NL1001123C2 - Activating metal surfaces for absorption and release of hydrogen - Google Patents

Abstract

The surface oxide of an iron-titanium (Fe-Ti), lanthanum-nickel (La-Ni) or magnesium-nickel (Mg-Ni) alloy is activated by conversion to halide through exposure to a halogen compound. This compound is chosen such that the metal halide contains a compound that evaporates between 100-700 deg C. This may be removed from the metal surface and an oxygen acceptor added that can take up O2 from the metal oxide.

Description

Method for activating hydrogen-absorbing alloys.

The invention relates to a process for activating surface oxide of an iron-titanium, lanthanum-nickel or magnesium-nickel alloy, comprising converting the metal oxides into metal halides by exposing them to a halogen compound

Generally, empty rings containing a less noble metal with a more noble metal are used for the storage of hydrogen. Examples are lanthanum-nickel, nickel-magnesium and titanium-iron. When exposed to atmospheric air, the alloy surface reacts to the corresponding metal oxides. As a result, the absorption and release capacity of hydrogen in the metal 15 is considerably limited or even rendered impossible, so that removal of the metal oxide is necessary.

The oxide of the more noble metal can generally be reduced by treatment with hydrogen at some elevated temperature. However, the oxide of the less noble metal does not react with hydrogen, even at strongly elevated temperatures. Hydrogen is included in the alloy as hydrogen atoms. As a result, dissociation of gaseous molecular hydrogen is required for absorption in the alloy. It is now possible that, upon thermal treatment, the oxide of the less noble component of the alloy 25 separates as a three-dimensional oxide on the surface of the alloy, such that the surface of the alloy is not completely covered by the oxide. In that case, dissociation of hydrogen on those parts of the surface where the more precious metal is present can occur. Migration through the surface layer to the underlying alloy then leads to uptake of {large amounts) of hydrogen. The removal or rendering harmless of oxides is indicated by 'activating'.

According to the prior art, hydrogen absorbing alloys and in particular titanium-iron alloys are activated by heat treatment at 400 to 500 ° C in hydrogen of elevated pressure. At the mentioned temperatures, a hydrogen pressure of 10 bar is usual. Such a high pressure is applied to shift the thermodynamic equilibrium to the side of the reduced metal. In general, such treatment is carried out in a number of cycles. This starts with a treatment at 400 to 500 ° C in hydrogen of 10 bar; at the elevated temperature, the hydrogen is then pumped off to a vacuum of, for example, 1 Pascal 5 and cooled to room temperature, after which hydrogen is admitted to a pressure of, for example, 50 bar. After this, the hydrogen pressure is lowered again to 1 Pascal and the temperature is raised to 400-500 ° C, after which hydrogen is allowed at a pressure of 10 bar. This cycle is repeated ten to thirty times before a maximum absorption of hydrogen is observed (JJReilly, RHWiswall, Inorganic Chemistry 13 (197 * 0218-222) and lasts several weeks. Treatment at high temperature is likely to separate the oxide of the base metal as a lower oxide that adheres well to the metal or alloy surface At lower temperature, the oxide is reoxidized by residual water vapor that is never completely removable, causing the adhesion to the alloy surface to disappear and the surface of the more precious metal is released The migration of the hydrogen through the more precious metal layer on the surface is promoted by the high hydrogen pressure.

However, activation by thermal pretreatment in hydrogen is difficult to control. The degree to which the alloy surface oxidizes upon exposure to atmospheric air varies widely and thus the degree to which the alloy surface is covered by the oxide of the less noble component. In general, activating by thermal treatment in hydrogen is therefore less suitable.

When absorbing and releasing hydrogen, the crystal lattice of the alloy expands and shrinks. This leads to disintegration of the alloy particles, resulting in a relatively fine powder (van Mal, dissertation Delft, 1976). Fresh alloy surface is exhibited here, which accelerates the absorption of hydrogen. This is one of the reasons that exposure to high pressure hydrogen at room temperature promotes activation of the alloy. At the high pressure, the alloy particles decompose and a clean alloy surface is formed. To promote molecular hydrogen activation, palladium is added to the alloy. Palladium can already be reduced with hydrogen at room temperature, so that already at low temperatures, centers are formed on the surface 100 1123 3 which can dissociate hydrogen. Nickel can also be reduced with hydrogen, albeit at a slightly higher temperature than palladium. In order to promote the separation of base oxides as three-dimensional oxide particles, base components, such as vanadium or manganese, are added to the alloy. However, little is known about the basic reactions that occur with such more complex alloys.

Disintegration of the alloy into particles of a few µm or less is common. The activation brought about by this is mentioned above. However, the surface-to-volume ratio of the alloy particles increases, and therefore the degree of oxidation when exposed to atmospheric air. In general, therefore, smaller alloy particles will no longer be able to be activated by a heat treatment in a hydrogen stream.

Because of the poorly controllable activation in the prior art, it has been proposed (X.-L. Wang and S. Suda, Journal of Alloys and Compounds 19 ^ (1993) 73 ~ 76) to the surface of alloys of less and more precious metals and in particular treating lanthanum-nickel alloys with fluorine-containing aqueous solutions.

According to this publication, the surface of lanthanum-nickel alloys is treated with aqueous solutions of a fluoride salt. According to this publication, a continuous layer of lanthanum fluoride then deposits on the surface of the alloy. This fluoride layer would be transparent to molecular hydrogen, which then dissociates on the underlying pure nickel surface and migrates through the nickel layer to the underlying alloy. An advantage of the continuous lanthanum fluoride layer would be that it prevents oxidation of the alloy surface. In the air, the fluoride layer gradually reacts again with water to give lanthanum oxide, which is impermeable to molecular hydrogen, thereby gradually losing the ability to absorb hydrogen when exposed to water vapor. However, for titanium iron alloys, which exhibit more favorable hydrogen absorption properties, the reaction of the titanium oxide on the surface to fluoride is less effective due to the lower stability and solubility of the titanium fluoride.

Due to the presence of water, the danger of oxidation persists if this water is not completely removed.

It has also been proposed (H. Imamura, T. Takahashi, R. Gal- 100 1123 4 leguillos, S. Tsuchiya, Journal of the Less-Common Metals 89 (1983) 251-256) to treat the surface of alloys with organic compounds . It is described that due to a high electron affinity of the organic compound hydrogen is activated and can penetrate through the surface layer. Treatment of the surface of a magnesium-nickel alloy with tetracyanoethylene or phthalonitrile leads to an accelerated absorption of hydrogen at relatively low temperatures. A drawback of this activation procedure, however, is that the surface remains partially covered by the organic molecules or the decomposition products, so that the desorption of hydrogen proceeds more slowly. Nor is this method applicable to titanium-iron alloys.

The object of the invention is therefore to carry out a more universal activation procedure for hydrogen-absorbing alloys and, in particular, for iron-titanium alloys, which can be carried out smoothly and easily at high temperatures.

In addition, it should be possible to incorporate hydrogen into the alloy at a pressure that is not too high. from it.

This object is accomplished in a process described above in that the halogen compound is selected such that the metal halide comprises an at least elevated temperature between 100 ° C and 700 ° C vaporous compound which is removed from the metal surface, and wherein an oxygen acceptor is added, which is capable of absorbing the oxygen from the metal oxide. In contrast to the above-described process in which a fluoride layer transparent to molecular hydrogen is formed, the halide layer is now removed. This is possible in a simple manner and without residues, because this halide layer easily evaporates. Numerous halogen compounds are conceivable which can be used. Chlorides are mentioned as an example because, for example, both ferric chloride and titanium chloride are volatile and boil away at a relatively low temperature. In such a method, it is not necessary to use an aqueous solution, so that the problem with residual water resp. water vapor is not present.

Obviously, the oxygen released during the conversion of metal oxide to metal halide should not be bound to the non-oxidized alloy underlying the oxide layer. Therefore, it is necessary that additionally an oxygen acceptor is added which is able to recycle the oxygen from the metal oxide. An example of this is a carbon-containing compound. Exposure to this carbon-containing compound can take place both before exposure to the halogen compound and simultaneously. Carbon is preferred because, when combined with oxygen, a gaseous product (CO or CO2) is formed which can easily desorb from the metal surface.

It is possible to apply carbon separately to the surface and to feed the halogen compound separately. It is also possible to combine both types of atom into a molecule.

When a chlorine-containing halogen compound is used, the carbon and halogen-containing compound preferably contains carbon tetrachloride.

It is hereby possible to expose the oxidized alloy surface at an elevated temperature to suitable gas molecules containing carbon and halogens, or to first adsorb these molecules on the surface at a lower temperature and then raise the temperature such that the reaction takes place.

According to a further embodiment of the invention, the halogen (carbon) compound is liquid at a reduced temperature and reacts relatively slowly with the metal oxides. This makes it possible to incorporate this compound in the alloy in the liquid state and by subsequently increasing the temperature, on the one hand, the reaction speed is increased and, on the other hand, the conditions are realized in which the metal halides are volatile and can be easily removed. Without limiting the scope of the invention, it is believed that the thermodynamic equilibrium is shifted in the desired direction by the stability of the metal chloride. Generally, the reduction of the surface oxides proceeds at a temperature of 300 to 500 ° C while the volatilization of the chlorides formed requires temperatures of 100 to 700 ° C.

The invention will be explained in more detail below with reference to an exemplary embodiment.

35

Example: 1 gram of titanium iron with a particle size between 5OO and 85Ο microns with an oxide layer on the surface was placed 100112 3 6 in a reactor. The reactor was heated in an inert gas stream to 350 ° C with a heating rate of 300 ° C / hour.

At this temperature, 20 ml of gaseous carbon tetrachloride were injected into the reactor through a septum, after which an inert gas was passed through the reactor for 15 minutes.

The reactor was then heated to 500 ° C at a heating rate of 300 ° C / hour in a 10% by volume hydrogen in argon stream and kept at this temperature for 1 hour.

The reactor was then evacuated at the above temperature 10 to a vacuum of 10-5 mbar and held at this temperature and pressure for one hour.

That the above-described process was effective was shown by subsequently subjecting it to a hydrogen pressure of 1 bar at a reaction temperature of 258 K. Within 2 hours an uptake of hydrogen in the metal alloy was observed, corresponding to the theoretical maximum.

Although it is assumed above that both the iron oxide and the titanium oxide react with the halogen compound to form the respective volatile halides, it should be understood that only one of the two metals reacts in this manner and that the other metal oxide is removed or rendered harmless in another manner known in the art. Especially in this case, instead of a chlorine-containing halogen compound, bromine, iodine or fluorine-containing compound can be used.

Although the invention has been described above with reference to a preferred embodiment, it is to be understood that numerous modifications can be made which include both the conditions under which the alloy is activated and the compounds used therefor.

100 1123

Claims (6)

Method for activating surface oxide of an iron-titanium, lanthanum-nickel or magnesium-nickel alloy, comprising converting the metal oxides into metal halides by exposing them to a halogen compound, characterized in that the halogen compound is such choose, that the metal halide comprises an at least elevated temperature between 100 ° C and 700 * 0 vaporous compound, which is removed from the metal surface, and which is added to an oxygen acceptor, which is capable of oxygenation of the metal oxide on to take. The method of claim 1, wherein the halogen compound comprises chlorine. 3. A method according to any one of the preceding claims, wherein the reducer comprises a carbon-containing compound. k. A method according to any of the preceding claims in combination with claim 3. wherein the halogen compound contains carbon. The method of claim k, wherein the carbon / halogen compound comprises carbon tetrachloride. Process according to any of the preceding claims, wherein the conversion is carried out at a temperature of up to 500 ° C. 7. A method according to any one of the preceding claims, wherein the halogen compound is adsorbed a substantially liquid state in the metal alloy at a relatively low temperature and then the temperature of the alloy containing halogen compound is increased. **** «# 100 1123 COOPERATION TREATY (PCT) REPORT ON NEWNESS RESEARCH OF INTERNATIONAL TYPE OF IDENTIFICATION OF NATIONAL APPLICATION Characteristic of the applicant o (of the authorized representative NO 39449 TM Dutch application no. Indianngsdaaim 1001123 1 Priority invoked) FOUNDATION FOR ENERGY RESEARCH CENTER THE NETHERLANDS Issue of the request for an investigation of intamasonaai type By the Insunae for Intemonaai Investigation (ISA) to the request for an investigation of international type Logeekand nr SN 26362 NL I. CLASSIFICATION OF THE SUBJECT (when using various badgerift caoes (specify all dassificaoe symbols) According to the Donate dassifieaoe (IPC) Int Cl.6: C 01 B 3/00 II RESEARCHED FIELDS OF TECHNIQUE __Purchased minimum documentation Classification system _Classifications ymDolen_ Int. Cl.6 C 01 B Other examined oocumemaoe then oe minimum documentaoe insofar as such documents are discussed Prayers Searched Have Been Recorded III. I 1 NO EXAMINATION FOR CERTAIN CONCLUSIONS (comments on supplementary sheet) /, IV. I 1 LACK OF UNIT OF INVENTION (Notes on Supplementary Sheet) I Form PC7 / ISA / 231 (aiC8 1994