Kinetics and reaction stoichiometry in the reductive dissolution of manganese(IV) dioxide and co(III) oxide by hydroquinone

Abstract

Oxidation of hydroquinone (QH 2) to p-benzoquinone (Q) in aqueous suspensions by manganese(IV) dioxide (MnO 1.97) and cobalt (III) oxide (CoOOH) and subsequent reduced metal ion release were examined in order to explore the kinetics and mechanisms of surface chemical redox reactions. When the ratio [Q(aq)] [Me 2+(aq)] exceeds the value predicted from reaction stoichiometry, reduced metal ions are accumulating on oxide surfaces. This behavior was observed for both oxides, and is especially pronounced early in the course of reaction and in suspensions of high pH. Sorption of added Mn 2+ onto Mn(IV) dioxide and Co 2+ onto Co(III) oxide in reductant-free suspensions resembles the retention of reduced metal ions observed during the reductive dissolution reactions. Malonate has been shown to facilitate Co 2+(aq) release during reaction of Co(III) oxide with hydroquinone. Rates of reduced metal ion adsorption/desorption relative to rates of prior surface chemical reactions are discussed using a numerical model. Comparisons are made with the reductive dissolution of Fe(III) oxides by hydroquinone.