Catalyst – solid sorbent – or support therefor: product or process – Catalyst or precursor therefor – Metal – metal oxide or metal hydroxide
Reexamination Certificate
1999-02-03
2001-01-09
Wood, Elizabeth D. (Department: 1755)
Catalyst, solid sorbent, or support therefor: product or process
Catalyst or precursor therefor
Metal, metal oxide or metal hydroxide
C502S302000, C502S325000, C502S326000, C502S327000, C502S332000, C502S339000, C502S334000
Reexamination Certificate
active
06171999
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of manufacturing a catalyst in which a catalytic metal is supported by a base material, the method being able to improve dispersibility of the catalytic metal in the base material and further able to restrain the catalytic metal from being oxidized during the manufacturing process of the catalyst.
2. Description of the Prior Art
In general, because exhaust gas discharged from automotive engines or the like contains atmospheric pollutants such as HC (hydrocarbon), CO (carbon monoxide), NOx (nitrogen oxides) and so on, regulations on the exhaust gas have been strengthened worldwide in recent years. For example, in the State of California, USA where air pollution causes serious social problems, the corporate average NMOG regulations have been introduced, and further the regulation level of the exhaust gas is going to be strengthened stepwise from now on. In order to cope with these regulations, low-pollution vehicles such as LEV or ULEV must be introduced into the market (to be introduced stepwise from year 1997-year 2000).
Consequently, to an exhaust system of the automotive engine or the like, in general, an exhaust gas purifying device (catalytic converter) using an exhaust gas purifying catalyst is installed to purify the exhaust gas. Hereupon, as the above-mentioned exhaust gas purifying catalyst, conventionally, a catalyst, in which a catalytic metal such as palladium, platinum, rhodium or the like is supported by a porous base material composed of ceria, alumina, zeolite or the like, has been popularly adopted.
Thus, in the specification of Japanese Patent Application No. Hei 9-57485, the applicant of the present application formerly disclosed an exhaust gas purifying catalyst including a lower catalytic layer in which palladium is supported as a catalytic metal by a base material composed of alumina, ceria etc. and an upper catalytic layer in which platinum and rhodium are supported as catalytic metals by a base material composed of ceria, the lower catalytic layer being disposed on a honeycomb-formed carrier, and the upper catalytic layer being disposed on the lower catalytic layer. In the exhaust gas purifying catalyst, due to the above-mentioned construction, its heat resistance is improved, and further its catalytic activity at lower exhaust gas temperature is improved, while improving its NOx purifying performance at higher exhaust gas temperature. In the exhaust gas purifying catalyst described above, palladium is principally used for improving catalytic activity of the catalyst at lower exhaust gas temperature.
However, in the exhaust gas purifying catalyst disclosed in the specification of Japanese Patent Application No. Hei 9-57485 according to the applicant of the present application, there is still such a problem that NOx purifying ratio at higher exhaust gas temperature is not sufficiently improved. That is, for example, as to the exhaust gas purifying catalyst after an aging treatment at high temperature of 1100° C., although its NOx purifying ratio at exhaust gas temperature of 500° C. (namely, C500 purifying ratio) reaches approximately 80% (rig test, that is simulated test), the NOx purifying ratio described above would be required to be a little more improved.
According to knowledge or consideration of the inventors of the present application, a reason why the NOx purifying ratio of the exhaust gas purifying catalyst at higher exhaust gas temperature is not sufficiently improved as described above, may be supposed such that the catalytic metal is not sufficiently atomized because dispersibility of the catalytic metal in the base material is relatively lower. Further, another reason may be supposed such that the catalytic metal is partially oxidized during the manufacturing process of the exhaust gas purifying catalyst (furnacing step etc.) so that catalytic activity of the catalyst is lowered. In consequence, it may be supposed that NOx purifying ratio of the exhaust gas purifying catalyst at higher exhaust gas temperature would be sufficiently improved and further purifying ratio of each of other atmospheric pollutants would be also sufficiently improved, if dispersibility of the catalytic metal in the base material is sufficiently raised and further oxidation of the catalytic metal is restrained or prevented during the manufacturing process of the exhaust gas purifying catalyst.
SUMMARY OF THE INVENTION
This invention has been achieved to solve the above conventional problems in view of the inventors' knowledge or consideration described above, and has an object to provide a method of manufacturing a catalyst, which is able to raise dispersibility of the catalytic metal in the base material and further able to restrain or prevent the catalytic metal from being oxidized during the manufacturing process of the catalyst.
A method of manufacturing a catalyst according to the present invention, which is achieved to solve the above-mentioned problems, includes the steps of (a) mixing a metal cluster complex (carbonyl cluster), which includes a first catalytic metal (for example, noble metal such as rhodium or the like) and has a carbonyl group (one of functional groups including multiple bonds) as a ligand, with methanol to prepare a methanol solution of metal cluster complex, (b) attaching (impregnating) the methanol solution of metal cluster complex to a base material (for example, cerium oxide, often called “ceria”), and (c) releasing (removing) the ligand from the metal cluster complex attaching to the base material to make the base material support (as very fine metal particles) the first catalytic metal.
It is preferable that the method further includes the steps of (d) drying the base material after the step of making the base material support the first catalytic metal, (e) mixing a second catalytic metal, which is different from the first catalytic metal, with water to prepare an aqueous solution of catalytic metal, and (f) attaching the aqueous solution of catalytic metal to the base material to make the base material support the second catalytic metal.
In this case, other organic solvents may be used instead of methanol.
Hereupon, the carbonyl group (namely, ═C═O) having a double bond composed of C and O, which is used as the above-mentioned ligand, abounds in both of the reactivity against metals and the variation of possible form of compounds including it. Meanwhile, methanol has a particularly strong agency to dissolve the carbonyl cluster and solubility of the carbonyl cluster for methanol is higher. In consequence, according to the above-mentioned combination, the catalytic metal (particularly, rhodium) may be supported in the form of very fine particles (namely, with highly dispersed state) by the base material. Further, when a plurality of catalytic metals are supported by the base material, they may be supported with more highly dispersed state.
According to the method of manufacturing the catalyst of the present invention, dispersibility of the catalytic metal in the base material is highly raised so that the catalytic metal is stably supported in the form of ultra-atomized particles by the base material. Therefore, when the catalyst is used as an exhaust gas purifying catalyst, NOx purifying ratio at higher exhaust gas temperature is of course highly improved, and further purifying ratio of each of other atmospheric pollutants is also highly improved. However, it goes without saying that the method of manufacturing the catalyst according to the present invention is not restricted to the manufacture of the exhaust gas purifying catalyst and in consequence may be broadly used for manufacturing various catalysts.
In the above-mentioned method of manufacturing the catalyst, the solvent for dissolving the catalytic metal cluster complex need not be methanol (for example, organic solvents such as acetone, toluene, pentane, dichloromethane or the like), if only it is a solvent except water.
In general, when water is used as the solvent
Ichikawa Satoshi
Misonoo Kazuo
Shigetsu Masahiko
Takemoto Takashi
Mazda Motor Corporation
Nixon & Peabody LLP
Studebaker Donald R.
Wood Elizabeth D.
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