Palladium hydride

The absorption of hydrogen gas by palladium was first noted by T. Graham in 1866 and absorption of electrolytically produced hydrogen, where hydrogen was absorbed into a palladium cathode, was first documented in 1939.[2] Graham produced an alloy with the composition PdH_0.75.[3]

Metals are arranged in lattices, and in forming metallic hydrides, the hydrogen atoms place themselves in interstitial sites in the lattice. This is also the case for palladium hydride. When the surface of a palladium lattice is brought in contact with a H₂ molecule the two hydrogen atoms split, each absorbed onto an interstitial site. The interstitial placing of hydrogen can lead to a non-stoichiometric mixture, i.e., the ratio of palladium and hydrogen cannot be represented by a natural number.

The ratio in which H is absorbed on Pd is defined by ${\displaystyle x={\frac {[H]}{[Pd]}}}$. When Pd is brought into a H₂ environment with a pressure of 1 atm, the resulting concentration of H reaches x ~ 0.7. However, the concentration of H to obtain superconductivity is higher. Therefore, the concentration of H should be increased, to x > 0.75.[4] This is done via three different routes. It is known that hydrogen easily desorbs from palladium; therefore, extra care should be taken to prevent H desorption from Pd.

The first route is loading from gas phase. A Pd sample is placed into a high-pressure cell of H₂, at room temperature. The H₂ is added through a capillary. As a result, H is loaded onto Pd. To maintain this bonding, the pressure cell will be cooled to liquid N₂ temperature (77 K). The resulting concentration is found to be [H]/[Pd] = 0.97.[4]

The second route is electrochemical bonding. This is a method where the critical concentration for superconductivity can easily be exceeded without using a high-pressure environment, via a reaction as equilibrium between H in an electrochemical phase and H in a solid phase. The hydrogen is added to Pd and Pd–Ni alloys by an H concentration of ~0.95.[4] Thereafter, it has been loaded into electrolysis of 0.1n-H₂SO₄ with a current density of 50 to 150 mA/cm³. Finally, after lowering the loading temperature to ~ 190 K, a H concentration of x ~ 1 has been reached.[4]

The third route is known as ion implantation. Before the implantation of H ions into Pd, the Pd foil was pre-charged with H. This is done in a H₂ high-temperature gas. This shortens the implantation time which follows. The concentration reached is about x ~ 0.7.[4] Afterwards the foil is cooled to a temperature of 77 K to prevent a loss of H before the implantation can take place. The implantation of H in PdH_x happens at a temperature of 4 K. The H ions penetrate in a H₂⁺-beam. This results in a high concentration layer of H in a Pd foil.[4]

Palladium is sometimes metaphorically called a "metal sponge" (not to be confused with more literal metal sponges) because it soaks up hydrogen "like a sponge soaks up water". At room temperature and atmospheric pressure (standard ambient 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. Researchers look for ways to extend the useful life of palladium storage.[6]

The absorption of hydrogen is reversible and is highly selective. Industrially, a palladium-based diffuser separator is used. Impure gas is passed through tubes of thin walled silver-palladium alloy as protium and deuterium readily diffuse through the alloy membrane. The gas that comes through is pure and ready for use. 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.[3]

Another use of Palladium-Hydride is increased adsorption of H₂-molecules with respect to pure Palladium. In 2009, a study was conducted which tested this fact.[16] At a pressure of 1 bar, the probability was measured of Hydrogen molecules sticking to the surface of Palladium versus the probability of sticking to surface of Palladium-hydride. 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 ${\displaystyle r_{a}}$ by virtue of the equation

${\displaystyle r_{a}=S\Phi _{H}}$

where ${\displaystyle S}$ is the aforementioned sticking probability and ${\displaystyle \Phi _{H}}$ is the flux of Hydrogen molecules in the toward the surface of the Palladium/Palladium-hydride.

When the system is in a steady state, we must have that the rate of adsorption and, oppositely, the rate of desorption (${\displaystyle r_{d}}$) are equal. This gives

${\displaystyle r_{a}=r_{d}}$

The rate of desorption is assumed to be given by a Boltzmannian distribution, i.e.

(*)${\displaystyle r_{d}=e^{-{\frac {E_{d}}{k_{B}T}}}}$

where ${\displaystyle C}$ is some unknown constant,${\displaystyle E_{d}}$ is the desorption energy, ${\displaystyle k_{B}}$ is Boltzmann’s constant and ${\displaystyle T}$ is the temperature.

The relation (*) can be fitted to find the value of ${\displaystyle E_{d}}$. It was found that, within the uncertainty of their experiment, the values for of Palladium and Palladium-hydride respectively were roughly equal. Thus Palladium-hydride has as higher average adsorption rate than Palladium, while the energy required for desorption is the same.

Density functional theory was performed to find an explanation for this fact. It was found that the bond of hydrogen with the Palladium-hydride surface is weaker than the bond with the Palladium surface and that the desorption activation barrier is lower by a small amount for Palladium-hydride than for Palladium, although the adsorption barriers are comparable in magnitude. Moreover, the heat of adsorption is lower for Palladium-hydride than for Palladium, which leads to lower equilibrium surface coverage of H. This means that the surface of Palladium-hydride would be less saturated, which leads to greater opportunity for sticking, i.e. a higher sticking probability.

The reversible absorption of Palladium is a means to store hydrogen, and the above findings indicate that even in the hydrogen-absorbed state of Palladium, there is further opportunity for hydrogen storing.

• Hydrogen sensor

• Grashoff, G. J.; Pilkington, C. E.; Corti, C. W. (1 October 1983). "The Purification of Hydrogen" (PDF). Platinum Metals Review. 27 (4): 157–169.
• Altunoglu, Abdulkadir (1994). Hydrogen Permeation Through Nickel And Nickel Alloys: Surface Reactions And Trapping (Thesis). doi:10.21954/ou.ro.00004d82.
• Breger, V.; Gileadi, E. (1 February 1971). "Adsorption and absorption of hydrogen in palladium". Electrochimica Acta. 16 (2): 177–190. doi:10.1016/0013-4686(71)80001-4.