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.
Bibliographical noteFunding Information:
This research was supported by Creative Materials Discovery Program through the National Research Foundation of Korea (NRF) funded by Ministry of Science and ICT (NRF-2017M3D1A1039377 and 2017M3D1A1039380), the NRF grant funded by the Korea government (MSIT) (NRF-2017R1A2B3012003), the Industrial Strategic Technology Development Program (10062218, Transition metal-based flexible electrode for hydrogen production using super-saturated alloy) funded By the Ministry of Trade, Industry & Energy (MOTIE, Korea), and the KIST-SNU Joint Research Lab project (2V06170) under the KIST Institutional Program funded by the Korea government (Ministry of Science, ICT & Future Planning). K.T.N. appreciates the support from Institute of Engineering Research, Research Institute of Advanced Materials (RIAM) and Soft Foundry at Seoul National University. S.P. appreciates the support from the POSCO Science Fellowship of POSCO TJ Park Foundation.
© 2020, The Author(s).
ASJC Scopus subject areas
- General Chemistry
- General Biochemistry,Genetics and Molecular Biology
- General Physics and Astronomy