Design And Electrocatalytic Properties Of Silver-based Single Atom Alloys:a Theoretical Study

Noble metal catalysts such as Pt and Pd play an important role in the fields of energy,chemical industry and environmental protection,etc.These excellent noble metal catalysts can also lead to problems such as poor reaction selectivity,easy poisoning and deactivation.Recently,single atom alloy(SAA)is an alloy catalyst which has received much attention.SAA is prepared by doping atomically dispersed low concentration of active metals atoms on the surface of host metals.Due to the existence of well-defined and isolated active metal sites on the surface of SAA,which leads to dissociation and decoupling of reaction sites,it is possible to escape from the linear scaling relationship that limits the performance of traditional catalysts.SAA is considered promising to overcome the shortcomings of traditional catalysts such as poor selectivity and easy poisoning,to form affordable alloy catalysts with excellent performance.However,the surface segregation phenomenon and the highly complex surface composition and atomic structure of alloy catalysts make it difficult to accurately characterize the internal structure and surface atomic distribution of the alloy experimentally,and it is also difficult to fully explore the influencing factors such as alloy composition,external temperature and reaction environment.Therefore,it is still an important and challenging task to clarify the composition-structure-property relationship of catalyst surfaces and design catalyst surfaces accordingly.In this thesis,a combination of first-principles calculations,cluster expansion(CE)and Monte Carlo(MC)simulations was used to investigate Ag-based SAAs and their behavior in electrocatalytic oxygen reduction reaction(ORR).At the atomic scale,the correlation effects of alloy composition,temperature and gas coverage on the surface atomic arrangement of SAA were investigated.The connection between surface structure and catalytic activity was explored.Besides,a simple,low-cost and efficient SAA preparation and surface modification strategy of the alloy catalyst was proposed to provide guidance for catalyst design,preparation and application from the theoretical direction.The main contents are as follows:(1)A new method for the design and preparation of SAA was developed.The alloy surfaces were innovatively constructed with the bulk structures of bimetallic elements and vacancies.The segregation behaviors of the(111)and(100)surfaces of AuAg,PtAg and PdAg alloys at different surface depths were studied by CE combined with MC simulations,and the simulation results were in good agreement with the experiments.The first layer of(111)and(100)surfaces of AuAg alloy does not appear monoatomic dispersion.The surfaces of the PtAg and PdAg alloy systems have similar elemental segregation,i.e.,Ag is strongly enriched in the first layer of the surface and Pt/Pd is enriched to a certain extent in the second layer.However,the element segregation behavior on the(111)and(100)surfaces of PdAg alloys has a better consistency.Accordingly,a casting combined quenching method for the preparation of PdAg SAA was proposed,and samples were successfully prepared.The consistency of the surface structure of the samples with the theoretical predictions was demonstrated by characterization methods such as depth-profile X-ray photoelectron spectroscopy and CO adsorption Fourier infrared spectroscopy.The design screening and preparation method was then confirmed,that is,the alloy properties are first calculated using CE combined with MC,and then directly by casting and quenching at specific temperatures to achieve control of the structure and chemical composition of the alloy surface,which is expected to be applied to the mass production of SAA.(2)Theoretical calculations show that PdAg SAA has excellent comprehensive performance in ORR.The adsorption behaviors of ORR intermediates on the PdAg(111)surfaces(with/without Pd subsurface layer)by various Pd ensembles were systematically investigated using the first-principles calculations.It was found that ORR intermediates on the surfaces with different Pd ensembles tend to bind at sites between heterogeneous atoms.PdM@Pd1LAg(111)with a Pd monomer and a Pd subsurface exhibits the best ORR performance,and its limiting potential(0.82 V)is higher than that of Pt(111)(0.80 V).Another important advantage of PdAg SAA is the significantly improved resistance to CO poisoning,where PdM@Ag(111)with Pd monomers exhibits the strongest resistance to CO poisoning.Thus,the specific surface structures of PdAg SAA have comprehensive performance with excellent catalytic properties and resistance to CO poisoning.(3)The control method and optimization scheme of CO on the distribution of Pt atoms on the PtAg SAA surface were established.It was found that heat treatment under CO atmosphere can induce reverse segregation to realize the modulation of the surface structure of PtAg alloy.MC results revealed that the competitive effects between CO-metal interaction and CO-CO repulsion can lead to the dispersion and aggregation of Pt ensembles.Two methods of tuning the surface Pt ensembles were established:for Pt bulk concentrations higher than 10%,the surface can be tuned to obtain more monomer(0.25 ML CO coverage)or pure Pt layers(1ML coverage)by adjusting the CO coverage;for Pt less than 10%,the surface Pt distribution can be controlled by adjusting the bulk concentration.An ordered structure was observed on the surface between 400 K and 600 K for 8%Pt bulk concentration and less than 1 ML CO coverage,which maximized the number of Pt monomers and the degree of the uniform distribution on the surface.The limiting potential of the ordered structure Pt3Ag(111)is 0.80 V,which is higher than that of pure Pt(111)(0.80 V),indicating its potential to become an ORR catalyst with abundant active sites and low overpotential.(4)The surface properties of the PdPtAg ternary alloy are different from those of the PtAg/PdAg binary alloy.The simulation results indicate that the local chemical environment of the alloy surface strongly depends on its bulk composition and ambient temperature,which has a significant impact on the catalytic properties of the alloy surfaces.Even at high temperatures,the doping metal(Pd or Pt)tends to remain in the bulk,while Ag tends to form an enriched layer in the topmost layer.The concentration of Pd is at its peak in the subsurface layer and increases with decreasing temperature.The atomic ensembles analysis of the surface doped elements shows that,unlike the consistent trend of the number of monomers and temperature on the surface of the binary alloy,the surface of the PdPtAg ternary alloy exhibits a diversified relationship.The segregation of Ag increases the number of unique catalytically active sites on the surface and the concentration of doped atoms by about 10%compared to PtAg SAA.