МЕХАНІЗМ ЕЛЕКТРООКИСНЕННЯ Mn2+ ІОНІВ

Abstract

In this work the mechanisms of electrooxidation of Mn 2+ to MnO2 were investigated in perchlorate, sulphate and acetate solutions. Density functional theory (DFT), as a quantum modeling method, was used for identification of red-ox potentials of one-electron oxidation of the aquacomplexes [Mn 2+ (H2O)6], [Mn 2+ (H2O)5(SO4 2-)]. The calculated values were significantly higher than the measured potentials of the initial stages of complexes oxidation on Pt electrode. The thermodynamical possibilities of formation of oxocomplexes and the kinetic measurements were analyzed. Based on this data it has been found that in perchlorate and sulphate solutions (pH 4) Mn 2+-iones were oxidized due to the interaction with adsorbed •OH-radicals, produced by the water-splitting reaction. For strongly acid sulphate solutions (pH 1) it was observed the convergence of values of the potential of water-splitting reaction (1,2 V) and the potential of oxidation of [Mn 2+ (H2O)5(НSO4-)] complex (1,13V). This points to simultaneous implementation of two reaction paths: the direct electrooxidation of Mn 2+-iones and the oxidation due to the interaction with •OH-radicals. The calculated value of potential of electrooxidation of monoacetate aquacomplex of Mn 2+-iones is notably low (0,66 V). This poin to the only electrooxidation path of the reaction. The calculated data have been confirmed by the kinetic measurements. The particles [Mn 3+ (H2O)5(Ас-)] rapidly disproportionate to MnO2 and [Mn 2+ (H2O)5(Ас-)] due to the features of carboxyl group.

Вплив Аніонів Ненасичених Органічних Кислот На Процес Електроокиснення Аквакомплексів Мангану (Іі)

“…As expected, the bidentate binding of maleic acid anions significantly affected the energy of the process Mn 2+ -e -= Mn 3+ and, thus, the value of the standard potential. Its value dropped to 0.32 V, which is almost twice less relative to acetate complexes (E0 = 0.66 V [13]).…”

“…The solvent was taken into account using the polarization continuum DSM model [20; 21]. Other details are given in [13].…”

“…1 shows their spatial structure. The octahedral structure, which is energetically the most advantageous for manganese aqua complexes, was taken as the basic model [13]. During the optimization process, it remained unchanged for all complexes, except for the compounds of manganese ions with M 2-, where the bidentate binding of fully deprotonated anions to the manganese cation leads to a natural decrease in the number of coordinated water molecules from 5 to 4 (z = 3), and to 3 (z = 2).…”

“…In [13], it was shown that when an electron is removed from homogeneous Mn 2+ aquacomplexes, the charge of the central atom increases by 0.446 e, and 0.554 e is distributed approximately evenly between the six water molecules of the inner coordination sphere. It can be seen from Table 2 that in monosubstituted manganese acidoacqua complexes the degree of increase of the charge of the central atom becomes smaller, apparently, due to the greater electron donation of the anions of dibasic organic acids compared to the molecules of H2O.…”

“…As it turned out [12; 13], due to the entry of acetate ions into the internal coordination sphere of manganese aquacomplexes, the standard potential of the reaction Mn 2+ -e -= Mn 3+ decreases to 0.66 V, which is approximately 0.5 V more negative than the E0 of the oxygen extraction process. However, since E0 of the process of the next electron extractionfrom the intermediate Mn(Aс) 2+ remains very high (2.13 B [13]), and the elimination of the reaction H2O -e -= •OH + H + removed the effective oxidant (OH-radicals) from the electrode surface, the formation of MnO2 in acetate solutions occurs due to the disproportionation of Mn(Aс) 2+ complexes and the hydrolysis of the highly oxidized form of Mn(Aс) 3+ . Trivalent manganese complexes Mn(Ас) 2+ are also prone to hydrolysis, therefore, the precipitate electrolytically released on the anode contains not only MnO2 crystals, but also MnO(OH) [14; 15].…”