Systematics and properties
There are stoichiometric metal hydrides (examples are the hydrides of the alkali and alkaline earth metals), polymeric metal hydrides (hydrides of aluminum, beryllium and magnesium), the so-called complex metal hydrides ( alanates , hydroborates ) and non-stoichiometric metal hydrides.
Metal hydrides can have predominantly ionic or predominantly covalent bonding relationships. Hydrogen in compounds with the elements of the first - (Li) or second period - (Be) of the periodic table shows both negative ( e.g. lithium hydride , beryllium hydride or borohydride ) and positive ( e.g. methane , ammonia , water and hydrogen fluoride ) Polarity. The oxidation number of hydrogen in a metal hydride is - I instead of + I. As in the normal case, hydrogen can therefore be understood in its compounds as either a hydride or a proton , whereby the transition is fluid. Accordingly, a subdivision into salt-like, covalent or metallic metal hydrides is common. The extension of this system to combinations of metals that are capable of forming salt-like (A) and metallic (M) hydrides leads to ternary hydrides A x M y H z , which can be understood as hydrido complexes or hydridometallates.
The salt-like metal hydrides include stoichiometrically composed hydrides of the alkali metals and the alkaline earth metals (with the exception of beryllium ). These are colorless solids and form typical salt-like (ionic) structures with hydridic hydrogen (H - ). The alkali metal hydrides lithium hydride, sodium hydride , potassium hydride , rubidium hydride and cesium hydride crystallize in the sodium chloride type and the alkaline earth metal hydrides calcium hydride , strontium hydride and barium hydride in the lead dichloride type. The less ionic magnesium dihydride crystallizes in the rutile type. Most salt-like metal hydrides are synthesized by heating the metals under hydrogen. When these hydrides come into contact with water, violent reactions occur with evolution of hydrogen, with heavy alkali metal hydrides igniting even in moist air. Compared to the strongly electropositive metals of the hydrides, the hydrogen behaves like an electronegative component (for example of the type of the chloride ion). In fact, hydride ions were found in the melts of these compounds, with lithium hydride being particularly thermally stable. Such hydrides are strong reducing agents and are protonated by water and Brønsted acids with the evolution of hydrogen.
To the covalent metal hydrides include hydrides of groups 11 and 12 ( copper hydride , Goldhydrid , zinc hydride , cadmium hydride , Quecksilberhydrid ), with the exception of Silberhydrid , as well as aluminum hydride , Galliumhydrid and beryllium. Since these covalent metal hydrides are only stable at low temperatures (gold hydride decomposes at room temperature), they are prepared by hydridolysis. For this purpose, metal halides are reacted with hydridic hydrogen in organic solvents (lithium hydride, sodium borohydride or lithium aluminum hydride are used for this ).
As metal-like metal hydrides , the hydrides of are the transition metals of groups 3-6 and 10 as well as metals of the lanthanides and actinides respectively. They form binary hydrides with hydrogen, which can be produced by direct reaction of high-purity metal powders with hydrogen at high temperatures and often under pressure. This results in non-stoichiometric metal hydrides with a large phase width. Most of them have a metallic appearance and metallic or semiconducting properties. The crystal structure of metal-like metal hydrides is based on the closest packing of metal atoms, the gaps of which are filled by hydrogen atoms. Hence, they can be considered as storage compounds. When hydrogen atoms are incorporated into a metal structure, a solid solution (α-phase) with a relatively low hydrogen content is initially created in which the metal structure remains unchanged. The boundary compositions MH (octahedron gaps), MH 2 (tetrahedron gaps) and MH 3 (tetrahedron and octahedron gaps) can be realized by filling further gaps . One cubic meter of iron can e.g. B. 19, one cubic meter of gold 46, one cubic meter of platinum 50 and one cubic meter of palladium even absorb 500–900 m 3 of hydrogen in an alloy-like manner. With hydrogen storage, the hydrogen molecules adsorbed on the metal surface are first split into hydrogen atoms and these are then absorbed into the lattice. The combination of two metal hydrides that form salt-like (or metal-like) hydrides again results in a salt-like (or metal-like) ternary metal hydride.
Metal hydrides are either salt-like or resemble solutions of hydrogen in metal or alloys . Hydrogen molecules are first adsorbed on the surface of the metal and then incorporated into the metal lattice as elementary hydrogen . This creates a very brittle metal hydride, which is, however, insensitive to air and water.
The mechanism of the uptake of hydrogen was unknown for a long time, because the uptake of hydrogen changed the crystal structure of the previously known metal hydrides, making modeling and theoretical calculations impossible. However, the alloy LaMg 2 Ni has a strictly ordered crystal structure that is retained even after hydrogen absorption. This made it possible to determine that the hydrogen atoms penetrate the metal lattice via the regular spaces and acquire one of the freely movable electrons in the alloy. In this way, the hydrogen atoms can chemically bond with the nickel atoms: insulating NiH 4 molecules are created. The concentration of the absorbed hydrogen depends strictly on the number of free electrons in the alloy.
Metal hydrides have important uses in synthesis, catalysis (palladium and nickel) and in technical applications. Lithium aluminum hydride is used as a selective hydrogenation reagent in organic synthesis. Thermally stable metal hydrides such as zirconium hydride are used as an alternative to elements with a low atomic number (beryllium, carbon) as neutron catchers (moderators) in nuclear power plants.
In general, metals absorb hydrogen gas at a certain temperature and pressure (the corresponding metal hydrides are formed) and release it again when the temperature is slightly increased or the pressure is decreased. For this reason, metal hydrides are used technically in metal hydride stores for hydrogen. Since metal hydrides will always be 10 to 20 times heavier than a hydrocarbon tank (petrol, heating oil) due to their high specific metal weight compared to carbon, it follows that the use of metal hydrides also involves the weight problem of all hydrogen storage media. Due to the reversible chemical bond of hydrogen in metals, the hydrides are not only hydrogen, but also heat storage. This heat-hydrogen coupling enables technical applications that make the use of hydrides both mobile and stationary appear interesting. In the case of metal hydrides, with the same amount of stored hydrogen, the equilibrium pressure above the hydride is one to three orders of magnitude lower than the values for gaseous storage in high-pressure cylinders. The volume-related hydrogen density in the hydride is generally greater than in the case of hydrogen in liquid form. Due to the density of the alloys used for storage, the weight-related energy densities are only between 2 and 8 MJ / kg hydride (compared to ~ 40 MJ / kg for gasoline and heating oil). Metal hydrides are also used in reversible chemical systems for energy storage, including numerous metal hydride-metal systems. A special feature of these metal hydride storage systems is that the hydrogen released during thermal loading can also be used as fuel. This means that metal hydride storage systems can be viewed as both heat and hydrogen storage (fuel storage). Depending on the decomposition temperature, metal hydrides are divided into low (50 ° C .. -30 ° C), medium (200 ° C .. 100 ° C) and high temperature hydrides (> 200 ° C). The low-temperature hydrides are suitable as working materials for heat pumps , air conditioning systems and for cooling. High-temperature hydrides based on magnesium or magnesium alloys are being investigated as heat storage systems for temperatures up to 400 ° C, for example in connection with decentralized solar thermal systems for generating electricity using a Stirling engine or steam turbine . Low temperature hydrides and medium temperature hydrides have a heat storage density of up to 0.3 MJ / kg (TiFeH 2 , CaNi 5 H 6 ) or 1.5 * 10 3 MJ / m 3 in a temperature range of -20 ° C to 200 ° C under a hydrogen pressure of about 10 bar. High temperature hydrides have a heat density of up to 3 MJ / kg (MgH 2 , Mg 2 NiH 4 ) or 6 * 10 3 MJ / m 3 in a temperature range from 200 ° C to 700 ° C (TiH 2 : 3 MJ / kg, 1, 5 10 4 MJ / m 3 , T = 500 ° C) and under a hydrogen pressure> = 1 bar. For hydrides of metallic alloys it follows from theoretical considerations that the upper limit of the storage capacity of low-temperature hydrides is between 2.3 and 2.5% by weight of hydrogen and for high-temperature hydrides it is around 8% by weight of hydrogen (MgH 2 ). In the crystalline state of the alloys, only slight improvements in terms of hydrogen storage densities can therefore be expected. A further increase in the hydrogen uptake of an alloy could be achieved by producing the metallic alloys in the amorphous or rapidly quenched state (not crystalline).
However, metal hydrides can also be found in metals that have been exposed to hydrogen for a long time because they are formed there unintentionally. The formation of metal hydrides in metals on contact with hydrogen can also lead to hydrogen embrittlement . For example, autoclaves for the synthesis of ammonia are lined with soft iron sheet according to the Haber-Bosch process and the pressure-resistant steel jacket is provided with drill holes to prevent this hydrogen embrittlement.
There are alloys (such as LaMg 2 Ni) that are electrical conductors, but become insulators as soon as they are saturated with hydrogen (then: LaMg 2 NiH 7 ). Because of these two properties, such alloys could be used in the development of sensitive hydrogen detectors.
Another application of metal hydride is, for example, the nickel-metal hydride accumulator .
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