hydrideDivisibleIonic hydride, molecular hydride and metal hydride.Metal hydride is divided into ionic hydride and metal hydride.
Ionic metal hydride
Hydrogen is the same as alkali metal and mostalkaline-earth metalWhen directly combining at a higher temperature, the hydrogen atom obtains an electron, which becomes H-Ion, to generate ionic metal hydride.Common ionic metal hydrides include LiH, NaH, KH and CaHtwo、BaHtwoEtc.
Ionic metal hydride can be generated at a higher temperature because of the reaction
Htwo====H-ΔrH> 0 needs to absorb heat.andhalogenCorresponding reaction of single substance
Xtwo====X-ΔrH<0 is an exothermic reaction, so the ionic hydride is far inferior to the correspondinghalideIt exists universally.
Ionic metal hydrides are white or grayish white crystals, in which LiH and BaHtwoThe thermal stability is relatively high, melting without decomposition at 688.7 ℃ and 1200 ℃ respectively, and other ionic metal hydrides are decomposed into corresponding simple substances before melting.The molten ionic metal hydride conducts electricity.Their properties are similar to those of salts, so they are sometimes called salt hydrides.
Ionic metal hydride can react violently with water to release hydrogen.In non-aqueous solvents (such as ether), ionic hydrides canElectron deficient compoundAssociative generationComplex hydride。Ionic metal hydride and composite hydride have strong reducibility, which can reduce metal chlorides, oxides and oxyacid salts at high temperatures, and can also reduce HtwoH in O, and COtwoRestore to CO.
Metal hydride
Most d and f elements can form metal hydride, while only Cr in VIB group can form hydride.Group VIII Pd can form stable loose phase with hydrogen under appropriate pressure, and its compound composition is PdHx（x<1）。Ni only forms metal hydride under high pressure.Pt cannot form hydride under any conditions, and hydrogen can only form chemisorbed hydride on the surface of Pt.Common metal type metal hydride is CrHtwo、NiH、CuH、ZnHtwo、PdHzero point eightEtc.
Metal type metal hydride basically retains the appearance characteristics of metal, includingMetallic lusterIt has conductivity, and the conductivity decreases with the increase of hydrogen content.Another property of metal type metal hydride is that when the temperature is slightly increased, the H atom diffuses rapidly through the solid.The ordinary hydrogen is diffused through Pd Ag alloy tubes to produce ultrapure hydrogen, which makes use of this feature.[1]
purpose
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Metal hydride, especially ionic metal hydride, is widely used as reducing agent andNegative hydrogen ionIt can be used as a hydrogen generating agent in the field, which is convenient to use but expensive.
In addition, research has proved that some metals have a strong ability to capture hydrogentemperatureUnder pressure and pressure, these metals can "absorb" a large amount of hydrogen, react to generate metal hydride, and release heat at the same time.Later, these metal hydrides will be heated, and they will decompose, releasing the hydrogen stored in them.These metals that will "absorb" hydrogen are calledHydrogen storage alloy。
Hydrogen storage alloys have strong hydrogen storage capacity.Hydrogen storage per unit volumedensityIt is 1000 times of gaseous hydrogen under the same temperature and pressure, which is equivalent to high-pressure hydrogen stored at 1000 atmospheres.
As hydrogen storage alloys are solid, they do not need large and bulky cylinders for storing high-pressure hydrogen or extremely low temperature conditions such as liquid hydrogen. When hydrogen storage is needed, the alloy reacts with hydrogen to generate metal hydride and release heat. When hydrogen is needed, the hydrogen stored in the alloy is released by heating or reducing pressure, just like the charging and discharging of batteries,Therefore, the use of hydrogen storage alloys is an ideal hydrogen storage method that is extremely simple and easy to operate.
The hydrogen storage alloys under research and development mainly include titanium based hydrogen storage alloys, zirconium based hydrogen storage alloys, iron based hydrogen storage alloys and rare earth based hydrogen storage alloys.
Hydrogen storage alloys not only have the ability to store hydrogen, but also can convert chemical energy in the process of hydrogen storage into mechanical energy or thermal energy.Hydrogen storage alloys emit heat when absorbing hydrogen and absorb heat when releasing hydrogen. With this exothermic endothermic cycle, heat can be stored and transmitted, and refrigeration or heating equipment can be manufactured.
Hydrogen storage alloy can also be used to purify and recover hydrogen, which can purify hydrogen to a very high purity[2]。For example, ordinary hydrogen can be obtained by diffusion through Pd Ag alloy tubes.With hydrogen storage alloy, ultrapure hydrogen with purity higher than 99.9999% can be obtained at a very low cost.
The rapid development of hydrogen storage alloys has opened up a broad way for the utilization of hydrogen.
Reaction process
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There are many metal hydride systems, and different types of metal hydride may have different control steps due to different composition elements and crystal structures.For example, for Mg2Fe alloy, the large amount of free Fe element can be used as a catalyst to accelerate the surface reaction rate. In addition, the change of the proportional relationship between the surface and volume will make the control steps mainly the diffusion of hydrogen atoms or the nucleation and growth of hydride.Even for the same metal alloy, its control mechanism is controversial.Taking the most common LaNi5 alloy as an example, the dynamic characteristics of hydrogen absorption between hydrogen and LaNi5 at the initial stage of the two phases were measured through experimentsChemical reaction process。The control mechanism of LaNi5 hydrogen absorption reaction is the internal diffusion process of hydrogen in the ash layer.Its control mechanism should be the adsorption and decomposition process of free hydrogen molecules[3]。
