Synthesis, characterisation and hydrogenation performance of ternary nitride catalysts

Synthesis of phase pure Co₃Mo₃N and Fe₃Mo₃N by temperature programmed ammonolysis has been established by XRD and elemental analysis. The ternary nitrides are characterised by a η-6 structure and low surface area (4–9 m² g⁻¹). Pseudomorphic transformation of cobalt molybdate prepared using cobalt nitrate generated rod-shaped crystals while the use of iron chloride resulted in Fe₃Mo₃N aggregates with irregular morphology and wide size distribution. XPS measurements have revealed surface N enrichment relative to the bulk where the passivated samples show a range of oxidation states; Co₃Mo₃N exhibited Mo²⁺ and Coⁿ⁺ (0 ≤ n ≤ 3) whereas Fe₃Mo₃N was characterised by higher oxidation states (Fe³⁺ and Mo³⁺). Temperature programmed reduction (TPR) to 823 K served to remove the passivation layer where subsequent H₂ chemisorption and temperature programmed desorption (TPD) has demonstrated greater uptake on Fe₃Mo₃N relative to Co₃Mo₃N, resulting in a higher nitrobenzene hydrogenation rate (to aniline). Fe₃Mo₃N promoted selective reduction of –NO₂ in p-chloronitrobenzene to generate p-chloroaniline as sole product whereas Co₃Mo₃N favoured C-Cl scission with the formation of nitrobenzene (in addition to p-chloroaniline). Hydrodechlorination properties were further established for Co₃Mo₃N in the conversion of chlorobenzene (to benzene) under conditions where Fe₃Mo₃N was inactive. A temporal deactivation of both nitrides is associated with Cl poisoning of Co₃Mo₃N and structural changes to Fe₃Mo₃N.