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

Abstract

Bimetallic nanoparticles are catalysts for reactions such as CO_x hydrogenation or propane dehydrogenation. Recently, gallium has been identified as a promoter, which enables dispersion of transition metal sites, increasing their activity and selectivity. However, quantitative information on alloying dynamics under reaction conditions is not readily available, and a general 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 the 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 states 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 α^min of the most stable alloying state; (ii) the curvature η of the free energy at α^min, referred to as the alloying hardness; and (iii) the skew κ of the free energy at α^min, 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 barriers of propane dehydrogenation. Since the alloying behavior of a catalyst is a critical parameter that is overlooked in catalyst design, quantitative descriptors are the first step in designing alloys with set catalytic properties.