(481c) Are All Single Atoms Created Equal? Surface Density Dependent Catalytic Activity of Single Pd Atoms Supported on Ceria

Being at the limit of single atoms, SACs are typically thought to present chemically uniform catalytically active sites, which are thus assumed to exhibit a single specific activity regardless of the metal surface density. However, we show here a general enough case that is in contradiction to this assumption to cast credible doubt on its generality. Using CO oxidation as a probe reaction, we show here that the specific activity of Pd/CeO₂ SACs increases linearly with Pd surface density, which we show is due to cumulative nonlocal effects.

In this work, specific activity experiments, H₂-temperature programed reduction (TPR), in-situ infrared (IR), Raman, and density functional theory (DFT) calculations are combined to determine the origin of the observed deviation from constant specific activity of Pd/CeO₂ (Figure 1A). H₂-TPR (Figure 1B) shows a progressive enhancement in Pd/CeO₂ reducibility as Pd surface density increases that mirrors the observed activity enhancement. The associated Raman and IR spectra corroborate this result. These results suggest that an increased Pd surface density leads to increased lattice oxygen mobility and thus to enhanced specific activity. DFT calculations confirm this, showing that the creation of oxygen vacancies is progressively made easier as Pd surface density increases (Figure 1C) while no associated change in CO or O₂ adsorption is found. This suggests the Pd single atoms cumulatively activate lattice oxygen. Pd charge and spin states reveal the active site to be a uniquely overoxidized but otherwise stable [PdO₄] square planar complex (Figure 1D). As Pd surface density increases, the overoxidation of the active site allows for increasingly effective charge scavenging upon formation of oxygen vacancies. Overall, this work shows that the constant-specific-activity assumption is not dogma and that nonlocal effects must be considered when studying SACs, especially ones supported on reducible oxides permitting charge transfer to the SAC active centers.