A First-Principles Study of Pd-Pt Nanoclusters and their Hydrogen Adsorption Properties
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A First-Principles Study of Pd-Pt Nanoclusters and their Hydrogen Adsorption Properties Teck L. Tan and Kewu Bai Institute of High Performance Computing, 1 Fusionopolis Way, #16-16 Connexis, Singapore 138632, Singapore. ABSTRACT To demonstrate the effects of particle size and alloying on hydrogen adsorption on metals, we explore stable configurations of Pd-Pt alloy using a 55-atom cubo-octahedron. Via first-principles based cluster expansion method, we obtained groundstate configurations and show how their hydrogen adsorption energies change with Pd-Pt composition. Comparison with surface adsorption energies further shows the effect of particle size. INTRODUCTION With the world pushing for clean energy sources and a hydrogen based economy, chemical reactions involving hydrogen are increasingly being researched on. For hydrogen fuel cells and electrochemical cells, the hydrogen evolution and oxidation reactions play important roles. Theoretically, there exists an optimal hydrogen adsorption energy for which these reactions are most efficient. Among transition metals surfaces, Pt is closest to this optimal energy. However, Pt is costly and there is room for improving its catalytic activity. The use of nanoclusters (large surface to volume ratio) offers a solution although adsorption properties are altered by size effect. Using first-principles calculations, we show the adsorption energies may be tuned back to the desired level by alloying with Pd. In our theoretical study, we shall focus on particle size of the order of 1nm (nanoclusters), for which commonly observed morphologies include the icosahedron, the truncated octahedron and the cubo-octahedron, with the latter two being derived from the FCC lattice. Nanoclusters are modeled using a 55-atom cubo-octahedron (~1nm in diameter) with each site being occupied by either a Pd or Pt atom. In addition to their common occurrence in experiments, the cubo-octahedron structure of Pt is also stabilized by adsorption of atomic species such as hydrogen [1]. Using first-principles calculations from density functional theory (DFT), we evaluate the energies of Pd-Pt cubo-octahedron for various alloy configurations. We then identify the groundstate configurations at each composition using the cluster expansion (CE) method and study their adsorption properties. THEORY We used the Vienna ab-initio package (VASP) [2,3] to obtain the DFT energies of the nanoclusters and surfaces. Calculations are done using a Projected Augmented Wave (PAW) basis [4], within the generalized-gradient approximation (GGA) using the PW91 exchangecorrelation functional. Plane wave cutoffs from 400 up to 460 eV are used. For nanoclusters, we use a large supercell (19.2 x 19.2 x 19.2 Å3), which is sufficient to prevent spurious interactions between repeating nanoclusters. We found that a 1 x 1 x 1 k-point mesh (at Τ point) is sufficient to give reliable results. The nanoclusters are fully relaxed so that the magnitude of the forces on
each atom is below 0.02eV/Å. For calculations involving hydrogen adsorpt
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