Single-atom Or Cluster Nitrogen Reduction Electrocatalyst:a First-principles Study

The conversion of nitrogen to ammonia by electrocatalyst under mild conditions is a valuable research direction.However,the Haber-Bosch process is still used in industrial ammonia synthesis.The high purity hydrogen（H₂）required by the Haber-Bosch process consumes large amounts of fossil fuels（mainly natural gas）and emits large amounts of greenhouse gases（CO₂）.In recent years,electrochemical nitrogen fixation is a very promising method,which uses N₂abundant in nature and renewable energy to convert N₂into a high value added product NH₃at the negative electrode.Electrocatalysis is a sustainable alternative to Haber-Bosch method.However,the high stability of N₂poses a major challenge to electrocatalytic nitrogen reduction reaction（NRR）.Single atom catalysts（SACs）with discrete metal centers have attracted wide attention in the field of catalysis due to their remarkable catalytic action and high theoretical atomic utilization rate of 100%.Supported metal catalysts with discrete metal centers can provide efficient and highly active surface sites for sustainable catalytic conversion.SACs not only maximizes the utilization of the active site,but also increases efficiency,reduces the use of metals,and improves selectivity in the conversion process.In addition,the nanocluster catalyst can not only provide multiple metal atoms for the catalytic site,but also maintain the high efficiency of the catalyst.However,most SACs and cluster catalysts still have low catalytic activity and selectivity.In order to solve the above problems,this paper investigates the NRR activity of graphene and h-BN in-plane heterostructure loaded transition metal atoms and Mo cluster loaded on C₂N using the method of first-principles calculation.The details are as follows:（1）Since the original graphene and hexagonal boron nitride（h-BN）have no catalytic activity,the transition metal doping on h-BN still has a large band gap,which is not conducive to the application of electrocatalysts.The in-plane heterostructure between graphene and h-BN has been extensively studied and prepared.The heterostructure interface is conducive to electron transfer,which is very favorable for electrocatalytic reaction.Therefore,we investigated the electrocatalytic NRR performance of doping a single transition metal atom（TM-B_xN_yC_z）at the graphene/h-BN heterostructure interface.The results showed that Mo-BN₂C and Re-B₂NC had the best NRR performance,and their limiting potentials were-0.42 V vs.RHE（reversible hydrogen electrode）and-0.31 V vs.RHE,respectively.The synergistic effect of B,N and C atoms on TM atoms increased the NRR activity.（2）Cluster catalysts have dispersed metal active sites and close to 100%atomic utilization.Compared with SAC,cluster catalysts have various types of active sites（Top,Bridge,and Hollow）.Mo has a partially filled d-orbitals with high intrinsic activity,and Mo is located at the top of the NRR active volcano curve.Therefore,we explored the NRR performance of Mo₁₂cluster on C₂N monolayer(Mo₁₂-C₂N).It was found that the diversity of active sites in Mo₁₂clusters provided a good reaction path for intermediates and reduced the reaction energy barrier of NRR.Mo₁₂-C₂N has good NRR performance,and its limit potential is-0.26V vs.RHE.