Design And Synthesis Of Crystalline Covalent Organic Framework Materials And Their Carbon Dioxide Reduction Properties

The continuous fossil fuel consumption has led to massive emissions of greenhouse gases（mainly carbon dioxide,CO₂）,causing severe environmental issues such as global warming and sea level rising.The development of an approach for high-efficient conversion of CO₂ to high value-added substances with the assistance of renewable energy sources is highly important for the sustainable development of human society.The use of light and electric energy to catalytically convert CO₂ has currently emerged as the most promising ways to address the above issues.Among this,using inexhaustible sunlight as driving force to reduce CO₂ to chemical fuels（CO,HCOOH,HCHO,CH₃OH,CH₄,etc.）is one of the most attractive ways.Secondly,the use of clean electric energy for electrocatalytic reduction of CO₂ has also been proved to be a particularly efficient method.Significantly,the exploration of suitable photo/electrocatalysts is critical for the effective execution of this catalytic process.Generally,a suitable CO₂ reduction photocatalyst should possess a stable chemical construction,a strong light-harvesting ability,an efficient electron transport and CO₂adsorption and activation ability.Among the reported photocatalysts,covalent organic frameworks（COFs）exhibit outstanding advantages in the photocatalytic reduction of CO₂,because they not only have the ability to absorb CO₂ molecules through their specifically functionalized channels but also can activate CO₂ molecule through their adjustable active sites.COFs are a class of crystalline porous polymers that consist of strong covalent bonds and conjugated layers with periodic skeletons and abundant regular pores.CO₂ adsorption and activation,electron transfer and structural stability can be achieved simultaneously by using rationally designed COF.In this paper,we have designed several new functional COFs based catalysts and further applied them to the photoelectrocatalytic reduction of CO₂,and main works are as follows.（1）In order to introduce highly reactive metal sites into COFs,we constructed a transition metal ion（TMI）modified COF system for photocatalytic CO₂ reduction that showed high efficiency and durability for converting CO₂ to CO and/or HCOOH under visible light irradiation.The successful incorporation of TMIs into DQTP COF realized through a post-treatment method was verified by exhaustive characterization combined with density functional theory（DFT）calculations.The feasibility of photoreaction was confirmed by electronic structure analysis.The core role of DQTP COF as a platform is that in the columnar orientation COFs provide a high-efficient charge carrier transport because of the orderedπ-electronic pathway,which improves electron transfer from COF to metal moieties,thereby increasing the reactivity.Different kinds of open metal active species on COFs have a strong influence on the activity and selectivity of CO₂reduction.As a result,DQTP COF-Co and Zn exhibited the highest CO and formic acid production rate of 1.02×10³ and 152.5μmol h^-1g^-1,respectively.A“two-pathway”mechanism for photocatalytic CO₂reduction over metal-modified COF was proposed to explain differences in activity and selectivity.This work demonstrated the great potential of COFs as platforms for effective heterogeneous photocatalytic conversion of CO₂,and we believe that the present strategy can open up a new avenue to the development of more efficient heterogeneous catalysts for photocatalytic CO₂ reduction.（2）Since the CO₂ reduction photocatalyst designed in the first work is only able to catalyze the reaction with a photosensitizer and a sacrificial agent,it is economically and environmentally unfavorable.In the next step of research,we expect to realize the catalytic reaction with water as the sacrificial agent.Herein,we designed a series of crystalline 2D rigid porphyrin-tetrathiafulvalene COFs（TTCOF-M,M=2H,Zn,Ni,Cu）for artificial photosynthesis including CO₂ reduction and H₂O oxidation.By adjusting the type of central metal in porphyrin,the band gap of TTCOF-M materials can be adjusted.The study found that TTCOF-Zn/Cu has suitable conduction band and valence band positions,which theoretically meets the requirements of CO₂ reduction and water oxidation reaction.Under the irradiation of visible light（420-800nm）,TTCOF-Zn exhibited the highest CO production of 12.33μmol within 60 h and selectivity（ca.100%）combined with excellent durability under our experimental conditions,along with H₂O oxidation to O₂.For the photocatalytic mechanism,it is proposed that the efficient electron-hole separation and transfer of electrons from tetrathiafulvalene to the acceptor porphyrin under light excitation is the main reason why this material can reduce CO₂ in water.It is confirmed that the photoexcited electrons（on porphyrin）and positive holes（on TTF）can be used for CO₂ reduction and H₂O oxidation reactions,respectively.Moreover,the corresponding DFT calculations agreed with the experimental results.This work creatively designed and synthesized a crystalline COF catalyst for selective photoreduction of CO₂ using water as the electron donor,also provides a new sight for designing more efficient artificial crystalline photocatalysts.（3）Although artificial photosynthesis has been achieved in the previous work,the overall catalytic efficiency still needs further improvement.Herein,a series of covalent linked COF-semiconductor Z-scheme photocatalysts were synthesized and the CO₂reduction under the condition of CO₂-water vapor was achieved.These composite materials realized the CO₂ reduction using H₂O as the electron donor,without the help of additional photosensitizers and sacrificial agents.As expected,COF-318-Ti O₂ Z-scheme catalyst showed the highest CO yield of 278.7μmol g^-1in 4 h with about 100%selectivity.The cycle stability test results show that the performance of the catalyst decreases only about 5%after five cycles.The mechanism study shows that this Z-scheme heterojunction catalyst can achieve efficient electron hole separation and transfer between COF and oxide semiconductors under visible light excitation,and effectively connect CO₂ reduction on COF with water oxidation on oxide semiconductors through covalent bonds.The combination of density of states（DOSs）calculation and in-situ XPS proves that the electron transport direction is from Ti O₂ to COF-318,and Z-scheme heterojunction is formed macroscopically.The photogenerated electrons redistribution on COF after excitation,resulting in the electrons accumulating at the cyano/pyridine active sites of the COF for the CO₂reduction while positively charged holes in semiconductor enable H₂O oxidation,thus mimicking natural photosynthesis.Finally,the DFT simulation based on the composite structure model confirms the above theory.In this work,a novel covalent bonding strategy between crystalline COF and inorganic semiconductors was developed and successfully used in artificial photosynthesis.This strategy represents a new insight for the future rational design of Z-scheme organic-inorganic heterojunctions for artificial photosynthesis.（4）In addition to driven by photo energy,electrical energy is considered as another promising approach to realize CO₂ reduction reaction.Based on the M-TTCOFs structure in the first work,we successfully used it for efficient electrocatalytic CO₂reduction（ECR）after rational modulation.Tetrathiafulvalene,serving as electron donator or carrier,can construct an oriented electron transmission pathway with metalloporphyrin.Thus obtained M-TTCOFs can serve as electrocatalysts with high FE_CO（91.3%,-0.7 V）and possess high cycling stability（>40 h）.In addition,after exfoliation,the FE_CO value of Co-TTCOF nanosheets（～5nm）is higher than 90%in a wide potential range from-0.6 to-0.9 V and the maximum FE_CO can reach up to almost100%（99.7%,-0.8 V）.The ECR mechanisms are discussed and revealed by density functional theory calculations.This work paves a new way in exploring porous crystalline materials in ECR.This study shows that the intramolecular multi-electron transfers of the catalyst itself is beneficial to improve the electrocatalytic performance of CO₂,and the exposure of active sites will also be beneficial to the electroreduction of CO₂.It provides a new strategy for the design and construction of CO₂electroreduction catalysts based on crystalline porous materials,and a new method for solving CO₂ environmental problems.（5）We proposed that the effective combination of light energy and electric energy will enhance the catalytic performance of ECR.Herein,a series of stable crystalline metallophthalocyanine covalent organic frameworks（MPc-TFPN COF,M=Ni,Co,Zn）were synthesized and used as photo-coupled electrocatalysts for CO₂ reduction.These COFs show high stability and crystallinity due to dioxin-linkage.And because of the presence of metal phthalocyanine（photosensitizer）,the COFs showed excellent photosensitivity.Firstly,the ECR properties of different metal phthalocyanine COF-M（M=Ni,Co,Zn）were studied.The Faradaic efficiency of CO(FE_CO)up to 99.8（±1.24）%for Ni Pc-TFPN COF and 96.1（±1.25）%for Co Pc-TFPN COF at-0.9 V vs.RHE,respectively,and remarkable cycle stability with neither an obvious current drop(an average of-16.3 m A cm^-2 for Ni Pc-TFPN COF and-12.1 m A cm^-2 for Co Pc-TFPN COF)nor large FE fluctuation was observed during 60 h ECR test.More importantly,with the assistance of light,the FE_CO and the current density were further enhanced over a wide potential range（-0.6 to-1.2 V vs.RHE）compared to the dark environment for Ni Pc-TFPN COF(j _CO increased from 14.1 to 17.5 m A cm^-2 at-0.9 V;FE_CO reached up to～100%at-0.8 to-0.9 V),which is one of the highest in reported COFs,suggesting that the photo coupled ECR（PECR）showed enhanced performance.Furthermore,an in-depth mechanism study with S₁ and T₁ excited states was established by DFT simulation and experimental characterization.A mechanism study revealed that external light-field can enhance the electron transfer to the adsorbed CO₂ in phthalocyanine COF,which is conducive to the reduction of CO₂.This is the first report of dioxin-linked metallophthalocyanine COF,which shows excellent performance for ECR and PECR.Our results also highlight that external light irradiation has a significant effect on the ECR activity and product selectivity for light-sensitive electrocatalysts.