Mechanistic study of the reduction of MoO2 to Mo2C under methane pulse conditions

Molybdenum carbide (Mo2C), an interstitial transition metal carbide, has been used in a myriad of industrial applications due to its refractory nature, extreme hardness and strength, and high electrical and thermal conductivity. It also possesses catalytic activity for many chemical processes such as hydrodeoxygenation, reforming, water-gas shift, and the Fischer-Tropsch reaction. Among the current synthesis methods available to produce beta-Mo2C, temperature-programmed reduction yields materials with the highest specific surface areas. The objective of the present work is to perform a detailed investigation of the carburization process and to determine the key intermediate phases that are formed during reduction. To achieve this objective, we performed the carburization process under pulse conditions wherein a small amount of CH4 in each pulse was reacted with a packed bed of MoO2. Our XRD and TEM results demonstrate that the solid-phase transformation from MoO2 to beta-Mo2C follows a "plum-pudding" mechanism where Mo metal crystallites are constantly formed as the key intermediate phase throughout the matrix.