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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• 作者：Ryan G. Hadt ; Dugan Hayes ; Casey N. Brodsky ; Andrew M. Ullman ; Diego M. Casa ; Mary H. Upton ; Daniel G. Nocera ; Lin X. Chen
• 刊名：Journal of the American Chemical Society
• 出版年：2016
• 出版时间：August 31, 2016
• 年：2016
• 卷：138
• 期：34
• 页码：11017-11030
• 全文大小：776K
• 年卷期：0
• ISSN：1520-5126

文摘

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 t_2g-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 E⁰ 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.