Identifying MnVII-oxo Species during Electrochemical Water Oxidation by Manganese Oxide

Abstract

SummaryIdentifying surface active intermediate species is essential to reveal the catalytic mechanism of water oxidation by metal-oxides-based catalysts and to develop more efficient catalysts for oxygen-oxygen bond formation. Here we report, through electrochemical methods and ex situ infrared spectroscopy, the identification of a MnVII = O intermediate during catalytic water oxidation by a c-disordered δ-MnOx with an onset-potential-dependent reduction peak at 0.93 V and an infrared peak at 912 cm−1. This intermediate is proved to be highly reactive and much more oxidative than permanganate ion. Therefore, we propose a new catalytic mechanism for water oxidation catalyzed by Mn oxides, with involvement of the MnVII = O intermediate in a resting state and the MnIV−O−MnVII = O as a real active species for oxygen-oxygen bond formation.

Why nature chose the Mn4CaO5 cluster as water-splitting catalyst in photosystem II: a new hypothesis for the mechanism of O–O bond formation

“…43 Very recently, we reported the presence of an essential Mn VII -dioxo intermediate in a synthetic c-disordered δ-MnO xbased water oxidation catalyst (i.e., MnO x -300), 45 where a Mn IV -O-Mn VII vO was proposed as the active species for O-O bond formation. 44 As shown in Fig. 4 , which is unique, i.e., distinguishable from other 3 Mn ions by being placed outside the cubane structure.…”

Uniform, Assembled 4 nm Mn3O4 Nanoparticles as Efficient Water Oxidation Electrocatalysts at Neutral pH

“…By forming MnN bonds through the introduction of nitrogen ligands on the MnO 2 surface, Mn(III) species were stabilized by suppressing the CD reaction, thus leading to the OER activity enhancement . Recently, there have been some insightful studies on the Mn based water oxidation catalysts regarding the mechanistic studies including OO bond formation and identification of Mn VII O intermediate during catalytic water oxidation, structural and electronic effects on superior OER activity of activated‐MnO 2 and the correlation between the stability of the intermediate species and crystal facet …”

Mono‐/Multinuclear Water Oxidation Catalysts

“…The two DFT structures exhibited in Figure 2b couldb ee asily interconverted under certain conditions, as demonstrated through DFT calculations. [53] Considering the high catalytic activitya nd stability,m ultinuclear Mn 12 DH [23] and mononuclear Mn-NG, [14] which exhibit excellent performances, have been synthesized.W OCs with low overpotential, high TOF (Table 1), and strong durability can bring some new insightsi nto the preparation of increasingly efficient catalysts for the OER. EPR spectroscopy can be used to monitor the catalytic process, and every Ss tate has itso wn EPR signal.…”

“…The redox potentials of [Mn 4 CaO 4 (tBuCO2) 8 (tBuCO 2 H) 2 (py)] were different from those of the Mn 3 CaO 4 complex, [7] which indicated that the fourth Mn could change the redox potential. [53] The following formation process for the active inter- [53] The following formation process for the active inter- …”

“…[6,7] The highest oxidation state of most manganese-based WOCs for the OER is Mn V .H owever,M n VII =Oh as also been proposed as an intermediate for the OER. [53] Considering the high catalytic activitya nd stability,m ultinuclear Mn 12 DH [23] and mononuclear Mn-NG, [14] which exhibit excellent performances, have been synthesized.W OCs with low overpotential, high TOF (Table 1), and strong durability can bring some new insightsi nto the preparation of increasingly efficient catalysts for the OER.…”