Electrochemical reduction mechanism of several oxides of refractory metals in FClNaKmelts

Abstract The dissolution characteristics and electrochemical reduction mechanism of oxides of refractory metals ZrO2, HfO2 and MoO3 in NaCl-KCl-NaF melts are studied. The results shows that there are no chemical reaction of ZrO2 and HfO2 in NaCl-KCl-NaF melts, the dissolution of MoO3 is chemically dissolved, and MoO3 reactwith melts to form Na2Mo2O7. The reduction process of zirconium in the NaCl-KCl-NaF-ZrO2 melts is a reversible process of one-step electron transfer controlled by diffusion. The electrochemical reduction process of ruthenium is a one-step reversible process and the product is insoluble; Electrochemical reduction of metallic molybdenum in melts is controlled by the diffusion and electron transfer process of active ion Mo2O27 − . The electrochemical reduction process of the metal molybdenum in the melts is carried out in two steps.


Introduction
Oxides of refractory metals have good high temperature strength, good corrosion resistance to molten alkali metals and vapors, and have wide applications in some special fields. The main research methods for refractory metal production are melts electrolysis, FFC (Fray-Farthing-Chen) [1], SOM (Solid oxygen-ion membrane) [2], OS (One-Suzuki) [3], etc. Melts electrolysis is mainly used to produce metal elements whose electrode potential is much more negative than hydrogen. These metals cannot be produced by aqueous solution electrolysis, including light metals, refractory metals, rare earth metals and certain non-metals. The traditional melts electrolysis method is to dissolve the mixture in melts for electrolysis, and the basic idea is derived from electrolytic aluminum [4]. Finding the electrolyte based on cryolite is the key to the ultimate industrialization of electrolytic aluminum. However, some metal oxides have low solubility in melts. For example, the solubility of Ta 2 O 5 in K 2 TaF 7 -Ta 2 O 5 /KCl-KF melts is only 0.2%~0.3% (mass fraction) [5]. Fray, Farthing, and Chen proposed the FFC method for the direct electrowinning of titanium by titanium oxide [6].
Based on the above studies, Chen et al. [6], Elena et al. [7], Chen et al. [8], and Schwandt and Fray [9] prepared metal chromium, titanium and titanium alloys; Yan et al. [10][11][12] prepared metal ruthenium; Hironori, Toshiyuki and Rika obtain a coating of metallic molybdenum from ZnCl 2 -NaCl-KCl-MoCl 3 of melts by electroplating, but the coating is thin, easy to fall off and there are a large number of cracks; under the same conditions, KF is added to the melts to obtain a thick and dense Metallic molybdenum plating [13]. Preparation of some metals by NaCl-KCl-NaF melts as a base salt have also been reported [14][15][16][17]. In this paper, the electrochemical behavior of ZrO 2 , HfO 2 and MoO 3 in the melts was analyzed by using NaCl-KCl-NaF melts as the base salt.

Experimental
Analytically pure NaCl, KCl, NaF, ZrO 2 , HfO 2 , and MoO 3 were selected for the experiment. The chemical reagents were first ground into powder in a mortar, and then dried in an electric oven at 200 ∘ C for 8 h to remove adsorbed water and crystal water, and then cooled for use.
Solubility experiments were carried out using a tubular resistance furnace. NaCl, KCl and NaF were well mixed and the molar ratio was 1:1:0.86. ZrO 2 , HfO 2 , MoO 3 was added at 10% of the total mass, and placed at the bottom of NaCl-KCl-NaF melts. Moved the salt-containing mash to the furnace and the temperature was heated to 700 ∘ C, and the furnace was protected by high purity argon gas (99.999%). At different times, the upper salt of the melts was sampled using a quartz tube. The extracted melts was quenched and ground to a powder, and subjected to X-ray fluorescence spectroscopy (XRF, AXIOS, PANalytical B.V.) analysis [18,19]. The amount of oxide dissolved in the meltswas calculated according to the formula [20]: Where w MeO2 is the amount of oxide dissolved; w Me is the amount of metal dissolved; M Me is the relative molecular mass of metal; M MeO2 is the relative molecular mass of oxide. NaCl, KCl and NaF were well mixed and the molar ratio was 1:1:0.86. Then, 0.044% ZrO 2 , 0.111% HfO 2 , and 10% MoO 3 were added in a mass ratioon the bottom of the file. The salt was placed in a high-purity graphite crucible and placed in a tubular resistance furnace at 200 ∘ C for 10 h. Then, the temperature was raised to 700 ∘ C for 4 h, protected by high purity argon gas, and temperature was measured by a S-type thermocouple. All electrochemical experiments were performed using a Zahner IM6ex electrochemical workstation using a three-electrode system with platinum wire (99.99%) for the working (Φ0.5 mm, immersion depth is 1 cm), auxiliary (Φ 1 mm, immersion depth is 2.5 cm) and reference (Φ0.5 mm, immersion depth is 1cm) electrodes. Figure 1 showed the dissolution characteristics of ZrO 2 and HfO 2 in a NaCl-KCl-NaF melts. It could be seen from Figure 1 that ZrO 2 and HfO 2 were substantially unchanged in the NaCl-KCl-NaF melts at 700 ∘ C for 3 h. The XRD results of the upper melts at 700 ∘ C were NaCl, KCl, NaF and HfO 2 .

Dissolution characteristics
No new material formation indicated that ZrO 2 and HfO 2 did not react chemically in the NaCl-KCl-NaF melts.
At 700 ∘ C, Figure 2 was an XRD and laser Raman spectroscopy analysis of MoO 3 in a NaCl-KCl-NaF melts. It couldbe seen from Figure 2(b), molybdenumoxy groups were present at 340 cm −1 , 375 cm −1 , 472 cm −1 , 665 cm −1 , 810 cm −1 , 844 cm −1 , 978 cm −1 , 992 cm −1 , and 1022 cm −1 . The stretching vibration peak was made of MoO 3 or Na 2 Mo 2 O 7 , which was consistent with the results of references [21][22][23][24]. It could be seen from Figure 2(a) that there were four substances of NaCl, KCl, NaF and Na 2 Mo 2 O 7 in the melts of NaCl-KCl-NaF-MoO 3 , but no MoO 3 existed. Combined with the thermodynamic analysis of the melts, the standard Gibbs free energy ∆G θ < 0 about the reaction of NaCl, NaF and MoO 3 at 700 ∘ C. The reaction could be spontaneously carried out, and Na 2 Mo 2 O 7 was formed. Therefore, the stretching vibration peak of the molybdenumoxy group was Na 2 Mo 2 O 7 . The dissolution of MoO 3 in the NaCl-KCl-NaF melts was mainly chemical dissolution.   Figure 3 was a cyclic voltammetry curve of ZrO 2 , HfO 2 and MoO 3 in NaCl-KCl-NaF melts at 700 ∘ C. Figure 3 (1) was a cyclic voltammetry curve of NaCl-KCl-NaF melts. When the scanning range was −2 V~0.3 V, an oxidation peak appears at −1.65 V during forward scanning, which corresponded to the oxidation process of sodium. As the potential changed, an increased in current occurs at 0.6 V, which corresponded to the precipitation of gas. In the reverse scan, the reduction peak that occurredat −2.0 V was the sodium reduction process. Sodium reduction peak at −2.0 V during reverse scan. Figure 3(2) showed the cyclic voltammetry curve of ZrO 2 in NaCl-KCl-NaF melts. A pair of redox peaks a'/a (−0.82 V/−0.96 V) appeared which corresponding to the redox process of zirconium. Figure 3(3)   Table 1 showed the relationship between theoretical electrolytic potentials E θ and T of ZrO 2 and HfO 2 at 700 ∘ C calculated by HSC 6.0 thermodynamics software. The more positive theoretical electrolysis potential occurred, the more reduction reaction happened. The ionic state of Hf was Hf 4+ and Hf 2+ . The Hf stepwise reactions were Hf 4+ /Hf 2+ (−4.80V) and Hf 2+ /Hf (−0.05V), respectively. The potential of Hf 4+ reduced to Hf (−2.43V) was more positive than the potential of Hf 4+ reduced to Hf 2+ (−4.80V), indicating that Hf 4+ was reduced by a one-step reduction process, that consistent with the cyclic voltammetry results of Hf showed in Figure 3(1). The ionic state of Zr was Zr 4+ , and the reduction of Zr 4+ to zirconium metal was a one-step reaction process, which was consistent with the cyclic voltammetry curve of Zr shown in Fig-ure

Electrochemical experiment and electrodeposition producted analysis
From the results of Figure 2, it showed that the dissolution of MoO 3 in the NaCl-KCl-NaF meltswas chemically dissolved, and the product was Na 2 Mo 2 O 7 . The reaction of Na 2 Mo 2 O 7 from Mo 6+ to metallic molybdenum was the formula (1)~(6). Figure 4 was a graph showing the relationship between E θ and T in reactions (1) to (6) at 500 to 1000 ∘ C. At 700 ∘ C, the potentials of Mo 6+ /Mo 4+ corresponding reactions (1) and (2) are −0.62V and −1.68V. The theoretical electrolysis potential of reaction (1) was more positive than reaction (2), so reaction (1) was more priority. And Mo 2 O 2− 7 was decomposed into MoO 2− 4 and Mo 4+ . The theoretical electrolysis potential of the one-step reduction (Mo 6+ /Mo) corresponding to reaction (3) was −1.27 V, which was more negative than the theoretical electrolysis potential of Mo 6+ /Mo 4+ corresponding to reaction (1), and the process of reduction of Mo 6+ to metallic molybdenum was a stepwise reaction. The reaction of MoO 2− 4 reduced to metallic molybdenum and MoO 2 was (4) and (5). The theoretical electrolytic potential of reaction (5) was −2.12 V, which was more negative than −1.68V of reaction (4). Therefore, MoO 2− 4 was first reduced to metallic molybdenum. The equilibrium electrode potential of the reaction (6) was Mo 4+ /Mo(−1.06V). As shown in Figure 3 (3 Figure 5(a) were cyclic voltammetry curves at different sweep speeds. The peak current gradually increased with the scanning speed increases, which was because the higher the scanning speed, the higher the electrochemical Figure 4: Relationship between the theoretical electrolysis potential E θ and T of MoO 3 at 500~1000 ∘ C reaction speeds. The relationship between ipc and v 1/2 was derived from the data in Figure 5(a). And ipc had a linear relationship with v 1/2 , and the reduction peak potential Epc did not change with the change of the scanning speed. As could be seen from Figure 5(a), ipa /ipc > 1. Therefore, the electrode reaction corresponding to the reduction peak on the cyclic voltammetry curve was a reversible reaction and the product was insoluble, that was, the reduction process of the cathode was completed under diffusion control. The relationship between the reversible reaction Epc and n satisfied the equation: Epc − E pc/2 = −0.77(RT/nF) Where Epc (V) is the peak potential and E pc/2 (V)is the half-peak potential; n is the electron transfer number and F (96485 C·mol −1 ) is a Faraday's constant; R (8.314 J·mol −1 ·K −1 ) is the molar gas constant, T(K) is the temperature.
The average value of the number n of reaction electrons corresponding to the reduction peak in Figure 5(a) was calculated to be about 4. Therefore, it could be concluded that the Zr 4+ ion in the meltswasreduced to elemental zirconium in one step, so the cathode reaction on the CV curve was: Zr 4+ + 4e=Zr.
4h constant potential electrolysis was carried out at −1.15V to obtain a material having a metallic luster on the surface, and the energy spectrum analysis product was mainly zirconium. It was indicated that the reduction peak appearing at a potential of −0.935V corresponded to the precipitation process of Zr.    Figure 7(a). It could be seen from Figure 7(b) that ipc had a linear relationship with v 1/2 , and the reduction peak potential Epc did not change with the change of the scanning speed. ipa /ipc > 1 was showed in Figure 7(a), therefore, the electrode reaction corresponding to the reduction peak of the cyclic voltammetry curve was reversible and the product was insoluble, that was, the reduction process of the cathode was completed under diffusion control. The calculated value of the number n of reaction electrons corresponding to the reduction peak in Figure 7(a) was about 4. Therefore, the reduction process of HfO 2 in NaCl-KCl-NaF meltswas a one-step electronic reaction, Hf 4+ +4e = Hf.
The potentiostatic electrolysis was carried out under the conditions of −1.3 V and 4 h, and the surface energy spectrum of the electrolysis product was analyzed. The re-sults were shown in Figure 8. From the energy spectrum, the main substance of the deposited layer was Hf. Figure 9 showed the cyclic voltammetry curve of NaCl-KCl-Na-MoO 3 as the scanning speed changes. As could be seen from Figure 9(a), as the scanning speed increased, the peak current density gradually increased because the electrochemical reaction rate was affected by the scanning speed. Using the data in Figure 9(a), the relationship between ipc~v 1/2 and Epc~v 1/2 was obtained, as shown in Figure 9(b). The ipc of peak c was linear with the change of v 1/2 , and the potential of peak c'/c moved to the positive and negative directions of the potential with the increase of scanning speed. The scanning speed increased, and the peak c' gradually disappeared and changed towards irreversible. Therefore, the number of electrons lost could be calculated from the irreversible reaction. As the scanning speed v increased, the Epa and Epc of the peak d'/d moved to the positive and negative directions respectively, and ipc and v 1/2 did not have a linear relationship. Therefore, the peak d was a quasi-reversible reaction process. The potential Epc of the peak e moved in the negative direction of the potential with the increase of the scanning speed v, ipc was linear with v 1/2 , and there was no corresponding obvious oxidation peak, so the peak e was an irreversible reaction process.
According to the values of current density ip and potential Epc corresponding to each scanning rate peak c in Figure 9(a), a log ip vs. Epc was obtained in Figure 10.
As the scanning speed increased, the cathode peak potential Epc gradually shifts to the negative direction and had a linear relationship with the sweep speed lnv, as shown in Figure 10. For the irreversible process Ep~lnv sat-isfied the following formula [25,26]: Where α is transfer coefficient, k 0 is standard rate constant of the reaction, n is electron transfer number involved in the rate-determining step, v is san rate, E 0 ′ is formal potential, Epc is the reduction peak potential, Epa is the oxidation peak potential (T=973K, R=8.314 J·mol −1 ·K −1 , F=96485 C·mol −1 ) According to the above equation and the slope of the linear relationship of Ep~lnv in Figure 10, the symmetry factor α was calculated as 0.2685. n was calculated as 2.23 ≈ 2. Therefore, the peak c corresponds to the 2 electron reduction process. It was proved that the reaction (1) corresponds to a 2-electron process in which Mo 2 O 2− 7 was reduced to MoO 2 . From the nonlinear relationship of the peak d in Figure 9(b), the number of electrons to be lost could not be calculated by the formula. However, molybdenum in MoO 2 was tetravalent, and therefore, the reaction corresponding to peak d was a 4 electron reduction process of Mo 4+ → Mo.
Using graphite as anode and low carbon steel as cathode, the temperature was set to 700 ∘ C, the molar ratio of melts system was NaCl: KCl: NaF = 1:1: 0.86, MoO 3 (mass fraction is 20%), and the mixture was evenly mixed. In pure corundum, the current density was 100mA·cm −2 and the electrodeposition time was 60 min. After deposition, the surface of the substrate was loose and porous. From the XRD results, it was found that metal molybdenum was formed after electrolysis.

Conclusions
The dissolution about oxides of refractory metals ZrO 2 , HfO 2 and MoO 3 in NaCl-KCl-NaF melts was studied at 700 ∘ C. The electrochemical reduction mechanism of oxides of refractory metals in NaCl-KCl-NaF melts obtained by cyclic voltammetry analysis, the following conclusions were obtained: 1. When the molar ratio of NaCl, KCl and NaF was 1:1:0.86, ZrO 2 and HfO 2 did not undergo chemical reaction; MoO 3 was chemically dissolved, and MoO 3 mainly formed Na 2 Mo 2 O 7 . 2. The reduction process of zirconium in melts was a reversible process of one step and four electron transfer controlled by diffusion, and the reduction reaction was: Zr 4+ +4e − =Zr; The electrochemical reduction process of hafnium in melts was a one-step reversible process, and the product was insoluble. The reduction mechanism was: Hf 4+ +4e − → Hf; In melts, the electrochemical reduction of metallic molybdenum was controlled by the diffusion and electron transfer process of the active ion Mo 2 O 2− 7 . The electrochemical reduction process of metallic molybdenum in the melts was divided into two steps. The process was as follows: Mo 2 O 2− 7 + 4e = 2MoO 2 + 3O 2− MoO 2 + 4e = Mo + 2O 2−