JP4876644B2 - Exhaust gas purification method - Google Patents

Description

  The present invention relates to a method for efficiently purifying exhaust gas discharged from an internal combustion engine such as an automobile, particularly NOx, with a catalyst.

  NOx is one of environmentally harmful substances that cause acid rain. NOx is contained in exhaust gas from automobiles and the like, and various treatment technologies are being studied. In the automobile industry, in accordance with exhaust gas regulations that are becoming stricter year by year, development of a catalyst that exhibits a high removal rate and a control technology that suppresses the generation of NOx are being promoted. In the case of a conventional gasoline catalyst, a three-way catalyst performs NOx reduction, HC combustion, and CO oxidation reaction. However, the number of vehicles driven by lean combustion with less fuel for the purpose of improving fuel consumption has been increasing year by year.

Since lean-burn gasoline engines, diesel engines, DI-direct-injection (DI) engines operated with lean combustion, etc. are operated above the stoichiometric air-fuel ratio, the exhaust gas from the engine will contain a lot of O ₂ , In terms of the original catalyst performance, NOx reduction performance was particularly insufficient. That is, the conventional three-way catalyst for a gasoline engine can remove HC and CO in the exhaust gas described above, but it has become difficult to purify NOx in a lean combustion atmosphere.

Since a catalyst capable of reducing NOx in a high oxygen concentration atmosphere has not yet been put into practical use, NO release is suppressed by devising engine operation control. That is, the air-fuel ratio is controlled in a pulse shape so as to change from the lean combustion side to the normal combustion side, and NOx trapped in the sorbent during lean combustion is purified by a reducing material such as HC and CO at the normal combustion side. This complicates engine control. The sorbent material here is one of an absorbent material, an adsorbent material, and a material that simultaneously absorbs and adsorbs.

Further, NOx removal with a relatively large amount of O ₂ is desired in exhaust gas from internal combustion engines such as power plants other than automobiles and boilers. As an example so far, there is a method of reducing and purifying NOx using a hydrocarbon gas generated by incomplete combustion of hydrocarbons as a reducing material (Japanese Patent Laid-Open No. 6-319953 (Patent Document 1) [0021]). In addition, it is described that the incomplete combustion state can be monitored from the CO generation amount. If CO is used as a reducing material , it becomes NO + CO → 1 / 2N ₂ + CO ₂ , so CO needs to be contained in an amount equal to or more than NO, but in Example 2, when CO is present on 450 ppm, An NO purification rate of 85% or more is obtained, suggesting a reduction reaction by hydrocarbons generated by incomplete combustion.

JP-A-6-319953

Challenge of the present application, in an atmosphere of high concentrations of O ₂ is present is to provide a method and cleaning apparatus for reducing the N ₂ at a higher efficiency of NOx in the exhaust gas. Moreover, it exists in internal combustion engines, such as a motor vehicle, a power plant, and a boiler provided with such a purification apparatus.

A feature of the present invention resides in a method of using a catalyst that removes NOx using CO contained in exhaust gas from an internal combustion engine as a reducing material , and removing hydrocarbons (HC) before the NOx removing catalyst. Further, the hydrocarbon removing means is in an apparatus arranged upstream of the NOx removing means.

In order to solve the above problems, we have developed a catalyst for purifying NOx discharged from an internal combustion engine using CO as a reducing material. As a result of detailed investigations, it has been found that a catalyst that reduces NOx with high efficiency using CO as a reducing material even in the presence of high concentration of O ₂ has reduced catalytic performance when HC coexists in exhaust gas. This is because HC combustion reaction occurs at the same time, and the active point of the catalyst is used competitively with the NOx removal reaction, and H ₂ O generated by HC combustion deteriorates the catalyst.

According to the present invention, it has been found that high NOx reduction performance is exhibited, and when using a catalyst that improves selectivity to N ₂ , it is found that the catalyst performance is prevented from being lowered. In the presence of high concentration of O _2, it is possible to maintain the performance of the catalyst for obtaining a high NOx removal rate and N ₂ selectivity for a long period of time.

Further, reaction condition control for NOx reduction becomes unnecessary. In addition, a decrease in activity can be suppressed even in a catalyst process used in the presence of a low concentration of O ₂ .

As described above, a feature of the present invention is that it has a hydrocarbon removal step for removing hydrocarbons in exhaust gas from an internal combustion engine between the internal combustion engine and a catalyst that purifies NOx using CO as a reducing material . Examples of the method for removing hydrocarbons include a method using a hydrocarbon combustion catalyst, a hydrocarbon sorbent, and the like.

  The inventors of the present invention have attempted to improve the performance of the NOx removal catalyst, and have studied the novel method, catalyst and apparatus in detail, and have reached the present invention.

When the performance of the active ingredient itself is low, NOx removal and selective reduction reaction to N ₂ are not likely to occur in the presence of high concentration O ₂ . On the other hand, a catalyst having high NOx removal performance even in the presence of high concentration O ₂ has very high performance of the active component. This also shows the same effect for gas types other than NOx. For this reason, when using a highly active catalyst, it is desirable to remove as much as possible other than the target substance of the reaction which should be performed.

In addition, when using a catalyst that purifies NOx that exhibits activity in the presence of a low concentration of O _2, it is possible to avoid performance degradation due to hydrocarbons or water vapor.

When the hydrocarbon combustion catalyst is used, since it can be burned at a low temperature, there is an effect that the influence on the apparatus equipment, particularly the catalyst on the downstream side is small. Normally, when hydrocarbons are burned, CO ₂ and moisture are generated, so that CO, which is a reducing material , can be supplied to the catalytic reaction on the downstream side of the gas flow by incomplete combustion.

<CH combustion catalyst>
A general combustion catalyst can be used as the hydrocarbon combustion catalyst. For example, a support such as CeO ₂ , ZrO ₂ , CeO ₂ —ZrO ₂ composite oxide is used as a transition metal (Mg, La, Cs,
Al, Ni, etc.) or noble metal (Pt, Pd, Rh) supported catalyst. In particular, those having a metal oxide formation enthalpy that is negatively smaller than Fe, such as Co, Ni, and noble metals, are preferred. This is because they easily hold O and have high combustibility. Metal oxide formation enthalpy is a value converted per MO bond with MO _x-1 + 1 / 2O ₂ = MO _x , and M + xO ₂ =
A value converted per MO bond by MOy can be used.

As a method for controlling combustion, a method for controlling the amount of O ₂ and a method for controlling the amount of fuel are preferable. For example, the amount of O ₂ may be controlled assuming an O ₂ utilization rate (ex: 70 to 80%), or the fuel may be controlled so that the amount of hydrocarbons is 20% or less, and if possible, approximately 0%. As a result, the NOx purification catalyst does not deteriorate in performance due to carbon deposition and the NOx purification rate is improved.

  In addition, when moisture is generated due to combustion of hydrocarbons and the catalytic activity is reduced, a means for removing water may be disposed at the subsequent stage of the combustion catalyst.

<Use of CH sorbent>
As the hydrocarbon sorbent , a porous carrier carrying Ag, Cu, Zn, Ba, Mg or the like is preferable. As the carrier, Si / Al compounds such as zeolite, mesoporous bodies and the like are desirable. In particular, a mesoporous body is preferred because it has a high specific surface area and the added metal component is less likely to aggregate.

Also, when using hydrocarbon sorbents, if NOx exclusion is required, the hydrocarbons in the exhaust gas are sorbed, and when NOx exclusion is no longer necessary, the hydrocarbons are released to another line. Those are preferred. Further, it is preferable to prepare a plurality of sorbent materials and use them by switching the flow path.

  It is also necessary to monitor the amount of hydrocarbons in the sorbent. This is because flammability increases as the amount of hydrocarbon increases, so it is desirable to measure the amount of inflowing hydrocarbons and desorb and release hydrocarbons in a specific period.

The position and components of the HC sorbent can be appropriately adjusted according to the NOx purification reaction temperature. The hydrocarbon sorbent is upstream from the NOx purification catalyst, and the exhaust gas flowing through the sorbent is higher than the NOx purification reaction temperature. Since the NOx purification reaction easily proceeds at 150 to 350 ° C., the NOx purification catalyst is installed at a position where the exhaust gas temperature becomes such a temperature. This is because it is not necessary to input extra energy for heating. Accordingly, it is preferable that the hydrocarbon sorbent sorbs a large amount of HC at 150 ° C. or higher. In particular, it is desirable that the enthalpy of formation of the metal oxide is larger than that of Fe and smaller than that of a noble metal.

<Reducing material>
In the present invention, NO is reacted with a reducing material on the surface of the NOx purification catalyst to reduce NO. As the reducing material for the reduction, a general reducing material can be used, but it is preferably a gas during the catalytic reaction, and CO is particularly desirable. When CO is contained in a large amount in the exhaust gas to be treated, it is not necessary to provide means for adding a reducing material . When the CO amount is insufficient with respect to the NO amount (the NO number is larger than the CO number), it is necessary to add a predetermined amount of CO before the exhaust gas contacts the NOx reduction catalyst. For example, after the hydrocarbon removal step, CO is supplied into a pipe through which the exhaust gas in front of the reactor in contact with the NOx reduction catalyst and the exhaust gas flows, or directly to the catalyst layer and the reactor separately from the exhaust gas supply port. A CO supply port can be provided.

  Of course, the same addition method can be used when the hydrocarbon removal step is unnecessary.

<NOx purification catalyst>
As an example of the catalyst for reducing NOx, a lean NOx catalyst, a three-way catalyst, or the like can be used, and the exhaust gas composition may be controlled in order to use them. The catalyst in the present invention can be used after being formed into a granular shape, a columnar shape, a pellet shape or the like. As the molding method, any method such as an extrusion molding method, a tableting molding method, and a rolling granulation method can be adopted depending on the purpose.

A transition metal or a noble metal can be used as the catalytically active component. As the carrier for dispersing the catalytically active component, SiO ₂ , TiO ₂ , ZrO ₂ , Al ₂ O _{3 and the} like are preferable.

〔Example〕
EXAMPLES Hereinafter, although an Example demonstrates this invention, it is not limited to these.

(Catalyst adjustment)
In this example, a NOx purification catalyst ( reference catalyst 1, catalyst 2) in which Co ₃ O ₄ and a Co cluster having 4 Co atoms were supported on mesoporous silica was prepared.
< Reference catalyst 1> Co ₃ O ₄ / MCM-41
MCM-41 using MCM-41 (commercial product, manufactured by Zude Chemie), which is mesoporous silica, was dried at 120 ° C. for 1 hour before use. 0.260 g of cobalt nitrate hexahydrate (manufactured by Wako Pure Chemical Industries, Ltd.) was dissolved in 3.236 g of pure water, and impregnated in 0.5 g of MCM-41 after drying. This powder was press-molded at 200 kgf / cm ² , crushed in a mortar, and sized to 0.5 to 1 mm.

  In addition, since mesoporous silica has a high specific surface area, the catalyst can be highly dispersed. The above MCM-41 (Mobil Catalytic Material) has a uniform hexagonal mesoporous laminated structure. In addition to the hexagonal type, there are mesoporous silicas having skeletons such as cubic type and planar type, and those having a high specific surface area can be used.

<Catalyst 2> Co4 cluster / MCM-41
MCM-41 was based on the method of D. Das et al. (Chem. Commun., (1999) 473-474). Cetyltrimethylammonium bromide (CTMABr) and tetrapropylammonium bromide (TPABr) were added to H ₂ O and stirred while heating to about 40 ° C. To this aqueous solution was added a sodium silicate sol. Containing about 3.7% NaOH and about 7.0% SiO ₂ . The SiO ₂ -containing gel was stirred at room temperature for about 10 minutes, and then the pH was adjusted to 9.5 to 10 by adding a 1.1 MH ₂ SO ₄ solution. The gel was stirred at room temperature for about 2 hours. After stirring, the gel was transferred to a polypropylene container and heated at 100 ° C. for 4 days at normal pressure. During the heating, the lid of the polypropylene container was loosened to evaporate the solvent. The heated gel was allowed to cool in a tap water stream as it was in a polypropylene container. The solid material was transferred onto a filter paper and washed with water until it was free of Br ^- ions while filtering under reduced pressure with a tap water stream. The solid was then dried and finally calcined at 550 ° C.

Next, Co ₄ (CO) ₁₂ which is a Co ₄ cluster was prepared. Co ₄ (CO) ₁₂ was prepared by thermal polymerization of Co ₂ (CO) ₈ . 1.00 g of Co ₂ (CO) ₈ was placed in a two-necked flask and heated to 60 ° C. in an oil bath with stirring under N ₂ . Heating was performed for 20 hours. After the heating, the flask was gradually cooled to room temperature to obtain solid Co ₄ (CO) ₁₂ .

Finally, Co4 cluster / MCM-41 was prepared by the following method. 200 mg of MCM-41 placed in a Schlenk tube was dried for 2 hours at 195 ° C. under vacuum with stirring. After drying, Co clusters were added to the Schlenk tube in an N ₂ glove box. The amount of Co cluster added was 10 wt% as the amount of Co with respect to the weight of MCM-41. The Co cluster and MCM-41 powder were stirred well and about 20 ml of anhydrous ether was added. When the cluster did not dissolve in anhydrous ether, 20 drops of a solvent for dissolving the cluster was added dropwise. The mixture was stirred at room temperature under N ₂ for 48 hours. After stirring, ether was sucked in vacuum to obtain catalyst powder.

The catalyst powder was press-molded at 200 kgf / cm ² , crushed in a mortar, and sized to a particle size of 0.5 to 1 mm. The clusters were dispersed as small aggregates of 5 nm or less.

A catalyst in which a metal cluster having a controlled number of atoms, a metal, a metal oxide, or a metal cluster having a ligand is supported on a carrier exhibits high NOx purification activity even in the presence of a high concentration of O ₂ . An example of a metal atom is cobalt. Originally, particularly active surfaces of the catalytically active component are likely to react with each other, and aggregates having a stable number of atoms are likely to be formed. However, by controlling the number of atoms, we believe that there are more reaction surfaces where reaction selectivity tends to occur.

  As a method for controlling the number of cobalt atoms, a cobalt cluster having a ligand is used. As the ligand, a general ligand can be used. In particular, in the case of CO, it is coordinated with cobalt and can be easily detached, and an active surface is likely to appear.

  By dispersing the cluster having the ligand on the support, the aggregation of the cobalt cluster can be suppressed to several nm or less. As the carrier, MCM-41 (mesoporous silica) having mesoporous is preferable. Since MCM-41 has a high specific surface area, clusters can be highly dispersed. MCM-41 (Mobil Catalytic Material) has a uniform hexagonal mesoporous laminate structure. In addition to the hexagonal type, there are mesoporous silicas having skeletons such as cubic type and planar type, and those having a high specific surface area can be used.

The reaction temperature used in the present invention is preferably about 150 to 400 ° C.
A temperature of 350 ° C. is desirable because it keeps the Co cluster form for a long time. More preferably, it is 150-275 degreeC. Above 400 ° C., the Co cluster decomposes and tends to form an agglomerated structure similar to that of a conventional oxidation catalyst, resulting in a rapid decrease in performance. Use at 150 ° C. or lower is unsuitable when the exhaust gas contains a large amount of water vapor because water condenses easily.

The amount of cobalt clusters supported on the support may be adjusted by the specific surface area and pore diameter of the support. If the amount is too large, the clusters tend to aggregate. Conversely, if it is small, the effect is small. In MCM-41 having a pore diameter of 2-3 nm used this time, 5 to 15 as Co atoms.
It is preferable to add so that it may become wt%. 8 to 12 wt% is particularly desirable.

  In order to prepare MCM-41 which is the catalyst carrier of the present invention, a method using a normal micelle can be used. For example, micelles can be formed using a solution containing cetylmethylammonium salt and tetraalkylammonium salt in a sodium silicate solution.

< Reference Catalyst 3> FeO / MCM-41
As the carrier, MCM-41 (commercial product, manufactured by Zude Chemie), which is mesoporous silica, was used. MCM-41 was dried at 120 ° C. for 1 hour before use. 0.408 g of iron nitrate nonahydrate (manufactured by Wako Pure Chemical Industries, Ltd.) was dissolved in 1.176 g of pure water and impregnated with 0.6 g of MCM-41 after drying. This powder was press-molded at 200 kgf / cm ² , crushed in a mortar, and sized to 0.5 to 1 mm.

< Reference Catalyst 4> NiO / MCM-41
As the carrier, MCM-41 (commercial product, manufactured by Zude Chemie), which is mesoporous silica, was used. MCM-41 was dried at 120 ° C. for 1 hour before use. 0.294 g of nickel nitrate hexahydrate (manufactured by Wako Pure Chemical Industries, Ltd.) was dissolved in 1.227 g of pure water and impregnated in 0.6 g of MCM-41 after drying. This powder was press-molded at 200 kgf / cm ² , crushed in a mortar, and sized to 0.5 to 1 mm.

< Reference Catalyst 5> SrO / MCM-41
As the carrier, MCM-41 (commercial product, manufactured by Zude Chemie), which is mesoporous silica, was used. MCM-41 was dried at 120 ° C. for 1 hour before use. 0.214 g of strontium nitrate (manufactured by Wako Pure Chemical Industries) was dissolved in 1.339 g of pure water and impregnated in 0.6 g of MCM-41 after drying. This powder was press-molded at 200 kgf / cm ² , crushed in a mortar, and sized to 0.5 to 1 mm.

In this reference example, the NO removal rate was examined using the reference catalyst 1 in the presence or absence of HC. The reaction gas composition was set as follows, and the space velocity was set to 20,000 h ⁻¹ . C ₃ H ₆ was used as HC.

[NO] = 0.10 (vol%)
[CO] = 0.10 (vol%)
[O ₂ ] = 5.00 (vol%)
[C ₃ H ₆ ] = 0.10 (vol%)
Balance; He
The NO removal rate was calculated by the following formula.

NO removal rate (%) = (1−outlet NO concentration / inlet NO concentration) × 100
The space velocity was calculated by the following formula.

Space velocity (h ⁻¹ ) = reaction gas amount (ml / h ⁻¹ ) / catalyst amount (ml)
In the test without HC, Ag-supported β zeolite was used before the reference catalyst 1, and C ₃ H ₆ was sorbed during the test.

  FIG. 1 shows the NO removal rate at 200 to 275 ° C. When HC coexists, the NO removal rate was almost the same as 7 and 8% at 200 and 250 ° C., but became almost zero when the temperature was raised to 275 ° C. On the other hand, when HC was sorbed in the previous stage, the NO purification rate improved to 17.4% at 250 ° C and 31% at 275 ° C. By removing the coexisting hydrocarbons, the NO removal rate was improved. That is, the NO removal rate reduction when HC coexists is considered to be caused by reaction inhibition due to simultaneous progress of HC combustion reaction.

In this example, the NO removal rate was examined using the catalyst 2 based on the presence or absence of HC. Reaction gas composition,
The conditions are the same as in Example 2 ( Reference catalyst 1).

FIG. 2 shows the NO removal rate at 200 to 275 ° C. When HC coexists, the NO removal rate is 29% at 200 ° C., which is higher than that of the reference catalyst 1. However, it decreased to 12% at 250 ° C and 8% at 275 ° C. On the other hand, when HC was sorbed in the previous stage, the NO purification rate improved to 51% at 200 ° C, 84% at 250 ° C, and 98% at 275 ° C.

In this reference example, the reference catalyst 1 was used to examine the dependency of the NO removal rate on the HC concentration. In the reaction gas composition described in Example 2, the C ₃ H ₆ concentration was changed. The total reaction gas amount was adjusted with He so as to be the same.

  FIG. 3 shows the NO removal rate at 250 ° C. When HC does not coexist, it is 25%, but when HC coexists, the NO removal rate changes greatly. At 100 ppm, it improved to 37.1%, and at 250 ppm, it reached 22.8%. The removal rate was lower than when HC was not added. Further increase to 17.1% at 750 ppm, and to 14.0 ppm when added at 1000 ppm.

In this reference example, the NO removal rate was examined by changing the metal component supported on the mesoporous body. The reaction gas composition and conditions are the same as in Example 2.

FIG. 4 shows the NO removal rate at 150 ° C. Whereas reference catalysts of 1, 7.1%, 11.9% in the reference catalysts 3, reference in the catalyst 4 28.4% and NO removal rate is improved. However, NO was not purified by the reference catalyst 5 to which Sr, which is a basic metal, was added.

From this result, it was suggested that the temperature range where a high NO purification rate is obtained can be expanded by adding a plurality of metal components. That is, the reference catalyst 4 containing Ni and the reference catalyst 1 containing Co may be physically mixed, and both metal components may be supported on the same carrier.

In this reference embodiment, FIG. 5 shows a CO supply method when CO required for NOx reduction is insufficient. The CO and NOx concentrations in the gas after passing through the hydrocarbon removal step are measured, CO equal to or greater than the equivalent ratio is calculated, and the shortage of CO is supplied from the CO supply unit. CO may be supplied before NOx comes into contact with the NOx reduction catalyst.

FIG. 6 is an example of a hydrocarbon removal method using a sorbent. First, NOx-containing gas is introduced into the sorbent of the hydrocarbon removal step 1. At this time, valves other than the valves 12 and 14 are closed. Next, when the hydrocarbon in the gas after passing through the valve 14 reaches a predetermined amount, for example, 20 vol% or more with respect to the NOx amount, it is determined that the sorbent is saturated, and the valves 12 and 14 are closed. Then, the valves 16 and 17 are opened to introduce the NOx-containing gas into the sorbent in the hydrocarbon removal step 2. While using the hydrocarbon removal step 2, the valve 11 of the hydrocarbon removal step 1 is opened, and an inert gas is circulated to desorb the adsorbed hydrocarbon. It is also possible to heat the sorbent and release hydrocarbons that are not easily released or promoted. Heating can be performed by heating the container containing the sorbent material from the outside with an electric furnace or circulating a heated inert gas. The hydrocarbons contained in the exhaust gas 13 can be sent to the combustion process for combustion treatment. By repeating this operation alternately, hydrocarbons can be efficiently removed for a long time.

FIG. 7 shows a reference embodiment of a hydrocarbon removal method using an apparatus for physically separating hydrocarbons. The porous tubes 20 and 22 are installed in the pipe 21 through which the NOx-containing gas flows, and the NOx-containing gas is introduced into the porous tubes 20 and 22. The outside of the perforated tubes 20 and 22 is controlled to be a negative pressure from the inside. The pore diameter of the perforated tubes 20 and 22 is set to a size that allows only CO, NO, and CH ₄ to circulate. Even if C ₂ hydrocarbons are present, the pore diameter may be such that C ₂ hydrocarbons can also pass through if they are 20 vol% or less based on the amount of NOx. Thereby, hydrocarbons can be easily and efficiently removed from the NOx-containing gas.

NO generated in a combustion reaction or the like can be converted into N ₂ with high efficiency by a catalyst.

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Claims (5)

In a method for purifying NOx in exhaust gas containing at least CO, O ₂ and NOx,
Sorbing hydrocarbons in the exhaust gas to a hydrocarbon sorbent;
Contacting the exhaust gas after sorbing the hydrocarbon with a NOx purification catalyst having catalytic ability for a reaction of reducing NOx using CO as a reducing agent and carrying a metal cluster of Group 9 supported on a mesoporous body. A NOx purification method comprising: In the NOx purification method according to claim 1,
In the step of sorbing onto the hydrocarbon sorbent, the NOx purification method is characterized in that the hydrocarbon concentration is 0-20 vol% or less of the NOx concentration. In the NOx purification method according to claim 1,
The hydrocarbon sorbent is a NOx purification method characterized in that at least one of Ag, Cu, Zn, Ba, and Mg is supported on a mesoporous body. In the NOx purification method according to claim 1,
The NOx purification method, wherein the NOx purification catalyst contains at least one of Co, Ni, Fe, Zn, Cu, Mo, W, V, Pt, Pd, and Rh in the mesoporous body. In the NOx purification method according to claim 4,
The mesoporous body is any one of SiO ₂ , TiO ₂ , ZrO ₂ , and Al ₂ O ₃ .

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