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

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.