Clarke Palmer

While low-cost natural gas remains abundant, the energy content of this fuel can be utilized without greenhouse gas emissions through the production of molecular hydrogen and solid carbon via methane pyrolysis. In the absence of a carbon tax, methane pyrolysis is not economically competitive with current hydrogen production methods unless the carbon byproducts can be valorized. In this work, we assess the viability of the carbon byproduct produced from methane pyrolysis in molten salts as high-value-added anode or conductive additive for secondary Li-ion and Na-ion batteries. Raman characterization and electrochemical differential capacity analysis demonstrate that the use of molten salt mixtures with catalytically-active FeCl3- or MnCl2 result in more graphitic carbon co-products. These graphitic carbons exhibit the best electrochemical performance (up to 272 mAh/g of reversible capacity) when used as Li-ion anodes. For all carbon samples studied here, disordered carbon domains and retained salt species trapped and/or intercalated into the carbon structure were identified by X-ray photoelectron and multinuclear solid-state nuclear magnetic resonance spectroscopy. The latter lead to reduced electrochemical activity and reversibility, and poorer rate performance compared to commercial carbon anodes. The electronic conductivity of the pyrolyzed carbons is found to be highly dependent on their purity, with the purest carbon exhibiting an electronic conductivity nearly on par with that of commercial carbon additives. These findings suggest that more effective removal of the salt catalyst could enable applications of these carbons in secondary batteries, providing a financial incentive for the large-scale implementation of methane pyrolysis for low-carbon hydrogen production.

The rate of methane pyrolysis in molten salt environments is increased with hydrocarbon feed additives which provide a low-cost means of improving the process which could allow for CO₂-free hydrogen production at industrial scales.

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 then stored or utilized. Compared to solid catalysts, there is a lack of fundamental understanding of the methane transformation processes in liquid environments which could inhibit progress to industrializing a low-cost commercial process. In this work, we have investigated methane pyrolysis in alkali-halide molten salts (i.e., NaCl, NaBr, KCl, and KBr) that are industrially attractive due to their low cost, high temperature stability, and non-toxicity. We measured first-order effective activation energies in differential bubble column reactors of ∼300 kJ mol⁻¹ for methane pyrolysis in all molten salts, which is lower than the reported value for the homogeneous uncatalyzed reaction of ∼420 kJ mol⁻¹. We also measured an effective activation energy of ∼230 kJ mol⁻¹ for single CH₄-D₂ exchange, which is consistent with some solid metal oxide catalysts. Calculations using combined molecular dynamics and density functional theory support the experimental findings with a calculated energy barrier of ∼3.29 eV (or ∼317 kJ mol⁻¹) on a molten KCl surface. The initial reaction rates may be sufficient for an industrial reactor; however, at high conversion, the hydrogen dependent reverse reaction limits single-pass conversion. We also find that the accumulated solid carbon is ‘wetted’ into the liquid salt medium and does not have significant contact with the gas phase or affect methane decomposition rates. The solid phase products are composed of both amorphous and graphitic carbon domains with no long-range ordering. We find that significant salt residue is retained on the carbon removed from the reactor which is inadequately removed by water washing. Only high temperature heat treatments were effective at cleaning the residual salt from the carbon, presumably due to selective evaporation of the salt.

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/mole for the eutectic NaCl-KCl to 171 kJ/mole 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 hours, 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.

Suspensions of platinum nanoparticles (PtNPs) were formed in molten LiCl–LiBr–KBr via thermal decomposition of H ₂ PtCl ₆ , and subsequently evaluated for thermal stability and CO oxidation activity.