Exhaust gas purification catalyst

Abstract

To provide an exhaust gas purification catalyst that can serve as a measure against P poisoning and Ca poisoning of exhaust gas purification catalysts.SOLUTION: Provided is an exhaust gas purification catalyst that is disposed in an exhaust passage of an engine and purifies P and Ca-containing exhaust gas of the engine, and the catalyst includes a support 2 and a catalyst layer 3 formed on the support 2 for purifying the exhaust gas, and the catalyst layer 3 contains Ba3P4O13 in a dispersed manner.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a catalyst for purifying exhaust gas from an engine. [Background technology]

[0002] A catalyst is placed in the exhaust passage of an automobile engine to purify harmful components such as HC (hydrocarbons), CO, and NOx (nitrogen oxides) in the exhaust gas. Meanwhile, 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. Therefore, if the catalyst is used for a long time, its performance will deteriorate due to poisoning by these substances.

[0003] Patent Document 1 describes that in order to suppress P poisoning of a catalyst made by supporting Rh on a CeZrNd composite oxide, Ba is supported on the CeZrNd composite oxide. Ba forms barium phosphate with P in the exhaust gas, and P poisoning of the catalyst is suppressed.

[0004] Patent Document 2 describes that the surface of a catalyst layer containing ceria and alumina is coated with barium phosphate. Since barium phosphate has a phosphate group and does not attract the P component in the exhaust gas, the phosphorus component is blocked by the barium phosphate and passes through the three-way catalyst, thereby suppressing P poisoning of the ceria and alumina in the catalyst layer.

[0005] Patent Document 3 describes that a catalyst layer containing oxides of Ce, Al, etc. is covered with a surface layer containing an unsaturated phosphate. The unsaturated phosphate combines with P in the exhaust gas to form a saturated phosphoric acid compound, thereby suppressing P poisoning of oxides of Ce, Al, etc. in the catalyst layer. Examples of unsaturated phosphate include a hydrogen phosphate compound containing at least one selected from Ca, Al, and Zn, and a phosphoric acid compound containing La. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] JP 2003-170047 A [Patent Document 2] Patent No. 4161722 [Patent Document 3] JP 2015-66529 A Summary of the Invention [Problem to be solved by the invention]

[0007] In addition to P, which has been the subject of various countermeasures, Ca is another catalyst poison, but there have been few reports on countermeasures against Ca poisoning. Ca in exhaust gas reacts with P to form calcium phosphate (β-Ca3(PO4)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 catalyst's exhaust gas purification performance.

[0008] An object of the present invention is to provide a countermeasure against P-poisoning and Ca-poisoning of exhaust gas purifying catalysts. [Means for solving the problem]

[0009] In order to solve the above problems, the present invention disperses a compound that traps Ca in exhaust gas and forms calcium phosphate in a catalyst layer.

[0010] The catalyst for purifying exhaust gas 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 is made of Ba3P4O 13 It is characterized by containing dispersed therein.

[0011] According to this, P and Ca in the exhaust gas are converted into Ba3P4O 13The inventors have formed β-Ca3(PO4)2 and Ba3P4O 13 By overlaying the electron diffraction patterns of each, it was confirmed that the crystallographic phases of the two are identical. This crystallographic relationship between the two indicates that β-Ca3(PO4)2 is Ba3P4O 13 Therefore, the catalytic components of the catalytic layer are prevented from being poisoned by P and Ca. In addition, although β-Ca3(PO4)2 is glassy, ​​its formation site is on the Ba3P4O of the catalytic layer. 13 Since the above is localized, it is possible to avoid impeding the diffusion of exhaust gas in the catalyst layer, and 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.

[0012] 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-Ca3(PO4)2 and is converted to Ba3P4O 13 Therefore, P in the exhaust gas forms AlPO4 to reduce the surface area of ​​the activated alumina, and P forms CePO4 to reduce the surface area of ​​the Ce-containing oxygen storage and release material. 3+ ⇔Ce 4+ This can avoid the inhibition of the valence change of the oxygen storage and release property.

[0013] 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 an Rh catalyst layer containing Rh-supported alumina in which Rh is supported as the precious metal on the activated alumina, and an Rh-supported oxygen storage-release material in which Rh is supported as the precious metal on the Ce-containing oxygen storage-release material, The above Ba3P4O 13 is contained in the outer catalyst layer.

[0014] 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 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. Thus, the outer Rh catalyst layer, which is in a more severe environment than the inner catalyst layer in terms of poisoning, is coated with Ba3P4O 13 By providing the above, the poisoning of the Rh catalyst layer by P and Ca is effectively suppressed, and therefore the catalytic performance can be maintained for a long period of time. Effect of the Invention

[0015] According to the present invention, the catalyst layer for purifying exhaust gas is Ba3P4O 13 Since it contains dispersed P and Ca in the exhaust gas, 13 The catalyst is trapped by forming β-Ca3(PO4)2 on the surface of the catalyst, effectively suppressing the poisoning and deterioration of the catalyst by P and Ca. [Brief description of the drawings]

[0016] [Figure 1] 3A and 3B are a perspective view and a partial cross-sectional view of a honeycomb carrier; [Diagram 2] FIG. [Diagram 3] FIG. 4 is a graph showing the light-off temperature T50 for HC and NOx purification in the examples and comparative examples. [Figure 4] 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. [Diagram 5] A figure showing the ED images of Ba3P4O13 and β-Ca3(PO4)2 and the superposition of the two ED images. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] Hereinafter, the embodiments of the present invention will be described with reference to the drawings. The following description of the preferred embodiments is merely illustrative in nature and is not intended to limit the present invention, its application, or its uses.

[0018] (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 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.

[0019] 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 using Pd as a catalyst metal (hereinafter also referred to as "Pd catalyst layer"). The outer catalyst layer 3b is formed on the inner catalyst layer 3a (on the exhaust gas passage side) and is a catalyst layer using Rh as a catalyst metal (hereinafter also referred to as "Rh catalyst layer").

[0020] The Pd in ​​the inner catalyst layer 3a is supported on activated alumina and a Ce-containing oxygen storage / release material. The inner catalyst layer 3a further contains activated alumina that does not support a catalytic metal as a promoter.

[0021] 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 Ba3P4O 13 It contains dispersed

[0022] Examples of Ce-containing oxygen storage and release materials that can be used include composite oxides of Ce and Zr, composite oxides of Ce, Zr, and rare earth elements (Nd, La, Y, Pr, etc.), and ceria. Such composite oxides can be prepared by coprecipitation or the like.

[0023] Ba3P4O as a catalyst poisoning inhibitor 13 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 air 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) are as follows:

[0024] [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 (12.7g / L) Catalyst poisoning inhibitor; Ba3P4O 13 (Ba loading equivalent to 30g / L) Binder: Zirconia binder (12.8g / L) The Rh-supported ZrCeNd composite oxide is a catalyst powder in which Rh is supported on ZrCeNd composite oxide by the evaporation-to-dryness method. The composition of the ZrCeNd composite oxide is ZrO2:CeO2:Nd2O3=80:10:10 (mass ratio). The Rh-supported activated alumina is a catalyst powder in which Rh is supported on activated alumina by the evaporation-to-dryness method. The activated alumina is La-stabilized alumina. This is the same for the cocatalyst (3) and other activated aluminas described below. "g / L" refers to the amount supported per 1 L of carrier. This is also the same for the inner catalyst layer described next.

[0025] [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 (7.69g / L) Pd-supported ZrCeNd composite oxide is a catalyst powder in which Pd is supported on ZrCeNd composite oxide by evaporation to dryness. The composition of ZrCeNd composite oxide is ZrO2:CeO2:Nd2O3=67:23:10 (mass ratio). Pd-supported activated alumina is a catalyst powder in which Pd is supported on activated alumina.

[0026] -Catalyst preparation method- The catalyst powders (4) and (5), the co-catalyst (6) and the binder are mixed to form a slurry, which is then coated onto the honeycomb carrier. The honeycomb carrier is then fired in air at 450°C for 2 hours to form the inner catalyst layer (Pd catalyst layer). Next, the catalyst powders (1) and (2), the co-catalyst (3) and the catalyst poisoning prevention agent Ba3P4O 13 The rhodium and binder are mixed to form a slurry, which is then coated onto the inner catalyst layer, and calcined in air at 450° C. for 2 hours to form the outer catalyst layer (Rh catalyst layer). <Evaluation of exhaust gas purification performance> The catalyst of the present embodiment was prepared by the above-mentioned component composition and preparation method. The honeycomb carrier used had a diameter of 25 mm and a length of 50 mm.

[0027] For the evaluation of 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 those of the Example catalysts.

[0028] -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 the 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).

[0029] -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. Thereafter, a barium hydroxide solution serving as a catalyst poisoning prevention agent 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.

[0030] --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 on 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).

[0031] -Evaluation method- The catalysts of the examples 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 simulated exhaust gas.

[0032] (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.

[0033] (Measurement of HC and NOx purification rate) Each of the catalysts of the examples 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.

[0034] For the simulated exhaust gas, two types of fluctuating gases were added in a pulsed manner to the base gas with an A / F ratio of 14.7, giving 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 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 fluctuation pattern was base gas only (0.25 seconds) → lean (0.25 seconds) → base gas only (0.25 seconds) → rich (0.25 seconds), and the A / F was fluctuated by repeating this cycle. 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 is 15°C / min.

[0035] (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 because Ba3P4O 13 This is the effect of dispersing the catalyst in the outer catalyst layer.

[0036] FIG. 4 shows a high-angle annular dark-field scanning transmission electron microscope (HAADF-STEM) 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 Ba3P4O 13The map image is due to Ba in the exhaust gas, and the Ca map image is due to Ca in β-Ca3(PO4)2 generated from the Ca and P components in the exhaust gas. According to the mapping data, the Ca map image and the Ba map image are adjacent to each other, so it is Ba3P4O 13 It can be said that β-Ca3(PO4)2 is formed on the

[0037] Figure 5 shows Ba3P4O 13 The electron diffraction (ED) images of Ba3P4O and β-Ca3(PO4)2 are shown, along with a superposition of the two ED images. The superposition of the two ED images shows that the Ca and P in the exhaust gas are Ba3P4O 13 It is suggested that the reason for the formation of β-Ca3(PO4)2 on Ba3P4O is the coincidence of crystallographic phases. 13 Therefore, the catalytic components of the catalytic layer are prevented from being poisoned by P and Ca. In addition, although β-Ca3(PO4)2 is glassy, ​​its formation is easily caused by the Ba3P4O 13 Since the above is localized, it is possible to avoid impeding the diffusion of exhaust gas in the catalyst layer.

[0038] Although the catalyst layer in the above embodiment has a two-layer structure, the catalyst layer may have 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. [Explanation of symbols]

[0039] 1. Exhaust gas purification catalyst 2. Honeycomb carrier 3 Catalyst layer 3a Inner catalyst layer 3b Outer catalyst layer

Claims

1. 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 catalyst layer is Ba ３ P ４ O １３ 1. A catalyst for purifying exhaust gas, comprising:

2. 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.

3. 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, The above Ba ３ P ４ O １３ The exhaust gas purifying catalyst is characterized in that the outer catalyst layer is comprised of:

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