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...
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