Design Of Molybdenum-based Catalysts And The Properties Of The Electrocatalytic Ammonia Synthesis

As a carbon-free energy carrier that can replace hydrogen and an important raw material for fertilizer,ammonia plays an indispensable role in the development of human civilization.Compared with the conventional Haber method for synthesizing ammonia,which consumes large amounts of energy and causes serious pollution to the environment,electrochemical synthesis of ammonia has the green advantage of using renewable resources for the reaction under mild conditions.In this study,a clean and environmentally friendly electrochemical method is used,but the current electrocatalytic synthesis of ammonia gas faces the problem of low synthesis efficiency.Boron（B）atom doping is considered as an effective modification strategy to improve the efficiency of catalysts for Nitrogen Reduction Reaction（NRR）.In this paper,molybdenum-based metal materials are used as substrates and B atom doping is used to enhance the ability of catalysts to capture nitrogen molecules,thus improving the efficiency and selectivity of electrocatalytic ammonia synthesis.（1）Two electrocatalysts,single B-atom-doped atomic-level Mo O₂（B/Mo O₂）and double B-atom-doped atomic-level Mo O₂（B2/Mo O₂）,were designed using PWmat software,and their reaction thermodynamics and selectivity in the nitrogen fixation reaction were calculated and analyzed based on density generalization theory.The results show that B2/Mo O₂ is superior for N₂ adsorption,activation and reduction,and can effectively activate nitrogen molecules by two adjacent boron atoms and achieve a very low-0.18 V overpotential and fast NRR kinetics through an enzymatic mechanism,which is difficult to achieve with conventional single-atom transition metal catalysts.Therefore,B2/Mo O₂ is a very promising NRR candidate catalyst that can catalyze N₂ reduction more efficiently than a single boron-doped molybdenum dioxide.（2）In this paper,B-doped Mo S₂/Mo O₂ composites（BM-0,BM-1 and BM-5）were successfully prepared with different mass ratios（0:1,1:1,5:1）of boric acid and Mo S₂/Mo O₂composites.Among them,BM-5,is a spherical nanoflower encapsulated with nanoparticles,a morphology with abundant active sites and high electrical conductivity.The ammonia yield of BM-5 was determined to be up to 42.23μg·h^-1·mg_cat^-1 at-0.6 V relative to the reversible hydrogen electrode（vs.RHE）and the Faraday efficiency was up to 2.78%at-0.5 V（vs.RHE）by electrochemical ammonia synthesis tests on the three materials.