U.S. patent number 3,616,445 [Application Number 04/690,407] was granted by the patent office on 1971-10-26 for titanium or tantalum base electrodes with applied titanium or tantalum oxide face activated with noble metals or noble metal oxides.
This patent grant is currently assigned to Electronor Corporation. Invention is credited to Giuseppe Bianchi, Vittorio DeNora, Patrizio Gallone, Antonio Nidola.
United States Patent |
3,616,445 |
Bianchi , et al. |
October 26, 1971 |
TITANIUM OR TANTALUM BASE ELECTRODES WITH APPLIED TITANIUM OR
TANTALUM OXIDE FACE ACTIVATED WITH NOBLE METALS OR NOBLE METAL
OXIDES
Abstract
Describes a titanium or tantalum base electrode having a
protective and electrocatalytic layer applied to the faces exposed
to the electrolyte, said protective and electrocatalytic layer
consisting of mixtures of solid solutions of valve metal oxides and
platinum group and noble metals as such or in the form of oxides
and/or oxyhalides.
Inventors: |
Bianchi; Giuseppe (Milan,
IT), DeNora; Vittorio (Nassau, BA),
Gallone; Patrizio (Milan, IT), Nidola; Antonio
(Milan, IT) |
Assignee: |
Electronor Corporation
(Chiasso, CH)
|
Family
ID: |
27538157 |
Appl.
No.: |
04/690,407 |
Filed: |
December 14, 1967 |
Current U.S.
Class: |
204/290.08;
204/290.09; 204/290.12; 204/290.13; 205/535 |
Current CPC
Class: |
C25B
1/46 (20130101); C25B 11/093 (20210101); C25B
11/04 (20130101); C25B 11/061 (20210101) |
Current International
Class: |
C25B
1/00 (20060101); C25B 11/00 (20060101); C25B
11/04 (20060101); C25B 1/46 (20060101); C01d
001/08 () |
Field of
Search: |
;204/29F,98,99 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Tung; T.
Claims
We claim:
1. An electrode comprised of a metal base from the group consisting
of titanium and tantalum having a coating thereon, the major
portion of which is an oxide of a valve metal from the group
consisting of titanium and tantalum and the minor portion is an
oxide of ruthenium, said minor portion also containing a material
from the group consisting of iridium and gold, said coating being
formed by applying onto said base a solution containing a metal
compound from the group consisting of a titanium compound and a
tantalum compound, a ruthenium compound and a metal compound from
the group consisting of an iridium compound and a gold compound and
heating the coating in air to bake said coating on said base.
2. The electrode of claim 1 in which the minor portion contains
iridium and the iridium is in substantially equal amounts with the
metal in the ruthenium oxide.
3. As a product of manufacture, a titanium base electrode in mesh
form having a titanium oxide coating with a mixture of ruthenium
oxide and one metal from the group consisting of iridium and gold
extending through the coating and capable of conducting current
from the titanium base through said coating to the exposed surface
of said coating, said coating being formed on said base from a
solution containing a titanium and a ruthenium compound and a
compound from the group consisting of an iridium compound and a
gold compound and baked on said base.
4. The product of claim 3 in which the metal in the ruthenium oxide
and the metal compound from the group consisting of iridium and
gold constitute from about 20 to about 85 percent of the weight of
the metal in the titanium oxide.
5. The product of claim 3 in which the metal is iridium, said metal
and the metal in the ruthenium oxide are in approximately equal
amounts and constitute from about 20 to about 85 percent of the
weight of the metal in the titanium oxide coating.
6. The product of claim 3 in which the coating is on all sides of
the titanium base electrode.
7. The product of claim 3 in which the coating is applied in a
plurality of layers and the electrode is heated in an oxidizing
atmosphere after each layer is applied.
8. As a product of manufacture, an electrode consisting of a core
of solid massive substantially nonporous valve metal from the group
consisting of titanium and tantalum in mesh form, a substantially
nonporous oxide layer of the same metal on said core in mixture
with ruthenium oxide and a metal from the group consisting of
iridium and gold, said layer being formed by applying a solution
containing a compound from the group consisting of a titanium
compound and a tantalum compound, a ruthenium compound and a
compound selected from the group consisting of an iridium compound
and a gold compound onto said core and baking on said core.
9. The product of claim 8 in which the oxide from the group
consisting of titanium and tantalum oxides, the ruthenium oxide and
the metal from the group consisting of iridium and gold are mixed
together as chloride and applied to the core and baked on the core
under air circulation.
10. As a product of manufacture, an electrode comprising a core of
titanium and a conducting cover coating over said core consisting
of (a) titanium oxide, (b) ruthenium oxide and (c) a metal from the
group consisting of iridium and gold, said coating being formed by
applying a solution containing a titanium compound, a ruthenium
compound and a compound from the group consisting of an iridium
compound and a gold compound onto said core and baking thereon,
said coating being in multiple layers on said core and the metal in
the titanium compound constituting more than 50 percent of the
weight of the metals in said coating.
Description
This invention relates to an improvement in electrodes for use in
anodic and cathodic reactions in cells used for example in the
manufacture of chlorine and caustic alkali by the electrolysis of
aqueous solution of alkali metal chlorides, for use in other
processes in which an electrolysis current is passed through an
electrolyte for the purpose of decomposing the electrolyte, for
carrying out organic oxidations and reductions and for other
purposes.
The invention is particularly valuable in flowing mercury cathode
cells in which the cathode consists of mercury flowing over the
cell base and in which the anodes are suspended above the flowing
mercury cathode and the electrolysis of the brine takes place in
the space between the anodes and the cathode. This, however, is
only one illustration of the application of the invention and it
may be used in diaphragm cells and in all other types of
electrolysis cells and for other electrolysis and oxidation
purposes.
With the advent of dimensionally stable anodes, based upon the use
of valve metals, such as titanium, tantalum, zirconium, molybdenum
and tungsten, which in service develop an oxide or barrier layer
which prevents the further flow of anodic current through the
anode, except at substantially higher voltage, it was considered
necessary to cover the entire face of the titanium or tantalum
anode with a conductive layer of noble metal from the platinum
group (i.e., platinum, palladium, iridium, osmium, rhodium,
ruthenium or alloys thereof), by electroplating or otherwise, which
completely covered the titanium or tantalum base, except for
inevitable pores through the coating metal, which pores were,
however, sealed by the development of the barrier layer above
referred to on the titanium or tantalum base.
The platinum group metals are, however, expensive, good adherence
and good conduction between the titanium base and the platinum
group metal coatings is difficult to secure, the platinum group
metal coatings are consumed or lost in the electrolysis process,
frequent cell shut downs are necessary to replace defective anodes,
and the cost of the coating, per ton of chlorine produced, is high.
Although many platinum group metal coated titanium base anodes have
been produced and tested, no platinum group metal coated titanium
base anodes have heretofore been produced either by the use of
coatings of platinum group metals themselves or by the use of
oxides of such metals, which gave reliable and reproducible results
and which could be produced in commercial quantities and used
satisfactorily in commercial electrolysis cells.
We have found that it is not necessary to cover the metal base of
anodes, such as titanium or tantalum, with a conductive layer of a
platinum group metal over the entire surface, but that a coating
made of a valve metal oxide or oxyhalide, such as, for instance,
titanium oxide, titanium oxychloride, tantalum oxide, tantalum
oxychloride, zirconium oxide or oxyfluoride, may also develop
electrocatalytic properties. if its structure is modified by the
addition of platinum group and noble metals, either as such or in
the form of oxides or oxyhalide. Furthermore, we have found that
the coatings so applied will give equal or greater conduction than
an entire anode surface coated with platinum or a platinum group
metal or oxide and that the coating so applied is more resistant to
the cell-operating conditions than platinum plated anodes and will
cost far less per ton of chlorine produced than the platinum-plated
titanium base anodes heretofore produced. The anodes of our
invention will resist current reversals and amalgam dips, and
resist the conditions of commercial operation in an electrolytic
cell better than anodes which have their entire active surface
coated with a platinum layer, and will produce a higher chlorine
discharge with less overvoltage and have less wear per ton of
chlorine produced than the previous anodes, completely covered with
platinum group metals.
It is, therefore, an object of our invention to produce an anode
for use in electrolysis processes which will have a long active
life before developing an overvoltage for chlorine discharge and
which will be resistive of the conditions of practical operation
encountered in an electrolysis cell.
Another object of our invention is to produce a titanium or
tantalum base anode provided with modified valve metal oxide or
valve metal oxyhalide coatings which will continue to conduct
electric current from the anode base to the electrolyte over long
periods of time, without spalling or peeling of the coating from
the anode base under practical cell conditions, and without
material loss of the platinum group metal conductor, in the
electrolysis process.
Another object of our invention is to provide a conductive coating
on a titanium anode base which will be economical in the use of the
platinum group and noble metals used for the coating and which will
produce a high yield of chlorine per gram of the platinum group and
noble metals used in the anode coating, or consumed in the
electrolysis process.
Another object of our invention is to produce a titanium or
tantalum base electrolysis anode provided with platinum group and
noble metals dissolved as such, or in the form of oxides or
oxyhalide, within the valve metal oxyhalide so that the platinum
group and noble metals or oxides are protected from operative cell
conditions by the presence of the valve metal oxide or oxyhalide
which firmly adheres to the titanium or tantalum base and protects
the platinum group and noble metals from destruction by current
reversals, amalgam dips, short circuits and corrosive action of
chlorine containing electrolyte encountered during operation of an
electrolysis cell.
Another object of our invention is to provide a titanium or
tantalum base anode having a coating comprising a mixture of
titanium or tantalum oxides or oxychlorides of ruthenium oxide or
oxychloride and of iridium metal, in which the weight ratio of
noble metals to valve metal is not lower than 20/100 20/100 and not
higher than 85/800.
Various other objects and advantages of our invention will appear
as this description proceeds.
While any platinum group metals or alloy or oxides thereof may be
used, we have secured our best results with iridium metal dissolved
mixtures of ruthenium oxide or oxychloride embedded in and
surrounded by a titanium or tantalum oxide or oxychloride coating
applied on a titanium or tantalum base or core, and our invention
will be described with reference to this preferred embodiment,
without, however, intending to limit the invention to these
embodiments.
Unlike prior platinum group metal coated titanium base anodes,
which depend upon the development of a barrier layer of titanium
oxide between the pores of the platinum group metal coating, our
invention provides a valve metal oxide or oxyhalide coating on the
titanium base which coating surrounds and protects the platinum
group metal or oxide from destruction and wear under operative
electrolysis cell conditions.
The electrodes of our invention are particularly useful as anodes
for the electrolysis of sodium chloride brines in horizontal
mercury cells and diaphragm cells as they have the ability to
liberate chlorine at low-anode voltages essentially throughout the
life of the platinum group metal or oxide conductors and conductive
points and have a low wear rate (loss of noble metal per ton of
chlorine produced).
Passivity of platinum or platinum group coated anodes of the prior
art in electrolysis of brines has been a problem. Passivity refers
to the rapid rise in potential in the said anodes after being used
for some time at sufficiently high-current density under chlorine
discharge. This rise in potential indicates that the anodic
oxidation of the dissolved chloride ion to molecular chloride gas
will proceed only at a higher overvoltage because of the diminished
catalytic activity of the electrode surface.
The anodes of our invention have a lower anode potential and
operate for a much longer time before reaching passivation than
platinum group metal coated anodes, which demonstrate the high
activity and long life of the anodes of our invention. This is
particularly advantageous in electrolytic cells for the
electrolysis of brines such as sodium chloride since the said cells
can be operated for much longer periods of time before the
electrodes have to be replaced, and shut down time of the cells is
greatly reduced. Our invention also reduces the amount of the
platinum group metal initially used, and eventually consumed, per
ton of chlorine, making the process more economical.
In addition to the platinum group metals or oxides our anodes may
be further improved by incorporating into the platinum group metal
or oxides a minor amount of an activating element, such as
antimony, as described in the copending application U.S. Ser. No.
506,852, filed Nov. 8, 1965, now U.S. Pat. No. 3,428,544.
The conductive coating of our invention may be applied in various
ways, and to various forms of titanium or tantalum base anodes,
such as solid rolled massive titanium plates, perforated plates,
slitted, reticulated, titanium plates, titanium mesh and rolled
titanium mesh, woven titanium wire, or screen or similar tantalum
plates, all of which will be referred to as "mesh form." Our
preferred method of application is by chemi-deposition in the form
of painted on, dipped, sprayed or curtain coatings baked on the
titanium anode base, but other methods of application, including
electrophoretic deposition or electrodeposition, may be used.
In all applications the titanium or tantalum base must be cleaned
and free of oxide or other scale. This cleaning can be done in any
way, by mechanical or chemical cleaning, such as by sand blasting,
etching, pickling or the like.
EXAMPLE I
An expanded titanium anode plate, with a surface of 50 cm..sup.2
projected area, was cleaned by boiling at reflux temperature of
110.degree. C. in a 20 percent solution of hydrochloric acid for 40
minutes. It was then given a liquid coating containing the
following materials:
Ruthenium as Ru Cl.sub.3 .sup.. H.sub.2 0-- 10 mg. (metal)
Iridium as (NH.sub.4 ).sub.2 Ir Cl.sub.6 10 mg. (metal)
Titanium as TiCl.sub.3 56 mg. (metal)
Formamide (HCONH.sub.2)-- 10 to 12 drops
Hydrogen peroxide (H.sub.2 O.sub.2 30 %)--3 to 4 drops
The coating was prepared by first blending or mixing the ruthenium
and iridium salts containing the required amount of Ru and Ir in a
2 molar solution of hydrochloric acid (5 ml. are sufficient for the
above amounts) and allowing the mixture to dry at a temperature not
higher than 50.degree.C. until a dry precipitate is formed.
FOrmamide is then added to the dry salt mixture at about
40.degree.C. to dissolve the mixture. The titanium chloride,
TiCl.sub.3, dissolved in hydrochloric acid (15 percent strength
commercial solution), is added to the dissolved Ru-Ir salt mixture
and a few drops of hydrogen peroxide (30% H.sub.2 O.sub.2) are
added, sufficient to make the solution turn from the blue color of
the commercial solution of TiCl.sub.3, to a brown-reddish
color.
The coating mixture, thus prepared was applied to both sides of the
cleaned titanium anode base, by brush, in eight subsequent layers.
After applying each layer, the anode was heated in an oven under
forced air circulation at a temperature between 300.degree.and
350.degree.C. for 10 to 15 minutes, followed by fast natural
cooling in air between each of the first seven layers, and after
the eighth layer was applied the anode was heated at 450.degree.C.
for 1 hour under forced air circulation and then cooled.
The amounts of the three metals in the coating correspond to the
weight ratios of 13.sup.. 15% Ir, 13.sup.. 15 % Ru and 73.sup.. 7%
Ti and the amount of noble metal in the coating corresponds to 0.2
mg. Ir and 0.2 mg. Ru per square centimeter of projected electrode
area. It is believed that the improved qualities of our anode are
due to the fact that although the three metals in the coating
mixture are originally present as chlorides they are codeposited on
the titanium base in other forms. Stoichiometric determinations
indicate that in the final coating the iridium chloride is reduced
to metallic Ir, whereas ruthenium chloride an titanium chloride are
converted into ruthenium oxide RuO.sub.2 and titanium oxide or
oxychloride TiOCl. In accelerated testing, the anode of this
example showed a weight loss of zero after three current reversals,
a loss of 0.152 mg./cm..sup.2 after three amalgam dips as against a
weight loss of 0.93 mg./cm..sup.2 of a similar titanium base anode
covered with ruthenium oxide. After 2,000 hours of operation this
anode showed a weight increase of 0.7 mg./cm..sup. 2, whereas
similar anodes covered with a layer of platinum or ruthenium oxide
showed substantial weight losses. The weight increase had
apparently become stabilized.
EXAMPLE II
The coating mixture was applied to a cleaned titanium anode base of
the same dimensions as in example I according to the same
procedure. The applied mixture consisted of the following
amounts:
Ru as RuCl.sub. 3 .sup.. H.sub.2 O--20 mg. (metal)
Ir as (NH.sub.4).sub.2 IrCl.sub. 6 --20 mg. (metal)
Ti as TiCl.sub.3 --48 mg. (metal)
Hconh.sub.2 --10 to 12 drops
H.sub.2 o.sub.2 30 % --3 to 4 drops
The procedure for compounding the coating and applying it to the
titanium base was the same as in example I. The quantities of the
three metals in this mixture corresponded to the weight ratios of
22.6 % Ir, 22.6 % Ru and 54.8 % Ti and the amount of noble metal
oxide in the active coating corresponded to 0.4 mg. Ir, and 0.4 mg.
Ru per square centimeter of the active electrode area. After 2300
hours of operation this anode showed a weight increase of 0.9
mg./cm..sup.2 which had apparently become stabilized.
EXAMPLE III
Before being coated, the titanium anode after pre-etching, as
described in example I, was immersed in a solution composed of 1
molar solution of H.sub.2 0 .sub.2 plus a 1 molar solution of NaOH
at 20 to 30.degree. C. for 2 days. The surface of the titanium was
thus converted to a thin layer of black titanium oxide.
The coating mixture of the same composition as given in example I
was used, except that isopropyl alcohol was used as the solvent in
place of formamide. The use of isopropyl alcohol resulted in a more
uniform distribution of the coating films on the black titanium
oxide substrate then when formamide was used as the solvent.
EXAMPLE IV
An expanded titanium anode plate of same size as in the former
examples was submitted to the cleaning and etching procedure as
described above and then given a liquid coating containing the
following materials:
Ru as RuCl.sub.3 .sup.. H.sub.2 O-- 10 mg. (metal)
In as IrCl.sub.4 --10 mg. (metal)
Ta as TaCl.sub. 5 --80 mg. (metal)
Isopropyl alcohol--5 drops
20 % HC1--5 ml.
The coating was prepared by first blending or mixing the ruthenium
and iridium salts in 5 ml. of 20 % HC1. The volume of this solution
was then reduced to about one-fifth by heating to a temperature of
85.degree. C. The required amount of TaCl.sub.5 was dissolved in
boiling 20 % HC1 so as to form a solution containing about 8 %
TaC1.sub.5 by weight. The two solutions were mixed together and the
overall volume reduced to about one-half by heating at 60.degree.
C. The specified quantity of isopropyl alcohol was then added.
The coating mixture was applied to both sides of the cleaned
titanium anode base in eight subsequent layers and following the
same heating and cooling procedure as described in example 1.
The amounts of the three metals in the coating correspond to the
weight ratios of 10 % Ru, 10 % Ir and 80 % Ta and the amount of
noble metal in the coating corresponds to 0.2 mg. Ir and 0.02 mg.
Ru. per square centimeter of projected electrode area. In
accelerated testing, this anode showed a weight loss of 0.0207
mg./cm..sup. 2 after three current reversals and a loss of 0.0138
after two amalgam dips. After 514 hours of operation this anode
showed a weight decrease of 0.097 mg./cm..sup.2.
EXAMPLE V
An expanded titanium anode plate of same size as in the former
examples, after cleaning and etching, was given a liquid coating
containing the following materials:
Ru as RuCl.sub.3 .sup. . H.sub.2 0-- 11.25 mg. (metal)
Au as HAuCl.sub.4 .sup. . nH.sub.2 O-- 3.75 mg. (metal)
Ti as TiCl.sub.3 --60 mg. (metal)
Isopropyl alcohol --5-10 drops
H.sub.2 o.sub.2 30 %--2-3 drops
The coating was prepared by first blending the ruthenium and gold
salts in the required amount in a 2 molar solution of hydrochloric
acid (5 ml.) and allowing the mixture to dry at a temperature of
50.degree. C. The commercial solution of TiCl.sub.3 was then added
to the Ru-Au salt mixture and a few drops of hydrogen peroxide were
stirred into the solution, sufficient to make the solution turn
from blue to brown reddish. Isopropyl alcohol was finally added in
the required amount. The coating mixture thus prepared was applied
to both sides of the cleaned titanium anode base in eight
subsequent layers, following the same heating and cooling procedure
as described in example I.
The amounts of the three metals in the coating correspond to the
weight ratios of 15 % Ru, 5 % Au, 80 % Ti and the amount of noble
metal in the coating corresponds to 0.225 mg. Ru and 0.075 mg. Au
per square centimeter of projected electrode area. In accelerated
testing this anode showed a weight loss of 0.030 mg./cm..sup.2
after three current reversals and a loss of 0.043 mg./cm..sup.2
after two amalgam dips. After 514 hours of operation this anode
showed a weight change of +0.2 mg./cm..sup.2.
Stoichiometric determinations indicate that the final deposit
contains the three metals in the following form: gold is reduced by
thermal decomposition to the metallic state Au, whereas ruthenium
chloride and titanium chloride are converted into ruthenium oxide
RuO.sub.2 and titanium oxychloride TiOCl, respectively.
Our tests so far have shown that when using the formulations and
deposition methods described above the presence of titanium or
tantalum oxide or oxychloride and iridium alone, i.e., without
ruthenium oxide, gives a deposit which does not withstand the
reducing action of current reversal (cathodic current) and amalgam
dips, whereas the presence of titanium or tantalum oxide,
oxychloride and ruthenium oxide, without iridium or iridium oxide,
gives a deposit of low activity with a high-chlorine discharge
voltage.
The anodes produced according to these examples showed the
following advantages when compared to titanium base anodes covered
with platinum group metals by electroplating or chemi-deposition.
##SPC1##
Weight losses on samples prepared according to the present
invention were determined under simulated operating conditions and
compared with weight losses determined under same conditions on
titanium base samples coated with a Pt-Ir alloy. The tests were
conducted in NaCl saturated solution at 65.degree. C. and under an
anodic current density of 1 A./cm..sup.2. Anode potentials were
measured by means of a Luggin tip against a saturated calomel
electrode and converted to the normal hydrogen electrode scale. The
relevant results are summarized in table II. The integrated weight
change, as shown in the next to last column, was positive, that is
increased, for most of the samples prepared according to the
present invention; this is an indication that the coating, instead
of gradually wearing off and thus decreasing its precious metal
content, tends to build up an additional amount of protective valve
metal oxide which reaches stability after a short period of
operation as shown by sample B.
On the contrary, the results summarized in table I show that even
the best noble metal alloy coatings suffer a greater wear rate,
during operation; while such wear rate is not necessarily to be
imputed exclusively to the spalling off of noble metals, it
certainly involves also a substantial decrease of the noble metal
content in the coating. The amount of noble metals in such noble
metal alloy coatings, which is the amount necessary to obtain a
satisfactory anode activity and a sufficiently long operating life,
is from five to ten times greater than in the coatings prepared
according to the present invention. ##SPC2##
Stoichiometric determinations indicate that the final coating
contains the three metals in different form than the starting
compounds. The iridium chloride apparently is reduced by thermal
decomposition to metallic Ir, whereas ruthenium chloride and
titanium chloride are converted into ruthenium oxide Ru0.sub.2 and
titanium oxychloride TiOCl, respectively, in the anodes of examples
I to IV, and in example V the gold is converted into metallic
Au.
The average thickness of the final coating is 1.45 microns or 57
micro inches and the ratio of precious metals to nonprecious metals
in the coatings may be between 20 to 100 and 85 to 100.
While we have set forth theories as to the final composition of our
improved electrodes we do not intend to be bound by these theories
but base our claim to invention on the procedures described to
produce these electrodes and the results obtained in their use.
The word oxide in the following claims is intended to cover oxides
of titanium and tantalum whether in the form of TiO.sub.2 and
Ta.sub.2 O.sub.5 or TiOCl and TaO.sub.2 C1, or other oxides of
these metals and the words noble metals is intended to include the
platinum group metals and gold and silver.
Various modifications and changes may be made in the steps
described without departing from the spirit of our invention or the
scope of the following claims.
* * * * *