Note: Descriptions are shown in the official language in which they were submitted.
203~092
IMPROVED ELECTROCATALYTIC COATING
BACKGROUND OF TH~ INVENTION
. . ~ ~
~ Electrodes for use in electrolytic processes have
been known which have a base or core metal bearing a
layer or coating of metal oxides. The core metal of the
electrode may be a valve metal such as titanium,
tantalum, zirconium, niobium or tungsten. Where the
coating is an oxide mixture, an oxide of the core or
substrate metal can contribute to the mixture. As taught
for example in U.S. Patent 3,711,385, such mixture can
include an oxide of the substrate metal p1us at least one
oxide of a metal such as platinum, iridium, rhodium,
pa~lladium, ruthenium, and osmium.
It has also been known that such mixture which can
be termed a noble metal oxide mixture, can be a mixture
of ruthenium oxide and iridium oxide. Such have been
taught generally in U.S. Patent 3,632,498 and examples
shown specifically, when combined with titanium oxide, in
U.S. Patent 3,948,751. Particularly for utilization as a
coating on an electrode used in an electrolysis of an
aqueous alkali-metal halide, e.g., sodium chloride, it
has been taught in U.S. Patent 4,005,004 that such noble
metal oxide mixture can be particularly serviceable when
in further mixture with both titanium oxide and zirconium
CA 02030092 1998-04-22
oxide. Such a mixture, as taught in the patent, yields a
solid solution coating that ostensibly enhances the
practical utilization of the electrodes for their
intended use.
More recently, it has been proposed for enhanced
wear resistance of an electrode, especially when utilized
in electrolysis producing oxygen and chlorine in
combination, to provide the molar amount of titanium
oxide equal to or greater than the moles of the total
oxides of iridium and ruthenium. Such has been disclosed
in U.S. Patent 4,564,434, wherein there is also taught
providing the molar amount of iridium oxide about the
same as, to greater than, the molar amount of ruthenium
oxide.
SUMMARY OF THE INVENTION
It would however be desirable to provide an
electrocatalytic coating which in electrolysis of
halogen-containing solutions, e.g., chlor-alkali
production from brine electrolysis, will achieve reduced
oxygen evolution. It would also be particularly
desirable to provide such a coating exhibiting retarded
weight loss when exposed to caustic. It would be most
beneficial if such characteristics could be achieved, not
only in combination, but without sacrifice to other
wanted features, e.g., no sacrifice in the chlorine
evolution potential for the anode. It would also be
advantageous to prepare an electrode using a coating
composition that is readily prepared, has a simplistic
formulation, and provides ease and safety in handling and
use.
CA 02030092 1998-04-22
The invention is broadly directed to an electrode
having reduced oxygen evolution during electrolysis of
halogen-containing solutions particularly at low pH, such
electrode comprising an electrically conductive metal
substrate having a coating of enhanced stability under
alkaline conditions, which coating comprises at least 15,
but less than 25, mole percent iridium oxide, 35-50 mole
percent ruthenium oxide and at least 30, but less than 45
mole percent titanium oxide basis 100 mole percent of the
oxides present in the coating. Thereby the coating has a
molar ratio of titanium oxide to the total of the oxides
of iridium and ruthenium of less than 1:1, and should
have a molar ratio of ruthenium oxide to iridium oxide of
greater than 1.5:1 and up to 3:1.
In another aspect, the invention is directed to a
coating composition adapted for providing the foregoing
described mixed metal oxide coating and in a still
further aspect is directed to the method of making an
electrode which is hereinbefore defined. The electrode
will be particularly useful as an anode in a membrane
cell used for the electrolysis of brine that is at a pH
within the range of from about 2 to about 4.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The coating composition of the present invention is
broadly applicable to any electrically conductive metal
substrate which will be sufficiently electrically
conductive to serve as an electrode in an electrolysis
process. Thus the metals of the substrate are broadly
contemplated, but in view of the application of an
electrocatalytic coating, the substrate metals more
typically may be such as nickel or manganese, or most
CA 02030092 1998-04-22
always the valve metals, including titanium, tantalum,
aluminum, tungsten, zirconium and niobium. Of particular
interest for its ruggedness, corrosion resistance and
availability is titanium. As well as the normally
available elemental metals themselves, the suitable
metals of the substrate can include metal alloys and
intermetallic mixtures. For example, titanium may
generally be alloyed with nickel, cobalt, iron, manganese
or copper. More specifically, Grade 5 titanium may
include up to 6.75 weight ~ aluminum and 4.5 weight ~
vanadium, grade 6 up to 6~ aluminum and 3~ tin, grade 7
up to 0.25 weight ~ palladium, grade 10, from 10 to 13
weight ~ molybdenum plus 4.5 to 7.5 weight ~ zirconium
and so on.
The coating composition applied to the coated metal
substrate will be aqueous, which will most always be
simply water without any blending with further liquid.
Preferably, deionized or distilled water is used to avoid
inorganic impurities. For economy of preparation and
utilization, the aqueous compositions that are
serviceable will be solutions of precursor constituents
in the aqueous medium, that is, precursors to the oxides
tat will be present in the coating. The precursor
constituents utilized in the aqueous solution are those
which can be solubilized in water efficiently and
economically, e.g., achieve solution without extensive
boiling condition. Moreover, the precursors must supply
the respective metal oxide on thermodecomposition. Where
they are all present in the same composition, they must
also be compatible with one another. In this regard,
they are advantageously non-reactive toward one another,
e.g., will not react so as to form products which will
lead to deleterious non-oxide substituents in the coating
or precipitate from the coating solution. Usually, each
203~092
precursor constituent will be a metal salt that most
often is a halide salt and preferably for economy coupled
with efficiency of solution preparation such will all be
the chloride salt. However, other useful salts include
iodides, bromides and ammonium chloro salts such as
ammonium hexachloro iridate or ruthenate.
In the individual or combination solutions, in
addition to the suitable precursor constituent, most
always with only one exception no further solution
ingre~ients will be present. Such exception will
virtually always be the presence of inorganic acid. For
example, a solution of iridium trichloride can further
~ contain strong acid, most always hydrochloric acid, which
will usually be present in an amount to supply about 5 to
20 weight percent acid. Typically, the individual or
combination solutions will have a pH of less than 1, such
as within the range of from about 0.2 to about 0.8.
When the coating composition is a solution of all
precursor constituents, such will contain at least 15,
but less than 25, mole percent of the iridium
constituent, 35-50 mole percent of the ruthenium
constituent, and at least 30, but less than 45, mole
percent of the titanium constituent, basis 100 mole
percent of these constituents. A composition containing
an iridium constituent in an amount of less than 15 mole
percent will be inadequate for providing an electrode
coating having the best caustic stability, such as when
the electrode is used in a chlor-alkali cell. On the
other hand, less than 25 mole percent for the iridium
precursor will be desirable for best low operating
potential efficiency for the coating. In regard to the
ruthenium, a constituent amount in the solution of less
than about 35 mole percent will be insufficient to
provide the most efficient l~w chlorine potential for
2030092
resulting coatings, while an amount not greater than 50
mole percent enhances coating stability. Also, for best
coating characteristics, the molar ratio of ruthenium
oxide to iridium oxide in the resulting coating will be
from greater than 1.5:1 up to 3:1.
For the titanium precursor in the coating
composition, an amount providing less than 30 mole
percent titanium is uneconomical while 45 mole percent
titanium or more can lead to higher operating potential
for e~ectrode coatings operating in chlor-alkali cells.
Preferably for best economy, coupled with the overall
most desirable coating characteristics, the coating
solution will contain constituents in a proportion such
as to provide from about 18-22 mole percent iridium, 35-
40 mole percent ruthenium, and 40-44 mole percent
titanium. The resulting coating will furthermore have a
molar ratio of titanium oxide to the total of the oxides
of iridium ruthenium of less than 1:1, but most always
above 0.5:1.
Before applying the coating composition to the
substrate metal, the substrate metal advantageously is a
cleaned surface. This may be obtained by any of the
t~eatments used to achieve a clean metal surface,
including mechanical cleaning. The usual cleaning
procedures of degreasing, either chemical or
electrolytic, or other chemical cleaning operation may
also be used to advantage. Where the substrate
preparation includes annealing, and the metal is grade 1
titanium, the titanium can be annealed at a temperature
of at least about 450~C. for a time of at least about 15
minutes, but most often a more elevated annealing
temperature, e.g., 600~-875~C. is advantageous.
After the foregoing operatlon, e.g., cleaning, or
cleaning and annealing, and including any desirable
.: :
2030092
rinsing and drying steps, the metal surface is then ready
for continuing operation. Where such is etching, it will
be with an active etch solution. Typical etch solutions
are acid solutions. These can be provided by
hydrochloric, sulfuric, perchloric, nitric, oxalic,
tartaric, and phosphoric acids as well as mixtures
thereof, e.g., aqua regia. Other etchants that may be
utilized include caustic etchants such as a solution of
potassium hydroxide/hydrogen peroxide in combination, or
a mel~ of potassium hydroxide with potassium nitrate.
For efficiency of operation, the etch solution is
advantageously a strong, or concentrated, aqueous
solution, such as an 18-22 weight ~ solution of
hydrochloric acid, or a solution of sulfuric acid.
Moreover, the solution is advantageously maintained
during etching at elevated temperature such as at 80~C. or
more for aqueous solutions, and often at or near boiling
condition or greater, e.g., under refluxing condition.
Preferably, the etching will prepare a roughened surface,
as determined by aided, visual inspection. Following
etching, the etched metal surface can then be subjected
to rinsing and drying steps to prepare the surface for
cOating.
The coating composition can then be applied to the
metal substrate by any means for typically applying an
aqueous coating compositicn to a substrate metal. Such
methods of application include brush, roller, and spray
application. Moreover, combination techniques can be
utilized, e.g., spray and brush technique. Spray
application can be either conventional compressed gas or
can be electrostatic spray application. Advantageously,
electrostatic spray application will be used for best
wrap around affect of the spray for coating the back side
of an article such as a mesh electrode.
2~3~92
Following application of the coating, the applied
composition will be heated to prepare the resulting mixed
oxide coating by thermodecomposition of the precursors
present in the coating composition. This prepares the
mixed oxide coating containing the mixed oxides in the
molar proportions as above discussed. Such heating for
the thermodecomposition will be conducted at a
temperature of at least about 440~C. peak metal
temperature for a time of at least about 3 minutes. More
typically the applied coating will be heated at a more
elevated temperature for a slightly longer time, but
usually a temperature of greater than about 550~C. is
~ avoided for economy and to avoid detrimental effects on
anode potential where the coated metai will serve as an
anode. Suitable conditions can include heating in air or
oxygen. Following such heating, and before additional
coating as where an additional application of the coating
composition will be applied, the heated and coated
substrate will usually be permitted to cool to at least
substantially ambient temperature. The resulting
finished coating has a smooth appearance to the unaided
eye, but under microscopic Px~r;n~tion is seen to be non-
homogeneous, having embedded crystallites in the field of
the coating. Although the application of coating
compositions other than as disclosed herein is then
contemplated, for best overall performance of the coated
substrate metal as an electrode, subsequently applied
coatings will be of those compositions of the invention
disclosed herein.
The following example shows a way in which the
invention has been practiced, but should not be construed
as limiting the invention.
2030092
EXAMPLE
A coating solution was prepared by combining 157 gms
of iridium, using a solution of iridium trichloride in
18% by weight HCl, 144 gms of ruthenium, using a solution
of ruthenium trichloride in 18% by weight HCl, 80 gms of
titanium, using titanium tetrachloride in 10% by weight
HCl solution, 331 gms HCl, using 36 weight % solution,
then diluting to 10 liters with deionized water. This
provi~ed a coating composition having 21 mole % iridium;
36.3 mole % ruthenium, and 42.7 mole % titanium. Four
liters of 93 grams per liter (gpl) HCl solution were then
~ added to make the final coating solution.
This solution was applied using a hand roller to a
titanium mesh substrate having a diamond-patterned mesh,
with each diamond pattern having about 8 millimeters
(mms.) long way of design plus about 4 mms. short way of
design. The titanium mesh had been annealed at 600~C. for
30 minutes and etched in 25 wt % sulfuric acid at
85-90~C., then water rinsed and air dried. The applied
coating was air dried then baked at 470~C. Eighteen (18)
coats were applied in this manner. After the final coat,
t~e anode was postbaked at 525~C. for 4 hours.
Operation of eight samples of the resulting coated
tltanium substrate, when utilized as an anode in 12
normal NaOH at 95~C. for 4 hours at 25 kA/m2 resulted in
an average weight loss of 5.27 gm/m2. Use of a sample as
an anode in a chlor-alkali membrane cell operating at 3.3
kA/m2 resulted in 0.06%, 0.22~, and 0.38~, by volume,
oxygen produced in the chlorine cell product at an
electrolyte pH of 2, 3 and 4, respectively. The
operating potential of this anode in the membrane cell
was 1.09 volts vs. a standard calomel reference
electrode.
2030092
The average caustic weight loss of 5.27 gm/m2 was
especially noteworthy since a comparative coating having
7.8 mole percent iridium oxide, 15 mole percent ruthenium
oxide and 77.2 mole percent titanium oxide exhibited such
weight loss of 8.9 gm/m2 when tested under the same
conditions. Moreover, again comparatively, but as the
mole percent changed to more closely approach the
invention composition, but still in a comparative
coating, the caustic weight loss increased to 19.2 gm/m2.
