Design Of Polyoxometalate-based Composite Nanomaterials And Its Study On Photocatalytic Fixation Nitrogen

In nature,the nitrogen(N2)fixation to generate ammoniais(NH3)is very important chemical process for inputting nitrogen(N)source into global biogeochemical cycle.NH3 is not only essential clean energy,but also an important industrial product for human life and one of the raw materials for nitrogen fertilizer.The Haber—Bosch method is currently widely used in the industry for synthetic NH3,but the problems it faces are highly energy consumption,harsh reaction conditions and severe pollution.Photocatalysis is considered to be an alternative technology for NH3 synthesis with the advantages of green,safety and sustainability.Therefore,it is important to seek efficient photocatalysts that can achieve continuous producing NH3 at normal temperature and pressure,and to completely solve the problems involved in industrial NH3 synthesis.Polyoxometalates(POMs)are a class of inorganic molecular clusters composed of high-yielding elements with outstanding property of uniform nano-size,unique physical and chemical properties,adjustable compositional structure,wide absorption spectrum,strong electron receiving and giving ability,and reversible multi-electron redox activity,etc.Catalysis is one of the important research fields for POMs.In this paper,POMs as the molecular pre-assembly platform,from the demand for photocatalytic materials with specific functions,a series of POMs-derived nanomaterials been constructed,which have realized a significant improvement of catalytic performance in homogeneous photocatalytic reduction of N2 and high stability.The research on these composite nanomaterials provides unprecedented opportunities and challenges for designing and synthesizing more efficient and stable POMs-based catalysts,and offers an important reference for further expanding the application research of POMs in photocatalytic reduction N2.The specific work is as follows:1.For the first time,we have reported three types of reduced state of POMs@graphene oxide(GO)composite nanomaterial with outstanding photocatalytic N2 fixation activity in pure water without any other electronic sacrificial agents and cocatalysts at atmospheric pressure and room temperature.Among them,r-H5[PMo10V2O40]@GO(r-PMo10V2@GO)exhibits the highest NH3 generation efficiency of 130.3 ?mol L-1 h-1,which is improved by 65.9%and 97.3%compared to the reduced PMo10V2 and PMo10V2.Experimental results show that the high photocatalytic N2 fixation effect of composite nanomaterials is attributed to the synergistic effect of reduced POMs(heteropoly blue HPBs)and reduced graphene oxide(rGO).The introduction of HPBs reduces the degree of accumulation of rGO,so that rGO exposes more active sites to adsorb N2.HPBs can be uniformly dispersed on rGO,which can expose more reactive sites.At the same time,HPBs have a wide absorption spectrum,which can effectively convert light energy into chemical energy,and stimulate abundant electrons to activate N2.This work provides a new perspective for the research of POMs-based composite nanomaterials in the field of photocatalytic fixation N2.2.In order to further explore the recyclability of POMs-based materials in photocatalytic N2 fixation reactions,for the first time,we investigate four different transition metal-substituted Dawson-type POMs and containing N-defect g-C3N4(V-g-C3N4)composite nanomaterials and apply into photocatalytic fixation N2,which all exhibit outstanding catalytic activities under mild conditions.Surprisingly,?2-K8P2W17O61(Co2+·OH2)·16H2O@V-g-C3N4,(P2W17Co@V-g-C3N4)shows the best photocatalytic N2 fixation efficiency of 214.6 ?mol L-1 h-1,which is increased by 91.05%and 95.99%as compared to that of P2W17Co and V-g-C3N4 only.The size and shape of the defect N in V-g-C3N4 is the same as that in the N2 molecule,and the N2 molecule can be captured firmly and accurately by V-g-C3N4.POMs can effectively promote the activation and dissociation of N2.POMs are easily reduced under the presence of electron sacrificial agent and light conditions.The HPBs have stronger reducing properties and can provide abundant electrons to activate N2.The HPBs further react with oxygen returning to the oxidized POMs.This reaction forms a self-repairing,recyclable photocatalytic N2-fixing system,which provides new ideas for the design of efficient,sustainable,and green-cycle photocatalytic N2-fixing nanomaterials.3.Based on the above two work,we strive to develop more stable and durable POMs-based materials for photocatalytic reduction N2.For the first time,we design five kinds of efficient and stable transition metal vanadium(V)substituted Keggin type POMs-based metal organic framework ZIF-67 composite nanomaterials,which are research on photocatalytic reduction N2.ZIF-67 can effectively fix N2 due to its porosity.Noting that the integration of POMs cluster contribute enormously advantages in terms of broadening absorption spectrum to improve the sunlight utilization,effectively inhibiting the recombination of photo-generated electron-hole pairs and reducing the charge transfer impedance.POMs are highly dispersed into ZIF-67 framework achieving heterogeneous catalysis at the molecular level,which are all beneficial to improve the photocatalytic activity of composite nanomaterials.After five cycles of the reaction,the catalyst activity remains stable basically,and the yields of NH3 are not significantly decreased.A series of experimental datas show that the photocatalytic activity enhances with the increasing number of substituted metals V in the POMs.Satisfactorily,ZIF-67@K 11[PMo4 V 8O40](ZIF-67@PMo4V8)displays the most significant photocatalytic N2 activity with the NH3 yield of 149.0 ?mol L-1 h-1,improving by 83.5%(ZIF-67)and 78.9%(PMo4V8).The introduction of POMs provides new route in design of high-performance photocatalyst nanomaterials to reduce N2.More importantly,it was revealed that this type of material can truly achieve POMs monodisperse heterogeneous catalytic reduction N2 at molecular level.