JP4626854B2 - Exhaust gas purification device for internal combustion engine - Google Patents

Description

  The present invention relates to an exhaust gas purification device for an internal combustion engine, and more particularly to an exhaust gas purification device for an internal combustion engine provided with exhaust gas purification means such as a catalyst for purifying exhaust gas.

In recent years, NOx catalysts have been put into practical use to purify nitrogen oxides (NOx) contained in the exhaust gas of lean combustion internal combustion engines. For example, in the case of a NOx storage reduction catalyst, the NOx catalyst is a catalyst in which an alkaline earth such as barium (Ba) and a noble metal such as platinum (Pt) are supported on alumina as a carrier, and NOx in exhaust gas is supported. Is stored in the NOx catalyst in the form of nitrate (Ba (NO ₃ ) ₂ ). The NOx catalyst occludes NOx in the exhaust gas when the internal combustion engine is operating at a lean air-fuel ratio, while it operates when the exhaust air-fuel ratio of the internal combustion engine is operated at a rich air-fuel ratio less than the stoichiometric air-fuel ratio. It has the function of releasing and reducing the stored NOx.

However, since sulfur (S) is contained in the fuel and engine lubricating oil, sulfur is also contained in the exhaust gas. For this reason, the NOx catalyst has the property of storing the sulfur component in the exhaust gas as a sulfate such as BaSO ₄ and being poisoned (S poison) by the sulfur component. Since sulfate stored in the NOx catalyst is more stable than nitrate, it is not released from the NOx catalyst even when the exhaust air-fuel ratio is made rich in fuel, and gradually accumulates in the NOx catalyst. When the amount of sulfate in the NOx catalyst increases, the amount of NOx that can be absorbed by the NOx catalyst gradually decreases, causing a problem that the NOx storage capacity of the NOx catalyst decreases.

Therefore, by increasing the temperature of the sulfur-poisoned NOx catalyst and placing the NOx catalyst in a reducing atmosphere, the stored sulfate is decomposed into sulfur oxide (SOx) and separated from the NOx catalyst, and the NOx is stored. It is known to restore or regenerate ability. However, when the NOx catalyst poisoned with S is regenerated, for example, SOx desorbed by the following chemical reaction formula reacts with hydrogen (H ₂ ) in the exhaust gas, and hydrogen sulfide (H ₂ S) is temporarily converted. Produced in large quantities.
BaSO ₄ + CO → BaCO ₃ + SO ₂
SO ₂ + H ₂ → H ₂ S + O ₂

  Such hydrogen sulfide has a property of generating a strong odor, and if released into the atmosphere, for example, an unpleasant odor is emitted around a vehicle equipped with an internal combustion engine, which is not preferable.

  Therefore, various measures for preventing such a strange odor caused by hydrogen sulfide have been proposed. For example, Patent Document 1 discloses a catalyst to which a specific component that suppresses the generation of hydrogen sulfide is added, and Patent Document 2 discloses hydrogen sulfide during the S poison recovery recovery of the NOx storage reduction catalyst. An apparatus that executes control for suppressing occurrence is disclosed.

  However, the catalyst disclosed in Patent Document 1 itself suppresses the generation of hydrogen sulfide, and the hydrogen sulfide discharged from the NOx catalyst at the time of S poison recovery recovery when a normal NOx catalyst is used. On the other hand, no measures are taken. Further, in the apparatus disclosed in Patent Document 2, hydrogen sulfide is discharged in a relatively small amount during the S poison recovery recovery of the NOx catalyst, and no measures are taken against this hydrogen sulfide.

  On the other hand, Patent Document 3 discloses that fine particles in exhaust gas collected by a particulate filter are oxidized by an active oxygen release agent carried on the particulate filter. In this process, the temperature of the particulate filter is raised during the S poison recovery recovery of the particulate filter, and the hydrogen sulfide generated thereby is oxidized and decomposed into SOx by the active oxygen generated from the active oxygen release agent.

Japanese Patent Publication No. 6-511117 JP 2004-108176 A JP 2002-97926 A

  According to the apparatus described in Patent Document 3, hydrogen sulfide generated during the S poison recovery recovery of the particulate filter can be oxidatively decomposed by the active oxygen released from the active oxygen releasing agent of the filter itself, For the time being, it is thought that the release of hydrogen sulfide into the atmosphere can be prevented or suppressed.

  However, since this apparatus oxidizes and decomposes hydrogen sulfide released from the particulate filter into SOx by the active oxygen release agent of the filter itself, there is no doubt that the decomposed SOx will adhere to the filter again. However, there remains a problem in the removal efficiency of sulfate during S poison recovery recovery. Thus, even if active oxygen is released from the exhaust gas purification means itself such as a catalyst and hydrogen sulfide is decomposed into SOx, the redeposition of SOx on the exhaust gas purification means becomes a problem and does not constitute a fundamental solution.

  On the other hand, as a result of earnest research, the inventors of the present invention supplied ozone having strong oxidizing power to a position downstream of the exhaust gas purification means such as a catalyst, and this ozone converted the hydrogen sulfide discharged from the exhaust gas purification means into SOx. I found a method of oxidative decomposition. According to this, since ozone is supplied at a position downstream of the exhaust purification means, it is possible to prevent SOx from reattaching to the exhaust purification means, and the above problem can be solved.

  However, SOx generated as a result of oxidative decomposition of hydrogen sulfide is a problem. That is, SOx has a strong oxidizing power and reacts with water in the exhaust gas to generate sulfuric acid and sulfurous acid. On the other hand, the exhaust pipe and muffler downstream from the ozone supply position are generally made of a material such as iron. Therefore, SOx, sulfuric acid, or sulfurous acid may corrode the exhaust pipe and muffler downstream from the ozone supply position.

  The present invention was created in view of the above circumstances, and the object thereof is generated as a result of oxidative decomposition when hydrogen sulfide generated during sulfur poisoning recovery regeneration of exhaust purification means such as a catalyst is oxidized and decomposed with ozone. An object of the present invention is to provide an exhaust emission control device for an internal combustion engine capable of suitably processing SOx.

  In order to achieve the above object, an aspect of an exhaust gas purification apparatus for an internal combustion engine according to the present invention is provided in an exhaust passage of the internal combustion engine, and purifies exhaust gas discharged from a combustion chamber, and the exhaust gas purification device. Ozone supply means capable of supplying ozone to the exhaust passage at a position downstream of the means, and an SOx trap provided in the exhaust passage at a position downstream of the ozone supply position and absorbing SOx in the exhaust gas And a catalyst.

  According to this aspect of the present invention, the hydrogen sulfide discharged from the exhaust purification unit during the sulfur poisoning recovery regeneration of the exhaust purification unit can be oxidized by the ozone supplied from the ozone supply unit. The SOx generated as a result of this oxidative decomposition can be absorbed by the SOx trap catalyst. Therefore, it is possible to further prevent SOx from being discharged to the downstream side and to prevent corrosion of the exhaust pipe and the muffler.

  Here, the SOx trap catalyst preferably contains an alkali metal element, an alkaline earth metal element, or a rare earth element.

The alkali metal element is preferably Li, Na or K.
The alkaline earth metal element is preferably Mg, Ba, Ca or Sr.
The rare earth element is preferably La.

  According to the present invention, when hydrogen sulfide generated during sulfur poisoning recovery regeneration of an exhaust purification means such as a catalyst is oxidatively decomposed with ozone, SOx generated as a result of the oxidative decomposition can be suitably treated. Excellent effect is exhibited.

Embodiments of the present invention will be described below with reference to the accompanying drawings.
FIG. 1 is a system diagram schematically showing an exhaust gas purification apparatus for an internal combustion engine according to an embodiment of the present invention. In the figure, reference numeral 10 denotes an internal combustion engine, that is, an engine. The engine of the present embodiment is a spark ignition internal combustion engine, more specifically, a direct injection gasoline engine. However, the engine may be a compression ignition type internal combustion engine, that is, a diesel engine. In short, the engine type and type are not limited as long as the exhaust gas contains a sulfur component. 11 is an intake manifold communicated with the intake port, 12 is an exhaust manifold communicated with the exhaust port, and 13 is a combustion chamber. In the present embodiment, fuel supplied from a fuel tank (not shown) to the high pressure pump 17 is pumped to the delivery pipe 18 by the high pressure pump 17 and accumulated in a high pressure state, and the high pressure fuel in the delivery pipe 18 is fuel injection valve. 14 is directly injected into the combustion chamber 13. Exhaust gas from the engine 10 passes from the exhaust manifold 12 through the turbocharger 19 and then flows into the exhaust passage 15 downstream thereof. After being purified as described later, the exhaust gas is discharged to the atmosphere.

  The exhaust passage 15 is provided with a NOx catalyst 20 for purifying NOx in the exhaust gas as an exhaust purification means for purifying the exhaust gas discharged from the combustion chamber 13. However, the exhaust purification means is not limited to the NOx catalyst 20, and any exhaustion purification means may be used as long as it is poisoned by the sulfur component contained in the exhaust gas and loses the inherent exhaust purification performance. Other examples of such exhaust purification means include three-way catalysts, HC adsorbents, NOx adsorbents and particulate matter oxidation catalysts. Further, the exhaust gas purification means may comprise a combination of two or more of these.

The NOx catalyst 20 of this embodiment is an NOx storage reduction catalyst (NSR: NOx Strage Reduction). In this case, the NOx catalyst 20 is configured such that a noble metal such as platinum Pt as a catalyst component and a NOx absorbing component are supported on the surface of a base material made of an oxide such as alumina Al ₂ O ₃ . The NOx absorbing component is at least one selected from, for example, an alkali metal such as potassium K, sodium Na, lithium Li, and cesium Cs, an alkaline earth such as barium Ba and calcium Ca, and a rare earth such as lanthanum La and yttrium Y. It consists of one. The NOx storage reduction catalyst 20 absorbs NOx when the air-fuel ratio of the exhaust gas flowing into it is leaner than a predetermined value (typically the theoretical air-fuel ratio), and the oxygen concentration in the exhaust gas flowing into the NOx catalyst 20 When NO is reduced, the absorbed NOx is released, that is, the absorbed NOx is released. In this embodiment, a direct-injection gasoline engine is used, and lean burn operation can be performed. Therefore, during the lean burn operation, the exhaust air-fuel ratio is lean, and the NOx catalyst 20 absorbs NOx in the exhaust. . Further, when the reducing agent is supplied on the upstream side of the NOx catalyst 20 and the air-fuel ratio of the inflowing exhaust gas becomes rich, the NOx catalyst 20 releases the absorbed NOx. The released NOx reacts with the reducing agent and is reduced and purified.

  Any reducing agent may be used as long as it generates a reducing component such as hydrocarbon HC or carbon monoxide CO in the exhaust gas. Gas such as hydrogen or carbon monoxide, liquid such as propane, propylene, or butane or carbonization of gas. Liquid fuels such as hydrogen, gasoline, light oil and kerosene can be used. In this embodiment, gasoline, which is a fuel, is used as a reducing agent in order to avoid complications during storage and replenishment. As a method for supplying the reducing agent, for example, fuel is injected from an injection valve (not shown) separately provided in the exhaust passage 15 upstream of the NOx catalyst 20, or a larger amount of fuel than normal is injected. A method of performing so-called post-injection in which fuel is injected from the fuel injection valve 14 to control the exhaust gas atmosphere richly, or fuel is injected from the fuel injection valve 14 in the later stage of the expansion stroke or the exhaust stroke is possible. Thus, the supply of the reducing agent for the purpose of releasing and reducing NOx in the NOx catalyst 20 is referred to as a rich spike.

By the way, the NOx catalyst 20 may be a selective reduction type NOx catalyst (SCR: Selective Catalitic Reduction). This selective reduction type NOx catalyst has a base material such as zeolite or alumina that supports a noble metal such as Pt, a base metal surface that supports a transition metal such as Cu by ion exchange, and the base material. Examples include those having a titania / vanadium catalyst (V ₂ O ₅ / WO ₃ / TiO ₂ ) supported on the surface. In this selective reduction type NOx catalyst, under the condition that the air-fuel ratio of the inflowing exhaust gas is lean, HC and NO in the exhaust gas are reacted steadily and simultaneously so that N ₂ , O ₂ , H ₂ O, etc. Purified. However, the presence of HC is essential for NOx purification. Even if the air-fuel ratio is lean, unburned HC is always contained in the exhaust gas, and this can be used to reduce and purify NOx. Further, the reducing agent may be supplied by performing rich spike like the NOx storage reduction catalyst. In this case, ammonia or urea can be used as the reducing agent in addition to those exemplified above.

Explaining other exhaust purification means, the three-way catalyst is formed by supporting a noble metal such as Pt, Pd and Rh on a porous oxide such as alumina and ceria, and in an atmosphere near the stoichiometric air-fuel ratio (stoichiometric). HC, CO and NOx in the exhaust gas can be purified at the same time. As the HC adsorbent, for example, a porous adsorbent mainly composed of silica (for example, SiO ₂ supported between layered crystals of SiO ₄ ), a porous material such as zeolite, and the like are used in a number of thin axial flows. HC formed in a cylindrical shape having a channel (cell), adsorbing HC components in the exhaust flowing in when the adsorbent temperature is low, into the porous pores and adsorbing when the adsorbent temperature is high Performs absorption and release of HC that releases components. This is particularly effective for reducing the so-called cold HC at the cold start of the engine. The NOx adsorbent is made of porous zeolite or the like, and holds NO or NO ₂ in the exhaust gas as it is, not in the form of nitrate. The particulate matter oxidation catalyst is supported on the surface of a particulate filter that mainly collects particulate matter (PM) emitted from diesel engines, and the collected particulate matter is kept at a relatively low temperature. Oxidation (combustion) removal, for example, noble metals such as platinum Pt, palladium Pd, rhodium Rh, alkali metals such as potassium K, sodium Na, lithium Li, cesium Cs, barium Ba, calcium Ca, strontium Sr At least one selected from alkaline earth metals such as lanthanum La, rare earth such as yttrium Y and cerium Ce, and transition metals such as iron Fe.

Returning to FIG. 1, in this embodiment, ozone supply means capable of supplying ozone (O ₃ ) to the exhaust passage 15 at a position downstream of the NOx catalyst 20 is provided. The ozone supply means includes an ozone supply member 40 inserted into the exhaust passage 15 and an ozone generator 41 connected to the ozone supply member 40 via an ozone supply passage 42. The ozone generated by the ozone generator 41 reaches the ozone supply member 40 through the ozone supply passage 42 and is injected and supplied into the exhaust passage 15 from the supply port 43 provided in the ozone supply member 40 toward the downstream side. The supply port 43 is plural (two) in this embodiment, but may be one. The ozone supply member 40 extends in the diameter direction of the exhaust passage 15, and the supply ports 43 are arranged at predetermined intervals in the longitudinal direction of the ozone supply member 40 so that ozone is evenly distributed in the exhaust passage 15.

  As the ozone generator 41, a form in which ozone is generated while flowing air or oxygen as a raw material in a discharge tube to which a high voltage can be applied, or any other type can be used. Here, the air or oxygen as the raw material is a gas taken from outside the exhaust passage 15, for example, a gas contained in the outside air, and is not a gas contained in the exhaust gas in the exhaust passage 15. In the ozone generator 41, ozone generation efficiency is higher when a low temperature source gas is used than when a high temperature source gas is used. Therefore, by generating ozone using the gas outside the exhaust passage 15 in this way, it is possible to improve the ozone generation efficiency.

  The ozone generator 41 is connected to an electronic control unit (hereinafter referred to as an ECU (Electrical Control Unit)) 100 as control means, and generates ozone when the ECU 100 is turned on, and generates ozone when the ECU 100 is turned off. Stop generating. The generated ozone is supplied into the exhaust passage 15 from the supply port 43 of the ozone supply member 40 as described above, whereby ozone supply is executed. In the present embodiment, ozone generated by turning on the ozone generator 41 is supplied immediately when ozone is supplied. However, ozone may be generated and stored in advance, and ozone may be supplied by switching valves. Good. It is also possible to supply ozone by pressurizing it with a pump or a compressor.

  The exhaust passage 15 is provided with an air-fuel ratio sensor 54 as exhaust air-fuel ratio detection means, and this air-fuel ratio sensor 54 is connected to the ECU 100. The air-fuel ratio sensor 54 is provided in the exhaust passage 15 at a position upstream of the NOx catalyst 20, and detects the oxygen concentration of the exhaust gas at the position. During normal operation of the engine, the ECU 100 calculates the exhaust air / fuel ratio of the engine based on the output signal of the air / fuel ratio sensor 54. The air-fuel ratio sensor 54 is a global air-fuel ratio sensor that can detect a relatively wide range of air-fuel ratios, and linearly outputs a voltage signal as an output signal in accordance with the detected air-fuel ratio. The ECU 100 feedback-controls the fuel injection amount of the engine 10 so that the exhaust air-fuel ratio detected by the air-fuel ratio sensor 54 approaches a predetermined target air-fuel ratio. At this time, the target air-fuel ratio is set to a value leaner than the stoichiometric air-fuel ratio (for example, A / F = 18) during lean burn operation, and the target air-fuel ratio is stoichiometric (for example, A / F = 14. 7).

  The exhaust passage 15 is provided with a temperature sensor 53 as exhaust temperature detection means, and the temperature sensor 53 is connected to the ECU 100. ECU 100 calculates the exhaust gas temperature based on the output signal of temperature sensor 53. The temperature sensor 53 is made of, for example, a thermocouple, and the tip as a temperature measuring unit is positioned at the center of the exhaust passage 15. Similarly, the temperature sensor 53 linearly outputs a voltage signal as an output signal in accordance with the detected exhaust gas temperature. The temperature sensor 53 is provided in the exhaust passage 15 at a position upstream of the NOx catalyst 20, but may be provided in the exhaust passage 15 at a position between the NOx catalyst 20 and the ozone supply member 40.

  ECU 100 executes rich spike control according to a predetermined program stored in advance. That is, when a predetermined rich spike execution condition is satisfied, the ECU 100 simultaneously injects fuel from a separately provided rich spike injection valve or injects a larger amount of fuel from the fuel injection valve 14 than usual. The rich spike is executed by controlling the exhaust gas atmosphere to be rich or by causing the fuel injection valve 14 to perform post injection. As a result, the air-fuel ratio of the exhaust gas flowing into the NOx catalyst 20 becomes richer than the stoichiometric air-fuel ratio, the NOx stored in the NOx catalyst 20 is released, and the unburned components (CO, HC) in the exhaust gas are released. It reacts and is reduced and purified. Thus, the rich spike control means is constituted by the ECU 100.

  In the exhaust gas purification apparatus according to this embodiment, sulfur components in the exhaust gas are absorbed or adsorbed on the NOx catalyst 20 in the form of sulfate during engine operation, and the NOx catalyst 20 is sulfur poisoned (S poisoned). The Specifically, the sulfur component in the exhaust gas is absorbed by the NOx absorption component such as barium of the NOx catalyst 20 in the form of sulfate. As the sulfate gradually accumulates in the NOx catalyst 20, the NOx storage capacity of the NOx catalyst 20 decreases.

  Therefore, in order to recover the NOx occlusion capacity of the NOx catalyst 20, sulfur poisoning recovery regeneration for desorbing or releasing sulfate from the NOx catalyst 20 is performed. This sulfur poisoning recovery regeneration is performed by setting the atmosphere of the NOx catalyst 20 to a reducing atmosphere and setting the atmosphere temperature to a relatively high predetermined temperature (for example, 400 ° C.) or higher. More specifically, when the air-fuel ratio of the exhaust gas flowing into the NOx catalyst 20 is stoichiometric or richer and the inflowing exhaust gas temperature is equal to or higher than the predetermined temperature, sulfur poisoning recovery regeneration is performed. Is called. At this time, the sulfate absorbed in the NOx catalyst 20 is decomposed into sulfur oxides (SOx) and desorbed from the NOx catalyst 20.

  The exhaust temperature at which the sulfate can be desorbed, for example, 400 ° C. or higher is a temperature that can be reached relatively easily in the case of a gasoline engine as in this embodiment, but in the case of a diesel engine that originally has a low exhaust temperature. Then, it is a temperature that is relatively difficult to reach. On the other hand, the temperature at which the stored NOx can be released and reduced in the NOx catalyst is lower than the temperature at which sulfate can be desorbed. For example, NOx can be released and reduced at about 200 to 300 ° C. Thus, sulfate is more stable than nitrate, and cannot be removed unless the ambient temperature is higher than that of nitrate. The exhaust temperature at which the sulfate can be desorbed varies depending on the material and structure of the exhaust purification means, and may be, for example, 500 ° C. or higher.

  The sulfur poisoning recovery regeneration is first performed simultaneously with the purification of the stored NOx released in the NOx catalyst. That is, the purification of the stored NOx release is realized by the above-described rich spike control, and at this time, the exhaust air-fuel ratio is rich. Therefore, at this time, if the exhaust temperature is a temperature at which sulfate can be desorbed, the sulfate is naturally desorbed from the NOx catalyst and decomposed into SOx. In the case of the gasoline engine as in this embodiment, such a high exhaust temperature is relatively frequent. Therefore, if rich spike control for NOx emission reduction is performed, in many cases, sulfur poisoning is simultaneously performed. Recovery playback is executed.

  Second, when the engine is for a vehicle, the air-fuel ratio is made rich at every predetermined timing in consideration of the travel distance, fuel consumption, fuel type (regular or high-octane), sulfur concentration in the fuel, etc. In this method, the exhaust gas temperature is raised to a predetermined temperature or higher, and sulfur poisoning recovery regeneration is executed. This is a control dedicated to sulfur poisoning recovery regeneration. For example, the air-fuel ratio is temporarily made rich every predetermined travel distance, and the exhaust temperature is raised to a predetermined temperature or higher by increasing the engine speed, etc. A poison recovery regeneration can be performed. In addition, like these first and second methods, sulfur poisoning recovery regeneration that is intentionally performed by performing predetermined control is referred to as forced regeneration.

  Third, even if it is not during forced regeneration, if the engine is in an operating state that satisfies the sulfur poisoning recovery and regeneration condition, sulfur poisoning recovery and regeneration is performed automatically. Specifically, for example, when the engine is operating at a high load when the vehicle is traveling uphill, the air-fuel ratio feedback control is stopped, the exhaust air-fuel ratio becomes richer than stoichiometric, and the exhaust temperature can be desorbed with sulfate. When the temperature is reached, the sulfate is naturally desorbed from the NOx catalyst and decomposed into SOx. Such sulfur poisoning recovery regeneration that is executed without special control is called natural regeneration.

By the way, as described above, during the sulfur poisoning recovery regeneration, the generated SOx reacts with hydrogen (H ₂ ) in the exhaust gas to generate hydrogen sulfide (H ₂ S). If this hydrogen sulfide is released from the exhaust passage into the atmosphere, it will cause an unpleasant odor, which is not preferable.

Therefore, in the exhaust purification apparatus of the present embodiment, the hydrogen sulfide discharged from the NOx catalyst 20 at the time of sulfur poisoning recovery regeneration is oxidized to SOx by ozone as an oxidizing gas before being released into the atmosphere. This prevents the release of hydrogen sulfide into the atmosphere and the generation of odors associated therewith. Specifically, ozone generated by the ozone generator 41 is supplied from the ozone supply member 40 into the exhaust passage 15 at a position downstream of the NOx catalyst 20. Thereby, the hydrogen sulfide discharged from the NOx catalyst 20 can be oxidized into SOx by ozone on the downstream side of the NOx catalyst 20, and the discharge of hydrogen sulfide into the atmosphere can be suppressed. The reaction between hydrogen sulfide H ₂ S and ozone O ₃ is represented by the following formula, for example.
H ₂ S + O ₃ → H ₂ O + SO ₂
As can be seen, the reaction between hydrogen sulfide H ₂ S and ozone O ₃ is an equimolar reaction.

  In particular, the ozone supply position (that is, the position of the supply port 43) in the exhaust passage 15 is a position downstream of the NOx catalyst 20, more specifically, a position separated from the NOx catalyst 20 by a predetermined distance downstream. . By doing so, SOx produced after the decomposition of hydrogen sulfide, which becomes a problem in the apparatus described in Patent Document 3, is not re-adsorbed to the NOx catalyst, and the sulfate is removed at the time of sulfur poisoning recovery regeneration. Efficiency can be increased.

  Here, at the time of sulfur poisoning recovery and regeneration, the NOx catalyst 20 is at a high temperature, and if ozone is supplied immediately after the vicinity of the NOx catalyst 20, the ozone is thermally decomposed, and the oxidizing ability of hydrogen sulfide decreases accordingly. Resulting in. In general, ozone is thermally decomposed at a temperature of about 250 ° C. Therefore, the distance L from the NOx catalyst 20 (especially its downstream end) to the ozone supply position is preferably set to a distance that does not substantially cause such ozone thermal decomposition, and is ideally as long as possible. It is preferable to do this.

  If ozone is supplied upstream of the NOx catalyst 20, ozone reacts with NOx and SOx in the exhaust gas and is consumed, so this should be avoided.

  Incidentally, it is preferable to supply ozone only at the time of sulfur poisoning recovery regeneration in which hydrogen sulfide is discharged from the NOx catalyst 20. This is because, in order to generate ozone with the ozone generator 41, electric power is required. Therefore, useless consumption of ozone leads to useless consumption of electric power, which leads to deterioration of fuel consumption. For example, even if the ozone generator 41 is always turned on and ozone supply is always performed, the discharge of hydrogen sulfide can be prevented, but this should be avoided.

  Therefore, in the present embodiment, based on the exhaust air-fuel ratio detected by the air-fuel ratio sensor 54 and the exhaust temperature detected by the temperature sensor 53, it is determined whether or not sulfur poisoning recovery regeneration is being performed, Only when the operation is in progress, the ozone generator 41 is turned on and the ozone supply is executed. Specifically, the ECU 100 recovers sulfur poisoning when the detected exhaust air-fuel ratio is a stoichiometric or richer value and the detected exhaust temperature is equal to or higher than the predetermined temperature (for example, 400 ° C.). It is determined that regeneration is being performed, and the ozone generator 41 is turned on. According to this, regardless of whether it is forced regeneration or natural regeneration, ozone is supplied only under a situation where hydrogen sulfide is discharged, and wasteful consumption of ozone and ozone-generated power can be prevented. In other words, the exhaust air-fuel ratio and the exhaust temperature are conditions that determine the ozone supply start timing.

  As shown in FIG. 1, the air-fuel ratio sensor 54 is provided on the upstream side of the temperature sensor 53 in this embodiment, but this arrangement may be reversed.

  Here, in view of the efficient use of ozone as described above, it is preferable to supply the minimum amount of ozone commensurate with the amount of hydrogen sulfide to be discharged. For this reason, for example, the amount of fuel consumed from the previous sulfur poisoning recovery regeneration is integrated and the amount of hydrogen sulfide generated during the current sulfur poisoning recovery regeneration is estimated based on this fuel consumption. It is conceivable to control the ozone supply 41 by controlling the ozone generator 41 according to the amount of hydrogen sulfide.

  By the way, when hydrogen sulfide is oxidized and decomposed with ozone, SOx is generated as a result. SOx has a strong oxidizing power and reacts with water in the exhaust gas to generate sulfuric acid and sulfurous acid. On the other hand, the exhaust pipe and muffler downstream from the ozone supply position are generally made of a material such as iron. Therefore, SOx, sulfuric acid, or sulfurous acid may corrode the exhaust pipe and muffler downstream from the ozone supply position.

  Therefore, in the present embodiment, the SOx trap catalyst 30 that absorbs SOx in the exhaust gas is provided in the exhaust passage 15 at a position downstream of the ozone supply position. According to this, SOx generated as a result of the oxidative decomposition of hydrogen sulfide by ozone can be absorbed or adsorbed by the SOx trap catalyst 30, SOx is discharged to the downstream side of the SOx trap catalyst 30, and sulfuric acid based on this SOx is used. Or the production | generation of sulfurous acid can be prevented and corrosion of an exhaust pipe, a muffler, etc. can be prevented.

  Here, in order to quickly adsorb SOx, the SOx trap catalyst 30 is preferably installed as close to the ozone supply nozzle 40 as possible. On the other hand, in order to completely oxidize the hydrogen sulfide discharged from the NOx catalyst 20 with ozone, a certain length of the exhaust passage 15, that is, a reaction time is required. Accordingly, the distance M from the ozone supply position to the upstream end of the SOx trap catalyst 30 is determined in consideration of the balance between the two.

  Here, in the portion between the ozone supply position and the downstream end of the SOx trap catalyst 30, since the exhaust gas contains SOx having a strong oxidizing power, there is a concern about corrosion. The material of the member is preferably a material having high corrosion resistance (for example, stainless steel). The members include an ozone supply nozzle 40, a casing (converter) 31 in which the SOx trap catalyst 30 is housed, and a connecting exhaust pipe 16 to which the ozone supply nozzle 40 is attached. As shown in the figure, the connecting exhaust pipe 16 connects a casing (converter) 21 in which the NOx catalyst 20 is stored and a casing (converter) 31 in which the SOx trap catalyst 30 is stored.

In FIG. 2, the enlarged view of the cell of the SOx trap catalyst 30 is shown. The SOx trap catalyst 30 has a base material 32 made of, for example, a cylindrical cordierite as a whole, and this base material 32 is formed in a mesh shape or a honeycomb shape to define a number of cells 33 as exhaust gas passage holes. To do. The cell 33 extends in the axial direction (front and back direction in FIG. 2) of the SOx trap catalyst 30, and both ends thereof are opened to form an inlet and an outlet for exhaust gas. On the inner wall of the cell 33, a carrier 34 as a washcoat layer is formed over the entire surface. The carrier 34 is made of alumina (Al ₂ O ₃ ), for example, and the thickness thereof is, for example, about 20 to 50 μm.

  FIG. 3 shows an enlarged view of the carrier 34. Microscopically, the carrier 34 is configured by aggregating countless particles 35, and pores 36 through which gas can diffuse are formed between the particles 35. The particle size of the particles 35 is about several tens of nm, for example. On the surface of the particles 35, a component that reacts with SOx to generate a sulfate, that is, a SOx reaction component 38, and a plurality of active sites 37 made of noble metals such as Pt and Pd are provided as necessary. The carrier 34 is sintered by mixing the material powder constituting the carrier 34 in a solution such as water and dispersing it, immersing the base material 32 in this solution, drying the base material 32 and firing it. .

  Here, when the exhaust gas passes through the SOx trap catalyst 30, the exhaust gas passes through the catalyst while being in contact with the carrier 34. More specifically, the exhaust gas contacts the surface of the particles 35, the SOx reaction component 38, and the active points 37 while entering the pores 36 between the particles 35. At this time, SOx in the exhaust gas reacts with the SOx reaction component 38 to form a sulfate, and this sulfate is adsorbed on the surface of the particle 35. In this way, SOx in the exhaust gas is absorbed by the SOx trap catalyst 30 and SOx discharge is prevented.

  Here, the SOx reaction component 38 is preferably made of an alkali metal element, an alkaline earth metal element, or a rare earth element. The alkali metal element is preferably Li, Na or K, the alkaline earth metal element is preferably Mg, Ba, Ca or Sr, and the rare earth element is preferably La.

In addition, when hydrogen sulfide is oxidized with ozone, SO ₃ having high oxidizing power may be generated as SOx generated as a result. This SO ₃ can react with the SOx reaction component 38 without the aid of the active point 37 to generate a sulfate. Therefore, when hydrogen sulfide is oxidatively decomposed with ozone as in the present embodiment, the active points 37 made of noble metals such as Pt and Pd may be omitted.

  By the way, when SOx is absorbed by the SOx trap catalyst 30, the amount of SOx absorbed eventually reaches the maximum absorption amount of the SOx trap catalyst 30, and the SOx trapping capacity of the SOx trap catalyst 30 is significantly reduced. This is a full state of the SOx trap catalyst 30. In this case, it is conceivable to replace the SOx trap catalyst 30. In order to notify the user of this replacement time, warning means such as a lamp or a buzzer may be provided. Further, a means for detecting the full state of the SOx trap catalyst 30 may be provided. For example, the ECU 100 integrates the fuel consumption based on the detected value of the fuel meter, and detects the full state of the SOx trap catalyst 30 based on the fuel consumption. The SOx trap catalyst 30 is replaced with the casing 31 that houses the SOx trap catalyst 30.

  On the other hand, in recent years, fuels with extremely low sulfur concentrations have been developed, and some have been put to practical use. In the case of vehicle engines using such fuels, the SOx trap catalyst does not fill up to the end of the vehicle's useful life, There is a possibility of continuing to exhibit sufficient SOx absorption ability. Therefore, in such a case, it is not necessary to consider the replacement of the SOx trap catalyst.

Next, the result of the experiment using the simulation gas (model gas) performed in connection with the present embodiment is shown below.
(1) Experimental apparatus FIG. 4 shows the whole experimental apparatus, and FIG. 5 shows the details of the V part of FIG. 61 is a plurality of gas cylinders, and each gas cylinder is filled with a raw material gas for making a simulated gas imitating the exhaust gas composition of a gasoline engine. The source gas here is a gas such as N ₂ , O ₂ , and CO. A simulation gas generator 62 includes a mass flow controller, and generates a simulation gas MG by mixing each raw material gas by a predetermined amount. As shown in detail in FIG. 5, the simulated gas MG passes through the three-way elbow 72, then passes through the SOx trap catalyst 64 disposed in the quartz tube 63, and is discharged to the outside from an exhaust duct (not shown).

As shown in FIG. 4, the gaseous oxygen O ₂ supplied from the oxygen cylinder 67 is branched into two, and the flow rate is controlled by the flow rate control unit 68 on one side and then supplied to the ozone generator 69. In the ozone generator 69, oxygen is selectively and partially converted into ozone O _3, and these oxygen and ozone (or only oxygen) reach the ozone analyzer 70. On the other side of the branch, the flow rate of oxygen is controlled by another flow rate control unit 71, and then mixed with the gas supplied from the ozone generator 69 to reach the ozone analyzer 70. The ozone analyzer 70 measures the ozone concentration of the gas that has flowed into it, that is, the supply gas, and then the flow rate of the supply gas is controlled by the flow rate control unit 71. Excess supply gas is discharged to the outside from an exhaust duct (not shown), and the supply gas whose flow rate is controlled is simulated gas MG at a three-way elbow 72 positioned upstream of the SOx trap catalyst 64 as shown in FIG. Mixed with. The mixed gas is processed by an exhaust gas analyzer 78 for measuring SOx, SO ₂ , and H ₂ S concentrations and an ozone analyzer 79 for measuring ozone concentrations, and then discharged outside through an exhaust duct (not shown). .

  An electric heater 73 is provided on the outer peripheral portion of the quartz tube 63 so that the temperature of the SOx trap catalyst 64 is controlled. Further, a temperature sensor 75 for measuring the catalyst bed temperature of the SOx trap catalyst 64 is provided.

(2) Experimental conditions Experiments were performed under the following conditions for each of the SOx trap catalysts 64 of Examples 1 to 4 described later. First, the electric heater 73 is controlled so that the temperature detected by the temperature sensor 75 is constant (200 ° C.). When the temperature is stabilized at a constant level, a simulated gas having the following composition is circulated and simultaneously The supply gas is mixed with the simulated gas at the position of the three-way elbow 72. When supplying ozone, the ozone generator 69 is turned on. Thereby, supply gas turns into a mixed gas of ozone and oxygen. Conversely, when ozone supply is not performed, the ozone generator 69 is turned off. As a result, the supply gas is only oxygen. The composition of the simulated gas is 50 ppm in H ₂ S, 3% in H ₂ O, and the balance is N ₂ in volume concentration. The flow rate of the simulation gas is 10 L (liter) / min. The composition of the supply gas containing ozone is 50000 ppm of ozone O ₃ and the balance is O ₂ . The flow rate of the supply gas is 2 L (liter) / min.

Then, after supplying the simulated gas, the time integrated value of the SOx concentration and the H ₂ S concentration of the gas that has passed through the SOx trap catalyst 64 and the time integrated value of the H ₂ S concentration of the simulated gas for one hour are measured, From the difference between these time integral values, the ratio of the sulfur content captured by the SOx trap catalyst 64 was calculated.

(3) Example / Example 1
Cordierite base with a diameter of 30 mm, length of 25 mm, cell wall thickness of 4 mil (milli inch length, 1/1000 inch) (approximately 0.1 mm), and 400 cpsi (cells per square inch) (approximately 62 cells per square centimeter) A material coated with γ-Al ₂ O ₃ as a carrier was used. The coating amount is 120 g / L (however, the denominator L (liter) means per 1 L of catalyst). To this, barium acetate was supported by water absorption and calcined at 500 ° C. for 2 hours. The amount of barium acetate supported is 0.2 mol / L. This catalyst was immersed in a solution containing ammonium hydrogen carbonate and dried at 250 ° C. Furthermore, Pt was supported using an aqueous solution containing dinitrodiammine platinum, dried, and then fired at 450 ° C. for 1 hour. The supported amount of Pt is 2 g / L.
Example 2
The difference from Example 1 is that Pt is not supported. The rest is the same as in the first embodiment.
Example 3
Cordierite base with a diameter of 30 mm, length of 25 mm, cell wall thickness of 4 mil (milli inch length, 1/1000 inch) (approximately 0.1 mm), and 400 cpsi (cells per square inch) (approximately 62 cells per square centimeter) A material coated with γ-Al ₂ O ₃ as a carrier was used. The coating amount is 120 g / L. This was supported with Pt using an aqueous solution containing dinitrodiammine platinum, dried, and calcined at 450 ° C. for 1 hour. The supported amount of Pt is 2 g / L. Furthermore, potassium acetate was supported on this by water absorption and calcined at 500 ° C. for 2 hours. The amount of potassium acetate supported is 0.2 mol / L.
Example 4
The difference from Example 3 is that Pt is not supported. Other than that is the same as Example 3.

(4) Experimental Results FIG. 6 shows a comparison of the ratio of sulfur content trapped (trapped) by the SOx trap catalysts of Examples 1 to 4, respectively. As can be seen, when ozone was supplied with the ozone generator 69 turned on, almost 100% of the sulfur content could be captured in any of the examples. Thereby, the effect of this invention provided with a SOx trap catalyst can be confirmed.

On the other hand, when the ozone generator 69 is turned off and ozone is not supplied, the ratio of the trapped sulfur content is much smaller than when ozone is supplied. The reason is that H ₂ S in the simulated gas is not decomposed into SOx by ozone. However, in Examples 1 and 3 having Pt, a larger amount of sulfur can be captured than in Examples 2 and 4 having no Pt. The reason is considered that Pt as a catalyst component can decompose H ₂ S in the simulated gas into SOx, although not as much as ozone. Therefore, from this result, it can be said that the SOx trap catalyst including active points such as Pt and Pd can absorb SOx relatively effectively even when ozone is not supplied, and is advantageous for exhaust gas purification.

  The embodiments of the present invention have been described above. However, the embodiments of the present invention are not limited to the above-described embodiments, and any modification examples included in the idea of the present invention defined by the claims. Application examples and equivalents are included in the present invention. Therefore, the present invention should not be construed as being limited, but can be applied to any other technique within the scope of the idea of the present invention.

1 is a system diagram schematically showing an exhaust gas purification apparatus for an internal combustion engine according to an embodiment of the present invention. It is an enlarged view of the cell of a SOx trap catalyst. It is an enlarged view of a support | carrier. It is a figure which shows the whole experimental apparatus of the experiment conducted in relation to this embodiment. It is the detail of the V section of FIG. It is a graph which shows the comparison of the ratio of the sulfur content trapped by the SOx trap catalyst of each Example.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Engine 13 Combustion chamber 15 Exhaust passage 20 NOx catalyst 30,64 SOx trap catalyst 31 Casing 32 Base material 33 Cell 34 Carrier 35 Particle 36 Hole 37 Active point 38 SOx reaction component 40 Ozone supply member 41 Ozone generator 43 Supply port 100 Electronic control unit (ECU)

Claims (5)

An exhaust purification means provided in an exhaust passage of the internal combustion engine for purifying exhaust gas discharged from the combustion chamber;
Ozone supply means capable of supplying ozone to the exhaust passage at a position downstream of the exhaust purification means;
An SOx trap catalyst provided in the exhaust passage at a position downstream of the ozone supply position and absorbing SOx in the exhaust gas ,
The exhaust gas purification apparatus for an internal combustion engine, wherein the ozone supply position is separated from the exhaust gas purification means by a distance at least so that the supplied ozone is not substantially thermally decomposed by the high temperature of the exhaust gas purification means. .   The exhaust purification apparatus for an internal combustion engine according to claim 1, wherein the SOx trap catalyst contains an alkali metal element, an alkaline earth metal element, or a rare earth element.   The exhaust gas purification apparatus for an internal combustion engine according to claim 2, wherein the alkali metal element is Li, Na, or K.   The exhaust gas purification apparatus for an internal combustion engine according to claim 2, wherein the alkaline earth metal element is Mg, Ba, Ca, or Sr.   3. The exhaust gas purification apparatus for an internal combustion engine according to claim 2, wherein the rare earth element is La.

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