Controllable Construction Of Active Sites On Photocatalysts Surface For Carbon Dioxide Reduction

With the continuous development of economy and society and the excessive use of fossil energy,a large amount of CO₂ gas is emitted into the air,which leads to a series of problems such as environmental damage and climate deterioration.Photocatalytic CO₂reduction is an effective means of carbon sequestration,which can not only solve environmental problems,but also alleviate the current situation of energy shortage,and realize artificial carbon cycle.Therefore,the research on photocatalytic CO₂ reduction is of great significance for the sustainable development of human beings.At present,the photocatalytic CO₂ reduction are still facing many problems:on the one hand,the intrinsic semiconductor materials which suffer from limited surface sites,in the process of the reaction surface active center,which directly affect the CO₂ molecules on the catalyst surface and adsorption activation,leading to the slow process of reaction,quantum yield is low;On the other hand,the gap width of some semiconductor catalysts is large,the utilization of sunlight is insufficient,and the internal electron transport ability is blocked,the electron holes tend to recombining,resulting in the low optical quantum yield.Therefore,how to design high performance catalysts is a major problem for researchers.In this paper,the impeded Lewis acid-base pair was introduced on the surface to achieve the controllable construction of the surface active site by improving the TiO₂ synthesis method.Furthermore,Co atoms were incorporated into the TiO₂crystal by in situ topological conversion method,which improved the internal local charge density and effectively promoted the rapid migration of electron holes.In addition,with the construction of C₃N₄ homojunction between crystalline and amorphous C₃N₄,the light absorption area was extended,which also avoided photogenerated electron-hole recombination.constructed an artificial photosynthetic system,and realized the selective generation of HCOOH from CO₂.The main research contents of this paper are as follows:1.The weak adsorption and activation of the active site on the catalyst surface for CO₂ largely affects the efficiency of photocatalytic reduction of CO₂.Firstly,fabricating cubic perovskite precursor（Ca TiO₃）and using in-situ topological conversion method to synthesize hollow TiO_2-x layered structure which possess rich surface frustrated Lewis acid-base pairs（FLPs）.The constructed surface frustrated Lewis acid-base pair is composed of O_Vs（Lewis acid sites）and adjacent surface-OH（Lewis base sites）.The photocatalytic carbon dioxide reduction test shows that under the condition of pure water system without sacrificial agent and co-catalyst,the photocatalytic CO₂ reduction yield to CO can reach 65.1μmol g^-1h^-1,which is 19.1 times of the TiO₂ sample without frustrated Lewis acid-base pair,and 36.6 times of the commercial TiO₂ P25.This work provides a novel strategy for the efficient reduction of CO₂ by constructing surface frustrated Lewis acid-base pairs through an in-situ topological conversion pathway.2.In the photocatalytic CO₂ reduction reaction,sacrificial agents must be introduced to consume the holes generated by photo-induced semiconductors due to the insufficient oxidation ability of most catalysts in water,to realize the efficient separation and transfer of photogenerated electrons for improving the efficiency of CO₂ reduction.In this work,based on the above method of in situ topological transformation to construct highly active photocatalyst,the ability of photocatalytic reduction of CO₂ in pure water environment without sacrificial agent was further improved by introducing cobalt element with strong water oxidation ability.With Co-Ti complex as the reaction precursor,under hydrothermal reaction conditions,TiO₂-Co samples with different doping concentrations were prepared by adjusted concentration of chelating reagent to regulate the content of Co element in TiO₂materials.The construction of Ti and Co bimetallic active sites was realized,and the absorption range of the spectrum was expanded.At the same time,the removal of oxygen vacancy introduced by Co element is conducive to the adsorption and activation of CO₂.The results of photocatalytic test show that when the doping amount of Co element is 3.8%,the yield of CO₂reduction to CO is as high as 114.2μmol g^-1h^-1.For the competitive hydrogen evolution,side reaction in the process of CO₂ reduction,the production of a large amount of H₂ affects the selectivity of CO₂ reduction products.Furthermore,the surface of the catalyst was hydrophobic,and different concentrations of PTFE dispersed solution were used to treat the TiO₂-Co catalyst.The hydrophobic of the surface was improved,and the contact area between the catalytic active site and H₂O during the reaction was reduced,so as to reduce the generation of H₂ by-products.This work provides a synthetic idea for the preparation of efficient hydrophobic catalysts,which can be used to inhibit the generation of H₂ by-products and improve the yield and selectivity of photocatalytic CO₂ reduction.3.The photocatalytic reduction of CO₂ reaction paths are difficult to control,reducing product sort is more,in view of the water environment,and use only the light catalyst reduction problem of poor selectivity of CO₂,inspired by natural photosynthesis in this paper,the advantages of using single enzyme reaction products and high selectivity,constructed the photoenzymatic catalytic systems for CO₂reduction.By grafting amorphous carbon nitride（PCN-A）onto the surface of crystalline carbon nitride（PCN-C）,A polymer semiconductor material with excellent visible light response and bio-compatibility was used to construct carbon nitride homojunction（PCN-H）.Abundant construction of surface active sites and efficient separation of photogenerated carriers were achieved,and coenzyme NADH was efficiently regenerated.The yield can be as high as 82%.Further coupling with formic acid dehydrogenase（FDH）,the photocatalytic reduction of CO₂ to formic acid can be achieved.The concentration of FDH is up to 8.4 m M,and the yield is significantly improved compared with PCN-A and PCN-C.In this work,by constructing the PCN homojunction and constructing the photo-enzyme coupling system,the highly selective photocatalytic reduction of CO₂ to HCOOH was realized,which provided ideas for the construction of the active site on the surface of the catalyst,the separation of photogenerated carriers and the improvement of the selectivity of photocatalytic CO₂reduction products.