JP6495811B2 - Selective reduction exhaust gas purification catalyst - Google Patents

Description

  The present invention relates to a selective reduction exhaust gas purification catalyst.

  The exhaust gas from an engine such as an automobile engine contains components to be purified such as carbon monoxide, hydrocarbons, nitrogen oxides (NOx), and particularly when combustion is performed in an oxygen-excess atmosphere. It becomes easy to generate NOx. A selective catalytic reduction (SCR) catalyst is known as a catalyst for reducing NOx discharged under such an oxygen-excess atmosphere using a reducing agent such as ammonia.

  As the SCR catalyst, an exhaust gas purification catalyst in which copper (Cu) is supported on chabazite-type zeolite, and an exhaust gas purification catalyst (Fe-supported β-type zeolite) in which iron (Fe) is supported on β-type zeolite are known. .

  In Patent Document 1, a dispersion obtained by mixing ferrous sulfate aqueous solution and crystalline silicoaluminophosphate particles is sprayed, dried, and calcined, so that crystalline silicoaluminoline carrying an iron component is supported. A method for producing acid salt particles is described (Example 11).

  Patent Document 2 describes a nitrogen oxide purifying catalyst in which iron, cobalt, palladium, copper or the like is supported on a zeolite containing at least silicon, aluminum, and phosphorus in a skeleton structure (claim 1, paragraph [0079). ]etc).

  Furthermore, Patent Document 3 describes a nitrogen oxide adsorbent in which paramagnetic iron (III) ions are supported on zeolite (claim 1 and the like).

JP 2012-140316 A JP 2012-148272 A JP 2007-24450 A

Although Cu-supported SAPO known as an SCR catalyst has a high activity for the reaction of reducing NOx with NH ₃ , for example, in a high temperature range of 500 ° C. or higher, NH ₃ is oxidized to generate NO by a reaction of generating NO _{3 3} is consumed, and there is a problem that the NOx purification rate decreases. For this reason, Cu-supported SAPO cannot be used effectively in an environment where the exhaust temperature is high.

On the other hand, Fe-supported β-type zeolite, which is also known as an SCR catalyst, consumes less due to the oxidation of NH ₃ at high temperatures, but its β-zeolite has an unstable crystal structure, so its hydrothermal durability is low. The heat resistance required for the exhaust gas purifying catalyst was not satisfied (Patent Document 3).

  Therefore, a selective reduction type exhaust gas purifying catalyst that exhibits high NOx purification activity even in a high temperature range and has high heat resistance has been desired.

  As a result of diligent efforts, the present inventors have reached the following present invention.

(1) Divalent iron ions are supported in the pores of the chabazite-type zeolite carrier, and the divalent iron is converted into metal on the basis of the total of the carrier and the divalent iron ion. The ion loading concentration is 5.0 wt% to 10.0 wt%.
Selective reduction type exhaust gas purification catalyst.

According to the selective reduction type exhaust gas purifying catalyst of the present invention, a chabazite-type zeolite carrier is used and Fe having a lower oxidizing power of NH ₃ than Cu is used as an active site to carry divalent Fe. an elevated NOx purification activity in the range, no nO ₂ Std. High activity can be obtained even under SCR reaction conditions. Further, according to the selective reduction type exhaust gas purifying catalyst of the present invention, high heat resistance can be provided by using a chabazite type zeolite carrier as a carrier.

FIG. 1 shows a standard SCR at 500 ° C. for the samples of Comparative Example 1, Comparative Example 3 (a), Comparative Example 3 (b), Example 1 (a) (Reference Example), and Example 1 (b). It is the graph which plotted steady-state SCR-NOx purification rate using Fast SCR. FIG. 2 is a graph plotting measurement results of the oxidation rate of NH ₃ at 400 ° C. and 500 ° C. for the samples of Comparative Example 1 and Example 1 (b). FIG. 3 shows the steady SCR-NOx purification rate at 500 ° C. for the samples of Example 1 (b), Comparative Example 1 and Comparative Example 2 using Standard SCR and Fast SCR after the endurance and durability. It is the graph which showed the deterioration degree. FIG. 4 is a graph showing a comparison between the pore size of various zeolites and the molecular size. FIG. 5 is a graph plotting the measurement results of the NOx purification rate (%) by Std SCR at 500 ° C. for the samples of Reference Example 1, Example 2, Comparative Example 6, Comparative Example 7, and Comparative Example 8. FIG. 6 is a graph plotting the measurement results of the NOx purification rate (%) by Fast SCR at 500 ° C. for the samples of Reference Example 1, Example 2, Comparative Example 4, Comparative Example 5, and Comparative Example 8. FIG. 7 shows the absorption energy at the FeK edge of the sample of (a) Example 2, (b) Comparative Example 6, (c) Comparative Example 5, and (d) Comparative Example 7 by XAFS. It is a graph which shows the value (= result of the oxidation state of Fe) of the X-ray absorption energy in the place where it is measured, normalized, and further has a spectral height of 0.2. FIG. 8 is a graph plotting the measurement results of the average oxidation number of Fe by XAFS for the samples of Reference Example 1, Example 2, Comparative Example 6, and Comparative Example 5. FIG. 9 shows the results of measuring the steady-state SCR-NOx purification rate using the Standard SCR at 400 ° C. and 500 ° C. for the samples of Comparative Example 8, Reference Example 2, Reference Example 3, Example 3 and Example 4. Is a graph in which is plotted. FIG. 10 is a graph showing the results of measuring NH ₃ / NOx [−] for the samples of Reference Example 2, Reference Example 3, Example 3, and Example 4. FIG. 11 is a graph plotting calculation results of the average oxidation number of Fe by XAFS for the samples of Example 3 and Example 4. FIG. 12 is a graph plotting the measurement results of the TCD voltage value (μV) against the sample temperature (° C.) for the sample of Example 3. FIG. 13 is a graph in which the measurement result of the TCD voltage value with respect to the sample temperature (° C.) is plotted as a relative value for the sample of Comparative Example 9. FIG. 14 is a graph plotting the results of measuring the ratio of NO ₂ -TPD high temperature peak area / low temperature peak area for the samples of Reference Example 2, Reference Example 3, Example 3, and Example 4.

《Selective reduction type exhaust gas purification catalyst》
In the selective reduction type exhaust gas purifying catalyst of the present invention, divalent iron ions are supported in the pores of the chabazite-type zeolite support, and the metal conversion is based on the sum of the support and the divalent iron ions. Thus, the supported concentration of divalent iron ions is 5.0 wt% to 10.0 wt%.

  In the present specification, “supporting iron ions on a carrier” means supporting iron ions on the carrier and / or ion exchange of iron ions into the pores of the carrier.

<Carrier>
In the selective reduction type exhaust gas purifying catalyst of the present invention, a chabazite-type zeolite carrier is used as a carrier. Specific examples of the zeolite include silicoaluminophosphates such as SAPO-34, aluminosilicates such as SSZ-13, and mixtures thereof. Among these, SAPO-34 is preferable from the viewpoint of heat resistance.

Chabasite (CHA) type zeolite has a pore diameter of about 3.8 mm and can efficiently concentrate NH ₃ having a molecular size of less than that. Therefore, according to the present invention, NOx reduction by NH ₃ can be further promoted by using chabazite-type zeolite as a carrier.

<Divalent iron ion>
In the selective reduction exhaust gas purifying catalyst of the present invention, divalent iron ions are supported in the pores of the chabazite-type zeolite carrier.

By using Fe having an oxidizing power of NH ₃ lower than that of Cu as an active point, consumption due to the oxidation of NH ₃ particularly in a high temperature region can be suppressed, so that the NOx purification activity in the high temperature region can be remarkably improved.

In the selective reduction type exhaust gas purifying catalyst of the present invention, the supported concentration of divalent iron ions is 5.0 wt% to 10.0 wt% in terms of metal based on the total of the support and divalent iron ions. .
In this specification, the metal conversion means an amount based only on a metal. For example, in the case of FeCl ₂ .4H ₂ O, it means an amount of only Fe not containing Cl ₂ .4H ₂ O.
(In this specification, “iron loading concentration” may be referred to as “Fe loading concentration”.)

  If the supported concentration of divalent iron ions is less than 5.0 wt%, the number of active sites decreases, and there may be a case where sufficient NOx purification activity cannot be achieved in the finally obtained selective reduction exhaust gas purification catalyst. On the other hand, even if the supported concentration is increased to more than 10.0 wt%, the number of active points of divalent iron ions in the SCR active pores is increased simply by increasing the number of iron ions that cannot be introduced into the pores of the carrier. May not be increased.

  The supported concentration may be, for example, 5.5 wt% or more, 6.0 wt% or more, or 6.5 wt% or more. Moreover, the carrying | support density | concentration of this bivalent iron ion may be 9.5 wt% or less, 9.0 wt% or less, and 8.5 wt% or less, for example.

When the concentration of the divalent iron ions is 7.0 wt% or more and 10.0 wt% or less, a better NOx purification rate can be obtained, and a low NH ₃ oxidation ratio with respect to the NOx purification rate is preferable.

In the present invention, the oxidation state of iron is determined by measuring an arbitrary part on a sample by X-ray absorption fine structure analysis (X-ray absorption fine structure: XAFS), and X-ray absorption at the FeK edge based on the transmitted light. The X-ray absorption near-edge structure (XANES) with a normalized intensity of X-rays has an X-ray absorption energy value of 0.2 when the spectrum intensity (intensity) is 0.2. A value obtained by converting the value based on a measured value in Fe foil, FeO, Fe ₃ O ₄ , Fe ₂ O ₃ or the like (this value is referred to as “average oxidation number” in the present specification).

In the present specification, iron ions having an average oxidation number of 1.5 or more and less than 2.3 are referred to as “divalent iron ions”.
In addition to the ions, the divalent iron ions may include oxides and hydrates.

As described in detail below, in the selective reduction exhaust gas purification catalyst of the present invention, by using a combination of divalent iron ions and chabazite-type zeolite having a predetermined inner diameter, by carrying in a solid phase, The ion diameter of the divalent iron ion raw material and the inner diameter of the chabasite-type zeolite are matched so that the divalent iron ion can be arranged at the stable site in the chabasite-type zeolite pores. Due to the small size of the NH ₃ molecule, it is possible to concentrate NH ₃ efficiently in the pores, and the combination of these stable divalent iron ions and concentrated NH ₃ makes it more oxidizing than copper. By using weak iron, it is possible to achieve both high NOx purification rate in the high temperature range while suppressing oxidation of NH ₃ and enter into the pores of the chabazite-type zeolite. A trivalent iron ion raw material that is difficult to achieve has an excellent effect that cannot be achieved.

In addition, in the selective reduction exhaust gas purifying catalyst of the present invention, the supported concentration of divalent iron ions is 5.0 wt% or more and 10.0 wt% or less, so that NH ₃ oxidation is suppressed, and NOx Is highly selective with respect to a reaction for reducing NH ₃ with NH ₃ and can have high heat resistance.

<< Method for producing selective reduction type exhaust gas purification catalyst >>
The selective reduction exhaust gas purifying catalyst of the present invention can be produced by any method, for example, by the method shown below.

Specifically, for example, the selective reduction type exhaust gas purifying catalyst of the present invention can be produced by a method including the following steps in the following order:
A mixing step of mixing iron (II) chloride and chabasite-type zeolite;
A heat treatment step of heat-treating the mixed iron (II) chloride and chabasite-type zeolite in an oxygen-free atmosphere;
A hydrogen reduction step in which the heat-treated iron (II) chloride and chabasite-type zeolite are subjected to hydrogen reduction, and, optionally, an oxidation in which the hydrogen-reduced iron chloride (II) and chabasite-type zeolite are oxidized in air Processing step.

<Mixing process>
In the mixing step, iron (II) chloride and the chabazite-type zeolite can be mixed with each other at a quantitative ratio so that a desired amount of iron is supported in the catalyst to be produced until it becomes uniform visually.

  The mode of mixing is not particularly limited, and can be simple physical mixing.

  As the chabazite-type zeolite according to the present invention, a commercially available product can be used without particular limitation, and as the iron (II) chloride, a commercially available product can be used without particular limitation.

  However, since iron is introduced into the pores of the chabasite-type zeolite in a later step, it is preferable that the amount of iron chloride is small and the proportion of iron chloride is high in iron chloride. In this example, the case where iron (II) chloride is used as a source of divalent iron ions has been specifically described. However, other sources can be used, for example, iron (II) nitrate. Iron (II) sulfate can be used.

<Heat treatment process>
Next, after the mixing step, a heat treatment step is performed in an oxygen-free atmosphere.

Although not wishing to be bound by any theory, this heat treatment step allows the oxidation state of iron to be iron (II) chloride in an oxygen-free atmosphere without being oxidized to iron (II) chloride, chabazite type. It is considered that iron (II) chloride is taken into the pores of the zeolite and chloride ions (Cl ⁻ ) are removed from the iron (II) chloride to form divalent iron ions in the pores. .

  An oxygen-free atmosphere refers to an atmosphere that does not contain oxygen. Therefore, the oxygen-free atmosphere gas of the present invention is not particularly limited as long as it does not contain oxygen, and can be performed under a stream of nitrogen, argon, or the like. Moreover, you may positively carry out by the mixed gas with reducing components, such as hydrogen.

  Also, since the hydrate of iron (II) is larger than the pore size of the chabasite-type zeolite, in the present invention, when iron (II) is taken into the pores, not as a hydrate, It is thought that it can be taken in as iron (II) chloride which does not have a hydrate.

  The heat treatment process consists of a (low temperature) pre-stage heat treatment and a (high temperature) post-stage heat treatment.

  The temperature of the pre-stage heat treatment can be about 100 ° C. or higher, about 120 ° C. or higher, about 140 ° C. or higher, or about 150 ° C. or higher. The temperature may be about 200 ° C. or less, about 180 ° C. or less, or about 160 ° C. or less.

  The pre-stage heat treatment time can be about 2 hours or more, about 4 hours or more, about 6 hours or more, about 8 hours or more, about 10 hours or more, about 12 hours or more. Also, this time can be about 26 hours or less, about 24 hours or less, about 22 hours or less, about 18 hours or less, about 16 hours or less, about 14 hours or less.

  The temperature of the post-stage heat treatment can be about 300 ° C. or more, about 350 ° C. or more, about 400 ° C. or more, about 450 ° C. or more, about 500 ° C. or more, about 520 ° C. or more. The temperature may be about 650 ° C. or less, about 600 ° C. or less, or about 570 ° C. or less.

  The time for the post-stage heat treatment can be about 2 hours or more, about 4 hours or more, about 6 hours or more, about 8 hours or more, about 10 hours or more, about 12 hours or more. Also, this time can be about 26 hours or less, about 24 hours or less, about 22 hours or less, about 18 hours or less, about 16 hours or less, about 14 hours or less.

<Hydrogen reduction process>
Then, after the heat treatment step, a hydrogen reduction step is performed. Hydrogen reduction refers to maintaining a predetermined temperature for a certain time in a hydrogen stream or a reducing stream containing hydrogen. Although not wishing to be bound by any theory, this process promotes the ion exchange of iron that has not been sufficiently ion exchanged outside or inside the zeolite pores, resulting in a negatively charged stable site within the pores. It is considered that a valent iron ion can be arranged, whereby a catalyst having a high NOx purification property can be produced even when used at a high temperature.

  The temperature of the hydrogen reduction process can be about 300 ° C. or more, about 350 ° C. or more, about 400 ° C. or more, about 450 ° C. or more, about 500 ° C. or more, about 550 ° C. or more, about 580 ° C. or more. The temperature may be about 900 ° C. or lower, about 850 ° C. or lower, about 800 ° C. or lower, about 750 ° C. or lower, about 700 ° C. or lower, about 650 ° C. or lower, or about 630 ° C. or lower.

  The duration of the hydrogen reduction step can be about 1 hour or more, about 1.5 hours or more, about 2 hours or more. Also, this time can be about 4 hours or less, about 3.5 hours or less, about 3 hours or less, or about 2.5 hours or less.

  In the manufactured iron zeolite catalyst, it is thought that it is arrange | positioned in the state of SCR active iron ion in a pore after a hydrogen reduction process as mentioned above.

(Oxidation process)
Furthermore, an oxidation treatment step can optionally be performed after the hydrogen reduction step. The oxidation treatment step refers to maintaining a predetermined temperature in air for a predetermined time. Although not wishing to be bound by any theory, it is believed that this process makes it possible to more reliably fix iron ions to the stable site and to adjust to a divalent state or a state close thereto.

  The temperature of the oxidation treatment step is not particularly limited, and can be about 300 ° C. or higher, about 350 ° C. or higher, about 400 ° C. or higher, about 450 ° C. or higher, or about 480 ° C. or higher. Also, this temperature is about 900 ° C or less, about 850 ° C or less, about 800 ° C or less, about 750 ° C or less, about 700 ° C or less, about 650 ° C or less, about 600 ° C or less, about 550 ° C or less, about 530 ° C or less. Can be.

  The time of the oxidation treatment process can be about 2 minutes or more, about 4 minutes or more, about 6 minutes or more, about 8 minutes or more, about 10 minutes or more. Also, the temperature can be about 20 minutes or less, about 18 minutes or less, about 16 minutes or less, about 14 minutes or less, about 12 minutes or less.

EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples, but the present invention is not limited to these examples.
In the examples, SAPO means SAPO-34.

[Example 1]
[Preparation of Fe / SAPO (Fe supported concentration: 7.4 wt%)]
(Step 1-1): Mixing step
2 g of powdered FeCl ₂ .4H ₂ O hydrate (Wako Pure Chemical Industries, Ltd., product number: 095-00912) and 7 g of SAPO-34 (Mitsubishi Resin Co., Ltd.) were placed in a mortar at room temperature. And mixed.

(Step 1-2): Heat treatment step The sample obtained in the above step 1-1 is preheated at 150 ° C. for 12 hours in a nitrogen stream using a high temperature furnace, and then subjected to heat treatment at 500 ° C. for 12 hours. went.

(Step 1-3): Hydrogen reduction step The sample obtained in the above step 1-2 was subjected to 2 hours at 600 ° C under a hydrogen stream using a high temperature furnace to obtain a sample.

[Comparative Example 1 (use of Cu / SAPO)]
Commercially available CuSAPO (Cu: 2.5 wt%, Si: 9 mol%) manufactured by Mitsubishi Resin Co., Ltd. was used.

[Comparative Example 2]
[Use of β-type zeolite (Fe supported concentration: 1 wt%)]
β-type zeolite (Zeochem, product name: ZEOCAT PB / 25, product number 3089999.900.900) was used. As the catalyst metal, about 1 wt% iron in terms of metal was used.

[Comparative Example 3]
[Preparation of Fe / SAPO (Fe supported concentration: 7.4 wt%)]
In step 1-1 of Example 1, FeCl ₃ · 6H ₂ O hydrate instead of FeCl ₂ (Wako Pure Chemical Industries, Ltd., product number: 091-00872) was used as a, similarly as in Example 1 Thus, a catalyst of Comparative Example 3 was obtained.

[Evaluation of catalyst activity (initial)]
About the catalyst of Example 1 and Comparative Examples 1, 2, and 3, the catalyst activity evaluation (initial stage) was evaluated using the fixed bed flow type reactor.

  Here, the catalysts of Example 1 and Comparative Example 3 were evaluated for both the catalyst before and after the treatment in Step 1-3. The catalyst before the process of Step 1-3 is used as the catalyst of Example 1 (a) (Reference Example) and Comparative Example 3 (a), respectively, and the catalyst after the process of Step 1-3 is set as Example 1 (b). ) And Comparative Example 3 (b).

  The catalyst activity evaluation (initial) was performed by the following step 2-1 and step 2-2.

(Step 2-1)
Each sample was treated with a mixed gas containing O ₂ (10%) + CO ₂ (10%) + H ₂ O (8%) + N ₂ (residue) at a temperature of 550 ° C. for 10 minutes, and then the following Standard SCR ( Hereinafter, it was pretreated for 5 minutes at a temperature of 550 ° C. using a mixed gas of Std SCR.

(Process 2-2)
Next, after the above step 2-1, each sample was processed under the following (condition 1) to (condition 4).
(Condition 1) 10 minutes at 500 ° C. using a mixed gas of Std SCR.
(Condition 2) Heat treatment in (Condition 1) at 400 ° C. for 10 minutes.
(Condition 3) Fast SCR mixed gas at 500 ° C. for 10 minutes.
(Condition 4) Heat treatment in (Condition 3) at 400 ° C. for 10 minutes.

In any one of (Condition 1) to (Condition 4), the average value of the NO _x purification rate from 9 minutes to 10 minutes in 10 minutes was measured as the steady SCR-NO _x purification rate.

Here, the Std SCR reaction and the Fast SCR reaction are as follows:
(Std SCR reaction)
4NO + 4NH ₃ + O ₂ = 4N ₂ + 6H ₂ O
(Fast SCR reaction)
NO + NO ₂ + 2NH ₃ = 2N ₂ + 3H ₂ O

  The composition of the gas used for the Std SCR and Fast SCR is as follows.

  The amount of catalyst was 1 g, and the gas flow rate was 10 liters / minute.

First, the Std SCR at 500 ° C. was evaluated for different active species and preparation methods.
The results were 62.6% for the sample of Comparative Example 1 and 8.4% without (Step 1-3) for the sample of Comparative Example 3 (with Comparative Example 3 (a)) and (Step 1-3) 43 6% (Comparative Example 3 (b)), whereas in the sample of Example 1, 52.4% (Step 1-3) without (Step 1-3) (Example 1 (a)) ( It was 66.6% (Example 1 (b)) with (Reference Example) and (Step 1-3) (FIG. 1).

For this reason, in the catalyst activity evaluation (initial stage), in Example 1, even when the Std SCR does not contain NO ₂ , the hydrogen reduction treatment of (Step 1-3) is performed to perform steady SCR-NOx purification. It was found that the rate was higher, showing 66.6% higher (Example 1 (b)) higher activity than 62.6% Cu / SAPO (Comparative Example 1).

And when it evaluated similarly using Fast SCR in 500 degreeC, the result was as follows (FIG. 1):
Catalyst of Comparative Example 1: 65.2%
Sample of Comparative Example 3 (a): 71.4%
Sample of Comparative Example 3 (b): 76.4%
Sample of Example 1 (a) (reference example): 81.8%
Sample of Example 1 (b): 82.0%

Therefore, in the initial stage, the Fast SCR containing NO ₂ has a higher temperature range of 500 ° C. than the conventional Cu / SAPO (Comparative Example 1), regardless of the presence or absence of the hydrogen reduction treatment in (Step 1-3). It was found that the samples in which the active species of Example 1 and Comparative Example 3 were Fe had a higher steady SCR-NOx purification rate.

From the above, as shown in FIG. 1, SAPO using FeCl ₂ became a catalyst showing Std SCR activity comparable to Comparative Example 1 by performing hydrogen reduction treatment, and more than trivalent iron salt. It is preferable to use a divalent iron salt, and SAPO using FeCl ₂ (Example 1) is faster in Fast SCR than SAPO using FeCl ₃ (Comparative Example 3) and Cu / SAPO (Comparative Example 1). It was found that the reaction activity was high.

[NH ₃ oxidation rate measurement]
For each sample, using a fixed bed flow type reactor, a mixed gas of NH ₃ (700 ppm) + O ₂ (10%) + CO ₂ (10%) + H ₂ O (8%) + N ₂ (residue) is used. The NH ₃ oxidation rate at 400 ° C. and 500 ° C. was measured under the conditions of a catalyst amount of 1 g and a mixed gas flow rate of 10 l / min.

The above [NH ₃ oxidation rate measurement] was performed on the samples of Example 1 (b) and Comparative Example 1 after the above (Step 2-2). As a result, as shown in FIG. 2, in the sample of Comparative Example 1, NH ₃ was oxidized at a high rate of 71.2% at 400 ° C. and 90.0% at 500 ° C. It was found that the oxidation of NH ₃ can be kept very low at 6.7% at 400 ° C and 19.8% at 500 ° C.

Therefore, it was confirmed that Example 1 (b) had an advantage that the rate of generating NO by oxidizing NH ₃ was much lower than that of Cu / SAPO (Comparative Example 1).

Therefore, the following is considered based on these results.
First, in general, due to the large number of functional groups and the like, iron (III) chloride has a larger molecular size than iron (II) chloride, and iron (II) chloride hydrate has more iron chloride ( The molecular size is larger than II). It is known that iron (II) chloride 2,4,6 hydrate decomposes when heated in the air and becomes iron chloride (III) and hydrogen chloride at 250 ° C.

  Although not wishing to be bound by any theory, when iron (III) chloride is used, it is difficult to diffuse into the pores, and in the heat treatment step, the salt decomposes and aggregates outside the pores. It is considered that iron is not easily ion-exchanged at the stable site in the hole, and the steady SCR-NOx purification rate is low (Comparative Example 3 (a)).

In contrast, while not wishing to be bound by any theory, in the catalyst according to the present invention, iron (II) chloride is subjected to heat treatment in an oxygen-free oxygen-free atmosphere after solid-phase ion exchange with zeolite, It is considered that divalent iron (II) could be put in the pore diameter. And since the hydrate of iron (II) chloride is larger than the pore diameter of the chabasite-type zeolite, when the iron (II) chloride is taken into the pores, the hydrate is not hydrated. It is thought that it was able to achieve being taken in as iron (II) chloride which does not have. As a result, due to the restriction due to the small pore size of the chabasite-type zeolite, even if chloride ions are removed, they are stabilized at the ion exchange sites in the pores and are not oxidized to Fe ³⁺ , but divalent. It is thought that high activity could be achieved by remaining iron ions.

Furthermore, although not wishing to be bound by any theory, the ions that are not sufficiently ion-exchanged by the hydrogen reduction treatment are then exchanged, and the divalent iron ions are arranged at stable sites in the pores. It is considered that this promotes the change in iron ion valence even during the NOx reduction reaction, and as a result, the NH ₃ -SCR activity of the catalyst is improved.

For these reasons, the catalyst according to the present invention supports Std. With no NO ₂ by supporting divalent iron ions. It is considered that a catalyst having both a high SCR activity under the SCR reaction conditions and at a high temperature and an advantage of a low oxidation rate of ammonia by using iron ions can be produced.

[Catalyst activity evaluation (durability)]
As durability evaluation, the following processes 2-3 and 2-4 were performed.

(Step 2-3): Hydrothermal durability treatment A sample was treated in an electric furnace at 750 ° C for 24 hours in an air atmosphere in which water was bubbled.

(Step 2-4)
The said process 2-2 was performed using the said (condition 1) and (condition 3).

  FIG. 3 shows the degree of deterioration of each catalyst calculated as follows from the NOx purification rate obtained by the above [catalytic activity evaluation (initial)] and [catalytic activity evaluation (durability)].

  Degree of degradation = [(initial NOx purification rate) − (NOx purification rate after endurance)] / (initial NOx purification rate) × 100

  In Example 1 (b), the zeolite species is CHA type, the active species is Cu (Comparative Example 1), or the active species is Fe, but the zeolite species is β type ( It was found that the Std SCR activity and the Fast SCR activity are less likely to deteriorate compared to Comparative Example 2).

The effect of hydrothermal durability treatment on the ionic state of iron was as follows.
First, as described in the above [catalytic activity evaluation (initial stage)], although not as much as Example 1 prepared with iron (II) chloride (FIG. 1, Example 1 (b)), iron chloride (III) When the hydrogen reduction treatment was performed, partial ion exchange proceeded and the steady SCR-NOx purification rate was improved (FIG. 1, Comparative Example 3 (b)).

When hydrothermal durability treatment is performed, the active species is Cu even if the zeolite species is CHA type (Comparative Example 1), or the zeolite species is β type even if the active species is Fe (Comparative Example) In 2), the active species are difficult to be stabilized at the ion exchange sites in the zeolite pores. In the case of β-type zeolite, since the pores are large, divalent iron ions outside and inside the pores are easily oxidized to Fe ³⁺ , and the zeolite itself has low thermal stability and durability. As a result of not being very good (FIG. 3, Comparative Example 2), it is considered that the degree of deterioration is larger than that in Example 1 (b).

  From these results, it was confirmed that the catalyst of the present invention in which divalent iron ions were supported on the pore diameter of zeolite had superior SCR activity and excellent hydrothermal durability.

  In the present embodiment, the ion exchange rate of the catalyst is calculated as necessary in relation to the hydrogen reduction treatment, and if necessary, the oxidation treatment step is performed after the hydrogen reduction step.

(Ion exchange rate)
The ion exchange rate is the rate at which H ⁺ on the ion exchange site (acid point) in the catalyst carrier is replaced with ions of the catalyst metal. When the catalyst metal is a divalent iron ion, the two acid points are Electrically equivalent one divalent iron ion will be replaced.
Specifically, the ion exchange rate can be calculated by the formula (number of metal ion atoms) × (metal ion valence) / (number of acid points in the catalyst support) × 100.

  The ion exchange rate can be adjusted to a desired value by adjusting the amount of Fe ions supported relative to the acid sites on the catalyst support, and is about 1% or more, about 5% or more, about 10% or more, about It can be 20% or more, about 30% or more, or about 35% or more, while it can be about 100% or less, about 90% or less, or about 85% or less.

(Oxidation process)
The following conditions were used as the oxidation process.

(Step 3-1)
The sample was placed in an electric furnace and oxidized in air at 500 ° C. for 10 minutes.

  An example in which an oxidation treatment process is further performed is shown below.

<Reference Example 1 and Example 2, and Comparative Examples 4 to 7>
As summarized in Table 2 below, in Example 1, whether (1) (Step 1-1) was used in which FeCl _{2 of} Example 1 or FeCl ₃ of Comparative Example 3 was used (2) ( Except for whether or not the process of step 1-3) was performed, and (3) the ion exchange rate was adjusted to be either 5% or 37%, the same procedure as in Example 1 was performed, Further, (Step 3-1) was performed to obtain a sample. (Fe loading concentration is 7.4 wt% in all cases.)

[Comparative Example 8]
[Cu / SAPO (Cu support concentration: 2.5 wt%)]
100 g of SAPO (Mitsubishi Resin Co., Ltd.) is added to 100 g of 2.5% by mass copper acetate aqueous solution, stirred at room temperature for 10 minutes, heated to 120 ° C. and evaporated to dryness, and then powdered. A sample was obtained by heat treatment in air at 800 ° C. for 10 minutes.

[Catalyst activity evaluation (with oxidation treatment step, Std SCR)]
For the samples of Reference Example 1, Example 2, and Comparative Examples 6, 7, and 8, Step 3-1: After performing the oxidation treatment step, Step 2-1 to Step 2-2 in the above catalytic activity evaluation (initial stage) (Condition 1) was used to evaluate the catalytic activity.

As shown in FIG. 5, the NOx purification rate was low in Reference Example 1 and Comparative Example 7, and higher in Comparative Examples 6, Comparative Example 8, and Example 2. In particular, the sample subjected to hydrogen reduction treatment using FeCl ₂ (in Example 2) showed a higher value than Cu / SAPO in Comparative Example 8. Even when FeCl ₃ was used, the value with the hydrogen reduction step (Comparative Example 6) was higher than that without the hydrogen reduction step (Comparative Example 7).

Specifically, when an oxidation treatment step is further performed after the hydrogen reduction step, when the ion exchange rate is 37%, the NOx purification rate is high even in the Std SCR in which NO ₂ does not exist, and the oxidizing power is high. (Example 2) / (Comparative Example 8) = 1.29 times that of Cu / SAPO.

  When the ion exchange rate becomes 5%, the NOx purification rate corresponding to the value of the ion exchange rate is obtained (Reference Example 1), but the ion exchange rate is 37%, there is no hydrogen reduction step, and the sample using iron (III) chloride is used. The NOx purification ability almost equivalent to (Comparative Example 7) was exhibited.

  Therefore, in FIG. 5 in which the oxidation treatment is performed after the hydrogen reduction step, particularly when iron (II) chloride is used, the NOx purification ability is improved as compared with the case where iron (III) chloride is used. Turned out to be possible.

[Catalyst activity evaluation (with oxidation process, Fast SCR)]
Similarly, for the samples of Reference Example 1, Example 2 and Comparative Examples 4, 5, and 8, after performing the oxidation treatment step of Step 3-1, Step 2-1 to Step 2 of the above catalytic activity evaluation (initial). -2 (Condition 3) was used to evaluate the catalytic activity.

When the above sample was evaluated under Fast SCR conditions, as shown in FIG. 6, even in a sample (Reference Example 1) having an ion exchange rate of only 5% using iron (II) chloride, the ion exchange rate was 37%. When using iron (III) chloride that shows a NOx purification rate close to that of the sample of Example 2 (Example 2) and Cu / SAPO (Comparative Example 8) and tends to improve the NOx purification ability due to the presence of NO ₂ (hydrogen NOx purification ability with and without reduction step was also shown.

  As shown in the above results, when the hydrogen reduction step is performed and the oxidation treatment step is further performed, the oxidation state of iron (II) chloride is maintained in a good state, and the produced catalyst has a further excellent effect. It was shown that.

[Evaluation of average oxidation number of Fe]
In FIG. 7, (a) Example 2, (b) Comparative Example 6, (c) Comparative Example 5, and (d) Comparative Example 7, each FeK end was measured at room temperature under the following measurement conditions using XAFS. The result of the X-ray absorption energy when the measured XANES spectrum height normalized to peak top is 0.2 is shown.

  In general, in calculating the average oxidation number of noble metals, it is common to use a standardized spectral height of 0.5, but in the case of Fe, it is derived from the structural coordination environment. Since the pre-edge peak is strong and the variation in the correlation with the oxidation state of Fe becomes large, the case of 0.2 was used avoiding 0.5.

(Measurement condition)
Absolute energy: 0.2eV

  As shown in FIG. 7, the measured values are 7106 (eV) ((a) in FIG. 7), 7108.8 (eV) ((b) in FIG. 7), and about 7112.2 (eV) ( (C) in FIG. 7) and about 7112.2 (eV) ((d) in FIG. 7).

These energy values are about 7102 (eV) for Fe foil, about 7107.8 (eV) for FeO, about 7109.8 (eV) for Fe ₃ O ₄ , and about 7112 for Fe ₂ O ₃ , measured by the same method. FIG. 8 shows the result of conversion to the average oxidation number of Fe based on .2 (eV).

  As shown in FIG. 8, the average oxidation number of Fe is found to be over 0 in Reference Example 1 and Example 2, less than 2, Comparative Example 6 is slightly larger than 2, and Comparative Example 5 is 3. did.

  When the average oxidation number of Fe based on the measurement results of these catalysts by XAFS was calculated, as shown in FIG. 8, at an ion exchange rate of 37%, a sample using iron (II) chloride was 1.5 (implemented). Example 2) (Reference Example 1 with an ion exchange rate of 5% is a value corresponding to the ratio of the ion exchange rate of Example 2) and a range of more than 0 and less than 2, while iron (III) chloride was used. In the sample (Comparative Example 6), the value was higher than 2 but even when iron (II) chloride was used, it was very high as 3 (Comparative Example 5) without the hydrogen reduction step. It was.

For this reason, it is not desired to be bound by any theory, but even if the divalent iron ions can be arranged at stable sites in the pores of the catalyst support, the oxidation must be carried out without a hydrogen reduction step. Accordingly, it seems divalent iron ion is oxidized to Fe ^3+, even if the hydrogen reduction step followed by the oxidation treatment process to Fe ³⁺ Conversely, a stable site of the catalyst support in the Fe ³⁺ pores Since it is not arranged, it is considered that sufficient purification ability could not be exhibited in the catalytic activity evaluation in spite of the initial stage.

  On the other hand, although not wishing to be bound by any theory, when a hydrogen reduction step and subsequent oxidation treatment are performed after placing divalent iron ions at stable sites in the pores, the average oxidation number of Fe Is maintained in a good state of more than 0 and less than or equal to 2.12 and shows NOx purification ability better than Cu / SAPO under the Std SCR condition as described above, even if the ion exchange rate is 5%, regardless of the number Depending on the quality of the catalyst, it is considered that the NOx purification ability equivalent to other catalysts could be exhibited under the Fast SCR condition.

  From the above, when iron (II) chloride is used, the divalent iron ions are placed in the stable sites in the pores of the catalyst support as they are, and the hydrogen reduction step is performed, and the oxidation treatment step is further performed from that state. It was found that Fe and Fe can be more appropriately oxidized in a proper state.

[Example 3]
[Preparation of Fe / SAPO (Fe supported concentration: 7.4 wt%)]
Samples were obtained according to the procedures of Step 1-1 and Step 1-2, except that FeCl ₂ was used as a raw material, and the carrying concentration of divalent iron ions was 7.4 wt%, and φ0.5 mm to 1 mm Into crushed pellets.

The obtained sample was mixed at 500 ° C. for 10 minutes with a mixed gas of O ₂ (10%) + CO ₂ (10%) + H ₂ O (8%) + N ₂ (residue)) using a fixed bed flow type furnace. The pre-oxidation heat treatment was performed.

Further, using a fixed bed flow type furnace, reduction heat treatment is performed at 600 ° C. for 30 minutes with a mixed gas of H ₂ (3%) + CO ₂ (10%) + H ₂ O (8%) + N ₂ (residue). It was.

[Example 4, Reference Example 2 to Reference Example 3]
[Preparation of Fe / SAPO (Fe loading concentration: 10.0 wt%, 1.0 wt%, and 3.0 wt%)]
Same as Example 3 except that the Fe carrying concentrations were 10.0 wt% (Example 4), 1.0 wt% (Reference Example 2), and 3.0 wt% (Reference Example 3), respectively. A sample was obtained by the following procedure.

(NOx purification rate measurement (Std SCR))
After performing the above (Step 2-1) on the samples of Example 3, Example 4, Reference Example 2, Example 3, and Comparative Example 8, (Step 2-2) ( The catalytic activity was evaluated using Condition 1) (Condition 2).

  As shown in FIG. 9, in the Std SCR at 500 ° C., the sample of Comparative Example 8 is 50.5%, whereas the sample of Example 3 is 65.9% and the sample of Example 4 is 67. It was 5%.

From these results, it was found that the Std SCR containing no NO ₂ in Examples 3 to 4 has a higher steady SCR-NOx purification rate and higher activity than Cu / SAPO (Comparative Example 8).

  FIG. 9 shows a case where the catalyst carrier is SAPO, and the active species (copper or iron) and the loading ratio are different. The catalyst according to the present invention (Example 3) has a higher reaction activity at 500 ° C. Std SCR than Cu / SAPO (Comparative Example 8), and the reaction activity is further increased by further increasing the loading ratio of divalent iron ions. It became high.

  On the other hand, when the Fe loading ratio was small (Reference Example 2 and Reference Example 3), the Std SCR at 400 ° C. and 500 ° C. did not reach the reaction activity of Cu / SAPO (Comparative Example 8).

Reference Example 2 after (Condition 1) of the step 2-1, Reference Example 3, Example 3, the sample of Example 4, at 500 ° C., NH in [NO _x purification rate Measurement of (condition 1) A ratio of ₃ consumption divided by NO _x conversion was calculated and plotted.

The result is shown in FIG. Referring to FIG. _10, NH 3 / NO _x ratio decreases with increasing Fe support concentration, the support concentration of particular divalent iron ions of NH ₃ with respect to the purification of the NO _x in the case of more than 5.0 wt% It can be seen that oxidation is significantly suppressed.

(Measurement of ion exchange rate)
For the samples of Reference Example 2, Reference Example 3, Example 3, and Example 4, the ion exchange rate was measured. The measurement results were 7% (Reference Example 2), 22% (Reference Example 3), 52% (Example 3), and 81% (Example 4).

(Evaluation of average oxidation number of Fe)
(A) Example 3 and (b) Example 4 For each FeK end, the XANES spectrum height measured with XAFS and normalized at the peak top (intensity) is 0.00. The result of the value of X-ray absorption energy in the case of 2 is shown.

(Normally, when calculating the oxidation state of noble metals, it is common to use a normalized spectral height of 0.5, but in the case of Fe, it is derived from a structural coordination environment. Since the pre-edge peak is strong and the variation in the correlation with the oxidation state of Fe becomes large, the case of 0.2 is used avoiding 0.5.)
(Measurement condition)
Absolute energy: 0.2eV

The measured values were 7106.5 (eV) and 7108 (eV), respectively.
These energy values are about 7102 (eV) for Fe foil, about 7107.8 (eV) for FeO, about 7109.8 (eV) for Fe ₃ O ₄ , and about 7112 for Fe ₂ O ₃ , measured by the same method. When converted to the oxidation number of Fe based on .2 (eV), they were 1.5 (Example 3) and 2.2 (Example 4) as shown in FIG.

  From this, it is considered that all of Examples 3 and 4 have a large amount of divalent Fe ions.

  Although the catalyst according to the present invention does not want to be bound by any theory, as shown in the value based on the measurement result of XAFS by the above heat treatment step, hydrogen reduction step, etc., the catalyst has a large amount of divalent iron ions. It is believed that iron is placed at better ionized ion exchange sites within the zeolite pores.

In the catalyst according to the present invention, as shown in the measurement results of Example 3 and Example 4, the divalent iron ions are arranged at the ion exchange sites in the zeolite pores as shown in FIG. As shown in FIG. 10, the NOx purification rate is high even at high temperatures such as Std SCR and 500 ° C. Further, as shown in FIG. 10, the NH ₃ oxidation rate is low, the catalyst heat resistance is high, and the ammonia oxidation rate is low. That can have advantages.

  Furthermore, in the iron zeolite catalyst produced by the catalyst according to the present invention, higher activity can be obtained by increasing the concentration of divalent iron ions.

[Comparative Example 9]
[(SAPO not supporting Fe)]
The support concentration of iron was 0 wt%, that is, SAPO not supporting iron was used.

(Measurement of the ratio of NO ₂ -TPD high temperature peak area / low temperature peak area)
The samples of Reference Example 2, Reference Example 3, Example 3, and Example 4 were subjected to NO ₂ -TPD measurement.
That is, after adsorbing NO ₂ , the sample was heated in a He stream, and the TCD voltage value [μV] detected corresponding to NO ₂ desorbed with respect to the sample temperature (° C.) was measured.
The high temperature peak and the low temperature peak in each sample were calculated from the profile of the TCD voltage value.
A graph plotting the measurement results for the sample of Example 3 is shown in FIG. In Example 3, two peaks, a low temperature peak and a high temperature peak, were observed.

  On the other hand, when the sample of Comparative Example 9 was measured in the same manner, as shown in FIG. 13, only the low temperature peak was observed in this sample, and it did not have the high temperature peak.

This high temperature peak depends on the amount of divalent iron ions ion-exchanged to the ion exchange sites in the pores, and the low temperature peak is considered to be NO ₂ weakly adsorbed on the zeolite.
Therefore, the ratio of the high temperature peak area to the low temperature peak area was calculated by setting the temperature at the time of measurement to more than 230 ° C. and 500 ° C. or less as the high temperature peak area and 50 ° C. to 230 ° C. as the low temperature peak area. .
It can be said that the ratio of the high temperature peak area / low temperature peak area indicates the number of active points per catalyst subjected to NO ₂ -TPD.

  As shown in FIG. 14, the measurement results show that the ratio of high temperature peak area / low temperature peak area increases as the Fe support concentration increases, and the number of active sites increases at 5 to 10 wt%. The maximum was in the vicinity of 7.4 wt%, and in the case of Example 4, it was the second largest. This result is in good agreement with FIGS.

  From the above, when a salt of divalent iron ions such as iron (II) chloride is used, the divalent iron ions remain in the stable site in the pores of the catalyst support by heat treatment process, hydrogen reduction process, etc. In addition to the arrangement, the supported concentration of divalent iron ions is preferably about 5.0 wt% to about 10.0 wt%, more preferably about 6.0 wt% to about 10.0 wt%, It has been found that very good catalysts can be obtained.

Claims (1)

Divalent iron ions are supported in the pores of the chabasite-type zeolite carrier, and based on the sum of the carrier and the divalent iron ions, the divalent iron ions are supported in terms of metal. The concentration is 5.0 wt% to 10.0 wt%;
Selective reduction type exhaust gas purification catalyst.

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