Upcycling Spent Selective-Catalytic-Reduction Catalyst to Produce Titanium Carbide Through Molten-Salt Electrolysis

Abstract

The molten-salt electrolytic method was employed to recycle spent SCR catalyst to prepare TiC compound. A systematic investigation has been carried out through thermodynamic calculation and experimental analysis. The effects of graphite content, cell voltage, electrolyzing temperature, and electrolyzing time on electrolytic products were explored. The results show that a suitable amount of graphite content, high cell voltage, and a high electrolyzing temperature are beneficial to promote the formation of TiC compounds. It has also been found that the electroreduction of spent SCR catalyst/graphite can completely transform it into TiC compound in a relatively short time. The final electrolytic product is confirmed to be a solid solution of (Ti, W, Si, V)C. Meanwhile, the electrolytic process and reaction mechanism were investigated through the analysis of intermediates and the thermodynamic calculation. The electrolytic product has a potential application as reinforcement in metal matrix, which is a high additional-value utilization for spent SCR catalysts.

1. Introduction

TiC, as a typical transition metal carbide, has attracted considerable attention due to its high melting point, high hardness, relatively low density, excellent chemical stability, and good electrical conductivity. It has been widely applied in many fields, such as cutting tools, aviation, high-temperature heat exchangers and microelectronics [1,2,3,4,5]. Generally, TiC can be used as the reinforcement of metal matrix composites to significantly improve the mechanical properties [6,7,8,9,10,11]. The metal matrix mainly includes a copper matrix, an aluminum matrix, and a steel matrix. For example, the introduction of TiC into an aluminum matrix can increase its hardness twofold and the yield strength threefold [6]. Also, TiC can be employed as an electrocatalyst or support in fuel cells and electrolyzers owing to its high conductivity and excellent thermal-chemical stability [2,12]. In addition, TiC can be applied in high-speed machining and aerospace as a high-temperature structural material [13,14]. Systematical investigations exhibit that the additives of WC and SiC can obviously improve their mechanical properties [14]. Therefore, it is significant to prepare TiC and improve its mechanical performance.

Conventionally, carbothermal reduction is the industrial approach used to synthesize TiC by using titanium powder or titanium dioxides with solid carbon [4,15]. Lv et al. prepared TiC through the carbothermal reduction of TiO₂ and carbon, and the process was investigated by thermodynamical analysis and experiments [4]. However, carbothermal reduction has some drawbacks, such as high temperature, long reaction time, and high energy consumption. The limited progress is due to the solid–solid diffusion of carbon. In order to efficiently and quickly prepare TiC, other synthesis methods have been explored and applied, including the chemical vapor deposition (CVD) technique [16], mechanical alloying [17], thermal plasma synthesis [18], self-propagating high-temperature synthesis [19], Mg-thermal reduction [20], and the sol–gel process [21]. Among them, the high cost of reaction agents, reaction devices, and the production output are the main limiting factors for further industrial production. The starting materials change into titanium metal, titanium alkoxide, and titanium chlorides, while the reducing agent becomes polymeric carbon, H₂, and Mg. These reaction materials have higher costs. Meanwhile, some special reaction devices are needed in these methods, such as plasma devices, chemical vapor deposition equipment, and high-energy ball-milling machines. Moreover, the production outputs of CVD, mechanical alloying, and thermal plasma synthesis are small. Hence, an environmentally friendly and low-cost method is needed to prepare TiC. Recently, the molten-salt electrolysis method has attracted considerable attention, which synthesizes alloy/metal carbides using metal oxide and C in a relatively short time (6–12 h) at a low temperature (700–1000 °C). It has been successfully applied to prepare TiC [22], TiC/SiC [23], Ni-TiC [24] and Fe-TiC [25].

Selective catalytic reduction (SCR) technology is commonly used for decreasing NO_x emissions in thermal power plants, in which SCR catalysts play a key role. The commercial SCR catalyst of V₂O₅–WO₃/TiO₂ has been widely used owing to its high denitrification efficiency and low cost [26,27,28]. However, adverse working environments inevitably result in SCR catalysts becoming deactivated, and a large amount of spent SCR catalysts are generated. Spent SCR catalysts are a typical bulk solid waste, of which 250,000–300,000 m³ is generated annually in China [29]. Although spent SCR catalysts are classified as hazardous waste, they contain many valuable metals, such as TiO₂ (80–85%), WO₃ (5–7%), and V₂O₅ (0.7–1.5%). If discarded randomly, they not only cause serious environmental problems but also produce resource waste. Therefore, it is meaningful to recycle spent SCR catalysts. Up to now, numerous studies have been devoted to investigating the recycling method of spent SCR catalysts, which can be divided into hydrometallurgy, pyrometallurgy, and electroreduction methods [30,31,32,33]. In the hydrometallurgy process, W and V are leached into the solution from the spent SCR catalyst through alkaline leaching, acid leaching, calcification roasting-leaching, and sodium roasting-leaching, and the filter slag is the titanium compound. The hydrometallurgy method usually has a long process flow and produces waste liquid, resulting in environmental problems. For the pyrometallurgy method, spent SCR catalysts are reduced to alloys by thermal reduction with C, Mg, and Al at 1400–1700 °C, which is a high-cost and high-energy consumption method owing to the reducing agent and reaction temperature. For the electroreduction method, molten-salt electrolysis is an effective method for recycling solid waste, which possesses the advantages of cleanness, controllability, and simplicity. Therefore, it is a better way to recycle spent SCR catalysts. In our previous work, the molten-salt electrolytic method was employed to treat spent catalysts to prepare titanium alloys of Ti(W) and Ti₅Si₃ [34]. Also, TiC/SiC composites can be synthesized with titanium slag and C using the molten-salt electrolytic method, providing a new direction for recycling spent SCR catalysts [23]. To prepare TiC-based composites, the molten-salt electrolytic method is used to treat the mixture of spent SCR catalysts and C, with high additional value.

Generally, spent SCR catalysts mainly contain TiO₂, WO₃, SiO₂, and V₂O₅, which would convert into TiC-WC-SiC-VC composites through molten-salt electrolysis with carbon. Compared with TiC, the additives of WC, SiC, and VC can improve their mechanical properties [13,14]. Fattahi et al. detected that the addition of 1.5 wt% WC can increase the relative density of TiC-10 vol% SiC_w ceramics systems. Chen et al. found that adding VC to TiC-based cermet not only acts as grain inhibitor but also improves its transverse rupture strength. Hence, the electrolytic product of the spent SCR catalyst and carbon would be more suitable to prepare high-temperature structural materials with high mechanical properties. In this paper, the mixture of spent SCR catalysts and graphite was treated to obtain TiC-WC-SiC-VC through the molten-salt electrolytic method. The influencing factors of graphite content, cell voltage, electrolyzing temperature, and electrolyzing time were explored to obtain the optimal preparation conditions. Meanwhile, the electroreduction process and reaction mechanism were investigated based on experimental results and theoretical analysis, providing guidance for the electroreduction and carbonization of complex solid waste. This research would pave a new way to recycle spent SCR catalysts.

2. Experimental Section

2.1. Reagents and Materials

The spent SCR catalyst was supplied by a thermal power plant in Hebei Province, China. In order to remove the fly ash, blown cleaning and washing were used to clean the spent SCR catalyst. Then, ball-milling was employed to grind the catalyst for subsequent experiments. The chemical compositions are presented in Table 1, which were determined by X-ray fluorescence spectroscopy (XRF). From the table, it can be seen that the main compositions of the spent SCR catalyst contain Ti, W, Si, Ca, and V, with weight ratios of 85.1%, 7.6%, 2.7%, 1.5% and 0.9%, respectively. The other elements are less than 1%, respectively. The phase composition and micro-morphology of the spent SCR catalyst have also been investigated, as exhibited in Figure 1. The diffraction peaks were identified with the standard PDF data of 21-1272, which is anatase TiO₂, and there is no other obvious phase detected. This suggests that the main phase of the spent SCR catalyst is anatase TiO₂. It can also be seen that the XRD peaks present broadening, which is closely related to the small particles of spent SCR catalysts, as shown in the inset of Figure 1. The molten salts of CaCl₂ and NaCl are of analytical grade and were purchased from Beijing Chemical Industry.

2.2. Electrolytic Process

Before electrolysis, the electrodes were prepared, as shown in Figure 2 (inset). The graphite sheets (100 mm × 15 mm × 5 mm) were applied as anodes, which were polished, and the impurities adhering to the surface were removed by deionized water and alcohol. Then, the processed graphite sheets were fixed onto stainless-steel rods. The cathode mainly contained the waste SCR catalyst and graphite powder with different ratios. The mixed powder was placed in the iron net and then fixed onto stainless-steel rods. The analytical-grade anhydrous CaCl₂ and NaCl with a molar ratio of 0.52:0.48 were mixed as the molten-salt electrolyte and placed in the corundum crucible. Subsequently, the corundum crucible was moved into the furnace tube. A schematic illustration of the electrolytic cell is exhibited in Figure 2. The tube furnace was sealed with vacuum-pumping and filled with argon to remove the air and H₂O. The electrolytic processes were performed with the following variables: graphite content, cell voltage, electrolyzing temperature, and electrolyzing time. The electrolytic products were washed with deionized water and then dried at 120 °C for 3 h for further testing.

2.3. Characterization

XRF was performed to investigate the chemical compositions of raw materials. During the electrolytic process, the electrochemical workstation of CHI 660E (Chenhua, Shanghai, China) was employed to control the experiment, the applied voltage was 3.2 V, and the current–time curve was recorded. The phase compositions of the electrolytic products were analyzed by X-ray diffraction (XRD), which was conducted using an X-ray diffractometer from Rigaku SmartLab SE (Rigaku, Tokyo, Japan) with Cu Kα at a scan speed of 10°/min. The scan range of 2θ was 10–80°. The morphology and surface element distribution of samples were characterized through a scanning electron microscope (JEOL JSM-IT-100, JEOL, Tokyo, Japan) with X-ray energy dispersive spectroscopy (EDX). An accelerating voltage of 20 kV was used. In order to improve conductivity, the samples were coated with a thin layer of gold, and different magnifications were applied to detect the samples.

3. Results and Discussion

3.1. Effect of Graphite Contents on Reduction

In electrolysis, there are two main effects of graphite content. On the one hand, the graphite is the reactant for the carbonization reaction. An adequate content of graphite is beneficial to carbonization. On the other hand, the graphite addition is in favor of electron transfer and enhancing the conductivity of the cathode. For the electroreduction of metal oxide, a three-phase interline (3PI) is usually to employed to elucidate the electroreduction process, which contains the following: a conductor (metal or graphite)/insulator (metal oxides)/electrolyte [35,36]. The electroreduction starts at the 3PI and makes the adjacent solid oxide convert to the corresponding metal. The addition graphite has a similar effect to metal, which acts as a current collector and promotes the electron transfer at the stage of electroreduction. At the carbonation stage, the graphite is used as carbon source to react. Hence, it is significant to investigate the influence of graphite contents on the electrolytic products.

The electrolytic experiments with different contents of graphite were performed under 3.2 V at 800 °C for 12 h. The electrolyzing parameters were selected according to the characteristics of the electrolyte and the previous literature [23,37], which concluded that the applicable temperature for CaCl₂-NaCl is 550–800 °C. After electrolyzing with different graphite contents, the compositions of electrolytic products were investigated, as shown in Figure 3. It is evident in Figure 3a that W and Ti₅Si₃ appear at a low content of graphite, indicating there is not enough graphite to form the carbide compound and titanium is the most easily carbonized material in the spent SCR catalyst. With increasing graphite content, W and Ti₅Si₃ disappear and completely convert into carbides. At the graphite content of 20 wt%, the main composition of the electrolytic product is TiC, as shown in Figure 3b, and there is no other phase detected. This suggests that the spent SCR catalyst/C was totally transformed to carbides and TiC is more stable than Ti₅Si₃. The graphite phase appears at the graphite content of 30 wt%, as exhibited in Figure 3c. This means an excess of graphite for the experiment. In addition, according to the peak intensities of (111) and (200), it can be seen that the XRD patterns of Figure 3c present a preferred orientation for TiC (111). The main reason is that TiC (111)//Ti (0001) is the primary orientation relationship for the TiC/Ti interface [38]. The excess graphite may be beneficial to stabilizing TiC (111). For further investigation, the graphite content of 20 wt% was employed.

3.2. Effects of Cell Voltage on Electrolytic Products

Cell voltage is a key factor for electrolysis. Figure 4 shows the XRD patterns of electrolytic products under different cell voltages. It can be seen that the main phase is TiC in the voltage range from 2.8 V to 3.2 V. However, there exists an impurity phase in Figure 4a, which is identified to be W₂C. This indicates that the low voltage is adverse to the diffusion of W. Meanwhile, the TiC peaks become sharper with increasing voltage, which is closely associated with the grain size. The smaller the particles are, the wider the XRD peaks. So, the micromorphology and size of electrolytic products were also investigated, as shown in Figure 5. At the cell voltage of 2.8 V, the fine TiC crystals agglomerate to form a large particle. The fine particles induce the XRD peaks of TiC broadening. As the cell voltage increases, the particle size increases and the XRD peaks become sharper, which is closely related to the nucleation and growth of TiC. The TiC particles grow with increasing cell voltage. Generally, high cell voltage can promote nucleation and form tiny particles, which contrasts with these results. The cell voltage is the main contributor to the growth of TiC in our experiment. According to the phase composition of different cell voltages, it can be concluded that the growth of TiC is a diffusion-controlled process. High cell voltage can enhance electroreduction-driven forces and accelerate the diffusion of substances, which can promote the growth of TiC.

3.3. Effect of Temperature on Electrolysis

Generally, working temperature is closely related to electroreduction sequences and electroreduction kinetics. Figure 6 shows the time–current curves of electrolysis at different temperatures. It is obvious that the value of the current is enhanced with the increase in temperature, which indicates that the increasing temperature accelerates the transfer of electrons and ions. This is because increasing temperature reduces the viscosity of the molten salt, enhances the ion mobility in the electrolyte, and improves the reaction kinetics. It is advantageous for electroreduction. The effects of temperature on phase composition were also investigated, as shown in Figure 7. The main phase of all cases is TiC, and there is no other phase observed. This indicates that spent SCR catalysts were completely electroreduced to form TiC. Meanwhile, the XRD peaks of TiC become sharper with the increasing temperature, which has the same changing trend as cell voltage. This is also tightly linked to grain size. The increasing temperature is beneficial to the diffusion of substances, which promotes the growth of TiC.

In the previous literature, TiC/SiC was prepared with high titanium slag and graphite powder through the molten-salt electrolytic method [23]. The XRD peaks of electrolytic products were identified as TiC with a space group of Fm$\overline{3}$m and SiC with a space group of F$\overline{4}$3m. No solid solution was formed. In order to explore the electrolytic product of our experiment, the electrolytic product of 700 °C with the broad XRD peaks was selected, which may contain TiC and SiC. The micromorphology and element distribution were investigated, as exhibited in Figure 8. The particle size is not uniform. On the surface of bulk particles, some tiny particles are present. From Figure 8b–f, it can be seen that Ti, C, Si, V, and W have a uniform distribution, and there is no obvious segregation appearing for the electrolytic product, indicating that a solid solution of (Ti, W, Si, V)C is formed. This result is consistent with Johnson et al. [14], where the (Ti, W, Si) C solid solution was also confirmed by SEM/EDS.

3.4. Electroreduction Process and Reaction Mechanism

In order to better understand the electroreduction process, the time–current curve of the electrolyzing process under 3.2 V at 800 °C with a graphite content of 20 wt% was explored, as shown in Figure 9. The current reduces quickly within 1 h and subsequently slows. According to the slope variation of the curve, the electroreduction process is divided into five stages. Also, electroreduction products with different durations of 5 min, 10 min, 20 min, 0.5 h, 1 h, 3 h, and 6 h are obtained to investigate the reaction mechanism. Also, an immersion test was conducted at 800 °C to reflect the chemical reaction between the spent SCR catalyst, graphite, and molten salts. The variations of phase compositions in the electroreduction process were explored, as presented in Figure 10.

After the immersion test for 0.5 h, the main phases remain anatase TiO₂ and graphite, as shown in Figure 10a, demonstrating that no obvious chemical reaction occurs. This is also confirmed by the thermodynamic calculation, as shown in Figure 11. Most of the standard Gibbs free energies $(∆G^{\theta })$ have a positive value, except for the carboreduction of WO₃. These positive values imply that the chemical reaction cannot take place, while the negative value means the reaction can occur in theory. Therefore, the carboreduction of WO₃ would take place in theory, but the actual experiment revealed that almost no reaction occurs between WO₃ and graphite at 800 °C [39], which is consistent with the immersion test.

Electrolysis with different electrolyzing times was performed to investigate the electroreduction process. After electrolyzing for 5 min, an amount of CaTiO₃ appears, and some titanium suboxides are formed, such as Ti₂O₃, TiO, and Ti₂O. There is no TiO₂ phase observed. In the meantime, some TiC is also generated, as exhibited in Figure 10b. This demonstrates TiO₂ has a rapid transformation in a short time. With increasing time, the peaks of CaTiO₃ significantly decrease, and the peaks belonging to TiC become increasingly evident, as presented in Figure 10c,d. This indicates that the transformation of CaTiO₃→Ti→TiC occurs. From Figure 10b–d, it can be seen that the electroreduction reaction is the dominant reaction at the first stage. In addition, CaCO₃ and Ca(OH)₂ exist in Figure 10b–d, owing to incomplete washing. When the electrolysis was carried out for 0.5 h, the phase compositions of the electrolytic product contain CaTiO₃, C, and TiC, as shown in Figure 10e. As the electrolysis duration is extended for 1 h, the contents of C and CaTiO₃ almost disappear, while the relative content of TiC significantly increases, which is presented in Figure 10f. Apparently, the solid solution of (Ti, W, Si, V)C can be successfully prepared in a relatively short time within 3 h, as shown in Figure 10g. Over time, there is no evident change in the phase composition. However, the XRD peaks become sharper owing to the growth of TiC. The morphology of electrolytic products of 0.5 h and 1 h are investigated in Figure 12. From Figure 12a, it can be seen that the tiny particles present a flocculent distribution on the bulk particles. The emergence of loose and porous structures is a typical feature for electrolysis owing to the deoxidization of spent SCR catalysts. The morphology of the electrolytic product of 1 h presents an obvious change compared with the sample of 0.5 h. There is no flocculence distribution, and bulk particles appear. The main reason for this is that the electroreduction and carbonation are almost complete, which is verified by the results of XRD. According to the variation of phase compositions with increasing time, it can be concluded that there are two electroreduction paths for TiO₂: one is TiO₂→Ti₂O₃→TiO→Ti₂O→Ti, and the other is TiO₂→CaTiO₃→TiO_x(0 < x < 2)→Ti, which agrees well with the results of Hu et al. [40]. Subsequently, the electrolytic products of W, V, Si, and Ti were carbonized by graphite. Owing to the limitations of the experiment, the details of the reaction mechanism are still unclear. Thus, theoretical calculations were employed for further exploration.

The theoretical analysis for electroreduction and carbonization is shown in Figure 13. Figure 13a exhibits the theoretical decomposition voltage ($∆E^{ϴ}$) of the possible reaction as a function of temperature. According to the values of $\Delta E^{ϴ}$, it can be concluded that the electroreduction sequence is W, V, Si, and Ti. Owing to the affinity of oxygen, the electroreduction of Ti is the most difficult process. Moreover, two reduction routes for Ti are exhibited. TiO₂ is directly reduced by stepwise reduction, which is TiO₂-Ti₇O₄-Ti₂O₃-TiO-Ti. Although some intermediates were not observed in our experiment, previous studies have confirmed this result [40,41]. Also, TiO₂ is preferentially forms the intermediate compound of CaTiO₃ during electrolysis. The reduction path of CaTiO₃ is CaTiO₃-TiO-Ti. In comparison to the values of $\Delta E^{ϴ}$ of TiO₂ and CaTiO₃, it can be seen that the complex oxide of CaTiO₃ needs higher voltage, indicating that the formation of CaTiO₃ increases the difficulty of the electrode oxidation reduction.

The standard Gibbs free energies of the carbonization reaction were calculated, as shown in Figure 13b. From Figure 13b, it can be seen that all displayed reactions have negative values, indicating that, in theory, these reactions can all occur at high temperatures. Among them, the carbonization reaction of Ti occurs first, owing to the lowest energy in all reactions. In addition, Ti can displace W, V, and Si from their carbides via substitution reactions. Besides the carbonization, the formation of Ti₅Si₃ also has a low energy, indicating that Ti₅Si₃ is preferentially formed as there is not enough carbon to react with the spent SCR catalyst, which agrees well with the results of electrolytic products with low content.

Combined with the experimental results and thermodynamic calculations, the reaction mechanism was generally summarized, as shown in Figure 14. According to the electroreduction sequence, WO₃, V₂O₅, and SiO₂ were first reduced and subsequently carbonized to form WC, VC, and SiC. Then, TiO₂ was electroreduced by two routes: one is TiO₂→Ti₂O₃→TiO→Ti₂O→Ti, and the other is TiO₂→CaTiO₃→ TiO→Ti₂O→Ti. The obtained Ti would react with graphite to form TiC. Meanwhile, Ti would also react with WC, VC, and SiC to form TiC and the corresponding metals. When the graphite is sufficient, the solid solution of (Ti, W, Si, V)C was formed. However, the insufficient graphite would lead to the formation of TiC, Ti₅Si₃, and W. The reaction mechanism reveals the electroreduction process and carbonization sequence, which provides guidance for upcycling spent SCR catalysts.

4. Conclusions

The (Ti, W, Si, V)C solid solution was obtained by the electroreduction of the spent SCR catalyst and graphite in molten salt. The utilization of the optimum graphite content of 20 wt% results in a complete electroreduction to (Ti, W, Si, V)C. The increase in cell voltage and temperature is conducive to the formation of the electrolytic product. Owing to the graphite addition, the carbides are achieved in a short electrolysis time. Also, we found that the intermediates of electrolysis are CaTiO₃, Ti₂O₃, TiO and Ti₂O. Combined with the thermodynamics calculation, the electroreduced sequence of the spent SCR catalyst is W, V, Si, and Ti. Among them, there are two routes for the electroreduction of TiO₂, which are TiO₂→Ti₂O₃→TiO→Ti₂O→Ti and TiO₂→CaTiO₃→TiO→Ti₂O→Ti. The carbonizations occur between graphite and metals. Ti would react with WC, VC, and SiC to form TiC and the corresponding metals. The present study provides a new recycling method for spent SCR catalysts, and the electrolytic product of an (Ti, W, Si, V)C solid solution can be used as enforcement and high-temperature structure material. One objective of future work is to prepare high-temperature ceramics using the electrolytic product.

Author Contributions

Conceptualization, Y.X.; Methodology, W.C., Q.Z., L.Z. and J.J.; Formal analysis, Q.Z., L.Z. and J.J.; Investigation, Q.Z., X.L. and Y.X.; Data curation, W.C., J.J. and X.L.; Writing—original draft, W.C.; Writing—review & editing, Y.X.; Funding acquisition, W.C. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

Data are contained within the article.

Conflicts of Interest

The authors declare that they have no competing financial interests or personal relationships.

Footnotes

Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Figures and Table

Figure 1. XRD pattern and SEM image (inset) of the spent SCR catalyst.

Figure 2. Schematic illustration of the electrolytic cell and assembled electrodes.

Figure 3. XRD patterns of products after 12 h of electrolysis under 3.2 V at 800 °C with different contents of graphite: 10 wt% (a), 20 wt% (b), and 30 wt% (c).

Figure 4. XRD pattern of products after 12 h of electrolysis under 2.8 V (a), 3.0 V (b), and 3.2 V (c) at 800 °C with graphite content of 20 wt%.

Figure 5. SEM images of products after 12 h of electrolysis under 2.8 V (a), 3.0 V (b), and 3.2 V (c) at 800 °C with graphite content of 20 wt%.

Figure 6. Time–current curves of electrolysis under 3.2 V with the graphite content of 20 wt% at 700 °C, 800 °C, and 900 °C.

Figure 7. XRD patterns of product after 12 h of electrolysis under 3.2 V with the graphite content of 20 wt% at 700 °C (a), 800 °C (b), and 900 °C (c).

Figure 8. SEM images (a) and element distributions (b–f) of the electrolytic product after 12 h electrolysis under 3.2 V with the graphite content of 20 wt% at 700 °C.

Figure 9. Time–current curves of electrolysis under 3.2 V with the graphite content of 20 wt% at 800 °C. I, II, III, IV and V represent different stages of electrochemical reduction.

Figure 10. XRD patterns of electrolytic products under 3.2 V at 800 °C with the graphite content of 20 wt% for different durations, immersions into molten salt for 0.5 h without electrolysis (a), electrolysis within 5 min (b), 10 min (c), 20 min (d), 0.5 h (e), 1 h (f), 3 h (g) and 6 h (h).

Figure 11. Relationships between standard Gibbs free energies[Forumla omitted. See PDF.] and temperature of possible carboreduction reactions.

Figure 12. SEM images of electrolytic products under 3.2 V at 800 °C with the graphite content of 20 wt% for 0.5 h (a) and 1 h (b).

Figure 13. Theoretical decomposition voltage ([Forumla omitted. See PDF.]) (a) and standard Gibbs free energies ([Forumla omitted. See PDF.]) (b) of possible reaction as a function of temperature.

Figure 14. Schematic diagram of the reaction mechanism.

Table 1

Chemical composition of spent SCR catalysts analyzed by XRF (wt%).

Component	Ti	W	Si	Ca	V	Others
Mass fraction	85.1	7.6	2.7	1.5	0.9	2.2

References

1. Larhlimi, H.; Ghailane, A.; Makha, M.; Alami, J. Magnetron sputtered titanium carbide-based coatings: A review of science and technology. Vacuum; 2022; 197, 110853. [DOI: https://dx.doi.org/10.1016/j.vacuum.2021.110853]

2. Saha, S.; Rajbongshi, B.M.; Ramani, V.; Verma, A. Titanium carbide: An emerging electrocatalyst for fuel cell and electrolyser. Int. J. Hydrogen Energy; 2021; 46, pp. 12801-12821. [DOI: https://dx.doi.org/10.1016/j.ijhydene.2021.01.116]

3. Galevsky, G.V.; Rudneva, V.V.; Garbuzova, A.K.; Valuev, D.V. Titanium Carbide: Nanotechnology, Properties, Application. IOP Conf. Ser. MSE; 2015; 91, 012017. [DOI: https://dx.doi.org/10.1088/1757-899X/91/1/012017]

4. Lv, T.; Tian, F.; Hu, T. Preparation of Titanium Carbide by Carburisation of Titanium Dioxide. Processes; 2024; 12, 102. [DOI: https://dx.doi.org/10.3390/pr12010102]

5. Mhadhbi, M. Titanium Carbide: Synthesis, Properties and Applications. J. Brill. Eng.; 2020; 2, pp. 1-11. [DOI: https://dx.doi.org/10.36937/ben.2021.002.001]

6. Zemtsova, E.G.; Morozov, N.F.; Semenov, B.N.; Smirnov, V.M.; Venatovskaya, L.A. Mechanical properties of aluminum matrix composite reinforced with titanium carbide. J Phys. Conf. Ser.; 2024; 2817, 012014. [DOI: https://dx.doi.org/10.1088/1742-6596/2817/1/012014]

7. Ravichandran, M.; Alagarsamy, S.V.; Dhinakaran, V.; Abdul Samad, M.; Katiyar, J.K. Optimization of tribological process parameters of titanium carbide reinforced copper matrix composites. Proc. Inst. Mech. Eng. Part J-J. Eng. Tribol.; 2022; 236, pp. 1737-1751. [DOI: https://dx.doi.org/10.1177/13506501221085055]

8. Janicki, D. The friction and wear behaviour of in-situ titanium carbide reinforced composite layers manufactured on ductile cast iron by laser surface alloying. Surf. Coat. Tech.; 2021; 406, 126634. [DOI: https://dx.doi.org/10.1016/j.surfcoat.2020.126634]

9. Xiang, S.; Hu, H.; Liang, Y.; Zhang, X. Fabrication of sub-stoichiometric titanium carbide-reinforced copper matrix composites using electropulsing-assisted flash sintering. Compos. Part A-Appl. S.; 2021; 151, 106664. [DOI: https://dx.doi.org/10.1016/j.compositesa.2021.106664]

10. Levy, A.; Miriyev, A.; Elliott, A.; Babu, S.S.; Frage, N. Additive manufacturing of complex-shaped graded TiC/steel composites. Mater. Des.; 2017; 118, pp. 198-203. [DOI: https://dx.doi.org/10.1016/j.matdes.2017.01.024]

11. Cho, S.; Lee, Y.-H.; Ko, S.; Park, H.; Lee, D.; Shin, S.; Jo, I.; Kim, Y.; Lee, S.-B.; Lee, S.-K. Enhanced high-temperature compressive strength of TiC reinforced stainless steel matrix composites fabricated by liquid pressing infiltration process. J. Alloys Compd.; 2020; 817, 152714. [DOI: https://dx.doi.org/10.1016/j.jallcom.2019.152714]

12. Shi, J.; Zhang, P.; Han, Y.; Wang, H.; Wang, X.; Yu, Y.; Sun, J. Investigation on electrochemical behavior and surface conductivity of titanium carbide modified Ti bipolar plate of PEMFC. Int. J. Hydrog. Energ.; 2020; 45, pp. 10050-10058. [DOI: https://dx.doi.org/10.1016/j.ijhydene.2020.01.203]

13. Mao, H.; Shen, F.; Zhang, Y.; Wang, J.; Cui, K.; Wang, H.; Lv, T.; Fu, T.; Tan, T. Microstructure and Mechanical Properties of Carbide Reinforced TiC-Based Ultra-High Temperature Ceramics: A Review. Coatings; 2021; 11, 1444. [DOI: https://dx.doi.org/10.3390/coatings11121444]

14. Guria, J.F.; Bansal, A.; Kumar, V.; Manoj Kumar, B.V. Effect of co-addition of SiC and WC on the densification behaviour and microstructural evolution of TiC-based composites. Ceram. Int.; 2022; 48, pp. 12675-12691. [DOI: https://dx.doi.org/10.1016/j.ceramint.2022.01.137]

15. Matsuda, T. Effect of C/TiO₂ ratio in raw materials on thermal conductivity of titanium carbonitrides synthesized by carbothermal reduction. J. Alloys Compd.; 2020; 816, 152541. [DOI: https://dx.doi.org/10.1016/j.jallcom.2019.152541]

16. Noel, M.S.; Kovar, D. Laser chemical vapor deposition of TiC on tantalum. J. Mater. Sci.; 2002; 37, pp. 689-697. [DOI: https://dx.doi.org/10.1023/A:1013871326880]

17. Xing, T.; Gieleciak, R.; Chen, J. Synthesis of nanostructured titanium carbide (TiC) from bitumen coke by mechanical alloying process. Can. J. Chem. Eng.; 2024; 102, pp. 2443-2454. [DOI: https://dx.doi.org/10.1002/cjce.25195]

18. Tong, L.; Reddy, R. Synthesis of titanium carbide nano-powders by thermal plasma. Scr. Mater.; 2005; 52, pp. 1253-1258. [DOI: https://dx.doi.org/10.1016/j.scriptamat.2005.02.033]

19. Zhang, M.; Hu, Q.; Huo, Y.; Huang, B.; Li, J. Formation and growth mechanism of TiC terraces during self-propagating high-temperature synthesis from a Fe-Ti-C system. J. Cryst. Growth.; 2012; 355, pp. 140-144. [DOI: https://dx.doi.org/10.1016/j.jcrysgro.2012.06.045]

20. Lee, D.; Ahn, J.; Kim, B. Preparation of nanostructured titanium carbonitride particles by Mg-thermal reduction. J. Mater. Res.; 2005; 20, pp. 844-849. [DOI: https://dx.doi.org/10.1557/JMR.2005.0118]

21. Zhang, H.; Li, F.; Jia, Q.; Ye, G. Preparation of titanium carbide powders by sol–gel and microwave carbothermal reduction methods at low temperature. J. Sol-Gel. Sci. Technol.; 2008; 46, pp. 217-222. [DOI: https://dx.doi.org/10.1007/s10971-008-1697-0]

22. Yan, X.Y.; Pownceby, M.I.; Cooksey, M.A.; Lanyon, M.R. Preparation of TiC powders and coatings by electrodeoxidation of solid TiO₂ in molten salts. Min. Proc. Ext. Met.; 2013; 118, pp. 23-34.

23. Zhou, Z.; Zhang, Y.; Dong, P.; Hua, Y.; Zhang, Q.; Wang, D.; Duan, J.; Zhang, Z. Electrolytic synthesis of TiC/SiC nanocomposites from high titanium slag in molten salt. Ceram. Int.; 2018; 44, pp. 3596-3605. [DOI: https://dx.doi.org/10.1016/j.ceramint.2017.11.087]

24. Song, Q.; Xu, Q.; Xu, L.; Ning, Z.; Lou, T.; Xie, H.; Qi, Y.; Yu, K. Synthesis of Ni-TiC composite powder electrochemically in molten chlorides. J. Alloys Compd.; 2017; 690, pp. 116-122. [DOI: https://dx.doi.org/10.1016/j.jallcom.2016.08.101]

25. Hu, M.; Tan, R.; Ma, T.; Hu, M. In situ synthesis of titanium carbide coating on foam titanium by molten salt electrolysis. Mater. Today Commun.; 2023; 35, 105531. [DOI: https://dx.doi.org/10.1016/j.mtcomm.2023.105531]

26. Li, J.; Chang, H.; Ma, L.; Hao, J.; Yang, R.T. Low-temperature selective catalytic reduction of NO_x with NH₃ over metal oxide and zeolite catalysts—A review. Catal. Today; 2011; 175, pp. 147-156. [DOI: https://dx.doi.org/10.1016/j.cattod.2011.03.034]

27. Tang, X.; Wu, P.; Wang, Y.; Liu, Y. Recent advances in heavy metal poisoning mechanism and regeneration methods of selective catalytic reduction (SCR) denitration catalyst. Fuel; 2024; 355, 129429. [DOI: https://dx.doi.org/10.1016/j.fuel.2023.129429]

28. Wei, Y.; Li, D.; Qiao, J.; Guo, X. Recovery of spent SCR denitration catalyst: A review and recent advances. J. Environ. Chem. Eng.; 2023; 11, 110104. [DOI: https://dx.doi.org/10.1016/j.jece.2023.110104]

29. Liu, J.; Wang, C.; Wang, X.; Zhao, C.; Li, H.; Zhu, G.; Zhang, J. Reconstruction and recovery of anatase TiO₂ from spent selective catalytic reduction catalyst by NaOH hydrothermal method. Chin. J. Chem. Eng.; 2023; 60, pp. 53-60. [DOI: https://dx.doi.org/10.1016/j.cjche.2023.03.019]

30. Qian, X.; Ao, W.; Ding, H.; Wang, X.; Sun, S. A Review on Resource Utilization of Spent V-W-Ti Based Selective Catalytic Reduction Catalysts. Materials; 2022; 15, 7984. [DOI: https://dx.doi.org/10.3390/ma15227984]

31. Ma, L.-W.; Xi, X.-L.; Chen, J.-P.; Guo, F.; Yang, Z.-J.; Nie, Z.-R. Comprehensive recovery of W, V, and Ti from spent selective reduction catalysts. Rare Met.; 2023; 42, pp. 3518-3531. [DOI: https://dx.doi.org/10.1007/s12598-023-02321-0]

32. Zhao, J.; Zhang, X.; Yang, F.; Ai, Y.; Chen, Y.; Pan, D. Strategy and Technical Progress of Recycling of Spent Vanadium-Titanium-Based Selective Catalytic Reduction Catalysts. ACS Omega; 2024; 9, pp. 6036-6058. [DOI: https://dx.doi.org/10.1021/acsomega.3c07019] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/38371753]

33. Li, Z.; Yang, L.; Ma, W.; Lei, Y. Eutectic Si–Ti brazing alloy prepared from spent selective catalytic reduction catalyst and diamond-wire sawing silicon waste. Chem. Eng. J.; 2024; 496, 154053. [DOI: https://dx.doi.org/10.1016/j.cej.2024.154053]

34. Zheng, L.; Jin, J.; Cai, Z.; Li, X.; Zhu, Q.; Cao, W. Electrosynthesis of Titanium Alloys from Spent SCR Catalysts. Crystals; 2025; 15, 83. [DOI: https://dx.doi.org/10.3390/cryst15010083]

35. Deng, Y.; Wang, D.; Xiao, W.; Jin, X.; Hu, X.; Chen, G.Z. Electrochemistry at conductor/insulator/electrolyte three-phase interlines: A thin layer model. J. Phys. Chem. B; 2005; 109, pp. 14043-14051. [DOI: https://dx.doi.org/10.1021/jp044604r]

36. Vishnu, D.S.M.; Sanil, N.; Shakila, L.; Panneerselvam, G.; Sudha, R.; Mohandas, K.; Nagarajan, K.J.E.A. A study of the reaction pathways during electrochemical reduction of dense Nb₂O₅ pellets in molten CaCl₂ medium. Electrochim. Acta; 2013; 100, pp. 51-62. [DOI: https://dx.doi.org/10.1016/j.electacta.2013.03.135]

37. Hu, B.; Ding, J.; Wei, X.; Peng, Q.; Liao, M. Test of thermal physics and analysis on thermal stability of high temperatyre molten salt. Inorg. Chem. Ind.; 2010; 42, pp. 22-24.

38. Liang, T.; Ashton, M.; Choudhary, K.; Zhang, D.; Fonseca, A.F.; Revard, B.C.; Hennig, R.G.; Phillpot, S.R.; Sinnott, S.B. Properties of Ti/TiC interfaces from molecular dynamics simulations. J. Phys. Chem. C; 2016; 120, pp. 12530-12538. [DOI: https://dx.doi.org/10.1021/acs.jpcc.6b02763]

39. Ali, M.; Basu, P. Mechanochemical synthesis of nano-structured TiC from TiO₂ powders. J. Alloys Compd.; 2010; 500, pp. 220-223. [DOI: https://dx.doi.org/10.1016/j.jallcom.2010.04.007]

40. Hu, M.; Ma, T.; Gao, L.; Lai, P.; Qu, Z.; Wen, L.; Li, D.; Zhang, S.; Hu, M. Phase Transformations and Deoxidation Kinetics during the Electrochemical Reduction of TiO₂ in Molten CaCl₂. Mater. Trans.; 2019; 60, pp. 416-421. [DOI: https://dx.doi.org/10.2320/matertrans.MA201802]

41. Dring, K.; Dashwood, R.; Inman, D. Voltammetry of titanium dioxide in molten calcium chloride at 900 °C. J. Electrochem Soc.; 2005; 152, E104. [DOI: https://dx.doi.org/10.1149/1.1860515]