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C: Physical Processes in Nanomaterials and Nanostructures
Structural and Thermodynamic Studies of Hydrogen Absorption/Desorption Processes on PdPt Nanoparticles Hiroshi Akiba, Hirokazu Kobayashi, Hiroshi Kitagawa, Kazutaka Ikeda, Toshiya Otomo, Tomokazu Yamamoto, Syo Matsumura, and Osamu Yamamuro J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b11380 • Publication Date (Web): 19 Mar 2019 Downloaded from http://pubs.acs.org on March 19, 2019
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The Journal of Physical Chemistry
Structural and Thermodynamic Studies of Hydrogen Absorption/Desorption Processes on PdPt Nanoparticles Hiroshi Akiba1, Hirokazu Kobayashi2,3, Hiroshi Kitagawa2, Kazutaka Ikeda4, Toshiya Otomo4, Tomokazu Yamamoto5, Syo Matsumura5,6, Osamu Yamamuro1* 1
Institute for Solid State Physics, University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba, 277-8581, Japan Department of Chemistry, Graduate School of Science, Kyoto University, Kitashirakawa-Oiwakecho, Sakyo-ku, Kyoto 6068502, Japan 3 JST, PRESTO, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan 4 Institute of Materials Structure Science, High Energy Accelerator Research Organization, 203-1 Shirakata, Tokai, Ibaraki 319-1106, Japan 5 Department of Applied Quantum Physics and Nuclear Engineering, Kyushu University, Motooka 744, Nishi-ku, Fukuoka 819-0395, Japan 6 The Ultramicroscopy Research Center, Kyushu University, Motooka 744, Nishi-ku, Fukuoka 819-0395, Japan 2
ABSTRACT: This work is motivated by an interesting phenomenon discovered by Kobayashi et al., whereby a phase-separated nanoparticle of Pd-core and Pt-shell is mixed into a solid solution alloy nanoparticle by repeating hydrogen absorption/desorption processes at 373 K. To investigate the structural change, including the positions of hydrogen atoms and the thermodynamic aspect, we measured the neutron diffraction and enthalpy of hydrogen absorption for Pd0.8Pt0.2 nanoparticles (diameter: 5.0 ± 1.1 nm). Rietveld and atomic pair distribution function (PDF) analyses revealed that D atoms are located at the interstitial octahedral (O) and tetrahedral (T) sites in the solid solution Pd0.8Pt0.2D0.36 nanoparticles, while D atoms are not located at the interstitial sites but trapped somewhere, probably at the surface and at the core-shell interface, in core-shell Pd0.8Pt0.2D0.50 nanoparticles. These results are consistent with the model that hydrogen atoms play a role in creating defects around the interface to lower the activation energy of the mixing process. The enthalpies of H2 and D2 absorptions in the solid solution Pd0.8Pt0.2 nanoparticles at 298 K and 0.1 MPa are – (20.7±0.1) kJ(H-mol)–1 and –(20.1±0.2) kJ(D-mol)–1, respectively. Both of these values are larger than the corresponding values in Pd nanoparticles, suggesting that the hydrogen absorption sites are stabilized by adding Pt atoms, even though Pt itself does not absorb hydrogen. This unusual and interesting effect is discussed on the basis of the structural and thermodynamic data obtained in this work.
Koyama12. Furthermore, Kofu et al. observed the characteristic diffusional and vibrational motions of H atoms in Pd nanoparticles by quasielastic13 and inelastic14 neutron scattering experiments, respectively. There are two primary motions of H atoms: one is largely the same motion as in the bulk sample, which originates from H atoms located at the O-sites, and the other is a new motion related to the T-site occupation. Various combinations of metals, which cannot be mixed in bulk scale, form solid solutions in nanoparticles15,16. Pd and Pt metals, both of which take the fcc structure, form a bulk solid solution only at a significantly higher temperature than room temperature17 (1043 K). However, Kobayashi et al.18 found that phase-separated nanoparticles of a Pd-core and Pt-shell are mixed into a solid solution alloy by repeating hydrogen absorption/desorption processes (HADP) at 373 K. An ab initio totalenergy calculation for PdxPt711–x nanoparticles consisting of 711 atoms (diameter: ca. 3 nm) demonstrated that the solid solution state without hydrogen is thermodynamically more stable than the core-shell state19. This result is due to the difference in surface energies between Pd and Pt; the bulk solid solution state is unstable. On the basis of this calculation, we propose a possible
1. INTRODUCTION Nanometer-sized materials attract significant attention since their physical and chemical properties are substantially different from those of bulk materials owing to their size and surface effects1,2. For the nanoparticles of palladium hydride (PdHx, 0 < x
