Department of Chemistry, SUNY-Potsdam                                          Slide 1 2 3 4 5 6 7 8 9 10

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The National Meeting of the Electrochemical Society

Symposium:  Nanotechnology 

Anodic Photooxidation of Remazol Black B Azo Dye on Transition Metal Oxide Electrodes

Abstract.  The dye pollutants discharged into industrial waste water are often toxic, carcinogenic, mutagenic, or teratogenic in living organisms. Many dyes, including the diazo dyes, are very stable and are resistant to chemical degradation as well as to a microbial attack. We have investigated the degradation kinetics of various azo dye pollutants (such as the Chicago Sky Blue, Brilliant Cresyl Yellow, Orange II, Naphthol Blue Black, Methylene Blue, and others) using photoelectrochemical methods. In this work, the anodic photo-decomposition of Remazol Black B (RBB), was investigated. Previous studies devoted to RBB degradation, were based on fungal peroxidase-catalyzed oxidation [1], and photooxidation by UV light in combination with a powerful oxidant such as H₂O₂ [2,3]. The RBB degradation in colloidal semi-conductor suspensions acting as photocatalysts have also been used [4]. We have found that photo-electrochemical degradation [5] is superior in terms of the degradation speed in comparison with other methods. We have observed faster degradation rates using photoelectrocatalysis on solid semiconductor electrodes as compared to the degradation rates obtained in colloidal photocatalysis systems.
    The preparation of semiconductor electrodes was based on electrochemical deposition of thin semiconductor films on Pt substrates as described in our previous papers [5,6]. Nanoparticulate films of TiO₂, WO₃, and MoO₃, with 50-100 nm thickness and an average size of nanocrystallites from 20 to 40 nm., were used in this work as the electrocatalysts for RBB degradation.
    The photocatalyst film morphology and crystalline structure are of paramount importance for the dye degradation effectiveness. Therefore, the catalyst film morphology was examined using SEM and AFM imaging techniques and structure determined using X-ray diffraction. The effect of annealing temperature on performance of these electrocatalysts have been investigated in the temperature range from room temperature to 600 ^oC. Typical plot of the dependence of RBB degradation rate on annealing temperature is shown in Figure 1. The optimum annealing temperature was found to be 450 ^oC.
The kinetics of RBB degradation process was followed by monitoring the decay of absorbance maximum at  l_m = 597 nm. The decay rate was strongly dependent on the applied electrode potential and thus can be controlled by changing the potential. The efficiency of the photoelectrochemical degradation process depends also on the dye concentration, selection of a suitable supporting electrolyte, and solution pH. 

    The mechanism of reactions taking place during the photodecomposition process of RBB is based on the electron-hole pair generation in the semiconductor film and heterogeneous oxidation with participation of valence-band holes. Using the ab-initio SCF Hartree-Fock calculations of molecular orbitals and electrostatic potential mapping, the mechanisms of nucleophilic and electrophilic attacks, which prevail in different media, are elucidated.

Acknowledgements
This work was supported by ACS-PRF grant No. 33190-B5.

References
1. Young L., Yu J., Wat. Res., 31 (1997) 1187-1193.
2. Ince N. H., Wat. Res., 33 (1999) 1080-1084.
3. Yang Y., Wyatt D. T. II, Bahorsky, M., Textile Chemist and Colorist, 30, (1998) 27-35.
4. Reutergrdh L. B., Iangphasuk, M., Chemosphere, 35 (1997) 585-596.
5. Hepel M., Hazelton S., Electrochim. Acta, 50 (2005) 5278.
6. Hepel M., Luo J., Electrochim. Acta, 47 (2001) 729.

Figure 1. Dependence of photoelectrocatalytic degradation rate of RBB on the annealing temperature of n-type WO₃ semiconductor film electrode at E = 1.06 V vs. SCE, in a 6x10^-5 M RBB solution.

MOTIVATION

Maintaining the natural human habitat in the era of global industrialization requires immense research efforts to develop pollution-free technologies and advise efficient methods for cleaning, already polluted, environment. Many industrial processes release every day tons of toxic, carcinogenic, and degenerative chemicals. Some of the pollutants remain in the environment for long time. One of the ubiquitous environmental pollutants are dyes used in textile and other industries. The textile dyes do not degrade easily. The dye degradation schemes have been widely investigated and, in recent years, considerable improvement in the degradation efficiency has been achieved. It has been reported that several organic pollutants can be degraded on illuminated semiconductor powders. On the other hand, photoelectrodes developed recently for water photoelectrolysis, can also be used as efficient catalysts for degrading pollutants.

MECHANISM

In a photooxidative degradation process, an n-type semiconductor is illuminated with radiation having energy at least equal to the band-gap (E_g) of the semiconductor. The photons absorbed excite electrons from the valence band (VB) to the conduction band (CB), while holes are left in the valence band. The holes in a semiconductor have a high oxidizing power and can oxidize organic molecules in an aqueous environment. These photoelectrons and photoholes may recombine producing thermal energy, but they may be separated and engaged in driving chemical reactions in photochemical processes. The rate of oxidation by holes has to be balanced by the rate of the reduction by electrons. The transfer of photogenerated holes from the valence band to organic molecules may be either isoenergetic or inelastic via band-gap surface states. Surface states can act as the recombination centers for photogenerated electron-hole pairs but may also participate in sub-bandgap excitation processes and help utilize more energy from the solar spectrum.

Bandgap excitation in nanostructured semiconductor film

Figure 2.

Figure 3.

n-WO₃ PHOTOELECTRODES

Tungsten trioxide is an important photoelectrocatalytic material. The photoelectrochemical behavior of WO₃ films was extensively studied in this laboratory for electrochromic applications [8,9], solar energy conversion, and degradation of pollutants. The electrodeposited Pt/WO₃ catalysts have also been found to have high activity toward oxidation of methanol and formic acid, higher than benchmark Pt catalysts. The WO_x films with Pt, Sn, and Ru centers were used by Bock and MacDougall for electrooxidation of HCOOH and (COOH)₂.  Kulesza and Faulkner described electrocatalysis of chlorate reduction on mixed valency WO_3-x electrodes.  The electrocatalytic activity of Pt/WO₃ electrode in phosphoric acid fuel-cells was studied by Savadogo. Augustynski et al., investigating photooxidation of methanol on thermal WO₃ films, have found that at high methanol concentrations, hole scavenging is the predominant process while at low methanol concentration, an indirect photooxidation with the formation of hydrogen peroxide species takes place. Modulation of absorbance spectra of composite films WO₃-TiO₂ were described by de Tacconi et al.  The photoelectrodes were synthesized by pulsed electrodeposition.  Krasnov and Kolbasov have found that index of refraction of electrodeposited a-WO₃ films increases during the film drying process, indicating on chemisorbed water content.
 

AZO DYE

Remazol Black B (RBB) is a complex textile diazo dye and has a high photo- and thermal stability. It cannot be efficiently degraded using conventional methods of oxidative degradation such as ozonation. Recently, a ZnO-assisted photocatalytic degradation of RBB has been reported. Various metal oxide semiconductors were used by Poulios and Tsachpinis in their RBB photodegradation investigations. The decolorization of RBB on TiO₂ and CdS photocatalysts in aqueous suspensions was also investigated. Reaction mechanisms for reduction of several diazo dyes on mercury electrode were elucidated by Zanoni and coworkers.  Biodegradability studies of Ganesh et al. have shown that RBB and other azo dyes are not biodegradable under aerobic conditions.
      Remazol Black B structure

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