Catalyst – solid sorbent – or support therefor: product or process – Catalyst or precursor therefor – Metal – metal oxide or metal hydroxide
Patent
1991-02-11
1992-09-15
Shine, W. J.
Catalyst, solid sorbent, or support therefor: product or process
Catalyst or precursor therefor
Metal, metal oxide or metal hydroxide
4232135, B01J 2104, B01J 2106, B01J 2310, B01J 2340
Patent
active
051478420
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
This invention relates to purifying catalysts for exhaust gas emitted from internal combustion engines such as automobiles and a process for preparation thereof. More detailedly, this invention relates to catalysts which exhibit excellent exhaust gas purification performance at lower temperature compared to usual catalysts and a process for preparation thereof.
BACKGROUND ART
Elements of the platinum group such as platinum and rhodium, and cerium oxide having an oxygen storage function for enhancement of their activity at low temperature are mainly used now as catalytic converter to simultaneously remove hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NOx) in exhaust gas exhausted from internal combustion engines such as automobiles.
However, it is said that in catalysts containing an element of the platinum group and cerium oxide the oxygen storage function of cerium oxide strikingly lowers at a high temperature exposure above 800.degree. C. and thus the catalysts are liable to deteriorate at those temperature. Thus, many methods have been disclosed comprising adding oxides of alkaline earth elements, or zirconium compounds for the purpose of inhibiting crystallization of cerium oxide and preserving its oxygen storage function (for example, Japanese Laid-Open Patent Publication Nos. 58347/1989 and 116741/1988).
Further, in the latest catalytic converter, catalysts having a two or more-layer structure and containing rhodium in the outer layer began to be used for effective utilization of expensive rhodium and are becoming the main stream. It is known in these catalysts that it is effective to add zirconium oxide for prevention of lowering of the catalytic performance due to interaction of rhodium with alumina. Further, in these catalysts, it is considered that the catalytic performance is enhanced by interaction between rhodium and zirconium oxide (for example, Japanese Laid-Open Patent Publication Nos. 222,539/1986 and 88,040/1988).
However, when zirconium oxide is exposed to high temperatures, its crystallite structure changes from the metastable tetragonal system having a catalytic activity to the inactive monoclinic system, contribution of rhodium and zirconium oxide to catalytic performance due to their interaction diminishes, and the catalytic performance is lowered. At present there is no disclosure about a catalyst to stabilize zirconium oxide in the catalyst and preserve the interaction between rhodium and zirconium oxide.
DISCLOSURE OF INVENTION
In this invention, it was aimed at to develop a catalyst which solves the above problems of the prior art and exhibits an excellent exhaust gas purification performance at lower temperature even after high temperature aging.
As a result of vigorous study for solution of the above problems, the present inventors found that it is extremely effective for enhancement of the activity of exhaust gas-purifying catalysts at low temperature after high temperature endurance to combine a coprecipitated ceria-stabilized zirconia (cerium oxide-stabilized zirconium oxides prepared by the coprecipitation method) having a specific oxygen storage performance as well as heat resistance with a usual catalyst containing at least one element of the platinum group, activated alumina, cerium oxide and optionally a zirconium compound.
Further, they found that it is more effective to add this coprecipitated ceria-stabilized zirconia to the outer layer of a catalyst having a structure consisting of at least two catalytic component (active component)-containing layers, and completed this invention.
This invention is detailedly described below.
In the invention, addition of a ceria-stabilized zirconium coprecipitated oxide gave a desirable result for enhancement of the activity at low temperature after high temperature aging.
As shown in FIG. 1, the structure of zirconium oxide changes from the metastable tetragonal system to the monoclinic system as the temperature becomes higher, whereas a coprecipitated ceria stabilized zirconia prepared by the
REFERENCES:
patent: 4316822 (1982-02-01), Fujitani et al.
patent: 4367162 (1983-01-01), Fujitani et al.
patent: 4806519 (1989-02-01), Chiba et al.
Funabiki Masaki
Kayano Kunihide
Yamada Teiji
N.E. Chemcat Corporation
Shine W. J.
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