Achieving Phase Control of Polymorphic Tungsten Carbide Catalysts.

Area of Science:

• Materials Science
• Catalysis
• Chemical Engineering

Background:

• Tungsten carbide (W_xC) exhibits phase polymorphism, complicating structure-property relationship studies in catalysis.
• Selective synthesis of pure W_xC phases is crucial for understanding and optimizing catalytic performance.

Purpose of the Study:

• To establish a framework for phase-selective synthesis of W_xC by controlling carburization kinetics.
• To investigate the mechanistic pathway of WO_x carburization and the role of particle size.
• To evaluate the catalytic activity and stability of synthesized β-W₂C for the reverse water-gas shift (RWGS) reaction.

Main Methods:

• Phase-selective synthesis of W_xC using temperature-programmed carburization (TPC) with gaseous carbon precursors (CH₄/H₂).
• Controlled particle size (<10 nm) to stabilize β-W₂C.
• Mechanistic elucidation of WO_x carburization via CH₄ activation.
• Reactor studies comparing *ex situ* and *in situ* synthesized β-W₂C for RWGS.

Main Results:

• Selective synthesis of β-W₂C achieved by controlling particle size and carburization kinetics.
• W₂C stabilization is primarily governed by particle size and kinetics, not support interactions.
• CH₄ activation during WO_x carburization occurs at mild temperatures via lattice oxygen.
• *In situ* synthesized β-W₂C shows higher CO selectivity and yield (STY) in RWGS compared to *ex situ* synthesized catalysts and other W_xC phases.

Conclusions:

• A tunable approach for synthesizing phase-pure tungsten carbide catalysts is established.
• The intrinsic activity of β-W₂C for RWGS is demonstrated, particularly when synthesized *in situ*.
• Understanding carburization kinetics and particle size control is key to optimizing W_xC catalyst design.