Compositions: ceramic – Ceramic compositions – Refractory
Patent
1996-10-03
1999-06-01
Marcantoni, Paul
Compositions: ceramic
Ceramic compositions
Refractory
423263, 502304, C04B 3548
Patent
active
059088001
DESCRIPTION:
BRIEF SUMMARY
Under the heading of new, useful industrial products, the present invention concerns composition precursors and mixed cerium and zirconium oxide-based compositions possessing, in particular, improved specific surface areas, especially large and thermally-stable specific surface areas.
The invention further concerns a process for obtaining these precursors and compositions, as well as the use of the latter, in particular in the field of catalysis, as catalysts themselves or catalyst carriers.
Cerium oxide and zirconium oxide are today considered to be two especially important and interesting constituents. As an example, they are both encountered with increasing frequency, either singly or in combination, in numerous so-called multifunctional catalysts, in particular catalysts deigned for the treatment of exhaust gases from internal combustion engines. The qualifier "multifunctional" designates catalysts capable of carrying out not only oxidation, especially of carbon monoxide and of hydrocarbons present in the exhaust gases, but also the reduction of nitrogen oxides also present in these gases ("three-way" catalysts). It will be noted that, as regards both their composition and their operating principle, these catalysts have been widely described in the literature and have been made the object of numerous patents and/or patent applications.
Even though the scientific reasons advanced to date to attempt an explanation of this fact still seem somewhat doubtful, and indeed sometimes contradictory, it now appears nevertheless certain that "three way" industrial catalysts containing both cerium oxide and zirconium oxide are, overall, more effective than catalysts which are either completely free of the two aforementioned oxides or free of one of them.
In the catalysts mentioned above, cerium oxide and zirconium oxide, which may, moreover, perform a catalytic function in their own right and/or serve as simple carriers for other catalytic elements, e.g., platinum, rhodium, and other precious metals, are normally present in an uncombined form; i.e., these two constituents are found in the final catalyst as a simple physical mixture of highly-individualized oxide particles. This results in part from the fact that these cerium and zirconium oxide-based catalysts are most often obtained by intimately mixing the corresponding oxide powders, or are produced from thermally-decomposable precursors in these oxides.
For different reasons, there emerges today from the state of the art an increasingly pronounced trend toward attempting to add and make functional the cerium and zirconium constituents in the catalyst composition, no longer in a separate, uncombined state, but rather directly as a true mixed oxide of the solid solution type CeO2--ZrO2.
However, in such a situation giving rise to an entirely conventional catalysis requirement, it becomes necessary to have available a mixed oxide possessing a specific surface area which is as large as possible and is also preferably thermally stable. In fact, given that the effectiveness of a catalyst normally increases as a function of the increased contact surface between the catalyst (catalytically-active phase) and the reagents, the catalyst should be kept in the most divided state possible, both when new and after prolonged use at relative high temperatures; that is, the solid particles, or crystallites, which compose it must remain as small and individualized as possible, a state that can be produced only from mixed oxides having large specific surface areas and being relatively stable when heated.
It will be noted at this stage of the description that some mixed oxides of the solid-solution type in the CeO2--ZrO2 system have already been described in the literature. However, the preparation thereof normally requires a calcination step carried out at relatively high temperature, so as to obtain a single cubic phase, as is revealed, for example, in the article of E. Tani, M. Yoshimura, and S. Somiya, entitled "Revised Phase Diagram of the System ZrO2--CeO2 Below 1,400.degree.
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Bonneau Lionel
Chopin Thierry
Touret Olivier
Vilmin Gabriel
Marcantoni Paul
Rhone-Poulenc Chimie
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