JP2016137458A - Exhaust gas purification catalyst - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an exhaust gas purification catalyst having a high NOx purification ratio, by obtaining a new index for developing a highly-active NOx catalyst.SOLUTION: There is provided an exhaust gas purification catalyst 1 including a carrier 13 containing a metal oxide, and at least one kind of active metal 12 selected from rhodium, palladium and platinum carried on the carrier or a compound of the active metal. In the exhaust gas purification catalyst 1, an oxide layer 11 of the active metal is provided on the surface of the exhaust gas purification catalyst 1, and the thickness of the oxide layer of the active metal is 2.8-12 atomic layers.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an exhaust gas purification catalyst. The present invention particularly relates to an exhaust gas purification catalyst having a predetermined structure specified by an X-ray absorption fine structure (XAFS) method and exhibiting a high nitrogen oxide (NOx) purification rate.

  Conventionally, a catalyst (lean NOx catalyst) for purifying NOx in an oxygen-excess atmosphere has been demanded, and various catalyst systems have been proposed. In general, platinum (Pt) catalysts are highly active, and it is known that NOx purification activity varies depending on the type of carrier. Specifically, a catalyst in which a noble metal such as platinum is supported on a carrier such as alumina is known as an exhaust gas purification catalyst (see, for example, Patent Documents 1 and 2).

  The active point of the NOx purification reaction is Pt in a metallic state, and it is proposed that when Pt is supported on a so-called acidic carrier, Pt is stabilized in the metallic state and a highly active catalyst can be obtained (for example, Non-Patent Document 1). See). Further, according to Non-Patent Document 1, it is disclosed that the existence ratio of metal Pt investigated by X-ray photoelectron spectroscopy (XPS) method correlates with NOx purification activity.

JP 2002-126533 A Japanese Unexamined Patent Publication No. 7-112131

Yuki Tanabe, Effect of Support and Pt Particle Size on Selective Reduction NOx Catalyst, Toyota Central R & D Review 33 (1998)

  In general, it is considered that the higher the dispersion ratio of the metal supported on the carrier, the better the catalytic activity. Non-Patent Document 1 suggests that there is a good correlation between the acid strength of the carrier and the NOx purification rate. In practice, however, they sometimes deviated from these trends. Therefore, another index was required to develop a highly active NOx catalyst.

  In order to solve the above problems, the present inventors have focused on the relationship between the thickness of the oxide layer of the active metal on the support surface and the NOx purification rate, and found that there is a very good correlation between them. . Furthermore, the present inventors have found that an exhaust gas purification catalyst having a specific active metal oxide layer thickness exhibits a particularly high NOx purification activity, and completed the present invention.

  According to one embodiment of the present invention, there is provided an exhaust gas purification catalyst, a support comprising a metal oxide, and at least one active metal selected from rhodium, palladium, and platinum supported on the support, or The active metal oxide layer is provided on the surface of the exhaust gas purification catalyst, and the thickness of the active metal oxide layer is 2.8 to 12 atomic layers.

  In the exhaust gas purifying catalyst, the active metal oxide layer preferably has a thickness of 4.0 to 7.4 atomic layers.

  In the exhaust gas purification catalyst, it is preferable that the active metal includes at least platinum, and the oxide layer of the active metal includes at least a platinum oxide layer.

In the exhaust gas purification catalyst, the carrier includes a silica-alumina composite oxide and a zirconium or zirconium compound, and a cation in the silica-alumina composite oxide in the carrier, and a cation in the zirconium or zirconium compound. The molar ratio is preferably 95: 5 to 50:50.
The molar ratio of the cation in the silica-alumina composite oxide to the cation in the zirconium or zirconium compound is more preferably 85:15 to 55:45.

In the exhaust gas purification catalyst, the silica-alumina composite oxide is preferably a hydrophobic zeolite having a silica / alumina ratio of 5 or more.
More preferably, the silica / alumina ratio is 23 or more.

  The exhaust gas purification catalyst is preferably used in an oxygen-excess atmosphere.

  According to the present invention, the thickness of the active metal oxide layer, which is an index uniquely developed by the present inventors, is set to 2.8 to 12 atomic layers, so that the exhaust gas purification preferably used in an oxygen-excess atmosphere. A high NOx purification rate can be achieved in the catalyst. In addition, by simply obtaining the value of the thickness of the oxide layer of the active metal by the XAFS method, it is advantageous for screening of exhaust gas purification catalysts and performance confirmation.

FIG. 1 (a) is a diagram schematically showing the sample surface of the exhaust gas purifying catalyst of the present invention when measuring the ratio of the active metal oxide, and FIG. 1 (b) shows the dispersion ratio of the active metal. It is a figure which shows typically the sample surface of the catalyst for exhaust gas purification of this invention at the time of measuring. FIG. 2 is a graph showing a model gas purification rate curve of the exhaust gas purification catalyst of Comparative Example 1. FIG. 3 is a diagram showing an XAFS measurement spectrum of the exhaust gas purifying catalyst of Comparative Example 1. FIG. 4 is a diagram showing an XAFS measurement spectrum after reducing the exhaust gas purifying catalyst of Comparative Example 1 at 500 ° C. for 1 hour with 3% H ₂ . FIG. 5 is a graph showing the relationship between catalyst activity and oxide layer thickness. FIG. 6 is a graph showing the catalyst activity and the ratio of oxidized Pt. FIG. 7 is a graph showing the relationship between catalyst activity and dispersion rate. FIG. 8 is an enlarged graph showing the vicinity of the maximum value of the NOx purification rate of FIG. FIG. 9 is a graph showing the relationship between the Zr / Zr ⁺ high silica zeolite ratio and the NOx purification rate in the Pt / Zr / high silica zeolite catalyst. FIG. 10 is a graph showing the relationship between the Zr / Zr ⁺ high silica zeolite ratio and the oxide layer thickness in a Pt / Zr / high silica zeolite catalyst.

  Hereinafter, embodiments of the present invention will be described. However, the present invention is not limited to the embodiments described below.

[First embodiment: Exhaust gas purification catalyst]
The present invention is an exhaust gas purifying catalyst according to the first embodiment. The exhaust gas purifying catalyst includes a support comprising a metal oxide, at least one active metal selected from rhodium (Rh), palladium (Pd), and platinum (Pt) supported on the support or the active metal. The active metal oxide layer is provided on the surface of the exhaust gas purification catalyst, and the thickness of the active metal oxide layer is 2.8 to 12 atomic layers.

  In general, an inorganic porous carrier made of a metal oxide is used as the carrier. For example, one or more compounds selected from the group consisting of silicon oxide, zirconium oxide, cobalt oxide, aluminum oxide, titanium oxide, chromium oxide, magnesium oxide, and tungsten oxide, or OSC (oxygen storage such as ceria-zirconia composite oxide) Inorganic porous carrier made of / release material). In addition to ceria-zirconia, silica-alumina, alumino-silicates, alumina-zirconia, alumina-chromia, alumina-ceria, and the like can be used as the composite oxide, but not limited thereto.

  As the carrier, amphoteric oxides are preferably used. If the support is an amphoteric oxide, the supported active metal is likely to be in an intermediate or mixed state between the metal state and the oxide state, and the active metal is likely to dissociate and adsorb nitrogen oxides. is there. As specific amphoteric oxides, zirconium oxide, silica-alumina composite oxide, alumina, silica, and titania are preferable. In particular, it is preferable to use a hydrophobic zeolite having a silica / alumina ratio of 5 or more (also referred to as high silica zeolite) as a carrier, and more preferably to use a hydrophobic zeolite having a silica / alumina ratio of 23 or more as a carrier. It is known that acid sites exist on the surface of zeolite. An active metal (such as platinum) in the vicinity of the acid point is likely to be in a metal state, but an active metal sufficiently away from the acid point is likely to be in an oxide state. For this reason, when an active metal is supported on zeolite, the active metal easily takes an intermediate or mixed state between a metal state and an oxide state, and this shows high catalytic activity.

  The active metal or the compound of the active metal constituting the exhaust gas purifying catalyst in the first embodiment is generally a platinum group element, and rhodium, palladium, or platinum having high NOx purification activity can be used. In the case of a compound, for example, a platinum-palladium alloy, a platinum-rhodium alloy, a platinum-iridium alloy, a platinum-ruthenium alloy, or a palladium-rhodium alloy can be used. Of these, only one type may be used, or two or more types may be used in combination. Of these, platinum, which has high NOx purification activity and is considered useful, is particularly preferable.

  The amount of the active metal supported on the carrier is, for example, 1.7% by mass to 5.2% by mass, and preferably 3.2% by mass to 4.0% by mass, but is not limited thereto.

  In the exhaust gas purifying catalyst according to the present embodiment, the carrier may contain a further metal element in addition to the metal oxide. In the exhaust gas purifying catalyst of the present invention, the further metal element constitutes a part of the support and modifies the surface state of the oxide support. In particular, by changing the solid acidity and the solid basicity, the electronic state of the active metal can be changed to improve the catalyst characteristics. As a further metal element, a metal element in which an oxide is formed in a practical atmosphere and the melting point of the oxide is high is preferable. Examples thereof include silicon (Si), cerium (Ce), and zirconium (Zr). It is not limited to. In particular, the further metal element preferably contains zirconium. This is because zirconium forms zirconium oxide in a practical atmosphere and has a very high melting point of about 2700 ° C., so that a catalyst having excellent heat resistance can be obtained. These metal elements may be in the form of metal elements or in the form of compounds containing metal elements. In the case of a compound containing a metal element, specifically, an oxide (zirconium oxide), an oxynitrate compound (zirconium oxynitrate), or an oxysulfate compound (zirconium oxysulfate) is preferable.

  The amount of the metal element supported on the metal oxide support is, for example, 5 mol% to 50 mol%, preferably 15 mol% to 45 mol%, when all the cation atoms contained in the catalyst are 100 mol%. However, the amount is not limited.

In a particularly preferred embodiment of the present invention, the exhaust gas purification catalyst contains Pt as an active metal, has a platinum oxide (PtO ₂ ) layer on the surface, and contains high silica zeolite and zirconium or a zirconium compound as a support. Zirconium compounds include, but are not limited to, zirconium oxide. At this time, the molar ratio of the cation in the silica-alumina composite oxide in the support to the cation in the zirconium compound is preferably 95: 5 to 50:50, and 85:15 to 55:45. More preferably. This is because high NOx purification activity is obtained within this range.

An active metal oxide layer is provided on the surface of the exhaust gas purification catalyst, and the thickness of the active metal oxide layer is 2.8 to 12 atomic layers. The provision of the active metal oxide layer on the surface of the exhaust gas purification catalyst can be confirmed by measuring the exhaust gas purification catalyst by the XAFS method described in detail later. In the present invention, the thickness of the oxide layer of the active metal is a unique parameter defined by the following formula (1).
Active metal oxide layer thickness = active metal oxide ratio ÷ active metal dispersion (1)

  In the formula (1), “the ratio of the active metal oxide” refers to the active metal element supported on the support obtained by measuring the exhaust gas purifying catalyst according to the present invention in the atmosphere by the XAFS method. The percentage (number of atoms / number of atoms) of the active metal element present as an oxide shall be said. Referring to FIG. 1A, an exhaust gas purifying catalyst according to an embodiment of the present invention has an active metal 12 and its oxide 11 supported on a carrier 13. In the prepared and sintered exhaust gas purifying catalyst, the active metal oxide 11 is roughly composed of three layers to constitute the surface of the catalyst. The “ratio of active metal oxide” in the formula (1) is represented by the number of active metal oxides 11 / the number of active metals 12 + the number of active metal oxides 11 × 100 (%) in FIG. .

  The measurement conditions for the ratio of the active metal oxide are as follows. The measurement sample is prepared by preparing an exhaust gas purification catalyst by a general impregnation method or coprecipitation method, calcining in air at 650 ° C. for 2 hours, and further cooling to room temperature at 10 ° C./min. by. The sample is measured in the atmosphere by the XAFS method to obtain the XAFS spectrum, and then analyzed by the spectrum separation method, whereby the active metal oxide ratio can be obtained.

The spectral separation method can be performed by using the absorption spectrum of the active metal flakes and the absorption spectrum of the active metal oxide as a basis, and expressing the absorption spectrum of the catalyst sample by a linear combination. Specifically, the analysis can be performed using “Athena” which is an XAFS analysis software developed by Bruce Ravel (NIST, USA), but other XAFS analysis software known to those skilled in the art is used. You can also. In the spectral separation method, the ratio of the active metal oxide can be derived from the XAFS spectrum by the following equation (2).
Ratio of active metal oxide = (absorption coefficient of sample catalyst−absorption coefficient of active metal flakes) /
(Absorption coefficient of active metal oxide-absorption coefficient of active metal flakes) (2)
In the formula (2), the absorption coefficients of the sample catalyst, the active metal flakes, and the active metal oxide can be obtained from the XAFS spectrum. The above equation (2) is an equation when the range of spectrum separation is narrowed to an arbitrary point, and the actual function equation is calculated by the above software.

  Next, in the formula (1), the “active metal dispersion ratio” means that the exhaust gas purifying catalyst according to the present invention is supported on a support obtained by reducing the exhaust gas purifying catalyst according to the invention under a predetermined condition and then measuring by the XAFS method. The percentage (number of atoms / number of atoms) of the active metal elements exposed on the surface among the active metal elements shall be said. Referring to FIG. 1B, in the exhaust gas purifying catalyst after hydrogen reduction, the active metal oxide 11 is composed of only one atomic layer and constitutes the surface of the catalyst. The “active metal dispersion ratio” in the formula (1) is represented by the number of active metal oxides 11 / the number of active metals 12 + the number of active metal oxides 11 × 100 (%) in FIG.

  The measurement conditions of the active metal dispersion rate are as follows. The measurement sample was reduced by reducing the sample used for the measurement of the ratio of the active metal oxide at 500 ° C. for 1 hour in a 3% hydrogen / nitrogen atmosphere and further cooling to room temperature at 10 ° C./min. Prepare sample. Similarly to the measurement of the ratio of the active metal oxide, the reduced sample is measured by the XAFS method in the atmosphere to obtain the XAFS spectrum, and then the active metal dispersion is obtained by analyzing the spectrum separation method. Can do.

  In the present invention, the thickness of the active metal oxide layer defined by “the ratio of active metal oxide (%)” / “active metal dispersion ratio (%)” obtained by XAFS analysis is 2.8 to 12. Atomic layer. This is because a high NOx purification rate can be obtained within this range. More preferably, the thickness of the active metal oxide layer is 4.0 to 7.4 atomic layers. This is because a higher NOx purification rate can be obtained within this range. The thickness of the active metal oxide layer varies depending on the type and composition percentage of the active metal, the support, and optionally the further metal element constituting the exhaust gas purification catalyst. Depending on the combination of these conditions, the thickness of the active metal oxide layer is 2.8-12. Atomic layers can be achieved. It can be said that the thickness of the active metal oxide layer obtained from FIGS. 1A and 1B is a triatomic layer, and an exhaust gas purification catalyst having such an atomic layer thickness constitutes the present invention. .

  Next, the present embodiment will be described from the viewpoint of a method for producing an exhaust gas purification catalyst. The method for producing an exhaust gas purification catalyst according to the present embodiment includes a step of preparing a catalyst by an impregnation method or a coprecipitation method, and a step of calcining the prepared catalyst, and optionally the obtained exhaust gas purification catalyst And obtaining the thickness of the active metal oxide layer by XAFS measurement.

  In the step of preparing the catalyst by impregnation or coprecipitation methods, according to conventional methods known to those skilled in the art, the active metal, the support, and optionally further metal elements are selected to achieve the desired composition, A catalyst is prepared. In the step of calcining the prepared catalyst, calcining is performed at a predetermined temperature, time, and number of times according to a conventional method known to those skilled in the art. Thereby, an exhaust gas purification catalyst can be manufactured.

  Optionally, the method may further include obtaining a thickness of the active metal oxide layer. Through this step, the NOx purification performance of the obtained exhaust gas purification catalyst can be predicted. The step of obtaining the thickness of the active metal oxide layer can be performed according to the XAFS measurement and analysis described above, and the above equations (1) and (2).

  According to the first embodiment of the present invention, an exhaust gas purification catalyst having a high NOx purification performance of an exhaust gas purification catalyst can be obtained by setting the thickness of the predetermined active metal oxide layer.

[Second Embodiment: Characteristic Evaluation and Screening Method of Exhaust Gas Purification Catalyst]
According to a second embodiment of the present invention, there is provided an exhaust gas purification catalyst characteristic evaluation and screening method, the step of obtaining an exhaust gas purification catalyst, and the thickness of the active metal oxide layer by XAFS measurement of the exhaust gas purification catalyst. Obtaining the step.

  The step of obtaining the exhaust gas purification catalyst is as described in the first embodiment, and can be performed by the step of preparing the catalyst by the impregnation method or the coprecipitation method and the step of calcining the prepared catalyst. The step of obtaining the thickness of the active metal oxide layer by the XAFS measurement of the exhaust gas purification catalyst is performed by performing XAFS measurement of the exhaust gas purification catalyst, the active metal oxide and the active metal as described in the first embodiment. The thickness of the active metal oxide layer can be calculated according to the definition formula: This process is performed automatically by creating software that can calculate from the XAFS analysis to the derivation of the thickness of the active metal oxide layer according to Equation (1), for example, by a predetermined programming technique. You can also Such software can be useful in catalyst development.

  According to the characteristic evaluation and screening method of the exhaust gas purification catalyst according to the second embodiment of the present invention, the thickness of the active metal oxide layer, which is an index newly invented by the present inventors, is used as an index for catalyst development. Can do. Note that the method according to the present embodiment can also be applied to a catalyst mounted on a vehicle such as a commercial automobile or a motorcycle, and / or a catalyst already mounted on the catalyst, and in this case, the activity As a pretreatment for measuring the ratio of metal oxide, a commercially available catalyst sample or a catalyst sample after use was calcined at 650 ° C. for 2 hours in air, and further cooled to room temperature at 10 ° C./min. obtain. In addition, as a pretreatment for measuring the active metal dispersion, the sample used for measuring the ratio of the active metal oxide was reduced at 500 ° C. for 1 hour in a 3% hydrogen / nitrogen atmosphere, and further 10 ° C. / A reduced sample is prepared by cooling to room temperature with min, and this is measured in the atmosphere by the XAFS method to obtain an XAFS spectrum. Thereafter, in the same manner as described in the first embodiment, spectral separation is performed, and the thickness of the active metal oxide layer can be obtained according to the formula (1).

  In the following, the present invention will be described in more detail with reference to examples. However, the following examples do not limit the present invention.

  The catalysts of Examples and Comparative Examples were prepared by a general impregnation method or coprecipitation method. The Pt carrying concentration was all 1.5 mol%.

[Example 1]
A mixture of 75 mmol of aluminum nitrate nonahydrate manufactured by Wako Pure Chemical Industries, 25 mmol of tetraethoxysilane manufactured by Wako Pure Chemical Industries, Ltd., and dinitrodiamine Pt solution manufactured by Tanaka Kikinzoku Kogyo Co., Ltd. corresponding to 1.5 mmol in terms of Pt, Was added to make 100 ml, and 30 ml of 25% aqueous ammonia manufactured by Wako Pure Chemical Industries, Ltd. was added to cause precipitation. The solution containing the precipitate was evaporated to dryness at 120 ° C. for 24 hours using an electric furnace, and then calcined at 650 ° C. for 2 hours to obtain a catalyst.

[Example 2]
100 mmol of nitric acid Ce solution made of the first rare element and dinitrodiamine Pt solution made by Tanaka Kikinzoku Kogyo Co., Ltd. corresponding to 1.5 mmol as the amount of Pt were used as raw materials. Other than that was the same as Example 1.
[Example 3]
1.5 mmol of a dinitrodiamine Pt solution manufactured by Tanaka Kikinzoku Kogyo Co., Ltd. was added dropwise to anatase-type TiO ₂ powder manufactured by Wako Pure Chemicals. After evaporating to dryness at 120 ° C. for 24 hours using an electric furnace, the catalyst was calcined at 650 ° C. for 2 hours.
[Example 4]
Example 3 was the same as Example 3 except that Tosoh high-silica zeolite (820NHA) was used as the base material.

[Example 5]
Tosoh high silica zeolite (820NHA) and first rare element Zr oxynitrate solution were mixed at a molar ratio of 95: 5 (however, the composition of 820NHA was (NH ₃ -AlO _1.5 ) _1/24 From (SiO ₂ ) _23/24 , the molecular weight was 60.42), and the sum of Al, Si and Zr ions was 100 mmol. Here, 1.5 mmol of a dinitrodiamine Pt solution manufactured by Tanaka Kikinzoku Kogyo Co., Ltd. was weighed as Pt amount, and ion-exchanged water was added to make 100 ml. Then, 30 ml of 25% ammonia water manufactured by Wako Pure Chemical Industries, Ltd. was added to cause precipitation ( So-called coprecipitation method). The solution containing the precipitate was evaporated to dryness at 120 ° C. for 24 hours using an electric furnace, and then calcined at 650 ° C. for 2 hours to obtain a catalyst.

[Example 6]
The same as Example 5 except that the molar ratio of high silica zeolite to Zr was 90:10.
[Example 7]
The same as Example 5 except that the molar ratio of high silica zeolite to Zr was 85:15.
[Example 8]
The same as Example 5 except that the molar ratio of high silica zeolite to Zr was 80:20.
[Example 9]
The same as Example 5 except that the molar ratio of high silica zeolite to Zr was 75:25.
[Example 10]
The same as Example 5 except that the molar ratio of high silica zeolite to Zr was 70:30.
[Example 11]
The same as Example 5, except that the molar ratio of high silica zeolite Zr was 65:35.
[Example 12]
The same as Example 5, except that the molar ratio of the high silica zeolite Zr was 60:40.
[Example 13]
The same as Example 5, except that the molar ratio of high silica zeolite Zr was 55:45.
[Example 14]
The same as Example 5 except that the molar ratio of high silica zeolite Zr was 50:50.

  [Comparative Example 1] 100 mmol of aluminum nitrate nonahydrate manufactured by Wako Pure Chemical Industries, Ltd. and dinitrodiamine Pt solution manufactured by Tanaka Kikinzoku Kogyo Co., Ltd. corresponding to 1.5 mmol as the amount of Pt were used as raw materials. Other than that was the same as Example 1.

[Comparative Example 2]
As a raw material, 75 mmol of aluminum nitrate nonahydrate manufactured by Wako Pure Chemical Industries, 25 mmol of Mg nitrate manufactured by Wako Pure Chemical Industries, Ltd., and a dinitrodiamine Pt solution manufactured by Tanaka Kikinzoku Kogyo Co., Ltd. corresponding to 1.5 mmol of Pt amount were used. Other than that was the same as Example 1.
[Comparative Example 3]
As a raw material, 75 mmol of aluminum nitrate nonahydrate manufactured by Wako Pure Chemical Industries, 25 mmol of Cr chloride manufactured by Wako Pure Chemical Industries, Ltd. and dinitrodiamine Pt solution manufactured by Tanaka Kikinzoku Kogyo Co., Ltd. corresponding to 1.5 mmol of Pt amount were used. Other than that was the same as Example 1.
[Comparative Example 4]
Using 75 mmol of aluminum nitrate nonahydrate manufactured by Wako Pure Chemical Industries, 25 mmol of Mn nitrate hexahydrate manufactured by Wako Pure Chemical Industries, and dinitrodiamine Pt solution manufactured by Tanaka Kikinzoku Kogyo Co., Ltd. corresponding to 1.5 mmol of Pt Using. Other than that was the same as Example 1.
[Comparative Example 5]
Using 75 mmol of aluminum nitrate nonahydrate manufactured by Wako Pure Chemical Industries, 25 mmol of Cu nitrate trihydrate manufactured by Wako Pure Chemical Industries, Ltd., and dinitrodiamine Pt solution manufactured by Tanaka Kikinzoku Kogyo Co., Ltd. corresponding to 1.5 mmol as the amount of Pt Using. Other than that was the same as Example 1.
[Comparative Example 6]
Using 75 mmol of aluminum nitrate nonahydrate manufactured by Wako Pure Chemical Industries, 25 mmol of Zn nitrate hexahydrate manufactured by Wako Pure Chemical Industries, Ltd., and dinitrodiamine Pt solution manufactured by Tanaka Kikinzoku Kogyo Co., Ltd. corresponding to 1.5 mmol as the amount of Pt Using. Other than that was the same as Example 1.
[Comparative Example 7]
The raw material was 75 mmol of aluminum nitrate nonahydrate manufactured by Wako Pure Chemical Industries, 25 mmol of Zr solution of oxynitrate made of the first rare element, and dinitrodiamine Pt solution of Tanaka Kikinzoku Kogyo Co., Ltd. corresponding to 1.5 mmol as the amount of Pt. . Other than that was the same as Example 1.
[Comparative Example 8]
As a raw material, 75 mmol of aluminum nitrate nonahydrate manufactured by Wako Pure Chemical Industries, 25 mmol of Ba nitrate manufactured by Wako Pure Chemical Industries, Ltd., and a dinitrodiamine Pt solution manufactured by Tanaka Kikinzoku Kogyo Co., Ltd. corresponding to 1.5 mmol in Pt amount were used. Other than that was the same as Example 1.
[Comparative Example 9]
As raw materials, 75 mmol of aluminum nitrate nonahydrate made by Wako Pure Chemical Industries, 25 mmol of Ce nitrate solution made of the first rare element, and dinitrodiamine Pt solution made by Tanaka Kikinzoku Kogyo Co., Ltd. corresponding to 1.5 mmol as the amount of Pt were used. Other than that was the same as Example 1.
[Comparative Example 10]
The same procedure as in Example 3 was performed except that 100 mmol of boron nitride manufactured by Wako Pure Chemical Industries, Ltd. was used as the base material.
[Comparative Example 11]
As a raw material, 100 mmol of Mn nitrate nonahydrate manufactured by Wako Pure Chemical Industries, Ltd. and dinitrodiamine Pt solution manufactured by Tanaka Kikinzoku Kogyo Co., Ltd. corresponding to 1.5 mmol as the amount of Pt were used. Other than that was the same as Example 1.
[Comparative Example 12]
The same procedure as in Example 3 was performed except that 100 mmol of yttria-stabilized zirconia (TZ-4Y) manufactured by Tosoh was used as a base material.
[Comparative Example 13]
100 mmol of nitric acid La solution manufactured by the first rare element and dinitrodiamine Pt solution manufactured by Tanaka Kikinzoku Kogyo Co., Ltd. corresponding to 1.5 mmol as the amount of Pt were used as raw materials. Other than that was the same as Example 1.
[Comparative Example 14]
The same procedure as in Example 3 was performed except that 100 mmol of Nb ₂ O ₅ powder made by Wako Pure Chemical Industries was used as the base material.
[Comparative Example 15]
Example 3 was the same as Example 3 except that 100 mmol of MoO ₃ powder made by Wako Pure Chemical Industries Ltd. was used as the base material.
[Comparative Example 16]
The same as Example 5 except that the molar ratio of high silica zeolite to Zr was 25:75.
[Comparative Example 17]
The same as Example 5 except that the molar ratio of high silica zeolite to Zr was 1: 9.
[Comparative Example 18]
100 mmol of oxynitrate Zr solution made of the first rare element and dinitrodiamine Pt solution made by Tanaka Kikinzoku Kogyo Co., Ltd. corresponding to 1.5 mmol as the amount of Pt were used as raw materials. Other than that was the same as Example 1.

[Experimental example]
The results of measuring various physical properties of the produced catalysts of Examples 1 to 14 and Comparative Examples 1 to 18 are described in detail in the following experimental examples.

[Model gas test]
The NOx purification activity of the produced catalyst was analyzed with a fixed bed flow type reactor (model gas test). Each catalyst was weighed so that the amount of Pt was 18 mg and filled in a quartz tube, and then the model shown in Table 1 was used while raising the temperature from room temperature to 500 ° C. at 10 ° C./m using an electric furnace (tubular furnace). Gas was allowed to flow under gas conditions. The relationship between the NOx purification rate and the temperature (purification rate curve) was determined by monitoring the exit gas of the quartz tube. The analysis of NOx was performed with Horiba VA-3012 (chemical fluorescence method).

  For reproducibility check, the model gas test was measured twice in succession for all samples, and the second result was adopted. Note that the difference in the NOx purification rate between the first time and the second time was within 5%.

As an example, a purification rate curve of a Pt / Al ₂ O ₃ catalyst (a catalyst of Comparative Example 1) is shown. According to the model gas conditions in Table 1, NO reduction activity using a small amount of coexisting hydrocarbon compound as a reducing agent in an oxygen-excess atmosphere is evaluated. This reaction is HC-SCR (hydrocarbon-selective reduction). Called. The NOx purification rate curve has a “mountain shape”, which is a characteristic of typical HC-SCR type lean NOx catalysts (Non-patent Document 1). The maximum value of the NOx purification rate was adopted as an index of the NOx purification performance. A model gas purification rate curve is shown in FIG. In FIG. 2, it was 38.9%. For other catalysts, NOx purification rates were obtained in the same manner. The results are shown in Table 2.

[Microstructure analysis]
In order to elucidate the oxidation / reduction state of Pt, X-ray absorption fine structure (XAFS) measurement was performed. The analysis experiment was performed with Aichi Synchrotron BL5S1. The samples subjected to XAFS measurement are the prepared catalyst sample (calcined in air at 650 ° C. for 2 hours) and a catalyst sample obtained by reducing them in a 3% hydrogen / nitrogen atmosphere at 500 ° C. for 1 hour.

An example of analyzing the Pt / Al ₂ O ₃ catalyst (the catalyst of Comparative Example 1) is shown in FIG. The XAFS spectrum (broken line) of the catalyst sample immediately after preparation is an intermediate spectrum between Pt foil, which is a standard sample, and oxidized Pt. This indicates that in the catalyst sample of Comparative Example 1, Pt is a mixed state of oxidized Pt and metal Pt. In general, it is known that Pt is reduced from PtO ₂ to Pt at 550 ° C. or higher in air. Since the catalyst sample is calcined in air at 650 ° C. for 2 hours, Pt becomes metal particles. However, since it oxidizes again at the time of cooling (especially when it cools from 550 degreeC to room temperature), it is thought that most became oxidation Pt. The cooling rate was 10 ° C./min. Analysis was performed by “spectral separation method”. The range of the spectral separation was changed from 20 electron volts before the absorption edge to 30 electron volts after the absorption edge. These analyzes were performed at “Linear Combination Fitting” using “Athena” which is XAFS analysis software developed by Bruce Ravel (NIST, USA). As a result, the existing ratio of oxidized Pt in the Pt / Al ₂ O ₃ catalyst sample was calculated to be 81%. For other catalysts, the proportion of oxidized Pt was obtained in the same manner. The results are shown in Table 2 (i).

XAFS measurement was also performed on the Pt / Al ₂ O ₃ catalyst after the model gas test. The resulting spectrum (dotted line in FIG. 3) was equivalent to that before the model gas test (broken line in FIG. 3). This shows that the redox state of Pt has not changed before and after the model gas test as far as the average information of the entire catalyst is considered. That is, Pt, which is the catalyst active point, is considered to produce an intermediate during the model gas test, but the state of Pt averaged over the entire catalyst is kept constant.

Next, FIG. 4 shows an XAFS spectrum after reducing the Pt / Al ₂ O ₃ catalyst (the catalyst of Comparative Example 1) at 500 ° C. for 1 hour with 3% H ₂ . At this time, the ratio of oxidized Pt of the catalyst was determined to be 6.4%. Pt is reduced by hydrogen reduction to form Pt particles in a metallic state, but is considered to have been naturally oxidized when taken out to the atmosphere for XAFS measurement. Since naturally oxidized Pt is one atomic layer on the surface of Pt particles, the ratio of oxidized Pt (6.4%) can be regarded as the Pt dispersion ratio (number of Pt atoms exposed on the surface / interface). For other catalysts, the Pt dispersion ratio was obtained in the same manner. The results are shown in Table 2 (ii).

In addition, the thickness of the oxidized Pt layer in the model gas tested catalyst can be estimated.
Thickness of oxidized Pt layer = ratio of oxidized Pt ÷ Pt dispersion rate In other words, if the proportion of oxidized Pt is the same as the dispersion rate, the thickness of oxidized Pt layer is one atomic layer, and the proportion of oxidized Pt is twice the dispersion rate. Then, the thickness of the oxidized Pt layer is a diatomic layer. Accordingly, the oxidized Pt layer thickness of the Pt / Al ₂ O ₃ catalyst (the catalyst of Comparative Example 1) is 81% ÷ 6.4% = 12.7 (atomic layer). For other catalysts, the oxidized Pt layer thickness was obtained in the same manner. The results are shown in Table 2 (iii).

[Comparison of XAFS method and model gas test]
Table 2 below shows the results of the model gas test (NOx purification rate) and the results of microstructural analysis ((i) to (iii)) for the exhaust gas purification catalysts of Examples and Comparative Examples.

  The analysis method will be further described. First, “(iii) thickness of oxidized Pt layer” is compared with the NOx purification rate. FIG. 5 is a graph showing the relationship between catalyst activity and oxide layer thickness. In the graph, the auxiliary line represents the average of two adjacent points. FIG. 5 shows that a clear correlation is recognized. That is, by obtaining “the thickness of the oxidized Pt layer” by the XAFS method, for example, when used in a screening method, the NOx purification activity can be predicted, and the model gas test can be substituted or omitted. Although the model gas test takes several hours per sample, since the XAFS analysis can be performed in a few minutes, there is an advantage that catalyst development can be made more efficient by omitting the model gas test by this method.

  “(I) Ratio of oxidized Pt” is compared with the NOx purification rate. FIG. 6 is a graph showing the catalyst activity and the ratio of oxidized Pt. In the graph, the auxiliary line represents the average of two adjacent points. FIG. 6 shows that there is no clear relationship between “(i) the ratio of oxidized Pt” and the NOx purification rate. Non-patent document 1 shows that “the state of Pt analyzed by the XPS method is highly active when it is metallic”, but such a result is not obtained. Perhaps the XPS method obtains information on several atomic layers on the surface of the catalyst material, so that only the surface of the Pt particle, which is the active point of the catalytic reaction, can be analyzed (surface sensitive). On the other hand, since the XAFS method obtains the average information of the entire catalyst material, the average information including the information inside the Pt particles that does not directly contribute to the catalytic reaction is obtained (bulk sensitive).

  “(Ii) Pt dispersion rate” is compared with the NOx purification rate. FIG. 7 is a graph showing the relationship between catalyst activity and dispersion rate. FIG. 7 shows that there is no clear relationship between the “Pt dispersion ratio” and the NOx purification rate. Intuitively, it can be imagined that the higher the Pt dispersion rate, that is, the finer Pt, the higher the activity, but such a result is not obtained.

  FIG. 8 is an enlarged graph showing the vicinity of the maximum value of the NOx purification rate of FIG. According to FIG. 8, it can be seen that the NOx purification rate changes abruptly with the thickness of the oxide layer as the boundary of 2.8 atomic layers. Inflection points are observed near the 12 atomic layer. Therefore, it is possible to specifically determine that the “catalyst characterized in that the thickness of the oxidized Pt layer determined by the XAFS method is 2.8 to 12 atomic layers” is a catalyst exhibiting a high NOx purification rate. did it. Note that the NOx purification rate increases qualitatively with Non-Patent Document 1 when the thickness of the oxide layer is 12 atomic layers or less (that is, the more Pt is metallic). On the other hand, when the thickness of the oxide layer is 2.8 atomic layers or less, the reduction in the NOx purification rate does not correspond to Non-Patent Document 1. This can be qualitatively explained that if the stability of Pt in the metal state is too high, no reaction intermediate is formed and the catalytic activity is lowered.

  In Table 2 and FIG. 8, Examples 1-14 all showed a high NOx purification rate of 40% or more, and the thickness of the oxidized Pt layer was in the range of 2.8-12 atomic layers. On the contrary, Comparative Examples 1 to 18 all showed NOx purification rates of less than 40%, and the thickness of the oxidized Pt layer was outside the range of 2.8 to 12 atomic layers. Note the results of Examples 4-14 and Comparative Examples 16-18. FIG. 9 is a graph showing the relationship between the Zr / Zr + high silica zeolite ratio (molar ratio of Zr to all cations) and the NOx purification rate in the Pt / Zr / high silica zeolite catalyst, and FIG. 4 is a graph showing the relationship between the ratio of Zr / Zr + high silica zeolite (molar ratio of Zr to all cations) and the oxide layer thickness in the / high silica zeolite catalyst. Among the catalysts composed of Pt, high silica zeolite, and Zr, the catalyst having a Zr ratio of 0% to 50% satisfies the “oxidized Pt layer thickness of 2.8 to 12 atomic layers”, and 40% A higher NOx purification rate was exhibited. However, paying attention to the relationship between the Zr ratio and the NOx purification rate, a particularly high NOx purification rate of 57% or more was obtained with a catalyst having a Zr ratio of 15 to 45%. At this time, the “thickness of the oxidized Pt layer” was “4.0 to 7.4 atomic layer”.

  That is, the catalyst having the “oxidized Pt layer thickness of 2.8 to 12 atomic layers” is a highly active lean NOx catalyst, and specifically corresponds to Examples 1 to 14. Of these, the catalyst having an “oxidized Pt layer thickness of 4.0 to 7.4 atomic layer” is particularly highly active. Specifically, Examples 7 to 13 (of the matrix composed of Zr and zeolite) , The ratio of Zr to all cations is 15 to 45 mol%, and the base material is a catalyst having Pt supported thereon.

  The exhaust gas purification catalyst according to the present invention can be widely used for applications used in an oxygen-excess atmosphere, and is particularly useful as an exhaust gas purification catalyst for vehicles such as motorcycles, light vehicles, and automobiles.

Claims (8)

  An exhaust gas purification catalyst comprising a carrier comprising a metal oxide, and at least one active metal selected from rhodium, palladium and platinum supported on the carrier, or a compound of the active metal, the exhaust gas purification catalyst The active metal oxide layer is provided on the surface of the catalyst, and the thickness of the active metal oxide layer is 2.8 to 12 atomic layers.   The exhaust gas-purifying catalyst according to claim 1, wherein the active metal oxide layer has a thickness of 4.0 to 7.4 atomic layers.   The exhaust gas purifying catalyst according to claim 1 or 2, wherein the active metal includes at least platinum, and the oxide layer of the active metal includes at least a platinum oxide layer.   The support includes a silica-alumina composite oxide and zirconium or a zirconium compound, and a molar ratio of a cation in the silica-alumina composite oxide in the support to a cation in the zirconium or zirconium compound is 95. The catalyst for exhaust gas purification according to claim 1, which is 5:50:50.   The exhaust gas purifying catalyst according to claim 4, wherein a molar ratio of the cation in the silica-alumina composite oxide to the cation in the zirconium or the zirconium compound is 85:15 to 55:45.   The exhaust gas-purifying catalyst according to claim 4 or 5, wherein the silica-alumina composite oxide is a hydrophobic zeolite having a silica / alumina ratio of 5 or more.   The exhaust gas-purifying catalyst according to claim 6, wherein the silica / alumina ratio is 23 or more.   The exhaust gas-purifying catalyst according to any one of claims 1 to 7, which is used in an oxygen-excess atmosphere.

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