Study by potential-modulated reflectance spectroscopy of the reactivity of ethanol with the passivating layer on Ni and Fe

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Journal of Electroanalytical Chemistry 395 (1995) 243-247

Study by potential-modulated reflectance spectroscopy of the reactivity of ethanol with the passivating layer on Ni and Fe A. Kowal 1, C. Guti6rrez lnstituto de Qulmica Fisica " Rocasolano' '. CS1C, C. Serrano, 110. 28006-Madrid, Spain

Received 6 March 1995; in revised form 11 May 1995

Abstract The influence of ethanol on the passivating films on Ni and Fe electrodes in 1 M NaOH was studied by cyclic voltammetry and potential-modulated reflectance (PMR) spectroscopy over a wide potential range. The decrease in the PMR maximum attributed to NiOOH which occurs when ethanol is added clearly shows that ethanol produces a decrease in the thickness of the NiOOH layer on the Ni electrode; this effect increases in a nearly exponential way with the potential, reaching near saturation at an ethanol concentration of 1 M. A similar, but much smaller, decrease in the FeOOH layer on iron is also produced by ethanol. In contrast, neither the passivating NiO on Ni nor the passivating Fe(III) oxides on Fe are attacked by ethanol. Since only the Fe and Ni oxyhydroxides are attacked by ethanol, while the passivating oxides are not, it is concluded that in order to react with ethanol the anodic oxides of Fe and Ni must have hydroxyl groups. Keywords: Passivation

1. Introduction The electro-oxidation of alcohols for industrial purposes has been actively investigated [1] in particular the oxidation of diacetone-L-sorbose to diacetone-2-ketogulonic acid, an intermediate in vitamin C synthesis, which can be carried out with both iron oxide and nickel oxide electrodes in alkaline media. The electro-oxidation of low molar mass alcohols is also of practical interest, because low molar mass ( C 2 - C 5) organic acids and their salts are used as bioassimilable preservatives for corn and green forage. A process has been developed [2,3] in which these salts are obtained by the electro-oxidation of faints, a waste product of ethanol distillation consisting of a mixture of C 2 - C 5 alcohols, on a Ni electrode in an alkaline medium. At the positive potentials used the surface of the Ni electrode is covered by anodic oxides a n d / o r oxyhydroxides, and the question therefore emerges as to whether a direct chemical reaction occurs between this anodic layer and the alcohols. Severals studies of the electro-oxidation of alcohols on Ni have been reported [4-6].

IOn leave of absence from the Institute of Catalysis and Surface Chemistry. Polish Academy of Sciences, Cracow, Poland. 0022-0728/95/$09.50 © 1995 Elsevier Science S.A. All rights reserved SSDI 0022-0728(95)04149-4

Potential-modulated reflectance (PMR) spectroscopy is a very sensitive in-situ technique which in favourable cases can distinguish between lower and higher oxides formed on the metal surface, and even between oxides with the same chemical formula but different structures (e.g. between o¢-Fe20 3 and y - F e : O 3) [7]. The purpose of this work is to establish, by means of this in-situ spectroscopic technique, whether ethanol, taken as a model compound for faints, reacts chemically with the anodic oxides and oxyhydroxides on the surface of Fe and Ni electrodes. Fe was also included in this study, both because iron oxide electrodes can be used in the electro-oxidation of diacetone-L-sorbose, as stated above, and because the anodic oxides of iron have been extensively studied by PMRS [7].

2. Experimental Fe sheet (99.998% pure) and Ni sheet (99.99% pure), both 1.0 mm thick, were obtained from Alfa-Johnson Matthey. Disks of diameter 15 mm were cut from these sheets, polished with SiC emery paper and with alumina powder down to 0.05 /zm, and sonicated in ultrapure water. AnalaR grade reagents and MilIi-RO + Milli-O ultra pure water were used. The solutions were deoxygenated

244

A. Kowal, C. Guti&rez/Journal of Electroanalytical Chemistry 395 (1995) 243-247

bubbling with vigorous stirring for 15 min just before starting the experiment. A saturated calomel electrode (SCE) was used as the reference. The electrochemical cell and PMR set-up are described in [7]. An Osram Xenophot HLX 150W halogen lamp was used for measurements in the range 3 2 0 - 8 0 0 nm. The Ni electrode was cathodically polarized at - 1 . 5 2 V / S C E for 5 min in order to reduce the oxide film formed in air, and then it was subjected to 15 repetitive cycles from - 1.52 to 0.48 V. After this the potential sweep was stopped at + 0.03 V during a positive ramp, a sinusoidal potential modulation of rms amplitude 20 mV and frequency 170 Hz was applied and the PMR spectrum was recorded in the range 3 2 0 - 8 0 0 nm. Then the potential was swept to 0.23, 0.28, 0.33, 0.38 or 0.43 V, and the PMR spectrum was recorded under the same conditions as mentioned above. The values of A R / R and R at the wavelength of the maximum in the PMR spectrum were determined after successive ethanol additions to the solution up to a final concentration of 1.5 M. Finally the whole PMR spectrum was recorded again. The experimental procedure followed with the Fe electrode was similar to that used with Ni. It was polarized at - 1.36 V for 30 s in order to reduce the film formed in air, and then the potential was swept to 0.64 V. After this the potential was swept down to - 0.86, - 0.66, - 0.51, 0.01, 0.49 or 0.59 V, and a sinusoidal potential modulation of rms amplitude 50 mV and frequency 170 Hz was applied. The PMR spectrum in the range 3 2 0 - 8 0 0 nm was recorded, the value of Areax w a s noted, and A R / R and R at this wavelength were determined for successive ethanol additions, up to 1.5 M. Finally the PMR spectrum in the range 3 2 0 - 8 0 0 nm at the final 1.5 M ethanol concentration was recorded. by N 2

i'll 4~

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...... __----~_-2--"~

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Fig. 1. First, fifth and fifteenth voltammograms of a Ni electrode in 1 M NaOH from - 1.52 to 0.48 V/SCE. Sweep rate, 40 mV s - ~.

0.43 V two peaks with maxima at 400 nm and 630 nm appear in the PMR spectra and their relative contributions change with the potential. Therefore the peak at 400 nm dominates at 0.23 and 0.28 V, but at 0.28 V a shoulder appears at 600 nm which at higher potentials becomes the main peak at 630 nm. Addition of ethanol (dashed curves) to the cell at potentials in the range 0.23-0.43 V, at which NiOOH is formed, produces a decrease in the absolute value of the PMR spectrum of Ni, and this decrease increases with increasing potential. In order to analyse this decrease, it is necessary to take into account the changes in the amount of nickel hydroxide formed on the electrode surface in different

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3.1. Nickel electrode Fig. 1 shows selected cyclic voltammograms (first, fifth and fifteenth sweeps) of a Ni electrode in the range from - 1 . 5 2 to 0.48 V. Two peaks can be observed in the positive sweep, one at - 0.77 V due to the oxidation of Ni to NiO, and a reversible one at 0.33 V due to the formation of Ni(III) oxyhydroxide [8]. The second peak increases with cycling, as is well known [9,10]. The PMR spectra recorded using the procedure given in the experimental section are shown in Fig. 2, where the dashed and solid curves are respectively the spectra in 1 M NaOH and in a 1 M NaOH solution containing ethanol at its final concentration of 1.5 M. It can be seen that the wavelength of the maximum Areax increases monotonically with the potential, from 400 nm at 0.23 and 0.28 V, to 570 nm at 0.33 V, to 620 nm at 0.38 V and finally to 630 nm at 0.43 V. Actually, over the potential range from 0.23 to

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Fig. 2. PMR spectra of a Ni electrode at several potentials, shown against each spectrum: - PMR spectra in 1 M NaOH; - - - PMR spectra in 1 M NaOH+ 1.5 M ethanol. Modulation frequency, 170 Hz; modulation amplitude, 20 mV rms.

A. Kowal, C. Guti&rez /Journal of Electroanalytical Chemistry 395 (1995) 243-247

245

experiments. For this purpose the changes in reflectance due to ethanol addition are normalized as follows: max

p :=

( aR/R),,

max

20

-- ( AR/R),.5 max

( AR/R)o

where P is the normalized reflectance change, and the subscripts 0 and 1.5 respectively denote measurements in 1 M NaOH and in 1 M NaOH containing 1.5 M ethanol; the superscript max indicates that the value of AR/R was measured at ~max" The dependence of P on the electrode potential is shown in Fig. 3. It can be seen that the changes in P are small up to {).33 V, and become greater at 0.38 and 0.43 V. The dependence of AR/R at 0.38 V and Ama x = 630 nm on ethanol concentration is shown in Fig. 4. A monotonic decrease in AR/R with increasing ethanol concentration can be seen. The peak at 630 nm has been attributed to NiOOH [7] and, assuming that the value of AR/R is proportional to the amount of NiOOH, we can conclude that the amount of NiOOH present on the surface of the Ni electrode decreases monotonically as the concentration of ethanol increases. The finding that ethanol reacts with the oxyhydroxide NiOOH, but not with the oxide NiO, cannot be interpreted in terms of Burke's incipient hydrous oxide adatom mediator (IHOAM) model [11,12]. As Burke himself says, the activity of Ni for the electro-oxidation of methanol, formaldehyde and formate ion is minimal in the potential range below about 0 V / R H E over which these incipient hydrous oxides are stable on Ni in base [13]. NiOOH, and not a Ni hydrous oxide, is the species that catalyses ethanol oxidation, as was clearly proved in experiments in which the height of the current plateau at about 1.5 V / R H E for the electro-oxidation of 0.05 M ethanol on Ni in 2 M NaOH in potential sweeps at 0.1 mV s -~ increased by up to a factor of 12 with increasing thickness of the NiOOH layer grown by previous potential cycling in base electrolyte [14]. No further increase was observed for values of the charge of the anodic transpassivation peak

of

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0;5 1.0 1.'5 Ethanol cone/M

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Fig. 4. Dependence on the ethanol concentration of the PMR signal intensity at the wavelenglh maximum (630 nm) for a Ni electrode in 1 M NaOH. Electrode potential, 0.38 V / S C E .

above 80 mC cm 2, which was attributed to accessibility for ethanol oxidation of only a fraction of the total NiOOH surface area. A much simpler explanation is that the maximum current obtained (16 mA c m - 2 ) approximately coincides with the calculated diffusion-limited current. 3.2. Iron electrode The cyclic voltammograms (first, fifth and fifteenth sweeps) of an Fe electrode in the range - 1.36 to 0.64 V show three anodic peaks (Fig. 5). The first peak is attributed to the oxidation of iron to Fe(II) oxide, and the next two are due to the progressive oxidation of FeUD oxide to Fe(II[) oxyhydroxide, the amount of which increases with cycling [7]. The PMR spectra of the Fe electrode in 1 M NaOH (solid curves) and in 1 M NaOH + 1.5 M ethanol (dashed curves) at selected potentials are shown in Fig. 6, and the wavelengths of the peak maxima in the PMR spectra are given in Table 1. At negative potentials ( - 0 . 8 6 to - 0 . 5 1 V) a PMR peak attributed to FeOOH, appears at 360-380 nm. At positive potentials two peaks appear at about 560 and 470 nm, which are attributed to a-Fe203 (hematite) and y-Fe203 (maghemite) respectively [7].

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Fig. 3. Dependence on the potential of the normalized change P in the PMR intensity of a Ni electrode at the wavelength maximum caused by the addition of 1.5 M ethanol to the 1 M NaOH base electrolyte.

Fig. 5. First, fifth and fifteenth cyclic voltammograms of an Fe electrode in 1 M NaOH from - 1.36 to 0.64 V / S C E . Sweep rate, 40 mV s - 1.

A. Kowal, C. Guti~rrez/ Journal of Electroanalytical Chemistry 395 (1995) 243-247

246

[ ~- o.I16v

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&/rim Fig. 6. P M R spectra of an Fe electrode at several potentials, shown in each plot: - which ethanol had been added up to a concentration of 1.5 M.

The spectra in Fig. 6 show that the addition of 1.5 M ethanol at - 0 . 8 6 and - 0 . 6 6 V produces a small decrease in the intensity of the 360-380 peak, while no change was observed at 0.01 and 0.49 V. On the contrary, an increase in the intensity of the peaks at 520 and 460 nm was observed upon ethanol addition at 0.59 V. The effect of successive ethanol additions, up to a final concentration of 1.5 M, on the intensity of the PMR signal at - 0 . 6 6 and 0.49 V is shown in Fig. 7. While the PMR signal intensity at the wavelength maximum of 370 nm (FeOOH) at - 0 . 6 6 V is decreased slightly by ethanol, the PMR intensity at the wavelength maximum of 560 nm (ot-Fe203) at 0.49 V remains unaffected. This potential dependence runs contrary to that obtained with the Ni

PMR spectra in 1 M NaOH; - - -

PMR spectra in 1 M NaOH to

electrode, for which a strong decrease in AR/R was observed at the more positive potentials only. We used the normalized reflectance change, P defined above in order to compare the relative changes in the intensities of the peaks in the PMR spectra of the Fe electrode due to the addition of ethanol to the 1 M NaOH solution. As can be seen in Fig. 8, the parameter P is small and positive at the potentials of the second and third

10

Es:-0.66 V 5

Table 1 Wavelengths of the m a x i m a in the PMR spectra of an Fe electrode in 1 M NaOH as a function of electrode potential

E / V (SCE) - 0.86 - 0.66 -0.51 0.01 0.49 0.59

Es= 0.49V ,7,

)tmax / n m 360 370 380 550, 425 560, 470 520, 460

The addition of ethanol up to a concentration of 1.5 M did not alter the position of the maxima since the PMR spectra were barely affected by it.

K

X

J{

0 0

05 110 Ethanol c o n c / M

1.5

Fig. 7. Dependence on the ethanol concentration of the PMR intensity for an Fe electrode in 1 M NaOH at two potentials, shown against each curve. The PMR signal intensity at the wavelength m a x i m u m of 370 nm (FeOOH) at - 0 . 6 6 V is decreased slightly by ethanol, while it remains unaffected at the wavelength m a x i m u m of 560 nm ( a - F e 2 0 3 ) at 0.49 V.

A. Kowal, C. Guti~rrez /Journal of Electroanalytical ChemistD' ~95 (1995) 243-247

0.1

0

- 0,1

-1

°'

°

Fig 8. Dependence on the potential of the normalizedchange P in the PMR intensity of an Fe electrode (at the wavelength maximum at each potential) caused by the additionof 1.5 M ethanol to the 1 M NaOH base electrolyte.

anodic peaks, usually attributed to the formation of FeOOH. Thus it can be concluded that ethanol produces a decrease in the amount of FeOOH, but to a much smaller extent than the large decrease in the amount of NiOOH on a Ni electrode produced by the same ethanol addition. This was to be expected, since the peak potential of NiOOH reduction is about 1.3 V more positive than that of FeOOH reduction, i.e. NiOOH is much more oxidizing than FeOOH. At 0.01 and 0.49 V, potentials at which the oxide layer on the Fe electrode is strongly passivating, probably owing to the presence of Fe(III) oxides, P = 1, i.e. the oxide layer was not attacked by ethanol, which is in perfect agreement with its passivating inert character. Finally, at 0.59 V, P is negative, i.e. ethanol produced an increase in the PMR signal. However, measurements at this potential should be viewed with caution, since visual inspection of the Fe electrode showed that generalized corrosion had taken place.

4. Conclusions From the results presented here the following conclusions can be reached. • Ethanol reacts with the anodic NiOOH layer formed in the transpassivation peak on a Ni electrode, destroying it to an extent that increases in an exponential way with increasing potential. The effect reaches near saturation for an

247

ethanol concentration of 1 M. On the contrary, the passivating NiO layer is not attacked by ethanol. • Ethanol also reacts with the anodic FeOOH layer formed in the passivation peak on an Fe electrode, although to a much smaller extent than with NiOOH. On the contrary, the Fe(III) oxides, probably c~-Fe20 3 and y-Fe20 ~, are not attacked by ethanol, in agreement with their strongly pasivating character. • The fact that with both Fe and Ni ethanol reacts with the anodic oxyhydroxides only, and not with the anodic oxides, attests to the higher reactivity of hydroxyl-containing anodic oxides compared with metallic oxides.

Acknowledgements One of us (AK) gratefully acknowledges a PECO Fellowship (ERB-CIPA-CT92-2103 (6793)) of the European Community. This work was carried out under Project PB90-0119 of the Spanish DGICYT.

References [1] D. Degner, Topics in Current Chemistry, Vol. 148, SpringerVerlag, Berlin, 1988, p. 32. [2] A. Kowal, J Haber, K. Niewiara and J. Gasior. Polish Patcnt 149513, 1990. [3] A. Kowal and J. Haber, 4th World Congr. on Chemical Engineering, "'Strategies 2001)", Karlsruhe, 1991. [4] G. Vcrtes, G. Horfinyiand F. Nagi, Acta Chim. Acad. Sci. Hung., 6S (1971) 145. [5] M. Fleischmann,K. Korinekand D. Pletcher, J. Electroanal. Chcm., 31 (1971) 39. [6] M. Fleischmann,K. Korinekand D. Plctchcr, J. Chem. Soc. Perkin Trans. II, (1972) 1396. [7] G. Larramona and C. Guti~rrez, J. Electrochem. Soc., 136 (1989) 2171, and references cited therein. [8] G. Larramona and C. Guti~rrez, J. Electrochem. Soc., 137 (1990) 428, and references cited therein. [9] L.D. Burke and T.A.M. Twomey, J. Electroanal. Chcm., 162 (1984) 101. [10] A. Visintin, A.C. Chialvo, W.E. Triaca and A.J. Arvia, J. Elcctroanal. Chem., 225 (1987)227. [11] LD. Burke, Electrochim.Acta, 39 (1994) 1841. [12] L.D. Burke, PlatinumMet. Rev., 38 (1994) 166. [13] L.D. Burke and B.H. Lec, J. Electrochem. Soc., 138 (1991) 2496. [14] T. Kessler and A.M. Castro Luna, Z. Phvs. Chem., 185 (1904) 79.