Structure And Electrocatalytic Properties Of Ag-M(M=3d TM,Pd) Clusters And Nanoalloys

Nanoalloys have an advantage of enhancing catalytic activity while reducing the loading of noble metals,which have attracted extensive attention in the field of catalysis.However,due to the lack of experimental technology,the relation of alloys composition,structure,and properties can be hardly solved through experimental research.Therefore,it is particularly important to study the alloys structure and their electrocatalytic reaction process on an atomic-molecular scale.At the same time,with the rapid development of modern computer technology,quantum computing has been more and more popularly applied in the study of electrochemical catalysis.According to previous research works,Ag has high stability in an alkaline environment,and Ag-based nanoalloy catalysts have great potential for the development of alkaline fuel cells.However,the principle of high catalytic activity of nanoalloys has not been adequately understood,and the mechanism of their application in catalytic reaction has not been fully clarified.In addition,the diversity of nanoparticles makes it difficult to obtain a low-energy structure,and the microstructure of alloy clusters is also controversial.Therefore,in view of these problems,carrying out further relevant research work would contribute to providing basic theoretical support for the design of alloy catalysts.In this study,the structure of 38 atoms AgCu cluster was studied by a genetic algorithm global optimization technique and density functional theory(DFT)calculations.The results show that the polyicosahedral(p Ih)Ag₃₂Cu₆ core-shell cluster is more stable than the truncated octahedral(TO)Ag₃₂Cu₆ core-shell cluster of the atomistic models.Therefore,the newfound p Ih-Ag₃₂Cu₆core-shell cluster would have a potential application for an oxygen reduction reaction(ORR).For the O₂ dissociation on p Ih-Ag₃₂Cu₆core-shell cluster,the density of states at the Fermi energy level is maximal at the favorable absorption site b2,the activation energy barrier for the O₂ dissociation is 0.715 e V,and the d-band center is-3.395e V.This indicates that the catalytic activity is attributed to a maximal charge transfer between an oxygen molecule and the p Ih-Ag₃₂Cu₆core-shell cluster.This work revises the earlier viewpoint that Ag₃₂Cu₆core-shell nanoparticles are not suitable as ORR catalysts and demonstrated that AgCu nanoalloy is a potential candidate to substitute noble Pt-based catalysts in the alkaline fuel cells.As for AgCu nanoalloy,the catalytic properties of ORR in core-shell and AgCu alloys for the oxygen reduction was investigated utilizing both theoretical and experimental methods.The activation energies on the pure Ag,core-shell Ag₃Cu@Ag and alloy Ag₃Cu are 0.532,0.443 and 1.137 e V,respectively,indicating that the ORR activity of core-shell Ag₃Cu@Ag is higher than that of pure Ag and alloy Ag₃Cu.The predicted working potentials of pure Ag,core-shell Ag₃Cu@Ag and alloy Ag₃Cu are 0.737,0.761 and 0.675 V,respectively,indicating that the core-shell Ag₃Cu@Ag nanoparticles have the highest working potential but the lowest overpotential 0.469 V.AgCu bimetallic catalysts were prepared by the pulse laser deposition(PLD).The core-shell Ag₃Cu@Ag catalysts showed greater ORR activity than the alloy Ag₃Cu catalysts,which is consistent with the DFT calculations.The results indicate that a core-shell atom order should be designed for AgCu bimetallic nanoparticles to enhance their ORR activity.To search for suitable doping elements for Ag-based ORR catalysts,the ORR catalytic properties of dilute alloy M₁Ag(111)doped with M(M=3d transition metal TM)on Ag(111)surface were studied.Furthermore,the catalytic performance of ORR was improved by optimizing AgCu nanoalloy to ternary Cu Mn Ag nanoalloy.Through an investigation into surface segregation and mixing energy,it is unexpected to find that the M₁Ag(111)dilute alloys exhibit higher stability when M is in the subsurface(2L)than that on the surface(1L)of Ag(111)facet.The atomic charge analysis reveals that this subsurface stability is attributed to the relative positive charge transfer of surface Ag atoms.The 2L-M₁Ag(111)catalysts were further investigated for their electronic structure and ORR activities,particularly,2L-Cu₁Ag(111),2L-Ni₁Ag(111)and 2L-Zn₁Ag(111)are dilute nanoalloys with free-atom-like electronic structure.2L-Mn₁Ag(111)and 2L-Cu₁Ag(111)exhibit an overpotential of 0.459 V and 0.468 V.The ternary 2L-Cu₁Mn₁Ag(111)is theoretically predicted with an overpotential of 0.450 V.Motivated by the prediction,the PLD method is proposed to prepare ternary Cu Mn Ag nanoalloys.Accordingly,these ternary Cu Mn Ag catalysts exhibit an overpotential of 0.50 V,which is close to the predicted overpotential and that of the commercial Pt/C catalyst.To explore the application of Ag-based alloy in anode catalysis,a series of DFT calculations were carried out to investigate the catalytic activity of Pd-doped Ag dilute nanoalloys in formate oxidation reaction(FOR).Compared with Pd₂Ag(111)and Pd₃Ag(111)dilute alloys,the Pd₁Ag(111)single-atom alloy(SAA)exhibits the highest FOR catalytic activity.The low limiting potential of 0.026 e V for direct association path and a value of0.084 e V for the direct dissociation path,and the lowest activation energy of 0.774 e V for the rate-determining-step in the direct dissociation path.Pd₁Ag(111)SAA exhibits an extremely narrow sharp peak in the partial density of state curves from-0.75 to-2.0 e V,which is due to the free-atom-like electronic structure of the single Pd atom.The isolated Pd single atom is more stable(-0.041 and-0.097 e V,respectively)than the aggregated Pd₂ and Pd₃ atom cluster on the Ag(111)surface.This verifies the potential application of Pd₁Ag(111)SAA in experiments.This work further elucidates the theoretical profile of FOR and provides a new strategy for designing the catalytic reaction at the atomic level.To further study the effect of surface structure on the ORR activity of Ag catalyst,the ORR catalytic performance of Ag(hkl)surfaces was investigated by computational modeling and experimental measurement.The reaction paths in three possible mechanisms of ORR on Ag(hkl)surfaces were analyzed by the transition-state searching method.The stepped Ag(220)surface exhibits an activation energy barrier of O₂ protonation reaction is 0.504 e V,which is the lowest activation energy barrier compared with the flat Ag(111)and Ag(200)surfaces.Furthermore,the theoretical calculations show that the ORR overpotential of stepped Ag(220)surface is 0.457 V,which is comparable to the Pt(111)catalyst with the overpotential of 0.441V.It indicates that the stepped Ag(220)surface is a high ORR active site on the silver surface.The ORR polarization curves of the textured silver surface were measured by the rotating disk electrode method.The experimental ORR activity trend is Ag(220)>Ag(111)>Ag(200)surface for the silver surface.Both the experimental and computational results indicate that the stepped Ag(220)surface is one of high ORR activity origin for pure Ag materials with desirable morphologies.This new insight provides a fundamental understanding of the ORR reaction mechanism of monometallic Ag and is beneficial for the design of advanced catalytic materials based on pure Ag.