JP2016513013A - Method for the synthesis of bimetallic catalyst particles made of platinum and another metal and its use in an electrochemical hydrogen production method - Google Patents

Abstract

The subject of the present invention is a method for synthesizing particles of a bimetallic catalyst mainly comprising platinum and at least one second metal, wherein the salt or complex mainly comprising the first platinum, and the second Of a pure reducing agent that is in liquid form under ambient temperature and pressure conditions (ATPC) comprising: chemical reduction with at least one second salt or complex based on In the presence of preparing a mixture comprising a salt or complex based on the first platinum as a main component and the second salt or complex based on the second metal. A step defined as being equal to 25 ° C. and 100 kPa, respectively; and allowing the mixture to reach a temperature approximately between the freezing temperature of water and the freezing temperature of the reducing agent. Is the method.

Description

The field of the invention is that of H ₂ / O ₂ fuel cells. The use of this type of fuel cell in the automotive industry as an alternative to an internal combustion engine still faces problems with hydrogen storage. Paper Schlapbach L. and Zuettel A.A. , Hydrogen-storage materials for mobile applications, Nature, 2001, 414, 353-8, pressurized hydrogen tanks (gas storage) are potentially dangerous and metal hydrides (in solid form) Storage) is inadequate because of their low energy density.

  Paper Basile A. Galluci F., et al. and Paturzo L. , "Hydrogen production from methanol by oxidative steam reforming carried out in a membrane reactor", as described in hydrogen by Catalysis Today, 2005, 104, 251-9. While this is another solution for direct storage, a purification step is required to supply the fuel cell.

For this type of application, gaseous biofuels or hydrocarbons, such as natural gas, or liquid biofuels and hydrocarbons, such as alcohol, gasoline, or diesel oil, may be a source of hydrogen and connected to a low power source Electrochemical cells can be used to extract hydrogen at low temperatures from gas mixtures containing carbon monoxide (CO), carbon dioxide (CO ₂ ), methane (CH ₄ ), and other gases.

Paper M.D. Ciureanu et al. , "Electrochemical Impedance Study of Electrode- Membrane Assemblies in PEM Fuel Cells I.Electro-oxidation of H 2 and H 2 / CO Mixtures on Pt-Based Gas-Diffusion Electrodes", Journal of the Electrochemical Society, 1999,146,4031- As described in 4040, by applying sufficient voltage to the terminals of the cell, electrolytic oxidation of hydrogen and carbon monoxide occurs at the anode:
H ₂ → 2H ⁺ + 2e ⁻ (E ° (H ⁺ / H ₂ ) = 0 on SHE basis, electrochemical equilibrium standard potential SHE is the potential of standard hydrogen electrode)
CO _ads + H ₂ O → CO ₂ + 2H ⁺ + 2e ⁻ .

Platinum is very often used as an electrochemical reaction catalyst and is the best material used at low temperatures for hydrogen electrooxidation reactions (H ₂ → 2H ⁺ + 2e ⁻ ), but purified as an anode material There are problems with its use in applications.

  This is because platinum (Pt) is expensive, rare, and carbon monoxide (present in the reformed hydrogen) has a property that inhibits its action, so its effectiveness is low at low potential. It is. Inhibition of this action occurs by CO irreversibly adsorbing on the Pt surface, closing available adsorption sites, thereby preventing hydrogen adsorption and its oxidation.

Actually, the electrolytic oxidation of CO on the surface of Pt supported on carbon, that is, the electrolytic oxidation of CO to CO ₂ (CO + H ₂ O → CO ₂ + 2H ⁺ + 2e ⁻ ) is around 0.7 to 0.8 V on the basis of SHE. Occurs at a relatively high potential, and this requires a significant energy contribution.

  In order to overcome the problems arising with platinum Pt inherently associated with low resistance to carbon monoxide CO, new anode catalysts are sought.

  To obtain these new types of catalysts, the preparation of alloys of Pt and transition metals can be envisaged, or a combination of highly resolved Pt and metal oxide type phases can be envisaged. . Several types of platinum such as platinum-ruthenium (Pt-Ru), platinum-tin (Pt-Sn), platinum-molybdenum (Pt-Mo) or platinum-cobalt (Pt-Co) supported on high specific surface area carbon black Alloys are the subject of research aimed at finding the best platinum-based alloys that have good resistance to carbon monoxide CO and that allow oxidation of the carbon monoxide CO at low potentials.

In addition, the performance of these anode catalysts for H ₂ / CO electrooxidation is known to depend on their structure, their chemical composition, the nanometer size of the particles, and the nature of their supports. Physicochemical properties can be controlled by making full use of these catalyst synthesis techniques and preparation methods. Metal nanoparticles are generally synthesized by chemical reduction of a metal salt or complex in the presence of a reducing agent. During synthesis, the mechanism by which nanoparticles are formed occurs by grain growth and growth. This step is important because the size of the particles is determined and consequently affects the electroactive surface of the catalyst.

Schlapbach L. and Zuettel A.A. , Hydrogen-storage materials for mobile applications, Nature, 2001, 414, 353-8. Basile A. Galluci F., et al. and Paturzo L. , “Hydrogen production from methanol by oxidative steam reforming carried out in a membrane reactor”, Catalysis Today, 2005, 104, 251-9. M.M. Ciureanu et al. , "Electrochemical Impedance Study of Electrode-Membrane Assemblies in PEM Fuel Cells I.Electro-oxidation of H2 and H2 / CO Mixtures on Pt-Based Gas-Diffusion Electrodes", Journal of the Electrochemical Society, 1999,146,4031-4040

  The production of catalytic nanoparticles by chemical reduction of a metal salt or complex in the presence of a reducing agent is an easy-to-implement method, but control and control of the grain growth and nanoparticle growth stages remains a major problem. It is.

  For this reason, and in this context, the subject of the present invention is to limit the growth stage of nanoparticles and promote the grain growth stage to allow an increase in the number of seeds. This is a method based on optimization of synthesis operation conditions.

  Compared to growing naturally during the synthesis of particles by chemical reduction (without changing the operating parameters), by limiting the growth stage, typically a small size of around 2-5 nm, and a larger number Nanoparticles having particles can be obtained.

  When these two aspects are integrated, a microstructured structure is obtained that optimizes the electroactive surface of the catalyst.

  Changing the temperature at which the chemical reduction of the metal salt and complex takes place is the most effective means for reducing the growth rate of the particles because the two parameters evolve in the same direction.

In particular, the subject of the present invention is a method for synthesizing particles of a bimetallic catalyst mainly comprising platinum and at least one second metal, the salt or complex mainly comprising the first platinum, Chemical reduction with at least one second salt or complex based on a second metal, the chemical reduction comprising:
The first platinum-based salt or complex and the second metal as the main component in the presence of a pure reducing agent in liquid form under ambient temperature and pressure conditions (ATPC); Preparing a mixture comprising two salts or complexes of which the conditions are defined as 25 ° C. and 100 kPa;
-Allowing the mixture to reach a temperature approximately between the freezing temperature of water and the freezing temperature of the reducing agent.

  According to another form of the invention, the reducing agent is formic acid and the chemical reduction is carried out at a temperature that may be between about 0 ° C. and 8 ° C., preferably about 4 ° C.

  According to another form of the invention, the reducing agent is hydrazine and the reduction is carried out at a temperature between 0 ° C and 2 ° C.

  According to another form of the invention, the reducing agent is formaldehyde and the reduction is carried out at a temperature between -19 ° C and 0 ° C.

  According to another aspect of the present invention, a method mixes a platinum salt or complex with a second metal salt or complex in the presence of carbon black or metal oxide or metal nitride or metal carbide particles. Including the steps of:

  According to another aspect of the invention, the amount of reducing agent is greater than that required to perform all chemical reductions of the platinum salt or complex and the second metal salt or complex.

  According to another aspect of the invention, the reaction is carried out in the presence of an additional energy source that facilitates the chemical reduction operation and does not promote the growth of the nanoparticles.

  According to another aspect of the invention, the additional energy source is an ultraviolet radiation source.

  According to another aspect of the invention, the ultraviolet radiation source emits within a wavelength range between about 200 nm and 300 nm.

  According to another aspect of the invention, the second metal is tin or ruthenium or molybdenum or cobalt.

  According to another aspect of the present invention, the bimetallic catalyst particles are based on platinum and tin, their size distribution is low dispersion and shows a median value of 4 nm, with a standard deviation of about 1.1.

  Another subject of the present invention is a method for producing particles of a bimetallic catalyst according to the invention in an electrochemical production process of hydrogen comprising a catalytic reforming reaction in the presence of the catalyst particles of the invention and a gas mixture comprising a hydrocarbon compound. The use of synthetic methods. This is because the metal particles obtained by the synthesis method of the present invention are considered less susceptible to contamination by hydrocarbon compounds present in the gas mixture than the particles obtained by the method described in the state of the art. It is.

  According to another form, the gas mixture comprises carbon monoxide, carbon dioxide, and methane.

  A better understanding of the present invention and other advantages will be apparent from the following description, provided without implying limitation, and with reference to the accompanying drawings, in which:

² shows the UV absorption spectrum of PtCl ₆ ^2- ion. 2 shows the particle size distribution of Pt ₃ Sn / C catalyst synthesized at ambient temperature and 4 ° C. by the method of the present invention. FIG. 5 shows a cyclic voltammogram obtained for Pt ₃ Sn / C synthesized at ambient temperature in an aqueous solution of H ₂ SO _{4 at} a concentration of 0.5 M at 25 ° C. FIG. FIG. 5 shows a cyclic voltammogram obtained for Pt ₃ Sn / C synthesized at 4 ° C. in an aqueous solution of H ₂ SO _{4 at} a concentration of 0.5 M at 25 ° C. FIG. After exposure of the catalyst to a gas mixture composed of 50 ppm CO in H ₂ at 0.24 V on an RHE basis, the change in hydrogen oxidation current over time reached a substantially steady state at a concentration of 0.5M. shows the change of the current in the electrolytic oxidation of _{H 2} that in H ₂ SO ₄ aqueous solution on ambient temperature and 4 ° C. with synthesized _Pt 3 Sn / C.

  For the synthesis of platinum-metal (Pt-M) alloy catalysts, Applicants I. Santiago et al. , “CO tolerance on PtMo / C electrocatalysts prepared by the formal acid method”, Electrochimica Acta, 48 (2003), 3527-3534, is used.

According to the present invention, high specific surface area carbon black, metal oxides _(such as _{TiO 2, ZrO 2, Al 2} O 3), metal nitrides (TiN, TaN, BN), or metal carbides (TiC, _{WC, W} 2 C Pt and metal alloy salts or complex solutions as precursors of Pt-M catalysts supported on or not supported on Mo ₂ C, etc.) are proposed.

  In the case of a carbon support, a solution of a salt or complex of Pt and similar alloying metal elements M is mixed with the carbon support and the combination is subjected to ultrasound for at least 30 minutes, which is the time required to obtain a homogeneous mixture. Stir vigorously.

  The volume of the solution is determined from the concentration of the solution and is determined so as to obtain the desired atomic composition of the metal alloy. The weight of the metal nanoparticle support used in the synthesis is determined to be 50% of the weight of the synthesized catalyst. Formic acid is used as a reducing agent and is added to the above mixture.

  The volume of formic acid needs to be in excess to complete the chemical reduction of the metal salt or complex.

  The entire mixture is then brought to a temperature between that of water and that of formic acid. After 12 to 72 hours, the chemical reduction is completed, and a metal powder serving as a catalyst is obtained.

  Lowering the temperature promotes a decrease in the growth rate of the nanoparticles, resulting in an extended time for the chemical reduction of the metal salt. In order to overcome this extended time, it is desirable to promote the chemical reduction reaction but not increase the growth rate.

  The flask containing the mixture can be advantageously exposed to an energy source, such as an ultraviolet (UV) radiation source, and this energy contribution promotes the reaction (grain growth of the particles), thereby increasing the number of nanoparticles in the nanoparticulate state. It is possible to promote seeds but not to promote their growth. The wavelength of the UV rays is preferably selected between 200 and 300 nm corresponding to the absorption region of the platinum complex as shown in FIG.

  A Pt—Sn / C catalyst with a molar composition of 3: 1 was synthesized by chemical reduction with formic acid.

A solution of Sigma-Aldrich's K ₂ PtCl ₆ · 6H ₂ O and SnCl ₂ · 2H ₂ O was supported on Pt— supported on high specific surface area carbon black (Vulcan XC-72R, Cabot Corp., 250 m ² / g). Used as a precursor for the catalyst formed from Sn.

An aqueous solution of 0.01M platinum and tin salts was mixed in the presence of carbon black and stirred vigorously under ultrasound for about 1 hour. The volume of the K ₂ PtCl ₆ · 6H ₂ O solution and SnCl ₂ · 2H ₂ O solution to be mixed is 15 ml and 5 ml, respectively, which gives an atomic ratio of 3: 1.

  To allow simultaneous reduction of platinum and tin salts, a large amount of formic acid HCOOH (ACS reagent, 98% or more, Sigma, molar ratio between formic acid and metal salt used as reducing agent is about 1000: 1. -Aldrich) is added to the mixture.

Perform two examples of composition work conditions:
The first example is 24 hours at ambient temperature;
-The second example was 72 hours at 4 ° C. After completion of the specified time, a metal powder was obtained. The weight of Pt + Sn is 50% by weight of the catalyst. The temperature of 4 ° C. was selected to be between 0 ° C. and 8 ° C.

  The purpose of synthesizing at this temperature is to reduce the growth rate of the nanoparticles, during which the temperature-dependent growth occurs.

The results obtained are shown in the following table, which relates to the physicochemical properties of the Pt ₃ Sn / C catalyst synthesized at ambient temperature and 4 ° C.

  An electroactive surface corresponds more specifically to a surface that is electrochemically active for the reaction under consideration that it is desirable to increase.

FIG. 2 shows the particle size distribution of the Pt ₃ Sn / C catalyst synthesized at ambient temperature (25 ° C.) and 4 ° C., which is typically as small as 3-4 nm using the synthesis method of the present invention. It shows that the size particles have a high percentage value.

  In particular, the size parameters listed in the table below are obtained.

  In the measurement performed by energy dispersive X-ray spectroscopy (EDS) analysis, the following results are also obtained.

  Thus, the electroactive surface of the catalyst prepared at 4 ° C. is much larger than the electroactive surface of the same catalyst synthesized at 25 ° C.

  This is necessary, for example, to fully operate a system for electrochemical production of hydrogen including catalytic reforming reaction in the presence of a gas mixture containing catalyst particles and hydrocarbon compounds obtained according to the present invention. A small amount of catalyst means that it can be advantageously reduced below the amount of catalyst used obtained by the prior art process.

  Applicants have made cyclic voltammograms of electrochemical half-cells at 25 ° C. at a scan rate of 10 mV / s for the various catalysts prepared.

  Cyclic voltammetry is based on the measurement of the current obtained by reduction or oxidation of a compound in contact with the working electrode (sample to be examined) under the influence of a controlled variation in the potential difference of an electrode with a defined potential known as the reference electrode. Note that this is an electrochemical analysis method. This makes it possible to identify and quantitatively measure a large number of compounds, and to study chemical reactions involving these compounds.

  The high adsorptive power can be characterized by the absence of an oxidation peak (positive current) on the voltammogram showing the current density measured as a function of the applied potential E at the working electrode. The absence of these peaks on the voltammogram reflects interference with the adsorption site of the catalyst by another factor.

FIG. 3 relates to the results obtained with Pt ₃ Sn / C particles prepared at 25 ° C. Curve 3a is related to the voltammetric curve obtained after adsorbing CO and contaminating the catalyst (under N ₂ in 0.5 M H ₂ SO ₄ at 25 ° C. and 10 mV · s ⁻¹ ).

  A comparison can be made using curve 3b (Voltammetry cycle: VC) obtained after desorption of CO.

  The catalyst is completely contaminated and the oxidation current on curve 3A is between 0.1 and 0.4 V on the RHE basis, which is the potential range corresponding to the desorption of electroadsorbed hydrogen (Hupd: underpotential deposited H). Not observed. CO oxidation begins when the electrode potential exceeds 0.4 V on an RHE basis.

FIG. 4 relates to the results obtained with Pt ₃ Sn / C particles s prepared at 4 ° C. For a catalyst prepared at 4 ° C., the same cyclic voltammetry curve (curve 4a) after fouling the catalyst by CO adsorption shows that the catalyst is not completely fouled, and the peak for hydrogen desorption is , Still observed on curve 4a between 0.1 and 0.4V on RHE basis, CO oxidation begins with an electrode potential of 0.3V on RHE basis.

  A comparison can be made using curve 4b (Voltammetry cycle: VC) obtained after desorption of CO.

30% increase in energy terms regarding the oxidation of hydrogen in the presence of a contaminant gas traces (4 ° C. of the catalyst synthesized in 0.72kWh / ^Sm _{3 H2} pairs 25 ° C. of the synthesized catalyst 0.96kWh / ^Sm _{3 H2} Close to).

Applicants supply the electrode with a gas mixture composed of 50 ppm CO in H ₂ and at an ambient temperature in the electrochemical half-cell device and 4 ° C. when a potential of 0.24 V is applied on a RHE basis. The change in current of electrolytic oxidation of H ₂ over the synthesized Pt ₃ Sn / C was also monitored.

  FIG. 5 shows, by curves 5a and 5b, the difference in behavior when a catalyst produced according to the invention at a temperature of 4 ° C. is compared with a catalyst produced at ambient temperature.

The current density measured over time (j) is based on the current density (j _max ) measured when pure hydrogen is supplied to the sample. Initially, since the catalyst is not contaminated, j = j _max and j / j _max = 1.

  In the case of the catalyst synthesized at 25 ° C. (curve 5a), the current density measured with time decreases rapidly and becomes 71% of the initial current after inhibition with CO for 1 hour. In the case of a catalyst synthesized at 4 ° C., the current measured after 1 hour of inhibition is 93% of the initial current. Thus, the catalyst synthesized at 4 ° C. is much more resistant to CO than the catalyst synthesized at 25 ° C. (FIG. 5b).

Claims (12)

In the method for synthesizing particles of a bimetallic catalyst mainly comprising platinum and at least one second metal, a salt or complex mainly comprising the first platinum and at least the second metal being a main component. Including chemical reduction with one second salt or complex, the chemical reduction comprising:
The first platinum-based salt or complex and the second metal as the main component in the presence of a pure reducing agent in liquid form under ambient temperature and pressure conditions (ATPC); Preparing a mixture comprising two salts or complexes, wherein the conditions are defined as equal to 25 ° C. and 100 kPa, respectively;
-Allowing the mixture to reach a temperature approximately between the freezing temperature of water and the freezing temperature of the reducing agent.   The method for synthesizing bimetallic catalyst particles according to claim 1, wherein the reducing agent is formic acid, and the step of reaching the temperature is performed between about 0 ° C and 8 ° C. Wherein a reducing agent is hydrazine (N 2 _{H 4),} synthesis of bimetallic catalysts particles according to claim 1, characterized in that steps to reach the temperature is carried out between about 0 ° C. to 2 ° C. .   2. A step of mixing a platinum salt or complex with said second metal salt or complex in the presence of particles of carbon black or metal oxide or metal nitride or metal carbide. The synthesis | combining method of the particle | grains of the bimetal catalyst as described in any one of -3.   2. The amount of reducing agent is greater than or equal to the amount necessary to perform all of the chemical reductions of the platinum salt or complex and the salt or complex of the second metal. The synthesis | combining method of the particle | grains of the bimetallic catalyst as described in any one of -4.   6. The reaction according to any one of the preceding claims, wherein the reaction is performed in the presence of an additional energy source that facilitates a chemical reduction operation and does not promote the growth of nanoparticles. A method for synthesizing the particles of the bimetal catalyst described.   7. The method of synthesizing bimetallic catalyst particles according to claim 6, wherein the additional energy source is an ultraviolet radiation source.   8. The method of synthesizing bimetallic catalyst particles according to claim 7, wherein the ultraviolet radiation source emits within a wavelength range between about 200 nm and 300 nm.   The method for synthesizing particles of a bimetallic catalyst according to any one of claims 1 to 8, wherein the second metal is tin, ruthenium, molybdenum, or cobalt.   10. The method for synthesizing bimetallic catalyst particles according to claim 9, wherein the particle size distribution shows a median size of 4 nm and a standard deviation of 1.1.   In the electrochemical production method of the hydrogen containing the catalytic reforming reaction in presence of the said catalyst particle and the gas mixture containing a hydrocarbon compound, the particle | grains of the bimetallic catalyst as described in any one of Claims 1-10. Use synthetic methods.   The use of the bimetallic catalyst particle synthesis method of claim 11, wherein the gas mixture comprises carbon monoxide, carbon dioxide, and methane.

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