Colloid systems and wetting agents; subcombinations thereof; pro – Continuous liquid or supercritical phase: colloid systems;... – Aqueous continuous liquid phase and discontinuous phase...
Reexamination Certificate
2000-08-31
2002-10-08
Lovering, Richard D. (Department: 1712)
Colloid systems and wetting agents; subcombinations thereof; pro
Continuous liquid or supercritical phase: colloid systems;...
Aqueous continuous liquid phase and discontinuous phase...
C075S721000, C106S001210, C429S047000, C502S173000, C502S339000
Reexamination Certificate
active
06462095
ABSTRACT:
Polymer-stabilized metal colloid solutions, process for preparing them and their use as catalysts for fuel cells.
The invention relates to metal colloid solutions which comprise one or more platinum compounds and, if desired, one or more compounds of Rh, Ru, Ir or Pd and are stabilized by polymeric protective colloids, and also a process for preparing them and their use as catalysts, in particular in fuel cells.
The use of a sol process for producing heterogeneous catalysts whose active centers consist of a metal, in particular a noble metal, for chemical and electrochemical processes is known. Here, a sol of the respective catalytically active metal or, if desired, two or more metals is first prepared in a separate process step and the dissolved or solubilized nanosize particles are subsequently immobilized on the support. General descriptions of this method may be found, for example, in (a) B. C. Gates, L. Guczi, H. Knözinger, Metal Clusters in Catalysis, Elsevier, Amsterdam, 1986; (b) J. S. Bradley in Clusters and Colloids, VCH, Weinheim 1994, pp. 459-544 or (c) B.C. Gates, Chem. Rev. 1995, 95, 511-522.
In general, the sols are prepared using a stabilizer, in particular when further-processable sols having a metal concentration of 0.1% or above are required. The stabilizer envelops the metal particle and prevents the agglomeration of the particles by means of electrostatic or steric repulsion. In addition, the stabilizer has some influence on the solubility of the particles.
Possible stabilizers include both low molecular weight compounds and polymeric compounds.
EP-A-0 672 765 describes the electrochemical preparation of platinum hydrosols using cationic and betaine-type stabilizers and also describes catalysts produced therefrom, which are said to be suitable, inter alia, for fuel cells.
Coated catalysts comprising platinum, which are prepared via a cationically stabilized hydrosol and are suitable for fuel cells, are described in, for example, DE-A44 43 701. In these, the particles of a noble metal form a shell which extends up to 200 nanometers into the support particle.
Platinum sols containing polymeric stabilizers such as polyacrylic acid, polyvinyl alcohol or poly(N-vinylpyrrolidone) and their use for producing catalysts, including ones for fuel cells, have likewise been described (J. Kiwi and M. Gratzel, J. Am. Chem. Soc. 101 (1979) 7214; N. Toshima et al., Chemistry Letters 1981 793). Apart from stabilizing the sol in question, the polymers mentioned have no functional importance.
To obtain a low internal resistance in a membrane fuel cell, it is of critical importance that the transport of protons and electrons within the cell can proceed with as little hindrance as possible. Every barrier between the catalytically active platinum centers and the conduction paths of the electrons and/or protons inhibits the process or brings it to a stop. Critical zones are the sections between the catalytically active centers and the current collectors or the membrane, since a plurality of phase transitions take place in these regions.
A difficult task is to bring about proton charge transport by contact between the catalytically active platinum centers and the membrane. In the methods known from the prior art, the platinum/carbon mixture is, for example, worked into the surface of the membrane by rolling or pressing. However, this way of establishing the contact is difficult to control and there is a risk of excessive hindrance to mass transfer from and to the platinum centers. Furthermore, there is a risk of some of the platinum particles losing contact to the current collector.
In another process, a certain amount of polymeric cation-exchange material is introduced into the platinum/carbon layer, for example by impregnating the platinum/carbon mixture with a solution of the polymeric cation-exchange material, before the platinum/carbon mixture is pressed onto the membrane. This process has the disadvantage that it can result in a reduction in the surface area of the catalyst or the carbon particles can be enveloped too much so that they become electrically insulated.
As a result of this unsatisfactory contact with the current collector or with the membrane, more platinum than would actually be necessary for achieving a particular electric output is needed. In practice, the amount of platinum used is from about 0.5 to 4 mg/cm
2
of membrane area. This corresponds to a number of 100 g platinum for a practical vehicle having a motor power of 40-50 kW.
A further, significant reason for the increased platinum requirement is the production process predominantly employed in the past for the platinum/carbon mixture. In this process, the solution of a reducible or precipitatable platinum compound is applied to the carbon support by impregnation or spraying. Subsequently, the platinum compound is converted into finely divided platinum or platinum oxide particles by precipitation and/or chemical reduction, frequently resulting in formation of relatively large particles having a diameter of up to a few 10 s to 100 nanometers. This causes a reduction in the catalytic activity as a result of the decrease in the specific surface area of the platinum. This can be illustrated by the following example: a cluster, which in the interests of simplicity can be thought of as a queue, made of up atoms of a metal having a given diameter of 0.25 nanometers contains approximately 87% surface atoms at an edge length of 1 nanometer, 49% surface atoms at an edge length of 2.5 nanometers and 0.14% surface atoms at an edge length of 10 nanometers.
It is also known that a platinum catalyst on a carbon support loses surface area under customary operation conditions, i.e. at elevated temperature. This loss is due to the fact that the platinum particles migrate on the support surface and can combine with other particles, i.e. recrystallize to form larger particles. This effect is more pronounced, the smaller the platinum particles. From this point of view it is desirable to reduce the migration velocity of the platinum particles by embedding them in a polar microenvironment which interacts strongly with the carbon support.
In summary, it may be said that in order to obtain a functional membrane fuel cell it is necessary, firstly, to achieve a high dispersion of the catalytically active metal centers, secondly to ensure unhindered transport of starting materials, products and also protons and electrons and thirdly to reduce the recrystallization of the metal particles to form larger particles on the carbon support.
It is an object of the present invention to provide water-soluble, stabilized metal colloids comprising ultrafine particles of platinum. or platinum metals and also a process for preparing them. These colloids should be suitable as catalysts, in particular for fuel cells. When using such metal colloids as catalysts for membrane-electrode units (MEAs), the platinum particles should be in good proton-conducting contact with the membrane and exhibit a reduced tendency to recrystallize.
The present invention achieves this object and thus provides water-soluble metal colloids comprising one or more platinum compounds and, if desired, one or more compounds of Rh, Ru, Ir or Pd, where the metal colloids are stabilized by a proton-conducting protective colloid. According to the invention, the protective colloids used are water-soluble or solubilizable cation-exchange polymers.
Furthermore, the present invention provides a process for preparing these metal colloid solutions by reacting a platinum compound and, if desired, one or more compounds of Rh, Ru, Ir or Pd with a reducing agent. To stabilize the metal colloid solutions, use is made of at least one cation-exchange polymer, and the reduction is either carried out in the presence of the cation-exchange polymer or the cation-exchanger polymer is added to the solution after the reduction step. The stabilized metal colloid (sol) can subsequently be purified by reprecipitation and/or be concentrated by evaporation.
When the metal colloids of the invention are
Bönsel Harald
Deckers Gregor
Frank Georg
Millauer Hans
Soczka-Guth Thomas
Axiva GmbH
Connolly Bove & Lodge & Hutz LLP
Lovering Richard D.
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