U.S. patent application number 10/669271 was filed with the patent office on 2004-03-25 for method for catalytic conversion of carbon monoxide in a hydrogen-containing gas mixture.
Invention is credited to Baumann, Frank, Wieland, Stefan.
Application Number | 20040058810 10/669271 |
Document ID | / |
Family ID | 7635710 |
Filed Date | 2004-03-25 |
United States Patent
Application |
20040058810 |
Kind Code |
A1 |
Baumann, Frank ; et
al. |
March 25, 2004 |
Method for catalytic conversion of carbon monoxide in a
hydrogen-containing gas mixture
Abstract
A method for catalytic conversion of carbon monoxide with water
to carbon dioxide and water in a hydrogen-containing gas mixture
(carbon monoxide conversion) by passing the gas mixture over a
shift catalyst that is at an operating temperature for the carbon
monoxide conversion. The method is carried out with a shift
catalyst based on noble metals that is applied to an inert support
element in the form of a coating.
Inventors: |
Baumann, Frank; (Alzenau,
DE) ; Wieland, Stefan; (Offenbach, DE) |
Correspondence
Address: |
Smith Gambrell & Russell, LLP
Beveridge, DeGrandi, Weilacher & Young
Intellectual Property Group
1850 M Street, N.W., Suite 800
Washington
DC
20036
US
|
Family ID: |
7635710 |
Appl. No.: |
10/669271 |
Filed: |
September 23, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10669271 |
Sep 23, 2003 |
|
|
|
09568814 |
May 11, 2000 |
|
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Current U.S.
Class: |
502/304 |
Current CPC
Class: |
Y02P 20/52 20151101;
C01B 3/16 20130101; Y02E 60/50 20130101; H01M 8/0662 20130101; B01J
35/04 20130101; B01J 23/894 20130101; B01J 37/0205 20130101 |
Class at
Publication: |
502/304 |
International
Class: |
B01J 023/10 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 21, 2000 |
DE |
100 13 895.0 |
Claims
1. A method for the catalytic conversion of carbon monoxide in a
hydrogen-containing gas mixture with water to form carbon dioxide
and hydrogen comprising passing said gas mixture over a shift
catalyst, which is at an operating temperature for the carbon
monoxide conversion, said shift catalyst being at least one noble
metal that is applied to an inert support in the form of a
coating.
2. The method according to claim 1, wherein the shift catalyst
contains at least one of the noble metals platinum, palladium,
rhodium, ruthenium, iridium, osmium and gold on an oxide support
material selected from the group and consisting of aluminum oxide,
silicon dioxide, titanium dioxide, rare earth oxides, mixed oxides
thereof and zeolites.
3. The method according to claim 2, wherein the shift catalyst
contains at least one rare earth metal as an additional
catalytically active component.
4. The method according to claim 2, wherein the shift catalyst
contains at least one non-noble metal of the subgroups of the
periodic system of elements as an additional catalytically active
component.
5. The method according to claim 4, wherein the oxide support
material is doped with a redox-active oxide of a metal selected
from the group consisting of cerium, zirconium, titanium, vanadium,
manganese and iron in an amount of 1 to 50 wt % with respect to the
total weight of the support material.
6. The method according to claim 5, wherein the shift catalyst
contains platinum and/or palladium together with iron or copper as
well as cerium oxide on finely divided aluminum oxide.
7. The method according to claim 1, wherein a honeycomb element of
ceramic or metal, open-cell, ceramic or metallic foam elements,
metal sheet, heat exchanger plates or irregularly shaped elements
is a carrier.
8. The method according to claim 7, further comprising passing the
gas mixture over the catalyst at a space velocity between an idling
space velocity and 100,000 h.sup.-1 and at a pressure between
atmospheric pressure and 10 bar, where the space velocity refers to
the volume of the carrier coated with the catalyst.
9. The method according to claim 8, wherein the temperature of the
shift catalyst lies between 180 and 300.degree. C.
10. The method according to claim 9, wherein the gas mixture
contains 2 to 15 vol % carbon monoxide.
11. The method according to claim 8, wherein the operating
temperature of the shift catalyst lies between 280 and 550.degree.
C.
12. The method according to claim 8, wherein the shift catalyst
with an operating temperature between 280 and 550.degree. C. is
another shift catalyst with an operating temperature between 180
and 300.degree. C. and that the gas mixture is cooled to the
operating temperature of the additional catalyst before contact
with it.
13. The method according to claim 11, wherein the gas mixture
contains 2 to 40 vol % carbon monoxide.
Description
INTRODUCTION AND BACKGROUND
[0001] The present invention relates to a method for catalytic
conversion of carbon monoxide with water to carbon dioxide and
hydrogen in a gas mixture that contains hydrogen and other
oxidizable components.
[0002] The conversion of carbon monoxide with water to carbon
dioxide and hydrogen in the presence of catalysts is a known method
for producing hydrogen-rich gas mixtures, which is based on the
following exothermic reaction:
CO+H.sub.2OH.sub.2+CO.sub.2 .DELTA.H>0 (1)
[0003] Here the following side reactions can occur:
1 CO methanation: CO + 3 H.sub.2 CH.sub.4+ H.sub.2O .DELTA.H > 0
(2) and CO.sub.2 methanation: CO.sub.2 + 4 H.sub.2 CH.sub.4 +
H.sub.2O .DELTA.H > 0 (3)
[0004] The reaction in accordance with reaction equation (1) is
called carbon monoxide conversion or CO conversion herein. The term
"water gas shift reaction" is commonly used for this in the
USA.
[0005] The production of hydrogen-rich gas mixtures from
hydrocarbons, or alcohols, by steam reforming, partial oxidation or
autothermic reforming is a known process. These gas mixtures
(reformates) contain 1 to 40 vol % carbon monoxide, depending on
the method that is used.
[0006] To use the reformate as fuel in fuel cells, it is necessary
to reduce the carbon monoxide contained in them as far as possible,
in order to avoid poisoning of the platinum-containing anode
catalyst of the fuel cell in the oxidation of the hydrogen. In
addition, the conversion of carbon monoxide in accordance with
reaction equation (1) leads to an increase of the hydrogen content
of the reformate and thus to an improvement of the efficiency of
the overall process.
[0007] For reasons of size and weight, catalysts for conversion of
carbon monoxide with very high activity and selectivity are
required for use in motor vehicles. The high space-time yields that
can be achieved by this allow the volume of the reactors that are
required to be kept small.
[0008] The known catalysts for the conversion of carbon monoxide
have chiefly been developed for stationary industrial applications.
The emphasis lay in the production of pure hydrogen, ammonia and
other large scale products that are based on the use of synthesis
gas mixtures (CO/H.sub.2). Catalysts for the conversion of carbon
monoxide in accordance with reaction equation (1) are also called
shift catalysts herein.
[0009] These known catalysts are complete catalysts that contain
non-noble metals. They are used in two-stage processes. In the
first process stage a so-called high temperature CO conversion
(high temperature water-gas shift, HTS) is carried out on Fe/Cr
catalysts at temperatures between 360 and 450.degree. C. In the
subsequent second stage a low-temperature CO conversion (low
temperature water-gas shift, LTS) is undertaken on Cu/ZnO catalysts
at temperatures between 200 and 270.degree. C. After the low
temperature process stage carbon monoxide concentrations of less
than 1 vol % in correspondence with the thermal equilibrium are
obtained.
[0010] The conventional catalysts for the conversion of carbon
monoxide have crucial disadvantages:
[0011] The described two stage conduct of the process is necessary
because of the properties of these catalysts. While
Cu/ZnO-containing catalysts become deactivated above 270.degree. C.
because of recrystallization, or sintering, of the copper, the
Fe/Cr-containing catalysts that are used in the high temperature
range cannot be used at low temperatures because of insufficient
activity. If the indicated temperature range of the high
temperature catalysts is exceeded, methanation reactions (reaction
equations (2) and (3)) can occur, which reduce the selectivity of
the high temperature catalyst and because of this lower the overall
efficiency of the hydrogen generation system.
[0012] Both the known high-temperature and the low-temperature
catalysts are bulk catalysts, in which the catalyst material is
pressed to form pellets or other molded bodies. Accordingly, they
consist entirely of catalytically active mass and are also called
complete catalysts. As a rule, they have a very high bulk
weight.
[0013] The known industrial methods for conversion of carbon
monoxide on catalysts according to reaction equation (1) operate at
space velocities of the gas mixture between 300 and 3000 h.sup.-1.
These low velocities are not sufficient for use in motor
vehicles.
[0014] High bulk weights and low space velocities lead to low
specific conversion rates R.sub.CO for the carbon monoxide, which
is understood within the scope of this invention to mean the amount
of carbon monoxide N.sub.CO converted per weight of the catalyst
m.sub.cat and reaction time .DELTA.t. The weight of the catalyst
here is given in grams, the reaction time in seconds and the amount
of carbon monoxide in mol: 1 R CO = n co m Cat t [ mo 1 g s ] ( 40
)
[0015] The known Cu/ZnO and Fe/Cr catalysts have to be activated by
reduction before they are used. The activated catalysts are
sensitive to oxygen. Upon contact with atmospheric oxygen they are
reoxidized and deactivated in an exothermic reaction.
[0016] In comparison with the just described industrial high
temperature and low temperature catalysts based on Fe/Cr or Cu/ZnO,
noble metal catalysts for these uses are also known, mainly from
the scientific literature.
[0017] D. C. Grenoble et al. describe in "The Chemistry and
Catalysis of the Water Gas Shift Reaction. 1. The Kinetics over
Supported Metal Catalysts," J. Catal. 67 (1980) 90-102, powdered
catalysts that contain Cu, Re, Co, Ru, Ni, Pt, Os, Au, Fe, Pd, Rh
or Ir as active components and that are deposited on aluminum oxide
(Al.sub.2O.sub.3) as a support material. The kinetic tests gave a
reaction order of about 0.2 for carbon monoxide and about 0.5 for
the water that was used.
[0018] In "Methanization and Water Gas Shift Reactions over
Pt/CeO.sub.2," J. Catal. 96 (1985), 285-287, Steinberg et al.
observed poor selectivities in view of the carbon monoxide
conversion according to reaction equation (1). Accordingly, the
product gas mixture contains high proportions of methane.
[0019] In "Water gas shift conversion using a feed with a low steam
to carbon monoxide ratio and containing sulfur," Catal. Today 30
(1996) 107-118, J. Ross et al. investigate a Pt/ZrO.sub.2 catalyst,
in addition to Fe/Cr, Cu/ZnO and Co/Cr catalysts. This catalyst
shows a carbon monoxide conversion of 50% at 320.degree. C. The
Pt/ZrO.sub.2 catalyst shows the highest tolerance for
sulfur-containing compounds among the tested compounds. It shows a
conversion of 25% at 300.degree. C. and a conversion of 70% at
350.degree. C. This corresponds to a specific carbon monoxide
conversion rate R.sub.CO (300.degree. C.)=7.00.times.10.sup.-6
mol/(g.sub.cat.multidot.sec), or R.sub.CO (350.degree.
C.)=1.95.times.10.sup.-5 mol/(g.sub.cat.multidot.sec).
[0020] FR 2567866 A describes a copper- and/or palladium-containing
catalyst on a support of ZnAl.sub.2O.sub.4 spinel, which is
obtained by impregnating the spinel formed into particles with
diameters between 0.4 and 0.6 mm with solutions of copper and/or
palladium and calcining it. A conversion of 86% is achieved with
this catalyst at pressures of 40 bar and a temperature of
250.degree. C. at a very high excess of water (H.sub.2O/CO=10).
[0021] The powdered catalyst systems that have been investigated in
the scientific literature are not suitable for industrial use.
[0022] The known complete catalysts in the form of tablets, pellets
or irregularly shaped particles are used as so called bulk
catalysts. Only unsatisfactory space-time yields are obtained with
such catalysts. In addition, the achievable specific conversion
rates with these catalysts are low.
[0023] Accordingly, an object of the present invention is to
provide a method for conversion of carbon monoxide in a
hydrogen-containing gas mixture that, under the conditions of
mobile use in motor vehicles with their rapidly changing power
requirements, a high specific conversion rate for carbon monoxide
with good selectivity, has high temperature stability and is
insensitive to oxygen in the educt gas mixture.
SUMMARY OF THE INVENTION
[0024] This above and other objects of the invention can be
achieved by a method for catalytic conversion of carbon monoxide to
carbon dioxide and hydrogen (carbon monoxide conversion) in a
hydrogen-containing gas mixture. For conversion of the carbon
monoxide, the gas mixture is passed over a shift catalyst, which is
at the operating temperature for carbon monoxide conversion. The
method features a shift catalyst based on noble metals that is
applied to an inert carrier in the form of a coating.
[0025] The method of the present invention is specifically directed
to mobile use in motor vehicles powered by fuel cells in order to
effectively removed carbon monoxide from the hydrogen-rich gas
mixture that is obtained by steam reforming, partial oxidation or
autothermic reforming (hereinafter also called reformate gas) under
all conditions of operation of the motor vehicle. The gas mixture
can contain up to 40 vol % carbon monoxide, depending on its
production.
[0026] The mobile use of the method imposes high requirements on
its efficiency and dynamics. During the operation of the motor
vehicle, the catalysts are loaded with very different space
velocities. They vary between a low space velocity at idling and
100,000 h.sup.-1 at full load.
[0027] The method of the invention enables a high efficiency, i.e.,
a high space-time yield through the application of the catalyst in
the form of a coating onto an inert carrier. Such a catalyst is
also called a coating catalyst herein. The monolithic honeycomb
elements of ceramic or metal with cell densities (number of flow
channels per area of cross section) of more than 10 cm.sup.-2 that
are known from auto exhaust treatment are suitable as carrier.
However, metal sheet, heat exchanger plates, open-cell ceramic or
metal foam elements and irregularly shaped elements formed in each
case according to requirements can also be used as carriers. The
thickness of the coating can vary between 10 and 100 .mu.m
according to application.
[0028] A carrier within the scope of this invention is
characterized as inert if the material of the carrier does not
participate or participates only negligibly in the catalytic
conversion. As a rule, these are bodies with low specific surface
and low porosity.
[0029] A catalyst that contains the elements of the platinum group
of metals, thus platinum, palladium, rhodium, iridium, ruthenium
and osmium, or gold as the catalytic active components on an oxide
support made from the group consisting of aluminum oxide, silicon
dioxide, titanium oxide, rare earth oxides or mixed oxides of these
or zeolites is suitable for the proposed method. In order to enable
distribution of the catalytically active components on the support
material that is as fine as possible, the support material should
have at least a specific surface (BET surface, measured in
accordance with DIN 66132) of more than 10 m.sup.2/g.
[0030] This noble metal catalyst exhibits a shift activity, i.e.,
it is capable, if the appropriate reaction conditions exist
(temperature, gas composition), of converting carbon monoxide with
water in accordance with reaction equation (1) to carbon dioxide
and hydrogen. For this reason it is also called a noble metal shift
catalyst herein. Its shift activity and selectivity can be improved
by the addition of other catalytically active components, or
promoters. Among these are elements of the rare earth metals, in
particular cerium and lanthanum, as well as the non-noble metals of
the subgroups of the periodic system of elements, especially iron
or copper.
[0031] The shift activity and selectivity can, moreover, also be
increased by doping the support material with redox-active oxides
of the metals cerium, zirconium, titanium, vanadium, manganese and
iron in an amount of 1 to 50 wt % with respect to the total weight
of the support material.
[0032] A preferred shift catalyst for the method in accordance with
the invention contains platinum and/or palladium together with iron
or copper as well as cerium oxide on a finely divided aluminum
oxide.
[0033] The use of the shift catalyst based on noble metals for the
method also has the advantage that this catalyst does not become
deactivated by contact with oxygen. For this reason no costly
measures to protect the catalyst from contact with air are
necessary in a motor vehicle.
DETAILED DESCRIPTION OF INVENTION
[0034] The present invention will now be described in further
detail.
[0035] In accordance with the invention, the described catalyst
material is not processed to complete catalysts, but rather is
applied in the form of a coating to inert supports. In this way the
disadvantages of complete catalysts that consist of the
catalytically active centers in the interior of the complete
catalyst being poorly accessible to the reactants are avoided in
this method. Poor accessibility reduces the specific conversion
rate for carbon monoxide and thus the achievable space-time yield.
This has the corresponding negative effects on the volume of the
required reactor. The vibrations caused by operation of the motor
vehicle additionally lead to undesired abrasion of complete
catalysts, which blocks the flow paths in the catalyst bed and thus
continuously increases the pressure difference in the reactor.
[0036] The process operates at gas mixture space velocities from
idling space velocity up to a value of 100,000 h.sup.-1 and at a
pressure between atmospheric pressure and 10 bar, where the space
velocity is given in reference to the volume of carrier coated with
the catalyst. The method can be used both for low-temperature CO
conversion as well a for high-temperature CO conversion.
[0037] A noble metal shift catalyst with an operating temperature
between 180 and 300.degree. C. is used for the low-temperature CO
conversion. The low operating temperature is achieved through a
relatively high charge of catalytically active noble metals on the
catalyst. In low-temperature CO conversion the reformate gas
usually contains 2 to 15 vol % carbon monoxide and has an input
temperature between 100 and 250.degree. C. which results from the
reforming process.
[0038] A noble metal shift catalyst with an operating temperature
between 280 and 550.degree. C. is used for the high temperature CO
conversion. In the high temperature CO conversion the reformate gas
usually contains 2 to 40 vol % carbon monoxide and has an input
temperature between 300 and 600.degree. C., which results from the
reforming process.
[0039] The method also allows a high temperature conversion stage
and a low temperature conversion stage to be connected in
succession. The gas mixture in this case leaves the high
temperature stage at a temperature that corresponds to the
operating temperature of the catalyst of the high temperature stage
and for this reason has to be cooled to the operating temperature
of the catalyst of the low-temperature stage before contact with
it.
[0040] There are various possibilities for production of a coating
catalyst suitable for the method, a few of which are discussed
here.
[0041] To produce a shift catalyst on a carrier element in
accordance with the invention, the support material for the
catalytically active components can be suspended in an aqueous
solution of soluble compounds of a noble metal selected from the
group consisting of platinum, palladium, rhodium, ruthenium,
iridium, osmium, gold and mixtures thereof and other soluble
compounds of non-noble metals of the subgroups. Then the acid
suspension is neutralized at elevated temperature with a base, for
example, a sodium carbonate, and then reduced at the same
temperature with an aqueous reducing agent (formaldehyde,
hydrazine), filtered, washed, dried, calcined in an oxidizing
atmosphere at temperatures between 300 and 550.degree. C., and then
reduced at temperatures between 300 and 600.degree. C. The catalyst
material is again suspended in water to produce a coating
suspension. The carrier element is coated with this suspension. For
this, the methods for coating carrier elements that are known from
auto exhaust catalysis can be used. To finish the production of the
coating catalyst the coating is dried, calcined at temperatures
between 300 and 600.degree. C. and reduced in a hydrogen-containing
gas at temperatures between 300 and 600.degree. C.
[0042] As an alternative to the described method, the carrier
element is first coated only with the support material, where the
support material can contain rare earth oxides and oxides of
non-noble metals of the subgroups. The coating on the carrier
element is then impregnated with a solution of at least one soluble
noble metal compound, soluble compounds of the rare earths and the
non-noble metals of the subgroups. To finish the production of the
coating catalyst, the coated carrier element is dried, calcined at
temperatures between 300 and 600.degree. C. and reduced in a
hydrogen-containing gas at temperatures between 300 and 600.degree.
C.
[0043] Another variation for making a coating catalyst in
accordance with the invention resides in first producing a
suspension of the support material, the soluble compounds of the
noble metals and optionally the soluble compounds of the non-noble
metals of the subgroups and the rare earths. The dissolved
components of the suspension are then precipitated onto the
suspended support material through the addition of a basic
precipitation agent such as sodium hydroxide. The suspension
prepared in this way is used directly for coating the carrier
element. To finish the production of the coating catalyst, the
coated carrier element is dried, calcined at temperatures between
300 and 600.degree. C. and reduced in a hydrogen-containing gas at
temperatures between 300 and 600.degree. C.
[0044] The invention is illustrated in more detail by means of the
following examples.
EXAMPLE 1
[0045] A noble metal shift catalyst (catalyst A) was produced as
follows:
[0046] A ceramic element honeycomb carrier with 93 cells per square
centimeter and a volume of 0.041 L was coated with 7.25 g
.gamma.-aluminum oxide by immersing it in an aqueous suspension of
.gamma.-aluminum oxide (specific surface 140 m.sup.2/g) and
calcining for 2 h at 600.degree. C. After calcination the coated
honeycomb element was impregnated with a solution of
Ce(NO.sub.3).sub.2.6H.sub.2O and then calcined for 2 h at
500.degree. C. The calcined molded element was then impregnated
with a solution of Pt(NO.sub.3).sub.2, Pd(NO.sub.3).sub.2 and
Fe(NO.sub.3).sub.3.
[0047] The catalytically active coating of the catalyst prepared in
this way had a total weight of 5.16 g, which corresponds to 126 g
per liter of volume of the honeycomb element. It contained 1.2 wt %
Pt, 1.2 wt % Pd, 2.4 wt % Fe, 35.7 wt % CeO.sub.2 and 59.5 wt %
Al.sub.2O.sub.3.
[0048] The catalyst was tested under the conditions of a high
temperature conversion with a synthetic reformate. Its CO.sub.2
selectivity S.sub.CO2, CO conversion, as well as specific
conversion rate R.sub.CO in accordance with equation (4) were
measured. The following gas composition was used for the high
temperature conversion: 27.0 vol % H2, 9.0 vol % CO, 9.0 vol %
CO.sub.2, 18.0 vol % HO, 37.0 vol % N.sub.2. The catalysts were
tested at a gas space velocity GHSV=10,000 h.sup.-1 and a pressure
of 2 bar (absolute).
[0049] The CO.sub.2 selectivity S.sub.CO2 of the conversion of
carbon monoxide was calculated by means of the partial pressures of
the carbon dioxide P.sub.CO2 and methane P.sub.CH4 that formed, as
2 S CO 2 = P CO 2 P CO 2 + P CH 4 ( 5 )
2TABLE 1 High-temperature CO conversion on catalyst A T [.degree.
C.] 3 S CO 2 [ % ] CO Conversion [%] 4 R CO [ mol g cat S ] 300 100
27 3.0 .multidot. 10.sup.-5 350 100 35 4.0 .multidot. 10.sup.-5 400
100 45 4.8 .multidot. 10.sup.-5
COMPARISON EXAMPLE 1
[0050] A commercial Fe/Cr catalyst (catalyst B; tablets 5.times.5
mm) was tested under the same conditions as catalyst A.
3TABLE 2 High-temperature CO Conversion on catalyst B T [.degree.
C.] 5 S CO 2 [ % ] CO Conversion [%] 6 R CO [ mol g cat S ] 300 100
30 3.0 .multidot. 10.sup.-5 350 100 37 4.0 .multidot. 10.sup.-5 400
100 45 4.8 .multidot. 10.sup.-5
[0051] As Tables 1 and 2 show, both catalysts exhibit comparable CO
conversions. However, catalyst A in accordance with the invention
shows a tenfold higher specific conversion rate R.sub.CO in
comparison with catalyst B, because of its higher activity.
[0052] Further variations and modifications of the foregoing will
be apparent to those skilled in the art and are intended to be
encompassed by the claims appended hereto.
[0053] German priority application 100 13 895.0 is relied on and
incorporated herein by reference.
* * * * *