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

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 Pt-8 clusters in a hydrogen gas environment to study the intracluster, cluster-support, and cluster-adsorbate interactions. Two alumina surfaces, alpha-Al2O3(0001) and gamma-Al2O3(100), and two conditions, T = 600 degrees C, pH(2) = 0.1 bar and T = 25 degrees C, pH(2) = 1.0 bar, were considered corresponding to low and high hydrogen chemical potential mu H, respectively. The low free energy ensemble of Pt-8 is decorated by a medium (2-12 H), respectively, high (20-30 H), number of hydrogen atoms under equilibrium at low mu H, respectively, high mu H, and undergoes different morphological transformations on the two surfaces. On alpha-Al2O3(0001), Pt-8 is mostly 3D but very fluxional in structure at low mu H and converts to open one-layer 2D structures with minimal fluxionality at high mu H, whereas on gamma-Al2O3(100), the exact opposite occurs: Pt-8 clusters present one-layer 2D shapes at low mu H and switch to compact 3D shapes under high mu H, during which the Pt-8 cluster preserves moderate fluxionality. Further analysis reveals a similar Pt-Pt bond length increase when switching from low mu H to high mu 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. Published under license by AIP Publishing.