Chemical apparatus and process disinfecting – deodorizing – preser – Chemical reactor – Waste gas purifier
Reexamination Certificate
1999-12-08
2004-01-27
Tran, Hien (Department: 1764)
Chemical apparatus and process disinfecting, deodorizing, preser
Chemical reactor
Waste gas purifier
C422S177000, C502S304000
Reexamination Certificate
active
06682706
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a catalytic converter for effectively cleaning the exhaust gas of an automotive internal combustion engine by removal of nitrogen oxide (NO
x
), carbon monoxide (CO) and hydrocarbons (HC). The present invention also relates to a process for making such a catalytic converter.
2. Description of the Related Art
As is well known, the exhaust gas of an automotive internal combustion engine inevitably contains harmful substances such as NO
x
, CO and HC. In recent years, particularly, the restrictions on exhaust gas cleaning are increasingly strict for environmental protection.
A so-called three-way catalytic converter has been most widely used for removing the above-described harmful substances. Typically, a three-way catalytic converter includes a honeycomb support made of a heat-resistant material such as cordierite, and a wash-coat formed on the surfaces of the respective cells of the honeycomb support. The wash-coat contains a catalytically active substance such as Pt, Pd and/or Rh, and carrier oxide powder such as zirconium oxide powder for supporting the catalytically active substance. The catalytically active substance reduces NO
x
to N
2
while oxidizing CO and HC to CO
2
and H
2
O, respectively.
However, it has been found that the grains or particles of zirconium oxide powder (as the carrier oxide) grows due to sintering at high temperature. Such grain growth of zirconium oxide results in a decrease of surface area, consequently lowering the catalytic activity of the catalytic converter as a whole. Particularly, if the catalytic converter is mounted near the engine, it may be frequently subjected to an extremely high temperature of no less than 900° C. (or sometimes even higher than 1,000° C.), which prompts the grain growth of zirconium oxide powder.
A conventional counter measure against such a problem is to lower the heat-treating temperature at the time of preparing zirconium oxide powder and/or at the time of coating the prepared zirconium oxide powder onto the honeycomb support for increasing the initial specific surface area of the zirconium oxide powder. It is true that such a counter measure allows for a subsequent decrease of the specific surface area, thereby prolonging the time needed until the specific surface area of the zirconium oxide powder drops to a certain level. On the other hand, however, an initial increase of the specific surface area results in a greater extent of sintering (i.e., a greater decrease of the specific surface area) upon lapse of a relatively long time, thereby causing the catalytically active substance (Pt, Rh and/or Pd) to be buried in the sintered zirconium oxide powder. As a result, the catalytic activity of the catalytic converter drops remarkably in the long run.
DISCLOSURE OF THE INVENTION
It is, therefore, an object of the present invention to provide a catalytic converter for cleaning exhaust gas which is capable of retaining its catalytic activity for a long time even under severe operating conditions above 900° C. for example.
Another object of the present invention is to provide a process for advantageously making such a catalytic converter.
According to one aspect of the present invention, a catalytic converter for cleaning exhaust gas comprises a heat-resistant support; and a coating formed on the support, the coating including at least one kind of catalytically active substance and a zirconium oxide; wherein the zirconium oxide having a pre-aging specific surface area I and a post-aging specific surface area A, the aging being performed in an atmosphere of 1,000° C. for 5 hours; and wherein A/I≧0.4 and I≧40 m
2
/g.
The zirconium oxide incorporated in the coating of the catalytic converter described above exhibits a relatively small decrease of specific surface area (i.e., a relatively high A/I value) even after the high temperature aging (1,000° C., 5 hours). Therefore, even if the catalytic converter is repetitively subjected to a high temperature of no less than 900° C., the zirconium oxide is subsequently sintered only to a limited extent. As a result, the catalytic activity of the catalytic converter can be maintained for a longer time than is conventionally possible.
The zirconium oxide, which experiences a relatively small decrease of specific surface area, may be prepared by suitably adjusting the composition of the zirconium oxide or by suitably adjusting the conditions for making the zirconium oxide.
More specifically, the zirconium oxide may be a zirconium complex oxide represented by the following formula,
Zr
1−(x+y)
Ce
x
R
y
Oxide
where R represents a rare earth element other than Ce or an alkaline earth metal, and where the zirconium complex oxide meets 0.12≦x≦0.25 and 0.02≦y≦0.15 in this formula.
Examples of rare earth elements “R” other than Ce include Sc, Y, La, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu. Of these examples, La and Nd are preferred. Examples of alkaline earth metals include Be, Mg, Ca, Sr and Ba.
Alternatively, the zirconium oxide may be subjected to a preliminary aging or baking step at a high temperature for positively causing grain growth (sintering). Such preliminary aging or sintering restrains or limits subsequent grain growth (a decrease of specific surface area) under high temperature operating conditions, thereby prolonging the service life of the catalytic converter.
Typically, the catalytically active substance contained in the coating may be a precious metal such as Ru, Rh, Pd, Ag, Os, Ir, Pt and Au. Preferably, however, the catalytically active substance may be selected from a group consisting of Pt, Rh and Pd. Each of these active substances may be used alone or in combination with another.
The coating may also contain at least one heat-resistant inorganic oxide selected from a group consisting of alumina (Al
2
O
3
), silica (SiO
2
), titania (TiO
2
) and magnesia (MgO). Particularly useful is activated alumina. Further, the coating may further comprise an oxygen-storing oxide such as cerium complex oxide.
The catalytically active substance may be supported selectively on the particles of the zirconium oxide or the heat-resistant inorganic oxide before the zirconium oxide or the inorganic oxide is coated on the heat-resistant support. Alternatively, the catalytically active substance may be coated on the heat-resistant support at the same time when the zirconium oxide (and optionally the inorganic oxide) is coated on the support. Further, the catalytically active substance may be supported at the surface of the coating after the zirconium oxide (and optionally the inorganic oxide) is coated first on the heat-resistant support.
The heat-resistant support, which may be made of cordierite, mullite, &agr;-alumina or a metal (e.g. stainless steel), should preferably have a honeycomb structure. In this case, the coating is formed in each cell of the honeycomb structure.
The zirconium complex oxide having the above formula may be prepared by using known techniques such as coprecipitation process or alkoxide process.
The coprecipitation process includes the steps of preparing a mixture solution which contains respective salts of Ce, Zr and other element (a rare earth element other than Ce or an alkaline earth metal) in a predetermined stoichiometric ratio, then adding an aqueous alkaline solution or an organic acid to the salt solution for causing the respective salts to coprecipitate, and thereafter heat-treating the resulting coprecipitate for oxidization to provide a target complex oxide.
Examples of starting salts include sulfates, nitrates, hydrochlorides, phosphates, acetates, oxalates, oxychloride, oxynitrate, oxysulfate and oxyacetate. Examples of aqueous alkaline solutions include an aqueous solution of sodium carbonate, aqueous ammonia and an aqueous solution of ammonium carbonate. Examples of organic acids include oxalic acid and citric acid.
The heat treatment in the coprecipitation process includes a heat-drying step for drying the coprecipita
Yamada Koji
Yamamoto Mari
Bednarek Michael D.
Daihatsu Motor Co. Ltd.
Shaw Pittman LLP
Tran Hien
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