JP4120862B2 - Catalyst for CO shift reaction - Google Patents
Catalyst for CO shift reaction Download PDFInfo
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- JP4120862B2 JP4120862B2 JP2002078426A JP2002078426A JP4120862B2 JP 4120862 B2 JP4120862 B2 JP 4120862B2 JP 2002078426 A JP2002078426 A JP 2002078426A JP 2002078426 A JP2002078426 A JP 2002078426A JP 4120862 B2 JP4120862 B2 JP 4120862B2
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- shift reaction
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- 238000006243 chemical reaction Methods 0.000 title claims description 76
- 239000003054 catalyst Substances 0.000 title claims description 65
- 239000011148 porous material Substances 0.000 claims description 61
- 239000002131 composite material Substances 0.000 claims description 46
- CETPSERCERDGAM-UHFFFAOYSA-N ceric oxide Chemical compound O=[Ce]=O CETPSERCERDGAM-UHFFFAOYSA-N 0.000 claims description 26
- 229910000422 cerium(IV) oxide Inorganic materials 0.000 claims description 26
- BASFCYQUMIYNBI-UHFFFAOYSA-N platinum Chemical compound [Pt] BASFCYQUMIYNBI-UHFFFAOYSA-N 0.000 claims description 25
- 229910000510 noble metal Inorganic materials 0.000 claims description 20
- PNEYBMLMFCGWSK-UHFFFAOYSA-N aluminium oxide Inorganic materials [O-2].[O-2].[O-2].[Al+3].[Al+3] PNEYBMLMFCGWSK-UHFFFAOYSA-N 0.000 claims description 13
- 229910052697 platinum Inorganic materials 0.000 claims description 6
- 238000001354 calcination Methods 0.000 claims description 5
- 238000002485 combustion reaction Methods 0.000 claims description 5
- UGFAIRIUMAVXCW-UHFFFAOYSA-N Carbon monoxide Chemical compound [O+]#[C-] UGFAIRIUMAVXCW-UHFFFAOYSA-N 0.000 description 63
- 229910002091 carbon monoxide Inorganic materials 0.000 description 63
- 239000000843 powder Substances 0.000 description 37
- 230000000694 effects Effects 0.000 description 32
- 239000007789 gas Substances 0.000 description 23
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Chemical compound O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 description 18
- 229910018072 Al 2 O 3 Inorganic materials 0.000 description 17
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- HSJPMRKMPBAUAU-UHFFFAOYSA-N cerium(3+);trinitrate Chemical compound [Ce+3].[O-][N+]([O-])=O.[O-][N+]([O-])=O.[O-][N+]([O-])=O HSJPMRKMPBAUAU-UHFFFAOYSA-N 0.000 description 12
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- XNDZQQSKSQTQQD-UHFFFAOYSA-N 3-methylcyclohex-2-en-1-ol Chemical compound CC1=CC(O)CCC1 XNDZQQSKSQTQQD-UHFFFAOYSA-N 0.000 description 4
- 229910017518 Cu Zn Inorganic materials 0.000 description 4
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- TVZPLCNGKSPOJA-UHFFFAOYSA-N copper zinc Chemical compound [Cu].[Zn] TVZPLCNGKSPOJA-UHFFFAOYSA-N 0.000 description 4
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- 239000002245 particle Substances 0.000 description 4
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- 230000004913 activation Effects 0.000 description 3
- -1 aluminum compound Chemical class 0.000 description 3
- 229910021529 ammonia Inorganic materials 0.000 description 3
- QVGXLLKOCUKJST-UHFFFAOYSA-N atomic oxygen Chemical compound [O] QVGXLLKOCUKJST-UHFFFAOYSA-N 0.000 description 3
- 230000015572 biosynthetic process Effects 0.000 description 3
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- 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 2
- ATRRKUHOCOJYRX-UHFFFAOYSA-N Ammonium bicarbonate Chemical compound [NH4+].OC([O-])=O ATRRKUHOCOJYRX-UHFFFAOYSA-N 0.000 description 2
- LFQSCWFLJHTTHZ-UHFFFAOYSA-N Ethanol Chemical compound CCO LFQSCWFLJHTTHZ-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
- VYPSYNLAJGMNEJ-UHFFFAOYSA-N Silicium dioxide Chemical compound O=[Si]=O VYPSYNLAJGMNEJ-UHFFFAOYSA-N 0.000 description 2
- CDBYLPFSWZWCQE-UHFFFAOYSA-L Sodium Carbonate Chemical compound [Na+].[Na+].[O-]C([O-])=O CDBYLPFSWZWCQE-UHFFFAOYSA-L 0.000 description 2
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- 230000003213 activating effect Effects 0.000 description 2
- 239000001099 ammonium carbonate Substances 0.000 description 2
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- 238000006386 neutralization reaction Methods 0.000 description 2
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- 229910052684 Cerium Inorganic materials 0.000 description 1
- VEXZGXHMUGYJMC-UHFFFAOYSA-M Chloride anion Chemical compound [Cl-] VEXZGXHMUGYJMC-UHFFFAOYSA-M 0.000 description 1
- 229910002651 NO3 Inorganic materials 0.000 description 1
- NHNBFGGVMKEFGY-UHFFFAOYSA-N Nitrate Chemical compound [O-][N+]([O-])=O NHNBFGGVMKEFGY-UHFFFAOYSA-N 0.000 description 1
- GRYLNZFGIOXLOG-UHFFFAOYSA-N Nitric acid Chemical compound O[N+]([O-])=O GRYLNZFGIOXLOG-UHFFFAOYSA-N 0.000 description 1
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- IXSUHTFXKKBBJP-UHFFFAOYSA-L azanide;platinum(2+);dinitrite Chemical compound [NH2-].[NH2-].[Pt+2].[O-]N=O.[O-]N=O IXSUHTFXKKBBJP-UHFFFAOYSA-L 0.000 description 1
- 239000010953 base metal Substances 0.000 description 1
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Images
Classifications
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
- Y02P20/00—Technologies relating to chemical industry
- Y02P20/50—Improvements relating to the production of bulk chemicals
- Y02P20/52—Improvements relating to the production of bulk chemicals using catalysts, e.g. selective catalysts
Description
【０００１】
【発明の属する技術分野】
本発明は、一酸化炭素（CO）と水蒸気（ H2O）から水素（H2）を生成するCOシフト反応を行う触媒に関し、詳しくは空間速度が大きい条件下でも活性が高く、燃料電池及び内燃機関の排ガス浄化などに利用できるCOシフト反応用触媒に関する。
【０００２】
【従来の技術】
アンモニアの合成、都市ガスなどにおけるCOの除去、あるいはメタノール合成、オキソ合成におけるCO／H2比の調整などに、COシフト反応が応用されている。また近年では、内部改質型燃料電池の燃料改質システムにおけるCOの除去などにも用いられている。このCOシフト反応は、COと H2OからH2を生成する反応であり、水性ガスシフト反応とも称されている。
【０００３】
COシフト反応を促進させる触媒としては、例えば1960年代に Girdler社や du Pont社からCu−Zn系触媒が発表され、現在まで主として工場におけるプラント用などに幅広く利用されている。また、W.Hongli et al, China-Jpn.-U.S. Symp. Hetero. Catal. Relat. Energy Probl.,B09C,213(1982)には、アナターゼ型チタニアよりなる担体にPtを担持した触媒を 500℃付近で還元処理した触媒が、さらに高いCOシフト反応活性を示すことが報告されている。
【０００４】
またγ-Al2O3にPt,Rh,Pdなどの貴金属を担持した触媒もCOシフト反応活性を有することが知られている。さらにγ-Al2O3にCuを担持した触媒は、γ-Al2O3にPt,Rh,Pdなどの貴金属を担持した触媒よりもCOシフト反応活性が高いことも報告されている。
【０００５】
ところで、自動車などの移動体に搭載する内部改質型燃料電池の燃料改質システム、あるいは自動車排ガス中のCOをH2に改質し、そのH2を用いて触媒上に吸蔵されたNOx を還元する排ガス浄化システムなどに用いられるCOシフト反応用触媒としては、触媒反応器の大きさに制約があるため、空間速度の大きな反応条件下でも高い活性を示すことが必要となる。
【０００６】
ところが従来のCu−Zn系触媒では、空間速度が大きな反応条件下では活性が低いという不具合がある。そのため内部改質型燃料電池の燃料改質システム、あるいは自動車排ガス浄化システムなどのように空間速度が大きな反応条件下では、COをH2に効率よく転換することが困難となる。またCOシフト反応は平衡反応であり、反応温度が高いほどCOからH2への転換に不利となる。したがってCu−Zn系触媒では、空間速度の大きな反応条件での活性を補うことを目的としていくら反応温度を上げても、COをH2に効率よく転換することはできない。
【０００７】
さらに、COシフト反応用触媒を内部改質型燃料電池の燃料改質システム、あるいは自動車排ガス浄化システムなどに用いた場合には、使用条件によって一時的に反応場が高温雰囲気となる場合があるため、その場合には、Cu−Zn系触媒の活性種であるCu、あるいはγ-Al2O3にCuを担持した触媒のCuが粒成長して活性が低下するという問題もあり、COをH2に効率よく転換することが一層困難となる。
【０００８】
さらにCOシフト反応用触媒を内部改質型電池の燃料改質システムに用いる場合は、COシフト反応では H2Oの濃度が高いほどH2を生成する反応が進行しやすいので、Cu−Zn系触媒などでは一般に H2O／CO比が２以上となる条件下で用いられる。
【０００９】
しかし、自動車のように限られた環境でこの反応を行うためには、多量の水を保存する水タンク及び大きな蒸発器などが必要となるため、装置が大きくなるという不具合がある。さらに水蒸気を供給するためには、水を蒸発させるための多量のエネルギーを必要とし、システム全体としてのエネルギー効率を低下させることになる。
【００１０】
そこで卑金属より活性が高く、高温雰囲気で安定であると予想される貴金属を用いることが想起される。しかしながら上記したように、γ-Al2O3にPt,Rh,Pdなどの貴金属を担持した触媒はγ-Al2O3にCuを担持した触媒よりも活性が低い。またアナターゼ型チタニアよりなる担体にPtを担持した触媒では、Ptと担体との間で強い相互作用（SMSI: strong metal support interaction）が生じることが知られている。そのため通常の使用条件に含まれる 200℃〜 400℃の低温域で反応ガスに曝されると、SMSIによってPtを担体成分の一部が覆うようになり、活性点の減少により活性が著しく低下するという不具合がある。
【００１１】
一方、特開2001−316682号公報の実施例には、アルミナ担体にセリアとPt，Pd及びFeを担持したCOシフト反応用触媒が記載されている。また Applied Catalysis B:Environmental 15(1998)107-114において、T.Bunluesin らは、セリアにPt，Pd及びRhを担持したCOシフト反応用触媒のCOシフト反応機構を解説している。
【００１２】
セリアと貴金属の間にはSMSIが生じにくく、SMSIによって貴金属の活性点が減少するような不具合が生じにくい。またセリアは水蒸気（ H2O）の活性化に寄与し、貴金属はCOの活性化に寄与するため、セリアと貴金属が共存する触媒とすることで、COシフト反応が促進されるという効果がある。
【００１３】
ところで固体触媒は、一般に一つの結晶子である一次粒子及びその集合体である二次粒子から構成され、一般的には二次粒子の内部すなわち一次粒子どうしの間隙に細孔が形成されている。一般に流通型反応において、低い空間速度の反応条件であれば反応物は細孔の内部まで拡散して反応するが、高い空間速度の反応条件では反応物が細孔内部まで拡散しにくくなり、反応に利用される固体表面が減少する傾向がある。そして細孔径が小さい細孔をもつ固体ほどその傾向が顕著である。
【００１４】
上記した従来技術における固体担体としてのセリアは、細孔径が 3.2nm以下の微細細孔が大部分を占めているために、空間速度が高い反応条件では、反応物が拡散し反応に利用される固体表面が少なく、COシフト反応活性が低いという不具合がある。したがって自動車の排ガス中で用いられるような環境下においては、空間速度が高いために十分なCOシフト反応の発現は困難であった。また高温下に曝されたり雰囲気が変動する条件下で使用された場合には、セリアが容易に粒成長し、それに伴ってセリアの酸素吸蔵放出能も低下する。そのため酸化雰囲気下における貴金属の粒成長が避けられない。したがってセリアと貴金属の両者の粒成長により、COシフト反応活性が低下するという問題もある。
【００１５】
【発明が解決しようとする課題】
本発明はこのような事情に鑑みてなされたものであり、高い空間速度の反応条件下においても高いCOシフト反応活性を示し、かつ高温下に曝されたり雰囲気が変動する条件下で使用されても活性の低下の少ない実用的な触媒とすることを目的とする。
【００１６】
【課題を解決するための手段】
上記課題を解決する本発明のCOシフト反応用触媒の特徴は、セリアとアルミナが nm スケールで分散し 800 ℃で５時間の焼成後に細孔径が 3.5 〜 100nm の細孔容積が 0.20cc ／ｇ以上となる特性を有する複合酸化物を含む担体に貴金属を担持してなることにある。
【００１７】
複合酸化物は、中心細孔径は15〜65nmであることが望ましい。また 800℃で５時間の焼成後に細孔径が 3.5〜 100nmの細孔容積が0.30cc／ｇ以上となる特性を有することが特に望ましく、中心細孔径は25〜65nmであることが特に望ましい。また複合酸化物にはセリアが50〜90重量％含まれていることが望ましい。
【００１８】
そして貴金属は少なくともPtを含むことが好ましい。
【００１９】
【発明の実施の形態】
本発明のCOシフト反応用触媒では、セリアとアルミナがnmスケールで分散した複合酸化物を含む担体に貴金属を担持している。セリアは塩基点を有しているため H2Oの活性化に優れ、貴金属はCOの活性化に優れている。これにより高いCOシフト反応活性が発現される。そして複合酸化物はセリアとアルミナがnmスケールで分散しているので、高温雰囲気に曝された場合にはアルミナがセリア粒子間の障壁となり、セリアの粒成長が抑制される。さらにこれによってセリアの酸素吸蔵放出能の低下が抑制されるため、雰囲気変動が緩和され貴金属の粒成長が抑制される。したがってセリアと貴金属の両者の粒成長が抑制されるため、高いCOシフト反応活性が保持される。
【００２０】
ここでnmスケールの分散とは、１nm程度の高分解能を有するミクロ分析装置を用いて測定しても、独立した粒子として観察されないレベルの分散状態のことをいう。このようなミクロ分析装置としては、例えば日立製作所（株）製の「HD−2000」などのFE−STEM走査形透過電子顕微鏡がある。
【００２１】
セリアとアルミナがnmスケールで分散している状態は、FE−STEMの EDSを用いた重なりのない一つの粒子の、直径 0.5nm程度のビーム径による微少範囲分析を行うことで確認することができる。そしてこの微小範囲分析による結果、複合酸化物の90％以上が、Ce及びAlの仕込み組成比±20％以内の組成比であることが望ましい。なおFE−STEMは、Field Effect-Scanning Transmission Electron Microscopyの略称であり、EDS は、Energy Dispersion Spectroscopyの略称である。
【００２２】
複合酸化物は、 800℃で５時間の焼成後に細孔径が 3.5〜 100nmの細孔容積が0.20cc／ｇ以上となる特性を有することが望ましい。また、 800℃で５時間の焼成後に細孔径が 3.5〜 100nmの細孔容積が0.30cc／ｇ以上となる特性を有することが特に望ましい。水銀ポロシメータを用いて原理上測定可能な下限値 3.5nmから 100nmの細孔径をもつ細孔はいわゆるメソ細孔と称されるが、このメソ細孔の細孔容積が0.20cc／ｇ以上であることにより、空間速度が高い反応条件下においても、CO及び H2Oが細孔内部まで拡散する。したがって反応に利用される表面積が大きいため、高いCOシフト反応活性が発現される。このような特性を有しない場合には、ガス拡散性が低下するため、空間速度が高い反応条件下におけるCOシフト反応活性が低下してしまう。
【００２３】
また複合酸化物の中心細孔径は、15〜65nmであることが望ましく、25〜65nmであることが特に望ましい。中心細孔径が15nm未満ではガス拡散性が低下するため、空間速度が高い反応条件下におけるCOシフト反応活性が低下してしまう。一方、中心細孔径が65nmを超えると、高温により一次粒子が再配列し、凝集し、メソ細孔の細孔容積が低下しやすくなると考えられる。
【００２４】
複合酸化物には、セリアが50〜90重量％含まれていることが望ましい。セリア量が50重量％未満では H2Oの活性化が困難となり、COシフト反応活性が低下する。またセリア量が90重量％を超えると、障壁となるアルミナ量が少ないためにセリアに粒成長が生じ易くなり、その結果雰囲気変動を緩和する作用が小さくなって貴金属も粒成長するようになる。したがってCOシフト反応活性が低下する。
【００２５】
本発明にいう複合酸化物を製造するには、先ずセリウム化合物とアルミニウム化合物が溶解した水溶液又は水を含む溶液からセリア前駆体とアルミナ前駆体又はそれらの前駆体の化合物の沈殿を析出させる。
【００２６】
セリウム化合物及びアルミニウム化合物としては、一般に塩が用いられ、塩としては、硝酸塩、塩化物、酢酸塩、硫酸塩などが利用できる。また塩を均一に溶解する溶媒としては、水、アルコール類が使用できる。さらに、例えば硝酸アルミニウムの原料として、水酸化アルミニウムと硝酸と水とを混合して用いてもよい。
【００２７】
沈殿の析出方法は、主にアンモニア水などの添加によってpHを調節して行うが、様々な調節方法により、さらに特徴的な複合酸化物の前駆体とすることができる。例えば、セリウム化合物及びアルミニウム化合物を含む水溶液又は水を含む溶液から、これらの酸化物前駆体又はそれらの前駆体の化合物の沈殿をほぼ同時に析出させる方法、又は、アルミナ前駆体が沈殿するよりも先にセリア前駆体を析出させる方法（又はその逆）がある。
【００２８】
前者のほぼ同時に析出させる方法については、アンモニア水などを瞬時に添加し強撹拌する方法や、過酸化水素などを加えることでセリア前駆体及びアルミナ前駆体の沈殿し始めるpHを調節した後、アンモニア水などで沈殿を析出させる方法などがある。
【００２９】
また後者については、アンモニア水などで中和させる際にかかる時間を十分に長くし、好ましくは10分以上で中和させる方法や、pHをモニターしながらセリア前駆体沈殿が析出するpH又はアルミナ前駆体の沈殿が析出するpHに、段階的に中和する又はそのようなpHに保つような緩衝溶液を添加する方法などがある。
【００３０】
なおアンモニア水以外に、炭酸アンモニウム、水酸化ナトリウム、水酸化カリウム、炭酸ナトリウムなどを溶解した水溶液、アルコール溶液が使用できる。焼成時に揮散するアンモニア、炭酸アンモニウムが特に好ましい。なお、アルカリ性溶液のpHは、９以上であることが前駆体の析出反応を促進するのでより好ましい。
【００３１】
水の存在下で沈殿を熟成することにより、より均一な複合酸化物を製造することができる。この熟成工程は、室温以上、好ましくは 100〜 200℃で、さらに好ましくは 100〜 150℃で行うことが望ましい。 100℃未満の加温では、熟成に要する時間が長大となる。また 200℃より高い温度では、水蒸気圧がきわめて高くなるために、高圧に耐える大がかりな装置が必要になり、製造コストが非常に高くなって好ましくない。
【００３２】
なお、沈殿物を含む溶液をそのまま加熱して蒸発乾固させ、さらに焼成すれば、蒸発乾固中に熟成工程を行うことができるが、室温以上好ましくは 100℃以上で保持して熟成する方がよい。
【００３３】
そして、このようにして得られた沈殿を、再度水中に分散させ、ホモジナイザーなどを用いて強力に分散させた後に焼成することが望ましい。これによって、 800℃で５時間の焼成後に細孔径が 3.5〜 100nmの細孔容積が0.20cc／ｇ以上、さらには0.30cc／ｇ以上となる特性を有する複合酸化物を製造することができる。強力に分散させる際には、界面活性剤を添加することが望ましい。界面活性剤の添加により、一次粒子の凝集を阻止し、メソ細孔の細孔容積を増大させることができる。
【００３４】
沈殿物の焼成は、大気中で行えばよく、その温度は 300〜 900℃の範囲が望ましい。焼成温度が 300℃より低いと、実質上、担体としての安定性に欠ける。また 900℃より高温での焼成は比表面積の低下をまねき、担体としての利用法から考えても不必要である。
【００３５】
本発明にいう担体は、上記複合酸化物を含めばよく、上記複合酸化物のみから構成してもよいし、上記複合酸化物の粉末と他の多孔質酸化物粉末を混合して構成することもできる。この他の多孔質酸化物としては、アルミナ、セリア、ジルコニア、チタニア、シリカなどの一種又は複数種を用いることができる。上記複合酸化物粉末とこの多孔質酸化物粉末とを混合した担体とする場合には、上記複合酸化物粉末が50重量％以上とすることが望ましい。担体中の上記複合酸化物粉末の量がこれより少ないと、COシフト反応活性が低下し実用的でない。
【００３６】
担体に担持される貴金属としては、Pt、Rh、Pd、Irなどから選択することができるが、少なくともPtを含むことが望ましい。少なくともPtを担持することにより、COシフト反応活性が特に向上する。この貴金属の担持量は、担体 100ｇあたり 0.1〜10ｇとするのが好ましい。担持量がこれより少ないとCOシフト反応活性が低く、これより多く担持してもCOシフト反応活性が飽和するとともに貴金属どうしの粒成長が生じる場合がある。
【００３７】
本発明のCOシフト反応用触媒は、少なくともCOと H2Oを含むガスと接触されることで、COシフト反応により高い活性で水素を生成する。そしてこのようなガスとして、自動車など内燃機関からの排ガスを用いることが特に好ましい。内燃機関からの排ガスは空間速度が高いが、本発明のCOシフト反応用触媒によれば、このように空間速度の高いガス中においても高いCOシフト反応活性が発現される。
【００３８】
【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。
【００３９】
（実施例１）
硝酸アルミニウム９水和物 0.2モル（75.1ｇ）を2000mlのイオン交換水に混合し、プロペラ撹拌器で５分間撹拌して溶解した。そこへ濃度28重量％の硝酸セリウム水溶液 304ｇ（CeO2換算で 0.5モル相当）を混合し、さらに５分間撹拌した。得られた混合水溶液に、25％アンモニア水 177ｇを加え、さらに10分間撹拌して沈殿物を含む水溶液とした。これを２気圧の加圧下にて 120℃で２時間熱処理する熟成工程を行い、沈殿物を熟成した。
【００４０】
その後、吸引濾過器を用いて濾過し、フィルタケーキを得た。このフィルタケーキを 500mlのイオン交換水中にプロペラ撹拌器を用いて再度分散させ、さらにアルキルアミン系イオン性界面活性剤（「アーマック T50」ライオン（株）製） 9.1ｇを添加し、ロータ径17mm、ギャップ１mmのホモジナイザーを用いて10000rpmで５分間混合した。次いで吸引濾過器を用いて濾過し、得られたフィルタケーキを 400℃で５時間焼成して、CeO2-Al2O3複合酸化物粉末を調製した。得られた複合酸化物粉末は、約89重量％のCeO2と約11重量％の Al2O3から構成されている。
【００４１】
この複合酸化物粉末に所定濃度のジニトロジアンミン白金硝酸溶液の所定量を含浸させ、大気中 300℃で３時間焼成することによりPtを５重量％担持した。これをさらに定法によって粒径 0.5〜 1.0mmのペレットに成形し、ペレット触媒を調製した。
【００４２】
（実施例２）
硝酸アルミニウム９水和物0.56モル（ 211ｇ）と、濃度28重量％の硝酸セリウム水溶液 304ｇ（CeO2換算で 0.5モル相当）と、25％アンモニア水 251ｇを用いたこと以外は実施例１と同様にして複合酸化物粉末を調製し、同様にPtを担持してペレット触媒を調製した。得られた複合酸化物粉末は、約75重量％のCeO2と約25重量％の Al2O3から構成されている。
【００４３】
（実施例３）
硝酸アルミニウム９水和物 0.2モル（75.1ｇ）を2000mlのイオン交換水に混合し、プロペラ撹拌器で５分間撹拌して溶解した。そこへ濃度28重量％の硝酸セリウム水溶液 304ｇ（CeO2換算で 0.5モル相当）を混合し、さらに５分間撹拌した。得られた混合水溶液に、25％アンモニア水 177ｇを加え、さらに10分間撹拌して沈殿物を含む水溶液とした。その後、沈殿物を含む水溶液を 100℃／hrの昇温速度で加熱し、 400℃で５時間焼成してCeO2-Al2O3複合酸化物粉末を調製した。得られた複合酸化物粉末は、約89重量％のCeO2と約11重量％の Al2O3から構成されている。
【００４４】
この複合酸化物粉末を用い、実施例１と同様にPtを担持して同様にペレット触媒を調製した。
【００４５】
（実施例４）
硝酸アルミニウム９水和物0.56モル（ 211ｇ）と、硝酸セリウム水溶液 304ｇ（CeO2換算で 0.5モル相当）と、25％アンモニア水 251ｇを用いたこと以外は実施例３と同様にして複合酸化物粉末を調製し、同様にPtを担持してペレット触媒を調製した。得られた複合酸化物粉末は、約75重量％のCeO2と約25重量％の Al2O3から構成されている。
【００４６】
（比較例１）
硝酸アルミニウム９水和物を用いず、濃度28重量％の硝酸セリウム水溶液 304ｇ（CeO2換算で 0.5モル相当）と25％アンモニア水 136ｇを用いたこと以外は実施例１と同様にして酸化物粉末を調製し、同様にPtを担持してペレット触媒を調製した。得られた酸化物粉末は、CeO2のみから構成されている。
【００４７】
（比較例２）
γ-Al2O3粉末（比表面積 200m2／ｇ）に所定濃度の硝酸セリウム水溶液の所定量を含浸させ、大気中 500℃で２時間焼成して複合酸化物粉末を調製した。この複合酸化物粉末は、約38重量％のCeO2と約62重量％の Al2O3から構成され、特開2001−316682号公報の実施例に記載の複合酸化物に相当する。
【００４８】
この複合酸化物粉末を用い、実施例１と同様にPtを担持してペレット触媒を調製した。
【００４９】
＜試験・評価＞
各実施例及び各比較例で調製された複合酸化物粉末について、大気中において 800℃で５時間の焼成を行い、その後水銀ポロシメータを用いて細孔分布を測定した。実施例１と実施例３で調製された複合酸化物粉末の細孔分布曲線を抜き出して図１に示す。図１より、実施例１及び実施例３で調製された複合酸化物粉末は細孔径 100nm以下の細孔の細孔容積が大きいことがわかり、特に実施例１で調製された複合酸化物粉末は細孔径 100nm以下の細孔がシャープな分布となっていることがわかる。
【００５０】
それぞれの試料の細孔分布曲線から算出した細孔径 3.5〜 100nmの細孔の細孔容積と、中心細孔径の値を図２及び図３に示す。通常の酸化物粉末は、高い温度で焼成されるほど一次粒子間で固相反応が起きて一次粒子が凝集するため、細孔径 3.5〜 100nmの細孔の細孔容積が減少し中心細孔径は小さくなる。しかし図１〜３より、実施例１〜４で調製された複合酸化物粉末、特に実施例１〜２で調製された複合酸化物粉末は、 800℃という高温での焼成後においても、比較例１〜２で調製された複合酸化物粉末又は酸化物粉末に比べて細孔容積が大きく中心細孔径も大きいことがわかる。また比較例２で調製された複合酸化物粉末では、細孔容積が小さすぎて中心細孔径が不明であった。
【００５１】
次に、各実施例及び各比較例で調製されたそれぞれのペレット触媒に対し、表１に示すリーンガス４分とリッチガス１分を交互に切り換えて流しながら、入りガス温度 700℃で５時間加熱保持する耐久試験を行った。耐久試験前後の各触媒の比表面積を BET法にて測定し、結果を図４に示す。またCOパルス吸着法を用いて耐久試験前後の各触媒におけるPt分散度を測定し、結果を図５に示す。
【００５２】
【表１】
【００５３】
次に、耐久試験後の各触媒をそれぞれ常圧固定床流通型触媒評価装置に配置し、CO： 1.8％， H2O：10％，残部N2よりなるモデルガスを触媒２ｇに対して10リットル／分となるように流しながら、 500℃で15分保持する前処理を行った。その後、同モデルガスを同様に流しながら、15℃／分の昇温速度で 100℃から 400℃まで昇温した。このときの各ペレット触媒の体積は約３mlであり、空間速度は約 200,000hr-1である。昇温の間、各触媒についてCO転化率を連続的に測定した。結果を図６に示す。なお別の実験により、CO転化率はH2生成率に等しいことが確認されている。
【００５４】
図６より、約 200,000hr-1という高い空間速度の反応条件において、各実施例の触媒は各比較例の触媒に比べて低温域から高いCOシフト反応活性を示していることが明らかである。そして図１〜５の結果を踏まえると、各実施例の触媒が高いCOシフト反応活性を示すのは、CeO2とPtの粒成長が抑制されていることに起因すると考えられる。また実施例１〜２の触媒が実施例３〜４の触媒より高いCOシフト反応活性を示すのは、細孔径の大きな細孔が大きな細孔容積で存在していることに起因することが明らかであり、 800℃で５時間の焼成後に細孔径が 3.5〜 100nmの細孔容積が0.20cc／ｇ以上となる特性を有し、かつ中心細孔径が15nm以上の複合酸化物を担体とするのが特に好ましいことが明らかである。
【００５５】
【発明の効果】
すなわち本発明のCOシフト反応用触媒によれば、空間速度が高い反応条件で高い活性が発現され、かつ高温条件あるいは雰囲気変動条件に曝されてもその活性を保持することができる。したがって自動車エンジンなどの内燃機関からの排ガス中での利用に最適であり、NOx を還元浄化する排ガス浄化用触媒あるいは燃料電池用の水素生成触媒としてきわめて有用である。
【図面の簡単な説明】
【図１】実施例１及び実施例３で製造された複合酸化物粉末の細孔分布曲線を示すグラフである。
【図２】実施例及び比較例で製造された複合酸化物粉末又は酸化物粉末の 800℃焼成後の細孔容積を示すグラフである。
【図３】実施例及び比較例で製造された複合酸化物粉末又は酸化物粉末の 800℃焼成後の中心細孔径を示すグラフである。
【図４】実施例及び比較例の各触媒の比表面積を示すグラフである。
【図５】実施例及び比較例の各触媒のPt分散度を示すグラフである。
【図６】実施例及び比較例の各触媒の温度とCO転化率との関係を示すグラフである。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a catalyst for performing CO shift reaction to produce hydrogen (H 2) from carbon monoxide (CO) and water vapor (H 2 O), more detail is highly active even under space velocity is large, the fuel cell and The present invention relates to a catalyst for CO shift reaction that can be used for exhaust gas purification of an internal combustion engine.
[0002]
[Prior art]
The CO shift reaction is applied to synthesis of ammonia, removal of CO in city gas, etc., or adjustment of the CO / H 2 ratio in methanol synthesis and oxo synthesis. In recent years, it has also been used for CO removal in fuel reforming systems for internal reforming fuel cells. This CO shift reaction is a reaction that generates H 2 from CO and H 2 O, and is also referred to as a water gas shift reaction.
[0003]
As a catalyst for promoting the CO shift reaction, for example, Cu-Zn-based catalysts were announced by Girdler and du Pont in the 1960s and have been widely used until now, mainly for plants in factories. In W. Hongli et al, China-Jpn.-US Symp. Hetero. Catal. Relat. Energy Probl., B09C, 213 (1982), a catalyst comprising Pt on a support made of anatase-type titania is used at 500 ° C. It has been reported that the catalyst reduced in the vicinity exhibits higher CO shift reaction activity.
[0004]
It is also known that a catalyst in which a noble metal such as Pt, Rh, Pd is supported on γ-Al 2 O 3 has CO shift reaction activity. Furthermore carrying the Cu on γ-Al 2 O 3 catalyst, Pt on γ-Al 2 O 3, Rh , a noble metal CO shift reaction activity than the catalyst supported such as Pd has been reported high.
[0005]
Incidentally, modified internal reforming type fuel cell in the fuel reforming system to be mounted in a mobile object such as an automobile, or the CO of the car in the exhaust gas to H 2, NO x occluded on the catalyst using the H 2 As a catalyst for a CO shift reaction used in an exhaust gas purification system for reducing the amount of catalyst, it is necessary to exhibit a high activity even under reaction conditions with a large space velocity, because the size of the catalytic reactor is limited.
[0006]
However, the conventional Cu-Zn-based catalyst has a disadvantage that its activity is low under reaction conditions with a large space velocity. Therefore, it is difficult to efficiently convert CO to H 2 under reaction conditions with a large space velocity, such as a fuel reforming system of an internal reforming fuel cell or an automobile exhaust gas purification system. The CO shift reaction is an equilibrium reaction, it becomes higher the reaction temperature is high from CO disadvantageous for conversion to H 2. Therefore, Cu—Zn-based catalysts cannot efficiently convert CO to H 2 no matter how much the reaction temperature is raised for the purpose of supplementing the activity under reaction conditions with a large space velocity.
[0007]
Furthermore, when a CO shift reaction catalyst is used in a fuel reforming system for an internal reforming fuel cell or an automobile exhaust gas purification system, the reaction field may temporarily become a high temperature atmosphere depending on the use conditions. In that case, there is also a problem that Cu, which is an active species of a Cu-Zn catalyst, or Cu of a catalyst supporting Cu on γ-Al 2 O 3 grows and drops in activity, and CO is reduced to H be converted efficiently into 2 becomes more difficult.
[0008]
Furthermore, when using a catalyst for CO shift reaction in a fuel reforming system for an internal reforming cell, the higher the H 2 O concentration in the CO shift reaction, the more likely the reaction to generate H 2 proceeds. In general, the catalyst is used under conditions where the H 2 O / CO ratio is 2 or more.
[0009]
However, in order to perform this reaction in a limited environment such as an automobile, a water tank for storing a large amount of water, a large evaporator, and the like are necessary. Further, in order to supply water vapor, a large amount of energy for evaporating water is required, and the energy efficiency of the entire system is lowered.
[0010]
Thus, it is recalled to use a noble metal that is more active than a base metal and is expected to be stable in a high temperature atmosphere. However, as described above, the γ-Al 2 O 3 Pt, Rh, a catalyst carrying a noble metal such as Pd is less active than catalyst supporting Cu on γ-Al 2 O 3. In addition, it is known that a strong metal support interaction (SMSI) occurs between Pt and a carrier in a catalyst in which Pt is supported on a carrier made of anatase type titania. Therefore, when exposed to the reaction gas in the low temperature range of 200 ° C to 400 ° C, which is included in normal use conditions, SMS will cover part of the carrier component with SMSI, and the activity will decrease significantly due to the reduction of active sites. There is a problem that.
[0011]
On the other hand, the examples of JP-A-2001-316682 describe a CO shift reaction catalyst in which ceria and Pt, Pd and Fe are supported on an alumina support. In Applied Catalysis B: Environmental 15 (1998) 107-114, T. Bunluesin et al. Describe the CO shift reaction mechanism of a catalyst for CO shift reaction in which Pt, Pd and Rh are supported on ceria.
[0012]
SMSI is unlikely to occur between ceria and noble metal, and SMSI is unlikely to cause problems such as reducing the active sites of noble metals. Ceria contributes to the activation of water vapor (H 2 O), and precious metals contribute to the activation of CO. By using a catalyst in which ceria and precious metals coexist, the CO shift reaction is promoted. .
[0013]
By the way, a solid catalyst is generally composed of primary particles that are one crystallite and secondary particles that are aggregates thereof, and generally pores are formed inside the secondary particles, that is, in the gaps between the primary particles. . In general, in a flow-type reaction, the reaction product diffuses and reacts inside the pores under low space velocity reaction conditions. However, the reaction product becomes difficult to diffuse into the pores under high space velocity reaction conditions. There is a tendency to reduce the solid surface utilized. And the tendency is so remarkable that the solid has a small pore diameter.
[0014]
Ceria as a solid support in the above-described prior art is mainly composed of fine pores having a pore diameter of 3.2 nm or less. Therefore, under reaction conditions with a high space velocity, reactants diffuse and are used for the reaction. There are defects that the solid surface is small and the CO shift reaction activity is low. Therefore, in an environment such as that used in automobile exhaust gas, it is difficult to develop a sufficient CO shift reaction due to the high space velocity. In addition, when used under conditions where the atmosphere is exposed to a high temperature or the atmosphere changes, ceria grows easily, and the oxygen storage / release capacity of ceria decreases accordingly. Therefore, noble metal grain growth is unavoidable in an oxidizing atmosphere. Therefore, there is also a problem that CO shift reaction activity decreases due to grain growth of both ceria and noble metal.
[0015]
[Problems to be solved by the invention]
The present invention has been made in view of such circumstances, and shows high CO shift reaction activity even under high space velocity reaction conditions, and is used under conditions where the atmosphere is exposed to high temperatures or the atmosphere changes. Is intended to be a practical catalyst with little decrease in activity.
[0016]
[Means for Solving the Problems]
The feature of the catalyst for CO shift reaction of the present invention that solves the above problems is that ceria and alumina are dispersed in nm scale, and the pore volume of pores having a pore size of 3.5 to 100 nm is 0.20 cc / g or more after firing at 800 ° C. for 5 hours. In other words, a noble metal is supported on a support containing a complex oxide having the following characteristics .
[0017]
The composite oxide desirably has a central pore diameter of 15 to 65 nm. In addition, it is particularly desirable that the pore volume having a pore diameter of 3.5 to 100 nm is 0.30 cc / g or more after firing at 800 ° C. for 5 hours, and the center pore diameter is particularly preferably 25 to 65 nm. The composite oxide preferably contains 50 to 90% by weight of ceria.
[0018]
The noble metal preferably contains at least Pt.
[0019]
DETAILED DESCRIPTION OF THE INVENTION
In the catalyst for CO shift reaction of the present invention, a noble metal is supported on a support containing a composite oxide in which ceria and alumina are dispersed on the nm scale. Ceria has a base point, so it is excellent in activating H 2 O, and noble metals are excellent in activating CO. Thereby, high CO shift reaction activity is expressed. In the composite oxide, ceria and alumina are dispersed on the nm scale. Therefore, when exposed to a high temperature atmosphere, alumina serves as a barrier between ceria particles, and ceria grain growth is suppressed. Furthermore, since this suppresses the decrease in the oxygen storage / release capability of ceria, the change in atmosphere is mitigated and the growth of noble metal grains is suppressed. Therefore, since the grain growth of both ceria and noble metal is suppressed, high CO shift reaction activity is maintained.
[0020]
Here, “nm-scale dispersion” refers to a dispersion state at a level that is not observed as independent particles even when measured using a microanalyzer having a high resolution of about 1 nm. An example of such a microanalyzer is a FE-STEM scanning transmission electron microscope such as “HD-2000” manufactured by Hitachi, Ltd.
[0021]
The state where ceria and alumina are dispersed on the nm scale can be confirmed by performing a micro-range analysis of one non-overlapping particle using a FE-STEM EDS with a beam diameter of about 0.5 nm. . As a result of this micro range analysis, it is desirable that 90% or more of the composite oxide has a composition ratio within 20% of the charged composition ratio of Ce and Al. Note that FE-STEM is an abbreviation for Field Effect-Scanning Transmission Electron Microscopy, and EDS is an abbreviation for Energy Dispersion Spectroscopy.
[0022]
The composite oxide desirably has a characteristic that the pore volume with a pore size of 3.5 to 100 nm becomes 0.20 cc / g or more after firing at 800 ° C. for 5 hours. Further, it is particularly desirable that the pore volume having a pore diameter of 3.5 to 100 nm becomes 0.30 cc / g or more after calcination at 800 ° C. for 5 hours. The lower limit value that can be measured in principle using a mercury porosimeter is referred to as a so-called mesopore. The pore volume of the mesopore is 0.20 cc / g or more. Thus, CO and H 2 O diffuse into the pores even under reaction conditions with a high space velocity. Therefore, since the surface area utilized for reaction is large, high CO shift reaction activity is expressed. Without such characteristics, the gas diffusibility decreases, and the CO shift reaction activity under reaction conditions with a high space velocity decreases.
[0023]
The central pore diameter of the composite oxide is preferably 15 to 65 nm, and particularly preferably 25 to 65 nm. If the central pore diameter is less than 15 nm, gas diffusibility is lowered, and therefore, CO shift reaction activity under reaction conditions with high space velocity is lowered. On the other hand, when the central pore diameter exceeds 65 nm, the primary particles are rearranged and aggregated at a high temperature, and the pore volume of the mesopores is likely to decrease.
[0024]
The composite oxide preferably contains 50 to 90% by weight of ceria. Amount ceria becomes difficult to activation of H 2 O is less than 50 wt%, CO shift reaction activity decreases. On the other hand, when the amount of ceria exceeds 90% by weight, the amount of alumina serving as a barrier is small, so that ceria easily grows, and as a result, the effect of alleviating the atmospheric fluctuation is reduced and the noble metal also grows. Therefore, the CO shift reaction activity decreases.
[0025]
In order to produce the composite oxide referred to in the present invention, first, a precipitate of a ceria precursor and an alumina precursor or a compound of those precursors is precipitated from an aqueous solution in which a cerium compound and an aluminum compound are dissolved or a solution containing water.
[0026]
As the cerium compound and the aluminum compound, a salt is generally used, and as the salt, nitrate, chloride, acetate, sulfate, etc. can be used. Moreover, water and alcohol can be used as a solvent which melt | dissolves a salt uniformly. Further, for example, aluminum hydroxide, nitric acid and water may be mixed and used as a raw material for aluminum nitrate.
[0027]
The precipitation is carried out mainly by adjusting the pH by adding ammonia water or the like, but it can be made a more characteristic complex oxide precursor by various adjustment methods. For example, a method in which precipitation of these oxide precursors or compounds of those precursors is almost simultaneously performed from an aqueous solution containing cerium compound and aluminum compound or a solution containing water, or before the alumina precursor is precipitated. There is a method of depositing a ceria precursor (or vice versa).
[0028]
As for the former method of precipitating almost simultaneously, ammonia water or the like is instantly added and stirred strongly, or hydrogen peroxide is added to adjust the pH at which ceria precursor and alumina precursor start to precipitate, and then ammonia. There is a method of depositing a precipitate with water or the like.
[0029]
For the latter, the time required for neutralization with ammonia water or the like is made sufficiently long, preferably a method of neutralization in 10 minutes or more, or the pH or alumina precursor at which ceria precursor precipitates while monitoring pH. There is a method of adding a buffer solution that neutralizes stepwise or maintains such a pH to the pH at which the body precipitates.
[0030]
In addition to aqueous ammonia, an aqueous solution or alcohol solution in which ammonium carbonate, sodium hydroxide, potassium hydroxide, sodium carbonate or the like is dissolved can be used. Particularly preferred are ammonia and ammonium carbonate which volatilize during firing. In addition, since pH of an alkaline solution is 9 or more, since the precipitation reaction of a precursor is accelerated | stimulated, it is more preferable.
[0031]
By aging the precipitate in the presence of water, a more uniform composite oxide can be produced. This aging step is desirably performed at room temperature or higher, preferably 100 to 200 ° C, more preferably 100 to 150 ° C. When heating at less than 100 ° C, the time required for aging becomes long. Further, when the temperature is higher than 200 ° C., the water vapor pressure becomes extremely high, and thus a large-scale apparatus that can withstand high pressure is required, which is not preferable because the manufacturing cost becomes very high.
[0032]
If the solution containing the precipitate is heated as it is to evaporate to dryness, and further baked, an aging step can be performed during evaporation to dryness. Is good.
[0033]
And it is desirable to disperse the precipitate obtained in this way in water again, and to disperse it strongly using a homogenizer or the like, followed by firing. This makes it possible to produce a composite oxide having the characteristics that the pore volume with a pore size of 3.5 to 100 nm is 0.20 cc / g or more, further 0.30 cc / g or more after calcination at 800 ° C. for 5 hours. When dispersing strongly, it is desirable to add a surfactant. By adding a surfactant, aggregation of primary particles can be prevented and the pore volume of mesopores can be increased.
[0034]
The precipitate may be fired in the air, and the temperature is preferably in the range of 300 to 900 ° C. When the calcination temperature is lower than 300 ° C., the stability as a carrier is substantially lacking. In addition, firing at a temperature higher than 900 ° C. leads to a decrease in the specific surface area, and is unnecessary from the viewpoint of utilization as a carrier.
[0035]
The carrier referred to in the present invention may include the above complex oxide, may be composed of only the complex oxide, or may be composed of a mixture of the complex oxide powder and another porous oxide powder. You can also. As the other porous oxide, one kind or plural kinds of alumina, ceria, zirconia, titania, silica and the like can be used. In the case of using a carrier in which the composite oxide powder and the porous oxide powder are mixed, the composite oxide powder is desirably 50% by weight or more. If the amount of the composite oxide powder in the support is less than this, the CO shift reaction activity is lowered, which is not practical.
[0036]
The noble metal supported on the carrier can be selected from Pt, Rh, Pd, Ir, etc., but it is preferable that at least Pt is contained. By carrying at least Pt, the CO shift reaction activity is particularly improved. The amount of the noble metal supported is preferably 0.1 to 10 g per 100 g of the carrier. If the supported amount is less than this, the CO shift reaction activity is low, and even if it is supported more than this, the CO shift reaction activity is saturated and grain growth of noble metals may occur.
[0037]
The catalyst for CO shift reaction of the present invention generates hydrogen with high activity by the CO shift reaction by contacting with a gas containing at least CO and H 2 O. As such a gas, it is particularly preferable to use exhaust gas from an internal combustion engine such as an automobile. Although the exhaust gas from the internal combustion engine has a high space velocity, the CO shift reaction catalyst of the present invention exhibits a high CO shift reaction activity even in a gas having such a high space velocity.
[0038]
【Example】
Hereinafter, the present invention will be specifically described with reference to Examples and Comparative Examples.
[0039]
(Example 1)
0.2 mol (75.1 g) of aluminum nitrate nonahydrate was mixed with 2000 ml of ion-exchanged water, and dissolved by stirring for 5 minutes with a propeller stirrer. Thereto, 304 g (corresponding to 0.5 mol in terms of CeO 2 ) of an aqueous cerium nitrate solution having a concentration of 28% by weight was mixed and further stirred for 5 minutes. To the obtained mixed aqueous solution, 177 g of 25% aqueous ammonia was added, and the mixture was further stirred for 10 minutes to obtain an aqueous solution containing a precipitate. This was subjected to a aging step in which heat treatment was performed at 120 ° C. for 2 hours under a pressure of 2 atm to age the precipitate.
[0040]
Then, it filtered using the suction filter and obtained the filter cake. This filter cake is dispersed again in 500 ml of ion exchange water using a propeller stirrer, and further 9.1 g of an alkylamine-based ionic surfactant (“Armac T50” manufactured by Lion Corporation) is added, and the rotor diameter is 17 mm. Mixing was performed at 10,000 rpm for 5 minutes using a homogenizer with a gap of 1 mm. Then filtered using a suction filter, the resulting filter cake was calcined for 5 hours at 400 ° C., was prepared CeO 2 -Al 2 O 3 composite oxide powder. The obtained composite oxide powder is composed of about 89% by weight of CeO 2 and about 11% by weight of Al 2 O 3 .
[0041]
The composite oxide powder was impregnated with a predetermined amount of a dinitrodiammine platinum nitric acid solution having a predetermined concentration, and calcined in the atmosphere at 300 ° C. for 3 hours to support 5% by weight of Pt. This was further formed into pellets having a particle diameter of 0.5 to 1.0 mm by a conventional method to prepare a pellet catalyst.
[0042]
(Example 2)
Example 1 except that 0.56 mol (211 g) of aluminum nitrate nonahydrate, 304 g of 28% strength by weight cerium nitrate aqueous solution (corresponding to 0.5 mol in terms of CeO 2 ), and 251 g of 25% aqueous ammonia were used. A composite oxide powder was prepared, and Pt was similarly supported to prepare a pellet catalyst. The obtained composite oxide powder is composed of about 75% by weight of CeO 2 and about 25% by weight of Al 2 O 3 .
[0043]
(Example 3)
0.2 mol (75.1 g) of aluminum nitrate nonahydrate was mixed with 2000 ml of ion-exchanged water, and dissolved by stirring for 5 minutes with a propeller stirrer. Thereto, 304 g (corresponding to 0.5 mol in terms of CeO 2 ) of an aqueous cerium nitrate solution having a concentration of 28% by weight was mixed and further stirred for 5 minutes. To the obtained mixed aqueous solution, 177 g of 25% aqueous ammonia was added, and the mixture was further stirred for 10 minutes to obtain an aqueous solution containing a precipitate. Thereafter, the aqueous solution containing the precipitate was heated at a temperature rising rate of 100 ° C./hr and calcined at 400 ° C. for 5 hours to prepare CeO 2 —Al 2 O 3 composite oxide powder. The obtained composite oxide powder is composed of about 89% by weight of CeO 2 and about 11% by weight of Al 2 O 3 .
[0044]
Using this composite oxide powder, Pt was supported in the same manner as in Example 1 to similarly prepare a pellet catalyst.
[0045]
Example 4
And aluminum nitrate 9-hydrate 0.56 mol (211 g), and aqueous solution of cerium nitrate 304g (0.5 mol equivalent in terms of CeO 2), the composite oxide powder in the same manner as in Example 3 except for using 25% aqueous ammonia 251g In the same manner, a pellet catalyst was prepared by supporting Pt. The obtained composite oxide powder is composed of about 75% by weight of CeO 2 and about 25% by weight of Al 2 O 3 .
[0046]
(Comparative Example 1)
Oxide powder in the same manner as in Example 1 except that 304 g of 28% by weight cerium nitrate aqueous solution (corresponding to 0.5 mol in terms of CeO 2 ) and 136 g of 25% aqueous ammonia were used without using aluminum nitrate nonahydrate. In the same manner, a pellet catalyst was prepared by supporting Pt. The obtained oxide powder is composed only of CeO 2 .
[0047]
(Comparative Example 2)
A γ-Al 2 O 3 powder (specific surface area 200 m 2 / g) was impregnated with a predetermined amount of a cerium nitrate aqueous solution having a predetermined concentration, and calcined in the atmosphere at 500 ° C. for 2 hours to prepare a composite oxide powder. This composite oxide powder is composed of about 38% by weight of CeO 2 and about 62% by weight of Al 2 O 3 and corresponds to the composite oxide described in the examples of JP-A-2001-316682.
[0048]
Using this composite oxide powder, Pt was supported in the same manner as in Example 1 to prepare a pellet catalyst.
[0049]
<Test and evaluation>
The composite oxide powders prepared in each Example and each Comparative Example were fired at 800 ° C. for 5 hours in the air, and then the pore distribution was measured using a mercury porosimeter. The pore distribution curves of the composite oxide powders prepared in Example 1 and Example 3 are extracted and shown in FIG. FIG. 1 shows that the composite oxide powders prepared in Example 1 and Example 3 have a large pore volume of pores having a pore diameter of 100 nm or less. In particular, the composite oxide powder prepared in Example 1 is It can be seen that pores having a pore diameter of 100 nm or less have a sharp distribution.
[0050]
2 and 3 show the pore volume of the pores having a pore diameter of 3.5 to 100 nm and the value of the central pore diameter calculated from the pore distribution curve of each sample. In normal oxide powders, the higher the temperature, the more the primary particles agglomerate due to the solid-phase reaction between the primary particles, so the pore volume of pores with pore diameters of 3.5 to 100 nm decreases and the central pore diameter is Get smaller. However, from FIG. 1 to FIG. 3, the composite oxide powders prepared in Examples 1 to 4, particularly the composite oxide powders prepared in Examples 1 and 2, are comparative examples even after firing at a high temperature of 800 ° C. It can be seen that the pore volume is larger and the central pore diameter is larger than the composite oxide powder or oxide powder prepared in 1-2. Further, in the composite oxide powder prepared in Comparative Example 2, the pore volume was too small and the central pore diameter was unknown.
[0051]
Next, with respect to each pellet catalyst prepared in each example and each comparative example, the lean gas 4 minutes and the
[0052]
[Table 1]
[0053]
Next, each catalyst after the endurance test was placed in an atmospheric pressure fixed bed flow type catalyst evaluation device, and a model gas consisting of CO: 1.8%, H 2 O: 10%, and the balance N 2 was 10 A pretreatment was carried out at 500 ° C. for 15 minutes while flowing at a rate of 1 liter / minute. Thereafter, the temperature was raised from 100 ° C. to 400 ° C. at a rate of 15 ° C./min while the same model gas was allowed to flow. At this time, the volume of each pellet catalyst is about 3 ml, and the space velocity is about 200,000 hr −1 . During the temperature increase, the CO conversion was continuously measured for each catalyst. The results are shown in FIG. Note by another experiment, CO conversion has been confirmed to be equal in H 2 production rate.
[0054]
From FIG. 6, it is clear that the catalyst of each example exhibits higher CO shift reaction activity from a low temperature region than the catalyst of each comparative example under the high space velocity reaction condition of about 200,000 hr −1 . Based on the results of FIGS. 1 to 5, the high CO shift reaction activity of the catalyst of each example is considered to be due to the suppression of grain growth of CeO 2 and Pt. Moreover, it is clear that the catalysts of Examples 1 and 2 exhibit higher CO shift reaction activity than the catalysts of Examples 3 and 4 due to the presence of pores having a large pore diameter and a large pore volume. And a composite oxide having a pore volume of 3.5 to 100 nm and a pore volume of 0.20 cc / g or more after firing at 800 ° C. for 5 hours and a center pore diameter of 15 nm or more is used as a carrier. It is clear that is particularly preferred.
[0055]
【The invention's effect】
That is, according to the catalyst for CO shift reaction of the present invention, high activity is exhibited under reaction conditions having a high space velocity, and the activity can be maintained even when exposed to high temperature conditions or atmospheric fluctuation conditions. Therefore, it is optimal for use in exhaust gas from an internal combustion engine such as an automobile engine, and is extremely useful as an exhaust gas purification catalyst for reducing and purifying NO x or a hydrogen generation catalyst for a fuel cell.
[Brief description of the drawings]
1 is a graph showing pore distribution curves of composite oxide powders produced in Example 1 and Example 3. FIG.
FIG. 2 is a graph showing pore volumes after firing at 800 ° C. of composite oxide powders or oxide powders produced in Examples and Comparative Examples.
FIG. 3 is a graph showing the central pore diameter after firing at 800 ° C. of composite oxide powders or oxide powders produced in Examples and Comparative Examples.
FIG. 4 is a graph showing specific surface areas of the catalysts of Examples and Comparative Examples.
FIG. 5 is a graph showing the Pt dispersion degree of each catalyst in Examples and Comparative Examples.
FIG. 6 is a graph showing the relationship between the temperature of each catalyst and the CO conversion rate in Examples and Comparative Examples.
Claims (7)
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| JP5595385B2 (en) * | 2009-05-15 | 2014-09-24 | 三菱重工業株式会社 | CO shift catalyst and method for producing the same, and CO shift reaction apparatus using CO shift catalyst |
| CN111215065B (en) * | 2018-11-26 | 2023-03-14 | 中国科学院大连化学物理研究所 | Water-gas shift catalyst, preparation and application thereof |
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