(581e) Structural Evolution of Au-Pd Single Atom Alloy Catalysts for H2O2 Synthesis

Single atom alloys (SAAs) can break the typical linear free energy scaling relationships found in other mono- and bi- metallic catalysts because there is an absence of homonuclear metal coordination. One of the main drawbacks of SAAs is their limited applications due to small per-volume and per-area of active sites. One reaction that makes use of these SAAs is the direct synthesis of H₂O₂ (from H₂ and O₂) on AuPd SAA catalysts. Prior work on large SAA AuPd materials have shown that isolated Pd atoms selectively make H₂O₂, while Pd ensembles form undesired H₂O. In this collaborative effort, our work uses density functional theory (DFT) to understand the structure of small AuPd catalysts (~2 nm) and how Pd atoms prefer to arrange as a function of nanoparticle size, composition, and reaction conditions. Pd most favorably exchanges into the subsurface sites of Au nanoparticles (in vacuum), and Pd exchange becomes less favorable when Pd is present at neighboring sites (Fig. 1a). Thus, Pd preferentially locates in the subsurface of the particle and isolated from each other. However, adsorption energies for O* and CO* become more favorable when Pd is present in the adsorption site (Fig. 1b); such species are likely to drive Pd atoms to the surface. Here, we use Monte Carlo methods to simulate how the particle might change in the presence of adsorbates. For example, CO (1 bar) changes the average Pd coordination number from 12 to ~8 and increases the composition of the surface from ~0% Au to ~20% Pd (Fig. 1c). These results will be expanded to include solvent and solvent-derived adsorbates (e.g., OH*) and compared to ex situ and in situ characterizations (FTIR, XAS) and used to inform kinetic studies exploring the effects of reaction conditions and pretreatments on SAA performance in H₂O₂ synthesis reactions.