Effect of the Reactive Gas on the Solid-State Transformation of Molybdenum Trioxide to Carbides and Nitrides

A temperature-programmed reaction method was employed to investigate the solid-state transformation of molybdenum oxides to carbides or nitrides by reaction with various reacting gases such as H 2 and NH 3 and the mixtures of CH 4 and H 2 (CH 4 /H 2 1/4), CO and H 2 (CO/H 2 1/4), and N 2 and H 2 (N 2 /H 2 1/3 and 3/1). The solid-state transformation of MoO 3 to nitride by reaction with NH 3 was topotactic. In contrast, MoO 3 took a nontopotactic route by reaction with the other gases or their mixtures, leading to products that were thermodynamically stable under the investigated conditions. For the synthesis to take a topotactic route it required that the carburization or nitridation occurs in the initial stage of reduction to form intermediate phases containing carbon or nitrogen at low temperatures.