Comparing and contrasting CO oxidation by Rh atoms and clusters on and under thin-film copper oxides

Single-atom catalysts, where isolated metal atoms are stabilized on oxide supports and excel in oxidation reactions, and single-atom alloys, where active atoms are dispersed in inert metal hosts and perform best under reducing conditions, have both gained significant attention in recent years. However, the catalytic behavior in the oxidation state space between these regimes remains poorly understood. Here, we compare CO oxidation with Rh under thin oxide films grown on RhCu(1 1 1) surface alloys and compare the results to the reactivity of Rh on thin oxide films grown on a Cu(1 1 1) surface. Temperature-programmed desorption shows two distinct CO₂ peaks for Rh on the “29” oxide – arising from clusters (lower temperature) and single atoms (higher temperature), whereas Rh under the Cu_xO exhibits a single higher temperature CO₂ peak that shifts to lower temperatures with increasing Rh coverage. The CO₂ yield is markedly lower for Rh under the oxide due to oxygen encapsulation, but adsorbed CO at exposed Rh sites is fully converted with no intact CO desorption above 300 K, in contrast to Rh on the “29” oxide. Isotopic labelling confirms a Mars-van Krevelen mechanism in both systems, though C-O bond scission is suppressed when Rh is encapsulated. X-ray photoelectron spectroscopy confirms the enhanced CO oxidation activity for Rh on the “29” oxide as well as shows that the reaction occurred between CO and O adatoms. Density functional theory further show that Rh under the oxide enforces a Mars-van Krevelen pathway whose barrier strongly depends on Rh nuclearity, with clusters more active than isolated atoms. Moreover, while Rh under the oxide can follow Eley-Rideal/Mars-van-Krevelen pathways, Rh on the oxide follows a strict Langmuir-Hinshelwood/Mars-van-Krevelen mechanism. These findings provide new insight into how surface oxidation modulates the accessibility, structure, and reactivity of Rh atom and cluster sites in Cu-based catalysts.