Heat storage exhaust gas purification catalyst

Abstract

The present invention provides a heat-storage exhaust gas purification catalyst that can slow down the rate at which the temperature of the exhaust gas purification catalyst decreases, thereby reducing the energy consumption required to maintain the catalyst temperature. [Solution] The heat storage exhaust gas purification catalyst of the present invention comprises a catalyst coating layer on the surface of a substrate containing catalyst particles and latent heat storage particles. Furthermore, the solidification point of the latent heat storage material is above the activation temperature of the catalyst particles. Since the catalyst coating layer has a two-layer structure consisting of a surface layer and an inner layer, and the content of the latent heat storage particles differs between the surface layer and the inner layer, it is possible to provide a heat storage exhaust gas purification catalyst that can maintain the activation temperature for a long period of time.

Description

Technical Field

[0001] The present invention relates to a heat storage exhaust gas purification catalyst.

Background Art

[0002] Exhaust gas from internal combustion engines contains hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NOx) as harmful components, and catalysts are used to purify these harmful components.

[0003] Patent Document 1 discloses an exhaust gas purification catalyst in which a concentration gradient of a noble metal is formed in the thickness direction in a catalyst coating layer having high aspect ratio pores with excellent gas diffusibility.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] Since the activation temperature at which an exhaust gas purification catalyst for an internal combustion engine can purify harmful components is generally 300 to 400°C or higher, it is preferable that the catalyst be maintained in a high temperature state.

[0006] However, in vehicles having a function of stopping the internal combustion engine during vehicle travel, such as hybrid vehicles, when the internal combustion engine stops, exhaust gas that serves as a heat source for the catalyst is not supplied from the internal combustion engine, and if the temperature of the catalyst drops, the catalyst does not function sufficiently when the internal combustion engine is restarted, and the purification ability cannot be obtained.

[0007]

[0008] This invention has been made in view of the problems of the prior art, and its objective is to provide a heat-storage exhaust gas purification catalyst that can slow down the rate at which the temperature of the exhaust gas purification catalyst decreases, thereby reducing the energy consumption required to maintain the temperature of the catalyst. [Means for solving the problem]

[0009] The inventors of this invention conducted extensive research to achieve the above objectives and discovered that these objectives could be achieved by unevenly distributing the content of the latent heat storage material between the surface and the interior, thus completing the present invention.

[0010] In other words, the heat storage exhaust gas purification catalyst of the present invention comprises a catalyst coating layer on the surface of a substrate containing catalyst particles and latent heat storage particles. Furthermore, the solidification point of the latent heat storage material is above the activation temperature of the catalyst particles. The catalyst coating layer has a two-layer structure comprising a surface layer and an inner layer, and is characterized in that the content of the latent heat storage particles differs between the surface layer and the inner layer. [Effects of the Invention]

[0011] According to the present invention, by unevenly distributing the content of the latent heat storage material between the surface and the interior, the latent heat from the latent heat storage material is transferred to the catalyst, slowing down the rate at which the catalyst temperature decreases, and providing a heat storage exhaust gas purification catalyst that can maintain its activation temperature for a long period of time. [Brief explanation of the drawing]

[0012] [Figure 1] This graph shows the temperature change when the heat storage exhaust gas purification catalyst is cooled. [Modes for carrying out the invention]

[0013] The heat storage exhaust gas purification catalyst of the present invention will be described in detail. The heat storage exhaust gas purification catalyst of the present invention comprises a catalyst coating layer on the surface of a honeycomb-shaped ceramic substrate, and this catalyst coating layer contains catalyst particles and latent heat storage particles.

[0014] The above-mentioned latent heat storage particles are a phase-change type latent heat storage material that stores and releases heat by utilizing a solid-liquid phase change, and consist of a core containing an aluminum alloy and a shell formed of a metal oxide, with the core covered by the shell.

[0015] These latent heat storage particles are heated by exhaust gases when the engine is running, causing their temperature to rise and melting the aluminum alloy core material, which then stores heat. When the engine stops, the temperature drops, but as the aluminum alloy solidifies, it releases latent heat, allowing it to release more heat than its specific heat capacity.

[0016] However, whether or not the temperature of the catalyst particles can be maintained above the activation temperature depends on the competition between the cooling rate of the catalyst particles and the heating rate due to the release of latent heat. The latent heat from the latent heat storage particles must be released all at once near the activation temperature of the catalyst particles to heat them.

[0017] In other words, since the cooling rate of the catalyst particles is almost constant, as shown by the solid line in Figure 1, the rate of latent heat release is slow, and if the amount of heat that the latent heat heats the catalyst particles per unit time is small, the ratio of the amount of heat removed from the catalyst particles by cooling to the amount of heat that the latent heat heats the catalyst particles per unit time becomes large, and the temperature maintenance effect of the latent heat storage particles on the catalyst particles is reduced.

[0018] In the heat storage exhaust gas purification catalyst of the present invention, instead of uniformly dispersing latent heat storage particles in the catalyst coating layer, they are unevenly distributed in the surface layer or inner layer, and the content of latent heat storage particles differs between the surface layer and the inner layer. As shown by the dashed and dotted lines in Figure 1, the latent heat released by the latent heat storage particles is transferred to the catalyst particles all at once, making it possible to maintain the temperature of the catalyst particles above the activation temperature for a long period of time.

[0019] In the present invention, the "activation temperature" refers to the temperature at which the purification rates of NOx and CO reach 95%.

[0020] The proportion of uneven distribution of latent heat storage particles in the catalyst coating layer is such that the latent heat storage particles contained in the surface layer and the inner layer do not have to be the same, and either the surface layer or the inner layer may contain more latent heat storage particles. However, it is preferable that the surface layer contains 60% or more of the latent heat storage particles, or the inner layer contains 67% or more of the latent heat storage particles, based on the total amount of the latent heat storage particles contained in the catalyst coating layer.

[0021] As described above, due to the uneven distribution of the latent heat storage particles in the catalyst coating layer, the latent heat of the latent heat storage particles is concentrated and transmitted to the catalyst particles, so it is possible to maintain the temperature of the catalyst particles above the activation temperature for a long time.

[0022] As the core of the latent heat storage particles, an alloy having a freezing point equal to or higher than the activation temperature of the catalyst particles can be used. Examples of the alloy include ZnAl alloy and SiAl alloy, and these alloys may further contain other metal components.

[0023] The ZnAl alloy includes, for example, a zinc alloy in which the zinc content is 83% by mass or more and 98% by mass or less, and the aluminum content is 2% by mass or more and 17% by mass or less. When the contents of zinc and aluminum are as described above, the effect of rapidly releasing the absorbed heat when becoming solid and being likely to cause a temperature rise in the catalyst coating layer is more easily obtained.

[0024] The type of other metal components is not particularly limited, and examples thereof include magnesium, tin, indium, and lead.

[0025] The shell of the latent heat storage particles prevents the core from melting and flowing out, and suppresses the oxidation of the core and the deterioration of the performance as a latent heat storage material.

[0026] As the shell mentioned above, metal oxides can be used, for example, aluminum oxide (Al2O3) or zinc oxide (ZnO). These metal oxides may also contain other metal oxides, such as P2O5, SiO2, and TiO2.

[0027] The diameter of the latent heat storage particles is not particularly limited, but a smaller diameter is preferable. For example, the diameter of the latent heat storage particles is preferably 50 μm or less, and more preferably 30 μm or less. The smaller the diameter of the latent heat storage particles, the faster the melting and solidification rates of the latent heat storage particles become, making it possible to store and release heat in a short time.

[0028] The above-mentioned latent heat storage particles can be produced by hydrothermal treatment. Specifically, core-shell latent heat storage particles can be produced by oxidizing core alloy particles by hydrothermal treatment and forming a metal oxide shell on their surface.

[0029] Examples of the catalyst particles mentioned above include particles of precious metals such as platinum (Pt), palladium (Pd), and rhodium (Rh). [Examples]

[0030] The present invention will be described in detail below with reference to examples, but the present invention is not limited to the following examples.

[0031] [Example 1] ZnAl alloy particles (Zn-5%Al: melting point 381℃: 38μm) were subjected to hydrothermal treatment at 60℃ for 3 hours to obtain latent heat storage particles.

[0032] The above latent heat storage particles, platinum-supported zirconia powder (activation temperature 280°C), and alumina sol were mixed, and water was added. By varying the concentrations of the latent heat storage particles and platinum, slurries for inner layer coating with the concentrations shown in Table 1 below were obtained.

[0033] The above latent heat storage particles, rhodium-supported zirconia powder (activation temperature 300°C), and alumina sol were mixed, and water was added. By varying the concentrations of the latent heat storage particles and platinum, surface layer coating slurries with the concentrations shown in Table 1 below were obtained.

[0034] A thermal energy storage exhaust gas purification catalyst was obtained by applying an inner layer coating slurry to a 2.1L base material (honeycomb) so that the amount of adhesion after firing was 63g / L, drying it, and then firing it in a durable furnace at 850°C for 1 hour to form the inner layer. Furthermore, a surface layer coating slurry was applied and dried so that the amount of adhesion after firing was 104g / L, and then firing it in a durable furnace at 850°C for 1 hour to form the surface layer.

[0035] <Rating> First, an exhaust gas purification catalyst that does not contain latent heat storage particles was heated to 600°C and then allowed to cool naturally in a muffle furnace, and the cooling rate was measured. The cooling rate was 2.3°C / min. Next, the thermal energy storage exhaust gas purification catalyst was heated and cooled under the same conditions, and the temperature change over time was measured. As shown in Figure 1, the heat generation area (amount of heat released by the thermal energy storage exhaust gas purification catalyst - amount of heat released by the latent heat of the exhaust gas purification catalyst without latent heat storage particles) was obtained from the difference in temperature change over time with that of the exhaust gas purification catalyst without latent heat storage particles. The evaluation results are shown in Table 1.

[0036] [Table 1]

[0037] The results in Table 1 confirm that the examples in which the latent heat storage particle content differed between the surface layer and the inner layer had a larger heat-generating area than Comparative Example 1, in which the latent heat storage particle was uniformly dispersed. Furthermore, Examples 1-3, in which the proportion of latent heat storage particles contained in the surface layer is 70% or more, and Examples 6-8, in which the proportion of latent heat storage particles contained in the inner layer is 78% or more, have a larger heat-generating area compared to Examples 4 and 5. Furthermore, in Examples 1 and 2, where the proportion of latent heat storage particles contained in the surface layer is 80% or more, and in Examples 7 and 8, where the proportion of latent heat storage particles contained in the inner layer is 90% or more, the heat-generating area is larger compared to Examples 6 and 7. This indicates that the proportion of latent heat storage particles is heavily skewed towards either the surface layer or the inner layer, allowing the temperature of the catalyst particles to be maintained above the activation temperature for a longer period of time.

Claims

1. A heat storage exhaust gas purification catalyst comprising a catalyst coating layer containing catalyst particles and latent heat storage particles on the surface of a substrate, The freezing point of the latent heat storage material is above the activation temperature of the catalyst particles. A heat-storage exhaust gas purification catalyst characterized in that the catalyst coating layer has a two-layer structure having a surface layer and an inner layer, and the content of the latent heat storage particles differs between the surface layer and the inner layer.

2. The heat storage exhaust gas purification catalyst according to claim 1, characterized in that the surface layer contains 60% or more of the latent heat storage particles, or the inner layer contains 67% or more of the latent heat storage particles, relative to the total amount of the latent heat storage particles.

3. The heat storage exhaust gas purification catalyst according to claim 1, characterized in that the latent heat storage particles include a ZnAl alloy or a SiAl alloy.

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