CA1058283A - Fuel cell electrodes with finely divided platinum catalyst - Google Patents

Abstract

Abstract of the Disclosure This disclosure deals with novel very fine, particulated colloidal platinum of the 15-25 Angstrom size range of unusual catalytic activity and particu-larly adapted for adsorption or other deposition upon carbon for use as fuel cell catalytic electrodes and the like and produced from platinum colloids and sols including complex platinum sulfite compounds and sols derived therefrom.

Description

~11 5~Z83 The present invention relates to new platinum com-pounds, sols and particulated platinum deposits derived therefrom and to methods of preparing the same, being specifically, though not exclusively, concerned with use in fuel cell electrode preparation and the like~
This application is related to Canadian Patent No.
982,783, issued February 3, 1976.
The art is, of course, replete with numerous compounds and processes employed to provide platinum deposits for use as catalysts in a myriad of applications including oxidation, hydrogenation, dehydrogenation, reforming, cracking, chemical reaction-aiding, contaminant burning, electrochemical cell electrode operation and the like, all hereinafter generically connoted by reference to "catalytic" usage. Particulated platinum has been employed to provide increased effective surface area, as by adherence to rough substrata, such as carbon, alumina and other substances, such deposits being obtained from compounds such as platinum tetrachloride chloro-platinic acid and the like. As described, for example, in Actes Du Deuxieme Congres International De Catalyse, Paris, 1960, pp. 2236, 2237, the average particle size of such particulated platinum lies in the range of from about 45 to 250 Angstroms, an~d it has not proven possible commercially to provide much smaller particles and thus obtain vastly increased catalytic efficiency.
In accordance with discoveries underlying the present invention, however, it has, in summary, now been found possible consistently to produce excellently adhering particulated platinum deposits in the much finer 15~25 Angstrom range:
cm/Jc ~L~S~IZ~33 and it is to new methods, compounds and sols for producing the same that the present invention is accordingly primarily directed.
In one particular aspect the present application is concerned with the provision of a catalytic electrode com-prising an electrically conductive high surface area substrate on which has been deposited platinum particles of the order of substantially 15 to ?5 Angstroms in particle size, said particles having been formed by one of an oxidative decom-position of a platinum complex comprising an oxidizable ligand, and hydrolysis of a non-complex platinum salt solution.
~n another particular aspect the present application is concerned with the provision of in the method of preparing electrodes for fuel cells and the like comprising a platinum-on-carbon electrocatalyst, the steps of subjecting a complex : platinum compound comprising the products therefrom an aqueousdispersion comprising the products of said oxidation, deposit-ing the platinum compound contained in said dispersion on an electrically-conducting carbon substrate, and decomposing said platinum compound thereon, thereby forming platinum particles on said carbon having an average of the order of substantially 15-25 Angstroms.
Other and further objects will be explained hereinafter and are more particularly delineated in the appended claimsO
A first discovery underlying a part of the invention resides in the rather unexpected fact that a novel comple~
platinum sulphite acid void of chlorine may be prepared from chloroplatinic acid and particularly adapted for the formation of a colloidal sol from which extremely finely particulated cm/J~ - 2 -~1~S8Z~

platinum may be deposited. While prior experience had led those skilled in the art to consider either that adding S2 to chloroplatinic acid would invariably result in reducing the platinum to the "2" state, ~7ithout replacing chloride in the complex with S03 , yielding chloroplatinous acid (see, for example, H. Remy, Treatise on Inorganic Chemistry, Vol. 2, p. 348), or that the reaction of S02 with a platinum compound resulted in its reduction to the metallic or zero valence state ("Applied Colloidal Chemistry", W.N. Bankcroft, McGraw Hill, 1926, p. 54), it has been discovered that through appropriate pH and other controls, a complex platinum acid containing sulphite (and to the complete exclusion of chloride) is decidedly achievable. And from such complex acid, unusual colloidal sols depositing particulate platinum in the 15-25 Angstrom range can readily be obtained, and thus vastly superior catalytic performance attained.
Specifically, one of the preferred methods for the preparation of this novel complex platinum acid .

cm/Jc~

~.~58~33 (represented substantially by a formula containing two mole of S03 per mole oE platinum) involves the neutralizing of chloroplatinic acid with sodium carbonate, forming orange-red Na2 Pt (C1)6.
Sodium bisulfite is then added, dropping the pH to about 4, and with the solution changing to pale yellow and then to a substantially colorless shade. Adding more sodium carbonate brings the pH back to neutral (7), and a white precipitate forms in which the platinum has been found to be contained in excess of 99~ of the platinum contained in the chloroplatinic acid starting sample. It was believed (now confirmed) that this precipitate contains six atoms of sodium and four moles of S03 per atom of platinum. It is slurried with water, and then enough strong acid resin is added (such as sulfonated styrene divinyl benzene in the hydrogen from ~-DOWE~-50, for example), to replace three of the Na atoms. The solution is filtered to remove resin and then passed through an ! ion-exchange column with sufficient of the said acid resin to re-place the other three Na atoms. Inherently9 during this two-step cation exchange, copious quantities of SO2 are liberated, amounting to a loss of substantially two moles of SO2/mole Pt. Boiling to con-centrate the solution, results in the novel complex sulfite platinum acid compound above discussed containing groups of (OH) and H3Pt(SO3) 2 ~ free of excess unbound SO2.
Proof of the above-stated complex character of this novel platinum acid has been obtained by reacting 0.0740 g-mole of chloroplatinic acid in the form of the commercial material containing 40~ by weight of Pt to form the "white precipitate" precisely in accordance with the method described above. The "white precipitate"
weighted 48.33 g, after filtering, washing and drylng at 150C
(to constant weight). The filtrate contained 40 ppm platinum, as , ~, .~, jl/ -4--- 1 051~283 determlned by atomic adsorption, showing that more than 99% of the original platinum contained in the sample of chloroplatinic acid was present in the precipitate. Thus, the precipitate has an empirical formula weight of about 653 based on one atom of Pt {o807430}~ 653. Chemical analysis showed that the salt contained 21.1% Na :by atomic adsorption), 29.9% Pr (by atomic adsorption) and 48.7% S03 (by oxidative fusion and BaS04 precipitation and by l~MnO4 titration), thereby confiriming the presence of substantially 6 Na and 4 S03 per Pt atom.
The precipitate was then converted to the complex acid solution in accordance with the precise procedure described above.
It was boiled to a concentra-tion approximately 2 molar in Pt (2 g atoms Pt/liter of solution).

I

jl/ -5-~.0~8283 When the acid was concentrated to this strength, S02 was no longer evolved.
(1) A sample of substantially water-free complex platinum acid, prepared by distillation under high vacuum, was found to contain 52% Pt by weight determined by thermo~
gravimetric analysis.

(2) A sample of complex platinum acid` (in solution) was found to have a sulfur content of 42.6~ by weight, as S03, determined by oxidative fusion and BaS04 pre-cipitation and by oxidometric titration with KMn04, i.e. ? moles of sulfite/mole Pt.

(3) Titration of a sample of the complex platinum acidwith standard base showed a characteristic titration curve with three titratable hydrogen ions per atom of Pt , amounting to 0.8% by weight, two of which were strongly acid (i.e. completely dissociated) and the third quite weakly acid (Ra 10 for the third H ion).
~4) A sample of complex platinum acid was ~ound to contain one OH group per atom Pt , or 4.54~ by weight OH, determined by neutralizing the three acid hydrogens with NaOH to pH 9.5, then reacting with excess sodium sulphite solution of natural pH = 9.5, thereby gradually reforming white precipitate having the above described composition, and raising the pH o~ the reaction mixture above 12, and back-titrating with H2S04 to pH 9.5.

cm/ ~, - 6 -~L~)S~Z~3 (5) A sample decomposed at about 400C in nitrogen yielded only oxides of sulfur (SO2 and SO3) and water in the gas phase, and Pt metal residue.
(6) Addition of silver nitrate to the acid yielded a yellow product insoluble in dilute sulfuric acid.
From these experiments, the following is concluded:
(1) The acid contains only H, O, Pt and S. (The replace-ment of Na by H in the ion exchange step cannot intro~duce any other element); Cl is absent.

(2) The acid contains Pt and S in the ratio o~ 1:2.
(3) The sulfur is present as sulfite as shown by the analy~is and by the high temperature decomposition of the acid in nitrogen.

(4) The sulfite has to be complexed because (a) the complex acid (no SO2 odor) is completely dissociated whereas the ionization constants of H2SO3 (which is odorous) are 1.54 x 10 2 and 1.02 x lO 7, respectively; (b) the complex acid is more soluble in water than ~2SO3 at the boiling - point (max. solubility of SO2 is 5.8g/l or 0.07 molar in H2SO3 at lO0 C vs. the 2 molar acid produced by the method of this invention); and (c) silver sulite is soluble in dilute sulfuric acid, whereas the silver salt of the new complex platinum acid is insoluble in dilute sulfuric acid.

(5) The acid is trivalent, having two strongly acidic and a third weakly acidic hydrogen as evidenced by a cm/~ _ 7 --` 105~3Zl~3 characteristic titration curve. An unusual kinetic effect occurring during titration of the third hydrogen suggests the possibility that it could be part of th~
sulfite ligant.
Turning back, now, to the said "white precipitate", attention is invited to "The Chemistry of the Co-ordination Compounds" r edited by John C. Bailar Jr., ACS Monograph, Reinhold Publishing Co., 1956, p. 57 5'8, where a compound of composition Na6Pt (SO3)4 is disclosed (with no reference to any utility), but as having to be prepared by the complicated process of making the appropriate isomer of a platinum ammine - chloride, Pt (NH3)2 C12, and then converting it to Na6Pt (5O3)4. This further points up the highly novel and greatly simplified high-yield technique of the present invention, ..
starting with chloroplatinic acid and preparing the sodium platinum sulfite complex "white precipitate" (~or which the present invention has found and taught important utility in the development of the novel complex plati.num acid of the invention), substantially quantitatively.
From this novel complex platinum acid, a new colloidal sol may be prepared by decomposing the acid by heating it to dryness in air (oxidizing) and holding the temperature at about 135C for about an hour, producing a black, glassy material which, when dispersed in water, yields a novel colloidal platinum-containing sol having an average finely divided platinum particle size of from about 15~25 Angstroms, .~ .

cm/J - 8 -~5~ 33 with substantially all the platinum particles consistently lying within this range. Some platinum metal and sulfuric acid may be present and may be respectively removed by filtering ~and re-cycling use of the metallic platinum) and by treating with hydroxide resin such as DOWEX or the like.
A jet black colloidal sol with these fine size particles is thus obtained.
From this novel product, a host of vastly improved catalytic surfaces have been obtained.
As a first example, the sol has been deposited or adsorbed on a carbon black substrata (such as electricall~ conductive Norit A) to form a catalytic electrode structure (by means well known in the art and comprising a conventional curr~nt collector). One of the uses of such an electrode structure for example, is as a cathode electrode in fuel cells and the like. This has been effected by reducing the adsorbed metal of the sol with hydrazine; forming on the carbon, platinum metal crystals o~ measured approximately 20-Angstrom size.
For use as an oxygen cathode electrode in an air-hydrogen 135C fuel cell with phosphoric acid electrolyte and a platinum anode, with both electrode sizes about 1 inch by 1 inch, about 2-10~ by wéight of adsorbed platinum was so reduced with about 10% solution of hydrazine to form and adhere the fine particulate platinum on the electrically conductive carbon substrate, the electrode structure exclusive ~f conventional components being about ~0% by weight of Norit A carbon and 30~ by weight of Teflon (i.e. 2 typical fluorinated hydro-carbon polymer) emulsion, such as TFE 30. Most remarkable cathode performance was obtained in this fuel cell, with cathode loading of only 0.25 milligrams/cm.2 of platinum, as follows:
cm/~ g _ ~582~33 .

Current oltage 100 amperes/Pt.2 660 milllvolts Thls improved performance is evident from the Pact that in an identically operating cell with the cathode formed by adhering to the carbon substrate platinum particles from platinum black of nominal surface area of 25 meters 2/gram, such cell performance could only be obtained with ~en times the platinum loading (i.e. 2 milligrams/cm.2).
Similar performance could also be obtained in the same cell with the platinum deposited on the carbon from platinum tetrachloride and chloroplatinic acid (approximately 40-80 Angstrom particles), but only with three to four times the platinum loading. Prior phosphoric acid Puel cell cperation with other platinum catalysts is described, for :-xample, by W. T. Grubb et al., ~. Electrochemical Society III, 1015, 1964, "A High Performance Propane Fuel Cell Operating in the Temperature Range of 150-200C".
Prlor methods of fabricating fuel cell electrodes are described, for example, in U. S. Letters Patent No.
3,388~004.
As another example, similar electrochemical cell electrodes were operated as air cathodes in the same cell as the first example with as little as 0.04 milligrams/cm.2 platinum loading, and with as much as 0.5 mllligrams/cm. . The respective cell performance characteristics were 100 amperes/ft.2 at 530 millivolts, and 100 amperes/Pt.2 at 690 millivolts.

~05~3Z~33 The above-described catalytlc electrode struc-tures have other advantages, for example when used as hydrogen anode electrodes in fuel cells and the like.
As an illustration, the electrode structure described above as a first example, was used as novel hydrogen anode electrode ln the above mentloned air-hydrogen ~uel cell ln lleu of the (conventional) platinum anode also above mentioned. Remarkable anode performance was obtained in thls fuel cell with low loadings between 0.05 and 0.25 mllligrams of platinum per cm2 of anode area, particularly wlth respect to improved tolerance of carbon monoxide.
One ~nown commercial method of producing low-cost hydrogen is by steam reforming of hydrocarbons followed by the shift reaction, which process yields an impure hydrogen containing typically of the order of 80% hydrogen, the remainder being CO2, excess steam and of the order of 1%-2% carbon monoxide. It is well known in the fuel cell art that carbon monoxide is a poison for anodic platinum and that such poisoning is temperature dependent, the loss of anode performance being the more drastic, the lower the temperature. Using such low cost hydrogen, it is thus generally advantageous to operate the above phosphoric acid fuel cell at higher temperatures, for example in the range Or 170C to 190C. Remarkable anode performance ln the presence of CO impurity, was obtained in this fuel cell, especially at high current densities, with an anode loading of 0.05 milligrams/cm2 of platinum when compared to the performance of an anode having a con-ventional platinum catalyst (prepared by reaction Or chloro-platinic acld and deposited ln substantially the same manner) and having the same loading Or 0.05 milligrams/cm2, as shown in the following table.

,,, , ,, ~ .. ... . r ~ ' ' ' ' Cell Current Density Loss of Voltage (millivolts) Temperature(Amps/sq ft) by Polarlzation Due to 1.6%
_ C0 ln Hydrogen Novel Anode Conventional _ _ _ Anode In connection with the examples above, moreover, not only has greatly improved catalytic efficiency been obtained as a result of the extremely high surface area provided by such fine colloidal particles, but this en-hanced activity was found to be maintainable ov&r several thousand hours of operation with no detectable decay in cell performance.
As a further example, such catalytic structures for electrode use have also been prepared without the step of converting the complex platinum sulfite acid to the sol. Specifically, the acid was adsorbed on the carbon substrate, decomposed with air, and reduced with hydrogen.
During such reduction, it was observed that H2S evolved, indicating the retention of sulfide materials; but the H2 reduction at 400C was found to remove substantially all sulfides. Again particles in the 20-Ai.gstrom range were produced with similar electrode performance to that above-presented.
A still additional example is concerned with deposition or adhering to a refractory non-conductive ~051~32~3 substrate of alumina. Sufflclent complex platlnum sulfite acld to oontain 200 milligrams Or platlnum was applied to 50 cc. of insulative eta-alumina pellets, about lJ8 lnch by 1/~ lnch. The mlxture was drled at 200C and, to effect decomposition and adsorption, was held at 600C
ln air for about fifteen minut~s. This resulted in a very uniform distrlbutlon of fine platinum particles (approxlmately 20 Angstroms) throughout the alumina sur-~ace structure, but not within the same. This was reduced by H2 at 500C for about half an hour, providing a signi-ficantly improved oxidation catalyst having the following properties, considerably improved from Houdry Platinum-on-Alumina Catalyst Series A, Grade 200 SR, a typical present-day commercial product, under exactly comparable conditions:
Ignition Temperature For _vention Houdry 1. Methane 355 C 445 C
2. Ethanol 85 C 125 C
3. Hexane 145 C 185 C
Another example, again bearing upon this oxida-tion catalyst application, involves the same preparation as in the immediately previous example, but with two and a half times the amount of particulated platinum (i.e. 500 milligrams). The following results were obtained:
Ignition Temperature ForInvention 1. Methane 3110 C
2. Ethanol30 C (room temperature) 3. Hexane 130 C

.. . . ,,, "" ,, ,, ".,. ~,___", .. _ ,.. ,.. ,.. ,.. ,., .,, ., ., .. ,. .. .. , .,, .. ,. ".. .. ... .. .... . .. ................. . ..... ..
. ..
, .

- 1C1 58Z~3 .
Stlll another example, identlcal to the prevlous one, but with 2 grams o~ platinum adhered to the 50 cc alumina, was found to produce the ~ollowing results:
Ignltion Temperature For Invention 1. MPthane 250 C
2. Ethanol 30 C (room temperature) 3. Hexane 90 C
Stlll another example, 200 milllgrams o~ the pre-formed sol was adsorbed on alumina, and reduced with H2 and ~ound to produce the ~ollowing results:
Ignition Temperature For Invention 1. Methane 310 C
2. Ethanol 45 C
3. Hexane 110 C
For the usage of the last four examples, a range of platinum o~ from about 0.01% to 5% may be most useful, depending upon the economics and applicatlon.
~ s stlll a ~urther example, the deposition or adsorption descrlbed in the last ~our examples, above, may also be e~fected on other refractory oxides in similar fashion, including silica and zirconia.
Lastly, other refractories, such as zeolites, calcium phosphate and barium sul~ate, may be similarly coated by the processes of the last ~our examples.
While the novel complex platinum compounds, acld and/or sol may be prepared by the pre~erred method previously described, it has been ~ound that the acid may also be prepared from hydroxyplatinic acid (H2P~C(OH~) . . ~

by dlssolvlng the same cold in about 6% aqueous 112S03, and evaporating to boil off excess S02. Thls appears to yield the complex platinum sulfite acid material, also ~dentified by lts characterlstlc titration curve). While this process lnvolves a lower pH, it should be noted that chlorlde is excluded by the starting materlal.
~ he above-described methods for the preparation Or several platinum compounds of unexpected utility as sources Or superior catalysts for fuel cells, oxidation catalysts, etc. have proven quite satisfactory; specifi-cally, for producing (I) the water-insoluble salt characterized to have the composition of Na6Pt~ ~S03)4: (II) the complex sulfite-platinum compound, soluble in water, and having an empirical formula and composition represented substan-tially by H3P~c (S03)20H; and (III) the colloidal disper-sion or sol of a platinum compound Or unknown composition, but formed by the oxidative, thermal decomposition of (II).
Among the important before-described uses for these compounds is the preparing of fuel cell catalysts, consisting of platinum supported on carbon, having superior electrocatalytic properties.
Subsequent work has revealed new, unexpected and simplified means and steps Or preparing such superior forms of fuel cell catalysts. The basis for all of the syntheses of a carbon-supported platinum fuel cell catalyst is the formation of a platinum colloid, capable of being deposited on carbon to yield platinum supported on carbon of average particles size range of substantially of the order of l5-25 Angstroms, either as a colloid, as before ~58~:83 described, which can be subsequently contacted wlth finely di~ided carbon3 or as hereinafter described, as colloid generated in the presence of such carbon, thereby causlng the colloidal platinum particles to be formed and deposited on the carbon in a single step. We will now describe in detail one especially advantageous technique whlch involves, typically, the step of oxidizing the sulfite ligand of the preferred complex platinum compounds (I) and (II) to sul~ate, ln aqueous solution, by means of a non-complexing oxidant, it being understood that other platinum complexes containing ligands capable of being oxidized to substan-tially non-complexing products are also suitable, as later dlscussed.
Techniques for preparing a fuel cell catalyst, equivalent to that found from the complexes (I) or (II), have been discovered, wherein chloroplatinic acid (CPA) and sul~ite are reacted, to yield (II), but wherein, unlike the `oefore-described methods, the complex acid (II) is never separately isolated, but is converted to a catalyst directly, and without isolation from by-products, such as HCl and NaC1.
An illustration of the synthesis of a carbon-supported platinum fuel cell catalyst is the observation - of the oxidizing reaction of the complex platinum sulfite acid (II) wlth H202.~ When H202 is added to a dilute solu-tion of the complex acid (II), the sul~ite present in the sulfite-platinum complex, is oxidized. Thc solution's color slowly changes from a faint yellow, to orange.
Following the appearance of the orange color, a ~aint Tyndale effect is noted. With time, this becomes more pronounced; the solution becomes cloudy, and finally, j .

l~Sl~Z83 preclpltation occurs. Whlle the materlal preclpitated ls Or unknown exact composition, it is believed to be a hydrated oxide of platinum, since it ls soluble in base much as is hydrated platinum hydroxide or platinic acid, H2P ~(OH)6. In any case, treatment o~ the complex platinum sul~ite acid (II) with H202 yields a meta-stable colloid of a platinum compound. ~he sequence o~ reactions described above are hastened with heat, and proceed more slowly with increasing acidity, as from the addition o~
sulfurlc acid.
Whereas in the earlier-described methods, the platinum colloidal sol is first formed and then applied to the carbon particle substrate, if the reaction described immediately above is per~ormed in the presence o~ the high sur~ace area carbon, the carbon particles act both as nuclei and as a support ~or the extremely small particles of the platinum compound, as they are formed, and they are deposited on the carbon rather than coalescing to yield a lower sur~ace area precipitate. It has been found that this carbon nucleation Or the platinum particles permits the restrictlon of the platinum deposits to particulate catalytic particles of the said preferred 15-25 Angstrom size range.
It has also been found that the same reaction occurs i~ the complex sodium platinum sulfite precipitate (I) is acidulated by dissolving in dilute sulfuric acid, and is then oxidized by treatment with H202; or if CPA
is reacted with NaHS03 or H2S03, to yield a sulfite-platinum complex, and then oxidizingly treated with H202.

, ~S~3283 Several examples of the use Or the reactions observed above are given below. Basically, however, they all depend upon the oxidatlon of ~he sulfite pre-sent in a platinum-sulfite complex, with H202 belng the prererred oxldant, although other non-complexing oxidants, ~uch as potassium permanganate, persulfuric acid and the like have been used. The term "non-complexing oxidant", as used in this specification and in appended claims~
means an oxidant which does not introduce groups capable of rorming strong complexing ligands with platinum. Also whlle any high surface area carbon is suitable, the carbon black, Vulcan XC-72 (Cabot Corp.), has been found to yield an excellent catalyst; but the fact that this carbon is used ln the examples to be cited does not imply that other carbons cannot be used. Nor, since the carbon is merely a support onto which to deposit the colloidal particles of platinum as they are ~ormed, should it be thought that carbon is the only support upon which the deposit can be made. Other materials such as A1203, BaS04~ SiO2, etc.
can be used as supports for a high surface area platinum, as previously described, but are, of course, useful ~or other catalytic properties rather than for ~uel cells, electrodes and the like, because of their hlgh electrical resistance. We shall now proceed to a further series of examples.
Example 1 To a liter of water, su~ficient of complex platinum sul~ite acid (II) is added to give a platinum concentra-tion o~ 2.5 g/l. To this solution is added 22.5 grams of Vulcan XC-72. The solution has an initial pH of about 1.8 la , .. _ .. .___ .. _., .. ... _. . _.. , ....... , .. .. .. , _.... . .... ........ .... .. . .... .... .... ......... .

~58Z83 which ls unaltered by the addltlon of carbon. The solu-tion is stlrred vlgorously, so as to keep the carbon well dlspersed. Add 50 ml of 30% H2O2, while continuing the vigorous stirrlng. Malntain the stirrlng for about one hour. The pH wlll drop slowly, lndicatlng that hydrogen ions are belng generated. Next, heat the solution to boillng, while maintaining the stirrlng. Filter the carbon, wash it well wlth water, and dry the carbon ln an oven set to 100-150C. This air-dried material is now ready ~or use wlthout further treatment. Platinum uptake is about 98% with the remainder being discharged to the fil-trate. The resulting carbon, containing 9.9 - 9.8% platinum shows platinum crystallites of 5-20 Angstroms in diameter A by electron ~icroscopy. Fuel cell performance was measured uslng Teflon bonded anodes and cathodes havlng platlnum loadings of 0.25mg/cm2 of electrode area. Performance with H2 and air, at 190C in a phosphoric acid fuel cell, was ~easured and found to give 200 Amperes per square foot (ASF) at .670-.680 V. The resistance loss was about 0.02 volts at this current density, so the IR-free per formance was about .700 Volts at 200 ASF.
Example 2 The reaction was conducted as in Example 1, - but rather than heating the solution after one hour, stirrlng was continued for 24 hours at amblent temperature.
Platlnum uptake was 97-98%, and physical and electrochemicai properties substantially identical to the produce described in Example 1 were obtained.

~5~32~3 Example 3 The reactlon of the complex platlnum sulfite acld (II) with H202 was conducted much as in Example 1, except the pH o~ the solution was ad~usted to 3 with NaOH, prior to the addition of H202. After the one hour reaction perlod, the pH was agaln brought to 3 with NaOH, and the solution boiled. The carbon was filtered, washed, and drled, as prevlously described. Platinum uptake was substantially quantitative, and the physical and electro-chemical properties of the product substantially identical to those described in Examples 1 and 2.
Example 4 In 100 ml of H20, sufficient of the complex sodium platinum sulfite salt tI) was dissolved to yield a platinum concentration of 25g~1. The salt was put in solution by the addition of sufficient H2S04 to drop the pH to 2. This solution was diluted with H20 to volume of one liter, and reacted as described in Example 3.
Platinum uptake was quantitative and the physical and electrochemical properties of the product substantially identical to those already described in the previous examples.
Before proceeding to Example 5, which describes a process that does not require the isolation of either of the complexes (I) or (II) but rather uses CPA heated with sulfite, it may be useful to hypothesize upon the meahanism of the reactions taking place in Examples 1-4, since they have a bearing on the reaction of Example 5, and will help to explain some of the difficulties of con-trol noted in Example 5; though the invention is not de-pendent upon the accuracy of such hypothesis, it being , , 1~5~2~3 su~icient to describe the steps that do indeed work and produce the results Or the lnvention.
It ls belleved, however, that when H202 is added to either the sodium platlnum sulflte complex (I) or the like, dlssolved in dllute H2S04, or to a solution of the platinum sol (II~), the sulfite or like ligand is destroyed.
Slnce it is the complexlng power of sulflte which is the stabilizing ~orce in maintaining an ionic platinum species, lts oxidation to sulfate destroys this stabilizing force.
Sulfate is, at best, a ~eeble complexing agent for platlnum, whether it is p~II or p~cIv. With the removal of the sul~ite, there does not exist a favorable environment for main~aining a soluble species of platinum, and the plati-num species just formed upon the destruction o~ the stabili-~ing sul~ite must slowly hydroli~e and in the process has a transient existence as extremely small colloidal particles. It is these particles which are deposited on the carbon yielding the active catalytic structure. It is believed that the reactions of ~xamples 1-3 can be adequately described as being substantially:
(1) and (2) H3P~ (S03)20H + 3H22-~2H2S4 p~c 2 2 (3) Na2HP~C (S03)2 OH + 3H202--~Na2S04 + p~c 2 + 3 2 2 4 Example 4 is somewhat di~ferent, in that the starting material is dif~erent. However, it would appear that when the complex salt of composition Na6P~ (SO3)4 is dissolved in H2SO4, the complex acid of composition H3P~ (S03)20H is formed, since there is a vigorous evolution of SO2, and when the SO2 is evolved, the characteristic titration curve Or H3P~ (S03)20H is observed. Hence, the reaction of Example 4 is apparently similar to that of Example 3.

3;2 83 In Example 5 presanted below, however, CPA ls reacted with NaHSO3 to yield a complex believed to be the complex acld of composition H3P~(SO3)20H, and HCl and NaCl are formed. One possible reaction is substantially as follows:
2 6 3 2H2O-~H3P~(S03)20H + Na2S04 ~ NaCl + 5HCl However, when this mixture is treated with H202, the presence of chloride, along with the hlgh acidity, leads to the formation in part, of H2P~C16, rather than the desired colloldal species. To minimize this effect, the platinum concentration must be kept low (in order to keep the chloride concentration low) and the pH closely controlled.
Example 5 Dissolve 1 gram of CPA (O.4 gm P~c) in 100ml water. Add 2 grams of NaHSO3 and heat until the solution turns colorless. Dilute to 1 liter with water and ad~ust the pH to 5 with NaOH. Add 3.6 grams of Vulcan XC-72, and while stirring add 50 ml of 30% H2O2. Continue to stir and as the pH changes, add NaOH to maintain the pH
be~we~n 4 and 5. When the pH has stabili~ed, heat the solution to boil, and filter and wash the carbon. Platinum pickup is variable, but in general is about 90%. Increasing the platinum concentration decreases the percentage of platinum deposited upon the carbon since the conversion of H2P~C16 is favored. The catalyst formed in this way, has been found to be substantially identical in per~ormance to that made in Examples 1-4.

.. . , . . _. __ __.. " _.. __ .. _.. _ .......... ....... . _ ..... ,. ,. . .. ... ,.. ................. _.. . .. ... ... ... _.. ~ . ....
.

~5~3Z~3 As compared wlth the earller descrlbed methods of said prlor applicatlon, also embodied herein, the additional methods, supra, avoid the converslon of the oompound having the composltlon of Na6p~c(sQ3)4 to that of composltion H3P~t(S03)20H, and then to the colloidal sol material. Thls latter colloid~ in turn, must then be applied to carbon, filtered, dried, and reduced ln H2, ln accordance with the earlier methods. As described in Example 4, however, the compound of compositlon Na6P~tS03)4 ls dissolved in acid, reacted with H202 in the presence of carbon, the product filtered, washed and dried and with no H2 reduction necessary~ slnce the sintering tempera-ture re~uired to prepare the electrodes is ample to de-compose the adsorbed species to the catalytically-actlve platinum particles.
Example 6 5 g of the precipltate having the composition corresponding to Na6P~(S03)4 ls suspended in about lOOcc o~ water and ~eacted with a large excess of the ammonlum form of Dowex 50 (a sulfonated copolymer of styrene and dlvinylbenzene) cation exchange resin in bead form until the precipitate ls dissolved. The pH of the resulting solution is about 4. After filt~ation, the solutlon is passed through a column of Dowex 50 in the ammonium form until all of the sodium is removed. The resulting platinum sul~ite complex in solution is then oxidlzed wlth hydrogen peroxide ln the presence of finely dlvided carbon, using the procedure of Example 1, yieldlng a nearly equlvalent electro-catalyst.

~51~33 Similar results are obtainable by first neutralizing to pH 9 a solution of the complex compound corresponding to H3Pt (SO3)2OH with aqueous ammonia which neutralization requires almost five moles of NH3 (instead oE only 3 moles in the case of neutralization by NaOH), then acidifying the solution of pH 3 with sulfuric acid, and oxidizing with H2O2 in the presence of carbon, again using the procedure of Example 1.
In both the earlier methods of the said applications and the additional methods supplementarily discussed herein, however, common over-all steps are involved of forming the complex sodium platinum sulfite precipitate from CPA, acidifying the same and developing the complex platinum sulfite acid and oxidizing such into a platinum colloidal sol, which is applied to the carbon particle substrate and reduced to form the conduction catalytic fuel cell or related electrode.
While the above examples relate to a complex platinum sulfite as the starting material for an appropriate platinum colloid, other platinum complexes comprising oxidizable ligands can be similaxly used, as before stated, to produce suitable platinum colloids by means of a non-complexing oxidant, as illustrated in the next Example 7.
Example 7 Four grams of platinic ac.id, H2Pt (OH)6, were dissolved in 25 milliliters of 1 molar NaOH. Six grams of sodium nitrite were dissolved in this solution and then the mixture was diluted to a volume of 800 millileters cm/JO - 24 -1~5~283 with water. The pH was then reduced from about 11 to pH of 2 with H2SO4. During this process, a precipitate formed and then re-dissolved as the pH approached 2, thereby forming a platinum nitrite complex. To~this solution, 18 grams of finely divided carbon (VU1Gan XC-72~ were added, and while vigorously stirring, 200 millileters of 3~ H2O2 were added~
The pH dropped to 1.4 substantially instantaneously. The resulting platinum-catalyzed carbon was filtered, washed and dried. Fuel cell performance for 0.25 milligram per square centimeter electrodes of this material in a phosphoric acid fuel cell at 190C, was 640 millivolts at 200 amperes per square foot, with hydrogen and air.
In this case, the lower performance of this platinum nitrite complex, as compared with the platinum sulfite complex, appears attributable to the fact that the colloidal state is rapidly produced and persists only for a very short time, followed by precipitation; whereas in the case of the platinum sulfite complex, the oxidation proceeds slowly and the colloid is stable over long periods of time.
As before explained, in general, suitable electro-catalysts are prepared by depositing platinum of the 15~25 Angstrom particle size on finely divided conducting carbon.
It has also been found possible to prepare colloidal solutions, though not quite so efficacious, by the use of solutions of non-complex platinum salts from which colloidal solutions can be made, for example, by the use of an appropriate hydrolysis technique, as illustrated by Examples 8 and 9.

cm/~O - 25 -~5~Z~33 Example 8 Four grams of platinic acid, H2Pt (OH)6, wexe dissolved in 10 millileters concentrated H NO3. This solution was slowly added to one liter of water containing 18 grams of finely divided carbon (Vulcan XC 72) while vigorous stirring was maintained for one hour, and then the pH was adjusted to 3 with Na OH, while continuing stirring. The dispersion was then boiled, while stirring. This colloid was thus produced by hydrolizing a non-complex platinum salt solution at the above appropriate pH. The resulting platinized carbon was filtered, washed and dried. Fuel cell electrodes were fabricated therefrom having a platinum loading of 0.25 milligrams per square centimeter and a phosphoric acid fuel cell constructed. Performance with hydrogen and air at 190C
was 660 millivolts at 200 amperes per square foot.
Ex mple 9 ~ The experiment of Example 8 was repeated except 6 molar H2SO4 was substituted for nitric acid, this time producing the colloid by hydrolizing the non-complex platinum salt resulting from the H2SO4 reaction at the same pH of about 3.
Fuel cell performance under similar conditions as in Example 8 was 667 millivolts at 200 amperes per square foot.
The platinized carbon electrodes produced with the no~-complex platinum sols of Examples 8 and 9, while most useful for the purposes described, have given somewhat lower fuel cell voltages at the same current densities than electrodes made from the preferred platinum sulfite complex, before discussed, apparently because of the difficulties involved in controlling the hydrolysis conditions required for the non-complex platinum salt processes.

cm/Jo - 26 -~5~28~3 As before stated, while only lllustratlve electrode and other catalytic uses have been described, the lnventlon is clearly appllcable to a wide variety of electrodes, oxldatlon, hydrogenatlon, de-hydrogenation, reforming, cracking, chemical reaction-aiding, contaminant burnin~ and other uses, as well. Further modi~ications wlll also occur to those skllled in this art and all such are considered to fall within the spirit and scope of the invention as defined in the appended claims.

Claims (23)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. Catalytic electrode comprising an electrically con-ductive high surface area substrate on which has been deposited platinum particles of the order of substantially 15 to 25 Angstroms in particle size, said particles having been formed by one of (a) oxidative decomposition of a platinum complex comprising an oxidizable ligand, and (b) hydrolysis of a non-complex platinum salt solution.

2. A catalytic electrode as claimed in 1 and in which said particles load the electrode surface in the range of from substantially 0.04 milligrams/cm2 to 0.5 milligrams/cm2.

3. A catalytic electrode adapted for use in fuel cells and the like comprising a composite of finely divided platinum particles deposited substantially uniformly on electrically-conducting high surface area carbon substrate, said particles having a particle size of substantially 15 to 25 Angstroms and having been formed by the oxidative decomposition of a platinum complex comprising an oxidizable ligand.

4. A catalytic electrode as claimed as in claim 3 and in which said complex is selected from the group consisting of platinum sulfite and platinum nitrite complexes.

5. A catalytic electrode as claimed in claim 4 and in which said particles have been reduced subsequent to said oxidative decomposition.

6. A catalytic electrode as claimed in claim 4 and in which said particles load the electrode surface in the range of from substantially 0.04 milligrams/cm2 to 0.5 milligrams/cm2.

7. A catalytic electrode as claimed in claim 4 and in which said platinum sulfite complex is the compound having the composition corresponding substantially to H3P?(SO3)2OH.

8. A catalytic electrode structure as claimed in claim 5 and in which the same is connected within a phosphoric acid electrolyte fuel cell with air-hydrogen electrode supply means, and said electrode is provided with means for enabling the drawing of current flowing through the cell.

9. A catalytic electrode structure as claimed in claim 8 and in which said structure comprises a catalytic anode, and in which said air-hydrogen electrode supply means comprise a source of hydrogen contain-ing carbon monoxide impurity.

10. A catalytic electrode structure as claimed in claim 9 wherein said catalytic anode has a platinum particle loading in the range of from substantially 0.04 milligrams/cm2 to 0.25 milligrams/cm2.

11. A catalytic electrode structure as claimed in claim 8 and in which the said carbon is composited with fluori-nated hydrocarbon polymer to provide the electrode structure.

12. In the method of preparing electrodes for fuel cells and the like comprising a platinum-on-carbon electrocatalyst, the steps of subjecting a complex platinum compound comprising an oxidizable ligand to oxidation, producing therefrom an aqueous dis-persion comprising the products of said oxidation, depositing the platinum compound contained in said dispersion on an electrically-conducting carbon substrate, and decomposing said platinum compound thereon, thereby forming platinum particles on said carbon having an average particle size of the order of substantially 15-25 Angstroms.

13. The method of Claim 12 wherein said complex platinum compound is platinum sulfite and it is subjected to air oxidation.

14. The method of Claim 13 wherein said complex platinum sulfite contains groups of (OH) and H3Pt(SO3)2.

15. The method of Claim 13 wherein said air oxidation is carried out at about 135°C.

16. The method of Claim 13 wherein said dispersion con-tains the product of said complex platinum sulfite and a non-complexing oxidant, said oxidation being carried out in said dis-persion.

17. The method of Claim 16 wherein said oxidant is selected from the group consisting of hydrogen peroxide, potassium permanganate and persulfuric acid.

18. The method of Claim 16 wherein said complex platinum sulfite is selected from the group of compounds having substantially the composition of Na6Pt(SO3)4 and H3Pt(SO3)2OH and mixtures thereof.

19. The method of Claim 16 wherein said complex platinum sulfite is the compound having the composition of Na6Pt(SO3)4 and wherein said compound is in an aqueous sulfuric acid solution.

20. The method of Claim 18 wherein said oxidation is effected with H2O2.

21. The method of claim 12 wherPin said oxidation iscarried out in the presence of said carbon substrate in finaly divided form.

22. The method of claim 12 wherein said complex platinum sulfite is formed in said dispersion by reacting a solution of chloroplatinic acid with a sulfiting agent.

23. The method of claim 22 wherein said oxidation is effected thermally in air, and said decomposing following depositing on the carbon is effected by reducing the platinum compound.