Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
~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
200 598
300 548
400 500
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.
-10-
~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 ~ ' ' ' '
283
Cell Current Density Loss of Voltage (millivolts)
Temperature(Amps/sq ft) by Polarlzation Due to 1.6%
_ C0 ln Hydrogen
Novel Anode Conventional
_ _ _ Anode
190C 500 1744
190C 400 1028
190C 300 914
175C 500 66118
175C 40 469
175C 300 2238
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
-12-
~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
-13-
.. . . ,,, "" ,, ,, ".,. ~,___", .. _ ,.. ,.. ,.. ,.. ,., .,, ., ., .. ,. .. .. , .,, .. ,. ".. .. ... .. .... . .. ................. . ..... ..
. ..
,
.
- 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~)
. . ~
-14-
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,
-16-
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.
-17-
,
~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.
--19--
~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
-20-
,
,
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.
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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.
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.. . , . . _. __ __.. " _.. __ .. _.. _ .......... ....... . _ ..... ,. ,. . .. ... ,.. ................. _.. . .. ... ... ... _.. ~ . ....
.
~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.
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~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.
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