JP2014172775A - Method for producing powder - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide powder which contains a reduced rare element and has high performance as a catalyst for exhaust gas purification.SOLUTION: Powder is produced using a solution containing a salt of an iron ion, and at least one of zirconium, silicon, titanium, and aluminum in a reaction field of supercritical water or subcritical water. Preferably, the produced powder comprises nanoparticles. The powder contains an iron oxide containing magnetite having a large oxygen storage and discharge capacity, and high heat-resistant silica, zirconia, titania, and alumina.

Description

  The present invention relates to a method for producing a powder, for example, a method for producing a powder using supercritical water or subcritical water.

  As an exhaust gas purifying catalyst, a catalyst in which a noble metal such as platinum (Pt) or rhodium (Rh) is supported on a carrier containing cerium (Ce) is used. For example, Patent Documents 1 and 2 describe catalyst carriers that do not contain cerium. For example, Patent Document 1 describes that a powder in which silica is coexisted with iron oxide powder is used as a catalyst. For example, Patent Document 2 describes using particles containing iron as a catalyst. Furthermore, for example, Patent Document 3 describes that oxide microcrystal particles generated using a reaction field of supercritical water are used as a support.

JP 2000-290018 A JP 2012-135716 A JP 2009-172593 A

  Cerium is a rare element, and cerium depletion is becoming a serious problem. In addition, cerium is expensive, and a catalyst using cerium is expensive. On the other hand, as in Patent Documents 1 and 2, a catalyst using a carrier that does not contain cerium has a lower oxygen storage / release capacity than a carrier that contains cerium. For this reason, the catalyst using the support | carrier which does not contain cerium will also be inferior in the performance as a catalyst.

  The present invention has been made in view of the above problems, and an object of the present invention is to provide a powder that reduces rare elements and has high performance as an exhaust gas purifying catalyst.

  The present invention uses a solution containing a salt of iron ions and at least one of zirconium, silicon, titanium and aluminum to produce a powder in a reaction field of supercritical water or subcritical water. It is a manufacturing method of a body.

  The said structure WHEREIN: The said powder can be set as the structure containing a zirconia. Moreover, the said structure WHEREIN: The said powder can be set as the structure containing a magnetite.

  The said structure WHEREIN: The reaction field of the said supercritical water or subcritical water can be set as the structure which is a reaction field of supercritical water. In the above configuration, the powder may be a powder for exhaust gas purification catalyst.

  ADVANTAGE OF THE INVENTION According to this invention, a rare element can be reduced and the powder with the high performance as a catalyst for exhaust gas purification can be provided.

FIG. 1 is a schematic diagram illustrating a manufacturing apparatus for manufacturing powder according to an embodiment. FIG. 2A and FIG. 2B are diagrams showing the XRD analysis results of Examples 1 and 3, respectively. FIGS. 3A and 3B are diagrams showing TEM images of Examples 1 and 3, respectively. FIG. 4 is a diagram showing the oxygen storage / release capability in each example. FIG. 5A and FIG. 5B are diagrams showing the removal rates with respect to temperature in Example 1 and Comparative Example 2. FIG. FIGS. 6A and 6B are diagrams showing the removal rate with respect to the temperature in the initial state. FIG. 7A and FIG. 7B are diagrams showing the removal rate with respect to the temperature after aging.

  The present inventor has studied a method for producing a powder for an exhaust gas purifying catalyst containing iron and having good characteristics such as oxygen storage / release capability.

  FIG. 1 is a schematic diagram illustrating a manufacturing apparatus for manufacturing powder according to an embodiment. With reference to FIG. 1, the manufacturing apparatus 10 includes pumps 12 and 13, a heating unit 14, a cylinder pump 16, a cooling unit 18, a recovery cylinder 20, and a pipe 22. The pump 12 supplies pure water to the pipe 22. The pump 13 is a pump for maintaining the pressure in the pipe 22 before introducing the raw material solution into the pipe 22. The heating unit 14 is a heater and heats pure water in the pipe 22. The cylinder pump 16 introduces the raw material solution into the pipe 22. The cooling unit 18 is a unit in which a pipe 22 is passed through a water bath, and cools water containing powder in the pipe 22 to approximately room temperature. The collection cylinder 20 adjusts the pressure in the pipe 22. Also, the powder is collected. The water in the pipe 22 from the heating unit 14 to the cooling unit 18 is in a supercritical state. A supercritical water reaction field is formed in the region 24 from the introduction of the raw material into the supercritical water in the pipe 22 to the cooling unit 18. Positions 22 a to 22 d indicate the position of the pipe 22.

In Example 1, the following conditions were used.

Flow rate of pure water supplied by the pump 12: 30 ml / min Pressure in the pipe 22 in the region 24: 30 MPa
Flow rate of raw material solution supplied by the cylinder pump 16: 10 ml / min Diameter of the pipe 22: 1/8 inch Length of the region 24: 1 m
Temperature at position 22a: 452 ° C
Temperature at position 22b: 435 ° C
Temperature at position 22c: 400 ° C
Temperature at position 22d: 24 ° C
Specific resistivity of pure water: 15-18 MΩ · cm
Reaction time: 1 second

Table 1 is a table showing the conditions of the manufacturing method according to Examples 1 and 2.

In Examples 1 and 2, iron sulfide (FeSO ₄ ) and sodium silicate (NaSiO ₃ ) were used as raw materials. The production method of the raw material solution is as follows.
12 g (Example 1) or 36 g (Example 2) of sodium silicate solution (water glass, 55% by weight) is dissolved in 500 g of pure water.
75 g (Example 1) or 225 g (Example 2) of iron sulfide hydrate is dissolved in 500 g of pure water.
The iron sulfide solution is added while stirring the sodium silicate solution using a magnetic stirrer.
A 3 mol / liter (mol / l) potassium hydroxide (KOH) solution is added to the stirring solution to adjust the pH to 7.
Pure water is added so that the volume of the solution is 1800 ml.

Table 2 is a table | surface which shows the conditions of the manufacturing method which concerns on Example 3 and 4. FIG.

In Examples 3 and 4, iron sulfide (FeSO ₄ ) and zirconium oxynitrate (ZrO (NO ₃ )) were used as raw materials. The production method of the raw material solution is as follows.
12.5 g (Example 3) or 37.5 g (Example 4) of iron sulfide hydrate is dissolved in 90 g of pure water.
4.8 g (Example 3) or 14.4 g (Example 4) of zirconium oxynitrate is dissolved in 90 g of pure water.
Add the iron sulfide solution to the zirconium oxynitrate solution.
To the stirring solution, 3 mol / liter potassium hydroxide (KOH) solution is added to adjust the pH to 7.
Pure water is added so that the volume of the solution is 300 ml.

Next, the reaction process will be described.
Pure water is supplied to the pipe 22 from the pumps 12 and 13. At this time, the flow rate of the pump 12 is the same as the flow rate during the reaction. The flow rate of the pump 13 is the same as the flow rate of the raw material solution supplied to the pipe by the cylinder pump 16.
Using the recovery cylinder 20, the pressure in the pipe 22 is adjusted.
The raw material solution is introduced into the cylinder pump 16.
Using the heating unit 14, pure water is heated to a predetermined temperature.
The pump 13 is stopped and the raw material solution is supplied into the pipe 22 using the cylinder pump 16.
While the raw material solution is supplied into the pipe 22 using the cylinder pump 16, the liquid after the reaction discharged from the recovery cylinder 20 is recovered.
After supplying the raw material solution, the temperature of pure water is lowered while supplying pure water using the pump 12.

Next, the process of recovering powder from the recovered reaction solution will be described.
Centrifuge the reaction and discard the supernatant liquid. The acceleration of centrifugation is 10,000 G, and the time is 30 minutes.
Add 250 ml of pure water to the precipitate and centrifuge again under the same conditions. Repeat three times.
In Examples 1 to 3, the precipitate is redispersed in 100 ml of pure water and then freeze-dried.
In Examples 3 and 4, the precipitate is redispersed in ethanol and then supercritical carbon dioxide drying is performed.
In Example 3, the freeze-dried powder and the supercritical carbon dioxide-dried powder were produced.

  As described above, the powders according to Examples 1 to 4 are collected.

The powder produced using Examples 1 and 3 was analyzed using X-ray diffraction (XRD). Example 3 is freeze-dried. FIG. 2A and FIG. 2B are diagrams showing the XRD diffraction results of Examples 1 and 3, respectively. 2A and 2B show the signal intensity (arbitrary unit) with respect to 2θ. The solid line arrows and the broken line arrows show the signals of Fe ₃ O ₄ and Fe ₂ O ₃ , respectively. The parentheses indicate the face index of Fe ₃ O ₄ . Figure 2 (a), the the powder of Example 1 includes magnetite _(Fe 3 _{O 4)} as the iron oxide. The powder of Example 1 also contains silica (SiO ₂ ), but the silica is not crystallized and no silica signal is observed. Figure 2 Referring to (b), the powder of Example 3 contains the iron oxide magnetite _(Fe 3 _{O 4)} and _Fe 2 _{O 3.} The powder of Example 3 also contains zirconia (ZrO ₂ ), but zirconia is not crystallized, and no zirconia signal is observed.

  The powder produced using Examples 1 and 3 was observed using a transmission electron microscope (TEM). Example 3 is freeze-dried. FIGS. 3A and 3B are diagrams showing TEM images of Examples 1 and 3, respectively. 3A and 3B, it can be seen that the powders according to Examples 1 and 3 have a particle size of 20 nm to 50 nm. Thus, the powder according to Examples 1 and 3 is a nanometer-sized fine powder, which is a so-called nanoparticle.

  Next, for Examples 1 to 4, the oxygen storage / release capacity was measured using a thermogravimetric analyzer. Example 3 is obtained by drying with supercritical carbon dioxide. The method for measuring the oxygen storage / release capacity is the same as in paragraph 0020 of Patent Document 1.

  FIG. 4 is a diagram showing the oxygen storage / release capability in each example. In each Example, the oxygen storage / release capacity (μmol-O / liter: μmol-O / l) measured in the first, second and third cycles from the left is shown. Comparative Example 1 illustrates the value with the highest oxygen storage / release capacity at 800 ° C. in FIG.

  In any of Example 1 to Example 4, the oxygen storage / release capacity is larger than that of Comparative Example 1. In Example 1 and Example 2, the oxygen storage / release capability is comparable, but in Example 3 and Example 4, Example 4 has a greater oxygen storage / release capability. By optimizing the concentration of the raw material in the raw material solvent, the oxygen storage / release capability may be further increased. The oxygen storage / release capacity is not significantly different from the drying method. As described above, any drying method such as freeze drying, supercritical drying, and vacuum drying may be used.

  Next, the catalytic activities of Example 1 and Example 3 were measured. A catalyst using a carrier containing cerium generally used for a vehicle exhaust gas purification catalyst is referred to as Comparative Example 2. The method for measuring the catalytic activity is the same as in paragraphs 0076 and 0077 of Patent Document 3.

FIG. 5A and FIG. 5B are diagrams showing the removal rates with respect to temperature in Example 1 and Comparative Example 2. FIG. FIG. 5 (a) shows the measurement result in the initial state, and FIG. 5 (b) shows the measurement result after aging at 1000 ° C. for 20 hours. Black circles, black triangles, and black squares indicate the removal rates of NO _x , CO, and C ₃ H ₆ in Example 1, respectively. The solid line is an approximate curve. White circles, white triangles, and white squares indicate the removal rates of NO _x , CO, and C ₃ H ₆ in Comparative Example 2, respectively. A broken line is an approximate curve.

  With reference to Fig.5 (a), in 300 degreeC or more, both Example 1 and the comparative example 2 have a high removal rate of each gas. In Example 1 and Comparative Example 2, the removal rate of each gas is substantially the same. Thus, in Example 1, the same gas removal rate as that of Comparative Example 2 can be obtained.

  Referring to FIG. 5B, in Comparative Example 2, the removal rate of each gas is substantially the same as that in FIG. 5A before aging. On the other hand, in Example 1, the removal rate is significantly lower than that of the comparative example in the range of 300 ° C. to 450 ° C. Thus, in Example 1, although the catalyst performance is comparable to that of Comparative Example 2, the catalyst performance is degraded by aging.

FIGS. 6A and 6B are diagrams showing the removal rate with respect to the temperature in the initial state. FIG. 7A and FIG. 7B are diagrams showing the removal rate with respect to the temperature after aging. The aging conditions are a temperature of 1000 ° C. and a time of 20 hours. Example 3 is obtained by drying with supercritical carbon dioxide. 6A and 7A show the measurement results of Comparative Example 2, and FIGS. 6B and 7B show the measurement results of Example 3. FIG. Since the measurement conditions are different between FIG. 6A and FIG. 7A, it cannot be simply compared with Comparative Example 2 in FIG. 5A and FIG. 5B. In FIG. 6A to FIG. 7B, black circles, black triangles, and black squares indicate the removal rates of NO _x , CO, and C ₃ H ₆ , respectively. The solid line is an approximate curve.

  Referring to FIGS. 6A and 6B, Comparative Example 2 shows a very high removal rate for any gas at 200 ° C. or higher, and Example 3 at 250 ° C. or higher. 7A and 7B, both Comparative Example 2 and Example 3 show a very high removal rate for any gas at 300 ° C. or higher. As described above, Example 3 was found to have a catalyst performance comparable to that of Comparative Example 2 even after aging.

  According to Examples 1 to 4, it contains iron oxide having a high oxygen storage / release capability and high heat-resistant silica or zirconia. As a result, as shown in FIGS. 5A to 7B, a catalyst powder having catalyst performance comparable to that of a general catalyst powder containing cerium can be obtained. Moreover, nanoparticles can be formed by using a reaction field of supercritical water or subcritical water. Thereby, as shown in FIG. 4, the oxygen storage and release capability can be further improved.

In addition to silica and zirconia, titania (TiO ₂ ) or alumina (Al ₂ O ₃ ) can be used as the high heat-resistant oxide. Therefore, the raw material solution may contain an iron ion salt and at least one of silicon, zirconium, titanium (Ti), and aluminum (Al). As a salt of iron ions, for example, iron nitrate, iron sulfide or iron chloride can be used. As a raw material containing silicon, zirconium, titanium and aluminum, for example, at least one of silicon alone, zirconium alone, titanium alone and aluminum alone may be used, or an oxide thereof may be used. Further, nitride may be used. Silica, zirconia, titania and alumina may have a stoichiometric composition, but may not have a stoichiometric composition.

  As in Examples 1 to 4, the catalyst powder is composed mainly of iron oxide containing magnetite and at least one of silica and zirconia. Thereby, oxygen storage and release capability can be improved.

  In particular, as in Examples 3 and 4, it is preferable that iron oxide and zirconia are the main components from the viewpoint of heat resistance, and it is more preferable that iron oxide containing magnetite and zirconia are the main components.

  The concentration of iron and silicon, zirconium, titanium, or aluminum in the raw material solution can be changed as appropriate within the concentration range where the reaction occurs. Each concentration can be in the range of, for example, 0.1 mol / liter or more and 1 mol / liter or less.

  The reaction temperature and pressure can be arbitrarily set within a range where the reaction occurs in the reaction field of supercritical water or the reaction field of subcritical water. The critical temperature of water is 374 ° C., and the critical pressure is 22.1 MPa. A state above the critical temperature and above the critical pressure is called a supercritical state. However, even if the temperature and / or pressure is below the critical point, a reaction field similar to that of supercritical water can be realized. This is called a subcritical state. For example, the temperature may be 250 ° C. or higher and the pressure may be 20 MPa or higher. The temperature and pressure are each preferably above the critical point. Furthermore, the temperature is preferably 350 ° C. or higher. The pressure is preferably 25 MPa or more. When the temperature is high, the heat resistance of the piping and the like is increased, so the temperature is preferably 450 ° C. or lower. Since the pressure resistance of piping and the like is increased when the pressure is increased, the pressure is preferably 40 MPa or less.

  Although 1 second was used as the reaction time, the reaction time can be arbitrarily set. For example, it may be 0.1 seconds or longer. Moreover, when reaction time is long, reaction piping will become long. Therefore, the reaction time is preferably 10 seconds or less.

The pH of the solution can be appropriately selected within the range where the reaction occurs. In order to produce FeOH as an intermediate reactant, it is preferable that the concentration of OH ⁻ ions is high. For example, the pH is preferably 6 or more. When the pH is increased, the alkali resistance of piping and the like is increased, and therefore the pH is preferably 9 or less. The solution for adjusting the pH only needs to have OH groups in addition to KOH. For example, NaOH can be used.

  The diameter of the powder is preferably 1 nm or more and 100 nm or less, more preferably 5 nm or more, and more preferably 10 nm or more in order to function as a nanoparticle. 70 nm or less is more preferable. The diameter of the powder is expressed by, for example, an average diameter or a median diameter.

  The form in the case of making the catalyst powder of the present invention into the form of an exhaust gas purifying catalyst is not particularly limited. For example, it may be molded into a predetermined shape such as a pellet. You may carry | support on the base material for catalysts. The substrate may adopt a honeycomb structure or the like.

  The preferred embodiments of the present invention have been described in detail above. However, the present invention is not limited to the specific embodiments, and various modifications and changes can be made within the scope of the gist of the present invention described in the claims. It can be changed.

Claims (5)

  A method for producing a powder, characterized in that a powder is produced in a reaction field of supercritical water or subcritical water using a solution containing an iron ion salt and at least one of zirconium, silicon, titanium and aluminum. .   The method for producing a powder according to claim 1, wherein the powder contains zirconia.   The method for producing a powder according to claim 1, wherein the powder contains magnetite.   The method for producing a powder according to any one of claims 1 to 3, wherein the reaction field of supercritical water or subcritical water is a reaction field of supercritical water.   The method for producing a powder according to any one of claims 1 to 4, wherein the powder is an exhaust gas purifying catalyst powder.

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