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

The analogy between single atom catalysts (SACs) and molecular catalysts predicts that the specific catalytic activity of these systems is constant. We provide evidence that this prediction is not necessarily true. As a case in point, we show that the specific activity over ceria5 supported single Pd atoms linearly increases with metal atom density, originating from the cumulative enhancement of lattice oxygen mobility. The long-range electrostatic fingerprints (~1.5 nm) around each Pd site overlap with each other as surface Pd density increases, resulting in the observed deviation from constant specific activity. These cooperative effects exhaust previously active O atoms above a certain Pd density, leading to their permanent 10 removal and consequent drop in reaction rate. The findings of our combined experimental and computational study show that the specific catalytic activity of reducible oxide-supported single atom catalysts can be tuned by varying the surface density of single metal atoms.