Palladium hydride

Palladium hydride is metallic palladium that contains a substantial quantity of hydrogen within its crystal lattice. At room temperature and atmospheric pressure, palladium can absorb up to 900 times its own volume of hydrogen. This process is reversible. This property has been investigated because hydrogen storage is of such interest and a better understanding of what happens at the molecular level could give clues to designing improved metal hydrides. A palladium based store would be however prohibitively expensive due to the cost of the metal. Palladium electrodes have been used in some cold fusion experiments, under the hypothesis that the hydrogen could be "squeezed" between the palladium atoms to help them fuse at lower temperatures than would otherwise be required. No cold fusion experiments have achieved conclusive positive results, however, and the theoretical ability of palladium to accomplish this is in dispute.

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.

Chemical structure and properties
The absorption of hydrogen produces two different phases, both of which contain palladium metal atoms in a face centred cubic (fcc) lattice, which is the same structure as pure palladium metal. At low concentrations up to PdH0.02 the palladium lattice expands slightly, from 3.889 A to 3.895 A. Above this concentration the second phase appears with a lattice constant of 4.025A. Both phases coexist until a composition of PdH0.58 when the alpha phase disappears. Neutron diffraction studies have shown that hydrogen atoms randomly occupy the octahedral interstices in the metal lattice. (In a fcc lattice there is one octahedral hole per metal atom). The limit of absorption at normal pressures is PdH0.7, indicating that approximately 70% of the octahedral holes are occupied. The absorption of hydrogen is reversible, and hydrogen rapidly diffuses through the metal lattice. Metallic conductivity reduces as hydrogen is absorbed, until at around PdH0.5 the solid becomes a semiconductor.

Surface absorption process
The process of absorption of hydrogen has been shown by scanning tunnelling microscopy to  require aggregates of at least three  vacancies on the  surface of the crystal to promote the dissociation of the hydrogen molecule. The reason for such a behaviour and the particular structure of trimers has been analyzed.

Uses of palladium hydride
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. Hydrogen 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. 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.