JP2012163059A - Exhaust emission control device for internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an exhaust emission control device for an internal combustion engine that can prevent the NOx purification performance of a NOx purification catalyst containing Fe as a catalyst metal from being deteriorated due to oxygen poisoning.SOLUTION: The NOx purification catalyst disposed in an exhaust passage of the internal combustion engine and carrying Au and Fe in a catalyst carrier, includes: a NOx purification catalyst in which at least a part of Au and Fe exist adjacent to each other; a catalyst temperature detector for detecting the temperature of the NOx purification catalyst; a catalyst heating device for heating the NOx purification catalyst; and an oxygen poisoning detector for detecting whether the NOx purification catalyst has suffered a predetermined oxygen poisoning. When the oxygen poisoning detector detects that the NOx purification catalyst has suffered the predetermined oxygen poisoning, the catalyst heating device heats the NOx purification catalyst up to equal to or more than 600°C.

Description

  The present invention relates to an exhaust emission control device for an internal combustion engine.

  A three-way catalyst that simultaneously oxidizes carbon monoxide (CO) and hydrocarbons (HC) and reduces nitrogen oxides (NOx), and NOx contained in the exhaust gas when the air-fuel ratio of the inflowing exhaust gas is lean There is known an internal combustion engine in which an NOx occlusion reduction type catalyst for reducing and purifying occluded NOx is disposed in an engine exhaust passage when the air-fuel ratio of the exhaust gas that is occluded and flows into becomes the stoichiometric air-fuel ratio or rich.

  In such a three-way catalyst or NOx occlusion reduction type catalyst, when a catalytic metal such as a noble metal contained as a catalyst component is exposed to an atmosphere having a particularly high oxygen concentration, the surface of the catalytic metal is covered with oxygen, that is, There is a problem that the surface of the catalytic metal is subjected to so-called oxygen poisoning, and as a result, the NOx purification performance of these catalysts is lowered.

  In Patent Document 1, fuel injection means for directly injecting fuel into a combustion chamber and an exhaust passage are provided to store NOx in exhaust during lean air-fuel ratio operation and store during stoichiometric air-fuel ratio operation or rich air-fuel ratio operation. A NOx catalyst for releasing and reducing the NOx (ie, NOx occlusion reduction type catalyst), an oxygen poisoning state detecting means for detecting that the noble metal contained in the NOx catalyst is in a predetermined oxygen poisoning state, and the NOx catalyst In addition to the NOx purge for releasing and reducing the NOx occluded by the catalyst, the oxygen poisoning state detecting means detects that the noble metal contained in the NOx catalyst has become a predetermined oxygen poisoning state during the lean air-fuel ratio operation. And by the fuel injection means, fuel injection by main injection performed in the intake stroke or compression stroke, and sub-injection performed in the expansion stroke or exhaust stroke. Exhaust purification apparatus for an internal combustion engine having an oxygen poisoning purge control means for Migihitsuji control are described. Further, in Patent Document 1, according to the exhaust purification apparatus, in addition to the NOx purge described above, an oxygen poisoning purge that desorbs oxygen adsorbed on the NOx catalyst is performed, so that when the NOx purge is performed, the oxygen content is reduced. It is described that the NOx purge can be completed in a short period since the poison purge is hardly performed, and the lean operation time becomes longer as a whole, and the fuel consumption reduction by the lean operation can be increased.

JP 2006-336614 A

  In Patent Document 1, for a NOx purification catalyst used in an exhaust gas purification apparatus for an internal combustion engine that burns an oxygen-rich lean air-fuel mixture, that is, a NOx occlusion reduction type catalyst, oxygen poisoning of a noble metal contained in the NOx occlusion reduction type catalyst Issues related to are being considered. However, even in an internal combustion engine such as a gasoline engine controlled mainly near the stoichiometric air-fuel ratio (stoichiometric), naturally, the exhaust gas contains oxygen. Therefore, also in the NOx purification catalyst generally disposed in the exhaust passage of such an internal combustion engine, for example, a three-way catalyst, the precious metal contained therein is oxygen-poisoned by oxygen in the exhaust gas, and as a result, the NOx The purification performance may be reduced.

  On the other hand, noble metals used as catalyst components in NOx storage reduction catalysts and three-way catalysts, especially platinum group elements such as platinum (Pt), rhodium (Rh), and palladium (Pd), are used together with stricter exhaust gas regulations for automobiles. The amount is increasing, so there is concern about the depletion of resources. For this reason, while reducing the usage-amount of a platinum group element, it is required in the future to substitute the role of a platinum group element with another metal. Thus, much research has been conducted on catalytic metals for reducing the amount of platinum group elements used or in place of them. Some possible alternative metals include base metals such as Fe. However, base metals such as Fe are more likely to be oxidized than platinum group elements and the like, and therefore are more susceptible to oxygen poisoning than platinum group elements and the like. Therefore, when such a NOx purification catalyst containing a base metal such as Fe as a catalyst metal is used in an exhaust gas purification apparatus for an internal combustion engine, a sufficient countermeasure against a decrease in NOx purification performance due to oxygen poisoning of the catalyst metal. It is necessary to apply.

  Accordingly, an object of the present invention is to provide an exhaust purification device for an internal combustion engine that can suppress a decrease in NOx purification performance due to oxygen poisoning of a NOx purification catalyst containing Fe as a catalyst metal with a novel configuration. And

The present invention for solving the above problems is as follows.
(1) A NOx purification catalyst which is disposed in an exhaust passage of an internal combustion engine and has Au and Fe supported on a catalyst carrier, and at least a part of Au and Fe is present in a state of being close to each other. A catalyst,
Catalyst temperature detecting means for detecting the temperature of the NOx purification catalyst;
Catalyst heating means for heating the NOx purification catalyst;
Oxygen poisoning detection means for detecting that the NOx purification catalyst has received a predetermined oxygen poisoning,
An internal combustion engine in which the NOx purification catalyst is heated to 600 ° C. or more by the catalyst heating means when the oxygen poisoning detection means detects that the NOx purification catalyst has received a predetermined oxygen poisoning. Exhaust purification device.
(2) Reducing agent introduction means disposed in the upstream exhaust passage of the NOx purification catalyst is further provided, and heating of the NOx purification catalyst is started by the catalyst heating means, and the NOx purification catalyst is started from the reducing agent introduction means. The exhaust gas purification apparatus for an internal combustion engine according to (1), wherein the reducing agent is introduced into the exhaust gas flowing into the catalyst.

  According to the exhaust gas purification apparatus for an internal combustion engine of the present invention, when it is determined that the NOx purification catalyst has received predetermined oxygen poisoning, the NOx purification catalyst is heated to 600 ° C. or higher, preferably 700 ° C. or higher. The surface of the NOx purification catalyst, particularly the surface of the catalytic metal contained in the NOx purification catalyst, can be recovered from the state that has been subjected to oxygen poisoning to the active state. Therefore, according to the exhaust gas purification apparatus for an internal combustion engine of the present invention, it is possible to remarkably suppress a decrease in NOx purification performance due to oxygen poisoning of the NOx purification catalyst.

It is the figure which showed typically the embodiment of the exhaust gas purification apparatus of this invention. It is a flowchart of the catalyst regeneration operation in the embodiment shown in FIG. It is a graph which shows the surface Fe density | concentration and NO purification rate of a NOx purification catalyst with respect to the fluctuation | variation of an air fuel ratio. It is a schematic diagram of a pulse laser deposition (PLD) apparatus. It is an XPS spectrum of N1s when NO gas is introduced into Au—Fe thin films A and B. It is a graph which shows the surface element density | concentration by XPS of the Au-Fe thin film C with respect to heat processing temperature.

  An exhaust purification device for an internal combustion engine according to the present invention is a NOx purification catalyst that is disposed in an exhaust passage of an internal combustion engine and carries Au and Fe on a catalyst carrier, and at least a part of Au and Fe are close to each other. NOx purification catalyst present in a state, catalyst temperature detection means for detecting the temperature of the NOx purification catalyst, catalyst heating means for heating the NOx purification catalyst, and the NOx purification catalyst is a predetermined oxygen Oxygen poisoning detection means for detecting that the poisoning has been received, and when the oxygen poisoning detection means detects that the NOx purification catalyst has received a predetermined oxygen poisoning, the catalyst heating The NOx purification catalyst is heated to 600 ° C. or higher by means.

In general, in NOx purification using an exhaust purification catalyst, NO adsorbed on the surface of the catalyst metal contained in the exhaust purification catalyst is selectively selected depending on HC in the exhaust gas or HC adsorbed on the catalyst carrier. The NOx in the exhaust gas is dissociated and adsorbed by N and O on the surface of the catalyst metal contained in the exhaust purification catalyst, and the NOx in the exhaust gas is purified by the action of being reduced to NO, that is, the selective reduction action of NOx. It is considered that a part of the NOx is purified by the action of dissociated and adsorbed N becoming N ₂ and desorbing from the surface of the catalytic metal, that is, the direct decomposition action of NOx.

In the specification of Japanese Patent Application No. 2009-244704, the applicant of the present application examined a catalyst in which at least a part of Au and Fe existed in close proximity to each other. NO is dissociated and adsorbed at a temperature much lower than that of conventionally known catalysts containing Rh as a metal, and the dissociated and adsorbed N and O atoms are desorbed as N ₂ and O ₂ , respectively, at a low temperature of about 450 ° C. Showed that you can. In particular, in a conventionally known catalyst containing Rh as a catalyst metal, NO dissociative adsorption occurs at a temperature of about 100 ° C. under the same conditions, and O ₂ desorption occurs at a temperature of about 800 ° C. This fact is extremely surprising. In addition, since a catalyst in which at least a part of Au and Fe are close to each other can dissociate and adsorb NOx even at a low temperature, such a catalyst can be used in an exhaust purification device of an internal combustion engine. It is considered that high NOx purification performance can be obtained by the direct decomposition action. On the other hand, since the catalyst can desorb oxygen at an extremely low temperature as compared with a conventionally known catalyst containing Rh as a catalyst metal, the catalyst metal can be obtained by using such a catalyst in an exhaust purification device of an internal combustion engine. It is considered that the reduction in NOx purification performance due to oxygen poisoning can be remarkably suppressed.

  The present invention provides an exhaust emission control device for an internal combustion engine provided with a NOx purification catalyst containing Au and Fe as catalyst components based on the above-mentioned knowledge shown in Japanese Patent Application No. 2009-244704.

  According to the present invention, as the NOx purification catalyst, a material is used in which Au and Fe are supported on a catalyst carrier and at least a part of the Au and Fe are present in close proximity to each other.

  Here, in the present invention, “the state in which at least part of Au and Fe are close to each other” means, for example, a state in which Au particles and Fe particles are in contact with each other in a region, A state in which a core part mainly composed of one element and a shell part mainly composed of the other element are formed, and the core part and the shell part are in contact with each other to form a core-shell structure; Alternatively, each element of Au and Fe is mutually melted to form a uniform solid solution as a whole, or each element of Au and Fe is not completely solid solution, but is at least partially solid solution The state (incomplete solid solution state) is included.

  The NOx purification catalyst used in the exhaust purification apparatus of the present invention can be prepared, for example, as follows. First, one or more reducing agents are added to a solution containing a predetermined amount of appropriate Au salt and Fe salt, and Au ions and Fe ions contained in the solution are reduced by the one or more reducing agents. A colloidal solution containing metal particles composed of Au and Fe is formed. In addition, it is preferable to perform such reduction | restoration operation, implementing heating, cooling, etc. as needed. The reducing agent is not particularly limited as long as it can reduce Au ions and Fe ions in the solution. For example, alcohols such as methanol, ethanol, propanol, and ethylene glycol, hydrazine, hydrogenation, etc. Sodium boron, citric acid, ascorbic acid, formaldehyde and the like can be used. In addition, the mixing order of Au salt, Fe salt, and one or several reducing agent is not specifically limited, These can be mixed in arbitrary orders. For example, after adding a solution containing Fe salt to a solution containing Au salt, one or more reducing agents may be added, or alternatively, after adding a first reducing agent to a solution containing Fe salt A solution containing an Au salt may be added thereto, and a second reducing agent different from the first reducing agent may be added to the mixed solution.

  When preparing a colloidal solution containing metal particles composed of Au and Fe as described above, for example, it is coordinated or adsorbed on the surface of the generated metal particles to suppress and stabilize the aggregation and particle growth of the metal particles. For the purpose, a protective agent may optionally be added to the solution containing Au salt, Fe salt and / or reducing agent. As such a protective agent, any known metal colloid protective agent can be used. For example, an organic polymer or a low molecular weight organic compound containing a heteroatom such as nitrogen, phosphorus, oxygen, sulfur or the like and having a high coordination power can be used as the protective agent. High molecular compounds such as polyamide, polypeptide, polyimide, polyether, polycarbonate, polyacrylonitrile, polyacrylic acid, polyacrylate, polyacrylamide, polyvinyl alcohol, heterocyclic polymer, and polyester are used as organic polymer protective agents. can do. Particularly preferably, polyvinylpyrrolidone, polyvinylpyridine, polyacrylamide and the like can be used. By adding such a protective agent, it is possible to control the size of the obtained metal particles to a nanometer size, and therefore, the activity of the NOx purification catalyst finally obtained can be significantly improved. it can.

  Next, after the above colloidal solution containing metal particles made of Au and Fe is purified by an operation such as centrifugation as necessary, it is further dispersed in an appropriate solvent such as ethanol. Next, the dispersion is used as an arbitrary metal oxide powder generally used as a catalyst carrier for a NOx purification catalyst so that the total amount of Au and Fe is generally in the range of 0.01 to 10 wt% with respect to the powder. In small amounts. This is then dried and calcined at a predetermined temperature and time, in particular at a temperature and time sufficient to decompose and remove the salt portion of the metal salt, an optional protective agent, etc., and support the metal particles on the catalyst support. By doing so, it is possible to obtain a NOx purification catalyst in which Au and Fe are supported on a catalyst carrier, and at least a part of the Au and Fe are present in proximity to each other.

  Hereinafter, a preferred embodiment of an exhaust gas purification apparatus for an internal combustion engine according to the present invention will be described in more detail with reference to the drawings. However, the following description is intended only to illustrate a preferred embodiment of the present invention. It is not intended that the invention be limited to such specific embodiments.

  FIG. 1 is a diagram schematically showing an embodiment of the exhaust emission control device of the present invention.

  Referring to FIG. 1, the exhaust side of the internal combustion engine 10 is connected to a casing 13 containing a NOx purification catalyst 12 via an exhaust pipe 11, and the outlet portion of the casing 13 is connected to an exhaust pipe 14. The casing 13 is provided with a temperature sensor 15 (catalyst temperature detection means) for detecting the temperature of the NOx purification catalyst 12. The temperature sensor 15 only needs to be able to detect the temperature of the NOx purification catalyst 12, and may be attached to the exhaust pipe 14 connected to the outlet of the casing 13, for example. Furthermore, in the embodiment of the present invention, the NOx sensor 17 (oxygen poisoning detection means) for detecting NOx in the exhaust gas flowing out from the casing 13 is attached to the exhaust pipe 14. An electric heater 16 is arranged on the upstream side of the NOx purification catalyst 12 in the casing 13 as catalyst heating means for heating the NOx purification catalyst 12. The electric heater 16 is, for example, the NOx sensor 17 described above. Can be controlled by an electronic control unit (ECU) 20 based on the amount of NOx in the exhaust gas detected by. Further, in the exhaust pipe 11 upstream of the NOx purification catalyst 12, a reducing agent supply valve 18 (reducing agent introduction means) for supplying a reducing agent made of, for example, hydrocarbons into the exhaust gas is optionally attached. ing. Although not shown in FIG. 1, in order to reliably purify other components other than NOx in the exhaust gas, that is, unburned HC (hydrocarbon) and CO, the NOx purification catalyst 12 is, for example, You may use together with the three way catalyst arrange | positioned in the exhaust pipe of the upstream of the NOx purification catalyst 12, or the downstream.

  As the catalyst heating means for heating the NOx purification catalyst 12, various means or methods other than the electric heater 16 can be used. For example, as the catalyst heating means in the present invention, a ribbon heater or the like is wound around the casing 13 containing the NOx purification catalyst 12, and the NOx purification catalyst 12 is heated by this, or the fuel injected into the combustion chamber of the internal combustion engine 10 A method may be used in which combustion is promoted by increasing the amount of NOx, thereby raising the temperature of the exhaust gas flowing into the NOx purification catalyst 12 and heating the NOx purification catalyst 12. Alternatively, as the catalyst heating means in the present invention, for example, a method of directly heating the NOx purification catalyst 12 by applying electricity to the honeycomb substrate or the like coated with the NOx purification catalyst 12 itself may be employed. Good.

  According to this embodiment, when it is detected by the NOx sensor 17 disposed downstream of the NOx purification catalyst 12 that the NOx purification catalyst 12 has been subjected to predetermined oxygen poisoning, the electric heater 16 causes the NOx purification catalyst to be used. By heating 12 to 600 ° C. or higher, it is possible to reliably eliminate oxygen poisoning of the NOx purification catalyst 12.

  FIG. 2 is a flowchart of the catalyst regeneration operation in the embodiment shown in FIG. The catalyst regeneration operation is performed as a routine that is executed by interruption every set time predetermined by the electronic control unit (ECU) 20.

Referring to FIG. 2, first, at step 100, it is determined whether or not the bed temperature T of the NOx purification catalyst 12 detected by the temperature sensor 15 has reached its activation temperature T ₀ , and T ≧ T ₀ . If so, go to Step 101. On the other hand, if the bed temperature T of the NOx purification catalyst 12 has not reached its activation temperature T ₀ , that is, if T <T ₀ , the routine ends without performing the heating process (that is, the catalyst regeneration process). The NOx purification catalyst 12 having Au and Fe supported on the catalyst carrier used in the present invention can exhibit its catalytic activity even under relatively low temperature conditions, but reliably exhibits its catalytic activity. Therefore, it is preferable to use at a temperature of about 300 ° C. or higher. Therefore, in this embodiment, the activation temperature T ₀ is preferably set to 300 ° C.

  In step 101, it is determined whether or not the NOx purification catalyst 12 is subjected to predetermined oxygen poisoning. Here, in this embodiment, when the NOx purification performance of the NOx purification catalyst 12 is reduced, that is, when the amount of NOx in the exhaust gas that has passed through the NOx purification catalyst 12 is increased, the NOx purification catalyst 12 has a predetermined oxygen content. The catalyst regeneration process is carried out based on the judgment that the poison has been received. Specifically, in step 101, it is determined whether or not the amount of NOx in the exhaust gas detected by the NOx sensor 17 has increased. When the amount of NOx in the exhaust gas has increased, that is, when ΔNOx> 0. Goes to step 102. Then, in step 102, power supply to the electric heater 16 is started, the NOx purification catalyst 12 is heated, and the process proceeds to step 103. On the other hand, if the amount of NOx detected by the NOx sensor 17 has not increased, that is, if ΔNOx ≦ 0, it is determined that the NOx purification catalyst 12 has not been subjected to predetermined oxygen poisoning, and no heat treatment is performed. The routine ends.

  Next, in step 103, it is determined whether or not the bed temperature T of the NOx purification catalyst 12 detected by the temperature sensor 15 has become 600 ° C. or higher. If T ≧ 600 ° C., the routine proceeds to step 104. In step 104, the power supply to the electric heater 16 (that is, the heating process) is stopped, and the catalyst regeneration process routine is ended. In order to perform the regeneration process of the catalyst more reliably, for example, after detecting that the bed temperature T of the NOx purification catalyst 12 is 700 ° C. or higher in Step 103, the heating process is stopped in Step 104. It may be.

  As described above, according to this embodiment, when it is determined that the NOx purification catalyst 12 has received the predetermined oxygen poisoning, the NOx purification catalyst 12 is heat-treated at 600 ° C. or higher, preferably 700 ° C. or higher. Thus, it is possible to eliminate the oxygen poisoning of the NOx purification catalyst 12 and regenerate it into an active state. In this way, the entire surface of the NOx purification catalyst 12 is subjected to oxygen poisoning, and the oxygen poisoning can be surely eliminated before the NOx purification performance is greatly reduced. Therefore, NOx in the exhaust gas can be eliminated. Can be stably reduced and purified. In addition, since such a catalyst regeneration process can be performed under an atmosphere richer than the stoichiometric air-fuel ratio (stoichiometric), the process temperature can be lowered. For example, the NOx purification catalyst 12 is heated by the electric heater 16. And a reducing agent made of hydrocarbon or the like may be introduced into the exhaust gas flowing into the NOx purification catalyst 12 from the reducing agent supply valve 18 shown in FIG.

  In this embodiment, a NOx sensor is used as the oxygen poisoning detection means for detecting that the NOx purification catalyst 12 has received a predetermined oxygen poisoning, and the exhaust gas detected by the NOx sensor 17 contains the NOx sensor. The heat treatment of the NOx purification catalyst 12 is performed based on the NOx amount. However, various means other than the above-mentioned NOx sensor 17 can be used as means for detecting oxygen poisoning of the NOx purification catalyst 12. For example, an oxygen sensor or an air-fuel ratio sensor may be used instead of using the NOx sensor. In this case, the oxygen amount W contained in the exhaust gas is detected by these sensors, and the integrated value ΣW of the oxygen amount W passing through the NOx purification catalyst 12 is calculated based on the detected data. Then, by comparing this integrated value ΣW with an allowable value WX that has been obtained in advance by experiments or the like regarding the NOx purification catalyst 12, it is determined whether or not the NOx purification catalyst 12 has been subjected to predetermined oxygen poisoning. That is, based on the flowchart of FIG. 2, when the integrated value ΣW> WX is satisfied in step 101, it is determined that the NOx purification catalyst 12 has received a predetermined oxygen poisoning, and the process proceeds to step 102. On the other hand, when the integrated value ΣW ≦ WX, it is determined that the NOx purification catalyst 12 has not received the predetermined oxygen poisoning, and the routine is terminated without performing the heating process. The operations after step 102 proceed in the same manner as in FIG.

  Alternatively, as means for detecting oxygen poisoning of the NOx purification catalyst 12, the electronic control unit (ECU) 20 in advance in the exhaust gas discharged per unit time from the internal combustion engine 10 according to the operating state of the internal combustion engine 10. A method of storing the oxygen concentration in the form of a map and calculating the oxygen amount W using this map may be employed. In this case, an integrated value ΣW of the oxygen amount W passing through the NOx purification catalyst 12 is calculated based on the oxygen amount W obtained using the map. Then, similarly to the case of using an oxygen sensor or an air-fuel ratio sensor, the integrated value ΣW is compared with an allowable value WX that has been previously obtained by experiments or the like regarding the NOx purification catalyst 12, whereby the NOx purification catalyst 12 is predetermined. It can be determined whether or not the patient has received oxygen poisoning.

  Hereinafter, regeneration of the NOx purification catalyst 12 used in the exhaust purification apparatus of the present invention will be described in more detail based on experimental results.

[Catalyst preparation]
[Synthesis of Au-Fe nanoparticles]
First, 1.1 g of poly-n-vinylpyrrolidone (PVP) as a protective agent was added to 120 ml of anhydrous ethylene glycol as a reducing agent in a two-necked round bottom flask, and iron (II) acetate as an Fe salt was added to this mixed solution. 0574g was added and it stirred at 80 degreeC for 3 hours (solution 1). Next, 0.1809 g of sodium chloroaurate (NaAuCl ₄ ) as Au salt was added to 50 ml of distilled water in another two-necked round bottom flask and dissolved with vigorous stirring for 2 hours. In addition, the color of the obtained solution was bright orange (solution 2). Next, the solution 1 was cooled to 0 ° C. using a cooling bath, and the solution 2 was poured into the solution with stirring to make it uniform. Next, 1M NaOH solution (about 7 ml) was added to the resulting mixture to adjust the pH of the mixture to 9-10. The resulting mixture was heated to 100 ° C. using an oil bath, and kept at this temperature for about 2 hours with stirring. Thereafter, the obtained colloidal suspension was pulled up from the oil bath and cooled to room temperature. In order to completely reduce all the ionic species in the flask, 0.038 g of sodium borohydride was added as a reducing agent. The suspension was then left for a while. The produced nanoparticles were purified by treating an aliquot containing a predetermined amount of nanoparticles with a large amount of acetone. As a result, the PVP polymer used as the protective agent was extracted into the acetone phase, and the metal nanoparticles were aggregated. The supernatant was then transferred or the nanoparticles were purified by centrifugation. After removing the acetone phase, the purified colloid was redispersed in absolute ethanol with gentle stirring.

[Supporting Au—Fe nanoparticles on catalyst support]
A 100 ml Schlenk tube was charged with 1 g of alumina (γ-Al ₂ O ₃ ) as a catalyst carrier. Next, the inside of the Schlenk tube was evacuated, and N ₂ was allowed to flow therethrough to clean the Schlenk tube, and the air in the Schlenk tube was completely replaced with N ₂ . Next, the colloidal suspension prepared above was added to the Schlenk tube through a rubber septum with a syringe so that the total amount of Au and Fe was 1 wt% with respect to alumina. The resulting mixture was stirred at room temperature for about 3 hours and then evacuated to remove the solvent. Subsequently, the obtained colloidal precipitate was heat-dried at about 200 ° C. under vacuum to decompose and remove the PVP polymer that was not completely extracted in the previous operation. Next, the obtained powder was compression-molded into pellets of about 2 mm to obtain a NOx purification catalyst of Au-Fe / Al ₂ O ₃ .

[Evaluation of catalyst]
The NOx purification catalyst prepared above was examined for its NOx purification performance and surface Fe concentration when the air-fuel ratio was varied. Specifically, first, the above NOx purification catalyst is set in a quartz glass tube, and then a model gas containing a predetermined concentration of NO is caused to flow through the catalyst bed, and the CO concentration in the model gas is set at a constant temperature of 400 ° C. The A / F value was varied from about 4 to about 18 by changing, and the NO purification rate was measured at each A / F value shown in FIG. Moreover, about each sample measured in each A / F value, those surfaces were measured by X-ray photoelectron spectroscopy (XPS) apparatus (ESCA1600 made from ULVAC-PHI) and Auger electron spectroscopy (AES) apparatus (KCMA2002 made from KITANO SEIKI). The Fe concentration was examined. The value of the surface Fe concentration indicates the surface concentration of Fe when the sum of the surface concentrations of Au and Fe is 100%. The obtained results are shown in FIG. FIG. 3 is a graph showing the surface Fe concentration and the NO purification rate of the NOx purification catalyst with respect to fluctuations in the air-fuel ratio. FIG. 3 shows the air-fuel ratio (A / F) on the horizontal axis, the surface Fe concentration (atomic%) on the left vertical axis, and the NO purification rate (%) on the right vertical axis.

  As is clear from the results of FIG. 3, the NO purification rate improves as the A / F value increases, that is, as the oxygen concentration in the model gas increases, and when A / F = about 11, the value is the maximum value. Reached. However, the NO purification rate decreased as the oxygen concentration further increased. On the other hand, the value of the surface Fe concentration increased monotonously with increasing oxygen concentration, and reached about 100 atomic% at A / F = about 18. From this experimental result, each surface concentration of Au and Fe in the NOx purification catalyst varies greatly depending on the atmosphere, and only one element of Au and Fe is the surface in an excessive reduction (rich) and oxidation (lean) atmosphere. It was found that the NOx purification performance was greatly reduced. Although it is not intended to be bound by any particular theory, the surface Fe concentration increased and the NO purification rate greatly decreased especially as the oxygen concentration increased. The state, preferably from the active state forming a uniform solid solution as a whole, Fe is attracted to the catalyst surface by oxygen present in the atmosphere and is oxidized on the catalyst surface, that is, has been subjected to oxygen poisoning. It is thought to be caused.

  Next, similarly to the Au—Fe thin film corresponding to the NOx purification catalyst having the surface Fe concentration of about 40 atomic% (that is, the surface Au concentration = about 60 atomic%) having the highest NO purification activity in the experiment of FIG. The Au—Fe thin films corresponding to the NOx purification catalyst having the surface Fe concentration of about 0 atomic% (ie, the surface Au concentration = about 100 atomic%) having the lowest NO purification activity in the experiment 3 were prepared. The NO adsorption characteristics of the -Fe thin film were examined.

Specifically, first, using an ion sputtering apparatus (HITACHI E101, energy 100 eV, ion current 15 mA), Au was sputtered on an Al ₂ O ₃ (sapphire) substrate in five portions for 2 minutes, and then the substrate. A uniform Au sputtered film having a thickness of about 30 nm was deposited thereon. Next, the obtained Au / Al ₂ O ₃ was set on a sample stage placed in a chamber of a pulse laser deposition (PLD) apparatus shown in FIG. Note that the pulse laser deposition (PLD) apparatus shown in FIG. 4 includes an Auger electron spectroscopy (AES) apparatus (KITANO SEIKI KCMA2002) and an X-ray photoelectron spectroscopy (XPS) apparatus (ULVAC-PHI ESCA1600) as analysis means. ing. Next, as a surface pretreatment of Au / Al ₂ O ₃ , Ar sputtering (0.5 eV) was performed for 30 minutes under a chamber pressure of 1.8 × 10 ⁻⁴ Torr, and then annealed at 450 ° C. for 25 minutes. Processed and repeated these steps a total of two times. Next, an excimer laser (manufactured by LAMBDA PHYSIK, 25-29 kV, 1-10 Hz, KrF 3000 mbar) is irradiated to the Fe target in the chamber until Fe has a surface composition of 100% on Au / Al ₂ O ₃ Deposited. The analysis of the surface composition was confirmed by AES. Finally, the obtained sample was heated to 350 ° C. with an infrared laser to obtain an Au—Fe thin film A having a surface Fe concentration of about 40 atomic%.

Note that Fe was deposited on Au / Al ₂ O ₃ until the surface composition reached 100% by the same operation as above, and then the obtained sample was subjected to an H ₂ pressure of 3 × 10 ⁻⁸ Torr. By heating to 500 ° C. with an infrared laser, an Au—Fe thin film B having a surface Fe concentration of about 0 atomic% (ie, surface Au concentration = about 100 atomic%) was obtained.

[NO adsorption characteristics]
Next, the NO adsorption characteristics of the Au—Fe thin films A and B prepared above were examined. Specifically, an NO gas amount of about 1 L (Langmuir) was introduced into the chamber at room temperature to adsorb the NO gas to each Au—Fe thin film. The result is shown in FIG. Here, when each Au—Fe thin film is exposed to 5.0 × 10 ⁻⁶ Pa NO gas for 44 seconds, that is, each Au—Fe thin film has a NO gas amount of 2.2 × 10 ⁻⁴ Pa · s. When exposed to 1L (Langmuir).

  FIG. 5 is an XPS spectrum of N1s when NO gas is introduced into the Au—Fe thin films A and B. From the result of FIG. 5 and the shift of the peak position of Fe2p similarly measured by XPS, the peak near 397 eV in the Au—Fe thin film A (with a surface Fe concentration of 40 atomic%) in FIG. It turned out to be the cause. That is, this result indicates that NO is dissociated and adsorbed by N and O on Fe. On the other hand, no dissociative adsorption of NO was detected for the Au—Fe thin film B having a surface Au concentration of 100 atomic%.

  In FIG. 3, the NOx purification catalyst having a surface Fe concentration of about 40 atomic% showed the highest NOx purification activity, and the Au—Fe thin film having the same composition adsorbs NO on Fe even at room temperature. In consideration of the above, in the NOx purification catalyst of the present invention, Au and Fe are uniformly present on the surface in a state of being close to each other, thereby promoting the direct decomposition action of NO, thereby improving the NOx purification performance. It is thought that there is.

[Catalyst regeneration]
In this experiment, the change in the surface Fe concentration of the NOx purification catalyst that had been subjected to oxygen poisoning when the heat treatment was performed was examined. Specifically, after depositing Au and then Fe on an Al ₂ O ₃ substrate according to the method for preparing an Au—Fe thin film described above, the obtained sample was placed at 350 ° C. in a vacuum atmosphere containing a small amount of oxygen. By performing the oxidation treatment, an Au—Fe thin film C subjected to a poisoning treatment with oxygen in a simulated manner was obtained. Next, the obtained Au—Fe thin film C was heat-treated in a vacuum atmosphere with an H ₂ pressure of 3 × 10 ⁻⁸ Torr. The result is shown in FIG.

FIG. 6 is a graph showing the surface element concentration by XPS of the Au—Fe thin film C with respect to the heat treatment temperature. In FIG. 6, the horizontal axis indicates the heating temperature (° C.), and the vertical axis indicates the surface concentration (atomic%) of each element. In this experiment, each peak of Fe2p3, Au4f and O1s related to the Au—Fe thin film C was measured by XPS, and thereby the surface concentration of each element of Fe, Au and O was calculated. From the surface concentration of Fe and O, the peak positions of Fe2p3 and O1s, etc., it is recognized that Fe in the Au—Fe thin film is exposed to the thin film surface as an oxide of Fe ₂ O ₃ by the above oxidation treatment. . In addition, referring to FIG. 6, when the heating temperature was 200 ° C. to 550 ° C., no significant change was observed with respect to the surface concentration of each element of Fe, Au, and O, but by heating at a temperature of 600 ° C. or more, Fe and The surface concentration of O greatly decreased, and the surface concentration of Au increased accordingly. Furthermore, by heat-treating the Au—Fe thin film C at 700 ° C., most of the oxygen present on the thin film surface was removed, and the surface composition with the highest NO purification activity in the experiment of FIG. It was possible to recover the surface composition of the Au—Fe thin film to the atomic composition of atomic%. This experimental result shows that when the NOx purification catalyst used in the exhaust purification apparatus of the present invention is subjected to oxygen poisoning, it is heated at a temperature of 600 ° C. or higher, preferably 700 ° C. or higher. It shows that oxygen poisoning can be reliably eliminated and the surface composition of the NOx purification catalyst can be reformed to a highly active state.

DESCRIPTION OF SYMBOLS 10 Internal combustion engine 11, 14 Exhaust pipe 12 NOx purification catalyst 13 Casing 15 Temperature sensor 16 Electric heater 17 NOx sensor 18 Reducing agent supply valve 20 Electronic control unit

Claims (2)

A NOx purification catalyst disposed in an exhaust passage of an internal combustion engine and carrying Au and Fe on a catalyst carrier, wherein at least a part of Au and Fe are present in proximity to each other;
Catalyst temperature detecting means for detecting the temperature of the NOx purification catalyst;
Catalyst heating means for heating the NOx purification catalyst;
Oxygen poisoning detection means for detecting that the NOx purification catalyst has received a predetermined oxygen poisoning,
An internal combustion engine in which the NOx purification catalyst is heated to 600 ° C. or more by the catalyst heating means when the oxygen poisoning detection means detects that the NOx purification catalyst has received a predetermined oxygen poisoning. Exhaust purification device.   The apparatus further comprises a reducing agent introduction means disposed in an exhaust passage upstream of the NOx purification catalyst, and starts heating the NOx purification catalyst by the catalyst heating means and flows into the NOx purification catalyst from the reducing agent introduction means. The exhaust gas purification apparatus for an internal combustion engine according to claim 1, wherein a reducing agent is introduced into the exhaust gas.

Images