The Metal Ions Modification And Photocatalytic Properties Of NH₂-MIL-125（Ti） And NH₂-MIL-101（Fe）

The global energy crisis and environmental pollution caused by limited reserves of fossil fuels and the harmful gas emissions from their combustion promoting the rapid development of sustainable and renewable energy.Hydrogen energy is considered as a promising energy carrier to solve the energy crisis.Photocatalytic hydrogen production is considered to be one of the most promising strategies to overcome energy shortage because it can directly convert inexhaustible solar energy into hydrogen energy.At the same time,photocatalytic technology can also be used to degrade various harmful chemical pollutants（such as NO_x）to purify the environment.The core of photocatalysis lies in photocatalysts,but most photocatalysts have high photo induced carrier recombination rate and low solar energy utilization efficiency.Therefore,it is particularly important to find an efficient and stable photocatalyst.Metal organic framework materials（MOFs）are a new type of porous materials with periodic network structure formed by the self-assembly of inorganic central metals and bridged organic ligands.MOFs have good applications in the field of catalysis because of their extremely ordered pore structure,high specific surface area and modifiable pore surface.The current common MOFs suffer from the disadvantages of low separation ability of photogenerated electron-hole pairs and low transfer rate of photogenerated charge carriers,resulting in poor photocatalytic performance.Therefore,it is of great significance to modify MOFs to enhance their photocatalytic performance.In this study,we modified NH₂-MIL-125（Ti）and NH₂-MIL-101（Fe）by introducing different metal ions,which improved the separation ability of their photogenerated electron hole pairs and the transfer rate of photogenerated charge carriers,thereby enhancing their photocatalytic performance:（1）The new bifunctional photocatalyst synthesized via Yb（NO₃）·5H₂O and NH₂-MIL-125（Ti）stirred at room temperature could enhance photocatalytic NO removal and hydrogen production,in which the rare earth metal Yb was coordinated with the-NH₂group of NH₂-MIL-125（Ti）.accelerate the transfer and separation rate of charge carriers.The NO removal rate of the optimal sample Yb-NM-6 was 66.3%（the original NH₂-MIL-125（Ti）was only 28.1%）,and the hydrogen production was 1883.80μmol·g^-1（the original NH₂-MIL-125（Ti）was almost no effect）.Comparative experiments and cyclic voltammetry（CV）tests showed that the transient Yb^II/III center was obtained through a noval ligand to linker metals charge transfer（LLCMT）pathway,which can improve the electron transfer and separation rate,so as to improve the photocatalytic performance.This work provides us a feasible avenue to obtain a novel LLMCT pathway through introducing various metal ions anchored on the MOFs as efficiently transient centers to further improve the photocatalytic performance.（2）The earth-abundant copper species coordinated with the-NH₂ of NH₂-MIL-101（Fe）as noble-metal-free cocatalysts via transient Cu^II/Cu^I centers were obtained via simply stirring Cu（OAc）₂ with NH₂-MIL-101（Fe）to achieve excellent photocatalytic hydrogen evolution performance in the dye sensitization system.The prepared sample6Cu-NM-101 exhibited the best hydrogen evolution activity of 5770.96μmol·g^-1·h^-1,which was 4.06 times higher than the original NH₂-MIL-101（Fe）.A series of characterization tests,especially the CV and the Auger Cu LMM,demonstrated that the transient Cu^II/Cu^I center played a significant part in the novel ligand to linker metal charge transfer（LLMCT）process.In other words,the electrons could transfer from the lowest unoccupied molecular orbital（LUMO）energy level of the excited state Eosin Y（EY*）to the transient Cu^II/Cu^I center to further accelerate the carrier transfer and separation rate.Our work provided a convenient and effective strategy to prepare various earth-abundant metal ions based on MOFs as low-cost cocatalysts via transient metal centers to boost the photocatalytic hydrogen evolution performance.（3）A novel bifunctional photocatalyst Au@NML-（Cu/Ti）was fabricated to improve the photocatalytic performance of hydrogen production and NO removal,in which the Cu ion was first doped into the titanium oxide clusters of NH₂-MIL-125（Ti）（NML）via hydrothermal synthesis,and uniformly dispersed gold nanoparticles with surface plasmon resonance effect in-situ generated under the constraint of NML-（Cu/Ti）framework.The introduction of copper ions reduced the band gap of NML and accelerated charge separation rate.Moreover,the NML-（Cu/Ti）framework could prevent the Au nanoparticles agglomeration as far as possible via confinement effect to obtain highly dispersed gold nanoparticles in very small size,which can effectively improve the atom utilization efficiency for enhancing photocatalytic performance.The photocatalytic hydrogen production of Au@NML-（Cu/Ti）was 5193.40μmol·g^-1,which was 11.8 times as that of the original NML.Meanwhile,the NO removal rate of Au@NML-（Cu/Ti）was about 25.6%higher than that of the pristine NML.These results showed that the synergistic combination of central metal cluster modification of MOFs with surface plasma resonance provided a feasible strategy for improving photocatalytic hydrogen production and NO removal.The experimental results show that the ligand modification of NH₂-MIL-125（Ti）and NH₂-MIL-101（Fe）improves their photocatalytic hydrogen production ability under visible light irradiation;NH₂-MIL-125（Ti）was modified via metal clusters and loaded with nanoparticles to prepare an efficient bifunctional photocatalyst,which enhanced its ability of photocatalytic removal of NO and hydrogen production.The mechanism of photocatalytic reaction was deeply studied by means of CV,ESR and in-situ infrared spectroscopy.This research finally produced efficient photocatalysts,which provided a strategy to solve the problems of energy and environment.