Electrochemical Behavior of Fe(VI)-Fe(III) System in Concentrated

Chapter 4

Electrochemical Behavior of Fe(VI)-Fe(III) System in Concentrated NaOH Solution 1,2

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Cun ZhongZhang *, HongBo Deng , TingtingZhao , Feng Wu *, Wei Liu , Shengmin Cai , Kai Yang , and Virender K. Sharma 3

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Downloaded by MIT on June 10, 2013 | http://pubs.acs.org Publication Date: July 25, 2008 | doi: 10.1021/bk-2008-0985.ch004

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School of Chemical Engineering and The Environment, Beijing Institute of Technology, Beijing 100081, People's Republic of China National Development Center of Hi-Tech Green Materials, Beijing 100081, People's Republic of China School of Environment Sciences and Engineering, Sun Yat-Sen University, Guangzhou 510275, People's Republic of China College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, People's Republic of China Chemistry Department, Florida Institute of Technology, 150 West University Boulevard, Melbourne, FL 32901

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The electrochemical behavior of a Fe(VI)/Fe(III) system was investigated with both a SnO -Sb O /electrode and a powder microelectrode. Results showed that ferrite ion (FeO ), formed in solution and solid phase, was a suitable Fe(III) species for the generation of Fe0 and for construction of a Fe(VI)/Fe(III) redox system under strong basic conditions. Solid ferrite was made by the method of molten melting a mixture of Fe O and NaOH at 973 ± 2K. The product was confirmed to be NaFeO by X-ray diffraction (XRD) measurements. Ferrate (Fe(VI)) was also synthesized from ferrite (solid), Fe(OH) , and Fe O using an hypochlorite oxidation method. In this method, it was demonstrated that ferrite was the most suitable material among these Fe(III) compounds for ferrate synthesis. Results also indicated: a) Fe0 /FeO is a suitable redox 2

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© 2008 American Chemical Society

In Ferrates; Sharma, V.; ACS Symposium Series; American Chemical Society: Washington, DC, 2008.

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82 system under studied experimental conditions, b) rate and the reversibility of the redox reaction between Fe0 " and Fe0 " were promoted markedly with the increase of temperature from 293 K to 333 K, c) the electro-oxidation of Fe0 " is a fast two-electron transfer reaction on both the Sn0 -Sb 03/Ti electrode and the NaFe0 powder microelectrode, and d) open-hearth dust of iron and steel industry could be used as a promising and valuable raw material for ferrate synthesis. 2

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Introduction VI

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Ferrate (Fe(VI), Fe 04 ") has been proposed in wastewater treatment in the past few decades because of its dual quality of oxidant and flocculent in a single dose (7,2). Ferrate has also been used in organic synthesis (3) since it has a strong but controllable oxidative potential. A new application of ferrate in a super-iron battery was first reported by Licht in 1999 (4) and since then many investigations have been carried out on this subject (5,6). Fabricating a safe and rechargeable super-iron battery is a potential application of ferrate because this battery is environmentally-friendly with high specific energy. Until recently, chemical, electrochemical, and high temperature methods (5,5-7) have been developed for ferrate synthesis. There are some variations on the chemical composition and formal potential (or standard potential) of the Fe(VI)/Fe(III) system in previous reports (8-10). Additionally these results showed dependence on experimental conditions. Moreover, the results indicated that the Fe(VI)/ Fe(III) system is not a simple redox system. Besides, it also indicated that the published Latimer profile of Fe(VI)/Fe(0) (9), especially the Fe(VI)/Fe(III) part, is incomplete. First of all, such information might play an illusive role in the development of selective electrochemical synthesis of ferrate and even in the manufacture of safe and rechargeable batteries. Furthermore, insufficient clarity may limit the communication between electrochemical research and research of other fields and also affects the application of the Marcus theory on the Fe(VI)/Fe(III) redox system (77). Thermodynamic data of different valence state Fe species indicate that Fe(IIl) is the most stable species under most of the environmental conditions.



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Therefore, synthesis of ferrate from pure iron by electrochemical methods did not completely exhibit the actual charge/discharge process of the positive electrode material of a super-iron battery. It should be noticed that the most published potential of ferrate generation is higher than that of Oxygen Evolution Reaction (OER). OER thus induce a sharp increase of the inner pressure of the battery and will possibly result in a dangerous blast during charging process of the secondary super-iron battery. This will also reduce the high efficiency electro-generation of ferrate in simple unsealed electrochemical equipment. Hence, the study of the selective electrochemical synthesis of ferrate is critically needed. In addition, the electrogeneration of ferrate will broaden our knowledge of the behaviors of such a high-valent oxidant under extreme conditions. This paper addresses issues in selective electro-generation of ferrate using different Fe(III) compounds. Special electrode materials with high over-potential of OER could be used as an effective tool for the exploration of electro-generation of strong oxidants in aqueous solution. In the last century, dimensionally stable anode (DSA) has played important role in many fields for its excellent properties. The electrochemical behavior of the molecule on different electrode materials provides useful and meaningful information. Therefore DSA should be used as an effective and helpful tool for extreme reaction conditions. The open-hearth dust in the iron and steel industry in Inner Mongolia of China has been widely used (12,13). Uses include manufacturing of catalysis and magnetic materials. However, the electrochemical property of the main ingredient, Fe 0 , of the dust has not been investigated. In this paper, we have used Sn02-Sb2C>3/Ti electrodes and powder microelectrode techniques (14) to understand the mechanism of the Fe (VI)/Fe (III) system. 2

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Experimental Electrochemical measurements were carried out using a CH1660A electrochemical workstation (CH Instruments). The conventional threeelectrode electrochemical cell was used for electrochemical experiments. A Hg/HgO (in 14M NaOH) electrode as a reference electrode (RE) was set in an independent compartment and introduced into electrolytic cell by a salt


84 bridge having layers of basic-resistant semi-permeable membranes and Luggin capillary. A sintered Pt/Ti sheet electrode was used as a counter electrode. Two types of microelectrodes, Sn02-Sb2C>3/Ti electrodes and powder microelectrode (14), were used as working electrodes. The coating number used was 20 times in order to keep the stability of the Sn02Sb20 /Ti electrodes in concentrated NaOH aqueous solution. A piece of platinum wire with diameter of 0.1mm was used to fabricate powder microelectrode. Each kind of investigated sample powder, Fe 0 , Fe (OH) , and NaFe0 , was loaded in the micro-hole at the very tip of powder microelectrode. A NaFe02 powder was made from the mixture of Fe20 and NaOH at a molar ratio of 1.2 : 1 (Na : Fe) at 973 ± 2 K . The Fe(OH) was prepared by mixing of Fe(N0 ) and NaOH solutions at pH = 7 - 8. The Fe(OH) deposition was washed thoroughly by redistilled water. Concentrated NaOH aqueous solutions having no Fe02* ion and 0.02 M Fe0 " ion (75) were used as blank and working electrolyte solutions, respectively. Saturated hypochlorite solution was prepared by dissolving Cb gas into 14.0 -14.5 M NaOH solution. All of the reagents were of analytic grade. The XRD analysis was carried out on a Dmax-2500 diffractometer using Cu Ka radiation. The UV-Visible spectroscopic measurements were carried out with the Hitachi U-2800 Spectrophotometer. 3

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Results and Discussion Chemical Behavior of Fe(III) Compounds Figure 1 shows the cyclic voltammograms (CVs) on Sn0 -Sb 0 /Ti electrodes in blank and working solutions at 293 K. The blank solution showed no peak and the value of the onset potential of O E R was - I V . It demonstrated that the over-potential of O E R on Sn0 -Sb 0 /Ti electrodes was considerably high in 14 M NaOH solution. In working solution, one anodic peak (P ) and one cathodic peak (P ) appeared in the CV curves. It is difficult to produce the integrated anodic peak in strong basic solution on an ordinary electrode due to the limitation of electrochemical windows. According to previous reports (5,15-17), the integrated anodic peak and cathodic peak are related to electro-generation and electro-reduction of ferrate, respectively. Addition of Fe 0 powder did not change the CV behavior in NaOH solution, possibly due to the low solubility of this powder in alkaline 2

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(pVvsHg/HgO Figure 1. Cyclic voltammograms ofSnOr~Sb20s/Ti electrodes in: (a) 14 MNaOHcontaining 0.02MFeOj, (b) 14 MNaOH blank solution, scan rate: 10 mV/s, the arrows indicate the scan direction.

solution. The soluble Fe0 " may thus be a suitable species for the electrogeneration of ferrate. Results using Fe0 " have reported that the onset potential and peak potential of the anodic peak, P , were almost kept at the same position, and no change occurred due to the surface component of electrode materials and scan rate (5,15,16). However, the peak potential of the cathodic peak, P , was easily affected by these variables (5,6,15-17). This indicates that the electro-reduction reaction of ferrate differs from the electro-oxidation of Fe0 ". In order to find a suitable ferric ions source for the electro-generation of ferrate, different Fe(III) compounds were investigated. The XRD profile of NaFe0 is shown in Figure 2. The reactions of Fe(III) compounds, Fe 0 , Fe(OH) , and NaFe0 with saturated hypochlorite solution were investigated by a UV-Visible spectrophotometer at 293 ± 0.5K. The results are shown in Figure 3. Both Fe(OH) and NaFe0 showed a maximum absorption peak at 505 nm and an absorption shoulder at 570 nm. However, the peak intensity obtained in the use of NaFe0 is more remarkable than that of Fe(OH) . Meanwhile, there was no absorption peak obtained in using Fe 0 . It appears that the F e 0 could not be dissolved in saturated hypochlorite basic aqueous solution to have any reaction with the hypochlorite ion. The spectral results of F e 0 in saturated hypochlorite solution are in agreement with the result of electro2

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Figure 3. Spectra of solution obtained in the mixture of solid state Fe(III) compounds (equi-molar of Fe atom) with saturated hypochlorite solution.


87 chemical investigation (see Figure 1). This comparative result demonstrated that Fe0 ' ion is the most active Fe(III) species for the generation of ferrate in concentrated basic solution. The use of Fe(OH) showed a weak acidity to form Fe0 " in a basic solution. The use of molten basic electrolyte may have a suitable to ensure activity of Fe C>3. The reaction (1) may occur at 973 ± 2 K to give sodium ferrite. 2

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2 NaOH + F e 0 -> 2 NaFe0 + H 0


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The equation (1) indicates that the ferrite could be generated by thermochemical reaction. The electrochemical behavior of different Fe(III) compounds was also investigated by using a powder microelectrode technique in 14 M NaOH solution at 293 K (Figure 4). The results indicate that no reaction occurred on the F e 0 electrode. OER was a unique obvious phenomenon until the potential exceeded 0.75 V. Under the same conditions, a weak hump and an obvious current peak in the potential range from 0.6 to 0.7 V, appeared on the segment of positive-going sweep of C V curves for both Fe(OH) and NaFe0 . However, obvious OER could also be found after the weak hump on the curve for Fe(OH) . There were weak and strong cathodic peaks from ~ 0.4 to 0.2 V for Fe(OH) and NaFe0 , respectively, during the segment of negative-going sweep on C V curves. Results clearly demonstrate that Fe0 " ion was the most suitable Fe(III) compound for the electro-generation of ferrate in strong basic solution. 2

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The electro-generation of ferrate from Fe(OH) and NaFe0 were also investigated by the cyclic voltammetry method in 14 M NaOH solution at different temperatures. The results are shown in Figure 5. The generation rate and reversible degree of the electrochemical redox reaction on either NaFe0 or Fe(OH) were remarkably promoted with the increase in temperature. In addition, electro-generation of ferrate using NaFe0 could be separated from OER in comparison with Fe(OH) at all studied temperatures. Thus, NaFe0 is the most suitable material to form ferrate. With the increase in temperature, an anodic peak also grew gradually on the Fe(OH) electrode. The acidic property and acidic ionization of Fe(OH) 3

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2 at three temperatures, scan rate: WOmV/s, arrows indicate scan direction (Reproduced with permission with J. Functional Material.,)


89 became stronger at higher temperature. This phenomenon is consistent with earlier reported results (15,17).

Mechanism of Electro-Generation of Ferrate In general, the apparent potential of the Fe (VI)/Fe(III) system (3,8,10) is higher than that of OER. This inhibits the selective electro-generation of ferrate in aqueous solution. It is thus necessary to understand the electrochemical mechanism for the reorganization of the Fe (VI)/Fe (III) system for selective electro-generation of ferrate. The parameter of | ^ _ ^ | o f the anodic peak, P in Figure 1 was investigated in different Downloaded by MIT on June 10, 2013 | http://pubs.acs.org Publication Date: July 25, 2008 | doi: 10.1021/bk-2008-0985.ch004

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NaOH solutions and results are given in Table 1.

Table. 1. L -

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In this investigation, the nature of P and the relationship between P and P were used to learn the cause of short-life of Fe0 " and to understand the mechanism of electro-generation of ferrate. The results are shown in Figure 6. In the potential range from 0.5 to 0.0 V, the cathodic peak of the C V curve of a mixed powder micro-electrode, containing NaFe0 and K F e 0 showed that P must arise from electro-reduction of the F e 0 ' ion on the NaFe0 electrode. This suggests that the disproportionation reaction of Fe0 " may be happening according to the equation (3). c

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Fe(V)

Fe(VI) + Fe0 "

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Reaction (3) is different from previous results (16), which reported disproportionation of Fe(V) species. However, reaction (3) is a fast homogenous reaction in the electrochemical step and is not affected by the electrode material. These combined results with the UV-Visible spectral measurements suggest that the amount of Fe0 " and electro-generated Fe0 " can be expected considerably different on Fe(OH)3 and NaFe0 electrodes. 3

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The amount of formed Fe0 " would thus be different on investigated electrodes. Thus weak and strong Faradaic current signals on Fe(OH)3 and NaFe0 electrodes, respectively, could be used as a reliable evidence to demonstrate the disproportionation reaction and mechanism on both used electrodes. Interestingly, the formal potential of C107C1" is larger than the

Electrochemical Behavior of Fe(VI)-Fe(III) System in Concentrated

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