Preparation Of MOF-Based Materials And Their Photocatalytic Removal Of Cr(Ⅵ) Properties

Cr（Ⅵ）is one of the most toxic heavy metals,posing multiple threats to both human beings and ecosystems.The removal of toxic Cr（Ⅵ）through photocatalysis is one of the effective methods to solve the problem of chromium in wastewater.Metal-Organic Frameworks（MOFs）materials have made some progress in the photocatalytic removal of Cr（Ⅵ）due to their advantages such as adjustable structure and pore size,large specific surface area,and adjustable absorption range.However,the catalytic performance of pure MOFs is still greatly limited due to the lack of sufficient active sites and the existence of drawbacks such as the recombination of photo-generated electron-hole pairs.How to make MOFs materials have more catalytic centers and enhance the separation ability of photo-generated electron-hole pairs to ultimately improve their photocatalytic performance still remains a major challenge.Composite functional motifs and post-functionalization treatment can make MOFs have more catalytic centers,and then improve the catalytic performance.In this study,Ti₃C₂/Ui O-66-NH₂ composite materials with different Ti₃C₂ mass fractions（5 wt%,10wt%,15 wt%,and 20 wt%）were synthesized by an in-situ solvent-thermal method,which significantly improved the photocatalytic removal rate of Cr（Ⅵ）.Ti₃C₂ MXene has abundant catalytic centers and rich coordination unsaturated sites at its terminus,which can form heterojunctions with MOFs to promote the separation of photo-generated electrons and holes,and thus the composite material can effectively enhance the performance to remove Cr（Ⅵ）.SEM images showed that Ui O-66-NH₂nanoparticles were uniformly dispersed on the accordion-like surface and slit of Ti₃C₂.X-ray photoelectron spectroscopy（XPS）further indicated that a strong interface interaction was formed between Ti₃C₂ and Ui O-66-NH₂,which was beneficial to the separation of photo-generated electron-hole pairs.Among them,Ti₃C₂/Ui O-66-NH₂composite material with 10 wt%Ti₃C₂（named 2-T/U）had the best visible light photocatalytic effect on Cr（Ⅵ）,which could be completely removed after 40 min of light irradiation,and were 3.8 times and 1.2 times removal rates of pure Ti₃C₂ and Ui O-66-NH₂,respectively.In addition,this study simultaneous increased the number of mesopores and active sites in MOFs by calcining pure MOFs at different temperatures,thus improving the performance of photocatalytic removal of Cr（Ⅵ）.Different-temperature-calcined Ui O-66-NH₂ was used firstly,and the performance of Ui O-66-NH₂ and its series of calcined catalysts for removing Cr（Ⅵ）was studied.The results showed that the material obtained by calcining Ui O-66-NH₂ in N₂ at 300℃for 2 h（named 300-U）had the best visible light removal efficiency for Cr（Ⅵ）,with a removal rate of 97%after 60 min of light irradiation,while the removal rate of Ui O-66-NH₂ was only 86%.At the same time,the apparent rate constant（k）of 300-U was 1.8 times that of Ui O-66-NH₂.The increase in the mesoporous proportion of 300-U promoted substrate mass transfer,and the newly added defect sites provided more catalytic centers,thus synergistically improving the photocatalytic efficiency.To further investigated the relationship between the performance of the calcined MOFs and their ability to photocatalytically reduce Cr（Ⅵ）,a series of defective MIL-125-NH₂ catalysts were prepared using different calcination temperatures,and their photocatalytic performance for Cr（Ⅵ）removal was studied.The results showed that the material obtained by calcining MIL-125-NH₂ at 300℃for 2 h in N₂（named300-M）had the best visible light removal effect on Cr（Ⅵ）,with a removal rate of 98%after 20 min of irradiation,which was 1.85-fold the removal rate of MIL-125-NH₂,and the k of 300-M was 5.1 times that of MIL-125-NH₂.The N₂ adsorption curve shows that300-M has a large specific surface area(910.77 m² g^-1),and has a certain proportion of mesopore,which can accelerate the substrate mass transfer.XPS further indicates that300-M has more coordination-unsaturated Ti-O clusters,which can provide more active sites for the photocatalytic reaction;and the Tauc diagram shows that 300-M has a smaller band gap that promotes light absorption,all of which can improve the efficiency of photocatal.