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, WC1-x (0.5 < x < 1), and stoichiometric WC.