DOI: https://doi.org/10.15421/081704

Quantum chemical analysis of Со2+ aqua complexes electrochemical reduction

Viktor F. Vargalyuk, Andrey O. Borschevich, Larisa V. Borschevich

Abstract


Based on the analysis of quantum chemical calculations results (GAMESS, density functional theory, B3LYP method) as to [Co(H2O)n]z(H2O)6–n clusters for z = 0, 1, 2 and n=1÷6, it has been demonstrated that electrochemical reduction of [Co(H2O)6]2+ aqua complexes runs stage-wise. At the first stage, an electron injected into the [Co(H2O)6]2+ complex is entirely located in the orbital of the central atom, as z(Co) herewith changes from +1.714 е to +0.777 е. The weakening of Со–ОН2 bonds leads to decomposition of resulting [Co(H2O)6]+ particles into two energetically related forms – [Co(H2O)4]+ and [Co(H2O)3]+. Further reduction of these intermediates runs differently. Electron injection into the [Co(H2O)3]+ intermediate terminatesthe transition of Со2+-ions to Со0 z(Co)= –0.264 е. This process is accompanied by rapid decomposition of [Co(H2O)3]0 product into monohydrate atom of cobalt Со(Н2О). On the contrary, electron injection into the [Co(H2O)4]+ intermediate leads to emergence of a specific structure – [Co+(H2O)(Н2О)3]¹0, whereby the electron is located in the atoms of cobalt only by 28%, and by 72% in cobalt-coordinated water molecules, clearly focusing on one of the. In this molecule, z(H2O) changes from +0.148 е to –0.347 е. There is an assumption that a non-equilibrium [Co+(H2O)(Н2О)3]0¹ form transits to [Co(ОH)(Н2О)3]0 hydroxo-form, which further disproportionates turning into Co(ОH)2 hydroxide. In order to reduce the impact of this unfavorable reaction pathway on the overall reaction rate Со2+ + 2ē = Со0, we suggest raising the temperature to ensure complete dissociation of [Co(H2O)4]+ to [Co(H2O)3]+.

Keywords


cobalt aqua complexes, electrochemical reduction, quantum chemical modeling

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