Corresponding Member of the Academy of Sciences of the USSR A. I. BRODSKII, V. A. LUNENOK-BURMAKINA

ON THE NATURE OF SURFACE OXIDES OF PLATINUM FORMED DURING THE ANODIC DISCHARGE OF HYDROGEN PEROXIDE

Electrolysis of aqueous solutions containing hydrogen peroxide or peroxo acids is accompanied by the evolution of anodic \(O_2\), which contains, as was found in earlier works \((1–3)\), oxygen of the peroxide compound.* In this process, peroxide discharge is the dominant anodic reaction, and only after the peroxide concentration in the solution becomes sufficiently small (for example, a few g/liter \(H_2O_2\) in 20% \(KHSO_4\) \((1)\)) does the discharge of water and \(HSO_4^-\) begin to predominate, with formation of \(O_2\) from the oxygen of water (and of persulfate, retaining the oxygen of sulfate). In another work \((2)\) it was shown that anodic ozone is formed from \(O_2\) and the oxygen of water in exact accordance with the stoichiometric relation:

\[ O_2 + H_2O = O_3 + 2H^+ + 2e. \tag{1} \]

These results are well explained by the ideas developed by V. M. Veselovskii \((6)\) and A. N. Frumkin \((7)\), according to which the primary stage of the anodic reaction is the formation of surface oxides of platinum, which, upon interaction with water, give anodic oxygen and ozone. However, our data \((2, 3)\) lead to the conclusion that, in an electrolyte containing peroxide compounds, higher surface oxides of platinum are formed not only from water:

\[ \mathrm{PtO} + 2H_2O = \mathrm{PtO}[OO] + 4H^+ + 4e, \tag{2} \]

but also from these peroxides:

\[ \mathrm{PtO} + H_2O_2 = \mathrm{Pt}[OO] + 2H^+ + 2e, \tag{3} \]

\[ \mathrm{PtO} + H_2O + HOOSO_3^- = \mathrm{PtO}[OO] + HSO_4^- + 2H^+ + 2e. \tag{4} \]

In studying the mechanism of the last reaction (4), cleavage of peroxomonosulfuric acid during its decomposition at the anode was established \((3)\).

To study the behavior of the peroxide bond during the anodic decomposition of \(H_2O_2\) and to elucidate the nature of the higher oxide of platinum formed in this process, we investigated the isotope-exchange reaction

\[ O_2^{18} + O_2^{16} = 2O^{16}O^{18} \tag{5} \]

under the conditions of the anodic process. Its equilibrium constant

\[ K = \frac{[O^{16}O^{18}]^2}{[O_2^{16}][O_2^{18}]} \]

corresponds practically exactly to the statistical distribution (\(K = 4\)), for which the ratio of isotopic molecules is:

\[ \frac{[O_2^{18}]}{[O^{16}O^{18}]} = \frac{u^2}{2u(1-u)} \quad \text{and} \quad \frac{[O_2^{18}]}{[O_2^{16}]} = \frac{u^2}{(1-u)^2}, \]

where

\[ u = \frac{2[O_2^{18}] + [O^{16}O^{18}]} {2[O_2^{16} + O_2^{18} + O^{18}O^{16}]} \]

is the total atomic fraction of \(O^{18}\) in the oxygen. If mixtures of \(O_2\) of different isotopic composition are taken, then the equilibrium of reaction (5) and the indicated ratio of isotopic varieties can be established only upon cleavage of the \(O—O\) bonds, without which no exchange of oxygen occurs between \(O_2\) molecules of different isotopic composition.

The experiments were carried out in a U-shaped electrolyzer at a current density from 0.05 to 0.2 a/cm\(^2\) on an anode made of platinum wire, at a potential difference of 12 V.

* Our data were confirmed in a later work by Devis et al. \((4)\). In it, also, in agreement with the earlier published work \((5)\), it was found that the hydrogen peroxide formed on a carbon cathode during electrolysis of alkaline solutions contains oxygen only from gaseous \(O_2\) washing this electrode. In \((4)\) it was also shown by the method described in the present article that, during the cathodic reduction of elemental oxygen to \(H_2O_2\), no destruction of the \(O—O\) bond occurs.

The electrolyte used was a solution of $\mathrm{H_2O_2^{18}}$ in $2N$ $\mathrm{H_2SO_4^{16}}$, dissolved in water of natural isotopic composition. The isotopic analysis of the oxygen was carried out by V. G. Golovatyi on an MX-1302 instrument. Heavy hydrogen peroxide was prepared from $\mathrm{H_2O^{18}}$ in a glow discharge by the method developed by I. I. Vol’nov and co-workers ($^8$). The content of $\mathrm{O^{18}}$ in the peroxide was determined from the oxygen evolved from it under the action of $\mathrm{KMnO_4}$. In experiments 1–3 (Table 1) the $\mathrm{H_2O_2}$ contained 11.1% $\mathrm{O^{18}}$, and in experiments 4–5 it was diluted approximately in a 1 : 1 ratio with $\mathrm{H_2O_2}$ of natural isotopic composition.

Table 1

Isotopic composition of anodic oxygen

Table 1 gives the mean values of $u$, found from the heights of the mass-spectrometric peaks [36], [34], and [32], as well as their ratios calculated for an equilibrium mixture.

In experiments 4–5, separate portions of $\mathrm{O_2}$ were successively collected and analyzed at different time intervals from the beginning of electrolysis.

The data obtained confirm earlier observations ($^{1,4}$), according to which, at a considerable content of $\mathrm{H_2O_2}$ in the electrolyte, the anodic oxygen has the isotopic composition of the oxygen in $\mathrm{H_2O_2}$. As the concentration of $\mathrm{H_2O_2}$ in the electrolyte decreases, increasing amounts of light oxygen from water are admixed with the anodic $\mathrm{O_2}$. It is significant that the initial ratio $[\mathrm{O_2^{18}}]/[\mathrm{O^{18}O^{16}}]$ of the oxygen in $\mathrm{H_2O_2^{18}}$ is retained in the anodic $\mathrm{O_2}$ even upon addition of 50% light $\mathrm{H_2O_2^{16}}$ and with a large decrease in the total $\mathrm{O^{18}}$ content in it. At the same time, as is seen from Table 1, this ratio differs sharply from the equilibrium one.

Thus, the data presented show that the anodic decomposition of hydrogen peroxide is not accompanied by rupture of the O—O bond, and that the peroxide group passes, without being destroyed, from $\mathrm{H_2O_2}$ into the anodic oxygen. This confirms the peroxide nature of the surface oxides of platinum formed during the discharge of $\mathrm{H_2O_2}$ in accordance with stoichiometric equation (3).

L. V. Pisarzhevskii Institute of Physical Chemistry
Academy of Sciences of the Ukrainian SSR

Received
27 V 1963

CITED LITERATURE

1. A. I. Brodskii, I. F. Franchuk, V. A. Lunenok-Burmakina, DAN, 115, No. 5, 934 (1957).
2. V. A. Lunenok-Burmakina, A. P. Potemskaya, A. I. Brodskii, DAN, 137, No. 6, 1402 (1961).
3. V. A. Lunenok-Burmakina, A. P. Potemskaya, DAN, 149, No. 6, 1343 (1963).
4. M. O. Davies, M. Clark et al., J. Electrochem. Soc., 106, No. 1, 56 (1959).
5. T. M. Abramova, I. L. Gankina, A. S. Fomenko, Dopov. AN URSR, No. 9, 974 (1958).
6. A. A. Rakov, V. I. Veselovskii et al., ZhFKh, 32, No. 12, 2702 (1958).
7. A. N. Frumkin, Comité intern. de thermodynamique et de cinétique électrochimiques. Réunion, 9, Paris, 1957, C. R., London, 1959, p. 396; M. A. Gerovich, R. I. Kaganovich et al., DAN, 137, No. 3, 634 (1961).
8. I. I. Vol’nov, A. B. Tsentsiper, V. I. Chamova, Izv. AN SSSR, OKhN, No. 3, 531 (1961).