Method for coating a porous electrode

Coating a porous electrode for electrochemical processes with an activation layer which covers the electrode surface at least in part and contains metals or compounds of metals of the platinum group by: PA1 (a) coating with a suspension containing particles of a metal of the platinum group and a dispersion agent in which the particles will dissolve at an elevated temperature, PA1 (b) heating the coated electrode to dissolve the particles, PA1 (c) heating the electrode to evaporate the agent, decompose the compound and deposit a layer of the metal of the platinum group on the electrode surface, PA1 (d) repeating the cycle to obtain a layer of desired thickness, PA1 (e) heating the electrode in an oxygen-containing atmosphere to a temperature between 400.degree. and 600.degree. C.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to a method for coating a porous electrode for 
electrochemical processes with an activation layer which covers the 
electrode surface at least in part and contains metals or compounds of 
metals of the platinum group. 
2. Description of the Prior Art 
For activating electrodes for electrochemical processes, for instance, of 
anodes for the chlorine-alkali electrolysis, of a material which is 
resistant to the products of the electrolysis and forms a passivating 
layer under the conditions of the electrolysis, numerous methods have 
become known, the purpose of which is essentially to anchor a platinum 
metal or compounds containing platinum metals mechanically firmly on the 
electrode or the electrode core in an electrochemically effective degree 
of dispersion. It is known, for instance, from German Published Prosecuted 
Application No. 11 55 762 to coat degreased and pickled titanium sheets by 
electroplating with a platinum metal and to heat the sheets in a first 
thermal cycle in an inert atmosphere and in a second cycle in an oxidizing 
atmosphere to a temperature of 800.degree. C. With this treatment, a 
firmly adhering activation layer and at the same time improved protection 
of the electrode material is obtained by reaction of the titanium core in 
rutile which core is exposed in the pores of the activation layer which is 
a thin barrier layer of titanium oxide. This and other coating methods 
that have become known are not as suitable, however, for porous 
electrodes, for instance, for sintered electrodes according to German 
Published Non-Prosecuted Application No. 23 05 175 or electrodes of 
titanium suboxide according to German Published Prosecuted Application No. 
24 05 010. The adhesion of the activation layers is particularly favorable 
with porous electrodes and for many electrochemical processes, the large 
surface of the electrode is an advantage. However, in coating the 
electrode with an activation layer, losses of activating agent occur if 
the known methods are used, since the platinum metals or platinum metal 
compounds are in part also deposited in the pores of the electrode which 
are away from the surface, and the surfaces of which do not participate in 
the electrochemical reactions. The loss is particularly great if the 
activating agent is precipitated from solutions and is not so great if the 
activation layer is electrodeposited. Electrodeposited layers, on the 
other hand, are less suitable because of their dense structure. It has 
also been proposed (for instance, German Published Prosecuted Application 
No. 16 71 422) to apply the metals or metal compounds to the electrode 
surface from finely dispersed suspensions. Because of the great difficulty 
in obtaining uniform distribution and good adhesion of the applied 
activating substances, the above-described coating methods are preferred 
on a technical scale. 
SUMMARY OF THE INVENTION 
An object of the invention is to provide a coating method for porous 
electrodes which gives highly effective activation layers with small 
amounts of platinum metals and does not have the above-described 
disadvantages, especially the relatively large requirement of activating 
agents. 
With the foregoing and other objects in view, there is provided in 
accordance with the invention a method for coating a porous electrode for 
electrochemical processes with an activation layer which covers the 
electrode surface at least in part and contains metals and compounds of 
metals of the platinum group, which comprises coating the electrode 
surface to be covered by an activation layer with a suspension containing 
particles of a compound of a metal of the platinum group and a dispersion 
agent in which the particles will dissolve at an elevated temperature, 
heating the coated electrode to an elevated temperature to dissolve the 
dispersed phase of the suspension in the dispersion agent, heating the 
electrode to deposit a layer of the metal of the platinum group on the 
electrode surface by evaporating the agent and decomposing the compound by 
heating the electrode to a temperature between 250.degree. and 350.degree. 
C.; repeating the cycle a plurality of times to obtain a layer of desired 
thickness; and then heating the electrode in an oxygen-containing 
atmosphere to a temperature between 400.degree. and 600.degree. C. 
Other features which are considered as characteristic for the invention are 
set forth in the appended claims. 
Although the invention is illustrated and described herein as embodied in a 
method for coating a porous electrode, it is nevertheless not intended to 
be limited to the details shown, since various modification may be made 
therein without departing from the spirit of the invention and within the 
scope and range of equivalents of the claims. 
The invention, however, together with additional objects and advantages 
thereof will be best understood from the following description. 
DETAILED DESCRIPTION OF THE INVENTION 
According to the invention, the electrode surface is coated with a 
suspension coating fine-grain compounds of metals of the platinum group 
and a dispersion agent dissolving the compounds at elevated temperatures. 
The dispersed phase of the suspension is dissolved in the dispersion agent 
by heating the coated electrode; is deposited on the electrode surface by 
evaporating the agent and is decomposed by heating the electrode to a 
temperature between 250.degree. and 350.degree. C. The cycle is repeated 
several times and the electrode is then heated in an oxygen-containing 
atmosphere to a temperature between 400.degree. and 600.degree. C. 
The invention is based on the discovery that particles dispersed in a 
suspension cannot get into pores which are accessible via narrow tubes or 
canals, while the dispersion agent fills these pores. The particles with a 
diameter corresponding to the pore diameter block the tubes or canals 
preventing solids from penetrating. Particles of larger diameter cannot 
enter the pores and particles of smaller diameter because the dispersion 
agent tends to retard their mobility together with other particles of 
different size, tend to cause the particles to pile up at the pore 
entrance and block passage of solids therein. If the electrode is heated 
after the coating, the dispersion agent leaves the pores and dissolves the 
particles concentrated at the pore entrance because the compounds of the 
platinum group used according to the invention are increasingly soluble 
with increasing temperatures. The solution which is present in a thin 
layer has relatively good viscosity and spreads uniformly across the outer 
electrode surface and the surface of larger pores accessible from the 
outer surface without penetrating into narrow canals or tube pores. The 
compounds which are deposited with uniform layer thickness through 
evaporation of the solvent, are then decomposed by heating the electrode 
to a temperature between 250.degree. and 350.degree. C., in the process of 
which a ragged metallic activation layer is formed which has a large 
specific surface. A layer thickness of about 1 .mu.m ordinarily required 
for technical purposes is obtained by repeating the coating cycle several 
times. Finally, the invention provides for heating the coated electrode in 
an oxidizing atmosphere of preferably air, to a temperature between 
400.degree. and 600.degree. C. The purpose of the heat treatment is 
primarily passivating the electrode surface exposed in pores of the 
activation layer, and anchoring the activation layer on this surface. 
Partial oxidation of the platinum metals contained in the activation layer 
is not detrimental since the growth of the metal crystals is inhibited and 
finely dispersed layers exhibit greater electrochemical activity. The 
treatment temperature should, therefore, not be less nor more than the 
temperature interval from 400.degree. to 600.degree. C. The heating time 
is advantageously 3 to 60 minutes and can be determined in detail readily 
by simple tests for each electrode material and each compound used as the 
activating agent. 
According to one advantageous embodiment of the method according to the 
invention, compounds of non-platinum metals are dispersed in the 
dispersion agent in addition to compounds of metals of the platinum group. 
Suitable non-platinum metals are tantalum, zirconium, niobium, aluminum 
and especially, titanium. The activation layer then contains after the 
oxidizing treatment a finely dispersed mixture of platinum metals, oxides 
of platinum metals, and oxides of non-platinum metals. According to 
another advantageous embodiment of the invention, thermally decomposable 
complex compounds are used, which contain free acid, as compounds of 
metals of the platinum group and of non-platinum metals, especially 
compounds selected from the group consisting of oxalate, formate, 
tartrate, and citrate complexes of metals selected from the group 
consisting of ruthenium, rhodium, palladium, iridium, and platinum and 
analgous compounds of the non-platinum metals. The dispersion agent used 
according to the invention dissolves the complex compounds at elevated 
temperature; solutions are formed which etch especially the electrode 
surface and, particularly, passivating layers. Especially well suited for 
this purpose are water and optionally, aqueous oxalic acid solutions. With 
this method, the adhesion background is improved without the formation of 
corrosive and noxious vapors as when hydrochloric platinum metal chloride 
solutions are used. 
All electrically conductive metals, alloys and compounds which are stable 
under the conditions of electrochemical processes are basically suited as 
electrodes. Passivating layer-forming metals such as titanium, tantalum, 
zirconium and niobium, and preferably electrodes which consist at least in 
part of titanium suboxide are used, for instance, as an anode for 
chlorine-alkali electrolysis. The electrodes according to the invention 
have a porosity of about 10 to 15% and are generally produced by sintering 
molded pieces of a metal powder or an oxide powder. 
The advantages of the method according to the invention for preparing an 
activation layer on a porous electrode are substantially the following: 
1. Only that part of the total surface is coated which participates in the 
electrochemical reactions, 
2. the substances used are not corrosive and detrimental to health, 
3. the activation layer produced has fine-grain structure and exhibits high 
electrochemical activity, and 
4. the activation layer is firmly anchored in the porous electrode. 
From this results better utilization of the expensive platinum metals, the 
availability of which is limited. 
The invention will be further illustrated by way of the following examples:

EXAMPLE 1 
4.14 parts by weight titanium powder with a grain size of less than 0.06 mm 
and 38.6 parts by weight rutile powder with a grain size of less than 0.01 
mm were mixed after 5 parts by weight of a 2% aqueous polyvinyl-alcohol 
solution have been added in a mixer and rapidly mixed. The mixture was 
pressed with a pressure of about 50 bar in a forging press into molded 
parts. The molded parts were dried, heated in an argon atmosphere to 
1250.degree. C. and then comminuted in a jaw crusher and milled in a ball 
mill to a grain size of less than 0.06 mm. The brittle powder had the 
color of grey iron and had a composition of TiO.sub.0.56. 
To 100 parts by weight powder were added 5 parts by weight of a 10% 
solution of hard paraffin in toluol, followed by 5 minutes of mixing in a 
rotary mixer. The mixture was molded in a forging press with a pressure of 
2 k bar to form sheet electrodes which were heated to 1250.degree. C. in a 
pusher furnace in an argon atmosphere. The sintered electrode sheets, the 
porosity of which is approximately 16%, were coated with a 10% aqueous 
sludge of H[Ru(C.sub.2 O.sub.4).sub.2 ].2.5H.sub.2 O (prepared in 
accordance with O. E. Zviagintsev and S. M. Starostin, Zh. Neorgan, Khim. 
2 (1957) 1281/8), first dried at room temperature and finally at 
105.degree. C. For decomposing the salt, the temperature was increased to 
300.degree. C. The cycle was repeated four times and a total quantity of 
rare metal of about 7 g Ru/m.sup.2 was deposited. The coated electrode was 
heated to 500.degree. C., the residence time at this temperature being 
five minutes. 
The electrode sheet was then tested as an anode in an amalgam test cell. 
The conditions were: 
______________________________________ 
Current density 20 kA/m.sup.2 
Temperature approx. 70.degree. C. 
Brine approx. 300 g/l NaCl 
______________________________________ 
After predetermined time intervals, the anode potential was measured with a 
Luggin capillary against the saturated calomel electrode. 
______________________________________ 
Running time Potential 
______________________________________ 
100 hours 1.102 Volts 
300 1.115 
500 1.112 
______________________________________ 
EXAMPLE 2 
In a die, 100 parts titanium sponge having a grain size of less than 2 mm 
were overlayed with 20 parts TiO.sub.0.56 powder, the preparation of which 
is described in Example 1, and pressed with a pressure of about 2 k bar 
into composite sheets which were sintered as in Example 1. The titanium 
suboxide side of the sheets was coated with a sludge of 66 parts 
H[Ru(C.sub.2 O.sub.4).sub.2 ].2.5H.sub.2 O and 100 parts Ti.sub.2 (C.sub.2 
O.sub.4).sub.3.10H.sub.2 O (prepared in accordance with A. Staehler, Ber. 
38 (1905) 2619/29) in 1000 parts water in which 25 parts oxalic acid were 
dissolved. The heat treatment corresponded to Example 1 except for the 
oxidizing anneal at 550.degree. C. and a holding time of 15 minutes. 
The following potentials were measured: 
______________________________________ 
Running time Potential 
______________________________________ 
100 hours 1.109 Volts 
200 1.113 
300 1.113 
______________________________________ 
EXAMPLE 3 
Composite sheets as in Example 2 were coated with a sludge of 66 parts 
H.sub.2 [Ru(C.sub.2 O.sub.4).sub.2 ].2.5H.sub.2 O, 70 parts H.sub.2 
[Ir.sub.2 (C.sub.2 O.sub.4).sub.3 ], and 100 parts Ti.sub.2 (C.sub.2 
O.sub.4).sub.3.10H.sub.2 O in 1000 parts water and 50 parts oxalic acid, 
were annealed and the potentials were measured. 
______________________________________ 
Running time Potential 
______________________________________ 
100 hours 1.106 Volts 
200 1.112 
300 1.110 
______________________________________