JP2004066003A - Catalyst for water gas shift reaction of fuel reformed gas - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a catalyst for water gas shift reaction for producing hydrogen from carbon monoxide and water vapor in a gas mixture containing carbon monoxide, carbon dioxide and water vapor as well as hydrogen. <P>SOLUTION: The catalyst for water gas shift reaction of a fuel reformed gas comprises gold and a metal oxide including a metal which of the cation has an electronegativity in the range of 10-14. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a water gas shift reaction catalyst for producing carbon dioxide and hydrogen from carbon monoxide and water in reformed hydrogen gas of various fuels.
[0002]
[Prior art]
The water gas shift reaction is a reaction that plays an important role in an industrial hydrogen production process, and is represented by the following reaction formula.
[0003]
CO + H ₂ O → CO ₂ + H ₂
This reaction is used in a methanol synthesis plant, an ammonia synthesis plant, or the like to increase the amount of hydrogen in the methane reformed gas (H ₂ + CO + CO ₂ ) and adjust the H ₂ / CO ratio.
[0004]
In particular, when high-purity hydrogen is required, this reaction is usually performed in the following two-stage reaction. In the one-stage reaction, the CO concentration can be reduced to 2 to 3% by performing the reaction in a temperature range of 310 to 450 ° C. using a high-temperature shift catalyst such as an Fe—Cr mixed oxide catalyst. In the two-stage reaction, the CO concentration can be reduced to 1% or less by performing the reaction in a temperature range of 210 to 240 ° C. using a low-temperature shift catalyst such as a Cu—Zn—Al composite oxide catalyst.
[0005]
In a fuel cell using various fuels such as methane and gasoline, hydrogen is produced by the following process.
[0006]
Fuel → [fuel reformer] → hydrogen → [fuel cell]
The reaction in the fuel reformer requires a catalyst, and is usually performed in the following three stages of catalytic reaction. Of these catalytic reactions, the latter two reactions (2) and (3) have a main purpose of removing CO in hydrogen gas. In particular, in the polymer electrolyte fuel cell, when the concentration of carbon monoxide in the electrode catalyst of the fuel cell becomes several tens of ppm or more, the electrode is poisoned, and the performance is greatly reduced. Therefore, CO removal is particularly important.
(1) Steam reforming reaction (or partial oxidation reaction): Various fuels are reacted with steam (or oxygen) to reform into a mixed gas of CO, CO ₂ , H ₂ , and H ₂ O.
(2) Water gas shift reaction (CO shift reaction): Hydrogen and CO ₂ are generated by the reaction of the remaining CO and water.
(3) CO selective oxidation reaction (or methanation reaction): The still remaining CO is reacted with oxygen or hydrogen and removed as CO ₂ or methane.
[0007]
The CO concentration after the steam reforming reaction varies greatly depending on the type of fuel. Typical values are 0.8% for methanol, 11% for methane, 10 to 14% for ethanol, and 10 to 14% for gasoline and other long-chain hydrocarbons. 20% (LF Brown, Int. J. Hydrogen Energy, 26 (2001) 381). In any case, the CO concentration in the second-stage water gas shift reaction is reduced to 1% or less, more preferably 0.5% or less, and the CO concentration is finally reduced to 10 ppm in the third-stage reaction. To be reduced.
[0008]
The fuel cell has been attempted to be applied to a small fuel cell for home use or the like, and for that purpose, it is necessary to reduce the size of the fuel reformer. However, the water gas shift reaction of the two-stage reaction (2) has a slower reaction rate than the one-stage reaction (1) and the three-stage reaction (3), and requires a large amount of catalyst, so that the fuel cell becomes large. Would. Therefore, a water gas shift catalyst that reacts at a high speed is desired.
[0009]
It is advantageous to carry out the reaction at a high temperature in order to react at a high speed. However, the water gas shift reaction is restricted by chemical equilibrium, and the higher the temperature, the lower the theoretically achievable upper limit of the CO removal rate. For this reason, in order to reduce the CO concentration, it is necessary to carry out the reaction at a low temperature, and a catalyst that reacts at a high speed at a low temperature is desired. For example, in order to reduce the CO concentration at the outlet of the shift reactor to 0.5% or less, it is necessary to set at least the reactor outlet temperature to around 200 ° C. A catalyst that can reach an equilibrium reaction rate is required.
[0010]
As a water gas shift catalyst capable of reducing the CO concentration in such a low temperature range, a copper catalyst is mentioned as a catalyst which has been used so far. However, in a copper-based catalyst, copper is kept in a reduced state when the catalyst is in operation, but when the operation of the fuel cell system is stopped, the catalyst comes into contact with air to become copper oxide, and when the fuel cell system is restarted, copper oxide is generated. However, there is a problem that copper oxide is reduced and heat is generated, so that the catalyst is thermally degraded.
[0011]
Recently, various catalysts based on a noble metal such as platinum have been studied for this purpose, and among them, catalysts supporting platinum on ceria have been widely studied as candidates. However, Zalc et al. In Journal of Catalysis 206, 169-171 (2002) point out the following important points. (1) Many researchers evaluate the activity of shift catalysts using a reaction gas consisting only of carbon monoxide, water vapor, and diluent gas. The composition of the reformed gas in which carbon dioxide coexists is significantly different from that of the reformed gas and is not suitable for practicality evaluation. (2) When the reaction of a platinum-ceria catalyst is actually evaluated in a reformed gas composition, for example, assuming that the catalyst is used for on-board reforming of an automobile, the reaction rate is not sufficient and the activity decreases with time. Poor practicality.
[0012]
Therefore, it is necessary to evaluate the activity of the water gas shift catalyst using a mixed gas having a composition of a reformed gas suitable for practicality evaluation.
[0013]
[Problems to be solved by the invention]
An object of the present invention is to provide a water gas shift reaction catalyst that generates hydrogen from carbon monoxide and water vapor in a mixed gas containing carbon monoxide, carbon dioxide, and water vapor in addition to hydrogen.
[0014]
[Means for Solving the Problems]
In view of the above-mentioned problems of the prior art, the present inventor has conducted intensive studies on a catalyst for a water gas shift reaction, and as a result, a catalyst comprising gold and a specific metal oxide has a low temperature of around 200 ° C. or lower. And has a high activity in a water gas shift reaction in which CO and water contained in a mixed gas composed of hydrogen, carbon monoxide, carbon dioxide, water vapor and a diluent gas corresponding to a fuel reforming gas are reacted to produce CO ₂ and hydrogen. To complete the present invention.
[0015]
That is, the present invention provides the following water gas shift catalyst.
[0016]
Item 1. A water gas shift reaction catalyst for a fuel reforming gas comprising a metal oxide containing a metal having a cation electronegativity in the range of 10 to 14 and gold.
[0017]
Item 2. Item 2. The catalyst according to item 1, wherein the fuel reformed gas is a mixed gas containing hydrogen, carbon monoxide, carbon dioxide, and water vapor, and having a carbon monoxide concentration of about 1 to 25 vol%.
[0018]
Item 3. The metal oxide is (i) a metal oxide of a single metal selected from the group consisting of titanium oxide, zirconium oxide, and cerium oxide, and (ii) two or more kinds selected from the group consisting of titanium, zirconium, and cerium. Item 3. The catalyst according to item 1 or 2, which is a mixed oxide of a metal or a mixture of at least one of (iii) and (i) and at least one of (ii).
[0019]
BEST MODE FOR CARRYING OUT THE INVENTION
The present invention relates to a water gas shift reaction catalyst which comprises a metal oxide and gold and which can achieve a reduction in the CO concentration of a fuel reformed gas.
[0020]
As the metal oxide in the present invention, among the oxide species generally selected for the purpose of carrying gold in a dispersed manner, those having an appropriate acid-base property are preferable. In order for formate species, which is a reaction intermediate of the water gas shift reaction, to be stably generated on the catalyst surface, the catalyst surface needs to be basic to some extent. On the other hand, in order to promote the desorption of CO ₂ generated from the catalyst surface and to weaken the re-adsorption of the CO ₂ component coexisting in the reaction gas to the catalyst surface, it is not preferable that the basicity of the catalyst surface is too strong. .
[0021]
When the electronegativity of metal cations proposed by Tanaka et al. (Tanaka, Ozaki, Tamaru, Catalysis, Vol. 6, p. 262 (1964)) is used as an index of the acid-base property of oxides, It has been found that a metal oxide composed of oxygen and a metal having an electronegativity in the range of 10 to 14 is suitable as a carrier. As the metal oxides corresponding to this, Al ₂ O ₃ (10.5), CeO ₂ (10.8), Cr ₂ O ₃ (11.2), Cr ₂ O ₃ (11.2), Ga ₂ O ₃ (11.2), In ₂ O ₃ (11.9), Co ₂ O ₃ (12.6), Fe ₂ O ₃ (12.6), NiO (12.6), ZrO ₂ (12. 6), Bi ₂ O ₃ (13.3), MnO ₂ (13.5), TiO ₂ (13.5) and the like. (The numbers in parentheses indicate the electronegativity of the metal cation in the oxide). These are suitable as carriers for the water gas shift gold catalyst, but oxides having a lower electronegativity (eg, ZnO (8.0)) or oxides having a higher electronegativity (eg, SiO 2 (16 When 2)) is used as a carrier, the activity is low.
[0022]
Among these oxide groups, a metal oxide containing at least one metal selected from the group consisting of titanium, zirconium, and cerium is more preferable. Specifically, a metal oxide of a single metal selected from the group consisting of titanium oxide, zirconium oxide, and cerium oxide, a composite oxide of two or more metals selected from the group consisting of titanium, zirconium, and cerium, and the like Is mentioned. If necessary, it is also possible to use a mixture of at least one of the above-mentioned single metal metal oxides and at least one of the composite oxides. Among them, preferably, titanium oxide, cerium oxide, zirconium oxide, cerium-zirconium composite oxide, cerium-lanthanum composite oxide, and the like, more preferably, titanium oxide, cerium oxide, cerium-zirconium composite oxide, etc. Titanium oxide, cerium oxide, and cerium-zirconium composite oxide are particularly preferable from the viewpoint of high catalytic activity and stability.
[0023]
The shape of the metal oxide is not particularly limited. For example, the metal oxide may be used in a powdered state, in a pre-molded state or in a state fixed to various supports.
[0024]
Further, as the water gas shift reaction catalyst of the present invention, a gold-immobilized oxide in which gold is immobilized on a metal oxide is particularly preferable. In the case where gold is fixed on the oxide in this manner, the contact area between the gold and the oxide increases, and excellent catalytic performance can be exhibited. Even when gold is immobilized on the oxide, the gold is preferably in the form of fine particles having an average particle size of about 2 to 10 nm, more preferably about 2 to 4 nm.
[0025]
As a method for immobilizing gold on the metal oxide, a known method can be used. For example,
Impregnation method (GC Bond and PA Sermon, Gold Bull. 102 (1973) 6, etc.)
・ Co-precipitation method (JP-A-60-238148)
-Precipitation precipitation method (JP-A-62-155937, JP-A-3-97623, JP-A-63-252908, JP-A-2-252610, etc.)
-Colloid mixing method (Tsubota S., Nakamura T., Tanaka K., and Haruta M., Catal. Lett., 56 (1998) 131)
-Gas phase grafting method (Japanese Patent Application Laid-Open No. 9-122478)
Liquid phase grafting (Okumura M., and Haruta M., Chem. Lett., (2000) 396)
And the like.
[0026]
The following compounds may be mentioned as starting materials. Examples of the gold precursor include compounds that are vaporized by heating, such as a water-soluble gold compound (eg, chloroauric acid) and an acetylacetonato complex (eg, a gold acetylacetonato complex).
[0027]
Examples of the raw material of the metal oxide include nitrates, sulfates, acetates, and chlorides of various metals. Specific examples include nitrates such as cerium nitrate and zirconium nitrate, sulfates such as titanium sulfate, and chlorides such as cerium chloride, titanium trichloride, and titanium tetrachloride.
[0028]
After depositing the precipitate by the known method described above, the precipitate is dried. In order to finally bring gold into a metallic state, the precipitate may be heat-treated in an oxygen atmosphere or a reducing gas. The term “under an oxygen atmosphere” refers to an atmosphere under air or a mixed gas obtained by diluting oxygen with nitrogen, helium, argon, or the like. As the reducing gas, for example, about 1 to 10 vol% of hydrogen gas diluted with nitrogen gas, carbon monoxide gas, or the like can be used. The heat treatment temperature may be appropriately selected from the range of known reduction conditions, and is usually preferably about room temperature to 600 ° C., and more preferably about 200 to 400 ° C. for obtaining stable and fine gold particles. The heat treatment time is preferably, for example, about 1 to 12 hours.
[0029]
Further, a method of reducing the precipitate by contacting the precipitate with a reducing agent in a liquid phase can also be adopted. Examples of the liquid phase include water, methanol and the like. Examples of the reducing agent include hydrazine and sodium citrate. The reaction temperature is preferably, for example, about room temperature to about 100 ° C. The reaction time is preferably, for example, about 1 minute to 10 hours.
[0030]
The content of gold in the catalyst of the present invention may be about 0.1 to 30% by weight based on the total amount of the catalyst, and is about 0.1 to 10% by weight from the viewpoint of activity per metal amount. It is preferred that
[0031]
In the present invention, in order to use the catalyst in a more practical form, the catalyst can be supported on a support having various shapes. Examples of the support carrier include alumina, silica, cordierite, zeolite, and titanium oxide. The shape of the carrier is not particularly limited, and for example, all shapes generally used as a catalyst carrier at present, such as powder, sphere, granule, honeycomb, foam, fiber, cloth, plate, and ring, can be used. Can be used.
[0032]
The catalyst of the present invention can be used mainly as a water gas shift catalyst for a reformed gas produced by reacting various fuels (eg, methane, methanol, ethanol, gasoline, etc.) with water vapor (or oxygen). Specific examples of the reformed gas include a mixed gas containing four kinds of gases of carbon monoxide, water vapor, carbon dioxide, and hydrogen, and further diluted with an inert gas (for example, nitrogen, helium, argon, or the like). May be. The catalyst of the present invention can be effectively used for the purpose of generating hydrogen by reducing the concentration of carbon monoxide (vol%) in the reformed gas. The catalyst of the present invention exhibits activity in a wide range of the concentration of carbon monoxide in the mixed gas from a low concentration to a high concentration. It shows activity in any composition up to the concentration range of about 20 to 25 vol% assumed for gas.
[0033]
The pressure of the water gas shift reaction using the catalyst of the present invention is not particularly limited, and it can be used from a normal pressure to a high pressure condition of 100 atm. The use temperature varies depending on the type of metal oxide used and other conditions, but may be, for example, about 100 to 450 ° C. The catalyst of the present invention exhibits a higher water gas shift reaction activity than a conventional catalyst at a reaction temperature of about 100 ° C to 200 ° C, and is effectively used for the purpose of reducing CO in a mixed gas. Also, the range of the space velocity is not particularly limited, and the space velocity may be set according to the target CO reduction rate.
[0034]
Although depending on the inlet CO concentration and the space velocity condition, the use of the catalyst of the present invention makes it possible to reduce the carbon monoxide concentration in the reformed gas to about 10 to 5000 ppm at a reaction temperature of about 100 to 200 ° C. .
[0035]
【The invention's effect】
The water gas shift reaction catalyst of the present invention has a higher conversion rate of CO in a low temperature range of about 100 ° C. to 200 ° C. than the conventional catalyst, and allows the water gas shift reactor to be downsized.
[0036]
Also, in household fuel cell systems and the like, it is known that the catalyst comes into contact with air when the equipment is stopped at night, and metal species such as copper are oxidized, leading to a decrease in activity. INDUSTRIAL APPLICABILITY The catalyst of the present invention has oxidation resistance, and can be effectively used for applications to fuel cell devices that repeatedly start and stop on a daily basis.
[0037]
【Example】
Hereinafter, examples will be shown to make the features of the present invention clearer, but the present invention is not limited thereto.
[0038]
Example 1
1 mmol of chloroauric acid [HAuCl ₄ .4H ₂ O] was dissolved in 500 ml of distilled water, and the pH was adjusted to 7 by dropwise addition of an aqueous KOH solution. To this, 6.5 g of cerium oxide powder was added, and the mixture was stirred at 70 ° C. for 1 hour. Thereafter, the precipitate was sufficiently washed with distilled water, dried, and calcined in air at 400 ° C. for 4 hours to obtain a gold-supported cerium oxide catalyst [Au / CeO ₂ , gold supported amount 3 wt%]. . Subsequently, using the obtained catalyst, the activity against the water gas shift reaction was examined by the following method.
[0039]
Using 0.12 g of the above catalyst, a raw material gas (CO 1.3%, H ₂ O 3.1%, CO ₂ 0.4%, H ₂ 5.0%, He 90.2%: mixed gas in a volume ratio of 90.2%) ) Was passed at normal pressure at a flow rate of 103 ml / min, the temperature was once raised to 350 ° C. in the reaction gas to reduce the catalyst, and the catalytic activity for the water gas shift reaction at each temperature was examined. The result is shown in FIG. FIG. 1 shows the CO conversion as an index of the catalyst activity with respect to the reaction temperature.
[0040]
Comparative Example 1
The reaction was carried out under the same conditions as in Example 1 using a commercially available Cu / ZnO / Al ₂ O ₃ catalyst. The results are shown in FIG.
[0041]
Comparative Example 2
Chloroplatinic acid _{[H 2 PtCl 6 · 6H 2} O] A catalyst was prepared in the same conditions as in Example 1 by using in place of chloroauric acid, a platinum-supporting ceric oxide catalyst [Pt / CeO _2, amount of platinum supported 3wt %]. Using the obtained catalyst, a reaction was conducted under the same conditions as in Example 1. The results are shown in FIG.
[0042]
Example 2
Using the gold-supported cerium oxide catalyst [Au / CeO ₂ , gold supported amount 3 wt%] obtained in Example 1, the activity to the water gas shift reaction was examined by the following method.
[0043]
Using 0.3 g of the above catalyst, a raw material gas (mixed gas having a volume ratio of 12.2% of CO, 26.6% of H ₂ O, 4.9% of CO ₂ , and 56.3% of H ₂ ) at a flow rate of 26 ml / min. At normal pressure, the temperature was once raised to 350 ° C. in the reaction gas to reduce the catalyst, and then the catalytic activity for the water gas shift reaction at each temperature was examined. The result is shown in FIG.
[0044]
Example 3
Using cerium oxide-zirconium oxide composite oxide instead of cerium oxide, a catalyst was prepared under the same conditions as in Example 1, and a gold-supported cerium oxide-zirconium oxide composite oxide catalyst [Au / CeO ₂ -ZrO ₂ , gold 3% by weight]. Using the obtained catalyst, a reaction was conducted under the same conditions as in Example 2. FIG. 2 shows the results.
[0045]
Example 4
Using zirconium oxide instead of cerium oxide, a catalyst was prepared under the same conditions as in Example 1 to obtain a gold-supported zirconium oxide catalyst [Au / ZrO ₂ , gold supported amount 3 wt%]. Using the obtained catalyst, a reaction was conducted under the same conditions as in Example 2. FIG. 2 shows the results.
[0046]
Comparative Example 3
The reaction was carried out under the same conditions as in Example 2 using the platinum-supported cerium oxide catalyst obtained in Comparative Example 2 [Pt / CeO ₂ , platinum supported amount 3 wt%]. FIG. 2 shows the results.
[0047]
From these results, the following features of the catalyst of the present invention were clarified.
(1) Au / CeO ₂ has a higher CO conversion rate than the Cu / ZnO / Al ₂ O ₃ catalyst widely used as an industrial shift catalyst.
(2) It can be seen that Au / CeO ₂ shows a higher CO conversion at a lower temperature (particularly 100 to 200 ° C.) than Pt / CeO ₂ . Although it depends on the reaction conditions, Au / CeO ₂ can achieve the same CO conversion at a reaction temperature lower by about 50 to 80 ° C. than Pt / CeO ₂ .
(3) Even when CeO ₂ —ZrO ₂ or ZrO ₂ is used as a carrier for the gold catalyst, a CO conversion higher than Pt / CeO ₂ can be achieved at 250 ° C. or lower.
[0048]
From the above, it can be seen that the catalyst of the present invention is effective as a catalyst for a water gas shift reaction.
[Brief description of the drawings]
FIG. 1 shows a reaction temperature and a CO conversion ratio when various water gas shift reaction catalysts are used for five kinds of mixed gases (CO, CO ₂ , H ₂ , H ₂ O, He) containing 1.3% of CO. It is a graph showing.
FIG. 2 is a graph showing a reaction temperature and a CO conversion ratio when various water gas shift reaction catalysts are used for four kinds of mixed gases (CO, CO ₂ , H ₂ , H ₂ O) containing 12% of CO. It is.

Claims (3)

A catalyst for a water gas shift reaction of a fuel reforming gas comprising a metal oxide containing a metal having a cation electronegativity in the range of 10 to 14 and gold. The catalyst according to claim 1, wherein the fuel reformed gas is a mixed gas containing hydrogen, carbon monoxide, carbon dioxide, and water vapor, and having a carbon monoxide concentration of about 1 to 25 vol%. The metal oxide is (i) a metal oxide of a single metal selected from the group consisting of titanium oxide, zirconium oxide, and cerium oxide, and (ii) two or more kinds selected from the group consisting of titanium, zirconium, and cerium. 3. The catalyst according to claim 1, which is a composite oxide of a metal or a mixture of at least one of (iii) and (i) and at least one of (ii).

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