From Kinetic Gateways to Thermodynamic Locking: Unveiling the Dynamic Adsorption Landscape of CO on Pt(111).

The adsorption of CO on Pt(111) serves as a benchmark system in surface science, yet resolving the discrepancies between theoretical predictions and experimental observations regarding site preference and structural evolution remains a challenge. Here, we present a comprehensive coverage-temperature study that combines generalized simulated annealing on a high-precision potential energy surface (PES) with molecular dynamics-based free energy calculations. Our statistical analysis reveals that surface relaxation plays a decisive role. It enhances the stability of t o p $$ top $$ sites at low coverage and accurately captures the adsorbate-induced surface distortion at saturation (0.750 ML), driving the densely packed adlayer into a symmetry-broken configuration proximal to the b r i d g e $$ bridge $$ sites. Crucially, free energy landscapes reveal a non-monotonic evolution of surface mobility. A critical "kinetic gateway" at ∼ 0 . 611 $$ \sim 0.611 $$  ML was identified, where migration barriers between t o p $$ top $$ , b r i d g e $$ bridge $$ , and h o l l o w $$ hollow $$ sites nearly vanish, creating a highly fluid phase that facilitates complex changes of adlayer structure. In contrast, at the saturation limit, the CO adlayer becomes thermodynamically locked into deep potential wells with high diffusion barriers, indicative of a rigid "catalyst poisoning" state. These findings bridge the gap between zero-temperature static models and finite-temperature experimental realities, offering a unified theoretical framework for understanding the dynamic interplay between thermodynamic site competition and kinetic accessibility.