In Situ X-ray Absorption Fine Structure Spectroscopy Measurement of Suspended Cobalt Oxide Nanoparticle Water Oxidation Catalyst

Megumi Okazaki,* Zi Lang Goo, Haruka Yamamoto, Kenta Ikegami, Issei Yamamoto, Naoki Tarutani, Fumitaka Takeiri, Takeshi Watanabe, Shunsuke Nozawa, Kiyofumi Katagiri, Kunihisa Sugimoto, Kazuhiko Maeda*

Water oxidation using a heterogeneous catalyst under near-neutral pH conditions is of importance in artificial photosynthesis. This work assessed photochemical water oxidation over a supported cobalt oxide (CoO_x) nanoparticle catalyst in the presence of Ru(II) trisdiimine as a photosensitizer and sodium persulfate as an electron acceptor, in both phosphate and borate buffer solutions at a pH of 7.9, using X-ray absorption fine structure spectroscopy (XAFS). The steady-state activity of the CoO_x was found to be increased by a factor of 3−4 in the phosphate buffer. Co-K edge in situ XAFS measurement revealed that this catalyst maintained a high-valence CoOOH-like local structure during photoirradiation in the phosphate buffer. In contrast, CoOOH-like species rapidly formed in the borate buffer but remained stable for less than 40 min, with the subsequent generation of a mixture of Co²⁺ and Co³⁺ states with 4 and 6 coordination numbers. These results indicate that the phosphate buffer evidently promoted the generation and stabilization of active CoOOH species, thus facilitating water oxidation. The borate buffer failed to sustain these active species, resulting in lower catalytic activity. These insights provide a basis for the rational design of catalytic systems, emphasizing the importance of buffer-controlled local environments in sustaining active species for efficient water oxidation.

J. Phys. Chem. Lett. 2026, in press.