Influence of Preparation and Activation on the Speciation of Carbon‐Supported Mo–W Carbides

ABSTRACT Carbon‐supported Mo–W carbides were synthesized by two different methods (co‐impregnation and sequential impregnation of W and Mo precursors) to unravel how precursor impregnation controls carburization pathways. In situ W L 3 ‐edge HERFD‐XAS and Mo K‐edge XAS, quantitatively analyzed by MCR‐ALS reveals that all samples follow a 3‐component sequence “oxide → reduced oxide → carbide” upon temperature‐programmed carburization. However, the temperature window of the reduced MoO 2 –WO 3‐x coexistence is route‐dependent. Co‐impregnation and preliminary carburization of W generate extended overlapping domains (300°C‐420°C) in which both reduced oxides are simultaneously formed, promoting cooperative oxide‐oxide transformations and leading to mixed Mo–W carbidic environments, consistent with XRD and TEM. In contrast, in the route where Mo is carburized first, pre‐reduced molybdenum domains trigger early W reduction and shorten the MoO 2 –WO 3‐x overlap window, leading to independent transformation paths for the two metal precursors and to the formation of separate Mo‐rich and W‐rich carbide phases. These results demonstrate that impregnation and carburization orders are decisive synthetic levers for directing the speciation of Mo–W carbide towards mixed bicarbides or phase‐segregated Mo‐rich/W‐rich carbides providing actionable guidelines for designing bifunctional carbide catalysts.