Assessing Metal Valence States of Iridium Single-Atom Catalysts by ROS-Mediated Luminol Electrogenerated...

Catalytic activities of single-atom catalysts (SACs) are strongly correlated to metal center valence states. Herein, we report a simple and facile electrogenerated chemiluminescence (ECL) method for the sensitive and quantitative assessment of iridium (Ir) valence states in UiO-67-supported SACs, using luminol as an ECL luminophore. Three UiO-67-Ir SACs (UiO-67-Ir-1/2/3) with tunable Ir valence states were synthesized by varying the solvent types during preparation. Atomic dispersion of Ir on UiO-67 was confirmed via atomic-resolution high-angle annular dark-field scanning transmission electron microscopy, with Ir anchored through Ir(III)-nitrogen coordination. To elucidate the influence of Ir valence states on the ECL behavior, we systematically investigated the catalytic effects of the three UiO-67-Ir catalysts on both cathodic (luminol-O₂) and anodic (luminol-H₂O₂) ECL systems. Different Ir valence states induced distinct types and amounts of reactive oxygen species (ROS), governing ECL responses. Compared to Ir(III), Ir(0) enhanced ROS generation, promoting both cathodic and anodic ECL intensities of luminol. Notably, the ECL intensity exhibits a positive correlation with the relative content of Ir(0): the higher the Ir(0) ratio, the greater the ECL signal. Furthermore, a linear relationship was established between the amount of Ir(0) and the ECL intensity, enabling the quantitative determination of the Ir(0) content in the SACs. This ROS-mediated ECL method offers a novel, sensitive, and quantitative alternative for assessing metal valence states in SACs, which is crucial for the rational design and optimization of high-performance SACs for catalytic applications.