Catalyst – solid sorbent – or support therefor: product or process – Catalyst or precursor therefor – Metal – metal oxide or metal hydroxide
Reexamination Certificate
2001-05-10
2004-12-14
Nguyen, Cam N. (Department: 1754)
Catalyst, solid sorbent, or support therefor: product or process
Catalyst or precursor therefor
Metal, metal oxide or metal hydroxide
C502S302000, C502S303000, C502S304000, C502S328000, C502S332000, C502S333000, C502S334000, C502S339000, C502S349000, C502S355000, C502S415000, C502S439000
Reexamination Certificate
active
06831036
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a co-catalyst for purifying an exhaust gas of an internal combustion engine, more particularly to the co-catalyst for efficiently converting the exhaust gas containing an environmental polluting substance such as carbon monoxide, hydrocarbons and nitrogen oxides into a nontoxic substance by means of oxidation and/or reduction.
BACKGROUND ART
A catalyst prepared by finely dispersing a precious metal such as platinum, rhodium and palladium on a support such as alumina having a larger specific surface area is employed for purifying an exhaust gas discharged from an internal combustion engine of an automobile. These precious metals have a role of converting the nitrogen oxides into nitrogen in addition to a role of converting the hydrocarbons and the carbon monoxide in the exhaust gas into carbon dioxide and water and into carbon dioxide, respectively, by oxidation. A simultaneous catalysis on both of the oxidation reaction and the reduction reaction enables simultaneous removal of the three components including the hydrocarbons, the carbon monoxide and the nitrogen oxides. It is required to maintain a ratio between fuel and air (air-fuel ratio) at constant (at a theoretical air-fuel ratio) for efficiently affecting the simultaneous catalysis on the both reactions.
However, the number of rotations of the internal combustion engine of the automobile is likely to be changed instantaneously, and the air-fuel ratio largely changes depending on driving situation such as speed-acceleration, speed-reduction, lower-speed driving and higher-speed driving. Accordingly, an amount of fuel supplied to an engine is controlled to make the air-fuel ratio constant by judging the change of an oxygen concentration in the exhaust gas by using an oxygen sensor.
The precious metal catalyst is employed with a co-catalyst for a purpose of preventing the reduction of the purifying catalysis generated due to the change of the air-fuel ratio by the chemical function of the catalyst itself. An example of the co-catalyst includes cerium oxide. The cerium oxide has a characteristic of eliminating and absorbing adhered oxygen and lattice oxygen in the cerium oxide depending on the degree of the oxygen partial pressure. Accordingly, when the exhaust gas is reductive, the cerium oxide eliminates the oxygen [CeO
2
→CeO
2-x
+(x/2)O
2
] to supply the oxygen into the exhaust gas for affecting the oxidation reaction. On the other hand, when the exhaust gas is oxidative, the cerium oxide taking the oxygen into its oxygen defects [CeO
2-x
+(x/2)O
2
→CeO
2
] to decrease the oxygen concentration in the exhaust gas for affecting the reduction reaction. In this manner, the cerium oxide acts as a buffering agent for decreasing the change of the oxidative property and the reductive property of the exhaust gas to maintain the purifying property of the catalyst.
However, the co-catalyst for purifying the exhaust gas is likely to be deteriorated because of the long-time exposure to a higher-temperature gas from the internal combustion engine. Especially, the resistance to heat of the cerium oxide is low, and the cerium oxide is sintered to reduce its specific surface area upon exposure to a higher-temperature gas, thereby resulting in the reduction of the initial properties as the co-catalyst. Accordingly, the elevation of the thermal stability by the addition of other elements such as zirconium or of the stability of the hexagonal by increasing the lattice constant by means of the addition of other elements is attempted. However, the satisfactory results have not jet been obtained.
Although the cerium oxide is mixed with aluminum oxide (&agr;-type, &ggr;-type, &thgr;-type) to prepare a co-catalyst in accordance with a widely-employed technology and the sintering of the mixture can be physically prevented by the mixing of the aluminum oxide in this case, the sintering of the cerium oxide itself cannot be prevented.
Even if the sintering of the cerium oxide could be prevented, the actively is lowered in a relatively short period of time not to obtain a substantially longer lifetime when the co-catalyst is continuously used with the precious metal catalyst in an actual internal combustion engine for purifying an exhaust gas.
An object of the present invention is to provide a co-catalyst for purifying an exhaust gas which can be used for a loner period of time as an actual catalyst and a process for treating an exhaust gas catalyst by using the cerium oxide in the conventional co-catalyst for purifying the exhaust gas as a cerium-containing complex oxide for elevating the resistance to heat and suppressing the performance reduction due to thermal deterioration and by making a specific surface area and an oxygen storage capacity over specified values.
DISCLOSURE OF INVENTION
The present invention is a co-catalyst for purifying an exhaust gas including; a composite oxide including (a) cerium; and (b) at least one element selected from the group consisting of zirconium, yttrium, strontium, barium and a rare-earth element supported on a particulate aluminum oxide support; a specific surface area of the co-catalyst after sintering being not less than 40 m
2
/g; an oxygen storage capacity at 700° C. being not less than 100 &mgr;mols/g or an oxygen storage capacity at 400° C. being not less than 10 &mgr;mols/g.
The present invention will be described in detail.
As described above, the sintering of the cerium oxide itself cannot be prevented even if the co-catalyst is formed by mixing the cerium oxide and the aluminum oxide. However, the repeated experiments by the present inventors revealed that a co-catalyst containing a cerium-based composite oxide having a higher stability to heat and an anti-sintering property prepared by supporting a composite oxide containing cerium, a specified element and oxygen on the particulate aluminum oxide. The co-catalyst can be prepared, for example, by reacting an aqueous solution dissolving therein a water-soluble salt of cerium and another water-soluble salt of a specified element in contact with the particulate aluminum oxide, with a specified precipitating agent (reducing agent) to deposit the reaction product on the particulate aluminum oxide and sintering the particulate aluminum oxide having the deposited reaction product thereon to support the composite oxide containing the cerium, the specified element and the oxygen on the particulate aluminum oxide.
It has been known that the use of the co-catalyst having the specific surface area and the specified oxygen storage capacity among those having the above particulate aluminum. oxide supporting the composite oxide can elevate the catalysis and increase the treating efficiency of the actual exhaust gas.
After vigorous experiments for elevating the catalysis, the present inventors have conceived based on the above knowledge that the co-catalyst can be prevented from the reduction of the initial activity even after exposure to a higher temperature for a longer period of time to maintain the higher catalysis by supporting the cerium-based composite oxide containing the cerium oxide and an oxide of another metal on the particulate aluminum oxide and by adjusting the specific surface area thereof after higher-temperature sintering to be 40 m
2
/g or more, and the oxygen storage capacities at 700° C. and 400° C. to be 100 &mgr;mols/g and 10 &mgr;mols/g, respectively, thereby reaching to the present invention.
A composition ratio between the aluminum oxide and the composite oxide, and a composition ratio between the cerium and the added element in the composite oxide are not especially restricted. However, the reduction of a relative amount of the cerium in the co-catalyst tends to decrease the effect of the co-catalyst. When, inversely, the relative amount of the cerium is excess, the cerium oxide separately exists in addition to the composite oxide containing the cerium or all the cerium oxide cannot be supported on the aluminum oxide. Accordingly, the excessive relat
Anno Yuichi
Shinoda Kiyoshi
Yamazaki Masatoshi
Yashima Isamu
Mitsui Mining & Smelting Co. Ltd.
Nguyen Cam N.
Sughrue & Mion, PLLC
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