| One of the most essential ways for mankind to break free from their reliance on fossil fuels and achieve sustainable development is to mimic natural photosynthesis and convert CO2 into chemicals or chemical fuels with commercial value.However,the low catalytic conversion efficiency has been the fundamental cause for its limited industrial utilization on a large scale.As a result,it is critical to create,design,and manufacture innovative efficient CO2 reduction photocatalysts.In general,the efficiency of photocatalytic CO2 reduction can be improved mainly by the following aspects:firstly,improving the visible light capture efficiency of the catalyst;secondly,improving the photogenerated electron-hole separation efficiency of the catalyst;thirdly,increasing the contact probability between the reaction substrates and the catalyst.However,traditional inorganic semiconductor photocatalysts,such as metal oxides,are difficult to improve in the three areas mentioned above.Unlike traditional inorganic semiconductor or inorganic metal oxide photocatalysts,porous framework materials,such as metal-organic framework materials(MOFs)and hydrogen-bonded organic framework materials(HOFs),with permanent pore channels and very high specific surface area allow better access of CO2 to catalytic sites.In addition,the designability of porous framework materials allows them to be used as catalytic platforms to carry catalytic reactions of different requirements.Furthermore,the highly crystalline nature of porous framework materials allows us to precisely understand the structure of catalysts and thus to more efficiently investigate the correlation between catalyst structure and activity.This dissertation explores the relationship between catalyst structure and activity through the modification of MOFs/HOFs and controlled synthesis to design and develop new efficient and stable photocatalysts,and through advanced characterization and comparative experimental studies.The research results of this dissertation provide some theoretical guidance and experimental basis for photocatalytic CO2 reduction reactions,and also broaden the horizon for the application of porous framework materials.The main contents of this dissertation are as follows:1.The electrical conductivity of porous materials is closely related to the carrier transfer efficiency in photocatalytic process,so it is crucial to study the electrical conductivity of porous materials to develop the photocatalytic and electrocatalytic performance of the materials.The conductivity of most MOFs materials is relatively poor,and how to enhance the conductivity of the materials is also one of the problems that need to be solved for designing photocatalysts and electrocatalysts.Therefore,we introduce anionic guest molecules(Zn-S2-)into the cationic backbone MOF(PFC-8)under electrostatic action to enhance the conductivity of the MOF by a simple postmodification in this chapter.The experimental results show that the conductivity of PFC-8 after loading anionic guest molecules is improved by 5 orders of magnitude.In order to further explore and study the mechanism of its conductivity improvement,we designed and synthesized a MOF(PFC-9)with the same topology and same metal node as PFC-8 but with an electrically neutral framework.The conductivity of PFC-9 does not increase significantly after loading the same conductive anionic guest molecules.Further through the characterization of XAFS,CV cycle curves,we found that electrostatic interaction has a great influence on the electron cloud density and electrochemical properties of Ni in PFC-8.At the same time,combined with theoretical calculations,we demonstrate that electrostatic interactions play a role in stabilizing conductive anionic guest molecules and expanding new conductive pathways in cationic framework MOFs.Using electrostatic effect to enhance the conductivity of MOFs framework can not only improve the conductivity while maintaining the inherent excellent properties of MOFs materials,but also provide potential possibilities for the application of MOFs materials in other photocatalytic fields.2.Since the hydroxyl group has outstanding affinity for CO2 and the trivalent metal cobalt has excellent catalytic activity,the MOF(Co-Cl)was immersed in KOH solution by post-modification method,and the chlorine atom connecting the central metal atom cobalt in the MOF framework was replaced by the hydroxyl group.The modified MOF(Co-OH)can effectively integrate the carbon dioxide capture and efficient catalytic center with the integration to achieve the highly active and selective conversion of CO2 photoreduction to CO.XPS,SEM,EDS and other experimental characterization have demonstrated that the chlorine atom can be completely replaced by the hydroxyl group,and at the same time,the central metal atom cobalt of the MOF material is partially oxidized from the original divalent to trivalent.BET gas adsorption experiments and photocatalytic CO2 reduction experiments illustrate that the modified MOF not only shows stronger adsorption affinity for CO2 molecules,but also shows higher catalytic activity and selectivity by the trivalent cobalt metal.This method to enhance the activity and selectivity of MOFs for CO2 photoreduction by simple ion exchange not only gives us a further understanding of the mechanism of the photocatalytic process,but also provides a new way to design efficient MOFs-based photocatalysts.3.As a kind of widely used building blocks for self-assembling biomimetic photocatalyst,porphyrin was found to exhibit varying catalytic properties with different central metals and oxidation state.However,very few studies have been focused on whether and how the degree of metalation in porphyrin moieties influence their catalytic performance.In this work,porphyrins were self-assembled into a highly porous hydrogen-bonded organic framework(HOF)for overall CO2 photoreduction.In this prototype structure,we discovered that changing the proportion of metalated porphyrin moieties in structure not only realizes a fine tuning on the ratio of photosensitizer to catalytic site,but also alters the microenvironment surrounding active site and the charge separation efficiency.Consequently,the HOF with the fewest metalation gave rise to the highest production rate,standing out from existing competitors.As such we show a subtle structure change in a close packing molecular assembly deliver pronounced influence on its photocatalytic performance,and the trend unveiled in this work accommodates some principles observed in natural photosynthesis system,providing new insight into the design of biomimetic heterogeneous photocatalyst.4.The deactivation of the catalyst due to agglomeration and the low catalytic activity caused by the inability of a large number of photogenerated electrons and holes in the catalyst to be effectively separated have always been the problems faced by HOFs-based photocatalysts.Therefore,how to alleviate these two problems to improve the photocatalytic CO2 reduction efficiency has always been the research direction of researchers.In view of this,based on the HOFs material,we integrated the P-type semiconductor Cu2O into the HOFs material to form a PN junction-like heterojunction photocatalyst.At the same time,the obtained HOF/Cu2O heterojunction photocatalyst was formed by a simple electrophoretic deposition(EPD)method.The heterojunction catalyst was deposited on the conductive substrate carbon paper(CP)to prepare the heterojunction catalyst film.The photocatalytic CO2 reduction experiments proved that the heterojunction catalyst film can not only convert CO2 to CO with high efficiency,but also exhibit ultra-high selectivity and stability.Further experimental tests such as XPS photoelectron spectroscopy,CO2 isothermal adsorption and CV cycling tests revealed that the HOF material and Cu2O formed a direct Z-mechanism heterostructure,which greatly improved the speed of electron transport and electron-hole separation efficiency.The comparative catalytic experiments of powder catalyst and catalysts film show that the stability and catalytic activity of the catalysts prepared into thin film have been greatly improved.This work not only provides new insights into the design of HOFs photocatalysts,but also broadens the application fields of HOFs materials. |
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