Metal oxide coated silica shells

The present invention discloses a novel high surface area powder composition in which the individual powder particles comprise hollow silica shells, e.g., amorphous hydroxylated silica, coated with finely distributed surface accessible metal oxides. The invention also discloses metallic coatings, which are obtained by converting at least a portion of the oxide coatings into the corresponding metals.

FIELD OF THE INVENTION 
The present invention relates to a novel high surface area powder 
composition in which the individual powder particles comprise silica 
shells, e.g., amorphous hydroxylated silica, coated with finely 
distributed surface accessible metal oxides. 
BACKGROUND OF THE INVENTION 
The production of silica shell structures is disclosed in U.S. Pat. No. 
5,024,826, which issued on Jun. 18, 1991. 
A process for coating silica shell structures with an antimony-containing 
tin oxide layer to produce electroconductive powders is described in 
European Patent Application Publication No. 0359569, which published on 
Mar. 21, 1990. Such electroconductive powders are useful in electrically 
conductive coatings, but generally not as primary conductors of 
electricity. 
The entire content of U.S. Pat. No. 5,024,826, and European Patent 
Application Publication No. 0359569, is hereby incorporated by reference. 
SUMMARY OF THE INVENTION 
The present invention relates to a high surface area powder composition. 
The individual powder particles comprise about 0.05 to 15 micron silica 
shells, e.g., hollow amorphous hydroxylated silica shells, which have a 
shell thickness of from about 5 to 50 nm. The silica shells typically have 
a surface area of about 25 to 350 m.sup.2 /g. 
At least a portion of the silica shell is coated with about 10 to 75% by 
eight of a finely distributed surface accessible metal containing species, 
e.g, a metal oxide. Surface accessible denotes that the metal oxide is 
situated on or about the outer surface of the silica shells. Suitable 
metal oxides comprise one or more members selected from the group of the 
oxides of Fe, Al, Zr, V, Nb, Ta, Cr, Mo, W, Co, Ni, Cu, Zn, Sn, Sb, 
mixtures thereof, among others. 
One aspect of the invention relates to metallic coatings, which are 
obtained by converting or reducing at least a portion of the oxide coating 
to its corresponding metal. The metal oxide coating may be reduced by 
being exposed to an environment containing hydrogen or carbon monoxide at 
an elevated temperature, e.g., between about 550.degree.-850.degree. C. 
For example, a metal oxide coating comprising iron oxide can be reduced to 
iron at a temperature of about 550.degree. C., whereas a nickel oxide 
coating can be reduced to nickel at a temperature of about 850.degree. C. 
The hollow shells can be prepared by any suitable process. One suitable 
process for preparing the shells is described in U.S. Pat. No. 5,024,826, 
which issued on Jun. 18, 1991, the teachings of which have been 
incorporated by reference. 
A hydroxide of the metal containing species is deposited on the silica 
shells by adding, to an aqueous slurry of silica shells, a water soluble 
salt of the desired metal. An alkali metal hydroxide solution, e.g., 
sodium hydroxide solution, is added to the slurry in order to convert the 
salt to a metal hydroxide which deposits upon the silica shells. The 
hydroxide coated silica shells can be recovered from the aqueous slurry by 
any suitable means such as filtration, centrifugation, among other 
recovery techniques. The recovered shells can be washed with water until 
substantially free from soluble residues. At least a portion of the metal 
hydroxide coating can then converted to a metal oxide by thermally 
dehydrating the hydroxide coating. 
Moreover, the metal oxide coating may be modified or reduced to tailor the 
characteristics of the powder to satisfy a particular end-use application. 
For example, a metal oxide coating comprising Fe.sub.2 O.sub.3 may be 
converted to magnetic Fe.sub.3 O.sub.4. In some cases, a metal oxide 
coating is reduced to obtain a metal coating by exposing the metal oxide 
to a reducing environment, e.g., hydrogen or carbon monoxide can be used 
to reduce iron oxide to Fe. 
As a result of the hollow shell structure, the density of the metal or 
metal oxide coated powders of the invention is much lower than 
conventional solid or bulk powders. The low-density hollow-shell structure 
affords greater economy because the quantity of material necessary for a 
particular application is reduced in comparison to using either bulk 
metals or metal oxides. The economic advantages of the inventive powder 
are particularly useful in applications such as catalyst, toners, carriers 
for toners, pigments such as an automotive finish, electrical conductors, 
magnetic applications, transparent products, among many others. 
When the powder coating upon the silica shell comprises, for example, 
Fe.sub.2 O.sub.3, the powder is an effective ultra-violet (UV) ray 
absorber. Such a UV absorber is desirable for use in protective coatings, 
e.g., wood preservatives, color pigments, cosmetics, among many others.

DETAILED DESCRIPTION OF THE INVENTION 
The present invention relates to a high surface area powder composition, 
and to a method for producing the powder. The individual powder particles 
comprise about 0.05 to 15 micron silica shells, e.g., amorphous 
hydroxylated silica, which have a shell thickness of from about 5 to 50 
nm, and a surface area of from about 25 to 350 m.sup.2 /g. The silica 
shells are coated with about 10 to 75% by weight of a finely distributed 
surface accessible metal containing species, e.g., a metal oxide. 
Whenever used in the specification and appended claims the terms below are 
intended to have the following definitions. 
"Finely distributed" as used herein refers to the characteristics of the 
metal containing species. Typically, metal containing species comprise 
crystallites which have an average size of about 50 to 200 Angstroms. The 
metal containing species may not completely surround the silica shell, but 
rather in some cases form an at least partially interconnected network 
about the silica shell. For example, the density or loading of metal 
containing species may range from trace amounts to a substantially 
complete coating or monolayer. 
"Metal containing species" as used herein refers to the composition and 
morphology of the metal and/or metal oxide which is present upon at least 
a portion of the silica shell. The metal and/or metal oxide containing 
species may include one or more compositions in a variety of morphologies. 
Typically, the morphology of the metal and/or metal oxide is crystalline. 
However, the presence of amorphous material is permissible, and may be 
desirable when the coated powder is employed as a catalyst. 
"Surface accessible" denotes that the metal containing species are situated 
on or about the outer surface of the silica shells. This term does not 
include species which are incorporated within the silica shell structure. 
Suitable metals and oxides thereof comprise at least one member selected 
from the group of Fe, Al, Zr, V, Nb, Ta, Cr, Mo, W, Co, Ni, Cu, Zn, Sn, 
Sb, mixtures thereof, among others. 
"Silica shell" as used herein refers to the characteristics and composition 
of the shell upon which the metal containing species are deposited. The 
silica shell is normally hollow, and can be employed in a wide range of 
sizes, shapes, and shell thicknesses. In some cases, the core material is 
not removed, and the silica shell is characterized by a skin which 
surrounds the core material. In such cases, the silica shell or skin may 
also include additional components such as alumina, boric oxide, among 
others. The additional components as well as the core material can be 
removed by acid extraction. 
Suitable hollow shells, which will support the metal containing species, 
can be prepared by the procedures described in U.S. Pat. No. 5,024,826 
which issued on Jun. 18, 1991; the teachings of which have been 
incorporated herein by reference. The metal containing species can be 
deposited on the silica shells by forming an aqueous slurry comprising 
previously formed silica shells, soluble salts of the desired metal 
containing species, and an alkali hydroxide. 
The average size and shape of the powder particles is controlled by the 
configuration of the silica shells. By appropriately selecting the silica 
shells upon which the metal containing species are to be deposited, the 
invention can tailor the physical characteristics of the powder. The 
silica shells may be 1) equiaxial particles which have an average diameter 
of from about 0.05 to 15 microns, 2) acicular particles that have an 
aspect ratio of from about 2 to 50, and an average diameter of from 0.1 to 
0.5 microns, and; 3) platelike particles which have an aspect ratio of 
from 10 to 150, and an average diameter of 2 to 15 microns, among others. 
The surface area of the silica shells typically ranges from about 25 to 
350 m.sup.2 /g, and the shell thickness is from about 5 to 50 nm; usually 
from about 10 to 20 nm. 
A hydroxide of the metal containing species can be deposited on the silica 
shells by adding, to an aqueous slurry of silica shells, at least one 
soluble salt of the desired metal oxide, and an alkali metal hydroxide 
solution. The silica shells, which are at least partially coated with a 
metal hydroxide, can be recovered from the aqueous slurry by filtration, 
centrifugation, vacuum filtration, among others, and washed with water 
until substantially free from soluble residues. At least a portion of the 
metal hydroxide upon the silica shells is then converted into a metal 
oxide by thermally dehydrating the hydroxide. The metal oxide coating may 
then be reduced further to obtain a coating of a reduced oxide and/or the 
corresponding metal. In this aspect of the invention, the metal oxide is 
reduced, for example, to a metal by being exposed to a high-temperature 
reducing environment, e.g., hydrogen or carbon monoxide. 
The metal oxide is typically present as finely distributed sub-micron size 
crystallites, which are deposited on or about at least a portion of the 
outer surface of the silica shells, e.g., refer to FIGURE 1 which is 
discussed below in greater detail. The average size of the metal oxide 
crystallites ranges from about 50 to 200 Angstroms. The amount of metal 
oxide which is present can range from about 10 to 75% by weight of the 
powder composition. The particular amount of metal containing species, 
e.g., metal oxide, which is provided upon the surface of the silica shell, 
will depend upon the intended end-use of the coated powder. For example, 
when employing a metal oxide as catalyst a relatively small amount of 
metal oxide is typically present upon the silica shell, whereas an 
ultraviolet light absorbing metal oxide may require a relatively large 
amount. In some cases, at least a portion of the finely distributed metal 
species are located within the pores of the silica shell, i.e., the pores 
which were formed during acid extraction to remove the core material. 
The physical characteristics of the coated powder can be better understood 
by reference to FIGURE 1, which is a cross-sectional schematic drawing 
that shows a coated hollow silica shell. Referring now to FIGURE 1, 1 
refers to a representative silica shell which has pores 2. Such pores 
typically result when the core material (not shown) is removed. A metal 
oxide and/or metal crystallites 3 are present generally on or about the 
outer surface of silica shell 1. In some cases, crystallites 3 are present 
within pore 2. 
For certain end-use applications, a silica shell coating can comprising 
oxides such as FeOOH, alpha Fe.sub.2 O.sub.3, among others. Such iron 
oxide coated shells are also useful as precursors for shell products which 
are coated further and/or converted to materials such as with Fe.sub.3 
O.sub.4, gamma Fe.sub.2 O.sub.3, Fe metal, among others. The precursor can 
be converted or reduced by being heated in a reducing environment, e.g., 
containing hydrogen, carbon monoxide, among others. These iron and iron 
oxide coated shells possess desirable dispersion characteristics, and low 
densities when compared with conventional iron oxide powders. Iron oxide 
coated powders of the invention are useful as toners, brown pigments for 
an automotive finish, among others. When the iron oxide coating is 
converted into Fe.sub.3 O.sub.4, the resultant magnetic properties of the 
coated silica shells enable the shells to be used in a variety of 
electronic applications, e.g, circuit boards, carrier for toner, among 
others. 
The size and shape of the coated powders can be controlled, e.g., by 
appropriately selecting a core material which is used to form the silica 
shell, to obtain a wide range of products, e.g., transparent coatings, 
films, among others. The thickness of the metal or metal oxide coating 
upon the silica shells can be varied by controlling the composition of the 
hydroxide reducing agent, process temperatures, reaction time, among 
others. Thus, the composition, configuration, size, among others, of the 
powder can be tailored to satisfy a wide range of end-use applications. 
In another aspect of the invention, at least a portion of the silica shells 
can be coated with an intermediate material before depositing the metal 
containing species. For example, an intermediate coating which comprises 
one or more members from the group of alumina, tin oxide, zirconium oxide, 
among others, can be deposited upon the silica shell. The intermediate 
coating can be applied by any suitable technique such as hydrolysis of a 
soluble salt. When employing an intermediate coating, it is normally 
expedient to deposit the coating using salt hydrolysis techniques which 
are similar to those used to deposit the metal containing species. In some 
cases, it may be desirable to deposit a plurality of layers upon the 
silica shell before, during, and/or after depositing the metal containing 
species. Such layers can be either chemically similar and/or distinct. 
Yet another aspect of the invention relates to forming metallic coatings 
upon the silica shells, which are obtained by converting at least a 
portion of the oxide coatings to its corresponding metal, e.g., converting 
iron oxide to iron. The metallic coatings can be formed by exposing the 
metal oxide to a high-temperature reducing environment, e.g., gaseous 
hydrogen, carbon monoxide, among others. For example, a metal oxide 
coating upon a silica shell can be reduced to a metal be being heated to a 
temperature of about 500.degree.-900.degree. C., and contacted with an 
atmosphere comprising at least one member of hydrogen, carbon monoxide, 
among others. 
A powder composition of the invention can be prepared by a process which 
generally comprises: 
(a) coating an aqueous slurry of a finely divided inert core material such 
as calcium and/or barium carbonate, among others, with silica, e.g., 
amorphous hydroxylated silica; 
(b) removing the core material, e.g., by acid extraction, thereby obtaining 
an aqueous slurry of hollow shells; 
(c) recovering the silica shells, washing the shells substantially free 
from soluble residues or species, and optionally drying; 
(d) preparing an aqueous slurry of the silica shells, and depositing at 
least one finely distributed metal hydroxide upon the shell, which is 
obtained by adding to the aqueous shell slurry at least one soluble salt 
of the desired metal, and an alkali metal hydroxide; 
(e) recovering the solids, washing the solids substantially free from water 
soluble residues or species and drying; 
(f) heating the solids to convert the coating of hydroxide to oxide; and 
(g) optionally heating the oxide coated particles in a reducing atmosphere 
to convert the oxide coating to a reduced oxide and/or the corresponding 
metal. 
In one aspect of the invention, the core material is not removed, and a 
silica skin is coated with a finely distributed metal containing species. 
Should the presence of the core material be desired, the pH, which is used 
in further processing, should be controlled in order to prevent 
dissolution of the core. In this aspect of the invention, the silica shell 
and/or skin composition can be modified to include additional components. 
Examples of suitable additional components comprise one or more members 
from the group of boric oxide, aluminum oxide, zirconium oxide, among 
others. For example, when employing a process which deposits silica upon a 
core material, one or more salts of an additional skin component can be 
deposited along with the silica. The additional component, which may be 
present as a complex oxide, mixture or solid solution with silica, becomes 
a part of the skin. If desired, at least a portion of the additional 
component and the core material can be removed by exposing such a skin to 
an appropriate acid. Whether or not the additional component is removed, 
by employing an effective quantity of an additional component when 
depositing silica upon the core material., the surface area of the 
resultant powder can be increased. 
The silica shells can be coated by preparing a suspension, which contains 
about 100 to 700 g/l of silica shells, that is heated to about 40.degree. 
C. and 90.degree. C., and typically continuously agitated. Any suitable 
means, such as a paddle stirrer, can be employed to agitate the 
suspension. An aqueous solution of at least one metal salt, e.g, an 
aqueous solution of a mixture of metal salts, and an aqueous alkali metal 
hydroxide, e.g., ammonium hydroxide solution, sodium hydroxide, among 
others, are typically added concurrently to the agitated slurry while 
maintaining the pH between about 2 and 8. After adding all the reagents to 
the slurry, agitating and heating are usually continued for about half an 
hour to one hour for ensuring substantially complete deposition of the 
metal hydroxide upon the silica shells. 
Any suitable water soluble salts can be employed as the source of the metal 
containing species or oxides. Suitable water soluble salts include 
chlorides, nitrates, among others. The concentration of the salt solutions 
typically ranges from about 50 g/l to 600 g/l, when the desired metal 
containing species of the product is between 10 to 75 wt %. When the 
desired quantity of metal containing species is greater, the concentration 
of the salt can be increased. 
During the deposition of the metal hydroxides, the pH of the suspension can 
be controlled. Typically, the pH will be decreased by adding an acid such 
as HCl. However, when the pH becomes too great a basic material such as 
NaOH is added to the suspension. By introducing the appropriate quantity 
of acid and/or basic material, the pH can be controlled to satisfy a 
desired range. 
The solids, which comprise metal hydroxide coated silica shells, can be 
recovered from the slurry by any suitable process such as filtration, 
centrifugation, vacuum filtration, among others. The recovered solids can 
be washed with water until substantially free from soluble residues, and 
normally dried at a temperature which ranges from about 110.degree. to 
150.degree. C. 
The dry solids can be calcined in an oxidizing atmosphere, for example, air 
at a temperature which ranges from about 550.degree. to 900.degree. C., 
and normally about 750.degree. C. Typically, the powder is calcined for 
about one to two hours, in order to convert at least a portion of the 
deposited hydroxides to the corresponding oxides. 
At least a portion of the oxide coating on the silica shells may be reduced 
to a lower oxide and/or to the corresponding metal. The oxide coating is 
reduced by being heated in a reducing atmosphere, such as hydrogen, carbon 
monoxide, among others, at a temperature which ranges from about 300 to 
800.degree. C. 
While particular emphasis in the above description has been placed upon 
silica shells which are coated with a metal containing species such as a 
metal and oxides thereof, the invention is capable of producing a wide 
range of products. For example, the composition of the silica skin, which 
surrounds a core material, may be modified, for example, to include other 
materials such as B.sub.2 O.sub.3, Al.sub.2 O.sub.3, ZrO.sub.2, among 
others. Further, one or more metal containing species may be deposited 
upon the silica shells either simultaneously and/or as sequential layers. 
Accordingly, the present invention can be employed to produce a product 
which has been tailored to satisfy a wide range of end-use applications. 
Compositions of the invention and processes for obtaining the same are 
illustrated in greater detail by the following Examples which are not to 
be construed as limiting in any way the scope of the invention. Unless 
specified otherwise, percentages are in weight percent, and the materials 
used in these Examples were commercially available. 
EXAMPLE 1 
This Example describes a process for preparing iron oxide coated silica 
shells. 
About three liters of de-ionized water was added to a 1-gallon Waring 
blender jar, and the pH was increased to about 10.0 by adding 20% NaOH. A 
stock solution which comprised potassium silicate was produced that had a 
SiO.sub.2 /K.sub.2 O molar ratio of about 3.3, and contained about 26 wt % 
SiO.sub.2. Approximately 100 g of this stock solution was added to the 
solution in the Waring Blender jar and thereafter about 1,700 g of 
CaCO.sub.3 powder, (Albacor H.O. Dry, available from Pfizer Corp.) was 
added to form a mixture. The mixture was blended at high speed for about 
two minutes to form a slurry. 
The slurry was transferred to a 18-liter, polyethylene beaker, steam heated 
to about 90.degree. C., stirred for about one half-hour (the pH was about 
9.5). Next about 1,027 g of the potassium silicate stock solution, 
described above, was diluted with water to 1 liter, and added to the 
slurry over a period of about 4 hours. The pH was maintained at about 9.0 
by the concurrent addition of hydrochloric acid. The hydrochloric acid 
used for controlling the pH consisted of about 255 ml 37% HCl diluted with 
water to 1 liter. 
The slurry, which had a pH of about 9.0, and a temperature of about 
90.degree. C., was stirred for about one half-hour. The pH was decreased 
to about 7.5 by adding hydrochloric acid, which caused solids to 
floculate. The solids were washed with de-ionized water to remove soluble 
species, and dried in an air oven at about 110.degree. C. The dry powder 
weighed about 1931 g, and had a nitrogen surface area of about 7.8 m.sup.2 
/g. 
Approximately 500 ml of deionized water, and 300 g of the dry solids were 
admixed in a Waring Blender for two minutes to form a slurry. The slurry 
was transferred to a polyethylene beaker which was agitated. After 
diluting the slurry with water to 1 liter, and heating to 90.degree. C. 
with steam, approximately 415 ml of 37% HCl was added to reduce the pH to 
2.0, which removed the CaCO.sub.3 cores and formed substantially hollow 
silica shells. 
A solution was prepared containing about 85 g FeCl.sub.3.5H.sub.2 O, 
(equivalent to 27 g Fe.sub.2 O.sub.3), within about 200 ml of water. This 
solution was added to the aqueous slurry of silica shells concurrently 
with 20% NaOH over a period of about two hours, at a temperature of about 
90.degree. C., while maintaining the pH at about 2.5. The slurry was 
stirred, and the resultant solids separated by filtration. The solids were 
washed with de-ionized water to remove soluble species and dried in an air 
oven at about 110.degree. C. Approximately 73 g of a light brown to yellow 
powder were recovered. When this powder was heated in a hydrogen 
environment at 550.degree. C. for about two hours, a black magnetic powder 
was obtained. 
EXAMPLE 2 
This Example describes a process for preparing aluminum oxide coated silica 
shells. 
A procedure substantially in accordance with Example 1 was used to obtain a 
dilute aqueous solution of potassium silicate. To this solution was added 
600 g of CaCO.sub.3 powder, (Albacor H. O. Dry, available from Pfizer 
Corp.), while mixed in a 1-gallon high speed Waring blender jar for about 
two minutes which formed a slurry. The slurry was transferred to a 
18-liter polyethylene beaker, steam heated to about 90.degree. C. in one 
half-hour, stirred (after which the pH was 9.8). Next about 1,027 g of a 
potassium silicate stock solution, which had a SiO.sub.2 /K.sub.2 O molar 
ratio of 3.3, and contained about 26 wt % SiO.sub.2, was diluted with 
water to 1 liter. The diluted potassium silicate was added to the slurry 
over a period of about 4 hours. 
The pH of the slurry was maintained at about 8.5 by the concurrent addition 
of a solution which comprised about 209 ml 37% HCl, 28 g 
CaCl.sub.2.2H.sub.2 O, and 1 liter of water. The slurry was then stirred 
at 90.degree. C. and pH 8.5 for 15 minutes. The pH of the slurry was 
adjusted to about 7.0 by adding HCl. The slurry was allowed to stand 
undisturbed overnight to permit the solids to settle. The solids were 
separated by decantation, and washed several times by slurrying with 
deionized water and decanting. 
The washed and decanted solids were re-slurried with one liter of 37% HCl, 
which changed the pH to about 2.0, and heated to about 90.degree. C., 
thereby removing the CaCO.sub.3 cores and forming hollow silica shells. 
The pH of silica shell slurry was adjusted to about 4.5 by adding 20% 
NaOH. About one liter of an aqueous solution containing approximately 110 
g of Al(NO.sub.3).sub.3.9H.sub.2 O was added, and stirred into the slurry. 
The pH was maintained at about 4.5 by adding 20% NaOH, and the temperature 
at 90.degree. C. The slurry was then stirred while at a temperature of 
about 90.degree. C., a pH of about 4.5, for one half-hour after which 
produced solids. The solids were recovered substantially in accordance 
with the procedures described in Example 1. 
EXAMPLE 3 
This Example describes a process for preparing zirconium oxide coated 
silica shells. 
A slurry comprising about 200 g CaCO.sub.3 powder (Albacor H.O. Dry 
available from Pfizer Corp.), and about 2500 ml of de-ionized water was 
prepared in a 4 liter beaker. The slurry was agitated, heated to about 
80.degree. C., and the pH adjusted to 9.5 by adding 20% NaOH. A solution 
comprising potassium silicate was prepared by diluting about 200 g of the 
potassium silicate stock solution described in Example 2 into about 200 ml 
of a solution comprising de-ionized water and calcium chloride. (The 
solution containing calcium chloride was prepared by dissolving about 50 g 
of CaCl.sub.2 into 1 liter of de-ionized water). All the potassium 
silicate solution and about 10 ml of the CaCl.sub.2 solution were added 
concurrently to the agitated CaCO.sub.3 slurry over a period of about 2 
hours, at a temperature of about 80.degree. C., while maintaining the pH 
at 9.5 by adding 20% HCl. The resultant slurry was then stirred, while at 
a temperature of about 80.degree. C. and a pH of about 9.5, for 30 
minutes. 
The pH of the agitated slurry was decreased to about 2.0 by adding 340 ml 
37% HCl, removing the CaCO.sub.3 cores and form silica shells. 
Approximately 1230 ml of a solution comprising ZrOCl.sub.2, which 
contained the equivalent of 20 wt % ZrO.sub.2, was then added to the 
slurry of silica shells over a two hour period, while maintaining the pH 
at about 2.0 by adding 20% NaOH. The slurry was stirred for about 30 
minutes after which the solids were separated by filtration. The solids 
were washed with de-ionized water to remove soluble species and dried in 
an air oven at about 120.degree. C. About 487g of powder was recovered. 
The nitrogen surface area of the recovered powder was about 189 m.sup.2 /g. 
Two samples of the recovered powder were calcined, respectively, at 
1000.degree. C. and 1200.degree. C., for about one hour. The calcined 
powders were analyzed by x-ray diffraction, and found to contain a major 
phase corresponding to tetragonal ZrO.sub.2 and a minor phase of 
monoclinic ZrO.sub.2. 
EXAMPLE 4 
This Example describes a process for preparing silica shells coated with 
chromium and antimony oxides. 
A CaCO.sub.3 slurry substantially as that described in Example 3 was 
prepared in a 2-liter beaker using about 100 g of CaCO.sub.3 and 1000 ml 
of de-ionized water. The slurry was agitated, heated to 90.degree. C., and 
the pH adjusted to about 9.5 by adding 20% NaOH. A solution of potassium 
silicate was prepared by diluting about 100 g of the potassium silicate 
stock solution, described in Example 2, to 200 ml with deionized water. 
The diluted stock solution was added to the agitated CaCO.sub.3 slurry 
over a period of about 2 hours, at 90.degree. C., while maintaining the pH 
at about 9.5 by adding 20% HCl. The slurry was stirred for about 30 
minutes. 
The pH of the agitated slurry was decreased to about 2.0 by adding about 
170 ml of 37% HCl, thereby removing the CaCO.sub.3 cores and forming 
hollow silica shells. A solution comprising chromium chloride was prepared 
by dissolving about 27 g of CrCl.sub.3.6H.sub.2 O into 100 ml of 37% HCl. 
A solution comprising antimony chloride was prepared by dissolving about 
500 g SbCl.sub.3 into 1000 ml of 37% HCl. Then, approximately 48 ml of the 
antimony chloride solution was admixed with the chromium chloride 
solution, and the mixture was added to the silica shell slurry over an 
about one hour period. The pH was maintained at about 2.0 by adding 20% 
NaOH. The slurry was stirred for about 30 minutes after which the solids 
were filtered, washed and dried substantially as described in Example 3. 
About 46 g of powder was recovered. 
The nitrogen surface area of the recovered powder was about 142 m.sup.2 /g. 
A sample of the powder was taken and calcined at about 500.degree. C. The 
calcined powder was analyzed by x-ray diffraction which indicated that the 
powder contained a major crystalline phase which corresponded to 
CrSbO.sub.4. Further analysis by EDAX showed Si, Cr and Sb to be the only 
metallic elements present in the calcined powder. 
EXAMPLE 5 
This Example describes a process for preparing silica shells coated with 
iron, nickel and zinc oxides. 
A CaCO.sub.3 slurry was prepared substantially in accordance with Example 
4, by using about 1200 ml of de-ionized water. Two solutions which, 
respectively, comprised potassium silicate and calcium chloride were 
prepared substantially in accordance with Example 3, by using about 100 g 
the potassium silicate stock solution described above. All the potassium 
silicate solution, which was prepared, and about 10 ml of the CaCl.sub.2 
were added concurrently to an agitated CaCO.sub.3 slurry over a period of 
2 hours. The temperature of the slurry was about 80.degree. C. The pH was 
maintained at about 9.5 by adding 20% HCl. The slurry was stirred for 
about 30 minutes. 
The pH of the agitated slurry was then decreased to about 2.0 by adding 
about 170 ml of 37% HCl, which removed the CaCO.sub.3 cores, thereby 
forming silica shells. 
A solution comprising iron chloride was prepared by dissolving about 100 g 
of FeCl.sub.3.6H.sub.2 O into 250 ml of 7.5% HCl. The iron chloride 
solution was added to the agitated silica shell slurry, which was at 
80.degree. C., over an approximately 2 hour period, while maintaining the 
pH at about 2.0 by adding 20% NaOH. The slurry was stirred for about 30 
minutes, after which the solids were separated by filtration. The solids 
were washed several times with de-ionized water to remove soluble species, 
which produced a filter cake. 
The washed filter cake was re-slurried in about 500 ml of de-ionized water 
within a 2 liter beaker, and heated to about 60.degree. C. A solution 
comprising nickel chloride was prepared by dissolving about 22 g of 
NiCl.sub.2.6H.sub.2 O into 50 ml of de-ionized water. Another solution 
which comprised zinc chloride was prepared by dissolving about 13 g of 
zinc chloride into 50 ml of de-ionized water. These two solutions were 
admixed together, and about two ml of 37% HCl was added. The solution 
mixture was added to the filter cake slurry over a period of about one 
hour, at a temperature of 60.degree. C., while maintaining the pH at about 
8.5 by adding 20% NaOH. The resultant slurry was stirred for about 30 
minutes after which the solids were separated by filtration, washed, and 
dried substantially in the manner described in Example 3. About 80 g of 
powder was recovered. 
A portion of the recovered powder was calcined in air for about 1 hour at a 
temperature of about 900.degree. C. The calcined sample was examined by 
X-ray diffraction analysis which indicated the presence of a phase that 
corresponded to magnetite, i.e., Fe.sub.3 O.sub.4. 
EXAMPLE 6 
This Example describes a process for preparing silica shells which are 
coated with iron, tin and antimony oxides. 
An aqueous slurry of silica shell particles was prepared substantially in 
the manner described in Example 5, with the exception that about 100 g of 
potassium silicate stock solution was diluted to about 100 ml with 
de-ionized water, and about 5 ml of CaCl.sub.2 solution, (50 g/l) was 
used. 
A solution comprising iron chloride was added to an agitated slurry of 
silica shells, which had a temperature of about 80.degree. C., over an 
approximately two hour period substantially in the manner described in 
Example 5. 
A solution comprising about 140 ml of a 37% HCl solution which contained 
SnCl.sub.4, (equivalent to 0.375 g SnO.sub.2 /ml), and SbCl.sub.3, 
(equivalent to 0.100 g Sb.sub.2 O.sub.3 /ml), was added to the agitated 
slurry. The solution was added over an approximately 2 hour period while 
maintaining the pH at 2.0 by adding 20% NaOH. The slurry was stirred for 
about 30 minutes, after which the solids were separated by filtration, 
washed, and dried substantially in the manner described in Example 3. 
About 123 g of powder was recovered. 
A portion of the powder was calcined in air at about 750.degree. C. for a 
period of about 2 hours. A sample of the calcined powder was examined by 
X-ray diffraction analysis which indicated the presence of a major 
crystalline phase that corresponded to SnO.sub.2 with a trace of 
crystalline Fe.sub.2 O.sub.3. 
EXAMPLE 7 
This Example describes a process for preparing iron coated silica shells. 
Approximately 100 g of CaCO.sub.3 powder was slurried into a 1 liter of 
de-ionized water within a stirred 2 liter beaker. The slurry was heated to 
about 90.degree. C., and the pH was increased to about 9.5 by adding 20% 
NaOH. 
About 100 g of potassium silicate stock solution substantially as described 
in Example 1 was diluted to 200 ml with de-ionized water. About 50 g of 
CaCl.sub.2 was dissolved into 1 liter of de-ionized water. Approximately 
200 ml of diluted K.sub.2 SiO.sub.3, and 10 ml of CaCl.sub.2 solution were 
added concurrently to the CaCO.sub.3 slurry, which was at a temperature of 
about 90.degree. C., and continuously agitated for about 2 hours while 
maintaining the pH at about 9.5 by adding 20% HCl. The slurry was stirred 
for an additional 30 minutes. 
About 172 ml of concentrated HCl was then added to the slurry for 
decreasing the pH of the slurry to about 2.0, which removed the CaCO.sub.3 
cores, thereby forming hollow silica shells. 
A solution was prepared which contained about 114 g of FeCl.sub.3.6H.sub.2 
O, (equivalent to 67.38 g of Fe.sub.2 O.sub.3), in 100 ml of water, and 
about 100 ml concentrated HCl. This solution was added to the aqueous 
slurry concurrently with 20% NaOH over a period of 90 minutes while 
maintaining the temperature at about 90.degree. C., and the pH at 2.0. The 
slurry was stirred for a further 30 minutes, and the solids separated by 
filtration. The separated solids were washed with de-ionized water to 
remove soluble species, and dried in an air oven at about 120.degree. C. 
Approximately 65 g of powder was recovered. 
The recovered powder was calcined in air at about 600.degree. C. for about 
2 hours. A sample of the calcined powder was taken. The nitrogen surface 
area of the calcined powder was about 117 m.sup.2 /g. The calcined powder 
was examined by X-ray diffraction analysis which indicated the presence of 
amorphous material that gave a weak broad peak pattern corresponding to 
FeOOH. 
Approximately 4 g of the calcined powder was placed in an alumina boat and 
loaded into a horizontal tube furnace. The powder was heated at about 
750.degree. C. for about 90 minutes within a reducing atmosphere which 
contained 50% hydrogen. After cooling the furnace to room temperature the 
reduced powder was examined by x-ray diffraction analysis, which indicated 
that the powder contained Fe.sub.2 SiO.sub.4 and Fe as the major 
crystalline phases. The iron content as determined by EDAX analysis was 
about 60%. The average crystallite size of the iron was about 1130 
Angstroms. 
EXAMPLE 8 
This Example describes a process for preparing nickel coated silica shells. 
Substantially in accordance with the procedure described in Example 1, 
about 100 g of CaCO.sub.3 powder was slurried into one liter of de-ionized 
water. The slurry was heated to about 80.degree. C., and the pH was 
increased to about 9.5 by adding 20% NaOH. 
Approximately 100 g of the stock K.sub.2 SiO.sub.3 solution, described in 
Example 1, was diluted to 200 ml by using de-ionized water. About 50 g of 
CaCl.sub.2 was dissolved in one liter of de-ionized water. The diluted 
K.sub.2 SiO.sub.3 solution and 10 ml of the CaCl.sub.2 solution were added 
concurrently over a period of two hours to the CaCO.sub.3 slurry. The 
temperature of the slurry was maintained at about 80.degree. C., and the 
pH at about 9.5 by adding 20% HCl. The slurry was stirred for about half 
hour. 
About 175 ml of concentrated HCl was slowly added to the slurry, and the pH 
was stabilized at about 2.0. Next the pH of the slurry was increased to 
about 8.0 by adding 20% NaOH. A solution comprising 120 g of nickel 
chloride, NiCl.sub.2.6H.sub.2 O, about 100 ml concentrated HCl, and 100 ml 
of de-ionized water, was added to the agitated slurry over a two hour 
period during which the pH was kept at about 8.0 by adding 20% NaOH. 
After a period, the solids from the slurry were recovered by filtration, 
washed, and dried in an air oven at 120.degree. C. The dry powder yield 
was about 77g, and the nitrogen surface area of the product was about 377 
m.sup.2 /g. 
The dried powder was charged into two 10".times.1" alumina boats, and 
loaded into a horizontal tube furnace. The powder was heated to about 
800.degree. C. for about 3 hours in an atmosphere consisting of about 50% 
hydrogen and 50% argon. After cooling the powder to room temperature, 
tinder argon a black powder was obtained. The black powder was examined by 
X-ray diffraction analysis which indicated that nickel was the major 
crystalline phase with a minor amount of NiO. 
EXAMPLE 9 
This Example describes a process for preparing copper coated silica shells 
which have an intermediate titania coating. 
A procedure substantially in accordance with Example 6 was used to obtain 
an aqueous agitated slurry of silica shells. Approximately 30 ml of a 
solution of SnCl.sub.4, (equivalent to 0.445 g of SnO.sub.2 /ml), was 
added to the slurry which was at a temperature of about 80.degree. C., 
over a period of about 20 minutes. 
Next a solution comprising about 100 ml of TiCl.sub.4 (28.2%), 2.5 ml of 
SnCl.sub.4 solution, and 25 ml of concentrated HCl was added over a period 
of 11/2 hours to the agitated slurry, while maintaining the temperature at 
about 80.degree. C. and the pH at about 2.0 by adding 20% NaOH. After 
about half an hour the solids from the slurry were recovered by 
filtration, washed, and dried in an air oven at about 120.degree. C. The 
dry product yield of TiO.sub.2 coated silica shell particles was about 58 
g. 
Approximately 40 g of the above product was slurried into about 350 ml of 
de-ionized water. About 2 g of copper acetate, Cu(CH.sub.3.COO).sub.2 !, 
was added to this slurry, and the pH was adjusted to about 8.0 with 20% 
NaOH. A solution of copper acetate, which comprised about 20 g of 
Cu(CH.sub.3.COO).sub.2 dissolved into 100 ml of de-ionized water, was 
added to the slurry over a period of about one hour. The slurry was 
stirred for about 30 minutes. 
The solids were recovered from the slurry by filtration, washed 
substantially free from soluble species, dried at about 120.degree. C., 
and calcined at about 800.degree. C. for two hours. The dry product yield 
was about 44 g. The product was examined by X-ray diffraction analysis 
which indicated that the major crystalline phases corresponded to 
SnO.sub.2, TiO.sub.2 and CuO. The X-ray pattern also indicated the 
presence of some amorphous material. 
A portion of the calcined powder was reduced by being exposed to a high 
temperature hydrogen-containing environment substantially in the manner 
described in Example 5. The reduction conditions were 600.degree. C. for 
two hours. The calcined powder was examined by X-ray diffraction analysis 
which indicated that the major crystalline phase corresponded to Cu, a 
minor phase of TiO.sub.2, and trace amounts of CuO and SnO.sub.2. The 
X-ray pattern also indicated the presence of some amorphous material. 
While certain desirable aspects of the invention have been described above 
in detail, a person in this art will recognize that a variety of 
variations and embodiments are encompassed by the appended claims.