Potential dependent surface structure of the Pt(1 1 1) electrolyte interface

Abstract

X-Ray scattering has been used to study the structure of the Pt(1 1 1) interface with 0.1 M KOH and 0.05 M H 2SO 4 electrolytes. The absence of in-plane reconstruction peaks and the intensity profiles of the crystal truncation rods (CTR) indicate that the surface is not reconstructed at any potential. At potentials just above hydrogen evolution a monotonic expansion of the top Pt layer occurs in the region where hydrogen is adsorbed on the surface. The maximum expansion was approximately 0.03 Å in KOH electrolyte and approximately half this magnitude in H 2SO 4 electrolyte. For both electrolytes this expansion is smaller than observed for the Pt(0 0 1) surface. The “anomalous voltammetric feature” observed in both electrolytes appears to be due to anion adsorption and/or an ordering transition in the inner ionic layer. At potentials approximately 1 V positive of hydrogen evolution, displacement of a fraction of the top Pt layer from bulk lattice positions occurs during anodic oxidation. This restructuring is reversible upon lowering the potential and reducing the oxide. We have investigated the structure of the Pt(1 1 1) surface with dilute KOH and H 2SO 4 electrolytes in the potential region of between hydrogen evolution and irreversible oxide formation. In the hydrogen adsorption region in both electrolytes the top Pt layer expands away from the second layer. Surprisingly, although the surface coverage by adsorbed hydrogen in acid was slightly higher than in alkaline electrolyte, in sulfuric acid the maximum expansion was less than half of the maximum expansion in KOH electrolyte. The most likely reason for this difference is the influence of the strongly adsorbed tetrahedral (bi) sulfate anions at 0.4 V, which appear to cause an expansion of the top Pt layer at potentials above the hydrogen adsorption region. In both electrolytes the anomalous electrochemical feature appears to be a consequence of anion adsorption and/or ordering in the inner ionic layer. In KOH electrolyte the adsorbed anions are presumably hydrated hydroxyl species (which do not change the position of the top Pt layer) and in H 2SO 4 solution they are (bi)sulfate anions. Incipient oxidation in KOH electrolyte is reversible between 1.0 and 1.3 V, with the data favoring a place-exchange model for the restructured surface. However, the place-exchange process at these potentials is relatively slow, and may not occur at all under normal cycling sweep rates. In contrast, oxidation of the Pt(1 1 1) surface in H 2SO 4 electrolyte results in irreversible restructuring, since specifically adsorbed (bi)sulfate anions appear to block adsorption of hydroxyl in the potential region where place-exchange of hydroxyl with Pt is reversible.