Carbothermal reduction assisted by hydrogenation treatment for preparing single-phase CeC2 nanoscale powders

Single-phase CeC2 powders can be successfully prepared through the carbothermal reduction assisted by hydrogenation treatment (CRHT) using 20–50 nm CeO2 as cerium source and 100–200 nm carbon black as carbon source. During the hydrogenation process, the interaction of CeO2 with H2 leads to a partial valence reduction of Ce ions, yielding a partially anoxic phase (CeO1.675) and a hydrogenated phase (CeH2). CeO1.675 can increase the oxygen vacancies required in the reaction process, while CeH2 can produce gas-phase reductant H2 at the reaction temperature, which together promotes the reduction reaction. The results show that the CRHT can obtain single-phase CeC2 powders at a temperature of 1550 °C, which is 100 °C lower than its theoretical temperature. For the preparation of CeC2 powders, the CRHT is a low-temperature and efficient method.


Introduction
Nuclear energy has attracted much attention as a clean and efficient energy source.Uranium (U) and plutonium (Pu) are commonly used metal elements in the field of nuclear engineering.Ceramic nuclear fuels, including oxides [1], carbides [2], nitrides [3], and silicides [4] have excellent nuclear combustion properties, and are considered as ideal candidates for nuclear fuels [1][2][3][4].However, U and Pu are radioactive, which can cause human damage and environmental hazards.To reduce the risks and costs of nuclear energy research, it is particularly important to develop nuclear simulation materials [5].The rare earth element cerium (Ce) has physicochemical properties and an electronic layer structure similar to those of U and Pu [6][7][8].Furthermore, Ce is non-radioactive, making it an ideal simulation element for nuclear engineering [6,9,10].Ce can be used in the preparation of nuclear simulated carbides.
Ce has 4f orbital electrons, which give it active chemical properties and complex chemical states (valence fluctuations of Ce 3+ and Ce 4+ ).At present, there are few reports on the preparation of CeC 2 powders, and existing relevant methods include arc melting [11], arc evaporation [12][13][14][15] and carbothermal reduction reaction(CRe) [16].Arc melting method [11] mainly uses metal elements and carbon rods for repeated melting, which is a tedious process and is mostly used to synthesize two-element carbides.DC arc evaporation [12][13][14][15] of carbonaceous deposits containing 3 wt% CeO 2 composite carbon rods enables the synthesis of nanoscale CeC 2 crystals encapsulated within within gigantic super fullerenes.This method has its limitations and is not suitable for widespread application.The most commonly used CRe method [16] for the preparation of CeC 2 involves the high-temperature reaction of CeO 2 with graphitic carbon at 1850 °C.However, CRe has a high synthesis temperature and low carbide purity.Therefore, it is necessary to improve the existing experimental protocols for the efficient preparation of CeC 2 .In recent years, hydrogen (H 2 ) owing to its strong reducibility, has often been used as a reducing gas to improve the preparation of target products [17][18][19][20][21].It can be inferred that H 2 can assist the CRe in improving the preparation efficiency of CeC 2 .
Herein, we report a method for the preparation of single-phase CeC 2 powders through carbothermal reduction assisted by hydrogenation treatment (CRHT).The effects of hydrogenation on the CRe process and its products are also explored.Meanwhile, the effects of the process parameters such as molar ratio of the raw material, synthesis temperature, holding time and argon (Ar) gas flow rate on the preparation process are studied.Our work provides a new approach for the single-phase preparation of CeC 2 and other carbides.

Materials and methods
Single-phase CeC 2 powders is prepared by CRHT.The schematic of preparing CeC 2 powders is illustrated in figure 1.Firstly, nano CeO 2 (purity of 99.95%, 20-50 nm), nano carbon black (purity of 99.5%, 100-200 nm) and anhydrous ethanol are mixed, followed by mechanical mixing (300 r min −1 , 6 h) and drying (60 °C, 3 h) to obtain the mixture powders.Subsequently, the mixture powders (50-100 g per batch) is subjected to hydrogenation treatment (H 2 , 1000 °C, 2 h) to prepare hydrogenation powders.Finally, the hydrogenation powders (1-5 g per batch) is subjected to CRe under different conditions to obtain CeC 2 powders.
The product powders at each stage (including mixture powders, hydrogenation powders, CRe powders, and CRHT powders) of the experimental process are stored in a glove box to prevent oxidation.All samples are sealed in the glove box, and the moisture and oxygen content in the glove box are less than 1 ppm.Subsequently, an appropriate amount of samples are taken from the glove box for testing, and the process is carried out in accordance with operating procedures and rapid action to reduce the possibility of oxidation.
The phase compositions of the prepared powder are characterized by an x-ray Diffraction (XRD, DX-2700) with Cu Kα radiation at 40 kV/30 mA in the range of 20°−90°.All specimens are characterized by XRD with Mylar Thin-Film as a protective film.The morphology of the products was observed by a Scanning Electron Microscopy (SEM, JOEL JSM-7900F), and the relevant components are analyzed by Energy Dispersive Spectroscopy (EDS, ULTMAX-65) using the JSM-7900F.The compositions and chemical states of the samples are analyzed by x-ray Photoelectron Spectroscopy (XPS, AXIS Ultra DLD).

Theoretical thermodynamic calculations of CeC 2
The Gibbs free energy variable ΔG is an important basis for determining the direction of chemical reactions.In the thermodynamic calculations, the errors in the first and second approximation equations are large at high temperatures.Therefore, the thermodynamics of high-temperature reactions are analyzed by the linear regression with the Gibbs free energy binomial for the standard reaction expressed as equation (1) [22].
Where T i is the temperature, T is the average temperature, G T D Q is the standard reaction Gibbs free energy, is the Gibbs free energy of reaction at a certain temperature and ḠT D Q is the average reaction Gibbs free energy of the reaction.
The equation for the CRe of CeC 2 is given in equation (2).Thermodynamic data for the corresponding reactants and products can be obtained according to the Practical Handbook of Inorganic [22].The thermodynamic data for 298-1300 K are listed in table 1 (corresponding thermodynamic data beyond 1300 K are not available).
Substituting the thermodynamic data in the range of 298-1300 K into equation (1) to obtain a linear regression fitting diagram of Gibbs free energy ΔG versus temperature T (as shown in figure 2) with the fitting equation in equation (3).In figure 2, K and R 2 represent the slope and accuracy of the linear fitting equation for temperature of 298-1300 K, respectively.The accuracy R 2 of the fitting equation is close to 1, which indicates that the CRe of CeC 2 yields good fitting results with high confidence.Thermodynamic data show that the thermodynamic enthalpy change ΔH f > 0 J for CeC 2 in the range of 298-1300 K [22], which implies that the reaction is heatabsorbing and requires external energy to promote the reaction.CRe of CeC 2 can occur under certain temperature conditions.According to the fitting equation in equation (3), it can be inferred that the reaction equilibrium temperature of CeC 2 is 1650 °C.In practice, the choice of the raw materials and process parameters can affect the reaction process.Therefore, the actual reaction temperature is generally higher than the theoretical temperature.

Mechanism of hydrogenation treatment and its effect on the product composition and morphology
CRe is a typical high-temperature solid-phase reaction that is significantly influenced by the morphology and particle size of the raw material.Clearly, with the decrease of the reactant particle size, the relative surface energy increased to improve the reaction rate.Figures 3(b)-(c) shows the morphology of the raw material powder.It can be seen that CeO 2 and carbon black are nanometer spherical powders with a uniform particle size, here, the particle size of CeO 2 is smaller than that of carbon black.The CeO 2 powder adhered to the carbon black powder, as shown in figure 3(d), and the mixture powder (without hydrogenation powder) is uniformly mixed, maintaining the spherical shape of the raw material powders.The homogeneous mixing of powder increases the    As can be seen in figure 3(e), during the hydrogenation treatment process Ce element is better dispersed on the carbon black and bonding occurs between the particles to form a larger size powders.This is because the increase in temperature during the hydrogenation stage enhanced the diffusion of the powder particles, which promoted the combination and homogenization between particles.
As shown in figure 4(a) and table 2, a small amount of CeC 2 was obtained under the CRe condition at 1550 °C.However, the phase of the product is complex, consisting of a mixture of Ce 11 O 20 as the main phase and a small amount of CeC 2 , CeO 2 C 2 and Ce 2 O 3 phases.From figures 4(b)-(c), it can be seen that the particle size of CRe powders is about 136-375 nm and the sphericity of the powders is poor.This indicates that the CRe is incomplete at 1550 °C, and the reaction temperature needs to be further increased.In contrast, only the CeC 2 phase is present in the powders under the CRHT at 1550 °C, which indicates that CRHT is complete and the CRHT powders is single-phase CeC 2 .Under this condition, the corresponding chemical reactions may be as follows.
As can be seen in figures 4(d)-(e), the powder size of CeC 2 is relatively uniform and sphere-like, and its particle size distribution is in the range of 160-300 nm.Clearly, the CRHT reaction temperature of 1550 °C is lower than its theoretical temperature value of 1650 °C, indicating that the hydrogenation treatment provides better reducing condition to reduce the conventional reaction temperature.Combining with figure 3(a) and table 2, it can be seen that a partial reduction of Ce 4+ occurred during the hydrogenation process, generating a partial anoxic phase (CeO 1.675 ) and a hydrogenated phase (CeH 2 ).Among them, CeO 1.675 increased the oxygen vacancy during the CRe process.Meanwhile, CeH 2 can generate H 2 at the temperature of CRe, which combines with carbon reduction to promote the reaction.In addition, the large specific surface area of the nano material powder can increase the contact between the powder, thus shortening the migration distance in the process to facilitate the reaction.Therefore, single-phase CeC 2 powders can be prepared under the CRHT conditions, and the actual synthesis temperature is lower than its theoretical synthesis temperature.
In figure 5(a), CeO 2 , CeO 1.675 and CeH 2 show typical peaks, indicating that the valence changes of Ce have changed during the process.To investigate the local bonding structure, XPS was performed to detect the   Under hydrogenation treatment conditions, the heterolytic cleavage of H 2 molecules results in the formation of one H − and one H + , which adsorb on Ce ion and O anion to form the corresponding Ce-H and O-H bonds, respectively, as shown in equation (5).Due to the opposite charges of Ce and O, an electric field is created on the outer surface that polarizes H 2 as it comes close [23].On the one hand, the polarization of the H-H bond is conducive to the dissociation and adsorption of the heterolytic.On the other hand, the H atoms in the Ce-H portion are transferred to the O ion sites close to the surface.Moreover, homolytic cleavage produces two H atoms that combine with two contiguous surface oxygen sites leading to the formation of two O-H groups, accompanied by the reduction of Ce ions, as shown in equation (6).That is, the interaction of CeO 2 with H 2 produces Ce-H and O-H through the H 2 mutually exclusive homolytic or heterolytic pathways, which is consistent with relevant reports [23][24][25].In this process, a part of CeO 1.675 and CeH 2 is obtained through the interaction of H 2 with CeO 2 , which increases oxygen vacancies in the process and provides a better reduction condition for the CRe.Therefore, the single-phase CeC 2 powders can be prepared by the CRHT at a lower temperature than the theoretical value.(

Effect of different parameters on the product
Figure 6 shows the XRD patterns of the CRHT powders at different parameters.In the CRe, carbon as a reducing agent significantly has a large influence on the product phase composition.In figure 6(a), the product phase obtained at the theoretical molar ratio (CeO 2 :C = 1:4) includes Ce 11 O 20 , CeO 2 C 2 and CeC 2 phases, which are not single-phase.With the increase of the carbon content in the raw material ratio, the degree of the CRe deepened, and single-phase CeC 2 could be obtained until the molar ratio of reactants increased to 1:6.As shown in figure 6(b), with the temperature increase, the process proceeded as follows the product evolution: CeO 2 →Ce 2 O 3 →Ce 11 O 20 →CeC 2 O 2 →CeC 2 .Furthermore, the synthesis temperature of the single-phase CeC 2 powders is 1550 °C, which is lower than its theoretical value and the CRe temperature [15,19].The contact area between the raw materials in the solid-phase reaction is limited, and sufficient reaction time is required for solidphase diffusion to ensure that the reaction proceeds.Figure 6(c), shows the effect of holding time on the product phases.With the increase of holding time, the product evolves from a mixed phase to single-phase CeC 2 , and single-phase CeC 2 can be obtained by holding for 4 h under the CRHT conditions.Ar is involved in the CRHT process as a protective gas.The flowing Ar gas can carry the gaseous CO generated in the reaction process out of  the reaction system to promote the reaction.As shown in figure 6(d), the Ar gas flow rate of 800 ml min −1 can effectively remove CO and promote the preparation of single-phase CeC 2 .Table 3 shows the XRD phase compositions of the CRHT powders under different conditions.Obviously, single-phase CeC 2 powders can be prepared under CRHT conditions with a raw material ratio of 1:6 for nano CeO 2 and nano carbon black, synthesis temperature of 1550 °C, holding time of 4 h, and Ar gas flow rate of 800 ml min −1 .Importantly, the synthesis temperature of the CRHT was significantly lower than its theoretical temperature.
Figure 7 shows the morphology and elemental distribution of CeC 2 powders under the optimal parameter conditions.The micromorphology of CeC 2 in figures 7(a)-(c) is a sphere-like nanoscale powder, which basically maintains the shape of the raw materials.Figures 7(d)-(e) shows the mapping of Ce and C elements in figure 7(c), respectively, where Ce and C are uniformly distributed.This indicated that the single-phase CeC 2 powders can be prepared by CRHT, and the CeC 2 powders is a sphere-like nanoscale powder with a uniform particle size.

Conclusions
The single-phase CeC 2 powders was successfully prepared by the CRHT using nano CeO 2 and nano carbon black as the raw materials.Firstly, the theoretical calculation of the reaction for the preparation of CeC 2 by CRe was performed.The mechanism of action of the hydrogenation treatment and its effect on the CRe process are investigated.In addition, the effects of different parameters such as the molar ratio of the raw materials, synthesis temperature, holding time and Ar gas flow rate on the product phases are also explored.
These results indicate that the corresponding Ce-H and O-H bonds can be formed in the interaction of CeO 2 with H 2 , where part of Ce 4+ undergoes valence reduction and both CeO 1.675 and CeH 2 are obtained.Among them, CeO 1.675 phase can increase the oxygen vacancies required in CRe.In addition, CeH 2 can be cleaved at high temperatures to produce H 2 , which can be used as a gas-phase reducing agent to promote reduction reactions.Meanwhile, the larger specific surface area of the raw material nano powder can increase the contact between the powder and shorten the atomic migration distance in the reaction.Based on these, the actual synthesis temperature of the CeC 2 powders prepared by CRHT was lower than its theoretical temperature value.In the CRHT, with the increase of nano carbon black (reducing agent) content in the molar ratio of the raw material, the raise of synthesis temperature, the extension of holding time, and the increase of Ar gas flow rate, the product generally show a trend of reduction from sub-reduced state to single-phase CeC 2 .The spherelike single-phase CeC 2 powders with a particle size distribution in the range of 160-300 nm was prepared under the CRHT conditions with a raw material ratio of 1:6 between nano CeO 2 and nano carbon black, a synthesis temperature of 1550 °C, a holding time of 4 h, and an Ar gas flow rate of 800 ml min −1 .

Figure 1 .
Figure 1.Schematic diagram of the CRHT used for the preparation of CeC 2 powders.

Figure 2 .
Figure 2. Gibbs free energy variable diagram of CeC 2 preparation by CRe.
contact area between particles, thus shortening the migration distance of C and O atoms during the reaction, which facilitates the reaction of the CRe. Figure 3(a) shows the XRD patterns of with and without the hydrogenation powder.The carbon black existed in the amorphous form, and only the CeO 2 phase diffraction peaks are present in the untreated powder.It is obvious that the diffraction peaks of CeO 1.675 phases and CeH 2 phases exist in addition to the main phase of CeO 2 in the hydrogenation treatment powder.This indicates that the chemical valence of Ce is changed according to the hydrogenation treatment, and part of Ce 4+ is reduced.The corresponding SEM patterns are shown in figures 3(d)-(e).

Figure 3 .
Figure 3. (a) XRD pattern of with and without hydrogenation treatment powders; SEM images of the raw material, with and without hydrogenation treatment powders: (b) CeO 2 powders, (c) carbon black powders, (d) CeO 2 and carbon black mixing powders, (e) hydrogenation treatment powders.

Figure 4 .
Figure 4. XRD and SEM images of the powders: (a) XRD patterns of the CRe powders and CRHT powders, (b) SEM image of the CRe powders, (c) particle size distribution of the CRe powders, (d) SEM image of the CRHT powders; (e) particle size distribution of the CRHT powders.
elements present in the hydrogenation treatment powder.Mainly three types of elements C, O, and Ce are present in the powder, as shown in figure 5(b).The C element can be entirely attributed to carbon black of the raw material.The characteristic peaks shown in figures 5(c)-(d) are due to O and Ce elements.There are two peaks observed in the O 1s spectrum in figure 5(c).The lower binding energy component peak (529.7 eV) is attributed to the lattice oxygen of Ce 4+ -O 2− , whereas the higher binding energy component peak (531.5 eV) is due to the defective oxygen of Ce 3+ -O 2− .As shown in figure 5(d), the Ce 3d spectrum is composed of multiple doublets (u + v) corresponding to the spin-orbit split of 3d 3/2 and 3d 5/2 .The peaks labeled as u″ (903.7 eV) and v″ (885.5 eV) can be indexed to Ce 3+ , while peaks labeled as u‴ (917.1 eV), u′ (907.1 eV), u (901.2 eV), v‴ (899.1 eV), v' (888.8 eV) and v (882.7 eV) are assigned to Ce 4+ .The different chemical valence states of Ce are due to the formation of oxygen vacancies caused by the escape of oxygen ions from the lattice.

Figure 6 .
Figure 6.XRD patterns of the CRHT products under different parameters: (a) molar ratio of raw materials, (b) reaction temperature, (c) holding time, (d) Ar gas flow rate.

Table 1 .
Thermodynamic data for CeC 2 preparation by CRe.

Table 2 .
Phase composition of the CRe and CRHT powders.

Table 3 .
Phase compositions of the CRHT powders under different parameters.