JP6201588B2 - Exhaust gas purification catalyst - Google Patents

Description

The present invention relates to a catalyst that purifies exhaust gas discharged from an engine, and more particularly to a catalyst that suppresses poisoning and deterioration of the catalyst due to catalyst poison contained in the exhaust gas.

In the catalyst that purifies the exhaust gas of the engine, if the usage time is prolonged, the catalyst poison (P, Mn, Zn, Ca, etc.) in the exhaust gas is deposited in combination with the catalyst precious metal and promoter that constitute the catalyst. It is known that the function as a catalyst is reduced.

The catalyst poison is contained in antioxidants and high-pressure lubricating oil additives in engine oil, but is released to the exhaust system only slightly during combustion and exhaust. It degrades the cocatalyst.

As an example of poisoning countermeasures, Patent Document 1 discloses that a poison that does not contain a catalyst precious metal that adheres and deposits a catalyst poison by utilizing the fact that the catalyst poison is deposited within 2.5 centimeters upstream of the catalytic converter. A catalyst with a supplementary region is described. According to this measure, it is described that the poisoning of the catalyst noble metal is suppressed because the catalyst poison in the exhaust gas is not brought into contact with the catalyst noble metal downstream.

  However, in the countermeasure of patent document 1, when the exhaust gas which flowed in contacts with the catalyst noble metal other than the area | region which the countermeasure is taking, the catalyst noble metal which exists there will be poisoned. In addition, providing a region having no catalyst noble metal component only at a specific location during catalyst production has a problem in that the number of steps is large as a production method.

  Patent Document 2 discloses measures against phosphorus poisoning. Specifically, by containing Ba, Sr, and Mg in the catalyst on the support by the impregnation method, phosphorus in the exhaust gas is collected when the oxygen concentration of the exhaust gas is high, and the oxygen concentration of the exhaust gas is reduced. It is described that the amount of phosphorus bound to ceria-based oxides or alumina is reduced by releasing the collected phosphorus when it is low. However, the above-mentioned countermeasure substances were only countermeasures against phosphorus poisoning suppression.

Special table 2009-501079 gazette Japanese Patent Laid-Open No. 2003-47848

In addition to phosphorus, it is generally known that calcium and zinc, which are catalyst poisons, degrade catalyst noble metals, but the degree of poisoning is small compared to phosphorus and sulfur, so it is negligible. It was. However, in recent years, exhaust gas temperature tends to be lowered due to higher efficiency of engine combustion. For this reason, it is expected that the purification rate as a whole decreases due to the fact that the exhaust gas temperature is less than the activation temperature of the catalyst noble metal catalyst and the activation takes time. At this time, since the inflow amount of the catalyst poison into the catalyst is the same, the ratio of poisoning to the purification rate increases, and the catalyst deterioration due to the catalyst poison cannot be ignored. That is, it is necessary to more reliably suppress poisoning due to the catalyst poison.

In order to solve this problem, the present invention includes a catalyst layer on a catalyst carrier containing a compound that preferentially reacts with calcium and zinc, which are one of catalyst poisons in exhaust gas, to thereby provide a catalyst noble metal and a promoter. The present invention provides an exhaust gas purification catalyst that suppresses poisoning of the catalyst poison.

That is, claim 1 of the present invention is an exhaust gas purifying catalyst that is disposed in an exhaust passage of an engine and purifies exhaust gas containing at least one of calcium and zinc. The catalyst is provided on a substrate. The exhaust gas flow path side surface layer of the catalyst layer contains Ca or Zn and contains a precursor compound that preferentially reacts with Ca or Zn that is a catalyst poison in the exhaust gas. A precursor compound-containing layer is provided, and the precursor compound is at least one of calcium hydrogen phosphate (Ca (HPO ₄ ) ₂ ) and zinc hydrogen phosphate (ZnHPO ₄ ). It is a purification catalyst.

According to this embodiment, calcium and zinc, which are catalyst poisons in the exhaust gas, are combined with the precursor, and gradually become saturated compounds during long-term use. At this time, the precursor is contained in the surface layer upstream of the exhaust gas flow, and the catalyst poison in the exhaust gas combines with the precursor before reaching the catalyst layer to become a saturated compound. Therefore, Ce in the catalyst layer Oxide such as Al (Ce-containing oxide or activated alumina) and the catalyst noble metal supported on these and the catalyst poison in the exhaust gas are less likely to combine, and the activity decrease of the catalyst noble metal can be suppressed. it can. In addition, since this measure is applied to the entire surface of the catalyst, poisoning can be stably suppressed even with changes per gas due to operating conditions. Moreover, since the precursor compound is at least one of calcium hydrogen phosphate (Ca (HPO ₄ ) ₂ ) and zinc hydrogen phosphate (ZnHPO ₄ ), it has high heat resistance and is a catalyst poison in exhaust gas for a long time. The function of combining with can be maintained.

The invention according to claim 2 is the exhaust gas purifying catalyst, wherein the precursor compound is contained in an amount of 0.22 mol / L or more with respect to the catalyst layer .

According to a third aspect of the present invention, the precursor compound reacts preferentially with Ca or Zn, which is a catalyst poison in exhaust gas, to cause Ca ₁₀ (PO ₄ ) ₆ (OH) ₂ and Zn ₃ (PO ₄ ) An exhaust gas purifying catalyst characterized in that at least one of the _two is formed .

    The above compound has high heat resistance, and the catalyst layer provided on the substrate side of the surface layer is exposed to high temperature exhaust gas, and the surface area of the support material supporting the catalyst noble metal is reduced over a long period of time. Can be suppressed.

The present invention utilizes the fact that the precursor is capable of coordinating the catalyst poison and has a high affinity with the catalyst poison in the exhaust gas, so that the catalyst is prevented from being deteriorated by poisoning of the catalyst poison. This is an invented catalyst.
That is, since a precursor having high affinity with calcium and zinc in exhaust gas, which is a catalyst poison, is provided on the surface layer, the precursor layer reacts with the catalyst poison in the exhaust gas to produce a compound. Therefore, the catalyst poison is less likely to flow downstream of the exhaust gas flow below the surface layer, and the poisoning of calcium and zinc to the catalyst layer containing the catalyst noble metal and the promoter can be suppressed.

In addition, the precursor of the surface layer and the catalyst layer are in close contact, but the precursor or compound of the surface layer is thermodynamically more stable than the catalyst layer. Poisoning deterioration due to zinc can be suppressed over a long period of time. Therefore, according to the catalyst of the present invention, the exhaust gas purification can suppress the poisoning of the catalyst layer to the catalyst noble metal or oxide and suppress the deterioration of the catalyst due to the engine lubricating oil even when used for a long time. A catalyst can be provided.

Although the preferable form of this invention is demonstrated in detail below, the following description is an example of this invention and this invention is not limited to these content, unless the summary is exceeded.

FIG. 1 is a cross-sectional view showing a catalyst layer configuration of an exhaust gas purifying catalyst according to the present invention. As shown in FIG. 1, in the exhaust gas purifying catalyst 6 according to this embodiment, the catalyst layer 2 is formed on the upstream side of the cell wall 1 of the honeycomb carrier, and the catalyst layer 3 is further formed on the upstream side thereof. . And the precursor compound content layer 4 which comprises a precursor is formed in the surface layer. The catalyst layer 2 and the catalyst layer 3 contain a catalyst noble metal and are represented by a catalyst layer 5. In this embodiment and FIG. 1, the catalyst layer 5 is composed of two layers, but the same effect can be obtained by a single layer or a plurality of layers. The honeycomb carrier is a quadrangle in this embodiment, but may be a polygon such as a hexagon.

<Composition of catalyst>
The configurations of the catalyst layer 2 and the catalyst layer 3 of the present invention are as follows.
[Catalyst layer 2]
Catalyst powder (1): Rh-supported first ZrCeNd composite oxide; 72.9 g / L (Rh; 0.32 g / L)
Catalyst powder (2): Rh-supported second active alumina; 29.5 g / L (Rh; 0.1 g / L)
Other cocatalyst (3): first activated alumina; 12.7 g / L
Binder: Zirconia binder; 12.8 g / L
The composition of the catalyst powder (1) is ZrO ₂ : CeO ₂ : Nd ₂ O ₃ = 80: 10: 10 (mass ratio).

[Catalyst layer 3]
Catalyst powder (4): Pd-supported second ZrCeNd composite oxide; 35 g / L (Pd; 0.69 g / L)
Catalyst powder (5): Pd-supported first active alumina; 45.1 g / L (Pd; 1.39 g / L)
Other co-catalyst (6): ZrCeNd composite oxide; 5.72 g / L, ceria; 5.72 g / L
Binder: Zirconia binder; 7.69 g / L
The composition of the catalyst powder (4) is ZrO ₂ : CeO ₂ : Nd ₂ O ₃ = 67: 23: 10 (mass ratio).

[Precursor compound-containing layer 4]
Precursor: 0.22 mol / L relative to catalyst (total amount of catalyst layer 2 and catalyst layer 3)

<Production method of catalyst layer>
The basic production method of the catalyst is as follows.

(Underlayer)
The catalyst powder (1) is formed by using a ZrCeNd composite oxide as a co-catalyst and supporting Rh as a catalyst noble metal by an evaporation to dryness method or the like. Further, the catalyst powder (2) is formed by supporting the second activated alumina as a co-catalyst and supporting Rh as a catalyst noble metal by an evaporation to dryness method or the like. If necessary, the catalyst powders (1) and (2) are pulverized by a mill or the like to adjust the particle size. Catalyst powder (1), catalyst powder (2), other cocatalyst (3), the above binder and water are mixed to prepare a slurry, and this slurry is coated on carrier 1 and dried and fired to form a lower layer. Catalyst 2 is formed.

(Upper layer)
The catalyst powder (4) is formed by using ZrCeNd composite oxide as a co-catalyst and carrying Pd as a catalyst noble metal by an evaporation to dryness method or the like. The catalyst powder (5) is formed by supporting the first activated alumina as a co-catalyst and carrying Pd as a catalyst noble metal by an evaporation-drying method or the like. If necessary, the catalyst powders (4) and (5) are pulverized with a mill or the like to adjust the particle size. The catalyst powder (4), the catalyst powder (5), the other promoter (6), the binder and water are mixed to prepare a slurry, and this slurry is washed on the catalyst layer 2 and dried and fired. Thus, the catalyst layer 3 is formed.

(Precursor layer)
The precursor compound-containing layer 4 is prepared so as to include a precursor in which Ca exists in an unsaturated state and becomes saturated by combining with the catalyst poison in the exhaust gas. Specifically, the precursor in slurry form and the binder are mixed by 10 wt% of the precursor, washed on the catalyst layer 3, dried and fired to form the precursor layer.

<Example 1>
The catalyst layer 5 carries 30 g / L of calcium hydrogen phosphate (Ca (HPO ₄ ) ₂ ) as a precursor compound-containing layer 4 on the surface layer.

<Example 2>
In the catalyst layer 5, as a precursor compound-containing layer 4, 33 g / L of zinc hydrogen phosphate (ZnHPO ₄ ) is supported on the surface layer.

<Comparative example>
The catalyst structure of Example 1 is a catalyst that does not include a precursor layer.

<Catalyst evaluation and cross-sectional surface analysis method>
―Bench aging―
Bench aging treatment was applied to the exhaust gas purifying catalysts of Examples 1 to 5 and Comparative Example. All used were honeycomb catalysts made of ceramics (capacity 1 L) having a cell wall thickness of 3.5 mil (8.89 × 10 ⁻² mm) and 600 cells per square inch (645.16 mm ² ). is there. Each honeycomb catalyst is attached to the engine exhaust system, (1) A / F = 14 exhaust gas flows for 15 seconds → (2) A / F = 17 exhaust gas flows for 5 seconds → (3A / F = 14.7 The engine speed was set so that the cycle of flowing the exhaust gas for 40 seconds was repeated for a total of 200 hours, and the catalyst inlet gas temperature was 930 ° C. About the catalyst of the above embodiment and comparative example The catalyst in a state of being poisoned by the catalyst poisons Ca and Zn was reproduced using engine oil containing P: 0.22 mass%, Ca: 0.49 mass%, and Zn 0.21 mass%.

The light-off temperature T50 relating to the purification of HC, CO and NOx was measured. T50 is the gas temperature (° C.) at the catalyst inlet when the temperature of the model exhaust gas flowing into the catalyst is gradually increased from room temperature and the purification rate reaches 50%. The exhaust gas purification rate C400 is the purification rate of each component of the gas when the model exhaust gas temperature at the catalyst inlet is 400 ° C. The model gas was A / F = 14.7 ± 0.9. That is, the A / F is forced at an amplitude of ± 0.9 by adding a predetermined amount of fluctuation gas in a pulse form at 1 Hz while constantly flowing the main stream gas of A / F = 14.7. Vibrated. The space velocity SV is 60000h-1, and the heating rate is 30 ° C./min. Table 1 shows the gas composition of the model exhaust gas when A / F = 14.7, A / F = 13.8, and A / F = 15.6.

-Evaluation of light-off performance-
A core sample obtained by cutting 25.4 mm × 50 mm from the catalyst inlet end after bench durability was set in the model exhaust gas flow reactor, and the light-off temperature T50 relating to the purification of HC and NOx was measured. T50 is the gas temperature (° C.) at the catalyst inlet when the purification rate reaches 50% by gradually increasing the temperature of the model exhaust gas flowing into the catalyst from room temperature, and represents the low temperature purification performance of the catalyst. is there. The model gas was A / F = 14.7 ± 0.9. That is, the A / F is forced at an amplitude of ± 0.9 by adding a predetermined amount of fluctuation gas in a pulse form at 1 Hz while constantly flowing the main stream gas of A / F = 14.7. Vibrated. The space velocity SV is 60000 h ⁻¹ , and the heating rate is 30 ° C./min.

FIG. 2 shows the results of measuring the purification rate after poisoning durability for Examples (1) and (2) and Comparative Examples.

As can be seen from FIG. 2, the light off (T50) performance of HC and NOx is superior to that of the comparative example in both Example (1) and Example (2). This is because, when a precursor containing unsaturated calcium as in Example (1) is used, calcium in the exhaust gas can be effectively taken in and the poisoning of the catalyst can be suppressed, and as in Example (2). This is because when a precursor containing unsaturated zinc is used, zinc in the exhaust gas can be effectively taken in and poisoning of the catalyst can be suppressed.

-Surface analysis of cross section of poisoned catalyst-
Surface analysis of the cross sections of the catalyst layers before and after the endurance with the catalysts of the comparative example and Example (1) was performed by an electron probe microanalyzer (EPMA). The element mapping results are shown in FIG. The difference in shading in FIG. 3 is that the characteristic X-rays generated by electron beam irradiation are dispersed with energy in the above EPMA, and shading is applied according to the detected value of phosphorus.

The upper part of FIG. 3 shows a surface analysis result of the cross section of the catalyst layer of the comparative example, which is a catalyst not containing a precursor. Moreover, the lower stage of FIG. 3 is a surface analysis result of the catalyst layer cross section of Example (1), and contains calcium hydrogen phosphate as a precursor.

(A) of FIG. 3 is a surface analysis result before poisoning durability. In the embodiment (1), the layer having an uneven surface on the side of the passage through which the exhaust gas flows is a calcium hydrogen phosphate layer.

FIG. 3 (b) shows the results of an area analysis in which the calcium distribution in the catalyst after poisoning durability was examined. From the upper part of FIG. 3B, it can be seen that in the comparative example after the poisoning endurance, calcium is deposited thinly over a wide range on the catalyst upper layer 13.

    On the other hand, from the lower part of FIG. 3B, in the example (1) after the poisoning endurance, the distribution of calcium stays in the precursor layer 24 mainly containing calcium hydrogen phosphate, and the distribution of calcium to the upper layer 3 of the catalyst layer. It can be seen that there is less compared to the comparative example. This indicates that since Example (1) has the precursor layer 24, calcium in the exhaust gas is preferentially deposited on the precursor layer 24 and not on the catalyst layer upper layer 23. That is, Example (1) indicates that the catalytic noble metal and the like are less likely to be poisoned because there is less contact between the catalytic noble metal contained in the catalyst layer 25 and calcium in the exhaust gas.

The above effect is that the presence of the precursor compound-containing layer 4 causes calcium in the exhaust gas to preferentially react with the precursor, and gradually form hydroxyapatite (Ca ₁₀ (PO ₄ ) ₆ (OH) ₂ ). This is considered to be because the poisoning of the catalyst layer to the catalyst noble metal and oxide is suppressed. Analyzing and confirming that the precursor of the precursor compound-containing layer 4 becomes hydroxyapatite (Ca ₁₀ (PO ₄ ) ₆ (OH) ₂ ) after bench aging is performed using XRD (X-ray diffusion). Can do.

The effect of this embodiment (1) is that the above precursor is contained in an amount of 0.22 mol / L or more with respect to the catalyst layer. It reacts to become hydroxyapatite (Ca ₁₀ (PO ₄ ) ₆ (OH) ₂ ) and can prevent calcium poisoning of the catalyst noble metal. Therefore, in order to provide exhaust gas purification performance equivalent to 240,000 km, which is the standard for exhaust gas regulations, calcium phosphate (Ca (HPO ₄ ) ₂ ) is contained in an amount of 0.22 mol / L or more with respect to the catalyst. Is preferred.

Similarly, in Example (2), the precursor compound-containing layer 4 contained zinc hydrogen phosphate as a precursor, and analysis was performed using EPMA. The same result was obtained. That is, zinc as a poisoning substance was deposited on the precursor compound-containing layer, and no zinc was deposited on the catalyst layer.

In Example (2), zinc zinc in the exhaust gas preferentially reacts with the precursor due to the presence of zinc hydrogen phosphate in the precursor compound-containing layer 4, and gradually zinc phosphate (Zn ₃ (PO ₄ It is considered that the formation of ₂ ) suppresses poisoning of the catalyst layer to the catalyst noble metal and oxide. It can be confirmed by analyzing using XRD (X-ray diffraction) that the precursor becomes zinc phosphate (Zn ₃ (PO ₄ ) ₂ ) after bench aging.

The effect of this embodiment (2) is that if the precursor is contained in an amount of 0.22 mol / L or more with respect to the catalyst layer, the catalyst poison and the precursor in the exhaust gas equivalent to traveling at 240,000 km are not insufficient. It reacts to become zinc phosphate (Zn ₃ (PO ₄ ) ₂ ) and can prevent zinc poisoning of the catalyst noble metal. Therefore, in order to provide an exhaust gas purification performance equivalent to 240,000 km, which is a standard for exhaust gas regulations, it is preferable that zinc hydrogen phosphate (ZnHPO ₄ ) is contained in the catalyst in an amount of 0.22 mol / L or more. .

1 is a perspective view and a partial sectional view showing an example of an exhaust gas purifying catalyst of the present invention. It is a figure which shows the light-off performance of HC and NOx of a present Example and a comparative example. It is a surface analysis result of a layer section of the catalyst of the present invention before and after an endurance experiment.

1 shows a honeycomb-shaped carrier carrying a catalyst.
2 Lower layer of catalyst layer 3 Upper layer of catalyst layer 4 Layer containing precursor 5 Catalyst layer (including lower layer 2 of catalyst layer and upper layer 3 of catalyst layer)
6 Exhaust gas purification catalyst 6 (including 1 to 5 above)
11 shows a carrier of a comparative example before endurance 12 shows a lower layer of a catalyst layer of a comparative example before endurance 13 shows an upper layer of a catalyst layer of a comparative example before endurance 21 shows a carrier of Example (1) after endurance 22 after endurance 23 shows the lower layer of the catalyst layer of Example (1) of 23 shows the upper layer of the catalyst layer of Example (1) after endurance 24 shows the precursor layer of Example (1) after endurance 25 Example after endurance (1 ) Shows the catalyst layer

Claims (3)

An exhaust gas purification catalyst that is disposed in an exhaust passage of an engine and purifies exhaust gas containing at least one of calcium and zinc,
The catalyst has a catalyst layer provided on a substrate,
The exhaust gas flow path side surface layer of the catalyst layer is provided with a precursor compound-containing layer that contains Ca or Zn and contains a precursor compound that reacts preferentially with the catalyst poison in the exhaust gas. and,
It said precursor compound, calcium hydrogen phosphate (Ca _{(HPO 4) 2)} and hydrogen phosphate of zinc (ZnHPO ₄₎ of at least one a is <br/> exhaust gas purifying catalyst, characterized in that. In claim 1,
The exhaust gas purifying catalyst , wherein the precursor compound is contained in an amount of 0.22 mol / L or more with respect to the catalyst layer . In claim 1 or claim 2,
The precursor compound reacts preferentially with Ca or Zn which is a catalyst poison in the exhaust gas to form at least one of Ca ₁₀ (PO ₄ ) ₆ (OH) ₂ and Zn ₃ (PO ₄ ) _2. An exhaust gas purifying catalyst characterized by comprising: