Synthesis of Ti(C, N, O) from TiO2 by CH4-H2-N2 Gas Mixture at Low Temperature

Titanium carbides, oxides, nitrides and carbonitrides possess many special and excellent properties. But the high production cost caused by the traditional carbothermic reduction process severely limits the wide applications. In this study, a novel synthesis process has been proposed by reducing and carbonitriding TiO 2 with CH 4 -H 2 -N 2 gas mixture at low temperatures. The synthesis of Ti(C, N, O), reaction mechanism and the products composition have been investigated. Thermodynamic analysis indicated that TiO 2 could be ultimately reduced and carbonitrided to Ti(C, N, O) by CH 4 -H 2 -N 2 gas mixture. Based on the predictions of thermodynamics, the effects of reduction time, temperature, and gas composition have been studied experimentally. The obtained results indicated that increasing reduction temperature and introduction of N 2 are beneficial to the synthesis of Ti(C, N, O). Finally, the optimum reaction conditions have been obtained. The density functional theory (DFT) results further demonstrated the reaction mechanism. This work provides a new approach to prepare metallic carbonitrides from their oxides.


Introduction
Titanium carbides, nitrides, oxides, and their solid solutions (TiC, TiN, Ti(C, N, O)) are the leading advanced engineering ceramics that can be used in metal-working, electrical and electronic, automotive, chemical, and refractory industries [1][2][3][4][5], due to their special properties such as low density, high melting points, and hardness, hightemperature strength retention, good conductivity, excellent oxidation and wear resistance, and low thermal expansion coefficient in coating and thin films [6]. Titanium carbide can also be used as catalytical and electrochemical material [7]. A potential application is that titanium carbide, oxide, nitride, and their solid solution can be used as the raw material (soluble anode) for producing metallic titanium [8][9][10][11].
Industrially, TiC is usually synthesized from TiO2 by carbothermic reduction process; while the synthesis of TiN is based on the reaction of Ti or TiH2 powders with N2 or NH3, at 1000 o C to 1400 o C; Ti(C, N) is usually made of the diffusion of TiC with TiN powders at 1700 o C to 1800 o C. The reaction process can be described as the following reactions: TiO2 (s) + 3C (s) = TiC (s) + 2CO (g) Ti (s) + 0.5N2 (g) = TiN (s) (2) TiH2 (s) + N2 (g) = TiN (s) + H2 (g) (3) Ti (s) + NH3 (g) = TiN (s) + 1.5H2 (g) (4) TiH2 (s) + NH3 (g) = TiN (s) + 2.5H2 (g) (5) TiC (s) + TiN (s) = Ti(C, N) (s) (6) Under the standard condition, reaction (1) occurs at a temperature of 1564 K (1291 o C). Besides, reaction (1) requires a large supply of heat to complete this reaction because of the strongly endothermic nature of the reaction (∆H(1) = 518.5 kJ/mol, T = 1291 o C). In industrial production, the reaction usually occurs at 1700 o C to 2100 o C for 10 to 24 hours, or even longer time. Even though ∆G(2)~(5) < 0, the high cost of raw materials (Ti powder and TiH2 powder) leads to the high cost of products. To obtain a high-quality product, the process usually needs to continue for more than 30 hours. The synthesis of Ti(C, N) is limited by the quality of TiC and TiN as well.
To address these issues, many researchers have put forward a lot of methods, such as the gas-solid reaction of TiCl4 with CaC2 [12], the gas-liquid reaction of TiCl4 with NaNH2 [13], the gas-gas reaction of TiCl4 with CH4 (N2) gas [14,15], the sol-gel process of TiCl3 with polyacrylonitrile and dimethylformamide [16], and other methods [17]. But none of them has been used for commercial production so far, due to the high cost and low quality of TiCl4 and TiCl3 as raw material, which need to go through a complex treatment process from TiO2 or titanium bearing mineral. Oyama et al [18] reported that the transition-metal oxides could be reduced into their carbides and nitrides by methane, hydrogen, and ammonia gas. Zhang and Ostrovski [19] also showed that TiC could be prepared from TiO2 and ilmenite with CH4-bearing gas mixture. Our previous researches [20][21][22][23][24][25] also exhibit the tremendous advantage of this method for titanium oxycarbide and titanium oxycarbonitride production compared with the traditional process. However, the reduction mechanism and the product composition have not been clarified clearly, and are needed to further study.
Therefore, based on our previous work, the effects of N2 addition and reaction temperature on the reduction of TiO2 with CH4-containing gas mixture, and the reduction products composition under different conditions have been studied in the present work. The thermodynamic and first-principles calculations have also been carried out to achieve a deep understanding of the reduction process.

Materials
TiO2 and Fe2O3 powders are both the analytical reagent (AR) with 99% purity (Chengdu Kelong Corporation, China). The CH4, H2, N2, and Ar gases used in this study as the reaction gas and protective gas are with 99.999% purity (Chongqing Ruike Corporation, China).

Experimental
The raw materials were mixed in a certain proportion after dried at 200 o C for more than 24 hours to remove the water in the raw materials. Around 0.5 g powders were loaded in a corundum crucible with a 30 mm diameter and 5 mm height, placed inside the vertical tube furnace. The furnace was then heated up to the desired temperature at a 5 o C/min rate. CH4, N2, and H2 gas mixtures were introduced into the furnace from the top at the desired temperature, while Ar gas was blown into the furnace during the heating and cooling procedures. The flow rate of gas was precisely controlled by gas flow controllers (Alicat, Model MC-500SCCM-D, and MC-1SLPM-D). The device and the schematic diagram have been described elsewhere [22].

Analytical methods
After the carbonitriding procedure, the product would be ground and mounted for the following detection.

Thermodynamic calculation
Thermodynamic calculations for the equilibrium phases of TiO2 in CH4-H2-N2 gas mixture were performed by using FactSage software based on Gibbs free energy minimization and the database of CaO et al. [29] The equilibrium phase diagrams were calculated in the temperature range from 900 o C to 1400 o C in different CH4-N2-H2 systems, and the equilibrium results of solid-phase were shown in Figure 1   Combining with equilibrium phase diagrams in different systems (described in ESI), it can be clearly found that the reduction of titania performs as the step by step with the gas/TiO2 ratio increasing. In summary, the reduction sequence could be inferred as the following: It illustrates that at high temperature TiC or Ti(C, O) is more stable than TiN. It can be explained by the reaction Gibbs free energy of TiO2 with CH4-N2-H2 gas mixture.
The possible reactions of TiO2 with gas mixture could be expressed as follows: TiO2 + 2CH4 + 0.5N2 = TiN + 4H2 + 2CO TiO2 + 3CH4 = TiC + 6H2 + 2CO TiO2 + CH4 = TiO + 2H2 + CO The reaction standard Gibbs free energies of equation (7) ~ (9) as a function of temperature is shown in Figure 1 (c). When the temperature ranges from 900 o C to 995 o C, the standard Gibbs free energies of equation (7) is negative and less than that of equation (8); when the temperature is higher than 995 o C, the standard Gibbs free energies of equation (8) is lower. The function of standard Gibbs free energies vs temperature can also indicate that the lower temperature is beneficial to the formation of TiN and the higher temperature is beneficial to the formation of Ti(C, O), consistent with the above equilibrium calculation. However, the standard Gibbs free energy of equation (9) is always higher than those of equations (7) and (8).
The effect of Fe2O3 additive on the reduction sequence and the phase transformation has been studied by equilibrium calculation as well. In FeTiO3 and FeTi2O5, the valence of iron is +2, which is reduced from Fe2O3 by the first step calculation with CH4 bearing gas mixture. In the first step calculation, the amount of CH4 is 0.005 mol, with 0.01 mol N2 and 0.0475 mol H2, which would reduce the ferric oxide (Fe2O3) to ferrous oxide (FeO). The ferrous oxide would react with TiO2 to form FeTiO3 and FeTi2O5. The difference of phases existing at different temperatures may be caused by thermodynamic stability. After the ferrous oxide phase being completely reduced, the subsequent reaction is the reduction of TiO2.

Reaction sequence
To study the phase transformation during the reduction process, samples were reduced for different times at 1200 o C with the total gas flow rate of 500 mL/min (8 vol.% CH4 -16 vol.% N2 -76 vol.% H2). The XRD patterns of reduced samples were shown in Figure 2 (a), and the detected phases were summarized in Table S1. During the first 10 minutes of the reduction, Ti3O5 as the major low-valent-titanium-bearing phase existed in the sample with residual TiO2 and metallic iron reduced from Fe2O3.   Table   1, and the reduction degree and conversion degree of Ti (from TiO2 to Ti(C, N, O)) could be calculated, as described by equations (10) and (11): Reduction degree: Conversion degree: Where , , and are the number moles of total Ti atom, residual TiO2, titanium suboxide and Ti(C, N, O), respectively   Figure 2 and Figure S3)  The conversion of Ti in the reduction process has also been listed in Table S1 and shown in Figure 2  The reduction degree has also been listed in Table 1. It can be clearly seen that the reduction degree increased drastically with the reaction time extending in the first 1 hour; however, after 1 hour, the increase rate of reduction degree lowered. The different rates indicate the different reduction stages: in the first 1 hour, it mainly contains the reduction of TiO2 into Ti(C, N, O); after 1 hour, the deeply deoxidization of Ti(C, N, O) occurs.
The particle morphology images of samples reduced at 1200 o C for different times were shown in Figure 2 (d). It can be seen that different morphologies were presented after different reduction times. Evidently, it presented as coralline with a porous structure for the product obtained for 10 min and 20 min, in which Ti3O5 is the main phase. With the reduction time extending, the particles size increased. When the reduction time was increased to 3 h, the particles aggregated and presented as spherical, and a similar morphology was observed in the sample reduced for 5 h. The deposited carbon from CH4 attached on the surface of particles, and impeded the gas transmission into the inside of particle, so a small amount of TiO2 is still remained.

Effect of Temperature
In our previous researches [22,24,25], the effect of reduction temperature on the product and reduction process has been studied, finding that higher temperature was helpful to the reduction reaction under certain conditions, but the deposited carbon cracked from methane and the sintering of particles could hinder the gas diffusion and slow down the reduction rate as well. The thermodynamic calculation also indicates that the reaction temperature has a serious impact on the reduction sequence and final product. In  Table S2.   Figure 3 and Figure S4) Figure S4 showed the Rietveld refinements of the experimental XRD patterns of samples reduced at different temperatures for 1 h, and the quantitation of titanium-bearing phases were listed in Table 2. The conversion of Ti has been determined from the results, and the function of Ti conversion fraction with reduction temperature has been summarized in Table S2 and also drawn as The reduction degree has been listed in Table 2. It can be seen that the reduction degree sharply increases with the reaction temperature increasing from 900 to 1100 o C.

Figure 3 (b) and
While from 1100 ~ 1200 o C, the reduction degree slightly increases with the increase of temperature.

Effect of N2 addition
As discussed by thermodynamic calculation, the content of N2 in the gas mixture has an important influence on the reduction. noting that the Magnéli phase was not detected, which was not consistent with the thermodynamic calculation. It may be that the amount of Magnéli was too little to be detected by XRD, which has a relatively high detection limit about 1 ~ 2% of the sample [30,31].
The quantitative results of Ti-bearing phases and the reduction degree have been listed in Table 3, and the conversion degree of Ti as a function with reduction temperature has been listed in Table S3 and shown in Figure 4(c), from the Rietveld refinement results. It is obvious that the conversion degree of Ti increases with the reduction temperature increasing in the range of 1000 ~ 1150 o C. When above 1150 o C, the conversion degree of Ti changes slowly. Compared with the CH4-H2-N2 system, the conversion degree of Ti in the CH4-H2 system is much smaller. The addition of N2 in the gas mixture can improve the carbonitriding process and lower the reaction temperature, as discussed above.  Table 3 Quantitative results of phases bearing titanium from the Rietveld refinement (corresponding to Figure 4 and Figure S5)

DFT study
For pure H2 reduction, TiO2 can only be reduced to Ti3O5 [32], while in addition CH4, titanium oxides can be deeply reduced. To understanding the reduction mechanism and why N2 can facilitate the reduction, the DFT calculation was carried out in this study. The adsorption of CH4, H2, and N2 on the surface of TiO2 is the prerequisite for the reduction occurring. Hence, the adsorption of gas molecules on the surface of TiO2 (110), which is the lowest energy surface and the active surface of TiO2 [33], has been calculated, and the most stable structures have been shown in Figure 5 (a) ~ (c), and the adsorption parameters are listed in Table 4 is -0.365 eV, which is close to the result of Adam Kubas' study [34]. In the stable structure of H2 adsorption, the H=H bond is parallel to the surface, and the most neighboring distance of H atom with Ti5c atom and O atom are 2.438 and 2.748 Å, respectively. The adsorption energy of H2 on the surface is -0.220 eV, which is higher than that of CH4 on the surface. Combined with the thermodynamic analysis, it can be inferred that the difference between the reduction of TiO2 with CH4 and H2 is caused by the thermodynamic and the adsorption: both the Gibbs free energies of CH4 reacting with TiO2 and the adsorption of CH4 on the surface are lower than those of H2. In the structure of N2 adsorption, the NN bond is perpendicular to the TiO2 (110) surface.
The proximal N atom is located on the Ti5c atom, agreeing to Xie's research [35], and the distance of the N-Ti atoms is 2.509 Å. The adsorption energy of N2 on the surface is -0.376 eV, which is the most negative value among the three adsorption energies. It can explain that the addition of N2 in the system can reduce the reaction temperature and improve the carbonitriding reaction.

Reduction mechanism
Based on the above discussion, the reduction mechanism of TiO2 in CH4-H2-N2 gas mixture system was step-by-step reduction, as shown in Figure 6. It could be divided into four stages: the reduction of ferric oxide, the formation of titanium suboxides, the formation of Ti(C, N, O) and the deep deoxygenation of Ti(C, N, O).
CH4 would act as a carbonizing agent and N2 would act as a nitridation agent in this stage. Last, the Ti(C, N, O) was deoxidized deeply.
In the whole process, H2 had two important functions: reduction agent for reduction of Fe2O3 and TiO2; balance gas for the CH4 pyrolysis reaction to avoid the fast pyrolysis.

Process evaluation
In order to produce the titanium carbonitride, many methods have been proposed, some of which have been summarized in Table 5. Commercially, titanium carbide is usually synthesized from TiO2 by carbothermic reduction process, at 1700 ~ 2100 o C for 10 ~ 24 h. This method is the most mature technology and widely used. But the large energy consumption and long production cycle lead to the high production cost.
The grades of TiO2 and graphite have an important impact on the product quality. Other carbothermic methods also have the same disadvantages. The magnetron sputtering and plasma spray methods are usually used to coat a film on the surface of material, but cannot be used for mass preparation of Ti(C, N). Dewan used ilmenite and graphite to react at low temperature (1000 ~ 1300 o C), but there exist a lot of titanium suboxides (Ti2O3, Ti3O5 …) in the product. Jiao and his colleagues could synthesize the pure TiCxNyOz, but pure TiC, TiN and TiO as the raw material are required. Bai's method is similar to the hydrothermal method, but the temperature is higher than that of the hydrothermal method, and the pressure in the device is very high. Ammonia reduction method can reduce the reaction temperature, but NH3 could corrode the equipment seriously and its cost is also very high. Zhang's group and our team have proposed the CH4-bearing gas mixture reduction method, which proves that titanium-bearing mineral can reduced and carbonized at low temperature comparing with the commercial process.
In this study, the CH4-H2-N2 gas mixture has been used to reduce and carbonitride the TiO2. It has been verified that the additive of N2 can further lower the reduction temperature and promote the reaction rate. This method can also be applied in the reduction of ilmenite and titanium-bearing BF slag. Compared with the commercial method, the source of gas mixture is very extensive, such as nature gas, coke oven gas.
The tail gas in this method is mainly composed of H2, N2, CO and a small amount of H2O which can be removed. However, the product by this method is also a mixture phase, so it needs to be further refined to produce the high-quality product.

Conclusion
This work describes the reduction and carbonitridation of TiO2 with CH4-H2-N2 gas mixture, through thermodynamic, experimental, and DFT studies. And some conclusions can be drawn as follows: 1. The reduction of TiO2 is a step-by-step process: TiO2 → Magnéli → Ti3O5 → Ti(N, C, O), and the gas composition and reaction temperature have significant impacts on the final products. 3. The reduction degree and the conversion degree of Ti increase with the reduction time, reduction temperature, and the addition of N2 in the gas system.

5.
The method proposed in this work has shown a huge advantage and feasibility.       Schematic reaction mechanism of TiO2 in CH4-H2-N2 system