Ab Initio Molecular Dynamics Study of Pd Nucleation on a γ-Al2O3 Surface

We perform ab initio molecular dynamics (AIMD) free energy simulations to investigate the stability and dynamics of dispersed Pd atoms and subnanometer clusters on γ-Al2O3. We consider nucleation for varying Pd loadings on dry γ-Al2O3 (100), dry γ-Al2O3 (110), and wet γ-Al2O3 (110) and examine temperature, entropy, and water coverage effects on the stability and dynamics of Pd clusters. At low Pd loading (1.07 Pd atoms/nm2 on the (100) and 0.75 Pd/nm2 on the (110)), association of the Pd atoms is thermodynamically unfavorable on the dry surfaces and almost thermoneutral on the wet (110) owing to water blocking of the AlIII defect sites. In agreement with past studies, the Pd atoms are quite mobile but the diffusion free energy barriers do not increase with the water coverage, as previously suggested, but rather remain similar in magnitude owing to translational entropy at finite temperatures. At these Pd loadings, oxidized Pd(II) atoms on the wet (110) surface are dispersed and immobile. At higher Pd loading (1.12 and 1.5 Pd atoms/nm2) on the (110) surface, Pd3 and Pd4 clusters are thermodynamically very stable and form fast. At the 1.12 Pd atoms/nm2 loading, the formation of the Pd3 cluster is two orders of magnitude slower when the surface is hydroxylated as the system becomes somewhat trapped before the dinuclear cluster Pd2 collides with a Pd atom.