Ceria-supported Pd catalysts with different size regimes ranging from single atoms to nanoparticles for the oxidation of CO.

[Display omitted] • CeO 2 -supported single Pd atoms are the most intrinsically active in CO oxidation. • The CO reaction rate on isolated Pd sites is exclusively promoted by H 2. • The oxidation of CO proceeds through a Langmuir-Hinshelwood mechanism. • The decomposition of formate species contribution dominantly in CO oxidation. • A stronger H-spillover effect was observed on isolated Pd sites. • Bridged-OH contributes to the consumptions of bicarbonate and formate species. Supported metal catalysts are the most widely used in industrial processes and the metal particle size plays a crucial factor in determining the catalytic performance. Herein, CeO 2 -supported Pd catalysts with different Pd size regimes ranging from single atoms, to nanoclusters (1–2 nm), and to nanoparticles (>2 nm) were used for both CO oxidation and preferential oxidation of CO in H 2 (CO-PROX). Compared to Pd nanoclusters and nanoparticles, CeO 2 -supported single Pd atoms (Pd SA /CeO 2) are the most intrinsically active in CO oxidation, with an apparent activation energy of ca. 40 kJ mol−1. Results of kinetic investigations and in situ diffuse reflectance infrared Fourier transformed spectroscopy demonstrate the CO oxidation proceeding through a Langmuir-Hinshelwood mechanism with the decomposition of formate species acting dominantly as the rate-determining step. The CO reaction rate is exclusively promoted on Pd SA /CeO 2 catalysts for the CO-PROX reaction, which could be ascribed to a stronger H-spillover effect on isolated Pd sites to produce bridged-OH on CeO 2 surface, simultaneously facilitating the consumptions of bicarbonate and formate species. There results greatly deepen the fundamental understanding of the Pd size regimes over Pd/CeO 2 catalysts for the oxidation of CO. [ABSTRACT FROM AUTHOR]