JP3345927B2 - How to clean diesel engine exhaust - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates are those which are harmful components in exhaust gas discharged from the diesel engine NO _X, an exhaust gas purification method for effectively reducing the CO and HC.

[0002]

2. Description of the Related Art As is well known in the art, at least one or more noble metal catalyst components such as platinum (Pt), palladium (Pd), and rhodium (Rh) are used in automobile engines, particularly gasoline engines. A three-way catalyst or an oxidation catalyst supported on a suitable carrier such as a monolith carrier is widely used. The three-way catalyst, if a gasoline engine is operated at the stoichiometric air fuel vicinity, HC are harmful components in the exhaust, CO, but it is possible to purify the NO _X respectively effectively, the main improvement of fuel consumption for so-called lean burn having an increased air-fuel ratio for the purpose, there is a drawback that reduction purification of the nO _X is not effectively performed to the oxygen content in the exhaust gas is increased. Also, in a diesel engine, a larger amount of oxygen, i.
Since the oxygen of about 0% contains, to purify Yotsute NO _X in the three-way catalysts are said to be more difficult.

Further, a reduction catalyst (hereinafter referred to as a zeolite catalyst) in which various natural or synthetic zeolites are used as carriers and at least a part of sodium (Na) and hydrogen ions in the components are exchanged for transition metal ions such as copper (Cu). ,
It is said that it is effective for purifying NO _X in exhaust gas of a diesel engine. However, various zeolite catalysts proposed so far have a relatively large amount of H in a high temperature range of about 400 ° C. where the activity of the catalyst is originally high.
Very good NO _X purification performance can be exhibited only when the exhaust gas having C is brought into contact with a given volume of catalyst at a relatively small flow rate per unit time. When exhaust gas having a much lower temperature, for example, 250 ° C. and a low HC content is brought into contact with a large volume of catalyst per unit time, the general tendency that the NO _X purification performance in exhaust gas is significantly reduced There is.

On the other hand, the exhaust of a diesel engine mounted on a truck or the like is 500 ° C. only in the full-power operation which is rarely generated, such as when climbing a steep hill with a full load of cargo, or in an operation state close to this. becomes higher, in urban driving and summer and NO _X environmental particular problem in the exhaust, the exhaust gas temperature is only normal 300 ° C before and after substantially, yet the exhaust diesel engine, activation of the zeolite catalyst Under the adverse conditions that the content of HC required for the process is essentially small, and conversely, the content of oxygen which hinders the reduction of NO _X is extremely large as described above. It was but connexion, the zeolite catalyst alone known conventionally to purify NO _X contained in the exhaust of the tracks, such as diesel engines for vehicles effectively, in practice it is extremely difficult.

[0005]

SUMMARY OF THE INVENTION It is an object of the present invention, conventional three-way catalyst despite the technical common knowledge that does not degrade the NO _X in the presence of oxygen, part of the three-way catalyst, in particular platinum (Pt)
Is based on the discovery that in the presence of about 10% oxygen, NO _X can be decomposed in a certain temperature range using HC as a reducing agent. On the basis of this new finding, the present invention provides a method for producing HC and C
And O oxidative capacity, by utilizing the NO _X decomposition ability at low temperatures, the conventional purification can be difficult to remove a contained in the exhaust been filed diesel engine NO _X and HC, and CO respectively effectively exhaust It is an object to provide a purification method.

[0006]

SUMMARY OF THE INVENTION The present invention has been made in order to achieve the above-mentioned object, and is provided in an exhaust passage in which an appropriate amount (including zero) of HC is added to an operating state of a diesel engine. A first catalyst comprising a crystalline silicate containing a transition metal such as Cu is disposed, and a three-way catalyst in which a catalyst component such as Pt, Pd, Rh or the like is supported on an appropriate carrier downstream of the first catalyst. A second catalyst comprising a catalyst, wherein the first catalyst has a maximum NO _X purification point in a temperature range of 200 to 500 ° C., and an HC light-off temperature of the second catalyst is lower than that of the second catalyst; A catalyst whose temperature is higher than the off-temperature and whose temperature difference is 150 ° C. or less;
The catalyst has an HC light-off temperature of 150 to 350 ° C.
The present invention also proposes a method of purifying diesel engine exhaust, wherein the catalyst has a CO light-off temperature lower than the HC light-off temperature.

[0007]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be specifically described below with reference to FIGS. 1 to 7 and Table 1. First, in FIG. 1, reference numeral 10 conceptually shows a cylinder formed in a crankcase of a truck diesel engine.
Is a piston slidably fitted in the cylinder, 14
Indicates a cylinder head.

An intake port 18 communicating with the intake pipe 16 and an exhaust port 22 communicating with the exhaust pipe 20 are provided in the cylinder head 14. The communication of the intake port 18 with the cylinder 10 is controlled by an intake valve 24, and the communication of the exhaust port 22 with the cylinder 10 is controlled by an exhaust valve 26. A catalytic converter 28 is interposed in the exhaust pipe 20. Inside the catalytic converter 28,
A zeolite catalyst, ie, a first catalyst A, is disposed on the upstream side, and a three-way catalyst, ie, a second catalyst B, is disposed on the downstream side in proximity.

An HC injector 30 is provided in the exhaust pipe 20 on the upstream side of the catalytic converter 28. The HC injector 30 sets the HC injection amount according to the operating state of the engine, for example, the rotational speed, the load, the exhaust temperature and the like. Driven by the controller 32, the HC in the HC source 34 is injected into the exhaust pipe 20. In the HC source 34, methane,
Proper HC such as propane, butane, or engine fuel
(Hydrocarbons) are stored.

[0010] The zeolite catalyst A is used for sodium (Na) in crystalline silicate such as natural or synthetic zeolite.
And a catalyst in which hydrogen (H) ions are at least partially replaced by copper (Cu) ions. Preferably, a catalyst described in Japanese Patent Application Laid-Open No. Hei 3-143546 is used. The catalyst is, for example, silica, an inorganic oxide such as alumina or a clay as a binder, and if necessary, adding a molding aid such as an organic substance, and is formed into an appropriate shape such as a granular shape or a honeycomb shape. Be charged.

The zeolite catalyst A has a purification rate-temperature characteristic as shown in FIG. 5, for example. FIG. 5
A typical track like imitating the exhaust diesel engine vehicles and practically preferable a small amount of the test gas I having the following ingredients was added HC (NO _X: 500ppm, C
O: 300 ppm, CO ₂ : 6%, O ₂ : 10%, H _{2
O: 6%, C 2 H} 4: 750ppm, N 2: using the balance), 15 × 15 × 60mm (carrier cordierite: volume of catalyst A 13.5cc), the test gas I prepared in various temperatures 161 / min corresponding to a practical engine
And the purification rates of NO _X , C ₂ H ₄ and CO were examined.

As shown in the figure, the zeolite catalyst A is
Exhaust gas temperature 200 at the operating region of the engine where the NO _X content in the exhaust gas is relatively high, that is
Those having a peak of the NO _X purification in the range of to 500 ° C, the illustrative case, a peak of the purification rate at around 350 ° C.

On the other hand, the above-mentioned three-way catalyst B is one widely used for purifying exhaust gas from gasoline engines, for example, a cordierite carrier, and platinum (Pt), palladium (Pd), rhodium (Rh), etc. A catalyst carrying a known catalyst component can be employed, and a catalyst in which a catalyst component such as Pt, Pd, Rh, or the like is carried on a metal carrier as described below (hereinafter sometimes referred to as a lithium metal carrier catalyst) Can be used.

An example of a method for producing the metal-supported catalyst will be briefly described. First, about 5% of aluminum and about 20%
% Chromium stainless steel is made by corrugating a thin sheet made of corrugated thin plate and combining a flat stainless steel sheet with one end as a core and winding it up into a scroll to form a large number of parallel small cross sections. After forming a cylindrical body having an exhaust passage, heating is performed at a predetermined temperature and for a predetermined time to deposit aluminum oxide whiskers on the surface of the stainless steel sheet. next,
The cylindrical body is immersed in a co-catalyst (oxygen storage component) solution widely used as a three-way catalyst, dried and calcined at a predetermined temperature, then immersed in an acidic aqueous solution of dinitrodiamine platinum, pulled up and dried. Firing at a predetermined temperature. Subsequently, it is immersed in an aqueous solution of rhodium chloride, pulled up and dried, and then fired at a predetermined temperature. The above steps are repeated an appropriate number of times, and finally a reduction treatment is performed at a predetermined temperature in a reducing atmosphere containing hydrogen or a large amount of hydrogen.

As an example, a diameter of 18.5 mm and a length of 50
mm, thus a volume of about 13.5 cc (mesh 400 cells), 40 g / l cocatalyst and 0.98 g / l Pt
Flowing the test gas 1 at various temperatures at a flow rate of 161 / min through the metal support catalyst supporting
_The results of examining the purification rates of _X , C ₂ H ₄ and CO are shown in FIG. As shown in the drawing, the HC light-off temperature (temperature corresponding to a 50% purification rate) of the three-way catalyst B (metal carrier catalyst) is included in the range of 150 to 350 ° C., and about 230 ° C. in the illustrated example. C. The CO light-off temperature is lower than the HC light-off temperature, and is approximately 165 ° C. in the example. On the other hand, since the HC light-off temperature of the zeolite catalyst A shown in FIG. 5 is about 325 ° C., the HC light-off temperature of the three-way catalyst B is lower than the HC light-off temperature of the zeolite catalyst A. ΔT
_HC is about 95 ° C.

When the test gas 1 flows through the zeolite catalyst A at a flow rate close to the practical flow rate with respect to the catalyst volume, as shown in FIG. 5, a vehicle equipped with a diesel engine is used. When the exhaust gas temperature is around 300 ° C. when traveling in a city, the activity of the catalyst is relatively low, so that the purification rates of NO _X , HC and CO are all low. There is a problem that the exhaust gas is even more increased, and therefore, the desired exhaust gas purifying effect cannot be achieved by using the zeolite catalyst A alone. Similarly, in the case where the three-way catalyst B is used alone, as shown in FIG.
At around 0 ° C., NO _X was purified despite the high oxygen concentration in the test gas I. At 250 ° C., about 25% purification performance was obtained, and HC and CO were removed from a low temperature of 250 ° C. Purification occurs. However, when the temperature is around 400 ° C., the purification rate of NO _X significantly decreases and is insufficient.

Next, the same zeolite catalyst A as described above is arranged on the upstream side, and the metal carrier catalyst (three-way catalyst) B is arranged close to the downstream side. flow, _NO X, the result of examining the purification rate of HC and CO are shown in Figure 7. As shown, the general purification rate of the exhaust gas temperature 300 ° C around in NO _X is increased to 40% substantially during urban driving of a vehicle equipped with a diesel engine, HC and CO purification rate approximately 95% of It was found that the above was reached. In other words, a relatively small amount of HC suitable for practical use is added to a large amount of exhaust gas having a flow rate close to the actual one in the exhaust gas temperature range during city driving where the activity of the catalyst is originally low, and HC and CO are substantially reduced. the perfectly, and because the NO _X been made difficult conventionally removed can be removed substantially about 40%, there is an advantage that can achieve an excellent exhaust gas purification effect at a practical driving range of diesel engine equipped vehicle.

FIGS. 2, 3 and 4 show the zeolite catalyst A and the metal carrier catalyst B shown in FIGS. 5, 6 and 7, respectively.
(Three-way catalyst), the conversion method of the purification rate-temperature characteristic of the combination of the zeolite catalyst A and the metal carrier catalyst B is converted, and the respective catalysts A and B and their combination (A
+ B) as a parameter, NO _X purification rate-temperature characteristic diagram, H
It is shown as a C purification rate-temperature characteristic diagram and a CO purification rate-temperature characteristic diagram.

Next, Table 1 below shows that the same zeolite catalyst A and metal carrier as those described above were used except that the test gas I was changed to a test gas II in which only HC was changed to C ₃ H ₆ with a concentration of 500 ppm. catalyst B (three-way catalyst), and a zeolite catalyst a on the upstream side, and for each of the combinations of arranging the metallic support catalyst B on the downstream side (a + B), the purification rate of NO _X, CO, HC at various gas temperatures It is a thing which investigated.

[0020]

[Table 1]

As is clear from Table 1, the zeolite catalyst A is used at a temperature around 300 ° C., which occurs most frequently when a vehicle equipped with a diesel engine such as a truck or a bus, for which reduction of NO _X is particularly required, runs in a city. It has been confirmed that the combination (A + B) can achieve a superior NO _X purification rate, and the HC and CO purification rates are extremely excellent, respectively, than when the metal carrier catalyst B is used alone. Was.

The zeolite catalyst A, that is, the first catalyst, as described above, is disclosed in Japanese Unexamined Patent Application Publication No.
Among the various catalysts disclosed in No. 47, 200 to 5
It has a maximum NO _X purification rate peak in the temperature range of 00 ° C., and the HC light-off temperature is lower than the three-way catalyst B, ie, the second
Higher than the HC light-off temperature of the catalyst and the temperature difference Δ
_THC is 150 ° C. or less, preferably 30 to 100 ° C.
Any catalyst may be used. The three-way catalyst B has a light-off temperature of HC in the range of 150 to 350 ° C.
If the light-off temperature of O is lower than the light-off temperature of HC, the reduction catalyst A is arranged upstream of the exhaust passage, and the three-way catalyst B is arranged downstream to combine the reduction catalyst A. It was confirmed that a harmful component purifying effect substantially the same as that described above, in particular, an excellent NO _X purifying effect was obtained.

The HC added to the exhaust of the vehicle diesel engine (accordingly, the corresponding test gases I and II) is preferably added in an amount equal to or larger than the NO _X concentration in the exhaust. Although improved NO _X purification rate with increasing amount of addition, the limit NO _X in the exhaust gas
Is about 10 times as high as the concentration of HC, and any further increase in the amount of added HC has almost no effect of improving the purification rate. Further, as is clear from FIGS. 2 and 7, addition of HC is practically useless in a low-temperature region where the activity of the catalyst is low and 200 ° C. or less, for example, during idling operation of the engine or low-load low-speed operation.
It is advantageous not to add it in this temperature range, since it increases the operating cost of the engine.

As described above, the method for purifying diesel engine exhaust gas according to the present invention provides a method for purifying crystalline silicate in an exhaust gas passage to which an appropriate amount (including zero) of HC is added according to the operating condition of the diesel engine. A first catalyst containing a transition metal such as Cu is disposed, and a third catalyst comprising a three-way catalyst in which a catalyst component such as Pt, Pd, and Rh is supported on an appropriate carrier is provided downstream of the first catalyst. 2 in which the first catalyst has a maximum N in a temperature range of 200 to 500 ° C.
Together with the O _X purification point, the HC light-off temperature is higher than the HC light-off temperature of the second catalyst, and a temperature difference is 0.99 ° C following catalysts, also the second catalyst, the HC light Off temperature is 150 to 350 ° C
In, and the CO light-off temperature is characterized by a low catalyst than the HC light-off temperature, harmful components in the exhaust of diesel engine, effectively removes NO _X which is a particular conventional purification difficult Is industrially beneficial.

[Brief description of the drawings]

FIG. 1 is a schematic sectional view showing one embodiment of the method of the present invention.

FIG. 2 is a diagram showing the relationship between the NO _X purification rate and the test gas temperature in the method of the present invention in comparison with a zeolite catalyst and a three-way catalyst.

FIG. 3 is a diagram showing the relationship between the HC purification rate and the test gas temperature in the method of the present invention in comparison with a zeolite catalyst and a three-way catalyst.

FIG. 4 is a diagram showing the relationship between the CO purification rate and the test gas temperature in the method of the present invention in comparison with a zeolite catalyst and a three-way catalyst.

FIG. 5: NO _X , HC, C when zeolite catalyst is used alone
FIG. 3 is a diagram illustrating a relationship between an O purification rate and a test gas temperature.

FIG. 6 is a graph showing the relationship between the NO _X , HC, and CO purification rates and the test gas temperature when the three-way catalyst is used alone.

FIG. 7 is a graph showing the relationship between the NO _x , HC, and CO purification rates and the test gas temperature when a zeolite catalyst and a three-way catalyst are arranged in series according to the method of the present invention.

[Explanation of symbols]

 Reference Signs List 10 Cylinder of diesel engine 12 Piston 20 Exhaust pipe 28 Catalytic converter A Zeolite catalyst B Three-way catalyst

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int.Cl. ⁷ , DB name) F01N 3/08-3/36

Claims (1)

(57) [Claims] 1. A first catalyst comprising a crystalline silicate containing a transition metal such as Cu in an exhaust passage to which an appropriate amount (including zero) of HC is added according to the operating state of a diesel engine. At the same time, on the downstream side of the first catalyst, a second catalyst composed of a three-way catalyst in which a catalyst component such as Pt, Pd, Rh or the like is supported on an appropriate carrier is arranged. It has a maximum NO _X purification point in the temperature range of 500 ° C. and its HC light-off temperature
The catalyst is a catalyst whose HC light-off temperature is higher than the HC light-off temperature and whose temperature difference is 150 ° C. or less, and the second catalyst has an HC light-off temperature of 150 to 350 ° C. and a CO light-off temperature of A method for purifying diesel engine exhaust, characterized in that the catalyst is lower than the HC light-off temperature.