JP2009162145A - Catalyst device for purification of exhaust gas - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To restrain unpurified HC from being discharged to the atmosphere at the cooling of an engine. <P>SOLUTION: An HC adsorption type catalyst 4 having a zeolite-based HC absorption material layer 7 and three-dimensional catalyst layers 8, 9 is provided in an exhaust gas passage, and a Zr-based basic composite oxide layer 6 containing alkaline earth metal is arranged in the exhaust gas passage on an upstream side of the HC adsorption type catalyst 4. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

    The present invention relates to an exhaust gas purifying catalyst device.

    In recent years, regarding engine emissions, reduction of HC (hydrocarbon) emissions when the engine is cold is strongly desired. In contrast, a three-way catalyst in which a catalyst metal is supported on alumina or the like has a relatively high temperature at which it becomes active, so that it can sufficiently purify HC discharged from the engine when the engine is cold. Can not.

    Therefore, an HC adsorption type catalyst combining an HC adsorbent and a three-way catalyst has been developed. That is, HC is adsorbed by the HC adsorbent when the three-way catalyst is not activated and the HC adsorbed from the HC adsorbent is subsequently purified by the three-way catalyst. That's it. However, HC adsorbents typified by zeolite tend to start desorbing HC adsorbed (trapped) in the pores when the engine is cold before the three-way catalyst is activated (HC desorption temperature is not high). ). Therefore, even if it is an HC adsorption type catalyst, the HC purification rate is not necessarily high at present.

On the other hand, for example, Patent Document 1 discloses that neutral or basic zeolite obtained by supporting alkali metal or alkaline earth metal on zeolite by ion exchange has an exhaust gas temperature of 150 ° C. as compared with acidic zeolite. It describes that the HC adsorption ability is high, and that the release temperature (desorption temperature) of the adsorbed HC is 175 ° C. or higher, and further 200 ° C. or higher. Patent Document 2 discloses a catalyst composition A containing a zeolite adsorbent, a desorption temperature improving component containing at least one of Ag, Cu, Co, Ni, Sr and Mg, and a combustion catalyst active component, A catalyst composition B containing the zeolite adsorbent and a combustion catalyst active component, and a mixture of the catalyst composition A and the catalyst composition B is supported on a honeycomb carrier, or the catalyst composition A and the catalyst composition By laminating B with the honeycomb carrier, the HC desorption temperature of the zeolite adsorbent is increased, and the above metal solution such as Sr and Mg is added to the zeolite adsorbent and mixed uniformly, followed by drying and firing. Thus, it is described that the desorption temperature improving component is supported on a zeolite adsorbent.
JP 2000-502282 Gazette JP 2004-8855 A

    However, when the HC desorption temperature of the HC adsorbent exceeds 175 ° C. or 200 ° C. as described in Patent Document 1, it is normal that the three-way catalyst has not yet been sufficiently active. There is no great expectation regarding the prevention of unpurified HC emissions.

    On the other hand, Patent Document 2 discloses Sr and Mg as desorption temperature improving components. However, in the catalyst composition A of Patent Document 2, since the metal solution is mixed with a zeolite adsorbent and dried and calcined, it is considered that the catalyst composition A is an oxide of SrO or MgO. Such an alkaline earth metal oxide becomes carbonate when the air-fuel ratio of the exhaust gas becomes rich, and becomes nitrate when the air-fuel ratio becomes lean. That is, the alkaline earth metal becomes a different compound depending on whether the air-fuel ratio of the exhaust gas when the engine is stopped is rich or lean, so it is effective as a desorption temperature improving component at the next cold engine start. It is thought that there will be cases that do not work.

    Furthermore, as already known, since zeolite as an HC adsorbent is bulky, when a catalyst layer containing this HC adsorbent is formed on a support, the catalyst layer becomes thick. In that case, when the desorption temperature improving component is mixed with the catalyst layer, the layer thickness is further increased, and the cross-sectional area of the exhaust gas flow path is reduced, the engine back pressure is increased, the engine output is reduced, and the fuel consumption is reduced. There is concern that the rate will deteriorate.

    Therefore, the present invention has an object to reliably increase the HC desorption temperature of the HC adsorbent without increasing the engine back pressure and to suppress the discharge of unpurified HC to the atmosphere when the engine is cold. To do.

    In order to solve such problems, the present invention employs a Zr-based basic complex oxide as a component for increasing the temperature at which HC is desorbed from the HC adsorbent, and mixes this with the HC adsorbent. Instead, it is arranged in the exhaust gas passage upstream of the HC adsorbent.

That is, the present invention provides an exhaust gas purifying catalyst device in which an HC adsorption type catalyst having a zeolite-based HC adsorbent layer and a three-way catalyst layer is provided in an exhaust gas passage.
A Zr-based basic complex oxide containing Zr as a main component and containing an alkaline earth metal is disposed in the exhaust gas passage upstream of the HC adsorption catalyst.

    With such an exhaust gas purification catalyst device, the amount of HC that is adsorbed by the HC adsorbent and then discharged without being purified by the three-way catalyst is reduced. This is presumably because the Zr-based composite oxide exhibits strong basicity by containing an alkaline earth metal. More specifically, this Zr-based basic complex oxide has a function of drawing out hydrogen from allylic carbon of HC having a double bond in exhaust gas to generate carbanion. And since the HC becomes carbanion, it is relatively strongly adsorbed on the Lewis acid sites (on Si) in the pores of the HC adsorbent (zeolite) and is difficult to desorb from the HC adsorbent (the desorption temperature increases). ). As a result, the proportion of HC desorbed from the HC adsorbent after the three-way catalyst becomes active increases, and the amount of HC oxidized and purified by the three-way catalyst increases, that is, remains unpurified. The amount of HC discharged into the atmosphere is reduced.

    In this case, what is important is that the alkaline earth metal is not merely an oxide, but constitutes a composite oxide containing Zr as a main component. The composition or structure of the Zr-based basic composite oxide containing an alkaline earth metal does not change even when the air-fuel ratio of the exhaust gas changes. Therefore, the effect of increasing the HC desorption temperature is not affected by the change in the air-fuel ratio of the exhaust gas, and the catalyst device can be expected to surely improve the HC purification performance in the cold state.

    The important thing is that the Zr-based basic composite oxide is not mixed with the HC adsorbent or the three-way catalyst of the HC adsorption catalyst, but the exhaust gas upstream of the HC adsorption catalyst. It is arranged in the passage. Therefore, the proportion of HC that is carbanionized by the Zr-based basic complex oxide and supplied to the HC adsorbent, that is, HC that strongly adsorbs to the HC adsorbent (HC having a high desorption temperature) increases. Further, since the Zr-based basic complex oxide is disposed upstream of the HC adsorption catalyst, the HC adsorbent layer or the three-way catalyst layer becomes a layer thickness due to the Zr-based complex oxide. This avoids the problem of increased back pressure of the engine.

    The Zr-based basic composite oxide can be arranged away from the HC adsorption catalyst upstream in the exhaust gas flow direction. For example, each of the Zr-based basic complex oxide and the HC adsorption catalyst is supported on separate honeycomb carriers, and the former is disposed upstream of the exhaust gas passage and the latter is disposed downstream thereof.

    Alternatively, the Zr-based basic composite oxide and the HC adsorption catalyst can be arranged on the upstream side and the downstream side of the same honeycomb carrier. That is, the Zr-based basic composite oxide is supported on the upstream portion of the honeycomb carrier in the exhaust gas flow direction, and the HC adsorbent layer and the three-way catalyst layer of the HC adsorption catalyst are formed on the downstream portion of the honeycomb carrier. It is the structure to do.

    In particular, when a configuration in which each of the Zr-based basic composite oxide and the HC adsorption catalyst is supported on separate honeycomb carriers is used, a longer honeycomb carrier is prepared than a case where both are supported on the same honeycomb carrier. There is no need, and the loading on each of the honeycomb carriers is facilitated. Moreover, even if a long honeycomb carrier is not used, a Zr-based basic composite oxide can be formed in a thin layer on the honeycomb carrier to facilitate contact with the exhaust gas, thereby achieving HC carbanionization. This is advantageous in increasing the HC desorption temperature from the HC adsorbent.

    As the HC adsorbent, β-type, mordenite-type, Y-type or pentasil-type zeolite is preferably employed.

    As the alkaline earth metal of the Zr-based composite oxide, at least one selected from Mg, Ca, Sr and Ba can be used. The content of the alkaline earth metal R in the Zr-based composite oxide is preferably 1.5 atomic mol% or more and 12 atomic mol% or less.

    As described above, according to the present invention, the HC adsorption type catalyst is arranged in the exhaust gas passage, and the Zr-based basic composite containing an alkaline earth metal is disposed in the exhaust gas passage upstream of the HC adsorption type catalyst. Since the oxide is disposed, the HC in the exhaust gas can be carbanionized without being affected by the air-fuel ratio of the exhaust gas, and the HC can be strongly adsorbed on the HC adsorbent. By increasing the ratio of HC desorbed from the HC adsorbent after the catalyst becomes active, the amount of HC discharged into the atmosphere without being purified by the three-way catalyst can be reduced, It is possible to avoid an increase in the passage resistance of the exhaust gas passage.

    Hereinafter, embodiments of the present invention will be described with reference to the drawings. It should be noted that the following description of the preferred embodiment is merely illustrative in nature, and is not intended to limit the present invention, its application, or its use.

In FIG. 1, reference numeral 1 denotes an engine exhaust gas passage. In the exhaust gas passage 1, a front three-way catalyst 2, a carbanionizing material 3, and an HC adsorption catalyst 4 are sequentially arranged from the upstream side to the downstream side in the exhaust gas flow direction. The carbanionizing material 3 includes a Zr-based basic complex oxide containing Zr as a main component and containing an alkaline earth metal, and dissociates hydrogen bonded to HC carbon in the exhaust gas as H ^+. Generate carbanions. The HC adsorption-type catalyst 3 includes a zeolite-based HC adsorbent and a three-way catalyst, adsorbs HC with the HC adsorbent when the engine is not activated and the three-way catalyst is cold, and then raises the exhaust gas temperature. Accordingly, HC desorbed from the HC adsorbent is purified by the three-way catalyst.

    FIG. 2 shows an example of the structure of the carbanionizing material 3 and the HC adsorption catalyst 4 (Example 1). In the carbanionized material 3, a Zr-based basic complex oxide layer 6 containing an alkaline earth metal is formed on the pore wall surface of the honeycomb carrier 5. In the HC adsorption catalyst 4, the HC adsorbent layer 7 and the three-way catalyst layers 8 and 9 are laminated on the pore wall surfaces of the honeycomb carrier 5 separate from the carbanionized material 3. In this example, the HC adsorbent layer 7 is disposed in the lowermost layer, and upper and lower two-way three-way catalyst layers 8 and 9 having different compositions are laminated thereon.

    With the above configuration, the Zr-based basic complex oxide layer 6 is disposed away from the HC adsorption catalyst 4 on the upstream side in the exhaust gas flow direction.

<Increase effect of HC desorption temperature by Zr-based basic complex oxide>
One of the features of the present invention is that HC is carbanionized by a Zr-based basic complex oxide, thereby increasing the desorption temperature of the HC when the HC is adsorbed on the HC adsorbent. is there.

The effect of this Zr-based basic complex oxide will be described based on the experimental results shown in FIG. In this experiment, as shown in FIG. 4, a gas containing toluene as HC (toluene concentration 2500 ppmC, remaining N ₂ ) was supplied to the test material at a temperature of 50 ° C. for 900 seconds, and then the flow gas was supplied. The temperature was increased at a rate of 30 ° C./min with N ₂ alone, and the change in the concentration of toluene desorbed from the test material at that time was measured.

    The test materials are those in which a SrZr composite oxide is supported on the upstream side of the honeycomb carrier in the exhaust gas flow direction, and β-zeolite as an HC adsorbent is supported on the downstream side thereof. There are three types: one in which β-zeolite is mixed and supported, and one in which β-zeolite is supported on a honeycomb carrier (no SrZr composite oxide). For supporting them, zirconyl nitrate was used as a binder. None of the test materials contained a three-way catalyst. The Sr content (Sr / (Sr + Zr)) of the SrZr composite oxide is 11.4 atomic mol%.

    According to FIG. 3, in the case of β-zeolite alone, the concentration of desorbed toluene peaks at around 200 ° C., and thereafter the toluene concentration decreases as the temperature rises, and the toluene concentration is almost higher at temperatures higher than 300 ° C. It is zero. On the other hand, in each of the test materials having the SrZr composite oxide, after the first toluene desorption peak appears around 200 ° C., the second desorption peak appears around 300 ° C. to 320 ° C. Further, desorption of toluene continues even at 420 ° C.

    It is recognized that the second desorption peak and toluene desorbed on the high temperature side are carbanionized by the SrZr composite oxide and adsorbed on β-zeolite. FIG. 3 shows that the effect of increasing the desorption temperature of toluene is higher when the SrZr composite oxide is arranged upstream of the β-zeolite than when the SrZr composite oxide and β-zeolite are mixed.

<Cold HC purification performance>
Example 1
In the catalyst device shown in FIG. 2 (the upstream side three-way catalyst 2 shown in FIG. 1 is not provided), the following material configuration was adopted. The unit (g / L) is the mass per 1 L of honeycomb carrier.

Zr-based basic composite oxide layer 6; SrZr composite oxide 150 g / L
(Sr content 11.4 atomic mol%)
HC adsorbent layer 7; β-zeolite 160 g / L
Upper three-way catalyst layer 8; CeO ₂ 5.5 g / L
ZrCeNd composite oxide 5.7 g / L
Pd / La containing alumina 45.45 g / L
(Pd 0.45 g / L)
Pd / CeZrLaY alumina composite oxide 22.85 g / L
(Pd 0.25 g / L)
Lower three-way catalyst layer 9; Rh / ZrCeNd composite oxide 70.1 g / L
(Rh 0.1g / L)
Rh / ZrLa composite oxide supported alumina 30.05 g / L
(Rh 0.05g / L)
La-containing alumina 13g / L
“Pd /” and “Rh /” mean that Pd or Rh is supported on the base material described next to “/”.

The composition of the ZrCeNd composite oxide of the upper three-way catalyst layer 8 is ZrO ₂ : CeO ₂ : Nd ₂ O ₃ = 55: 35: 10 (mass ratio), and the composition of the ZrCeNd composite oxide of the lower three-way catalyst layer 9 Is ZrO ₂ : CeO ₂ : Nd ₂ O ₃ = 80: 10: 10 (mass ratio). The composition of the CeZrLaY alumina composite oxide was Al ₂ O ₃ : CeO ₂ : ZrO ₂ : La ₂ O ₃ : Y ₂ O ₃ = 80.1: 10.4: 7.4: 1.7: 0.4 (mass Ratio). The La ₂ O ₃ content of the La-containing alumina is 4% by mass in both of the three-way catalyst layers 8 and 9. The composition of the ZrLa composite oxide-supported alumina is ZrO ₂ : La ₂ O ₃ : Al ₂ O ₃ = 38: 2: 60 (mass ratio). Further, zirconyl nitrate was used as a binder for forming each of the layers 6 to 9 such as the Zr-based basic composite oxide layer 6.

-Example 2-
Instead of the catalyst device shown in FIG. 2, this example has the catalyst device configuration shown in FIG. That is, the carbanionizing material 3 is the same as that of Example 1, but in the HC adsorption type catalyst 4, the HC adsorbing material layer 7 is arranged on the upstream side of the honeycomb carrier 5 in the exhaust gas flow direction, and the three way catalyst is arranged on the downstream side. Layers 8 and 9 were placed. Accordingly, in this example as well, the Zr-based basic complex oxide layer 6 is disposed away from the HC adsorption catalyst 4 on the upstream side in the exhaust gas flow direction. The material composition of the Zr-based basic complex oxide of the carbanionized material 3, the HC adsorbent of the HC adsorption catalyst 4 and the upper and lower three-way catalysts, and the binder used are the same as those in Example 1.

Example 3
Instead of the catalyst device shown in FIG. 2, this example has the catalyst device configuration shown in FIG. That is, the carbanionizing material 3 and the HC adsorption catalyst 4 are arranged on the upstream side and the downstream side in the exhaust gas flow direction in the same honeycomb carrier 5. The material composition of the Zr-based basic complex oxide of the carbanionized material 3, the HC adsorbent of the HC adsorption catalyst 4 and the upper and lower three-way catalysts, and the binder used are the same as those in Example 1.

-Comparative example-
Instead of the catalyst device shown in FIG. 2, this example has the catalyst device configuration shown in FIG. That is, a mixed layer 11 of Zr-based basic complex oxide and HC adsorbent and upper and lower three-way catalyst layers 8 and 9 were laminated on the honeycomb carrier 5. The mixed layer 11 is disposed in the lowermost layer, and upper and lower two-way three-way catalyst layers 8 and 9 are laminated thereon. The material composition of the Zr-based basic complex oxide, the HC adsorbent and the upper and lower three-way catalysts, and the binder used are the same as those in Example 1.

As shown in FIG. 4, a toluene-containing gas (toluene concentration 2500 ppmC, residual N ₂ ) was allowed to flow for 900 seconds at a temperature of 50 ° C. to adsorb toluene to each of the catalyst devices of Examples 1 to 3 and the comparative example. . The amount of toluene flowed for 900 seconds at that time is A, and the amount of toluene adsorbed on the catalyst device (HC adsorbent) is B. The toluene adsorption amount B was determined from the amount of toluene passed through the catalyst device. Next, simulated exhaust gas with A / F = 14.7 (CO ₂ : 13.9%, O ₂ : 0.6%, CO: 0.6%, H ₂ : 0.2%, NO: 1000 ppm, H The gas temperature was increased at a rate of 30 ° C./min while flowing ₂ O: 10% and the balance: N ₂ (without HC) into the catalyst device, and the amount of toluene C flowing out from the catalyst device was measured. . Then, [(BC) × 100 / A] was determined as the cold HC purification rate of each test catalyst device. The results are shown in Table 1.

    In each of Examples 1 to 3 in which the Zr-based basic composite oxide is disposed upstream of the HC adsorbent and the three-way catalyst, the cooling is less than the comparative example in which the Zr-based basic composite oxide and the HC adsorbent are mixed. The HC purification rate is high. This is because in Examples 1 to 3, the amount of toluene supplied to the HC adsorbent in a state carbanionized by the Zr-based basic composite oxide increased, that is, strongly adsorbed to the HC adsorbent. This is because the amount of HC desorbed from the HC adsorbent and purified by the three-way catalyst after the three-way catalyst became active increased.

    The cold HC purification rate in Examples 1 and 2 is higher than that in Example 3. In Examples 1 and 2, the Zr-based basic composite oxide layer 6 is formed on a honeycomb carrier 5 different from the HC adsorption catalyst 4. It is recognized that it was formed to be a thin layer. That is, the Zr-based basic composite oxide layer 6 of Examples 1 and 2 is thinner than the Zr-based basic composite oxide layer 6 of Example 3, and the contact area with the toluene-containing gas is widened. This is thought to be due to the high efficiency of toluene being carbanionized.

    The reason why the cold HC purification rate of Example 1 is higher than that of Example 2 is that all toluene desorbed from the HC adsorbent layer 7 in the lowermost layer passes through the upper three-way catalyst layers 8 and 9. It is considered that the three-way catalyst layers 8 and 9 work efficiently for purification of the desorbed toluene.

<Influence of alkaline earth metal content of Zr-based basic complex oxide>
A plurality of SrZr composite oxides having different Sr contents are prepared as Zr-based basic composite oxides, and the effect of the Sr content on HC carbanionization (HC desorption characteristics in the HC adsorbent) is shown in FIG. It investigated by the test method shown in. That is, the test material had a configuration in which the SrZr composite oxide was supported on the upstream side of the honeycomb carrier in the exhaust gas flow direction and β-zeolite (HC adsorbent) was supported on the downstream side. Then, after supplying toluene-containing gas (toluene concentration 2500 ppmC, residual N ₂ ) at a temperature of 50 ° C. for 900 seconds to adsorb toluene, the temperature is increased at a rate of 30 ° C./min using only N ₂ as the flow gas. The amount of toluene desorbed from the HC adsorbent in the temperature range of 300 ° C. or higher was measured.

The results are shown in FIG. According to the figure, as the Sr content increases, the toluene desorption amount at 300 ° C. or higher increases, and the increase in the Sr content indicates that HC is carbanionized (the HC desorption temperature from the HC adsorbent). It can be seen that it is advantageous to increase the Further, it can be said that the Sr content is preferably 2 atomic mol% or more, and more preferably 6.25 atomic mol% or more. The upper limit of the Sr content may be set to 12 atomic mol% as a guide from the relationship of the solid solution amount of SrO to ZrO ₂ .

    In the above embodiment, the three-way catalyst has a structure composed of two upper and lower layers having different compositions. However, a three-way catalyst having the same composition may have a single-layer structure.

It is a figure which shows the structure of the catalyst apparatus for exhaust gas purification which concerns on embodiment of this invention. 1 is a cross-sectional view illustrating a configuration of a catalyst device according to Example 1. FIG. It is a graph which shows the temperature characteristic of toluene desorption of various test materials. It is a graph which shows the time-dependent change of the gas temperature which flows into a test material in the adsorption / desorption (purification) test of toluene, and the time-dependent change of the toluene concentration which flows out from a test material. 6 is a cross-sectional view showing a configuration of a catalyst device according to Example 2. FIG. 6 is a cross-sectional view showing a configuration of a catalyst device according to Example 3. FIG. It is sectional drawing which shows the structure of the catalyst apparatus which concerns on a comparative example. It is a graph which shows the relationship between Sr content of Zr type | system | group basic complex oxide, and the toluene desorption amount in 300 degreeC or more of HC adsorbent.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Exhaust gas passage 2 Pre-placement three way catalyst 3 Carbanion-ized material 4 HC adsorption type catalyst 5 Honeycomb support 6 Zr system basic complex oxide layer 7 HC adsorbent layer 8 Three way catalyst layer 9 Three way catalyst layer

Claims (3)

In the exhaust gas purification catalyst device in which an HC adsorption type catalyst provided with a zeolite-based HC adsorbent layer and a three-way catalyst layer is provided in the exhaust gas passage,
A Zr-based basic complex oxide containing Zr as a main component and containing an alkaline earth metal is disposed in the exhaust gas passage upstream of the HC adsorption catalyst. Catalytic device. In claim 1,
The exhaust gas purifying catalyst device, wherein the Zr-based basic complex oxide is disposed away from the HC adsorption catalyst upstream in the exhaust gas flow direction. In claim 1,
The exhaust gas purifying catalyst device, wherein the Zr-based basic composite oxide and the HC adsorption catalyst are arranged on the upstream side and the downstream side in the exhaust gas flow direction of the same honeycomb carrier.

Images