Achieving Phase Control of Polymorphic Tungsten Carbide Catalysts

The polymorphism of tungsten carbide (W _x C) and the challenge of selectively synthesizing pure phases have impeded a precise understanding of catalytic structure-property relationships. This study establishes a framework for phase-selective synthesis of W _x C through controlling carburization kinetics. By maintaining particle sizes below 10 nm, β-W₂C is selectively synthesized using gaseous carbon precursors (CH₄/H₂) via temperature-programmed carburization (TPC). Our findings reveal that W₂C stabilization is predominantly dictated by particle size and carburization kinetics rather than support interactions, providing a tunable approach to synthesize tungsten carbide catalysts. We elucidate the mechanistic pathway of WO _x carburization, demonstrating that CH₄ activation occurs at mild temperatures via lattice oxygen. Our reactor studies establish ex situ synthesized β-W₂C as an active and stable catalyst for the reverse water-gas shift (RWGS) reaction. However, the need for passivation and reduction pretreatment leads to a complex surface structure with diminished intrinsic activity. In contrast, our in situ synthesis protocol for β-W₂C eliminates the need for passivation and exhibits increased CO STY during RWGS, illustrating the intrinsically higher activity compared to metallic W, WC_1-x (0.5 < x < 1), and stoichiometric WC.