J. Electroanal. Chem., 69 (1976) 423--427 423 © Elsevier Sequoia S.A., Lausanne -- Printed in The Netherlands

Preliminary note

OXIDATION AND REDUCTION OF SOME SELENIUM AND SULFUR COMPOUNDS ON DIMENSIONALLY STABLE ANODE MATERIALS

D. CIPRIS* and D. POULI**

Allied Chemical Corporation, Corporate Research Center, Morristown, N.J. 07960 (U.S.A.)

(Received 17th February 1976)

I n t r o d u c t i o n

Recent publications [1] related to the evolution of oxygen on dimensional- ly stable anode (DSA) materials repor t exchange currents that are significantly higher than those observed for platinum. These electrode materials have recent- ly found wide application as a replacement for graphite in the electrolysis of brine. Similar results have also been reported for the electrolytic product ion of sodium chlorate [2]. The catalytic activity of DSA materials is, therefore, not restricted to the reaction involving chlorine evolution. Some time ago we were interested in the oxidation of selenium in acidic media and since oxidat ion/ reduction reactions involving sulfur and selenium compounds in different oxidation states are of ten facile, it seemed to us that DSA materials containing selenium as one of its components might facilitate the reaction of interest. Electrodes of this general nature had already been described in the patent literature [3, 4]. We shall show that these electrodes are catalytically active for oxidat ion/reduct ion reactions involving not only selenium but also sulfur compounds.

The electrochemical studies were carried out in a conventional manner using commercially available potent iostat ic equipment and voltage sweep generators. Potentials were measured with respect to a saturated calomel electrode. The method of electrode preparat ion was similar to that described earlier [3, 4]. A mixture of metallic salts of the desired coating components was dissolved in di- methyl formamide; for example, 80 mg H2SeO3 was added to 10 ml of 0.1 M IrC13 • H20 in DMF. The ti tanium screen electrode was coated by dipping into this solution and subsequently heat treating in a furnace at 350°C for 10 min, The coating and heat treating procedures were repeated 10 times followed by a final heat t rea tment at 350--450°C for 1--2 h. The furnace atmosphere was air. The composit ions cited later refer to the composi t ion of the coating mixtures. Determinations of surface concentrat ions and coating thickness were not made.

*To whom all correspondence should be addressed. **Present address: Central Research Labs, Hooker Chemical, Grand Island, N.Y. 14302.

424

Results and discussion

Typical current-voltage curves for a platinum and the selenium containing DSA in molar sulfuric acid are shown in Fig.1. The curves are the same, for all practical purposes, between -0.5 and +1.25 V vs. SCE. There do not appear to be any significant faradaic processes occurring over this range of potentials. The evolution of oxygen on the DSA occurs at potentials some 200 mV less positive than those required for platinum. This is in agreement with published data for the evolution of 02 on DSA and on platinum. The data of specific interest are those shown in Fig.2. The difference in behavior between platinum and the DSA containing selenium is most striking. The curve for platinum ap- pears to be unaffected by the presence of selenous acid in the electrolyte when the potential is cycled between -0.3 and +1.5 V. It should be noted that the standard potentials for the couples Se/Se 4+ and Se 4 +/Se 6+ are +0.74 and +1.15 V vs. SHE, respectively. These curves indicate that the oxidation of SeO~- -* SeO~- and the reduction of SeO~--+ Se occur only at high over- voltages on platinum electrodes. The DSA on the other hand readily supports reduction of selenous acid at relatively low overvoltages and the deposited selenium is readily oxidized on the reverse anodic sweep. Thus, the current peak shown in Fig.2 is due to the Se -~ SeO~- oxidation reaction. The process responsible for this peak is clearly due to a process involving selenium rather than to a double layer charging process since the charge under the curve is 1A--1A C cm -2 . Furthermore, the charge under the anodic peak corresponds to the amount of Se deposited on the electrode during cathodic discharge. The DSA electrode, therefore, supports both oxidation and deposition of selenium. Interestingly, selenium can be deposited on platinum, but only at large over-

9.0

6.0

~' 3.0

E '~- 0

- 3 . 0

- 0 . 5 0 0.5 1.0 1 .5 E/V vs. SCE

Fig.1. V o l t a m m e t r y curves for Pt p la ted Ta(1 ) and Ir-Se based DSA (2) in 1 M H2SO 4. Sweep ra te 10 m V s -1 ; e lec t rode area 1 cm2; Ir-Se a t o m i c ra t io 1:2, t emp. 65°C.

425

20.0

lo.o 1

? E

< 5.O E

--5.0

• 1 ~ + I

--0.5 0 1.0 1.5

r T I I I I I I

I ¼ / !

! ! I

/ k

/

/

I !

0.5 E/V vs. SCE

Fig.2. V o l t a r n m e t r y curves for Ir-Se based DSA in 1 M H=SO 4 (1) and 1 M H2SO ~ wi th 1.5 × 10 -2 M H2SeO 3 (2). Sweep ra te 10 m V s - ' ; e l ec t rode area 1 cm~; Ir-Se a t o m i c ra t io 1:2 t emp. 65°C.

voltages and it occurs in the presence of H2 evolution. If after sufficient deposition of Se on platinum, an anodic voltage sweep is applied, we then ob- serve the i--V curve in Fig.3. The deposited selenium is therefore re-oxidizable on platinum, as well as on the DSA, although at somewhat higher overvoltages (~100 mV). The major difference between the DSA and platinum is that the exchange current for SeO~- reduction is much higher on the DSA material.

9.0

6 . 0

E ~ 3.0

¢

-3 .0

- 0 , 5

I I ! I I I

l I I

I I f ~

I x

I 1 0 0.5 110 1'.5

E/V vs. SCE

Fig.3. V o l t a m m e t r y curves for P t p la ted Ta in 1 M H2SO + (1) and 1 M H2SO + w i t h 1.5 x 10 -2 M H~SeO3 af te r po la r i za t ion in the h y d r o g e n evo lu t ion region. Sweep rate 10 mVs-1; e lec t rode area 1 cm2; t emp. 65°C.

426

The rates of oxidation of selenium are reasonably similar. The data presented here are very preliminary. We have no comparable data for other DSA com- positions and hence are unable to assess the importance of coating com- positions.

It is interesting to note that neither the DSA nor platinum catalyzes the oxidation of seo~- -~ SeO~-. The catalytic process responsible for the reduc- tion of SeO~- is not general for selenium compounds. It is rather specific for the SeO~- species, actually H2 SeO3 considering the pH of M H: SO4. Since no quantitative studies were performed, it is premature to speculate on the mechanistic reasons for these facts. If the explanation simply involved adsorp- tion of the H2 SeO3 on the DSA and not on platinum then one would expect catalysis of the oxidation reaction as well as an enhanced rate of reduction. The fact that this is not observed leads us to discount the importance of ad- sorption as the rate determining step in either process.

The fact that the reaction SeO~--~ SeO~- does not proceed on Pt can per- haps be explained by classical inhibition of the oxide species covering platinum at the relevant potential range. The absence of an SeO~--+ SeO~- oxidation current on DSA cannot be ascribed in an a priori manner to a similar mechan- ism since nothing has been published about the difference in surface composi- tion of the DSA over the range in question. In other words, it is not known whether or not "passivating oxides" are able to form on DSA in the same manner as they do on noble metal electrodes. The sweep rate used was too slow to detect charge transfer processes involving the DSA materials in the potential range of relevance. It is this type of data that is generally used to determine oxide concentrations on noble metals. Our investigation did not in-

20

15

E 10

E

i I I

0 I

0 015 2.0

ej I I

1.0 1.5 E/V vs. SCE

Fig.4. Voltammetry curves for Pt plated Ta (1) and Ir-Se based DSA (2) in 1 M H2SO J DMSO solution (1:1). Sweep rate 10 mVs-1; electrode area 1 cm2; Ir-Se atomic ratio 1:1, temp. 65°C.

427

clude such a detailed mechanistic study. As a pertinent comment, we should mention that the reactions reported here are not affected by corrosion involv- ing the DSA material. The material remained stable for at least several days under all conditions studied.

In addition to our studies with selenium systems we also investigated the oxidation of dimethyl sulfoxide (DMSO). Current-voltage curves for the oxida- tion of DMSO are shown in Fig.4 for both platinum and a DSA containing iridium and selenium. The platinum electrode is obviously much less active for the oxidation than the DSA material. On platinum DMSO is oxidized, presum- ably to dimethyl sulfone, at potentials near those required for oxygen evolu- tion. However, with the DSA material containing selenium, oxidation occurs at potentials near those of the reversible oxygen electrode. Even though the potential required with the DSA material lies some 500 mV below that needed to oxidize DMSO on platinum, the overvoltage is still very high. Hence, the catalytic effect of the DSA electrode does not provide a reversible couple based on DMSO. As observed earlier for the reduction of selenous acid, the oxidation of DMSO is not catalyzed by all DSA materials. However, detailed correlation between the composition of the surface (DSA coating) and its electrocatalytic properties were not made. Interestingly, DMSO is not reduced on either platinum or the DSA at potentials above 0 V vs. SCE, whereas in the case of electrochemistry of selenous acid, the reduction reaction was catalyzed. It is difficult to understand why DMSO is not reducible on either platinum or the DSA materials whereas it is readily reducible by sulfur compounds such as thiols.

The rate of adsorption of DMSO from the solution onto the substrate does not appear to be rate determining. It appears, therefore, that the configuration of the adsorbed species must be critical in its effect upon the rate of oxidation/ reduction processes.

In conclusion, we have shown that DSA materials have specific catalytic activities that are different from those of noble metals. We can also conclude that those materials containing chalcogen components enhance the rate of some of the oxidation/reduction reactions involving chalcogenides. However, a more detailed study of surface properties of the DSA materials would be re- quired to develop theories having any real predictive value.

REFERENCES

1 G. Bianchi, J. Appl. Electrochem°, 1 (1971) 231. 2 R.T. Atanasoski, B.Z. Nikolic, M.M. Jaksic and A.R. Despic, J. Appl. Electrochem.,

5 (1975) 155. 3 German Offen, 2,136,391 (1972). 4 S. Africa Appl., 7482 (1968).