Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Carbon monoxide component
Reexamination Certificate
2000-03-27
2002-02-26
Griffin, Steven P. (Department: 1754)
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
Carbon monoxide component
C422S168000, C422S177000, C423S437200, C502S326000, C502S329000, C502S337000, C502S340000
Reexamination Certificate
active
06350423
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a carbon monoxide concentration reduction apparatus, a method of reducing the concentration of carbon monoxide, and a carbon monoxide selective oxidation catalyst. More specifically the present invention pertains to a carbon monoxide concentration reduction apparatus that reduces the concentration of carbon monoxide included in a hydrogen rich gas, a corresponding method of reducing the concentration of carbon monoxide, and a carbon monoxide selective oxidation catalyst used therefor.
2. Description of Related Art
Proposed carbon monoxide concentration reduction apparatuses for reducing the concentration of carbon monoxide included in a hydrogen rich gas utilize a ruthenium catalyst carried on a carrier, such as alumina (for example, JAPANESE PATENT LAID-OPEN GAZETTE No. 8-133701, No. 8-133702, and No. 8-217406). When the hydrogen rich gas and a predetermined quantity of oxygen are fed into any of these apparatuses, the ruthenium catalyst accelerates a carbon monoxide selective oxidation reaction, which oxidizes carbon monoxide, in preference to the oxidation of hydrogen and thereby reduces the concentration of carbon monoxide included in the hydrogen rich gas.
Such a carbon monoxide concentration reduction apparatus is used in a fuel cells system including, for example, polymer electrolyte fuel cells or phosphate fuel cells. The following electrochemical reactions proceed in these fuel cells:
H
2
→2H
+
+2e
−
(1)
2H
+
+2e
−
+(½)O
2
→H
2
O (2)
H
2
+(½)O
2
→H
2
O (3)
Equation (1) shows the reaction proceeding on the anodes of the fuel cells, Equation (2) shows the reaction proceeding on the cathodes of the fuel cells, and Equation (3) shows the reaction proceeding in the whole fuel cells. As clearly understood from these equations, for the progress of the reaction in the fuel cells, it is required to feed a supply of a hydrogen-containing gaseous fuel to the anodes and a supply of an oxygen-containing oxidant gas to the cathodes. Carbon monoxide that is present in these supplies of the gases is adsorbed on a platinum catalyst included in the fuel cells and deteriorates its catalytic ability. The air, which is generally used as the oxidant gas, does not contain carbon monoxide to the level that lowers the catalytic ability. The gaseous fuel, on the other hand, generally contains a little quantity of carbon monoxide, which may interfere with the dissociation of hydrogen proceeding on the anodes and lower the performance of the fuel cells.
The presence of carbon monoxide in the gaseous fuel is ascribed to the production of the gaseous fuel by reforming a hydrocarbon. The problem of carbon monoxide described above accordingly arises when the gaseous fuel used is not the gaseous hydrogen of a high purity but the hydrogen rich gas produced by reforming a hydrocarbon. The fuel cells system, which utilizes the reformed gas as the gaseous fuel supplied to the fuel cells, generally has a fuel reformer unit that reforms a hydrocarbon to produce a hydrogen rich gaseous fuel, which is fed to the anodes of the fuel cells. The following shows an example of the reforming reactions to produce the hydrogen rich gas, in which methanol is steam reformed:
CH
3
OH→CO+2H
2
(4)
CO+H
2
O→CO
2
+H
2
(5)
CH
3
OH+H
2
O→CO
2
+3H
2
(6)
In the steam reforming reaction of methanol, the dissociation of methanol shown by Equation (4) and the reforming of carbon monoxide shown by Equation (5) simultaneously proceed. As a whole, the reaction shown by Equation (6) occurs to produce a hydrogen rich gas containing carbon dioxide. No carbon monoxide is present in the final stage if these reactions completely proceed. In the actual fuel reformer unit, however, it is practically impossible to shift the reaction of Equation (5) completely to the right. The gaseous fuel reformed by the fuel reformer unit accordingly contains a trace amount of carbon monoxide as a side product.
The steam reforming reaction generally proceeds in the presence of a known reforming catalyst like a Cu-Zn catalyst. In the presence of the reforming catalyst, however, a reverse shift reaction shown by Equation (7) given below proceeds with the steam reforming reaction discussed above, so as to generate a trace amount of carbon monoxide in the reformed gas:
CO
2
+H
2
→CO+H
2
O (7)
The reverse shift reaction shown by Equation (7) produces carbon monoxide from hydrogen and carbon dioxide, which are obtained in the process of the steam reforming reaction. The reverse shift reaction proceeds only slightly, compared with the steam reforming reaction. In the case where an extremely low concentration of carbon monoxide is required, for example, when the reformed gas is used as a supply of gaseous fuel fed to the fuel cells, however, carbon monoxide produced by the reverse shift reaction is not negligible but may have a significant influence.
The carbon monoxide concentration reduction apparatus is accordingly used to reduce the concentration of carbon monoxide included in the gaseous fuel, prior to the supply of the gaseous fuel to the fuel cells. In the carbon monoxide concentration reduction apparatus, the selective oxidation of carbon monoxide proceeds in preference to the oxidation of hydrogen, which is affluently present in the reformed gas as mentioned above. The oxidation reaction of carbon monoxide is shown by Equation (8) given below. The allowable concentration of carbon monoxide included in the gaseous fuel fed to the fuel cells is not greater than several percents in the case of phosphate fuel cells and not greater than several ppm in the case of polymer electrolyte fuel cells. The reformed gas is introduced into the carbon monoxide concentration reduction apparatus including the ruthenium catalyst, and the selective oxidation of carbon monoxide shown by Equation (8) proceeds in the apparatus. This lowers the concentration of carbon monoxide included in the reformed gas and ensures the supply of the gaseous fuel having a sufficiently low concentration of carbon monoxide to the fuel cells.
CO+(½)O
2
→CO
2
(8)
The effective temperature range of the ruthenium catalyst, in which the carbon monoxide selective oxidation reaction is sufficiently accelerated, is about 140 to 200° C. The carbon monoxide concentration reduction apparatus with the ruthenium catalyst incorporated in the fuel cells system may not sufficiently lower the concentration of carbon monoxide included in the gaseous fuel fed to the fuel cells. When the temperature in the carbon monoxide concentration reduction apparatus becomes lower than the effective temperature range, the catalytic activity lowers and does not sufficiently accelerate the oxidation of carbon monoxide. This results in the insufficient reduction of the concentration of carbon monoxide. When the temperature in the carbon monoxide concentration reduction apparatus becomes higher than the effective temperature range, on the other hand, hydrogen that is affluently present in the gaseous fuel is oxidized. This interferes with the selective oxidation of the trace amount of carbon monoxide co-existing in the gaseous fuel. In order to lower the concentration of carbon monoxide sufficiently, it is required to regulate the internal temperature of the carbon monoxide concentration reduction apparatus according to the amount of the reformed gas, which is subjected to the selective oxidation of carbon monoxide and to make the selective oxidation of carbon monoxide proceed in the effective temperature range mentioned above.
Especially in the event that the load to be processed in the carbon monoxide concentration reduction apparatus (that is, the quantity of the reformed gas fed to the carbon monoxide concentration reduction apparatus) remarkably varies, it is difficult to keep the internal temperature of the c
Griffin Steven P.
Medina Maribel
Oliff & Berridg,e PLC
Toyota Jidosha & Kabushiki Kaisha
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