Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
1
PLATINUM-GROUP-METAL ACTIVATED MULTI-LAYER ELECTRODE
FOR ELECTROLYTIC CELL
FIELD OF THE INVENTION
The invention relates to an electrode suitable for operating as anode in
electrolysis cells, for instance
as chlorine-evolving anode in chlor-alkali cells.
BACKGROUND OF THE INVENTION
The use of metal electrodes equipped with catalytic coatings in electrolytic
applications is known in
the art: electrodes consisting of a metal substrate equipped with a coating
based on noble metals or
oxides thereof are for instance utilised as cathodes for hydrogen evolution in
water or alkali chloride
electrolysis processes, as anodes for oxygen evolution in electrometallurgical
processes of various
kinds or for chlorine evolution in alkali chloride electrolysis. Electrodes of
such kind can be produced
via thermal route, i. e. by suitable thermal decomposition of solutions
containing the precursors of
metals to be deposited; by galvanic electrodeposition from suitable
electrolytic baths; by direct
metallisation via flame or plasma spraying processes or chemical or physical
phase vapour deposition.
The electrolysis of sodium chloride brine directed to the production of
chlorine and caustic soda, for
instance, is often carried out with anodes consisting of a titanium or other
valve metal substrate
activated with a superficial layer or ruthenium dioxide (RU02) in order to
lower the overvoltage of
the anodic chorine evolution reaction. For this type of electrolysis, catalyst
formulations based on
mixtures of oxides of ruthenium, iridium and titanium are also known, all
capable of lowering the
overvoltage of the anodic chorine evolution reaction.
Electrodes of such kind are generally produced via thermal route.
Catalytic formulations can be deposited on the substrate by phase vapour
deposition techniques,
having the advantage of allowing an extremely accurate control of coating
deposition parameters.
However, these are fundamentally characterised by being
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batch-type processes, requiring the loading of the substrate in a suitable
deposition chamber, which
has to undergo a slow depressurisation process, lasting several hours, in
order to be able to treat a
single piece. Besides the remarkable duration of the process (several hours
being usually necessary,
depending on the required noble metal loading), the application of high
amounts of catalytic coatings
leads to coatings having a very limited lifetime.
SUMMARY OF THE INVENTION
Under a first aspect, the present invention relates to an electrode for
evolution of gaseous products
in electrolysis cells consisting of a valve metal substrate coated with at
least one first catalytic
composition and with an outer catalytic composition, said at least one first
catalytic composition
comprising a mixture of oxides of a valve metal or of tin and of noble metals
selected from the group
of platinum metals (PM) or oxides thereof taken alone or in admixture, said at
least one first catalytic
composition obtained by thermal decomposition of precursors, said outer
catalytic composition
comprising noble metals selected from the group of platinum metals or oxides
thereof taken alone or
in admixture, said outer catalytic composition being deposited by means of a
chemical or physical
phase vapour deposition technique, the amount of noble metal on said first
catalytic composition
being higher than 5 g/m2 of surface and the amount of noble metal in said
outer catalytic composition
ranging between 0.1 and 3.0 g/m2 of surface.
The inventors have surprisingly found out that the deposition of one last
catalytic layer, with the
specified characteristics, through chemical or physical phase vapour allows
obtaining an electrode
with unexpected features both in terms of duration and of potential decrease.
In one embodiment, the first catalytic composition of the electrode according
to the invention
comprises titanium, iridium, ruthenium in form of metals or oxides.
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In one embodiment, the outer catalytic composition comprises ruthenium and/or
iridium
in form of metals or oxides.
In one embodiment, the specific noble metal loading in the first catalytic
composition
ranges between 6 and 8 g/m2 and the specific metal loading in the outer
catalytic
composition ranges between 1.5 and 2.5 g/m2.
Under another aspect, the invention relates to a method of manufacturing an
electrode
comprising the deposition of an outer catalytic composition by chemical or
physical
phase vapour deposition, preferably by reactive sputtering of noble metals
selected in
the group of platinum metals.
Under a further aspect the invention relates to the reactivation of a used
electrode
comprising the chemical or physical phase vapour deposition of an outer
catalytic
composition including noble metals selected from the group of platinum metals
or
oxides thereof taken alone or in admixture.
Under a further aspect, the invention relates to an electrolysis cell of
alkali chloride
solutions, for instance a sodium chloride brine electrolysis cell directed to
producing
chlorine and caustic soda, which effects the anodic evolution of chlorine on
an electrode
as hereinbefore described.
The following examples are included to demonstrate particular embodiments of
the
invention, whose practicability has been largely verified in the claimed range
of values.
It should be appreciated by those of skill in the art that the compositions
and techniques
disclosed in the examples which follow represent compositions and techniques
discovered by the inventors to function well in the practice of the invention;
however,
those of skill in the art should, in light of the present disclosure,
appreciate that many
changes can be made in the specific embodiments which are disclosed and still
obtain a
like or similar result without departing from the scope of the invention.
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COUNTEREXAMPLE 1
A sample of titanium mesh of 10 cm x 10 cm size was blasted with corundum,
cleaning
the residues with a jet of compressed air. The sample was then degreased using
acetone in a ultrasonic bath for about 10 minutes. After drying, the sample
was dipped
into an aqueous solution containing 250 g/I of NaOH and 50 g/I of KNO3 at
about 100 C
for 1 hour. After the alkaline treatment, the sample was rinsed in deionised
water at
60 C for three times, changing the liquid every time. The last rinse was
carried out
adding a small quantity of HCI (about 1 ml per litre of solution). An air
drying was
effected, observing the formation of a brown hue due to the growth of a thin
film of TiOx.
100 ml of a hydroalcoholic solution containing RuCI3*3H20, H2IrCle6H20, TiCI3
in a
mixture of water and 2-propanol acidified with HCI were then prepared, having
a molar
composition of 36% Ru, 20% Ir, 44% Ti referred to the metals.
The solution was applied to the sample of titanium mesh by brushing in five
coats; after
each coat, a drying at 100-110 C for about 10 minutes was carried out,
followed by a
thermal treatment of 15 minutes at 450 C. The sample was cooled in air each
time prior
to applying the subsequent coat.
At the end of the whole procedure, a total noble metal loading of 9 g/m2,
expressed as
the sum of Ru and Ir referred to the metals, was obtained.
The thus obtained electrode was identified as sample No. 1.
COUNTEREXAMPLE 2
A sample of titanium mesh of 10 cm x 10 cm size was blasted with corundum,
cleaning
the residues with a jet of compressed air. The sample was then degreased using
acetone in a ultrasonic bath for about 10 minutes. After drying, the sample
was dipped
into an aqueous solution containing 250 g/I of NaOH and 50 g/I of KNO3 at
about 100 C
for 1 hour. After the alkaline treatment, the sample was rinsed in deionised
water at
60 C for three times, changing the liquid every time. The last rinse was
carried out
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adding a small quantity of HCI (about 1 ml per litre of solution). An air
drying was
effected, observing the formation of a brown hue due to the growth of a thin
film of TiOx.
The mesh sample was then introduced into the vacuum chamber of the reactive
5 sputtering equipment.
Upon establishing a dynamic vacuum of about 50 E-4 mbar feeding an oxygen
mixture
with 20% argon, the sputtering targets were polarised at the following powers:
ruthenium 35 W, iridium 24 W, titanium 250 W. The target-electrode substrate
gap was
about 10 centimetres.
The process of deposition was carried out, at the same conditions,
alternatively on the
two sides of the titanium mesh for an overall duration of 220 minutes. The
thus obtained
electrode presented a catalytic coating of about 1 micron and a total noble
metal loading
of about 9 g/m2, expressed as the sum of Ru and Ir referred to the metals.
The thus obtained electrode was identified as sample No. 2.
EXAMPLE 1
A sample of titanium mesh of 10 cm x 10 cm size was blasted with corundum,
cleaning
the residues with a jet of compressed air. The sample was then degreased using
acetone in a ultrasonic bath for about 10 minutes. After drying, the sample
was dipped
into an aqueous solution containing 250 g/I of NaOH and 50 g/I of KNO3 at
about 100 C
for 1 hour. After the alkaline treatment, the sample was rinsed in deionised
water at
60 C for three times, changing the liquid every time. The last rinse was
carried out
adding a small quantity of HCI (about 1 ml per litre of solution). An air
drying was
effected, observing the formation of a brown hue due to the growth of a thin
film of TiOx.
100 ml of a hydroalcoholic solution containing RuCI3*3H20, H2IrCI6*6H20, TiCI3
in a
mixture of water and 2-propanol acidified with HCI were then prepared, having
a molar
composition of 36% Ru, 20% Ir, 44% Ti referred to the metals.
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The solution was applied to the sample of titanium mesh by brushing in five
coats; after
each coat, a drying at 100-110 C for about 10 minutes was carried out,
followed by a
thermal treatment of 15 minutes at 450 C. The sample was cooled in air each
time prior
to applying the subsequent coat.
At the end of the whole procedure, a total noble metal loading of 7 g/m2,
expressed as
the sum of Ru and Ir referred to the metals, was obtained.
The semi-finished electrode was then introduced into the vacuum chamber of the
reactive sputtering equipment.
Upon establishing a dynamic vacuum of about 100 E-4 mbar feeding an oxygen
mixture
with 20% argon, the sputtering targets were polarised at the following powers:
ruthenium 30 W, iridium 35 W. The target-electrode substrate gap was about 10
centimetres. To confer optimal properties to the resulting coating, the
substrate was
also subjected to a residual polarisation of about 150 V.
The process of deposition was carried out, at the same conditions,
alternatively on the
two sides of the electrode for an overall duration of 40 minutes. The thus
obtained
electrode had an outer catalytic coating about 0.1 pm thick and a total noble
metal
loading of about 9 g/m2, expressed as the sum of Ru and Ir referred to the
metals.
The thus obtained electrode was identified as sample No. 3.
The samples of the previous examples were characterised as anodes for chlorine
evolution in a lab cell fed with a sodium chloride brine at a concentration of
200 g/I,
strictly controlling the pH at 3. Table 1 reports chlorine overvoltage
measured at a
current density of 4 kA/m2 and the volume percentage of oxygen in product
chlorine.
TABLE 1
Sample No. r1C12 (mV)
1 73
2 63
3 44
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The samples of the previous examples were also subjected to a duration test.
Said
duration test is the simulation in a separated cell of industrial electrolysis
conditions as
regards electrolyte concentration and temperature, but at a current density
conveniently
increased up to a value 2-3 times higher than the nominal one for the sake of
accelerating the experimental response. Table 2 reports the precious metal
lost per unit
current.
TABLE 2
Sample No. Loss of precious metal per
unit current (mgpm/kAh)
1 0.016
2 0.5
3 0.005
The previous description is not intended to limit the invention, which may be
used
according to different embodiments without departing from the scopes thereof,
and
whose extent is univocally defined by the appended claims.
Throughout the description and claims of the present application, the term
"comprise"
and variations thereof such as "comprising" and "comprises" are not intended
to
exclude the presence of other elements or additives.
The discussion of documents, acts, materials, devices, articles and the like
is included
in this specification solely for the purpose of providing a context for the
present
invention. It is not suggested or represented that any or all of these matters
formed part
of the prior art base or were common general knowledge in the field relevant
to the
present invention before the priority date of each claim of this application.
