(4f) Deciphering the Multiphase Challenge of Tungsten Carbide for Carbon Dioxide Hydrogenation

Transition metal carbides (TMCs) are promising alternatives to precious metals for catalytic applications, particularly CO₂ conversion due to the intrinsic hydrogenation and C=O bond breaking activity. Orthorhombic molybdenum carbide (β-Mo₂C) is an established active catalyst for this reaction, and similarly, orthorhombic tungsten carbide (β-W₂C) possesses similar electronic properties and is therefore hypothesized to exhibit comparable performance. However, the significant polymorphism of tungsten carbide (W_xC) complicates the isolation of pure phases, thus hindering accurate evaluation of structure-property relationships.

This study tackles the polymorphic complexity of W_xC by refining synthetic approaches through temperature-programmed carburization (TPC). Phase-pure β-W₂C is selectively synthesized on a silica support at 835 °C. TEM analysis shows uniformly dispersed β-W₂C nanoparticles, aligning with computational studies that β-W₂C is thermodynamically favorable at particle sizes ≤10 nm. X-ray photoelectron spectroscopy (XPS) indicates that the surface primarily consists of W₂C (64%) and WO₃ (36%), with the formation of tungsten oxide due to passivation, a necessary step because of the pyrophoric nature of TMCs. We also synthesize phase-pure hexagonal δ-WC by increasing the tungsten loading and carburization time, leading to larger agglomerates and more favorable conditions for accessing the thermodynamically favored bulk W_xC phase. These findings are supported by X-ray diffraction (XRD) of the synthesized phases, highlighting successful control of synthesis parameters to achieve specific W_xC phases.

Catalytic tests in a packed bed reactor support our hypothesis that β-W₂C is the active phase for CO₂ hydrogenation, evidenced by the performance of ex-situ synthesized β-W₂C compared to δ-WC catalysts, after normalizing for tungsten content, Figure. Furthermore, in situ synthesized β-W₂C exhibits higher catalytic activity due to the absence of inactive surface oxides formed during passivation. Our results address the challenges associated with the complex polymorphism in TMC-based catalysts, which are fundamentally and industrially significant and can likely be extended to the full TMC library.