Dynamic Variation of Ni and Pt Single Atom Oxidation States on CeO₂ Nanocubes during CO Oxidation

Palladium single atoms supported on ceria nanocubes demonstrate unique catalytic behavior during CO oxidation, allowing the metal atom to access two redox cycles and oscillate between four oxidation states. Ni and Pt single atoms supported on ceria nanocubes are active for CO oxidation but do not exhibit the same unique behavior as Pd/CeO2 single-atom catalysts (SACs). Experimentally measured CO reaction orders for Ni/CeO2 and Pt/CeO2 SACs are less than one, indicating the catalytic cycle accesses only three oxidation states. IR spectroscopy reveals a narrow range of Ni (Ni2+ and Ni4+) and Pt (Pt2+) oxidation states under lean and rich CO conditions. Density functional theory calculations demonstrate oxygen vacancies cannot form adjacent to adsorbed Ni atoms, and the formation of PtO2 is kinetically infeasible, limiting these SACs to a simpler redox cycle relative to Pd single atoms. Microkinetic modeling, utilizing Bayesian inference to allow the elementary energetics to vary, successfully matches experimental reaction orders and apparent barriers for Ni/CeO2 and Pt/CeO2 SACs with a mechanism using a single redox cycle. The kinetic behavior of single metal atoms supported on ceria nanocubes highlights how different metals have mechanistic differences during CO oxidation. The delineation of the oxidation states accessible to each metal atom may also guide their use in other catalytic chemistries.