JPWO2004000458A1 - Selective oxidation catalyst for carbon monoxide in reformed gas - Google Patents

Abstract

It is a catalyst that selectively oxidizes carbon monoxide in the reformed gas with oxygen gas. One carbon monoxide selective oxidation catalyst comprises ruthenium and / or platinum supported on an alumina carrier containing α-alumina, and sodium contained in the alumina carrier is less than 0.07% in terms of Na 2 O. . The other carbon monoxide selective oxidation catalyst comprises ruthenium and / or platinum supported on an alumina support containing α-alumina, and further supported on a support containing magnesium, and ruthenium and / or Magnesium contained in the alumina carrier carrying platinum is less than 0.04% in terms of MgO.

Description

The present invention relates to a catalyst that selectively oxidizes carbon monoxide in a reformed gas. More specifically, the present invention relates to a catalyst in a reformed gas used in a fuel cell operating at a low temperature, particularly a polymer electrolyte fuel cell. The present invention relates to a catalyst that selectively oxidizes carbon oxide.
According to the catalyst of the present invention, carbon monoxide in the reformed gas is selectively oxidized, so that the fuel cell can be effectively operated even at a low temperature.

Conventionally, as a fuel gas for a fuel cell, in view of cost, a reformed gas obtained by steam reforming a natural gas hydrocarbon such as methane or propane, an alcohol such as methanol or naphtha is widely used. It has been. Such reformed gas contains carbon monoxide in addition to hydrogen, carbon dioxide, etc., and is known to contain about 1 vol% carbon monoxide even after being processed by a shift reaction. It has been.
Such by-product 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. Catalytic poisoning to the catalyst, especially in polymer electrolyte fuel cells that are operated at lower temperatures than phosphoric acid fuel cells, the catalyst poisoning due to carbon monoxide coexisting in the reformed gas is significant, and power generation efficiency There was a problem of decline.
For such problems, conventionally, alumina catalysts using various platinum group metals have been proposed.
However, in the alumina catalyst using such a platinum group metal, since the selectivity and activity of the oxidation reaction with oxygen is low, hydrogen that is the main component of the reformed gas and the fuel gas is wasted and oxidized at the same time. There was a problem of causing a decrease in fuel utilization efficiency.
In the polymer electrolyte fuel cell, in order to obtain the required power generation efficiency using the reformed gas, the coexisting carbon monoxide is reduced to about 1/100 or less from the initial about 1 vol%, and then supplied. However, in the conventional platinum-alumina-based catalyst, the carbon monoxide is not sufficiently reduced in oxidation, and the remaining carbon monoxide causes deterioration in power generation efficiency.
Furthermore, the present inventors have studied and added a carbon monoxide selective oxidation catalyst using alumina as a carrier. If the alumina carrier contains a certain amount or more of an oxide of a specific element, the activity of the selective oxidation catalyst is remarkably reduced. I found out that

The present invention has been made in view of such problems of the prior art, and an object of the present invention is to selectively oxidize and reduce carbon monoxide in the reformed gas, and to make good use of fuel. The object is to provide a carbon monoxide selective oxidation catalyst capable of realizing efficiency and power generation efficiency.
As a result of intensive studies to achieve the above object, the present inventors have supported ruthenium and platinum on an alumina carrier whose content of specific impurities is appropriately controlled, so that oxygen gas can be reduced relative to carbon monoxide. The inventors have found that excellent selective oxidation of carbon monoxide is performed under an excessively existing condition, and the present invention has been completed.
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,
Ruthenium and / or platinum is supported on an alumina support containing α-alumina, and sodium contained in the alumina support is less than 0.07% in terms of Na ₂ O.
Furthermore, another carbon monoxide selective oxidation catalyst of the present invention is a catalyst that selectively oxidizes carbon monoxide in the reformed gas with oxygen gas,
Ruthenium and / or platinum is supported on an alumina carrier containing α-alumina, and further this alumina carrier is supported on a support containing magnesium,
Magnesium contained in the alumina carrier supporting ruthenium and / or platinum is less than 0.04% in terms of MgO.
Furthermore, in another preferred form of the carbon monoxide selective oxidation catalyst of the present invention, sodium is contained in the above-mentioned ruthenium and / or platinum-supported alumina support, and the sodium content is 0.002 in terms of Na ₂ O. The reason why the selective oxidation catalyst of the present invention exhibits the excellent selective oxidation property of carbon monoxide (CO), which is characterized by being less than 07%, is not necessarily clear, but at present, it is presumed as follows: Is done.
That is, in the present invention, by using a support containing α-alumina, the catalytic metals ruthenium (Ru) and platinum (Pt) are made to be present in the vicinity of the outermost surface of the support.
Thus, by localizing the catalytic metal on the surface of the support, the temperature at which CO oxidation occurs can be shifted to the low temperature side, and the selectivity for other reactions can be improved. It seems that the concentration of CO in the gas can be reduced and the consumption of hydrogen can be prevented.
In general, when water vapor is mixed into the gas, water vapor adsorption occurs, and the temperature at which CO oxidation occurs is shifted to the high temperature side. By using α-alumina, the reaction temperature due to the adsorption is shifted to the high temperature side. 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.
Furthermore, in the CO selective oxidation catalyst of the present invention, by using highly pure α-alumina as a support, high CO oxidation activity can be expressed. If a metal such as sodium or a metal oxide thereof is contained, it is considered that the adsorption form of the reaction gas changes and the activity is greatly inhibited.
In the CO selective oxidation catalyst of the present invention, it is possible to use a so-called honeycomb-shaped support. However, magnesium or the like derived from such a honeycomb-shaped support is a causative substance that reduces the catalytic activity. It is desirable to control the amount of alpha alumina mixed to a certain value or less.
Hereinafter, the carbon monoxide selective oxidation catalyst of the present invention will be described in detail. In the present 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 the 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 the methanol reformed gas typically includes hydrogen gas. As components, carbon dioxide (CO ₂ ), methane (CH ₄ ), water (H ₂ O) and CO are included.
Of these, what is effective as an application object of the present invention is a reformed gas after the shift reaction, and has a CO concentration of about 1 vol%.
Next, the oxygen gas is not particularly limited as long as it exists in excess of the reaction equivalent with CO, but typically, oxygen of 1.1 to 5 times the reaction equivalent with CO is present. It is preferable.
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.
The selective oxidation catalyst of the present invention preferably contains Ru and Pt in a proportion of 0.01 to 10%. That is, the supported amount of the mixture of Ru and Pt is preferably 0.01 to 10% of the total obtained catalyst, and desirably 0.02 to 0.5%.
If the amount of the mixture supported is less than 0.02%, the CO oxidation activity may not be sufficient, and if it exceeds 0.5%, Ru and Pt may not be used effectively.
Furthermore, in the selective oxidation catalyst of the present invention, it is preferable that the particle diameter of at least one of Ru and Pt supported is 200 mm or less, desirably 5 to 200 mm.
When the particle diameter exceeds 200 mm, the oxidation activity of CO may not be sufficient, which is not preferable.
In the selective oxidation catalyst of the present invention, an alumina carrier is used. However, α alumina is contained, and the Ru, Pt, Ru-Pt mixture is within 100 μm, preferably within 20 μm from the surface of the alumina particles constituting the carrier. The shape is not limited as long as it is an alumina carrier that can be present.
If Ru or the like cannot be supported within 100 μm from the alumina particle surface, the Ru concentration on the catalyst surface becomes thin and the desired effect may not be obtained.
In the selective oxidation catalyst of the present invention, α-alumina can be suitably used as a support, because α-alumina does not have fine pores, so that the above localization can be easily realized. This is because the influence of water vapor can be reduced as described above.
Note that α-alumina may be used alone, but may be used in combination with other crystalline alumina.
In addition, γ-alumina transitions to α-alumina if kept at a temperature of 1000 ° C. or higher. However, if the temperature is kept at that temperature, the catalytic metals Ru and Pt cause sintering, and sufficient activity cannot be obtained. It is not suitable for use alone in the catalyst of the invention. However, before supporting catalytically active species such as Ru and Pt, alumina such as γ, θ, and η is heat-treated and phase-converted to α-alumina and used as a support, and various types of materials satisfying the above-mentioned characteristics. It is possible to use crystalline high-purity alumina and α-alumina in combination.
In the selective oxidation catalyst of the present invention, the alumina support preferably contains α-alumina at a very high concentration. When an impurity such as sodium (Na) or magnesium (Mg) is contained in a certain amount or more, the catalytic activity is increased. May interfere.
For example, the content of Na in the alumina carrier needs to be controlled to less than 0.07% in terms of Na ₂ O.
Further, the selective oxidation catalyst of the present invention can be in the form of particles or pellets, and further, a honeycomb-shaped support can be used. For example, a honeycomb-shaped support made of cordierite or metal can be used. It is also possible to coat and use a monolithic support.
However, in order to secure high concentration or high purity of the above-mentioned α-alumina and to exhibit high activity, magnesium (Mg) derived from cordierite, the content in the alumina carrier is less than 0.04% in MgO, respectively. It needs to be controlled.
The selective oxidation catalyst of the present invention has the above-described configuration and excellent CO selective oxidation properties. Typically, about 1 vol% CO coexisting in the reformed gas is oxidized and removed to about 100 ppm. To do.
The use conditions are not particularly limited, but a remarkable effect can be obtained if the space velocity (SV) is 30,000 h ⁻¹ or less and the catalyst temperature is 100 to 200 ° C.

  FIG. 1 is a graph showing the dependence of the catalytic performance of the CO selective oxidation catalyst on the Na mixing rate, and FIG. 2 is a graph showing the catalytic performance of the CO selective oxidation catalyst containing Mg.

EXAMPLES Hereinafter, although an Example and a comparative example demonstrate this invention further in detail, this invention is not limited to these Examples.
[Performance evaluation]
In the following examples and comparative examples, the performance of the obtained catalyst was evaluated by the following method.
(Evaluation conditions, etc.)
Evaluation device; fixed bed flow type SV; 50000h ^-1
CO concentration: 1 vol%
O _{2 /} CO; 2.0, 1.5 or 1.0 (volume ratio)
Hydrogen concentration: 40%
Water vapor concentration: about 30%
Carbon dioxide concentration; 20%
Methane concentration; 2%
Reaction temperature: 100-250 ° C
(Catalyst metal support particle size)
The catalyst was pulverized and the supported metal was directly observed with a transmission electron microscope to confirm the particle size.
(Localization of catalyst metal support surface)
The catalyst was divided almost in half, and the cross section was observed with EPMA to confirm the loading width.
Further, the surface of the catalyst was observed by XPS to confirm the noble metal concentration on the alumina surface.

About 0.2% of Ru was supported on high-purity α-alumina (alumina concentration: 99.995%) having an average particle diameter of about 2 mm to obtain the selective oxidation catalyst A of this example. In this catalyst, Ru was present at a depth of 50 μm from the outer surface of the α-alumina particles. The average particle size of Ru was about 100 mm.
Through this selective oxidation catalyst, a test gas (O ₂ /CO=1.5) in which 1.5 vol% of oxygen was similarly charged into a reformed gas containing 1 vol% of carbon monoxide was passed at SV = 50000 h ⁻¹ . The CO concentration was measured in the catalyst temperature (center temperature) range of 100 ° C to 250 ° C. The obtained results are shown in FIG.

A selective oxidation catalyst of this example was obtained by repeating the same operation as in Example 1 except that Na was mixed in the high-purity α-alumina at a ratio of 0.014% in terms of Na ₂ O. The CO concentration was measured in the same manner as in Example 1, and the results obtained are shown in FIG.
(Comparative Example 1)
A selective oxidation catalyst of this example was obtained by repeating the same operation as in Example 1 except that Na was mixed in the high-purity α-alumina at a rate of 0.07% in terms of Na ₂ O. The CO concentration was measured in the same manner as in Example 1, and the results obtained are shown in FIG.
1 that the selective oxidation catalysts of Examples 1 and 2 exhibit excellent CO selective oxidation activity. In particular, in the selective oxidation catalyst of Example 1, CO could be reduced to a target concentration in a wide temperature range from a low temperature to a high temperature range. On the other hand, in the selective oxidation catalyst of Comparative Example 1, the CO concentration was 10000 ppm unchanged from the time of supply, and the CO concentration was 3000 ppm in the range of 150 ° C. to 200 ° C. and the activity was low.
Na mixed in this selective oxidation catalyst is thought to affect the adsorption and desorption of gases involved in the CO selective oxidation reaction, resulting in a decrease in CO removal ability, but details are unknown at this time. It is.

The selective oxidation catalyst of this example is prepared by mixing 0.02% of Mg with high purity α-alumina (alumina concentration: 99.995%) with an average particle size of about 2 mm and supporting 0.2% of Ru. Obtained. A test gas (O ₂ /CO=2.0, 2.0 vol%, 1.5 vol%, and 1.0 vol%) of the selective oxidation catalyst in which oxygen is added to a reformed gas containing 1 vol% carbon monoxide. 1.5 and 1.0) are passed at SV = 50000 h ⁻¹ , the CO concentration is measured in the catalyst temperature (center temperature) range of 100 ° C. to 250 ° C., and the obtained results are shown in FIG.
(Comparative Example 2)
The selective oxidation catalyst of this example is prepared by mixing 0.04% of Mg with high purity α-alumina (alumina concentration 99.995%) with an average particle diameter of about 2 mm and further supporting about 0.2% of Ru. Obtained.
This selective oxidation catalyst was tested as in Example 3. The obtained results are shown in FIG.
From FIG. 2, regardless of O ₂ / CO, the CO selective oxidation ability of the selective oxidation catalyst of Comparative Example 2 is significantly lower than that of the selective oxidation catalyst of Example 3, and the temperature range in which the target CO concentration can be reached is narrow. It was.
As mentioned above, although this invention was demonstrated in detail by the preferred embodiment, this invention is not limited to these Examples, A various deformation | transformation implementation is possible within the range of this indication.
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, and can be used to reduce CO in the reformed gas. It can also be applied to various processes such as synthesis of ammonia that requires high-purity hydrogen gas.

As described above, according to the present invention, ruthenium and platinum are supported on an alumina support whose specific impurity content is appropriately controlled, so that carbon monoxide in the reformed gas is selectively oxidized. Therefore, it is possible to provide a carbon monoxide selective oxidation catalyst that can be reduced and realize good fuel utilization efficiency and power generation efficiency.
For example, by using the catalyst of the present invention, if carbon monoxide present in the reformed gas at about 1 vol% is reacted at about 150 ° C. in the presence of excess oxygen, the carbon monoxide concentration is 0.1 vol%. The following can be reduced.

Claims (3)

A catalyst that selectively oxidizes carbon monoxide in the reformed gas with oxygen gas,
Carbon monoxide comprising ruthenium and / or platinum supported on an alumina support containing α-alumina, and the sodium contained in the alumina support being less than 0.07% in terms of Na ₂ O Selective oxidation catalyst. A catalyst that selectively oxidizes carbon monoxide in the reformed gas with oxygen gas,
Ruthenium and / or platinum is supported on an alumina support containing α-alumina, and the alumina support is supported on a support containing magnesium,
A carbon monoxide selective oxidation catalyst, wherein magnesium contained in the alumina carrier supporting ruthenium and / or platinum is less than 0.04% in terms of MgO. 3. The carbon monoxide selection according to claim 2, wherein sodium is contained in the alumina carrier carrying ruthenium and / or platinum, and the sodium content is less than 0.07% in terms of Na ₂ O. Oxidation catalyst.

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