Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
  • B01J35/00—Catalysts, in general, characterised by their form or physical properties
  • B01J35/20—Catalysts, in general, characterised by their form or physical properties characterised by their non-solid state
  • B01J35/23—Catalysts, in general, characterised by their form or physical properties characterised by their non-solid state in a colloidal state
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
  • B01J35/00—Catalysts, in general, characterised by their form or physical properties
  • B01J35/40—Catalysts, in general, characterised by their form or physical properties characterised by dimensions, e.g. grain size
  • B01J35/45—Nanoparticles
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B82—NANOTECHNOLOGY
  • B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
  • B82Y30/00—Nanotechnology for materials or surface science, e.g. nanocomposites
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M4/00—Electrodes
  • H01M4/86—Inert electrodes with catalytic activity, e.g. for fuel cells
  • H01M4/8647—Inert electrodes with catalytic activity, e.g. for fuel cells consisting of more than one material, e.g. consisting of composites
  • H01M4/8657—Inert electrodes with catalytic activity, e.g. for fuel cells consisting of more than one material, e.g. consisting of composites layered
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M4/00—Electrodes
  • H01M4/86—Inert electrodes with catalytic activity, e.g. for fuel cells
  • H01M4/90—Selection of catalytic material
  • H01M4/92—Metals of platinum group
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M4/00—Electrodes
  • H01M4/86—Inert electrodes with catalytic activity, e.g. for fuel cells
  • H01M4/90—Selection of catalytic material
  • H01M4/92—Metals of platinum group
  • H01M4/925—Metals of platinum group supported on carriers, e.g. powder carriers
  • H01M4/926—Metals of platinum group supported on carriers, e.g. powder carriers on carbon or graphite
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M4/00—Electrodes
  • H01M4/86—Inert electrodes with catalytic activity, e.g. for fuel cells
  • H01M4/90—Selection of catalytic material
  • H01M4/9008—Organic or organo-metallic compounds
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M4/00—Electrodes
  • H01M4/86—Inert electrodes with catalytic activity, e.g. for fuel cells
  • H01M4/90—Selection of catalytic material
  • H01M4/92—Metals of platinum group
  • H01M4/921—Alloys or mixtures with metallic elements
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M4/00—Electrodes
  • H01M4/86—Inert electrodes with catalytic activity, e.g. for fuel cells
  • H01M4/90—Selection of catalytic material
  • H01M4/92—Metals of platinum group
  • H01M4/923—Compounds thereof with non-metallic elements
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
  • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
  • Y02E60/30—Hydrogen technology
  • Y02E60/50—Fuel cells
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
  • Y10S—TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10S977/00—Nanotechnology
  • Y10S977/70—Nanostructure
  • Y10S977/773—Nanoparticle, i.e. structure having three dimensions of 100 nm or less
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
  • Y10S—TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10S977/00—Nanotechnology
  • Y10S977/70—Nanostructure
  • Y10S977/81—Of specified metal or metal alloy composition
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
  • Y10S—TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10S977/00—Nanotechnology
  • Y10S977/902—Specified use of nanostructure
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
  • Y10S—TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10S977/00—Nanotechnology
  • Y10S977/902—Specified use of nanostructure
  • Y10S977/932—Specified use of nanostructure for electronic or optoelectronic application
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
  • Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
  • Y10T428/00—Stock material or miscellaneous articles
  • Y10T428/25—Web or sheet containing structurally defined element or component and including a second component containing structurally defined particles
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
  • Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
  • Y10T428/00—Stock material or miscellaneous articles
  • Y10T428/29—Coated or structually defined flake, particle, cell, strand, strand portion, rod, filament, macroscopic fiber or mass thereof
  • Y10T428/2982—Particulate matter [e.g., sphere, flake, etc.]
  • Y10T428/2991—Coated

Definitions

• the present invention relates to the use of nanoparticles having a metallic core, more precisely based on a platinoid or an alloy of a platinoid, and a double organic coating, as catalysts. It also relates to nanoparticles thus formed.
• the nanoparticles targeted by the present invention combine remarkable catalytic and, in particular, electrocatalytic properties with very satisfactory properties of dispersibility and stability in liquid medium. They are therefore capable of being used in all fields in which catalytic and, in particular, electrocatalytic processes are involved and, in particular, in devices for producing electrical energy such as fuel cells.
• the coatings initially proposed were formed of polymers or surfactants, they are, more and more often, made up of molecules which have a chemical function (acid, thiol, phosphate, isocyanate, ...) allowing their attachment to the surface nanoparticles.
• the approach consists in coating them optimally to stabilize them in a durable way and to be able to handle them easily in medium liquid appears a priori for the skilled person in contradiction with the maintenance of an availability of the metal surface of these nanoparticles as favorable as possible to the electrocatalytic process that one seeks to exploit.
• organic coatings conventionally proposed to date for stabilizing metallic nanoparticles intended to serve as catalysts are polymers which stabilize these nanoparticles essentially by steric effects, such as polyvinyl alcohol, polyacrylic acid and poly (n-vinylpyrrolidone) •
• the chemical bonds ensuring the fixing of these polymers on the metal surface nanoparticles are neither numerous nor very strong, so that they are liable to be destroyed by media adapted to the expression of electrochemical phenomena and which are characterized by very acidic pH or, on the contrary, very basic and by high ionic forces. Such destruction obviously results in the loss of the stabilizing effect initially sought.
• Organic coatings consisting of surfactants, ionic or neutral, are also known.
• the molecules of surfactant must be long enough to have a stabilizing effect and have chains of methylene groups (CH 2) n a priori unsuited to electronic or ionic charge transfers involved in electrocatalytic processes.
• CH 2 methylene groups
• the chemical bonds ensuring the attachment of the surfactant molecules to the metal surface of the nanoparticles are relatively weak and therefore sensitive to the particular conditions of pH and ionic strength of the media suitable for electrochemistry.
• some authors have recommended the use of stabilizing polymers or copolymers provided with chemical groups capable of improving charge transfers or mass involved in electrocatalytic processes, and in particular for fuel cell applications.
• 6,462,095 [1] describes platinum nanoparticles stabilized by a polymer or copolymer of cation exchangers of the polyarylether sulfone ketone type, sulfonated polyether sulfone, poly (acrylo-nitrile / butadiene / styrene) or still poly- ( ⁇ , ⁇ , ⁇ -trifluorostyrene / sulfonic acid).
• stabilizing molecules have been proposed capable of forming strong interactions with the metallic surface of the nanoparticles. These are molecules carrying thiol functions which establish iono-covalent bonds with many metals.
• the second organic ring of nanoparticles ensures the properties of selective interaction of thin films with the species or chemical species to be detected;
• the metallic core of the nanoparticles provides the possibility of measuring a variation in the electrical conductivity of thin films, while the first organic core serves essentially to ensure the attachment of the second organic core to said metallic core.
• nanoparticles comprising, like those described in reference [5], a platinum core and a double organic coating, are endowed with remarkable catalytic, and in particular electrocatalytic, properties without it being necessary to subject them to any activation treatment.
• nanoparticles have very satisfactory properties of dispersibility, stability in a liquid medium and resistance to very acidic or very basic media and with high ionic forces conventionally used in the field of electrochemistry. It is this observation which is the basis of the present invention.
• the invention therefore relates, firstly, to the use of nanoparticles comprising: a metallic core containing at least one platinoid or an alloy of a platinoid, a first organic coating formed of molecules fixed on the surface of the metallic core, and a second organic coating formed of molecules different from the molecules of the first organic coating and which are grafted onto molecules of the first organic coating, as catalysts.
• platinum means a metal chosen from platinum, iridium, palladium, ruthenium and osmium, and by “alloy of a platinoid”, an alloy comprising at least one platinoid, this alloy possibly being natural like osmiridium (natural alloy of iridium and osmium) or unnatural like an alloy of platinum and iron, platinum and cobalt or else platinum and nickel.
• the metallic core of the nanoparticles consists of platinum or a platinum alloy or a mixture of the two.
• the molecules of the first organic coating serve mainly to allow the grafting of the molecules of the second organic coating, while the molecules of the second organic coating provide a significant improvement in the stability of the nanoparticles in suspension in a liquid medium, the two coatings allowing the phenomena of charge and material transport as well as the accessibility of the surface of the metallic core necessary for the expression of the catalytic properties of nanoparticles.
• the molecules of the first organic coating are preferably fixed on the surface of the metallic core by a chemical bond with a strong covalent character, that is to say by a covalent or iono-covalent bond.
• the molecules of the first organic coating which are preferably identical to each other for the same particle, are the remains of at least bifunctional compounds, that is to say that is to say of compounds which have at least two free chemical functions: a first function called, hereinafter, "FI function” and capable of forming a chemical bond of strong covalent character with the surface of the metal core for their attachment to this surface , and a second function called, hereinafter, "F2 function” and capable of reacting with at least one function carried by the compounds chosen to form the molecules of the second organic coating for their grafting by the latter.
• FI function first function
• F2 function a second function
• the molecules of the second organic coating which are also preferably identical to one another for the same particle, are the remains of compounds which comprise at least one free chemical function which is hereinafter called “function F3 "and which is capable of reacting with the function F2 of the above-mentioned bifunctional compound.
• the expression “residue of compounds” means the part of these compounds which remain on the nanoparticles when they are: • either fixed on the surface of the metallic core and, possibly, grafted with a molecule of the second organic coating, if they are residues forming the first organic coating; • or grafted onto a molecule of the first organic coating, if they are residues forming the second organic coating.
• the formation of a covalent or iono-covalent chemical bond between the F1 function of the at least bifunctional compounds chosen to form the molecules of the first organic coating and the surface of the metallic core can be obtained by any one methods used in the prior art to establish this type of bond between an organic compound and a metal.
• it can be obtained by synthesizing the nanoparticles by reduction of a metal salt corresponding to the metal which must constitute the core thereof by means of said at least bifunctional compounds.
• it can also be obtained by replacing, on nanoparticles formed from a metallic core covered with a labile compound, this compound with the at least bifunctional compounds.
• the reaction between the function F2 of the at least bifunctional compounds and the function F3 of the compounds chosen to form the molecules of the second organic coating - which will be designated, in what follows, "grafting reaction” for convenience - may be, it, any reaction of organic chemistry making it possible to link, by any type of bond, preferably covalent, two organic compounds to each other from their respective chemical functions.
• the molecules of the first organic coating of the nanoparticles are capable of degrading on the surface of the metallic core when they are not grafted with molecules of the second organic coating.
• This degradation capacity can be either spontaneous, that is to say intrinsically linked to the nature of the molecules used, or result from a treatment of the nanoparticles, for example by means of an appropriate reagent, it being understood that it is appropriate that this treatment does not lead to degradation of the molecules forming the second organic coating.
• 4-mercaptoaniline which has both the characteristic of being bifunctional, since it comprises a thiol function and an amin function in the para position of a phenyl group, and that of degrading spontaneously, represents an example of compound particularly suitable for carrying out the first organic coating.
• the molecules of the second organic coating are grafted onto the molecules of the first organic coating by a grafting reaction at the end of which the grafting rate of said molecules of the first organic coating, it is to say the proportion of these molecules on which molecules of the second organic coating are grafted, is less than 100%. It follows that the nanoparticles obtained at the end of the grafting reaction have a first organic coating of which certain molecules are not grafted with molecules of the second organic coating. This is illustrated in FIG.
• each F1-F2 represents a molecule of the first organic coating which is not grafted with a molecule of the second organic coating
• • F3-D represents a compound chosen to form the molecules of the second organic coating
• each Fl-D represents a molecule of the first organic coating grafted by a molecule of the second organic coating.
• the degradation of the molecules of the first organic coating, which were not grafted by molecules of the second organic coating during the grafting reaction, should quickly lead to the elimination of these molecules from the surface of the metallic core and therefore to the liberation of the zones previously occupied by them which would thus become accessible.
• the grafting rate of the molecules of the first organic coating can be modulated by varying the respective amounts of nanoparticles and of the compounds chosen to form the molecules of the second organic coating which are reacted during the grafting reaction.
• this way of operating is not the one that is preferred in the context of the present invention because it risks leading to too partial grafting of the nanoparticles, which is likely to lead, in turn, taking into account degradation of the molecules of the first organic coating, at an insufficient or even zero dispersibility of the nanoparticles in a liquid medium and by instability of the suspensions prepared from these nanoparticles.
• it is preferred to modulate this grafting rate by playing on the geometric characteristics of the compounds chosen to form the molecules of the second organic coating, and in particular on the steric bulk generated by these molecules.
• the molecules of the second organic coating are the remains of compounds which, while being able to be of very varied nature (oligomers, polymers, etc.), are capable of: • confer on nanoparticles properties of dispersibility, stability in a liquid medium and resistance to mediums conventionally used in the field of electrochemistry, and this, in a sustainable manner to avoid any phenomenon of aggregation and migration of these nanoparticles in the short , medium and long term, especially when they are involved in electrochemical processes; • preserve the accessibility of the core surface of the nanoparticles; • exempt the nanoparticles from prior activation treatment; and possibly • optimize charge transfers (electronic and ionic) and mass, in the case where the nanoparticles are intended to be used in applications which involve such transfers (fuel cells for example).
• these molecules are the remains of compounds which have one or more of the following properties: 1. be able to preserve electronic transfers from one nanoparticle to another. To do this, these compounds must have as few saturated CC bonds as possible, since these are unfavorable to electronic transfers, and avoid leading to an excessive increase in the thickness formed by the two organic coatings, the probability of transfers electronic, decreasing quickly with distance. In this regard, it is preferable that the thickness formed by the two organic coatings does not exceed ten nm.
• compounds capable of preserving electronic transfers mention may be made of polycyclic compounds, and in particular polycyclic anhydrides such as tetraphenylphthalic anhydride, diphenic anhydride or diphenylmaleic anhydride. 2.
• These compounds can in particular be compounds of small dimensions which have a certain rigidity by the presence of aromatic rings and in which the function F3 is positioned so that the major axis of the molecules of the second organic coating, once grafted onto the molecules of the first coating, is oriented perpendicular rather than parallel to the axis of the covalent bond formed between said function F3 and the function F2 of the molecules of the first organic coating.
• Examples of such compounds are polyparaphenylenes substituted with at least one function for their grafting on the molecules of the first organic coating. 3.
• ionizable functional groups capable of relaying and thus making possible the transfer of ionic species.
• Such compounds are, for example, cyclic anhydrides such as glutaric anhydride, which can optionally be perfluorinated beforehand to exacerbate the ionizable nature of said functional group or groups. 4. have specific characteristics, in terms of chemical affinity, to favor the association of nanoparticles with a particular support, chosen according to the application for which they are intended. Thus, for example, these compounds may be more or less hydrophilic or hydrophobic depending on whether the support will itself be hydrophilic or hydrophobic, or include a polymerizable or copolymerizable species such as a thiophene or a pyrrole. 5.
• the molecules of the second organic coating are the residues of compounds chosen from thiophenes comprising at least one function for their grafting onto the molecules of the first organic coating, and the mono- and polycyclic anhydrides.
• the molecules forming the second organic coating are the residues of compounds chosen from thiophene acid chloride, glutaric anhydride, sulfobenzoic anhydride, diphenic anhydride, tetrafluorophthalic anhydride, anhydride tetraphenylphthalic and diphenylmaleic anhydride.
• the nanoparticles have a size of the order of 1.5 to 10 nm in diameter, and preferably of the order of 1.5 to 5 nm in diameter.
• the nanoparticles useful according to the invention are capable of being preserved before use and / or of being used in suspension in a solvent suitably chosen according to the degree of polarity of the molecules forming the second organic coating.
• the solvent used for this purpose is generally a polar aprotic solvent of the dimethyl sulfoxide, dimethylformamide or dimethylacetamide type, but it can also be an apolar solvent such as chloroform or dichloromethane, if it turns out that the nanoparticles are not dispersible in a polar aprotic solvent.
• the nanoparticles can be stored before use in the form of suspensions, of concentrations of the order of 0.3 to 1 mg / ml, which are then diluted according to the use for which these nanoparticles are intended.
• these suspensions it is possible to produce thin films, formed from one or more layers of nanoparticles, by deposition on supports of very varied nature and characteristics.
• these supports can be insulating, ionic conductors, conductors or semiconductors of electricity; they can be made of very diverse materials (metal, glass, carbon, plastic, textile, ...) and present itself as well in finely divided form as in massive form.
• they can be provided with electrodes.
• These thin films can be prepared by any of the techniques known to a person skilled in the art for manufacturing such films such as the Langmuir-Blodgett technique, the sequential deposition of self-assembled layers, spontaneous adsorption by chemical grafting or electrochemical, spinning deposition, surface impregnation deposition, electroplating or electrografting, the mechanism of which has been described by Bureau et al. in Macromolecules, 1997, _30, 333 [6] and in Journal of Adhesion, 1996, 58_, 101 [7] as well as by Bureau and Delhalle in Journal of Surface Analysis, 1999, 6 (2), 159 [8].
• the Langmuir-Blodgett technique the sequential deposition of self-assembled layers, spontaneous adsorption by chemical grafting or electrochemical, spinning deposition, surface impregnation deposition, electroplating or electrografting, the mechanism of which has been described by Bureau et al. in Macromolecules, 1997, _30, 333 [6] and
• the Langmuir-Blodgett technique which has been widely described in the literature, is, for example, well suited to depositing nanoparticles, monolayer by monolayer, on rigid supports, while surface impregnation is better suited, for example, to deposition of nanoparticles on flexible supports such as textile supports.
• the term "monolayer” means a layer whose thickness does not exceed the diameter of a nanoparticle when the latter is assimilated to a sphere.
• nanoparticles As previously described have numerous advantages as catalysts. Indeed, although these nanoparticles have a double organic coating, they demonstrate very interesting catalytic properties and show, in particular, a very high electro-catalytic activity with respect to the reduction of oxygen and oxidation of hydrogen. The same is true of the materials prepared from these nanoparticles.
• the mass activities measured for monolayers of nanoparticles in accordance with the invention can reach 500 A / g of platinum, a value of 5 times higher than the best results obtained (89.6 A / g of platinum) for a powder formed from platinum dispersed on carbon and introduced into polytetrafluoroethylene [9].
• Another advantage of nanoparticles lies in the fact that, if the molecules forming their second organic coating are suitably chosen, their catalytic properties are manifested without it being necessary to subject them beforehand to any activation treatment. In other words, they are immediately active.
• nanoparticles although spontaneously active, appear not to exhibit optimal performance in an acid medium, it turns out that it is possible to significantly improve their performance by subjecting them beforehand to a treatment in medium basic, which may in particular consist in immersing the nanoparticles, possibly already in the form of a film, in a solution of a strong base such as a 1M sodium hydroxide solution for several minutes, even several tens of minutes.
• medium basic which may in particular consist in immersing the nanoparticles, possibly already in the form of a film, in a solution of a strong base such as a 1M sodium hydroxide solution for several minutes, even several tens of minutes.
• nanoparticles also demonstrate remarkable properties of dispersibility, stability in a liquid medium and resistance to very acidic or very basic media and with high ionic strengths. As a result, these nanoparticles are particularly easy to handle and, above all, that their catalysis properties are very stable over time. So the nanoparticles can be preserved or.
• the use of nanoparticles as electrocatalysts is of particular interest in devices for producing electrical energy and, in particular, in fuel cells.
• the present invention therefore also relates to a device for producing electrical energy, which comprises nanoparticles as previously defined.
• this device is preferably a fuel cell.
• the use of nanoparticles as catalysts is also of great interest in the field of detection and determination of chemical or biological species, in particular in solution, and in particular in sensors or multisensors.
• the molecules forming the second organic coating are chosen so as to specifically recognize the species or the chemical or biological species to be detected or measured and to interact with them.
• the invention therefore also relates to a nanoparticle which comprises a metallic core containing at least one platinoid or an alloy of a platinoid, a first organic coating formed of molecules fixed on the surface of the metallic core and a second organic coating formed of molecules different from the molecules forming the first organic coating and which are grafted onto the molecules of the first organic coating, and in which the molecules forming the second organic coating are residues of a compound chosen from mono- and polycyclic anhydrides.
• the molecules forming the second organic coating of this nanoparticle are the remains of a compound chosen from glutaric anhydride, sulfobenzoic anhydride, diphenic anhydride, tetrafluorophthalic anhydride, tetraphenylphthalic anhydride and diphenylmaleic anhydride.
• the metallic core of this nanoparticle consists of platinum, a platinum alloy or a mixture of the two, while, according to yet another preferred arrangement of the invention, the molecules forming the first organic coating are residues of 4-mercaptoaniline.
• FIG. 1 already commented on, schematically represents a nanoparticle before and after a grafting reaction.
• FIGS. 2A, 2B, 2C, 2D, 2E, 2F and 2G schematically illustrate 7 different grafting reactions applied to nanoparticles with a platinum core functionalized by a first coating consisting of residues of 4-mercaptoaniline.
• FIG. 3 represents a photograph taken with an electron microscope in transmission of a Langmuir film produced from a suspension, two years old, of nanoparticles grafted by the reaction illustrated in FIG. 2A.
• FIG. 1 already commented on, schematically represents a nanoparticle before and after a grafting reaction.
• FIGS. 2A, 2B, 2C, 2D, 2E, 2F and 2G schematically illustrate 7 different grafting reactions applied to nanoparticles with a platinum core functionalized by a first coating consisting of residues of 4-mercaptoaniline.
• FIG. 3 represents a photograph taken with an electron microscope in transmission of a
• FIG. 4 represents the voltammograms obtained by cyclic voltammetry, in an acid medium initially saturated with oxygen, for Langmuir-Blodgett films respectively composed of nanoparticles grafted by the reactions illustrated in FIGS. 2A and 2B.
• FIG. 5 represents the voltammograms obtained by cyclic voltammetry, in basic medium initially saturated with oxygen, for Langmuir-Blodgett films respectively composed of nanoparticles grafted by the reactions illustrated in FIGS. 2C and 2D.
• FIG. 5 represents the voltammograms obtained by cyclic voltammetry, in basic medium initially saturated with oxygen, for Langmuir-Blodgett films respectively composed of nanoparticles grafted by the reactions illustrated in FIGS. 2C and 2D.
• FIG. 6 represents the voltammogram obtained by cyclic voltammetry, in an acid medium initially saturated with hydrogen, for a Langmuir-Blodgett film composed of nanoparticles grafted by the reaction illustrated in FIG. 2A.
• FIGS. 7A and 7B represent the spectra obtained by X-ray induced photoelectron spectroscopy for a Langmuir-Blodgett film composed of nanoparticles grafted by the reaction illustrated in FIG. 2A, before and after application to this film of extended electrochemical cycles in the medium acid.
• FIG. 7A and 7B represent the spectra obtained by X-ray induced photoelectron spectroscopy for a Langmuir-Blodgett film composed of nanoparticles grafted by the reaction illustrated in FIG. 2A, before and after application to this film of extended electrochemical cycles in the medium acid.
• FIG. 8 represents the voltammograms obtained by cyclic voltammetry, in an acid medium initially saturated with oxygen, for Langmuir-Blodgett films composed of nanoparticles grafted by the reaction illustrated in FIG. 2F, with and without polarization of these films.
• FIG. 9 represents the voltammograms obtained by cyclic voltammetry, in an acid medium initially saturated with oxygen, for Langmuir-Blodgett films respectively composed of nanoparticles grafted by the reactions illustrated in FIGS. 2F, 2A and 2G.
• FIG. 9 represents the voltammograms obtained by cyclic voltammetry, in an acid medium initially saturated with oxygen, for Langmuir-Blodgett films respectively composed of nanoparticles grafted by the reactions illustrated in FIGS. 2F, 2A and 2G.
• Nanoparticles are prepared comprising a platinum core and a first organic coating resulting from the binding of 4-mercaptoaniline molecules to this core by following the following operating protocol.
• solution 1, solution 2 and solution 3 three solutions are prepared, hereinafter called solution 1, solution 2 and solution 3.
• Solution 1 is obtained by dissolving 300 mg of platinum tetrachloride in 75 ml of hexylamine. It is orange in color.
• Solution 2 is obtained by dissolving 300 mg of sodium borohydride in 40 ml of a water / methanol mixture (50/50) followed, after complete dissolution of the sodium borohydride, by adding 20 ml of hexylamine.
• Grafted nanoparticles are prepared by subjecting nanoparticles functionalized according to Example 1 and freshly prepared (taking into account the ability presented by 4-mercaptoaniline to degrade spontaneously) to one of the reactions grafting illustrated in Figures 2A to 2G. These reactions are all carried out in the presence of a large excess of compound to be grafted relative to the amount of amino functions carried by functionalized nanoparticles.
• the grafting reactions are carried out as follows. We start by dispersing the functionalized nanoparticles in a volume of a suitable solvent and the suspension obtained is maintained under magnetic stirring and under nitrogen for about fifteen minutes. The compound to be grafted is then directly introduced into this suspension, optionally with a compound capable of trapping the by-products of the grafting reaction. The reaction medium is stirred for 12 hours under nitrogen, then transferred to a centrifuge tube in which there is added a large excess of a solvent intended to cause precipitation of the nanoparticles. Once this has been produced, the reaction medium is centrifuged and the supernatant, which most often contains a large part of the excess of grafting molecules used in the reaction, is discarded.
• the precipitate of nanoparticles is then washed and centrifuged from one to four times with a solvent capable of solubilizing the molecules of the grafting compound which, although unreacted, would be likely to remain with the nanoparticles, without however dispersing the latter. It is sometimes necessary to use a mixture of solvents for washing the precipitate (s) or to redisperse the nanoparticles and have them precipitate again. Finally, in some cases, the solvent used for the reaction should be evaporated in vacuo before washing (s) of the precipitate.
• the powder of grafted nanoparticles thus obtained is dried under vacuum or under nitrogen. It can then be used to prepare suspensions of grafted nanoparticles of desired concentrations, in general from 0.3 to 2 mg / ml.
• the solvent used for this purpose is generally a polar aprotic solvent of the dimethyl sulfoxide (DMSO), dimethylformamide (DMF) or dimethylacetamide (DMA) type, or an apolar solvent of the chloroform or dichloromethane type, if the grafted nanoparticles are not dispersible in the solvents polar aprotics. More precisely, the grafting reactions illustrated in FIGS. 2A to 2G are carried out using: Grafting reaction in FIG.
• a so-called “spreading" suspension by adding 1 ml of chloroform or dichloromethane to 0.5 ml of the suspension for which it is desired to check the stability over time. Then, 1.2 ml of the spreading suspension is spread on the surface of the water contained in a Langmuir tank measuring, for example, 45 cm long by 6.5 cm wide. The nanoparticles are compressed laterally until a surface pressure previously chosen, for example 4 mN / m, is reached. The length of the film of nanoparticles thus obtained is measured and its area is calculated by multiplying this length by the width of the tank.
• the area occupied on average by each mass unit of nanoparticles in the monolayer is determined.
• FIG. 3 represents a photograph taken with an electronic microscope in transmission of a Langmuir film produced from a two-year-old suspension of nanoparticles grafted by the reaction illustrated in FIG. 2A, and taken at l air / water interface of a Langmuir tank. This film was obtained by applying a surface pressure of 4 mN / m.
• EXAMPLE 4 Electrochemical Activity of Grafted Nanoparticles with Respect to the Reduction of Oxygen in an Acid Medium
• the electrochemical activity of grafted nanoparticles with respect to the reduction of oxygen in an acid medium is appreciated in subjecting Langmuir-Blodgett films (monolayers on a support) - hereinafter "LB films" - composed of grafted nanoparticles to cyclic voltammetry tests in a 1M sulfuric acid solution.
• a spreading suspension is prepared by adding 0.5 ml of a suspension containing 0.5 mg of grafted nanoparticles per ml of DMSO, 0.82 ml of dichloromethane or chloroform, and 0.18 ml of a 5.4.10 -4 M solution of behenic acid in chloroform, this acid being intended to facilitate the vertical transfer of the film of nanoparticles grafted on the support. Then, 1 ml of the spreading suspension is spread on the surface of the water contained in a Langmuir tank (45 cm x 6.5 cm) and a film is formed by lateral compression at a surface pressure of 28 mN / m.
• the surface occupied by the nanoparticles is of the order of 50% of the total surface of the film, the rest being occupied by behenic acid.
• the film is then transferred vertically onto a support, at 0.5 cm / min, by the Langmuir-Blodgett technique, the support being provided with a gold electrode. Voltammetry tests are carried out, in a conventional manner, on LB films of grafted nanoparticles without prior electrochemical treatment of these films and after saturation with 0 2 of the acid solution.
• FIG. 4 represents the voltammograms, recorded at a scanning speed of 20 mV / s, for two films of different grafted nanoparticles, composed respectively of nanoparticles grafted by the reaction illustrated in FIG. 2A (curve 1) and nanoparticles grafted by the grafting reaction illustrated in FIG. 2B (curve 2).
• the potentials are expressed in mV relative to a standard hydrogen electrode (ESH).
• FIG. 4 shows that the electrochemical activities of the two types of grafted nanoparticles are extremely close to each other, the electric current density of the reduction peak being, for the two films, between 230 and 240 ⁇ A / cm 2 .
• the reduction process disappears when the acid solution is oxygenated by a stream of nitrogen.
• EXAMPLE 5 Electrochemical Activity of Grafted Nanoparticles with Respect to the Reduction of Oxygen in Basic Medium
• the electrochemical activity of grafted nanoparticles with respect to the reduction of oxygen in Basic medium is assessed by submitting LB films of grafted nanoparticles, prepared as described in Example 4, in voltampero- tests cyclic measurement which is carried out under the same conditions as those of Example 4, except that the solution used does not contain sulfuric acid, but 1M sodium hydroxide.
• FIG. 5 represents the voltammograms, recorded at a scanning speed of 20 mV / s, for two films of different grafted nanoparticles, composed respectively of nanoparticles grafted by the reaction illustrated in FIG.
• FIG. 5 shows that the electrochemical activities of the two types of grafted nanoparticles are extremely close to one another. the other, the electric current density of the reduction peak being, for the two films, between 570 and
• Example 6 Electrochemical Activity of nanoparticles grafted vis-à-vis the oxidation of hydrogen in an acid medium
• the electrochemical activity 'nanoparticles grafted vis-à-vis the oxidation of hydrogen in an acid medium is appreciated by subjecting LB films of grafted nanoparticles, prepared as described in Example 4, to cyclic voltammetry tests which are carried out under the same conditions as those of Example 4, except that one uses an acid solution initially saturated with H 2 .
• FIG. 6 represents the voltammogram, recorded at a scanning speed of 20 mV / s, for a film composed of nanoparticles grafted by the reaction illustrated in FIG. 2A.
• the potentials are expressed in this figure in mV relative to a standard hydrogen electrode (ESH).
• Example 7 Stability of the electrochemical activity of grafted nanoparticles The stability of the electrochemical activity of grafted nanoparticles with respect to the reduction of oxygen in acidic and basic media is verified by subjecting LB films of grafted nanoparticles, prepared as described in Example 4, to voltammetric tests which are carried out under the same conditions as those of Examples 4 and 5, but, on the one hand, by varying the age of the suspensions of grafted nanoparticles from which are directed the films, and, on the other hand, by submitting or not previously said films to electrochemical cycles.
• EXAMPLE 8 Stability in Acid Medium of the Second Organic Coating of Grafted Nanoparticles The stability in acid medium of the second organic coating of grafted nanoparticles is assessed by subjecting LB films of grafted nanoparticles, prepared as described in Example 4, to electrochemical cycles. prolonged, in a solution of sulfuric acid IM, and by analyzing these films by spectroscopy of photoelectrons induced by X-rays before and after these cycles. In this experiment, the electrochemical cycles are carried out at the speed of 50 mV / s under an oxygen atmosphere, and between 800 and -50 mV / ESH.
• FIGS. 7A and 7B show the spectra recorded for films of nanoparticles grafted by the reaction illustrated in FIG.
• FIG. 7A relates to the electrons 4f of platinum
• FIG. 7B relates to the electrons S2p of sulfur.
• the shapes of the spectra obtained respectively before (spectra 1 and 3) and after the cycles (spectra 2 and 4) show, for each type of electron, a remarkable similarity, testifying to a remarkable stability of the second organic coating.
• the peak centered on 163 eV corresponds to the second organic coating of the nanoparticles
• the peak centered on 169 eV corresponds to the sulfate ions which are inserted in the films.
• the semi-quantitative analyzes resulting from these characterizations before and after treatment of the films in acid medium made it possible to show that the relationship between the intensities of the platinum peaks and of the sulfur peaks of the second organic coating does not change in a way significant (1.72 after the cycles versus 1.44 before the cycles), thus testifying to a remarkable stability of the overall composition of the nanoparticles.
• Example 10 Modulation of the electrochemical activity of grafted nanoparticles by the choice of the second organic coating LB films respectively composed of nanoparticles grafted by the reactions illustrated in FIGS. 2F, 2A and 2G, and prepared as described in Example 4, are subjected to cyclic voltammetry tests in acid medium (H 2 S0 IM) under the same conditions as those of Example 4. The results are presented in FIG. 9 which shows the voltammograms, recorded at a scanning speed of 20 mV / s, for a film of grafted nanoparticles by the reaction illustrated in FIG. 2F (curve 1), for a film of grafted nanoparticles by the reaction illustrated in FIG.
• EXAMPLE 11 Influence of a Treatment in Basic Medium on the Electrochemical Performance of Grafted Nanoparticles The Influence of a Treatment in Basic Medium on the Electrochemical Performance of Grafted Nanoparticles is Assessed by Comparing the Electrochemical Activities Observed in Basic Medium respectively (IM NaOH ) and in an acid medium (H 2 S0 IM) for LB films of nanoparticles grafted by the reaction illustrated in FIG. 2A, before and after a treatment consisting in immersing these nanoparticle films for 30 min in sodium hydroxide in the presence of oxygen.
• the electrochemical activities are evaluated by cyclic voltammetry tests which are carried out on LB films prepared from said nanoparticles as described in Example 4, the tests in basic medium being carried out under the same conditions as those of Example 5, while the tests in an acid medium are carried out under the same conditions as those of Example 4.
• the results are presented in FIG.

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