Catalyst – solid sorbent – or support therefor: product or process – Catalyst or precursor therefor – Silicon containing or process of making
Reexamination Certificate
2002-02-22
2004-07-13
Bos, Steven (Department: 1754)
Catalyst, solid sorbent, or support therefor: product or process
Catalyst or precursor therefor
Silicon containing or process of making
C423S593100, C423S598000, C423S592100, C423S600000, C501S103000, C501S127000, C501S134000, C502S302000, C502S304000, C502S349000, C502S355000
Reexamination Certificate
active
06762147
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to composite oxide powder having oxygen storage ability and a process for producing the same, and a catalyst employing the composite oxide powder of this invention as a catalyst support and a process for producing the same. This catalyst can be used for hydrogen generation, exhaust gas purification, etc.
2. Description of the Related Art
As catalysts for purifying automotive exhaust gases, there have been employed 3-way catalysts so far which oxidize carbon monoxide (CO) and hydrocarbons (HC) and reduce nitrogen oxides (NO x) at the same time to purify exhaust gases. For example, 3-way catalysts are known widely which comprise a heat-resistant honeycomb-shaped supporting base material formed of cordierite, a catalyst support layer formed of &ggr;-Al
2
O
3
and disposed on the supporting base material, and a noble metal such as platinum (Pt) and rhodium (Rh) loaded on the catalyst support layer.
Catalyst supports used for catalysts for purifying exhaust gases are required to have a large specific surface area and a high heat resistance, and are generally composed of Al
2
O
3
, SiO
2
, ZrO
2
, TiO
2
or the like. In order to reduce variations in exhaust gas atmospheres, it is also known to add CeO
2
, which has oxygen storage ability, or a CeO
2
—ZrO
2
solid solution, which has improved oxygen storage ability and heat resistance of CeO
2
.
By the way, owing to the recent severe regulations on exhaust gases, there is an extremely strong need to purify exhaust gases even in a very short time from an engine start. To meet these regulations, it is necessary to activate a catalyst to purify regulated exhaust gas components at lower temperatures. Particularly, a catalyst formed by loading Pt, etc. on CeO
2
has a superior ability of converting CO from a low temperature. By using this type of catalysts in combination, CO is ignited at a low temperature and CO adsorption poisoning of Pt is lessened and HC ignitability improves. Also by using this type of catalysts in combination and igniting CO at a low temperature, warming of catalyst surfaces is promoted, whereby HC can be converted from a low temperature range. Besides, when this type of catalysts are employed in combination, H
2
is generated in a low temperature range by water gas shift reaction and the generated H
2
can be used as a reducing agent for a reaction with NO
x
, whereby NO
x
can be reduced and purified from a low temperature range.
However, the conventional catalyst comprising CeO
2
and a noble metal loaded thereon is poor in durability in actual exhaust gases, so CeO
2
is sintered by heat and its oxygen storage ability deteriorates. The noble metal loaded on CeO
2
grows granularly due to the sintering of CeO
2
and its catalytic activity deteriorates, so the conventional catalyst has a problem in practical use. Therefore, it is essential to improve heat resistance without damaging the oxygen storage ability of CeO
2
and to stabilize the noble metal on the catalyst support.
For example, Japanese Unexamined Patent Publication (KOKAI) No.H4-292480 discloses a composite catalyst support in which CeO
2
is contained in an Al
2
O
3
matrix in a highly dispersed state. This composite catalyst support has a CeO
2
particle diameter of 2.5 to 6 nm after subjected to a thermal treatment at 350° C. for 4 hours, and a CeO
2
particle diameter of 15 to 35 nm after subjected to a thermal treatment at 1100° C. for 4 hours.
Japanese Patent No.2893648 discloses catalyst support raw materials composed of Al
2
O
3
—CeO
2
composite oxide prepared by a co-precipitation method. This catalyst support attains a CeO
2
particle diameter of not more than 9 nm even after calcination at 1000° C. for 2 hours.
In the case of a catalyst in which a noble metal is loaded on the composite catalyst support described in Japanese Unexamined Patent Publication No.H4-292480, however, when the CeO
2
particle diameter exceeds 15 nm, contact surface area of the noble metal and CeO
2
decreases and oxygen storage ability and noble metal grain growth suppression effect deteriorate. Therefore, the CeO
2
particle diameter after thermal treatment at 1100° C. is desirably not more than 15 nm.
Japanese Unexamined Patent Publication No.H4-292480 and Japanese Patent No.2893648 disclose that a catalyst support preferably has a pore volume of 0.15 to 0.5 cc/g. With this small pore volume, however, gas diffusion into the inside of the catalyst support is insufficient, which results in a low efficiency of using a catalyst metal loaded on the catalyst support as catalytic activity points.
SUMMARY OF THE INVENTION
The present invention has been conceived in view of the aforementioned circumstances. It is an object of the present invention to provide composite oxide powder having a large specific surface area and a large pore volume even after exposed to high temperature for a long time without losing oxygen storage ability of such a single metal oxide as CeO
2
. It is another object of the present invention to provide a catalyst with excellent durability by suppressing noble metal grain growth when exposed to high temperature for a long time.
Composite oxide powder of the present invention, which attains the above objects, is characterized in that a first metal oxide having oxygen storage ability is held as ultrafine particles in the form of islands by a second metal oxide which is different from the first metal oxide; the composite oxide powder has a pore volume of not less than 2 cc/g; and the first metal oxide has a particle diameter of not more than 30 nm even after exposed to high temperature of 900° C. or more.
The first metal oxide is preferably at least one of CeO
2
, Pr
2
O
3
, Eu
2
O
3
and Tb
2
O
3
. The second metal oxide is preferably at least one of Al
2
O
3
, SiO
2
, TiO
2
, SiO
2
—Al
2
O
3
and TiO
2
—Al
2
O
3
. It is desirable that the second metal oxide includes Al
2
O
3
, and more desirably is principally composed of Al
2
O
3
. It is also preferable that at least one of the first metal oxide and the second metal oxide further contains a third metal oxide which is at least one of La
2
O
3
, Y
2
O
3
and ZrO
2
.
The composite oxide powder of the present invention preferably has a shell shape, and desirably the shell shape is hollow. It is also desirable that the particles of the first metal oxide are exposed on the surface of a shell composed of the second metal oxide or the second metal oxide and the third metal oxide.
Moreover, it is preferable that the composite oxide powder has an outside particle diameter of 20 to 5000 nm, and it is desirable that the ratio of an inner hole diameter to an outside particle diameter is 0.5 to 0.99. The composite oxide powder desirably has a specific surface area of 20 m
2
/g or more.
A catalyst of the present invention is characterized in comprising a catalyst support composed of the composite oxide powder of the present invention and a catalyst metal loaded on the catalyst support.
A production process of the present invention, which is most suitable to produce the composite oxide powder of the present invention, is characterized in comprising the steps of preparing a W/O type emulsion by mixing an organic solvent and a dispersing agent in an aqueous solution in which a water-soluble compound of a first metal element whose oxide has oxygen storage ability and a water-soluble compound of a second metal element which is different from the first metal element are dissolved; and spraying and burning the W/O type emulsion, thereby obtaining composite oxide powder. Spraying and burning temperature is desirably in the range from 700 to 1200° C. It is also possible to apply a thermal treatment to the obtained composite oxide powder at 700 to 1200° C., after the spraying and burning.
The first metal element is preferably at least one of Ce, Pr, Eu and Tb, and the second metal element is preferably at least one of Al, Si and Ti. Moreover, it is particularly desirable that the second metal is principally composed of Al. It is also preferable
Hatanaka Miho
Morikawa Akira
Suda Akihiko
Tani Takao
Bos Steven
Kabushiki Kaisha Toyota Chuo Kenkyusho
Wright, Sr. William G.
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