Theoretical Design Of High-loading Two-dimensional Catalysts For Electrocatalytic N₂ And CO Reduction

Global requirement of chemicals increases everlastingly with expanding industrialization and increasing population.Since traditional production of chemicals relies on nonrenewable fossil fuels including oil and coal,developing green and sustainable strategies of chemicals production is of great significance and urgency.Currently,electrocatalytic N₂ reduction reaction(NRR)and CO reduction reaction(CORR)with the energy supply of renewable electricity have attracted worldwide interest,and they have been regarded as eco-friendly strategies for the production of two important chemicals,NH₃ and CH₄.For NRR and CORR,efficient catalysts are indispensable,and efficient catalysts should possess high-loading active sites with excellent inherent activity and selectivity.In the field of catalysis,numbers of nano-catalysts have been fabricated recently,since they can provide more active sites than conventional catalysts.Catalytic reactions initiate with the adsorption of reactants onto catalysts surface,and two-dimension materials thus have been the research hotspots in the field of nano-catalysis due to the remarkable advantage in surface/volume ratio and good stability.However,though preparation techniques are becoming more and more mature and great progress has been made in the characterization and testing techniques,the underlying mechanism is still unclear,hindering the development of catalysis.Nowadays,due to the rapid promotion of computational power and the gradual perfection of computational models,some theoretical results regarding catalysis are close to the real cases,which makes it possible to uncover the underlying mechanism.Meanwhile,systematically theoretical investigation before experiment can decrease the waste of time and cost caused by the conventional trial-and-error method.Unfortunately,most theoretical works regarding the design of catalysts only focus on the inherent activity and selectivity of active sites,ignoring the loading of active sites,and the as-designed catalysts thus suffer from a lack of practical value.Here,with the help of density function theory(DFT)calculations,monolayer transition metal dichalcogenides and transition metal-porphyrins(TM-PPs)sheets were explored as two-dimensional catalysts with high-loading active sites for NRR and CORR.The main contents are divided into three parts:(1)Single-atom catalysts with the active sites of transition metal-nitrogen(TMN_x)centers are usually efficient for NRR,a sustainable strategy of NH₃ synthesis,but the loading of active sites is relatively low due to the clustering of transition metal heteroatoms.Herein,two-dimensional TM-PPs sheets possessing high-loading intrinsic TMN₄ centers were investigated to catalyze NRR with DFT calculations.The results suggest that 5d TM-PP sheets are more efficient than 3d and 4d TM-PP sheets owing to their better capabilities of electron transfer,which is related to the screening effect induced by inner-shell electrons.Among them,as the most promising one,W-PP sheet possesses high mass loading of 38.33%and low overpotential of 0.15 V vs.reversible hydrogen electrode(RHE),beneficial for catalyzing NRR with good selectivity.(2)Ever-increasing demand of NH₃ requires more advanced atomic catalysts with high-activity and high-loading active sites for NRR.Owing to the“accept and backdonate mechanism”,transition metal atoms are preferred active sites.However,because of the fast self-nucleation of transition metal heteroatoms,the loading of active transition metal heteroatoms is relatively low.Investigating intrinsic transition metal active sites is thus of great worth.Herein,using DFT calculations,intrinsic defects in the basal plane of WS₂,including vacancy defects and antisite defects,were created with high-loading transition metal active sites for boosting NRR.As expected,antisite W_S with one W atom replacing one S atom can catalyze NRR with a low potential of-0.24 V vs.RHE.(3)CO methanation from CORR is significant for sustainable development of society,but catalysts with excellent selectivity and activity are still lacking.Selectivity is sensitive to the structure of active sites,and activity can be tailored by work function.Moreover,intrinsic active sites usually possess relatively high loading compared to artificial ones.Herein,antisite Mo _S2 and W_S2,intrinsic atomic defects of Mo S₂ and WS₂ with a transition metal atom substituting a S₂ column,were investigated for CORR by DFT calculations.The steric hindrance from the special bowl structure of Mo _S2 and W_S2 ensures good selectivity towards CO methanation.Coordination environment variation of the active sites,the under-coordinated Mo or W atoms,effectively lowers the work function,making Mo _S2 and W_S2highly active for CO methanation with the required potentials of-0.47 and-0.49 V vs.RHE,respectively.Moreover,high loading of active sites and little structural deformation during the catalytic process of Mo _S2 and W_S2 enhance their attraction for future commercial application.