JP4285095B2 - Exhaust gas purification catalyst and exhaust gas purification method - Google Patents

Description

【００４４】
耐久試験後の各ペレット触媒を評価装置にそれぞれ同量配置し、表２に示す評価モデルガスを用いてNO_x 吸蔵量を測定した。先ず 400℃でリッチガス定常状態からリーンガス定常状態にガスを切り替え、触媒出ガス中のNO_x 濃度が定常になるまでリーンガスを流した後、リッチガスを10秒間流し、再びリーンガスに切り替えた際の飽和NO_x 吸蔵量をそれぞれ測定した。結果を図３に示す。
【００４５】
【表２】

【００４６】
図３より、実施例１及び参考例１の触媒は各比較例の触媒に比べて耐久試験後のNO_x 吸蔵能が高いことがわかり、これは表面にTiO ₂ 層を有した効果であることが明らかである。
【００４７】
次に、耐久試験後の各ペレット触媒を評価装置にそれぞれ２cm³ 充填し、表３に示すモデルガスを流量10Ｌ／分で流しながら、室温から 500℃まで20分かけて昇温し、その間のHC及びCOの浄化率を連続的に測定した。そしてそれぞれ50％浄化温度を求め、結果を図４に示す。
【００４８】
【表３】

【００４９】
図４より、実施例１の触媒は比較例２の触媒に比べて高い浄化活性を示し、参考例１の触媒は比較例１の触媒に比べて高い浄化活性を示していることがわかり、これは表面にTiO₂層３を有した効果であることが明らかである。また比較例どうしの比較、及び実施例１と参考例１との比較から、PtとＫとを分離して担持することが望ましいことも明らかである。
【００５０】
【発明の効果】
すなわち本発明の排ガス浄化用触媒及び浄化方法によれば、貴金属の硫黄被毒を効果的に抑制することができ、ひいてはNO_x 吸蔵能の低下が抑制される。
【図面の簡単な説明】
【図１】本発明の一実施例の触媒粒子を示す模式的断面図である。
【図２】本発明の参考例１の触媒粒子を示す模式的断面図である。
【図３】実施例１、参考例１及び比較例の触媒のNO_x 吸蔵量を示すグラフである。
【図４】実施例１、参考例１及び比較例の触媒の50％浄化温度を示すグラフである。
【符号の説明】
１：ZrO₂粒子（酸化物粒子） ２： Al₂O₃層（NO_x 吸蔵材被覆酸化物）
３：TiO₂層（酸性酸化物層）[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a catalyst and an exhaust gas purifying method for purifying exhaust gas, in particular oxygen excess occludes NO _x in _the NO _x storage material in an exhaust gas in a lean atmosphere, NO _x released from _the NO _x storage material in a rich atmosphere from the stoichiometric It relates _the NO _x storage-reduction type catalyst and purification method reduces and purifies.
[0002]
[Prior art]
As an exhaust gas purifying catalyst capable of purifying exhaust gas from a lean burn engine, an NO _x storage reduction type catalyst is used. The NO _x storage-and-reduction type catalyst, an alkali metal, _the NO _x storage material such as an alkaline earth metal oxide support such as alumina, which is formed by loading a noble metal such as Pt, oxygen excess lean atmosphere occludes NO _x in _the NO _x storage material in the exhaust gas. And by letting exhaust gas in rich atmosphere from stoichiometrically pulse, NO _x is released from the NO _x storage material, and the released NO _x is reduced and purified by reducing components such as HC and CO that are abundant in the atmosphere. .
[0003]
In addition, in order to improve the NO _x storage capacity in a high temperature range, ZrO ₂ which is a basic oxide is used as a carrier. A catalyst having Kr or the like supported on ZrO ₂ is strong in basicity, so that it easily adsorbs acidic NO _{x and} exhibits a high NO _x storage capacity. However in such a NO _x storage-and-reduction type catalyst, SO _x in the exhaust gas tends to react with _the NO _x storage material, there is a problem that thereby _the NO _x storage ability is lowered. This phenomenon is called sulfur poisoning. In a highly basic NO _x storage-reduction catalyst, acidic SO _x tends to approach, and the NO _x storage material becomes sulfate by reaction with SO _x . The sulfates, since it is difficult to decompose compared to nitrates, _the NO _x storage material became sulfate is lost _the NO _x storage capacity, it is difficult to recover _the NO _x storage capacity.
[0004]
Therefore, it is recalled that SO _x is prevented from approaching the NO _x storage material, and various proposals have been made. For example JP-A 10-258232, JP-A No. 11-114422 and JP JP 2000-176286, carrying _the NO _x storage material in the core portion of the oxide particles composed of a core portion covering layer, the core NO _x storage-and-reduction type catalyst obtained by supporting a noble metal is disclosed in the coating layer covering the parts. With such a NO _x storage-and-reduction type catalyst, since the coating layer proximity is suppressed to _the NO _x storage material of the SO _x functions as a shielding layer, it is possible to suppress the sulfur poisoning.
[0005]
Also JP-A-2001-009279 discloses, it is described that the composite particles to form a coating layer made of titania on the surface of the alumina particles and the carrier of the NO _x storage reduction catalyst, the effect of the proximity of the SO _x by titania It is described that it can be suppressed.
[0006]
[Patent Document 1]
Japanese Patent Laid-Open No. 10-258232 [Patent Document 2]
Japanese Patent Laid-Open No. 11-114422 [Patent Document 3]
JP 2000-176286 [Patent Document 4]
JP 2001-009279 A
[Problems to be solved by the invention]
However, in the NO _x storage-and-reduction type catalyst disclosed in the above publication, the noble metal since come into contact directly with the flue gas, the sulfur poisoning is caused by the SO _x of the noble metal. In the NO _x storage-reduction catalyst, NO _x in the exhaust gas is oxidized by noble metal to make NO _x occluded in the NO _x storage material, and it becomes highly active NO _x. The activity decreases, and the ability to generate NO _x by oxidizing NO to produce NO _x decreases. When NO _x production ability decreases, highly active NO _x decreases, so NO _x occlusion ability decreases. That is, in the NO _x storage-and-reduction type catalyst disclosed in the above publication, the noble metal sulfur poisoning, _the NO _x storage capacity has a drawback of a decrease.
[0008]
The present invention has been made in view of such circumstances, and an object thereof is to suppress sulfur poisoning of noble metals and suppress a decrease in NO _x storage capacity.
[0009]
[Means for Solving the Problems]
Features of the NO _x storage reduction catalysts of the present invention for solving the above-mentioned problems, NO _x and storage material _the NO _x storage material bearing oxide carrying, _the NO _x storage material supported oxide was coated supporting the noble metal NO _x Oxide particles comprising an occlusion material coating oxide,
It is to include composite particles comprising an acidic oxide layer that is coated on the surface of the oxide particles and is made of an acidic oxide having a higher acidity than the oxide particles.
[0010]
Further, the exhaust gas purification method of the present invention is characterized by using the exhaust gas purification catalyst of the present invention, storing NO _x in the NO _x storage material in the exhaust gas in an oxygen-excess lean atmosphere, and storing the NO _{x in a} rich atmosphere from stoichiometry. In reducing and purifying NO _x by releasing it from the material,
The lean exhaust gas that has passed through the acidic oxide layer is brought into contact with a noble metal to oxidize NO to generate NO _x .
[0011]
The acidic oxide layer preferably contains titania. The NO _x storage material-supported oxide is preferably a basic oxide, and particularly preferably zirconia. The NO _x storage material-covered oxide is preferably an amphoteric oxide, and desirably contains alumina.
[0012]
DETAILED DESCRIPTION OF THE INVENTION
NO _x storage-and-reduction type catalyst of the present invention, the surface and the interior of the oxide particles supporting the noble metal and NO _x storage material in at least one, is coated on the surface of the oxide particles oxide particles from acidity high acidity oxide And a composite particle comprising an acidic oxide layer made of a material.
[0013]
Therefore, in the exhaust gas purifying catalyst of the present invention, the proximity of SO _x to the noble metal supported on the oxide particles by the acidic oxide layer can be suppressed, and sulfur poisoning of the noble metal can be suppressed. When the sulfur poisoning of the noble metal is suppressed, the decrease in the oxidation activity of the noble metal is suppressed, and the decrease in the NO _x generation ability for generating NO _x by oxidizing NO in the exhaust gas can be suppressed. By suppressing the decrease in NO _x generation ability, the decrease in highly active NO _x can be suppressed, and the decrease in NO _x storage ability can be suppressed.
[0015]
The acidic oxide layer can use an acidic oxide having a higher acidity than the oxide particles. When typical oxides are arranged in order from low to high acidity, MgO → ZrO ₂ → CeO ₂ → Al ₂ O ₃ → TiO ₂ → SiO ₂ , so the oxide particles that form the core Accordingly, an oxide having a higher acidity may be selected. Since SiO ₂ becomes glass at a high temperature, it is not suitable as a catalyst for exhaust gas purification, and TiO ₂ is a particularly preferred acidic oxide. For example, in order to form an acidic oxide layer from TiO ₂ , a titanate coupling agent may be used.
[0016]
The coupling agent generally has one hydrophilic group per molecule, and the hydrophilic group is chemically bonded to the surface of the oxide particle which is an inorganic substance. And the metal atom contained in a coupling agent turns into an oxide and is formed in the oxide particle surface by baking the couple | bonded coupling agent. By doing so, the acidic oxide layer is extremely thin and can be coated with oxide particles carrying a noble metal and a NO _x storage material in a substantially monomolecular film. Since the acidic oxide layer is thin, there is no adverse effect on the NO _x storage capacity.
[0017]
As the noble metal, one or more of Pt, Rh, Pd, Ir, and Ru can be used. In the case of Pt and Pd, the supported amount is preferably 0.1 to 20.0 g, particularly preferably 0.5 to 10.0 g with respect to 120 g of composite particles. In the case of Rh, 0.01 to 80 g is preferable with respect to 120 g of composite particles, and 0.05 to 5.0 g is particularly preferable. In terms of 1 liter of catalyst volume, 0.1 to 20 g is preferable in the case of Pt and Pd, and 0.5 to 10 g is particularly preferable. In the case of Rh, 0.01 to 10 g is preferable, and 0.05 to 5 g is particularly preferable.
[0018]
As the NO _x storage material, at least one of alkali metals, alkaline earth metals and rare earth elements can be used, and at least alkali metals and alkaline earth metals are preferably used. Alkaline earth metals have a high NO _x storage capacity at low temperatures, and alkali metals have an excellent NO _x storage capacity at high temperatures. The supported amount of the NO _x storage material is preferably in the range of 0.01 to 2.0 mol per liter of the catalyst.
[0019]
Both the noble metal and the NO _x storage material are supported on at least one of the surface and the inside of the oxide particles. However, when the noble metal and the NO _x storage material come into contact with each other, the activity of the noble metal may be reduced by the NO _x storage material.
[0020]
Accordingly, oxide particles constitute _the NO _x storage material and _the NO _x storage material supported oxide that carries the _the NO _x storage material-coated oxide carrying precious metal coated with _the NO _x storage material supported oxide, from, The surface is covered with an acidic oxide layer . In this way, since the NO _x storage material and the noble metal are separated and supported, it is possible to prevent the oxidation activity of the noble metal from being impaired by the NO _x storage material, and the oxidation activity of HC and the like is improved, while NO Since the oxidation activity of NOx is improved, the NO _x storage capacity is further improved. In addition, since the NO _x storage material is covered with both the NO _x storage material covering oxide and the acidic oxide layer, the shielding performance against SO _x is improved, and the sulfur poisoning of the NO _x storage material is further suppressed. Can do.
[0021]
_The NO _x storage material supported oxide is preferably basic oxide in order to improve _the NO _x storage ability, it is particularly desirable to include zirconia. Further, since the NO _x storage material-covered oxide has higher acidity than the NO _x storage material-supported oxide and preferably lower acidity than the acidic oxide layer, it is preferable to use amphoteric oxides. A typical amphoteric oxide is Al ₂ O ₃ .
[0022]
From the viewpoint of NO _x storage capacity, it is desirable that the precious metal and the NO _x storage material are close to each other. Therefore, the NO _x storage material coating oxide is also preferably thin, and the coupling agent is similar to the acidic oxide layer. It is desirable to form from. The _the NO _x storage material coated oxide when forming the Al ₂ O ₃ can be used an aluminum-based coupling agent.
[0023]
The exhaust gas purifying method of the present invention, by using the exhaust gas purifying catalyst of the present invention, NO and NO _x in exhaust gas of excess oxygen lean atmosphere is occluded in _the NO _x storage material, from _the NO _x storage material in a rich atmosphere from the stoichiometric Upon reduction purification by releasing _x, it is made to produce a NO _x by the exhaust gas of lean atmosphere that has passed through the acidic oxide layer by oxidizing NO in contact with the noble metal.
[0024]
Accordingly, in a state of being inhibited from SO _x in the exhaust gas is close to the acidic oxide layer, exhaust gas comes into contact with the noble metal passes through the acidic oxide layer, it is possible to suppress the noble metal sulfur poisoning. Moreover, since the generation of the NO _x with high activity is suppressed to reduce the decrease in activity of the noble metal, it is possible to suppress the reduction of the NO _x storage capacity. The noble metal with suppressed sulfur poisoning efficiently oxidizes NO to generate NO _x, and highly active NO _x is efficiently stored in the NO _x storage material.
[0025]
【Example】
Hereinafter, the NO _x is occluded in _the NO _x storage material, in a rich atmosphere from the stoichiometric to release NO _x from the _the NO _x storage material in an exhaust gas in a lean atmosphere specific excess oxygen of the present invention through examples and comparative examples reduction In purifying,
Acidic oxide layer of the exhaust gas in the lean atmosphere is described in manner to produce a NO _x is oxidized to NO in contact with the noble metal that has passed through the.
[0026]
Example 1
50 g of ZrO ₂ powder was put into an aqueous solution containing 0.1 mol of potassium acetate, heated with stirring and evaporated to dryness, and K was supported. This was dried and calcined at 500 ° C. for 2 hours to carry K as potassium carbonate to prepare K / ZrO ₂ powder.
[0027]
The total amount of the obtained K / ZrO ₂ powder was put into a mixer, and 50 g of an n-propanol solution in which 12 g of an Al coupling agent (“Plenact AL-M” manufactured by Ajinomoto Fine Techno Co., Ltd., equivalent to 1.23 g as Al ₂ O ₃ ) was dissolved. The solution was sufficiently stirred while gradually dropping.
[0028]
An appropriate amount of ethanol was added thereto and stirred well, and then the solvent was separated by decantation and centrifugation. This was repeated three times to remove unreacted coupling agent and alcohol, and after drying, calcination was carried out at 500 ° C. for 2 hours. As a result, the organic group of the coupling agent adhered and fixed to the K / ZrO ₂ particles was oxidized and decomposed to form a thin Al ₂ O ₃ layer on the surface of the K / ZrO ₂ particles.
[0029]
The total amount of Al ₂ O ₃ / K / ZrO ₂ powder obtained is impregnated with an acetone solution of platinum acetate (containing 1 g of Pt), heated to 40 ° C. with good stirring, and evaporated to dryness to carry Pt. did. Since acetone is used as a solvent, the supported potassium carbonate does not elute and the state of being supported on the ZrO ₂ particles is maintained. This was dried and calcined at 500 ° C. for 2 hours to prepare a Pt / Al ₂ O ₃ / K / ZrO ₂ powder.
[0030]
The whole amount of the obtained Pt / Al ₂ O ₃ / K / ZrO ₂ powder was put into a mixer, and 3.2 g of a Ti coupling agent (“Preneact KR44” manufactured by Ajinomoto Fine-Techno Co., Ltd., equivalent to 0.61 g as TiO ₂ ) was dissolved. The mixture was sufficiently stirred while gradually adding 50 ml of n-propanol solution dropwise.
[0031]
An appropriate amount of ethanol was added thereto and stirred well, and then the solvent was separated by decantation and centrifugation. This was repeated three times to remove unreacted coupling agent and alcohol, and after drying, calcination was carried out at 500 ° C. for 2 hours. As a result, the organic group of the coupling agent attached and fixed to the Pt / Al ₂ O ₃ / K / ZrO ₂ particles is oxidized and decomposed to form a thin TiO ₂ layer on the surface of the Pt / Al ₂ O ₃ / K / ZrO ₂ particles. did.
[0032]
The obtained catalyst is a powder in which TiO ₂ / Pt / Al ₂ O ₃ / K / ZrO ₂ catalyst particles shown in FIG. 1 are assembled. The catalyst particles include a ZrO ₂ particle 1 as a core part, an Al ₂ O ₃ layer 2 formed on the surface of the ZrO ₂ particle 1, and a TiO ₂ layer 3 formed on the surface of the Al ₂ O ₃ layer 2. The core portion 1 carries potassium carbonate 10 and the Al ₂ O ₃ layer 2 carries Pt20. Since the Al ₂ O ₃ layer 2 is extremely thin, Pt20 and potassium carbonate 10 are supported in close proximity while being separated from each other. Then potassium carbonate 10 is covered with the Al ₂ O ₃ layer 2 and TiO ₂ layer 3, since the Al ₂ O ₃ layer 2 is TiO ₂ layer 3 amphoteric oxide is an acidic oxide, ZrO ₂ particles 1 and carbon Proximity of SO _x to potassium 10 is suppressed, and sulfur poisoning can be sufficiently suppressed.
[0033]
The catalyst powder was pelletized by a conventional method and subjected to a test.
[0034]
( Reference Example 1 )
50 g of ZrO ₂ powder was put into an aqueous solution containing 0.1 mol of potassium acetate, heated with stirring and evaporated to dryness, and K was supported. This was dried and calcined at 500 ° C. for 2 hours to carry K as potassium carbonate.
[0035]
The obtained K / ZrO ₂ powder was impregnated with an acetone solution of platinum acetate (containing 1 g of Pt), heated to 40 ° C. with good stirring, and evaporated to dryness to carry Pt. Since acetone is used as a solvent, the supported potassium carbonate does not elute and the state of being supported on the ZrO ₂ particles is maintained. This was dried and calcined at 500 ° C. for 2 hours to prepare Pt—K / ZrO ₂ powder.
[0036]
The total amount of the obtained Pt-K / ZrO ₂ powder was put into a mixer, and 50 g of an n-propanol solution in which 3.2 g of a Ti coupling agent (“Plenact KR44” manufactured by Ajinomoto Fine-Techno Co., Ltd., equivalent to 0.61 g as TiO ₂ ) was dissolved. The solution was sufficiently stirred while gradually dropping.
[0037]
An appropriate amount of ethanol was added thereto and stirred well, and then the solvent was separated by decantation and centrifugation. This was repeated three times to remove unreacted coupling agent and alcohol, and after drying, calcination was carried out at 500 ° C. for 2 hours. As a result, the organic group of the coupling agent adhered and fixed to the Pt—K / ZrO ₂ particles was oxidized and decomposed to form a thin TiO ₂ layer on the surface of the Pt—K / ZrO ₂ particles.
[0038]
The obtained catalyst is a powder in which TiO ₂ / Pt—K / ZrO ₂ catalyst particles shown in FIG. 2 are assembled. The catalyst particles include a core portion 1 made of ZrO ₂ particles and a TiO ₂ layer 3 formed on the surface of the core portion 1, and potassium carbonate 10 and Pt20 are supported on the core portion 1. Since potassium carbonate 10 is covered with the TiO ₂ layer 3 and TiO ₂ is an acidic oxide, the proximity of SO _x can be suppressed and sulfur poisoning can be sufficiently suppressed.
[0039]
The catalyst powder was pelletized by a conventional method and subjected to a test.
[0040]
(Comparative Example 1)
Pt-K / ZrO ₂ powder prepared in the same manner as in Reference Example 1 was pelletized by a conventional method and subjected to the test.
[0041]
(Comparative Example 2)
Pt / Al ₂ O ₃ / K / ZrO ₂ powder prepared in the same manner as in Example 1 was pelletized by a conventional method and subjected to the test.
[0042]
<Test and evaluation>
Sulfur is processed at 550 ° C for 3 hours by placing the same amount of each pellet catalyst in the evaluation device and flowing the endurance model gas shown in Table 1 alternately in a cycle of lean / rich = 55 seconds / 5 seconds. A poisoning endurance test was conducted.
[0043]
[Table 1]

[0044]
The same amount of each pellet catalyst after the durability test was placed in the evaluation device, and the NO _x storage amount was measured using the evaluation model gas shown in Table 2. First, the gas is switched from the rich gas steady state to the lean gas steady state at 400 ° C., and after flowing the lean gas until the NO _x concentration in the catalyst outgas becomes steady, the rich gas is allowed to flow for 10 seconds, and then the saturated NO when switching to the lean gas again. _x occlusion amount was measured respectively. The results are shown in FIG.
[0045]
[Table 2]

[0046]
FIG. 3 shows that the catalysts of Example 1 and Reference Example 1 have a higher NO _x storage capacity after the endurance test than the catalysts of the comparative examples, and this is the effect of having a TiO ₂ layer on the surface. Is clear.
[0047]
Next, 2 cm ³ of each pellet catalyst after the durability test was filled in the evaluation device, and the temperature was raised from room temperature to 500 ° C. over 20 minutes while flowing the model gas shown in Table 3 at a flow rate of 10 L / min. The purification rate of HC and CO was measured continuously. And 50% purification temperature was calculated | required, respectively, and a result is shown in FIG.
[0048]
[Table 3]

[0049]
From FIG. 4, it can be seen that the catalyst of Example 1 shows higher purification activity than the catalyst of Comparative Example 2, and the catalyst of Reference Example 1 shows higher purification activity than the catalyst of Comparative Example 1. Is clearly the effect of having the TiO ₂ layer 3 on the surface. It is also clear from the comparison between the comparative examples and the comparison between the example 1 and the reference example 1 that it is desirable to carry Pt and K separately.
[0050]
【The invention's effect】
That is, according to the exhaust gas purifying catalyst and the purifying method of the present invention, sulfur poisoning of noble metals can be effectively suppressed, and as a result, a decrease in NO _x storage capacity is suppressed.
[Brief description of the drawings]
FIG. 1 is a schematic cross-sectional view showing catalyst particles according to an embodiment of the present invention.
FIG. 2 is a schematic cross-sectional view showing catalyst particles of Reference Example 1 of the present invention.
FIG. 3 is a graph showing the NO _x storage amount of the catalysts of Example 1, Reference Example 1 and Comparative Example.
FIG. 4 is a graph showing 50% purification temperatures of catalysts of Example 1, Reference Example 1 and Comparative Example.
[Explanation of symbols]
1: ZrO ₂ particles (oxide particles) 2: Al ₂ O ₃ layer (NO _x storage material coated oxide)
3: TiO ₂ layer (acidic oxide layer)

Claims (7)

And _the NO _x storage material bearing oxide carrying _the NO _x storage material, and _the NO _x storage material-coated oxide carrying precious metal covers the _the NO _x storage material supported oxide, and oxide particles consisting of,
A catalyst for exhaust gas purification comprising composite particles comprising an acidic oxide layer made of an acidic oxide having a higher acidity than the oxide particles and coated on the surface of the oxide particles.   The exhaust gas purifying catalyst according to claim 1, wherein the acidic oxide layer contains titania. NO _x  The exhaust gas-purifying catalyst according to claim 1 or 2, wherein the storage material-supported oxide is a basic oxide. NO _x  The exhaust gas-purifying catalyst according to any one of claims 1 to 3, wherein the storage material-supporting oxide contains zirconia. NO _x  The exhaust gas-purifying catalyst according to any one of claims 1 to 4, wherein the occluding material-covering oxide is an amphoteric oxide. NO _x  The exhaust gas-purifying catalyst according to any one of claims 1 to 5, wherein the storage material-covered oxide contains alumina. Using the exhaust gas purifying catalyst according to any one of claims 1 to 6, NO is contained in the exhaust gas in an oxygen-excess lean atmosphere. _x  NO _x  Occluded in the occluding material, the NO in stoichiometric rich atmosphere _x  NO from occlusion material _x  To reduce and purify
The lean exhaust gas that has passed through the acidic oxide layer is contacted with a noble metal to oxidize NO and NO _x  An exhaust gas purification method characterized by producing

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