Theoretical Design Of Efficient Catalysts For Electrocatalytic Ammonia Synthesis And Ammonia Oxidation

The utilization of clean and renewable carbon-free energy is critical to address global energy demands and achieve a strict goal of the net-zero CO₂ emission.Ammonia（NH₃）,as a carbon-free energy,has attracted substantial attention due to the advantages of safe,economical,high energy density(3 k Wh kg^-1)and extensive infrastructures dedicated to the storage,transport and distribution of NH₃ have been in place.Presently,NH₃ synthesis mainly relies on the traditional Haber-Bosch process,which consumes approximately 1-2%of the world’s annual energy and contributes serious CO₂ emissions.Electrocatalytic nitrogen reduction reaction（NRR）is a promising alternative to the Haber-Bosch process as it uses carbon-neutral and low-cost renewable electricity to fix nitrogen under ambient temperature and pressure.However,there are still some challenges in realizing high reaction selectivity and low overpotential for NRR electrocatalysts.Moreover,the AOR at the anode of DAFC is often kinetically sluggish.Recently,the challenges in realizing high efficiency NRR catalyst are the activation of N₂ and inhibition of hydrogen evolution（HER）.While the main challenge of designing AOR catalyst is the difference of adsorption energy requirements between NH₃ dehydrogenation and N-N coupling.Herein,we designed a series of high efficiency catalysts for NRR and AOR.The catalytic properties,electronic structure and reaction mechanism were investigated based on density functional theory（DFT）,which includes the following three parts:（1）Os₁B₁₁N₁₂/C₂N as an efficient electrocatalyst for nitrogen reduction reactionSingle-atom catalysts（SAC）have become one of the most rapidly developing catalytic systems due to its high catalytic activity and metal utilization.Accordingly,single Os atom-doped B₁₂N₁₂ supported by C₂N(Os₁B₁₁N₁₂/C₂N)as electrocatalyst for NRR was designed.The evenly distributed holes and nitrogen atoms of C₂N exert remarkable effects on Os₁B₁₁N₁₂ stabilization.A further investigation of electronic property reveals that the gain or loss of electrons（"acceptance-donation"process of electrons）take place on both Os atom and N₂,which promotes the activation of N₂.The BN cluster appropriately alters the upper d-band edge of Os for obtaining an optimal adsorption strength to intermediates,thereby improving the NRR catalytic activity of Os₁B₁₁N₁₂/C₂N.From Hirshfeld charge analysis,positive charges accumulate in Os atom of Os₁B₁₁N₁₂/C₂N,which leads to the electrostatic repulsion with proton,thus suppressing HER and improving the efficiency and selectivity of NRR electrocatalysts.This work is not only beneficial for understanding the mechanism of NRR but also provides a fundamental guidance for rational design of catalysts for NRR.（2）Mo doped Pt（100）as an efficient electrocatalyst for ammonia oxidation reactionThe separation of active site is a very effective catalyst design strategy,which can avoid the volcano curve relationship between intermediate adsorption energy and catalytic performance to improve catalyst activity.According to the requirements of adsorption energy of intermediates in different stages of ammonia oxidation,Mo doped Pt（100）surface[Mo-Pt（100）]as electrocatalyst for AOR was designed to separate the active sites of NH₃dehydrogenation and N-N coupling.Our results show that the formation of*NH is the potential determining step（PDS）with?G=0.56 e V.While the energy barrier of*NH-NH coupling is 0.35 e V,which is below the threshold for fast kinetics based on the basic transition state theory（0.75 e V）.The investigation of AOR mechanism shows that the doped Mo atoms offer active sties for NH₃ dehydrogenation with strong adsorption,which reduces the?G of PDS.While Pt（100）offers active sties for NH-NH coupling with weak adsorption,which accelerate the kinetics of the reaction.Moreover,the adsorption of additional NH₃ on T_Mo site triggers NH spillover from B_Mo-Pt to B_Pt-Pt,enabling facile N₂ formation and desorption.Therefore,the synergistic effect of T_Mo and B_Pt-Pt sites greatly facilitates the AOR.This work not only benefits for understanding the mechanism of AOR,but also provides a fundamental guidance for rational design of AOR catalysts.（3）N-modified Co₃Mo₃C electrocatalyst with separated active sites for ammonia oxidation reactionThe practical application of AOR catalysts will be limited by the high price and limited reserves of Pt.Herein,the N-modified Co₃Mo₃C（N-Co₃Mo₃C）has been designed.Our results show that the formation of*N is the potential determining step（PDS）with?G=0.53e V.While the energy barrier of*NH-NH coupling is 0.46 e V,which is below the threshold for fast kinetics based on the basic transition state theory（0.75 e V）.The investigation of AOR mechanism shows that the hollow site of Co-Mo-Mo(H_Co-Mo-Mo)and top site of Mo(T_Mo)play essential roles in NH₃ dehydrogenation,while top site of Co(T_Co)and the diffusion of N play significant roles in N-N coupling and active site separation strategy,respectively.The synergistic effect of various active sites makes N-Co₃Mo₃C the highly efficient AOR electrocatalysts.This work greatly reduces the cost of efficient AOR electrocatalyst and promotes the practical application of AOR electrocatalyst.