GB2071157A - Catalytic electrode and combined catalytic electrode and electrolytic structure - Google Patents

Abstract

An electrocatalytic material comprising at least one reduced platinum group metal oxide is subsequently heated in the presence of oxygen at a temperature high enough to stabilized the catalyst in acidic and halogen environments. The catalyst optionally contains other thermally stabilized, reduced platinum group metal oxides, electroconductive extenders of the group consisting of graphite and oxides of valve metals. A novel electrode structure includes the catalyst and a polymeric binder. A unitary electrolyte/electrode structure has an electrode containing the electrocatalytic material bonded to at least one side of a membrane-electrolyte.

Description

1 GB 2 071 157 A 1

SPECIFICATION

Catalytic electrode and combined catalytic electrode and electrolyte structure The instant invention relates to a catalytic elec trode and a combined catalytic electrode and electro lyte structure. More particularly, it relates to elec trodes and electrode/electrolyte structures which are particularly useful in the electrolysis of halides.

Generating gas by electrolyzing a chemical com pound into its constituent elements, one of which may be a gas, is, of course, an old and well known technique. One recently developed form of such gas evolving electrolyzer involves the use of a cell which utilizes an electrolyte in the form of a solid polymer, ion-exchanging membrane. In an arrangement of this sort, catalytic electrodes using a suitable catalyst are positioned on opposite sides of an ion transport ing membrane medium such as a sulfonated perf luorocarbon ion-exchange membrane. Through an oxidation reaction, the ionic form of one of the con stituent elements (hydrogen ions, for example, when H,O or HCI is electrolyzed, or sodium ions when an alkali metal halide such as sodium chloride is elec trolyzed) is produced at one electrode. The ion is transported across the ion-exchanging membrane to the other electrode where it is reduced to form an electrolysis product such as molecular hydrogen, NaOH, etc. Solid polymer ion-exchange membranes electrolysis units are particularly advantageous because they are efficient, small in size, and do not utilize any corrosive liquid electrolytes.

Various metal and alloys have been utilized in the past as part of the catalytic electrodes associated with such electrochemical electrolyzing cells. The performance of the catalyst atthe gas evolving elec trodes is obviously crucial in determining the effec tiveness and efficiency of the cefl, and consequently of the economics of the process. The choice of a catalyst in an electrochemical cell and its effective ness depends upon a complex set of variables, such as surface area of a catalyst, availability of oxides of its species on the catalyst surface, contaminants in the reactants, and the nature of the conversion tak ing place in the cell. Consequently, it is, and always has been, difficult to predict the applicability of a catalyst useful in one electrochemical cell to a differ ent system. US Patent No. 3,992,271 entitled "Methods and Apparatus for Gas Generation" describes an improved oxygen evolving catalytic electrode utilizing a platinum-iridium alloy, a mix ture which was found to provide much improved performance and efficiency. U.S. Patent No.

4,039,490 describes another oxygen evolving cataly- 120 tic electrode which utilizes reduced oxides of platinum-ruthenium. The Olatinum-ruthenium catal yst not only is substantially less expensive than the reduced platinum-iridium catalyst, because it uses a less expensive material such as ruthenium to alloy with the platinum, but it also turns out to be more efficient because it has a lower oxygen overvoltage than a platinum-iridium electrode.

However, attempts to use reduced ruthenium oxide electrocatalysts for evolution of halogens by electrolysis of aqueous halide solutions have not been entirely successful due to the harsh electrolysis conditions in the cell. There can be a substantial loss of catalyst from the membrane during chlorine evolution since these reduced platinum metal oxides are susceptible to dissolution in acidic environments which are present in the electrolysis of hydrogen halides or in the electrolysis of alkali metal halide solutions which are often acidifed. Not only is there a tendency to dissolution of the platinum metals resulting in a loss of a catalytic material, but the overvoltage of the electrodes also tends to increase so that the efficiency of the cell decreases, and in many instances does not permit prolonged periods of operation.

The advantages of the invention will become apparent as the description thereof proceeds.

In accordance with the invention, the novel electrode comprises catalytic material including at least one reduced platinum group metal oxide which is subsequently treated in the presence of oxygen at a temperature high enough to stabilize the oxide thermally to increase the resistance of the catalyst against the corrosive electrolysis conditions. The catalytic, reduced platinum group metal oxide may optionally contain other reduced platinum group metal oxides and optionally up to fifty (50) percent by weight of the electroconductive extenders such as graphite, valve metal oxides, and nitrides, car- bides and sulfides.

Examples of useful platinum group metals are platinum, palladium, iridium, rhodium, ruthenium, and osmium with the preferred reduced metal oxide for chlorine and other halogen production being thermally stabilized, reduced oxides of ruthenium. Reduced oxides of ruthenium are preferred because they are found to have extremely low chlorine overvoltages as well as their stability in the eletrolysis environment.

As pointed out above, the electrocatalytic material may be a single reduced platinum group metal oxide such as ruthenium oxide, or platinum oxide, or iridium oxide, etc. It has been found, however, that mixtures or alloys of thermally stabilized, reduced platinum group metal oxides are even more stable. One such mixture or alloy of ruthenium oxide containing up to twenty five (25) percent of iridium oxide, with the preferred range being five (5) to twenty-five (25) percent by weight calculated as metal, even though iridium is somewhat more expensive than ruthenium alone.

Electroconductive extenders such as graphite have low overvoltages for halogens and are substantially less expensive than the platinum metal oxides and may readily be incorporated without reducing the effectiveness of the catalyst. In addition to graphite, oxides of valve metals such as titanium, tantalum, niobium, tungsten, vanadium, zirconium, and hafnium may be added to further stabilize the electrocatalyst and increase its resistance against adverse electrolysis conditions.

The thermally stabilized, reduced platinum metal oxides and the extenders are formed into an electrode by bonding polymeric particles, e.g. fluorocar- bon resin particles such as those sold by Dupon Z 1 GB 2 071 157 A 2 under its trade designation Teflon (Registered Trade Mark). The catalytic particles and resin particles are mixed, placed in a mold and heated until the cornposition is sintered into a suitable form which is bonded to at least one surface of the membrane by application of heat and pressure to provide an elec trode structure and a unitary membrane/electrode structure.

A process for the preparation of the electrocatalyst comprises forming oxides of at least one platinum group metal along with one or more extenders such as graphite, valve metals, reducing the oxide to a partially oxidized state and then heating the latter in the presence of oxygen at a temperature which is sufficiently high to stabilize the reduced oxides.

The novel features which are believed to be characteristic of this invention are set forth with par ticularity in the appended claims. The invention itself, however, both as to its organization and method of operation, together with further objects and advantages thereof, may best be understood by reference to the following description taken in con junction with the accompanying drawings in which:

Figure 1 is a schematic illustration of an electrolysis cell in accordance with the invention utilizing a solid polymer electrolyte membrane and novel catalyst bonded to the surface thereof.

Figure 2 is a schematic illustration showing the reactions taking place in various portions of the cell during electrolysis of an aqueous halide solution.

Catalytic material which includes thermally stabil ized, reduced oxides of a platinum group metal along or in combination with other platinum group metals or optional valve metals may be prepared in any convenient fashion whereby an oxide catalyst is 100 permanent, partially reduced and thermally stabil ized.

The preferred manner of reduction is by a modifi cation of the Adams method of platinum group metal preparation by the addition of a thermally 105 decomposable platinum group metal halide, such as ruthenium chloride, either alone or, if desired, along with an appropriate quantity of otherthermally decomposable halides of other platinum group met als or valve metals to an excess of sodium nitrate.

The Adams method of platinum group metal prep aration is disclosed in an article published in 1923 by R. Adams and R. L. Schriner in the Journal of the American Chemical Society, Volume 45, Page 217. It is convenient to mix the finely divided halide salts of 115 the platinum group metals, such as Chloroplatinic acids in the case of platinum ruthenium chloride in the case of ruthenium, titanium chloride, tantalum chloride, in the case of titanium and tantalum in the same weight ratio of the metals as desired in the final alloy mixture. An excess of sodium nitrate is incorporated and the mixture is fused in a silica dish at 500 to 600'C forthree (3) hours. The residue is washed thoroughly to remove nitrates and halides still present, leaving a residue of the desired 125 platinum metal oxide, i.e., ruthenium oxide, platinum-ruthenium oxide, ruthenium-iridium oxide, ruthenium-titanium oxide, etc. The resulting sus pension of mixed and alloyed oxides is then partially reduced. The reduction of the platinum group metal 130 oxides may be effected by any convenient known reducing method, such as an electrochemical reduction or by bubbling hydrogen through the mixture at room temperature as long as the oxides are notto be completely reduced to the free metal form. In a preferred embodiment, oxides are reduced by using an electrochemical reduction technique, i.e., electrochemical reduction in an acid medium. The product which is now a reduced platinum metal oxide, either alone or as a mixed alloy oxide, is dried thoroughly, such as by the use of a heat lamp, ground, and then sieved through a 400 mesh nylon screen (having openings of about 0.037 mm) to produce a fine powder of the reduced platinum metal oxide.

The resulting reduced platinum metal oxides are then stabilized thermally by the heating in the presence of oxygen for a sufficient time to ensure a catalytic material which is stable in an acidic hyd- rogen halide environment and in the presence of halogens. In a manner to be described subsequently, thermal stabilization of the catalyst results in a catalyst which has much better corrosion characteristics in halogens, such as chlorine, etc., and in the pres- ence of halides solutions such a hydrochloric, etc., acids. It is believed thatthermal stabilization results in the formation of a catalytic particle having a large mean pore diameter and stable thin oxide film on the outside of the reduced oxide particle. This stabilizes the reduced oxide particles so thatthey have better mechanical properties for bonding to the solid polymer electrolyte membrane, and in their resistance characteristics to dissolution in hydrochloric acid or other halide acid solutions or to the evolved halogens. Thus, preferably, the reduced oxides are heated at 300 to 7500C from thirty (30) minutes to six (6) hours with the preferable thermal stabilization procedure being accomplished by heating the reduced oxides for one (1) hour at 550 to 6000C.

It has also been found that the electrocatalytic activity of the catalyst and of the electrode including the catalyst is optimized by providing the catalytic particles in as fine a powder form as possible. Thus, it has been found that the surface area of the parti- cles, as observed both by the BET nitrogen absorption method, should be at least 24 meters square per gram of catalyst (MI/g). The preferred range is 50 to 150MI/g.

The gas permeable electrode structure of catalytic particles and fluorocarbon polymer particles is produced by blending the catalytic particles with a Teflon dispersion to produce a bonded electrode structure in the manner described in U.S. Patent No. 3,297,484 assigned to the assignee of the present invention. In the process of bonding the electrode, it is desirable to blend the catalyst with Teflon dispersions in such a mannerthat the dispersion contains few or no hydrocarbons. If the fluorocarbon Teflon composition contains hydrocarbon organic surface active agents, it results in loss of surface area of the reduced oxide catalyst. Any reduction on the surface area of the catalyst is obviously undesirable, since it has potentially deleterious effect on the efficiency and effectiveness of the catalyst. Hence, fabrication of the electrode should be by the use of a Teflon 3 GB 2 071 157 A 3 polytetrafluoroethylene particle composition which contains few, if any, hydrocarbons. One suitable for of these particles which may be utilized in fabricating the electrode is sold by Dupont under its designation Teflon T-30.

The mixture of platinum group metal particles and Teflon particles or of graphite and the reduced oxide particles are placed in a mold and heated until the composition is formed into a decal which is then bonded and embedded in the surface by the applica- 75 tion of pressure and heat. As described, for example, in U.S. Patent No. 3,297,484, above, the electrode structure is bonded to the surface of the ion exchange membrane thus integrally bonding the gas absorbing particle mixture and, in some instances, 80 preferably embedding it into the surface of the membrane.

The novel membrane/electrode structure thus fab ricated comprises a solid polymer electrolyte mem brane capable of selective ion transport having a thin, porous, gas permeable electrode of the above described electrocatalytic reduced platinum group metal oxides bonded to at least one side of the membrane. A second electrode may be bonded to the other side of the membrane and may include the same electrocatalytic material, or any other suitable cathodic material. The selective ion transporting membrane is preferably a stable, hydrated, cationic membrane which is characterized by ion transport selectivity. The cation exchange membrane allows passage of positively charged cations such as hyd rogen ions in the case of the electrolysis of a halide such as hydrogen chloride or sodium cations in the case of the electrolysis of aqueous alkali metal halides, and thus minimizes passage of negatively charged anions.

There are various types of ion exchange resins which may be fabricated into membranes to provide selective transport of the cations. Two classes of such resins are the so-called sulfonic acid cation exchange resins and the carboxylic cation exchange resins. Sulfonic acid exchange resins, which are the preferred type, include ion-exchange groups in the form of hydrated, sulfonic acid radicals (SO3H x H,O) attached to the polymer backbone by sulfonation.

The ion exchanging acid radicals in the membrane are fixedly attached to the backbone of the polymer ensuring that the electrolyte concentration does not vary. As pointed out previously, perfluorocarbon sul fonic acid cation membranes are preferred. One specific class of cation polymer membranes in this category is sold by the Dupont Company under its trade designation "Nafion". These "Nafion" mem branes are hydrated, copolymers of polytetraf luoroethylene (PTFE) and polysulfonyl fluoride vinyl ether containing pendant sulfonic acid groups.

The ion-exchange capacity (IEC) of a given sulfonic cation exchange membrane is dependent upon the milliequivalent weight (MEW) of the S03 radical per gram of dry polymer. The greaterthe concentration of the sulfonic acid radicals, the greaterthe ion exchange capacity and hence the capability of the membrane to transport cations. However, as the ion-exchange capacity of the membrane increases, so does the water content and the ability of the membrane to reject salt decreases. Thus in electrolysis of alkali metal halide solutions, caustic is generated atthe cathode side and the rate atwhich the sodium hydroxide migrates from the cathode to the anode side thus increases with IEC. Such back migration reduces the cathodic current efficiency (CE) and also results in oxygen generation at the anode which have undesirable consequences in its effect on the catalytic anode electrode. Consequently, the preferred ion-exchange membrane for use in brine electrolysis is a laminate consisting of a thin (0.05mm or so) film of fifteen hundred (1500) MEW, low water content (515%) cation exchange membrane which has high salt rejection, bonded to a 4 mil or so film of high ion-exchange capacity, 1100 MEW, bonded together with a Teflon cloth. One form of such a laminated construction sold by the Dupont Company is Nafion 315. Other forms of laminates or constructions in which the cathode side layer con- sists of a thin layer of film of low water content resin (5-15%) to optimize salt rejections are also available. Typical of such other laminates are Naflon 355,376, 390, 227,214. In the case of a laminated membrane bonded together by a Teflon cloth, it may be desir- able to clean the membrane and Teflon cloth by refluxing it in seventy (70) percent HN03forthree to four (3 to 4) hours in addition to soaking in caustic preferred to previously.

In the case of electrolysis hydrogen halides such as hydrochloric acid, there is no problem of back migration of caustic or other salts, so that simpler forms of membranes such as Nafion 120 may be utilized asthe ion transporting medium.

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

Referring now to Figure 1, the halogen electrolysis cell is shown generally at 10 and consists of a cathode compartment 11 and an anode compartment 12 separated by a solid polymer electrolyte membrane 13 which is preferably a hydrated, perm- selective, cationic membrane. Bonded to opposite surfaces of membrane 13 are electrodes comprising particles of a fluorocarbon such as Teflon bonded to thermally stabilized, reduced oxides of ruthenium (RuOJ or iridium (IrOx), or stabilized, reduced oxides of ruthenium-iridium (RuIr), ruthenium-titanium (RuTi), ruthenium- tantalum (RuTa), ruthenium -tantalum - iridium (RuTalr), or ruthenium- graphite or combinations of the above with graphite and other valve and transition metal oxides. The cathode, shown at 14, is bonded to and preferably embedded in one side of the membrane and a catalytic anode, not shown, is bonded, and preferably embedded in the opposite side of the membrane. Current collectors in the form of metallic screens 15 and 16 are pressed against the electrodes. The whole membrane/electrode assembly is firmly supported between the electrode compartments 11 and 12 by means of gaskets 17 and 18 which are made of any material resistant or inert to the cell environment, namely caustic chlorine, oxygen, aqueous sodium 4 chloride in the case of brine electrolysis and HU, HBr, in the case of other hydrogen halides. One form of such a gasket is a filled rubber gasket sold by Irving Moore Company of Cambridge, Mass. under its trade designation EPDM.

An aqueous alkali metal halide such as brine or hydrogen halides such as HCI is introduced through an electrolyte inlet 19 which communicates with chamber 20. Spent electrolyte and halogens such as chlorine are removed through an outlet conduit 21. A 75 cathode inlet conduit 22 is provided in the case of brine electrolysis and communicates with cathode chamber 11 to permit the introduction of the catholyte, water, or aqueous NaOH (more dilute than that formed electrochemically at the electrode/electrolyte 80 interface). In the case of electrolysis of hydrogen halides such as hydrogen chloride, no catholyte need be provided and the cathode inlet conduit 22 may be dispensed with.

In a brine electrolysis cell, the water serves two 85 separate functions. A portion of the water is electrol yzed to produce hydroxyl (OH-) anions which com bined with the sodium cations transported across the membrane to form caustics (NaOH). The water also sweeps across the porous, bonded cathode 90 electrode to dilute the highly concentrated caustic formed at the membrane/electrode interface to minimize diffusion of the caustic back across the membrane into the anolyte chamber. Cathode outlet conduit 21 communicates with the cathode chamber 95 11 to remove excess catholyte and the electrolysis products such as caustic in the case of brine elec trolysis, plus any hydrogen discharge atthe cathode both in brine electrolysis and in hydrogen chloride electrolysis. A power cable 24 is brought into the 100 cathode chamber and a comparable cable not shown is brought into the anode chamber. The cables con nect the current conducting screens 15 and 16 or any other suitable kind of collector as source of electrical power. 105 Figure 2 illustrates diagrammatically the reactions taking place in the cell during the electrolysis of an aqueous alkali metal halide such as brine and is use ful in understanding the electrolysis process in the manner in which the cell functions. Thus, an aque- 110 ous solution of sodium chloride is brought into the anode compartment which is separated from the cathode compartment by the cationic membrane 13.

Membrane 13 is a composite membrane comprising a high water content (20-35% based on dry weight of 115 membrane) layer 26, on the anode side and a low water content high MEW cathode side layer 27, (5-15% based on dry weight of membrane) sepa rated by a Teflon cloth 28. The cathode side barrier layer may also be chemically modified on the 120 cathode side to form a thin layer of a low water con tent polymer. In one form this is achieved by modify ing the polymerto form a substituted sulfonamide membrane layer. By converting the cathode side layer to a weak acid form (sulfonamide), the water 125 content of this portion of the membrane is reduced and the salt rejecting capability of the film is increased. As a result, diffusion of sodium hydroxide back across the membrane to the anode is minim ized. While laminated membrane constructions are 130 GB 2 071 157 A 4 preferred in brine electrolysis to block migration of sodium hydroxide, other homogeneous films of low water content may be utilized, (viz., Nafion 150, perfluorocarboxylates, etc.). Obviously, in the case of the electrolysis of hydrogen halides such as HCI, HBr, etc., the ion transporting membrane may be a simple, homogeneous film such as the Nafion 120 referred to previously.

The Teflon-bonded, reduced platinum group metal oxide catalysts contains at least one thermally stabilized, reduced platinum group metal oxide, such as ruthenium, iridium, or ruthenium-iridium with or without reduces oxides of titanium, niobium, or tantalurn and particles of graphite are, as shown, pressed into the surface of membrane 13 in an amount up to 50% by weight of graphite. Current collectors 15 and 16, shown only partially, for the sake of clarity, are pressed against the surface of the catalytic electrodes and are connected, respectively, to the positive and negative terminals of the power source to provide the electrolyzing potential across the cell electrodes. The aqueous halide ion solution, such as an aqueous sodium chloride solution, is brought into the anode chamber, is electrolyzed at anode 29 to produced chlorine as shown diagrammatically by the bubble formation 30. The chlorine actually is principally evolved at the interface of the electrode and the membrane, but passes through the porous electrode to the electrode surface. The sodium ions (Na -) are transported across membrane 13 to cathode 14. A stream of water or aqueous NaOH shown at 31 is brought into the cathode chamber and acts as a catholyte. The aqueous stream is swept across the surface of the Teflon-bonded catalytic cathode 14 to dilute the caustic formed atthe membrane/cathodic interface and thereby reduce diffusion of the caustic back across the membrane to the anode.

A portion 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 across the membrane to produce sodium hydroxide atthe membrane/electrode interface. The sodium hydroxide readily wets the Teflon forming part of the bonded electrode and migrates to the surface where it is diluted by the aqueous stream sweeping across the surface of the electrode. Even with a cathode water sweep, concentrated sodium hydroxide in the range of 4.5-6.5M is produced at the cathode. Some sodium hydroxide, as shown by the arrow 33, does migrate back through membrane 13 to the anode. NaOH migration is a diffusion process caused by the concentration gradient and electrochemical negative ion transport to the anode. Sodium hydroxide transported to the anode is oxidized to produce water and oxygen as shown by bubble formation at 34. This of course, is a parasitic reaction which reduces the cathode current efficiency and should be minimized by the utilization of membranes which have high salt rejection characteristics on the cathode side. Aside from its effect on current efficiency, production of oxygen at the anode is undesirable since it can have troublesome effects on the electrode and membrane, particularly if the electrode includes graphite. In addition, the oxygen 0 D dilutes the chlorine produced at the anode so that processing is required to remove the oxygen. Oxygen formation may be minimized further by acidifying the aqueous anolyte so that back migrat- GB 2 071 157 A 5 ing hydroxide is converted to water rather than generating oxygen. The reactions in various portions of the cell for electrolysis of NaCI is as follows:

Anodefleaction: 2 Cl --> C12 T + 2e- (1) (Principal) Membrane Transport: 2W + H20 (2) Cathode Reaction: 2H20 - 201-1- + H2 2e- (3) 2Na4 + 201-1- 2NaOH (4) Anode Reaction: 40H- --> 02 + 2H20 + 4e- (5) Overall 2NaCI + 2H20 - 2NaOH + C12 1 + H2 (6) (Principal) The reactions for electrolysis of a hydrogen halide, such as HCI, are very similar:

Anode Reaction: 21-IC1 - 2H 4 + C12 1 + 2e- (1) Membrane Transport: 2 H' (H20, HC 1) (2) Cathode Reaction: 2W + 2e- - H2 (3) Overall Reaction: 2HCI - H2 + C12 (4) The novel arrangement for electrolyzing aqueous solutions of brine or of HCI which is described herein 55 is characterized by the fact that the catalytic sites in the electrodes are in direct contact with the cation membrane and the ion exchanging acid radicals attached to the polymer backbone (whether these radicals are theS03H X HO suffonic radicals or the 60 COOH X HO carboxylic acid radicals). Consequently, there is no IR drop to speak of in the anolyte orthe catholyte fluid chambers (this IR drop is usually referred to as "Electrolyte IR drop"). "Electrolyte IR drop" is characteristic of existing systems and processes in which the electrode and the membrane are separated and can be in the order of 0.2 to 0.5 volts.

The elimination or substantial reduction of this vol tage drop is, of course, one of the principal advan tages of this invention since it has an obvious and 70 very significant effect on the overall cell voltage and the economics of the process. Furthermore, because chlorine is generated directly at the anode and membrane interface, there is no IR drop due to the so-called "Bubble effect" which is a gas blending and mass transport loss due to the interruption of blockage of the electrolyte path between the elec trode and the membrane. As pointed out previously, in prior art systems, the chlorine discharging cataly tic electrode is separated from the membrane. The 80 gas is formed directly atthe electrode and results in a gas layer in the space between the membrane and the electrode. This in effect breaks up the electolyte path between the electrode-co I lector and the mem brane blocking passage of Na' ions and thereby, in 85 effect, increasing the IR drop.

In a preferred embodiment, the Teflon-bonded platinum group metal electrode contains reduced oxides of ruthenium, iridium or ruthenium-iridium in orderto minimize chlorine overvoltage atthe anode. The reduced ruthenium oxides are stabilized against chlorine and oxygen

evolution to produce an anode which is stable. Stabilization is effected initially by temperature stabilization; i.e., by heating the reduced oxides of ruthenium for one hour at temp eratures in the range of 550 to 600'C. The Teflon bonded reduced oxides of ruthenium anode is further stabilized by mixing it with graphite and/or alloying or mixing with reduced oxides of iridium (1r)O. in the range of 5 to 25% of iridium, with 25% being preferred, or with reduced oxides or titanium (Ti)Ox, with 25-50% of TiOx preferred. It has also been found that a ternary alloy of reduced oxides of titanium, ruthenium and iridium (Ru, Ir, Ti)O,, or tantalum, ruthenium and iridium (Ru, Ir, Ta)O,, bonded with Teflon is very effective in producing a stable, long-lived anode. In case of the ternary alloy, the composition is preferably 5% to 25% by weight of reduced oxides or iridium, approximately 50% by weight reduced oxides of ruthenium, and the remainder a valve metal such as titanium. For a binary alloy of reduced oxides of ruthernium and titanium, the preferred amount is 50% by weight of titanium with the remainder ruthenium. Titanium, of course, has the additional advantage of being much less expensive than either ruthenium or iridlum, and thus is an effective extender which reduces cost while at the same time stabilizing the electrode in an acid environment and against HCI, chlorine and oxygen evolution. Other valve metals, such as niobium (Nb), tantalum (Ta), zirconium (Zr) or hafnium (Hf) can readily be substituted for Ti in the electrode structures. In addition to valve metals, valve metal carbides, nitrides and sulfides may also be utilized as catalyst extenders.

The alloys of the reduced platinum group metal oxides along with the reduced oxides of titanium or other valve metals are blended with Teflon to form a homogeneous mix. The anode Teflon content may be 15 to 50% by weight of the Teflon, although 20 to 30% by weight is preferred. The Teflon is of the type as sold by the DuPont Corporation under its designation T-30, although other fluorocarbons may be used go with equal facility. Typical platinum group metal, etc., loadings forthe anode are at least 0.6 mg/cml of the electrode surface with the preferred range being 1-2 Mg/CM2. The current collector forthe anode electrode may be a platinized niobium screen of fine mesh which makes good contact with the electrode surface. Alternatively, an expanded titanium screen coated with ruthenium oxide, iridium oxide, valve 6 GB 2 071 157 A 6 metal oxide and mixtures thereof may also be used as an anode collector structure. Yet another anode collector structure may be in the form of a titaniumpalladium plate with a platinum clad screen attached 5 to the plate by welding or bonding.

The cathode is preferably a bonded mixture of Tef- 20 lon particles and platinum black with platinum black loading of 0.4 to 4 mg/cm'. The cathode electrode, like the anode, is bonded to and embedded in the surface of the cation membrane. The cathode is made quite thin, 0.05-0. 075 mm or less, and preferably approximately 0.012mm, is porous and has a low Teflon content.

The thickness of the cathode can be quite signific- antin that it can be reflected in reduced water or aqueous NaOH sweeping and penetration of the cathode and thus reduces cathodic current efficiency. Cells were constructed with thin (approximately 0.012-0.05 mm) Pt black-15% Teflon bonded cathodes. The current efficiencies of thin cathode cells were approximately 80% at 5M NaOH when operated at 88-91'C with a 290g/L NaCl anode feed. With a 0.075mm Ru-graphite cathode, the current efficiency was reduced to 54% at 5M NaOH. Table A shows the relationship of CE to thickness, and indicates that thicknesses not exceeding 0.05-0.075 mm give the best performance.

TABLEA

Cathode Current Efficiency Cell Cathode Thickness fmm) %(MNaOH) 1 Pt Black 0.05-0.075 64 (4.0 M) 2 Pt Black 0.05-0.075 73 (4.5 M) 1 Pt Black 0.025-0.05 75 (3.1 M) 4 Pt Black 0.025-0.05 82 (5 M) Pt Black 0.012 78 (5.5 M) 6 5% Pt Black 0.075 78 (3.0 M) on Graphite 7 15% Ru 0., on 0.075 54 (5.0 M) Graphite 8 Platinized 0.25-0.37 57 (5 M) Graphite Cloth The electrode is made gas permeable to allow gases evolved at the electrode/membrane interface 60 to escape readily. It is made porous to allow penetra tion of the sweep water to the cathode elec trode/membrane interface where the NaOH is formed and to allow brine feedstock ready access to the membrane and the electrode catalytic sites. The former aids in diluting the highly concentrated NaOH when initially formed before the NaOH wets the Tef lon and rises to the electrode surface to be further diluted by water sweeping across the electrode sur face. It is importantto dilute atthe membrane inter face where the NaOH concentration is the greatest.

In orderto maximize water penetration at the cathode, the Teflon content should not exceed 15% to 30% weight, as Teflon is hydrophobic. With good porosity, a limited Teflon content, a thin cross section, and a water or diluted caustic sweep, the NaOH concentration is controlled to reduce migra tion of NaOH across the membrane.

The current collector forthe cathode must be care fully selected since the highly corrosive caustic pres ent at the cathode attacks many materials, especially during shutdown. The current collector may take the form of a nickel screen since nickel is resistant to caustic. Alternatively, the current collector may be constructed of a stainless steel plate with a stainless steel screen welded to the plate. Another cathode current structure which is resistant to or inert in the caustic solution is graphite or graphite in combina tion with a nickel screen pressed to the plate and 1 1 1 against the surface of the electrode.

Cells incorporating ion exchange membranes having Teflon-bonded reduced platinum group metal oxide electrodes embedded in the membrane were built and tested to illustrate the effect of various parameters on the effectiveness of the cell in brine electrolysis and to illustrate particularly the operating voltage characteristics of the cell.

Table 1 illustrates the effect on cell voltage of the various combinations of the reduced platinum group metal oxides. Cells were constructed with electrodes containing various specific combinations of reduced noble metal oxides bonded to Teflon particles and embedded into a cationic membrane 0.1 5mm thick. The cell was operated with a current density of 325 mA/cM2 at 900C, at feed rates of 200 to 2000 ml/m in., with feed concentration of 5M.

One cell was constructed in accordance with the teachings of the prior art and contained a dimensionally stabilized anode spaced from the membrane and a stainless steel cathode screen similarly spaced. This control cell was operated under the same conditions.

It can readily be observed from this data that in the process of the instant invention, the cell operating potentials are in the range of 2.93.6 volts. When compared to a typical prior art arrangement (Control Cell No. 4), under the same operating conditions, a voltage improvement of 0.6V-1.5V is realized. The operating efficiencies and economic benefits which result are clearly apparent.

7 GB 2 071 157 A 7 TABLEI

Current Cell Cell Brine Density Voltage Membrane No. Anode Cathode Feed (MA 1cm 1) (V) T'LCO C.E. f5M Na OH) 1 6 Mg/CM2 4 Mg/Cml -5M 323 3.2-3.3 90' 85% DuPont Nafion 315 (Ru 25% Ir)Ox Pt Black (290g/L) Laminate 2 6 Mg/CM2 4 Mg/CM2 323 3.3-3.6 90' 78% DuPont 1500 EW (Ru 25% Ir)Ox Pt Black Nafion 3 6 Mg/CM2 323 2.9 90 66% DuPont 1500 Ew (Ru 25% Ir)Ox Nafion 4 Dimensionally Stable Stainless 323 4.2-4.4 900 81% DuPont 1500 EW Screen Anode-Spaced Steel Nafion from Membrane Screen Spaced from Membrane 4 Mg/CM2 4 Mg/CM2 323 3.6-3.7 90 85% DuPont Nafion 315 (Ru 50% Ti)Ox Pt Black Laminate 6 4 Mg/CM2 4 Mg/CM2 323 3.5-3.6 90' 86% DuPont Nafion 315 (Ru 25% Ir-25% Ta)Ox Pt Black Laminate 7 6 Mg/CM2 2 Mg/CM2 323 3.0 90' 89% DuPont Nafion 315 (Ru Ox-Graphite) Pt Black Laminate 8 6 Mg/CM2 4 Mg/CM2 323 3.4 80' 83% DuPont 1500 EW (Ru Ox) Pt Black Nafion 9 6 Mg/CM2 4 Mg/CM2 323 3.4-3.7 90 73% DuPont 1500 EW (Ru-5 100x Pt Black Nafion 2 Mg/CM2 4 Mg/CM2 323 3.1-3.5 90 80% DuPont Naflon 315 (Ir Ox) Pt Black Laminate 11 2 Mg/CM2 4 Mg/CM2 323 3.2-3.6 90 65% DuPont Nafion 315 (I r Ox) Pt Black Laminate Acell similarto Cell No. 7 of Table] was constructed and operated at WC in a saturated brine feed. The cell potential (V) as a function of current density (mAJcm2) was observed and is shown in 5 Table 11.

Cell Voltage (V) 3.2 2.9 2.7 2.4 TABLE11 Current Density (mAIcml) 430 323 215 108 This data shows that cell operating potential is reduced as current density is reduced. Current density vs. cell voltage is, however, a trade- off between operating and capital costs of a chlorine electrolysis. It is significant, however, that even at very high current densities (325 and 430mA/cml), significant improvements (in the order of a volt or more) in cell voltages are realized in the chlorine generating process of the instant invention.

Table lit illustrates. the effect of cathodic current efficiency on oxygen evolution. A cell having Teflon-bonded reduced noble metal oxides catalytic anodes and cathodes embedded in a cationic membrane were operated at 900C with a saturated brine concentration, with a current density of 323mA/cm' and a feed rate of 2- 5mi/min/6.25Cm2 of electrode area. The volume percent of oxygen in the chlorine was determined as a function of cathodic current efficiency.

Cathodic Current Efficiency (%) 89 86 84 80 TABLEN

Oxygen Evolution (Volume 0/6) 2.2 4.0 5.8 8.9 Table IV illustrates the controlling effectthat acidifying the brine has on oxygen evolution. The volume percent of oxygen in the chlorine was measured for various concentration of HCI in the brine.

TABLE Ill

Acid (HCl) Concentration (M) 0.05 0.75 0.10 0.15 0.25 Oxygen Volume % 2.5 1.5 0.9 0.5 0.4 It is clear from this data that oxygen evolution due to electro-chemical oxidation of the back migrating OH- is reduced by preferentially reacting the OW chemically with H' to form H,O.

A cell similar to Cell No. 1 of Table 1 was constructed and operated with a saturated NaC] feeds- tock acidified with 0.2M HCI and at323 mAlcM2. The cell voltage was measured at various operating temperatures from 35-90'C.

A cell similar to Cell No. 7 of Table 1 was constructed and operated with 290g/L (- 5M)/L NaC] 8 GB 2 071 157 A 8 Cel[No. 1 Voltage 3.65 3.38 3.2 3.15 3.10 3.05 3.02 stock (not acidified) at 215 mA/cm2.

The cell voltage was measured at various operating temperatures from 35900C. The data was nor- malized for 323 mA/cM2.

TABLE V

Cell No. 7 Voltage Normalized to 323MAICM2 Temperature 20 (215MA1CM2 Data) 3.50(3.15) 3.30(2.98) 3.20(2.9) 3.12(2.78) 3.05(2.72) 2.97(2.65) 2.95(2.63) 35' 45' 55' 65' 75' 85 900 This data shows thatthe best operating voltage is obtained in the 80-90' range. It is to be noted, however, that even at 350C, the voltage with the instant catalyst and electrolyzer is at least 0.5 volts better than prior art chlorine electrolyzers operating at 10 900C.

When the NaCI electrolysis is carried out in a cell in Cell Anode 1 RuO.-Graphite (Bonded) 2 Platinized Niobium Screen (Not Bonded) 3 Platinized Niobium Screen (Not Bonded) 4 RuO.-Graphite (Bonded) Ru 0.

(Bonded) 6 Platinized Niobium Screen (Not Bonded) which both electrodes are bonded to the surface of an ion transporting membrane, the maximum improvement is achieved. However, improved pro- cess performance is achieved for all structures in which at least one of the electrodes is bonded to the surface of the ion transporting member (hybrid cell). The improvement in such a hybrid structure is somewhat less than is the case with both electrodes bonded. Nevertheless, the improvement is quite significant (0.3-0.5 volts better than the voltage requirements for known processes).

A number of cells were constructed and brine electrolysis carried outto compare the results in a fully bonded cell (both electrodes) with the results in hybrid cell constructions (anode only bonded and cathode only bonded) and with the results a prior art non-bonded construction (neither electrode bonded). All of the cells were constructed with membranes of Naflon 315, the cell was operated at 90'C with a brine feedstock of approximately 290 g/L. The bonded electrode catalyst loadings were 2g/930 cm2 at the cathode for Pt Black and 4g/930 CM2 atthe anode for RuO,,-graphite and RuOx. The current effi- ciency at 300 ASF was essentially the same for all cells (84-85% for 5M NaOH). Table VI shows the cell voltage characteristics for the various cells:

TABLE W

Cathode Pt Black (Bonded) Pt Black (Bonded) Pt Black (Bonded) Ni Screen (NotBonded) Ni Screen (NotBonded) Ni Screen (NotBonded) It can be seen thatthe cell voltage of the fully Teflon-bonded cell No. 1 is almost a volt betterthan the voltage forthe prior art, completely non-Teflon bonded, control cell No. 6. Hybrid cathode bonded cells 2 and 3 and hybrid anode bonded cells 4 and 5 are approximately 0.4-0.6 volts worse than the fully Teflon-bonded cell but still 0.3-0.5 volts betterthan the prior art processes which are carried out in a cell without any Teflon bonded electrodes.

It will be appreciated that a vastly superior process 70 for generating chlorine and other halides from brine and, as will be shown hereafter, from HCI and other halides, has been made possible by reacting the ano lyte and the catholyte at catalytic electrodes bonded directly to and embedded in the cationic membrane. 75 By virtue of this arrangement, the catalytic sites in the electrodes are in direct contact with the mem brane and the acid exchanging radicals in the mem brane resulting in a much more voltage efficient pro cess in which the required cell potential is signific- 80 antly better (up to a volt or more) than known pro cesses. The use of highly effective fluorocarbon bonded thermally stabilized, reduced noble metal Cell Voltage Mat323mAIcm' 2.9 3.5 3.4 3.5 3.3 3.8 oxide catalysts, as well as fluorocarbon graphitereduced noble metal oxide catalysts with low overvoltages, further enhance the efficiency of the process. 65 Electrodes containing thermally stabilized, reduced platinum group metal oxides, etc., embedded in ion-exchange membranes were built and tested to illustrate the effect of various parameters on the effectiveness of the cell and catalyst in the electrolysis of hydrochloric acid. Table Vil illustrates the Effect on Cell Voltage of various combinations of reduced platinum group metal oxides. Cells were constructed with Teflonbonded, graphite electrodes containing various specific combinations of thermally stabilized, reduced platinum metal oxides and reduced oxides of titanium embedded into a hydrated cationic membrane, 0.3mm thick. The cell was operated with a current density of 430 mA/cM2, at 300C, at a feed rate of 70 cc per minutes, (46.5cml active cell area) ith feed normalities of 9-11 N. wl Table Vill and IX illustrate the effect of time forthe same cells and under the same conditions, on cell i J r 1 i GB 2 071 157 A 9 operating voltages.

Table X shows the effect of acid feed concentration ranging from 7.5-10. 5N. A cell, like cell No. 5 in Table 11, was constructed with reduced (Ru, 25% 100,, platinum group metals added to the Teflon-bonded graphite electrode. The cell was operated at fixed feed rate of 150 mi/min, (46.5cm' active cell area) at 300C and 430 mA/cM2.

TABLE V111 cell Cell Voltage (V) Voltage (V) at 100 Hrs. At Operating Current Cell Operating Time From Density No. Time Table 1 (mAIcm2) C) c 1 1.85 2.10 430 C! a? q OD W CM Cli CM 2 1.84 2.01 430 P 3 1.78 1.97 430 4 1.80 1.91 430 1.75 2.07 430 1,:'- C c> 0 0 0 C) C) 0 (1.9) to: ' 1 -1 m m m % R 4 Q W W W W W 6 1.70 1.80 430 a 1Z c See note for Table V11 Intermediate Cell Operating No. Time-(Hrs.) c,, W, j:

G m (D 0 0 0 0 0 Ct C; C; 0i C 1 3900 0::E ---4 TABLEIX

Current Density Cell (mAlCM2) Voltages (V) 108 215 323 1.70 1.93 2.00 z 0 0 Z CO) 4 ': t! t! 3 M 50, Q.0 X.0 X L4 0 0 X +T 0 ' (D U) = U) = 0 +, - F- SCO W:3 cr cc cc i J, (R m - c) 0 rj) i:

X M -0 -0 C) -0 X (U (D a) X (D a) (D 2 ti.!%! C) N ti -, 0 -0 J2 0 -0 cc 00_ 0 cc LD cc LD U) 0 U) L 0 U) F- U) ', C14 cc -3 Cc:3 CO:: M R q R R C C14 C14 C:

0 X -Q -0 -0 m M -0 C) -0 X ,t 1.; (t " (D (L) (D (1) X G) - 0 0 CJ: N T L4.! 0 N -- N - Q. 0 a- '-= - - - _= = m C z (,t L. - -0 X:a X:a X:3 -!=:a.0:5 1- 0 R (D:3 M 0 2 0 2 c) 9 -o so L.m, 0-0 X iZ 0 (n (1) = U) = U) L0 U) r_ U) LC) P cm C14 Cc = Cc CO Cc S (0:3 (L) cc (D cc (D cr (D W_ (D = (D cc 9 = -:E - X - = - = = - M C: p) 1: 0 C 0 0 C) 0 0 0 C1 00 q) ce) CV) a) Q) 9 (D L0 c - i:z C) 0 0 Q 0 0 0 C4 - c a cm CV) nt L0 (D a; cn 2 3400 0 i 3 1900 ts a) g- 0 r_ 0 9 p CR z m E X 2 0. CL m a) M 2 :3 0 cn W 00 T a) W 2 c) a) W a 108 215 323 1.57 1.70 1.83 108 215 323 1.58 1.70 1.81 4 1000 1200 108 215 323 1.47 1.60 1.72 108 215 323 Feed Normality (Eq/L) 7 7.5 8 8.5 10.5 11.5 TABLEX

Volume 02 0.4 0.15 0.04 0.015 0.007 0.004 0.003 1.32 1.45 1.55 GB 2 071 157 A 10 From the above examples, it will be clear that HC1 is electrolyzed to produce chlorine gas, substantially 15 free of oxygen. The catalyst used in the electrolyzer cell is characterized by low cell voltage and low temperature (- 300C) operation resulting in econom ical operation of such electrolyzer cells. Further more, this data shows excellent performance at var- 20 ious current densities, particularly at 323-430 mA/cm'. This has a positive and beneficial effect on capital costs for chlorine electrolyzers embodying the instant invention.

To show the effect of thermal stabilization on 25 reduced platinum group metal and valve metal TABLEXI oxides, certain tests were carried out. These tests show the impact on the resistance of the catalyst to harsh electrolysis environments. Thermally stabilized as well as non-stabilized, reduced oxide catalysts were exposed to highly concentrated HC1 solutions which represent extremely harsh environmental 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 resuits of these corrosion resistance and stability tests for catalyst batches ranging from.5 to 20 gms.

Corrosion Time Observation Stability Catalyst Treatment Ternp."C Medium (Hours) (Color) Evaluation Ru 0. None 240C 12N HCI 24 Light Brown Modest Corrosion Color Thermally Stabilized 240C 12N HCI 744 Very Pale Yellow Very Little Corrosion 550C for one (1) Hour Good Stability (Ru 25Nb)O, None 240C 12N HCI 24 Light Brown Modest Corrosion 912 Amber (Ru 50Ta)O. None 240C 12N HCI 168 Pale Amber Modest Corrosion 5500C for one (1) Hour 24C 12N HCI 96 VeryPaleYellow )Fully Stable

5500 for one (1) More 72 No change in Hour Color (Ru 5100 None 24PC 12N HCI 168 Amber Substantial Corrosion Unstable

5500C for one (1) Hour 240C 12N HCI 96 No Change in FullyStable Color (Ru 25100. None 240C 12N HCI 168 Amber Substantial Corrosion Unstable

550"C for one (1) Hour 240C 12N HCI 96 Very Pale Yellow )Fully Stable

5500C for one (1) More 24"C 12N HCI 72 No Color Change Hour It will be obvious from this data that thermal stabilization of the reduced oxides enhances the corrosion resistance of the catalyst in very concentrated HCI and, in fact, provides very good stability. It is obvious that resistance of the catalysts in the much less corrosive chlorine or brine environments is excellent and attributable to thermal stabilization of the reduced oxide catalyst.

Having observed the improved corrosion characteristics of the thermally stabilized, reduced, platinum group metal oxides, physical and chemical tests were conducted to determine the effect of thermal stabilization on various characteristics of the catalysts which might account forthe improved corrosion characteristics. The oxide content, surface area in M11g of catalyst, the pore volume and pore size distribution of the catalyst were measured after fabrication of the catalyst by the modified Adams method; after reduction of the catalyst; and after thermal stabilization of the reduced catalyst. The result of these tests, which will be set forth in detail below, show that the surface area of the catalyst is reduced somewhat after the catalyst is reduced, and quite substantially after thermal stabilization. A drop in oxide content after the reduction step is believed to account for part of the decrease of the surface area. A substantial change in the internal pore size distribution of the catalyst after thermal stabilization without a corresponding change in the poor volume is believed responsible for the very substantial decrease (ratio of 2 to 1) in surface area accompanying thermal stabilization and would account forthe improved corrosion characteristics as corrosion is directly related to the area exposed to attack by any corrosive agents.

Initially, Sample #1, a ruthenium - 25% by weight iridium catalyst was prepared by the modified Adams method. A portion of this catalyst was reduced eiectrochemicallyto form Sample #2. A reduced (Ru 25 lr)O,, sample was thermally stabilized for one (1) hour at 550 - 600'C. The surface area of the unreduced (Sample #1) catalyst, the reduced catalyst (Sample #2) and the thermally stabilized, reduced (Ru 25 lr)O,, catalyst (Sample #3), as measured by the three point BET (BRUNAUER-EMMETTELLER) nitrogen adsorption method, is shown in Table XII.

GB 2 071 157 A 11 TABLEX11

Catalyst (Ru 25 1r) Treatment Sample #1 None Sample#2 Reduced Sample#3 Reducedand Thermal Stabiliza tion - 550 - 600OC; One (1) Hour Surface Area 127.6 M2/9 123.5 M2/g 62.3 M2/9 The oxide content of Samples #1, #2, and #3 was then measured as well as that of a Sample (#4) thermally stabilized at 700 - 75WC for one (1) hour, In addition the oxide content of Pt lr catalysts contain- ing respectively 5 and 50% by weight of Iridium was measured. The results are shown in Table XIII.

TABLEX111

Catalyst (Ru251r) Treatment Sample #1 None Sample #2 Reduction Sample#3 Thermal Stabilization; 550 - 600C - One (1) Hour Sample#4 Reduction and Thermal Stabilization; 700 - 750C One (1) Hour Catalyst (Ru-25 1r) Treatment Sample#l None Sample #2 Reduction Sample#3 Reduction and Thermal Stabilization; 500 - 6000C One (1) Hour % Oxide Content 24.4 24.3 22.6 21.5 The data shows that the total pore volume is relatively unchanged. Thus the porosity, in terms of g/m[ or if converted to void volume (knowing the spheri- cal size and density), is essentially the same and is in the range of 0.7 - 0.8 g/mi for Ru - 25 1r.

Simultaneously, the pore size distribution was measured to obtain the pore diameter distributions in the 40K- 10 micron range. Capillary condensation was used in the 40 - 500A' range. In this method TABLEXIV (Pt-5010 Sample #5 None 16.5 Sample#6 Reduction 15.2 Sample#7 Reduction and Thermal 13.0 Stabilization; 550 - 600'C One (1) Hour The data from Table XIII shows a decrease in oxide content with reduction and thermal stabilization just as surface area decreases after reduction and ther- mal stabilization.

Decrease of oxide content (i.e., unreduced catalyst) will have a corresponding effect on surface area since the surface area of oxides is normally greater than that of the non-oxide form. This reduction in oxide content in part explains surface area reduction but does notwholly explain the dramatic reduction in surface area after thermal stabilization.

The porosity of the catalyst was therefore measured to determine whether thermal stabilization of the catalyst causes a change in porosity thereby decreasing the surface area and increasing its corrosion resistance. Catalyst samples were taken from the same batches as Samples #1, #2, and #3 and the porosity of the samples and particle size distribu- tion measured. Particle size distribution was measured by a sedimentation method and showed that the equivalent spherical diameter at 50% mass distribution was 3.7 micros (1i) after reduction of the catalyst and 3.1 microns (tz) after thermal stabiliza- tion. This indicates that the external surface of the particles is reduced but again does not account for all of the surface area reduction after stabilization.

Total pore volume data (ml/g) was obtained by capillary condensation and mercury intrusion methods. Data for Samples #1, #2, and #3 is shown in Table XIV.

Range 40A - 1 Og 40A - 1 Og 40A - 1 Og Total Pore Volume M11g 0.80 m [/g 0.72 m l/g 0.76 milg liquid condensation for a given vapor pressure is measured to obtain pore size distribution. The capillary condensation method has a lower resolution limit of 40Kand an upper limit of 500A0. For pores in 50 excess of 500A0 (i.e., 50OK- 1 Og) a mercury intrusion method is utilised to obtain pore size distribution. Pore diameter distribution measurements for the two ranges is shown in Tables XV and XVI respectively.

Catalyst Treatment Sample #1 None Sample#2 Reduction Sample #3 Reduction and Thermal Stabilization; 550 600'C; One (1) Hour Catalyst Sample #1 Sample #2 Sample #3 Treatment None Reduction Reduction and Thermal Stabilization; 500 - 600OC; One (1) Hour This data indicates that thermal stabilization of the catalyst results in a change in pore size diameter.

This change seems to be accompanied by a change in the number of pores so thatthe overall surface area decreases. With the pore volume staying sub stantially constant and the pore diameter distribu tion changing so that the distribution shows a max imum at 20OA0, and 1.5 g it seems quite clear that many of the pores below 40A0 coalesce, reducing the overall number of pores. Above 50OA0 the porediameter increases. The pore size diameter and internal pore surface area thus changes with thermal stabilization. In summary, the internal porosity and 50 thus the pore surface area is reduced as the catalyst is thermally stabilized. It is believed that this is a result of changing the morphology to provide a smaller number of pores with a larger pore diameter distribution. With the pore volume relatively unchanged and a change in pore diameter distribu tion to a larger pore diameter, it seems clear that the surface area reduction associated with thermal stabilization of the catalyst is attributable to the change in internal pore surface area.

The porosity (in terms of pore volume (ml) per unit weight (g) of catalyst) of the thermally stabilized, reduced p!atinum metal oxide catalyst ties in the 0.4 - 1.5 ml/g range with the preferred porosity for a thermally stabilized Ru - 25 Ir catalyst being 0.7 - 0.8 m l/g.

The pore diameter distribution of the thermally stabilized catalyst shows the principal distribution below 50OA0 in the 100 - 300A' range, with a max imum at 200A'. Above 500A'the pore diameter dis tribution shows the greatest pore volumes and hence the principal distribution is in 0.4 - 9 p range with a maximum at 1.5 g's (representing the 50% point is distribution).

TABLEXI1

TABLEXV1

Size Range 40 - 500A' - 500A' - 500A' Size Rane 50OK- 10microns 50OK- 1 Omicrons 50OK- 10microns Pore Diameter Distribution Pore Distribution below 40A' Pore diameter distribution below 40A' Distribution in the range of 100 - 300A' with maximum at 200A' Pore Diameter Distribution (50%Point in distribution) 0.46 0.82 1.5 GB 2 071 157 A 12 The catalyst surface area range for thermally stabilized, reduced platinum metal oxide catalysts should be such that for any given platinum metal, or combination of platinum metals with or without transition metals, etc., the surface area should be as low as possible (to reduce corrosion) for the required catalytic activity. Thus, the surface area should be in excess of 1 0M2/9m ranges from 24 - 165M21g, with a preferred range being from 24 - 165M2/g and from 60 - 70M2/g for a thermally stabilized Ru - 25 lr catalyst. The reduced thermally stabilized platinum group metal oxide catalysts are, as may be seen, large surface area catalyst as compared to powders, blacks, etc., which normally have surface areas around 10-1 5M21g.

The oxide content of the catalyst can range from 2 - 25% by weight, withthe preferred range being 13 to 23%byweight.

Claims (18)

Reference is made to our British Patent Application No. 42324178 which claims catalysts as described hereinbefore. CLAIMS

1. A combination electrolyte and electrode strueture comprising a gas and hydraulically impervious polymeric ion transporting membrane having at least one gas and 5quid permeable catalytic electrode bonded to a surface of the membrane to form a unitary electrode-electrolyte structure, said electrode comprising electroconductive thermally stabilized reduced oxides of at least one platinum group metal consisting of platinum, iridium, ruthenium, palladium, osmium, or alloys thereof.

2. An electrode and electrolyte structure according to claim 1 wherein said electrode also contains graphite.

3. An electrolyte and electrode structure according to claim 1 or claim 2 wherein said gas permeable C; k 1 13 GB 2 071 157 A 13 electrode comprises a plurality of electroconductive thermally stabilized particles of oxides of said platinum group metals bonded together by polymeric particles.

4. An electrolyte and electrode structure according to Claim 3 wherein said particles include the thermally stabilized oxides of at leasttwo metals taken from the reduced oxides of said platinum group metal and thermally stabilized, reduced valve metal oxides of tantalum, titanium, niobium, zirconium, or hafnium, with at least one kind being a thermally stabilized reduced platinum group metal oxide.

5. An electrolyte and electrode structure accord- ing to Claim 4 wherein said particles include thermally stabilized, electroconductive, reduced oxides of ruthenium.

6. An electrolyte and electrode structure according to Claim 5, wherein the thermally stabilized, elec- troconductive particles are thermally stabilized, reduced oxides of ruthenium and reduced oxides of iridium.

7. An electrolyte and electrode structure according to Claim 6 wherein the particles include 5% to 25% by weight of reduced oxides of iridium.

8. An electrolyte and electrode structure according to claims 5 or 6 wherein the particles include 25% by weight of iridium.

9. An electrolyte and electrode structure accord- ing to anyone of Claims 4to 6 wherein the plurality of particles include thermally stabilized, electroconductive particles of reduced platinum group metal oxides and thermally stabilized reduced oxides of a valve metal.

10. An electrolyte and electrode according to Claim 9 wherein the reduced platinum group metal oxide is reduced iridium oxide.

11. An electrolyte and electrode structure accordingto anyone of claims 1 to 10wherein the pore diameter distribution of the reduced, thermally stabilized platinum group metal is modified by thermal stabilization to have a pore diameter dis tribution maxima centered at 200A0 and a 50% pore distribution point at 1. 5 p_

12. A combination electrolyte and electrode structure for halogens comprising an ion transporting membrane having at least one gas permeable electrode bonded to one surface of said membrane, said electrode comprising thermally stabilized, electro-conductive, catalytically active reduced oxide particles of at least one platinum group metal, the surface area of the electrode being at least 60 sq. meters per gram of particles and the pore diameter distribution has maxima centered at 200A0 and a 50% pore distribution point at 1.5 g.

13. A catalytic electrode structure comprising an agglomerate of electroconductive catalytic particles bonded together with particles of a resinous binder, said catalytic particles including electroconductive, thermally stabilized, reduced oxides of at least one platinum group metal of platinum, iridium, ruthenium, palladium, osmium or alloys thereof said electrode structure being gas and liquid permeable, electronically conductive and catalytically active for 65 electrolysis of halides.

14. A catalytic electrode structure according to claim 13 wherein said particles include the oxides of at least two metals taking from said electroconductive thermally stabilized, reduced platinum group metal oxides and thermally stabilized, reduced valve metal oxides of tantalum, titanium, niobium, zirconium, or hafnium with at least one of said oxides being a reduced thermally stabilized platinum group metal oxide.

15. A catalytic electrode structure according to Claims 13 or 14 wherein said catalytic particles are thermally stabilized, reduced oxides of ruthenium.

16. A catalytic electrode structure according to C laims 13-15 which further includes particles of another thermally stabilized, reduced oxides of a platinum group metal.

17. A combination electrolyte and electrode structure according to claim 1 or claim 12, and substantially as hereinbefore described.

18. A catalytic electrode as claimed in claim 13 and substantially as hereinbefore described.

Printed for Her Majesty's Stationery Office by The Tweeddale Press Ltd., Berwick-upon-Tweed, 1981. Published atthe Patent Office, 25 Southampton Buildings, London, WC2A lAY, from which copies may be obtained.