JP3644572B2 - Exhaust gas purification catalyst and method for producing the same - Google Patents
Exhaust gas purification catalyst and method for producing the same Download PDFInfo
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- 239000003054 catalyst Substances 0.000 title claims description 53
- 238000000746 purification Methods 0.000 title claims description 25
- 238000004519 manufacturing process Methods 0.000 title claims description 11
- 239000010948 rhodium Substances 0.000 claims description 120
- 239000002245 particle Substances 0.000 claims description 55
- PNEYBMLMFCGWSK-UHFFFAOYSA-N aluminium oxide Inorganic materials [O-2].[O-2].[O-2].[Al+3].[Al+3] PNEYBMLMFCGWSK-UHFFFAOYSA-N 0.000 claims description 47
- 239000011247 coating layer Substances 0.000 claims description 25
- 239000012298 atmosphere Substances 0.000 claims description 18
- 239000006104 solid solution Substances 0.000 claims description 18
- 229910052703 rhodium Inorganic materials 0.000 claims description 17
- 230000009467 reduction Effects 0.000 claims description 12
- MHOVAHRLVXNVSD-UHFFFAOYSA-N rhodium atom Chemical compound [Rh] MHOVAHRLVXNVSD-UHFFFAOYSA-N 0.000 claims description 12
- 239000010419 fine particle Substances 0.000 claims description 10
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- 238000007254 oxidation reaction Methods 0.000 claims description 10
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- 230000001590 oxidative effect Effects 0.000 claims description 7
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- 239000007789 gas Substances 0.000 description 24
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- 150000003839 salts Chemical class 0.000 description 9
- UIIMBOGNXHQVGW-UHFFFAOYSA-M Sodium bicarbonate Chemical compound [Na+].OC([O-])=O UIIMBOGNXHQVGW-UHFFFAOYSA-M 0.000 description 8
- 230000003197 catalytic effect Effects 0.000 description 7
- 230000000694 effects Effects 0.000 description 7
- BASFCYQUMIYNBI-UHFFFAOYSA-N platinum Chemical compound [Pt] BASFCYQUMIYNBI-UHFFFAOYSA-N 0.000 description 7
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- MCMNRKCIXSYSNV-UHFFFAOYSA-N Zirconium dioxide Chemical compound O=[Zr]=O MCMNRKCIXSYSNV-UHFFFAOYSA-N 0.000 description 4
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- 235000017557 sodium bicarbonate Nutrition 0.000 description 4
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 description 4
- 238000002441 X-ray diffraction Methods 0.000 description 3
- AZDRQVAHHNSJOQ-UHFFFAOYSA-N alumane Chemical class [AlH3] AZDRQVAHHNSJOQ-UHFFFAOYSA-N 0.000 description 3
- DIZPMCHEQGEION-UHFFFAOYSA-H aluminium sulfate (anhydrous) Chemical compound [Al+3].[Al+3].[O-]S([O-])(=O)=O.[O-]S([O-])(=O)=O.[O-]S([O-])(=O)=O DIZPMCHEQGEION-UHFFFAOYSA-H 0.000 description 3
- 238000006243 chemical reaction Methods 0.000 description 3
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- ATRRKUHOCOJYRX-UHFFFAOYSA-N Ammonium bicarbonate Chemical compound [NH4+].OC([O-])=O ATRRKUHOCOJYRX-UHFFFAOYSA-N 0.000 description 2
- UFHFLCQGNIYNRP-UHFFFAOYSA-N Hydrogen Chemical compound [H][H] UFHFLCQGNIYNRP-UHFFFAOYSA-N 0.000 description 2
- MHAJPDPJQMAIIY-UHFFFAOYSA-N Hydrogen peroxide Chemical compound OO MHAJPDPJQMAIIY-UHFFFAOYSA-N 0.000 description 2
- 238000010521 absorption reaction Methods 0.000 description 2
- 238000004220 aggregation Methods 0.000 description 2
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- VSCWAEJMTAWNJL-UHFFFAOYSA-K aluminium trichloride Chemical compound Cl[Al](Cl)Cl VSCWAEJMTAWNJL-UHFFFAOYSA-K 0.000 description 2
- 239000001099 ammonium carbonate Substances 0.000 description 2
- 238000011156 evaluation Methods 0.000 description 2
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- SONJTKJMTWTJCT-UHFFFAOYSA-K rhodium(iii) chloride Chemical compound [Cl-].[Cl-].[Cl-].[Rh+3] SONJTKJMTWTJCT-UHFFFAOYSA-K 0.000 description 2
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- RPAJSBKBKSSMLJ-DFWYDOINSA-N (2s)-2-aminopentanedioic acid;hydrochloride Chemical class Cl.OC(=O)[C@@H](N)CCC(O)=O RPAJSBKBKSSMLJ-DFWYDOINSA-N 0.000 description 1
- BNGXYYYYKUGPPF-UHFFFAOYSA-M (3-methylphenyl)methyl-triphenylphosphanium;chloride Chemical compound [Cl-].CC1=CC=CC(C[P+](C=2C=CC=CC=2)(C=2C=CC=CC=2)C=2C=CC=CC=2)=C1 BNGXYYYYKUGPPF-UHFFFAOYSA-M 0.000 description 1
- QTBSBXVTEAMEQO-UHFFFAOYSA-M Acetate Chemical compound CC([O-])=O QTBSBXVTEAMEQO-UHFFFAOYSA-M 0.000 description 1
- 229910000013 Ammonium bicarbonate Inorganic materials 0.000 description 1
- VHUUQVKOLVNVRT-UHFFFAOYSA-N Ammonium hydroxide Chemical compound [NH4+].[OH-] VHUUQVKOLVNVRT-UHFFFAOYSA-N 0.000 description 1
- IJGRMHOSHXDMSA-UHFFFAOYSA-N Atomic nitrogen Chemical compound N#N IJGRMHOSHXDMSA-UHFFFAOYSA-N 0.000 description 1
- LFQSCWFLJHTTHZ-UHFFFAOYSA-N Ethanol Chemical compound CCO LFQSCWFLJHTTHZ-UHFFFAOYSA-N 0.000 description 1
- QAOWNCQODCNURD-UHFFFAOYSA-L Sulfate Chemical compound [O-]S([O-])(=O)=O QAOWNCQODCNURD-UHFFFAOYSA-L 0.000 description 1
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- 229940009827 aluminum acetate Drugs 0.000 description 1
- 235000012538 ammonium bicarbonate Nutrition 0.000 description 1
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- QVGXLLKOCUKJST-UHFFFAOYSA-N atomic oxygen Chemical compound [O] QVGXLLKOCUKJST-UHFFFAOYSA-N 0.000 description 1
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- BVKZGUZCCUSVTD-UHFFFAOYSA-N carbonic acid Chemical class OC(O)=O BVKZGUZCCUSVTD-UHFFFAOYSA-N 0.000 description 1
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- JSKIRARMQDRGJZ-UHFFFAOYSA-N dimagnesium dioxido-bis[(1-oxido-3-oxo-2,4,6,8,9-pentaoxa-1,3-disila-5,7-dialuminabicyclo[3.3.1]nonan-7-yl)oxy]silane Chemical compound [Mg++].[Mg++].[O-][Si]([O-])(O[Al]1O[Al]2O[Si](=O)O[Si]([O-])(O1)O2)O[Al]1O[Al]2O[Si](=O)O[Si]([O-])(O1)O2 JSKIRARMQDRGJZ-UHFFFAOYSA-N 0.000 description 1
- 229910001873 dinitrogen Inorganic materials 0.000 description 1
- 238000010894 electron beam technology Methods 0.000 description 1
- 238000001704 evaporation Methods 0.000 description 1
- 238000010304 firing Methods 0.000 description 1
- 239000011261 inert gas Substances 0.000 description 1
- 230000003993 interaction Effects 0.000 description 1
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- 229910052751 metal Inorganic materials 0.000 description 1
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- VXNYVYJABGOSBX-UHFFFAOYSA-N rhodium(3+);trinitrate Chemical compound [Rh+3].[O-][N+]([O-])=O.[O-][N+]([O-])=O.[O-][N+]([O-])=O VXNYVYJABGOSBX-UHFFFAOYSA-N 0.000 description 1
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- Catalysts (AREA)
Description
【０００１】
【発明の属する技術分野】
本発明は、担体としてα−アルミナを用い、その担体に触媒貴金属としてのロジウム（Rh）を担持した排ガス浄化用触媒とその製造方法に関する。本発明の排ガス浄化用触媒は、排ガス中のNOx を効率よく浄化することができる。
【０００２】
【従来の技術】
従来より自動車の排ガス浄化用触媒として、排ガス中のCO及びHCの酸化とNOx の還元とを同時に行って浄化する三元触媒が用いられている。このような三元触媒としては、例えばコーディエライトなどからなる耐熱性ハニカム基材にγ−アルミナからなる担体層を形成し、その担体層に白金（Pt）などの触媒貴金属を担持させたものが広く知られている。
【０００３】
また従来の排ガス浄化用触媒を製造するには、γ−アルミナよりなる担持層をもつ担体をPt塩やRh塩の水溶液に浸漬し引き上げて乾燥・焼成することで担持する吸着担持法、あるいはPt塩やRh塩の水溶液の所定量を含浸させ蒸発・乾固させて担持する吸水担持法などによりPtやRhを担持している。
γ−アルミナは活性アルミナとも称され、高い比表面積をもつため触媒担体として広く用いられている。また触媒貴金属としては、HCやCOを酸化する活性の高いPtと、排ガス中のHCなどの還元性物質によりNOx を還元する活性に優れたRhとを併用するのが一般的である。
【０００４】
ところがγ−アルミナ担体にRhを担持した触媒では、初期の浄化活性には優れるものの、高温の酸素過剰雰囲気においてRhが担体中に固溶し、担体表面における触媒活性点の減少によりNOx 浄化性能が大きく低下するという問題があることが明らかとなった。
そこでRhが固溶しない担体にRhを担持することが想起され、ジルコニアを用いたもの（米国特許第233189号）や、α−アルミナを用いたもの（特開昭55−41894号公報）などの提案がある。
【０００５】
特開昭55−41894号公報には、Rhは高温の酸素過剰雰囲気下においてγ−アルミナに固溶するが、担体としてα−アルミナを用いれば固溶が抑制され、Rhの触媒活性が長期間維持されることが記載されている。
【０００６】
【発明が解決しようとする課題】
ところがジルコニアやα−アルミナは、γ−アルミナに比べて比表面積が著しく低い。したがって従来の吸着担持法や吸水担持法による製造方法では、α−アルミナにRhを高分散で担持することが困難であり、そのため触媒活性点が少なくなって初期から触媒活性が低いという不具合がある。
【０００７】
そこで鋭意研究の結果、本願発明者らは、α−アルミナにRhを含浸担持させ、それを酸化・還元処理することにより微粒子状のRh粒子を担体表面に析出させる方法を開発した。この方法によれば、Rhはその担持量の50％以上が粒子径２nm以下の微粒子として担持されるため、触媒の活性点が増大し高いNOx 浄化性能が得られる。
【０００８】
そしてα−アルミナは熱安定性に優れ、かつRhの固溶が生じにくい。したがって使用中に高温の酸素過剰雰囲気に曝されたとしてもRhの凝集が起こりにくく、Rhは高分散状態が維持され、NOx 浄化性能の耐久性にも優れている。
ところが上記方法で得られた触媒では、特にRhの担持量が多い場合などにRhの分散性が低下し、初期からの触媒活性が低いという不具合があることが明らかとなった。この理由は、比表面積の小さいα−アルミナからなる担体を用いたのでは、高温酸化処理時の担体中へのRhの固溶量に限界があり、固溶されないRhは担体表面に凝集粒子として付着する。これを還元処理すると、担体中に固溶しているRhは数nmの微細粒子として担体表面に析出するが、担体表面に存在するRhの凝集粒子にはさらに粒成長が生じ分散性がさらに低下することに起因している。
【０００９】
本発明はこのような事情に鑑みてなされたものであり、α−アルミナを担体として酸化・還元処理時のRhの凝集を抑制し、これによりRhの担持量を多くしかつ高分散担持することで触媒活性を一層高めることを目的とする。
【００１０】
【課題を解決するための手段】
上記課題を解決する請求項１に記載の本発明の排ガス浄化用触媒の製造方法の特徴は、α−アルミナ粒子の表面に非晶質アルミナ及び／又は遷移アルミナよりなる被覆層を形成した担体にロジウムを含浸担持して Rh 担持担体とする担持工程と、 Rh 担持担体を酸化雰囲気中で熱処理し担体にロジウムを固溶させて Rh 固溶担体とする酸化工程と、 Rh 固溶担体を還元雰囲気中で熱処理し固溶したロジウムを担体表面に微粒子として析出させるとともに被覆層を結晶性のα−アルミナとする還元工程と、を有することにある。
【００１１】
そして請求項２に記載の本発明の排ガス浄化用触媒の特徴は、請求項１に記載の製造方法により製造され、α−アルミナよりなる担体と、担体に担持されたロジウムとを有する排ガス浄化用触媒であって、ロジウムはその担持量の 80 ％以上が粒子径５ nm 以下の微粒子として担体表面に表出していることにある。
【００１２】
【発明の実施の形態】
本発明の排ガス浄化用触媒では、Rhはその担持量の80％以上が粒子径５nm以下の微粒子としてα−アルミナ担体に担持されている。したがってRhはきわめて微細な状態で高分散状態で担持されているため、触媒の活性点が増大し高いNOx 浄化性能を示す。
【００１３】
そしてα−アルミナは熱安定性に優れ、かつRhの固溶が生じにくい。したがって使用中に高温の酸素過剰雰囲気に曝されたとしてもRhの凝集が起こりにくく、Rhは高分散状態が維持され、NOx 浄化性能の耐久性にも優れている。
担持されているRhのうち、粒子径が５nmを超えるものが担持量の20％以上となると、触媒の活性点が減少するためNOx 浄化性能が低下してしまう。粒子径は５nm以下の微細であるほど望ましく、その微細粒子が多いほど望ましい。
【００１４】
Rhの担持量は、担体 120ｇ当たり0.01〜 2.0ｇ、重量％では 0.008〜1.66重量％が好適な範囲である。Rhの担持量がこの範囲より少ないと十分なNOx 浄化性能が得られず、この範囲より多く担持してもNOx 浄化性能が飽和して高価なRhが無駄になってしまう。また担持量がこの範囲より多くなると、粒子径が10nm以上の粗大粒子が多くなる。
【００１５】
なお、Rhの担持量を担体120 ｇ当たり 1.0ｇ以下とすれば、ほとんどのRhを有効利用できるとともに、ほとんどのRhの粒子径が５nm以下となり、特に高いNOx 浄化性能が得られる。
請求項２に記載の排ガス浄化用触媒の製造方法では、α−アルミナ粒子の表面に非晶質アルミナ及び／又は遷移アルミナよりなる被覆層を形成した担体が用いられる。この担体にRhを担持させ、それを酸化雰囲気中で熱処理すると、Rhは非晶質アルミナ及び／又は遷移アルミナに固溶しやすいため、担持されているRhはほとんど全量が担体に固溶し、次の還元雰囲気中での熱処理により固溶したRhは数nmの微細粒子となって担体表面に析出する。これにより、Rhが高分散担持された触媒が得られる。
【００１６】
α−アルミナ粒子の表面に非晶質アルミナ及び／又は遷移アルミナよりなる被覆層を形成するには、例えば、核となるα−アルミナ粒子を水溶性アルミニウム塩の水溶液中に懸濁させ、そこへ中和剤水溶液を滴下する中和共沈法によって沈殿を形成し、それを乾燥・焼成する方法がある。また気相法や、機械的な方法を採用することもできる。
【００１７】
水溶性アルミニウム塩としては、アルミニウムの硝酸塩、硫酸塩、塩化アルミニウム、酢酸塩などの無機塩・有機塩が例示される。また中和剤としては、アンモニア水、炭酸水素アンモニウム、炭酸アンモニウム、炭酸水素ナトリウムなどが例示されるが、中和反応の進行が緩やかで被覆層を形成しやすい炭酸塩や炭酸水素塩が適している。
【００１８】
懸濁液中の水溶性アルミニウム塩濃度と中和剤水溶液中の中和剤濃度は、ともに 0.01mol/L以上、１mol/L 以下の範囲が望ましい。１mol/L を超える濃度では、中和剤溶液を滴下した時に滴下した局所で反応が起こってα−アルミナ粒子表面に析出せず自発核が発生するようになる。また0.01mol/L より低い濃度では、被覆層の形成が困難となる。
【００１９】
なお反応液中には、反応液中の金属イオンを安定化するために、酸、過酸化水素などの錯形成剤、あるいは粒子の凝集を抑制するための界面活性剤などを添加してもよい。
非晶質アルミナ及び／又は遷移アルミナよりなる被覆層の厚さは、 0.5〜 100nmの範囲が望ましい。 0.5nm未満では、Rhの担持効率が低くなるため10nm以上の粗大粒子が多くなり、得られる触媒の活性が低下する。また 100nmを超えると、還元工程における熱処理時にRhの析出が困難となり、得られる触媒の活性が低下する。特に望ましいのは10〜50nmの範囲である。
【００２０】
担持工程では、上記被覆層をもつ担体にRh塩溶液を含浸してRh担持担体が製造される。Rh塩としては、硝酸ロジウム（Rh(NO3 )3 ）、塩化ロジウム（RhCl3 ）などが用いられ、これらの塩の場合には水溶液として用いられる。またRh塩の種類によっては、アルコールなどの有機溶媒を用いることもできる。
Rh担持担体のRh担持量は、0.01〜 2.0重量％の範囲が望ましい。Rh担持量が0.01重量％未満では、得られた触媒が充分なNOx 浄化能を示さず、 2.0重量％を超えて担持すると、得られた触媒では粒径が10nm以上の粗大Rh粒子が多くなるとともにNOx 浄化性能が飽和し、効果なRhが無駄になってしまう。
【００２１】
このRh担持担体は、酸化工程において例えば大気中などの酸化雰囲気下にて熱処理され、Rh固溶担体となる。α−アルミナはRhとの相互作用が低くRhが固溶しにくいが、非晶質アルミナよりなる被覆層へはRhが固溶しやすく、Rh担持担体に多量のRhが担持されていてもそのほとんどが固溶する。
酸化工程の熱処理温度は、 700〜1000℃の範囲とすることが好ましい。熱処理温度が 700℃未満ではRhの固溶が困難となり、次工程の還元工程において10nm以上の粗大粒子が多くなるためRhの分散性が低下する場合がある。また熱処理温度が1000℃を超えると、Rh粒子が粗大化し固溶しにくくなりやすい。
【００２２】
なお酸化工程における熱処理時間は、１〜３時間で十分である。熱処理時間が１時間より少ないとRhの固溶が不十分となり、次工程の還元工程における担体表面へのRhの析出も困難となるため、Rhの微細化が困難となる。また、３時間以上行うとRhが担体の内部に拡散し、次工程の還元工程でRhが析出しにくくなる。また熱エネルギーも無駄になる。
【００２３】
酸化工程で熱処理されRhが固溶したRh固溶担体は、還元工程において、還元雰囲気中で熱処理される。これにより固溶していたRhが粒径５nm以下の微粒子となって担体表面に析出し、高分散担持される。また被覆層では、非晶質アルミナ及び／又は遷移アルミナが変態してα−アルミナ構造となる。したがって得られる触媒は、α−アルミナ担体に微細なRhが高分散担持された状態であり、高温の酸素過剰雰囲気におけるRhの担体への固溶が少ないため、浄化性能の耐久性に優れた触媒となる。
【００２４】
還元雰囲気としては、例えば不活性ガス中に水素ガスを含むガスなどが用いられる。そして、熱処理温度は 800〜1100℃の範囲が好ましい。熱処理温度が 800℃未満ではRhの析出が困難となり、熱処理温度が1100℃を超えると析出した微細Rh粒子が粗大化する。
なお還元工程における熱処理時間は、 0.5〜２時間で十分である。熱処理時間が 0.5時間より少ないとRhの析出が不十分となり、２時間以上行うと析出した微細Rh粒子が粗大化する。また熱エネルギーも無駄になる。
【００２５】
【実施例】
以下、実施例及び比較例により本発明を具体的に説明する。
（実施例１）
図１に本実施例の製造方法の概要と、本実施例で製造される粉末の模式的な構造を示す。
（１）担体の調製工程
市販のα−アルミナ粉末（比表面積５m2/g、平均粒子径 0.3μｍ）の 5.0ｇを濃度0.026mol/Lの硫酸アルミニウム水溶液 300mlに混合し、それを攪拌しながら、濃度0.134mol/Lの炭酸水素ナトリウム水溶液 300mlを定量ポンプを用いて２時間かけて滴下した。得られた沈殿物を濾過・水洗し、80℃にて10時間乾燥後、大気中にて 500℃で２時間熱処理して担体粉末を得た。
【００２６】
得られた担体粉末の電子顕微鏡写真を図２に示す。図２から明らかなように、α−アルミナ粒子１の表面に厚さ約30nmの被覆層２が形成されており、電子線回折の結果、被覆層２の構造は結晶性の低いγ−アルミナであった。
（２）担持工程
この担体粉末に所定濃度の塩化ロジウム水溶液の所定量を含浸させ、水分を蒸発乾固させた後、大気中にて 120℃で12時間乾燥後、大気中にて 600℃で３時間焼成してRhを担持したRh担持担体を調製した。Rhは主として被覆層２に担持され、その担持量は金属Rhとして 0.6重量％である。
（３）酸化工程
次に、得られたRh担持担体を大気中にて 800℃で３時間熱処理し、担持したRhを被覆層２に固溶させてRh固溶担体を得た。このRh固溶担体の電子顕微鏡写真を図３に、被覆層20の電子回折像を図４に示す。
【００２７】
図３及び図４より、Rh固溶担体には、この時点ではまだ非晶質でRhを固溶した被覆層20が存在していることがわかる。
（４）還元工程
酸化工程で得られたRh固溶担体は、水素ガスを10％含む窒素ガス中にて1000℃で１時間熱処理された。得られたRh析出担体の電子顕微鏡写真を図５に、電子回折像を図６に、さらに倍率を上げた電子顕微鏡写真を図７に、Ｘ線回折チャートを図８に示す。
【００２８】
図５の暗視野像から、被覆層は結晶化し、その構造は図６の電子回折から結晶質のα−アルミナ構造となっていることがわかる。また図７より、得られたRh析出担体では粒子径が１〜８nmの微細なRh粒子３が高分散担持されていることがわかり、あまりに微細なため図８には明瞭なピークとしては現れていない。なおRhの粒径分布を電子顕微鏡写真のRh粒子を画像解析することで測定したところ、５nm以下のものが86％を占めていた。
【００２９】
すなわち還元工程では、被覆層21表面に微細なRh粒子３が析出するとともに、被覆層21はα−アルミナ構造に変態している。このRh析出担体からなる粉末を実施例１の触媒粉末とした。
（実施例２）
担体の調製工程における硫酸アルミニウム水溶液を 150mlとし、炭酸水素ナトリウム水溶液 150mlを定量ポンプを用いて１時間かけて滴下したこと以外は実施例１と同様にして、実施例２の触媒粉末を調製した。なお、担持工程を行う前の担体の被覆層の厚さは10〜15nmであった。またRhの粒径分布を電子顕微鏡写真のRh粒子を画像解析することで測定したところ、５nm以下のものが80％を占めていた。
【００３０】
（実施例３）
担体の調製工程における硫酸アルミニウム水溶液を 450mlとし、炭酸水素ナトリウム水溶液 450mlを定量ポンプを用いて３時間かけて滴下したこと以外は実施例１と同様にして、実施例３の触媒粉末を調製した。なお、担持工程を行う前の担体の被覆層の厚さは約45nmであった。またRhの粒径分布を電子顕微鏡写真のRh粒子を画像解析することで測定したところ、５nm以下のものが90％を占めていた。
【００３１】
（比較例１）
担体の調製工程を行わなず、α−アルミナ粉末をそのまま担体として用いたこと以外は実施例１と同様にして、比較例１の触媒粉末を調製した。
比較例１で用いたα−アルミナ担体の電子顕微鏡写真を図９に示す。図９より、担体にはもちろん被覆層は形成されていない。
【００３２】
得られた触媒粉末の電子顕微鏡写真を図10に、Ｘ線回折チャートを図８に示す。図10より粒子径が１〜５nmの微細なRh粒子が僅かに存在するものの、Rhは大部分が10nm以上の粗大粒子として担持され、図８には明瞭なピークとして現れている。なおRhの粒径分布を電子顕微鏡写真のRh粒子を画像解析することで測定したところ、10nm以上のものが40％を占めていた。
【００３３】
（試験・評価）
実施例及び比較例それぞれの触媒粉末を圧粉成形し、0.5 〜 1.0mmの粒径のペレット触媒とした。そして各ペレット触媒について、表１に示す評価ガスを用い、触媒量 1.0ｇ、ガス流量3.3L/minの条件で、入りガス温度を 100℃から 500℃まで５℃／min の速度で昇温させながら、NOx 浄化率を測定した。そしてそれぞれのNOx 50％浄化温度（初期）を求め、結果を表３に示す。
【００３４】
【表１】
【００３５】
【表２】
【表３】
【００３７】
また耐久試験によるNOx 浄化率の低下度合いは、実施例及び比較例ともに小さく、担体にα−アルミナを用いた効果はどちらも良好に奏され、実施例においてRhの粒子径が微細化したことによる不具合はみられない。
【００３８】
【発明の効果】
すなわち本発明の排ガス浄化用触媒によれば、α−アルミナ担体を用いながらRhを高分散担持できるため、触媒の活性点が多く効率良くNOx を浄化することができる。また高温酸化雰囲気で使用してもRhの担体への固溶がほとんど生じないので、きわめて耐久性に優れている。
【００３９】
また本発明の排ガス浄化用触媒の製造方法によれば、上記した優れた特性をもつ排ガス浄化用触媒を容易にかつ安定して製造することができる。
【図面の簡単な説明】
【図１】本発明の一実施例の製造方法の概要と、製造される粉末の模式的な構造を示す説明図である。
【図２】実施例１で製造した担体の粒子構造を示す電子顕微鏡写真である。
【図３】実施例１で製造した還元工程前の粉末の粒子構造を示す暗視野電子顕微鏡写真である。
【図４】実施例１で製造した還元工程前の粉末の粒子構造を示す電子回折像の写真である。
【図５】実施例１で製造した触媒粉末の粒子構造を示す暗視野電子顕微鏡写真である。
【図６】実施例１で製造した触媒粉末の粒子構造を示す電子回折像の写真である。
【図７】実施例１で製造した触媒粉末の粒子構造を示す電子顕微鏡写真である。
【図８】実施例１の触媒と比較例１の触媒のＸ線回折チャートを示す線図である。
【図９】比較例１で製造した担体の粒子構造を示す電子顕微鏡写真である。
【図１０】比較例１で製造した触媒粉末の粒子構造を示す電子顕微鏡写真である。
【符号の説明】
１：α−アルミナ粒子 ２（20，21）：被覆層 ３：Rh粒子[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an exhaust gas purifying catalyst in which α-alumina is used as a carrier and rhodium (Rh) as a catalyst noble metal is supported on the carrier, and a method for producing the same. The exhaust gas purifying catalyst of the present invention can efficiently purify NOx in exhaust gas.
[0002]
[Prior art]
As an exhaust gas purifying catalyst conventionally automobiles, three-way catalyst for purifying performing the reduction of the oxidized and NO x CO and HC in the exhaust gas simultaneously is used. As such a three-way catalyst, for example, a carrier layer made of γ-alumina is formed on a heat-resistant honeycomb substrate made of cordierite or the like, and a catalyst noble metal such as platinum (Pt) is supported on the carrier layer. Is widely known.
[0003]
In addition, in order to produce a conventional exhaust gas purification catalyst, an adsorption support method in which a support having a support layer made of γ-alumina is immersed in an aqueous solution of Pt salt or Rh salt, pulled up, dried and calcined, or Pt Pt and Rh are supported by a water absorption method in which a predetermined amount of an aqueous solution of salt or Rh salt is impregnated and evaporated and dried.
γ-alumina is also referred to as activated alumina and is widely used as a catalyst support because it has a high specific surface area. Further, as the catalytic noble metal, it is common to use Pt having a high activity for oxidizing HC and CO and Rh having an excellent activity for reducing NO x by a reducing substance such as HC in exhaust gas.
[0004]
However γ- The catalyst carrying Rh on alumina support, although excellent in the initial purification activity, Rh is dissolved in the carrier in the hot oxygen-rich atmosphere, NO x purification performance by a decrease in the catalytic active sites on the support surface It has become clear that there is a problem of a significant drop.
Therefore, it is recalled that Rh is supported on a carrier in which Rh does not form a solid solution, such as those using zirconia (US Pat. No. 233189) and those using α-alumina (Japanese Patent Laid-Open No. 55-41894). I have a suggestion.
[0005]
In JP-A-55-41894, Rh is dissolved in γ-alumina in a high-temperature oxygen-excess atmosphere. However, if α-alumina is used as a carrier, the solid solution is suppressed, and the catalytic activity of Rh is prolonged. It is described that it is maintained.
[0006]
[Problems to be solved by the invention]
However, zirconia and α-alumina have a significantly lower specific surface area than γ-alumina. Therefore, in the production method using the conventional adsorption loading method or water absorption loading method, it is difficult to carry Rh on α-alumina in a highly dispersed state, so that there is a problem that the catalytic activity is low and the catalytic activity is low from the beginning. .
[0007]
As a result of intensive research, the present inventors have developed a method in which α-alumina is impregnated with Rh and oxidized and reduced to precipitate fine Rh particles on the surface of the carrier. According to this method, since 50% or more of the supported amount of Rh is supported as fine particles having a particle diameter of 2 nm or less, the active point of the catalyst is increased and high NO x purification performance is obtained.
[0008]
Α-alumina is excellent in thermal stability and hardly causes Rh solid solution. Therefore, even when exposed to a high-temperature oxygen-excess atmosphere during use, Rh does not easily aggregate, Rh maintains a highly dispersed state, and is excellent in durability of NO x purification performance.
However, it has been clarified that the catalyst obtained by the above-described method has a disadvantage that the dispersibility of Rh is lowered and the catalytic activity from the beginning is low particularly when the amount of Rh supported is large. The reason for this is that when a carrier made of α-alumina having a small specific surface area is used, there is a limit to the solid solution amount of Rh in the carrier during high-temperature oxidation treatment, and Rh that is not solid-solubilized is agglomerated on the carrier surface. Adhere to. When this is reduced, Rh dissolved in the carrier precipitates on the surface of the carrier as fine particles of several nanometers, but the aggregated particles of Rh present on the surface of the carrier further grow and further reduce dispersibility. Is due to
[0009]
The present invention has been made in view of such circumstances, and suppresses the aggregation of Rh during oxidation / reduction treatment using α-alumina as a carrier, thereby increasing the loading amount of Rh and carrying it in a highly dispersed manner. The purpose is to further increase the catalytic activity.
[0010]
[Means for Solving the Problems]
The feature of the method for producing an exhaust gas purifying catalyst of the present invention according to claim 1 for solving the above-mentioned problem is that a carrier having a coating layer made of amorphous alumina and / or transition alumina formed on the surface of α-alumina particles. A loading process in which rhodium is impregnated to form an Rh- supported carrier, an oxidation process in which Rh- supported carrier is heat-treated in an oxidizing atmosphere and rhodium is dissolved in the carrier to form an Rh solid-supported carrier, and an Rh solid-solved carrier is reduced to an atmosphere And a reduction step of depositing rhodium which is heat-treated and dissolved in the form of fine particles on the surface of the carrier and converting the coating layer into crystalline α-alumina.
[0011]
A feature of the exhaust gas purifying catalyst of the present invention as set forth in claim 2 is the exhaust gas purifying catalyst produced by the manufacturing method according to claim 1 and comprising a carrier made of α-alumina and rhodium supported on the carrier. The catalyst is that rhodium is exposed on the support surface as fine particles having a particle diameter of 5 nm or less in 80 % or more of the supported amount .
[0012]
DETAILED DESCRIPTION OF THE INVENTION
In the exhaust gas purifying catalyst of the present invention, 80% or more of the supported amount of Rh is supported on the α-alumina carrier as fine particles having a particle diameter of 5 nm or less. Therefore, since Rh is supported in a very fine state and in a highly dispersed state, the active point of the catalyst is increased and high NO x purification performance is exhibited.
[0013]
Α-alumina is excellent in thermal stability and hardly causes Rh solid solution. Therefore, even when exposed to a high-temperature oxygen-excess atmosphere during use, Rh does not easily aggregate, Rh maintains a highly dispersed state, and is excellent in durability of NO x purification performance.
Among the supported Rh, those having a particle diameter of more than 5 nm are 20% or more of the supported amount, so that the active point of the catalyst is decreased and the NO x purification performance is degraded. The particle diameter is preferably as fine as 5 nm or less, and the more fine particles are as desirable.
[0014]
The loading amount of Rh is preferably 0.01 to 2.0 g per 120 g of carrier, and 0.008 to 1.66% by weight in weight percent. If the loading amount of Rh is less than this range, sufficient NO x purification performance cannot be obtained. Even if the loading amount is larger than this range, the NO x purification performance is saturated and expensive Rh is wasted. When the loading amount is larger than this range, coarse particles having a particle diameter of 10 nm or more increase.
[0015]
If the amount of Rh supported is 1.0 g or less per 120 g of carrier, most Rh can be used effectively, and most of Rh has a particle size of 5 nm or less, and a particularly high NO x purification performance can be obtained.
In the method for producing an exhaust gas purifying catalyst according to claim 2, a carrier in which a coating layer made of amorphous alumina and / or transition alumina is formed on the surface of α-alumina particles is used. When Rh is supported on this support and heat-treated in an oxidizing atmosphere, Rh easily dissolves in amorphous alumina and / or transition alumina, so almost all of the supported Rh is dissolved in the support. The Rh solid-dissolved by the heat treatment in the subsequent reducing atmosphere is deposited as fine particles of several nm on the surface of the carrier. Thereby, a catalyst in which Rh is highly dispersed and supported can be obtained.
[0016]
In order to form a coating layer made of amorphous alumina and / or transition alumina on the surface of α-alumina particles, for example, α-alumina particles serving as nuclei are suspended in an aqueous solution of a water-soluble aluminum salt, and then there. There is a method in which a precipitate is formed by a neutralization coprecipitation method in which an aqueous neutralizing agent solution is dropped, and then dried and fired. A gas phase method or a mechanical method can also be employed.
[0017]
Examples of the water-soluble aluminum salt include inorganic salts and organic salts such as aluminum nitrate, sulfate, aluminum chloride, and acetate. Examples of the neutralizing agent include ammonia water, ammonium hydrogen carbonate, ammonium carbonate, sodium hydrogen carbonate, and the like, but carbonates and hydrogen carbonates that are easy to form a coating layer with a slow progress of the neutralization reaction are suitable. Yes.
[0018]
Both the concentration of the water-soluble aluminum salt in the suspension and the concentration of the neutralizing agent in the aqueous neutralizing agent solution are preferably in the range of 0.01 mol / L to 1 mol / L. When the concentration exceeds 1 mol / L, when the neutralizing agent solution is dropped, a reaction takes place locally, and no spontaneous precipitation occurs on the surface of the α-alumina particles. At a concentration lower than 0.01 mol / L, it is difficult to form a coating layer.
[0019]
In order to stabilize metal ions in the reaction solution, a complexing agent such as acid or hydrogen peroxide, or a surfactant for suppressing particle aggregation may be added to the reaction solution. .
The thickness of the coating layer made of amorphous alumina and / or transition alumina is preferably in the range of 0.5 to 100 nm. If it is less than 0.5 nm, the loading efficiency of Rh becomes low, so that coarse particles of 10 nm or more increase, and the activity of the resulting catalyst decreases. On the other hand, if it exceeds 100 nm, it becomes difficult to deposit Rh during the heat treatment in the reduction step, and the activity of the resulting catalyst is lowered. Particularly desirable is a range of 10-50 nm.
[0020]
In the supporting step, the Rh-supported carrier is manufactured by impregnating the carrier having the coating layer with the Rh salt solution. As the Rh salt, rhodium nitrate (Rh (NO 3 ) 3 ), rhodium chloride (RhCl 3 ) or the like is used, and these salts are used as an aqueous solution. Depending on the type of Rh salt, an organic solvent such as alcohol can also be used.
The amount of Rh supported on the Rh supported carrier is preferably in the range of 0.01 to 2.0% by weight. When the amount of Rh supported is less than 0.01% by weight, the obtained catalyst does not exhibit sufficient NO x purification ability. When the amount of supported catalyst exceeds 2.0% by weight, the resulting catalyst has many coarse Rh particles having a particle size of 10 nm or more. At the same time, the NO x purification performance is saturated, and effective Rh is wasted.
[0021]
This Rh-supported carrier is heat-treated in an oxidizing atmosphere such as in the air in the oxidation step to become an Rh solid solution carrier. α-Alumina has low interaction with Rh and Rh is not easily dissolved, but Rh is easily dissolved in the coating layer made of amorphous alumina. Even if a large amount of Rh is supported on the Rh-supported carrier, Most of them dissolve.
The heat treatment temperature in the oxidation step is preferably in the range of 700 to 1000 ° C. When the heat treatment temperature is less than 700 ° C., it is difficult to dissolve Rh, and in the subsequent reduction step, the coarse particles having a size of 10 nm or more increase, which may reduce the dispersibility of Rh. On the other hand, when the heat treatment temperature exceeds 1000 ° C., the Rh particles become coarse and are difficult to dissolve.
[0022]
In addition, the heat processing time in an oxidation process is enough for 1-3 hours. If the heat treatment time is less than 1 hour, the solid solution of Rh becomes insufficient, and it becomes difficult to precipitate Rh on the surface of the carrier in the subsequent reduction step, so that it is difficult to refine Rh. If it is carried out for 3 hours or more, Rh diffuses into the inside of the carrier, and Rh is difficult to precipitate in the subsequent reduction step. Also, heat energy is wasted.
[0023]
The Rh solid solution carrier that is heat-treated in the oxidation step and in which Rh is dissolved is heat-treated in a reducing atmosphere in the reduction step. As a result, the dissolved Rh becomes fine particles having a particle size of 5 nm or less and is deposited on the surface of the carrier and is supported in a highly dispersed state. In the coating layer, amorphous alumina and / or transition alumina is transformed into an α-alumina structure. Therefore, the resulting catalyst is a state in which fine Rh is highly dispersed and supported on an α-alumina carrier, and the Rh carrier has a low solid solution in a high-temperature oxygen-excess atmosphere, so that it has excellent purification performance durability. It becomes.
[0024]
As the reducing atmosphere, for example, a gas containing hydrogen gas in an inert gas is used. And the heat processing temperature has the preferable range of 800-1100 degreeC. When the heat treatment temperature is less than 800 ° C, Rh precipitation becomes difficult, and when the heat treatment temperature exceeds 1100 ° C, the precipitated fine Rh particles become coarse.
In addition, 0.5 to 2 hours is sufficient as the heat treatment time in the reduction step. If the heat treatment time is less than 0.5 hours, the precipitation of Rh is insufficient, and if it is performed for 2 hours or more, the precipitated fine Rh particles become coarse. Also, heat energy is wasted.
[0025]
【Example】
Hereinafter, the present invention will be specifically described with reference to Examples and Comparative Examples.
(Example 1)
FIG. 1 shows an outline of the production method of this example and a schematic structure of the powder produced in this example.
(1) Carrier preparation process 5.0 g of commercially available α-alumina powder (specific surface area 5 m 2 / g, average particle size 0.3 μm) is mixed with 300 ml of an aqueous aluminum sulfate solution having a concentration of 0.026 mol / L, while stirring. Then, 300 ml of an aqueous sodium hydrogen carbonate solution having a concentration of 0.134 mol / L was added dropwise over 2 hours using a metering pump. The resulting precipitate was filtered and washed with water, dried at 80 ° C. for 10 hours, and then heat-treated in air at 500 ° C. for 2 hours to obtain a carrier powder.
[0026]
An electron micrograph of the obtained carrier powder is shown in FIG. As is apparent from FIG. 2, a coating layer 2 having a thickness of about 30 nm is formed on the surface of the α-alumina particles 1, and as a result of electron beam diffraction, the structure of the coating layer 2 is γ-alumina having low crystallinity. there were.
(2) Loading process After impregnating this carrier powder with a predetermined amount of rhodium chloride aqueous solution of predetermined concentration and evaporating water to dryness, it is dried in the atmosphere at 120 ° C for 12 hours and then in the atmosphere at 600 ° C. An Rh-supported carrier carrying Rh was prepared by firing for 3 hours. Rh is mainly supported on the coating layer 2, and the supported amount is 0.6% by weight as metal Rh.
(3) Oxidation step Next, the obtained Rh-supported carrier was heat-treated at 800 ° C. for 3 hours in the atmosphere, and the supported Rh was dissolved in the coating layer 2 to obtain an Rh solid-solution carrier. An electron micrograph of this Rh solid solution carrier is shown in FIG. 3, and an electron diffraction image of the
[0027]
3 and 4 that the Rh solid solution carrier still has a
(4) Reduction Step The Rh solid solution carrier obtained in the oxidation step was heat-treated at 1000 ° C. for 1 hour in nitrogen gas containing 10% hydrogen gas. FIG. 5 shows an electron micrograph of the obtained Rh precipitate carrier, FIG. 6 shows an electron diffraction image, FIG. 7 shows an electron micrograph obtained by further increasing the magnification, and FIG. 8 shows an X-ray diffraction chart.
[0028]
From the dark field image of FIG. 5, the coating layer is crystallized, and the structure is a crystalline α-alumina structure from the electron diffraction of FIG. In addition, it can be seen from FIG. 7 that the obtained Rh precipitation support has
[0029]
That is, in the reduction process,
(Example 2)
The catalyst powder of Example 2 was prepared in the same manner as in Example 1 except that the aluminum sulfate aqueous solution in the carrier preparation step was 150 ml and the sodium hydrogen carbonate aqueous solution 150 ml was added dropwise over 1 hour using a metering pump. Note that the thickness of the coating layer of the carrier before the carrying step was 10 to 15 nm. Further, when the particle size distribution of Rh was measured by analyzing the image of Rh particles in an electron micrograph, those having a size of 5 nm or less accounted for 80%.
[0030]
(Example 3)
The catalyst powder of Example 3 was prepared in the same manner as in Example 1 except that the aluminum sulfate aqueous solution in the carrier preparation step was 450 ml and the sodium hydrogen carbonate aqueous solution 450 ml was dropped using a metering pump over 3 hours. Note that the thickness of the coating layer of the carrier before the carrying step was about 45 nm. Further, when the particle size distribution of Rh was measured by analyzing the image of Rh particles in an electron micrograph, 90% of the particles were 5 nm or less.
[0031]
(Comparative Example 1)
A catalyst powder of Comparative Example 1 was prepared in the same manner as in Example 1 except that the carrier preparation step was not performed and α-alumina powder was used as a carrier as it was.
An electron micrograph of the α-alumina carrier used in Comparative Example 1 is shown in FIG. From FIG. 9, it is obvious that no coating layer is formed on the carrier.
[0032]
FIG. 10 shows an electron micrograph of the obtained catalyst powder, and FIG. 8 shows an X-ray diffraction chart. Although Rh particles having a particle diameter of 1 to 5 nm are slightly present from FIG. 10, most of Rh is supported as coarse particles having a particle diameter of 10 nm or more, and appears as a clear peak in FIG. The particle size distribution of Rh was measured by image analysis of Rh particles in an electron micrograph, and 40% of the particles were 10 nm or more.
[0033]
(Examination / Evaluation)
Each of the catalyst powders of Examples and Comparative Examples was compacted to obtain a pellet catalyst having a particle size of 0.5 to 1.0 mm. Then, for each pellet catalyst, using the evaluation gas shown in Table 1, the inlet gas temperature was raised from 100 ° C to 500 ° C at a rate of 5 ° C / min under the conditions of a catalyst amount of 1.0 g and a gas flow rate of 3.3 L / min. The NO x purification rate was measured. Each NO x 50% purification temperature (initial) was determined, and the results are shown in Table 3.
[0034]
[Table 1]
[0035]
[Table 2]
[Table 3]
[0037]
The degree of decrease of the NO x purification rate by the durability test is smaller in both Examples and Comparative Examples, the effect of using an α- alumina carrier is achieved in both good, the particle size of Rh in the embodiment is miniaturized There is no defect due to.
[0038]
【The invention's effect】
That is, according to the exhaust gas purifying catalyst of the present invention, alpha-for-alumina carrier can highly dispersed supports Rh while, can be used to purify the active sites are many efficient NO x catalyst. In addition, even when used in a high-temperature oxidizing atmosphere, there is almost no solid solution of Rh in the carrier, which makes it extremely excellent in durability.
[0039]
Moreover, according to the method for producing an exhaust gas purifying catalyst of the present invention, the exhaust gas purifying catalyst having the above-described excellent characteristics can be produced easily and stably.
[Brief description of the drawings]
BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is an explanatory view showing an outline of a production method of one embodiment of the present invention and a schematic structure of a produced powder.
2 is an electron micrograph showing the particle structure of the carrier produced in Example 1. FIG.
3 is a dark-field electron micrograph showing the particle structure of the powder produced in Example 1 before the reduction step. FIG.
4 is a photograph of an electron diffraction image showing the particle structure of the powder before the reduction process produced in Example 1. FIG.
5 is a dark-field electron micrograph showing the particle structure of the catalyst powder produced in Example 1. FIG.
6 is a photograph of an electron diffraction image showing the particle structure of the catalyst powder produced in Example 1. FIG.
7 is an electron micrograph showing the particle structure of the catalyst powder produced in Example 1. FIG.
8 is a diagram showing X-ray diffraction charts of the catalyst of Example 1 and the catalyst of Comparative Example 1. FIG.
9 is an electron micrograph showing the particle structure of the carrier produced in Comparative Example 1. FIG.
10 is an electron micrograph showing the particle structure of the catalyst powder produced in Comparative Example 1. FIG.
[Explanation of symbols]
1: α-alumina particles 2 (20, 21): coating layer 3: Rh particles
Claims (2)
該Rh担持担体を酸化雰囲気中で熱処理し該担体にロジウムを固溶させてRh固溶担体とする酸化工程と、
該Rh固溶担体を還元雰囲気中で熱処理し固溶したロジウムを該担体表面に微粒子として析出させるとともに該被覆層を結晶性のα−アルミナとする還元工程と、を有することを特徴とする排ガス浄化用触媒の製造方法。a supporting step in which rhodium is impregnated and supported on a carrier in which a coating layer made of amorphous alumina and / or transition alumina is formed on the surface of α-alumina particles to form a Rh carrier;
An oxidation step in which the Rh-supported carrier is heat-treated in an oxidizing atmosphere and rhodium is dissolved in the carrier to form a Rh solid-solution carrier;
A reduction step of heat treating the Rh solid solution carrier in a reducing atmosphere to precipitate the solid solution rhodium as fine particles on the surface of the carrier, and reducing the coating layer to crystalline α-alumina. A method for producing a purification catalyst.
該ロジウムはその担持量の80％以上が粒子径５nm以下の微粒子として該担体表面に表出していることを特徴とする排ガス浄化用触媒。An exhaust gas purification catalyst produced by the production method according to claim 1, comprising a support made of α-alumina and rhodium supported on the support,
A catalyst for exhaust gas purification, wherein 80% or more of the supported amount of rhodium is exposed on the surface of the carrier as fine particles having a particle diameter of 5 nm or less.
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| JP33737297A JP3644572B2 (en) | 1997-12-08 | 1997-12-08 | Exhaust gas purification catalyst and method for producing the same |
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| JP3760717B2 (en) * | 2000-02-29 | 2006-03-29 | 株式会社豊田中央研究所 | Low temperature harmful gas purification catalyst |
| JP4274826B2 (en) * | 2003-03-17 | 2009-06-10 | 株式会社豊田中央研究所 | Exhaust gas purification catalyst and method for producing the same |
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