Palladium-hydrogen System Research Articles

Phase transformation in the palladium hydrogen system: Effects of boundary conditions on phase stabilities

Different elastic constraint conditions affect the phase stabilities of metal-hydrogen systems. This is studied considering the free energy density within a chemo-mechanically coupled approach with linear elastic deformations and homogeneous concentrations. Utilizing the palladium-hydrogen alloy as a model system, the effects of various mechanical constraints in 1D, 2D and 3D are investigated. These constraints change occurring mechanical deformations and strongly influence the systems chemical potential compared to the unconstrained system. With increasing dimensionality of the constraints, large compressive mechanical stresses occur, which destabilize the hydride phase. This yields a reduced critical temperature of hydride formation. Spinodal and equilibrium miscibility gaps of the system are deduced as a function of the boundary conditions. Notably, the critical temperature of the ideal palladium-hydrogen system with 2D constraints is predicted to be ▪, revealing a driving force for hydride formation at room temperature even under these constraints.

Endergonic Hydrogenation at Ambient Conditions Using an Electrochemical Membrane Reactor.

Here, we determine how the hydrogen loading (x) of an electrochemical palladium membrane reactor (ePMR) varies with electrochemical conditions (e.g., applied current density, electrolyte concentration). We detail how x influences the thermodynamic driving force of an ePMR. These studies are accomplished by measuring the fugacity (P) of hydrogen desorbing from the palladium-hydrogen membrane and subsequently relating P to pressure-composition isotherms to determine x. We find that x increases with both applied current density and electrolyte concentration, but plateaus at a loading of x ≅ 0.92 in 1.0 M H2SO4 at -200 mA·cm-2. The validity of the fugacity measurements is supported experimentally and computationally by: (a) electrochemical hydrogen permeation studies; and (b) a palladium-hydrogen porous flow finite element analysis (FEA) model. Both (a) and (b) agree with the fugacity measurements on the following x-dependent properties of the palladium-hydrogen system during electrolysis: (i) the onset for spontaneous hydrogen desorption; (ii) the point of steady-state hydrogen loading; and (iii) the function describing hydrogen desorption between (i) and (ii). We proceed to detail how x defines the free energy of palladium-hydrogen alloy formation (ΔG(x)PdH), which is a descriptor for the thermodynamic driving force of hydrogenation at the PdHx surface of an ePMR. A maximum value ΔGPdH of 11 kJ·mol-1 is observed, suggesting that an ePMR is capable of driving endergonic hydrogenation reactions. We empirically demonstrate this capability by reducing carbon dioxide to formate (ΔGCO2/HCO2H = 3.4 kJ·mol-1) at ambient conditions and neutral pH.

The high-pressure lithium–palladium and lithium–palladium–hydrogen systems

The lithium–palladium and lithium–palladium–hydrogen systems are investigated at high pressures at and above room temperature. Two novel lithium–palladium compounds are found below {18.7},{mathrm{GPa}}. An ambient temperature phase is tentatively assigned as F{bar{4}}3m,hbox {Li}_{17}hbox {Pd}_{4}, with a = 17.661(1) Å at 8.64 GPa, isostructural with hbox {Li}_{17}hbox {Sn}_{4}. The other phase occurs at high-temperature and is I{bar{4}}3m, hbox {Li}_{11}hbox {Pd}_{2}, a = 9.218(1) Å at 3.88 GPa and 200 ^circ {mathrm{C}}, similar to hbox {Li}_{11}hbox {Pt}_{2}, which is also known at high pressure. The presence of hydrogen in the system results in an I{bar{4}}3m structure with a = 8.856(1) Å at 9.74 GPa. This persists up to {13.3},mathrm{GPa}, the highest pressure studied. Below {2},{mathrm{GPa}} an fcc phase with a large unit cell, a = 19.324(1) Å at 0.39 GPa, is also observed in the presence of hydrogen. On heating the hydrogen containing system at 4 GPa the I{bar{4}}3m phases persists to the melting point of lithium. In both systems melting the lithium results in the loss of crystalline diffraction from palladium containing phases. This is attributed to dissolution of the palladium in the molten lithium, and on cooling the palladium remains dispersed.

On the Mechanism of the Growth of Electronic Conductivity upon Reduction in the Excessive Deformation Volume of a Diluted Palladium–Hydrogen System

On the Mechanism of the Growth of Electronic Conductivity upon Reduction in the Excessive Deformation Volume of a Diluted Palladium–Hydrogen System

Sulfuric Acid Electrolyte Impacts Palladium Chemistry at Reductive Potentials

We report herein that sulfuric acid electrolyte affects the kinetics of hydrogen sorption and desorption, the amount of absorbed hydrogen, and the electrocatalytic activity of palladium using X-ray photoelectron spectroscopy (XPS) and in situ temperature-programmed desorption (TPD). This study utilized a custom TPD instrument tailored to the in situ characterization of palladium films electrochemically saturated with absorbed hydrogen (PdHx for x > 0.6), and we present the first experimental data for the formation of sulfates on palladium hydride at E < 0 V (vs RHE). The surface adsorption of sulfates leads to a retardation of hydrogen sorption and desorption kinetics by increasing the desorption energy of absorbed hydrogen by 92 kJ mol–1 relative to a clean palladium surface. Although previous studies have reported that the SO42– anions of H2SO4 only adsorb on palladium under oxidative potentials E > 0.5 V (vs RHE), the differing chemistry of hydrogen-free and hydrided palladium was not considered. The sulfate adsorbates affect the reaction chemistry through both a competitive binding of surface sites and by altering the electronic structure of surface atoms. The influence of the electrolyte is evidenced by a sluggish kinetics of H absorption and desorption in H2SO4 electrolyte relative to HCl, concomitant with a lower hydrogen uptake in H2SO4 (x = 0.64 ± 0.01) versus HCl (x = 0.697 ± 0.007). The findings of this study emphasize the critical and often overlooked role of the electrolyte utilized in electrochemical studies of the palladium–hydrogen system.

Classifying hydrogen-rich superconductors

The era of near-room-temperature superconductivity started after experimental discovery by Drozdov et al (2015 Nature 525 73) who found that compressed H3S exhibits superconducting transition at Tc = 203 K. To date, the record near-room-temperature superconductivity stands with another hydrogen-rich highly compressed compound, LaH10 (Somayazulu et al 2019 Phys. Rev. Lett. 122 027001), which has critical temperature of In this paper, we analyse available upper critical field, Bc2(T), data for LaH10 (Drozdov et al 2019 Nature 569 528) and report that this compound in all considered scenarios has the ratio of Tc to the Fermi temperature, TF, 0.009 < Tc/TF < 0.038, which is typical range for unconventional superconductors. In attempt to extend our finding, we examined experimental Bc2(T) data for superconductors in the palladium-hydrogen system and surprisingly find that PdHx compounds have the ratio of 0.008 < Tc/TF < 0.012. Taking in account that H3S has the ratio of 0.012 < Tc/TF < 0.039 (Talantsev 2019 Modern Phys. Lett. B 33 1950195) we come to conclusion that in the Uemura plot all discovered to date hydrogen-rich superconductors, i.e. PdHx, H3S and LaH10, lie in same band as all unconventional superconductors, particularly heavy fermions, fullerenes, pnictides, and cuprates, and former should be classified as a new class of unconventional superconductors.

The Electronic State of Hydrogen in the α Phase of the Hydrogen‐Storage Material PdH(D)x: Does a Chemical Bond Between Palladium and Hydrogen Exist?

AbstractThe palladium–hydrogen system is one of the most famous hydrogen‐storage systems. Although there has been much research on β‐phase PdH(D)x, we comprehensively investigated the nature of the interaction between Pd and H(D) in α‐phase PdH(D)x (x&lt;0.03 at 303 K), and revealed the existence of Pd−H(D) chemical bond for the first time, by various in situ experimental techniques and first‐principles theoretical calculations. The lattice expansion, magnetic susceptibility, and electrical resistivity all provide evidence. In situ solid‐state 1H and 2H NMR spectroscopy and first‐principles theoretical calculations revealed that a Pd−H(D) chemical bond exists in the α phase, but the bonding character of the Pd−H(D) bond in the α phase is quite different from that in the β phase; the nature of the Pd−H(D) bond in the α phase is a localized covalent bond whereas that in the β phase is a metallic bond.

The Electronic State of Hydrogen in the α Phase of the Hydrogen-Storage Material PdH(D)x : Does a Chemical Bond Between Palladium and Hydrogen Exist?

The palladium-hydrogen system is one of the most famous hydrogen-storage systems. Although there has been much research on β-phase PdH(D)x , we comprehensively investigated the nature of the interaction between Pd and H(D) in α-phase PdH(D)x (x<0.03 at 303 K), and revealed the existence of Pd-H(D) chemical bond for the first time, by various in situ experimental techniques and first-principles theoretical calculations. The lattice expansion, magnetic susceptibility, and electrical resistivity all provide evidence. In situ solid-state 1 H and 2 H NMR spectroscopy and first-principles theoretical calculations revealed that a Pd-H(D) chemical bond exists in the α phase, but the bonding character of the Pd-H(D) bond in the α phase is quite different from that in the β phase; the nature of the Pd-H(D) bond in the α phase is a localized covalent bond whereas that in the β phase is a metallic bond.

Tritium aging effects on separation factors in palladium-hydrogen system

Abstract The separation factors of the palladium-hydrogen system were experimentally measured at different temperatures ranging from 248 to 303 K after aging for up to 3.5 years by tritium. The results showed a remarkable hydrogen isotope effect in the aged palladium-hydrogen system compared to the raw system. The hydrogen-deuterium separation factor of the palladium-hydrogen system aged by tritium was correlated to temperature and hydrogen/deuterium ratio in the solid phase of palladium hydride.

Change in the crystalline structure during the phase transition of the palladium-hydrogen system.

We performed an X-ray diffraction experiment while a palladium bulk absorbed and desorbed hydrogen to investigate the behavior of the crystalline lattice during the phase transition between the α phase and the β phase. Fast growth of the β phase was observed at around x = 0.1 and x = 0.45 of PdHx, and the phase transition rate has an exponential behavior in between. In addition, slight compression of the lattice at a high hydrogen concentration, an increase in the lattice constant, and broadening of the line width of the α phase after a cycle of absorption and desorption of hydrogen were observed. These behaviors correlated with the change in the sample length, which may infer that the change in shape was related to the phase transition.

Catalytic activity of the palladium-hydrogen system in copper sulfate solutions

The currentless deposition of Cu2+, Ag+, and other cations from solutions of their salts takes place on the surface of hydrogen-saturated palladium (Pd-H). It is hypothesized that this process occurs via a catalytic chemical mechanism. It is demonstrated that 80–90% of the copper deposits onto Pd-H from a CuSO4 solution in the first hour of their contact. The Pd-H system with a thin palladium coating can finely purify solutions from Cu2+ when the amount of hydrogen in Pd exceeds the amount of Cu2+ in the solution.

Nanoindentation of palladium–hydrogen

Nanoindentation has been utilised to track the mechanical effects of hydrogen on palladium foils over a range of hydrogen concentrations. The miscibility gap in the palladium–hydrogen system yields discrete phases over a range of compositions. It is shown that nanoindentation can measure the extent of hydrogen-induced phase transformations across the film thickness after hydrogen removal, with the α → β → α phase transformations yielding a ∼50% increase in local hardness. Interstitial hydrogen was observed to promote work hardening in β phase regions, and a ∼75% increase in hardness was observed in regions where the α phase was saturated with hydrogen.

Tuning the plasmon energy of palladium–hydrogen systems by varying the hydrogen concentration

First-principles calculations are performed to obtain the dielectric function and loss spectra of bulk PdHx. Hydrogen concentrations between x = 0 and 1 are considered. The calculated spectra are dominated by a broad peak that redshifts in energy with x. The obtained bulk dielectric function is employed to compute the loss spectra of PdHx spherical nanoparticles as a function of x. The dominant plasmon peak in the spherical nanoparticle is lowered in energy with respect to the bulk case. However, the dependence of the resonance energy on the hydrogen concentration is roughly similar to that in bulk.

ChemInform Abstract: The Palladium-Hydrogen System

In this review an attempt is made to highlight some of the important properties of the palladium-hydrogen system. (The term hydrogen will be used as a collective term when referring to all three isotopes, but otherwise the names of the specific isotopes, protium, deuterium, and tritium, will be used.) Most of the data in the literature are for the palladium-protium system; generally the three isotopes behave similarly, however, the thermo­ dynamic and kinetic (diffusion) behavior of the isotopes differ quantita­ tively and these differences are discussed below. Lasser recently published a monograph entitled Tritium and Helium-3 in Metals (1), which contains a compilation of the thermodynamic prop­ erties of all three isotopes in palladium and in palladium-silver alloys. Lewis presented a comprehensive review in 1966 (2), certain aspects of which he has updated (3). Wicke & Brodowsky (4) reviewed the thermo­ dynamic and kinetic behavior of palladium-hydrogen and palladium alloy­ hydrogen systems in 1978. In view of the interest in this system, kindled by the recent discovery of possible anomalous effects in the Pd-D system, it appears to be a timely occasion to review the palladium-hydrogen system. The palladium-hydrogen system has long held a key role in metal-

First-principles calculations and tight-binding molecular dynamics simulations of the palladium-hydrogen system

We present a study of palladium-hydrogen system by the linearized augmented plane-wave (LAPW) and Naval Research Laboratory (NRL) tight-binding (TB) methods. We constructed a TB Hamiltonian by fitting to first-principles LAPW data for the electronic energies of a large range of palladium and palladium hydride structures differing in symmetry and compositions as a function of volume. This TB Hamiltonian was then used to calculate phonon frequencies and elastic constants. Our calculations show good agreement with experiments and demonstrate the efficiency of the NRL-TB scheme. In addition, we performed tight-binding molecular dynamics simulations to calculate the density of states, mean-squared displacement, and the formation energy as a function of hydrogen content. We found a relative dip in the lattice energy of structures near the experimental limit of hydrogen content. We calculated the nearest hydrogen-hydrogen distance for various compositions of palladium hydride and confirmed the Switendick criterion.

Hydrogen Storage Mediated by Pd and Pt Nanoparticles

The hydrogen storage properties of metal nanoparticles change with particle size. For example, in a palladium-hydrogen system, the hydrogen solubility and equilibrium pressure for the formation of palladium hydride decrease with a decrease in the particle size, whereas hydrogen solubility in nanoparticles of platinum, in which hydrogen cannot be stored in the bulk state, increases. Systematic studies of hydrogen storage in Pd and Pt nanoparticles have clarified the origins of these nanosize effects. We found a novel hydrogen absorption site in the hetero-interface that forms between the Pd core and Pt shell of the Pd/Pt core/shell-type bimetallic nanoparticles. It is proposed that the potential formed in the hetero-interface stabilizes hydrogen atoms rather than interstitials in the Pd core and Pt shells. These results suggest that metal nanoparticles a few nanometers in size can act as a new type of hydrogen storage medium. Based on knowledge of the nanosize effects, we discuss how hydrogen storage media can be designed for improvement of the conditions of hydrogen storage.

A REVIEW OF HIGH TEMPERATURE SUPERCONDUCTING PROPERTY OF PDH SYSTEM

The discovery of superconductivity in palladium-hydrogen ( PdH ) and its isotopes ( D , T ) at low temperature, brought about extensive study of this system. These studies have shown that the critical transition temperature is a function of the H concentration x in the PdHxsystem with Tc=9 K for x =1. In the last decade we defined a room temperature and room pressure technique to load H and maintain stable the stoichiometry in Pd lattice at levels higher than unit. Several magnetic and electric transport measurements have been performed showing transition temperature in the range of [18 K &lt; Tc&lt; 273 K ]. Moreover in a typical critical current measurement configuration, current density greater than 6*104Acm-2⊐ has been measured at liquid nitrogen temperature. The 263.5K superconducting transition after a week of sample storage at room pressure and temperature, decreased down to 261.5K and after 2 years it became 160.5K, demonstrating a fairly good stability of the sample. Evidences of the flux exclusion (ZFC measurements) and the flux expulsion (FC measurements) have been found at very high transition temperature ( Tc =235 K ) for the PdH system.

The Impact of the Layer Thickness on the Thermodynamic Properties of Pd Hydride Thin Film Electrodes

Recently, a lattice gas model was presented and successfully applied to simulate the absorption/desorption isotherms of various hydride-forming materials. The simulation results are expressed by parameters corresponding to several energy contributions, e.g., interaction energies. However, the use of a model system is indispensable in order to show the strength of the simulations. The palladium-hydrogen system is one of the most thoroughly described metal hydrides found in the literature and is therefore ideal for this purpose. The effects of decreasing the thickness of Pd thin films on the isotherms have been monitored experimentally and subsequently simulated. An excellent fit of the lattice gas model to the experimental data is found, and the corresponding parameters are used to describe several thermodynamic properties. It is analyzed that the contribution of H-H interaction energies to the total energy and the influence of the host lattice energy are significantly and systematically changing as a function of Pd thickness. Conclusively, it has been verified that the lattice gas model is a useful tool to analyze thermodynamic properties of hydrogen storage materials.

H2 cold plasma on Pd∕H system at low hydrogen pressure

The effect of hydrogen cold plasmas, in negative corona regime, was investigated on the palladium-hydrogen system in the α+β region at room temperature and H2 pressures below 0.037bar. Three independent experimental quantities have been simultaneously measured: hydrogen concentration, electrical resistance, and elongation of the samples. An increase in the above quantities was always observed when plasma was lighted. The ability of an external electric field (the corona discharge) to perturb the thermodynamic state of the Pd∕H system was experimentally demonstrated.

Relationship between Palladium Morphology and Thermodynamics in Palladium-Hydrogen System

The pressure-composition (P-C) isotherms of protium and deuterium in Pd sponge particles have been measured over a temperature range from 278K to 323K. Based on these data and combined with literature data, the relationship between the Pd crystal type, particle size and thermodynamic properties in the Pd-H system was investigated. The saturation solubility of hydrogen in solid solution region ( a-phase) and the absorption plateau pressure increase with the decreasing Pd particle size at ambient temperature, but the desorption plateau pressure does not change with Pd morphology. The effect of Pd morphology on above two parameters gets weaker at higher temperature and the difference of plateau pressure among several different Pd morphologies disappears at higher temperature, such as 373K. The absolute value of phase transformation enthalpy and entropy from solid solution phase to hydride phase decrease with the decreasing particle size of Pd, but which are the smallest in single crystal Pd. The degree of hysteresis effect in Pd-H system depends on the background density in the sample, so it is the strongest in Pd sponge.