Palladium hydride is metallic
palladiumPalladium is a chemical element with the chemical symbol Pd and an atomic number of 46. Palladium is a rare and lustrous silvery-white metal that was discovered in 1803 by William Hyde Wollaston, who named it after the asteroid Pallas, which in turn, was named after the epithet of the Greek...
that contains a substantial quantity of
hydrogenHydrogen is the chemical element with atomic number 1. It is represented by the symbol H. At standard temperature and pressure, hydrogen is a colorless, odorless, nonmetallic, tasteless, highly flammable diatomic gas with the molecular formula H
2...
within its crystal lattice. At
room temperatureRoom temperature is a common term to denote a certain temperature within enclosed space at which humans are accustomed. Room temperature is thus often indicated by general human comfort, with the common range of 68°F to 77°F , though climate may acclimatize people to higher or lower...
and
atmosphericThe Earth's atmosphere is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention , and reducing temperature extremes between day and night...
pressurePressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...
, palladium can
absorbAbsorption, in chemistry, is a physical or chemical phenomenon or a process in which atoms, molecules, or ions enter some bulk phase - gas, liquid or solid material. This is a different process from adsorption, since molecules undergoing absorption are taken up by the volume, not by the surface...
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.
Palladium hydride is metallic
palladiumPalladium is a chemical element with the chemical symbol Pd and an atomic number of 46. Palladium is a rare and lustrous silvery-white metal that was discovered in 1803 by William Hyde Wollaston, who named it after the asteroid Pallas, which in turn, was named after the epithet of the Greek...
that contains a substantial quantity of
hydrogenHydrogen is the chemical element with atomic number 1. It is represented by the symbol H. At standard temperature and pressure, hydrogen is a colorless, odorless, nonmetallic, tasteless, highly flammable diatomic gas with the molecular formula H
2...
within its crystal lattice. At
room temperatureRoom temperature is a common term to denote a certain temperature within enclosed space at which humans are accustomed. Room temperature is thus often indicated by general human comfort, with the common range of 68°F to 77°F , though climate may acclimatize people to higher or lower...
and
atmosphericThe Earth's atmosphere is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention , and reducing temperature extremes between day and night...
pressurePressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...
, palladium can
absorbAbsorption, in chemistry, is a physical or chemical phenomenon or a process in which atoms, molecules, or ions enter some bulk phase - gas, liquid or solid material. This is a different process from adsorption, since molecules undergoing absorption are taken up by the volume, not by the surface...
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 fusionCold fusion refers to a proposed nuclear fusion process of unknown mechanism offered to explain a group of disputed experimental results first reported by electrochemists Martin Fleischmann and Stanley Pons...
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. GrahamThomas Graham FRS was a nineteenth-century Scottish chemist who is best-remembered today for his pioneering work in dialysis and the diffusion of gases.- Life and work :...
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 cubicIn geometry, close-packing of spheres is the construction of an infinite regular arrangement of identical spheres so that they take up the greatest possible fraction of an infinite 3-dimensional space...
(fcc) lattice, which is the same structure as pure palladium metal. At low concentrations up to PdH
0.02 the palladium lattice expands slightly, from 3.889 Å (
ångströmThe ångström or angstrom is an internationally recognized unit of length equal to 0.1 nanometre or 1 metres. It is named after Anders Jonas Ångström...
s) to 3.895 Å. Above this concentration the second phase appears with a lattice constant of 4.025 Å. Both phases coexist until a composition of PdH
0.58 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
octahedralIn geometry, an octahedron is a polyhedron with eight faces. A regular octahedron is a Platonic solid composed of eight equilateral triangles, four of which meet at each vertex.It is a 3-dimensional cross polytope.-Dimensions:...
hole per metal atom). The limit of absorption at normal pressures is PdH
0.7, 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 PdH
0.5 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.