surface and the reactant.The effective contact between catalyst surface and reactant is a prerequisite for the catalytic reaction,and the subsequent charge transfer process can only proceed smoothly after the two are fully adsorbed and activated.Therefore,optimizing the adsorption and activation process between catalyst surface and reactant is one of the effective ways to further improve the conversion efficiency and product selectivity of photocatalytic organic synthesis.Two-dimensional(2D)materials have the characteristics of large specific surface area,abundant active sites,easy regulation of electronic structure and short carrier transport distance,which show unique advantages in enhancing the adsorption and activation of reactant on catalyst surface,and stabilizing the reaction intermediates.In this paper,we aim at the current problems of low conversion efficiency and poor product selectivity in photocatalytic organic synthesis.Based on the unique physicochemical properties of 2D materials,the adsorption and activation process between catalyst and reactant may be further optimized by rationally designing and regulating the electronic structure,defect and surface/interface state of semiconductor,thereby improving the photocatalytic activity.The main research contents of this thesis are as follows:In chapter 1,the research background,basic principle and application of photocatalysis,as well as the significance,application,evaluation indicators,basic process,key factors and regulation strategies of photocatalytic selective organic synthesis were briefly summarized.Subsequently,2D materials were introduced and their advantages in photocatalytic selective organic synthesis were analyzed.Finally,combined with the current research status,the opportunities and challenges of 2D materials in organic synthesis were analyzed,and the significance and research content of this thesis were clarified.In chapter 2,the effect of metal-like 2D material Ti3C2(MXene)in composite on photocatalytic selective oxidation performance was mainly studied.The TiO2/Ti3C2 composite was prepared by in-situ partial oxidation and applied in the selective oxidation of benzyl alcohol to benzaldehyde,which showed higher conversion efficiency and product selectivity than single TiO2.The test results confirmed that the existence of Ti3C2 in the composite can stabilize the oxygen vacancies and Ti3+ to promote the generation of ·O2-,improving the conversion efficiency.At the same time,Ti3C2 can shift the energy band of TiO2 up to weak the oxidation ability of photogenerated holes,improving the product selectivity.In chapter 3,g-C3N4 with excellent product selectivity was selected as the research object,and the effect of trace B doping on the adsorption and activation of O2 was explore.The Bdoped g-C3N4 prepared by mixing calcined boronic acid and urea,which exhibited higher conversion efficiency and product selectivity than pure g-C3N4 under visible light.XPS and NMR results showed that the B atom was incorporated into the g-C3N4 framework by substituting the sp2-hybridized C atom in the tri-s-triazine rings.And the test found that the B atom at this position can not only enhance the absorption and utilization of visible light by gC3N4,but also provide more specific sites for the adsorption and activation of O2,accelerating the transfer of photogenerated electrons to O2.Therefore,the introduction of B promoted the generation of key reactive oxygen species·O2-,thereby enhancing the conversion efficiency of photocatalytic selective oxidation of benzyl alcohol.In chapter 4,the effect of defects in g-C3N4 on the adsorption and activation of O2 and benzyl alcohol was further investigated.In this work,DFT calculation first confirmed that the nitrogen vacancies(NVs)in g-C3N4 can act as specific active sites to enhance the adsorption of O2 and lower the generation barrier of ·O2-.Meanwhile,the in-situ FTIR tests indicated that NVs can also enhance the adsorption of benzyl alcohol and promote its dehydrogenation for oxidation.Subsequently,the g-C3N4 samples containing NVs were prepared by alkali-assisted calcination and applied in the photocatalytic selective oxidation of benzyl alcohol.The presence of NVs may accelerate both reduction and oxidation half-reactions,so the NVs-containing samples exhibited better conversion efficiency and product selectivity under visible light irradiation.In chapter 5,the interaction between the complex biomass(5-Hydroxymethylfurfural,HMF)and flexible ultrathin graphitic carbon nitride nanosheet(UCNT)was studied in detail.There are poor stability and the production of nonstoichiometric reduction products and oxidation products during sulfides photocatalytic HMF oxidation coupled with H2 evolution.Combining the results of previous chapters,we propose a conjecture that differs from the reported papers:UCNT may be an ideal candidate for the above reaction.It was found that UCNT exhibited higher catalytic activity,product selectivity and cycle stability than the reported catalysts,and the excellent performance could be attributed to the strong specific interaction between HMF and UCNT.The unique 2D layered conjugated structure makes UCNT own a large flexible surface to ensure close contact with HMF and enhance the interaction between them,thereby reducing the dehydrogenation energy barrier of HMF and speeding up the whole catalytic process.In chapter 6,the research contents of this paper were summarized,the main innovation points of this paper were further clarified,the existing problems and deficiencies were analyzed and put forward,and the follow-up work was prospected.