Palladium hydride
Palladium hydride is metallic palladium that contains a substantial quantity of hydrogen within its crystal lattice. It is an alloy presented by a chemical formula PdHx with x < 1. At room temperature, palladium hydrides may contain two crystalline phases, α and β (sometimes called α'). Pure α phase exists at x < 0.017 whereas pure β phase is realised for x > 0.58; intermediate x values correspond to α-β mixtures.[1]
Hydrogen absorption by palladium is reversible and therefore has been investigated for hydrogen storage.[2] 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.
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[edit] 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.[2] Graham produced alloy with composition PdH0.75.[3]
[edit] Chemical structure and properties
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 PdH0.02 the palladium lattice expands slightly, from 3.889 Å 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 PdH0.58 when the alpha phase disappears.[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 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.[3]
[edit] Superconductivity
PdHx is a superconductor with a transition temperature Tc 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.[4][5][6] These results have been questioned[7] and have not been confirmed thus far.
[edit] 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.[8] The reason for such a behaviour and the particular structure of trimers has been analyzed.[9]
[edit] 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. 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. 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]
[edit] See also
[edit] References
- ^ a b Manchester, F. D.; San-Martin, A.; Pitre, J. M. (1994). "The H-Pd (hydrogen-palladium) System". Journal of Phase Equilibria 15: 62. doi:10.1007/BF02667685. Phase diagram for Palladium-Hydrogen System
- ^ a b W. Grochala, P. P. Edwards (2004). "Thermal Decomposition of the Non-Interstitial Hydrides for the Storage and Production of Hydrogen". Chem. Rev. 104 (3): 1283–1316. doi:10.1021/cr030691s. PMID 15008624.
- ^ a b c Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Oxford: Butterworth-Heinemann. pp. 1150–151. ISBN 0080379419.
- ^ Tripodi, P (2003). "Possibility of high temperature superconducting phases in PdH". Physica C 388–389: 571–572. doi:10.1016/S0921-4534(02)02745-4. http://www.heraphysics.it/PhysC388p571.pdf.
- ^ Tripodi, P; Digioacchino, D; Vinko, J (2004). "Superconductivity in PdH: phenomenological explanation". Physica C: Superconductivity 408-410: 350. doi:10.1016/j.physc.2004.02.099.
- ^ Tripodi, Paolo; Di Gioacchino, Daniele; Vinko, Jenny Darja (2007). "A review of high temperature superconducting property of PdH system". International Journal of Modern Physics B 21 (18&19): 3343–3347. Bibcode 2007IJMPB..21.3343T. doi:10.1142/S0217979207044524.
- ^ Baranowski, B.; Dębowska, L. (2007). "Remarks on superconductivity in PdH". Journal of Alloys and Compounds 437: L4. doi:10.1016/j.jallcom.2006.07.082. http://144.206.159.178/FT/537/584536/12434643.pdf.
- ^ T. Mitsui, M. K. Rose, E. Fomin, D. F. Ogletree & M. Salmeron (2003). "Dissociative hydrogen adsorption on palladium requires aggregates of three or more vacancies". Nature 422 (6933): 705–7. doi:10.1038/nature01557. PMID 12700757.
- ^ N. Lopez, Z. Lodziana, F. Illas & M. Salmeron (2004). "When Langmuir is too simple: H2 dissociation on Pd(111)". Physical Review Letters 93 (14): 146103. doi:10.1103/PhysRevLett.93.146103.
[edit] External links
- The Purification of Hydrogen. Platinum Metals Review
