JP2013203609A - Oxygen storable ceramic material, method for producing the same, and catalyst - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a material having a good oxygen storage capacity and exhaust gas purification performance, a method for producing the material, and a catalyst for gas purification.SOLUTION: There is provided a ceramic material: CeFeO, x is 0.03-0.4, containing cerium and iron, and comprising a composite metal oxide and/or a solid solution, the ceramic material having an oxygen storage capacity of at least 10 mL per g in terms of oxygen (O) gas. A method for producing the ceramic material includes a process in which a precipitate, after being formed from a solution containing cerium and iron by being added with a precipitant and separated, is dried and fired at a temperature of 600-1,000°C in air, and the fired precipitate, after being heated at a temperature of 800°C or below in a reducing atmosphere, is further heat-treated in an oxidizing atmosphere. In addition, a catalyst for exhaust gas purification and mixed gas treatment using the ceramic material is provided.

Description

The present invention relates to a material having an oxygen storage capacity, a method for producing the same, and a catalyst that makes use of the characteristics thereof, which are used in an exhaust treatment technique.

The exhaust gas from an internal combustion engine such as an automobile engine contains hydrocarbon (HC) and the like, but these substances can be removed by a combustion catalyst or an exhaust gas purification catalyst. Further, in the engine exhaust purification catalyst, carbon monoxide (CO) and hydrocarbons can be oxidized and simultaneously removed by a three-way catalyst that can reduce nitrogen oxides (NOx). In recent years, a solid solution or composite oxide of ceria (cerium oxide, CeO _x ) and zirconia (zirconium oxide, ZrO _x ) absorbs fluctuations in oxygen concentration in the exhaust gas and enhances the exhaust gas purification ability ( It is widely known that it is a material having OSC), and is used as a catalyst containing this or as an exhaust gas purification catalyst carrier.

In a three-way catalyst, the air-fuel ratio of the internal combustion engine needs to be the stoichiometric air-fuel ratio, so it is high exhaust gas purification ability to absorb oxygen concentration fluctuations in the exhaust gas and maintain the oxygen concentration near the stoichiometric air-fuel ratio. It is preferable to exhibit It is also said that this OSC provides an oxygen activation function and can be purified by an oxidation reaction in removing volatile organic compounds (VOC). Furthermore, such a property is also used for processing and reforming of a mixed gas containing hydrocarbons.

In addition, the oxygen storage capacity material has various potential uses derived from oxygen storage itself, for example, there are proposals such as a property that stores oxygen and a useful catalyst in a process of generating hydrogen in a chemical reaction, Although it is not as widely industrialized as an automobile catalyst, its use is expected as a catalyst for treating mixed gas.

As described above, the oxygen storage capacity material and the use thereof are technologies that have been realized and expected in the future, but ceria or ceria zirconia is said to have the most preferable properties. On the other hand, there is a problem that it is difficult to cope with the reduction in price that is often required in that cerium is expensive. In addition, the use of high-quality cerium raw materials is limited in terms of resources and economy, and it is necessary to use alternative materials and reductions.By combining with other elements and components having characteristic properties, substitution, There is a need for new compositions to replace this.

Among the catalysts for oxygen storage capacity (OSC) and their materials, cerium-containing materials are particularly excellent, and their usage is large. Therefore, the improvement of OSC performance by other elements exhibiting the same properties as these is various. Exhaust purification technology has become a problem from the viewpoint of resource utilization. Furthermore, the performance improvement is anticipated by the manufacturing method which can achieve control of the material using other elements, its form, and structure | tissue.

For example, Japanese Patent Publication No. 6-44999 describes that ceria zirconia solid solution is used as a catalyst having a high oxygen storage capacity, and that oxygen storage capacity materials are effectively used in automobile exhaust gas purification catalysts. It is an example. In terms of performance, it is widely known that the oxygen storage capacity of this composition is excellent, and it has great utility. However, since cerium and zirconium are used, the product price cannot be reduced, so that it is difficult to solve the problem that the cost of the produced catalyst is high.

Japanese Patent Laid-Open No. 2007-83126 states that as a novel oxygen storage capacity material, calcium aluminosilicate is used as an oxygen storage capacity material, and a part of this Ca may be replaced with a transition metal such as copper or iron. A material that exhibits oxygen storage capacity has been proposed, and it can be considered that the cost can be reduced by a general-purpose composition. However, this material has a problem in that the amount of oxygen itself is not large and the performance is not sufficient in the characteristics relating to repeated use.

Furthermore, Japanese Patent Application Laid-Open No. 2011-41912 proposes a material that maintains high catalytic activity of iron without solid-dissolving iron in ceria zirconia by mixing iron with ceria or ceria zirconia and performing oxidation-reduction treatment. Yes. In this catalyst, it has been proposed to replace the catalytic function with an iron component, but mainly for the purpose of improving oxidation activity, and the state of iron by reduction treatment is supported on ceria particles and the surface of the iron itself is supported. Since it exhibits a catalytic action, it is not suitable for improving the oxygen storage capacity itself or for its utilization. Further, in the manufacturing method, iron uses a known process such as an impregnation method.

JP 6-44999 JP2007-83126A JP2011-41912

  The present invention has been made in view of the above-described conventional situation, and the amount of cerium is reduced, and the resource can be effectively replaced without any problem in terms of resources and price. A catalyst is provided. In addition, an oxygen storage capacity material using a ceramic iron component, which is a general-purpose component, which is effective from a resource problem, is possible.

As a result of intensive studies, the present inventors have found that the above problems can be solved. That is, according to the present invention, an oxygen storage capacity ceramic material, a method for producing the same, and a catalyst are provided.

[1] It contains cerium and iron, is Ce _1-x Fe _x O _y , x is 0.03 to 0.4, and oxygen storage capacity is 10 ml or more in terms of oxygen (O ₂ ) gas per gram. A ceramic material comprising a composite metal oxide and / or a solid solution.

[2] A precipitant is added from a solution containing cerium and iron to form and separate a precipitate, which is then dried, calcined at 600 ° C. to 1000 ° C. in air, heated in a reducing atmosphere of 800 ° C. or lower, and further in an oxidizing atmosphere. A method for producing a ceramic material, comprising a step of heat treatment.

[3] A catalyst for exhaust gas purification and mixed gas treatment comprising the ceramic material of [1] or the ceramic material obtained by the production method of [2].

It is a figure which shows the oxygen release characteristic of the ceramic material obtained by this invention.

Hereinafter, embodiments of the present invention will be described with reference to the drawings. The present invention is not limited to the following embodiments, and changes, modifications, and improvements can be added without departing from the scope of the invention.

The present invention includes the cerium and _iron, with _{Ce 1-x Fe x O y} , x is 0.03 to 0.4, an oxygen storage capacity per gram of oxygen _{(O 2)} or 10ml with gas equivalent It is a composite metal oxide and / or solid solution catalyst material. Also, a precipitant is added from a solution containing cerium and iron to form a precipitate, the precipitate is separated, dried, and calcined in air at 600 ° C to 1000 ° C. Later, it is manufactured by a process of heating in a reducing atmosphere of 800 ° C. or lower and further heat-treating in an oxidizing atmosphere, and the material is used for exhaust gas purification and treatment of various mixed gases.

The oxygen storage capacity ceramic material according to the first aspect of the present invention includes a composite metal oxide containing cerium and iron and having an oxygen storage capacity of 10 ml or more, more preferably 18 ml or more in terms of O ₂ gas per gram, and / or or a solid solution catalyst material, in the chemical composition formula _{Ce 1-x Fe x O y} , x is 0.03 to 0.4, more preferably x is 0.05 to 0.3, more preferably x 0.1 to 0.25. Although y is not particularly limited, it is 1.1 to 2.0.

At least a part of the material of the present invention is characterized in that it is a solid solution or composite oxide of ceria and iron oxide, and the ratio is preferably 80% or more by volume ratio. Furthermore, it is preferable that a catalyst for exhaust gas purification and various mixed gas treatments is constituted by the composition containing this material.

Regardless of whether the form of the composite material is particulate or massive, the particle diameter of the composite material is preferably 0.1 to 100 μm, more preferably 0.05 to 50 μm. Moreover, although it does not prevent what is called a bulk state, such as a film form or on a thick plate, in that case, they are aggregates of particles.

Furthermore, when the oxygen storage capacity material of the present invention includes a ceria-iron oxide solid solution, these include other metals selected from the group consisting of, for example, zirconium (Zr), rare earth elements, and alkaline earth metals. You may go out. The rare earth metal is selected from the group of yttrium, scandium, lanthanum, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, and lutetium, and oxides of these rare earth metals are added. In particular, it is more preferable to contain zirconia (Zr), lanthanum (La), neodymium (Nd), praseodymium (Pr), and yttrium (Y) alone or in a mixed state. As the alkaline earth metal, magnesium and calcium are preferable. The addition of zirconium is particularly preferred.

Depending on the structure of the solid solution or composite oxide, taking advantage of the high oxygen storage capacity that is derived from ceria and iron oxide components but not expressed alone, when this is used as a catalyst carrier or catalyst component, large amounts of oxygen are released and absorbed. The effect of is expressed. The manifestation mechanism regarding the material structure is not clear, but the state of the iron cerium complex oxide or solid solution is considered to be a stable structure suitable for oxygen storage by suppressing the reduction of iron oxide to metallic iron. Yeah.

According to this composite oxide particle, iron oxide provides resistance to reduction of iron oxide by forming a solid solution or composite oxide with ceria. There is an effect of imparting stable OSC and oxidation activity.

In the method for producing ceria zirconia composite particles according to the second invention of the present invention, a precipitant is added from a solution containing cerium and iron to form a precipitate, the precipitate is separated, dried, and then heated to 600 ° C. in air. After being fired at ˜1000 ° C., it is produced by a process of heating in a reducing atmosphere of 800 ° C. or lower and further heat-treating in an oxidizing atmosphere.

In the aqueous solution containing iron and cerium ions, further adjusting to an alkaline solution by addition or coexistence of a precipitating agent, and precipitating iron oxide or cerium-containing particles, including a firing step at 600 ° C. to 1000 ° C. in air, And it is manufactured by the process of heating in a reducing atmosphere below 800 degreeC after that, and also heat-processing in an oxidizing atmosphere. The mixing process in an aqueous solution or other solvent, the solid-liquid separation process, the drying process, and the baking process may be performed by a normal method. In particular, heat treatment under oxidation / reduction conditions at 800 ° C. or lower is important, and characteristic oxygen It is preferable from the viewpoint of improving its activity as a catalyst material that exhibits storage ability characteristics.

The iron and cerium raw materials used in the production of the material of the present invention are not particularly limited, and a wide range of water-soluble salts can be used. The inclusion of each metal component in the aqueous solution can be realized by dissolving a salt in water, and the metal salt can be used without particular limitation such as nitrate, chloride, acetate, ammonium nitrate. Furthermore, it is good also as a metal ion containing aqueous solution by melt | dissolving a metal or a metal oxide, and each other metal containing solid using nitric acid or another solution with high solubility. Furthermore, organic acids containing metal components such as various carboxylic acids and water-soluble complexes can be used as raw materials.

When preparing a precipitate from the above aqueous solution, an additive for realizing an alkaline solution is added to the aqueous solution containing iron and cerium ions so that they coexist in the aqueous solution. Aqueous ammonia can be used typically and simply, and many additions such as adding organic amines such as dimethylamine and trimethylamine, and hexamethylenetetramine and urea that decompose to express alkalinity in the aqueous solution It can be used in the form of an agent or a mixture thereof, and can be used in cold conditions other than room temperature. Further, known metal-containing aqueous solutions showing alkalinity, for example, solutions of alkali metals or alkaline earth hydroxides can be used in the same manner as ammonia.

The phenomenon of precipitation formation is somewhat adjusted and separated depending on the physical properties of the precipitate, but it is sufficient that the pH is approximately 10 or so. The pH may be adjusted at any point in the process and at any concentration because the pH may be adjusted according to the amount of the alkaline additive such as ammonia water and the amount generated.

In the precipitation production process, application of a so-called sol-gel method using an organic solvent or another solvent or an inorganic derivative synthesis method from an organometallic complex is not hindered.

The precipitate thus obtained is solid-liquid separated by operations such as centrifugation and filtration, and then dried at room temperature to 200 ° C. or calcined without drying. The drying conditions are not particularly limited, but may be air drying at room temperature, freeze drying, a heat drying method at a temperature in the vicinity of 80 ° C. to 150 ° C., or the like. The present invention includes a step of baking at 600 ° C. to 1000 ° C., and then heating in a reducing atmosphere of 800 ° C. or lower and further heat-treating in an oxidizing atmosphere. In this step, heat treatment under oxidation / reduction conditions is particularly important, and it is preferable from the viewpoint of improving its activity as a catalyst material that expresses characteristic OSC. The heat treatment under oxidation / reduction at 800 ° C. or lower is important for activating the solid solution or / and composite oxide of ceria and iron oxide, preferably 600 to 800 ° C. When the temperature is higher than 800 ° C., the catalyst activity and the catalyst carrier performance become insufficient due to the aggregation phenomenon. The temperature on the low temperature side is not particularly limited, but if it is lower than 600 ° C., it is necessary to take a longer treatment time. The heat treatment time is not particularly limited, but the treatment may be usually performed for 30 to 300 minutes.

For reducing conditions, an inert gas mixed with dilute hydrogen gas is usually suitable, and hydrogen / argon, hydrogen / helium, hydrogen / nitrogen mixed gas, or the like can be used. The conditions are not limited, such as carbon monoxide, carbon monoxide / carbon dioxide mixture, hydrocarbon mixture, steam / hydrogen coexisting gas, or reducing gas under incomplete combustion conditions with oxygen-containing gas in the presence of carbon. The oxidation treatment condition may be simply in the air. The time for these oxidation-reduction treatments may be about the above-mentioned time, and the oxidation-reduction can be maintained in a preferable state of the material as long as it is at least once each other.

In addition, the step of processing into powder suitable for the catalyst by appropriately performing processing such as pulverization and ball milling, that is, mixing the raw materials and stirring sufficiently during precipitation to maintain the uniformity of ceria-added zirconia in water In some cases, various mill operations and mixing operations such as a bow mill may be performed, and further, after washing, filtration or drying, or after heat treatment, mixing operations such as a mill operation or a bow mill are not prevented.

In the present invention, it is important that a large amount of oxygen storage capacity is stably expressed at a low temperature by performing oxidation-reduction treatment. Although the mechanism is not clear, it is conceivable that the state of the oxide material made of cerium iron becomes a structure more suitable for oxygen storage by such heat treatment.

In the third aspect of the present invention, a catalyst for purifying exhaust gas or the like, the oxygen storage capacity material achieved by the first and second aspects of the present invention is directly molded or other metal oxidation It can be used as a catalyst by being supported on a material and also by coexisting a noble metal such as platinum, rhodium or palladium. The mixing of the various components into the composite oxide particles can be performed using a known method. For example, in the production method according to the second invention of the present invention, the raw material solution is used to make the powder coexist with the components. Examples of the method include addition, drying and firing. Preferably, alumina or zirconia is preferably allowed to coexist, and metal oxides such as titania, aluminum hydroxide, hydrated zirconia, zirconium hydroxide, boehmite, and other catalyst components are not prevented from coexisting.

The catalyst of the present invention exhibits an excellent oxygen storage capacity, and can be used as a catalyst for purifying automobile exhaust gas, a purifying catalyst for contaminated air containing organic gas, and a catalyst for treating various mixed gases. The purification catalyst of the present invention can be used not only by molding itself but also by coating a monolithic carrier such as a ceramic honeycomb.

(Example _{1: Ce 1-x Fe x} O y composite composition dependence)
The powder composition of _{Ce 1-x Fe x O y} (x = 0,0.05,0.1,0.2,0.3) was prepared in the coprecipitation method. In 2 L (liter) of distilled water, Ce (NH ₄ ) ₂ (NO ₃ ) ₆ (Wako Pure Chemicals 95%) and Fe (NO ₃ ) ₃ (Wako Pure Chemicals 99%) After weighing and adding enough to the above composition, 25 wt% aqueous ammonia (manufactured by Wako Pure Chemical Industries, Ltd.) diluted 3-fold was added to this while stirring until pH 10. A precipitate aged in water for 3 hours was filtered, washed, dried at 110 ° C. for 24 hours, calcined at 600 ° C., and further heat treated at 800 ° C. for 3 hours to obtain a series of samples. The produced phase was evaluated by powder X-ray diffraction (XRD) for each of the above samples. From the XRD pattern, when x is in the range of 0 to 0.3, a diffraction pattern similar to CeO ₂ can be observed, and the formation of a solid solution Ce _1-x Fe _x O _y in which iron oxide is dissolved in cerium oxide was observed. .

In order to evaluate the oxygen storage capacity of the composite powder sample, the following operation was performed using a gas adsorption amount measuring device BP-1S (manufactured by Okura Riken, special specification). A 0.1 gram sample is placed in a quartz sample tube, exhausted at room temperature, and then heated from room temperature to 800 ° C at a flow rate of 30 ml / min. The temperature was raised at 0 ° C. and held at 800 ° C. for 20 minutes for reduction. During this time, the gas composition change was monitored with a TCD detector. Further, the gas was circulated in Ar (argon), and the temperature was lowered to 600 ° C. and held, and oxygen gas was introduced in pulses, and the oxygen absorption amount was measured with a TCD detector.
These results are shown in Table 1.

From Table 1, as compared to cerium oxide C1 (x = _0), in the _{Ce 1-x Fe x O y} (x = 0.05,0.1,0.2,0.3), significant oxygen An increase in storage capacity is observed, indicating that the material is an excellent oxygen storage capacity. Further, these characteristics were maintained even after repeated heat treatment with reduction or oxidation up to 800 ° C., and thus had the same OSC properties as cerium oxide.

Iron oxide as a comparative example (x = 1.0) and _{Ce 1-x Fe x O y} (x = 0.5, x = 1.0) A sample of iron oxide, Example 1 without using the starting cerium It produced similarly. Comparative Sample C2, _also the powder of the composition of _{Ce 1-x Fe x O y} (x = 0.5) was prepared by the same co-precipitation method as in Example 1 (comparative sample C3). When the generated phase was evaluated by powder X-ray diffraction (XRD), iron oxide (Fe ₂ O ₃ ) was present when x = 1.0, and coexistence of the solid solution and Fe ₂ O ₃ was observed when x = 0.5. Admitted. In order to evaluate the oxygen storage capacity of the sample, the same operation as in Example 1 was performed using a gas adsorption amount measuring device BP-1S (manufactured by Riken Okura, special specification).
The results are shown in Table 2.

It can be seen that sufficient OSC is not observed in the comparative samples C2 and C3. Further, in Fe ₂ O ₃ of the comparative sample C2, a change in the sample was observed after the reduction treatment at 800 ° C. When examined by XRD, metal iron was generated, and a phenomenon was observed in which metal oxide was reduced and decomposed into metal rather than release of oxygen. Therefore, a sufficiently reproducible result was not obtained in the oxygen pulse measurement. Further, in the sample C3 where x = 0.5, metallic iron was similarly generated, and the oxygen storage capacity could not be measured with good reproducibility. These causes were considered that iron oxide changed to metallic iron under reducing conditions and did not return to the original oxide due to thermal change at that time. Therefore, it was proved unsuitable as an oxygen storage capacity material that needs to be repeatedly used under redox.

(Example _2: heat treatment of _{Ce 1-x Fe x O y} )
In this example, the invention relating to the improvement of oxygen storage capacity in the reduction and oxidation treatment at 800 ° C. is shown by the result of the sample of x = 0.2 produced in Example 1. In order to evaluate the oxygen storage capacity at the time of repeated use, the same operation as in Example 1 was performed. The same operation is repeated 6 times, that is, the reduction treatment in a 5% hydrogen / Ar 95% gas stream up to 800 ° C. and the subsequent oxygen pulse oxidation treatment (oxygen absorption) at 600 ° C. are repeated. The TCD signal during each temperature reduction was monitored (this figure is called a temperature reduction spectrum). The result is shown in FIG.

The peak indicated by the TCD signal on the vertical axis in the temperature-programmed reduction spectrum of FIG. 1 shows that hydrogen introduced onto the sample reacts with oxygen in the solid and is consumed at each temperature, that is, oxygen is absorbed from within the solid. It shows the degree of reaction at that temperature in the phenomenon of released hydrogen being oxidized. In the initial sample Ce _0.8 Fe _0.2 O _y produced by treatment in air at 800 ° C. (lowermost curve), oxygen release was observed at 400 to 500 ° C. and around 700 to 800 ° C. When this was subjected to a reduction treatment at 800 ° C. and then an oxidation treatment at 600 ° C., a broad peak was observed at 500 to 700 ° C., mainly in the vicinity of 600 ° C., from the first time to the sixth time. This result shows that, by reduction at 800 ° C. and subsequent oxidation treatment (change to the first curve), the main oxygen release temperature is lowered from around 700 ° C. to around 500 ° C. and continuously from 300 ° C. to 700 ° C. It shows that the characteristics that oxygen can be released are improved. Furthermore, this characteristic shows that the oxygen storage capacity material has good properties because oxygen release and absorption continue without deterioration even after repeated oxidation-reduction (from the first time to the sixth time). That is, it can be seen that the comparative examples C2 and C3 containing iron oxide itself and iron oxide show properties that cannot be realized, there is no problem due to reduction to metallic iron, and it can be used repeatedly even at high temperatures. Furthermore, it has an oxygen storage capacity higher than that of cerium oxide and has excellent properties as a mass oxygen storage material. As mentioned above, it turns out that the composite material of this invention is excellent in performance.

(Example 3: Hydrocarbon purification activity)
In order to examine the catalyst activity, the propylene (C ₃ H ₆ ) purification catalyst activity was evaluated by a fixed bed type catalyst evaluation apparatus designed in the laboratory. In _Ce 1-x _Fe x _{O y} prepared in Example 1, x = 0 (comparative sample), 0.2 (present invention), using powder of 0.3 (present invention), each 100mg in a glass tube And the C ₃ H ₆ concentration is 3345 (ppmC) on the carbon basis, 0.5% O ₂ concentration and the balance nitrogen gas, the flow rate is 500 ml per minute, and the SV is circulated at about 300,000 l · kg ⁻¹ h ⁻¹ Then, a purification performance test was performed in which the hydrocarbon combustion activity at 550 ° C. was determined by the amount of CO 2 produced. As a result, when x = 0, the purification rate was 10%, when x = 0.2, the purification rate was 96%, and when x = 0.3, the purification rate was 98%. From these results, it can be seen that when this material is used as a catalyst, a high gas processing capacity is developed.

In general, iron oxide is easily reduced to agglomerate into metallic iron and does not recover its activity. However, in the present invention, the iron component is stable by forming a solid solution or composite oxide with cerium oxide having excellent oxidizing power. Because it becomes a state, it is difficult to reduce to metallic iron, and it can be a catalyst having reduction resistance, it can withstand repeated use in oxidation and reduction, and is useful as an oxygen storage capacity material for automobile catalysts and a hydrocarbon purification catalyst by oxidation reaction . Furthermore, in the production method of the present invention, when the oxidation-reduction treatment is performed, the temperature of oxygen release is lowered, and the oxygen release temperature is continuously expressed over a wide range of temperatures, so that a useful oxygen storage ability (release ability) is exhibited. . Furthermore, it has high activity as a catalyst. Thus, a low-cost catalyst material using iron that can reduce cerium and can be used abundantly as a resource is provided. Furthermore, despite the reduced amount of cerium, it is possible to provide a catalyst that has a very high oxygen storage capacity and can withstand repeated use of redox.

The ceramic material of the present invention can be used as an oxygen storage capacity material for storing or releasing oxygen as a catalyst for oxygen storage, mixed gas treatment, or purification of exhaust gas from an internal combustion engine such as an automobile engine.

Claims (3)

Composite metal oxide containing cerium and iron, Ce _1-x Fe _x O _y , x is 0.03 to 0.4, and oxygen storage capacity is 10 ml or more in terms of oxygen (O ₂ ) gas per gram. Ceramic material comprising a product and / or a solid solution. A precipitant is added from a solution containing cerium and iron to form a precipitate, the precipitate is separated, dried, fired in air at 600 ° C. to 1000 ° C., heated in a reducing atmosphere of 800 ° C. or lower, and further heat-treated in an oxidizing atmosphere. The manufacturing method of the ceramic material including the process to do. A catalyst for exhaust gas purification and mixed gas treatment comprising the ceramic material of claim 1 or the ceramic material obtained by the production method of claim 2.