Process for preparing a platinum surface on tin oxide substrate

A process for coating tin oxide with platinum which comprises soaking a tin oxide substrate in a strong caustic solution to activate the substrate, washing the substrate to remove all caustic solution, immersing the washed substrate in an alkaline solution containing Pt(OH).sub.6.sup..dbd. ions applying an electric potential of +0.1 to +0.7 volt to the immersed substrate, and recovering the substrate having a coating of platinum. This process is useful in preparing active platinum-coated catalysts using very small amounts of platinum.

BACKGROUND OF THE INVENTION 
The preparation of platinum coatings on various substrates for use as 
catalysts have been the subject of many patents. A particularly important 
combination is platinum coated on tin oxide which is optically 
transparent. 
In the prior art there is described in U.S. Pat. No. 4,273,624 a process 
for coating tin oxide with platinum by immersive tin oxide as an electrode 
in an electrolyte containing platinum ions and applying a potential which 
varies with time from positive to negative to positive. Another method of 
preparing a coating of platinum on tin oxide is described in an article 
entitled "Preparation of Dispersed Platinum On Conductive Tin Oxide And 
Its Catalytic Activity For Oxygen Reduction" by Watanabe et al. J. 
Electrochemical Society, Vol. 130, pages 59-64, January, 1983. In this 
method a tin oxide substrate is treated with a strong caustic solution to 
activate the substrate, immersed in a buffered solution of a platinum 
complex for a time sufficient for some of the platinum complex to bind to 
the substrate, and heating the substrate and bound platinum complex to a 
temperature of about 200.degree.-300.degree. C. 
It has been found that the platinum coating prepared by the Watanabe et al. 
process is an agglomeration of platinum crystallites not approaching the 
ideal of a layer of platinum atoms. While the Watanabe et al. process is 
better than that previously known because it employs less platinum, there 
is much room for improvement to approach the ideal of an atomic layer. 
An object of this invention is to provide an improved process for 
depositing a thin layer of platinum of a tin oxide substrate. It is 
another object of this invention to provide an improved process for 
depositing platinum on a tin oxide substrate by using a controlled 
potential chemisorption. Still other objects will appear from the more 
detailed description which follows. 
BRIEF DESCRIPTION OF THE INVENTION 
This invention relates to a process for depositing platinum on a tin oxide 
substrate which comprises soaking a tin oxide substrate in a strong 
caustic solution at 70.degree.-100.degree. C. for 10-60 minutes, removing 
the substrate from the solution and washing substantially all of the 
caustic therefrom, immersing the caustic treated substrate in 0.001M to 
0.1M alkaline solution of Pt(OH).sub.6.sup..dbd., applying an electric 
potential of +0.1 to +0.7 volt to the immersed substrate, and recovering 
said substrate with a thin coating of platinum. The strong caustic 
solution may be a solution of metal hydroxide having a concentration of 
5-15 molar. The potential may be applied for 0.5 to 10 hours while the 
alkaline solution is maintained at 70.degree.-90.degree. C. 
In a preferred embodiment of this invention the tin oxide substrate is 
soaked in a 10M. NaOH solution for 15-60 minutes (more specifically, for 
30 minutes at 90.degree. C.), washed to remove all hydroxide, immersed in 
an electrolyte of Na.sub.2 Pt(OH).sub.6 dissolved in 0.001M to 0.01M KOH 
and subjected to an electric potential of +0.1 to +0.7 volt, and the tin 
oxide substrate is recovered having a very thin platinum coating. The 
applied potential may be in the range of +0.1 to +0.5 volt and the 
solution to which the potential is applied may be at about 80.degree. C.

DETAILED DESCRIPTION OF THE INVENTION 
The present invention concerns a new method, controlled-potential 
chemisorption, for the preparation of platinized tin oxide. The term 
"chemisorption" is used to describe a process involving a chemical 
reaction at a surface. In the prior art of catalyst literature it has 
sometimes been used to describe the preparation of a metallic catalyst by 
soaking a porous or granular support material with a solution containing a 
salt of the metal, and heating to effect a reaction between the surface 
and the solution. The amount of metal is determined by the amount in 
solution, because it is not rinsed away. Precious metal catalysts prepared 
in this way consist of crystallites produced by thermal decomposition of 
the compound in solution. In contrast, the present procedure consists of 
controlling the amount of platinum deposition by controlling the solution 
composition, the platinum salt concentration, the temperature, and in 
particular, applying a controlled potential to control the reaction rate. 
The platinum compound remaining unreacted in solution is removed, and can 
be re-used after replenishment of the platinum. The procedure is designed 
especially for the application of very small amounts of platinum (for 
example, 0.01 to 0.10 micrograms of platinum per cm.sup.2 of true surface 
area), less than the equivalent of a compact monolayer, which would 
correspond to about 0.3 micrograms for a smooth surface. It has been found 
that these small amounts show catalytic activity. 
In the Watanabe et al. article cited above, a process is described which 
involves chemisorption, but which does not differ greatly from other prior 
art procedures. The amount of platinum deposited is controlled by the 
solution composition, the platinum concentration, the temperature and time 
of reaction, but no control of electrical potential is involved, and the 
platinum deposited is not a fractional monolayer. It is believed that the 
platinum solutions used in this process were metastable with respect to 
precipitation of hydrous oxides or basic salts of platinum, with the 
surface constituting a nucleation site for precipitation. The platinum 
crystallites resulting from this process are very similar to those 
resulting from conventional methods, and the only real difference is that 
the amount of platinum is determined by the conditions of the step of 
immersing the tin oxide substrate in a platinum complex solution rather 
than the total amount of platinum in the solution. 
In contrast, the present invention involves the use of a solution which is 
thermodynamically stable against precipitation, and from which no platinum 
is deposited onto the surface by simple dipping or soaking. It is 
necessary to apply an anodic (positive) potential and the rate of 
deposition depends strongly upon the applied potential. It is believed 
that the process involves the application of a potential gradient in the 
electric double layer at the solid-solution interface to overcome the 
electrostatic repulsion between a negatively charged surface and 
negatively charged ions in solution. But the reaction is not a simple 
electrostatic interaction because it is slow and irreversible. It is 
believed that it involves a condensation between hydroxy groups at the 
surface and hydroxy groups coordinated to platinum to form a covalent bond 
such as Sn-O-Pt at the surface. The first step in the present invention is 
fundamentally different also, in that it does not involve a thermal 
decomposition but an electrochemical reduction at ambient temperature. The 
reaction conditions for this step do not appear critical, but it is 
desirable to avoid appreciable hydrogen evolution, which might be 
destructive of the surface. 
In this invention the platinum is in the form of hexahydroxyplatinate (IV) 
ion, Pt(OH).sub.6.sup..dbd., in alkaline solution. This solution may be 
conveniently prepared by dissolving the commercially available sodium salt 
Na.sub.2 Pt(OH).sub.6 in KOH (which is preferred over NaOH because of 
greater solubility). The resulting solution is indefinitely stable against 
precipitation. Alternatively, the solution can be prepared from other 
complexes of platinum (IV), such as H.sub.2 PtCl.sub.6, by allowing a 
reaction to proceed with excess KOH. 
A series of experiments were undertaken to determine how the amount of 
deposited platinum is influenced by time and by the applied potential. The 
data were collected from the use of a three electrode system including a 
platinum counter electrode, the tin oxide electrode described above, and a 
saturated calomel reference electrode. The electrolyte was a solution of 
0.01M KOH containing 500 micrograms per ml of Pt as 
Pt(OH).sub.6.sup..dbd.. The temperature of the electrolyte was maintained 
at 80.degree. C. and the time and potential varied from 30 minutes at +0.1 
volt to 10 hr. at +0.7 volt. The results indicated an optimum deposition 
at about +0.3 volt (measured with reference to the saturated calomel 
electrode). When no potential was applied there was a negligible amount of 
platinum deposited. 
In Table I there are listed data concerning the catalytic activity of the 
fractional monolayer quantities of platinum deposited under the conditions 
of the present invention as described above with respect to the 
accompanying drawing. Hydrogen evolution was measured at an arbitrary 
small current density of about 50 microamperes per square centimeter in 
1M. sulfuric acid, with the potential measured against the saturated 
calomel electrode. 
TABLE I 
______________________________________ 
Estimated Potential For 
Surface Coverage 
H.sub.2 Evolution 
Reaction Conditions 
nanograms Pt/cm.sup.2 
Volts 
______________________________________ 
No platinization 
none -0.64 
30 min at 80.degree. C., +0.6 volt 
Less than 3 -0.247 
10 hr at 80.degree. C., +0.7 volt 
70 -0.220 
bright platinum -0.210 
______________________________________ 
It is clear that tin oxide has a higher overpotential for hydrogen 
evolution than bright platinum to the extent of about 0.43 volt (the 
difference between -0.64 and -0.21). The smallest amount of platinization, 
reported as less than 3 nanograms or 0.003 micrograms/cm.sup.2, was below 
the detection limit of the electrochemical estimation method. Yet this 
surface showed an overpotential of only 0.037 (the difference between 
-0.247 and -0.210) as compared with bright platinum. Using the more 
drastic conditions of 10 hours at +0.7 volt, with 70 nanograms of Pt 
deposited, the overpotential compared with bright platinum was further 
decreased to 0.01 volt. 
The maximum permissible applied potential varies with the concentration of 
hydroxide, because it is limited by the evolution of oxygen. It is 
advantageous to apply a potential just below oxygen evolution to maximize 
the reaction rate but to pass only a very small current, so that no 
special precautions need be taken to ensure uniform current distribution 
throughout a porous bed. 
Another special feature of this invention is the method of activating the 
deposited platinum. Instead of using a thermal decomposition procedure, an 
electrochemical reduction is accomplished by applying a small cathodic 
current to the surface in an alkaline solution not containing platinum. 
This can be readily done by rinsing off the platinum-containing solution 
with a solution of KOH. For example, using 0.5M KOH, a cathodic potential 
of -1.2 volts with reference to the saturated calomel electrode will 
activate the platinum, presumably by reducing it to the metallic state. 
In studies employing a new method of chemical analysis, electron 
spectroscopy also known as X-ray photoelectron spectroscopy, to determine 
the amount of platinum deposited on a substrate, it was confirmed that no 
appreciable amount of platinum is deposited by merely soaking the tin 
oxide surface in an alkaline solution of hexahydroxyplatinum (IV) ion, 
Pt(OH).sub.6.sup..dbd.. When the surface was connected to a source of 
potential, no appreciable deposition was observed at applied potentials 
more negative than +0.1 volt with respect to the saturated calomel 
electrode. The greatest rate of deposition was observed at +0.30 volt. At 
+0.50 volts, a lesser amount of platinum was found. At higher anodic 
polarizations, negligible amounts of deposit were formed. The reason for 
the decrease at excessive polarizations is not known. A possible 
explanation is that precursors to molecular oxygen may be present at the 
surface at excessively positive potentials, and that these species might 
interfere with the chemisorption reaction. In any case, in repeated 
experiments is has been found that there is an optimum potential, which 
may depend upon the solution composition, for example, the concentration 
of hydroxide. As mentioned above, hydroxide is essential to stabilize the 
platinum complex against hydrolysis, which could precipitate the platinum 
from solution. Preferred potentials are between +0.1 and +0.7 using 0.001M 
to 0.1M hydroxide. 
While the invention has been described with respect to certain specific 
embodiments, it will be appreciated that many modifications and changes 
may be made by those skilled in the art without departing from the spirit 
of the invention. It is intended, therefore, by the appended claims to 
cover all such modifications and changes as fall within the true spirit 
and scope of the invention.