JP2013233541A - Catalyst for purifying exhaust gas - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an exhaust gas purifying catalyst having excellent gas purification performance as well as burning purifying particulates in exhaust gas discharged from an internal combustion engine of an automobile or the like.SOLUTION: The present invention contains a complex oxide comprising a crystal having a DyMnOstructure, in which an A site contains Y and a B site contains Mn, the composition ratio between the B site and the A site (B/A) being greater than 2.

Description

  The present invention relates to an exhaust gas purifying catalyst used for purifying exhaust gas discharged from an internal combustion engine such as an automobile.

  The exhaust gas discharged from an internal combustion engine such as an automobile contains harmful components such as hydrocarbons, carbon monoxide, and nitrogen oxides.

  Further, exhaust gas discharged from a diesel engine contains particulates (PM; particulate matter), and if these materials are released into the atmosphere as they are, they cause air pollution. As an effective means for removing particulates, there is a diesel exhaust gas trap system using a diesel particulate filter (DPF) for collecting soot.

  On the other hand, in order to efficiently purify diesel exhaust gas, attempts have been made to install a diesel oxidation catalyst (DOC) as close as possible to the engine and promote the gas purification reaction using the heat of the engine. However, since the space in the engine room is limited, it is necessary to reduce the size of the DOC, and there is a problem that a trade-off occurs between the efficiency improvement due to temperature and the reduction in the absolute reaction area due to the size reduction.

  As one of the solutions, it is considered that not only PM combustion but also a function of gas purification performance is added to the DPF to improve the performance of the entire exhaust gas purification system. For example, a particulate oxidant that combusts and purifies particulates and simultaneously reduces and purifies NOx has been proposed (see Patent Document 1), but further performance improvement is required.

JP 2003-334443 A

  An object of the present invention is to provide an exhaust gas purifying catalyst that combusts and purifies particulates of exhaust gas discharged from an internal combustion engine such as an automobile and has excellent gas purification performance.

As a result of various experiments using various substances in order to achieve the above object, the present inventors have found that from a crystal having a DyMn ₂ O ₅ structure in which the A site contains Y and the B site contains Mn. Thus, the present inventors have found that a complex oxide having a B / A site composition ratio B / A larger than 2 is excellent in gas purification performance as well as combustion purification of particulates.

That is, the exhaust gas purifying catalyst of the present invention comprises a crystal having a DyMn ₂ O ₅ structure in which the A site contains Y and the B site contains Mn, and the composition ratio B / A> 2 of the B site and the A site. It is characterized by including a double oxide.

Here, a double oxide consisting of a crystal having a DyMn ₂ O ₅ structure and having a composition ratio B / A> 2 of the B site and the A site is B / A which is a stoichiometric ratio among the elements of the B site. = Excess B-site element that is a portion exceeding = 2 means that at least a part is in solid solution in the crystal. In addition, being dissolved in the crystal | crystallization is interpreted as containing in the crystal | crystallization lattice, and is synonymous.

  Further, the exhaust gas purifying catalyst of the present invention preferably contains Ag, and at least a part of Ag is preferably dissolved in the double oxide crystal.

  Moreover, it is preferable that the catalyst layer supported on the catalyst support contains the double oxide, and Ag is supported on the catalyst layer.

  Furthermore, it is preferable that the catalyst layer supported on the catalyst support contains the double oxide, and at least one element selected from Pt, Au, Pd and Rh is supported on the catalyst layer.

  The exhaust gas purifying catalyst of the present invention is effective in purifying exhaust gas discharged from an internal combustion engine such as an automobile, because the exhaust gas purifying catalyst of the present invention combusts and purifies particulates and is excellent in gas purification performance.

2 is a chart showing XRDs of exhaust gas purifying catalysts of Examples 1 to 4 and 7 and Comparative Example 1; 7 is a chart showing XRDs of exhaust gas purifying catalysts of Examples 8 to 11 and Comparative Example 1. 7 is a chart showing XRD of exhaust gas purifying catalysts of Examples 12 to 16 and Example 4. It is a chart which shows XRD of the exhaust gas purification catalyst of Comparative Examples 1-6.

The exhaust gas purifying catalyst of the present invention will be described below.
The exhaust gas purifying catalyst of the present invention is composed of a crystal having a DyMn ₂ O ₅ structure in which the A site contains Y and the B site contains Mn, and the composition ratio B / A> 2 of the B site and the A site. Includes double oxides. Such a double oxide is one in which an excess of B-site elements, which are portions exceeding the stoichiometric ratio B / A = 2, of the B-site elements are dissolved in the crystal.

Here, the double oxide composed of a crystal having a DyMn ₂ O ₅ structure is a dysprosium manganate structure whose XRD pattern is included in the space group Pbam (see DyMn ₂ O ₅ structure, ICSD (Inorganic crystal structure database)). It is identified as a crystal that takes Furthermore, mixed oxide of crystalline taking DyMn ₂ O ₅ structure is shown as a general formula _AB 2 _{O 5.}

  In the double oxide of the present invention, the A site contains Y, the B site contains Mn, the B site and the A site have a composition ratio B / A> 2, and the B site is excessive from the stoichiometric ratio. And at least a part of the excess B-site element is dissolved in the crystal.

Here, the element in which the excess B-site element is dissolved in the crystal means that the B-site element is excessively shifted from the stoichiometric ratio, but in the XRD pattern, the DyMn ₂ O ₅ structure And the BRD element having an ionic radius smaller than that of the A-site element is dissolved in a solid solution, so that a shift is observed in 2θ of the XRD peak. In this case, even if the peak can not be attributed to and DyMn ₂ O ₅ comprises an element in the B site was observed, as long observed deviation of 2θ of XRD peak attributed to DyMn ₂ O _5, of excess B-site It is considered that at least a part of the element is dissolved in the crystal lattice.

  The exhaust gas purifying catalyst of the present invention may coexist with an oxide of an element constituting the B site, for example, manganese oxide, together with the double oxide in which the B site is excessive. Are also encompassed by the present invention.

  Further, the exhaust gas purifying catalyst of the present invention may contain Ag. “Containing Ag” means that Ag coexists with the double oxide, but at least a part of Ag may be dissolved in the crystal of the double oxide.

  Here, the fact that at least a part of Ag is dissolved in the crystal means that the peak intensity of metal Ag in the XRD pattern is clearly smaller than the intensity estimated from the absolute value of the content.

  Such a double oxide of the present invention can be produced, for example, as follows.

  An example of the production method will be described by taking as an example the production of a complex oxide in which the A site is Y and the B site is Mn.

As an example of the manufacturing method, a precipitant is added to a solution containing Y and Mn to obtain a precipitate containing Y and Mn at an atomic ratio Mn / Y> 2, and this is dried and fired. Thus, a method of obtaining a double oxide YMO can be mentioned. Here, in addition to the stoichiometric ratio YMn ₂ O ₅ , those having a composition deviating from the stoichiometric ratio are also collectively referred to as YMO.

  Alternatively, the composite oxide may be produced by performing the Mn / Y ratio in a stoichiometric ratio, and thereafter, an excess amount of Mn may be supported.

  The support of Ag may be performed by a method such as evaporation to dryness using silver nitrate or dry mixing using metal Ag.

Y compound used in each of the above production methods is a group consisting of La compound, Sr compound, Ce compound, Ba compound, Ca compound, Sc compound, Ho compound, Er compound, Tm compound, Yb compound, Lu compound and Bi compound. The compound (Y _1-x A _x ) Mn _{2 + y} O ₅ (wherein A is La) is carried out by replacing the Y atom of the Y compound with one or more compounds selected from , Sr, Ce, Ba, Ca, Sc, Ho, Er, Tm, Yb, Lu, or Bi, and 1>x> 0, y> 0).

The Mn compound used in each of the above production methods is a compound selected from the group consisting of Co compound, Fe compound, Ni compound, Cr compound, Mg compound, Ti compound, Nb compound, Ta compound, Ru compound and Cu compound. By carrying out the above production method by substituting one or more kinds so that the amount of atoms is less than half the amount of Mn atoms of the Mn compound, double oxide Y (Mn _1-z B _z ) _{2 + y} O ₅ (wherein , B is Co, Fe, Ni, Cr, Mg, Ti, Nb, Ta, Ru, or Cu, and y> 0 and 0.5 ≧ z> 0).

Further, the Y compound used in the above production method is composed of La compound, Sr compound, Ce compound, Ba compound, Ca compound, Sc compound, Ho compound, Er compound, Tm compound, Yb compound, Lu compound and Bi compound. The Y atom of the Y compound is replaced with one or more compounds selected from the group, and the Mn compound is replaced with a Co compound, Fe compound, Ni compound, Cr compound, Mg compound, Ti compound, Nb compound, Ta compound, Ru compound. and at least one compound selected from the group consisting of Cu compounds, double oxide by replacing such that the amount of half or less of atoms of Mn atoms of Mn compound implementing the above manufacturing method (Y ₁ during _{-x A x) (Mn 1-} z B z) 2 + y O 5 ( wherein, A is La, Sr, Ce, Ba, Ca, Sc, Ho, Er, Tm, Yb, Lu or i, B is Co, Fe, Ni, Cr, Mg, Ti, Nb, Ta, Cu or Ru, 1>x> 0, y> 0, 0.5 ≧ z> 0 Can be manufactured).

  Here, La, Sr, Ce, Ba, Ca, Sc, Ho, Er, Tm, Yb, Lu, or Bi listed as atoms replacing Y at the A site have an ionic radius that can replace Y. However, among these, La, Ce, Ca, Sc, Ho, Er, Tm, Yb, Lu, or Bi having an ionic radius within ± 10% with respect to the ionic radius of Y can be stably substituted. .

  Further, Co, Fe, Ni, Cr, Mg, Ti, Nb, Ta, Cu or Ru listed as atoms replacing Mn at the B site have an ionic radius that can replace Mn. Among these, Co, Fe, Ni, Cr, Mg, Ti or Cu having an ionic radius within ± 10% with respect to the ionic radius of Mn is particularly preferable because it can be stably replaced.

In the general formula _{(Y 1-x A x)} (Mn 1-z B z) 2 + y O 5 ( wherein, A is La, Sr, Ce, Ba, Ca, Sc, Ho, Er, Tm, Yb, Lu , or Bi And B is Co, Fe, Ni, Cr, Mg, Ti, Nb, Ta, Ru or Cu, 1> x ≧ 0, y> 0, and 0.5 ≧ z ≧ 0. ) Includes all the above-mentioned double oxides.

  The double oxide of the present invention is preferably used as a catalyst layer supported on a catalyst support.

Here, the catalyst support is made of, for example, ceramics or a metal material. The shape of the catalyst support is not particularly limited, but is generally a honeycomb shape, a plate, a pellet, a DPF or the like, and preferably a honeycomb or a DPF. The material of such a catalyst support, e.g., alumina _{(Al 2 O} 3), mullite _{(3Al 2 O 3 -2SiO 2)} , cordierite _{(2MgO-2Al 2 O 3 -5SiO} 2), titanate Examples thereof include ceramics such as aluminum (Al ₂ TiO ₅ ) and silicon carbide (SiC), and metal materials such as stainless steel.

  Further, by supporting at least one element selected from the group consisting of Ag, Pt, Au, Pd, Rh, Cu, and Mn on the catalyst layer, the combustion purification performance and gas purification performance of the particulates are further improved. Further, the amount of the supported metal element is 0.01 to 20%, preferably 0.1 to 10%, based on the total mass of the metal element and the support, so that the exhaust gas purification performance is improved.

  Further, a catalyst layer carrying at least one element selected from the group consisting of Ag, Pt, Au, Pd, Rh, Cu and Mn can be provided on the surface of the catalyst support. That is, a catalyst support made of a ceramic or metal material, a double oxide supported on the catalyst support, and Ag, Pt, Au, Pd, Rh, Cu and Mn supported on the double oxide An exhaust gas purifying catalyst excellent in combustion purification performance and gas purification performance of particulates having a structure having at least one element selected from the group consisting of

  Hereinafter, the present invention will be described in detail based on examples and comparative examples.

Examples 1-7 and Comparative Example 1
An yttrium nitrate solution with a known metal concentration and a manganese nitrate solution are mixed so that Y and Mn are Mn / Y = 2, respectively, and ion-exchanged water is obtained so that the final YMO is 50 g / L. The solution whose concentration was adjusted in step 1 was used as a raw material solution. Ion-exchanged water was added to an aqueous solution obtained by mixing 26.59 mL of 2.5% NH ₃ aqueous solution and 11.33 mL of 30% hydrogen peroxide solution to prepare a precipitant. Thereafter, a precipitant was added dropwise to the raw material liquid to form a precipitate. The obtained precipitate was filtered, washed, and then heated to obtain a powder.

  37.5 g of water was added to 0.124 g of silver nitrate and stirred to obtain an aqueous silver nitrate solution. Further, manganese nitrate was added so as to contain the excess Mn shown in Table 1 to obtain an aqueous solution. The powder was added in an amount of 1.5 g and heated to obtain exhaust gas purifying catalysts of Examples 1 to 7 and Comparative Example 1. The amount of Ag supported on the obtained exhaust gas purification catalyst was 5.57% by mass based on the total mass of metal Ag + carrier.

  Here, the excess amount of Mn (at%) is the amount of manganese contained excessively with respect to the amount of manganese in the case of Y: Mn = 1: 2, for example, Y: Mn = 1: 2.2 In this case, the excess amount of Mn is 10 at%.

Examples 8-11
An yttrium nitrate solution and a manganese nitrate solution with known metal concentrations are prepared and mixed so that Y and Mn are in a proportion of excess Mn as shown in Table 1, and the final YMO obtained is 50 g / A solution whose concentration was adjusted with ion-exchanged water so as to be L was used as a raw material liquid. On the other hand, ion-exchanged water was added to an aqueous solution in which 26.59 mL of 2.5% NH ₃ aqueous solution and 11.33 mL of 30% hydrogen peroxide solution were mixed to prepare 265.9 mL, which was used as a precipitant.

  Thereafter, a precipitant was added dropwise to the raw material solution to form a precipitate, the precipitate was filtered, washed, and then heated to obtain a powder.

  Add 37.5 g of ion-exchanged water to 0.124 g of silver nitrate, stir into a silver nitrate aqueous solution, add 1.5 g of each of the above powders, and heat to obtain exhaust gas purifying catalysts of Examples 8-11. It was. The amount of Ag supported on the obtained exhaust gas purification catalyst was 5.57% by mass based on the total mass of metal Ag + carrier.

<XRD measurement 1>
FIG. 1 shows XRD patterns after the exhaust gas purifying catalysts of Examples 1 to 4 and 7 and Comparative Example 1 were subjected to an endurance treatment at 700 ° C. for 30 hours in the atmosphere. Table 1 also shows the ratio of the Ag (220) plane peak intensity and the YMO (211) plane peak intensity of each of Examples 1 to 11 and Comparative Example 1.

As a result, the exhaust gas purifying catalysts of Examples 1 to 4 and 7 and Comparative Example 1 were identified as double oxides having a DyMn ₂ O ₅ structure. Omitted XRD for Example 5 and 6, was identified as a mixed oxide taking Similarly DyMn ₂ O ₅ structure.

Moreover, in Examples 1-4, since the peak resulting from excess Mn was not observed and the peak resulting from the DyMn ₂ O ₅ structure was shifted to the high angle side, all excess Mn It was confirmed that it was dissolved in the crystal. In Examples 5 and 6 where the composition ratio Mn / Y = 3.2 and Mn / Y = 3.6 and Example 7 where the composition ratio Mn / Y = 3.7, the peak of manganese oxide (Mn ₂ O ₃ ) It was confirmed that a part of the excess Mn coexisted with the double oxide as manganese oxide, but the shift to the high angle side of the peak due to the DyMn ₂ O ₅ structure remains. Therefore, it is considered that at least a part of the excess Mn is dissolved in the double oxide crystal. Therefore, in this case, Mn / Y in the double oxide is slightly smaller than the blending ratio.

  Further, in Examples 1 to 7, the peak attributed to Ag is smaller than the peak of Comparative Example 1 having the same amount of Ag. From this, at least a part of Ag is dissolved in the crystal. It is judged that

  Moreover, the XRD pattern after performing the endurance process for 30 hours at 700 degreeC in air | atmosphere about the exhaust gas purification catalyst of Examples 8-11 and the comparative example 1 is shown in FIG. Table 1 also shows the ratio of the Ag (220) plane peak intensity and the YMO (211) plane peak intensity of each Example and Comparative Example.

As a result, the exhaust gas purifying catalysts of Examples 8 to 11 were identified as Y / Mn> 2, but a double oxide having a DyMn ₂ O ₅ structure.

Also, in Examples 8 to 11, no peak due to excess Mn was observed, and the peak due to DyMn ₂ O ₅ structure was shifted to the high angle side. It was confirmed that the solid solution was dissolved.

  Furthermore, also in Examples 8 to 11, the peak attributed to Ag is smaller than the peak of Comparative Example 1 having the same amount of Ag. From this, at least a part of Ag is a crystal of a double oxide. It is judged that it is in solid solution.

Examples 12-20
Using an yttrium nitrate solution, a manganese nitrate solution, and a lanthanum nitrate solution, Y and La have the ratio shown in column a of Table 2, and (Y + La) and Mn have the ratio shown in column b of Table 2. except that mixed respectively, in the same manner as in example 1 _to give the _{(Y 1-x La x)} Mn 2 + y O 5 exhaust gas purifying catalyst of example 12 to 20 comprising a mixed oxide of. The amount of Ag supported on the obtained exhaust gas purification catalyst was 5.57% by mass based on the total mass of metal Ag + carrier.

Comparative Examples 2-6
Using an yttrium nitrate solution, a manganese nitrate solution, and a lanthanum nitrate solution, Y and La are the same as in Examples 12 to 16, and (Y + La) and Mn are mixed so that Mn / (Y + La) = 2. In the step of adding the aqueous silver nitrate solution, Comparative Example 2 comprising a double oxide of (Y _1-x La _x ) Mn ₂ O ₅ was carried out in the same manner as in Examples 12 to 16, except that excess Mn was not added. ~ 6 exhaust gas purifying catalysts were obtained. The amount of Ag supported on the obtained exhaust gas purification catalyst was 5.57% by mass based on the total mass of metal Ag + carrier.

<XRD measurement 2>
FIG. 3 shows XRD patterns after endurance treatment at 700 ° C. for 30 hours in the atmosphere for the exhaust gas purifying catalysts of Examples 12 to 16 and Example 4. Moreover, the XRD pattern measured similarly about the exhaust gas purification catalyst of Comparative Examples 1-6 is shown in FIG.

As a result, a peak attributable to the DyMn ₂ O ₅ structure was confirmed for Examples 12 to 16 in which Mn was excessive. Similar results were confirmed for Examples 17-20.

As for Comparative Examples 2 to 6 Mn is not excessive, DyMn ₂ O ₅ peaks due to the structure is confirmed that smaller compared with Example 12 to 16 of the same La substitution amount, increased amorphous component I found out that This indicates that it is difficult to grow into a crystal having a DyMn ₂ O ₅ structure unless Mn is excessive.

Furthermore, it was confirmed that the peaks due to the DyMn ₂ O ₅ structures of Examples 12 to 16 remained shifted to the high angle side. From this, it is considered that at least a part of the excess Mn is dissolved in the double oxide crystal. Therefore, in this case, Mn / (Y + La) in the double oxide is somewhat smaller than the notation of the blending ratio in Table 2. Moreover, the same result was confirmed also about Examples 17-20.

  Furthermore, in Examples 12 to 20, the peak attributed to Ag is the same Ag loading, and is smaller than the peaks of Comparative Examples 2 to 6 having the same La substitution amount. It is judged that at least a part of Ag is dissolved in the crystal.

<Fixed bed simulation gas purification performance evaluation test 1>
Regarding the exhaust gas purifying catalysts of Examples 1 to 20 and Comparative Examples 1, 5, and 6, the catalytic activity of the exhaust gas purifying catalyst after the endurance treatment at 700 ° C. for 30 hours in the atmosphere is as follows. evaluated.

First, using a fixed bed flow type reactor, 0.1 g of catalyst powder was set in a reaction tube, simulated exhaust gas having the composition shown in Table 3 below was passed at 1 L / min, heated to 500 ° C. and held for 10 minutes. Then, pretreatment was performed. Then, after cooling, the temperature was raised from 100 ° C. to 500 ° C. at 10 ° C./min, and the outlet gas component at 100 to 500 ° C. was measured using a CO / HC / NO analyzer. From the obtained purification performance evaluation results, the temperature at which the CO and HC 50% purification rate (T50) and the NO purification rate at 400 ° C. were obtained. The results were as shown in Tables 1 and 2. Incidentally, most of the clarified NO was found to be converted to NO _2.

  Tables 1 and 2 also show specific surface areas (measured by the BET method) after endurance treatment at 700 ° C. for 30 hours.

  As a result, it was found that the exhaust gas purifying catalysts of Examples 1 to 11 in which Mn was excessive had improved catalytic activity as compared with Comparative Example 1. Further, the catalyst activity improves as the excess amount increases until Mn exceeds 50 at%. However, in Examples 5, 6, and 7 in which 60 at%, 80 at%, and 85 at% excess, the catalyst activity is lower than that in Example 4. The excess amount of Mn was found to be 2 to 85 at%, preferably 5 to 50 at%.

  Moreover, also about Examples 12-20 which substituted a part of Y of A site with La, it turned out that the catalyst activity is improving compared with the comparative example 2-comparative example 6 which is the same La substitution amount. . Table 2 shows values of Examples 15 and 16 and Comparative Examples 5 and 6 as representative examples.

<Soot flammability evaluation test>
The particulate flammability of the exhaust gas purifying catalysts of Examples 1 to 20 was superior to that of conventionally used Pt-supported alumina.

Examples 21 and 22
Exhaust gas purifying catalysts of Examples 21 and 22 were obtained in the same manner as in Example 4 except that the amount of Ag supported was 0% by mass and 2% by mass based on the total mass of Ag + carrier.

<Fixed bed simulation gas purification performance evaluation test 2>
The exhaust gas purification catalysts of Examples 21 and 22 were subjected to an endurance treatment at 700 ° C. for 30 hours in the atmosphere, and then the catalytic activity of the exhaust gas purification catalyst was evaluated by a fixed bed simulated gas purification performance evaluation test. The results are shown in Table 4. From this result, it was found that the exhaust gas purification performance is improved by the support of Ag. It has also been found that the supported amount of Ag is 1 to 20% by mass, preferably 1 to 10% by mass, based on the total mass of Ag + carrier.

Claims (5)

The A site includes Y and the B site includes Mn. The crystal has a DyMn ₂ O ₅ structure, and includes a multiple oxide having a B / A composition ratio B / A larger than 2. Exhaust gas purification catalyst.   The exhaust gas purifying catalyst according to claim 1, comprising Ag.   The exhaust gas purifying catalyst according to claim 2, wherein at least a part of Ag is dissolved in the double oxide crystal.   The exhaust gas purifying catalyst according to any one of claims 1 to 3, wherein a catalyst layer supported on a catalyst support includes the double oxide, and Ag is supported on the catalyst layer. .   The catalyst layer supported on the catalyst support includes the double oxide, and at least one element selected from Pt, Au, Pd, and Rh is supported on the catalyst layer. 5. The exhaust gas purifying catalyst according to any one of 4 above.

Images