U.S. patent application number 10/039447 was filed with the patent office on 2002-10-03 for enhanced stability water-gas shift reaction catalysts.
Invention is credited to Farrauto, Robert J., Liu, Xinsheng, Ruettinger, Wolfgang F..
Application Number | 20020141938 10/039447 |
Document ID | / |
Family ID | 27100048 |
Filed Date | 2002-10-03 |
United States Patent
Application |
20020141938 |
Kind Code |
A1 |
Ruettinger, Wolfgang F. ; et
al. |
October 3, 2002 |
Enhanced stability water-gas shift reaction catalysts
Abstract
The invention provides processes for producing hydrogen that
include contacting an input gas stream comprising steam and carbon
monoxide with water-gas shift catalysts. The water-gas shift
catalysts are copper-based catalysts containing low concentrations
of platinum group metals. In some embodiments, the processes of the
invention are conducted using water-gas shift catalysts having a an
oxide support on which is dispersed copper or an oxide thereof, a
platinum group metal and a reducible metal oxide. In other
embodiments, the processes of the invention are conducted with a
water-gas shift catalysts having a cerium oxide support on which is
dispersed copper or an oxide thereof and a platinum group
metal.
Inventors: |
Ruettinger, Wolfgang F.;
(East Windsor, NJ) ; Liu, Xinsheng; (Edison,
NJ) ; Farrauto, Robert J.; (Princeton, NJ) |
Correspondence
Address: |
Stephen I. Miller
Chief Patent Counsel
Engelhard Corporation
101 Wood Avenue
Iselin
NJ
08830-0770
US
|
Family ID: |
27100048 |
Appl. No.: |
10/039447 |
Filed: |
November 9, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10039447 |
Nov 9, 2001 |
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09771812 |
Jan 29, 2001 |
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09771812 |
Jan 29, 2001 |
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09669044 |
Sep 25, 2000 |
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Current U.S.
Class: |
423/652 |
Current CPC
Class: |
B01J 23/63 20130101;
B01J 37/0205 20130101; B01J 23/56 20130101; C01B 2203/0283
20130101; B01J 37/0248 20130101; C01B 2203/047 20130101; C01B
2203/066 20130101; B01J 23/76 20130101; Y02E 60/50 20130101; C01B
2203/044 20130101; C01B 2203/0205 20130101; C01B 2203/146 20130101;
H01M 8/0662 20130101; Y02P 20/52 20151101; B01J 23/83 20130101;
B01J 23/894 20130101; C01B 3/16 20130101; B01J 23/868 20130101;
C01B 3/583 20130101; H01M 8/0612 20130101 |
Class at
Publication: |
423/652 |
International
Class: |
C01B 003/26 |
Claims
What is claimed:
1. A process for producing hydrogen, comprising: contacting an
input gas stream comprising steam and carbon monoxide with a
water-gas shift catalyst below 350.degree. C., wherein the
water-gas shift catalyst comprises: at least 50 wt. % of an oxide
support selected from the group consisting of activated alumina,
zirconia, titania, silica, zeolites, and combinations thereof;
copper or an oxide thereof dispersed on the oxide support; 0.01 to
0.5 wt. % of a platinum group metal selected from the group
consisting of platinum, palladium, rhodium, osmium, iridium,
ruthenium and combinations thereof dispersed on the oxide support;
and a reducible metal oxide selected from the group consisting of
the oxides of chromium, vanadium, molybdenum, cerium, praseodymium,
neodymium, titanium nickel, manganese, cobalt and dispersed on the
oxide support.
2. The process of claim 1, wherein the reducible metal oxide
comprises cerium oxide.
3. The process of claim 1, wherein the oxide support comprises
activated alumina.
4. The process of claim 1, wherein the platinum group metal
comprises platinum.
5. A process for producing hydrogen, comprising: contacting an
input gas stream comprising steam and carbon monoxide with a
water-gas shift catalyst below 350.degree. C., wherein the
water-gas shift catalyst comprises: at least 50 wt. % of an oxide
support selected from the group consisting of activated alumina,
zirconia, titania, silica, zeolites, zinc oxide and combinations
thereof; copper or an oxide thereof dispersed on the oxide support;
0.01 to 0.5 wt. % of a platinum group metal selected from the group
consisting of platinum, palladium, rhodium, osmium, iridium,
ruthenium and combinations thereof dispersed on the oxide support;
and cerium oxide dispersed on the oxide support.
6. The process of claim 5, wherein the platinum group metal of the
water-gas shift catalyst comprises platinum.
7. The process of claim 5, wherein the support of the water-gas
shift catalyst comprises activated alumina.
8. The process of claim 5, wherein there is 10 wt. % or more cerium
oxide in the water-gas shift catalyst.
9. The process of claim 5, wherein the water-gas shift catalyst is
in the form of particles having a mesh size of 12 or greater, and a
BET surface area of 10 m.sup.2/g or greater.
10. The process of claim 5, wherein the input gas stream further
comprises 10% by volume or more of hydrogen.
11. The process of claim 5, wherein there is 10% by volume or more
of steam in the input gas stream.
12. The process of claim 5, wherein the input gas stream further
comprises up to 2% by volume oxygen.
13. A process for producing hydrogen, comprising: contacting an
input gas stream comprising steam and carbon monoxide with a
water-gas shift catalyst below 300.degree. C., wherein the
water-gas shift catalyst comprises: at least 50 wt. % of an alumina
support; 6 to 12 wt. % of copper or an oxide thereof dispersed on
the alumina support; about 0.01 to about 0.5 wt. % platinum
dispersed on the alumina support; and 10 to 25 wt. % cerium oxide
dispersed on the alumina support.
14. The process of claim 13, wherein the alumina support of the
water-gas shift catalyst is in the form of particles having a mesh
size of 12 or greater, and a BET surface area of 10 m.sup.2/g or
greater.
15. A process for producing hydrogen, comprising: contacting an
input gas stream comprising steam and carbon monoxide with a
water-gas shift catalyst below 450.degree. C.: wherein the
water-gas shift catalyst comprises: a cerium oxide support; copper
or an oxide thereof dispersed on the cerium oxide support; and 0.1
wt. % or more of a platinum group metal selected from the group
consisting of platinum, palladium, rhodium, osmium, iridium,
ruthenium and combinations thereof dispersed on the cerium oxide
support.
16. The process of claim 15, wherein the platinum group metal of
the water-gas shift catalyst comprises platinum.
17. The process of claim 15, wherein a concentration of the copper
or oxide thereof in the water-gas shift catalyst is about 4 wt. %
to 12 wt. %.
18. The process of claim 15, wherein the input gas stream further
comprises 10% by volume or more of hydrogen.
19. The process of claim 15, wherein there is 10% by volume or more
of steam in the input gas stream.
20. The process of claim 15, wherein the input gas stream further
comprises up to 2% by volume of oxygen.
Description
[0001] This application is a continuation-in-part of U.S. patent
application Ser. No. 09/771,812, filed Jan. 29, 2001, which is a
continuation-in-part of U.S. patent application Ser. No. 09/669,044
filed Sep. 25, 2000, now abandoned. The disclosures of both of
these applications are incorporated herein by reference as if fully
set forth herein.
[0002] The present invention relates to copper-based water gas
shift catalysts comprising a stabilizing species that retards the
deactivation rate of the catalysts during operational exposure to
steam or water at low temperatures, such as below 220.degree. C.
The present invention also relates to methods for the use of these
catalysts for generating hydrogen by reaction of carbon monoxide
(CO) and steam (gaseous H.sub.2O), and in particular to generating
hydrogen from a gas stream comprising hydrogen, water, and carbon
monoxide. The catalysts and methods of the invention are useful,
for example, in generating hydrogen in the gas stream supplied to
fuel cells, particularly to proton exchange membrane (PEM) fuel
cells.
[0003] 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 two to three 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 cell generate less carbon
dioxide (green house gas) and have enhanced fuel efficiency.
[0004] Fuel cells, including PEM fuel cells [also called solid
polymer electrolyte or (SPE) fuel cells], 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 includes 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, catalytic particles supported on the 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 reaction catalysts 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.sub.2O 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, 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.
[0005] The water-gas shift reaction is a well known catalytic
reaction which is used, among other things, to generate hydrogen in
a gas-borne stream by chemical reaction of carbon monoxide with
water vapor (H.sub.2O) according to the following
stoichiometry:
CO+H.sub.2O.fwdarw.CO.sub.2+H.sub.2
[0006] The reaction requires catalysts that are typically based on
combinations of iron oxide with chromia for catalysis at high
temperatures (about 350.degree. C.), or mixtures of copper and zinc
materials at lower temperatures (about 200.degree. C.).
[0007] A problem encountered in water-gas shift processes using
non-pyrophoric, copper-based water gas shift catalysts as disclosed
in copending U.S. Pat. application Ser. No. 09/771,812, filed Jan.
29, 2001, is the decline in catalytic activity at lower
temperatures in the presence of steam (H.sub.2O). For example, at
or below temperatures of about 220.degree. C., copper-based
water-gas shift catalysts are deactivated by the presence of steam
(H.sub.2O). The problem of catalytic deactivation is of particular
importance during startup and shutdown of water-gas shift reactors
containing copper-based catalysts, wherein the temperature is
dropped below the dew point of the input gas stream. Steam induced
deactivation can also occur gradually, whereby the levels of
unconverted carbon monoxide (CO) remaining downstream from the
catalyst gradually rise with time. This gradual rise in downstream
CO concentration while carrying out the water-gas shift reaction is
indicative of a catalytic instability found in many different
copper-based, low temperature water-gas shift catalysts that
operate below 220.degree. C. In an industrial setting, where
reaction conditions can be carefully monitored controlled,
appropriate controls and protocols serve to prevent the
deactivating effects of low temperature steam exposure. For
example, the deactivation of certain copper-based water gas shift
reaction catalysts can be reversed by oxidation under dry
conditions, followed by reduction of the catalyst with dry
hydrogen. These procedures, however, are not easily implemented in
a fuel processor that would be used in a vehicle, or in a
residential setting.
SUMMARY OF THE INVENTION
[0008] In one embodiment the invention relates to a catalyst having
at least 50 wt. % of an oxide support on which is dispersed: at
least 5 wt. % copper or an oxide thereof, 0.01 to 0.5 wt. % of a
platinum group metal, and at least 10 wt. % of a reducible metal
oxide. The oxide support is selected from the group consisting of
activated alumina, zirconia, titania, silica, zeolites, and
combinations thereof. In one preferred embodiment, the oxide
support is activated alumina. The platinum group metal is selected
from platinum, palladium, rhodium, osmium, iridium, ruthenium and
combinations thereof. In one preferred embodiment, the platinum
group metal is platinum. The reducible metal oxide is selected from
the group consisting of the oxides of chromium, vanadium,
molybdenum, cerium, praseodymiun, neodymium, titanium, nickel,
manganese, cobalt and combinations thereof. In a preferred
embodiment, the reducible metal oxide is cerium oxide.
[0009] In another embodiment, the invention relates to a catalyst
having at least 50 wt. % of an alumina support; at least 5 wt. %
copper or an oxide thereof dispersed on the alumina support; 0.01
to 0.5 wt. % of a platinum group metal, preferably platinum,
dispersed on the alumina support; and at least 10 wt. % cerium
oxide dispersed on the alumina support. In a preferred aspect of
this catalyst, there is at least 65 wt. % of the alumina support; 6
to 12 wt. % of copper or an oxide thereof, 0.01 to 0.5 wt. % of
platinum, and 10 to 25 wt. % of cerium oxide dispersed on the
alumina support.
[0010] In one embodiment the catalyst is in the form of particles
having a mesh size of 12 or greater, and a BET surface area of 10
m.sup.2/g or greater. In another embodiment, the catalyst is in the
form of a washcoat composition deposited on a monolith
substrate.
[0011] In another aspect, the invention relates to a water-gas
shift catalyst for converting carbon monoxide and steam into
hydrogen and carbon dioxide. The water-gas shift catalyst has at
least 50 wt. % of an alumina support, on which is dispersed at
least 5 wt. % copper or an oxide; 0.01 to 0.5 wt. % of a platinum
group metal, preferably platinum, dispersed on the alumina support;
and at least 10 wt. % cerium oxide dispersed on the alumina
support.
[0012] In a preferred embodiment, there is at least 65 wt. % of the
alumina support in the water-gas shift catalyst. Dispersed on the
alumina support are 6 to 12 wt. % of copper or an oxide thereof;
0.01 to 0.5 wt. % of platinum, and 10 to 25 wt. % of cerium oxide.
In a preferred embodiment, the alumina support is in the form of
particles having a mesh size of 12 or greater, and a BET surface
area of 10 m.sup.2/g or greater.
[0013] In another embodiment, the invention relates to a catalyst
having a cerium oxide support, copper or an oxide thereof dispersed
on the cerium oxide support, and 0.1 wt. % or more of a platinum
group metal dispersed on the cerium oxide support. The platinum
group metal is selected from the group consisting of platinum,
palladium, rhodium, osmium, iridium, ruthenium and combinations
thereof. A preferred platinum group metal is platinum. Preferably,
there is 4 to 12 wt. % of copper or an oxide thereof dispersed on
the cerium oxide support; and 0.1 wt. % to 2 wt. % platinum
dispersed on the cerium oxide support. The catalyst is preferably
in the form of a washcoat composition deposited on a monolith
substrate.
[0014] In another aspect, the invention relates to a water-gas
shift catalyst for converting carbon monoxide and steam into
hydrogen and carbon dioxide. This water-gas shift catalyst has a
cerium oxide support on which is dispersed copper or an oxide
thereof and 0.1 wt. % or more of a platinum group metal, preferably
platinum. In a preferred embodiment of the water-gas shift
catalyst, there is 4 to 12 wt. % of copper or an oxide thereof, and
0.1 to 2 wt. % platinum dispersed on the cerium oxide support.
Preferably the water-gas shift catalyst is in the form of a
washcoat composition deposited on a monolith substrate.
[0015] The inyention also relates to a process for producing
hydrogen by contacting an input gas stream containing steam and
carbon monoxide with a water-gas shift catalyst below 350.degree.
C. The water-gas shift catalyst has at least 50 wt. % of an oxide
support which can be activated alumina, zirconia, titania, silica,
or zeolites; and is preferably activated alumina. Dispersed on the
oxide support are copper or an oxide thereof, 0.01 to 0.5 wt. % of
a platinum group metal, and a reducible metal oxide. The platinum
group metal is platinum, palladium, rhodium, osmium, iridium,
ruthenium and combinations thereof. In one preferred embodiment of
the process, the platinum group metal is platinum. The reducible
metal oxide selected from the group consisting of the oxides of
chromium, vanadium, molybdenum, cerium, praseodymium, neodymium,
titanium, nickel, manganese, cobalt. Preferably, the reducible
metal oxide is cerium oxide.
[0016] In another embodiment, the invention relates to a process
for producing hydrogen, by contacting an input gas stream
containing steam and carbon monoxide with a water-gas shift
catalyst below 350.degree. C. The water-gas shift catalyst has at
least 50 wt. % of an oxide support selected from the group
consisting of activated alumina, zirconia, titania, silica,
zeolites, zinc oxide and combinations thereof. Preferably the oxide
support includes activated alumina. Dispersed on the oxide support
are copper or an oxide thereof, 0.01 to 0.5 wt. % of a platinum
group metal (preferably platinum), and cerium oxide. In a preferred
embodiment of the process, the water-gas shift catalyst contains 10
wt. % or more cerium oxide. In another preferred process, the
water-gas shift catalyst is in the form of particles having a mesh
size of 12 or greater, and a BET surface area of 10 m.sup.2Ig or
greater.
[0017] In one embodiment of this process, the input gas stream has
10% by volume or more of hydrogen, in addition to carbon monoxide
and steam. In another embodiment, there is 10% by volume or more of
steam in the input gas stream. In yet another embodiment, the input
gas stream contains up to 2% by volume oxygen, in addition to
carbon monoxide and steam.
[0018] The invention also relates to a preferred process for
producing hydrogen by contacting an input gas stream containing
steam and carbon monoxide with a water-gas shift catalyst below
300.degree. C. In this preferred process, the water-gas shift
catalyst has at least 50 wt. % of an alumina support. Dispersed on
the alumina support are 6 to 12 wt. % of copper or an oxide
thereof, about 0.01 to 0.5 wt. % platinum, and 10 to 25 wt. %
cerium oxide dispersed on the alumina support. Preferably, the
alumina support of the water-gas shift catalyst is in the form of
particles having a mesh size of 12 or greater, and a BET surface
area of 10 m.sup.2/g or greater.
[0019] In an additional process aspect, the invention relates to
another process for producing hydrogen, by contacting an input gas
stream containing steam and carbon monoxide with a water-gas shift
catalyst below 450.degree. C. In this process, the water-gas shift
catalyst has a cerium oxide support on which is dispersed copper or
an oxide thereof and 0.1 wt. % or more of a platinum group metal,
preferably platinum. Preferably, the concentration of the copper or
oxide thereof in the water-gas shift catalyst is about 4 to 12 wt.
%.
[0020] In one embodiment of the process using this cerium
oxide-supported, copper-based catalyst, the input gas stream
contains 10% by volume or more of hydrogen, in addition to carbon
monoxide and steam. In another embodiment, there is 10% by volume
or more of steam in the input gas stream. In yet another
embodiment, the input gas stream further has up to 2% by volume
oxygen, in addition to carbon monoxide and steam.
[0021] In another aspect, the invention relates to an apparatus for
supplying hydrogen to a PEM fuel cell with a hydrocarbon reformer
reactor, a selective carbon monoxide oxidation reactor and a
water-gas shift reactor. The hydrocarbon reformer reactor is
upstream and in train with the water-gas shift reactor, and the
preferential oxidation catalyst is downstream and in train with the
water-gas shift reactor.
[0022] In one embodiment of the apparatus, the water-gas shift
reactor contains a water-gas shift catalyst that has at least 50
wt. % of an oxide support selected from the group consisting of
activated alumina, zirconia, titania, silica, zeolites and
combinations thereof. Dispersed on the oxide support are copper or
an oxide thereof, 0.01 to 0.5 wt. % of a platinum group metal, and
at least 10 wt. % of a reducible metal oxide. The reducible metal
oxide is preferably selected from the group consisting of the
oxides of chromium, vanadium, molybdenum, cerium, praseodymium,
neodymium, titanium, nickel, manganese, cobalt and dispersed on the
oxide support.
[0023] In another embodiment of the apparatus, the water-gas shift
reactor contains a water-gas shift catalyst that has a cerium oxide
support, on which is dispersed copper or an oxide thereof and 0.1
wt. % or more of a platinum group metal.
BRIEF DESCRIPTION OF THE DRAWINGS
[0024] FIG. 1 is a depiction of one embodiment of fuel processor
unit for supplying hydrogen to a fuel cell.
[0025] FIG. 2 is a graphic representation the a single repeating
subunit of a temperature profile used to analyze the steam induced
deactivation of various copper-based water-gas shift catalysts.
[0026] FIG. 3 is a graphic representation of test results obtained
for the steam induced deactivation of various copper-based
water-gas shift catalysts.
DEFINITIONS
[0027] The definitions of certain terms used herein are as
follows:
[0028] "activated alumina" means a high BET surface area alumina,
for example greater than 10 m.sup.2/g, preferably greater than 150
m.sup.2/g having primarily one or more of gamma, theta and delta
aluminas.
[0029] "BET surface area" means the Brunauer, Emmett, Teller method
for determining surface area by N.sub.2 adsorption. Unless
otherwise specifically stated, all references herein to the surface
area refer to the BET surface area.
[0030] "Catalyst A" refers to a catalyst of the invention that
includes a refractory inorganic oxide support (e.g., alumina), a
copper catalytic agent, a reducible metal oxide (e.g., cerium
oxide) and one or more platinum group metals.
[0031] "Catalyst B" refers to a catalyst of the invention that
includes a cerium oxide support, a copper catalytic agent and one
or more platinum group metals.
[0032] "catalytically effective amount" means that the amount of
material present is sufficient to affect the rate of reaction of
the water gas shift reaction in the sample being treated.
[0033] "cerium oxide" includes all oxides of cerium including ceria
(CeO.sub.2).
[0034] "cerium oxide support" refers to a particulate support
material wherein cerium oxide is present, preferably in at least a
concentration of 50 wt. % of the total catalyst weight.
[0035] Other composite materials may be present along with the
cerium oxide, including for example, other rare earth oxides (e.g.,
oxides of lanthanum, praseodymium, neodymium), zirconium oxide and
gallium oxide.
[0036] "copper-based catalyst" refers to a catalyst that includes a
copper catalytic agent. While platinum group metals or oxides
thereof may be included in the composition, their concentrations
are less, preferably at least five times lower (on a weight basis)
than the copper or an oxide thereof.
[0037] "high heat capacity support" means support materials with a
heat capacity that is approximately equal to or, preferably,
greater than that of the reducible metal oxide in the catalyst.
[0038] "high surface area support" means support materials with a
BET surface area that is approximately greater than 10 m.sup.2/g,
preferably greater than 150 m.sup.2/g.
[0039] "incipient wetness impregnation" means the impregnation of
the catalyst support with a volume of metal salt solution
substantially equal to the pore volume of the support material.
[0040] "inlet temperature" shall mean the temperature of the
hydrogen, water, and carbon monoxide stream, test gas, fluid sample
or fluid stream being treated immediately prior to initial contact
of the hydrogen stream, test gas, fluid sample or fluid stream with
a catalyst composition.
[0041] "percent by volume" refers to the amount of a particular gas
component of a gas stream, unless otherwise indicated, means the
mole percent of the gas component of the gas stream as expressed as
a volume percent.
[0042] "wt. %." or "percent by weight", means weight percent based
on the weight of an analyte as a percentage of the total catalyst
weight, including the support and any material impregnated therein,
including without limitation the copper catalytic agent and any
metal oxide material impregnated therein. The calculation does not
include the monolith substrate in embodiments where the catalyst is
in the form of a washcoat composition deposited on a monolith
substrate.
[0043] "input gas stream" means a gas stream prior to passing
through a catalytic region or prior to initial contact with a
catalyst composition.
[0044] "supports" or "catalyst support" refer to particulate
materials that are part of the catalyst composition including
inorganic oxides including oxide support selected from the group
consisting of activated alumina, zirconia, titania, silica,
zeolites and combinations thereof for Catalyst A. Cerium oxide
serves as the support for Catalyst B.
[0045] "VHSV" means volume hourly space velocity; that is, the flow
of a reactant gas in liter per hour per liter of catalyst volume at
standard temperature and pressure. In embodiments of the invention
that include a monolith substrate, the determination includes the
volume of the monolith substrate.
[0046] "WHSV" means weight hourly space velocity; that is the flow
of a reactant gas in liter per hour per kilogram of catalyst.
DETAILED DESCRIPTION OF THE INVENTION
[0047] Applicants have found that addition of low concentrations of
platinum group metals to copper-based catalysts can reduce or
prevent the deactivation of the catalysts that would otherwise
occur upon exposure to steam at temperatures of about 220.degree.
C. and lower. These conditions are prevalent during startup and
shutdown of reactors incorporating copper-based water gas shift
catalysts. Applicants have also found methods of producing the new
catalysts, that include low concentrations of platinum group
metals, that are more resistant to steam-deactivation at lower
temperatures. In other embodiments, the invention provides methods
for the use of the water gas shift reaction catalysts in high steam
environments at low temperatures. In addition, the invention
provides apparatus for supplying hydrogen to a fuel cell that
incorporates the catalysts of the invention.
[0048] It has been found that the degradation in catalytic activity
of copper-based catalysts that typically occurs upon exposure to
steam at low temperatures, e.g., below 220.degree. C., can be
abated, or in some cases prevented, by adding low concentrations of
platinum group metals to the catalyst. As mentioned above, these
lower temperatures are typically encountered during startup or
shutdown of water-gas shift reactors.
[0049] While it will be apparent to those of ordinary skill in the
art that certain platinum group metals can serve as effective
water-gas shift catalytic agents themselves, their use in the
instant invention is believed to serve primarily as a stabilizing
species to the copper catalytic agents. Certain copper-based
water-gas shift catalysts display a gradual decline in the rate of
carbon monoxide conversion (and production of hydrogen) over time
when exposed to high steam concentrations at lower temperatures,
e.g., below 220.degree. C. The inclusion of low concentrations of
one or more platinum group metals in the copper-based catalyst
compositions, however, lowers the observed deactivation rate under
these conditions. In other words, the catalysts of the invention
display an enhanced stability upon exposure to high steam
concentrations at low temperatures.
[0050] In addition, the utilization of a base metal, copper, in the
catalysts of the invention offers significant cost advantages over
certain conventional, platinum-based water-gas shift catalysts that
utilize higher concentrations of platinum group metals as catalytic
agents. Only low concentrations of platinum group metals in the
catalysts of the invention are necessary to provide the catalysts
their enhanced stability to high steam, low temperature
environments.
[0051] The catalysts of the invention of the invention can be
employed to catalyze reactions. For example, the catalysts of the
invention can be employed to oxidize chemical feedstocks, e.g.,
feedstocks containing carbon monoxide. In other embodiments the
catalysts serve as water-gas shift catalysts for generating
hydrogen and carbon dioxide from carbon monoxide and steam.
[0052] The catalysts of the invention include a catalyst support,
copper or an oxide thereof as a catalytic agent, and one or more
platinum group metals. The catalyst can be in any form, including
tablets, extrudates, washcoat compositions deposited on monolith
substrates and high-strength, high heat capacity particulate
catalysts.
[0053] In one embodiment, the invention relates to a catalyst that
includes a refractory inorganic oxide support (e.g., alumina), a
copper catalytic agent, a reducible metal oxide (e.g., cerium
oxide) and one or more platinum group metals. For economy of
expression this catalyst will be referred to herein as "Catalyst
A".
[0054] In a second embodiment, the invention relates to a catalyst
that has a cerium oxide support, a copper catalytic agent and one
or more platinum group metals. Here again, for reasons of
convenience, this catalyst will be referred to herein as "Catalyst
B".
[0055] Catalyst A
[0056] Catalyst A contains a refractory inorganic oxide support
(e.g., alumina). Dispersea on the support are a copper catalytic
agent, a reducible metal oxide (e.g., cerium oxide) and one or more
platinum group metals. The catalyst can be in any form including
extrudates, tablets, washcoat compositions deposited on monolith
substrates and high-strength particulate catalysts. A preferred
form of the catalyst is as high mechanical strength, high-heat
capacity particulate catalysts.
[0057] Typically, there is at least 5 wt. % of copper in the
catalyst composition for Catalyst A, to serve as a catalytic agent.
Preferably, there is at least 6 to 12 wt. % of copper in the
catalyst composition. The copper catalytic agent can be in the form
of copper (II) oxide (CuO), copper (I) oxide (Cu.sub.2O) or as
metallic copper depending on the conditions that the catalyst is
exposed to. The copper is generally dispersed on the inorganic
oxide support by contacting the support with a water-soluble or
water-dispersible salt of copper for sufficient time to impregnate
the support, followed by a drying step. The support material
containing the copper is then calcined, preferably at a temperature
above about 400.degree. C.
[0058] The catalysts of the invention include a platinum group
metal as a stabilizing agent, which is selected from the group
consisting of platinum, palladium, rhodium, iridium and ruthenium.
Preferably, the platinum group metal is platinum, palladium or
rhodium, with platinum being particularly preferred. The platinum
group metals are typically dispersed on the support by contacting
the support with a water-soluble or water-dispersible salt of the
platinum group metal for sufficient time to impregnate the support.
Non-limiting examples of such platinum group metal salts include
nitrates, acetates, Pt(NH.sub.3).sub.4(NO.sub.3).sub.- 2, palladium
and rhodium nitrate. The supported platinum group metal is then
calcined to fix it to the support as the metal or as an oxide.
Preferably, the amount of the platinum group metal salts is chosen
so that the final concentration of the platinum group metal on the
support is about 0.01 wt. % to about 0.5 wt. %.
[0059] In embodiments of Catalyst A wherein the support is a high
heat capacity support with high mechanical strength, the platinum
group metal is preferably impregnated into the support using
methods that lead to a substantially uniform distribution of the
platinum group metal on the support. For example, when the catalyst
contains a high heat capacity, high mechanical strength alumina
particulate support, a cationic platinum salt such as
Pt(NH.sub.3).sub.4(NO.sub.3).sub.2 is preferably used to impregnate
the alumina. It has been observed that anionic salts such as
platinum nitrate typically achieve a less uniform platinum
distribution on the alumina particulate support.
[0060] Suitable reducible metal oxides for Catalyst A include the
oxides of chromium, vanadium, molybdenum, cerium, praseodymium,
neodymium, titanium, nickel, manganese, cobalt, as well as
combinations thereof. Typically, there is at least 10 wt. % of the
reducible metal oxide in the catalyst composition.
[0061] A preferred reducible metal oxide is cerium oxide. While
cerium oxide alone as the reducible oxide component, in certain
applications it is desired to include certain composite materials
of cerium oxide. These cerium oxide composites additionally contain
other rare earth metal oxides component such as oxides of
lanthanum, praseodymium, and neodymium. In addition to these rare
earth metal oxide composites, other composite materials such as
zirconia-cerium oxide, gallium oxide-cerium oxide, titania-cerium
oxide are also useful as a reducible metal oxide components in
Catalyst A. In both types of composite materials, it is believed
that the additional component acts as a stabilizer for the cerium
oxide component, although in some cases, the additional component
may also serve to promote the water-gas shift reaction. In
composites comprising cerium oxide with the additional components,
the additional component may comprise up to about 30 wt. % of the
reducible metal oxide component of the composition.
[0062] Inorganic oxide supports for Catalyst A include high surface
area refractory oxide supports. These refractory oxide supports
include, for example, activated alumina, zirconia, titania, silica,
zinc oxide and zeolites. These supports also include combinations
of these inorganic oxides such as stabilized forms of alumina
including composites of zirconia, or silica with alumina, for
example, alumina-zirconia, silica-alumina, and
alumino-silicates.
[0063] In preferred embodiments of Catalyst A, the support is
substantially comprised of alumina which preferably includes the
members of the gamma or activated alumina family, such as gamma and
eta aluminas, and, if present, a minor amount of other refractory
oxides, e.g., up to about 20 wt. % of silica, zirconia and titania.
Preferably, the activated alumina has a specific surface area of at
least 10 m.sup.2/g. More preferably, the activated alumina has a
specific surface area of at least 150 m.sup.2/g.
[0064] In other preferred embodiments of Catalyst A, the catalyst
supports possess high heat capacity and high mechanical strength.
Conventional, copper-based water-gas shift catalysts are prone to
extremely rapid temperature rises upon exposure of the activated
(reduced) catalysts to atmospheric air. The temperature rise is due
to the rapid and exothermic oxidation of the reduced copper
catalysts. Such rapid temperature rises can damage that catalyst
due to sintering that occurs at high temperatures. In industrial
settings, reactors are equipped with sophisticated process controls
and protocols that can control the temperature rise that occurs
upon oxidation. In fuel cell processors in residential or vehicular
applications, however, such controls and protocols are impractical
to implement. The incorporation of copper-based catalysts that are
less prone to the temperature rises in fuel cell processors is
therefore desirable.
[0065] As described in copending U.S. patent application Ser. No.
09/771,812, the pyrophoricity of copper-based water-gas shift
catalyst having supports with high heat capacity and high
mechanical strength, are lower as compared to conventional
copper-based water-gas catalysts formed on catalyst supports of
lower heat capacity. The "low pyrophoricity" copper catalysts
described in the application show comparable or improved catalytic
activity to conventional copper based water-gas shift
catalysts.
[0066] The low pyrophoricity catalysts in this embodiment include a
structurally strong support of any suitable durable high heat
capacity material, such as alumina, which is in a particulate form
preferably having a longest dimension of about {fraction
(1/32)}-inch (0.78 mm) to about 1/2inch (1.25 cm) in cross section.
Preferably, the support particle is at least {fraction (1/16)}-inch
(1.56 mm) in cross section or, in other terms, has a mesh size of
12 (sieve opening of 1.52 mm) or above. For example, the support
particle preferably has a mesh size of 12, 11, 10, etc. The
catalytic supports in this embodiment are impregnated, with a
suitable precursor of a reducible metal oxide, the copper catalytic
agent and one or more platinum group metals. Preferred support
materials have a heat capacity that is preferably higher than that
of the reducible metal oxide. Examples of supports are silica,
zeolites, zirconia, titania and alumina. Activated alumina is a
particularly preferred support.
[0067] The high heat capacity particulate support in this preferred
embodiment can take the form of any suitable high strength support
such as a particle, pellet, extrudate, tablet and the like. The
support is preferably in a durable, rigid form. A number of
supports that are suitable for preparing the catalysts of the
invention and practicing the methods of the invention are readily
commercially available. For example, 1/8-inch diameter alumina
particles available from ALCOA as DD-443 (with 327 m.sup.2/g BET
surface area measured as received) can be used to practice the
invention. Desirable characteristics for preferred supports
include: having a high mechanical strength (resistance to
crumbling), being readily available; having the capacity for being
impregnated to high loadings with copper catalytic agents,
reducible metal oxides and other catalyst additives; and possessing
a high heat capacity. Supports with a heat capacity of at least the
heat capacity of the reducible metal oxide are preferred. Supports
with a heat capacity greater than the heat capacity of the
reducible metal oxide are particularly preferred.
[0068] Due to their low pyrophoricity, high mechanical strength,
high heat capacity particulate catalysts are preferred forms of
Catalyst A. These catalysts can be prepared by first impregnating
the particulate support with the reducible metal oxide precursor,
followed by drying and calcination. For example, in a preferred
preparation calcined 1/8-inch alumina support particles are
impregnated in an aqueous solution of cerium nitrate (or any other
suitable CeO.sub.2 precursor such as cerium acetate, chloride,
etc.). The particles are then dried and calcined at 500.degree. C.
in air to prepare the cerium oxide-impregnated alumina support (or
"CeO.sub.2/alumina particles").
[0069] After preparation of the particulate support with the
reducible metal oxide, the support can be impregnated with a
precursor of the copper catalytic agent. The CeO.sub.2/alumina
particles are impregnated with an aqueous solution of a
water-soluble copper salt, e.g., copper (II) nitrate, at a weakly
acidic pH. The particles are then dried and calcined to provide
CuO/CeO.sub.2/alumina particles. Finally, the CuO/CeO.sub.2/alumina
particles are similarly impregnated with a desired concentration of
a water-soluble salt of the platinum group metal [e.g.,
Pt(NH.sub.3).sub.4(NO.sub.3).sub.2] followed by a calcination step
to provide the CuO/CeO.sub.2/alumina particles impregnated with
platinum.
[0070] In other embodiments of Catalyst A, wherein the catalyst is
in the form of washcoat compositions, extrudates and tablets, the
catalyst is preferably formed from powdered supports impregnated
with the copper catalytic agent, the platinum group metal and
reducible metal oxide.
[0071] The impregnated, powdered supports can be prepared by
incipient wetness impregnation of the oxide support (e.g.,
activated alumina) with soluble salts of the reducible metal (e.g.,
cerium). For example, soluble salt forms of the reducible metal
such as acetates, halides, nitrates, sulfates and the like can be
utilized. This step is followed by drying and calcination steps to
fix the reducible metal component as its oxide to the refractory
oxide support.
[0072] The calcined support is then impregnated in analogous
fashion with a desired concentration of a water-soluble salt of the
copper (e.g., copper (II) nitrate) followed by drying and
calcination steps. Finally, the resulting oxide support is
impregnated with a desired concentration of a water-soluble salt of
the platinum group metal (e.g., platinum nitrate) followed by a
calcination step. The calcined, impregnated powdered oxide support
containing the copper catalytic agent, the platinum group metal,
and reducible metal oxide can then be formed into either an
extrudate, tablet or washcoat composition for deposition on a
monolith substrate.
[0073] In embodiments of the invention wherein the catalyst is in
the form of extrudates, the calcined, powdered oxide support
containing the copper catalytic agent, the platinum group metal,
and reducible metal oxide is typically mixed with a binder and
extrudated through a die of the desired shape. Typical useful
binders include hydrated forms of alumina (e.g., pseudoboehmite),
silica binders, clay binders, zirconia binders and the like.
[0074] Tablets can be prepared by: (1) combining the calcined,
powdered oxide support with a binder; (2) shaping the combined
powder and binder into the desired shape which could include
tablets, pellets, beads, cylinders or hollow cylinders; and (3)
calcining the shaped catalyst.
[0075] Washcoat compositions of the catalyst for deposition on
monolith substrates (or "slurries") are prepared using methods
known in the art. Preferably, the impregnated oxide support is ball
milled as a suspension using sufficient water to prepare a slurry
of a desired concentration. The concentration of the solids in the
washcoat slurry can be used as a method to control the thickness of
the catalyst coating ultimately deposited on the monolith
substrate. For example, increasing the weight percentage of solids
in the aqueous slurry will result in a thicker catalytic coat.
[0076] It is also generally advantageous to prepare slurries having
particles of small particle sizes, e.g., less than 10 .mu.m, to
maximize the surface area of the catalyst upon deposition on the
monolith substrate. Therefore, the particle size distribution of
the slurry is typically measured, and milling is continued until
the desired particle size has been achieved. Here again, binders
are optionally included in the slurries to improve adherence of the
washcoat to the monolith substrate walls.
[0077] The washcoat slurries are deposited on the monolith
substrate by methods well-known to those of ordinary skill. Thus,
for example, a layer of the supported copper catalyst can be
prepared by dipping the substrate in a reservoir containing a
sufficient volume of the slurry so that the substrate is fully
immersed. The coated substrate can be subsequently dried and
calcined.
[0078] As mentioned above, the washcoat catalyst compositions of
the invention are disposed on monolith substrates to form coated
monolith substrates. Although a variety of monolith substrates
could be used, the monolith substrate is preferably of the type
with one or more monolithic bodies having a plurality of finely
divided gas flow passages (channels) extending there through.
Preferably, the monolith substrate is of the type having a
plurality of fine, parallel gas flow passages extending across the
longitudinal axis of the substrate from an inlet or an outlet face,
so that the channels are open to fluid flow there through (often
referred to as a "honeycomb substrate"). The passages, which are
essentially straight from the inlet and outlet of the substrates,
are defined by walls on which the catalyst composition can be
coated in washcoat compositions so that the gases flowing through
the passages contact the catalyst material.
[0079] Monolith substrates are commercially available in various
sizes and configurations. The flow passages of the monolithic
substrate are thin-walled channels which can be of any suitable
cross-sectional shape and size such as trapezoidal, rectangular,
square, sinusoidal, hexagonal, oval, circular. Such monolithic
substrates may contain up to about 700 or more flow channels
("cells") per square inch of cross section, although far fewer may
be used. For example, the substrate can have from about 60 to 600,
more usually from about 200 to 400, cells per square inch
("cpsi").
[0080] Various types of materials of construction for monolith
substrates are known. The monolith substrate can be made from a
variety of materials, including metal or ceramic monoliths. In some
embodiments, the monolith substrate can be made from a ceramic
porous material composed of one or more metal oxides, e.g.,
alumina, alumina-silica, alumina-silica-titania, mullite,
cordierite, zirconia, zirconia-ceria, zirconia-spinel,
zirconia-mullite, silicon-carbide, and like. Some non-limiting
examples of ceramic monoliths can include those made of: zirconium,
barium titanate, porcelain, thorium oxide, magnesium oxide,
steatite, boron or silicon carbonates, cordierite-alpha alumina,
silicon nitride, spodumene, alumina-silica magnesia, zircon
silicate, sillimanite, magnesium silicates, zircon, petalite, alpha
alumina and aluminosilicates. One example of a commercially
available material for use as the substrate for the present
invention is cordierite, which is an alumina-magnesia-silica
material.
[0081] The metallic monolith substrate can be a honeycomb substrate
made of a refractory metal such as a stainless steel or other
suitable iron based corrosion resistant alloys (e.g., iron-chromium
alloy). Metal monoliths can be produced, for example, from alloys
of chromium, aluminum and cobalt, such as those marketed under the
trademark KANTHAL, or those produced from alloys of iron, chromium,
aluminum and yttrium, marketed under the trademark of FECRALLOY.
The metal can also be carbon steel or simple cast iron. Monolith
substrates are typically fabricated from such materials by placing
a flat and a corrugated metal sheet one over the other and rolling
the stacked sheets into a tubular configuration about an axis
parallel to the configurations, to provide a cylindrical-shaped
body having a plurality of fine, parallel gas flow passages, which
can range, typically, from about 200 to about 1,200 per square inch
of face area. Heat exchangers, which are typically formed from
metallic materials, can also be used as the monolith
structures.
[0082] In other embodiments, the monolith substrate can be made of
a ceramic or metal foam. Monolith substrates in the form of foams
are well known in the prior art, e.g., see U.S. Pat. No. 3,111,396
and SAE Technical Paper 971032, entitled "A New Catalyst Support
Structure For Automotive Catalytic Converters" (February,
1997).
[0083] Catalyst B
[0084] Catalyst B contains a copper catalytic agent and one or more
platinum group metals dispersed on a cerium oxide support The
catalyst can be in the form of extrudates, tablets, or washcoat
compositions deposited on monolith substrates. Preferred forms of
Catalyst B are as washcoat compositions deposited on monolith
substrates.
[0085] Typically, there is at least 4 wt. % of copper in the
Catalyst B composition. Preferably, there is about 4 to 12 wt. % of
copper in Catalyst B. The copper is preferably dispersed on the
inorganic oxide support by contacting the cerium oxide support with
a water-soluble or water-dispersible salt of copper for sufficient
time to impregnate the cerium oxide, followed by a drying step. The
support material containing the copper can then be calcined,
preferably at a temperature above about 400.degree. C.
[0086] Catalyst B includes a platinum group metal, which is
selected from the group consisting of platinum, palladium, rhodium,
iridium and ruthenium. Preferably, the platinum group metal is
platinum, palladium or rhodium, with platinum being particularly
preferred. Analogous to the methods for preparation of Catalyst A,
the platinum group metals are typically dispersed on the cerium
oxide support by impregnation followed by calcination. Preferably,
the amount of the platinum group metal salts is chosen so that the
final concentration of the platinum group metal on the support is
about 0.1 wt. % to 2 wt. %, more preferably about 0.1 wt. % to 0.8
wt. %.
[0087] Cerium oxide serves as the support of Catalyst B. While
cerium oxide alone can be used for the support, in certain
applications it may be desirable to include additives to the cerium
oxide that stabilize the support. For example, certain composite
materials of cerium oxide that additionally contain other rare
earth metal oxides component such as oxides of lanthanum,
praseodymium, and neodymium are useful. In addition to these rare
earth metal oxide composites, certain composite materials such as
zirconia-cerium oxide, gallium oxide-cerium oxide, titania-cerinum
oxide are also useful as composite cerium-oxide support materials
for Catalyst B. In both types of composite materials, it is
believed that the additional component acts as a stabilizer for the
cerium oxide support, although in some cases, the additional
component may also serve to promote the water-gas shift reaction.
With composite support materials comprising cerium oxide with
additional stabilizing component, typically the stabilizing
component may comprise up to about 30 wt. % of the cerium oxide
support of the composition.
[0088] Washcoat compositions, extrudates and tablets of Catalyst B
are preferably formed from powdered cerium oxide impregnated with
the copper catalytic agent and platinum group metal.
[0089] In a preferred embodiment, Catalyst B is prepared by
impregnating cerium oxide powder with a solution of a water-soluble
salt of copper, e.g., copper (II) nitrate, followed by drying at,
for example 120.degree. C., and calcination at, for example,
500.degree. C. Preferably, the impregnation is accomplished using
incipient wetness impregnation wherein minimal volumes of copper
salt solutions are employed to soak the cerium oxide support. The
resulting cerium oxide support is then impregnated with a
water-soluble salt of a platinum group metal, e.g., platinum
nitrate. A drying and a calcination step follow to provide powdered
Catalyst B. The powdered Catalyst B composition can be formed into
extrudates, tablets and washcoat compositions using the methods
described above for forming the Catalyst A compositions.
[0090] As an alternative to the impregnation method described
above, catalyst B can also be prepared by utilizing an aqueous
solution containing a mixture of a water-soluble cerium salt, e.g.,
cerium nitrate, and a water-soluble copper salt, e.g., copper (II)
nitrate. The pH of the solution is then raised by addition of a
base, e.g., sodium carbonate, to the solution to precipitate a
material comprising a mixture of the corresponding hydroxides of
copper and cerium. Preferably the solution is stirred during the
precipitation step. The resulting coprecipitate is collected, dried
and calcined to provide mixed metal oxide material comprising
copper oxide and cerium oxide. The mixed metal oxide material is
then impregnated with an aqueous solution of a platinum group salt,
dried and calcined to incorporate the platinum group metal
component into the Catalyst B composition.
[0091] The invention also relates to processes for using the
catalysts of the invention, i.e., Catalysts A and B. In a preferred
embodiment the catalysts of the invention can be used in processes
for producing hydrogen via the water-gas shift reaction. For
example the catalyst of the invention can be incorporated in a
reactor that is charged with an input gas stream containing carbon
monoxide and steam to produce hydrogen and carbon dioxide as
products in the output gas stream.
[0092] The catalysts of the invention are reduced (activated) prior
to their use in the water-gas shift reaction. The reduction can be
effected using a reducing gas stream, preferably containing
hydrogen, at 150.degree. C. to 250.degree. C. Typically, the
catalyst is heated to a predetermined temperature and kept at that
temperature, for a predetermined time, with the hydrogen-containing
gas stream being passed therethrough. For operational simplicity
and decreased startup time, it is preferred that the process gas
itself, containing carbon monoxide, hydrogen and steam, serve as
the reducing gas stream to activate the catalysts of the
invention.
[0093] The composition of the input gas stream for the process can
vary depending on the source of the reactant carbon monoxide.
Although higher proportions of carbon monoxide can be accommodated
in the process, the process of the invention is particularly
effective wherein the carbon monoxide concentration is less than
10% by volume. Typically, molar excesses of steam are used relative
to the amount of carbon monoxide introduced into the input gas
stream. Generally, H.sub.2O:CO molar ratios of between 1:1 (i.e.,
"1.0") and 20:1 (i.e. "20.0") are preferred in the input gas
stream, with higher ratios being particularly preferred for high
conversion of carbon monoxide.
[0094] In fuel cell applications of the inventive process, input
gas streams typically contain at least 10% by volume of hydrogen in
addition to the carbon monoxide and steam. Higher volumes, e.g.,
greater than 30-40% by volume, are often utilized in fuel cell
applications.
[0095] In addition to carbon monoxide, steam and hydrogen, the
input gas stream can contain carbon dioxide, nitrogen, and minor
amounts of olefins, alcohols, aldehydes and/or other hydrocarbons.
Preferably, the input gas stream contains not more than 4-5% by
volume of hydrocarbons and not more than 25% by volume carbon
dioxide.
[0096] The reaction temperature of the water-gas shift process is
dependent on the identity of the catalyst. For instance, processes
conducted with water-gas shift Catalyst A the reaction temperature
is preferably below 350.degree. C., with temperatures of about
220-300.degree. C. being particularly preferred. The optimal
temperature ranges for Catalyst A make it an ideal candidate for
incorporation into reactors serve as the "low-temperature"
component in fuel processors. Catalyst A may be used, for example,
in place of conventional low temperature copper-zinc based
catalysts such as CuO/ZnO/Al.sub.2O.sub.3 which typically operate
at about 200.degree. C. It should be noted that lower temperatures
could also be used, for example, when lower equilibrium carbon
monoxide concentrations in the outlet gas stream are desired.
[0097] Where the water-gas shift process is conducted with Catalyst
B, the reaction temperature is preferably below 450.degree. C.,
with temperatures of about 350-400.degree. C. being particularly
preferred. For example, water-gas shift Catalyst B can be used to
replace conventional high temperature iron-chromium based water-gas
shift catalysts such as Fe.sub.2O.sub.3/Cr.sub.2O.sub.3 that
typically operate at about 350.degree. C.
[0098] Reaction zone pressure is preferably maintained below the
dew point pressure of the reaction mixture. It should be recognized
that lower or higher reaction zone pressures can be used such as
from atmospheric up to about 500 psig.
[0099] Preferably, the water-gas shift reaction process is carried
out in a continuous mode with the reactants being passed over the
catalyst contained in one or more reaction zones. Gaseous hourly
space velocities of about 500 to about 50,000 hr.sup.-1 VHSV
measured on the basis of dry gas under standard conditions are
particularly suitable for most fuel cell operations. In embodiments
wherein the catalysts are in the form of washcoat compositions
deposited on monolith substrates, space velocities of up to 100,000
hr.sup.-1 VHSV may be possible. One skilled in the art would
recognize that lower gas reactant flow rates favor more complete CO
conversion.
[0100] In certain embodiments of the process it may be preferable
to include low concentrations of O.sub.2 in the input gas stream as
an additional measure to prevent the deactivation due to the
presence of high concentrations of steam at lower temperatures.
While not being bound by theory, Applicants believe that the
oxidation of small portions of carbon monoxide serve to heat the
catalyst and prevent catalyst deactivation. Preferably less than 2%
by volume oxygen is included in the input gas stream in this
embodiment.
[0101] Although the water-gas shift catalysts and processes of the
invention can be used in any application where hydrogen production
is needed, a particularly useful application is in apparatus such
as fuel processors that supply hydrogen to fuel cells. These
systems typically comprise a series of reactors that convert
hydrocarbon fuels (e.g., natural gas, gasoline, fuel oil, liquid
petroleum gas, and the like) into hydrogen fuel. The conversions
that take place in the reactors typically include reforming
reactions and water gas shift reactions to produce hydrogen. Other
reactors and trapping devices can also be included in the apparatus
that reduce unwanted components in the hydrogen feed streams (e.g.,
carbon monoxide and sulfur components), that are ultimately
supplied to the fuel cell.
[0102] As illustrated in the fuel processor (1) of FIG. 1, the
hydrocarbon reformer reactor (2) converts hydrocarbons (e.g.,
methane) and steam into hydrogen, carbon monoxide, and carbon
dioxide. For example, methane is converted by the two reactions
below:
CH.sub.4+H.sub.2O.fwdarw.3H.sub.2+CO
CH.sub.4+2H.sub.2O.fwdarw.4H.sub.2+CO.sub.2
[0103] The resulting gas is then reacted in the water-gas shift
reactor (3) to further enrich the process gas in hydrogen, through
the reaction of carbon monoxide with steam. Residual carbon
monoxide in the process is then removed by selective carbon
monoxide oxidation (with minimal hydrogen consumption) to carbon
dioxide in the preferential oxidation reactor (4) according to the
reaction:
CO+.sub.1/2O.sub.2.fwdarw.CO.sub.2.
[0104] The resulting process stream comprising high levels of
hydrogen is then supplied to the fuel cell.
[0105] The following examples further illustrate the present
invention, but of course, should not be construed as in any way
limiting its scope.
EXAMPLE 1
Preparation of Low-Pyrophoricity Copper/Cerium
Oxide/Platinum/Alumina Water-Gas Shift Catalysts (Exemplifies
Preparation of High Mechanical Strength, High Heat Capacity
Particulate Embodiments of Catalyst A)
[0106] To prepare a low-pyrophoricity catalyst wherein the support
is alumina, the reducible metal oxide is ceria and the catalytic
agent is CuO, ceria impregnated alumina support particles were
prepared by incipient wetness impregnation of alumina beads or
particulates.
[0107] 1/8-inch alumina support particles (ALCOA DD-443) were dried
for 2 hours at 200.degree. C. and then calcined for 2 hours at
500.degree. C. The calcined 1/8-inch particles were then
impregnated (i.e., impregnated at 55% incipient wetness to obtain
about 15 wt. % CeO.sub.2) in an aqueous solution of cerium nitrate
(i.e., Ce(NO.sub.3).sub.3; 33.44 g Ce(NO.sub.3).sub.3 dissolved in
55 g water, per 100 g alumina). The particles were then dried at
120.degree. C. for 8 hours. The sample was then calcined at
500.degree. C. for 2 hours.
[0108] The ceria/alumina particles were subsequently impregnated
(i.e., impregnated at 44% incipient wetness to obtain 8.25 wt. %
CuO) with Cu-nitrate solution (i.e., 26.6 mL of SM
Cu(NO.sub.3).sub.2 solution in 32 mL deionized water) at a pH of 6,
dried at 120.degree. C. for 8 hours and then calcined at
500.degree. C. for 2 hours to prepare catalyst precursor particles
(CuO/ceria/alumina particles).
[0109] The CuO/ceria/alumina particles were then impregnated with
Pt(NH.sub.3).sub.4(NO.sub.3).sub.2. 40 mg of
Pt(NH.sub.3).sub.4(NO.sub.3)- .sub.2 in 35 mL per 100 g of catalyst
precursor particles of water was used to prepare a catalyst
containing 0.02 wt. % Pt. 200 ng of
Pt(NH.sub.3).sub.4(NO.sub.3).sub.2 in 35 mL per 100 g of catalyst
precursor particles of water was used to prepare a catalyst
containing 0.1 wt. % Pt. After impregnation, both catalysts were
dried at 120.degree. C. for 4 hours then calcined at 500.degree. C.
for 4 hours.
EXAMPLE 2
Determination of Water-Gas Shift Catalyst Deactivation Rate at
250.degree. C.
[0110] 40 cc each of the 0.02 wt. % Pt-(Catalyst Al) and the 0.1
wt. % Pt-(Catalyst A2) impregnated copper catalysts of Example 1
were evaluated along with a control catalyst (Catalyst C) prepared
using the same procedure as described in Example 1 omitting the
platinum impregnation step and the accompanying drying and
calcining steps.
[0111] 40 cc of each catalyst were loaded in 1 in diameter quartz
reactor. A test gas composition containing 5.92% (v/v) of carbon
monoxide, 7.40% CO.sub.2, 31.82% H.sub.2, 28.86% N.sub.2 and 26%
H.sub.2O were passed over the catalyst at a flow rate of 1.5 L/min.
The following temperature program for the catalyst was
implemented:
[0112] (1) Heat the reactor to 200.degree. C. and hold for 2
hours.
[0113] (2) Heat to 250.degree. C. and hold for 2 hours.
[0114] (3) Cool to 100.degree. C. and hold for five minutes.
[0115] The program was repeated continuously. A graphic
representation of a single repeating unit of the temperature
program is shown in FIG. 2.
[0116] The activity of the catalyst is measured by the amount of CO
converted to CO.sub.2 in the gas stream at each segment at
200.degree. C. and 250.degree. C. The concentration of CO in the
output gas was detected using an infrared gas analyzer (California
Analytical Instruments). The decline in activity of the catalyst
over time is measured as the increase in CO concentration at
250.degree. C. per minute of experiment time. A graphic
representation of the decline in CO conversion (as measured by the
outlet concentration of CO) over time as measured for catalysts A2
(+0.1 wt. % Pt) and C (0 wt. % Pt) are depicted in the graph in
FIG. 3. The data points observed Catalyst A2 are indicated by the
triangles, and the data points observed with Catalyst C (the
control catalyst) are indicated by diamonds. Best fit lines for the
data points for the data point are also seen for Catalyst A2 and C.
As can be seen in the graph, gradually higher CO concentrations are
observed in the output gas stream over time for the control
Catalyst C. The gradual increase in CO concentration in the output
gas reflects the catalyst's instability to the high steam, low
temperature conditions. In contrast, the decline in Catalyst A2's
activity over time (observed as higher CO concentration in the
output gas) is significantly less pronounced.
[0117] Table 1 summarizes the results of the experiment for all
three catalysts.
1 TABLE 1 Deactivation Rate @ 250.degree. C. Initial Activity
Catalyst Sample (ppm CO/min) (% conversion) Catalyst C (0 wt. % Pt)
2.03 82.5% Catalyst A1 (0.02 wt. % Pt) 0.95 83.3% Catalyst A2 (0.1
wt. % Pt) 0.37 85.0%
[0118] As can be seen in Table 1, the inclusion of the low
concentration of platinum to the copper catalyst composition
(Catalyst A1 and A2) has little significant impact on the initial
activity of the catalyst. However, the rates at which the carbon
monoxide conversion rate decline under these high-steam, low
temperature conditions is significantly different. The inclusion of
as little as 0.02 wt. % Pt in the copper-based catalyst composition
provides a catalyst whose deactivation rate is approximately half
of the deactivation rate to the control catalyst (Catalyst C).
[0119] In summary, the graph shown in FIG. 3 and the data in Table
1 demonstrate that the catalysts of the invention are significantly
less prone to deactivation to high steam, low temperature
conditions than other copper-based water-gas shift catalysts. The
increased stability is observed without penalty to the water-gas
shift catalyst's initial activity.
EXAMPLE 3
Preparation of a Copper Ceria-Supported Water-Gas Shift Reagent
Impregnated with 0.5 wt. % Pt (Exemplifies Preparation of Catalyst
B)
[0120] 205 g of cerium oxide powder (HSA-15 cerium oxide available
from Rhodia, Inc.) was impregnated with 80.6 g of copper nitrate
solution in water using the incipient wetness method. The powder
was calcined using a two-step process; holding the temperature at
120.degree. C. for 2 hours, and then at 500.degree. C. for 2 hours.
The powder was cooled to yield an intermediate powder of 215 g
cerium oxide impregnated with 8 wt. % copper oxide (CuO).
[0121] The intermediate powder was then impregnated with an aqueous
solution of alkali-free amine-solubilized platinum hydroxide
containing 1.08 g of platinum. After 1 hour, acetic acid was added
to immobilize the platinum on the cerium oxide support. The
resulting material was calcined using the two step procedure
described above to yield cerium oxide support impregnated with 8
wt. % copper and 0.5 wt. % platinum.
[0122] A second portion of the intermediate cerium oxide powder
(281 g) impregnated with 8 wt. % copper was prepared as described
above. This portion was impregnated with an aqueous solution of
alkali-free amine-solubilized platinum hydroxide salt that
contained 0.70 g of platinum, treated with acetic acid (15.15 g),
and calcined using the two step calcination procedure to provide
cerium oxide powder impregnated with 8 wt. % copper and 0.25 wt. %
platinum.
[0123] 3 in .times.0.75 in 400 cpsi cordierite monolith pieces were
coated with the two slurries, dried and calcined; a dry gain of
.about.2 g/in.sup.3 was achieved.
* * * * *