Despite its name, it is not an ionic hydride but rather an alloy of palladium with metallic hydrogen that can be written PdHx.
At room temperature, palladium hydrides may contain two crystalline phases, α and β (also called α′).
[3] The hydrogen atoms occupy interstitial sites in palladium hydride.
When Pd is brought into a H2 environment with a pressure of 1 atm, the resulting concentration of H reaches x ≈ 0.7.
[4] This is done via three different routes, with measures to prevent the ready desorption of the hydrogen from the palladium.
A Pd sample is placed into a high-pressure cell of H2, at room temperature.
To maintain the high absorption, the pressure cell is cooled to liquid N2 temperature (77 K).
[4] Afterwards the foil is cooled to a temperature of 77 K to prevent a loss of H before the implantation can take place.
At standard temperature and pressure, palladium can absorb up to 900 times its own volume of hydrogen.
[5] Hydrogen can be absorbed into the metal hydride and then desorbed back out for thousands of cycles.
[6] The absorption of hydrogen produces two different phases, both of which contain palladium metal atoms in a face-centered cubic (fcc, rocksalt) lattice, which is the same structure as pure palladium metal.
[1] Neutron diffraction studies have shown that hydrogen atoms randomly occupy the octahedral interstices in the metal lattice (in an fcc lattice there is one octahedral hole per metal atom).
The limit of absorption at normal pressures is PdH0.7, indicating that about 70% of the octahedral holes are occupied.
Metallic conductivity reduces as hydrogen is absorbed, until at around PdH0.5 the solid becomes a semiconductor.
[3] This formation of the bulk hydride does depend on the size of the catalyst palladium.
[7] The most important property of the band structure of PdH(oct) is that filled Pd states are lowered with the presence of hydrogen.
Also, the lowest energy levels, which are the bonding states, of PdH are lower than that of Pd.
[8] Additionally, the hydride formation raises the fermi level above the d-band.
This results in filled p-states and shifts the ‘edge’ to a higher energy level.
The α-phase of PdH lies in the same range of the fermi surface as Pd itself, therefore 𝛼-phase does not influence the susceptibility.
[4] When H is added to the pure Pd, the electronic heat coefficient drops.
However, Zimmerman et al. also measured the heat coefficient γ for a concentration of x = 0.96.
This can be explained by a hardening of the phonon spectrum, which includes a decrease in the electron–phonon constant λ.
Palladium is alloyed with silver to improve its strength and resistance to embrittlement.
To ensure that the formation of the beta phase is avoided, as the lattice expansion noted earlier would cause distortions and splitting of the membrane, the temperature is maintained above 300 °C.
The sticking probability of Palladium was found to be greater at temperatures where the phase of the used Palladium and hydrogen mixture was pure β-phase, which is in this context corresponds to palladium hydride (at 1 bar this means temperatures greater than roughly 160 degrees Celsius), as opposed to temperatures where β- and α-phases coexist and even lower temperatures where there is pure α-phase (α-phase here corresponds to a solid solution of Hydrogen atoms in Palladium).
Knowing these sticking probabilities enables one to calculate the rate of adsorption
Density functional theory was performed to find an explanation for this fact.