U.S. patent application number 10/217398 was filed with the patent office on 2003-01-09 for shift converter having an improved catalyst composition.
Invention is credited to Silver, Ronald G..
Application Number | 20030007912 10/217398 |
Document ID | / |
Family ID | 25313061 |
Filed Date | 2003-01-09 |
United States Patent
Application |
20030007912 |
Kind Code |
A1 |
Silver, Ronald G. |
January 9, 2003 |
Shift converter having an improved catalyst composition
Abstract
A shift converter (16) in a fuel processing subsystem (14, 16,
18) for a fuel cell (12) uses an improved catalyst composition (50)
to reduce the amount of carbon monoxide in a process gas for the
fuel cell (12). The catalyst composition (50) is a noble metal
catalyst having a promoted support of mixed metal oxide, including
at least both ceria and zirconia. Cerium is present in the range of
30 to 50 mole %, and zirconium is present in the range of 70 to 50
mole %. Additional metal oxides may also be present. Use of the
catalyst composition (50) obviates the requirement for prior
reducing of catalysts, and minimizes the need to protect the
catalyst from oxygen during operation and/or shutdown.
Inventors: |
Silver, Ronald G.; (Tolland,
CT) |
Correspondence
Address: |
Stephen A. Schneeberger
49 Arlington Road
West Hartford
CT
06107
US
|
Family ID: |
25313061 |
Appl. No.: |
10/217398 |
Filed: |
August 13, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10217398 |
Aug 13, 2002 |
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09852333 |
May 9, 2001 |
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6455182 |
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Current U.S.
Class: |
422/211 ;
422/177; 422/600 |
Current CPC
Class: |
C01B 3/16 20130101; B01J
23/63 20130101; B01J 21/066 20130101; Y02P 20/52 20151101; B01J
8/025 20130101; Y02E 60/50 20130101; H01M 8/0662 20130101 |
Class at
Publication: |
422/211 ;
422/190; 422/177 |
International
Class: |
B01J 008/02 |
Claims
What is claimed is:
1. A shift converter (16) for reducing the amount of carbon
monoxide in a process gas (20, 24) using a water gas shift
reaction, the shift converter (16) having a catalyst chamber (32),
the chamber (32) having an inlet (36) for entry of the process gas
into the chamber, an outlet (38) downstream of the inlet for exit
of effluent from the chamber, and a catalyst composition (50)
located between the inlet (36) and the outlet (38) for converting
at least a portion of the carbon monoxide and water in the process
gas into carbon dioxide and hydrogen, the improvement wherein the
catalyst composition (50) in the catalyst chamber (32) comprises: a
noble metal catalyst having a promoted support, said promoted
support comprising a mixed metal oxide of at least cerium oxide
(ceria) and zirconium oxide (zirconia).
2. The shift converter (16) of claim 1 wherein, in the mixed metal
oxide, the amount of cerium present is in the range of 30 to 50
mole % and the amount of zirconium present is in the range of 70 to
50 mole %.
3. The shift converter (16) of claim 1 wherein the noble metal of
the catalyst composition (50) is selected from the group consisting
of platinum, palladium, rhodium, and gold.
4. The shift converter (16) of claim 3 wherein the noble metal of
the catalyst composition (50) is platinum.
5. The shift converter (16) of claim 1 wherein the metal oxide
comprising the promoted support additionally includes at least a
third metal oxide.
6. The shift converter (16) of claim 5 wherein the third metal
oxide is selected from the group consisting of praseodymium oxide,
lanthanum oxide, neodymium oxide, and hafnium oxide.
7. The shift converter (16) of claim 1 wherein at least the
promoted support of the catalyst composition (50) is wash-coated
onto a supporting substrate, and further including alumina mixed
with the promoted support to facilitate adhesion of the wash-coat
onto the supporting substrate.
8. The shift converter (16) of claim 1 wherein the catalyst
composition (50) operates independent of any requirement for
prereduction, a shutdown purge, or an inerting atmosphere.
9. The shift converter (16) of claim 1 wherein the shift converter
(16) is operatively connected in a fuel processing subsystem (14,
16, 18) for a fuel cell (12).
10. The shift converter (16) of claim 8 wherein the shift converter
(16) is operatively connected in a fuel processing subsystem (14,
16, 18) for a fuel cell (12) and the process gas includes hydrogen.
Description
[0001] This application is a division of U.S. patent application
Ser. No. 09/852,333 filed May 9, 2001.
TECHNICAL FIELD
[0002] This invention relates to hydrocarbon fuel processing, and
more particularly to an improved shift converter and the catalysts
used therein. More particularly still, the invention relates to
improved catalyst compositions in, and used in, shift converters
for processing hydrogen-rich gas streams, as for use in fuel
cells.
BACKGROUND ART
[0003] Fuel cell power plants that utilize a fuel cell stack for
producing electricity from a hydrocarbon fuel are well known. In
order for the hydrocarbon fuel to be useful in the fuel cell
stack's operation, it must first be converted to a hydrogen-rich
stream. Hydrocarbon fuels that are used by the fuel cell stack pass
through a reforming process to create a process gas having an
increased hydrogen content that is introduced into the fuel cell
stack. The resultant process gas contains, primarily, water,
hydrogen, carbon dioxide, and carbon monoxide. The process gas has
about 10% carbon monoxide (CO) upon exit from the reformer.
[0004] Anode electrodes, which form part of the fuel cell stack,
can be "poisoned" by a high level of carbon monoxide. Thus, it is
necessary to reduce the level of CO in the process gas, prior to
flowing the process gas to the fuel cell stack. This is typically
done by passing the process gas through a shift converter, and
possibly additional reactors, such as a selective oxidizer, prior
to flowing the process gas to the fuel cell stack. The shift
converter also increases the yield of hydrogen in the process
gas.
[0005] Shift converters for reducing the CO content of process gas
are well known, and typically comprise a chamber having an inlet
for entry of the process gas into the chamber, an outlet downstream
of the inlet for exit of effluent from the chamber, and a catalytic
reaction zone between the inlet and the outlet. The catalytic
reaction zone typically contains a catalyst, or catalyst
composition, for converting at least a portion of the carbon
monoxide in the process gas into carbon dioxide. In operation a
shift converter carries out an exothermic shift conversion reaction
represented by the following equation:
CO+H.sub.2O.fwdarw.CO.sub.2+H.sub.2 (1)
[0006] The reaction (1) between the CO and water concurrently
reduces the CO content and increases the CO.sub.2 and H.sub.2
content of the process gas. The generation of additional hydrogen
from this reaction is advantageous to the power plant inasmuch as
hydrogen is consumed at the fuel cell anode to produce power. A
discussion of one such shift converter is contained in PCT
Application US97/08334 for "Shift Converter", published on Nov. 27,
1997 as WO 97/44123. In the shift converter of that application, a
catalyst bed contains a catalyst composition of copper and zinc
oxide, or copper, zinc oxide, and alumina. Such catalyst
composition is further disclosed in U.S. Pat. No. 4,308,176 to
Kristiansen, and has been used for a number of years to promote the
shift reaction in the shift converters associated with fuel cell
power plants. However, reactors using these catalyst compositions
have the limitation that they must be purged with a flow of
hydrogen to initially reduce them, and steps must be taken
subsequent to operation to prevent significant oxidation or
exposure to oxygen. In fact, the required reaction does not work,
or occur, unless the catalyst is reduced. Exposure of these
catalyst compositions to oxygen is, or may be, detrimental to the
catalyst. This is because the catalyst is self-heating in the
presence of oxygen, and it can easily heat itself to the point
where catalyst particles will sinter, and thus lose surface area
and decrease activity. This need to provide a reducing atmosphere
and to minimize the possibility of oxygen leaks to the catalyst
with a special shutdown purge and the maintenance of an inert
atmosphere during shutdown, results in additional hardware and
process control considerations that add to the complexity and cost
of the fuel cell power plant system, particularly with regard to
the shift converter.
[0007] Recent studies show that cerium oxide, or "ceria"
(CeO.sub.2), can be used in combination with a noble metal to
promote the shift reaction and to eliminate the requirement that
the catalyst be reduced. The combination of ceria and platinum
provide a catalyst that is more oxygen tolerant than the prior
catalysts. However, such ceria-promoted platinum catalysts have not
demonstrated sufficient activity for the shift reaction to be
useful in a reactor of a reasonable size. Rather, an unreasonably
large catalyst bed would be required, particularly for mobile fuel
cell power plants. Moreover, water levels typical of water-gas
shift reactions may promote sintering of the ceria support.
[0008] It is thus an object of the present invention to provide a
shift converter having an improved catalyst composition for
efficiently converting carbon monoxide to carbon dioxide and
hydrogen using a water-gas shift reaction without the need for
special catalyst preconditioning.
[0009] It is a further object to provide and use an improved
catalyst composition having increased activity in a shift
conversion reactor for converting carbon monoxide to carbon dioxide
and hydrogen using a water-gas-shift reaction without the need to
protect the catalyst from exposure to air.
[0010] It is a still further object of the invention to provide and
use an improved catalyst composition providing improved activity
and durability over existing noble metal catalysts for the
water-gas-shift reaction.
DISCLOSURE OF INVENTION
[0011] A shift converter for reducing the amount of carbon monoxide
in a process gas, as for a fuel cell power plant, uses an improved
catalyst composition in accordance with the invention. The shift
converter includes an inlet for entry of the process gas, an outlet
downstream of the inlet for exit of effluent from the chamber, and
a catalytic reaction zone between the inlet and outlet. The
catalyst composition of the invention resides in the catalytic
reaction zone of the shift reactor and is active to convert at
least a portion of the carbon monoxide and water in the process gas
into carbon dioxide and hydrogen. The operation of the shift
reactor with the improved catalyst composition obviates the prior
requirements for pre-reducing the catalyst, providing a special
post-shutdown purge, and maintaining an inert atmosphere during
shutdown.
[0012] The improved catalyst composition used in the shift
converter comprises a noble metal catalyst having a promoted
support, which promoted support comprises a mixed metal oxide of at
least cerium oxide (ceria) and zirconium oxide (zirconia). The
inclusion of the zirconia with the ceria promoter increases the
number of oxygen vacancies, and thus the composition's activity.
Moreover, the zirconia increases the resistance of ceria to
sintering, thereby improving the durability of the catalyst
composition. The mixed metal oxides, in addition to the ceria and
zirconia, may include a third metal oxide, selected from the group
consisting of praseodymium oxide, lanthanum oxide, neodymium oxide,
and hafnium oxide, to form a ternary mix of the metal oxides.
Additionally, alumina may be added to the catalyst composition,
particularly if the latter is in the powder form, to improve its
suitability for washcoating onto a supporting substrate.
[0013] The noble metal catalyst on the promoted support is selected
from the metals of groups VIIb, VIII, and Ib of the second and
third transition series of the periodic table, with platinum,
palladium, rhodium, and gold being generally preferred, and
platinum being particularly preferred.
[0014] The invention further includes the method of removing carbon
monoxide from a process fuel gas for a fuel cell via the
utilization of a shift converter which employs the improved
catalyst composition.
[0015] The foregoing and other features and advantages of the
present invention will become more apparent in light of the
following detailed description of exemplary embodiments thereof as
illustrated in the accompanying drawings.
BRIEF DESCRIPTION OF DRAWINGS
[0016] FIG. 1 is a simplified functional schematic diagram of a
representative fuel cell power plant, depicting a shift converter
employing the improved catalyst composition in accordance with the
invention; and
[0017] FIG. 2 is a graph depicting plots of the shift conversion
activity of the improved catalyst of the invention vs. that of the
Cu/ZnO catalyst previously used.
BEST MODE FOR CARRYING OUT THE INVENTION
[0018] Referring to FIG. 1, there is depicted in functional
schematic form, a fuel cell power plant 10. The power plant 10
includes a fuel cell stack assembly 12 of conventional design and
construction, and a fuel processing subsystem which includes a
reformer 14, a shift converter 16 and an optional selective
oxidizer 18. The fuel processor converts a hydrocarbon fuel source
into a hydrogen-rich stream of fuel which is supplied as the fuel
to the fuel cell stack assembly 12. Typically, the hydrocarbon fuel
source is a liquid, such as gasoline, or a gas, such as methane,
natural gas, or the like, and is supplied to the Fuel inlet 20 of
reformer 14. Air and/or steam is supplied to the Air/Steam inlet 22
of reformer 14. The reformer 14 reacts hydrocarbon fuel and steam
and/or air to reform the hydrocarbon (and steam) to yield hydrogen
(H.sub.2), carbon monoxide (CO), carbon dioxide (CO.sub.2), and
residual steam/water (H.sub.2O), in a well known manner. However,
to further reduce or minimize the presence of carbon monoxide (CO)
which otherwise "poisons" the anodes of the fuel cell stack
assembly, and to increase the yield of hydrogen in the
hydrogen-rich fuel source for the fuel stack assembly 12, the
effluent process gas from the reformer 14 is conducted, via conduit
24, to the shift converter 16, where it is processed to reform the
carbon monoxide to carbon dioxide.
[0019] The shift converter 16 carries out an exothermic shift
reaction as noted in the formula (1) expressed in the Background
Art above. The desired reaction in the shift reactor 16 is the
conversion of carbon monoxide and water to carbon dioxide and
hydrogen. To the extent necessary, an optional selective oxidizer
18 may also be provided, and receives effluent process gas from the
shift reactor 16 via conduit 26, to further convert carbon monoxide
to carbon dioxide through the addition of air (O.sub.2). The
resultant effluent gas stream is sufficiently rich in hydrogen and
depleted of carbon monoxide to meet the needs of the fuel cell
stack assembly 12, and is extended thereto via conduit 30.
[0020] The shift converter 16 includes a housing having a catalyst
chamber 32 containing one or more catalyst beds or functionally
equivalent structures, 34, for promoting the desired shift
reaction. The process gas from the reformer 14 enters the shift
reactor 16 at inlet 36, flows through and across the catalyst
bed(s) 34 in the catalyst chamber 32, and exits via outlet 38. Each
catalyst bed 34 contains a catalyst composition, or simply,
catalyst, 50, formulated particularly for improving the performance
of the shift reactor 16 in accordance with the invention. Although
the catalyst 50 is depicted here as a bed within the catalyst
chamber 32, it will be appreciated, that other arrangements for
supporting the catalyst 50 within the catalyst chamber 32 are well
known and are contemplated as alternatives. For instance, a
preferred arrangement may be that of a honeycomb-type structure of
ceramic, alumina, cordierite (alumina/magnesia/silica), or the
like, mounted in the catalyst chamber 32 and containing the
catalyst as a coating thereon.
[0021] The catalyst 50 is a formulation of a noble metal on a
promoted support of mixed metal oxides, in which at least two of
the metal oxides include cerium oxide, or ceria, (CeO.sub.2) and
zirconium oxide, or zirconia, (ZrO.sub.2). The literature suggests
that ceria acts to promote noble metal catalysts for the water-gas
shift reaction, by serving as a source of oxygen vacancies.
Increasing the oxygen vacancies is thought to correspond to an
improved water-gas shift reaction rate. Importantly, the addition
of one or more additional metal oxides, of which an essential one
is zirconia, to the ceria to create a mixed metal oxide promoted
support (i.e., the support is a promoter) for the noble metal has
been found to give the resulting catalyst composition 50 improved
resistance to sintering at the higher operating temperatures
(400-700.degree. F.) of the shift converter 16, as well as to
further enhance the number of oxygen vacancies of the promoted
catalyst.
[0022] The ceria and the zirconia are present in the catalyst
composition in relation to one another in the range of about 50.0
to 30.0 mole % (mole percent) zirconium to 50.0 to 70.0 mole %
(mole percent) cerium. A third metal oxide may be present in the
range of 0.0 to 10.0 mole % of the total oxide. The noble metal is
in the range of 0.1 to 2.0 mole %, with 0.3 mole % being the value
in a representative example. The quantity of zirconium should not
be less than 30.0% in order to assure the enhanced stability it
provides to the catalyst 50, nor should it be greater than 50.0% in
order to prevent phases in the system which are only zirconia
and/or only ceria.
[0023] An exemplary formulation of and for the catalyst composition
50 for shift reactor 16 is provided in the following Example, in
which the MEI 01023 pellet material is a metal oxide mix of ceria
and zirconia, and serves as the catalyst support for the noble
metal catalyst. The noble metal catalyst is platinum. The ceria is
present in the pellet in the amount of 58 mole % Ce, and the
zirconia is present in the amount of 42 mole % Zr. The MEI 01023 is
available from Magnesium Elektron Inc. of Flemington, N.J. The
promoted support material, MEI 01023, of ceria and zirconia, was
provided in the form of small pellets of {fraction (1/16)} inch
diameter, but might also have been provided as a powder or the
like. The following Example uses the method of incipient wetness to
apply the platinum to the supports. Other methods of adding the
noble metal are well known.
EXAMPLE
[0024]
1 Support 36.600 g pellets (50 cc) Pore volume 0.700 g water/g
catalyst Amount of solution 25.620 g liquid solution containing Pt
(see below) fills all the pores of the pellets Amount of Pt 0.247
Pt/Pt (NH.sub.3).sub.2 (NO.sub.3).sub.2 Diamminodinitrite - 61% Pt
(labeled) Pt Solution 0.247 Pt (NH.sub.3).sub.2(NO.sub.2).sub.2 DI
water 15.372 Nitric Acid 10.248
[0025] Steps:
[0026] 1. Weigh out and dry the pellet support (MEI 01023) for 2
hours at 100.degree. C.
[0027] 2. Dissolve Pt(NH.sub.3).sub.2(NO.sub.2).sub.2 in 10.248 ml
concentrated nitric acid as indicated, stirring constantly.
[0028] 3. Once Pt dissolved, add Pt acid solution to the DI
water.
[0029] 4. Pour resulting solution over dried pellets, then stir
with glass or Teflon.RTM. stirrer until support is uniformly
coated.
[0030] 5. Dry resulting mixture for 1 h at 100.degree. C., then
calcine 4 h at 400.degree. C.
[0031] 6. Weigh dried and calcined mixture to assure/determine
complete mass balance.
[0032] This formulation uses pellets of the ceria and zirconia, and
coats it with the platinum. The resulting dried and calcined
mixture represents the catalyst composition 50.
[0033] Alternatively, powders of the mixed metal oxide may be
wash-coated onto an appropriate supporting substrate of alumina or
cordierite, or such, and then the platinum can be applied to the
wash-coated support in a manner similar to the preparation of the
pellets. Further, it may be desirable to add alumina to the powder
to improve its suitability for wash-coating onto a supporting
substrate. The alumina facilitates the adhesion of the wash-coat to
the supporting substrate.
[0034] FIG. 2 is an Arrhenius plot showing the shift conversion
activity of catalyst composition 50, prepared in accordance with
the Example above, in graphic comparison with that of pellets of
copper/zinc oxide catalyst of the type previously used as the
catalyst in shift reactors for this water-gas shift reaction. The
parameter measured logarithmically along the y-axis is a reaction
rate constant, k, at a given temperature, for the water-gas shift
reaction. The parameter measured linearly along the x-axis is the
inverse of the temperature at which reactivity is measured, or
1000/T It is seen that the Cu/ZnO of the prior art increases the
reaction rate at a 1.sup.st slope as the temperature increases from
300.degree. F. to about 400.degree. F., and there after at a much
lower 2.sup.nd slope as the temperature increases further from
400.degree. F. to about 600.degree. F. However, it will be noted
that the Pt on CeO.sub.2/ZrO.sub.2 catalyst of the invention
increases its reaction rate at a substantially constant slope,
comparable to the 1.sup.st slope above, as the temperature
increases from about 380.degree. F. to 600.degree. F. It will be
observed that at the cross-over region of about 580.degree. to
600.degree. F., the catalyst composition of the invention exhibits
activity that is equivalent to the activity of the Cu/ZnO. Thus,
for such level of activity, a reactor utilizing the noble metal
catalyst 50 of the invention would be approximately the same size
as a reactor using the conventional Cu/ZnO catalyst, yet would not
require the additional cost, volume, and complexity of the
reducing/purging/inerting system(s) presently associated with the
latter.
[0035] Although zirconia is the second metal oxide in the mix with
ceria, further advantages, such as lower overall cost, may be
derived by including a third metal oxide in a ternary mix of such
oxides. The third metal oxide may conveniently be selected from the
group consisting of praseodymium oxide, lanthanum oxide, neodymium
oxide, and hafnium oxide. The addition of one or more of these
metal oxides serves to assist ZrO.sub.2 in its stabilization and
promotion of ceria.
[0036] The noble metal, or metals, that comprise(s) the catalyst
supported by the mixed metal oxides of at least ceria and zirconia,
is/are selected from the metals of groups VIIb, VIII, and Ib of the
second and third transition series of the periodic table. That
group of noble metals includes rhenium, platinum, palladium,
rhodium, ruthenium, osmium, iridium, silver, and gold. Platinum,
palladium, rhodium, and/or gold, alone or in combination, are
generally preferred, and platinum is the noble metal that is
particularly preferred. Platinum is preferred because it provides
the level of activity required to obtain the desired reaction rate
in a reactor of reasonable size/volume.
[0037] Although the invention has been described and illustrated
with respect to the exemplary embodiments thereof, it should be
understood by those skilled in the art that the foregoing and
various other changes, omissions and additions may be made without
departing from the spirit and scope of the invention.
* * * * *