(246e) Elucidation of the Site Structures and CO Oxidation Mechanisms of the Ir1/TiO2 Single-Atom Catalysts

While single-atom catalysts (SACs) show promise to reduce usage of noble metals and maximize intrinsic catalytic activity for CO oxidation, their active sites and reaction mechanisms remain elusive. Herein, by combining DFT calculations and microkinetic modeling (MKM) with experiments, we employ Ir₁/TiO₂ catalysts as a specific case to elucidate the dynamic surface intermediates and CO oxidation mechanisms under operating conditions.

The adsorbed Ir₁ is more plausible to represent the experimental structures because it can form a gem-dicarbonyl in the CO atmosphere (Figure a), in line with the DRIFT spectra (Figure b). The adsorbed Ir₁ with a gem-dicarbonyl is the dominant species in the CO atmosphere, which can be transformed into the active Ir(CO)(O)₃ structure as the gas flow shifts to CO and O₂ (Figures a,b).

To provide further insight into the catalytic activity and ligand configurations of the catalysts, the CO oxidation mechanism at 423 K starting from the Ir(CO)(O)₃ species is proposed (Figure c). Interestingly, the reaction mechanism has two competing rate determining steps (RDSs) because of their similar free energy barriers. The first RDS is CO adsorption on the Ir(CO)(O)₃ species (Eley-Rideal-type), which has +1 CO order and has a Gibbs-free activation energy (G_a) increasing dramatically with T. In contrast, the second one is O₂ dissociation on the Ir(CO)(O)(O₂)₂ (Langmuir-Hinshelwood-type) that is +0 order in both CO and O₂ and has a stable G_a. Consistent with the proposed mechanism, CO order increases with T and E_app increases with P_co (Figures d, e) in both experiments and theory. Accordingly, by varying reaction conditions, three Ir₁ states along the reaction cycle can be isolated and identified (Figure f). This study provides atomic-level details on the intermediate states and reaction cycle of SACs, which is critical for rational design of improved site motifs on TiO₂ and beyond.