Palladium hydride is palladium metal with hydrogen within its crystal lattice. Despite its name, it is not an ionic hydride but rather an alloy of palladium with metallic hydrogen that can be written PdH. At room temperature, palladium hydrides may contain two crystalline phases, α and β (also called α′). Pure α-phase exists at x < 0.017 while pure β-phase exists at x > 0.58; intermediate values of x correspond to α–β mixtures.
Hydrogen absorption by palladium is reversible and therefore has been investigated for hydrogen storage. Palladium electrodes have been used in some cold fusion experiments, under the theory that hydrogen can be "squeezed" between palladium atoms to help it fuse at lower temperatures than normal.
History
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. 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.
Size effect
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. At low concentrations up to PdH the palladium lattice expands slightly, from 388.9 pm to 389.5 pm. Above this concentration the second phase appears with a lattice constant of 402.5 pm. Both phases coexist until a composition of PdH when the alpha phase disappears. The absorption of hydrogen is reversible, and hydrogen rapidly diffuses through the metal lattice. Metallic conductivity reduces as hydrogen is absorbed, until at around PdH the solid becomes a semiconductor.
Additionally, empty Pd states, that are below the fermi energy, are also lowered with the presence of H.
Superconductivity
PdH is a superconductor with a transition temperature T of about 9 K for x = 1. (Pure palladium is not superconducting.) Drops in resistivity vs. temperature curves were observed at higher temperatures (up to 273 K) in hydrogen-rich (x ≈ 1), nonstoichiometric palladium hydride and interpreted as superconducting transitions. These results have been questioned and have not been confirmed thus far.
A great advantage of palladium hydride over many other hydride systems is that palladium hydride does not need to be highly pressurized to become superconducting. This makes measurements easier and gives more opportunity for different kinds of measurements (many superconducting materials require extreme pressure in order to superconduct, on the order of 100 GPa). The reason for such a behaviour and the particular structure of trimers has been analyzed.
Uses
The absorption of hydrogen is reversible and is highly selective. A palladium-based diffuser separator is used, although they are not employed industrially. 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.
Another use of palladium hydride is increased adsorption of H-molecules with respect to pure palladium. In 2009, a study was conducted which tested this fact. 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 <math>r_\text{a}</math> by virtue of the equation
: <math>r_\text{a} = S\Phi_\text{H}</math>
where <math>S</math> is the aforementioned sticking probability and <math>\Phi_\text{H}</math> 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 (<math>r_\text{d}</math>) are equal. This gives
: <math>r_\text{a} = r_\text{d}</math>
The rate of desorption is assumed to be given by a Boltzmannian distribution, i.e.
: (*) <math>r_\text{d} = e^{-\frac{E_\text{d{k_\text{B} T</math>
where <math>C</math> is some unknown constant,<math>E_\text{d}</math> is the desorption energy, <math>k_\text{B}</math> is the Boltzmann constant and <math>T</math> is the temperature.
The relation (*) can be fitted to find the value of <math>E_\text{d}</math>. 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.
See also
- Hydrogen sensor