Ti electrodes modified with WO3 in electro- and photoelectrochemical oxidation of Acid Orange 7 dye

Accepted Manuscript Title: Application of TiO2 –RuO2 /Ti electrodes modified with WO3 in electro- and photoelectrochemical oxidation of Acid Orange 7 dye Author: Elzbieta Kusmierek Ewa Chrzescijanska PII: DOI: Reference:

S1010-6030(15)00022-2 http://dx.doi.org/doi:10.1016/j.jphotochem.2015.01.009 JPC 9825

To appear in:

Journal of Photochemistry and Photobiology A: Chemistry

Received date: Revised date: Accepted date:

18-7-2014 4-12-2014 22-1-2015

Please cite this article as: Elzbieta Kusmierek, Ewa Chrzescijanska, Application of TiO2ndashRuO2/Ti electrodes modified with WO3 in electro- and photoelectrochemical oxidation of Acid Orange 7 dye, Journal of Photochemistry and Photobiology A: Chemistry http://dx.doi.org/10.1016/j.jphotochem.2015.01.009 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Application of TiO2-RuO2/Ti electrodes modified with WO3 in electro- and photoelectrochemical oxidation of Acid Orange 7 dye Elzbieta Kusmierek* [email protected], Ewa Chrzescijanska

Lodz University of Technology, Institute of General and Ecological Chemistry 90-924 Lodz, ul. Zeromskiego 116, Poland

*

Corresponding author. Tel.: +48 42 631 31 30, fax: +48 42 631 31 28.

Highlights

 TiO2-RuO2/Ti electrodes modified with WO3 were applied in AO7 degradation.  Electrochemical behaviour of the electrodes was tested using cyclic voltammetry.  Dye photoelectrochemical degradation was carried out under UV and VIS irradiation.  The best results were achieved at TiO2(91%)-RuO2(3%)-WO3(6%)/Ti electrode.  TiO2-RuO2/Ti electrodes modified with WO3 can be applied under VIS irradiation.

Abstract TiO2-RuO2/Ti electrodes modified with WO3 were applied in electro- and photoelectrochemical degradation of a monoazo dye (Acid Orange 7) under UV and VIS irradiation. Their electrochemical behaviour was investigated by recording cyclic voltammetry curves in the dye and supporting electrolyte solution. The highest oxidation power was stated in the case of the tested electrodes with WO3 content of 3% and various contents of other oxides. Electrolytic degradation of the monoazo dye coupled with the photocatalytic process resulted in higher conversion of the dye. Application of VIS irradiation was also advantageous but the efficiency of the degradation was slightly lower than under UV irradiation. Taking into consideration not only decolouration but also mineralization of the dye solution, the application of the electrode with 6% content of WO3 and the content of TiO2 higher than 90% resulted in highest degradation of the dye solution. TiO2-RuO2/Ti electrodes modified with WO3 seems to be applicable not only under UV irradiation but also under VIS irradiation, e.g. sunlight or even room light lamps.

Keywords:

oxide

electrodes,

TiO2,

WO3,

electrolytic

oxidation,

photoelectrochemical oxidation, azo dyes 1. Introduction

Industrial wastewater often contain large amounts of recalcitrant organic pollutants and may be treated with the application of Advanced Oxidation Processes (AOPs). These processes include various methods involving degradation of organics by hydroxyl and superoxide radicals species [1-3]. One of AOPs, photocatalytic treatment with wide band gap semiconductors, e.g. TiO2, can destroy pollutants under UV irradiation, however it requires post-

treatment in order to separate photocatalyst particles unless they are immobilized on solid substrates [4-7]. Last years, Electrochemical Advanced Oxidation Processes (EAOPs) involving generation of hydrogen peroxide and hydroxyl radicals species at anode surface has drawn great attention [8-13]. Electrochemical oxidation of organics can be performed by direct anodic oxidation (poor efficiency) or chemical reactions with electrogenerated “active oxygen” (e.g. hydroxyl radicals) [14]. Electrochemical or electrocatalytic treatment results in total decoloration but complete mineralization is difficult to achieve [15, 16] and requires application of high voltages and long time electrooxidation resulting in high costs and making it uneconomical [10]. An improvement of the treatment efficiency can be achieved in indirect oxidation with active chlorine [17-20] but it is usually connected with necessity of chlorides addition to treated wastewater resulting in more corrosive medium. The other solution of electrooxidation improvement of organic pollutants is combination of this process with photocatalytic degradation. The photoelectrochemical process can be performed with the application of TiO2RuO2/Ti electrodes. TiO2 is characterized by the band gap of 3.2 eV (anatase) and 3.0 eV (rutile) [19]. Generally, anatase form is photocatalytically more active than rutile due to lower rates of electron-hole recombination rates, but a mixture of these two forms is often applied in photocatalytic processes [21-24]. Photoexcitation of TiO2 requires irradiation from UV region due to its wide band gap. A decrease in process costs can be achieved by application of inexpensive and renewable sunlight or even room light lamps emitting visible photons instead of harmful UV irradiation. Therefore, TiO2 should be modified in such a way to make it possible to be photoexcited by irradiation with the wavelength higher than 380 nm. One of modification methods is coupling of TiO2 with another semiconductor with narrow band gap able to absorb visible photons. WO3 is a n-type semiconductor and is often coupled with TiO2 due to its band gap of 2.8 eV [25, 26]. This semiconductor is characterized by low

photoactivity due to high rate of electron-hole recombination because the lower conduction band edge does not provide a sufficient potential to reduce O2 (Fig. 1) [27]. This bi-component photocatalyst is characterized also by lower electronhole recombination rate and higher photoactivity than single-component TiO2 or WO3 photocatalyst [21, 30]. The photoactivity of WO3/TiO2 bi-component semiconductor strongly depends on its preparation method and the content of WO3. The optimum content of WO3 in bi-component photocatalyst was proved to be 3-5% [31-34]. Due to the position of valence and conduction bands in TiO2 and WO3 (Fig. 1), it was stated that the photogenerated electrons are transferred from the conduction band of TiO2 to the WO3 conduction band. The holes accumulate in the TiO2 valence band [31, 33, 34]. It was assumed that in the bi-component photocatalyst (WO3/TiO2), a complex of WxTi1-xO2 is formed [31]. This complex has lower energy level than that of TiO2. If this photocatalyst is irradiated by photons with the energy of hν equal or higher than (Ec-Ev) then photoelectrons can be excited from the valence band of TiO2. If the energy of photons (hν) is lower than (Ec-Ev) but equal or higher than band gap in the WxTi1-xO2 complex then photoelectrons can be excited only from the WxTi1-xO2 energy level. Under UV irradiation both TiO2 and WO3 can be excited simultaneously [33]. If VIS irradiation is applied then electrons in WO3 can be excited from HOMO to LUMO followed by transfer to the conduction band of TiO2 [30]. Simultaneously, the photogenerated holes still remains in WO3 which creates a temporary separation of charge carriers. Coupling of TiO2 with WO3 results in higher photoactivity of the bi-component photocatalyst. The enhancement of the photoactivity can be attributed to improved charge-carrier separation and increased surface adsorption affinity [35, 36]. The increased surface adsorption affinity might be the predominant effect under some conditions.

Photoanodes covered with WO3 or WO3/TiO2 were applied in degradation of organic compounds, especially dyes [21, 25, 37]. It was proved that the composite film demonstrated higher photoelectrochemical activity than WO3 alone [38]. Modification of the oxide layer with WO3 can change the electrochemical and chemical characteristic of the electrode. Standard composition of the electrodes (30%RuO2 and 70%TiO2) ensures highly developed porous surface, a high catalytic activity, selectivity and corrosion resistance in chlorine electrolysis [39]. It was reported that the films containing less than 25 mol % RuO2 were semiconductors, and the films with a higher content of RuO2 were metallic conductors. The magnitude of currents determined in cyclic voltammograms recorded in various solutions decreases with decreasing Ru content, especially for the less than 15% RuO2 in the oxide layer [40, 41]. The substitution of WO3 for TiO2 was reported as leading to degradation of electrocatalytic characteristic of anodes [39]. Taking into consideration the fact that WO3 is thermodynamically stable in acidic solutions, the electrodes covered only with WO3 should be applied in acidic media, but experiments can be also performed in neutral and alkaline solutions since the rate of its dissolution is relatively low [37]. The aim of this work was to investigate an effect of different content of oxides in the film layer at TiO2-RuO2/Ti electrodes modified with WO3 oxide on degradation of an azo dye. Acid Orange 7 was chosen as a representative dye because azo dyes are used in various industries and they are often components of industrial wastewater. Degradation of

this dye was carried out in an

electrolytic and photoelectrochemical oxidation under UV and VIS irradiation. Electrochemical behaviour of the tested electrodes in the dye solution was also investigated by cyclic voltammetry method.

2. Materials and methods

2.1 Chemicals

Acid Orange 7 (AO7) - a monoazo dye was used in electroanalytical measurements and degradation processes in order to compare behaviour of the tested electrodes. This compound (Fig. 2) was obtained from dyestuff plant Boruta-Zachem SA in Zgierz (Poland). The dye solutions were prepared by dissolution of the substrate in 0.1 mol L-1 Na2SO4 (Sigma-Aldrich, ACS reagent) which was applied as a supporting electrolyte. The concentration of AO7 was 200 mg L-1. Determination of COD in solutions before and after degradation experiments were carried out using the following reagents: H2SO4 (Chempur, Poland) with Ag2SO4 (POCH Gliwice, Poland) and Fe(NH4)2(SO4)2 (POCh Gliwice, Poland). Purity of these chemicals was analytical grade.

2.2 Electrodes

TiO2-RuO2/Ti electrodes modified with WO3 were prepared by thermal decomposition (500oC) of proper metal precursors deposited on titanium support. Precursors mixture concentration was chosen in such a way to obtain electrodes with the following nominal composition: TiO2(67%)-RuO2(30%)WO3(3%)/Ti,

TiO2(64%)-RuO2(30%)-WO3(6%)/Ti,

WO3(3%)/Ti,

TiO2(91%)-RuO2(3%)-WO3(6%)/Ti,

TiO2(94%)-RuO2(3%)TiO2(94%)-WO3(6%)/Ti,

TiO2(97%)-WO3(3%)/Ti and TiO2(98,5%)-WO3(1,5%)/Ti. The composition of the tested electrodes was established taking into consideration that the optimum WO3 content in bi-component WO3/TiO2 photocatalysts was in the range 3-5% [31-35]. RuO2 has electroctalytic activity and ensures electrical conductivity. Its content was 30 or 3% in the oxide film.

In order to compare the results of the investigation, non-modified electrodes, i.e. TiO2(70%)-RuO2(30%)/Ti and TiO2(97%)-RuO2(3%)/Ti, with the same geometric surface area were also applied in electroanalytical measurements and degradation experiments. The surface area of electrodes used in electroanalytical experiments was 2 cm2 while their surface in degradation experiments was 20 cm2. Pt electrode in the form of wire was used as a counter electrode in recording voltammetric curves. Degradation experiments were carried out with a platinum cathode in the form of sheet (20 cm2).

2.3 Voltammetric analysis

Voltammetric measurements were carried out using a cyclic voltammetry method. Voltammetric curves were recorded in a dye solution using a three electrode cell connected to AUTOLAB electrochemical workstation - AutoLab III (Methrom Autolab B.V., The Netherlands). The tested TiO2-RuO2/Ti electrodes modified with WO3 were used as a working electrode. Potential of the working electrode was measured vs. saturated calomel electrode (SCE). Pt in the form of wire was used as a counter electrode. Voltammetric curves were recorded with the scan rate of 10 mV s-1. The volume of tested solutions was about 10 mL. All solutions were purged with argon before measurements in order to remove dissolved oxygen. During the measurements argon blanket was kept over the solution.

2.4 Electrolysis and photelectrolysis

Electrochemical degradation of AO7 solution at the tested electrodes was performed under galvanostatic conditions in a two electrode cell with undivided electrode compartments made of quartz glass. The tested electrodes with the surface area of 20 cm2 were applied as anodes. Pt in the form of sheet (20 cm2)

was used as a cathode. The electrochemical cell was connected to laboratory power supply (Matrix, Poland). Current intensity applied during experiments was 0.2 A and corresponded to current density of 0.01 A cm-2. The duration of electrolysis was 2 hour. Photoelectrochemical degradation experiments were performed in the same electrochemical cell inserted into a photochemical reactor RPR 200 (Southern New England Ultraviolet Co., USA). This reactor was equipped with 16 lamps emitting irradiation with the wavelength of 254 nm (51 W m-2) or 420 nm (57 W m-2).The experimental conditions of photoelectrolysis, i.e. current intensity and experiment duration, were the same as in the case of electrolysis. The

effectiveness

of

electrochemical

and

photoelectrochemical

degradation of AO7 with the application of the tested electrodes was controlled by determination of chemical oxygen demand (COD), total organic carbon (TOC) and absorbances from UV-VIS spectra recorded in solutions before and after degradation. TOC was analyzed with the application of TOC 5050AShimadzu Total Organic Carbon Analyser 5050A (Shimadzu, Japan). COD was determined in tested solutions according to procedures described in Refs. [42, 43]. UV-VIS spectra were recorded within wavelength range from 190 to 800 nm using UV-VIS Spectrophotometer Shimadzu UV 2401 PC (Shimadzu, Japan).

3. Results and discussion

3.1 Cyclic voltammetry

The electrochemical behaviour of TiO2-RuO2/Ti electrodes modified with WO3 was investigated by recording voltammetric curves (E vs. I) in AO7 and Na2SO4 solutions in the potential range from 0 to the potential of oxygen Fig. 3

evolution. These curves were compared with voltammograms recorded at nonmodified electrodes and platinum electrode with the same geometric area. Fig. 3 shows curves recorded at TiO2(70%)-RuO2(30%)/Ti and Pt electrodes. It results from Fig. 3 that AO7 dye is oxidized in at least one electrode step at both tested electrodes before the potential at which oxygen evolution starts. This process is probably irreversible. In the case of TiO2(70%)RuO2(30%)/Ti electrode, oxidation current of AO7 is significantly higher than at Pt electrode. Higher current values in the potential range from 0 to 0.6 V recorded at the oxide electrode in AO7 solution in comparison with Na2SO4 solution indicate that in this potential range the dye is adsorbed at the surface electrode. Moreover, the peak potential of AO7 oxidation at the oxide electrode is by 130 mV lower than at Pt electrode. This means that oxidation of AO7 at the oxide electrode proceeds easier. The peak current density of AO7 oxidation is almost 10 times higher at the oxide electrode than at Pt, and indicates the higher rate of the electrode reaction. The comparable current and potential values were expected in the case of TiO2(70%)-RuO2(30%)/Ti

electrodes

modified

with

WO3.

Cyclic

voltammograms recorded at the electrodes with WO3 content of 3 and 6% in the oxide layer are presented in Fig. 4. Fig. 4 shows that introduction of WO3 into oxide layer causes a slight shift of the peak potential towards more positive values. Simultaneously, the peak current decreases slightly. The oxide electrodes with WO3 show lower current values in the potential range from 0 to 0.6 V than non-modified electrode. Moreover, in this potential range, current values recorded in AO7 solution are close to values recorded in the supporting electrolyte. This means that AO7 adsorption at the surface electrode proceeds in lower degree if the oxide electrode is modified with WO3. Modification of TiO2(70%)RuO2(30%)/Ti electrode with WO3 also has an influence on the activity of the

electrode towards oxygen evolution. The onset potential of oxygen evolution is slightly shifted to higher potentials. TiO2(97%)-RuO2(3%)/Ti electrodes were also modified by introduction of WO3 into the oxide layer. Cyclic voltammograms recorded at these electrodes in AO7 solution are presented in Fig. 5. Fig. 5 shows that modification of the oxide layer with WO3 causes a shift of the oxidation peak towards less positive values (by 32 mV - 3%WO3 and 74 mV - 6%WO3) in comparison with non-modified TiO2-RuO2/Ti. Moreover, modification of the oxide electrodes with WO3 caused an increase in the oxidation peak current. Generally, the oxide electrodes with higher content of TiO2 and lower content of RuO2 shows higher oxidation peak potentials of AO7 than the oxide electrodes with lower content of TiO2 and higher content of RuO2. The third group of the tested electrodes included the oxide electrodes without RuO2 and with high content (above 90%) of TiO2. Cyclic voltammograms recorded at these electrodes in AO7 solution do not show the oxidation peak of the dye (Fig. 6). However, an increase in current in the potential range from 0.6 to 1.1 V is obvious in comparison with the supporting electrolyte and confirms that AO7 oxidation proceeds although no oxidation peak is shaped. The lower content of WO3 in the oxide layer results in lower current values. This means that WO3 content is very important if the oxide layer does not contain RuO2 and effects efficiency of the process. The electrode with the lowest (1.5%) contents of WO3 seems to be not applicable in the oxidation of this monoazo dye. In order to compare the electrochemical behaviour of tested electrodes towards AO7 electrooxidation, the peak potential and current for AO7 oxidation was determined from voltammetric curves. Moreover, values of oxidation currents at the same potential (0.8 V) were determined and are presented in Table 1.

Results presented in Table 1 show that in the case of the electrodes with lower (ca. 70%) content of TiO2, the introduction of WO3 into the oxide layer results in no improvement of AO7 oxidation. The peak potential shifts towards more positive potentials with the increase in WO3 content while the peak current decreases. Quite opposite situation is observed in the case of electrodes with higher (above 90%) content of TiO2. The increase in WO3 content in the oxide layer of the tested electrodes results in a shift of Ep towards less positive values and Ip increases. That means that modification of these electrodes with WO3 improved AO7 electrooxidation. However, values of Ep determined at the electrodes with higher TiO2 content are higher than at the electrodes with lower TiO2 content, what means that AO7 electrooxidation proceeds more difficult. In the third group of the tested electrodes, it was impossible to determine values of Ep and Ip . The current values determined at the constant potential (0.8 V) were at least one order lower than in the case of the electrodes from the first and second group and comparable with values determined at Pt electrode except for TiO2(98.5%)-RuO2(1.5%)/Ti. The latter electrode should not be applied in AO7 oxidation. Lower values of oxidation current observed at the electrodes in the third group can be also attributed to the lack of RuO2 (electrocatalyst) in the oxide layer. In the case of all tested electrodes, the onset potential of oxygen evolution was compared (Table 2). As it is described in the paper [44], oxidation of organic compounds by electrogenerated hydroxyl radicals is in competition with the reaction of these radicals anodic discharge to oxygen and the activity of these radicals strongly depends on their interactions with electrode surface. One general rule is important, in comparison of reactivity of electrogenerated hyroxyl radicals at various anodes - the weaker interaction of hydroxyl radicals with anode surface, the lower is electrochemical activity toward oxygen evolution and the higher is chemical reactivity toward organics oxidation.

Modification of TiO2-RuO2/Ti electrodes with WO3 caused a decrease of the onset potential of oxygen evolution (EO2) determined in the supporting electrolyte. The higher WO3 content (6%) resulted in lower EO2 value. This decrease was much higher in the third group of the electrodes without RuO2. This means that modification of the electrodes caused lower their oxidation power due to the rule that the higher is the O2 overvoltage the higher is the oxidation power of the electrode material. However, values of EO2 were higher in the dye solution for all tested electrodes except for TiO2(70%)RuO2(30%)/Ti. This means that AO7 has an influence on the activity of the electrodes towards oxygen evolution. An increase in EO2 suggests the higher oxidation power of the electrode. In the first, second and third group of the electrodes, the highest oxidation power was observed in the case of TiO2(67%)RuO2(30%)-WO3(3%)/Ti, TiO2(94%)-RuO2(3%)-WO3(3%)/Ti and TiO2(97%)WO3(3%)/Ti electrode, respectively. Taking into consideration the oxidation power of the electrode, the WO3 content of 3% seems to be the most favourable.

3.2. Galvanostatic electrolysis Galvanostatic electrolysis of AO7 (200 mg L-1) solution were performed for 2 h at 0.01 A cm2 with the application of all tested electrodes except for the third group electrodes without RuO2 oxide. The electrodes from the third group were the least stable during electrolytic process which resulted in rapid increase in voltage. The process efficiency was controlled by recording UV-VIS spectra and determination of COD and TOC values in solutions after electrolysis. A decrease in COD and TOC values is attributed to mineralization of the dye solution and proves not only its discolouration but also degradation to CO2 and H2O. UV-VIS spectra of AO7 solution presents three bands at the wavelength of 228, 310 and 483 nm. The peak in the visible region is attributed to n-*

transition of the azo group. A decrease in the absorption at 483 nm results from decolouration of the solution. Two peaks in UV region are attributed to -* transition of naphthalene (310 nm) and benzene (228 nm) rings [45-47] connected by azo bond in the dye molecule. Exemplary UV-VIS spectra recorded in AO7 initial solution before and after photochemical, electrochemical and photoelectrochemical degradation with the application of TiO2(94%)RuO2(3%)-WO3(3%)/Ti electrode are presented in Fig. 7. A comparison of absorbances decrease observed at different wavelengths during electrolytic degradation of AO7 at TiO2-RuO2/Ti electrodes modified with WO3 is presented in Table 3. Decolouration of AO7 solution as well as degradation of benzene and naphthalene rings was comparable in electrolytic processes carried out at tested electrodes. However, higher conversion of AO7 calculated as a change in the absorbances at 483, 310 and 228 nm was observed in the second group of electrodes (the highest value - TiO2(91%)-RuO2(3%)-WO3(6%)/Ti). That means that lower content of RuO2 did not decrease the dye conversion as it could be expected taking into consideration the fact that RuO2 is an electrocatalyst [48]. The content of three oxides influenced also the conversion of AO7 calculated as a change in TOC and COD value (Fig. 8). The highest decrease in TOC and COD values (mineralization) of the dye solution was observed in the electrolytic process with the application of TiO2(64%)-RuO2(30%)-WO3(6%)/Ti (except for TOC) and TiO2(91%)RuO2(3%)-WO3(6%)/Ti electrodes. The lowest decrease in TOC was observed while TiO2(70%)-RuO2(30%)/Ti and TiO2(97%)-RuO2(3%)/Ti electrode was used. Generally, the decrease in COD values was several times higher than the decrease in TOC. This can be explained by the fact that TOC reduction corresponds to conversion of organic compounds to carbon dioxide and water (mineralization) while COD reduction may be attributed to total oxidation as well as to the conversion of organic compounds to more oxidized intermediates.

If higher amount of the oxidized intermediates is formed in the degradation process then the lower amount of oxygen is required for their oxidation in COD determination. Simultaneously, the higher decrease in absorbances at 228, 310 and 483 nm in comparison with TOC and COD reduction was observed. This means that bonds which are attributed to the chosen wavelengths were broken. Moreover, a decrease in the absorbance at 483 nm was higher than at 230 and 310 nm what means that the azo bond (chromophore bond) responsible for the absorption at 483 nm was more easily broken than the aromatic bonds which are attributed to 230 and 310 nm. Comparison of all results obtained in the electrolytic process of AO7 degradation proves that the application of TiO2(91%)-RuO2(3%)-WO3(6%)/Ti enables the highest efficiency of this process. However, further improvement of the efficiency can be achieved if the electrolytic degradation is combined with the photocatalytic process under UV or even VIS irradiation.

3.3 Galwanostatic photoelectrolysis under UV irradiation

The electrolytic degradation of AO7 solutions was carried at the tested electrodes with simultaneous irradiation at the wavelength of 254 nm. The irradiation at the wavelengths below than 380 nm is enough for photoexcitation of TiO2 [49-51]. The results of photoelectrochemical degradation of AO7 dye under UV irradiation are presented in Table 4 and Fig. 9. For comparison, photochemical and photocatalytic degradation of AO7 in the presence of the tested electrodes was performed in 2h experiments under irradiation at the wavelength of 254 nm. A decrease in absorbances at 228, 310 and 483 nm is also presented in Table 4. The decoloration of AO7 in the photochemical process was about 6%. Slight increase in the decoloration was achieved in the presence of the tested electrodes. Then, the decoloration was about 8%. Simultaneously, a decrease in absorbance at 228 and 310 nm was very lowin the

photochemical process, and slightly increased in the presence the tested electrodes. Significant changes in absorbances in these three bands were obeserved in the photoelectrochemical process under UV irradiation. A decrease in COD value in the photochemical process was only 1.91%. In the presence of the tested electrodes under UV irradation, a decrease in COD value was as follows:

TiO2(70%)-RuO2(30%)/Ti

WO3(3%)/Ti

-

3.98%,

-

4.02%,

TiO2(67%)-RuO2(30%)-

TiO2(64%)-RuO2(30%)-WO3(6%)/Ti

-

2.78%,

TiO2(97%)-RuO2(3%)/Ti - 4.11%, TiO2(94%)-RuO2(3%)-WO3(3%)/Ti - 2.86%, TiO2(91%)-RuO2(3%)-WO3(6%)/Ti - 2.63% and TiO2(94%)-WO3(6%)/Ti 3.01%. In the case of TOC values, no decrease was observed in the photochemical and photocatalytic process. Significantly higher changes in TOC and COD values were achieved in the photoelectrochemical degradation of AO7. Conversion of AO7 achieved in the photoelectrochemical process calculated as a change in absorbances determined at three main bands in UVVIS region shows no significant differences at the tested electrodes. Decolouration of AO7 solution was above 96% and higher than in the electrolytic process (Table 3). The decrease in the absorbances in UV region was also comparable for all tested electrodes except for TiO2(70%)RuO2(30%)/Ti electrode. In the case of this electrode without WO3 in the oxide film, AO7 conversion calculated as a change in the absorbance at 228 and 310 nm was slightly lower than at other electrodes. However, changes in TOC and COD in the photoelectrochemical process at the tested electrodes were clear (Fig. 9). Application of UV irradiation caused at least 5 times and twice higher AO7 conversion calculated as a change in TOC and COD, respectively. The highest conversion

was

observed

at

TiO2(64%)-RuO2(30%)-WO3(6%)/Ti

and

TiO2(91%)-RuO2(3%)-WO3(6%)/Ti. In both groups of the tested electrodes, introduction of WO3 into the oxide layer was advantageous due to higher

efficiency of the photoelectrochemical process. Independently on the content of TiO2 in the oxide layer, an increase in WO3 content caused higher AO7 conversion.

3.4 Galvanostatic photoelectrolysis under VIS irradiation Combination of the electrolytic degradation with photocatalytic process at TiO2RuO2/Ti electrodes should be carried out under UV irradiation due to the fact that TiO2 is photoexcited at the wavelengths lower than 380 nm, i.e. by photons with higher energy. A decrease in process costs can be achieved by application of photons with lower energy (irradiation with higher wavelengths) even by solar light. However, VIS irradiation can cause lower degradation of organic compounds due to very low photoactivity of TiO2. Thus, the third oxide (WO3) was introduced to the oxide layer of TiO2-RuO2/Ti electrodes. The experiments were carried out under the same conditions as previously but the electrolytic process was coupled with irradiation at the wavelength of 420 nm. The results of experiments are presented in Table 5 and Fig. 10. For comparison, photochemical and photocatalytic degradation of AO7 in the presence of the tested electrodes was performed in 2h experiments under irradiation at the wavelength of 420 nm. No decoloration and no decrease in absorbances at 228 and 310 nm was observed in these processes. Moreover, no decrease in TOC and COD value was also observed in both processes. In the photoelectrochemical process, changes in absorbances at three main bands in UV-VIS spectra are noticeable comparing all tested electrodes. The lowest dye conversion was obtained at TiO2(70%)-RuO2(30%)/Ti electrode. In the first group of the electrodes, modification with WO3 was advantageous because caused higher decolouration and decrease in absorbances related to naphthalene and benzene rings. In the second group of the electrodes, results are more comparable without noticeable effect of WO3 content. Application of the tested electrodes (the first and second group) modified with WO3 resulted in higher

dye conversion in comparison with the electrolytic process and a little lower conversion than in the photoelectrochemical process under UV irradiation (Table 4). The highest dye conversion calculated as a change in TOC (17 and 13%) was observed at TiO2(91%)-RuO2(3%)-WO3(6%)/Ti and TiO2(67%)-RuO2(30%)WO3(3%)/Ti electrodes, respectively (Fig. 10). This means that these two electrodes should be used if not only decolouration but also mineralization of the dye solution should be achieved. Introduction of WO3 into the oxide layer of the electrodes from the first group caused higher AO7 conversion. The increase in WO3 amount in the oxide layer from 3 to 6% decreases the dye solution mineralization. Opposite situation was observed in the second group of the electrodes with higher content of TiO2 (above 90%). The increase in WO3 content resulted in higher mineralization of the dye. The dye conversion calculated as the change in absorbances, TOC and COD achieved at the photoelectrochemical process under VIS irradiation was higher

than

in

the

electrolytic

process

but

lower

than

in

the

photoelectrochemical process in the presence of UV (254 nm) irradiation. Application of VIS (420 nm) irradiation caused 3 times and twice higher conversion of AO7 calculated as a change in TOC and COD, respectively.

3.5 An effect of electrode oxide layer on AO7 degradation in the photoelectrochemical process An effect of oxide layer composition on AO7 degradation in the photoelectrochemical process carried out at 254 and 420 nm was compared for the tested electrodes by determination of the difference between the dye conversion achieved in the photoelectrochemical and electrolytic process (C parameter) calculated according to the following equation: C = FE - E

where FE and E is AO7 conversion in the photoelectrolytic and electrolytic process, respectively, calculated as a change in the absorbance, TOC and COD. Changes in C values should enable estimate an effect of oxide layer composition, especially WO3, on the effectiveness of AO7 degradation. The higher value of C the more advantageous is the oxide layer composition of the tested electrode and its application in the photoelectrochemical process especially under VIS irradiation. Values of C parameter achieved in the photoelectrochemical degradation of AO7 under UV and VIS irradiation are presented in Table 6 and Fig. 11. In the first group of tested electrodes, modification of the oxide layer with WO3 caused higher C value taking into consideration a decrease in absorbances at various wavelengths. The best results were achieved at TiO2(64%)-RuO2(30%)WO3(6%)/Ti and TiO2(67%)-RuO2(30%)-WO3(3%)/Ti electrode while the irradiation at 254 and 420 nm was applied, respectively. In the second group of tested electrodes, the best results were observed at TiO2(94%)-RuO2(3%)WO3(3%)/Ti electrode independently on the wavelength of the irradiation. The highest values of C parameter calculated by TOC change were achieved at TiO2(67%)-RuO2(30%)-WO3(3%)/Ti and TiO2(91%)-RuO2(3%)WO3(6%)/Ti electrodes in the photoelectrochemical processes with the UV and VIS irradiation, respectively. However, the higher content of TiO2 and WO3 (TiO2(91%)-RuO2(3%)-WO3(6%)/Ti)

is

more

advantageous

in

the

mineralization of the dye. Generally, C values decrease while VIS irradiation is applied, in comparison with the process under UV irradiation. Quite opposite situation is observed in the case of C calculated on the basis of COD values. This is due to the fact that the dye conversion calculated as a change in COD in the photoelectrochemical process with VIS irradiation was sometimes higher than in the process with UV irradiation. It can be explained by formation of intermediates which are relatively resistant to oxidation in

determination of COD. Probably, higher amount of these intermediates is formed while UV irradiation is applied. If the photoelectrochemical process is coupled with VIS irradiation then degradation of the dye is lower and lower amount of these intermediates is formed resulting in apparently higher decrease in COD (higher AO7 conversion). Such situation was observed in the case of all tested electrodes except for TiO2(91%)-RuO2(3%)-WO3(6%)/Ti electrode at which AO7 conversion calculated as a change in COD under UV and VIS irradiation was relatively high and the same and thus C parameter achieved also the same value. Generally, the higher content of TiO2 and WO3 in the oxide layer at the electrodes the higher value of C is observed and the lower difference between C for the process with UV and VIS irradiation was found. Taking into consideration all results achieved in the degradation processes (electrolytic and photoelectrolytic at 254 and 420 nm) it can be concluded that 6% content of WO3 is more advantageous for the tested electrodes. Application of the electrodes covered with TiO2 in 91% and containing 6% of WO3 in the oxide layer results in the highest degradation of the dye. The higher conversion of the dye at the electrode with 6% content of WO 3 can be attributed to higher acidity of the electrode surface and higher surface adsorption affinity. Formation of WO3 monolayer at TiO2 can significantly increase the surface acidity due to the fact that WO3 is 15 times more acidic than TiO2 and has a higher adsorption affinity to reactant molecules (organic compounds) as well as to hydroxyl groups [33, 34]. It is attributed to an excess positive charge in the system resulting in generation of additional acidity [52]. It was found that an increase in WO3 content in the bi-component photocatalyst up to 3% caused significant increase in surface acidity and simultaneous increase in photocatalytic activity [35, 53]. Further increase in WO3 content resulted in almost constant surface acidity (a plateau level) but the photocatalytic activity decreased due to loading of WO3 beyond the monolayer coverage. The monolayer coverage of WO3 on the surface of TiO2 was reported enhanced the

photocatalytic activity by 3-4 times in decomposition of gaseous 2-propanol, benzene, acetaldehyde and propionaldehyde [54]. This enhancement is related to much higher adsorption of organic compounds on the bi-component photocatalyst. However too much loading of WO3 in the bi-component photocatalyst can result in quite adverse effect [36]. In case of the tested electrodes, the WO 3 content of 6% seems to be optimum independently on TiO2 content in the oxide layer. Probably, the increase in WO3 content from 3 to 6% in the oxide layer causes an increase in the electrode surface acidity and affinity to adsorption of the dye and hydroxyl groups. This means that loading of WO3 is not too high and only a monolayer of this oxide is formed. However, the explanation if this WO3 content is optimum in the case of TiO2-RuO2-WO3/Ti electrodes needs further investigations including the characterization of the tested electrodes surface, determination of electroactive surface and determination of an effect of the WO3 content in the oxide layer on the band gap energy.

4. Conclusions

Titanium electrodes covered with TiO2 and RuO2 as well as modified by WO3 can be applied in degradation of the monoazo dye (AO7) carried out in the electrolytic and photoelectrochemical process. Cyclic voltammograms recorded at the tested electrodes in the dye solution and supporting electrolyte proved that the lowest oxidation currents were observed at electrodes without RuO2 in the oxide layer. Further experiments showed that these electrodes were characterized by the lowest stability. If the electrodes contained higher amount of TiO2 (above 90%) in the oxide layer, the increase in WO3 content from 3 to 6% resulted in facilitation of AO7 oxidation. Simultaneously, the highest oxidation power was observed in the case of these electrodes in the solution of AO7.

Taking into consideration of AO7 degradation in the electrolytic process, the presence of WO3 in the oxide layer of the tested electrodes did not affected much changes in absorbances at various wavelengths. However changes in TOC and COD values were quite clear and proved that 6% content of WO3 was the most advantageous in the case of the tested electrodes. The highest conversion of AO7 was achieved at the electrode with the higher content of TiO2 (91%) and 6% content of WO3. Enhancement of the electrolytic process efficiency can be achieved by combination of this process with the photocatalytic degradation under UV and VIS irradiation. If photons with lower energy (VIS irradiation) were applied then the efficiency of AO7 degradation was lower the under UV irradiation but this still improved AO7 degradation in comparison with the electrolytic process. Under UV and VIS irradiation the best results were achieved while TiO2(91%)RuO2(3%)-WO3(6%)/Ti electrodes were applied. Application of UV and VIS irradiation was the most reasonable in the case of TiO2(94%)-RuO2(3%)-WO3(3%)/Ti

electrode

taking

into

consideration

decolouration. However, the highest mineralization of AO7 solution was achieved

at

TiO2(91%)-RuO2(3%)-WO3(6%)/Ti

electrode.

Considering

mineralization of the dye solution, the increase in WO3 content to 6% seems to be advantageous and probably results in higher acidity of the electrode surface as well as in higher affinity to adsorption of the dye and hydroxyl groups. The TiO2-RuO2/Ti modified with WO3 seems to be applicable under inexpensive and renewable sunlight or even room light lamps emitting visible photons instead of harmful UV irradiation. However, their application in degradation of organic compounds require further investigations leading to explanation of WO3 effect on the band-gap of three-component oxide layer as well as on stability of the electrodes, their electroactivity and photoactivity.

References 1. C. Comninellis, A. Kapalka, S. Malato, S.A. Parsons, I. Poulios, D. Mantzavinos, Perspective. Advaced oxidation processes for water treatment: advances and trends for R&D, J. Chem. Technol. Biotechnol. 83 (2008) 769776. 2. A.S. Stanikis, Use of selected advanced oxidation processes (AOPs) for wastewater treatment - A mini review, Global NEST Journal 10 (2008) 376385. 3. M. Swaminathan, M. Muruganandham, M. Sillanpaa, Advanced oxidation processes for wastewater treatment, Int. J. Photoenergy 2013 (2013), Article DOI.ORG/10.1155/2013/ 683682. 4. M.N. Rashed, A.A. El-Amin, Photocatalytic degradation of methyl orange in aqueous TiO2 under different solar irradiation sources, Int. J. Phys. Sci. 2 (2007) 73-81. 5. L. Andronic, A. Duta, The influence of TiO2 powder and film on the photodegradation of methyl orange, Mater. Chem. Phys. 112 (2008) 1078– 1082. 6. V. Augugliaro, M. Bellardita, V. Loddo, G. Palmisano, L. Palmisano, S. Yurdakal, Overview on oxidation mechanisms of organic compounds by TiO 2 in heterogeneous photocatalysis, J. Photochem. Photobiol. C 13 (2013) 224– 245. 7. D. Mukherjee, S. Barghi, A.K. Ray, Preparation and Characterization of the TiO2 Immobilized Polymeric Photocatalyst for Degradation of Aspirin under UV and Solar Light, Processes 2 (2014) 12-23. 8. K. Esquivel, L.G. Arriaga, F.J. Rodríguez, L. Martínez, L.A. Godínez, Development of a TiO2 modified optical fiber electrode and its incorporation into a photoelectrochemical reactor for wastewater treatment, Water Res. 43 (2009) 3593-3603.

9. Y. Wang, B. Gu, W. Xu, Electro-catalytic degradation of phenol on several metal-oxide anodes, J. Hazard. Mater. 162 (2009) 1159-1164. 10. C.A. Martínez-Huitle,

E. Brillas, Decontamination of wastewaters

containing synthetic organic dyes by electrochemical methods: A general review, Appl. Catal. B 87 (2009) 105-145. 11. M. Skoumal, R.M. Rodriguez, P.L. Cabot, F. Centellas, J.A. Garrido, C. Arias, E. Brillas, Electro-Fenton, UVA photoelectro-Fenton and solar photoelectro-Fenton degradation of the drug ibuprofen in acid aqueous medium using platinum and boron-doped diamond anodes, Electrochim. Acta 54 (2009) 2077-2085. 12. M.A. Oturan, E. Brillas, Electrochemical advanced oxidation processes (EAOPs) for environmental applications, Portugaliae Electrochim. Acta 25 (2007) 1-18. 13. B.P. Chaplin, Critical review of electrochemical advanced oxidation processes for water treatment applications, Environ. Sci.: Processes Impacts 16 (2014) 1182-1203. 14. G. Chen, Electrochemical technologies in wastewater treatment, Sep. Purif. Technol. 38 (2004) 11-41. 15. E. Kusmierek, E. Chrzescijanska, Photoelectrochemical degradation of the reactive dye - Reactive Blue 81, Pol. J. Chem. 83 (2009) 1337-1351. 16. E. Kusmierek, E. Chrzescijanska, M. Szadkowska-Nicze, J. KaluznaCzaplinska, Electrochemical discolouration and degradation of reactive dichlorotriazine dyes: Reaction pathways, J. Appl. Electrochem. 41 (2011) 51-62. 17. L. Szpyrkowicz, C. Juzzolino, S.N. Kaul, S. Daniele, M.D. De Faveri, Electrochemical Oxidation of Dyeing Baths Bearing Disperse Dyes, Ind. Eng. Chem. Res. 39 (2000) 3241-3248.

18. L. Szpyrkowicz, R. Cherbanski, G.H. Kelsall, Hydrodynamic Effects on the Performance of an Electrochemical Reactor for Destruction of Disperse Dyes, Ind. Eng. Chem. Res. 44 (2005) 2058-2068. 19. D. Rajkumar, J.G. Kim, Oxidation of various reactive dyes with in situ electro-generated active chlorine for textile dyeing industry wastewater treatment, J. Hazard. Mater. 136 (2006) 203-212. 20. D. Rajkumar, B.J. Song, J.G. Kim, Electrochemical degradation of Reactive Blue 19 in chloride medium for the treatment of textile dyeing wastewater with identification of intermediate compounds, Dyes Pigments 72 (2007) 17. 21. J. Georgieva, E. Valova, S. Armyanov, N. Philippidis, I. Poulios, S. Sotiropoulos, Bi-component semiconductor oxide photoanodes for the photoelectrocatalytic oxidation of organic solutes and vapours: A short review with emphasis to TiO2–WO3 photoanodes, J. Hazard. Mater. 211-212 (2012) 30-46. 22. A. Fujishima, X. Zhang, D.A. Tryk, TiO2 photocatalysis and related surface phenomena, Surf. Sci. Rep. 63 (2008) 515-582. 23. S. Somasundaram, N. Tacconi, C.R. Chenthamarakshan, K. Rajeshwar, N.R. de Tacconi, Photoelectrochemical behavior of composite metal oxide semiconductor films with a WO3 matrix and occluded Degussa P 25 TiO2 particles, J. Electroanal. Chem. 577 (2005) 167-177. 24. M. Ju, G. Sun, J. Wang, Q. Meng, W. Liang, Origin of high photocatalytic properties in the mixed-phaseTiO2: A First-principles Theoretical Study, ACS Appl. Mater. Interfaces (2014) DOI: 10.1021/am502830m. 25. S.J. Hong, H. Jun, P.H. Borse, J. S. Lee, Size effects of WO3 nanocrystals for

photooxidation

of

water

in

particulate

suspension

and

photoelectrochemical film systems, Inter. J. Hydrogen Energy 34 (2009) 3234-3242.

26. X. Luo, F. Liu, X. Li, H. Gao, G. Liu, WO3/TiO2 nanocomposites: Salt– ultrasonic assisted hydrothermal synthesis and enhanced photocatalytic activity, Mater. Sci. Semicond. Process. 16 (2013) 1613-1618. 27. S. Yamazaki, T. Yamate, K. Adachi, Photocatalytic activity of aqueous WO3 sol for the degradation of Orange II and 4-chlorophenol, Appl. Catal. A 454 (2013) 30-36. 28. M. Grätzel, Photoelectrochemical cells, Nature 414 (2001) 338-344. 29. N. Serpone, Brief introductory remarks on heterogeneous photocatalysis, Sol. Energy Mater. Sol. Cells 38 (1995) 369-379. 30. D. Su, J. Wang, Y. Tang, C. Liu, L. Liua, X. Hana, Constructing WO3/TiO2 composite structure towards sufficient use of solar energy, Chem. Commun. 47 (2011) 4231-4233. 31. X.Z. Li, F.B. Li, C.L. Yang, W.K. Ge, Photocatalytic activity of WOx-TiO2 under visible light irradiation, J. Photochem. Photobiol. A 141 (2001) 209217. 32. V. Puddu, R. Mokaya, G. L. Puma, Novel one step hydrothermal synthesis of TiO2/WO3 nanocomposites with enhanced photocatalytic activity, Chem. Commun. (2007) 4749-4751. 33. K.K. Akurati, A. Vital, J-P. Dellemann, K. Michalowa, T. Graule, D. Ferri, A. Baiker, Flame-made WO3/TiO2 nanoparticles: Relation between surface acidity, structure and photocatalytic activity, Appl. Catal. B 79 (2008) 53– 62. 34. A.K.L. Sajjad, S. Shamaila, B. Tian, F. Chen, J. Zhang, One step activation of WOx/TiO2 nanocomposites with enhanced photocatalytic activity, Appl. Catal. B 91 (2009) 397–405. 35. Y.T. Kwon, K.Y. Song, W.I. Lee, G. J. Choi, Y.R. Do, Photocatalytic behavior of WO3-loaded TiO2 in an oxidation reaction, J. Catal. 192 (2000) 192–199.

36. H.

Zhang,

G.

Chen,

D.W.

Bahnemann,

Environmental

photo(electro)catalysis: Fundamental principles and applied electrochemistry for the environment, in: C. Comninellis, G. Chen (Eds.), Electrochemistry for the environment, Ch. 16, Springer, 2010, pp. 371-442. 37. M. Hepel, J. Luo, Photoelectrochemical mineralization of textile diazo dye pollutants using nanocrystalline WO3 electrodes, Electrochim. Acta 47 (2001) 729–740. 38. J. Georgieva, S. Armyanov, E. Valova, Ts. Tsacheva, I. Poulios, S. Sotiropoulos, Photoelectrochemical behaviour of electrodeposited tungsten trioxide and electrosynthesised titanium dioxide single component and bilayer coatings on stainless steel substrates, J. Electroanal. Chem. 585 (2005) 35–43. 39. S.V. Evdokimov, Electrochemical and corrosion behaviour of electrode materials based on compositions of ruthenium dioxide nd base-metal oxides, Russ. J. Electrochem. 38 (2002) 583-588. 40. B.V. Tilak, C.P. Chen, V.I. Birss, J. Wang, Capacitive and kinetic characteristics of Ru-Ti oxide electrodes : influence of variation in the Ru content, Can. J. Chem. 75 (1997) 1773-1781. 41. J.L.S. Duarte, W.M.G. Soares, L.M. Gomes, J. Tonholo, C.L.P.S. Zanta, Electrochemical oxidation of safrole using Ti/RuxTi(1-x)O2 Preparation,

characerization,

and

role

of

electrode

system:

composition,

Electrocatalysis 4 (2013) 320-328. 42. M. Tomar, Quality Assessment of Water and Wastewater, CRC Press LLC, Lewis Publisher, 1999. 43. M.L. Leo, C.R.C. Nollet, Handbook of Water Analysis, Taylor & Francis Group, London, New York, 2007. 44. A. Kapalka, G. Foti, C. Comninellis, Basic principles of the electrochemical mineralization of organic pollutants for wastewater treatment, in: C.

Comninellis, G. Chen (Eds.), Electrochemistry for the environment, Ch. 1, Springer, 2010, pp. 1-24. 45. W. Feng, D. Nansheng, H. Helin, Degradation mechanism of azo dye C. I. reactive red 2 by iron powder reduction and photooxidation in aqueous solutions, Chemosphere 41 (2000) 1233-1238. 46. M. Stylidi, D.I. Kondarides, X.E. Verykios, Visible light-induced photocatalytic degradation of Acid Orange 7 in aqueous TiO2 suspensions, Appl. Catal. B 47 (2004) 189-201. 47. L. Xu, H. Zhao, S. Shi, G. Zhang, J. Ni, Electrolytic treatment of C.I. Acid Orange 7 in aqueous solution using a three-dimensional electrode reactor, Dyes Pigments 77 (2008) 158-164. 48. R. Pelegrini, P. Peralta-Zamora, A.R. de Andrade, J. Reyes, N. Durán, Electrochemically assisted photocatalytic degradation of reactive dyes, Appl. Catal. B 22 (1999) 83-90. 49. H. Song, H. Jiang, X. Liu, G. Meng, Efficient degradation of organic pollutant with WOx modified nano TiO2 under visible irradiation, J. Photochem. Photobiol. A 181 (2006) 421–428. 50. W. Ho, J.C. Yu, Sonochemical synthesis and visible light photocatalytic behavior of CdSe and CdSe/TiO2 nanoparticles, J. Mol. Catal. A: Chem. 247 (2006) 268–274. 51. S.B. Rawal, D.P. Ojha, Y.S. Choi, W.I. Lee, Coupling of W-Doped SnO2 and TiO2 for Efficient Visible-Light Photocatalysis, Bull. Korean Chem. Soc. 35 (2014) 913-918. 52. J. Papp, S. Soled, K. Dwight, A. Wold, Surface acidify and photocatalytic activity of TiO2, WO3/TiO2, and MoO3/TiO2 Photocatalysts, Chem. Mater. 6 (1994) 496-500. 53. K.Y. Song, M.K. Park, Y.T. Kwon, H.W. Lee, W.J. Chung, W.I. Lee, Preparation of Transparent Particulate MoO3/TiO2 and WO3/TiO2 films and their photocatalytic properties, Chem. Mater. 13 (2001) 2349-2355.

54. A.K. Chakraborty, S.Y. Chai, W.I. Lee, Photocatalytic behavior of WO3/TiO2 in decomposing volatile aldehydes, Bull. Korean Chem. Soc. 29 (2008) 494-496.

Figure captions Fig. 1. Band position (valence band top and conduction band bottom) of TiO2 and WO3 semiconductors in contact with aqueous electrolyte at pH 1, compared with some selected redox potential of processes occurring at semiconductor surface [28, 29]. Fig. 2. Structure of AO7 dye. Fig. 3. Cyclic voltammograms recorded at TiO2(70%)-RuO2(30%)/Ti in AO7 (curve 1) and Na2SO4 solution (curve 2), and at Pt electrode (curve 3) in AO7 solution. Fig. 4. Cyclic voltammograms recorded at TiO2(70%)-RuO2(30%)/Ti (curve 1), TiO2(67%)-RuO2(30%)-WO3(3%)/Ti

(curve

2),

TiO2(64%)-RuO2(30%)-

WO3(6%)/Ti (curve 3) electrodes in AO7. Fig. 5. Cyclic voltammograms recorded at TiO2(97%)-RuO2(3%)/Ti (curve 1), TiO2(94%)-RuO2(3%)-WO3(3%)/Ti (curve 2) and TiO2(91%)-RuO2(3%)WO3(6%)/Ti (curve 3) electrodes in AO7. Fig. 6. Cyclic voltammograms recorded at TiO2(94%)-WO3(6%)/Ti (curve 1), TiO2(97%)-WO3(3%)/Ti (curve 2) and TiO2(98.5%)-WO3(1.5%)/Ti (curve 3) electrodes in AO7 solution. Fig. 7. UV-VIS spectra recorded in AO7 initial solution (curve 1), after photochemical (curve 2), electrochemical (curve 3) and photoeletrochemical (curve 4) degradation at TiO2(94%)-RuO2(3%)-WO3(3%)/Ti electrode.

Fig. 8. An effect of electrode material on AO7 conversion calculated as a change in COD and TOC in the electrolytic process (COD and TOC in AO7 initial solution - 257 mg dm-3 O2 and 88 mg dm-3 C, respectively). Fig. 9. An effect of electrode material on AO7 conversion calculated as a change in COD and TOC in the photoelectrochemical process with UV irradiation (COD and TOC in AO7 initial solution - 257 mg dm-3 O2 and 88 mg dm-3 C, respectively). Fig. 10. An effect of electrode material on AO7 conversion calculated as a change in COD and TOC in the photoelectrochemical process with VIS irradiation (COD and TOC in AO7 initial solution - 257 mg dm-3 O2 and 88 mg dm-3 C, respectively). Fig. 11. Effect of the oxide layer composition on C parameter in the photoelectrochemical process of AO7 degradation under UV and VIS irradiation, calculated as a change in TOC and COD.

Table 1. Values of peak potential (Ep) and current (Ip) as wells values of oxidation current (I) determined at 0.8 V for AO7 oxidation at the tested electrodes. Electrode

Ep (V)

Ip (A)

I (A) at E=0.8V

0.803 2.67  10-4

3.33  10-4

TiO2(67%)-RuO2(30%)-WO3(3%)/Ti 0.820 1.97  10-4

2.16  10-4

TiO2(64%)-RuO2(30%)-WO3(6%)/Ti 0.832 1.61  10-4

2.01  10-4

TiO2(97%)-RuO2(3%)/Ti

0.911 1.55  10-4

1.53  10-4

TiO2(94%)-RuO2(3%)-WO3(3%)/Ti

0.879 2.23  10-4

1.87  10-4

TiO2(91%)-RuO2(3%)-WO3(6%)/Ti

0.837 1.88  10-4

2.09  10-4

TiO2(98.5)-WO3(1.5%)/Ti

n.d.

1.55  10-6

TiO2(97%)-WO3(3%)/Ti

n.d.

1.07  10-5

TiO2(94%)-WO3(6%)/Ti

n.d.

1.64  10-5

0.940 2.94  10-5

1.03  10-5

TiO2(70%)-RuO2(30%)/Ti

Pt

n.d. - not detected.

Table 2. Onset potential of oxygen evolution (EO2) at the tested electrodes determined in the solution of Na2SO4 and AO7. Electrode

EO2 (V)

EO2

Na2SO4

(V) AO7

TiO2(70%)-RuO2(30%)/Ti

0.967

0.952

TiO2(67%)-RuO2(30%)-WO3(3%)/Ti

0.950

1.006

TiO2(64%)-RuO2(30%)-WO3(6%)/Ti

0.948

0.983

TiO2(97%)-RuO2(3%)/Ti

0.998

1.015

TiO2(94%)-RuO2(3%)-WO3(3%)/Ti

0.945

1.040

TiO2(91%)-RuO2(3%)-WO3(6%)/Ti

0.941

0.974

TiO2(98.5)-WO3(1.5%)/Ti

0.985

1.055

TiO2(97%)-WO3(3%)/Ti

0.963

1.091

TiO2(94%)-WO3(6%)/Ti

0.886

1.042

Table 3. AO7 conversion calculated as a change in absorbances determined at various wavelengths achieved at the tested electrodes in the electrolytic process. Electrode TiO2(70%)-RuO2(30%)/Ti

Dye conversion (%) 228 nm 310 nm 483 nm 43.84 60.04 86.55

TiO2(67%)-RuO2(30%)-WO3(3%)/Ti

43.01

65.39

85.39

TiO2(64%)-RuO2(30%)-WO3(6%)/Ti

44.20

65.14

85.83

TiO2(97%)-RuO2(3%)/Ti

45.86

61.23

89.25

TiO2(94%)-RuO2(3%)-WO3(3%)/Ti

43.99

64.20

85.17

TiO2(91%)-RuO2(3%)-WO3(6%)/Ti

47.29

68.09

88.65

Table 4. AO7 conversion calculated as a change in absorbances determined at various wavelengths achieved in photolysis and at the tested electrodes in the photocatalytic and photoelectrochemical process under irradiation at 254 nm. Electrode

Dye conversion (%) Photocatalysis

Photoelectrochemistry

228nm 310nm 483nm 228nm 310nm 48 nm Photolysis

0.51

1.78

6.23

-

-

-

TiO2(70%)-RuO2(30%)/Ti

2.36

4.15

7.65

62.70

69.50

97.18

TiO2(67%)-RuO2(30%)-

2.68

5.88

8.29

64.31

73.45

96.86

2.08

5.38

8.57

65.64

73.74

97.89

TiO2(97%)-RuO2(3%)/Ti

2.34

5.55

8.32

68.63

74.86

98.63

TiO2(94%)-RuO2(3%)-

2.73

5.60

8.02

65.12

73.13

97.42

1.00

2.72

8.03

68.00

76.10

98.03

WO3(3%)/Ti TiO2(64%)-RuO2(30%)WO3(6%)/Ti

WO3(3%)/Ti TiO2(91%)-RuO2(3%)WO3(6%)/Ti

Table 5. AO7 conversion calculated as a change in absorbances determined at various

wavelengths

achieved

at

the

tested

electrodes

in

photoelectrocatalytic process under irradiation at 420 nm. Electrode

Dye conversion (%) 228 nm 310 nm 483 nm

TiO2(70%)-RuO2(30%)/Ti

54.25

70.56

89.39

TiO2(67%)-RuO2(30%)-WO3(3%)/Ti

54.62

75.66

96.67

TiO2(64%)-RuO2(30%)-WO3(6%)/Ti

55.13

74.33

95.80

TiO2(97%)-RuO2(3%)/Ti

54.58

76.18

96.40

TiO2(94%)-RuO2(3%)-WO3(3%)/Ti

58.58

77.47

96.93

TiO2(91%)-RuO2(3%)-WO3(6%)/Ti

57.00

78.27

96.30

the

Table 6. Effect of the oxide layer composition on C parameter in the photoelectrocatalytic process of AO7 degradation under UV and VIS irradiation, calculated as a change in absorbances at different wavelenghts. Electrode

C254nm (%)

C420nm (%)

228

310

483

228

310

483

nm

nm

nm

nm

nm

nm

TiO2(70%)-RuO2(30%)/Ti

18.86

9.46

10.63

10.41

10.52

2.84

TiO2(67%)-RuO2(30%)-

21.30

8.06

11.47

11.61

10.27

11.28

21.44

8.60

12.06

10.93

9.16

9.97

TiO2(97%)-RuO2(3%)/Ti

19.26

11.90

8.17

8.72

14.95

7.15

TiO2(94%)-RuO2(3%)-

24.64

10.66

13.46

14.59

13.27

11.76

17.83

5.04

8.77

9.71

10.18

7.65

WO3(3%)/Ti TiO2(64%)-RuO2(30%)WO3(6%)/Ti

WO3(3%)/Ti TiO2(91%)-RuO2(3%)WO3(6%)/Ti

Fig. 1

Fig. 2

Fig. 3

Fig. 4

Fig. 5

Fig. 6

Fig. 7

Fig. 8

Fig. 9

Fig. 10

Fig. 11