JP7619155B2 - Exhaust gas purification catalyst - Google Patents

Description

The present invention relates to a catalyst for purifying engine exhaust gas.

Catalysts are placed in the exhaust passage of an automobile engine to purify harmful components in the exhaust gas, such as HC (hydrocarbons), CO, and NOx (nitrogen oxides). Incidentally, engine oil with added antioxidants is necessary for the engine to operate. This engine oil contains substances such as P and Ca that act as catalyst poisons, and these are mixed into the exhaust gas. As a result, if the catalyst is used for a long time, its performance will deteriorate due to poisoning by these substances.

Patent Document 1 describes the use of Ba supported on a CeZrNd composite oxide to suppress P poisoning of the catalyst, which is made by supporting Rh on the CeZrNd composite oxide. Ba forms barium phosphate with P in the exhaust gas, which suppresses P poisoning of the catalyst.

Patent Document 2 describes coating the surface of a catalyst layer containing ceria and alumina with barium phosphate. Since barium phosphate has phosphate groups and does not attract the P components in the exhaust gas, the phosphorus components are blocked by the barium phosphate and pass through the three-way catalyst, thus suppressing P poisoning of the ceria and alumina in the catalyst layer.

Patent Document 3 describes covering a catalyst layer containing oxides of Ce, Al, etc. with a surface layer containing unsaturated phosphate. The unsaturated phosphate combines with P in the exhaust gas to form a saturated phosphate compound, which suppresses P poisoning of oxides of Ce, Al, etc. in the catalyst layer. Examples of unsaturated phosphate include hydrogen phosphate compounds containing at least one element selected from Ca, Al, and Zn, and phosphate compounds containing La.

JP 2003-170047 A Patent No. 4161722 JP 2015-66529 A

In addition to P, which has been the subject of various countermeasures, Ca is another catalyst poisoning substance, but there have been few reports on countermeasures against Ca poisoning. Ca in exhaust gas reacts with P to form calcium phosphate (β-Ca ₃ (PO ₄ ) ₂ ). This calcium phosphate is glassy, and by covering the catalyst layer, it inhibits the diffusion of exhaust gas into the catalyst layer. This reduces the exhaust gas purification performance of the catalyst.

The objective of this invention is to deal with P and Ca poisoning of exhaust gas purification catalysts.

To solve the above problem, the present invention disperses a compound in the catalyst layer that traps Ca in the exhaust gas and forms calcium phosphate.

The exhaust gas purifying catalyst disclosed herein is disposed in an exhaust passage of an engine and purifies exhaust gas containing P and Ca from the engine _. The catalyst comprises a carrier and a catalyst layer formed on the carrier for purifying the exhaust gas, the catalyst layer containing dispersed _{Ba3P4O13 .}

According to this, P and Ca in the exhaust gas are trapped by forming β-Ca ₃ (PO ₄ ) ₂ on Ba ₃ P ₄ O ₁₃ of the catalyst layer. The inventors have confirmed that the crystallographic phases of β-Ca ₃ (PO ₄ ) ₂ and Ba ₃ P ₄ O ₁₃ are identical to each other by superimposing their electron beam diffraction images. Due to this crystallographic relationship between the two, β-Ca ₃ (PO ₄ ) ₂ is easily formed continuously on Ba ₃ P ₄ O _13. Therefore, the catalytic components of the catalyst layer are prevented from being poisoned by P and Ca. In addition, although β-Ca ₃ (PO ₄ ) ₂ is glassy, its formation is localized on Ba ₃ P ₄ O ₁₃ of the catalyst layer, so that the diffusion of the exhaust gas in the catalyst layer is prevented from being hindered. Therefore, according to the present invention, the poisoning and deterioration of the catalyst caused by P and Ca in the exhaust gas can be effectively suppressed.

In one embodiment, the catalyst layer contains a noble metal-supported alumina in which a noble metal is supported on activated alumina, and a Ce-containing oxygen storage and release material. As described above, P in the exhaust gas forms β-Ca ₃ (PO ₄ ) ₂ and is trapped in Ba ₃ P ₄ O _13. This prevents P in the exhaust gas from forming AlPO ₄ to reduce the surface area of the activated alumina, and prevents P from forming CePO ₄ to inhibit the valence change of Ce ³⁺ ⇔ Ce ⁴⁺ in the Ce-containing oxygen storage and release material, thereby reducing the oxygen storage and release performance.

In one embodiment, the catalyst layer comprises an inner catalyst layer and an outer catalyst layer;
the inner catalyst layer is a Pd catalyst layer containing Pd-supported alumina in which Pd is supported as the precious metal on the activated alumina, and a Pd-supported oxygen storage-release material in which Pd is supported as the precious metal on the Ce-containing oxygen storage-release material,
the outer catalyst layer is a Rh catalyst layer containing Rh-supported alumina in which Rh is supported as the precious metal on the activated alumina, and a Rh-supported oxygen storage-release material in which Rh is supported as the precious metal on the Ce-containing oxygen storage-release material,
The Ba ₃ P ₄ O ₁₃ is contained in the outer catalyst layer.

According to this, the decomposition of NOx in the exhaust gas is mainly carried out in the outer Rh catalyst layer, and the oxidative decomposition of HC and the oxidation of CO in the exhaust gas are mainly carried out in the inner Pd catalyst layer. Since the Pd catalyst layer is on the inside and the Rh catalyst layer is on the outside, the barrier effect of the Rh catalyst layer makes it possible to avoid thermal deterioration of Pd and Pb _poisoning and S poisoning. And since _Ba3P4O13 is placed in the outer Rh catalyst layer, which is in a _more severe environment than the inner catalyst layer in terms of poisoning, poisoning of this Rh catalyst layer by P and Ca is effectively suppressed, and therefore the catalytic performance can be maintained for a long period of time.

According to the present invention, the catalyst layer for purifying exhaust gas contains dispersed Ba ₃ P ₄ O ₁₃ , so that P and Ca in the exhaust gas are trapped by forming β-Ca ₃ (PO ₄ ) ₂ on Ba ₃ P ₄ O ₁₃ , and the poisoning and deterioration of the catalyst by P and Ca is effectively suppressed.

3A and 3B are a perspective view and a partial cross-sectional view of a honeycomb carrier; FIG. FIG. 4 is a graph showing the light-off temperature T50 for HC and NOx purification in the examples and comparative examples. FIG. 1 shows an HAADF-STEM image of an example catalyst and STEM-EDS mapping of Ba and Ca in the same field of view. 1 shows the ED images of Ba ₃ P ₄ O ₁₃ and β-Ca ₃ (PO ₄ ) ₂ , and the overlapping of both ED images.

The following describes the embodiments of the present invention with reference to the drawings. The following description of the preferred embodiment is merely exemplary in nature and is not intended to limit the present invention, its applications, or its uses.

(Configuration of exhaust gas purification catalyst)
An exhaust gas purifying catalyst 1 according to an embodiment of the present invention shown in Fig. 1 is disposed in an exhaust passage of an automobile engine and purifies HC, CO, and NOx in the exhaust gas of the engine. The exhaust gas purifying catalyst 1 is formed by forming a catalyst layer 3 on the cell walls (exhaust gas passage walls) of a honeycomb carrier 2 made of cordierite. In Fig. 1, the cross-sectional shape of the cells of the honeycomb carrier is shown as rectangular, but is not limited thereto and may be, for example, a hexagonal cell honeycomb structure.

As shown in FIG. 2, the catalyst layer 3 has a two-layer structure consisting of an inner catalyst layer 3a and an outer catalyst layer 3b. The inner catalyst layer 3a is formed on the cell walls of the honeycomb carrier 2 and is a catalyst layer that uses Pd as the catalyst metal (hereinafter also referred to as the "Pd catalyst layer"). The outer catalyst layer 3b is formed on top of the inner catalyst layer 3a (on the exhaust gas passage side) and is a catalyst layer that uses Rh as the catalyst metal (hereinafter also referred to as the "Rh catalyst layer").

The Pd in the inner catalytic layer 3a is supported on activated alumina and a Ce-containing oxygen storage and release material. The inner catalytic layer 3a further contains activated alumina that does not support a catalytic metal as a promoter.

The Rh in the outer catalyst layer 3b is supported on activated alumina and a Ce-containing oxygen storage/release material. The outer catalyst layer 3b further contains an oxygen storage/release material that does not support _{a catalytic metal as a promoter, and Ba3P4O13 is} dispersed as a catalyst poisoning prevention _agent .

As the Ce-containing oxygen storage and release material, for example, a composite oxide of Ce and Zr, a composite oxide of Ce, Zr and a rare earth element (Nd, La, Y, Pr, etc.), or ceria can be used. Such composite oxides can be prepared by a coprecipitation method, etc.

_Ba3P4O13 as a catalyst poisoning inhibitor can be prepared as follows: Barium carbonate _BaCO3 and diammonium phosphate ( _NH4 ) _2HPO4 aqueous solution are mixed so that the molar ratio of _Ba : _P is 1: ₁ . This mixture is dried at 120°C in an air atmosphere, and then fired at 800°C for 2 hours in _N2 gas containing 0.6 vol% CO. The fired product is pulverized to form fine particles.
<Example>
The components of the outer catalyst layer (Rh catalyst layer) and the inner catalyst layer (Pd catalyst layer) were as follows:

[Outer catalyst layer (Rh catalyst layer)]
Catalyst powder (1): Rh-supported ZrCeNd composite oxide (72.9 g/L (Rh; 0.32 g/L))
Catalyst powder (2): Rh-supported activated alumina (29.5 g/L (Rh; 0.1 g/L))
Cocatalyst (3): Activated alumina (12.7 g/L)
Catalyst poisoning prevention agent: Ba ₃ P ₄ O ₁₃ (Ba loading amount: 30 g/L)
Binder: Zirconia binder (12.8 g/L)
The Rh-supported ZrCeNd composite oxide is a catalyst powder in which Rh is supported on the ZrCeNd composite oxide by evaporation to dryness. The composition of the ZrCeNd composite oxide is ZrO ₂ :CeO ₂ :Nd ₂ O ₃ =80: The ratio is 10:10 (mass ratio). Rh-supported activated alumina is a catalyst powder in which Rh is supported on activated alumina by evaporation to dryness. The activated alumina is La-stabilized alumina. 3) and other activated aluminas described below are the same. "g/L" refers to the amount supported per 1 L of carrier. This point is also the same for the inner catalyst layer described next.

[Inner catalyst layer (Pd catalyst layer)]
Catalyst powder (4): Pd-supported ZrCeNd composite oxide (35 g/L (Pd; 0.69 g/L))
Catalyst powder (5): Pd-supported activated alumina (45.1 g/L (Pd; 1.39 g/L))
Cocatalyst (6): ZrCeNd composite oxide (5.72 g/L), ceria (5.72 g/L)
Binder: Zirconia binder (7.69 g/L)
The Pd-supported ZrCeNd composite oxide is a catalyst powder in which Pd is supported on the ZrCeNd composite oxide by evaporation to dryness. The composition of the ZrCeNd composite oxide is ZrO ₂ :CeO ₂ :Nd ₂ O ₃ =67:23. The mass ratio of Pd-supported activated alumina is 10. The Pd-supported activated alumina is a catalyst powder in which Pd is supported on activated alumina.

-Catalyst preparation method-
The catalyst powders (4), (5), the co-catalyst (6) and the binder are mixed to form a slurry, which is then coated onto the honeycomb carrier. The mixture is then fired in air at 450°C for two hours to form an inner catalyst layer (Pd catalyst layer). Next, the catalyst powders (1), (2), the co-catalyst ( _{3), the catalyst poisoning prevention agent Ba3P4O13 and the} binder are mixed to form a slurry, which is then coated onto the inner catalyst layer. The mixture is then fired in air at 450°C for two hours to form an outer catalyst layer (Rh catalyst layer).
<Evaluation of exhaust gas purification performance>
The example catalyst was prepared using the above-mentioned component composition and preparation method. The honeycomb carrier used had a diameter of 25 mm and a length of 50 mm.

To evaluate the purification performance, the following catalysts of Comparative Examples 1-3 were prepared. The honeycomb carriers of Comparative Examples 1-3 also had the same dimensions as the example catalysts.

--Comparative Example 1--
In the same manner as in the example, an inner catalyst layer (Pd catalyst layer) was formed on a honeycomb carrier. Next, the catalyst powders (1), (2), the co-catalyst (3) and a binder were mixed to form a slurry, which was then coated on the inner catalyst layer and then fired in air at 450°C for 2 hours to form an outer catalyst layer (Rh catalyst layer, no measures were taken to prevent catalyst poisoning).

--Comparative Example 2--
An inner catalyst layer (Pd catalyst layer) and an outer catalyst layer (Rh catalyst layer) were formed in the same manner as in Comparative Example 1. Then, a barium hydroxide solution serving as a catalyst poisoning inhibitor was impregnated into the inner and outer catalyst layers so that the total Ba loading amount was 30 g/L, and the layers were subjected to a calcination treatment in air at 450° C. for 2 hours.

--Comparative Example 3--
An inner catalyst layer (Pd catalyst layer) was formed in the same manner as in Comparative Example 1. Next, catalyst powders (1), (2), cocatalyst (3), 36 g/L of _LaPO4 (unsaturated phosphate) as a catalyst poisoning inhibitor, and a binder were mixed to form a slurry, which was then coated onto the inner catalyst layer. Then, the slurry was fired in air at 450°C for 2 hours to form an outer catalyst layer (Rh catalyst layer).

-Evaluation method-
The catalysts of the Example and Comparative Examples 1 to 3 were subjected to the following bench aging, and then the purification performance of HC and NOx was examined using a simulated exhaust gas.

(Bench aging)
Using an engine bench system, the catalyst was aged by repeating the following engine operating modes: idle operation (A/F (air-fuel ratio) = 14.7) for 1 minute → acceleration operation (engine speed 3560 rpm, A/F = 13.5) for 1 minute → steady operation (engine speed 3300 rpm, A/F = 14.7) for 2 minutes → fuel cut operation (A/F > 20) → return to idle operation. The catalyst temperature was 930°C, and the aging time was 100 hours. During this aging, engine oil was supplied to the engine from the intake manifold at a flow rate of 30 mL/h.

(Measurement of HC and NOx purification rates)
Each of the catalysts of the Example and Comparative Examples 1 to 3 after bench aging was attached to a gas flow reactor, and the temperature of the simulated exhaust gas flowing into the catalyst was gradually increased from room temperature, while the concentrations of HC and NOx in the gas flowing out of the catalyst were detected to determine the purification rates of HC and NOx.

For the simulated exhaust gas, two types of fluctuating gases were added in a pulsed manner to a base gas with an A/F ratio of _14.7 , to give the A/F a fluctuation of ±0.9 at 1 Hz. The base gas composition was HC= _C3H6 ; 1500 ppmC, CO; 0.56%, _O2 ; 0.6%, _H2 ; 1800 ppm, _CO2 ; 13.9%, _H2O ; 10.0%, and the balance was _N2 . 1.05% _O2 was added to this base gas as a fluctuating gas to give an A/F ratio of 15.6 (lean), and 2% CO and 5400 ppm _H2 were added as fluctuating gases to give an A/F ratio of 13.6 (rich). The A/F ratio was varied in a repeating pattern of base gas only (0.25 seconds) → lean (0.25 seconds) → base gas only (0.25 seconds) → rich (0.25 seconds). The flow rate of the simulated exhaust gas was 26.1 L/min (space velocity SV = approximately 60,000 h ^-1 ), and the temperature rise rate of the simulated exhaust gas was 15°C/min.

(Measurement results)
The gas temperature T50(HC) at the catalyst inlet when the HC purification rate reaches 50% and the gas temperature T50(NOx) at the catalyst inlet when the NOx purification rate reaches 50% are shown in Fig. 3. According to the figure, the effect of suppressing catalyst poisoning is observed in Comparative Example 2 in which the catalyst layer is impregnated with barium hydroxide and Comparative Example 3 in which _LaPO4 is added to the outer catalyst layer, but the embodiment shows a more remarkable effect of suppressing _{catalyst poisoning than Comparative Examples 2 and 3. This is the effect of dispersing Ba3P4O13 in the} outer catalyst layer.

4 shows a HAADF-STEM (high angle annular dark field scanning transmission electron microscope) image of the outer catalyst layer of the example catalyst, and mapping data of Ba and Ca measured in the same field of view by STEM-EDS (energy dispersive X-ray spectrometry). The Ba map image of the mapping data is due to Ba in Ba ₃ P ₄ O ₁₃ , and the Ca map image is due to Ca in β-Ca ₃ (PO ₄ ) ₂ generated from the Ca and P components in the exhaust gas. According to the mapping data, since the Ca map image and the Ba map image are adjacent to each other, it can be said that β-Ca ₃ (PO ₄ ) ₂ is formed on Ba ₃ P ₄ O ₁₃ .

Figure 5 shows the ED (electron beam diffraction) images of Ba ₃ P ₄ O ₁₃ and β-Ca ₃ (PO ₄ ) ₂ , and the overlap of the two ED images. The overlapping of the two ED images shows that they match, suggesting that the reason why Ca and P in the exhaust gas form β-Ca ₃ (PO ₄ ) ₂ on Ba ₃ P ₄ O ₁₃ is due to the coincidence of crystallographic phases. In other words, due to the crystallographic relationship between the two, β-Ca ₃ (PO ₄ ) ₂ is likely to be formed continuously on Ba ₃ P ₄ O _13. Therefore, the catalytic components of the catalytic layer are prevented from being poisoned by P and Ca. In addition, although β-Ca ₃ (PO ₄ ) ₂ is glassy, its formation is localized on Ba ₃ P ₄ O ₁₃ of the catalyst layer, so that the diffusion of exhaust gas in the catalyst layer is not hindered.

Although the catalyst layer in the above embodiment has a two-layer structure, the catalyst layer may be a single layer or a laminated structure of three or more layers. The catalyst metal is not limited to Rh or Pd, but may be other noble metals such as Pt or other transition metals.

Reference Signs List 1: exhaust gas purification catalyst 2: honeycomb carrier 3: catalyst layer 3a: inner catalyst layer 3b: outer catalyst layer

Claims (3)

An exhaust gas purifying catalyst that is disposed in an exhaust passage of an engine and purifies exhaust gas containing P and Ca from the engine, comprising:
a catalyst layer formed on the carrier for purifying the exhaust gas;
The exhaust gas purifying catalyst is characterized in that the catalyst layer contains Ba ₃ P ₄ O ₁₃ in a dispersed state. In claim 1,
The catalyst layer is characterized in that it contains a noble metal-supported alumina in which a noble metal is supported on activated alumina, and a Ce-containing oxygen storage/release material. In claim 2,
The catalyst layer comprises an inner catalyst layer and an outer catalyst layer,
the inner catalyst layer contains Pd-supported alumina in which Pd is supported as the precious metal on the activated alumina, and a Pd-supported oxygen storage-release material in which Pd is supported as the precious metal on the Ce-containing oxygen storage-release material,
the outer catalyst layer contains Rh-supported alumina in which Rh is supported as the precious metal on the activated alumina, and a Rh-supported oxygen storage-release material in which Rh is supported as the precious metal on the Ce-containing oxygen storage-release material,
An exhaust gas purifying catalyst, characterized in that the Ba ₃ P ₄ O ₁₃ is contained in the outer catalyst layer.

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