| Energy and environmental issues have become increasingly prominent with the advancement of industrialization in countries around the world.How to deal with the increasingly scarce energy issues and the increasing environmental degradation has attracted great attention from the international community.Among them,the massive combustion and excessive consumption of non-renewable resources such as oil,coal,and natural gas have caused excessive CO2 greenhouse gas emissions,melting glaciers,frequent extreme weather,and many other problems,which have had a huge impact on the earth’s ecological environment and human production and life.Among many countermeasures,the catalytic conversion of CO2 into fuels or carbon-containing chemicals with higher added value has attracted widespread attention,and it is expected to alleviate energy and environmental problems caused by non-renewable energy consumption.Among them,the method of non-redox CO2 chemical fixation has great application value due to its flexibility of reaction conditions and relatively low requirements on reaction equipment,such as converting it into value-added carbonates by means of photocatalysis and thermal catalysis are extremely attractive.However,due to the chemical inertness and stability of the CO2 molecule,and the too many and too complex elementary reaction steps involved in the reaction process of synthesizing carbonates,the unclear structure-effect relationship between the coordination structure of the catalytic site and the catalytic activity and unidentified reaction mechanism have hindered the rapid development of this field.Therefore,based on the regulation of the atomic structure of the catalyst surface,combined with various techniques of in-situ characterizations and density functional theory(DFT)calculations,this paper aims to achieve photocatalytic and thermally efficient CO2 chemical conversion to prepare carbonates,revealing the structure-activity relationship between the microstructure of the catalyst and the macroscopic catalytic activity,and the in-depth understanding of catalytic reaction mechanism will provide new ideas for the realization of efficient CO2 chemical conversion to synthesize carbonates and for the field of solar energy conversion.The main research work of this paper is as follows:1.Construction of steric hindered Lewis acid-base pair(FLP)sites on the surface of heterogeneous catalytic materials through surface defect engineering,and employing it for the efficient synthesis of dimethyl carbonate:In order to realize the efficient conversion of CO2 to dimethyl carbonate(DMC),the CeO2 was used as a model system to design a highly efficient catalyst(P-CeO2-NR)with FLP mediated by vacancy defect clusters as the catalytic center,and the widely distributed FLP sites on the surface of PCeO2-NR were confirmed by XPS,ESR and positron annihilation characterizations,and further confirmed the FLP sites on the CeO2 surface by high-angle annular dark field patterning(HADDF).Synchrotron radiation in situ infrared spectroscopy characterized all the intermediates involved in the DMC synthesis reaction.Based on this,DFT calculation verification was carried out,the results showed that the FLP sites on the CeO2 surface can effectively accelerate the formation and conversion of the key reaction intermediate(CH3OCOO*),thereby promoting the formation of the product DMC.Benefiting from the abundant surface FLP sites,the DMC yield of P-CeO2-NR was 15.3 mmol/g,significantly higher than that of other catalytic materials reported so far under similar reaction conditions.This work proposed an efficient strategy based on vacancy cluster-mediated construction of FLP and reveals the in-depth reaction mechanism of FLP-promoted DMC synthesis for the first time,providing new insights into the chemical conversion of CO2 to long-chain chemicals through defect engineering.2.Unsaturated coordinated Mg-MOF structure for a new mechanism of photocatalytic CO2 cycloaddition:in order to achieve photocatalytic CO2 synthesis of cyclic carbonates under mild conditions,we propose a mechanism based on photoexcited single electron transfer to promote the formation of CO2-radicals,enables the green synthesis of cyclic carbonates.By taking Mg-MOF-74 with abundant unsaturated-coordinating Mg active sites as a model system,we propose a deep understanding of the photocatalytic cycloaddition of CO2 with epoxides and reveal the key elementary reaction step.In situ infrared spectroscopy by synchrotron radiation intuitively reveals the photo-excitation-promoted formation and transformation process of carbonate intermediates.And based on DFT calculations,the CO2 attacking step is the rate-determining step of the entire cycloaddition reaction step.Furthermore,constrained DFT calculation results show that the charged CO2-radicals are more likely to occupy the active center of Mg,thereby significantly reducing the energy barrier of the rate-determining step and promoting the progress of the entire reaction.More importantly,the in situ ESR spectroscopy results verified the efficient single-electron transfer from Mg-MOF-74 to CO2.Therefore,thanks to the photoexcited singleelectron transfer mechanism,Mg-MOF-74 achieves high-efficiency photocatalytic CO2 cycloaddition reaction activity under mild conditions(the reaction rate reaches 4.69 mmolg-1·h-1,and the yield is 97%).This work systematically elucidates the reaction mechanism behind the photocatalytic CO2 cycloaddition based on coordinatively unsaturated Mg-MOF-74.This strategy of photo-excited single-electron transfer is expected to be extended to more other fields of CO2 chemical conversion.3.Artificial imitation of the chlorophyll-like magnesium porphyrin structure for efficient photocatalytic CO2 cycloaddition:In order to realize the photocatalytic efficient CO2 cycloaddition under mild conditions to synthesize cyclic carbonates,we designed and synthesized a nitrogenous carbon material with chlorophyll-like magnesium porphyrin structure.Inspired by the magnesium porphyrin active unit in chlorophyll in nature to convert CO2 and water into carbohydrates through photosynthesis,we propose a simple method to synthesize nitrogen-doped graphitebased materials with Mg-like coordination structures for efficient photocatalytic CO2 cycloaddition.We first predicted the stability and feasibility based on the Mg-N4 coordination structure in the nitrogen-doped graphite support through DFT calculations,and used phenanthroline as the chelating ligand of Mg,through the staged heating coordination pyrolysis strategy,the target sample was synthesized successfully.XPS,spherical aberration-corrected HADDF and XANES confirmed the Mg-N4 unsaturated coordination structure in which Mg was dispersed on the entire material surface in the form of single atom and had a magnesium porphyrin-like structure.In situ DRIFTS clearly reveals the formation and diffusion of intermediates in the light-promoted CO2 cycloaddition process.The results of in-situ ESR test show that the light mainly promotes the electron transfer from the active center of Mg to the epoxide and weakens the C-O bond in it,thereby promoting its ring opening and subsequent reaction process.Through the DFT calculation of the complete reaction path of the CO2 cycloaddition reaction process,it is found that the ring opening of the epoxide is the rate-determining step of the entire reaction process,and the activation energy barrier is 1.20 eV.Therefore,thanks to the electron injection and activation of epoxides promoted by light irradiation,Mg-NC exhibited excellent catalytic activity,realizing the photocatalytic reaction rate as 9.67 mmol·g-1·h-1(～99%yield),and its catalytic activity was significantly higher than other previously reported catalyst materials.This work provides new insights into the design and construction of biomimetic enzyme heterogeneous photocatalysts,and provides new insights into the development of photocatalytic CO2 cycloaddition technology with high catalytic activity... |
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