CO Adsorption on Pt(111): From Isolated Molecules to Ordered High-Coverage Structures

Carbon monoxide (CO) adsorption on Pt(111) is a bellwether system for surface science, a puzzle for computational catalysis, and a model reaction step in catalysis. While CO can form a nearly infinite number of structures and coverages on Pt(111), only a limited number of ordered structures have been experimentally observed. Here, we used first-principles thermodynamic calculations to evaluate the stability of a few hundred structures for CO on Pt(111), with coverages ranging from 0.06 to 1.0 monolayer. At low coverages, vdW-DF density functional theory calculations confirm the experimentally observed (√3 × √3)R30°-CO structure with CO at top sites. At medium coverages, the system switches to the c(4 × 2)-4CO structure with equal top and bridge site population. The next stable structure is the c(√3 × 5)rect-6CO structure with a bridge:top ratio of 1:2, followed by the c(√3 × 3)rect-4CO structure with a bridge:top ratio of 1:3. This structure corresponds to the saturation coverage under typical reaction conditions. The increase and decrease in the bridge:top ratio with coverage, from 0 to 1:1 and back to 0, is correctly predicted by vdW-DF and is driven by the change in the Pt surface charge upon CO adsorption at top and bridge sites.