METHOD FOR PRODUCING CATALYSTS WITH NANOPARTICLES OF PLATINUM AND ITS ALLOYS WITH METALS

Abstract

The method allows to produce catalysts with nanoparticles of platinum and its alloys with metals of a given composition, with high values of catalytic activity in an oxygen electroreduction reaction, and with predetermined values of structural characteristics. The method comprises preparation of a solution of chloroplatinic acid or a mixture of chloroplatinic acid with metal salts, mixing thereof with dispersed carbon or non-carbon carriers, their mixtures and compositions with specific surface area of more than 60 m.sup.2/g, dispersion of the obtained mixture, chemical reduction of compounds of platinum and a metal salt with subsequent deposition of nanoparticles of metallic platinum or its alloys on a dispersed carrier being carried out by purging gases selected from: nitrogen oxides (N.sub.2O, NO, NO.sub.2), carbon oxides (CO, CO.sub.2), sulfur oxide (SO.sub.2), ammonia (NH.sub.3) or their mixtures through the solution at a temperature of the solution in the range from 5 to 98 C.

Claims

1. A method for producing catalysts with nanoparticles of platinum and its alloys with metals for a cathode and an anode of low-temperature fuel cells and electrolyzers, comprising preparation of a solution of chloroplatinic acid or a mixture of chloroplatinic acid with metal salts in water or in an aqueous organic solvent, mixing thereof with dispersed carbon or non-carbon carriers, their mixtures and compositions with specific surface area of more than 60 m.sup.2/g, dispersing the obtained mixture, comprising chemical reduction of compounds of platinum and a metal salt with subsequent deposition of nanoparticles of metallic platinum or its alloys on a dispersed carrier is carried out by purging gases selected from: nitrogen oxides (N.sub.2O, NO, NO.sub.2), carbon oxides (CO, CO.sub.2), sulfur oxide (SO.sub.2), ammonia (NH.sub.3) or their mixtures through the solution at a temperature of the solution in the range from 5 to 98 C.

Description

TABLES AND DRAWINGS

[0021] The tables and drawings described herein are for illustrative purposes only of selected embodiments and not all possible implementations, and are not intended to limit the scope of the present disclosure.

[0022] Table 1 illustrates synthesis conditions and composition characteristics;

[0023] Table 2 illustrates comparative characteristics of the activity of Pt/C catalysts;

[0024] FIG. 1a shows X-ray diffraction patterns of Pt/C samples produced in Examples 1-7.

[0025] FIG. 1b shows X-ray diffraction patterns of Pt/C samples produced in Examples 14-20 on various carbon carriers: 1VulcanXC-72R; 2KetjenblackEC 600JD; 3VulcanXC-72 held at the temperature of 200 C. in an air atmosphere for 3 hours; 4Ketjenblack EC 600JD processed at the temperature of 200 C. in an air atmosphere for 3 hours; 5VulcanXC-72 treated with an excess of an aqueous solution of NaBH.sub.4 for 24 hours; 6Ketjenblack EC 300J; 7BlackPearl 2000.

[0026] FIG. 1c shows X-ray diffraction patterns of commercial Pt/C catalysts HiSPEC 3000 (1) and HiSPEC 6000 (2); Pt/C samples produced with the use of formic acid (FA) as the reducing agent (3, 4), ethylene glycol (EG) (5, 6), formaldehyde (F) (7, 8) in an NO.sub.2 atmosphere (3, 5, 7) and in an air atmosphere (4, 6, 8).

[0027] FIG. 2 shows TEM (transmission electron microscopy) high resolution photographs of samples corresponding to Examples 5, 8, 16, 24, 26.

[0028] FIG. 3 shows TEM microphotographs of samples of FA (a, d, g), FA.sub.NO2 (b, d, h), EG.sub.NO2 (c, f, i) and histograms of nanoparticle size distribution.

[0029] FIG. 4 shows cyclic voltammograms of Pt/C samples produced with the use of EG (1), F (2), FA (3) as the reducing agent. Samples produced in an air atmosphere are shown by a dot line, those produced in an NO.sub.2 atmosphere are shown by a solid line. Potential sweep speed is 20 mV/s. 2.sup.nd cycle. The electrolyte is a 0.1 M solution of HClO.sub.4 saturated with argon.

[0030] FIG. 5 illustrates dependence of an average size of platinum crystallites and an area of the electrochemically active surface of Pt/C catalysts on time purged from the start of synthesis to the start of purging NO.sub.2. The reducing agent is formic acid.

DETAILED DESCRIPTION

[0031] Example embodiments will now be described more fully with reference to accompanying drawings.

[0032] The synthesis of a catalysts was carried out by chemical reduction of platinum from a solution of H.sub.2PtCl.sub.6.Math.6H.sub.2O (Aurat, Russia). When preparing a solution of chloroplatinic acid as an organic component, ethylene glycol, ethanol, isopropanol, dimethyl sulfoxide, dimethylformamide, propylene glycol, glycerol, acetone can be used. Vulcan XC-72 graphitized carbon black (Cabot Corp., S.sub.BET=250-280 m.sup.2/g) was used as the carrier material for nanoparticles of platinum or its alloys, but dispersed carbon and non-carbon materials with a specific surface area more than 60 m.sup.2 /g could be used, including carbon black, carbon nanofibers and nanotubes, graphene, fullerenes, oxides of tin, titanium, zirconium, cerium and other metals, nitrides and oxynitrides of titanium, zirconium, molybdenum and other metals, carbides and carbonitrides of tungsten, tantalum, zirconium and other metals, as well as other types of dispersed materials.

[0033] The carrier material can be pre-treated by holding it in a solution of acids, alkalis or salts in different concentrations and at different temperatures, subject to heat treatment in an atmosphere of air, argon, nitrogen, hydrogen, ammonia or other gases and mixtures thereof.

[0034] As a reducing agent, it is possible to use formic acid, sodium borohydride, potassium borohydride, formaldehyde, ethylene glycol, hydrazine, hydroxylamine and other reducing agents.

[0035] The synthesis was carried out in an air atmosphere and by purging gases, such as nitrogen oxides (N.sub.2O, NO, NO.sub.2), carbon oxides (CO, CO.sub.2), sulfur oxide (SO.sub.2) or ammonia (NH.sub.3), as well as mixtures thereof through a suspension (reaction mixture) at a suspension temperature from 5 to 98 C.

[0036] Example 1. 0.1 g of Vulcan XC-72 carbon carrier (specific surface area 250-280 m.sup.2/g) is placed in a solution containing 10 mL of water, and 10 mL of ethylene glycol, 0.066 g of chloroplatinic acid hexahydrate (H.sub.2PtCl.sub.6.Math.6H.sub.2O) is added. The resulting suspension is dispersed by ultrasound. The reaction mixture is purged with an NO.sub.2 stream. Without stopping the purge of NO.sub.2, 25 mL of 0.1 M formic acid is added. The synthesis is carried out for 1 hour: 0.5 hours at the temperature of 90 C. and 0.5 hours without heating.

[0037] In the result, a catalyst was produced with the platinum content of 15.2 wt %, the average particle size of 1.3 nanometers, and the electrochemically active surface area of 135 m.sup.2/g (Pt).

[0038] Example 2. The process is similar to that described in Example 1, but 0.114 g of chloroplatinic acid hexahydrate (H.sub.2PtCl.sub.6.Math.6H.sub.2O) is added. In the result, a catalyst was produced with the platinum content of 27.0 wt %, the average particle size of 1.8 nanometers, and the electrochemically active surface area of 93 m.sup.2/g (Pt).

[0039] Example 3. The process is similar to that described in Example 1, but 0.397 g of chloroplatinic acid hexahydrate (H.sub.2PtCl.sub.6.Math.6H.sub.2O) and 40 mL of 0.1 M formic acid are added.

[0040] In the result, a catalyst was produced with the platinum content of 57.3 wt %, the average particle size of 3.0 nanometers, and an electrochemically active surface area of 68 m.sup.2/g (Pt).

[0041] Example 4. The process is similar to that in Example 1, but the Vulcan XC-72 carbon carrier is placed in 10 mL of a 10% solution of isopropanol in water and 60 mL of ethylene glycol is added which acts as a reducing agent in the synthesis process. The mixture obtained after dispersion and containing chloroplatinic acid is alkalized to adjust the pH of the solution to 6. The synthesis is carried out at a temperature of 98 C.

[0042] In the result, a catalyst was produced with the platinum content of 16.5 wt %, the average particle size of 1.2 nanometers, and the electrochemically active surface area of 130 m.sup.2/g (Pt).

[0043] Example 5. The process is similar to that in Example 1, but 0.178 g of chloroplatinic acid hexahydrate (H.sub.2PtCl.sub.6.Math.6H.sub.2O) is added.

[0044] In the result, a catalyst was produced with the platinum content of 38.1 wt %, the average particle size of 2.4 nanometers, and the electrochemically active surface area of 79 m.sup.2/g (Pt).

[0045] Example 6. The process is similar to that described in Example 1, but 0.030 g of chloroplatinic acid hexahydrate (H.sub.2PtCl.sub.6.Math.6H.sub.2O) is added.

[0046] In the result, a catalyst was produced with the platinum content of 8.4 wt %, the average particle size of 1.0 nanometers, and the electrochemically active surface area of 152 m.sup.2/g (Pt).

[0047] Example 7. The process is similar to that described in Example 1, but the synthesis is carried out at a temperature of 65 C.

[0048] In the result, a catalyst was produced with the platinum content of 18 wt %, the average particle size of 1.4 nanometers and the electrochemically active surface area of 105 m.sup.2/g (Pt).

[0049] Example 8. The process is similar to that described in Example 1, but the purging of NO.sub.2 through the reaction mixture is started 5 minutes after the latter is heated to 90 C. (i.e. 5 minutes after the start of synthesis).

[0050] In the result, a catalyst was produced with the platinum content of 15.9 wt %, the average particle size of 1.5 nanometers, and the electrochemically active surface area of 128 m.sup.2/g (Pt).

[0051] Example 9. The process is similar to that described in Example 1, but the purging of NO.sub.2 through the reaction mixture is started 10 minutes after the latter is heated to 90 C. (i.e. 10 minutes after the start of synthesis).

[0052] In the result, a catalyst was produced with the platinum content of 15.9 wt %, the average particle size of 3.6 nanometers, and the electrochemically active surface area of 69 m.sup.2/g (Pt).

[0053] Example 10. The process is similar to that described in Example 1, but the purging of NO.sub.2 through the reaction mixture is started 20 minutes after the latter is heated to 90 C. (i.e. 20 minutes after the start of synthesis).

[0054] In the result, a catalyst was produced with the platinum content of 18.6 wt %, the average particle size of 4.7 nanometers, and the electrochemically active surface area of 52 m.sup.2/g (Pt).

[0055] Example 11. The process is similar to that described in Example 1, but the purging of NO.sub.2 through the reaction mixture is started 30 minutes after the latter is heated to 90 C. (i.e. 30 minutes after the start of synthesis).

[0056] In the result, a catalyst was produced with the platinum content of 18.8 wt %, the average particle size of 4.9 nanometers, and the electrochemically active surface area of 50 m.sup.2/g (Pt).

[0057] Example 12. The process is similar to that in Example 10, but CO is purged through the reaction mixture.

[0058] In the result, a catalyst was produced with the platinum content of 17.2 wt %, the average particle size of 4.6 nanometers, and the electrochemically active surface area of 55 m.sup.2/g (Pt).

[0059] Example 13. The process is similar to that described in Example 4, but 0.212 g of chloroplatinic acid hexahydrate is added and CO.sub.2 was purged through the reaction mixture during the synthesis.

[0060] In the result, a catalyst was produced with the platinum content of 44 wt %, the average particle size of 3.0 nanometers, and the electrochemically active surface area of 68 m.sup.2/g (Pt).

[0061] Example 14. The process is similar to that described Example 13, but VulcanXC-72R was used as the carbon carrier.

[0062] In the result, a catalyst was produced with the platinum content of 42.6 wt %, the average particle size of 3.1 nanometers, and the electrochemically active surface area of 67 m.sup.2/g (Pt).

[0063] Example 15. The process is similar to that described Example 2, but Ketjenblack EC 600JD was used as the carbon carrier.

[0064] In the result, a catalyst was produced with the platinum content of 28.0 wt %, the average particle size of 1.6 nanometers, and the electrochemically active surface area of 109 m.sup.2/g (Pt).

[0065] Example 16. The process is similar to that described in Example 2, but VulcanXC-72 was used as the carbon carrier that was held in the air atmosphere at the temperature of 200 C. for 3 hours.

[0066] In the result, a catalyst was produced with the platinum content of 28.7 wt %, the average particle size of 1.8 nanometers, and the electrochemically active surface area of 92 m.sup.2/g (Pt).

[0067] Example 17. The process is similar to that described in Example 2, but CO.sub.2 was purged through the reaction mixture during the synthesis, and Ketjenblack EC 600JD was used as the carbon carrier that was treated in the air atmosphere at 200 C. for 3 hours.

[0068] In the result, a catalyst was produced with the platinum content of 29.7 wt %, the average particle size of 3.2 nanometers, and the electrochemically active surface area of 66 m.sup.2/g (Pt).

[0069] Example 18. The process is similar to that described in Example 2, but VulcanXC-72 was used as the carbon carrier that was treated with excess of a NaBH.sub.4 aqueous solution for 24 hours.

[0070] In the result, a catalyst was produced with the platinum content of 28 wt %, the average particle size of 1.9 nanometers, and the electrochemically active surface area of 88 m.sup.2/g (Pt).

[0071] Example 19. The process is similar to that described in Example 3, but Ketjenblack EC 300J was used as the carbon carrier.

[0072] In the result, a catalyst was produced with the platinum content of 60 wt %, the average particle size of 2.8 nanometers, and the electrochemically active surface area of 74 m.sup.2/g (Pt).

[0073] Example 20. The process is similar to that described in Example 3, but BlackPearl 2000 was used as the carbon carrier.

[0074] In the result, a catalyst was produced with the platinum content of 58.5 wt %, the average particle size of 2.7 nanometers, and the electrochemically active surface area of 79 m.sup.2/g (Pt).

[0075] Example 21. The process is similar to that described in Example 1, but the Vulcan XC-72 carbon carrier is placed in a solution containing 15 mL of water and 5 mL of glycerol, and formaldehyde is used as the reducing agent which is added to the reaction mixture in the quantity of 1 mL

[0076] In the result, a catalyst was produced with the platinum content of 18 wt %, the average particle size of 1.4 nanometers, and the electrochemically active surface area of 107 m.sup.2/g (Pt).

[0077] Example 22. The process is similar to that described in Example 4, but NO is purged through the reaction mixture.

[0078] In the result, a catalyst was produced with the platinum content of 15 wt %, the average particle size of 1.2 nanometers, and the electrochemically active surface area of 127 m.sup.2/g (Pt).

[0079] Example 23. The process is similar to that described in Example 22, but titanium dioxide doped with niobium (specific surface area 70 m.sup.2/g) is used as the dispersed carrier. In the result, a catalyst was produced with the platinum content of 19 wt %, the average particle size of 1.2 nanometers and the electrochemically active surface area of 72 m.sup.2/g (Pt).

[0080] Example 24. The process is similar to that described in Example 22, but tin dioxide doped with antimony (specific surface area 102 m.sup.2/g) is used as the dispersed carrier. In the result, a catalyst was produced with the platinum content of 18 wt %, the average particle size of 1.2 nanometers, and an electrochemically active surface area of 81 m.sup.2/g (Pt).

[0081] Example 25. The process is similar to that described in Example 22, but a mixture containing H.sub.2PtCl.sub.6.Math.6H.sub.2O is alkalized with a 0.1 M solution of NaOH in water-ethylene glycol solution (1:1) to pH=11, and then 5 mL of a 0.5 M sodium borohydride solution is added as the reducing agent. In the result, a catalyst was produced with a platinum content of 18 wt %, the average particle size of 2.8 nanometers, and the electrochemically active surface area of 72 m.sup.2/g (Pt).

[0082] Example 26. The process is similar to that described in Example 22, but 0.066 g of chloroplatinic acid hexahydrate and 0.01 g of nickel nitrate hexahydrate (Ni(NO.sub.3).sub.2).Math.6H.sub.2O are introduced into the solution as the precursors.

[0083] In the result, a Pt.sub.3Ni/C catalyst is produced with the platinum content of 17 wt %, the nickel content of 2 wt%, the average alloy nanoparticles size of 2.2 nanometers and the electrochemically active surface area of 90 m.sup.2/g (Pt).

[0084] All the experimental results are summarized in Table 1 where the synthesis conditions and the characteristics of the composition/structure of the produced catalysts are provided. Table 2 presents the comparative characteristics of the activity of the Pt/C catalysts produced in the atmosphere of the corresponding gas and analogs (produced in the air atmosphere, that is without purging other gases), as well as two commercial Pt/C HiSPEC catalysts (Johnson Matthey) with different mass fractions of platinum.

[0085] The catalytic activity in the oxygen electroreduction reaction was determined by the results of voltammetry at a potential sweep at a speed of 20 mV/s in the range from 0.04V to 1.20V relative to a reversible hydrogen electrode (RHE). The measurements were conducted on a rotating disk electrode at rotational speeds of 400, 900, 1,600 and 2,500 rpm at the room temperature in a 0.1 M HClO.sub.4 solution which was purged with oxygen before measurements for 40 minutes. A thin catalyst layer on the electrode was preliminarily formed by deposition and subsequent drying of a drop of a suspension containing 36 g of Pt/C. To fix the Pt/C layer, 7 L of a 0.05% Nafion solution was applied over the catalyst layer remaining on the electrode after the drop was dried.

[0086] The experimental voltammograms were normalized as follows: the electrode potential was determined by the equation E=E.sub.regJ.sub.i*R, where: E.sub.reg is the measured potential value; J.sub.i*R is the ohmic potential drop. The electrolyte resistance R was 26 ohms. A correction for background was introduced into the value of current by subtracting the current of a similar voltammogram measured in an argon atmosphere: J=.sub.O2J.sub.Ar. The catalytic activity of the Pt/C catalyst in the oxygen electroreduction reaction (kinetic current J.sub.k) was determined by extrapolating the straight lines obtained on the basis of the normalized voltammograms in the coordinates of the Koutetsky-Levich equation 1/J1/.sup.0.5to the ordinate axis.

J.sub.k=J.sub.dJ/(J.sub.dJ),

where: J is the current strength on the voltammogram at the potential of 0.90V (relative to RHE); J.sub.d is the current limited by diffusion; J.sub.k is the kinetic current not limited to slowed mass transfer. The calculated kinetic current values were related to the weight of platinum deposited on the electrode (A/g (Pt)).

[0087] As follows from a comparison of the X-ray diffraction patterns shown in FIG. 1a and FIG. 1b, and the results of purging electron microscopy shown in FIG. 2 and FIG. 3, the average size of platinum nanoparticles in Pt/C materials produced with gas purging is smaller, and the dispersion of the particle size distribution is narrower than in analogs produced under the same conditions but without purging the corresponding gases (Tables 1, 2).

[0088] As follows from the results of the calculations performed for the cyclic voltammograms taken on Pt/C catalysts and presented in FIG. 4, the area of the electrochemically active surface of platinum in Pt/C materials produced in an NO.sub.2 atmosphere is larger than that in a commercial HiSPEC 3000 catalyst with comparable loading of platinum and in analogs synthesized in the same conditions but without purging NO.sub.2 (Tables 1, 2). The specific activity of the catalysts in the oxygen electroreduction reaction (A/g (Pt)) is also greater than that of the commercial Pt/C catalyst and analogs with a similar mass fraction of platinum produced in the same conditions (Table 2).

[0089] According to the results of calculating the average nanoparticles size, as performed according to the obtained X-ray diffraction patterns, the longer a time interval between the start of synthesis (bringing the reaction mixture to a predetermined temperature) and the start of purging the NO.sub.2 stream through the reaction mixture, the larger the size of platinum nanoparticles (Table 1).

[0090] As follows from the results of the calculations performed on cyclic voltammograms taken on Pt/C catalysts, the smaller is the area of the electrochemically active surface of platinum in the produced Pt/C materials, the longer is a time interval between the start of synthesis (bringing the reaction mixture to a given temperature) and the start of purging a stream of NO.sub.2 through the reaction mixture (FIG. 5).

[0091] The proposed method allows producing catalysts with nanocrystals of platinum or its alloys fixed to a foreign carrier with an average nanoparticles size of 15 nm, a narrow size distribution of nanoparticles, a platinum mass fraction of 860%, and a combined electrochemically active surface area of 50185 m.sup.2/g (Pt) of platinum nanoparticles with uniform distribution of platinum nanoparticles on the surface of the carbon carrier. The activity of the produced Pt/Cu PtM/C catalysts (A/g (Pt)) in the oxygen electroreduction reaction exceeds the activity of the HiSPEC3000 and HiSPEC4000 commercial analogs (JohnsonMatthey).

[0092] The foregoing description of the embodiments has been provided for purposes of illustration and description. It is not intended to be exhaustive or to limit the disclosure. Individual elements or features of a particular embodiment are generally not limited to that particular embodiment, but, where applicable, are interchangeable and can be used in a selected embodiment, even if not specifically shown or described. The same may also be varied in many ways. Such variations are not to be regarded as a departure from the disclosure, and all such modifications are intended to be included within the scope of the disclosure.