Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Carbon monoxide component
Reexamination Certificate
2000-01-14
2004-04-27
Dunn, Tom (Department: 1725)
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
Carbon monoxide component
C423S246000, C423S437200, C502S064000, C502S066000, C502S074000, C502S078000
Reexamination Certificate
active
06726890
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a catalyst for oxidizing a reformed gas, which catalyst is used for removing carbon monoxide contained in the reformed gas, which serves as a source of hydrogen for a solid polymer fuel cell.
2. Description of the Related Art
Conventionally, hydrogen gas reformed from methane gas has been widely used as a fuel for a polyelectrolyte fuel cell (hereinafter referred to as a “PEFC”) containing a platinum electrode catalyst. Such a reformed gas is used in consideration of cost.
However, the reformed gas contains carbon monoxide which is inevitably formed during reformation. The carbon monoxide content of the reformed gas is as low as 1%, but it has been known that the small amount of carbon monoxide acts as a catalytic poison for a platinum electrode catalyst and that poisoning of the catalyst results in seriously deteriorated performance of the PEFC.
In order to solve this problem, the carbon monoxide content of the reformed gas must be reduced by a factor of 100 or more. To meet such a demand, Gottesfeld et al. have proposed “a method for oxidizing existing carbon monoxide into carbon dioxide in advance by use of a platinum-on-&ggr;-alumina catalyst” and “a method for oxidizing existing carbon monoxide into carbon dioxide on an electrode catalyst of a fuel cell” by mixing the reformed gas supplied to the cell with 2% or thereabout oxygen gas.
However, these methods proposed by Gottesfeld et al. cause reduction in efficiency of fuel use, because in the course of oxidation of carbon monoxide into carbon dioxide a large amount of hydrogen gas which has to serve as a fuel is also oxidized. A possible explanation for this phenomenon is that catalyst particles carried by carbon black (which serves as an electrode catalyst) or by &ggr;-alumina are exposed at the surface of the carrier, and therefore, not only carbon monoxide but also hydrogen which should serve as a fuel is simultaneously adsorbed onto the catalyst and oxidized. Generally, the rate of adsorption is proportional to the partial pressure of gas. In the present case, hydrogen, a primary component of the reformed gas, has a high partial pressure, and therefore the consumption rate of hydrogen becomes high.
In order to solve this problem, the present inventor has previously proposed a catalyst for oxidizing the reformed gas, which can selectively oxidize carbon monoxide in the reformed gas serving as a fuel for a fuel cell, and can thereby suppress the loss of hydrogen caused by oxidation, wherein the catalyst is supported by a carrier having pores of molecule size (Japanese Patent Application Laid-Open (kokai) No. 7-256112). In that publication, the inventor disclose that a mixture or alloy of one or more species selected from among platinum, palladium, rhodium, iridium, ruthenium, nickel, cobalt, and iron is preferably used as the catalyst.
The PEFC, employing the above-described reformed gas as a fuel, may also greatly contribute toward realizing practical use of an electric automobile, which may find itself a mainstream of a ZEV (Zero Emission Vehicle) emitting no toxic substances, including nitrogen oxides. An electric vehicle employing the PEFC may run for a drastically prolonged distance as compared with a conventional electric automobile employing a lead storage battery. If electric automobiles employing nickel-hydrogen storage batteries so as to overcome, to some extent, the problem of short running distance become popular, an enormous amount of charging power will be required, and the amount of carbon dioxide and nitrogen oxides discharged in the course of thermal power generation for charging the electric automobiles will increase, which may make the overall contribution toward reducing environmental pollution insufficient.
In contrast, the PEFC, having high efficiency in power generation and capable of reducing the discharge of carbon dioxide, employs hydrogen as a fuel (a reformed gas), which is obtained by conversion from methanol serving as a starting material of a fuel. Obtaining hydrogen from methanol is not difficult in consideration of modern technology, and the conversion is performed in an apparatus that can be loaded on a vehicle. When the technique is applied to an electric automobile, existing gas station facilities can be used, and an electric automobile having a long running distance can be provided.
In order to make use of the PEFC in an electric automobile, the performance of the cell should not vary with the flow rate of hydrogen gas serving as a fuel. When the flow rate of hydrogen gas varies during acceleration or deceleration, insufficient removal of carbon monoxide from a reformed gas may adversely affect the running performance of an electric automobile.
In order to realize stable performance of the PEFC, carbon monoxide in the reformed gas serving as a fuel must be oxidized to thereby eliminate poisoning by carbon monoxide. The present inventor confirmed that zeolite serving as a catalyst carrier is a candidate for solving the above problem, and that a mixture or alloy of one or more species selected from among platinum, palladium, rhodium, iridium, ruthenium, nickel, cobalt, and iron is used as a catalyst for selectively oxidizing carbon monoxide. However, they did not know which type of catalyst is best suited for selective oxidization of carbon monoxide in a reformed gas, which is employed in the PEFC of an electric automobile.
Particularly, a catalyst for oxidizing the reformed gas is demanded to have performance differing from that of general catalysts, and has a purpose of selectively oxidizing carbon monoxide. Therefore, the structure or characteristics of a carrier are very important, and a catalyst element must be appropriately chosen. However, suitable combination of the carrier and catalyst element has not been determined so far.
In view of the foregoing, in an attempt to obtain a more improved selectivity in oxidization of carbon monoxide contained in a reformed gas, the present inventor has studied on possible combinations of a catalyst carrier and a catalyst element which had not been conceived at the time of filing of Japanese Patent Application Laid-Open (kokai) No. 7-256112. Therefore, an object of the present invention is to provide a catalyst for oxidizing a reformed gas with quality that permits use of the gas in a fuel cell of an electric automobile.
SUMMARY OF THE INVENTION
As described above, in order to obtain a catalyst for oxidizing the reformed gas, the catalyst being capable of selectively oxidizing carbon monoxide with high accuracy, the present inventor has proceeded the following thinking process. When zeolite serving as a carrier has pores of molecule size and supports a predetermined species of catalyst in the pores, the catalyst can selectively oxidize carbon monoxide in the reformed gas. The catalyst utilizes the difference in the passage rate of elements of the reformed gas; i.e., hydrogen, oxygen, and carbon monoxide, when these species pass through pores of zeolite serving as the carrier of the catalyst.
Accordingly, elements having a smaller size with respect to the pore size of zeolite pass through pores rapidly, and those having a larger size pass through the pores slowly. Therefore, in consideration of the constitutional elements of reformed gas; i.e., hydrogen, oxygen, and carbon monoxide, it is readily understood that hydrogen, the smallest of these, passes through the pores very rapidly as compared with oxygen and carbon monoxide. In addition, regarding polarity of these species, a hydrogen, molecule has no polarity and a carbon monoxide molecule has polarity. In this case, zeolite has a large amount of polar groups in the pores, and therefore, a polar molecule species such as carbon monoxide is easily adsorbed onto the inside walls of the pores and oxidized. This means that hydrogen, oxygen, and carbon monoxide undergo different contact times when they contact with catalyst particles supported on inner walls of the pores of zeolite.
As is described above, in t
Dunn Tom
Ildebrando Christina
Rothwell Figg Ernst & Manbeck
Tanaka Kikinzoku Kogyo K.K.
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