Theoretical Studies On Two-Dimensional Conjugated Microporous Polymers As Catalysts For Overall Water Splitting And Nitrogen Reduction

Using clean and renewable energy to achieve low-carbon conversion of small molecules such as H2O and N2 is not only an important way to solve the energy and environmental crisis,but also a huge challenge to scientific researchers,and the core of which is the design and search for efficient and stable photo/electrocatalysts.Among numerous photo/electrocatalysts,two-dimensional(2D)conjugated microporous polymers(CMPs)show broad application prospects in the field of photo/electrocatalysis due to their excellent properties such as stable physical and chemical properties,tunable structural properties,uniform and controllable pore sizes,and easy surface modification,which is one of the important catalytic systems in theoretical and experimental studies.The density functional theory(DFT)calculation method can be used to calculate the basic properties of materials and obtain internal reactions that are difficult to be monitored experimentally and has advantages in the screening,design,regulation and mechanism exploration of catalysts,which can be used to guide the relevant experimental research.In this paper,based on the DFT calculation methods,aiming at the challenges of water splitting under visible light and nitrogen reduction(NRR)at room temperature and pressure,2D CMPs photo/electrocatalysts with chemical stability and excellent catalytic performance were designed,and the influence of group modification and strain regulation on the electronic structures and catalytic performance of catalysts were studied.The main research contents and results are as follows:1.Based on the two-coordinate nitrogen and the edge carbon in the benzene ring,they are favorable for the hydrogen evolution reaction(HER)and the oxygen evolution reaction(OER),respectively,17 two-dimensional conjugated microporous heptazinebased polymers(CHFs)were theoretically designed using heptazine with abundant dicoordinate N atoms and benzene-containing linkers for photocatalytic watersplitting.The band gaps of 17 CHFs range from 2.26 eV to 3.25 eV.The band gaps of 12 CHFs less than 3.0 eV,and the energy level positions match well with the redox potential of water.Gibbs free energy calculations demonstrated that 3 CHFs could achieve HER and 8 CHFs could achieve OER spontaneously.Among them,H@DBTD can achieve overall water splitting under visible light with a band gap of 2.47 eV,and the prediction was verified by the experiment.This study indicates that 2D CHFs are potential photocatalysts for overall water splitting under visible light,which provides guidance for further experimental research.2.The regulation of the modified groups on the photocatalytic properties of 2D CMPs were further studied theoretically.Diphenyl linked CHF(H@2Bz)was used as the research object,the influence of the position,type and number of the introducing groups on the electronic structure of the system were studied by substituting the hydrogen atoms on the linker with the electron-donating groups(hydrocarbon groups)or the electron-gaining groups(halogen atoms).The introduction of hydrocarbon groups could increase the valence band maximum(VBM)and conduction band minimum(CBM)of the system,destroy the conjugation of the system and lead to the increase of the band gap,while the introduction of halogen atoms could reduce the VBM and CBM of the system.The regulation effect of the groups on the energy level positions are enhanced with the increase of the number of groups.The introduction of groups could change the energy level positions and thus change the the bias provided photogenerated electrons and holes,thereby accomplishing visible-light overall water splitting.This study demonstrates that functional groups modulation is an effective regulation strategy to motivate the catalytic performance of CMPs.3.2D CMP photocatalyst C15N3H3 for water splitting was designed,and the effects of the biaxial strain on its electronic structure and catalytic performance were studied.The conclusions are as follows:The biaxial tensile strain could increase the band gap of C15N3H3,reduce the energy level position and the free energy change of the ratelimiting step of of HER and OER.The biaxial compression strain can reduce the band gap of C15N3H3,raise the energy level position,increase the free energy change of the rate-limiting step of HER,and reduce free energy change of the rate-limiting step of OER.Additionally,it was theoretically demonstrated that C15N3H3 could realize overall water splitting under visible light when the tensile strains range from 3%to 5%.This study could provide guidance for strain-tuning catalytic activities of photocatalysts.4.Using the properties of holes in 2D CMPs can be loaded with metal atoms,the catalytic performance of 30 2D metal phthalocyanines(TM-Pc)for NRR were theoretically investigated.The rate-determining step of NRR is the first or last protonation reaction,and the free energy changes of the two steps show a negative linear relationship.Mechanism analysis prove that the activation mechanism of nitrogen(N2)is bidirectional feedback,and the NRR activity is affected by the electron transfer ability of the metal atoms and the number of d electrons.The adsorption of NH2 on the substrate can be used as an excellent descriptor to describe the catalytic behavior of TM-Pc.Based on stability,activity and selectivity,7 potential NRR catalysts were screened out.This study provides theoretical guidance for the experimental exploration of TM-Pc catalysts and the rapid screening and designing of other efficient NRR catalysts.5.In view of the current research status that metal-supported CMPs NRR catalysts mainly focus on d-block metals but rarely involve other metals,the NRR activity of metal without d electrons anchored to g-C3N4 to form M@g-C3N4 was theoretically investigated.The results are as follows:The free energy changes of Be@g-C3N4 and Al@g-C3N4 for the first-step hydrogenation reaction of N2 are less than 0.98 eV and have remarkable activation ability for N2.Mechanism analysis show that both s electrons and p electrons of the metal atoms affect the activation of N2,and two systems show different activation mechanisms due to the different coupling between the electrons in metal atoms and the N2 orbitals.The rate-limiting steps of nitrogen reduction for Be@g-C3N4 and Al@g-C3N4 are the first and last protonation steps,respectively,with limiting potentials of-0.29 and-0.53 V,respectively.This study provides a theoretical basis for the exploration of non-d-block metal NRR catalysts.