Emergent Behavior in Oxidation Catalysis over Single-Atom Pd on a Reducible CeO2 Support via Mixed Redox Cycles

Experimental kinetics demonstrate catalytic lean CO oxidation over single Pd atoms with a CO reaction order greater than unity. A CO order greater than unity is unique to Pd single atoms supported on CeO2 nanocubes and is not observed for Pd nanoparticles on the same support nor other supported single-atom systems. A reaction mechanism including eight elementary steps, in which Pd dynamically samples three formal oxidation states, is unable to capture a reaction order in CO greater than unity. Vibrational spectroscopy of CO adsorption ranging from lean to rich conditions reveals Pd accesses an extensive array of formal oxidation states, ranging from Pd4+ to Pd0. Density functional theory calculations demonstrate Pd atoms supported on CeO2 can access four formal oxidation states during lean CO oxidation. Two catalytic redox cycles are possible, with cross-over between them leading to a reaction mechanism yielding reaction orders in CO greater than unity. A microkinetic model, utilizing Bayesian inference to adjust elementary energetics to fit the experimentally measured activation barrier and reaction orders, confirms the reaction order in CO of >1 arises from the ability of the Pd active site to access four formal oxidation states, spanning from +4 (PdO2) to -2 (Pd adjacent to a CeO2 lattice oxygen vacancy). This work demonstrates the emergent redox behavior of Pd single atoms on CeO2, which may be important to explain their high activity for low-temperature catalytic combustion of hydrocarbons.