Cu-Sn-Bi ternary liquid metal catalyst for methane pyrolysis to hydrogen and carbon

Methane pyrolysis using liquid metal catalysts provides a promising pathway for low-emission hydrogen production. To synergistically optimize both reaction kinetics and hydrodynamics, this study introduces a novel Cu-Sn-Bi ternary catalyst system. Within this ternary system, Cu contributes intrinsic catalytic activity toward methane pyrolysis, while Sn serves primarily as the liquid-metal solvent and Bi acts mainly as a hydrodynamic modifier. At 1050°C and a CH4 flow rate of 20 mL·min−1, the optimal Cu0.375Sn0.250Bi0.375 system achieved a CH4 conversion of 63.28% and an H2 selectivity of 98.57%. Experimental kinetic analyses indicate that while Bi exhibits limited intrinsic catalytic activity, it functions effectively as a physical hydrodynamic modifier. Specifically, by lowering the surface tension and increasing the melt density, Bi may favor smaller bubbles and a larger gas-liquid interfacial area. Furthermore, the incorporation of Bi mitigates the conversion decline typically observed at high gas flow rates. The accumulated carbon-metal “slurry-like” layer may contribute to this behavior by increasing the resistance to bubble motion and prolonging the effective gas residence time, although this effect was not directly measured. However, the low surface tension and high vapor pressure of Bi promote interfacial enrichment and evaporation, contributing to metal contamination in the solid carbon by-products. Additionally, operating above 1000°C with a Bi content exceeding 37.5 at.% results in pronounced accumulation of the surface carbon-metal layer, posing a critical risk of reactor clogging. These findings provide experimental insights into the trade-offs in liquid metal catalyst design and the formation and evolution mechanisms of carbon by-products.