Identification of the active complex for CO oxidation over single-atom Ir-on-MgAl2O4 catalysts

Supported single atoms provide an opportunity to design new heterogeneous catalysts while optimizing the utilization of noble metals. However, identification of the active single-atom structure is required for understanding the reaction mechanism and guiding catalyst design. Here, we use in situ infrared spectroscopy, operando X-ray absorption spectroscopy and quantum chemical calculations to identify the active single-atom complex as well as the resting state of the Ir/MgAl₂O₄ catalysts during the low-temperature CO oxidation. In contrast to poisoning of iridium nanoparticles by CO, here we show that the formation of Ir(CO) on single atoms results in a different reaction mechanism and high activity for low-temperature CO oxidation. This is due to the ability of single atoms to coordinate with multiple ligands, where Ir(CO) provides an interfacial site for facile O₂ activation between Ir and Al and lowers the reaction barrier between gas-phase CO(g) and *O in Ir(CO)(O) through an Eley–Rideal mechanism.