Transition metal carbides have numerous applications and are known to excel in terms of hardness, thermal stability and conductivity. In particular, the Pt-like behavior of Mo and W carbides has led to a popularization of metal carbides in catalysis, ranging from electrochemically-driven reactions to thermal methane coupling. Herein, we show the dynamics of Mo and W carbides and the active participation of carbidic carbon in the formation of C2 products during methane coupling. A detailed mechanistic study reveals that the catalyst performance of these metal carbides can be traced back to its carbon diffusivity and exchange capability upon interaction with gas phase carbon (methane). A stable C2 selectivity over time on stream for Mo carbide (Mo2C) can be rationalized by fast carbon diffusion dynamics, while W carbide (WC) shows loss of selectivity due to slow diffusion leading to surface carbon depletion. This finding showcases that the bulk carbidic carbon of the catalyst plays a crucial role and that the metal carbide is not only responsible for methyl radical formation. Overall, this study supports the presence of a carbon equivalent to the Mars-Van Krevelen type mechanism for non-oxidative coupling of methane, thus introducing guiding principles to design and develop associated catalysts.
Supplementary Information Role and Dynamics of Transition Metal Carbides in Methane Coupling