Pt₈ cluster on alumina under a pressure of hydrogen: Support-dependent reconstruction from first-principles global...

Alumina supported Pt nanoclusters under a hydrogen environment play a crucial role in many heterogeneous catalysis applications. We conducted grand canonical genetic algorithm simulations for supported Pt₈ clusters in a hydrogen gas environment to study the intracluster, cluster-support, and cluster-adsorbate interactions. Two alumina surfaces, α-Al₂O₃(0001) and γ-Al₂O₃(100), and two conditions, T = 600 °C, p_H = 0.1 bar and T = 25 °C, p_H = 1.0 bar, were considered corresponding to low and high hydrogen chemical potential μ_H, respectively. The low free energy ensemble of Pt₈ is decorated by a medium (2-12 H), respectively, high (20-30 H), number of hydrogen atoms under equilibrium at low μ_H, respectively, high μ_H, and undergoes different morphological transformations on the two surfaces. On α-Al₂O₃(0001), Pt₈ is mostly 3D but very fluxional in structure at low μ_H and converts to open one-layer 2D structures with minimal fluxionality at high μ_H, whereas on γ-Al₂O₃(100), the exact opposite occurs: Pt₈ clusters present one-layer 2D shapes at low μ_H and switch to compact 3D shapes under high μ_H, during which the Pt₈ cluster preserves moderate fluxionality. Further analysis reveals a similar Pt-Pt bond length increase when switching from low μ_H to high μ_H on both surfaces although morphological transformations are different. Electronic structure analysis shows the existence of bonding interactions between Pt and Lewis acidic Al³⁺ sites along with the Pt-O interaction, which implies the necessity to include Al neighbors to discuss the electronic structure of small Pt clusters.