Study On Catalytical Properties And Mechanism Of Eletrocatalysts For Ammonia Synthesis

Ammonia is an important raw material for agriculture,the demand would be continuously increasing.Currently,the artificial ammonia is synthesized mainly by the Haber-Bosch(H-B)process.However,the harsh conditions of the H-B method bring the cascade of problems,such as energy crisis,environmental pollutions and economic risks.The electrocatalytic ammonia synthesis is recognized as a promising alternative approach.For the electrocatalytic nitrogen reduction(NRR),a suitable catalyst is the key factor for improving reaction efficiency.Recently,intensive researches have been attracted to finding potential NRR catalysts under ambient conditions.These studies also pointed out some difficulties and challenges for NRR industrial application.These reported electro-catalysts possess low intrinsic NRR activity and selectivity.Besides,the neglected ammonia pollution in experiments make the claimed results controversial.Aiming toward better NRR performance of electro-catalysts,as well as more insightful understanding of experimental results,the theoretical calculations is a favorable research method.Based on density functional theory(DFT)calculations,we have studied the catalytic performance of some single-atom catalysts(SACs).First,we systematically explored the catalytic performance of a novel two-dimensional material Pt Te sheet for NRR and hydrogen evolution reaction(HER)by using frst-principles calculation.Pristine Pt Te possess weak adsorption capacity and thus shows poor NRR and HER activity.doping transition metal(TM)atoms into the lattice can effectively enhance the catalytic performance.The enhanced catalytic activity owing to the TMs with the occupied and empty d orbitals can coupling strongly with dinitrogen and hydrogen.Calculations show that four TM doped structures,including W-Pt₁₈Te₁₇,Ru-Pt₁₈Te₁₇,Mo-Pt₁₈Te₁₇ and Cr-Pt₁₈Te₁₇,are promising NRR catalysts on the prerequisite of whose HER activities are effectively suppressed.In additions,Cu,Cr,Co,Ni,Mo,Rh,Ru,and Tc atoms supported by the Pt Te sheet with Te-vacancy are promising HER electro-catalysts.Moreover,Mo-Pt₁₈Te₁₇,Cr-Pt₁₈Te₁₇,and Ru-Pt₁₈Te₁₇ sheets boost catalytic activity for both NRR and HER processes.Second,Bismuth(Bi),a hydrogen-insensitive metal crystal,has recently received attention as a potential catalyst for NRR.We investigated NRR performance of Bi-based catalysts by performing DFT calculations.Our results indicate that the NRR activity of pristine Bi(012)surface is slightly higher than that of the(001)surface.Bi catalysts achieve enhanced catalytic performance with defect engineering strategies,such as the introduction of vacancies and heteroatoms into the lattice.After introducing Bi vacancy,the dangling bond can enhance binding strength of the intermediate,optimizing the NRR activity.However,both of the NRR and HER activities of Bi site would be increased simultaneously,that resulted in NRR selectivity decreasing.On the other hand,heteroatom doping is another kind of method to change NRR and HER activity for Bi catalysts.Remarkably,single boron atom doping on Bi surface could give rise to impressive catalytic performance,which can potentially act as a promising NRR catalyst.Finally,we studied the NRR catalytic behavior of graphene-based catalysts with DFT calculations.We investigated the influence of P-block dopants and coordination environments(C,N/C,O/C,P/C and S/C)on the NRR performance.Among these dopants,B,Al and Si atoms can enhance the adsorption capacity of catalysts,and thus them exhibit increased NRR activity.In particular,the catalytic performance of B and Al sites would be significantly affected by the coordination environment.For example,the B site with pure C coordination show weak adsorption capacity,which cause extremely poor NRR activity.In the B,N co-doped graphene,the B site can exhibit satisfactory activity and selectivity in the NRR process.For Al-doped catalysts,Al sites with pure carbon coordination would preferentially bind to N₂ molecules.After introducing Al and N atoms,Al site can achieve the improved activity without sacrificing its N₂ adsorption advantage.In this thesis,we studied the NRR performance of Pt Te-based,Bi-based,carbon-based catalysts.We investigated the NRR and HER activities of these catalysts and established the relationship between structure and catalytic performance.Our work deepens the understanding of the catalytic mechanism of NRR catalysts,and also provide feasible strategies to improve NRR performance for experimental studies.