(532cw) Trends in Catalytic Activity of Single-Atom M/TiO2 Catalysts Towards CO Oxidation

Recently, single-atom catalysts (SACs) show promise to reduce usage requirements of noble metals and maximize intrinsic catalytic activity per metal atom for low temperature CO oxidation. Unlike extended metal surfaces, different reaction mechanisms of SACs for CO oxidation were proposed. However, even with these advances, underlying kinetic parameters governing catalytic activity of these SACs remains mysterious. Herein, by combining DFT calculations and microkinetic modeling with experiments, we employ M₁/TiO₂ catalysts as a specific case to develop descriptors to understand trends in their catalytic activity for CO oxidation.

The preferred reaction pathway of Ir₁/TiO₂ catalysts is firstly calculated with 5 elementary steps. Interestingly, the rate-determining step could be CO oxidation (R1) or O₂ dissociation (R5) because the two steps have similar activation energies, while the other steps are fast enough. Herein, we assume this mechanism also fits CO oxidation over other M₁/TiO₂. As gas phase CO attacks an *O in R1, the binding strength of *O to M1 matters. Thus, a positively linear relationship between vacancy formation energies of *O (E_vo) and DFT-calculated activation energies of reaction R1 is developed. In contrast, the activation barriers of reaction R5 inversely correlate with the E_vo, indicating there is a trade-off between the catalytic activity of M₁/TiO₂ for O₂ dissociation (R5) and that for CO oxidation (R1). Thus, the E_vo is regarded as the descriptor to characterize the activity of these SACs for CO oxidation in a simple microkinetic modeling. Finally, general trends in catalytic activity of M₁/TiO₂ are identified, with reasonable agreement with experimental results. In summary, our study unravels the general trends in catalytic activity on single-atom M/TiO₂ catalysts for CO oxidation, which is critical for screening high-performance single-atom active sites on TiO₂ and even other oxides.