Electrocatalysis Of Nitrogen Reduction On The Surface Of Graphdiyne-supported Atomic Catalysts

Ammonia（NH₃）plays an important role in the modern life of humans and is essential to the global economy because it is used not only as a basic substance to produce chemicals including fertilizers,pharmaceuticals,dyes,explosives,etc.,but also as a hydrogen carrier for potential future clean energy.Currently,NH₃ is mainly produced by the Haber-Bosch process.However,this method consumes a lot of energy and produces a lot of greenhouse gases.Therefore,it is necessary to develop some new synthetic methods to prepare and produceNH₃.The synthesis of NH₃ by electrochemical reduction of N₂ is considered as a potential alternative to the Haber-Bosch process.In order to improve the efficiency of electrochemical nitrogen reduction reaction（eNRR）,it is very important to design and prepare efficient eNRR electrocatalysis.This thesis provides a solution to the problem of activity and selectivity in eNRR.The main work is as follows:1.Graphdiyne（GDY）supported single Fe catalyst（Fe-GDY）was constructed,and the stability of the catalyst,the reaction process,activity and selectivity of eNRR on the catalyst were investigated in detail.Because of its unique structure and electron configuration,GDY provides many possible sites for iron atom to binding.Theoretical calculation result shows that there are four Fe-GDY structures,（ie.the top of the hexatomic ring,the corner of the acetylenic ring,the top of C-C single and triple bond.）It is found that Fe is the most stable when it binds to the corner position（H1）of the alkyne ring on GDY by comparing the binding energy（E_b）and local density of state（LDOS）of these four structures,the E_b reaches-3.29 e V.Through the charge analysis of Fe-GDY,it was found that a large number of electrons were transferred to GDY through Fe,making Fe positively charged,which can promote the adsorption of N₂ in the eNRR process.After determining the structure of Fe-GDY,the performance of its eNRR was further studied.The adsorption mode of N₂ on Fe-GDY was determined by DFT calculation,and the possible reaction path of eNRR was designed.All reaction intermediates were optimized and the Gibbs free energy changes during the reaction were calculated and the reaction process of eNRR on Fe-GDY catalyst was described in detail.The results showed that compared with the bulk metal Fe catalyst,the selectivity of Fe-GDY catalyst for eNRR was greatly improved,but its activity was not greatly optimized.This work not only provides theoretical guidance for the construction of diatomic catalyst（DAC）on GDY,but also provides a scheme for the exploration of the selectivity and activity of DAC for eNNR.2.In order to solve the problem of low activity in Fe-GDY single-atom catalyst（SAC）,a series of Fe-based double-atom catalysts supported by GDY were constructed,（all 3d,4d,and 5d group metal atoms 26 metals,except for lanthanides,radioactive Tc,and toxic Cd and Hg）and by means of systematic density functional theory（DFT）calculations,the challenge of eNRR can be understood and addressed by building a full profile of the stability,activity,and selectivity of the eNRR on a model electrocatalyst system,i.e.,graphdiyne（GDY）-supported Fe-based transition metal（M）double-atom catalysts（FeM-GDYs）.Based on the studies on 26 FeM-GDYs,the activity trends of the catalysts are constructed using the adsorption energy of the*NH₂ intermediate(E_ads（*NH₂）)as a descriptor;this displays a well-defined volcano-shaped relationship,which enables us to identify 11 FeM-GDYs that show improved activities in comparison with that of the Ru（0001）stepped surface,which is used as a benchmark.Employed the difference in the free energies of the H and N₂ adsorption processes,i.e.,ΔG（*N₂）–ΔG（*H）,as a selectivity descriptor,3 FeM-GDYs（M=Ni,Mo,and Cr）are finally demonstrated to exhibit a strong ability to suppress the competitive hydrogen evolution reaction（HER）.Their improved activity and selectivity have been discussed and explained based on the alteration of the electronic structures and the coordination environments of the metal atoms upon FeM-GDY formation.More importantly,these FeM-GDY catalysts have a high potential for experimental synthesis.These results not only contribute to more efficient electrocatalysts toward the eNRR but also provide an effective way to find new substrates for catalyst synthesis and a guideline to improve the activity and selectivity of the resulting catalysts.