Catalyst – solid sorbent – or support therefor: product or process – Catalyst or precursor therefor – Organic compound containing
Reexamination Certificate
2000-08-31
2002-05-21
Wood, Elizabeth D. (Department: 1755)
Catalyst, solid sorbent, or support therefor: product or process
Catalyst or precursor therefor
Organic compound containing
Reexamination Certificate
active
06391818
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to soluble nanosize particles which comprise platinum alone or platinum together with other metals of the platinum group and are stabilized by protective colloids, and also to a process for preparing them by the sol process, wherein the protective colloids consist completely or partly of polymers which bear side chains containing a sulfobetaine group and can be degraded by hydrolysis. The invention further relates to catalysts produced from the abovementioned nanosize particles and to their use for electrodes of membrane fuel cells.
2. Description of the Prior Art
For reasons of declining energy reserves and of environmental protection, electric drives for the operation of motor vehicles are of great importance as a future-oriented alternative to conventional internal combustion engines. A considerable technical problem is still presented by the provision of electric energy onboard the vehicle. Vehicles powered by rechargeable batteries have only a small storage capacity and therefore allow only limited ranges. In contrast, fuel cells which generate the electric energy onboard the vehicle from a chemical fuel offer, due to the high storage density of the chemical energy carrier and because of their superior efficiency in energy conversion, comparable ranges to those of present-day internal combustion engines.
Fuel cells based on platinum catalysts and on polymeric solid electrolyte membranes have considerable advantages for powering vehicles. Due to their mode of construction, membrane fuel cells are referred to as “polymer electrode membrane fuel cells”, PEMFC or PEFC.
Although a highly developed prior art for membrane fuel cells already exists, a further improvement in the performance together with a reduced usage of the expensive noble metal platinum is required for economic use of production-line electric drives in motor vehicles.
The key part of a PEMFC is a gastight but proton-permeable membrane of a cation-exchange polymer, i.e. a polymer to which negatively charged, acid groups are bound. Both sides of the membrane are covered with a thin layer of a mixture of nanosize platinum particles and fine particles of electrically conductive carbon. The platinum acts as catalyst for the two electrochemical substeps, namely oxidation of the fuel at the anode and reduction of oxygen at the cathode. The outer covering of the platinum/carbon layer in each case comprises the current collector, namely a gas-permeable nonwoven or paper made of electrically conductive carbon fibers. Together with the platinum/carbon layer, the current collector forms the anode or cathode, respectively, of the fuel cell. The complete assembly of all components, anode-membrane-cathode, is referred to as a “membrane electrode assembly” (MEA).
The transport of the gaseous reactants to the electrodes, i.e. of the fuel to the anode and of the atmospheric oxygen to the cathode, occurs backward through the gas-permeable current collector. The anode current collector serves to carry away the electrons which are liberated in the oxidation of the fuel. The cathode current collector serves to supply electrons which are required at the cathode for reduction of the oxygen. The external electronic charge flow from the anode to the cathode corresponds to the external electric circuit with the power consumer located in between. Internal, protic charge transport occurs by protons formed at the anode due to oxidation of the fuel being transported by means of the negatively charged solid-state ions through the cation-exchange membrane to the cathode and there combining with the reduction products of the oxygen to form water.
J. Electrochem. Soc. 143 (1996) L7 describes platinum colloids on a highly porous carbon support. The platinum particles are generated on the support by the impregnation method.
WO 91/19 566 discloses alloys of noble metals with cobalt, chromium and/or vanadium on carbon supports, which are produced by stepwise deposition of the metals on the support and subsequent calcination.
EP-A-0 106 197 discloses catalysts comprising, inter alia, thin, flat platinum crystallites on a graphite support and a process for preparing them in which they are deposited electrochemically on the support.
DE-A-27 19 006 claims catalysts in which the cations of the catalytically active metal are bound to the carbon support via acid groups and the catalyst is used without prior reduction in the process to be catalyzed.
It is also known that heterogeneous catalysts for chemical and electrochemical processes, whose active centers consist of a metal, in particular a noble metal, can be prepared on the basis of a sol process. Here, a sol of the appropriate catalytically active metal or, if desired, a plurality of metals is firstly produced in a separate process step and the dissolved or solubilized nanosize particles are subsequently immobilized on the support. The general advantage of the sol process is the high dispersion of the particles which can be achieved, with the lower limit at present extending, for example, to about 1 nanometer in the case of platinum.
General descriptions of these methods may be found, inter alia, in (a) B. C. Gates, L. Guczi, H. Knozinger, Metal Clusters in Catalysis, Elsevier, Amsterdam, 1986; (b) J. S. Bradley in Clusters and Colloids, VCH, Weinheim 1994, p. 459-544; (c) B. C. Gates, Chem. Rev. 1995, 95, 511-522.
The sols are generally produced using a stabilizer, in particular when further-processable sols having a metal concentration of 0.1% or more are required. The catalyst envelopes the metal particles and prevents agglomeration of the particles by means of electrostatic or steric repulsion. In addition, the stabilizer influences the solubility of the particles to some degree.
As stabilizers, it is possible to use both low molecular weight compounds and polymeric compounds.
Platinum sols comprising low molecular weight, mainly surface-active stabilizers and their use for producing catalysts for fuel cells have been described many times:
EP-A-0 672 765 discloses the electrochemical preparation of platinum hydrosols using cationic and betainic stabilizers and also catalysts produced therefrom which are said to be suitable, inter alia, for fuel cells.
DE-A-44 43 701 discloses platinum-containing coated catalysts which are said to be suitable for fuel cells. Here, the Pt particles form a shell which Th extends into the support particle to a depth of up to 200 nm. A process for producing them via a cationically stabilized hydrosol is also claimed.
DE-A44 43 705 claims the preparation of surfactant-stabilized metal colloids as precursors for heterogeneous catalysts.
Platinum sols comprising polymeric stabilizers and their use for producing catalysts, inter alia for fuel cells, have likewise been described. These involve the use of, for example, polyacrylic acid, polyvinyl alcohol or poly(N-vinylpyrrolidone). Apart from the purpose of stabilizing the sol concerned, the polymers mentioned have achieved no functional importance.
J. Am. Chem. Soc. 101 (1979) 7214 describes platinum colloids for the photolysis of water which have a hydrodynamic diameter of from 22 to 106 nm and are stabilized by means of polyvinyl alcohol.
Chemistry Letters 1981 793 describes polymer-supported platinum colloids which have a particle size of from about 1.5 to 3.5 nm and have been prepared using poly(N-vinylpyrrolidone) or polyvinyl alcohol.
Science 272 (1996) 1924 describes platinum particles stabilized with sodium polyacrylate. It has been found that the edge length and the crystal shape of the particles depends on the ratio of the amounts of stabilizer and platinum.
Furthermore, it has also been shown that the presence of a stabilizer can be unnecessary if the sol is produced in the presence of a support. DE-A-25 59 617 discloses the production of catalysts by converting a platinum salt into a metastable colloid in the presence of a support so that the colloid is deposited on the support.
U.S. Pat. No. 4,937,220 discloses a process for reducing
Bönsel Harald
Deckers Gregor
Frank Georg
Millauer Hans
Soczka-Guth Thomas
Celanese Ventures GmbH
Connolly Bove & Lodge & Hutz LLP
Wood Elizabeth D.
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