A study on mechanochemical behavior of MoO₃–Mg–C to synthesize molybdenum carbide

Highlights

• Molybdenum carbide has been synthesized starting with Mg, MoO₃ and C.

• Mg content plays a key role in the mechanochemical behavior of the ternary system MoO₃–Mg–C.

• By increasing milling time, an exothermic reaction occurs before magnesium melting.

• Magnesiothermic reaction provided enough heat for activating carbothermic reaction.

Abstract

The influence of Mg value in the MoO₃–Mg–C mixture on the molybdenum carbide formation and the mechanism of reactions during mechanochemical process were investigated. In keeping with this aim, magnesium and carbon contents of the mixture were changed according to the following reaction: 2MoO₃ + (6 − x) Mg + (1 + x) C = (6 − x) MgO + Mo₂C + x CO. The value of x varied from 0 to 6. Differential thermal analysis (DTA) results for sample with stoichiometric ratio (x = 0) revealed that in the early stage, carbon reduced the MoO₃ to MoO₂ and subsequently highly exothermic magnesiothermic MoO₂ reduction occurred after magnesium melting. Also, it was indicated that the exothermic reaction temperature shifted to before magnesium melting in the 11 h-milled sample (x = 0) and all the exothermic reactions happened, simultaneously. According to the experimental findings, molybdenum carbide (Mo₂C) was synthesized in the mixture powder with stoichiometric ratio (x = 0) after 12 h milling process and the type of reactions was mechanically induced self-sustaining reaction (MSR). However, at lower Mg content in the MoO₃–Mg–C mixture (0 < x ≤ 2), the magnesiothermic reduction occurred in MSR mode and activated the carbothermal reaction. Further decrease in Mg value (2 < x ≤ 3) resulted in MSR mode magnesiothermic reaction and gradual carbothermal reduction. In samples with lower magnesium contents, partial molybdenum oxide reduction proceeded through a gradual mode magnesiothermic reaction.

Introduction

In recent years, transition metal carbides are widely used in industry because of some outstanding properties in mechanical hardness, thermal stability, and especially catalytic performance, which has been extensively studied over the past three decades [1], [2]. Among these transition metal carbides, molybdenum carbide (Mo₂C) is receiving considerable attention because of catalytic behavior. Some interesting examples of the type of reactions carried out using it are: the removal of sulfur (hydrodesulfurization) and the removal of nitrogen (hydrodenitrogenation), hydrogenation, dehydrogenation, and the Fischer–Tropsch reaction [1], [2], [3].

Currently, many processes are available for the synthesis of molybdenum carbide and each process varies in the characteristics of the powder produced and the processing cost. Mo₂C can be conventionally prepared by temperature-programmed reaction (TPR) method via carbonization of molybdenum trioxide with gas hydrocarbon, such as C₂H₂, CH₄ and their gas mixtures [1], [2], [3], [4]. In addition, many other chemical methods were reported for synthesis of molybdenum carbide including chemical vapor deposition (CVD) [5], high temperature during dc discharge [6], solution reactions [7], and Microwave Induced Alloying (MIA) [8]. Solid state carbothermal reaction has been investigated earlier in several papers with various carbon resources and performed at different temperatures [1], [2], [4]. Moreover, metallothermic reduction of molybdenum oxide has been studied, with aluminum, calcium, magnesium or Al–Si alloy as a reducing agent [9], [10]. These active metals generate a great amount of heat, which can be used for activation of carbothermal reaction. In this regard, K. Manukyan et al. in two papers [11], [12] reported self-sustaining reduction and reaction pathway in the MoO₃ + Mg + C mixtures may yield molybdenum or molybdenum carbide, depending on the process conditions.

Mechanochemical activation is a solid state powder processing method which involves inducing chemical reactions in a mixture of reaction powders at room temperature or at least much lower temperatures. An increase in the kinetic of reaction during high energy milling can be a result of microstructural refinement, repeated cold deformation and fracture of particles [13], [14]. A. Ataie et al. [3] synthesized nanocrystalline molybdenum by mechanical activation of MoO₃ powder and its subsequent hydrogen reduction. In several attempts [3], [15], [16], [17], nanocrystalline molybdenum carbide was prepared through the reduction of molybdenum oxide by carbon using high energy ball milling. Also, Mo₂C was mechanosynthesized by ball milling mixtures (MoO₃ + 2Al + C) with different molar graphite contents at room temperature by Z.Q. Li et al. [10]. Our researches indicated that the synthesis of molybdenum carbide in the MoO₃–Mg–C mixtures was done by thermal processes and heretofore the mechanochemical behavior of this system has not been reported elsewhere.

In our previous works [18], [19], the mechanical activation process was used to synthesize Al₂O₃/B₄C and MgO/B₄C nanocomposite by using commercial pure materials, such as Al/Mg, B₂O₃ and C (graphite). As reported by authors in previous papers, Al/Mg content plays a key role in the mechanochemical behavior of the ternary system Al/Mg–B₂O₃–C.

The aim of this work is to explore the possibility of in-situ synthesis of molybdenum carbide powder via mechanochemical method by using the MoO₃–Mg–C mixture as precursor materials at room temperature. The influence of Mg content in this system on the carbide formation phenomena and the mechanism of reactions during the ball milling process were studied. Thermodynamic calculations and thermal analysis were performed to get an insight about the probable reactions.