JP4371241B2 - Exhaust gas purification system - Google Patents

Description

  The present invention relates to a system for purifying exhaust gas containing nitrided oxide (NOx) discharged from an internal combustion engine of a moving body, particularly a vehicle.

Theoretical air-fuel ratio (Air / Fuel) is used to purify carbon monoxide (CO), hydrocarbons (HC), nitrided oxides (NOx), etc. contained in exhaust gas discharged from an internal combustion engine of a moving body such as an automobile. A three-way catalyst working under the exhaust gas and an exhaust purification system using the same are used.
On the other hand, in order to improve fuel efficiency, lean burn engines that cause combustion in an air-fuel ratio range higher than the stoichiometric air-fuel ratio have been put into practical use. As a means for purifying exhausted nitrogen oxides (NOx) under lean burn conditions, a NOx purification catalyst having a function of trapping NOx is used (see, for example, Patent Document 1).
JP 2000-257417 A

This NOx purification catalyst for lean burn includes a NOx trap agent (NOx adsorbent) in addition to a catalyst that reduces and purifies NOx, and is discharged when the air-fuel ratio is lean (fuel ratio is small). Is adsorbed by the NOx trapping agent, and when the air-fuel ratio becomes rich (the fuel ratio is large), the trapped NOx is reduced by the reducing gas component in the exhaust gas and released and purified. That is, in this NOx purification catalyst, when the air-fuel ratio is rich, reducing gas components such as hydrocarbons (HC), carbon monoxide (CO), and hydrogen (H ₂ ) discharged from the internal combustion engine enter the NOx reduction purification catalyst. Supplied, NOx is purified by a reduction reaction, and released as N ₂ . Note that the reducing function of each reducing gas depends on temperature conditions. A temperature condition of 250 ° C. or higher is necessary for HC and CO to exhibit an excellent reducing action in the NOx purification reaction.

However, in general, the exhaust gas temperature under lean burn conditions is low, and it is difficult to reach a temperature of 250 ° C. or higher at which HC and CO exhibit an excellent reducing action for NOx emission purification. Particularly, since the exhaust gas temperature at the time of starting the engine, which is called a cold region, is a temperature of 180 ° C. to 200 ° C., NO reduction purification by the NOx purification catalyst cannot be performed efficiently.
Further, in the case of a vehicle using a diesel engine, the exhaust gas temperature is further lowered, so that the reduction purification efficiency of the NOx purification catalyst becomes worse.
On the other hand, vehicles subject to exhaust gas regulations include not only NOx but also HC gas. As described above, when the exhaust gas temperature is low, in the NOx purification catalyst, unless HC or CO in the exhaust gas is used for the reduction reaction, it may not be consumed and may flow out to the outside as it is. is there.

In order to solve the problem of HC outflow at a low temperature in an internal combustion engine that performs lean burn, Japanese Patent Laid-Open No. 2000-257417 discloses an HC trapping agent that traps HC having a high carbon number at the downstream of the NOx purification catalyst and a low An exhaust emission control device in which an oxidation catalyst for purifying HC having carbon number is arranged is disclosed.
However, the exhaust purification device does not perform NOx purification at a low temperature, and NOx purification is performed by increasing the temperature, and HC trapped in the HC trapping agent in the subsequent stage is also released and purified by this temperature increase. That is, in this apparatus, since the temperature heating means is always used, the fuel consumption is increased. Therefore, good fuel consumption, which is the greatest advantage of lean burn, cannot be obtained.

  An object of the present invention is to provide a vehicle purification system capable of more efficiently purifying NOx and HC in exhaust gas under lean burn conditions.

  In order to achieve the above object, a vehicle exhaust gas purification system according to an aspect of the present invention is disposed in a combustion gas or exhaust gas flow path, and increases a hydrogen component ratio with respect to a reducing gas component in the exhaust gas. Means, a NOx purification catalyst disposed after the hydrogen enrichment means on the exhaust gas flow path, and an HC trap catalyst disposed after the NOx purification catalyst on the exhaust gas flow path, Exhaust gas always flows directly from the hydrogen enrichment means to the NOx purification catalyst, and the hydrogen enrichment means controls at least one of the fuel injection amount of the internal combustion engine, fuel injection timing, ignition timing, and intake / exhaust valve opening / closing timing. By doing so, it has a combustion control means for generating hydrogen.

  According to the present invention, the hydrogen enrichment means is provided upstream of the NOx purification catalyst, and the HC trap catalyst is disposed downstream of the NOx purification catalyst. It is possible to provide an exhaust gas purification system that reduces the outflow amount of HC into the atmosphere.

The inventors of the present application have so far conducted various studies on NOx purification at a low temperature of less than 250 ° C. Based on these studies, the present inventors have proposed an exhaust gas purification system that effectively utilizes hydrogen for NOx purification, as disclosed in US patent application 09 / 692,470. This exhaust gas purification system is provided with hydrogen enriching means in front of the NOx purification catalyst on the exhaust gas flow path.
The hydrogen enrichment means placed in front of the NOx purification catalyst increases the relative hydrogen concentration in the exhaust gas entering the NOx purification catalyst by generating hydrogen or selectively oxidizing a reducing gas other than hydrogen. Since hydrogen can exhibit a high reducing action even at a low temperature of less than 250 ° C., NOx trapped in the NOx purification catalyst can be efficiently reduced and released. By using this purification system, it is possible to purify NOx even if the exhaust gas temperature, which has been considered impossible in the past, is in the cold region.

However, as a result of further studies by the present inventors, even in the above exhaust gas purification system, some HC that was not involved in the reaction by the hydrogen enrichment means or the NOx purification catalyst may flow out into the atmosphere at a low temperature. It has been found that it is necessary to reduce the concentration of HC flowing into the atmosphere.
An exhaust gas purification system according to an embodiment of the present invention is an improvement of the above purification system, and more efficiently not only NOx in exhaust gas discharged from an internal combustion engine but also HC under lean burn conditions. It is a vehicle purification system that can be purified. This will be specifically described below. In the present specification, “%” represents mass percentage unless otherwise specified.

First, as shown in FIG. 1, in the exhaust gas purification system of the present embodiment, the hydrogen enrichment means 20, the NOx purification catalyst 30, and the HC trap catalyst 40 are disposed on the exhaust gas flow path from the engine 10. They are arranged inline in this order. The hydrogen enrichment means is always exposed to the exhaust gas and is arranged in the flow of exhaust gas or combustion gas. Therefore, in this system, there is no need to provide a bypass to circulate the exhaust gas, to provide a switching valve in the exhaust gas passage, or to supply hydrogen from the outside of the exhaust gas passage. This system becomes a more compact and high performance system and is excellent for vehicles.
In the engine 10, the fuel injection conditions and the like are controlled by the control device 15 in accordance with the operating state, and the air-fuel ratio becomes lean or rich.

First, the hydrogen enrichment means 20 will be outlined. The hydrogen enrichment means is means for increasing the hydrogen component ratio to the reducing gas component in the combustion gas and / or exhaust gas. Specifically, (a) hydrogen in the combustion gas and / or exhaust gas. Or (b) means for reducing reducing components other than hydrogen in the combustion gas and / or exhaust gas. In addition, although said (a) is used as a hydrogen enrichment means, you may use (b) together.
Specifically, the hydrogen generation means (a) is a means for generating hydrogen in combustion gas or exhaust gas, and actively increasing the hydrogen concentration by increasing the amount of hydrogen present in the gas. Combustion control means (combustion system) relating to fuel injection timing, ignition timing, and the like, and a hydrogen generation catalyst (catalyst system) containing a noble metal such as rhodium (Rh) can be exemplified. In the present invention, at least the above combustion system is used. To do. The reducing component reducing means (b) selectively reduces reducing components such as HC and CO (or more preferentially than hydrogen), and increases the hydrogen existing ratio in the gas. Examples thereof include a CO / HC selective oxidation catalyst and a catalyst containing a solid acidic oxide that reduces the consumption ratio of hydrogen compared to the consumption ratio of other gas components.

FIG. 2 is a diagram for explaining a purification mechanism in the NOx trap type NOx purification catalyst 30. The NOx purification catalyst 30 includes a noble metal catalyst 35 such as platinum (Pt) and Rh that is a NOx reduction purification catalyst, and an adsorbent 36 such as Ba that traps NOx as nitrate. When the air-fuel ratio is lean, the NOx trapped in the adsorbent 36 is reduced and purified by the reducing gas in the exhaust gas under the noble metal catalyst 35 when the air-fuel ratio becomes rich, and becomes N ₂ Released.
Reducing components such as HC and CO other than hydrogen contained in the exhaust gas develop a reducing function under a temperature condition of 250 ° C. to 500 ° C., but 250 like the exhaust gas temperature discharged under lean burn conditions. Does not exhibit reducing function at low temperatures below ℃. In addition, as shown in FIG. 2, particularly at a temperature of less than 250 ° C., CO gas tends to adhere to the surface of the NOx purification catalyst 35 and inhibit the NOx purification reaction.

On the other hand, since hydrogen exhibits a reducing function even at a low temperature of 200 ° C. or lower and in a cold region, the NOx purification reaction by the NOx purification catalyst 30 is promoted if the amount of hydrogen in the exhaust gas increases. In addition, the reduction component other than hydrogen in the exhaust gas (HC and CO) is reduced, and even when the relative hydrogen concentration increases, the amount of CO that inhibits the purification reaction decreases, so the purification reaction by hydrogen is accelerated. Is done.
Specifically, when the NOx purification catalyst of the present embodiment performs NOx purification, hydrogen enrichment is performed by the above-described hydrogen enrichment means, and the hydrogen concentration [H2] in the exhaust gas and the total reducing component concentration [TR] ] Preferably satisfies the following formulas (f1) and (f2).
[H2 / TR] d> [H2 / TR] u (f1)
[H2 / TR] d ≧ 0.3 (f2)
Here, [H2 / TR] u is the ratio of the hydrogen concentration [H2] u to the total reducing component concentration [TR] u before or upstream of the hydrogen enrichment by the hydrogen enrichment means, and [H2 / TR] d is The ratio of the hydrogen concentration [H2] d at the inlet of the NOx purification catalyst and the total reducing component concentration [TR] d is shown.

When the above formulas (f1) and (f2) are satisfied, hydrogen is effectively used as a reducing component in the NOx purification reaction, and NOx is efficiently purified.
According to the study by the inventors of the present application, when the hydrogen enrichment means is not used, and when the exhaust gas from the automobile engine or the exhaust gas purification catalyst is used, [H2 / TR] d <0.3. When the above formula (f2) is satisfied, the effect of the hydrogen enrichment means can be clearly obtained. More preferably, [H2 / TR] d ≧ 0.5. In order to improve safety, the hydrogen content is preferably less than 4% with respect to the total exhaust gas.
The above-described hydrogen enrichment means can always perform the hydrogen enrichment, but it is desirable from the viewpoint of improving the NOx purification efficiency to make the execution of the hydrogen enrichment coincide with the time of the NOx purification by the NOx purification catalyst.

Furthermore, in the purification system of the present embodiment, the hydrogen concentration [H2] d and the carbon monoxide concentration [CO] d in the total reducing component concentration [TR] d at the inlet of the NOx trap purification catalyst during NOx purification It is preferable that the ratio satisfies the following formula (f3).
[H2 / CO] d> 1 (f3)
When (f3) is satisfied, the concentration of CO, which tends to adhere to the NOx catalyst surface in the reducing gas and inhibit the purification reaction between NOx and hydrogen, is reduced. The reactivity between H ₂ and NOx can be increased, and the NOx purification efficiency can be further improved.

Next, the HC trap catalyst 40 according to the present embodiment will be described. FIG. 3 is an external view and a partial sectional view showing the configuration of the HC trap catalyst 40 according to the present embodiment.
As shown in FIG. 1, in the exhaust gas purification system according to the present embodiment, the HC trap catalyst 40 is disposed downstream of the NOx purification catalyst described above. The HC trap catalyst 40 contains an adsorbent capable of adsorbing HC at a low temperature and releasing HC as the temperature rises (when the activation temperature is reached). HC remaining in the exhaust gas is trapped without being consumed by the NOx purification catalyst. The released HC is purified by a three-way catalyst provided integrally with the HC trap catalyst 40 or a three-way catalyst provided separately from the HC trap catalyst 40.

Although an HC purification catalyst such as a three-way catalyst may be provided separately from the HC trap catalyst, the HC trap catalyst itself has a structure that includes an HC adsorbent layer and an HC purification catalyst. It can release and purify well.
For example, as shown in FIG. 3, the HC trap catalyst of the present embodiment has a hydrocarbon adsorbent layer 42 and a metal catalyst layer that is a three-way catalyst on a monolith support 41 having a plurality of cells having a polygonal cross section. 43 is formed by stacking. The exhaust gas passes through the center of the cell inside the metal catalyst layer 43. As the monolith carrier 41, for example, a ceramic material such as cordierite or a metal material such as ferritic stainless steel can be used.
Examples of the hydrocarbon adsorbent of the hydrocarbon adsorbent layer 42 include zeolite or activated carbon. Moreover, as a metal catalyst of the metal catalyst layer 43, the metal which has what is called a three-way catalyst function, such as Pd, Pt, or Rh, can be used. In addition, when using a zeolite as a hydrocarbon adsorbent, it is desirable not to contain a metal catalyst in a zeolite layer, in order to raise durability.

As described above, in the exhaust gas purification system according to the present embodiment, the hydrogen enrichment means is provided upstream of the NOx purification catalyst and the HC trap catalyst is disposed downstream of the NOx purification catalyst. The HC remaining in the exhaust gas that is not used in the NOx purification reaction can be trapped by the HC trap catalyst to prevent the outflow of HC into the atmosphere. That is, the outflow of HC generated in the cold region can be suppressed.
Note that the type of the hydrocarbon adsorbent layer 42 of the HC trap catalyst 40 according to the present embodiment is preferably changed as appropriate according to the type of fuel of the target internal combustion engine.

FIG. 4 is a graph showing the molecular size distribution of HC in the exhaust gas. In FIG. 4, line A is for a gasoline vehicle using gasoline fuel, and line B is for a diesel vehicle using light oil fuel.
As shown in the line A in FIG. 4, when the fuel is light oil such as gasoline, unpurified HC contains many HCs having 3 or more carbon atoms and a molecular diameter of 4 mm (0.4 nm) or more. . As a hydrocarbon adsorbing material for this, it is preferable to use β-zeolite, ZSM-5, or the like that efficiently adsorbs these molecular diameters. In addition, since there is a correlation between the average molecular diameter to be adsorbed and the pore diameter of the zeolite, it is preferable to use a zeolite having a pore diameter equal to or larger than the molecular diameter to be adsorbed.

On the other hand, as shown by the line B in FIG. 4, when the fuel is heavy oil such as light oil, the exhaust gas contains 3 carbon atoms from HC having a molecular diameter of less than 3 and a molecular diameter of less than 4 cm. Various HCs having a molecular diameter of 4 mm or more are included. Therefore, as a hydrocarbon adsorbent, in addition to β-zeolite and ZSM-5 that can efficiently adsorb 4 HC or more of HC, ferrierite, A type, B type zeolite and the like that can adsorb less than 4 HC of HC efficiently It is preferable to use it.
The HC trap catalyst is disposed after the hydrogen enrichment means and the NOx purification catalyst. When a CO / HC selective oxidation catalyst is used as the hydrogen enrichment means, HC having 3 or more carbon atoms and a molecular diameter of 4 mm or more is oxidized and purified by this CO / HC selective oxidation catalyst. Accordingly, when diesel oil is used as a fuel as in a diesel vehicle and a CO / HC selective oxidation catalyst is used as the hydrogen enrichment means, as shown by line b in FIG. The exhaust gas that has passed through mainly contains HC having a carbon number of less than 3 and a molecular diameter of less than 4 cm. Therefore, in this case, it is preferable to use ferrierite, A-type zeolite, B-type zeolite, or the like that can efficiently adsorb HC less than 4 mm (0.4 nm) as the hydrocarbon adsorbent.
In general, since methane (CH ₄ ) is not subject to exhaust gas regulations, the hydrocarbon trapped by the HC trap catalyst may be any that can trap at least molecules larger than CH ₄ . In other words, any hydrocarbon that has a boiling point of −50 ° C. or higher can be mainly trapped.

  As described above, in the exhaust gas purification system according to the present embodiment, the specific hydrogen enriching means is disposed upstream of the NOx purification catalyst disposed in the exhaust gas flow path, and the NOx trap purification catalyst is An HC trap catalyst is disposed downstream, and the hydrogen enrichment means supplies combustion gas and exhaust gas enriched with hydrogen to the NOx purification catalyst, so that the NOx purification process can be performed efficiently and the HC trap The catalyst can prevent HC from flowing out to the atmosphere in the cold region.

Again, the hydrogen enrichment means according to the present embodiment will be described in more detail.
As described above, the hydrogen enrichment means is roughly divided into a hydrogen generation means (a) and a means (b) for reducing reducing components other than hydrogen. Further, examples of the hydrogen generation means (a) include combustion control means (I) (combustion system) and hydrogen generation catalyst (II) (catalyst system). Examples of means (b) for reducing reducing components other than hydrogen include a CO / HC selective oxidation catalyst (III) (catalyst system) and a solid acidic oxide-containing catalyst (IV) (catalyst system).
These means are not completely distinguished, and one catalyst may have a plurality of catalyst functions. As for the means (I) to (IV), the means (I) is essential, but other means can be used in combination. As a structure of a suitable hydrogen enrichment means, a combination of the combustion control means (I), a hydrogen generation catalyst (II), an HC / CO selective oxidation catalyst (III) and a solid acidic oxide-containing catalyst (IV) Can be mentioned.

Specifically, examples of the combustion control means (I) include means for controlling the fuel injection amount, fuel injection timing, ignition timing or intake / exhaust valve opening / closing timing, and any combination thereof. Some combustion control means include means for partially oxidizing HC in the combustion gas and / or exhaust gas to cause CO generation.
As the hydrogen generation catalyst (II), a catalyst having a function of generating hydrogen from HC and CO in combustion gas and / or exhaust gas is sufficient, but Pt, Pd or Rh and any mixture thereof can be used. A noble metal catalyst containing such a noble metal can be exemplified. In addition, in this hydrogen production | generation catalyst, as a thing using a single noble metal, the catalyst containing Rh is the most preferable.

The CO / HC selective oxidation catalyst (III) has an effect of reducing reducing components other than H ₂ . For example, examples of the catalyst include a catalyst containing a zirconium oxide having a function of generating H ₂ . In this case, it is preferable to use a zirconium oxide containing an alkaline earth metal and having a composition satisfying the following general formula (f4).
[X] aZrbOc (f4)
Here, X in the formula is at least one alkaline earth metal selected from the group consisting of magnesium, calcium, strontium and barium, a and b are atomic ratios of the respective elements, and c is a valence of X and Zr. The number of oxygen atoms necessary to satisfy the above is shown, and a = 0.01 to 0.5, b = 0.5 to 0.99, and a + b = 1 are satisfied.
In the above formula (f4), if a is less than 0.01, the effect of modifying the additive element (alkaline earth metal) with respect to zirconium oxide cannot be sufficiently obtained. Conversely, if a exceeds 0.5, the heat resistance May deteriorate and the catalytic activity may decrease. On the other hand, if a + b exceeds 1.0, the structural stability of the zirconium oxide may decrease, which is not preferable.

As described above, the CO / HC selective oxidation catalyst related to this zirconium oxide can be used in combination with a hydrogen generation catalyst, preferably Rh, and the combined use maintains the electronic state of Rh in a suitable state. Since H ₂ is efficiently generated, hydrogen enrichment can be further promoted.
In this case, the amount of Rh used is desirably 0.01 to 10 g / L per liter of the catalyst, and if it is less than 0.01 g / L, the effect of increasing the H ₂ component ratio due to Rh cannot be sufficiently obtained. If it exceeds 10 g / L, the increase effect is saturated.
In the combined use, in the zirconium oxide represented by the above formula (f4), if a + b exceeds 1.0, the added alkaline earth metal is precipitated on the catalyst surface to reduce the catalytic activity of Rh. Sometimes.

Further, such a CO / HC selective oxidation catalyst can contain Pd and cerium oxide in order to selectively oxidize and remove unburned HC and CO and increase the H ₂ component ratio. It is desirable that 20 to 80% of the Pd amount is supported on the cerium oxide.
If the amount of Pd supported on the cerium oxide is less than 20%, the effect of increasing the H ₂ production ratio cannot be obtained sufficiently. Conversely, if it exceeds 80%, the dispersibility of Pd deteriorates and the catalytic activity decreases.
The amount of Pd used is desirably 0.01 to 50 g / L per liter of the catalyst, and if it is less than 0.01 g / L, Pd selectively oxidizes and removes unburned HC and CO to form H. _The improvement effect which raises a ₂ component ratio is not fully acquired, but when 50 g / L is exceeded conversely, the improvement effect will be saturated.

Then, the solid acidic oxide-containing catalyst (IV) is to reduce the consumption of H _2. A catalyst containing a solid acidic zirconium oxide which preferably has a CO / HC selective oxidation function can also be mentioned.
Such a solid acidic zirconium oxide contains at least one element selected from the group consisting of titanium, aluminum, tungsten, molybdenum and zinc, and its composition is represented by the following general formula (f5). Things are desirable.
[Y] dZreOf (f5)
Here, Y in the formula is at least one element selected from the group consisting of titanium, aluminum, tungsten, molybdenum and zinc, d and e are atomic ratios of each element, and f satisfies the valences of Y and Zr. This indicates the number of oxygen atoms necessary to achieve d = 0.01 to 0.5, e = 0.5 to 0.99, and d + e = 1.
In the above (f5), if d is less than 0.01, the effect of modifying the additive element such as titanium with respect to zirconium oxide cannot be sufficiently obtained. Conversely, if d exceeds 0.5, the heat resistance deteriorates and the catalytic activity May decrease. On the other hand, when a + b exceeds 1.0, the structural stability of the zirconium oxide may be lowered.

Further, the solid acidic oxide-containing catalyst (IV) is a hydrogen generating catalyst, in particular can be used in combination with Pt, by such combination, effectively NOx purification of H ₂ in the composition in the gas H ₂ component ratio is increased The catalyst can be supplied.
In this case, in order to suppress the consumption of Pt by H ₂ , it is preferable to support Pt on the zirconium oxide represented by the above formula (f5), and 10 to 30% of the total amount of Pt is supported on this zirconium oxide. It is desirable to do so. If the amount of Pt supported on such zirconium oxide is less than 10%, the effect of suppressing the consumption of the generated H ₂ cannot be sufficiently obtained. Conversely, if the amount exceeds 30%, the suppressing effect is saturated.
Further, the amount of Pt used is desirably 0.01 to 25 g / L per liter of the catalyst, and if it is less than 0.01 g / L, Pt selectively oxidizes and removes unburned HC and CO to form an H ₂ component. The improvement effect which raises a ratio cannot fully be acquired, but when 25 g / L is exceeded conversely, the improvement effect will be saturated.
In addition, in the combined use, in the zirconium oxide represented by the above formula (f5), if a + b exceeds 1.0, an additive element such as titanium is precipitated on the catalyst surface to reduce the catalytic activity of Pt. This is not preferable.

Here, the catalyst structure of the combined catalyst with the hydrogen generation catalyst and the CO / HC selective oxidation catalyst described above will be described in detail.
In the exhaust gas purification system of the present embodiment, the hydrogen generation catalyst which is an example of the hydrogen enrichment means and the combined catalyst with other components are an integral structure type catalyst using a Molinos carrier or the like, and the exhaust gas of the carrier A catalyst component that oxidizes HC and CO to reduce oxygen is arranged on the upstream side of the flow path, and a catalyst component that generates hydrogen is arranged on the downstream side so that the amount of oxygen in contact with the hydrogen generation catalyst component is reduced. It is preferable to adopt a simple configuration.

In the above catalyst, it is preferable to use Pd and / or Pt as a catalyst component that oxidizes HC / CO on the upstream side and reduces O ₂ in order to quickly cope with fluctuations in conditions such as space velocity and temperature. , Pd and / or Pt content is preferably 0.1 to 50 g / L per catalyst volume. In addition, you may use an alumina as a support | carrier as needed.
If the content of Pd and / or Pt is less than 0.1 g / L, sufficient catalytic activity may not be obtained. Conversely, if it exceeds 50 g / L, the catalytic activity is saturated.
In addition, in order to efficiently generate H ₂ from the residual HC and CO, it is preferable to contain Rh and zirconium oxide as downstream hydrogen generation catalyst components.
In order to quickly cope with fluctuations in conditions such as space velocity and temperature, the Rh content is preferably 0.1 to 50 g / L and the zirconium oxide content is preferably 10 to 300 g / L per catalyst volume. Zirconium oxide is used as a carrier.
If the Rh content is less than 0.1 g / L, sufficient catalytic activity may not be obtained. Conversely, if it exceeds 50 g / L, the catalytic activity is saturated. On the other hand, if the zirconium oxide content is less than 5 g / L, the effect of improving the rhodium catalyst performance cannot be sufficiently obtained. Conversely, if it exceeds 100 g / L, the catalytic activity is saturated.

Moreover, if the zirconium oxide containing the alkaline earth metal represented by the formula (f4) is used as the zirconium oxide and the electronic state of Rh is maintained in a suitable state, H ₂ is efficiently generated. be able to.
Further, as described above, it is also possible to contain Pd and cerium oxide to maintain the electronic state of Pd in a suitable state and promote more efficient generation of H ₂ .
In this case, the Pd content is preferably 0.01 to 50 g / L, and the cerium oxide (support) content is preferably 10 to 300 g / L. The Pd content of less than 0.1 g / L is sufficient. In some cases, the catalytic activity may not be obtained. On the other hand, if it exceeds 50 g / L, the catalytic activity is saturated. On the other hand, if the content of cerium oxide is less than 10 g / L, the effect of improving the catalytic performance of Pd may not be sufficiently obtained. Conversely, if it exceeds 300 g / L, the catalytic activity is saturated.

In the exhaust gas purification system of the present embodiment, the various hydrogen enriching means can be combined. In particular, the combustion control means (I) and the catalyst system hydrogen enriching means ((II) to (II) to Combination with (IV)) is effective.
That is, the combustion flow that flows into the hydrogen enriching means of the catalyst system by controlling the fuel injection amount of the internal combustion engine, the fuel injection timing, the ignition timing or the opening / closing timing of the intake / exhaust valve and any combination thereof by the combustion control means. If the gas or exhaust gas is adjusted to a reducing component excess atmosphere (rich region) such as hydrocarbon such that the Z value is intermittently 1.0 or less, the effect of generating H _{2 and} the like is further improved. It becomes easy to execute the gas composition control defined in the equations f1) to (f3).
The Z value described above represents the stoichiometric ratio between the oxidizing agent and the reducing agent, and is defined by the following equation.
Z = ([O2] × 2 + [NO]) / ([H2] × 2 + [CO] + [HC] × α)
Here, [O2], [NO], [H2], [CO] and [HC] indicate the concentrations of oxygen, nitric oxide, hydrogen, carbon monoxide and hydrocarbon, respectively, and α is the type of HC component Is a coefficient determined by.

Next, the NOx purification catalyst will be described in detail.
This NOx purification catalyst is arranged at the lower stage of the various hydrogen enriching means described above, and it is sufficient that NOx can be reduced by a reducing component such as hydrogen, and is not particularly limited. Or a NOx trap type NOx purification catalyst containing alkaline earth metal and any mixture thereof and Pt, Pd or Rh and any mixture thereof, and copper (Cu), cobalt (Co), nickel (Ni) , Iron (Fe), gallium (Ga), lanthanum (La), cerium (Ce), zinc (Zn), titanium (Ti), calcium (Ca), barium (Ba) or silver (Ag) and any of these It is classified as a selective reduction type NOx purification catalyst containing a mixture and Pt, iridium (Ir) or Rh and any mixture thereof.

As the catalyst that can use H ₂ as a reducing component with high efficiency, the former NOx trap type catalyst is desirable, and the exhaust gas purification system according to the present embodiment uses the NOx trap type.
In this case, cesium (Cs) is used as the alkali metal, magnesium (Mg), Ca, strontium (Sr) and / or Ba is used as the alkaline earth metal, and these are in a ratio of 10 to 70 g / L per 1 L of the catalyst in terms of oxide. It is preferable to contain. If the total of these contents is less than 10 g / L, sufficient performance cannot be obtained. Conversely, if the content exceeds 70 g / L, the NOx purification performance may deteriorate.
Further, the amount of noble metal supported such as Pt or Rh is preferably 0.01 to 25 g / L. If the amount of noble metal supported is less than 0.01 g / L, sufficient NOx purification performance may not be obtained. Conversely, if it exceeds 25 g / L, the catalytic activity is saturated.

In such a NOx trap type NOx purification catalyst, alkali metal or alkaline earth metal is supported by an impregnation method using a solution of a water-soluble salt such as acetate, or a poorly soluble salt such as carbonate or sulfate / Any method such as a mixing method of mixing an insoluble salt into a water-soluble slurry can be used.
In addition, as another example of the NOx purification catalyst, a catalyst containing at least Rh and having an activation temperature of 260 to 380 ° C. can be used. If this catalyst is used, NOx in a relatively low temperature exhaust gas can be used. Can also be effectively purified.
In various catalysts, as the porous substrate used for supporting the catalyst component, alumina, silica alumina or zeolite and any mixture thereof are suitable, and the specific surface area is particularly about 50 to 300 m ² / g. The activated alumina is preferred.
Furthermore, for the purpose of increasing the specific surface area of alumina, a rare earth element, zirconium or the like may be added thereto. The amount of the porous carrier used is preferably 50 to 300 g per liter of the catalyst.

Examples of the present invention will be described below.
In the exhaust gas purification system of the embodiment, as shown in FIG. 1, the hydrogen enrichment means 20, the NOx purification catalyst 30, and the HC trap catalyst 40 are arranged on the exhaust gas passage connected to the internal combustion engine 10. . Further, as the hydrogen enrichment means 20, a hydrogen generation / permeation catalyst 21 and an HC / CO selective oxidation catalyst 22 are arranged in series.
Here, the hydrogen generation / permeation catalyst 21 is a catalyst having a structure in which a hydrogen generation catalyst (II), a CO / HC selective oxidation catalyst (III), and a solid acidic oxide-containing catalyst (IV) are combined.
FIG. 5 shows an example of the structure. A hydrogen generation catalyst layer 220 containing Pd and cerium oxide is formed on the support 210, and CO / HC selective oxidation containing Pd supported on alumina or the like on the upstream side of the exhaust gas on the hydrogen generation catalyst layer 220. A catalyst layer 230 is formed, and on the downstream side of the exhaust gas on the hydrogen generation catalyst 220, a solid acidic oxide-containing catalyst layer 240 in which Rh is supported on a zirconium oxide support containing an alkaline earth metal is formed.

HC, CO, and H ₂ O in the exhaust gas are oxidized into CO by passing through the CO / HC selective oxidation catalyst layer 230. Further, when HC and CO reach the hydrogen generation catalyst layer 220, they react with H ₂ O to generate hydrogen. The produced hydrogen is discharged outside without being consumed by the solid acidic oxide-containing catalyst 240. Thus, this structure shows a function of generating hydrogen and permeating the generated hydrogen.
Note that most of the HC and CO not consumed by the hydrogen generation / permeation catalyst 21 is oxidized and consumed by the next HC / CO selective oxidation catalyst 22.

The internal combustion engine 10 can perform lean combustion. For example, a lean burn engine, a direct injection engine, a diesel engine, or the like is used. In the internal combustion engine 10, the fuel injection conditions and the like are controlled by the control device 15 in accordance with the operating state, and the air / fuel ratio is lean or rich.
Among the exhaust gas components generated when the air-fuel ratio of the internal combustion engine 10 is rich in the cold region where the exhaust gas temperature is low, some HC selects the H ₂ generation / permeation catalyst 21 and HC / CO. It is partially oxidized by the oxidation catalyst 22 and flows into the NOx purification catalyst 30 disposed in the subsequent stage. The NOx purification catalyst 30 disposed in the subsequent stage is a trap type catalyst and has a function of trapping NOx discharged when the air-fuel ratio of the internal combustion engine 10 is lean.

In the cold region where the exhaust gas temperature is low, NOx trapped when the air-fuel ratio is lean is reduced and purified by H ₂ flowing into the NOx purification catalyst 30 when the air-fuel ratio of the internal combustion engine 10 is rich. The
At this time, the HC flowing into the NOx purification catalyst 30 together with H ₂ is not consumed as a reducing agent for releasing and purifying NOx trapped in the NOx purification catalyst 30 because the exhaust gas temperature is low. pass. The passed HC is trapped by the hydrocarbon adsorbent of the HC trap catalyst 40 disposed at the subsequent stage of the NOx purification catalyst 30. The HC trapped in the HC trap catalyst 40 is released as the exhaust gas temperature rises and is purified by the three-way catalyst layer formed on the hydrocarbon adsorbent layer.

In the HC trap catalyst 40, the HC adsorbent that traps the inflowing HC is a porous material, and for example, zeolite is used. Depending on the size of the HC component molecules that flow in, the size of the pores of the HC trapping agent, that is, the trapping agent having a different pore diameter can be appropriately changed.
As described above, this exhaust gas purification system can prevent the outflow of HC into the atmosphere while purifying NOx in a cold region at a low temperature.

FIG. 6A shows an example of the HC trap characteristic when the exhaust gas purification system of this embodiment is used in a gasoline vehicle using a lean burn engine. Here, gasoline is used as the fuel gas.
As shown in FIG. 6A, the HC component generated in the internal combustion engine fueled with gasoline has 86% of the HC component having 3 or more carbon atoms (C number), and the C number also exists at the inlet of the HC trap catalyst 40. Is present in 82% of three or more components. Therefore, in this case, in order to prevent the outflow of HC in the cold region, it is necessary to reduce the HC component of C3 or higher.
Therefore, in this embodiment, β zeolite that effectively traps C3 or higher HC components is used as the adsorbent for the HC trap catalyst 40. As a result, as shown in FIG. 6A, HC trap catalyst 40 can reduce HC by 60%. It can also be seen that the reduction effect is due to trapping of HC components having a C number of 3 or more.

FIG. 6B shows an example of HC trap characteristics when the exhaust gas purification system of this embodiment is used in a diesel vehicle. Here, light oil is used as the fuel gas.
In FIG. 6B, the HC component generated in the internal combustion engine using light oil is different from the internal combustion engine using gasoline, and the HC component having a carbon number (C number) of less than 3 is as high as 40%. In contrast to gasoline, 60% of components with a C number of less than 3 are present at the inlet. Therefore, in an internal combustion engine that uses light oil as a fuel, it is necessary to reduce the HC component having a C number of less than 3 in order to prevent the outflow of HC in the cold region.
Therefore, in this embodiment, ferrierite that efficiently traps HC components less than C3 is used as the adsorbent for the HC trap catalyst 40. As a result, as shown in FIG. 6B, the HC trap catalyst 40 can reduce HC by 50%. Further, it can be seen that the reduction effect is due to trapping of HC components having a C number of less than 3.

  As described above, due to the difference in the internal combustion engine using the exhaust gas purification system of the present invention, by appropriately changing the HC trapping agent of the HC trap catalyst, the outflow of HC into the atmosphere in the cold region can be prevented in advance. It becomes possible to do.

It is a block diagram of the exhaust gas purification system which has a hydrogen enrichment means, an NOx purification catalyst, and an HC trap catalyst according to an embodiment of the present invention. It is a figure which shows the mechanism of NOx purification in the NOx purification catalyst which concerns on embodiment of this invention. It is the external view which shows an example of the HC trap catalyst which concerns on embodiment of this invention, and a partial expanded sectional view. It is a figure which shows molecular diameter distribution of HC contained in exhaust gas. It is a figure which shows the structural example of the hydrogen production | generation and permeation | transmission catalyst which is one of the hydrogen enrichment means based on the Example of this invention. A is a graph showing the HC concentration in the exhaust gas at each place when the gasoline engine is used, and B is a graph showing the HC concentration in the exhaust gas at each place when the diesel engine is used.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Engine 15 Control apparatus 20 Hydrogen enrichment means 21 H2 production | generation / permeation catalyst 22 HC / CO selective oxidation catalyst 30 NOx purification catalyst 35 Noble metal catalyst 36 Adsorbent 40 HC trap catalyst 41 Monolith support 42 Hydrocarbon adsorbent layer 43 Metal catalyst Layer 210 Support 220 Hydrogen generation catalyst layer 230 CO / HC selective oxidation catalyst layer 240 Solid acidic oxide-containing catalyst layer

Claims (16)

A hydrogen enrichment means arranged in the combustion gas or exhaust gas flow path to increase the hydrogen component ratio to the reducing gas component in the exhaust gas;
A NOx purification catalyst disposed after the hydrogen enrichment means on the exhaust gas flow path;
And an HC trap catalyst disposed after the NOx purification catalyst on the exhaust gas flow path,
Exhaust gas always flows directly from the hydrogen enrichment means to the NOx purification catalyst,
The hydrogen enriching means includes combustion control means for generating hydrogen by controlling at least one of a fuel injection amount, fuel injection timing, ignition timing, and intake / exhaust valve opening / closing timing of an internal combustion engine. Exhaust gas purification system.   The exhaust gas purification system according to claim 1, wherein the hydrogen enriching means includes hydrogen generating means.   The exhaust gas purification system according to claim 1, wherein the hydrogen enriching means includes means for reducing a reducing component other than hydrogen in the exhaust gas.   The exhaust gas purification system according to claim 1, wherein the hydrogen enriching means includes a hydrogen generation catalyst that promotes a reaction of generating hydrogen from an exhaust gas component.   The exhaust gas purification system according to claim 4, wherein the hydrogen generation catalyst contains a noble metal related to Pt, Pd or Rh and any mixture thereof. The exhaust gas purification system according to claim 1, wherein the hydrogen enriching means includes a catalyst containing zirconium oxide represented by the following general formula (f5).
[Y] dZreOf (f5)
Here, Y in the formula is at least one element selected from the group consisting of titanium, aluminum, tungsten, molybdenum and zinc,
d and e are atomic ratios of the respective elements, f is the number of oxygen atoms necessary to satisfy the valences of Y and Zr, and d = 0.01 to 0.5, e = 0.5 to 0.99 , D + e = 1 is satisfied.   The exhaust gas purification system according to claim 1, wherein the HC trap catalyst has a hydrocarbon adsorbent layer and a three-way catalyst layer formed on the hydrocarbon adsorbent layer.   The exhaust gas purification system is mounted on a gasoline vehicle, and the hydrocarbon adsorbent layer of the HC trap catalyst includes zeolite that mainly traps HC having a molecular diameter of 0.4 nm or more as a main component. The exhaust gas purification system according to claim 7.   The exhaust gas purification system according to claim 8, wherein the zeolite is any one of β-zeolite and ZSM-5.   The exhaust gas purification system is mounted on a diesel vehicle, and the hydrocarbon adsorbent layer of the HC trap catalyst comprises, as a main component, zeolite that mainly traps HC having a molecular diameter of less than 0.4 nm. The exhaust gas purification system according to claim 7, comprising:   The exhaust gas purification system according to claim 10, wherein the zeolite is any one of ferrierite, A-type zeolite, and B-type zeolite. The exhaust gas purification system is mounted on a diesel vehicle,
The hydrogen enriching means has an HC / CO oxidation catalyst that promotes a reaction of oxidizing HC or CO in the exhaust gas,
8. The exhaust gas purification system according to claim 7, wherein the hydrocarbon adsorbent layer of the HC trap catalyst contains, as a main component, zeolite that mainly traps HC having a molecular diameter of less than 0.4 nm. The exhaust gas purification system according to claim 12, wherein the HC / CO oxidation catalyst includes a catalyst containing a zirconium oxide represented by the following chemical formula (f4).
[X] aZrbOc (f4)
Here, X in the formula is at least one alkaline earth metal selected from the group consisting of magnesium, calcium, strontium and barium,
a and b are atomic ratios of the respective elements, c is the number of oxygen atoms required to satisfy the valences of X and Zr, and a = 0.01 to 0.5, b = 0.5 to 0.99 A + b = 1.   The exhaust gas purification system according to claim 12 or 13, wherein the zeolite is any one of ferrierite, A-type zeolite, and B-type zeolite. When the NOx purification catalyst performs NOx purification, hydrogen enrichment is performed by the hydrogen enriching means, and the hydrogen concentration [H2] and the total reducing component concentration [TR] in the exhaust gas are expressed by the following equation (f1) and Formula (f2)
[H2 / TR] d> [H2 / TR] u (f1)
[H2 / TR] d ≧ 0.3 (f2)
(Where [H2 / TR] u is the ratio of the hydrogen concentration [H2] u to the total reducing component concentration [TR] u before or upstream of the hydrogen enrichment of the hydrogen enriching means, [H2 / TR] d 15 represents the ratio of the hydrogen concentration [H2] d and the total reducing component concentration [TR] d at the inlet of the NOx purification catalyst). Exhaust gas purification system. The ratio of the hydrogen concentration [H2] d to the carbon monoxide concentration [CO] d in the total reducing component concentration [TR] d at the inlet of the NOx purification catalyst during NOx purification is expressed by the following equation (f3).
[H2 / CO] d> 1 (f3)
The exhaust gas purification system according to any one of claims 1 to 15, wherein the exhaust gas purification system is satisfied.

Images