Hydrogen storage in spherical and platelet palladium nanoparticles

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 m²/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 (α + β-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 α + β 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.