2.1 Reagents

Phenol (Prolabo), catechol, hydroquinone (Aldrich) and p-benzoquinone (Across) were of analytical grade and were used as received. All the solutions were freshly prepared with bidistilled water, using 0.5 M H₂SO₄ (Prolabo) as supporting electrolyte.

2.2 Electrochemical measurements

Runs were made in a three-electrode compartment Pyrex glass cell (100 cm³) thermostated at 25 ± 0.5 °C. A platinum disk electrode (Tacussel) with a diameter of 0.2 cm was used, after its treatment, as working electrode. The counter electrode is a bar of graphite (∅ = 0.2 cm, L = 12 cm immersed). The working potential electrode was measured versus an Hg/Hg₂SO₄/K₂SO_4(sat) electrode (E = 0.61 V/SHE).

Polarisation measurements were carried out using a Tacussel potentiostat model PJT 35-2. The signal for triangular potential sweeps was produced by Tacussel scanner model GSTP 4. Voltammograms were recorded with the aid of a GSTP4 Tacussel X–Y recorder. The amount of electricity (Q) was measured by an IG 5-LN Tacussel-type integrator.

2.3 Pre-treatment of the platinum electrode

The working electrode was a polycrystalline platinum disk of ca. 0.0314 cm² exposed geometric area. This electrode was mechanically polished with fine alumina powder and washed with bidistilled water before transferring to the cell. The electrode was then cycled between −0.61 and +0.89 V in 0.5 M H₂SO₄ at a scan rate of 5 V s⁻¹. By subjecting the electrode to repeated cycles, there was an increase in the charge of the voltammogram, suggesting an increase in thickness of the oxide film adherent to Pt. An examination of the treated platinum surface by electronic scan microscopy suggested that the oxide layer deposited on Pt electrode is amorphous with sponge-like morphology (Fig. 1).

2.4 Electrolyses

The electrolyses of the phenol aqueous solutions (H₂SO₄, pH 2) were carried out in a single-compartment Pyrex glass cell of 500 mL volume at 25 °C. The anode is a cylindrical polycrystalline platinum grid (∅ = 4.3 cm, L = 5 cm) covered with platinum oxide layer (Q = 480 mC) by CV under the conditions described in the previous paragraph. The counterelectrode was a rectangular platinum plate with a geometrical area of 15 cm². The electrolysis was carried out by CV at 50 mV s⁻¹.

2.5 Analyses

The analyses of phenol and its oxidation intermediate products were made by high-performance liquid chromatography using a Hewlett-Packard 1090 HPLC apparatus. The products formed are separated on a Hamilton PRP X300 column and then analyzed using a diode array detector measuring the optical density at 220 nm. The mobile phase was a mixture of methanol and 0.05 M H₂SO₄ with the percentage by volume of methanol varying linearly with time as follows: from 2 to 25% for the first 10 min then from 25 to 40% up to 20 min and finally from 40 to 60% up to 40 min. The mobile flow rate was fixed at 1.3 mL min^−l.

3.1 Bulk reactions

The electrochemical response of the treated electrode is presented in Fig. 2 (curve Pt). The cyclic voltammogram was characterized by a large peak beginning at +0.10 V and corresponding to the platinum electro-oxidation, while the reduction peak of the oxide was observed at +0.02 V. The charge Q considered as the main characteristic of the electro-active surface was measured for a single cathodic scan at 50 mV s⁻¹ from +0.89 to −0.30 V. The treatment period varied from some minutes to 70 h to get surface states characterized by a charge between 0.3 and 4.8 mC. For Q values higher than 4.8 mC, the platinum oxide layer does not adhere to the Pt substrate.

The cyclic voltammogram of 0.1 M phenol + 0.5 M H₂SO₄ solution at a pre-treated platinum electrode (Q = 3.4 mC) is shown in Fig. 2. During the first forward positive scan, phenol exhibits two anodic peaks P₁ and P₂ at approximately 0.50 and 0.78 V, respectively, within the region corresponding to the oxidation of the platinum surface. On the reverse scan, a broad reduction peak (P₃), probably due to the reduction of the adsorbed oxidation product and of platinum oxide, appears near 0 V. Moreover, it can be observed that most of the adsorption sites of the platinum surface are blocked, as indicated by the low electric charge recorded in the potential region between −0.61 and +0.30 V. During the second and successive potential cycles, the cyclic voltammogram displays two features in the positive scan P₄ and P₅, located at 0.02 and 0.22 V, respectively. These peaks are, respectively, ascribed to the oxidation of hydroquinone and catechol by comparison with their authentic cyclic voltammograms recorded under identical experimental conditions. The current densities of peaks P₁, P₂ and P₃ depend on experimental conditions, particularly the charge Q.

Fig. 3 shows the voltammograms obtained during the oxidation of 0.1 M phenol solution for Q values in the range 0.9–4.0 mC. The current intensity of the peak P₁ increases linearly with the value of Q. It is multiplied by a factor 46 when Q increases from 0.3 to 4.0 mC (inset of Fig. 3). If the acidic medium was changed by an alkaline one using 0.5 M K₂CO₃ as electrolyte, all other experimental conditions were similar (Fig. 4), this current is multiplied by a factor 21 (inset of Fig. 4). These important factors could result from one or the two following causes:

• (1) an increase in the active surface of the electrode after pre-treatment;
• (2) an increase in the catalytic activity of the electrode surface.

The increase in surface due to pre-treatment was evaluated by CV. The electrode was immersed in a 0.1 M ferrocyanide + 0.5 M K₂CO₃ solution and then cycled for different values of Q (inset of Fig. 5). It is shown that when Q increases from 0.3 to 4.0 mC, the surface increases only by a factor of about 1.9, so the amplification of the phenol current according to Q is not mainly due to an increase in the active electrode surface. It is therefore necessary to consider the increase of the electrocatalytic effect of the platinum surface.

For comparison purposes, cyclic voltammograms were also recorded at the pre-treated Pt electrodes in p-benzoquinone and hydroquinone aqueous solutions.

Fig. 6 shows the first cyclic voltammograms for 10⁻² M p-benzoquinone + 0.5 M H₂SO₄ solution at pre-treated platinum electrodes at different values of Q. These voltammograms were carried out between 0.3 and 0.9 V. In each voltammogram, only one oxidation peak at around 0.62 V was observed. The inset of Fig. 6 shows a linear dependence of the current of this peak on the Q values. It results in the amplification of about 21 when Q increases from 0.3 to 4.0 mC. Thus, as for phenol, one can admit that the catalytic activity of the platinum surface was increased by the electrochemical pre-treatment.

In similar experiments, the pre-treated electrode was immersed in 0.1 M hydroquinone + 0.5 M H₂SO₄ solution and then recorded between −0.61 and +0.89 V at various Q values. As can be seen from the cyclic voltammograms in Fig. 7, a quasi-reversible peak appear at approximately 0 V, which corresponds to the transformation of hydroquinone to p-benzoquinone and vice versa within a quasi-reversible two-electron process. The oxidation current densities slightly increase with the Q values. It results in an amplification of about 3 when Q increases from 0.3 to 4.0 mC (inset of Fig. 7). This value is much lower than that of both phenol and p-benzoquinone. One can see that this factor is close to that of the electrode surface and thus the electrocatalytic effect of the Pt surface is not important in this case.

It is generally believed that the degree of oxidation of aromatic compounds, irreversibly adsorbed from aqueous solutions onto platinum electrodes, was dependent on the chemical structure, the type of adsorption and the surface electrode composition [11,24–31].

With regard to the oxidation potential of phenol, p-benzoquinone and hydroquinone presented in Table 1, it should be noted that contrarily to both phenol and p-benzoquinone, the oxidation of hydroquinone begins at potentials lower than + 0.12 V where the Pt oxidation wave begins. So the oxidation of hydroquinone is carried out directly in the platinum double-layer region where the surface of the electrode is covered with a monolayer of adatoms Pt^∗ (reduction product of Pt oxide). This oxidation involves a small number of electron transfers per mole while both phenol and p-benzoquinone are oxidized at higher potential values than that of the platinum. This oxidation is attributed to chemical reaction with reversibly deposited OH at open platinum metal sites as it has been explained by Conway [32].

The importance of PtOH species on the catalytic surface is underlined by the data shown in Fig. 8. In this figure, curve (Pt) shows the platinum voltammogram in solution containing only 0.5 M H₂SO₄. During the drawing of this voltammogram, if scanning is stopped at −0.61 V, the electrode surface will be covered by a layer of adatoms Pt^∗; on the other hand, if scanning is stopped at 0.89 V, the electrode will be covered by an oxide film essentially formed from PtO species following Eq. (3) [7,8]. Curves (1) and (2) show the first scans between 0.30 and 0.89 V in 0.5 M H₂SO₄ + 10⁻² M p-benzoquinone solution, respectively, for these two surface qualities of platinum. It is clear that the p-benzoquinone oxidation peak on the PtO surface disappears (curve 2), whereas this oxidation (curve 1) takes place on the incipient hydrous oxide PtOH readily formed according to the following equation:

It appears from the preceding results that when Q increases by pre-treatment of platinum, the average coordination of the atoms on the surface of the electrode decreases and the energy level of these atoms increases. Consequently, the active sites with respect to the discharge of water (Eq. (4)) become much more numerous and thus the concentration of PtOH species on the surface of the electrode increases.

3.2 Pre-adsorbed phenol oxidation

The adsorption of organic substrates at a platinum electrode in aqueous medium was widely investigated [11,33–35]. In the present work, oxidation of the pre-adsorbed layer of phenol was studied as a function of charge Q. The pre-treated platinum electrode was immersed in a 4 × 10⁻² mol L⁻¹ phenol solution for 1 min, then rinsed with bidistilled water. After that, it was transferred to the working solution (0.5 M H₂SO₄), which was free of phenol, held for 30 s at the adsorbed potential and cycled between −0.61 and 0.89 V. The resulting voltammogram is shown in Fig. 9 along with the results for clean platinum. It can be observed that the hydrogen adsorption reaction between −0.61 and 0.3 V was partially blocked by the adsorbed species. During the first anodic sweep to 0.89 V, the oxidation of the adsorbed layer of phenol gives an oxidation wave in the potential zone where the formation of a surface oxide layer on the platinum electrode occurs.

Integration of the voltammetric current for anodic and cathodic reactions yielded a coulometric charge Q. In Fig. 9, Q_Pt and Q_Ph + Pt designed the charge of the anodic waves in the absence and in the presence of adsorbed phenol, respectively. Q_PtO and QPt′ are those corresponding to the reduction peaks (P₃) of the platinum oxide. The charge QPt′ which would correspond to the platinum oxide formation in the presence of adsorbed phenol could be deduced from the following equation:

The charge Q_Ph corresponding only to the oxidation of the adsorbed phenol layer could be evaluated:

As can be seen in the inset of Fig. 9, Q_Ph increased approximately up to 2.5 when Q increased from 0.3 to 4.0 mC and tended to stabilize for higher values of Q. This result suggests that the increase of Q_Ph was essentially due to the increase of the active surface of the electrode which results from the formation of Pt adatoms (Pt^∗). Thus, electro-oxidation is limited by the amount of the adsorbed layer of phenol.

On the other hand, the degree of coverage θ of the electrode surface by the adsorption of the phenol and its own oxidation products can be calculated by the following formula:

The degree of coverage θ decreased as a function of Q; it passed from 74 to 55% when Q increased from 0.3 to 4.0 mC (inset of Fig. 9). In this case, if one admits that the number of exchanged electrons remains constant, the observed decrease of θ is due to the increase of the specific surface area.

3.3 Degradation of phenol

At first, electrolyses of 0.1 M phenol solutions (V = 0.5 L) on platinum oxide electrodes were tested in potentiostatic mode between 400 and 900 mV. However, in all cases, after about 1 min, the reaction was inhibited by the formation of a passivating film which avoids further phenol oxidation and the formation of platinum surface oxide. To overcome this problem, electrolyses were carried out by CV under the same experimental conditions which were used in cyclic voltammograms of phenol. This technique has the advantage of allowing the reactivation of the electrode during the negative sweep [11,36,37]. Conversely, this technique presents a disadvantage: in the cathodic sweep, many phenol oxidation products can be reduced. It results in the reduction of phenol degradation rate and a useless consumption of the electric energy. Consequently, it will be advisable to optimize the relative values of the positive and negative scan rates.

Thus, during an electrolysis test, the phenol solution presents a yellowish color after about 30 min, and a brownish one after 4 h characteristic of the presence of quinonic structures. When the electrolysis was prolonged, these substances were dissolved and the brown color disappeared. Moreover, the oxidation process involves the formation of insoluble product which could be dimers and/or polymers. The electrolytic medium was regularly assayed by liquid chromatography. The analyses were conducted with reference to commercially available products used as standards. Thus, catechol, hydroquinone, p-benzoquinone, oxalic and maleic acids were detected.