JP2008229459A - Exhaust gas cleaning device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To prevent a filter base material from being thermally damaged by enhancing regeneration efficiency in a forced regeneration, and preventing accelerated temperature rising and thermal runaway caused thereby. <P>SOLUTION: The exhaust gas cleaning device is provided with a base metal carrying part 20 carrying a PM oxidation catalyst comprising a base metal and oxidizing PM in a catalyst layer of the inner circumferential part in the radius direction, and a noble metal carrying part 21 carrying a noble metalcatalyst in a catalyst layer of the outer circumferential part in the radius direction. Heat generation by the reaction at the base metal carrying part 20 is small, and that of the noble metal carrying part 21 is large. Therefore, a temperature difference in the inner and outer circumferential parts becomes small, unburning is prevented at the outer circumferential part, and thermal damage can be prevented at the downstream part of the inner circumferential part. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an exhaust gas purification apparatus that can purify particulate matter (hereinafter referred to as PM) contained in diesel exhaust gas and the like from a low temperature range and suppress thermal damage of a filter base material during forced regeneration.

  As for gasoline engines, toxic components in exhaust gas are steadily decreasing due to strict regulations on exhaust gas and technological advances that can cope with it. On the other hand, in the case of diesel engines, exhaust gases are more purified than in the case of gasoline engines because harmful components are emitted as PM (carbon particulates, sulfur particulates such as sulfate, high molecular weight hydrocarbon particulates (SOF), etc.). Is difficult.

  Therefore, a ceramic plug-type honeycomb body (diesel particulate filter (hereinafter referred to as DPF)) has been known. This DPF is formed by alternately sealing both ends of the openings of the cells of the ceramic honeycomb structure, for example, in a checkered pattern, and is adjacent to the inflow side cells and the inflow side cells clogged on the exhaust gas downstream side. It consists of an outflow side cell clogged on the exhaust gas upstream side and a cell partition wall that partitions the inflow side cell and the outflow side cell, and collects PM by filtering the exhaust gas through the pores of the cell partition wall.

  However, in DPF, pressure loss (hereinafter referred to as pressure loss) increases due to PM accumulation, so it is necessary to periodically remove and regenerate PM (mainly soot) accumulated by some means. Therefore, conventionally, when pressure loss increases, DPF is regenerated by burning high-temperature exhaust gas and burning PM. For example, an oxidation catalyst is placed upstream of the DPF, exhaust gas rich in HC and CO is supplied, the exhaust gas temperature is raised by the reaction heat in the oxidation catalyst, and the high temperature exhaust gas is supplied to the DPF to oxidize the deposited PM. How to do is known. However, in this case, if the amount of PM deposited is large, acceleration combustion occurs, and sometimes thermal runaway occurs, causing thermal damage to the center or downstream end of the DPF.

  Therefore, in recent years, for example, as described in Japanese Patent Publication No. 07-106290, a coating layer is formed on the surface of the cell partition wall of DPF from alumina or the like, and a catalytic metal such as platinum (Pt) is supported on the coating layer. Filter catalysts have been developed. According to this filter catalyst, the collected PM is oxidized and burned by the catalytic reaction of the catalytic metal, so that the filter catalyst can be regenerated continuously by burning simultaneously with the collection or continuously with the collection. . Since the catalytic reaction occurs at a relatively low temperature and can be burned while the amount collected is small, there is an advantage that the thermal stress acting on the filter catalyst is small and breakage is prevented.

  Japanese Patent Application Laid-Open No. 09-094434 describes a filter catalyst in which a coating layer supporting a catalyst metal is formed not only in the cell partition walls but also in the pores of the cell partition walls. By supporting the catalyst metal also in the pores, the probability of contact between the PM and the catalyst metal is increased, and the PM collected in the pores can be oxidized and burned.

  However, even if a filter catalyst is used, it is difficult to oxidize PM in a low temperature range below the activation temperature of the catalytic metal, and forced regeneration is indispensable because pressure loss increases due to PM deposition. Japanese Patent Laid-Open No. 2003-097251 proposes a filter catalyst in which a noble metal such as Pt is supported on the inner periphery and potassium is supported on the outer periphery. Since potassium can oxidize PM at a lower temperature than noble metals such as Pt, the combustion start at the outer periphery is accelerated, and variation in the combustion state of PM at the inner and outer periphery can be corrected. Therefore, the disparity in the degree of regeneration between the inner and outer periphery can be corrected.

JP-A-2003-161138 proposes a filter catalyst configured such that the PM oxidizing power decreases stepwise or continuously from upstream to downstream or from the outer periphery to the inner periphery. According to this technique, the catalyst on the downstream side or the inner peripheral portion has a weak oxidizing power, so that rapid oxidative combustion of PM is avoided, and a rapid increase in temperature of that portion is prevented. Therefore, thermal damage to the filter base material can be prevented.
JP2003-097251 JP 2003-161138 A

However, it has become clear that thermal runaway cannot be stopped only by controlling the oxidizing power of the catalyst as described in the above publication. In other words, if the temperature required for regeneration (approximately 600 ° C) is secured on the upstream side of the filter catalyst, the temperature on the downstream side exceeds 700 ° C due to the oxidation combustion of PM. Runaway occurs. In particular, when a noble metal such as Pt is supported on the inner peripheral portion where the temperature tends to be high, since PM is oxidized to CO ₂ , the calorific value becomes extremely large and thermal runaway tends to occur.

  The present invention has been made in view of the above circumstances, and improves the regeneration efficiency at the time of forced regeneration, prevents accelerated temperature rise and thermal runaway associated therewith, and prevents thermal damage to the filter base material. It is a problem to be solved.

The features of the exhaust gas purification apparatus of the present invention that solves the above-described problems are characterized in that an inflow side cell clogged on the exhaust gas downstream side, an outflow side cell adjacent to the inflow side cell and clogged on the exhaust gas upstream side, and an inflow side cell And a filter substrate having a wall flow structure having a porous cell partition wall partitioning the outflow side cell and having a large number of pores,
An exhaust gas purification device comprising a catalyst layer formed on a cell partition wall,
The catalyst layer in the radially inner peripheral portion has a base metal supporting portion made of a base metal and supporting a PM oxidation catalyst capable of oxidizing PM, and the catalyst layer in the radially outer peripheral portion has a noble metal supporting portion supporting a noble metal catalyst. There is.

  It is desirable that the base metal supporting part is formed in a range of 1/4 to 3/4 with respect to the diameter of the filter base material.

  In a honeycomb body such as a DPF or a filter catalyst, the outer peripheral portion is more easily cooled than the inner peripheral portion. If the exhaust gas is regarded as a laminar flow, the exhaust gas has a larger flow velocity in the inner peripheral portion. Therefore, during forced regeneration, unburned residue is likely to occur in the outer peripheral portion, and thermal damage is likely to occur in the downstream portion of the inner peripheral portion.

In view of this, the exhaust gas purifying apparatus of the present invention has a base metal supporting portion supporting a base metal capable of oxidizing PM on the inner peripheral portion. A PM oxidation catalyst made of a base metal oxidizes PM, but stays in the oxidation to CO, and it is considered that the oxidation does not progress to CO _2, and the heat generated by the reaction at the base metal supporting portion is small. On the other hand, since the noble metal supported on the outer peripheral portion oxidizes PM to CO ₂ , the noble metal supporting portion generates a large amount of heat. Accordingly, the temperature difference between the inner and outer peripheral portions is reduced, and unburned residue is prevented at the outer peripheral portion, and thermal damage is prevented from occurring at the downstream portion of the inner peripheral portion.

  The exhaust gas purification apparatus of the present invention comprises a filter base material and a catalyst layer formed on the cell partition walls of the filter base material. Among these, the filter base material is divided into an inflow side cell clogged on the exhaust gas downstream side, an outflow side cell adjacent to the inflow side cell and clogged on the exhaust gas upstream side, and an inflow side cell and an outflow side cell. The wall flow structure is the same as that of the conventional DPF, which has a porous cell partition wall having a number of pores.

  The filter substrate can be formed from a metal foam, a heat-resistant nonwoven fabric, or the like, or can be manufactured from a heat-resistant ceramic such as cordierite or silicon carbide. For example, when manufacturing from heat-resistant ceramics, a clay-like slurry containing cordierite powder as a main component is prepared, formed by extrusion molding or the like, and fired. Instead of cordierite powder, each powder of alumina, magnesia, and silica can be blended so as to have a cordierite composition. Thereafter, the cell opening on one end face is sealed in a checkered pattern with the same clay-like slurry, and the cell opening of the cell adjacent to the cell sealed on the one end face is sealed on the other end face. Thereafter, a filter substrate having a honeycomb structure can be manufactured by fixing the plugging material by firing or the like. The shapes of the inflow side cell and the outflow side cell are not particularly limited, such as a cross-sectional triangle, a cross-sectional square, a cross-sectional hexagon, and a cross-sectional circle.

  The cell partition wall has a porous structure through which exhaust gas can pass. In order to form pores in the cell partition walls, carbon powder, wood powder, starch, resin powder and other combustible powders are mixed in the above-mentioned slurry, and the combustible powder disappears during firing to form pores. The diameter and pore volume of the pores can be controlled by adjusting the particle size and the amount of the combustible powder. The inflow side cell and the outflow side cell communicate with each other through this pore, and PM is collected in the pore, but gas can pass through the pore from the inflow side cell to the outflow side cell.

  The porosity of the cell partition walls is preferably 40 to 70%, and the average pore diameter is preferably 10 to 40 μm. When the porosity and average pore diameter are in this range, an increase in pressure loss can be suppressed even when the catalyst layer is formed at 100 to 200 g / L, and a decrease in strength can be further suppressed. And PM can be collected more efficiently.

  The exhaust gas purification apparatus of the present invention has a catalyst layer on the cell partition wall of the filter base material. The catalyst layer may be formed only on the surface of the cell partition wall, but it is desirable to form the catalyst layer also on the pore inner surface of the cell partition wall. This catalyst layer uses as a support a porous oxide selected from alumina, titania, zirconia, ceria, or one or a mixture of complex oxides selected from these, and supports the catalyst metal on this support. Become.

  The catalyst layer has a base metal supporting portion made of a base metal and supporting a PM oxidation catalyst capable of oxidizing PM on the radially inner peripheral portion, and has a noble metal supporting portion supporting a noble metal catalyst on the radially outer peripheral portion.

  It is desirable that the base metal supporting part is formed in a range of 1/4 to 3/4 with respect to the diameter of the filter base material. In the range where the base metal carrying part is less than 1/4 with respect to the diameter of the filter substrate, the range of the noble metal carrying part is widened, so that thermal damage is likely to occur in the downstream part of the inner peripheral part during forced regeneration. Also, if the base metal carrying part exceeds 3/4 of the diameter of the filter base, the range of the noble metal carrying part becomes narrow, so that unburned PM tends to occur during forced regeneration, and unreacted during normal use. A lot of HC may be discharged.

  As the PM oxidation catalyst comprising a base metal capable of PM oxidation, at least one selected from alkali metals and lanthanoid elements is preferable, and K, Cs, Rb, Ce, Nd and the like are particularly preferable.

  The loading amount of the PM oxidation catalyst made of a base metal in the base metal carrying portion is preferably in the range of 0.1 to 0.5 mol per liter of the volume of the base metal carrying portion. If the amount of the PM oxidation catalyst supported is less than 0.1 mol / L, the PM purification rate decreases, and PM remains unburned in the inner periphery during forced regeneration. On the other hand, when the amount of the PM oxidation catalyst supported is more than 0.5 mol / L, the strength of the filter base material decreases due to the reaction with the filter base material. The PM oxidation catalyst can be supported by an impregnation supporting method.

  The noble metal carrying part is formed on the outer periphery of the base metal carrying part. The noble metal catalyst supported on the noble metal supporting part preferably contains at least Pt having high oxidation activity. Further, other noble metal catalysts such as Pd and Rh may be further supported. The amount of the noble metal catalyst supported is preferably in the range of 0.1 to 5 g per liter of the noble metal supporting portion. If the loading amount is less than this, the activity is too low to be practical, and if the loading amount exceeds this range, the activity is saturated and the cost is increased. In order to support the noble metal, a solution in which a nitrate of noble metal is dissolved can be used and supported by an adsorption support method, an impregnation support method, or the like.

  In order to form the catalyst layer, the porous oxide powder is made into a slurry together with a binder component such as alumina sol and water, and the slurry is attached to the cell partition wall and then baked to form a coat layer, and masking unnecessary portions. And a coating layer may be loaded with a noble metal catalyst and a PM oxidation catalyst made of a base metal. A normal dipping method can be used to attach the slurry to the cell partition walls. However, the slurry is forcibly filled into the pores of the cell partition walls by air blowing or suction, and the excess slurry contained in the pores is filled. It is desirable to remove things.

  In this case, the formation amount of the coat layer or the catalyst layer is preferably 30 to 200 g per liter of the filter base material. When the coat layer or the catalyst layer is less than 30 g / L, the durability of the noble metal is inevitably lowered. When the coat layer or catalyst layer exceeds 200 g / L, the pressure loss becomes too high to be practical.

  By the way, at the time of forced regeneration, as explained in the prior art, an oxidation catalyst is arranged upstream of the filter catalyst, and exhaust gas rich in HC and CO is supplied by a method such as adding fuel to the exhaust gas. In many cases, the exhaust gas temperature is raised by the reaction heat in the step and the high temperature exhaust gas is supplied to the filter catalyst. In this case, exhaust gas having a high concentration of HC and CO flows into the filter catalyst. However, in the inner peripheral portion that is the base metal supporting portion, the oxidation activity thereof is low, and the amount of HC and the like discharged may increase.

  Therefore, in the exhaust gas purifying apparatus of the present invention, it is desirable to have an additional noble metal carrying portion further carrying a noble metal catalyst at the end of the base metal carrying portion on the exhaust gas downstream side. In this case, the noble metal catalyst is supported on the entire exhaust gas downstream end from the inner periphery to the outer periphery. By doing in this way, since HC etc. which were not oxidized by the base metal carrying part are oxidized by the additional noble metal carrying part, discharge of HC etc. can be controlled.

  It is desirable that the additional noble metal supporting portion is formed within 50 mm from the end face on the downstream side of the exhaust gas to the upstream side and within 1/3 or less of the total length of the filter base. If the additional noble metal supporting portion is formed to exceed 50 mm from the end surface on the exhaust gas downstream side to the upstream side, or if it is formed to exceed 1/3 of the total length of the filter substrate, thermal runaway may occur during forced regeneration. Further, it is desirable that the additional noble metal supporting portion does not support a PM oxidation catalyst made of a base metal. This is because the PM oxidation catalyst may reduce the activity of the noble metal catalyst.

  Note that the length of the additional noble metal support may be 50 mm or less no matter how long the filter base is. This is because the range within 50 mm is sufficient to oxidize excess HC. The lower limit of the formation length of the additional noble metal supporting portion can oxidize HC and the like as long as it is formed, but generally 20 mm is sufficient.

  The loading amount of the noble metal catalyst in the additional noble metal supporting portion is preferably in the range of 0.1 to 5 g per liter of the additional noble metal supporting portion, similarly to the loading amount of the noble metal supporting portion. Further, the supported noble metal catalyst species may be the same as the noble metal supporting portion or a different type of noble metal catalyst.

  Hereinafter, the present invention will be specifically described with reference to Examples and Comparative Examples.

Example 1
1 and 2 show an exhaust gas purification apparatus of the present embodiment. This exhaust gas purification device is a filter catalyst, an inflow side cell 10 clogged on the exhaust gas downstream side, an outflow side cell 11 adjacent to the inflow side cell 10 and clogged on the exhaust gas upstream side, and an inflow side cell 10 It consists of a filter substrate 1 comprising a cell partition wall 12 that defines the outflow side cell 11, and a catalyst layer (not shown) formed on the surface of the cell partition wall 12 and the pore inner surface.

The catalyst layer has a base metal supporting part 20 supporting potassium as a PM oxidation catalyst on its inner peripheral part, and has a noble metal supporting part 21 supporting Pt on its outer peripheral part. The base metal support 20 is formed in a range of 40 mm in diameter around the central axis of the filter base 1 and is formed in a range of 40/130 (1.23 / 4) with respect to the diameter of the filter base 1 The method for producing the catalyst layer composed of the base metal supporting part 20 and the noble metal supporting part 21 will be described, and a detailed description of the configuration will be given.

A cordierite filter substrate 1 (wall flow structure, 12 mil / 300 cpsi) having a diameter of 130 mm and a length of 150 mm was prepared. Next, γ-Al ₂ O ₃ powder was mixed with alumina sol and ion-exchanged water so that the viscosity was 100 cps or less to prepare a slurry, and milled so that the average particle size of the solid particles was 1 μm or less. Then, the filter base material 1 is immersed in this slurry, the slurry is poured into the cell, and the excess slurry is removed by pulling up and sucking from the end surface opposite to the immersion side. Baked at 2 ° C. for 2 hours. This operation was performed twice, and adjustment was performed so that the same amount of coat layer was formed in the inflow side cell 10 and the outflow side cell 11. The formation amount of the coat layer is 100 g per liter of the filter substrate 1.

  Next, both end faces are masked in the range of φ40 mm from the center of the filter substrate 1 on which the coating layer is formed, and a predetermined amount of a dinitrodiammine platinum aqueous solution having a predetermined concentration is absorbed in the outer peripheral portion that is not masked, After drying with warm air, it was dried by ventilation at 120 ° C. for 2 hours to carry Pt, and a noble metal carrying part 21 was formed. The amount of Pt supported is 1 g per liter of the volume of the noble metal support 21.

  After removing the masking, both end surfaces of the outer peripheral portion excluding the range of φ40 mm from the center of the filter substrate 1 were masked, and a predetermined amount of a potassium acetate aqueous solution having a predetermined concentration was absorbed in the range of φ40 mm from the center. After removing the masking, dry with warm air at 60 ° C, then air dry at 120 ° C for 2 hours, and further heat at 300 ° C for 1 hour to decompose the acetate and carry potassium (K) to form base metal carrier 20 Formed. The loading amount of K is 0.3 mol per liter of the volume of the base metal supporting portion 20.

(Example 2)
The base metal carrier 20 is formed in a range of φ65 mm from the center (65/130 (2/4) with respect to the diameter of the filter base 1), and the precious metal carrier 21 is formed on the outer periphery. Similar to Example 1.

(Example 3)
The base metal carrier 20 is formed in a range of φ97.5 mm from the center (97.5 / 130 (3/4) of the diameter of the filter base 1), and the noble metal carrier 21 is formed on the outer periphery. Is the same as in Example 1.

Example 4
The base metal carrier 20 is formed in a range of φ20mm from the center (20/130 (0.62 / 4) of the filter base 1 diameter), and the noble metal carrier 21 is formed on the outer periphery. Similar to Example 1.

(Example 5)
Example 2 is the same as Example 2 except that a neodymium nitrate aqueous solution is used instead of the potassium acetate aqueous solution, and heating is performed at 500 ° C. for 1 hour instead of heating at 300 ° C. for 1 hour. The amount of Nd supported is 0.3 mol per liter of the volume of the base metal support 20.

(Example 6)
A coating layer was formed in the same manner as in Example 1 except that the same filter substrate 1 as in Example 1 was used and CeO ₂ powder was used instead of γ-Al ₂ O ₃ powder. The formation amount of the coat layer is 100 g per liter of the filter substrate 1. Subsequently, Pt and K were supported in the same manner as in Example 2.

(Example 7)
Example 6 is the same as Example 6 except that a cesium acetate aqueous solution was used instead of the potassium acetate aqueous solution. The amount of Cs supported is 0.3 mol per liter of the volume of the base metal support 20.

(Comparative Example 1)
Using the filter base material 1 on which the same coating layer as in Example 1 was formed, Pt was supported on the whole without masking. The amount of Pt supported is 1 g per liter of filter substrate volume.

(Comparative Example 2)
Using the filter base material 1 on which the same coating layer as in Example 1 was formed, K was supported on the whole without masking. The amount of K supported is 0.3 mol per liter of the filter substrate volume.

<Test and evaluation>
Each of the above filter catalysts is attached to the exhaust system of a common rail diesel engine (displacement of 2.2 L), and is operated for 1 hour at 1800 rpm, 60 Nm, inlet gas temperature of 260 ° C. PM was collected. The PM trapping amount was calculated by measuring the PM trapping efficiency and the inflow PM amount with a smoke meter attached before and after the filter catalyst.

  Next, as shown in FIG. 2, a flow-through oxidation catalyst 3 having a diameter of 130 mm and a length of 150 mm is disposed upstream of each filter catalyst 1 ′ in the exhaust system of the same engine, and the following thermal runaway test is performed. went.

  First, after stabilizing the engine 4 at 2000 rpm and 100 Nm, a predetermined amount of light oil is added to the exhaust gas from the injector 5 installed upstream of the exhaust pipe, and the exhaust gas temperature is raised by burning with the oxidation catalyst 3 on the upstream side. . When the center temperature of the filter catalyst 1 ′ reaches 700 ° C., the addition of light oil is stopped, the torque is released, and the engine speed is lowered to idle.

  By this operation, the bed temperature of the filter catalyst rises under conditions where oxygen concentration is low and PM combustion does not occur. When the addition of light oil is stopped, the captured PM is ignited and at the same time, combustion is propagated and the filter catalyst is abnormally heated. . At this time, the temperature inside each filter catalyst was measured at 15 points, and the maximum temperature was recorded. The results are shown in Table 1.

  The filter catalyst 1 'after the test was removed from the exhaust system and treated in an electric furnace at 300 ° C for 2 hours to volatilize the SOF content in PM, and the weight after that was measured. Further, PM was burned by treating at 600 ° C. for 2 hours in an electric furnace, and then the weight was measured to calculate the PM burning rate during the thermal runaway test. The results are shown in Table 1.

  Since the filter catalyst of Comparative Example 1 carries Pt having high oxidation activity as a whole, the maximum temperature is extremely high but thermal runaway occurs although the PM combustion rate is high. In addition, since the filter catalyst of Comparative Example 2 carries K having a low oxidizing ability as a whole, although the thermal runaway does not occur, the PM combustion rate is extremely low, and unburned residue is generated.

  However, it is clear that the filter catalyst of each Example has a high PM combustion rate and a maximum temperature lower than that of Comparative Example 1, and it is clear that regeneration efficiency during forced regeneration is high and thermal runaway is suppressed. . And since the filter catalyst of Example 4 has the highest temperature compared with other Examples, the range of the base metal carrying | support part 20 is 0.62 / 4 or more with respect to the diameter of the filter base material 1, ie, 1/4 or more. Obviously it is desirable to do.

1 is a perspective view including a partial cross section of an exhaust gas purification apparatus according to an embodiment of the present invention. It is explanatory drawing which shows the state which mounted the exhaust gas purification apparatus which concerns on one Example of this invention in the exhaust system of a diesel engine.

Explanation of symbols

1: Filter base material 3: Oxidation catalyst 5: Injector
20: Base metal carrying part 21: Precious metal carrying part

Claims (5)

An inflow side cell clogged on the exhaust gas downstream side, an outflow side cell adjacent to the inflow side cell and clogged on the exhaust gas upstream side, the inflow side cell and the outflow side cell are partitioned, and a large number of pores are formed. A filter substrate having a wall flow structure having a porous cell partition wall, and
An exhaust gas purification device comprising a catalyst layer formed on the cell partition wall,
The catalyst layer in the radially inner peripheral portion has a base metal supporting portion that is made of a base metal and supports a PM oxidation catalyst capable of oxidizing PM, and the catalyst layer in the radially outer peripheral portion has a noble metal supporting portion that supports a noble metal catalyst. An exhaust gas purification apparatus comprising:   The exhaust gas purifying apparatus according to claim 1, wherein the base metal supporting part is formed in a range of ¼ to ¾ with respect to a diameter of the filter base material.   The exhaust gas purification apparatus according to claim 1 or 2, wherein the PM oxidation catalyst is at least one selected from alkali metals and lanthanoid elements.   The exhaust gas purifying apparatus according to claim 1, wherein an end of the base metal supporting portion on the exhaust gas downstream side has an additional noble metal supporting portion further supporting a noble metal catalyst.   The exhaust gas purifying apparatus according to claim 4, wherein the additional noble metal supporting portion is formed within a range of 50 mm from the end face on the exhaust gas downstream side to the upstream side and within 1/3 or less of the total length of the filter base material.

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