The influence of counter anions on the underpotential deposition of mercury(II) on Au(1 1 1): temperature-dependent studies

We have studied the temperature dependence of the underpotential deposition (upd) of mercury(II) onto Au(1 1 1) electrodes from a series of electrolytes in which anion interactions could be varied from essentially zero (i.e., HClO4) to strongly interacting with either the Au(1 1 1) substrate (i.e., H2SO4) or with solution phase Hg2+ ions (i.e., HC2H3O2). Both isothermal and non-isothermal cells were used to make temperature-dependent cyclic voltammetry measurements. Analysis of the temperature-dependent cyclic voltammetric curves at a constant ΓHg yielded values for the half-reaction entropy and enthalpy, ΔS0rc(upd) and ΔH0rc(upd), for the Hg upd reaction. ΔS0rc(upd) values of −30.9 and −18.3 J/mol K were measured for perchloric acid and sulfuric acid electrolytes, respectively. The respective ΔH0rc(upd) values for these electrolytes were −182 and −184 kJ/mol. In acetic acid media, thermodynamic functions were found to depend on the electrolyte pH. ΔS0rc(upd) values increased monotonically from +16.4 to +83.5 J/mol K as the pH was increased from 2.7 to 5.3 at a constant formal acetate concentration of 0.10 M. Under the same experimental conditions, ΔH0rc(upd) increased from −150 to −102 kJ/mol.