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Studies of the reduction mechanism of selenium dioxide and its impact on the microstructure of manganese electrodeposit
Electrochimica Acta 56 (2011) 8305–8310
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Studies of the reduction mechanism of selenium dioxide and its impact on the microstructure of manganese electrodeposit Yan Sun a , XiKe Tian a,b,∗ , BinBin He a , Chao Yang a , ZhenBang Pi a , YanXin Wang b , SuXin Zhang c a b c
Faculty of Material Science and Chemistry Engineering, China University of Geosciences, Wuhan, Hubei 430074, People’s Republic of China School of Environmental Studies, China University of Geosciences, Wuhan, Hubei 430074, People’s Republic of China State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan, Hubei 430074, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 20 January 2011 Received in revised form 4 May 2011 Accepted 30 June 2011 Available online 7 July 2011 Keywords: SeO2 Electrochemical techniques Hydrogen evolution Adsorption Multi-selenium ions
a b s t r a c t The influence of selenium dioxide (SeO2 ) on the microstructure and electrodeposition of manganese coatings obtained from a sulfate based neutral solution was investigated by material characterization methods and electrochemical techniques. The crystal structure and surface morphology of these coatings were studied by scanning electron microscopy (SEM) and powder X-ray diffraction spectroscopy (XRD), respectively. The SEM and XRD data showed that SeO2 could effectively accelerate phase transformation, and facilitate leveled and fine grain growth. The electrochemical results indicated that SeO2 could inhibit hydrogen evolution reaction and promote manganese deposition. The action of selenium dioxide in manganese deposition was found to be a reduction and adsorption mechanism. The process could be explained as following: First, Se (IV) was reduced to Se (0), and part of Se (0) future reduce to selenide, which then combined with the remainder Se (0) forming a complicate compound (multi-selenium ions). © 2011 Elsevier Ltd. All rights reserved.
1. Introduction Metallic manganese (Mn) is used in certain types of steel, particularly low-carbon steels, and in nonferrous alloys. In various steelmaking processes, approximately 90% Mn is consumed. Industrially, manganese and its alloys are obtained by cathodic reduction of manganese sulfate and chloride, together with corresponding ammonium salts on stainless steel sheets [1,2]. In order to promote the deposition of metal manganese and maintain production process, SeO2 and selenium compounds were commonly added as an important additive to the electrolyte. Unfortunately, SeO2 has a hazardous effect on the environment and disposal of selenium compounds was regulated. Extensive use of SeO2 will not only lead to the employees of manganese production which is exposed to the toxic effects of SeO2 [3] but also lower the quality of manganese products. Selenium pollution has become a global environmental safety issue [4–6]. The best way to curtail selenium pollutants for the electrolytic manganese metal (EMM) industry is to apply the selenium-free technology. Efforts have been taken to find alternatives of selenium dioxide [7,8]. As the complexity of the electrolysis
∗ Corresponding author at: Faculty of Material Science and Chemistry Engineering, China University of Geosciences, Wuhan, Hubei 430074, People’s Republic of China. Tel.: +86 27 67884574; fax: +86 27 67884574. E-mail address: [email protected] (X. Tian). 0013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2011.06.111
process, the mechanism of additive in electrodeposition was still unclear. Therefore how to find out the mechanism of manganese electrolysis additives, especially SeO2 , became a challenge. The influence of inorganic selenium compounds such as selenic acid, H2 SeO3 , and ammonium selenate, (NH4 )2 SeO4 , on electrodeposition of manganese has been known for a long time [9–11]. When a small amount of SeO2 is dissolved in a sulfate manganesebath, light, semi-lustrous, good quality and very hard ␣-manganese coatings, are obtained, with current efficiencies reaching 82.49% [12]. Radhakrishnamurthy and Reddy [13] found that selenic acid affected the manganese deposition indirectly by influencing the hydrogen evolution reaction. In Zn-Mn alloys system, A. Sulcius and coworkers have done a series of research about the influence of several additives including analysis of the micro mechanism [14–16]. At the initial moments of electrolysis manganese atoms, which are more thermodynamically active than zinc atoms, formed at the cathode have a greater role in the reduction of selenate ions to elemental selenium and selenides. The film of the reduction products of selenate ions (elemental selenium and selenides) on the cathode surface increases the hydrogen overvoltage and facilitates further electrochemical reduction of Mn2+ ions [14]. Unfortunately, the existing theories which could explain the mechanism of Secontaining additives in manganese electrodeposition system are insufficient. The purpose of this paper is to investigate the mechanism of the action of selenium dioxide in manganese electrowinning by various
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Fig. 1. XRD patterns of Mn electrodeposits obtained from a sulfate bath electrolysis for 30 min with different SeO2 concentrations.
electrochemical methods which include cyclic voltammetry (CV), linear sweep voltammetry (LSV), chronopotentiometry and crosspotential electrolysis. The effect of SeO2 on the crystal structure and morphology of manganese has also been studied. 2. Experimental The electrodeposition experiments were carried out in a two electrode, two-compartment prismatic cell. The counter electrode was a Pb–Sb–Sn–Ag alloy plate placed in the anodic compartment and the diaphragm was used to avoid the contamination of solution following formation of manganese oxides. Manganese was plated on stainless-steel plates with an active surface area of 14 cm2 (4 cm × 3.5 cm). The substrates were first mechanically polished with various grades of metallographic sandpaper and degreased with alkaline solution (23 g/L NaOH, 22 g/L Na2 CO3 , 10 g/L Na2 SiO3 (anhydrous), 10 g/L Na3 PO4 , 1 g/L sodium lauryl sulfate, 90–95 ◦ C, 3–5 min) and acetone. Subsequently, they were electropolished in concentrated phosphoric acid (85%) and pickled in mixed nitric (5%) and hydrochloric acid (25%) just before used. Simple sulfate electrolytes with addition of ammonium sulfate were used. pH was adjusted by adding concentrated ammonium hydroxide or sulfuric acid, but no attempt was made to control it during deposition. However, pH was measured and, if necessary, adjusted, after each deposition experiment. The basic bath contained 15 g/L MnSO4 , 100 g/L (NH4)2 SO4 , and the solutions were prepared with analytical grade reagents and double-distilled water. All experiments were carried out at 30 ◦ C and N2 was poured to solutions for 15 min in order to get rid of the dissolved oxygen before experiment. Electrochemical tests were all operated on CHI660 C electrochemical workstation. A conventional three electrode system was employed with a platinum foil as a counter electrode and a saturated Hg/Hg2 SO4 as the reference electrode [17]. The working electrode consisted of a stainless-steel rod designed to expose to the electrolyte an area of 0.4 cm2 . The cyclic voltammograms scans were measured at a rate of 20 mV/s and the scan rate of steady polarization was controlled at 1 mV/s. To minimize iR-drop effects, a Luggin capillary was employed to connect the reference electrode compartment to the working electrode. The current efficiency was calculated from the weight gained from the sample. Surface morphology was examined by scanning electron microscopy (SEM) using a FEI Quanta 200 instrument. Crystallographic structure was determined by X-ray diffraction
Fig. 2. SEM microphotographs of Mn electrodeposits obtained from a sulfate bath electrolysis for 30 min containing (a) 0 g/L SeO2 ; (b) 30 mg/L SeO2 and (c) 0.1 g/L SeO2 .
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Fig. 3. Electrochemical behaviors of manganese electrodeposition. (a) Curves of steady polarization, scan rate 1 mV/s. (b) Curves of cyclic voltammetry, scan rate 20 mV/s. (1) MnSO4 + (NH4 )2 SO4 ; (2) MnSO4 + (NH4 )2 SO4 + SeO2 .
(XRD) using X’ Pert PRO DY2198 diffractometer with Cu K␣ radiation. 3. Results and discussion 3.1. Crystal structure and morphology investigations The Mn electrodeposits were analyzed using XRD and SEM to determine the influence of SeO2 on the crystalline structure and morphology of the deposits. Manganese films were electrodeposited at different SeO2 concentrations, from 0 g/L to 0.1 g/L, at 360 A/m2 current density. With the increase of SeO2 concentration, the current efficiency increases from 50% to 83%, and the film thickness increases from 34 to 43 m. XRD was used to determine the crystalline structure and its patterns of manganese deposits were shown in Fig. 1. The main diffraction peaks due to ␥-Mn phase when selenium dioxide was absent. With increasing the concentration of SeO2 , the diffraction peaks of ␥-Mn become broader and the peak intensities of ␣-Mn increase obviously. At 0.1 g/L SeO2 , good ␣-Mn is obtained. At room temperature the ductile ␥-Mn gradually transforming into brittle and hard ␣-Mn was not exhibited for a relatively long period [18]. It can be concluded that increasing the amount of SeO2 could be a great boost for the conversion of manganese electrodeposit crystal from ␥-phase into ␣- phase. SEM surface micrographs of manganese deposits are shown in Fig. 2. Fine cubic crystallites and some pinholes are visible. With the increase of SeO2 , the quantity of the protrusions decreases and the grain size becomes smaller. It can be seen as the concentration of SeO2 increases to 0.1 g/L, the coatings are silvery leveling in appearance and refining, grain size about 2 m. The pinholes are attributed to the evolution of hydrogen bubbles. The relatively leveled coating and low grain size may be induced by the comparatively higher SeO2 concentration. The presented results indicate that SeO2 could be used as a leveling and refining agent. It may be due to the fact that Se content can increase cathodic overpotential.
different for the solutions with and without selenium dioxide as shown in Fig. 3a. An increase in the cathodic polarization was recorded when selenium dioxide was present in the electrolyte, in accordance with previously reported observations [11]. This could be explained by the increase in the hydrogen-evolution reaction overpotential induced by the presence of this additive, followed by the increase in the partial current densities for the electrodeposition of manganese. The cyclic voltammetry measurements for the electrolyte in the presence and absence of SeO2 are reported in Fig. 3b. The obvious difference is the large increase of the oxidation peak when SeO2 is present, which indicates that selenium dioxide could promote the deposition of manganese metal. On the basis of CV and LSV results, we can conclude that addition of selenium dioxide could promote manganese electrodeposition and inhibit hydrogen evolution reaction. 3.3. Hydrogen evolution reaction To determine how SeO2 can increase the hydrogen overvoltage, we carried out linear sweep voltammetry studies in the potential range of −0.6 to −1.8 V using solutions without manganese. Fig. 4
3.2. Electrochemical behaviors To evaluate the effect of SeO2 on the electrochemical behaviors of Mn, a study using cyclic voltammetry and linear sweep voltammetry in the Mn solutions without (curve 1) and with SeO2 (curve 2) was carried out. The cathodic linear polarization curves are
Fig. 4. Effect of SeO2 on the overvoltage of hydrogen, scan rate 1 mv/s. (A) SeO2 : 0 g/L, (B) SeO2 : 0.1 g/L and (C) electrode adsorbent material in SeO2 solution.
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Fig. 5. CV curve of SeO2 solution, scan rate 20 mV/s.
shows the steady polarization curves of iron electrodes in Solution A, Solution B and special work electrode in Solution A (Curve A, Curve B and Curve C respectively). Solution A was composed of 100 g/L−1 (NH4 )2 SO4 , while Solution B was obtained by adding SeO2 in Solution A and its concentration of SeO2 was 0.1 g/L−1 . To investigate whether SeO2 was adsorbed, the iron electrode was constant-potential electrolyzed for 3 min in Solution B at −1.6 V, removed and washed by double-distilled water. After the above series of processing, the electrode was used to make steady polarization curve in solution A which was defined Curve C. From the composition of the solutions, it is clear that the curves are the steady polarization curves of hydrogen evolution. Comparison of Curve B with Curve A indicates with the adding of SeO2 the potential of hydrogen evolution moves from −1.3 V to −1.5 V. Curve C is similar to curve B although the electrolytes are different. Hence, there can be no doubt Se species were adsorbed on the cathode surface and increase further the hydrogen overvoltage. 3.4. Cathodic reduction mechanism To study detailed mechanism of action of SeO2 in the electrodeposition process, several electrochemical methods including cyclic voltammetry, chronopotentiometry and cross-potential electrolysis were used. Cyclic voltammogram was run for a solution of SeO2 (5 g/L) in ammonium sulfate but without the presence of manganese(II) (Fig. 5). The major feature of the voltammogram is the presence of two reduction peaks (at −1.0 V and at −1.48 V respectively) and two oxidation peaks (at −0.9 V and at −1.47 V respectively). The reduction peak at −1.0 V is larger than the reduction peak at −1.48 V; the oxidation peak at −1.47 V is weaker than the oxidation peak at −0.9 V. Presumably the process of SeO2 reduction takes two steps, and the reduction product of the first step has not fully transformed into the product of the second step. This view is supported by the following fact of chemical analysis. When the stainless steel cathode was constant-potential electrolyzed at −1.0 V in Solution B, a kind of red deposit was generated. As a substitute −1.48 V for −1.0 V, orange-red solutions generated near the cathode. When adding a small amount of sulfuric acid in the solution and heating it, there would appear the same red deposit and in the meantime the solution became colorless gradually and released a nasty smell of H2 Se gas. XRD image of the red deposit is shown in Fig. 6. The peak data confirm the deposit is Se (0) crystalline phase (JCPDS 00-42-1425). Therefore, it is indicated that Se (IV) would first reduce to Se (0) and part of Se (0) future reduce to selenide at more negative potentials and that, the selenide might combine with the remainder Se (0) to form complicate compounds
Fig. 6. XRD patterns of red deposit generated by constant-potential electrolyzed Solution B at −1.0 V.
(multi-selenium ions). SeO2 dissolved in water quickly transforms into SeO3 2− . The overall reaction equation of SeO2 reduction is suggested as follows + 3xOH− 3xH+ + xSeO3 2− + 4(x + n/2)e = Se2n− x
(1)
3.4.1. Adsorption behaviors of the reduction products of SeO2 According to Fig. 5, we conclude the first step reaction is irreversible due to the fact that the potential range between its reduction peak (−1.0 V) and its oxidation peak (−0.9 V) is 100 mV which is far larger than that of reversible process (59.2/4 = l4.75 mV)[19]. The following reaction is proposed: 3xH+ + xSeO3 2− + 4xe = Sex + 3xOH−
(2)
If Sex has an adsorption behavior and complies with Langmuir isotherm, the Faraday current of the electrochemical reaction is as follows [20]:
˛nF(ϕ+vt) i ˇnF(ϕ+vt) −kc0 exp − = ka0 [SeO3 2− ](1−)exp RT RT nF
(3) where is the Sex coverage on the cathode; ϕ is the original potential; v is the scan rate. As the first step reaction is irreversible, the second item could be neglected. Eq. (3) is simplified to: i = nFka0 [SeO3 2− ](1 − )exp
˛nF(ϕ + vt)
(4)
RT
The expression of the reciprocal of the time derivative response of Eq. (4) is given by:
˛nF(ϕ + vt) di = nFka0 [SeO3 2− ]exp RT dt
˛nF RT
(1 − )v −
d dt
(5)
with the boundary conditions:At the peak current (di/dt = 0), Eq. (6) is obtained. ˛nF d (1 − )v = RT dt
(6)
If r is presumably for the demand power of the form of monolayer adsorption per unit area, the Faraday current of the electrochemical reaction is as follows: i=r
d dt
(7)
From Eqs. (6) and (7) ip =
˛nF (1 − )r v RT
(8)
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The oxidation peak at −1.0 V is presumably for the reaction Se (0) → Se (IV): (z + y)Se − 4e = (z + y)Se(IV),
(11)
where z is the molar of Se (0) in the adsorptions. When the transition time is 2 , the required electricity is i 2 . 2 contains the oxidation transition time of both Se (0) oxidized from Se2− and original Se (0). 1 and 20 are supposed to be the oxidation transition times of Se (0) oxidized from Se2− and original Se (0), respectively. Then at any time, one has 2 = 20 + 1
(12)
z follows z=
i 0 it = 2 nF 4F
(13)
The oxidation of Se2− follows the law of mass conservation i1 i 1 = 2F 4F
Fig. 7. The first peak current curve of SeO2 reduction affected by scan rate.
(14)
hence In order to investigate whether Sex has an adsorption behavior, the curve of the first peak current of SeO2 reduction (ip ) with different scan rate (v) was plotted (Fig. 7). The ip vs. v plot is a straight line through the origin which is linked to the absorption of irreversible electrochemical product on the cathode [21]. 3.4.2. Composition of adsorptions [22] In order to study the second step reaction (the reduced product’s adsorption behavior and its composition), electrochemical chronopotentiometry and cross-potential electrolysis were carried out. Fig. 8 shows the anode potential–time curve which is obtained in Solution A by iron electrode which has been electrolyzed for 3 min in Solution B at constant −1.6 V. As the current sets 0.04 mA, there are two plateaus on the potential–time curve. The first platform is located at −1.48 V with the transition time 1 ( 1 = 0.33 s); the second one is located at −1.0 V with the transition time 2 ( 2 = 3.67 s). The oxidation peak at −1.48 V is assumed for the reaction Se2− → Se (0): ySe
2−
− 2e = ySe,
(9)
where y is the molar of Se2−
in the adsorptions. When the transition time is 1 , the required electricity is i 1 . Then y follows y=
i1 it = . nF 2F
(10)
1 = 21
(15)
Then, by substituting 1 by the value 2 1 in Eq. (12), one has 2 = 20 + 1 = 20 + 21
(16)
20 = 2 − 21
(17)
Eq. (13) has the general solution: z=
i20 4F
=
i (2 − 21 ) 4F
(18)
z 2 − 21 = 4.5 = 21 y
(19)
z = 4.5y
(20)
Therefore, the abbreviation of the adsorption composition is Se4.5y Sey 2y− . When y = 1, Se4.5y Sey 2y− becomes Se5.5 2− . 4. Conclusion Several material characterization and electrochemical methods were employed for the goal of understanding the mechanism of the action of SeO2 additive in manganese electrodeposition. Research indicates that SeO2 follows the following reduction and adsorption mechanism: Firstly, SeO2 is reduced to Se (0) and Se (0) has an adsorption effect on the cathode. Then a portion of Se (0) is consumed by the formation of Se2− ; the remainder is available to combine with Se2− to form multi-selenium ions (Se4.5y Sey 2y− ) which also have an adsorption effect on the cathode. The process of SeO2 has a marked influence on the electrochemical behaviors of manganese electrodeposition. The influence includes inhibiting hydrogen evolution reaction and promoting the deposition of manganese. Simultaneously, SeO2 could accelerate the conversion of manganese electrodeposit from ␥-phase crystal into ␣-phase crystal and facilitate manganese coatings into leveled and refined. Acknowledgments
Fig. 8. The anode potential–time curve of adsorbed substance on cathode.
We are grateful to the National Natural Science Foundation of China (Grant no. 50904054) for the financial support. The project was also supported by the “Fundamental Research Funds for the Central Universities”.
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J. Gong, I. Zana, G. Zangari, J. Mater. Sci. Lett. 20 (2001) 1921. L.Y. Zhao, A.C. Siu, K.T. Leung, Chem. Mater. 19 (2007) 6414. C. Reilly, Selenium in Food and Health , Aspen Publishers, 1996. P. Wei, O.E. Hileman, M.R. Bateni, X.H. Deng, A. Petric, Surf. Coat. Technol. 201 (2007) 7739. D. Ning, F. Wang, C.B. Zhou, C.L. Zhu, H.B. Yu, Resour. Con. 54 (2010) 506. A.D. Lemly, Ecotoxicol. Environ. Safety 59 (2004) 44. T.W. Coleman, R.A. Griffin, United States Patents: US4478697, 1984. J.B. Goddard, D.J. Hansen, United States Patents: US4149944, 1979. I. Lewis, P. Scaife, D. Swinkels, J. Appl. Electrochem. 6 (1976) 453. M. Gonsalves, D. Pletcher, J. Electroanal. Chem. 285 (1990) 185. P. Ilea, I.C. Popescu, M. Urda, L. Oniciu, Hydrometallurgy 46 (1997) 149.
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Q.F. Wei, X.L. Ren, J. Du, S.J. Wei, S.R. Hu, Miner. Eng. 23 (2010) 578. P. Radhakrishnamurthy, A. Reddy, J. Appl. Electrochem. 7 (1977) 113. E. Griskonis, A. Sulcius, Bull. Electrochem. 21 (2005) 561–570. B. Bozzini, E. Giskonis, A. Sulcius, P.L. Cavallotti, Plat. Surf. F. 88 (2001) 64. A. Sulcius, E. Griskonis, P. Diaz-Arista, G. Trejo, Trans. Inst. Met. Finish. 87 (2009) 254. Y.Y. Wang, W.J. Peng, L.Y. Chai, Y.D. Shu, J. Cent. S. U. 10 (2003) 183. J. Gong, G. Zangari, J. Electrochem. Soc. 149 (2002) 209. P.T. Kissinger, W.R. Heineman, J. Chem. Educ. 60 (1983) 702. E. Gileadi (Ed.), Electrosorption, Plenum Press, New York, 1967. H. Jiang, Metallurgical Electrochemistry , Metallurgy Industry Press, Beijing, 1989. A.J. Bard, L.R. Faulkner, Electrochemical Methods: Fundamentals and Applications , A1bazaar, 2006.
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Studies of the reduction mechanism of selenium dioxide and its impact on the microstructure of manganese electrodeposit Yan Sun a , XiKe Tian a,b,∗ , BinBin He a , Chao Yang a , ZhenBang Pi a , YanXin Wang b , SuXin Zhang c a b c
Faculty of Material Science and Chemistry Engineering, China University of Geosciences, Wuhan, Hubei 430074, People’s Republic of China School of Environmental Studies, China University of Geosciences, Wuhan, Hubei 430074, People’s Republic of China State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan, Hubei 430074, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 20 January 2011 Received in revised form 4 May 2011 Accepted 30 June 2011 Available online 7 July 2011 Keywords: SeO2 Electrochemical techniques Hydrogen evolution Adsorption Multi-selenium ions
a b s t r a c t The influence of selenium dioxide (SeO2 ) on the microstructure and electrodeposition of manganese coatings obtained from a sulfate based neutral solution was investigated by material characterization methods and electrochemical techniques. The crystal structure and surface morphology of these coatings were studied by scanning electron microscopy (SEM) and powder X-ray diffraction spectroscopy (XRD), respectively. The SEM and XRD data showed that SeO2 could effectively accelerate phase transformation, and facilitate leveled and fine grain growth. The electrochemical results indicated that SeO2 could inhibit hydrogen evolution reaction and promote manganese deposition. The action of selenium dioxide in manganese deposition was found to be a reduction and adsorption mechanism. The process could be explained as following: First, Se (IV) was reduced to Se (0), and part of Se (0) future reduce to selenide, which then combined with the remainder Se (0) forming a complicate compound (multi-selenium ions). © 2011 Elsevier Ltd. All rights reserved.
1. Introduction Metallic manganese (Mn) is used in certain types of steel, particularly low-carbon steels, and in nonferrous alloys. In various steelmaking processes, approximately 90% Mn is consumed. Industrially, manganese and its alloys are obtained by cathodic reduction of manganese sulfate and chloride, together with corresponding ammonium salts on stainless steel sheets [1,2]. In order to promote the deposition of metal manganese and maintain production process, SeO2 and selenium compounds were commonly added as an important additive to the electrolyte. Unfortunately, SeO2 has a hazardous effect on the environment and disposal of selenium compounds was regulated. Extensive use of SeO2 will not only lead to the employees of manganese production which is exposed to the toxic effects of SeO2 [3] but also lower the quality of manganese products. Selenium pollution has become a global environmental safety issue [4–6]. The best way to curtail selenium pollutants for the electrolytic manganese metal (EMM) industry is to apply the selenium-free technology. Efforts have been taken to find alternatives of selenium dioxide [7,8]. As the complexity of the electrolysis
∗ Corresponding author at: Faculty of Material Science and Chemistry Engineering, China University of Geosciences, Wuhan, Hubei 430074, People’s Republic of China. Tel.: +86 27 67884574; fax: +86 27 67884574. E-mail address: [email protected] (X. Tian). 0013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2011.06.111
process, the mechanism of additive in electrodeposition was still unclear. Therefore how to find out the mechanism of manganese electrolysis additives, especially SeO2 , became a challenge. The influence of inorganic selenium compounds such as selenic acid, H2 SeO3 , and ammonium selenate, (NH4 )2 SeO4 , on electrodeposition of manganese has been known for a long time [9–11]. When a small amount of SeO2 is dissolved in a sulfate manganesebath, light, semi-lustrous, good quality and very hard ␣-manganese coatings, are obtained, with current efficiencies reaching 82.49% [12]. Radhakrishnamurthy and Reddy [13] found that selenic acid affected the manganese deposition indirectly by influencing the hydrogen evolution reaction. In Zn-Mn alloys system, A. Sulcius and coworkers have done a series of research about the influence of several additives including analysis of the micro mechanism [14–16]. At the initial moments of electrolysis manganese atoms, which are more thermodynamically active than zinc atoms, formed at the cathode have a greater role in the reduction of selenate ions to elemental selenium and selenides. The film of the reduction products of selenate ions (elemental selenium and selenides) on the cathode surface increases the hydrogen overvoltage and facilitates further electrochemical reduction of Mn2+ ions [14]. Unfortunately, the existing theories which could explain the mechanism of Secontaining additives in manganese electrodeposition system are insufficient. The purpose of this paper is to investigate the mechanism of the action of selenium dioxide in manganese electrowinning by various
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Fig. 1. XRD patterns of Mn electrodeposits obtained from a sulfate bath electrolysis for 30 min with different SeO2 concentrations.
electrochemical methods which include cyclic voltammetry (CV), linear sweep voltammetry (LSV), chronopotentiometry and crosspotential electrolysis. The effect of SeO2 on the crystal structure and morphology of manganese has also been studied. 2. Experimental The electrodeposition experiments were carried out in a two electrode, two-compartment prismatic cell. The counter electrode was a Pb–Sb–Sn–Ag alloy plate placed in the anodic compartment and the diaphragm was used to avoid the contamination of solution following formation of manganese oxides. Manganese was plated on stainless-steel plates with an active surface area of 14 cm2 (4 cm × 3.5 cm). The substrates were first mechanically polished with various grades of metallographic sandpaper and degreased with alkaline solution (23 g/L NaOH, 22 g/L Na2 CO3 , 10 g/L Na2 SiO3 (anhydrous), 10 g/L Na3 PO4 , 1 g/L sodium lauryl sulfate, 90–95 ◦ C, 3–5 min) and acetone. Subsequently, they were electropolished in concentrated phosphoric acid (85%) and pickled in mixed nitric (5%) and hydrochloric acid (25%) just before used. Simple sulfate electrolytes with addition of ammonium sulfate were used. pH was adjusted by adding concentrated ammonium hydroxide or sulfuric acid, but no attempt was made to control it during deposition. However, pH was measured and, if necessary, adjusted, after each deposition experiment. The basic bath contained 15 g/L MnSO4 , 100 g/L (NH4)2 SO4 , and the solutions were prepared with analytical grade reagents and double-distilled water. All experiments were carried out at 30 ◦ C and N2 was poured to solutions for 15 min in order to get rid of the dissolved oxygen before experiment. Electrochemical tests were all operated on CHI660 C electrochemical workstation. A conventional three electrode system was employed with a platinum foil as a counter electrode and a saturated Hg/Hg2 SO4 as the reference electrode [17]. The working electrode consisted of a stainless-steel rod designed to expose to the electrolyte an area of 0.4 cm2 . The cyclic voltammograms scans were measured at a rate of 20 mV/s and the scan rate of steady polarization was controlled at 1 mV/s. To minimize iR-drop effects, a Luggin capillary was employed to connect the reference electrode compartment to the working electrode. The current efficiency was calculated from the weight gained from the sample. Surface morphology was examined by scanning electron microscopy (SEM) using a FEI Quanta 200 instrument. Crystallographic structure was determined by X-ray diffraction
Fig. 2. SEM microphotographs of Mn electrodeposits obtained from a sulfate bath electrolysis for 30 min containing (a) 0 g/L SeO2 ; (b) 30 mg/L SeO2 and (c) 0.1 g/L SeO2 .
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Fig. 3. Electrochemical behaviors of manganese electrodeposition. (a) Curves of steady polarization, scan rate 1 mV/s. (b) Curves of cyclic voltammetry, scan rate 20 mV/s. (1) MnSO4 + (NH4 )2 SO4 ; (2) MnSO4 + (NH4 )2 SO4 + SeO2 .
(XRD) using X’ Pert PRO DY2198 diffractometer with Cu K␣ radiation. 3. Results and discussion 3.1. Crystal structure and morphology investigations The Mn electrodeposits were analyzed using XRD and SEM to determine the influence of SeO2 on the crystalline structure and morphology of the deposits. Manganese films were electrodeposited at different SeO2 concentrations, from 0 g/L to 0.1 g/L, at 360 A/m2 current density. With the increase of SeO2 concentration, the current efficiency increases from 50% to 83%, and the film thickness increases from 34 to 43 m. XRD was used to determine the crystalline structure and its patterns of manganese deposits were shown in Fig. 1. The main diffraction peaks due to ␥-Mn phase when selenium dioxide was absent. With increasing the concentration of SeO2 , the diffraction peaks of ␥-Mn become broader and the peak intensities of ␣-Mn increase obviously. At 0.1 g/L SeO2 , good ␣-Mn is obtained. At room temperature the ductile ␥-Mn gradually transforming into brittle and hard ␣-Mn was not exhibited for a relatively long period [18]. It can be concluded that increasing the amount of SeO2 could be a great boost for the conversion of manganese electrodeposit crystal from ␥-phase into ␣- phase. SEM surface micrographs of manganese deposits are shown in Fig. 2. Fine cubic crystallites and some pinholes are visible. With the increase of SeO2 , the quantity of the protrusions decreases and the grain size becomes smaller. It can be seen as the concentration of SeO2 increases to 0.1 g/L, the coatings are silvery leveling in appearance and refining, grain size about 2 m. The pinholes are attributed to the evolution of hydrogen bubbles. The relatively leveled coating and low grain size may be induced by the comparatively higher SeO2 concentration. The presented results indicate that SeO2 could be used as a leveling and refining agent. It may be due to the fact that Se content can increase cathodic overpotential.
different for the solutions with and without selenium dioxide as shown in Fig. 3a. An increase in the cathodic polarization was recorded when selenium dioxide was present in the electrolyte, in accordance with previously reported observations [11]. This could be explained by the increase in the hydrogen-evolution reaction overpotential induced by the presence of this additive, followed by the increase in the partial current densities for the electrodeposition of manganese. The cyclic voltammetry measurements for the electrolyte in the presence and absence of SeO2 are reported in Fig. 3b. The obvious difference is the large increase of the oxidation peak when SeO2 is present, which indicates that selenium dioxide could promote the deposition of manganese metal. On the basis of CV and LSV results, we can conclude that addition of selenium dioxide could promote manganese electrodeposition and inhibit hydrogen evolution reaction. 3.3. Hydrogen evolution reaction To determine how SeO2 can increase the hydrogen overvoltage, we carried out linear sweep voltammetry studies in the potential range of −0.6 to −1.8 V using solutions without manganese. Fig. 4
3.2. Electrochemical behaviors To evaluate the effect of SeO2 on the electrochemical behaviors of Mn, a study using cyclic voltammetry and linear sweep voltammetry in the Mn solutions without (curve 1) and with SeO2 (curve 2) was carried out. The cathodic linear polarization curves are
Fig. 4. Effect of SeO2 on the overvoltage of hydrogen, scan rate 1 mv/s. (A) SeO2 : 0 g/L, (B) SeO2 : 0.1 g/L and (C) electrode adsorbent material in SeO2 solution.
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Fig. 5. CV curve of SeO2 solution, scan rate 20 mV/s.
shows the steady polarization curves of iron electrodes in Solution A, Solution B and special work electrode in Solution A (Curve A, Curve B and Curve C respectively). Solution A was composed of 100 g/L−1 (NH4 )2 SO4 , while Solution B was obtained by adding SeO2 in Solution A and its concentration of SeO2 was 0.1 g/L−1 . To investigate whether SeO2 was adsorbed, the iron electrode was constant-potential electrolyzed for 3 min in Solution B at −1.6 V, removed and washed by double-distilled water. After the above series of processing, the electrode was used to make steady polarization curve in solution A which was defined Curve C. From the composition of the solutions, it is clear that the curves are the steady polarization curves of hydrogen evolution. Comparison of Curve B with Curve A indicates with the adding of SeO2 the potential of hydrogen evolution moves from −1.3 V to −1.5 V. Curve C is similar to curve B although the electrolytes are different. Hence, there can be no doubt Se species were adsorbed on the cathode surface and increase further the hydrogen overvoltage. 3.4. Cathodic reduction mechanism To study detailed mechanism of action of SeO2 in the electrodeposition process, several electrochemical methods including cyclic voltammetry, chronopotentiometry and cross-potential electrolysis were used. Cyclic voltammogram was run for a solution of SeO2 (5 g/L) in ammonium sulfate but without the presence of manganese(II) (Fig. 5). The major feature of the voltammogram is the presence of two reduction peaks (at −1.0 V and at −1.48 V respectively) and two oxidation peaks (at −0.9 V and at −1.47 V respectively). The reduction peak at −1.0 V is larger than the reduction peak at −1.48 V; the oxidation peak at −1.47 V is weaker than the oxidation peak at −0.9 V. Presumably the process of SeO2 reduction takes two steps, and the reduction product of the first step has not fully transformed into the product of the second step. This view is supported by the following fact of chemical analysis. When the stainless steel cathode was constant-potential electrolyzed at −1.0 V in Solution B, a kind of red deposit was generated. As a substitute −1.48 V for −1.0 V, orange-red solutions generated near the cathode. When adding a small amount of sulfuric acid in the solution and heating it, there would appear the same red deposit and in the meantime the solution became colorless gradually and released a nasty smell of H2 Se gas. XRD image of the red deposit is shown in Fig. 6. The peak data confirm the deposit is Se (0) crystalline phase (JCPDS 00-42-1425). Therefore, it is indicated that Se (IV) would first reduce to Se (0) and part of Se (0) future reduce to selenide at more negative potentials and that, the selenide might combine with the remainder Se (0) to form complicate compounds
Fig. 6. XRD patterns of red deposit generated by constant-potential electrolyzed Solution B at −1.0 V.
(multi-selenium ions). SeO2 dissolved in water quickly transforms into SeO3 2− . The overall reaction equation of SeO2 reduction is suggested as follows + 3xOH− 3xH+ + xSeO3 2− + 4(x + n/2)e = Se2n− x
(1)
3.4.1. Adsorption behaviors of the reduction products of SeO2 According to Fig. 5, we conclude the first step reaction is irreversible due to the fact that the potential range between its reduction peak (−1.0 V) and its oxidation peak (−0.9 V) is 100 mV which is far larger than that of reversible process (59.2/4 = l4.75 mV)[19]. The following reaction is proposed: 3xH+ + xSeO3 2− + 4xe = Sex + 3xOH−
(2)
If Sex has an adsorption behavior and complies with Langmuir isotherm, the Faraday current of the electrochemical reaction is as follows [20]:
˛nF(ϕ+vt) i ˇnF(ϕ+vt) −kc0 exp − = ka0 [SeO3 2− ](1−)exp RT RT nF
(3) where is the Sex coverage on the cathode; ϕ is the original potential; v is the scan rate. As the first step reaction is irreversible, the second item could be neglected. Eq. (3) is simplified to: i = nFka0 [SeO3 2− ](1 − )exp
˛nF(ϕ + vt)
(4)
RT
The expression of the reciprocal of the time derivative response of Eq. (4) is given by:
˛nF(ϕ + vt) di = nFka0 [SeO3 2− ]exp RT dt
˛nF RT
(1 − )v −
d dt
(5)
with the boundary conditions:At the peak current (di/dt = 0), Eq. (6) is obtained. ˛nF d (1 − )v = RT dt
(6)
If r is presumably for the demand power of the form of monolayer adsorption per unit area, the Faraday current of the electrochemical reaction is as follows: i=r
d dt
(7)
From Eqs. (6) and (7) ip =
˛nF (1 − )r v RT
(8)
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The oxidation peak at −1.0 V is presumably for the reaction Se (0) → Se (IV): (z + y)Se − 4e = (z + y)Se(IV),
(11)
where z is the molar of Se (0) in the adsorptions. When the transition time is 2 , the required electricity is i 2 . 2 contains the oxidation transition time of both Se (0) oxidized from Se2− and original Se (0). 1 and 20 are supposed to be the oxidation transition times of Se (0) oxidized from Se2− and original Se (0), respectively. Then at any time, one has 2 = 20 + 1
(12)
z follows z=
i 0 it = 2 nF 4F
(13)
The oxidation of Se2− follows the law of mass conservation i1 i 1 = 2F 4F
Fig. 7. The first peak current curve of SeO2 reduction affected by scan rate.
(14)
hence In order to investigate whether Sex has an adsorption behavior, the curve of the first peak current of SeO2 reduction (ip ) with different scan rate (v) was plotted (Fig. 7). The ip vs. v plot is a straight line through the origin which is linked to the absorption of irreversible electrochemical product on the cathode [21]. 3.4.2. Composition of adsorptions [22] In order to study the second step reaction (the reduced product’s adsorption behavior and its composition), electrochemical chronopotentiometry and cross-potential electrolysis were carried out. Fig. 8 shows the anode potential–time curve which is obtained in Solution A by iron electrode which has been electrolyzed for 3 min in Solution B at constant −1.6 V. As the current sets 0.04 mA, there are two plateaus on the potential–time curve. The first platform is located at −1.48 V with the transition time 1 ( 1 = 0.33 s); the second one is located at −1.0 V with the transition time 2 ( 2 = 3.67 s). The oxidation peak at −1.48 V is assumed for the reaction Se2− → Se (0): ySe
2−
− 2e = ySe,
(9)
where y is the molar of Se2−
in the adsorptions. When the transition time is 1 , the required electricity is i 1 . Then y follows y=
i1 it = . nF 2F
(10)
1 = 21
(15)
Then, by substituting 1 by the value 2 1 in Eq. (12), one has 2 = 20 + 1 = 20 + 21
(16)
20 = 2 − 21
(17)
Eq. (13) has the general solution: z=
i20 4F
=
i (2 − 21 ) 4F
(18)
z 2 − 21 = 4.5 = 21 y
(19)
z = 4.5y
(20)
Therefore, the abbreviation of the adsorption composition is Se4.5y Sey 2y− . When y = 1, Se4.5y Sey 2y− becomes Se5.5 2− . 4. Conclusion Several material characterization and electrochemical methods were employed for the goal of understanding the mechanism of the action of SeO2 additive in manganese electrodeposition. Research indicates that SeO2 follows the following reduction and adsorption mechanism: Firstly, SeO2 is reduced to Se (0) and Se (0) has an adsorption effect on the cathode. Then a portion of Se (0) is consumed by the formation of Se2− ; the remainder is available to combine with Se2− to form multi-selenium ions (Se4.5y Sey 2y− ) which also have an adsorption effect on the cathode. The process of SeO2 has a marked influence on the electrochemical behaviors of manganese electrodeposition. The influence includes inhibiting hydrogen evolution reaction and promoting the deposition of manganese. Simultaneously, SeO2 could accelerate the conversion of manganese electrodeposit from ␥-phase crystal into ␣-phase crystal and facilitate manganese coatings into leveled and refined. Acknowledgments
Fig. 8. The anode potential–time curve of adsorbed substance on cathode.
We are grateful to the National Natural Science Foundation of China (Grant no. 50904054) for the financial support. The project was also supported by the “Fundamental Research Funds for the Central Universities”.
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