Surface Segregation Of Ag-Au-Pt Nanoparticles:Atomistic Simulations

Alloy nanoparticles (NPs) containing noble metals have received enormous attention and already been utilized in/for many technologically important reactions applied in chemical and petrochemical industry, electrochemical fuel cells, and automobile exhaust purification, owing to their excellent catalytic activity and selectivity. Considering the high surface-to-volume ratio of alloy NPs, the component with lower surface energy will segregate onto the surface to reduce the free energy, which determines whole structural features and surface atomic distribution of alloy NPs. Experimentally, it is still a challenge to glean the precise structural and compositional information for small NPs. On the other hand, computer simulations have recently evolved to effective tools to study the properties of NPs and complement ongoing experimental efforts.In this work, quench molecular dynamics and Metropolis Monte Carlo (MC) simulations coupled with modified analytic embedded atom model (MAEAM) potentials have been used to study the influence of surface segregation on structural features and thermodynamics stability of Au-Pt, Au-Ag, Ag-Pt, and Au-Ag-Pt NPs, as well as their dependence on particle size, morphology or alloy composition. The present objective is to explore general trends of surface segregation and structural features as well as thermodynamics stability, and get insight into the surface structure-catalytic properties relationship. These atomic-scale understandings of geometric features, energetic stabilities, and atomic arrangement would play a role in intelligently designing robust and active nanocatalysts with a low cost.The geometric analyses show that site-to-surface and surface-to-volume ratio as well as average coordination number (CN) are all morphology-dependent. Using macroscopic considerations as a guide, energetic analyses show that the average cohesive energy decreases linearly with increasing particle sizes and average CN. As a result of the competition and balance between the surface energy and the strain energy, icosahedral (ICO) NP is favorable at small size but the least favorable at large size. Better truncated octahedral (TOC) and Marks decahedral (MDEC) NPs are comparable with ICO at small sizes, and the most favorable for a range of particle sizes.It is found that Ag strongly segregates to the surface of Ag-Pt NPs, regardless of the particle sizes, compositions and temperatures. And the surface segregation of Ag was promoted by increasing the particle sizes or global Ag compositions and by decreasing the temperatures. At low Ag global compositions, the core-shell structures were preferred, while onion-like structures with Pt enriched subsurface were formed at high Ag compositions. Generally, surface Ag concentration is higher in more open particles. For large NPs, however, the closest ICO show the highest surface Ag concentration as the lowering of strain energy.For Au-Pt NPs, the surface segregation and structural features and their dependence on particle sizes and alloy compositions are similar to those of Ag-Pt NPs. The site-preference segregation on particle surface was predicted where Au atoms favor sites at the vertices, edges, and facets. The reverse temperature dependency of segregation for different surface sites has been discussed in terms of configuration energy differences.For Ag-Au NPs, the dependencies of surface Ag enrichment and their atomic-scale structural features on the particle sizes, compositions and temperatures are similar to those of Au-Pt and Ag-Pt NPs. However, their core-shell and onion-like mixing patterns are less obvious than those of Au-Pt and Ag-Pt NPs, due to the alloying effects between Ag and Au. Accordingly, the calculated results indicate more obvious alloying features in Ag-Au NPs with larger sizes or at higher temperatures, and more obvious segregated features in particles under the opposite conditions.For Au-Ag-Pt NPs, it is found that Ag strongly segregates to the surface, similar to Ag-Pt and Au-Ag systems. While the surface enrichment or depletion of Au depends on its global composition, dissimilar to any binary system. For Au0.125Ag0.125Pt0.75particles, Ag and Au co-segregates to the surface, but Ag prefers lower-coordinated surface sites. At higher Au and Ag global compositions, the strong surface enrichment of Ag suppress Au enrichment, even resulting in surface Au depletion. Generally, the structural features of Au-Ag-Pt NPs are similar to those of Ag-Pt and Au-Ag ones. In the core of Auo.25Ag0.25Pt0.5nanoparticles, however, Pt atoms and excess Ag and Au formats complex short-range-order structures, dissimilar to any binary systems.On the basis of ligand effects and ensemble effects as well as bifunctional mechanism, the correlation between catalytic properties and specific atomic arrangement on particles’surface were discussed. It is predicted that high catalytic activities of low temperature CO oxidation on Ag-Au and Au-Ag-Pt NPs can be obtained by creating or destroying special active sites. For Ag-Pt system, the presence of partly alloyed facets along with vertices and edges blocked by Ag would not only greatly enhance the catalytic activity of ORR and CO electro-oxidation in alkaline media, but also improve the CO poisoning with respect to Pt nanocatalysts. Similarly, the specific coupling between Ag or Au and Pt on the surface of Au-Ag-Pt NPs will also improve the catalytic activity of glucose electro-oxidation. Furthermore, Ag-Pt and Au-Ag-Pt NPs with low Ag or Au composition might be promising candidates for selective hydrogenation of the C=O group in a,p-unsaturated aldehydes.