Chemistry of inorganic compounds – Hydrogen or compound thereof – Elemental hydrogen
Reexamination Certificate
2002-10-30
2003-05-13
Langel, Wayne A (Department: 1754)
Chemistry of inorganic compounds
Hydrogen or compound thereof
Elemental hydrogen
C423S655000, C502S302000, C502S304000, C502S325000, C502S326000, C502S327000, C502S329000
Reexamination Certificate
active
06562315
ABSTRACT:
The present invention relates to improved water gas shift catalysts and methods of their use for lowering the undesirable methane formation that can accompany water gas shift reaction processes, and in particular to lowering methane production in high temperature water gas shift reaction processes in a gas stream comprising hydrogen, steam and carbon monoxide. The catalysts and methods of the invention are useful, for example, in inhibiting methane production in a water gas shift reaction used to produce a hydrogen gas stream supplied to a fuel cell, particularly to proton exchange membrane (PEM) fuel cells.
Fuel cells directly convert chemical energy into electricity thereby eliminating the mechanical process steps that limit thermodynamic efficiency, and have been proposed as a power source for many applications. The fuel cell can be 2 to 3 times as efficient as the internal combustion engine with little, if any, emission of primary pollutants such as carbon monoxide, hydrocarbons and nitric oxides. Fuel cell-powered vehicles which reform hydrocarbons to power the fuel cells generate less carbon dioxide. (green house gas) and have enhanced fuel efficiency.
Fuel cells, including PEM fuel cells [also called solid polymer electrolyte or (SPE) fuel cells], as known in the art, generate electrical power in a chemical reaction between a reducing agent (hydrogen) and an oxidizing agent (oxygen) which are fed to the fuel cells. A PEM fuel cell comprises an anode and a cathode separated by a membrane which is usually an ion exchange resin membrane. The anode and cathode electrodes are typically constructed from finely divided carbon particles and proton conductive resin intermingled with the catalytic and carbon particles. In typical PEM fuel cell operation, hydrogen gas is electrolytically oxidized to hydrogen ions at the anode composed of platinum catalytic particles deposited on a conductive carbon electrode. The protons pass through the ion exchange resin membrane, which can be a fluoropolymer of sulfonic acid called a proton exchange membrane. H
2
O is produced when protons then combine with oxygen that has been electrolytically reduced at the cathode. The electrons flow through an external circuit in this process to do work, for example creating an electrical potential across the electrodes. Examples of membrane electrode assemblies and fuel cells are described in U.S. Pat. No. 5,272,017.
Fuel cells require both oxygen and a source of hydrogen to function. The oxygen can be readily obtained in pure form (i.e., O
2
) or from the air. However, hydrogen gas is not present in sufficient quantities in the air for fuel cell applications. The low volumetric energy density of isolated hydrogen gas compared to conventional hydrocarbon fuels makes the direct supply of hydrogen gas to fuel cells impractical for most applications because a very large volume of hydrogen gas would be required to provide an equivalent amount of energy stored in a much smaller volume of conventional hydrocarbon fuels such as natural gas, alcohol, oil or gasoline. Accordingly, the conversion of known hydrocarbon based fuel stocks to hydrogen gas is an attractive source of hydrogen for fuel cells and other applications.
Removal of impurities such as sulfur from the starting materials and lowering the concentration of oxidative products generated in the conversion process, such carbon monoxide, are major challenges in hydrogen production. Fuel cells are generally incapacitated by the presence of even low concentrations of CO, which poisons that catalyst at the anode. Despite development of more CO-tolerant Pt/Ru anodes, fuel cells are still susceptible to compromised function, for example when used with hydrogen sources with a CO concentration above 5 ppm.
The production of hydrogen gas from natural hydrocarbon sources is widely practiced in the chemical industry, for example in the production of ammonia and alcohol. A variety of reaction steps employing different carefully designed catalysts are used in the industrial production of hydrogen. A series of several reaction steps is typically required to reduce CO concentrations to below required levels, for example below 5 ppm. Many of these reaction steps require high pressures (for example, in excess of 1,000 psig), high reaction temperatures (for example, in excess of 800 deg. C.) and use self-heating pyrophoric catalysts. The scale and weight of machinery required to safely carry out such processes is too large for many fuel cell applications, such as automobile or residential applications. Furthermore, while the hazards presented by such reaction conditions can be effectively managed in an industrial production setting, similar hazards present unacceptable levels of risk for most fuel cell applications.
The water gas shift (WGS) reaction is a well known catalytic reaction which is used, among other things, to generate hydrogen by chemical reaction of CO with water vapor (H
2
O) according to the following stoichiometry:
CO+H
2
O→CO
2
+H
2
wherein the reaction requires a catalyst. Typical catalysts employed in this reaction are based on combinations of iron oxide with chromia at high temperatures (about 350 deg. C.) or mixtures of copper and zinc materials at lower temperatures (about 200 deg. C.).
When used at temperatures above about 300 degrees C., water gas shift reaction catalysts also cause the formation of methane (CH
4
) by catalyzing the reaction of CO or CO
2
with hydrogen according to the reaction stoichiometries:
CO+3 H
2
→CH
4
+H
2
O
CO
2
+4 H
2
→CH
4
+2 H
2
O
The production of methane during the water gas shift reaction, also known as “methanation”, is a side reaction that consumes hydrogen gas in an exothermic reaction. Thus, for applications where the water gas shift reaction is used to produce hydrogen gas and reduce CO concentration, the methanation reaction is a major disadvantage related primarily to precious metal containing water gas shift reaction catalysts. Methanation can reduce the hydrogen yield from the water gas shift reaction by consuming hydrogen to form methane, and increase the temperature of the catalyst thereby lowering the efficiency hydrogen production.
What is needed is a water gas shift reaction catalyst that inhibits or eliminates the methanation side reaction, that can be integrated with existing catalytic systems without significantly reducing the activity of commercially available catalysts and without significantly increasing the cost of catalyst synthesis and production. The present invention overcomes these deficiencies in the prior art by providing an improved precious metal water-gas shift reaction catalyst and methods for the use thereof.
SUMMARY OF THE INVENTION
In one embodiment, the invention relates to a process for carrying out the water gas shift reaction employing a methane production suppressing water gas shift reaction catalyst. The methane production suppressing water gas shift reaction catalyst comprises a methane production suppressing effective amount of a basic metal oxide. The basic metal oxide can be one or more of MgO, CaO, SrO, BaO, or ZnO. In one preferred embodiment of the process the basic metal oxide is zinc oxide calculated as ZnO.
In another embodiment the process employs a methane production suppressing water gas shift reaction catalyst that also has a support and a catalytic agent. In one embodiment the support is activated alumina. Preferably, the support has a BET effective surface area of at least 10 m
2
/g. In one preferred embodiment, the process employs a methane production suppressing water gas shift reaction catalyst that also has a promoter. The promoter can be one or more of CeO
2,
Nd
2
O
3
, Pr
2
O
3
, TiO
2
, Fe
2
O
3
, NiO, MnO
2,
or Co
2
O
3
. Preferably, the promoter is ceria calculated as CeO
2
.
In another preferred embodiment, the catalytic agent has one or more of Rh, Pd, or Pt.
In one aspect of the invention the process also employs a monolith, wherein the methane production suppressing water gas shift reaction
Farrauto Robert Joseph
Korotkikh Olga
Ruettinger Wolfgang Friedrich
Engelhard Corporation
Langel Wayne A
Lindenfeldar Russell G.
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