CA1150231A - Process for electrode fabrication having a uniformly distributed catalyst layer upon a porous substrate - Google Patents

Abstract

48,694 ABSTRACT OF THE DISCLOSURE
This is a method for preparing electrodes for the oxidation of sulfur dioxide (including the oxidation of sulfur dioxide in a sulfur cycle process for hydrogen generation) in which a platinum group metal containing catalyst material is coated evenly onto a high-surface-area porous substrate. A solution is applied onto the substrate by a vacuum filtration method. A pressure dif-ferential of at least 10 mm-Hg is applied across the sub-strate. The substrate is uniformly heated (preferably by radiant heating).

Description

1 48,694 PROCESS FOR ELECTRODE FA~RI~ATION HAVING
; A UNIFORMLY DISTRIBUTED CATALYST LAYER

CROSS-REF ~ CE TO RELATED PATENTS
In U.S. Patent No. 4,306,950 lssued December 22, 1981, entitled "Palladium Electrode For U~e In ~ul~ur Cycle Hydrsgen Operat~on Process", by the in~entor herein and assigned to the same a~signee, there i8 de~cribed a palladium electrode for u~e in the anodic oxidation of ~ulfur dioxide. me use oi a palladlum catalyst rather than the platlnum provides ~or a signific2nt voltage reduction and there~ore a greatly in-crea~ed electrolyzer ef~iciency. me lnvention de~cribed herein can be, but not need necessarily be, used to fabri-cate the palladlum electrode of th$~ copending lnvention.
BACKGROUND OF THE I~VENTION
This inventlon relate~ to electrodes for oxida-tion o~ sulfur dioxide, and in particular to porou~ sub-strates coated with a platinum group metal (or compoundsthereof) catalyst.
0~ all o~ the advanced concepts proposed gor the large sc~1e production of hydrogen, the procesq described in U.S. PAtent 3,888,750, is~ued June 10, 1975, to Brecher and Wu, appearæ to be the most economical. That process is a two-step cycle which decomposes water wlth the hydro-gen being generatsd ln an electrolyzer and the oxygen being generated in a separate step in a thermochemical ..~

~1 z~

2 48,694 apparatus. Sulfur dioxide is electrochemically oxidized at a relatively low tcmperature (less than about 150~C) to produce sulfuric acid on the anode while hydrogen gas is simultaneously generated on the cathode. The sulfuric acid is then catalytically reduced at higher temperatures (generally above 870C) into sulfur dioxide and oxygen.
Subsequently, the sulfur dioxide is recycled as a reactant in the first step. The use of sulfur dioxide as an anode depolarizer reduces the thermo-dynamic reversible voltage of an electrolyzer from 1.23 volts (for the conventional electrolysis of water) to only 0.17 volts.
SUMMARY OF_THE INVENTION
While the sulfur cycle reduces the electric power required in electrolysis, it has been found that conventional electrode coating processes have not resulted in uniform coatings of a catalyst on the porous sub-strates. Such non-uniformity has resulted in increased electrode voltage at a given production rate and thus a loss of efficiency.
It has been discovered that extremely uniform coatings of catalyst can be applied to electrodes for oxidation of sulfur dioxide by using a pressure differ-ential across the substrate (which when coated will form the electrode). In particular, a solution containing at least one platinum group metal is applied to one side of the substrate, a pressure differential of 10-100 milli-meters of mercury is applied across the substrate with the side to which the solution has been applied being the higher pressure side and uniformly (prererably radiantly) heating the substrate.
BRIEF DESCRIPTION OF THE DRAWING
A better understanding of the invention may be had by reference to the drawings in which:
Figure 1 shows a block diagram illustrating the sulfur cycle hydrogen generation system in which the elec-trode made by the process of this invention can be used;
Fig. 2 is a diagram of an electrolytic cell - (electrolyzer) in which the electrode fabricated by this ~. J

- ~5~3~

3 48,694 process can be used;
Fi~. 3 is a cross-section of a substrate holder and subsLrate in which a vacuum is used to produce the pressure differential;
Fig. 4 is a cross-section of a substrate holder and substrate where positive pressure is used to produce the pressure differential;
Fig. 5 shows the appearance of a commercially available platinum-coated carbon electrode;
Fig. 6A shows the general appearance and Figure 6B shows a scanning electron micrograph of a typical carbon substrate prior to coating by the method of this invention; and Fig. 7A illustrates the general appearance and Figure ~B shows a scanning electron micrograph of a re-sulting electrode prepared by the process of this inven-ti.on.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
` The sulfur cycle hydrogen generation system of Fig. 1 is a typical use of the sulfur dioxide oxidation electrode of this invention. In Figs. 1 and 2, an elec-~` trolyzer 1 contains an aqueous solution of sulfuric acid 2 which is saturated with SO2. Direct current is applied to the solution through an anode 3 (made by the process ; 25 described herein) and a cathode 4. Sulfuric acid and ~ hydrogen gas are generated at the anode 3 and the cathode '~ 4, respectively. Inlets 5 and 6 are provided for the addition of more dilute sulfuric acid and additional ~; sulfur dioxide. The hydrogen product leaves by outlet 7 where it separates from the sulfuric acid. Unconsumed sulfur dioxide leaves by outlet 8 with the more concen-trated sulfuric acid solution, and both are recycled. A
portion of the sulfuric acid from outlet 8 passes to vaporizer 9 where water is evaporated and its concentra-tion is increased. The concentrated sulfuric acid then passes to oxygen generator 10 where the sulfuric acid is ;`~ heated over a catalyst, for example, of platinum or vana-dium pentoxide, to decompose it into water, sulfur diox--' ~ ~ ` ' :

, .
~ ~ .

4 ~8,694 ide, ~nd oxygen whi,ch pass to oxygen recovery unit 11. In oxyxen recove~ry unit :ll, I,he sulfur dioxide is separated ~rom the oxygen by lowering t,he lemperat,ure to condense t:he sulfur dioxide into a l:iquid. The sulfur dioxide and the water arc then returned t:o inlet 6 of the electrolytic ce`ll l, thu.s completing the cycle. A hydrogen-ion-perme-able meml)rclne 12 xeparales ~he fluid around the anode 3 ~'rom l~he flui(l around ~he cathode l~, (`yclic vol~ammel:ric studies have revealed that he anodic ox.i.datic)n o~' sulfur dioxide is highly irrever-xi.b'le. Tlle overvol.tage on the anode contributes a signif-icant. proport,ion to the overall potential of an electro-'Lyæer, and ~hus is one o~ t.he major sources of efficiency loss in the cel.'L. Methods to reduce the anode overvoltage .1.5 inclucle the use of appropriate electrode catalyst and the maximization of active surface areas of the electrode. To maximize the surface area, are electro-catalysts supported on highly porous substrates (electrodes made completely of a palladium, ~or example, rather than as a palladium coat-,~) ing on a substrate would, of course, be exLremely expen-sive). Carbon substrates formed from extremely fine carbon powder provides a porous substrate wi.th a very high surface area (a specific surface area of at least 200 and typically about 450 square meters per gram). The appear-ance and microstructures of a typical carbon (plate~substrate are i.llustrated in Fig. 6. These inexpensive carbon substrates provide good porosity, electrical con-ductivity and mechanical strengt'h. While anodes of carbon ~graphite) catalyzed wi.th fine platinum particles have been used for preparation of sulfuric acid from sulfur dioxide, commercially available platinum-coated carbon electrodes have been found to have an extremely non-uniform coating of platinum. All such electrodes had, as shown in Fi.g. 5, areas which were clearly uncoated.
Early experiments to improve the uniformity of catalyæed carbon plate electrodes which, like the commer-cial'ly available electrodes, when visually inspected, had little or no catalyst in the center. When these elec-3~
48,694 trodes were disected, there was also little or no cata-lysts on interior surfaces. In this early work, the ~ eous ~ollltion or noble metal colllpound (xuch as dihy(lrc)-gen hexachloroplatinate or palladium acelate) was applied to the surface of a porous carbon substrate by painting.
'I`he substrate doped with catalyst was slowly dried in a furnace at about 50C. The resultant non-uniform coatings may have been caused by the pressure of the water vapor in the pores increasing during the evaporation process and reP~e/~t9 gradually ~cpcaling the solution out of the pores (the aqueous solution also apparently migrated toward low temperature regions at the edges).
In order to avoid the problems of those early techniques, a new method was developed in which a pressure differential across the substrate was used to push solu-tion through the substrate and radiant heating of one side of the substrate was used to heat the substrate uniformly (note that uniformity on the surface and on planes paral-lel to the surface is ~ cri~ical but that temperature differences through the thickness of the material - e.g.
between planes parallel to the surface - can be toler-- ated).
This invention is illustrated by the following example:
A. A five by five centimeter porous carbon substrate (mean pore size approximately nine microns) was activated by oxidation in a concentrated nitric acid (13.5 ~- normal at 80C).
B. 0.255 grams of palladium acetate was dis-solved in 30 milliliter of distilled water and the solu-tion was then heated at 80C for 30 minutes.
C. The oxidized carbon substrate 13 was mounted (as shown in Fig. 3) in a lucite holder 14 using a seal around the perimeter of the substrate. The substrate was then positioned in a horizontal plane with its underside exposed to a cavity 18 that was connected to a vacuum pump.
D. A small amount of solution was poured onto 6 ~l8,694 the upper side of the substrate and the vacuum was applied to maintain a pressure differential of approximately 10 millimeters of mercury across the subs~rate 13. rrhe substrate 13 was heated in situ to 4~-60C using an over-head infrared lamp 20 (palladium acetate was thus uniform-ly deposited over the electrode 13 as the solution filter-ed through the pores). This treatment (adding solution, applying pressure differential, and reheating) can be re-peated to increase the thickness of compound applied.
E. After the deposition of palladium acetate was completed, the thermal decomposition process was performed in a nitrogen or hydrogen atmosphere at 600C
for 2 hours, which resulted in the formation of a thin layer of palladium completely covering the electrode surface (as shown in Fig. 7, the density of palladium particles is high and the distribution is uniform).
When a palladium-oxide electrode is to be used, the palladium-covered carbon substrate prepared as above is further treated, for example, at a temperature of 40~-500C in a stream of helium gas containing 5% oxygen.
While the electrode is preferably heated from above (as apparently the solution being applied tends to migrate towards lower temperature regions), the substrate could be heated from below, as shown in Fig. 4. Fig. 4 also illustrates the use of a positive pressure cavity 22 (as opposed to the vacuum cavity of Fig. 3) as a means for applying the pressure differential across the substrate 13. Whether heated from above as in Fig. 3, or below as in Fig. 4 (or both as can be done by sticking a radiant heating source in either the vacuum cavity 18 of Fig. 3 or the pressure cavity 22 of Fig. 4) or whether the pressure dif~erential is applied by a positive pressure or by a vacuum (or both), it is critical that one uses a pressure differential to uniformly move the solution through the substrate.
Experiments in which uniform heating alone was used (a variation of the above process without a pressure differential), failed to produce uniform coatings. When a 3~
7 48,6~4 pressure differential was used with non-uniform heating, poor results were obtained. Only by using both the uni-form heating and the pressure differential were even coatings obtained. Although uniform heating in an oven was accomplished experimentally, oven heating tended to be non-uniform unless extreme care was taken and radiant heating is preferred.
Again with reference to Fig. 4, it should be noted that the pressure could be applied by a pump direct-ly to the coating solution (thus the pressure chamber 22would be completely filled with fluid) or by using suffi-cient depth of solution to provide the pressure hydrostat-ically. Neither of these techniques, however, lend them-selves to radiant heating from the side to which the solution is applied and thus the arrangement of Fig. 3 is preferred.
It has been found that at least ten millimeter of mercury's differential pressure is required to move the fluid through the substrate at a practical rate and that the differential pressure should be less than about 100 millimeters of mercury if carbon substrates are used to avoid damage to the substrate. Preferably a differential pressure of 10-30 millimeters of mercury is used with car-bon substrates. Other types of substrates (e.g. a sub-strate sintered from finely divided titanium powder) canalso be used and the upper limit of the pressure differen-tial is determined by the strength of the substrate. As described in the aforementioned copending application, the palladium or palladium oxide catalyst is preferred (al-though other platinum group of metals can also be uniform-ly deposited by the techniques described herein) and pal-ladium is preferably deposited using radiant heating to 40-60C.
The invention is not to be construed as limited to the particular forms described herein, since these are to be regarded as illustrative rather than restrictive.
The invention is intended to cover all configurations which do not depart from the spirit and scope of the invention.
, ,

Claims (6)

8 48,694 What is claimed is:

1. In combination with a process for preparing electrodes for the anodic oxidation of sulfur dioxide of the type in which a platinum group metal containing catalyst solution is coated onto a high-surface-area porous substrate and dried, thus forming a film on the substrate surface, the improvement which comprises:
a) applying a solution containing at least one platinum group metal compound to a first side of said sub-strate;
b) applying a pressure differential of at least 10 mm-Hg across said substrate which said first side of said substrate being the higher pressure side;
c) uniformly heating at least one side of said substrate while applying said pressure differential; and d) depositing said platinum group metal compound on said high surface area porous metal substrate during said uniform heating while applying said differential pressure.

2. The process of claim 1, wherein said differ-ential pressure across the substrate is 10-100 mm-Hg.

3. The process of claim 2, wherein the differ-ential pressure is 10-30 mm-Hg.

4. The process of claim 3, wherein said sub-strate is radiantly heated.

5. me process of claim 4, wherein the platinum group metal is palladium and the substrate is radiantly heated to 40-60°C.

6. The process of claim 1, wherein said porous substrate has a composition consisting essentially of carbon.