RESUMO
Detailed density functional theory calculations have been performed to investigate the adsorption and diffusion of the Rh and Au adatom on the graphene moiré superstructure on Ru(0001). The adsorption energies of each adatom in all of the non-equivalent C-top and C6 ring center sites on the graphene moiré have been calculated. The resulting potential energy surfaces encompass the entire graphene moiré unit cell and shows that the adsorption of both Rh1 and Au1 is most stable in the fcc region on the graphene moiré. The minimum-energy diffusion path between adjacent moiré cells is identified to run mostly directly between the fcc and hcp regions for Au1, but deviates toward the mound region for Rh1. The global diffusion barrier is estimated to be 0.53 eV for Rh1 and 0.71 eV for Au1, corresponding to a hopping rate between adjacent moiré cells of ~10(3) s(-1) and ~1 s(-1) at 298 K, respectively. The consequences of different hopping rates to cluster nucleation have been explored by performing Monte Carlo-based statistical analysis, which suggests that diffusing species other than adatoms need to be taken into account to develop an accurate description of cluster nucleation and growth on this surface.
RESUMO
Au deposited on Ru(0001)-supported extended, continuous graphene moiré forms large 2-D islands at room temperature that are several nanometers in diameter but only 0.55 nm in height, in the apparent absence of typical binding sites such as defects and adsorbates. These Au islands conform to the corrugation of the underlying graphene and display commensurate moiré patterns. Several extended Au structure models on graphene/Ru(0001) are examined using density functional theory calculations. Close-packed Au overlayers are energetically more stable, but all interact weakly with the support. Preliminary tests found the Au islands/graphene/Ru(0001) surface to be active for CO oxidation at cryogenic temperature, which suggests that the Au itself is the locus of catalytic activity.
RESUMO
First-principles calculations offer a useful complement to experimental approaches for characterizing hydrogen permeance through dense metal membranes. A challenge in applying these methods to disordered alloys is to make quantitative predictions for the net solubility and diffusivity of interstitial H based on the spatially local information that can be obtained from first-principles calculations. In this study, we used a combination of density functional theory calculations and a cluster expansion method to describe interstitial H in alloys of composition Pd96M4, where M=Ag, Cu, and Rh. The cluster expansion approach highlights the shortcomings of simple lattice models that have been used in the past to study similar systems. We use Sieverts' law to calculate H solubility and a kinetic Monte Carlo scheme to find the diffusivity of H in PdAg, PdCu, and PdRh alloys at a temperature range of 400
