Construction Of Ti₃C₂T_x（MXene） Heterophase Junction Photocatalysts For NADH Regeneration And Application

Enzymes possess excellent catalytic properties such as fast reaction speed,good activity,high stereoselectivity and mild reaction conditions,and their applications in the fields of chemical synthesis,pharmaceutical,environment and energy utilization are growing exponentially.Reduction of carbon dioxide（CO₂）through the activation of formate dehydrogenase（FDH）by niacinamide adenine dinucleotide（NADH）requires a huge stostoic ratio and cost,hence there is an urgent need for efficient NADH regeneration.Inspired by the photosynthesis of chloroplasts to regenerate NADH under sunlight through photogenic electron transfer,photocatalytic NADH regeneration has become a research hotspot.Traditional semiconductor photocatalyst materials,including TiO₂,Cd S and g-C₃N₄,have attracted extensive attention,but their low carrier separation efficiency and weak electron conduction ability limit their further development.Ti₃C₂T_xMXene has abundant photogenerating carrier trap sites and good electrical conductivity to achieve effective charge separation when combined with photocatalyst materials.The construction of phase junction helps to enhance the efficiency of carrier separation and transfer in semiconductor.Based on the properties of Ti₃C₂T_xMXene,the combination with phase junction semiconductor can enhance photocatalytic activity and photogenerated electron transfer.The contents and results of this study are as follows:（1）A series of anatase/rutile TiO₂@Ti₃C₂T_x（ARTM）photocatalysts were prepared by using Ti₃C₂T_xMXene in situ to grow anatase/rutile TiO₂,and by regulating the concentration of hydrochloric acid and hydrothermal reaction time.When the concentration of hydrochloric acid was 0.25 mol L^-1and the hydrothermal time was 36 h,ARTM36 with rich anatase/rutile TiO₂heterojunction（ART-PJ）and TiO₂@Ti₃C₂T_xheterojunction（TM-HJ）was prepared,and the yield of NADH was up to 85%within 50 min.The selectivity of 1,4-NADH was 93.1%,and the TOF of 1,4-NADH was 0.166 h^-1.More importantly,the yield of NADH reached65%without the addition of the electron-mediated（Rh complex）.The high regeneration rate of 1,4-NADH(0.712 mmol g^-1h^-1)provided the basis for the construction of photoenzyme cascade catalytic system,and the photoconversion efficiency of CO₂to formic acid was 767μmol g^-1h^-1.Detailed mechanism analysis shows that the synergistic effect between ART-PJ and TM-HJ promotes the efficient separation of photogenerated carriers and improves the photocatalytic activity.In addition,the addition of Ti₃C₂T_xpromoted the selective regeneration of 1,4-NADH.（2）A hexagonal/tetragonal Cd S sulfide phase junction photocatalysts were prepared with cadmium sulfate and thiourea,and modified with a few layers of Ti₃C₂T_x.The yield of NADH regenerated by 1%Ti₃C₂T_x@h/c-Cd S reached 72%in 25 min,the selectivity of 1,4-NADH was up to 85.8%,the TOF of 1,4-NADH was up to 0.407 h^-1,and the regeneration rate was1.116 mmol g^-1h^-1.A photoenzyme cascade system was constructed to promote the photoconversion of CO₂to formate(1940μmol g^-1h^-1).Detailed mechanism analysis showed that the efficiency of NADH regeneration is related to the construction of phase junction and the loading capacity of fewer layers of Ti₃C₂T_x,which can improve the efficiency of carrier separation,enhance photoexcited electron migration,and promote the hydrogenation of NAD⁺.In addition,possibly due to the high conductivity of Ti₃C₂T_xand the interfacial interaction with h/c-Cd S,1%Ti₃C₂T_x@h/c-Cd S showed good selectivity for 1,4-NADH.（3）A series of g-C₃N₄phase junction photocatalysts were prepared by adjusting the ratio of supramolecular（calcining by melamine）precursor to thiourea,and modified with a few layers of Ti₃C₂T_x.5-MX-TM-CN-3 achieved NADH re-life and 1,4-NADH selectivity of 84%and 81.6%.The yield and selectivity reached 48%and 41.1%,respectively,in the absence of electron mediator.Systematic characterization showed that the abundant TM-CN phase junction in 5-MX-TM-CN-3 could improve the carrier separation efficiency,while the high conductivity of Ti₃C₂T_xcontributed to the hydrogenation of NAD⁺.The excellent activity and selectivity of 5-MX-TM-CN-3 prompted the reduction of CO₂to formate by FDH(1023μmol g^-1h^-1).