Abstract
High surface area (>170 m2 g−1) molybdenum nitride was prepared by the temperature-programmed nitridation of α-MoO3 with pure ammonia. The process was optimized by adjusting the experimental variables: the reaction temperature, heating rate, and molar flow rate of ammonia. The physicochemical properties of the as-formed molybdenum nitride were characterized by X-ray diffraction, N2 sorption, transmission electron microscopy, temperature-programmed oxidation/reduction, and X-ray photoelectron spectroscopy. Of the experimental variables, the nitridation temperature was found to be the most critical parameter determining the surface area of the molybdenum nitride. When the prepared molybdenum nitride was exposed to air, the specific surface area rapidly decreased because of the partial oxidation of molybdenum nitride to molybdenum oxynitride. However, the surface area recovered to 90% the initial value after H2 treatment. The catalyst with the highest degree of nitridation showed the best catalytic activity, superior to that of unmodified α-MoO3, for the decomposition of ammonia because of its high surface area.
Highlights
Transition metal oxides, carbides, nitrides, and phosphides are widely used as catalysts or catalyst supports [1]
Analyses after the nitridation of α-MoO3 at temperatures over 800 ◦ C, and this phase transformation caused a rapid decrease in the specific surface area
When the prepared molybdenum nitride was exposed to air, the diffusion of oxygen into the bulk nitride resulted in a volume expansion, and a corresponding gradual reduction in the surface area and total pore volume arising from pore blocking was observed
Summary
Transition metal oxides, carbides, nitrides, and phosphides are widely used as catalysts or catalyst supports [1]. The application of transition metal nitrides and carbides has been limited because of their low specific surface areas, which are a result of sintering during their high-temperature synthesis [4,5]. Transition metal nitrides have been widely applied in chemical reactions, such as ammonia synthesis [24,25,26], carbazole hydrodenitrogenation [27], indole hydrodenitrogenation [28], hydrazine decomposition [29], and direct NO decomposition [30]. Molybdenum nitride having a high surface area was prepared by the topotactic conversion of α-MoO3 by ammonia treatment at high temperature, and the catalytic activity of these materials for the decomposition of ammonia were investigated. The prepared molybdenum nitride was characterized by various techniques, and its stability under an oxidizing, reducing, and inert atmosphere was examined
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