JP5506292B2 - Exhaust gas purification catalyst - Google Patents

Description

  The present invention relates to a catalyst composition, and more particularly to a catalyst composition suitably used as an exhaust gas purifying catalyst.

Exhaust gas discharged from an internal combustion engine such as an automobile engine includes hydrocarbon (HC), carbon monoxide (CO), nitrogen oxide (NOx), and the like. As a three-way catalyst for purifying these, various exhaust gas purification catalysts in which noble metals (Pt (platinum), Rh (rhodium), Pd (palladium), etc.) as active components are supported on a catalyst carrier are known. Yes.
For example, various catalysts made of Pd / alumina, in which Pd is supported on an alumina carrier, which is a general heat-resistant oxide, are known (see, for example, Patent Document 1).

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, Patent Document 1 proposes a catalyst containing no transition metal, in which Cu as a transition metal is supported on a catalyst carrier (alumina, zeolite, etc.) as an active component.

JP-A-5-96132

  However, in the catalyst of Patent Document 1, since Cu is only supported on the catalyst carrier, the crystallite diameter of Cu in the catalyst carrier is relatively large. For this reason, the catalyst active points in the catalyst carrier are reduced by agglomeration of Cu fine particles that are supported only at high temperatures or under oxidation-reduction fluctuations, and also during long-term use. As a result, there is a problem that the catalytic activity is lowered and the exhaust gas purification performance (particularly, NOx purification performance) is lowered.

  An object of the present invention is to provide a catalyst composition capable of exhibiting excellent catalytic activity of Cu under high temperature or oxidation-reduction fluctuations, and further during long-term use, while reducing the use of noble metal elements. That is.

In order to achieve the above object, the catalyst composition of the present invention is characterized by containing a spinel type complex oxide represented by the following general formula (1).
_{(M 1-x Cu x)} O · nAl 2 O 3 (1)
(In the formula, M represents at least one element selected from Mg, Fe, Co and Ni, x represents an atomic ratio of 0 <x ≦ 1, and n represents 0.08 to 5) .)
In the catalyst composition of the present invention, it is preferable that x = 1 in the general formula (1).

  According to the catalyst composition of the present invention, Cu, which is an active component, is contained as a composition rather than being supported on a spinel composite oxide having a specific composition. Therefore, the crystallite diameter of Cu in the oxide can be reduced, and the dispersion state with respect to the oxide of Cu is well maintained. As a result, the aggregation of Cu fine particles can be suppressed even at high temperatures or under redox fluctuations and even after long-term use.

Therefore, it is possible to prevent a decrease in catalytic activity due to Cu grain growth over a long period of time, and to maintain a high catalytic activity of Cu.
Therefore, if the catalyst composition of the present invention is used, Cu can be used as an active ingredient, so that excellent catalytic activity is exhibited over a long period of time at a low cost and under high temperature or oxidation-reduction fluctuations while reducing noble metal elements. be able to.

The catalyst composition of the present invention contains a spinel type complex oxide represented by the following general formula (1).
_{(M 1-x Cu x)} O · nAl 2 O 3 (1)
(In the formula, M represents at least one element selected from Mg, Fe, Co and Ni, x represents an atomic ratio of 0 <x ≦ 1, and n represents 0.08 to 5) .)
In the said General formula (1), x shows the atomic ratio of Cu of 0 <x <= 1. That is, Cu is an essential component, and preferably x = 1. In that case, the spinel-type composite oxide is, for example, a so-called copper spinel represented by the following general formula (1 ′).

CuO.nAl ₂ O ₃ (1 ′)
(In the formula, n represents 0.08 to 5.)
When the catalyst composition of this invention contains the copper spinel represented by the said general formula (1 '), the crystallite diameter of Cu in a catalyst composition can be made still smaller, and also cost can be reduced.

On the other hand, the atomic ratio of M is 1-x, that is, the atomic ratio of the remainder obtained by subtracting the atomic ratio of Cu from 1 (0 <x ≦ 1). That is, in the general formula (1), M is an optional component and may or may not be included.
When M is included, M represents at least one element selected from Mg, Fe, Co, and Ni. These elements may be used alone or in combination of two or more. When M is contained, M is preferably Co and / or Ni.

Moreover, in the said General formula (1), n shows 0.08-0.5, Preferably, 0.16-5 is shown.
As such a spinel type complex oxide represented by the general formula (1), specifically, (Co _0.3 Cu _0.2 Mg _2.5 ) O ₃ .0.5Al ₂ O ₃ , Co _0.3 Cu _0.2 Mg _2.5 AlO _4.5 , (Ni _0.3 Cu _0.2 Mg _2.5 ) O ₃ .0.5Al ₂ O ₃ , that is, Ni _0.3 Cu _0.2 Mg _2.5 AlO _4.5 , (Fe _0.1 Cu _0.4 Mg _2.5 ) O ₃ .0.5Al ₂ O ₃ , that is, Fe _0.1 Cu _0.4 Mg _2.5 AlO _4.5 , CuO.Al ₂ O _{3 and the} like.

The spinel-type composite oxide represented by the general formula (1) is not particularly limited, and an appropriate method for preparing the composite oxide, for example, a coprecipitation method, a citric acid complex method, an alkoxide method It can manufacture by.
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 tetramethylammonium hydroxide, triethylamine, and pyridine, such as sodium hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate, and ammonium carbonate. An inorganic base is mentioned. In addition, a neutralizing agent is added so that the pH of the solution after adding the neutralizing agent may become about 6-10.

And a precursor is obtained by filtering mixed salt aqueous solution and washing a coprecipitate if necessary. Next, the precursor is dried by, for example, vacuum drying or ventilation drying, and then heat-treated at, for example, 500 to 1000 ° C., preferably 600 to 950 ° C., whereby the spinel represented by the above general formula (1) is obtained. Type complex oxide is obtained.
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. In the preliminary firing, for example, heating is performed at 250 to 350 ° C. in a vacuum or an inert atmosphere. Thereafter, for example, heat treatment is performed at 500 to 1200 ° C., preferably 600 to 1000 ° C., to obtain the spinel complex oxide represented by the general formula (1).
In the alkoxide method, for example, a mixed alkoxide solution containing the alkoxide of each element described above in the above stoichiometric ratio is prepared, and water is added to the mixed alkoxide solution to hydrolyze the precipitate, thereby producing a precipitate. obtain.

Examples of the alkoxide of each element include (mono, di, tri) alcoholate formed from each element and alkoxy such as methoxy, ethoxy, propoxy, isopropoxy, butoxy, and the following general formula (2). Examples include (mono, di, tri) alkoxy alcoholates of each element.
_{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.

  Then, the obtained precipitate is evaporated to dryness, and then dried by, for example, vacuum drying or ventilation drying, and then heat-treated at, for example, 500 to 1000 ° C., preferably 600 to 950 ° C. The spinel type complex oxide represented by the general formula (1) is obtained. The content of Cu in the spinel-type composite oxide represented by the above general formula (1) thus obtained is appropriately determined depending on the purpose and application. For example, with respect to the spinel-type composite oxide (total amount) For example, it is 1.5 to 14% by weight, preferably 6.5 to 13.3% by weight.

The specific surface area of the spinel-type complex oxide (e.g., BET specific surface area) is, for example, 0.5~90m ² / g, preferably, 50~90m ² / g.
Moreover, the crystallite diameter of Cu in the spinel type complex oxide represented by the general formula (1) is, for example, 2 to 60 nm, and preferably 2 to 50 nm. The crystallite diameter is obtained by, for example, measuring the catalyst composition with an X-ray diffraction (XRD) apparatus and applying the Scherrer equation to the Cu peak in the XRD data obtained by the measurement. Can do.

If the crystallite diameter of Cu is within the above range, even if the specific surface area of the spinel composite oxide is within the above range, it is possible to prevent a decrease in catalytic activity due to Cu grain growth, and the high Cu content. The catalytic activity can be maintained.
The spinel composite oxide of the present invention 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.

  Examples of the catalyst carrier include known catalyst carriers such as a honeycomb monolith carrier made of cordierite. In order to make it carry | support on a catalyst support | carrier, for example, water is first added to the spinel type complex oxide obtained by the above, and it is set as a slurry. Then, this is coated on a catalyst carrier, dried, and then heat-treated at 300 to 800 ° C., preferably 300 to 600 ° C. Thereby, the spinel type complex oxide can be supported on the catalyst carrier.

  According to the catalyst composition of the present invention, Cu, which is an active component, is contained as a composition rather than being supported on the spinel complex oxide having a specific composition represented by the general formula (1). . Therefore, the crystallite diameter of Cu in the spinel complex oxide can be reduced, and the dispersion state of the Cu spinel complex oxide is well maintained. As a result, the aggregation of Cu fine particles can be suppressed even at high temperatures or under redox fluctuations and even after long-term use.

Therefore, it is possible to prevent a decrease in catalytic activity due to Cu grain growth over a long period of time, and it is possible to maintain a high catalytic activity of Cu even when the specific surface area of the spinel composite oxide is within the above-described range.
Therefore, if the catalyst composition of the present invention is used, Cu can be used as an active ingredient, so that excellent catalytic activity is exhibited over a long period of time at a low cost and under high temperature or oxidation-reduction fluctuations while reducing noble metal elements. be able to.

  The catalyst composition 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, for example, as an exhaust gas purification catalyst for purifying exhaust gas discharged from internal combustion engines such as gasoline engines and diesel engines, boilers, etc. Can be used.

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 Co _0.3 Cu _0.2 Mg _2.5 AlO _4.5 powder)
Cobalt nitrate 0.03 mol copper nitrate in terms of Co 0.02 mol in terms of Cu magnesium nitrate 0.25 mol in terms of Mg Aluminum nitrate 0.10 mol in terms of Al The above ingredients were added to a 500 mL round bottom flask, A mixed salt aqueous solution was prepared by adding 100 mL of ultrapure water and dissolving with stirring for about 30 minutes. Subsequently, the mixed salt aqueous solution was dropped into the stirring 10% tetramethylammonium hydroxide aqueous solution at a rate of 20 drops per minute to obtain a coprecipitate. After completion of the dropwise addition, stirring of the 10% tetramethylammonium hydroxide aqueous solution was continued for 1 hour, and then left overnight.

And the precursor was taken out by filtering aqueous solution. During filtration, the coprecipitate was washed with a large amount of ultrapure water to remove the ammonia component remaining in the coprecipitate. Thereafter, the precursor was dried at 110 ° C. for 12 hours. After drying, the precursor was pulverized into a powder form and heat-treated at 850 ° C. for 5 hours in an air atmosphere to obtain Co _0.3 Cu _0.2 Mg _2.5 AlO _4.5 powder.

Incidentally, in this _Co 0.3 _Cu 0.2 _Mg 2.5 _{AlO 4.5} powder, the content of Cu was 6.7% by weight.
Example 2 (Production of Ni _0.3 Cu _0.2 Mg _2.5 AlO _4.5 powder)
Nickel nitrate 0.03 mol copper nitrate in terms of Ni 0.02 mol in terms of Cu magnesium nitrate 0.25 mol in terms of Mg Aluminum nitrate 0.10 mol in terms of Al Except for using the above components, Example 1 and A precursor was prepared by a similar method. Thereafter, the precursor was dried, pulverized, and heat-treated in the same manner as in Example 1 to obtain Ni _0.3 Cu _0.2 Mg _2.5 AlO _4.5 powder.

Incidentally, in this _Ni 0.3 _Cu 0.2 _Mg 2.5 _{AlO 4.5} powder, the content of Cu was 6.7% by weight.
Reference Example 3 (Production of Fe _0.1 Cu _0.4 Mg _2.5 AlO _4.5 powder)
Iron nitrate 0.01 mol in terms of Fe 0.04 mol in terms of Cu magnesium nitrate 0.25 mol in terms of Mg Aluminum nitrate 0.10 mol in terms of Al Except for using the above components, Example 1 A precursor was prepared by a similar method. Thereafter, the precursor was dried, pulverized, and heat-treated in the same manner as in Example 1 to obtain Fe _0.1 Cu _0.4 Mg _2.5 AlO _4.5 powder.

Incidentally, in this _Fe 0.1 _Cu 0.4 _Mg 2.5 _{AlO 4.5} powder, the content of Cu was 6.7% by weight.
Reference Example 4 (Production of CuAl ₂ O ₄ powder)
Copper nitrate 0.10 mol in terms of Cu Aluminum nitrate 0.20 mol in terms of Al A precursor was prepared in the same manner as in Example 1 except that the above components were used. Thereafter, in the same manner as in Example 1, the precursor dried, pulverized, and then heat-treated _to obtain _CuA l2 _{O 4} powder.

Incidentally, in the CuAl ₂ O ₄ powder, the content of Cu was 35.0 wt%.
Comparative Example 1 (Production of Cu / Al ₂ O ₃ powder)
A commercially available θ-Al ₂ O ₃ powder was impregnated with an aqueous copper nitrate solution, dried overnight at 110 ° C., and then heat-treated (fired) in the air at 650 ° C. for 1 hour in an electric furnace to obtain Cu / Al ₂ A θ-alumina powder carrying Cu represented by O ₃ was obtained. In the Cu / Al ₂ O ₃ powder, the supported amount (content) of Cu was 3.0% by weight.
Comparative Example 2 (Production of FeAl ₂ O ₄ powder)
Iron nitrate 0.10 mol in terms of Fe Aluminum nitrate 0.20 mol in terms of Al A precursor was prepared in the same manner as in Example 1 except that the above components were used. Thereafter, the precursor was dried, pulverized, and heat-treated in the same manner as in Example 1 to obtain FeAl ₂ O ₄ powder.
Comparative Example 3 (Production of CoAl ₂ O ₄ powder)
Cobalt nitrate Co-converted 0.10 mol Aluminum nitrate Al-converted 0.20 mol A precursor was prepared in the same manner as in Example 1 except that the above components were used. Thereafter, the precursor was dried, pulverized, and heat-treated in the same manner as in Example 1 to obtain CoAl ₂ O ₄ powder.
1 Measurement of crystallite diameter of Cu Each powder obtained by the above Examples and Comparative Examples was measured using an X-ray diffraction (XRD) apparatus. Then, with respect to the Cu peak in the XRD data obtained by the measurement, the crystallite diameter of Cu in each powder was obtained by applying the Scherrer equation. The results are shown in Table 1 below.
2 Oxidation reduction endurance test 5 minutes of inert atmosphere, 10 minutes of oxidizing atmosphere, 5 minutes of inert atmosphere and 10 minutes of reducing atmosphere are set as one cycle, and this cycle is repeated for 10 cycles for a total of 5 hours. The powder after the evaluation was 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 2 containing high temperature steam at the flow volume of 300 * 10 < ^-3 > m < ³ > / hr. The ambient temperature was maintained at about 1000 ° C.
3 NOx purification rate Each powder after the oxidation-reduction durability test was placed in an atmospheric pressure fixed bed flow reactor. A model gas having the composition shown in Table 3 below was circulated through the catalyst bed, and as a pretreatment, it was kept in a rich gas with an air fuel ratio (A / F) of 14.0 shown in Table 3 at 600 ° C. for 10 minutes, Cooled once until.

Next, after raising the catalyst bed temperature from room temperature to 600 ° C. in 1800 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 3. The NOx purification rate during that time was continuously measured. The purification rate at A / F = 14.6 for each powder is shown in Table 1 below.
4. Measurement of specific surface area The specific surface areas of the powders obtained in the above examples and comparative examples were measured before and after the oxidation-reduction durability test according to the BET method. The results are shown in Table 1 below.

Claims (1)

An exhaust gas purifying catalyst comprising a spinel type complex oxide represented by the following general formula (1):
_{(M 1-x Cu x)} O · nAl 2 O 3 (1)
(Wherein, M is a Mg, shows at least one element selected from C o and Ni, x denotes the 0 <x <1 in atomic ratio, n is a 0.08 to 5 Show.)