JP2018015708A - Exhaust gas purification catalyst and exhaust gas purification apparatus using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an exhaust gas purification catalyst exhibiting high NOpurification performance and an apparatus thereof.SOLUTION: There is provided an exhaust gas purification catalyst which comprises a carrier containing AlO, a first catalyst provided on the carrier and a second catalyst provided on the carrier, wherein in an internal-combustion engine exhaust gas purification catalyst in which the first catalyst contains at least Ga as a first catalyst element and the second catalyst contains any one of Rh, Ce and Ti as a second catalyst element, the amount of the second catalyst element is 0.05 to 0.23 wt.% relative to the AlO.SELECTED DRAWING: Figure 3

Description

The present invention relates to a novel exhaust gas purification catalyst suitable for purifying NO _x contained in lean burn exhaust gas, an apparatus therefor, and exhaust gas purification in an internal combustion engine that is operated in a lean burn state where the fuel is leaner than the stoichiometric air-fuel ratio. Regarding the method.

In recent years, a lean burn engine in which the air-fuel ratio is a lean fuel has attracted attention. Here, the air-fuel ratio represents the ratio of air to fuel in the gas. Exhaust gas of lean-burn engine, it is difficult to purify NO _x in the three-way catalyst which has been conventionally used in purification of exhaust gas the stoichiometric air-fuel ratio (stoichiometric) for the engine. For this reason, exhaust gas purification catalysts for lean burn engines have been studied.

As an exhaust gas purification catalyst for lean burn engines, it is known that a catalyst in which Rh is supported on a metal oxide carrier has NO _x purification performance even in an oxygen-excess atmosphere.

  In patent document 1, the catalyst performance is improved by adding Ga to the catalyst containing Rh, Al, and Zr.

Patent Document 2 describes a catalyst containing at least one of Pt, Rh, and Pd and at least one of Ga ₂ O _3, Al ₂ O ₃ , CeO ₂ , and ZrO ₂ .

Also, in the exhaust gas purifying catalyst for lean-burn engine, the noble metal are contained not only the Rh, resource savings, in terms of cost reduction, while maintaining _the NO _x purification performance, it is required to reduce the use of precious metals ing.

JP-A-11-138000 JP-A-6-269668

In Patent Document 1 and Patent Document 2, NO _x is purified using hydrocarbon as a reducing agent, but the combustion reaction of hydrocarbon proceeds preferentially, and the NO _x purification rate may be reduced.

An object of the present invention is to improve NO _x purification performance for a catalyst used in an oxygen-excess atmosphere.

To achieve the above object, the present invention comprises a support containing Al ₂ O ₃ , a first catalyst provided on the support, and a second catalyst provided on the support, one catalyst comprises at least Ga as the first catalytic element, the second catalyst is at least Rh, Ce, in an internal combustion engine exhaust gas purifying catalyst comprising any of Ti as a second catalytic element, wherein the Al ₂ The amount of the second catalytic element with respect to O ₃ is characterized by using an internal combustion engine exhaust gas purification catalyst in the range of 0.05 to 0.23% by weight.

  According to the present invention, harmful substances, particularly nitrogen oxides can be purified with high efficiency even in an oxygen-excess atmosphere.

It is a figure showing an example of the exhaust gas purification system of an engine. It is a figure showing the state of the support | carrier and catalyst active component on the catalyst surface. 4 is a graph showing the relationship between the amount of Rh and the activity of a catalyst.

  FIG. 1 is an example of an exhaust gas purification system for an engine.

  The exhaust gas purification system includes an engine 1 and an exhaust gas purification mechanism 2 provided downstream of the engine 1. The exhaust gas purification mechanism 2 has a passage through which the exhaust gas 3 passes. For example, a honeycomb structure or the like is used for the passage.

A catalyst for purifying the exhaust gas 3 is provided in the passage. As a catalyst, a three-way catalyst is used for exhaust gas purification of a stoichiometric engine, and for exhaust gas purification of a lean burn engine that operates at an air / fuel ratio (14.7 or more) that is leaner than the theoretical air / fuel ratio. The exhaust gas purification catalyst for lean burn engine is used. The lean burn engine exhaust gas purification catalyst contains precious metals such as Rh, and for example, an NO _x purification catalyst that purifies NO _x in the exhaust gas can be used.

FIG. 2 is a conceptual diagram showing the structure of the NO _x purification catalyst.

The NO _x purification catalyst coats a support 4 containing Al ₂ O ₃ on a substrate 7 having a honeycomb structure, and the support 4 contains at least one of a first catalyst 5 containing Ga and at least one of Rh, Ce, and Ti. What carried the 2nd catalyst 6 can be used.

It is considered that the NO _x purification reaction by the NO _x purification catalyst proceeds by the following mechanism. First, NO is adsorbed on the first catalyst 5, and hydrocarbon as a reducing agent is adsorbed on the second catalyst 6. NO reacts with hydrocarbons as in Equation (2) is oxidized to NO ₂ by _{O 2} as in Equation _(1), eliminated as N _2, CO 2, _{H 2} O.

However, since a large amount of O ₂ is contained in the exhaust gas in the lean state, the oxidation (combustion reaction) of hydrocarbons by O ₂ proceeds simultaneously as shown in Equation (3). Further, when NO _x purification and combustion reaction by hydrocarbons is insufficient, it is considered that the reactions of formulas (4) and (5) proceed, and conversion to CO and unreacted hydrocarbons are discharged. NO _x may not be sufficiently purified.

(1) NO + O ₂ → NO ₂ Formula (1)
(2) NO ₂ + hydrocarbon → N ₂ + CO ₂ + H ₂ O Formula (2)
(3) Hydrocarbon + O ₂ → CO ₂ + H ₂ O Formula (3)
(4) Hydrocarbon + O ₂ → CO + CO ₂ + H ₂ O Formula (4)
(5) Hydrocarbon → Unreacted hydrocarbon (5)
Therefore, the amount of the second catalytic element is preferably in the range of 0.05 to 0.23% by weight with respect to Al ₂ O ₃ contained in the support 4. (Here, “wt%” represents the content ratio of each component in terms of g. For example, the loading amount of the B component with respect to the A component is 0.5 wt%.) This means that A is 1 and B is supported at a ratio of 0.5 regardless of the amount of A.)
When the amount of the second catalytic element is less than 0.05% by weight, the formula (2) hardly reacts and NO _x is not sufficiently purified. When the amount of the second catalytic element is larger than 0.23% by weight, the formula (3) proceeds with priority over the formula (2), and NO _x may not be sufficiently purified. It is considered that when the amount of the second catalytic element is in the range of 0.05 to 0.23% by weight, NO _x can be purified by the formula (2) before the hydrocarbon is consumed by the formula (3). The purification rate of _x can be increased.

In addition, if even a part of the NO _x purification catalyst has a portion in which the amount of the second catalytic element is 0.05 to 0.23% by weight with respect to Al ₂ O ₃ contained in the support 4, the above effect is efficiently achieved. Can be expressed. For example, the following cases [1] and [2] are conceivable as examples of the form of the catalyst. [1] There is a plate-like or granular Al ₂ O ₃ as a layer not containing the second catalyst 6 on the substrate 7, and the carrier 4 containing the second catalyst 6 is further present thereon. [2] The support 4 containing the second catalyst 6 is present on the surface of the granular Al ₂ O ₃ not containing the second catalyst 6. In any case of [1] and [2], the amount of the second catalyst element is 0.05 to 0.23 weight with respect to Al ₂ O ₃ contained in the support 4 including the second catalyst 6. It is preferable that the part which becomes% exists.

Furthermore, from the viewpoint of improving the hydrocarbon purification rate and suppressing the conversion to CO, the amount of the second catalytic element is preferably 0.16 to 0.23% by weight with respect to Al ₂ O ₃ . If the amount of the second catalytic element is less than 0.16% by weight, the formulas (4) and (5) proceed, and there is a possibility that the conversion amount of hydrocarbons into CO and the discharge amount of unreacted hydrocarbons increase. . On the other hand, when the amount of the second catalytic element is in the range of 0.16 to 0.23% by weight, it is considered that the remaining hydrocarbon that has reacted with NO _{x in} the formula (2) is purified by the formula (3). Further, by providing a hydrocarbon removal catalyst or a CO removal catalyst in the preceding stage or the subsequent stage of the catalyst of the present invention, the hydrocarbon purification rate can be improved and the conversion to CO can be suppressed.
A first catalyst, the second catalyst is preferably in contact with the carrier containing Al ₂ O _3, a first catalyst, the second catalyst is provided on the surface of the carrier comprises Al ₂ O ₃ The state obtained is preferable.

As the second catalytic element, in addition to Rh, Ce and Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Mo, Ru, Pd, Ag, Ce, W, Ir, Au Such metals can be used. On the other hand, when Pt is used, NO _x may be converted to N ₂ O, and the substantial NO _x purification rate may decrease.

The ratio of the amount of the first catalyst element and second catalyst element is preferred for _the NO _x purification performance when the first catalytic element / the second catalyst element to 6 to 25. If there are more than 25 first catalyst elements / second catalyst elements, the first catalyst elements are likely to cover the second catalyst elements, and the second catalyst elements may not work effectively. If it is less than 6, the activity may decrease due to a reduction in the amount of the first catalytic element.

As the catalytic active component, in addition to the first catalytic element and the second catalytic element, other transition metals can be further added. As other transition metals, Fe, Co, Ni, Pd, Ag, Au, etc. are particularly suitable. Since these metals promote the oxidation of NO to NO ₂ and the adsorption of hydrocarbons, the NO _x purification performance can be improved.

The carrier 4 is considered to play a role of enhancing the dispersibility of the catalytically active component. Carrier 4 may be supported on the base 7, preferably for _the NO _x purification performance when the supporting amount of a carrier 4 relative to the substrate 7 of the case 1L and 30g or 400g or less. If the loading amount of the carrier 4 is less than 30 g, the effect of the carrier 4 is insufficient, and if it is more than 400 g, the specific surface area of the carrier 4 itself decreases, which is not preferable.

The support 4 contains Al ₂ O _3, and the specific surface area of Al ₂ O ₃ is preferably 100 to 300 m ² / g for NO _x purification performance. In this case, the amount of the second catalytic element is preferably in the range of 1.7 × 10 ^{−6 to} 2.3 × 10 ⁻⁵ g / m ² with respect to the specific surface area of Al ₂ O ₃ . Also preferred for _the NO _x purification performance When 1~25μm the particle size of _Al 2 _{O 3.}

The support 4 is most preferably Al ₂ O _3, but other metal oxides or composite oxides such as TiO ₂ , SiO ₂ , SiO ₂ —Al ₂ O ₃ , ZrO ₂ , MgO, CoO, NiO, CuO, etc. Can also be used. The reason why Al ₂ O ₃ is most preferable is that Ga—O—Al is formed by supporting the first catalyst 5 containing Ga, and this becomes a catalyst active point. In addition, zeolite such as β-zeolite, mordenite, ferrierite, and Y-type zeolite can be added to the carrier 4 and used.

The NO _x purification catalyst can be used after being coated on a substrate 7 having a honeycomb structure or the like in addition to being supported. The base material 7 is optimally cordierite, but good results can be obtained even if a metal material is used.

The shape of the NO _x purification catalyst can be applied in various shapes depending on the application. The present invention can be applied to pellets, plates, granules, powders, etc., including honeycombs obtained by coating catalyst powder carrying various components on honeycomb structures made of various materials such as cordierite and stainless steel.

As a method for preparing the NO _x purification catalyst, any of a physical preparation method such as an impregnation method, a kneading method, a coprecipitation method, a sol-gel method, an ion exchange method, and a vapor deposition method and a preparation method utilizing a chemical reaction can be applied. As the starting material for the exhaust gas purification catalyst, various compounds such as nitric acid compounds, acetic acid compounds, complex compounds, hydroxides, carbonate compounds, organic compounds, metals, and metal oxides can be used.

When the hydrocarbon concentration of the exhaust gas 3 flowing into the NO _x purification catalyst is 100 ppm or more, preferably 500 ppm or more, the NO _x purification performance is enhanced. If the hydrocarbon concentration is less than 100 ppm, the reaction of formula (2) may not proceed sufficiently.

  EXAMPLES Hereinafter, although this invention is demonstrated with a specific example, this invention is not restrict | limited by these Examples.

In the example catalysts 1 to 3 and the comparative example catalysts 1 to 4, the NO _x purification rate at 450 ° C. was measured. These results are shown in Table 1.

Example 1
A cordierite honeycomb (400 cells / inc ² ) coated with a slurry made of Al ₂ O ₃ powder and Al ₂ O ₃ precursor and adjusted to nitric acid acidity was dried and fired, and the apparent volume of the honeycomb was 1 liter. An Al ₂ O ₃ coated honeycomb coated with 170 g of Al ₂ O ₃ was obtained. The Al ₂ O ₃ coated honeycomb was impregnated with Ga nitrate solution and Rh nitrate solution as impregnation components, dried at 120 ° C., and then fired at 600 ° C. for 1 hour.

Thus, Example Catalyst 1 containing 170 g of Al ₂ O ₃ with respect to 1 L of honeycomb and 1.3 wt% Ga and 0.076 wt% Rh in terms of element with respect to Al ₂ O ₃ was obtained.

The obtained catalyst was subjected to an exhaust gas purification performance test under the following conditions. A honeycomb catalyst having a capacity of 6 cc was fixed in a reaction tube made of quartz glass. This reaction tube was introduced into an electric furnace, and the heating was controlled so that the temperature of the gas introduced into the reaction tube was 450 ° C. In the reaction tube, model gas was introduced at a space velocity of 30000 h ⁻¹ assuming exhaust gas when the automobile engine is performing lean burn operation. The composition of the model gas was NO _x : 150 ppm, C ₃ H ₆ : 750 ppm, O ₂ : 10%, H ₂ O: 2.8%, N ₂ : residual. At this time, the NO _x purification rate, the C ₃ H ₆ purification rate, and the CO conversion rate were calculated by the following equations.

_The NO _x purification rate (%) = (NO _x amount flowing into the catalyst - NO _x amount flowing out from the catalyst)
÷ NO _x amount flowing into the catalyst × 100 ... Formula (6)
C ₃ H ₆ purification ratio (%) = (amount _C 3 _{H 6} flowing into the catalyst - _C 3 _{H 6} amount flowing out from the catalyst)
÷ C ₃ H ₆ amount flowing into the catalyst × 100 (7)
CO conversion rate (%) = CO amount flowing out from the catalyst / (C ₃ H ₆ amount flowing into the catalyst × 3)
× 100 ... Formula (8)

(Example 2)
Except for changing the addition amount of the Ga nitrate solution and the Rh nitrate solution, the same operation as in Example 1 was carried out, and the element containing Ga 2.6% by weight and Rh 0.15% by weight with respect to Al ₂ O ₃ was obtained. Example catalyst 2 was obtained. The obtained catalyst was subjected to an exhaust gas purification performance test in the same manner as in Example 1.

(Example 3)
Except changing the addition amount of the Ga nitrate solution and the Rh nitrate solution, the same operations as in Example 1 were carried out, and contained 3.9 wt% Ga and 0.23 wt% Rh in terms of elements with respect to Al ₂ O ₃ . Example catalyst 3 was obtained. The obtained catalyst was subjected to an exhaust gas purification performance test in the same manner as in Example 1.

(Comparative Example 1)
Comparative Example Catalyst 1 containing 10% by weight of Ga in terms of element with respect to Al ₂ O ₃ was performed in the same manner as in Example 1 except that the addition amount of the Ga nitrate solution was changed and no Rh nitrate solution was used. Obtained. The obtained catalyst was subjected to an exhaust gas purification performance test in the same manner as in Example 1.

(Comparative Example 2)
A comparison containing 0.64 wt% Ga and 0.038 wt% Rh in terms of element with respect to Al ₂ O ₃ by the same operation as in Example 1 except that the addition amounts of the Ga nitrate solution and the Rh nitrate solution were changed. Example catalyst 2 was obtained. The obtained catalyst was subjected to an exhaust gas purification performance test in the same manner as in Example 1.

(Comparative Example 3)
Comparison containing Ga 5.1 wt% and Rh 0.30 wt% in terms of elements with respect to Al ₂ O ₃ by the same operation as in Example 1 except that the addition amounts of Ga nitrate solution and Rh nitrate solution were changed. Example catalyst 3 was obtained. The obtained catalyst was subjected to an exhaust gas purification performance test in the same manner as in Example 1.

(Comparative Example 4)
A comparative example catalyst containing 10 wt% Ga and 0.61 wt% Rh in terms of element with respect to Al ₂ O ₃ by the same operation as in Example 1 except that the addition amounts of the Ga nitrate solution and the Rh nitrate solution were changed. 4 was obtained. The obtained catalyst was subjected to an exhaust gas purification performance test in the same manner as in Example 1.

(Test results)
The results of the NO _x purification rates of Example Catalysts 1 to 3 and Comparative Example Catalysts 1 to 4 are shown in Table 1 and FIG. About Example catalyst 1-3, activity is higher than the comparative example catalysts 3 and 4 with much Rh usage-amount. On the other hand, in the comparative example catalysts 1 and 2 in which the amount of Rh used is further reduced as compared with the example catalysts 1 to 3, the activity is lowered. From the above results, it can be seen that Example Catalysts 1 to 3 having an Rh amount of 0.05 to 0.23% by weight show high NO _x purification performance with a small amount of Rh used. The reason for this is thought to be that by reducing the amount of Rh used, the oxidation (combustion reaction) of C ₃ H ₆ by O ₂ was suppressed, and C ₃ H ₆ worked effectively as a reducing agent for NO _x .

Table 2 shows the results of C ₃ H ₆ purification rates of Example Catalysts 1 to 3 and Comparative Example Catalysts 1 to 4. From Table 2, it can be seen that when the Rh amount is 0.16 to 0.23% by weight, not only the NO _x purification performance is improved but also the C ₃ H ₆ purification rate is improved.

Table 3 shows the results of the CO conversion rates of Example Catalysts 1 to 3 and Comparative Example Catalysts 1 to 4. From Table 3, when the Rh amount is from 0.16 to 0.23 wt%, not only improves the _the NO _x purification performance, it is understood that it is also possible to suppress the conversion to CO.

1 Engine 2 Exhaust gas purification mechanism 3 Exhaust gas 4 Carrier 5 First catalyst 6 Second catalyst 7 Base material

Claims (5)

A support containing Al ₂ O ₃ , a first catalyst provided on the support, and a second catalyst provided on the support, wherein the first catalyst is at least as a first catalyst element In the internal combustion engine exhaust gas purification catalyst that contains Ga, and the second catalyst contains at least one of Rh, Ce, and Ti as the second catalyst element, the second catalyst for the Al ₂ O ₃ is 0.8. An internal combustion engine exhaust gas purification catalyst in the range of 05 to 0.23 wt%.   The internal combustion engine exhaust gas purification catalyst according to claim 1, wherein a ratio of the amount of the first catalyst element to the amount of the second catalyst element (first catalyst element / second catalyst element) is in the range of 6-25. The internal combustion engine exhaust gas purification catalyst according to claim 2, wherein an amount of the second catalytic element with respect to the Al ₂ O ₃ is 0.16 to 0.23% by weight.   4. The exhaust gas purification catalyst according to claim 3, wherein the internal combustion engine exhaust gas purification catalyst has a third catalyst provided on the carrier, and the third catalyst has at least Fe, Co, Ni, Pd, Ag as a third catalyst element. An exhaust gas purification catalyst for an internal combustion engine containing any one of Au and Au.   The exhaust gas purification catalyst for an internal combustion engine according to claim 4, further comprising zeolite as the carrier of the exhaust gas purification catalyst.

Images