Designed Synthesis Of Organic Photocatalysts And Their Applications

With the establishment and rapid implementation of the goals of "carbon peaking"and "carbon neutrality",China is accelerating its march towards the new energy era.Solar energy is the most abundant one among sustainable energy sources.The utilization and transformation of solar energy is therefore the focus of this new energy revolution.Converting solar energy into chemical energy through photocatalysis is one of the advanced technologies and enjoys the reputation of "artificial photosynthesis".Using solar energy as the only energy input,the technology is attractive in current industrial production architectures not only for its ability of producing chemical fuels such as carbon monoxide and hydrogen,but also for its ability of synthesizing various basic chemicals,such as hydrogen peroxide.After developing for half a century,the conversion efficiency of photocatalytic reactions has been greatly improved.It can reach or even exceed the photosynthetic efficiency of plants(～1%),but still cannot meet the requirements of commercial applications.The development of novel photocatalysts is the key to improving the efficiency of solar energy conversion and promoting its march towards practical applications.Compared with the classical inorganic semiconductor materials,organic materials possess the advantages of structural and functional diversity as well as low cost,which have gradually emerged in recent years and become star materials in this field,such as carbon nitride,covalent organic framework materials and organic molecular catalysts.Based on the above background,this dissertation explored selective 2e-oxygen reduction as the main research subject and carbon dioxide reduction as a more challenging research subject.Focusing on the two research subjects,we designed and explored efficient organic photocatalysts to seek performance breakthroughs.The controlled synthesis of functional photocatalysts is realized by means of molecular-level structural assembly and high-temperature polymerization.The morphology,crystallinity and molecular structure of the material are explored in combination with a series of characterization methods.Through the effective regulation of the photoelectric properties and active sites of the materials,the structure-activity relationship between the molecular structure and photocatalytic properties of the materials was deeply explored.Through the combined use of photochemical and electrochemical characterization,we explored the catalytic reaction path,proposed possible reaction models and deepened our understanding of the catalytic mechanism,which provides an experimental and theoretical basis for the further design and development of novel high-performance photocatalysts.The main research contents and results of this paper are as follows:1.Through multi-scale structural engineering,we prepared defective C3N4 spheres(DCNS)as high-performance 2e-oxygen reduction photocatalysts.At the microscopic scale,DCNS presents a porous spherical morphology with an open network of large specific surfaces,which is conducive to the exposure of the more catalytic active site and fast mass transfer of reactants;at the atomic scale,the nitrogen defects in the C3N4 skeleton and the doping of alkali metal Na+ can promote the separation of photogenerated electron holes within and between layers,thereby improving the efficiency of solar energy utilization.The results show that DCNS can achieve a cumulative rate of H2O2 concentration of up to 3.08 mM h-1 in the presence of ethanol as the sacrificial agent,,which is an order of magnitude higher than that of bulk C3N4.In addition,the catalytic system can work continuously for 15 h with H2O2 concentration accumulating up to 45 mM,which can be directly applied to H2O2 fuel cells or bacterial disinfection.2.By using the covalent organic framework structure as a modular design platform,we organically combined the C3N-inspired catalytically active triazine/heptazine(TA/HA)with the photosensitive unit-1,3,5-trialdehyde resorcinol(Tp)-to obtain a covalent triazine/heptazine frameworks(CTF-TA/CHF-HA).Compared to C3N4,which is entirely composed of repeated TA/HA units,CTF-TA/CHF-HA possess a higher degree of structural conjugation and therefore broader light absorption and faster charge separation rates.The results show that in the presence of ethanol as the sacrificial agent,the H2O2 production rates of CTF-TA and CHF-HA reach 96 μmol h-1 and 30 μmol h-1,respectively,which is 10-40 times higher than C3N4,and far exceeds other reported organic or inorganic catalysts.When we used the benzene ring unit to replace the triazine/heptazine unit,it was found that the photocatalytic performance of the corresponding product decreased significantly,indicating that the triazine/heptazine unit played an important role in the oxygen reduction reaction.In addition to superior activity,CTF-TA and CHFHA exhibit excellent stability of up to 30 h and great selectivity of up to 90%.3.To mitigate the fast charge recombination in the organic polymer photocatalyst,we designed the donor-acceptor(D-A)structure based on conjugated microporous polymer(CMP)using the thiophene-benzene pairing,and systematically regulate the structure from the atomic level by changing the number of nitrogen heteroatoms in the benzene ring.The work demonstrates the role of the D-A structure in improving the photoelectroelectric properties of the organic polymer and further,in improving the photocatalytic 2e-oxygen reduction performance.The results show that with the increase of the number of nitrogen atoms in the benzene ring,the electron withdrawing ability of the acceptor unit continues to increase,resulting in a continuous decrease in the band gap width of the polymer,which broadens the optical absorption range of the material.However,the results of steady-state fluorescence spectroscopy show that the changes in charge separation efficiency do not follow the same tendency.In performance assessment,the product with one-nitrogen substitution in benzene ring exhibits the most superior H2O2 production rate.We will seek further explanations in the following studies.4.Inspired by the sulfur-containing structure of carbon monoxide dehydrogenase in nature,we synthesized the Ni-N3S2 molecular catalyst by introducing sulfur atoms into the ligand structure of nickel-based molecular catalysts through a series of substitution reactions.The molecular catalyst exhibited excellent photocatalytic CO2 reduction performance in the presence of bipyridine ruthenium as a photosensitizer and triethanolamine as a sacrificial agent:Under visible light irradiation,it produced up to 25 tmol of CO within 7 hours,corresponding to a turnover number of 63 and a CO selectivity of 91%.This activity is superior to most other nickel-based catalysts,while the catalyst is also stable enough to maintain structural stability during a reaction cycle.Mechanism studies have shown that Ni-N3S2 can quickly capture the photogenerated electrons of the excited photosensitizer and is reduced to Ni0-N3S2 to undergo efficient CO2 reduction reaction.