Hydrogen-metal systems are often prototypes in the study of a number of fundamental physical properties. The extreme simplicity of electronic properties and the low mass of hydrogen atoms make it possible to analyze phenomena at the microscopic level. The following tasks are considered:
- Rearrangement of the electron density near a proton in an alloy with low hydrogen concentrations, including strong electron-ion interaction
- Determination of indirect interaction in a metal matrix through the perturbation of the "electronic fluid" and the deformation of the crystal lattice.
- At high hydrogen concentrations, the problem arises of the formation of a metallic state in alloys with a nonstoichiometric composition.
Alloys hydrogen - metal
Hydrogen localized in the interstices of the metal matrix slightly distorts the crystal lattice. From the point of view of statistical physics, the model of an interacting "lattice gas" is realized. Of particular interest is the study of thermodynamic and kinetic properties near phase transition points. At low temperatures, a quantum subsystem is formed with a high energy of zero-point oscillations and with a large displacement amplitude. This makes it possible to study quantum effects during phase transformations. The high mobility of hydrogen atoms in a metal makes it possible to study diffusion processes. Another area of research is the physics and physical chemistry of surface phenomena of the interaction of hydrogen with metals: the decay of a hydrogen molecule and adsorption on the surface of atomic hydrogen. Of particular interest is the case when the initial state of hydrogen is atomic and the final state is molecular. This is important when creating metastable metal-hydrogen systems.
Application of hydrogen-metal systems
- Hydrogen purification and hydrogen filters
- The use of metal hydrides in nuclear reactors as moderators, reflectors, etc.
- Isotope separation
- Fusion reactors - extracting tritium from lithium
- Water dissociation devices
- Electrodes for fuel cells and batteries
- Hydrogen storage for automotive engines based on metal hydrides
- Heat pumps based on metal hydrides, including air conditioners for vehicles and homes
- Energy converters for thermal power plants
Intermetallic metal hydrides
Hydrides of intermetallic compounds have found wide application in industry. The main part of rechargeable batteries and accumulators, for example, for cell phones, portable computers (laptops), cameras and video cameras contains a metal hydride electrode. Such batteries are environmentally friendly, as they do not contain cadmium.
Typical NiMH Batteries
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Compounds of hydrogen with metals and with non-metals having a lower electronegativity than hydrogen. Sometimes compounds of all elements with hydrogen are classified as hydrides. Classification Depending on the nature of the hydrogen bond, they distinguish ... ... Wikipedia
Compounds of hydrogen with metals or non-metals less electronegative than hydrogen. Sometimes G. is referred to Comm. all chem. elements with hydrogen. Distinguish simple, or binary, G., complex (see, for example, Aluminum hydrides, Metal borohydrides ... Chemical Encyclopedia
Compounds of hydrogen with other elements. Depending on the nature of the hydrogen bond, three types of hydrogen are distinguished: ionic, metallic, and covalent. Ionic (salt-like) minerals include alkali metals and alkaline earth metals. This… …
- (metallides), possess metallic. St. you, in particular electric. conductivity, which is due to metallic. the nature of the chem. connections. To M. s. include Comm. metals with each other intermetallides and many others. conn. metals (mainly transitional) with non-metals. ... ... Chemical Encyclopedia
Boron hydrides, boranes, boron compounds with hydrogen. B. are known that contain from 2 to 20 boron atoms in a molecule. The simplest B., BH3, does not exist in the free state; it is known only in the form of complexes with amines, ethers, and the like. Character… … Great Soviet Encyclopedia
Simple substances that under normal conditions have characteristic properties: high electrical and thermal conductivity, negative temperature coefficient of electrical conductivity, the ability to reflect electromagnetic waves well ... ... Great Soviet Encyclopedia
SUB-GROUP VA. PHOSPHORUS NITROGEN FAMILY The trend of changing properties from non-metallic to metallic, which was revealed in subgroups IIIA and IVA, is also characteristic of this subgroup. The transition to metallicity (although not sharp) begins with arsenic, in ... ... Collier Encyclopedia
- (from lat. inter between and metal) (intermetallic compounds), chemical. conn. two or several metals among themselves. Relate to metallic compounds, or metallides. And. are formed as a result of interaction. components during fusion, condensation from steam ... Chemical Encyclopedia
- (from the Greek metallon originally, mine, mine), in wa, which under normal conditions have characteristic, metallic, high electrical properties. conductivity and thermal conductivity, negative. temperature coefficient. electric conductivity, ability ... ... Chemical Encyclopedia
Metal- (Metal) Definition of metal, physical and chemical properties of metals Definition of metal, physical and chemical properties of metals, application of metals Contents Contents Definition Finding in nature Properties Characteristic properties ... ... Encyclopedia of the investor
It is characteristic that the product of the interaction of hydrogen with thorium, in comparison with the hydrogen derivatives of all other metals, contains the largest amount of hydrogen and corresponds in composition to the ratio ThH 3.75, i.e., approaches the composition corresponding to the maximum valence of the elements of group IV. The density of hydrogen-containing thorium is almost 30% less than the density of the metal, while for the remaining elements of the titanium subgroup, the change in density upon interaction with hydrogen is approximately 15%.
The simplest hydrides of elements of the carbon subgroup - carbon, silicon, germanium, tin, lead are tetravalent and correspond to the general formula MeH 4. The thermal stability of hydrides of group IV elements gradually decreases with increasing atomic weight of these elements and atomic radius.
Vanadium subgroup V groups . The interaction of hydrogen with vanadium, niobium and tantalum is similar in many respects. Chemical compounds of exact stoichiometric composition have not been found in these systems. Since the absorption and desorption of hydrogen cause irreversible changes in the structure of metallic tantalum, the presence in the tantalum-hydrogen system and, apparently, in the niobium-hydrogen system of a certain fraction of chemical bonds of an intermediate type is possible.
Simple hydrides of nitrogen, phosphorus, arsenic, antimony and bismuth have the general formula MeH3. Hydrides of elements of group V are less resistant than elements of groups IV and VI. Most elements of the V group, in addition to simple hydrides of the NH 3 type, also form more complex compounds with hydrogen.
From the elements of the chromium subgroup Group VI - chromium, molybdenum, tungsten and uranium, only uranium hydride UH 3 has been studied. The chemical bond in this compound is explained, possibly, by the presence of hydrogen bridges, but by no means by covalence, which is consistent with the properties of UH 3 . The formation of uranium hydride is accompanied by a sharp (almost 42%) decrease in the density of uranium. This degree of decrease in density is the highest among the studied hydrogen derivatives of metals and, in order of magnitude, corresponds to the increase in density observed during the formation of group I alkali metal hydrides. There is no reliable information about obtaining chemical compounds of exact stoichiometric composition by the interaction of hydrogen with chromium, molybdenum and tungsten.
Hydrides of elements of this group can be obtained by direct interaction of elements with hydrogen. In the series H 2 O, H 2 S, H 2 Se, H 2 Te and H 2 Po, the thermal stability of hydrides decreases rapidly.
Concerning the chemical interaction of hydrogen with elements Group VIII periodic system - iron, nickel and cobalt - there are conflicting data in the literature. Naturally, there are doubts about the real existence of hydrides of these elements. The interaction of hydrogen with iron, cobalt and nickel at elevated temperatures is not a chemical process in the conventional sense. However, this does not yet prove the impossibility of the existence of hydrides of these elements.
Many investigators report obtaining products which they believe are hydrides. So, there is information about the indirect production of iron hydrides - FeH, FeH 2 and FeH 3, stable at temperatures below 150 ° C, above which they decompose. Nickel and cobalt hydrides have also been reported. The resulting products were dark finely dispersed pyrophoric powders. According to some authors, substances of this type, in fact, are not hydrides, but finely dispersed reduced metals containing significant amounts of hydrogen physically adsorbed on the surface. Others believe that the adsorbed hydrogen is on the surface of the metal in an atomic state and forms a chemical bond with the metal atoms.
There is very little consistent data on the chemical interaction of hydrogen with the other elements of group VIII (with the exception of palladium).
In table. Figure 5 shows the available data on the change in the density of metals upon interaction with hydrogen.
While the theory of plate tectonics was celebrating its “victory”, simultaneously gaining cons in the course of further research into the structure of the interior and moving towards its collapse, the theory of the expansion of the Earth solved its two main problems, and at the same time, a variant of such an expansion mechanism was found, which removes all questions along the way. according to "outrageous" pressures in the core.
A way out of the long impasse was proposed about three decades ago by the Soviet scientist Vladimir Larin (now Doctor of Geological Sciences), who, as often happens, approached this problem from a completely different angle.
Rice. 69. Scheme of metal and hydrogen atoms
First of all, the dissolution of hydrogen in a metal turns out to be not just mixing it with metal atoms - at the same time, hydrogen gives its electron to the common piggy bank of the solution, which it has only one, and remains an absolutely “bare” proton. And the size of the proton is 100 thousand times (!) Smaller than the size of any atom, which ultimately (together with the enormous concentration of charge and mass of the proton) allows it even to penetrate deep into the electron shell of other atoms (this ability of a bare proton has already been proven experimentally).
But penetrating inside another atom, the proton, as it were, increases the charge of the nucleus of this atom, increasing the attraction of electrons to it and thus reducing the size of the atom. Therefore, the dissolution of hydrogen in a metal, no matter how paradoxical it may seem, can lead not to the friability of such a solution, but, on the contrary, to compaction of the parent metal. Under normal conditions (that is, at normal atmospheric pressure and room temperature), this effect is negligible, but at high pressure and temperature, it is very significant.
Thus, the assumption that the outer liquid core of the Earth contains a significant amount of hydrogen, firstly, does not contradict its chemical properties; secondly, it already solves the problem of deep storage of hydrogen for ore deposits; and thirdly, which is more important for us, allows a significant compaction of a substance without an equally significant increase in pressure in it.
“At Moscow University, they created a cylinder based on ... an intermetallic compound [an alloy of lanthanum and nickel]. Turn the tap - and a thousand liters of hydrogen are released from a liter cylinder! (M. Kuryachaya, "Hydrides that were not").
But it turns out that all this is “seeds” ...
In metal hydrides - that is, in chemical compounds of a metal with hydrogen - we have a different picture: it is not hydrogen that gives up its electron (to a common rather loose electronic piggy bank), but the metal gets rid of its outer electron shell, forming a so-called ionic bond with hydrogen. At the same time, the hydrogen atom, accepting an additional electron to the same orbit, in which the electron it already has, practically does not change its size. But the radius of an ion of a metal atom - that is, an atom without its outer electron shell - is much less than the radius of the atom itself. For iron and nickel, the ionic radius is about 0.6 of the radius of a neutral atom, and for some other metals, the ratio is even more impressive. Such a reduction in the size of the metal ions allows them to be compacted in the hydride form by several times without any increase in pressure as a consequence of such compaction!..
Moreover, this ability to hypercompact the packing of hydride particles is experimentally detected even under ordinary normal conditions (see Table 1), and at high pressures it increases even more.
Density, g/cm
Metal
hydride
Compaction, %
Tab. 1. The ability to compact some hydrides (under normal conditions)
In addition, the hydrides themselves are also capable of dissolving additional hydrogen in themselves. At one time, they even tried to use this ability of theirs in the development of hydrogen automobile engines for fuel storage.
“... for example, one cubic centimeter of magnesium hydride contains hydrogen by weight one and a half times more than it is contained in a cubic centimeter of liquid hydrogen, and seven times more than in a gas compressed to one hundred and fifty atmospheres!” (M. Kuryachaya, "Hydrides that were not").
One problem - under normal conditions, hydrides are very unstable ...
But we do not need normal conditions, since we are talking about the possibility of their existence deep in the bowels of the planet - where the pressure is much higher. And with increasing pressure, the stability of hydrides increases significantly.
Now experimental confirmation of these properties has already been obtained, and more and more geologists are gradually inclined to believe that the hydride core model may turn out to be much closer to reality than the former iron-nickel model. Moreover, the refined calculations of the conditions in the bowels of our planet reveal the unsatisfactoriness of the "pure" iron-nickel model of its core.
“Seismological measurements indicate that both the inner (solid) and outer (liquid) cores of the Earth are characterized by a lower density compared to the value obtained on the basis of a core model consisting only of metallic iron with the same physicochemical parameters ...
The presence of hydrogen in the core has long been controversial due to its low solubility in iron at atmospheric pressure. However, recent experiments have made it possible to establish that iron hydride FeH can form at high temperatures and pressures and, plunging deep, is stable at pressures exceeding 62 GPa, which corresponds to depths of ~1600 km. In this regard, the presence of significant amounts (up to 40 mol.%) of hydrogen in the core is quite acceptable and reduces its density to values consistent with seismological data"(Yu. Pushcharovsky, "Tectonics and geodynamics of the Earth's mantle").
But the most important thing is that under certain conditions - for example, when the pressure is reduced or when heated - hydrides are able to decompose into components. Metal ions pass into the atomic state with all the ensuing consequences. There is a process in which the volume of matter increases significantly without changing the mass, that is, without any violation of the law of conservation of matter. A similar process also occurs when hydrogen is released from a solution in a metal (see above).
And this already gives a completely understandable mechanism for increasing the size of the planet !!!
“The main geological and tectonic consequence of the hypothesis of the originally hydride Earth is a significant, possibly multiple over the course of geological history increase in its volume, which is due to the inevitable decompression of the planet's interior during the degassing of hydrogen and the transition of hydrides into metals ”(V. Larin,“ Hypothesis of the originally hydride Earth ”).
So, Larin proposed a theory that not only solves some of the problems of ore deposits and explains a number of processes in the history of the Earth (to which we will return), but also provides serious ground for the hypothesis of the expansion of our planet - as a side effect.
Larin did the main thing - he removed all the main problems of the theory of the expansion of the Earth! ..
Only "technical details" remained.
For example, it is absolutely not clear how much our planet has increased over the entire period of its existence, and at what rate its expansion has taken place. Different researchers gave estimates that were very different from each other, in addition, at the same time, they strongly resembled simple finger sucking.
"... in the Paleozoic, according to this hypothesis, the radius of the Earth was about 1.5 - 1.7 times less than the modern one and, therefore, since then the volume of the Earth has increased by about 3.5 - 5 times" (O. Sorokhtin, "The Expanding Earth Catastrophe").
“The most probable ideas seem to me about a relatively moderate scale of the expansion of the Earth, at which from the early Archean (that is, over 3.5 billion years) its radius could increase by no more than one and a half to two times, from the late Proterozoic (that is, over 1, 6 billion years) - no more than 1.3 - 1.5 times, and since the beginning of the Mesozoic (that is, over the last 0.25 billion years) no more than 5, maximum 10 percent ”(E. Milanovsky,“ Earth expanding? Does the earth pulsate?").
Alas. Larin's hypothesis also does not provide a direct answer to this question.
Moreover, all researchers proceeded from the fact that the process proceeds from the very beginning of the formation of the Earth more or less evenly (the author of the hydride theory V. Larin also adheres to this hypothesis). And this leads to such low expansion rates that it is practically impossible to fix it with modern instruments. And the verification of the validity of the theory seems to be only a matter of the distant future.
In the case of hydrogen storage in the hydride form, there is no need for bulky and heavy cylinders required when storing hydrogen gas in a compressed form, or complex and expensive vessels for storing liquid hydrogen. When hydrogen is stored in the form of hydrides, the volume of the system is reduced by about 3 times compared to the volume of storage in cylinders. Simplifies the transport of hydrogen. There are no costs for the conversion and liquefaction of hydrogen.
Hydrogen can be obtained from metal hydrides by two reactions: hydrolysis and dissociation:
By hydrolysis, you can get twice as much hydrogen as it is in the hydride. However, this process is practically irreversible. The method of obtaining hydrogen by thermal dissociation of a hydride makes it possible to create hydrogen accumulators, for which a slight change in temperature and pressure in the system causes a significant change in the equilibrium of the hydride formation reaction.
Stationary devices for storing hydrogen in the form of hydrides do not have strict restrictions on mass and volume, so the limiting factor in the choice of one or another hydride will, in all likelihood, be its cost. For some applications, vanadium hydride may be useful because it dissociates well at a temperature close to 270 K. Magnesium hydride is relatively inexpensive, but has a relatively high dissociation temperature of 560-570 K and a high heat of formation. The iron-titanium alloy is relatively inexpensive, and its hydride dissociates at temperatures of 320-370 K with a low heat of formation.
The use of hydrides has significant safety benefits. A damaged hydrogen hydride vessel is much less dangerous than a damaged liquid hydrogen tank or pressure vessel filled with hydrogen.
It is essential that the binding of hydrogen to the metal proceeds with the release of heat. The exothermic process of hydride formation from hydrogen M metal (charging) and the endothermic process of hydrogen release from hydride (discharging) can be represented as the following reactions:
For the technical use of hydrides, of particular interest are the temperatures at which the pressure of hydrogen dissociation in the hydride reaches a value above 0.1 MPa. Hydrides, in which the dissociation pressure above 0.1 MPa is reached at a temperature below the freezing point of water, are called low-temperature hydrides. If this pressure is reached at a temperature above the boiling point of water, then such hydrides are considered high-temperature.
For the needs of road transport, hydrides are created, which theoretically can contain up to 130-140 kg of hydrogen per 1 m 3 of metal hydride. However, the realizable capacity of the hydride is unlikely to exceed 80 kg/m 3 But even such a hydrogen content in a tank with a capacity of 130 dm 3 is sufficient for 400 km of a car run. These are real indicators for the application, but the increase in the mass of the tank filled with hydride should be taken into account. For example, the mass of latan-nickel hydride reaches 1 ton, and magnesium hydride - 400 kg.
To date, metal hydrides with a wide range of properties have been synthesized and studied. Data on the properties of some hydrides, which are of the greatest potential interest for industrial use, are given in table. 10.3 and 10.4. As can be seen from Table. 10.3, for example, magnesium hydride makes it possible to store 77 g of H 2 per 1 kg of hydride mass, while in a cylinder under a pressure of 20 MPa there are only 14 g per 1 kg of capacity. In the case of liquid hydrogen, 500 g per 1 kg container can be stored.
The Comprehensive Program for Exploration, Research and Development in Hydrogen Energy and Fuel Cells plans to study palladium. The platinum group metal palladium is one of the main materials for fuel cells and all hydrogen energy. Catalysts, membrane devices for producing pure hydrogen, materials with enhanced functional characteristics, fuel cells, electrolyzers, and sensors for determining hydrogen are manufactured on its basis. Palladium can efficiently store hydrogen, especially palladium nanopowder.
In addition to hydrogen energy, palladium is used in catalysts for post-treatment of exhaust gases from conventional vehicles; electrolyzers for obtaining hydrogen and oxygen by decomposition of water; portable fuel cells, in particular methanol; solid oxide electrolyzers with palladium-based electrodes; devices for obtaining oxygen from the air, including for medical purposes; sensors for the analysis of complex gas mixtures.
It is important to note that our country controls about 50% of the world production of this metal necessary for hydrogen production. At present, the Institute of Problems of Chemical Physics of the Russian Academy of Sciences in Chernogolovka is working on the creation of hydrogen accumulators based on metal hydrides.
Properties of some hydrides
Table 10.3
