Catalytic molten Zn-Bi alloys for methane pyrolysis

Abstract

Methane pyrolysis with molten alloy catalysts enables the production of large-scale, CO₂-free hydrogen and valuable carbon byproducts. This work systematically screens molten alloy catalysts, determines reaction kinetics, elucidates detailed surface reaction mechanisms, and analyzes the structure of carbon byproducts using both computational and experimental methods. Several essential factors for designing Bi-based molten alloys suggest that Zn_0.45-Bi_0.55 is a promising candidate among 20 binary alloys. We calculate the accurate free energy of activation for the initial C-H activation of methane using ab initio molecular dynamics and metadynamics simulations. The computed barrier is lower than those of molten binary alloys reported in the literature, and this has been validated by our reaction kinetics measurements on the Zn-Bi alloy. In methane activation, active metals (Zn) contribute to changing the charge states of base metals (Bi), facilitating C-H dissociation. Methane activation is more likely to occur through a surface-stabilized-like pathway rather than a radical-initiated pathway, which could provide crucial information for developing a descriptor to predict C-H activation energies on molten catalysts. A distinct feature of the surface-stabilized-like pathway, compared to solid surfaces, is that methyl does not necessarily bind to the active site immediately after C-H dissociation. We also investigate methane decomposition and carbon formation pathways using density functional theory calculations. The initial C-C bond can form either through CH₃(g) radicals in the gas phase or via coupling reactions involving CH₂* and CH*. Transmission electron microscopy of the carbon products shows a partially crystalline structure, suggesting their potential usage as high-value carbon.