Note: Descriptions are shown in the official language in which they were submitted.
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PROCESS FOR SELECTIVE OXIDATION OF CARBON MONOXIDE
IN A HYDROGEN CONTAINING STREAM
Field of the Invention
The invention relates to the catalytic oxidation of carbon monoxide. In
another of its aspects the invention relates to the selective oxidation of
carbon
monoxide in the presence of hydrogen. In still another aspect the invention
relates to
catalyst compositions effective in the oxidation of carbon monoxide. In yet
another
aspect the invention relates to removing as much carbon monoxide as possible,
preferably all carbon monoxide, from a stream containing carbon monoxide and
hydrogen, particularly, to provide hydrogen feedstock for fuel cells.
Background of the Invention
The selective oxidation of carbon monoxide in hydrogen-rich streams
has been of considerable technical interest for the purification of reformed
hydrogen
used in feed gas in ammonia synthesis. Recently, this selective oxidation
process,
1 S sometimes referred to as preferential oxidation, has attracted interest
due to the
possibility of using this technology in providing suitable hydrogen fuel for
fuel cells.
Since carbon monoxide is also oxidized to provide carbon dioxide for carbon
dioxide
lasers, the use of catalyst which previously had been found useful in the
oxidation of
carbon monoxide for use in carbon dioxide lasers has also been investigated
for
adaptation for use in providing carbon monoxide-free hydrogen for fuel cell
feedstock.
A fuel cell is an electrochemical device that enables converting the
chemical energy of fuels directly to electricity. A hydrogen-air polymer
electrolyte
membrane (PEM) fuel cell stack is currently considered the best means for
adapting
this technology to most uses. The PEM fuel cell is most efficient using
gaseous
hydrogen for fuel. Use of a fuel processor to generate a hydrogen-rich
feedstock at the
point of use eliminates problems of storage and distribution of the hydrogen
fuel.
A fuel processor can convert fuels such as alcohol, gasoline, liquid
petroleum gas, or natural gas to a hydrogen-rich stream. By a process of steam
reforming a stream consisting primarily of hydrogen, carbon dioxide and carbon
monoxide can be produced, but the product is generally saturated with water.
Processing this stream in a shift reactor reduces the carbon monoxide content
to
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provide relatively more hydrogen by means of the well known water-gas-shift
reaction.
This reaction provides a product that contains from 0.2 to 2 percent carbon
monoxide
by volume which is sufficient to poison the platinum-based catalyst at the PEM
anode.
It has now been found that, among other possibilities for removing carbon
monoxide
to the level necessary to prevent poisoning of the PEM catalyst, the same
catalyst that
is used to recombine carbon monoxide and oxygen in carbon dioxide lasers can
be
used to provide hydrogen feedstock for fuel cells on a level of carbon
monoxide
removal that is commercially viable. The operating conditions for the
processes are
essentially different. The removal of carbon monoxide by selective oxidation
of a
stream containing both carbon monoxide and hydrogen can be accomplished using
the
same catalyst as used in carbon dioxide lasers by controlling an increased
oxygen flow
to the oxidation process, raising the operating temperature of the oxidation
process and
avoiding reaction between oxygen and hydrogen as compared to the conditions
used to
recombine carbon monoxide and oxygen in carbon dioxide lasers.
Summary of the Invention
It is desirable to provide a process that is effective for catalytically
oxidizing carbon monoxide with free oxygen. It is another object of this
invention to
provide a process for converting carbon monoxide (CO) to carbon dioxide (C02)
in the
presence of hydrogen. Again it is desirable to provide a process for producing
hydrogen fuel for a fuel cell in which carbon monoxide (CO) is converted to
carbon
dioxide (COZ) in the presence of hydrogen on a scale that is commercially
viable.
In accordance with this invention a process is provided for the selective
oxidation of carbon monoxide to carbon dioxide in a gaseous mixture comprising
hydrogen and carbon monoxide. In the process an amount of free oxygen is mixed
with the gaseous mixture comprising hydrogen and carbon monoxide to provide a
second gaseous mixture having an enhanced oxygen to carbon monoxide mol ratio.
The second gaseous mixture is contacted with a catalyst comprising platinum
and iron
impregnated on a support material. The carbon monoxide in the second gaseous
mixture is thereby substantially completely converted to carbon dioxide. When
used
herein the terminology "substantially complete conversion of carbon monoxide
to
carbon dioxide" or similar terminology means that the amount of carbon
monoxide
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present in a fuel cell feed stream is sufficiently low so as not to materially
affect the
functioning of a PEM catalyst.
Detailed Description of the Invention
According to this invention the process for oxidizing carbon monoxide
in a feed stream that also contains hydrogen can be carried out so that the CO
is
selectively oxidized in preference to the oxidation of the hydrogen thereby
providing a
means to deliver a highly pure hydrogen stream for fuel cell operation in
which the
oxidation of carbon monoxide in a hydrogen fuel can be integrated into a total
package
for generating a hydrogen-rich feedstock at the point of use.
The feed gas to the oxidation process can be formed in any suitable
manner, such as by mixing the hydrogen that contains carbon monoxide
contaminant
with the O~ containing air at any point before contact with the catalyst.
The process for oxidizing a feed containing carbon monoxide and
hydrogen gas can be earned out at any pressure conditions, for any length of
time, any
gas hourly space velocity and any volume ratio of OZ to CO that is suitable
for
selective oxidation of CO in the presence of hydrogen specified in a
temperature range
of about 0°C to about 300°C, preferably in a range of about
25°C to about 250°C, and
most preferably in a range of about 50°C to about 200°C.
The pressure during the oxidation process generally is in the range of
about 68.9 kPa to about 6890 kPa (about 10 psia to about 1000 psia),
preferably about
96.4 kPa to about 1378 kPa (about 14 psia to about 200 psia).
The ratio of mols of OZ in the feed gas to the mols of CO in the feed gas
will generally be in the range of about 0.5 to 8.0 mol OZ/mol CO, preferably
0.5 to 4.0
mol O,/mol CO, most preferably 0.5 to 1.5 mol OZ/mol CO.
The gas hourly space velocity (cc feed gas per cc catalyst per hour) can
be in the range of about 100 to about 200,000, preferably from about 5,000 to
about
50,000.
The hydrogen will generally be in the range of about 50-90 volume
percent and the inlet CO will generally be in the range of about 0.1 to about
5 volume
percent.
The preparation of the catalyst useful in this invention can be carried
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out by the process discloses in USPN 5,017,357 and USPN 4,943,550, which
disclose
processes using the catalyst for the. recombination of carbon monoxide and
oxygen for
carbon dioxide lasers.
Any of the well known support materials containing metal oxide can be
used as support material for the composition of matter used as catalyst in the
process
of this invention. Presently preferred are substantially pure alumina
(aluminum oxide),
titanic and/or magnesium aluminate spinet, More preferably, the support
material can
contain at least 95 weight percent A1z03 or magnesium aluminate. These
materials are
readily available commercially.
Generally the surface area of the support material, which can be
determined by the BET/N~ method (ASTM D3037), is in the range of about 10 mZ/g
to
about 350 mz/g. The support can be spherical, cylindrical, trilobal,
quadrilobal, ring-
like or irregular in shape. Spherical support material generally has a
diameter in the
range of from about 0.2 mm to about 20 mm, preferably from about 1 mm to about
5
mm.
The support can also be an inert porous, ceramic material in any of the
shapes cited above and coated with aluminum oxide and/or magnesium aluminate
spinet.
The impregnation of the support material with platinum and iron can be
carned out in any suitable manner. Generally, compounds of platinum and
compounds
of iron are dissolved in a suitable solvent, preferably water, to prepare a
solution of
suitable concentration, generally containing from about 0.005 g to about 5.0 g
platinum
per cc of solution and about 0.005 g to about 5.0 g iron per cc of solution.
Suitable
compounds of both platinum and iron are nitrates, carboxylates and
acetylacetonates,
among others, with acetylacetonates currently preferred. Organic solvents,
such as
methanol, ethanol, acetone, ethyl acetate, toluene and the like can be used as
solvents
for platinum or iron according to this invention. Currently, acetone is
preferred.
The support material can be soaked in a solution containing platinum
compounds and/or iron compounds or can be sprayed with such a solution to
impregnate the support. The ratio of impregnating solution to support material
is
generally such that the final composition of the catalyst contains 0.05 to
about 10
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weight percent platinum, preferably about 0.1 to about 5 weight percent
platinum and
about 0.05 to about 20 weight percent iron, preferably from about 0.1 to about
4
weight percent iron. It is in the scope of this invention to use any weight
percentage of
platinum and iron at which they act as copromoters of the oxidation of CO with
OZ. It
is presently preferred to spray a solution containing compounds of both metals
onto the
support, but the metal compounds can also be added separately in any order.
After impregnation the impregnated support material is heated to a
temperature sufficient to drive off the solvent used in the impregnation. A
flow of
inert gas across the support material can be used. A temperature in the range
of up to
about 250°C applied for about an hour is usually sufficient for the
purpose.
The dried catalyst is heat treated in an oxidizing atmosphere, preferably
in an atmosphere containing free oxygen (such as air) generally at a
temperature
ranging from about 80°C to about 700°C for a time ranging from
about 0.5 hr to about
10 hours. The heat treatment is preferably done in incremental substeps.
Currently,
the heat treatment is carried out at 150°C for 1 hour, 200°C for
2 hours and 400°C for
3 hours. Any combination of heating at a temperature for a time sufficient to
calcine
the impregnated support material to obtain at least one platinum oxide,
optionally
mixed with metallic platinum, and at least one iron oxide satisfies the
requirements of
this invention.
After the oxidation the calcined, platinum/iron impregnated support is
subjected to a reduction reaction which can be carried out in any suitable
manner,
preferably at a temperature in the range of about 20°C to about
650°C, more
preferably from about 200°C to about 500°C. Any reducing gas can
be used, such as a
gas containing hydrogen, CO, gaseous hydrocarbons such as methane, mixtures of
the
above and the like. Preferably a free hydrogen containing gas, more preferably
a gas
stream of substantially pure hydrogen, is employed. The reducing step can be
carried
out for any suitable period of time from about 1 minute to about 20 hours,
preferably
from about 1 hour to about 5 hours.
The reduced, platinum/iron impregnated support can be further treated
by contact with any suitable organic or inorganic acid having a pH of less
than about 7.
Preferably, an aqueous solution of nitric acid or of a carboxylic acid
(preferably acetic
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acid) is used. The previously reduced platinum/iron impregnated support is
preferably
soaked in about 0.01-16 mole/L of HN03 generally at a temperature of about
10°C to
about 80°C for a period of about 0.01 to about 1 hour, but sufficiently
to obtain
incipient wetness.
After the acid treatment the impregnated support material is heated to a
temperature sufficient to drive off the solvent used in the acid treatment. A
flow of
inert gas across the support material can be used. A temperature in the range
of up to
about 250°C applied for about an hour is usually sufficient for the
purpose.
The dried, acid treated catalyst is heat treated in an oxidizing
atmosphere, preferably in an atmosphere containing free oxygen (such as air)
generally
at a temperature ranging from about 80°C to about 700°C for a
time ranging from
about 0.5 hr to about 10 hours. The heat treatment is preferably done in
incremental
substeps. Currently, the heat treatment is carried out at 150°C for 1
hour, 200°C for 2
hours and 400°C for 3 hours. Any combination of heating at a
temperature for a time
sufficient to calcine the impregnated support material to obtain at least one
platinum
oxide, optionally mixed with metallic platinum, and at least one iron oxide
satisfies the
requirements of this invention.
Before use in the process of oxidizing carbon monoxide the oxidized,
acid-treated, supported platinum/iron catalyst can be activated by a reduction
step that
can be carried out in any suitable manner, preferably at a temperature of
about 20°C to
about 650°C, more preferably about 200°C to about 500°C
for about 0.5 hour to about
20 hours, preferably about 1 hour to about S hours to enhance the activity of
the
catalyst composition for catalyzing a low temperature oxidation of CO with Oz
in the
presence of hydrogen: Any reducing gas can be used: hydrogen, CO, paraffins
and the
like and mixtures thereof.
The following examples are presented in further illustration of the
invention and are not to be construed as limiting the scope of the invention.
EXAMPLE I
A catalyst precursor was prepared by weighing about 500 grams of 1/8
inch alumina spheres (Alcoa S-100 activated alumina) into two medium porcelain
bowls and calcined at 800 ° C for 16 hours in a an air-purged muffle
furnace. 400
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grams of the dry, calcined alumina was placed in a large porcelain bowl and-
using a
conventional, plastic, hand spray bottle- was sprayed with an impregnating
solution
prepared by dissolving 8.07 grams of platinum (II) acetylacetonate (platinum
(II) 2,4
pentanedionate) and 10.13 grams of iron (III) acetylacetonate in about 650 cc
of
S acetone. The support was stirred frequently to assure an even distribution
of the
solution. When about 1/4 of the impregnation solution had been applied to the
support
the catalyst was placed in a draft oven and heated at 175 °C for 45
minutes to an hour
thereby driving off the acetone and partially decomposing the metal
acetylacetonates.
The processes of spraying, stirnng and heating were repeated three more times.
When
all the impregnating solution had been used the catalyst was divided equally
into
portions of about 202 grams each and placed in an air-purged muffle furnace
heated at
150 ° C for 1 hour, 200 ° C for 2 hours and 400 ° C for 3
hours. This heat treatment
provided two 202 gram portions of oxidized 1.0 weight percent platinum/ 0.4
weight
percent iron on alumina as catalyst precursor.
EXAMPLE II
A 202 gram portion of catalyst precursor was transferred to a 2 inch
diameter quartz reactor which was then mounted in a vertical tube furnace. The
catalyst was activated by reducing at 300°C with about 200 cc/min
hydrogen gas
downflow at atmospheric pressure for three hours. The catalyst and reactor
were
cooled under hydrogen flow followed by nitrogen purge thereby providing an
activated
catalyst. This is Catalyst A.
EXAMPLE III
Another 202 gram portion of the catalyst precursor was transferred to a
2 inch diameter quartz reactor and mounted in a vertical tube furnace. The
catalyst
was reduced at 300°C with about 200 cc/min hydrogen gas downflow at
atmospheric
pressure for three hours. The catalyst and reactor were cooled under hydrogen
flow
followed by nitrogen purge. The freshly reduced catalyst was poured into a
large bowl
and impregnated in a ventilated hood with about 60 cc of concentrated nitric
acid. The
acid impregnation was done dropwise with stirnng. The impregnation was done as
quickly as possible to minimize oxidation by exposure to atmospheric oxygen.
The
acid treated catalyst was dried and calcined in an air-purged muffle furnace
heated at
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150 ° C for 1 hour, 200 ° C fo: 2 hours and 400 ° C for 3
hours. The catalyst was
transferred to a 2 inch diameter qL~artz reactor which was then mounted in a
vertical
tube furnace. The catalyst wa~~ activated by reducing at 300°C with
about 200 cc/min
hydrogen gas downflow at atmospheric pressure for three hours. The catalyst
and
reactor were cooled under hydrogen flow followed by nitrogen purge thereby
providing
an activated, acid-treated catalyst. This is catalyst B.
EXAMPLE IV
For these conversion runs the following equipment was used. There
were two separate Brooks 5850E mass flow controllers- one for the CO feed
blend and
one for airflow. The CO blend was held in a 30-liter, aluminum, high-pressure
cylinder. The CO blend normally was 1.0 percent CO with the balance being
hydrogen. The air was supplied by an in house system. The CO blend and air-
streams
were joined at the inlet to a jacketed glass tube with an outer jacket for
circulating
coolant. The glass tube was cooled by a temperature controlled circulating
bath which
could control the temperature to a specific temperature chosen from within a
range of
5°C to about 100°C. The catalyst was loaded inside the glass
tube.
The catalyst was prepared as set out in Examples I-III. A quantity of
2.0 grams of the treated catalyst was loaded into the glass tube with mesh
quartz chips
packed into the void space. Each catalyst was pretreated by heating to
97°C for one
hour with 100 cc/min of hydrogen flow through the catalyst bed. The tests were
run
with conditions as shown in the table below. All runs were carried out at
ambient
pressure, at 10,000 cc feed gas per cc catalyst per hour, GHSV (gas hourly
space
velocity) using a feed of 1 percent CO in hydrogen. Data were taken every 15
minutes
with the 30 minute results recorded as the result of the test run.
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TABLE 1
Pt/Fe
and
Pt/Fe(Acid
Treated)
in
the
Presence
of
Hydrogen
Target Actual CO
React /oConv /oConv /Select
at p O:CO O:CO Survival
Tem C mol ratiomol ratioO'- CO (vol%) to COz
A 80 1 0.8 100.1 59.3 0.4270 75.0
A 80 2 1.6 99.9 94.9 0.0519 61.1
A 80 2.5 1.9 99.8 99.9 0.0006 51.6
A 80 2.75 2.1 99.6 100.0 0.0000 47.2
A 80 3 2.3 99.4 100.0 0.0000 43.3
B 80 1 0.8 100.0 59.5 0.4290 76.6
B 80 2 1.5 100.0 97.0 0.0314 62.7
B 80 2.5 1.9 100.0 100.0 0.0000 51.7
B 80 2.75 2.1 100.0 100.0 0.0000 47.0
B 80 3 2.3 99.9 100.0 0.0000 43.1
A 25 1 0.8 90.3 85.0 0.1570 119.2
A 25 1.5 1.2 87.6 96.5 0.0359 93.1
A 25 2 1.6 84.7 98.0 0.0202 74.4
A 25 3 2.3 80.5 98.9 0.0114 52.9
A 25 4 3.1 78.3 99.0 0.0093 41.1
B 25 1 0.8 94.6 88.7 0.1200 120.7
B 25 1.5 1.2 91.5 98.9 0.0116 91.4
B 25 1.75 1.4 88.3 99.1 0.0089 82.3
B 25 2 1.5 85.5 99.3 0.0072 75.1
B 25 2.5 1.9 82.6 99.6 0.0045 62.3
B 25 3 2.3 77.1 99.4 0.0057 55.6
In the table above cat is catalyst, react temp is reaction temperature, cony
is
conversion and select is selectivity.
The data show that both Pt/Fe and the Pt/Fe (acid treated) were
effective as catalyst in the conversion of CO to CO2. In oxygen conversion
data the
acid treated catalyst had a higher activity than the catalyst that was not
acid treated.
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This higher activity occurred under all conditions, including the very high
space
velocities. At 80°C and 10,000 GHSV the acid treated catalyst had
slightly higher CO
conversion which resulted in less CO survival and a higher selectivity to COz.
At
25 ° C and 10,000 GHSV the acid treated catalyst had higher CO
conversion than the
untreated catalyst resulting in much lower CO survival and higher selectivity
to CO2.
Reasonable variations, modifications and adaptations for various
conditions and uses can be made within the scope of this disclosure and the
appended
claims.
