Sequential H<SUB>2</SUB> Chemisorption and H Desorption on Icosahedral Pt<SUB>13</SUB> and Pd<SUB>13</SUB> Clusters: A Density Functional Theory Study

Abstract

We present a density functional theory (DFT) study on the sequential H-2 dissociative chemisorption and H desorption on icosahedral Pt-13 and Pd-13 clusters. The coverage dependence of the sequential adsorption and desorption energies are given along with the charge transfer from metals to H. At low H coverage, the dissociation takes place at an atop site before diffusion redistributes the atoms to well-separated edge sites. At higher coverage, the edge sites become filled and the cluster undergoes a structure-transition from icosahedral to an fcc-like structure (these transitions occur at 10 H atoms for Pt-13 and 24 H atoms for Pd-13). Upon further H loading, the dissociated H atoms may reside at the cluster surface or be pulled inside to interact with the core metal atom. Ab initio molecular dynamics simulation and bond-distance distribution analysis shows that H-saturation occurs at 44 H atoms for Pt-13 and 30 H atoms for Pd-13. The calculated H-2 dissociative chemisorption energy and H desorption energy at saturation is 0.90 eV and 2.02 eV for Pt-13, and 0.76 eV and 2.04 eV for Pd-13, respectively. These values are comparable with what was reported for smaller close-packed clusters, which suggests that the catalytic performance of these transition metal clusters may not vary significantly with particle size or shape but instead depends primarily on the available surface metal atoms which can be accessed by H atoms.