Mechanisms for the formation of active sites in single-atom alloys.

Single-atom alloys (SAAs) show great promise in heterogeneous catalysis, yet their synthesis remains challenging. To address this challenge, we elucidate the fundamental surface mechanisms by which dopant adatoms deposited on Cu and Ag surfaces become embedded active sites in the host metal. Using density functional theory, we identify periodic trends across the transition metal (TM) series. Adatoms diffuse nearly freely across terraces due to very low diffusion barriers, whereas direct incorporation into terraces is unfavourable. In line with conventional wisdom, step edges and kink sites facilitate dopant incorporation, confirming their critical role in alloy formation. Attachment of adatoms to steps and kinks from the lower terrace is energetically favoured, and incorporation proceeds either from this attached state or when adatoms approach a step edge from above, where reactions frequently occur without an activation barrier. Incorporation barriers are generally lowest for early and central TMs, increase towards late TMs, and are slightly higher on Cu than on Ag surfaces. Our simulations further reveal how embedded dopants influence the behaviour of diffusing adatoms. They rationalise experimental observations for Pd on Cu, where repulsive adatom-dopant interactions repel diffusing adatoms from dopant-rich regions near steps, suppressing otherwise dominant incorporation pathways, and predict dopant-anchored adatom islands for attractive elements such as Ru on Cu. Overall, this work provides a unified perspective on how specific surface sites and adatom-dopant interactions govern dopant incorporation, offering guidance on the surface environments most conducive to SAA formation across different dopant elements.