Metadynamics Simulations Reveal Alloying-Dealloying Processes During CO2 Hydrogenation with Bimetallic PdGa Nanoparticle Catalysts

Supported bimetallic nanoparticles (NPs) are of great interest to industry and academia due to their high catalytic activity, but structure–activity relationships are difficult study due to the multitude of possible compositions, interfaces and alloys possible in these systems. This is particularly true for bimetallic NP catalysts involved in the selective hydrogenation of CO2 to methanol where the NP structure responds dynamically to the chemical potential of the reactants and prod-ucts. Herein, we use a recently developed combined computational and experimental approach that leverages ab-initio Molecular Dynamics (AIMD) and Metadynamics (MTD) in conjunction with in situ X ray absorption spectroscopy, chemisorption and CO-IR, to explore the dynamic structures and interactions with adsorbates under various CO2 hydrogenation conditions in well defined state-of-the-art silica-supported PdGa NPs on an atomic level. We find that PdGa alloying generates isolated Pd sites at the NP surface, changing the dominant binding modes of relevant adsorbates compared to pure Pd NPs: CO molecules mainly adopt terminal binding modes and hydrides switch from mainly internal to terminal and μ2-bridging in PdGa NPs. Under more oxidizing conditions, akin to CO2 hydrogenation for PdGa NPs, Ga is partially oxidised, forming a GaOX layer on the surface of the NP, with a partially dealloyed PdGa core, that retains some isolated Pd sites at the surface. Overall, these bimetallic NPs show high structural dynamics and a variable extent of alloying in the presence of different adsorbates relevant for CO2 hydrogenation. This work highlights that AIMD/MTD is a powerful approach to elucidate structural dynamics at a single particle level in complex catalytic systems