Unraveling Sequential Oxidation Kinetics and Determining Roles of Multi-Cobalt Active Sites on CoO 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 CoO nanoparticles as model catalysts. When OER initiates from fast oxidation of surface Co ions, both surface Co and Co ions are active sites of the multi-cobalt centers for water oxidation. In the sequential kinetics (Co → Co → Co), the key characteristic is fast oxidation and slow consumption for all the cobalt species. Due to this characteristic, the Co 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.