WO2003070371A2

WO2003070371A2 - Process for preparing supported metal catalysts

Info

Publication number: WO2003070371A2
Application number: PCT/IT2003/000091
Authority: WO
Inventors: Massimo De Francesco; Francesco Cardellini; Alessia Cemmi; Leonardo Giorgi; Alfonso Pozio
Original assignee: Massimo De Francesco; Francesco Cardellini; Alessia Cemmi; Leonardo Giorgi; Alfonso Pozio
Priority date: 2002-02-20
Filing date: 2003-02-18
Publication date: 2003-08-28
Also published as: AU2003222448A8; WO2003070371A3; AU2003222448A1; ITRM20020087A1

Abstract

The present invention relates to the preparation of supported nanocomposite catalysts of noble metals and not and alloys thereof, by means of low-temperature heat treatments. They consist of one or more elements among which platinum, palladium, gold, ruthenium, rhodium, osmium, silver, iridium, nickel, copper, etc. and are employed in the chemical industry of catalysis being particularly useful in fuel cells for oxidation of a fuel at the anode and reduction of oxygen at the cathode where, as known, high reaction kinetics are required to convert chemical energy into electric energy.

Description

PROCESS FOR PREPARING SUPPORTED METAL CATALYSTS

D e s c r i p t i o n

The invention relates to formation of crystallites of reduced metal obtained by quick low-temperature heat treatment of a suspension containing the powdered carrier material, characterized by a large specific surface area, dispersed in a solution comprising the compounds being the precursors of the metals to be deposited that are intimately mixed with an appropriate reducer of the latter. This for the purpose of minimizing the heat and concentration gradients normal in these preparations, giving rise to a product having homogeneous specifications, adapted to be easily reproduced and of reduced production costs as compared with similar catalysts already on the market.

It is known that nanocomposites comprise very tiny particles of a carrier material having sizes smaller than 100 nanometers, on the surface of which crystallites of usually metallic elements are deposited.

In recent years nanocomposites have become a topic of great interest as they can exhibit particular non-linear electronic and optical properties, and if they have a large surface area, they show a high chemical reactivity and catalyze a great variety of chemical reactions.

It is known that the average diameter of the metal particles deposited on the carrier material can be controlled by varying the crystallite-formation conditions such as: reactant concentrations, pH at which reduction takes place, oxidation state of the metal to be reduced, stability of the metal complexes, metal load in relation to the carrier material. Generally, the diameter of the particles deposited onto the carrier material increases on -increasing of the temperature of the heat treatments employed in the preparative operations.

Several different carrier materials can be selected for chemical use purposes.

For example, the material can have a high porosity and an electronic conductivity, or a transparent matrix may be required for optical applications.

Finally, due to their particular structural features and specific electrochemical qualities, nanocomposites are used in fuel cells for production of electric energy.

It is known that a fuel cell directly converts the chemical energy of a fuel and an oxidizing reactant into a direct electric current of low voltage by means of electrochemical reactions taking place spontaneously at the electrodes thereof, in particular on the surface of the catalysts that are deposited on the electrodes.

Being similar to batteries, fuel cells do not use up the materials composing the electrodes, but convert into electric energy the chemical energy of a fuel placed out of the reaction area. Atmospheric oxygen is the most used oxidizing reactant. Fuel and oxygen are conveyed to the electrodes, anode and cathode respectively, only when energy delivery is required. A fuel cell comprises an anode and a cathode separated by an electrolyte for inner ion conduction. The electric current is carried from the anode to an electric load and therefrom to the cathode by an outer circuit. A mere fuel cell using hydrogen and oxygen has a theoretical potential of 1.229 volts at standard pressure and temperature and generates direct current to a voltage of 0.6-0.8 volts. Obviously, if greater potentials are required the appropriate number of cells will have to be connected in series. In order to obtain high current densities, electrode surfaces provided with favorable catalytic properties can be utilized or better electrochemical-conversion outputs can be achieved by heating the fuel cell, the catalysts themselves being at the same time protected (e.g. CO poisoning of platinum) .

The supported metal catalysts have been widely used in different chemical reactions. In the case of use of expensive catalysts containing precious metals it is indispensable for the catalytically active metals to have very reduced particle sizes so that they can be distributed on the carrier in a homogeneous manner.

As described in US Patent Nos. 3,992,512 and 4,136,059 a method of depositing platinum particles of a diameter lower than 2 nanometers on a carrier was proposed. It comprised the reaction of sulphur-containing compounds, such as sodium sulphite and sodium dithionate, with chloroplatinic acid. The resultant colloid, absorbed on the carrier, was similar to a platinic acid complex compound containing sulphur. Therefore, as regards oxidation, it was treated with hydrogen peroxide thereby obtaining finely divided platinum particles. Finally, to avoid poisoning of platinum by the residual sulphur compounds, the catalyst was submitted to a high temperature heat treatment in a hydrogen atmosphere in order to decompose said compounds and make it substantially free from sulphur and with few agglomerations of metal particles.

In another preparation described in US Patent No. 5,350,727, in which catalysts of 10%-by-weight platinum supported on carbon were treated, use of sulphur- containing compounds was avoided, said compounds being replaced by allylalcohol and hydrated zinc hydroxide, but the final heat treatment at 250 °C for 30 minutes in a 10%-hydrogen admixture with nitrogen was still present, so that at all events dispersed particles of approximately 2-nanometer diameters were obtained.

Finally, in US Patent No. 6,232,264 a preparation was described for producing nanocompounds of a supported platinum-ruthenium alloy by thermal degradation of the precursors of an organometallic linkage. In addition to the high costs of the precursors used, it is to be pointed out that also the related heat treatments at varying temperatures, operating at least at 350 °C, preferably at 650 °C, were carried out in a controlled atmosphere and in various phases which involved complexity and great care during execution of same. The metal particles' diameter was between 3 and 15 nanometers.

In addition, in an example described in US Patent No. 6,232,264 an attempt to cause degradation of the organometallic precursor by heat treatment is disclosed in which use of a microwave oven is provided. The Pt/Ru catalyst, atomic ratio 50:50, consisted of nanoparticles of about 2.8 nanometers, but in this case too the operating conditions were rather complicated and delicate due to use of various atmospheres in the different phases and to the different controls to avoid explosions .and obtain a true metal reduction.

Therefore there is a need for a process designed to prepare supported metal catalysts that are devoid of the drawbacks present in the known art. In particular there is a need for a process designed to prepare supported metal catalysts involving noble metals and not, and alloys of same.

Accordingly, it is an object of the invention to provide an alternative method of preparing supported catalysts of noble metals and not and alloys of same having features of more inexpensiveness, simplicity, quickness and repeatability as compared with the existing ones - that utilizes low-temperature heat treatments (maximum temperature: 150 °C) and for reduced periods of time included between 1 and 20 minutes;

- in which the operating pressure during heat treatments is the same as the atmospheric temperature; - in which the metal crystallites formed are of a diameter included between 1 and 10 nanometers, preferably between 2 and 5 nanometers;

- in which formation of the intimately-bonded metal alloys is in compliance with the expected atomic ratios; - in which distribution and homogenization of the crystallites deposited on the carrier is the maximum one in order to obtain high catalytic surface areas;

- that utilizes the minimum metal load required for the specific catalytic reaction; - that, in the fuel cell field, obtains both anodic and cathodic electrocatalysts, performance of which is comparable with that of the catalysts already on the market but with remarkably lower costs due to the absence of high-temperature heat treatments and to non-use of particular atmospheres, making the preparative operations of easier accomplishment and with less risks as regards the operator's safety.

The principle on which the method of this patent is based rely on the high reduction kinetics of the compounds being the metal precursors. It is a further object of the invention to obtain metal or metal-alloy crystallites of appropriate sizes and an appropriate distribution of same on the carrier so as to promote high specific catalytic areas.

Some preliminary remarks are made necessary to better understand the intimate essence of the method and the orientation that is wished to be followed.

Advantageously, the metal precursor compounds, the reducer and the powdered carrier that will receive the metal crystallites are already intimately mixed and homogenized (monobath) , (condition "a") . At room temperature or a lower one the reduction kinetics is null or negligible.

Advantageously, concentration of the reducer in said monobath is the highest possible one, still in compliance

Advantageously, the finely divided carrier dispersed in the monobath is in a solid/liquid phase ratio included in the range of between 1:1 and 1:1000; in particular 1:100, still in compliance with condition "a".

Advantageously, the reduction kinetics of the metal precursors must be very quick, the thermal gradients within the system being minimized; this is obtained by increasing the monobath temperature in a homogeneous and very quick manner until the boiling temperature is reached (about 100 °C for aqueous-phase systems, for example) .

The Applicant has surprising found that a quick heat treatment carried out within very reduced periods of time with heat absorption, can activate and accelerate the reduction kinetics by the reducer on metal precursors.

Preferably, avoiding thermal gradients and concentration gradients within the suspension mass is advisable, for the purpose of obtaining homogeneity in the sizes of the metal grains on the particles of the carrier material.

Preferably, the boiling temperature of the monobath must be lower than 150 °C both to limit the size increase of the metal crystallites and to reduce the energy cost of the process.

Finally, it is desirable for the heat treatment used to produce high surface defects on the crystallites thereby increasing the catalytic sites, to supply the carrier- metal interface with stronger chemical bonds and to make deposits more stable in terms of sizes.

A process for preparation of a supported metal catalyst is described hereinafter, in particular a process for preparation of electrocatalysts for fuel cells.

The detailed description of the invention will be best understood with the aid of accompanying drawings, in which:

- Fig. 1 shows the XRD spectra of the catalysts obtained in accordance with the three examples given below;

- Fig. 2 shows the potentials (V) versus the related current densities (mA/cm²) of a fuel cell provided at the anode with a first catalyst obtained with the process in accordance with the present invention and of a cell provided with a catalyst of known type; and

- Fig. 3 shows the potential (V) and current density (ma/cm²) versus time (time/min) of a fuel cell provided at the anode with a second catalyst obtained with the process in accordance with the present invention. The process comprises a step in which the finely-divided powdered carrier material, preferably of high porosity and being a good electronic conductor, is dispersed in liquid solvents such as: water, low molecular weight alcohols (methanol, ethanol, isopropanol) , fat acids, aliphatic or aromatic hydrocarbons, aldehydes, ketones. Said dispersion is obtained by treatment with ultrasonic waves followed by heating of the formed suspension to a temperature included between 50 and 100 °C, preferably included between 60 and 90°C, for about 30 minutes, to eliminate air within the particle pores.

In a preferred embodiment carrier materials are employed that give rise to surface areas bigger than 10 m²/gram. Some electronically conductive carriers are available on the market such as acetylene black from Gulf Oil Corporation and Vulcan XC-72 or XC-72R, two naphtha- furnace blacks from Cabot Corporation. These carbon blacks, the particle sizes of which are of about 30 nanometers, can be used as such or alternatively can be combined with graphite to increase the resistance to oxidation of same before depositing the metals thereon or, alternatively, can be previously surface-treated with oxidating agents to promote better anchoring of the metals themselves.

The metal precursor compounds to be deposited are added to the carrier/solvent dispersion. Said metal precursor compounds to be deposited have been previously dissolved with an amount of the same solvent as used for preparing the dispersion.

The metals taken into account for producing the catalysts being the object of the invention concern one or more metals selected from the groups IVa to Vila, the elements from group VIII, the elements from groups lb and lib and finally tin and lead elements from group IVb of the periodic table. All organic or inorganic compounds of the above stated elements can be utilized, provided they are soluble in the selected solvent.

The carrier/solvent/metal precursor suspension can require the pH to be corrected to improve dissolution thereof and promote subsequent reduction. The necessary amount of the appropriate reducer is dissolved separately in the same solvent. Reducers that can be used are: hydrazine, boron hydrides, hydroxylamines, sulfites, phosphites, metal hydrides, formaldehyde, formic acid, low molecular weight alcohol, reducer sugars. The previous list is not exhaustive and does not exclude the possibility of using other reducers. The reducer choice and concentration are dictated by the fact that at room temperature or a lower one there is no considerable degradation and reduction of metal precursors in a lapse of time varying, depending on the process, from 5 minutes to some hours. The suspension and the reducer solution are joined together and weakly stirred: thus the "monobath" is obtained. Then the heat treatment is carried out and the latter must comply with the preliminary considerations set out above.

The technological embodiments to impart the right heat with the necessary features described at the above points can utilize steam currents blown into the suspension, or inert gases such as nitrogen, argon, etc. that are heated and blown thereinto as well, or in particular the suspension can be put into a microwave oven offering the possibility of programming radiation times and powers. It is known that a microwave oven enables different advantages to be achieved as compared with traditional thermal-conduction heating: reduced treatment times (for example, 100 ml of H₂0 are brought from a temperature of 20 °C to the boiling temperature within 35 seconds in a microwave oven with a power density of the microwaves of 32 W/l) ; very quick and homogeneous heating of the whole volume with minimum thermal gradients; privileged absorption of the microwave radiation by specific molecules and partial or overall reflection of the same radiation by other molecules (selective absorption) ; carbon and sensitive solvents such as water for example give rise to much absorption; almost instantaneous stopping of thermal radiation from the oven when the latter is turned off. The maximum power utilized in the heat treatment with microwaves and the treatment time depend on different parameters: boiling temperature and volume of the monobath, power density and uniformity inside the oven, specific microwave absorption (2.45 GHz) by the monobath components (solvents, reactants, carriers) .

In the following examples for preparation of typical anodic electrocatalysts for oxidation of hydrogen and methanol that can be utilized in a fuel cell, use of a heat treatment with microwave radiation of the carrier/precursor/reducer monobaths is highlighted. The catalysts thus produced are characterized, from the morphological-structural point of view, by X ray diffraction (XRD) and electronic microscopy (SEM, TEM) ; finally, from the physico-chemical point of view in electrochemical half-cell (cyclic voltammetry) , performance is evaluated upon use of same as anodic catalysts in a fuel cell.

Example 1

A 20%-by-weight platinum electrocatalyst on carbon is prepared as follows: 2.400 grams of carbon black Vulcan XC-72 from Cabot International having a specific surface area (BET) of 250 m²g^_1 are weighed and dispersed in a 240 ml solution of 2M sodium formate, contained in an appropriate beaker, by treatment with ultrasonic waves for 2 minutes followed by bain-marie heating at 60-90°C for about 30 minutes until complete elimination of air from the pores of the carbon particles. Then cooling at room temperature takes place. The carbon/formate suspension is called "CR".

Simultaneously, a solution of H₂PtCl₆ made acidic by 1 M HCl, is prepared which contains 0.600 grams of platinum. The pH of the solution is brought to 5.0 by addition of powdered sodium carbonate to yield a final volume of about 20 ml; then cooling at room temperature is carried out. The platinum solution is called "M" .

After treating the suspension "CR" with ultrasonic waves for 2 minutes and bringing it back to room temperature, solution "M" is added, and mild stirring is carried out for about 10 minutes to obtain the monobath "CRM".

The monobath "CRM" is treated in a microwave oven (2.45 GHz) and with a rotating plate depending on the following thermal schedule: a) oven "on" for 5 minutes' at 900 W; b) oven "off" for 5 minutes.

The beaker is removed from the oven and quickly cooled under running water to room temperature. After filtering and washing with water to obtain a conductivity of the waste water less of 10 μS, the solids are put in a crystallizer to cause slow evaporation of the residual solvent. Finally the powder is dried at 110 °C. The catalyst prepared with this method has a platinum composition by weight ranging between 19.7% and 20.2%. The residue is carbon. Example 2

An electrocatalyst of a binary alloy consisting of platinum and ruthenium with an atomic ratio of 50:50, supported on the same carbon black as in Example 1 in a percentage of 20% by weight is prepared as follows:

2.400 grams of Vulcan XC-72 are weighed and dispersed in a 240 ml solution of 2M sodium hydroxide, contained in a beaker, first by treatment with ultrasonic waves for 2 minutes then by bain-marie heating at 60-90 °C for at least 30 minutes. Then cooling to 25°C is carried out. The carbon/sodium hydroxide suspension is called "C".

Concurrently, two solutions are prepared. The first one is obtained by dissolving 0.600 grams of sodium hydroboron in 30 ml of 2M sodium hydroxide; this solution is identified as "R". The second solution is obtained by dissolving 0.719 grams of tetra amino hydrate platinum chloride (Pt (NH₃) ₄Cl₂xH₂0) in 30 ml of 2M sodium hydroxide and then adding 13.653 grams of a 1.5%-by-weight ruthenium solution of ruthenium nitrosyl nitrate (Ru NO(N0₃)_x(OH)_y) ; this solution is identified as "M" .

Suspension "C" is treated with ultrasonic waves for 2 minutes, then solution "R" is added, and stirring is carried out for 30 minutes followed by cooling at room temperature, if necessary; "CR" is obtained. Solution "M" is added to suspension "CR" and mild stirring is carried out for 2 minutes; the monobath "CRM" is obtained.

The monobath "CRM" is immediately treated in a microwave oven following the same schedule and modalities and in accordance with the same steps in succession as in the preceding example.

Example 3

Another platinum-ruthenium electrocatalyst on carbon is obtained but with an atomic ratio of 85:15 respectively, following the same procedure as in Example 2.

The catalysts of the two last examples have a composition by weight of the metals ranging from 19.5% to 20.3%. The remaining part is carbon black.

The catalysts prepared in accordance with the three examples herein reproduced are compared, as regards their features and where possible, with catalysts available on the market from E-TEK Inc. The latter use the same support (Vulcan XC-72) and have the same percentage by weight of total metals (20%) .

In Table 1 below the electrochemically active surfaces (EAS) expressed in m² per gram and relating to adsorption of hydrogen and carbon monoxide measured following the cyclic voltammetric technique are reproduced.

Table 1 - Catalytic surface area by cyclic voltammetry

Data relating to hydrogen adsorption- shows that the inventive catalysts have values well matching the corresponding reference E-TEK values; as regards carbon monoxide adsorption, in particular of catalysts Pt-Ru, the inventive preparations show quite lower values as compared with the respective E-TEK values, which means an important difference in the physico-chemical and electronic surface properties.

The X-ray diffraction analysis (XRD) shows values of the crystallite sizes for Pt/C in the range of 2.4-2.8 nanometers, for Pt-Ru 85:15 in the range of 2.4-3.4 nanometers and for Pt-Ru 50:50 in the range of 2.6-3.3 nanometers. The three catalysts submitted to heat treatment at 300 °C for 30 minutes in an inert atmosphere appear to have the crystallite sizes increased, which does not occur for E-TEK catalysts, which means that the latter have already been submitted to a high-temperature heat treatment during their preparation. Graph in Fig. 1 shows the XRD spectra of the catalysts of the three examples described above: in particular, it is highlighted that in catalysts Pt-Ru, formation of a solid solution with a lower grating parameter than that of platinum alone occurs, as proved by displacement of the diffraction peaks.

An important difference between the inventive catalysts Pt-Ru and commercially available catalysts E-TEK is represented by stability to ultrasonic treatment in an alcohol suspension. It is known that in many preparations of supported metal catalysts, phases with high-energy treatments are carried out by use of ultrasonic waves; these waves are used in particular during dispersion of the carrier in the solvent; in homogenization of the metal precursors with the same carrier; often when a chemical agent is added to the suspension for complete and homogeneous integration; in dispersion of the catalyst in a solvent to form an "ink" for the purpose of depositing it in thin layers on other materials or to make catalytic electrodes therefrom.

Based on these considerations, tests have been carried out with the inventive Pt-Ru 50:50 and the corresponding E-TEK, in which a known amount of catalyst is put in a known volume of ethanol (30 mg of catalytic powder Pt- Ru/C in 10 ml of alcohol) ; the suspension is submitted to ultrasonic treatment in a typical ultrasonic-wave cleaning tank (20 kHz) for 14 minutes; then after filtering, content of the metals dissolved in ethanol is analyzed. Identical tests (30 mg of catalytic powder Pt- Ru/C in 10 ml of alcohol) were carried out in batch at 65°C both in ethanol and in methanol, for the purpose of simulating chemical corrosion in the operating conditions of a methanol fuel cell; finally the ruthenium loss by the two catalysts was examined, under the same conditions as in the preceding tests but at room temperature and for 11 days. The results summarized in Table 2 show a stability to chemical corrosion in alcohol greatly higher with the inventive catalyst than with E-TEK.

Table 2 - Chemical Stability tests Ruthenium Loss in Ethanol and Methanol

In addition, measurements were carried out in a fuel cell by depositing the catalyst Pt/C at the anode having a 50 cm² area. The fuel and oxidizing agent utilized, both at a 2 bar absolute pressure, were hydrogen and air respectively, the cell temperature was 70°C. Figure 2 shows the cell potentials (V) and related current densities (mA/cm²) ; by the term "Patent" it is intended the inventive Pt/C catalyst performance, by E-TEK the Pt/C ETEK catalyst performance. It appears that they operatively behave in a similar manner.

Finally, the inventive 20%-by-weight Pt-Ru/C catalyst was tested as the anode of a fuel cell both in pure H2 and in H2 with 100 ppm (parts per million) of CO

p(0₂ and H₂)=1.4 abs bar, Current Density 477 mA/cm²,

[Pt] cathode=0.96 mg/cm² [Pt-Ru] anode=0.46 mg/cm ,

Electrolyte: Nafion 115 Membrane - Electrode Area 106 cm²) .

Performance in cell potential (V) and related current densities (mA/cm²⁾ are reproduced in Fig. 3.

Claims

C L A I M S

1. A process for preparing supported metal catalysts, characterized in that the reduction heat treatment of the metals on the carrier which are contained in a monobath consists in supplying heat in a quick and homogeneous manner, in particular utilizing a heat treatment with a microwave oven.

2. A process as claimed in claim 1, characterized in that heating of the monobath and reduction of the metals contained therein is carried out at a maximum temperature of 300°C, preferably of 150°C at most.

3. A process as claimed in claims 1 and 2, characterized in that the operating pressure during the heat treatment is the atmospheric pressure.

4. A process as claimed in claims 1 to 3, characterized in that the microwave oven can operate at a power density included between 10 and 200 watt/1, preferably between 20 and 50 watt/1.

5. A process as claimed in claims 1 to 4, characterized in that the powdered carrier material has a surface area included between 10 m² and 1500 m²/gram, preferably between 100 m² and 500 m²/gram.

6. A process as claimed in claims 1 to 5, characterized in that the metal elements of which the catalysts consist are selected individually or in a mixture from groups IVa, Va, Via, Vila, the elements from group VIII, the elements from groups lb and lib and finally the tin and lead elements from group IVb of the periodic table.

7. A process as claimed in claims 1 to 6, characterized in that a monobath is used which is stable at room temperature and consists of the precursor compounds of the metals to be deposited intimately admixed with the appropriate solvent, the finely dispersed carrier and the selected reducer.

8. A process as claimed in claims 1 to 7, characterized in that concentration of the reducer utilized for decomposition of the metal precursor compounds is included between 1 and 1000 times the necessary equivalents for stoichiometric reduction of the metals.

9. A process as claimed in claims 1 to 8, characterized in that the finely divided carrier dispersed in the monobath is in a solid/liquid phase ratio in the range between 1:1 and 1:1000, in particular 1:100.

10. A process as claimed in claims 1 to 9, characterized in that the complete reduction of the metals on the carrier takes place exclusively by heat treatment of the monobath.

11. A process as claimed in claims 1 to 10, characterized by catalysts obtained with one or more metals alloyed with each other in the established atomic ratios and percentages by weight, the residue being carrier material .

12. Catalysts obtained with the process of claims 1 to 11.