A probable solution? Knowledge of the species present at an electrode's surface is crucial for understanding its electrochemical activity. Electrochemical microcalorimetry is used to investigate a Au(111) electrode in H2SO4 and K2SO4 solutions and determine the reaction entropy of the adsorption process. This quantity offers an answer to an ongoing debate about the nature of the adsorbed species: It is SO4*.
We investigate compositional changes of an electrochemical interface upon polarization with electrochemical microcalorimetry. From the heat exchanged at a Au(111) electrode upon sulfate adsorption, we determine the reaction entropy of the adsorption process for both neutral and acidic solutions, where the dominant species in solution changes from SO4
2− to HSO4
−. In neutral solution, the reaction entropy is about 40 J mol−1 K−1 more positive than that in acidic solution over the complete sulfate adsorption region. This entropy offset is explicable by a deprotonation step of HSO4
− preceding sulfate adsorption in acidic solution, which shows that the adsorbing species is SO4* in both solutions. The observed overall variation of the reaction entropy in the sulfate adsorption region of ca. 80 J mol−1 K−1 indicates significant sulfate-coverage dependent entropic contributions to the Free Enthalpy of the surface system.Zum Volltext