(RuO2)x

Research Note pubs.acs.org/IECR

Unexpected Enhancement of Electrocatalytic Nature of Ti/(RuO2)x− (Sb2O5)y Anodes Prepared by the Ionic Liquid-Thermal Decomposition Method Tarciso É. S. Santos,† Ronaldo S. Silva,‡ Cristiano T. Meneses,§ Carlos A. Martínez-Huitle,⊥ Katlin I. B. Eguiluz,† and Giancarlo R. Salazar-Banda*,† †

Instituto de Tecnologia e Pesquisa/Programa de Pós-Graduaçaõ em Engenharia de Processos, Universidade Tiradentes, 49032-490, Aracaju, SE Brazil ‡ Laboratório de Materiais Cerâmicos Avançados, Departamento de Física, Universidade Federal de Sergipe, 49100-000 São Cristóvão, SE Brazil § Departamento de Física, Universidade Federal de Sergipe, Campus Itabaiana, 49500-000 Itabaiana, SE Brazil ⊥ Instituto de Química, Universidade Federal do Rio Grande do Norte, Lagoa Nova, CEP 59072-970, RN Brazil ABSTRACT: Titanium electrodes covered with ruthenium and antimony oxides with different compositions (Ti/(RuO2)0.5−(Sb2O5)0.5 and Ti/ (RuO2)0.8−(Sb2O5)0.2) were synthesized by using the ionic liquid (IL) and Pechini methods. These electrodes were characterized by X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy, and electrochemical techniques, showing that the electrodes prepared by the IL method (green chemistry route) presented grains, a noncracked morphology, rutile structure, and significant electroactive area. The electrocatalytic nature of both electrodes was evaluated by electrochemical degradation of a persistent organic compound (atrazine herbicide) by applying 10 and 20 mA cm−2, and compared, for the first time, with a nonactive electrode with higher oxidation power (boron-doped diamond (BDD)). Results clearly showed that electrodes obtained by the IL method achieved higher degradation efficiencies. Compared with BDD anode, the Ti/(RuO2)0.8− (Sb2O5)0.2 electrode showed a similar performance to remove atrazine (98% in 100 min), evidencing an improvement in its oxidation power as active anode.

1. INTRODUCTION Few studies have reported the elimination of persistent organic pollutants (POPs) from water by adsorption,1 chemical oxidation,2 and bioreactors.3 However, these methodologies require long treatment times (up to days) or generate other residues.4 Consequently, the application of other advanced technologies has been encouraged. In this context, electrochemical advanced oxidation processes (EAOPs) are emerging methods developed as an eco-friendly and efficient alternative to mineralize low contents of POPs in water.4−6 EAOPs are mainly based on the in situ electrogeneration of reactive oxygen species and other oxidant species.6 The simplest and most popular EAOP is electrochemical oxidation (EO) where organic pollutants in solution are oxidized by direct charge transfer at the anode (M), or extensively destroyed by physisorbed hydroxyl radicals (M(•OH)) produced as intermediate from water oxidation (M + H2O → M(•OH) + H+ + e−).6,7 Hydroxyl radicals have a very high standard reduction potential that can react with most organics up to complete combustion and their production depends strongly on the nature of anode material used.6 BDD electrodes are preferred because of their weak BDD−•OH © 2016 American Chemical Society

interaction and greater O2-overpotential that leads to the generation of higher amounts of reactive physisorbed BDD(•OH) radicals that mineralize more effectively POPs in the effluents than other anodes.7−9 However, the material acquisition cost and their higher energy consumption incentivize the use of alternative cheaper materials which besides present lower operational costs, such as Ti/Pt and mixed metal oxide electrodes (i.e., IrO2, RuO2, SnO2, or their mixtures).6 These anodes are popularly known as dimensionally stable anodes (DSAs) and present a lower oxidation power than BDD,10−13 favoring preferentially the electrochemical conversion. DSAs are constituted of a cheap metallic support, normally titanium, where metallic oxides are deposited by thermal decomposition.14−16 The adequate methodology for DSA synthesis is of fundamental importance, because the physical characteristics (mechanical resistance, dimensions, thickness, etc.) and the electrocatalytic properties of these Received: Revised: Accepted: Published: 3182

December 8, 2015 February 11, 2016 March 14, 2016 March 14, 2016 DOI: 10.1021/acs.iecr.5b04690 Ind. Eng. Chem. Res. 2016, 55, 3182−3187

Research Note

Industrial & Engineering Chemistry Research

Figure 1. SEM images with EDX analyses of the Ti/(RuO2)0.5−(Sb2O5)0.5 (a,b) and Ti/(RuO2)0.8−(Sb2O5)0.2 (c,d) electrodes with a 2000× enhancement synthesized by ionic liquid at 600 °C (a,c) and Pechini at 400 °C (b,d) methods. Insets: EDX analysis for each one of the DSA electrodes prepared.

2. EXPERIMENTAL SECTION For precursor solution preparation, chemical treatment of the Ti substrates and herbicide degradation the following chemicals were used: ethylene glycol (98%), oxalic acid (99%), H2SO4 (96%), acetonitrile (98%), and isopropyl acid (99.5%) from Vetec; HCl (98%), ruthenium chloride (III) (99.5%), antimony chloride (III) (99.5%), and atrazine (99.5%) from Aldrich. Ultrapure water was used (Milli-Q systemMillipore). For electrode synthesis, the Ti support received a pretreatment, as reported elsewhere.17−22 The electrode preparation by the IL method recovers the Ti substrate with a precursor solution prepared by ruthenium chloride (RuCl3) and antimony chloride (SbCl3) dissolutions in molar proportion of 50:50 in 1 mL of methylimidazolium hydrogensulfate IL (0.1 mol L−1). This precursor solution was sonicated and then heated to 90 °C. After recovering titanium support by brushing, the electrodes were transferred to an oven and thermally treated at 600 °C for 5 min with a heating ramp of 5 °C min−1, this procedure was repeated around five times to achieve around 1.20 mg cm−2. Finally, they were calcined at 600 °C for 1 h under atmospheric air. IL was previously synthesized by mixing concentrated H2SO4 (Merck, 98%) and 1-methylimidazole (Aldrich, ≥99%) in a stoichiometric ratio in a thermostated bath and characterized by Fourier transform infrared spectros-

materials (active or nonactive anode nature) are directly related to the synthesis method. The most important procedure used to synthesize DSAs is the polymeric precursor method (socalled, Pechini method).17−21 Nevertheless, a promising synthesis approach has been recently developed: the IL method.22 It tends to reduce the amount of chemical precursors used, preparation-time and cost, but the loading of catalyst oxide is maintained. Moreover, Carlesi and co-workers22 demonstrated that a series of antimony-doped tin oxide electrodes showed comparatively higher stability and longer service lifetime than the anodes prepared by alcoholic and sol−gel routes. Thus, this may alter the economic analysis of the whole pilot- or full-scale treatment of wastewater effluents compensating the probable high cost of the IL compared with the other more traditional solvent used in synthesis. However, the oxidation power6 of the novel DSA electrodes, active or nonactive, prepared by the IL approach has been not compared with other anodes. Thus, we synthesized and characterized active electrodes based on Ru−Sb oxides, Ti/ (RuO2)0.5−(Sb2O5)0.5, and Ti/(RuO2)0.8−(Sb2O5)0.2, by IL and Pechini methods. After that, for the first time, the oxidation efficiency of this kind of electrodes was compared with the BDD anode for the EO of atrazine pesticide. 3183

DOI: 10.1021/acs.iecr.5b04690 Ind. Eng. Chem. Res. 2016, 55, 3182−3187

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Industrial & Engineering Chemistry Research copy and nuclear magnetic resonance.22 The Pechini method already reported in the literature21,22 was employed to prepared Ti/(RuO2)0.5−(Sb2O5)0.5 and Ti/(RuO2)0.8−(Sb2O5)0.2 electrodes, with a specific mass of 1.2 mg cm−2 after calcination at 400 °C for 1 h.23 In the case of the IL method, a final calcination temperature (600 °C) was attained, avoiding the use of intermediary temperatures used in the Pechini method;22,23 thus reducing the synthesis time. Ti/(RuO2)0.5−(Sb2O5)0.5 and Ti/(RuO2)0.8−(Sb2O5)0.2 were calcinated in an electric oven EDG 3P-S 3000, with temperature, time, and heat ramp control, and with gas flow facilities. A potenciostat/galvanostat AUTOLAB 302N with a glass electrochemical cell containing three electrodes (a hydrogen electrode prepared in H2SO4 solution 0.5 mol L−1, immersed in a Luggin capillary, a counter-electrode of platinum of 1 cm2, and a DSA as working electrode) was used for electrochemical experiments. Cyclic voltammetry was used to characterize the electrodes synthesized by both methods, considering the fourth cycle aiming to obtain a stable profile with a scan rate of 50 mV s−1 in a 0.1 mol L−1 Na2SO4 solution, from 0.4 to 1.4 V. Galvanostatic electrolysis measurements were made during 120 min in 40 mL of 0.1 mol L−1 Na2SO4 containing 10 mg L−1 of atrazine herbicide at 25 °C, from which samples were taken. Atrazine degradation was followed by spectrophotometric analysis and chemical oxygen demand (COD) measurements. Total organic carbon (TOC) measurements were carried out by injecting 50 μL of samples into a Shimadzu TOC-L analyzer (precision ±1%).

3. RESULTS AND DISCUSSION Figure 1 shows scanning electronic microscopy (SEM) images and energy-dispersive X-ray spectroscopy (EDX) analysis for Ti/(RuO2)0.8−(Sb2O5)0.2 and Ti/(RuO2)0.5−(Sb2O5)0.5 electrodes prepared by IL (Figure 1a,c) and Pechini methods (Figure 1b,d). Images clearly showed that electrodes morphology is different, depending on the preparation method used. Electrodes synthesized by the IL method presented the formation of grains with a homogeneous coverage and porous surfaces. For the Pechini method, an apparently compact morphology was achieved, with some microcracks and several grains (so-called “mud-cracked” surface), which is characteristic of higher density metals.21,24 However, the amounts of Ru precursor as well as the preparation time were minor when DSAs were prepared by the IL method. Additionally, EDX analyses (insets in Figure 1) prove the existence of Ru and Sb on the electrodes surfaces. Meanwhile, a comparison of the peaks of the patterns displayed in the XRD analysis with the relevant International Centre for Diffraction Data (ICDD) database; RuO2 of rutile structure (ICDD card no. 40-1290) and Sb2O5 (ICDD card no. 71-0587) were formed by both methods (Figure 2a), but a significant crystallinity was observed for DSAs prepared by using IL. It is important to remark that RuO2 is inserted in the composition due to its catalytic properties and Sb2O5 has doping and stabilizing functions.24 The presence of metallic Ti (ICDD card no. 44-12880) due to the supporting substrate was also detected. To understand the electrocatalytic properties of each one of the anode materials, cyclic voltammetry was used. Figures 2 panels b and c show cyclic voltammograms for Ti/(RuO2)0.5− (Sb2O5)0.5 and Ti/(RuO2)0.8−(Sb2O5)0.2 electrodes, respectively, a similar electrochemical behavior was observed for the electrodes prepared by both methods. These anodes presented

Figure 2. (a) XRD patterns obtained for the DSA electrodes synthesized by IL and Pechini methods. Cyclic voltammograms obtained for (b) Ti/(RuO2)0.5−(Sb2O5)0.5 and (c) Ti/(RuO2)0.8− (Sb2O5)0.2 electrodes at scan rate of 50 mV s−1 in a 0.1 mol L−1 Na2SO4 solution.

the typical behavior of a ternary thermally prepared oxide layer. In fact, in the region 0.7−1.2 V, it is possible to observe a peak typical of the Ru(III)/Ru(IV) transition. However, it is wide and not well-defined because the ternary electrodes have heterogeneous surface sites, and the superposition of the redox processes for the transition of lower metal oxide/higher metal oxide, is seen.6,14,24,25 The results from Figure 2 are in agreement with the XRD data, confirming the insertion of RuO2 in its composition. Another interesting feature is that a significant voltammetric area was registered for the electrodes synthesized by the IL method (black curves in Figure 2b,c), indicating that a higher electrical charge is obtained (5.45 × 10−5 and 8.19 × 10−4 coulombs for Ti/(RuO2)0.5−(Sb2O5)0.5 and Ti/(RuO2)0.8−(Sb2O5)0.2 electrodes, respectively).24 Also, it can be inferred that an increase on Ru loading promotes an increase on the voltammetric charge (Figure 2c), increasing the electroactive area and consequently, the electrical charge.21,25 The last characteristic is directly related to the synthesis method.24 In fact, porous surfaces were obtained when the IL approach was used (Figure 1a,c), favoring an increase on the surface area.22 At the same time, it can promote an enhancement on electrocatalytic activity of the DSAs during their use on energy conversion, oxidants production, or environmental applications (e.g., EO of organic com3184

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Industrial & Engineering Chemistry Research pounds).26−32 This fact was verified via the EO of atrazine following its concentration decay. According to the UV−vis spectra during EO, the maximum absorption band in the range of visible light (λMAX 225 nm) changed as a function of time (data not shown) when 10 and 20 mA cm−2 of current density were applied using DSA anodes (synthesized by IL and Pechini methods). The intensity of the visible band decreases rapidly until about 120 min of electrolysis, indicating that during the first treatment stages there are mechanisms involving atrazine oxidation to other organics33,34 (e.g., formation of aliphatic carboxylic acids and carbon dioxide).35−38 Another feature that it is important to note is the difference of the concentration decay obtained (Figure 3); these

decrease.24,29,39 In fact, this outcome is in agreement with the results obtained in Figure 3a. The mineralization of 40 mL of solution containing 10 mg dm−3 of atrazine using the DSA synthesized by the Pechini method does not exceed 70% after 60 min of electrolysis by applying 20 mA cm−2 for Ti/(RuO2)0.5−(Sb2O5)0.5, while 90% of atrazine was degraded with Ti/(RuO2)0.8−(Sb2O5)0.2 under similar conditions (Figure 3a). This effect is also observed when different current densities were applied (Figure 3b). Conversely, when DSAs synthesized by the IL approach were used, it was observed that the efficiency of the treatment was significantly higher regarding the atrazine concentration removal (Figure 3a). This behavior occurs because on these anodes the electrogenerated •OH are weakly adsorbed and consequently more reactive than those produced on the surface of DSA electrodes prepared by the Pechini method. No significant alterations on the atrazine decay were observed at different applied current densities (Figure 3b). Comparing DSA synthesized by the IL method with a BDD electrode (nonactive anode)38,39 during EO of atrazine, prominent performances were obtained using Ti/(RuO2)0.8− (Sb2O5)0.2, removing 100% of atrazine concentration by applying 20 mA cm−2 (Figure 3b). This can be explained because with Ti/(RuO2)0.8−(Sb2O5)0.2, the •OH radicals formed by water oxidation have a weaker interaction with the electrode surface, remaining free to react with the organic pollutants in the reaction cage,6 promoting a complete mineralization.34−38 Nevertheless, the higher electroactive area of the Ti/(RuO2)0.8−(Sb2O5)0.2 anode plays an important role on parallel adsorption processes during electrolysis24 influencing the kinetics of heterogeneous M(•OH) radical with organics due to the coexistence of adsorption steps.30,38 It indicates that, if any byproducts are formed or some of them are adsorbed on the electrode surface, the efficient production of hydroxyl radicals on an anode surface promotes a rapid elimination of these intermediates from solution or electrode surface, respectively. In fact, almost total mineralization (in terms of TOC) and COD removal were achieved for Ti/ (RuO2)0.8−(Sb2O5)0.2 and BDD anodes. TOC values of 90% and 95% were obtained, while approximately 93% and 97% of COD for Ti/(RuO2)0.8−(Sb2O5)0.2 and BDD were attained, respectively, at 20 mA cm−2. It promotes an unexpected enhancement of electrocatalytic activity of Ti/(RuO2)0.8− (Sb2O5)0.2 anode due to the heterogeneous surface (porous observed at SEM analysis) as well as the possible uniform distribution of active sites of Ru that favor the production of strong oxidants.24 In the case of DSA electrodes prepared by the Pechini method, the cracked mud coating gradually undergoes erosion and their adsorption capacity is favored by the irregular electrode surface.24,38 But it involves undesired strong adsorption of hydroxyl radicals as well. Strong interaction between the electrode surface and hydroxyl radicals decreases the electrochemical activity of the electrode toward the oxidation of organics (e.g., EO of atrazine), resulting in oxygen evolution.13,24 On the contrary, the oxide surface formed by the IL approach assisted the structural alignment and produced more agglomerated coating.22 The nature of the precursor solution can significantly affect the host metal and dopant content in the coating. It was found that viscous precursor solutions aid in the formation of agglomerated uniform coating and accumulation of the dopant on the surface of the coating.

Figure 3. (a) Effect of DSA-synthesis method (IL and Pechini) during EO of atrazine by applying 20 mA cm−2. (b) Comparison of elimination performances between DSA electrodes synthesized by the IL approach and BDD anodes by applying different current densities (40 mL of 0.1 mol L−1 Na2SO4 containing 10 mg L−1 of atrazine herbicide at 25 °C).

efficiencies depend basically on the anode used. It was confirmed that the DSA electrodes with higher electrocatalytic activity (IL method) promoted an increase on atrazine removal rate and removal efficiency by applying 20 mA cm−2 (79% and 100% of atrazine concentration elimination for Ti/(RuO2)0.5− (Sb2O5)0.5 and Ti/(RuO2)0.8−(Sb2O5)0.2, respectively, see Figure 3a) with respect to the results obtained when DSAs synthesized by the Pechini approach were used (67% and 90% of atrazine concentration elimination for Ti/(RuO2)0.5− (Sb2O5)0.5 and Ti/(RuO2)0.8−(Sb2O5)0.2, respectively). This behavior of DSA anodes is related to the undesired or desired electrochemical reactions,24,38,39 in the former case, the •OH radicals formed by water oxidation can be either electrochemically oxidized to dioxygen (M(•OH) → M + (1/2)O2 + H+ + e−), while in the latter case, •OH radicals contribute to the complete oxidation of the pollutant (atrazine + •OH → intermediates → →→ CO2 + H2O). However, the production of •OH radicals depends on the electrocatalytic activity of the anode.38,39 Ru−Sb oxide electrodes are classified as active anodes thus favoring the oxygen evolution reaction which causes the efficiency in the degradation of organic pollutants to 3185

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(7) Cavalcanti, E. B.; Garcia-Segura, S.; Centellas, F.; Brillas, E. Electrochemical Incineration of Omeprazole in Neutral Aqueous Medium Using a Platinum or Boron-Doped Diamond Anode: Degradation Kinetics and Oxidation Products. Water Res. 2013, 47 (5), 1803. (8) Jeong, J.; Kim, C.; Yoon, J. The Effect of Electrode Material on the Generation of Oxidants and Microbial Inactivation in the Electrochemical Disinfection Processes. Water Res. 2009, 43 (4), 895. (9) Garcia-Segura, S.; Vieira dos Santos, E.; Martínez-Huitle, C. A. Role of sp3/sp2 Ratio on the Electrocatalytic Properties of BoronDoped Diamond Electrodes: A Mini Review. Electrochem. Commun. 2015, 59, 52. (10) Bagastyo, A. Y.; Batstone, D. J.; Rabaey, K.; Radjenovic, J. Electrochemical Oxidation of Electrodialysed Reverse Osmosis Concentrate on Ti/Pt-IrO2, Ti/SnO2-Sb and Boron-Doped Diamond Electrodes. Water Res. 2013, 47 (1), 242. (11) Aquino, J. M.; Rocha-Filho, R. C.; Ruotolo, L. A. M.; Bocchi, N.; Biaggio, S. R. Electrochemical Degradation of a Real Textile Wastewater Using β-PbO2 and DSA® Anodes. Chem. Eng. J. 2014, 251, 138. (12) Rajkumar, D.; Song, B. J.; Kim, J. G. Electrochemical Degradation of Reactive Blue 19 in Chloride Medium for the Treatment of Textile Dyeing Wastewater with Identification of Intermediate Compounds. Dyes Pigm. 2007, 72 (1), 1. (13) de Moura, D. C.; de Araújo, C. K. C.; Zanta, C. L. P. S.; Salazar, R.; Martínez-Huitle, C. A. Active Chlorine Species Electrogenerated on Ti/Ru0.3Ti0.7O2 Surface: Electrochemical Behavior, Concentration Determination and Their Application. J. Electroanal. Chem. 2014, 731, 145. (14) Trasatti, S. Electrocatalysis: Understanding the Success of DSA®. Electrochim. Acta 2000, 45 (15−16), 2377. (15) Comninellis, C. Environmental Oriented Electrochemistry. Studies in Environmental Sciences; Vol. 59, C.A.C. Sequeira; Elsevier: Amsterdam, 1994. (16) Malpass, G. R. P.; Miwa, D. W.; Miwa, A. C. P.; Machado, S. A. S.; Motheo, A. J. Study of Photo-Assisted Electrochemical Degradation of Carbaryl at Dimensionally Stable Anodes (DSA). J. Hazard. Mater. 2009, 167 (1−3), 224. (17) Santana, M. H. P.; Da Silva, L. M.; De Faria, L. A. Investigation of Surface Properties of Ru-Based Oxide Electrodes Containing Ti, Ce and Nb. Electrochim. Acta 2003, 48 (13), 1885. (18) Burke, L. D.; Murphy, O. J.; O’Neill, J. F.; Venkatesan, S. The Oxygen Electrode. Part 8.Oxygen Evolution at Ruthenium Dioxide Anodes. J. Chem. Soc., Faraday Trans. 1 1977, 73, 1659. (19) Bernardi, M. I.; Soledade, L.; Santos, I.; Leite, E.; Longo, E.; Varela, J. Influence of the Concentration of Sb2O3 and the Viscosity of the Precursor Solution on the Electrical and Optical Properties of SnO2 Thin Films Produced by the Pechini Method. Thin Solid Films 2002, 405, 228. (20) Profeti, D.; Lassali, T. A. F.; Olivi, P. Preparation of Ir0.3Sn(0.7‑x)Tix O2 Electrodes by the Polymeric Precursor Method: Characterization and Lifetime Study. J. Appl. Electrochem. 2006, 36, 883. (21) Pechini, M. Method of preparing lead and alkaline earth titanates and niobates and coating method using the same to form a capacitor. U.S. Patent 3,330,697, 1967. (22) Carlesi Jara, C.; Salazar-Banda, G. R.; Arratia, R. S.; Campino, J. S.; Aguilera, M. I. Improving the Stability of Sb Doped Sn Oxides Electrode Thermally Synthesized by Using an Acid Ionic Liquid as Solvent. Chem. Eng. J. 2011, 171, 1253. (23) Santos, M. C.; Cogo, L.; Tanimoto, S. T.; Calegaro, M. L.; Bulhões, L. O. A Nanogravimmetric Investigation of the Charging Processes on Ruthenium Oxide Thin Films and Their Effect on Methanol Oxidation. Appl. Surf. Sci. 2006, 253, 1817. (24) Subba Rao, A. N.; Venkatarangaiah, V. T. Metal Oxide-Coated Anodes in Wastewater Treatment. Environ. Sci. Pollut. Res. 2014, 21, 3197.

4. CONCLUSION In this study, the synthesis and characterization of DSA electrodes by IL and Pechini methods was successfully achieved. Physical characterization showed that both methods were efficient in the homogeneous deposition of RuO2 and Sb2O5 films on the substrate surface. However, the electrodes synthesized by the IL method showed grains without a cracked morphology. Consequently, the method used contributes significantly in controlling the characteristics of a metal oxide coating (resistance, large surface area, and roughness). By using Ti/(RuO2)0.5−(Sb2O5)0.5 and Ti/(RuO2)0.8−(Sb2O5)0.2 electrodes synthesized by the IL method, atrazine degradation was efficiently enhanced with respect to the DSA prepared by the Pechini method. Compared with the BDD anode, Ti/ (RuO2)0.8−(Sb2O5)0.2 showed a similar performance to remove atrazine (98% in 100 min). It provided clear evidence of the improvement of oxidation power of the active anode due to the preparation method used. Finally, the fabrication technique can modulate both the chemical and physical nature of the coating and consequently its electrocatalytic and electrochemical activities. For this reason, further experiments are in progress to determine the effect of the temperature, IL, precursor concentration on the electrocatalytic nature as well as on the concentration of hydroxyl radicals produced, hydrogen peroxide, stability, and power oxidation efficiency during electrochemical degradation with other pollutants in solution.

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AUTHOR INFORMATION

Corresponding Author

*Tel./Fax: + 55 79 32182190. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank CNPq (303630/2012-4, 310282/2013-6, 477797/2012-1, 474261/2013-1, 446846/2014-7, and 401519/ 2014-7), CAPES, and FAPITEC from Brazil for the scholarships and financial support provided for this work.

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REFERENCES

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