Study of the oxidation of solutions of p-chlorophenol and p-nitrophenol on Bi-doped PbO 2 electrodes by UV-Vis and FTIR in situ spectroscopy

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1 Electrochimica Acta 49 (2004) Study of the oxidation of solutions of p-chlorophenol and p-nitrophenol on Bi-doped PbO 2 electrodes by UV-Vis and FTIR in situ spectroscopy C. Borrás, T. Laredo, J. Mostany, B.R. Scharifker Departamento de Química, Universidad Simón Bolívar, Apartado 89000, Caracas 1080A, Venezuela Received 9 July 2003; received in revised form 3 September 2003; accepted 14 September 2003 Abstract The oxidation of p-chlorophenol (pcp) and p-nitrophenol (pnp) was studied at Bi-doped PbO 2 (Bi PbO 2 ) electrodes. The mass balance between solution composition and flowing charge was obtained from deconvolution of UV-Vis spectra recorded during electrolysis at constant potential. It is shown that the time-dependent production of CO 2 is different for the oxidation of pcp and pnp, indicating different reaction mechanisms for the oxidation of each of these compounds. The reaction kinetics was also followed under thin layer conditions by SNIFTIRS; the similarly increasing signals associated to the generation of CO 2 obtained during oxidation of both compounds indicates that under conditions of restricted mass transfer the mineralization rates of pcp and pnp are similar. The results show that benzoquinone (bq) formed from oxidation of phenols desorbs prior to further oxidation to yield maleic acid (ma), in turn oxidizing further to CO Elsevier Ltd. All rights reserved. Keywords: p-chlorophenol; p-nitrophenol; Electrocatalysis; Oxidation of organics; Metal oxide anode 1. Introduction The electrochemical oxidation of organic compounds has been a subject of scientific research during the past few years with the aim of identifying suitable ways to treat wastewaters. The oxidation of phenolic compounds has been of particular interest because of their abundant presence in industrial effluents. The general purpose of the electrochemical oxidation of these compounds is to mineralize them to CO 2, thus providing a method to decontaminate the effluent. The kinetics of the oxidation reactions during electrolysis are usually conveniently derived from measurements of total organic carbon (TOC) [1 3] or chemical oxygen demand (COD) [4,5], or more distinctively using analytical tools such as UV-Vis spectroscopy [6] or liquid or gas chromatography [7 10], allowing determination of the oxidation efficiency of the phenolic compounds as well as monitoring the solution composition during oxidation, particularly in relation to the formation and subsequent decay of interme- Corresponding author. Tel.: ; fax: address: cborras@usb.ve (C. Borrás). diate products that can be even more toxic than the starting compounds. Because of this, it is of interest to know under which conditions either mineralization or the formation of a less toxic compound occurs, so that the impact on the environment is diminished. The overall mechanism for the electrochemical oxidation of phenols involves three consecutive irreversible steps [1]: (1) Oxidation of the phenolic compound to a quinoid compound. (2) Ring opening reaction with formation of aliphatic acids. (3) Mineralization to CO 2. The appearance and accumulation of intermediates depend on the relative rate of each of the consecutive steps. In this way, with HPLC determinations it has been shown that during oxidation of p-chlorophenol on Ti/PbO 2 electrodes the quinoid compound accumulates in solution, whereas it is practically absent when the reaction is carried out on Ti/SnO 2 electrodes [4]. Also, it has been reported that when platinum or carbon electrodes are used for the oxidation of phenols, then ring opening is the rate determining step [1], evidencing the influence of the electrode material on the reaction kinetics. Other factors affecting the kinetics of the /$ see front matter 2003 Elsevier Ltd. All rights reserved. doi: /j.electacta

2 642 C. Borrás et al. / Electrochimica Acta 49 (2004) reaction are the concentration of the reacting species [4], temperature and ph [6]. In this work we have studied the oxidation of aqueous solutions of p-chlorophenol and p-nitrophenol on Bi-doped PbO 2 electrodes, as a way for treating wastewaters. The composition of the solution during electrolysis at constant potential was quantified by deconvolution of UV-Vis spectra of the solution, and related to the electric charge passed through the electrode/solution interface in order to establish the reaction mechanism as well as the efficiency of the process with respect to CO 2 generation. 2. Experimental A three-compartment glass cell, with glass-frit separators, was used throughout. This allowed maintaining the solution in the working compartment separated from that contacting the 15 cm 2 platinum gauze used as auxiliary electrode, and also from that contacting the saturated calomel electrode (SCE) used as reference. All potentials are reported with respect to the SCE unless otherwise stated. Reagents were of analytical grade (Aldrich or Sigma), used as received, and all solutions were prepared with distilled and ultrafiltered (Nanopure ) water, using 0.1 M Na 2 SO 4 as supporting electrolyte. Bi PbO 2 electrodes with large surface area (ca cm 2 real area) were used for extensive electrolysis of pcp and pnp. These were prepared depositing Bi PbO 2 films onto platinum gauze electrodes from 45 mm Pb mM Bi M HClO 4 solutions, as reported by Johnson et al. [11], at 1.5 A during 10 min. When necessary, the platinum surfaces were recovered dissolving the deposits in concentrated HCl. Long-time electrolyses were carried out using an EG&G PAR model 273 potentiostat, and a second platinum gauze with larger geometrical surface area as auxiliary electrode. In these experiments the working electrode compartment was filled with 60 ml of phenol-containing solution. The ph of the solutions was 6.7 and decreased during electrolysis. Spectra of the solutions were acquired with a Hewlett- Packard 8452A diode-array spectrometer under HP MS-DOS UV-Vis operating software. Using an Eppendorf micropipette, 100 l of solution were extracted from the electrolysis cell at different times during electrolysis, and these aliquots were diluted in 10 ml of water before obtaining the spectra in a quartz cell with 1 cm optical path. The experimental UV-Vis spectra thus obtained were deconvoluted into Lorentzian bands with Jandel Peakfit v The concentrations of the different compounds present in solution were determined using the Lambert Beer law, with molar absorption coefficients ε obtained from reported data at the wavelengths of maximum absorption for each compound, as indicated in Table 1. Formation of oxygen bubbles on the electrode surface was observed during electrolysis at potentials more positive Table 1 Wavelengths of maximum absorption and molar absorption coefficients of analyzed compounds Compound λ (nm) ε ( 10 3 cm 1 M 1 ) Reference p-chlorophenol (pcp) [12] p-nitrophenol (pnp) [13] p-benzoquinone (bq) [14] Maleic acid (ma) [13] than 1.65 V. From analysis of UV-Vis spectra it was also determined that the rate of oxidation of both pcp and pnp did not increase significantly at more positive potentials. Therefore, long-time electrolyses were carried out at 1.65 V (SCE). In situ infrared spectra of the Bi PbO 2 /solution interface during electrolysis were acquired with a Bruker Equinox IFS-55 spectrometer, with a medium band (MIR, cm 1 ) globar source and liquid nitrogen-cooled MCT detector, on a mirror-polished Bi PbO 2 film deposited onto a flat 1 cm diameter Au disc, set against a CaF 2 window to form a thin layer cell. Each spectrum was obtained by Fourier transformation after averaging 150 interferograms during 75 s at 8 cm 1 resolution, using p-polarized radiation, at constant potential of 1.65 V versus SCE. 3. Results and discussion Fig. 1 shows the evolution of the UV-Vis spectra during electrolysis of pcp and pnp at a constant potential of 1.65 V versus SCE. In both cases the current initially rose, then stabilized at ca. 0.4 A, and then increased again after oxidation of most of the organic in solution was accomplished. Fig. 1a shows spectra during oxidation of 10 3 M pcp + 0.1MNa 2 SO 4 aqueous solution. The inset shows the deconvoluted spectrum after 30 min of electrolysis. While the bands between nm (I) and nm (II) decrease with electrolysis time and are associated to the starting compound, the band between 230 and 265 nm (III) increases as pcp is oxidized. This band arises from the oxidation of phenol to a quinoid compound, turning the solution yellow due to the increased conjugation of double-bonds in the molecule. The concentration of chloride ions liberated during oxidation of pcp has been determined by potentiometric titration [15] and confirms that the generated product is benzoquinone (bq). The band centered at 210 nm (IV) appears after the passage of larger charges through the electrode/solution interface and corresponds to maleic acid (ma), as confirmed by other authors [16,17]. Fig. 1b shows UV-Vis spectra obtained at different times during electrolysis of 10 3 M pnp and 0.1 M Na 2 SO 4 aqueous solution. The inset shows the deconvolution of the spectrum after 10 min of electrolysis; the bands located between 210 and 250 nm (I), and between 260 and 370 nm (II), are attributed to the reacting species and decrease as oxidation

3 C. Borrás et al. / Electrochimica Acta 49 (2004) Absorbance I III IV II Absorbancia λ / nm 0.05 (a) λ / nm Absorbance III I II Absorbancia λ / nm (b) λ / nm Fig. 1. UV-Vis spectra of 10 3 M pcp (a) and pnp (b) solutions during electrolysis at 1.65 V vs. SCE on Bi PbO 2 electrodes, obtained at 600 s (a) and 300 s (b) intervals. The inset shows deconvoluted spectra after 30 min (a) and 10 min (b) of electrolysis, respectively. proceeds. In contrast to the result shown above related to the oxidation of pcp, the band centered at 210 nm (III) due to formation of ma increases from the start of the experiment during oxidation of pnp, and the absence of a band at 250 nm indicates that bq or other quinoid compounds do not accumulate in solution. Fig. 2a shows the concentration of pcp and the intermediates formed during its anodic oxidation, obtained through deconvolution of spectra such as that shown in Fig. 1a, as a function of the charge passed per mole of pcp initially present in solution, n pcp,0. As pcp oxidizes, the bq concentration increases to a maximum at a charge of Cdm 3 ; for higher charges the concentration of bq falls while the concentration of ma increases. Oxalic (oa) or formic (fa) acids are also produced during oxidation of bq to ma, but these are quickly oxidized further to CO 2 [1] and hence do

4 644 C. Borrás et al. / Electrochimica Acta 49 (2004) (a) 10 4 c / mol dm (b) 80 f pcp Q / C dm -3 Fig. 2. pcp ( ), bq ( ) and ma ( ) concentrations (a) and fraction of organic content oxidized (b) as a function of the electric charge passed, during oxidation of pcp in aqueous 0.1 M Na 2 SO 4 solution on Bi PbO 2 electrode. not accumulate in solution. The following scheme of consecutive reactions describes the results obtained: where R 1 = pcp, R 2 = bq, R 3 = ma, R 4 = oa and/or fa, and R 5 = CO 2. Since the UV-Vis intensities obtained from the spectra account for the concentrations of pcp and the intermediates involved in its mineralization accumulated in solution, bq and ma, then the fraction of organic content oxidized after passing a certain charge may be obtained from the spectral response, and is given by the relation, [ ] cpcp + c bq + c ma f pcp = 1 (2) c pcp,0 (1) where c pcp, c bq and c ma represent the concentration of pcp, bq and ma, respectively, at each value of the charge passed, and c pcp,0 is the initial concentration of pcp at the beginning of electrolysis. In Fig. 2b it can be seen that there is no loss of organic content up to charges lower than Cdm 3, since during this stage bq is the only product formed. The increase in concentration of bq in solution observed experimentally and the delay for the appearance of ma indicate that oxidation of pcp occurs faster than further oxidation reactions involving bq or ma. Due to accumulation of ma in solution, a stationary value in the loss of organic content is observed up to Cdm 3. As further charge is passed then bq ring opening occurs, generating maleic acid with generation of CO 2. Larger charges produce higher losses of organic content by oxidation of ma; this matches the decrease of ma shown in Fig. 2a. Extrapolation of the data in Fig. 2b indicate

5 C. Borrás et al. / Electrochimica Acta 49 (2004) that the total mineralization of the pcp can be considered to occur after the passage of Cdm 3 through the cell. Fig. 3 shows the variation of pnp and ma concentrations as a function of the charge passed, obtained from deconvolution of spectra such as that shown in Fig. 1b. Fig. 3a illustrates the degradation of pnp down to 10% of its initial concentration after passing a charge of Cdm 3. This decrease is accompanied by an increase of the concentration of ma as the only detected intermediate. The results obtained here agree with those reported by other authors during the photocatalytic degradation of nitrophenols [18,19], in which a very fast initial step destroys the cyclic species, followed by a slower reaction where the aliphatic chain is mineralized. The nitro group is a good leaving group in aromatic compounds, thus it can be readily eliminated, favoring electrophilic substitution with OH radical in the para position with respect to the hydroxyl group [20]. This may occur by elimination of NO 2 radical which oxidizes to nitric acid either by electron removal and addition of water with loss of a proton or by reaction with an OH radical, or via the initial formation of nitrite ion which rapidly oxidizes to nitrate [21]. Previous work carried out in our laboratory [15] shows that pnp is strongly adsorbed on the surface of the Bi PbO 2 electrode. This strong adsorption of the nitro compound modifies the electronic density distribution of the molecule and favors the oxidation of 10 (a) c / mol dm (b) f pnp Q / C dm -3 Fig. 3. pnp ( ) and ma ( ) concentrations (a) and fraction of organic content oxidized (b) as a function of the electric charge passed, during pnp oxidation in aqueous 0.1 M Na 2 SO 4 solution on Bi PbO 2 electrodes.

6 646 C. Borrás et al. / Electrochimica Acta 49 (2004) intermediate species, thus preventing accumulation of bq in solution. The fraction of organic content oxidized during anodic oxidation of pnp was obtained from the UV-Vis intensities corresponding to pnp and ma in solution using Eq. (3), f pnp = 1 c pnp + c ma (3) c pnp,0 Fig. 3b shows that pnp oxidizes readily to 10% of its initial concentration after passing a charge of Cdm 3, producing ma as the only intermediate, and approaching a steady organic content as the concentration of pnp diminishes, due to the slow degradation of ma. The very short life-time in solution of the OH radicals produced on the substrate limit the reaction zone to the surface of the catalyst [22]. Therefore, the rate of each stage of oxidation depends on the surface concentration of the respective organic compound, in turn depending on their corresponding adsorption equilibrium constants and bulk concentrations. Mineralization to CO 2 requires then that intermediates either remain adsorbed on the electrode surface or attain sufficient bulk concentration. Hence, the amounts of produced intermediates, as well as the rate of each of the consecutive steps of the reaction, are strongly dependent on the interactions of the starting compounds with the electrode surface. We have recently shown that pcp adsorption Fig. 4. SNIFTIRS spectra during pcp (a) and pnp (b) oxidation in aqueous 0.1 M Na 2 SO 4 solution on Bi PbO 2 electrodes. Spectra were acquired at constant potential of 1.65 V vs. SCE at 75 s intervals.

7 C. Borrás et al. / Electrochimica Acta 49 (2004) Peak Area / Arbitrary Units t / s Fig. 5. Area of CO 2 peaks of SNIFTIRS spectra obtained during pcp ( ) and pnp ( ) electrolysis at 1.65 V vs. SCE on Bi PbO 2, as a function of time. on Bi PbO 2 surfaces is affected by the presence of pnp in solution due to stronger adsorption of pnp [15]. Fig. 4 shows SNIFTIRS spectra of Bi PbO 2 electrodes during pcp and pnp electrolysis at a constant potential of 1.65 V versus SCE. The spectra corresponding to pcp oxidation (Fig. 4a) shows the appearance of a band at 2342 cm 1, attributed to CO 2 produced from oxidation of the organic compound, which grows with electrolysis time. The spectra also show a bipolar band between 1600 and 1800 cm 1 due to the scissor mode vibration of water. No other signals corresponding to the starting species or intermediates were identified; this may indicate weak adsorption of pcp or may be due to surface selection rules related to flat orientation of the adsorbed pcp molecules on the surface of the electrode. Fig. 4b shows the spectra corresponding to pnp oxidation. The negative band at 1290 cm 1 is associated to the NO 2 group of the pnp molecule. A wider positive-going band centered around 1330 cm 1 can also be seen in these spectra. This signal corresponds to the nitrate ion [23] and maintains a constant intensity after the second spectrum, indicating fast degradation of pnp to ma during the initial stages of the reaction, releasing the nitrite ion into solution, followed by fast oxidation to nitrate. Successive spectra show the growth of the band at 2342 cm 1 associated to CO 2 production, due to oxidation of the generated ma. Mineralization of pcp and pnp during these thin-layer experiments was assessed from integration of the CO 2 bands obtained during pcp (Fig. 4a) and pnp (Fig. 4b) oxidation. Fig. 5 shows that the integrated intensities obtained during oxidation of both compounds grew at comparable rates, confirming complete mineralization in both cases with similar kinetics and no accumulation of intermediates in solution under thin layer conditions. 4. Conclusions The kinetics of anodic oxidation of pcp and pnp on Bi PbO 2 electrodes was followed by deconvolution of UV-Vis spectra of the solutions during electrolysis at constant potential. The fraction of organic content oxidized to CO 2 is a function of the charge passed through the electrode/solution interface. Comparison of the amounts of produced intermediates during oxidation of pcp and pnp revealed that the rates of each of the consecutive steps of the reaction are strongly dependent on the interactions of the starting compounds and intermediates with the electrode surface. Mineralization to CO 2 requires that intermediates generated during anodic oxidation attain a sufficiently high concentration, or that they remain adsorbed on the electrode surface. SNIFTIRS studies allowed us to determine that under thin layer conditions the mineralization rate was similar for both pcp and pnp, in spite of their different interactions with the electrode surface, since under these conditions the organic species are confined to a region close to the electrode surface, thus increasing the concentration of intermediates and their mineralization to CO 2. Acknowledgements We gratefully acknowledge the members of the Electrochemistry Group at Universidad Simón Bolívar for

8 648 C. Borrás et al. / Electrochimica Acta 49 (2004) discussions, Michele Milo for technical and managerial assistance, and financial support from Fonacit (grant no. S ) and Decanato de Investigación y Desarrollo, USB. References [1] N.B. Tahar, A. Savall, J. Electrochem. Soc. 145 (1998) [2] O.J. Murphy, G.D. Hitchens, L. Kaba, C.E. Verostko, Water Res. 26 (1992) 443. [3] Ch. Comninellis, C. Pulgarin, J. Appl. Electrochem. 21 (1991) 703. [4] A.M. Polcaro, S. Palmas, F. Renoldi, M. Mascia, J. Appl. Electrochem. 29 (1999) 145. [5] M. Panizza, P.A. Michaud, G. Cerisola, Ch. Comninellis, Electrochem. Commun. 3 (2001) 336. [6] G. Saracco, L. Solarino, R. Aigoti, V. Specchia, M. Maja, Electrochim. Acta 46 (2000) 373. [7] G. Fóti, D. Gandini, Ch. Comninellis, A. Perret, W. Haenni, Electrochem. Solid State Lett. 2 (1999) 228. [8] A.M. Polcaro, S. Palmas, F. Renoldi, M. Mascia, Electrochim. Acta 46 (2000) 389. [9] J.D. Rodgers, W. Jedral, N.J. Bunce, Environ. Sci. Technol. 33 (1999) [10] A.M. Polcaro, S. Palmas, Ind. Eng. Chem. Res. 36 (1997) [11] W.R. LaCourse, Y. Hsiao, D.C. Johnson, J. Electrochem. Soc. 136 (1989) [12] W.W. Simons (Ed.), The Sadtler Handbook of Ultraviolet Spectra, QC459 S25, Philadelphia, [13] J.W. Robinson, Handbook of Spectroscopy, vol. II, CRC Press, Ohio, [14] E.A. Braude, J. Chem. Soc. 45 (1945) [15] C. Borras, T. Laredo, B.R. Scharifker, Electrochim. Acta 48 (2003) [16] H. Sharifian, D.W. Kirk, J. Electrochem. Soc. 133 (1986) 921. [17] C. Bock, B. MacDougall, J. Electroanal. Chem. 491 (2000) 48. [18] A. Di Paola, V. Augugliaro, L. Palmisano, G. Pantaleo, E. Savinov, J. Photochem. Photobiol. A Chem. 155 (2003) 207. [19] M.A. Oturan, J. Peiroten, P. Chartrin, A.J. Acher, Environ. Sci. Technol. 34 (2000) [20] K.H. Wang, Y.H. Hsieh, L.J. Chen, J. Hazard. Mater. 59 (1998) 251. [21] V. Maurino, C. Minero, E. Pelizzeti, P. Piccinini, N. Serpone, H. Hidaka, J. Photochem. Photobiol. A Chem. 109 (1997) 171. [22] C.S. Turchi, D.F. Ollis, J. Catal. 122 (1990) 178. [23] K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds, fifth ed., Wiley, New York, 1997.