Catalyst – solid sorbent – or support therefor: product or process – Catalyst or precursor therefor – Metal – metal oxide or metal hydroxide
Reexamination Certificate
1999-12-13
2002-11-05
Silverman, Stanley S. (Department: 1754)
Catalyst, solid sorbent, or support therefor: product or process
Catalyst or precursor therefor
Metal, metal oxide or metal hydroxide
C502S302000, C502S303000
Reexamination Certificate
active
06475949
ABSTRACT:
BACKGROUND OF THE INVENTION
(a) Field of the Invention
The present invention relates to a promoter useful for purifying the exhaust gases discharged from the internal combustion engines of, for instance, motorcars as well as a method for the preparation thereof. More specifically, the present invention pertains to a promoter for purifying the internal combustion engine exhaust gas as well as a method for preparing the same, wherein a cerium-containing double oxide is substituted for cerium oxide which has conventionally been used as a promoter for purifying the internal combustion engine exhaust gas and the former is supported on particulate aluminum oxide to thus improve the heat resistance of the resulting promoter and to hence prevent any reduction of the quality thereof due to thermal deterioration.
(b) Description of the Prior Art
As a catalyst for purifying the exhaust gas discharged from internal combustion engines of, for instance, motorcars, there have been used, for instance, those obtained by finely distributing noble metals such as platinum, rhodium and palladium on a carrier having a large specific surface area such as alumina. These noble metals can serve to oxidize and convert the hydrocarbons present in the exhaust gas into carbon dioxide and water and to oxidize and convert the carbon monoxide into carbon dioxide, while they also have an ability of reducing the nitrogen oxides present in the exhaust gas into nitrogen gas. In other words, they can exert a catalytic effect on both oxidation and reducing reactions to thus permit the simultaneous removal of these three components of hydrocarbons, carbon monoxide and nitrogen oxides present in the exhaust gas. It has been well known that the ratio of air to fuel (or air-fuel ratio) should be maintained constant (at the theoretical air-fuel ratio) in order to ensure this effective catalytic effect on the both reactions simultaneously.
However, the internal combustion engines of, for instance, motorcars are apt to cause rapid change in the number of revolutions and the air-fuel ratio greatly varies depending on the traveling conditions such as the accelerated conditions, reduced speed conditions and low speed-traveling and high speed-traveling conditions. For this reason, oxygen concentration changes in the exhaust gas are continuously detected or monitored using an oxygen sensor to thus always control the amount of fuel to be supplied to an engine so that the air-fuel ratio is always kept constant.
On the other hand, it is common to use a combination of a noble metal catalyst and a promoter for the purpose of preventing any reduction of the purifying ability of the catalyst due to the change in the air-fuel ratio by the chemical action of the catalyst by itself. As such a promoter, there has been used, for instance, cerium oxide. Cerium oxide has such characteristic properties that it can release and/or absorb the oxygen attached thereto and the lattice oxygen present in the cerium oxide crystals depending on the level of the oxygen partial pressure in the exhaust gas. Accordingly, if the exhaust gas has a reducing ability, cerium oxide undergoes release of oxygen (CeO
2
→CeO
2−x
+0.5×O
2
) and supplies oxygen to the exhaust gas to cause an oxidation reaction. On the other hand, if the exhaust gas has an oxidation ability, cerium oxide conversely incorporates oxygen into the oxygen-deficient sites of the crystals (CeO
2−x
+0.5×O
2
→CeO
2
) to reduce the oxygen concentration in the exhaust gas and to thus cause a reducing reaction. As has been described above, cerium oxide serves as a buffering agent which can adjust any change in the extent of oxidizing and/or reducing ability of the exhaust gas and can thus maintain purifying quality of the catalyst.
However, the promoter for purifying the exhaust gas discharged from the internal combustion engines is exposed to high temperature gases generated from the engines over a long period of time and therefore, it is apt to be easily deteriorated in its ability. In particular, cerium oxide has a low heat resistance and accordingly, undergoes sintering upon exposure to high temperature gases and the specific surface area thereof is correspondingly reduced. As a result, the initial characteristic properties peculiar to the promoter is greatly impaired or reduced. For this reason, there have been proposed various means for eliminating the foregoing drawbacks of cerium oxide as a promoter, for instance, a method in which other elements such as zirconium are added to cerium oxide to improve the heat stability thereof and a method wherein the lattice-constant of cerium oxide crystals is enlarged by the incorporation of other elements into the cerium oxide crystals to thus improve the stability of the cubic system thereof. However, there has not yet been obtained any satisfactory results.
In addition, there have widely been used a technique, in which a promoter is prepared by mixing cerium oxide with aluminum oxide (such as &agr;-, &ggr;- and &thgr;-type ones). In this case, the coexistence of aluminum oxide surely has an effect of physically suppressing any sintering of the resulting mixture, but cannot suppress, at all, any sintering of cerium oxide per se.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a promoter for purifying the exhaust gas discharged from internal combustion engines, whose heat resistance is improved and whose quality reduction due to thermal deterioration can be suppressed, by the use of a cerium-containing double oxide in place of cerium oxide conventionally used.
As has been discussed above in detail, any sintering of cerium oxide per se cannot be suppressed even if a promoter is prepared by mixing cerium oxide with aluminum oxide. However, as a result of various repeated investigations of the inventors of this invention, it has been found that a cerium-containing double oxide having excellent heat resistance and free of any sintering can be prepared by bringing an aqueous solution containing a water-soluble salt of cerium and a water-soluble salt of a specific element, dissolved therein and keeping contact with particulate aluminum oxide, into contact with a specific precipitant to cause a reaction therebetween and to thus deposit the reaction product on the particulate aluminum oxide; and then firing the aluminum oxide to which the reaction product is adhered, to thus give the double oxide constituted by cerium, the specific element and oxygen supported on the particulate aluminum oxide.
Under such circumstances, the inventors of this invention have conducted various studies to achieve the foregoing object, have found that a promoter can be prepared by using a cerium-containing double oxide instead of cerium oxide and supporting the double oxide on the particulate high-heat-resistant aluminum oxide, the resulting promoter does not cause any reduction of its initial activity as a promoter even if it is exposed to a high temperature over a long period of time and can maintain its high quality and thus have completed the present invention on the basis of the foregoing findings.
According to an aspect of the present invention, there is thus provided a promoter for purifying internal combustion engine exhaust gas, which comprises (A) particulate aluminum oxide and (B) a double oxide of (i) cerium and (ii) at least one member selected from the group consisting of zirconium, yttrium, strontium, barium and rare earth elements, supported on the aluminum oxide.
According to another aspect of the present invention, there is provided a method for preparing a promoter for purifying internal combustion engine exhaust gas, which comprises the steps of mixing a dispersion, which contains (i) a water-soluble salt of cerium and (ii) a water-soluble salt of at least one member selected from the group consisting of zirconium, yttrium, strontium, barium and rare earth elements, dissolved therein and particulate aluminum oxide dispersed therein, with an aqueous solution of ammonium hydrog
Yamaguchi Yasuhide
Yamazaki Masatoshi
Mitsui Mining and Smelting Co. Ltd.
Nguyen Cam N.
Silverman Stanley S.
Young & Thompson
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