Catalyst – solid sorbent – or support therefor: product or process – Catalyst or precursor therefor – Metal – metal oxide or metal hydroxide
Reexamination Certificate
2000-01-18
2002-01-15
Hendrickson, Stuart L. (Department: 1754)
Catalyst, solid sorbent, or support therefor: product or process
Catalyst or precursor therefor
Metal, metal oxide or metal hydroxide
C502S339000, C502S185000, C429S047000, C429S047000, C429S047000, C429S047000, C429S047000
Reexamination Certificate
active
06339038
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a catalyst for a fuel cell containing a polymer solid electrolyte; and to a method for producing the catalyst. More particularly, the present invention relates to such a catalyst exhibiting excellent resistance to catalyst poisoning attributed to carbon monoxide and containing platinum and ruthenium in combination on a carrier therefor; and to a method for producing the catalyst.
BACKGROUND ART
A fuel cell containing a polymer solid electrolyte has become of interest as a power source of an electric automobile or a spacecraft, because such a cell is compact and provides higher current density as compared with a phosphoric acid fuel cell.
The fuel cell containing a polymer solid electrolyte has a layered structure in which a polymer solid electrolyte is sandwiched between a hydrogen electrode (anode) and an air electrode (cathode). Each the hydrogen electrode and the air electrode comprises a mixture of a precious metal-on-carrier catalyst and a solid electrolyte. In this structure, hydrogen gas supplied to the hydrogen electrode passes through micropores in the electrode, during which time the hydrogen gas contacts the catalyst and releases electrons by the action of the catalyst, to thereby be transformed into hydrogen ions. The hydrogen ions pass through the electrolyte in the hydrogen electrode and a solid electrolyte between the hydrogen and air electrodes, and reach the air electrode. At the air electrode, the hydrogen ions produce water as they react with oxygen supplied to the air electrode and electrons which flow into the air electrode from an outside circuit. Meanwhile, the electrons released from hydrogen pass through the catalyst carrier in the hydrogen electrode to the outside circuit and then flow into the air electrode. As a result, in the outside circuit, electrons flow from the hydrogen electrode to the air electrode, to thereby enable utilization of electric power.
As the hydrogen gas supplied to the hydrogen electrode, a hydrogen gas obtained through conversion from a liquid fuel such as methanol is considered promising, from the viewpoints of easy handling and high energy density. However, the hydrogen gas obtained through conversion contains a trace amount of carbon monoxide, which acts as a catalyst poison. Deactivation of a catalyst by poisoning adversely affects the characteristics of the fuel cell.
The catalyst which contains platinum and ruthenium in combination on a carrier has conventionally been known to exhibit excellent resistance to catalyst poisoning attributed to carbon monoxide. A possible explanation for the platinum-ruthenium catalyst's resistance to catalyst poisoning attributed to carbon monoxide is that the poisonous carbon monoxide is eliminated by the following mechanism: OH ions are bonded to ruthenium because ruthenium is a hydrophilic substance, and the OH ions on ruthenium oxidize carbon monoxide adsorbed onto platinum. Accordingly, in platinum-ruthenium catalysts, in order to maximize the effect of ruthenium; i.e., resistance to catalyst poisoning attributed to carbon monoxide, platinum particles and ruthenium particles on the carrier are preferably brought as close to one another as possible.
Conventionally, catalysts used for a fuel cell containing a polymer solid electrolyte in which metallic platinum particles and metallic ruthenium particles are carried in combination by a carrier have been prepared by the following method: an aqueous solution of a platinum compound and an aqueous solution of a ruthenium compound are mixed, and carbon powder serving as a carrier and a reducing agent such as ethyl alcohol are added thereto, to thereby reduce platinum ions and ruthenium ions so as to precipitate platinum particles and ruthenium particles on the carbon powder. The proportions of platinum and ruthenium to be carried on a carrier has typically been 1:1.
However, precious metal particles are very small, having diameters on the order of angstroms. Therefore, it is quite difficult to obtain, over the entire surface of the carrier, a regular, orderly arrangement of the particles such that platinum particles and ruthenium particles are in proximity to one another. Especially, when the proportions of platinum and ruthenium on a carrier are 1:1, platinum particles and ruthenium particles are not necessarily in proximity to one another, and upon occurrence of even the slightest segregation of the particles there may be produced a portion where platinum particles and ruthenium particles are present apart from one another on the carrier. In such a case, in the region where ruthenium particles are sparsely present, the catalyst cannot exhibit sufficient resistance to catalyst poisoning attributed to carbon monoxide, resulting in catalyst deactivation with failure to exhibit satisfactory performance as an electrode catalyst for a fuel cell containing a polymer solid electrolyte.
DISCLOSURE OF THE INVENTION
An object of the present invention is to provide a platinum/ruthenium catalyst for a fuel cell containing a polymer solid electrolyte, wherein platinum particles and ruthenium particles are carried on a carrier such that the particles of the two species exist in proximity to one another without forming an aggregate, which catalyst exhibits excellent resistance to catalyst poisoning attributed to carbon monoxide. Another object of the present invention is to provide a method for producing the catalyst.
In an attempt to attain these objects, the present inventors have examined conventional catalysts for a fuel cell containing a polymer solid electrolyte, as well as methods of producing the same, from the following two viewpoints; proportions of platinum and ruthenium carried on a carrier, and a manner of carrying platinum and ruthenium, and in connection with each viewpoint have encountered a new finding.
The catalyst for a fuel cell containing a polymer solid electrolyte, which is a first invention disclosed by the present inventors, comprises platinum and ruthenium, and a carrier therefor, wherein the proportions of platinum and ruthenium, as measured in the completed catalyst product, are 2-4:8-6 on a mol basis.
In the present invention, the probability at which ruthenium particles exist in proximity to platinum particles is increased by means of incorporating ruthenium at a proportion higher than in the conventional catalysts, and as a result, ruthenium and platinum can be held on a carrier such that ruthenium particles are found in the vicinity of every platinum particle.
Within the above-described range, the proportions of platinum and ruthenium are most preferably 4:6, in order to easily achieve the conditions in which platinum and ruthenium are in proximity to one another on the carrier and to avoid excessive consumption of platinum, which is an active species in an electrode reaction of a fuel gas. A platinum/ruthenium catalyst produced at this ratio exhibits improved resistance to catalyst poisoning attributed to carbon monoxide and has the same catalytic activity as that of a conventional catalyst containing platinum and ruthenium in equal proportions on a carrier.
In view of application of the catalyst to a fuel cell containing a polymer solid electrolyte, the carrier bearing platinum and ruthenium at the above-described proportions is preferably a carbon powder which satisfies the definitions provided in claim
2
; that is, the carrier is preferably carbon powder having micropores of a diameter of 60 Å or less in an amount of 20% or less with respect to the entirety of micropores and a specific surface area of 600-1200 m
2
/g.
The micropore distribution is such that micropores of a diameter of 60 Å or less are limited to 20% or less of the entirety of micropores, because a solid electrolyte cannot enter micropores of a diameter of 60 Å or less. Therefore, even if platinum particles are held in such micropores, hydrogen ions released through electrode reaction are not transferred to a solid electrolyte in the electrode and hydrogen ions cannot reach an air el
Inoue Masahiko
Tada Tomoyuki
Yamamoto Yumi
Arent Fox Kintner & Plotkin & Kahn, PLLC
Hendrickson Stuart L.
Nguyen Cam N.
Tanaka Kikinzoku Kogyo K. K.
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