WO2009144568A1

WO2009144568A1 - Exhaust purification catalyst

Info

Publication number: WO2009144568A1
Application number: PCT/IB2009/005749
Authority: WO
Inventors: Naoto Miyoshi; Takeshi Nobukawa; Kenji Sakurai; Kenji Katoh; Hiroto Imai
Original assignee: Cataler Corp; Toyota Motor Corp
Current assignee: Cataler Corp; Toyota Motor Corp
Priority date: 2008-05-30
Filing date: 2009-05-28
Publication date: 2009-12-03
Anticipated expiration: 2010-11-30
Also published as: JP2009285604A

Abstract

An exhaust purification catalyst has a catalyst coat layer having a two-layer structure including a lower layer, and a upper layer formed on a surface of the lower layer. At least one of platinum or palladium is present in at least the upper layer, and 60% by mass or more of the total amount of rhodium in the catalyst coat layer is present in the lower layer.

Description

EXHAUST PURIFICATION CATALYST

BACKGROUND OF THE INVENTION

1. Field of the Invention

[0001] The present invention relates to an exhaust purification catalyst that purifies exhaust gas from an internal combustion engine. More specifically, the present invention relates to a NOx storage-reduction exhaust purification catalyst that stores NOx when the air-fuel ratio of the exhaust gas is lean and reduces the stored NOx when the air-fuel ratio of the exhaust gas is stoichiometric or rich.

2. Description of the Related Art

[0002] In recent years, the NOx storage-reduction catalyst is in practical use as a catalyst to purify exhaust gas from a lean-burn engine. The NOx storage-reduction catalyst is composed of a porous carrier made of a material such as alumina (AI₂O₃) and a NOx storing material such as an alkaline metal or alkaline-earth metal and a noble metal supported on the carrier. In such a NOx storage-reduction catalyst, if the fuel injection is controlled in a pulse manner so that the air-fuel ratio between a lean air-fuel ratio and a stoichiometric to rich air-fuel ratio, NOx is stored in the NOx storing material when the air-fuel ratio is lean. The stored NOx is released when the air-fuel ratio is rich and reacted with and purified by reducing components such as HC and CO by the catalytic action of the noble metal. Therefore, emission of NOx may be reduced on the lean side as well, and a high NOx purification capability can be achieved as a whole.

[0003] Among noble metals, platinum (Pt) and palladium (Pd) primarily contribute to oxidation purification of carbon monoxide (CO) and hydrocarbons (HC), and rhodium (Rh) primarily contributes to reduction purification of NOx. Rhodium also prevents sintering of Pt or Pd. Thus, it has been found that combined use of Rh with Pt or Pd can prevent defects where the activity decreases because of a decrease in active spots caused by sintering and improves heat resistance.

[0004] However, it has also been found that combined use of Pt and Rh may lead to a defect where the oxidation capability of Pt decreases because Pt and Rh form an alloy at a high temperature.

[0005] In addition, there are some combinations between a noble metal species and a type of carrier that should be avoided under certain usage conditions. For example, a catalyst composed of Rh supported on alumina may have a defect that the Rh forms a solid solution in the alumina and significantly deteriorates in performance in an oxidation atmosphere at a temperature of 900 ⁰C or higher. In addition, Rh is a very rare resource and it is therefore desired to use Rh efficiently and prevent deterioration of Rh to improve the heat resistance properties.

[0006] Exhaust gas of motor vehicles contains SO₂ generated from the combustion of sulfur (S) contained in the fuel, and it is oxidized by the noble metal into SO3 when it passes through the NOx storage-reduction catalyst in an oxygen-excess atmosphere. It has been found that the SO3 and water vapor contained in exhaust gas easily form sulfuric acid, which in turn reacts with the NOx storing material to form sulfites and sulfates that poison and deteriorate the NOx storing material. This phenomenon is called sulfur poisoning. When the NOx storing material is poisoned by sulfur as described above, it can no longer store NOx, resulting in deterioration in the NOx purification capability.

[0007] One solution for the problem is to form a coating from a mixture of a catalyst powder obtained by depositing Rh on zirconia and alumina. For example, Japanese Patent Application Publication No. 11-226404 (JP-A- 11 -226404) describes a NOx storage-reduction catalyst having a catalyst coating formed from a mixture of a first powder obtained by depositing Pt and a NOx storing material on alumina and titania and a second powder obtained by depositing Rh on stabilized zirconia. By using zirconia on which Rh is deposited as described above, formation of a solid solution of Rh in alumina may be avoided and therefore deterioration of Rh may be inhibited. Also, a decrease in the activity of Pt caused by alloying of Pt with Rh may also be inhibited because Pt and Rh are separated from each other, and sintering of Pt may be inhibited because Pt and Rh are located relatively close to each other.

[0008] In addition, Rh deposited on zirconia exhibits an ability to generate hydrogen through a steam-reforming reaction. Therefore, according to the catalyst described in JP-A-11-226404, NO is oxidized by the high oxidation activity of Pt and stored in the NOx storing material in a lean atmosphere, and the stored NOx is released and efficiently reduced by the hydrogen generated in a stoichiometric to rich atmosphere. Also, the sulfites and sulfate on the NOx storing material are reduced by the hydrogen, and the NOx storing material recovers its NOx storage capacity. Therefore, the NOx purification performance significantly improves.

[0009] However, with the recent tightening of exhaust gas regulations, the NOx purification performance of the catalyst disclosed in JP-A-11-226404 has become insufficient. The reason for this is believed to be that alloying of Pt and Rh occurs and decreases the activity of Pt because Pt and Rh are located in close proximity to each other.

[0010] Japanese Patent Application Publication No. 06-039292 (JP-A-06-039292) and Japanese Patent Application Publication No. 2001-182527 (JP-A-2001-182527) describe an exhaust purification catalyst having a catalyst coating composed of two, upper and lower, layers or three layers, in which Pt or Pd and Rh are contained in different layers. When Pt and Rh are contained separately in different layers, alloying of Pt and Rh may be inhibited and therefore a decrease in the activity of Pt can be inhibited.

[0011] However, the catalysts described in JP-A-06-039292 and JP-A-2001-182527 are not NOx storage-reduction catalysts and do not contain a NOx storing material. Thus, even if the technology described in the official gazettes is applied to a NOx storage-reduction catalyst, those skilled in the art cannot come up to the idea to solve sulfur poisoning, and it is not obvious at all what effect it has on the NOx purification performance.

SUMMARY OF THE INVENTION

[0012] The present invention enables Pt and Rh to fulfill their functions to the maximum and further improves the NOx purification performance.

[0013] A NOx storage-reduction exhaust purification catalyst according to an aspect of the present invention, wherein the exhaust purification catalyst includes a NOx storing material which stores NOx when the air-fuel ratio of the exhaust gas is leans, and reduces the stored NOx when the air-fuel ratio of the exhaust gas is stoichiometric or rich, the exhaust purification catalyst including: a carrier substrate; and a catalyst coat layer formed on a surface of the carrier substrate and containing an oxide carrier on which at least one NOx storing material selected from alkaline metal, alkaline-earth metal or rare earth elements, at least one of platinum and palladium, and rhodium are supported, wherein the catalyst coat layer has a two-layer structure including a lower layer formed on a surface of the carrier substrate, and an upper layer formed on a surface of the lower layer, and the at least one of platinum and palladium is present in at least the upper layer, and the proportion of Rh contained in the lower layer is higher than the proportion of Rh contained in upper layer.

[0014] In the first aspect, 40% by mass or less of the total amount of rhodium contained in the catalyst coat layer may be present in the upper layer and 60% by mass or more of the total amount of rhodium contained in the catalyst coat layer may be present in the lower layer.

[0015] In the exhaust purification catalyst according to this aspect of the present invention, at least one of Pt and Pd is contained at least in the upper layer. The NO contained in exhaust gas is efficiently oxidized into NOx by Pt or Pd in the upper layer in a lean atmosphere and stored in the NOx storing material contained at least in the upper layer. The proportion of Rh contained in the lower layer is higher than that contained in the upper layer. Therefore, hydrogen is generated primarily in the lower layer in a stoichiometric to rich atmosphere, and the hydrogen unavoidably passes through the upper layer. Thus, the NOx released from the NOx storing material at least in the upper layer is efficiently reduced and purified by the hydrogen. In addition, even when the NOx storing material undergoes sulfur poisoning, the sulfur compounds are reduced by the hydrogen and the NOx storing material recovers its NOx storage capacity.

[0016] In addition, most of Pt or Pd and Rh can be separately contained in the upper layer and the lower layer, alloying of Pt or Pd with Rh can be inhibited and therefore a decrease in the activity can be inhibited.

[0017] Therefore, according to the exhaust purification catalyst of the aspect of the present invention, high NOx purification performance can be achieved even after endurance.

BRIEF DESCRIPTION OF THE DRAWINGS [0018] The foregoing and further features and advantages of the invention will become apparent from the following description of example embodiments with reference to the accompanying drawings, wherein like numerals are used to represent like elements and wherein:

FIG 1 is a schematic explanatory view illustrating a catalyst according to one embodiment of the present invention;

FIG 2 is a graph showing the relation between the proportion of Rh contained in a lower layer and the NOx purification efficiency;

FIG 3 is a bar graph showing the NO conversion rate; and

FIG 4 is a bar graph showing the amount of hydrogen generation.

DETAILED DESCRIPTION OF EMBODIMENTS [0019] An exhaust purification catalyst according to an embodiment of the present invention is composed of a carrier substrate, and a catalyst coat layer. Preferably, the carrier substrate has a configuration such as a porous configuration or honeycomb configuration, and may be made of cordierite, ceramics such as SiC, or a metal, for example. When the exhaust purification catalyst has a honeycombconfiguration, it may be either of a straight-flow structure or a wall-flow structure.

[0020] The embodiment of the present invention is characterized by the configuration of the catalyst coat layer. That is, the catalyst coat layer is a two-layer structure that includes a lower layer formed over a surface of the carrier substrate, and an upper layer formed over the surface of the lower layer. The oxide carrier, of which most of the lower layer and the upper layer are composed, may be selected from alumina, silica, ziiconia, silica-alumina, ceria, zeolite and the like. These materials may be used singly, or in combination with each other as part of a mixture or a composite.

[0021] Rh is preferably supported on zirconia. In this case, it is also preferred to use a stabilized zirconia stabilized with Ca, La, Ba or the like.

[0022] A NOx storing material is contained at least in the upper layer. The reason for this is to increase the amount of NOx storing material contained in the vicinity of Pt or Pd. With this configuration, NOx generated by oxidation catalyzed by the Pt or Pd may be stored in the NOx storing material efficiently. It is needless to say that a portion of the NOx storing material may be contained in the lower layer.

[0023] As the NOx storing material, at least one element selected from alkaline metals, alkaline-earth metals and rare earth metals may be used. It is preferred that the NOx storing material contains both an alkaline metal and an alkaline-earth metal. Suitable examples of alkaline metal include lithium (Li), sodium (Na), potassium (K), and cesium (Cs). Suitable examples of alkaline-earth metals include magnesium (Mg), calcium (Ca), strontium (Sr), and barium (Ba).

[0024] The amount of NOx storing material contained is preferably in the range of 0.01 to 5 moles, more preferably in the range of 0.1 to 0.5 moles, per liter of the carrier substrate in the entire catalyst coat layer. When the amount is less than 0.01 moles/L, the NOx storage capacity is too low for practical use, and when NOx storing material is contained in an amount more than 5 moles/L, the activity of Pt is lowered. It is preferred that the NOx storing material is contained in the upper layer in an amount of at least 0,01 moles/L or more. When the NOx storing material in the upper layer is smaller than that, the NOx purification performance is lowered.

[0025] In the exhaust purification catalyst according to this embodiment, at least one of Pt and Pd is contained at least in the upper layer. With this configuration, the contact between exhaust gas and Pt or Pd is improved, and NO₂ generated by oxidation of NO may be stored in the storing material efficiently. Thereby increasing NOx storage capacity.

[0026] In addition, it is preferred that the density of Pt or Pd in the upper layer is highest in the surface zone, which tends to contact with the exhaust gas. With this configuration, the oxidation activity of NOx is further improved and the NOx storage capacity further increases. To increase the amount of Pt or Pd in the surface zone, an absorption-deposition method can be used.

[0027] It is preferred that the proportion (amount) of at least one of Pt and Pd contained in the upper layer is greater than half of the total amount thereof contained in the entire catalyst coat layer. It is desirably to maximize the amount of Pt or Pd contained in the upper layer, and it is permissible to have all the Pt or Pd contained in the upper layer.

[0028] In the exhaust purification catalyst of this embodiment, 40% by mass or less of the total amount of Rh contained in the catalyst coat layer is contained in the upper layer, and 60% by mass or more of the total amount of Rh contained in the catalyst coat layer is contained in the lower layer.

[0029] When the proportion of Rh contained in the lower layer is smaller than 60% by mass of the total amount of Rh contained in the catalyst coat layer, more than 40% by mass of the total amount of Rh is contained in the upper layer, in which case alloying of Rh with Pt or Pd causes lowering of the NOx purification performance after endurance.

[0030] However, when the upper layer is free from Rh, the effect of preventing sintering of Pt is lost. In addition, the steam-reforming reaction activity is too low to generate a sufficient amount of hydrogen, and the recovery of the NOx storage capacity of the NOx storing material poisoned by sulfur is adversely affected. Therefore, 40% by mass or less of the total amount of Rh contained in the catalyst coat layer is contained in the upper layer.

[0031] The amounts of at least one of Pt and Pd and Rh contained are both preferably in the range of 0.1 to 10 g per liter of the carrier substrate in the entire catalyst coat layer. When the amounts are less than 0.1 g/L, the NOx purification activity is insufficient. When at least one of Pt and Pd and R are contained in an amount greater than 10 g/L, sintering tends to occur during endurance.

[0032] The catalyst coat layer is preferably in the range of 50 to 300 g per liter of the carrier substrate. When the catalyst coat layer is less than 50 g/L, the Pt tends to undergo sintering. A catalyst coat layer greater than 300 g/L is not preferred due to increases in the pressure loss of exhaust gas. While the coating amounts of the lower layer and the upper layer may be generally equal to each other, it is preferable for the upper layer containing a larger amount of Pt, which is apt to undergo sintering, is slightly thicker than the lower layer.

[0033] The following examples, comparative examples and test examples describe the present invention in further detail below. (Example 1) FIG. 1 depicts a catalyst according to this embodiment. The NOx storage- reduction catalyst includes a honeycomb substrate 1, and a catalyst coat layer 2 formed over surfaces of cell walls 10 of the honeycomb substrate 1. The catalyst coat layer 2 is composed of a lower layer 20 formed over the surfaces of the cell walls 10 and an upper layer 21 formed over a surface of the lower layer 20. The method for producing the catalyst is described in substitution for detailed description of the configuration thereof.

[0034] First, an aqueous solution of rhodium nitrate was added to zirconia powder, and the mixture was stirred for one hour. The mixture was then evaporated and dried, and the dried mixture was baked, thereby obtaining RhZZjO₂ powder having 0.8% by mass of Rh supported thereon.

[0035] The Rh/Zrθ₂ powder of 100 parts by mass, an alumina sol (Al₂O₃: 10% by mass) of 30 parts by mass as a binder, and distilled water were mixed to prepare a slurry. A 1.3 L cordierite honeycomb substrate 1 was immersed in and lifted out of the slurry, and excess slurry was blown off. Then, the honeycomb substrate 1 was dried and baked to form a lower layer 20. The lower layer 20 was formed in an amount of 50 g per liter of the honeycomb substrate 1. The lower layer 20 contains 0.4 g of Rh per liter of the carrier substrate.

[0036] The above RhZZrO₂ powder of 10 parts by mass, Al₂O₃ powder of 85 parts by mass, Ceθ2-Zrθ2 composite oxide powder of 20 parts by mass, ZτO₂-Tiθ₂ composite oxide powder of 85 parts by mass, an alumina sol (AI2O3: 10% by mass) of 60 parts by mass, and distilled water were mixed to prepare a slurry. The honeycomb substrate 1, on which the lower layer 20 had been formed, was immersed in and lifted out of the slurry, and excess slurry was blown off. Then, the honeycomb substrate 1 was dried and baked to form an upper layer 21. The upper layer 21 was formed in an amount of 200 g per liter of the honeycomb substrate 1. The upper layer 21 contains 0.1 g of Rh per liter of the honeycomb substrate 1. That is, 80% by mass of Rh was contained in the lower layer 20, and 20% by mass of Rh was contained in the upper layer 21.

[0037] The honeycomb substrate 1, on which the lower layer 20 and the upper layer 21 had been formed, was immersed in an aqueous solution of dinitrodiammine platinum having a prescribed concentration to cause Pt to be adsorbed and deposited in the honeycomb substrate 1. Because an absorption-deposition method was used, 90% or more of the Pt was contained in the surface zone of the upper layer 21. The amount of Pt contained was 2 g per liter of the carrier substrate.

[0038] The honeycomb substrate 1 was dried, and then impregnated with a prescribed amount of a mixed aqueous solution of barium acetate and potassium acetate having a prescribed concentration. The solution was evaporated and dried, and the honeycomb substrate 1 was baked so that Ba and K were deposited in the honeycomb substrate 1. About 80% of the total amount of Ba and K was deposited in the upper layer 21, and about 20% of the total amount of Ba and K was deposited in the lower layer 20. Ba and K were contained in an amount of 0.2 moles and 0.1 moles, respectively, per liter of the honeycomb substrate 1 in the entire catalyst coat layer.

[0039] (Example 2) A catalyst of Example 2 was prepared in the same manner as in Example 1 except that the amount of the Rh/ZrCh powder was controlled so that the lower layer 20 contained 0.3 g/L of Rh and the upper layer 21 contained 0.2 g/L of Rh. In other words, 60% by mass of IUi was contained in the lower layer 20, and 40% by mass of Rh was contained in the upper layer 21.

[0040] (Example 3) A catalyst of Example 3 was prepared in the same manner as in Example 1 except that the an aqueous solution of palladium nitrate was used to cause Pd to be adsorbed and deposited in the honeycomb substrate 1 after the deposit of Pt. Pt and Pd were contained in an amount of 1.2 g and 0.8 g respectively, per liter of the honeycomb substrate 1.

[0041] (Example 4) A catalyst of Example 4 was prepared in the same manner as in Example 1 except that the amount of Rh/Zrθ2 powder was controlled so that the lower layer 20 contained 0.5 g/L of Rh and that ZrO₂ powder was used instead of RhZZrO₂ powder to form the upper layer 21. That is, 100% by mass of Rh was contained in the lower layer 20, and the upper layer 21 was free of Rh.

[0042] (Comparative Example 1) A catalyst of Comparative Example 1 was prepared in the same manner as in Example 1 except that the amount of Rh/ZrCh powder was controlled so that the lower layer 20 contained 0.25 g/L of Rh and the upper layer 21 contained 0.25 g/L of EIh. That is, 50% by mass of Rh was contained in the lower layer 20, and 50% by mass of Rh was contained in the upper layer 21.

[0043] (Test Example 1) The properties of the catalysts described above are summarized in Table 1.

[Table 1]

[0044] Each of the above catalysts was installed in the exhaust system of an engine bench equipped with a lean-burn engine, and an endurance test in which a catalyst bed temperature of 750 ^βC was maintained for 50 hours under the condition that a lean control and a rich control were alternately repeated was conducted using gasoline containing 100 ppm of sulfur.

[0045] Each of the catalysts after the endurance test was installed in the exhaust system of the same engine bench as above, and the NOx storage capacity at a time when a lean atmosphere was produced after a rich spike was measured under the condition that rich spike was introduced for one second after a lean atmosphere was maintained for one minute, and it was defined as NOx purification efficiency. The catalyst bed temperature was 400 ⁰C. The results are shown in FIG 2, in which the horizontal axis represents the amount of Rb in terms of percent by mass in the lower layer.

[0046] FIG 2 indicates that, when the amount of Rh contained in the lower layer 20 is 60% by mass or greater, the NOx purification efficiency after the endurance test becomes 90% or higher. This is believed that because alloying and sulfur poisoning are inhibited. It is also indicated that Rh is preferably contained in the upper layer 21 as well because the NOx purification efficiency was slightly lower when the proportion of Rh contained in the lower layer 20 is 100% by mass, in other words, when the upper layer 21 was free of Rh. The proportion of Rh contained in the upper layer 21 is preferably at least 1% by mass based on the total amount of Rh in the catalyst coat layer 2.

[0047] (Test Example 2) Using the catalysts of Example 1 and Comparative Example 1, after an endurance test was conducted in the same manner as in test example 1, the NO conversion rates at 300 ⁰C and 400 ⁰C were measured using a model gas containing NO and having an air-fuel ratio (A/F) equivalent to 20. The result is shown in FIG 3.

[0048] FIG 3 indicates that the catalyst of Comparative Example 1 had a lower NO conversion rate than the catalyst of Example 1, which means that the catalyst of Comparative Example had lower oxidation activity than the catalyst of Example 1. The reason for this is believed that the oxidation activity of Pt was inhibited by the formation of an alloy of Rh and Pt in the upper layer of the catalyst in Comparative Example 1.

[0049] (Test Example 3) Using the catalysts of Example 4 and Comparative Example 1, after an endurance test was conducted in the same manner as in Test Example 1, the amount of hydrogen generated (the concentration of hydrogen in reaction gas) at 400 ⁰C was measured using a rich atmosphere model gas containing 0,1% of C₃Hg and 4% of H₂O. The result is shown in FIG 4.

[0050] FIG 3 indicates that the amount of hydrogen generated was smaller when the catalyst of Example 4 was used than when the catalyst of Comparative Example 1 was used. That is, when the upper layer 21 is free of Rh, the steam-reforming reaction is less likely to occur. Thus, it is apparent that Rh is preferably contained in the upper layer 20 as well.

[0051] While the exhaust purification catalyst of this embodiment is primarily used in a straight flow structure NOx storage-reduction catalyst of an exhaust system for a gasoline engine, it may also be used for a filter catalyst, having a catalyst coat layer in the fine pores in cell walls, in the exhaust system of a diesel engine.

i8

Claims

1. A NOx storage-reduction exhaust purification catalyst, wherein the exhaust purification catalyst includes a NOx storing material that stores NOx when the air-fuel ratio of the exhaust gas is lean and reduces the stored NOx when the air-fuel ratio of the exhaust gas is stoichiometric or rich, the exhaust purification catalyst comprising: a carrier substrate; and a catalyst coat layer that is formed on a surface of the carrier substrate, wherein the catalyst coat layer contains an oxide carrier on which at least one NOx storing material selected from alkaline metal, alkaline-earth metal or rare earth elements, at least one of platinum and palladium, and rhodium are supported, wherein the catalyst coat layer has a two-layer structure including a lower layer formed on a surface of the carrier substrate, and an upper layer formed on a surface of the lower layer, and the at least one of platinum and palladium is present in at least the upper layer, and the proportion of rhodium contained in the lower layer is higher than the proportion of rhodium contained in upper layer.

2. The exhaust purification catalyst according to claim 1, wherein 40% by mass or less of the total amount of rhodium contained in the catalyst coat layer present in the upper layer and 60% by mass or more of the total amount of rhodium contained in the catalyst coat layer present in the lower layer.

3. The exhaust purification catalyst according to claim 1 or 2, wherein at least 1% by mass of the total amount of rhodium contained in the catalyst coat layer is contained in the upper layer.

4. The exhaust purification catalyst according to any one of claims 1 to 3, wherein the platinum in the upper layer is deposited in the upper layer by an absorption-deposition method.

5. The exhaust purification catalyst according to any one of claims 1 to 4, wherein the concentration of at least one of platinum and palladium present in the upper layer is highest in the surface zone thereof.

6. The exhaust purification catalyst according to any one of claims 1 to 5, wherein the amount of at least one of platinum and palladium present in the upper layer is greater than half of the total amount thereof contained in the entire catalyst coat layer.

7. The exhaust purification catalyst according to any one of claims 1 to 6, wherein the concentration of at least one of platinum and palladium in the catalyst coat layer is 0.1 to 10 g per liter of the carrier substrate in the entire catalyst coat layer, and the concentration of rhodium in the catalyst coat layer is 0.1 to 10 g per liter of the carrier substrate in the entire catalyst coat layer.

8. The exhaust purification catalyst according to any one of claims 1 to 7, wherein the amount of the catalyst coat layer is 50 to 300 g per liter of the carrier substrate.

9. The exhaust purification catalyst according to any one of claims 1 to 8, wherein the amount of catalyst coating in the lower layer and the amount of catalyst coating in the upper layer are substantially equal to each other, or the amount of catalyst coating in the upper layer is greater than that of the lower layer.