Abstract
The structure at the interface between elements or phases that exhibit different hydrogen (H) binding energies exerts a profound influence on the thermodynamics of H in nanophase materials. In this paper, we study the local structure at the Mg/Pd interface in Mg nanoparticles with partial Pd coating, and we map its evolution in response to annealing and H sorption. This task is accomplished by x-ray photoelectron spectroscopy and x-ray absorption spectroscopy, also including in situ experiments, with the support of crystallographic information from x-ray diffraction. It is shown that the initial Pd surface layer reacts with Mg at relatively low temperatures, leading to irreversible formation of a Mg-rich intermetallic phase Mg${}_{6}$Pd. Due to the high Mg-H binding energy, this phase reversibly transforms, upon H absorption, into a nanophase mixture of magnesium hydride and a Pd-rich intermetallic with H in solid solution, MgPdH${}_{\mathrm{\ensuremath{\delta}}}$. These reversible structural changes are discussed with reference to recent calculations that highlight their relevance to the thermodynamics of the metal-hydride transition. The picture drawn here might be relevant to other multiphase materials presently investigated in the field of hydrogen-related science and technology.
Highlights
The metal-hydride reversible transformation stands out as a prominent example of first-order phase transition in solids.Many elements exhibit this phenomenon at temperatures and hydrogen (H) pressures whose equilibrium values are mostly determined by the metal-H bond energy
In order to track the local structure at the Mg/MgO/Pd interface, we studied three samples obtained from the same NP batch: Sample 1 consists of the as-prepared Mg-Pd NPs; sample 2 was obtained after 1-h annealing at 623 K under dynamic vacuum (1 Pa); and sample 3, representing the hydrogenated state, was subjected to one activation treatment at 638 K under 1.8-MPa H2 pressure in a volumetric Sieverts apparatus, followed by two H release and uptake cycles at 593 K
We have found that the approach which provided good fits of the experimental data while maintaining a good physical insight in the local structure and correspondence to the crystallographic structure of Mg6Pd consisted in grouping the radial distribution function (RDF) in a small number of separate contributions, each representing the average signal from similar atomic configurations; a Gaussian RDF was assumed for each average contribution
Summary
The metal-hydride reversible transformation stands out as a prominent example of first-order phase transition in solids. Many elements exhibit this phenomenon at temperatures and hydrogen (H) pressures whose equilibrium values are mostly determined by the metal-H bond energy. The corresponding enthalpies of hydride formation fH hyd span from the large negative values of extremely stable hydrides such as TiH2, ZrH2, and LaH2 (−126 to −210 kJ/mol H2), to the typical values of interstitial metallic hydrides PdH0.5, VH2, and NbH2 (−40 to −60 kJ/mol H2), up to the slightly negative enthalpies of high-pressure hydrides such as NiH0.5 and AlH3 (−6 to −11 kJ/mol H2). In order to tailor the thermodynamics and the stability of hydride formation, intermetallic compounds (IMCs) of metals with different fH hyd were developed and extensively studied in the past.
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
