First principles computational studies of spontaneous reduction reaction of Eu(III) in eutectic LiCl‐KCl molten salt. (26th March 2018)

• Record Type: Journal Article
• Authors: Kwon, Choah; Noh, Seung Hyo; Chun, Hoje; Hwang, Il Soon; Han, Byungchan
• Is Part Of: International journal of energy research. Volume 42:Number 8(2018)
• Page Start: 2757
• Page End: 2765
• Publication Date: 2018-03-26
• Subjects: coordination number -- diffusion -- europium -- Gibbs free energy -- first principles -- molten salt -- radial distribution function -- vibrational entropy; Power resources -- Periodicals; Power (Mechanics) -- Periodicals; Power resources -- Research -- Periodicals; 621.042
• DOI: 10.1002/er.4064
• Languages: English
• ISSNs: 0363-907X

Abstract

Summary: Using first principles calculations, we study fundamental mechanism of spontaneous reduction reaction of Eu 3+ to Eu 2+ in eutectic LiCl‐KCl molten salt. We decouple the reaction Gibbs free energy into enthalpy and entropy contributions by using rigorous thermodynamic formalism. Key structural features of the solvation shell are characterized by the radial distribution function and the coordination number. Compared with Eu 2+, the Eu 3+ ion has a more rigid framework of the solvation shells, corroborating its stronger electrostatic interaction with neighboring ligands of Cl − ions and a more favorable state on the aspect of enthalpy. Computations on vibrational frequency, however, pose significant contribution of vibrational entropy to the reaction Gibbs free energy for the reduction. Vibration frequency of Eu 2+ is smaller than that of Eu 3+, driving a more positive change of the entropy in the reduction reaction. Furthermore, an Eu 2+ diffuses more quickly than an Eu 3+ in the LiCl‐KCl molten salt with switching mechanism of ligand Cl − ions in the solvation shell. Our results propose that the spontaneity of the reduction reaction is driven by the entropic contribution by overcoming the penalty of the reaction enthalpy. Abstract : In this paper, combined with ab initio molecular dynamic simulations, we calculated solvation shell structures in the molten salt and decoupled each contribution of reaction enthalpy and vibrational entropy to the free energy. The structures of Eu 2+ and Eu 3+ in an LiCl‐KCl molten salt were analyzed via radial distribution functions and coordination numbers calculated by ab initio molecular dynamic simulations. The first peak positions in the radial distribution functions for Eu 2+ and Eu 3+ ions, corresponding to their first solvation shells, were calculated as 2.85 and 2.65 Å, respectively. On the basis of the results, we concluded that the solvation shell structure of Eu 3+ is more rigid than that of Eu 2+ in an LiCl‐KCl molten salt. Our results indicated that the spontaneous reduction is driven not by the enthalpy but the positive generation of vibrational entropy in the reaction. It was consistent with our calculation that Eu 2+ has a higher diffusion coefficient via switching mechanism with Cl ‐ ion nearby in the molten salt.