Theoretical Study Of Hydrogen Peroxide And Ammonia Synthesis On Nitrogen-doped Porous Carbon And Its Supported Single/Double Atoms Catalysts

An increasing energy demand,rising global environmental pollution and impending climate change pose serious threat for the security of human survival future.Therefore,developing sustainable pathways to produce basic chemicals is of great value and plays a key role in the field of environment and energy.Meanwhile,it also provides the raw material needed to produce the products that we use in our daily life.One promising method is to convert molecules in the atmosphere（e.g.,oxygen and nitrogen）into higher-value products（e.g.,hydrogen peroxide and ammonia）by developing electrochemical conversion technology.In this paper,the design and screening strategies for SACs and associated electrocatalysts for two state-of-the-art electrochemical ractions involving oxygen reduction reaction（ORR）and nitrogen reduction reaction（NRR）are discussed here.However,the great challenge is still how to design and develop highly efficient electrocatalysts.Single-atom catalysts（SACs）have emerged as a novel star in the area of heterogeneous and homogeneous catalysis since they have the features of maximum atom utilization efficiency,superior catalytic activity,and outstanding selectivity in various catalysis fields.By using density functional theory（DFT）,ab initio molecular dynamics（AIMD）and reaction molecular dynamics（Rx MD）calculations,a series of SACs and Double-atom catalysts for the synthesis of H₂O₂ and NH₃ were designed,and the fundamental explorations have been performed regarding how different supports and active centers affect the catalytic performance and reaction mechanism for H₂O₂ and NH₃ synthesis.The main findings are as follows:（1）As for the issue of how the different types of water molecules affect the stnthesis of H₂O₂,the reaction mechanism of H₂O₂ synthesis on g-C₅N₂ with different stabilities by a series of DFT,AIMD and Rx MD calculations are studied,and the role of different water in the formation of H₂O₂ was clarified.Firstly,the stability of g-C₅N₂with different hydrogen coverages was studied by thermodynamic phase diagram.Key findings for the reaction mechanism of H₂O₂ formation include:a pristine g-C₅N₂has the high catalytic activity for the synthesis of H₂O₂in all models.The adsorption and activation of water molecules at the unsaturated carbon site of of g-C₅N₂ is a key step in the initiation of H₂O/O₂ reaction.Proton transfer between water molecules plays an important role in the formation of H₂O₂.To the best of the authors’knowledge,this study provides the theoretical details of the direct synthesis of H₂O₂ from H₂O/O₂,which can further guide the optimization of carbon based 2e^–ORR catalysts.（2）As for the issue that how the different catalytic active centers affect the catalytic activity and selectivity of H₂O₂ formation,g-C₃N₄was selected as the support of SACs,and Pd-Cu double atoms were screened out as active centers by two-step screening strategy.More importantly,relationship between the charge of surface metal and the activity and selectivity of H₂O₂ was also constructed.The unique“slope”configuration on the Pd Cu@V＿C₃N₄ surface and the charge of metal atoms regulate the O₂ adsorption configuration and strength,thus improving the performance of synthesizing H₂O₂.Sabatier analysis indicated that Pd Cu@V＿C₃N₄ is located at the top of 2e^–ORR volcanic plot with an extremely low overpotential of 0.02 V.Furthermore,the dynamic rate-determining is OOH*intermediate formation with a dynamics activation barrier of 0.64 e V.This work not only predicts a potential high efficiency 2e^–ORR catalyst for the experimental researchers,but also provides a general screening method for the design of other types of 2e^–ORR catalysts.（3）As for the issue that the supports might affect the stability of SACs and the performance of NRR,the DFT calculations and thermodynamic approach were combined to study the supporting effect of the NRR.The thermodynamic properties of different Ru doped supports and their effects on the electrocatalytic synthesis of ammonia were studied.The binding energy of single atom Ru doped C₂N、T-C₃N₄andγ-graphynes are-5.47 e V,-4.39 e V and-3.43 e V,respectively,indicates that there is a strong interaction between Ru atom and its supports.AIMD calculations indicates that Ru atom can steadily reside on the anchored position without migration and agglomeration.There is strong electron transfer and feedback between Ru and N₂,which makes N₂ active.The reaction mechanism of NRR all follows the associate pathway in three kinds of SACs.The potential determining step（PDS）of NRR is mainly determined by the position of Ru atom on the support,while the overpotential is mainly controlled by the charge on the surface of Ru atom.For the NRR on Ru@C2N、Ru@T-C₃N₄and Ru@γ-graphynes,PDS are NNH*,NH₃*and NHNH₂*species formation with the overpotential of 0.80 V,0.78 V and 0.82 V,respectively.This study provides an insight for the synthesis of high efficient and stable SACs NRR catalyst.（4）As for the issue that the metal–free SACs and the number of active sites might affect the performance of NRR,single and double boron atoms doped C₂N-h2D electrocatalysts（B@C₂N and B₂@C₂N）were designed,respectively.The nature of metal-free activation of N₂ was studied by using B@C₂N as the model catalyst.The B and N atoms are hybridized by sp³ in B@C₂N.Therefore,B atom contains both empty p-orbitals that can receive the lone pair electrons of N₂,and p-orbitals that can be filled into the anti-bond orbitals of N₂.The“acception-donation”of electrons between B atom and N₂ molecule reduce the bond order of N₂,resulting in the activation of N₂.The study on reaction mechanism shows that the NRR on B@C₂N and B₂@C₂N both follow the enzyme pathway.This study also shows that B₂@C₂N is a more efficient electrocatalyst with extremely low overpotential of 0.19 V than B@C₂N.A new design strategy of double atom catalyst is helpful for the development of a series of efficient metal-free NRR catalysts.