Chemistry of inorganic compounds – Oxygen or compound thereof – Metal containing
Reexamination Certificate
2001-04-09
2004-08-03
Bos, Steven (Department: 1754)
Chemistry of inorganic compounds
Oxygen or compound thereof
Metal containing
C423S253000, C423S263000, C423S594100, C423S594200, C423S594300, C423S594400, C423S594500, C423S594600, C423S594700, C423S594800, C423S594900, C423S594120, C423S594130, C423S594150, C423S594160
Reexamination Certificate
active
06770256
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a process for synthesizing a metal oxide having a perovskite or perovskite-like crystal structure by high energy milling. More particularly, a mixture of starting powders are subjected to a high energy milling sufficient to induce chemical reaction of the components and thereby directly mechanosynthesize a metal oxide in the form of a perovskite or perovskite-like nanocrystalline structure as determined by X-ray diffractometry.
BACKGROUND OF THE INVENTION
In general, mixed metal oxides are crystalline compounds and they are classified by general formulas and certain structural-type characteristics of naturally occurring minerals. Perovskite is a well-known type of mixed metal oxides. Perovskites have the general formula ABO
3
where A and B stand for cations. More than one cation for each A and B may be present.
Another type of metal oxide includes “perovskite-like” materials which comprises basic perovskite cell separated by intervening oxide layers. Perovskite-like materials have the general formula [(ABO
3
)
n
+C
y
O
z
] where A, B and C stand for cations. More than one cation for each A, B and C may be present.
Are also known compounds derived from perovskite or perovskite-like materials by substitution and deviations to stoichiometry but maintaining their perovskite or perovskite-like crystal structure. Non-stoichiometric compounds derived from perovskites have the general formula (ABO
3−x
) and non-stoichiometric compounds derived from perovskite-like materials have the general formula [(ABO
3−x
)
n
+C
y
O
z
]. In all these non-stoichiometric compounds, metal ions with a different valence may replace both A and B ions thereby generating non-integral numbers of oxygen atoms in the formula. La
0.8
Sr
0.2
CoO
3−x
and La
0.8
Sr
0.2
MnO
3−x
are examples of non-stoichiometric compounds derived from perovskites and Sr
2
FeO
4−x
and Sr
3
Fe
2
O
7−x
are examples of non-stoichiometric compounds derived from perovskite-like materials. Other examples of such deviation to stoichiometry are obtained by making a perovskite or a perovskite-like material deficient in oxygen. For example, the brownmillerite structure (ABO
2.5
) is formed from perovskites (ABO
3
).
It is at once apparent that there is quite a large number of compounds which fall within the scope of the term perovskite and perovskite-like materials. The compounds and their structure can be identified by X-ray diffraction.
In prior art, perovskite and perovskite-like compounds have been commonly used in the following fields: electrocatalysis, hydrogenation, dehydrogenation and auto-exhaust purification. One drawback with the metal oxides having the perovskite and perovskite-like structure produced in prior art is that, in general, they show a very low BET specific surface area (SS) in the order of 1 m
2
/g. Therefore despite the fact that perovskite and perovskite-like structure metal oxides are not expensive to produce, that they usually show good catalytic oxidation activities, that they are thermally stable and that they show a good resistance to poisoning, they have found to date very limited application in place of noble metal based catalysts used in the field of industrial pollution abatement or automobile emission control. Higher specific surface area perovskite and perovskite-like compounds could thus have a great potential as catalysts, particularly in the selective reduction of nitrogen oxides (NO
x
) and as electrocatalysts in the cathodic reduction of oxygen.
The known methods for preparing perovskites and perovskite-like materials include sol-gel process, co-precipitation, citrate complexation, pyrolysis, spray-drying and freeze-drying. In these, precursors are prepared by a humid way such as in a mixed gel or in the co-precipitation of metallic ions under the form of hydroxides, cyanides, oxalates, carbonates or citrates. These precursors can thus be submitted to various treatments such as evaporation or combustion (SS~1-4 m
2
/g), to the method of explosion (SS<30 m
2
/g), plasma spray-drying (SS~10-20 m
2
/g) and freeze-drying (SS~10-20 m
2
/g). However, the drawbacks with all of these methods are that either low specific surface area values are reached or that they are complicated and expensive to put into practice.
The most common method for preparing perovskite and perovskite-like catalysts is the traditional method called “ceramic”. This method simply consists in mixing constituent powders (oxides, hydroxides or carbonates) and sintering the powder mixture thus formed to high temperature. The problem with this method is that calcination at high temperature (generally above 1000° C.) is necessary to obtain the crystalline perovskite or perovskite-like crystalline structure. Another drawback lo is that low specific surface area value is obtained (SS around 1 m
2
/g). An example of such a high temperature heating method is disclosed in U.S. Pat. No. 5,093,301 where a perovskite structure to be used in a catalyst is formed after heating a ground dry powder mixture at 1300° C.
U.S. Pat. No. 4,134,852 (Volin et al.) issued in 1979 disclosed a variant to the ceramic method by “mechanically alloying”, in the old sense of that expression, the constituent powders necessary for the preparation of perovskite catalysts. Indeed, it refers to a conventional grinding in order to obtain a more or less homogenous mixture of particles but not infer any chemical reaction between the components. It can be read in column 7, lines 5-8 of this patent that “[a] mechanically alloyed powder is one in which precursor components have been intimately intradispersed throughout each particle . . . ”. Therefore a necessary step of the process disclosed therein to obtain the desired perovskite structure is by heating the “mechanically alloyed” powder composition to an elevated temperature greater than 800° C. (column 7, lines 61-62).
Today, the use of the expression “mechanical alloying” or “mechanosynthesis” refers among other things to a high energy milling process wherein nanostructural particles of the compounds milled are induced. Therefore it also refers to the production of metastable phases, for example high temperature, high pressure or amorphous phases, from crystalline phases stable under ambient temperature and pressure. For example, the structural transformation of alumina (Al
2
O
3
), the preparation of ceramic oxides and the preparation of stabilized zirconias by high energy milling or mechanical alloying have already been respectively disclosed in the following references: P. A. Zielinski et al. in J. Mater. Res., 1993, Vol. 8. p 2985-2992; D. Michel et al., La revue de métallurgie-CIT/Sciences et Génies des matériaux, February 1993; and D. Michel et al., J. Am. Ceram. Soc., 1993, Vol 76, p 2884-2888. The publication by E. Gaffet et al. in Mat. Trans., JIM, 1995, Vol 36, (1995) p 198-209) gives an overview of the subject.
However, even if these papers disclosed the use of high energy milling, their authors have only been able to transform their starting product from one phase to another phase. The product resulting from the milling thus still has the same structure. Furthermore, none of them discloses the preparation of perovskite or perovskite-like materials.
There is still presently a need for a simple process, low in cost for producing a metal oxide having the perovskite or the perovskite-like crystal structure. Furthermore, the perovskite and perovskite-like metal oxides produced according to all of the above mentioned methods known in the art does not have a nanocrystalline structure. Therefore, there is also a need for a metal oxide having a perovskite or a perovskite-like nanocrystalline structure with a high specific surface area and need for a process for synthesizing such compounds.
SUMMARY OF THE INVENTION
An object of the present invention is to propose a process for producing a metal oxide that will satisfy the above-mentioned needs.
According to the present invention, th
Kaliaguine Serge
Van Neste Andre
Bos Steven
Darby & Darby
Universite Laval
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