Density functional theory (DFT) calculations are used to examine hydrogen and hydroxide adsorption on stepped Pt(553) and Pt(533) surfaces and to simulate the associated cyclic voltammograms in both basic and acidic electrolytes. Hydrogen and hydroxide surface species are active intermediates or spectator species in many important electrocatalytic reactions, such as hydrogen oxidation, oxygen reduction, and methanol oxidation. We examine the adsorption of hydrogen, hydroxide, water, and a sodium cation onto the stepped platinum surfaces, Pt(553) and Pt(533). Owing to the strong adsorption of both hydrogen and hydroxide (with co-adsorbed water) at the steps of Pt(553) and Pt(533), they will competitively adsorb on the step at low potentials. The presence of co-adsorbed sodium near the step weakens the adsorption of solvated hydroxide, which we identify as a possible cause of the non-Nernstian shift of the sharp step-associated peaks in changing from acidic to basic electrolytes. We also examine hydrogen adsorption at the terrace of Pt(553) and Pt(533), and simulate a low-potential cyclic voltammogram on Pt(553) and Pt(533). The experimental voltammogram is well-represented as a sum of step and terrace features, and the (553) and (533) cyclic voltammetry features closely resemble those from the low-index (111), (100), and (110) surface facets that match the local atom arrangements.
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