Structure Synthesis And CO₂ Photoreduction Properties Of Titanium Oxide Cluster Based Crystalline Materials

As we all know,TiO2 as one kind of famous photocatalyst materials has been produced commercially,and used widely in many photocatalytic reactions(including pollutant degradation,water decomposition,CO2 reduction reactions,etc),due to its low toxicity,easy preparation,simple structure,stable properties and low cost.However,the long-range order and ultraviolet light absorption of the traditional TiO2 structure make them have many limitations in terms of structural changes,solar light utilization efficiency and functional modification.Although the TiO2 structure can be modified through a lot of strategies such as surface-interface reactions,recombination,oxygen vacancy manufacturing,it still inevitably limits their effective application and development in photocatalytic reactions.In recent decades,titanium oxide cluster(TOC)-based crystalline materials(including TOC,TOC-based organic framework materials,etc.)based on coordination chemistry have developed rapidly,which can be attributed to two main aspects.On the one hand,TOC-based crystalline materials exhibit similar structure as that of traditional TiO2.On the other hand,they have great flexibility in regulating structural design,synthesis,composition,type(involving structure,nuclearity and dimension),light absorption range,functional modification,charge transport,etc.Based on the above characteristics,TOC-based crystalline materials have more advantages and potentials compared to traditional TiO2 in terms of structure change,band gap adjustment and functional applications.Therefore,they have been gradually applied to traditional photocatalytic reactions,and showed superior excellent performance.Especially for TOCs,their inherent zero-dimensional molecular structures belong to short-range order,and then can behave as heterogeneous catalysts in the insoluble state but can behave as homogeneous catalysts in the dissolved state.However,for the photocatalytic CO2 reduction reaction(one of the most promising green ways for CO2 recycling in the future),although TiO2 and its composite catalysts have been explored,there are still few reports on the use of TOC-based crystalline materials in this important reaction.In this thesis,we designed and synthesized a series of new TOC-based crystalline materials,and regulated their structural composition and type,nuclearity,light response range and charge transport capacity through the reasonable selection of structural building unit and functional modification strategies.Finally,we systematically studied their application as homogeneous or heterogeneous catalysts in the CO2 photoreduction.The specific works are as follows:(1)Synthesis of the new UV-responsed titanium oxide clusters and their CO2 photoreduction performanceWe usd the heteroatom sources,tetrahedral phosphate(PO43-)and triangular pyramid sulfate(SO42-),to construct four new TOC-based heteropolyoxometalates(Ti6S4,Ti16S4,Ti7P6 and Ti12P3),and the structure diversity,nuclearity,light absorption capacity and band gaps of these TOCs were adjusted.According to the HOMO-LUMO energy level distribution obtained from the experimental characterization results,Ti6S4,Ti16S4 and Ti12P3 can be used as homogeneous catalysts for the CO2 photoreduction reaction.These three TOCs can all exhibit very high CO2-to-HCOOH conversion activity in water under ultraviolet light irradiation.Among them,Ti16S4,which contains more active sites,has a photocatalytic activity as high as 443.33 ?mol g-1 h-1 and selectivity close to 100%after 12 h reaction.The research content in this chapter effectively expands the TOC-based heteropolyoxometalate system and provides more insights for these materials applications in the photocatalytic CO2 reduction reaction.(2)Synthesis of the UV-Vis-responsed titanium oxide clusters and their CO2 photoreduction performanceAiming at the limited UV response capability of traditional TiO2 materials,we used macrocyclic molecular calix[4]arene as functionalized ligand to successfully construct four new TOCs with the increasing nuclearity.Based on the modification of the hydrophobic benzene rings in the molecular structure of calix[4]arene,these TOCs can exhibit high structural stability in aqueous solutions and photocatalytic systems.Moreover,due to the strong coordination interaction between the titanium-oxo core and the calix[4]arene hydroxyl groups can induce a strong charge transfer effect,their light absorption range can be extended from the traditional ultraviolet region to the visible region.Based on these advantages and matched energy level distribution,Ti-C4A,Tiz-C4A and Ti16-C4A can be used as heterogeneous molecular catalysts to perform photocatalytic CO2 reduction reactions in water and can convert CO2 into HCOOH selectively.Among them,Ti16-C4A is the best crystalline TOC-based photocatalyst materials reported so far,with very high photocatalytic activity(2265.5 ?mol g-1 h-1,12 h)and HCOOH selectivity(100%)under the ultraviolet light.At the same time,its photocatalytic activity induced by visible light can reach 211.0 ?mol g-1 h-1(12 h).This research work provides a new strategy for the future development of more stable and photosensitive TOCs for artificial photocatalytic CO2 reduction reaction.(3)Synthesis of the stable UV-vis-responsed titanium oxide clusters and their CO2 photoreduction performanceIn order to further improve the light absorption,stability and charge transfer ability of the calix[4]arene-modified titanium oxide clusters,we used functionalized calix[8]arenes with higher molecular freedom and larger volume to construct two stable TOCs.Interestingly,the crystal structures of these two TOCs can achieve thermodynamic interconversion under specific solvent and temperature conditions,and Ti7-C8A can obtain different crystal morphologies by changing the mixed solvents ratio.In addition,given that the molecular structure of calix[8]arene has more hydrophobic benzene rings and hydroxyl groups,it can be used as a functionalized armor to make these TOCs exhibit higher structural stability,chemical stability and strong charge transfer effect.At the same time,the visible light absorption intensity of TOCs was also greatly improved.Finally,these two TOCs can also be used as heterogeneous molecular photocatalysts to convert CO2 into HCOOH with high activity(488.35 ?mol g-1 h-1)and high selectivity(99.7%)in water.It is worth noting that this work is the first report of crystalline coordination molecular compounds as heterogeneous photocatalysts to complete CO2 reduction in water.(4)Synthesis of the stable UV-vis-responsed titanium oxide clusters with strong charge transfer capability and their CO2 photoreduction performancewe constructed three new TOCs with increasing nuclearity by using ferrocene carboxylic acid ligands with good electron storage and donating capabilities.Since the overall structure of the three TOCs are surrounded by different numbers of functionalized conductive ferrocene units,they finally exhibit good structural and thermal stabilities,a wide range of light absorption capabilities(even to the entire UV-vis light absorption area,such as Ti13Fc6)and excellent photo-generated charge transfer ability.Based on their structural characteristics,functionalized advantages,and matching energy band structures,they can be used as heterogeneous molecular catalysts to photocatalytic reduce CO2 to HCOOH with high selectivity(nearly 100%)in water.In particular,the photocatalytic performance(412.0 ?mol g-1 h-1)induced by visible light is significantly improved compared to TOC-based heteropolyoxometalates and calixarene-modified TOCs,indicating the unique role of functionalized ferrocene motif in improving the performance of the photocatalytic CO2 reduction reaction.More importantly,this research work is the first use of conductive group modification to construct a highly efficient crystalline material catalyst for the photocatalytic CO2 reduction reaction,which provides a new way for the future development of more stable,photosensitive,and conductive crystalline material catalysts.(5)Synthesis of the stable UV-vis-responsed titanium oxide cluster-based MOF with strong charge transfer capability and their CO2 photoreduction performanceWe further applied the functionalized ferrocene units through chemical bond grafting and loading in pores to the structure modification of stable crystalline NH2-MIL-125(Ti)material catalyst for the first time,in order to improve its intrinsic photocatalytic CO2 reduction activity.Through post-synthesis functionalized grafting(aldimine condensation reaction),the host MOF structure realized the bifunctional groups(conductivity and photosensitivity)modification(-NH2 auxochromic group and ferrocene conductive group),thereby effectively increasing the light harvesting(even to almost the entire UV-vis absorption region)and charge transfer capacities.Finally,the ferrocene-modified MOF materials can perform photocatalytic CO2 reduction reactions in the water phase and exhibited the improved CO2-to-HCOOH conversion activity progressively.Among them,the photocatalytic activity of Fca-NH2-MIL-125(Ti)-4,which was grafted with the highest amount of ferrocene groups,under ultraviolet light and light irradiation reaches up to 293.40 ?mol g-1 h-1 and 266.33 ?mol g-1 h-1,respectively,and the selectivity of HCOOH production is close to 100%.It is worth noting that this is also the first report of using conductive groups-modified crystalline MOF material catalyst to improve the CO2 photoreduction reaction activity,further expanding the application potential of functionalized ferrocene unit in more TOC-based crystalline material catalysts.