Intrinsic reactivity and competitive ligand binding at an isolated Cu⁺ site: implications for single-atom CO...

Understanding the intrinsic reactivity of isolated metal centers is essential for defining the fundamental limits of single-atom catalysis. Here, we combine laser-vaporization high-pressure mass spectrometry with density functional theory to investigate the gas-phase chemistry of isolated Cu⁺ interacting with CO, H₂O, O₂, N₂, and their mixtures. Under multicollisional conditions approaching thermodynamic control, Cu⁺ undergoes sequential ligand coordination and saturates at a fourfold coordination limit dictated by competitive ligand binding rather than gas-phase composition. A consistent hierarchy of ligand affinities, CO > H₂O > N₂ > O₂, is established by both experiment and theory and drives extensive ligand substitution in mixed atmospheres. Although Cu⁺ forms O₂ adducts at low pressure, O₂ binding is intrinsically weak, involves minimal charge transfer, and results in negligible O-O bond activation. All computed pathways for CO oxidation at an isolated Cu⁺ site are strongly endothermic, rendering CO₂ formation thermodynamically inaccessible under multicollisional conditions. These results establish an atomically resolved thermodynamic baseline for Cu⁺ single-site reactivity, demonstrate that O₂ activation in copper-based catalysts necessarily requires cooperative mechanisms beyond those of an isolated Cu⁺ center, in marked contrast to Au⁺ systems, and highlight the importance of accounting for trace gases and realistic multicomponent environments when modeling active-site behavior.