Solvation Effects on OH Adsorbates on Stepped Pt Surfaces

Density functional theory calculations were applied to OH formations on stepped Pt electrodes of Pt[n(111) x (111)] (n = 3, 4, and 6) for examining solvation effects on the OH adsorbates. Results indicated that OH adsorbates at terrace sites are slightly destabilized by water molecules adsorbed at step sites forming 1-dimensional water chains whereas OH adsorbates at step sites are significantly destabilized by water molecules adsorbed at terrace sites forming 2-dimensional honeycomb structures. On stepped Pt surfaces with narrow terrace widths, water molecules cannot exist at terrace sites, and therefore, the solvation effects on OH adsorbates at step sites disappear. Hence, OH adsorbates are formed at step sites at a low potential region, ca. 0.3 V (standard hydrogen electrode (SHE)). When high-coverage CO adsorbates are present on the stepped Pt surfaces, water molecules cannot exist at the terrace sites either because strongly bound CO molecules exclude the water molecules. In such conditions, OH formation potentials decrease significantly, too. Thermodynamic stabilities of OH adsorbates are, therefore, controlled not only by the local surface morphology but also by long-ranged interfacial solvation environments. In other words, the stability and presence/absence of OH adsorbates should be considered to be totally different with water adsorbates (like in inert conditions) and without them (like in the CO oxidation).