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Hydrides of transition elements. Storage of hydrogen in metals Metals that do not dissolve hydrogen

Nickel hydride

NiH (g). The thermodynamic properties of gaseous nickel hydride in the standard state at temperatures of 100 - 6000 K are given in Table. NiH.

The IR spectrum of NiH and NiD molecules in a low-temperature matrix [78WRI/BAT, 97LI/VAN] was studied. The fundamental frequencies of molecules in matrices of Ne, Ar, Kr, as well as the transitions X 2 2 ∆ 3/2 - X 1 2 Δ 5/2 (928 and 916 cm -1 , respectively in Ar and Kr) and 2 Π 3/2 - X 1 2 Δ 5/2 (2560cm -1 in Ar). The vibrational-rotational [ 88NEL/BAC, 89LIP/SIM ] and rotational [ 88BEA/EVE, 90STE/NAC ] spectra of NiH and NiD molecules have been studied. The photoelectron spectrum of NiH - and NiD - [ 87STE/FEI ] was obtained. The spectrum is interpreted as transitions from the ground state of the anion to the ground and several excited states of the neutral molecule: X 2 Δ, B 2 Π, A 2 Σ and states with energies of 7400 and 11600 cm -1, which are considered as 4 D and overlapping 4 P and 4 S predicted in [ 82BLO/SIE ].

There are a number of abinitio calculations [82BLO/SIE, 86CHO/WAL, 86ROH/HAY, 90HAB, 90MAR] describing the electronic structure of NiH. Calculations [ 90HAB, 82BLO/SIE, 86CHO/WAL ], as well as a study of the dipole moment [ 85GRA/RIC ], showed that the bond in the ground X 2 Δ state of the NiH molecule arises mainly from the asymptote 3d 9 4s with a small admixture of character 3 d 8 4s 2. Most of the calculations are devoted to the study of three states X 2 Δ, A 2 Σ, B 2 Π, forming, according to the latest interpretation (Ni + 3 d 9 2 D)-supermultiplet [ 82BLO/SIE, 86ROH/HAY, 90MAR, 91GRA/LI2 ] and are in good agreement with the experimental data. The calculation of [ 82BLO/SIE ] in accordance with the experimental study [ 91KAD/SCU ] showed that in the energy range above 5000 to ~ 32000 cm-1 there are superconfiguration states d 8 σ 2 σ * (σ and σ * - bonding and loosening molecular orbitals formed by 1 s atom H and 4 s Ni atom). In the energy range from 32000 cm -1 to 40000 cm -1, the calculation [ 82BLO/SIE ] gives the states (with a total statistical weight p=20) belonging to the superconfiguration d 9 σσ * . The experimentally observed states were included in the calculation of the thermodynamic functions X 2 Δ, BUT 2 Σ, B 2 a. The energies of states above 5000 cm -1 were taken from the calculation data [ 82BLO/SIE ], taking into account that the calculation gives energies underestimated by 2000 - 3000 cm -1, and the statistical weights of all excited states are grouped at fixed energies. At energy levels above the dissociation energy, the statistical weight estimated from the [82BLO/SIE] data was halved under the assumption that only half of the states are stable. The error in the energies of the estimated states is assumed to be 10%.

The vibrational constants in the ground X 2 Δ state were calculated from the values ​​of ΔG 1/2 and ΔG 3/2 found in [90KAD/SCU] based on an analysis of the rotational structure of the bands associated with transitions to X 2 Δ 5/2 (v = 0, 1 and 2).

The rotational constants in the ground state are calculated from the values B 0 and D 0 [ 87KAD/LOE ], determined by the Hill and Van Vleck formula for doublet states when processing state terms X 2 Δ (v = 0, J < 12.5), и постоянной α, полученной в работе [ 88NEL/BAC ] в результате анализа колебательно-вращательного спектра. Принятые значения хорошо согласуются с приведенными в [ 84ХЬЮ/ГЕР ]. Небольшое различие с результатами последних работ [ 88NEL/BAC, 91GRA/LI2 ] связано с различными методами обработки данных.

The molecular constants in the A 2 S and B 2 P states were taken from the data of [91GRA/LI2], where they were obtained as a result of joint processing of all experimental data on the vibrational-rotational levels of the states forming the (Ni + 3d 9 2 D)-supermultiplet [ 88NEL/BAC, 90KAD/SCU, 91KAD/SCU, 90HIL/FIE].

The thermodynamic functions of NiH(g) were calculated using equations (1.3) - (1.6) , (1.9) , (1.10) , (1.93) - (1.95) . The values ​​of Q and its derivatives were calculated according to equations (1.90) - (1.92) taking into account eleven excited states (Ω-components of X 2 Δ and В 2 P states were considered as separate states of the case from Gunda) on the assumption that Q no.vr ( i) = (pi/p X)Q no.vr ( X) . Vibrational-rotational partition function of the state X 2 D 5/2 and its derivatives were calculated by equations (1.70) - (1.75) by direct summation over energy levels. The calculations took into account all energy levels with values J < J max ,v , where J max ,v was found from conditions (1.81) . Vibrational-rotational levels of state X 2 D 5/2 were calculated by equations (1.65) , (1.41) , coefficients Y kl in these equations were calculated using relations (1.66) for the isotopic modification corresponding to the natural mixture of nickel isotopes from the 58 Ni 1 H molecular constants given in Table Ni.7 . Coefficient values Y kl , as well as the quantities v max and J lim are given in Table Ni.8.

The main errors in the calculated thermodynamic functions of NiH(g) at temperatures of 1000–6000 K are due to the error in the fundamental constants. At temperatures above 3000 K, errors become noticeable due to the uncertainty in the energies of the excited electronic states. Errors in the values ​​of Φº( T) at T= 298.15, 1000, 3000 and 6000 K is estimated at 0.02, 0.06, 0.2, and 0.6 J× K -1 × mol -1 , respectively.

The thermodynamic functions of NiH(g) were previously calculated without taking into account excited states up to 5000 K [74SCH], before 2000 To[ 76MAH/PAN ] and up to 1000 K[ 81XAR/KRA ]) in the rigid rotator-harmonic oscillator approximation. In this regard, the comparison of the calculated functions is not carried out.

The equilibrium constant of the reaction NiH(g) = Ni(g) + H(g) is calculated from the value:

D° 0 (NiH) = 254 ± 8 kJ × mol -1 = 21300 ± 700 cm.

The value was taken based on the results of mass spectrometric measurements by Kant and Moon (Ni(g) + 0.5H 2 (g) = NiH(g), 1602-1852K, 21 measurements, D r H° (0) = -38.1 ± 8 kJ× mol -1 (III law of thermodynamics) [79KAN/MOO]). The error is associated with the inaccuracy of the ionization cross sections and with the inaccuracy of the thermodynamic functions of NiH (approximately 5-6 kJ × mol -1 due to each). Processing using the II law leads to the value D° 0 (NiH) = 254 ± 20 kJ × mol -1.

The available spectral data do not allow us to reliably estimate the dissociation energy by extrapolation of vibrational levels: only 3 levels of the ground state were observed for NiH. X 2 D 5/2 states, for NiD - 2 levels (a rough estimate of the number of levels: N = w e / w e x e / 2 = 2003 / 2 / 37 = 27). Linear extrapolation results in a value D° 0 = 26100 cm. In the spectrum of NiH, broadening begins at J ~ 12.5 and J ~ 11.5 in the bands 0-0 2 D 5/2 - X 2 D 5/2 and 2 D 3/2 - X 2 D 3/2 , respectively subbands 1-0 at J ~ 9.5). The authors believe that this is due to predissociation by rotation. According to them, the energy of the corresponding limit E< 26000 см -1 . Состояние С 2 D является третьим состоянием такой симметрии и может коррелировать только с третьим пределом диссоциации Ni(1 D) + H(2 S), что дает верхнюю границу для энергии диссоциации, равную ~ 26000-3400 = 22600 см -1 . С другой стороны начальные линии нормальные, что позволяет предположить, что уровень v = 0 NiH лежит ниже предела диссоциации и принять T 0 (2 D 5/2 - X 2 D 5/2) = 20360 cm -1 beyond the lower limit of the corresponding limit. From here we get 20360< D° 0 < 22600 см ‑1 . Теоретические вычисления приводят к величинам энергии диссоциации, заключенным в интервале 220 - 265 кДж× моль ‑1 [ 82BLO/SIE, 86CHO/WAL, 90HAB ].

The accepted dissociation energy corresponds to the values:

D f H° (NiH, g, 0) = 383.996 ± 8.2 kJ× mol -1 .

D f H° (NiH, g, 298.15) = 383.736 ± 8.2 kJ× mol -1 .

Nickel hydride describes an alloy made by combining nickel and hydrogen. The hydrogen content in nickel hydride is up to 0.002% by weight.

Hydrogen acts as a hardening agent, preventing dislocations in the nickel atom's crystal lattice from sliding past each other. Changing the amount of hydrogen alloy production and the form of its presence in nickel hydride (accelerated phase) controls qualities such as hardness, ductility and tensile strength of the resulting nickel hydride. High hydrogen nickel hydride can be made harder and stronger than nickel, but such nickel hydride is also less malleable than nickel. The loss of ductility is due to cracks supporting sharp points due to the suppression of elastic deformation by hydrogen and voids forming under tension due to decomposition of the hydride. Hydrogen embrittlement can be a problem in nickel for use in turbines at high temperatures.

In the narrow range of concentrations that make up nickel hydride, mixtures of hydrogen and nickel can only form a few different structures with very different properties. Understanding such properties is important to create high quality nickel hydride. At room temperature, the most stable form of nickel is the face-centered cubic (FCC) α-nickel structure. It is a fairly soft metallic material that can only dissolve a very small concentration of hydrogen, no more than 0.002 wt.% w, and only 0.00005 wt.%. The solid solution phase with dissolved hydrogen, which maintains the same crystal structure as the original nickel, is called the α-phase. At 25°C, 6 kbar of hydrogen pressure is needed to break up into b=nickel, but hydrogen will return from solution if the pressure drops below 3.4 kbar.

Surface

The atoms hydrogen bond strongly to the surface of the nickel, with the hydrogen molecules severing to do so.

Decoupling of the dihydrogen requires enough energy to cross the barrier. On Ni(111) the crystal surface barrier is 46 kJ/molecular weight, while on Ni(100) the barrier is 52 kJ/molecular weight. The Ni(110) crystal plane surface has the lowest activation energy to break a hydrogen molecule at 36 kJ/molecular weight. The surface layer of hydrogen on nickel can be released by heating. Neither (111) lost hydrogen between 320 and 380 K. Neither (100) lost hydrogen between 220 and 360 K. Neither (110) crystal surfaces lost hydrogen between 230 and 430 K.

To disintegrate in nickel, hydrogen must migrate from on the surface through the face of the nickel crystal. This does not occur in a vacuum, but may occur when other molecules interfere with the hydrogen-nickel plated surface. The molecules don't have to be hydrogen, but they seem to work like hammers, punching hydrogen atoms through the surface of the nickel to the interior. An activation energy of 100 kJ/molecular weight is required to penetrate the surface.

High pressure phases

A true crystallographically distinct nickel hydride phase can be produced with a high pressure hydrogen gas of 600 MPa. Alternatively, it can be produced in an electrolytic manner. The crystalline form is face centered cubic or β-nickel hydride. Hydrogen to nickel atomic ratios up to one, with hydrogen occupying an octahedral position. The density of β-hydride is 7.74 g/cm. It's dyed grey. At a current density of 1 ampere per square decimeter in 0.5 mol/liter of sulfuric acid and thiourea, the nickel surface layer will be converted to nickel hydride. This surface is crowded cracks up to millimeters long. The cracking direction is in the (001) plane of the original nickel crystals. The lattice constant of nickel hydride is 3.731 Å, which is 5.7% larger than that of nickel.

The usual methods of storing (in cylinders) compressed or liquefied hydrogen is a rather dangerous occupation. In addition, hydrogen penetrates very actively through most metals and alloys, which makes shut-off and transport valves very expensive.

The property of hydrogen to dissolve in metals has been known since the 19th century, but only now the prospects for the use of metal hydrides and intermetallic compounds as compact hydrogen storage facilities have become visible.

Hydride types

Hydrides are classified into three types (some hydrides may have multiple bond properties, such as being metal-covalent): metallic, ionic, and covalent.

Ionic hydrides - as a rule, they are created at high pressures (~100 atm.) and at temperatures above 100°C. Typical representatives are alkali metal hydrides. An interesting feature of ionic hydrides is a greater degree of atomic density than in the original substance.

covalent hydrides- practically do not find application due to the low stability and high toxicity of the metals and intermetallic compounds used. A typical representative is beryllium hydride, obtained by the “wet chemistry” method by the reaction of dimethylberyllium with lithium aluminum hydride in a solution of diethyl ether.

Metal hydrides- can be considered as alloys of metallic hydrogen, these compounds are characterized by high electrical conductivity, like the parent metals. Metal hydrides form almost all transition metals. Depending on the types of bonds, metal hydrides can be covalent (for example, magnesium hydride) or ionic. Almost all metal hydrides require high temperatures for dehydrogenation (hydrogen release reaction).

Typical metal hydrides

  • Lead hydride - PbH4 - a binary inorganic chemical compound of lead with hydrogen. Very active, in the presence of oxygen (in air) ignites spontaneously.
  • Zinc hydroxide - Zn (OH) 2 - amphoteric hydroxide. It is widely used as a reagent in many chemical industries.
  • Palladium hydride is a metal in which hydrogen is located between the palladium atoms.
  • Nickel hydride - NiH - is often used with additives of lanthanum LaNi5 for battery electrodes.

Metal hydrides can form the following metals:
Ni, Fe, Ni, Co, Cu, Pd, Pt, Rh, Pd-Pt, Pd-Rh, Mo-Fe, Ag-Cu, Au-Cu, Cu-Ni, Cu-Pt, Cu-Sn.

Metals-record holders in terms of the volume of stored hydrogen

The best metal for storing hydrogen is palladium (Pd). One volume of palladium can contain almost 850 volumes of hydrogen. But the viability of such a repository raises strong doubts due to the high cost of this platinum group metal.
On the contrary, some metals (for example copper Cu) dissolve only 0.6 volume of hydrogen per volume of copper.

Magnesium hydride (MgH2) can store up to 7.6% mass fractions of hydrogen in the crystal lattice. Despite the tempting values ​​and low specific gravity of such systems, an obvious obstacle is the high temperatures of the direct and reverse charge-discharge reactions and high endothermic losses during the dehydrogenation of the compound (about a third of the stored hydrogen energy).
Crystal structure of the β-phase of MgH2 hydride (figure)

Accumulation of hydrogen in metals

The reaction of hydrogen absorption by metals and intermetallics occurs at a higher pressure than its release. This is determined by the residual plastic deformations of the crystal lattice during the transition from a saturated α-solution (original substance) to a β-hydride (substances with stored hydrogen).

Metals that do not dissolve hydrogen

The following metals do not absorb hydrogen:
Ag, Au, Cd, Pb, Sn, Zn
Some of them are used as valves for storing compressed and liquefied hydrogen.

Low-temperature metal hydrides are among the most promising hydrides. They have low losses during dehydrogenation, high rates of charge-discharge cycles, are almost completely safe and have low toxicity. The limitation is the relatively low specific density of hydrogen storage. The theoretical maximum is the storage of 3%, but in reality 1-2% of the mass fraction of hydrogen.

The use of powdered metal hydrides imposes restrictions on the rate of "charge-discharge" cycles due to the low thermal conductivity of powders and requires a special approach to the design of containers for their storage. It is typical to introduce areas into the storage vessel to facilitate heat transfer and to produce thin and flat cylinders. Some increase in the rate of discharge-charge cycles can be achieved by introducing an inert binder into the metal hydride, which has a high thermal conductivity and a high threshold of inertness to hydrogen and the base substance.

Intermetallic hydrides

In addition to metals, storage of hydrogen in the so-called "intermetallic compounds" is promising. Such hydrogen storage facilities are widely used in household metal hydride batteries. The advantage of such systems lies in the relatively low cost of reagents and low environmental impact. At the moment, metal hydride batteries are almost universally replaced by lithium energy storage systems. The maximum stored energy of industrial designs in nickel-metal hydride batteries (Ni-MH) is 75 Wh / kg.

An important property of some intermetallic compounds is their high resistance to impurities contained in hydrogen. This property allows such compounds to be used in polluted environments and in the presence of moisture. Multiple "charge-discharge" cycles in the presence of impurities and water in hydrogen do not poison the working substance, but reduce the capacity of subsequent cycles. The decrease in useful capacity occurs due to contamination of the base substance with metal oxides.

Separation of intermetallic hydrides

Intermetallic hydrides are divided into high-temperature (dehydrogenating at room temperature) and high-temperature (more than 100°C). The pressure at which decomposition of the hydride phase occurs) is usually not more than 1 atm.
In real practice, complex intermetallic hydrides are used, consisting of three or more elements.

Typical intermetallic hydrides

Nickel lanthanum hydride, LaNi5, is a hydride in which one unit of LaNi5 contains more than 6 H atoms. Hydrogen desorption from nickel lanthanum is possible at room temperatures. However, the elements included in this intermetallic compound are also very expensive.
A unit volume of lanthanum-nickel contains one and a half times more hydrogen than liquid H2.

Features of intermetallic-hydrogen systems:

  • high hydrogen content in the hydride (wt. %);
  • exo (endo)-thermicity of the reaction of absorption (desorption) of hydrogen isotopes;
  • change in the volume of the metal matrix in the process of absorption - desorption of hydrogen;
  • reversible and selective absorption of hydrogen.

Areas of practical application of intermetallic hydrides:

  • stationary storages of hydrogen;
  • storage mobility and transportation of hydrogen;
  • compressors;
  • separation (purification) of hydrogen;
  • heat pumps and air conditioners.

Application examples of metal-hydrogen systems:

  • fine purification of hydrogen, all kinds of hydrogen filters;
  • reagents for powder metallurgy;
  • moderators and reflectors in nuclear fission systems (nuclear reactors);
  • separation of isotopes;
  • thermonuclear reactors;
  • water dissociation installations (electrolyzers, vortex chambers for producing gaseous hydrogen);
  • electrodes for batteries based on tungsten-hydrogen systems;
  • metal hydride batteries;
  • air conditioners (heat pumps);
  • converters for power plants (nuclear reactors, thermal power plants);
  • transportation of hydrogen.

The article mentions metals:


Let's start with the composition of interstitial compounds. Let us consider this issue using the example of transition element hydrides. If, during the formation of the interstitial phase, hydrogen atoms fall only into tetrahedral voids in the metal lattice, then the limiting hydrogen content in such a compound should correspond to the formula MeH 2 (where Me is a metal whose atoms form a close packing). After all, there are twice as many tetrahedral voids in the lattice as there are atoms forming a dense packing. If, on the other hand, hydrogen atoms fall only into octahedral voids, then it follows from the same considerations that the limiting hydrogen content should correspond to the formula MeH, - there are as many octahedral voids in a close packing as there are atoms that make up this packing.

Usually, during the formation of compounds of transition metals with hydrogen, either octahedral or tetrahedral voids are filled. Depending on the nature of the initial substances and the conditions of the process, complete or only partial filling may occur. In the latter case, the composition of the compound will deviate from the integer formula, will be indefinite, for example, MeH 1-x; MeH 2-x. Embedding connections, therefore, by their very nature must be compounds of variable composition, i.e., those whose composition, depending on the conditions for their preparation and further processing, varies within fairly wide limits.

Let us consider some typical properties of interstitial phases using the example of compounds with hydrogen. To do this, we compare the hydrides of some transition elements with the hydride of an alkali metal (lithium).

When lithium is combined with hydrogen, a substance of a certain composition LiH is formed. In terms of physical properties, it has nothing to do with the original metal. Lithium conducts electric current, has a metallic luster, plasticity, in a word, the whole complex of metallic properties. Lithium hydride does not have any of these properties. It is a colorless salt-like substance, not at all like a metal. Like other alkali and alkaline earth metal hydrides, lithium hydride is a typical ionic compound, where the lithium atom has a significant positive charge, and the hydrogen atom has the same negative charge. The density of lithium is 0.53 g / cm 3, and the density of lithium hydride is 0.82 g / cm 3 - occurs noticeable increase in density. (The same is observed in the formation of hydrides of other alkali and alkaline earth metals).

Palladium (a typical transition element) undergoes completely different transformations when interacting with hydrogen. There is a well-known demonstration experiment in which a palladium plate coated on one side with a gas-tight varnish is bent when blown with hydrogen.

This is because the density of the resulting palladium hydride decreases. Such a phenomenon can take place only if the distance between the metal atoms increases. The introduced hydrogen atoms "push" the metal atoms, changing the characteristics of the crystal lattice.

The increase in the volume of metals during the absorption of hydrogen with the formation of interstitial phases is so noticeable that the density of the metal saturated with hydrogen turns out to be significantly lower than the density of the original metal (see Table 2)

Strictly speaking, the lattice formed by the atoms of a metal usually does not remain completely unchanged after absorption of hydrogen by this metal. No matter how small the hydrogen atom, it still introduces distortions into the lattice. In this case, usually there is not just a proportional increase in the distances between atoms in the lattice, but also some change in its symmetry. Therefore, it is often said, only for simplicity, that hydrogen atoms are introduced into voids in close packing - the dense packing of metal atoms itself is nevertheless violated when hydrogen atoms are introduced.

Table 2 Changes in the density of some transition metals during the formation of interstitial phases with hydrogen.

This is far from the only difference between typical and transition metal hydrides.

During the formation of interstitial hydrides, such typical properties of metals as metallic luster and electrical conductivity are preserved. True, they can be less pronounced than in the parent metals. Thus, interstitial hydrides are much more similar to parent metals than alkali and alkaline earth metal hydrides.

Such a property as plasticity changes much more strongly - hydrogen-saturated metals become brittle, it is often difficult to turn the original metals into powder, and it is much easier to do this with hydrides of the same metals.

Finally, a very important property of interstitial hydrides should be noted. When transition metals interact with hydrogen, the metal sample is not destroyed. Moreover, it retains its original shape. The same happens during the reverse process - the decomposition of hydrides (loss of hydrogen).

A natural question may arise: can the process of formation of interstitial phases be considered chemical in the full sense of the word? Perhaps the formation of aqueous solutions - a process that has much more "chemistry"?

The answer is to use chemical thermodynamics.

It is known that the formation of chemical compounds from simple substances (as well as other chemical processes) is usually accompanied by noticeable energy effects. Most often, these effects are exothermic, and the more energy is released, the stronger the resulting connection.

Thermal effects are one of the most important signs that not just a mixture of substances is taking place, but a chemical reaction is taking place. Since the internal energy of the system changes, therefore, new bonds are formed.

Let us now see what energy effects are caused by the formation of interstitial hydrides. It turns out that the spread here is quite large. In metals of secondary subgroups III, IV and V of groups of the periodic system, the formation of interstitial hydrides is accompanied by a significant release of heat, of the order of 30–50 kcal / mol (when lithium hydride is formed from simple substances, about 21 kcal / mol is released). It can be recognized that interstitial hydrides, at least of the elements of the indicated subgroups, are quite "real" chemical compounds. However, it should be noted that for many metals located in the second half of each transition row (for example, for iron, nickel, copper), the energy effects of the formation of interstitial hydrides are small. For example, for a hydride with an approximate composition of FeH 2, the thermal effect is only 0.2 kcal / mol .

The small value of DN arr of such hydrides dictates the methods of their preparation - not the direct interaction of the metal with hydrogen, but an indirect way.

Let's look at a few examples.

Nickel hydride, whose composition is close to NiH 2, can be obtained by acting on an ethereal solution of nickel chloride with phenylmagnesium bromide in a stream of H 2:

The nickel hydride obtained as a result of this reaction is a black powder, which easily gives off hydrogen (which is generally characteristic of interstitial hydrides), and ignites when slightly heated in an oxygen atmosphere.

In the same way, hydrides of nickel's neighbors in the periodic system, cobalt and iron, can be obtained.

Another method for obtaining transition hydrides is based on the use of lithium alanate LiAlH. When the chloride of the corresponding metal reacts with LiAlH 4 in an ether solution, an alanate of this metal is formed:

MeCl 2 + LiAlH 4 > Me(AlH 4 ) 2 + LiCl(5)

For many metals, alanates are fragile compounds that decompose with increasing temperature.

Me(AlH 4 ) 2 >MeH 2 + Al + H 2 (6)

But for some metals of the secondary subgroups, a different process occurs:

Me(AlH 4 ) 2 >MeH 2 +AlH 3 (7)

In this case, instead of a mixture of hydrogen and aluminum, aluminum hydride is formed, which is ether-soluble. By washing the reaction product with ether, pure transition metal hydride can be obtained as a residue. In this way, for example, low-stable hydrides of zinc, cadmium and mercury were obtained.

It can be concluded that the preparation of hydrides of elements of secondary subgroups is based on typical methods of inorganic synthesis: exchange reactions, thermal decomposition of fragile compounds under certain conditions, etc. These methods were used to obtain hydrides of almost all transition elements, even very fragile ones. The composition of the obtained hydrides is usually close to stoichiometric: FeH 2 , CoH 2 , NiH 2 ZnH 2 , CdH 2 , HgH 2 . Apparently, the achievement of stoichiometry is facilitated by the low temperature at which these reactions are carried out.

Let us now analyze the effect of reaction conditions on the composition of the resulting interstitial hydrides. It follows directly from Le Chatelier's principle. The higher the pressure of hydrogen and the lower the temperature, the closer to the limiting value of saturation of the metal with hydrogen. In other words, each specific temperature and each pressure corresponds to a certain degree of saturation of the metal with hydrogen. And vice versa, each temperature corresponds to a certain equilibrium pressure of hydrogen over the metal surface.

This leads to one of the possible applications of transition element hydrides. Suppose, in some system, it is necessary to create a strictly defined pressure of hydrogen. A metal saturated with hydrogen is placed in such a system (titanium was used in the experiments). By heating it to a certain temperature, it is possible to create the required pressure of hydrogen gas in the system.

Any class of compounds is interesting for its chemical nature, the composition and structure of the particles of which it consists, and the nature of the bond between these particles. Chemists devote their theoretical and experimental work to this. They are no exception to the implementation phase.

There is no final point of view on the nature of interstitial hydrides yet. Often different, sometimes opposing points of view successfully explain the same facts. In other words, so far there are no unified theoretical views on the structure and properties of interstitial compounds.

Let's consider some experimental facts.

The process of absorption of hydrogen by palladium has been studied in most detail. This transition metal is characterized by the fact that the concentration of hydrogen dissolved in it at a constant temperature is proportional to the square root of the external pressure of hydrogen.

At any temperature, hydrogen to some extent dissociates into free atoms, so there is an equilibrium:

The constant of this equilibrium is:

where R H -- pressure (concentration) of atomic hydrogen.

From here (11)

It can be seen that the concentration of atomic hydrogen in the gas phase is proportional to the square root of the pressure (concentration) of molecular hydrogen. But the concentration of hydrogen in palladium is also proportional to the same value.

From this we can conclude that palladium dissolves hydrogen in the form of individual atoms.

What, then, is the nature of the bond in palladium hydride? A number of experiments have been carried out to answer this question.

It was found that when an electric current is passed through hydrogen-saturated palladium, non-metal atoms move towards the cathode. It must be assumed that the hydrogen found in the metal lattice completely or partially dissociates into protons (i.e., H + ions) and electrons.

Data on the electronic structure of palladium hydride were obtained by studying the magnetic properties. The change in the magnetic properties of the hydride depending on the amount of hydrogen included in the structure was studied. Based on the study of the magnetic properties of a substance, it is possible to estimate how many unpaired electrons are contained in the particles that make up this substance. On average, there are approximately 0.55 unpaired electrons per atom of palladium. When palladium is saturated with hydrogen, the number of unpaired electrons decreases. And in a substance with the composition PdH 0.55, unpaired electrons are practically absent.

Based on these data, we can conclude that the unpaired electrons of palladium form pairs with the unpaired electrons of hydrogen atoms.

However, the properties of interstitial hydrides (in particular, electrical and magnetic) can also be explained on the basis of the opposite hypothesis. It can be assumed that interstitial hydrides contain H - ions, which are formed due to the capture by hydrogen atoms of part of the semi-free electrons present in the metal lattice. In this case, the electrons received from the metal would also form pairs with the electrons present on the hydrogen atoms. This approach also explains the results of magnetic measurements.

It is possible that both types of ions coexist in interstitial hydrides. Metal electrons and hydrogen electrons form pairs and, therefore, a covalent bond occurs. These electron pairs can be shifted to one degree or another to one of the atoms - metal or hydrogen.

The electron pair is more strongly biased towards the metal atom in the hydrides of those metals that are less likely to donate electrons, such as palladium or nickel hydrides. But in the hydrides of scandium and uranium, apparently, the electron pair is strongly shifted towards hydrogen. Therefore, hydrides of lanthanides and actinides are in many respects similar to hydrides of alkaline earth metals. By the way, lanthanum hydride reaches the composition LaH 3 . For typical interstitial hydrides, the hydrogen content, as we now know, is not higher than that corresponding to the formulas MeH or MeH 2 .

Another experimental fact shows the difficulty in determining the nature of the bond in interstitial hydrides.

If hydrogen is removed from palladium hydride at a low temperature, then it is possible to preserve the distorted ("expanded") lattice that was found in hydrogen-saturated palladium. The magnetic properties (note this), electrical conductivity and hardness of such palladium are generally the same as those of hydride.

Hence it follows that, during the formation of interstitial hydrides, the change in properties is caused not only by the presence of hydrogen in them, but also simply by a change in the interatomic distances in the lattice.

We have to admit that the question of the nature of interstitial hydrides is very complicated and far from a final solution.

Mankind has always been famous for the fact that, even without fully knowing all the aspects of any phenomena, it was able to practically use these phenomena. This fully applies to interstitial hydrides.

The formation of interstitial hydrides in some cases is deliberately used in practice, in other cases, on the contrary, they try to avoid it.

Interstitial hydrides release hydrogen relatively easily when heated, and sometimes at low temperatures. Where can this property be used? Of course in redox processes. Moreover, the hydrogen given off by interstitial hydrides is in the atomic state at some stage of the process. This is probably related to the chemical activity of interstitial hydrides.

It is known that Group VIII metals (iron, nickel, platinum) are good catalysts for reactions in which hydrogen is added to some substance. Perhaps their catalytic role is associated with the intermediate formation of unstable interstitial hydrides. Further dissociating, hydrides provide the reaction system with a certain amount of atomic hydrogen.

For example, finely dispersed platinum (the so-called platinum black) catalyzes the oxidation of hydrogen with oxygen - in its presence, this reaction proceeds at a noticeable rate even at room temperature. This property of platinum black is used in fuel cells - devices where chemical reactions are used to directly produce electrical energy, bypassing the production of heat (combustion stage). The so-called hydrogen electrode, an important tool for studying the electrochemical properties of solutions, is based on the same property of finely dispersed platinum.

The formation of interstitial hydrides is used to obtain highly pure metal powders. Metallic uranium and other actinides, as well as very pure titanium and vanadium, are ductile, and therefore it is practically impossible to prepare powders from them by grinding the metal. To deprive the metal of plasticity, it is saturated with hydrogen (this operation is called "embrittlement" of the metal). The resulting hydride is easily triturated into powder. Some metals, when saturated with hydrogen, themselves pass into a powder state (uranium). Then, when heated in a vacuum, the hydrogen is removed and pure metal powder remains.

Thermal decomposition of some hydrides (UH 3 , TiH 2) can be used to produce pure hydrogen.

The most interesting areas of application of titanium hydride. It is used for the production of foam metals (for example, aluminum foam). To do this, the hydride is introduced into molten aluminum. At high temperatures, it decomposes, and the resulting hydrogen bubbles foam liquid aluminum.

Titanium hydride can be used as a reducing agent for some metal oxides. It can serve as a solder for joining metal parts, and as a substance that accelerates the process of sintering of metal particles in powder metallurgy. The last two cases also use the reducing properties of the hydride. A layer of oxides usually forms on the surface of metal particles and metal parts. It prevents adhesion of adjacent metal sections. Titanium hydride, when heated, reduces these oxides, thereby cleaning the metal surface.

Titanium hydride is used to produce some special alloys. If it is decomposed on the surface of a copper product, a thin layer of copper-titanium alloy is formed. This layer gives the surface of the product special mechanical properties. Thus, several important properties (electrical conductivity, strength, hardness, abrasion resistance, etc.) can be combined in one product.

Finally, titanium hydride is a very effective protection against neutrons, gamma rays and other hard radiation.

Sometimes, on the contrary, one has to struggle with the formation of interstitial hydrides. In metallurgy, chemical, oil and other industries, hydrogen or its compounds are under pressure and at high temperatures. Under such conditions, hydrogen can diffuse to a noticeable extent through the heated metal, simply "leave" the equipment. In addition (and this is perhaps the most important thing!), due to the formation of interstitial hydrides, the strength of metal equipment can be greatly reduced. And this is already fraught with a serious danger when working with high pressures.