The Surface/Interface Regulations On Metal Oxide And Mechanism Of Photocatalytic Reduction Of Carbon Dioxide

It is one of the most effective ways to solve environmental pollution and energy crisis by using sunlight to drive CO₂ reduction to high-value products through photocatalysis.In this system,photocatalyst converts sunlight energy into chemical energy,and then facilitates the reduction of CO₂ to high-value products,such as CO,CH₄ and CH₃OH etc.However,there are several disadvantages that limit the efficiency of the photocatalytic process.Firstly,the CO₂ molecule is exceptionally stable and therefore requires a large energy input to be reduced.Secondly,CO₂ mainly exists in the form of gas,so its interaction with photocatalyst is very difficult,thus causing low reduction efficiency.Although many materials can achieve excellent CO₂absorption,but in most cases,they do not have photocatalytic activity.Finally,the CO₂ reduction process is very complex and involves a variety of intermediates,so it is difficult to control product selectivity.In conclusion,the surface/interface design of efficient photocatalyst is a key scientific issue in the field of photocatalytic reduction of CO₂.In this thesis,surface functionalized metal oxides are synthesized in a controlled manner and their structure regulation is studied.The influence methods of surface/interface interaction on carrier separation efficiency are systematically investigated,the mechanism of CO₂ adsorption activation and structure are elucidated,and the surface state change of CO₂ reaction process at the surface/interface reaction process is investigated in detail.The relationship between the structure of metal oxides and the high activity and selectivity of photocatalytic CO₂ reduction is revealed.We optimize the surface structure,interfacial stability and composition change process of metal oxides and explore the guiding law and mechanism of highly selective reduction of CO₂ to CO or CH₄.The research will provide a basis for the efficient conversion and storage of solar energy and recycling of CO₂,which is of great significance to solve the current energy shortage problem and help resource utilization.The research results include the following several points:1.In response to the drawback of the high recombination rate of photogenerated carriers of Co₃O₄ materials,we investigate the 2D heterostructure comprised of Co₃O₄/2D g-C₃N₄,which can provide enhanced photocatalytic performance of reducing CO₂ to CO.The CO production rate is 419?mol g^-1 h^-1 with selectivity of89.4%,which is 13.5 and 2.6 times higher than those of pure 2D g-C₃N₄ and Co₃O₄.The enhanced photocatalytic performance arises from:(i)enhanced light absorption and charge separation originated from the unique 2D heterostructure connected through specifically-exposed facet interface and(ii)favorable CO₂ adsorption.The study may provide insights for the establishment of heterostructure-based photocatalytic systems toward CO₂ reduction.2.In the above system,although the carrier separation is effectively improved,the CO₂ reduction mechanism has not been thoroughly studied,so In₂O₃ is chosen as a model in this work to investigate the influence of oxygen vacancies in the photocatalytic CO₂ reduction reaction.We explored the effects of oxygen vacancies on activating CO₂ photoreduction via coupling theoretical calculations and experimental results.In a broad sense,oxygen vacancies improved the transport and separation of the photogenerated electron-hole pairs and enhanced the photocatalytic CO₂ reduction activity.The underlying effects,however,were that for n-type semiconductor,the introduced oxygen vacancies could increase the Fermi level,optimize the band structure,boost the reduction capability of the photogenerated electrons,enhance the reactants adsorption of the photocatalyst for the photocatalytic CO₂ reduction reaction.We highlighted the rational design of the photocatalysts,and how the fundamental theory deserves to be integrated into the application of the photocatalysts to accelerate the development of photocatalysis.3.Based on the above In₂O₃ studies,the influence of oxygen vacancies in the design of photocatalysts is explored,but excellent photocatalysts must not only satisfy the thermodynamic conditions of for the redox reaction but also have the ability to accelerate the reaction kinetics to proceed.Here,we report a strategy using grain-boundary surface twinning and oxygen vacancies to synergistically and selectively boost photocatalytic CO₂ reduction activity.Thereinto,grain boundaries as bulk defects create high-energy surfaces by stabilizing dislocations that are kinetically trapped for catalysis owing to the lattice strain of the photocatalyst.Oxygen vacancies are used to tailor the band structure and enhance the adsorption ability of reactants or intermediates.High-energy surface structures arisen from these bulk defects may be more resistant to the relaxation effect,resulting in excellent stability for photocatalytic CO₂ reduction.Considering the increase for photocatalytic CO₂reduction activity,this work provides a strategy for broader unitization of bulk defects in heterogeneous catalysis.4.Although the kinetic and thermodynamic basis of twinned structure and the oxygen vacancy in the photocatalytic CO₂ reduction reaction has been investigated,direct evidence that twin boundaries produce surfaces with enhanced activity is lacking.In order to investigate the mechanism of the influence of twinned structure on the active surface,Cu₂O nanotwins(GB-Cu₂O)were controllably synthesized in this work using microwave radiation twinning technique,and the parallel twin crystal surfaces promote charge-directed migration.CO₂ molecules adsorbed on the surface of GB-Cu₂O undergo reduction reactions in the twin crystals at the ends of the twinned boundaries,while H₂O undergoes oxidation reactions in the twinned boundaries.This spatial separation of the oxidation/reduction reactions provides the kinetic basis for the photocatalytic reduction of CO₂to CH₄.Twinned boundaries create regions of strain in polycrystalline materials by stabilizing dislocations and provide a way to create high-energy surfaces for kinetics improvement.Thanks to the advantages of the nanotwin structure,the rate of conversion to CH₄ in the photocatalytic reduction of CO₂ reached 14.5?mol g^-1 h^-1 with a selectivity of 90%.5.In the present work,the factors influencing the selectivity and activity of catalytic sites in the photocatalytic reduction of CO₂ were also investigated.We design a hierarchical electron harvesting system on Co-Ni-P hollow nano-millefeuille,which enables the charge enrichment on Co-Ni dual active sites and selective conversion of CO₂ to CH₄.The Co-Ni-P serves as an electron harvester and photonic"black hole"accelerating the kinetics for CO₂ reduction reactions.Moreover,the dual-sites form from highly stable Co-O-C-Ni intermediates,which thermodynamically not only lowers the reaction energy barrier but also transforms the reaction pathways,thus enabling the highly selective generation of CH₄ from CO₂.As an outcome,the Co-Ni-P NH/black phosphorus with dual-sites leads to a tremendously improved photocatalytic CH₄ generation with a selectivity of 86.6%and an impressive activity of 38.7?mol g^-1 h^-1.