Issue 9, 2024

Tracing mechanistic pathways and reaction kinetics toward equilibrium in reactive molten salts

Abstract

In the dynamic environment of multi-component reactive molten salts, speciation unfolds as a complex process, involving multiple competing reaction pathways that are likely to face free energy barriers before reaching the reaction equilibria. Herein, we unravel intricate speciation in the AlCl3–KCl melt compositions with rate theory and ab initio molecular dynamics simulations. We find that the compositions with 100 and 50 mol% AlCl3 exclusively comprise neutral Al2Cl6 dimers and charged AlCl4 monomers, respectively. In intermediate AlCl3–KCl compositions, the chemical speciation proves to be a very complex process, requiring over 0.5 nanosecond to reach an equilibrium distribution of multiple species. It is a consequence of the competitive formation and dissociation of additional species, including charged Al dimers, trimers, and tetramers. Here, the species formation occurs through ion exchange events, which we explain by computing free energy landscapes and employing a Marcus-like rate theory. We show that both interspecies and intraspecies ion exchanges are probable and are dictated by the local structural reorganization reflected in the change of local coulombic fields. The species distributions are validated by comparing computed Raman spectra and neutron structure factors with the available experimental data. We find an excellent simulation-experiment agreement in both cases. Nevertheless, Raman spectroscopy turns out to be particularly advantageous for distinguishing between unique species distributions because of the distinct vibrational signatures of different species. The mechanistic insight into reaction dynamics gained in this study will be essential for the advancement of molten salts as reactive media in high-temperature energy applications.

Graphical abstract: Tracing mechanistic pathways and reaction kinetics toward equilibrium in reactive molten salts

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Article information

Article type
Edge Article
Submitted
07 Dec 2023
Accepted
17 Jan 2024
First published
22 Jan 2024
This article is Open Access

All publication charges for this article have been paid for by the Royal Society of Chemistry
Creative Commons BY-NC license

Chem. Sci., 2024,15, 3116-3129

Tracing mechanistic pathways and reaction kinetics toward equilibrium in reactive molten salts

L. D. Gibson, S. Roy, R. Khanal, R. Chahal, A. Sedova and V. S. Bryantsev, Chem. Sci., 2024, 15, 3116 DOI: 10.1039/D3SC06587A

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