Journal of Alloys and Compounds 389 (2005) 234–242 Hydrogen storage in spherical and platelet palladium nanoparticles S. Kishore a , J.A. Nelson b , J.H. Adair b , P.C. Eklund a,b,∗ a Department of Physics, 104 Davey Laboratory, The Pennsylvania State University, University Park, PA 16802, USA b Materials Research Institute and Department of Materials Science and Engineering, The Pennsylvania State University, University Park, PA 16802, USA Received 6 May 2004; accepted 8 June 2004 Abstract The PdH x system is used to investigate possible benefits of hydrogen storage in nanoparticulate matter. Particles of different morphology (platelets/tabular or spherical) were synthesized with minimum dimension in the 4–10 nm range via reverse and bilayer-micellular techniques. High specific surface area (SSA ∼ 40–80 m 2 /g) was obtained for all Pd samples studied. For most particles, it was found that the SSA can only be maintained if processing or H-adsorption temperatures did not exceed T ∼ 50–100 ◦ C. The isothermal hydrogen uptake (to 10 bar) of the nanoparticles was measured gravimetrically at 50 ◦ C and compared with that of bulk powders (∼micron grain Pd). It was noticed that the nanoparticulate isotherm plateaus ( + -phases) were not as flat, or as wide, as in the bulk. Several samples were observed to store 10–20% more than the bulk at 10 bar, suggesting that surface and subsurface sites in nanoparticulate matter provide an additional and significant set of adsorption sites. In fact, using the width of the  +  plateau as a measure of the normal bulk (octahedral, O) site concentration, we can estimate that many of the nanoparticulate samples studied exhibit a larger fraction of subsurface sites than bulk-like O-sites. Post-synthesis hydrazine washing has been observed to be a crucial factor in enhancing the hydrogen uptake performance of the nanomaterials studied—a marked improvement in the washed samples over the unwashed ones suggests a possible removal of some of the disadvantageous organics from the sample surfaces. © 2004 Elsevier B.V. All rights reserved. Keywords: Palladium; Hydrogen storage; Micellular synthesis; Isotherms; Subsurface sites 1. Introduction The Palladium–Hydrogen system is one of the most widely and thoroughly investigated bulk metal–hydrogen systems studied to date, mainly because of the availability of pure samples and the fact that no special surface treatments of the bulk samples are required [1,2]. Palladium belongs to the second transition metal series, with 10 electrons in the 4d electron shell and exhibits high magnetic susceptibility and high electronic specific heat, since its electronic density of states at the Fermi level is the highest of any pure metal. Experimental and theoretical investigations have proved that in transition metals like Pd, the electron accompanying the proton of the dissociated hydrogen atom enters the s- and ∗ Corresponding author. Tel.: +1 814 865 5233; fax: +1 814 865 9851. E-mail address: pce3@psu.edu (P.C. Eklund). d-bands of the host metal, changing the density of states at the Fermi surface and causing shifts of the energy bands [3]. These electronic reactions produce both short-range and long-range interactions between the electronic species. Be- sides bringing about changes in local and global electronic structure of the host metal, hydrogen also induces changes in the cohesive forces between atoms in the host matrix, as evidenced by several theoretical investigations [4,5]. Neu- tron diffraction studies in various metal–hydrogen systems have shown that hydrogen occupies interstitial sites in the metal lattice. This process causes the nearest-neighbor metal atoms to move farther apart from each other, causing an in- crease in the nearest-neighbor separation between atoms. In palladium, the lowest energy sites that are occupied by hy- drogen are the interstitial sites with octahedral symmetry [6]. However, as all the octahedral interstitials are progressively filled by hydrogen, a ‘spill-over’ occurs as hydrogen occu- 0925-8388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2004.06.105