JP4812359B2 - Exhaust gas purification catalyst composition - Google Patents

Description

The present invention relates to an exhaust gas purification catalyst composition used as an exhaust gas purification catalyst .

As a catalyst for removing nitrogen oxides NO _x (NO and NO ₂ ) contained in exhaust gas, for example, an ilmenite type complex oxide represented by a general formula ABO ₃ is known.
As such an ilmenite type complex oxide, for example, NiMnO ₃ , NiMnO ₃ impregnated with Pt (platinum) and the like have been proposed (see Patent Document 1).
JP 2001-170446 A

  However, in the ilmenite type composite oxide described in Patent Document 1, the noble metal is only supported on the ilmenite type composite oxide, and is supported at high temperatures, under oxidation-reduction fluctuations, and during long-term use. As much noble metal as possible moves and coalesces on the surface of the ilmenite type complex oxide, thereby causing grain growth, and reducing the effective surface area of the noble metal reduces the catalytic activity.

An object of the present invention is to provide a catalyst composition for exhaust gas purification that can prevent a decrease in catalytic activity due to grain growth of noble metal at high temperatures, under oxidation-reduction fluctuations, or during long-term use, and can realize excellent catalytic activity over a long period of time. It is to provide.

In order to achieve the above object, the exhaust gas purifying catalyst composition of the present invention (hereinafter referred to as catalyst composition) is an ilmenite complex oxide containing a noble metal, and the noble metal is ilmenite type in an oxidizing atmosphere. solid solution in the composite oxide, in a reducing atmosphere to precipitate from ilmenite-type complex oxide represented by the general formula (1)
AB _1-x M _x O ₃ (1)
(In the formula, A represents Fe and / or Mg, B represents Ti, M represents at least one element selected from precious metals, and x represents a range of 0 <x <1. )
It is characterized by being indicated by.

  In the catalyst composition of the present invention, it is preferable that the noble metal is at least one element selected from Pt, Pd, and Rh.

According to the catalyst composition of the present invention, a solid solution regeneration in which noble metals such as Pt, Pd, and Rh are efficiently dissolved in an oxidizing atmosphere and precipitated in a reducing atmosphere with respect to the ilmenite type composite oxide. Since (self-regeneration) is repeated, these can be kept in a dispersed state. Therefore, it is possible to prevent a decrease in catalytic activity due to grain growth and maintain a high catalytic activity over a long period of time. As a result, it is possible to realize a superior exhaust gas purification performance for a long time, for example, a gasoline engine, or an internal combustion engine such as a diesel engine, as an exhaust gas purifying catalyst for purifying exhaust gas discharged from such a boiler, It can be used suitably.

The catalyst composition of the present invention is an ilmenite type composite oxide containing a noble metal, and the noble metal is dissolved in the ilmenite type composite oxide in an oxidizing atmosphere, and is precipitated from the ilmenite type composite oxide in a reducing atmosphere. .
In the present invention, the ilmenite complex oxide is a complex oxide having an ilmenite crystal structure.

Further, the catalyst composition of the present invention, Ru is represented by the general formula (1).
AB _1-x M _x O ₃ (1)
(In the formula, A represents Fe and / or Mg, B represents Ti, M represents at least one element selected from precious metals, and x represents a range of 0 <x <1. )
In the catalyst composition of the general formula (1), a noble metal is coordinated to the ilmenite complex oxide of the general formula ABO ₃ . That is, in this catalyst composition, Fe (iron) and / or Mg (magnesium ) represented by A is always coordinated at the A site . These elements may be used alone, or may be used in combination of two or more.

Further, in the B site, indicated by B, T i (titanium) is always coordinated.

In addition, the noble metal represented by M together with the above-described elements is always coordinated with the B site.
Examples of the noble metal include Pt (platinum), Ir (iridium), Os (osmium), Ag (silver), Pd (palladium), Rh (rhodium), and Ru (ruthenium). Pt, Pd and Rh are preferable. These noble metals may be used alone or in combination of two or more.

The above-described catalyst composition is not particularly limited, and can be prepared by an appropriate method for preparing an ilmenite-type composite oxide, for example, a coprecipitation method, a citric acid complex method, an alkoxide method, or the like. it can.
In the coprecipitation method, for example, a mixed salt aqueous solution containing the salt of each element described above at a predetermined stoichiometric ratio is prepared, and a neutralizing agent is added to the mixed salt aqueous solution to perform coprecipitation. The precipitate 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. Further, the mixed salt aqueous solution can be prepared, for example, by adding the salt of each element to water at a ratio that gives a predetermined stoichiometric ratio and stirring and mixing.
Thereafter, a neutralizing agent is added to the mixed salt aqueous solution to cause coprecipitation. Examples of the neutralizing agent include ammonia, for example, organic bases such as amines such as triethylamine and pyridine, and inorganic bases such as caustic soda, caustic potash, potassium carbonate, and ammonium carbonate. The neutralizing agent is added so that the pH of the solution after adding the neutralizing agent is about 6 to 10.

The obtained coprecipitate is washed with water if necessary, and dried by, for example, vacuum drying or ventilation drying, and then heat-treated at, for example, about 500 to 1200 ° C., preferably about 600 to 1000 ° C. A catalyst composition can be produced.
Further, in the citric acid complex method, for example, citric acid and a salt of each element described above are added to each element described above by adding a slightly excessive citric acid aqueous solution than the stoichiometric ratio to prepare a citric acid mixed salt aqueous solution. After this citric acid mixed salt aqueous solution is dried to form a citric acid complex of each element described above, the obtained citric acid complex is pre-baked and then heat-treated.

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.

The formed citric acid complex is subjected to heat treatment after calcination. Temporary baking should just heat at 250-350 degreeC in a vacuum or an inert atmosphere, for example. Thereafter, for example, the catalyst composition can be produced by heat treatment at about 500 to 1200 ° C., preferably about 600 to 1000 ° C.
Further, in the alkoxide method, for example, a mixed alkoxide solution containing the alkoxide of each element described above excluding the noble metal in the above stoichiometric ratio is prepared, and water is added to the mixed alkoxide solution and precipitated by hydrolysis. Thereafter, the obtained precipitate is used as a slurry, and an aqueous solution containing a salt of a noble metal is mixed and supported on the slurry so that each element and the noble metal have the above stoichiometric ratio. Get. The obtained precursor composition is then heat-treated (fired).

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 (2). Can be mentioned.
^{_{E [OCH (R 1) -}} (CH 2) i -OR 2] j (2)
(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, examples of the alkoxy alcoholate include 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 stoichiometric 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 mixed alkoxide solution is hydrolyzed to form a precipitate. Then, an organic solvent is distilled off from the mixed alkoxide solution to prepare a slurry, and an aqueous solution containing a noble metal salt is added to the slurry at a predetermined stoichiometric ratio.
Examples of the aqueous solution containing a noble metal salt include nitrate aqueous solution, chloride aqueous solution, hexaammine chloride aqueous solution, dinitrodiammine nitric acid aqueous solution, hexachloro acid hydrate, potassium cyanide salt and the like.

And after distilling water off by the obtained slurry, for example by vacuum drying, ventilation drying, etc., and drying, a precursor is obtained, for example, about 500-1200 degreeC, Preferably, it is about 600-1000 degreeC. A catalyst composition can be produced by heat treatment (firing).
Further, in the alkoxide method, an aqueous solution containing a noble metal salt is added to the mixed alkoxide solution and precipitated by hydrolysis, and then the obtained precipitate is dried to obtain a precursor, followed by heat treatment, A catalyst composition can also be produced.

Examples of the noble metal organic metal salts include, for example, the above-mentioned noble metal carboxylates formed from acetates, propionates, and the like, for example, β-diketone compounds or β-ketoester compounds represented by the following general formula (3), and / Or the metal chelate complex of the said noble metal formed from the (beta) -dicarboxylic acid ester compound shown by following General formula (4).
R ³ COCHR ⁵ COR ⁴ (3)
(In the formula, R ³ is an alkyl group having 1 to 6 carbon atoms, a fluoroalkyl group having 1 to 6 carbon atoms or an aryl group, R ⁴ is an alkyl group having 1 to 6 carbon atoms, and a fluoro having 1 to 6 carbon atoms. An alkyl group, an aryl group, or an alkyloxy group having 1 to 4 carbon atoms and R ⁵ represents a hydrogen atom or an alkyl group having 1 to 4 carbon atoms.)
R ⁷ CH (COOR ⁶ ) ² (4)
(Wherein R ⁶ represents an alkyl group having 1 to 6 carbon atoms, and R ⁷ represents a hydrogen atom or an alkyl group having 1 to 4 carbon atoms.)
In the general formula (3) and the general formula (4), examples of the alkyl group having 1 to ⁶ carbon atoms of R ³ , R ⁴ and R ⁶ include methyl, ethyl, propyl, isopropyl, n-butyl, s -Butyl, t-butyl, t-amyl, t-hexyl and the like. The alkyl group having 1 to 4 carbon atoms of ^{R 5} and ^{R 7,} for example, methyl, ethyl, propyl, isopropyl, n- butyl, s- butyl, t- butyl. Moreover, as a C1-C6 fluoroalkyl group of R < ³ > and R < ⁴ >, trifluoromethyl etc. are mentioned, for example. The aryl group of ^{R 3} and ^{R 4,} for example, phenyl. Examples of the alkyloxy group having 1 to ⁴ carbon atoms of R ⁴ include methoxy, ethoxy, propoxy, isopropoxy, n-butoxy, s-butoxy, t-butoxy and the like.

  More specifically, β-diketone compounds are, for example, 2,4-pentanedione, 2,4-hexanedione, 2,2-dimethyl-3,5-hexanedione, 1-phenyl-1,3-butanedione. 1-trifluoromethyl-1,3-butanedione, hexafluoroacetylacetone, 1,3-diphenyl-1,3-propanedione, dipivaloylmethane and the like.

More specific examples of the β-keto ester compound include methyl acetoacetate, ethyl acetoacetate, t-butyl acetoacetate, and the like.
Specific examples of the β-dicarboxylic acid ester compound include dimethyl malonate and diethyl malonate.
Further, in the catalyst composition of the present invention, a noble metal may be further supported on the ilmenite type composite oxide thus obtained. By supporting the noble metal, the catalytic activity can be further improved.

Examples of the noble metal include the above-described Pt, Ir, Os, Ag, Pd, Rh, and Ru. Pt, Pd, and Rh are preferable. These noble metals may be used alone or in combination of two or more.
And in order to carry | support a noble metal on an ilmenite type complex oxide, it does not restrict | limit in particular, A well-known method can be used. For example, a solution of a salt containing a noble metal is prepared, and the noble metal salt solution is impregnated in the ilmenite type complex oxide obtained above, and then fired. The amount of the noble metal supported on the ilmenite-type composite oxide is, for example, 20 parts by weight or less, preferably 0.2 to 10 parts by weight with respect to 100 parts by weight of the ilmenite-type composite oxide.

  As the noble metal salt solution, the salt solution exemplified above may be used, and practically, an aqueous nitrate solution, an aqueous dinitrodiammine nitrate solution, an aqueous chloride solution, or the like is used. More specifically, as rhodium salt solution, for example, rhodium nitrate solution, rhodium chloride solution, etc., as palladium salt solution, for example, palladium nitrate solution, palladium chloride solution, etc., as platinum salt solution, for example, dinitrodiammine platinum nitrate solution Chloroplatinic acid solution, tetravalent platinum ammine solution and the like.

Then, after impregnating the ilmenite type complex oxide with the noble metal salt solution, for example, drying is performed at 50 to 200 ° C. for 1 to 48 hours, and further, baking is performed at 500 to 1200 ° C. for 1 to 12 hours.
The ilmenite-type composite oxide thus obtained can be used as it is as a catalyst composition, but is usually prepared as a catalyst composition by a known method such as loading on a catalyst carrier.

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 ilmenite type composite oxide to form a slurry, which is then coated on the catalyst carrier, dried, and then about 300 to 800. The heat treatment is performed at a temperature of about 300 to 600 ° C.

In the catalyst composition of the present invention thus obtained, the noble metal is coordinated in the crystal structure of the ilmenite complex oxide, and the coordinated noble metal is precipitated from the crystal structure in a reducing atmosphere. It dissolves in the crystal structure in an oxidizing atmosphere.
As a result, the catalyst composition of the present invention effectively suppresses noble metal grain growth even in long-term use by such a self-regenerative function that repeats solid solution in an oxidizing atmosphere and precipitation in a reducing atmosphere. These dispersed states are maintained. As a result, even if the amount of noble metal used is significantly reduced, high catalytic activity can be realized over a long period of time.

Therefore, the catalyst composition of the present invention, it is possible to realize a superior exhaust gas purification performance for a long time, for example, a gasoline engine, or an internal combustion engine such as a diesel engine, for purifying exhaust gas discharged from such a boiler As an exhaust gas purification catalyst, it can be suitably used.

EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples and comparative examples. However, the present invention is not limited to these examples and comparative examples.
Example 1
(Production of FeTi _0.95 Rh _0.05 O ₃ )
Iron n-butoxide 0.100 mol Titanium isopropoxide 0.095 mol was added to a 500 mL capacity round bottom flask, and 200 mL of toluene was added and dissolved by stirring to prepare a mixed alkoxide solution. When 200 mL of deionized water was dropped into the mixed alkoxide solution and hydrolyzed, a viscous precipitate was formed. Toluene was distilled off from the mixed alkoxide solution to form a slurry, and an aqueous rhodium nitrate solution (Rh content: 0.005 mol) was added to the slurry, followed by stirring at room temperature for 1 hour.

Next, water was distilled off to dryness under reduced pressure to obtain a precursor. This was heat-treated (fired) at 950 ° C. for 2 hours in an electric furnace in the atmosphere, and an Rh-containing ilmenite-type composite oxide (Rh content: 3.33) composed of FeTi _0.95 Rh _0.05 O _3. % By weight).
As a result of X-ray diffraction, this powder was confirmed to have a single crystal phase of an Rh-containing ilmenite complex oxide composed of FeTi _0.95 Rh _0.05 O ₃ .

Example 2
(Production of MgTi _0.95 Rh _0.05 O ₃ )
Magnesium methoxypropylate 0.100 mol Titanium isopropoxide 0.095 mol was added to a 500 mL capacity round bottom flask, and 200 mL of toluene was added and dissolved by stirring to prepare a mixed alkoxide solution. When 200 mL of deionized water was dropped into the mixed alkoxide solution and hydrolyzed, a viscous precipitate was formed. Toluene was distilled off from the mixed alkoxide solution to form a slurry, and an aqueous rhodium nitrate solution (Rh content: 0.005 mol) was added to the slurry, followed by stirring at room temperature for 1 hour.

Next, water was distilled off to dryness under reduced pressure to obtain a precursor. This was heat-treated (fired) at 950 ° C. for 2 hours in an electric furnace in the atmosphere, and an Rh-containing ilmenite-type composite oxide (Rh content: 4.19) composed of MgTi _0.95 Rh _0.05 O _3. % By weight).
As a result of X-ray diffraction, this powder was confirmed to have a single crystal phase of an Rh-containing ilmenite complex oxide composed of MgTi _0.95 Rh _0.05 O ₃ .

Comparative Example 1
(Production of LaAl _0.95 Rh _0.05 O ₃ )
Lanthanum isopropoxide 0.100 mol Aluminum isopropoxide 0.095 mol was added to a 500 mL capacity round bottom flask, and 200 mL of toluene was added and dissolved by stirring to prepare a mixed alkoxide solution. When 200 mL of deionized water was dropped into the mixed alkoxide solution and hydrolyzed, a viscous precipitate was formed. Toluene was distilled off from the mixed alkoxide solution to form a slurry, and an aqueous rhodium nitrate solution (Rh content: 0.005 mol) was added to the slurry, followed by stirring at room temperature for 1 hour.

Next, water was distilled off to dryness under reduced pressure to obtain a precursor. This was heat-treated (fired) at 950 ° C. for 2 hours in an electric furnace in the atmosphere, and an Rh-containing ilmenite-type composite oxide (Rh content: 2.36) composed of LaAl _0.95 Rh _0.05 O _3. % By weight).
As a result of X-ray diffraction, this powder was confirmed to have a single crystal phase of an Rh-containing ilmenite type complex oxide composed of LaAl _0.95 Rh _0.05 O ₃ .

Test example 1
Observation by TEM (Transmission Electron Microscope) The powder obtained in Example 2 was subjected to an oxidation treatment (heat treatment at 800 ° C. for 1 hour in the air), followed by reduction treatment (800% in N ₂ gas containing 10% H ₂ ). And heat treatment at 800 ° C. for 1 hour). The powder was observed using a TEM (transmission electron microscope) after the oxidation treatment, the reduction treatment, and the re-oxidation treatment. The results are shown in FIGS.

  As can be seen from FIGS. 1 to 3, after the oxidation treatment, it was observed that Rh was dissolved in the ilmenite type crystal structure. In addition, after the reduction treatment, it was observed that Rh was precipitated from the ilmenite type crystal structure. Furthermore, after re-oxidation treatment, it was observed that Rh was again dissolved in the ilmenite type crystal structure.

From these results, it was confirmed that the powder obtained in Example 2 can exhibit a self-regenerating function that repeats solid solution in an oxidizing atmosphere and precipitation in a reducing atmosphere.
Measurement of precious metal solid solution amount The powder obtained in each Example and Comparative Example was subjected to an oxidation treatment and a reduction treatment, in Example 1, in a 7 wt% hydrofluoric acid aqueous solution, in Example 2 and Comparative Example. In No. 1, the sample was dissolved in a 5% by weight aqueous hydrochloric acid solution and allowed to stand at room temperature for 20 hours, and then each solution was filtered through a 0.1 μmφ filter. The amount of precious metal dissolved in the filtrate is quantitatively analyzed by ICP (radio frequency inductively coupled plasma) emission spectrometry, and the precious metal in the residue is qualitatively analyzed by XRD (X-ray diffraction) -SEM (scanning electron microscope) analysis. did. From these results, the amount of noble metal solid solution after the oxidation treatment and after the reduction treatment was calculated. Further, the amount of precious metal deposited was calculated from the difference between the amount of precious metal solid solution after the oxidation treatment and the amount of precious metal solid solution after the reduction treatment. The results are shown in Table 1.

  In the above method, a residue of fluoride (calcium fluoride or the like) was generated when the powder was dissolved in a 7 wt% hydrofluoric acid aqueous solution. However, the noble metal dissolved in the ilmenite crystal structure was Therefore, by measuring the concentration of the noble metal in the solution, the ratio of the noble metal dissolved in the ilmenite type crystal structure could be obtained.

It is the image processing figure which observed the powder (after oxidation process) of Example 2 by TEM. It is the image processing figure which observed the powder (after reduction process) of Example 2 by TEM. It is the image processing figure which observed the powder (after re-oxidation process) of Example 2 by TEM.

Claims (2)

An ilmenite-type composite oxide containing a noble metal, wherein the noble metal is solid-dissolved in the ilmenite-type composite oxide in an oxidizing atmosphere, and is precipitated from the ilmenite-type composite oxide in a reducing atmosphere ,
General formula (1)
AB _1-x M _x O ₃ (1)
(In the formula, A represents Fe and / or Mg, B represents Ti, M represents at least one element selected from precious metals, and x represents a range of 0 <x <1. )
The catalyst composition for exhaust gas purification characterized by these. The exhaust gas purifying catalyst composition according to claim 1 , wherein the noble metal is at least one element selected from Pt, Pd, and Rh.