Catalyst – solid sorbent – or support therefor: product or process – Catalyst or precursor therefor – Metal – metal oxide or metal hydroxide
Reexamination Certificate
2000-12-04
2004-10-19
Silverman, Stanley S. (Department: 1754)
Catalyst, solid sorbent, or support therefor: product or process
Catalyst or precursor therefor
Metal, metal oxide or metal hydroxide
C423S239100, C423S213500, C502S327000, C502S328000, C502S330000
Reexamination Certificate
active
06806225
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a catalyst for purifying an exhaust gas, a process for producing the same, and a method using the catalyst for purifying an exhaust gas, in particular, to an NO
x
storage and reduction type catalyst which can efficiently purify nitrogen oxides (NO
x
) in an exhaust gas which contains oxygen excessively in an amount more than necessary for oxidizing carbon monoxide (CO) and hydrocarbons (HC) which are contained in the exhaust gas, a process for producing the same and a method for purifying the exhaust gas by using the catalyst.
BACKGROUND ART
Conventionally, as a catalyst for purifying an automobile exhaust gas, a 3-way catalyst has been employed which carries out the oxidation of CO and HC and the reduction of NO
x
simultaneously to purify an exhaust gas. With regard to such a catalyst, for example, a catalyst has been known widely in which a loading layer comprising &ggr;-alumina is formed on a heat-resistant support, such as cordierite, and a noble metal, such as Pt, Pd and Rh, is loaded on the loading layer.
By the way, the purifying performance of such a catalyst for purifying an exhaust gas depends greatly on the air-fuel ratio (A/F) of an engine. For example, when the air-fuel ratio is large, namely on a lean side where the fuel concentration is lean, the oxygen amount in the exhaust gas increases so that the oxidation reactions of purifying CO and HC are active, on the other hand, the reduction reactions of purifying NO
x
are inactive. Conversely, for example, when the air-fuel ratio is small, namely on a rich side where the fuel concentration is high, the oxygen amount in the exhaust gas decreases so that the oxidation reactions are inactive and the reduction reactions are active.
Whilst, in order to suppress the recent global warming, it is required to control the CO
2
emission in automobiles. In order to meet the requirement, the lean burn is effective in which the burning is carried out in an oxygen-rich lean atmosphere having a large air-fuel ratio, engines, which are appropriate for the lean burn, have been made practicable. However, when purifying the exhaust gas emitted from the lean burn engines, there arises a problem in that it is difficult to purify the NO
x
as aforementioned.
Hence, an NO
x
storage and reduction type catalyst has been proposed in which an alkaline-earth metal and Pt are loaded on a porous support, such as alumina (Al
2
O
3
), (Japanese Unexamined Patent Publication (KOKAI) No. 5-317,652, etc.). In accordance with this catalyst, since the NO
x
are absorbed in the alkaline earth metal, serving as the NO
x
storage member, and since they are reacted with the reducing components, such as HC, and are purified, it is possible to control the emission of the NO
x
even on the lean side.
In the catalyst disclosed in Japanese Unexamined Patent Publication (KOKAI) No. 5-317,652, it is believed that barium, for example, is loaded as the carbonate, and the like, on the support, and it reacts with NO
x
to generate barium nitrate (Ba(NO
3
)
2
) in a lean atmosphere, thereby storing the NO
x
. And, when the exhaust gas is in the range of from the stoichiometric point to the rich atmosphere, the stored NO
x
are released and are reacted with the reducing components, such as HC and CO, and are thereby reduced. And, in order to enhance the NO
x
conversion by carrying out these reactions of the NO
x
storage member efficiently, an engine control method has been developed, in which the air-fuel ratio is usually controlled on an oxygen-excessive lean side and it is intermittently controlled in the range of from the stoichiometric point to the rich atmosphere in a pulsating manner, and has been put into practical applications.
In accordance with this engine control method, since the used amount of the fuel is less on the lean side, the emission of the CO
2
is suppressed, and the NO
x
are stored in the NO
x
storage member. Accordingly, the emission of the NO
x
is suppressed on the lean side as well. And, the stored NO
x
are released in the range of from the stoichiometric point to the rich side, and are reacted with the reducing components, such as HC and CO, by the catalytic action of Pt, and so on. Therefore, a high NO
x
purifying capability is exhibited as a whole.
However, in the exhaust gas, SO
2
is contained which is generated by burning sulfur (S) contained in the fuel, it is further oxidized to SO
3
, by the noble metal in an oxygen-rich atmosphere. Then, they are easily reacted with the barium, etc., to generate sulfites and sulfates, and it is understood that the NO
x
storage member is thus poisoned and degraded. This phenomenon is referred to as sulfur poisoning. Moreover, the porous support, such as alumina, has a property that it is likely to adsorb the SO
x
, and there is a problem in that the aforementioned sulfur poisoning is facilitated.
And, when the NO
x
storage member is turned into the sulfites and the sulfates, it cannot store the NO
x
any more, and, as a result, there is a drawback in the aforementioned NO
x
storage and reduction type catalyst in that the NO
x
purifying ability decreases after a durability test.
Moreover, since TiO
2
, which is an acidic oxide, does not adsorb SO
2
, it was thought of using a TiO
2
support, and an experiment was carried out. As a result, SO
2
was not adsorbed by the TiO
2
and flowed downstream as it was, since only the SO
2
which contacted directly with the noble metal, was oxidized, it was revealed that the sulfur poisoning occurred to a lesser extent. However, in the TiO
2
support, a drawback was revealed that the initial activity was low. This reason is believed that the NO
x
purifying capability of the NO
x
storage member decreases because TiO
2
reacts with the NO
x
storage member to form composite oxides (BaTiO
3
, etc.) in the temperature range of the exhaust gas.
Hence, in Japanese Unexamined Patent Publication (KOKAI) No. 6-327,945, it is proposed to use a support in which Al
2
O
3
is mixed with a composite oxide, such as a Ba—Ce composite oxide and a Ba—Ce—Nb composite oxide. In addition, in Japanese Unexamined Patent Publication (KOKAI) No. 8-99,034, it is proposed to use at least one composite support selected from the group consisting of TiO
2
—Al
2
O
3
, ZrO
2
—Al
2
O
3
and SiO
2
—Al
2
O
3
.
By thus using the support in which the composite oxide is mixed, or by using the composite support, the NO
x
storage member is inhibited from the sulfur poisoning, and the NO
x
purifying capability after durability is improved.
However, a further reduction of the CO
2
emission is required against the recent issue of the global warming, and the lean burn driving range tends to increase. Accordingly, since the sulfur poisoning tends to further increase, and since the exhaust gas emission control tends to be further strengthened, the exhaust gas purifying catalysts are required to furthermore improve their durability.
The present invention has been developed in view of the aforementioned circumstances, and it is an object of the present invention to make an exhaust gas purifying catalyst, in which the sulfur poisoning of the NO
x
storage member can be further inhibited, and which can maintain a high NO
x
conversion even after a durability test.
DISCLOSURE OF INVENTION
A characteristic of an exhaust gas purifying catalyst according to the present invention, solving the aforementioned assignments, is that the catalyst comprises: a support including a porous oxide including TiO
2
at least and ZrO
2
on which Rh is loaded in advance; an NO
x
storage member including at least one member selected-from the group consisting of alkali metals, alkaline-earth metals and rare-earth elements and loaded on the support; and a noble metal including at least one member selected from the group consisting of Pt, Pd and Rh and loaded on the support.
In the aforementioned exhaust gas purifying catalyst, it is preferred that the porous oxide includes a composite oxide of Al
2
O
3
and TiO
2
.
Further, a characteristic of a process for producing
Hirayama Hiroshi
Ikeda Yasuo
Tanizawa Riemi
Tsuji Sinji
Oliff & Berridg,e PLC
Silverman Stanley S.
Strickland Jonas N.
Toyota Jidosha & Kabushiki Kaisha
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