Alloy Reorganization and Dynamics in Group-10-metal-Gallium Nanoparticles under Reactive Atmospheres: Impact on Local Environment and Reactivity.

Bimetallic nanoparticles are catalysts for reactions as COx hydrogenation or propane dehydrogenation. Recently, gallium has been identified as a promoter which enables dispersion of transition metal sites, raising their activity and selectivity. However, quantitative information on alloying dynamics under reaction conditions are not readily available and a gen-eral computational method to access such information is lacking. Here, an ab initio molecular dynamics workflow with enhanced sampling methods is used to probe the alloying behavior of Ni-, Pd-, and Pt-Ga nanoparticles under operating conditions (T = 600°C) in presence of H2 or CO. The three metals display different alloying behaviors with Ga: Ni forms a core surrounded by gallium, while Pd and Pt form different alloyed structures. Both H2 and CO shift the alloying state to different extents. A set of three descriptors is then proposed to compare and quantify the alloying behavior of these catalyst models: (i) the position αmin of the most stable alloying state; (ii) the curvature ηα of the free energy at αmin, re-ferred to as the alloying hardness; (iii) the skew κα of the free energy at αmin, which relates to its propensity to alloy or segregate. The cost of alloy reorganization, which correlates with alloy hardness, is a major part of the free energy barri-ers of propane dehydrogenation. Seeing as the alloying behavior of a catalyst is a critical parameter that is overlooked in catalyst design, quantitative descriptors are a first step in designing alloys with set catalytic properties.