JP3188011B2 - Exhaust gas purification method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a nitrogen oxide (hereinafter referred to as NO
x), carbon monoxide (CO) and hydrocarbons (hereinafter, referred to as x).
Abbreviated as HC).

[0002]

2. Description of the Related Art In the treatment of exhaust gas from automobiles and the like, a three-way catalyst (NOx, NOx,
Purification is generally performed using a catalyst (composition: Pt, Rh / Al ₂ O ₃ system) called a catalyst that simultaneously removes three substances of CO and HC, but in a very narrow range around the stoichiometric air-fuel ratio. Only NOx is purified. In recent years, with increasing global environmental problems, there is a strong demand for low fuel consumption of automobiles, and a lean burn engine that burns at a stoichiometric air-fuel ratio or more has attracted attention as a key technology. Recently N in lean area
A crystalline silicate catalyst supporting copper as a catalyst capable of purifying Ox has been spotlighted as a catalyst having high performance and durability.

[0003] However, the above catalyst has a problem that the initial activity has a sufficient performance but the durability is poor. In addition, in the case of normal lean burn engine exhaust gas, the exhaust gas temperature ranges as high as 300 to 700 ° C., and at high temperatures, in the presence of steam, sintering of the supported active metal and dealumination of the crystalline silicate occur, and the catalyst deteriorates. Has been found to be.

Heretofore, the present inventors have developed a catalyst excellent in heat resistance and steam resistance by applying a crystalline silicate with a low dealumination, and have developed a catalyst for sintering copper as an active metal. I have found that the degree is small. In other words, the crystalline silicate in the dehydrated state has a molar ratio of oxide of (1 ± 0.6)
R ₂ O [aM ₂ O ₃ .bAl ₂ O ₃ .cMeO] .yS
iO ₂ (wherein R is an alkali metal ion and / or hydrogen ion, M is at least one element selected from the group consisting of group VIII elements, rare earth elements, titanium, vanadium, chromium, niobium, antimony, and gallium) Ions and Me are alkaline earth elements, a ≧ 0, b ≧ 0, c ≧ 0, a + b = 1,
y / c> 12, y> 12) and an X-ray diffraction pattern shown in Table 1 below.

Further, a crystalline silicate made of Si and O having the same crystal structure as that of the mother crystal is grown on the outer surface of the mother crystal using the above crystalline silicate as a mother crystal. It has been found that when layered crystalline silicate having a line diffraction pattern is used and copper is supported on this silicate, the durability of the catalyst is dramatically improved (Japanese Patent Application No. 03-319195, 03-19282).
9).

[0006]

[Table 1] VS: Very strong S: Strong M: Intermediate W: Weak

On the other hand, lean burn engines frequently switch between the lean and stoichiometric regions during actual running, and the NOx purification efficiency is not sufficient with a single catalyst. Has also been proposed. For example, in JP-A-01-139145, a reduction catalyst in which a transition metal is ion-exchanged and supported on zeolite is disposed on the exhaust inflow side, and an oxidation catalyst or a three-way catalyst in which a catalyst component is supported on alumina is disposed on the exhaust outflow side. It is stated that an organic substance is necessary for reacting NOx.

However, since the zeolite catalyst (a kind of catalyst having a molecular sieve structure) in the first stage removes most of the organic substances and the reducing agent such as CO, NOx is not or completely eliminated in the second stage catalyst. The problem is that you can't.

[0009]

The above-mentioned disadvantages can be said to be the same even if the catalyst developed by the present inventors is installed at the preceding stage. Especially when the exhaust gas changes from lean to stoichiometric, CO and HC work effectively in the denitration reaction of the three-way catalyst.
Is reacted with oxygen adsorbed on the catalyst in the preceding stage and is consumed, so that the three-way catalyst does not work and there is a problem that the amount of NOx emission instantaneously increases.

The present invention has been made in view of the above-mentioned state of the art, and has as its object to provide a method for allowing a subsequent three-way catalyst to work at any time.

[0011]

Means for Solving the Problems The present inventors have intensively studied a method in which the three-way catalyst works effectively even if the exhaust gas changes from lean to stoichiometric. As a result, when the exhaust gas changes from lean to stoichiometric, It was found that the oxygen adsorbed on the former catalyst was reacted and removed in a lean atmosphere by instantaneously adding excess fuel to the catalyst, and that the latter three-way catalyst worked effectively. It was completed. That is, the present invention relates to a method for purifying exhaust gas of an internal combustion engine or the like containing nitrogen oxides, carbon monoxide and hydrocarbons, wherein a crystalline silicate supporting copper is provided in a stage preceding the exhaust gas passage, and a three-way catalyst is arranged in a stage subsequent to the catalyst. Placed in
An exhaust gas purification method characterized by instantaneously and excessively adding fuel when the composition of exhaust gas changes from a lean state to a stoichiometric state.

A lean burn engine operates in a lean atmosphere during idling or constant speed running, and in a stoichiometric atmosphere during acceleration. Usually, when a crystalline silicate supporting copper is arranged in front of the exhaust passage and a three-way catalyst is arranged in series in the latter stage, when accelerating from idling, the three-way catalyst does not act instantaneously even in stoichio, so a large amount of NOx Is released. However, when changing from lean to stoichio, fuel (gasoline) is excessively added and instantaneously enriched, whereby O ₂ adsorbed on the pre-stage catalyst is reduced.
Reacts and is removed, so that NOx and the reducing agent (CO, H
The response delay of C) is not seen, and the three-way catalyst works effectively. In general, it has been found that fuel efficiency can be reduced by 10 to 15% by installing a lean burn engine as compared with a conventional engine. On the other hand, when adopting a lean burn engine and purifying exhaust gas by placing a crystalline silicate containing copper in the front stage of the exhaust passage and a three-way catalyst in the rear stage, excess fuel is added during acceleration to reduce fuel consumption. It is about 1 wt% worse at the maximum. Therefore, excessive addition of fuel in the present invention hardly leads to deterioration of fuel efficiency.

[0013]

The denitration reaction using a catalyst used in the preceding stage of the exhaust passage opening is schematically described as follows.

[0014]

Embedded image The above represents an example of HC in C ₃ H _6, represents a C ₃ H ₆ was activated with CH ₂ O ^*. In the above reaction, HC is activated by oxygen on the catalyst (formula), the activated HC burns (formula), and is also used as a reducing agent for NO removal (formula). Usually, the equations are competitive reactions, and the consumption of activated HC differs depending on the reaction conditions.

As described above, the catalyst used in the first step of the present invention has a property that the denitration reaction proceeds sufficiently even when oxygen is excessively present, and the catalyst has a higher performance in a lean state with a large amount of oxygen than in a stoichiometric state. Have. On the other hand, the three-way catalyst arranged in the latter stage has extremely high denitration performance in the stoichiometric state,
Performance in lean condition is poor. When the above catalysts are combined in series, both lean and stoichiogas have extremely excellent denitration performance in a steady state. In the present invention, when the gas atmosphere changes from lean to stoichio, the fuel such as gasoline is excessively injected, and the reducing agent that is consumed by the above equation is supplied instantaneously to reduce the gas atmosphere before the three-way catalyst. It is always in a stoichiometric state.

The timing of excessively injecting the fuel is detected by the oxygen concentration sensor at the moment when the air-fuel ratio changes from lean to stoichiometric, and the fuel is slightly excessively injected. Therefore, the current fuel supply system can be sufficiently controlled. is there.

[0017]

【Example】

(Embodiment) FIG. 1 shows a schematic configuration diagram of an exhaust gas purifying apparatus for a lean burn engine. The crystalline silicate denoted by reference numeral 3 disposed in the former stage of the exhaust gas has a molar ratio of CuO ·
[0.2Fe ₂ O ₃ · 0.8Al ₂ O ₃ · 0.1Ca
O] .30SiO ₂ and has a crystal structure having an X-ray diffraction pattern shown in Table 1. Also,
The three-way catalyst denoted by reference numeral 4 arranged at the latter stage of the exhaust gas contains Pt (0.5 wt%) and Rh (0.5 wt%) on an alumina carrier.
Is carried. 1, 1 is a lean burn engine, 2 is an oxygen concentration sensor, 5 is a throttle, 6
Indicates an air cleaner.

FIG. 2 shows a model running pattern of the lean burn engine 1. Changes in vehicle speed and air-fuel ratio during 22 seconds are as follows.

[0019]

[Table 2]

The basic running pattern is the oxygen concentration sensor 2
Full control is possible by making full use of.

In order to check the operating condition of the catalyst in the above pattern, a NOx meter was attached to the exhaust gas outlet,
The concentration was monitored. The exhaust gas temperature is 400 at the catalyst inlet side.
° C, GHSV value is 10,000 h ⁻¹ , 1 for 0 to 10 seconds.
0 to 18 seconds were performed at 30,000 h ^-1 . FIG. 3 shows the catalyst inlet NOx concentration and the catalyst outlet NOx concentration. The results show that the exhaust gas purifying apparatus according to the method of the present invention can sufficiently remove NOx under any exhaust gas conditions.

(Comparative Example 1) In the basic running pattern shown in FIG. 2, the air-fuel ratio was increased to 10-1 when accelerating at 0-40 km / h.
The exhausted NOx concentration when operating in the stoichiometric state at 14.7 for 2.5 seconds was measured. Other operation methods are the same as those in the embodiment. FIG. 4 shows the measurement results. From this result, when the air-fuel ratio is kept in the stoichiometric state during acceleration of 0 to 40 km / h, a large amount of NOx is discharged in the instant of 10 to 10.5 seconds, and the purification of NOx becomes insufficient. It has been found.

[0023]

According to the present invention, as shown in the examples,
In order to purify the exhaust gas for lean burn engines, a crystalline silicate catalyst carrying copper is arranged in the front stage of the exhaust passage, and a three-way catalyst is arranged in series in the rear stage.When the exhaust gas composition changes from lean to stoichiometric, fuel By instantaneously adding excess (enrichment), it was possible to efficiently purify NOx under any gas conditions.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram of one embodiment of the present invention.

FIG. 2 is a table showing a model running pattern by the lean burn engine of the present invention.

FIG. 3 is a table showing a change in concentration of inlet NOx and outlet NOx according to one embodiment of the present invention.

FIG. 4 shows an inlet N of one embodiment of a conventional lean burn engine.
The chart which shows the concentration change of Ox and exit NOx.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ identification code FI F01N 3/28 301 B01D 53/36 ZAB ZAB 103B (58) Fields investigated (Int.Cl. ⁷ , DB name) F02D 41/04 330 F01N 3/24 ZAB F01N 3/28 301

Claims (1)

(57) [Claims] 1. A method for purifying exhaust gas of an internal combustion engine or the like containing nitrogen oxides, carbon monoxide and hydrocarbons, wherein a crystalline silicate supporting copper is provided in a stage preceding an exhaust gas passage, and a three-way catalyst is arranged in a stage subsequent to the exhaust gas passage. Exhaust gas purification method, wherein fuel is instantaneously and excessively added when the composition of exhaust gas changes from a lean state to a stoichiometric state.