Methane Pyrolysis with a Molten Cu–Bi Alloy Catalyst

Current methods of hydrogen productionfrom methane generate more than 5 kg of CO₂ for every 1 kg of hydrogen. Methane pyrolysison conventional solid heterogeneous catalysts produces hydrogen without CO₂, but the carbon coproduct poisons the catalyst. This can be avoided by using a molten metal alloy catalyst. We present here a study of methane pyrolysis using mixtures of molten Cu–Bialloys as the catalyst. We find that molten Cu–Bi is an active catalyst, even though pure molten Bi and Cu are not. Surface tension measurements and constant-temperature ab initio molecular dynamics simulations indicate that the surface is enriched in Bi and that the catalytic activity is correlated with the concentration of Bi at the surface. Bader charge analysis indicates that bismuth donates charge to copper. In the most stable configuration of dissociated methane on these liquid surfaces, CH₃ binds to a bismuth surface atom and H to Cu. The energy barriers for the dissociative adsorption of methane, calculated using the nudged elastic band (NEB) method, are between 2.5 and 2.6 eV, depending on the binding site on the surface of the Cu₄₅Bi₅₅ alloy. The computed barriersare in rough agreement with the experimental apparent activation energy of 2.3 eV.