JP2004181425A - Selective oxidation catalyst for carbon monoxide in reformed gas - Google Patents

Abstract

An object of the present invention is to provide a carbon monoxide selective oxidation catalyst capable of selectively oxidizing and reducing carbon monoxide in a reformed gas and realizing good fuel use efficiency and power generation efficiency.
The catalyst selectively oxidizes carbon monoxide in a reformed gas with oxygen gas. Ruthenium or platinum is supported on an alumina support containing α-alumina. The pregelatinized ratio of this alumina carrier is 10 to 85%. Ruthenium and platinum are contained at a ratio of 0.01 to 2%.
By using this selective oxidation catalyst, the carbon monoxide concentration can be reduced to 100 ppm or less by reacting carbon monoxide present in the reformed gas at about 1 vol% in the presence of an excessive amount of oxygen.
[Selection diagram] None

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a catalyst for selectively oxidizing carbon monoxide in a reformed gas, and more particularly, to a catalyst in a reformed gas used for a fuel cell operating at a low temperature, particularly a polymer electrolyte fuel cell. The present invention relates to a catalyst for selectively oxidizing carbon oxide.
According to the catalyst of the present invention, carbon monoxide in the reformed gas is selectively oxidized, so that such a fuel cell can be effectively operated even at a low temperature.
[0002]
[Prior art]
Conventionally, as a fuel gas for a fuel cell, a reformed gas obtained by steam reforming a hydrocarbon of natural gas such as methane or propane, an alcohol such as methanol or naphtha, etc. is widely used in consideration of cost. Have been. Such a reformed gas contains carbon monoxide in addition to hydrogen and carbon dioxide, and even after being subjected to the shift reaction, it may contain about 1% by volume of carbon monoxide. Are known.
[0003]
Such by-produced carbon monoxide is also used as a fuel in a high-temperature operation type fuel cell such as a molten carbonate type, but in a phosphoric acid type or solid polymer type low temperature operation type fuel cell, a platinum-based electrode catalyst is used. It exhibits catalytic poisoning effect on the catalyst, especially in polymer electrolyte fuel cells that operate at lower temperatures than phosphoric acid fuel cells, and the poisoning of the catalyst by the carbon monoxide coexisting in the reformed gas is remarkable, resulting in power generation efficiency. The problem of the decrease in the temperature has arisen.
To cope with such a problem, conventionally, an alumina catalyst in which various platinum group metals are supported on various alumina carriers has been proposed (for example, see Patent Document 1).
[0004]
[Patent Document 1]
JP-A-5-201702
[Problems to be solved by the invention]
However, in such an alumina catalyst using a platinum group metal, since the selectivity and activity of the oxidation reaction by oxygen are low, hydrogen which is a main component of the reformed gas and becomes a fuel gas is oxidized and wasted at the same time. There is a problem that fuel efficiency is reduced.
In the case of a polymer electrolyte fuel cell, in order to obtain the required power generation efficiency while using a reformed gas, the coexisting carbon monoxide is reduced from about 1 vol% at the beginning to about 1/100 or less thereof, and then supplied. However, in the conventional platinum-alumina-based catalyst, the reduction of carbon monoxide oxidation is not sufficient, and the remaining carbon monoxide causes deterioration of power generation efficiency.
[0006]
The present invention has been made in view of such problems of the related art, and an object of the present invention is to selectively oxidize and reduce carbon monoxide in a reformed gas to achieve good fuel utilization. An object of the present invention is to provide a carbon monoxide selective oxidation catalyst capable of realizing efficiency and power generation efficiency.
[0007]
[Means for Solving the Problems]
The present inventors have conducted intensive studies to achieve the above object, and as a result, have found that by appropriately controlling the amount of α-alumina in an alumina carrier, excellent selective oxidation of carbon monoxide can be realized. Thus, the present invention has been completed.
[0008]
That is, the carbon monoxide selective oxidation catalyst of the present invention is a catalyst that selectively oxidizes carbon monoxide in the reformed gas with oxygen gas,
It is characterized in that ruthenium and / or platinum are supported on an alumina carrier containing α-alumina, and the alumina carrier has an α conversion of 10 to 85%.
[0009]
Further, a preferred embodiment of the carbon monoxide selective oxidation catalyst of the present invention is characterized in that it contains ruthenium and / or platinum at a ratio of 0.01 to 2%.
[0010]
[Action]
The details of the reason why the selective oxidation catalyst of the present invention exhibits excellent selective oxidation property of carbon monoxide (CO) are not necessarily clear, but at the present time, it is presumed as follows. That is, in the present invention, by using a carrier containing α-alumina, ruthenium (Ru) and platinum (Pt) as catalyst metals are present near the outermost surface of the carrier.
Thus, by localizing the catalytic metal on the surface of the support, the temperature at which CO oxidation occurs can be shifted to a lower temperature side, and the selectivity to other reactions can be improved. It is considered that the CO concentration in the raw gas can be reduced and the consumption of hydrogen can be prevented.
[0011]
In general, when water vapor is mixed into a gas, water vapor adsorption occurs and the temperature at which CO oxidation occurs is shifted to a high temperature side. However, by using α-alumina, the reaction temperature is shifted to a high temperature side due to the adsorption. Can be avoided. As a result, it is considered that the selectivity of CO oxidation can be improved, the CO concentration in the reformed gas after the reaction can be reduced, and the consumption of hydrogen can be prevented.
[0012]
Further, in the present invention, the abundance ratio of useful α-alumina in the carrier as described above was controlled by the α-conversion rate described later.
[0013]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, the carbon monoxide selective oxidation catalyst of the present invention will be described in detail. In addition, in this specification, "%" indicates a mass percentage unless otherwise specified.
As described above, the carbon monoxide selective oxidation catalyst of the present invention is a catalyst that selectively oxidizes CO in reformed gas with oxygen gas.
Here, the reformed gas generally refers to a gas obtained by steam reforming a hydrocarbon such as methane or propane, an alcohol such as methanol, or naphtha, and typically, a methanol reformed gas is mainly a hydrogen gas. As a component, carbon dioxide (CO ₂ ), methane (CH ₄ ), water (H ₂ O) and CO are included.
It is to be noted that an effective object to which the present invention is applied is a reformed gas after the shift reaction, and a CO concentration of about 1 vol%.
[0014]
Next, the oxygen gas is not particularly limited as long as it is present in excess of the reaction equivalent with CO, but typically, oxygen is present in an amount of 1.1 to 5 times the reaction equivalent with CO. Is preferred.
If it is less than 1.1 times, unoxidized CO remains, and if it exceeds 5 times, the consumption of hydrogen may increase, which is not preferable.
[0015]
The selective oxidation catalyst of the present invention preferably contains Ru and Pt at a ratio of 0.01 to 2%. That is, the supported amount of the mixture of Ru and Pt is preferably 0.01 to 2%, and more preferably 0.02 to 0.5% of the whole obtained catalyst.
If the supported amount of the mixture is less than 0.02%, the oxidation activity of CO may not be sufficient, and if it exceeds 0.5%, Ru and Pt may not be effectively used.
[0016]
Further, in the selective oxidation catalyst of the present invention, the particle size of at least one of Ru and Pt carried is preferably 200 ° or less, more preferably 5 to 200 °. If the particle size exceeds 200 °, the oxidation activity of CO may not be sufficient, which is not preferable.
[0017]
In the selective oxidation catalyst of the present invention, an alumina carrier is used. This alumina carrier must contain α-alumina, and its α-formation ratio must be 10 to 85%.
Here, the “α conversion rate” means a relative peak intensity of a peak inherent to α alumina (corresponding to the (113) plane) that appears in an X-ray diffraction pattern when γ alumina undergoes a phase transition to α alumina by heating. The peak intensity at the time when the α dislocation is completed is expressed as an α conversion rate of 100%.
If the ratio of α becomes out of the above range, the intended CO reduction effect cannot be obtained.
[0018]
Further, as the above alumina carrier, it is preferable that Ru, Pt, and Ru-Pt mixture can be present within 100 μm, preferably within 20 μm from the outer surface of the carrier, but the shape is not particularly limited. .
If Ru or the like cannot be supported within 100 μm from the outer surface of the carrier, the Ru concentration on the catalyst surface becomes too low, and the desired effect may not be obtained.
[0019]
In the selective oxidation catalyst of the present invention, γ-alumina is converted to α-alumina when kept at a temperature of 1000 ° C. or higher, but when kept at that temperature, the catalytic metals Ru and Pt cause sintering, and sufficient activity is obtained. It is not suitable for use alone in the catalyst of the present invention since it is no longer obtainable. However, prior to supporting catalytically active species such as Ru and Pt, alumina is subjected to heat treatment such as γ, θ, η, etc., and is converted to α-alumina using the above-mentioned α conversion as a scale. It is possible.
[0020]
Further, the selective oxidation catalyst of the present invention can be in the form of granules or pellets, and further, it is possible to use a honeycomb-shaped support, for example, a honeycomb-shaped honeycomb made of cordierite or metal. It is also possible to coat and use an integral structure type support.
[0021]
The selective oxidation catalyst of the present invention has the above-described configuration and has excellent CO selective oxidation properties. However, typically, about 1 vol% of CO coexisting in the reformed gas is removed by oxidation to about 100 ppm. I do.
The use conditions are not particularly limited, but a remarkable effect can be obtained when the space velocity (SV) is 30,000 h ⁻¹ or less and the catalyst temperature is 100 to 200 ° C.
[0022]
【Example】
Hereinafter, the present invention will be described in more detail with reference to Examples and Comparative Examples, but the present invention is not limited to these Examples.
[0023]
[Performance evaluation]
In the following Examples and Comparative Examples, the performance of the obtained catalyst was evaluated by the following method.
[0024]
(CO selective oxidation performance)
・ Evaluation device: fixed bed flow type ・ Simulated gas composition CO: 1 vol%
CO ₂ : 22 vol%
CH ₄ : 3 vol%
H ₂ : 74 vol%
-Simulated gas flow rate: 830 ml / min
・ Air flow rate: 108ml / min
・ H ₂ O flow rate: 0.43 ml / min
· _O 2 / CO ratio: was varied from 1 to 3.
・ Amount of catalyst used: 1.67 ml
・ Reaction temperature: 110-210 ° C (20 ° C increments)
[0025]
(Example 1)
After heating γ-alumina having an average particle size of about 2 mm in air at 1100 ° C. for 3 hours, about 2% of Ru was supported thereon to obtain a selective oxidation catalyst of this example. X-ray diffraction analysis of the selective oxidation catalyst revealed that the degree of pregelatinization was 16%.
This selective oxidation catalyst was subjected to the above-described performance evaluation, and the obtained results are shown in FIG. In FIG. 1, numerical values and symbols such as “110” on the right side indicate the reaction temperature.
[0026]
(Example 2)
After heating γ-alumina having an average particle size of about 2 mm in air at 1100 ° C. for 6 hours, about 2% of Ru was supported thereon to obtain a selective oxidation catalyst of this example. X-ray diffraction analysis of the selective oxidation catalyst revealed that the degree of pregelatinization was 80%.
This selective oxidation catalyst was subjected to the above-described performance evaluation, and the obtained results are shown in FIG. In FIG. 2, numerical values and symbols such as “110” on the right side indicate the reaction temperature.
[0027]
(Comparative Example 1)
The same operation as in Example 1 was repeated except that the heating time was set to 0 hour, to obtain a catalyst of this example. The α conversion was 0%.
The same performance evaluation as above was performed, and the obtained results are shown in FIG.
[0028]
(Comparative Example 2)
The same operation as in Example 1 was repeated except that the heating time was set to 24 hours, to obtain a catalyst of this example. The α conversion was 100%.
The same performance evaluation as above was performed, and the obtained results are shown in FIG.
[0029]
1 to 4 show that the selective oxidation catalysts of Examples 1 and 2 belonging to the scope of the present invention exhibit higher selective oxidation activity than the oxidation catalysts of Comparative Examples 1 and 2.
[0030]
As described above, the present invention has been described in detail with reference to the preferred embodiments. However, the present invention is not limited to these embodiments, and various modifications can be made within the scope of the disclosure of the present invention.
For example, the use of the selective oxidation catalyst of the present invention is not limited to the reformed gas supplied to the polymer electrolyte fuel cell, but can also be used to reduce CO in other reformed gases, The present invention is also applicable to various processes such as synthesis of ammonia requiring high-purity hydrogen gas.
[0031]
【The invention's effect】
As described above, according to the present invention, since the amount of α-alumina in the alumina carrier is appropriately controlled, carbon monoxide in the reformed gas is selectively oxidized and reduced, and It is possible to provide a carbon monoxide selective oxidation catalyst that can realize high fuel use efficiency and power generation efficiency.
For example, by using the catalyst of the present invention, if carbon monoxide present in about 1 vol% in the reformed gas is reacted at about 150 ° C. in the presence of an excessive amount of oxygen, the carbon monoxide concentration can be reduced to 100 ppm or less. can do.
[Brief description of the drawings]
FIG. 1 is a graph showing catalytic performance of a CO selective oxidation catalyst of Example 1.
FIG. 2 is a graph showing catalytic performance of a CO selective oxidation catalyst of Example 2.
FIG. 3 is a graph showing the catalytic performance of a CO selective oxidation catalyst of Comparative Example 1.
FIG. 4 is a graph showing the catalytic performance of a CO selective oxidation catalyst of Comparative Example 2.

Claims (2)

A catalyst for selectively oxidizing carbon monoxide in a reformed gas with oxygen gas,
A selective oxidation catalyst for carbon monoxide, comprising ruthenium and / or platinum supported on an alumina carrier containing α-alumina, wherein the alumina carrier has an α conversion of 10 to 85%. The carbon monoxide selective oxidation catalyst according to claim 1, wherein ruthenium and / or platinum are contained at a ratio of 0.01 to 2%.

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