JP2018003811A - Oxidation catalyst and exhaust emission control system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an oxidation catalyst device which is easy in manufacturing, and can improve the exhaust emission control performance of an exhaust emission control device such as a PM collection filter device and an NOx purification catalyst device which is arranged at a downstream side, and an exhaust emission control system.SOLUTION: In an oxidation catalyst device 20 for carrying an oxidation catalyst at a honeycomb structure 21, cells 22a, 22b through which an exhaust gas G passes are formed into square shapes, the cell 22a of an external peripheral part Ra is formed into a shape which is obtained by eliminating a cross-shaped bulkhead of the cell 22b of a center part Rb with respect to a face vertical to a flow direction of the exhaust gas G, a circulation cross section area of the cell 22a of the external peripheral part Ra becomes four times of a circulation cross section area of the cell 22b of the center part Rb, and a cell concentration of the external peripheral part Ra becomes one fourth of a cell concentration of the center part Rb.SELECTED DRAWING: Figure 2

Description

  The present invention relates to an oxidation catalyst device for purifying exhaust gas such as an internal combustion engine mounted on a vehicle, and an exhaust gas purification system.

  An internal combustion engine mounted on a vehicle is provided with an exhaust gas purification system that combines an oxidation catalyst device, a PM collection filter device, an SCR catalyst device for NOx purification, and the like to purify exhaust gas. This oxidation catalyst device oxidizes hydrocarbons and the like in exhaust gas, and is usually arranged in the forefront of an exhaust gas purification system in many cases.

  In this oxidation catalyst device, a noble metal catalyst such as platinum is often supported on a carrier of a honeycomb structure, and in order to efficiently purify exhaust gas while achieving both high temperature rise performance and high heat capacity, the partition walls of the outer peripheral cells There has also been proposed a honeycomb structure in which the thickness of the honeycomb structure is made larger than the thickness of the partition wall of the cell on the center side (see, for example, Patent Document 1).

JP 2012-200265 A

  On the other hand, in the oxidation catalyst device, the exhaust gas flowing into the outer peripheral portion of the oxidation catalyst device is slower than the central portion side at the outer peripheral portion (wall surface side) of the pipe flow channel due to the influence of the frictional resistance of the wall surface of the exhaust pipe. The amount of is reduced. As a result, the amount of HC deposited is lower than that at the center, and the amount of heat generated by the oxidation of HC is reduced. Therefore, the downstream exhaust gas purification device passes through the oxidation catalyst device and the flow velocity and temperature are reduced at the outer periphery. Exhaust gas will flow in, and there is a problem that exhaust gas purification performance cannot be fully exhibited.

  In addition, when clogging due to HC accumulation on the front surface of the oxidation catalyst occurs, the oxidation catalyst cannot sufficiently oxidize HC, and the regeneration temperature of the PM collecting filter device on the downstream side and the NOx purification catalyst device There is a problem that the activation temperature cannot be ensured and regeneration failure occurs.

  An object of the present invention is an oxidation catalyst device that is easy to manufacture and that can improve the exhaust gas purification performance of an exhaust gas purification device such as a PM collection filter device or a NOx purification catalyst device arranged on the downstream side, and an exhaust gas It is to provide a gas purification system.

  Furthermore, it is possible to improve the flow in the honeycomb shape against clogging from the outer peripheral part and improve the HC purification performance at low temperature for avoiding the adhesion of HC causing clogging, and the oxidation that can avoid clogging of the front surface of the oxidation catalyst device A catalyst device and an exhaust gas purification system are provided.

  To achieve the above object, an oxidation catalyst device of the present invention is an oxidation catalyst device in which an oxidation catalyst is supported on a honeycomb structure, the cells through which exhaust gas passes have a square shape, and the flow direction of the exhaust gas With respect to the surface perpendicular to the outer peripheral cell, the cross-sectional partition wall of the central cell is omitted from the central cell, and the flow cross-sectional area of the peripheral cell is four times that of the central cell. The cell density at the outer periphery is ¼ of the cell density at the center.

  Further, in the above oxidation catalyst device, with respect to the flow direction of the exhaust gas, a region in front of the position of 15% to 25% of the entire length of the front side of the oxidation catalyst device is defined as a front region, and The ratio of the amount of palladium supported with respect to platinum is set to be larger than the ratio of the amount of palladium supported with respect to platinum in the rear side region on the rear side of the front region.

  In addition, an exhaust gas purification system of the present invention for achieving the above object includes the above oxidation catalyst device provided on the upstream side of the exhaust gas purification system.

  According to the oxidation catalyst device and the exhaust gas purification system of the present invention, the cell shape is a square shape, and the outer peripheral cell has a shape in which the cross-shaped partition wall of the central cell is omitted. Furthermore, since the cell density in the outer peripheral portion is ¼ of the cell density in the central portion, it is easy to manufacture and the exhaust gas from the PM collection filter device, the NOx purification catalyst device, etc. disposed on the downstream side. The exhaust gas purification performance of the gas purification device can be improved.

  Furthermore, it is possible to improve the flow in the honeycomb shape against clogging from the outer peripheral portion and improve the HC purification performance at a low temperature for avoiding the adhesion of HC causing clogging, thereby avoiding the front clogging of the oxidation catalyst device.

It is a figure showing typically composition of an exhaust-gas purification system of an embodiment concerning the present invention. It is a cross-sectional view which shows the structure of the oxidation catalyst apparatus of embodiment which concerns on this invention. It is a cross-sectional view which shows the division of the outer peripheral part and center part in the oxidation catalyst apparatus of FIG. FIG. 3 is a side sectional view schematically showing the configuration of the oxidation catalyst device of FIG. 2 without the cell structure. It is a sectional side view which shows typically the structure of the oxidation catalyst apparatus formed separately, without a cell structure. It is a cross-sectional view which shows typically the structure of the oxidation catalyst apparatus of a uniform cell.

  Hereinafter, an oxidation catalyst device and an exhaust gas purification system according to embodiments of the present invention will be described with reference to the drawings.

  As shown in FIG. 1, an exhaust gas purification system 1 according to an embodiment of the present invention includes an oxidation catalyst device (DOC) 20 and an exhaust passage 11 through which an exhaust gas G discharged from an engine (internal combustion engine) 10 passes. , PM collection filter device (DPD) 30 for collecting particulate matter (PM), SCR catalyst device (SCR) 40 for purifying components such as nitrogen oxide (NOx) in exhaust gas G, etc. In this exhaust gas purification system 1, the oxidation catalyst device 20 is disposed in the uppermost stream of the exhaust gas purification system 1.

The oxidation catalyst device 20 uses oxygen (O ₂ ) in the exhaust gas G to oxidize hydrocarbons (HC) and carbon monoxide (CO) contained in the exhaust gas G, or particulate matter (PM). ) Is an exhaust gas purifying device that oxidizes unburned combustible material (SOF) contained in) and converts it into water (H ₂ O) and carbon dioxide (CO ₂ ). The full-throw type honeycomb structure 21 is configured to carry a noble metal such as platinum (Pt), palladium (Pd), rhodium (Rh) as an oxidation catalyst (not shown).

Then, the oxidation catalyst device 20 raises the temperature of the PM collection filter device 30 arranged on the downstream side, and at the time of forced regeneration in which the PM collected by the PM collection filter device 30 is burned and removed. The fuel is supplied into the exhaust gas G by post-injection of in-cylinder fuel injection of the engine 10 or direct injection in the exhaust pipe through which fuel is directly injected into the exhaust pipe from a fuel injection nozzle (not shown) provided in the exhaust passage 11. The role of increasing the unburned fuel in the exhaust gas G, oxidizing the unburned fuel by a catalytic reaction in the oxidation catalyst device 20, and increasing the temperature of the exhaust gas G by the heat generated by the oxidation, nitrogen monoxide in the exhaust gas G the (NO) by oxidation to nitrogen dioxide (NO ₂₎ in the exhaust gas G NO: the ratio of NO ₂ 1: to close to 1, the particle in the PM collection filter 30 The combustion of Jo substance has a role or to promote.

  The PM collection filter device 30 is for collecting particulate matter in the exhaust gas G. For example, the inlet and outlet of a porous ceramic honeycomb cell (channel) are alternately plugged. It consists of a monolith honeycomb type wall flow type filter.

  Exhaust gas G flows from the inlet of the unsealed cell of the PM collection filter device 30 and passes through the cell wall for collecting PM formed at the boundary between the adjacent outlet and the unsealed cell. Then, the adjacent outlet flows out from the outlet of the unsealed cell. Since there is a limit to the amount of PM that can be collected by the PM collection wall, before the PM collection amount is saturated, the high temperature exhaust gas G is passed through the PM collection filter device 30 to capture the PM. Forced PM regeneration control for burning and removing collected particulate matter is periodically performed.

The SCR catalyst device 40 is obtained by supporting a catalyst zeolite such as iron ion exchange aluminosilicate on a carrier such as a ceramic honeycomb, and urea injected by a urea water supply device 41 provided in the exhaust passage 11 on the upstream side thereof. This is an apparatus for purifying nitrogen oxide (NOx) contained in the exhaust gas G into nitrogen (N ₂ ) using ammonia (NH ₃ ) generated by hydrolysis of water by the heat of the exhaust gas G as a reducing agent.

  The exhaust gas purification system 1 includes a urea water injection system for supplying urea water into the exhaust gas G for NOx purification in the SCR catalyst device 40, a fuel injection system for direct fuel injection in the exhaust pipe, An ammonia adsorption unit for preventing ammonia outflow, a differential pressure sensor for detecting the differential pressure across the PM collection filter device 30, a temperature sensor for detecting the temperature of the exhaust gas G, NOx concentration, A gas concentration sensor or the like for detecting the oxygen concentration is arranged. However, since these components are not directly related to the present invention, they are omitted for simplification of description.

  In the present invention, as shown in FIGS. 2 and 3, the honeycomb structure 21 of the oxidation catalyst device 20 is formed so that the cells through which the exhaust gas G passes have a quadrangular shape, preferably a square shape. In addition, with respect to a plane perpendicular to the flow direction of the exhaust gas G, the cell 22a of the outer peripheral portion Ra (hatched portion by hatching in FIG. 3) is the center portion Rb (white portion surrounded by the hatching portion by hatching in FIG. 3). The cross-sectional area of the cell 22a in the outer peripheral portion Ra is four times the distribution cross-sectional area of the cell 22b in the central portion Rb, and the cell density of the outer peripheral portion Ra is The cell density is configured to be 1/4 of the cell density of the central portion Rb. As shown in FIGS. 2 to 4, the honeycomb structure 21 is fixed in a ring shape or the like on a case 23 having front and rear tapered portions and forming a flow path for the exhaust gas G. The member 24 is fixedly supported.

  According to this configuration, the shape of the cells 22a and 22b is a quadrangular shape, preferably a square shape, and the cell 22a of the outer peripheral portion Ra is a shape in which the cross-shaped partition wall of the cell 22b of the central portion Rb is omitted. Since the cell density of Ra is ¼ of the cell density of the central portion Rb, as shown in FIGS. 2 and 3, the wall surfaces of the cells 22 a and 22 b are displaced as seen from the flow direction of the exhaust gas G. Does not occur. Therefore, manufacture by extrusion molding becomes easy, and manufacture of the oxidation catalyst device 20 becomes easy. In this configuration, in order to facilitate manufacture, it is preferable that the partition walls of the cell 22a in the outer peripheral portion Ra and the cell 22b in the central portion Rb have the same thickness.

  Further, in the cell 22a of the outer peripheral portion Ra, the flow passage cross-sectional area is larger than that of the cell 22b of the central portion Rb, so that the flow resistance of the cell 22a of the outer peripheral portion Ra is the flow resistance of the cell 22b of the central portion Rb. The flow rate of the exhaust gas G in the cell 22a of the outer peripheral portion Ra increases. Therefore, in the oxidation catalyst device 20X of the honeycomb structure 21X having a uniform channel cross-sectional area as shown in FIG. 6 of the prior art, the flow rate of the exhaust gas G is slower on the outer peripheral side than on the central side, so that from the outer peripheral side. Particulate matter deposition occurs and proceeds toward the center, but in the oxidation catalyst device 20 of this embodiment, the particulate matter deposition can be avoided.

  As a result, the difference between the flow rate of the exhaust gas G after passing through the outer peripheral portion Ra and the central portion Rb and the calorific value become small, and the flow rate of the exhaust gas G flowing into the PM collection filter device 30 disposed on the downstream side Distribution and temperature distribution are made uniform. Therefore, since the particulate matter can be collected almost uniformly in the entire PM collection filter device 30, the temperature distribution during the forced regeneration of PM can be made uniform, and damage to the filter due to a difference in thermal expansion can be avoided. In addition, the amount of collected PM as a whole until the PM forced regeneration control can be increased. Thereby, collection of particulate matter and forced PM regeneration can be performed more efficiently, and the exhaust gas purification performance in the PM collection filter device 30 can be improved.

  Further, when clogging due to HC accumulation occurs on the front surface of the oxidation catalyst device 20, the oxidation catalyst device 20 cannot sufficiently oxidize HC, and the regeneration side PM collection filter device 30 is regenerated. Since the temperature and the activation temperature of the SCR catalyst device 40 cannot be ensured and a regeneration failure occurs, the oxidation catalyst device 20 has a problem with respect to the flow direction of the exhaust gas G from the front end. A region in front of the position (Pa) of 15% to 25% of the entire length Lt is defined as a front region Aa of length La, and the amount of palladium (Pd) supported on platinum (Pt) in this front region Aa The proportion αa is increased to the proportion αb of the amount of palladium (Pd) supported with respect to platinum (Pt) in the rear region Ab of the length Lb on the rear side of the front region Aa.

  More specifically, the ratio αa of the supported amount of palladium with respect to platinum in the front region Aa is 5% to 10% on a weight basis (20: 1 to 10: 1 in the Pt: Pd loading rate), and the rear region The ratio αb of the amount of palladium (Pd) supported on platinum in Ab is preferably 20% to 33% (Pt: Pd 5: 1 to 3: 1) on a weight basis.

In other words, by adding a very small amount of palladium to only platinum, the purification rate of HC on the low temperature side is improved, but the cost of precious metals is large for use as a whole, and more palladium is added in terms of performance. Since a large amount of nitrogen dioxide (NO ₂ ) is generated with respect to (3: 1 or the like), a difference is made in the ratios αa and αb of the amount of palladium supported with respect to platinum in the front region Aa and the rear region Ab.

  As a result, it is possible to improve the flow in the honeycomb shape against clogging from the outer peripheral portion and to improve the HC purification performance at a low temperature for avoiding adhesion of HC causing clogging. Can be avoided.

  The oxidation catalyst device 20 having a difference in the supported amount ratios αa and αb of palladium with respect to platinum in the front region Aa and the rear region Ab is used in the first catalyst solution in which the ratio of the supported amount of palladium to platinum is the rate αa. It can be manufactured easily by dipping the front area Aa and dipping the rear area Ab in the second catalyst solution in which the ratio of the supported amount of palladium to platinum is αb.

  Further, as in the oxidation catalyst device 20A shown in FIG. 5, the front region Aa and the rear region Ab are formed of separate front honeycomb structures 21Aa and rear honeycomb structures 21Ab, respectively, and the first catalyst solution and It is also possible to manufacture by immersing them in the second catalyst solution and then joining them together. In this case, the front region Aa and the rear region Ab may be closely integrated to each other, but a gap (space) C or a heat insulating material is provided between the front region Aa and the rear region Ab. Is preferred.

  As a result, the amount of heat transfer from the front region Aa to the rear region Ab is reduced, so that the front region Aa having a small heat capacity is quickly heated and activated by the exhaust gas G when the engine 10 is started. Therefore, combustion of hydrocarbons (HC) and carbon monoxide (CO) in the exhaust gas G in the front region Aa can be promoted. As a result, the temperature in the front region Aa rises, so that the efficiency of oxidation of hydrocarbons and carbon monoxide increases, and the heated exhaust gas G can be sent to the rear region Ab. This rear region Ab The catalyst can be activated by raising the temperature.

  As shown in FIG. 1, the exhaust gas purification system 1 according to the embodiment of the present invention is configured to include the oxidation catalyst devices 20 and 20A on the upstream side of the exhaust gas purification system 1. The effects exhibited by the oxidation catalyst devices 20 and 20A can be similarly exhibited.

1 exhaust gas purification system 10 engine (internal combustion engine)
11 Exhaust passages 20, 20A, 20X Oxidation catalyst devices 21, 21A, 21X Honeycomb structure 21Aa Front honeycomb structure 21Ab Rear honeycomb structure 22a Cell 22b in the outer peripheral part Cell 23 in the central part Case 24 Fixing member 30 PM collection filter Device 40 SCR catalyst device Aa Front region Ab Rear region La Length of front region Lb Length of rear region Lt Overall length Ra Outer portion Rb Center portion

Claims (4)

  In an oxidation catalyst device in which an oxidation catalyst is supported on a honeycomb structure, the cells through which the exhaust gas passes have a quadrangular shape, and the cells in the outer peripheral portion of the cell perpendicular to the flow direction of the exhaust gas In a shape that excludes the cross-shaped partition wall, the flow cross-sectional area of the outer peripheral cell is four times the flow cross-sectional area of the central cell, and the cell density in the outer peripheral portion is 1/4 of the cell density in the central portion. An oxidation catalyst device characterized by the above.   With respect to the flow direction of the exhaust gas, a region in front of the position 15% to 25% of the entire length of the front side of the oxidation catalyst device is defined as a front region, and the ratio of the amount of palladium supported to platinum in this front region is 2. The oxidation catalyst device according to claim 1, wherein the ratio is larger than the ratio of the amount of palladium supported with respect to platinum in the rear region on the rear side of the front region.   The ratio of the supported amount of palladium to platinum in the front region is 5% to 10% on a weight basis, and the ratio of the supported amount of palladium to platinum in the rear region is 20% to 33% on a weight basis. The oxidation catalyst device according to claim 2, wherein   An exhaust gas purification system comprising the oxidation catalyst device according to any one of claims 1 to 3 on an upstream side of the exhaust gas purification system.

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