Chemistry of inorganic compounds – Hydrogen or compound thereof – Elemental hydrogen
Reexamination Certificate
2001-03-30
2003-09-23
Silverman, Stanley S. (Department: 1754)
Chemistry of inorganic compounds
Hydrogen or compound thereof
Elemental hydrogen
C252S373000
Reexamination Certificate
active
06623720
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to fuel cells, and more particularly to transition metal carbides, nitrides and borides, and their oxygen containing analogs (e.g. oxycarbides) useful as water gas shift catalysts for use in producing hydrogen for chemical processing and petroleum refining, and reducing the carbon monoxide content of feeds to fuel cells.
BACKGROUND OF THE INVENTION
The water gas shift (WGS) is an important reaction in the conversion of fossil fuels into hydrogen for use in processing chemicals and refining petroleum. An important emerging application is in the production of hydrogen for fuel cells. Fuel cells electrochemically convert fuel and oxidant directly into electricity. Because of their inherent high efficiencies and low emissions, fuel cells have gained significant interest from automobile manufacturers and their suppliers. Many manufacturers favor the use of proton exchange membrane (PEM) fuel cells operating with hydrogen from the processing of fossil fuels. The key fuel processing steps are (1) steam reforming and/or partial oxidation and (2) water gas shift.
Hydrocarbon steam reforming and partial oxidation are the principal reactions used to generate hydrogen. Hydrocarbon steam reforming is highly endothermic and usually requires temperatures in excess of 700° C. to be effective (eqn. 1). Performance of the reformer is very sensitive to the composition of the fuel, consequently steam reforming is not considered to be very fuel flexible.
C
n
H
m
+n
H
2
O→
n
CO+(
n+m/
2)H
2
eqn. 1
Hydrogen can also be extracted from hydrocarbons via partial oxidation reactions (see for example eqn. 2). Partial oxidation reactions are exothermic; however, because the reaction is not catalyzed, temperatures in excess of 1000° C. are required to achieve the necessary rates. The product composition is regulated by controlling the amount of O
2
.
2C
n
H
m
+n
O
2
→2
n
CO+
m
H
2
eqn. 2
In autothermal reforming, partial oxidation is coupled with steam reforming. The relative contribution of steam reforming versus partial oxidation can be controlled by choice of catalyst and operation conditions. For a given feed, the reaction temperature is lower than that for partial oxidation alone. Compared to steam reforming, autothermal reforming can be carried out in a smaller reactor volume, starts faster, and responds more quickly to control actions or changes in feed conditions.
The water gas shift reaction (eqn. 3) is well established for producing hydrogen and decreasing the CO content to less than 1%.
CO+H
2
O→CO
2
+H
2
eqn. 3
Carbon monoxide removal is critical because many catalysts are poisoned by CO. For example, the noble metal electrocatalysts in PEM fuel cells are susceptible to poisoning by as little as 10-100 ppm CO. The poisoning problem is exacerbated by the operating constraints imposed by commercial membrane materials. Present PEM fuel cells must be operated under conditions which avoid drying out the membrane. This essentially excludes operating the fuel cell at the higher temperatures where Pt oxidizes CO. The water gas shift reaction is typically carried out in two stages using Fe—Cr catalysts in the high temperature stage and Cu—Zn—Al catalyst in the low temperature stage.
Presently employed catalysts lack sufficient activity and durability for many portable and automotive applications. Furthermore, presently available catalysts are very sensitive to sulfur compounds, a common contaminant in modern transportation fuels.
Therefore, there exists a pressing need for water gas shift catalysts that are highly active, durable, and sulfur tolerant. These materials would be especially well suited for use in conjunction with PEM fuel cells for automotive applications.
BACKGROUND REFERENCES
U.S. Pat. No. 3,666,682 to Muenger, the entire specification of which is incorporated herein by reference, discloses a water gas shift conversion process in which a feed gas mixture is subjected to successive contacts with catalyst and the temperature of the reacting gases contacting the shift conversion catalyst is controlled by indirect concurrent heat exchange with the feed gas mixture.
U.S. Pat. No. 3,974,096 to Segura et al., the entire specification of which is incorporated herein by reference, discloses that hydrogen is produced by reacting carbon monoxide with steam at a temperature of at least 200° F. in the presence of a supported catalyst containing: (1) at least one alkali metal compound derived from an acid having an ionization constant below 1×10
−3
, (2) a metallic hydrogenation-dehydrogenation material, and (3) a halogen moiety. The ratio of metal component to alkali metal compound, each calculated on the basis of the oxide thereof, ranges from 0.0001 to about 10 parts by weight per part by weight of the alkali metal compound. The halide constituent is present in amounts in excess of about 0.01 weight %, based on total catalyst. A preferred catalyst composition comprises potassium carbonate, a mixture of cobalt and molybdenum oxides and combined chlorine contained on an alumina support.
U.S. Pat. No. 4,172,808 to Böhm et al., the entire specification of which is incorporated herein by reference, discloses a process for the production of a tungsten carbide catalyst by carburization of tungsten oxides, comprises, directing a mixture of carbon monoxide and carbon dioxide over tungsten oxide while heating it in a heated reactor at a heating rate and gas flow rate such that the reduction of the tungsten oxide occurs more slowly than the diffusion of the carbon into the tungsten and into tungsten carbide which is formed during the reaction with the diffusion being faster than the separation of carbon from the gaseous phase according to the rate of adjustment of the Boudouard equilibrium. The carbon monoxide is charged at a rate of 560 l/h and the carbon dioxide is charged at a rate of 40 l/h and, after a reactor containing the sample of tungstic acid is positioned in a closed reactor, the reactor is flushed with the gases for around ten minutes and then placed into a muffle furnace. The reactor is heated to a temperature of 670° C. in the furnace and the temperature is then reduced to a reaction temperature of 620° C. First, all of the water is eliminated, and then there is a reduction of the tungsten oxides and a diffusion of the carbon into tungsten or into tungsten carbide which is formed. The reduction of the tungsten oxides occurs more slowly than the diffusion of the carbon, but faster than the deposition of the carbon from the gaseous phase.
U.S. Pat. No. 4,219,445 to Finch, the entire specification of which is incorporated herein by reference, discloses a process of preparing methane-containing gas comprising contacting carbon monoxide and hydrogen in the presence of a catalyst containing tungsten carbide. Various tungsten carbide-containing alumina gel catalysts are also disclosed.
U.S. Pat. No. 4,271,041 to Boudart et al., the entire specification of which is incorporated herein by reference, discloses a high specific surface area molybdenum oxycarbide catalyst. They are prepared by the vapor condensation of molybdenum hexacarbonyl and catalyze the reaction of hydrogen and carbon monoxide to form hydrocarbons. Carburization of the molybdenum oxycarbides increases their activity in the carbon monoxide-hydrogen reaction.
U.S. Pat. No. 4,325,842 to Slaugh et al., the entire specification of which is incorporated herein by reference, discloses a process for preparing a supported molybdenum carbide composition which comprises impregnating a porous support with a solution of hexamolybdenum dodecachloride, drying the impregnated support and then heating in a carbiding atmosphere at a temperature of about 650°-750° C.
U.S. Pat. No. 4,325,843 to Slaugh et al., the entire specification of which is incorporated herein by reference, discloses a process for preparing a supported tungsten carbide composition which comprises first forming a supported
Moon Dong Ju
Patt Jeremy
Phillips Cory
Thompson Levi T.
Medina Maribel
Silverman Stanley S.
The Regents of the University of Michigan
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