Abstract: A NOx reduction system comprises NOx treatment catalyst and air-fuel ratio control switch for changing the air-fuel ratio of an exhaust gas to a lean or rich state. The NOx treatment catalyst comprises at least a first catalyst layer containing a solid acid catalyst capable of adsorbing ammonia and a second catalyst layer containing a noble metal and a cerium oxide material. The second and the first catalyst layer are stacked in that order on a carrier, and the first catalyst layer constitutes the uppermost layer. NOx is passed through the first catalyst layer in the lean state and is temporarily stored in the second catalyst layer. Thereafter, in the rich state, NOx is converted to NH3 and is restored in the first catalyst layer. When the state is again returned to the lean state, NH3 is converted to nitrogen which is finally released from the first catalyst layer.

Abstract: The present invention provides an NOx purifying catalyst that removes NOx with sufficient efficiency even under low temperature operation such as diesel engine automobiles. An NOx purifying catalyst for processing NOx in exhaust gas by rich/lean control of the air-fuel ratio of the exhaust gas, includes a first catalytic layer containing ?-zeolite having iron and/or cerium elements, and a second catalytic layer including a noble metal and cerium oxide-based material; in which the second and the first catalytic layers are coated on a support in that order so that the first catalytic layer is the uppermost layer. The NOx purifying catalyst purifies NOx as showed in Reaction Formulations (1) to (4). Lean condition 1: NO+½O2?NO2??(1) Rich condition: CO+H2O?CO2??(2) NOx+H2?NH3??(3) Lean condition 2: NOx+NH3+O2?N2+H2O??(4).

Abstract: To provide a method for controlling a NOx reduction system which can remove NOx with a satisfactorily high efficiency even under low-temperature operating conditions as in diesel cars. The NOx reduction system comprises NOx treatment means disposed in an exhaust passage in an internal combustion engine, and air-fuel ratio control means for bringing the air-fuel ratio of an exhaust gas to a lean or rich state. The NOx treatment means comprises at least a first catalyst layer containing a solid acid catalyst capable of adsorbing ammonia and a second catalyst layer containing a noble metal and a cerium oxide material. The second catalyst layer and the first catalyst layer are stacked in that order on a carrier, and the first catalyst layer constitutes the uppermost layer. The air-fuel ratio control means controls so that the lean state and the rich state are repeated at predetermined intervals.

Abstract: The present invention provides an NOx purifying catalyst that removes NOx with sufficient efficiency even under low temperature operation such as diesel engine automobiles. An NOx purifying catalyst for processing NOx in exhaust gas by rich/lean control of the air-fuel ratio of the exhaust gas, includes a first catalytic layer containing ?-zeolite having iron and/or cerium elements, and a second catalytic layer including a noble metal and cerium oxide-based material; in which the second and the first catalytic layers are coated on a support in that order so that the first catalytic layer is the uppermost layer. The NOx purifying catalyst purifies NOx as showed in Reaction Formulations (1) to (4).

Abstract: A NOx removing device 4 having NOx absorbing capacity and ammonia retaining capacity is provided in an exhaust pipe 2 of an internal combustion engine 1. A reforming catalyst 3 is disposed upstream of the NOx removing device 4. The reforming catalyst 3 generates hydrogen and carbon monoxide by the steam reforming reaction, when the exhaust gases are in the reducing state. Hydrogen and carbon monoxide generated by the reforming catalyst 3 are supplied to the NOx removing device 4, and contribute to generation of ammonia. The generated ammonia is retained in the NOx removing device 4 and reduces NOx in exhaust gases when the exhaust gases are in the oxidizing state.

Abstract: A catalyst for purifying exhaust gas is disclosed which can attain a high percentage of NO purification in an oxygen-excess atmosphere even after exposure to a hydrothermal environment. The catalyst for purifying exhaust gas comprises a combination of a carrier having platinum deposited thereon and CeO2 having an average crystallite diameter D satisfying D≧13.0 nm. The carrier is made of a gallosilicate having an SiO2/Ga2O3 molar ratio M satisfying 100≦M≦1,230.

Abstract: An exhaust emission control catalyst having an excellent NOx converting capability includes a modified alumina which has a .theta.-phase and an .alpha.-phase and has an .alpha.-conversion rate R in a range of 0.5 %.ltoreq.R.ltoreq.95 %, and a catalytic metal carried in the modified alumina, e.g., platinum (Pt). In addition, a catalyst including a catalyst material which is the above-described catalyst including the modified alumina and the catalytic metal, and an aluminosilicate or cerium oxide (CeO.sub.2), also has an excellent NOx converting capability.

Abstract: An adsorbent exhibiting an excellent adsorption ability to nitrogen oxides (NO) is made from an oxide-based ceramics having an average crystallite grain size D in a range of D<500 .ANG., e.g., CeO.sub.2. A catalyst is formed from, for example, an cerium oxide (CeO.sub.2) and a zeolite. A content C of CeO.sub.2 in the catalyst is set in a range of 10% by weight .ltoreq.CA.ltoreq.80% by weight. The zeolite adsorbs hydrocarbon (HC). The hydrocarbon (HC) adsorbed is oxidized into active CHO which acts to reduce the nitrogen oxides (NO) adsorbed by the CeO.sub.2.