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.
Highlights
High-valent metal-oxo moieties have been implicated as key intermediates preceding various oxidation processes
While many experimental and computational results imply the existence of a high-valent metal-oxo species on the surface of heterogeneous catalysts that serves as an active reaction intermediate during the water oxidation reaction, very few results offering direct evidence of the chemical identity of intermediates have been reported to date for heterogeneous catalysts
Electron energy loss spectroscopy (EELS) line scans were used to examine the composition of the synthesized catalysts and revealed that the nickel species was uniformly distributed within the assembled NPs from the bottom interface between the Mn3O4 NPs layer and the NiO layer to the top surface of the electrode (Fig. 1c)
Summary
High-valent metal-oxo moieties have been implicated as key intermediates preceding various oxidation processes. High-valent transition metal-oxo species are involved in the rate-determining steps of various oxygen atom transfer (OAT) reactions, including C–H bond activation, hydroxylation, and O–O bond formation[1,2,3,4]. Characterization and manipulation of high-valent transition metal-oxo species are crucial to designing heterogeneous catalysts for water oxidation reaction, where O–O bond formation is the key elementary step. We demonstrated that Mn(III) species can be stably generated on the surface of manganese oxide NPs via a proton-coupled electron transfer pathway in a neutral phosphate electrolyte[29] and further suggested that the rate-determining step involves Mn(IV)-oxo species rather than Mn(III) based on the in-situ/ex-situ spectroscopy analysis[30]. We find that the Ni substitution enables the compression of surface Mn octahedron, resulting in low-spin Mn(IV)-oxo intermediate species during the water oxidation reaction
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