JP4450984B2 - Exhaust gas purification catalyst - Google Patents
Exhaust gas purification catalyst Download PDFInfo
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- JP4450984B2 JP4450984B2 JP2000392666A JP2000392666A JP4450984B2 JP 4450984 B2 JP4450984 B2 JP 4450984B2 JP 2000392666 A JP2000392666 A JP 2000392666A JP 2000392666 A JP2000392666 A JP 2000392666A JP 4450984 B2 JP4450984 B2 JP 4450984B2
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- 239000003054 catalyst Substances 0.000 title claims description 60
- 238000000746 purification Methods 0.000 title claims description 12
- 239000011148 porous material Substances 0.000 claims description 99
- 239000007789 gas Substances 0.000 claims description 39
- 239000011247 coating layer Substances 0.000 claims description 31
- 239000010410 layer Substances 0.000 claims description 30
- 239000011232 storage material Substances 0.000 claims description 22
- 229910000510 noble metal Inorganic materials 0.000 claims description 17
- 239000002245 particle Substances 0.000 claims description 10
- 238000010304 firing Methods 0.000 claims description 7
- 229910052783 alkali metal Inorganic materials 0.000 claims description 6
- 150000001340 alkali metals Chemical class 0.000 claims description 6
- 229910052784 alkaline earth metal Inorganic materials 0.000 claims description 6
- 150000001342 alkaline earth metals Chemical class 0.000 claims description 6
- 238000002485 combustion reaction Methods 0.000 claims description 6
- 229910052761 rare earth metal Inorganic materials 0.000 claims description 6
- QVGXLLKOCUKJST-UHFFFAOYSA-N atomic oxygen Chemical compound [O] QVGXLLKOCUKJST-UHFFFAOYSA-N 0.000 claims description 5
- 229910052760 oxygen Inorganic materials 0.000 claims description 5
- 239000001301 oxygen Substances 0.000 claims description 5
- 239000008187 granular material Substances 0.000 claims description 4
- 230000001590 oxidative effect Effects 0.000 claims description 4
- 239000000203 mixture Substances 0.000 claims description 3
- BASFCYQUMIYNBI-UHFFFAOYSA-N platinum Substances [Pt] BASFCYQUMIYNBI-UHFFFAOYSA-N 0.000 description 19
- 230000000052 comparative effect Effects 0.000 description 17
- 238000003860 storage Methods 0.000 description 12
- PNEYBMLMFCGWSK-UHFFFAOYSA-N aluminium oxide Inorganic materials [O-2].[O-2].[O-2].[Al+3].[Al+3] PNEYBMLMFCGWSK-UHFFFAOYSA-N 0.000 description 10
- 239000000243 solution Substances 0.000 description 9
- 239000000758 substrate Substances 0.000 description 9
- OKTJSMMVPCPJKN-UHFFFAOYSA-N Carbon Chemical compound [C] OKTJSMMVPCPJKN-UHFFFAOYSA-N 0.000 description 8
- 239000000843 powder Substances 0.000 description 7
- 239000007864 aqueous solution Substances 0.000 description 6
- 239000000446 fuel Substances 0.000 description 6
- 239000011236 particulate material Substances 0.000 description 5
- GWEVSGVZZGPLCZ-UHFFFAOYSA-N Titan oxide Chemical compound O=[Ti]=O GWEVSGVZZGPLCZ-UHFFFAOYSA-N 0.000 description 4
- 230000007423 decrease Effects 0.000 description 4
- 238000009826 distribution Methods 0.000 description 4
- 230000000694 effects Effects 0.000 description 4
- 238000004453 electron probe microanalysis Methods 0.000 description 4
- 229910052697 platinum Inorganic materials 0.000 description 4
- 239000002002 slurry Substances 0.000 description 4
- KDLHZDBZIXYQEI-UHFFFAOYSA-N Palladium Chemical compound [Pd] KDLHZDBZIXYQEI-UHFFFAOYSA-N 0.000 description 3
- 239000000463 material Substances 0.000 description 3
- CURLTUGMZLYLDI-UHFFFAOYSA-N Carbon dioxide Chemical compound O=C=O CURLTUGMZLYLDI-UHFFFAOYSA-N 0.000 description 2
- LFQSCWFLJHTTHZ-UHFFFAOYSA-N Ethanol Chemical compound CCO LFQSCWFLJHTTHZ-UHFFFAOYSA-N 0.000 description 2
- 229910002651 NO3 Inorganic materials 0.000 description 2
- NHNBFGGVMKEFGY-UHFFFAOYSA-N Nitrate Chemical compound [O-][N+]([O-])=O NHNBFGGVMKEFGY-UHFFFAOYSA-N 0.000 description 2
- VYPSYNLAJGMNEJ-UHFFFAOYSA-N Silicium dioxide Chemical compound O=[Si]=O VYPSYNLAJGMNEJ-UHFFFAOYSA-N 0.000 description 2
- MCMNRKCIXSYSNV-UHFFFAOYSA-N Zirconium dioxide Chemical compound O=[Zr]=O MCMNRKCIXSYSNV-UHFFFAOYSA-N 0.000 description 2
- AYJRCSIUFZENHW-UHFFFAOYSA-L barium carbonate Chemical compound [Ba+2].[O-]C([O-])=O AYJRCSIUFZENHW-UHFFFAOYSA-L 0.000 description 2
- 239000002585 base Substances 0.000 description 2
- 229910052878 cordierite Inorganic materials 0.000 description 2
- 238000009792 diffusion process Methods 0.000 description 2
- 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 2
- 238000011156 evaluation Methods 0.000 description 2
- 238000010438 heat treatment Methods 0.000 description 2
- 229910052744 lithium Inorganic materials 0.000 description 2
- IIPYXGDZVMZOAP-UHFFFAOYSA-N lithium nitrate Chemical compound [Li+].[O-][N+]([O-])=O IIPYXGDZVMZOAP-UHFFFAOYSA-N 0.000 description 2
- 238000011068 loading method Methods 0.000 description 2
- 238000005259 measurement Methods 0.000 description 2
- 229910052751 metal Inorganic materials 0.000 description 2
- 239000002184 metal Substances 0.000 description 2
- 239000011812 mixed powder Substances 0.000 description 2
- 238000002156 mixing Methods 0.000 description 2
- NWAHZABTSDUXMJ-UHFFFAOYSA-N platinum(2+);dinitrate Chemical compound [Pt+2].[O-][N+]([O-])=O.[O-][N+]([O-])=O NWAHZABTSDUXMJ-UHFFFAOYSA-N 0.000 description 2
- 229910052700 potassium Inorganic materials 0.000 description 2
- FGIUAXJPYTZDNR-UHFFFAOYSA-N potassium nitrate Chemical compound [K+].[O-][N+]([O-])=O FGIUAXJPYTZDNR-UHFFFAOYSA-N 0.000 description 2
- 239000010948 rhodium Substances 0.000 description 2
- 239000000126 substance Substances 0.000 description 2
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 description 2
- QTBSBXVTEAMEQO-UHFFFAOYSA-M Acetate Chemical compound CC([O-])=O QTBSBXVTEAMEQO-UHFFFAOYSA-M 0.000 description 1
- ATRRKUHOCOJYRX-UHFFFAOYSA-N Ammonium bicarbonate Chemical compound [NH4+].OC([O-])=O ATRRKUHOCOJYRX-UHFFFAOYSA-N 0.000 description 1
- 229910000013 Ammonium bicarbonate Inorganic materials 0.000 description 1
- OYPRJOBELJOOCE-UHFFFAOYSA-N Calcium Chemical compound [Ca] OYPRJOBELJOOCE-UHFFFAOYSA-N 0.000 description 1
- 229910052684 Cerium Inorganic materials 0.000 description 1
- 229910052692 Dysprosium Inorganic materials 0.000 description 1
- VEXZGXHMUGYJMC-UHFFFAOYSA-N Hydrochloric acid Chemical compound Cl VEXZGXHMUGYJMC-UHFFFAOYSA-N 0.000 description 1
- DGAQECJNVWCQMB-PUAWFVPOSA-M Ilexoside XXIX Chemical compound C[C@@H]1CC[C@@]2(CC[C@@]3(C(=CC[C@H]4[C@]3(CC[C@@H]5[C@@]4(CC[C@@H](C5(C)C)OS(=O)(=O)[O-])C)C)[C@@H]2[C@]1(C)O)C)C(=O)O[C@H]6[C@@H]([C@H]([C@@H]([C@H](O6)CO)O)O)O.[Na+] DGAQECJNVWCQMB-PUAWFVPOSA-M 0.000 description 1
- WHXSMMKQMYFTQS-UHFFFAOYSA-N Lithium Chemical compound [Li] WHXSMMKQMYFTQS-UHFFFAOYSA-N 0.000 description 1
- FYYHWMGAXLPEAU-UHFFFAOYSA-N Magnesium Chemical compound [Mg] FYYHWMGAXLPEAU-UHFFFAOYSA-N 0.000 description 1
- 229910052779 Neodymium Inorganic materials 0.000 description 1
- ZLMJMSJWJFRBEC-UHFFFAOYSA-N Potassium Chemical compound [K] ZLMJMSJWJFRBEC-UHFFFAOYSA-N 0.000 description 1
- 229910052777 Praseodymium Inorganic materials 0.000 description 1
- 229910052769 Ytterbium Inorganic materials 0.000 description 1
- 229910021536 Zeolite Inorganic materials 0.000 description 1
- 235000012538 ammonium bicarbonate Nutrition 0.000 description 1
- 239000001099 ammonium carbonate Substances 0.000 description 1
- 238000004458 analytical method Methods 0.000 description 1
- 229910052788 barium Inorganic materials 0.000 description 1
- ITHZDDVSAWDQPZ-UHFFFAOYSA-L barium acetate Chemical compound [Ba+2].CC([O-])=O.CC([O-])=O ITHZDDVSAWDQPZ-UHFFFAOYSA-L 0.000 description 1
- DSAJWYNOEDNPEQ-UHFFFAOYSA-N barium atom Chemical compound [Ba] DSAJWYNOEDNPEQ-UHFFFAOYSA-N 0.000 description 1
- 229910052790 beryllium Inorganic materials 0.000 description 1
- ATBAMAFKBVZNFJ-UHFFFAOYSA-N beryllium atom Chemical compound [Be] ATBAMAFKBVZNFJ-UHFFFAOYSA-N 0.000 description 1
- 239000011230 binding agent Substances 0.000 description 1
- 229910052792 caesium Inorganic materials 0.000 description 1
- TVFDJXOCXUVLDH-UHFFFAOYSA-N caesium atom Chemical compound [Cs] TVFDJXOCXUVLDH-UHFFFAOYSA-N 0.000 description 1
- 229910052791 calcium Inorganic materials 0.000 description 1
- 239000011575 calcium Substances 0.000 description 1
- 239000001569 carbon dioxide Substances 0.000 description 1
- 229910002092 carbon dioxide Inorganic materials 0.000 description 1
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- GWXLDORMOJMVQZ-UHFFFAOYSA-N cerium Chemical compound [Ce] GWXLDORMOJMVQZ-UHFFFAOYSA-N 0.000 description 1
- 238000006243 chemical reaction Methods 0.000 description 1
- 230000008094 contradictory effect Effects 0.000 description 1
- 150000004696 coordination complex Chemical class 0.000 description 1
- HNPSIPDUKPIQMN-UHFFFAOYSA-N dioxosilane;oxo(oxoalumanyloxy)alumane Chemical compound O=[Si]=O.O=[Al]O[Al]=O HNPSIPDUKPIQMN-UHFFFAOYSA-N 0.000 description 1
- 238000001035 drying Methods 0.000 description 1
- KBQHZAAAGSGFKK-UHFFFAOYSA-N dysprosium atom Chemical compound [Dy] KBQHZAAAGSGFKK-UHFFFAOYSA-N 0.000 description 1
- 238000011049 filling Methods 0.000 description 1
- 229910052730 francium Inorganic materials 0.000 description 1
- KLMCZVJOEAUDNE-UHFFFAOYSA-N francium atom Chemical compound [Fr] KLMCZVJOEAUDNE-UHFFFAOYSA-N 0.000 description 1
- 229910052741 iridium Inorganic materials 0.000 description 1
- GKOZUEZYRPOHIO-UHFFFAOYSA-N iridium atom Chemical compound [Ir] GKOZUEZYRPOHIO-UHFFFAOYSA-N 0.000 description 1
- 229910052746 lanthanum Inorganic materials 0.000 description 1
- FZLIPJUXYLNCLC-UHFFFAOYSA-N lanthanum atom Chemical compound [La] FZLIPJUXYLNCLC-UHFFFAOYSA-N 0.000 description 1
- 229910052749 magnesium Inorganic materials 0.000 description 1
- 239000011777 magnesium Substances 0.000 description 1
- 238000004519 manufacturing process Methods 0.000 description 1
- QSHDDOUJBYECFT-UHFFFAOYSA-N mercury Chemical compound [Hg] QSHDDOUJBYECFT-UHFFFAOYSA-N 0.000 description 1
- 229910052753 mercury Inorganic materials 0.000 description 1
- QEFYFXOXNSNQGX-UHFFFAOYSA-N neodymium atom Chemical compound [Nd] QEFYFXOXNSNQGX-UHFFFAOYSA-N 0.000 description 1
- 229920000620 organic polymer Polymers 0.000 description 1
- 229910052763 palladium Inorganic materials 0.000 description 1
- 239000012188 paraffin wax Substances 0.000 description 1
- 239000012466 permeate Substances 0.000 description 1
- 239000011591 potassium Substances 0.000 description 1
- 239000004323 potassium nitrate Substances 0.000 description 1
- 235000010333 potassium nitrate Nutrition 0.000 description 1
- PUDIUYLPXJFUGB-UHFFFAOYSA-N praseodymium atom Chemical compound [Pr] PUDIUYLPXJFUGB-UHFFFAOYSA-N 0.000 description 1
- 230000003578 releasing effect Effects 0.000 description 1
- 229910052703 rhodium Inorganic materials 0.000 description 1
- MHOVAHRLVXNVSD-UHFFFAOYSA-N rhodium atom Chemical compound [Rh] MHOVAHRLVXNVSD-UHFFFAOYSA-N 0.000 description 1
- 229910052701 rubidium Inorganic materials 0.000 description 1
- IGLNJRXAVVLDKE-UHFFFAOYSA-N rubidium atom Chemical compound [Rb] IGLNJRXAVVLDKE-UHFFFAOYSA-N 0.000 description 1
- 229920006395 saturated elastomer Polymers 0.000 description 1
- 229910052706 scandium Inorganic materials 0.000 description 1
- SIXSYDAISGFNSX-UHFFFAOYSA-N scandium atom Chemical compound [Sc] SIXSYDAISGFNSX-UHFFFAOYSA-N 0.000 description 1
- 239000000377 silicon dioxide Substances 0.000 description 1
- 238000005245 sintering Methods 0.000 description 1
- 229910052708 sodium Inorganic materials 0.000 description 1
- 239000011734 sodium Substances 0.000 description 1
- 239000007787 solid Substances 0.000 description 1
- 229910052712 strontium Inorganic materials 0.000 description 1
- CIOAGBVUUVVLOB-UHFFFAOYSA-N strontium atom Chemical compound [Sr] CIOAGBVUUVVLOB-UHFFFAOYSA-N 0.000 description 1
- NAWDYIZEMPQZHO-UHFFFAOYSA-N ytterbium Chemical compound [Yb] NAWDYIZEMPQZHO-UHFFFAOYSA-N 0.000 description 1
- 229910052727 yttrium Inorganic materials 0.000 description 1
- VWQVUPCCIRVNHF-UHFFFAOYSA-N yttrium atom Chemical compound [Y] VWQVUPCCIRVNHF-UHFFFAOYSA-N 0.000 description 1
- 239000010457 zeolite Substances 0.000 description 1
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- Catalysts (AREA)
- Exhaust Gas After Treatment (AREA)
Description
【０００１】
【発明の属する技術分野】
本発明は、排ガス中の有害成分を一層効率よく浄化できるNOx 吸蔵還元型の排ガス浄化用触媒に関する。本発明の排ガス浄化用触媒は、常時は酸素過剰のリーン雰囲気で燃焼させ間欠的にストイキ〜リッチ雰囲気となるように混合気の比率を制御する燃焼システムの排気系に用いられ、間欠的にストイキ〜リッチ雰囲気とされた時のNOx 浄化率が大きく向上する。
【０００２】
【従来の技術】
近年、地球環境保護の観点から、自動車などの内燃機関から排出される排ガス中の二酸化炭素（CO2 ）が問題とされ、その解決策として酸素過剰雰囲気において希薄燃焼させるいわゆるリーンバーンが有望視されている。
【０００３】
このリーンバーンにおいては、排ガスが酸素過剰雰囲気であるため、通常の三元触媒ではNOx の還元浄化が困難である。そこで常時は酸素過剰のリーン条件で燃焼させ、間欠的にストイキ〜リッチ条件とすることにより排ガスを還元雰囲気としてNOx を還元浄化するシステムが開発された。そしてこのシステムに最適な触媒として、リーン雰囲気でNOx を吸蔵し、ストイキ〜リッチ雰囲気で吸蔵されたNOx を放出するNOx 吸蔵材を用いたNOx 吸蔵還元型の排ガス浄化用触媒が開発されている。
【０００４】
このNOx の吸蔵・放出作用をもつNOx 吸蔵材としては、アルカリ土類金属、アルカリ金属及び希土類元素が知られ、例えば特開平5-317652号公報には、Baなどのアルカリ土類金属とPtをアルミナなどの多孔質担体に担持したNOx 吸蔵還元型触媒が提案されている。また特開平 6-31139号公報には、Ｋなどのアルカリ金属とPtをアルミナなどの多孔質担体に担持したNOx 吸蔵還元型触媒が提案されている。さらに特開平5-168860号公報には、Laなどの希土類元素とPtをアルミナなどの多孔質担体に担持したNOx 吸蔵還元型触媒が提案されている。
【０００５】
これらのNOx 吸蔵還元型触媒を用いれば、空燃比をリーン側からパルス状にストイキ〜リッチ側となるように制御することにより、排ガスもリーン雰囲気からパルス状にストイキ〜リッチ雰囲気となる。したがって、リーン側ではNOx がNOx 吸蔵材に吸蔵され、それがストイキ又はリッチ側で放出されてHCやCOなどの還元性成分と反応して浄化されるため、リーンバーンエンジンからの排ガスであってもNOx を効率良く浄化することができる。また排ガス中のHC及びCOは、貴金属により酸化されるとともにNOx の還元にも消費されるので、HC及びCOも効率よく浄化される。
【０００６】
なお、排ガスをパルス状にストイキ〜リッチ条件とするには、空燃比をパルス状にストイキ〜リッチ雰囲気とすることで行われ、空燃比をパルス状にストイキ〜リッチ雰囲気とすることは「リッチスパイクを打つ」と称されている。
【０００７】
【発明が解決しようとする課題】
ところで従来のシステムにおいては、リッチスパイクの時間は数秒程度である。これは、リッチスパイクの時間が長いとその分 CO2の生成量が増大し、かつ燃費が悪化するからである。
【０００８】
ところが従来のNOx 吸蔵還元型触媒においては、リッチスパイクの時間が短い場合には吸蔵されているNOx を十分に還元浄化することが困難であるために、リッチスパイクの時間を短縮するには限界があった。
【０００９】
つまり CO2の生成の面ではリッチスパイクの時間は短いほど好ましいのに対し、NOx の還元浄化の面ではリッチスパイクの時間は長いほど好ましく、両者は背反事象の関係にある。
【００１０】
本発明はこのような事情に鑑みてなされたものであり、吸蔵されているNOx を短時間のリッチスパイクで効率よく還元浄化すること、あるいはより短いリッチスパイクで高いNOx 浄化効率が得られるようにすることを目的とする。
【００１１】
【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排ガス浄化用触媒の特徴は、多孔質担体よりなるコート層と、コート層に担持された貴金属と、アルカリ金属、アルカリ土類金属及び希土類元素から選ばれコート層に担持されたNOx 吸蔵材とよりなり、常時は酸素過剰のリーン雰囲気で燃焼させ間欠的にストイキ〜リッチ雰囲気となるように混合気の比率を制御する燃焼システムの排気系に用いられるNOx 吸蔵還元型の排ガス浄化用触媒において、
コート層は、孔径 0.1〜20μｍの細孔が全体の細孔容積の60％以上を占め、孔径10〜20μｍの細孔が全体の細孔容積の20％以上を占めることにある。
【００１２】
そして細孔は、粒径0.08〜30μｍの可燃性粒状物質をコート層中に10〜50重量％添加し、それを酸化性雰囲気中にて焼成することで形成されたものであることが望ましい。
【００１３】
【発明の実施の形態】
従来のNOx 吸蔵還元型触媒としては、ハニカム基材の表面にアルミナなど多孔質担体からなるコート層が形成され、そのコート層に貴金属とNOx 吸蔵材とが担持されたものが一般的である。しかし従来のNOx 吸蔵還元型触媒では、担持方法と金属薬液の特性から、NOx 吸蔵材はコート層の表面から内部までほぼ均一に担持されているのに対し、貴金属はコート層の表面に多く担持されている。そのためコート層内部のNOx 吸蔵材に吸蔵されたNOx の還元効率が低いという問題がある。
【００１４】
また従来のNOx 吸蔵還元型触媒に用いられている多孔質担体の細孔の孔径は、約0.01μｍ程度ときわめて微細である。またコート層は担体粉末が緻密に充填して構成されている。そのためコート層内のガス拡散性が低く、リッチスパイク時に排ガスがコート層内部まで十分に拡散できていないことが考えられる。
【００１５】
そこで本発明の排ガス浄化用触媒では、孔径 0.1〜20μｍの細孔が全体の細孔容積の60％以上を占め、孔径10〜20μｍの細孔が全体の細孔容積の20％以上を占めるコート層としている。
【００１６】
細孔の孔径が0.01〜20μｍの領域では、ガス拡散速度係数が連続的に変化し、その変化量は細孔径に比例する。したがって本発明におけるコート層では、孔径が 0.1〜20μｍの細孔が大部分を占めているので、孔径が0.01μｍの細孔が大部分を占める従来のコート層に比べて細孔径が10〜2000倍となり、細孔中をガスが拡散する速度も10〜2000倍となる。したがって短時間のリッチスパイクでも、排ガスがコート層の内部まで到達可能となり、コート層内部のNOx 吸蔵材に吸蔵されているNOx を還元浄化することが可能となる。
【００１７】
また孔径10〜20μｍの細孔には、毛細管現象によって貴金属薬液の溶液が十分に浸透する。すなわちコート層の内部にまで細孔に沿った形で貴金属が担持されるので、コート層内部においてもNOx 吸蔵材と貴金属とが近接している。したがってコート層内部におけるNOx 吸蔵材の利用効率が高まるとともに、コート層内部のNOx 吸蔵材に吸蔵されたNOx の還元効率がさらに向上する。
【００１８】
つまり孔径10〜20μｍの細孔は、ガスの拡散通路となるとともに貴金属及びNOx 吸蔵材の担持場でもあるので、排ガスと触媒成分との接触確率が著しく高まり、短時間のリッチスパイクで高いNOx 浄化効率が得られる。したがって本発明の排ガス浄化用触媒によれば、リッチスパイクの時間をさらに短縮することが可能となり、燃費が向上するとともに CO2の排出を一層抑制することができる。
【００１９】
孔径 0.1〜20μｍの細孔が全体の細孔容積の60％未満であると、ガス拡散性が低下してリッチスパイク時のNOx 浄化能が低下する。また孔径10〜20μｍの細孔が全体の細孔容積の20％未満では、コート層内部の貴金属担持量が低下するため、NOx 吸蔵材の利用効率が低下しNOx 浄化能が低下する。
【００２０】
多孔質担体としては、アルミナ、シリカ、ジルコニア、チタニア、シリカ−アルミナ、ゼオライトなどが例示され、このうちの一種でもよいし複数種類を混合あるいは複合化して用いることもできる。中でも活性の高いγ−アルミナを用いるのが好ましい。
【００２１】
コート層に担持される貴金属としては、白金（Pt）、ロジウム（Rh）、パラジウム（Pd）、イリジウム（Ir）などが例示される。中でも活性の高いPtが特に好ましい。またこの貴金属の担持量は、触媒１リットル当たり 0.1〜10ｇとすることが好ましい。これより少ないと浄化活性が不足し、これより多く担持しても効果が飽和するとともに高価となる。
【００２２】
コート層に担持されるNOx 吸蔵材は、アルカリ金属、アルカリ土類金属及び希土類元素から選ばれる少なくとも一種である。アルカリ金属としては、リチウム、ナトリウム、カリウム、ルビジウム、セシウム、フランシウムが例示される。アルカリ土類金属としては、バリウム、ベリリウム、マグネシウム、カルシウム、ストロンチウムなどが例示される。また希土類元素としては、スカンジウム、イットリウム、ランタン、セリウム、プラセオジム、ネオジム、ジスプロシウム、イッテルビウムなどが例示される。
【００２３】
このNOx 吸蔵材の担持量は、触媒１リットル当たり0.01〜１モルの範囲とすることが望ましい。担持量がこの範囲より少ないとNOx 吸蔵量が低下するためNOx 浄化能が低下し、この範囲より多くなると貴金属がNOx 吸蔵材に覆われて活性が低下するようになる。
【００２４】
コート層の厚さには特に制限がないが、一般には基材１リットル当たり 120〜 300ｇの付着量とされる。またコート層が形成される基材としては、コージェライトなどの耐熱性セラミックスあるいはメタルなどが用いられ、その形状はハニカム形状ばかりでなく粒状とすることも可能である。
【００２５】
孔径 0.1〜20μｍの細孔が全体の細孔容積の60％以上を占め、孔径10〜20μｍの細孔が全体の細孔容積の20％以上を占めるようにコート層を形成するには、粒径0.08〜30μｍの可燃性粒状物質をコート層中に10〜50重量％添加し、それを酸化性雰囲気中にて焼成することで行うことができる。
【００２６】
つまり、多孔質担体粉末に粒径0.08〜30μｍの可燃性粒状物質を10〜50重量％混合した混合粉末を用い、従来と同様にしてバインダ成分などとともにスラリー化する。これをハニカム基材などの表面にウェットコートし、大気中などの酸化性雰囲気中にて焼成してコート層を形成する。焼成時には可燃性粒状物質が焼失し、その跡に細孔が形成される。そして可燃性粒状物質が焼失して生成したガスは、コート層を貫通して表面から外部へ排出されるので、細孔は外部と連通する。そして焼結により細孔径が若干縮小しあるいはスラリー中で可燃性粒状物質が凝集又は破砕して、その結果、孔径 0.1〜20μｍの細孔が全体の細孔容積の60％以上を占め、孔径10〜20μｍの細孔が全体の細孔容積の20％以上を占めるようになる。
【００２７】
用いられる可燃性粒状物質としては、燃焼によって完全に焼失するものが望ましく、パルプ粉末、カーボン粉末、パラフィン粉末、有機高分子体粉末などを用いることができる。この可燃性粒状物質は、コート層中に10〜50重量％となるように混合される。この量が10重量％未満あるいは粒径が0.08μｍ未満では、孔径 0.1〜20μｍの細孔が全体の細孔容積の60％以上を占めることが困難となり、粒径が30μｍを超えあるいは50重量％を超えて混合すると焼成後のコート層の強度が低下して使用時に剥離などの不具合が発生する場合がある。
【００２８】
コート層に貴金属を担持するには、貴金属の錯体あるいは硝酸塩などの薬液の水溶液あるいはアルコール溶液を用い、コート層に所定量含浸させた後乾燥・焼成することが望ましい。貴金属はコート層の表面に吸着して担持するとともに、コート層表面に開口する孔径10〜20μｍの細孔内にも水溶液が含浸するため、その細孔に沿う形で細孔内にも貴金属を担持することができる。
【００２９】
またコート層にNOx 吸蔵材を担持するには、NOx 吸蔵材の酢酸塩、塩酸塩あるいは硝酸塩などの水溶液あるいはアルコール溶液を用い、貴金属と同様にして担持することができる。NOx 吸蔵材は担体に対する吸着性が低いので、どのようにして担持してもコート層の表面から内部までほぼ均一に担持することができる。
【００３０】
【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。
【００３１】
（実施例１）
アルミナ粉末とチタニア粉末を重量比で１対１で混合した混合粉末 100重量部と、アルミナゾル（アルミナ40重量％）65重量部と、水 190重量部を混合した。さらに粒径 0.085μｍのカーボン粉末を固形分の30重量％となるように混合し、スラリーを調製した。
【００３２】
次にコージェライト製のハニカム基材（容量35cc）を用意し、上記スラリー中に浸漬し引き上げて余分なスラリーを吹き払った後、大気中にて 600℃で１時間焼成してコート層を形成した。コート層はハニカム基材１Ｌあたり 270ｇ形成された。
【００３３】
このコート層の中心細孔径と、細孔容積を水銀ポロシメータにて測定し、孔径 0.1〜20μｍの細孔が全体の細孔容積に占める割合と、孔径10〜20μｍの細孔が全体の細孔容積に占める割合を算出した。結果を表１に示す。
【００３４】
得られたコート層をもつハニカム基材を所定濃度の酢酸バリウム水溶液に浸漬して吸水させ、 250℃で15分間乾燥し 500℃で30分間焼成してBaを担持した。さらに濃度15ｇ／Ｌの重炭酸アンモニウム水溶液に15分間浸漬し、250℃で15分間乾燥してBaを炭酸バリウムとした。Baの担持量は、ハニカム基材１Ｌあたり 0.2モルである。
【００３５】
次に、Baが担持されたコート層をもつハニカム基材を所定濃度の硝酸白金水溶液に浸漬し、引き上げて余分な液滴を吹き払った後、 300℃で15分間乾燥し 500℃で30分間焼成してPtを担持した。Ptの担持量は、ハニカム基材１Ｌあたり２ｇである。
【００３６】
さらに硝酸カリウム水溶液と硝酸リチウム水溶液を用い、同様にしてＫとLiをそれぞれハニカム基材１Ｌあたり 0.1モル担持した。
【００３７】
（実施例２）
粒径が６μｍのカーボン粉末を用いたこと、及び焼成条件を 700℃で７時間としたこと以外は実施例１と同様にしてコート層を形成し、同様に中心細孔径と、孔径 0.1〜20μｍの細孔が全体の細孔容積に占める割合と、孔径10〜20μｍの細孔が全体の細孔容積に占める割合を算出した。結果を表１に示す。そして実施例１と同様にPtとNOx 吸蔵材を担持した。
【００３８】
（実施例３）
粒径が25μｍのカーボン粉末を用いたこと、及び焼成条件を 700℃で12時間としたこと以外は実施例１と同様にしてコート層を形成し、同様に中心細孔径と、孔径 0.1〜20μｍの細孔が全体の細孔容積に占める割合と、孔径10〜20μｍの細孔が全体の細孔容積に占める割合を算出した。結果を表１に示す。そして実施例１と同様にPtとNOx 吸蔵材を担持した。
【００３９】
（比較例）
カーボン粉末を用いなかったこと以外は実施例１と同様にしてコート層を形成し、同様に中心細孔径と、孔径 0.1〜20μｍの細孔が全体の細孔容積に占める割合と、孔径10〜20μｍの細孔が全体の細孔容積に占める割合を算出した。結果を表１に示す。そして実施例１と同様にPtとNOx 吸蔵材を担持した。
【００４０】
＜試験・評価＞
【００４１】
【表１】
表１より、カーボン粉末を添加することで中心細孔径が 0.1〜20μｍの範囲となり、孔径 0.1〜20μｍの細孔が全体の細孔容積に占める割合が65％以上であり、孔径10〜20μｍの細孔が全体の細孔容積に占める割合が20％以上となっていることがわかる。またカーボン粉末の粒径が大きいほど細孔径と上記割合も大きくなり、カーボン粉末の粒径が0.08μｍ以上の範囲が好ましいことがわかる。
【００４３】
次に、実施例１〜３及び比較例の触媒を評価装置にそれぞれ配置し、表２に示すリッチガスとリーンガスをそれぞれ入りガス温度 600℃で、 2.5分毎に交互に流すのを５時間行う熱処理を行った。
【００４４】
【表２】
熱処理後の各触媒について、表３に示すリッチガスを10分間流した後、リーンガスに切り換え、NOx が飽和するまでリーンガスを流した。その後リッチスパイクガスを３秒間打ち、再びリーンガスに切り換えて、リッチスパイク時に触媒に吸蔵されたNOx 量を測定した。入りガス温度が 300℃と 400℃の場合についてそれぞれ測定した。結果を図１に示す。
【００４６】
【表３】
また実施例１の触媒と比較例の触媒について、リッチスパイク後にリーンガスに切り換えてから、出ガス中のNOx 濃度を時間の経過とともに連続的に測定して、NOx 吸蔵曲線を作成した。測定条件は、入りガス温度が 300℃であり、ガス流量は50Ｌ／分（SV＝86,000hr-1）である。結果を図２に示す。
【００４８】
図１より、比較例の触媒に比べて各実施例の触媒はリッチスパイク後のNOx 吸蔵量が多いことが明らかである。各実施例の触媒と比較例の触媒とは同じだけNOx を吸蔵できる容量があるにも関わらず、このような差が生じたのは、実施例の触媒では吸蔵されていたNOx がリッチスパイク時に十分還元されたのに対し、比較例の触媒では短時間のリッチスパイクでは還元しきれなかったためと考えられる。
【００４９】
また図２より、実施例１の触媒では、リーンガスに切り換えてから約20秒間はNOx の放出がなく、NOx は完全に触媒に吸蔵されている。一方比較例の触媒では、リーンガスに切り換えた直後からNOx 濃度が高くなっている。
【００５０】
実施例１の触媒と比較例の触媒とは同じだけNOx を吸蔵できる容量があるにも関わらず、このような差が生じたのは、比較例の触媒ではNOx の吸蔵速度が遅いために触媒に流入するNOx を吸蔵しきれないのに対し、実施例１の触媒ではNOx が速やかに吸蔵されたからと考えられる。つまり実施例１の触媒では、孔径の大きな細孔を十分に有しているために、ガス拡散性に優れていることが示されている。
【００５１】
なお図２から初期のNOx 吸蔵速度を算出した結果、実施例１の触媒では 0.006mmol／秒、比較例の触媒が 0.003mmol／秒であって、実施例１の触媒は比較例の触媒に比べて２倍の速度でNOx を吸蔵している。
【００５２】
次に、実施例２〜３の触媒と比較例の触媒について、コート層の断面のPtの分布状態をEPMA分析により解析した。結果を図３〜図５に示す。
【００５３】
図３〜図５より、比較例の触媒ではコート層の表面のみにPtが担持されているのに対し、各実施例の触媒ではコート層の内部にまでPtが担持されていることが観察される。つまり各実施例の触媒では、コート層に中心細孔径が 0.2〜３μｍの細孔を多く有しているため、硝酸白金水溶液がその細孔内にも含浸されたことによって、細孔に沿うようにコート層内部までPtが担持されたと考えられる。
【００５４】
したがって各実施例の触媒では、コート層の内部でもPtによる反応が生じ、またガス拡散性の向上も相まって、図１及び図２に示されたようにリッチスパイク後に高いNOx 吸蔵量を示し、大きなNOx 吸蔵速度が発現されたと考えられる。
【００５５】
【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、高いNOx 吸蔵能を示すとともにリッチスパイク時のNOx の還元効率が向上する。また従来と同等のNOx 浄化能とすれば、リッチスパイクの時間を短縮することができ、 CO2の生成が抑制されるとともに燃費も向上する。
【図面の簡単な説明】
【図１】実施例及び比較例の触媒のリッチスパイク後のNOx 吸蔵量を示すグラフである。
【図２】実施例１及び比較例の触媒のNOx 吸蔵能を示すグラフである。
【図３】実施例２の触媒におけるコート層のPt分布を示すEPMA写真である。
【図４】実施例３の触媒におけるコート層のPt分布を示すEPMA写真である。
【図５】比較例の触媒におけるコート層のPt分布を示すEPMA写真である。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a NO x storage-and-reduction type catalyst for purifying an exhaust gas which can be purified more efficiently the harmful components in the exhaust gas. The exhaust gas purifying catalyst of the present invention is used in an exhaust system of a combustion system in which the ratio of the air-fuel mixture is controlled so that it is always burned in a lean atmosphere rich in oxygen and intermittently becomes a stoichiometric to rich atmosphere. ~ NO x purification rate when the rich atmosphere is greatly improved.
[0002]
[Prior art]
In recent years, carbon dioxide (CO 2 ) in exhaust gas discharged from internal combustion engines such as automobiles has become a problem from the viewpoint of protecting the global environment, and so-called lean burn that performs lean combustion in an oxygen-rich atmosphere is promising as a solution. ing.
[0003]
In this lean burn, since the exhaust gas has an oxygen-excess atmosphere, it is difficult to reduce and purify NO x with an ordinary three-way catalyst. In view of this, a system has been developed in which NO x is reduced and purified by using exhaust gas as a reducing atmosphere by always burning under lean conditions with excess oxygen and intermittently changing to stoichiometric or rich conditions. And as the best catalysts for this system, occludes NO x in lean atmosphere, stoichiometric ~ the NO x storage-reduction type exhaust purifying catalyst is developing with the NO x storage material that releases occluded NO x in a rich atmosphere Has been.
[0004]
As the NO x storage material with absorbing and releasing action of the NO x, the alkaline earth metals, known alkali metal and rare earth elements, for example, JP-A-5-317652, an alkaline earth metal such as Ba Pt was supported on a porous carrier such as alumina NO x storage-and-reduction type catalyst has been proposed. Also JP-A-6-31139, NO x storage reduction catalysts of the alkali metal and Pt, such as carrying on a porous support such as alumina K has been proposed. More Hei 5-168860 discloses, NO x storage-reduction catalyst carrying a rare earth element and Pt, such as La on a porous support such as alumina have been proposed.
[0005]
With these NO x storage-and-reduction type catalyst, by controlling so that the stoichiometric-rich side air-fuel ratio from the lean side in a pulsed manner, the exhaust gas becomes stoichiometric-rich atmosphere from a lean atmosphere in pulses. Therefore, on the lean side, NO x is occluded in the NO x occlusion material, and it is released on the stoichiometric or rich side and reacts with reducing components such as HC and CO to be purified, so the exhaust gas from the lean burn engine Even if it exists, NO x can be purified efficiently. Further, HC and CO in the exhaust gas are oxidized by the noble metal and consumed for the reduction of NO x , so that HC and CO are also efficiently purified.
[0006]
In addition, in order to make exhaust gas into a stoichiometric to rich condition in a pulsed manner, the air-fuel ratio is changed to a stoichiometric to rich atmosphere in a pulsed manner. It is called.
[0007]
[Problems to be solved by the invention]
By the way, in the conventional system, the time of the rich spike is about several seconds. This is because if the rich spike time is long, the amount of CO 2 produced increases and the fuel consumption deteriorates.
[0008]
However, in the conventional NO x storage reduction catalyst, when the rich spike time is short, it is difficult to reduce and purify the stored NO x sufficiently. There was a limit.
[0009]
In other words, the shorter the rich spike time is preferable in terms of CO 2 production, while the longer the rich spike time is preferable in terms of NO x reduction purification, the two are in a contradictory relationship.
[0010]
The present invention has been made in view of such circumstances, and it is possible to efficiently reduce and purify the stored NO x with a short rich spike, or to obtain a high NO x purification efficiency with a shorter rich spike. The purpose is to do so.
[0011]
[Means for Solving the Problems]
The exhaust gas purifying catalyst of the present invention that solves the above problems is characterized in that the coating layer is selected from a coating layer made of a porous carrier, a noble metal supported on the coating layer, an alkali metal, an alkaline earth metal, and a rare earth element. more becomes supported the NO x storage material, normally the NO x storage used in an exhaust system of a combustion system for controlling the ratio of the mixture so as to be intermittently stoichiometric-rich atmosphere is burned with oxygen excess lean atmosphere In a reduction type exhaust gas purification catalyst,
The coating layer is such that pores having a pore diameter of 0.1 to 20 μm occupy 60% or more of the entire pore volume, and pores having a pore diameter of 10 to 20 μm occupy 20% or more of the entire pore volume.
[0012]
The pores are preferably formed by adding 10 to 50% by weight of a combustible particulate material having a particle size of 0.08 to 30 μm to the coating layer and firing it in an oxidizing atmosphere.
[0013]
DETAILED DESCRIPTION OF THE INVENTION
As a conventional NO x storage reduction catalyst, a coating layer made of a porous carrier such as alumina is formed on the surface of a honeycomb substrate, and a noble metal and a NO x storage material are supported on the coating layer. is there. However, in the conventional NO x storage-reduction catalyst, the NO x storage material is supported almost uniformly from the surface of the coat layer to the inside due to the loading method and the characteristics of the metal chemical solution, whereas noble metal is deposited on the surface of the coat layer. Many are supported. Therefore reduction efficiency of the NO x occluded in the NO x storage material of the inner coating layer is low.
[0014]
Further, the pore diameter of the porous carrier used in the conventional NO x storage-reduction catalyst is as extremely small as about 0.01 μm. The coat layer is constituted by densely filling the carrier powder. Therefore, the gas diffusibility in the coat layer is low, and it is considered that the exhaust gas is not sufficiently diffused to the inside of the coat layer at the time of rich spike.
[0015]
Therefore, in the exhaust gas purifying catalyst of the present invention, a pore having a pore diameter of 0.1 to 20 μm occupies 60% or more of the entire pore volume, and a pore having a pore diameter of 10 to 20 μm occupies 20% or more of the entire pore volume. It is as a layer.
[0016]
In the region where the pore diameter is 0.01 to 20 μm, the gas diffusion rate coefficient changes continuously, and the amount of change is proportional to the pore diameter. Therefore, in the coat layer in the present invention, the pores having a pore diameter of 0.1 to 20 μm occupy most of the pores, so that the pore diameter is 10 to 2000 compared with the conventional coat layer having the pores of 0.01 μm mostly. The speed at which the gas diffuses in the pores is also 10 to 2000 times. Therefore, even in a short time of the rich spike, the exhaust gas is reachable to the inside of the coat layer, the a NO x occluded in the the NO x storage material of the inner coat layer can be reduced and purified.
[0017]
In addition, the noble metal solution sufficiently permeates into pores having a pore diameter of 10 to 20 μm by capillary action. That is, since the noble metal is supported in the form along the pores to the inside of the coat layer, the NO x storage material and the noble metal are close to each other also inside the coat layer. With increasing the utilization efficiency of the NO x storage material in the inner coat layer therefore the reduction efficiency of the occluded NO x is further improved in the NO x storage material of the inner coating layer.
[0018]
In other words, pores with a pore diameter of 10 to 20 μm serve as gas diffusion passages and are also places for supporting noble metals and NO x storage materials, so that the contact probability between the exhaust gas and the catalyst component is remarkably increased, and a high NO with a short rich spike. x purification efficiency can be obtained. Therefore, according to the exhaust gas purifying catalyst of the present invention, it is possible to further shorten the rich spike time, improve fuel efficiency and further suppress CO 2 emissions.
[0019]
If the pores having a pore diameter of 0.1 to 20 μm are less than 60% of the total pore volume, the gas diffusibility is lowered and the NO x purification ability at the time of rich spike is lowered. On the other hand, if the pores having a pore diameter of 10 to 20 μm are less than 20% of the total pore volume, the amount of noble metal supported in the coat layer is reduced, so that the use efficiency of the NO x storage material is lowered and the NO x purification ability is lowered.
[0020]
Examples of the porous carrier include alumina, silica, zirconia, titania, silica-alumina, zeolite, and the like. One of them may be used, or a plurality of types may be mixed or combined. Of these, highly active γ-alumina is preferably used.
[0021]
Examples of the noble metal supported on the coating layer include platinum (Pt), rhodium (Rh), palladium (Pd), iridium (Ir) and the like. Of these, highly active Pt is particularly preferable. The amount of the noble metal supported is preferably 0.1 to 10 g per liter of the catalyst. If it is less than this, the purification activity will be insufficient, and even if it is supported more than this, the effect will be saturated and it will be expensive.
[0022]
The NO x storage material supported on the coat layer is at least one selected from alkali metals, alkaline earth metals, and rare earth elements. Examples of the alkali metal include lithium, sodium, potassium, rubidium, cesium, and francium. Examples of the alkaline earth metal include barium, beryllium, magnesium, calcium and strontium. Examples of rare earth elements include scandium, yttrium, lanthanum, cerium, praseodymium, neodymium, dysprosium, ytterbium, and the like.
[0023]
The amount of the NO x storage material supported is desirably in the range of 0.01 to 1 mol per liter of catalyst. If the loading amount is less than this range, the NO x storage amount decreases, so the NO x purification ability decreases, and if it exceeds this range, the noble metal is covered with the NO x storage material and the activity decreases.
[0024]
Although there is no restriction | limiting in particular in the thickness of a coating layer, Generally it is set as the adhesion amount of 120-300g per liter of base materials. Further, as the base material on which the coating layer is formed, heat-resistant ceramics such as cordierite or metal is used, and the shape thereof can be not only a honeycomb shape but also a granular shape.
[0025]
To form a coat layer so that pores with a pore size of 0.1-20 μm occupy 60% or more of the total pore volume, and pores with a pore size of 10-20 μm occupy 20% or more of the total pore volume, A combustible granular material having a diameter of 0.08 to 30 μm is added to the coating layer in an amount of 10 to 50% by weight, and it is fired in an oxidizing atmosphere.
[0026]
That is, a mixed powder obtained by mixing 10 to 50% by weight of a combustible granular material having a particle size of 0.08 to 30 μm with a porous carrier powder is slurried together with a binder component and the like in the same manner as in the past. This is wet-coated on the surface of a honeycomb substrate or the like, and fired in an oxidizing atmosphere such as the air to form a coat layer. At the time of firing, the combustible particulate material is burned off, and pores are formed in the trace. The gas generated by burning the combustible particulate material passes through the coating layer and is discharged from the surface to the outside, so that the pores communicate with the outside. Then, the pore diameter is slightly reduced by sintering, or the combustible granular material is aggregated or crushed in the slurry. As a result, pores having a pore diameter of 0.1 to 20 μm occupy 60% or more of the total pore volume. ˜20 μm pores occupy 20% or more of the total pore volume.
[0027]
As the combustible particulate material to be used, those which are completely burned off by combustion are desirable, and pulp powder, carbon powder, paraffin powder, organic polymer powder and the like can be used. This combustible particulate material is mixed so that it may become 10 to 50 weight% in a coating layer. If this amount is less than 10% by weight or the particle size is less than 0.08 μm, it becomes difficult for pores with a pore size of 0.1 to 20 μm to occupy 60% or more of the total pore volume, and the particle size exceeds 30 μm or 50% by weight. If it is mixed beyond the range, the strength of the coating layer after firing may decrease, and problems such as peeling may occur during use.
[0028]
In order to carry the noble metal on the coat layer, it is desirable to impregnate the coat layer with a predetermined amount of an aqueous solution of a chemical solution such as a noble metal complex or nitrate, or an alcohol solution, followed by drying and baking. The noble metal is adsorbed and supported on the surface of the coat layer, and the aqueous solution is also impregnated into pores having a pore diameter of 10 to 20 μm that are open on the surface of the coat layer. It can be supported.
[0029]
In order to support the NO x storage material on the coat layer, an aqueous solution such as acetate, hydrochloride or nitrate of the NO x storage material or an alcohol solution can be used and supported in the same manner as the noble metal. Since the NO x storage material has a low adsorptivity to the carrier, it can be supported almost uniformly from the surface to the inside of the coating layer, regardless of how it is supported.
[0030]
【Example】
Hereinafter, the present invention will be specifically described with reference to Examples and Comparative Examples.
[0031]
Example 1
100 parts by weight of a mixed powder obtained by mixing alumina powder and titania powder in a weight ratio of 1: 1, 65 parts by weight of alumina sol (40% by weight of alumina), and 190 parts by weight of water were mixed. Further, a carbon powder having a particle size of 0.085 μm was mixed so as to have a solid content of 30% by weight to prepare a slurry.
[0032]
Next, a cordierite honeycomb substrate (capacity 35 cc) was prepared, immersed in the above slurry and pulled up to blow off the excess slurry, and then fired in the atmosphere at 600 ° C. for 1 hour to form a coat layer did. The coating layer was formed in an amount of 270 g per 1 L of honeycomb substrate.
[0033]
The central pore diameter and pore volume of this coat layer are measured with a mercury porosimeter, the proportion of pores with a pore diameter of 0.1-20 μm in the total pore volume, and the pores with a pore diameter of 10-20 μm are the whole pores. The ratio to the volume was calculated. The results are shown in Table 1.
[0034]
The obtained honeycomb substrate having a coating layer was immersed in an aqueous barium acetate solution having a predetermined concentration to absorb water, dried at 250 ° C. for 15 minutes, and fired at 500 ° C. for 30 minutes to carry Ba. Further, it was immersed in an aqueous solution of ammonium bicarbonate having a concentration of 15 g / L for 15 minutes and dried at 250 ° C. for 15 minutes to obtain Ba as barium carbonate. The amount of Ba supported is 0.2 mol per liter of honeycomb substrate.
[0035]
Next, a honeycomb substrate having a coating layer on which Ba is supported is immersed in a platinum nitrate aqueous solution of a predetermined concentration, pulled up and blown off excess droplets, and then dried at 300 ° C. for 15 minutes and then at 500 ° C. for 30 minutes. Firing and supporting Pt. The amount of Pt supported is 2 g per 1 L of honeycomb substrate.
[0036]
Further, an aqueous potassium nitrate solution and an aqueous lithium nitrate solution were used in the same manner, and 0.1 mol of K and Li were supported per liter of the honeycomb substrate.
[0037]
(Example 2)
A coating layer was formed in the same manner as in Example 1 except that a carbon powder having a particle size of 6 μm was used and the firing conditions were set to 700 ° C. for 7 hours. Similarly, the central pore size and the pore size of 0.1 to 20 μm were formed. The ratio of the total pore volume to the total pore volume and the ratio of the pore diameter of 10 to 20 μm to the total pore volume were calculated. The results are shown in Table 1. In the same manner as in Example 1, Pt and NO x storage materials were supported.
[0038]
(Example 3)
A coating layer was formed in the same manner as in Example 1 except that carbon powder having a particle size of 25 μm was used and the firing conditions were set to 700 ° C. for 12 hours. Similarly, the central pore size and the pore size of 0.1 to 20 μm were formed. The ratio of the total pore volume to the total pore volume and the ratio of the pore diameter of 10 to 20 μm to the total pore volume were calculated. The results are shown in Table 1. In the same manner as in Example 1, Pt and NO x storage materials were supported.
[0039]
(Comparative example)
A coating layer was formed in the same manner as in Example 1 except that no carbon powder was used. Similarly, the central pore diameter, the ratio of pores having a pore diameter of 0.1 to 20 μm to the total pore volume, and the pore diameter of 10 to The ratio of 20 μm pores to the total pore volume was calculated. The results are shown in Table 1. In the same manner as in Example 1, Pt and NO x storage materials were supported.
[0040]
<Test and evaluation>
[0041]
[Table 1]
From Table 1, by adding carbon powder, the center pore diameter is in the range of 0.1-20 μm, the proportion of pores of 0.1-20 μm in the total pore volume is 65% or more, and the pore diameter is 10-20 μm. It can be seen that the ratio of the pores to the total pore volume is 20% or more. It can also be seen that the larger the particle diameter of the carbon powder is, the larger the pore diameter and the above ratio are, and it is preferable that the carbon powder has a particle diameter of 0.08 μm or more.
[0043]
Next, heat treatment is performed in which the catalysts of Examples 1 to 3 and the comparative example are respectively placed in the evaluation apparatus, and the rich gas and the lean gas shown in Table 2 are respectively entered and the gas temperature is 600 ° C. and alternately flow every 2.5 minutes for 5 hours. Went.
[0044]
[Table 2]
For each of the catalysts after the heat treatment, after flowing the rich gas shown in Table 3 for 10 minutes, switched to lean, NO x is flowed lean to saturation. Then, rich spike gas was struck for 3 seconds and switched to lean gas again, and the amount of NO x stored in the catalyst during the rich spike was measured. Measurements were made for inlet gas temperatures of 300 ° C and 400 ° C, respectively. The results are shown in FIG.
[0046]
[Table 3]
Further, for the catalyst of Example 1 and the catalyst of the comparative example, after switching to lean gas after rich spike, the NO x concentration in the outgas was continuously measured over time to create a NO x storage curve. The measurement conditions are an incoming gas temperature of 300 ° C. and a gas flow rate of 50 L / min (SV = 86,000 hr −1 ). The results are shown in FIG.
[0048]
From FIG. 1, the catalyst of each example in comparison with the catalyst of Comparative Example It is clear that the NO x storage amount after the rich spike is large. Although the catalyst of each example and the catalyst of the comparative example had the same capacity to store NO x , this difference was caused by the rich NO x stored in the catalyst of the example. This is probably because the catalyst of the comparative example was not fully reduced by a short rich spike while it was sufficiently reduced at the time of spike.
[0049]
Also from Figure 2, in the catalyst of Example 1, about 20 seconds after switching to lean gas has no release of NO x, NO x is completely stored in the catalyst. On the other hand, in the catalyst of the comparative example, the NO x concentration is high immediately after switching to the lean gas.
[0050]
Although the catalyst of Example 1 and the catalyst of Comparative Example have the same capacity to store NO x , such a difference occurred because the NO x storage rate was slow in the catalyst of Comparative Example. It is considered that NO x flowing into the catalyst cannot be fully occluded, whereas NO x was occluded rapidly in the catalyst of Example 1. That is, it is shown that the catalyst of Example 1 is excellent in gas diffusibility because it has sufficiently large pores.
[0051]
As a result of calculating the initial NO x storage rate from FIG. 2, the catalyst of Example 1 was 0.006 mmol / second, the catalyst of Comparative Example was 0.003 mmol / second, and the catalyst of Example 1 was replaced with the catalyst of Comparative Example. It is storing NO x at twice the speed.
[0052]
Next, regarding the catalysts of Examples 2 and 3 and the catalyst of the comparative example, the distribution state of Pt in the cross section of the coat layer was analyzed by EPMA analysis. The results are shown in FIGS.
[0053]
From FIG. 3 to FIG. 5, it is observed that Pt is supported only on the surface of the coat layer in the catalyst of the comparative example, whereas Pt is supported up to the inside of the coat layer in the catalyst of each example. The That is, in the catalyst of each example, the coating layer has many pores having a central pore diameter of 0.2 to 3 μm, so that the aqueous solution of platinum nitrate is impregnated in the pores, so that the pores are aligned along the pores. It is considered that Pt was supported up to the inside of the coating layer.
[0054]
Therefore, in the catalyst of each example, a reaction due to Pt also occurs inside the coating layer, and combined with improvement of gas diffusivity, as shown in FIGS. 1 and 2, a high NO x occlusion amount is shown after the rich spike, It is thought that a large NO x storage rate was expressed.
[0055]
【The invention's effect】
That is, according to the exhaust gas purifying catalyst of the present invention, high NO x storage ability is exhibited and NO x reduction efficiency at the time of rich spike is improved. Further if the conventional equivalent of the NO x purification performance, it is possible to shorten the time of the rich spike, fuel consumption increases with the generation of CO 2 is suppressed.
[Brief description of the drawings]
FIG. 1 is a graph showing NO x occlusion amounts after rich spikes of catalysts of Examples and Comparative Examples.
FIG. 2 is a graph showing NO x storage capacity of catalysts of Example 1 and Comparative Example.
3 is an EPMA photograph showing the Pt distribution of a coating layer in the catalyst of Example 2. FIG.
4 is an EPMA photograph showing the Pt distribution of a coat layer in the catalyst of Example 3. FIG.
FIG. 5 is an EPMA photograph showing a Pt distribution of a coating layer in a catalyst of a comparative example.
Claims (2)
該コート層は、孔径 0.1〜20μｍの細孔が全体の細孔容積の60％以上を占め、孔径10〜20μｍの細孔が全体の細孔容積の20％以上を占めることを特徴とする排ガス浄化用触媒。A coating layer of a porous carrier, a noble metal supported on the coat layer, an alkali metal, selected from alkaline earth metals and rare earth elements and more becomes supported the NO x storage material on the coat layer, is always oxygen in excess it is combusted in a lean atmosphere intermittently NO x storage-and-reduction type exhaust gas purifying catalyst used in an exhaust system of a combustion system for controlling the ratio of the mixture so that the stoichiometric-rich atmosphere,
The coating layer is characterized in that pores having a pore diameter of 0.1 to 20 μm occupy 60% or more of the entire pore volume, and pores having a pore diameter of 10 to 20 μm occupy 20% or more of the entire pore volume. Purification catalyst.
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| JP2000392666A JP4450984B2 (en) | 2000-12-25 | 2000-12-25 | Exhaust gas purification catalyst |
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| JP2006110485A (en) | 2004-10-15 | 2006-04-27 | Johnson Matthey Japan Inc | Exhaust gas catalyst and exhaust gas trteatment apparatus using the catalyst |
| JP2007275704A (en) * | 2006-04-03 | 2007-10-25 | Johnson Matthey Japan Inc | Exhaust gas catalyst and exhaust gas treating device using the same |
| JP2008168278A (en) * | 2006-12-15 | 2008-07-24 | Nissan Motor Co Ltd | Catalyst for cleaning exhaust gas and its manufacturing method |
| JP4888470B2 (en) | 2007-11-08 | 2012-02-29 | 日産自動車株式会社 | Method for producing noble metal-supported powder and exhaust gas purifying catalyst |
| EP2543439A4 (en) | 2010-03-02 | 2014-05-07 | Nippon Sheet Glass Co Ltd | Catalyst loaded with fine noble metal particles, method for producing same, and purification catalyst |
| JP5873731B2 (en) | 2012-02-07 | 2016-03-01 | 本田技研工業株式会社 | Exhaust gas treatment catalyst structure |
| US9561495B2 (en) | 2013-03-06 | 2017-02-07 | Basf Corporation | Porous catalyst washcoats |
| EP3045227B1 (en) * | 2013-09-11 | 2019-12-11 | Mitsui Mining & Smelting Co., Ltd | Exhaust gas purification catalyst |
| JP6358902B2 (en) * | 2014-09-02 | 2018-07-18 | 株式会社アルバック | Method for producing exhaust gas purification catalyst |
| DE102017106374A1 (en) | 2016-04-01 | 2017-10-05 | Johnson Matthey Public Limited Company | exhaust gas purifying filter |
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