No abstract

Current methods of hydrogen production from methane generate more than 5 kg of CO2 for every 1 kg of hydrogen. Methane pyrolysis on 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–Bi alloys 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 barriers are in rough agreement with the experimental apparent activation energy of 2.3 eV.

Mixtures of molten iron–sodium-potassium chloride salts are found to be catalytic for methane pyrolysis. In a differential bubble column reactor, the apparent activation energy of the molten salt decreases from 301 kJ/mol for the eutectic NaCl-KCl to 171 kJ/mol for 3 wt % of iron-added as FeCl3. The solid carbon produced in the iron-containing salt mixture has a graphitic structure which is distinct from the more disordered carbon produced in the iron-free eutectic, suggesting a different solid carbon formation pathway. Results from H–D exchange investigations are consistent with a different reaction pathway for methane pyrolysis in the iron-containing NaCl-KCl melt than in the melt without Fe. The activity of the salt mixture was stable for over 50 h, producing molecular hydrogen and separable solid carbon. It is likely that the activity is due to the presence of Fe in molecular ions stabilized in the NaCl-KCl melt that facilitate the C–H bond activation in methane.

The catalytic decomposition of methane, propane, benzene, and crude petroleum was investigated between 900 and 1000 °C in molten metal bubble column reactors. The conversion to gas phase products and solid carbon was measured after introducing the gas phase reactants into a bubble column reactor containing a catalytic molten mixture of 27 mol % Ni and 73 mol % Bi. The conversions of propane, benzene, and crude oil are 100% at temperatures >950 °C at a reactor residence time of ∼1 s. Equilibrium selectivity of 100% H 2 and carbon was not achieved in the short residence time, but can be achieved at longer residence times. The solid carbon products obtained from methane pyrolysis were more graphitic than those produced from the other, highermolecular weight reactants; the latter were more amorphous, as measured by Raman spectroscopy and electron microscopy and resembled carbon black. A model is proposed for carbon formation in bubble column reactors, in which amorphous carbon products are derived from the gas-phase decomposition and graphitic carbon products are formed from dissolution and reprecipitation of carbon into and out of the molten metal.

Molten salts have received renewed attention as potential reaction media for methane pyrolysis, in which CO2-free hydrogen gas can be produced and the solid carbon can be continuously removed and...