JP2014213269A - Exhaust gas purification catalyst and exhaust gas purification device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an exhaust gas purification catalyst and an exhaust gas purification device capable of particularly improving an HC purification rate in a lean atmosphere, when an LNT and a noble metal/an acidic carrier are supported on the same support.SOLUTION: There is provided an exhaust gas purification catalyst for purifying an exhaust gas of an internal combustion engine which comprises a first catalyst and a second catalyst supported on the same support, wherein the first catalyst has a carrier having a solid acid and a noble metal supported on the carrier and the second catalyst has an NOx adsorbent which comprises: CeOand substantially contains neither an alkaline earth metal nor an alkali metal; and a noble metal.

Description

  The present invention relates to an exhaust purification catalyst and an exhaust purification device.

  Conventionally, stoichiometric combustion is performed on the high load side and lean combustion is performed on the low load side, such as an internal combustion engine (hereinafter referred to as “engine”) that performs premixed compression ignition combustion (hereinafter referred to as “HCCI”). Engines that perform are known. In such an engine, it is necessary to purify the exhaust gas both in a stoichiometric atmosphere and a lean atmosphere. In a stoichiometric atmosphere, exhaust gas can be purified by using a three-way catalyst, but in a lean atmosphere, exhaust gas cannot be sufficiently purified even if such a three-way catalyst is used. In this case, in particular, nitrogen oxides (hereinafter referred to as “NOx”) and hydrocarbons (hereinafter referred to as “HC”) contained in the exhaust gas cannot be sufficiently purified.

  Therefore, a lean NOx purification catalyst (hereinafter referred to as “LNT”) containing a NOx adsorbent composed of alkali metal or alkaline earth metal is known as a catalyst that can efficiently purify NOx even in a lean atmosphere (for example, “LNT”). , See Patent Document 1). According to this NOx purification catalyst, NOx in exhaust gas is adsorbed by a NOx adsorbent in a lean atmosphere, and the adsorbed NOx is released in a stoichiometric or rich atmosphere, so that NOx can be purified.

  Further, as a catalyst capable of efficiently purifying HC in a lean atmosphere, an exhaust purification catalyst (hereinafter referred to as “noble metal / acidic carrier”) in which a noble metal such as Pt is supported on a carrier having solid acidity is known (hereinafter referred to as “noble metal / acidic carrier”). For example, see Patent Document 2). According to this noble metal / acidic carrier, HC can be efficiently purified even in a lean atmosphere due to the high acidity of the acidic carrier.

  Therefore, by arranging the support supporting LNT and the support supporting the noble metal / acidic carrier in series in the exhaust passage, NOx and HC can be efficiently purified even in a lean atmosphere. However, it is desirable to support LNT and a noble metal / acidic carrier on the same support from the viewpoints of restrictions on the exhaust layout and cost increase due to the use of two general honeycomb supports as the support.

Japanese Patent No. 2600492 Special table 2010-506712 gazette

  However, even if the conventional LNT and the noble metal / acidic carrier are simply supported on the same support, the effect of the combination is not sufficiently exhibited, and the improvement of the HC purification rate in a lean atmosphere is not particularly recognized. there were. This is an important issue to be solved in view of recent strict regulations on HC emissions.

  The present invention has been made in view of the above, and an object of the present invention is to provide an exhaust purification catalyst and an exhaust capable of improving the HC purification rate particularly in a lean atmosphere when LNT and a noble metal / acidic carrier are supported on the same support. It is to provide a purification device.

To achieve the above object, the present invention provides an exhaust purification catalyst for purifying exhaust gas from an internal combustion engine, comprising a first catalyst and a second catalyst carried on the same support, wherein the first catalyst is a solid catalyst. An NOx adsorbent containing CeO ₂ and substantially free of alkaline earth metal and alkali metal, and a noble metal having a support having acidity and a noble metal supported on the support; And an exhaust purification catalyst characterized by comprising:

Conventionally, when a noble metal / acid carrier is used alone, the reason why a high HC purification rate can be obtained in a lean atmosphere is as follows.
That is, in the noble metal / acidic carrier, the electron density on the surface of the noble metal is lowered by the electron withdrawing action based on the high acidity of the acidic carrier. Then, the binding force between oxygen and noble metal present in a large amount in the lean atmosphere is reduced, and adsorption of oxygen on the surface of the noble metal is suppressed. By suppressing the adsorption of oxygen, HC can be directly adsorbed on the surface of the noble metal, so that a high HC purification rate can be obtained.

On the other hand, conventionally, even if LNT and noble metal / acidic carrier are simply supported on the same support, the effect of the combination is not sufficiently exhibited, and the improvement of the HC purification rate especially in a lean atmosphere is not recognized. Is as follows.
That is, conventionally, since the alkali metal and alkaline earth metal constituting the NOx adsorbent in LNT are basic, the electron donating action of these substances inhibits the electron withdrawing action of the acidic carrier, and the electron density on the surface of the noble metal Becomes higher. Then, the bonding force between the noble metal and oxygen is increased, and the adsorption of oxygen to the surface of the noble metal is promoted. In addition, for example, exhaust discharged from an engine during lean operation contains more paraffin as HC than during stoichiometric operation, and paraffin is structurally less reactive than olefins and aromas. For this reason, the purification rate of paraffin is greatly reduced. Thereby, the purification rate of HC falls.

Therefore, in the present invention, as the noble metal / acid carrier, the first catalyst in which the noble metal is supported on the carrier having solid acidity, CeO ₂ as the LNT, and substantially alkaline earth metal and alkali metal are contained. And an NOx adsorbent and a second catalyst having a noble metal to constitute an exhaust purification catalyst. That is, the first catalyst and the second catalyst are mixed or stacked, and the alkaline earth metal and the alkali metal are not contained in the NOx adsorbent constituting the second catalyst as the LNT. Instead, CeO ₂ Containing.
As a result, CeO ₂ has a lower basicity than alkali metals and alkaline earth metals, so that it does not significantly affect the electron withdrawing action of the acidic carrier and can reduce the electron density on the surface of the noble metal. . As a result, since the bonding force between the noble metal and oxygen can be reduced and the adsorption of oxygen to the surface of the noble metal can be suppressed, the purification rate of paraffin can be particularly improved, and a high HC purification rate can be obtained.
CeO ₂ also has an oxygen storage capacity (OSC) in addition to the NOx adsorption capacity, and releases active oxygen at a predetermined temperature or higher. Since this active oxygen is very reactive, according to the present invention, oxidation purification of paraffin can be promoted even in a lean atmosphere, and the HC purification rate can be improved.

  In this case, it is preferable to include a first catalyst layer formed by the first catalyst and a second catalyst layer adjacent to the first catalyst layer and formed by the second catalyst.

  In the present invention, the first catalyst layer is formed using the first catalyst, and the second catalyst layer is formed using the second catalyst adjacent to the first catalyst layer. As a result, the above-described effects are exhibited particularly at the interface between the first catalyst layer and the second catalyst layer.

  In this case, the first catalyst layer is preferably formed on the second catalyst layer.

In the present invention, the first catalyst layer is formed on the second catalyst layer. That is, the first catalyst layer is formed as an upper layer and the second catalyst layer is formed as a lower layer. Thereby, compared with the case where the first catalyst layer containing the noble metal / acidic carrier is used as the lower layer, the upper layer has better gas diffusibility, so that paraffin can be purified efficiently and the HC purification rate can be improved. Further, by using the first catalyst layer containing the noble metal / acidic carrier as the upper layer, NO in the exhaust is easily oxidized to NO ₂ and easily adsorbed to the LNT, so that the NOx purification rate is improved.

  In this case, the solid acid support is preferably at least one selected from the group consisting of ZrSi composite oxide, SiAl composite oxide, ZrSiYW composite oxide, ZrSiCeW composite oxide, mesoporous silica, and zeolite.

In the present invention, as the carrier having solid acidity, at least one selected from the group consisting of ZrSi composite oxide, SiAl composite oxide and ZrSiYW composite oxide, ZrSiCeW composite oxide, mesoporous silica and zeolite is contained in the first catalyst. To contain. Here, the ZrSi composite oxide is a composite oxide of ZrO ₂ and SiO ₂ , and the SiAl composite oxide is a composite oxide of SiO ₂ and Al ₂ O ₃ . The ZrSiYW composite oxide is one in which a part of Zr and Si in the ZrSi composite oxide is substituted with Y or W, and the ZrSiCeW composite oxide is a composition of Zr and Si in the ZrSi composite oxide. A part thereof is substituted with Ce or W.
These acidic carriers have both high acidity (the value obtained by subtracting 2.7 from the average value of the Pauling electronegativity of the constituent elements constituting the acidic carrier) and the specific surface area, and further reduce the electron density on the surface of the noble metal. Thus, the bonding strength between the noble metal and oxygen can be further reduced. Therefore, according to the present invention, the adsorption of oxygen to the surface of the noble metal can be further suppressed, and a higher HC purification rate can be obtained.

  In this case, the carrier having solid acidity is preferably at least one of ZrSiYW composite oxide and ZrSiCeW composite oxide.

In the present invention, at least one of ZrSiYW composite oxide and ZrSiCeW composite oxide is contained in the first catalyst as a carrier having solid acidity.
These acidic carriers have a particularly high acidity (a value obtained by subtracting 2.7 from the average value of the Pauling electronegativity of the constituent elements constituting the acidic carrier) and a specific surface area, which greatly reduces the electron density on the surface of the noble metal. In addition, the binding force between the noble metal and oxygen can be greatly reduced. Therefore, according to the present invention, the adsorption of oxygen to the surface of the noble metal can be further suppressed, and a higher HC purification rate can be obtained.

  In this case, it is preferable that the first catalyst and the second catalyst have Pt as the noble metal.

  According to this invention, Pt is used as the noble metal contained in each of the first catalyst and the second catalyst. Thereby, the effect of the said invention is show | played more reliably.

  Further, the present invention provides an exhaust gas purification apparatus for an internal combustion engine that is operated while controlling the air-fuel ratio to be lean and stoichiometric, and includes the above-described exhaust gas purification catalyst.

  According to this invention, the same effect as the invention of the exhaust purification catalyst is exerted.

  ADVANTAGE OF THE INVENTION According to this invention, when carrying | supporting LNT and a noble metal / acid support | carrier on the same support body, the exhaust gas purification catalyst and exhaust gas purification apparatus which can improve the HC purification rate in a lean atmosphere can be provided.

It is a figure which shows HC composition in exhaust_gas | exhaustion, (A) shows HC composition in exhaust_gas | exhaustion discharged | emitted from the engine in stoichiometric operation, (B) is in exhaust_gas | exhaustion discharged | emitted from the engine in lean (HCCI) operation | movement It is a figure which shows HC composition. It is a figure which shows the HC purification rate by a three way catalyst. It is a figure which shows the adsorption | suction behavior of HC with respect to the oxygen which adsorb | sucked to the noble metal surface, (A) shows the adsorption | suction behavior of an olefin and an aroma, (B) is a figure which shows the adsorption | suction behavior of a paraffin. Is a diagram showing the adsorption behavior of oxygen and paraffin in a noble metal surface, (A) is a case of a conventional exhaust gas purifying catalyst in which the noble metal is supported on Al ₂ O ₃ carrier, (B) is supported on the noble metal is acidic carrier It is the case of the exhaust gas purification catalyst which concerns on one Embodiment of this invention. It is a figure which shows the relationship of the acidity x specific surface area of an acidic support | carrier, and HC purification rate. It is a figure which shows the relationship between the electronegativity of an acidic support | carrier, and a specific surface area. It is a X-ray diffraction spectrum figure of ZrSi complex oxide, ZrSiW complex oxide, and ZrSiYW complex oxide. Is a diagram showing the results of performance evaluation al ₂ O ₃ carrier and ZrSiYW composite oxide support, (A) is a diagram showing the oxygen adsorption amount of supported noble metal surface on each carrier, (B) is according to the temperature It is a figure which shows the paraffin adsorption rate. Pt-Rh / ZrSiYW composite oxide is a diagram illustrating _Pt / _Al 2 O 3 + AlBa composite oxides and HC average purification rate Ita300 ° C. These combinations -400 ° C. (percent). Pt-Rh / ZrSiYW composite oxide is a diagram illustrating _{Pt / Al 2 O 3 + CeO} 2 and HC average purification rate Ita300 ° C. These combinations -400 ° C. (percent). In the exhaust purification catalyst which concerns on one Embodiment of this invention, it is a figure which shows HC average purification rate (eta) 300 degreeC-400 degreeC (%) when the lamination | stacking order of Pt / acid support and LNT is changed. In the exhaust purification catalyst according to one embodiment of the present invention, it is a diagram showing the NOx adsorption rate η300 ℃ (%) when the stacking order of Pt / acid carrier and LNT is changed.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
The exhaust purification catalyst according to an embodiment of the present invention is preferably used in an exhaust purification device of an internal combustion engine that is operated with the air-fuel ratio being controlled to be lean and stoichiometric. The exhaust purification catalyst according to the present embodiment includes a first catalyst and a second catalyst carried on the same support.

  As the support, for example, a cordierite honeycomb support is preferably used. In the present embodiment, the first catalyst and the second catalyst supported on different supports are not arranged in the exhaust passage in series, for example, but the first catalyst and the second catalyst are supported on the same support. It is.

The first catalyst and the second catalyst may be supported on the same support in a mixed state, and the first catalyst layer containing the first catalyst and the second catalyst layer containing the second catalyst are adjacent to each other. And may be supported in a laminated state. In the present embodiment, it is preferable that the first catalyst layer and the second catalyst layer are supported on the same support in a state where they are laminated adjacently.
Moreover, it is preferable that the 1st catalyst layer is formed on the 2nd catalyst layer. The reason will be described in detail later.

The first catalyst includes a carrier having solid acidity and a noble metal supported on the carrier.
As the carrier having solid acidity, that is, the acidic carrier, at least one selected from the group consisting of ZrSi composite oxide, SiAl composite oxide, ZrSiYW composite oxide, ZrSiCeW composite oxide, mesoporous silica and zeolite is preferably used. The reason why these are preferable will be described in detail later. Among these, at least one of ZrSiYW composite oxide and ZrSiCeW composite oxide is more preferably used.
As the noble metal, various noble metals such as Pt, Pd, and Rh may be used alone, or these may be used in combination. Among these, Pt is preferably used.

The second catalyst includes a NOx adsorbent containing CeO ₂ and substantially free of alkaline earth metal and alkali metal, and a noble metal.
“Substantially free of alkaline earth metals and alkali metals” means that these alkaline earth metals and alkali metals are not actively added, and is intended to exclude cases where they are inevitably contained. Absent.
The noble metal is supported on a carrier such as alumina or the above NOx adsorbent. As the precious metal, similarly to the first catalyst, various precious metals such as Pt, Pd, and Rh may be used alone or in combination. Among these, Pt is preferably used.

The exhaust purification catalyst according to the present embodiment is prepared by a conventionally known method. For example, a slurry containing constituent materials of each catalyst is prepared, a honeycomb support is immersed therein, and then dried and fired. It is obtained with.
When the first catalyst layer is formed on the second catalyst layer, first, the honeycomb support is immersed in a slurry containing the constituent material of the second catalyst, and then dried and fired, whereby the second catalyst is formed. Form a layer. Next, the honeycomb support on which the second catalyst layer is formed is immersed in the slurry containing the constituent material of the first catalyst, and then dried and fired, whereby the first catalyst layer is formed on the second catalyst layer. The formed exhaust purification catalyst according to the present embodiment is obtained.

A ZrSiXW (X = Ce or Y) composite oxide can be prepared by the following method. However, the method for preparing the ZrSiXW composite oxide according to the present embodiment is not limited to the following method.
First, zirconium nitrate, cerium nitrate or yttrium nitrate is dissolved in pure water so as to have a desired Zr / X ratio. Thereafter, sodium silicate containing a desired amount of Si is added. Next, a precipitate is obtained by adding sodium hydroxide dropwise so that the pH becomes, for example, 10 if necessary. Thereafter, the solvent is evaporated by filtering the solution containing the precipitate under reduced pressure while being heated to 60 ° C., for example. Next, after extracting the residue, a ZrSiX composite oxide is obtained by performing calcination, for example, at 500 ° C. for 2 hours in a muffle furnace.
Next, the ZrSiX composite oxide obtained as described above is added to nitric acid to adjust the pH to 3, for example, and then sodium metatungstate (Na ₂ WO ₄ ) containing a desired amount of W is added. Next, sodium hydroxide is added dropwise so that the pH of the solution becomes 10, for example, to obtain a precipitate. Thereafter, the solvent is evaporated by filtering the solution containing the precipitate under reduced pressure while being heated to 60 ° C., for example. Next, after extracting the residue, calcination is performed at 500 ° C. for 2 hours in a muffle furnace to obtain a ZrSiXW composite oxide.

The effect of the exhaust purification catalyst according to the present embodiment having the above configuration will be described in detail below.
FIG. 1 is a diagram showing the HC composition in the exhaust, where (A) shows the HC composition in the exhaust discharged from the engine during stoichiometric operation, and (B) shows the exhaust from the engine during lean (HCCI) operation. It is a figure which shows HC composition in the exhaust_gas | exhaustion to be performed. In FIG. 1, the horizontal axis indicates the number of HC carbon atoms, and the vertical axis indicates the HC concentration (ppmC) in the exhaust gas.
As shown in FIG. 1 (A), the exhaust discharged from the engine during stoichiometric operation contains a lot of short carbon atoms, and most contains olefins. On the other hand, as shown in FIG. 1 (B), the exhaust discharged from the engine during lean operation contains a lot of carbon having a longer carbon number than during stoichiometric operation, It contains a lot of paraffin.

FIG. 2 is a graph showing the HC purification rate by the three-way catalyst. Specifically, the HC purification rate η350 when the exhaust discharged from the engine during the stoichiometric operation and the exhaust discharged from the engine during the HCCI lean operation are purified at 350 ° C. using a conventional three-way catalyst. It is a figure which shows (%).
As shown in FIG. 2, there is no difference between the stoichiometric atmosphere and the lean atmosphere with respect to the purification rates of olefin and aroma, which are almost the same. On the other hand, the purification rate of paraffin is greatly reduced in the lean atmosphere as compared with the stoichiometric atmosphere. From this, it can be seen that the cause of the decrease in the HC purification rate in the lean atmosphere is the decrease in the purification rate of paraffin.

FIG. 3 is a diagram showing the adsorption behavior of HC with respect to oxygen adsorbed on the surface of the noble metal (Pt), (A) shows the adsorption behavior of olefin and aroma, and (B) shows the adsorption behavior of paraffin. . FIG. 3 shows a state where a large amount of oxygen is adsorbed on the surface of the noble metal (Pt) in an oxygen-rich lean atmosphere.
As shown in FIG. 3A, since olefins and aromas have weak bonds, unstable and highly reactive π bonds, oxygen having low activity is adsorbed on the surface of the noble metal (Pt). Even so, the double bond is cleaved to react and adsorb. Thereby, the purification rate of olefin and aroma does not decrease even in a lean atmosphere.
On the other hand, as shown in FIG. 3B, paraffin has only a strong, stable, and low-reactivity σ bond, so that oxygen with low activity is adsorbed on the surface of the noble metal (Pt). In this case, the adsorbed oxygen inhibits the adsorption of paraffin to the noble metal, and the purification rate is greatly reduced in a lean atmosphere.

FIG. 4 is a view showing the adsorption behavior of oxygen and paraffin on the surface of the noble metal (Pt, Pd), and FIG. 4A is a case of a conventional exhaust purification catalyst in which the noble metal is supported on an Al ₂ O ₃ support. B) shows an exhaust purification catalyst according to an embodiment of the present invention in which a noble metal is supported on an acidic carrier.
As shown in FIG. 4 (A), precious metals (Pt, Pd) in the conventional exhaust gas purification catalyst supported on _Al 2 _{O 3} carrier, since the _Al 2 _{O 3} carrier is a carrier ampholyte, the noble metal surface electron There is no change in density, and a large amount of oxygen is adsorbed on the surface of the noble metal in a lean atmosphere. Therefore, the adsorption of paraffin to the noble metal is inhibited by the adsorbed oxygen, and the purification rate of paraffin is reduced.
On the other hand, as shown in FIG. 4B, in the exhaust purification catalyst according to this embodiment in which the noble metals (Pt, Pd) are supported on the acidic carrier, the electron withdrawing action based on the high acidity of the acidic carrier is used. , The electron density on the surface of the noble metal decreases. Then, the binding force between oxygen and noble metal present in a large amount in the lean atmosphere is reduced, and adsorption of oxygen on the surface of the noble metal is suppressed. Thereby, since the paraffin with low reactivity also reacts and adsorbs, the purification rate of paraffin improves.

FIG. 5 is a diagram showing the relationship between the acidity × specific surface area of the acidic carrier and the HC purification rate. Specifically, in FIG. 5, for a catalyst in which a noble metal (Pt) is supported on an acidic carrier, the HC purification rate at 300 ° C. and the HC at 400 ° C. when the acidity × specific surface area value of the acidic carrier is changed. The average purification rate η300 ° C.-400 ° C. (%) with the purification rate is shown.
Here, the acidity of the acidic carrier in the present embodiment means a value obtained by subtracting 2.7 from the average value of the Pauling electronegativity of the constituent elements constituting the acidic carrier. The specific surface area in the present embodiment is the ratio after aging is performed in an oxygen-excess atmosphere at 750 ° C. for 20 hours (O ₂ = 10% by volume, H ₂ O = 6% by volume, remaining N ₂ balance gas). It means surface area.
As shown in FIG. 5, there is a correlation between the acidity × specific surface area value of the acidic carrier and the HC purification rate, and the HC purification rate increases as the acidity × specific surface area value of the acidic carrier increases. Tend to be. From this result, it is understood that the product of the acidity and the specific surface area is an important index when selecting an acidic carrier.
It has been confirmed that there is not much correlation between the acidity of the acidic carrier and the HC purification rate, and there is not much correlation between the specific surface area of the acidic carrier and the HC purification rate.

FIG. 6 is a diagram showing the relationship between the electronegativity of the acidic carrier and the specific surface area. In FIG. 6, the average value of the Pauling electronegativity of the constituent elements is shown as the electronegativity of each acidic carrier.
As shown in FIG. 6, ZrO ₂ has both a small electronegativity and a small specific surface area, whereas Al ₂ O ₃ has a large specific surface area although it has a low electronegativity, while SiO ₂ has a small specific surface area but has an electronegativity. The degree is great. However, ZrSi composite oxide, which is a composite oxide of ZrO ₂ and SiO ₂ , and SiAl composite oxide, which is a composite oxide of SiO ₂ and Al ₂ O ₃ , are 2.7 from the average electronegativity of the constituent elements. It can be seen that large values can be obtained for both the acidity and the specific surface area, which are values obtained by subtracting. For this reason, ZrSi composite oxide or SiAl composite oxide is preferably used as an acidic carrier having a large value of acidity × specific surface area.
In addition, as shown in FIG. 6, a ZrSiYW composite oxide in which a part of Zr and Si in the ZrSi composite oxide is substituted with Y or W, or a part of Zr and Si in the ZrSi composite oxide is Ce or The ZrSiCeW composite oxide substituted with W has a larger value of acidity × specific surface area. Therefore, these ZrSiYW composite oxide and ZrSiCeW composite oxide are more preferably used as acidic carriers.

The ZrSiYW composite oxide and ZrSiCeW composite oxide as acidic carriers will be described in more detail.
First, as described above, in the acidic carrier, the higher the electronegativity (the average value of the electronegativity of the constituent elements), the higher the acidity. Therefore, the electron density on the surface of the noble metal is further reduced, and the noble metal and oxygen As a result, the HC purification rate can be increased. However, even if the electronegativity is high and the acidity is high, if the specific surface area decreases after aging, the HC purification rate decreases due to the burying of noble metals and the reduction of active sites. Therefore, both improvement of the acidity and maintenance of the specific surface area after aging are key to improving the HC purification rate.

By the way, as will be described later in Examples, a higher HC purification rate can be obtained when the ZrSiCeW composite oxide is used as the acidic carrier than when the ZrSiYW composite oxide is used. The reason is presumed as follows.
That is, as shown in FIG. 6, the electronegativity of the composite oxide as a whole is lower in the ZrSiCeW composite oxide than in the ZrSiYW composite oxide. This is because Ce has a lower electronegativity than Y. On the other hand, regarding the specific surface area after aging, the ZrSiCeW composite oxide is higher than the ZrSiYW composite oxide.
In view of the above fact, the HC purification rate in the lean atmosphere is higher in the ZrSiCeW composite oxide than in the ZrSiYW composite oxide. This is presumably because the effect of lowering the HC purification rate due to the reduction was exceeded.

FIG. 7 is an X-ray diffraction spectrum diagram of a ZrSi composite oxide, a ZrSiW composite oxide, and a ZrSiYW composite oxide. The horizontal axis indicates the 2θ angle (°), and the vertical axis indicates the X-ray intensity.
As shown in FIG. 7, in the ZrSiW composite oxide formed by adding W having high electronegativity to the ZrSi composite oxide, in addition to the peak derived from the ZrSi composite oxide on the X-ray diffraction spectrum, , A peak derived from ZrO ₂ is confirmed. This means that a part of the crystal structure has collapsed.
Therefore, in the ZrSiYW composite oxide obtained by adding Y to the ZrSiW composite oxide, it was confirmed that the peaks derived from ZrO ₂ disappeared, while other new peaks were confirmed. Not. This means that the crystal structure is stabilized and the ZrSiYW composite oxide is well formed.
Therefore, from the results of FIGS. 6 and 7, it can be said that the ZrSiYW composite oxide having a stabilized structure in addition to a large value of the acidity × specific surface area is more preferable as the acidic carrier. The same can be said about the ZrSiCeW composite oxide in which Y in the ZrSiYW composite oxide is substituted with Ce.

FIG. 8 is a view showing the performance evaluation results of the Al ₂ O ₃ support and the ZrSiYW composite oxide support, (A) is a view showing the oxygen adsorption amount on the surface of the noble metal supported on each support, and (B) These are figures which show the paraffin adsorption rate according to temperature.
The oxygen adsorption amount (μmol / g) in FIG. 8 (A) was raised after adsorbing oxygen by letting an exhaust gas in a lean atmosphere flow for a predetermined time with respect to each carrier on which noble metal (Pt) was supported. The amount of oxygen desorption is shown. Specifically, the oxygen desorption amount is measured as follows. First, each carrier on which a noble metal (Pt) is supported is heated to 500 ° C. in a He atmosphere. Next, oxygen is adsorbed by holding at 500 ° C. for 15 minutes in an atmosphere with an oxygen concentration of 7%. Next, after cooling to room temperature, the adsorbed oxygen is desorbed by heating to room temperature to 800 ° C. in a He atmosphere, and the amount of oxygen desorbed at this time is measured.

  Further, the paraffin adsorption rate (%) in FIG. 8B indicates the paraffin adsorption rate when exhaust gas in a lean atmosphere containing a lot of paraffin is circulated for a predetermined time at each temperature. Specifically, the paraffin adsorption rate is measured as follows. First, each support on which a noble metal (Pt) is supported is held for 15 minutes in an atmosphere having an oxygen concentration of 7% at each measurement temperature. Next, after replacing with a He atmosphere, 1000 ppmC of n-pentane is injected in pulses. At this time, the n-pentane concentration after passing through the catalyst (each carrier) is measured to determine the adsorption amount. A pulse is injected 6 times, and the paraffin adsorption rate is calculated using the total as the adsorption amount.

As shown in FIG. 8A, the oxygen adsorption amount on the surface of the noble metal supported on the ZrSiYW composite oxide support is approximately 20% of the oxygen adsorption amount on the surface of the noble metal supported on the Al ₂ O ₃ support. According to the composite oxide support, it can be seen that the oxygen adsorption amount is dramatically reduced.
Further, as shown in FIG. 8B, it can be seen that the paraffin adsorption rate of the ZrSiYW composite oxide carrier is larger than that of the Al ₂ O ₃ carrier, and this tendency becomes more remarkable as the temperature increases.
From these results, it can be seen that by using the ZrSiYW composite oxide support as the acidic support, a higher HC purification rate than in the prior art can be obtained in a lean atmosphere. The same can be said about the ZrSiCeW composite oxide in which Y in the ZrSiYW composite oxide is substituted with Ce.

FIG. 9 is a diagram showing the HC average purification rate η300 ° C.-400 ° C. (%) of the Pt—Rh / ZrSiYW composite oxide, Pt / Al ₂ O ₃ + AlBa composite oxide, and combinations thereof. FIG. 10 is a diagram showing the HC average purification rate η300 ° C.-400 ° C. (%) of the Pt—Rh / ZrSiYW composite oxide, Pt / Al ₂ O ₃ + CeO ₂ and combinations thereof. 9 and 10, the calculated combined purification rate indicates the total purification rate when HC that has not been purified by the upper layer catalyst can be purified by the lower layer catalyst purification rate. .
As shown in FIG. 9, in the conventional LNT using the alkaline earth metal Ba as the NOx adsorbent, the purification rate is actually lower than the calculated purification rate even when combined with the noble metal / acidic carrier. You can only get it. The reason is as follows.
That is, since the alkali metal or alkaline earth metal constituting the NOx adsorbent in LNT is basic, the electron donating action of these substances inhibits the electron withdrawing action of the acidic carrier, and the electron density on the surface of the noble metal increases. . Then, the bonding force between the noble metal and oxygen is increased, and the adsorption of oxygen to the surface of the noble metal is promoted. In addition, for example, exhaust discharged from an engine during lean operation contains more paraffin as HC than during stoichiometric operation, and paraffin is structurally less reactive than olefins and aromas. For this reason, the purification rate of paraffin is greatly reduced. Thereby, the purification rate of HC falls.

On the other hand, as shown in FIG. 10, according to the exhaust purification catalyst according to this embodiment in which LNT using CeO ₂ as a NOx adsorbent is laminated and combined with a noble metal / acid support, the purification rate almost as calculated. You can see that The reason is as follows.
That is, CeO ₂ has a lower basicity than alkali metals and alkaline earth metals, so that it does not significantly affect the electron withdrawing action of the acidic carrier and can reduce the electron density on the surface of the noble metal. As a result, since the bonding force between the noble metal and oxygen can be reduced and the adsorption of oxygen to the surface of the noble metal can be suppressed, the purification rate of paraffin can be particularly improved, and a high HC purification rate can be obtained.
CeO ₂ also has an oxygen storage capacity (OSC) in addition to the NOx adsorption capacity, and releases active oxygen at a predetermined temperature or higher. Since this active oxygen is very reactive, according to this embodiment, oxidation purification of paraffin can be promoted even in a lean atmosphere, and the HC purification rate can be improved.

FIG. 11 is a diagram showing an HC average purification rate η300 ° C.-400 ° C. (%) when the stacking order of Pt / acidic carrier and LNT is changed in the exhaust purification catalyst according to the present embodiment. FIG. 12 is a graph showing the NOx adsorption rate η300 ° C. (%) when the stacking order of Pt / acidic carrier and LNT is changed in the exhaust purification catalyst according to this embodiment.
As shown in FIG. 11, a higher HC purification rate is obtained when the Pt / acid carrier is the upper layer and the LNT is the lower layer. Also, as shown in FIG. 12, it can be seen that the NOx adsorption rate is higher when the Pt / acidic carrier is the upper layer and the LNT is the lower layer. The reason is as follows.
That is, compared with the case where the first catalyst layer containing the noble metal / acidic carrier is used as the lower layer, the upper layer has better gas diffusibility, so that paraffin can be purified efficiently and the HC purification rate can be improved. In addition, by using the first catalyst layer containing the noble metal / acidic carrier as the upper layer, NO in the exhaust is easily oxidized to NO ₂ and is easily adsorbed by LNT. As a result, the NOx adsorption rate is improved.

The exhaust purification catalyst according to this embodiment has the following effects.
In the present embodiment, the first catalyst in which a noble metal is supported on a carrier having solid acidity as a noble metal / acid carrier, and CeO ₂ as LNT and substantially free of alkaline earth metal and alkali metal. An exhaust purification catalyst was configured including a second catalyst having a NOx adsorbent and a noble metal. That is, the first catalyst and the second catalyst are mixed or stacked, and the alkaline earth metal and the alkali metal are not contained in the NOx adsorbent constituting the second catalyst as the LNT. Instead, CeO ₂ Was included.
As a result, CeO ₂ has a lower basicity than alkali metals and alkaline earth metals, so that it does not significantly affect the electron withdrawing action of the acidic carrier and can reduce the electron density on the surface of the noble metal. . As a result, since the bonding force between the noble metal and oxygen can be reduced and the adsorption of oxygen to the surface of the noble metal can be suppressed, the purification rate of paraffin can be particularly improved, and a high HC purification rate can be obtained.
CeO ₂ also has an oxygen storage capacity (OSC) in addition to the NOx adsorption capacity, and releases active oxygen at a predetermined temperature or higher. Since this active oxygen is very reactive, according to the present invention, oxidation purification of paraffin can be promoted even in a lean atmosphere, and the HC purification rate can be improved.

  In the present embodiment, the first catalyst layer is formed using the first catalyst, and the second catalyst layer is formed using the second catalyst adjacent to the first catalyst layer. As a result, the above-described effects are exhibited particularly at the interface between the first catalyst layer and the second catalyst layer.

In the present embodiment, the first catalyst layer is formed on the second catalyst layer. That is, the first catalyst layer was formed as an upper layer and the second catalyst layer was formed as a lower layer. Thereby, compared with the case where the first catalyst layer containing the noble metal / acidic carrier is used as the lower layer, the upper layer has better gas diffusibility, so that paraffin can be purified efficiently and the HC purification rate can be improved. Further, by using the first catalyst layer containing the noble metal / acidic carrier as the upper layer, NO in the exhaust is easily oxidized to NO ₂ and easily adsorbed to the LNT, so that the NOx purification rate is improved.

  In the present embodiment, as the carrier having solid acidity, at least one selected from the group consisting of ZrSi composite oxide, SiAl composite oxide, ZrSiYW composite oxide, ZrSiCeW composite oxide, mesoporous silica, and zeolite is used as the first support. It was contained in the catalyst. These acidic carriers have both high acidity and specific surface area, and can lower the electron density on the surface of the noble metal to further reduce the bonding force between the noble metal and oxygen. Therefore, according to this embodiment, the adsorption of oxygen to the surface of the noble metal can be further suppressed, and a higher HC purification rate can be obtained.

  In the present embodiment, at least one of ZrSiYW composite oxide and ZrSiCeW composite oxide is contained in the first catalyst as a carrier having solid acidity. These acidic carriers have particularly high acidity and specific surface area, and can further reduce the electron density on the surface of the noble metal and further reduce the bonding force between the noble metal and oxygen. Therefore, according to this embodiment, the adsorption of oxygen to the surface of the noble metal can be further suppressed, and a higher HC purification rate can be obtained.

  In this embodiment, Pt is used as a noble metal contained in each of the first catalyst and the second catalyst. Thereby, the above-mentioned effect is more reliably achieved.

  Further, the present embodiment provides an exhaust gas purification apparatus for an internal combustion engine that is operated with the air-fuel ratio being controlled to be lean and stoichiometric, and includes the above-described exhaust gas purification catalyst. Thereby, the same effect as the invention of the exhaust purification catalyst is exhibited.

  It should be noted that the present invention is not limited to the above-described embodiment, and modifications and improvements within the scope that can achieve the object of the present invention are included in the present invention.

  Next, the present invention will be described in more detail based on examples, but the present invention is not limited thereto.

<Example 1>
(1) A mixed solution obtained by mixing 100 g of γ-Al ₂ O ₃ powder with 43.0 g of a 5% dinitrodiammine platinum solution and 400 g of ion-exchanged water is dried at 200 ° C. for 12 hours after removing water with a rotary evaporator. I let you. Then, by performing the calcination of 500 ° C. × 2 hours in a muffle furnace to obtain a 2.1 wt% Pt supported _Al 2 _{O 3} powder A.

(2) 2.1% by mass Pt-supported Al ₂ O ₃ powder A19g obtained in (1) above and 19g of CeO ₂ powder were added to 10 g of Al ₂ O ₃ sol (Al ₂ O ₃ 20% by mass), 60 g of alumina balls, and Mixed with 80 g of water. Next, slurry B was obtained by wet pulverization with a ball mill for 15 hours.

  (3) The slurry B obtained in the above (2) is immersed in a cordierite honeycomb support (diameter φ25.4 mm × length L30 mm (capacity 15 cc), 900 cells / inch, 3.5 mil), and extra Slurry was removed by air jet. This operation was repeated until a predetermined amount was supported, and the honeycomb support that had reached the predetermined amount was fired in a muffle furnace at 500 ° C. for 2 hours to obtain a lower layer coat catalyst C. The lower layer coating amount was 200 g / L, and the Pt carrying amount was 2 g / L.

(4) First, zirconium nitrate and yttrium nitrate were dissolved in pure water so that the molar ratio of Zr to Y was Zr: Y = 77%: 10%. Thereafter, sodium silicate containing a desired amount of Si was added. Subsequently, sodium hydroxide was added dropwise so that the pH was 10 as necessary, thereby obtaining a precipitate. Then, the solvent was evaporated by filtering under reduced pressure the solution containing a precipitate at 60 degreeC. Next, after extracting the residue, a ZrSiY composite oxide was obtained by calcining at 500 ° C. for 2 hours in a muffle furnace.
Next, the ZrSiY composite oxide obtained as described above was added to nitric acid to adjust the pH to 3, and then sodium metatungstate (Na ₂ WO ₄ ) containing a desired amount of W was added. Subsequently, sodium hydroxide was added dropwise so that the pH of the solution was 10, thereby obtaining a precipitate. Then, the solvent was evaporated by filtering under reduced pressure the solution containing a precipitate at 60 degreeC. Next, after extracting the residue, by carrying out calcination at 500 ° C. for 2 hours in a muffle furnace, ZrSiYW composite oxide (Zr: Si: Y: W = 77%: 3%: 10%: 10%) Molar ratio)).
Next, 42.3 g of a ZrSiYW composite oxide (Zr: Si: Y: W = 77%: 3%: 10%: 10% (molar ratio)) powder was added to 44.5 g of a 5% dinitrodiammine platinum solution and ion exchange. The mixed solution mixed with 400 g of water was dried at 200 ° C. for 12 hours after removing water with a rotary evaporator. Subsequently, the powder D of 5.3 mass% Pt carrying | support ZrSiYW complex oxide was obtained by baking at 500 degreeC * 2 hours in a muffle furnace.

(5) The powder D19 g of the 5.3 mass% Pt-supported ZrSiYW composite oxide obtained in the above (4) is mixed with 5 g of Al ₂ O ₃ sol (Al ₂ O ₃ 20 mass%), 30 g of alumina balls, and 80 g of water. Then, slurry E was obtained by wet pulverization with a ball mill for 15 hours.

  (6) The slurry E obtained in (5) above was immersed in the lower layer coat catalyst C, and excess slurry was removed by air injection. This operation is repeated until a predetermined amount is supported, and the honeycomb support that has reached the predetermined amount is fired in a muffle furnace at 500 ° C. for 2 hours to purify the exhaust gas of Example 1 in which the upper layer coat F is formed. A catalyst was obtained. The upper layer coat amount was 50 g / L, and the Pt carrying amount was 5 g / L.

<Example 2>
(1) The ZrSiYW composite oxide of the powder D of Example 1 was replaced with a ZrSiCeW composite oxide (Zr: Si: Ce: W = 77%: 3%: 10%: 10% (molar ratio)). The exhaust purification catalyst of Example 2 was obtained by performing the same operation as in Example 1.
The ZrSiCeW composite oxide (Zr: Si: Ce: W = 77%: 3%: 10%: 10% (molar ratio)) was used in Example 1 except that yttrium nitrate was changed to cerium nitrate. A ZrSiYW composite oxide (Zr: Si: Y: W = 77%: 3%: 10%: 10% (molar ratio)) was prepared by the same preparation method.

<Comparative Example 1>
(1) Except that CeO ₂ added to the slurry B of Example 1 was replaced with an alumina barium (80:20) mixed powder, the exhaust purification catalyst of Comparative Example 1 was obtained by performing the same operation as in Example 1. Obtained.

<HC purification rate evaluation 1>
Aging was performed on each exhaust purification catalyst obtained in Example 1 and Comparative Example 1. Aging was performed in an oxygen excess atmosphere (O ₂ = 10% by volume, remaining N ₂ balance gas) at 750 ° C. for 20 hours. In addition, the HC purification rate of each exhaust purification catalyst was evaluated. Specifically, an HC average purification rate η300 ° C.-400 ° C. (%) obtained by averaging the HC purification rates at 300 ° C. and 400 ° C. was obtained. The evaluation conditions for the HC purification rate were as follows. The evaluation results are shown in Table 1.

[Evaluation conditions]
Model gas composition: CO = 0.3%, C ₅ H ₁₂ = 1200 ppmC, NO = 50 ppm, O ₂ = 7%, CO ₂ = 8%, H ₂ O = 7%, N ₂ = balance gas linear velocity SV: 100,000 / hour Temperature range: Room temperature to 450 ° C
Temperature increase rate: 20 ° C / min

<NOx purification rate evaluation 1>
Aging was performed on each exhaust purification catalyst obtained in Example 1 and Comparative Example 1. Aging was performed in an oxygen excess atmosphere (O ₂ = 10% by volume, remaining N ₂ balance gas) at 750 ° C. for 20 hours. In addition, the NOx purification rate of each exhaust purification catalyst was evaluated. Specifically, the NOx average adsorption rate η300 ° C. (%) for 1 minute at 300 ° C. was determined. In addition, all NOx adsorbed on the catalyst was desorbed by circulating a gas in a rich atmosphere before the measurement. The evaluation conditions for the NOx purification rate were as follows. The evaluation results are shown in Table 2.

[Evaluation conditions]
Model gas composition: CO = 0.3%, C ₃ H ₈ = 1200 ppmC, NO = 70 ppm, O ₂ = 7%, CO ₂ = 8%, H ₂ O = 7%, N ₂ = balance gas linear velocity SV: 100,000 / hour Temperature range: 300 ° C

  As shown in Table 1 and Table 2, compared with Comparative Example 1, Example 1 is about 6% at an HC average purification rate η300 ° C.-400 ° C. (%) and about 3 at an NOx average adsorption rate η300 ° C. (%). It turned out to be expensive. From these results, it was confirmed that the exhaust purification catalyst according to the present invention can improve the HC purification rate and NOx adsorption rate (NOx purification rate) in a lean atmosphere as compared with the conventional case.

<HC purification rate evaluation 2>
Aging was performed on each exhaust purification catalyst obtained in Examples 1 and 2. Aging was performed in an oxygen-excess atmosphere at 750 ° C. for 20 hours (O ₂ = 10% by volume, remaining N ₂ balance gas). In addition, the HC purification rate of each exhaust purification catalyst was evaluated. Specifically, an HC purification rate η300 ° C. of 300 ° C. was obtained. The evaluation conditions for the HC purification rate were as follows. The evaluation results are shown in Table 3.

[Evaluation conditions]
Model gas composition: CO = 0.3%, C ₅ H ₁₂ = 1200 ppmC, NO = 50 ppm, O ₂ = 7%, CO ₂ = 8%, H ₂ O = 7%, N ₂ = balance gas space velocity SV: 100,000 / hour Temperature range: Room temperature to 450 ° C
Temperature increase rate: 20 ° C / min

<NOx purification rate evaluation 2>
Aging was performed on each exhaust purification catalyst obtained in Examples 1 and 2. Aging was performed in an oxygen-excess atmosphere at 750 ° C. for 20 hours (O ₂ = 10% by volume, remaining N ₂ balance gas). In addition, the NOx purification rate of each exhaust purification catalyst was evaluated. Specifically, the NOx average adsorption rates η300 ° C (%) and η400 ° C (%) for 1 minute at 300 ° C and 400 ° C were determined. In addition, all NOx adsorbed on the catalyst was desorbed by circulating a gas in a rich atmosphere before the measurement. The evaluation conditions for the NOx purification rate were as follows. The evaluation results are shown in Table 4.

[Evaluation conditions]
Model gas composition: CO = 0.3%, C ₃ H ₈ = 1200 ppmC, NO = 70 ppm,
O ₂ = 7%, CO ₂ = 8%, H ₂ O = 7%, N ₂ = balance gas space velocity SV: 100,000 / hour Temperature range: 300 ° C., 400 ° C.

  As shown in Tables 3 and 4, it was found that Example 2 has an improved HC purification rate, although the NOx purification rate is equivalent to Example 1. From this result, it was confirmed that both the ZrSiCeW composite oxide and the ZrSiYW composite oxide can obtain a high NOx purification rate and HC purification rate by using them as acidic carriers. It was also confirmed that the HC purification rate in the lean atmosphere can be further improved when the ZrSiCeW composite oxide is used as the acidic carrier, compared with the case where the ZrSiYW composite oxide is used.

Claims (7)

An exhaust purification catalyst for purifying exhaust gas from an internal combustion engine,
Comprising a first catalyst and a second catalyst supported on the same support,
The first catalyst has a carrier having solid acidity and a noble metal supported on the carrier,
The exhaust gas purifying catalyst, wherein the second catalyst includes a NOx adsorbent containing CeO ₂ and substantially free of alkaline earth metal and alkali metal, and a noble metal.   The first catalyst layer formed by the first catalyst, and a second catalyst layer adjacent to the first catalyst layer and formed by the second catalyst, according to claim 1, Exhaust purification catalyst.   The exhaust purification catalyst according to claim 2, wherein the first catalyst layer is formed on the second catalyst layer.   The support having solid acidity is at least one selected from the group consisting of ZrSi composite oxide, SiAl composite oxide, ZrSiYW composite oxide, ZrSiCeW composite oxide, mesoporous silica, and zeolite. The exhaust purification catalyst according to any one of 1 to 3.   The exhaust purification catalyst according to any one of claims 1 to 3, wherein the carrier having solid acidity is at least one of a ZrSiYW composite oxide and a ZrSiCeW composite oxide.   The exhaust purification catalyst according to any one of claims 1 to 5, wherein the first catalyst and the second catalyst have Pt as the noble metal. An exhaust gas purification device for an internal combustion engine operated by controlling the air-fuel ratio to lean and stoichiometric,
An exhaust purification device comprising the exhaust purification catalyst according to any one of claims 1 to 6.

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