JP3896223B2 - Exhaust gas purification device for internal combustion engine - Google Patents

Description

【００２４】
放出したＮＯｘを速やかに還元して装置外へ排出される排ガス中のＮＯｘ濃度を低減すべく還元剤の濃度を高めても、気相においてはＮＯ₂ とＨＣの反応はあまり進まない。したがって、還元剤の増量でＮＯｘ放出量を十分に減ずることができない。また、ＮＯｘ吸収量が少ない段階で還元反応による操作を行うことも考えられるが、ＮＯｘ吸収剤の再生頻度が増し、実用的でない。
【００２５】
本発明の吸着触媒は、その表面近傍でのみＮＯｘを捕捉するため発熱の絶対量としては少なく、且つ速やかに排ガスに伝達されるため吸着触媒の温度上昇は少ない。したがって一旦捕捉したＮＯｘの放出を防止することができる。
【００２６】
本発明のＮＯｘ吸着触媒は、ＮＯｘをその表面で化学吸着により捕捉しＮＯｘの還元に際しての発熱反応でＮＯｘの放出を生起しない材料として特徴付けられる。また、本発明のＮＯｘ吸着触媒は、ＮＯｘをその表面で化学吸着によりもしくは表面近傍で化学結合により捕捉し、ＮＯｘの還元に際しての発熱反応でNOx の放出を生起しない材料として特徴付けられる。
【００２７】
本発明者等は、少なくともカリウム（Ｋ），ナトリウム（Ｎａ），マグネシウム（Ｍｇ），ストロンチウム（Ｓｒ）及びカルシウム（Ｃａ）から選ばれる一種以上の元素を成分の一部として含むＮＯｘ吸着触媒で上記特徴を実現し得ることを見出した。
【００２８】
本発明の内燃機関の排ガス浄化装置は、少なくともカリウム（Ｋ），ナトリウム（Ｎａ），マグネシウム（Ｍｇ），ストロンチウム（Ｓｒ）及びカルシウム
（Ｃａ）から選ばれる一種以上の元素を成分の一部として含むＮＯｘ吸着触媒を排ガス流路に配置し、排ガス中の各成分間の酸化還元化学量論関係において還元剤に対して酸化剤が多い状態をつくって吸着触媒上にＮＯｘを化学吸着させ、次に酸化剤に対し還元剤が同量以上の状態をつくり、吸着触媒上に吸着したＮＯｘを還元剤と接触反応させてＮ₂ に還元して無害化することを特徴とする。
【００２９】
本発明の内燃機関の排ガス浄化装置は、また、少なくともカリウム（Ｋ），ナトリウム（Ｎａ），マグネシウム（Ｍｇ），ストロンチウム（Ｓｒ）及びカルシウム（Ｃａ）から選ばれる一種以上の元素を成分の一部として含むＮＯｘ吸着触媒を排ガス流路に配置し、酸化還元化学量論関係においてＨＣ等の還元剤に対してＯ₂ 等の酸化剤が多い状態をつくって吸着触媒表面及び表面近傍にＮＯｘを化学結合により捕捉し、次に酸化剤に対し還元剤が同量かもしくは多い状態をつくり、吸着触媒に捕捉されたＮＯｘを還元剤と接触反応させてＮ₂ に還元して無害化することを特徴とする。
【００３０】
本発明におけるＮＯｘ吸着触媒としては特に以下が好適に適用できる。
【００３１】
カリウム（Ｋ），ナトリウム（Ｎａ），マグネシウム（Ｍｇ），ストロンチウム（Ｓｒ）及びカルシウム（Ｃａ）から選ばれる少なくとも一種と、セリウム等からなる希土類から選ばれる少なくとも一種と、白金，ロジウム，パラヂウム等からなる貴金属から選ばれる少なくとも一種の元素を含む、金属および金属酸化物（もしくは複合酸化物）からなる組成物、該組成物を多孔質耐熱性金属酸化物に担持してなる組成物。本組成物は、優れたＮＯｘ吸着能に加え優れた耐ＳＯｘ性を有する。
【００３２】
本発明の方法における、酸化剤に対し還元剤が同量かもしくは多い状態は以下の方法で作る事ができる。
【００３３】
内燃機関における燃焼条件を理論空燃比もしくは燃料過剰（リッチ）とする。また、リーンバーン排ガスに還元剤を添加する。
【００３４】
前者は以下の方法で達成することができる。
【００３５】
排気ダクトに設けられた酸素濃度センサー出力及び吸気流量センサー出力等に応じて燃料噴射量を制御する方法。本法では、複数の気筒の一部を燃料過剰とし残部を燃料不足とし、全気筒からの混合排ガス中の成分が酸化還元化学量論関係において酸化剤に対して還元剤が同量かもしくは多い状態をつくる方法をも含む。
【００３６】
後者は以下の各方法で達成することができる。
【００３７】
排ガス流の吸着触媒上流に還元剤を投入する方法。還元剤には内燃機関の燃料としてのガソリン，軽油，灯油，天然ガス、これらの改質物，水素，アルコール類，アンモニア等が適用できる。ブローバイガス及びキャニスターパージガスを吸着触媒上流に導きこれらに含まれる炭化水素等の還元剤を投入することも有効である。燃料直噴式内燃機関においては、排気行程で燃料を噴射し還元剤としての燃料を投入することが有効である。
【００３８】
本発明における吸着触媒は、各種の形状で適用することができる。コージェライト，ステンレス等の金属材料からなるハニカム状構造体に吸着触媒成分をコーティングして得られるハニカム形状を始めとし、ペレット状，板状，粒状，粉末として適用できる。
【００３９】
本発明における、酸化剤に対し還元剤が同量かもしくは多い状態を作るタイミングは以下の各方法によることができる。
【００４０】
ＥＣＵ(Engine Control Unit）で決定される空燃比設定信号，エンジン回転数信号，吸入空気量信号，吸気管圧力信号，速度信号，スロットル開度，排ガス温度等からリーン運転時におけるＮＯｘ排出量を推定し、その積算値が所定の設定値を超えたとき。
【００４１】
排気流路の吸着触媒上流または後流に置かれた酸素センサー（もしくはＡ／Ｆセンサー）の信号により累積酸素量を検出し累積酸素量が所定の量を超えたとき。
【００４２】
その変形態様として、リーン運転時の累積酸素量が所定の量を超えたとき。
【００４３】
排気流路の吸着触媒上流に置かれたＮＯｘセンサー信号により累積ＮＯｘ量を算出し、リーン運転時における累積ＮＯｘ量が所定の量を超えたとき。
【００４４】
排気流路の吸着触媒後流に置かれたＮＯｘセンサーの信号によりリーン運転時におけるＮＯｘ濃度を検出し、ＮＯｘ濃度が所定濃度を超えたとき。
【００４５】
本発明における、酸化剤に対し還元剤が同量かもしくは多い状態を維持する時間もしくは維持すべく投入する還元剤量は、前述のごとく、予め吸着触媒の特性，内燃機関の諸元と特性等を考慮して決めることができるが、これらは、燃料噴射弁のストローク，噴射時間及び噴射間隔を調整して実現できる。
【００４６】
【発明の実施の形態】
本発明の具体的実施態様を挙げて本発明を詳細に説明する。なお、本発明は以下の実施態様及び実施例に限定されるものでなく、その思想範囲内において各種の実施態様があることは言うまでもない。
【００４７】
［吸着触媒］
本発明の方法による吸着触媒の特性について説明する。アルカリ金属として Ｎａを含むＮ−Ｎ９とＫを含むＮ−Ｋ９の特性は次の様である。
【００４８】
《吸着触媒調製法》
吸着触媒Ｎ−Ｎ９を以下の方法で得た。
【００４９】
アルミナ粉末とベーマイトを硝酸邂逅して得たバインダーとしてのアルミナゾルを混合し硝酸酸性アルミナスラリーを得た。該コーティング液にハニカムを浸漬した後速やかに引き上げ、セル内に閉塞した液をエアブローして除去した後、乾燥、続いて４５０℃で焼成した。この操作を繰返しハニカムの見掛け容積１Ｌあたり１５０ｇのアルミナをコーティングした。該アルミナコートハニカムに触媒活性成分担持しハニカム状吸着触媒を得た。例えば、硝酸セリウム(硝酸Ｃｅ)溶液を含浸し乾燥後６００℃で１時間焼成した。続いて硝酸ナトリウム（硝酸Ｎａ）溶液とチタニアゾル溶液と硝酸マグネシウム（硝酸Ｍｇ）溶液の混合溶液を含浸し、同様に乾燥，焼成した。さらにジニトジアンミンＰｔ硝酸溶液と硝酸ロジウム（硝酸Ｒｈ）溶液の混合溶液に含浸し、乾燥後４５０℃で１時間焼成した。最後に硝酸Ｍｇ溶液を含浸し４５０℃で１時間焼成した。以上によりアルミナ(Ａｌ₂Ｏ₃)にＣｅ，Ｍｇ，Ｎａ，Ｔｉ，Ｒｈ，Ｐｔを担持したハニカム状吸着触媒、２Ｍｇ−(０.２Ｒｈ，２.７Ｐｔ)−(１８Ｎａ，４Ｔｉ，２Ｍｇ)−２７Ｃｅ／Ａｌ₂Ｏ₃を得た。ここで、／Ａｌ₂Ｏ₃は活性成分がＡｌ₂Ｏ₃上に担持されたことを示し、元素記号前の数値はハニカム見掛け容積１Ｌ当たりに担持した表示金属成分の重量（ｇ）である。表記順序は担持順序を示しており、Ａｌ₂Ｏ₃に近く表記される成分から離れる成分の順で担持し、（ ）で括られた成分は同時に担持した。ちなみに各活性成分の担持量は含浸溶液中の活性成分濃度を変化させることにより変えることができる。
【００５０】
吸着触媒Ｎ−Ｋ９を以下の方法で調製した。
【００５１】
吸着触媒Ｎ−Ｎ９調製における硝酸Ｎａ溶液に代わり硝酸カリウム（硝酸Ｋ）溶液を用い、その他は吸着触媒Ｎ−Ｎ９と同様の方法でＮ−Ｋ９ ２Ｍｇ− (０.２Ｒｈ，２.７Ｐｔ)−(１８Ｋ，４Ｔｉ，２Ｍｇ)−２７Ｃｅ／Ａｌ₂Ｏ₃を得た。また同様の方法で比較触媒Ｎ−Ｒ２ ２Ｍｇ−(０.２Ｒｈ，２.７Ｐｔ）− ２７Ｃｅ／Ａｌ₂Ｏ₃を得た。
【００５２】
《性能評価法》
上記方法で得た吸着触媒を７００℃で５時間酸化雰囲気で熱処理した後、以下の方法で特性を評価した。
【００５３】
排気量１.８Ｌ のリーンバーン仕様ガソリンエンジンを搭載した乗用車に本発明の方法により調製した容積１.７Ｌ のハニカム状吸着触媒を搭載しＮＯｘ浄化特性を評価した。
【００５４】
《吸着触媒の特性》
吸着触媒Ｎ−Ｎ９を搭載し、Ａ／Ｆ＝１３.３ のリッチ運転３０秒間とＡ／Ｆ＝２２のリーン運転約２０分間を交互に繰り返し図２のＮＯｘ浄化率経時特性を得た。同図から本吸着触媒によりリーン運転期間中のＮＯｘが浄化されることが伺える。リーン運転中ＮＯｘ浄化率は徐々に低下し初期に１００％あった浄化率は２０分後には約４０％となる。しかしこの低下した浄化率は３０秒間のリッチ運転で１００％にまで回復する。再びリーン運転を行うとＮＯｘ浄化能は回復して前述の経時変化を繰り返す。リーン運転とリッチ運転を複数回繰返してもリーン運転中のＮＯｘ浄化率の経時低下の速度は不変であり、これはリッチ運転によりＮＯｘ吸着性能が十分に再生されたことを示している。
【００５５】
車速を約４０ｋｍ／ｈ一定（排ガスの空間速度(ＳＶ)約２０,０００／ｈ一定)とし点火時期を変化させて排ガス中のＮＯｘ濃度を変え、ＮＯｘ濃度とリーン排ガス中のＮＯｘ浄化率の関係を求めて図３を得た。ＮＯｘ浄化率は経時的に低下するがＮＯｘ濃度が低いほど低下速度は小さい。ＮＯｘ浄化率５０％及び３０％に至るまでに捕捉されたＮＯｘ量を同図から求めると表１となる。
【００５６】
【表１】

【００５７】
ＮＯｘ捕捉量はＮＯｘ濃度に依らずほぼ一定である。吸着量が吸着質の濃度 （圧力）に寄らないのは化学吸着の特徴である。
【００５８】
供試吸着触媒中でＮＯｘ吸着楳として先ず考えられるのはＰｔ粒子である。露出Ｐｔ量を評価する手段として多用されるＣＯ吸着量評価を行ったところＣＯ吸着量（ａｔ １００℃)は４.５×１０^-4molであった。この値は上記ＮＯｘ吸着量の約１／１００でありＰｔがＮＯｘ吸着楳の主役でないことは明らかである。
【００５９】
一方、本吸着触媒のコーディェライトごと測定したＢＥＴ比表面積（窒素吸着で測定）は約２５ｍ²／ｇでハニカム１.７Ｌ当たり２８,０５０ｍ²であった。また、本発明の吸着触媒のＮａの化学構造について検討したところ、鉱酸にＣＯ₂ ガスを発生して溶解すること及び鉱酸による中和滴定曲線における変曲点の値から判断して主にＮａ₂ＣＯ₃として存在すると判断できた。仮に全ての表面が Ｎａ₂ＣＯ₃で占められているとすると表面には０.２７５molのＮａ₂ＣＯ₃が露出していることになる（Ｎａ₂ＣＯ₃の比重が２.５３３ｇ／ｍｌであることから Ｎａ₂ＣＯ₃１分子の体積が求まる（Ｎａ₂ＣＯ₃を立方体と仮定してその１面の面積を求めこれを表面Ｎａ₂ＣＯ₃の占有面積とした）。前出の反応式に従えば ０.２７５molのＮａ₂ＣＯ₃は０.５５molのＮＯ₂ を吸着する能力がある。しかし、実際に本発明の吸着触媒が除去したＮＯｘ量はその１／１０以下の０.０４molのオーダーである。この相違はＢＥＴ法が物理表面積を評価するものでＡｌ₂Ｏ₃等のＮａ₂ＣＯ₃以外の表面積も評価していることによる。以上の評価は、吸着 ＮＯｘ量はＮａ₂ＣＯ₃バルクのＮＯｘ捕捉能よりはるかに少なく、少なくとも ＮＯｘがＮａ₂ＣＯ₃表面か表面近傍の限られた領域で捕捉されていることを示している。
【００６０】
なお、図３においてＮＯｘ浄化率２０％前後から浄化率低下の速度が低下しているが、これは触媒機能による還元反応が生じていることを示すものである。
【００６１】
図４は、リーン運転からストイキ運転に切替えた直後のＮＯｘ浄化率を示す。本吸着触媒では、ストイキ運転への切替え直後から９０％以上のＮＯｘ浄化率が得られることが分かる。
【００６２】
図５，図６に、リーンからストイキあるいはリッチへの切替え前後における ＮＯｘ浄化特性を示した。図５は吸着触媒Ｎ−Ｎ９の入口と出口のＮＯｘ濃度を示したもので、図(ａ）はＡ／Ｆ＝２２のリーンからＡ／Ｆ＝１４.２のリッチへ空燃比を切替えた場合である。リッチ切替え直後の再生の開始時点においてはＡ／Ｆ＝１４.２ の排ガスＮＯｘ濃度が高いためリッチ運転の入口ＮＯｘ濃度が大きく増加し、これに伴い過渡的に出口ＮＯｘ濃度は増加するが、常時出口ＮＯｘ濃度は入口ＮＯｘ濃度を大きく下回る。再生は速やかに進み短時間で出口ＮＯｘ濃度は０近傍に到達する。図(ｂ）はＡ／Ｆ＝２２のリーンからＡ／Ｆ＝１３.２のリッチへ空燃比を切替えた場合であるが、図（ａ）と同様に、常時出口ＮＯｘ濃度は入口ＮＯｘ濃度を大きく下回り、且つ、より短時間で出口ＮＯｘ濃度は０近傍に到達する。
【００６３】
以上から明らかであるが、再生条件としてのＡ／Ｆ値は再生に要する時間に影響する。再生に適したＡ／Ｆ値，時間、さらには還元剤量は、吸着触媒の組成，形状，温度，ＳＶ値，還元剤の種類，排気流路の形状や長さの影響を受ける。従って、再生条件はこれらを考慮して総合的に決められるものである。
【００６４】
図６は吸着触媒Ｎ−Ｋ９の入口と出口のＮＯｘ濃度を示したもので、図(ａ)はＡ／Ｆ＝２２のリーンからＡ／Ｆ＝１４.２ のリッチへ空燃比を切替えた場合、図(ｂ）はＡ／Ｆ＝２２のリーンからＡ／Ｆ＝１３.２のリッチへ空燃比を切替えた場合であるが、上述の吸着触媒Ｎ−Ｎ９の場合と同様に常時出口ＮＯｘ濃度は入口ＮＯｘ濃度を大きく下回り、且つ、短時間で吸着触媒の再生が進んでいる。《吸着触媒の基礎特性》
モデルガスを用い基礎特性、特にＮＯｘ浄化率に与える酸素濃度の影響を評価した。
【００６５】
Ｎ−Ｎ９の６ｍｌ小ハニカムを内径２８mmφの石英半応管内に充填し、モデルガスを流通させた。モデルガス中の酸素濃度を変化させＮＯｘ浄化率に与える影響を検討した。反応温度は触媒入り口ガス温度で３００℃とした。
【００６６】
先ず、Ｏ₂ ５％（体積分率；以下同じ），ＮＯ６００ppm，Ｃ₃Ｈ₆５００ppm （１５００ppm as Ｃｌ)，ＣＯ１０００ppm，ＣＯ₂ １０％，Ｈ₂Ｏ１０％及び Ｎ₂ Balance なる組成のガスを流通させＮＯｘ浄化率が安定した１０分後に酸素濃度のみを所定の値まで低下させて２０分間保持し、最後に再び初期のガス組成に戻した。この間のＮＯｘ濃度変化を、低下させる酸素濃度を０％，０.５％， ０.７％，１％，２％ 及び３％の６種変化させ図１９を得た。図１９では、リーンガス中の酸素濃度が低くなるとすなわち酸化雰囲気が弱まり還元雰囲気が強くなるとＮＯｘ浄化率が高くなる傾向が認められ、本吸着触媒がＮＯｘを還元により浄化していることを示唆している。また、図１９では、ＮＯｘ浄化率は常に正の値であり本吸着触媒では酸素濃度の如何に因らず吸着触媒通過後ＮＯｘ濃度が増すことはない。
【００６７】
次に、Ｏ₂ ５％，ＮＯ６００ppm，Ｎ₂ Balanceなる組成のガスを流通させつつこのガス中の酸素濃度を同様に変化させて図２０を得た。本検討ではガス中に還元剤が含まれていないことに特徴がある。図２０において酸素濃度を低下させて酸化雰囲気が弱めてもＮＯｘ浄化率は向上しない。この事は本吸着触媒がＮＯｘを還元により浄化していることを示唆している。図２０において、ＮＯｘ浄化率が負になることはなく、本吸着触媒では酸化還元の雰囲気によって、一旦捕捉したＮＯｘを放出することはない。
【００６８】
［排気浄化装置］
図１は本発明の排ガス浄化装置の一実施態様を示す装置の全体構成である。
【００６９】
本発明の装置は、リーンバーン可能なエンジン９９，エアフローセンサー２，スロットルバルブ３等を擁する吸気系，酸素濃度センサー(or Ａ／Ｆセンサー）１９，排気温度センサー２１，ＮＯｘ吸着触媒１８等を擁する排気系及び制御ユニット（ＥＣＵ）等から構成される。ＥＣＵは入出力インターフェイスとしてのＩ／Ｏ ＬＳＩ，演算処理装置ＭＰＵ，多数の制御プログラムを記憶させた記憶装置ＲＡＭおよびＲＯＭ，タイマーカウンター等より構成される。
【００７０】
以上の排気浄化装置は、以下のように機能する。エンジンへの吸入空気はエアクリーナー１により濾過された後エアフローセンサー２により計量され、スロットルバルブ３を経て、さらにインジェクター５から燃料噴射を受け、混合気としてエンジン９９に供給される。エアフローセンサー信号その他のセンサー信号はＥＣＵ（Engine Control Unit）へ入力される。
【００７１】
ＥＣＵでは後述の方法によって内燃機関の運転状態及びＮＯｘ吸着触媒の状態を評価して運転空燃比を決定し、インジェクター５の噴射時間等を制御して混合気の燃料濃度を所定値に設定する。シリンダーに吸入された混合気はＥＣＵ２５からの信号で制御される点火プラグ６により着火され燃焼する。燃焼排ガスは排気浄化系に導かれる。排気浄化系にはＮＯｘ吸着触媒が設けられ、ストイキ運転時にはその三元触媒機能により排ガス中のＮＯｘ，ＨＣ，ＣＯを浄化し、また、リーン運転時にはＮＯｘ吸着能によりＮＯｘを浄化すると同時に併せ持つ燃焼機能により、ＨＣ，ＣＯを浄化する。さらに、ＥＣＵの判定及び制御信号により、リーン運転時にはＮＯｘ吸着触媒のＮＯｘ浄化能力を常時判定して、ＮＯｘ浄化能力が低下した場合燃焼の空燃比等をリッチ側にシフトして吸着触媒のＮＯｘ吸着能を回復させる。以上の操作により、本装置では、リーン運転，ストイキ（含むリッチ）運転の全てのエンジン燃焼条件下における排ガスを効果的に浄化する。
【００７２】
エンジンに供給される混合気の燃料濃度（以下空燃比）は次の様に制御される。図７に空燃比制御方法をブロック線図で示した。
【００７３】
アクセルペダルの踏み込みに応じた信号を出力する負荷センサー出力，エアフローセンサーにより計量された吸気量の出力信号，クランク角センサーにより検出されるエンジン回転数信号，排ガス温度信号，スロットル開度を検出するスロットルセンサー信号，エンジン冷却水温信号，スターター信号等の情報から ＥＣＵ２５は空燃比（Ａ／Ｆ）を決定し、さらにこの信号は酸素センサーからフィードバックされる信号に基づき補正され、燃料噴射量を決定する。なお、低温時，アイドル時，高負荷時等では各センサー及びスイッチの信号によりフィードバック制御を停止する。また、空燃比補正学習機能により空燃比の微妙な変化や急な変化にも正確に対応できるよう空燃比補正学習機能で対応する。
【００７４】
決定された空燃比がストイキ(Ａ／Ｆ＝１４.７)及びリッチ(Ａ／Ｆ＜１４.７)のときＥＣＵの指示によりインジェクタの噴射条件が決定されストイキ及びリッチ運転が行われる。一方、リーン(Ａ／Ｆ＞１４.７）運転が決定された場合、 ＮＯｘ吸着触媒のＮＯｘ吸着能の有無の判定を行い吸着能があると判定された場合に指示通りのリーン運転を行うべく燃料噴射量が決定され、吸着能がないと判定された場合には空燃比を所定期間リッチシフトしてＮＯｘ吸着触媒を再生する。
【００７５】
図８に空燃比制御のフローチャートを示した。ステップ１００２で各種の運転条件を指示するあるいは運転状態を検出する信号を読み込む。これらの信号に基づきステップ１００３で空燃比を決定、ステップ１００４では決定された空燃比を検出する。ステップ１００５で決定された空燃比と理論空燃比との大小を比較する。ここでの比較対象となる理論空燃比は、正確には吸着触媒においてＮＯｘの接触還元反応の速度が吸着による捕捉速度を上回る空燃比であり、予め吸着触媒の特性を評価して決定されるもので、理論空燃比近傍の空燃比が選定される。ここで、設定空燃比≦理論空燃比の場合ステップ１００６に進み吸着触媒の再生操作を行うことなく指示通りの空燃比運転を行う。設定空燃比＞理論空燃比の場合ステップ１００７に進む。ステップ１００７ではＮＯｘ吸着量の推定演算を行う。推定演算方法については後述する。続いてステップ１００８で推定ＮＯｘ吸着量が所定限界量以下であるか否かを判定する。限界吸着量は予め実験等により吸着触媒のＮＯｘ捕捉特性を評価して、また排ガス温度や吸着触媒温度等を考慮して、排ガス中のＮＯｘが十分に浄化できる値に設定される。ＮＯｘ吸着能がある場合にはステップ１００６に進み、吸着触媒の再生操作を行うことなく指示通りの空燃比運転を行う。ＮＯｘ吸着能がない場合にはステップ１００９に進み、空燃比をリッチ側にシフトする。ステップ１０１０ではリッチシフト時間をカウントし、経過時間Ｔｒが所定の時間（Ｔｒ）ｃを超えればリッチシフトを終了する。
【００７６】
ＮＯｘ吸着能の判定は次のように行うことができる。
【００７７】
図９はリーン運転時の各種運転条件からＮＯｘ排出量を積算し判定する方法である。
【００７８】
ステップ１００７−Ｅ０１で排ガス温度等のＮＯｘ吸着触媒の作動条件に関する信号と排ガス中のＮＯｘ濃度に影響する各種の機関運転条件に関する信号とを読み込み単位時間に吸着するＮＯｘ量Ｅ_N を推算する。ステップ１００７−E02 でＥ_N を積算し、ステップ１００８−Ｅ０１で積算値ΣＥＮと吸着量の上限値 （Ｅ_N )ｃとの大小を比較する。ΣＥＮ≦(Ｅ_N )ｃの場合は積算を継続し、ΣＥ_N＞(Ｅ_N )ｃの場合ステップ１００８−Ｅ０２で積算を解除しステップ１００９に進む。
【００７９】
図１０はリーン運転の積算時間で判定する方法である。
【００８０】
ステップ１００７−Ｈ０１でリーンの運転時間Ｈ_L を積算し、ステップ1008−Ｈ０１で積算値ΣＨ_L と積算時間の上限値(Ｈ_L )ｃとの大小を比較する。ΣＨ_L ≦(Ｈ_L )ｃの場合積算を継続し、ΣＨ_L ＞(Ｈ_L )ｃの場合ステップ１００８− Ｈ０２で積算を解除しステップ１００９に進む。
【００８１】
図１１はリーン運転時の酸素センサー信号で判定する方法である。
【００８２】
ステップ１００７−Ｏ０１でリーン運転における酸素量Ｑ_O を積算し、ステップ１００８−Ｏ０１で積算値ΣＱ_O と積算酸素量の上限値(Ｑ_O )ｃとの大小を比較する。ΣＱ_O ≦(Ｑ_O )ｃの場合積算を継続し、ΣＱ_O ＞(Ｑ_O )ｃの場合ステップ１００８−Ｏ０２で積算を解除しステップ１００９に進む。
【００８３】
図１２はリーン運転時のＮＯｘ吸着触媒入口で検出したＮＯｘ濃度センサー信号で判定する方法である。
【００８４】
ステップ１００７−Ｎ０１でＮＯｘ濃度センサー信号に基づきＮＯｘ吸着触媒入口におけるＮＯｘ量Ｑ_N を積算する。ステップ１００８−Ｎ０１で積算値 ΣＱ_N と積算ＮＯｘ量の上限値(Ｑ_N )ｃとの大小を比較する。ΣＱ_N ≦(Ｑ_N )ｃの場合積算を継続し、ΣＱ_N ＞(Ｑ_N )ｃの場合ステップ１００８−Ｎ０２で積算を解除しステップ１００９に進む。
【００８５】
図１３はリーン運転時のＮＯｘ吸着触媒出口で検出したＮＯｘ濃度センサー信号で判定する方法である。
【００８６】
ステップ１００７−Ｃ０１でＮＯｘ濃度センサー信号に基づきＮＯｘ吸着触媒入口におけるＮＯｘ濃度ＣＮを検出する。ステップ１００８−Ｃ０１でＣ_N と Ｃ_N の上限値(Ｃ_N )ｃとの大小を比較する。Ｃ_N ≦(Ｃ_N )ｃの場合検出を継続し、Ｃ_N ＞(Ｃ_N )ｃの場合ステップ１００９に進む。
【００８７】
図１４に本発明の排ガス浄化装置の他の実施態様を示す。図１の態様との相違は、エンジン近くの排気ダクトにマニホールド触媒１７を設けた点にある。自動車排ガスの排出規制の強化は、エンジン起動直後に排出されるＨＣ等の有害物の浄化を必要としている。すなわち従来は触媒が作動温度に達するまで未処理で排出されていたが、この量を大幅に低減する必要がある。これには、触媒を作動温度まで急速に昇温する方法が有効である。図１４はエンジン起動時のＨＣ，ＣＯ排出量低減と、リーン及びストイキ（含むリッチ）運転における排ガス浄化に対応できる装置構成である。図１４の構成においてマニホールド触媒１７にはＰｔ，Ｒｈ，ＣｅＯ₂ を主たる成分とするいわゆる三元触媒やこれらにＰｄを添加したりあるいはＰｄ等の燃焼活性成分を中心成分とした燃焼触媒が適用できる。本構成では、起動時にはマニホールド触媒１７が短時間で昇温してＨＣやＣＯの浄化を起動直後から行い、ストイキ運転時にはマニホールド触媒と吸着触媒１８の双方が機能してＨＣ，ＣＯ，ＮＯｘの浄化を行い、リーン運転時は吸着触媒が ＮＯｘを吸着浄化する。吸着触媒の再生にあたり空燃比をリッチシフトすると還元剤としてのＨＣ，ＣＯはマニホールド触媒で大きな化学変化を受けることなく吸着触媒に到達し、これを再生する。このような構成を可能とするのは吸着触媒の大きなな特長である。
【００８８】
図１５に本発明の排ガス浄化装置のさらに他の実施態様を示す。図１の態様との相違は、エンジン９９が筒内噴射方式である点にある。本発明の装置は筒内噴射方式エンジンにも良好に適用することができる。
【００８９】
図１６に本発明の排ガス浄化装置のさらに他の実施態様を示す。図１及び１５の態様との相違は、吸着触媒の下流に後触媒２４を設けたことにある。たとえば後触媒に燃焼触媒を置くことによりＨＣ浄化能を向上させた装置が、三元触媒を置くことによりストイキ時の三元機能を強化させた装置が、実現する。
【００９０】
図１７に本発明の排ガス浄化装置のさらに他の実施態様を示す。図１及び１４−１６の図との相違は、リッチシフトの指示により、還元剤インジェクタ２３を通じて吸着触媒上流に燃料を添加することにある。本方式ではエンジンの運転状態を吸着触媒の状態と無関係に設定することができるという大きな利点がある。
以下、具体例を挙げて本発明の効果を説明する。
【００９１】
本発明の吸着触媒及び装置の排ガス浄化性能を評価した。排気量１.８Ｌ のリーンバーン仕様車に吸着触媒及び比較触媒を搭載し、シャシダイナモメータ上で走行させた。両供試触媒は容積１.７Ｌのハニカム状（４００cell／ｉｎ²）で、７００℃で５時間酸化雰囲気で熱処理したものを床下に置いた。走行は、定常走行及び国内排ガス規制測定法に基づく１０−１５モード走行とした。排ガス分析は自動車排気ガス測定装置を用いダイレクト分析で排ガス中のＮＯｘ，ＨＣ， ＣＯの濃度を測定する方法と、自動車定容量希釈サンプリング装置でＣＶＳ （Constant Volume Sampling）を求める方法を適用した。
【００９２】
なお、１０−１５モード走行においては、１０モード及び１５モードの定常走行時と１０モードの２０ｋｍ／ｈから４０ｋｍ／ｈへの加速時及び１５モードの４０ｋｍ／ｈから６０ｋｍ／ｈへの加速時と５０ｋｍ／ｈから７０ｋｍ／ｈへの加速時をリーン（Ａ／Ｆ＝２２〜２３）走行としその他をストイキ走行とした。図１８に、本発明の方法による吸着触媒Ｎ−Ｎ９を搭載した場合、吸着触媒Ｎ−Ｋ９を搭載した場合、また比較の触媒Ｎ−Ｒ２を搭載した場合の、３度繰返される１０モードの最後の１０モードとそれに続く１５モードにおける吸着触媒前後のＮＯｘ濃度を示した。比較触媒は後掲の表２に示す組成のものとした。
【００９３】
図１８において、吸着触媒Ｎ−Ｎ９およびＮ−Ｋ９を搭載した場合全運転域において出口ＮＯｘ濃度は入口ＮＯｘ濃度を下回り、リーン運転とストイキ運転が繰返されることにより吸着触媒が効果的に再生されＮＯｘ浄化機能を保持し続けていることが分かる。一方、比較触媒Ｎ−Ｒ２においては出口ＮＯｘ濃度が入口ＮＯｘ濃度を上回る部分が生じている。
【００９４】
各種吸着触媒および比較触媒で得たＣＶＳ値を吸着触媒組成とともに表２及び３に示した。吸着触媒および比較触媒の調製は前述の方法によったが、調製原料として、シリコン（Ｓｉ）にはシリカゾルを用いた。Ｓｉはシリカ（ＳｉＯ₂ ）もしくはその複合酸化物として存在すると推定される。
【００９５】
【表２】

【００９６】
【表３】

【００９７】
【発明の効果】
以上から明らかな様に、本発明の装置によれば、排ガス流路にＮＯｘ吸着触媒を設け、リーン排ガスの酸化雰囲気でＮＯｘを吸着捕捉し還元雰囲気をつくって吸着触媒を再生することにより、リーンバーン排ガス中のＮＯｘ等を、燃費に大きな影響を与えることなく高効率で浄化できる。
【図面の簡単な説明】
【図１】本発明の代表的な実施態様を示す本発明の方法による排ガス浄化装置の構成図。
【図２】本発明の方法によりリッチ運転とリーン運転を交互に繰返したときのＮＯｘ浄化率の経時特性。
【図３】リーン排ガス中のＮＯｘ濃度とＮＯｘ浄化率の関係。
【図４】ストイキ排ガス中のＮＯｘ浄化率。
【図５】リッチ（ストイキ）運転からリーン運転に切替えたときの吸着触媒入口ＮＯｘ濃度と出口ＮＯｘ濃度の関係。
【図６】リッチ（ストイキ）運転からリーン運転に切替えたときの吸着触媒入口ＮＯｘ濃度と出口ＮＯｘ濃度の関係。
【図７】空燃比の制御方法を示すブロック線図。
【図８】空燃比の制御方法を示すフローチャート。
【図９】リーン運転時のＮＯｘ排出量の積算方法を示すフローチャート。
【図１０】図８のフローチャートにおけるＮＯｘ量推算部分。
【図１１】図８のフローチャートにおけるＮＯｘ量推算部分。
【図１２】図８のフローチャートにおけるＮＯｘ量推算部分。
【図１３】図８のフローチャートにおけるＮＯｘ量推算部分。
【図１４】マニホールド触媒を設けた実施態様を示す装置の構成図。
【図１５】筒内噴射エンジンにおける実施態様を示す装置の構成図。
【図１６】後触媒を設けた実施態様を示す装置の構成図。
【図１７】吸着触媒の上流に還元剤を添加する実施態様を示す装置構成図。
【図１８】モード運転したときのＮＯｘ浄化特性図。
【図１９】モデルガスを用いて酸素濃度を変化させたときのＮＯｘ浄化特性を示すグラフ。
【図２０】モデルガスを用いて酸素濃度を変化させたときのＮＯｘ浄化特性を示すグラフ。
【符号の説明】
１…エアクリーナー、２…エアフローセンサー、３…スロットルバルブ、５…インジェクタ、６…点火プラグ、７…アクセルペダル、８…負荷センサー、９…吸気温度センサー、１２…燃料ポンプ、１３…燃料タンク、１７…マニホールド触媒、１８…吸着触媒、１９…酸素センサー、２０…吸着触媒温度センサー、
２１…排ガス温度センサー、２２…ＮＯｘ濃度サンサー、２３…還元剤インジェクター、２４…後触媒、２５…ＥＣＵ、２６…ノックセンサー、２８…水温サンサー、２９…クランク角センサー、９９…エンジン。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an apparatus for purifying exhaust gas discharged from an internal combustion engine such as an automobile, and more particularly to purification of exhaust gas discharged from an internal combustion engine that can be operated at a lean air-fuel ratio (lean burn) and an automobile equipped with the internal combustion engine. Relates to the device.
[0002]
[Prior art]
Carbon monoxide (CO), hydrocarbons (HC: Hydrocarbon), nitrogen oxides (NOx), etc. contained in exhaust gas discharged from internal combustion engines such as automobiles are harmful to the human body as air pollutants. It causes problems such as preventing growth. Therefore, a great deal of effort has been made to reduce these emissions, and in addition to reducing the amount generated by improving the combustion method of internal combustion engines, development of a method to purify the exhaust gas using a catalyst etc. Has been steadily achieved. With respect to gasoline engine vehicles, a method using a catalyst that uses Pt and Rh, which are three-way catalysts, as active main components and detoxifies them by simultaneously oxidizing HC and CO and reducing NOx has become the mainstream.
[0003]
By the way, the three-way catalyst effectively works only on the exhaust gas generated by burning near the theoretical air fuel ratio called a window. Conventionally, the air-fuel ratio fluctuates in accordance with the driving condition of the automobile, but the fluctuation range is in principle the theoretical air-fuel ratio gasoline A (weight of air) / F (weight of fuel) = about 14.7; In the book, the theoretical air twist ratio is represented by A / F = 14.7, but this value varies depending on the fuel type. ) Has been adjusted in the vicinity. However, if the engine can be operated at a leaner air / fuel ratio than the stoichiometric air / fuel ratio, fuel efficiency can be improved. Therefore, the development of lean burn combustion technology has been promoted, and recently an internal combustion engine in a lean region with an air / fuel ratio of 18 or more. It is not uncommon for cars to burn. However, when the lean burn exhaust gas is purified with the current three-way catalyst as described above, the HC and CO can be oxidized and purified, but NOx cannot be effectively reduced and purified. Therefore, in order to promote the application of lean burn to large vehicles and the expansion of lean burn combustion time (expansion of lean burn applied operating range), lean burn compatible exhaust gas purification technology is required. Therefore, lean-burn exhaust purification technology, that is, oxygen (O₂), A technology for purifying HC, NO, NOx in exhaust gas containing a large amount of NOx, especially a technology for purifying NOx, has been energetically advanced.
[0004]
In Japanese Patent Laid-Open No. 63-61708, HC is supplied upstream of lean burn exhaust gas, and O in the exhaust gas is supplied.₂A method has been proposed in which the concentration of the catalyst is lowered by reducing the concentration to a concentration range where the catalyst functions effectively.
[0005]
JP-A-62-297630, JP-A-62-106826, and JP-A-62-117620 disclose NOx in exhaust gas (NO is oxidized and easily absorbed by NO).₂After conversion to NO), the catalyst is absorbed and removed by contacting with a catalyst having NOx absorption ability.₂, A method of reducing and removing NOx accumulated using a reducing agent such as HC such as methane and gasoline to regenerate the NOx absorption capacity of the catalyst is shown.
[0006]
In addition, in PCT / JP92 / 01279 and PCT / JP92 / 01330, a NOx absorbent that absorbs NOx when the exhaust gas is lean and releases the absorbed NOx when the oxygen concentration in the exhaust gas is reduced is installed in the exhaust passage, NOx is absorbed when the exhaust gas is lean, and the absorbed NOx flows into the NOx absorbent.₂There has been proposed an exhaust emission control device that lowers the concentration and discharges it.
[0007]
However, in Japanese Patent Laid-Open No. 63-61708, the exhaust gas composition (O / F) corresponding to about 14.7 (A / F) which is the air-fuel ratio at which the catalyst functions (O / F)₂A large amount of HC is required to achieve a concentration of about 0.5%. Although the use of blow-by gas of the present invention is effective, it is not an amount sufficient for treating exhaust gas during operation of the internal combustion engine. Although it is not technically impossible to add fuel, it results in a reduction in fuel consumption that was saved by the lean burn method.
[0008]
In JP-A-62-297630, JP-A-62-106826, and JP-A-62-117620, in the regeneration of the NOx absorbent, the circulation of the exhaust gas is stopped to bring the reducing agent such as HC into contact with the NOx absorbent. , O in the exhaust gas of the reducing agent₂Combustion consumption due to is greatly suppressed and the amount of reducing agent used is drastically reduced. However, it is undeniable that the structure of the exhaust gas treatment apparatus becomes complicated because two exhaust gas switching mechanisms for providing two NOx absorbents and alternately circulating the exhaust gas are required.
[0009]
Further, in PCT / JP92 / 01279 and PCT / JP92 / 01330, exhaust gas is always circulated through the NOx absorbent, and when the exhaust gas is lean, it absorbs NOx, and O in the exhaust gas.₂Since the absorbent is regenerated by releasing the absorbed NOx by reducing the concentration, it is not necessary to switch the exhaust gas flow, and the problem of the above method is solved. However, NOx is absorbed when the exhaust gas is lean and O in the exhaust gas.₂The premise is the application of a material that can release NOx when the concentration is lowered. In the case of this material, absorption and release of NOx inevitably repeats periodic changes in the crystal structure of the absorbent, and careful consideration of durability is required. Further, when it is necessary to treat the released NOx and a large amount of it is released, it is necessary to consider post-treatment with a three-way catalyst.
[0010]
[Problems to be solved by the invention]
In view of the above-described problems of the prior art, the present invention has a simple exhaust treatment device structure, consumes a small amount of reducing agent, and is excellent in durability. An object of the present invention is to provide a device capable of effectively removing and detoxifying harmful components.
[0011]
[Means for Solving the Problems]
The above problems can be solved by the following methods of the present invention.
[0012]
In the present invention, in the redox stoichiometric relationship between the components in the exhaust gas, NOx is chemisorbed in a state where the oxidizing agent is more than the reducing agent, and the reducing agent is adsorbed in the same amount or more to the oxidizing agent. A NOx adsorption catalyst for catalytic reduction of NOx is placed in the exhaust gas flow path, and in the oxidation-reduction stoichiometric relationship between the components in the exhaust gas, a state in which the oxidizing agent is more than the reducing agent is used to chemistry NOx on the adsorption catalyst. Next, a state in which the amount of the reducing agent is equal to or more than that of the oxidizing agent is made to adsorb, and NOx adsorbed on the adsorption catalyst is contacted with the reducing agent to react with N.₂Reduced to harmless.
[0013]
Here, the adsorption catalyst refers to a material having the ability to adsorb a substance such as NOx and simultaneously having a catalytic function. In the present invention, it refers to a material having the ability to adsorb and capture NOx, the ability to catalytically reduce NOx, and the ability to catalytically oxidize HC, CO and the like.
[0014]
The oxidizing agent is O₂, NO, NO₂Etc., mainly oxygen. The reducing agent is HC supplied to an internal combustion engine, HC (including oxygen-containing hydrocarbons), CO, H as its derivative produced by combustion excessive₂Furthermore, it is a reducing substance such as HC which is added to the exhaust gas as a reducing component described later.
[0015]
As mentioned above, HC, CO, H as reducing agents for reducing lean exhaust gas and NOx to nitrogen₂And so on, these are O as an oxidant in the exhaust gas.₂Causes a combustion reaction. NOx (NO and NO₂) Also reacts with these and is reduced to nitrogen. Usually, both reactions proceed in parallel, so the utilization rate of the reducing agent is low in the presence of oxygen. In particular, the proportion of the latter is considerably increased at a high temperature of 500 ° C. or higher (depending on the catalyst material). Therefore, NOx is separated from the exhaust gas by the adsorption catalyst (at least O in the exhaust gas)₂NOx N by contact reaction with a reducing agent.₂Can be effectively reduced. In the present invention, NOx in the lean exhaust gas is adsorbed and removed by the NOx adsorption catalyst to remove NOx in the exhaust gas as O.sub.x.₂Separate from.
[0016]
In the present invention, next, an oxidizing agent (O₂, NOx, etc.) and reducing agents (HC, CO, H₂Etc.) in the redox system composed of the same amount or excellent condition, NOx adsorbed on the adsorption catalyst is contacted with a reducing agent such as HC to react with N₂To reduce.
[0017]
By the way, NOx in exhaust gas is almost NO and NO.₂Consists of. NO₂Is more reactive than NO. Therefore NO₂Adsorption removal and reduction are easier than NO. Therefore NO to NO₂If it is oxidized, NOx adsorption removal and reduction in the exhaust gas are facilitated. In the present invention, Ox coexists with NOx in lean exhaust gas.₂NO₂This includes a method of oxidizing and removing the catalyst, and an oxidation means for that purpose, for example, providing the adsorption catalyst with a NO oxidation function or providing an oxidation catalyst in the preceding stage of the adsorption catalyst.
[0018]
In the present invention, the reduction reaction of chemisorbed NOx can be roughly described by the following reaction formula.
[0019]
M-NO_Three+ HC → MO + N₂+ CO₂+ H₂O → MCO_Three+ N₂+ H₂O
Where M is a metal element
(MCO as reduction product_ThreeThe reason for adopting the
The above reaction is an exothermic reaction. Taking the alkali metal and the alkaline earth metal as the metal M, and evaluating the heat of reaction by representing Na and Ba respectively, the following is obtained in the standard state (1 atm, 25 ° C.).
[0020]
2NaNO_Three(s) + 5 / 9C_ThreeH₆→ Na₂CO_Three(s) + N₂+ 2 / 3CO₂
+ 5 / 3H₂O
[-ΔH = 873 kjule]
Ba (NO_Three)₂+ 5 / 9C_ThreeH₆→ BaCO_Three(s) + N₂+ 2 / 3CO₂
+ 5 / 3H₂O
[-ΔH = 751 kjule]
Where s: solid g: gas
The corresponding solid value was used as the thermodynamic quantity of the adsorbed species.
[0021]
By the way, C_ThreeH₆The combustion heat of 5/9 mole is 1070 kjule, and each of the above reactions is a calorific value comparable to that of HC. As a matter of course, this heat generation is transmitted to the exhaust gas in contact with it, and a local temperature rise on the surface of the adsorption catalyst is suppressed.
[0022]
When the NOx trapping agent is a NOx absorbent, NOx trapped in the bulk of the absorbent is also reduced, resulting in a large amount of heat generation, and there is a limit to transmission to the exhaust gas, leading to an increase in the temperature of the absorbent. This exotherm shifts the equilibrium of the absorption reaction shown in the following equation to the release side.
[0023]

[0024]
Even if the concentration of the reducing agent is increased to reduce the released NOx quickly and reduce the NOx concentration in the exhaust gas discharged out of the apparatus, NO in the gas phase₂The reaction between HC and HC does not progress much. Therefore, the amount of NOx released cannot be reduced sufficiently by increasing the reducing agent. Although it is conceivable to perform a reduction reaction operation at a stage where the amount of NOx absorbed is small, the regeneration frequency of the NOx absorbent increases, which is not practical.
[0025]
Since the adsorption catalyst of the present invention captures NOx only in the vicinity of its surface, the absolute amount of heat generation is small, and since it is quickly transmitted to the exhaust gas, the temperature rise of the adsorption catalyst is small. Therefore, once trapped NOx can be prevented from being released.
[0026]
The NOx adsorption catalyst of the present invention is characterized as a material that captures NOx on its surface by chemical adsorption and does not cause NOx release by an exothermic reaction during NOx reduction. The NOx adsorption catalyst of the present invention is characterized as a material that captures NOx on its surface by chemical adsorption or by a chemical bond near the surface and does not cause NOx release by an exothermic reaction during NOx reduction.
[0027]
The inventors of the present invention are NOx adsorption catalysts containing at least one element selected from potassium (K), sodium (Na), magnesium (Mg), strontium (Sr), and calcium (Ca) as a component. We found that the feature can be realized.
[0028]
The exhaust gas purifying apparatus for an internal combustion engine according to the present invention includes at least potassium (K), sodium (Na), magnesium (Mg), strontium (Sr), and calcium.
A NOx adsorption catalyst containing at least one element selected from (Ca) as a component is disposed in the exhaust gas flow path, and the oxidizing agent is present with respect to the reducing agent in a redox stoichiometric relationship between the components in the exhaust gas. The NOx is chemisorbed on the adsorbing catalyst by creating a large number of states, and then the reducing agent is in the same amount or more with respect to the oxidizing agent.₂It is characterized by being rendered harmless by reduction.
[0029]
The exhaust gas purifying apparatus for an internal combustion engine of the present invention also includes at least one element selected from potassium (K), sodium (Na), magnesium (Mg), strontium (Sr) and calcium (Ca) as a part of the component. NOx adsorbing catalyst contained in the exhaust gas passage is placed in the exhaust gas flow path, and O in the redox stoichiometric relationship with respect to a reducing agent such as HC.₂NOx trapped by the adsorption catalyst by creating a state where there is a large amount of oxidant such as NOx trapped by chemical bonding on the surface of the adsorption catalyst and in the vicinity of the surface, and then creating a state where the amount of reducing agent is the same or greater than the oxidant Is contacted with a reducing agent to form N₂It is characterized by being rendered harmless by reduction.
[0030]
As the NOx adsorption catalyst in the present invention, the following can be suitably applied.
[0031]
From at least one selected from potassium (K), sodium (Na), magnesium (Mg), strontium (Sr) and calcium (Ca), at least one selected from rare earths such as cerium, platinum, rhodium, palladium, etc. A composition comprising a metal and a metal oxide (or composite oxide) containing at least one element selected from the above-mentioned noble metals, and a composition obtained by supporting the composition on a porous heat-resistant metal oxide. The present composition has excellent SOx resistance in addition to excellent NOx adsorption ability.
[0032]
In the method of the present invention, the state in which the amount of reducing agent is the same or larger than the oxidizing agent can be produced by the following method.
[0033]
The combustion condition in the internal combustion engine is the stoichiometric air-fuel ratio or excess fuel (rich). A reducing agent is added to the lean burn exhaust gas.
[0034]
The former can be achieved by the following method.
[0035]
A method of controlling the fuel injection amount in accordance with an oxygen concentration sensor output, an intake flow rate sensor output, etc. provided in the exhaust duct. In this method, a part of the plurality of cylinders is excessively fueled and the remaining part is insufficiently fueled, and the components in the mixed exhaust gas from all the cylinders have the same or more reducing agent than the oxidant in the redox stoichiometric relationship. Also includes a method of creating a state.
[0036]
The latter can be achieved by the following methods.
[0037]
A method of introducing a reducing agent upstream of the adsorption catalyst in the exhaust gas flow. As the reducing agent, gasoline, light oil, kerosene, natural gas, reformed products thereof, hydrogen, alcohols, ammonia and the like as fuel for the internal combustion engine can be applied. It is also effective to introduce blow-by gas and canister purge gas upstream of the adsorption catalyst and introduce a reducing agent such as hydrocarbon contained therein. In a direct fuel injection internal combustion engine, it is effective to inject fuel in the exhaust stroke and to inject fuel as a reducing agent.
[0038]
The adsorption catalyst in the present invention can be applied in various shapes. The present invention can be applied to pellets, plates, granules, and powders, including honeycombs obtained by coating an adsorption catalyst component on a honeycomb structure made of a metal material such as cordierite and stainless steel.
[0039]
In the present invention, the timing for producing a state in which the amount of the reducing agent is the same or larger than that of the oxidizing agent can be determined by the following methods.
[0040]
Estimate NOx emissions during lean operation from air-fuel ratio setting signal determined by ECU (Engine Control Unit), engine speed signal, intake air amount signal, intake pipe pressure signal, speed signal, throttle opening, exhaust gas temperature, etc. When the integrated value exceeds a predetermined set value.
[0041]
When the accumulated oxygen amount exceeds the predetermined amount when the accumulated oxygen amount is detected by the signal of the oxygen sensor (or A / F sensor) placed upstream or downstream of the adsorption catalyst in the exhaust passage.
[0042]
As a variation thereof, when the cumulative oxygen amount during lean operation exceeds a predetermined amount.
[0043]
When the accumulated NOx amount is calculated from the NOx sensor signal placed upstream of the adsorption catalyst in the exhaust passage, and the accumulated NOx amount during the lean operation exceeds a predetermined amount.
[0044]
When the NOx concentration during lean operation is detected by a signal from the NOx sensor placed in the downstream of the adsorption catalyst in the exhaust passage, and the NOx concentration exceeds a predetermined concentration.
[0045]
In the present invention, as described above, the time for maintaining the amount of reducing agent equal to or greater than the oxidizing agent or the amount of reducing agent to be added to maintain the properties of the adsorption catalyst, the specifications and characteristics of the internal combustion engine, etc. However, these can be realized by adjusting the stroke of the fuel injection valve, the injection time, and the injection interval.
[0046]
DETAILED DESCRIPTION OF THE INVENTION
The present invention will be described in detail with reference to specific embodiments of the present invention. In addition, this invention is not limited to the following embodiment and an Example, It cannot be overemphasized that there are various embodiments within the thought range.
[0047]
[Adsorption catalyst]
The characteristics of the adsorption catalyst according to the method of the present invention will be described. The characteristics of N-N9 containing Na as an alkali metal and N-K9 containing K are as follows.
[0048]
《Adsorption catalyst preparation method》
Adsorption catalyst N-N9 was obtained by the following method.
[0049]
Alumina sol as a binder obtained by nitric acid-separating alumina powder and boehmite was mixed to obtain a nitric acid acidic alumina slurry. The honeycomb was dipped in the coating solution and then immediately pulled up, and the liquid blocked in the cell was removed by air blowing, followed by drying and subsequent firing at 450 ° C. This operation was repeated to coat 150 g of alumina per liter of apparent volume of the honeycomb. A catalytically active component was supported on the alumina-coated honeycomb to obtain a honeycomb-like adsorption catalyst. For example, it was impregnated with a cerium nitrate (Ce nitrate) solution, dried, and then fired at 600 ° C. for 1 hour. Subsequently, it was impregnated with a mixed solution of a sodium nitrate (Na nitrate) solution, a titania sol solution, and a magnesium nitrate (Mg nitrate) solution, and similarly dried and fired. Furthermore, it was impregnated with a mixed solution of dinitodiammine Pt nitric acid solution and rhodium nitrate (nitric acid Rh) solution, dried and calcined at 450 ° C. for 1 hour. Finally, it was impregnated with an Mg nitrate solution and calcined at 450 ° C. for 1 hour. As described above, alumina (Al₂O_Three) Honeycomb-like adsorption catalyst carrying Ce, Mg, Na, Ti, Rh, Pt, 2Mg- (0.2Rh, 2.7Pt)-(18Na, 4Ti, 2Mg) -27Ce / Al₂O_ThreeGot. Where / Al₂O_ThreeThe active ingredient is Al₂O_ThreeThe numerical value before the element symbol is the weight (g) of the display metal component supported per 1 L of honeycomb apparent volume. The notation order indicates the loading order, Al₂O_ThreeThe components listed in parentheses are carried in the order of the components that are separated from the components indicated in the order of (). Incidentally, the loading amount of each active ingredient can be changed by changing the active ingredient concentration in the impregnation solution.
[0050]
Adsorption catalyst NK9 was prepared by the following method.
[0051]
Instead of the sodium nitrate solution in the preparation of the adsorption catalyst N-N9, a potassium nitrate (nitric acid K) solution was used, and the other methods were the same as those of the adsorption catalyst N-N9. , 4Ti, 2Mg) -27Ce / Al₂O_ThreeGot. In the same manner, the comparative catalyst N-R2 2Mg- (0.2Rh, 2.7Pt) -27Ce / Al₂O_ThreeGot.
[0052]
《Performance evaluation method》
The adsorption catalyst obtained by the above method was heat-treated at 700 ° C. for 5 hours in an oxidizing atmosphere, and then the characteristics were evaluated by the following method.
[0053]
A passenger car equipped with a 1.8 L lean burn gasoline engine was loaded with a 1.7 L honeycomb adsorbent catalyst prepared by the method of the present invention, and the NOx purification characteristics were evaluated.
[0054]
<Characteristics of adsorption catalyst>
The adsorption catalyst N-N9 was mounted, and the NOx purification rate aging characteristics of FIG. 2 were obtained by alternately repeating the rich operation for 30 seconds with A / F = 13.3 and the lean operation for about 20 minutes with A / F = 22. From this figure, it can be seen that this adsorption catalyst purifies NOx during the lean operation period. During the lean operation, the NOx purification rate gradually decreases, and the purification rate that was 100% in the initial stage becomes approximately 40% after 20 minutes. However, this reduced purification rate recovers to 100% after 30 seconds of rich operation. When the lean operation is performed again, the NOx purification ability is restored and the above-mentioned change with time is repeated. Even if the lean operation and the rich operation are repeated a plurality of times, the rate of decrease in the NOx purification rate with time during the lean operation remains unchanged, which indicates that the NOx adsorption performance has been sufficiently regenerated by the rich operation.
[0055]
Relationship between NOx concentration and NOx purification rate in lean exhaust gas by changing the ignition timing and changing NOx concentration in exhaust gas with vehicle speed constant at about 40 km / h (exhaust gas space velocity (SV) about 20,000 / h constant) To obtain FIG. The NOx purification rate decreases with time, but the lower the NOx concentration, the lower the rate of decrease. Table 1 shows the amounts of NOx trapped up to NOx purification rates of 50% and 30%.
[0056]
[Table 1]

[0057]
The NOx trapping amount is almost constant regardless of the NOx concentration. It is a characteristic of chemisorption that the amount of adsorption does not depend on the concentration (pressure) of the adsorbate.
[0058]
Pt particles are first considered as NOx adsorption soot in the test adsorption catalyst. When CO adsorption amount evaluation frequently used as a means for evaluating the amount of exposed Pt was performed, the CO adsorption amount (at 100 ° C.) was 4.5 × 10.^-Fourmol. This value is about 1/100 of the NOx adsorption amount, and it is clear that Pt is not the main role of NOx adsorption soot.
[0059]
On the other hand, the BET specific surface area (measured by nitrogen adsorption) measured for each cordierite of this adsorption catalyst is about 25 m.²/ G at 28,050m per 1.7L of honeycomb²Met. In addition, when the chemical structure of Na of the adsorption catalyst of the present invention was examined, CO was added to the mineral acid.₂Judging from the value of the inflection point in the neutralization titration curve by the generation and dissolution of gas and the neutral acid, mainly Na₂CO_ThreeI was able to judge that it exists. Suppose all surfaces are Na₂CO_ThreeThe surface is 0.275 mol Na₂CO_ThreeWill be exposed (Na₂CO_ThreeSince the specific gravity of 2.533 g / ml is Na₂CO_ThreeThe volume of one molecule is obtained (Na₂CO_ThreeAssuming that the surface is a cube, the area of one surface is obtained and this is the surface Na₂CO_ThreeOccupied area). According to the above reaction formula, 0.275 mol of Na₂CO_ThreeIs 0.55 mol of NO₂Has the ability to adsorb. However, the amount of NOx actually removed by the adsorption catalyst of the present invention is on the order of 0.04 mol, which is 1/10 or less. This difference is because the BET method evaluates the physical surface area.₂O_ThreeNa₂CO_ThreeIt is because the surface area other than is evaluated. The above evaluation shows that the amount of adsorbed NOx is Na₂CO_ThreeMuch less than bulk NOx trapping capacity, at least NOx is Na₂CO_ThreeIt shows that it is captured in a limited area on or near the surface.
[0060]
In FIG. 3, the rate of reduction of the purification rate is reduced from about 20% of the NOx purification rate, which indicates that a reduction reaction by the catalytic function occurs.
[0061]
FIG. 4 shows the NOx purification rate immediately after switching from lean operation to stoichiometric operation. It can be seen that with the present adsorption catalyst, a NOx purification rate of 90% or more can be obtained immediately after switching to the stoichiometric operation.
[0062]
5 and 6 show the NOx purification characteristics before and after switching from lean to stoichiometric or rich. FIG. 5 shows the NOx concentration at the inlet and outlet of the adsorption catalyst N-N9. FIG. 5A shows the case where the air-fuel ratio is switched from lean at A / F = 22 to rich at A / F = 14.2. It is. At the start of regeneration immediately after the rich switching, the exhaust gas NOx concentration at A / F = 14.2 is high, so that the inlet NOx concentration in the rich operation greatly increases. With this, the outlet NOx concentration increases transiently. The outlet NOx concentration is much lower than the inlet NOx concentration. The regeneration proceeds promptly and the outlet NOx concentration reaches near 0 in a short time. Fig. (B) shows the case where the air-fuel ratio is switched from lean (A / F = 22) to rich (A / F = 13.2). As in Fig. (A), the outlet NOx concentration is always equal to the inlet NOx concentration. The outlet NOx concentration reaches near 0 in a shorter time and in a shorter time.
[0063]
As is apparent from the above, the A / F value as the reproduction condition affects the time required for reproduction. The A / F value, time, and amount of reducing agent suitable for regeneration are affected by the composition, shape, temperature, SV value, type of reducing agent, shape and length of the exhaust passage of the adsorption catalyst. Therefore, the reproduction conditions are determined comprehensively in consideration of these.
[0064]
FIG. 6 shows the NOx concentration at the inlet and outlet of the adsorption catalyst N-K9. FIG. 6A shows the case where the air-fuel ratio is switched from lean at A / F = 22 to rich at A / F = 14.2. FIG. 5B shows the case where the air-fuel ratio is switched from lean at A / F = 22 to rich at A / F = 13.2. However, as in the case of the adsorption catalyst N-N9, the outlet NOx concentration is always constant. Is far below the inlet NOx concentration, and the regeneration of the adsorption catalyst proceeds in a short time. <Basic characteristics of adsorption catalyst>
Model gas was used to evaluate the influence of oxygen concentration on basic characteristics, particularly NOx purification rate.
[0065]
A 6 ml small honeycomb of N-N9 was filled in a quartz semi-responding tube having an inner diameter of 28 mmφ, and a model gas was circulated. The effect of changing the oxygen concentration in the model gas on the NOx purification rate was examined. The reaction temperature was 300 ° C. at the catalyst inlet gas temperature.
[0066]
First, O₂5% (volume fraction; the same applies below), NO600ppm, C_ThreeH₆500ppm (1500ppm as Cl), CO1000ppm, CO₂10%, H₂O10% and N₂A gas having a composition of Balance was circulated, and after 10 minutes when the NOx purification rate was stabilized, only the oxygen concentration was reduced to a predetermined value and maintained for 20 minutes, and finally returned to the initial gas composition. During this period, the NOx concentration change was made by changing the oxygen concentration to be reduced to six types of 0%, 0.5%, 0.7%, 1%, 2% and 3%, thereby obtaining FIG. In FIG. 19, when the oxygen concentration in the lean gas decreases, that is, when the oxidizing atmosphere is weakened and the reducing atmosphere is strengthened, the NOx purification rate tends to increase, suggesting that the present adsorption catalyst purifies NOx by reduction. Yes. In FIG. 19, the NOx purification rate is always a positive value, and in this adsorption catalyst, the NOx concentration after passing through the adsorption catalyst does not increase regardless of the oxygen concentration.
[0067]
Next, O₂5%, NO600ppm, N₂FIG. 20 was obtained by changing the oxygen concentration in the gas in the same manner while circulating the gas having the composition of Balance. This study is characterized by the fact that no reducing agent is contained in the gas. In FIG. 20, even if the oxygen concentration is lowered to weaken the oxidizing atmosphere, the NOx purification rate is not improved. This suggests that the present adsorption catalyst purifies NOx by reduction. In FIG. 20, the NOx purification rate does not become negative, and in the present adsorption catalyst, once trapped NOx is not released by the oxidation-reduction atmosphere.
[0068]
[Exhaust gas purification device]
FIG. 1 is an overall configuration of an apparatus showing an embodiment of an exhaust gas purification apparatus of the present invention.
[0069]
The apparatus of the present invention includes a lean burnable engine 99, an air flow sensor 2 having an air flow sensor 2, a throttle valve 3, and the like, an oxygen concentration sensor (or A / F sensor) 19, an exhaust temperature sensor 21, a NOx adsorption catalyst 18, and the like. It consists of an exhaust system and a control unit (ECU). The ECU includes an I / O LSI as an input / output interface, an arithmetic processing unit MPU, a storage device RAM and ROM that store a large number of control programs, a timer counter, and the like.
[0070]
The above exhaust purification device functions as follows. The intake air to the engine is filtered by the air cleaner 1 and then measured by the air flow sensor 2. The fuel is injected from the injector 5 through the throttle valve 3 and supplied to the engine 99 as an air-fuel mixture. The airflow sensor signal and other sensor signals are input to an ECU (Engine Control Unit).
[0071]
The ECU evaluates the operating state of the internal combustion engine and the state of the NOx adsorption catalyst by a method described later to determine the operating air-fuel ratio, and controls the injection time of the injector 5 to set the fuel concentration of the air-fuel mixture to a predetermined value. The air-fuel mixture sucked into the cylinder is ignited and burned by a spark plug 6 controlled by a signal from the ECU 25. The combustion exhaust gas is led to an exhaust purification system. The exhaust purification system is equipped with a NOx adsorption catalyst. During the stoichiometric operation, the three-way catalyst function purifies NOx, HC, CO in the exhaust gas, and during the lean operation, the NOx is adsorbed by the NOx adsorption capacity and simultaneously has a combustion function. To purify HC and CO. Further, the NOx purification ability of the NOx adsorption catalyst is always judged by the ECU's judgment and control signal during lean operation, and if the NOx purification capacity is reduced, the combustion air-fuel ratio, etc. is shifted to the rich side and the NOx adsorption of the adsorption catalyst. Restore the ability. With the above operation, the present apparatus effectively purifies exhaust gas under all engine combustion conditions of lean operation and stoichiometric (including rich) operation.
[0072]
The fuel concentration (hereinafter referred to as air-fuel ratio) of the air-fuel mixture supplied to the engine is controlled as follows. FIG. 7 is a block diagram showing the air-fuel ratio control method.
[0073]
A load sensor output that outputs a signal corresponding to the depression of the accelerator pedal, an output signal of the intake air measured by the airflow sensor, an engine speed signal detected by the crank angle sensor, an exhaust gas temperature signal, and a throttle that detects the throttle opening The ECU 25 determines the air-fuel ratio (A / F) from information such as the sensor signal, the engine coolant temperature signal, the starter signal, and the like, and this signal is corrected based on the signal fed back from the oxygen sensor to determine the fuel injection amount. It should be noted that feedback control is stopped by signals from the sensors and switches at low temperatures, idle times, high loads, and the like. In addition, the air-fuel ratio correction learning function is used to accurately cope with subtle or sudden changes in the air-fuel ratio.
[0074]
When the determined air-fuel ratio is stoichiometric (A / F = 14.7) and rich (A / F <14.7), the injection conditions of the injector are determined by an instruction from the ECU, and stoichiometric and rich operation are performed. On the other hand, when the lean (A / F> 14.7) operation is determined, it is determined whether or not the NOx adsorption catalyst has the NOx adsorption capability, and if it is determined that the adsorption capability is present, the lean operation as instructed should be performed. When the fuel injection amount is determined and it is determined that there is no adsorption capacity, the air-fuel ratio is richly shifted for a predetermined period to regenerate the NOx adsorption catalyst.
[0075]
FIG. 8 shows a flowchart of air-fuel ratio control. In step 1002, signals for instructing various operating conditions or detecting an operating state are read. Based on these signals, the air-fuel ratio is determined in step 1003, and the determined air-fuel ratio is detected in step 1004. The air-fuel ratio determined in step 1005 is compared with the theoretical air-fuel ratio. The theoretical air-fuel ratio to be compared here is precisely the air-fuel ratio at which the NOx catalytic reduction reaction speed exceeds the trapping speed by adsorption in the adsorption catalyst, and is determined in advance by evaluating the characteristics of the adsorption catalyst. Thus, an air-fuel ratio in the vicinity of the theoretical air-fuel ratio is selected. Here, when the set air-fuel ratio ≦ theoretical air-fuel ratio, the routine proceeds to step 1006, and the air-fuel ratio operation is performed as instructed without performing the regeneration operation of the adsorption catalyst. If set air-fuel ratio> stoichiometric air-fuel ratio, the routine proceeds to step 1007. In step 1007, the NOx adsorption amount is estimated. The estimation calculation method will be described later. Subsequently, at step 1008, it is determined whether or not the estimated NOx adsorption amount is equal to or less than a predetermined limit amount. The limit adsorption amount is set to a value that can sufficiently purify NOx in the exhaust gas in advance by evaluating the NOx trapping characteristics of the adsorption catalyst through experiments or the like and considering the exhaust gas temperature, the adsorption catalyst temperature, and the like. If there is NOx adsorption capability, the routine proceeds to step 1006, and the air-fuel ratio operation is performed as instructed without performing the regeneration operation of the adsorption catalyst. When the NOx adsorption ability is not present, the routine proceeds to step 1009 and the air-fuel ratio is shifted to the rich side. In step 1010, the rich shift time is counted, and if the elapsed time Tr exceeds a predetermined time (Tr) c, the rich shift is terminated.
[0076]
Determination of NOx adsorption ability can be performed as follows.
[0077]
FIG. 9 shows a method of integrating and determining NOx emission from various operating conditions during lean operation.
[0078]
In step 1007-E01, a signal related to the operating condition of the NOx adsorption catalyst such as the exhaust gas temperature and a signal related to various engine operating conditions that affect the NOx concentration in the exhaust gas are read, and the NOx amount E adsorbed per unit time._NIs estimated. In step 1007-E02_NIn step 1008-E01, the integrated value ΣEN and the upper limit value (E_N) Compare the size with c. ΣEN ≦ (E_N) In case of c, the integration is continued and ΣE_N> (E_N) In the case of c, the integration is canceled in step 1008-E02 and the process proceeds to step 1009.
[0079]
FIG. 10 shows a method of determining based on the accumulated time of lean operation.
[0080]
In step 1007-H01, the lean operation time H_LIs integrated, and in step 1008-H01, the integrated value ΣH_LAnd the upper limit of accumulated time (H_L) Compare the size with c. ΣH_L≦ (H_L) In case of c, continue integration and ΣH_L> (H_L) In the case of c, the integration is canceled in step 1008-H02 and the process proceeds to step 1009.
[0081]
FIG. 11 shows a method of determination based on an oxygen sensor signal during lean operation.
[0082]
Step 1007-O01 Oxygen amount Q in lean operation_OIs integrated, and in step 1008-O01, the integrated value ΣQ_OAnd the upper limit of accumulated oxygen (Q_O) Compare the size with c. ΣQ_O≦ (Q_O) In case of c, continue integration and ΣQ_O> (Q_O) In the case of c, the integration is canceled in Step 1008-O02 and the process proceeds to Step 1009.
[0083]
FIG. 12 shows a method of determination based on the NOx concentration sensor signal detected at the NOx adsorption catalyst inlet during the lean operation.
[0084]
In step 1007-N01, the NOx amount Q at the NOx adsorption catalyst inlet based on the NOx concentration sensor signal._NIs accumulated. In step 1008-N01, the integrated value ΣQ_NAnd the upper limit of accumulated NOx amount (Q_N) Compare the size with c. ΣQ_N≦ (Q_N) In case of c, continue integration and ΣQ_N> (Q_N) In the case of c, the integration is canceled at step 1008-N02 and the process proceeds to step 1009.
[0085]
FIG. 13 shows a method for determination based on the NOx concentration sensor signal detected at the NOx adsorption catalyst outlet during lean operation.
[0086]
In step 1007-C01, the NOx concentration CN at the NOx adsorption catalyst inlet is detected based on the NOx concentration sensor signal. In step 1008-C01, C_NAnd C_NUpper limit (C_N) Compare the size with c. C_N≦ (C_N) If c, continue detection and C_N> (C_N) If c, go to Step 1009.
[0087]
FIG. 14 shows another embodiment of the exhaust gas purifying apparatus of the present invention. 1 is that a manifold catalyst 17 is provided in an exhaust duct near the engine. The tightening of emission regulations for automobile exhaust gas requires purification of harmful substances such as HC discharged immediately after engine startup. That is, conventionally, the catalyst was discharged untreated until the operating temperature was reached, but this amount needs to be greatly reduced. For this purpose, a method of rapidly raising the temperature of the catalyst to the operating temperature is effective. FIG. 14 shows an apparatus configuration that can cope with HC and CO emission reduction at engine startup and exhaust gas purification in lean and stoichiometric (including rich) operation. In the configuration of FIG. 14, the manifold catalyst 17 has Pt, Rh, CeO.₂A so-called three-way catalyst having a main component as a catalyst, a Pd added thereto, or a combustion catalyst having a combustion active component such as Pd as a central component can be applied. In this configuration, the temperature of the manifold catalyst 17 is raised in a short time at the time of start-up, and the purification of HC and CO is performed immediately after the start-up. During lean operation, the adsorption catalyst adsorbs and purifies NOx. When the air-fuel ratio is richly shifted during regeneration of the adsorption catalyst, HC and CO as reducing agents reach the adsorption catalyst without undergoing a large chemical change in the manifold catalyst and regenerate it. This configuration is a major feature of the adsorption catalyst.
[0088]
FIG. 15 shows still another embodiment of the exhaust gas purifying apparatus of the present invention. 1 is that the engine 99 is a cylinder injection system. The apparatus of the present invention can be applied well to a cylinder injection engine.
[0089]
FIG. 16 shows still another embodiment of the exhaust gas purifying apparatus of the present invention. 1 and 15 is that a post-catalyst 24 is provided downstream of the adsorption catalyst. For example, a device that improves the HC purification performance by placing a combustion catalyst in the rear catalyst, and a device that enhances the three-way function during stoichiometry by placing a three-way catalyst are realized.
[0090]
FIG. 17 shows still another embodiment of the exhaust gas purifying apparatus of the present invention. The difference from FIGS. 1 and 14-16 is that fuel is added upstream of the adsorption catalyst through the reducing agent injector 23 in response to a rich shift instruction. This method has a great advantage that the operating state of the engine can be set irrespective of the state of the adsorption catalyst.
Hereinafter, the effects of the present invention will be described with specific examples.
[0091]
The exhaust gas purification performance of the adsorption catalyst and apparatus of the present invention was evaluated. An adsorption catalyst and a comparative catalyst were mounted on a lean-burn vehicle with a displacement of 1.8L and run on a chassis dynamometer. Both test catalysts are in the form of honeycomb with a capacity of 1.7 L (400 cell / in²) And heat treated in an oxidizing atmosphere at 700 ° C. for 5 hours was placed under the floor. The traveling was 10-15 mode traveling based on steady traveling and domestic exhaust gas regulation measurement method. For exhaust gas analysis, a method of measuring NOx, HC, CO concentration in exhaust gas by direct analysis using an automobile exhaust gas measuring device and a method of obtaining CVS (Constant Volume Sampling) by an automobile constant volume dilution sampling device were applied.
[0092]
In 10-15 mode driving, during steady driving in 10 mode and 15 mode, acceleration from 20 km / h to 40 km / h in 10 mode, and acceleration from 40 km / h to 60 km / h in 15 mode During acceleration from 50 km / h to 70 km / h, lean (A / F = 22 to 23) travel was used, and the rest was stoichiometric travel. In FIG. 18, when the adsorption catalyst N-N9 according to the method of the present invention is installed, when the adsorption catalyst N-K9 is installed, or when the comparative catalyst N-R2 is installed, the last of 10 modes repeated three times The NOx concentrations before and after the adsorption catalyst in the 10 mode and the subsequent 15 mode are shown. The comparative catalyst had a composition shown in Table 2 below.
[0093]
In FIG. 18, when the adsorption catalysts N-N9 and N-K9 are mounted, the outlet NOx concentration is lower than the inlet NOx concentration in the entire operation range, and the adsorption catalyst is effectively regenerated by repeating the lean operation and the stoichiometric operation. It can be seen that the purification function is maintained. On the other hand, in the comparative catalyst N-R2, there is a portion where the outlet NOx concentration exceeds the inlet NOx concentration.
[0094]
The CVS values obtained with various adsorption catalysts and comparative catalysts are shown in Tables 2 and 3 together with the adsorption catalyst composition. The adsorption catalyst and the comparative catalyst were prepared according to the method described above, but silica sol was used as the preparation raw material for silicon (Si). Si is silica (SiO₂) Or its complex oxide.
[0095]
[Table 2]

[0096]
[Table 3]

[0097]
【The invention's effect】
As is clear from the above, according to the apparatus of the present invention, a NOx adsorption catalyst is provided in the exhaust gas flow path, and NOx is adsorbed and captured in the oxidizing atmosphere of the lean exhaust gas, and a reducing atmosphere is created to regenerate the adsorption catalyst. NOx and the like in the burned exhaust gas can be purified with high efficiency without greatly affecting fuel consumption.
[Brief description of the drawings]
FIG. 1 is a configuration diagram of an exhaust gas purifying apparatus according to the method of the present invention showing a typical embodiment of the present invention.
FIG. 2 is a time-dependent characteristic of NOx purification rate when a rich operation and a lean operation are alternately repeated by the method of the present invention.
FIG. 3 shows the relationship between the NOx concentration in the lean exhaust gas and the NOx purification rate.
FIG. 4 shows the NOx purification rate in stoichiometric exhaust gas.
FIG. 5 shows the relationship between the adsorption catalyst inlet NOx concentration and the outlet NOx concentration when the rich (stoichiometric) operation is switched to the lean operation.
FIG. 6 shows the relationship between the adsorption catalyst inlet NOx concentration and the outlet NOx concentration when the rich (stoichiometric) operation is switched to the lean operation.
FIG. 7 is a block diagram showing an air-fuel ratio control method.
FIG. 8 is a flowchart showing a method for controlling the air-fuel ratio.
FIG. 9 is a flowchart showing a method of integrating NOx emissions during lean operation.
10 is a NOx amount estimation portion in the flowchart of FIG.
11 is a NOx amount estimation portion in the flowchart of FIG.
12 is a NOx amount estimation portion in the flowchart of FIG.
13 is a NOx amount estimation portion in the flowchart of FIG.
FIG. 14 is a configuration diagram of an apparatus showing an embodiment in which a manifold catalyst is provided.
FIG. 15 is a configuration diagram of an apparatus showing an embodiment in a cylinder injection engine.
FIG. 16 is a configuration diagram of an apparatus showing an embodiment in which a post-catalyst is provided.
FIG. 17 is an apparatus configuration diagram showing an embodiment in which a reducing agent is added upstream of an adsorption catalyst.
FIG. 18 is a NOx purification characteristic diagram when the mode operation is performed.
FIG. 19 is a graph showing NOx purification characteristics when the oxygen concentration is changed using a model gas.
FIG. 20 is a graph showing NOx purification characteristics when the oxygen concentration is changed using a model gas.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Air cleaner, 2 ... Air flow sensor, 3 ... Throttle valve, 5 ... Injector, 6 ... Spark plug, 7 ... Accelerator pedal, 8 ... Load sensor, 9 ... Intake temperature sensor, 12 ... Fuel pump, 13 ... Fuel tank, 17 ... Manifold catalyst, 18 ... Adsorption catalyst, 19 ... Oxygen sensor, 20 ... Adsorption catalyst temperature sensor,
21 ... exhaust gas temperature sensor, 22 ... NOx concentration sensor, 23 ... reducing agent injector, 24 ... post catalyst, 25 ... ECU, 26 ... knock sensor, 28 ... water temperature sensor, 29 ... crank angle sensor, 99 ... engine.

Claims (10)

Redox stoichiometry relationship between the components in the exhaust gas chemisorption as NO ₂ in the vicinity of the surface of the NOx contained in the exhaust gas when in the oxidizing agent is more state against reducing agents (provided that nitrate ions NO ₃ ^- was captured by excluding those held in the form of absorption), the NOx adsorbing catalyst for reducing NO ₂ to a reducing agent to oxidizing agent is trapped in the vicinity of the surface when it is in the same amount or more states in N ₂ The air-fuel ratio A / F is disposed in an exhaust gas flow path of an internal combustion engine that is operated in a lean state of 18 or more, and the air-fuel ratio A / F in which the oxidation-reduction stoichiometry is greater than the reducing agent is 18 Estimate NOx emissions during lean operation using the air / fuel ratio setting signal, engine speed signal, intake air amount signal, intake pipe pressure signal, speed signal, throttle opening, and exhaust gas temperature. The integrated value is When the set value is exceeded, the state in which the reducing agent is the same or larger than the oxidizing agent is created, and NO ₂ trapped in the vicinity of the NOx adsorption catalyst surface is brought into contact with the reducing agent and reduced to N ₂ . An exhaust gas purification apparatus for an internal combustion engine characterized by the above. Redox stoichiometry relationship between the components in the exhaust gas chemisorption as NO ₂ to NOx in the exhaust gas in the vicinity of the surface when it is in a state oxidizing agent is more to the reducing agent (where nitrate ions NO ₃ ^- in was captured by excluding those which absorb held in the form), the NOx adsorbing catalyst for reducing NO ₂ to a reducing agent to oxidizing agent is trapped in the vicinity of the surface when it is in the same amount or more states in N _2, The air-fuel ratio A / F is disposed in an exhaust gas flow path of an internal combustion engine that is operated in a lean state of 18 or more, and the air-fuel ratio A / F in which the oxidation-reduction stoichiometry is greater than the reducing agent is 18 or more. The amount of accumulated oxygen during lean operation in the state of is detected by a signal from an oxygen sensor or air-fuel ratio sensor placed upstream or downstream of the NOx adsorption catalyst in the exhaust gas flow path, and the accumulated oxygen amount is a predetermined amount. The same as the reducing agent. An exhaust gas purifying apparatus for an internal combustion engine characterized in that NO ₂ trapped in the vicinity of the surface of the NOx adsorption catalyst is produced in an amount or large state and is brought into contact with a reducing agent to reduce it to N ₂ . Redox stoichiometry relationship between the components in the exhaust gas chemisorption as NO ₂ to NOx in the exhaust gas in the vicinity of the surface when it is in a state oxidizing agent is more to the reducing agent (where nitrate ions NO ₃ ^- in was captured by excluding those which absorb held in the form), the NOx adsorbing catalyst for reducing NO ₂ to a reducing agent to oxidizing agent is trapped in the vicinity of the surface when it is in the same amount or more states in N _2, The air-fuel ratio A / F is disposed in an exhaust gas flow path of an internal combustion engine that is operated in a lean state of 18 or more, and the air-fuel ratio A / F in which the oxidation-reduction stoichiometry is greater than the reducing agent is 18 or more. The amount of accumulated NOx at the time of lean operation in the state of is calculated by a NOx sensor signal placed upstream of the NOx adsorption catalyst in the exhaust gas flow path, and the oxidizer when the accumulated NOx amount exceeds a predetermined value The same or more reducing agent An exhaust gas purifying apparatus for an internal combustion engine, wherein NO ₂ trapped in the vicinity of the NOx adsorption catalyst surface is brought into contact with a reducing agent and reduced to N ₂ . Redox stoichiometry relationship between the components in the exhaust gas chemisorption as NO ₂ to NOx in the exhaust gas in the vicinity of the surface when it is in a state oxidizing agent is more to the reducing agent (where nitrate ions NO ₃ ^- in was captured by excluding those which absorb held in the form), the NOx adsorbing catalyst for reducing NO ₂ to a reducing agent to oxidizing agent is trapped in the vicinity of the surface when it is in the same amount or more states in N _2, The air-fuel ratio A / F is disposed in an exhaust gas flow path of an internal combustion engine that is operated in a lean state of 18 or more, and the air-fuel ratio A / F in which the oxidation-reduction stoichiometry is greater than the reducing agent is 18 or more. The NOx concentration present in the exhaust gas flow path downstream of the NOx adsorption catalyst during the lean operation in the state is detected by a signal of the NOx sensor placed in the downstream flow of the NOx adsorption catalyst, and the NOx concentration exceeds a predetermined concentration Sometimes the same amount of reducing agent to oxidizing agent Alternatively, an exhaust gas purifying apparatus for an internal combustion engine characterized in that NO ₂ trapped in the vicinity of the surface of the NOx adsorption catalyst is produced and brought into contact with a reducing agent to reduce it to N ₂ . Exhaust gas by capturing and flows ^- fuel ratio of the exhaust gas flowing chemical adsorbing NOx in the exhaust gas as NO ₂ in the vicinity of the surface at the time of lean (excluding those which absorb held in the form of However, the nitrate ions NO ₃₎ An internal combustion engine in which a NOx adsorption catalyst that reduces NO ₂ trapped by chemical adsorption near the surface to N ₂ when the air-fuel ratio of the engine is stoichiometric or rich is operated in a lean state in which the air-fuel ratio A / F is 18 or more. NOx adsorption per unit time of the NOx adsorption catalyst using at least one of exhaust gas temperature, NOx adsorption catalyst temperature, engine speed, air-fuel ratio, intake air amount and ignition timing An internal combustion engine comprising an air-fuel ratio control unit including NOx adsorption capacity determination means for estimating the amount and determining whether or not the NOx adsorption catalyst has NOx adsorption performance Seki's exhaust gas purification equipment. Chemisorption fuel ratio of the exhaust gas flowing is as NO ₂ in the vicinity of the surface of the NOx in the exhaust gas when higher than the stoichiometric air-fuel ratio (provided that nitrate ions NO ₃ ^- excluding those which absorb held in the form of) a capture When the air-fuel ratio of the inflowing exhaust gas is the stoichiometric air-fuel ratio or below, the NOx adsorption catalyst for reducing NO ₂ trapped by chemical adsorption near the surface to N ₂ is in a lean state with an air-fuel ratio A / F of 18 or more. Whether or not the NOx adsorption catalyst possesses NOx adsorption performance by integrating the time during which the internal combustion engine is operated in a state higher than the stoichiometric air-fuel ratio. An exhaust gas purifying apparatus for an internal combustion engine, comprising an air-fuel ratio control unit including a NOx adsorption capacity determining means for determining. Exhaust gas by capturing and flows ^- fuel ratio of the exhaust gas flowing chemical adsorbing NOx in the exhaust gas as NO ₂ in the vicinity of the surface at the time of lean (excluding those which absorb held in the form of However, the nitrate ions NO ₃₎ An NOx adsorption catalyst for reducing NO ₂ trapped by chemical adsorption near the surface to N ₂ when the air-fuel ratio of the engine is stoichiometric or rich is an internal combustion engine operated in a lean state where the air-fuel ratio A / F is 18 or more. The NOx adsorption catalyst retains NOx adsorption capacity based on the accumulated oxygen amount during lean operation calculated from a signal from an oxygen sensor installed upstream of the NOx adsorption catalyst in the exhaust gas flow path. An exhaust gas purifying apparatus for an internal combustion engine, comprising an air-fuel ratio control unit including a NOx adsorption capacity determining means for determining whether or not Exhaust gas by capturing and flows ^- fuel ratio of the exhaust gas flowing chemical adsorbing NOx in the exhaust gas as NO ₂ in the vicinity of the surface at the time of lean (excluding those which absorb held in the form of However, the nitrate ions NO ₃₎ An NOx adsorption catalyst for reducing NO ₂ trapped by chemical adsorption near the surface to N ₂ when the air-fuel ratio of the engine is stoichiometric or rich is an internal combustion engine operated in a lean state where the air-fuel ratio A / F is 18 or more. The NOx adsorption catalyst has a NOx adsorption capacity based on an integrated NOx amount during lean operation calculated from a signal from a NOx sensor installed on the NOx adsorption catalyst inlet side of the exhaust gas flow path. An exhaust gas purifying apparatus for an internal combustion engine, comprising an air-fuel ratio control unit including a NOx adsorption capacity determining means for determining whether or not it is held. Exhaust gas by capturing and flows ^- fuel ratio of the exhaust gas flowing chemical adsorbing NOx in the exhaust gas as NO ₂ in the vicinity of the surface at the time of lean (excluding those which absorb held in the form of However, the nitrate ions NO ₃₎ An NOx adsorption catalyst for reducing NO ₂ trapped by chemical adsorption near the surface to N ₂ when the air-fuel ratio of the engine is stoichiometric or rich is an internal combustion engine operated in a lean state where the air-fuel ratio A / F is 18 or more. The NOx adsorption catalyst retains the NOx adsorption capability based on the NOx concentration during lean operation calculated from the signal from the NOx sensor installed in the exhaust gas flow path and on the NOx adsorption catalyst outlet side of the exhaust gas flow path An exhaust gas purifying apparatus for an internal combustion engine, comprising an air-fuel ratio control unit including a NOx adsorption capacity determining means for determining whether or not Exhaust gas by capturing and flows ^- fuel ratio of the exhaust gas flowing chemical adsorbing NOx in the exhaust gas as NO ₂ in the vicinity of the surface at the time of lean (excluding those which absorb held in the form of However, the nitrate ions NO ₃₎ When the air-fuel ratio of the engine is stoichiometric or rich, the exhaust gas passage of the internal combustion engine is provided with a NOx adsorption catalyst for reducing NO ₂ trapped by chemical adsorption to N ₂ in the vicinity of the surface, and the state of the internal combustion engine is further monitored An exhaust gas purification system for an internal combustion engine, comprising an air-fuel ratio control unit for determining whether or not the NOx adsorption catalyst maintains NOx adsorption capacity based on signals from various sensors and controlling the air-fuel ratio stoichiometrically or richly. apparatus.

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