U.S. patent application number 10/630146 was filed with the patent office on 2004-06-10 for electrochemical production of nanoscale metal (mixed) oxides.
Invention is credited to Dierstein, Andrea, Hempelmann, Rolf, Kropf, Christian, Natter, Harald, Stephan, Hans-Oskar.
Application Number | 20040108220 10/630146 |
Document ID | / |
Family ID | 7672333 |
Filed Date | 2004-06-10 |
United States Patent
Application |
20040108220 |
Kind Code |
A1 |
Stephan, Hans-Oskar ; et
al. |
June 10, 2004 |
Electrochemical production of nanoscale metal (mixed) oxides
Abstract
A process for the production of amorphous, crystalline, or mixed
oxides of metals having mean particle diameters of 1 to 500 nm, by
providing an electrolysis cell having a cathode in a cathode
half-cell and an anode in an anode half cell, providing in the
electrolysis cell a solution comprising ions of a metal or metals
from which the oxide particles are to be formed dissolved in an
organic electrolyte, and electrochemically reducing the metal ions
at the cathode in the presence of an oxidizing agent while impeding
or preventing passage of halogens from the anode half cell to the
cathode half cell, to form the oxide or oxides.
Inventors: |
Stephan, Hans-Oskar;
(Duisburg, DE) ; Kropf, Christian; (Hilden,
DE) ; Hempelmann, Rolf; (St. Ingbert, DE) ;
Dierstein, Andrea; (Schiffweiler, DE) ; Natter,
Harald; (Saarbruecken, DE) |
Correspondence
Address: |
HENKEL CORPORATION
THE TRIAD, SUITE 200
2200 RENAISSANCE BLVD.
GULPH MILLS
PA
19406
US
|
Family ID: |
7672333 |
Appl. No.: |
10/630146 |
Filed: |
July 30, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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10630146 |
Jul 30, 2003 |
|
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PCT/EP02/00826 |
Jan 26, 2002 |
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Current U.S.
Class: |
205/538 |
Current CPC
Class: |
Y02E 60/368 20130101;
Y02E 60/36 20130101; C25B 1/00 20130101 |
Class at
Publication: |
205/538 |
International
Class: |
C25B 001/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 30, 2001 |
DE |
101 04 302.3 |
Claims
1. A process for the production of amorphous and/or crystalline
(mixed) oxides of metals, more particularly metals of the third to
fifth main group or the secondary groups of the periodic system,
with mean particle diameters of the (mixed) oxides in the range
from 1 to 500 nm and more particularly 1 to 100 nm, characterized
in that, in an electrolysis cell equipped with a cathode and an
anode, ions dissolved in organic electrolyte of that metal or those
metals of which the (mixed) oxide is to be produced are
electrochemically reduced at the cathode in the presence of an
oxidizing agent, characterized in that the cathode compartment
(cathode half-cell) and the anode compartment (anode half cell) are
separated from one another by a porous partition or separating wall
(diaphragm) which allows current to pass through, i.e. is permeable
to cations and anions, but impedes and in particular prevents the
passage of elemental halogen from the anode to the cathode
compartment.
2. A process as claimed in claim 1, characterized in that the
porous partition or separating wall in particular has a high
halogen retention capacity.
3. A process as claimed in claim 1 or 2, characterized in that the
porous partition or separating wall is a porous glass frit, more
particularly with a pore diameter of about 1 to 4 .mu.m.
4. A process as claimed in claim 1 or 2, characterized in that the
porous partition or separation wall is a polytetrafluoroethylene
filter or a polytetrafluoroethylene membrane or a porous
polyethylene film or polyethylene disk, more particularly with a
pore diameter of about 1 to 4 .mu.m.
5. A process as claimed in claim 1 or 2, characterized in that the
porous partition or separation wall is a proton-conducting
membrane, preferably with a pore diameter of about 1 to 4
.mu.m.
6. A process as claimed in claim 5, characterized in that the
proton-conducting ion exchanger membrane is in particular a
perfluorinated ion exchanger membrane, preferably with a pore
diameter of about 1 to 4 .mu.m.
7. A process as claimed in any of claims 1 to 6, characterized in
that organic electrolyte used is a substance which is liquid at
temperatures in the range from about -78.degree. C. to about
+260.degree. C. and more particularly in the range from about
0.degree. C. to about 60.degree. C. at normal pressure and is
preferably selected from the group consisting of ketones, alcohols
and polyalcohols, ethers, nitrites and aromatic compounds.
8. A process as claimed in claim 7, characterized in that the
organic electrolyte is selected from the group consisting of
n-propanol, i-propanol, glycerol and mixtures thereof with acetone,
tetrahydrofuran, acetonitrile and toluene.
9. A process as claimed in any of claims 1 to 8, characterized in
that the organic electrolyte additionally contains small quantities
of water, more particularly about 0.01 to about 2% by weight and
preferably about 0.05 to about 1% by weight, based on the total
quantity of the organic electrolyte and water.
10. A process as claimed in any of claims 1 to 9, characterized in
that the organic electrolyte additionally contains a dissolved
supporting electrolyte which may be selected in particular from the
group consisting of electrolyte-soluble hexafluorophosphates,
sulfonates, acetyl acetonates, carboxylates and quaternary
phosphonium and/or ammonium salts, more particularly with organic
groups at the phosphorus and/or nitrogen, the supporting
electrolyte being used together with surfactants which stabilize
the metal oxide particles produced and preferably being selected
from the entire class of surfactants, the following compounds being
particularly preferred: fatty alcohols, fatty alcohol ethoxylates,
polyoxyethylene carboxylic acids and/or fatty acid ethoxylates.
11. A process as claimed in claim 10, characterized in that the
supporting electrolyte used is a quaternary ammonium compound which
may optionally carry one or more aryl and/or alkyl groups at the
nitrogen and which is preferably present as a halide, such as
tetrabutyl ammonium bromide.
12. A process as claimed in any of claims 1 to 11, characterized in
that the organic electrolyte has a temperature of 30 to 120.degree.
C.
13. A process as claimed in one or more of claims 1 to 12,
characterized in that the oxidizing agent is selected from the
group consisting of oxygen, hydrogen peroxide, peroxo compounds and
oxo anions of the halogens chlorine, bromine or iodine where the
halogen in particular has an oxidation number of +1 to +5.
14. A process as claimed in claim 13, characterized in that the
oxidizing agent is air (atmospheric oxygen), the oxidizing agent
preferably being supplied to the reaction system by introduction of
air into the electrolyte solution.
15. A process as claimed in any of claims 1 to 14, characterized in
that the electrical d.c. voltage between cathode and anode is
preferably adjusted so that a current density of 0.05 to 10
mA/cm.sup.2, based on the anode area, and preferably in the range
from 1 to 5 mA/cm.sup.2, is obtained.
16. A process as claimed in claim 15, characterized in that an
electrical d.c. voltage of 1 to 100 volts is applied between
cathode and anode.
17. A process as claimed in any of claims 1 to 16, characterized in
that the electrolyte is agitated, more particularly by stirring,
introduction of a gas and/or ultrasonication.
18. A process as claimed in any of claims 1 to 17, characterized in
that the anode and/or cathode material is a material which is inert
under the selected electrolysis conditions and which is selected in
particular from the group consisting of platinum and other platinum
metals, gold, niobium, tantalum, tungsten, graphite and glassy
carbon.
19. A process as claimed in any of claims 1 to 18, characterized in
that the metal ions dissolved in the electrolyte are produced by
dissolving in the electrolyte a salt of the metal or salts of those
metals whose oxide or mixed oxide is to be produced.
20. A process as claimed in claim 19, characterized in that a
halide, nitrate, acetate, sulfonate, carboxylate or
hexafluorophosphate is used as the metal salt.
21. A process as claimed in any of claims 1 to 20, characterized in
that, in the production of mixed oxides of at least two metals, the
metal ions dissolved in the electrolyte are produced by dissolving
a salt of one metal in the electrolyte and using an anode
containing the other metal as a sacrificial anode.
22. A process as claimed in any of claims 1 to 21, characterized in
that the metal (mixed) oxide formed is separated from the
electrolyte and dried.
23. A process as claimed in claim 22, characterized in that the
dried metal (mixed) oxide is subsequently calcined, more
particularly at a temperature in the range from about 300 to about
1200.degree. C. and preferably at a temperature in the range from
about 400 to about 1,000.degree. C., or subjected to a hydrothermal
treatment.
24. The use of the process claimed in any of claims 1 to 23 for the
production of amorphous and/or crystalline oxides and mixed oxides
of metals of the third to fifth main group or the secondary groups
of the periodic system, more particularly of titanium, zirconium,
chromium, molybdenum, iron, cobalt, nickel, indium, tin, lead
and/or aluminium.
Description
[0001] This invention relates to a process for the production of
amorphous and/or crystalline oxides and mixed oxides of metals,
more particularly metals of the third to fifth main group or the
secondary groups, the (mixed) oxides thus produced having mean
particle diameters in the nanometer range. In the context of the
present invention, the "nanometer range" is understood in
particular to be the range from about 1 nm to about 500 nm and
preferably the range from about 1 nm to about 100 nm.
[0002] Metal oxides such as these may be used for various
industrial applications, for example as dielectrics for
miniaturized multilayer capacitors, as catalysts, as additives in
paints and cosmetics, as additives in plastics to stabilize them
against thermal or photochemical decomposition and/or to modify
their dielectric and/or magnetic properties and as polishes.
[0003] Metal oxides with particle diameters in the nanometer range
may be obtained, for example, by dissolving alkoxides of the metals
in a water-immiscible solvent, preparing an emulsion of the
resulting solution in water using suitable surfactants, the
emulsified droplets of the solvent having diameters in the
nanometer range, and hydrolyzing the metal alkoxides to the oxides.
The disadvantages of this process lie in particular in the fact
that the metal alkoxides are expensive starting materials, in the
fact that emulsifiers also have to be used and in the fact that the
preparation of the emulsion with droplet sizes in the nanometer
range is a complicated process step.
[0004] It is also known that metal particles (not metal oxide
particles!) with a particle size below 30 nm can be produced by
cathodically reducing suitable metal salts in organic solvents or
mixtures thereof with water in the presence of a stabilizer and
optionally in the presence of a supporting electrolyte. Instead of
dissolving metal salts in the electrolyte, the metal ions to be
cathodically reduced can also be dissolved by using anodes of the
corresponding metals which dissolve during the electrolysis. One
such process is described in DE-A-44 43 392 and the corresponding
EP-A-0 672 765.
[0005] In addition, DE-A-44 08 512 and the corresponding EP-A-0 672
765 describes a process for the electrolytic production of metal
colloids in which one or more metals are anodically dissolved in
aprotic organic solvents in the presence of a supporting
electrolyte and cathodically reduced in the presence of stabilizers
to colloidal metal solutions or redispersible metal colloid powders
with a particle size below 30 nm. The supporting electrolyte and
the stabilizer may be identical. If the cathodic reduction is
carried out in the presence of suitable supports, the metal
colloids are precipitated onto those supports.
[0006] In addition, according to Chemical Abstracts Report
110:65662, fine-particle zirconium oxide powder can be obtained by
electrochemically producing a base in a solution of zirconyl
nitrate, the zirconyl nitrate being hydrolyzed by the base with
precipitation of hydrated zirconium oxide. Crystalline zirconium
oxide can be obtained from the hydrated zirconium oxide by
calcination.
[0007] According to Chemical Abstracts Report 114:31881, mixed
oxides of iron, nickel and zinc can be produced by
electrochemically precipitating a hydroxide mixture of those metals
from metal salt solutions and calcining the isolated hydroxides to
the mixed oxides.
[0008] C. Pascal et al., "Electrochemical Synthesis for the Control
of .gamma.-Fe.sub.2O.sub.3 Nanoparticle Size. Morphology,
Microstructure and Magnetic Behavior" in Chem. Mater. 1999, 11,
pages 141-147, describe the electrochemical synthesis of
nanoparticles of maghemite (.gamma.-Fe.sub.2O.sub.3) in organic
media by control of the current density and using cationic
surfactants to stabilize the colloidal suspension of the particles
obtained.
[0009] R. M. Nyffenegger et al., "A Hybrid Electrochemical/Chemical
Synthesis of Zinc Oxide Nanoparticles and Optically Intrinsic Thin
Films" in Chem. Mater. 1998, 10, pages 1120-1129, describe a
chemical/electrochemical hybrid route for the production of zinc
oxide nanoparticles and films, metallic zinc being
electrochemically deposited onto a graphite electrode in a first
step and the metallic zinc thus deposited being oxidized and
dehydrated in a subsequent step.
[0010] E. P. Reddy et al., "Preparation and Characterization of
Cobalt Oxide Nanosized Particles Obtained by an Electrochemical
Method" in NanoStructured. Materials, Vol. 12, pages 61-64, 1999,
describe a two-stage process for the electrochemical/chemical
production of cobalt oxide particles in which, in a first stage,
cobalt cations are precipitated in the presence of tetraalkyl
ammonium salts as stable cobalt metal clusters which are then
oxidized under the effect of oxygen to form nanoscale cobalt oxide
particles which are stabilized with an ammonium salt.
[0011] S. Mahamuni et al., "Spectroscopic and Structural
Characterization of Electrochemically Grown ZnO Quantum Dots" in J.
Appl. Phys., Vol. 85, No. 5, pages 2861-2864, describe a process
for the electrochemical synthesis of stable, hydroxide-free zinc
oxide using a zinc sacrificial electrode which is reacted during
electrolysis to form ZnO particles, the electrolysis being carried
out under oxygen in acetonitrile and tetrahydrofuran in the
presence of tetraoctyl ammonium bromide.
[0012] DE 198 408 42 A1 describes the electrochemical production of
amorphous and crystalline nanoscale metal oxides and metal mixed
oxides. In this process, metal ions are produced in situ, more
particularly from a metal or alloy anode by electrochemical
oxidation in an organic solvent, and the metal ions thus produced
are reductively deposited as metal clusters onto the anode and
converted in situ into the metal oxide under oxidizing conditions
(for example purging with air). Additives and supporting
electrolytes, preferably quaternary ammonium salts, present in the
solution stabilize the nanostructure. Unfortunately, the process
according to DE 198 408 42 A1 is attended by a number of problems.
The price of certain metal plates as sacrificial electrode material
(for example Mg, Zr, In, V) is very high and hence uneconomical. In
addition, certain metals (for example Mn, Cr) are very difficult to
produce as plates. For the deposition of mixed oxides, alloys have
to be produced or several anodes have to be used which is both
expensive and very time-consuming. In addition, where halides are
used, the process according to DE 198 408 42 A1 is attended by the
problem of anodic halogen deposition. The metal clusters formed are
re-dissolved immediately after their formation by the chemically
aggressive halogen and no product can be isolated. Accordingly,
halogen-containing compounds can only be used to a limited extent
in this process. However, metal salt halides and the halogen
compounds of the stabilizers (for example Dehyquart.RTM. A) are
very low in price and often show extremely high solubility in
organic media.
[0013] The problem addressed by the present invention was to
provide a process for the production of amorphous and/or
crystalline metal oxides or mixed oxides with mean particle
diameters in the nanometer range. More particularly, this process
would avoid the disadvantages of the prior art. It would even be
possible to use metal halides and halogen-containing supporting
electrolytes without any adverse effect on the formation of the
metal oxides. In addition, the metal (mixed) oxides produced in
particular would not be able to be contaminated by incorporation of
anionic constituents emanating from the metal salts and/or
supporting electrolytes.
[0014] Another problem addressed by the present invention was in
particular to further develop the process described in DE 198 408
42 A1 to the extent that the disadvantages and problems mentioned
above would be avoided.
[0015] Applicants have now surprisingly found that the problem
stated above can be solved by the use of electrolysis cells with
separate electrode compartments.
[0016] Accordingly, the present invention relates to a process for
the production of amorphous and/or crystalline (mixed) oxides of
metals, more particularly metals of the third to fifth main group
or the secondary groups of the periodic system, with mean particle
diameters of the (mixed) oxides in the range from 1 to 500 nm and
more particularly 1 to 100 nm, characterized in that, in an
electrolysis cell equipped with a cathode and an anode, ions
dissolved in organic electrolyte of that metal or those metals of
which the (mixed) oxide is to be produced are electrochemically
reduced at the cathode in the presence of an oxidizing agent, the
cathode compartment (cathode half-cell) and the anode compartment
(anode half cell) being separated from one another by a porous
partition or separating wall (diaphragm) which allows current to
pass through, i.e. is permeable to cations and anions, but impedes
and in particular prevents the passage of elemental halogen from
the anode to the cathode compartment.
[0017] For example, a porous glass frit, more particularly with a
pore diameter of about 1 to 4 .mu.m, may be used as the partition
or separating wall. In addition, a polytetrafluoroethylene filter
or a polytetrafluoroethylene membrane, more particularly with a
pore diameter of about 1 to 4 .mu.m, may be used as the partition
or separating wall. A porous polyethylene film or polyethylene disk
(for example Vyon.RTM., a material of low-pressure polyethylene),
more particularly with a pore diameter of about 1 to 4 .mu.m, may
also be used as the partition or separating wall. Equally, a
proton-conducting membrane, more particularly a perfluorinated ion
exchanger membrane (for example Nafion.RTM. from DuPont or
Aldrich), preferably with a pore diameter of about 1 to 4 .mu.m,
may be used as the partition or separating wall. The Nafion.RTM.
membrane is a membrane with a Teflon-like structure. The advantage
of Nafion.RTM. membranes for example lies in their high chemical
resistance, their high ion conductivity and the high halogen
retention capacity. In addition, these films are very easy to
process in a thickness of 128 .mu.m and thus provide for
uncomplicated production of the electrolysis cells.
[0018] Through the separation of the electrode compartments by the
diaphragm used in accordance with the invention, electrical
conductivity is maintained but no halide is able to enter the
cathode compartment.
[0019] In the process according to the invention, the mean particle
diameter of the (mixed) oxides can be adjusted by varying the
temperature of the electrolyte, the electrical voltage or current
intensity and through the nature of the supporting electrolyte
optionally used. The process according to the invention is
preferably carried out in such a way that the metal oxides obtained
have mean particle diameters in the range from 1 to about 500 nm
and preferably in the range from about 1 to about 100 nm.
[0020] Using the process according to the invention, it is possible
in particular to produce metal oxides which do not react with
moisture to form hydroxides at a temperature below about
100.degree. C. Accordingly, the process according to the invention
is not suitable for the production of oxides and mixed oxides of
alkali or alkaline earth metals. It is also particularly suitable
for the production of oxides of metals which are oxidized by
atmospheric oxygen at temperatures below about 100.degree. C. Where
metals such as these are used, the process according to the
invention may be carried out at temperatures below 100.degree. C.
using air as the oxidizing agent. This enables the process to be
carried out in an uncomplicated manner. The process according to
the invention is particularly suitable for the production of
amorphous and/or crystalline oxides and mixed oxides of Ti, Zr, Cr,
Mo, Fe, Co, Ni, Pb, In, Sn and/or Al.
[0021] The organic electrolyte used is preferably a substance which
is liquid at temperatures in the range from about -78.degree. C. to
about +260.degree. C. at normal pressure. In one particularly
preferred embodiment, a substance which is liquid at temperatures
in the range from about 0 to about 60.degree. C. at normal pressure
is used. The organic electrolyte is preferably selected from
alcohols (for example isopropanol or n-propanol) or polyalcohols
(for example glycerol or polyglycols) or mixtures and derivatives
thereof, ketones, ethers (for example tetrahydrofuran or diethyl
ether), nitrites, organic carbonates (for example propylene
carbonate or diethyl carbonate) and aromatic compounds, those which
are liquid at temperatures in the ranges mentioned being preferred.
Other particularly suitable electrolytes are tetrahydrofuran,
acetone, acetonitrile, toluene and mixtures thereof with
alcohols.
[0022] Depending on the metal (mixed) oxide to be produced, it can
be favorable if the electrolyte contains small quantities of water.
For example, the water content of the organic electrolyte may be in
the range from about 0.01 to about 2% by weight and, more
particularly, is in the range from about 0.05 to about 1% by
weight, the percentages by weight being based on the total quantity
of organic electrolyte and water.
[0023] Should the electrolyte not of itself have an adequate
electrical conductivity or acquire an adequate electrical
conductivity by dissolution in salts of the metals whose (mixed)
oxides are to be produced, it is advisable to dissolve a supporting
electrolyte in the electrolyte. The usual supporting electrolytes
which are normally used to give the electrolytes mentioned an
electrical conductivity sufficient for electrochemical processes
may be employed. Suitable supporting electrolytes are, for example,
electrolyte-soluble hexafluorophosphates, sulfonates, acetyl
acetonates, carboxylates and in particular quaternary phosphonium
and/or ammonium salts with organic groups at the phosphorus or at
the nitrogen. Preferred supporting electrolytes are quaternary
ammonium compounds which bear aryl and/or alkyl groups at the
nitrogen and which are preferably present as halides. One example
of a particularly suitable supporting electrolyte is tetrabutyl
ammonium bromide. If necessary, these supporting electrolytes may
be used together with surfactants which stabilize the metal oxide
particles produced. The surfactants are selected in particular from
the entire class of surfactants, the following compounds preferably
being used: fatty alcohols, fatty alcohol ethoxylates,
polyoxyethylene carboxylic acids and/or fatty acid ethoxylates.
[0024] The process according to the invention is preferably carried
out in a temperature range in which the supporting electrolyte is
sufficiently soluble in the organic electrolyte. The process
according to the invention is preferably carried out in such a way
that the organic electrolyte has a temperature in the range from
about 30.degree. C. to about 50.degree. C. If tetrahydrofuran is
used as the electrolyte and tetrabutyl ammonium bromide as the
supporting electrolyte, the process is preferably carried out at
temperatures above 35.degree. C., for example in the range from
35.degree. C. to 40.degree. C.
[0025] The supporting electrolytes have the additional effect that
they protect the oxide particles formed against agglomeration. A
very narrow particle size distribution can be obtained in this way.
If no importance is attached to a narrow particle size
distribution, there is no need to add the supporting electrolytes
providing the electrolyte has an adequate electrical conductivity
from the dissolved salts of the metal to be precipitated as
oxide.
[0026] According to the invention, the metal (mixed) oxides are
formed by electrochemical reduction of the ions of the metals at a
cathode in the presence of an oxidizing agent. The easiest
oxidizing agent to use is oxygen or air. Accordingly, oxygen or air
is preferably used. In a preferred embodiment, therefore, the
process is carried out by introducing air into the electrolyte
during the electrochemical reduction of the metal ions.
Accordingly, the metal particles deposited in the first stage are
preferably oxidized by atmospheric oxygen. To this end, the air is
introduced in the form of small bubbles which, on the one hand,
provide for fine distribution of the oxidizing agent oxygen in the
cathode compartment and, on the other hand, ensure through the
constant mixing of the electrolyte that no solid metal oxide layer
is formed on the cathode, but instead that the metal oxide
particles are flushed from the cathode and dispersed in the
electrolyte. A device for introducing air in the form of small
bubbles is shown in FIG. 3. In this device, compressed air is
passed through a flat frit material. Vyon.RTM. (pore diameter 5 to
40 .mu.m), porous Teflon.RTM. and even ceramic frits, for example,
may be used for this purpose. The frits are between 2 and 5 mm in
thickness. The frit may be screwed by a ring onto a wall (see FIG.
3 in particular). The seal between the tank and the frit may be
provided by the frit material itself. In the case of ceramic frits,
it is advisable for example to use a rubber seal. The body of this
device may be made, for example, of a solvent-resistant plastic,
such as PEK, PVC or Teflon.RTM.. The device may also be made of
metal. To support oxidation in the case of highly surface-active
additives, such as fatty alcohols for example, pure oxygen may be
added (up to ca. 70% by volume). If desired, oxygen-enriched air or
substantially pure oxygen may also be introduced into the
electrolyte. If highly inflammable electrolytes are used, it may be
advisable for reasons of operational safety to enrich the air with
nitrogen to such an extent that the oxygen partial pressure remains
below the value required for ignition of the gas mixture. Other
suitable but less preferred oxidizing agents are hydrogen peroxide,
organic or inorganic peroxo compounds or oxo anions of the halogens
chlorine, bromine or iodine where the halogen has an oxidation
number of +1 to +5. However, if stronger oxidizing agents than
atmospheric oxygen are used, it is important to ensure that there
is no peroxide formation with the electrolyte.
[0027] The electrical d.c. voltage between cathode and anode is
preferably adjusted so that a current density of 0.05 to 10
mA/cm.sup.2, based on the anode area, and preferably in the range
from 1 to 5 mA/cm.sup.2 is obtained. Given a sufficiently
conductive electrolyte, this can be achieved by applying a d.c.
voltage of about 1 to about 100 volts between the cathode and
anode.
[0028] In a preferred embodiment, the electrolyte is vigorously
agitated throughout the process according to the invention. This
can be done, for example, by stirring the electrolyte. In addition
or alternatively, the electrolyte may be ultrasonicated for this
purpose. It is also possible, particularly where atmospheric oxygen
is used as the oxidizing agent, to use the gas stream to be
introduced in the form of small gas bubbles for convection of the
electrolyte. The advantage of electrolyte agitation and/or
ultrasonication is that the metal oxides formed do not adhere to
the cathode, i.e. do not cover it with an insulating layer.
[0029] The ions of the metal or metals whose oxides or mixed oxides
are to be produced can enter the electrolyte in various ways.
[0030] In a preferred embodiment, the process according to the
invention may be carried out by using an inert anode and dissolving
in the electrolyte a salt of the metal or salts of the metals of
which the oxide or mixed oxide is to be produced. In this case, the
salts selected must of course be sufficiently soluble in the
electrolyte used. The metal salts used may generally be halides
(preferably fluorides, chlorides, bromides and iodides), nitrates,
acetates, sulfonates, carboxylates and hexafluorophosphates of the
metals and mixtures thereof. Where tetrahydrofuran, for example, is
used as the electrolyte, chlorides or nitrates of the particular
metals, for example, are generally suitable. In this embodiment,
mixed oxides may also be produced where metal salt mixtures are
used. A material which is inert under the electrolysis conditions
selected is preferably used as the cathode material and optionally
the anode material. In view of the aggressive conditions in the
anode compartment, chemically resistant anode materials are
generally used. Suitable electrode materials are, for example,
electrodes of platinum or other platinum metals, gold, niobium,
tantalum, tungsten, graphite or glassy carbon. The materials are
equally suitable as cathode and anode materials.
[0031] Another particular embodiment for the production of mixed
oxides is a "hybrid process" between the process using a
sacrificial anode according to DE 198 408 42 A1 and the "salt
route" previously described. This embodiment may be used in
particular in cases where, in the production of mixed oxides,
solubility problems exist for a corresponding metal salt component.
In addition, doping material could be introduced into deposited
products in this way. In this special embodiment, the process
according to the invention--in the production of mixed oxides of at
least two metals--may be carried out, for example, by producing the
metal ions dissolved in the electrolyte by dissolving a salt of one
metal in the electrolyte and using an anode (sacrificial anode)
containing the other metal. For example, the anode used may contain
or consist of the metal whose oxide is to be produced and may
dissolve anodically during the production of the oxides. Equally,
the anode used may be an anode of an inert material coated
beforehand with the metal whose oxide is to be produced. In the
latter case, the corresponding metal separates anodically from the
anode during the electrochemical production of the metal oxide.
[0032] The oxides or the mixed oxides are obtained in X-ray
amorphous or crystalline form, depending on the metal and the
electrolysis conditions. Accordingly, they show either an X-ray
diffractogram which resembles that of a liquid and has only a few
broad maxima (X-ray amorphous) or which consists of individual
clearly contrasting X-ray reflexes (X-ray crystalline). The X-ray
amorphous or X-ray crystalline metal oxides obtained are separated
from the electrolyte either continuously or in batches, for example
by continuous or discontinuous filtration or centrifugation. If
necessary, the metal oxides separated from the electrolyte are
washed, preferably with the organic solvent used as electrolyte,
optionally at elevated temperature, in order to remove any salt
residues and stabilizer residues present. The metal oxides are then
dried, for example at a temperature of 100.degree. C.
[0033] If it is intended to produce crystalline metal oxides or
mixed oxides and if they do not accumulate in the desired form
during the electrolysis process, the metal (mixed) oxides separated
from the electrolyte may be thermally aftertreated. For example,
they may be converted into an X-ray crystalline form by calcination
at a temperature in the range from about 300 to about 1200.degree.
C. and more particularly at a temperature in the range from about
400 to about 1,000.degree. C. The calcination time will depend on
the rate at which the amorphous (mixed) oxides are converted into
the crystalline (mixed) oxides and may be, for example, between
about 5 minutes and about 4 hours. Depending on the metal (mixed
oxide selected, the size of the crystallites may increase with
increasing calcination time. Alternatively, the amorphous samples
may be subjected to a hydrothermal treatment. To this end, a
solvent, preferably water, is added to the samples which are then
treated for a few hours in a closed vessel under autogenous
pressure and at a temperature of 100 to 250.degree. C. The organic
constituents adhering to the particles are removed by this
procedure. At the same time, the growth of the nanoscale primary
particles is seriously restricted and preferably avoided.
[0034] FIGS. 1 and 2 are each sections through an arrangement 1
suitable for carrying out the process according to the invention.
In this particular embodiment, the electrolysis cell 1--as shown in
FIG. 1--is in the form of a system of concentric tubes. The outer
tube 2, which consists for example of 0.5 mm thick stain steel
plate, is preferably the cathode. The middle tube 4 is formed by a
cation-conducting membrane, for example a Nafion.RTM. membrane. The
outer tube 2 and the middle tube 4 with the membrane delimit the
space of the cathode half cell 6. The counter-electrode 3, for
example an electrode of platinum, titanium or graphite which is
preferably used as the anode, represents the inner of the
concentric tubes. The inner tube 3 (anode) and the membrane layer 4
delimit the space of the anode half cell 5. The individual tubes 2
(cathode), 4 (membrane), 3 (anode) are held in placed by mutually
opposite holders 8, 9 and 10 which are held together by a fixing
element 7, for example in the form of a screw, for example of an
inert material, such as a suitable plastic. The above-described
arrangement for carrying out the process according to the invention
may be connected to a suitable current and voltage source. FIG. 1
shows a particular embodiment of an arrangement for carrying out
the process according to the invention. The process according to
the invention is not of course confined to that particular
embodiment. The expert will know of numerous possible modifications
and variants as to how the anode half cell and the cathode half
cell can be separated from one another by a diaphragm without
departing from the scope of the present invention (see FIG. 2 for
example).
[0035] The process according to the invention has a number of
advantages over the prior art.
[0036] The process according to the invention provides for the
relatively simple production of nanoscale, amorphous and
crystalline metal oxides and mixed oxides from metal salt
solutions. In contrast to the process according to DE 198 408 42
A1, the metal ions required do not have to be formed in situ from
metal plates during the process, but may be directly added to the
starting electrolyte as metal salt. Where halide salts (metal salts
and/or supporting electrolytes) are used, the problem of halogen
deposition where the metal clusters formed are re-dissolved
immediately after their formation does not arise. Accordingly,
metal salt halides and the halogen compounds of the stabilizers
(for example Dehyquart.RTM. A), which are often very low in price
and generally show extremely high solubility in organic media, may
readily be used. In addition, contamination of the metal (mixed)
oxides produced by the incorporation of anionic constituents
emanating from the metal salts and/or supporting electrolytes can
be avoided because they remain in the anode compartment.
[0037] Further embodiments and variations of the present invention
will be readily apparent to the expert on reading the specification
and could be put into practice without departing from the scope of
the present invention.
[0038] The following Examples are intended to illustrate the
invention without limiting it in any way.
EXAMPLES
[0039] In the following Examples, solutions of various metal salts
or mixtures thereof were electrolyzed under the particular process
conditions according to the invention indicated. The arrangement
shown in FIG. 2 was used for the tests. The results obtained are as
follows:
Example 1
[0040]
1 Electrolyte cathode compartment: isopropanol, tetrabutyl ammonium
bromide (TBAB) (0.1 M), 1 g ZnCl.sub.2 Electrolyte anode
compartment: isopropanol, TBAB (0.1 M) Electrodes: 2 .times. Pt
Current density: 3 mA/cm.sup.2 Diaphragm material: porous glass
frit Product: ZnO, crystalline
Example 2
[0041]
2 Electrolyte cathode compartment: isopropanol, TBAB (0.1 M), 1 g
Electrolyte anode compartment: MnCl.sub.2 isopropanol, TBAB (0.1 M)
Electrodes: 2 .times. Pt Current density: 3 mA/cm.sup.2 Diaphragm
material: porous glass frit Product: .gamma.-Mn.sub.3O.sub.4
(tetragonal), crystalline
Example 3
[0042]
3 Electrolyte cathode compartment: 200 ml n-propanol, TBAB (0.1 M),
2.0 g NiCl.sub.2 .times. 6H.sub.2O Electrolyte anode compartment:
200 ml n-propanol, TBAB (0.1 M) Electrodes: 2 .times. Pt Current
density: 3 mA/cm.sup.2 Diaphragm material: porous glass frit
Product: X-ray amorphous, light green Calcination (45
mins./300.degree. C.): NiO, black
Example 4
[0043]
4 Electrolyte cathode compartment: 200 ml n-propanol, TBAB (0.1 M),
1.5 g FeCl.sub.2 Electrolyte anode compartment: 200 ml n-propanol,
TBAB (0.1 M) Electrodes: 2 .times. Pt Current density: 3
mA/cm.sup.2 Diaphragm material: porous glass frit Product: X-ray
amorphous Calcination (15 mins./500.degree. C.):
.alpha.-Fe.sub.2O.sub.3 + .gamma.-Fe.sub.2O.sub.3
Example 5
[0044]
5 Electrolyte cathode compartment: 200 ml n-propanol, TBAB (0.1 M),
1.5 g CoCl.sub.2 .times. 6H.sub.2O Electrolyte anode compartment:
200 ml n-propanol, TBAB (0.1 M) Electrodes: 2 .times. Pt Current
density: 3 mA/cm.sup.2 Diaphragm material: porous glass frit
Product: X-ray amorphous Calcination (35 mins./500.degree. C.):
Co.sub.3O.sub.4
Example 6
[0045]
6 Electrolyte cathode compartment: 200 ml n-propanol, cetylmethyl
ammonium chloride (Dehyquart A) (0.1 M), 1.5 g FeCl.sub.3, 0.6 g
CoCl.sub.2 .times. 6H.sub.2O (Co:Fe = 1:2) Electrolyte anode
compartment: 200 ml n-propanol, Dehyquart A (0.1 M) Electrodes: 2
.times. Pt Current density: 3 mA/cm.sup.2 Diaphragm material:
porous glass frit Product: X-ray amorphous Calcination (30
mins./500.degree. C.): CoFe.sub.2O.sub.4
Example 7
[0046]
7 Electrolyte cathode compartment: 200 ml n-propanol, TBAB (0.1 M),
SnCl.sub.2 (0.015 M) Electrolyte anode compartment: 200 ml
n-propanol, TBAB (0.1 M) Electrodes: In anode, stainless steel
cathode Current density: 3 mA/cm.sup.2 Diaphragm material: porous
glass frit Product: X-ray amorphous Calcination (30
mins./700.degree. C.): In.sub.2O.sub.3 + SnO.sub.2
Example 8
[0047]
8 Electrolyte cathode compartment: n-propanol, TBAB (0.1 M);
ZnCl.sub.2 (0.05 M) Electrolyte anode compartment: n-propanol, TBAB
(0.1 M) Cathode: stainless steel Anode: graphite Current density: 1
mA/cm.sup.2 Diaphragm material: Vyon .RTM. F Product: white
crystalline ZnO
Example 9
[0048]
9 Electrolyte cathode compartment: isopropanol, TBAB (0.1 M);
MnCl.sub.2 (0.05 M) Electrolyte anode compartment: isopropanol,
TBAB (0.1 M) Electrodes: 2 .times. Pt Current density: 3
mA/cm.sup.2 Diaphragm material: Teflon .RTM. Product: crystalline,
.gamma.-Mn.sub.3O.sub.4 (tetragonal)
Example 10
[0049]
10 Electrolyte cathode compartment: 1-propanol, Dehyquart A (0.05
M), 6.8 g ZnCl.sub.2 (0.1 M), CaCl.sub.2 (0.1 M) Electrolyte anode
compartment: 1-propanol, Dehyquart A (0.05 M) CaCl.sub.2 (0.1 M)
Electrodes: graphite anode, platinized Ti cathode Current density:
3 mA/cm.sup.2 Diaphragm material: Nafion .RTM. Product: crystalline
ZnO
Example 11
[0050]
11 Electrolyte cathode compartment: 1-propanol, Dehyquart A (0.1
M), FeCl.sub.3 (anhydrous) Electrolyte anode compartment:
1-propanol, Dehyquart A (0.1 M) Electrodes: graphite anode,
stainless steel cathode Current density: 0.5 mA/cm.sup.2 Diaphragm
material: Nafion .RTM. Product: amorphous Calcination (30
mins./500.degree. C.): crystalline: .alpha.-Fe.sub.2O.sub.3 +
.gamma.-Fe.sub.2O.sub.3
Example 12
[0051]
12 Electrolyte cathode compartment: 1-propanol, TBAB (0.1 M),
CoCl.sub.2 .times. 6H.sub.2O (0.025 M) Electrolyte anode
compartment: 1-propanol, TBAB (0.1 M) Electrodes: graphite anode,
stainless steel cathode Current density: 0.5 mA/cm.sup.2 Diaphragm
material: Nafion .RTM. Product: amorphous Calcination (30
mins./500.degree. C.): crystalline: Co.sub.3O.sub.4
* * * * *