JP5177685B2 - Exhaust gas purification catalyst and exhaust gas purification honeycomb catalyst structure - Google Patents

Description

The present invention relates to a NOx purification catalyst and NOx purification honeycomb catalyst structure in exhaust gas of a combustion engine.

  With regard to gases emitted from combustion engines in factories such as thermal power plants, various boilers, and heating furnaces, and combustion engines using internal combustion engines such as gasoline engines and diesel engines, as the global environmental problems become more serious in recent years, further exhaust gas Improvement of purification technology is desired.

For example, in exhaust gas purification of internal combustion engines such as automobiles, the exhaust gas contains carbon monoxide (CO), nitrogen oxides (NO _x ), and hydrocarbons (HC). Catalytic technology is generally known in which CO (NO ₂ ), nitrogen (N ₂ ), and water (H ₂ O) are converted to reduce CO, NO _x , and HC emissions.

There is a three-way catalyst as a catalyst that purifies all CO, NO _x , and HC in the gas discharged from engines such as automobiles, and the catalyst includes platinum (Pt), palladium (Pd), rhodium (Rh), etc. Metal catalysts combined with precious metals are widely used.

As the catalyst to selectively reduce and purify NO _x, there is storage reduction _the NO _x purification catalyst and _the NO _x selective reduction catalyst has been used in purification of exhaust gas lean-burn engines and diesel engines. In the storage-reduction type NO _x purification catalyst, NO _x is temporarily stored, and the stored NO _x is reduced by switching from lean to rich. In the NO _x selective reduction catalyst, urea, ammonia, hydrocarbon (HC) or the like is used as the NO _x reducing agent. In particular, since the exhaust gas of a diesel engine has a high oxygen concentration, the NO _x reduction reaction in the exhaust gas must be performed in an oxidizing atmosphere, and the progress of the reaction becomes difficult. A NO _x selective reduction catalyst using urea as a reducing agent is being put into practical use. However, the catalyst has a problem that urea as a reducing agent must be periodically replenished. On the other hand, when hydrocarbon is used as the reducing agent, fuel can be used as the reducing agent, so there is no need to replenish the reducing agent periodically. However, the NO _x selective reduction catalyst (HC-SCR) using hydrocarbon as a reducing agent has a high oxygen concentration (for example, 6 to 16%) like exhaust gas from a diesel engine and a low exhaust temperature (for example, 160 to 400 ° C). ) There is a problem that NO _x cannot be sufficiently reduced and purified.

In general, an exhaust gas purifying catalyst is used in a state where a noble metal is supported on an oxide carrier having a high specific surface area. As the oxide serving as the carrier, γ-alumina is used. The specific surface area of γ-alumina is usually 100 to 200 m ² / g. If the specific surface area of the carrier supporting the noble metal is large, the noble metal can be highly dispersed, so that improvement in catalytic activity and durability can be expected.

As an oxide having a specific surface area higher than that of γ-alumina, zeolite (aluminosilicate) and silica mesoporous material are known. Here, according to IUPAC (International Union of Pure and Applied Chemistry), the pores of the porous body are micropores having a pore diameter of 2 nm or less, and mesopores having 2 to 50 nm (also referred to as “mesopores”). And macropores of 50 nm or more. In particular, the silica mesoporous material has a pore peak at a position of several nm and a specific surface area of, for example, a very large value of 500 to 1200 m ² / g.

Therefore, attempts have been made to use zeolite or silica mesoporous material as a catalyst carrier. For example, in Patent Documents 1 to 13, zeolite or silica mesoporous material is used in the exhaust gas purification catalyst. In particular, Patent Document 2～8,10～11,13, in _the NO _x purification catalyst such as a diesel engine exhaust, Shirikamezo porous body (mesoporous silica) is used. When a silica meso porous material is used in an exhaust gas purification catalyst such as a NO _x purification catalyst, usually, a noble metal such as Pt, Pd, Rh is supported on the silica meso porous material.

Further, focusing on precious metals, Non-Patent Documents 1 to 4 are listed as research examples of NO _x purification catalysts using Pt. In Non-Patent Document 1, zeolite (ZSM-5) is used as a Pt carrier, and in Non-Patent Document 2, ordinary silica is used as a carrier. Non-patent documents 3 and 4 are examples of research using a silica meso porous material as a Pt support.

  Regarding the silica mesoporous material, in Patent Document 7, the lanthanoid group metal silicate is changed to a mesoporous material to improve the constituent material itself of the silica mesoporous material. In Patent Document 9, mesoporous silica is also modified by arranging a modifying substance containing silica, aluminum, titanium, zirconium, or the like at the entrance or inside of the pores of a silica mesoporous material (mesoporous silica). Furthermore, there are examples in which gold (Au) and iridium (Ir) are arranged in the pores of the silica mesoporous material (Patent Document 11) or a catalytically active substance is supported (Patent Document 12).

Further, in Non-Patent Document 4, as Pt / V / MCM-41, vanadium (V) is supported on the silica mesoporous material of MCM-41, and further Pt is supported thereon to serve as a NO _x purification catalyst. The characteristics of have been studied.

Japanese Patent Laid-Open No. 10-2022107 JP-A-8-257407 JP 2008-161812 A JP 2008-161811 A JP 2007-244934 A JP 2007-209866 JP JP 2006-297348 A JP 2006-081957 A Japanese Patent Laid-Open No. 2003-320245 JP 2008-279352 A Japanese Patent Laid-Open No. 2002-210369 JP 2002-320850 A JP 2007-069095 JP JP 2000-002111 A

H. Hirabayashi, H. Yahiro, N. Mizuno, M. Iwamoto, Chem. Lett., 2235-2236 (1992). R. Burch, P. K. Loader, F. J. Urbano, Catal. Today, 37, 37-42 (1996). S. -C. Shen, S. Kawi, Appl. Catal. B, 45, 63-76 (2003). J. Y. Jeon, H. Y. Kim, S. I. Woo, Appl. Catal. B, 44, 301-310 (2003).

As described above, there are many examples in which a silica meso porous material is used as an exhaust gas purification catalyst, and a silica meso porous material is also used as a NO _x purification catalyst.

Further, lanthanide metals in the silica framework constituting the Shirikamezo porous body (e.g., lanthanum, cerium, samarium, gadolinium) and lanthanoid metal silicate was introduced, it has been used in _the NO _x purification catalyst for exhaust gas from a diesel engine ( Patent Document 7). Also, it is disclosed that aluminum, titanium, zirconium, or the like is contained in the form of an oxide in the pore inlet or inside of a silica mesoporous body so as to be used as an HC adsorbent in exhaust gas (Patent Document 9). . In this way, the silica mesoporous material is modified with an oxide other than silica.

  On the other hand, a catalyst active material such as a noble metal supported on a silica meso porous body is also disposed inside the pores (Patent Documents 11 and 12).

Further, in Non-Patent Document 4, as a NO _x purification catalyst, a silica mesoporous material is modified by supporting V and further, Pt is supported.

  As described above, various devices have been made for supporting the silica mesoporous material and the noble metal, but the catalyst for exhaust gas purification is required to further improve the catalyst performance along with the strengthening of environmental regulations. The technology has a problem that a catalyst having sufficient performance has not been developed.

Further, although not subject to current emission regulations, dinitrogen monoxide (N ₂ O) which occurs as an intermediate product during the reduction purification of NO _x in _the NO _x purification catalyst even in an exhaust gas purifying catalyst capable of reducing Development is an issue, and as a solution to that problem, Patent Document 14 discloses an exhaust gas purification catalyst that combines an NO _x purification catalyst and an N ₂ O decomposition catalyst. However, there is a problem that a high-performance exhaust gas purifying catalyst having only a NO _x purifying catalyst and reducing the generation ratio of N ₂ O has not been developed. That is, as the NO _x purification catalyst, an exhaust gas purification catalyst having a high N ₂ yield is desired.

The present invention has been made in view of the above problems, and the object of the present invention is to improve the catalyst performance of the NOx purification catalyst in the exhaust gas using the silica meso porous material, and efficiently improve the NOx in the exhaust gas. purifying it a catalyst for NOx purification, and to provide a NOx purification honeycomb catalyst structure.

The inventors include a specific metal ion on the pore inner wall of the silica mesoporous material, and platinum is supported on the outer wall of the pore, so that NOx having a high N2 yield even in a high oxygen atmosphere. The present invention has been completed by finding out that it is possible to perform excellent NOx purification having a purification ability.

That is, the present invention has the following gist.
(1) The silica mesoporous material includes one or more metal ions selected from Al, Ti, V, Zr, Nb, Mo, Ce, Mg, Fe, or La on the pore inner wall of the silica mesoporous material, A catalyst for purifying NOx in exhaust gas, characterized in that platinum is supported on the outer walls of the pores.
(2) The NOx purification in exhaust gas according to (1), wherein the metal ion is at least one selected from Al, V, Nb, Mo, Ce, Mg, Fe, or La. Catalyst.
(3) The catalyst for purifying NOx in exhaust gas according to (1), wherein the metal ion is one or more selected from Al, V, Zr, Nb, Mo, Ce, or Mg. .
(4) The catalyst for purifying NOx in exhaust gas according to the above (1), wherein the metal ion is at least one selected from Al, V, Nb, Ce, or Mg.
(5) A honeycomb catalyst structure for NOx purification in exhaust gas , characterized in that the NOx purification catalyst in exhaust gas according to any one of (1) to (4) is coated on the inner wall of a metal or ceramic honeycomb.

As described above, according to the present invention, as a NOx purification catalyst in exhaust gas, a remarkable effect is obtained that it has excellent catalytic performance and can efficiently purify NOx .

The schematic diagram of the pore cross section of a silica meso porous body. The schematic diagram of the pore cross section of the silica meso porous body of the structure based on this invention. Explanatory drawing of a template ion exchange method.

The catalyst for purifying NOx in the exhaust gas of the present invention has at least one kind selected from Al, Ti, V, Zr, Nb, Mo, Ce, Mg, Fe, or La on the inner wall of the pores of the silica mesoporous material. It contains metal ions and platinum is supported on the pore outer wall. With such a configuration, a high N ₂ yield can be obtained for NOx purification at a high oxygen concentration. Here, the N ₂ yield is based on the reaction formula [1] assuming that NOx in NOx is almost NO (nitrogen monoxide).
2NO → N ₂ + O ₂ ... [1]
The N ₂ increase concentration (N ₂ generation amount) in NOx in the exhaust gas after treatment is twice the NO concentration (NO introduction amount) in NOx in the exhaust gas before treatment. That is, it is represented by the following expression {1}.

Therefore, simply purify NO _x (decomposition) and instead of determining the ability of the catalyst from the viewpoint of regulation so far that it can reduce the extent NO _x, the complete reaction of the NO _x to a N ₂ The ability of the catalyst is judged from the viewpoint of whether the catalyst can be advanced.

Compared to a conventional catalyst in which platinum or the above metal ions are simply supported on a silica mesoporous material, the present inventors have arranged the above metal ions on the inner wall of the pore of the silica mesoporous material, and placed the platinum on the outer wall of the same pore. It has been found that a high N ₂ yield can be obtained by using a supported structure.

In addition, among the above metal ions, one or more metal ions selected from Al, V, Nb, Mo, Ce, Mg, Fe, or La are included in the pore inner wall of the silica mesoporous material, By making platinum supported on the outer wall of the same pore, an excellent N ₂ selectivity can be obtained. Here, the N ₂ selectivity is defined as the N ₂ yield relative to the sum of the N ₂ yield and the N ₂ O yield. That is, it is represented by the following expression {2}.

The N ₂ O yield in the formula {2} is based on the reaction formula [2] on the assumption that NO _{x in} the exhaust gas is almost NO (nitrogen monoxide).
2NO → N ₂ O + 1 / 2O ₂ ... [2]
The N ₂ O generation amount (N ₂ O concentration) in the exhaust gas after the treatment is doubled with respect to the NO concentration (NO introduction amount) in the exhaust gas before the treatment. That is, it is represented by the following expression {3}.

Therefore, being excellent in the N ₂ selectivity means that the ratio of reduction to N ₂ is high in the NO _x decreased in the process.

In addition, among the above metal ions, one or more metal ions selected from Al, V, Zr, Nb, Mo, Ce, or Mg are contained in the pore inner wall of the silica mesoporous material. A high NO conversion rate (NO _x conversion rate) can be obtained by making platinum supported on the pore outer walls. Here, the NO conversion rate is the sum of the N ₂ yield and the N ₂ O yield, and corresponds to the ratio of the NO reduction amount after the treatment to the NO introduction amount. That is, it is represented by the following expression {4}.

Therefore, a high NO conversion means that a large amount of NO _x can be treated.

As described above, one or more metal ions selected from the above metal ions are included in the pore inner wall of the silica mesoporous material, and platinum is supported on the outer wall of the pore, The reason why the NOx purification performance in the exhaust gas is excellent is not clear, but for example, the following reasons can be considered.

The pores of the silica mesoporous material have a very small pore diameter of several nanometers to several tens of nanometers. Therefore, the silica forming the pore walls differs from the usual silica siloxane bond angle and bond distance. . As schematically shown in FIG. 1, outside the pores, the siloxane bond angle and bond distance tend to increase, and as a result, the electron density decreases and the Si and O polarization δ increases. On the other hand, inside the pores, the siloxane bond angle and the bond distance tend to decrease, and the electron density increases. Regarding catalyst design using silica mesoporous materials, the special reaction space of nano-space and the properties of the inner wall of the pore, which has higher electron density than the normal silica surface (surface properties shifted to the basic side), have been used so far. It was that. In the NO _x purification catalyst, a basic solid surface is preferable from the viewpoint of adsorbing NO _x which is an acidic gas. Although the normal silica surface is acidic, as described above, the inner wall of the pores of the silica mesoporous material is shifted to the basic side (acidity is weakened), so that the silica mesoporous material has a high specific surface area, Used as NO _x purification catalyst. Therefore, Patent Documents 3 and 4 show examples in which an anion is added to the silica mesoporous material to further increase the basicity.

However, in the catalyst of the present invention, it is considered that the NO _x purification reaction occurs mainly with the platinum supported on the pore outer wall of the silica mesoporous material. That is, outside the mesopores, as described above, the siloxane bond angle and bond distance are increased and the electron density is decreased, but the polarization of each of Si and O is increased, and in Si, positive polarization ( (δ +) increases, and in O, negative polarization (δ−) increases. Further, as shown in FIG. 2, when the metal ion (M ^{n +} ) according to the present invention is present on the inner wall of the pore, the siloxane bond electrons on the inner wall of the pore are attracted to the metal ion side (electron attraction by the metal ion). ). Since the pore wall thickness is thin in the silica mesoporous material, when the electrons of the siloxane bond on the inner wall of the pore are attracted by the metal ions, the polarization of the siloxane bond outside the pore is affected through the siloxane bond on the pore wall. That is, the polarization δ of Si and O becomes larger, and in O, the negative polarization becomes larger and acts as a base point, so that NO _x that is an acid gas is easily adsorbed. The adsorbed NO _x is reductively decomposed by the dissociation of NO bonds under platinum supported on the outer wall of the pore. Further, when the polarization of the siloxane bond is further increased, the positive polarization is further increased in Si and acts as an acid point. At the acid point, the hydrocarbon as the reducing agent is activated, so that O generated by dissociation of NO bonds can be efficiently removed. As described above, it is presumed that NO _x purification occurs efficiently by the configuration of the present invention. Among the metal ions according to the present invention, a large metal ion having a valence of 4 or more makes the above effect remarkable, and it is considered that a catalyst having a high NO conversion rate can be obtained.

Accordingly, among the above metal ions, one or more metal ions selected from Al, V, Nb, Ce, or Mg are contained in the pore inner wall of the silica mesoporous material, and platinum is present in the pore outer wall. By supporting the catalyst, the N ₂ yield is further increased, and the NOx purification performance in the exhaust gas can be improved.

The reason why platinum is used in the catalyst of the present invention is that, compared with other noble metals such as palladium and rhodium, oxygen generated by decomposition of NO _x is dissociated to easily regenerate the metal surface. For the purpose of suppressing platinum sintering, palladium or the like may be added as long as the effects of the present invention are not impaired. The amount of platinum supported on the silica mesoporous material is not particularly limited, but may be appropriately determined according to the conditions under which the catalyst is used, such as the amount of processing gas and the volume of the catalytic converter. For example, if the amount of platinum supported is small, sufficient NOx purification can be achieved if a large amount of catalyst is used, and if the amount of platinum supported is large, the amount of catalyst can be reduced and the volume of the catalytic converter can be reduced. Usually, the platinum loading is often 0.2 mass% to 20 mass%. This is because if the amount of platinum supported is less than 0.2 mass%, the amount of platinum is too small and a large amount of catalyst may be required. If the amount of platinum supported exceeds 20 mass%, the treatment efficiency may not be greatly increased even if the amount of platinum supported is increased, which may not be economical. A particularly preferable range of the amount of platinum supported is 0.5 mass% to 5 mass% from the viewpoint of efficiency and economy. Moreover, if the said conditions are satisfy | filled, platinum may exist in the inside of the pore of a silica meso porous body in the range which does not impair the effect of this invention.

  The amount of the metal ion M according to the present invention is not particularly limited as long as the above-described action of the present invention is obtained, but is preferably 5 to 500 in terms of Si / M molar ratio. When the Si / M molar ratio is less than 5, the metal ions may increase excessively with respect to the amount of silica constituting the mesoporous material, and metal ions overflowing from the pores may come out. If the Si / M molar ratio exceeds 500, the metal ion on the inner wall of the pores of the silica mesoporous material is too small, and the above-described action may not occur sufficiently. A more preferable range of the Si / M molar ratio is 10 to 50 from the viewpoint of efficiently obtaining the catalyst performance improvement effect by metal ions.

The silica mesoporous material (mesoporous silica) of the present invention is a porous material of silica (SiO ₂ ) having mesopores by IUPAC. That is, it is a silica porous body having pores having a pore diameter of 2 to 50 nm.

The method for synthesizing the silica mesoporous material is not particularly limited. For example, a known method for synthesizing a surfactant and a silica precursor as raw materials can be used. The surfactant is a micelle-forming surfactant used in the preparation of conventional silica mesoporous materials, such as a long-chain quaternary ammonium salt, a long-chain alkylamine N-oxide, and a long-chain sulfonate. , Polyethylene glycol alkyl ether, polyethylene glycol fatty acid ester and the like. As a specific example, alkyltrimethylammonium halide cationic surface activity represented by CH ₃ (CH ₂ ) _n N (CH ₃ ) ₃ .X (n is an integer of 7 to 21, X is a halogen ion) Examples thereof include n-octyltrimethylammonium bromide, n-decyltrimethylammonium bromide, n-dodecyltrimethylammonium bromide, n-tetradecyltrimethylammonium bromide, and n-octadecyltrimethylammonium bromide.

  Types of silica precursors include colloidal silica, silica gel, alkali silicates such as sodium silicate and potassium silicate, silicon alkoxides such as tetramethyl orthosilicate and tetraethyl orthosilicate, oligomers of the above silicon alkoxides, alone or Can be used as a mixture.

The specific surface area of the silica mesoporous material is preferably as high as possible unless there are special circumstances. The specific surface area of the silica mesoporous material that can be suitably used in the present invention is 100 to 1400 m ² / g, preferably 200 to 1200 m ² / g, and more preferably 400 to 1200 m ² / g. When the specific surface area is less than 100 m ² / g, platinum dispersibility becomes small, and sufficient catalyst performance may not be obtained. If the specific surface area exceeds 1400 m ² / g, the porous material strength may be insufficient. The specific surface area in the present invention is a surface area per 1 g of a substance obtained from the BET equation using physical adsorption of argon or nitrogen gas.

  As the silica mesoporous material of the present invention, a silica mesoporous material in which the pores are regularly arranged in a hexagonal structure or a cubic structure (regularity) can be preferably used. For example, a silica mesoporous material having through-type pores regularly arranged in a hexagonal structure of a type called “MCM (Mobil Catalytic Material-41)”. In the MCM-41 silica mesoporous material, the pore diameter (opening diameter) is usually 2 to 10 nm.

In the catalyst for purifying NOx in the exhaust gas of the present invention, as described above, the inner wall of the pores of the silica meso porous body is selected from Al, Ti, V, Zr, Nb, Mo, Ce, Mg, Fe, or La. It contains one or more selected metal ions. In order to include the metal ions in the pore inner walls of the silica mesoporous material, a template ion exchange method is used in the production process of the silica mesoporous material, as schematically shown in FIG. Specifically: In a solution containing a silica precursor, a surfactant, and a solvent, the surfactant forms micelles, and the silica precursor takes a mesoporous structure using the micelle structure of the surfactant as a template. A normal silica mesoporous material is obtained by firing a mesoporous structure containing the surfactant and removing the surfactant. In the present invention, the mesoporous structure containing the surfactant is added to the mesoporous structure. Metal ions are added to exchange ions with the surfactant (template). By firing after ion exchange, a silica mesoporous material containing the metal ions on the pore inner walls can be obtained.

  As a raw material to be added as the metal ion, an inorganic acid salt, an organic acid salt, a metal oxo acid salt, or a metal complex of a metal ion can be used. For example, nitrate, acetate, chloride, bromide, sulfate, hydroxide, oxalate, ammonium salt and the like can be mentioned. These raw materials can be used alone or in admixture of two or more.

  As the solvent, one or more of water, alcohols, and diols are usually used, and an aqueous solvent is preferable.

  The procedure for producing the silica mesoporous material containing the metal ions on the pore inner walls is, for example, as follows.

  First, a solution containing a surfactant and a solvent is prepared. On the other hand, a solution containing a silica precursor and a solvent is prepared, and an alkali solution in which an inorganic alkali such as sodium hydroxide or potassium hydroxide is dissolved is added to the solution. Instead of the alkaline solution, an acid solution having an inorganic acid such as hydrochloric acid, sulfuric acid or nitric acid, an organic acid such as citric acid, and the pH adjusted to 1 to 4 can be used. The concentration of the silica source in the solution is not particularly limited, but is usually preferably 0.1 to 3 mol / L, more preferably 0.4 to 1.5 mol / L.

Further, the two solutions are mixed and stirred at room temperature (20 ° C.) to 60 ° C. for 1 hour to 50 hours. Here, the surfactant is usually 2.0 to 6.0 mol, preferably 2.5 to 4.0 mol, with respect to 1 mol of silica (SiO ₂ ).

  After stirring, heat treatment is performed to improve the regularity of the structure. As specific heating conditions, the mixed liquid is subjected to hydrothermal treatment at 80 to 160 ° C., preferably 100 to 150 ° C., for 30 to 60 hours, preferably 40 to 50 hours. Precipitation nuclei having a mesoporous structure are formed by this hydrothermal treatment.

  Next, the precipitated product is filtered off, washed with water, and dried in the range of room temperature to 150 ° C.

  The dried product is dispersed in a solvent to form a suspension, and the suspension is added to a solution obtained by dissolving the above metal ion raw material in a solvent, and stirred for 0.5 to 5 hours. Leave for 1 to 48 hours. Thereby, the surfactant and metal ions in the pores of the mesoporous structure are ion-exchanged. The amount of ion exchange can be varied by adjusting the amount of metal ions in the solution, the concentration, and the temperature and time of standing. Until the amount of ion exchange reaches saturation, the amount of metal ion exchange increases as the amount of metal ions increases and the concentration of metal ions increases. In addition, the higher the temperature to be left and the longer the time to be left, the greater the amount of metal ion exchange. The amount of metal ion exchange can be calculated by analyzing the metal ions remaining in the solution.

  Further, the solid material after the treatment is separated by filtration and washed, and then the remaining surfactant is removed. The surfactant can be removed by thermal decomposition (combustion) at a high temperature. When the remaining surfactant is removed by combustion, it is preferable to perform a heat treatment in an atmosphere in which oxygen is present.

  Although heat processing temperature is not specifically limited as long as surfactant can be removed, Preferably it is 200-800 degreeC, More preferably, it is performed at 300-600 degreeC. When the heat treatment temperature is 200 ° C. or higher, the combustion of the surfactant is promoted to facilitate removal. This effect becomes more remarkable when the heat treatment temperature is 300 ° C. or higher. When the heat treatment temperature is 800 ° C. or lower, the silica constituting the pore walls can be prevented from collapsing. This effect becomes more remarkable when the heat treatment temperature is 600 ° C. or lower.

  The heat treatment time is not particularly limited as long as the surfactant can be removed, but is usually about 4 to 20 hours, preferably about 4 to 10 hours. Any atmosphere in which oxygen is present may be used, but it is preferably performed in air (21% oxygen) to oxygen (purity 95% or more).

  The content of the silica mesoporous material obtained as described above can be determined by analyzing metal ions by chemical analysis, ICP (Inductively Coupled Plasma) emission analysis, or the like. In addition, the pores of the silica mesoporous material can be observed with a TEM (transmission electron microscope), and the regularity of the pores can be determined by a TEM image and XRD (X-ray diffraction method). Moreover, the metal ion which exists in the pore inner wall can be confirmed by STEM (scanning transmission electron microscope). The presence of metal ions in the examples described later has been confirmed by STEM.

By impregnating a silica mesoporous material containing metal ions in the pore inner wall with a platinum precursor, the catalyst for purifying NOx in the exhaust gas of the present invention in which platinum is supported on the outer wall of the pore can be produced. Examples of platinum precursors include H ₂ PtCl ₄ , (NH ₄ ) ₂ PtCl ₄ , H ₂ PtCl ₆ , (NH ₄ ) ₂ PtCl ₆ , Pt (NH ₃ ) ₄ (NO ₃ ) ₂ , Pt (NH ₃ ) ₄ (OH) ₂ , PtCl ₄ , platinum acetylacetonate, Pt colloid, and the like can be used. In the impregnation treatment, the silica mesoporous material is dispersed in a solvent to prepare a suspension. On the other hand, a solution in which a platinum precursor is dissolved in a solvent is prepared. A platinum precursor solution is added to the suspension and stirred at room temperature to 95 ° C. Thereafter, the solvent is removed by evaporation. The remaining solid content is heat-treated at 300 to 800 ° C., whereby platinum can be supported, and the NOx purification catalyst of the present invention can be obtained. The heat treatment is performed in air, in an inert atmosphere, or in a reducing atmosphere.

  As the solvent, one or more of water, alcohols, and diols are usually used, and water or alcohol solvents are preferable.

The NOx purification catalyst in the exhaust gas of the present invention can be coated on the inner wall of a metal or ceramic honeycomb to form a NOx purification honeycomb catalyst structure. As a method for coating the inner wall of the honeycomb, for example, wash coating can be performed. The ceramic honeycomb that can be used in the present invention is not particularly limited, and examples thereof include a cordierite honeycomb and a silicon carbide honeycomb. The metal honeycomb that can be used in the present invention is not particularly limited, and examples thereof include a stainless steel honeycomb and an Al-enriched stainless steel honeycomb.

When the NOx purification catalyst in the exhaust gas of the present invention is wash-coated on the honeycomb, first, a slurry in which the catalyst, the binder and the like are dispersed is prepared, and the honeycomb is immersed in the slurry. Examples of the binder include aluminum nitrate, colloidal alumina, and an organic binder. Next, excess slurry on the honeycomb surface is removed by a method such as blowing off, and after drying, heat treatment is performed at a temperature of 500 to 800 ° C. in the atmosphere for several hours. It is also possible to suck up the slurry by devising a jig for mounting the honeycomb so that the slurry is applied only to the inner wall of the honeycomb.

Examples of the present invention will be described below, but the present invention is not limited to these examples.
Example 1
225.00 g of n-dodecyltrimethylammonium bromide C ₁₂ H ₂₅ N (CH ₃ ) Br (surfactant) and 644.94 g of ion-exchanged water were stirred at 40 ° C. until a uniform solution was obtained. A solution prepared by dissolving 11.6 g of sodium hydroxide in 126.30 g of ion-exchanged water and 306.32 g of colloidal silica (manufactured by Nissan Chemical Co., Snowtex 20, SiO ₂ : 20 mass%, particle size 10 ˜20 nm) and the mixture was stirred for 2 hours. The mixed solution was hydrothermally treated at 140 ° C. for 48 hours in an autoclave. After hydrothermal treatment, the solid content of the contents was filtered, washed with ion-exchanged water, and dried at 80 ° C. The solid part obtained after drying was 87.15 g.

  The solid content (powder) thus obtained was measured with a powder X-ray diffractometer of CuKα X-ray source. The diffraction pattern 2θ = 2.54 degrees (100), 4.40 degrees (110). At 5.05 degrees (200), a diffraction peak due to the mesopore structure was observed, confirming the formation of a silica mesoporous material.

Since the solid content powder is in a dry state, the surfactant as a template remains in the pores. 20 g of the solid powder was dispersed in 30 times mass of ion-exchanged water, and 2N hydrochloric acid was added with stirring to adjust the pH of the suspension to 6.5 ± 0.5. After stirring at room temperature for 1 hour, it was left in a water bath at 80 ° C. for 20 hours. Then, it washed with filtration and ion-exchange water (pH swing). The solid powder was fired in air at 600 ° C. for 6 hours. When the pore diameter of the baked powder was determined by a nitrogen adsorption method, the average was 2.24 nm (BET specific surface area: 979 m ² / g).

To 1 g of the unfired powder (dry powder), 10 mL of ion-exchanged water was added, stirred and suspended. A predetermined amount of metal salt was dissolved in 10 mL of ion exchange water to prepare an aqueous solution of metal ions. While stirring the suspension, an aqueous solution of the metal ions was added and stirred at room temperature for 1 hour. The container after stirring was immersed in a water bath set at 80 ° C. and held for 20 hours. Thereafter, the solution was suction filtered to separate a solid powder, washed with ion-exchanged water and dried at 80 ° C. The solid powder after drying was heated in air at 1 ° C./min and calcined at 600 ° C. for 6 hours. In this manner, silica mesoporous materials containing various metal ions in the pore inner walls were prepared.
Here, each metal salt is nitrate (Al (NO ₃ ) ₂ · 6H ₂ O, ZrO (NO ₃ ) ₂ · 2H ₂ O, Ce (NO ₂ ) ₃ · 6H ₂ O, Mg (NO ₃ ) ₂ · 6H ₂ O, Fe (NO ₃ ) ₂・ 9H ₂ O, Ni (NO ₃ ) ₂・ 6H ₂ O, Cu (NO ₃ ) ₂・ 3H ₂ O, Co (NO ₃ ) ₂・ 6H ₂ O, La ( NO ₃ ) ₃・ 6H ₂ O, Cr (NO ₃ ) ₃・ 9H ₂ O), Sulfate (VOSO ₄・ nH ₂ O), Oxalate ((NH ₄ ) ₂ TiO (C ₂ O ₄ ) ₂・2H ₂ O) and acetate (Mn (CH ₃ COO) ₂ · 4H ₂ O) were used.

NH ₄ NbO (C ₂ O ₄ ) ₂ · xH ₂ O oxalate, (NH ₄ ) ₆ Mo ₇ O ₂₄ · 4H ₂ O ammonium salt, WCl ₆ chloride metal salts are anions in solution. Therefore, a metal ion was included in the pores using the silica mesoporous material obtained by firing at 600 ° C. for 6 hours. 30 mL of ion exchange water was added to 1.5 g of the silica mesoporous material, and the mixture was stirred and suspended. A predetermined amount of metal salt was dissolved in 15 mL of ion exchange water to prepare an aqueous solution of metal ions. While stirring the suspension, an aqueous solution of the metal ions was added at room temperature. Thereafter, the mixture was stirred at 80 ° C. for 2 hours and then evaporated to dryness while stirring at the same temperature. Then, it dried at 80 degreeC for 12 hours, heated up at 1 degree-C / min in the air, and baked at 600 degreeC for 6 hours. For the case of the WCl _6, because WCl ₆ is insoluble in water, ethanol was used as solvent instead of ion-exchanged water. With respect to the metal species that become anions in the solution, a silica mesoporous material containing metal ions in the pore inner walls was prepared in this way.

Further, as a comparative example, Ce (NO ₂ ) ₃ · 6H ₂ O was used, and a silica mesoporous material obtained by firing at 600 ° C. for 6 hours was used in the same manner as described above. In this case, cerium is a cation in an aqueous solution, and since there is no replaceable surfactant in the pores of the silica mesoporous material, a silica mesoporous material in which cerium is present outside the pores can be prepared.

  The metal ions contained in the silica mesoporous material were analyzed by ICP, and the Si / M molar ratio was calculated.

Pt was supported by an impregnation method on the fired silica mesoporous material containing the metal ions and the fired silica mesoporous material not containing the metal ions. The Pt raw material used in the impregnation method is chloroplatinic acid H ₂ PtCl ₆ · 6H ₂ O. 1 g of chloroplatinic acid was dissolved in 100 g of ion-exchanged water. On the other hand, 1.5 g of each silica mesoporous material powder was added to 30 mL of ion-exchanged water, stirred and suspended. While stirring the suspension, the chloroplatinic acid aqueous solution was added at room temperature, and further stirred at 80 ° C. for 2 hours. Here, the ratio of chloroplatinic acid and silica mesoporous material powder was set to 2 mass% in terms of the amount of Pt supported. The solution was then evaporated to dryness if stirred at the same temperature. The obtained powder was heated in air at 1 ° C./min and calcined at 600 ° C. for 6 hours to prepare a catalyst powder.

The catalyst powder produced as described above is filled in a reaction tube of a normal pressure fixed bed flow type reactor, and under He gas flow (GHS, Gas Hourly Space Velocity): 5,000 h ^−1. The catalyst activity was evaluated after pre-treatment at 500 ° C. for 4 hours. The reaction gas is NO: 0.1%, C ₂ H ₄ : 0.3%, O ₂ : 14%, and He balance. The GHSV at this time was set to 10,000 h- ¹ . The measurement temperature is 100 ° C to 600 ° C. A gas chromatograph with TCD (Thermal Conductivity Detector) -FID (Flame Ionization Detector) was used for gas analysis. From the analysis results, the N ₂ yield was determined based on the above formula {1}, the N ₂ selectivity was determined based on the above formula {2}, and the NO conversion rate was determined based on the above formula {3}.

The evaluation results of each catalyst are shown in Table 1. In Table 1, on the basis of N ₂ yield, N ₂ selectivity, and NO conversion rate of a sample (No. 1-18) in which platinum is supported on a silica mesoporous material containing no metal ions, No. 1-1 to The catalyst performance evaluation result of each sample up to 17 is shown. For the N ₂ yield, relative to N ₂ yields samples which carries platinum Shirikamezo porous body containing no metal ions (No.1-18), "×" 1.0 times or less, 1.0 More than double and less than 1.1 times are indicated by “Δ”, 1.1 times or more and less than 1.2 times are indicated by “◯”, and 1.2 times or more are indicated by “◎”. For the N ₂ selectivity for N ₂ selectivity of the sample supporting platinum on Shirikamezo porous body containing no metal ions (No.1-18), "×" 1.0 times or less, 1.0 More than double and less than 1.3 times are indicated by “Δ”, 1.3 times or more and less than 1.5 times are indicated by “◯”, and 1.5 times or more are indicated by “◎”. Concerning the NO conversion rate, 0.8 times or less is “x” and more than 0.8 times the NO conversion rate of the sample (No. 1-18) in which platinum is supported on a silica mesoporous material containing no metal ions. Less than 1.0 times are indicated by “Δ”, 1.0 times or more and less than 1.05 times are indicated by “◯”, and 1.05 times or more are indicated by “◎”.

As shown in Table 1, in the case of metal ions of Al, Ti, V, Zr, Nb, Mo, Ce, Mg, Fe, or La, which are examples of Nos. 1-1 to 1-10, exhibited superior N ₂ yields, Al, V, Nb, Ce, or in the case of Mg metal ions, exhibited superior N ₂ yield. Furthermore, in the case of Al, V, Nb, Mo, Ce, Mg, Fe, or La metal ions, which are examples of No. 1-1, 1-3, 1-5 to 1-10, excellent N ₂ selectivity was shown, and in the case of Mg, Fe, or La metal ions, better N ₂ selectivity was shown. In the case of Al, V, Zr, Nb, Mo, Ce, or Mg metal ions, which are examples of Nos. 1-1 and 1-3 to 1-8, an excellent NO conversion rate is shown. In the case of metal ions of V, Zr, Nb, or Ce, better NO conversion was shown.

Moreover, although Example 1-7 and Comparative Example 1-17 are the same cerium as a metal ion, in Example 1-7, a metal ion has in the pore inner wall of a silica meso porous body, In Comparative Example 1-17, This is the case where there are no metal ions on the inner walls of the pores of the silica mesoporous material, and the metal ions are present only outside the pores. From Table 1, it can be seen that Example 1-7 is superior in N ₂ yield.

The above evaluation results show that the NOx purification catalyst in exhaust gas of the present invention enables excellent NOx purification in exhaust gas in NOx in exhaust gas with high oxygen concentration such as diesel engine and lean burn gasoline engine. In addition, it shows that it can be applied to NOx purification in exhaust gas from various combustion engines.

(Example 2)
Using the unfired powder or fired powder of the silica mesoporous material prepared in Example 1, silica mesoporous materials containing various metal ions shown in Table 2 in the pore inner wall were prepared in the same manner as in Example 1. . However, in this case, the amount of metal ions was changed as shown in Table 2. Further, platinum was impregnated and supported by the same method as in Example 1 so that the amount of Pt supported was 2 mass%.

  The catalyst performance evaluation was performed in the same manner as in Example 1. The evaluation result was based on a sample in which platinum was supported on a silica mesoporous material containing no metal ions No. 1-18 (Example 1) (Table 1). ). The results are shown in Table 2.

  With respect to the amount of metal ions, the effects of the present invention were obtained in all examples in Table 2. In particular, when the Si / M molar ratio was in the range of 5 to 500, a remarkable effect was obtained with no difference from Table 1.

(Example 3)
Using the unfired powder or fired powder of the silica mesoporous material prepared in Example 1, silica mesoporous materials containing various metal ions shown in Table 3 in the inner walls of the pores were prepared in the same manner as in Example 1. . Using the silica mesoporous material and the fired silica mesoporous material containing no metal ions prepared in the example, platinum was supported in the same manner as in Example 1 to prepare a catalyst sample. However, here, the Pt loading was impregnated so as to be 10 mass%.

Each catalyst sample obtained was evaluated for catalyst performance in the same manner as in Example 1. The evaluation results of each catalyst sample are shown in Table 3. In Table 3, on the basis of the N ₂ yield, N ₂ selectivity, and NO conversion rate of a sample (No. 3-17) in which platinum is supported on a silica mesoporous material containing no metal ions, No. 3-1 to The catalyst performance evaluation result of each sample up to 16 is shown. For the N ₂ yield, relative to N ₂ yields samples which carries platinum Shirikamezo porous body containing no metal ions (No.3-17), "×" 1.0 times or less, 1.0 Over double and less than 1.2 times are indicated by “◯”, and 1.2 times or more are indicated by “◎”. For the N ₂ selectivity for N ₂ selectivity of the sample supporting platinum on Shirikamezo porous body containing no metal ions (No.1-17), "×" 1.0 times or less, 1.0 More than double and less than 1.1 times are indicated by “Δ”, 1.1 times or more and less than 1.2 times are indicated by “◯”, and 1.2 times or more are indicated by “◎”. Concerning the NO conversion rate, 0.8 times or less is “x” and 0.8 times or more than the NO conversion rate of the sample (No. 3-18) in which platinum is supported on a silica mesoporous material containing no metal ions. Less than 1.0 times are indicated by “Δ”, 1.0 times or more and less than 1.05 times are indicated by “◯”, and 1.05 times or more are indicated by “◎”.

  As shown in Table 3, even when the Pt loading was 10 mass%, it was the same as the result of Example 1 (Table 1), and the effect of the present invention was recognized. Furthermore, with respect to the amount of Pt supported, sufficient catalyst performance was not obtained when platinum was not supported, but the silica meso containing no metal ions at least in the range of 0.2 mass% to 20 mass% with the same Pt supported amount. The same effect of being superior to the porous body was recognized.

  Further, each catalyst powder was made into a slurry and washed on the honeycomb to prepare a honeycomb catalyst structure, and the catalyst performance was evaluated. Here, the used honeycomb was a cylindrical stainless steel honeycomb having a predetermined shape diameter of 25 mm, a height of 60 mm, and a cell density of the honeycomb of 300 cells per inch (25.4 mm) square. Similar to the evaluation results in the powder form of Table 3, the catalyst performance results were superior to the silica mesoporous material containing no metal ions.

1 Silica wall of silica mesoporous material 2 Surfactant (template) ion 3 Proton 4 Metal ion

Claims (5)

The inner wall of pores of the silica mesoporous material contains one or more metal ions selected from Al, Ti, V, Zr, Nb, Mo, Ce, Mg, Fe, or La, and the pores of the silica mesoporous material A catalyst for purifying NOx in exhaust gas , characterized in that platinum is supported on the outer wall. 2. The catalyst for purifying NOx in exhaust gas according to claim 1, wherein the metal ion is one or more selected from Al, V, Nb, Mo, Ce, Mg, Fe, or La. 2. The catalyst for purifying NOx in exhaust gas according to claim 1, wherein the metal ion is one or more selected from Al, V, Zr, Nb, Mo, Ce, or Mg. 2. The catalyst for purifying NOx in exhaust gas according to claim 1, wherein the metal ion is at least one selected from Al, V, Nb, Ce, or Mg. A honeycomb catalyst structure for purifying NOx in exhaust gas, wherein the NOx purification catalyst in exhaust gas according to any one of claims 1 to 4 is coated on a metal or ceramic honeycomb inner wall.

Images