CH645552A5 - CATALYST. - Google Patents

Description

The invention relates to a catalyst, to its use and to a process for its production. More particularly, the invention relates to catalysts and electrodes that are particularly useful for the electrolysis of halides.

Gas generation by electrolysis of a chemical compound into its elements, one of which can be a gas, is of course an old and well known technique.

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A recently developed form of such a gas evolving electrolyser involves the use of a cell that uses an electrolyte in the form of a solid ion-exchanging polymer membrane. In an arrangement of this type, catalytic electrodes with a suitable catalyst are arranged on the opposite sides of the ion transport membrane, such as a sulfonated perfluorocarbon ion exchange membrane. An oxidation reaction produces the ionic form of one of the elements forming the compound on an electrode for what Hydrogen ions are given as an example when H20 or HCl are electrolyzed and sodium ions are another example when a sodium halide such as sodium chloride is electrolyzed. The ion is transported through the ion exchange membrane to the other electrode, where it is reduced to form an electrolysis product, such as molecular hydrogen, the NaOH. Electrolysers with solid ion exchange polymer membranes are particularly advantageous because they are effective, small and do not use corrosive liquid electrolytes.

Various metals and alloys have been used in the past as part of the catalytic electrodes of such electro-chemical electrolysis cells. The performance of the catalyst on the gas-generating electrodes is obviously of fundamental importance in determining the effectiveness and performance of the cell and thus the economy of the process. The selection of a catalyst for an electrochemical cell and its effectiveness depends on a complex set of variables such as the surface area of the catalyst, the accessibility of the oxides thereof at the catalyst surface, impurities in the reactants and the type of conversion taking place in the cell . As a result, it is and has always been difficult to predict the applicability of a catalyst that was useful in an electrochemical cell to another system.

US Pat. No. 3,992,271 describes an improved oxygen-generating catalytic electrode using a platinum / iridium alloy which provides very improved performance and effectiveness. US Pat. No. 4,039,490 describes another catalytic electrode for developing hydrogen, in which reduced oxides of platinum / ruthenium are used. The platinum / ruthenium catalyst is not only significantly cheaper than the reduced platinum / iridium catalyst because it uses a cheaper material, such as ruthenium, to alloy with the platinum, but it is also more effective because it uses less oxygen. Surge voltage, as a platinum Iridi around the electrode.

Attempts to use reduced ruthenium oxide electrocatalysts for the development of halogens by electrolysis of aqueous halide solutions have not been entirely successful, however, since the electrolysis conditions in such cells are harsh. There may be a significant loss of catalyst from the membrane during chlorine evolution because these reduced platinum metal oxides are amenable to dissolution in an acidic environment, such as are encountered in the electrolysis of hydrogen halides or in the electrolysis of alkali metal halide solutions, which are common become angry. And there is not only this tendency to dissolve the platinum metals, which leads to a loss of catalytic material, but the overvoltage of the electrodes also increases, so that the efficiency of the cell decreases and in many cases does not permit longer operating times.

To overcome these disadvantages, the catalyst according to the invention is characterized by that in claim 1

specified characteristics. A method for its manufacture is given in claim 26. Advantageous further developments of the catalyst and types of implementation of the method and uses of the catalyst result from the dependent claims.

The electrocatalytic material can be a single reduced platinum group metal oxide, such as ruthenium oxide or platinum oxide or iridium oxide. However, it has been found that mixtures or alloys of thermally stabilized, reduced platinum group metal oxides are even more stable. Such a mixture or alloy of the ruthenium oxide contains up to 25% by weight of iridium oxide, the preferred range being from 5 to 25% by weight, calculated on a metal basis, although iridium is somewhat more expensive than ruthenium alone.

Electrically conductive extenders, such as graphite, have low halogen overvoltages and are considerably cheaper than platinum metal oxides and can be easily incorporated without reducing the effectiveness of the catalyst. In addition to graphite, oxides of valve metals (the term "valve metal" is defined in U.S. Patent No. 3,948,451 and refers to a subset of transition metals that includes, for example, Ti, Ta, Zr, Mo, Nb, and W.) Conducts these valve metals the current in the anodic direction and resist the passage of current in the cathodic direction, they are sufficiently resistant to the electrolyte and the conditions in an electrolytic cell (e.g. for the production of chlorine and NaOH to be used as electrode material), such as Titanium, tantalum, niobium, tungsten, vanadium, zirconium and hafnium can be added to further stabilize the electrocatalyst and to increase its resistance to adverse electrolysis conditions.

The thermally stabilized, reduced platinum metal oxides and the extenders are formed into an electrode by bonding with fluorocarbon resin particles, such as the material sold by DuPont under the trade name Teflon. The catalytic particles and the resin particles are mixed, the mixture is placed in a mold and the whole is heated until the mass is sintered into a suitable shape which is bonded to at least one surface of the membrane by application of heat and pressure to form an electrode structure and to create a uniform structure of membrane and electrode.

The electrocatalyst, which includes the thermally stabilized, reduced oxides of a platinum group metal together or in combination with other platinum group metals or optionally valve metals, can be made in any suitable manner by which an oxide catalyst is permanently, partially reduced and thermally stabilized.

A preferred mode of reduction is by modifying the Adams platinum manufacturing process by adding a thermally decomposable platinum halide, such as ruthenium chloride, either alone or, if desired, together with an appropriate amount of other thermally decomposable platinum metal or valve metal halides Excess of sodium nitrate. The Adams process of platinum production is described in an article in the Journal of the American Chemical Society, Volume 45, page 217 (1923). It is appropriate to use the finely divided halide salts of platinum metals, such as the chloroplatinic acids in the case of platinum, the ruthenium chloride in the case of ruthenium, titanium chloride, tantalum chloride in the case of titanium and tantalum, in the same weight ratio. Mix metals as desired in the final alloy mix. An excess of sodium nitrate is incorporated and the mixture in a silicon5

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Dioxide shell melted for 3 hours at a temperature of 500 to 600 ° C. The rest is washed thoroughly to remove any nitrates and halides still present, leaving a residue of the desired platinum metal oxide, e.g. Ruthenium oxide, platinum-ruthenium oxide, ruthenium-iridium oxide, ruthenium-titanium oxide etc. The suspension of the mixed and alloyed oxides obtained is then partially reduced. The reduction of the platinum group metal oxides can be carried out by any suitable known reduction method, such as by electrochemical reduction, or by passing hydrogen through the mixture at room temperature, as long as the oxides are not to be reduced completely to the metal. In a preferred embodiment, the oxides are reduced electrochemically in an acidic medium. The product, which is a reduced platinum metal oxide either alone or a mixed alloy oxide, is thoroughly dried, e.g. by using a heating lamp, ground and then sieved through a nylon sieve with a mesh size of about 0.037 mm (corresponding to 400 mesh) to produce a fine powder from the reduced platinum metal oxide.

The resulting reduced platinum metal oxides are then thermally stabilized by heating in the presence of oxygen for a sufficient time to obtain a catalytic material which is stable in the acidic hydrogen halide environment and in the presence of the halogens. When the catalyst is thermally stabilized as described below, much better corrosion properties are obtained in halogens, such as chlorine, etc. and in the presence of halide solutions, such as hydrochloric acid, etc. It is believed that the thermal stabilization to form a catalytic particle with a large average pore diameter and a stable thin oxide film on the outside of the reduced oxide particle. This stabilizes the reduced oxide particles so that they have better mechanical properties for connection to the solid polymer electrolyte membrane, and their resistance to dissolution in hydrochloric acid or other halide acids or to developed halogens is also increased. The reduced oxides are preferably heated to a temperature of 350 to 750 ° C. for a period of 30 minutes to 6 hours, the preferred thermal stabilization being carried out by heating the reduced oxides to a temperature of 550 to 600 ° C. for one hour.

It has also been found that the electrocatalytic activity of the catalytic converter and the electrode with the catalytic converter is optimized by producing the catalytic particles in the finest possible powder form. Thus, it was found that the surface area of the particles, as observed by the BET nitrogen absorption method, should be at least 25 mz / g catalyst. The preferred surface area is between 50 and 150 m2 / g.

The gas permeable electrode structure of the catalytic particles and the fluorocarbon polymer particles is created by blending the catalytic particles with a Teflon dispersion to produce a bonded electrode structure in the manner described in U.S. Patent 3,297,484. In the method of connecting the electrode, it is appropriate to mix the catalyst with Teflon dispersion in such a way that the dispersion contains little or no hydrocarbons. If the fluorocarbon polymer composition contains hydrocarbon surfactants, the surface area of the reduced oxide catalyst is reduced. Any reduction in the surface area of the catalyst is apparently undesirable because it has a potentially adverse effect on the performance and effectiveness of the catalyst. The electrode should therefore be made using a polytetrafluoroethylene particle composition that contains little, if any, hydrocarbons. A suitable form of these particles for the manufacture of the electrode is available from DuPont under the trade name Teflon T-30.

The mixture of the noble metal particles and Teflon particles or of graphite and the reduced oxide particles is placed in a mold and heated until the mass is formed into a decal which is then embedded in and bonded to the surface by application of pressure and heat. For example, as described in U.S. Patent 3,297,484, the electrode structure is bonded to the surface of the ion exchange membrane, thereby integrally bonding the gas absorbing particle mixture and, in some cases, preferably embedding it in the surface of the membrane.

The new membrane / electrode structure thus produced comprises a solid polymer electrolyte membrane which is capable of selective ion transport, a thin porous gas-permeable electrode composed of the electrocatalytic reduced platinum group metal oxides described above being connected to at least one side of the membrane. A second electrode can be connected to the other side of the membrane, and this electrode can either have the same electrocatalytic material or any other suitable cathodic material. The selective ion transport membrane is preferably a stable, hydrogenated, cationic membrane characterized by ion transport selectivity. The cation exchange membrane thus allows the passage of positively charged cations, such as hydrogen ions in the case of the electrolysis of a halide, such as hydrogen chloride or sodium ions in the case of the electrolysis of aqueous sodium halides. The passage of negatively charged anions through a cation exchange membrane is kept to a minimum.

There are various types of ion exchange resins that can be made into membranes that allow the selective transport of the cations. Two classes of such resins are the so-called sulfonic acid cation exchange resins and the carboxyl cation exchange resins. Sulfonic acid exchange resins that are preferred include ion-exchanging groups in the form of hydrated sulfonic acid residues (S03H x H20) that are attached to the polymer chain by sulfonation. The ion-exchanging acid residues in the membrane are firmly connected to the polymer chain, which ensures that the electrolyte concentration does not change. The perfluorocarbon sulfonic acid cation exchange membranes are preferred. A specific class of cation exchange polymer membranes of this type is sold by DuPont under the trade name Nafion. These Nafion membranes are hydrogenated copolymers of polytetrafluoroethylene and polysulfonyl fluoride vinyl ether, which contain pendant sulfonic acid groups.

The ion exchange capacity of a given sulfonic acid cation exchange membrane depends on the milliequivalent weight of the C03 residues per gram of dry polymer. The greater the concentration of the sulfonic acid residues, the greater the ion exchange capacity and thus the ability of the membrane to transport cations. However, as the ion exchange capacity of the membrane increases, so does the water content, and with it the ability of the membrane to repel salt. In the electrolysis of alkali metal halide solutions, alkali is generated on the cathode side and the speed5

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The rate at which the sodium hydroxide migrates from the cathode to the anode side increases with the ion exchange capacity. Such migration back reduces the effectiveness of the cathode current and also leads to the generation of oxygen at the anode, which has undesirable consequences in its effect on the catalytic anode. The preferred ion exchange membrane for use in brine electrolysis is therefore a laminate made from a thin film (about 0.05 mm thick) with a mil equivalent weight of 1500 and a low water content (from about 5 to 15%) with high Salt repellency combined with a film approximately 0.1 mm thick with high ion exchange capacity, a milliequivalent weight of 1100 combined with a Teflon fabric. One form of such a layer construction is marketed by DuPont under the trade name Nafion 315. Other forms of laminates or constructions in which the cathode side layer consists of a thin resin film layer of low water content (about 5 to 15%) in order to optimize salt rejection are also available. Typical of such other laminates are those available under the trade names Nafion 355, 376, 390, 227 and 214. In the case of a laminate membrane connected to a Teflon fabric, it may be desirable to reflux the membrane and the Teflon fabric in addition to soaking in alkali, as was previously preferred, for cleaning in 70% nitric acid for 3 to 4 hours heat.

In the case of the electrolysis of hydrogen halides, such as hydrochloric acid, there is no problem with the migration of alkali or other salts back, so that simpler membrane forms, such as Nafion 120, can be used as the ion-transporting medium.

In the case of brine electrolysis, the nation-side barrier layer, which must be characterized by a low water content, can include laminates in which the cathode side layer is a thin (0.05 to 0.1 mm thick) chemically modified film with sulfonamide groups or carboxylic acid groups.

In the following reference is made to the drawing in which

1 shows a schematic representation of an electrolytic cell according to the invention using a solid polymer electrolyte membrane and the new catalyst connected to the surface of the membrane

Figure 2 is a schematic representation of the reactions that take place in the various parts of the cell during the electrolysis of an aqueous halide solution.

In Fig. 1, 10 denotes a halogen electrolysis cell, which consists of a cathode compartment 11 and an anode compartment 12, which are separated from one another by a solid polymer electrolyte membrane 13, the membrane being preferably a hydrated, permeability-selective cation exchange membrane. Connected to the opposite surfaces of membrane 13 are electrodes comprising particles of fluorocarbon such as Teflon coated with thermally stabilized, reduced oxides of ruthenium (RuOx) or iridium (IrOx) or stabilized, reduced oxides of ruthenium-iridium (Rulr ), Ruthenium titanium (RuTi), ruthenium tantalum (RuTa), ruthenium tantalum iridium (RuTalr) or ruthenium graphite or combinations of the aforementioned oxides with graphite and other valve metal oxides. The cathode 14 is connected to and preferably embedded in one side of the membrane, and a non-illustrated catalytic anode is connected to and preferably embedded in the opposite side of the membrane. Current collectors in the form of metal sieves 15 and 16 are pressed against the electrodes. The entire membrane and electrode assembly is firmly supported by the housing members 11 and 12 by means of the seals 17 and 18, which seals are made of any material that is resistant to the cell environment, namely alkali, chlorine, oxygen, aqueous sodium chloride in the case of electrolysis of brine and HCl, HBr in the case of other hydrogen halides, stable or inert. is. One form of such a seal is a filled rubber. Rubber seal made by the Irving Moore Company, Cambridge, Mass. is marketed under the trade name EPDM.

An aqueous alkali metal halide such as brine or a hydrogen halide such as HCl is introduced through an electrolyte inlet 19 which communicates with the chamber 20. Used electrolyte and halogens, such as chlorine, are removed through an outlet line 21. A cathode inlet line 22 is provided for the case of brine electrolysis and is connected to the cathode chamber 11 in order to introduce cathode chamber electrolyte, hereinafter abbreviated «catholyte», water or aqueous NaOH, which is more dilute than the electrolytic one. allow chemically formed at the interface electrode / electro-lyt. In the case of electrolysis of hydrogen halide such as HCL, no catholyte is required, and the cathode inlet line 22 can therefore be omitted.

In a brine electrolytic cell, the water serves two separate functions. Part of the water is electrolyzed to produce hydroxyl anions, and these hydroxyl anions combine with the sodium cations transported through the membrane to form lyes (NaOH). However, the water also rinses through the porous bound cathode electrode in order to dilute the highly concentrated lye formed at the membrane / electrode interface and thus diffuse the lye through the membrane back into the anode electrolyte (hereinafter referred to as «anolyte») ) Chamber to be kept as small as possible. The cathode outlet line 21 is in communication with the cathode chamber 11 for removing excess catholyte and the electrolysis products such as alkali in the case of brine electrolysis plus any hydrogen release at the cathode in both the electrolysis of brine and hydrogen chloride. An energized cable 23 is inserted into the cathode chamber and a comparable cable, not shown, is inserted into the anode chamber. These cables connect the current carrying screens 16 or 15 or any other suitable type of collector to the power source.

Figure 2 diagrammatically illustrates the reactions taking place in the cell during the electrolysis of an aqueous alkali metal halide solution, such as brine, and is used to understand the electrolysis process in the manner in which the cell operates. An aqueous solution of sodium chloride is introduced into the anode chamber, which is separated from the cathode chamber by the cationic membrane 13. The membrane 13 is a composite membrane with a layer 26 of high water content (20 to 35%, based on the dry weight of the membrane) on the anode side and a layer 27 of low water content (5 to 15%, based on the dry weight of the membrane) and high milliequivalent weight on the cathode side, both of which are separated from one another by a Teflon fabric 28.

The cathode side layer can also be chemically modified on the cathode side to form a thin layer of a polymer with a low water content. In one embodiment, this is done by modifying the

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Polymers to form a substituted sulfonamide membrane layer. By converting the cathode side layer to a weak acid form (sulfonamide), the water content of this part of the membrane is reduced and the film's ability to repel salt is increased. As a result, the back diffusion of sodium hydroxide through the membrane to the anode is minimized. While layered membrane constructions are preferred in brine electrolysis to block the back migration of sodium hydroxide, other homogeneous films with low water content can also be used, such as Nafion 150, perfluorocarboxylates etc. In the case of electrolysis of hydrogen halides such as HCl, HBr etc. the ion transport membrane can obviously be a simple homogeneous film, such as Nafion 120.

The Teflon-bound catalysts made from reduced precious metal oxides contain at least one thermally stabilized, reduced platinum metal oxide, such as one of ruthenium, iridium or ruthenium-iridium with or without the addition of reduced oxides of titanium, niobium or tantalum and particles of graphite. These catalysts are pressed into the surface of the membrane 13. The current collectors 15 and 16, which are only partially shown in FIG. 2 for the sake of clarity, are pressed against the surface of the catalytic electrodes and connected to the positive or negative connection of the energy source in order to create the electrolysis potential between the cell electrodes. The aqueous halide ion solution, such as aqueous sodium chloride solution, which is introduced into the anode chamber, is electrolyzed at the anode 29 to produce chlorine, as illustrated by the bubbles 30 in FIG. 2. The chlorine will in principle arise at the interface from the electrode to the membrane, but will reach the electrode surface through the porous electrode, which is facing the electrolyte. The sodium ions Na + are transported through the membrane 13 to the cathode 14. A stream of water or aqueous NaOH, designated 31, is introduced into the cathode chamber and acts as a catholyte. This aqueous stream rinses the surface of the Teflon-bound catalytic cathode 14 to dilute the caustic formed at the membrane to cathode interface and thereby reduce the back diffusion of the caustic through the membrane to the anode.

Part of the water catholyte is electrolyzed at the cathode to form hydroxyl ions and gaseous hydrogen. The hydroxyl ions combine with the sodium ions transported through the membrane to form sodium hydroxide at the interface from the membrane to the electrode. The sodium hydroxide slightly wets the Teflon portion of the bound electrode and migrates to the surface where it is diluted by the aqueous stream rinsing over the surface of the electrode. Even using the cathode's water rinse, concentrated sodium hydroxide in the range of 4.5 to 6.5 molar is formed on the cathode. However, some of the sodium hydroxide, which is indicated by the arrow 33, migrates back through the membrane 13 to the anode. This NaOH migration is a diffusion process caused by the concentration gradient and the electrochemical transport of negative ions to the anode. The sodium hydroxide transported to the anode is oxidized to produce water and oxygen as shown by bubble 34. This is a parasitic reaction which reduces the effectiveness of the cathode current and should be kept as low as possible by using membranes which have a strong salt repellency on the cathode side. In addition to the effect on current efficiency, oxygen generation at the anode is undesirable because it can have adverse effects on the electrode and membrane, especially if the electrode contains graphite. In addition, the oxygen dilutes the chlorine generated at the anode, so additional treatment is required to remove the oxygen. The formation of oxygen can be further reduced by acidifying the aqueous anolyte, so that the hydroxide which migrates back is converted into water by neutralization rather than being oxidized with the formation of oxygen. The reactions in the different parts of the cell during the electrolysis of NaCl are as follows:

15 anode reaction:

(Principle)

Membrane transport: cathode reaction:

20 anode reaction: overall reaction:

2 Cl- * Cl2î + 2e ~ (1)

2Na ++ H20 (2)

2H20-> 20H ~ + H2T-2e- (3)

2Na + + 20H -> 2Na0H (4)

40H ~ -> 02 + 2H20 + 4e ~ (5) 2NaCl + 2H20-> 2Na0H +

C12Î + H2Î (6)

In the electrolysis of hydrogen halide, such as HCl, the electrolysis reactions are very similar:

Anode reaction: 2HC1- »2H + + C12Î + 2e ~ (1) membrane transport: 2H + (H20, HCl) (2)

Cathode reaction: 2H + + 2e ~ -> H2î (3)

Overall reaction: 2HCl-> H2t + C12Î (4)

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The new arrangement for the electrolysis of aqueous solutions of salt or HCl, which is described in the present application, is characterized by the fact that the catalytic sites of the electrodes are in direct contact with the cation exchange membrane and the ion-exchanging acid residues are connected to the polymer chain, these acid-exchanging residues being sulfonic acid residues S03H x H20 or carboxylic acid residues COOH x H20. There is therefore no significant voltage drop in either the anolyte or catholyte chamber. Such an electrolyte voltage drop is characteristic of the existing systems and methods in which the electrode and membrane are separated from one another, and it can be of the order of magnitude 45 from 0.2 to 0.5 volts. Eliminating or significantly reducing this voltage drop is one of the main advantages of the present invention because it has a very significant effect on the overall cell voltage and the economics of the process.

so Since chlorine is generated directly at the interface of the anode to the membrane, there is also no voltage drop due to the so-called “bubble effect”, which means a loss of gas mixture and mass transport due to the interruption or blocking of the electrolyte path between the electrode and the membrane is.

In systems according to the prior art, the chlorine-releasing catalytic electrode is separated from the membrane. The gas is formed directly on the electrode and results in a gas layer in the space between the membrane and the 6o electrode. This interrupts the electrolyte path between the electrode collector and the membrane and blocks the passage of Na + ions and thus increases the voltage drop.

In a preferred embodiment, the Te-65 flon-bound noble metal electrode contains reduced oxides of ruthenium, iridium or ruthenium-iridium in order to keep the chlorine overvoltage at the anode as low as possible. The reduced ruthenium oxides are compared to chlorine

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and oxygen evolution stabilized to get a stable anode. The stabilization takes place initially through temperature stabilization, i.e. by heating the reduced ruthenium oxides for one hour at temperatures in the range from 550 to 600 ° C. The Teflon-bonded reduced oxides of the ruthenium anode are further stabilized by mixing with graphite and / or alloys or mixing with the reduced oxides of iridium IrOx in a range from 5 to 25% iridium, with 25% being preferred or with reduced oxides of Titanium TiOx, with 25 to 50% of TiOx being preferred. It has also been found that a ternary alloy of reduced oxides of titanium, ruthenium and iridium (Ru, Ir, Ti) Ox or tantalum, ruthenium and iridium (Ru, Ir, Ta) Ox bonded with Teflon is very effective in that Production of a stable, long-life anode. In the case of the ternary alloy, the composition preferably consists of 5 to 25% by weight of reduced oxides of iridium, about 50% by weight of reduced oxides of ruthenium and the rest is a valve metal such as titanium. For a binary alloy of reduced oxides aux ruthenium and titanium, the preferred electrode contains 50% by weight titanium and the rest is ruthenium. Titanium has the additional advantage that it is much cheaper than ruthenium or iridium, making it an effective extender that reduces costs while keeping the electrode in an acidic environment and against HCl, chlorine and oxygen evolution stabilized. Other valve metals such as niobium, tantalum, zirconium or hafnium can be used in the electrode structure instead of titanium. In addition to the valve metals, valve metal carbides, nitrides and sulfides can also be used as catalyst extenders.

The alloys of the reduced noble metal oxides are mixed with the reduced oxides of titanium or other valve metals with Teflon to form a homogeneous mixture. The anode content of Teflon can range from 15 to 50% by weight, although 20 to 30% by weight is preferred. The Teflon used is the material sold by DuPont Corporation under the trade name T-30, although other fluorocarbons can also be used equally. Typical amounts of noble metal for the anode are 0.6 mg / cm 2 of the electrode surface, the preferred range being from 1 to 2 mg / cm 2. The current collector for the anode can be a platinized niobium mesh with fine mesh, with which a good contact to the electrode surface is obtained. However, an expanded titanium mesh, which is coated with ruthenium oxide, iridium oxide, valve metal oxide or mixtures thereof, can also be used as the collector structure for the anode. Yet another possible anode collector structure can be a titanium-palladium plate with a platinum-plated mesh attached to it by welding or joining.

The cathode is preferably a bound mixture of Teflon particles and platinum black with a platinum black amount of 0.4 to 4 mg / cm 2. Similar to the anode, the cathode is connected to and embedded in the surface of the cation exchange membrane. The cathode is made very thin with a thickness of about 0.05 to 0.075 mm or less and preferably with a thickness of about 0.012 mm, it is porous and has a low teflon content.

The thickness of the cathode can be quite significant since the cathode can be reflected in reduced water or by flushing and penetrating aqueous NaOH into the cathode, reducing the effectiveness of the cathode current. The cells were made with thin cathodes of approximately 0.012 to 0.05 mm thick made of platinum black with 15%

Teflon manufactured. The storm efficiencies of cells with thin cathodes were about 80% with 5 molar NaOH, at 88 to 91 ° C. and a feed of 290 g / l to the anode. With a 0.075 mm thick cathode made of ruthenium and graphite, the current efficiency was reduced to 54% with 5 molar NaOH. The following Table A shows the relationship of current efficiency to thickness and indicates that thicknesses that do not exceed 0.05 to 0.075 mm have the best performance.

Table A

Cell cathode cathode thickness current efficiency

(mm) in% (with n-NaOH)

1

Platinum black

0.05-0.075

64 (4.0)

2nd

Plantin black

0.05-0.075

73 (4.5)

3rd

Platinum black

0.025-0.05

75 (3.1)

4th

Platinum black

0.025-0.05

82 (5)

5

Platinum black

0.012

78 (5.5)

6

5% platinum black

0.075

78 (3.0)

on graphite

7

15% Ru Ox on

graphite

0.075

54 (5.0)

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Platinized

Graphite fabric

0.25-0.37

57 (5)

The electrode is made permeable to gas so that the gases developed at the interface between the electrode and the membrane can easily escape. It is made porous in order to allow the rinsing water to penetrate to the interface between the electrode and membrane at which the NaOH is formed and to allow the supplied brine to access the membrane and the catalytic electrode sites. This supports the dilution of the highly concentrated NaOH formed before it wets the Teflon and reaches the electrode surface, where it is further diluted by the water flowing over the electrode surface. It is important to dilute the NaOH at the membrane-electrode interface, as the concentration is greatest there. In order to maximize the water penetration of the cathode, the teflon content should not exceed 15 to 30% by weight, since teflon is hydrophobic. With a good porosity, a limited teflon content, a thin cross-section and a rinsing liquid made of water or dilute alkali, the NaOH concentration is controlled in order to reduce the migration of NaOH through the membrane.

The current collector for the cathode must be carefully selected because the highly corrosive alkali present on the cathode attacks many materials, especially while the cell is not working. The current collector can be a nickel network because nickel is resistant to alkali. However, the current collector can also be made from a plate made of corrosion-resistant steel, onto which a mesh made of corrosion-resistant steel is welded. Another current collector structure for the cathode that is resistant or inert to alkaline solution is graphite or graphite in combination with a nickel mesh that is pressed onto the plate and against the surface of the electrode.

The invention is explained in more detail below with the aid of examples.

Examples

Cells were built and tested, which had ion exchange membranes and Teflon-bound electrodes with reduced noble metal oxide, which s

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were embedded to show the effect of the various parameters on the effectiveness of the cell in brine electrolysis and in particular to illustrate the operating voltage characteristics of the cell.

Table I illustrates the effect of the various combinations of reduced noble metal oxides on the cell voltage. The cells were equipped with electrodes which had various specific combinations of reduced noble metal oxides, which were bound with Teflon particles and embedded in a 0.15 mm thick cationic ion exchange membrane. The cell was operated with a current density of approximately 325 mA / cm 2 at 90 ° C., feed rates of 200 to 2000 ml / min and a concentration of the feed of 5 mol.

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One cell was constructed according to the prior art and contained a dimensionally stabilized anode at a distance from the membrane and a cathode network made of corrosion-resistant steel in a similar manner, s This control cell was operated under the same conditions.

It can be seen from the data given in Table I that the cell operating potentials in the process according to the invention were in the range from 2.9 to 3.6 volts. Compared to a cell according to the state of the art, control cell No. 4, under the same operating conditions a voltage improvement of 0.6 to 1.5 V was realized. The resulting operational efficiencies and economic benefits are clearly evident.

Table I

Cellular

anode

Salt supplied to the cathode

Current density

Cell voltage

Temp. ° C

Current effect

membrane

No.

alkali m A / cm2

(V)

totality

(5 molar NaOH)

1

6 mg / cm2 (Ru, 25% lr) 0 *

4 mg / cm2 platinum black

5 molar (290g / l

323

3.2-3.3

90

85%

Dupont Nafion 315 laminate

2nd

6 mg / cm2 (Ru, 25% Ir) Ox

4 mg / cm2 platinum black

5 molar (290 g / 1)

323

3.3-3.6

90

78%

'DuPont 1500 EW Nafion

3rd

6 mg / cm2 (Ru, 25% lr) 0.

4 mg / cm2 platinum black.

5 molar (290 g / 1)

323

2.9

90

66%

Dupont 1500 EW Nafion

4th

dimensionally

Corro mesh

5 molar

323

4.2-4.4

90

81%

DuPont 1500

stable mesh anode sion resistant (290 g / 1)

EW Nafion

at a distance of

Steel in the distance

the membrane from the membrane

5

4 mg / cm2

4 mg / cm2

5 molar

323

3.6-3.7

90

85%

DuPont

(Ru, 50% Ti) Ox

Platinum black

(290 g / 1)

Nafion 315

Laminate

6

4 mg / cm2

4 mg / cm2

5 molar

323

3.5-3.6

90

86%

DuPont

(Ru, 25% Ir-25%

Platinum black

(290 g / 1)

Nafion 315

Ta) Ox

Laminate

7

6 mg / cm2

2 mg / cm2

5 molar

323

3.0

90

89%

DuPont

(RuOjt graphite)

Platinum black

(290 g / 1)

Nafion 315

Laminate

8th

6 mg / cm2

4 mg / cm2

5 molar

323

3.4

80

83%

DuPont

(Rucg

Platinum black

(290 g / 1)

1500 PE

Nafion

9

6 mg / cm2

4 mg / cm2

5 molar

323

3.4-3.7

90

73%

DuPont

Ru - 5Ir) Ox

Platinum black

(290 g / 1)

1500 PE

Nafion

10th

2 mg / cm2

4 mg / cm2

5 molar

323

3.1-3.5

90

80%

DuPont

(Ir O *)

Platinum black

(290 g / 1)

Nafion 315

Laminate

11

2 mg / cm2

(Ircg

4 mg / cm2 platinum black

5 molar (290 g / 1)

323

3.2-3.6

90

65%

DuPont Nafion 315

Laminate

A cell similar to cell No. 7 of Table I was constructed and operated at 90 ° with the addition of saturated brine. The cell potential (V) as a function of the current density is given in Table II below.

Table II

Cell voltage (V) Slrom density (mA / cm2)

3.2 430

2.9 323

2.7 215

2.4 108

ss These results show that the cell operating potential decreases with reduced current density. However, the relationship between current density and cell voltage determines the relationship between operating and capital costs in chlorine electrolysis. However, it becomes clear that even at 6 o very high current densities (about 325 and 430 mA / cm 2), significant improvements in the cell voltage are realized in the process for chlorine production according to the invention.

Table III illustrates the effect of cathodic current efficiency on oxygen evolution. A cell with Teflon-bound catalytic anode and cathode with reduced noble metal oxides, embedded in a cation exchange membrane, was at 90 ° C below

9

645 552

Supply of a saturated brine with a current density of 323 mA / cm2 and a feed rate of 2 to 5 ml / min / 6.25 cm2 of the electrode area operated. The volume percent of oxygen in the chlorine was determined as a function of the cathodic current efficiency.

Table III

Cathodic oxygen

Current efficiency (%) development (vol%)

89 2.2

86 4.0

84 5.8

80 8.9

Table IV illustrates the controlling effect of the acidification of the brine on the evolution of oxygen. The volume percentage of oxygen in the chlorine was determined for different HCl concentrations in the brine.

Table IV

Acid (HCl) - 02 (vol%)

Concentration (moles)

0.05 2.5

0.075 1.5

0.10 0.9

0.15 0.5

0.25 0.4

From these data it is clear that the oxygen evolution due to the electro-chemical oxidation of the migrated OH "is reduced by preferential reaction of the OH ~ with the H + with the formation of H20.

A cell similar to cell # 1 of Table I was constructed and operated at 323 mA / cm2 with the addition of saturated NaCl solution which had been acidified with 0.2N HCl. The cell voltage was measured at various operating temperatures from 35 to 90 ° C.

A cell similar to cell No. 7 of Table I was constructed and operated at 215 mA / cm2 with the addition of a solution (approximately 5 molar) containing 290 g NaCl / 1, which was not acidified. The cell voltage was measured at various operating temperatures from 35 to 90 ° C. The results were converted to a current density of 323 mA / cm2.

Table V

Cell # 1 Cell # 7 Voltage (V) temperature ° C

Voltage (V) converted to 323 mA / cm *

(measured at 215 mA / cm2)

3.65 3.50 (3.15) 35

3.38 3.30 (2.98) 45

3.2 3.20 (2.9) 55

3.15 3.12 (2.78) 65

3.10 3.05 (2.72) 75

3.05 2.97 (2.65) 85

3.02 2.95 (2.63) 90

These results show that the best operating voltage is obtained in the range of 80 to 90 ° C. However, it should be pointed out that even at 35 ° C. the voltage with the catalyst and electrolyser according to the invention is at least 0.5 volt better than with the chlorine electrolysers according to the prior art, which were operated at 90 ° C.

If the NaCl electrolysis is carried out in a cell in which both electrodes are connected to the surface of an ion-transporting membrane, a maximum improvement is obtained. However, the improved performance of the method is achieved for all structures in which at least one of the electrodes is connected to the surface of the ion-transporting membrane. Such a cell is called a hybrid cell. The improvement in such a hybrid cell is somewhat less than in a cell in which both electrodes are connected to the membrane. Nevertheless, the improvement in a hybrid cell is clear, namely 0.3 to 0.5 volt better than in the cells according to the prior art.

A series of cells were constructed and brine electrolysis performed therein to compare the results of cells with two electrodes connected to the membrane with the results in hydride cells in which either only the anode or only the cathode was connected to the membrane to be compared with a cell according to the prior art, in which none of the electrodes was connected to the membrane.

All of these cells were made with membranes from Nafion 315, the cells were operated at 90 ° C., and an alkali was introduced at a concentration of about 290 g / l. The amount of catalyst in the bound electrode for platinum black was 2 g / 930 cm2 at the cathode and 4 g / 930 cm2 at the anode for RuOx graphite and RuOx. The current efficiency at 323 mA / cm2 was essentially the same for all cells (84 to 85% for 5 molar NaOH). Table VI shows the cell voltages for the different cells.

Table VI

Cell anode cathode cell voltage (V)

at 323 mA / cm2

1

Ru graphite

Platinum black

2.9

(bound)

(bound)

2nd

Platinized

Platinum black

3.5

Niobium net

(bound)

(not bound)

3rd

Platinized

Platinum black

3.4

Niobium net

(bound)

(not bound)

4th

Ru graphite

Nickel mesh

3.5

(bound)

(not bound)

5

Ru Ox

Nickel mesh

3.3

(bound)

(not bound)

6

Platinized

Nickel mesh

3.8

Niobium net

(not bound)

(not bound)

The cell voltage of cell # 1 equipped with two Teflon-bound electrodes is almost 1 volt better than the prior art cell in which none of the electrodes was a Teflon-bound electrode and which is listed under cell # 6. The hybrid cells with bound cathode Nos. 2 and 3 and the hybrid cells with bound

5

10th

15

20th

25th

30th

35

40

45

50

55

60

65

645 552

10th

Anode # 4 and 5 are 0.4 to 0.6 volts worse than those with two bonded electrodes, but still 0.3 to 0.5 volts better than cell # 6 without a Teflon bonded electrode.

By using catalytic electrodes that are directly connected to and embedded in the cation exchange membrane, a greatly improved process for producing chlorine and other halogens from brine and, as will be shown below, from HCl and other hydrogen halides is possible.

With the aforementioned arrangement of the catalytic electrodes, the catalytic sites of the electrodes are in direct contact with the membrane and the cation-exchanging acid residues in the membrane, and this leads to a process which is considerably better in terms of the required cell potential (up to 1 volt or more) than the known methods. The use of the very effective fluorocarbon-bound, thermally stabilized, reduced noble metal oxide catalysts, such as the fluorocarbon / graphite-reduced noble metal oxide catalysts with the low overvoltages, has further increased the effectiveness of the process. Electrodes were built and tested that had the thermally stabilized, reduced noble metal oxides and so on embedded in ion exchange membranes to illustrate the effect of the various parameters on the effectiveness of the cell and catalyst in the electrolysis of hydrochloric acid.

Table VII shows the effect of different combinations of reduced noble metal oxides on the cell voltage. The cells were provided with Teflon-bonded graphite electrodes, which contained various specific combinations of thermally stabilized, reduced platinum metal oxides and reduced oxides of titanium, embedded in a hydrated ionic membrane with a thickness of 0.3 mm. The cell was operated at a current density of 430 mA / cm 2 at 30 ° C. and a feed rate of 70 ml / min with a normality of the solution supplied from 9 to 11. The active cell area was 46.5 cm2. 15 Tables VIII and IX show the effect of the operating time on the cell operating voltages for the same cells and under the same conditions.

Table X shows the effect of the acid concentration of the supplied solution in the range from 7.5 to 11.5 n. A cell similar to cell No. 5 in Table I was constructed with reduced (Ru, 25% Ir) Ox, the was added to the Teflon bonded graphite electrode. The cell was operated at a fixed feed rate of 150 ml / min at 30 ° C and 430 mA / cm2. The active cell surface was 46.5 cm2.

Table VII

Cell no.

Operating time (h)

Anode graphite /

oxide

Cathode graphite /

oxide

Normality of

electricity

Cell

Fluorocarbon plus quantity

Fluorocarbon plus amount of supplied voltage

mg / cm2

mg / cm2

MA / cm2 solution

(V)

1

6300

(Ru) Ox

0.6

(Ru) Ox

0.6

9-11.

430

2.10

heat stabilized

heat stabilized

2nd

5300

(Ru, Ti) Ox

0.6

(Ru, Ti) Ox

0.6

9-11

430

2.01

heat stabilized

heat stabilized

3rd

4900

(Ru, Ti) Ox

1.0

(Ru, Ti) Ox

1.0

9-11

430

1.97

heat stabilized

heat stabilized

4th

1800

(Ru Ti) Ox

1.0

(Ru) Ox

1.0

9-11

430

1.91

heat stabilized

heat stabilized

5

4000

(Ru, 25% lr) 0

1.0

(Ru, 25% Ir) Ox

1.0

9-11

430

2.07 *

heat stabilized *

heat stabilized

(1.9)

6

200

(Ru, Ti, 5% lr) 0

2.0

(Ru, Ti, 5% Ir) Ox

2.0

9-11

430

1.80

heat stabilized

heat stabilized

7

100

(Ru, 25% Ta) Ox

2.2

(Ru, 25% Ta) Ox

2.0

9-11

430.

1.64

* The cell voltage of this cell was around 1.9 volts after 3800 hours. It was removed from the test due to leakage.

Table VIII

50

Table IX

Cellular

Cell voltage

Cell voltage

Current density

Cellular

Between

Current density

Cell voltages

No.

(V) after 100 h

(V) for the Be

(mA / cm2)

No.

operating time (h)

(mA / cm2)

(V)

Operating time drive time from

55 1

Tables

3900

108

1.70

215

1.93

1

1.85

2.10

430

323

2.00

2nd

1.84

2.01

430

2nd

3400

108

1.57

3rd

1.78

1.97

430

60

215

1.70

4th

1.80

1.91

430

323

1.83

5

1.75

2.07 *

430

3rd

1900

108

1.58

(1.9)

215

1.70

430

323

1.81

6

1.70

1.80

65

4th

1000

108

1.47

215

1.60

* See footnote in Table VII.

323

1.72

11

645 552

Table IX (continued)

Cell no.

Intermediate operating time (h)

Current density (mA / cm2)

Cell voltages (V)

5

1200

108

5

1.32

215 '

1.45

323

1.55

Table X

10th

Normality of the solution supplied

7

7.5

10th

10.5

11.5

Vol .-%

02

0.4 0.15 0.04 0.015

0.007 0.004 0.003

From the examples above it follows that HCl is electrolyzed to chlorine gas which is essentially free of

Oxygen. The catalyst used in the electrolysis cell is characterized by a low cell voltage and a low operating temperature (approximately 30 ° C.), which enables such electrolysis cells to be operated economically. Furthermore, the data show the excellent performance at different current densities, especially at 323 to 430 mA / cm2. This has a positive and beneficial effect on the cost of capital for the chlorine electrolysers according to the present invention.

Various tests were carried out to show the effect of thermal stabilization on reduced precious metal and valve metal oxides. These tests show the influence on the resistance of the catalyst to rough electrolysis conditions. Thermally stabilized and non-stabilized, reduced oxide catalysts were exposed to highly concentrated HCl solutions, which represent the extremely harsh conditions. The color of the solution was observed since darkening of the solution indicated loss of catalyst. Increasing loss of catalyst was accompanied by more pronounced color changes.

Table XI shows the results of these corrosion resistance and stability tests for catalyst mixtures of 0.5 to 20 g.

Table XI

catalyst

treatment

Temp.

Corrosion

Watching time

Stability evaluation

° C

medium

(H)

colour

RuOx none

24th

12n HCl

24th

slightly brown moderate corrosion

for 1 hour at 550 ° C

24th

12n HCl

744

very pale yellow very little corrosion,

thermally stabilized

good stability

(Ru 25Nb) Ox none

24th

12n HCl

24 912

light brown amber color moderate corrosion

(Ru 50Ta) Ox none

24th

12n HCl

168

pale amber stone color moderate corrosion

for 1 hour at 550 ° C

24th

12n HCl

96

very pale yellow, completely stable

thermally stabilized

for 1 more hour at

72

no color completely stable

550 ° C thermal

modification

stabilized

(Ru 5Ir) Ox none

24th

12n HCl

168

amber considerable corrosion,

colors unstable

for 1 hour at 550 ° C

24th

12n HCl

96

no color completely stable

thermally stabilized

modification

(Ru 25Ir) Ox none

24th

12n HCl

168

amber considerable corrosion,

colors unstable

for 1 hour at 550 ° C

24th

12n HCl

96

very pale yellow, completely stable

thermally stabilized

for 1 more hour at

24th

12n HCl

72

no color

completely stable

550 ° C thermal change stabilized

From these data it follows that the thermal stabilization of the reduced oxides improves the corrosion resistance of the catalyst in very concentrated HCl and gives very good stability. The durability of the catalysts in much less corrosive chlorine or brine environments is excellent and is due to the thermal stabilization of the reduced oxide catalysts.

After observing the improved corrosion properties of the thermally stabilized, reduced platinum

60 penmetall oxides were subjected to physical and chemical tests to determine the effect of thermal stabilization on various properties of the catalysts, which could determine the improved corrosion properties. The oxide content, the surface area in m2 / g of the catalyst, the pore volume and the pore size distribution of the catalyst were measured after it had been produced using the modified Adams process. After the reduction of the catalytic converter

645 552

12

tors and after the thermal stabilization of the reduced catalyst. The results of these tests, which are detailed below, show that the surface area of the catalyst is somewhat reduced after the reduction and that it is considerably reduced after the thermal stabilization. A reduction in the oxide content after the reduction is partly responsible for the decrease in the surface. A significant change in the internal pore size distribution of the catalyst after thermal stabilization or a corresponding change in pore volume appears to be responsible for the significant decrease (2: 1 ratio) of the surface that accompanies thermal stabilization, and this would explain the improved corrosion properties, since the corrosion is directly related to the area that is exposed to the attack by the corrosion agents.

First, Sample No. 1, a ruthenium-25% by weight iridium catalyst was prepared by the modified Adams method. Part of this catalyst was electrochemically reduced to form Sample No. 2. A sample of the reduced (Ru 25Ir) Ox was thermally stabilized for one hour at 550 to 600 ° C. The surface of the non-reduced catalyst of Sample No. 1, the reduced catalyst of Sample No. 2, and the thermally stabilized, reduced (Ru 25 Ir) Ox catalyst of Sample No. 3 were prepared by the three-point BET nitrogen absorption method measured. The results follow in Table XII.

Table XIII

Table XII

Catalyst (Ru 25 Ir)

treatment

surface

Sample No. 1 Sample No. 2 Sample No. 3

none reduced reduced and thermally stabilized for 1 hour at 550-600 ° C

127.6 m2 / g 123.5 m2 / g 62.3 m2 / g

Then, the oxide content of Sample Nos. 1,2 and 3 was measured as well as that of Sample No. 4 which had been thermally stabilized at 700 to 750 ° C for one hour. In addition, the oxide content of piatin-iridium catalysts with 5 to 50 wt .-% iridium was measured. The results can be found in the following Table XIII.

Catalyst (Ru 25 Ir)

treatment

% Oxide content

Sample No. 1 Sample No. 2 Sample No. 3

io Sample No. 4

(Pt-50Ir) Sample No. 5 i5 Sample No. 6 Sample No. 7

no 24.4

Reduction 24.3

thermal stabilization 22.6

1 hour at 550-600 ° C

Reduction and thermal 21.5

Stabilization for 1 hour at 700-750 ° C

no 16.5

Reduction 15.2

Reduction and thermal 13.0

Stabilization for 1 hour at 550-600 ° C

20 The results of this table XIII show a decrease in the oxide content with the reduction and the thermal stabilization, just as the surface after the reduction and the thermal stabilization decreases.

The decrease in the oxide content has a corresponding effect on the surface, since the surface of oxides is normally larger than that of the non-oxide form. This reduction in oxide content partly explains the reduction in surface area, but not fully the dramatic reduction in surface area after thermal stabilization.

The porosity of the catalyst was therefore measured to determine whether the thermal stabilization of the catalyst caused a change in the porosity and thus a reduction in the surface and an increase in the corrosion resistance. Catalyst samples were taken from the same mixtures as samples No. 1, 2 and 3 and the porosity of these samples and their particle size distribution were measured. The particle size distribution was determined using a sedimentation method 4o and found that the equivalent sphere diameter at 50% mass distribution was 3.7 µm after reduction of the catalyst and 3.1 µm after thermal stabilization. This shows that the outer surface the particle size is reduced, but does not provide a sufficient basis for the overall reduction in surface area after stabilization.

The total pore volume in ml / g was measured by capillary condensation and mercury introduction methods. The results for Sample Nos. 1, 2 and 3 are summarized in Table XIV.

Table XIV

Catalyst (Ru-25 Ir)

treatment

Area

Total pore volume in ml / g

Sample No. 1 Sample No. 2 Sample No. 3

no 40 Â-lOp.

Reduction 40 A-lOp.

Reduction and thermal 40 Â-loyt stabilization for 1 hour at 500-600 ° C

0.80 0.72 0.76

These results show that the total pore volume remains relatively unchanged. The porosity in g / ml or, if it is converted into the void volume with knowledge of the sphere size and density, is therefore essentially the same and is in the range from 0.7 to 0.8 g / ml for Ru-25 Ir.

At the same time, the pore size distribution was measured in order to obtain the pore diameter distributions in the range from 40 65 angstroms to 10 μm. Capillary condensation was used in the range of 40 to 500 angstroms. In this method, the liquid condensation is measured at a given vapor pressure to determine the pore size

to determine division. The capillary condensation method has a lower resolution limit of 40 angstroms and an upper limit of 500 angstroms. For pores of more than 500 angstroms (e.g. from 500 angstroms to 10 µm) is used for

645 552

Determination of the pore size distribution uses a mercury penetration method. The results for both ranges of pore sizes are summarized in Tables XV and XVI below.

Table XV

Catalyst treatment

Size range

Pore diameter distribution

Sample No. 1 no 40-500 A

Sample No. 2 reduction 40-500 A.

Sample No. 3 Reduction and thermal 40-500 A stabilization for 1 hour at 500-600 ° C

Pore distribution below 40 A

Pore diameter distribution below 40 A Distribution in the range of 100-300 A with a maximum at 200 A

Table XVI

Catalyst treatment

Size range

Pore diameter distribution (50% point in the distribution)

Sample No. 1 no sample No. 2 reduction Sample No. 3 reduction and thermal stabilization for 1 hour at 500-600 ° C

500 A-10 around 500 A-10 around 500 A-10 [im

0.46 Jim 0.82 by 1.5 [im

These results show that the thermal stabilization of the catalyst leads to a change in the pore diameter. This change appears to be accompanied by a change in the number of pores, so that the total surface is reduced. Since the pore volume remains essentially constant and the pore diameter distribution changes, so that the distribution shows a maximum at 200 angstroms and 1.5 µm, it seems quite clear that many of the pores with a diameter below 40 angstroms merge and so the total number reduce the pores. The pore diameter increases above 500 angstroms. The pore size diameter and the inner pore surface thus change with the thermal stabilization. Overall, the internal porosity and thus the pore surface is reduced when the catalyst is thermally stabilized. This is believed to be a result of changing the morphology to create a smaller pore number with a larger pore diameter distribution. Since the pore volume remains relatively unchanged and there is a change in the pore diameter distribution towards a larger pore diameter, it seems to be clear that the reduction in surface area which is associated with the thermal stabilization of the catalyst is attributable to the change in the inner pore surface.

The porosity, expressed in pore volume in ml / g of the catalyst, of the thermally stabilized, reduced platinum metal oxide catalyst is in the range from 0.4 to

1.5 ml / g, the preferred porosity for a thermally stabilized Ru-25 Ir catalyst being 0.7 to 0.8 ml / g.

The pore diameter distribution of the thermally stabilized catalyst shows the basic distribution below 500 angstroms as in the 100 to 300 angstroms range with a maximum at 200 angstroms. Above 500 angstroms, the pore diameter distribution shows the largest pore volume and therefore the main distribution in the range from 0.4 to 9 μm, with a maximum being 1.5 μm, which represents the 50% point in the distribution.

The surface area for the thermally stabilized, reduced platinum metal oxide catalysts should be such that for any given platinum metal or combination of platinum metals with or without valve metals, etc., the surface area should be as small as possible (to reduce corrosion (for the required catalytic The surface area should therefore be more than 10 m2 / g, with 24 to 165 m2 / g being a preferred range and for a thermally stabilized Ru-25-Ir catalyst the range being 60 to 70 m2 / g with the reduced, thermally stabilized platinum group metal oxides, compared to powders, soot, etc., are large-area catalysts that normally have a surface area of 10 to 15 m2 / g.

The oxide content of the catalyst can range from 2 to 25% by weight and preferably from 13 to 23% by weight.

40

45

50

55

s

1 sheet of drawings

Claims (29)

645 552 1. Catalyst, characterized in that it contains at least one electrically conductive, thermally stabilized, reduced metal oxide of the platinum group. 2. Catalyst according to claim 1, characterized in that the metal oxide is ruthenium or iridium oxide. 2nd PATENT CLAIMS 3. Catalyst according to claim 1 or 2, characterized in that it contains electrically conductive graphite in addition to the thermally stabilized metal oxide. 4. Catalyst according to claims 1 to 3, characterized in that it also contains thermally stabilized oxides of a valve metal. 5. Catalyst according to claims 2 to 4, characterized in that the metal oxide is ruthenium oxide, which includes at least one further thermally stabilized, reduced metal oxide of iridium, tantalum, titanium, niobium, zirconium or hafnium. 6. Catalyst according to claim 5, characterized in that it contains 5 to 25 wt .-% reduced iridium oxides. 7. Catalyst according to claim 5 or 6, characterized in that it contains 25% by weight of reduced iridium oxides. 8. Catalyst according to claim 5 to 7, characterized in that it contains reduced oxides of iridium and either tantalum or titanium. 9. Catalyst according to claims 2 to 4, characterized in that the metal oxide is iridium oxide, which contains reduced oxides of tantalum or titanium. 10 m2 / g includes, the porosity of the catalyst in the range of 0.4 to 1.5 cm3 / g of the catalyst. 10. A catalyst according to claim 1, characterized in that the catalyst has a surface area in the range from 24 to 165 m2 / g. 11. A catalyst according to claim 1, characterized in that the catalyst has a surface area of more than 12. Catalyst according to claim 11, characterized in that the pore diameter distribution has a maximum at 20 nm and has a 50% pore distribution point at 1.5 µm. 13. A catalyst according to claim 1, characterized in that the surface of the catalyst is at least 60 m2 / g. 14. Use of the catalyst according to claim 1 in a combined electrolyte and electrode structure with an ion-transporting membrane, to the surface of which at least one gas-permeable catalytic electrode is connected, characterized in that the gas-permeable catalytic electrode has at least one thermally stabilized, electrically conductive, reduced Metal oxide covered by the platinum group. 15. Use according to claim 14, characterized in that the gas-permeable catalytic electrode comprises a plurality of particulate, thermally stabilized, electrically conductive reduced metal oxides of the platinum group. 16. Use according to claim 15, characterized in that the particles of the gas-permeable catalytic electrode include the oxides of at least two metals of the group consisting of thermally stabilized, electrically conductive, reduced platinum group metal oxides and reduced valve metal oxides, at least one of which is a reduced metal oxide the platinum group. 17. Use according to claim 16, characterized in that the particles of the gas-permeable, catalytic electrode include thermally stabilized, electrically conductive, reduced ruthenium oxides. 18. Use according to claim 16 or 17, characterized in that the particles of the gas-permeable, catalytic electrode include thermally stabilized, electrically conductive, reduced oxides of ruthenium and at least one reduced metal oxide which is selected from the reduced oxides of iridium, tantalum , Titanium, niobium, zirconium or hafnium. 19. Use according to claim 18, characterized in that the thermally stabilized, electrically conductive particles of the gas-permeable, catalytic electrode are reduced oxides of ruthenium and reduced oxides of iridium. 20. Use according to claim 19, characterized in that the particles of the gas-permeable, catalytic electrode include 5 to 25% by weight of reduced iridium oxides. 21. Use according to claim 18 or 19, characterized in that the particles of the gas-permeable, catalytic electrode include 25 wt .-% iridium. 22. Use according to claim 16 to 19, characterized in that the majority of the particles of the gas-permeable, catalytic electrode include thermally stabilized, electrically conductive particles of reduced platinum metal oxides and reduced oxides of titanium or tantalum. 23. Use according to claim 22, characterized in that the reduced platinum metal oxide is reduced iridium oxide. 24. Use according to claims 14 to 23, characterized in that the particles of the gas-permeable, catalytic electrode additionally include electrically conductive graphite particles. 25. Use according to claim 14, characterized in that the surface of the gas-permeable, catalytic electrode is at least 60 m2 per gram of particles. 26. A process for producing the catalyst according to claim 1, characterized in that at least one metal oxide from the platinum group is produced, the oxides are reduced to a partially oxidized state, the reduced oxides are heated to a temperature and for a period in the presence of oxygen, which is sufficient to stabilize the reduced oxides. 27. The method according to claim 26, characterized in that a valve metal oxide is additionally added to the metal oxide of the platinum group and that up to 50% graphite is added after the reduced oxides have been stabilized. 28. The method according to claim 26 or 27, characterized in that the reduced oxides are heated to 300 to 750 ° C. 29. The method according to claim 28, characterized in that the reduced oxides are heated to 550 to 600 ° C.