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

Palmer, C. - Presenter, University of California
Kang, D., University of California
Mannini, D., University of California
Gordon, M., University of California
Metiu, H., University of California - Santa Barbara
McFarland, E., University of California
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.


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  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