JP2012187560A - Catalyst and method for producing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a catalyst capable of exhibiting excellent catalyst performance and promoting an efficient reaction even with a small carried amount of a catalyst component, and a method for producing such a catalyst.SOLUTION: The catalyst is formed in a curved double thin film structure with a curvature, and uses silica with a diameter of 0.1-10 μm to carry the catalyst component. The silica in use preferably contains a methyl group and has the content of C derived from the methyl group being 3 mass% or more and less than 25 mass%.

Description

  The present invention relates to a catalyst containing a component called a noble metal element or a rare element, and a method for producing the catalyst, and more particularly to a catalyst capable of exhibiting high performance even with a small amount of catalyst and a method for producing such a catalyst. It is.

In recent years, against the backdrop of expanding global demand for automobiles, due to consideration for the global environment, clean exhaust gas and reduction of carbon dioxide (CO ₂ ) emissions have been screamed. Development is underway.
In any case, the core technology is a catalyst, and since the required performance is high, components called noble metal elements and rare elements are used relatively frequently.

For example, with respect to exhaust purification catalysts, the level of exhaust required is increasing year by year, and the amount of noble metal used is also increasing in order to achieve its performance. Precious metals are not only expensive, but also valuable in terms of resources, and techniques for reducing the amount of use are important.
In addition, with the widespread use of automobiles, the importance of energy issues is becoming more prominent, and the introduction of more efficient electric vehicles such as hybrid vehicles, fuel cell vehicles, and electric vehicles is also being promoted.

For a fuel cell vehicle, a highly efficient hydrogen production method is required, and a reforming catalyst is mainly used. In particular, when hydrogen is produced from a conventional fuel containing bioethanol on a vehicle, it is necessary to promote the reforming reaction at a lower temperature, and a noble metal catalyst becomes indispensable.
Furthermore, in the case of a solid oxide fuel cell (SOFC), fuels other than hydrogen are effective, but in that case, preliminary fuel reforming is indispensable. Used.

Also, the application of a catalyst has been proposed for a heat management system including an internal combustion engine.
That is, heat is released from the internal combustion engine together with the exhaust gas, and energy is wasted. If the exhaust gas and exhaust heat energy can be recovered effectively, CO ₂ can be directly reduced and a great fuel efficiency improvement effect can be obtained.

  For this reason, many proposals have been made to perform an endothermic fuel reforming reaction using exhaust heat on an automobile with the aim of improving fuel efficiency. As an example, Patent Document 1 below can be cited.

JP 2004-92520 A

In this case, it is possible to recover energy chemically by performing an endothermic fuel reforming reaction such as steam reforming or CO ₂ reforming reaction (dry reforming) using moisture or CO ₂ in the exhaust gas. Become. That is, when the fuel absorbs the exhaust heat during the reaction, a hydrogen-containing reformed gas fuel having a larger amount of heat than the supplied fuel can be obtained. In addition, the obtained hydrogen can be used for an internal combustion engine to promote combustion, and an improvement in fuel consumption due to improved combustion can be expected. Therefore, a large CO ₂ reduction effect is possible.
A catalyst is also required to promote the endothermic reforming reaction, and a high supported amount of catalyst is required to sufficiently promote the reaction even at a lower temperature of 600 ° C. or lower. In this case as well, reducing the amount of precious metal used is an important issue.

  As mentioned above, even when dealing with catalysts used in automobiles, such as exhaust catalysts, fuel cells, fuel reforming catalysts, etc., catalysts are used as the core in the environment and energy fields to meet the required high performance. The amount of precious metal components and rare high-performance element components used is increasing. There is an urgent need for effective measures to reduce the amount of catalyst used while maintaining high performance.

  The present invention has been made in view of such problems of the prior art, and the object of the present invention is to provide a catalyst capable of exhibiting high performance even with a small amount of catalyst and a method for producing such a catalyst. There is to do.

  As a result of intensive investigations to achieve the above object, the present inventors have used a silica material having a curved double thin film structure with a curvature and a diameter of 0.1 to 10 μm, thereby providing a catalyst. It has been found that the effective utilization rate of the components can be increased to efficiently promote the reaction, and the amount of the catalyst used can be greatly reduced, and the present invention has been completed.

That is, the present invention is based on the above knowledge, and the catalyst of the present invention includes a catalyst component and silica, and the silica has a curved double thin film structure and a diameter of 0.1. 10 to 10 μm.
Further, in the catalyst production method of the present invention, when the silica and the catalyst component are combined, the content of C having a methyl (CH ₃ ) group and derived from this methyl group is 3% by mass or more and 25% by mass. It is characterized by using silica which is less than.

  According to the present invention, a double thin film structure having a curved shape with a curvature, silica having a diameter of 0.1 to 10 μm is contained in the catalyst, and the base carrier is used, so that the reaction can be efficiently performed even with a small amount of catalyst. Further, since the catalyst life can be extended by suppressing side reactions, the amount of catalyst used can be greatly reduced.

It is a TEM (transmission electron microscope) photograph of the hemispherical cup-shaped double thin film structure silica used for the catalyst of this invention. It is a SEM and TEM (transmission electron microscope) photograph of the catalyst by Example 1 of this invention. It is a SEM and TEM photograph of the catalyst by the comparative example 1 of this invention. It is the graph which calculated and compared those critical areas from the particle diameter of Co oxide based on the TEM photograph of the catalyst by Example 1 of this invention, and the comparative example 1. FIG. It is the schematic which shows the structure of the microreactor used for the Example of this invention. It is a graph which compares and shows the hydrogen production ability in the steam reforming reaction by the catalyst of the Example of this invention, and a comparative example.

  In the following, the catalyst of the present invention will be described in more detail together with the structure of silica used in the catalyst, the catalyst components supported thereon, the loading method, and the like. In the present specification, “%” for concentration, content, blending amount, etc. means mass percentage unless otherwise specified.

As described above, the catalyst of the present invention has, for example, a double thin film structure having a curvature curved in a hemispherical cup shape, a bowl shape, or a dish shape, and contains silica having a diameter of 0.1 to 10 μm. A catalyst component such as metal or metal oxide is compounded (supported).
The thickness of the silica is, for example, 10 to 100 nm, but has a high mechanical strength due to the double structure.

As will be described later, such silica can be obtained by adding organosilicon halides having different chain lengths to a W / O type emulsion and calcining an organosiloxane obtained by polycondensation.
The organosiloxane before firing is a hollow sphere made of a thin film with countless pores, and the water in the hollow part is removed by firing and the thin film adheres, as if the air is extracted from the rubber balls In this way, a cup-shaped or dish-shaped double thin film structure is formed.

Thus, since the hollow part of a spherical thin film is crushed and the membranes are in close contact with each other, the silica has a substantially circular maximum outer diameter surface. Therefore, in the present invention, “diameter” means this outer diameter.
Moreover, since the “film thickness” means the thickness of one thin film of silica having a double thin film structure, the thickness of the entire silica is more than twice the above film thickness. .

The hemispherical methylsiloxane thin film obtained as described above is finally converted to silica by removing pores and methyl (CH ₃ ) groups as firing proceeds.
Therefore, the “silica” referred to in the present invention is not necessarily limited to perfect silica (SiO ₂ ), and those having a methyl group include those in an intermediate state between methylsiloxane and silica.

Since the silica used in the present invention has a film thickness of 10 to 100 nm and has a thin film structure, it has a relatively high specific surface area of about ^{20 to} 100 m ² / g even after firing at 500 ° C. Since it has a small pore volume of 05 cm ² / g or less and the ratio of the outer surface to the front surface is high, the catalyst component can be exposed to the reaction field. For this reason, it is considered that the reaction molecule can easily approach the catalyst component, the effective utilization rate of the catalyst is increased, and the amount of the catalyst used can be greatly reduced.

In addition, for example, in the fuel reforming reaction, fuel that is oil is reacted with H ₂ O or CO ₂ . That is, a hydrophobic molecule and a hydrophilic molecule are reacted. Conventional catalysts have various catalyst components supported on oxide surfaces such as alumina, ceria, and magnesia.
The oxide surface also acts as a co-catalyst for activating H ₂ O or CO ₂ , but since it is hydrophilic, it may suppress the approach of fuel. As will be described later, the silica used in the present invention can have a methyl group on the particle surface and can control the affinity for fuel, so that it can effectively act to promote the reaction.

The specific surface area of silica used in the present invention can be controlled by firing, it can have a value of about several m ^{2 /} g~400m ^{2 / g} according to the firing temperature. A preferable specific surface area value as a catalyst carrier is ²⁰ to 100 m ² / g as described above, and the corresponding firing temperature is 480 to 600 ° C.

Moreover, although it is as above-mentioned that it has a methyl group on the surface of a silica originating in the manufacturing method, The quantity can also be controlled by a calcination temperature.
If the calcination temperature is 700 ° C. or higher, most of the methyl groups are decomposed / desorbed to form extremely dense silica particles, but in this case, the effect of the methyl groups as described above cannot be expected.

In the catalyst of the present invention, Pt (platinum), Pd (palladium), Rh (rhodium), Ni (nickel), Co (cobalt), Cu (copper), Mn (manganese), Fe (iron) are used as catalyst components. At least one metal selected from the group consisting of Al (aluminum), Zr (zirconium), La (lanthanum), Ce (cerium), Mg (magnesium) and Ca (calcium), and at least one of these metals It is effective to include one or both of these oxides. In particular, Rh is an important catalyst component in a fuel reforming catalyst for automobiles, which has severe use conditions.
As a result, the catalyst metal and the metal oxide component are dispersed on the outer surface, and contact with the reaction molecule is facilitated, so that the effective utilization rate of the catalyst is increased. Furthermore, since the pore volume is small and the reaction product is immediately desorbed and removed from the catalyst surface, the progress of the side reaction can be suppressed. In this case, coking is suppressed and the catalyst life can be improved. Therefore, the frequency of catalyst regeneration and catalyst replacement can be reduced, which saves valuable resources and is economical.

  In the catalyst of the present invention, the content of the metal oxide is preferably 1 to 50%. If the amount of these components is small, the effect cannot be obtained. If the amount is too large, the silica support is coated with the metal oxide and the effect cannot be exhibited.

In the catalyst production method of the present invention, the methyl group on the silica surface plays an important role. That is, in the step of supporting a catalyst component, that is, a metal catalyst and a metal oxide on silica, it is effective to support the catalyst component in a state where the silica contains a methyl group (as C) of 3% or more and less than 25%. .
At this time, if the number of methyl groups is small, the base point for supporting the catalyst disappears and the components aggregate. On the other hand, when there are too many methyl groups, it becomes difficult to control the distance between the catalyst components and the degree of dispersion. When there are too many methyl groups, surface hydrophobicity becomes strong, and in the case of a catalytic reaction in which H ₂ O or CO ₂ is used as a counter reaction product, the approach thereof is hindered.

Further, in the process for producing the catalyst of the present invention, it is desirable to use an organic acid salt, an organic complex salt, or an organic compound such as a metal alkoxide as a starting salt of one or both of the metal catalyst and the metal oxide constituting the catalyst component. .
In general, the silica surface is relatively inactive, and it is difficult to disperse and hold the metal catalyst or metal oxide component in a fine particle state. However, in the present invention, the starting material for the metal catalyst or metal oxide is as described above. By using such an organic compound, the catalyst component can be effectively dispersed and supported with the methyl group on the silica surface as a base point, and high performance can be exhibited.

As a form of the catalyst of the present invention, for example, it can be applied to a ceramic or metal honeycomb or foam monolith support. Further, the “offset honeycomb” in which offset fins are provided inside the cells of the honeycomb-shaped carrier is effective in increasing the effective utilization rate of the catalyst.
As the number of cells of the monolith carrier, those having about 400 to 900 cells per square inch can be suitably used.

  The amount of the catalyst applied to the monolith support is preferably about 50 to 200 g / L, although it depends on the material of the monolith. In the case of a metal carrier having no pores, the catalyst particles can be effectively used even with a relatively small amount of catalyst applied, but the adhesion of the catalyst particles to the surface of the monolith carrier becomes a problem.

  On the other hand, in the case of a ceramic carrier such as cordierite having pores, a relatively large amount of catalyst is required for the catalyst particles to enter and bury the pores on the surface of the monolith carrier, but the catalyst particles are attached to the surface of the monolith carrier. The wearing power is relatively strong. However, if the amount of catalyst applied is too large, fuel molecules will hardly reach the inner particles, resulting in waste. In particular, when the coating amount exceeds 200 g / L, the ratio of the catalyst layer that cannot be reached by the reactive molecules cannot be ignored in the catalyst layer coated on the inner side, and the pressure loss increases, which is not preferable.

  Hereinafter, the present invention will be described more specifically based on examples and comparative examples. The present invention is not limited to these examples.

[Example 1]
(1) Preparation of silica Ion exchange water 7.5mL and isooctane (2,2,4-trimethylpentane) 500mL were mixed, and water was disperse | distributed in oil by stirring well. Thereto, 5.8 g of octyltrichlorosilane (OTCS) was added and ultrasonically stirred to prepare a water-in-oil (W / O) emulsion.
Furthermore, by adding 2.7 g of methyltrichlorosilane (MTCS), stirring well, filtering, washing and drying, followed by firing in the atmosphere at 400 ° C. for 3 hours and further at 450 ° C. for 2 hours, A silica carrier having a double thin film structure and a hemispherical cup shape was obtained.

A TEM (transmission electron microscope) photograph of the obtained silica is shown in FIG. The specific surface area of the silica is 160 m ² / g, the pore volume is 0.049 cm ³ / g, the average diameter by TEM observation is 1.2 μm, the film thickness is about 20 nm, and the methyl group content is as follows: The amount of C was 13%.

(2) Support of metal oxide catalyst Cobalt acetate (Co (CH ₃ COO) ₂ ) was dissolved in ethanol, the thin film silica obtained above was added and mixed, stirred for 2 hours, dried, and dried with ethanol and Removed moisture.
Next, the reforming catalyst 1 of the present invention in which 10% of Co oxide was supported as Co on the silica support was obtained by firing at 500 ° C. for 2 hours in the atmosphere.

  FIG. 2 shows an SEM (scanning electron microscope) photograph and a TEM photograph of the reforming catalyst 1 obtained as described above. It can be seen that the fine points in the TEM photograph are Co oxide particles and are well dispersed on the silica surface. The average particle size is about 10 nm.

[Example 2]
As a metal oxide source, zirconyl acetate (ZrO (CH ₃ COO) ₂ ) and lanthanum acetate hemihydrate (La (CH ₃ COO) ₃ · 1.5H ₂ O) were dissolved in ethanol. The silica support having a hemispherical cup-shaped double thin film structure obtained in 1 was added and mixed, stirred for 2 hours, and then dried to remove ethanol and moisture.
Then, by firing in air 500 ° C. 2 _h, ZrO 2 _-La 2 _{O 3} (weight ratio 90:10) to obtain 12% supported catalyst precursor with respect to the silica support.

Then, the catalyst precursor 1 is impregnated in an aqueous solution containing rhodium nitrate (Rh (NO ₃ ) ₃ ), dried, and then calcined at 500 ° C. for 2 hours, whereby Rh is supported at 1.0%. An inventive reforming catalyst 2 was obtained.

Example 3
By repeating the same operation as in Example 2 using cerium acetate (Ce (CH ₃ COO) ₃ .H ₂ O) as a metal oxide source, 36% of CeO ₂ was supported on the silica support. The obtained catalyst precursor was obtained.
Next, the same Rh supporting step as in Example 2 was performed to obtain the reforming catalyst 3 of the present invention in which 1.0% Rh was supported.

Example 4
Using cerium acetate (Ce (CH ₃ COO) ₃ .H ₂ O) and zirconyl acetate (ZrO (CH ₃ COO) ₂ ) as the metal oxide source, the same operation as in Example 2 is repeated. Thus, a catalyst precursor in which 28% of CeO ₂ —ZrO ₂ (mass ratio 80:20) was supported on the silica support was obtained.
Next, the same Rh supporting step was performed to obtain the reforming catalyst 4 of the present invention in which 1.0% Rh was supported.

[Comparative Examples 1-4]
In place of the above-described hemispherical cup-shaped double thin film structure silica support, operations identical to those in Examples 1 to 4 were performed except that amorphous silica powder having a specific surface area of 160 m ² / g was used as the support. By repeating, the reference catalysts 1-4 were obtained as Comparative Examples 1-4, respectively.

The SEM photograph and TEM photograph of the reference catalyst 1 obtained in Comparative Example 1 are shown in FIG.
Compared with the reforming catalyst 1 according to Example 1 shown in FIG. 2, the Co oxide particles are aggregated into large particles, which shows that the catalyst component loading method according to the present invention is effective.

FIG. 4 shows the result of calculating the specific surface area of Co oxide particles based on the average particle diameter of Co oxide based on the TEM photographs of FIGS. 2 and 3. With the catalyst of the present invention, a specific surface area of about 3 times that of the comparative reference catalyst was obtained.
In the present invention, a methyl group is present on the surface of the thin-film silica used as the carrier, and it is considered that the dispersibility of the Co oxide can be enhanced by using the methyl group as a base point.

[Fuel reforming performance test]
Using a microreactor as shown in FIG. 5, 0.01 g of the powder catalysts of Examples and Comparative Examples obtained as described above were filled in a quartz reaction tube, respectively, and steam reforming using isooctane as a gasoline simulated fuel. Reaction was performed. The reaction conditions were S / C = 3.6 and LHSV = 20 h ⁻¹ . GHSV condition, the catalyst evaluation of Example 1 and Comparative Example 1 and 100,000 h ^-1, Examples 2-4, the catalyst evaluation of Comparative Example 2-4 was 200,000 ^-1. The reaction temperature was 590 ° C.

FIG. 6 shows the result of comparing the performance of each catalyst using the H ₂ concentration in the resulting reformed gas as an index of hydrogen production ability. The reforming catalysts 1 to 4 of the examples obtained based on the special thin-film silica have higher hydrogen production with the same weight and the same standard as compared with the reference catalysts 1 to 4 using an amorphous silica carrier as a comparative example. It was confirmed to show the ability.
In the catalyst of the present invention, since silica having a curved double thin film structure is used as a base carrier, the silica carrier acts as a micro lashing ring, so that the reaction product is effective in a micro-order space in the catalyst layer. It is considered that the effective utilization rate of the supported and supported catalyst is improved. In addition, high catalyst performance is exhibited by taking into account the effect of the dispersion support method of the catalyst component utilizing the methyl group on the silica surface.

  Here, although the catalyst of the present invention has shown effectiveness as a fuel reforming catalyst, a catalyst having the same form such as a combustion catalyst and an exhaust purification catalyst can be expected to have the same effect. However, the present invention is not limited to the examples.

Claims (7)

  A catalyst having a curved double thin film structure and containing silica having a diameter of 0.1 to 10 μm and a catalyst component.   The catalyst according to claim 1, wherein the silica has a thickness of 10 to 100 nm.   The catalyst component is at least one metal selected from the group consisting of Pt, Pd, Rh, Ni, Co, Cu, Mn, Fe, Al, Zr, La, Ce, Mg, and Ca, and / or the group The catalyst according to claim 1 or 2, comprising an oxide of at least one kind of metal selected from the above.   Content of the said metal oxide is 1-50 mass%, The catalyst as described in any one of Claims 1-3 characterized by the above-mentioned. The catalyst according to any one of claims 1 to 4, wherein the silica has a methyl (CH _3- ) group.   The method for producing a catalyst according to claim 5, wherein the silica having a content of C derived from a methyl group of 3% by mass or more and less than 25% by mass is used when the silica and the catalyst component are combined. And a catalyst production method.   7. The method for producing a catalyst according to claim 6, wherein an organic compound is used as a starting material for the metal catalyst and / or metal oxide.

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