Identification of Electrochemically Adsorbed Species via Electrochemical Microcalorimetry: Sulfate Adsorption on Au(111)

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 SO$_4$$^{2−}$ to HSO$_4$$^{−}$. In neutral solution, the reaction entropy is about 40 J mol$^{−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 HSO$_4$$^{−}$ preceding sulfate adsorption in acidic solution, which shows that the adsorbing species is SO$_4$* 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.