Catalyst – solid sorbent – or support therefor: product or process – Catalyst or precursor therefor – Metal – metal oxide or metal hydroxide
Reexamination Certificate
2001-10-22
2003-05-13
Silverman, Stanley S. (Department: 1754)
Catalyst, solid sorbent, or support therefor: product or process
Catalyst or precursor therefor
Metal, metal oxide or metal hydroxide
C502S328000, C502S330000, C502S340000, C502S344000, C502S400000, C502S514000
Reexamination Certificate
active
06562753
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a device for purifying an exhaust gas, a method for purifying an exhaust gas, a catalyst for purifying an exhaust gas, and a method for manufacturing the catalyst, arranged in such a manner that a NOx (nitrogen oxide) absorber that absorbs NOx in an excess oxygen atmosphere is disposed in an exhaust gas passage of an engine or the like, thereby making it possible to remove NOx from exhaust even in a lean air-fuel ratio condition.
BACKGROUND ART
Generally, there has been a known arrangement to dispose, in an exhaust gas passage of an engine, a NOx absorber that absorbs NOx in an exhaust gas when an air-fuel ratio mixture is lean, and hence, when an oxygen concentration in the exhaust gas is high, and releases NOx when the oxygen concentration drops, thereby purifying released NOx by means of reduction.
However, in a case where fuel or engine oil contains a slight amount of sulfur component (S), when the sulfur component are burned and exhausted, a conventional NOx absorber absorbs SOx (sulfur oxide) more readily than NOx in an exhaust gas, and further, once it has absorbed SOx, it hardly releases SOx even when the oxygen concentration in the exhaust gas drops. As a result, an amount of absorbed SOx increases with a lapse of time while the NOx absorption ability gradually deteriorates.
In regard to a problem of the S poisoning (sulfur poisoning), Japanese Patent Laid-Open Publication No. 6-142458 describes that combining Ba serving as a NOx absorber with at least one of alkali metal, Fe, Ni, Co, and Mg and supporting the combination on a substrate is advantageous in improving resistance to the S poisoning of Ba, and that in the case of a combination of Ba and K, SO
2
in the exhaust gas is incorporated in the form of complex sulfate with Ba and K, and when the oxygen concentration drops, the complex sulfate decomposes or is reduced at low temperatures to BaO and K
2
O that are active to NOx, which provides an advantage in improving resistance to the S poisoning of Ba. It should be noted, however, that the Ba support amount per 1 L (an apparent volume, and the same applies to the description below) is 13.7 to 27.4 g and the K support amount per 1 L is 0.39 to 7.8 g of the substrate.
However, when the K support amount is too little, the NOx absorption ability is not improved, and when the K support amount is increased to improve the NOx absorption ability, HC is not purified adequately in an atmosphere with reductants (excess oxygen ratio &lgr;≦1), that is, during a theoretical air-fuel ratio combustion operation or a rich combustion operation of the engine. The reason for this is not obvious, but it is assumed that a great deal of K is placed in the periphery of precious metal and prevents HC from approaching in close proximity to the precious metal.
Japanese Patent Laid-Open Publication No. 7-51544 describes that when a combination of at least two kinds of alkaline-earth metal is supported on a substrate as a NOx absorber, the NOx absorber incorporates SO
2
in the exhaust gas as complex sulfate and the complex sulfate readily decomposes at low temperatures when the oxygen concentration drops, thereby providing an advantage in improving resistance to the S poisoning of Ba. It should be noted, however, that, in the case of a combination of Ba and Mg, the Ba support amount per 1 L of the substrate is 41 to 69 g and the Mg support amount per 1 L is 2.4 to 4.8 g, and in the case of a combination of Ba and Sr, the Ba support amount per 1 L of the substrate is 41 to 69 g and the Sr support amount per 1 L is 8.7 to 42 g.
Also, Japanese Patent Laid-Open Publication No. 10-118494 describes that, in regard to a NOx purifying catalyst for purifying NOx by means of reduction in an excess oxygen atmosphere, by supporting Pt and Rh as catalytic metal on an alumina substrate and by also supporting Sr or Mg in addition to K having high affinity to NOx, it is possible to achieve high NOx purifying performance even in the presence of SOx. It is preferable to arrange in such a manner that, in the case of a combination of K and Sr, the K support amount per 1 L of the substrate is 20 to 40 g and the Sr support amount per 1 L is 0 to 50 g, and in the case of a combination of K and Mg, the K support amount per 1 L of the substrate is 5 to 20 g and the Mg support amount per 1 L is 0 to 5 g.
Also, Japanese Patent Laid-Open Publication No. 10-274031 describes that SOx absorbed in the NOx absorber is detached by injecting fuel in an expansion stroke of an in-cylinder direct injection engine and thereby raising a temperature of the exhaust gas.
The object of the present invention is to control absorption of sulfur component in an exhaust gas by the NOx absorber.
Another object of the present invention is to improve the heat resistance of the NOx absorber.
A further object of the present invention is to achieve a balance between NOx purifying performance during a lean combustion operation after being exposed to a high temperature atmosphere and HC purifying performance during a theoretical air-fuel ratio combustion operation or a rich combustion operation by adequately setting the K support amount.
Still another object of the present invention is to regenerate deteriorated NOx absorption ability of the NOx absorber resulted from absorption of sulfur component in the exhaust gas by causing the NOx absorber to detach the sulfur component, and more particularly to make an arrangement of the NOx absorber such that it can readily regenerate by detaching the sulfur component when predetermined sulfur detaching means is activated.
DISCLOSURE OF THE INVENTION
The present invention provides a catalyst for purifying an exhaust gas, disposed in an exhaust gas passage of an engine, for lowering NOx concentration in an exhaust gas containing NOx, sulfur, and oxygen, the catalyst including:
a substrate; and
a catalyst layer formed on the substrate by supporting, on alumina, a NOx absorber that absorbs NOx when an oxygen concentration in the exhaust gas is high and releases NOx when the oxygen concentration drops, and a precious metal for reducing NOx,
which is characterized in that the NOx absorber includes Ba, K, Sr, and Mg.
According to the invention, deterioration of the NOx absorption ability of the NOx absorber caused by the S poisoning can be suppressed, and the heat resistance of NOx absorber can also be improved. The reason for this is not obvious, but it is assumed as follows.
Firstly, it is assumed that the elements (K, Sr, and Mg) other than Ba are more susceptible to the S poisoning than Ba, and for this reason, the S poisoning of Ba is relatively small. In other words, because Ba has the higher NOx absorption ability than the other elements, the presence of the other elements makes the S poisoning of Ba relatively small, the deterioration of the NOx absorption ability is lessened.
According to an analysis, it appears that Ba and Sr (at least a part of each) form one compound (a mixed oxide or double salt) with both the elements being constituent elements. It is assumed that such a Ba—Sr compound (hereinafter, referred to as a double compound as needed) is less susceptible to the S poisoning compared with the case of Ba alone, and for this reason, deterioration of the NOx absorption ability can be suppressed.
Also, according to an analysis, it appears that Ba and Mg (at least a part of each) do not form a crystal, but are in an almost amorphous state by coming in close proximity to each other or bonding to each other. It is assumed that such a Ba—Mg coexisting substance suppresses the S poisoning of Ba (formation of barium sulfate) compared with the case of Ba alone, and for this reason, deterioration of the NOx absorption ability can be suppressed.
Also, it is affirmed from analysis that K neither forms a complex nor undergoes affinity with Ba, Sr, and Mg, and resides dispersedly in the periphery of the Ba—Sr compound or Ba—Mg coexisting substance. K with the foregoing nature is relatively highly reactive with sulfur, and therefore, is assumed to prevent t
Kooda Yuki
Kyougoku Makoto
Miyoshi Seiji
Okamoto Kenji
Takami Akihide
Madza Motor Corporation
Nixon & Peabody LLP
Silverman Stanley S.
Studebaker Donald R.
Vanoy Timothy C.
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