Synthesis Of Two-Dimensional Inorganic/Organic Framework Porous Materials And Their Performance In Photocatalytic Reduction Of CO₂

Over the past decades,rapid industrial development has led to a drastic depletion of fossil fuels,raising concerns about energy demand and environmental issues.For the sake of these issues,various green technologies have been actively sought for carbon recycling.Photocatalytic CO₂ reduction,with its clean and renewable energy source and mild reaction conditions,is considered one of the most promising technologies to address the dual challenges of energy and environment.In addition to solar energy as a driving force,photocatalytic CO₂ reduction technology also requires suitable catalysts.Inorganic/organic framework porous materials are considered to be suitable catalysts for photocatalytic CO₂ reduction due to their tunable porous structure,good physical/chemical stability,high specific surface area.At the same time,this type of materials can be constructed through the construction of heterostructures,tuning of architectural units and post-modification of transition metal elements to achieve the purpose of charge separation,energy level adjustment and CO₂ adsorption enhancement,thus providing opportunities for efficient CO₂ photoreduction.Meanwhile,these materials can realize charge separation,energy level modulation,light absorption and CO₂ adsorption enhancement by means of constructing heterostructures,adjusting building units,and post-modification of transition metal elements,thus providing new opportunities for efficient CO₂ photoreduction.In this paper,a variety of inorganic/organic framework porous materials have been prepared and applied to the photocatalytic reduction of CO₂,and the main research content includes the following aspects:1.Aiming at the scientific challenges of rapid self-compounding of catalyst electron-hole pairs and secondary pollution caused by overuse of sacrificial reagents,we have combined chalcogenide material（Cs Pb Br₃）,which has a long carrier lifetime,with g-C₃N₄ to form the Cs Pb Br₃@g-C₃N₄ heterojunction material.In CO₂ reduction experiments conducted in ethyl acetate solvent without excess sacrificial reagents,the results showed that Cs Pb Br₃@g-C₃N₄composite materials achieved a CH₄ yield of up to 274μmol g^-1 under simulated sunlight conditions,which is six times higher than pure g-C₃N₄.Fluorescence steady-state emission intensity reduction and enhanced photocurrent response demonstrated the effective suppression of charge carrier recombination by the heterojunction structure.This work provides a new strategy for designing and synthesizing cost-effective,efficient,and low-pollution catalysts based on g-C₃N₄.2.In the first work we used composite materials to solve the problem of fast self-compounding of electron-hole pairs and effectively improve the catalytic efficiency of catalysts.However,at the same time,the composite materials still have problems such as unclear structure and difficult to understand the catalytic mechanism.Moreover,the preparation process is time-consuming and energy-intensive.Therefore,we propose an ideal method to precisely control catalyst structures by pre-designing suitable building units,thus directly synthesizing catalyst materials with appropriate energy level structures.Using 2,4,6-Trihydroxy-benzene-1,3,5-tricarbaldehyde（Tp）and 5,5’-diamino-2,2’-bipyridine（bpy）as monomers,we synthesized a COF（Tp-COF）with interconvertible enol and keto isomers and applied it to CO₂ photocatalytic reduction.Compared to COF without keto groups（Tb-COF）,Tp-COF exhibited a five-fold increase in CO yield.Band structure calculations and Mott-Schottky measurements indicated that the presence of interconvertible isomers led to energy level differences between the two COFs,enhancing the catalytic performance of Tp-COF.This study achieved precise control of catalysts through simple building unit adjustments,providing a new strategy for preparing efficient and straightforward catalysts.3.In the first two works,the optimal yield of the reduction reaction occurred in a high-purity CO₂ gas atmosphere.At the same time,the current photocatalysts are also mostly suitable for high purity CO₂ atmosphere,while actual industrial emissions and daily life feature low CO₂ concentrations,posing a challenge for catalyst practicality.Therefore,we synthesized a COF（TPBD）and introduced two metal ions,Ni and Co（TPBD@Ni and TPBD@Co）,as heterogeneous catalysts for photocatalytic CO₂ reduction.The results showed that TPBD had poor catalytic performance,but the introduction of Ni²⁺into TPBD increased the CO yield by22 times,reaching 2.93 mmol g^-1 h^-1 with a selectivity of 89.6%.Importantly,the introduction of Ni²⁺in TPBD enhanced CO₂ adsorption capacity and reached adsorption equilibrium at low CO₂ partial pressures,allowing TPBD@Ni to achieve CO yields close to those under pure CO₂concentrations even in low CO₂ atmospheres（15%CO₂/N₂）.This research provides valuable insights into the direct conversion of low CO₂ concentrations and offers a promising photocatalyst.4.The industrial exhaust gases emitted in actual production are accompanied by a variety of toxic gases（acid gases）in addition to low concentrations of CO₂,so the tolerance of the catalyst becomes critical.POMOF materials with polyoxometalate-based extended connectivity nodes enhance resistance to acidic gases while improving structural stability.Furthermore,there is controversy regarding the identification of catalytic active sites on the catalysts.Based on these issues,we synthesized two new POMOF materials with[W₁₀O₃₂]^4-as the polyoxometalate extended connectivity node,named Compound 1 and Compound 2.Compound 1 featured mononuclear cobalt clusters with a synthesis gas yield of 72.7μmol h^-1,surpassing the synthesis gas yield of Compound 2 with dinuclear cobalt clusters(54.2μmol h^-1).In a simulated industrial flue gas system,Compound 1 achieved a synthesis gas yield of 42μmol h^-1,indicating good catalyst tolerance to exhaust gases.Through DFT calculations,we identified that the lowest unoccupied molecular orbital（LUMO）of Compounds 1 and 2 were primarily distributed on the cobalt ion（Co1）directly connected to[W₁₀O₃₂]^4-.These cobalt ions serve as catalytic active centers.Analysis using Multiwfn software revealed that the active center Co1 in Compound 1 experienced weaker repulsion with the surrounding environment,facilitating catalytic reactions.This work provides a promising photocatalyst for synthesizing synthesis gas under low CO₂ concentrations and exhaust gas conditions,while also offering new insights into determining the reduction sites of catalysts.In our research in the field of photocatalytic CO₂ reduction,not only focused on the design and synthesis of efficient catalysts,but also considered the practicality and tolerance in practical applications.These studies have provided new ideas for the development of green technologies.