Abstract
Lateral interactions between co–adsorbed species can produce non-random spatial organization that governs the availability of reactive motifs relevant to catalytic behavior. For CO* and H* co–adsorbed on Pt(111), these interactions determine how the two species distribute across the surface and therefore which local CO*–H* configurations are available under reaction conditions. In this work, we combine density functional theory, cluster expansions, and grand canonical Monte Carlo simulations to determine how CO* and H* organize on Pt(111), and subsequently use electronic surface charge as a controlled perturbation to examine how this organization responds to changes in the underlying energetics. On neutral Pt(111), CO*–H* first-nearest-neighbor interactions are strongly repulsive (0.42 eV), CO*–CO* first-nearest-neighbor interactions are moderately repulsive (0.12 eV), and H*–H* interactions are comparatively weak. These interactions cause CO* to nucleate into islands that grow and coalesce with increasing CO* coverage, while H* preferentially occupies regions outside CO*-rich domains. Consequently, the abundance of CO*–H* nearest-neighbor configurations varies non-monotonically with CO* coverage, reaching a maximum at intermediate coverage before decreasing as CO* islands grow and the adsorbates become increasingly segregated. Positive surface charge strengthens CO* adsorption but also increases CO*–CO* repulsion, producing higher CO* coverages, a broader distribution of CO* island sizes, and larger low-coverage regions between islands where H* can adsorb away from CO*. This suppresses CO*–H* nearest-neighbor configurations relative to the neutral surface. Negative charging weakens CO* adsorption, producing lower CO* coverages, higher H* coverages, and less consolidated CO* islands, thereby increasing the prevalence of mixed CO*–H* local environments. At intermediate CO* coverage, CO* islands enter a regime of accelerated growth and coalescence that coincides with a shallow plateau in H* coverage. This coincidence suggests that the onset of CO* island coalescence is accompanied by a reorganization of the co-adsorbed layer. Further, it illustrates the strong influence of lateral interactions on the formation of potentially reactive motifs, highlighting how they become influential in determining catalytic reactivity.