Modelling the Impact of Alloying State Dynamics on-the Reactivity of Group-10-Metal-Gallium Nanoparticles.

Bimetallic nanoparticles are catalysts for reactions, such as COx hydrogenation or propane dehydrogenation. Recently, gallium has been identified as a promoter which enables dispersion of group-X-metal sites, raising 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 missing. 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 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, referred 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 influence of the alloying behavior on the propane dehydrogenation activity of NiGa and PtGa is assessed: the energetic cost of alloy reorganization in the activation energy has been quantified. Extracting quantitative alloying descriptors from ab initio molecular dynamics is a promising tool to take alloy reorganization into account, both for mechanistic studies and for rational catalyst design.