Catalyst – solid sorbent – or support therefor: product or process – Catalyst or precursor therefor – Inorganic carbon containing
Reexamination Certificate
1999-09-30
2001-03-27
Bell, Mark L. (Department: 1755)
Catalyst, solid sorbent, or support therefor: product or process
Catalyst or precursor therefor
Inorganic carbon containing
C502S182000, C501S087000, C423S345000, C423S414000, C423S439000, C423S440000
Reexamination Certificate
active
06207609
ABSTRACT:
BACKGROUND OF THE INVENTION
Synthesis gas is a mixture of hydrogen and carbon monoxide, which is formed from methane reforming and has a variety of different applications in organic reactions. This can be formed by combining steam and oxygen with methane at high temperatures. Another method of forming synthesis gas from methane is the methane dry reforming reaction. In this reaction, carbon dioxide is mixed with methane and the blend is subjected to high temperature in the presence of a catalyst. This in turn forms hydrogen and carbon monoxide. The hydrogen from the reforming process is particularly suitable for use in fuel cell power systems.
The typical catalyst for use in the methane dry reforming reaction is a noble metal such as gold, platinum or the like. However, these catalysts tend to be relatively expensive. Molybdenum carbide is known as a catalyst for such reaction. However, this can be difficult to form. Further for use as a catalyst, high surface area is critical. Molybdenum carbide tends to form larger grains having reduced surface areas which in turn reduces its effectiveness as a catalyst. Thus, because of this problem and the high temperature and time required to form molybdenum carbide, it has not been used commercially as a catalyst for the methane dry reforming reaction.
SUMMARY OF THE INVENTION
The present invention is premised on the realization that a molybdenum carbide catalyst suitable for use in the methane dry reforming reaction as well as other reactions can be formed at relatively low temperatures and in relatively short periods of time. These reactions also include fuel processing as applicable in fuel cell uses.
More particularly the present invention is premised on the realization that molybdates such as ammonium molybdate can be directly formed into a high surface area molybdenum carbide by direct reaction with a mixture of hydrogen and carbon monoxide. The molybdate is heated from a temperature below 300° C. to a temperature below 850° C. at a ramp rate of about 2-20° C./min in the presence of the hydrogen, carbon monoxide mixture. This permits the molybdate to be reduced and then carburized directly to molybdenum carbide.
The formed molybdenum carbide has a high surface area in the range of 35-100 m
2
/g and has a metastable Mo
2
C structure with very diffused X-ray diffraction peaks. The carbide powder can be used as a catalyst in the methane dry reforming reaction to obtain nearly theoretical yields of hydrogen and carbon monoxide at a temperature of 850° C.
DETAILED DESCRIPTION
According to the present invention, a molybdenum carbide powder is formed from a molybdate by reacting it under 750° C. with a mixture of a reducing and carburizing gases, which are specifically hydrogen and carbon monoxide. The formed carbide has a structure of Mo
2
C
y
, wherein y represents 0.95 to 1.05.
The starting molybdate can be any molybdate wherein the counter ion is not a metal. Generally, the counter ion will be an organic compound or ammonium which is preferred due to its availability. Other molybdenum compounds such as molybdenum oxides can also be used.
The reaction gas would be a blend of hydrogen and carbon monoxide at a ratio between 3:1 to 1:1 and preferably at 1:1 ratio (by volume). Other carburizing gases such as methane or ethylene can also be used.
The reaction can be conducted in any suitable furnace which permits control of the gaseous atmosphere and temperature. A rotary kiln is particularly suitable due to its ability to ensure adequate mixing of the solid and gaseous reactants. The ammonium molybdate powder is simply loaded into a quartz liner and placed into the rotary kiln.
The system is purged with nitrogen first and then the hydrogen, carbon monoxide mixture is introduced. The temperature can be quickly raised to 300° C. and thereafter the temperature ramp rate should not exceed 20° C./min and preferably is 2-20° C./min. The reaction temperature and the furnace temperature ramp rate are critical for practicing the present invention. The molybdate starts decomposition at about 300° C. It decomposes into oxides, which at some stage can be amorphous. The reduction and carburization of the resulting oxide occurs at the same time when the temperature goes above 400° C. A 3- to 5-hour soak at a temperature between 550° to 600° C. provides enough time for molybdenum carbide to nucleate and a higher temperature will expedite the formation of molybdenum carbide.
The maximum temperature should not exceed 850° C. and most preferably be less than 750° C. At 700° C., the reaction can be completed in 2 to 4 hours. The reaction time can be shortened by increasing the reaction temperature at the expense of increasing grain size and reducing surface area.
During the reaction, the feeding gas composition can be changed if desired in order to adjust the total carbon content of the powder. Subsequently, the reactor is cooled down with flowing hydrogen, carbon monoxide mixture, hydrogen alone, or nitrogen. Because of the high surface area, the powder should be passivated with diluted oxygen or air after the powder cools down to room temperature.
The molybdenum carbide formed in this manner is a metastable Mo
2
C
y
. X-ray diffraction on the carbide powder shows some missing peaks in the diffraction pattern. At this stage, the powder samples exhibit specific surface areas of over 35 m
2
/g. Catalysis tests on the carbide powder for the methane dry reforming reaction indicate that the defect Mo
2
C
y
crystal structure may be responsible for the high catalysis activity of the powder.
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patent: 5427761 (1995-06-01), Grindatto et al.
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Gao Lin
Seegopaul Purnesh
Bell Mark L.
Hailey Patricia L.
N.V. Union Miniere S.A.
Wood Herron & Evans LLP
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