Chemical modification of graphite felts for efficient H 2 O 2 production: Influence of operational parameters

Transcription

1 Chemical modification of graphite felts for efficient H 2 O 2 production: Influence of operational parameters LEI ZHOU, MINGHUA ZHOU* College of Environmental Science and Engineering Nankai University Weijin Road 94 CHINA zhoumh@nankai.edu.cn Abstract: - Electro-Fenton process (EF) is a promising method for degradation of refractory pollutants in aquatic environment. The suitable cathode which produces hydrogen peroxide by oxygen reduction still plays a key role in the efficiency improvement of the EF. In this study, hydrazine hydrate-ethanol system was used to modify graphite felts, and the influence of the concentration of hydrazine hydrate on the production and efficiency of hydrogen peroxide was discussed based on the SEM and CV characterizations. The results showed that at a constant potential of -.65 V (vs. SCE) the modified electrodes had much higher catalytic activity than the unmodified one. Operational parameters such as cathodic potential, ph and aeration amount were also investigated. The maximum yields of H 2 O 2 obtained at -.75 V (vs. SCE) in.5m Na 2 SO 4 aqueous solution with oxygen mass flow rate at.4 L/min was mg/l after 9 min. Thus, this chemical modification was a promising approach to promote the H 2 O 2 electro-generation of carbon cathodes. Key-Words: - Graphite felts; Hydrazine hydrate; Chemical modification; H 2 O 2 ; Oxygen reduction reaction; Electro-Fenton 1 Introduction As an environmentally friendly electrochemical technology, Electro-Fenton process (EF) is a promising method for degradation of refractory pollutants in aquatic environment [1,2]. The EF process is based on the continuous in-situ electrogeneration of H 2 O 2, which can eliminate acquisition, shipment and storage, along with the addition of iron catalysts to produce oxidant OH. Therefore, the major concern with the EF system relates to the improvement of H 2 O 2 production. Carbonaceous materials are widely used as cathodes due the advantages such as nontoxic, good stability, conductivity and chemical resistance, high overpotential of hydrogen evolution, and low catalytic activity of H 2 O 2 decomposition [2]. Recently, several carbonaceous electrodes were reported such as graphite [3], reticulated vitreous carbon [4], activate carbon fiber [5,6], carbon sponge [7], carbon/ graphite felt [8-11], carbonaceous PTFE combined electrode [12-14], and metal-modified carbonaceous electrode [15,16]. The carbon/ graphite felt electrode has a high specific surface area favoring the fast generation of H 2 O 2, mechanical integrity and easily acquisition, which make it a promising cathode material. Chemical modification is an efficient way to improve the electrochemical activity of the carbonaceous electrodes by changing their surface functional groups. In the present work, hydrazine hydrate-ethanol system was used to modify graphite felts, and the influence of the concentration of hydrazine hydrate on the production and efficiency of hydrogen peroxide was discussed based on the SEM and CV characterizations. Operational parameters such as cathodic potential, ph and aeration amount were also investigated, and the experimental results were presented. 2 Experimental 2.1 Preparation of cathode materials All chemicals used in this study were analytical grade and used without further purification. The commercial graphite felts (Shanghai Qijie Carbon material Co., LTD) with a specific surface area of -1 about.6 m 2 g were degreased in an ultrasonic bath with acetone and deionized water in sequence, dried at 8 C for 16 h, and then annealed at 15 C for 2 h. These pretreated materials were marked as CF. A series of modified electrodes were prepared as follow. The pretreated graphite felts were immersed in 1 ml mixture of ethanol and hydrazine hydrate, and after refluxing at 6 C for 6h, the samples were annealed at 15 C for 2 h. ISBN:

2 Recent Advances in Energy, Environment and Economic Development Since the volume concentration of the hydrazine hydrate in the mixture were 5%, 1%, 15% and 2%, the modified electrodes were marked as CFHA-5%, CF-HA-1%, CF-HA-15% and CF-HA2%, respectively. 3 Results and discussions 3.1 Morphologies and properties of graphite felts a 2.2 Characterization For a morphology characterization of graphite felt electrodes, a field-emission scanning electron microscopy (FE-SEM, LEO153VP) was used. The contact angle of water on the electrode surface is examined by a contact angle meter (OCA15, Dataphysics). Electrochemical measurements were carried out with CHI66D workstation (CH Instruments, Chenhua, Shanghai, China) in a threeelectrode cell system at ambient temperature. b 2.3 Electro-generation of H2O2 The H2O2 electro-generation experiments were performed in a.13 L undivided three-electrode cell using CHI66D electrochemical workstation as power supply. The prepared cathode (5 cm 2 cm.5 cm) was used as working electrode, a platinum wire as counter electrode and a saturated calomel electrode (SCE) as reference electrode. The distance between the working electrode and counter electrode was 3.5 cm. Prior to the electrolysis, oxygen (96% purity) was bubbled near the cathode through the.5 M Na2SO4 aqueous solutions for 1 min, and then oxygen was reduced at a desired potential (vs. SCE) on the working electrode for 9min, with a constant magnetic stirring of 3 rpm. The concentration of H2O2 during electro-generation process (C) was monitored by UV-vis spectrophotometer (UV759, Shanghai instrument analysis instrument Co., LTD) using the potassium titanium (IV) oxalate method [17]. The current efficiency (CE) for H2O2 production was defined as follow [18]: nfch O V CE = t % Fig. 1 SEM images of (a) CF and (b) CF-1% Fig. 1a and 1b show the SEM images of CF and CF-1%. It was observed that the graphite felts were composed of an entangled network of carbon microfilaments with diameter around 2 µm, and this structure form could contribute to the large gasliquid contact interfaces. After chemical modification, the longitudinal etching trace on the fiber increased, and many carbon particles and clusters with an average diameter of about 5 nm were appeared on the surface. The changes in the microstructures of modified samples could effectively increase the specific surface areas and the number of active sites, which were considered to be conducive to the catalytic process. Idt Where n is the number of electrons transferred for oxygen reduction to H2O2, F is the Faraday constant ( C/mol), CH2O2 is the concentration of H2O2 (mg/l), V is the bulk volume (L), I is the current (A), and t is the time (s). ISBN: Effect of the concentration of hydrazine hydrate 155

3 Concetration of hydrazine hydrate (%) Fig. 2 The effects of the precursors on H 2 O 2 production. Conditions: E= -.65 V vs. SCE,.5M Na 2 SO 4, ph=7, O 2 flow rate at.4l/min. The concentrations of hydrogen dioxide and current efficiencies for cathodes before and after modification were shown in Fig. 2. After 9min, the concentrations of H 2 O 2 for CF, CF-HA-5%, CF- HA-1%, CF-HA-15% and CF-HA-2% were 52.7, 122.4, 144.7, 139. and mg/l, respectively. The enhanced yields of H 2 O 2 after modification indicated the positive effects of precursors on H 2 O 2 electro-generation. In addition, the optimum concentration of the hydrazine hydrate was detected, and among the modified electrodes the CF-HA-1% showed the highest yield of H 2 O 2. When the concentration of the hydrazine hydrate exceeded the optimum, a little decrease in the catalytic activity was observed under the constant potential of -.65 V vs. SCE. The current efficiencies of CF, CF-HA- 5%, CF-HA-1%, CF-HA-15% and CF-HA-2% were 89.1%, 86.8%, 85.1%, 8.3% and 73.7%, respectively. It was seen that the current efficiencies declined with the increasing concentration of hydrazine hydrate used in chemical modification To further investigate the effects of chemical modification on electrocatalytic activity of cathodes toward ORR, cyclic voltammetry was carried out at graphite felts before and after modification as Fig. 3 shown. Standard voltammograms with total irreversibility were obtained for all samples, and the modified cathodes exhibited stronger current responses and more negative hydrogen evolution potentials than unmodified one, suggesting that modified samples had higher activities for oxygen reduction, which promoted the H 2 O 2 production. Moreover, with the increasing concentration of hydrazine hydrate the current response for cathodes became stronger. It was also seen that the modification not only encouraged the two-electron transfer process towards ORR (i.e. H 2 O 2 electrogeneration, Eq. 1), but also enhance the other ORR processes, which might be competition for the H 2 O 2 generation (Eqs. 2, 3), resulting in a drop of current efficiencies for modified electrodes. O + 2H + 2e H (1) 2 2O2 O2+ 4H + 4 2H2O H2O2+ 2H + 2 2H2 e (2) e O (3) With the increasing concentration of hydrazine hydrate in modification, the ORR tended to the higher number electron transfer for each oxygen molecule under the same potential. In addition, the enhanced current in the system promoted the parasitic reactions at anode (Equ. 4, 5), which resulted in a decreased H 2 O 2 accumulation and lower current efficiency [19]. H2 O2 HO2 + H + e (4) HO2 O2(g) + H + e (5) 3.3 Effect of cathodic potential j (ma/cm 2 ) CF CF-HA-5% CF-HA-1% CF-HA-15% CF-HA-2% Potential (V) Fig. 3 Cyclic voltammograms of the cathodes obtained in the potential range from -1.4 to V (vs. SCE), in.5m Na 2 SO 4 solution, at the scan rate of 5mV s Potential (V) Fig. 4 The effects of the potentials (vs.sce) on H 2 O 2 production. Conditions: using CF-HA-1% as cathode,.5m Na 2 SO 4, ph=7, O 2 flow rate at.4l/min. ISBN:

4 The effects of the potential on H 2 O 2 production at CF-HA-1% cathode were shown in Fig. 4. The accumulations of H 2 O 2 at the potential ranging from -.35 V to -.85 V were 15.7, 46.4, 16., 144.7, and mg/l, respectively. It can be seen that the H 2 O 2 electro-generation initially enhanced with the increasing cathodic potential ( E ), and then declined when exceeded the potential of -.75 V, at which the maximum yield of H 2 O 2 was obtained. The current efficiencies of H 2 O 2 production at the potential ranging from -.35 V to -.85 V were 97.2%, 89.7%, 87.1%, 82.%, 64.3% and 41.7%, respectively. As CV curves confirmed, the competing reactions were also enhanced after modification, which could become more and more remarkable with the cathodic potential increasing. Hence, the current efficiencies went downhill quickly, although the yields of H 2 O 2 were improved. When the cathodic potential became more negative than -.75 V, the side reactions corresponding to the H 2 O 2 decomposition and higher number electron transfer became dominant, which resulted in a decreased yields and current efficiencies of H 2 O Effect of ph ph Fig. 5 The effects of the ph on H 2 O 2 production. Conditions: using CF-HA-1% as cathode, E= -.65 V vs. SCE,.5M Na 2 SO 4, O 2 flow rate at.4l/min. The effects of the ph on H 2 O 2 production at CF- HA-1% cathode were shown in Fig. 5. When ph=3~9, the concentrations of H 2 O 2 were 158.9, 152.1, and 138. mg/l, and the current efficiencies were 68.9%, 75.5%, 85.% and 86.1%, respectively. There was a slight increase of the electro-generated H 2 O 2 with the increasing ph. Since H 2 O 2 was electro-generated at cathode surface by reduction of dissolved oxygen in acidic medium, as Equ. 1 shown, from this point of view, the lower ph was beneficial to the H 2 O 2 production. However, the higher level of hydrogen ion also promoted the decomposition of H 2 O 2 (Eq. 2). As a result, the obtained results showed that ph did not dramatically influence the accumulation of H 2 O 2, which was similar with the results of previous studies [7,2], but the current efficiencies declined due to the increasing side reactions in acid solution. 3.5 Effect of oxygen mass flow rate Aeration Amount (L/min) Fig. 6 The effects of the aeration amount on H 2 O 2 production. Conditions: using CF-HA-1% as cathode, E= -.65 V vs. SCE,.5M Na 2 SO 4, ph=7. The effects of the aeration amount on H 2 O 2 production at CF-HA-1% cathode were shown in Fig.6. The accumulation of H 2 O 2 at O 2 flow rate of,.2,.4 and.6 L/min were 16.8, 1.2, and mg/l, and the current efficiencies were 35%, 77.5%, 85.1% and 94.%, respectively. The increasing aeration amount could enhance the concentration of dissolved O 2 and promote the mass transfer rate of dissolved O 2 in solution, which were conducive to the H 2 O 2 electro-generation. However, when O 2 flow rate reached to.6 L/min, the resistance of the medium increased with a mass of bubble in the constant potential system, and then the current declined, inducing a little drop in the yields of H 2 O 2. The result indicated that too much aeration could impede the H 2 O 2 production. 4 Conclusion In this study, hydrazine hydrate-ethanol system was used to modify graphite felts. After modification, a larger specific surface area was observed by SEM, which was considered to be conducive to the catalytic process. The modified samples had much higher electro-catalytic activity of oxygen reduction than the bare one, and the concentration of H 2 O 2 generated was doubled after modification. Besides, there was an optimum ISBN:

5 concentration for the precursor. The maximum yields of H 2 O 2 obtained at -.75 V (vs. SCE) in.5m Na 2 SO 4 aqueous solution with oxygen mass flow rate at.4 L/min was mg/l after 9 min, and the results indicated a dramatically influence of the applied potentials on the H 2 O 2 production. There was a slight increase of the electro-generated H 2 O 2 with the increasing ph, but the current efficiencies declined due to the increasing side reactions in acid solution. The increasing aeration amount was conducive to the H 2 O 2 electro-generation, but too much aeration would impede the H 2 O 2 production. Acknowledgements This work was financially supported by Natural Science Foundation of China (No and ) and Fund for the Doctoral Program of Higher Education of China ( ). References: [1] P.V. Nidheesh, R. Gandhimathi, Trends in electro-fenton process for water and wastewater treatment: An overview, Desalination, Vol.299, 212, pp [2] E. Brillas, I. Sires, M.A. Oturan, Electro- Fenton process and related electrochemical technologies based on Fenton's reaction chemistry, Chemical Review, Vol.19, No.12, 29, pp [3] J.S. Do, C.P. Chen, In situ oxidative degradation of formaldehyde with hydrogen peroxide electrogenerated on the modified graphites, Journal of Applied Electrochemistry, Vol.24, No. 9, 1994, pp [4] A. Alverez-Gallegos, D. Pletcher, The removal of low level organics via hydrogen peroxide formed in a reticulated vitreous carbon cathode cell. Part 2: The removal of phenols and related compounds from aqueous effluents, Eletrochimica Acta, Vol. 44, No. 14, 1999, pp [5] Fan Y, Ai Z, Zhang L. Design of an electro- Fenton system with a novel sandwich film cathode for wastewater treatment, Journal of hazardous materials, Vol.176, No.1-3, 21, pp [6] J. Li, Z.H. Ai, L. Zhang, Design of a neutral electro-fenton system with Fe@Fe 2 O 3 /ACF composite cathode for wastewater treatment, Journal of hazardous materials, Vol.164, No.1, 29, pp [7] A. Özcan, Y. Şahin, A. Savaş Koparal, M.A. Oturan, Carbon sponge as a new cathode material for the electro-fenton process: Comparison with carbon felt cathode and application to degradation of synthetic dye basic blue 3 in aqueous medium, Journal of Electroanalytical Chemistry, Vol.616, No.1-2, 28, pp [8] M. Panizza, M.A. Oturan, Degradation of Alizarin Red by electro-fenton process using a graphite-felt cathode, Electrochimica Acta, Vol. 56, No.2, 211, pp [9] S. Hammami, N. Oturan, N. Bellakhal, M. Dachraoui, M.A. Oturan, Oxidative degradation of direct orange 61 by electro- Fenton process using a carbon felt electrode: Application of the experimental design methodology, Journal of Electroanalytical Chemistry, Vol.61, No.1, 27, pp [1] M.H. Zhou, Q.Q. Tan, Q. Wang, et al., Degradation of organics in reverse osmosis concentrate by electro-fenton process, Journal of Hazardous Materials, Vol , 212, pp [11] M. Pimentel, N. Oturan, M. Dezotti, et al. Phenol degradation by advanced electrochemical oxidation process electro- Fenton using a carbon felt cathode, Applied Catalysis B: Environmental, Vol.83, No.1-2, 28,pp [12] M.H. Zhou, Q. Yu, L.C. Lei, The preparation and characterization of a graphite- PTFE cathode system for the decolorization of C.I. Acid Red 2, Dyes and Pigments, Vol.77, No.1, 28, pp [13] X.W. Zhang, L.C. Lei, B. Xia, et al., Oxidization of carbon nanotubes through hydroxyl radical induced by pulsed O 2 plasma and its application for O 2 reduction in electro- Fenton, Electrochimica Acta, Vol.54, No.1, 29, pp [14] Y. Sheng, S. Song, X. Wang, et al., Electrogeneration of hydrogen peroxide on a novel highly effective acetylene black-ptfe cathode with PTFE film, Electrochimica Acta, Vol.56, No.24, 211, pp [15] M.H.M.T. Assumpção, A. Moraes, R.F.B. De Souza, et al., Low content cerium oxide nanoparticles on carbon for hydrogen peroxide electrosynthesis, Applied Catalysis A: General, Vol , 212, pp.1-6. [16] J.L. Fu, X.W. Zhang, L.C. Lei. Fe-modified multi-walled carbon nanotube electrode for production of hydrogen peroxide, Acta Physico-Chimica Sinica, Vol.23, No.8, 27, pp [17] R.M. Sellers. Spectrophotometric determination of hydrogen peroxide using ISBN:

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