FR2938456A1 - Metal particles, useful for the hydrolysis reaction of a chemical hydride into hydrogen, comprises elements comprising group IIIA-VIIB of the periodic table of elements and their mixtures, carrying hydrophobic groups on their surface - Google Patents

Abstract

Metal particles (5) comprises one or more elements comprising group IIIA-VIIB of the periodic table of elements and their mixtures, carrying hydrophobic groups on their surface. Independent claims are included for: (1) preparing metal particles, where a surface treatment intended to make the hydrophobic catalyst is carried out by plasma-enhanced chemical vapor deposition; (2) a reaction chamber (1) for implementation of a hydrolysis reaction of a chemical hydride into hydrogen, where the reaction chamber comprises the metal particles; and (3) fuel cell comprising the reaction chamber.

Description

The invention relates to new materials for the production of hydrogen by hydrolysis of chemical hydrides and a process for producing hydrogen using these new materials. More particularly, the invention relates to catalyst materials for the hydrolysis reaction of chemical hydrides in hydrogen. Hydrogen is a so-called clean fuel because it reacts with oxygen in appropriate devices to provide energy and water. It is used as fuel in some combustion engines and in fuel cells. The use of hydrogen as a fuel is therefore interesting from an environmental point of view. It is therefore desirable to have methods of producing hydrogen that are satisfactory and that can be implemented in devices of reduced size or miniaturized so as to integrate this production of hydrogen in microsystems. US Pat. No. 6,534,033 discloses metal catalysts which make it possible to carry out the reduction of chemical hydrides, and in particular of chemical borohydrides, in hydrogen, this reaction being carried out in the presence of water, according to the diagram below. below:

NaBH4 + 2H20. NaBO2 + 4H2 + Catalyst energy

This hydrolysis reaction is exothermic, it does not require the use of a heat source or high pressure to start and it can

produce hydrogen even at very low temperatures (0 ° C). The borate produced is

non-toxic and can be regenerated to borohydride.

A theoretical mass yield in hydrogen can be calculated: it is

equal to 10.8%. But at room temperature, the stoichiometry of the reaction requires twice more water and generates a di-hydrogenated borate NaBO 2, 2H 2 O (or NaB (OH) 4). The

theoretical yield is then equal to 7.3%.

In addition to sodium borohydride (NaBH4), other chemical borohydrides can be used to carry out this reaction, such as, for example, zinc borohydride (ZnBH4), potassium borohydride (KBH4),

Calcium (CaBH4), but also other chemical hydrides such as lithium aluminum hydride (LiAlH4), potassium aluminum hydride (KA1H4), sodium gallium hydride (NaGaH4), lithium gallium hydride (LiGaH4), gallium potassium hydride (KGaH4), sodium trimethoxyborohydride (NaBH (OCH3) 3). And one can include in the list of hydrides usable as reagent in the above reaction ammonium borohydride (NH3BH3).

The catalysts which can be used for carrying out the hydrolysis reaction and which are taught by US Pat. No. 6,534,033 include metals ranging from group IB to group VIIIB of the periodic table of elements, or compounds obtained from these metals. Mention may in particular be made of ruthenium, copper, cobalt, iron, nickel, manganese, rhodium, rhenium, platinum, palladium, chromium, silver, osmium and iridium. A limiting factor of the reaction is the solubility of the starting hydrides. The solubility of sodium borohydride in water is only 35%, and it is lower for the other hydrides. Since the reaction consumes twice as many moles of water as moles of hydride chemical, the concentration of the latter tends to increase as the progress of the reaction. It is therefore necessary to provide either a very dilute solution of sodium borohydride or to supply water to the reaction as it progresses. But to maximize the energy density of the system we try to work with sodium borohydride concentrations as high as possible.

Another factor limiting the development of hydrogen production processes based on this reaction is the formation of borate, much less soluble in water than sodium borohydride. The metaborate formed (NaBO2.xH2O) has, at room temperature, a solubility in water of 22.3%. Also, at room temperature, if one wants to avoid the formation of solid deposits, it is necessary to use a solution of NaBH4 with a maximum concentration of 11.4% by weight of NaBH4 relative to the body of water. And the yield of H2 obtained under these conditions (without taking into account the mass of sodium hydroxide necessary to stabilize the aqueous solution of borohydride) is then only 2.6% by weight of the H2 produced, relative to the mass of NaBH4 + H2O. And when borate forms in the reaction medium in solid form, it tends to deposit on the catalyst, which hinders access of the reactants to the catalyst and blocks the progress of the reaction. Given the exothermic nature of the hydrolysis reaction and the temperature rise within the reactor, which makes it possible to better solubilize the products, it is possible to reach a mass concentration of NaBH 4 of 20% and a yield of H 2 of 4. , 2%. But when it is desired to produce hydrogen on demand, that is to say as and when consumption, if it is irregular, it may happen that the system stops working, and that the reactor cools, which then causes a precipitation of the by-products of the reaction. This precipitate tends to deposit on the surface of the catalyst and prevents the reaction from being resumed once all available hydrogen has been consumed. Different systems have been designed to prevent this clogging of the reactor: The application WO2007 / 142679 describes a multi-compartment reactor in which a constant flow of fuel is maintained in the catalyst chamber, the fuel solution is very diluted and the reactor is purged. on a regular basis to prevent fouling by solid residues.

In the device described in US Pat. No. 7,323,148, the catalyst chamber is made of a porous material on which the catalyst particles are arranged. Gradients in the properties of the support material make it possible to control the flow of materials within the catalyst chamber. In particular hydrophilic or hydrophobic membranes can be used to control the flow of reagents and reaction products. Washing the catalyst chamber with water can remove the solid residues formed. However, these devices have the disadvantage of being very complex and therefore bulky, which results in a yield of hydrogen in relation to the occupied volume which is reduced. Thus, there remained the need for a process and means for making the hydrolysis reaction of hydrides in hydrogen at high reactant concentrations so as to have high hydrogen yields and it was desired that The reaction can be carried out in simple, compact reactors, and passively (without the need to use operating auxiliaries), so as to promote the hydrogen yield in relation to the size of the device. And this process must be able to operate on demand, without a temporary stop causing the catalyst to block and prevent the reaction from being resumed. The solution to this problem lies in the development of new materials which will be described below. A first object of the invention consists of metal particles bearing on their surface hydrophobic groups. These metal particles consist of one or more elements selected from group IIIA atoms in group VIIB of the periodic table of elements and their mixtures. Preferably, one or more atoms are chosen for the catalyst in the following group: Pt, Ru, Pd, Co, Cr, Ni, Fe. Among these compounds, mention may be made of metal borides such as CoiBi, with i, j two integers or decimals 0 <i <25, 1 <j <8, and preferably CoB and Co2B. By hydrophobic group is meant more particularly a group chosen from: siloxane chains such as SiORR 'chains, with R, R' identical or different selected from: H, a C1-C30 alkyl or alkenyl group, or aryl C6-C30, and especially dimethylsiloxanes [(SiO (CH3) 2] r, and n> 1 or diphenyl siloxanes [(SiO (C6H5) 2] n and n> 1 and preferably SiO (CH3) 2 chains halogen, preferably fluorine, perfluorinated carbon chains, where n, m, p are integers or decimals: ## EQU1 ## As illustrated in the examples, in particular, nanoparticles of the CokFk type may be formed, with k and k 'non-zero integers, metal nanoparticles carrying a hydrophobic nanometric coating layer of the C 1, H 1, HP type, such as CF 2 or CF 3, nanoparticles of metal carrying a nanometric hydrophobic coating layer of the SiOR 2 type such as SiO (CH 3) 2. metal nanoparticles, that is to say particles having a size between 1 and 100 nm. In fact, small metal particles are more effective as catalysts. However, it is also possible to provide larger particles that are made porous by means of a suitable treatment so as to promote the efficiency of the catalysis (for example by specific deposition, by acid attack, by addition of blowing). The hydrophobic groups are either grafted by a covalent bond to the metal particles or deposited on these metal particles to which they adhere by covalent or metal bonding. When a hydrophobic coating is deposited on the metal particles, this deposit is either discontinuous, and in this case it may be porous or non-porous, or continuous and then it is porous, so as to allow access to the catalyst and to allow the latter to fulfill its role vis-à-vis the hydrogen precursors. The materials of the invention may be prepared: in a single step, during which a surface treatment for rendering the catalyst hydrophobic is carried out simultaneously with the synthesis of the catalyst; in two stages: on the catalyst, a hydrophobic coating is deposited or hydrophobic molecules are grafted. The deposition of a hydrophobic coating on metal particles can be done by any method known to those skilled in the art: PECVD (Plasma Enhanced Chemical Vapor Deposition or Plasma Enhanced Chemical Vapor Deposition). The grafting of a layer of fluorine atoms on the surface of the metal particles can be done by treatment with hydrofluoric acid either on the metal atoms before forming the particles, or on the particles already formed.

The preparation of these materials may comprise an additional step of shaping before use, such as for example the attachment of catalyst particles on a support, whether it be a plate, the walls of a tube, metal or glass balls.

US Pat. No. 7,323,148 discloses hydrophobic porous supports on which are deposited metal catalysts which can be used for carrying out the hydrolysis reaction of chemical hydrides in hydrogen, but these metal catalysts do not carry hydrophobic groups on their surface and are themselves directly exposed to the reaction medium. And the presence of a hydrophobic environment prevents borate deposits from forming on their surface. The materials of the invention have advantages over the catalysts of the prior art: they have a hydrophobic surface, and therefore the borate formed during the hydrolysis reaction of sodium borohydride does not occur. does not deposit on the surface of the catalyst. This is still available for the hydrogen precursor reagents and in particular the stopping and restarting steps of the hydrogen generation in the process flow do not cause a blockage of the system, despite the cooling of the reactor. This material thus makes it possible to catalyze hydrolysis reactions of chemical hydrides in hydrogen with very high concentrations of chemical hydrides. This results in an increase in the mass energy density of the fuel and therefore in the complete fuel source. And this system does not require the use of additional auxiliaries to purge the catalytic system. The invention further relates to the use of a material as described above for carrying out the hydrolysis reaction of a metal hydride to hydrogen according to the scheme below which illustrates the reaction with NaBH 4 as a precursor: NaBH4 + 2H2O NaBO2 + 4H2 + Energy Catalyst In addition to sodium borohydride (NaBH4), other chemical borohydrides can be used to carry out this reaction, such as, for example, zinc borohydride (ZnBH4) or potassium borohydride (KBH4). ), calcium borohydride (CaBH4), magnesium borohydride (MgBH4), but also other hydrides such as lithium aluminum hydride (LiAlH4), potassium aluminum hydride (KA1H4), hydride of sodium gallium (NaGaH4), lithium gallium hydride (LiGaH4), gallium potassium hydride (KGaH4), sodium trimethoxyborohydride (NaBH (OCH3) 3). And may also include in the list of hydrides usable as reagents in the above reaction ammonium borohydride (NH3BH3), calcium hydride CaH2, lithium hydride LiH, sodium hydride NaH, MgH2 magnesium hydride. The invention also relates to a reaction chamber for carrying out a hydrolysis reaction of a hydride chemical hydrogen, the reaction chamber comprising particles as described above and being adapted to allow their use: it must allow a flow of reactants entering and leaving the chamber while retaining the material of the invention in the reaction chamber. Such a reaction chamber (1) can consist as illustrated in FIG. 1, in a space delimited by walls (2), open at two ends (3) and (4) and comprising a grafting of at least a portion of the wall by particles (5) of material of the invention. The reagents: water and chemical hydride, with sometimes, and in a known manner, stabilizing agents to prevent the spontaneous decomposition of the hydride in water, are introduced by one end (3) and the products of the reaction come out through the other end (4). The hydrogen can be selectively recovered at the outlet of the reaction chamber by means of a suitable membrane. The borate can be recovered and recycled. It is also possible, as illustrated in FIG. 2, to use a reaction chamber (11) consisting of a space delimited by walls (12) and closed at its ends: on one side by a permeable membrane (13) to water and impervious to metal nanoparticles, on the other side by a membrane (14) permeable to water, hydrogen and by-products of the reaction and impermeable to metal nanoparticles. This reaction chamber (11) further comprises a material of the invention in the form of nanoparticles (15) which are in suspension in the reaction medium (16). According to one variant, the nanoparticles may be supported by a suitable support such as a porous Ni or carbon foam. The first membrane makes it possible to supply the reaction chamber with reagent and the second membrane makes it possible to recover the hydrogen and the borate. The invention further relates to a fuel cell comprising a hydrogen generating device comprising a material as described above for catalyzing the hydrogen production reaction. And in particular a fuel cell comprising a reaction chamber as described above. EXPERIMENTAL PART Example 1: Step of fluorination of cobalt-based catalyst A fluorination solution (F-solution) is prepared by mixing 0.6 ml of hydrofluoric acid with 6 g of potassium fluoride in 1 liter of water. 5 g of cobalt nanoparticles are immersed in 200 ml of F-solution with stirring (500 rpm) for 30 minutes and at 30 ° C. After treatment, the catalyst is washed several times with distilled water, filtered by centrifugation and then dried under vacuum. This fluorination step makes it possible first of all to remove all traces of oxides on the surface of the catalyst, and also to obtain a cobalt fluoride of the CokFk 'type. This step can be carried out in situ during the synthesis of the catalyst or once the catalyst is synthesized. Example 2: Deposition of a Hydrophobic Coating of CFA Type by Plasma Enhanced Chemical Vapor Deposition (PECVD) on a Nickel-Based Catalyst The catalytic Ni powder is placed in a chamber under a vacuum of 10-3 bar. A flow rate of 200 cc / min of Helium and 150 cc / min of C4F8 (octafluorobutene) is added to the chamber. Equipped with a 150 watt RF generator, a 13.56 MHz plasma is then applied between the two electrodes 20 mm apart for 3 minutes. The catalytic powder is then partially covered with a hydrophobic nanometric deposition of CFX type. Example 3: Deposition of a coating with a hinge of tv s e SiO C ~ .HZ by PECVD on a catalyst based on ruthenium. The Ru catalytic powder is placed in a chamber under a vacuum of 10-3 bar, a flow rate of 475 cc / min of Helium and 25 cc / min of C8H24O4Si4 (octamethylcyclotetrasiloxane) is added to the chamber. A RF generator of power 100 W, a plasma of 13.56 MHz is then applied between the two electrodes 30 mm apart for 1 minute, the catalytic powder is then partially covered with a nanometric hydrophobic deposition of SiOXCyHZ type.

Claims (4)

REVENDICATIONS1. Metallic particles consisting of one or more elements chosen from the group IIIA atoms in group VIIB of the periodic table of elements and their mixtures, bearing on their surface hydrophobic groups. 2. Metallic particles according to claim 1, wherein the group IIIA to group VIIB atoms are selected from: Pt, Ru, Pd, Co, Cr, Ni, Fe. 3. Metallic particles according to claim 1, wherein the precursors of the group IIIA to group VIIB atoms are chosen from metal borides and preferably Co; Bj, with i, j two integers or decimals 0 <i <25, 1 < j <8. 4. Metallic particles according to any one of claims 1 to 3 wherein the hydrophobic group is selected from: - siloxane chains such as SiORR 'chains, with R, R' identical or different selected from: H, an alkyl group or C1-C30 alkenyl, or C6-C30 aryl, - halogen, - perfluorinated carbon chains CnFmHp, with n, m, p integers or decimals: 0 <n, 0 <m, 0 p 8. Particles metal according to any one of claims 1 to 4 which have a size between 1 and 100 nm. 9. Particles according to any one of claims 1 to 5, which are nanoparticles of CokFk type, with k and k 'non-zero integers. 10. Particles according to any one of claims 1 to 5, which are metal nanoparticles carrying a hydrophobic nanometric coating layer selected from CF2 and CF3. 11. Particles according to any one of claims 1 to 5, which are metal nanoparticles carrying a hydrophobic nanometric layer of SiO (CH3) 2 type. A method of manufacturing metal particles according to any one of claims 1 to 8, wherein a surface treatment for rendering the hydrophobic catalyst is carried out by PECVD 13. A process for producing metal particles according to any one of Claims 1 to 8, wherein a grafting of a layer of fluorine atoms on the surface of the metal particles is done by treatment with hydrofluoric acid. 11. The use of particles as claimed in any one of claims 1 to 8 for carrying out the hydrolysis reaction of a chemical hydride to hydrogen. A reaction chamber for carrying out a hydrolysis reaction of a chemical hydride to hydrogen, said reaction chamber comprising particles according to any one of claims 1 to 8. fuel comprising a reaction chamber according to claim 12.