Note: Descriptions are shown in the official language in which they were submitted.
J~12~8~9
1 ELECTRODES FOR ELECTROLYTIC PROCESSE,S, ESPECIALLY
PERCHLOR~TE PRODUCTION.
TECHNICAL FIELD
The invention relates to electrodes for use
in electxol~tic processes, of the type comprising an
electrically~conductive and corrosion-resistant sub-
strate having an electrocatalytically-active surface
coating, and to electrolytic processes using such
electrodes, especially (~ut not exclusively) as anodes
for the production of chlorates, perchlorates and other
persalts and percompounds including organic peroxyacids.
BACKGROUND ART
For tile pxoduction of perchlorate, various
anode materials have ~een ~lsed com~ercially, including
smooth massive platinum, platinized t~tanium or tantalum
(despite a tendency to produce excess oxygen) and lead
dioxide coated on titanium or graphite, althou~h t~ese
lead dioxide anodes have a high overvoltage and wear
rapidly,
Some proposals have already been made to
con~ine platinum group metals and tin dioxide in electrode
coatin~ materials. For example, U.S. Patent Specifica-
tion 3,701,724 mentioned an anode for chlorine
- production having a coating consisting essentially of
';
.. ~
~L29~3~9
1 a minor amount o~ a platinum group metal and/or platinum
group metal oxides with a major amount of Sn02, Sbz05,
Sb203 or Ge02 and mixtures thereof. However, the claims
and examples of this patent are directed solely to such
coatings containing platinum group metal oxides and there
is no enabling disclosure of a coating containing a
platinum group metal. Also, IJ.S. Patent Sp~cification
- 3,882,002 proposed an anode for c,llorine production
havin~ a valve metal substrate coated with an intermediate
layer of tin dioxide which ~as covered with an outer
la~er of a platinum group metal or oxide thereof. Neither
of these proposals was directed to impro~ing electrolytic
performance in the production of percompounds.
DISCLO~UR~ OF INVENTION
An object of the invention therefore is
to provide an improved electrode suitable for use as an
anode for the production of perchlorates and other
persalts, but which may also be used in other applications,
such as chlorate production.
2Q According to a main aspect of the invention,
an electrode comprises an electrically-conductive
corrosion-resistant substrate having an electrocatalytic
coating and is characterized in -that the coating contains
a mixture of at least one platinum group metal and tin ~`
dioxide dis~ersed in one another throughout the coating
in the ratio of 8.5:1 to 3:2 by weight of the platinum
group metals to the tin (as metal) of the tin dioxide.
The platinum group metal/tin dioxide coating
may also contain a sta~ilizer/binder, for example a
compound such as titanium dioxide r zirconium dioxide or
s~licon dioxide. Additionally, the coating may include
a filler, e.g. particles or fibres of an inert material
such as silica or alumina, particles of titanium, or
~9~
1 z~rconium silicate. Furthermoxe, the coating may also
contain, e.g, as ~ dopant the tin dioxide in a quantity
up to about 30~ b~ welyht ~as metal) of the tin dioxide,
of at least one additional metal or oxide of zinc, cadmium,
arsenic, antimony, bismuth, selenium and tellurium.
Sucn stabilizers or binders, ~illers and dopants generally
do not account for more tnan 70% of the total weight of
the coating, usually far less. In the case of antimony
trioxide or bismuth trioxide as dopant, the preferred
amount corre$ponds to a ratio expressed as parts by
weight of Sb~Bi:Sn ~as metal) of at most ahout 1:4 to
about l:lO or even as low as 1:100.
The platinum group metals are ruthenium,
rihodium/ palladium, osmium, iridium and platinum.
Platinum is the preferred platinum group metal in the
coating, when a single metal is present, especially
in anodes for perchlorate production. However, it is
understood that alloys such as platinum-iridium and
platinum-rhodium, also are useful for other applications.
An allo~ of platinum-palladium containing up to 20%
palladium by weight of the alloy has given very satis-
factory results for perchlorate production. Also, in
some instances, it may be advantageous to alloy the
platinum group metal(s) with one or more non-platinum
group metals, for example an alloy or an intermetallic
compound witn one of the valve metals titanium, ~irconium,
hafnium~ vanadium, niobium and tantalum, or with another
transition metal, for example a metal such as tungsten,
manganese or cobalt.
The substrate may consist of any of the afore-
mentioned valve metals or alloys thereof, porous sintered
titanium being preferred. However, other electrically-
conductive and corrosion-resistant substrates may be used,
such as expanded graphite.
The platinum grou~ metal(s) and tin dioxide
1 with possible additional dopants, such as antimon~
trioxide or bismuth trio~ide, may be co-deposited
chemically from solutions of appropriate salts ~lhlch
are painted, sprayed or otherwise applied on the substrate
and then subjected to heat treatment, this process
being repeated until a suffi~iently thick layer has been
built up.
Alternatively, thin layers of different
components te.g. alternate platinum or Pt/Pd alloy
~0 layers and layers of pure or doped tin dioxide) ~an
be built up in such a way that the components are
effectively mixed and dis~ersed in one another through-
out the coating, possibly witll diffusion between the
layers, in contrast to the known prior art coatin~s
such as that of U. S. Patent Specification 3,882,002,
in which the tin dioxide was applied as a separate
intermediate layer covered by a platinum group metal.
Using this procedure of applying alternate layers, it
is possible to deposit thin layers of platinum
galvanically, which is advantageous, because gal-
vanically-deposited platinum has a lower oxygen evolution
potential than chemi-deposited platinumu
The platinum-group metal or alloy/-tin dioxide
layer may be applied directly to the substrate, or to
an intermediate layer, e.g. of co-deposited tin and
antimony oxides or tin and bismuth oxides, or to inter-
mediate layers consisting of one or more platinum group
metals or their oxides, mixtures or mixed crystals of
platinum group metals and valve metal oxides, inter-
metallics of platinum group metals and non-platinum
group metals, and so for-tn.
In a preferred embodiment, the coating
comprises 4Q to 85 parts by weight of platinum, 0 to 20
parts by wei~ht of palladium and 15 to 40 parts by weight
(as Sn metal~ of tin dioxide on a titanium, tantalum or
. .
` ~29~
1 titanium~tantalum alloy substrate. This emBodiment
of an electrode o~ the inventionJ ~hen used as anode
for perchlorate or persulphate production, has been
found to have selective properties favouring the persalt
production while hindering oxygen evolution. The
platinum metal acts as a catalyst ~or persalt production.
The tin dioxide acts as an oxygen evolution inhibitor
by blocking perox~de decomposition, which can be reyarded
as the intermediate step of the un~anted oxygen evolution
reaction. Finall~, ~he palladium acts as a diluent for
the relatively more expensive platinum, without
adversely affecting the ox~gen inhibition effect of the
tin dioxide.
Another aspect of the 'invention is a process
for the production of chlorates, perchlorates and other
percompounds, e.y. persul~hates, which is characterised
by using as anode an electrode according to the invention,
as defined above.
BRIEF DESCRIpTION OF DRAWINGS
In the accompanying drawings:
Fig. 1 shows a graph of the faraday
efficiency of oxygen evolution as ordinate plotted against
the tin content of the electrode coating as abscissa,
the electrode being that described below in detail in
Example I;
Fig. 2 shows a graph of the faraday efficiency
of oxygen evolution as ordinate plotted against the
palladium content of the electrode coating as abscissa,
the electrode being that described below in detail in
Example II.
BEST MODES FOR CARRXING OUT HE INVENTION
The following Examples are given to illustrate
- ,
, ~ . , , ~ ,
1 the invention,
E~AMPLE I
Titanium coupons measurin~ 10 x 10 x 1 mm
were sandblasted and etched in 20% hydrochloric acid and
were thoroughly washed in water. The coupons were then
coated with an aqueous solution of chlorides of platinum
and tin in different weight ratios~ dried at 95 to 100C
and then heated at 450C for 15 minutes in an oven ~7ith
forc d air ventilation. The procedure was repeated five
times and tlle coupons were given a final heat treatment
at 450C for 60 minutes. The coatings so produced con-
tained SnQ2 and platinum metal dispersed in one another.
The coated coupons were tested as anodes
for the production of sodium perchlorate by the electroly-
sis of a solution consisting of lOOg/l NaC103, 400g/1
NaC104 and 5g/1 Na2CrO4 at 30C using a stainless stPel
cathode and a current densit~ of 2KA/m . Sodium chlorate
was supplied and sodium perchlorate removed to maintain
the concentrations in the electrolyte at a steady state.
2Q The faraday efficiency of the oxygen evolution reaction(i.e. the unwanted side reaction in perchlorate production)
was measured as a function of the percentage by weight of
~in (as metal) in the mixe'd Pt-SnO2 coating. The results
obtained are shown in Fig. 1, from which it can be seen
that there is an optimum oxygen-inhibition effect for a
tin content in the range of about 25%~35% of the total
weight of tin and platinum metals, and a very appreciable
inhibition of oxygen evolution for a tin content in the
larger ran~e from about 15% to about 40%.
EXi~MPLE_II
Titanium coupons were coated as in Example I,
but using various coating solutions containin~ platinum,
.
~z~
-- 7 --
1 palladium and tin chlorides, to produce mixed Pt~Pd~SnO2
coatings having compositions as fol low5:
.
Coatins Composition
(% wei~ht of metal)
Pt ¦ Pd ~ SnO2
_ . .. ~
0 ~ 30
j 30
1 30
! 30
?0 , 30
' 25 ' 30
These coupons were tested as anodes for
perchlorate production under the same conditions as used in
Example I~ The faraday efficiency of the unwanted
lS oxygen evolution reaction was measured as a function of
the palladium metai content, and the results are shown
in Fig. 2. This graph shows that, for a palladium
content up to 20%~ the faraday efficiency remained low,
i.e. the palladium did not.adversely affect the performance
of the coating to inhibit oxygen evolution. However,
above the critical Pd content of 20%~ the faraday
efficiency abruptly increasedr the stahility of the coating
was lowered and some electrochemical corrosion took place.
The coatings of Examples I and II were tested
at different current densities, and it was found that the
oxygen evolution faraday efficiency decreased with in-
- creasing current density up to a~out 2 KA/m , then
remained sta~le above 2 KA/m .
,.:
