(206a) Catalytic Methane Pyrolysis in Polyvalent Molten Salt Mixtures

Methane pyrolysis (MP) for hydrogen production is one potentially cost-effective approach for the utilization of low-cost, natural gas without generation of CO₂. A major roadblock to industrial implementation is management of the stoichiometric solid carbon product; solid carbon quickly deactivates solid catalysts and there is currently no effective method for CO₂-free regeneration. Molten metal and molten salt environments do not suffer this deactivation by providing a continuously-renewed gas-liquid reaction interface and a means for continuous removal of the carbon via solid-liquid separations [1-3]. Molten salts (e.g., NaCl and KCl) are more attractive to industry; they are low-cost, non-toxic, and a trace residue in the solid carbon product may be acceptable for certain applications or disposal strategies. However, these monovalent molten alkali-halide salts (NaCl, KCl, NaBr, KBr), were found to have minimal activity for MP. Mixtures of these simple salts with higher-valency salts such as MnCl₂ and FeCl₃ resulted in considerable catalytic activity, which we attribute to the formation of polyvalent molecular ions, such as [MnCl4]^2- and [FeCl₄]^-, which are present at the gas-liquid interface and catalyze methane activation and conversion.

In this talk we will report on the activity of salt mixtures such as MnCl₂-KCl, FeCl₃-NaCl-KCl, and others. We demonstrate with CH₄-D₂ exchange experiments that the catalytic transformation pathways on these complex salt surfaces is unique from established routes for either the gas-phase or on solid catalysts. We confirm the formation of polyvalent ions using molar volume (density) measurements as a function of bulk concentration, which reveal an excess molar volume when a polyvalent ion is formed in salt mixtures. More importantly, we confirm their presence at the gas-liquid interface in molten salt bubble column reactors using surface tension measurements. Anionic speciation is carried out using spectroscopy such as Raman, UV-vis, and XPS. Continued work is focused on developing an overall understanding of which molten salt polyatomic ions are active for MP based on their valency (acidity) and surface concentration.

References

1. Upham, D. C., Agarwal, V., Khechfe, A., Snodgrass, Z. R., Gordon, M. J., Metiu, H., McFarland, E. W. Science 358, 6365 (2017).
2. Palmer, C.; Tarazkar, M.; Kristoffersen, H. H.; Gelinas, J.; Gordon, M. J.; McFarland, E. W.; Metiu, H. ACS Catal. 9, 8337 (2019).
3. Kang, D.; Rahimi, N.; Gordon, M. J.; Mectiu, H.; McFarland, E. W. Catal. B-Environ. 254, 659 (2019).c