Review—Electroreduction of Peroxodisulfate: A Review of a Complicated Reaction

Abstract

The electroreduction of peroxodisulfate has been studied on various electrode materials for decades. However, the reduction of peroxodisulfate is not fully understood to date. Many research groups have searched for an electrode material that exhibits a diffusion-controlled response in the electroreduction of peroxodisulfate. However, the voltammetric responses obtained on most electrode materials have shown a kinetically-controlled reaction. Reaction mechanisms of peroxodisulfate reduction have also been proposed by a few groups. This review paper focuses on the electrochemical responses for the reduction of peroxodisulfate obtained on platinum, gold, platinized platinum, and bismuth-adsorbed platinum electrodes. This paper will not review the polarograms obtained on mercury drop electrodes for the reduction of peroxodisulfate anions because we believe that the results do not contribute to the understanding of the electrochemical reactions. We propose a simplified version of the reaction mechanisms. This paper also reviews the kinetic models proposed by several groups which could assist the readers to apprehend the kinetic behaviors of peroxodisulfate reduction. This paper discusses several issues related to peroxodisulfate from the natural decomposition of peroxodisulfate to the reaction mechanisms of peroxodisulfate reduction.

Understanding Surface Charge Effects in Electrocatalysis. Part I: Peroxodisulfate Reduction at Pt(111)

“…The PDS reduction begins with a dissociative chemisorption step. ,,,, The adsorptive dissociation S 2 O 8 2– forms two adsorbed SO 4 – , each blocking up to three surface sites denoted as (SO 4 – ) ad , namely where * A denotes adsorption sites for (SO 4 – ) ad , and k f and k b denote the rate constants of the forward and backward reactions of the first dissociative chemisorption step, respectively. In this model, we distinguish adsorption sites for (SO 4 – ) ad and those for adsorbed hydroxyl (OH ad ), and experiments have proved the absence of site-blocking effect of OH ad on PDS reduction at Pt(111) …”

“…Equation is followed by two one-electron reduction steps, ,,,, occurring at the IHP2 where k O and k R are the oxidation and reduction reaction rate constants, respectively.…”

“…In this model, we distinguish adsorption sites for (SO 4 − ) ad and those for adsorbed hydroxyl (OH ad ), and experiments have proved the absence of site-blocking effect of OH ad on PDS reduction at Pt(111). 19 Equation 5 is followed by two one-electron reduction steps, 12,16,17,20,26 occurring at the IHP2…”

“…), and transition metals (Au ,, and Pt). Two parallel reaction pathways exist. ,, The first belongs to the inner-sphere mechanism involving an adsorbed anion, consisting of two steps: S 2 O 8 2– + 2* → 2­(SO 4 – ) ad and (SO 4 – ) ad + e – → SO 4 2– + *. The second belongs to the outer-sphere mechanism, consisting of two successive electron-transfer steps: S 2 O 8 2– + e – → SO 4 2– + SO 4 – and SO 4 – + e – → SO 4 2– .…”

“…Two parallel reaction pathways exist. 12,17,20 The first belongs to the inner-sphere mechanism involving an adsorbed anion, consisting of two steps: S . A remarkable feature of the polarization curve is that the current decays as the potential is swept to more negative values.…”