JP5607891B2 - Exhaust gas purification catalyst - Google Patents

Description

The present invention relates to an exhaust gas purifying catalyst that purifies hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NOx) contained in exhaust gas such as automobile engines.

Exhaust gas discharged from an internal combustion engine such as an automobile contains hydrocarbon (HC), carbon monoxide (CO), nitrogen oxide (NOx), and the like.
As a three-way catalyst for purifying these, noble metals (Rh (rhodium), Pd (palladium), Pt (platinum), etc.), which are active components, are ceria complex oxides, zirconia complex oxides, perovskite complex oxides. Various exhaust gas purifying catalysts that are supported or dissolved in heat-resistant oxides such as aluminum or alumina are known.

For example, a ceria-based composite oxide represented by Ce _0.60 Zr _0.30 Y _0.10 O _1.95 supporting Pd is known (see, for example, Patent Document 1).

Japanese Patent Laid-Open No. 10-216509

However, since noble metal elements such as Pd are generally expensive, industrially, it is required to effectively develop gas purification performance with as little amount as possible.
On the other hand, transition elements other than noble metal elements have lower catalytic activity than noble metal elements. In addition, since deterioration due to oxidation or aggregation is remarkable in an exhaust gas atmosphere, it is difficult to use a transition element other than a noble metal element as an active component.

  An object of the present invention is to provide an exhaust gas purifying catalyst capable of expressing excellent catalytic activity over a long period of time under high temperature or oxidation-reduction fluctuation while reducing the use of noble metal elements.

In order to achieve the above object, the exhaust gas purifying catalyst of the present invention is characterized by containing hexaaluminate represented by the general formula SrAl ₁₂ O ₁₉ supporting Pd and Fe.
In the exhaust gas purifying catalyst of the present invention, the Pd content is 0.01 to 10.00% by weight and the Fe content is 0.01 to 10.00% by weight relative to the total amount of hexaaluminate. Is preferred.

According to the exhaust gas purifying catalyst of the present invention, Fe is easily reduced from the interaction of Fe and Pd under fluctuations in the oxidation-reduction atmosphere by supporting Pd and Fe together with hexaaluminate having a specific composition. Become. Therefore, it is possible to prevent a decrease in catalytic activity due to Fe deterioration over a long period of time, and to maintain a high catalytic activity.
As a result, if the exhaust gas purifying catalyst of the present invention is used, Fe can be used as an active component, so that it has excellent catalytic activity over a long period of time at low temperature, under high temperature or under oxidation-reduction fluctuations, while reducing noble metal elements. Can be expressed.

It is a bar graph which shows the THC, NOx, and CO purification rate of the exhaust gas purification catalyst of Example 1 and Comparative Examples 1-5.

The exhaust gas purifying catalyst of the present invention contains hexaaluminate represented by the general formula SrAl ₁₂ O ₁₉ supporting Pd and Fe.
In this exhaust gas-purifying catalyst, Pd and Fe are supported on the surface of Pd and Fe without being dissolved in the hexaaluminate type oxide represented by the above formula. .

  In the exhaust gas purification catalyst, Pd is preferably contained in an amount of 0.01 to 10.00% by weight, more preferably 0.01 to 1.00% by weight, based on the total amount of hexaaluminate. Yes. Further, Fe is preferably contained in an amount of 0.01 to 10.00% by weight, more preferably 0.01 to 5.00% by weight, based on the total amount of hexaaluminate. If the content of Pd and Fe is in the above range, the amount of Pd used can be reduced while maintaining the catalytic activity of the exhaust gas purifying catalyst, so that the cost can be further reduced.

The hexaaluminate of the present invention is not particularly limited, and may be prepared by an appropriate method for preparing an oxide, for example, an alkoxide method, a coprecipitation method, a citric acid complex method, or the like. Can do.
In the alkoxide method, for example, a mixed alkoxide solution containing the alkoxide of each of the above elements (excluding palladium and iron) in a molar ratio of, for example, strontium: aluminum = 1: 12 is prepared.

Examples of the alkoxide of each element include an alcoholate formed from each element and an alkoxy such as methoxy, ethoxy, propoxy, isopropoxy, butoxy, and an alkoxy alcoholate of each element represented by the following general formula (1). Can be mentioned.
^{E [OCH (R 1) -} (CH 2) i -OR 2] j (1)
(In the formula, E represents each element, R ¹ represents a hydrogen atom or an alkyl group having 1 to 4 carbon atoms, R ² represents an alkyl group having 1 to 4 carbon atoms, and i represents 1 to 1) (An integer of 3 and j represents an integer of 2 to 4.)
More specifically, the alkoxy alcoholate includes, for example, methoxyethylate, methoxypropylate, methoxybutyrate, ethoxyethylate, ethoxypropylate, propoxyethylate, butoxyethylate and the like.

And a mixed alkoxide solution can be prepared by, for example, adding the alkoxide of each element to an organic solvent so that it may become said molar ratio, and stirring and mixing.
The organic solvent is not particularly limited as long as the alkoxide of each element can be dissolved, and examples thereof include aromatic hydrocarbons, aliphatic hydrocarbons, alcohols, ketones, and esters. Preferably, aromatic hydrocarbons such as benzene, toluene and xylene are used.

  Thereafter, the organic solvent is distilled off from the mixed alkoxide solution under reduced pressure to dryness to obtain a hexaaluminate precursor (gel). Then, the obtained hexaaluminate precursor is subjected to, for example, vacuum distillation or ventilation drying to distill water off, for example, vacuum drying or ventilation drying, and then heat treatment at, for example, about 1200 to 1400 ° C. By performing (primary firing), a hexaaluminate type oxide can be produced.

  Next, the obtained hexaaluminate type oxide is converted into a palladium salt aqueous solution and an iron salt hydrate aqueous solution in a hexaaluminate type oxide particle to be described later. 0.000% by weight, preferably 0.01 to 1.00% by weight, and Fe content is, for example, 0.01 to 10.00% by weight, preferably 0.01 to 5.00. Disperse and evaporate the dispersion to dryness.

  Examples of the palladium salt include inorganic salts such as sulfate, nitrate, chloride, and phosphate, and organic acid salts such as acetate and oxalate. The palladium salt aqueous solution can be prepared, for example, by adding a palladium salt to water and mixing with stirring. Practically, nitrate aqueous solution, dinitrodiammine nitric acid aqueous solution, chloride aqueous solution and the like can be mentioned. Specific examples include an aqueous palladium nitrate solution, an aqueous dinitrodiammine palladium nitric acid solution, and an aqueous tetravalent palladium ammine nitric acid solution.

  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.

  The order of dispersing the hexaaluminate type oxide is not particularly limited. Preferably, the hexaaluminate type oxide is first dispersed in an aqueous palladium salt solution and then dispersed in an aqueous iron salt hydrate solution. Thereafter, water is distilled off, for example, by vacuum drying or ventilation drying, and Pd contained in the palladium salt aqueous solution is subjected to heat treatment (secondary firing) at 300 to 800 ° C., preferably 400 to 700 ° C., for example. And the total amount of Fe contained in the aqueous solution of iron salt hydrate is supported on the hexaaluminate type oxide.

Thus, particles made of hexaaluminate type oxide supporting Pd and Fe are obtained.
Moreover, in the coprecipitation method, for example, after preparing a mixed salt aqueous solution containing the salt of each element (excluding palladium and iron) in the molar ratio described above and adding a neutralizing agent to the mixed salt aqueous solution to coprecipitate The obtained coprecipitate is dried and then heat-treated.

Examples of the salt of each element include inorganic salts such as sulfate, nitrate, chloride, and phosphate, and organic acid salts such as acetate and oxalate. The mixed salt aqueous solution can be prepared, for example, by adding the salt of each element to water at a ratio such that the above molar ratio is obtained, and stirring and mixing.
Thereafter, a neutralizing agent is added to the mixed salt aqueous solution to cause coprecipitation. Although it does not restrict | limit especially as a neutralizing agent, For example, organic bases, such as ammonia, for example, amines, such as a triethylamine and a pyridine, For example, inorganic bases, such as caustic soda, caustic potash, potassium carbonate, and ammonium carbonate, are used. Moreover, a neutralizing agent is dripped so that pH of the solution after adding the neutralizing agent may become about 6-10. If dripped in this way, the salt of each element can be efficiently coprecipitated.

The obtained coprecipitate is washed with water if necessary, and dried by, for example, vacuum drying or ventilation drying, and then heat-treated (primary firing) at about 1200 to 1400 ° C., for example, to obtain hexaaluminate. A type oxide can be produced.
Subsequently, the obtained hexaaluminate type oxide is dispersed in an aqueous solution of palladium salt and an aqueous solution of iron salt hydrate in the same manner as in the alkoxide method, and this dispersion is evaporated to dryness. Then, for example, Pd contained in the palladium salt aqueous solution is obtained by distilling off water by vacuum drying or ventilation drying, and performing heat treatment (secondary firing) at 300 to 800 ° C., preferably 400 to 700 ° C. And the total amount of Fe contained in the aqueous solution of iron salt hydrate is supported on the hexaaluminate type oxide.

Thus, particles made of hexaaluminate type oxide supporting Pd and Fe are obtained.
Further, in the citric acid complex method, for example, citric acid is added to the mixed salt aqueous solution so that the citric acid is slightly in excess of the above-described molar ratio with respect to each of the above elements (excluding palladium and iron). An aqueous solution was added to prepare a citric acid mixed salt aqueous solution, this citric acid mixed salt aqueous solution was dried to form a citric acid complex of each element described above, and then the obtained citric acid complex was calcined, Heat treatment.

Examples of the salt of each element include the same salts as described above. For the citric acid mixed salt aqueous solution, for example, a mixed salt aqueous solution is prepared in the same manner as described above, and an aqueous citric acid solution is added to the mixed salt aqueous solution. It can be prepared by adding.
Thereafter, the citric acid mixed salt aqueous solution is dried to form a citric acid complex of each element described above. Drying removes moisture at a temperature at which the formed citric acid complex does not decompose, for example, from room temperature to 150 ° C. Thereby, a citric acid complex of each element described above can be formed.

And the formed citric acid complex is subjected to a heat treatment (primary firing) after provisional firing. Temporary baking should just heat at 250-350 degreeC in a vacuum or an inert atmosphere, for example. Then, a hexaaluminate-type oxide can be manufactured by heat-processing at about 1200-1400 degreeC after that, for example.
Subsequently, the obtained hexaaluminate type oxide is dispersed in an aqueous solution of palladium salt and an aqueous solution of iron salt hydrate in the same manner as in the alkoxide method, and this dispersion is evaporated to dryness. Then, for example, Pd contained in the palladium salt aqueous solution is obtained by distilling off water by vacuum drying or ventilation drying, and performing heat treatment (secondary firing) at 300 to 800 ° C., preferably 400 to 700 ° C. And the total amount of Fe contained in the aqueous solution of iron salt hydrate is supported on the hexaaluminate type oxide.

Thus, particles made of hexaaluminate type oxide supporting Pd and Fe are obtained.
The hexaaluminate carrying Pd and Fe of the present invention thus obtained can be used as it is as a catalyst composition, but is usually supported by a known method such as carrying on a catalyst carrier. As prepared.

The catalyst carrier is not particularly limited, and for example, a known catalyst carrier such as a honeycomb monolith carrier made of cordierite or the like is used.
In order to carry it on the catalyst carrier, for example, first, water is added to the obtained hexaaluminate carrying Pd and Fe to form a slurry, which is then coated on the catalyst carrier and dried, and then about 300 It heat-processes at -800 degreeC, Preferably, it is about 400-700 degreeC.

And, in the exhaust gas purifying catalyst containing hexaaluminate supporting Pd and Fe of the present invention thus obtained, the dispersion state of Pd and Fe with respect to the hexaaluminate represented by the general formula SrAl ₁₂ O ₁₉ is excellent. Retained. Therefore, it is possible to prevent a decrease in catalytic activity due to the growth of Pd and Fe grains over a long period of time, and a high catalytic activity can be maintained.

As a result, if the exhaust gas purifying catalyst of the present invention is used, Fe can be used as an active component, so that it has excellent catalytic activity over a long period of time at low temperature, under high temperature or under oxidation-reduction fluctuations, while reducing noble metal elements. Can be expressed.
Therefore, the exhaust gas purifying catalyst containing hexaaluminate supporting Pd and Fe of the present invention can be widely used as a gas phase or liquid phase reaction catalyst. In particular, since excellent exhaust gas purification performance can be realized over a long period of time, it can be suitably used, for example, for purifying exhaust gas discharged from internal combustion engines such as gasoline engines and diesel engines, boilers, and the like.

Next, although this invention is demonstrated based on an Example and a comparative example, this invention is not limited by the following Example.
Example 1 (Production of Pd / Fe / SrAl ₁₂ O ₁₉ powder)
A mixed alkoxide solution was prepared by placing 4.12 g (0.020 mol) of strontium isopropoxide and 49.02 g (0.240 mol) of aluminum isopropoxide in a 500 ml round bottom flask, adding 200 ml of toluene and stirring to dissolve. Was prepared.

Subsequently, the solvent was distilled off from the mixed alkoxide solution under reduced pressure to dryness, thereby obtaining a SrAl ₁₂ O ₁₉ precursor gel.
Next, the precursor gel was transferred to a petri dish, dried by ventilation at 60 ° C. for 24 hours, and then heat-treated at 1000 ° C. for 1 hour. Further, heat-treated for 1 hour at 1300 ° _C., to obtain a dark brown powder 8.91g of hexaaluminate type oxide consisting of _SrAl 12 _{O 19.}

To this powder, 1.03 g of palladium nitrate aqueous solution (corresponding to Pd content of 4.40% by weight) (corresponding to 0.0452 g in terms of Pd) and ferric nitrate nonahydrate (corresponding to Fe content of 13.69% by weight) ) 0.661 g (corresponding to 0.0905 g in terms of Fe) was impregnated.
After impregnation, the material was air-dried at 80 ° C. for 24 hours, and then heat-treated in the atmosphere using an electric furnace at 650 ° C. for 1 hour, and Pd and Fe supported Pd and Fe represented by Pd / Fe / SrAl ₁₂ O ₁₉ 9.05 g of powder of aluminate (Pd content: 0.50 wt%, Fe content: 1.00 wt%) was obtained.
Comparative Example 1 (Production of Pd / SrAl ₁₂ O ₁₉ powder)
A mixed alkoxide solution was prepared by placing 4.12 g (0.020 mol) of strontium isopropoxide and 49.02 g (0.240 mol) of aluminum isopropoxide in a 500 ml round bottom flask, adding 200 ml of toluene and stirring to dissolve. Was prepared.

Subsequently, the solvent was distilled off from the mixed alkoxide solution under reduced pressure to dryness, thereby obtaining a SrAl ₁₂ O ₁₉ precursor gel.
Next, the precursor gel was transferred to a petri dish, dried by ventilation at 60 ° C. for 24 hours, and then heat-treated at 1000 ° C. for 1 hour. Further, heat-treated for 1 hour at 1300 ° _C., to obtain a dark brown powder 8.91g of hexaaluminate type oxide consisting of _SrAl 12 _{O 19.}

This powder was impregnated with 1.02 g (corresponding to 0.0448 g in terms of Pd) of an aqueous palladium nitrate solution (corresponding to 4.40% by weight of Pd).
After the impregnation, the material was air-dried at 80 ° C. for 24 hours, and then heat-treated in the atmosphere using an electric furnace at 650 ° C. for 1 hour, and Pd-supported hexaaluminate represented by Pd / SrAl ₁₂ O ₁₉ (Pd 8.95 g of powder having a content of 0.50% by weight was obtained.
Comparative Example 2 (Production of Pd / BaAl ₁₂ O ₁₉ powder)
A mixed alkoxide solution was prepared by placing 5.11 g (0.020 mol) of barium isopropoxide and 49.02 g (0.240 mol) of aluminum isopropoxide in a 500 ml round bottom flask, adding 200 ml of toluene and stirring to dissolve. Was prepared.

Next, a BaAl ₁₂ O ₁₉ precursor gel was obtained by evaporating the solvent from the mixed alkoxide solution to dryness under reduced pressure.
Next, the precursor gel was transferred to a petri dish, dried by ventilation at 60 ° C. for 24 hours, and then heat-treated at 1000 ° C. for 1 hour. Further, heat-treated for 1 hour at 1300 ° _C., to obtain a dark brown powder 15.30g of hexaaluminate type oxide consisting of _BaAl 12 _{O 19.}

This powder was impregnated with 1.75 g of a palladium nitrate aqueous solution (corresponding to Pd content of 4.40% by weight) (corresponding to 0.0769 g in terms of Pd).
After impregnation, the material was air-dried at 80 ° C. for 24 hours, and then heat-treated in the atmosphere using an electric furnace at 650 ° C. for 1 hour, and Pd-supported hexaaluminate represented by Pd / BaAl ₁₂ O ₁₉ (Pd 15.38 g of powder having a content of 0.50% by weight was obtained.
Comparative Example 3 (Production of Pd / Fe / BaAl ₁₂ O ₁₉ powder)
A mixed alkoxide solution was prepared by placing 5.11 g (0.020 mol) of barium isopropoxide and 49.02 g (0.240 mol) of aluminum isopropoxide in a 500 ml round bottom flask, adding 200 ml of toluene and stirring to dissolve. Was prepared.

Next, a BaAl ₁₂ O ₁₉ precursor gel was obtained by evaporating the solvent from the mixed alkoxide solution to dryness under reduced pressure.
Next, the precursor gel was transferred to a petri dish, dried by ventilation at 60 ° C. for 24 hours, and then heat-treated at 1000 ° C. for 1 hour. Further, heat-treated for 1 hour at 1300 ° _C., to obtain a dark brown powder 15.30g of hexaaluminate type oxide consisting of _BaAl 12 _{O 19.}

To this powder, 1.77 g of palladium nitrate aqueous solution (corresponding to Pd content of 4.40% by weight) (corresponding to 0.0777 g in terms of Pd), ferric nitrate nonahydrate (corresponding to Fe content of 13.69% by weight) ) 1.14 g (corresponding to 0.155 g in terms of Fe) was impregnated.
After impregnation, the material was air-dried at 80 ° C. for 24 hours, and then heat-treated in the atmosphere using an electric furnace at 650 ° C. for 1 hour, and Pd and Fe supported Pd / Fe represented by Pd / Fe / BaAl ₁₂ O ₁₉ 15.53 g of powder of aluminate (Pd content: 0.50% by weight, Fe content: 1.00% by weight) was obtained.
Comparative Example 4 (Production of Pd / Ce _0.45 Zr _0.50 Y _0.05 O ₂ Powder)
Mixing by weighing cerium nitrate, zirconium oxynitrate, and yttrium nitrate so that the atomic ratio of cerium, zirconium, and yttrium is 45: 50: 5, and adding them to 500 mL of deionized water. An aqueous salt solution was prepared.

After sufficiently stirring, a 10% by weight ammonium hydroxide aqueous solution was dropped into the mixed aqueous solution at room temperature to obtain a coprecipitate.
Next, the aqueous solution containing the coprecipitate was stirred for 60 minutes and then filtered. Next, the filter cake was thoroughly washed with deionized water and dried at 110 ° C. to obtain a dried product.
Next, the dried product was calcined at 500 ° C. for 3 hours in an air atmosphere. The obtained calcined product was pulverized in a mortar, and further calcined at 800 ° C. for 5 hours in an air atmosphere. Thereby, a composite oxide represented by Ce _0.45 Zr _0.50 Y _0.05 O ₂ was obtained.

To 14.93 g of the obtained composite oxide, 1.71 g (corresponding to 0.0750 g in terms of Pd) of an aqueous palladium nitrate solution (corresponding to Pd content of 4.40% by weight) was added, and the mixture was stirred and mixed for 1 hour. It was.
After impregnation, drying at 80 ° C. for 24 hours, followed by heat treatment at 650 ° C. for 1 hour, Pd-supported heat-resistant oxide represented by Pd / Ce _0.45 Zr _0.50 Y _0.05 O ₂ ( 15.00 g of powder having a Pd content of 0.50% by weight was obtained.
Comparative Example 5 (Production of Pd / Fe / Ce _0.45 Zr _0.50 Y _0.05 O ₂ Powder)
Mixing by weighing cerium nitrate, zirconium oxynitrate, and yttrium nitrate so that the atomic ratio of cerium, zirconium, and yttrium is 45: 50: 5, and adding them to 500 mL of deionized water. An aqueous salt solution was prepared.

After sufficiently stirring, a 10% by weight ammonium hydroxide aqueous solution was dropped into the mixed aqueous solution at room temperature to obtain a coprecipitate.
Next, the aqueous solution containing the coprecipitate was stirred for 60 minutes and then filtered. Next, the filter cake was thoroughly washed with deionized water and dried at 110 ° C. to obtain a dried product.
Next, the dried product was calcined at 500 ° C. for 3 hours in an air atmosphere. The obtained calcined product was pulverized in a mortar, and further calcined at 800 ° C. for 5 hours in an air atmosphere. Thereby, a composite oxide represented by Ce _0.45 Zr _0.50 Y _0.05 O ₂ was obtained.

To the obtained composite oxide, 1.71 g of palladium nitrate aqueous solution (corresponding to Pd content of 4.40 wt%) (corresponding to 0.0750 g in terms of Pd), ferric nitrate nonahydrate (Fe content of 13.69). 1.10 g (corresponding to wt%) (corresponding to 0.150 g in terms of Fe) was added, and the mixture was impregnated by stirring for 1 hour.
After impregnation, after drying at 80 ° C. for 24 hours, heat treatment was performed at 650 ° C. for 1 hour, and heat resistance carrying Pd and Fe represented by Pd / Fe / Ce _0.45 Zr _0.50 Y _0.05 O ₂ 15.00 g of a powder of a conductive oxide (Pd content: 0.50 wt%, Fe content: 1.00 wt%) was obtained.
Test Example (1) Durability Test 5 minutes of inert atmosphere, 10 minutes of oxidizing atmosphere, 5 minutes of inert atmosphere and 10 minutes of reducing atmosphere are defined as one cycle, and this cycle is repeated 10 times for a total of 5 hours. The powders obtained in the examples and comparative examples were alternately exposed to an oxidizing atmosphere and a reducing atmosphere, and then cooled to room temperature in the reducing atmosphere.

In addition, each atmosphere was prepared by supplying the gas of the composition shown in following Table 1 containing high temperature water vapor | steam at the flow volume of 300 * 10 < ^-3 > m < ³ > / hr. The ambient temperature was maintained at about 1000 ° C.

(2) Activity evaluation Each powder subjected to the durability test of (1) was placed in an atmospheric pressure fixed bed flow reactor. A model gas having the composition shown in Table 2 below was circulated through the catalyst bed, and was maintained as a pretreatment in a rich gas having an air fuel ratio (A / F) of 14.0 shown in Table 2 at 500 ° C. for 10 minutes, and then to room temperature. Once cooled.

  Next, after the catalyst bed temperature was raised from room temperature to 430 ° C. in 1200 seconds, the A / F was changed from 14.0 to 15.2 with each A / F holding time being 300 seconds as shown in Table 2. The exhaust gas purification rate during that time was continuously measured. FIG. 1 and Table 3 show the purification rates of THC (total hydrocarbons), NOx, and CO at A / F = 14.6 (stoichiometry) in each powder.

Claims (2)

A catalyst for purifying exhaust gas containing hexaaluminate represented by P d and F e the general formula _SrAl 12 _{O 19} was responsible di,
Hexaluminate is first dispersed in an aqueous palladium salt solution, then dispersed in an aqueous solution of iron salt hydrate, and then obtained by drying and heat treating the resulting dispersion.
An exhaust gas purifying catalyst characterized in that Pd and Fe are not dissolved in hexaaluminate but are held on the surface thereof .   The content of Pd with respect to the total amount of hexaaluminate is 0.01 to 10.00 wt%, and the content of Fe is 0.01 to 10.00 wt%. Exhaust gas purification catalyst.

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