JP2003117398A - Wc carrying catalyst and production method thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a catalyst which does not use noble metal such as platinum and expresses catalytic action equal to noble metal such as platinum. SOLUTION: This catalyst features that WC is carried on a carrier. Preferably, average grain size of WC is 0.5-10 nm and transition metal is additionally carried. Such a catalyst is produced in such a manner that hollow carbon particles introducing tungsten into fine pores thereof are carried on the carrier and, subsequently, are sintered. This catalyst can be used for the catalyst for cleaning waste gas of internal combustion engine and the electrode catalyst of fuel cell.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a catalyst for substituting a noble metal for a catalyst component, and more particularly to a catalyst using tungsten carbide (WC) as a catalyst component.

[0002]

2. Description of the Related Art Exhaust gas emitted from an internal combustion engine such as an automobile engine contains carbon monoxide (CO), hydrocarbon (H
C), nitrogen oxides (NO _x ), etc., and these toxic substances generally have a catalyst component mainly composed of a noble metal such as platinum (Pt) on a carrier such as alumina, zirconia or ceria-zirconia. It is purified by the carried exhaust gas purification catalyst.

Fuel cells such as phosphoric acid electrolyte fuel cells and polymer electrolyte fuel cells are provided with a negative electrode for ionizing a fuel such as hydrogen and a positive electrode for burning the ionized fuel. Generally, a catalyst component containing a noble metal such as platinum as a main component is supported on a conductive carbon material or the like.

However, although noble metals such as platinum act well as these exhaust gas purification catalysts and catalyst components of fuel cells, they are expensive and the amount of mining is limited. Also,
It is necessary to further improve the performance of the exhaust gas purifying catalyst for further environmental protection, but this will lead to an increase in the amount of precious metal used.

On the other hand, it is expected that the exhaust gas of the automobile will be cleaned by using the fuel cell as the power source of the automobile. However, for example, the amount of platinum as a catalyst component used in electrodes of hydrogen fueled fuel cells is considerably larger than the amount of platinum used in exhaust gas purification catalysts of gasoline vehicles that generate the same power. Is.

[0006]

As described above, both of the catalyst component used for the exhaust gas purifying catalyst of the internal combustion engine and the catalyst component used for the electrode of the fuel cell use the noble metal of the catalyst component. The challenge is to reduce the amount or replace the catalyst component with a noble metal. Therefore, an object of the present invention is to provide a catalyst that does not use a noble metal such as platinum and exhibits a catalytic action equivalent to that of a catalyst using a noble metal such as platinum.

[0007]

The above object can be achieved by a catalyst characterized in that WC (tungsten carbide) is supported on a carrier. That is, the catalyst of the present invention is
The catalyst uses WC as a catalyst component. It is considered that the reason why such a catalyst can exert the catalytic action is that the outer shell electronic state of WC is close to the outer shell electronic state of Pt.

In a preferred embodiment, the WC on the carrier has an average particle size of 0.5 to 10 nm (nanometer). WC
It is considered that the catalyst activity is further enhanced by having such a fine particle size.

In a preferred embodiment, WC and a transition metal are further supported on a carrier to form a catalyst. It is believed that such loading optimizes the outer shell electronic state of WC for catalytic function. The structure of the catalyst of the present invention in this embodiment is shown in model form in FIG. A catalyst in which WC is supported on such a carrier can be suitably used as an exhaust gas purifying catalyst or an electrode catalyst of a fuel cell.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION The catalyst of the present invention is characterized in that WC is supported on a carrier. When the catalyst of the present invention is applied to an exhaust gas purifying catalyst for an internal combustion engine, the carrier includes, for example, oxides such as alumina, silica and zirconia, as well as silica-alumina, zirconia-ceria, alumina-ceria-zirconia. Preferred are those composed of composite oxides such as, ceria-zirconia-yttria, and zirconia-calcia. Further, when the catalyst of the present invention is applied to an electrode of a fuel cell, the carrier is preferably conductive carbon black such as acetylene black.

To carry WC on such a carrier, for example, the powder of WC and the above-mentioned powder of alumina or the like are dispersed in water to form a slurry, which is mixed using a ball mill or the like, and then the slurry is dried. Can be done by Preferably, the WC carried on the carrier is fine particles having an average particle diameter of 0.5 to 10 nm.

To support such fine WC, for example, a tungsten compound is used as a WC precursor,
This can be done by depositing C as fine particles on the carrier. More specifically, for example, a gas of tungsten hexacarbonyl W (CO) ₆ is brought into contact with the heated carrier, or a mixed gas of tungsten chloride WCl ₆ and a hydrocarbon such as methane is brought into contact therewith, and then, if necessary, an inert gas. W by heating to 600-2000 ° C in the atmosphere
C fine particles can be supported.

The loading of such fine WC can be carried out by loading the hollow carbon particles having the tungsten introduced into the pores on the support and then firing. More specifically, hollow carbon particles having fine pores and a tungsten metal lump are prepared, and both are placed in a reaction tube kept in a vacuum, and then the tungsten metal lump is heated so that the tungsten becomes carbon in a gaseous state. Form a state of contact with the particles. If this state is maintained for a certain period of time, once the gaseous tungsten enters the pores, it does not escape to the outside of the pores. Can be introduced at.

Next, the hollow carbon particles having tungsten introduced into the pores are supported on a carrier by, for example, mixing and drying with a carrier such as alumina described above, and then calcined in an inert atmosphere. As a result, carbon and tungsten are reacted with each other to generate WC, and at the same time W is added to the carrier.
C can be supported. The firing temperature is 600 to
1300 ° C is a rough standard.

According to this method, WC having a fine particle size of 0.5 to 10 nm, more preferably 0.5 to 5 nm can be easily supported on the carrier. The reason for this is that in the above firing process, the tungsten clusters are blocked from contacting each other by the walls of the carbon particles surrounding them, and the hollow carbon particles into which tungsten has been introduced are not supported on the carrier. In this case, it is presumed that the WC is likely to be fixed to the structural defect site of the carrier, which suppresses the enlargement of WC due to the movement during firing.

Such hollow carbon particles have a pore size of about 0.5 to 10 nm and preferably 1 to 10 n.
Those having a length of m are suitable, and for example, so-called carbon nanotubes and carbon nanohorns can be preferably used. Such carbon nanotubes and carbon nanohorns can be produced by a method of thermally decomposing a hydrocarbon such as methane in the presence of a catalyst such as iron oxide, or irradiating solid carbon with a laser. As a prior art of such a manufacturing method, there is, for example, Japanese Patent Laid-Open No. 2001-64004.

Preferably, the catalyst of the present invention is on a support,
In addition to WC, a transition metal is supported and configured. This transition metal is supported by transition metal nitrates, chlorides, acetates,
It can be carried out by using a carbonate or the like by an evaporation dryness method, an impregnation method, a precipitation method, an ion exchange method, an adsorption method, a reduction precipitation method or the like. Preferably, the transition metal is supported by adding a reducing agent to an aqueous solution of a compound that produces a transition metal ion in an aqueous solution, reducing the transition metal ion to make it insoluble, and reducing the transition metal particles to precipitate. It is performed by precipitation.

Specifically, transition metal nitrates, sulfates,
Chloride, acetate, a compound such as carbonate, was dissolved in an aqueous slurry of the support carrying the above WC, to the slurry, trisodium citrate dihydrate _{C 6 H 5 Na 3 O 7} · 2
H ₂ O, hydrazine N ₂ H ₄ , sodium thiosulfate Na ₂ S
₂ O ₃ , potassium thiosulfate K ₂ S ₂ O ₃ , ammonium thiosulfate (NH ₄ ) ₂ S ₂ O ₃ , sodium sulfite Na ₂ SO ₃ , borohydride, hypophosphite, citrate, formic acid CH
A reducing agent such as ₂ O ₂ or oxalic acid CH ₂ O ₄ and a buffering agent such as sodium L-ascorbate C ₆ H ₇ O ₆ Na or ethylenediaminetetraacetic acid salt are added to reduce the transition metal ion and transition. Precipitate fine metal particles.

According to such reduction precipitation, the diameter is about 1 nm.
It is relatively easy to deposit substantially all of the following transition metal particles on WC. This transition metal and WC
Is not limited, but the transition metal / WC mass ratio is 0.01 to 0.2, and more preferably 0.01 to 0.2.
0.1 is a rough standard. Here, even if the metal component other than the transition metal is contained in an amount of several mass% or less based on the mass of WC, it is acceptable in the object and effect of the present invention.

The term "transition metal" in the present invention is used.
Is d- of the groups 3A to 7A, 8 and 1B of the periodic table.
An element containing a block element and an f-block element, titanium (Ti), vanadium (V), chromium (Cr), manganese
(Mn), iron (Fe), cobalt (Co), nickel (Ni),
Copper (Cu), yttrium (Y), zirconium (Zr), niobium (Nb), molybdenum (Mo), ruthenium (Ru), lanthanum (La), cerium (Ce), praseodymium (P).
r) and neodymium (Nd) are exemplified, and more preferably manganese, iron, cobalt, nickel, yttrium, molybdenum, ruthenium, copper, cerium, and neodymium.
Hereinafter, the present invention will be described more specifically with reference to Examples.

[0021]

Example 1 100 g of γ-alumina powder (specific surface area of about 2
00 m ² / g) was placed and heated and maintained at 200 ° C., and WC precursor W (CO) ₆ (manufactured by Pure Chemical Co.) was introduced together with nitrogen gas into the reaction tube to introduce W (CO) ₆ It was contacted and decomposed on γ-alumina powder. Next, this γ-alumina powder was heat-treated at 800 ° C. for 5 hours in a nitrogen gas atmosphere to obtain a catalyst of the present invention in which WC was supported on the γ-alumina powder. The amount of WC supported on this catalyst was 2% by mass based on the total mass of γ-alumina, and the amount of WC was carried by a transmission electron microscope.
The average particle size of the WC particles on γ-alumina as observed by (TEM) was 4 nm.

Example 2 0.5 g of carbon nanohorns having an average pore diameter of 3 nm and an average length of 7 nm were placed inside the reaction tube, and 0.5 g of a tungsten metal block was placed at a position approximately 2 cm from the carbon nanohorns. The inside of the reaction tube was maintained at a vacuum degree of 10 ⁻⁵ Pa, the carbon nanohorn was heated to a temperature of about 600 ° C., the tungsten metal block was heated to a temperature of about 2000 ° C., and the tungsten vapor was brought into contact with the carbon nanohorn. . This state was maintained for 24 hours, and tungsten was introduced into the pores of the carbon nanohorn.

Next, 0.5 g of the above-mentioned carbon nanohorn in which tungsten is introduced and 10 g of γ-alumina.
Was mixed in 200 g of ion-exchanged water for 2 hours and then evaporated to dryness. Then, this mixture is fired in a nitrogen gas atmosphere at 1000 ° C. for 2 hours to give γ-alumina powder by WC.
Thus, the catalyst of the present invention carrying The supported amount of WC on this catalyst is 2% by mass based on the total mass of γ-alumina.
And the average particle size of the WC particles on γ-alumina as observed by TEM was 2 nm.

Example 3 20 g of the WC-supporting γ-alumina powder obtained in the same manner as in Example 1 was dispersed in 1300 cc of ion-exchanged water at 60 ° C., and the reagents were successively added in the following concentrations. And manganese nitrate Mn (NO ₃ ) ₂ was deposited for 24 hours under mild stirring to reduce and precipitate Mn. _{Mn (NO 3) 2 2 ×} 10 -2 _{wt% Na 2 S 2 O 3 ·} 5H 2 O 1 × 10 -1 _{wt% Na 2 SO 3 2 × 10} -1 _{wt% C 6 H 7 O 6 Na} 1 mass %

After this reduction precipitation, the slurry is filtered and washed, and dried in air at 120 ° C. for 2 hours, and then,
Heat treatment was performed at 500 ° C. for 2 hours in the atmosphere. As a result, the catalyst of the present invention was obtained in which 2% by mass of WC and 0.2% by mass of Mn were supported on the γ-alumina powder.

[0026] Instead of Examples 4-6 manganese nitrate, iron nitrate Fe (NO _{3) 2} · 6H
₂ O, cobalt nitrate Co (NO ₃ ) ₂ , nickel nitrate Ni (N
In the same manner as in Example 3 except that O ₃ ) ₂ was used, WC was formed on the γ-alumina powder whose summary is shown in Table 1.
A catalyst of the present invention on which Fe, Co, and Ni were respectively supported was obtained.

Comparative Example 1 Table 1 is summarized in the same manner as in Example 6 except that nickel nitrate Ni (NO ₃ ) ₂ was used to reduce and precipitate Ni on γ-alumina powder on which WC was not supported. A catalyst of Comparative Example in which Ni was supported on the γ-alumina powder was obtained.

Comparative Example 2 γ-alumina was mixed with platinum dinitrodiammine Pt (NH ₃ ).
_{A 2} (NO ₂ ) ₂ solution was impregnated, dried and then calcined at 600 ° C. for 2 hours to obtain a catalyst of Comparative Example in which 2% by mass of Pt was supported on γ-alumina.

Examples 7 to 12 Examples 1 to 12 respectively, except that the carrier of the γ-alumina powder was replaced by acetylene black (particle size 20 to 40 nm).
In the same manner as in 6, a catalyst of the present invention whose summary is shown in Table 2 was obtained.

Comparative Examples 3 to 4 Comparative Examples 1 to 4 except that the carrier of γ-alumina powder was changed to acetylene black (particle size 20 to 40 nm).
In the same manner as in 2, a catalyst of Comparative Example whose summary is shown in Table 2 was obtained. The "supported amount" in Tables 1 and 2 is a value based on the total mass of WC, transition metal, or Pt of the catalyst component and the carrier, and "average particle size" is a value obtained from a TEM image. is there.

-Evaluation of catalyst performance (1) -The above-mentioned catalysts of Examples 1 to 6 and Comparative Examples 1 and 2 were compressed and crushed into pellets having a diameter of about 1.5 mm. 2 cc of each of these pellet catalysts was placed in a fixed bed flow reactor, a model exhaust gas of the following composition was circulated, and the 50% purification temperature (T50) of C ₃ H ₆ was measured while increasing the catalyst temperature. Model exhaust gas composition: 0.7% CO + 1000ppm C ₃ H ₆ + 1500ppm NO +
10.0% CO ₂ + 0.7% O ₂ + 10.0% H ₂ O + 5.0
% H ₂ (residual N ₂ )

In this exhaust gas purification performance test, the flow rate of the model exhaust gas was 5 liters / minute, and the temperature rising rate of the catalyst was 25 ° C./minute. The result is shown in FIG. From the results shown in FIG. 2, the catalyst of the example in which the fine WC was supported was inferior in catalytic performance to the catalyst of the comparative example in which Pt was supported, but showed a certain level of catalytic performance. In addition, it can be seen that the catalytic performance of Pt is approached by supporting the transition metal.

-Evaluation of catalyst performance (2) -Compression of each catalyst of Examples 7 to 12 and Comparative Examples 3 to 4
It was crushed into pellets having a diameter of about 1.5 mm. 2 cc of each of these pellet catalysts was placed inside the reaction tube, and the catalytic performance in the following hydrogenation reaction of acetone, which has been simply performed as a performance evaluation of the electrode catalyst, was evaluated. The results are shown in FIG. 3 based on the catalytic performance of the Pt catalyst of Comparative Example 2. (CH ₃ ) ₂ CO + H ₂ → CH ₃ CH (OH) CH ₃

The results shown in FIG. 3 are the same as those in FIG. 2, and the catalyst of the example in which the fine WC is supported shows some catalytic performance, and the transition metal is supported in addition to WC. As a result, it can be seen that the catalyst performance is improved.

-Morphological observation of alloy catalyst- For each catalyst of the above-mentioned examples, morphological observation by TEM and energy dispersive X-ray spectroscopic analysis with a resolution of about 1 nm
Elemental analysis was performed in the spot area of the electron microscope image by (EDX).

From these TEM observations, Examples 3 to 6 were obtained.
And no transition metal supported in Examples 9 to 12 was observed. However, by elemental analysis in the spot area by EDX, the transition metal was not detected on γ-alumina and acetylene black, but only on the WC particles. From this, it is judged that the transition metal has a diameter of less than about 1 nm and is loaded on the WC in a state as shown in FIG.

[0037]

By using WC as a substitute for a noble metal such as platinum, it is possible to provide a catalyst that exhibits a catalytic action as an exhaust gas purification catalyst or an electrode catalyst of a fuel cell.

[0038]

[Table 1]

[0039]

[Table 2]

[Brief description of drawings]

FIG. 1 is a model view showing the structure of a catalyst of the present invention.

FIG. 2 is a graph comparing the performance of exhaust gas purifying catalysts.

FIG. 3 is a graph comparing the performance of electrode catalysts.

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Claims (6)

[Claims] 1. A catalyst in which WC is supported on a carrier. 2. The average particle diameter of the WC is 0.5 to 10 n.
The catalyst according to claim 1, which is m. 3. The catalyst according to claim 1, which further comprises a transition metal. 4. The method for producing a catalyst according to claim 1, wherein hollow carbon particles having tungsten introduced into the pores are supported on a carrier and then calcined. 5. The catalyst according to claim 1, which is used as an exhaust gas purification catalyst. 6. The catalyst according to claim 1, which is used as an electrode catalyst for a fuel cell.