(230d) Identification of the Local Configurations and Reaction Mechanism of Single-Atom Ir/TiO2 Catalysts for CO Oxidation

While single-atom catalysts (SACs) show promise to reduce usage of noble metals and maximize intrinsic catalytic activity per atom for low temperature CO oxidation, their active sites and reaction mechanisms are still under intense debate. Herein, by combining DFT calculations and microkinetic modeling with experiments, we employ single atom Ir/TiO₂ catalysts as a specific case to draw a comprehensive picture of the dynamic evolution of surface species and to develop reaction mechanisms for CO oxidation.

The adsorbed Ir₁ was employed as model system. The gem-dicarbonyl structure is the dominant species in CO but can be transformed into the Ir(CO)(O)₃ structure as the gas stream shifts from CO only to CO and O₂. The reaction mechanism of Ir/TiO₂ towards CO oxidation starting from the Ir(CO)(O)₃ is proposed based on both experiments and theory. Interestingly, the reaction mechanism has competing rate determining steps (RDS). The first RDS is CO adsorption on the Ir(CO)(O)₃ (E-R step), which has +1 CO order and has an activation energy increasing dramatically with T. In contrast, the second one is O₂ dissociation on the Ir(CO)(O)(O₂)₂ (L-H step) that is +0 order in both CO and O₂. Thus, the dominant species in the reaction condition is a mixture of the Ir(CO)(O)₃ and Ir(CO)(O)(O₂)₂. In addition, CO order increases obviously as the temperature increases, while O₂ order keeps as 0, because the E-R step becomes more important at higher temperature. When P_co increases, the RDS switches from the E-R step to the L-H step, resulting in the increase of E_app. More interestingly, there is an inert *CO that is not depleted during reaction, which opens the door for us to adjust the activity of the Ir₁/TiO₂ catalysts by using other ligands. Therefore, our study is critical for designing high-performance catalysts atom-by-atom on TiO₂ and even other oxides.