Theoretical Study Of The Structure Of Metal Nanoparticles Under The Real Reaction Conditions

Heterogeneous catalysis is the cornerstone of the modern chemical industry.It participates and plays a key role in a number of important processes like synthesis,solar energy,pollution abatement,electronics,medicine,and engineering.The low cost and environmentally friendly catalysts with high reaction activity and selectivity are highly desired for energy conversion and green chemistry.Among the multitudinous catalysts,the metal nanoparticles?NPs?have attracted intensive interest due to their novel physical and chemical properties.Generally,the reactivity of the NP is mainly determined by its size,shape,composition,and surface structure.Especially,the shape of a NP has a direct influence on its number of surface active sites.Therefore,many studies have been focused on the shape control of the metal NPs to achieve higher catalysis performance.However,in the recent 20 years,in situ studies have evidenced that the nanoparticles?NPs?may change their shapes and surface structures dramatically under the reactive conditions,which could totally change their catalytic activities.How to understand and precisely predict the shape evolution of the nanoparticles in response to the environments from theoretical point of view is still unclear due to the lack of an efficient simulation approach.The main obstacle is that it is difficult to study the nanoparticles larger than 300 atoms by first principle calculation due to the limitation of computational capacity.Other classical methods like molecular dynamic and Monte Carlo simulation are not precise enough.In this work,we have developed a Multiscale Structure Reconstruction?MSR?model,consisting of the Wulff construction theorem,thermodynamic adsorption isotherms,and the first principle calculations to investigate the structure of the nanoparticles under the real reaction conditions.This MSR model can perfectly reproduce the shape evolution of nanoparticles in gas mixture,liquid,or on supports.It can handle nanoparticles with large size?1000?100000 atoms?and perform high-throughput prediction?temperature ranges from 1 to 1000?,and pressure ranges from1 to10⁷Pa?with low computational costs.We have studied the interaction between the metal?Pd,Pt,Rh,Cu,Au?and the gas molecules?CO,NO?,and quantitively described the equilibrium structures of metal nanoparticles under CO,NO,and NO+CO gas mixture environments different temperature,pressure or partial pressure conditions.Based on the MSR model,we have coded and developed the Multiscale Operando Simulation Package?http://www.mosp.top/?.By this package,one can visualize the reconstructed nanoparticles at given environmental conditions?gas mixture,liquid,support,etc.?in one second.Collaborated with experimental workers,we have studied the reshaping of the Pd NPs under reactive gas?O₂,H₂,N₂?environment at atmospheric pressure by in situ transition electronic microscopy?TEM?and the MSR model.in situ TEM observations showed that the morphology of Pd NP changed in each of the three gas environments,and the MSR simulations gave a completely consistent prediction.For example,we found that the Pd NPs changed from a round shape to a faceted shape under pure N₂environment,evidenced by the in situ TEM observation and the MSR modeling.It has been fully understood that the reshaping phenomenon is driven by the anisotropically changed surface energies under reaction conditions.Meanwhile,we studied the structure of Au NPs in O₂,H₂ at atmospheric pressure.Both in situ TEM observations and MSR simulations results indicate that the Au NPs undergo structural changes only in the O₂ environment.In another work,we have studied the adsorbed water molecules on the interface of the Cu₂O and the water induced oriented attachment?OA?growth mechanism of Cu₂O nanowires in the aqueous solution.All the research experiences have deepened the understanding of how the reactive environment factors change the structures of the catalysts.Further,we have extended the MSR model to study the surface segregation behavior of binary alloy systems.Combining thermodynamic adsorption isotherms with first principle calculations,we proposed and derived the environmental segregation energy to describe the surface segregation tendency of solute atoms under the reactive gas environment.By this new definition,we have studied the surface segregation tendencies of 72 binary alloys under various CO conditions and revealed insights on the flexible alloy segregation trends under changing gas conditions and perfectly explained the existing experimental results.In summary,this work provides a developed multiscale structure reconstruction model to efficiently study the structure of nanoparticles and the alloy surfaces under the operando conditions.Combined with the experimental techniques,we discovered the reshaping of nanoparticles under the reactive gas environments.The accuracy and reliability of the MSR model have been evidenced by the experimental results.These in situ studies have brought new ways for the rational design of efficient nanocatalysts in real reactions and deepen the understanding of the microscopy mechanism of heterogeneous catalysis.