Ab Initio Molecular Dynamics Study of Pt Clustering on γ-Al2O3 and Sn-Modified γ-Al2O3

We have conducted AIMD free energy simulations to examine the dynamics of Pt atoms and Pt n (n = 2-3) species on dry gamma-Al2O3(100), dry gamma-Al2O3(110), and wet gamma-Al2O3(110) surfaces, with OH coverages corresponding to 500 K (11.8 OH/nm(2)) and 800 K (5.9 OH/nm(2)), while varying the Pt and Sn loading. Under the same dry conditions and temperature, comparing the (100) and (110) surface terminations revealed that the interactions between Pt and the surface play a crucial role in determining whether the potential of mean force between reduced Pt atoms is repulsive, as observed on the (100) surface, or if it can support a bound Pt-Pt state, as observed on the (110) surface. The hydration of the (110) surface had a significant impact. At a Pt loading of 0.75 Pt/nm(2), with hydration of 5.9 OH/nm(2), the energy of the potential of mean force increases. Although a Pt-Pt bound state is still supported, it becomes kinetically less accessible from the dispersed state. At an even higher water loading of 11.8 OH/nm(2), the Pt-Pt potential of mean force becomes predominantly repulsive and can no longer sustain the Pt-Pt bound state. Higher Pt loadings of 1.12 Pt atoms/nm(2) promote the aggregation of Pt into progressively larger clusters, but high levels of hydration can kinetically impede particle growth. On Sn-modified gamma-Al2O3(110), Pt tends to associate with Sn, except at high levels of surface hydration where the potential of mean force between Pt and Sn atoms becomes repulsive. The presence of Sn inhibits the aggregation of Pt particles, and the Pt-Pt potential of mean force becomes increasingly repulsive with higher Sn loading.