Process for fabricating crystalline metal oxide material

Details Process for fabricating crystalline metal oxide material Process for fabricating crystalline metal oxide material

1999-07-28

2001-01-02

Beck, Shrive (Department: 1762)

Coating processes

Particles, flakes, or granules coated or encapsulated

Inorganic base

C427S255180, C427S255393, C427S397700

Utility Patent

active

06168830

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates a process for fabricating crystalline metal oxide materials comprising tin oxide, zirconia, or titania in form as powder, monolith gel or film by first forming a metal oxide material containing —OH surface group through a sol-gel process, and then treating them with [SiR
m
]
n
X
z
H
y
compound which reacts with the —OH group to form —OSiR
m
surface group, and heating the thus-treated oxide material at a high temperature to crystallize them into a crystalline metal oxide material with high crystallinity, small crystal size (<100 Å) and high surface area (>100 m
2
/g).
2. Description of the Prior Art
Transitional and non-transistional metal oxides have been widely used in fields of catalyst and air detection. For example, catalysts based on tin dioxide can be used in organic oxidation (U.S. Pat. Nos. 4,701,437 and 3,947,474) and for controlling quality of waste gases discharged from automobile (U.S. Pat. No. 5,051,393). Monolith gel or film based on tin dioxide can be used for detecting gases such as carbon monooxide and alcohol (U.S. Pat. No. 4,592,967). Catalysts based on titanium dioxide can increase the conversion rate for producing dimer and higher polymer from butylene as starting material (U.S. Pat. No. 5,073,658) and can reduce carbon dioxide and water (J. of Molecular Catalysis, 144 (1997) 207). Titanium dioxide has been used for detecting gases such as carbon monooxide (J. Electrochem. Soc., 144 (1997) 1750-1753) or oxygen (U.S. Pat. No. 4,713,646). Catalysts based on zirconium dioxide (ZrO
2
) can be used for producing hydrocarbons from synthetic gases (U.S. Pat. No. 5,391,362). On the other hand, zirconium dioxide is able to detect O
2
(J. Electrochem. Soc. 144 (1997) 4158-4160).
In the above-listed applications, specific surface area (i.e., the surface area per unit weight of oxide), crystal size and crystallinity are critical material microstructural characteristics that can have effect on their performance in the application. In general, catalyst requires high specific surface area (>100 m
2
/g) to increase the contact area between reactants and catalyst. As for the gas sensing application, increasing crystallinity can decrease electric power consumption, and reducing crystal size will increase sensitivity of detection. In summary, for applications as catalyst and for gas detection, high specific surface area, small crystal size and high crystallinity (i.e., low lattice defect density) are the preferred microstructural features of the oxides.
Sol-gel process has been extensively used for producing oxide materials of different forms, including powder, monolith gel and film. Sol-gel process involves dissolving a metal alkoxide having a general formula of M(OR′)
x
where M is metal element, R′ is an alkyl group, x is an integer of 1~6, or a water soluble metal salt in an aqueous solution, and carrying out hydrolysis and condensation reactions to obtain a sol:
M—OH+M—OH→M—O—M+H
2
O.
Then, the sol was washed, dried and calcined to yield metal oxide powder or form a monolith xerogel. On the other hand, a thin film can be formed first through coating, and then the film is subject to drying and clacining to yield a metal oxide film. In the course of the sol-gel process, compounds that can form hydrogen ion (H
+
) or hydroxyl ion (OH
−
) may be added to change the pH of the solution so as to alter the sol forming rate and the particle dispersion. For example, as described in J. of Non-Crystalline solids 147&148 (1992) 340-345, addition of ammonia water in the aqueous solution of tin tetrachloride can result immediately in a precipitation of sol which was dialyzed or washed repeatedly with fresh water to remove chlorine and ammonium ions. Thereafter, part of water was evaporated by heating to reach a critical concentration, and the residue was then dried to result in an integrally formed monolith xerogel of tin dioxide. Also, as reported in Powder Technology, 92 (1997) 233-239, tetra-isopropyl titanate (Ti(OCH(CH
3
))
4
) was dissolved in isopropyl alcohol (or n-hexane) and then the solution thus-formed was added with ammonia water (25%) under stirring at 400 rpm. A sol suspension was yielded. After standing for three days, it was filtered and air dried overnight followed by drying at 60° C. one day. The sample obtained was thereafter subjected to heat treatment at 400° C. to yield a titanium dioxide powder. Further, as described in J. Am. Ceram. Soc. 78(1995) 1329-1334, a method for preparing zirconium dioxide powder by sol-gel process comprised of mixing zirconium n-propoxide, 2-propanol, and deionized water in a predetermined ratio at 50° C. for one hour to form an opaque sol precipitate. The precipitate was dried at 120° C. for 24 hours and then washed with fresh water followed by drying to obtain a powder.
Material prepared through sol-gel process, including powder, xerogel or thin film, has, in general, a feature of high specific surface area (>100 m
2
/g), but has poor mechanical strength and crystallinity. Such material comprises amorphous substances having a large amount of hydroxyl group (—OH). Under heating, such amorphous substance will begin to dehydrate and crystallize wherein its microstructure will change with time, and hence various physical and chemical characteristics thereof varies correspondingly. Therefore, material prepared through sol-gel process should be subjected to a heat treatment to form a crystalline material such that its thermal stability in a practical application can be assured. For example, Goodman et al. in J. Chem. Soc. 237, 1162-67, 1960, stated the preparation of tin dioxide powder by a sol-gel process, wherein, sample just prepared had a specific surface area of 172 m
2
/g which, after subjecting to a heat treatment at 500° C., was lowered to 25 m
2
/g. Nae-Lih Wu et al in J. Am Ceram. Soc.82,67-73 (1999) pointed out that tin dioxide powder prepared by using SnCl
4
as the starting material via a sol-gel process contained more than 50% of an amorphous substance whose average crystal size was less than 20 Å, but, after heating at 500° C. for one hour, increased to become 240 Å. Mercera et al. mentioned in Applied Catalysis 57, 127-48 that zirconium dioxide, when used as catalyst support, required a high specific surface area and heat stability. They prepared zirconium dioxide powder by a sol-gel process to obtain an initial specific surface area of 289 m
2
/g. After heating at 450° C. for 15 hours, due to the growth of crystal, the specific surface area was lowered to be 111 m
2
/g.
In view of the above-mentioned prior art techniques, it can be clearly seen that the most directly negative effect of the heat treatment process is to cause the overgrowth of crystal and to decrease drastimaticaly the specific surface area. Accordingly, there is a need for a process for fabricating crystalline metal oxide materials, which can lower effectively the negative effect on crystal growth and loss of specific surface area associated with the prior art described above. Specifically, there is a need for a new process to fabricate crystalline metal oxide materials through a sol-gel process which can increase the crystallinity of the material obtained while maintaining the features of small crystal and high specific surface area thereof.
SUMMARY OF THE INVENTION
Accordingly, the object of the invention is to provide a process for fabricating crystalline metal oxide materials comprising tin oxide, zirconia, or titania in form as powder, monolith gel or film by first forming a metal oxide material containing —OH surface group through a sol-gel process, and then treating them with [SiR
m
]
n
X
z
H
y
compound which reacts with the —OH group to form —OSiR
m
surface group, and heating the thus-treated oxide material at a high temperature to crystallize them into a crystalline metal oxide material with high crystallinity, small crystal size (<100 Å) and high surface area (>100 m

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