CA1152451A - Electrolytic membrane and electrode structure including reduced platinum group metal oxide - Google Patents
Electrolytic membrane and electrode structure including reduced platinum group metal oxideInfo
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- CA1152451A CA1152451A CA000315518A CA315518A CA1152451A CA 1152451 A CA1152451 A CA 1152451A CA 000315518 A CA000315518 A CA 000315518A CA 315518 A CA315518 A CA 315518A CA 1152451 A CA1152451 A CA 1152451A
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Abstract
ABSTRACT OF THE DISCLOSURE
A novel, 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 stabilize 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 transition or valve metals. A
novel method of preparing the electrocatalytic material is described as well as a unitary electrolyte electrode structure which has a bonded electrode containing the novel electrocatalytic material, bonded to at least one side of a membrane-electrolyte.
A novel, 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 stabilize 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 transition or valve metals. A
novel method of preparing the electrocatalytic material is described as well as a unitary electrolyte electrode structure which has a bonded electrode containing the novel electrocatalytic material, bonded to at least one side of a membrane-electrolyte.
Description
~152451 52-EE-0-299 The instant invention relates to an electro-catalyst, a catalytic electrode, and a membrane/electrode assembly. More particularly, it relates to catalysts and electrodes which are particularly useful in the electrolysis of halides.
Generating gas by electrolyzing a chemical compound into it~ con~tituent elements, one of which may be a gas, is, of cour~e, an old and well known techni~ue. One recently developed form of ~uch gas evolving electrolyzer involves the u~e of a cell which utilizes an electrolyte in the form of a ~olid polymer, ion-exchanging membrane. In an an arrangement of this ~ort, catalytic electrode~ u~ing a ~uitable cat~lyst are positioned on oppo~ite ~ide~ of an ion tranQporting membrane medium such as a sulfonated perfluorocarbon ion-exchange membrane. Through an oxidation reaction, the ionic form of one of the con~tituent elements (hydrogen ion~, for example, when H20 or HCl is electrolyzed, or sodium ions _~ r,~
/l .
1~52451 52-EE-0-299 A. B. LaConti, et al ¦¦when an alkali metal halide such as sodium chloride is electrolyzed) is producedl ¦at one electrode. The ion is transported across the ion-exchangin~ membrane to ' the other electrode where it is reduced to form an electrolysis product such as Imolecular hydrogen, NaOH, etc. Solid polymer ion-exchange membranes electrolysis 5 l~units are part1cularly advantageous because they are efficient, small in size, ¦and do not utillze any corrosive liquid electrolytes.
I Yarious 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 at the gas evolving electrodes is obviously crucial in determining the effectiveness and efficiency of the cell, and con-~sequently of the economics of the process. The choice of a catalyst in an electrochem1cal cell and 1ts eff~ct~veness depends upon a complex set of var1-ables, such as surface area of a catalyst, availability of oxides of lts specieson the catalyst surface, contaminants in the reactants, and the nature of the convers10n taking place in the cell. Consequently, it ~s, and a1ways has been, dif~1cult to predict the applicab{l{ty of a catalyst useful in one electro-chemical cell to a dtfferent system. A commonly ass19ned Patent No. 3,99Z,271 entitled "Methods and Apparatus for Gas Generation" describes an improved oxygenevolvtng catalyt1c electrode util~zing a platlnum-tr~d~um alloy, a mixture whichwas ~ound to provide much improved performance and efficiency. Another commonlyassigned U-S- Patent No- 4,039,490 describes another oxygen evolving catalytic electrode whlch utillzes reduced ox~des of platinum-ruthenium. The platinum- j ruthenium catalyst not only is substantially less expensive than the reduced plat1num-iridium catalyst, because it uses a less expens{ve material such as ruthenium to alloy with the platinum, but it also turns out to be more eff kientbecause tt has a lower oxygen overvoltage than a platinum-1ridium electrode.
However, attempts to use reduced ruthenium oxide electrocatalysts for evolution of halogens by electrolysis of aqueous halide solutions have not been j entirely successful due to the harsh electrolysis conditions In the cell. There30 ¦can be substantial loss of catalyst from the membr~ne during chlorine evolution ¦
3L~L~24S1 52-EE-0-299 A. B. LaConti, et al ¦
since these reduced platinum metal oxides are susceptible to dissolution in acidic environments which are present in the electrolysis of hydrogen halides orin the electrolysis of alkali metal halide solutions which are of~en acidified.
Not only is there a tendency to d~ssolution of the platinum metals resulting in a loss of a catalytic mater~al, but the overvoltage of the electrodes also tendsto tncrease so that the efficiency of the cell decreases, and in many instances does not permit prolonged periods of operation.
It is, therefore, an object of the invention to provide a novel electro-catalytic material espec1ally useful for the electrolysis of aqueous solutions of hal~de ions and to a novel process for the preparation of said catalytic material.
Another ob~ect of the invention 1s to provide a novel membrane/electrode structure in which a sol1d polymer electrolyte membrane has a catalyt1c electrode including the said electrocatalytic material bonded to at least one side of the membrane.
An add~tional ob~ect of the 1nvent10n is to provide a novel, bonded elec-trode structure which includes the said electrocatalytlc mtter~al which is bonded w1th a polymerlc b1nter.
St111 another ob~ect o~ the 1nvention is to provide a novel electrolysis cell where~n the anode and cathode compartments are separated by a solid polymerelectrolyte membrane hav~ng a coat1ng of the novel electrocatalyt~c material bonded to at least one sur~ace of the membrane.
Other ob~ects and advantages of the invent10n will become apparent as the description thereof proceeds.
In accordance with the invention, the novel electrocatalyst comprises at least one reduced platinum group metal oxide which is subsequently treated ~n the presence of oxygen at a temperature high enough to stabilize the oxide thermally to increase the resistance of the catalyst against the corrosive electr~lysis conditions. The catalytic, reduced plàtinum group metal ox1de may optionally contain other reduced platinum group metal oxides such JS iridium and ~24S1 52-EE-0-299 A~s. LaConti, et al c ionally up to fifty (50) percent by weight of the electroconductive extende:s such as graphite, valve metal oxides, transition metal oxides, and nitrides, carbides 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 the other halogen production being thermally-stabilized reduced oxides of ruthenium. Reduced oxides of ruthenium are pre-ferred, because they are found to have extremely low chlorine over-voltages as well as having stability in the electrolysis 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 of thermally-stabilized reduced platinum group metal oxides are even more stable, as are thermally-stabilized reduced oxides of platinum group metal alloys. One such mixture of ruthenium oxide contains up to twenty-five (25) percent of iridium oxide, with the preferred range being five (5) to twenty-five (25) percent by weight, even though iridium is somewhat more expensive than ruth-enium alone.
Electroconductive extenders such as graphite have low over-voltages ~or halogens and are substantially less expensive than the platlnum metal ox~des and may readily ~e incorporated without reducing the effectiveness of the catalyst. In addition to graphite, oxides of a valve metal such as titanium, tantalum, niobium, tung~ten, 25 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 thereto are formed into an electrode by bonding with 30 fluorocarbon resin particles such as those sold by Dupont under i~s trademark Teflon. The catalytic particles and resin particles are mixed, placed in a mold and heated until the composition 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 electrode 35 structure and a unitary membrane/e~ectrode structure.
The novel 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 particularity in the app~nded 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 conjunction 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.
The novel electrocatalyst which includes thermally stabilized, reduced oxides of a platinum group metal alone or in c xbination with other platlnum group metalc or optionally valve metals may be prepared in any convenient fa~hion whereby an oxide catalyst is permanently, partially reduced and thermally stabilized.
The preferred manner of reduction ~s by a modification of the Adams method of platinum preparation by the addition of a thermally decomposable pla-tinum halide, such as ruthenium chloride, either alone or, if desired along with an appropriate quantity of other thermally decomposable halides of platinum metals or valve metals to an excess of sodium nitrate. m e Adams method of platinum preparation 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 con~enient to mix the finely divided halide salts of the platinum metals, suchas Ch~roplatinic acids in thecase of platinum, ruthenium chloride inthe case of ruthenium, titanium chloride, tantalum chloride, in the case of titanium ar.dtantalum inthesame weight ratio of the metals as desired in the final alloy ~l5Z45~
52-EE-0-299 A. B. LaConti, et al mixture. An excess of sodium nitrate is incorporated and the mixture is fused in a silica dish at 500 to 600C for three (3) hours. The residue is washed thoroughly to remove nitrates and halides still present, leaving a residue of the desired platinum metal ox~de, i.e., ruthenium oxide, plat~num-ruthenium ox1de, ruthen1um-1r~d1um oxide, ruthenium-titanium oxide, etc. The resulting suspens~on of mixed and alloyed oxides is then partially reduced. The reductio of the platinum group metal oxides may be effected by any convenient known reduc~ng method, such as an electrochemical reduction or by bubbling hydrogen through the mixture at room temperature as long as the oxides are not to be completely reduced to the free metal form. In a preferred embodiment, oxides are reduced by uslng an electrochemical reduction technique, ~.e., electrochemi cal retuction in an ac1d medium. The product which is now a reduced platinum metal ox1de, 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 to produce a f1ne powder of the reduced platinum metal oxide.
The resulting reduced plattnum metal ox1des are then stab11ized thermally by the heating in the presence of oxygen for a suffic1ent t1me to ensure a catalyt1c material wh kh 1s stable in an ac1d1c hydrogen halide env1ronment and 1n the pr~sence of halogens. In a manner to be described subsequently, thermal stab111zat1On of the catalyst results in a catalyst which has much better corros1On characterist1cs 1n halogens, such as chlorine, etc., and 1n the presence o~ halites solutlons such as hydrochloric, etc., acids. It 1s bel1eve that thermal stabilization results in the formation of a catalytic particle having a large mean pore diameter and stable thin ox1de f~lm on the outside of the reduced oxide particle. This stabil~zes the reduced oxide particles so tha they have better mechanical propert~es for bonding to the solid polymer electro lyte me~brane, and in thelr resistance characteristics to dissolution in hydro-chloric acid or other halide acid solutions or to the evolved halogens. Thus, preferabl~, the reduced oxides are heated at 350 to 750C from thirty (30) minutes to si~ (6) hours with the preferable thermal stabilization procedure ~2451 52-E-0-299 A. 8. LaConti, et al being accomplished ~y heating the reduced oxides for one ~l) hour at 550 to 600C.
I It has also been found that the electrocatalytic activity of the catalyst and of the electrode including the catalyst is opt~m~zed by prov~ding the S catalyttc part1cles 1n as fine a powder form as poss~ble~ Thus, it has been foùnd that the sur~ace area of the part1cles, as observed both by the BET nitro-gen absorption method, should be at least 25 meters square per gram of catalyst (M /g). The preferred range 1s 50 to 150M /9.
The gas permeable electrode structure of catalytic particles and fluoro-carbon polymer particles ~s produced by blending the catalytic particles with a Teflon dispers10n to produce a bonded electrode structure 1n the manner des-cribed in U.S. Patent No. 3,297,484 ass~gned to the assignee of the present 1nvention. In the process of bonding the electrode, ~t ~s desirable to blend the catalyst w~th Teflon d~spers10ns ~n such a manner that the dispersion con-tains little or no hydrocarbons. If the f1uorocarbon Teflon compos~t~on conta~n ihydrocarbon organ~c surface act~ve agents, it results 1n loss of surface area of the reduced ox1de catalyst. Any reduction on the surface area of the catalyst 1s obv10usly undes1rable, s1nce 1t has potent1ally deleter10us effect on the et~1c1ency and e~fect1vcness of the catalyst. Hence, fabr1cat10n of the electrode should be by the use of a Teflon polytetrafluoroethylene part1cle composition wh1ch contains few, ~f any, hydrocarbons. One suitable form of thespart1cles wh1ch may be ut~l~zed ~n fabrlcat1ng the electrode 1s sold by Dupont under its des1gnat10n Teflon T-30.
The m~xture of noble metal partlcles and Ten on particles or of graphite and the reduced oxide particles are placed in a mold and heated unt~l the compo-s~t10n is formed into a decal wh~ch is then bonded and embedded in the surface ¦ by the appltcat10n of pressure and heat. As described, for example, in U.S.
¦Patent No.3,2g7,484 above, the electrode structure is bonded to the surface of l the ~on-exchange membrane thus ~ntegrally bonding the gas absorb~ng part~cle 30 1 mixture and, in some instances, preferably embedding it into the surface of the membrane, 1~52451 52-EE-0-299 A. B. LaConti, et al The novel membrane/electrode structure thus fabricated comprises a solid polymer electrolyte membrane capable of selective ion transport having a thin, orous, gas permeable electrode of the above-described electrocata1ytic reduced latinum group metal oxides bonded to at least one side of the membrane. A
econd electrode may be bonded to the other side of the membrane and may include he same electroc~,talyttc material, or any other suitable cathodic material. Th~
elective ~on transporting membrane is preferably a stable, hydrated, cationic embrane which is characterized by ion transport selectivity. The cation ex-hange membrane allows passage of posit~vely charged cations such as hydrogen ons in the case of the electrolysis of a halide Such as hydrogren chloride or odium cations In the case of the electrolysis of aqueous alkal~ metal halides, nd thus min~mizes passage of negat1vely charged anions.
There are vartous types of ion exchange resins which may be fabricated int embranes to provide selective transport of the Cations. Two classes of such 15 Ires1ns are the so-called sulfonic acid cation exchange res~nS ant the carboxylic katton exchange resins. Sulfonic acid exchange restns, whiCh are the preferred type, include ion-exchange groups 1n the form of hydrated, sulfonic acid rad k al (S03H X H20) attached tO the polymer backbone by sulfonat~on. The ion exchangin ac~d rad1cals in the membrane are fixedly attached to the backbone of the polyme ensuring that the electrolyte concentration does not vary. As pointed out previ ously, perfluorocarbon sulfon1c acid cation membranes are preferred, One specif C
class Of cation polymer membranes in this category 1s sold by the Dupont Company m arff C under its trade dcsignet~on "Nafion". These "Nafion" membranes are hydrated, copolymers of polytetrafluoroethylene (PTFE) and polysulfonyl fluoride vinyl ether containing pendant sulfonic acid groups.
The ion-exchange capacity (IEC) of a given sulfonic cation exchange mem-brane is dependent upon the milliequivalent weight (MEW) of the 503 radical per I
~ram of dry polymer. The greater the concentration of the sulfontc acid radicals, the greater the ion-exchange capacity and hence the capability of the membrane tC
transport cations. However, as the ion-exchange capacity of the membrane 3L~5Z451 S2 EE-0-299 A. B. LaConti, et al increases, so does the water content and the ability of the membrane to reject alt decreases. Thus in electrolysis of alkali metal halide solutions, caustic is generated at the cathode side and the rate at which the sodium hydroxide mtgrates from the cathode to the anode side thus lncreases with IEC. Such back migrat10n reduces the cathod~c current efficiency (CE) and also results in oxygen generation at the anode whlch have undesirable consequences in its effecton the catalytic anode electrode. Consequently, the preferred lon-exchange membrane for use in brine electrolysis is a laminate consisting of a thin (2 milor so) film of fifteen hundred (1500) MEW, low water content (5-15%) cation exchange membrane which has high salt rejection, bonded to a 4 m11 or so film ofhigh ion-exchange capacity, 1100 MEW, bonded together w~th a Teflon cloth. One form of such a laminated construction sold by the Dùpont 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 re~ection are also available. Typical of such other laminates are Nafion 355, 376, 390, 227, 214. In the case of a laminated memb n ne bonded together bya Teflon cloth, tt may be des1rable to clean the membrane and Teflon cloth by refluxing It in seventy (70) percent HN03 for three to four (3 to 4) hours in add1~10n to soak~ng in caustic preferred to previously.
In the case of electrolysis hydrogen halides such as hydrochloric acid, there ls no problem of back migrat~on of caustic or other salts, so that simpler~orms of membranes such as Naf~on 120 may be util~zed as the 10n 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 slde layer is a thin, (2-4 mil~ chemically modif~ed film of sulfonamide I
groups or carboxylic acid groups.
Referring now to Figure l, the halogen electrolysis cell is shown generall at 10 and consists of a cathode compartment 11 and an anode c~mpartment 12 separated by a solid polymer electrolyte memberane 13 which is preferably a ll 3L~ 2451 52 EE-0-299 A. B LaCoAtl ~t al hydrated, permselective, cationic membrane. Bonded to opposite surfaces of membrane 13 are electrodes comprising particles of a fluorocarbon such as Teflonbonded to thermally stabilized, reduced oxides of ruthenium (RUOX) or iridium ¦(IrOx), or stabilized, reduced oxides of ruthenium-iridium (RuIr), ruthenium-5 ¦t1tan1um (RuTi), ruthenium-tantalum (RuTa), ruthenium-tantalum-iridium (RuTaIr), lor 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 prefer-¦ably embedded in one side of the membrane and a catalytic anode, not shown, is ¦bonded to and preferably embedded in the opposite side of the membrane. Current 10 Icollectors in the form of metallic screens 15 and 16 are pressed against theelectrodes. The whole membrane/electrode assembly is firmly supported between the housing elements 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 chlorineoxysen, aqueous sodium chloride in the case of brine electrolysis and HCI, 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 designat10n ~PDM.
An aqueous alkal~ metal hal1de such as brine or hydrogen halides such as HCl is 1ntroduced through an electrolyte inlet l9 whlch communicates with chamber 20. Spent electrolyte and halogens such as chlorine are removed throughan outlet condult 21. A cathode inlet conduit 22 is provided in the case of brine electrolysis and communicates with cathode chamber ll to perm1t the intro-duction of the catholyte, water, or aqueous NaOH (more dilute than that formed electrochemically at the electrode/electrolyte interface). In the case of electrolysis of hydrogen halides such as hydrogen chlorlde, no catholyte need beprovided and the cathode inlet conduit 22 may be dispensed with.
In a brine electrolysis cell, the water serves two separate functions. A
portion of the water is electrolyzed to produce hydroxyl (OH ) anions which combine with the sodium cations transported across the membrane to ~orm caustics~NaOH). The water also sweeps across the porous, bonded cathode electrode to i ~SZ4S1 52-tE-O-Z99 A. J LaConti, et a;
dilute the highly concentrated caustic formed at the membrane/electrode inter-face to minimize diffusion of the caustic back across the membrane into the anolyte chamber. Cathode outlet conduit 21 communicates with the cathode chambe~
ll to remove excess catholyte and the electrolys~s products such as caustic in the case of brine electrolysis, plus any hydrogen discharge at the cathode both 1n brtne electrolysis and in hydro~en chloride electrolysis. A power cable 24 is brought into the cathode chamber and a comparable cable not shown is brought intc the anode chamber. The cables connect the current conducting screens 15 and 16 r any other suitable kind of collector as source of electrical power.
Figure 2 illustrates diagrammatically the reactions taking place in the cell during the electrolysis of an aqueous alkali metal halide such as brlne and1s useful In understanding the eiectrolysis process tn the manner in which the ell funct10ns. Thus, an aqueous solution of sod1um chloride is brought into th~node compartment which is separated from the cathode compartment by the cation1 membrane 13. Membrane 13 fs a composite membrane comprising a h19h water conten(20-35% based on dry weight of membrane) layer 26, on the anode side and a low ¦water content high MEW cathode s~de layer, (5-15% based on dry we~ght of membran~) separated by a Teflon cloth 28. The cathode s1de barrier layer may also be chem1cally mod1~1ed on the cathode s1de to form a thin layer of a low water con-tent polymer. In one form this 1s achieYed by modifying the polymer to form asubstituted sulfonamide membrane layer. By converting the cathode s~de layer to a weak actd form (sulfonamide), the water content of thls portlon of the membran1s reduced and the salt re~ecting capability of She film is increased. As a result, dlffus~on of sodium hydroxide back across the membrane to the anode is minimized. While laminated membrane constructions are preferred in brine electrolysis to block migration of sodium hydroxide, other homogeneous films of low water content may be utilized, (viz. 7 Nafion 150, perfluorocarboxylates, etc )-Obviously, in the case of the eiectrolysis of hydrogen halides such as HCl, HBr,: etc., the ion transport~ng membrane may be a simple, homogeneous film such as the Na ff on 120 referred to previously.
ll 115Z451 52-EE-0-299 A. B. LaConti, et al The Teflon-bonded, reduced noble metal oxide ca~alysts contains at least one therma11y stabilized, reduced platinum metal oxide, such as ruthenium, ~ridium, or ruthenium-iridium with or without reduced oxides of t~tanium, i niobium, or tantalum and particles of graph~te are, as shown, pressed into the5 llsur~ace of membrane 13. 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 tenminals 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, isbrought into the anode chamber, is electrolyzed at anode 29 to produce chlorine las shown diagrammatically by the bubble formation 30. The chlorlne actually is¦princlpally evolved at the interface of the electrode and the membrane, but Ipasses 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 16 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 caustlc formed at the membrane/cathode inter-tace and thereby reduce d1ffusion of the caustic back across the membrane to the¦~node.
20ll A portion o~ the water catholyte is electrolyzed at the cathode to fo m hydroxyl ions and gaseous hydrogen. The hydroxyl ions combine with the sodium ions transported across the membrane to produce sodium hydroxide at the 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 251 aqueous stream sweeping across the surface of the electrode. Even with a cath-¦lode 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 j~migrate 6ack through membrane 13 to the anode. NaOH migration is a diffusion l process caused by the concentration gradien~ and electrochemical negative ion transport to the anode. Sodium hydroxide transported to the anode is oxidi~ed ~ ~2~Sl 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 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 anoltye 80 that back migrating hydroxide i8 converted to water rather than generating oxygen. The reactions in various portions of the cell for electrolysis of NaCl is as follows:
Anode Reaction: 2 Cl ' C12 P + 2e (1) (Principal) Membrane Transport: 2Na + H20 (2) Cathode Reaction: 2H20 -~ 20H + H2 r - 2e (3) 2Na + 20H ~ 2NaOH (4) Anode Reaction: 40H ~~ 2 + 2H20 + 4e (5) Overall 2NaCl + 2H20 ~ 2NaOH + C12 ~ + H2~ (6) (Principal) The reactions for electroly~is of a hydrogen halide, such as HCl, are very similar:
Anode Reaction: 2HC1 ~ 2H + C12 ~ + 2e (1) Membrane Transport: 2H+ (H20, HCl) (2) Cathode Reaction: 2H + 2e _ H2 ~ (3) Overall Reaction: 2HCl ~ H2 + C12 (4) ~524Sl 52-EE-0-299 A. B. LaConti, et al The novel arrangement for electrolyzing aqueous solutions of brine or of HCl which is described herein is characterized by the fact that the catalytic sites in the electrodes are in direct contact with the cation membrane and the ion exchanging ac1d radicals attached to the polymer backbone (whether these S rad1cals are the S03 H X H20 sulfonic radicals or the COOH X H20 carboxylic ac1d rad1cals). Consequently, there is no IR drop to speak of in the anolyte or the catholyte fluid chambers (this IR drop is usually referred to as "Electrolyte IR drop"). "Electrolyte IR drop~ ~s character~stic of existing systems and processes in which the electrode and the membrane are separated and ¦
can be 1n the order of 0.2 to 0.5 volts. The elim~nation or substantial re-duction of thts voltage drop is, of course, one of the principal advantages of this invention since ~t has an obv~ous and very s~gn~ficant effect on the over-¦all cell voltage and the economics of the process. Furthermore, because ¦chlorine is generated directly at the anode and membrane lnterface, there is no15 ¦IR drop due to the ss-catled "bubble effect" wh1ch is a gas blending and mass ¦transport loss due to the interruption or blockage of the electrolyte path ¦between the electrode and the membrane. As pointed out previously, 1n prior lart systems, the chlor~ne d1scharging catalytic electrode is separated from the¦membrane. The gas ls formed directly at the electrode and results in a gas 20 ¦ layer tn the space between the membrane and the electrode. This in effect brea~s up the electrolyte path between the electrode-collector and the membrane ~loc~tng passage o~ Na~ tons and there~y, in effect, increasing the IR drop.
In a pre~erred embodiment, the Teflon-bonded noble metal electrode con-tatns reduced oxtdes of ruthentuml irtdtum or ruthenium-trid~um in order to m~nimtze chlortne overvoltage at the anode. The reduced ruthenium oxtdes are stabtl~zed agatnst chlorine and oxygen evolution to produce an anode which is stable. Stabtlfzation ts effected tnitially by temperature stabilization; i.e.,by heating the reduced oxides of ruthenium for one hour at temperatures in the l range of 550 to -600C~ The Te~lon-bonded reduced oxides of ruthenium anode is furt~er stabilized by mixing it with graphite and/or alloying or mixing with 3L~L ~ Z 4 5 1 52-EE-0-299 A. B. LaConti, et al reduced oxides of iridium (Ir)Ox in the range of 5 to 25~ of iridium, with 25 being preferred, or with reduced oxides of 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)Ox or tantalum, ruthenium and ~r1dium ~Ru, Ir, Ta)Ox bonded with Teflon is very effective in producing a stab1e~ long-lived anode. In case of the ternary alloy, the composition is preferably 5% to 25% by weight of reduced oxides of iridium, approximately 50%
by weight reduced oxides of ruthenium, and the remainder a transition metal suchas titanium. For a binary alloy of reduced oxides of ruthenium and titanium, the preferred amount is ~0% by weight of titanium with the remainder ruthenium.
Titanium~ of course, has the addftional advantage of being much less expensive than either ruthenium or 1ridium, and thus is an effective extender which re-duces cost while at the same time stabilizing the electrode in an acid environ-ment and against HCl, chlor1ne and oxygen evolut~on. Other transition metals, such as niobium (Nb), tantalum (Ta), zirconfum (Zr) or hafnium (Hf) can readily be substituted for Ti fn the electrode structures. In addition to transit~on metals, transition metal carbides, nitrides and sul~ides may also be ut111zed ascatalyst extenders.
The alloys of the reduced noble metal oxides along with the reduced ox~des o~ t1tan1um or other trans1tion metals are blended with Teflon to form a homo-geneous m~x, The anode Teflon content may be 15 to 50% by weight of the Teflon,although 20 to 30Z by weight is preferred. The Teflon is of the type as sold by the DuPont Corporatfon under its designation T-30, although other fluoro-car~ons may be used wfth equal facflity. Typical noble metal, etc., loadings for the anode are 0.6 mg/cm2 of the electrode surface with the preferred range being 1 - 2 mg/cm . The current collector for the anode electrode may be a platinized nfobium screen of fine mesh which makes good contact with the elec-trode surface. Alternatively, an expanded titanium screen coated with ruth-enium oxide, iridium oxide, transition metal oxide and mixtures thereof may als 3a be used as an anode collector structure. Yet another anode collector structure ~ 2451 52-EE-0-299 A. B. LaConti, et al may be in the form of a titanium-palladium plate with a platinum clad screen attached to the plate by welding or bonding.
The cathode is preferably a bonded mixture of Teflon particles and plati- , ~ 'num black with platinum black loading of 0.4 to 4 m~/cm2. The cathode electrode, 5 ¦jllke the anode, ~s bonded to and embedded in the surface of the cation membrane.
The cathode 1s made qu~te th1n, 2-3 m~ls or less, and preferably approximately 0.5 m11s, ~s porous and has a low Teflon content.
The th~ckness of the cathode can be qu~te signif1cant in 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 w1th th~n (approxlmately .5 to 2.a m~l) pt black - 15% Teflon bonded cathodes.
The current eff1c1enc1es of th1n cathode cells were approximately 80% at 5M
NaOH when operated at 88 - 91C with a 290g/L NaCl anode feed. W1th a 3.0 m~l ¦Ru - graph1te cathode, the current effic~ency was reduced to 54% at 5M NaOH.
15~ Table A shows the relationsh1p to CE to th~ckness, and ~nd~cates that thick- nesses not exceed~ng 2-3 m11s g~ve the best performance.
TABLE A
Cathode Current Eff~c1ency Cell Cathode Th1ckness (m11) % (M NaOH) 1 Pt Black 2 - 3 64 (4.0 M)
Generating gas by electrolyzing a chemical compound into it~ con~tituent elements, one of which may be a gas, is, of cour~e, an old and well known techni~ue. One recently developed form of ~uch gas evolving electrolyzer involves the u~e of a cell which utilizes an electrolyte in the form of a ~olid polymer, ion-exchanging membrane. In an an arrangement of this ~ort, catalytic electrode~ u~ing a ~uitable cat~lyst are positioned on oppo~ite ~ide~ of an ion tranQporting membrane medium such as a sulfonated perfluorocarbon ion-exchange membrane. Through an oxidation reaction, the ionic form of one of the con~tituent elements (hydrogen ion~, for example, when H20 or HCl is electrolyzed, or sodium ions _~ r,~
/l .
1~52451 52-EE-0-299 A. B. LaConti, et al ¦¦when an alkali metal halide such as sodium chloride is electrolyzed) is producedl ¦at one electrode. The ion is transported across the ion-exchangin~ membrane to ' the other electrode where it is reduced to form an electrolysis product such as Imolecular hydrogen, NaOH, etc. Solid polymer ion-exchange membranes electrolysis 5 l~units are part1cularly advantageous because they are efficient, small in size, ¦and do not utillze any corrosive liquid electrolytes.
I Yarious 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 at the gas evolving electrodes is obviously crucial in determining the effectiveness and efficiency of the cell, and con-~sequently of the economics of the process. The choice of a catalyst in an electrochem1cal cell and 1ts eff~ct~veness depends upon a complex set of var1-ables, such as surface area of a catalyst, availability of oxides of lts specieson the catalyst surface, contaminants in the reactants, and the nature of the convers10n taking place in the cell. Consequently, it ~s, and a1ways has been, dif~1cult to predict the applicab{l{ty of a catalyst useful in one electro-chemical cell to a dtfferent system. A commonly ass19ned Patent No. 3,99Z,271 entitled "Methods and Apparatus for Gas Generation" describes an improved oxygenevolvtng catalyt1c electrode util~zing a platlnum-tr~d~um alloy, a mixture whichwas ~ound to provide much improved performance and efficiency. Another commonlyassigned U-S- Patent No- 4,039,490 describes another oxygen evolving catalytic electrode whlch utillzes reduced ox~des of platinum-ruthenium. The platinum- j ruthenium catalyst not only is substantially less expensive than the reduced plat1num-iridium catalyst, because it uses a less expens{ve material such as ruthenium to alloy with the platinum, but it also turns out to be more eff kientbecause tt has a lower oxygen overvoltage than a platinum-1ridium electrode.
However, attempts to use reduced ruthenium oxide electrocatalysts for evolution of halogens by electrolysis of aqueous halide solutions have not been j entirely successful due to the harsh electrolysis conditions In the cell. There30 ¦can be substantial loss of catalyst from the membr~ne during chlorine evolution ¦
3L~L~24S1 52-EE-0-299 A. B. LaConti, et al ¦
since these reduced platinum metal oxides are susceptible to dissolution in acidic environments which are present in the electrolysis of hydrogen halides orin the electrolysis of alkali metal halide solutions which are of~en acidified.
Not only is there a tendency to d~ssolution of the platinum metals resulting in a loss of a catalytic mater~al, but the overvoltage of the electrodes also tendsto tncrease so that the efficiency of the cell decreases, and in many instances does not permit prolonged periods of operation.
It is, therefore, an object of the invention to provide a novel electro-catalytic material espec1ally useful for the electrolysis of aqueous solutions of hal~de ions and to a novel process for the preparation of said catalytic material.
Another ob~ect of the invention 1s to provide a novel membrane/electrode structure in which a sol1d polymer electrolyte membrane has a catalyt1c electrode including the said electrocatalytic material bonded to at least one side of the membrane.
An add~tional ob~ect of the 1nvent10n is to provide a novel, bonded elec-trode structure which includes the said electrocatalytlc mtter~al which is bonded w1th a polymerlc b1nter.
St111 another ob~ect o~ the 1nvention is to provide a novel electrolysis cell where~n the anode and cathode compartments are separated by a solid polymerelectrolyte membrane hav~ng a coat1ng of the novel electrocatalyt~c material bonded to at least one sur~ace of the membrane.
Other ob~ects and advantages of the invent10n will become apparent as the description thereof proceeds.
In accordance with the invention, the novel electrocatalyst comprises at least one reduced platinum group metal oxide which is subsequently treated ~n the presence of oxygen at a temperature high enough to stabilize the oxide thermally to increase the resistance of the catalyst against the corrosive electr~lysis conditions. The catalytic, reduced plàtinum group metal ox1de may optionally contain other reduced platinum group metal oxides such JS iridium and ~24S1 52-EE-0-299 A~s. LaConti, et al c ionally up to fifty (50) percent by weight of the electroconductive extende:s such as graphite, valve metal oxides, transition metal oxides, and nitrides, carbides 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 the other halogen production being thermally-stabilized reduced oxides of ruthenium. Reduced oxides of ruthenium are pre-ferred, because they are found to have extremely low chlorine over-voltages as well as having stability in the electrolysis 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 of thermally-stabilized reduced platinum group metal oxides are even more stable, as are thermally-stabilized reduced oxides of platinum group metal alloys. One such mixture of ruthenium oxide contains up to twenty-five (25) percent of iridium oxide, with the preferred range being five (5) to twenty-five (25) percent by weight, even though iridium is somewhat more expensive than ruth-enium alone.
Electroconductive extenders such as graphite have low over-voltages ~or halogens and are substantially less expensive than the platlnum metal ox~des and may readily ~e incorporated without reducing the effectiveness of the catalyst. In addition to graphite, oxides of a valve metal such as titanium, tantalum, niobium, tung~ten, 25 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 thereto are formed into an electrode by bonding with 30 fluorocarbon resin particles such as those sold by Dupont under i~s trademark Teflon. The catalytic particles and resin particles are mixed, placed in a mold and heated until the composition 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 electrode 35 structure and a unitary membrane/e~ectrode structure.
The novel 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 particularity in the app~nded 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 conjunction 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.
The novel electrocatalyst which includes thermally stabilized, reduced oxides of a platinum group metal alone or in c xbination with other platlnum group metalc or optionally valve metals may be prepared in any convenient fa~hion whereby an oxide catalyst is permanently, partially reduced and thermally stabilized.
The preferred manner of reduction ~s by a modification of the Adams method of platinum preparation by the addition of a thermally decomposable pla-tinum halide, such as ruthenium chloride, either alone or, if desired along with an appropriate quantity of other thermally decomposable halides of platinum metals or valve metals to an excess of sodium nitrate. m e Adams method of platinum preparation 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 con~enient to mix the finely divided halide salts of the platinum metals, suchas Ch~roplatinic acids in thecase of platinum, ruthenium chloride inthe case of ruthenium, titanium chloride, tantalum chloride, in the case of titanium ar.dtantalum inthesame weight ratio of the metals as desired in the final alloy ~l5Z45~
52-EE-0-299 A. B. LaConti, et al mixture. An excess of sodium nitrate is incorporated and the mixture is fused in a silica dish at 500 to 600C for three (3) hours. The residue is washed thoroughly to remove nitrates and halides still present, leaving a residue of the desired platinum metal ox~de, i.e., ruthenium oxide, plat~num-ruthenium ox1de, ruthen1um-1r~d1um oxide, ruthenium-titanium oxide, etc. The resulting suspens~on of mixed and alloyed oxides is then partially reduced. The reductio of the platinum group metal oxides may be effected by any convenient known reduc~ng method, such as an electrochemical reduction or by bubbling hydrogen through the mixture at room temperature as long as the oxides are not to be completely reduced to the free metal form. In a preferred embodiment, oxides are reduced by uslng an electrochemical reduction technique, ~.e., electrochemi cal retuction in an ac1d medium. The product which is now a reduced platinum metal ox1de, 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 to produce a f1ne powder of the reduced platinum metal oxide.
The resulting reduced plattnum metal ox1des are then stab11ized thermally by the heating in the presence of oxygen for a suffic1ent t1me to ensure a catalyt1c material wh kh 1s stable in an ac1d1c hydrogen halide env1ronment and 1n the pr~sence of halogens. In a manner to be described subsequently, thermal stab111zat1On of the catalyst results in a catalyst which has much better corros1On characterist1cs 1n halogens, such as chlorine, etc., and 1n the presence o~ halites solutlons such as hydrochloric, etc., acids. It 1s bel1eve that thermal stabilization results in the formation of a catalytic particle having a large mean pore diameter and stable thin ox1de f~lm on the outside of the reduced oxide particle. This stabil~zes the reduced oxide particles so tha they have better mechanical propert~es for bonding to the solid polymer electro lyte me~brane, and in thelr resistance characteristics to dissolution in hydro-chloric acid or other halide acid solutions or to the evolved halogens. Thus, preferabl~, the reduced oxides are heated at 350 to 750C from thirty (30) minutes to si~ (6) hours with the preferable thermal stabilization procedure ~2451 52-E-0-299 A. 8. LaConti, et al being accomplished ~y heating the reduced oxides for one ~l) hour at 550 to 600C.
I It has also been found that the electrocatalytic activity of the catalyst and of the electrode including the catalyst is opt~m~zed by prov~ding the S catalyttc part1cles 1n as fine a powder form as poss~ble~ Thus, it has been foùnd that the sur~ace area of the part1cles, as observed both by the BET nitro-gen absorption method, should be at least 25 meters square per gram of catalyst (M /g). The preferred range 1s 50 to 150M /9.
The gas permeable electrode structure of catalytic particles and fluoro-carbon polymer particles ~s produced by blending the catalytic particles with a Teflon dispers10n to produce a bonded electrode structure 1n the manner des-cribed in U.S. Patent No. 3,297,484 ass~gned to the assignee of the present 1nvention. In the process of bonding the electrode, ~t ~s desirable to blend the catalyst w~th Teflon d~spers10ns ~n such a manner that the dispersion con-tains little or no hydrocarbons. If the f1uorocarbon Teflon compos~t~on conta~n ihydrocarbon organ~c surface act~ve agents, it results 1n loss of surface area of the reduced ox1de catalyst. Any reduction on the surface area of the catalyst 1s obv10usly undes1rable, s1nce 1t has potent1ally deleter10us effect on the et~1c1ency and e~fect1vcness of the catalyst. Hence, fabr1cat10n of the electrode should be by the use of a Teflon polytetrafluoroethylene part1cle composition wh1ch contains few, ~f any, hydrocarbons. One suitable form of thespart1cles wh1ch may be ut~l~zed ~n fabrlcat1ng the electrode 1s sold by Dupont under its des1gnat10n Teflon T-30.
The m~xture of noble metal partlcles and Ten on particles or of graphite and the reduced oxide particles are placed in a mold and heated unt~l the compo-s~t10n is formed into a decal wh~ch is then bonded and embedded in the surface ¦ by the appltcat10n of pressure and heat. As described, for example, in U.S.
¦Patent No.3,2g7,484 above, the electrode structure is bonded to the surface of l the ~on-exchange membrane thus ~ntegrally bonding the gas absorb~ng part~cle 30 1 mixture and, in some instances, preferably embedding it into the surface of the membrane, 1~52451 52-EE-0-299 A. B. LaConti, et al The novel membrane/electrode structure thus fabricated comprises a solid polymer electrolyte membrane capable of selective ion transport having a thin, orous, gas permeable electrode of the above-described electrocata1ytic reduced latinum group metal oxides bonded to at least one side of the membrane. A
econd electrode may be bonded to the other side of the membrane and may include he same electroc~,talyttc material, or any other suitable cathodic material. Th~
elective ~on transporting membrane is preferably a stable, hydrated, cationic embrane which is characterized by ion transport selectivity. The cation ex-hange membrane allows passage of posit~vely charged cations such as hydrogen ons in the case of the electrolysis of a halide Such as hydrogren chloride or odium cations In the case of the electrolysis of aqueous alkal~ metal halides, nd thus min~mizes passage of negat1vely charged anions.
There are vartous types of ion exchange resins which may be fabricated int embranes to provide selective transport of the Cations. Two classes of such 15 Ires1ns are the so-called sulfonic acid cation exchange res~nS ant the carboxylic katton exchange resins. Sulfonic acid exchange restns, whiCh are the preferred type, include ion-exchange groups 1n the form of hydrated, sulfonic acid rad k al (S03H X H20) attached tO the polymer backbone by sulfonat~on. The ion exchangin ac~d rad1cals in the membrane are fixedly attached to the backbone of the polyme ensuring that the electrolyte concentration does not vary. As pointed out previ ously, perfluorocarbon sulfon1c acid cation membranes are preferred, One specif C
class Of cation polymer membranes in this category 1s sold by the Dupont Company m arff C under its trade dcsignet~on "Nafion". These "Nafion" membranes are hydrated, copolymers of polytetrafluoroethylene (PTFE) and polysulfonyl fluoride vinyl ether containing pendant sulfonic acid groups.
The ion-exchange capacity (IEC) of a given sulfonic cation exchange mem-brane is dependent upon the milliequivalent weight (MEW) of the 503 radical per I
~ram of dry polymer. The greater the concentration of the sulfontc acid radicals, the greater the ion-exchange capacity and hence the capability of the membrane tC
transport cations. However, as the ion-exchange capacity of the membrane 3L~5Z451 S2 EE-0-299 A. B. LaConti, et al increases, so does the water content and the ability of the membrane to reject alt decreases. Thus in electrolysis of alkali metal halide solutions, caustic is generated at the cathode side and the rate at which the sodium hydroxide mtgrates from the cathode to the anode side thus lncreases with IEC. Such back migrat10n reduces the cathod~c current efficiency (CE) and also results in oxygen generation at the anode whlch have undesirable consequences in its effecton the catalytic anode electrode. Consequently, the preferred lon-exchange membrane for use in brine electrolysis is a laminate consisting of a thin (2 milor so) film of fifteen hundred (1500) MEW, low water content (5-15%) cation exchange membrane which has high salt rejection, bonded to a 4 m11 or so film ofhigh ion-exchange capacity, 1100 MEW, bonded together w~th a Teflon cloth. One form of such a laminated construction sold by the Dùpont 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 re~ection are also available. Typical of such other laminates are Nafion 355, 376, 390, 227, 214. In the case of a laminated memb n ne bonded together bya Teflon cloth, tt may be des1rable to clean the membrane and Teflon cloth by refluxing It in seventy (70) percent HN03 for three to four (3 to 4) hours in add1~10n to soak~ng in caustic preferred to previously.
In the case of electrolysis hydrogen halides such as hydrochloric acid, there ls no problem of back migrat~on of caustic or other salts, so that simpler~orms of membranes such as Naf~on 120 may be util~zed as the 10n 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 slde layer is a thin, (2-4 mil~ chemically modif~ed film of sulfonamide I
groups or carboxylic acid groups.
Referring now to Figure l, the halogen electrolysis cell is shown generall at 10 and consists of a cathode compartment 11 and an anode c~mpartment 12 separated by a solid polymer electrolyte memberane 13 which is preferably a ll 3L~ 2451 52 EE-0-299 A. B LaCoAtl ~t al hydrated, permselective, cationic membrane. Bonded to opposite surfaces of membrane 13 are electrodes comprising particles of a fluorocarbon such as Teflonbonded to thermally stabilized, reduced oxides of ruthenium (RUOX) or iridium ¦(IrOx), or stabilized, reduced oxides of ruthenium-iridium (RuIr), ruthenium-5 ¦t1tan1um (RuTi), ruthenium-tantalum (RuTa), ruthenium-tantalum-iridium (RuTaIr), lor 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 prefer-¦ably embedded in one side of the membrane and a catalytic anode, not shown, is ¦bonded to and preferably embedded in the opposite side of the membrane. Current 10 Icollectors in the form of metallic screens 15 and 16 are pressed against theelectrodes. The whole membrane/electrode assembly is firmly supported between the housing elements 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 chlorineoxysen, aqueous sodium chloride in the case of brine electrolysis and HCI, 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 designat10n ~PDM.
An aqueous alkal~ metal hal1de such as brine or hydrogen halides such as HCl is 1ntroduced through an electrolyte inlet l9 whlch communicates with chamber 20. Spent electrolyte and halogens such as chlorine are removed throughan outlet condult 21. A cathode inlet conduit 22 is provided in the case of brine electrolysis and communicates with cathode chamber ll to perm1t the intro-duction of the catholyte, water, or aqueous NaOH (more dilute than that formed electrochemically at the electrode/electrolyte interface). In the case of electrolysis of hydrogen halides such as hydrogen chlorlde, no catholyte need beprovided and the cathode inlet conduit 22 may be dispensed with.
In a brine electrolysis cell, the water serves two separate functions. A
portion of the water is electrolyzed to produce hydroxyl (OH ) anions which combine with the sodium cations transported across the membrane to ~orm caustics~NaOH). The water also sweeps across the porous, bonded cathode electrode to i ~SZ4S1 52-tE-O-Z99 A. J LaConti, et a;
dilute the highly concentrated caustic formed at the membrane/electrode inter-face to minimize diffusion of the caustic back across the membrane into the anolyte chamber. Cathode outlet conduit 21 communicates with the cathode chambe~
ll to remove excess catholyte and the electrolys~s products such as caustic in the case of brine electrolysis, plus any hydrogen discharge at the cathode both 1n brtne electrolysis and in hydro~en chloride electrolysis. A power cable 24 is brought into the cathode chamber and a comparable cable not shown is brought intc the anode chamber. The cables connect the current conducting screens 15 and 16 r any other suitable kind of collector as source of electrical power.
Figure 2 illustrates diagrammatically the reactions taking place in the cell during the electrolysis of an aqueous alkali metal halide such as brlne and1s useful In understanding the eiectrolysis process tn the manner in which the ell funct10ns. Thus, an aqueous solution of sod1um chloride is brought into th~node compartment which is separated from the cathode compartment by the cation1 membrane 13. Membrane 13 fs a composite membrane comprising a h19h water conten(20-35% based on dry weight of membrane) layer 26, on the anode side and a low ¦water content high MEW cathode s~de layer, (5-15% based on dry we~ght of membran~) separated by a Teflon cloth 28. The cathode s1de barrier layer may also be chem1cally mod1~1ed on the cathode s1de to form a thin layer of a low water con-tent polymer. In one form this 1s achieYed by modifying the polymer to form asubstituted sulfonamide membrane layer. By converting the cathode s~de layer to a weak actd form (sulfonamide), the water content of thls portlon of the membran1s reduced and the salt re~ecting capability of She film is increased. As a result, dlffus~on of sodium hydroxide back across the membrane to the anode is minimized. While laminated membrane constructions are preferred in brine electrolysis to block migration of sodium hydroxide, other homogeneous films of low water content may be utilized, (viz. 7 Nafion 150, perfluorocarboxylates, etc )-Obviously, in the case of the eiectrolysis of hydrogen halides such as HCl, HBr,: etc., the ion transport~ng membrane may be a simple, homogeneous film such as the Na ff on 120 referred to previously.
ll 115Z451 52-EE-0-299 A. B. LaConti, et al The Teflon-bonded, reduced noble metal oxide ca~alysts contains at least one therma11y stabilized, reduced platinum metal oxide, such as ruthenium, ~ridium, or ruthenium-iridium with or without reduced oxides of t~tanium, i niobium, or tantalum and particles of graph~te are, as shown, pressed into the5 llsur~ace of membrane 13. 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 tenminals 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, isbrought into the anode chamber, is electrolyzed at anode 29 to produce chlorine las shown diagrammatically by the bubble formation 30. The chlorlne actually is¦princlpally evolved at the interface of the electrode and the membrane, but Ipasses 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 16 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 caustlc formed at the membrane/cathode inter-tace and thereby reduce d1ffusion of the caustic back across the membrane to the¦~node.
20ll A portion o~ the water catholyte is electrolyzed at the cathode to fo m hydroxyl ions and gaseous hydrogen. The hydroxyl ions combine with the sodium ions transported across the membrane to produce sodium hydroxide at the 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 251 aqueous stream sweeping across the surface of the electrode. Even with a cath-¦lode 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 j~migrate 6ack through membrane 13 to the anode. NaOH migration is a diffusion l process caused by the concentration gradien~ and electrochemical negative ion transport to the anode. Sodium hydroxide transported to the anode is oxidi~ed ~ ~2~Sl 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 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 anoltye 80 that back migrating hydroxide i8 converted to water rather than generating oxygen. The reactions in various portions of the cell for electrolysis of NaCl is as follows:
Anode Reaction: 2 Cl ' C12 P + 2e (1) (Principal) Membrane Transport: 2Na + H20 (2) Cathode Reaction: 2H20 -~ 20H + H2 r - 2e (3) 2Na + 20H ~ 2NaOH (4) Anode Reaction: 40H ~~ 2 + 2H20 + 4e (5) Overall 2NaCl + 2H20 ~ 2NaOH + C12 ~ + H2~ (6) (Principal) The reactions for electroly~is of a hydrogen halide, such as HCl, are very similar:
Anode Reaction: 2HC1 ~ 2H + C12 ~ + 2e (1) Membrane Transport: 2H+ (H20, HCl) (2) Cathode Reaction: 2H + 2e _ H2 ~ (3) Overall Reaction: 2HCl ~ H2 + C12 (4) ~524Sl 52-EE-0-299 A. B. LaConti, et al The novel arrangement for electrolyzing aqueous solutions of brine or of HCl which is described herein is characterized by the fact that the catalytic sites in the electrodes are in direct contact with the cation membrane and the ion exchanging ac1d radicals attached to the polymer backbone (whether these S rad1cals are the S03 H X H20 sulfonic radicals or the COOH X H20 carboxylic ac1d rad1cals). Consequently, there is no IR drop to speak of in the anolyte or the catholyte fluid chambers (this IR drop is usually referred to as "Electrolyte IR drop"). "Electrolyte IR drop~ ~s character~stic of existing systems and processes in which the electrode and the membrane are separated and ¦
can be 1n the order of 0.2 to 0.5 volts. The elim~nation or substantial re-duction of thts voltage drop is, of course, one of the principal advantages of this invention since ~t has an obv~ous and very s~gn~ficant effect on the over-¦all cell voltage and the economics of the process. Furthermore, because ¦chlorine is generated directly at the anode and membrane lnterface, there is no15 ¦IR drop due to the ss-catled "bubble effect" wh1ch is a gas blending and mass ¦transport loss due to the interruption or blockage of the electrolyte path ¦between the electrode and the membrane. As pointed out previously, 1n prior lart systems, the chlor~ne d1scharging catalytic electrode is separated from the¦membrane. The gas ls formed directly at the electrode and results in a gas 20 ¦ layer tn the space between the membrane and the electrode. This in effect brea~s up the electrolyte path between the electrode-collector and the membrane ~loc~tng passage o~ Na~ tons and there~y, in effect, increasing the IR drop.
In a pre~erred embodiment, the Teflon-bonded noble metal electrode con-tatns reduced oxtdes of ruthentuml irtdtum or ruthenium-trid~um in order to m~nimtze chlortne overvoltage at the anode. The reduced ruthenium oxtdes are stabtl~zed agatnst chlorine and oxygen evolution to produce an anode which is stable. Stabtlfzation ts effected tnitially by temperature stabilization; i.e.,by heating the reduced oxides of ruthenium for one hour at temperatures in the l range of 550 to -600C~ The Te~lon-bonded reduced oxides of ruthenium anode is furt~er stabilized by mixing it with graphite and/or alloying or mixing with 3L~L ~ Z 4 5 1 52-EE-0-299 A. B. LaConti, et al reduced oxides of iridium (Ir)Ox in the range of 5 to 25~ of iridium, with 25 being preferred, or with reduced oxides of 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)Ox or tantalum, ruthenium and ~r1dium ~Ru, Ir, Ta)Ox bonded with Teflon is very effective in producing a stab1e~ long-lived anode. In case of the ternary alloy, the composition is preferably 5% to 25% by weight of reduced oxides of iridium, approximately 50%
by weight reduced oxides of ruthenium, and the remainder a transition metal suchas titanium. For a binary alloy of reduced oxides of ruthenium and titanium, the preferred amount is ~0% by weight of titanium with the remainder ruthenium.
Titanium~ of course, has the addftional advantage of being much less expensive than either ruthenium or 1ridium, and thus is an effective extender which re-duces cost while at the same time stabilizing the electrode in an acid environ-ment and against HCl, chlor1ne and oxygen evolut~on. Other transition metals, such as niobium (Nb), tantalum (Ta), zirconfum (Zr) or hafnium (Hf) can readily be substituted for Ti fn the electrode structures. In addition to transit~on metals, transition metal carbides, nitrides and sul~ides may also be ut111zed ascatalyst extenders.
The alloys of the reduced noble metal oxides along with the reduced ox~des o~ t1tan1um or other trans1tion metals are blended with Teflon to form a homo-geneous m~x, The anode Teflon content may be 15 to 50% by weight of the Teflon,although 20 to 30Z by weight is preferred. The Teflon is of the type as sold by the DuPont Corporatfon under its designation T-30, although other fluoro-car~ons may be used wfth equal facflity. Typical noble metal, etc., loadings for the anode are 0.6 mg/cm2 of the electrode surface with the preferred range being 1 - 2 mg/cm . The current collector for the anode electrode may be a platinized nfobium screen of fine mesh which makes good contact with the elec-trode surface. Alternatively, an expanded titanium screen coated with ruth-enium oxide, iridium oxide, transition metal oxide and mixtures thereof may als 3a be used as an anode collector structure. Yet another anode collector structure ~ 2451 52-EE-0-299 A. B. LaConti, et al may be in the form of a titanium-palladium plate with a platinum clad screen attached to the plate by welding or bonding.
The cathode is preferably a bonded mixture of Teflon particles and plati- , ~ 'num black with platinum black loading of 0.4 to 4 m~/cm2. The cathode electrode, 5 ¦jllke the anode, ~s bonded to and embedded in the surface of the cation membrane.
The cathode 1s made qu~te th1n, 2-3 m~ls or less, and preferably approximately 0.5 m11s, ~s porous and has a low Teflon content.
The th~ckness of the cathode can be qu~te signif1cant in 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 w1th th~n (approxlmately .5 to 2.a m~l) pt black - 15% Teflon bonded cathodes.
The current eff1c1enc1es of th1n cathode cells were approximately 80% at 5M
NaOH when operated at 88 - 91C with a 290g/L NaCl anode feed. W1th a 3.0 m~l ¦Ru - graph1te cathode, the current effic~ency was reduced to 54% at 5M NaOH.
15~ Table A shows the relationsh1p to CE to th~ckness, and ~nd~cates that thick- nesses not exceed~ng 2-3 m11s g~ve the best performance.
TABLE A
Cathode Current Eff~c1ency Cell Cathode Th1ckness (m11) % (M NaOH) 1 Pt Black 2 - 3 64 (4.0 M)
2 Pt 81ack 2 - 3 73 (4.5 M)
3 Pt Black l - 2 75 (3.1 M)
4 Pt Black l - 2 82 (5 M) l 5 Pt Black 0.5 78 (5.5 M) 25 ¦ 6 5% Pt Black 3 78 (3.0 M) on Graphite 7 15X Ru O on 3 54 (5.0 M) GraphiteX
1~ 8 Platinized 10 - 15 57 (5 M) 30~l Graphite Cloth ~52451 52-EE-0-299 A. B. LaConti, et al The electrode is made gas pe~mea~le to allow gases evolved at the elec-trode/membrane interface to escape readily~ It is made porous to allow pene-! tration of the sweep water to the cathode electrode/membrane interface wherethe NaOH is formed and to allow brlne feedstock ready access to the membrane and the electrode catalytic sites. The former aids tn d~luting the highly concentrated NaO~ when in~tlally formed before the NaOH wets the Teflon and rlses to the electrode surface to be further dfluted by water sweeping across the electrode surface. It is important to dilute at the membrane interface here the NaOH concentration is the greatest. In order to maximize water pene-trat10n at the cathode, the Teflon content should not exceed 15~ to 30% weight,as Teflon 1s hydrophob1c. With good porosity, a llmited Teflon content, a thin cross-section, and a water or diluted caustic sweep, the NaOH concentrat10n is controlled to reduce migrat10n of NaOH across the membrane.
The current collector for the cathode must be carefully selected since the highly corrosive caustic present at the cathode attacks many matertals, especial-ly durtng shutdown. ~he current collector may take the form of a nlckel screen ~nce nlckel is res~stant to caustlc. Alternat~vely, the current collector may e constructed of a st~inless steel plate wlth a sta~nless steel screen welded o the plate. Another cathode current structure which is .esistant to or inert n the caustic solution ts graph1te or graphite in combination with a nickel creen pressed to the plate and aga~nst the surface of the electrode.
EXAMPLES
Cells incorporating ion exchange membranes having Teflon-bonded reduced noble metal oxide electrodes embedded ln the membrane were built and tested to llustrate the effect of various parameters on the effectiveness of the cell in brine electrolysis and to illustrate particularly the operating vottage characteristics of the cell.
Table I illustrates the effect on cell voltage of the various combinations of the reduced noble metal oxides. Cells were constructed with electrodes con-taining various specific combinations of reduced noble metal ox~des bonded to ~ ~s~s~
52-EE-0-299 A. B. LaConti, et al Teflon particles and embedded into a cationic membrane 6 mils thick. The cell was operated with a current density of 300 amperes per square foot at 90C, at feed rates of 200 to 2~00 CC per minute, with feed concentration of 5M.
One cell was constructed in accordance with the teachings of the prior art and conta1ned a d1mens10nally stab11~zed anode spaced from the membrane and a stainless steel cathode screen similarly spaced. Th~s control cell was operated under the same cond1tions.
It can readily be observed from this data that in the process of the 1nstant invent10n, the cell operating potentials are in the range of 2.9 - 3.6 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 1s real1zed. The operat1ng effictenc1es and econom1c benefits whlch result are lS cl~rly apparen~.
52-EE-0-299 A. B. LaConti, et al u~
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52-EE-0-299 A. B. LaConti, et al A ce11 similar to Cell No. 7 of Table I was constructed and operated at 90C in a saturated brine feed~ The cell potential (V) as a function of currentl density (ASF) was observed and is shown in Table II.
¦I TABLE II
Cell Voltage (V) Current Dens~ty (ASF~
3.2 400 2.9 300 2.7 200 I 2.4 lOO
lO ~ This data shows that cell operating potent1al is reduced as current density is reduced. Current dens1ty vs, cell voltage 1s, however, a trade-off bebween operating and cap1tal costs of a chlor1ne electrolys~s. It is s1gnif1-cant, however, that even at very h1gh current dens1t1es (300 and 400 ASF~.
s1gn1f1cant improvements (1n the order of a volt or more) in cell voltages are 16 ¦realized 1n the chtor1ne generating process of the 1nstant inventlon, Table III 111ustrates the e~ect o~ cathod1c current e~r1c~ency on oxygen evolut1cn, A cell having Teflon-bonded reduced noble metal ox1des catalyt1c ~nodes and c~thodes embedded ln a cation1c membrane were operated at 90C with a s~tur~t~d brtne concentr~tton, w1th a current dens~ty of 300 ASF and a feed rate ot 2~5 CC/M~n/ln2 of electrode aréa, The volume percent of oxygen in the chlor1ne was determined as a function of cathodic current eff~c1ency.
Cathod~c Current Oxygen Evolution Efficiency (%) (Yolume %) 89 2.2 86 4.0 84 5,8 8.9 Table IY illustrates the controlling effect that acidifying the br1ne has 1, on oxygen evolution. The volume percent of oxygen in the chlorine was measured for various concentration of HCl in the brine.
~1524S~
52-EE-0-29~ A. B. LaConti, et al TABLE IY
Acid (HCl) Oxygen Concentration (M) Volume 0.05 2.5 0.75 1.5 0.10 0.9 0.15 Q.5 0.25 0.4 It ~s clear from this data that oxygen evo1ut~on due to electrochemical ox~dat~on of the back m~grat1ng OH- is reduced by preferentially reacting the ¦OH chem1cally w~th H+ to form H20.
¦ A cell s~milar to Cell No. 1 of Table I was constructed and operated w1thla satur~ted NaCl feedstock ac1d~f~et w1th 0.2M HCl and at 300 ASF. The cell ¦voltage was méasured at var~ous operat~ng temperatures from 35 - 90C.
15 ¦ A cell similar to Cell No. 7 of Table I was constructed and operated with ¦2909/L (~ 5M)/L NaCl stock (not ac1t1f1ed) at 200 ASF. The cell voltage was measured at various operating temperatures ~rom 35 - 90C. The data was normal1zed for 300 ASF.
l ~ABLE Y
201 Cell No, 7 Voltage Normalized to 300 ASFTempe~ature Cell No. 1 Voltaqe (200 ASF Data) C
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 Th1s data shows that the 6est operattng volt-age ~s obtatned tn the 80 - ga range. It is to be noted, however, that even at 35C~ the ~oltage with the instant catalyst and electrolyzer 1s at least 0.5 volts better than prtcr art chlor~ne electrolyzers operating at 90C.
52-EE-0-299 A. B. LaConti, et al When the NaCl electrolysis is carried out in a cell in which both elec-trodes are bonded to the surface of an ion transporting membrane, the maximum improvement is achieved. However, improved process performance is achieved for !all structures in which at least one of the electrodes is bonded to the surface5 ~ of the ion transporting member (hybrid cell). The improvement in such a hybr~d structure is somewhat less than ~s the case w~th both electrodes bonded.
Nevertheless~ the ~mprovem~nt is quite signi~icant (0.3 - 0.5 volts better than the voltage requirements for known processes).
A number of cells were constructed and brine electrolysis carrled out to compare the results in a ~ully bonded cell (both electrodes) wlth the results ~n hybrid ce11 constructions (anode on7y bonded and cathode only bonded) and w1th the results a pr~or art non-bonded construction (neither electrode bonded).IA11 of the cells were constructed w1th membranes of Naf~on 315, the cell was Ioperated at 90C with a brine feedstock of approximately 290g/L, The bonded electrode catalyst load1ngs were 2g/ft2 at the cathode for Pt Black and 49/ft2 at the anode for RuOx-graphtte and RuOx. The current effic1ency at 300 A~F was ¦
essent1ally the same for all cells (84-85% ~or 5M NaOH). Table ~I shows the cell voltage character1stics for the var~ous cells:
TABLE YI
Cell Voltage (Y) Cell Anode Cathode at 300 ASF
1 Ru-Graph1te Pt Black 2.9 (Bonded) (Bonded) 2 Platin~zed Niobium Pt Black 3.5 Screen (Not Bonded) (Bonded) 3 Platinized Niob~um Pt. Black 3,4 Screen (Not Bonded) (Bonded) 4 Ru~Graphite Ni Screen 3.5 I (Bonded) (Not Bonded) Ru x Ni Screen 3.3 (Bonded) (Not Bonded) 6 Platinized Niobium Ni Screen 3.8 Screen (Not Bonded) (Not Bonded)
1~ 8 Platinized 10 - 15 57 (5 M) 30~l Graphite Cloth ~52451 52-EE-0-299 A. B. LaConti, et al The electrode is made gas pe~mea~le to allow gases evolved at the elec-trode/membrane interface to escape readily~ It is made porous to allow pene-! tration of the sweep water to the cathode electrode/membrane interface wherethe NaOH is formed and to allow brlne feedstock ready access to the membrane and the electrode catalytic sites. The former aids tn d~luting the highly concentrated NaO~ when in~tlally formed before the NaOH wets the Teflon and rlses to the electrode surface to be further dfluted by water sweeping across the electrode surface. It is important to dilute at the membrane interface here the NaOH concentration is the greatest. In order to maximize water pene-trat10n at the cathode, the Teflon content should not exceed 15~ to 30% weight,as Teflon 1s hydrophob1c. With good porosity, a llmited Teflon content, a thin cross-section, and a water or diluted caustic sweep, the NaOH concentrat10n is controlled to reduce migrat10n of NaOH across the membrane.
The current collector for the cathode must be carefully selected since the highly corrosive caustic present at the cathode attacks many matertals, especial-ly durtng shutdown. ~he current collector may take the form of a nlckel screen ~nce nlckel is res~stant to caustlc. Alternat~vely, the current collector may e constructed of a st~inless steel plate wlth a sta~nless steel screen welded o the plate. Another cathode current structure which is .esistant to or inert n the caustic solution ts graph1te or graphite in combination with a nickel creen pressed to the plate and aga~nst the surface of the electrode.
EXAMPLES
Cells incorporating ion exchange membranes having Teflon-bonded reduced noble metal oxide electrodes embedded ln the membrane were built and tested to llustrate the effect of various parameters on the effectiveness of the cell in brine electrolysis and to illustrate particularly the operating vottage characteristics of the cell.
Table I illustrates the effect on cell voltage of the various combinations of the reduced noble metal oxides. Cells were constructed with electrodes con-taining various specific combinations of reduced noble metal ox~des bonded to ~ ~s~s~
52-EE-0-299 A. B. LaConti, et al Teflon particles and embedded into a cationic membrane 6 mils thick. The cell was operated with a current density of 300 amperes per square foot at 90C, at feed rates of 200 to 2~00 CC per minute, with feed concentration of 5M.
One cell was constructed in accordance with the teachings of the prior art and conta1ned a d1mens10nally stab11~zed anode spaced from the membrane and a stainless steel cathode screen similarly spaced. Th~s control cell was operated under the same cond1tions.
It can readily be observed from this data that in the process of the 1nstant invent10n, the cell operating potentials are in the range of 2.9 - 3.6 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 1s real1zed. The operat1ng effictenc1es and econom1c benefits whlch result are lS cl~rly apparen~.
52-EE-0-299 A. B. LaConti, et al u~
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52-EE-0-299 A. B. LaConti, et al A ce11 similar to Cell No. 7 of Table I was constructed and operated at 90C in a saturated brine feed~ The cell potential (V) as a function of currentl density (ASF) was observed and is shown in Table II.
¦I TABLE II
Cell Voltage (V) Current Dens~ty (ASF~
3.2 400 2.9 300 2.7 200 I 2.4 lOO
lO ~ This data shows that cell operating potent1al is reduced as current density is reduced. Current dens1ty vs, cell voltage 1s, however, a trade-off bebween operating and cap1tal costs of a chlor1ne electrolys~s. It is s1gnif1-cant, however, that even at very h1gh current dens1t1es (300 and 400 ASF~.
s1gn1f1cant improvements (1n the order of a volt or more) in cell voltages are 16 ¦realized 1n the chtor1ne generating process of the 1nstant inventlon, Table III 111ustrates the e~ect o~ cathod1c current e~r1c~ency on oxygen evolut1cn, A cell having Teflon-bonded reduced noble metal ox1des catalyt1c ~nodes and c~thodes embedded ln a cation1c membrane were operated at 90C with a s~tur~t~d brtne concentr~tton, w1th a current dens~ty of 300 ASF and a feed rate ot 2~5 CC/M~n/ln2 of electrode aréa, The volume percent of oxygen in the chlor1ne was determined as a function of cathodic current eff~c1ency.
Cathod~c Current Oxygen Evolution Efficiency (%) (Yolume %) 89 2.2 86 4.0 84 5,8 8.9 Table IY illustrates the controlling effect that acidifying the br1ne has 1, on oxygen evolution. The volume percent of oxygen in the chlorine was measured for various concentration of HCl in the brine.
~1524S~
52-EE-0-29~ A. B. LaConti, et al TABLE IY
Acid (HCl) Oxygen Concentration (M) Volume 0.05 2.5 0.75 1.5 0.10 0.9 0.15 Q.5 0.25 0.4 It ~s clear from this data that oxygen evo1ut~on due to electrochemical ox~dat~on of the back m~grat1ng OH- is reduced by preferentially reacting the ¦OH chem1cally w~th H+ to form H20.
¦ A cell s~milar to Cell No. 1 of Table I was constructed and operated w1thla satur~ted NaCl feedstock ac1d~f~et w1th 0.2M HCl and at 300 ASF. The cell ¦voltage was méasured at var~ous operat~ng temperatures from 35 - 90C.
15 ¦ A cell similar to Cell No. 7 of Table I was constructed and operated with ¦2909/L (~ 5M)/L NaCl stock (not ac1t1f1ed) at 200 ASF. The cell voltage was measured at various operating temperatures ~rom 35 - 90C. The data was normal1zed for 300 ASF.
l ~ABLE Y
201 Cell No, 7 Voltage Normalized to 300 ASFTempe~ature Cell No. 1 Voltaqe (200 ASF Data) C
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 Th1s data shows that the 6est operattng volt-age ~s obtatned tn the 80 - ga range. It is to be noted, however, that even at 35C~ the ~oltage with the instant catalyst and electrolyzer 1s at least 0.5 volts better than prtcr art chlor~ne electrolyzers operating at 90C.
52-EE-0-299 A. B. LaConti, et al When the NaCl electrolysis is carried out in a cell in which both elec-trodes are bonded to the surface of an ion transporting membrane, the maximum improvement is achieved. However, improved process performance is achieved for !all structures in which at least one of the electrodes is bonded to the surface5 ~ of the ion transporting member (hybrid cell). The improvement in such a hybr~d structure is somewhat less than ~s the case w~th both electrodes bonded.
Nevertheless~ the ~mprovem~nt is quite signi~icant (0.3 - 0.5 volts better than the voltage requirements for known processes).
A number of cells were constructed and brine electrolysis carrled out to compare the results in a ~ully bonded cell (both electrodes) wlth the results ~n hybrid ce11 constructions (anode on7y bonded and cathode only bonded) and w1th the results a pr~or art non-bonded construction (neither electrode bonded).IA11 of the cells were constructed w1th membranes of Naf~on 315, the cell was Ioperated at 90C with a brine feedstock of approximately 290g/L, The bonded electrode catalyst load1ngs were 2g/ft2 at the cathode for Pt Black and 49/ft2 at the anode for RuOx-graphtte and RuOx. The current effic1ency at 300 A~F was ¦
essent1ally the same for all cells (84-85% ~or 5M NaOH). Table ~I shows the cell voltage character1stics for the var~ous cells:
TABLE YI
Cell Voltage (Y) Cell Anode Cathode at 300 ASF
1 Ru-Graph1te Pt Black 2.9 (Bonded) (Bonded) 2 Platin~zed Niobium Pt Black 3.5 Screen (Not Bonded) (Bonded) 3 Platinized Niob~um Pt. Black 3,4 Screen (Not Bonded) (Bonded) 4 Ru~Graphite Ni Screen 3.5 I (Bonded) (Not Bonded) Ru x Ni Screen 3.3 (Bonded) (Not Bonded) 6 Platinized Niobium Ni Screen 3.8 Screen (Not Bonded) (Not Bonded)
5 Z 4 5 1 52-EE-0-299 A. B. LaConti, et al It can be seen that the cell voltage of the fully Teflon-bonded cell No. 1 ¦is almost a volt better than the voltage for the prior art, completely non-Teflon bonded, control cell No. 6. Hybrid cathode bonded cells 2 and 3 and llhybrid anode bonded cells 4 and 5 are approx~mately 0.4 - 0.6 volts worse than5 l¦the fully Teflon-~onded cell but st~ll 0.3 - 0.5 volts better than the prior art processes whlch are carr~ed out in a cell w~thout any Teflon bonded electrodes.
¦ It w~ll be apprec~ated that a vastly superior process for generating ¦chlor~ne ant other hal1des from br~ne and, as will be shown hereafter, from ¦HC1 and other halides, has been made possible by reacting the anolyte and the lO icatholyte at catalyt~c electrodes bonded directly to and embedded in the cat1On1c membrane. By v~rtue of th1s arrangement, the catalyt~c s1tes 1n the electrodes are 1n d1rect contact with the membrane and the ac1d exchang1ng rat~cals in the membrane result~ng 1n a much more voltage effic~ent process ln wh~ch the reguired cell potential is s~gn1f kantly better (up to a volt or more)15 ¦ than known processes, The use of h~ghly effect~ve fluorocarbon bonded thenmally stab11~zed, reduced noble metal ox1de catdlysts, as well as fluorocarbon graphite-reduced noble metal ox~de catalysts w~th low overvoltages, further enh~nce the eff1clency of the process.
EXAMPLES
Electrodes contatning thermally stab111zed, reduced noble metal ox~des, etc., embedded 1n ~on-exchange membranes were bu11t and tested to ~llustrate the effect o~ various parameters on the effect~veness of the cell and catalyst In the electrolys1s of hydrochlor~c ac1d.
Ta~l~ VII ~llustrates the Effect on Cell Voltage of var~ous comb~nat1Ons of reduced noble metal oxtdes~ Cells were conetructed w~th Teflon-bondet, Igraphtte electrodes contatn~ng var~ou~ specif1c comblnations of thermally ¦¦stabtlized, reduced platinum metal ox~des and reduced oxtdes of titanium em-bedded into a bydrated cationic membrane, 12 mils thick. The cell was operated with a current dens~t~ of 400 amps per Square, at 3QC, at a feed rate of 70 cc per minutes, ~0~05 ft active cell area) with feed normalîties of 9 - llN.
~Z451 52-EE-0-299 A. B. LaConti, et al Tables VIrI and IX illustrate the effect of time for the same cells and under the same conditions, on cell operating voltages~
Table X shows the ef~ect of acid feed concentration ranging from 7.5 - 10.5N, A cell, like cell No. 5 in Ta~le II, was constructed with reduced (Ru, 25% Ir)0x noble metals added to the Teflon-bonded graphite electrode. The cell was oper~ted at f~xed feed rate o~ 150 cc/min, (0.05 ft2 actlve cell area~
-24- \
~2451 ~2-EE-0=299 A. B. LaConti, et al I
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~2451 52-EE-0-299 A. B. LaConti, et al ¦ TABLE Vlll Cell Cetl Current Voltage ~V) Yoltage (Y) Dens~ty , at 100 Hrs. At Operat1ng Amperes Cell Operat1ng T~me From Per Square No. T~me Table I _ Foot (ASF) 1 1.85 2.10 400 2 1.84 2.0~ 400 3 1.78 1.97 400 4 1.80 1.91 400 1.75 2.07* 400 (1.9)
¦ It w~ll be apprec~ated that a vastly superior process for generating ¦chlor~ne ant other hal1des from br~ne and, as will be shown hereafter, from ¦HC1 and other halides, has been made possible by reacting the anolyte and the lO icatholyte at catalyt~c electrodes bonded directly to and embedded in the cat1On1c membrane. By v~rtue of th1s arrangement, the catalyt~c s1tes 1n the electrodes are 1n d1rect contact with the membrane and the ac1d exchang1ng rat~cals in the membrane result~ng 1n a much more voltage effic~ent process ln wh~ch the reguired cell potential is s~gn1f kantly better (up to a volt or more)15 ¦ than known processes, The use of h~ghly effect~ve fluorocarbon bonded thenmally stab11~zed, reduced noble metal ox1de catdlysts, as well as fluorocarbon graphite-reduced noble metal ox~de catalysts w~th low overvoltages, further enh~nce the eff1clency of the process.
EXAMPLES
Electrodes contatning thermally stab111zed, reduced noble metal ox~des, etc., embedded 1n ~on-exchange membranes were bu11t and tested to ~llustrate the effect o~ various parameters on the effect~veness of the cell and catalyst In the electrolys1s of hydrochlor~c ac1d.
Ta~l~ VII ~llustrates the Effect on Cell Voltage of var~ous comb~nat1Ons of reduced noble metal oxtdes~ Cells were conetructed w~th Teflon-bondet, Igraphtte electrodes contatn~ng var~ou~ specif1c comblnations of thermally ¦¦stabtlized, reduced platinum metal ox~des and reduced oxtdes of titanium em-bedded into a bydrated cationic membrane, 12 mils thick. The cell was operated with a current dens~t~ of 400 amps per Square, at 3QC, at a feed rate of 70 cc per minutes, ~0~05 ft active cell area) with feed normalîties of 9 - llN.
~Z451 52-EE-0-299 A. B. LaConti, et al Tables VIrI and IX illustrate the effect of time for the same cells and under the same conditions, on cell operating voltages~
Table X shows the ef~ect of acid feed concentration ranging from 7.5 - 10.5N, A cell, like cell No. 5 in Ta~le II, was constructed with reduced (Ru, 25% Ir)0x noble metals added to the Teflon-bonded graphite electrode. The cell was oper~ted at f~xed feed rate o~ 150 cc/min, (0.05 ft2 actlve cell area~
-24- \
~2451 ~2-EE-0=299 A. B. LaConti, et al I
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~2451 52-EE-0-299 A. B. LaConti, et al ¦ TABLE Vlll Cell Cetl Current Voltage ~V) Yoltage (Y) Dens~ty , at 100 Hrs. At Operat1ng Amperes Cell Operat1ng T~me From Per Square No. T~me Table I _ Foot (ASF) 1 1.85 2.10 400 2 1.84 2.0~ 400 3 1.78 1.97 400 4 1.80 1.91 400 1.75 2.07* 400 (1.9)
6 1~70 . 1.80 400 * See note for Table U~I
~1~2451 52-EE-0-299 A. B. LaConti, et al TABLE IX
,, Current Intermediate Density Ce11 Operating Amperes/Sq. Cell No. Time - (Hrs.) Ft. (ASf) Voltages (V) 1 3900 lOO 1.70 300 2.00 1- .
2 3400 100 1.57 200 1.70 300 1.83 3 1900 100 1.58 200 1.70 300 1.81 .
4 1000 lOOO 1.47 2000 1.60 300 1.72 1200 100 1.32 200 1.45 300 1.55 ~1~2451 ¦ 52-EE-0-299 A. 8~ LaConti, et al I
TA~LE X
¦ Feed Normal~ty Vollune %
l (eQ/L) 2 I
~1~2451 52-EE-0-299 A. B. LaConti, et al TABLE IX
,, Current Intermediate Density Ce11 Operating Amperes/Sq. Cell No. Time - (Hrs.) Ft. (ASf) Voltages (V) 1 3900 lOO 1.70 300 2.00 1- .
2 3400 100 1.57 200 1.70 300 1.83 3 1900 100 1.58 200 1.70 300 1.81 .
4 1000 lOOO 1.47 2000 1.60 300 1.72 1200 100 1.32 200 1.45 300 1.55 ~1~2451 ¦ 52-EE-0-299 A. 8~ LaConti, et al I
TA~LE X
¦ Feed Normal~ty Vollune %
l (eQ/L) 2 I
7.5 0.15
8 0.04 , 8.5 0.015 0.007 10.5 0'004 11.5 0,003 ~ 2451 52-EE-0-299 A. B. LaCont~, et al From the above examples, it will be clear that HCl is electrolyzed to produce chlorine gas, substantially free of oxygen. The catalyst used in the electrolyzer cell is characterized by low cell voltage and low temperature (~ 30C) operation resulting in economical operation of such electrolyzer cells.Furthermore, th~s data shows excellent performance at varlous current densities,partlcularly at 300 - 400 ASF. Thls has a posit~ve and benefic1al effect on cap1tal costs for chlor1ne electrolyzers embodying the ~nstant inventlon.
To show the effect of thermal stab~lizat~on on reduced noble metal and trans~t~on metal ox~des, certa1n tests were carried out. These tests show the 1mpact on the res1stance of the catalyst to harsh electrolysis environments.
thermally stabil~zed as well as non-stab~l~zed, reduced oxide catalysts were exposed to h~ghly concentrated HCl solut~ons wh~ch represent extremely harsh env~ronmental cond1t10ns. The color of the solut~on was observed since darken-1ng of the solut10n 1nd~cated loss of catalyst. Increasing loss of catalyst was accompan1ed by more pronounced color changes.
Table XI shows the results of these corros~on res~stance and stabil~ty ~c~ f ~ e ~n~S r~glr~ from .5 to 20 gms 52-EE-0-299 A. B. LaConti, et al Ij c o o o 0 0,~, ,o ,o O o. I
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52-EE-0-299 A. B. LaConti, et al It will be ovbious from this data that thermal stabilization of the re-reduced oxides enhances the corrosion resistance of the catalyst in very con-~centrated HCl and, ln fact, provides very good stability. It is obvious that resistance of the cat~lysts in the much less corrosive chlorine or brine en-S vironments is excellent and attributable to thermal stabilizatibn of the re-ducet oxlde catalyst.
Havtng observed the ~mproved corrosion characteristics of the thermally stabilized, reduced, platinum ~roup metal oxides, physical and chemical tests were conducted to determine the effect of thermal stabilization on various tO characteristics of the catalysts which might account for the improved corrosion characteristics. The ox~de content, surface area ~n M2/~ of catalyst, the pore volume and pore slze distribution of the catalyst were measured after fabrica-t10n o~ the catalyst by the mod1~1ed Adams method; after reduct10n of the catalyst; and after thermal stabtlization of the reduced catalyst. The result f these tests, wh1ch will be set forth in deta~l below, show that the surface rea of the catalyst 1s reduced somewhat after the catalyst is reduced, and u1te substantially after thermal stabilization. A drop in oxide content after the reduction step 1s bel1eved to account for part of the decrease of the surface area, A substantlal change in the internal pore size d1str1but10n of ~0 the c~talyst after thermal stab111zat10n w1thout a correspond1ng change in the ore volume ls bel1eved responsible for the very substantial decrease (ratio of to 1) in surface area accompanying thermal stab11~zation and would account for he tmproved corrosion character1st1cs as corros10n 1s d1rectly related to the rea exposed to attack by any corros~ve agents.
Initially, Sample #1, a ruthenium - 25Z by weight ir1dium catalyst was repared by the mod~fied Adams method. A portion of this catalyst was reduced lectrochemically to form Sample #2. A reduced (Ru 25 Ir)0x sample was thermally tabillzed for one (l) hour at 550 - 600C. The surface area of the unreduced (Sample #l) catalyst, the reduced catalyst (Sample #2) and the thenmally stabi-ize reduced (Ru 25 Ir)Ox catalyst (Sample d3), a5 measured by the three point l I
4 53l 52-EE-0-299 A. B. LaConti, et al BET (BRUNAUER-EMMET-TELLER) nitrogen adsorption method, is shown in Table XII.
TABLE XII
Catalyst (Ru 25 Ir) Treatment Surface Area Sample #1 None 127.6 M2/g Samp1e ~2 Reduced 123.5 M2/g Sample #3 Reduced and Thermal 62.3 M2/g Stab~liza~ion -550 - 600 C; One (1) 1 a Hour The ox1de content of Samples #i, #2, and #3 was then measured as well as that of a Ssmple (#4) thermally stab111zed at 700 - 750C for one ~1) hour. In add1t10n the ox1de content of Pt Ir catalysts conta~n~ng respect1vely 5 and 50%
by we1ght 1f Ir1d~um was measured. The results are shown 1n Table XII~.
TABLE XIII
Catalyst (Ru 25 Ir) Treatment X Ox~de Content Sample #1 None 24.4 Sample ~2 Reduct10n 24.3 Sample ~3 Thermal S~ab~11zat10n; 22.6 550 - 600 C - One (1) Hour Sample #4 Reduct10n ant ThermHl 21.5 Stab~11za~on;
700 - 750 C - One (1) Hour SPmtp50I#) None 16.5 Sample #6 Reduct10n 15.2 Sample #7 Reduction and Thermal 13.0 Stab~112a~ion;
550 ~ 600 C - One (1) Hour 5~451 52-EE-0-299 A. B. LaConti, et al 1~ the data from Table XIII shows a decrease in oxide content with reduction ¦,and thenmal stabilization just as surface area decreases after reduction and thenmal stabiliZation.
~! Decrease o~ oxide content (l.e., unreduced catalyst) w111 have a corre-5 ~spondtng e~ect on sur~ace area s~nce the surface area of ox~des is normally ¦9reater than that o~ the non-ox~de form. This reductton ~n ox~de content in Ipart explains surface area reduction but does not wholly explain the dramatic ¦reduction 1n surface area after thermal stab111zat~on.
I The poros~ty of the catalyst was therefore measured to determine whether 10 Ithermal stab~lizat~on of the catalyst causes a change in porosity thereby de-creas1ng the surface area and ~ncreaslng ~ts corros~on res~stance Catalyst samples were taken ~rom the same batches as Samples #1, #2 and #3 and the poros1ty o~ the samples and partlcle S~Ze d1strlbutlon measured. Part~cle s ke d~strtbut~on was measured by a sedimentat~on method and showed that the equ1va-lent spherlcal d1ameter at 50% maSs d~str~but~on was 3.7 m~crons (~) a~terreduct10n of the catalyst and 3.1 m~crons (~) after thermal stab111zat~on. Th1s 1nd1cates that the external sur~ace of the partte1es ~S reduced but aga1n does not account ~or all o~ the surface area reduct10n a~ter stab11 kat10n, Total pore volume data (CC/g) was obta~ned by cap111ary condensat~on and 20 mercury 1ntrus10n methods. Data for Samples #1, #2 and #3 and ~s shown 1n Table XIY.
TABLE XIY
Catalyst Total Pore Yolume (Ru ^ 25 Tr) Treatment Range cc/g 25 ! Sample #1 None 40A - 10~0.80 cc/gm Sample #2 Reduct~on ~0,72 cc/gm Sample #3 Reduction and " 0,76 cc/gm Thermal Stab11i~a-tlon; 500 - 600 C -One (l) Hour I _33_ ~ 24S~ I ' 52-EE-0-299 A. B. LaConti, et a1 The data shows that the total pore volume is relatively unchanged. Thus the porosity, in tenms of gms/cc or if converted to void volume (knowing the spherical s~ze and density), is essentially the same and is in the range of 0.7 - 0.8 gms/cc for Ru - 25 Ir.
S1multaneously, the pore s ke dlstr~bution was measured to obtain the pore d1ameter dtstr1but~ons ~n the 40A - 10 micron range. Capillary condensa-tton was uset ~n the 40 - 500A range. ~n th~s method llquid condensation for a g~ven vapor pressure ~s measured to obtafn pore size distribution. The capillar condensat~on method has a lower resolution lfmft of 40A and an upper limit of 500A. For pores ~n excess of 500A (i.e., 500A - lO~) a mercury intrusion method 1s ut111zed to obtaln pore s1ce d1str~butlon. Pore dlameter distribut1On measurements ~or the two ranges 1s shown 1n Tables XV and XVI respect~vely.
TABLE XV
Catalyst Treatment S~ze Range Pore D1ameter D1str1but1On Sample #1 None 40 - 500A Pore D1str1but~on below 40A
Sample #2 Reduct~on " Pore dtam8ter dlstr~button below 40A
Sample ~3 Reductton and Thermal " D1strtbut1On In the range l Stabtl1z~10n; of 100 - 300A w1th max1mum 20l 550 - 600 C; One (1) at 200A
Hour TABLE XVI
Pore D1ameter D1str1but1On Catalyst Treatment S~ze Range Sample #l None 500A-lOmicrons 0.46u Sample #2 Reduct~on " 0.82 Sample ~3 Reduct1On and Thermal " 1.5 Stabil1za~ton;
500 - 600 C; One (1) Hour ~15~45~
- 52-EE-0-29~ A. B. LaConti, et al This data indicates that thermal stabilization of the catalyst resu1ts in a change in pore size diameter. This change seems to be accompanied by a changein the number of pores so that the overall surface area decreases. With the pore volume staying substantially constant and the pore diameter distribution changing so that the dlstribution shows a max~mum at 200A, and 1.5~ it seems qulte clear that many of the pores below 40A coalesce, reducing the overall number of pores. Above 500A the pore diameter increases. The pore size dia-meter and internal pore surface area thus changes with thermal stabilization.
In summary, the Internal porosity and thus the pore surface area is reduced as the catalyst is thermally stabilized. It ~s believed that th;s is a result of changing the morphology to provide a smaller number of pores with a larger pore d1ameter dtstribut~on. With the pore volume relatively unchanged and a change ipore diameter distr1button to a larger pore diameter, it seems clear that the surface area reduction assoc~ated w~th thermal stabilization of the catalyst is attributable to the change in internal pore surface area.
The porosity (in terms of pore volume (cc) per un1t weight (9) of catalyst of the thermally stabilized, reduced platinum metal oxide catalyst lies in the 0.4 - 1.5 cc/gm range w1th the preferred poros~ty ~or a thermally stabilized u - 25 Ir catalyst belng 0.7 - 0.8 cc/gm.
The pore d1ameter distr1but1On of the thermally stabilized catalyst shows the pr1nc1pal d1str1but1On below SOOA in the 100 - 300A range, with a maximum at 200A. Above 500A the pore diameter distribution shows the greatest pore volume and hence the princ1pal d1stribution is in .04 - 9 ~ range with a max1mumat 1.5~'s (representing the 50~ point in distribution).
The catalyst surface area range for thermally stabilized, reduced platinum etal oxide catalysts shou1d be such that for any given platinum metal, or combination of platinum metals with or without transition metals, etc., the sur-face area should be as low as possible (to reduce corrosionl fcr the required catalytic activity. Thus, the surface area should be in excess of lOM2/gm ranges from 24 - 165M2/g, with a preferred range being from 24 - 165M2/9 and fr~
~ ~`Z45~
60 - 70 M /g for a thermally stabilized Ru - 25 Ir catalyst.
The reduced, thermally stabilized platinum group metal oxide catalysts are, as may be seen, large surface area catalysts as compared to powders, blacks, etc., which normally have surface areas around 10 - 15M /g.
The oxide content of the catalyst has a range of 13 - 24% by weight and a preferred range of 13 to 23 by weight.
While the instant invention has been shown in connection with a preferred embodiment thereof, the invention is by no means limited thereto, since other modifications of the instrumentalities, material and articles employed may be made and fall within the true scope and spirit of this invention.
. . .
To show the effect of thermal stab~lizat~on on reduced noble metal and trans~t~on metal ox~des, certa1n tests were carried out. These tests show the 1mpact on the res1stance of the catalyst to harsh electrolysis environments.
thermally stabil~zed as well as non-stab~l~zed, reduced oxide catalysts were exposed to h~ghly concentrated HCl solut~ons wh~ch represent extremely harsh env~ronmental cond1t10ns. The color of the solut~on was observed since darken-1ng of the solut10n 1nd~cated loss of catalyst. Increasing loss of catalyst was accompan1ed by more pronounced color changes.
Table XI shows the results of these corros~on res~stance and stabil~ty ~c~ f ~ e ~n~S r~glr~ from .5 to 20 gms 52-EE-0-299 A. B. LaConti, et al Ij c o o o 0 0,~, ,o ,o O o. I
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52-EE-0-299 A. B. LaConti, et al It will be ovbious from this data that thermal stabilization of the re-reduced oxides enhances the corrosion resistance of the catalyst in very con-~centrated HCl and, ln fact, provides very good stability. It is obvious that resistance of the cat~lysts in the much less corrosive chlorine or brine en-S vironments is excellent and attributable to thermal stabilizatibn of the re-ducet oxlde catalyst.
Havtng observed the ~mproved corrosion characteristics of the thermally stabilized, reduced, platinum ~roup metal oxides, physical and chemical tests were conducted to determine the effect of thermal stabilization on various tO characteristics of the catalysts which might account for the improved corrosion characteristics. The ox~de content, surface area ~n M2/~ of catalyst, the pore volume and pore slze distribution of the catalyst were measured after fabrica-t10n o~ the catalyst by the mod1~1ed Adams method; after reduct10n of the catalyst; and after thermal stabtlization of the reduced catalyst. The result f these tests, wh1ch will be set forth in deta~l below, show that the surface rea of the catalyst 1s reduced somewhat after the catalyst is reduced, and u1te substantially after thermal stabilization. A drop in oxide content after the reduction step 1s bel1eved to account for part of the decrease of the surface area, A substantlal change in the internal pore size d1str1but10n of ~0 the c~talyst after thermal stab111zat10n w1thout a correspond1ng change in the ore volume ls bel1eved responsible for the very substantial decrease (ratio of to 1) in surface area accompanying thermal stab11~zation and would account for he tmproved corrosion character1st1cs as corros10n 1s d1rectly related to the rea exposed to attack by any corros~ve agents.
Initially, Sample #1, a ruthenium - 25Z by weight ir1dium catalyst was repared by the mod~fied Adams method. A portion of this catalyst was reduced lectrochemically to form Sample #2. A reduced (Ru 25 Ir)0x sample was thermally tabillzed for one (l) hour at 550 - 600C. The surface area of the unreduced (Sample #l) catalyst, the reduced catalyst (Sample #2) and the thenmally stabi-ize reduced (Ru 25 Ir)Ox catalyst (Sample d3), a5 measured by the three point l I
4 53l 52-EE-0-299 A. B. LaConti, et al BET (BRUNAUER-EMMET-TELLER) nitrogen adsorption method, is shown in Table XII.
TABLE XII
Catalyst (Ru 25 Ir) Treatment Surface Area Sample #1 None 127.6 M2/g Samp1e ~2 Reduced 123.5 M2/g Sample #3 Reduced and Thermal 62.3 M2/g Stab~liza~ion -550 - 600 C; One (1) 1 a Hour The ox1de content of Samples #i, #2, and #3 was then measured as well as that of a Ssmple (#4) thermally stab111zed at 700 - 750C for one ~1) hour. In add1t10n the ox1de content of Pt Ir catalysts conta~n~ng respect1vely 5 and 50%
by we1ght 1f Ir1d~um was measured. The results are shown 1n Table XII~.
TABLE XIII
Catalyst (Ru 25 Ir) Treatment X Ox~de Content Sample #1 None 24.4 Sample ~2 Reduct10n 24.3 Sample ~3 Thermal S~ab~11zat10n; 22.6 550 - 600 C - One (1) Hour Sample #4 Reduct10n ant ThermHl 21.5 Stab~11za~on;
700 - 750 C - One (1) Hour SPmtp50I#) None 16.5 Sample #6 Reduct10n 15.2 Sample #7 Reduction and Thermal 13.0 Stab~112a~ion;
550 ~ 600 C - One (1) Hour 5~451 52-EE-0-299 A. B. LaConti, et al 1~ the data from Table XIII shows a decrease in oxide content with reduction ¦,and thenmal stabilization just as surface area decreases after reduction and thenmal stabiliZation.
~! Decrease o~ oxide content (l.e., unreduced catalyst) w111 have a corre-5 ~spondtng e~ect on sur~ace area s~nce the surface area of ox~des is normally ¦9reater than that o~ the non-ox~de form. This reductton ~n ox~de content in Ipart explains surface area reduction but does not wholly explain the dramatic ¦reduction 1n surface area after thermal stab111zat~on.
I The poros~ty of the catalyst was therefore measured to determine whether 10 Ithermal stab~lizat~on of the catalyst causes a change in porosity thereby de-creas1ng the surface area and ~ncreaslng ~ts corros~on res~stance Catalyst samples were taken ~rom the same batches as Samples #1, #2 and #3 and the poros1ty o~ the samples and partlcle S~Ze d1strlbutlon measured. Part~cle s ke d~strtbut~on was measured by a sedimentat~on method and showed that the equ1va-lent spherlcal d1ameter at 50% maSs d~str~but~on was 3.7 m~crons (~) a~terreduct10n of the catalyst and 3.1 m~crons (~) after thermal stab111zat~on. Th1s 1nd1cates that the external sur~ace of the partte1es ~S reduced but aga1n does not account ~or all o~ the surface area reduct10n a~ter stab11 kat10n, Total pore volume data (CC/g) was obta~ned by cap111ary condensat~on and 20 mercury 1ntrus10n methods. Data for Samples #1, #2 and #3 and ~s shown 1n Table XIY.
TABLE XIY
Catalyst Total Pore Yolume (Ru ^ 25 Tr) Treatment Range cc/g 25 ! Sample #1 None 40A - 10~0.80 cc/gm Sample #2 Reduct~on ~0,72 cc/gm Sample #3 Reduction and " 0,76 cc/gm Thermal Stab11i~a-tlon; 500 - 600 C -One (l) Hour I _33_ ~ 24S~ I ' 52-EE-0-299 A. B. LaConti, et a1 The data shows that the total pore volume is relatively unchanged. Thus the porosity, in tenms of gms/cc or if converted to void volume (knowing the spherical s~ze and density), is essentially the same and is in the range of 0.7 - 0.8 gms/cc for Ru - 25 Ir.
S1multaneously, the pore s ke dlstr~bution was measured to obtain the pore d1ameter dtstr1but~ons ~n the 40A - 10 micron range. Capillary condensa-tton was uset ~n the 40 - 500A range. ~n th~s method llquid condensation for a g~ven vapor pressure ~s measured to obtafn pore size distribution. The capillar condensat~on method has a lower resolution lfmft of 40A and an upper limit of 500A. For pores ~n excess of 500A (i.e., 500A - lO~) a mercury intrusion method 1s ut111zed to obtaln pore s1ce d1str~butlon. Pore dlameter distribut1On measurements ~or the two ranges 1s shown 1n Tables XV and XVI respect~vely.
TABLE XV
Catalyst Treatment S~ze Range Pore D1ameter D1str1but1On Sample #1 None 40 - 500A Pore D1str1but~on below 40A
Sample #2 Reduct~on " Pore dtam8ter dlstr~button below 40A
Sample ~3 Reductton and Thermal " D1strtbut1On In the range l Stabtl1z~10n; of 100 - 300A w1th max1mum 20l 550 - 600 C; One (1) at 200A
Hour TABLE XVI
Pore D1ameter D1str1but1On Catalyst Treatment S~ze Range Sample #l None 500A-lOmicrons 0.46u Sample #2 Reduct~on " 0.82 Sample ~3 Reduct1On and Thermal " 1.5 Stabil1za~ton;
500 - 600 C; One (1) Hour ~15~45~
- 52-EE-0-29~ A. B. LaConti, et al This data indicates that thermal stabilization of the catalyst resu1ts in a change in pore size diameter. This change seems to be accompanied by a changein the number of pores so that the overall surface area decreases. With the pore volume staying substantially constant and the pore diameter distribution changing so that the dlstribution shows a max~mum at 200A, and 1.5~ it seems qulte clear that many of the pores below 40A coalesce, reducing the overall number of pores. Above 500A the pore diameter increases. The pore size dia-meter and internal pore surface area thus changes with thermal stabilization.
In summary, the Internal porosity and thus the pore surface area is reduced as the catalyst is thermally stabilized. It ~s believed that th;s is a result of changing the morphology to provide a smaller number of pores with a larger pore d1ameter dtstribut~on. With the pore volume relatively unchanged and a change ipore diameter distr1button to a larger pore diameter, it seems clear that the surface area reduction assoc~ated w~th thermal stabilization of the catalyst is attributable to the change in internal pore surface area.
The porosity (in terms of pore volume (cc) per un1t weight (9) of catalyst of the thermally stabilized, reduced platinum metal oxide catalyst lies in the 0.4 - 1.5 cc/gm range w1th the preferred poros~ty ~or a thermally stabilized u - 25 Ir catalyst belng 0.7 - 0.8 cc/gm.
The pore d1ameter distr1but1On of the thermally stabilized catalyst shows the pr1nc1pal d1str1but1On below SOOA in the 100 - 300A range, with a maximum at 200A. Above 500A the pore diameter distribution shows the greatest pore volume and hence the princ1pal d1stribution is in .04 - 9 ~ range with a max1mumat 1.5~'s (representing the 50~ point in distribution).
The catalyst surface area range for thermally stabilized, reduced platinum etal oxide catalysts shou1d be such that for any given platinum metal, or combination of platinum metals with or without transition metals, etc., the sur-face area should be as low as possible (to reduce corrosionl fcr the required catalytic activity. Thus, the surface area should be in excess of lOM2/gm ranges from 24 - 165M2/g, with a preferred range being from 24 - 165M2/9 and fr~
~ ~`Z45~
60 - 70 M /g for a thermally stabilized Ru - 25 Ir catalyst.
The reduced, thermally stabilized platinum group metal oxide catalysts are, as may be seen, large surface area catalysts as compared to powders, blacks, etc., which normally have surface areas around 10 - 15M /g.
The oxide content of the catalyst has a range of 13 - 24% by weight and a preferred range of 13 to 23 by weight.
While the instant invention has been shown in connection with a preferred embodiment thereof, the invention is by no means limited thereto, since other modifications of the instrumentalities, material and articles employed may be made and fall within the true scope and spirit of this invention.
. . .
Claims (14)
1. A combination electrolyte and electrode structure comprising a gas and hydraulically impervious polymeric ion transporting membrane having at least one gas and liquid permeable catalytic electrode bonded to a surface of the membrane to form a unitary electrode-electrolyte structure, said electrode comprising 13 to 24% by weight of electroconductive particles of thermally-stabilized reduced oxides of at least one metal taken from the platinum group consisting of platinum, palladium, iridium, rhodium, ruthenium, osmium, and alloys thereof, and up to 50% by weight of an electroconductive extender, said particles being bonded together by polymeric particles.
2. The electrolyte and electrode structure of claim 1, wherein said particles include the thermally-stabilized reduced oxides of at least two metals taken from said platinum group and the group consisting of titanium, tantalum, niobium, tungsten, vanadium, zirconium and hafnium, with at least one of said metals being from said platinum group.
3. The electrolyte and electrode structure of claim 2, wherein said particles include thermally-stabilized reduced oxides of ruthenium.
4. The electrolyte and electrode structure of claim 2, wherein said thermally stabilized reduced oxide particles include thermally-stabilized reduced oxides of ruthenium and thermally-stabilized reduced oxides of iridium.
5. The electrolyte and electrode structure of claim 4, wherein said thermally stabilized reduced oxide particles are thermally-stabilized reduced oxides of iridium.
6. The electrolyte and electrode structure of claim 4, wherein said thermally stabilized particles include 24% by weight of thermally-stabilized reduced oxides of iridium.
7. The electrolyte and electrode structure of claim 2, wherein said particles include thermally-stabilized reduced oxides of a platinum group metal and thermally-stabilized reduced oxides of titanium.
8. The electrolyte and electrode structure of claim 7, wherein the platinum group metal is ruthenium.
9. The electrolyte and electrode structure of claim 7, wherein the platinum group metal is iridium.
10. The electrolyte and electrode structure of claim 2, wherein said particles include thermally-stabilized reduced oxides of a platinum group metal and thermally-stabilized reduced oxides of tantalum.
11. The electrolyte and electrode structure of claim 10, wherein the platinum group metal is ruthenium.
12. The electrolyte and electrode structure of claim 10, wherein the platinum group metal is iridium.
13. The electrolyte and electrode structure of claim 4, wherein said particles further include thermally-stabilized reduced oxides of tantalum.
14. The electrolyte and electrode structure of claim 1, wherein said electrode includes graphite as an electroconductive extender.
Applications Claiming Priority (6)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US85895977A | 1977-12-09 | 1977-12-09 | |
| US85894277A | 1977-12-09 | 1977-12-09 | |
| US858,959 | 1977-12-09 | ||
| US858,942 | 1977-12-09 | ||
| US93141978A | 1978-08-08 | 1978-08-08 | |
| US931,419 | 1978-08-08 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1152451A true CA1152451A (en) | 1983-08-23 |
Family
ID=27420398
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000315518A Expired CA1152451A (en) | 1977-12-09 | 1978-10-31 | Electrolytic membrane and electrode structure including reduced platinum group metal oxide |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1152451A (en) |
-
1978
- 1978-10-31 CA CA000315518A patent/CA1152451A/en not_active Expired
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