A single batch synthesis of pure phase Mo₂C from ammonium molybdate: pathway and properties

Abstract

This study presents an original, effective, and environmentally friendly method for synthesizing pure molybdenum carbide (Mo₂C) from ammonium molybdate tetrahydrate (AMT) without generating carbon dioxide, a greenhouse gas. The process involves the sequential transformation of AMT to Mo₂C, which follows the reaction pathway of (NH₄)₆Mo₇O₂₄→MoO₃→MoO₂→Mo→Mo₂C. This transformation is achieved by strategically altering the gas atmosphere, switching from Ar to H₂ at 800 K and then from H₂ to CH₄ at 1000 K. Thermal analysis, X-ray diffraction (XRD), and scanning electron microscopy (SEM) techniques were used to characterize AMT and the products. Mass measurements were used to follow the conversion of AMT to intermediate products and to the final product (Mo₂C). It was found that 57.67% of AMT was converted to Mo₂C, in agreement with the theoretical value (57.74%). Differential scanning calorimetry/thermogravimetry curves revealed four steps at 401 K, 495 K, 507 K, and 595 K during AMT decomposition to MoO₃. XRD patterns revealed the formation of phase-pure Mo₂C, with characteristic diffraction peaks 2θ = 34.176°, 2θ = 37.712°, and 2θ = 39.197° assigned to the (100), (002), and (101) crystal planes, respectively. SEM images showed that fine Mo₂C particles with a thickness of 0.1 μm was obtained from very coarse AMT particles (>50 μm). In order to determine the solid and gaseous phases likely to form during the reaction, thermodynamic analysis using Gibbs’ free energy minimization method was also carried out prior to synthesis. The reduction reactions and the resulting morphologies of the synthesized materials were discussed in terms of thermodynamic results and density changes associated with the conversions. This study demonstrates a novel reaction pathway that sequentially converts the molybdenum species from Ammonium Molybdate Tetrahydrate (AMT) to the final Mo₂C phase without the release of CO₂.

1. Introduction

In the 21^st century, the environmental impact of greenhouse gas emissions has become a growing concern, intensifying the need for eco-friendly approaches in material synthesis [1]. Developing innovative methods to minimize the release of greenhouse gases, such as methane and carbon dioxide, has emerged as an increasingly crucial priority. Recently, numerous studies have focused on exploring alternatives to carbon-based reductants like coal and natural gas and have investigated the use of hydrogen as a means to reduce greenhouse gas emissions, particularly in energy-intensive industries like iron and steel manufacturing [2].

Molybdenum carbide (Mo₂C) is a highly versatile material that offers a wide range of promising applications across various fields. Its exceptional physical properties, such as high hardness (1800 kgf/mm²), high melting point (2960 K), and exceptional chemical stability, make it an attractive additive for the development of hard coatings and wear-resistant components [3]. Moreover, Mo₂C has demonstrated significant potential in thermoelectric and electrochemical catalysis, as well as energy storage applications, making it a sought-after material in the development of advanced technologies and devices [4].

Traditionally, Mo₂C has been produced through the carburization of molybdenum powders using solid carbon as the carbon source. This process typically requires elevated temperatures ranging from 1673 K to 1773 K [5]. However, the use of solid carbon reactants necessitates such high temperatures, which has prompted researchers to explore alternative synthesis methods that can potentially operate at lower temperatures or with different carbon precursors.

In recent years, researchers have explored the use of gaseous carbon-containing precursors, such as methane, as alternative carbon sources for the synthesis of molybdenum carbide (Mo₂C). These gaseous precursors offer potential advantages over traditional solid carbon sources, as they can enable the synthesis of Mo₂C at lower temperatures and potentially reduce the emissions of greenhouse gases like carbon dioxide during the production process. For instance, Mo₂C has been synthesized from MoO₃ at 900 K using an H₂-CH₄ gas mixture [6]. The production of Mo₂C powders typically involves the reaction of MoO₃ with gaseous reactants comprising a mixture of H₂ and hydrocarbons [6–9].

The use of methane and other hydrocarbon gases as carbon sources has been investigated to develop more environmentally friendly and energy-efficient methods for the fabrication of this versatile ceramic material [10]. However, this direct reaction often leads to the generation of harmful gases such as CO and CO₂, which contribute to the greenhouse effect. The methods used to synthesize Mo₂C are not environmentally friendly or cost-effective. To mitigate these environmental concerns, direct reactions of hydrocarbons and CO with MoO₃ should be avoided in the fabrication of Mo₂C. Therefore, the synthesis of Mo₂C from AMT necessitates the implementation of intricate and costly multi-step procedures. In a recent study on the synthesis of pure Mo₂C by a one-pot technique with AMT as the starting material and hexamethylenetetramine (C₆H₁₂N₄) as the reducing agent, the AMT/hexamethylenetetramine molar ratio was set at 1:8, the reaction temperature was maintained at 1073 K, and the reaction time was set at 8 h. These experimental parameters were employed in a specially designed autoclave. It has been reported that in the carburization of MoO₂ with C₆H₁₂N₄, the formation of H₂O, CO₂, and N₂ gases has been observed as gaseous products in addition to the reaction product Mo₂C [11].

This study presents an innovative approach to produce pure phase molybdenum carbide (Mo₂C) from ammonium molybdate tetrahydrate as precursor material in a shorter reaction time using an efficient single batch synthesis method with the aim of minimizing greenhouse gas emission by preventing CO and CO₂ gas formation. The key aspect of this approach is that it does not generate carbon dioxide (CO₂) as a byproduct during the synthesis process. Previous research has shown that gaseous carbon-containing compounds, such as methane, can be used as potential carbon sources for Mo₂C synthesis. Additionally, using hydrogen as a reducing agent has been demonstrated to minimize the formation of greenhouse gases like CO₂. Building upon these earlier findings, the study presents a novel reaction pathway that sequentially converts the molybdenum species from AMT to the final Mo₂C phase without the release of CO₂.

2. Materials and methods

AMT (H₂₄Mo₇N₆O₂₄.4H₂O) powder with a purity of 99.98%, supplied by Sigma Aldrich, was used as the Mo source. The hot-wall furnace employed for the experiments consisted of Fe-Cr-Al alloy heating elements, a quartz tube (diameter 20 mm × length 50 mm), gas lines (Ar, H₂, and CH₄), and gas flow meters. Prior to the experiment, the furnace was cleaned and purged with Ar to remove any residues that may lead to CO₂ formation. An alumina boat filled with AMT powder (0.15 g) was heated at a rate of 15 K/min to 800 K under an Ar flow (42.5 standard cubic centimeters per minute, sccm). The gas was then switched to H₂ (370 sccm) at 800 K, followed by heating at 15 K/min from 900 K to 1000 K. After holding for 15 min in H₂, only CH₄ flowed (13.4 sccm) for 45 min at 1000 K. This was followed by furnace cooling under Ar flow (42.5 sccm). The experiment was interrupted at points B, C, and Dto characterize the intermediate products.

The extents of the reactions were calculated by

$$\text{Mass\hspace{0.17em}}(\%)=({\text{m}}_{\text{p}}/{\text{m}}_{\text{o}})\times 100,$$ (1)

where m_p is the product mass and m_o is the original AMT mass. Mass measurements were conducted at room temperature via an electronic balance (Sartorius) with a sensitivity of ±10⁻⁴ g. Differential Scanning Calorimetry (DSC) coupled with Thermogravimetric Analysis (TGA) curve of AMT powder (51.168 mg) was determined using equipment (Perkin Elmer STA6000 model) in dried air at a flow rate of 100 sccm during heating at a rate of 10 K/min to 1000 K. X-ray diffraction (XRD) analysis was used to identify phases via a Rigaku D/Max-2200/PC instrument with a Cu radiation tube (λ K_α1 = 0.15406 nm). Morphologies were examined via a scanning electron microscope (SEM) (FEI Quanta FEG250) operated at 5 kV under low vacuum.

A computer program utilizing the Gibbs free energy minimization method [12,13] was employed to forecast the compositions in the Mo-O-H (MoO₃-H₂) and Mo-C-H (Mo-CH₄) system’s condensed and gaseous phases at a specified temperature, input composition, and pressure (1 atm). Previous to computation, a separate input file was prepared for each temperature. Gibbs free energy values for species are derived from the thermochemical tables [14,15]. An input data file for the Mo-O-H system contained 14 gaseous species (e.g., Mo, MoO₂, MoO₃, H₂MoO₄, H, H₂, H₂O, H₂O₂, O₂) and 6 solid (Mo, MoO₂, MoO₃, Mo₄O₁₁, Mo₈O₂₃, and Mo₉O₂₆) components at temperatures ranging from 800 to 1000 K. The input data file for the Mo-C-H system included 39 gaseous species (e.g., Mo, CH, CH₂, CH₃, CH₄, C₂H, C₂H₂) and 4 solid phases (Mo, C, MoC, and Mo₂C) at a temperature of 1000 K.

3. Results and discussion

3.1. Thermodynamic analysis

Thermodynamic analysis provides a fundamental understanding of phase transformations and reaction equilibria in the synthesis process. This analysis is crucial for determining the optimal conditions required to achieve the desired Mo and Mo₂C phase. In this study, thermodynamic calculations were conducted to examine the reduction and carburization steps in the Mo-O-H and Mo-C-H systems. The equilibrium states of solid and gaseous phases were predicted at different temperatures and gas compositions, offering valuable insights into the reaction mechanisms that govern the formation of pure-phase Mo₂C [12,13]. Figure 1a illustrates the equilibrium solid phase diagram computed in the Mo-O-H system, showing the phase stability as a function of temperature and input H₂ content. Four solid phase regions are revealed, which are MoO₃+MoO₂, MoO₂, MoO₂+Mo, and Mo. The reduction of molybdenum oxide to Mo is complete under the H₂ atmosphere at various H₂ amounts, including over 4.43 atm at 800 K and over 1.32 atm at 1000 K. Figures 1b–d display the variations in the partial pressures of gaseous species at (b) 800 K, (c) 900 K, and (d) 1000 K as a function of input H₂ content. As a result of the calculations, only gas components with a partial pressure of over 10⁻¹⁰ atm are considered for the graphical representation of the gas components in the system. Previous experimental studies on the volatilization of molybdenum in water vapor atmospheres have suggested the formation of gaseous hydrated molybdenum species. Suggested and necessary major volatilization reactions in the literature are given in Eqs. 2–4[14,16].

Figure 1.

(a) Equilibrium solid phase diagram computed in the MoO₃-H₂ system as a function of temperature and input H₂ content. Variations of the partial pressures of the species with the input H₂ content at (b) 800 K, (c) 900 K, and (d) 1000 K.

$${\text{MoO}}_3(\text{s})+{\text{H}}_2\text{O}(\text{g})\to {\text{MoO}}_2{(\text{OH})}_2(\text{g})$$ (2)

$${\text{MoO}}_2(\text{s})+2{\text{H}}_2\text{O}(\text{g})\to {\text{MoO}}_2{(\text{OH})}_2+{\text{H}}_2(\text{g})$$ (3)

$${\text{MoO}}_2(\text{s})+2{\text{H}}_2\text{O}(\text{g})\to {\text{MoO}}_3.{\text{H}}_2\text{O}+{\text{H}}_2(\text{g})$$ (4)

As seen in Figure 1b–c, the abundance of both H₂ and H₂O remains notably high across the studied temperature range, as indicated by the green and blue curves. The thermodynamic calculations indicate that the formation of gaseous MoO₂(OH)₂ species increases as the temperature rises, as indicated by the black curve. As the temperature increases, the thermodynamic stability of MoO₂(OH)₂ shifts, leading to an increase in its partial pressure under equilibrium conditions. This trend is supported by Gibbs free energy minimization method, which indicates that MoO₂(OH)₂ formation becomes more favorable at higher temperatures within the given reaction atmosphere. Additionally, its presence is influenced by the reduction pathways of MoO₃ and MoO₂, where hydrogen plays a critical role in determining the intermediate species present in the system. For example, at temperatures of 800 K and 1000 K, the partial pressures of MoO₂(OH)₂ are around 5.58 × 10⁻⁶ atm and 3.18 × 10⁻³ atm, respectively, with H₂ gas at 0.09 atm. These results align well with previous experimental findings, which suggest that the presence of water vapor enhances the volatility of molybdenum by forming hydrated gas-phase species such as MoO₂(OH)₂ and MoO₃.H₂O [17–19]. Moreover, the relevant molybdenum gas species is identified as H₂MoO₄ in the JANAF thermochemical tables. However, in line with previous experimental studies, this species will be referred to as MoO₂(OH)₂ throughout this manuscript to ensure consistency with observed volatilization mechanisms. Figures 1c–1d also shows that the partial pressure of H gas is significantly lower, as indicated by the red curve.

Figure 2a presents the equilibrium solid-phase diagram for the Mo–C–H system, illustrating the transformation of solid phases as a function of input CH₄ content. The diagram highlights the sequential formation of different molybdenum-containing phases, demonstrating the stability regions of Mo, Mo₂C, MoC, and C under varying CH₄ content at a given temperature. As seen from the diagram, there are five phase fields in the Mo–C–H system at 1000 K: Mo + Mo₂C, Mo₂C, Mo₂C + MoC, MoC, and MoC + C. As CH₄ content is raised to about 0.5 mol, Mo₂C phase is increasingly formed and a single Mo₂C is obtained in the range of 0.5–0.57 mole of CH₄. MoC phase appears at higher CH₄ contents, free C forms along with MoC above 1.19 mol CH₄. Figure 2b illustrates the variations in the gas content as a function of input CH₄ amount at 1000 K. The graph provides insights into the equilibrium distribution of different gas-phase species in the system, highlighting the relative abundance of key components. Among these, H₂ appears to be the major species and has the highest partial pressure close to 1 atm. The other species, CH₄, CH₃, H, C₂H₄, and C₂H₆ are predicted to have much lower partial pressures. As the amount of CH₄ increases above 0.5 mol, the partial pressures of hydrocarbon species slightly rise. The increase in the partial pressures of carbonaceous species when Mo₂C is present can be attributed to the continuous decomposition of methane and the limited capacity of Mo₂C to incorporate additional carbon atoms. Once the Mo₂C phase is fully formed, further dissociation of CH₄ leads to the formation of excess carbon-containing gas-phase species such as CH₃, C₂H₄, and C₂H₆, which accumulate in the gas phase as they are no longer consumed in carbide formation. This phenomenon has also been observed in studies investigating methane decomposition over molybdenum carbide catalysts, where the limited carbon uptake capacity of the carbide led to an increase in hydrocarbon gas species in the gas phase [20]. Moreover, in-situ studies have demonstrated that molybdenum carbides can form dynamically under reactive environments, influencing the interaction between CH₄ and metal surfaces, further affecting gas-phase composition [21]. The phase transitions occur due to the progressive carburization of molybdenum by methane decomposition. The system is modeled under thermodynamic equilibrium conditions in the Mo-CH₄ system at 1000 K, where CH₄ is introduced as the carbon source, and its interaction with molybdenum leads to the formation of carbide structures. The primary reactions governing these transformations are given in Equations 5–6. These reactions result in the sequential formation of Mo₂C, MoC, and free carbon as a function of increasing CH₄ amount.

Figure 2.

(a) Equilibrium solid phase diagram computed in the Mo-CH₄ system as a function of input CH₄ content, (b) variations of partial pressures of the gaseous products with the input CH₄ content at 1000 K.

$$\text{Mo}(\text{s})+\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.{\text{CH}}_4(\text{g})\to \raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.{\text{Mo}}_2\text{C}(\text{s})+{\text{H}}_2(\text{g})$$ (5)

$${\text{Mo}}_2\text{C}(\text{s})+3{\text{CH}}_4(\text{g})\to 2\text{MoC}(\text{s})+\text{C}(\text{s})+6{\text{H}}_2(\text{g})$$ (6)

The thermodynamic model assumes that CH₄ undergoes dissociation, producing intermediate hydrocarbon species and hydrogen gas. These gaseous species play a crucial role in the overall carburization mechanism, influencing the formation of molybdenum carbide phases at different pressure regimes. Some decomposition reactions of methane into solid carbon and hydrogen are described by Equations 7–9.

$${\text{CH}}_4\to \text{C}(\text{s})+2{\text{H}}_2$$ (7)

$${\text{CH}}_4\to {\text{CH}}_3+\text{H}$$ (8)

$${\text{CH}}_3\to \text{C}(\text{s})+{\text{H}}_2$$ (9)

Thermodynamic calculations suggest that the MoC+C phase can be obtained as an end product at methane gas pressures higher than 1.18 mole. Furthermore, this compound undergoes significant decomposition at 1100 K, generating the gaseous species H₂ and C_xH_y [22]. However, based on the X-ray diffraction (XRD) findings, the experimental results confirm that the Mo₂C phase was formed at 1000 K in the methane atmosphere. Thermodynamic approximations contribute to the thermochemistry of the system by calculations in closed systems under constant pressure and provide predictions for experimental studies. However, it should be considered that under experimental conditions, some of the reactant gas reacts with the powder, and some flows over the powder bed and is discharged through the furnace outlet. Due to this situation, there may be a discrepancy between the thermodynamic approach and the experimental results.

3.2. DSC/TGA analysis of AMT

Figure 3 shows the DSC/TGA curves of the AMT powder used. The TGA curve indicates that the mass of the sample powder decreases as the temperature is raised from 360 K to 610 K. The mass loss occurs in four steps, which are marked with arrows, in this temperature range. There are also four DSC peaks associated with each step. The first step with a mass loss of 3.1% and an endothermic peak at 401 K occurs in the temperature range 360–420 K. This step was attributed to removing some of the weakly bonded water molecules from AMT [23]. The second and third steps, with the corresponding endothermic peaks at 495 K and 507 K, were observed between 460 K and 520 K. It was reported that the endothermic peaks were ascribed to the evolution of intercalated crystalline water and ammonia [24]. Since there is a mass loss at this stage, as shown in the figure, it is plausible to suggest that some of the species formed are removed from the solid to the gas phase. The fourth step appears to be at temperatures between 540 K and 610 K, where an endothermic peak is observed at 595 K. The last peak is reported to be assigned to the loss of remaining crystalline water and ammonia [24]. Beyond 610 K, the mass remains almost constant at 81.79%, corresponding to a total mass loss of 18.21%. This mass loss is close to the theoretical value (18.45%) calculated in accordance with Equation 10, which expresses the decomposition of AMT into solid MoO₃ and the gaseous reaction products NH₃ and H₂O. This suggests that NH₃ and H₂O species are removed from AMT during heating in air. The reaction expressed in Equation 10 is given.

Figure 3.

DSC/TGA curves of AMT powder in air.

$${({\text{NH}}_4)}_6{\text{MO}}_7{\text{O}}_{24}.\hspace{0.17em}4{\text{H}}_2\text{O}\to 7{\text{MoO}}_3(\text{s})+6{\text{NH}}_3\uparrow +7{\text{H}}_2\text{O}\uparrow$$ (10)

3.3. Synthesis of Mo₂C

The masses of AMT held in the tubular furnace in Ar at 900 and 1000 K for 2 h were measured to be 81.44% and 78.60%, which are below the theoretical mass (81.55%) of the conversion of AMT to MoO₃. These results indicate that under the conditions studied, there is a slight Mo loss into the gas phase at 900 K and 1000 K. At 800 K for 4.5 h, it was 81.55%, which is the same as the theoretical value, suggesting no Mo loss to the gas phase. Hence, the AMT sample held at 800 K for 4.5 h was used for the synthesis of metallic Mo and Mo₂C.

Figure 4 shows the variation in AMT mass with time under Ar, H₂, and CH₄ atmospheres represented by the black curve. The red curve illustrates the variation in temperature with time. The points on the curves represent the type of gaseous atmosphere. The AMT precursor (NH₄)₆Mo₇O₂₄.4H₂O undergoes thermal decomposition under an Ar atmosphere as the temperature reaches 800 K. After heating in Ar to 800, the mass is measured to be 81.44% at Point B, which is close to the theoretical mass (81.55%) expected for the conversion of AMT to MoO₃. This stage marks the formation of MoO₃, a crucial intermediate phase before reduction. The decomposition process releases volatile ammonium species, leaving behind the molybdenum oxide as the solid phase. At Point C, when the system is under H₂ flow, the solid curve in Figure 4 shows a gradual mass decrease as the sample is heated to 1000 K due to the reduction of MoO₃ to MoO₂ and Mo. After 15 min of H₂ flow at 1000 K, the mass is stabilized at 54.08%, which closely matches the theoretical value for complete AMT to Mo conversion (54.36%), as indicated by the horizontal dashed line. The H₂-reduction of MoO₃ can be expressed by Eq. 12. The sample is held at 1000 K in H₂ for 15 min at Point D, leading to the complete reduction of Mo oxides to metallic Mo. Hence, the mass loss is attributed to the removal of oxygen from MoO₃. This means that Mo is available in its pure form before carburization in the next stage. The mass increases to 57.67% at Point E within 45 min of CH₄ flow at 1000 K, which is close to the theoretical value for AMT to Mo₂C conversion mass (57.74%). This result indicates that Mo gained mass by C uptake. This final transformation corresponds to the carburization of Mo to form Mo₂C, expressed by Eq. 14. This step marks the completion of the reaction sequence, where Mo reacts with CH₄ to form molybdenum carbide (Mo₂C) as the dominant phase. The successful conversion of Mo to Mo₂C, marking the completion of the reaction sequence, was confirmed by XRD data. The reaction equations mentioned are given in the following section after the discussion of the XRD results.

Figure 4.

Variation in the AMT mass with temperature and time in Ar, H₂ and CH₄ atmospheres. The red line represents the temperature-time profile.

The reaction kinetics and phase transition dynamics of Mo₂C formation involve sequential reduction and carburization steps, which are strongly dependent on temperature, gas composition, and precursor reactivity. The reduction of MoO₃ to Mo occurs through an intermediate MoO₂ phase, as confirmed in previous studies on the thermochemical transformation of molybdenum oxides [25,26]. The complete reduction to metallic Mo is a critical step before carburization, where CH₄ decomposition facilitates carbon incorporation into the Mo lattice, leading to Mo₂C formation. Thermodynamic calculations predict that the reaction follows a stepwise reduction pathway, in agreement with previous reports detailing Mo₂C synthesis from molybdenum oxides [27].

Figure 5 shows the XRD patterns of the AMT powder and the products derived from it. Pattern a presents the characteristic peaks of AMT with a monoclinic crystal structure reported in powder diffraction files (PDFs) no. 00-027-1013 [28]. Pattern b belongs to the product obtained when the AMT was heated to 800 K in Ar. All the diffraction peaks correspond to MoO₃ with an orthorhombic crystal structure (PDF 00-05-0508). Pattern c reveals that MoO₃ is reduced to a mixture of MoO₂ and Mo phases during heating to 1000 K. After 15 min of holding at 1000 K in H₂, the product exhibits only Mo peaks (pattern d). Subsequent holding at 1000 K for 45 min under a CH₄ atmosphere yields pure-phase Mo₂C, as revealed by pattern e. The diffraction peaks in this pattern are assigned to closed-packed hexagonal Mo₂C as the interplanar spacing, and the intensities of the peaks agree with those listed in PDFs 035-0787. For example, the first three peaks at 2θ = 34.176°, 2θ = 37.712°, and 2θ = 39.197° are assigned to the (100), (002), and (101) crystal planes, respectively. The calculated interplanar spacings (0.2621 nm, 0.2383 nm, and 0.2296 nm) and their relative intensities (18, 23, 100) match well with the published standard values (0.2608 nm, 0.2367 nm, 0.2285 nm, 20, 25, 100). The XRD patterns obtained in this study confirm the sequential phase transformation from AMT to Mo₂C. The diffraction peaks observed for Mo₂C closely match those reported in previous studies, for example, the peaks at 2θ = 34.3°, 37.9°, and 39.5°, which correspond to the (100), (002), and (101) planes of Mo₂C [29]. These findings align with studies reporting Mo₂C synthesis using CH₄/H₂ gas mixtures, where similar peak positions and relative intensities were observed [25]. Furthermore, phase purity is a crucial aspect of Mo₂C synthesis. Some studies have reported the formation of secondary phases such as MoO₂ and MoC when reaction conditions deviate from thermodynamically favorable regions [30]. However, the present study confirms that the optimized reaction conditions successfully yield phase-pure Mo₂C, as indicated by the absence of MoO₂ or MoC peaks in the XRD patterns. These results demonstrate that the proposed synthesis method effectively achieves high purity comparable to established Mo₂C synthesis techniques.

Figure 5.

XRD patterns of AMT (a) and the products obtained at the conditions marked (b) Heating in Ar to 800 K – AMT to MoO₃ conversion (point B), c) Heating in H₂ from 800 K to 1000 K – Formation of MoO₂ and Mo (point C) d) Isothermal holding in H₂ at 1000 K – Complete reduction to metallic Mo (point D), e) Isothermal holding in CH₄ at 1000 K to complete carburization to Mo₂C (point E)

Based on the XRD patterns, it is plausible to suggest the primary reaction steps as follows: ${({\text{NH}}_4)}_6{\text{Mo}}_7{\text{O}}_2\stackrel{Ar}{⃗}{\text{MoO}}_3\stackrel{H_2}{⃗}>{\text{MoO}}_2\stackrel{H_2}{⃗}\text{Mo}\stackrel{C{H}_4}{⃗}{\text{Mo}}_2\text{C}$. The overall reactions leading to the MoO₃, MoO₂, Mo, and Mo₂C phases are expressed by Eqs.11–14.

$${({\text{NH}}_4)}_6{\text{Mo}}_7{\text{O}}_{24}.\hspace{0.17em}4{\text{H}}_2\text{O}\to 7{\text{MoO}}_3+6{\text{NH}}_3+7{\text{H}}_2\text{O}$$ (11)

$${\text{MoO}}_3+{\text{H}}_2\to {\text{MoO}}_2+{\text{H}}_2\text{O\hspace{0.28em}}\mathrm{\hspace{0.17em} }\mathrm{\hspace{0.17em} }\mathrm{\Delta }{{\text{G}}^{\text{o}}}_{\text{r}}=-104140\hspace{0.17em}\text{J}/\text{mol\hspace{0.17em}at\hspace{0.17em}}1000\hspace{0.17em}\text{K}$$ (12)

$${\text{MoO}}_2+2{\text{H}}_2\to \text{Mo}+2{\text{H}}_2\text{O\hspace{0.28em}}\mathrm{\hspace{0.17em} }\mathrm{\hspace{0.17em} }\mathrm{\Delta }{{\text{G}}^{\text{o}}}_{\text{r}}=20217\hspace{0.17em}\text{J}/\text{mol\hspace{0.17em}at\hspace{0.17em}}1000\hspace{0.17em}\text{K}$$ (13)

$$\text{Mo}+\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.C{\text{H}}_4\to \raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.{\text{Mo}}_2\text{C}+{\text{H}}_2\mathrm{\hspace{0.17em} }\mathrm{\hspace{0.17em} }\mathrm{\hspace{0.17em} }\mathrm{\Delta }{{\text{G}}^{\text{o}}}_{\text{r}}=-38885\hspace{0.17em}\text{J}/\text{mol\hspace{0.17em}at\hspace{0.17em}}1000\hspace{0.17em}\text{K}$$ (14)

where ΔG^o_r is the Gibbs’ free energy change of the reaction. Reactions expressed by Equations (12) and (14) are thermodynamically favorable owing to the negative ΔG^o_r values. ΔG°_r value for Equation 13 is positive at 1000 K, suggesting that the reaction is not favorable. However, the reduction of MoO₂ by H₂ is possible when the partial pressure of water is relatively low [31].

A complex series of reactions during CH₄ decomposition leads to the formation of intermediate hydrocarbons (e.g., CH₃, CH₂, CH, and C) that gradually become hydrogen-poor [32]. The reaction between C adsorbed on the particle surface and the Mo particle may yield Mo₂C. Rather than producing CO₂, reactions expressed by equations (11–14) and thermodynamic calculations do not yield CO₂ as a product, but gaseous products such as NH₃, H₂O, and H₂. Furthermore, CH₄ was introduced to the furnace after single-phase elemental Mo was used to avoid CO₂ formation during the carburization process.

Figure 6a-e shows SEM images of the AMT, MoO₃, Mo+MoO₂, Mo, and Mo₂C powders, respectively. The AMT powder mostly consists of large particles (>50 μm) with highly stacked features (image a). When AMT is converted to MoO₃, finer particles appear, as seen in image b. The morphology is disturbed, as revealed by the rough surfaces when MoO₃ is partially reduced to Mo (image c). The Mo powder typically consists of platelets with thicknesses ranging from approximately 160 to 600 nm (image d). After the carburization process, the Mo₂C product has a morphology substantially similar to that of Mo, as revealed by image e. A finer morphology (thickness ≈100 nm) is observed as the transformation takes place in the path of AMT (monoclinic) > MoO₃ (orthorhombic), MoO₂ (monoclinic) >Mo (b.c.c.), and Mo₂C (hexagonal). The changes in morphology and size can be attributed to cracks, pores, fissures, and delamination due to vacancy coalescence and stresses formed during the volumetric changes induced by the structural transformations. The densities are 2.50, 4.69, 6.47, 10.28, and 8.9 g/cm³ for AMT, MoO₃, MoO₂, Mo, and Mo₂C, respectively. The AMT to MoO₃, MoO₃ to MoO₂, and MoO₂ to Mo transformations all create tensile stresses in the transformed layers when the ratio of the volume of the product to that of the parent phase is less than unity. Tensile stresses can fragment the particles into smaller ones. The conversion of Mo to Mo₂C creates compressive stress in Mo₂C (volumetric ratio > 1), which may lead to delamination of the platelets. Some defects in the Mo₂C particles are possibly inherited from the thermal decomposition of AMT and oxide reduction processes. Rapid reduction/carburization occurs due to direct contact of the reactive gases (H₂ and CH₄) with fresh parent surfaces through surface cracks and through highly porous, cracked layers of Mo and Mo₂C. The SEM images presented in this study illustrate the morphological transformations occurring throughout the synthesis, from AMT to Mo₂C. The formation of intermediate phases, such as MoO₃ and MoO₂, results in notable changes in particle morphology, as observed in previous studies on Mo₂C synthesis [7]. The transition from MoO₃ to Mo results in the formation of platelet-like structures, which are retained upon carburization to Mo₂C. This is consistent with previous findings, where Mo₂C synthesized via C₃H₈ carburization exhibited similar morphologies [33]. Additionally, carburization conditions significantly impact the particle size and surface roughness of Mo₂C. Studies indicate that an increase in CH₄ concentration leads to the development of finer Mo₂C grains, whereas lower CH₄/H₂ ratios promote the formation of larger, more faceted particles [27]. The SEM observations in the present study suggest that the selected synthesis conditions lead to a morphology that aligns well with those reported in previous literature.

Figure 6.

SEM images of (a) the AMT and (b) MoO₃, (c) Mo plus MoO₂, (d) Mo and (e) Mo₂C powders.

Previous studies have explored various approaches to Mo₂C synthesis, including carburization using different carbon precursors, thermochemical reduction, and vapor-phase synthesis. While traditional methods such as solid-state carburization require high temperatures (~1273 K) and long reaction times (several hours), alternative techniques, including gas-phase carburization using CH₄/H₂ mixtures, have demonstrated reductions in temperature and reaction duration [34] However, many of these approaches still result in the formation of CO₂ as a byproduct, contributing to environmental concerns [27]. In contrast, the present study introduces a single batch gas-phase synthesis approach using AMT as the precursor, which enables Mo₂C formation at 1000 K in a significantly shorter reaction time (~1 hour). Compared to other methods, this approach eliminates CO₂ emissions by leveraging CH₄ decomposition into hydrocarbon species instead of CO/CO₂-producing pathways. Additionally, thermodynamic calculations confirm that the reaction pathway follows a stepwise reduction and carburization process, aligning with previous theoretical studies [35] This approach also ensures a highly controlled synthesis environment, yielding uniform and high-purity Mo₂C platelets. Regarding energy efficiency, the ability to synthesize Mo₂C at a lower temperature and within a single batch minimizes energy consumption, making the process more sustainable than conventional high-temperature carburization techniques [34]. Furthermore, in comparison to solution-phase synthesis of Mo₂C, which involves complex reaction media and additional post-processing steps [29]. The current method offers a direct and scalable approach to Mo₂C production, making it suitable for industrial applications.

4. Conclusions

Mo₂C was synthesized in a single batch under the guidance of thermodynamic predictions by strategically switching the gas atmosphere. Thermodynamic calculations carried out in Mo-O-H and Mo-C-H systems predict that Mo and Mo₂C syntheses are feasible at 1 atm and 800–1000 K and 1000 K, respectively. The initial stage of the reaction was conducted under an argon environment, which was then switched to a hydrogen (H₂) atmosphere at 800 K to obtain Mo. Subsequently, the gas flow was switched from H₂ to methane at 1000 K for Mo₂C synthesis. The changes in crystal structures during the transformation follow the path of AMT (monoclinic) > MoO₃ (orthorhombic) > MoO₂ (monoclinic) > Mo (b.c.c.), and Mo₂C (hexagonal). Fine Mo₂C particles with a thickness of 0.1 μm were obtained from very coarse AMT particles (>50 μm). The study demonstrated the feasibility of this straightforward single batch synthesis method for obtaining pure-phase Mo₂C powder avoiding carbon dioxide formation, which is an important consideration for the development of eco-friendly and energy-efficient material processing techniques in terms of synthesis reaction time.