JP2010240597A - Apparatus for cleaning exhaust - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an apparatus for cleaning exhaust capable of effectively removing NO<SB>x</SB>without enlarging its size. <P>SOLUTION: The apparatus 1 for cleaning exhaust including a gas flow path 2 for exhaust to flow therethrough includes a converter A wherein a catalyst including a composite oxide of a perovskite type comprising Pt and/or Pd represented by formula: ABO<SB>3</SB>is stored, and a converter B wherein a catalyst including a catalyst of selectively reducing NO<SB>x</SB>that comprises Fe and selectively reduces NO<SB>x</SB>in an atmosphere of a reductive gas generated by the action of the catalyst in the converter A is stored, which converter B is disposed downstream of the converter A. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an exhaust gas purifying apparatus for purifying exhaust gas such as an automobile engine.

In recent years, attention has been focused on internal combustion engines such as a diesel engine and a lean burn gasoline engine that operate on a lean burn in order to reduce CO ₂ emitted from the internal combustion engine.
Since exhaust gas discharged from this type of internal combustion engine contains excessive oxygen, a three-way catalyst aimed at purifying exhaust gas generated by fuel at the stoichiometric air-fuel ratio reduces and purifies NOx in the exhaust gas. Is difficult.

Therefore, a NOx selective reduction purification catalyst that promotes selective reduction of NOx by a reducing gas such as H ₂ or NH ₃ has been proposed.
For example, a NOx selective reduction catalyst that is composed of a mixture of MnO ₂ —CeO ₂ complex oxide and Fe-zeolite and promotes a reduction reaction using NH ₃ as a reducing agent has been proposed (see, for example, Patent Document 1). .

JP 2006-136776 A

However, even if it is a NOx selective reduction catalyst like patent document 1, depending on the flow volume of the exhaust gas which passes a gas flow path, it may not fully purify NOx.
On the other hand, in order to contain NH ₃ as a reducing agent in the exhaust gas, it is necessary to inject urea aqueous solution into the exhaust gas flow path to hydrolyze urea to generate NH ₃ in the flow path. For this reason, a tank for storing the urea aqueous solution and a device for injecting the urea aqueous solution into the flow path are required, and there is a problem that the facility is enlarged.

  The objective of this invention is providing the exhaust gas purification apparatus which can purify NOx effectively, suppressing the enlargement of an installation.

In order to achieve the above object, the exhaust gas purifying apparatus of the present invention is disposed so as to be in contact with exhaust gas at a relatively upstream side of the gas flow path through which the exhaust gas passes and Pt and / or Pd. An upstream catalyst containing a perovskite complex oxide represented by the general formula ABO ₃ containing, and an upstream catalyst disposed in contact with the exhaust gas relatively downstream in the gas flow path, containing Fe And a downstream side catalyst including a NOx selective reduction catalyst that selectively reduces NOx under the atmosphere of the reducing gas generated by the action of the above.

In the exhaust gas purifying apparatus of the present invention, the perovskite complex oxide may further include a general formula CaTi _1-x N _x O ₃ (wherein N represents Pt and / or Pd, and x is 0 <x < It is preferable that the NOx selective reduction catalyst is a zeolite carrying Fe.

According to the exhaust gas purifying apparatus of the present invention, on the upstream side of the gas flow path, the reducing component (for example, H _{2) in the} exhaust gas is generated by the action of the upstream catalyst containing the perovskite complex oxide containing Pt and / or Pd. Etc.) and NOx are promoted to purify NOx. Along with the reduction of NOx, reducing gas is generated upstream of the gas flow path.
On the other hand, on the downstream side of the upstream side catalyst in the gas flow path, the action of the downstream side catalyst promotes the oxidation-reduction reaction between the reducing gas generated on the upstream side and NOx, thereby purifying NOx.

In this way, since a reduction reaction independent of the reduction reaction on the upstream side occurs on the downstream side of the upstream catalyst, even if the NOx in the exhaust gas cannot be completely purified on the upstream side, the NOx is removed from the downstream catalyst. It can be purified by action.
Moreover, since the reducing gas used for the reduction reaction on the downstream side is generated by the reduction reaction on the upstream side, the reducing gas can be effectively utilized on the downstream side. Moreover, since it is not necessary to separately provide an apparatus for generating the reducing gas in the gas flow path, an increase in the size of the facility can be suppressed.

  Further, when the downstream catalyst is a NOx selective reduction catalyst that contains Fe and does not contain a noble metal, the use of the noble metal in the downstream catalyst can be eliminated, so that the catalyst cost can be reduced.

1 is a layout diagram of an exhaust gas purification apparatus showing an embodiment of the present invention. It is a graph which shows the relationship between the gas temperature in the exhaust gas purification apparatus of Example 1, and a NOx purification rate. It is a graph which shows the relationship between the gas temperature in the exhaust gas purification apparatus of Example 2, and a NOx purification rate. 5 is a graph showing the relationship between the gas temperature and the NOx purification rate in the exhaust gas purification apparatus of Comparative Example 1. It is a graph which shows the relationship between the gas temperature in the exhaust gas purification apparatus of the comparative example 2, and a NOx purification rate. 10 is a graph showing the relationship between the gas temperature and the NOx purification rate in the exhaust gas purification apparatus of Comparative Example 3.

FIG. 1 is a layout diagram of an exhaust gas purifying apparatus showing an embodiment of the present invention.
The exhaust gas purifying apparatus 1 is for purifying exhaust gas containing excess oxygen discharged from an internal combustion engine such as a diesel engine or a lean burn gasoline engine that operates on a lean burn, for example. A gas flow path 2 communicating with the port and a catalyst unit 3 interposed in the middle of the gas flow path 2 are provided.

  In the gas flow path 2, a hydrogen supply line 4 for supplying hydrogen gas into the gas flow path 2 is connected upstream of the catalyst unit 3. A hydrogen supply device (not shown) (for example, a high-pressure hydrogen tank, a hydrogen reformer, etc.) is connected to the upstream side of the hydrogen supply line 4, and hydrogen is supplied from this device at a pressure corresponding to the exhaust gas flow rate. The hydrogen gas can be well mixed with the exhaust gas passing through the gas flow path.

The catalyst unit 3 includes a converter A in which a catalyst containing a perovskite complex oxide represented by the general formula ABO ₃ is accommodated, and a converter B in which a NOx selective reduction catalyst is accommodated.
Converter A and converter B are arranged in series in this order along the gas flow direction so that each catalyst contacts exhaust gas.

The catalyst of converter A is a substance that selectively accelerates the oxidation-reduction reaction between NOx and H ₂ in the exhaust gas to promote the reduction of NOx, and contains Pt and / or Pd. . In other words, the catalyst of converter A contains a perovskite type complex oxide represented by the general formula ABO _{3 in} which Pt and / or Pd is dissolved and / or supported.
The fact that Pt and / or Pd is dissolved in the perovskite type complex oxide means that Pt and / or Pd is coordinated in the crystal lattice of the perovskite type complex oxide, And / or Pd forms a solid solution.

On the other hand, the phrase “Pt and / or Pd is supported on the perovskite complex oxide” means that Pt and / or Pd is not dissolved in the perovskite complex oxide but held on the surface thereof.
A perovskite complex oxide in which Pt and / or Pd is dissolved is represented by the following general formula (1).

ABNO ₃ (1)
(In the formula, A represents at least one element selected from rare earth elements and alkaline earth metals, B represents at least one element selected from transition elements excluding noble metals and Al, and N represents Pt. And / or Pd.)
In the general formula (1), examples of the rare earth element represented by A include Sc (scandium), Y (yttrium), La (lanthanum), Ce (cerium), Pr (praseodymium), Nd (neodymium), Pm ( Promethium), Sm (samarium), Eu (europium), Gd (gadolinium), Tb (terbium), Dy (dysprosium), Ho (holmium), Er (erbium), Tm (thulium), Yb (ytterbium), Lu ( Lutetium).

  Examples of the alkaline earth metal represented by A include Be (beryllium), Mg (magnesium), Ca (calcium), Sr (strontium), Ba (barium), and Ra (radium). These rare earth elements and alkaline earth metals can be used alone or in combination of two or more. Of these, an alkaline earth metal is preferable, and Ca is more preferable.

  In the general formula (1), as the transition element excluding the noble metal represented by B and Al, for example, in the periodic table (IUPAC, 1990), atomic number 21 (Sc) to atomic number 30 (Zn), atomic number 39 (Y) to atomic number 48 (Cd) and atomic number 57 (La) to atomic number 80 (Hg) (except for noble metals (atomic numbers 44 to 47 and 76 to 78)), Al Preferably, Ti (titanium), Cr (chromium), Mn (manganese), Fe (iron), Co (cobalt), Ni (nickel), Cu (copper), Zn (zinc), Zr (zirconium) ) And Al (aluminum), and more preferably Ti. These can be used alone or in combination of two or more.

  Such a perovskite-type composite oxide in which Pt and / or Pd is dissolved (contained as a composition) is not particularly limited, and examples thereof include, for example, paragraph numbers [0039] to [0039] of JP-A-2004-243305. In accordance with the description of [0059], it can be produced by an appropriate method for preparing a composite oxide, for example, a production method such as a coprecipitation method, a citric acid complex method, or an alkoxide method.

The content of Pt and / or Pd in the perovskite complex oxide thus obtained is, for example, generally 20 parts by weight or less with respect to 100 parts by weight of the perovskite complex oxide, 0.2 to 5 parts by weight.
More specifically, the perovskite complex oxide in which Pt and / or Pd is dissolved is represented by, for example, the following general formula (2).

AB _1-x N _x O ₃ (2)
(In the formula, A represents at least one element selected from rare earth elements and alkaline earth metals, B represents at least one element selected from transition elements excluding noble metals and Al, and N represents Pt. And / or Pd, and x represents an atomic ratio in a numerical range of 0 <x <1.)
In the B site of the general formula (2), x represents an atomic ratio of N in a numerical range of 0 <x <1, and preferably 0 <x ≦ 0.5. That is, in the B site, Pt and / or Pd is necessarily contained at an atomic ratio of 0.5 or less.

  Therefore, at the B site, the transition element excluding the noble metal represented by B and at least one element selected from Al are contained at an atomic ratio in the numerical range of 1-x. That is, the B site contains the atomic ratio of the residual 1-x of the atomic ratio x of Pt and / or Pd. Since x is 0 <x ≦ 0.5, 0.5 ≦ 1-x <1, and at the B site, at least one element selected from Al and a transition element excluding the noble metal represented by B is , And is necessarily contained in an atomic ratio of 0.5 or more and less than 1.

The perovskite complex oxide supporting Pt and / or Pd is represented by the following general formula (3).
N / ABO ₃ (3)
(In the formula, A represents at least one element selected from rare earth elements and alkaline earth metals, B represents at least one element selected from transition elements excluding noble metals and Al, and N represents Pt. And / or Pd.)
Such a perovskite type composite oxide carrying Pt and / or Pd is, for example, a general formula ABO ₃ produced according to the description of paragraph numbers [0039] to [0059] of JP-A-2004-243305. The perovskite type complex oxide shown can be produced by supporting Pt and / or Pd in accordance with the description in paragraph No. [0063] of JP-A No. 2004-243305.

The amount of Pt and / or Pd supported in the perovskite complex oxide thus obtained is, for example, generally 20 parts by weight or less, preferably 0 parts by weight based on 100 parts by weight of the perovskite complex oxide. 2 to 5 parts by weight.
And the catalyst of converter A can be formed as a coat layer on a catalyst carrier, for example.

The catalyst carrier is not particularly limited, and a known catalyst carrier such as a honeycomb monolith carrier made of cordierite or the like is used.
In order to form a catalyst layer as a coating layer on the catalyst support, for example, first, water is added to a perovskite type composite oxide containing Pt and / or Pd to form a slurry, and then coated on the catalyst support. What is necessary is just to dry at 50-200 degreeC for 1 to 48 hours, and also to calcinate at 350-1000 degreeC for 1 to 12 hours. Moreover, after adding water to each of the above-described components to form a slurry, these slurries are mixed, coated on the catalyst support, dried at 50 to 200 ° C. for 1 to 48 hours, and further 350 to 1000 You may bake at 12 degreeC for 1 to 12 hours.

The NOx selective reduction catalyst of the converter B selectively increases the speed of the oxidation-reduction reaction between NOx and a reducing gas (for example, NH ₃ ) generated by the catalytic action of the converter A disposed on the upstream side, It is a substance that promotes reduction.
The catalyst of converter B contains Fe, for example, contains zeolite carrying Fe.

  As the zeolite, for example, natural zeolite such as calcite, rhyolite, molybdenite, soda zeolite, mordenite, pyroxene, zeolitic, zeolitic, etc., A type zeolite, Y type zeolite, X type zeolite, L type zeolite, Synthetic zeolites such as erionite, mordenite, β zeolite, and ZSM-5 type zeolite are listed. These can be used alone or in combination of two or more. Moreover, among these, Preferably, a synthetic zeolite is mentioned, More preferably, a ZSM-5 type zeolite is mentioned.

Such a zeolite carrying Fe is not particularly limited, and can be produced by a known method. For example, in the zeolite in which the zeolite is an iron salt hydrate aqueous solution and the Fe content is Fe, for example, 0.01 to 10.00 wt%, preferably 0.10 to 5.00 wt% And the dispersion is evaporated to dryness.
Examples of the iron salt hydrate include, for example, ferric nitrate hexahydrate, ferric nitrate 9 hydrate, ferric nitrate hexahydrate, and ferric nitrate 9 hydrate as iron nitrate hydrate. Examples of the iron sulfate hydrate include ferric sulfate monohydrate, ferric sulfate tetrahydrate, ferric sulfate hexahydrate, and the like. Examples of the hydrate include ferric chloride dihydrate, ferric chloride tetrahydrate, ferric chloride dihydrate, ferric chloride hexahydrate, and the like. The aqueous solution of iron salt hydrate can be prepared, for example, by adding the iron salt hydrate to water and stirring and mixing. Moreover, practically, an aqueous solution of iron nitrate hydrate and the like can be mentioned, and specifically, an aqueous solution of ferric nitrate nonahydrate and the like can be mentioned.

Thereafter, the total amount of Fe contained in the aqueous solution of iron salt hydrate is removed by evaporating water, for example, by vacuum drying or ventilation drying, and heat treatment at, for example, 300 to 800 ° C., preferably 400 to 700 ° C. Is supported on zeolite. Thus, a zeolite carrying Fe is obtained.
The amount of Fe supported on the zeolite thus obtained is, for example, usually 10.00 parts by weight or less, preferably 0.10 to 5.00 parts by weight with respect to 100 parts by weight of zeolite.

And the catalyst of the converter B can be formed as a coat layer on the catalyst carrier in the same manner as the catalyst of the converter A, for example.
That is, in order to form a catalyst layer as a coating layer on a catalyst carrier, for example, first, water is added to a zeolite carrying Fe to form a slurry, which is then coated on the catalyst carrier, and 1 at 50 to 200 ° C. What is necessary is just to dry for -48 hours, and also baked for 1 to 12 hours at 350-1000 degreeC. Moreover, after adding water to each of the above-described components to form a slurry, these slurries are mixed, coated on the catalyst support, dried at 50 to 200 ° C. for 1 to 48 hours, and further 350 to 1000 You may bake at 12 degreeC for 1 to 12 hours.

According to this exhaust gas purification apparatus 1, an exhaust gas containing H ₂ for the gas flow passage 2, supplied from the gas (H _2, NOx and O _2, etc.) and the hydrogen supply line 4 as required generated by operation of the internal combustion engine Pass through.
Then, on the upstream side of the gas flow path 2, that is, the upstream converter 5 (converter A), the oxidation-reduction reaction between H ₂ and NOx in the exhaust gas is promoted by the action of the catalyst containing the perovskite complex oxide. NOx is purified. Along with the reduction of NOx, N ₂ , N ₂ O, reducing gas (for example, NH ₃ ) and the like are generated in the pre-stage converter 5.

On the other hand, in the latter stage converter 6 (converter B), the action of the zeolite carrying Fe promotes the oxidation-reduction reaction between the reducing gas (for example, NH ₃ ) generated upstream and NOx, and purifies NOx. .
As described above, in the rear stage converter 6, a reduction reaction that is independent of the reduction reaction in the front stage converter 5 occurs. Therefore, even if the NOx in the exhaust gas cannot be completely purified by the front stage converter 5, the NOx is passed through the rear stage converter 6. Can be purified.

In addition, since the reducing gas used for the reduction reaction in the post-stage converter 6 is generated by the reduction reaction in the front-stage converter 5, the reducing gas can be effectively used in the post-stage converter 6. Moreover, since it is not necessary to separately provide an apparatus for generating the reducing gas in the gas flow path 2, it is possible to suppress an increase in the size of the facility.
Furthermore, since noble metal elements are generally expensive, it is necessary to reduce the use of noble metal elements as much as possible in industrial production. In the exhaust gas purification apparatus 1, the catalyst of converter B contains Fe, In the case where it contains no noble metal, the converter B can eliminate the use of noble metal. Therefore, an increase in catalyst cost can be suppressed.

  Moreover, the exhaust gas purification apparatus 1 is not particularly limited, and can be used in various industrial fields. In particular, it purifies exhaust gas emitted from internal combustion engines that generate a combustion reaction in a lean atmosphere, such as diesel engines and lean-burn / gasoline engines, and bi-fuel engines of fossil fuel and hydrogen that are supplied with hydrogen as fuel. It is suitable as a device for this.

Next, although this invention is demonstrated based on an Example and a comparative example, this invention is not limited by the following Example.
Production Example 1 (Production of Ca _1.00 Ti _0.98 Pt _0.02 O ₃ powder)
Calcium isopropoxide 15.83 g (0.100 mol in terms of Ca)
Titanium isopropoxide 27.85 g (0.098 mol in terms of Ti)
Platinum acetylacetonate 0.787 g (0.002 mol in terms of Pt)
A platinum-containing mixed alkoxide solution was prepared by putting the above components into a 500 mL round bottom flask, adding 200 mL of toluene and stirring and dissolving.

Next, 150 mL of deionized water was dropped into the mixed alkoxide solution to cause hydrolysis, thereby obtaining a white viscous precipitate. The precipitated solution was stirred at room temperature for 1 hour to complete the reaction, and then heated under reduced pressure to evaporate the solvent and water to dryness. As a result, a yellowish white perovskite complex oxide precursor was obtained.
Next, this perovskite complex oxide precursor was heat-treated (calcined) at 800 ° C. for 2 hours in the atmosphere using an electric furnace. In this manner, 13.89 g of a perovskite complex oxide (Pt content = 2.809 wt%) represented by Ca _1.00 Ti _0.98 Pt _0.02 O ₃ and containing Pt as a composition was obtained. Obtained.

The Ca _1.00 Ti _0.98 Pt _0.02 O ₃ powder was pressure-molded into a pellet and the resulting molded product was pulverized. The particles obtained by the pulverization were sequentially passed through a sieve having a mesh width of 1 mm and a sieve having a mesh width of 750 μm, and the particles that passed through the 1 mm sieve and remained on the sieve having a mesh size of 750 μm were used as test samples.
Production Example 2 (Production of Ca _1.00 Ti _0.98 Pd _0.02 O ₃ powder)
Calcium isopropoxide 15.83 g (0.100 mol in terms of Ca)
Titanium isopropoxide 27.85 g (0.098 mol in terms of Ti)
Palladium acetylacetonate 0.609 g (0.002 mol in terms of Pd)
A palladium-containing mixed alkoxide solution was prepared by putting the above components into a 500 mL round bottom flask, adding 200 mL of toluene, and dissolving by stirring.

Next, 150 mL of deionized water was dropped into the mixed alkoxide solution to cause hydrolysis, thereby obtaining a white viscous precipitate. The precipitated solution was stirred at room temperature for 1 hour to complete the reaction, and then heated under reduced pressure to evaporate the solvent and water to dryness. As a result, a yellowish white perovskite complex oxide precursor was obtained.
Next, this perovskite complex oxide precursor was heat-treated (calcined) at 800 ° C. for 2 hours in the atmosphere using an electric furnace. In this manner, 13.71 g of a perovskite complex oxide (Pd content = 1.552 wt%) of a perovskite type composite oxide containing Pd as a composition represented by Ca _1.00 Ti _0.98 Pd _0.02 O ₃ was obtained. Obtained.

The Ca _1.00 Ti _0.98 Pd _0.02 O ₃ powder was pressure-molded into pellets, and the resulting molded product was pulverized. The particles obtained by the pulverization were sequentially passed through a sieve having a mesh width of 1 mm and a sieve having a mesh width of 750 μm, and the particles that passed through the 1 mm sieve and remained on the sieve having a mesh size of 750 μm were used as test samples.
Production Example 3 (Production of Fe / Zeolite Powder)
After impregnating 19.40 g of β-type zeolite powder with 4.384 g of ferric nitrate nonahydrate (corresponding to Fe content of 13.685 wt%) (0.60 g in terms of Fe), Air dried for hours. After drying, heat treatment is performed in the atmosphere using an electric furnace at 650 ° C. for 1 hour, whereby 20.00 g of Fe-supported zeolite (Fe content: 3.00 wt%) powder represented by Fe / zeolite is obtained. Obtained.

This Fe / zeolite powder was pressure-molded into pellets, and the resulting molded product was pulverized. The particles obtained by the pulverization were sequentially passed through a sieve having a mesh width of 1 mm and a sieve having a mesh width of 750 μm, and the particles that passed through the 1 mm sieve and remained on the sieve having a mesh size of 750 μm were used as test samples.
Examples 1-2 and Comparative Examples 1-3
Each sample obtained in Production Examples 1 to 3 was placed in a normal pressure fixed bed flow reactor in the sample amount / arrangement form shown in Table 1 below. When the two types of catalysts are arranged separately on the upstream side and the downstream side, the two samples are separated by glass wool in order to prevent mixing of the upstream sample and the downstream sample.

Then, while flowing the model gas through the catalyst bed, the catalyst bed temperature was raised from 30 ° C. to 450 ° C. at a rate of 20 ° C./min, and the exhaust gas purification rate during that time was continuously measured. The NOx purification rate at each catalyst bed temperature is shown in Table 1 and FIGS. In Table 1, the NOx maximum purification rate is the highest purification rate in the measured temperature range.
As the model gas, an oxidizing component excess gas in which 5% oxygen, 3000 ppm hydrogen, 333 ppm propane, 8% carbon dioxide, and 10% water vapor were mixed was used. This measurement result describes the NOx purification rate with respect to the temperature.

According to FIGS. 4 and 5, a sample of Ca _1.00 Ti _0.98 Pt _0.02 O ₃ (Comparative Example 1) and a sample of Ca _1.00 Ti _0.98 Pd _0.02 O ₃ (Comparative Example 2) ) By itself, the maximum NOx purification rates were 63% and 41%, respectively. In addition, as shown in FIG. 6, the Fe / zeolite sample (Comparative Example 3) alone did not exhibit catalytic activity against the above model gas and could not substantially purify NOx.

On the other hand, as in Example 1, in the form in which the Fe / zeolite sample is arranged on the downstream side of the Ca _1.00 Ti _0.98 Pt _0.02 O ₃ sample, Fe / zeolite alone has catalytic activity. Although NO was shown, it was confirmed that the maximum NOx purification rate was improved to 66%.
Moreover, in the embodiment in which the Fe / zeolite sample is arranged downstream of the Ca _1.00 Ti _0.98 Pd _0.02 O ₃ sample as in Example 2, the maximum NOx purification rate is improved to 54%. It was confirmed that

  Further, in Examples 1 and 2, since no noble metal is contained in the downstream catalyst, an increase in catalyst cost can be suppressed.

DESCRIPTION OF SYMBOLS 1 Exhaust gas purification apparatus 2 Gas flow path 5 Previous stage converter (converter A)
6 Rear stage converter (converter B)

Claims (2)

A gas flow path through which the exhaust gas passes;
An upstream catalyst including a perovskite-type composite oxide represented by the general formula ABO ₃ that is disposed so as to be in contact with the exhaust gas at a relatively upstream side in the gas flow path and contains Pt and / or Pd;
NOx selective reduction, which is arranged so as to be in contact with the exhaust gas relatively downstream in the gas flow path, selectively contains NOx in an atmosphere of a reducing gas containing Fe and generated by the action of the upstream catalyst. An exhaust gas purification apparatus comprising a downstream catalyst including a catalyst. The perovskite-type composite oxide further has a general formula CaTi _1-x N _x O ₃ (wherein N represents Pt and / or Pd, and x represents an atomic ratio in a numerical range of 0 <x <1. )
The exhaust gas purifying apparatus according to claim 1, wherein the NOx selective reduction catalyst is a zeolite carrying Fe.

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