Molybdenum carbide catalysts II. Topotactic synthesis of unsupported powders

Metastable cubic α-MoC1−x(x ~ 0.5) unsupported powders with specific surface area ca. 200 m2 g−1 and particle size ca. 3 nm were prepared by temperature-programmed reaction (TPR) of MoO3 with ammonia producing first Mo2N which was subsequently converted to α-MoC1−x by TPR with a CH4/H2 mixture. Alternatively, a direct TPR of MoO3 with a CH4/H2 mixture also produced α-MoC1−x, provided that a small amount of platinum was impregnated on MoO3 prior to TPR. Reactions involved in both processes were topotactic, with formation of metastable α-MoC1−x crystals exhibiting pseudomorphism with the original platelets of MoO3, the starting material. Without platinum, the reaction between MoO3 and CH4/H2 required a higher temperature of initial reduction and the reaction was not topotactic, leading to hexagonal Mo2C, the thermodynamically stable phase of molybdenum carbide.