Theoretical Insight into the Performance of Mn^II/III-Monosubstituted Heteropolytungstates as Water Oxidation Catalysts

Abstract

The performance of MnII/III-monosubstituted heteropolytungstates [MnIII(H2O)­GeW11O39]5– ([GT-MnIII-OH2]5–, where GT = GeW11O39) and [MnII(H2O)­GeW11O39]6– ([GT-MnII-OH2]6–) as water oxidation catalysts at pH 9 was explored using density functional theory calculations. The counterion effect was fully considered, in which five and six Na+ ions were included in the calculations for water oxidation catalyzed by [GT-MnIII-OH2]5– and [GT-MnII-OH2]6–, respectively. The process of water oxidation catalysis was divided into three elemental stages: (i) oxidative activation, (ii) O–O bond formation, and (iii) O2 evolution. In the oxidative activation stage, two electrons and two protons are removed from [Na5-GT-MnIII-OH2] and three electrons and two protons are removed from [Na6-GT-MnII-OH2]. Therefore, the MnIV-O• species [Na5-GT-MnIV-O•] is obtained. Two mechanisms, (i) water nucleophilic attack and (ii) oxo–oxo coupling, were demonstrated to be competitive in O–O bond formation triggered from [Na5-GT-MnIV-O•]. In the last stage, the O2 molecule could be readily evolved from the peroxo or dinuclear species and the catalyst returns to the ground state after the coordination of a water molecule(s).

Activation by oxidation and ligand exchange in a molecular manganese vanadium oxide water oxidation catalyst

“…As in Siegbahn's studies of the OEC in photosystem II, 28,29 calculations were carried out using the high-spin conguration and assuming that antiferromagnetic effects do not signicantly affect either the reaction barriers or the geometry of the complex. 30 Geometries were optimized using DFT and the B3LYP functional 31,32 as widely used in other studies of OEC model systems [33][34][35][36][37] and polyoxometalate WOCs 24,[38][39][40][41][42][43][44][45] combined with the def2-SVP basis set. 46 Local minima were conrmed by the absence of imaginary frequencies.…”

Activation by oxidation and ligand exchange in a molecular manganese vanadium oxide water oxidation catalyst

“…As in Siegbahn's studies of the OEC in photosystem II, 28,29 calculations were carried out using the highspin configuration and assuming that antiferromagnetic effects do not significantly affect either the reaction barriers or the geometry of the complex. 30 Geometries were optimized using DFT and the B3LYP functional 31,32 as widely used in other studies of OEC model systems [33][34][35][36][37] and polyoxometalate WOCs - 24,[38][39][40][41][42][43][44][45] combined with the def2-SVP basis set. 46 Local minima were confirmed by the absence of imaginary frequencies.…”

The Reactivity and Stability of Polyoxometalate Water Oxidation Electrocatalysts

“…To sum up, we have seen that theoretical methods are capable of handling a wide variety of problems in the study of POM-WOCs, demonstrating that fundamental insights into the mechanistic details of water oxidation on POM catalysts that can be gained through their applications. It appears that DFT is the method of choice, as with few exceptions, all mechanistic studies presented here adopt hybrid functionals [32,49,[51][52][53][55][56][57][58][59] and implicit solvent models [32,[49][50][51][52][53][54][57][58][59] for their simulations. For a more comprehensive overview of theoretical methods used in POM research, the reader is referred to the excellent review by López et al [64] Considerable progress has been made in elucidating the water oxidation cycles of some WOCs in great detail, especially those of {Ru 4 } and {Co 4 }.…”