In situ time-resolved X-ray diffraction study of the synthesis of Mo₂C with different carburization agents

Toluene, n-heptane, and methane were studied as carbon sources during the synthesis of Mo₂C. Ammonium heptamolybdate ((NH₄)₆Mo₇O₂₄·4H₂O), molybdenum dioxide (MoO₂), orthorhombic molybdenum trioxide (α-MoO₃), and a low crystallinity molybdenum oxide (MoO_x) were employed as metal precursors and their conversion into the carbide phase was followed by time-resolved X-ray diffraction from synchrotron radiation. Thermal treatment was carried out from 298 K up to 1023 K and the evolution of gases analyzed by mass spectrometry. The resulting carbidic phase obtained using methane/H₂ was hexagonal β-Mo₂C, while in contrast, cubic molybdenum carbide (α-MoC_1-x) was obtained when toluene/H₂ or n-heptane/H₂ were employed, which occurred independently of the original metal precursor. Molybdenum carbide was first detected, on average, at about 923 K. Mass spectrometry results show the formation of CO when methane was used as a carbon source, while methyl radicals and benzene and mainly methyl radicals were detected when toluene and n-heptane were employed, respectively. (author)