J . Chem. SOC., Furuduy Trans. I, 1982, 78, 2937-2945 Kinetics of the Electrochemical Evolution of Isotopically Enriched Gases Part 1.-180160 Evolution on Platinum in Acid and Alkaline Solution BY CHRISTINE R. CHURCHILL AND D. BRYNN HIBBERT* Department of Chemistry, Bedford College, Regent's Park, London NW 1 4NS Received 1 1 th December, 198 1 A method is described which follows, by mass spectrometry, the kinetics of la0160 evolution from an electrode surface enriched with lSO. The kinetics on a platinum surface in acid and alkaline electrolyte were consistent with a mechanism of oxygen evolution which involves successive oxidations on a single platinum atom. The total number of platinum sites responsible for oxygen evolution was less than the total number of atoms at the surface.The electrochemical evolution of oxygen from metals occurs on an oxide-covered surface,l but the exact nature and participation of this surface in the reaction has yet to be fully interpreted. Rosenthal and Veselovski9 first demonstrated the existence of surface oxides during oxygen evolution by means of .an l80 tracer. They showed that whilst surface oxygen compounds formed between 0.8 and 1.2 V (us. SHE) do not take part in the process, higher oxides formed at potentials > 1.4 V do participate. Several authors have postulated schemes involving reactions of higher oxides but no direct evidence has been forthcoming. Tseung and JasemlO have correlated, with some success, the potentials of the metal/metal-oxide couple or lower-metal-oxide/higher-metal-oxide couple with the onset of oxygen evolution.Further work on NiCo,O," has confirmed that Ni3+ (possibly Ni4+) and Co4+ are formed at potentials at which oxygen is evolved. The importance of this approach to electrolytic devices is obvious; no matter how efficiently oxygen is evolved on a particular surface, if the highest metal oxide is not formed until a potential is reached which is substantially above E* for oxygen evolution (1.23 V at 25 "C), then a voltage loss is inevitable. In the case of platinum, Hoarel agreed that unless the platinum electrode is anodically polarised beyond the equilibrium potential of the PtO/PtO, couple, oxygen evolution cannot proceed. In an earlier work12 the use of I*O to observe the kinetics of evolution of oxygen on NiCo,O, was demonstrated. Here, using an improved experimental technique and an extended kinetic interpretation which allows for two intermediate oxides, information is obtained regarding the total number of sites, the extent of higher oxide formation and the general mechanism.THEORY FRACTION OF IN THE EVOLVED GAS A T THE SURFACE A simple mechanism for oxygen evolution on a metal in oxidation state, N , MN via higher oxidation states N + I, N + IT is MN+-OH -+ MN+IOH+e MNflOH+-OH -+ MN+110+e+H20 2M A'+ I 1 0 --+ 2MN+0, 29372938 1s0160 EVOLUTION ON PLATINUM for reactions in alkaline media, and MN + H,O -+MN+IOH+H++e (1 b) MN+IOH+H20 -+ MN+IIO+H,O++e (2 b) 2MN+110 -+2MN+0, (3 b) for reactions in acid media. The initial oxidation state of the metal ( N ) may refer to a bare metal surface ( N = 0), or to an oxide covered surface ( N > 0).For example, on platinum PtO, (i.e. N = 4) is the stable oxide on which oxygen is evolved. If the reactions are carried out in electrolyte containing a fractionf, of l80, in the steady state a fractionf-, of MlsOH and MlsO will be found at the surface. Evolution of oxygen on such an enriched electrode in an electrolyte having a normal ls0 content leads to a progressive depletion of la0 from the surface and the appearance of ls0l6O and 1s0180 in the gas phase by reactions such as MlsOH+-OH -+ M180+H20+e (4) MlsO + ML60 -+ 2M + la0160. ( 5 ) 2M180 + 2M + lsO1sO (5’) may be neglected. In addition, if the overpotential for oxygen evolution is high, the back reactions of (4) and (5) may be neglected.Thus the reaction scheme comprising reactions (4) and ( 5 ) is one of consecutive first-order processes giving rise to 180160 in the gas phase. Let a and b be the initial enrichment of l80 in the surface oxide layer as M180H and M180, respectively, and let x, y and z be the enrichment of ls0 at time t as M180H, M180 and l80l6O, respectively. The kinetic equations are thus If the surface enrichment is small, reactions such as dx/dt = - k , ~ dy/dt = k,x- k,y dz/dt = k, y where k, and k, are first-order rate constants for reactions (4) and (5). In this derivation no account is taken of kinetic isotope effects and single values of k, and k, are assumed. The solution for y is y = exp (-k5t) [k,a/(k,-k,)I ([exP ((k5-k4)t)-lI+b} ( 6 ) for k4 # k5 and y = (kat+b) exp (-kt) (7) for k, = k, = k. The fraction of l80 in the evolved oxygen (fi8) is the ratio of the rate of evolution of l80 as l80l60 and the total rate of evolution of oxygen atoms.Thus (8) fis = (d~/dt)/(Li/2F) = k5y/(Li/2F) where i is the galvanostatic current, F is the Faraday constant and L is Avogadro’s number. The rate constants k, and k, are dependent on the current flowing and the total number of sites at the electrode surface. The rate of reaction (4) for all oxygen atoms must be iL/2F atom s-l and therefore the rate of reaction of M180H is (iLI2F) (fMoH), where fMoa is the fraction of MOH which is MlsOH. In terms of MlaOH (x) and the total MOH (x,), the rate is (iL/2F) (x/x,). Thus dx/dt = - iLx/(2FxT). (9)C. R. CHURCHILL AND D. B. HIBBERT 2939 Comparison with the treatment above shows that k , = iL/(2F+).If yT is the total MO, then by similar reasoning k , = iL/(2FyT). Substituting values of k , into eqn (8) gives where y is given by eqn (6) or (7). (10) f 1 8 = Y / y , FRACTION OF "0 IN THE EVOLVED GAS AT THE MASS SPECTROMETER In an experiment the fraction of lS0 in the evolved gas cannot be measured at the electrode surface but some distance away where it is diluted by a purging gas. It is possible to allow for this in the kinetic equations and obtain an expression for the experimental rn/e 34 signal. Let the evolved oxygen and a purge gas expand into a volume which may accommodate n moles of an ideal gas at a total rate v mol s-l. The rate at which all l80 is introduced into the volume is k,y [eqn ( 5 ) ] plus a background component vb.The natural abundance of l80 is 0.002, so vb = ( i L / 2 F ) x 0.002 atom s-l. The rate at which l80 is removed from the volume is the rate at which all gas flows through (v) multiplied by the fraction of l80 in the volume, which is n18/n, if 4 8 is the total in the volume. Therefore dn,,/dt = k , y + vb - (n,,/n) (1 1) where u/n (= k,) is a first-order rate constant which determines the rate at which the mass spectrometer 'sees' changes in l80 at the electrode surface. Eqn (11) may be solved using the expression for y , eqn (6), IS = (Vb/kn) [1 -exP ( - k n t)I 4- [ak4k5/(k5-k4)1 [exP (-k4 t)-eXP ( - k n t)I/(kn-k4) (12) +[kti/(kn-ktJI ( b - [ak,/(k,-k,)l} [exp (-k,t)-exP ( - k , t ) l for k , # k, and n,, = (vb/kn)[ 1 -exp (- k , t)] + [ k / ( k , - k)] {akt - [ka/(kn - k)] + b} exp (- k t ) + [k/(kn - k)] [ka/(kn - k ) - b] exp ( - k , t ) (1 2') for k, = k , = k .The mass spectrometer signal of m/e 34 ( S ) is proportional to the fraction of l80 in the volume at constant pressure = K(n18/n> (1 3) where K is the constant of proportionality. In a background experiment in which there is no enrichment (a and b both zero) (14) Data from enriched and background experiments will thus allow calculation of the unknowns in eqn (12)-(14). S = ( K / n ) ( ~ , / k n ) [1 - ~ X P (-knt)l* EXPERIMENTAL MATERIALS Platinum black electrodes on 80 gauge platinum mesh screens were prepared by electrolysis of a solution which contained 5 g PtC1, in 0.1 dm3 water.13 The electrodes were formed into a cylinder to provide the maximum area in a given volume of cell.The surface areas of the platinum electrodes were measured by hydrogen stripping14 and oxygen charging curves.I6 All reagents were of AnalaR grade. The acid electrolyte was 1 mol dmV3 sulphuric acid and the alkaline electrolyte was 5 mol dmF3 potassium hydroxide, made up in doubly distilled, deionised water. Enrichment to ca. 3% l80 in alkaline electrolyte and 4% l80 in acid electrolyte was accomplished by the addition of suitable amounts of 20% l80 H,O (B.D.H.) to a stock solution2940 l8ol60 EVOLUTION ON PLATINUM of appropriate concentration of either the acid or the alkali. The isotopic content of the electrolyte was determined by evolving oxygen in the electrolyte and measuring the ratio of m/e 32 and 34 at the steady state.APPARATUS The enrichment of a surface with lSO was performed in a thermostatted (20f 1 "C) two-compartment cell, continuously purged with nitrogen. The cell used to monitor the sub- sequent evolution of l80 in normal electrolyte is shown in fig. 1. The aim of the experiment is to detect l80 near to the electrode with as little dead space as possible. The probe to the mass spectrometer (P) was placed in the path of the outgoing gases just above the surface of the FIG. 1 .-The electrochemical cell. For description see text. electrolyte. Calcium chloride in the probe removed water vapour and also cut down the volume of the probe. Nitrogen was introduced into the working electrode compartment at (I). The arrangement shown allows the majority of the gases in the cell to pass out through an air-lock (11); only a small fraction was diverted into the mass spectrometer via a leak valve (L).The counter-electrode was a high-surface-area platinum electrode (C) and the potential of the working electrode (W) was measured against a dynamic hydrogen electrode (DHE) reference electrode (R) via a Luggin capillary. The DHE itself was standardised against a bubbling hydrogen electrode. No account was taken of the ohmic drop in the electrolyte. METHOD The platinum electrode was first reduced by evolving hydrogen on it at 0.5 A. The current was reversed and oxygen was then evolved in enriched electrolyte at constant current (usually 0.5 A) for periods of time between 15 min and 6 h. The electrode was removed, washed in distilled water for periods up to 1 h and introduced into the cell of fig.1 at open-circuit voltage. Nitrogen purging was maintained at a constant rate (ca. 5 times the rate of oxygen evolution) throughout the experiment. When the m/e 34 peak had fallen to an acceptably low value (< 1 % of the normal background), showing that the small amount of air introduced with the electrodeC. R. CHURCHILL A N D D. B. HIBBERT 294 1 had been purged away, a constant current (0.25-0.75 A) was passed through the cell by a Weir constant current supply or a Thompson Associates potentiostat operating in a galvanostatic mode. The quadrupole mass spectrometer (Vacuum Generators Anavac 2) operating at to mbar continuously monitored m/e 34 which was displayed on a Y-t recorder.This procedure will be referred to as an enriched run. The peak reached a steady state corresponding to the normal background value in < 10 min. The current was switched off and when the peak height had fallen back to its zero value the current was switched on again and m/e 34 monitored with time. From this second run the background 1s0160 evolution was determined. It is possible to ignore the small amount of l80 which can be introduced into the electrolyte during the enriched experiment. This can be no more than of the background and would be expected to be some orders of magnitude less. During the experiment the potential of the working electrode was continually monitored. The accuracy of the method was tested by a series of blank experiments in which two background runs were performed consecutively, and others in which the electrode was simply soaked in enriched electrolyte before being washed etc.in the manner described above. A computer program written in FORTRAN v fitted background data, taken at 3 s intervals, to eqn (14) by a least-squares procedure. Using the values of k , and K / n so obtained, plus a calculated value of ub, the data of an enriched run were fitted to eqn (12) and (1 3). Thus a, b, xT and y , were determined. In addition, an experiment was performed in which the potential of the electrode was measured during oxygen evolution in the cell of fig. 1 and at open circuit for some time after the current was switched off. Potential measurements were also made for an electrode after it had been washed in the manner described above, following oxygen evolution in enriched electrolyte. RESULTS SURFACE AREA OF THE ELECTRODE The surface area of the platinum electrode determined by hydrogen stripping and by oxygen charging curves agreed within 10%.A typical surface area of a 1 cm x 1 cm mesh determined by these methods was 2000 cm2. l80 ENRICHMENT A typical fitted background run is given in fig. 2. k , was of the order of 0.0142 s-l for an evolution current of 0.25 A. The values obtained for k , in a series of experiments were consistent with the estimated volume of the dead space (n) and the flow rate of the gases (u). A typical enrichment curve (enriched run minus background run) for evolution in alkali is given in fig. 3. The results obtained from acid and alkali solutions were of the same orders of magnitude.For the same 5.0 m2 platinum electrode, values of the constants in eqn (6) were: in alkali, xT = (1.09 k 0.12) x 1019, yT = (1.02k0.13) x lOl9, a = (1.48k0.46) x 10l6 and b = (1.42+ 1.26) x The errors in these values represent the standard errors of six experiments. For experiments in acid the constants were xT = (0.99k0.08) x 1019, yT = (0.80+0.03) x 1019, a = (1.48 kO.19) x 10l6 and b = 6.02 x 10l2. The number of platinum atoms on this electrode from oxygen charging experiments was 5.76 x 1019. All blank experiments showed that no enrichment of lSO could be imparted to an electrode by soaking in enriched electrolyte alone. VOLTAGE MEASUREMENTS Fig. 4 shows the potential of an electrode during oxygen evolution and at open circuit in the cell for up to 90 min.The continuous line shows the effect on the open circuit potential of removing the electrode from the cell and washing it for 90 s before re-introduction into the cell.2942 l8ol60 EVOLUTION O N PLATINUM ’ j{ , , , , , , 20 40 60 80 100 120 140 160 time/s FIG. 2.-Mass spectrometer signal (circles) for m/e 34 after the start of oxygen evolution on the Pt electrode in normal alkaline electrolyte. The continuous line is a fit to eqn (14). DISCUSSION TOTAL NUMBER OF SITES Two facts emerge from these experiments concerning the number of sites on the platinum surface responsible for evolution of oxygen in alkali. First, the numbers of sites generating the higher oxides MOH and MO [x, and y , of eqn (9) and (lo)] are the same.This is consistent with consecutive oxidation on a single platinum atom. Secondly, the values of xT and yT are ca. 1/5 of the value determined for the total number of sites by oxygen charging in alkaline solution. It may be impossible to form a further complete layer of oxygen on an already oxidised surface or reaction between certain high oxide pairs [reaction (3)] may be unfavourable on particular crystal planes. Arguments similar to the latter were proposed by Yeung for the very low surface oxygen coverage found during oxygen reduction on N ~ , ~ , S ~ , , , C O O , . ~ ~ RESIDUAL “0 I N THE SURFACE It was found that after the electrode was taken out of the enriched electrolyte, whatever the period elapsed (practically this was not less than 3 min), no MIsO remained in the surface, i.e.b = 0. ‘MO’ is considered to be an unstable oxide which is only formed at high anodic potentials and which decomposes by reaction (3). Its lifetime on removing the anodic potential would be short, in accordance with the experimental findings. In the absence of any mechanism which removes Ml80H, the initial MlsOH [a of eqn (6), (7) etc.] should be ca. 3% of xT in alkaline electrolyte and ca. 4% of xT in acid electrolyte. The values for a found in experiments with both acid and alkaline electrolyte were twenty times less than this figure. The loss of MlsOl b L 13 12 11 10 h 9 - 8 - - .d c e v d ; 7- ._ 6 - 5 - QJ --. 4 - C. R. CHURCHILL AND D. B. HIBBERT - - - - 3 - 2 - 1 - 0 2943 I,,,,,,,, 0 20 40 60 80 100 120 140 160 time/s FIG.3.-Difference between m / e 34 signal for oxygen evolution on the Pt electrode enriched with l80 and the background experiment (circles). The continuous line is a fit to eqn (1 2) and (1 3). Washing time was 90 s. The total elapsed time between evoltution in enriched and normal electrolyte was 3 min. may arise from chemical decomposition of MOH, exchange of I 8 0 with an underlying oxide layer or exchange of la0 with the electrolyte. It is also possible, as Laitinen and Enke have suggested, that evolution of oxygen, whilst being preceded by oxide formation, occurs via unstable adsorbed intermediates on the strongly bound oxide 1ayer.I' The potential of the electrode some minutes after cessation of evolution of oxygen and in a nitrogen purged solution (fig.4) is well above the ultimate steady-state potential. The chemical decomposition of the intermediate oxide MOH is thus relatively slow and would not cause the observed loss of M-180H. Exchange with an underlying oxide layer may be facile, but if the evolution of oxygen takes place on a metal surface, any oxide formed would have the isotopic content of the enriched solution. Exchange of oxygen with the normal electrolyte offers the most likely explanation of the apparent loss of l80. Reaction between the intermediate oxide MOH and adsorbed water by proton hopping M1*OH + M"OH, + M180H, + M160H (1 5 ) would diminish adsorbed M180H.2944 1s0160 EVOLUTION ON PLATINUM \ \ \ \ \ \ \ 11 ' * 3 t --- - - --- 1 . 2 1 2 1 0 60 120 180 240 jj 5LOO FIG.4.-Potential (us. DHE) of the platinum electrode in alkaline solution at 20 O C . I, Potential during steady-state evolution of oxygen at 0.25 A. 11, Open-circuit potential of electrode in cell purged with N, (dashed line). The continuous line shows the effect of removal from the cell for 90 s and washing in distilled water. timels THE NATURE OF THE OXIDES ON PLATINUM The potentials associated with the formation of oxides of platinum, measured against a hydrogen electrode in the same solution, arels PtO 0.98 V, PtO, 1.05 V and PtO, ca. 2.0 V. The stable form of the oxide in the presence of oxygen is PtO,. There is some doubt about the exact potential of the formation of PtO3,l8g l9 but it is known to decompose to PtO, and oxygen. We therefore assign the initial state of the platinum surface as PtO, with transition to PtO, as the mechanism of oxygen evolution.The fall in open-circuit potential with time shown in fig. 4 occurs as the intermediate oxide PtO, -OH decomposes back to PtO,. The mechanism of oxygen evolution is therefore PtrVO, + -OH -+ PtVO,OH + e (16) PtVO,OH + -OH + PtVIO,O + e + H,O (17) 2PtVI0,O + 2PtIV0, + 0,. (1 8) Another question concerning this mechanism is to what extent is the reactive oxygen atom of eqn (1 8) part of a recognisable species PtO,? A highly mobile adsorbed atom may not be correctly represented as PtO,. Further isotopic studies on the exchange between the oxygen atoms in PtO,-O may give the answer. HoareZ0 has shown that the formation of a Pt-0 'alloy' during strong anodisation results in a reversible oxygen potential of 1.229 V.He ascribes irreversible behaviour on platinum which is not saturated with oxygen to the action of a local cell at the platinum surface. It may be, therefore, that it is not correct to write the surface species as distinct oxides, although the dissolution of oxygen into the platinum lattice may occur concurrently with oxygen evolution. SENSITIVITY The use of l80 as a tracer for oxygen evolution studies reveals the exceptional sensitivity of the method. In this work an enrichment of ca. 10l6 atoms was determined. The lower limit for the enrichment (a) was found experimentally to beC. R. CHURCHILL AND D. B. HIBBERT 2945 ca. 10% of the normal abundance of ls0 in the total number of sites (xT). For xT = 1.1 x 1019 sites the limit would be 2.2 x 1015 atoms.(The sensitivity of the mass spectrometer eventually limits the total amount of l80). In terms of the rate at which l 8 0 is evolved, it is desirable for k, and k , to be < 0.2 s-l. For a current of 0.1 A, k, = 0.2 s-l gives xT = 1.5 x lOls atoms. The absolute sensitivity of the mass spectro- meter is not a limiting factor, and if the evolution of oxygen occurred from an electrolyte containing no l80, several orders of magnitude would be added to the sensitivity. C. C . is supported by an S.E.R.C. CASE award with British Gas. D.B.H. thanks the University of London Central Research Fund for an equipment grant. J. P. Hoare, The Electrochemistry of Oxygen (Wiley Interscience, New York, 1968). A. Damjanovic and B. Jovanovic, J. Electrochem. Soc., 1976, 123, 374. A. C. C. Tseung and S. Jasem, 151st Meeting of the Electrochemical Society, Philadelphia, 1977, Extended Abstracts no. 351. S. E. S. El Wakkad and S. H. Emara, J . Chem. Soc., 1952, 461. A. Damjanovic, A. T. Ward and M. O’Jea, J . Electrochem. Soc., 1974, 121, 1186. T. R. Hoar, Proc. R. Soc. London, Ser. 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Sci., 1907, 145, 117. *O J. P. Hoare, J . Electrochem. Soc., 1978, 125, 1768. 1 966). (PAPER 1 / 1924)