JP2014066149A - NOx ELIMINATION SYSTEM - Google Patents

NOx ELIMINATION SYSTEM Download PDF

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JP2014066149A
JP2014066149A JP2012210724A JP2012210724A JP2014066149A JP 2014066149 A JP2014066149 A JP 2014066149A JP 2012210724 A JP2012210724 A JP 2012210724A JP 2012210724 A JP2012210724 A JP 2012210724A JP 2014066149 A JP2014066149 A JP 2014066149A
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catalyst
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Taiji Nagaoka
大治 長岡
Teruo Nakada
輝男 中田
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Isuzu Motors Ltd
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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J23/00—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00
    • B01J23/38—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of noble metals
    • B01J23/40—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of noble metals of the platinum group metals
    • B01J23/42—Platinum
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
    • B01D53/34—Chemical or biological purification of waste gases
    • B01D53/92—Chemical or biological purification of waste gases of engine exhaust gases
    • B01D53/94—Chemical or biological purification of waste gases of engine exhaust gases by catalytic processes
    • B01D53/9404—Removing only nitrogen compounds
    • B01D53/9409—Nitrogen oxides
    • B01D53/9413—Processes characterised by a specific catalyst
    • B01D53/9422—Processes characterised by a specific catalyst for removing nitrogen oxides by NOx storage or reduction by cyclic switching between lean and rich exhaust gases (LNT, NSC, NSR)
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
    • F01N—GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR MACHINES OR ENGINES IN GENERAL; GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR INTERNAL COMBUSTION ENGINES
    • F01N13/00—Exhaust or silencing apparatus characterised by constructional features ; Exhaust or silencing apparatus, or parts thereof, having pertinent characteristics not provided for in, or of interest apart from, groups F01N1/00 - F01N5/00, F01N9/00, F01N11/00
    • F01N13/009—Exhaust or silencing apparatus characterised by constructional features ; Exhaust or silencing apparatus, or parts thereof, having pertinent characteristics not provided for in, or of interest apart from, groups F01N1/00 - F01N5/00, F01N9/00, F01N11/00 having two or more separate purifying devices arranged in series
    • F01N13/0093—Exhaust or silencing apparatus characterised by constructional features ; Exhaust or silencing apparatus, or parts thereof, having pertinent characteristics not provided for in, or of interest apart from, groups F01N1/00 - F01N5/00, F01N9/00, F01N11/00 having two or more separate purifying devices arranged in series the purifying devices are of the same type
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
    • F01N—GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR MACHINES OR ENGINES IN GENERAL; GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR INTERNAL COMBUSTION ENGINES
    • F01N13/00—Exhaust or silencing apparatus characterised by constructional features ; Exhaust or silencing apparatus, or parts thereof, having pertinent characteristics not provided for in, or of interest apart from, groups F01N1/00 - F01N5/00, F01N9/00, F01N11/00
    • F01N13/009—Exhaust or silencing apparatus characterised by constructional features ; Exhaust or silencing apparatus, or parts thereof, having pertinent characteristics not provided for in, or of interest apart from, groups F01N1/00 - F01N5/00, F01N9/00, F01N11/00 having two or more separate purifying devices arranged in series
    • F01N13/0097—Exhaust or silencing apparatus characterised by constructional features ; Exhaust or silencing apparatus, or parts thereof, having pertinent characteristics not provided for in, or of interest apart from, groups F01N1/00 - F01N5/00, F01N9/00, F01N11/00 having two or more separate purifying devices arranged in series the purifying devices are arranged in a single housing
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
    • F01N—GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR MACHINES OR ENGINES IN GENERAL; GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR INTERNAL COMBUSTION ENGINES
    • F01N3/00—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust
    • F01N3/08—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous
    • F01N3/0807—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous by using absorbents or adsorbents
    • F01N3/0828—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous by using absorbents or adsorbents characterised by the absorbed or adsorbed substances
    • F01N3/0842—Nitrogen oxides
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2255/00—Catalysts
    • B01D2255/10—Noble metals or compounds thereof
    • B01D2255/102—Platinum group metals
    • B01D2255/1021—Platinum
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2255/00—Catalysts
    • B01D2255/10—Noble metals or compounds thereof
    • B01D2255/102—Platinum group metals
    • B01D2255/1025—Rhodium
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2255/00—Catalysts
    • B01D2255/20—Metals or compounds thereof
    • B01D2255/202—Alkali metals
    • B01D2255/2022—Potassium
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2255/00—Catalysts
    • B01D2255/20—Metals or compounds thereof
    • B01D2255/204—Alkaline earth metals
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2255/00—Catalysts
    • B01D2255/20—Metals or compounds thereof
    • B01D2255/206—Rare earth metals
    • B01D2255/2065—Cerium
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2255/00—Catalysts
    • B01D2255/90—Physical characteristics of catalysts
    • B01D2255/904—Multiple catalysts
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2255/00—Catalysts
    • B01D2255/90—Physical characteristics of catalysts
    • B01D2255/91—NOx-storage component incorporated in the catalyst
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2257/00—Components to be removed
    • B01D2257/40—Nitrogen compounds
    • B01D2257/404—Nitrogen oxides other than dinitrogen oxide

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  • Engineering & Computer Science (AREA)
  • Chemical & Material Sciences (AREA)
  • Combustion & Propulsion (AREA)
  • General Engineering & Computer Science (AREA)
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  • General Chemical & Material Sciences (AREA)
  • Oil, Petroleum & Natural Gas (AREA)
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  • Exhaust Gas Treatment By Means Of Catalyst (AREA)
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  • Exhaust Gas After Treatment (AREA)

Abstract

PROBLEM TO BE SOLVED: To provide an inexpensive NOx elimination system using NOx storage-reduction type catalysts for eliminating NOx from exhaust gas, in which the amount of precious and expensive rhodium to be used can be reduced by devising the composition of the catalysts.SOLUTION: In a NOx elimination system 1 which includes NOx storage-reduction type catalysts 2, 3, the high-temperature NOx storage-reduction type catalyst 2 supporting a NOx storage material formed of potassium and the low-temperature NOx storage-reduction type catalyst 3 supporting a NOx storage material formed of alkaline earth metal are arranged in series to each other on the upstream side and on the downstream side, respectively. The amount of potassium supported by the high-temperature NOx storage-reduction type catalyst 2 is four to twelve times the amount of platinum supported by the high-temperature NOx storage-reduction type catalyst 2 in terms of "g/L", and the mole ratio of platinum to rhodium supported by the high-temperature NOx storage-reduction type catalyst 2 is 20:1 to 1:2.

Description

本発明は、排気ガスの後処理装置に使用されるＮＯｘ吸蔵還元型触媒の触媒組成とその配置に関し、特に、低温側の浄化率を高め、広い温度域で高い浄化率が得られる触媒組成と配置を備えたＮＯｘ浄化システムに関する。   The present invention relates to a catalyst composition and arrangement of a NOx occlusion reduction catalyst used in an exhaust gas aftertreatment device, and in particular, a catalyst composition that increases the purification rate on the low temperature side and obtains a high purification rate in a wide temperature range. The present invention relates to a NOx purification system having an arrangement.

ディーゼルエンジンや一部のガソリンエンジン等の内燃機関や様々な燃焼装置の排気ガス中からＮＯｘを還元除去するためのＮＯｘ触媒について種々の研究や提案がなされている。その一つに、ディーゼルエンジン用のＮＯｘ低減触媒としてＮＯｘ吸蔵還元型触媒があり、有効に排気ガス中のＮＯｘを浄化できる。   Various studies and proposals have been made on NOx catalysts for reducing and removing NOx from internal combustion engines such as diesel engines and some gasoline engines and exhaust gases from various combustion devices. One of them is a NOx occlusion reduction type catalyst as a NOx reduction catalyst for diesel engines, which can effectively purify NOx in exhaust gas.

このＮＯｘ吸蔵還元型触媒は、基本的に、アルミナ等の触媒担体上に、酸化・還元反応を促進する白金（Ｐｔ），ロジウム（Ｒｈ），パラジウム（Ｐｄ）等の貴金属類の触媒金属と、バリウム（Ｂａ）等のアルカリ土類金属やカリウム（Ｋ）等のアルカリ金属等で形成されるＮＯｘを吸蔵・放出する機能を有するＮＯｘ吸蔵材（ＮＯｘ吸蔵物質）を担持した触媒である。   This NOx occlusion reduction type catalyst basically has a catalyst metal such as platinum (Pt), rhodium (Rh), palladium (Pd) or the like that promotes an oxidation / reduction reaction on a catalyst carrier such as alumina, It is a catalyst carrying a NOx occlusion material (NOx occlusion material) having a function of occluding and releasing NOx formed with alkaline earth metals such as barium (Ba) and alkali metals such as potassium (K).

このＮＯｘ吸蔵還元型触媒は、流入する排気ガスの空燃比がリーン（酸素過多）状態であって雰囲気中に酸素（Ｏ2）が存在する場合には、排気ガス中の一酸化窒素（ＮＯ）が触媒金属により酸化されて二酸化窒素（ＮＯ2）となり、この二酸化窒素はＮＯｘ吸蔵材に硝酸塩（Ｂａ2ＮＯ4）等として蓄積される。 This NOx occlusion reduction type catalyst has a nitrogen (NO) concentration in the exhaust gas when the air-fuel ratio of the inflowing exhaust gas is lean (oxygen-rich) and oxygen (O 2 ) is present in the atmosphere. Is oxidized by the catalytic metal to form nitrogen dioxide (NO 2 ), and this nitrogen dioxide is accumulated in the NOx storage material as nitrate (Ba 2 NO 4 ) or the like.

また、流入する排気ガスの空燃比が理論空燃比やリッチ（低酸素濃度）状態になって雰囲気中の酸素濃度が低下すると、バリウム等のＮＯｘ吸蔵材は一酸化炭素（ＣＯ）と結合し、硝酸塩から二酸化窒素が分解放出され、この放出された二酸化窒素は貴金属類の三元機能により排気ガス中に含まれている未燃炭化水素（ＨＣ）や一酸化炭素等で還元され窒素（Ｎ2）となり、排気ガス中の諸成分は、二酸化炭素（ＣＯ2），水（Ｈ2Ｏ），窒素等の無害な物質として大気中に放出される。 In addition, when the air-fuel ratio of the inflowing exhaust gas becomes the stoichiometric air-fuel ratio or rich (low oxygen concentration) state and the oxygen concentration in the atmosphere decreases, the NOx storage material such as barium combines with carbon monoxide (CO), Nitrogen dioxide is decomposed and released from nitrate, and this released nitrogen dioxide is reduced by unburned hydrocarbons (HC), carbon monoxide, etc. contained in the exhaust gas by the ternary function of precious metals, and nitrogen (N 2 The various components in the exhaust gas are released into the atmosphere as harmless substances such as carbon dioxide (CO 2 ), water (H 2 O), and nitrogen.

そのため、ＮＯｘ吸蔵還元型触媒を備えたＮＯｘ浄化システムでは、ＮＯｘ吸蔵能力が飽和に近くなると、排気ガスの空燃比をリッチにして、流入する排気ガスの酸素濃度を低下させるＮＯｘ吸蔵能力回復用のリッチ制御を行い、これにより吸収したＮＯｘを放出させて、この放出されたＮＯｘを貴金属触媒により還元させるＮＯｘ再生操作を行っている。   Therefore, in the NOx purification system provided with the NOx occlusion reduction type catalyst, when the NOx occlusion capacity is close to saturation, the air-fuel ratio of the exhaust gas is made rich, and the oxygen concentration of the inflowing exhaust gas is reduced, so that the NOx occlusion capacity is recovered. A rich control is performed to release the absorbed NOx, and a NOx regeneration operation is performed in which the released NOx is reduced by a noble metal catalyst.

しかしながら、このＮＯｘ吸蔵還元型触媒においては、触媒の組成により、低温活性が高い低温型ＮＯｘ吸蔵還元型触媒と、高温活性が高い高温型ＮＯｘ吸蔵還元型触媒に分かれるが、低温型ＮＯｘ吸蔵還元型触媒は、貴金属の活性を阻害しないバリウム等のアルカリ土類金属を主体にしたＮＯｘ吸蔵材を使用しているため、貴金属の活性が阻害されず、低温時のＮＯｘ還元性能に優れている。しかし、このアルカリ土類金属は、高温時にＮＯｘ吸蔵能力が低下するという問題がある。   However, this NOx occlusion reduction type catalyst is divided into a low temperature type NOx occlusion reduction type catalyst having a high temperature activity and a high temperature type NOx occlusion reduction type catalyst having a high temperature activity, depending on the composition of the catalyst. Since the catalyst uses a NOx occlusion material mainly composed of an alkaline earth metal such as barium that does not inhibit the activity of the noble metal, the activity of the noble metal is not inhibited and the NOx reduction performance at a low temperature is excellent. However, this alkaline earth metal has a problem that the NOx occlusion ability decreases at high temperatures.

一方、高温型ＮＯｘ吸蔵還元型触媒は、吸蔵材にバリウム等のアルカリ土類金属とは逆の特性を持つカリウム等のアルカリ金属を使用しており、このアルカリ金属は、高温時のＮＯｘ吸蔵能力は高いが、低温時に貴金属（酸化触媒）の活性を阻害するため、低温域のＮＯｘ還元性能が低下するという問題がある。   On the other hand, the high-temperature NOx storage-reduction catalyst uses an alkali metal such as potassium, which has the opposite characteristics to alkaline earth metals such as barium, as the storage material, and this alkali metal has NOx storage capacity at high temperatures. However, since the activity of the noble metal (oxidation catalyst) is inhibited at a low temperature, there is a problem that the NOx reduction performance in a low temperature range is lowered.

このＮＯｘ吸蔵還元型触媒の温度ウィンドウを広げる試みとして、上流側の高温型ＮＯｘ吸蔵還元型触媒と下流側の低温型ＮＯｘ吸蔵還元型触媒を排ガス中に配置した排ガス浄化システムや排ガス浄化装置が提案されたり、リーン雰囲気でのＮＯｘ活性温度範囲の異なる複数の触媒を直列に近接配置し、ＮＯｘ活性温度範囲の高いものほど、触媒容量の配分を大きく設定し、また、上流側に配置する内燃機関の排気浄化用触媒装置が提案されたりしている（例えば、特許文献１〜３参照。）。   As an attempt to widen the temperature window of this NOx storage reduction catalyst, an exhaust gas purification system and an exhaust gas purification device in which an upstream high-temperature NOx storage reduction catalyst and a downstream low-temperature NOx storage reduction catalyst are arranged in exhaust gas are proposed. The internal combustion engine in which a plurality of catalysts having different NOx active temperature ranges in a lean atmosphere are arranged close to each other in series, and the higher the NOx active temperature range is, the larger the catalyst capacity distribution is set, and the higher the NOx active temperature range is. The exhaust gas purification catalyst device has been proposed (see, for example, Patent Documents 1 to 3).

また、その一方、このＮＯｘ吸蔵還元型触媒においては、熱劣化（主に、シンタリング）によるＮＯｘ浄化率の低下という問題もある。低温型ＮＯｘ吸蔵還元型触媒においては、熱劣化による低温域（２００℃付近）のＮＯｘ浄化率の低下は非常に少ないが、高温域（５００℃付近）のＮＯｘ浄化率は徐々に低下する。この理由は、低温域はＮＯｘ吸着性能を有効に利用しているので、貴金属劣化による「ＮＯ→ＮＯ2」の活性の低下に伴う吸蔵効率の低下の影響を受け難いためと考えられる。また、高温型ＮＯｘ吸蔵還元型触媒においては、反対に、熱劣化による高温域（５００℃付近）のＮＯｘ浄化率の低下は非常に少ないが、低温域（２００℃付近）のＮＯｘ浄化率は急激に低下する。 On the other hand, this NOx occlusion reduction type catalyst also has a problem that the NOx purification rate decreases due to thermal deterioration (mainly sintering). In the low-temperature NOx occlusion reduction type catalyst, the NOx purification rate in the low-temperature region (around 200 ° C.) decreases very little due to thermal deterioration, but the NOx purification rate in the high-temperature region (around 500 ° C.) gradually decreases. This is probably because the low temperature region effectively uses the NOx adsorption performance and is not easily affected by the decrease in storage efficiency due to the decrease in the activity of “NO → NO 2 ” due to noble metal degradation. On the other hand, in the high-temperature NOx occlusion reduction type catalyst, on the contrary, the NOx purification rate in the high temperature range (around 500 ° C.) is very small due to thermal degradation, but the NOx purification rate in the low temperature range (around 200 ° C.) To drop.

これらの熱劣化の特性を考慮しながら、システム全体として、低温域から高温域まで熱劣化の影響が少ないＮＯｘ浄化システムにする必要がある。   In consideration of these thermal deterioration characteristics, the entire system needs to be a NOx purification system that is less affected by thermal deterioration from a low temperature range to a high temperature range.

本発明者らは、上記の問題に鑑みて、触媒の組成及び配置（レイアウト）を工夫することにより、幅広いＮＯｘ活性温度ウィンドウを持つＮＯｘ浄化システムを提供することを目的に、排気ガスの空燃比が、リーン状態の場合にＮＯｘを吸蔵し、かつ、リッチ状態の場合に吸蔵していたＮＯｘを放出するＮＯｘ吸蔵材と触媒金属を備えたＮＯｘ吸蔵還元型触媒を備えたＮＯｘ浄化システムにおいて、上流側にアルカリ金属からなるＮＯｘ吸蔵材を担持した高温型ＮＯｘ吸蔵還元型触媒を、下流側にアルカリ土類金属からなるＮＯｘ吸蔵材を担持した低温型ＮＯｘ吸蔵還元型触媒を直列に配置すると共に、前記高温型ＮＯｘ吸蔵還元型触媒が担持する白金とロジウムのモル比を２：１以上で１：２以下の範囲とするＮＯｘ浄化システムを提案した（例えば、特許文献４参照）。   In view of the above problems, the present inventors have devised the composition and arrangement (layout) of the catalyst to provide a NOx purification system having a wide NOx activation temperature window. In the NOx purification system including the NOx occlusion reduction catalyst having the NOx occlusion material and the catalyst metal that occludes NOx in the lean state and releases the NOx occluded in the rich state, A high temperature type NOx occlusion reduction type catalyst carrying an NOx occlusion material made of alkali metal on the side and a low temperature type NOx occlusion reduction type catalyst carrying an NOx occlusion material made of alkaline earth metal on the downstream side are arranged in series, A NOx purification system is proposed in which the molar ratio of platinum and rhodium supported by the high-temperature NOx storage reduction catalyst is in the range of 2: 1 to 1: 2. (E.g., see Patent Document 4).

しかしながら、この提案を行った２００４年当時と比べると、ロジウム（Ｒｈ）の市場価格が高騰し、コスト面で、ロジウムの量を大きくすることが困難となってきており、白金とロジウムのモル比を２：１以上とすることが難しく、ロジウムの使用量を低減する必要性が生じてきている。   However, compared with the time when this proposal was made in 2004, the market price of rhodium (Rh) has soared and it has become difficult to increase the amount of rhodium in terms of cost. It is difficult to make the ratio 2: 1 or more, and there is a need to reduce the amount of rhodium used.

特開平１０−４７０４２号公報Japanese Patent Laid-Open No. 10-47042 特開２０００−１６７３５６号公報JP 2000-167356 A 特開平１０−２０５３２６号公報JP-A-10-205326 特許第３８５２４６６号公報Japanese Patent No. 3852466

一方、上記のＮＯｘ浄化システムの提案当時に比べて、その後、貴金属分散化技術やシンタリング熱劣化抑制技術が発達してきており、本発明者らは、これらの技術的発展も踏まえて、再度、ロジウムの担持量を減少できないかを検討し、実験等から、カリウム等のアルカリ金属が白金のＮＯｘ浄化活性を抑制する作用に関連して、ＮＯｘ吸蔵還元型触媒が担持する貴金属総量と、同じくＮＯｘ吸蔵還元型触媒が担持するカリウムの量とのバランスが重要であるとの知見を得た。   On the other hand, compared to the time when the NOx purification system was proposed, noble metal dispersion technology and sintering heat deterioration suppression technology have been developed. We examined whether the amount of rhodium supported could be reduced, and from experiments, etc., in relation to the action of alkali metals such as potassium to suppress the NOx purification activity of platinum, the total amount of noble metals supported by the NOx storage reduction catalyst and NOx It was found that the balance with the amount of potassium supported by the storage reduction catalyst is important.

本発明は、上記のことを鑑みてなされたものであり、その目的は、排気ガス中のＮＯｘの浄化のためにＮＯｘ吸蔵還元型触媒を用いるＮＯｘ浄化システムにおいて、触媒の組成を工夫することにより、貴重で価格の高いロジウムの使用量を減少できて、コスト的に安価となるＮＯｘ浄化システムを提供することにある。   The present invention has been made in view of the above, and an object of the present invention is to devise a composition of a catalyst in a NOx purification system using a NOx occlusion reduction type catalyst for purification of NOx in exhaust gas. An object of the present invention is to provide a NOx purification system which can reduce the amount of rhodium used, which is valuable and expensive, and which is low in cost.

以上のような目的を達成するためのＮＯｘ浄化システムは、排気ガスの空燃比が、リーン状態の場合にＮＯｘを吸蔵し、かつ、リッチ状態の場合に吸蔵していたＮＯｘを放出するＮＯｘ吸蔵材と触媒金属を備えたＮＯｘ吸蔵還元型触媒を備えたＮＯｘ浄化システムにおいて、上流側にカリウムからなるＮＯｘ吸蔵材を担持した高温型ＮＯｘ吸蔵還元型触媒を、下流側にアルカリ土類金属からなるＮＯｘ吸蔵材を担持した低温型ＮＯｘ吸蔵還元型触媒を直列に配置し、前記高温型ＮＯｘ吸蔵還元型触媒が担持するカリウムの量を、同じく前記高温型ＮＯｘ吸蔵還元型触媒が担持する白金の量に対して、「ｇ／Ｌ」単位で、４倍以上でかつ１２倍以下の範囲とすると共に、前記高温型ＮＯｘ吸蔵還元型触媒が担持する白金とロジウムのモル比を２０：１以上でかつ１：２以下の範囲とするように構成される。   The NOx purification system for achieving the above object is a NOx occlusion material that occludes NOx when the air-fuel ratio of the exhaust gas is lean and releases NOx that is occluded when it is rich. In the NOx purification system provided with the NOx occlusion reduction catalyst provided with the catalyst metal, the high temperature type NOx occlusion reduction type catalyst carrying the NOx occlusion material made of potassium on the upstream side and the NOx made of alkaline earth metal on the downstream side A low-temperature NOx storage-reduction catalyst supporting a storage material is arranged in series, and the amount of potassium supported by the high-temperature NOx storage-reduction catalyst is changed to the amount of platinum similarly supported by the high-temperature NOx storage-reduction catalyst. On the other hand, the molar ratio of platinum and rhodium supported by the high-temperature NOx storage reduction catalyst is set to a range of 4 times or more and 12 times or less in the unit of “g / L”. : 1 or more and 1: 2 is configured to the following range.

即ち、本発明では、低温型ＮＯｘ吸蔵還元型触媒が触媒金属に例えば白金（Ｐｔ）のみを使用して低温活性を向上させると共に、前段（上流側）に設置する高温型ＮＯｘ吸蔵還元型触媒において触媒金属とカリウム（Ｋ）の担持割合を変える。この高温型ＮＯｘ吸蔵還元型触媒では吸蔵材にアルカリ金属のカリウムを使用して、高温活性を向上させ、更に、このカリウムが白金等の貴金属のＮＯｘ浄化活性を抑制する作用を有しているため、カリウムの量を白金の量に対して、「ｇ／Ｌ」単位で、４倍以上でかつ１２倍以下の範囲として、両者のバランスをとる。   That is, in the present invention, the low temperature type NOx storage reduction catalyst uses only platinum (Pt) as a catalyst metal to improve the low temperature activity, and in the high temperature type NOx storage reduction catalyst installed upstream (upstream side). The loading ratio of catalyst metal and potassium (K) is changed. In this high temperature type NOx occlusion reduction type catalyst, alkali metal potassium is used as the occlusion material, so that the high temperature activity is improved, and further, this potassium has the action of suppressing the NOx purification activity of noble metals such as platinum. The amount of potassium is in the range of 4 times or more and 12 times or less in the unit of “g / L” with respect to the amount of platinum, and the balance is obtained.

そして、カリウムの担持量と白金の担持量とのバランスを取った上で、カリウムからなるＮＯｘ吸蔵材を担持した触媒において、白金に対するロジウム（Ｒｈ）のモル比の範囲を２０：１から１：２の範囲にまで拡張して、ロジウムの担持量を減量する。このロジウムの減少と、カリウムの担持量と白金の担持量のバランスとにより、ＮＯｘ浄化性能を低下させることなく、貴重で価格の高いロジウムの使用量を減少できて、コスト的に安価なＮＯｘ浄化システムとすることができる。   Then, after balancing the loading amount of potassium and the loading amount of platinum, in the catalyst supporting the NOx storage material made of potassium, the range of the molar ratio of rhodium (Rh) to platinum is from 20: 1 to 1: Extend to the range of 2 to reduce the loading of rhodium. This reduction in rhodium and the balance between the loading amount of potassium and the loading amount of platinum can reduce the amount of precious and expensive rhodium used without reducing the NOx purification performance, and the cost is low. It can be a system.

また、この白金とロジウムの担持割合は、白金の担持量とロジウムの担持量の和に対するＲｈの担持量の割合を１／２１〜２／３、即ち、Ｐｔ：Ｒｈ＝２０：１〜１：２（Ｐｔ／Ｒｈ＝２０〜０．５）の範囲とする。言い換えれば、モル比で、Ｒｈの量をＰｔの量の０．０５〜２．０までの範囲とする。この範囲は、高温型ＮＯｘ吸蔵還元型触媒においてロジウムの割合を変化させて、ＮＯｘ浄化率を計測した図２の実施例Ａの、ＮＯｘ浄化率の高い領域Ｒ１の範囲に対応する範囲となる。   The supported ratio of platinum and rhodium is 1/21 to 2/3 of the ratio of the supported amount of Rh to the sum of the supported amount of platinum and the supported amount of rhodium, that is, Pt: Rh = 20: 1 to 1: 2 (Pt / Rh = 20 to 0.5). In other words, by the molar ratio, the amount of Rh is in the range of 0.05 to 2.0 of the amount of Pt. This range is a range corresponding to the range of the region R1 where the NOx purification rate is high in Example A of FIG. 2 in which the NOx purification rate is measured by changing the ratio of rhodium in the high-temperature NOx storage reduction catalyst.

そして、この高温型ＮＯｘ吸蔵還元型触媒を低温型ＮＯｘ吸蔵還元型触媒の前段（上流側）に配置すると、ロジウムは炭化水素（ＨＣ）酸化活性が高いため、極低温時でも一酸化炭素（ＣＯ）を主成分とする炭化水素の部分酸化物を生成する。この一酸化炭素を主成分とする炭化水素の部分酸化物は良好なＮＯｘ浄化用還元剤となるため、後段の低温型ＮＯｘ吸蔵還元型触媒の極低温性能が大きく向上する。なお、この部分酸化物には、一酸化炭素の他にも、酸、ケトン、アルデヒド等の含酸素炭化水素があり、部分酸化されない炭化水素に比べて、ＮＯｘとの反応活性が強く、ＮＯｘをより選択的に還元する。   When this high-temperature NOx storage-reduction catalyst is placed upstream (upstream) of the low-temperature NOx storage-reduction catalyst, rhodium has a high hydrocarbon (HC) oxidation activity, so carbon monoxide (CO ) As a main component. Since the hydrocarbon partial oxide mainly composed of carbon monoxide is a good reducing agent for NOx purification, the cryogenic performance of the low-temperature NOx occlusion reduction catalyst in the latter stage is greatly improved. In addition to carbon monoxide, this partial oxide includes oxygen-containing hydrocarbons such as acids, ketones, and aldehydes. Compared with hydrocarbons that are not partially oxidized, the reaction activity with NOx is strong, and NOx is reduced. Reduce more selectively.

そして、この前段の高温型ＮＯｘ吸蔵還元型触媒では、酸化活性が高いロジウムのため、低温での一酸化窒素から二酸化窒素への反応に対する活性が促進され、吸蔵能力が向上する。また、酸化活性が高いロジウムのため、極低温時でも炭化水素の部分酸化が促進され、この部分酸化で発生した良好なＮＯｘ浄化用還元剤となる部分酸化物により、後段の低温型ＮＯｘ吸蔵還元型触媒の極低温性能が大きく向上する。更に、前段の高温型ＮＯｘ吸蔵還元型触媒における排気ガス温度の昇温により後段の低温型ＮＯｘ吸蔵還元型触媒の温度が昇温するので、浄化性能が向上する。従って、低温域から高温域まで幅広いＮＯｘ活性温度ウィンドウを持つＮＯｘ浄化システムとなる。   And in this upstream high temperature type NOx occlusion reduction type catalyst, rhodium having high oxidation activity, the activity for the reaction from nitric oxide to nitrogen dioxide at a low temperature is promoted, and the occlusion ability is improved. In addition, because rhodium has high oxidation activity, partial oxidation of hydrocarbons is promoted even at extremely low temperatures, and the low-temperature NOx occlusion reduction in the latter stage is achieved by the partial oxide that becomes a good reducing agent for NOx purification generated by this partial oxidation. The cryogenic performance of the catalyst is greatly improved. Further, the temperature of the exhaust gas temperature in the upstream high-temperature NOx storage reduction catalyst increases the temperature of the downstream low-temperature NOx storage reduction catalyst, thereby improving the purification performance. Therefore, the NOx purification system has a wide NOx activation temperature window from the low temperature range to the high temperature range.

更に、上記のＮＯｘ浄化システムにおいて、前記高温型ＮＯｘ吸蔵還元型触媒の白金の担持量とロジウムの担持量の和を０．０５ｇ／Ｌ以上でかつ５．０ｇ／Ｌ以下とすると共に、白金の担持量を０．１ｇ／Ｌ以上でかつ３．０ｇ／Ｌ以下とするように構成する。   Furthermore, in the above NOx purification system, the sum of the supported amount of platinum and the supported amount of rhodium in the high-temperature NOx storage reduction catalyst is set to 0.05 g / L or more and 5.0 g / L or less, and The loading amount is set to 0.1 g / L or more and 3.0 g / L or less.

白金の担持量とロジウムの担持量の和が、０．０５ｇ／Ｌより小さいと浄化活性が不足し、５．０ｇ／Ｌより大きいとこの効果が飽和し、効果の割にコスト高となる。また、白金の担持量が、０．１ｇ／Ｌより小さいと浄化活性が不足し、３．０ｇ／Ｌを超えるとアルカリ金属が白金のＮＯｘ浄化活性を抑制するため、効果の割にコスト高となる。   If the sum of the supported amount of platinum and the supported amount of rhodium is less than 0.05 g / L, the purification activity is insufficient, and if it is greater than 5.0 g / L, this effect is saturated, and the cost is increased for the effect. Further, if the amount of platinum supported is less than 0.1 g / L, the purification activity is insufficient, and if it exceeds 3.0 g / L, the alkali metal suppresses the NOx purification activity of platinum. Become.

そして、上記のＮＯｘ浄化システムにおいて、ＮＯｘの吸着量は、モル当たりで分子量の多い方が多くなるが、重量が多くなるので、重量当たりに換算すると少なくなる。一方、カリウムは分子量がアルカリ金属の中で中位であるので、モル当たり、重量当たりのバランスを勘案すると、カリウムが適切なものとなる。従って、このカリウムを高温型ＮＯｘ吸蔵還元型触媒のＮＯｘ吸蔵材に使用することにより、高温型ＮＯｘ吸蔵還元型触媒のＮＯｘ吸蔵における高温活性を向上できる。   In the above NOx purification system, the amount of NOx adsorbed increases as the molecular weight increases per mole, but increases in weight and decreases in terms of weight. On the other hand, since potassium has a medium molecular weight among alkali metals, potassium is appropriate considering the balance per mole and weight. Therefore, by using this potassium as the NOx occlusion material for the high temperature NOx occlusion reduction catalyst, the high temperature activity in the NOx occlusion of the high temperature NOx occlusion reduction catalyst can be improved.

その上、上記のＮＯｘ浄化システムにおいて、前記高温型ＮＯｘ吸蔵還元型触媒に、セリウムを担持させると共に、該セリウムの担持量を０．１ｇ／Ｌ以上でかつ２．０ｇ／Ｌ以下とする。この酸素の吸収及び放出を行う酸素吸収剤となるセリウム（Ｃｅ）の担持により、この酸素吸収剤はリッチ時に酸素（Ｏ2）を供給して、還元剤の部分酸化を促進するので、低温活性が向上する。そして、このセリウムの担持量が０．１ｇ／Ｌより少ないと、酸素の吸蔵及び放出効果が少なく、２．０ｇ／Ｌより多くても、量の割に効果が少なく、コスト高となる。 Moreover, in the above NOx purification system, the high-temperature NOx occlusion reduction catalyst is loaded with cerium, and the loaded amount of cerium is set to 0.1 g / L or more and 2.0 g / L or less. By supporting cerium (Ce) as an oxygen absorbent that absorbs and releases oxygen, this oxygen absorbent supplies oxygen (O 2 ) when rich and promotes partial oxidation of the reducing agent. Will improve. If the amount of cerium supported is less than 0.1 g / L, the effect of occlusion and release of oxygen is small, and if it is more than 2.0 g / L, the effect is small and the cost is high.

これらの作用効果により、それぞれの触媒を単独で使用する場合よりも、温度に関する浄化ウィンドウが低温から高温まで広い範囲となっていて、多様な試験モードにも対応できる幅広いＮＯｘ活性温度ウィンドウを持つＮＯｘ浄化システムにおいて、触媒の組成の工夫により、貴重で価格の高いロジウムの使用量を減少できるので、コスト的に安価なＮＯｘ浄化システムとすることができる。   Due to these effects, the temperature-related purification window is wider from low to high than when each catalyst is used alone, and the NOx has a wide NOx activation temperature window that can accommodate various test modes. In the purification system, the amount of precious and expensive rhodium used can be reduced by devising the composition of the catalyst, so that the NOx purification system can be made inexpensive in terms of cost.

本発明に係るＮＯｘ浄化システムによれば、排気ガス中のＮＯｘの浄化のためにＮＯｘ吸蔵還元型触媒を用いるＮＯｘ浄化システムにおいて、カリウムと白金の量的バランスをとることで、ＮＯｘ浄化率を維持したまま、従来技術よりも貴重で価格の高いロジウムの使用量を減少できて、コスト的に安価なＮＯｘ浄化システムとすることができる。   According to the NOx purification system of the present invention, in the NOx purification system that uses a NOx occlusion reduction type catalyst for the purification of NOx in the exhaust gas, the NOx purification rate is maintained by taking a quantitative balance between potassium and platinum. In this way, the amount of rhodium used is more valuable and expensive than the prior art, and the NOx purification system can be reduced in cost.

本発明に係る実施の形態のＮＯｘ浄化システムの構成を示す図である。It is a figure which shows the structure of the NOx purification system of embodiment which concerns on this invention. 高温型触媒の白金の担持量とロジウムの担持量の合計量に占めるロジウムの担持量の割合とＮＯｘ浄化率（％）との関係を示す実験結果の図である。It is a figure of the experimental result which shows the relationship between the ratio of the supported amount of rhodium to the total amount of the supported amount of platinum and the supported amount of rhodium of the high temperature catalyst and the NOx purification rate (%).

以下、本発明に係る実施の形態のＮＯｘ吸蔵還元型触媒について、図面を参照しながら説明する。なお、ここでいう排気ガスの空燃比状態とは、必ずしもシリンダ内における空燃比の状態を意味するものではなく、ＮＯｘ吸蔵還元型触媒に流入する排気ガス中に供給した空気量と燃料量（シリンダ内で燃焼した分も含めて）との比のことをいう。   Hereinafter, the NOx occlusion reduction type catalyst according to the embodiment of the present invention will be described with reference to the drawings. Here, the air-fuel ratio state of the exhaust gas does not necessarily mean the state of the air-fuel ratio in the cylinder, but the amount of air and the amount of fuel supplied to the exhaust gas flowing into the NOx storage reduction catalyst (cylinder) (Including the amount burned inside).

図１に、本発明の実施の形態のＮＯｘ吸蔵還元型触媒１の構成を示す。このＮＯｘ吸蔵還元型触媒１は、高温型ＮＯｘ吸蔵還元型触媒（以下、高温型触媒という）２と低温型ＮＯｘ吸蔵還元型触媒（以下、低温型触媒という）３の少なくとも２つのタイプの触媒からなり、ケース４に上流側に高温型触媒２を、下流側に低温型触媒３を直列に配置する。なお、高温型触媒２と低温型触媒３は、当接した状態でも、間に間隔を設けた状態でもよいが、間隔が大きすぎると、この間で排気ガスが冷却されるので好ましくなく、低温型触媒３への排気ガスの均等流入という面から多少間隔を開けるのが好ましい。   FIG. 1 shows the configuration of a NOx storage reduction catalyst 1 according to an embodiment of the present invention. The NOx occlusion reduction type catalyst 1 includes at least two types of catalysts, a high temperature type NOx occlusion reduction type catalyst (hereinafter referred to as a high temperature type catalyst) 2 and a low temperature type NOx occlusion reduction type catalyst (hereinafter referred to as a low temperature type catalyst) 3. Thus, a high temperature catalyst 2 is arranged upstream of the case 4 and a low temperature catalyst 3 is arranged in series downstream. The high-temperature catalyst 2 and the low-temperature catalyst 3 may be in contact with each other or may be provided with an interval therebetween. However, if the interval is too large, the exhaust gas is cooled during this period. It is preferable to leave a slight gap in terms of the uniform inflow of exhaust gas to the catalyst 3.

この高温型触媒２は、モノリス触媒（ハニカム触媒）で形成され、酸化アルミニウム（アルミナ）、酸化チタン（チタニア）、ゼオライト等の担持体に触媒コート層を設け、この触媒コート層に、排気ガスの空燃比が、リーン状態の場合にＮＯｘを吸蔵し、かつ、リッチ状態の場合に吸蔵していたＮＯｘを放出する吸蔵材と、触媒金属を担持させて構成する。   The high-temperature catalyst 2 is formed of a monolith catalyst (honeycomb catalyst), and a catalyst coat layer is provided on a carrier such as aluminum oxide (alumina), titanium oxide (titania), or zeolite, and the exhaust gas of the exhaust gas is provided on the catalyst coat layer. The air-fuel ratio is configured to occlude NOx when it is in a lean state and to absorb NOx that has been occluded when it is in a rich state and a catalyst metal.

このＮＯｘ吸蔵材（ＮＯｘ吸蔵物質）としては、高温時にＮＯｘ吸蔵能力が低下しない、アルカリ金属のカリウム（Ｋ）を用いる。一般に、ＮＯｘの吸着量はモル当たりで分子量の多い方が多くなるが、しかし、重量が大きくなるので、重量当たりに換算するとＮＯｘの吸着量は少なくなる。一方、カリウムは分子量がアルカリ金属の中で中位であるので、モル当たりと重量当たりのバランスを勘案して、カリウムを用いる。   As this NOx occlusion material (NOx occlusion material), alkali metal potassium (K), which does not lower the NOx occlusion ability at high temperatures, is used. In general, the amount of NOx adsorbed increases as the molecular weight increases per mole, but the weight increases, so the amount of NOx adsorbed decreases per weight. On the other hand, since potassium has a medium molecular weight among alkali metals, potassium is used in consideration of the balance per mole and weight.

また、触媒金属としては、通常は、白金（Ｐｔ）、ロジウム（Ｒｈ）、パラジウム（Ｐｄ）、イリジウム（Ｉｒ）等の白金族元素（その酸化物を含む）を使用するが、本発明では、白金とロジウムを使用する。そして、高温型触媒２が担持するカリウムの量を、同じく高温型触媒２が担持する白金の量に対して、「ｇ／Ｌ」単位で、４倍以上でかつ１２倍以下の範囲とすると共に、担持する白金とロジウムのモル比を２０：１以上でかつ１：２以下の範囲とする。   Further, as the catalytic metal, platinum group elements (including oxides thereof) such as platinum (Pt), rhodium (Rh), palladium (Pd), iridium (Ir) are usually used. Use platinum and rhodium. Then, the amount of potassium carried by the high temperature catalyst 2 is in the range of 4 times or more and 12 times or less in the unit of “g / L” with respect to the amount of platinum carried by the high temperature catalyst 2. The molar ratio of supported platinum and rhodium is in the range of 20: 1 or more and 1: 2 or less.

図２は、高温型触媒２に関しての白金とロジウムの担持割合を変化させた時のＮＯｘ浄化率の変化を示す図であり、実施例Ａと従来例Ｂの実験結果（排気ガス温度２００℃）を示す図である。なお、実施例Ａでは、貴金属の分散化技術やシンタリング熱劣化の抑制技術等の触媒技術の改良で、白金を減少しても高温域では従来例Ｂと同等な性能を得つつ、カリウムの量を白金の量とバランスを取りながら減少することで、従来例Ｂよりも低温活性の向上を図っており、これにより、ロジウムの量が少ない部分でも高い浄化率を得ることができている。   FIG. 2 is a graph showing the change in the NOx purification rate when the platinum and rhodium loading ratio is changed for the high-temperature catalyst 2, and the experimental results of Example A and Conventional Example B (exhaust gas temperature 200 ° C.). FIG. In Example A, improvement in catalyst technology such as precious metal dispersion technology and sintering heat deterioration suppression technology, and even if platinum is reduced, while obtaining performance equivalent to that of Conventional Example B in a high temperature range, By reducing the amount in balance with the amount of platinum, the low-temperature activity is improved as compared with the conventional example B, whereby a high purification rate can be obtained even in a portion where the amount of rhodium is small.

即ち、吸蔵材に使用されているカリウムの量によって、ＮＯｘ浄化活性が抑制される白金の担持量とのバランスを取った実施例Ａでは、ＮＯｘ浄化率が高い最適な範囲（領域Ｒ１：モル比が１／２０〜２／１）が、従来例ＢのＮＯｘ浄化率が高い範囲（領域Ｑ１：モル比が１／２〜２／１）に比べて、ロジウムの担持割合が小さく、白金の占める割合が大きい部分まで拡がっている。つまり、従来例Ｂに比べて、少ない担持量のロジウムでＮＯｘ浄化率を向上することができる。この構成により、図２の実施例Ａで示すように、従来例Ｂよりも低温域側で高いＮＯｘ浄化性能を得ることができる。   That is, in Example A in which the amount of potassium used in the occlusion material is balanced with the amount of platinum supported to suppress the NOx purification activity, the optimum range where the NOx purification rate is high (region R1: molar ratio). Is lower than the range in which the NOx purification rate of Conventional Example B is high (region Q1: molar ratio is 1/2 to 2/1), the rhodium loading ratio is small, and platinum occupies It spreads to the part where the ratio is large. That is, compared with the conventional example B, the NOx purification rate can be improved with a small amount of rhodium. With this configuration, as shown in Example A of FIG. 2, higher NOx purification performance can be obtained on the low temperature region side than Conventional Example B.

特に、図２に示すように、従来例Ｂでは、ロジウムの担持割合が小さく、白金の占める割合が大きい領域Ｑ２（モル比が１／２未満）ではＮＯｘ活性が低くなりＮＯｘ浄化率が低いが、実施例Ａでは、ＮＯｘ浄化率が向上し、高いＮＯｘ浄化率を示す領域Ｒ１がロジウムのモル比が小さい範囲まで拡がっている。本発明では、ＮＯｘ浄化率が７０％程度以上を目安に、ロジウムの担持割合の範囲（モル比が１／２０〜２／１）を領域Ｒ１として選んでいる。   In particular, as shown in FIG. 2, in Conventional Example B, the NOx activity is low and the NOx purification rate is low in the region Q2 (molar ratio is less than 1/2) in which the rhodium loading ratio is small and the platinum ratio is large. In Example A, the NOx purification rate is improved, and the region R1 showing a high NOx purification rate extends to a range where the molar ratio of rhodium is small. In the present invention, the range of the rhodium loading ratio (molar ratio is 1/20 to 2/1) is selected as the region R1, with the NOx purification rate being about 70% or more as a guide.

なお、この図２から分かるように、実施例Ａにおいてにも、更に、ロジウムの担持割合が小さく、白金の占める割合が大きい領域Ｒ２（モル比が１／２０未満）では、従来例Ｂに比べればＮＯｘ浄化率は高いものの、白金のカリウム被毒の影響により高温型触媒２のＮＯｘ浄化率が低くなる。また、ロジウムの担持割合が大きく、白金の占める割合が小さい領域Ｒ３（モル比が２／１より大）では、ロジウムは低温活性が低いので、白金が少ないとＮＯｘ活性が従来例Ｂと同様に低くなる。   As can be seen from FIG. 2, also in Example A, the region R2 (molar ratio is less than 1/20) in which the rhodium loading ratio is small and the platinum occupying ratio is larger than that in the conventional example B. Although the NOx purification rate is high, the NOx purification rate of the high-temperature catalyst 2 is lowered due to the influence of potassium poisoning of platinum. Further, in the region R3 (molar ratio is larger than 2/1) in which the rhodium loading ratio is large and the platinum occupying ratio is small, rhodium has low low-temperature activity. Lower.

更に、この高温型触媒２の白金の担持量とロジウムの担持量の和を０．０５ｇ／Ｌ以上でかつ５．０ｇ／Ｌ以下とすると共に、白金の担持量を０．１ｇ／Ｌ以上でかつ３．０ｇ／Ｌ以下とする。   Further, the sum of the supported amount of platinum and the supported amount of rhodium in the high-temperature catalyst 2 is set to 0.05 g / L or more and 5.0 g / L or less, and the supported amount of platinum is set to 0.1 g / L or more. And 3.0 g / L or less.

白金の担持量とロジウムの担持量の和が、０．０５ｇ／Ｌより小さいと浄化活性が不足し、５．０ｇ／Ｌより大きいと効果が飽和し、効果の割にコスト高となる。また、白金の担持量が、０．１ｇ／Ｌより小さいと浄化活性が不足し、３．０ｇ／Ｌを超えると効果の割にコスト高となる。   If the sum of the supported amount of platinum and the supported amount of rhodium is smaller than 0.05 g / L, the purification activity is insufficient, and if it is larger than 5.0 g / L, the effect is saturated, and the cost is increased for the effect. Further, if the amount of platinum supported is less than 0.1 g / L, the purification activity is insufficient, and if it exceeds 3.0 g / L, the cost becomes high for the effect.

更に、高温型触媒２にセリウム（Ｃｅ）を担持させると共に、このセリウムの担持量を０．１ｇ／Ｌ以上で２．０ｇ／Ｌ以下とする。このセリウムを担持する構成により、酸素を吸蔵及び放出できるようになるので、リーン状態とストイキ又はリッチ状態との間の酸素濃度差が縮小し、三元活性が発現し易くなり浄化性能が向上する。セリウムを担持した場合には、担持しない場合よりも、ＮＯｘ浄化率が高くなる。なお、セリウムの担持量が、０．１ｇ／Ｌより小さいと、酸素の吸蔵・放出効果が少なく、２．０ｇ／Ｌを超えると、リッチ深さが阻害される。   Further, cerium (Ce) is supported on the high-temperature catalyst 2, and the supported amount of cerium is set to 0.1 g / L or more and 2.0 g / L or less. This cerium-supporting structure allows oxygen to be stored and released, so that the difference in oxygen concentration between the lean state and the stoichiometric or rich state is reduced, ternary activity is easily developed, and purification performance is improved. . When cerium is supported, the NOx purification rate is higher than when cerium is not supported. If the amount of cerium supported is less than 0.1 g / L, the effect of occluding and releasing oxygen is small, and if it exceeds 2.0 g / L, the rich depth is inhibited.

次に、低温型触媒３について説明する。この低温型触媒３は、高温型触媒２と同様に、触媒モノリス触媒で形成され、酸化アルミニウム、酸化チタン等の担持体に触媒コート層を設け、この触媒コート層に、排気ガスの空燃比が、リーン状態の場合にＮＯｘを吸蔵し、かつ、リッチ状態の場合に吸蔵していたＮＯｘを放出する吸蔵材と、触媒金属を担持させて構成する。   Next, the low temperature catalyst 3 will be described. Like the high temperature catalyst 2, the low temperature catalyst 3 is formed of a catalyst monolith catalyst, and a catalyst coat layer is provided on a carrier such as aluminum oxide or titanium oxide. The catalyst coat layer has an air-fuel ratio of exhaust gas. In this case, the NOx is stored in the lean state and the NOx stored in the rich state is released, and the catalyst metal is supported.

このＮＯｘ吸蔵材（ＮＯｘ吸蔵物質）としては、高温型触媒２とは異なり、低温時にＮＯｘ吸蔵能力が低下しないバリウム（Ｂａ）、ベリリウム（Ｂｅ）、マグネシウム（Ｍｇ）、カルシウム（Ｃａ）、ストロンチウム（Ｓｒ）等のアルカリ土類金属を用いるのが好ましい。   As this NOx occlusion material (NOx occlusion substance), unlike the high temperature catalyst 2, barium (Ba), beryllium (Be), magnesium (Mg), calcium (Ca), strontium (NOx occlusion capacity does not decrease at low temperatures). It is preferable to use an alkaline earth metal such as Sr).

また、触媒金属としては、高温型触媒２とは異なり、アルカリ金属によって、ＮＯｘ浄化活性が抑制されることが無いので、白金（Ｐｔ）、ロジウム（Ｒｈ）、パラジウム（Ｐｄ）、イリジウム（Ｉｒ）等の白金族元素（その酸化物を含む）を使用することができる。   Further, as the catalyst metal, unlike the high temperature catalyst 2, the NOx purification activity is not suppressed by the alkali metal, so platinum (Pt), rhodium (Rh), palladium (Pd), iridium (Ir) Platinum group elements such as oxides thereof can be used.

上記の構成により、高温型触媒２において、吸蔵材にアルカリ金属のカリウムを使用して、高温活性を向上させると共に、このカリウムによってＮＯｘ浄化活性が抑制される白金の担持量とカリウムの量のバランスを取りつつ、ロジウムの担持量を減少し、それと共に、白金とロジウムの担持割合を最適な範囲としたので、高いＮＯｘ浄化性能を得ることができる。   With the above configuration, in the high-temperature catalyst 2, the alkali metal potassium is used as the occlusion material to improve the high-temperature activity, and the balance between the amount of platinum supported and the amount of potassium for which the NOx purification activity is suppressed by this potassium. In addition, the supported amount of rhodium is reduced and the loading ratio of platinum and rhodium is set to an optimum range, so that high NOx purification performance can be obtained.

更に、この前段の高温型触媒２では、酸化活性が高いロジウムのため、低温での一酸化窒素から二酸化窒素への反応に対する活性が促進され、吸蔵能力が向上する。また、酸化活性が高いロジウムのため、極低温時でも炭化水素の部分酸化が促進され、この部分酸化で発生した良好なＮＯｘ浄化用還元剤となる部分酸化物により、後段の低温型触媒３の極低温性能が大きく向上する。更に、前段の高温型触媒２における排気ガス温度の昇温により後段の低温型触媒３の温度が昇温するので、浄化性能が向上する。   Further, in the upstream high-temperature catalyst 2, rhodium having a high oxidation activity promotes the activity for the reaction from nitrogen monoxide to nitrogen dioxide at a low temperature and improves the storage capacity. Further, since rhodium has high oxidation activity, partial oxidation of hydrocarbons is promoted even at an extremely low temperature, and the partial oxide that is a good reducing agent for purifying NOx generated by this partial oxidation allows the low-temperature catalyst 3 of the latter stage to be Cryogenic performance is greatly improved. Furthermore, since the temperature of the downstream low-temperature catalyst 3 rises due to the temperature increase of the exhaust gas in the upstream high-temperature catalyst 2, the purification performance is improved.

従って、本発明の高温型触媒２を前段に低温型触媒３を後段にしたＮＯｘ浄化システム（高温型触媒＋低温型触媒）１は、１５０℃の低温域でも浄化率が向上し、幅広いＮＯｘ活性温度ウィンドウを持つＮＯｘ浄化システムとなる。この低温時における浄化性能向上の理由としては、前段の高温型触媒２による低温でのＮＯ→ＮＯ2反応の活性の促進による吸蔵能力の向上、前段の高温型触媒２による低温でのＨＣ→ＣＯ反応による還元剤の部分酸化効果による還元性能の向上、前段の高温型触媒２における排気ガス温度の昇温により後段の低温型触媒３の温度が昇温することによる浄化性能の向上等により、後段の低温型触媒３の性能向上の促進が考えられる。 Therefore, the NOx purification system (high temperature type catalyst + low temperature type catalyst) 1 having the high temperature type catalyst 2 of the present invention as the preceding stage and the low temperature type catalyst 3 as the latter stage has an improved purification rate even in a low temperature region of 150 ° C., and has a wide range of NOx activity. The NOx purification system has a temperature window. Reasons for improving the purification performance at this low temperature include the improvement of the storage capacity by promoting the activity of NO → NO 2 reaction at a low temperature by the high temperature catalyst 2 in the previous stage, and the HC → CO at a low temperature by the high temperature catalyst 2 in the previous stage. By improving the reduction performance due to the partial oxidation effect of the reducing agent by the reaction, improving the purification performance by raising the temperature of the downstream low-temperature catalyst 3 by raising the exhaust gas temperature in the upstream high-temperature catalyst 2, etc. The improvement of the performance of the low-temperature catalyst 3 can be promoted.

また、実験で、高温域では、略前段の高温型触媒２の機能だけで排気ガスが浄化されることが、この高温型触媒２の後のＮＯｘセンサの検出値で確認されている。そのため、高温域では、特に、前段の高温型触媒２の特性が重要となる。   Further, it has been confirmed by experiments that the exhaust gas is purified only by the function of the high-temperature catalyst 2 in the preceding stage in the high-temperature region, based on the detection value of the NOx sensor after the high-temperature catalyst 2. Therefore, in the high temperature range, the characteristics of the high temperature catalyst 2 in the previous stage are particularly important.

ちなみに、本発明の配置を入れ替えて、前段に低温型触媒３、後段に高温型触媒２を配置すると、低高温における性能が低下する。その理由は、低温域では、上記した低温型触媒３の上流側の効果が無くなり、高温域では、前段の低温型触媒３により還元剤が消費されてしまうため、後段の高温型触媒２の還元機能が低下し、また、排気ガス温度が上昇して後段の高温型触媒２の吸蔵機能が低下するためと推測される。   By the way, if the arrangement of the present invention is changed and the low temperature catalyst 3 is arranged at the front stage and the high temperature catalyst 2 is arranged at the rear stage, the performance at low and high temperatures is lowered. The reason is that, in the low temperature range, the upstream effect of the low temperature type catalyst 3 is lost, and in the high temperature range, the reducing agent is consumed by the low temperature type catalyst 3 in the previous stage. It is presumed that the function is lowered, and the exhaust gas temperature is raised, so that the occlusion function of the high-temperature catalyst 2 in the latter stage is lowered.

本発明のＮＯｘ浄化システムは、カリウムと白金の量的バランスをとることで、ＮＯｘ浄化率を維持したまま、従来技術よりも貴重で価格の高いロジウムの使用量を減少できて、コスト的に安価なＮＯｘ浄化システムとすることができるので、多くの内燃機関の排気通路の設置するＮＯｘ浄化システムとして利用できる。   The NOx purification system of the present invention can reduce the amount of rhodium used, which is more valuable and expensive than the prior art, while maintaining the NOx purification rate by maintaining a quantitative balance between potassium and platinum, and is inexpensive in terms of cost. Therefore, it can be used as a NOx purification system in which an exhaust passage of many internal combustion engines is installed.

１ ＮＯｘ浄化システム
２ 高温型ＮＯｘ吸蔵還元型触媒（高温型触媒）
３ 低温型ＮＯｘ吸蔵還元型触媒（低温型触媒）
４ ケース 1 NOx purification system 2 High-temperature NOx storage reduction catalyst (high-temperature catalyst)
3 Low-temperature NOx storage reduction catalyst (low-temperature catalyst)
4 cases

Claims (3)

排気ガスの空燃比が、リーン状態の場合にＮＯｘを吸蔵し、かつ、リッチ状態の場合に吸蔵していたＮＯｘを放出するＮＯｘ吸蔵材と触媒金属を備えたＮＯｘ吸蔵還元型触媒を備えたＮＯｘ浄化システムにおいて、
上流側にカリウムからなるＮＯｘ吸蔵材を担持した高温型ＮＯｘ吸蔵還元型触媒を、下流側にアルカリ土類金属からなるＮＯｘ吸蔵材を担持した低温型ＮＯｘ吸蔵還元型触媒を直列に配置し、
前記高温型ＮＯｘ吸蔵還元型触媒が担持するカリウムの量を、同じく前記高温型ＮＯｘ吸蔵還元型触媒が担持する白金の量に対して、「ｇ／Ｌ」単位で、４倍以上でかつ１２倍以下の範囲とすると共に、
前記高温型ＮＯｘ吸蔵還元型触媒が担持する白金とロジウムのモル比を２０：１以上でかつ１：２以下の範囲とすることを特徴とするＮＯｘ浄化システム。 NOx having a NOx occlusion reduction type catalyst comprising a NOx occlusion material and a catalyst metal that occludes NOx when the air-fuel ratio of the exhaust gas is lean and releases NOx occluded when it is rich. In the purification system,
A high-temperature NOx occlusion-reduction catalyst carrying a NOx occlusion material made of potassium on the upstream side, and a low-temperature NOx occlusion reduction catalyst carrying an NOx occlusion material made of an alkaline earth metal on the downstream side are arranged in series,
The amount of potassium supported by the high-temperature NOx storage reduction catalyst is 4 times or more and 12 times in terms of “g / L” with respect to the amount of platinum supported by the high-temperature NOx storage reduction catalyst. With the following range,
The NOx purification system, wherein a molar ratio of platinum and rhodium supported by the high temperature NOx occlusion reduction catalyst is in a range of 20: 1 or more and 1: 2 or less. 前記高温型ＮＯｘ吸蔵還元型触媒の白金の担持量とロジウムの担持量の和を０．０５ｇ／Ｌ以上でかつ５．０ｇ／Ｌ以下とすると共に、白金の担持量を０．１ｇ／Ｌ以上でかつ３．０ｇ／Ｌ以下とすることを特徴とする請求項１記載のＮＯｘ浄化システム。   The sum of the supported amount of platinum and the supported amount of rhodium in the high-temperature NOx storage reduction catalyst is 0.05 g / L or more and 5.0 g / L or less, and the platinum supported amount is 0.1 g / L or more. The NOx purification system according to claim 1, wherein the NOx purification system is not more than 3.0 g / L. 前記高温型ＮＯｘ吸蔵還元型触媒に、セリウムを担持させると共に、該セリウムの担持量を０．１ｇ／Ｌ以上でかつ２．０ｇ／Ｌ以下とすることを特徴とする請求項１又は２に記載のＮＯｘ浄化システム。   The cerium is supported on the high-temperature NOx storage-reduction catalyst, and the supported amount of cerium is 0.1 g / L or more and 2.0 g / L or less. NOx purification system.
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