JP2012166115A - Exhaust gas purifying catalyst - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an exhaust gas purifying catalyst, wherein both performances of heat resistance and catalytic activities are improved.SOLUTION: The exhaust gas purifying catalyst contains CoO, and CeO-ZrOcomposite oxide shown by general formula xCeO-(1-x) ZrO, where 0.3<x<1.0.

Description

The present invention relates to an exhaust gas purification catalyst, and more particularly, to an exhaust gas purification catalyst containing cobalt oxide (II, III) (Co ₃ O ₄ ) as a catalyst component.

Conventionally, a three-way catalyst that simultaneously oxidizes carbon monoxide (CO) and hydrocarbons (HC) and reduces nitrogen oxides (NOx) is used as an exhaust gas purification catalyst for automobiles. As such a catalyst, a catalyst in which a platinum group element such as platinum (Pt), rhodium (Rh) or palladium (Pd) is supported on a porous oxide carrier such as alumina (Al ₂ O ₃ ) is widely known. It has been.

  However, the amount of these platinum group elements is increasing with the tightening of exhaust gas regulations for automobiles, and therefore there is a concern about the depletion of resources. For this reason, while reducing the usage-amount of a platinum group element, it is required to substitute the role of a platinum group element with another metal etc. in the future.

Thus, much research has been conducted on catalyst components for reducing or replacing the amount of platinum group elements used. One such catalyst component is cobalt oxide (II, III) (Co ₃ O ₄ ), and several proposals have been made on exhaust gas purification catalysts using the same.

Patent Document 1 includes a catalyst powder made of a transition metal oxide having an average particle diameter of 1 nm to 2 μm and a binding energy of oxygen electrons shifted to a lower energy side than 531.3 eV. A purification catalyst characterized in that it further comprises an oxygen-releasing material supported on the surface of the metal oxide or forming a solid solution with the transition metal oxide is described. Specifically, a purification catalyst using cobalt oxide (II, III) (Co ₃ O ₄ ) as the above and cerium oxide (CeO ₂ ) as the oxygen release material is specifically disclosed. Moreover, in patent document 1, according to the purification catalyst which has such a structure, it describes that a transition metal oxide can function more effectively as an active point.

  In Patent Document 2, a catalyst obtained by mixing and molding alumina powder and nickel oxide and / or cobalt oxide powder having a particle size of less than 150 μm, and containing 1 to 10% by weight of nickel oxide and / or cobalt oxide. A catalyst for purifying nitrogen oxides in exhaust gas is described. Moreover, in patent document 2, according to the catalyst for purifying nitrogen oxides in the exhaust gas having such a configuration, nitrogen oxides present in the exhaust gas in an oxygen-excess atmosphere are decomposed into nitrogen gas at a high conversion rate. In addition, it is described that it is possible to completely suppress the by-product of dinitrogen monoxide.

JP 2010-104973 A Japanese Patent Laid-Open No. 7-080308

  In automotive exhaust gas purification catalysts, the temperature at which the catalyst is exposed repeatedly fluctuates between room temperature and about 1000 ° C. Therefore, even under such conditions of use, high catalyst activity can be achieved by suppressing thermal degradation of the catalyst. It is extremely important to maintain.

In Patent Documents 1 and 2, as described above, a catalyst containing cobalt oxide such as Co ₃ O ₄ as a catalyst component is studied. However, in these patent documents, sufficient examination is not necessarily made from two viewpoints of suppression of thermal deterioration of the catalyst, that is, improvement of heat resistance and improvement of catalyst activity. Therefore, the catalysts described in these patent documents still have room for improvement with respect to the catalyst performance.

  Therefore, an object of the present invention is to provide an exhaust gas purifying catalyst in which both performances of heat resistance and catalytic activity are improved by a novel configuration.

The present invention for solving the above problems is as follows.
(1) Co ₃ O ₄ and a CeO ₂ —ZrO ₂ composite oxide represented by the general formula xCeO _2. (1-x) ZrO ₂ , wherein 0.3 <x <1.0 An exhaust gas purifying catalyst, characterized in that it exists.
(2) The exhaust gas-purifying catalyst according to (1), wherein the CeO ₂ —ZrO ₂ composite oxide is a CeO ₂ —ZrO ₂ solid solution.
(3) The exhaust gas purifying catalyst as described in (1) or (2) above, wherein 0.4 ≦ x <1.0.
(4) The exhaust gas-purifying catalyst as described in (3) above, wherein x = 0.5.

According to the present invention, cobalt oxide (II, III) (Co ₃ O ₄ ) as a catalyst component is a CeO ₂ —ZrO ₂ composite in which the molar ratio of cerium to zirconium (Ce / Zr ratio) is greater than 3/7. Combining with oxides, especially CeO ₂ —ZrO ₂ solid solution, the heat resistance of the resulting exhaust gas purification catalyst is remarkable compared to conventional catalysts made by mixing Co ₃ O ₄ powder with Al ₂ O ₃ powder. In addition, it is possible to achieve excellent catalytic activity, particularly excellent CO oxidation activity.

It is a graph showing the relationship between the ratio and the catalyst heat resistance of Ce contained in the CeO ₂ -ZrO ₂ composite oxide.

The exhaust gas purifying catalyst of the present invention includes Co ₃ O ₄ and a CeO ₂ —ZrO ₂ composite oxide represented by a general formula xCeO _2. (1-x) ZrO ₂ , wherein 0.3 < It is characterized by x <1.0.

Cobalt oxide (II, III) (Co ₃ O ₄ ) has Co ³⁺ ions arranged in the octahedral gap of the cubic close-packed unit cell of oxide ions, and Co ²⁺ ions arranged in the tetrahedral gap. It is generally known that the metal oxide has a spinel structure and has high oxidation activity due to the presence of Co ³⁺ ions.

Further, by mixing this Co ₃ O ₄ powder with alumina (Al ₂ O ₃ ) powder generally used in, for example, an exhaust gas purification catalyst, carbon monoxide (CO) or hydrocarbon (HC) It is possible to prepare a catalyst having oxidation activity against However, such a catalyst may not always have sufficient heat resistance, and therefore, the exhaust gas purification of an automobile in which the temperature at which the catalyst is exposed repeatedly fluctuates between room temperature and about 1000 ° C. It is difficult to apply in the use of the catalyst.

No Without intending to be bound by any particular theory, the catalyst comprising a mixture of Co ₃ O ₄ powder and Al ₂ O ₃ powder, at a high temperature of use of Co ₃ O ₄ and Al ₂ O ₃ It is considered that the solid solution progresses and a solid solution composed of CoAl ₂ O ₄ is formed. By forming such a solid solution, it is considered that Co ³⁺ ions contained in the original Co ₃ O ₄ are changed to Co ²⁺ ions, and as a result, the activity of the catalyst is lowered.

The present inventors have, Co ₃ O ₄ powder, combined with the powder of CeO ₂ -ZrO ₂ composite oxide having a specific Ce / Zr ratio (molar ratio), Co ₃ O ₄ powder Al ₂ O ₃ Compared with conventional catalysts mixed with powder, the heat resistance of the obtained exhaust gas purification catalyst can be remarkably improved, and further, excellent catalytic activity, particularly excellent CO oxidation activity can be achieved. I found out that I can do it.

As Co ₃ O ₄ used in the exhaust gas purifying catalyst of the present invention, any commercially available Co ₃ O ₄ can be used. Alternatively, Co ₃ O ₄ may be prepared by any method known to those skilled in the art. For example, Co ₃ O ₄ can be prepared by heating and firing metallic cobalt, its oxide, carbonate, nitrate, hydroxide, etc. in air at a temperature of 800 ° C. or lower.

The above Co ₃ O ₄ is not particularly limited as long as it is used in an amount sufficient to exhibit catalytic activity against harmful components in exhaust gas, particularly CO and HC, but in general, It can be used in such an amount that the amount of Co in Co ₃ O ₄ is 0.1 to 60 wt% with respect to the total mass of the exhaust gas purifying catalyst.

The exhaust gas purifying catalyst of the present invention, together with Co ₃ O ₄ above, is represented by the general formula _{xCeO 2 · (1-x)} ZrO 2, wherein, 0.3 <x <1.0 in which CeO ₂ - Includes ZrO ₂ composite oxide.

When x in the above formula is 0.3 or less, that is, when the molar ratio of cerium and zirconium contained in the CeO ₂ —ZrO ₂ composite oxide (Ce / Zr ratio) is 3/7 or less, A sufficient effect cannot be achieved with respect to the improvement of the heat resistance of the exhaust gas purifying catalyst. On the other hand, when x in the above formula is 1.0, that is, when a metal oxide of CeO ₂ alone containing no ZrO ₂ is used together with Co ₃ O ₄ , the heat resistance of the obtained exhaust gas purification catalyst Although the performance is improved, high catalyst performance cannot always be achieved in terms of catalyst activity. According to the present invention, the CeO ₂ —ZrO ₂ composite wherein x in the above formula is 0.3 <x <1.0, preferably 0.4 ≦ x <1.0, more preferably x = 0.5. By using the oxide together with Co ₃ O ₄ , it is possible to obtain an exhaust gas purifying catalyst that is remarkably improved not only in heat resistance but also in catalytic activity, particularly in CO oxidation activity. Furthermore, it is preferable to add at least Zr to CeO ₂ from the viewpoint of purification performance particularly at low temperatures. That is, by using a CeO ₂ -ZrO ₂ composite oxide prepared by adding at least Zr in CeO ₂ with Co ₃ O _4, remarkable purification performance at low temperatures as compared with the case of using CeO ₂ alone metal oxides Can be improved.

As described above, Co ₃ O ₄ is a metal oxide containing both Co ³⁺ ions and Co ²⁺ ions. The presence of Co ³⁺ ions is considered to be extremely important for the catalyst using Co ₃ O ₄ to exhibit sufficient catalytic activity for oxidation of CO, HC and the like. However, using this Co ₃ O ₄ at high temperatures, for example, Co ₃ O ₄ Co ³⁺ ions contained in is reduced to Co ²⁺ ions, or Co ₃ O ₄ is CoO (cobalt oxide (II )) Etc., the amount of Co ³⁺ ions having oxidation activity is considered to decrease. As a result, the activity of the catalyst containing such a metal oxide as a catalyst component is reduced.

Although not intended to be bound by any particular theory, the heat resistance of the exhaust gas purifying catalyst of the present invention is improved because Co ₃ O ₄ is used together with CeO ₂ —ZrO ₂ composite oxide. Thus, it is considered that the reduction from the Co ³⁺ ion to the Co ²⁺ ion as described above is suppressed.

More specifically, the CeO ₂ —ZrO ₂ composite oxide occludes oxygen when the oxygen concentration in the exhaust gas is high in order to enhance the exhaust gas purification ability of a three-way catalyst or the like, and absorbs oxygen when the oxygen concentration in the exhaust gas is low. It is generally known in the art as a material to be released, so-called oxygen storage / release capability (OSC capability). Therefore, by using a combination of such materials with Co ₃ O _4, for example, even at a high temperature use conditions, the oxygen released from the CeO ₂ -ZrO ₂ composite oxide in Co ₃ O ₄ Co ^It is considered that the reduction of ³⁺ ions to Co ²⁺ ions is suppressed, or the change from Co ₃ O ₄ to CoO or the like can be suppressed.

The CeO ₂ —ZrO ₂ composite oxide in the present invention includes any commercially available CeO ₂ —ZrO ₂ composite oxide or a CeO ₂ —ZrO ₂ composite oxide prepared by any method known to those skilled in the art. Preferably, a CeO ₂ —ZrO ₂ solid solution in which ceria and zirconia are in solid solution with each other can be used.

By using such a CeO ₂ —ZrO ₂ solid solution as the CeO ₂ —ZrO ₂ composite oxide in the present invention, both the heat resistance and catalytic activity of the resulting exhaust gas purifying catalyst can be remarkably improved. It is considered possible. More specifically, in an oxygen storage / release material containing CeO ₂ , oxygen is generally absorbed and released by the following reaction formula.
CeO ₂ ⇔ CeO _1.5 + 1 / 4O ₂

As is apparent from the above reaction formula, the oxygen is released from the CeO _2, Ce ⁴⁺ ions in CeO ₂ is reduced to Ce ³⁺ ions. However, the increase of the ionic radius due to the reduction from Ce ⁴⁺ ions to Ce ³⁺ ions is a very large change in crystallography, and hence causes distortion of the crystal lattice of CeO ₂ , which makes the lattice unstable. Make it. Therefore, such a reaction is very disadvantageous in terms of energy. However, in the CeO ₂ —ZrO ₂ solid solution, part of the Ce ions in the CeO ₂ crystal lattice is replaced with Zr ions having a smaller ion radius, so that the distortion of the crystal lattice accompanying the increase in the ion radius of the Ce ions can be reduced. Is possible. For this reason, the CeO ₂ —ZrO ₂ solid solution has a much higher OSC ability than CeO ₂ . Therefore, by using CeO ₂ —ZrO ₂ solid solution having such high OSC ability in combination with Co ₃ O ₄ , the reduction of Co ³⁺ ions having oxidation activity to Co ²⁺ ions can be more remarkably suppressed. As a result, it is considered that both the heat resistance and catalytic activity of the obtained exhaust gas purifying catalyst can be further improved.

Such a CeO ₂ —ZrO ₂ solid solution can be prepared by any method known to those skilled in the art. For example, an alkaline substance such as aqueous ammonia is coprecipitated in a mixed solution obtained by dissolving each salt of cerium and zirconium constituting the solid solution in an amount corresponding to a desired Ce / Zr ratio (molar ratio). A CeO ₂ —ZrO ₂ solid solution in which ceria and zirconia are solid-solved with each other can be prepared by heat-treating.

Further, according to one embodiment of the present invention, the CeO ₂ —ZrO ₂ composite oxide is physically mixed in a powder state with the catalyst component Co ₃ O ₄ and, if necessary, a pellet or the like. Used after being molded into a shape. However, the exhaust gas purifying catalyst of the present invention is not limited to such an embodiment. For example, CeO ₂ —ZrO ₂ composite oxide is used as a catalyst carrier, and Co ₃ O as a catalyst component is used. ₄ may be supported, or a CeO ₂ —ZrO ₂ composite oxide and Co ₃ O ₄ forming a solid solution may be used as the exhaust gas purifying catalyst of the present invention.

  EXAMPLES Hereinafter, although an Example demonstrates this invention in detail, this invention is not limited to these Examples at all.

[Example 1]
[Preparation of CeO ₂ —ZrO ₂ Composite Oxide]
First, cerium ammonium nitrate ((NH ₄ ) ₂ Ce (NO ₃ ) ₆ ) as a cerium salt and zirconium oxynitrate dihydrate (ZrO (NO ₃ ) ₂ .2H ₂ O) as a zirconium salt are used as Ce / It was dissolved in a predetermined amount of distilled water in such an amount that the Zr ratio (molar ratio) = 1.0. Next, aqueous ammonia as a neutralizing agent was added to this solution to coprecipitate, and the obtained coprecipitate was dried at 120 ° C. for 8 hours and calcined at 500 ° C. for 2 hours to dissolve CeO and zirconia in a solid solution. A powder of _2- ZrO ₂ composite oxide (Ce / Zr ratio = 1.0, specific surface area: 50 to 60 m ² / g) was obtained.

[Preparation of catalyst]
The Co ₃ O ₄ nanopowder (crystallite diameter: 24 nm) manufactured by Aldrich and the CeO ₂ —ZrO ₂ composite oxide powder prepared above were compared with the mass of Co in the Co ₃ O ₄ nanopowder with respect to the total mass of the catalyst. Thus, it is physically mixed in the mortar in an amount of 5.0 wt% (Co ₃ O ₄ is 6.8 wt%), and consists of a mixed powder of Co ₃ O ₄ and CeO ₂ —ZrO ₂ composite oxide. An exhaust gas purification catalyst was obtained.

[Comparative Example 1]
Exhaust gas purification catalyst comprising a mixed powder of Co ₃ O ₄ and CeO ₂ was obtained in the same manner as in Example 1 except that CeO ₂ was used instead of CeO ₂ —ZrO ₂ composite oxide.

[Comparative Example 2]
Exhaust gas purification catalyst comprising a mixed powder of Co ₃ O ₄ and Al ₂ O ₃ was obtained in the same manner as in Example 1 except that Al ₂ O ₃ was used instead of CeO ₂ —ZrO ₂ composite oxide. .

[Evaluation of catalyst]
Regarding the exhaust gas purifying catalysts of Example 1 and Comparative Examples 1 and 2, their CO oxidation activity and HC oxidation activity were evaluated. A sample for evaluation was prepared by pressing each exhaust gas-purifying catalyst powder prepared above at a pressure of 196 MPa into a pellet. Next, with respect to 2.0 g of each of the pellet catalysts, the temperature of the catalyst bed was raised from 100 ° C. to 20 ° C./min while flowing each model gas for evaluation shown in Table 1 at a flow rate of 10 L / min. The temperature was raised at a temperature rate, and the temperatures at which the CO or C ₃ H ₆ purification rate was 50% were measured as the CO 50% purification temperature and the HC 50% purification temperature, respectively. The results are shown in Table 2.

  In order to investigate the heat resistance of each exhaust gas purifying catalyst in Example 1 and Comparative Examples 1 and 2, the CO 50% purification temperature and HC 50% purification of each exhaust gas purifying catalyst after calcination at 700 ° C. for 5 hours in air. Each temperature was also measured. And the heat resistance of each exhaust gas purification catalyst was evaluated by calculating the temperature difference between the CO 50% purification temperature and the HC 50% purification temperature when such heat treatment was not performed. These results are also shown in Table 2.

Referring to Table 2, regarding the initial catalytic activity not subjected to heat treatment at 700 ° C., in the exhaust gas purifying catalyst of Comparative Example 2 made of a mixed powder of Co ₃ O ₄ and Al ₂ O ₃ , Example 1 and Compared with the exhaust gas purifying catalysts of Comparative Example 1, the CO and HC 50% purification temperature was low, and therefore the oxidation activity of CO and HC was high. However, by performing the heat treatment at 700 ° C., the CO and HC 50% purification temperature of the exhaust gas purification catalyst of Comparative Example 2 greatly increased, that is, the oxidation activity of CO and HC decreased greatly, whereas Example 1 and In each exhaust gas purification catalyst of Comparative Example 1, the decrease in CO and HC oxidation activity after heat treatment at 700 ° C. was very small, and therefore the heat resistance of the catalyst was greatly improved.

On the other hand, in the exhaust gas purification catalyst of Comparative Example 1 made of a mixed powder of Co ₃ O ₄ and CeO ₂ , although the heat resistance of the catalyst was improved, the CO and HC 50% purification was compared with the exhaust gas purification catalyst of Comparative Example 2. The temperature was very high and therefore the oxidation activity of CO and HC was low. In contrast, the exhaust gas purifying catalyst of Example 1 composed of a mixed powder of Co ₃ O ₄ and CeO ₂ —ZrO ₂ composite oxide not only improved the heat resistance of the catalyst but also was a comparative example from the viewpoint of catalytic activity. It exhibited oxidation activity equivalent to or higher than that of No. 2 exhaust gas purification catalyst, and particularly showed high CO oxidation activity after heat treatment at 700 ° C. As is clear from the results of Example 1 and Comparative Example 1 shown in Table 2, CeO ₂ —ZrO ₂ composite oxide is used together with Co ₃ O ₄ instead of CeO ₂ alone. The purification performance at low temperature was significantly improved.

[Effect of Ce content in CeO ₂ —ZrO ₂ composite oxide]
Next, regarding the exhaust gas purification catalyst of Example 1, the heat resistance of each exhaust gas purification catalyst when the ratio of Ce contained in the CeO ₂ —ZrO ₂ composite oxide was changed from 0 to 100 mol% was examined. It was. The test is performed in the same manner as described above, that is, each exhaust gas purification catalyst is measured for CO50% purification temperature before and after 700 ° C. heat treatment, and the temperature difference between them is calculated to calculate each exhaust gas purification. The heat resistance of the catalyst was evaluated. The result is shown in FIG.

FIG. 1 is a graph showing the relationship between the ratio of Ce contained in the CeO ₂ —ZrO ₂ composite oxide and the heat resistance of the catalyst. FIG. 1 shows the ratio (mol%) of cerium to the total molar amount of cerium and zirconium contained in the CeO ₂ —ZrO ₂ composite oxide on the horizontal axis, and the vertical axis of the CO 50% purification temperature before and after 700 ° C. heat treatment. The temperature difference (° C.) is shown. As is apparent from the results of FIG. 1, it can be obtained by increasing the cerium ratio in the CeO ₂ —ZrO ₂ composite oxide to more than 30 mol%, particularly by setting the cerium ratio to 40 mol% or more. The heat resistance of the exhaust gas purifying catalyst was significantly improved.

Claims (4)

Co ₃ O ₄ and a CeO ₂ —ZrO ₂ composite oxide represented by the general formula xCeO _2. (1-x) ZrO ₂ , wherein 0.3 <x <1.0 An exhaust gas purifying catalyst that is characterized. It characterized in that the CeO ₂ -ZrO ₂ composite oxide is CeO ₂ -ZrO ₂ solid solution, a catalyst for purification of exhaust gas according to claim 1.   The exhaust gas-purifying catalyst according to claim 1, wherein 0.4 ≦ x <1.0.   The exhaust gas-purifying catalyst according to claim 3, wherein x = 0.5.

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