Platinum surfaces in perchloric acid: Pt(111), Pt(100), Pt(110) – perchlorate
In this chapter the experimental results on adsorption of perchlorate ion on Pt(111), Pt(100) and Pt(110) surfaces are described.
The difference between different anion adsorption behavior and structure is a consequence of two fundamental properties: interaction strength, which on all low-index surfaces increases in the order ClO4− < SO42− < Cl < Br < I, and the symmetry of the anion and the surface, e.g., for the tetrahedral (bi)sulfate anions, the interaction strength increases from Pt(100) ≈ Pt(110) to Pt(111), while the spherical halide anions are more strongly bound on (100) than on (111) sites.
No specific ClO4− adsorption/desorption peaks are found with all three low-index planes of Pt in 0.01 M aqueous perchlorate solution. Instead, on Pt(111) hydrogen upd [under-potential deposition] (at 0.0–0.3 V vs. Pd/H), OH adsorption (at 0.5–0.8 V vs. Pd/H), and finally surface oxidation (at 0.95–1.1 V vs. Pd/H) are observed. Both the OHad and oxide states are perturbed significantly as the perchlorate concentration increases suggesting a competition between OH− and ClO4− adsorption. The H-upd region remains unchanged, but ClO4− decomposition to Cl− is reported (see Cu-ClO4 in Chap. 192[ https://doi.org/10.1007/978-3-662-53908-8_192]).
On Pt(100) from 0 to 0.1 V (Pd/H), the oxidation (2H → 2H+) of hydrogen gas, previously formed in a more negative going sweep, is recorded. Between 0.1 and 0.4 V, a broad range of hydrogen upd starts to overlap with the adsorption of OH up to 0.6 V. At potentials >0.8 V, the Pt(100) surface is oxidized. So, H upd on Pt(100) differs from the narrower range of H upd on Pt(111). In contrast to Pt(111), no variation of the onset of electrooxidation is found.
On Pt(100) the upd region consists of two states at 0.09 V and 0.22 V. Electrochemical oxide formation happens between 0.7 and 0.9 V with a large oxide peak at 0.96 V (Pd/H). Also on Pt(110) and Pt(311), marginal changes in the onset of electrooxidation are recorded, again suggesting the competitive adsorption of ClO4− anions.