Theoretical Design Of Effective Catalysts For Electrochemical Ammonia Synthesis And Photo-Thermal Catalytic Ammonia Decomposition

The excessive exploitation and consumption of fossil fuels have brought two major problems to human society:energy crisis and environmental pollution,which force people to develop new sustainable energy sources.H₂ is a very efficient fuel that produces only H₂O without any polluting gas emissions.Thus,it is a sustainable energy source and a good alternative to fossil energy.However,H₂ is very unstable,flammable and explosive,making it difficult and costly to store and transport.NH₃ has more stable chemical properties than H₂,so NH₃ can be used as a H₂ carrier for storage and transport.The current NH₃ production method is mainly Haber-Bosch method,which requires reaction conditions of high temperature and high pressure,and brings a lot of energy consumption and CO₂ emission.Besides,the NH₃ decomposition reaction（ADR）to H₂ needs to be carried out at a certain high temperature,and the reaction rate is far from the requirements of industrial applications.Therefore,it is very important to find high-energy efficient and environmentally friendly methods to replace Haber-Bosch method and improve the reaction rate of ADR for H₂ production to alleviate energy crisis and environmental pollution.In this paper,density functional theory（DFT）was used to explore strategies to address the three problems of electrochemical NRR and ADR:（ⅰ）the linear scaling relationship among the adsorption energies of NRR reaction intermediates limits the catalytic activity;（ⅱ）The proton transfer rate from solution to NRR intermediate is slow;（ⅲ）The reaction temperature is high and the reaction rate is slow of ADR to H₂,and corresponding improvement strategies are proposed as follows:（1）Synergistic effect of active sites of double atomic catalysts for N₂ reductionThe transition metal dimers are embedded in the nitrogen-doped graphene layer to form diatomic catalysts to study how the synergistic effect between the two active atoms promotes the catalytic performance of NRR.Fifteen transition metal dimers are randomly composed of Ti,V,Mo,Mn,and Fe,and coordinate with N in N-doped graphene to form TM1TM2-N₆ active centers,where TM1 and TM2 represent two identical or different transition metal atoms.After a series of activity screening,we found that VFe-N-C has the best catalytic activity of NRR,and NRR proceeds spontaneously at U_L=-0.36 V.Competing HER is hindered by coordination effects and electrostatic repulsion,which improves the NRR selectivity of diatomic catalysts.By studying the adsorption behavior of the intermediates,we found that different NRR intermediates are adsorbed by different active sites,and the adsorption energies change with adsorption configurations.This can overcome the limitation of linear scaling relationship on NRR performance and promote catalytic performance.Due to the synergistic effect of the double transition metal atoms,the U_L of the NRR on VFe-N-C exceeds the highest point of the volcanic plots,indicating that the limitation of linear relationship is broken.In addition,by studying the PDOS and charge variation,an electronic elevator model of transition metal d band during N₂ activation and reduction is proposed,which provides theoretical guidance for the design of electrochemical NRR catalysts in the future.（2）Hydrogen spillover in alkaline solutions for effective N₂ reductionThe electrochemical NRR consists of six proton-electron coupling transfer steps,and the proton transfer rate from solution to NRR intermediates is very slow.Furthermore,the easier happened HER will compete with the NRR for the proton.These issues result in the slow kinetics of the electrochemical NRR.The nature of possessing multiple proton-electron coupling transfer steps is determined by the reaction process of NRR from reactant to product,which cannot be changed.Therefore,we can improve the activity of NRR only by promoting proton transfer rate from solution to NRR intermediates.Herein,Fe-doped Mo₂N with single atomic vacancy（Fe/SV-Mo₂N）is constructed for simultaneous adsorbing N₂ and OH^-through Fe atom doping and vacancy,respectively.The competing HER is suppressed by proceeding NRR in alkaline solution.Strong adsorption makes the chemical bonds of N₂and OH^-fully activated,which is conducive to the hydrogen spillover from*OH to*N₂.The H₂O in the solution will replenish the H atom lost by*OH due to hydrogen spillover process,thus promoting the hydrogen transfer rate from the solution to*N₂.Due to the hydrogen spillover process,the reaction kinetics of NRR is obviously improved,and its limiting potential is-0.22 V.（3）Lewis acid-base pairs on TiO₂ surface promote NH₃ decomposition to H₂Using NH₃ as H₂ carrier for storage and transport requires effective method to achieve efficient extraction of H₂ from NH₃.However,the ADR to H₂ needs to be carried out at a certain high temperature,and the reaction rate is slow.We use photocatalysis to assist thermal catalysis to reduce the reaction temperature and increase the reaction rate.The specific process is to doping Ru atom into the semiconductor material TiO₂,so that the coordination environment and chemical valence of O atoms around Ru dopant is changed,thus O vacancy can be easily formed around Ru atom,so as to construct the Lewis acid-base pair that can provide and accept electrons.The free energy barrier of the rate determining step of ADR on Ru/OV-TiO₂ is reduced from 4.55 e V to 1.51 e V by the catalytic strategy of constructing Lewis acid-base pairs,and the free energy difference of the thermodynamic control step is only 0.49 e V.The reaction rate constant reach 1.26×10³ s^-1 at 327℃,suggest that the reaction can quickly proceed at this temperature.The charge variation analysis of the intermediates shows that the surface frustrated Lewis acid-base pairs on Ru/OV-TiO₂ can effectively separate the positive and negative charges in the catalyst,so as to promote the redox performance of the catalyst.