A photoelectrochemical and spectroscopic study of phenol and catechol oxidation on titanium dioxide nanoporous electrodes

Electrochimica Acta 55 (2010) 4661–4668

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A photoelectrochemical and spectroscopic study of phenol and catechol oxidation on titanium dioxide nanoporous electrodes Damián Monllor-Satoca, Roberto Gómez ∗ Institut Universitari d’Electroquímica i Departament de Química Física, Universitat d’Alacant, Apartat 99, E-03080 Alacant, Spain

a r t i c l e

i n f o

Article history: Received 12 December 2009 Received in revised form 4 March 2010 Accepted 13 March 2010 Available online 24 March 2010 Keywords: Phenol Catechol Photoelectrocatalysis Titanium dioxide nanoporous electrodes Infrared spectroscopy

a b s t r a c t The photoelectrochemical behavior of TiO2 nanoporous thin film electrodes has been studied in the presence of phenol and catechol. Voltammograms, both in the dark and under illumination, as well as photocurrent transients were combined with Attenuated Total Reflection Infrared measurements. Particular attention has been paid to the way that photocurrent is sensitive to the organics concentration. A non-monotonic, unusual tendency has been observed in all cases. For low organics concentration (lower than 0.1 mM), there is a deactivation of the electrode toward the water photooxidation process. Parallel spectroscopic measurements prove that, in this concentration range, the amount of adsorbed phenol (or catechol) is very small (undetectable), which supports the existence of a low coverage of surface active sites for the water photooxidation process. In the high concentration range, an increase of the organics concentration leads to an unexpected decrease in the stationary photocurrent. This behavior, attributed to both the build-up of an intermediate acting as a recombination center and a fouling of the oxide surface, has rarely been observed in TiO2 heterogeneous photocatalysis. © 2010 Elsevier Ltd. All rights reserved.

1. Introduction Phenolic compounds are widely used in industry, both as solvents and reagents. They pose an environmental threaten, being the most common pollutants in industrial wastewaters [1–4]. Phenol is a toxic and slowly degradable pollutant [4], present in different wastewaters coming from paper mills, textile and petrochemical industries, paints, pesticide plants, etc. [4,5]. Catechol may also be considered as a model contaminant and an important intermediate in the phenol photooxidation mechanism [3,6]. The use of photocatalysts, particularly titanium dioxide, has grown over the last two decades as a promising and effective way of effluent decontamination [3,7–9]. Many authors have stated that the oxidation kinetics is first order, both for phenol [1,10] and catechol [11] molecules. Other hydroxilated aromatic substrates also follow first order kinetics [12]. However, it is not clear yet whether the Langmuir–Hinshelwood mechanism is appropriate for describing the oxidation process, existing an on-going controversy [12–17]. Photocatalysts can be either suspended or supported (conventional photocatalysis) or immobilized on conducting substrates (photoelectrocatalysis). An inherent problem arising from using suspended semiconductor particles is their separation and recovery from the liquid phase; this can be done by either using large photocatalyst particles or supporting them onto a solid substrate

∗ Corresponding author. Tel.: +34 965903748; fax: +34 965903537. E-mail address: [email protected] (R. Gómez). 0013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2010.03.045

[18]. More fundamentally, suspensions show a low quantum yield coming from a high recombination rate between the photogenerated charge carriers [19,20]. In addition, an electron scavenger (i.e., oxygen) is needed. Instead, in a film supported on a conducting substrate, the application of a positive bias can effectively withdraw the electrons from the semiconductor particles, reducing the recombination rate with holes [19–21]. Therefore, electrochemically assisted photocatalysis with supported films efficiently avoids the limitation coming from the electron scavenging process typical of suspensions [22]. In this sense, Liu et al. [23] showed that the phenol oxidation first order rate constant was 0.100 h−1 using a suspension of TiO2 nanotubes, while it doubled its value to 0.215 h−1 employing a film electrode made from the same nanotubes. More recently, Longo and co-workers [24] showed that the first order rate constant for phenol degradation increased from 0.201 h−1 to 0.470 h−1 when changing the potential applied to a TiO2 electrode from 0.7 to 1.1 V vs. (Ag/AgCl). The photocatalyst (titanium dioxide) features determine to a great extent the efficiency and performance of the degradation process. Under illumination, the photogenerated holes (free in the valence band or trapped at surface states) will be responsible for oxidizing the organic substrates. The direct reaction of the organic compounds with free holes can only compete with their reaction with surface-trapped ones if the organic molecules are strongly adsorbed at the catalyst surface [17,25–29]. On the other hand, it is commonly accepted that only the anatase phase is active for the photodegradation of phenol, whereas the rutile phase is completely inactive for this reaction [2,30].

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Not only the photocatalyst properties are important in the photoinduced process, but also the organic reductant features. Catechol is a weak diprotic acid, undissociated in a wide pH range (pK1 = 9.2, pK2 = 13.0), which forms very stable complexes with Ti(IV) ions [31–33]. Catechol is adsorbed through two non-competing adspecies at different adsorption sites, molecularly or forming a chelating species (most stable adsorbate) [6,34]. The catecholate anions (chelating species) bind to the surface Ti ions in an OH group substitution process [31]. In comparison, however, phenol is reported to exhibit a weak interaction with the TiO2 surface [14,35] although its adsorption has been demonstrated from both aqueous [36,37] and gaseous phases [38,39]. Two mechanistic aspects have been established concerning the photooxidation mechanism of phenol on TiO2 photocatalysts. There is almost a consensus that the first step is the phenol ring activation by means of an adduct formation with OH radicals, which evolves into a phenoxy [2,3] or a dihydroxylcyclohexadienyl [14] radical. These radicals could either further react with more OH radicals, in the former case, or directly evolve, in the latter, yielding catechol, hydroquinone, benzoquinone, hydroxyhydroquinone or other (poly)hydroxylated substrates [40]. In the case of catechol, an adsorbed cation-radical is generated, probably an o-benzosemiquinone, which initiates an oxidative polymerization leading to a humic acid like polymer (polycatechol). The resulting polymer also remains adsorbed at the surface as confirmed by means of Resonance Raman Spectroscopy [6]. During catechol or phenol degradation, the existence of intermediates would alter the photooxidation path [11], as these compounds could compete with the substrates for the semiconductor surface active sites and the photogenerated surface-trapped holes [13]. The intermediates could reduce the substrate degradation rate because either there is a blocking of the active sites (strong adsorption) or they behave as recombination centers [14,33,41]. In general, the photogenerated cation radical at the catalyst surface could recombine with conduction band electrons (back-reaction) [14]. On the other hand, (photo)electrochemical measurements employing polarized electrodes have furnished information complementing that coming from the classical photocatalysis studies. The published reports have focused on the electrochemical phenol oxidation mechanism on metallic and metal oxide electrodes [42–44], and the photoelectrochemistry of its oxidation over different photocatalysts [23,45–50]. In contrast, not many studies have been published on catechol photoelectrocatalysis [48]. Panic´ et al. [43] studied the phenol oxidation mechanism using RuO2 -TiO2 /Ti anodes and proposed that the first step in the electrooxidation mechanism was the formation of phenoxy radicals [22,42–44,46]. Phenoxy radicals remained adsorbed at the surface [42] and could further react [44]. It has been admitted that phenoxy polymerization (to polyoxyphenylene) is the second reaction step [43]. The inhibition of the first reaction step is due to continuous covering of the active sites at the oxide surface by the polymer film generated by radical polymerization. This polymer appears as a tarry deposit [42], inhibiting further electrocatalytic activity of the electrodes (surface fouling), but not completely passivating them [43]. In this report, we have employed titanium dioxide nanoporous thin film electrodes to perform photoelectrocatalytic and spectroscopic studies (infrared) aiming to shed light on some aspects of the photocatalyzed oxidation of water, phenol, and catechol.

MilliQ). Sodium perchlorate monohydrate (extra pure) was supplied by Scharlau. Perchloric acid (p.a., 60%), sodium hydroxide (p.a.) and phenol (p.a., +99%) were supplied by Merck. Pyrocatechol (99%) was supplied by Sigma Aldrich. Solution pH was adjusted to 3.5 using either 1 M NaOH or 1 M HClO4 stock solutions. Titanium dioxide nanoparticles were obtained from Alfa Aesar (anatase, 32 nm, 99.9%), PI-KEM Ltd. (anatase:rutile 1:1, 35 nm, 99%), and Degussa-Hüls P25 (anatase:rutile 4:1, 20 nm). The oxide slurry for thin film preparation was made using Triton X-100 and acetylacetone 99% from Aldrich. 2.2. Preparation of nanoporous thin films The semiconductor oxide slurry was prepared as follows. A total of 330 ␮L of MilliQ distilled water was mixed with 30 ␮L of acetylacetone in a mortar; 1 g of commercial nanoscopic powder was added and ground with the solution for 15 min until a homogeneous slurry was obtained. Then, 1.3 mL of distilled water were added dropwise. A dispersion with a 60 wt% of the oxide was formed, and 20 ␮L of the tensioactive TritonX-100 were added to stabilize it. Then, 15–20 ␮L of the suspension were spread over around 2 cm2 of a FTO substrate (U-type Asahi Glass Co., Japan) using the doctor blade method. Finally, the nanoporous thin film was thermally annealed in air for 1 h at 450 ◦ C. The resulting film average thickness was 3 ␮m for anatase and 11 ␮m for PI-KEM. 2.3. Apparatus The electrochemical measurements were performed using a standard photoelectrochemical setup. It was composed of a computer-controlled potentiostat, ␮AUTOLAB type III, and a 150 W Bausch&Lomb Xe arc lamp as illumination source. The electrochemical cell was a conventional three-electrode cell with a 1 mm thick fused silica window. A Pt wire was used as counter-electrode and a Ag/AgCl/KCl(sat) electrode as a reference electrode, to which all potential values are referred. The electrode was illuminated at the front side (EE, Electrolyte-Electrode). The light was focused through a quartz lens, illuminating a 1 cm diameter spot on the electrode surface. The working solutions were 0.5 M NaClO4 with or without phenol or catechol at different concentrations, buffered to pH 3.5 and purged with nitrogen. Incident light intensity was measured with an optical power meter (Oriel model 70310) equipped with a thermopile head (Ophir Optronics 71964), attaining a value of 0.28–0.33 W. The ATR-IR experiments were carried out with a Nicolet Magna 850 Spectrometer equipped with a MCT detector and a variable angle Veemax specular reflectance accessory (Pike Technologies). The ATR-IR cell was adapted to a semicylindrical ZnSe prism. The spectra were obtained at a resolution of 8 cm−1 and at an angle of incidence of 45◦ . ATR-IR spectra of phenol or catechol in the presence of the nanoporous thin film were referred to the single beam spectrum obtained for each film in the organic-free NaClO4 solution. TiO2 films were coated onto the ZnSe window by applying 0.4 ␮L mm−2 (anatase) or 1.2 ␮L mm−2 (P25) of a 0.4 M TiO2 suspension. It was left to dry in air overnight at 60 ◦ C, generating a 3–5 ␮m thick film. Solution spectra were obtained using an uncoated ZnSe prism. The electrode thickness was determined by SEM measurements, using a HITACHI S-3000N microscope. 3. Results and discussion

2. Experimental

3.1. Phenol photooxidation

2.1. Chemicals

ATR-IR spectroscopic measurements were first performed to examine the interaction of phenol with a titanium dioxide (anatase or P25) thin film surface. For these experiments, we used P25 as a mixed phase catalyst (anatase:rutile 4:1) instead of PI-KEM

All chemicals were used as received without further purification. Working solutions were prepared with ultrapure water (Millipore

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Fig. 1. ATR-IR spectra for a 4 mM phenol solution with (dotted line) and without (solid line) a nanoporous anatase thin film. Spectra were averaged over 100 repeated scans. Solution: 0.5 M NaClO4 , buffered at pH 3.5. Thin film average thickness: 3 ␮m.

because the latter did not adhere well to the ATR ZnSe prism. In Fig. 1, the ATR-IR spectra for a 4 mM phenol solution in the presence and the absence of an anatase thin film are shown. The bands at 1242 cm−1 , ascribed to a combination of C–O and C–C stretchings, and that at 1380 cm−1 , associated with the in plane C–O–H bending appear irrespective of the presence of the TiO2 overlayer. The bands at 1477 and 1500 cm−1 appearing in the spectrum associated to dissolved phenol (protonated form) in acidic medium are attributed to aromatic ring C–C stretchings; the latter is also considered as a combination band with C–H bending [37,51,52]. In the presence of the particles, minor but significant differences are noticed. On the one hand, a shoulder is developed at around 1270 cm−1 , which suggests the existence of adsorbed phenolate. Likewise, the appearance of an intense band at 1485 cm−1 would add more credit to the presence of such adsorbed form. It should be mentioned that previous adsorption studies from gas phase have confirmed that adsorption of the anionic form is predominant. Wu et al. [38] showed that, upon adsorption, the OH stretching disappeared, pointing to a phenolate-type adsorption.

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Recent measurements by Cropek et al. [39] confirmed the presence of such an adsorbate. In solution, not many adsorption studies have been conducted [36,37]. Palmisano et al. [36] concluded that wet TiO2 samples adsorbed both dissociated and undissociated phenol, which is compatible with our observations. In addition to phenolate adsorption, in solution, molecular adsorption could occur as a result of a weak interaction between TiO2 and the phenol adsorbate, probably through either a hydrogen bond or the ␲-electronic cloud of the aromatic moiety. Kim and Choi have suggested that phenolate adsorbed on wet TiO2 samples enables visible light absorption through ligand-to-metal charge transfer [53]. In any case, adsorption is weak as deduced from the similar intensity of the spectral features in the IR spectra, irrespective of the presence of the TiO2 film. In Fig. 2a, the ATR-IR spectra for phenol at different concentrations in the presence of a P25 thin film are gathered. In Fig. 2b we compare the integrated intensity of the band at 1242 cm−1 and different phenol concentrations, both in the presence and the absence of the P25 thin film: there is no significant difference in the whole concentration range studied. As anticipated in the case of anatase, phenol does not show a strong interaction with the P25 surface [14,35]. In fact, the adsorption seems to be weaker on P25 than on anatase as deduced from the absence of the strong band at 1485 cm−1 observed in the case of anatase. Both photocurrent transients and voltammetric measurements were performed to study the phenol photooxidation process. In Fig. 3, we show voltammetric profiles for anatase and PI-KEM electrodes either in the presence or in the absence of phenol, both in the dark and under EE illumination. In the case of anatase (Fig. 3a and b), the addition of phenol does not significantly change the photocurrent magnitude at positive potentials (in the plateau region at E > +0.4 V, where photocurrent saturates), but for the PI-KEM electrode (Fig. 3c and d) there is around a 50% photocurrent reduction in the plateau region. In addition, the voltammetric profile changes in the anatase case, showing a peak at −0.14 V. This peak could be attributed to the substrate (phenol) transport limitation by diffusion, or to the generation of species that could either recombine with electrons, or block the photoactive surface sites. No significant shift in the onset potential (−0.65 V for anatase and −0.55 V for PI-KEM) was observed upon the addition of phenol. This result indicates that the presence of phenol does not significantly change the recombination rate at the electrode, which is in stark contrast with the photooxidation of easily oxidizable organic substances,

Fig. 2. (a) ATR spectra for a solution of phenol with different concentrations in contact with a nanoporous P25 thin film. (b) Integrated intensity for the 1242 cm−1 phenol band vs. phenol concentration, with (open circle) and without (full square) the P25 thin film. In all cases, spectra were averaged over 50 repeated scans. Solution: 0.1 M NaClO4 , buffered at pH 3.5. Thin film average thickness: 5 ␮m.

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Fig. 3. Voltammetry in the dark (dotted line) and under EE illumination (solid line) of anatase (a, b) and PI-KEM (c, d) nanoporous electrodes, in the absence (a, c) and the presence (b, d) of 0.01 M phenol. Scan rate: 20 mV/s. Electrode exposed area: 1.5 cm2 . Electrolyte: N2 -purged 0.5 M NaClO4 solution, buffered at pH 3.5. Incident (white) light power: 0.28–0.33 W.

such as methanol or formic acid. In such cases, a clear shift of the photocurrent onset potential toward more negative values is commonly observed [26,27]. In Fig. 4, we present the photocurrent transients for four different phenol concentrations and at two different potential values (−0.2 and 0.6 V) for an anatase nanoparticulate electrode. The first potential corresponds to the onset of the photooxidation wave, whereas the second corresponds to the photocurrent plateau. These potential values lead to different driving forces (maximum at 0.6 V) for the electrons to diffuse toward the substrate. As the transport of electrons is slowed down by the application of a less positive potential, recombination is enhanced and photocurrent decreases. We observe a decrease in photocurrent at very low phenol concentration (compare Fig. 4a and b) and for both potentials, but it increases at higher concentrations (Fig. 4c and d), attaining a value close to the initial photocurrent without phenol (Fig. 4a). The initial spike formation points to the occurrence of surface inactivation (fouling), an increase of recombination as the phenol photooxidation proceeds, or both. It is significant that, as the phenol concentration is increased, so does the magnitude of the spike at the beginning of the transient for both potentials. A brownish spot was visible after the illumination of the electrode for highly concentrated phenol solutions, which could be attributed to polymer formation [1,4]. Importantly, repeating the transient after a few minutes in the dark led anew to the appearance of the initial spike. This points to the higher importance of the concentration build-up of the intermediate acting as a recombination center. The prevalence of surface fouling would mainly yield a lower photocurrent without the appearance of the spike. A second negative-going spike, which corresponds to a reduction process appears when illumination is interrupted (at −0.2 V), and could be assigned to a decay (reduction) of photogenerated intermediates (including surface

Fig. 4. Photocurrent transients obtained for EE illumination of an anatase electrode at 0.6 V (solid line) and −0.2 V (dotted line) constant potential, in contact with a (a) 0 M, (b) 0.01 mM, (c) 4.25 mM, (d) 10.25 mM phenol solution. Other experimental conditions as in Fig. 3.

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trapped holes) or reaction products from water photooxidation, as it appears clearly in the absence of phenol. The accumulation of intermediates would be favored by relatively fast surface trapping of holes and a relatively slow water photooxidation process. In this respect, it has been reported that the overall oxidation of water to oxygen at TiO2 requires 4 holes and occurs on one second timescale, whereas the trapping of a hole at the TiO2 surface proceeds within some nanoseconds [54]. In Fig. 5 we show the photocurrent transients obtained under similar conditions to these of Fig. 4, but for a PI-KEM electrode. As in the anatase case, the addition of phenol at low concentration (Fig. 5b) drastically reduces the photocurrent at both potentials. However, upon increasing the phenol concentration, the changes observed in the photocurrent are less defined than in the case of anatase, although the same trends hold (Fig. 5c and d). It is worth noting that, even for high phenol concentrations, no significant spike is observed upon illumination, pointing to a smaller accumulation of intermediates (acting as recombination centers). In the same way, the cathodic overshoot is now absent, which points to the fact that water photooxidation proceeds more efficiently on PIKEM than on anatase. This is in agreement with the substantially larger photocurrents recorded in the absence of organics for PIKEM electrodes. In Fig. 6, the stationary photocurrent values obtained after 1 min illumination of an anatase electrode at either −0.2 V or +0.6 V, are plotted as a function of phenol concentration. Three different ranges can be distinguished at both potentials. For phenol concentrations in the ␮M range, the photocurrent value dramatically decreases with concentration, attaining a minimum value at 10 ␮M, which corresponds to half its initial value (Fig. 6a). As the photocurrent in this low phenol concentration range should be attributed to water photooxidation, this decrease can be understood as the blocking of the water photooxidation surface active sites by phenol adsorption. Such reduced phenol surface coverage significantly affects water photooxidation, which seems to be in agreement with the role that Nakato and co-workers [55] attributed to surface defects in the photogenerated oxygen evolution. It is worth pointing out that the surface density of such sites should be low, as an important fraction appears to be blocked with phenol and no adsorbate could be detected by IR in the low concentration range. From 0.01 mM to 4.25 mM, once these sites are blocked, there is a progressive increase of the photocurrent until a maximum value is

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Fig. 5. Photocurrent transients for EE illumination of a PI-KEM electrode at 0.60 V (solid line) and −0.20 V (dotted line) constant potential, in contact with a (a) 0 M, (b) 0.05 mM, (c) 1.25 mM, (d) 10.25 mM phenol solution. Other experimental conditions as in Fig. 3.

achieved at 4.25 mM. This increase should be linked to phenol photooxidation. From 4.25 mM to 10.25 mM, the photocurrent starts to decrease again until it reaches a value 15% lower than that of the maximum. As indicated above, this diminution can be ascribed to the phenolic radical generated upon oxidation, which could act as a recombination center, polymerize with more phenol molecules or both [44]. The generated polymer would strongly adsorb at the titanium dioxide surface thus blocking (poisoning) its surface active

Fig. 6. Photocurrent vs. phenol concentration curve, obtained after 1 min of EE illumination of an anatase electrode at 0.6 V (full circle) and −0.2 V (open square) constant potential. Subparts (a) and (b) correspond to different ranges of concentration. I0 is the initial photocurrent prior to the addition of organics; Im is the photocurrent at the minimum; IM is the photocurrent at the maximum; If is the final photocurrent at high organics concentrations (10 mM); Cm and CM are the organic concentrations at the minimum and maximum, respectively.

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Fig. 7. Photocurrent vs. phenol concentration curve, obtained after 1 min of EE illumination of a PI-KEM electrode at +0.60 V (full circles) and −0.20 V (open squares) constant potential. Subparts (a) and (b) correspond to different ordinate and abscissa ranges.

sites, which would lead to a drop in photocurrent. The presence of such optimum concentration values points to the fact that the degradation rates may not follow a simple Langmuir–Hinshelwood type mechanism. Minero et al. proposed, on the basis of a kinetic model, that the appearance of a maximum in the quantum yield vs. concentration curves, may be due to the back-reaction of the photogenerated organic radical, which would consume electrons regenerating the reactant molecule and thus reducing the photocatalytic oxidation rate [56,57]. Similarly, Cunningham et al. [58] studied the degradation of chlorophenols and showed that a maximum appeared in the photocatalyzed decomposition rate vs. organics concentration plot. They proposed that the rate could be inhibited by depletion of surface OH groups, mass transport limitations or adsorbate enhanced hole–electron recombination. In this respect, if we plot the initial value of the photocurrent when the light is turned on at 0.6 V (not shown), we observe that upon increasing the phenol concentra-

tion, the photocurrent follows a similar tendency as in Fig. 4, but it does not give rise to a maximum in the high concentration range, indicating that not enough polymer or intermediate has been generated yet. For the sake of comparison, we repeated the transient measurements with a mixed phase electrode. In Fig. 7 we display the stationary photocurrent of a PI-KEM electrode as a function of phenol concentration. As in the case of the anatase electrode, three concentration ranges can be distinguished: from 0 M to 0.05 mM (Fig. 7a, initial decay region), from 0.05 mM to 1.25 mM (photocurrent increase range) and from 1.25 mM to 10.25 mM (second photocurrent decrease region). It is remarkable that past the maximum the photocurrent decrease is not as significant as in the case of anatase; this behavior parallels that observed in the photocurrent transients (Figs. 4 and 5), which show a significant drop in the photocurrent with time only for anatase electrodes. Experiments were also performed with P25 thin film electrodes (not shown) and the same trends were observed. Further comparison is precluded by

Fig. 8. (a) ATR spectra for a solution of catechol at different concentrations in contact with a nanoporous P25 thin film. (b) ATR spectra for a 10 mM catechol solution with (solid line) and without (dotted line) a P25 thin film. (c) Integrated intensity for the 1265 cm−1 catechol band vs. catechol concentration, with (open circle) and without (full square) a P25 thin film. Other experimental conditions as in Fig. 2.

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Fig. 9. Voltammetry in the dark (dotted line) and under illumination (solid line) for a PI-KEM electrode in contact with a 0.01 M solution of (a) phenol and (b) catechol. Other experimental conditions as in Fig. 3. Table 1 Significant photocurrent ratios and concentration values obtained from Figs. 6, 7 and 10. See Fig. 6 for the meaning of the different quantities. The ratio (Im /I0 ) is linked to the degree of inhibition of the water photooxidation reaction. The ratio (If /IM ) informs about the degree of self-inhibition of the photocatalyzed organic oxidation. Reactant, catalyst

Cm (mM)

CM (mM)

Im /I0

If /IM

Phenol, anatase Phenol, PI-KEM Catechol, PI-KEM

0.01 0.05 0.01–0.04

4.3 1.3 1.0

0.50 0.25 0.22

0.85 0.93 0.63

the fact that the BET surface area for P25 (51 m2 g−1 ) significantly differs from the values corresponding to anatase (30 m2 g−1 ) and PIKEM (27 m2 g−1 ) [59]. 3.2. Catechol photooxidation In Fig. 8a, the ATR-IR spectra for different catechol solutions in contact with a P25 film are shown. A similar series for an anatase nanoporous electrode can be found in ref. [6]. Different bands appear, the most prominent ones located at 1265 cm−1 and 1485 cm−1 , corresponding to the C–O stretching mode and an aromatic C–C stretching vibration, respectively. The bands at 1203 cm−1 and 1381 cm−1 correspond both to OH group bending modes. At 1331 cm−1 a combination band of C–O and C–C stretching modes appears; finally, at 1504 cm−1 there is another combination band of the C–C stretching and C–H bending modes. The bands at 1203, 1381 and 1504 cm−1 are ascribed to molecularly adsorbed catechol, whereas the band at 1485 cm−1 is characteristic of cat-

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echolate in a chelate configuration [6]. As in the case of anatase samples, both species seem to be present on P25. In Fig. 8b we show the differences between the 10 mM catechol spectra in the presence and in the absence of the P25 thin film. In Fig. 8c, the integrated absorbance of 1265 cm−1 band is plotted against catechol concentration, both in the presence and the absence of a P25 thin film. For all concentrations, the signal intensity is significantly higher in the presence of the film than in its absence, indicating the specific adsorption of catechol at the TiO2 surface. The behavior observed here is markedly different from the phenol case (Fig. 2). In Fig. 9, voltammograms for a PI-KEM electrode in contact with either a phenol or a catechol solution are given. The photocurrent is smaller in the case of catechol, which suggests that the stronger catechol adsorption leads to accelerated surface fouling. In Fig. 10, the stationary photocurrent values for different added catechol concentrations are gathered. The same general behavior reported for phenol on both P25 and anatase electrodes is found here. 3.3. General comparison Let us compare the different results, obtained for the photocurrent vs. concentration curves (Figs. 6, 7 and 10). With this purpose, we define some experimental parameters: I0 , as the initial photocurrent prior to the addition of organics, corresponding only to water photooxidation; Im , as the photocurrent at the minimum; IM , as the photocurrent at the maximum; If , as the final photocurrent at high organics concentration (10 mM); finally, Cm and CM , are defined as the organic concentrations at the minimum and maximum, respectively (see Fig. 6). In Table 1, we summarize all parameters taken from Figs. 6, 7 and 10 (at 0.6 V). Some observations are remarkable. We first compare the same organic substance (phenol) with different photocatalysts. The concentration value corresponding to the minimum in photocurrent (Cm ) is lower in the case of anatase. Accordingly, the ratio If /IM is also smaller for anatase than for PIKEM. A higher surface concentration would favor either recombination via adsorbed intermediates or surface fouling by product condensation. These data seem to reflect the higher degree of interaction of phenol with the anatase sample deduced from infrared measurements. Let us compare now different organics (phenol and catechol) with the same catalyst (PI-KEM). The data points to the fact that catechol adsorbs more strongly than phenol. Again, the values of Cm and If /IM are smaller in the case of catechol. It is noteworthy that the ratio Im /I0 is similar

Fig. 10. Photocurrent vs. catechol concentration curve, obtained after 1 min of EE illumination of a PI-KEM electrode at +0.60 V (full circles) and −0.20 V (open squares) constant potential. Subparts (a) and (b) correspond to different ranges of concentration. Other experimental conditions as in Fig. 3.

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in both cases, although slightly smaller for catechol than for phenol. This suggests that both species preferentially adsorb on surface active sites for water photooxidation. 4. Conclusions We have studied the effect of phenol and catechol on the photoelectrochemical behavior of anatase and mixed-phase (rutile/anatase) nanoporous electrodes. Upon varying the concentration of the organics, a similar and unusual trend is observed in all cases. Interestingly, addition of minute amounts of both compounds (of the order of 10−5 M) leads to a sudden drop in the stationary photocurrent, even when parallel infrared perusal of the oxide/solution interface do not reveal extensive adsorption of any of the organics. This may be taken as a proof for the existence of active sites for the water photooxidation process, which is in line with the idea put forward by Nakato and co-workers that certain surface defects may trap the surface holes, which would evolve following a complex mechanism toward oxygen as final product [55]. Another interesting feature of this system is that a maximum in the photocurrent vs. concentration curve is defined for both phenol and catechol photooxidation processes. An equivalent observation, theoretically predicted by Minero [56], has been previously reported for the photocatalyzed oxidation of chlorophenols [58]. The reason underlying this experimental behavior has been proposed to be the formation of an active intermediate that could act as a recombination center. Although some photocurrent transients seem to support this idea, a simple visual inspection of the electrodes after performing the experiments indicates that a concurrent poisoning (fouling) of the surface contributes to deactivation for high phenol concentration values. In a more general vein, this study illustrates the advantages of combining electrochemical and spectroscopic methods to elucidate processes occurring at the solid/solution interface. Acknowledgements Financial support from the Spanish Ministry of Education and Science (MEC) through Projects HOPE CSD2007-00007 (ConsoliderIngenio2010) and CTQ2006-06286 (Fondos FEDER) as well as from the Generalitat Valenciana (ACOMP/2009/132) is gratefully acknowledged. D.M.-S. is grateful to MEC for the award of a FPI grant. We thank Dr. A. Rodes for useful assistance during IR spectra acquisition. References [1] S. Cheng, S.-J. Tsai, Y.-F. Lee, Catal. Today 26 (1995) 87. [2] S.-J. Tsai, S. Cheng, Catal. Today 33 (1997) 227. [3] A.M. Peiró, J.A. Ayllón, J. Peral, X. Doménech, Appl. Catal. B: Environ. 30 (2001) 359. [4] M. Andersson, L. Österlund, S. Ljungström, A. Palmqvist, J. Phys. Chem. B 106 (2002) 10674. [5] R. Alnaizy, A. Akgerman, Adv. Environ. Res. 4 (2000) 233. [6] T. Lana-Villarreal, A. Rodes, J.M. Pérez, R. Gómez, J. Am. Chem. Soc. 127 (2005) 12601. [7] M.R. Hoffmann, S.T. Martin, W. Choi, D. Bahnemann, Chem. Rev. 95 (1995) 69. [8] G. Sivalingam, M.H. Priya, G. Madras, Appl. Catal. B: Environ. 51 (2004) 67. [9] V. Brezová, S. Stasko, J. Catal. 147 (1994) 156. ´ [10] A. Dobosz, A. Sobczynski, Water Res. 37 (2003) 1489.

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