Preparation Of Covalent Organic Polymers And Its Application Of Photocatalytic Hydrogen Evolution And Nitrogen Reduction

Recently,photocatalytic hydrogen evolution and nitrogen reduction on basis of particulate photocatalysts have been widely regarded as a cost-effective and potential technology for converting solar energy into green and clean hydrogen energy or other high value-added chemicals,which is expected to replace traditional fossil fuels for effectively alleviating growing serious energy crisis and environmental pollution.Inorganic photocatalysts have already gained some crucial achievements,but been confronted with some challenges of poor stability,single structure and photo-corrosion etc.Covalent organic polymers(COPs)have attracted ongoing attention due to the excellent merits of large π-conjugated system,versatile structures,easy-tune optical and electronic properties,well-defined chemical composites.However,the photocatalytic performance of polymers is still low,which is ascribed to high exciton binding energy,the recombination of electron-holes and the sluggish surface catalytic activity.Here,we synthesize a series of efficient and stable COPs for photocatalytic hydrogen evolution and nitrogen reduction via molecular design,carbon-encapsulated strategy,localized electron density modulation and constructing organic heterojunctions.The main researches are as follows:(1)The photocatalytic hydrogen evolution efficiency of COPs is still low owing to the easy combination of photo-induced electron-holes.Here,a donor-acceptor(D-A)based covalent organic polymer(termed as COP-TP3:1)was prepared by the irreversible kinetic coupling reaction,i.e.,Yamamoto-type Ullmann cross-coupling,using pyrene as electron donor and counterpart,e.g.,phenanthrolene,as electron acceptor.The D-A structures contribute to electron transfer from donor to acceptor molecules,improving the separation efficiency of photo-induced electron-holes.The optimal D-A based polymer(COP-TP3:1)has a 14-fold improvement in the photocatalytic H2 evolution rate from 300 to 4200 μmol g-1 h-1 compared with the sample without donor-acceptor structure.Moreover,the pyridine nitrogen from COP-TP3:1 can be coordinated with platinum conducing to anchor Pt(as the cocatalyst)onto its surface,which contributes to improve the photocatalytic stability.The photocatalytic performance of COP-TP3:1 is only reduced by about 7%even after discontinuous cycles of photocatalysis and storage for a month.Furthermore,COP-TP3:1 also performs excellent photocatalytic activity under different water quality(deionized water,municipal water,commercial mineral water,and simulated seawater(NaCl 3 wt%)).(2)Currently,most reported photocatalytic hydrogen evolution reaction is carried out in deionized water.However,97%of the water resource on the earth is seawater,developing stable and efficient photocatalytic H2 production from seawater is significantly important.Meanwhile,most polymer photocatalysts employed precious Pt as a cocatalyst to boost the catalytic activity,which impedes its future development and practical application owing to the scarcity and expensiveness.Herein,we have synthesized a carbon-encapsulated nickel phosphide(CNi2P)replacing Pt as a cocatalyst loaded on the surface of covalent organic polymer(COP-TF),which is applied for stable H2 generation from seawater.On one hand,the carbon layers covered around nickel phosphide can protect nickel phosphide from seawater erosion.On the other hand,the intense π-π interactions between the carbon layers and COP-TF can increase the adsorption capacity,which contributes to make CNi2P tightly loaded on the surface of COP-TF.As a result,the COP-TF@CNi2P presents an efficient photocatalytic H2 evolution efficiency up to 2500 μmol g-1 h-1 from seawater and even maintains 92%of initial efficiency after 16 intermittent cycles,which remarkably exceeds the state-of-the-art g-C3N4,CdS and TiO2 under similar conditions.(3)To solve the sluggish intrinsic catalytic activity of conjugated organic polymers,in this work,we newly develop a localized electron density modulation strategy to enhance the photocatalytic proton reduction efficiency by incorporating symmetric dual pyridine nitrogen(N)atoms into polymer skeleton.The sp2 occupied orbital of pyridine N with lone-pair electrons can interact with the empty 1s orbital of H which makes pyridine N serve as reactive sites for photocatalytic H2 evolution.In addition,compared with introducing single pyridine N in polymer COP-PB-N1,the doped symmetric dual N atoms in COP-PB-N2 can increase the localized electron density around N site,contributing to strengthen the electron density of states at Fermi level;which will be beneficial to lower the energy barrier of H adsorption and activation during photocatalytic hydrogen evolution process.Resultingly,the photocatalytic hydrogen evolution rate of newly developed covalent organic polymer(COP-PB-N2)achieves 10.3 mmol g-1 h-1 under visible light(λ≥ 400 nm)which is 5-fold higher than undoped COP-PB.Furthermore,COP-PB-N2 exhibits a high apparent quantum yield(AQY)of 35.5%at 400 nm,exceeding the state-of-the-art conjugated microporous polymer photocatalysts to date.(4)In order to solve the issue of activating triple bond in photocatalytic nitrogen reduction owing to the lack of reactive sites for polymer based photocatalysts;herein,we introduced transition metal Fe into polymer photocatalyst as reactive site via constructing heterojunction strategy.Based on the existence of d occupied and empty orbitals,the Fe atoms can not only donate electrons into the antibonding orbitals of N2 but also accept the lone-pair electrons of N2,which is conducive to activate the triple bond of N2.To accelerate photocatalytic efficiency of polymers,we have developed a novel covalent organic polymer(COP-Fe)with atomically dispersed Fe,which contribute to increase the amount of exposed active sites in polymers.Moreover,the organic heterojunction is successfully constructed combining carbon nitride polymer(CN)and COP-Fe for boosting the separation of photo-induced electron-holes.Therefore,the composite of CN/COP-Fe has an efficient photocatalytic nitrogen reduction performance.The photocatalytic NH3 evolution rate under visible light(λ≥ 400 nm)has achieved 69 μmol g-1 h-1 without adding any other sacrificial agent.