Unraveling Sequential Oxidation Kinetics and Determining Roles of Multi-Cobalt Active Sites on Co 3 O 4 Catalyst for Water Oxidation.

The multi-redox mechanism involving multi-sites has great implications to dictate the catalytic water oxidation. Understanding the sequential dynamics of multi-steps in oxygen evolution reaction (OER) cycles on working catalysts is a highly important but challenging issue. Here, using quasi-operando transient absorption (TA) spectroscopy and a typical photosensitization strategy, we succeeded in resolving the sequential oxidation kinetics involving multi-active sites for water oxidation in OER catalytic cycle, with Co ₃ O ₄ nanoparticles as model catalysts. When OER initiates from fast oxidation of surface Co ²⁺ ions, both surface Co ²⁺ and Co ³⁺ ions are active sites of the multi-cobalt centers for water oxidation. In the sequential kinetics (Co ²⁺ → Co ³⁺ → Co ⁴⁺ ), the key characteristic is fast oxidation and slow consumption for all the cobalt species. Due to this characteristic, the Co ⁴⁺ intermediate distribution plays a determining role in OER activity and results in the slow overall OER kinetics. These insights shed light on the kinetic understanding of water oxidation on heterogeneous catalysts with multi-sites.