Nanometer-Size Effect on Hydrogen Sites in Palladium Lattice

Abstract

Nanometer-sized materials attract much attention because their physical and chemical properties are substantially different from those of bulk materials owing to their size and surface effects. In this work, neutron powder diffraction experiments on the nanoparticles of palladium hydride, which is the most popular metal hydride, have been performed at 300, 150, and 44 K to investigate the positions of the hydrogen atoms in the face-centered cubic (fcc) lattice of palladium. We used high-quality PdD0.363 nanocrystals with a diameter of 8.0 ± 0.9 nm. The Rietveld analysis revealed that 30% of D atoms are located at the tetrahedral (T) sites and 70% at the octahedral (O) sites. In contrast, only the O sites are occupied in bulk palladium hydride and in most fcc metal hydrides. The temperature dependence of the T-site occupancy suggested that the T-sites are occupied only in a limited part, probably in the subsurface region, of the nanoparticles. This is the first study to determine the hydrogen sites in metal nanoparticles.

Hydrogen diffusion in bulk and nanocrystalline palladium: A quasielastic neutron scattering study

“…It is thus likely that the new fast process arises from the hydrogen atoms in this region. The lattice constant of the Pd nanoparticles is larger than that of the bulk sample [45,49,55], contrary to other metals. It implies that the Pd lattice expands near the surface.…”

“…Such strain and distortion presumably affect the relative stability of interstitial hydrogen sites. Indeed, recent ND work suggests that the D atoms partially occupy the T-sites in a limited area, probably the subsurface region [55].…”

“…Metal nanoparticles show promise as advanced materials with novel electronic, magnetic, optical, and catalytic properties. As for Pd nanoparticles, their properties were investigated by pressure composition isotherm (PCT) [44][45][46][47], heat capacity [33,42], X-ray diffraction (XRD) [48][49][50][51][52], extended X-ray absorption fine structure (EXAFS) [53,54], and neutron scattering measurements [55][56][57][58], inter alia. The PCT measurements for PdH x nanoparticles [44][45][46][47] demonstrated that the miscibility gap between the α and β phases is narrowed on reducing the particle size; the higher absorption ability is achieved in the α phase but the β phase is destabilized.…”

“…Most recently, Akiba and co-workers performed ND measurements for PdD 0.363 with a particle size of 8 nm [55]. Their Rietveld analysis revealed that a relatively large fraction (≈ 31 %) of hydrogen atoms occupy the T-sites even at T = 300 K. It was also shown that the T-site occupation occurs only in a limited area, probably the subsurface region, and its fraction decreases upon cooling.…”

“…Therefore no grain boundaries exist. The nanocrystals used in this work are basically the same as those used in the ND [55] and heat capacity [33] measurements. It is often discussed that the properties near the surface depend on the shape of the particles, the surface facets, etc.…”