Molybdenum nitrides: a study of synthesis variables and catalytic performance in acetylene hydrogenation

We have examined the catalytic action of β- and γ-Mo₂N in the partial hydrogenation of acetylene. The influence of variations in GHSV (230–1600 min⁻¹), feed composition (5–30% v/v N₂/H₂), heating rate (0.1–5 K min⁻¹) and isothermal hold (1–7 h at 933 K) on nitride structural properties has been assessed. At ≥ 2 K min⁻¹, β-Mo₂N (≤ 15 m² g⁻¹) consisting of small crystallites (< 5 μm) was generated. At ≤ 0.5 K min⁻¹, γ-Mo₂N with a platelet morphology and surface area ≥ 45 m² g⁻¹ was formed. High GHSV, low N₂ feed content and a prolonged isothermal hold served to increase γ-Mo₂N area (to 135 m² g⁻¹). Lower alkene selectivity and a twofold higher specific (per m²) acetylene hydrogenation rate were recorded for β-Mo₂N and linked to higher surface Mo/N ratio (from XPS). Olefin selectivity for both nitrides was greater than that reported for Pd catalysts. Moreover, we recorded negligible green oil formation in reactions over γ-Mo₂N.