Spectroscopic capture of a low-spin Mn(IV)-oxo species in Ni–Mn3O4 nanoparticles during water oxidation catalysis

Abstract

High-valent metal-oxo moieties have been implicated as key intermediates preceding various oxidation processes. The critical O–O bond formation step in the Kok cycle that is presumed to generate molecular oxygen occurs through the high-valent Mn-oxo species of the water oxidation complex, i.e., the Mn4Ca cluster in photosystem II. Here, we report the spectroscopic characterization of new intermediates during the water oxidation reaction of manganese-based heterogeneous catalysts and assign them as low-spin Mn(IV)-oxo species. Recently, the effects of the spin state in transition metal catalysts on catalytic reactivity have been intensely studied; however, no detailed characterization of a low-spin Mn(IV)-oxo intermediate species currently exists. We demonstrate that a low-spin configuration of Mn(IV), S = 1/2, is stably present in a heterogeneous electrocatalyst of Ni-doped monodisperse 10-nm Mn3O4 nanoparticles via oxo-ligand field engineering. An unprecedented signal (g = 1.83) is found to evolve in the electron paramagnetic resonance spectrum during the stepwise transition from the Jahn–Teller-distorted Mn(III). In-situ Raman analysis directly provides the evidence for Mn(IV)-oxo species as the active intermediate species. Computational analysis confirmed that the substituted nickel species induces the formation of a z-axis-compressed octahedral C4v crystal field that stabilizes the low-spin Mn(IV)-oxo intermediates.

Quinol-containing ligands enable high superoxide dismutase activity by modulating coordination number, charge, oxidation states and stability of manganese complexes throughout redox cycling

“…Very recently, an unprecedented signal at g ∼ 1.8 was observed during water oxidation reactions involving manganese-containing heterogeneous catalysts; the feature was assigned to a low-spin Mn( iv )-oxo species with S = 1/2. 95 Although the signal at ∼2 described in the present work could be also generally ascribed to a low-spin Mn( iv )-oxo species, we could not adequately fit the data using a S = 1/2 model; a S = 3/2 model provides a superior fit.…”

Tailoring the Electronic Structures of the La2NiMnO6 Double Perovskite as Efficient Bifunctional Oxygen Electrocatalysis

“…Park et al reported that surface Ni-doping on Mn 3 O 4 introduced high-valent Mn 4+ states. The Ni-doped Mn 3 O 4 catalyst exhibited excellent ORR activity because the adsorption of O 2 , which is recognized as the rate-limiting step for the ORR, has an optimum strength on Mn­(IV) cations (Mn 4+ –O 2 ) for better ORR activity . However, these Mn-based oxides usually show limited OER activity .…”

“…The Nidoped Mn 3 O 4 catalyst exhibited excellent ORR activity because the adsorption of O 2 , which is recognized as the rate-limiting step for the ORR, has an optimum strength on Mn(IV) cations (Mn 4+ −O 2 ) for better ORR activity. 40 However, these Mn-based oxides usually show limited OER activity. 41 Previous works suggested that Ni 3+ -based oxides with strong Ni 3d−O 2p hybridization are efficient OER electrocatalysts.…”

An overview of solid-state electron paramagnetic resonance spectroscopy for artificial fuel reactions

“…Reprinted with permission from Park et al. 14 Copyright 2020 Springer Nature. (C and D) Depictions of how the dopant atom added to the base material in both cases yields EPR-visible paramagnetic species Ni 3+ and low-spin Mn 4+ .…”

“…Alternatively, Ni doping into spinel Mn 3 O 4 nanoparticles also can result in a more active Mn-Ni oxide OER catalyst. 14 By tracing the behavior of the catalyst on polarization using ex situ freeze-quenching, it was observed that a new, low-spin Mn 4+ species was found to emerge, leading the authors to conclude that Ni stabilized low-spin Mn 4+ in Mn 3 O 4 within a compressed crystal field ( Figures 6 B and 6D). Notably, at no time were any EPR spectra for Ni 3+ observed, demonstrating that the oxidation state of Ni remained Ni 2+ .…”