Anodic Dissolution of Titanium Oxycarbide TiC_xO_1-x with Different O/C Ratio

TiC_xO_1-x with different O/C ratio (TiC_0.7O_0.3, TiC_0.5O_0.5, TiC_0.45O_0.55, TiC_0.4O_0.6, TiC_0.33O_0.67 and TiC_0.2O_0.8) were synthesized from titanium monoxide (TiO, Beijing Mengtai Corp.) and titanium carbide(TiC, Beijing Mengtai Corp.) with different TiC:TiO ratio (7:3, 1:1, 9:11, 2:3, 1:2 and 1:4). Titanium monoxide and titanium carbide were homogenously mixed and pressed to form a pellet with 10 mm in diameter and 10 mm in height. The pellet was then sintered at 1600 °C for 4 hrs under Ar atmosphere. The schematic diagram of the whole process is shown in Fig. S1.

The composition, crystal structure, and the morphology of the product before and after being sintered were analyzed by X-ray diffraction (XRD, JEOL JDX-3500, Cu Ka radiation) and scanning electron microscopy (SEM, JEOL JXA-8900M), respectively.

The electrochemical measurement was carried out in an equimolar NaCl - KCl solution (reagent grade, Beijing Chemical Industries). The salt was pre-dried in a graphite crucible under vacuum at the elevated temperature of 300°C for 4 hours to remove moisture. All electrochemical experiments were conducted in a sealed vessel under a dry helium atmosphere at 750°C.

The electrolysis and electrochemical experiments were performed using a potentiostat /galvanostat (EG&G PAR 263 A). A pellet of TiC_xO_1-x with 3 mm in diameter is served as a working electrode, and a graphite (6 mm in diameter) rod is served as a counter electrode. The reference electrode was the Ag/AgCl electrode, which consisted of a silver wire (1mm in diameter) in contact with the solution of AgCl (4 wt%) in a NaCl-KCl molten-salt mixture, contained in a Mullite tube. The reference electrode was calibrated against chlorine evolution, and all the potentials were recorded with respect to the potential of Cl₂/Cl⁻ electrode.

During the potentiostatic electrolysis, the emitted gas from the anode was analyzed online using a mass spectrometer. Electrochemical experiments including linear polarization and square-wave voltammetry were performed under the same conditions as the electrolysis. For these measurements a tungsten disk electrode with 0.1 mm in diameter was used as a working electrode. The tungsten disk electrode was prepared by sealing a tungsten wire into a quartz tube, and polishing the bottom part as the electrode surface. A schematic diagram of the electrochemical cell and the tungsten disk electrode is shown in Fig. S2(a) and S2(b). The anodic gas generated during dissolution was monitored by mass spectrometer with a special instrument shown in Fig. S2(c) and (d). The TiC_xO_1-x solid solution pellet was connected to a platinum wire as working electrode. A quartz sheath with 10mm in diameter and 50mm in height was used as a shell of the anode, and the anodic gas generated during the electrolysis was collected inside the shell. An alumina tube (3.2mm in diameter, 99.8%, McDanel Advanced Ceramic Technologies) with double bores (1mm in diameter) was used to hold the working electrode. One bore is used for the passing through of platinum wire as the current lead whereas the other one for the anodic gas to flow to the mass spectrometer. As shown in Fig. S2(c), some space between the quartz sheath and the alumina tube allows the helium gas to flow into the anode chamber. The anodic gas released during electrolysis was sampled through the alumina tube. The sampled anodic gas was analyzed for the concentration of CO, CO₂, and O₂ by online gas mass spectrum (MS, HPR20, Hiden Analytical).

XRD was used to provide composition and assignment of crystal structure information of TiC_xO_1-x. The XRD patterns of TiC_xO_1-x with different O/C ratio are shown in Fig. 1a and Fig. S3. From Fig. 1a, it is clear that the peak positions of TiC, TiC_0.7O_0.3, TiC_0.5O_0.5, TiC_0.45O_0.55, TiC_0.4O_0.6, TiC_0.33O_0.67 and TiC_0.2O_0.8 shifted with the changing of O/C ratio in TiC_xO_1-x. With the increasing of O/C ratio in TiC_xO_1-x, the peaks shifted step by step from that of TiC to TiO. It indicates that all these TiC_xO_1-x solid solution are formed after sintering, and they have the same structure as TiC and TiO. This is confirmed by the SEM analysis (Fig. 1, and Fig. S4). Figure 1 (b1) and (c1) presented SEM images of the raw materials (TiC-TiO mixture) with different ratio, 7:3, and 1:4, respectively. Figure 1 (b2) and (c2) are the corresponding samples after the sintering. Compared with the comparatively coarse and heterogeneous raw materials, the TiC_xO_1-x solid solutions are homogenous and compact, indicating the complete formation of TiC_xO_1-x solid solution. And the significant particle size growth was also observed from the raw materials TiC and TiO to the TiC_xO_1-x solid solution.

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Linear sweep voltammetry was used to explore the anodic dissolution behavior of TiC_xO_1-x solid solution, and the corresponding results are shown in Fig. 2. The anodic polarization curves of Ti metal, TiC_0.7O_0.3, TiC_0.5O_0.5, TiC_0.45O_0.55, TiC_0.4O_0.6, TiC_0.33O_0.67 and TiC_0.2O_0.8 indicated the similar dissolving phenomena. With the swept of potential less than −1.2 V vs. Cl₂/Cl⁻, the current were almost zero, indicating no electrochemical reaction taking place. As the potential is raised beyond −1.2 V vs. Cl₂/Cl⁻, the current density increased rapidly, attributed to the occurrence of electrochemical dissolution of TiC_xO_1-x solid solution. With respect to TiC_xO_1-x, the current density on glassy carbon electrode gradually increases when the potential is higher than 0 V vs. Cl₂/Cl⁻, which corresponds to the generating of Cl₂ gas. These results proved that TiC_xO_1-x has the similar anodic dissolution behavior as titanium metal. However, the slopes of anodic polarization curves of Ti metal, TiC_0.7O_0.3, TiC_0.5O_0.5, TiC_0.45O_0.55, TiC_0.4O_0.6, TiC_0.33O_0.67 and TiC_0.2O_0.8 are different. The slopes of the anodic polarization curves of Ti metal, TiC_0.7O_0.3, TiC_0.5O_0.5 are similar, whereas from TiC_0.45O_0.55 the slope decreases gradually with the increase of oxygen content in the oxycarbide.

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In order to investigate the effect of O/C ratio in TiC_xO_1-x on the dissolving product including titanium ions and the anodic gas, the electrochemical methods and mass spectrometer were both used. Because titanium ions can exist in multiple valence state, titanium ions with different valence states could be dissolved into melt under various potentials. Square wave voltammetry (SWV) was adopted to analyze the valence of dissolved titanium ions during electrolysis. TiC_xO_1-x solid solutions were electrochemically dissolved into melts at different potentials of −0.6 V, −0.4 V and −0.2 V vs Cl₂/Cl⁻, respectively, and each dissolution of the TiC_xO_1-x solid solutions lasts the same time, i.e.,1.5 hrs. Figure 3 presents the square wave voltammograms recorded in the salts after electrochemically dissolving TiC_xO_1-x solid solutions under various potentials. The exchange electron number calculated according the square wave voltammograms is shown in Table I. It is evident that the average exchange electron number changes from 2.0 to 2.3 with the increase of electrolysis potential. This implies that TiC_xO_1-x has been dissolved into the melt during electrolysis mostly as Ti²⁺ ions at −0.6 V, and partially as Ti³⁺ when the electrode potential is raised. Clearly, the electrolysis potential demonstrated significant effect on the valence state of titanium ion, whereas the composition of TiC_xO_1-x (O/C ratio) does not provide any contribution to that of titanium ion. However, the O/C ratio might affect the anodic tail gas. According to our previous results,^19–21 it was discovered that the anode reaction of TiC_0.5O_0.5 can be described as Eq. 1, during which Ti dissolves into chloride melt as Tiⁿ⁺ (n = 2∼3), while C and O forms CO without any C or O left in anode. It would be ideal if during the anode reaction of TiC_xO_1-x with different composition (C/O ratio), all Ti dissolves as titanium ion, Tiⁿ⁺, while C and O leaves as C, CO, CO₂, and O₂ without any residual. Based on this ideal assumption, the anode reaction of TiC_xO_1-x could be described as follows, depending on the O/C ratio.

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O/C≤1, x≥0.5 (typically as TiC_0.7O_0.3 and TiC_0.5O_0.5)

1<O/C≤2, 0.33≤x<0.5 (typically as TiC_0.45O_0.55, TiC_0.4O_0.6 and TiC_0.33O_0.67)

O/C>2, x<0.33 (typically as TiC_0.2O_0.8)

To test the above assumption on the anodic dissolution mechanism, the anode gas during the electrolysis was analyzed by the mass spectrometer online when the TiC_xO_1-x was under electrolysis by applied a constant potential step. Figure 4 shows the monitored concentration of CO and CO₂ in anode chamber before, during and after the electrolysis at the potential of −0.4V vs Cl₂/Cl⁻, for the TiC_xO_1-x. In Fig. 5, the average concentration of CO and CO₂ during the electrolysis is plotted against the applied potential. CO was the only anode gas during dissolution for TiC_0.7O_0.3 and TiC_0.5O_0.5. With the change of O/C ratio in TiC_xO_1-x, both of CO and CO₂ were detected for TiC_0.45O_0.55, TiC_0.4O_0.6, TiC_0.33O_0.67 and TiC_0.2O_0.8. During the electrolysis of all TiC_xO_1-x solid solution, no O₂ was found in the anodic gas. It is important to note that both CO and CO₂ were found simultaneously even at very low potential. The concentrations of both CO and CO₂ in the anode increase with the increase of the applied potential. This corresponds well with the current rise in the polarization curve in Fig. 2. As we only see the gas generation of CO in TiC_0.7O_0.3 and TiC_0.5O_0.5, the anodic reaction of high carbon content TiC_xO_1-x (O/C≤1, x≥0.5) most likely follow the Eq. 3. However, for the high oxygen content TiC_xO_1-x (O/C>2, x<0.33), the anode reaction obviously does not follow the Eq. 5 since we have never seen O₂ generation. In the middle range of carbon content, 1<O/C≤2, 0.33≤x<0.5, both CO and CO₂ were detected, corresponding with Eq. 4. However, the concentration of CO₂ in the anode gas was far lower than that of CO. CO was always the major component in the anodic gas in the entail range of oxygen content in TiC_xO_1-x, even for TiC_0.33O_0.67 and TiC_0.2O_0.8. Obviously, for the high oxygen content TiC_xO_1-x (O/C>1, x<0.5) part of oxygen content does not leave as the gas product.

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In Fig. 6a the concentration ratio of CO₂ to CO in anodic gas is shown as the function of the applied potential during the electrolysis. It is interesting that the CO₂/CO ratio is always kept around the same value for a certain carbon (oxygen) content TiC_xO_1-x, regardless of the applied potential. However, the O/C ratio in TiC_xO_1-x played a significant role on the CO₂/CO ratio in the anodic gas. CO₂/CO ratio increases with the increase of O/C ratio in TiC_xO_1-x anode material itself. In Fig. 6b, O/C ratio in anodic gas (CO, CO₂) is plotted as the function of the O/C ratio in TiC_xO_1-x anode material. In this figure, the blue dot line shows the theoretical value if the anode reaction follows the Eqs. 3–5. Other than the point of O/C = 1:1, the O/C ratio in anodic gas is always smaller than that in TiC_xO_1-x anode material, and such deviation becomes even larger as the ratio increases further. This implies that O in TiC_xO_1-x is consumed incompletely in the range of O/C ratio higher than 1:1. In order to analyze the by-product other than the gas species during the anode reaction, a four-hour electrolysis was carried out on TiC_0.7O_0.3, TiC_0.5O_0.5, TiC_0.4O_0.6 and TiC_0.2O_0.8. The anode after electrolysis was analyzed by XRD, and the results are shown in Fig. 7. Carbon was seen after long time electrolysis for TiC_0.7O_0.3. This result confirms again that for high carbon content TiC_xO_1-x (O/C≤1, x≥0.5), the anode reaction follows the Eq. 3, and that all Ti dissolves as titanium ion Tiⁿ⁺, while carbon and oxygen form as CO and C. For the low carbon (high oxygen) content anode TiC_xO_1-x (O/C>1, TiC_0.4O_0.6 and TiC_0.2O_0.8), Ti₂O₃ was observed as the by-production after the electrolysis. This means that part of Ti in TiC_xO_1-x has not dissolved as ion, and, at the same time, part of oxygen has not left as gas species. The anode reaction for low carbon content TiC_xO_1-x (O/C>1) does not follow Eqs. 4 and 5. According to the results above and the mass balance of the elements, the anode reaction for low carbon, high oxygen content TiC_xO_1-x (O/C>1) could be described as follow, where z is the dissolution ratio of titanium as ion.

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The valance of dissolved titanium ion, n is electrode potential dependent, the higher the potential applied, the higher the valance ion dissolved. However, n is not the function of the anode material composition. On the other hand, the anode material composition, and O/C ratio in TiC_xO_1-x, shows the significant effect on the anode gas composition. As seen in Fig. 6a, the concentration ratio of CO₂ to CO, is not potential dependent, but is a function of composition of the anode TiC_xO_1-x. Figure 8a shows CO₂/CO ratio as the function of O (TiO) concentration in TiC_xO_1-x. The CO₂/CO ratio increases almost linearly with O concentration in the anode material. The dissolution ratio, z can be calculated from the CO₂/CO ratio, and is shown in Fig. 8b. The dissolution ratio decreases linearly with the increase of the oxygen content.

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TiC_xO_1-x solid solutions with different O/C ratio demonstrated the similar dissolution behavior under electrolysis in NaCl-KCl. During the electrolysis, Ti was dissolved into molten salt as titanium ion, Tiⁿ⁺, and the valance of the dissolved titanium ion, n is dependent on the applied potential. Carbon and oxygen were released as carbon, carbon monoxide and carbon dioxide depending on the O/C ratio in TiC_xO_1-x. For high carbon content TiC_xO_1-x (x≥0.5, O/C≤1), all titanium dissolved as the titanium ion, whereas carbon and carbon monoxide CO were the by-product of the anode reaction. For low carbon content TiC_xO_1-x (x<0.5, O/C>1), the anode gas contained CO and CO₂, most of the titanium dissolved as titanium ion, whereas part of titanium formed as Ti₂O₃ without dissolution. For the same content, TiC_xO_1-x, the concentration ratio of carbon dioxide to carbon monoxide, CO₂/CO, stayed constant regardless of the applied potential. With the increase of O in TiC_xO_1-x the CO₂/CO ratio increases and the dissolution ratio of titanium as titanium ion, z, decreases linearly.

We thank the support from National Nature Science Foundation of China (51725401, 51874228, 51401157).

Shuqiang Jiao 0000-0001-9600-752X

Hongmin Zhu 0000-0001-9747-4533