JP2536104B2 - Exhaust gas purification catalyst - Google Patents

Description

The present invention relates to a catalyst for purifying exhaust gas of automobiles, and more particularly to a catalyst capable of purifying NOx at a high rate even in an oxygen excess atmosphere where the air-fuel ratio is lean. is there.

[Conventional technology]

Carbon monoxide (CO) as a catalyst for automobile exhaust gas purification
Also, a catalyst that simultaneously oxidizes hydrocarbon (HC) and reduces nitrogen oxide (NOx) is widely used. Such a catalyst is basically one in which a γ-alumina slurry is applied to a refractory carrier such as cordierite and baked, and then a metal such as Pd, Pt, Rh or a mixture thereof is supported. Further, many proposals have been made to enhance the catalytic activity thereof, for example, JP-A-61-11147 discloses a catalyst of a type in which a noble metal or the like is dispersed on γ-alumina particles stabilized with a rare earth oxide. , Rh on particles that are substantially free of rare earth oxides
Dispersed catalysts are disclosed.

By the way, the catalysts used or proposed so far are:
The purification characteristics are greatly affected by the set air-fuel ratio of the engine, and on the lean side where the air-fuel ratio is lean, that is, the lean side has a large amount of oxygen (O ₂ ) even after combustion, the oxidation effect becomes active and the reduction effect becomes inactive. Become. On the contrary, on the rich side where the air-fuel ratio is small, the oxidizing action becomes inactive and the reducing action becomes active.
The stoichiometric air-fuel ratio (A / F = 1
The catalyst works most effectively around 4.6).

Therefore, in an automobile equipped with an exhaust gas purifying apparatus that uses a catalyst, feedback control is performed to detect the oxygen concentration in the exhaust system and maintain the air-fuel mixture near the stoichiometric air-fuel ratio.

On the other hand, low fuel ratio is also required for automobiles,
For that purpose, it is known that the air-fuel mixture with excess oxygen should be burned as much as possible during normal traveling. However Then the air-fuel ratio becomes an oxygen-excess atmosphere leaner, HC of the harmful components in the exhaust gas, CO can be be removed oxide, NOx is prevented contact between the active metal by O ₂ adsorbed on the catalyst bed Therefore, it becomes difficult to reduce and remove it. Therefore, as a catalyst for purifying exhaust gas used in the exhaust system of a lean burn engine, a transition metal / zeolite catalyst in which a transition metal such as Cu is carried on a zeolite by ion exchange has been proposed.

Zeolite is a crystalline aluminosilicate represented by the general formula: xM ₂ / n · Al ₂ O ₃ · ySiO ₂ and is a tunnel in the crystal structure due to the difference in M (n-valent metal), x, and y. Many different types are commercially available with different structures (pore sizes). Also, since a part of Si ⁴⁺ is replaced with Al ³⁺ , the positive charge is insufficient, and in order to compensate for the lack, it has the property of holding cations such as Na ⁺ and K ⁺ in the crystal, which is high. Has cation exchange capacity.

JP-A-60-125250 discloses a crystalline aluminosilicate having a predetermined lattice spacing (d value) in powder X-ray diffraction and a SiO ₂ / Al ₂ O ₃ molar ratio of 20 to 100. Disclosed are nitrogen oxide catalytic cracking catalysts containing ions and methods of using the same.

Further, the present applicant has proposed an exhaust gas purifying catalyst characterized in that zeolite ion-exchanged with a transition metal is supported on a refractory carrier.

As the above transition metal, Cu, Co, Cr, Ni, Fe, Mg, and Mn are preferable, and Cu is particularly preferable.

Zeolite, which is also called a molecular sieve, has pores of a few Å units that are aligned with the size of the molecule. for that reason
HC is selectively incorporated into the pores. Since the active site of the transition metal introduced by ion exchange exists in the pores, HC is adsorbed there and reacts with NOx. For this reason,
Even on the lean side, NOx can be efficiently removed.

However, there are various zeolites having different structures, and various coordination points exist on the same type of zeolite. Therefore, even when Cu, which is the most preferable transition metal to be loaded on the zeolite for ion exchange, is selected, the performance of the obtained exhaust gas-purifying catalyst varies depending on the type of the zeolite and its coordination point. However, the conventional zeolite-based exhaust gas-purifying catalyst is one in which the transition metal is ion-exchanged and supported on the zeolite without sufficiently considering the properties of the active sites, so that the performance of the catalyst has not been sufficiently brought out.

For this reason, the applicant of the present invention has found that in an exhaust gas purifying catalyst in which a copper ion-exchanged zeolite is supported on a refractory carrier, the ion-exchange point is present on the surface of the super cage of the zeolite, and the distribution of oxygen atoms to copper ions is We have proposed an exhaust gas purifying catalyst characterized in that the seat is a tetracoordinate tetragonal type.

The super cage of zeolite means pores of zeolite.

[Problems to be Solved by the Invention]

Although the exhaust gas purifying catalyst has a high NOx purifying ability even on the lean side, for example, in a large vehicle, since the NOx emission amount is large, it is necessary to further increase the purifying rate in order to perform lean burn. Further, in order to expand the applications of the zeolite catalyst (lean NOx catalyst) used on the lean side, improvement in activity has been desired.

The present invention is to solve the above-mentioned problems in the prior art, and its object is to eliminate the conventional lean NOx.
An object of the present invention is to provide a catalyst for purifying exhaust gas, which has a higher NOx purification capacity than the catalyst.

[Means for solving the problem]

That is, the exhaust gas purifying catalyst of the present invention, a zeolite ion-exchanged with copper is supported on a refractory carrier, and one of the zeolites having a large super cage inlet diameter is arranged on the upstream side in the flow direction of exhaust gas, It is characterized in that a super cage with a small inlet diameter is arranged on the downstream side in the exhaust gas flow direction.

The super cage inlet diameter means the diameter of the inlet of the pores. Since molecules such as hydrocarbons enter the inside of the pores of the zeolite through the super cage inlet of the zeolite, the super cage inlet diameter limits the molecules that can be put inside the pores (that is, only the molecules smaller than the super cage inlet diameter are included. Inside the pores). Since most of the active sites of the exhaust gas purifying catalyst of the present invention are in the super cage (pores) of zeolite, it is difficult for molecules that cannot be placed in the super cage to react with copper (catalyst metal).

There are various types of zeolite as shown in Table 1 below. Some zeolites have one type of oxygen ring member and some have two types of oxygen ring member. or,
The super cage inlet diameter is represented by one representative value (in this case, the super cage can be regarded as almost circular) and two representative values (in this case,
Super cage can be regarded as almost oval).

Molecules such as hydrocarbons are basically linear and move freely, and can take various forms. Therefore, when there are two kinds of zeolite super cage inlet diameters used in the exhaust gas purifying catalyst of the present invention or when the super cage inlet diameters are represented by two representative values, the zeolite super cage inlet diameter is determined by the largest value. Determine the size of the caliber (because molecules such as hydrocarbons can deform and pass through the widest part).

The super cage structure can be one-dimensional, two-dimensional, three-dimensional, or a combination of these. In the exhaust gas purifying catalyst of the present invention, basically any super cage structure zeolite can be used, provided that, for example,
Zeolites having a one-dimensional super cage structure may cause problems such as saving the flow of exhaust gas and easily causing pore clogging. Therefore, suitable zeolites are appropriately selected.

Examples of zeolites that can be used in the catalyst of the present invention include
ZSM-5, ferrierite and mordenite are mentioned. ZSM-5 and ferrierite are particularly preferred. ZSM-5
For example, Gee. tea. Coco Teiro (GTKoko
tailo), S. El. SL Lawton and Dee. Etch. DHOlson “Structure of Synthetic Zeolite ZSM-5 (Str
uctureof Synthetic zeolite ZSM-5) ", Nature, Volume 272, March 30, 1978, p. 437. Further, regarding ferrierite, for example, R. Gramlich-Meier (R. Gramlich). -Meier),
Wu. M. W Meer and B.
K. BKSmith, “On faults in the framework
structure of zeolite ferrierit) ", Zeitschrift
frkristallographie) 169, 201~210 (1984) and Sea. El. Kibi (CLKiBBy), A. J. Perrotta (AJPerrotta) and F. E. Massos (FE
Massoth), “Composition and Catalytic Properties of Sy
nthetic Ferrierite ”” Journal of catalysis 35 , 256-272 (1974).

As a Cu ion exchange point, it is effective that the Cu atom exists on the surface of the super cage of the zeolite and the conformation of oxygen atom with respect to the copper ion is a tetracoordinate tetragonal type. A. buoy.
AV Kucherov et al. “Cu 2 ⁺ -kaochin”
Location and Reactivity in Mordenite and ZSM-5 (Cu ²⁺ -cation location
and reactivity in mordenite and ZSM-5): E.
S. Earl - Study (esr-study) ", Zeoraitsu (Zeolites), 5 (9 May 1985) Cu ^2+, of Cu ⁰
Analysis by ESR revealed that the independent Cu ²⁺ ions have four-coordinate tetragonal type (planar) shown in Fig. 3 and five-coordinate regular pyramid type (pyramid) shown in Fig. 4. There is. Regarding the reactivity with CO and O ₂ , it has been clarified that tetracoordinate Cu ²⁺ reacts selectively.

Highly reactive tetracoordinate tetragonal Cu ²⁺ is present on the inner surface of the supercages. On the other hand, pentacoordinate regular pyramidal Cu with low reactivity
²⁺ is contained in a cage other than the super cage.

Each zeolite catalyst constituting the catalyst of the present invention can be obtained by ion exchange. At this time, tetra-coordinated tetragonal Cu ²⁺ is selectively obtained by a technique such as sterically increasing the anion; not increasing dissociation (acid strength of the anion) so much; and performing rapid ion exchange.

Examples of the refractory carrier used in the catalyst of the present invention include ceramic carriers such as cordierite and metal carriers. The amount of zeolite applied to the refractory carrier and the properties such as size and shape of the refractory carrier are selected according to the properties required for the catalyst.

When one refractory carrier is used, one side carries a zeolite catalyst with a large super cage diameter and the other side carries a zeolite catalyst with a small super cage diameter. The ratio of zeolite catalysts with different super cage diameters is selected according to the required characteristics.

When two or more refractory carriers are used, zeolite catalysts having different super cage diameters are loaded on each, and they are arranged in the order of the super cage diameter. The size and shape of each refractory carrier are appropriately selected.

The catalyst of the present invention may of course be used in combination with other types of exhaust gas-purifying catalysts.

[Work]

Although the detailed mechanism is unknown, in the experiment using model gas, NOx reduction activity varies depending on the coexisting HC components (eg C ₃ H ₆ > C ₃ H ₈ ), and it is active with a small amount of oxygen. There the like can be improved, likely due to NOx-HC-O ₂ reaction, for example, the following reactions a and B can be considered.

Zeolite easily adsorbs due to the diameter of the super cage
HC is different. In the exhaust gas purifying catalyst of the present invention, HC having a large molecular weight is trapped by the zeolite on the upstream side and NOx
HC reacts with and purifies it, and HC having a small molecular weight is captured by zeolite on the downstream side and reacts with NOx to purify it, so that the purification rate of NOx is improved as a whole.

〔Example〕

The present invention will be described in more detail in the following examples and comparative examples. The present invention is not limited to the examples below.

Cordierite monolith carrier (diameter 107 mm, length 79 m
m) is coated with ZSM-5 from one end to 4/7 of the entire length with a thickness of about 50μ by the wash coat method, and ferrierite is applied to the remaining length (3/7) from the other end in the same manner. The same thickness was applied. Then, this was immersed in a copper salt solution (0.01 M copper acetate solution), the solution was stirred, taken out, thoroughly washed with water, dried, and calcinated at 500 to 700 ° C. by passing air. The amount of copper supported was about 5 g /.

FIG. 1 is a perspective view of the catalyst of the present invention, in which 1 is a monolith support, 2 is Cu-ZSM-5 (ZSM ion-exchanged with copper).
-5), 3 is Cu-F (ferrierite ion-exchanged with copper).

The catalyst of the present invention can be obtained by sequentially loading mordenite and ferrierite ion-exchanged with copper, or mordenite, ZSM-5 and ferrierite on the monolith carrier from the upstream side to the downstream side.

Comparative Example 1 A catalyst was obtained in the same manner as in Example except that ZSM-5 was supported on the entire monolith carrier. The same applies to the thickness of ZSM-5 and the amount of copper supported.

Comparative Example 2 A catalyst was obtained in the same manner as in Example except that ferrierite was supported on the entire monolith carrier. The same applies to the thickness of ferrierite and the amount of copper supported.

FIG. 5 is an explanatory diagram showing Cu ²⁺ ion exchange points (100 planes) in ZSM-5. In the figure, ○ indicates 4 in Super Cage 4.
The coordination tetragonal (planar) Cu ²⁺ ion exchange points are shown, and the triangles indicate the pentacoordinate regular pyramid (pyramid) Cu ²⁺ ion exchange points. Fig. 6 shows Cu ²⁺ exchange points in ferrierite (face 001)
FIG. In the figure, ◯ indicates the tetracoordinate tetragonal Cu ²⁺ ion exchange point in the super cage 4, and □ indicates the other tetracoordinate Cu ²⁺ ion exchange points.

[Performance evaluation test]

Performance evaluation tests were conducted on the exhaust gas purifying catalysts of the present invention and comparative examples under the following conditions. The air-fuel ratio (A / F) is 21.
Is.

Test conditions Engine: 4A-ELU, LCS; 2000 rpm x 3 Kgm as the basis.

Catalyst: Manifold type 7R, 300 cells / inch Analysis: MEXA-2400 (manufactured by Horiba), heated NOx meter (manufactured by Yanagimoto, heating the sample line to 120 ° C to eliminate NOx adsorption) The results are shown in Fig. 2. . There was no difference in output between the MEXA-2400 used for analysis and the heating NOx meter. From FIG. 2, it can be seen that the catalyst of the present invention is superior in NOx purification rate to the catalysts of Comparative Examples 1 and 2.

〔The invention's effect〕

As described above, the exhaust gas purifying catalyst of the present invention is a zeolite ion-exchanged with copper is supported on a refractory carrier, and the zeolite having a large super cage inlet diameter is located upstream of the exhaust gas flow direction. Since the one with a small super cage inlet diameter is placed on the downstream side in the exhaust gas flow direction, HC with a large molecular weight should be purified on the upstream side, and HC with a small molecular weight should be purified on the downstream side. As a result, the NOx purification capacity is increased as a whole. Further, the temperature inside the catalyst is higher on the downstream side. For example, if ferrierite ion-exchanged with high-temperature-active copper is disposed on the downstream side, the NOx purification capacity as a whole is further enhanced.

[Brief description of drawings]

FIG. 1 is a perspective view of an embodiment of the exhaust gas purifying catalyst of the present invention, and FIG. 2 shows the relationship between the incoming gas temperature and the NOx purification rate when the catalyst of the present invention and the catalyst of the comparative example are used. Fig. 3 is a diagram showing the tetracoordinate tetragonal conformation of Cu ²⁺ ions, Fig. 4 is a diagram showing the pentacoordinate tetragonal conformation of Cu ²⁺ ions, and Fig. 5 is ZSM. explanatory view showing a Cu ²⁺ ion exchange points in -5, 6 is an explanatory diagram showing a Cu ²⁺ ion exchange point in the ferrierite. In the figure, 1 ... Monolith carrier, 2 ... Cu-ZSM-5 3 ... Cu-F, 4 ... Super cage

Claims (1)

(57) [Claims] 1. A zeolite ion-exchanged with copper is supported on a refractory carrier, and one of the zeolites having a large super cage inlet diameter is arranged on the upstream side in the exhaust gas flow direction, An exhaust gas purifying catalyst characterized in that a small one is arranged on the downstream side in the exhaust gas flow direction.