Exploring the molecular features of cationic catalysis phenomenon: Peroxodisulfate reduction at a mercury electrode

Quantum chemical models are employed to get molecular level insight into the cationic catalysis phenomenon occurring at the peroxodisulfate electroreduction on mercury. A sharp increase of the reaction rate in cesium-containing solutions as compared with sodium containing media is considered, which is known to be one of the striking manifestations of cationic catalysis. The important feature under discussion is enhancement of the cationic catalysis with increasing the negative electrode charge. Special attention is paid to the ion pairing in both the solution bulk and the interfacial reaction layer. A conspicuous structural difference is predicted for Na+ center dot S2O82- (solvent-separated ion pair) and Cs+ center dot S2O82- (contact ion pair). The rough estimations of the bulk association constants do not afford to explain the experimentally observed effect even qualitatively. The partial contributions of S2O82- and ion pairs (Na+ center dot S2O82- or Cs+ center dot S2O82-) to the resulting current are compared with account of their molecular features which affect the work terms and transmission coefficients. The results of analysis are in satisfactory agreement with the experimentally observed trends. To check a possibility of an additional acceleration of the reaction due to local electrostatic effects, the specific adsorption of a cesium cation on the mercury surface was modeled as well. It is concluded that the partial contribution of ion pairs in the vicinity of metal/ solution interface is higher at the more negative electrode charges. Local electrostatic interactions with a sodium cation are also involved in the model calculations. It has been demonstrated that the cationic catalysis results from a number of specific features characterizing cesium-containing electrolytes: (1) higher association degree in the solution bulk; (2) stronger overlap between the electrode wave functions and acceptor orbital of ion pairs; (3) the charge-induced shift of specifically adsorbed cations towards the electrode surface. (C) 2005 Elsevier B.V. All rights reserved.