EP1912891A1

EP1912891A1 - Method for production of a hydrogen storage material

Info

Publication number: EP1912891A1
Application number: EP06762856A
Authority: EP
Inventors: Maximilian Fichtner; Christoph Frommen
Original assignee: Forschungszentrum Karlsruhe GmbH
Current assignee: Karlsruher Institut fuer Technologie KIT
Priority date: 2005-08-10
Filing date: 2006-07-27
Publication date: 2008-04-23
Also published as: JP2009504548A; DE102005037772B3; US20100167917A1; US8084386B2; WO2007017129A1; JP5015929B2

Abstract

The invention relates to a method for production of a hydrogen storage material which comprises a metal hydride and a non-hydrogenated material and which is doped with a metal as catalyst, having the steps: a) mixing a catalyst precursor which contains the metal with the non-hydrogenated material and if appropriate a solvent, and stirring this mixture, b) heat-treating the mixture, as a result of which the metal from the catalyst precursor forms a composite with the non-hydrogenated material, c) mixing the composite with the metal hydride and grinding the mixture, whereupon the hydrogen storage material is obtained. The method according to the invention permits the use of inexpensive TiCl<SUB>4</SUB> as catalyst precursor for production of a hydrogen storage material, the loading and discharging kinetics of which correspond to those of a hydrogen storage material which was produced using the same amount of the much more expensive TiCl<SUB>3</SUB> as catalyst precursor.

Description

Process for the preparation of a hydrogen-gas mixture

The invention relates to a method for producing a hydrogen storage material comprising a metal hydride and a non-hydrogenated material doped with a metal as a catalyst.

DE 195 26 434 A1 discloses a method for the reversible storage of hydrogen, in which sodium, potassium, sodium-lithium, sodium-potassium or lithium-potassium-alanate are used as reversible hydrogen storage materials.

The example of the sodium alanate NaAlH ₄ was used to show that the reversible hydrogen storage proceeds according to the following multi-step scheme:

NaAlH ₄ £ »1/3 Na ₃ AlH ₆ + 2/3 Al + H ₂ (1)

1/3 Na ₃ AlH ₆ t; NaH + 1/3 Al + 1/2 H ₂ (2)

Seam; Na + 1/2 H ₂ (3)

The hydrogen exchange of the steps according to equations (1) and (2) takes place at temperatures around 100 ^° C. and pressures of several MPa, which is of particular importance for low-temperature fuel cells. Of the total 7.6% by weight of H in the compound NaAlH ₄ , it is theoretically possible to exchange 5.6% by weight of H in equations (1) and (2). However, assuming pure NaAlH ₄ , the reaction times will take several days as kinetic barriers delay the conversion of the material.

Prerequisite for moderate working pressures and temperatures is therefore according to DE 195 26 434 Al, the addition of a catalytically acting dopant, typically in amounts of 0.2 to 10 mol%, based on the alkali metal alanate. For this purpose, are preferably suitable Ver ^¬ compounds of transition metals of the 3rd to 5th group of the Periodic Table (Sc, Y, Ti, Zr, Hf, V, Nb, Ta), iron, nickel, or a rare earth metal, preferably alcoholates, haldgenides, hydrides,

4 organometallic or intermetallic compounds.

DE 101 63 697 A1 discloses hydrogen storage materials comprising the abovementioned alanates or mixtures of aluminum metals with: alkali metals and / or alkali metal hydrides, which are reacted with metal catalysts having particle sizes of from 0.5 to 1000 nm and specific surface areas of from 50 to 1000 are used for which transition metals of groups 3 to 11 of the Periodic Table or aluminum and alloys, mixtures or compounds of these metals, in particular titanium, titanium-iron and titanium-aluminum are used.

B. Bogdanovic, M. Schwickardi, J. Alloy Compd. 253-254 (1997), p. 1 and D.L. Anton, J. Alloys Compd. 356-357 (2003) p. 400-404, found that titanium has the best properties as a dopant for accelerating the hydrogen exchange reaction. Likewise, J. Wang, A.D. Ebner, R. Zidan, J.A. Ritter, J. Alloys Compd. 391 (2005) pp. 245-255, it has been shown that doping with a mixture of different transition metals, in particular Ti with Zr, Fe or a mixture thereof, is advantageous.

As the doping method, a wet impregnation method in which a transition metal compound is stirred by adding a solvent, or a solvent-free doping method according to US Pat. No. 6,471,935 in which the transition metal compound is mechanically added to the hydrogen support material in a ball mill are currently used. In both cases, a chemical reaction of the high-quality transition metal with the alanate occurs with formation of the reduced metal. Depending on the type and quantity of catalyst precursor added (precursor), a certain amount of metal hydride is consumed (oxidized) for the reaction. In addition to finely divided and catalytically active Ti ⁰ , either gaseous organic by-products form, which may have harmful effects on the fuel cell, or according to the equation (1-x) NaAlH ₄ + x TiCl ₃ →

(l-4x) NaAlH ₄ + 3x NaCl + x Ti ⁿ + 3x Al ⁰ + 6x H _; (4)

Solids such as Al ⁰ and NaCl, which do not store hydrogen and are therefore detrimental to the gravimetric storage capacity; of the material.

EH Mazozoub and KJ Gross, J. Alloys Compd. 2003, 356-357, p. 363 and P. Wang and CM. Jensen, J. Alloys Compd. 2004, 379, pp. 99-102 attempted to circumvent this disadvantage by using expensive TiCl ₃ and instead using finely divided metallic Ti or a cubic TiAl ₃ alloy as dopant for NaAlH ₄ . However, the preparation required very long Kugelmahldauern and the material thus prepared exhibited a loading time of z. B. 12 h at a working temperature of 120 ⁰ C and a H ₂ loading pressure of 12 MPa only a very slow kinetics.

Since the production of alanate is chemically associated with high costs, according to DE 100 12 794 Al, the alanate by the jerk reaction according to Eq. (2) and (1), ie produced using inexpensive starting materials such as NaH and Al and transition metal or rare earth metal catalysts as dopants. However, using TiCl ₃ as catalyst precursor for the catalyst at a cost of approximately 15-20 EUR / g leads to catalyst costs of around 50,000 EUR for 100 kg of storage material, which significantly limits commercial use ,

Attractive would be the use of the much cheaper TiCi ₄ with a price of about 0.02 EUR / g. The price for an equivalent amount of Ti for 100 kg of storage material would only be around 50 EUR. However, this compound is tetravalent and contains only about 25% by weight of Ti, the remainder being inactive chloride, which lowers the gravimetric storage capacity of the material. When doping a metal hydride with a polyvalent Ti compound such as TiCl, a part of the hydrogen storage material is consumed, forming metallic Ti ⁰ by a redox reaction. According to Eq. (4) In addition to metallic Al ^υ , which stores no hydrogen, other memory-inactive by-products such as in particular NaCl arise.

Also in the production of the memory material via the jerk reaction of Eq. (2) after addition of the catalyst precursor (Ti precursor) according to

4 NaH + TiCl ₄ → 4 NaCl + Ti + 2H,; 5)

memory-inactive material formed.

Table 1: Theoretical proportion of storage-inactive material when using Ti, TiCl ₃ and TiCl ₄ as catalyst precursor (precursor)

According to Table 1, the theoretical proportion of memory-inactive material in the use of TiCl ₃ or TiCl ₄ as a catalyst precursor or metallic Ti, each with a concentration of 2 mol%, depending on the preparation process is between 1.9 and 14.4 wt. %. The use of the inexpensive TiCl ₄ was thus the approximately 6-8 times the amount of inactive storage material produce compared to metalli ^¬ cal Ti, which is by far the lowest proportion memory inactive Causes substances. Em use of metallic Ti-Pu ^ Lver is, however, as already described above, not advantageous due to the long Kugelmahldauern in the production and the slow kinetics of the product.

On this basis, it is the object of the invention to provide a process for the preparation of hydrogen storage materials, which does not have the disadvantages and limitations mentioned. In particular, a method is to be provided which enables the production of a hydrogen storage material which has a low content of inactive substances and at the same time has a fast kinetics. Such a method should be able to resort to an inexpensive catalyst precursor (precursor) and be characterized by the shortest possible process times.

This object is achieved by a method having the features of claim 1. The dependent claims describe advantageous embodiments of the invention.

The process according to the invention for producing a hydrogen storage material which comprises at least one metal hydride and at least one nonhydrogenated material and which is doped with a catalytically active metal as catalyst, comprises the steps a) to c) explained in detail below. ,

First, a catalyst precursor (precursor) is usually provided in the form of a powder containing the metal, which serves as a catalytically acting dopant for the produced water ^¬ material storage material. As the metal is preferably a transition metal of groups 3, 4, 5, 6, 7, 8, 9, 10 and 11 of the Periodic Table or a rare earth metal, wherein titanium, zirconium, iron, cobalt, nickel or cerium and a mixture or alloy of min ^¬ least two of these metals is particularly preferred. The catalyst precursor itself represents a metal compound, preferably in Form of a hydride, carbide, nitride, oxide, Alkohςats or halide, in particular a chloride, is present.

The metal compound provided is then used in step a) as a catalyst precursor with the non-hydrogenated material of the hydrogen storage material, which is usually also present as a powder, and preferably with an organic solvent such. Tetrahydrofuran or diethyl ether intimately mixed and preferably stirred for several hours. As the non-hydrogenated material of the hydrogen storage material, an element of the 3rd main group of the periodic table such as boron or aluminum is preferably used, with aluminum being particularly preferred.

By mixing and then stirring this mixture, the metal contained in the catalyst precursor is already partially reduced. From the catalyst precursor an intermediate product is generally formed, which is not volatile and therefore does not remove before the heat treatment.

In the subsequent step b), this mixture is subjected to a heat treatment, preferably at a temperature between 250 ⁰ C and 900 ⁰ C, more preferably between 350 ° C and 600 ⁰ C, in particular between 450 ⁰ C and 550 ⁰ C, however always below the melting point of the unhydrogenated material. This results in a thermally induced solid state reaction between the metal contained in the catalyst precursor and the non-hydrogenated material, in which, if the non-hydrogenated material is a metal, intermetallic phases are formed. The catalyst precursor or the non-volatile intermediate product formed therefrom thus decompose and the metal thus released forms a composite with the non-hydrogenated material which is catalytically active.

Finally, the composite prepared in this way according to step c) with a metal hydride, which is used for the hydrogen storage material is suitable, mixed and the mixture thus obtained preferably in a ball mill for several hours. Preferred metal hydrides are alkali metal hydrides such as NaH, KH, LiH or a mixture of these metal hydrides. After a time of about 1-3 hours, the finished hydrogen storage material can be removed from the (ball) mill.

The inventively produced catalytically active metal-doped hydrogen storage material consisting of the metal hydride used and according to step a) and b) pretreated non-hydrogenated material such as aluminum powder, represents itself a nano-composite, which is hydrogen ( in the case of NaH and aluminum powder with the formation of NaAlH ₄ ) loaded and discharged again. By changing the initial amounts of metal hydride and pretreated non-hydrogenated material (composite), different storage amounts and loading and unloading kinetics can be achieved.

The inventive method is based on the surprising finding that in a process based on the jerk reaction of equations (2) and (1), the chloride is almost completely removed when the Al powder is mixed with inexpensive TiCl ₄ and reacted is brought and then the reaction product between 350 ⁰ C and 600 ° C in the inert gas stream is heat treated. In this way, a finely divided titanium phase is formed, which is catalytically active. The chlorine gas produced at the same time can be neutralized at the exit of the furnace eg with potassium hydroxide solution.

The inventive method thus allows the use of inexpensive TiCl ₄ as a catalyst precursor (precursor) for the preparation of a hydrogen storage material based on NaH and Al, whose loading and discharge kinetics of that of a hydrogen storage material corresponds, using the far more expensive TiCl _{3 was} prepared. Despite the higher chlorine content in TiCl ₄ compared to TiCl _3, the hydrogen storage material only contains a small fraction of the chlorine from the catalyst precursor,

4 i. a lower proportion of inactive substances.

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

Fig. 1 desorption behavior of a hydrogen storage material produced according to the invention.

Fig. 2 Rontgenpulverdiffraktogramm one produced by the inventive method AlTi _{0 02} composite.

To a slurry of 27 g AI powder (1 πiol, 325 mesh, 99.8% purity) in 200 mL THF was added 3.79 g TiCl ₄ (0.02 mol) and stirred under inert conditions for several hours. This resulted in a turquoise blue color, suggesting a partial reduction of titanium.

By this pretreatment of the Al powder in the Flussigphase initially formed by partial reduction, the trivalent titanium compound TiCl ₃ - (THF), in contrast to TiCl ₄ , which has a boiling point K _p = 136 ° C, is not volatile and when removing the solvent, the surface of the aluminum particles as a thin layer covered.

The gray-green residue was then heat-treated in an inert gas stream at ca. 5OU ⁰ C for 90 minutes. The residue took on a dark gray color. The elemental analysis showed a content of 1.9 ± 0.1 mol% titanium and 1.0 ± 0.1 mol% chlorine in the composite. The Heat Treatment ^¬ lung in the inert gas causes a decomposition of the material and surface ^¬ cal reactions with the aluminum, whereby the intermetallic titanium-aluminum layers formed, which are catalytically active.

1.034 g of the composite material was ball milled with 0.888 g of NaH for 120 minutes under an argon atmosphere (planetary ball mill, 600 rpm, Si nitride beaker and balls, B / P ratio 20: 1). *

With 2 g of the resulting material, hydrogen loading and unloading cycles were carried out in a Sievert's apparatus. In Fig. 1, the Desorpcionsverhalten of this exemplary embodiment, prepared according to the hydrogen storage material that had been annealed for 90 minutes at 500 ⁰ C, shown in the first desorption (right curve) and in the 5th desorption (left curve). It turns out that an improvement in the conversion kinetics occurs when the material is cycled several times. The loading of the hydrogen was carried out at 100 ⁰ C and a total pressure of 10 MPa H ₂ ; the discharge was carried out at a temperature of 150 ⁰ C at a total pressure of 0.04 MPa H ₂ . The charge and discharge kinetics roughly correspond to those of a hydrogen storage material prepared using the same amount of far more expensive TiCl ₃ . By varying the starting amounts of NaH (+ y MH with y = 0-1, M = metal) and AlTi _x different storage amounts and charging and discharge kinetics can be provided.

Fig. 2 shows a Roncgenpuiver diffractogram of an AlTi _{0. o:} composite material prepared according to the invention. The symbols indicate which elements or connections the signals marked with are returned to. This result can be considered as evidence for both the formation of metallic Ti and Al ₃ Ti and Al ₂ Ti, wherein the proportions of the newly formed crystalline phases vary depending on the temperature and treatment time.

From likewise obtained REM / EDX images of the element distribution of Al and Ti in the micrograph of a 90 minutes at 500 ⁰ C treated Al-Ti composite (AlTi _{0 02} ) it can be seen that the thus pretreated Al particles surrounded by a titanium-containing layer are only up to about 1/10 of the chlorine from the catalyst precursor ent ^¬ halt.

Claims

Patent claims:

A process for producing a hydrogen storage material comprising a metal hydride and a non-hydrogenated material doped with a metal catalyst, comprising the steps of: a) mixing a catalyst precursor containing the metal with the catalyst; non-hydrogenated material, and stirring this mixture, b) heat treating the mixture, whereby the metal from the Kata ^¬ lyst precursor with the non-hydrogenated material is a compo- forms sit, c) mixing the composite with the metal hydride and grinding the mixture, after which the hydrogen storage material is obtained.

2. The method of claim 1, wherein a transition metal from group 3 to 11 of the periodic table or a rare earth metal is selected as the metal of the catalyst precursor.

3. The method of claim 2, wherein titanium, zirconium, iron, cobalt, nickel, cerium or a mixture or alloy of these metals is selected as the metal of the catalyst precursor.

A process according to any one of claims 1 to 3, wherein the catalyst precursor is in the form of a halide, hydride, carbide, nitride, oxide or alcoholate.

The process of claim 4 wherein the catalyst precursor is in the form of a chloride.

6. The method according to any one of claims 1 to 5, wherein an element of the 3rd main group of the periodic table is used as a non-hydrogenated material of the hydrogen storage material.

7. The method of claim 6, wherein aluminum or boron used as a non-hydrogenated material of the hydrogen storage material becomes .

8. The method according to any one of claims 1 to 7, wherein LiH, NaH, KH or a mixture thereof is used as the metal hydride.

A process according to any one of claims 1 to 8 wherein an organic solvent is added to mix the catalyst precursor with the non-hydrogenated material.

10. The method of claim 9, wherein tetrahydrofuran or diethyl ether is used as the organic solvent.

11. The method according to any one of claims 1 to 10, wherein the heat treatment at a temperature between 250 ⁰ C and 900 ⁰ C, but always below the melting point of the non-hydrogenated material takes place.

12. The method of claim 11, wherein the heat treatment is carried out at a temperature between 350 ⁰ C and 600 ⁰ C.