PdGa Alloying-Dealloying Processes under Reducing and CO2 Hydrogenation Reaction Conditions from Metadynamics Simu-lations

Silica-supported PdGa nanoparticles (NPs) prepared via Surface Organometallic Chemistry are selective catalysts for the hydrogenation of CO2 to methanol. However, despite their notable catalytic performances, that exceed the corresponding Cu-based systems, little is understood regarding the local structure of the PdGa NPs, their adsorption properties, and their behaviour under CO2 hydrogenation reaction conditions, making the development of structure–activity relationships challenging. Here, we use ab-initio Molecular Dynamics and Metadynamics at the density-functional theory level combined with in situ X-ray absorption spectroscopy to explore the structures and the dynamics of the alloyed PdGa NPs under various conditions. We look in particular at the impact of the SiO2 surface and adsorbates (H*, CO*, O*), expected under CO2 hydrogenation conditions, onto the structure of the NPs. Overall, addition of Ga to Pd generates alloyed PdGa NP with isolated Pd sites at the surface. This structural change decreases the amount of adsorbed hydrogen or CO on the NPs and changes the dominant binding mode of the adsorbates to the metal, from mainly bridging to terminal CO and from mainly internal hydrides to terminal and μ2-bridging hydrides. 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.