Kinetics and mechanism of the reduction–carburization processes of MoO₂ to Mo₂C with CO–15% CO₂ mixed gases

Molybdenum carbide (Mo₂C), as an alternative to platinum group metals, has been widely used in the hydrocarbon and hydrogen evolution reactions due to its excellent catalytic performance. The exploitation of its preparation method with high efficiency and low cost, therefore, received increasing attention in recent decades. In the current work, the preparation method of Mo₂C by reducing MoO₂ with CO–15%CO₂ mixed gases was proposed, in which the main focus was laid in the reaction kinetics and reduction mechanism studies of the reduction-carburization processes. To determine the isothermal reaction temperature, the nonisothermal reactions of MoO₂ in CO–15%CO₂ mixed gases under different heating rates (2, 5, 10, and 15 K·min^–1) were conducted first. After that, the isothermal reactions in the temperature range from 993 to 1153 K were carried out. Different analytical technologies, such as the thermodynamic calculation, Field emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), Thermogravimetric (TG), Brunauer-Emmett-Teller (BET), and model fitting methods were adopted to analyze the experimental data. The results revealed that both the beginning (953 to 997, 1015, and 1031 K) and ending reaction temperatures (1100 to 1201, 1318, and 1383 K) were gradually increased with the increase of the heating rate (2 to 5, 10, and 15 K·min^–1); besides, the reaction rate increased with increasing the temperature was also obtained. Phase transformation process of MoO₂ to Mo₂C was found to proceed by a one-step reaction (MoO₂→Mo₂C) without the formation of intermediate product Mo. The study also discovered that both Mo₂C and MoO₂ maintained the similar platelet-shaped morphology during the reaction process, but partial micro-pores and cracks were formed on the product surface because of the entry of reaction gases and escape of the product gases as well as the shrinking of the molar volume, increasing the specific surface area of the as-obtained Mo₂C by nearly 20 times when compared to that of the raw material. Kinetics analysis revealed that the reduction-carburization process of MoO₂ to Mo₂C were not controlled by a one-step reaction mechanism but by the co-action of nucleation growth and interfacial chemical reactions. It was also discovered that the nucleation growth accounted for 68.9% and the chemical reaction accounted for 31.1%, with the extracted activation energies of 80.651 and 121.002 kJ·mol^–1, respectively. The work would make a better understanding of the reaction processes of MoO₂ to Mo₂C in CO–15%CO₂ mixed gases.