Patent Application
20240328335
The present invention relates to an exhaust purification device.
Exhaust restrictions on automobiles are advancing further to reduce the negative impact on the global environment. For this reason, an exhaust purification device which purifies the exhaust gas is installed in the exhaust passage of an internal combustion engine such as a gasoline engine or diesel engine.
As the exhaust purification device, for example, a metal honeycomb catalyst has been known (for example, refer to Patent Document 1). Herein, the metal honeycomb catalyst is the finished article of a metal foil catalyst, and the metal foil catalyst includes metal foil and a catalyst layer provided on this metal foil. In addition, the catalyst layer contains a noble metal, and the thickness Ts (nm) of the metal foil and the thickness Tc (nm) of the catalyst layer satisfy Formula (1).
Furthermore, the catalyst layer is formed on the metal foil by vapor depositing a catalyst layer formation material containing the noble metal by arc discharge.
However, when using the exhaust purification device disclosed in Patent Document 1 for a long period of time under the high temperature environment of an internal combustion engine, the exhaust purification performance declines. This is assumed to be due to the noble metal agglomerating, starting at the gap of the grain boundary generated by the crystals of noble metal growing, upon forming the catalyst layer.
The present invention has an object of providing an exhaust purification device capable of improving durability.
According to an aspect of the present invention, an exhaust purification device is provided in an exhaust passage of an internal combustion engine, and purifies exhaust gas of the internal combustion engine, the exhaust purification device including: a metal support having a flow path in which the exhaust gas flows; and a catalyst layer formed on a surface of the metal support, in which the catalyst layer includes a noble metal, and a metal having a different crystal structure than the noble metal.
The metal having a different crystal structure than the noble metal may have a melting point of 1700° C. or higher.
The noble metal may be rhodium, and the metal having a different crystal structure than the noble metal may be zirconium and/or tungsten.
The above-mentioned exhaust purification device may further include a zirconium layer between the metal support and the catalyst layer,
The catalyst layer may be formed by vacuum depositing the noble metal and the metal having a different crystal structure than the noble metal.
The above-mentioned exhaust purification device may further include an energizing part which energizes the metal support, in which the metal support may be able to be heated by way of energizing.
The catalyst layer may have a content of the metal having a different crystal structure than the noble metal of at least 5% by mass and no more than 55% by mass.
According to the present invention, it is possible to provide an exhaust purification device capable of improving durability.
Hereinafter, an embodiment of the present invention will be explained while referencing the drawings.
The exhaust purification device of the present embodiment is provided in an exhaust passage of an internal combustion engine, and purifies the exhaust gas of the internal combustion engine. Herein, the internal combustion engine can be exemplified by a gasoline engine or diesel engine, for example.
A metal honeycomb catalyst 10 includes: a metal support 11 having a flow path 11a in which exhaust gas E flows, and a catalyst layer 12 formed on the surface of the metal support 11. Herein, the catalyst layer 12 includes: a noble metal 12a, and a metal 12b having a different crystal structure than the noble metal 12a. For this reason, the durability of the metal honeycomb catalyst 10 improves. This is assumed to be due to agglomeration of the noble metal 12a being suppressed by the metal 12b having a different crystal structure than the noble metal 12a existing in the gap of the grain boundary generated by the crystal of noble metal 12a growing, upon forming the catalyst layer 12. At this time, the gap of the grain boundary serving as the starting point of agglomeration of the noble metal 12a decreases by the metal 12b, and migration of noble metal 12a accompanying agglomeration of the noble metal 12a is suppressed. In addition, the metal 12b does not form a solid solution with the noble metal 12a due to having a different crystal structure than the noble metal 12a.
The shape of the metal support 11 is not particularly limited; however, a circular column shape and elliptical column shape can be exemplified. In addition, the cross-sectional shape of the flow path 11a is not particularly limited; however, a triangular shape, square shape, hexagonal shape and octagonal shape can be exemplified.
The material constituting the metal support 11 is not particularly limited so long as being able to use under the high-temperature environment of an internal combustion engine; however, stainless steel can be exemplified, for example.
The thickness of the metal support 11 is not particularly limited; however, it is 10 μm or more and 100 μm or less.
The noble metal 12a is not particularly limited so long as able to purify exhaust gas; however, Pt, Pd and Rh (face-centered cubic lattice structure: fcc) can be exemplified. Thereamong, Rh is preferable in the point of exhaust purification performance of the metal honeycomb catalyst 10.
The metal 12b is not particularly limited so long as the crystal structure is differing from the noble metal 12a; however, W (melting point 3407° C.), Nb (2477° C.), Zr (1852° C.), Fe (1538° C.) (above, body-centered cubic lattice structure: bcc), Hf (2233° C.), Ti (1666° C.), Zn (419° C.) (above, hexagonal closest packing structure: hcp), etc. can be exemplified. Thereamong, Zr and/or W are preferable in the point of the durability of the metal honeycomb catalyst 10.
The melting point of the metal 12b is preferably at least 1700° C., and is more preferably at least 1800° C. When the melting point of the metal 12b is 1700° C. or higher, the durability of the metal honeycomb catalyst 10 improves. It should be noted that, although the melting point of the metal 12b is not particularly limited; however, it is no higher than 3500° C., for example.
The content of the metal 12b in the catalyst layer 12 is preferably at least 5% by mass and no more than 55% by mass. If the content of the metal 12b in the catalyst layer 12 is at least 5% by mass, the durability of the metal honeycomb catalyst 10 improves, and if no more than 55% by mass, the exhaust purification performance of the metal honeycomb catalyst 10 improves.
The thickness of the catalyst layer 12 is not particularly limited; however, it is at least 1 nm and no more than 1 μm, for example.
The metal honeycomb catalyst 10 can be produced, for example, by forming the catalyst layer 12 on a metal foil by vacuum depositing the noble metal 12a and metal 12b, followed by processing into a predetermined shape.
The metal honeycomb catalyst 20 is the same as the metal honeycomb catalyst 10 other than further including an intermediate layer 21 between the metal support 11 and catalyst layer 12, and further including the power source 22 as an energizing part that energizes the metal support 11. Herein, the intermediate layer 21 includes Zr, and the metal support 11 can be heated by energizing.
The metal honeycomb catalyst 20 includes the intermediate layer 21, and thus durability improves. In addition, the metal honeycomb catalyst 20 includes the power source 22, and thus can heat the metal support 11, a result of which the exhaust purification performance improves.
The thickness of the intermediate layer 21 is not particularly limited; however, it is at least 50 nm and no more than 1 μm.
The material capable of heating by energizing constituting the metal support 11 is not particularly limited; however, stainless steel can be exemplified.
Although an embodiment of the present invention has been explained above, the present invention is not limited to the above described embodiment, and the above embodiment may be modified where appropriate within the scope of the gist of the present invention.
Examples of the present invention will be explained below; however, the present invention is not to be limited to the Examples. It should be noted that the test pieces corresponding to the exhaust purification device are used in the present examples, and the durability was evaluated.
A Zr film of 250 nm thickness and Rh—W film of 20 nm are formed on a stainless steel film of 40 μm thickness by vacuum deposition under conditions of 5.28×108 to 5.92×108 plasma density to obtain a test piece. At this time, vacuum deposition was done so that the content of W in the Rh—W film becomes 20% by mass.
Other than forming an Rh—Zr film in place of the Rh—W film, a test piece was obtained similarly to Example 1.
Other than forming an Rh film in place of the Rh—W film, a test piece was obtained similarly to Example 1.
On the test piece, durability treatment for 20 hours at 980° C. was conducted while alternately switching between a lean atmosphere (20 seconds) and rich atmosphere (80 seconds).
Using a catalyst reactor BELREA (manufactured by Microtrac BEL), the exhaust purification performances (conversion rates of NO and HC) of test pieces not subjected to durability treatment and test pieces subjected to durability treatment were evaluated. Herein, the composition of the model gas used upon evaluating the exhaust purification performance was NO (500 ppm), CO (5000 ppm), HC (propylene) (400 ppm), H2O (10%), O2 (4900 ppm), H2 (1700 ppm), N2 (balance), and the flowrate of model gas was set to 400 mL/min. In addition, before evaluating the exhaust purification performance, and after oxidizing the test pieces under an oxygen atmosphere at 500° C. for 15 minutes, pre-treatment reducing at 500° C. for 15 minutes under a hydrogen atmosphere was conducted.
It should be noted that the conversion rates of NO and HC are the average values of conversion rates at 300° C., 350° C., 400° C. and 450° C., respectively. In addition, the conversion rate of NO at each temperature was calculated by a formula:
[(inlet side NO concentration)−(outlet side NO concentration)]/(inlet side NO concentration)×100,
and the conversion rate of HC at each temperature was calculated by a formula:
It is understood from
Therefore, the test pieces of Examples 1 and 2 are considered to have higher durability than the test piece of Comparative Example 1.
Using the NO conversion rate (refer to
However, the decline ratio in exhaust purification performance when the value of z is less than 0 is set to 0. Therefore, the conversion rate of NO (y) of the test piece subjected to durability treatment is represented by a formula:
From
