Molybdenum nitride and carbide catalysts for ammonia synthesis Original Research Article

High surface area molybdenum nitride (γ-Mo2N) and two kinds of carbides (α-MoC1−x or β-Mo2C) were prepared by the temperature programmed reaction with NH3 for the former sample, and with CH4+H2 mixed gas for the latter samples. To prepare γ-Mo2N, the precursor MoO3 was heated up to 973 K and held for 2 h under NH3 flow. To prepare α-MoC1−x, the precursor γ-Mo2N was heated up to 973 K and held for 2 h under CH4+4H2 flow. To prepare β-Mo2C, the precursor MoO3 was heated up to 973 K and held for 2 h under CH4+4H2 flow. The surface areas of γ-Mo2N and α-MoC1−x were about 140 m2 g−1; on the other hand, that of β-Mo2C was 32 m2 g−1. However, β-Mo2C was most active among those samples and was as active as the doubly-promoted iron catalyst. The stability of those catalysts treated with the reactant gas at 873 K was examined under the reaction condition. The activity of α-MoC1−x was decreased because this phase was not stable. CN atomic exchange occurred. γ-Mo2N and β-Mo2C were more stable than α-MoC1−x. To improve the catalytic activity, alkali was added. Addition of caesium did not affect the formation of γ-Mo2N and α-MoC1−x, and the activities were somewhat improved. On the other hand, Mo metal was formed instead of β-Mo2C by the carburization from MoO3 with Cs added.