X-ray Spectroscopic Characterization of Co(IV) and Metal–Metal Interactions in Co4O4: Electronic Structure Contributions to the Formation of High-Valent States Relevant to the Oxygen Evolution Reaction
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Abstract
The formation of high-valent states is a key factor in making highly active transition-metal-based
catalysts of the oxygen evolution reaction (OER). These high oxidation states will
be strongly influenced by the local geometric and electronic structures of the metal
ion, which are difficult to study due to spectroscopically active and complex backgrounds,
short lifetimes, and limited concentrations. Here, we use a wide range of complementary
X-ray spectroscopies coupled to DFT calculations to study Co(III)4O4 cubanes and their
first oxidized derivatives, which provide insight into the high-valent Co(IV) centers
responsible for the activity of molecular and heterogeneous OER catalysts. The combination
of X-ray absorption and 1s3p resonant inelastic X-ray scattering (Kβ RIXS) allows
Co(IV) to be isolated and studied against a spectroscopically active Co(III) background.
Co K- and L-edge X-ray absorption data allow for a detailed characterization of the
3d-manifold of effectively localized Co(IV) centers and provide a direct handle on
the t2g-based redox-active molecular orbital. Kβ RIXS is also shown to provide a powerful
probe of Co(IV), and specific spectral features are sensitive to the degree of oxo-mediated
metal-metal coupling across Co4O4. Guided by the data, calculations show that electron-hole
delocalization can actually oppose Co(IV) formation. Computational extension of Co4O4
to CoM3O4 structures (M = redox-inactive metal) defines electronic structure contributions
to Co(IV) formation. Redox activity is shown to be linearly related to covalency,
and M(III) oxo inductive effects on Co(IV) oxo bonding can tune the covalency of high-valent
sites over a large range and thereby tune E(0) over hundreds of millivolts. Additionally,
redox-inactive metal substitution can also switch the ground state and modify metal-metal
and antibonding interactions across the cluster.