IRON CARBIDE NANOPARTICLES, METHOD FOR PREPARING SAME AND USE THEREOF FOR HEAT GENERATION

Abstract

Disclosed are iron nanoparticles, in which at least 70% of the iron atoms they contain are present in an Fe2,2C crystalline structure. In particular, these nanoparticles can be obtained via the carburization of zero-valent iron nanoparticles, by contacting the iron nanoparticles with a gas mixture of dihydrogen and carbon monoxide. The iron carbide nanoparticles are particularly suitable to be used for hyperthermia and for catalyzing Sabatier and Fischer-Tropsch reactions.

Claims

1-25. (canceled)

26. An iron carbide nanoparticle, wherein at least 70% of the iron atoms that it comprises are present in an Fe.sub.2.2C crystalline structure.

27. The iron carbide nanoparticle as claimed in claim 26, wherein at least 80% of the iron atoms that it comprises are present in an Fe.sub.2.2C crystalline structure.

28. The iron carbide nanoparticle as claimed in claim 26, having a size of between 1 and 20 nm.

29. The iron carbide nanoparticle as claimed in claim 26, having a size equal to 15 nm1 nm.

30. The iron carbide nanoparticle as claimed in claim 26, covered on at least part of its surface with a coating of a catalytic metal.

31. The iron carbide nanoparticle as claimed in claim 30, wherein said catalytic metal is chosen in the group consisting of nickel, ruthenium, cobalt, copper, zinc, platinum, palladium, rhodium, manganese, molybdenum, tungsten, vanadium, iridium, gold, or any one of the alloys thereof, alone or as a mixture.

32. The iron carbide nanoparticle as claimed in claim 26, obtainable by means of a step of carburization of a zero-valent iron nanoparticle by bringing said zero-valent iron nanoparticle into contact with a gas mixture of dihydrogen and carbon monoxide.

33. The iron carbide nanoparticle as claimed in claim 26, supported on a solid support.

34. A preparation method for preparing iron carbide nanoparticles as claimed in claim 26, comprising a step of carburization of zero-valent iron nanoparticles by bringing said zero-valent iron nanoparticles into contact with a gas mixture of dihydrogen and carbon monoxide.

35. The preparation method as claimed in claim 34, wherein said carburization step is carried out at a temperature of between 120 and 300 C.

36. The preparation method as claimed in claim 34, wherein said carburization step is carried out for a period of between 72 and 200 h.

37. The preparation method as claimed in claim 34, wherein said carburization step comprises the removal of the water formed during the reaction of said zero-valent iron nanoparticles and of said gas mixture, as said water is formed.

38. The preparation method as claimed in claim 34, comprising a prior step of preparing the zero-valent iron nanoparticles by decomposition of an organometallic precursor corresponding to general formula (I):
Fe(NR.sup.1R.sup.2)(NR.sup.3R.sup.4)(I) wherein R.sup.1, R.sup.2, R.sup.3 and R.sup.4, which may be identical or different, each represent an alkyl, aryl, trimethylsilyl or trimethylalkyl group, in the presence of dihydrogen and of a ligand system comprising a carboxylic acid and an amine, at least one of said carboxylic acid and of said amine comprising a C.sub.8 to C.sub.20 hydrocarbon-based chain.

39. The preparation method as claimed in claim 38, wherein said ligand system comprises palmitic acid and/or hexadecylamine.

40. The preparation method as claimed in claim 39, wherein said carburization step is carried out directly on the zero-valent iron nanoparticles obtained at the end of said decomposition step.

41. The preparation method as claimed in claim 38, wherein said decomposition step is carried out at a temperature of between 120 and 300 C.

42. The preparation method as claimed in claim 38, wherein said decomposition step is carried out for a period of between 1 and 72 h.

43. The preparation method as claimed in claim 34, comprising a subsequent step of treating the iron carbide nanoparticles obtained at the end of said carburization step, by bringing said iron carbide nanoparticles into contact with a precursor of a catalytic metal, so as to form a coating of said catalytic metal at the surface of said iron carbide nanoparticles.

44. A method for heat production comprising a step of using iron carbide nanoparticles as claimed in claim 26.

45. A method for the catalysis of chemical reaction comprising a step of using iron carbide nanoparticles as claimed in claim 26.

46. The method as claimed in claim 45, comprising a step of using said iron carbide nanoparticles for the catalysis of a reaction for reduction of carbon dioxide or of carbon monoxide into hydrocarbon(s).

47. A catalysis method for catalyzing a chemical reaction by means of iron carbide nanoparticles as claimed in claim 26, wherein said nanoparticles are introduced into a reaction medium containing one or more reagents for said chemical reaction, and said reaction medium is subjected to a magnetic field capable of causing an increase in the temperature of said nanoparticles up to a temperature of greater than or equal to a temperature required for carrying out said chemical reaction.

48. The catalysis method as claimed in claim 47, wherein the magnetic field is applied at a first amplitude for a first period of time, then at a second amplitude, of less than said first amplitude, for a second period of time, said second period of time being longer than said first period of time.

49. The catalysis method as claimed in claim 47, wherein the magnetic field is applied to said reaction medium in a pulsed manner.

50. The catalysis method as claimed in claim 47, wherein said nanoparticles are supported on a solid support, and said chemical reaction is carried out in a flow of continuous reagent(s).

Description

[0094] The characteristics and advantages of the invention will emerge more clearly in light of the examples of implementation below, given simply by way of illustration and which are in no way limiting with regard to the invention, with the support of FIGS. 1 to 25, wherein:

[0095] FIG. 1 shows the results of tests to characterize 9.0 nm Fe.sup.0 nanoparticles prepared in accordance with the invention, (a) by transmission electron microscopy, (b) by X-ray diffraction;

[0096] FIG. 2 shows the results of tests to characterize 12.5 nm Fe.sup.0 nanoparticles prepared in accordance with the invention, (a) by transmission electron microscopy, (b) by X-ray diffraction;

[0097] FIG. 3 shows the results of tests to characterize 13.0 nm iron carbide nanoparticles containing 80 mol % of Fe.sub.2.2C in accordance with the invention, (a) and (b) by transmission electron microscopy with two different magnifications, (c) by X-ray diffraction, (d) by Mssbauer spectroscopy;

[0098] FIG. 4 shows the results of tests to characterize 15.0 nm iron carbide nanoparticles containing 80 mol % of Fe.sub.2.2C in accordance with the invention, (a) by transmission electron microscopy, (b) by X-ray diffraction, (c) by Mssbauer spectroscopy;

[0099] FIG. 5 shows the results of tests to characterize 9.7 nm iron carbide nanoparticles containing 59 mol % of Fe.sub.2.2C in accordance with the invention, (a) by transmission electron microscopy, (b) by X-ray diffraction, (c) by Mssbauer spectroscopy;

[0100] FIG. 6 shows the results of tests to characterize iron carbide nanoparticles containing 80 mol % of Fe.sub.2.2C, said particles being covered with nickel, in accordance with the invention, (a) by transmission electron microscopy, (b) by X-ray diffraction, (c1) by STEM, (c2) by iron-targeted STEM-EDX and (c3) by nickel-targeted STEM-EDX;

[0101] FIG. 7 represents a graph showing the specific absorption rate (SAR), as a function of the amplitude of the magnetic field, for a hyperthermia test by magnetic induction at 100 kHz carried out on iron oxide nanoparticles of the prior art (FeONP), iron nanoparticles prepared according to a method in accordance with the invention (FeNP2), iron carbide nanoparticles in accordance with the invention (FeCNP2) and iron carbide nanoparticles according to the prior art (FeCcomp1),

[0102] FIG. 8 represents a graph showing the specific absorption rate (SAR), as a function of the amplitude of the magnetic field, for a hyperthermia test by magnetic induction at 100 kHz carried out on iron nanoparticles prepared according to a method in accordance with the invention (FeNP1), iron carbide nanoparticles in accordance with the invention (FeCNP1, FeCNP3, FeCNP4 and FeCNP5) and comparative iron carbide nanoparticles (FeCcomp2, FeCcomp3, FeCcomp4, FeCcomp6, FeCcomp7);

[0103] FIG. 9 represents a graph showing the specific absorption rate (SAR), as a function of the amplitude of the magnetic field, for a hyperthermia test by magnetic induction at 100 kHz carried out on iron nanoparticles prepared according to a method in accordance with the invention (FeNP2), iron carbide nanoparticles in accordance with the invention (FeCNP2) and comparative iron carbide nanoparticles (FeCcomp8, FeCcomp9, FeCcomp10),

[0104] FIG. 10 represents a graph showing the specific absorption rate (SAR), as a function of the amplitude of the magnetic field, for a hyperthermia test by magnetic induction at 100 kHz carried out on iron carbide nanoparticles in accordance with the invention of various sizes, FeCNP1 and FeCNP2;

[0105] FIG. 11 represents a graph showing the specific absorption rate (SAR), as a function of the amplitude of the magnetic field, for a hyperthermia test by magnetic induction at 100 kHz carried out on iron carbide nanoparticles in accordance with the invention (FeCNP2) and nickel-covered iron carbide nanoparticles in accordance with the invention (FeC@Ni);

[0106] FIG. 12 represents a graph showing, on the one hand, the conversion rate of CO.sub.2 and, on the other hand, the hydrocarbon yield, as a function of the amplitude of the magnetic field applied, during the use of iron carbide nanoparticles in accordance with the invention for the catalysis of the Sabatier reaction;

[0107] FIG. 13 shows the mass spectrum of the gas phase obtained following the use of iron carbide nanoparticles in accordance with the invention for the catalysis of the Sabatier reaction, under a magnetic field of 30 mT at 300 kHz;

[0108] FIG. 14 shows the mass spectrum of the gas phase obtained following the use of iron carbide nanoparticles in accordance with the invention for the catalysis of the Sabatier reaction, under a magnetic field of 40.2 mT at 300 kHz;

[0109] FIG. 15 shows the X-ray diffractogram of iron carbide nanoparticles in accordance with the invention following a reaction for catalysis of the Sabatier reaction, under a magnetic field of 40.2 mT at 300 kHz for 8 h;

[0110] FIG. 16 shows the Mssbauer spectra obtained for nanoparticles according to the invention (b/ obtained with a carburization time of 96 h, c/ obtained with carburization time of 140 h) and for iron carbide nanoparticles prepared according to the same protocol, but with a carburization time of 48 h (a/), not in accordance with the invention;

[0111] FIG. 17 represents a graph showing the specific absorption rate (SAR), as a function of the amplitude of the magnetic field, for a hyperthermia test by magnetic induction at 100 kHz carried out on zero-valent iron nanoparticles (Fe.sup.0), and iron carbide nanoparticles prepared from these zero-valent iron nanoparticles: according to a method in accordance with the invention using a carburization time of 96 h (NP96) or a carburization time of 140 h (NP140), or according to a method using a carburization time of 48 h (NP48);

[0112] FIG. 18 represents a graph showing the specific absorption rate (SAR), as a function of the amplitude of the magnetic field, for a hyperthermia test by magnetic induction at 100 kHz carried out on iron carbide nanoparticles prepared: according to a method in accordance with the invention using, for the carburization step, a molecular sieve and a carburization time of 40 h (NP40TM) or no molecular sieve and a carburization time of 140 h (NP140S), or according to a method using a carburization time of 48 h without molecular sieve (NP48S);

[0113] FIG. 19 represents a graph showing the specific absorption rate (SAR), as a function of the amplitude of the magnetic field, for a hyperthermia test by magnetic induction at 100 kHz carried out on iron carbide nanoparticles prepared: according to a method in accordance with the invention using, for the carburization step, a molecular sieve and a carburization time of 24 h (NP24TM, experiment carried out in duplicate) or no molecular sieve and a carburization time of 120 h (NP120S), or according to a method using a carburization time of 24 h without molecular sieve (NP24S);

[0114] FIG. 20 shows a graph representing, as a function of the amplitude of the magnetic field applied, the degree of conversion of carbon dioxide (CO.sub.2) and the degree of formation of methane (CH.sub.4) and degree of formation of carbon monoxide (CO) during the implementation of a method for catalysis of the Sabatier reaction according to the invention, carried out in continuous flow of reagents, using nickel-covered iron carbide nanoparticles supported on a solid support, in accordance with the invention;

[0115] FIG. 21 represents a gas chromatogram obtained at the outlet of the reactor during the implementation of the method of FIG. 20, for a magnetic field amplitude of 40 mT;

[0116] FIG. 22 shows an electron microscopy image of a grain of ruthenium-doped solid support supporting iron carbide nanoparticles in accordance with the invention;

[0117] FIG. 23 shows a graph representing, as a function of the amplitude of the magnetic field applied, the degree of conversion of carbon dioxide (CO.sub.2) the degree of formation of carbon monoxide (CO) and the selectivity of formation of methane (CH.sub.4) during the implementation of a method for catalysis of the Sabatier reaction according to the invention, carried out in continuous flow of reagents, using iron carbide nanoparticles covered and supported on a ruthenium-doped solid support, in accordance with the invention;

[0118] FIG. 24 represents a gas chromatogram obtained at the outlet of the reactor during the implementation of the method of FIG. 23, for a magnetic field amplitude of 28 mT;

[0119] and FIG. 25 represents a graph showing, as a function of time, the temperature change in the reactor, the degree of conversion of carbon dioxide (CO.sub.2), the degree of formation of carbon monoxide (CO) and the selectivity of formation of methane (CH.sub.4) during the implementation of the method of FIG. 23, at a magnetic field of amplitude 28 mT.

A/ MATERIALS AND METHODS

[0120] All the syntheses of non-commercial compounds were carried out under argon using Fischer-Porter bottles, a glove box and a vacuum/argon line. The mesitylene (99%), toluene (99%) and tetrahydrofuran (THF, 99%) were purchased from VWR Prolabo, purified on alumina and gassed by means of three freezing-pumping-liquefying cycles. The commercial products hexadecylamine (HDA, 99%) and palmitic acid (PA) were purchased from Sigma-Aldrich. The bis(amido)iron(II) dimer {Fe[N(SiMe.sub.3).sub.2].sub.2}.sub.2 and the (1,5-cyclooctadiene)(1,3,5-cyclooctatriene)ruthenium(0) (Ru(COD)(COT)) were purchased from NanoMePS. The nickel(II)acetylacetonate was purchased from Sigma Aldrich. The 9.0 nm Fe(0) nanoparticles were either synthesized, or purchased from NanoMePS. All these compounds were used without additional purification.

[0121] The molecular sieve (0.4 nm pores, 2 mm diameter) was purchased from Merck (CAS 1318-02-1) and was activated under vacuum at 200 C. for 3 h. The Siralox support was obtained from the company Sasol.

[0122] Characterization

[0123] The size and the morphology of the samples synthesized were characterized by transmission electron microscopy (TEM). The conventional microscopy images were obtained using a JeoL microscope (model 1400) operating at 120 kV. The X-ray diffraction (XRD) measurements were carried out on a PANalytical Empyrean diffractometer using a Co-K source at 45 kV and 40 mA. These studies were carried out on powdered samples prepared and sealed under argon. The mass spectrometry analyses were carried out on a Pfeiffer Vacuum Thermostar Gas Analysis System GSD 320 spectrometer. The state of the iron atoms and their environment was determined by Mssbauer spectroscopy (Wissel, 57Co source).

[0124] The gas chromatography coupled to mass spectrometry (GC-MS) analyses were carried out on a PerkinElmer 580 gas chromatograph coupled to a Clarus SQ8T mass spectrometer. The carbon dioxide CO.sub.2 conversion, the methane CH.sub.4 yield, the carbon monoxide CO yield and the methane selectivity were calculated from the gas chromatograms, after calibration of the TCD detector. The magnetic measurements were carried out on a vibrating sample magnetometer (Quantum Device PPMS EverCool-II). The nanoparticles were prediluted (100-fold) in tetracosane in order to eliminate the magnetic interactions.

B/ SYNTHESIS OF ZERO-VALENT IRON NANOPARTICLES

[0125] General Protocol

[0126] In a glove box, the {Fe[N(SiMe.sub.3).sub.2].sub.2}.sub.2 iron precursor, the palmitic acid and the hexadecylamine are weighed separately in 15 ml sample holders and dissolved in mesitylene. The green solution containing the iron precursor is introduced into a Fischer-Porter bottle, followed by the palmitic acid and the hexadecylamine. The Fischer-Porter bottle is removed from the glove box and placed with stirring in an oil bath at 32 C. It is then purged of its argon and placed under a dihydrogen pressure of between 1 and 10 bar. The mixture is vigorously stirred at a temperature of between 120 and 180 C. for 1 to 72 h.

[0127] Once the reaction has ended, the Fischer-Porter bottle is removed from the oil bath and left to cool with stirring. Once at ambient temperature, it is placed in a glove box and degassed. The iron nanoparticles obtained are washed by magnetic decanting, three times with toluene and three times with THF. To finish, the iron nanoparticles are dried under a vacuum line. They are then characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), vibrating sample magnetometry (VSM) and elemental analysis (TGA).

Example 1Synthesis of 9.0 nm Fe.SUP.0 .Nanoparticles

[0128] The general protocol above is applied with the following parameters: the {Fe[N(SiMe.sub.3).sub.2].sub.2}.sub.2 iron precursor (1.0 mmol; 753.2 mg), the palmitic acid (1.2 equivalents/Fe, 2.4 mmol; 615.4 mg) and the hexadecylamine (1 equivalent/Fe, 2.0 mmol; 483.0 mg) are dissolved respectively in 5 ml, 10 ml and 5 ml of mesitylene.

[0129] The green solution containing the iron precursor is introduced into a Fischer-Porter bottle (rinsing of the sample holder with 5 ml of mesitylene), followed by the palmitic acid (rinsing of the sample bottle with 10 ml of mesitylene) and the hexadecylamine (rinsing of the sample holder with 5 ml of mesitylene). The Fischer-Porter bottle is closed, removed from the glove box and placed with stirring in an oil bath at 32 C. It is then purged of its argon and placed under a hydrogen pressure (2 bar). The mixture is vigorously stirred at 150 C. for 48 h.

[0130] The nanoparticles obtained, hereinafter referred to as FeNP1, are characterized. The results obtained by TEM and XRD are shown in FIG. 1, respectively in (a) and (b). It is observed that they are spherical, monodisperse, with a diameter D=9.0 nm+/0.5 nm, and formed of an Fe.sup.0 bcc crystalline phase.

Example 2Synthesis of 12.5 nm Fe.SUP.0 .Nanoparticles

[0131] The general protocol above is applied with the following parameters: the {Fe[N(SiMe.sub.3).sub.2].sub.2}.sub.2 iron precursor (1.0 mmol; 753.2 mg), the palmitic acid (1.3 equivalents/Fe, 2.6 mmol; 666.4 mg) and the hexadecylamine (1 equivalent/Fe, 2.0 mmol; 483.0 mg) are dissolved respectively in 5 ml, 10 ml and 5 ml of mesitylene. The subsequent steps are carried out in accordance with example 1 above. The remainder of the synthesis and also the purification and the characterization are continued as in example 1.

[0132] The nanoparticles obtained, hereinafter referred to as FeNP2, are characterized. The results obtained by TEM and XRD are shown in FIG. 2, respectively in (a) and (b). It is observed that they are spherical, monodisperse, with a diameter D=12.5 nm+/0.7 nm, and formed of an Fe.sup.0 bcc crystalline phase.

C/ SYNTHESIS OF IRON CARBIDE NANOPARTICLES

[0133] General Protocol

[0134] In a glove box, the Fe.sup.0 nanoparticles are placed in a Fischer-Porter bottle and redispersed in mesitylene. The Fischer-Porter bottle is closed and removed from the glove box, purged of its argon and then placed under a carbon monoxide (between 1 and 10 bar) and hydrogen (between 1 and 10 bar) pressure. The mixture is then vigorously stirred at 120-180 C. for a period of between 1 min and 200 h.

[0135] Once the reaction has finished, the Fischer-Porter bottle is removed from the oil bath and left to cool with stirring. Once at ambient temperature, it is placed in a glove box and degassed. The nanoparticles obtained are washed, by magnetic washing, 3 times with toluene, then dried under a vacuum line. The black powder obtained is analyzed by TEM, XRD, VSM, Mssbauer spectroscopy and elemental analysis.

Example 3Synthesis of 13.0 nm Iron Carbide Nanoparticles Containing 83% of the Fe.SUB.2.2.C Crystalline Structure

[0136] The general protocol above is applied with the following parameters: the Fe.sup.0 nanoparticles obtained in example 2 above (1 mmol Fe; 100 mg) are placed in a Fischer-Porter bottle and redispersed in mesitylene (20 ml). The Fischer-Porter bottle is placed under a carbon monoxide (2 bar) and hydrogen (2 bar) pressure. The mixture is then vigorously stirred at 150 C. for 120 h.

[0137] The nanoparticles obtained, hereinafter referred to as FeCNP1, are characterized. The results obtained by TEM (at two different magnifications), XRD and Mssbauer spectroscopy are shown in FIG. 3, respectively in (a), (b), (c) and (d). It is observed that the nanoparticles are spherical, monodisperse, with a diameter D=13.1 nm+/1.1 nm, and that they comprise a monocrystalline Fe.sub.2.2C core. It emerges from the Mssbauer spectroscopy that their content is 83 mol % Fe.sub.2.2C and 17 mol % Fe.sub.5C.sub.2. The results of VSM at 300 K also indicate a magnetization at saturation Ms of approximately 151 emu/g.

Example 4Synthesis of 15.0 nm Iron Carbide Nanoparticles Containing 82% of the Fe.SUB.2.2.C Crystalline Structure

[0138] The general protocol above is applied with the following parameters: the Fe.sup.0 nanoparticles obtained in example 2 above (1 mmol Fe; 100 mg) are placed in a Fischer-Porter bottle and redispersed in mesitylene (20 ml). The Fischer-Porter bottle is closed and removed from the glove box, purged of its argon and then placed under a carbon monoxide (2 bar) and hydrogen (2 bar) pressure. The mixture is then vigorously stirred at 150 C. for 140 h.

[0139] The nanoparticles obtained, hereinafter referred to as FeCNP2, are characterized. The results obtained by TEM, XRD and Mssbauer spectroscopy are shown in FIG. 4, respectively in (a), (b) and (c). It is observed that the nanoparticles are spherical, monodisperse, with a diameter D=15.0 nm+/0.9 nm, and that they comprise a monocrystalline Fe.sub.2.2C core. It emerges from the Mssbauer spectroscopy that their molar content is 82% Fe.sub.2.2C and 18% Fe.sub.5C.sub.2. The results of VSM at 300 K also indicate a magnetization at saturation Ms of approximately 170 emu/g.

Examples 5-7

[0140] Iron carbide nanoparticles are prepared according to the general protocol above, for different carburization times.

[0141] The operating parameters used are the following: the Fe.sup.0 nanoparticles obtained in example 1 or example 2 above (1 mmol Fe; 100 mg) are placed in the Fischer-Porter bottle and redispersed in mesitylene (20 ml). The Fischer-Porter bottle is placed under a carbon monoxide (2 bar) and hydrogen (2 bar) pressure. The mixture is then vigorously stirred at 150 C. for a carburization time t.

[0142] The nanoparticles obtained are characterized. It is verified, by XRD analysis, that the core consists exclusively of Fe.sub.2.2C. Their molar content of Fe.sub.2.2C is, moreover, determined by Mssbauer spectroscopy.

[0143] The characteristics of the nanoparticles thus prepared and the operating parameters used for the preparation thereof are reiterated in table 1 below.

TABLE-US-00001 TABLE 1 characteristics of nanoparticles according to the invention and operating parameters for the preparation thereof Carburization Nanoparticle Reference Fe.sup.0 nanoparticles time t (h) diameter (nm) FeCNP3 FeNP1 72 12.1 FeCNP4 FeNP1 96 13.1 FeCNP5 FeNP1 144 13.3

Comparative Example 1Synthesis of 9.7 nm Iron Carbide Nanoparticles Containing 59% of the Fe.SUB.2.2.C Crystalline Structure

[0144] The general protocol above is applied with the following parameters: the Fe0 nanoparticles obtained in example 1 above (1 mmol Fe; 100 mg) are placed in a Fischer-Porter bottle and redispersed in mesitylene (20 ml). The Fischer-Porter bottle is placed under a carbon monoxide (2 bar) and hydrogen (2 bar) pressure. The mixture is then vigorously stirred at 150 C. for 24 h.

[0145] The nanoparticles obtained, hereinafter referred to as FeCcomp2, are characterized. The results obtained by TEM, XRD and Mssbauer spectroscopy are shown in FIG. 5, respectively in (a), (b) and (c). It is observed that the nanoparticles are spherical, monodisperse, with a diameter D=9.7 nm+/0.5 nm, and that they comprise a monocrystalline Fe.sub.2.2C core. It emerges from the Mssbauer spectroscopy that their molar content is 59% Fe.sub.2.2C, 16% Fe.sub.5C.sub.2, 21% Fe.sup.0 and 4% paramagnetic phase (amorphous). The results of VSM at 300 K also indicate a magnetization at saturation Ms of approximately 150 emu/g.

Comparative Examples 2 to 9

[0146] Iron carbide nanoparticles are prepared according to the general protocol above, for carburization times of less than 72 h.

[0147] The operating parameters used are the following: the Fe.sup.0 nanoparticles obtained in example 1 or example 2 above (1 mmol Fe; 100 mg) are placed in the Fischer-Porter bottle and redispersed in mesitylene (20 ml). The Fischer-Porter bottle is placed under a carbon monoxide (2 bar) and hydrogen (2 bar) pressure. The mixture is then vigorously stirred at 150 C. for a carburization time t.

[0148] The nanoparticles obtained are characterized. It is established by XRD analysis that the core consists of Fe.sub.2.2C or of a mixture of Fe.sub.2.2C and Fe.sup.0. Their molar content of Fe.sub.2.2C is, moreover, determined by Mssbauer spectroscopy.

[0149] The characteristics of the nanoparticles thus prepared and the operating parameters used for the preparation thereof are reiterated in table 2 below.

TABLE-US-00002 TABLE 2 characteristics of comparative nanoparticles and operating parameters for the preparation thereof Fe.sup.0 Carburization Core Molar % of Reference nanoparticles time t (h) composition Fe.sub.2.2C FeCcomp3 FeNP1 48 Fe.sub.2.2C FeCcomp4 FeNP1 16 Fe.sub.2.2C + Fe.sup.0 FeCcomp5 FeNP1 8 Fe.sub.2.2C + Fe.sup.0 FeCcomp6 FeNP1 4 Fe.sub.2.2C + Fe.sup.0 FeCcomp7 FeNP1 2 Fe.sub.2.2C + Fe.sup.0 FeCcomp8 FeNP2 48 Fe.sub.2.2C + Fe.sup.0 67 FeCcomp9 FeNP2 15 Fe.sub.2.2C + Fe.sup.0 FeCcomp10 FeNP2 4 Fe.sub.2.2C + Fe.sup.0

D/ SYNTHESIS OF NICKEL-COVERED IRON CARBIDE NANOPARTICLES

[0150] General Protocol

[0151] In a glove box, the iron carbide nanoparticles are placed in a Fischer-Porter bottle and redispersed in mesitylene. Palmitic acid is added in order to facilitate the redispersion of the nanoparticles and to improve their stability in solution. The Ni(acac).sub.2 (bis(acetylacetonate)nickel) nickel precursor previously dissolved in mesitylene is introduced into the Fischer-Porter bottle. The bottle is closed and removed from the glove box, then passed through ultrasound for 15 s to 10 min (preferably 1 min). The mixture is vigorously stirred under argon at 120-180 C. (preferably 150 C.) for 10 min to 4 h (preferably 1 h) in order to homogenize the solution. Finally, the Fischer-Porter bottle is placed under a hydrogen pressure of between 1 and 10 bar (preferably 3 bar). The mixture is vigorously stirred at 150 C. for a period of between 1 and 48 h (preferably for 24 h).

[0152] Once the reaction has ended, the Fischer-Porter bottle is removed from the oil bath and left to cool with stirring. Once at ambient temperature, it is passed through ultrasound for 15 s to 10 min (preferably 1 min) and then placed in a glove box. The nanoparticles are washed, by magnetic washing, three times with toluene and then dried under a vacuum line. The nanoparticles obtained are analyzed by TEM, XRD, VSM, high resolution transmission electron microscopy (HRTEM) and scanning transmission electron microscopy coupled to energy-dispersive X-ray spectroscopy (STEM-EDX).

Example 8

[0153] The general protocol above is applied using the iron carbide nanoparticles obtained in example 4 above, with the operating parameters described below.

[0154] The nanoparticles (1 mmol; 80 mg) are redispersed in mesitylene (15 ml). Palmitic acid (0.5 mmol; 128.4 mg) is added. The Ni(acac).sub.2 nickel precursor (0.5 mmol; 129.3 mg), previously dissolved in mesitylene (10 ml+5 ml rinsing), is introduced into the Fischer-Porter bottle. The latter is closed, removed from the glove box and then passed through ultrasound for 1 min. The mixture is vigorously stirred under argon at 150 C. for 1 h. Finally, the Fischer-Porter bottle is placed under a hydrogen pressure (3 bar). The mixture is vigorously stirred at 150 C. for 24 h.

[0155] Once at ambient temperature, the Fischer-Porter bottle is passed through ultrasound for 1 min.

[0156] The nanoparticles obtained, hereinafter referred to as FeC@Ni1, are characterized. The results obtained by TEM, XRD and STEM-EDX are shown in FIG. 6, respectively in (a), (b) and (c1) (crude STEM image), (c2) (iron-targeted STEM-EDX image) and (c3) (nickel-targeted STEM-EDX image). It is observed that the nanoparticles are spherical, monodisperse, with a diameter D=15.2 nm+/1.1 nm. The XRD analysis confirms the presence of the crystalline core consisting of Fe.sub.2.2C, and shows the growth of nickel in metal form at the surface of the nanoparticles, and also the absence of nickel oxides. The STEM-EDX analysis confirms that the iron is concentrated in the core of the nanoparticles, and that the nickel is for its part present at the surface.

E/ HYPERTHERMIA MEASUREMENTS BY MAGNETIC INDUCTION

[0157] General Protocol

[0158] In a glove box, 10 mg of nanoparticles are placed in a tube to which 0.5 ml of mesitylene is added. The tube is removed from the glove box and treated for 1 min with ultrasound in order to obtain a colloidal solution of nanoparticles. The tube is then placed in a calorimeter containing 2 ml of deionized water. The calorimeter is exposed to an alternating magnetic field (100 kHz, amplitude adjustable between 0 and 47 mT) for 40 seconds and the heating of the water is measured using two optical temperature probes. The temperature increase is determined by the mean slope of the function T/t. To finish, the SAR (Specific Absorption Rate) is calculated by means of the following equation:

[00001]$S.Math..Math.A.Math..Math.R=\frac{\underset{i}{.Math.}.Math.C_{\mathrm{pi}}.Math.m_i}{m_{\mathrm{Fe}}}.Math.\frac{.Math..Math.T}{.Math..Math.t}$

[0159] wherein: C.sub.pi represents the heat capacity of the compound i (C.sub.p=449 J kg.sup.1K.sup.1 for the Fe nanoparticles, C.sub.p=1750 J kg.sup.1K.sup.1 for the mesitylene, C.sub.p=4186 J kg.sup.1K.sup.1 for the water and C.sub.p=720 J kg.sup.1K.sup.1 for the glass); m.sub.i represents the mass of the compound i; m.sub.Fe represents the mass of iron in the sample.

[0160] Experiment 1

[0161] A first experiment is carried out for the Fe.sup.0 nanoparticles of example 2 (FeNP2) and the iron carbide nanoparticles of example 4 (FeCNP2).

[0162] By way of comparison, also tested, in parallel, are the iron carbide nanoparticles prepared in accordance with the protocol described in the publication by Meffre et al, 2012, Nanoletters, 4722-4728, of composition 43% Fe.sub.2.2C, 43% Fe.sub.5C.sub.2, 14% paramagnetic species (FeCcomp1).

[0163] The results obtained are also compared to those presented for iron oxide nanoparticles in the publication by Pellegrino et al, 2014, J. Mater. Chem. B, 4426-4434, described for presenting a high SAR (FeONP).

[0164] The results obtained are shown in FIG. 7.

[0165] It is observed that the FeCNP2 nanoparticles in accordance with the invention exhibit hyperthermia performance levels which are very much better than those of the other nanoparticles.

[0166] Experiment 2

[0167] The maximum SARs obtained at 100 kHz, for a magnetic field of amplitude 47 mT, for the various nanoparticles below, are indicated in table 3 below.

TABLE-US-00003 TABLE 3 maximum SARs obtained at 100 kHz for nanoparticles in accordance with the invention and comparative nanoparticles Nanoparticle SARmax (W/g) FeNP1 425 FeCcomp6 1070 FeCcomp5 450 FeCcomp4 95 FeCcomp3 890 FeCcomp2 45 FeCNP1 1630 FeCNP3 1485 FeNP2 1220 FeCcomp9 660 FeCcomp8 100 FeCNP2 3300

[0168] It is observed that the nanoparticles in accordance with the invention all have SARs much higher than those of the comparative nanoparticles, and than those of the zero-valent iron nanoparticles having served to prepare them.

[0169] Experiment 3

[0170] The FeNP1 zero-valent iron nanoparticles, the iron carbide nanoparticles in accordance with the invention (FeCNP1, FeCNP3, FeCNP4 and FeCNP5) and the comparative iron carbide nanoparticles (FeCcomp2, FeCcomp3, FeCcomp4, FeCcomp6, FeCcomp7), obtained using these FeNP1 nanoparticles, are subjected to the test protocol below. The SAR is measured for various magnetic field amplitudes.

[0171] The results obtained are shown in FIG. 8.

[0172] It is observed that not only do the nanoparticles according to the invention have much higher SARs than the comparative nanoparticles, but in addition, their hyperthermia performance levels are exerted at magnetic fields of low amplitude, as early as 25 mT for some of them. These performance levels are high starting from approximately 38 mT for all the nanoparticles in accordance with the invention.

[0173] Experiment 4

[0174] The FeNP2 zero-valent iron nanoparticles, the iron carbide nanoparticles in accordance with the invention (FeCNP2) and the comparative iron carbide nanoparticles (FeCcomp8, FeCcomp9, FeCcomp10), obtained from these FeNP2 nanoparticles, are subjected to the test protocol above. The SAR is measured for various magnetic field amplitudes.

[0175] The results obtained are shown in FIG. 9.

[0176] It is observed that not only do the nanoparticles according to the invention have a much higher SAR than the comparative nanoparticles, but in addition, their hyperthermia performance levels are exerted at magnetic fields of low amplitude, and are particularly high as early as approximately 25 mT.

[0177] Experiment 5Influence of the Nanoparticle Size

[0178] The SARs of the FeCNP1 (diameter approximately 13 nm) and FeCNP2 (diameter approximately 15 nm) nanoparticles in accordance with the invention are subjected to the test protocol above.

[0179] The results obtained are shown in FIG. 10.

[0180] It is observed that the two types of nanoparticles have a high heating capacity, the nanoparticles of approximately 15 nm in size being, however, more effective than the nanoparticles of approximately 13 nm in size.

[0181] Experiment 6

[0182] The SARs of the FeCNP2 and FeC@Ni nanoparticles in accordance with the invention are subjected to the test protocol above.

[0183] The results obtained are shown in FIG. 11.

[0184] It is observed that the nickel-covered nanoparticles have a heating capacity substantially equivalent to that of the non-covered nanoparticles for magnetic fields above 40 mT.

F/ CATALYSIS OF THE SABATIER REACTION BY MAGNETIC INDUCTION

[0185] General Protocol The iron carbide nanoparticles are used to catalyze the Sabatier reaction, according to the reaction scheme:

##STR00001##

[0186] in which FeC NPs represents the iron carbide nanoparticles.

[0187] For this purpose, in a glove box, the catalyst in powder form (10 mg) is placed in a Fischer-Porter bottle fitted at its head with a manometer in order to monitor the pressure variation during the reaction, without any solvent. The Fischer-Porter bottle is closed, removed from the glove box, emptied of its argon and placed under a CO.sub.2 (1 equivalent; 0.8 bar: from 1 bar to 0.2 bar) and H.sub.2 (4 equivalents; 3.2 bar: from 0.2 bar to 3 bar) pressure. The Fischer-Porter bottle is then exposed to an alternating magnetic field (300 kHz, amplitude adjustable between 0 and 64 mT) for 8 h. At the end of the reaction, the gas phase is analyzed by mass spectrometry in order to identify the compounds formed.

[0188] Experiment 1

[0189] The FeCNP2 iron carbide nanoparticles in accordance with the invention are used in this experiment. The results obtained, in terms, on the one hand, of degree of CO.sub.2 conversion and, on the other hand, of yield of hydrocarbon(s), as a function of the amplitude of the magnetic field applied, are shown in FIG. 12. These results show that the degree of CO.sub.2 conversion is close to 100% at magnetic field amplitudes even of less than 30 mT. The yield of hydrocarbon(s) is very high, about 80% above 30 mT, this being without having recourse to doping with another element such as cobalt or ruthenium.

[0190] The mass spectrum obtained for the gas phase at 30 mT is shown in FIG. 13. It is observed therein that methane (CH.sub.4) is the compound very predominantly formed.

[0191] Thus, the iron carbide nanoparticles in accordance with the invention have a very good catalytic activity at a field greater than or equal to 30 mT. The methane selectivity is also very high, approximately 80%.

[0192] Experiment 2

[0193] The FeC@Ni nickel-covered iron carbide nanoparticles in accordance with the invention and the FeCNP2 iron carbide nanoparticles in accordance with the invention are used in this experiment.

[0194] The operating protocols differ from that previously described in regard to the duration of application of the magnetic field and the amplitude of the latter.

[0195] The exact operating parameters and the associated results, in terms of degree of CO.sub.2 conversion, of yield of hydrocarbon(s) and of selectivity with respect to methane, are indicated in table 4 below.

TABLE-US-00004 TABLE 4 operating parameters and results of the catalysis of the Sabatier reaction by magnetic induction of nanoparticles in accordance with the invention Duration of induction Degree of and amplitude CO.sub.2 conver- Hydrocarbon Selectivity/ Nanopart. of the field sion (%) yield (%) methane (%) FeC@Ni 3 h at 64 mT 84 74 97 FeC@Ni activation for 5 s 36 27 >99 at 64 mT then 8 h at 25 mT FeCNP2 8 h at 64 mT >98 76 80

[0196] It is deduced therefrom that: [0197] for the nickel-covered nanoparticles, the reaction is virtually quantitative in 3 h at 64 mT, and the methane selectivity is virtually total. It also emerges from the mass spectrum (not shown) that the carbon dioxide is quantitatively converted, on the one hand, into methane and, on the other hand, into a small amount of carbon monoxide; [0198] at the same magnetic field amplitude, the nickel-covered nanoparticles make it possible to achieve similar hydrocarbon yields, and a greater methane selectivity, compared with the non-covered nanoparticles, this being in much shorter times; [0199] for the nickel-covered nanoparticles, after activation for a few seconds at 64 mT, then 8 h at 25 mT, a catalytic activity is observed, although at the value of 25 mT, the SAR of the nanoparticles is zero (cf. FIG. 11). This demonstrates that it is possible to catalyze the Sabatier reaction with a low energy consumption, by means of a very short first phase of activation at a high magnetic field, followed by a phase at a magnetic field of much lower amplitude.

[0200] Thus, in the case of the nickel-covered iron carbide nanoparticles, it is possible to activate the reaction at a strong field (64 mT) for a few seconds and then to work at a weak field (25 mT) for several hours. Since the reaction is exothermic, once initiated, it is advantageously possible to maintain it at low energy cost.

[0201] By way of comparison, the same protocol was applied for iron carbide nanoparticles of the prior art (containing 43% of Fe.sub.2.2C, prepared according to the publication by Meffre et al., 2012, Nanoletters, 12, 4722-4728). No catalytic activity was observed for these nanoparticles.

[0202] Experiment 3

[0203] The FeCNP1 iron carbide nanoparticles in accordance with the invention are used in this experiment. The amplitude of the magnetic field is set at 40.2 mT.

[0204] The mass spectrum obtained for the gas phase at the end of the reaction is shown in FIG. 14. A degree of CO.sub.2 conversion of approximately 55% and a hydrocarbon yield of approximately 37% are deduced from said spectrum, methane being the compound principally formed.

[0205] At the end of the reaction, the nanoparticles are analyzed by DRX. The diffractogram obtained is shown in FIG. 15. When it is compared with the XR refractogram of the nanoparticles before catalysis, shown in FIG. 3(c), it is noted that the nanoparticles have undergone only a very slight modification of their structure during the reaction. The catalyst comprising the nanoparticles can be reused several times without loss of activity.

[0206] The description above clearly demonstrates that the iron carbide nanoparticles in accordance with the invention have the capacity to catalyze the Sabatier reaction by magnetic induction. For the nanoparticles tested, a total conversion of carbon dioxide at 30 mT and 300 kHz is obtained, this being without using any additional catalyst. Under the conditions tested, only a slight modification of the catalyst is observed.

G/ COMPARATIVE MEASUREMENTS OF HYPERTHERMIA BY MAGNETIC INDUCTION

[0207] Synthesis of Iron Carbide Nanoparticles

[0208] Iron carbide nanoparticles are prepared, with the following various carburization times, from the same batch of 12.5 nm Fe.sup.0 nanoparticles, obtained in example 2 above.

[0209] The general protocol is that described in example 4 above, with only the carburization time varying, it being equal to 48 h (NP48 nanoparticles), 96 h (NP96 nanoparticles) or 140 h (NP140 nanoparticles).

[0210] The Mssbauer spectra for each of these nanoparticles are shown in FIG. 16. The compositions for the nanoparticles indicated in table 5 below are deduced from said Mssbauer spectra. The NP96 and NP140 nanoparticles are in accordance with the invention, while the NP48 nanoparticles are not in accordance with the invention, because they have an insufficient molar content of Fe.sub.2.2C.

[0211] The hyperthermia properties of these nanoparticles were analyzed as described in example E/ above. The curves showing, for each one, the SAR as a function of the amplitude of the magnetic field, are shown in FIG. 17. The maximum SARs for each are indicated in table 5 below.

TABLE-US-00005 TABLE 5 Composition and maximum SAR for iron carbide nanoparticles in accordance (NP96, NP140) or not in accordance with the invention Nanoparticle Composition SARmax (W/g) Fe.sup.0 650 NP48 54% Fe.sub.2.2C, 23% Fe.sub.5C.sub.2, 460 18% Fe(0), 5% others NP96 72% Fe.sub.2.2C, 24% Fe.sub.5C.sub.2, 2120 4% Fe(0) NP140 83% Fe.sub.2.2C, 17% Fe.sub.5C.sub.2 3220

[0212] It is observed that the NP96 and NP140 nanoparticles in accordance with the invention exhibit hyperthermia performance levels that are much better than those of other nanoparticles (NP48).

H/ SYNTHESIS OF IRON CARBIDE NANOPARTICLES WITH WATER REMOVAL

[0213] General Protocol

[0214] In a glovebox, the Fe.sup.0 nanoparticles are placed in a Fischer-Porter bottle and redispersed in mesitylene. The upper part of the Fischer-Porter bottle used has a cartridge of glass grafted to the wall (Fischer-Porter bottle manufactured by the company Avitec) into which is introduced pre-activated molecular sieve (approximately 1.5 g). The molecular sieve is not in contact with the nanoparticle solution, and remains at a moderate temperature. The Fischer-Porter bottle is closed and removed from the glovebox, purged of its argon and then placed under a carbon monoxide (between 1 and 10 bar) and hydrogen (between 1 and 3 bar) pressure in order to obtain an overpressure of 3 bar in the bottle. The mixture is then vigorously stirred at 120-180 C. for 1 min to 200 h.

[0215] Once the reaction has ended, the Fischer-Porter bottle is removed from the oil bath and left to cool with stirring. Once at ambient temperature, it is placed in a glovebox and degassed. The nanoparticles are washed, via magnetic washing, three times with toluene and then dried under a vacuum line. The black powder obtained is analyzed by TEM, XRD, VSM and elemental analysis.

Example 9Starting from 12.5 nm Fe.SUP.0 .Nanoparticles

[0216] In a glovebox, the Fe.sup.0 nanoparticles (12.5 nm; 1 mmol Fe; 100 mg), prepared in example 2, are placed in the Fischer-Porter bottle and redispersed in mesitylene (20 ml). The Fischer-Porter bottle is placed under a carbon monoxide (2 bar) and hydrogen (2 bar) pressure. The mixture is then vigorously stirred at 150 C. for 40 h.

[0217] By way of comparison, the same experiment is carried out in parallel without molecular sieve, over the course of periods of 48 h and 140 h.

[0218] After a reaction time of 16 h, a sample of the nanoparticles is taken and analyzed by XRD. No presence of Fe(0) is any longer seen. For the same experiment carried out in parallel without molecular sieve, after 16 h, a composition of approximately 50% of Fe(0) and 50% of Fe.sub.2.2C is obtained. This confirms that the use of the molecular sieve, making it possible to remove the water as it is formed in the reaction, has the effect of accelerating the carburization reaction.

[0219] After 40 h of reaction, nanoparticles comprising a molar content, determined by Mssbauer analysis, of greater than 75% of Fe.sub.2.2C are obtained for the experiment with molecular sieve.

[0220] For each of the experiments with and without molecular sieve, the hyperthermia properties of the nanoparticles obtained are analyzed as described in example E/ above. The curves presenting, for each one, the SAR as a function of the amplitude of the magnetic field, are shown in FIG. 18. It is observed that, for the experiment with molecular sieve (NP40TM), performance levels are obtained over the course of 40 h that are as high as those obtained over the course of 140 h in the absence of molecular sieve (NP140S).

Example 10Starting from 9.0 nm Fe.SUP.0 .Nanoparticles

[0221] In a glovebox, Fe.sup.0 nanoparticles (9.0 nm; 1 mmol Fe; 100 mg) (of commercial origin) are placed in the Fischer-Porter bottle and redispersed in mesitylene (20 ml). The Fischer-Porter bottle is placed under a carbon monoxide (2 bar) and hydrogen (2 bar) pressure. The mixture is then vigorously stirred at 150 C. for 24 h. This experiment is carried out in duplicate.

[0222] By way of comparison, the term experiment is carried out in parallel without molecular sieve, over the course of periods of 24 h and 120 h.

[0223] After 24 h of reaction, nanoparticles comprising a molar content, determined by Mssbauer analysis, of greater than 75% of Fe.sub.2.2C are obtained for the experiment with molecular sieve.

[0224] For each of the experiments with and without molecular sieve, the hyperthermia properties of the nanoparticles obtained are analyzed as described in example E/ above. The curves showing, for each one, the SAR as a function of the amplitude of the magnetic field are shown in FIG. 19. It is observed that, for the experiments with molecular sieve (NP24TM), performance levels are obtained over the course of 24 h that are as high as those obtained over the course of 120 h in the absence of molecular sieve (NP120S).

I/ SYNTHESIS OF SIRALOX-SUPPORTED NICKEL-COVERED IRON CARBIDE NANOPARTICLES

[0225] Synthesis of Ruthenium-Doped Siralox

[0226] In a glovebox, the Siralox (800 mg) is added to an orange solution of Ru(COD)(COT) (120.0 mg, 0.38 mmol) in mesitylene (5 ml). The mixture is stirred under argon at ambient temperature for 1 h, then placed under a dihydrogen (3 bar) pressure and stirred at ambient temperature for 24 h. At the end of the reaction, the powder is recovered by decanting and washed three times with toluene (35 ml). The ruthenium Ru-doped Siralox is then dried under vacuum. According to the elemental analyses, it contains 1% by weight of Ru.

[0227] Synthesis of the Siralox-Supported Nanoparticles

[0228] In a glovebox, the magnetic nanoparticles (FeC@Ni1 of example 8 or FeCNP2 of example 4) (100 mg, i.e. approximately 75 mg of Fe) are dispersed in toluene (5 ml). The Siralox (Sir) or the Ru-doped Siralox as described above (RuSir) (700 mg) is added to the solution, and the resulting mixture is exposed to ultrasound (outside the glovebox) for approximately 1 min. The Fischer-Porter bottle is again placed in the glovebox, and the supernatant is removed after decanting. The black powder obtained is dried under vacuum. A load at approximately 10% by weight of Fe on the Siralox is obtained.

J/ CATALYSIS OF THE SABATIER REACTION IN A CONTINUOUS-FLOW REACTOR

[0229] General Protocol

[0230] In a glovebox, the supported catalyst (800 mg) is loaded, in powder form, into a glass reactor (1 ml, 1 cm0.8 cm). The reactor is then connected to the catalysis equipment, placed at the center of an induction coil, and fed with a flow of H.sub.2 and CO.sub.2 controlled by mass flows (H.sub.2/CO.sub.2 ratio=4, stoichiometric). For a standard test, the total flow rate is set at 25 ml.Math.min.sup.1 (hourly space velocity HSV=1500 h.sup.1). The reactor, placed at the center of the induction coil, is exposed to alternating magnetic fields of frequency 300 kHz and of amplitudes of between 0 and 64 mT. At the reactor outlet, the gases are directly analyzed by GC-MS.

Example 11Sir-Supported FeC@Ni1 Nanoparticles

[0231] The degree of CO.sub.2 conversion, of CH.sub.4 formation and of CO formation, as a function of the amplitude of the magnetic field, which are measured after 2 hours of reaction, are shown in FIG. 20. It is observed that the CO.sub.2 conversion occurs at a high degree of conversion. In addition, from the point of view of the CH.sub.4 formation, the optimum amplitude is in the region of 40 mT (shaded area on the figure). It is associated with a CH.sub.4 yield of approximately 14%. Beyond this, the more the amplitude of the magnetic field increases, the more the CO yield increases and the CH.sub.4 yield decreases.

[0232] The gas chromatogram obtained at the outlet of the reactor, for the amplitude of 40 mT, is shown in FIG. 21. It confirms the presence of CH.sub.4 at the outlet of the reactor.

Example 12RuSir-Supported FeCNP2 Nanoparticles

[0233] In this example, the FeCNP2 nanoparticles according to the invention, on the ruthenium-doped Siralox support, are used. An electron microscopy image of a grain of powder obtained as described in example I/ is shown in FIG. 22. It confirms the presence of the iron carbide nanoparticles at the surface of the grain, in the form of black balls, one of which is indicated by the arrow on the figure.

[0234] The degree of CO.sub.2 conversion, of selectivity with respect to CH.sub.4 and of CO formation, as a function of the amplitude of the magnetic field, which are measured after 2 hours of reaction, are shown in FIG. 23. A high degree of CO.sub.2 conversion is observed as soon as there is a magnetic field amplitude of 25 mT. In addition, the selectivity with respect to CH.sub.4 formation is particularly strong, close to 100%. The optimum amplitude is in the range of from 25 to 30 mT (shaded area on the figure). It is associated with a CH.sub.4 yield of approximately 86%.

[0235] The gas chromatogram obtained at the outlet of the reactor, for the amplitude of 28 mT, is shown in FIG. 24. It confirms the very predominant presence of CH.sub.4 at the outlet of the reactor.

[0236] These results demonstrate that the nanoparticles in accordance with the invention make it possible to carry out the catalysis of the Sabatier reaction, in a continuous-flow operating mode, with high yields and total selectivity for methane formation.

[0237] Similar results are obtained for total flow rates in the range of from 25 to 125 ml/min.

[0238] The experiment is continued for 150 h under a magnetic field of 28 mT, while regularly evaluating the degrees of CO.sub.2 conversion, of selectivity with respect to CH.sub.4 and of CO formation, and also the temperature in the reactor. The curves obtained for these various parameters, as a function of time, are shown in FIG. 25. They confirm that the heating capacity of the nanoparticles according to the invention, and the catalytic activity, are stable after a period of time as long as 150 hours of continuous-flow reaction. This proves to be all the more advantageous since the operation of the magnetic field inducer was interrupted several times during the experiment, demonstrating an ability of the nanoparticles according to the invention to operate intermittently.