Note: Descriptions are shown in the official language in which they were submitted.
CA 02353759 2001-07-25
OXIDIZING CATALYSTS, CARBON MONOXIDE SENSOR, AND HYDROGEN
SENSOR
FIELD OF THE INVENTION
This invention relates to oxidizing catalysts, a carbon
monoxide sensor, and a hydrogen sensor using the catalyst
respectively, and more specifically t.o a catalyst selective-
ly oxidizing carbon monoxide, a catalyst selectively oxidiz-
1D ing hydrogen, an carbon monoxide sensor not having the
sensitivity to hydrogen existing together with the carbon
monoxide, a hydrogen sensor not having the sensitivity to
carbon monoxide existing together with the hydrogen, and a
hydrogen purification catalyst for a fuel cell.
CONVENTIONAL TECHNOLOGY
Carbon monoxide (CO) and hydrogen (H2) coexist in
exhaust gases generated when a coal gas, an water gas, a
city gas, an LPG and other types of gases are incompletely
2D combusted. As the carbon monoxide contained in the exhaust
gases as described above is highly toxic, the following
practice is applied for inspection of gas incomplete combus-
tion alarm units in Japan.
In the practice for inspection of a city gas incomplete
combustion alarm unit, it is required that the alarm unit
can correctly give an alarm when carbon monoxide is present
at 200 ppm, and also that the alarm unit should not give a
false alarm when hydrogen is present: at 500 ppm and ethyl
alcohol at 1,000 ppm. In the practice for inspection of a
3D high pressure gas incomplete combustion gas alarm unit, it
is required that the alarm unit can dive a first alarm when
carbon monoxide is present at 250 ppm and hydrogen at 125
ppm and also can give a second alarm when carbon monoxide is
present at 550 ppm and hydrogen at 275 ppm, and further that
the alarm should not give a false alarm when ethyl alcohol
is present at 1,000 ppm.
However, the prior sensors can n.ot cause an alarm unit
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CA 02353759 2001-07-25
to give an alarm by selectively sensing carbon monoxide or
hydrogen existing at the low concf~ntration. A tin (II)
oxide (Sn02) semiconductor sensor is excellent in its sen-
sitivity at the low concentration, but the sensor can not
sense any gas selectively at all, and even if the sensor is
temporally corrected with an activated carbon filter or the
like, its repeatability is lost dues to degradation of the
activated carbon, and the sensor becomes unavailable. The
contact combustion sensor detects temperature increase
caused when a combustible gas reacts with a catalyst and
burns on a heated coil as a change in the electric resist
ance, and there is not carbon monoxide sensor not having the
sensitivity to hydrogen even among those in which the sen
sitivity to a gas (output) is proportional to the gas con
centration.
It has generally been recognized that the hopcalite
catalyst comprising Mn02 by 70~ and Cu03 by 30~, or Mn02 by
50~, Cu0 by 30~, Co203 by 15~ and Ag~,O by 5~ is promising as
a catalyst capable of selectively oxidizing and detecting
carbon monoxide in a gas mixture comprising carbon monoxide
and hydrogen. As a carbon monoxide sensor, however, it has
low selectivity to the gas with poor sensitivity, and fur-
ther the catalyst is easily affected by poisoning and humid-
ity and the performance substantially changes as time pass-
es, so that the catalyst can not actually be used for any
industrial purposes.
In the fixed high molecular types of fuel cell used in a
fuel cell car now under development as an environment-
compatible car, hydrogen is used as the fuel. As a method
for production of hydrogen as the fuel, the hydrogen absorb-
ing alloys method, catalytic decomposition of gasoline,
methyl alcohol catalytic decomposition method, or other
methods have been used. Of these methods, the hydrogen
absorbing alloys method has the defect that the alloy used
in the method being a rare earth alloy such as La-Ni5 is
expensive and heavy, and that the alloy is depleted.
Although high purity hydrogen can be obtained by this meth-
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CA 02353759 2001-07-25
od, the price of the hydrogen fuel is expensive. In the
catalytic decomposition of gasoline, as the ratio of carbon
atoms to hydrogen atoms contained in gasoline is higher in
comparison to that in methanol, the method has the defect
that a large quantity of hydrogen can not be produced. What
is most expected among the methods listed above is the
method of catalytically decomposing methanol allowing easy
treatment of reactants in which methanol can be decomposed
with a catalyst at a low temperature. Unless carbon monox-
ide contained in the hydrogen gas obtained through catalytic
decomposition of methanol is removed,, electrodes of the fuel
cell are chemically degraded with the life becoming shorter,
and there has been the defect in the prior art that carbon
monoxide can not efficiently be removed from a gas mixture
comprising carbon monoxide and hydrogen.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a
catalyst capable of selectively oxidizing carbon monoxide in
a gas mixture comprising carbon monoxide and hydrogen as
well as a catalyst capable of select5.vely oxidizing hydrogen
in the gas mixture. It is another object of the present
invention to provide contact combustion carbon monoxide
sensor not having the sensitivity to hydrogen as well as a
contact combustion hydrogen sensor not having the sensitiv-
ity to carbon monoxide.
The present invention provides a catalyst obtained by
dispersing platinum black over a surface of a base metal
compound oxide containing Co203, Mr~02, CuO, and Cr203 to
form an Ag20 layer on the platinum black layer and charac-
terized in that the catalyst is capable of selectively
oxidizing carbon monoxide in a gas mixture comprising carbon
monoxide and hydrogen. In addition,, the present invention
provides a catalyst obtained by dispersing Ag20 over a
surface of the base metal compound oxide to form a platinum
black layer on the Ag20 layer and characterized in that the
catalyst is capable of selecting oxidizing hydrogen in a gas
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CA 02353759 2001-07-25
mixture comprising carbon monoxide and hydrogen. Further
the present invention provides a contact combustion carbon
monoxide sensor using the catalyst selectively oxidizing
carbon monoxide in its active section as well as a contact
combustion hydrogen sensor using the catalyst selectively
oxidizing hydrogen in the active section.
DETAILED DESCRIPTION OF THE INVENTION
In this invention, a base metal compound oxide contain
ing Co203, Mn02, CuO, and Cr203 each generating a small
heat of oxide formation (- D Ho) functions as a source to
AG2o, and activates the Ag catalyst. There is no specific
limitation aver composition of the base metal, but it is
preferable that the content of Mn02 is in the range from 44
to 55 weight ~, 2Cu0-Cr203 in the range from 25 to 40 weight
and Co203 in the range from 15 to 20 weight ~.
A catalyst capable of selectively oxidizing carbon
monoxide in a gas mixture comprising carbon monoxide and
hydrogen was obtained by dispersing platinum black parti-
cles over a surface of the base metal. compound oxide to form
an Ag2o layer on the surface. Also a catalyst capable of
selectively oxidizing hydrogen in the gas mixture above was
obtained by dispersing Ag20 over a surface of the base metal
compound oxide to form a platinum black layer on the sur-
face. These catalysts could repeatedly be used, and were
stable in use for a long time.
The catalyst produced by homogeneously milling the base
metal compound oxide, molding, activation, and then dis-
persing platinum black particles on. a surface and forming
an Ag20 layer on platinum black is capable of selectively
oxidizing carbon monoxide in a gas mixture comprising carbon
monoxide and hydrogen. The catalyst is more stable as
compared to the hopcalite. When the catalyst selectively
oxidizing only carbon monoxide is used as a fuel cell, the
catalyst oxidizes only carbon monoxide contained in the
hydrogen gas of the fuel obtained by decomposing methanol
with a catalyst such as Zn0-Cr203 or the like to carbon
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CA 02353759 2001-07-25
dioxide, and the resultant gas mixture does not give any
damage to electrodes of the fixed high molecular type of
fuel cell used for vehicles with a long life of the fuel
cell insured.
The catalyst selectively oxidizing carbon monoxide
according to the present invention is used in an active sec-
tion of a carbon monoxide sensor based on the contact com-
bustion system to form a sensor which can selectively detect
carbon monoxide. A catalyst was obtained by dispersing
platinum black particles on a surface formed by electrodepo-
siting an electrodeposition coating containing the base
metal compound oxide by 60 to 70 weight ~ and Y-A1203 by
30 to 40 weight ~ on a Pt line coil. or an Fe-Pd line coil
and calcinating the electrodeposited coating to form an Ag20
layer thereon. The contact combustion carbon monoxide
sensor having a bridge circuit in which a coil having this
catalyst is used in the active section is a contact combus-
tion carbon monoxide sensor providing output for carbon
monoxide in the range from 20 to 22 mV when the carbon
monoxide concentration is at 500 ppm or output for hydrogen
in the range from 0 to 1 mV when the hydrogen concentration
is at 500 ppm. This carbon monoxide sensor is that not
having the sensitivity to hydrogen even when carbon monoxide
and hydrogen coexist. The carbon monoxide sensor having the
sensitivity only to carbon monoxide can be used in applica-
tions such as city gas incomplete combustion alarm units,
alarm units based on practices in foreign countries, and
other security-related devices.
The hydrogen sensor according to the present invention
3~ is a contact combustion hydrogen sensor having a bridge
circuit in which a coil having a catalyst formed by dispers
ing Ag20 over a surface formed by electrically depositing an
electrodeposition coating comprising the base metal compound
oxide by 60 to 70 weight ~ and Y-A1203 by 30 to 40 weight
on a Pt or Fe-Pd line coil and then calcinating the compound
oxide is used, and have the sensitivity (output) of 0 to 1
mV for carbon monoxide at the concentration of 500 ppm and
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CA 02353759 2001-07-25
also of 35 to 40 mV for hydrogen at the concentration of 500
ppm. This hydrogen sensor does not show its sensitivity to
carbon monoxide even when carbon monoxide and hydrogen
coexist. This hydrogen sensor can detect hydrogen generated
when the trans-oil is degraded.
With the catalyst selectively o;~idizing carbon monoxide
and that selectively oxidizing hydrogen each according to
the present invention, it is possible to selectively detect
and oxidize carbon monoxide or hydrogen at the low concen-
tration, and the catalysts are extremely useful for provid-
ing a carbon monoxide sensor not having the sensitivity to
hydrogen which has been a defect of carbon monoxide sensors
based on the conventional technology as well as for reform-
ing hydrogen for a fuel cell.
The present invention is described in detail below with
reference to the embodiments. The ernbodiments are described
below only for the illustrative-purpose, and are not intend-
ed to limit the present invention in any means.
Example 1
Mn02 52 weight $
2Cu0-Cr203 32 weight ~
Co203 16 weight ~
A-mixture of the compounds above was mixed and pul-
verized with a ball mill and was then molded into a cylin-
drical body with the diameter of 3.0 mm and length of 3 mm,
which was calcinated under the temperature of 500 °C to
activate the mixture as an oxide catalyst. After the sur-
face was fully cleaned, the [Pt(NH3)4](N03)2(1:60) solution
was applied to the surface by dipping or blowing, the sur-
face was dried and subjected to thermal decomposition to
obtain platinum black, and then the AgN03 0.7 g/L solution
was applied to the surface by spraying or the like, which
was dried, subjected to thermal decomposition, washed with
water, and dried to obtain a catalyat. 20 L of mixed gas
comprising carbon monoxide at the 500 ppm and hydrogen at
500 ppm was passed through a vessel filled with this cata-
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CA 02353759 2001-07-25
lyst and having the diameter of 1 cm and length of 31 cm at
the flow rate of 22.5 L/min, and no carbon monoxide was
detected from the exhausted gas, and a gas mixture compris
ing carbon dioxide at the concentration of 500 ppm and
hydrogen at the concentration of 500 ppm was obtained.
Example 2
Mn02 30 weight ~
2Cu0-Cr203 18 weight ~
Co203 9 weight ~
y-A1203 43 weight ~
An electrodeposition coating containing the base
metal oxide and y-A1203 as described above was electrically
deposited on a coil formed by coiling an Fe-Pd line with the
diameter of 30 ~zm and having 22 turns each with the inner
diameter in the range from 0.8 to 1.0 mm. The compound
oxide was formed at the rate of about 0.02 g per coil.
After died under a low temperature, the coil was dried for
two hours under the temperature of 120 °C, and then was
calcinated for 15 to 20 minutes under the temperature of 500
°C to obtain a compound oxide. About 0.1 cc of the
[Pt(NH3)4](N03)2(1:60) solution was applied to a surface of
the compound oxide, which was dried and decomposed under the
temperature of 500 °C to give platinum black. The surface
was cleaned and dried, and then about 0.1 cc of the AgN03
0.7 g/L solution was applied to the platinum black with the
surface dried, subjected to thermal decomposition, and
washed, and the coil having the resu:Ltant catalyst was used
in an active section of a bridge circuit of the contact
combustion carbon monoxide sensor. when this sensor was
used as a 6V sensor based on the contact combustion system
under the conditions of sensor temperature in the range from
150 to 160 °C and bridge voltage of 6V for D.C 25 mA, the
sensor provided the output of 20 to :?2 mV for carbon monox-
ide at the concentration of 500 ppm, but it provided only
the output of 0 to 2 mV for hydrogen at the concentration of
500 ppm.
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Example 3
Mn02 29 weight ~
Cu0 17 weight ~
Co203 11 weight ~
Y-A1203 43 weight ~
The electrodeposition coating containing the base
metal oxide and y-A1203 as described above was adjusted and
was electrically deposited on a Fe-Pd 30 lzm line coil. More
specifically, the coating was electrically deposited on a
coil formed by coiling the Fe-Pd line with the diameter of
30 um with 22 turns each having they inner diameter in the
range from 0.8 to 1.0 mm so that a cfuantity of the compound
oxide was about 0.02 g for each coil. After dried under a
low temperature, further the coil was dried for two hours
under the temperature of 120 °C and calcinated for 15 to 20
minutes under the temperature of 500 °C to obtain an com-
pound oxide. After about 0.1 cc of the AgN03 0.7 g/L was
applied to a surface of the compound oxide and was dried,
the [Pt(NH3)4](N03)2(1:60) solution was adjusted, and about
p , 1 cc of the solution was applied to a surface of the
compound oxide, the surface was died and subjected to decom-
position under the temperature of 500 °C to form platinum
black, and a coil having the resultant catalyst was used in
an active section of a bridge circuit of a hydrogen sensor
based on the contact combustion system. When this sensor
was used as a 6 V sensor based on the contact combustion
system under the conditions of senior temperature in the
range from 150 to 160 °C, D.C voltage of 6 V and 25 mA, the
sensor provided the output of about 0 mV for carbon monoxide
at the concentration of 500 ppm, .and also provided the
output of 35 to 40 mV for hydrogen at the concentration of
500 ppm. When only the active section was subjected to the
aging test by applying the voltage of 3.5 V for 99 days
thereto, the average value of the sensitivity to hydrogen
(output) at the concentration of 500 ppm was 32.5 mV, and
that to carbon monoxide at the concentration of 500 ppm was
0.1 mV.
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