Exhaust gas purification catalyst

Abstract

To provide an exhaust gas purification catalyst having excellent oxygen absorbing / releasing performance and purification performance even when exposed to high temperatures.SOLUTION: There is provided an exhaust gas purification catalyst containing a catalyst carrier which comprises an alumina-based powder having a Ba surface enriched phase on the surface, has a surface Ba enriched degree β represented by the following expression (1) of 110 to 200 (wherein,[Ba(at%) / Al(at%)]XPS represents an elemental concentration ratio of Ba and Al obtained by X-ray photoelectron spectroscopy quantitative analysis at a photoelectron take-off angle of 45 degrees and [Ba(at%) / Al(at%)]Bulk represents an elemental concentration ratio of the total content of Ba and Al in the carrier particles and has a BET specific surface area of 20 to 200 m2 / g-carrier.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to an exhaust gas purification catalyst, and more particularly to an exhaust gas purification catalyst containing a catalyst carrier made of an alumina-based powder. [Background technology]

[0002] Exhaust gas purification catalysts have been used to remove harmful components such as hydrocarbons (HC) and nitrogen oxides (NOx) contained in exhaust gas from internal combustion engines such as automobile engines. Three-way catalysts that simultaneously purify HC and NOx in exhaust gas are known as such exhaust gas purification catalysts, and it is also known that Ba compounds are added to these catalysts to improve low-temperature purification activity and heat resistance.

[0003] For example, Japanese Patent Laid-Open No. 2008-23482 (Patent Document 1) discloses an exhaust gas purification catalyst comprising composite oxide particles A containing alumina and an alkaline earth metal element, composite oxide particles B containing zirconia and an alkaline earth metal element, and a precious metal supported on the composite oxide particles B. Also, Japanese Patent Laid-Open No. 2010-12459 (Patent Document 2) discloses an exhaust gas purification catalyst in which an active metal is supported on a support containing BaAl2O4 and BaZrO3. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2008-23482 [Patent Document 2] Japanese Patent Application Laid-Open No. 2010-12459 Summary of the Invention [Problem to be solved by the invention]

[0005] However, while adding Ba to three-way catalysts is effective in improving low-temperature purification activity, exposure to high temperatures causes a solid-phase reaction between Ba and coexisting materials such as oxygen storage catalysts (OSC materials), accelerating thermal degradation of the catalyst's oxygen absorption / release and purification performance. Furthermore, when the catalyst support is made of a Ba solid solution such as BaAl2O4 or BaZrO3, as in the exhaust gas purification catalysts described in Patent Documents 1 and 2, Ba is stabilized by forming a solid solution, suppressing solid-phase reactions with coexisting materials such as OSC materials, but the specific surface area of ​​the catalyst support becomes extremely small. As a result, sintering of the precious metal occurs, reducing contact with exhaust gas, and thus reducing the purification performance of the exhaust gas purification catalyst.

[0006] The present invention has been made in view of the problems associated with the prior art, and has an object to provide an exhaust gas purification catalyst that has excellent oxygen absorbing / releasing performance and purification performance even when exposed to high temperatures. [Means for solving the problem]

[0007] As a result of extensive research to achieve the above object, the inventors have discovered that an exhaust gas purification catalyst having excellent oxygen absorbing / releasing performance and purification performance (particularly purification performance at low temperatures) can be obtained by causing a solid-phase reaction of Ba on the surface of an alumina-based powder, thereby forming a Ba surface-enriched phase on the surface of the alumina-based powder while maintaining the specific surface area of ​​the alumina-based powder, and have thus completed the present invention.

[0008] That is, the present invention provides the following aspects. [1] Alumina-based powder having a Ba-enriched surface phase on its surface, The following formula (1):

[0009]

number

[0010] [In the above formula, [Ba (at%) / Al (at%)] XPSrepresents the element concentration ratio of Ba to Al obtained by X-ray photoelectron spectroscopy quantitative analysis at a photoelectron take-off angle of 45°, and [Ba (at%) / Al (at%)] Bulk represents the element concentration ratio of the total content of Ba and Al in the catalyst support particle.] The surface Ba concentration β represented by is 110 to 200, BET specific surface area is 20 to 200 m 2 / g-support, a catalyst support An exhaust gas purification catalyst comprising: [2] The exhaust gas purifying catalyst according to [1], wherein the Ba content in the catalyst support is 3 to 30 mass % in terms of BaO. [3] In the X-ray diffraction spectrum of the catalyst carrier, peaks attributable to BaAl2O4, BaAl 12 O 19 and a peak derived from BaCO3. [4] The exhaust gas purifying catalyst according to any one of [1] to [3], wherein at least one peak derived from γ-alumina, a peak derived from δ-alumina, and a peak derived from θ-alumina is present in the X-ray diffraction spectrum of the catalyst support. [5] The exhaust gas purifying catalyst according to any one of [1] to [4], further containing a ceria-zirconia solid solution oxide.

[0011] In addition, since the condition of a photoelectron take-off angle of 45° in XPS quantitative analysis corresponds to an analysis depth of 3 nm, the "element concentration ratio of Ba to Al obtained by XPS quantitative analysis at a photoelectron take-off angle of 45°" according to the present invention corresponds to the element concentration ratio of Ba to Al in the region from the surface of the catalyst support particle to a depth of 3 nm.

[0012] Furthermore, although the reason why the exhaust gas purification catalyst of the present invention has excellent oxygen adsorption / release performance and purification performance even when exposed to high temperatures is not entirely clear, the inventors speculate as follows: The exhaust gas purification catalyst of the present invention contains a catalyst support made of an alumina-based powder having a Ba-enriched surface phase on its surface, and in the Ba-enriched surface phase, Ba forms a solid solution with alumina.

[0013] In an exhaust gas purification catalyst in which a precious metal such as Pd is supported on a catalyst support, when a support containing Ba and alumina is used as the catalyst support, the oxygen affinity of the precious metal is improved due to electron donation from Ba to the precious metal, hydrocarbon (HC) poisoning of the precious metal is suppressed, and the oxidation reaction of HC is promoted, so purification performance tends to be improved compared to when a support that does not contain Ba is used. Furthermore, the improved oxygen affinity of the precious metal makes the interaction between the support and the precious metal stronger than when the precious metal is in a metallic state, restricting the movement of the precious metal on the support, so sintering of the precious metal tends to be less likely to occur compared to a support that does not contain Ba.

[0014] However, when the entire catalyst support is formed from a solid solution of Ba and alumina, the specific surface area of ​​the catalyst support is extremely small compared to the specific surface area of ​​the alumina powder, and therefore the above-mentioned effects of Ba (improvement of purification performance and suppression of sintering of precious metals) are not fully realized, and the purification performance of the exhaust gas purification catalyst is not sufficiently improved. On the other hand, in alumina-based powder having a Ba-enriched surface phase on the surface, the solid solution of Ba and alumina is formed near the surface of the alumina-based powder (Ba-enriched surface phase) and is hardly formed in the core, so the core maintains the large specific surface area of ​​the alumina powder. In the exhaust gas purification catalyst of the present invention, since an alumina-based powder having such a Ba-enriched surface phase on the surface is used as the catalyst support, the specific surface area of ​​the entire catalyst is large, and the above-mentioned effects of Ba (improvement of purification performance and suppression of sintering of precious metals) are fully realized, and it is presumed that the catalyst exhibits excellent purification performance.

[0015] Furthermore, because Ba is stabilized by forming a solid solution with alumina, in the exhaust gas purification catalyst of the present invention, which contains a catalyst support in which Ba forms a solid solution with alumina near the surface (Ba surface-enriched phase), it is unlikely to undergo a solid-phase reaction with the coexisting oxygen storage material (OSC material) even when exposed to high temperatures, and the oxygen absorption / release performance of the OSC material is not impaired, and it is presumed that this catalyst exhibits excellent oxygen absorption / release performance. On the other hand, in the exhaust gas purification catalyst containing a catalyst support in which a Ba compound is added to alumina powder or a catalyst support in which Ba is supported on the surface of alumina powder, when exposed to high temperatures, Ba undergoes a solid-phase reaction with the coexisting OSC material, reducing the specific surface area of ​​the OSC material and therefore presumably reducing the oxygen absorption / release performance of the OSC material and the oxygen absorption / release performance of the exhaust gas purification catalyst as well. [Effects of the Invention]

[0016] According to the present invention, it is possible to obtain an exhaust gas purification catalyst that has excellent oxygen absorbing / releasing performance and purification performance (particularly purification performance at low temperatures) even when exposed to high temperatures. [Brief explanation of the drawings]

[0017] [Figure 1] 1 is a graph showing X-ray diffraction patterns (XRD spectra) of the Pd-supported catalyst powders obtained in the examples and comparative examples. [Figure 2] 1 is a graph showing X-ray photoelectron spectroscopy (XPS) spectra of the Pd-supported catalyst powders obtained in the examples and comparative examples. [Figure 3] 1 is a graph showing NO purification rates at various temperatures of exhaust gas purification catalysts obtained in Examples and Comparative Examples. DETAILED DESCRIPTION OF THE INVENTION

[0018] The present invention will be described in detail below based on preferred embodiments thereof.

[0019] The exhaust gas purification catalyst of the present invention contains a catalyst support made of an alumina-based powder having a Ba-enriched surface phase on its surface. The alumina-based powder having a Ba-enriched surface phase on its surface is an alumina-based powder having a region (Ba-enriched phase) with a higher Ba concentration than the core.

[0020] In the Ba-enriched surface phase, Ba forms a solid solution with alumina. Examples of the solid solution of Ba and alumina include BaAl2O4 and BaAl 12 O 19 By using an alumina-based powder having the Ba-enriched surface phase on its surface as a catalyst support, an exhaust gas purification catalyst having excellent oxygen absorption / release performance and purification performance (particularly, purification performance at low temperatures) can be obtained. The Ba-enriched surface phase may contain BaCO3.

[0021] In the present invention, BaAl2O4 and BaAl 12 O 19 The formation of BaAl2O4 can be confirmed by, for example, peaks at the main diffraction angles 2θ = 19.6°, 28.3°, 34.3°, and 40.1° attributable to BaAl2O4 and BaAl 12 O 19 Therefore, from the viewpoint that the formation of a solid solution of Ba and alumina reduces BaO and Ba(OH)2, suppresses solid-phase reaction with the coexisting OSC material, and suppresses deterioration of the oxygen adsorption / release performance of the exhaust gas purification catalyst, in the exhaust gas purification catalyst of the present invention, in the X-ray diffraction spectrum of the catalyst support, there are peaks derived from BaAl2O4, BaAl 12 O 19 and a peak derived from BaCO3. The presence of BaCO3 can be confirmed, for example, by the presence of peaks derived from BaCO3 (major diffraction angles 2θ = 23.9°, 24.3°, 33.1°, 34.6°) in the X-ray diffraction (XRD) spectrum of the catalyst support.

[0022] In the present invention, the thickness of the Ba-enriched surface phase is not particularly limited. However, considering the conditions for the X-ray photoelectron spectroscopy (XPS) quantitative analysis of the catalyst support described below (photoelectron take-off angle: 45° (corresponding to an analysis depth of 3 nm)), the surface region from the surface of the catalyst support particle to a depth of 3 nm may be the Ba-enriched surface phase.

[0023] The core of the catalyst support (region other than the Ba-enriched surface phase) may contain a solid solution of Ba and alumina as long as the Ba concentration is lower than that of the Ba-enriched surface phase, but from the viewpoint of suppressing a decrease in the specific surface area of ​​the catalyst support, it is preferable that the core of the catalyst support has an alumina (Al2O3) composition. The fact that the core of the catalyst support has an alumina (Al2O3) composition can be confirmed by the presence of at least one peak derived from γ-alumina, δ-alumina, and θ-alumina in the X-ray diffraction spectrum of the catalyst support.

[0024] The catalyst carrier used in the present invention is represented by the following formula (1):

[0025]

number

[0026] [In the above formula, [Ba (at%) / Al (at%)] XPS represents the element concentration ratio of Ba to Al obtained by X-ray photoelectron spectroscopy (XPS) quantitative analysis at a photoelectron take-off angle of 45°, and [Ba (at%) / Al (at%)] Bulk represents the element concentration ratio of the total content of Ba and Al in the catalyst support particle.] It is necessary that the surface Ba concentration β represented by the following formula be 110 to 200. The surface Ba concentration β is an index relating to the Ba surface-enriched phase, and when β exceeds 100, it means that the Ba concentration near the surface of the catalyst support is higher than the Ba concentration throughout the catalyst support, and the Ba surface-enriched phase is present on the catalyst support, and when β is 100 or less, it means that the Ba concentration near the surface of the catalyst support and the Ba concentration throughout the catalyst support are equal (when β is 100), or the Ba concentration near the surface of the catalyst support is lower than the Ba concentration throughout the catalyst support (when β is less than 100), and the Ba surface-enriched phase is not present on the catalyst support.

[0027] In the present invention, if the surface Ba concentration β is less than the lower limit, a large amount of Ba is also contained in the core of the catalyst support, thereby reducing the specific surface area of ​​the catalyst support and reducing the purification performance of the exhaust gas purification catalyst. On the other hand, if the surface Ba concentration β exceeds the upper limit, a solid-phase reaction between the coexisting OSC material and Ba is likely to occur, reducing the oxygen absorption / release performance of the OSC material and the oxygen absorption / release performance of the exhaust gas purification catalyst. Furthermore, from the viewpoint of achieving both the specific surface area of ​​the catalyst support and the thermal durability of the oxygen absorption / release performance, the surface Ba concentration β is preferably 120 to 200, more preferably 150 to 200.

[0028] The above [Ba (at%) / Al (at%)] XPS is the element concentration ratio of Ba to Al in the surface region from the surface of the catalyst support particle to a depth of 3 nm. The XPS spectrum of the catalyst support is measured under the conditions of an X-ray source: Al-Kα, a photoelectron take-off angle: 45° (corresponding to an analysis depth: 3 nm). For the obtained XPS spectrum, the areas of the peaks Al2p (near 74.6 eV of binding energy) and Ba3d (near 780 eV of binding energy) are determined, and the ratio of the Ba3d peak area to the Al2p peak area is taken as the element concentration ratio of Ba to Al in the surface region from the surface of the catalyst support particle to a depth of 3 nm.

[0029] Also, [Ba(at%) / Al(at%)] Bulkis the element concentration ratio of the total content of Ba and Al in the catalyst support particles, and can be calculated from the amounts of Ba and Al charged, but can also be determined by X-ray fluorescence (XRF) analysis or high-frequency inductively coupled plasma (ICP) analysis.

[0030] The catalyst carrier has a BET specific surface area of ​​20 to 200 m 2 / g. If the BET specific surface area of ​​the catalyst support is less than the lower limit, the purification performance of the exhaust gas purification catalyst will decrease. On the other hand, if the BET specific surface area of ​​the catalyst support exceeds the upper limit, the catalyst will become bulkier than necessary, causing excessive gas permeation resistance of the catalyst and possibly preventing the catalyst from being coated with the required amount of catalyst weight when it is coated on a catalytic converter (due to clogging of honeycomb holes, etc.). In addition, from the viewpoint of ensuring the area for supporting the precious metal and not increasing the bulk of the catalyst more than necessary, the BET specific surface area of ​​the catalyst support is 40 to 190 m 2 / g is preferred, and 60 to 180m 2 / g is more preferable, and 80 to 170m 2 / g is more preferable, and 100 to 160m 2 / g is particularly preferred.

[0031] Furthermore, in the catalyst carrier, the Ba content is preferably 3 to 30 mass %, more preferably 3.5 to 20 mass %, and even more preferably 5 to 15 mass %, calculated as BaO. If the Ba content is less than the lower limit, the purification performance of the exhaust gas purification catalyst tends to decrease, while if the Ba content exceeds the upper limit, the oxygen absorption / release performance of the exhaust gas purification catalyst tends to decrease.

[0032] In the present invention, such a catalyst support can be prepared as follows. For example, it can be obtained by adding alumina powder to an aqueous solution in which a Ba compound (e.g., barium acetate, barium nitrate, barium chloride, or barium hydroxide) has been dissolved, allowing the Ba compound to adhere to the surface of the alumina powder, followed by drying and calcining the resulting dried powder. The calcination temperature is preferably 800 to 1400°C, more preferably 900 to 1200°C, and even more preferably 950 to 1100°C. If the calcination temperature is below the lower limit, a solid solution of Ba and alumina tends not to be sufficiently formed. On the other hand, if the calcination temperature exceeds the upper limit, the entire catalyst support becomes a solid solution of Ba and alumina, and the Ba-enriched surface phase tends not to be formed.

[0033] In the exhaust gas purification catalyst of the present invention, a precious metal is typically supported on the catalyst support. Such a precious metal is not particularly limited as long as it is a precious metal used in a three-way catalyst, and examples thereof include Pd, Rh, and Pt. The amount of the precious metal supported is also not particularly limited, and is preferably 0.5 to 10 mass% and more preferably 0.5 to 2 mass% relative to the catalyst support. Furthermore, the particle size of the precious metal is not particularly limited, but from the viewpoint of the purification performance of the exhaust gas purification catalyst after exposure to high temperatures, the particle size after a heat resistance test at 1050°C for 25 hours is preferably 20 to 70 nm and more preferably 30 to 65 nm.

[0034] Furthermore, the exhaust gas purification catalyst of the present invention preferably further contains an oxygen storage material (OSC material). There are no particular limitations on the OSC material, but examples thereof include ceria (CeO), ceria-zirconia solid solution oxide, praseodymium (PrO 11 The amount of the OSC material is not particularly limited, but from the viewpoint of the balance between the oxygen adsorption / release performance and purification performance of the exhaust gas purification catalyst, the mass ratio of the catalyst in which a precious metal is supported on the catalyst carrier (hereinafter also referred to as "precious metal supported catalyst") to the OSC material (precious metal supported catalyst:OSC material) is preferably 95:5 to 30:70, and more preferably 90:10 to 50:50.

[0035] Furthermore, in the exhaust gas purification catalyst of the present invention, the BET specific surface area of ​​the entire catalyst (for example, the noble metal supported catalyst + the OSC material) after a heat resistance test at 1050°C for 25 hours is 20 to 200 m 2 / g, and 30 to 160m 2 / g is more preferable, and 40 to 120m 2 / g is more preferable, and 50 to 100m 2 If the BET specific surface area of ​​the catalyst as a whole after the heat resistance test is less than the lower limit, the purification performance of the exhaust gas purification catalyst tends to decrease. [Example]

[0036] The present invention will be described in more detail below based on examples and comparative examples, but the present invention is not limited to the following examples.

[0037] Example 1 2.62 g of barium acetate was dissolved in 50 g of ion-exchanged water and stirred for at least 15 minutes. 20 g of Al2O3 powder ("MI307" manufactured by Solvay) was then added and stirred for at least 15 minutes. The mixture was then heated to 200 °C using a hot stirrer while stirring and evaporated to dryness. The resulting dried powder was dried overnight at 110 °C and then calcined in a muffle furnace at 500 °C for 5 hours. It was then crushed and sized to a particle size of 75 μm or less, and calcined in a muffle furnace in air at 900 °C for 10 hours to obtain BaAl2O4 powder. 19 g of the BaAl2O4 powder was added to 50 ml of distilled water and stirred. 2.34 g of an aqueous palladium nitrate solution (containing 8.2% by mass of Pd metal) was added dropwise and stirred for at least 15 minutes. The mixture was then heated to 200 °C using a hot stirrer while stirring and evaporated to dryness. The obtained dry powder was dried overnight at 110°C and then calcined in a muffle furnace at 500°C for 3 hours to obtain Pd / BaAl2O4 powder.

[0038] This Pd / BaAl2O4 powder and ceria-zirconia powder (CeO2: 30 mass%, ZrO2: 60 mass%, La2O3: 5 mass%, Y2O3: 5 mass%) were mixed in a mortar at a mass ratio of 1:1 to obtain an exhaust gas purification catalyst powder.

[0039] Example 2 BaAlO powder (catalyst support powder) was prepared in the same manner as in Example 1, except that the amount of barium acetate was changed to 1.26 g, and after pulverization and sieving, the powder was calcined at 1000°C for 10 hours in an oxidizing atmosphere. Furthermore, Pd / BaAlO powder and an exhaust gas purification catalyst powder were prepared.

[0040] Example 3 BaAl2O4 powder (catalyst support powder) was prepared in the same manner as in Example 2 except that the amount of barium acetate was changed to 2.62 g, and further, Pd / BaAl2O4 powder and exhaust gas purification catalyst powder were prepared.

[0041] Example 4 BaAl2O4 powder (catalyst support powder) was prepared in the same manner as in Example 2 except that the amount of barium acetate was changed to 5.68 g, and further, Pd / BaAl2O4 powder and exhaust gas purification catalyst powder were prepared.

[0042] Example 5 BaAl2O4 powder (catalyst support powder) was prepared in the same manner as in Example 2 except that the amount of barium acetate was changed to 8.33 g, and further, Pd / BaAl2O4 powder and exhaust gas purification catalyst powder were prepared.

[0043] (Comparative Example 1) 50 ml of distilled water was added to 19 g of Al2O3 powder ("MI307" manufactured by Solvay) as a catalyst support powder and stirred. 2.34 g of an aqueous palladium nitrate solution (containing 8.2 mass% Pd metal) was then added dropwise and stirred for 15 minutes or more. The mixture was then heated to 200°C using a hot stirrer while stirring and evaporated to dryness. The resulting dried powder was dried overnight at 110°C and then calcined in a muffle furnace at 500°C for 3 hours to obtain Pd / Al2O3 powder.

[0044] An exhaust gas purifying catalyst powder was obtained in the same manner as in Example 1, except that this Pd / Al2O3 powder was used instead of the Pd / BaAl2O4 powder.

[0045] (Comparative Example 2) BaAl2O4 powder was obtained in the same manner as in Example 1, except that the amount of barium acetate was changed to 50.48 g, the amount of ion-exchanged water was changed to 200 g, and after pulverization and sieving, the mixture was fired in air at 1500°C for 10 hours.

[0046] 1.95 g of the BaAl2O4 powder was added to 18.05 g of Pd / Al2O3 powder prepared in the same manner as in Comparative Example 1, and the mixture was stirred and pulverized in a mortar for 5 minutes or more to obtain a mixed powder of the Pd / Al2O3 powder and the BaAl2O4 powder.

[0047] An exhaust gas purifying catalyst powder was obtained in the same manner as in Example 1, except that this mixed powder was used instead of the Pd / BaAl2O4 powder.

[0048] (Comparative Example 3) BaAl2O4 powder (catalyst support powder) was prepared in the same manner as in Example 2 except that the amount of barium acetate was changed to 22.21 g, and further, Pd / BaAl2O4 powder and exhaust gas purification catalyst powder were prepared.

[0049] Comparative Example 4 Pd / BaAl2O4 powder was prepared in the same manner as in Example 1, except that 19.5 g of BaAl2O4 powder prepared in the same manner as in Comparative Example 2 was used as the catalyst support powder and the amount of palladium nitrate aqueous solution was changed to 2.341 g. Further, Pd / BaAl2O4 powder and further, an exhaust gas purification catalyst powder were prepared.

[0050] <Heat resistance test> Each exhaust gas purification catalyst powder obtained in the examples and comparative examples was molded by applying a pressure of 1 t using a hydrostatic press, and the obtained molded body was pulverized and sized to form pellets with a diameter of 0.5 mm to 1.0 mm. 2.0 g of the obtained catalyst pellets were filled into a reaction tube to prepare a catalyst bed, and a rich gas [CO2 (10 vol%) + H2 (2.0 vol%) + H2O (3.0 vol%) + N2 (balance)] and a lean gas [CO2 (10 vol%) + O2 (1.0 vol%) + H2O (3.0 vol%) + N2 (balance)] were alternately passed through this catalyst bed at a flow rate of 500 ml / min for 5 minutes each, and a heat resistance test was carried out at 1050 °C for 25 hours.

[0051] <Ba content (in terms of BaO)> When assuming that all Ba contained in the carrier exists as BaO, the Ba content in the carrier was determined as a BaO conversion value from the molar fraction of Ba atoms contained in the carrier calculated from the charging ratio of the raw materials. The results are shown in Table 1.

[0052] <X-ray diffraction> The X-ray diffraction (XRD) spectra of the Pd / BaAl2O4 powder obtained in the examples and Comparative Examples 3 to 4, the Pd / Al2O3 powder obtained in Comparative Example 1, and the mixed powder obtained in Comparative Example 2 were measured using a powder X-ray diffractometer ("ULTIMA IV" manufactured by Rigaku Corporation) under the conditions of X-ray source: Cu-Kα, 2θ = 10 to 60°, scanning speed: 10° / min, voltage: 40 V, and current: 30 A. The results are shown in Fig. 1.

[0053] <Surface Ba enrichment degree> The X-ray photoelectron spectroscopy (XPS) spectra of the Pd / BaAl2O4 powder obtained in Examples and Comparative Examples 3 to 4 and the mixed powder obtained in Comparative Example 2 were measured by XPS wide scan under the conditions of X-ray source: Al-Kα, photoelectron extraction angle: 45° (corresponding to an analysis depth of 3 nm), and an analysis area of 200 μm square using an X-ray photoelectron spectroscopy (XPS) analyzer ("PHI5000VersaProve" manufactured by ULVAC-PHI, Inc.). The results are shown in Figure 2. Regarding the obtained XPS spectra, using the spectral analysis software "PHI MultiPak", for each peak of Al2p (binding energy = around 74.6 eV) and Ba3d (binding energy = around 780 eV), after removing the background, the peak area was determined, and the ratio of the peak area of Ba3d to the peak area of Al2p was calculated as the elemental concentration ratio of Ba and Al in the surface region from the surface of the catalyst support particles to a depth of 3 nm by the following formula (1) to obtain the Ba enrichment degree β on the surface of the catalyst support particles. The results are shown in Table 1.

[0054]

Number

[0055] In the above formula, [Ba(at%) / Al(at%)] XPS represents the elemental concentration ratio of Ba and Al in the region from the surface of the catalyst support particles to a depth of 3 nm obtained by XPS analysis under the above conditions, and [Ba(at%) / Al(at%)] Bulk represents the elemental concentration ratio of the total contents of Ba and Al in the catalyst support particles. In Examples and Comparative Examples, the elemental concentration ratio of the total contents of Ba and Al in the catalyst support particles was calculated from the charged amounts of Ba and Al, but it may also be measured by fluorescence X-ray (XRF) analysis or high-frequency inductively coupled plasma (ICP) analysis.

[0056] <BET specific surface area> The Pd / BaAl2O4 powder obtained in Examples and Comparative Examples 3 to 4, the Pd / Al2O3 powder obtained in Comparative Example 1, the mixed powder obtained in Comparative Example 2, and the catalyst pellets after the heat resistance test were set in a fully automatic specific surface area measuring device ("Ms-4232" manufactured by Micro Data Co., Ltd.). After performing dehydration pretreatment at 250°C, the BET specific surface areas of the catalyst support powders (BaAl2O4 powder (Examples and Comparative Examples 3 to 4), Al2O3 powder (Comparative Example 1), mixed powder of Al2O3 powder and BaAl2O4 powder (Comparative Example 2)) and the catalyst pellets were measured by the BET one-point method using liquid nitrogen. The results are shown in Table 1.

[0057] <Pd particle size> 0.5 g of the catalyst pellets after the heat resistance test was filled into a U-shaped quartz sample tube to prepare a catalyst bed. This catalyst bed was subjected to an O2 oxidation pretreatment at 300°C for 15 minutes, and further subjected to a H2 reduction pretreatment at 400°C for 15 minutes. Then, while cooling the catalyst bed to -78°C with dry ice, a CO pulse gas was circulated multiple times to measure the CO adsorption amount, and the Pd particle size was calculated from this CO adsorption amount. The results are shown in Table 1.

[0058] <Oxygen absorption and release performance> 0.5 g of the catalyst pellets after the heat resistance test was sealed in a 10 mm diameter sample holder to prepare a catalyst bed, and set in a fixed bed flow type catalyst activity evaluation device. A rich gas [CO (2.0 vol%) + N2 (the rest)] and a lean gas [O2 (1.0 vol%) + N2 (the rest)] were circulated at a flow rate of 10000 ml / min while switching every 3 minutes, and pretreatment was performed at 600°C. Then, at 400°C, the CO2 generation amount when switching from the lean gas to the rich gas was measured, and the oxygen absorption and release amount was calculated. The results are shown in Table 1.

[0059] <NO and HC purification performance> After the heat resistance test, 1.0 g of the catalyst pellets was sealed in a 10 mm diameter sample holder to prepare a catalyst bed, which was then set in a fixed-bed flow-type catalytic activity evaluation apparatus. A rich gas [CO (10 vol%) + O (0.646 vol%) + CO (1.121 vol%) + NO (1200 ppm) + CH (1600 ppmC) + H (0.374 vol%) + H O (3.0 vol%) + N (balance)] and a lean gas [CO (10 vol%) + O (0.748 vol%) + CO (0.7 vol%) + NO (1200 ppm) + CH (1600 ppmC) + H (0.233 vol%) + H O (3.0 vol%) + N (balance)] were flowed through the catalyst bed at a flow rate of 10,000 ml / min, with the flow rate switched every 10 seconds, and the catalyst bed was pretreated at 600 °C. After cooling the catalyst bed, the temperature was increased from 100°C to 600°C at a rate of 25°C / min. A test gas (CO2 (14 vol%) + O2 (0.55 vol%) + CO (0.52 vol%) + NO (3000 ppm) + CH3 (3000 ppmC) + HO (3.0 vol%) + N2 (balance)) was passed through the bed at a flow rate of 10,000 ml / min. The NO and CH6 conversion efficiencies were measured at each temperature. Figure 3 shows the NO conversion efficiencies at each temperature. Table 1 shows the temperature T50 at which the NO and CH6 conversion efficiencies reached 50%.

[0060] [Table 1]

[0061] From the XRD spectrum shown in FIG. 1 and the surface Ba enrichment β shown in Table 1, it was found that the Pd / BaAl2O4 powders obtained in Examples 1 to 5 contained catalyst support powders having a shell made of BaAl2O4 and a core made of Al2O3. It is believed that BaCO3 particles are present on the surface of the Pd / BaAl2O4 powder obtained in Example 1. On the other hand, it was confirmed that the Pd / Al2O3 powder obtained in Comparative Example 1 contained catalyst support powder made of Al2O3, and the mixed powder obtained in Comparative Example 2 contained catalyst support powder made of Al2O3 and BaAl2O4 powder. It was also confirmed that the Pd / BaAl2O4 powder obtained in Comparative Example 3 contained catalyst support powder having a shell made of BaAl2O4 and a core made of Al2O3, and the Pd / BaAl2O4 powder obtained in Comparative Example 4 contained catalyst support powder made of BaAl2O4. Table 2 shows images of each catalyst support and each exhaust gas purification catalyst.

[0062] [Table 2]

[0063] Furthermore, as shown in Table 1, it was found that exhaust gas purification catalysts (Examples 1 to 5) containing catalyst supports whose surface Ba concentration β and specific surface area are within the predetermined ranges have high oxygen absorption / release performance and excellent HC and NO purification activity at low temperatures. On the other hand, exhaust gas purification catalysts (Comparative Examples 1 to 4) containing catalyst supports whose surface Ba concentration β is smaller than the predetermined range are inferior in HC and NO purification activity at low temperatures. In particular, it was found that exhaust gas purification catalysts (Comparative Examples 3 and 4) containing catalyst supports whose specific surface area is smaller than the predetermined range are also inferior in oxygen absorption / release performance. [Industrial Applicability]

[0064] As explained above, according to the present invention, it is possible to obtain an exhaust gas purification catalyst that has excellent oxygen absorption / release performance and purification performance (particularly purification performance at low temperatures) even when exposed to high temperatures. Therefore, the exhaust gas purification catalyst of the present invention exhibits excellent purification performance at low temperatures, such as at the start of an internal combustion engine such as an automobile engine, even after being exposed to high temperatures, and is therefore useful as a catalyst for removing harmful components, such as hydrocarbons (HC) and nitrogen oxides (NOx), contained in exhaust gas from internal combustion engines such as automobile engines.

Claims

1. The alumina powder has a Ba surface-enriched phase on its surface. The following formula (1): [Equation 1] [In the above formula, [Ba (at%) / Al (at%)] ＸＰＳ represents the element concentration ratio of Ba to Al obtained by X-ray photoelectron spectroscopy quantitative analysis at a photoelectron take-off angle of 45°, and [Ba (at%) / Al (at%)] Ｂｕｌｋ represents the element concentration ratio of the total content of Ba and Al in the catalyst support particle.] The surface Ba concentration β represented by is 110 to 200, BET specific surface area is 20 to 200 m ２ / g-carrier, An exhaust gas purification catalyst comprising:

2. 2. The exhaust gas purification catalyst according to claim 1, wherein the Ba content in the catalyst support is 3 to 30 mass % in terms of BaO.

3. In the X-ray diffraction spectrum of the catalyst support, BaAl ２ O ４ The peaks due to BaAl １２ O １９ and peaks due to BaCO ３ 2. The exhaust gas purifying catalyst according to claim 1, wherein at least one peak derived from the following is present:

4. 2. The exhaust gas purification catalyst according to claim 1, wherein at least one peak selected from a peak derived from γ-alumina, a peak derived from δ-alumina, and a peak derived from θ-alumina is present in the X-ray diffraction spectrum of the catalyst support.

5. 2. The exhaust gas purification catalyst according to claim 1, further comprising a ceria-zirconia solid solution oxide.

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