RU2446009C1 - Method of preparing platinum-ruthenium electrocatalysts - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: invention relates to methods of preparing electrocatalysts. Described is a method of preparing platinum-ruthenium electrocatalysts which are finely dispersed platinum-ruthenium alloy nanoparticles with atomic ratio of metals from 1:4 to 4:1 with total content thereof on a support from 15 to 70 wt %, said nanoparticles being deposited on the surface of a porous carbon support and said method involving co-deposition of hydroxo-chloride complexes of platinum and ruthenium on the surface of the carbon support with neutralisation of suspension thereof in aqueous solution of H₂PtCl₆ and RuCl₃ to pH=5.5-7.0 via gradual addition of an alkaline agent for more than 40 minutes, with moderate heating to temperature 55-70°C, followed by ageing the adsorbed metal compounds at pH=5.5-7.0, partial liquid-phase reduction thereof with an organic reducing agent and final gas-phase reduction in a hydrogen current at 150-250°C.

EFFECT: highly active electrocatalyst is obtained.

2 cl, 6 tbl, 1 dwg, 21 ex

Description

The invention relates to the field of preparation of metal-carbon compositions, which are bimetallic nanoparticles based on platinum and ruthenium attached to the surface of porous carbon carriers (CN) and which can be used as effective anode catalysts in fuel cells (TE) with polymer or liquid electrolyte , as well as catalysts for the implementation of electrochemical synthesis.

Metal-carbon compositions used as electrocatalysts are obtained by applying metal compounds - precursors of the active component - to the surface of a support taken in the form of a fine powder and restoring them to a metallic state. Since the required high content of Pt and Ru (from 20 to 60 wt.%) On the surface of the CN cannot be ensured by its adsorption capacity with respect to any of the traditional catalyst precursors (H ₂ PtCl ₆ and its salts, K ₂ PtCl ₄ , ammonia and nitrite complexes Pt ^II , RuCl ₃ and ammonia or nitrite complexes Ru ^III ), then at one of the stages of preparation of these catalysts some precipitant is usually introduced, the interaction with which leads to the formation of insoluble compounds of these metals and their deposition on the CN surface in the form of fine particles .

The most common method for the preparation of PtRu / C electrocatalysts is the redox deposition of metals on a carbon support in the form of metallic Pt-Ru nanoparticles. Recovery is carried out in aqueous and organic solvents, including immiscible aqueous-organic media (microemulsion methods). Inorganic (Na ₂ S ₂ O ₄ , NaBH ₄ , N ₂ H ₄ × H ₂ O, NaH ₂ PO ₂ ) and organic (formic acid, alcohols, formaldehyde, glycols, etc.) reducing agents are used as precipitants. taken in excess, as well as atomic hydrogen, which is generated during the decay of water caused by the action of γ-radiation or cavitation during ultrasonic treatment (C. A. Angelucci et al., Electrochim. Acta 52 (2007) 7293-7299).

Since metallic colloids are formed in this case, surface-active substances (SAS) are usually introduced to stabilize their particles, in most cases of an organic nature. The use of organic surfactants makes it easy to control the size of the deposited metal nanoparticles, however, the high bond strength of its molecules with the metal surface can change the electrocatalytic properties of the latter and creates certain difficulties with the removal of such contaminants from electrocatalysts. Moreover, metal colloids stabilized by means of surfactants, as a rule, have less affinity for the CN surface than “unprotected” colloids. In some cases, in order to improve the adsorbability of stabilized sols on a support, additional procedures must be resorted to, for example, such as compressing the ionic atmosphere around the sol particle (i.e., decreasing the Debye length 1 / χ) by increasing the ionic strength of the solution by adding simple electrolytes or destroying its protective "fur coats" by oxidation. Apparently, colloidal particles stabilized by means of a surfactant tend to be sorbed in places with the greatest adsorption potential — in pores of close size in which the force fields of the opposite walls overlap on the metal particle. The localization of metal sol particles in narrow pores, on the one hand, creates conditions for blocking the metal surface by pore walls, and on the other hand, complicates the diffusion of reagent molecules to the surface of the active component during the electrocatalytic process, as well as the removal of reaction products, especially gaseous ones. Nevertheless, despite these shortcomings, these methods of synthesis of electrocatalysts are currently the most popular.

Another approach to the synthesis of PtRu / C electrocatalysts is the combined hydrolysis of chloride complexes of platinum and ruthenium followed by adsorption of colloids of mixed metal oxides on CNs. A known method of preparing catalysts (M.T. Reetz, MGKoch, J. Amer. Chem. Soc. 121 (1999) 7933-7934), which consists in neutralizing a solution of RuCl ₃ + H ₂ PtCl ₆ (or PtCl ₄ ) with sodium carbonate with subsequent heating at 80 ° C for 18 h in the presence of surfactants, for example sulfobetaines, which leads to the formation of stable mixed PtRuO _x colloids 1.5 ± 0.4 nm in size, capable of adsorption onto carriers, and after application, the particle sizes remain practically unchanged. As can be seen, the authors of the work also did not avoid the use of surfactants for the synthesis of electrocatalysts.

A known method of preparing deposited Pt-Ru electrocatalysts (JP10334925 (A), H01M 4/88; B01J 23/46; H01M 4/92, 1998.12.18). In the examples in the prototype examples, the catalysts were prepared as follows. A solution of Pt (IV) and Ru (III) compounds, for example, Н ₂ РТСl ₆ + RuСl ₃ or Pt (NO ₃ ) ₄ + Ru (NO) (NO ₃ ) _{3, is} poured into a suspension of a conductive carrier (carbon black, graphite, activated carbon) then an alkali solution is added to pH 7-9 and any aldehyde, for example formaldehyde, is introduced as a reducing agent. In this case, the deposition of metals on the carrier is carried out simultaneously or in any sequence at a temperature of 50-80 ° C, and the suspension is heated before or after the introduction of a solution of metal salts into it. The result is a PtRu / C catalyst with a gross metal content of 10-40 wt.% At an atomic ratio of Pt: Ru from 1: 4 to 4: 1. According to the conclusions based on the interpretation of X-ray phase analysis data, the active component of the catalyst is represented by individual Pt crystallites with a size of <2 nm and Ru with a size of <1 nm. The authors of the patent particularly indicate the absence of alloy particles from these metals as a feature of the prepared catalyst.

Unfortunately, from all the data cited in the patent, it is generally not possible to judge the real state of the applied active component. The correctness of the conclusions of the only X-ray phase analysis method used is doubtful. First, the assignment of lines at 2θ = 40 ° to Pt (110) and at 2θ = 44 ° to Ru (111) is erroneous (in fact, when using CuK _α radiation, these are the Pt (111) and Ru (101) lines , respectively). Secondly, the particle sizes estimated from the broadening of these diffraction lines are about 1.5 nm for Pt and 1 nm for Ru, which generally goes beyond the scope of the X-ray diffraction method, since for such particles their reflections should be so broad that will overlap not only with each other, but also with other also wide lines located in the region 2θ = 38 - 46 °, namely with the line (10) from the CN at 2θ≈42 °, with the Ru (100) lines at 2θ≈ 38.3 °, Ru (002) at 2θ≈42.1 ° and Pt (200) at 2θ≈46.2 °. In addition, finely dispersed metal particles of Pt and Ru are almost completely oxidized upon contact with air, as a result of which the appearance of diffraction patterns from such samples changes drastically (see, for example, V. Rao et al., J. Power Sources 145 (2005) 178). In this regard, it would be correct only to take their diffraction patterns in a reducing atmosphere in a special flow cell reactor, which the authors of the above patent did not do. Finally, it is known that, unlike ruthenium chloride complexes, the kinetics of hydrolysis of platinum chloride complexes is a rather slow process (A.A. Grinberg. Introduction to the chemistry of complex compounds, 4th ed., Rev., L .: Chemistry, 1971 ), and the formation of their hydrolyzed forms, accompanied by their precipitation, proceeds rapidly according to a heterogeneous mechanism - on the surface of oxides and hydroxides of other metals present in the system (S. I. Pechenyuk. Sorption-hydrolytic deposition of platinum metals on the surface of inorganic sorbents. L. : Science, 1991). The latter suggests the inevitability of self-assembly of mixed platinum-ruthenium polynuclear hydroxocomplexes, subsequently leading to alloy metal particles. In this regard, it is not surprising that other authors, unlike the authors of the cited patent, always detected alloyed PtRu particles in the catalysts obtained by alkalizing solutions of their precursors in the presence of CNs with subsequent reduction.

A close technical solution to the claimed is the method of preparation of the deposited Pt-Ru electrocatalyst described in (JB Woodough et al., J. Electroanal. Chem. 240 (1988) 133.) To this end, a pre-neutralized solution of RuCl ₃ + H ₂ PtCl ₆ poured into an aqueous suspension of soot Vulcan XC-72 and then boiled for 2 hours, after which the catalyst is filtered off, washed with water, dried and reduced to H ₂ at 200 ° C for 12 hours. The resulting electrocatalyst 20% Pt ₁ Ru ₁ / Vulcan XC-72 is characterized by a uniform distribution of bimetallic particle sizes having an average The dimensions of about 6.5 nm, which is close to that of the PtRu particles deposited in the same conditions in the absence of carrier.

The disadvantage of this electrocatalyst is the relatively low dispersion of the deposited metal at a relatively low (for electrocatalysts used in fuel cells) content on CN. Apparently, the coarse dispersion of the resulting catalysts is due to the fact that the authors adhered to rather stringent conditions for the deposition of metal compounds, when their recrystallization is possible. Thus, the rapid neutralization of a solution of chloride complexes of platinum and ruthenium in the absence of a carbon carrier can lead to the fact that colloidal particles of hydroxyl complexes of these metals formed in large quantities do not have time to adsorb and stabilize on the surface of the adsorbent, but coagulate and undergo further growth, and boiling will only contribute to intensification of the latter. In this regard, in order to achieve a higher dispersion of the active component of the catalyst, it would be necessary to maintain a sufficiently low rate of formation of its colloidal precursors in solution in comparison with the rate of their adsorption on the carrier. This is, in principle, achievable by reducing the temperature of the suspension and the dosing rate of the alkaline agent.

It is generally recognized that PtRu / C electrocatalysts can exhibit high efficiency in the composition of a fuel cell with a metal content of more than 20 wt.% In the form of nanoparticles of a PtRu alloy 3-4 nm in size (N. Liu et al., J. Power Sources 155 (2006) 95 )

The invention solves the problem of preparing metal-carbon compositions, which are mixed on the surface of carbon in an amount of from 15 to 70 wt.% Mixed platinum-ruthenium nanoparticles, characterized by high dispersion at an atomic ratio of metals from 1: 4 to 4: 1, and which are intended for use as anode catalysts of TE with polymer electrolyte.

The problem is solved by the method of preparation of platinum-ruthenium electrocatalysts, which are superfine alloyed platinum-ruthenium nanoparticles deposited on the surface of a porous carbon carrier with an atomic ratio of metals from 1: 4 to 4: 1 with their total content on the carrier from 15 to 70 wt.%, Which is gidroksohloridnyh coprecipitation of complexes of platinum and ruthenium on the carbon support surface while neutralizing its suspension in H ₂ PtCl ₆ solution and an aqueous RuCl ₃ to pH = 5.5-7.0 by the gradual introduction of alkalis th agent with moderate heating to a temperature of 55-70 ° C, followed by aging the adsorbed metal compounds at pH 5.5-7.0, their partial liquid phase reduction of organic reducing agent and a final gas phase reduction in a stream of hydrogen at 150-250 ° C.

In this case, the total time of gradual introduction of an alkaline agent with moderate heating to a temperature of 55-70 ° C to the final point pH = 5.5-7.0 should be at least 40 minutes.

The porous carbon carrier has a specific surface area of 50 to 1000 m ² / g.

The gradual (slow) alkalization of the suspension at a relatively low temperature provides a low rate of hydrolysis of metal chloride complexes. Due to this, a sufficiently low concentration of colloidal particles will be maintained, and the process of their adsorption on CNs will dominate the process of their coagulation in solution. In addition, a more uniform composition of the supported PtRu particles and their size distribution can be achieved. The aging of precipitates in the pH range corresponding to the lowest solubility of hydrolyzed platinum complexes complicates the recrystallization of precipitation particles of hydroxocomplexes and contributes to a higher dispersion of the final catalysts. The gas-phase reduction of the catalysts along with the efficient formation of a metal alloy can also lead to an increase in the metal-carrier interaction, since at higher temperatures metal particles, due to their greater mobility, quickly reach the desired orientation on the substrate for optimal epitaxial and chemical interaction with its surface atoms, which increases the stability of supported catalysts .

Distinctive features of the present invention in comparison with the prototype are:

- creating a suspension of carbon carrier CN in an acidic solution

predecessors - RuCl ₃ + H ₂ PtCl ₆ ,

- the gradual addition of a solution of an alkaline agent directly to a suspension of a carbon carrier (CN) in a solution of RuCl ₃ + H ₂ PtCl ₆ to the desired pH value,

- moderate heating of the suspension (55-70 ° C) during the deposition of compounds of these metals under the action of an alkaline agent,

- aging of the suspension at pH = 5.5-7.0.

EFFECT: obtaining platinum-ruthenium electrocatalysts deposited on carbon carriers, which ceteris paribus surpass known analogues and prototype in dispersion of metal nanoparticles.

The invention is illustrated by the following examples.

Examples 1-11

Preparation of Pt-Ru / C electrocatalysts with an atomic ratio of Pt: Ru = 1: 1 by varying the nature of the carbon carrier and the metal content on it.

Carry out the following sequence of procedures.

I. Grinding of the UN.

5 g of dried powder of carbon material of the Sibunit brand (US 4978649, СВВ 31/10, B01J 20/20, 12/18/1990) (fraction less than 100 microns) are ground using a Pulverizette planetary mill at a speed of 400 min ^-1 for 4 hours ( a glass of 0.4 l, an agate headset, 50 balls with a diameter of 10 mm). The final powder is represented by particles of 0.1 ... 5 microns, the main fraction is 0.2 ... 2 microns. Soot Vulcan XC-72 (Cabot) is not subjected to grinding.

II. Obtaining RuCl ₃ from Ru (OH) Cl ₃ and preparing a solution of precursors.

5.400 cm ^{3 of a} 0.500 M solution of Ru (OH) Cl ₃ in 6 M HCl and 3.380 cm ^{3 of a} 0.200 M solution of N ₂ H ₄ × 2HCl (molar ratio of Ru ^IV / N ₂ H ₄ = 4: 1 are added to a round-bottom flask with a capacity of 100 cm ³ ), heated until gas evolution ceases, then to a weak boil and incubated for 10 minutes. The solution, initially intensely brown, becomes transparent red-brown due to the reaction:

4Ru (OH) Cl ₃ + N ₂ H ₄ × 2HCl = 4RuCl ₃ + N ₂ + 2HCl + 4H ₂ O

The flask is connected to the water-jet pump and the water and excess HCl are carefully distilled off, periodically shaking and slightly warming the flask in a water bath. After drying, the flask is cooled to room temperature and the pump is turned off. 5.400 cm ^{3 of a} 0.500 M solution of H ₂ PtCl _{6 are} poured into the flask and shaken until RuCl _{3 is} dissolved.

III. Precipitation of mixed metal oxides on a carbon carrier.

In a glass reactor with a capacity of 250 cm ³ equipped with a jacket, add a sample of M g of support and add 10 cm ^{3 of} water and stir for at least 15 minutes using a magnetic stirrer (200 min ^-1 ). The mass M of the CN sample depends on the required total metal content in the catalyst and is calculated by the formula:

where W is the total content (wt.%) of metals in the Pt ₁ Ru ₁ / C catalyst, and m is the mass (g) of a portion of metals used in its synthesis, which is equal to 0.8 g in these experiments.

Prepared according to claim II, a solution of metal-complex precursors is poured into a suspension of CN with stirring and its volume is brought with 60-70 cm ^{3 of} water. The suspension is stirred at room temperature for 15 minutes, then heated to 60 ° C and stirred for another 15 minutes.

To precipitate the metals on the support, slowly, over 40-60 minutes, a 15.00 cm ³ 1.00 M Na ₂ CO ₃ solution is poured with a peristaltic pump and the suspension is left to age with stirring for 30-40 minutes. As a result, pH 5.5-7 is established. If necessary, the pH correction is carried out by adding small excess amounts of a solution of Na ₂ CO ₃ . A qualitative analysis of the brine for platinum ions is carried out (reaction with a 10% solution of SnCl ₂ in 2 M Hcl), and if it turns out to be positive, aging is prolonged.

Then 2.750 cm ^{3 of} 1.00 M NaOOCH are dosed with the same pump and the suspension is left to mix for another 30 minutes. It should be noted that the amount of the injected liquid phase reducing agent — the sodium formate solution — is only about 30% of the stoichiometric amount theoretically required to restore all the active component precursors to the metallic state. The rationale for such an incomplete (partial) liquid-phase reduction of metals during their deposition on CN is given in Example 20.

IV. Filtering, washing and drying the catalyst. The suspension of the catalyst is cooled to 30-40 ° C and the catalyst is separated from the brine on a paper filter, rinsed abundantly with distilled water until the washing water reacts negatively with Cl ^- ions (for this, 1-2 cm ^{3 of the} filtrate is taken and 1-2 are dropped into it drops of a 5% solution of AgNO ₃ ).

The wet catalyst is dried at 110 ° C. in air and the cake obtained from the dried catalyst powder is crushed into small (about 3-4 mm) pieces.

V. Recovery and fusion of catalyst components.

The recovery and fusion of the catalyst components is carried out in a flow reactor in a stream of hydrogen. The reactor is purged with an inert gas (nitrogen or helium) at room temperature, then the temperature is raised to 100 ° C at a speed of 4 ° / min. At this temperature, the catalyst is kept in an inert gas stream for 20 minutes, after which hydrogen is additionally turned on and left for another 20 minutes. The inert gas flow is turned off, leaving only hydrogen, and the reactor is continued to heat up first to 150 ° C (with holding at this temperature for 30 minutes) and then to 200 ° C (with holding at this temperature for 60 minutes).

At the end of the reduction procedure, the reactor is cooled and flushed with an inert gas.

VI. Passivation of the catalyst.

The reduced catalyst is pyrophoric, therefore, it is passivated directly in the reactor by introducing small portions of air into the inert gas stream, preventing significant heating of the catalyst, and then unloading it.

Tables 1-3 show the characteristics (dispersion, chemical composition and structure) of the active component of Pt ₁ Ru ₁ / C catalysts prepared according to the described procedure on various supports with different supported metal contents. From these tables it follows that the proposed method for the preparation of catalysts allows to obtain catalysts with a higher dispersion of the alloyed particles of deposited metals in comparison with the prototype even with a higher content of metals on the surface of carbon carriers.

Table 1 The results of the study by various physicochemical methods * of the dispersion of the particles of the active component of the Pt ₁ Ru ₁ / C catalysts depending on the type of carbon carrier, its specific surface area and supported metal content (examples 1-11). Example Catalyst composition The specific surface of the UN (m ² / g) The average particle size (nm) Chemisorption of CO TEM Electrooxidation of Adsorbed CO No. S _BET d _chem. d _s d _electrohem. one 20% Pt ₁ Ru ₁ / Sibunit 19 72 4.2 3.9 - 2 30% Pt ₁ Ru ₁ / Sibunit 19 72 6.0 - 5.9 3 18% Pt ₁ Ru ₁ / Sibunit 85 82 2.4 - - four 30% Pt ₁ Ru ₁ / Sibunit 85 82 4.0 2.8 3.3 5 20% Pt ₁ Ru ₁ / Sibunit 20 292 2.4 - - 6 30% Pt ₁ Ru ₁ / Sibunit 20 292 3.1 3.9 - 7 40% Pt ₁ Ru ₁ / Sibunit 20 292 4.0 4.5 4.2 8 50% Pt ₁ Ru ₁ / Sibunit 20 292 4.8 5.1 - 9 20% Pt ₁ Ru ₁ / Vulcan XC-72 210 2.5 - 2.9 10 30% Pt ₁ Ru ₁ / Vulcan XC-72 210 3.4 - 3.4 eleven 20% Pt ₁ Ru ₁ / Sibunit 619 415 2.2 2.1 - 20% Pt ₁ Ru ₁ / Vulcan XC-72
(prototype**) 250 - 6.5 - 30% Pt ₁ Ru ₁ / Vulcan XC-72 (BASF, C13-30, lot # El370731) - 5.9 - *** 4.8 * Chemisorption of carbon monoxide in a H ₂ current (pulsed gas titration), transmission electron microscopy (TEM), potentiodynamic (10 mV s ^-1 ) electrooxidation of a CO monolayer adsorbed at a potential of 0.1 V (relative to a reversible hydrogen electrode) in a standard three-electrode cell in electrolyte 0.1 M H ₂ SO ₄ at 25 ° C. ** (J. B. Goodenough et al., J. Electroanal. Chem. 240 (1988) 133.) *** Strong aggregation of particles with a size of 3-4 nm with the formation of polycrystalline dendrites.

table 2 The chemical composition of metal particles detected in electrocatalysts Pt ₁ Ru ₁ / C (according to the microprobe of catalysts by the EDX method). Example No. Catalyst The atomic ratio of Ru: Pt in metal particles at different parts of the surface of the catalysts 2 30% Pt ₁ Ru ₁ / Sibunit 19Р 44:56, 48:52, 43:57, 48:52, 47:53, 54:46 7 40% Pt ₁ Ru ₁ / Sibunit 20Р 63:37, 48:52 8 50% Pt ₁ Ru ₁ / Sibunit 20Р 58:42, 64:36, 61:39

Table 3 The values of the parameter gratings (A) for metal particles in Pt ₁ Ru ₁ / C electrocatalysts (according to X-ray phase analysis of samples reduced by hydrogen in situ in a diffractometer chamber). Example No. Catalyst A, nm one 20% Pt ₁ Ru ₁ /Sib.l9P 0.3811 four 30% Pt ₁ Ru ₁ / Sibunit 85 0.3875 5 20% Pt ₁ Ru _{1 /} Sib.20P 0.3841 6 30% Pt ₁ Ru _{1 /} Sib.20P 0.3850 Pt (PDF # 040802) 0.3923

Examples 12-16

Preparation of 20% Pt-Ru / C electrocatalysts with an atomic ratio of Pt: Ru = 1: 1 with variation of the metal deposition temperature.

The synthesis of the catalysts is carried out using a carbon carrier Sibunit 111 (US 4978649) S _BET = 64 m ² / g, as in examples 1-11, except that at the stage III vary the deposition temperature of platinum and ruthenium.

Table 4 shows the characteristics of the prepared according to examples 12-16 catalysts 20% Pt ₁ Ru ₁ / Sibunit 111.

Table 4 The dispersion of the particles of the active component of the catalysts is 20% Pt ₁ Ru ₁ / Sibunit 111, depending on the temperature of the deposition of metals on the carrier (examples 12-16). Example No. Catalyst composition Precipitation Temperature, ° C The average size of PtRu particles (by chemisorption of CO), nm 12 20% Pt ₁ Ru ₁ / Sibunit 111 fifty 3.8 13 20% Pt ₁ Ru ₁ / Sibunit 111 55 3.5 fourteen 20% Pt ₁ Ru ₁ / Sibunit 111 60 3.4 fifteen 20% Pt ₁ Ru ₁ / Sibunit 111 70 3.7 16 20% Pt ₁ Ru ₁ / Sibunit 111 80 4.1

Example 17

Preparation of electrocatalysts 20% Pt-Ru / C with an atomic ratio of Pt: Ru = 1: 1 by varying the deposition rate of metals.

The synthesis of the catalysts is carried out using a carbon carrier Sibunit 111 (US 4978649) S _BET = 64 m ² / g, as in example 14, except that in stage III, the deposition of metals on the carrier is carried out quickly, i.e. the entire Na ₂ CO ₃ solution is added with a peristaltic pump to the suspension in just 5 minutes.

Table 5 shows the characteristics of the catalysts prepared according to examples 14 and 17 of 20% Pt ₁ Ru ₁ / Sibunit 111.

Table 5 The dispersion of the particles of the active component of the catalysts 20% Pt ₁ Ru ₁ / Sibunit 111 depending on the time of dispensing a solution of an alkaline agent (Na ₂ CO ₃ ) for the deposition of metal hydroxocomplexes on the carrier (examples 14 and 17). Example No. Catalyst composition The deposition temperature, ° C The precipitation time of the precipitant (Na ₂ CO ₃ ), min The average size of PtRu particles (by chemisorption of CO), nm fourteen 20% Pt ₁ Ru ₁ / Sibunit 111 60 40 3.4 17 20% Pt ₁ Ru ₁ / Sibunit 111 60 5 4.5

Examples 14 and 17 show that the rapid establishment of the final pH of the suspension with the addition of a precipitant impairs the dispersion of the catalyst, apparently due to a mismatch between the rates of formation of colloidal particles of metal hydroxocomplexes and their adsorption on the carrier, which leads to their enlargement. This explains the fact that the preliminary mixing of the solution of salts of platinum and ruthenium with a solution of an alkaline agent in the absence of a carbon carrier, as is done in the prototype, leads to coarse catalysts.

Examples 18-19

Preparation of electrocatalysts 20% Pt-Ru / Sibunit 111 by varying the atomic ratio from Pt: Ru = 4: 1 to Pt: Ru = 1: 4.

The synthesis of the catalysts is carried out analogously to examples 1-11, except that the atomic ratio of platinum and ruthenium in the catalyst varies. A solution of Na ₂ CO _{3 is} dosed to the desired pH of the suspension.

Table 6 shows the characteristics of the 20% Pt-Ru / Sibunit 111 catalysts prepared according to examples 13, 18, 19.

Table 6 The dispersion of particles of the active component of the catalysts 20% PtRu / Sibunit 111 depending on the atomic ratio of metals in the catalyst (examples 18, 19). Example No. Catalyst composition The average size of PtRu particles (by chemisorption of CO), nm 13 20% Pt ₁ Ru ₁ / Sibunit 111 3.4 eighteen 20% Pt ₄ Ru ₁ / Sibunit 111 3.8 19 20% Pt ₁ Ru ₄ / Sibunit 111 3.3

As can be seen from the above examples and tables 1-6, the present invention allows to obtain PtRu / C catalysts having a higher dispersion of alloyed particles of supported metals in comparison with the prototype and commercial reference sample from BASF even with a higher content of metals in the catalyst or a lower specific surface area of carbon carriers.

Example 20

Preparation of an electrocatalyst 20% Pt-Ru / C with an atomic ratio of Pt: Ru = 1: 1 by varying the amount of added NaOOCH.

The synthesis of the catalyst is carried out using a carbon carrier Sibunit 111 (US 4978649) S _BET = 64 m ² / g, as in example 16, except that in step III, 9.450 cm ³ 1.00 M NaOOCH are dosed, i.e. the amount of injected liquid phase reducing agent — a sodium formate solution — is equal to the stoichiometric amount formally necessary to restore platinum and ruthenium to a metallic state.

Despite the fact that the completeness of metal deposition is also achieved, subsequent filtration with washing the sample with water causes the desorption of metal colloids and their transition to the filtrate, probably due to a strong isoelectric shift of the potential of the surface of the carrier when replacing the brine with water (in this case, equilibrium the surface potential is established by removing the excess charge by desorption of charged particles - metal colloids). Although this undesirable loss of metals is small and amounts to only about 3-5% of the total mass of supported metals, the fineness of the final catalyst is noticeably coarser and amounts to 4.6 nm versus 4.1 nm (Example 16), which is explained by both an increase in the surface mobility of supported metal particles and their coagulation in the specified conditions.

As can be seen from examples 16 and 20, carrying out a complete liquid-phase reduction of the catalyst with sodium formate at a high content of supported metals on the CN surface is impractical.

Example 21

Comparison of the specific activity of the Pt ₁ Ru ₁ / C catalysts in the methanol electrooxidation reaction.

On a glassy carbon rod (diameter of the working surface 5 mm), mounted in a brass collector and insulated with teflon so that only the end surface of the glassy carbon rod can be in contact with the liquid electrolyte, 20 μl of the catalyst suspension in water (1.5 mg / ml) is applied and dried under argon current. The electrode is immersed in 0.1 M H ₂ SO ₄ and oxygen is removed from the electrolyte by purging with argon for 90 minutes. After this, the comparison of catalytic activity is carried out in a potentiostatic mode, recording the dependence of current on time.

Figure 1 shows chronoamperograms of methanol electrooxidation at potentials 0.40 (a) and 0.55 V (b) relative to a reversible hydrogen electrode (OEC) recorded at a temperature of 50 ° C in an electrolyte of 0.1 M CH ₃ OH + 0.1 M H ₂ SO ₄ for catalysts 1) 30% Pt ₁ Ru ₁ / Vulcan XC-72, 2) 30% Pt ₁ Ru ₁ / Sibunit 85, 3) 30% Pt ₁ Ru ₁ / Sibunit 19P, 4) 40% Pt ₁ Ru ₁ / Sibunit 20P and 5) commercial catalyst 30% Pt ₁ Ru ₁ / Vulcan XC-72 grade C13-30 BASF. The ordinates show the values of current densities i, referred to the total mass of noble metals in the working electrode, and the abscissa shows the time of electrooxidation.

Figure 1 shows that the specific catalytic activity of bimetallic catalysts for fuel cells of the composition Pt ₁ Ru ₁ / C prepared by the proposed method in the electrooxidation of methanol, referred to the mass of metals contained therein, significantly exceeds the specific activity of the commercial catalyst Pt ₁ Ru ₁ / Vulcan XC-72 grade C13-30 (BASF) due to the greater dispersion of bimetallic particles.

Claims (2)

1. The method of preparation of platinum-ruthenium electrocatalysts, which are superfine alloy platinum-ruthenium nanoparticles deposited on the surface of a porous carbon carrier with an atomic ratio of metals from 1: 4 to 4: 1 with their total content on the carrier from 15 to 70 wt.%, Which consists in the coprecipitation of hydroxyl chloride complexes of platinum and ruthenium on the surface of a carbon carrier while neutralizing its suspension in an aqueous solution of H ₂ PtCl ₆ and RuCl ₃ to a pH of 5.5-7.0 by gradually introducing an alkaline agent into for more than 40 min with moderate heating to a temperature of 55-70 ° C, followed by aging of the adsorbed metal compounds at pH 5.5-7.0, their partial liquid-phase reduction with an organic reducing agent and final gas-phase reduction in a stream of hydrogen at 150-250 ° C . 2. The method according to claim 1, characterized in that the porous carbon carrier has a specific surface area of from 50 to 1000 m ² / g

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