Theoretical Study Of Typical Transition Metal Crystal Phases Regulating The Reaction Of Nitrogen To Ammonia

Ammonia has always played an important role in the development of human society as a fertilizer,energy carrier,and precursor of various chemical substances.However,the industrial synthesis method has always relied on the energy-consuming and polluting Haber Bosch method.The electrocatalytic nitrogen reduction reaction for ammonia production is regarded as the ideal method for green,environmentally friendly and sustainable ammonia production.However,the complexity of nitrogen activation and protonation in the reaction process and the competing side reactions make electrocatalytic ammonia production difficult for widespread industrial application.With the help of computational simulation,we can effectively avoid the disadvantages of the trial-and-error method of designing and screening catalysts with long lead times,and can facilitate the development and application of new catalysts,thus enabling the accelerated industrialization of electrocatalytic ammonia production.In this paper,the reaction mechanism of typical transition metal unconventional phases in ammonia production by nitrogen reduction reaction is investigated based on first principles calculations,and suitable unconventional phases are screened to provide a theoretical basis for the experimental use of unconventional phases for catalytic ammonia production from nitrogen.The main contents of this paper are as follows.First,the phase transition mechanism of the precious metal ruthenium(Ru)from the conventional crystalline phase 2H structure to the unconventional crystalline phase structure under strain is systematically investigated.The strain determines the degree of crystal structure distortion and changes the type of crystalline phase structure.Calculations show that when a certain biaxial strain is applied along the a-and b-axis directions of the 2H structure of ruthenium,neither tensile nor compressive strains can induce the transformation to the unconventional phase structure.When a certain strain is applied along the c-axis,the tensile strain increases the energy of the 2H-structured ruthenium,destabilizing its structure and lowering the phase transition potential,causing it to shift to the unconventional phase.Second,the mechanism of the unconventional crystalline phase structure of ruthenium regulating the dissociation of nitrogen molecules is further explored.Since strain can change the interatomic interactions and thus the catalytic properties of the material,the transformation of the crystalline phase structure is bound to exhibit different catalytic properties due to the rearrangement of the atoms.Theoretical calculations show that strain is not effective in increasing the catalytic activity of nitrogen decomposition for ruthenium with a conventional phase structure,limited by the BEP relationship.In spite of this,the catalytic activity of ruthenium with an unconventional phase structure is significantly improved as a result of the change in structure,which results in an upward shift of the d-band center position and enhances the interaction with nitrogen atoms.The adsorption energy of nitrogen molecules,however,is not directly related to the crystal phase structure.The unconventional phase structure can break the BEP linear relationship between the activation energy and the reaction energy during nitrogen dissociation,and thus effectively promote the nitrogen dissociation reaction.Finally,the catalytic performance of the unconventional phases of noble metals ruthenium,osmium,rhodium and iridium in the electrocatalytic nitrogen reduction reaction to ammonia was predicted theoretically.The results show that the unconventional crystalline phases have a stronger ability to activate nitrogen molecules.This in turn reduces the limiting potential during the nitrogen reduction reaction and promotes the reaction.The nitrogen molecules are most likely to undergo the reduction reaction to produce ammonia molecules when following the distal hydrogenation reaction mechanism on 4H-Os(110),and the maximum reaction free energy becomes 0.598 eV throughout the process,which is comparable to 0.43 eV for the nitrogen reduction reaction of nitrogen molecules on the noble metal ruthenium step surface.Moreover,for the 4H-Os(110)system,the difference between the limiting potential of NRR and that of HER is close to 0,which indicates that the system has a relatively high selectivity and the selectivity is also significantly better than that of the conventional phase structure.Therefore,the transition to the unconventional phase structure can be achieved by modulating the crystallographic spacing of the conventional phase of ruthenium;the strain induced phase transition of the conventional phase can break the BEP relationship and promote the occurrence of nitrogen dissociation reaction.The unconventional phase structure of noble metals has a stronger ability to activate nitrogen molecules.This makes it more active and selective in nitrogen reduction reactions,facilitating the electrocatalytic ammonia reaction and improving the efficiency of ammonia synthesis.Therefore,crystal phase engineering is an effective strategy to rationally design noble metal nanocatalysts to improve the chemical activity of ammonia synthesis by electrochemistry.