COMPOSITION FOR REVERSABLY ABSORBING AND DESORBING HYDROGEN

Abstract: A substance for reversibly absorbing and desorbing hydrogen is disclosed. The substance consists of 0.8 - 20% by weight of one or more oxides of elements selected from La, Ce, Nd, Pr, Sm and Eu dispersed into Zr(FeV) x (0.01 ≦ x ≦ 0.88).

Description: This invention relates to a composition for reversibly absorbing (or adsorbing) and desorbing hydrogen and isotopes thereof. In particular, the invention relates to a substance for reversibly absorbing-desorbing hydrogen and isotopes thereof which is easy to hydrogenate and has a high hydrogen absorbing and desorbing rate. This composition may also be used as a 'getter' for other gases. Hydrogen getting substances, such as Ti, Zr, Hf and the like, showing a hydrogen dissociation pressure lower than atmospheric pressure at temperatures less than 300°C have been known. However, these metals are generally covered with strong oxide and/or nitride films, so that to obtain a hydrogen-gettable clean active surface, pretreatment is required, for example, by sublimation in a sputtering method, or heat-treatment at a high temperature e.g. 700 - 1,000°C. It has been proposed to improve pretreatment by activation with one or more additional elements to form an alloy, and so control the hydrogen dissociation pressure characteristic, or increase the reversible hydrogen absorbing-desorbing rate. However, Zr-Al and Zr-Ni alloys, for example, require heat-treatment or activation at a high temperature such as at 450 - 750°C. On the other hand, a Zr-V alloy can reversibly absorb and desorb hydrogen without an initial activation treatment, but if such an alloy (e.g. Zr V 0.15 ) is exposed to hydrogen gas at a pressure of about 0.5 kg/cm² , a high temperature for initially absorbing hydrogen of about 230°C is required and the absorption rate is slow. Further, the physical properties of Zr-V alloys are such that it is difficult to prepare these alloys with a desired size, e.g. by cutting or crushing. Intermetallic compounds, Zr-V-O and the like, stabilized by oxygen, have also been proposed, but these require an activation treatment and the control of sample grain size is difficult because of the hardness of the alloys. It has also been proposed to control hydrogen dissociation pressure, sample grain size and characteristics to gases other than hydrogen, by adding 4th and 5th elements to an alloy stabilized with oxygen and forming an intermetallic compound such as Zr-Ti-V-Fe-O. However, by increasing the kinds of additional elements, it has been difficult to regulate the composition of the substance. We have now found it possible to provide reversibly hydrogen absorbing-desorbing materials showing a hydrogen equilibrium pressure lower than atmospheric pressure with a comparatively high hydrogenation composition at a temperature of less than 300°C, the materials having a low temperature for initially absorbing hydrogen and a sufficiently fast rate of absorbing and desorbing hydrogen. In addition, the materials do not generally require any initial activation treatment. These properties are achieved by dispersing specified amounts of one or more particular rare earth oxides into Zr(FeV) having a specified composition to form a complex. Fe-V is substantially cheaper than V, and surprisingly the properties of the complex are generally equivalent to those of a complex obtained by dispersing a rare earth oxide into Zr-­ V. Therefore, we have found it possible to provide a low cost reversible hydrogen absorbing-desorbing substance. According to the invention, there is provided a composition for reversibly absorbing (or adsorbing) and desorbing hydrogen comprising 0.8 - 20% by weight of one or more oxides of one or more elements selected from La, Ce, Nd, Pr, Sm and Eu, in Zr(FeV) x (wherein x is in the range 0.01 ≦ × ≦ 0.88). In the compositions of the invention, the zirconium-­ iron-vanadium which forms a matrix should have a composition of Zr(FeV) 0.01-0.88 . As the amount of FeV in the composition of the invention is increased, the hydrogen gas activity increases and the temperature characteristic, hydrogen equilibrium pressure, and composition characteristics vary. When the FeV content is lower than the above range, the effect of increasing the hydrogen gas activity is not so conspicuous, so that the desired properties are not attained, and an FeV content exceeding the above range is not preferable because the hydrogen activity does not increase. The oxides of the elements which may be dispersed in the Zr(FeV) x matrix are one or more oxides selected from La, Ce, Nd, Pr, Sm and Eu oxides, and the amount thereof is 0.8 - 20% by weight. The hydrogen gas activity increases as the amount of added rare earth oxide increases, while the temperature-hydrogen equilibrium pressure characteristic is scarcely affected. However, when the amount added does not fall within the above range, the effect of increasing activity is not noticable, and when the amount exceeds the above range, there is no more effect on the activity and the hydrogen absorption amount is only reduced, so that neither cases are preferable. For consistent properties, it is preferable that the elements of the oxides are in their most stable oxidation states, that is, La₂O₃, CeO₂, Ce₂O₃, Nd₂O₃, Pr₆O₁₁, Sm₂O₃ and Eu₂O₃ ,but oxides with other oxidation states may be used. Thus, oxides with lower, mixed or non-stoichiometric oxidation states may also be used. Various methods may be employed to manufacture the compositions of the invention. The most preferable method is to prepare highly pure zirconium and highly pure iron-vanadium and a mixture of the one or more rare earth oxides mentioned above (inclusive of Misch metal oxide) to form respective predetermined compositions and to combine them in an inert atmosphere such as argon and the like. Alternatively, the rare earth oxides are not directly mixed, but rare earth pure metals are mixed and melted in an oxidizing atmosphere, or a small amount of oxide such as of Fe-V or Zr is mixed and melted. The compositions according to the invention may also readily act as a getter for impure gases such as oxygen, nitrogen, carbon monoxide, carbon dioxide, methane, steam and the like other than hydrogen gas. In addition, the compositions according to the invention may be easily ground as compared with a Zr(FeV) matrix, and are generally not too brittle, and have high density. The compositions according to the invention generally do not require any activation treatment whatever the initial hydrogen content, and their initial hydrogenation rates are large while the reversible absorbing-desorbing rate is advantageously large. Furthermore, the compositions of the invention has various advantages in that they may be readily ground; the temperature condition and the like in initial hydrogenation is optimised almost without changing the temperature-pressure-­ composition characteristic of the mother alloy in comparison with alloy improvement by the conventional 3rd element addition; the reaction rate is increased; and they may be used as getters for other gases. With these advantages, the reversible hydrogen absorbing-desorbing compositions of this invention may be advantageously used for refining and separating hydrogen gas; for hydrogen gas pressure control of vacuum machines and the like; as a hydrogen gas getter, including isotope and non-volatile getter; as well as in devices utilizing the high absorbing-desorbing rate at near atmospheric pressure. The invention is further illustrated by way of non-­ limiting example with reference to the accompanying drawings, in which:- Figures 1 and 2 are charcteristic diagrams showing the initial activity of the composition for reversibly absorbing and desorbing hydrogen according to the invention; and Figure 3 is a diagram showing equilibrium hydrogen pressure to temperature characteristics of the composition of the invention in case of changing the hydrogenation composition. The invention will be illustrated by referring to examples hereinafter. These examples describe preferred specified compositions and other conditions, but these examples are for illustration only and the invention is not limited by these examples. Examples 1 - 31 In order to form the compositions identified by Table 1 below, the following were used: Zr metal powder (purity : more than 99.7%); Fe-V metal pieces (purity : more than 99.7%); CeO₂ (purity : more than 99.0%; Nd₂O₃ (purity : more than 99.0%; Pr₆O₁₁(purity : more than 99.0%); Sm₂O₃ (purity : more than 99.0%); Eu₂O₃ (purity : more than 99.0%); and a mixed rare earth oxide (consisting of 30 wt% of La₂O₃, 50 wt% CeO₂, 15 wt% of Nd₂ O₃, 4 wt% of Pr₆O₁₁ and 1 wt% of Sm₂ O₃). The appropriate amounts of the components were arc melted in an argon atmosphere and formed into a reversible hydrogen absorbing-desorbing material. The thus obtained materials were ground to 20 - 40 mesh, and 4 g of each charged to a hydrogen absorbing-desorbing reactor, vacuum exhausted to 10⁻² Torr. Hydrogen of 99.9999% purity was introduced into the reactor at a pressure of about 0.6 kg/cm² , the material temperature was maintained at 30°C for 10 minutes, and then the temperature raised from 30°C to 300°C at a rate of 5°C/min. By measuring the sample temperature and the pressure in the reactor, the temperature that the sample started to absorb hydrogen and the initial hydrogen absorbing rate were determined. Thus, the indices of the hydrogen absorption starting temperature and the absorbing rate at the time of the initial hydrogen absorption are as shown in Table 1. Further, the saturated hydrogen absorbing amount at the temperature of starting absorption of initial hydrogen in each sample (converted into STP) is as shown in Table 1. Figure 1 is a graph showing the change with time of hydrogen pressure in a reactor when the materials of Example No. 22 (solid line (I), Example No. 14 (solid line (II)) and Zr(FeV) 0.19 (solid line (III)) as mother alloy thereof and ground to 20 - 40 mesh are charged in the above reactor. After vacuum exhaustion to 10⁻⁴Torr in the reactor, a hydrogen pressure of 0.6 kg/cm² is provided, 30°C is maintained for 10 minutes, and the temperature is raised to 300°C at a rate of 5°C/min. In Figure 1, the abscissa indicates time, the ordinate indicates the pressure in a reactor at that time by a solid line and the temperature in the vicinity of the sample at that time by a broken line. Figure 2 is a graph showing the change with time of hydrogen pressure when testing materials of Example No. 30 (solid line A), Example No. 27 (solid line B), Example No. 22 (solid line C), Example No. 5 (solid line D) and respective mother alloys Zr(FeV) 0.88 (broken line a) and Zr(FeV) 0.55 (broken line b), Zr(FeV) 0.19 (broken line c) and Zr(FeV) 0.05 (broken line d) under the same condition as in the case shown in Figure 1. In Figure 2, the abscissa indicates time, the ordinate indicates the pressure in a reactor at that time by a solid-­ broken line, and the sample temperature by one-dot line. Figure 3 is a graph showing the hydrogen pressure temperature characteristic of a sample measured by grinding the material of Example No. 22 to 20 - 40 mesh, weighing 4 g into a vacuum measuring hydrogen absorbing-desorbing container, continuously heating and vacuum exhausting the whole container at 250°C for 48 hours with a cryopump, thereafter sealing a sample system from the exhaust system, introducing a predetermined amount of hydrogen gas at room temperature for absorption by the sample, raising the temperature from room temperature to about 350°C by balancing the hydrogen pressure, and returning to room temperature in the same manner. In Figure 3, the abscissa indicates the hydrogen pressure in the container charged with the sample, that is, it shows the equilibrium hydrogen pressure of said sample at that temperature, and the ordinate indicates the sample temperature at that time (shown by 1000/T (1/k)) and (°C)). In Figure 3, the solid line shows the time of absorbing hydrogen and the broken line shows the time of desorbing hydrogen, wherein (1) shows that the hydrogen content in the hydrogenated product is 2.5%of the saturated hydrogen absorbing amount, (2) shows 5%, (3) shows 10%, and (4) shows 15%, respectively. In each example, hydrogen is extremely quickly absorbed and desorbed to reach equilibrium, and shows the hydrogen pressure-temperature characteristic as shown in Figure 3. Further, in this pressure measurement, the pressure range from 10⁻⁵ to 10⁻² Pa is the partial pressure measurement of hydrogen gas, and the pressure range higher than 10⁻² Pa is the total pressure measurement with the use of an ionizer vacuum meter. Absorption of Co-N₂ The material of Example No. 22 was ground to 20 - 40 mesh, 4 g placed in a stainless vacuum measurement hydrogen absorbing-desorbing containing (A), while a control container (B) was free of the material. Both (A) and (B) were continuously heated and vacuumed at 150°C for 24 hours with a cryopump and back to room temperature, sealed from the exhaust system, and the change with time of respective CO-N₂ partial pressures within said containers (A) and (B) determined. At a room temperature of 20°C, the container (A) shows an increase of CO-N₂ component at a rate of 2.19 × 10⁻¹⁴ Torr ℓ/sec, and the container (B) shows an increase of CO-N₂ at a rate of 3.51 × 10⁻¹⁰ Torr ℓ/sec. Therefore, even at a room temperature of 20°C, said sample absorbs CO-N₂ at an absorbing rate of 3.51 × 10⁻¹⁰ Torr ℓ/sec. The temperatures of both the containers (A) and (B) was raised from 25°C to 250°C at a rate of 15°C/min., and the change with time of CO-N₂ partial pressure was examined in the same manner as described above. The container (A) has 1.50 × 10⁻⁵ Pa at room temperature and a partial pressure of 6.50 × 10⁻⁶ Pa at 250°C, and the container (B) has 2.30 × 10⁻⁴ Pa at room temperature and 2.30 × 10⁻³ Pa at 250°C. Therefore, it means that said sample absorbs CO-N₂ at 1.75 × 10⁻⁹ Torr ℓ/sec during this period. Further, the temperature when the CO-N₂ partial pressure was reduced in the container (A) was about 40°C. Absorption of CO₂ The material of Example No. 22 was ground to 20 - 40 mesh, 4 g placed in a stainless vacuum measurement hydrogen absorbing-desorbing container (A), while a control container (B) was free of the material. Both (A) and (B) were continuously heated and vacuumed at 150°C for 24 hours with a cryopump and back to room temperature, sealed from the exhaust system, and the change with time of respective CO₂ partial pressures within containers (A) and (B) was determined. At a room temperature of 20°C, container (A) shows an increase of CO₂ component at a rate of 2.89 × 10⁻¹⁵ Torr ℓ/sec, and container (B) shows an increase of CO₂ at a rate 2.34 × 10⁻¹² Torr ℓ/sec. Therefore, even at a room temperature of 20°C, the sample absorbs CO₂ at an absorbing rate of 2.34 × 10⁻¹² Torr ℓ/sec. The temperatures of both containers (A) and (B) was raised from 25°C to 250°C at a rate of 15°C/min, and the change with time of CO₂ partial pressure was examined in the same manner as described above. The container (A) has 8.10 × 10⁻⁷ Pa at room temperature and a partial pressure of 1.60 × 10⁻⁶ Pa at 250°C, and container (B) has 1.00 × 10⁻⁶ Pa at a room temperature and 2.40 × 10⁻⁵ Pa at 250°C. Therefore, it means that said sample absorbs CO₂ at 1.83 × 10⁻¹¹ Torr ℓ/sec during this period. Zr(FeV) 0.19 Nd₂O₃ 10 wt% and Zr(FeV) 0.19 Pr₆O₁₁ 10 wt% are not shown in Table 1 as examples, but are expected to give the same effect as the examples shown in Table 1. The following materials may also be expected to give the same effect as the examples shown in Table 1. Zr(FeV) 0.19 La₂O₃ p wt%, CeO₂ q wt% Zr(FeV) 0.19 La₂O₃ p wt%, Nd₂O₃ q wt% Zr(FeV) 0.19 La₂O₃ p wt%, Pr₆O₁₁ q wt% Zr(FeV) 0.19 CeO₂ p wt%, Sm₂O₃ q wt% Zr(FeV) 0.19 CeO₂ p wt%, Eu₂O₃ q wt% Zr(FeV) 0.19 Nd₂O₃ p wt%, Pr₆O₁₁ q wt% Zr(FeV) 0.19 Sm₂O₃ p wt%, Eu₂O₃ q wt% (In the above materials, 0,8 ≦ p + q ≦ 20) Zr(FeV) 0.19 La₂O₃p wt%, CeO₂ q wt% Sm₂O₃ r wt% (In this material, 0.8 ≦ p + q + r ≦ 20) Zr(FeV) 0.19 La₂O₃ p wt%, CeO₂ q wt% Sm₂O₃ r wt%, Eu₂O₃ s wt% (In this material, 0.8 ≦ p + q + r + s ≦ 20) Zr(FeV) 0.19 La₂O₃ p wt%, CeO₂ q wt% Nd₂O₃ r wt%, Sm₂O₃ s wt% Eu₂O₃ t wt% (In this material, 0.8 ≦ p + q + r + s + t ≦ 20) Furthermore, the following compositions show similar excellent properties as those indicated above. These compositions comprise a complex containing 0.8 - 10 % by weight of an oxide of more than one element selected from La, Ce, Nd, Pr, Sm and Eu in a material of formula (Zr 1-x L x )(FeV) y , (Zr(FeV) 1-z M z ) y or (Zr 1-x L x )(FeV) 1-Z M z ) y .(In the materials, L is one or two elements selected from Ti and Hf, M is one or more elements selected from Cr, Mn, Fe, Co, Ni, Cu and A1 and further 0.05 ≦ x ≦ 0.95 0.01 ≦ y ≦ 0.88 0.05 ≦ z ≦ 0.5)

Claims: 1. A composition for reversibly absorbing and desorbing hydrogen, comprising 0.8 - 20% by weight of one or more oxides of one or more elements selected from La, Ce, Nd, Pr, Sm and Eu, in Zr(FeV)x (wherein x is in the range 0.01 ≦ x ≦ 0.88). 2. A composition as claimed in claim 1, wherein the oxide component comprises one or more of La₂O₃, CeO₂, Ce₂O₃, Nd₂O₃, Pr₆ O₁₁, Sm₂O₃ and Eu₂O₃. 3. A composition as claimed in either of claims 1 and 2, wherein the oxide component comprises La₂O₃ (p% by weight) and any one of CeO₂, Nd₂O₃ and Pr₆O₁₁ (q% by weight) where 0.8 ≦ (p + q) ≦ 20. 4. A composition as claimed in either of claims 1 and 2, wherein the oxide component comprises CeO₂ (p% by weight) and any one of Sm₂O₃ and Eu₂O₃ (q% by weight) where 0.8 ≦ (p + q) ≦ 20. 5. A composition as claimed in either of claims 1 and 2, wherein the oxide component comprises Nd₂O₃ (p% by weight) and Pr₆ O₁₁ (q% by weight) where 0.8 ≦ (p + q) ≦ 20. 6. A composition as claimed in either of claims 1 and 2, wherein the oxide component comprises Sm₂O₃ (p% by weight) and Eu₂ O₃ (q% by weight) where 0.8 ≦ (p + q) ≦ 20. 7. A composition as claimed in either of claims 1 and 2, wherein the oxide component comprises La₂O₃(p% by weight), CeO₂ (q% by weight) and Sm₂O₃(r% by weight), where 0.8 ≦ (p + q + r) ≦ 20. 8. A composition as claimed in either of claims 1 and 2, wherein the oxide component comprises La₂O₃ (p% by weight), CeO₂ (q% by weight), Sm₂O₃ (r% by weight) and Eu₂O₃ (s% by weight), where 0.8 ≦ (p + q + r + s) ≦ 20. 9. A composition as claimed in either of claims 1 and 2, wherein the oxide component comprises La₂O₃ (p% by weight), CeO₂ (q% by weight), Nd₂O₃ (r% by weight), Sm₂O₃ (s% by weight) and Eu₂ O₃ (t% by weight), where 0.8 ≦ (p + q + r + s + t) ≦ 20. 10. A method of reducing the hydrogen content of a hydrogen-containing atmosphere which comprises contacting said atmosphere with a composition according to any one of the preceding claims. 11. A method of purifying an impure gas which comprises contacting said gas with a composition according to any one of claims 1 to 9.

Inventors: HIROSAWA, KIMIHIKO, SAITO, TAKUYA, SUZUKI, JO, TERAZAWA, SHOTARO, YAMAGUCHI, TAMOTU
Applicants: SUZUKI SHOKAN CO. LTD.
Application year: 1987
Main classification: C01B3
IPC codes: C22C16, B01J20, C01B3