Decomposition of molybdate–hexamethylenetetramine complex: One single source route for different catalytic materials

Decomposition of ammonium heptamolybdate–hexamethylentetramine (HMTA) complex (HMTA)₂(NH₄)₄Mo₇O₂₄·2H₂O was studied as a function of treatment conditions in the range 300–1173 K. The evolution of solid products during decomposition was studied by thermal analysis and in situ EXAFS. Depending on the nature of the gas used for treatment, single phases of highly dispersed nitrides Mo₂N, carbide Mo₂C, or oxide MoO₂ can be obtained. The nature of the products obtained was explained by qualitative thermodynamical considerations. Morphology of the solids considerably depends on such preparation parameters as temperature and mass velocity of the gas flow. For the nitride-based materials, catalytic activity was evaluated in the model thiophene HDS reaction. It was demonstrated that NH₃-treated samples showed better catalytic activity than N₂-treated ones due to cleaner surface and better morphology. Transmission microscopy, XRD and XPS studies showed that MoS₂ is formed on the surface during HDS reaction or sulfidation with H₂S. Optimized nitride-derived catalysts showed mass activity several times higher than unsupported MoS₂ or MoS₂/Al₂O₃ reference catalyst.