Structural and Thermodynamic Studies of Hydrogen Absorption/Desorption Processes on PdPt Nanoparticles

This work is motivated by an interesting phenomenon discovered by Kobayashi et al., whereby a phase-separated nanoparticle of the 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 Pd_0.8Pt_0.2 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 Pd_0.8Pt_0.2D_0.36 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 Pd_0.8Pt_0.2D_0.50 nanoparticles. These results are consistent with the model in which 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 Pd_0.8Pt_0.2 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.