JP2004337681A - Method for regenerating exhaust gas cleaning catalyst - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To easily recover the activity of a catalyst, in which at least Rh is deposited as an active species, even when the activity is deteriorated after its durability limit. <P>SOLUTION: The exhaust gas cleaning catalyst, in which at least Rh is deposited on a carrier containing at least Al<SB>2</SB>O<SB>3</SB>and its activity is deteriorated because of being used in a high-temperature oxygen-excessive atmosphere, is treated at a high-temperature stoichiometric amount of air or in a reducing atmosphere. The activity of the catalyst is deteriorated since the Rh deposited on the carrier is subjected to the solid-phase reaction with Al<SB>2</SB>O<SB>3</SB>in the high-temperature oxygen-excessive atmosphere. Therefore, the activity-deteriorated catalyst is exposed to the high-temperature stoichiometric point or the reducing atmosphere so that metal Rh is precipitated again and consequently the activity of this catalyst can be recovered. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

【００３６】
触媒Ａを固定床流通型反応装置に搭載し、表１に示すストイキモデルガスの流通下、 １００℃〜 ５００℃まで１２℃／分の速度で昇温し、ＮＯ、ＣＯ及びＣ_３Ｈ_６の浄化率を連続的に測定した。そして各５０％浄化温度を求め、結果を初期５０％浄化温度として表３及び図１に示す。
【００３７】
次に、触媒Ａを大気中にて ８００℃で２０時間保持する劣化試験を行った。空間速度ＳＶは１０，０００^−１である。劣化試験後の触媒について、初期５０％浄化温度の測定と同様にして各５０％浄化温度を測定し、結果を劣化試験後５０％浄化温度として表３及び図１に示す。
【００３８】
【表２】

【００３９】
上記劣化試験後の触媒Ａを固定床流通型反応装置に搭載し、表２に示すリッチガスとリーンガスを５分間ずつ交互に流通させる雰囲気下、 ８００℃で５時間保持する再生試験を行った。空間速度ＳＶは１０，０００^−１である。再生試験後の触媒について、初期５０％浄化温度の測定と同様にして各５０％浄化温度を測定し、結果を再生試験後５０％浄化温度として表３及び図１に示す。
【００４０】
またＮＯ、ＣＯ及びＣ_３Ｈ_６についてそれぞれ劣化試験後５０％浄化温度と再生試験後５０％浄化温度との差を算出し、結果を回復度として表３に示す。
【００４１】
【表３】

【００４２】
次に、触媒Ｂ〜Ｆのそれぞれについて、触媒Ａと同様にして劣化試験後５０％浄化温度と再生試験後５０％浄化温度を測定し、それぞれ回復度を算出した。結果を触媒Ａの結果とともに表４に示す。
【００４３】
【表４】

【００４４】
表３及び図１より、劣化試験によって活性が劣化した触媒Ａは、再生試験によって活性が回復し、しかも初期より再生試験後の方が活性が向上していることがわかる。
【００４５】
また表４より、触媒Ｂでも回復現象が認められるものの、回復度は実施例１より小さい。また触媒Ａと同じ担体を用いＰｔを担持した触媒Ｆでは、再生試験による回復はほとんど認められない。したがって再生試験によって活性が再生するのは少なくともＲｈを活性種とし、担体には Ａｌ_２Ｏ_３単味よりもＣｅＯ_２及びＺｒＯ_２が共存するのが望ましいことが明らかである。
【００４６】
すなわち触媒Ａ，Ｃ，Ｄ，Ｅでは、 Ａｌ_２Ｏ_３とＣｅＯ_２とＺｒＯ_２とが共存しているため、それぞれの酸化物が互いに障壁となって粒成長が抑制され、しかもＣｅＯ_２表面にＲｈが微細に再析出したため、触媒Ｂに比べ再生試験後に活性が大きく回復したと考えられる。また触媒Ａ，Ｃ，Ｄ，Ｅの比較から、 Ａｌ_２Ｏ_３とＣｅＯ_２とＺｒＯ_２との組成比には最適範囲が存在することが示唆される。
【００４７】
【発明の効果】
すなわち本発明の排ガス浄化用触媒の再生方法によれば、高温の酸素過剰雰囲気で使用されることで劣化した触媒の活性を容易に、しかも劣化する前以上に回復させることができる。
【図面の簡単な説明】
【図１】触媒Ａの初期、劣化試験後及び再生試験後の各５０％浄化温度を示すグラフである。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method for recovering and regenerating the activity of an exhaust gas purifying catalyst whose activity has been reduced by being used in a high-temperature oxygen-rich atmosphere.
[0002]
[Prior art]
Various catalysts such as three-way catalysts and oxidation catalysts have been used as exhaust gas purification catalysts for automobiles. In these catalysts, noble metals such as Pt, Pd, and Rh are used as active species, and are used by being supported on a carrier such as Al ₂ O ₃ , ZrO ₂ , TiO ₂ , and CeO ₂ . In order for the purification reaction to proceed efficiently, it is essential to increase the surface area of the active species. Therefore, an aqueous solution of a water-soluble salt of a noble metal or the like has been conventionally used, and has been highly dispersed and supported on a carrier by an adsorption supporting method, an impregnation supporting method, or the like.
[0003]
However, the noble metal supported in a highly dispersed state has a problem in that the heat during use causes grain growth or solid solution in the carrier, and the activity of the catalyst decreases accordingly. For example, in a high-temperature oxygen-rich atmosphere, Pt is oxidized to volatile PtO ₂ , detached and diffused from the supporting position, and trapped by other Pt atoms or Pt fine particles, causing grain growth. Rh loses its activity by undergoing a solid phase reaction with Al ₂ O ₃ or the like in a high-temperature oxygen-rich atmosphere. Further, Pd easily sinters in an atmosphere near a high-temperature stoichiometric state.
[0004]
Therefore, Japanese Patent Application Laid-Open No. 09-075755 discloses a method of recovering the activity by oxidizing at least a catalyst supporting Pd in a high-temperature oxygen-rich atmosphere. According to this method, the surface of the sintered metal Pd becomes a Pd oxide, moves on the carrier, and is reduced during use to become metal Pd. The activity is restored.
[0005]
Japanese Patent Application Laid-Open No. 2000-202309 discloses a method of recovering the activity by treating a catalyst in which at least Pt is supported on a basic carrier in an oxidizing atmosphere and then in a reducing atmosphere. According to this method, the surface of the coarse Pt particles is easily oxidized because the basic carrier has a high affinity for Pt. Then, the generated Pt oxide is separated and diffused from the surface of the coarse Pt particles and moves on the carrier, so that the coarse Pt particles gradually decrease in particle diameter. The transferred Pt oxide is reduced in a reducing atmosphere to become Pt metal. Therefore, Pt is again in a state of being highly dispersed and supported, and the activity is restored.
[0006]
[Patent Document 1] JP-A-09-075755 [Patent Document 2] JP-A-2000-202309
[Problems to be solved by the invention]
However, the above-mentioned patent document only discloses a method for regenerating a catalyst supporting Pd and Pt, and does not say about a catalyst supporting Rh. Since Rh is the most active species for the reaction of reducing and removing NO in exhaust gas, it is desired to establish a regeneration method for a catalyst carrying Rh.
[0008]
The present invention has been made in view of such circumstances, and an object of the present invention is to easily recover the activity of a catalyst supporting Rh as an active species even if the activity decreases after durability.
[0009]
[Means for Solving the Problems]
The feature of the method for regenerating an exhaust gas purifying catalyst of the present invention that solves the above-mentioned problems is that at least Rh is supported on a carrier containing at least Al ₂ O ₃ , and the exhaust gas whose activity has been reduced by being used in a high-temperature oxygen-rich atmosphere It is to treat the purification catalyst in a high temperature stoichiometric or reducing atmosphere.
[0010]
The support preferably further contains CeO ₂ and ZrO _2, and more preferably, Al ₂ O ₃ is mixed with CeO ₂ and ZrO ₂ at a nano level.
[0011]
BEST MODE FOR CARRYING OUT THE INVENTION
In the method for regenerating an exhaust gas purifying catalyst of the present invention, a catalyst having at least Rh supported on a carrier containing at least Al ₂ O ₃ and having reduced activity is treated in a high-temperature stoichiometric or reducing atmosphere. Rh supported on a carrier containing at least Al ₂ O ₃ undergoes a solid-phase reaction with Al ₂ O ₃ at a high temperature and in an oxygen-excess atmosphere, resulting in loss of activity. However, when subsequently exposed to a high-temperature stoichiometric or reducing atmosphere, the metal Rh is re-deposited and the activity is recovered, and the catalytic function is regenerated.
[0012]
The amount of Rh supported in the catalyst is preferably in the range of 0.05 to 1% by weight. If the supported amount of Rh is less than this range, sufficient activity as an exhaust gas purifying catalyst cannot be obtained, and even if the supported amount is larger than this range, the activity is saturated and the supported density increases, so that Rh itself is not regenerated during the regeneration treatment. The activity may be reduced due to grain growth. Note that at least Rh must be supported, but other noble metals such as Pt and Pd may coexist.
[0013]
By the way, a phenomenon is observed in Al ₂ O ₃ in which phase transformation occurs at a high temperature and grain growth is remarkable. Therefore, when a part of the supported Rh is trapped inside the agglomerated particles of Al ₂ O ₃ , a part where the metal Rh cannot be reprecipitated on the outermost surface even if the treatment is performed in a high-temperature stoichiometric or reducing atmosphere. Also exists.
[0014]
Therefore, it is preferable to use a carrier in which Al ₂ O ₃ and other components that do not normally react in a solid phase are mixed at the nano level. If such a carrier is used, Al ₂ O ₃ and other components act as barriers to each other, so that grain growth of Al ₂ O ₃ is suppressed and Rh is not trapped inside the aggregated particles. Therefore, by performing the treatment under a high-temperature stoichiometric or reducing atmosphere, the metal Rh can be reprecipitated on the outermost surface, and the regeneration can be performed.
[0015]
Other components that do not normally react in a solid phase with Al ₂ O ₃ include ZrO ₂ , CeO ₂ , and TiO ₂ . When the metal element of the other component is M, the mixing ratio of the other component is preferably in the range of Al: M = 4: 1 to 1: 4 in atomic ratio. If the amount of the other components exceeds this range, the amount of Al ₂ O ₃ is relatively reduced, and as a result, the heat resistance of the carrier is reduced and the regeneration becomes difficult. If the amount of other components is less than this range, it is difficult to suppress the grain growth of Al ₂ O ₃ , and the effect of mixing other components will not be exhibited.
[0016]
In order to prepare a carrier in which Al ₂ O ₃ and other components are mixed at a nano level, a precipitate of an oxide precursor is precipitated from an aqueous solution in which an Al compound and a metal M compound are both dissolved, and the resultant is calcined. Can be prepared. In this case, it is preferable to bake the precipitate in a state where sufficient moisture is present in the system. It is also preferable that the precipitate is aged in a suspended state using water as a dispersion medium, and then calcined. Aging is preferably performed at 100 to 200 ° C, more preferably at 100 to 150 ° C. It is also possible to hydrolyze a mixture of an alkoxide of Al and an alkoxide of metal M to form a precursor.
[0017]
A salt is generally used as the compound of Al and the compound of the metal M, and as the salt, sulfate, nitrate, hydrochloride, acetate and the like can be used. Water and alcohols can be used as a solvent for uniformly dissolving the salt. Furthermore, for example, aluminum hydroxide, nitric acid and water may be mixed and used as a raw material of aluminum nitrate.
[0018]
Then, by adding an alkaline solution to this solution, a precipitate of the oxide precursor is deposited. As the alkaline solution, an aqueous solution in which ammonia, ammonium carbonate, sodium hydroxide, potassium hydroxide, sodium carbonate or the like is dissolved, or an alcohol solution can be used. Ammonia and ammonium carbonate which volatilize during firing are particularly preferred. It is more preferable that the pH of the alkaline solution is 9 or more, since the precipitation reaction of the precursor is promoted.
[0019]
When Rh is supported on such a composite oxide carrier, Rh is also supported on components other than Al ₂ O ₃ . At high temperatures and under an oxygen rich atmosphere, such as metal Rh supported on CeO ₂ is active is reduced by solid phase reaction and Rh ₂ O ₃ next to CeO _2. However on CeO ₂ is, _Rh 2 _{O 3} is in a state wet with CeO ₂ surface due to the strong affinity with CeO _2. When treated under a high temperature stoichiometric or reducing atmosphere in such a state, Rh ₂ O ₃ is decomposed and metal Rh is reprecipitated, but it is reprecipitation from Rh ₂ O ₃ wet on the surface of CeO _2. Then, the metal Rh is reprecipitated into fine particles. Thus, the regenerated catalyst has increased active sites and sometimes exhibits higher activity than at the beginning.
[0020]
However, Rh supported on CeO ₂ alone has too high an affinity for CeO ₂ , so that reprecipitation becomes difficult unless a treatment under a stoichiometric or reducing atmosphere is performed at a high temperature of 900 ° C. or more. There is. Therefore, the carrier preferably contains CeO ₂ and ZrO ₂ . CeO ₂ and easily form a solid solution and ZrO _2, considered basicity of CeO ₂ is moderately suppressed by containing ZrO ₂ is weak solid basicity compared to CeO _2, metal from a low temperature range Rh Can be reprecipitated, and the activity is easily recovered. The inhibition of grain growth of Al ₂ O ₃ by _CeO 2 -ZrO ₂ solid _solution, CeO 2 -ZrO ₂ solid solution because of its high heat resistance than the CeO ₂ plain is CeO ₂ PLAIN least equivalent.
[0021]
The ratio of CeO ₂ to ZrO ₂ is preferably in the range of Ce: Zr = 19: 1 to 1:19 in atomic ratio. If the amount of ZrO ₂ is larger than this range, the amount of CeO ₂ will be relatively small, and it will be difficult to exhibit the effect of depositing metal Rh in fine particles. In addition, the oxygen storage / release capacity (OSC) due to CeO ₂ also decreases. On the other hand, if the amount of ZeO ₂ is less than this range, the effect of appropriately adjusting the solid basicity of CeO ₂ is reduced, and the degree of activity recovery is reduced.
[0022]
Further, as other metal elements other than Al, alkaline earth metals, rare earth elements, and the like can be included. When such a component is contained, the effect of improving the heat resistance of the Al ₂ O ₃ or CeO ₂ —ZrO ₂ solid solution and further improving the oxygen storage / release capability (OSC) may be obtained. There is also an advantage that the acid-base degree of the carrier can be adjusted according to the purpose. Such a component is preferably in the range of 0.1 to 5 mol% based on the carrier. If the amount is less than this, the added effect is not exhibited, and if it is too large, the activity is reduced.
[0023]
In the regeneration method of the present invention, the catalyst whose activity has been reduced by being used in a high-temperature oxygen-rich atmosphere is treated in a high-temperature stoichiometric or reducing atmosphere. The high temperature in the oxygen-excess atmosphere refers to a temperature at which Rh undergoes a solid-phase reaction with a carrier containing at least Al ₂ O ₃ or becomes an oxide state, and is about 800 ° C. or higher. The oxygen-excess atmosphere means an atmosphere equivalent to λ> 1.0.
[0024]
The temperature for the treatment in a high-temperature stoichiometric or reducing atmosphere is preferably in the range of 500 to 1000 ° C, and particularly preferably in the range of 600 to 900 ° C. If the treatment temperature is lower than 500 ° C., reprecipitation of metal Rh becomes difficult, and if the treatment temperature exceeds 1000 ° C., grain growth of the carrier or metal Rh occurs, which is not preferable. As described above, when a CeO ₂ -ZrO ₂ solid solution is included, the metal Rh can be reprecipitated by performing the treatment in a low temperature range of 600 to 800 ° C.
[0025]
The degree of the stoichiometric or reducing atmosphere is preferably λ ≦ 1.0, more preferably λ ≦ 0.95. If the corresponding λ value exceeds 1.0, it becomes difficult to redeposit Rh. The processing time varies depending on the degree of activity reduction and the degree of the reducing atmosphere. For an oxygen-excess atmosphere of several tens of seconds due to fuel cut during high-speed running, a treatment of several seconds is sufficient, and even a catalyst that has been used for a long time without a recovery operation and has been completely degraded for 5 hours or less is sufficient.
[0026]
【Example】
Hereinafter, the present invention will be specifically described with reference to examples. First, catalysts A to F were prepared as follows.
[0027]
(Catalyst A)
A predetermined amount of aluminum nitrate nonahydrate, cerium nitrate hexahydrate, and zirconyl nitrate dihydrate was dissolved in pure water to prepare a mixed aqueous solution. After adding 1.1 times equivalent of hydrogen peroxide solution of Ce ion to this, neutralized by adding 1.2 times mole of ammonia water to neutralization equivalent of nitrate of each salt while stirring vigorously, A precursor precipitate was obtained by a coprecipitation method. This precursor precipitate is dried together with the solution by heating at 150 ° C., dried at 300 ° C. in the air for 3 hours, calcined at 500 ° C. for 1 hour, and further heat-treated at 700 ° C. in the air for 5 hours to obtain a composite oxide powder. Was prepared. The composition of the composite oxide is Al ₂ O ₃ : CeO ₂ : ZrO ₂ = 1: 1: 1 in molar ratio, and at least a part of CeO ₂ and ZrO ₂ is a solid solution.
[0028]
The obtained composite oxide powder was impregnated with a predetermined amount of an aqueous solution of rhodium nitrate having a predetermined concentration, evaporated and dried, and then calcined at 300 ° C. for 3 hours in the atmosphere to prepare a catalyst powder. The supported amount of Rh is 0.2 g per 100 g of the composite oxide powder. The obtained catalyst powder was pulverized after compacting, and sized to obtain a 0.5 to 1 mm pellet catalyst.
[0029]
(Catalyst B)
Using aluminum nitrate 9 hydrate and lanthanum nitrate hexahydrate in the same manner as in catalyst A except that no aqueous hydrogen peroxide was added, the molar ratio of Al ₂ O ₃ : La ₂ O ₃ = 1 : 0.025 composite oxide powder was prepared. Using this composite oxide powder, catalyst B was prepared in the same manner as catalyst A.
[0030]
(Catalyst C)
A composite oxide powder having a molar ratio of Al ₂ O ₃ : CeO ₂ : ZrO ₂ = 1: 1.9: 0.1 was prepared in the same manner as the catalyst A except that the amount of the metal salt was changed. Using this composite oxide powder, catalyst C was prepared in the same manner as catalyst A.
[0031]
(Catalyst D)
A composite oxide powder having a molar ratio of Al ₂ O ₃ : CeO ₂ : ZrO ₂ = 1: 0.1: 1.9 was prepared in the same manner as in the catalyst A except that the amount of the metal salt was changed. Using this composite oxide powder, catalyst D was prepared in the same manner as catalyst A.
[0032]
(Catalyst E)
A composite oxide powder having a molar ratio of Al ₂ O ₃ : CeO ₂ : ZrO ₂ = 0.5: 1: 1 was prepared in the same manner as in the catalyst A except that the amount of the metal salt was changed. Catalyst E was prepared in the same manner as Catalyst A using the oxide powder.
[0033]
(Catalyst F)
A catalyst E was prepared in the same manner as the catalyst A except that an aqueous dinitrodiammine platinum solution was used instead of the aqueous rhodium nitrate solution.
[0034]
<Test / Evaluation>
[0035]
[Table 1]

[0036]
The catalyst A was mounted on a fixed bed flow type reactor, and heated at a rate of 12 ° C./min from 100 ° C. to 500 ° C. under the flow of the stoichiometric model gas shown in Table 1 to obtain NO, CO and C ₃ H ₆ . Purification rates were measured continuously. Then, each 50% purification temperature was determined, and the result is shown in Table 3 and FIG. 1 as an initial 50% purification temperature.
[0037]
Next, a deterioration test was conducted in which the catalyst A was kept at 800 ° C. in the atmosphere for 20 hours. The space velocity SV is 10,000 ^-1 . For the catalyst after the deterioration test, each 50% purification temperature was measured in the same manner as the measurement of the initial 50% purification temperature, and the results are shown in Table 3 and FIG. 1 as the 50% purification temperature after the deterioration test.
[0038]
[Table 2]

[0039]
The catalyst A after the deterioration test was mounted on a fixed bed flow type reaction apparatus, and a regeneration test was performed at 800 ° C. for 5 hours under an atmosphere in which rich gas and lean gas alternately flowed for 5 minutes as shown in Table 2. The space velocity SV is 10,000 ^-1 . Regarding the catalyst after the regeneration test, each 50% purification temperature was measured in the same manner as the measurement of the initial 50% purification temperature, and the results are shown in Table 3 and FIG. 1 as the 50% purification temperature after the regeneration test.
[0040]
Further, for NO, CO and C ₃ H ₆ , the difference between the 50% purification temperature after the deterioration test and the 50% purification temperature after the regeneration test was calculated, and the results are shown in Table 3 as the degree of recovery.
[0041]
[Table 3]

[0042]
Next, for each of the catalysts B to F, the 50% purification temperature after the deterioration test and the 50% purification temperature after the regeneration test were measured in the same manner as the catalyst A, and the degree of recovery was calculated for each. The results are shown in Table 4 together with the results of Catalyst A.
[0043]
[Table 4]

[0044]
From Table 3 and FIG. 1, it can be seen that the activity of Catalyst A, whose activity has been deteriorated by the degradation test, has been recovered by the regeneration test, and the activity has been improved after the regeneration test from the initial stage.
[0045]
Further, from Table 4, although a recovery phenomenon is observed also in the catalyst B, the degree of recovery is smaller than that in Example 1. Further, in the case of the catalyst F in which Pt is supported using the same carrier as the catalyst A, almost no recovery is observed in the regeneration test. Therefore, it is clear that it is preferable that the activity is regenerated by at least Rh as the active species in the regeneration test, and that CeO ₂ and ZrO ₂ coexist in the carrier rather than Al ₂ O ₃ alone.
[0046]
That catalyst A, C, D, in E, because it coexist with _Al 2 _{O 3} and CeO ₂ and ZrO _2, the grain growth respective oxides becomes a barrier to one another is suppressed, moreover the CeO ₂ surface It is considered that the activity was largely recovered after the regeneration test as compared with the catalyst B due to the fine precipitation of Rh. Also, comparison of catalysts A, C, D, and E suggests that there is an optimal range for the composition ratio of Al ₂ O ₃ , CeO _2, and ZrO ₂ .
[0047]
【The invention's effect】
That is, according to the method for regenerating an exhaust gas purifying catalyst of the present invention, the activity of a catalyst that has been deteriorated by being used in a high-temperature oxygen-rich atmosphere can be easily recovered more than before the deterioration.
[Brief description of the drawings]
FIG. 1 is a graph showing 50% purification temperatures of catalyst A at an initial stage, after a deterioration test, and after a regeneration test.

Claims (3)

An exhaust gas purifying catalyst having at least Rh supported on a carrier containing at least Al ₂ O ₃ and having reduced activity by being used in a high-temperature oxygen-rich atmosphere is treated in a high-temperature stoichiometric or reducing atmosphere. A method for regenerating an exhaust gas purifying catalyst. The carrier, the method of regeneration exhaust gas purifying catalyst according to claim 1, further comprising a CeO ₂ and ZrO _2. The carrier, _Al 2 _{O 3} and CeO ₂ and ZrO ₂ is the method of regeneration exhaust gas purifying catalyst according to claim 2, which is mixed at the nano level.

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