Theoretical Study On Construction Of Single-atom Catalysts With Synergic Micropore Trapping And Nitrogen-dopant Anchoring

As the limit of supported catalysts,single-atom catalysts(SACs)can make use of precious metal resources economically,and also bring different catalytic activity and product distribution selectivity compared with metal nanoparticles.It is a hot research topic in recent years.However,the preparation methods of single-atom catalysts are still complex,such as atomic layer deposition(ALD),pyrolysis of organometallic compounds and wet chemistry.The equipment and reaction precursors required by the ALD are expensive,which the production capacity is not high and the mass production can not be achieved;the pyrolysis of organometallic compounds requires high temperature pyrolysis of organic ligand materials to prepare carbon carriers,which is easy to cause metal single-atom aggregation to form clusters,and is not easy to obtain uniformly distributed single-atom site catalysts;the process of preparing single-atom catalysts by wet chemical method is simple.However,the anchoring conditions between metal and carrier and the reduction conditions of metal atoms are not clear,so the preparation process and experimental conditions are difficult to determine.In addition,due to the high surface energy of metal atoms,it is easy to agglomerate and deactivate in the use process.Therefore,how to conveniently obtain stable and highly active single-atom catalysts has become a challenge in this field.Recently,our group found that nitrogen-doped carbon nanocages can be easily impregnated and dried in chloroplatinic acid solution to obtain high-loading Pt single-atom catalysts,which show record high activity and extraordinary stability in hydrogen evolution reaction,far superior to commercial Pt/C benchmark test.The experimental results indicate a new formation mechanism of single-atom catalysts and a new way to improve their stability,which is of great theoretical value for further study.Based on the research status of single-atom catalysts and the experimental result of our group,this paper focuses on the preparation mechanism of Pt single-atom catalysts and the mechanism of hydrogen evolution reaction stability,and the capturing ability of different size microporous and edge defects on multi-layer graphene carriers for different size metal coordination anions.Relevant theoretical research has been carried out and the following important progress has been made.1.Density functional theory(DFT)was used to systematically study the role of 6A micropore size and nitrogen-dopant on the surface of carbon nanocages in the formation of Pt single-atom sites.Theoretical studies show that pyridine nitrogen on the edge of carbon nanocage micropore can be protonated in acidic solution,and[PtCl6]2-can be trapped by electrostatic force and van der Waals action to form stable ionic salt structure.In the subsequent drying process,Pt single-atom catalysts can easily form on the heteroatomic nitrogen at the edge of the pore,and it is not easy to agglomerate or lose due to the anchoring effect of nitrogen-dopant.This method is simple and made the efficient utilization of Pt precious metal resources.2.In addition,the reasons for the high stability of hydrogen evolution of nitrogen-dopant carbon nanocage Pt single-atom catalysts were further studied:there were two competing processes in the process of hydrogen evolution:hydrogenation and hydrogen precipitation of Pt on carbon-based carriers,which destroyed the anchoring effect between Pt and carbon-based carriers,resulting in easier diffusion of Pt atoms and agglomeration into nanoclusters or particles.Nitrogen doping can significantly change the free energy changes of these two competitive reactions.Unlike Pt monoatomic site catalyst without nitrogen-dopant,Pt-C bond is more easily broken by H atom hydrogenation coupling in the process of hydrogen evolution reaction.Pt-N bond has strong interaction and is more stable and not easy to break.That is to say,the anchoring of nitrogen atom in nitrogen-dopant material can inhibit the hydrogenation of Pt site to support.The reaction pathway is more thermodynamically advantageous,resulting in excellent hydrogen evolution stability.3.The universality of the strategy of synergistic effect of micropore trapping and nitrogen-dopant anchoring for the preparation of single-atom catalysts was also studied.Five typical metal anions were calculated:[AgCl2]-,[AuCl4]-,[PtCl6]2-,[Ni(CN)4]2-,[Fe(SCN)6]3-in double layers with the nitrogen-dopant graphene microporous structure and Zigzag defect,which pore size is 2A,4A,6A,6-10A,10A,and 14A.The effects of micropore size and metal anion size on the capture efficiency were investigated theoretically.The calculation results show that the rule"similar size capture best",graphene micropore is easier to capture ions with similar size,and the micropore dopant with N can capture metal anions better.Thus,a simple and extensive strategy is proposed.By adjusting the size of micropore on carbon materials,combined with the synergistic effect of micropore capture and nitrogen-dopant anchoring,the target metal anions can be best adsorbed and captured,which facilitates the preparation of various metal single-atom catalysts.