Kinetics and mechanism of iridium(III) catalysed oxidation of ethanol by cerium (IV) in aqueous sulphuric acid media

Abstract

Kinetics and mechanism of iridium(III) (<em>ca</em>. 10^-6 mol dm^-3 ) catalysed oxidation of ethanol by cerium(IV) in aqueous sulphuric acid medium have been investigated at different temperatures (25 - 40°C) under the conditions [C₂H₅OH]_T >>[Ce(IV)_T>> [Ir]_T. The rate law conforms to: <img src='http://www.niscair.res.in/jinfo/ijcaequation.gif' border=0> where A<em>I</em>. B<em>I</em>. C<em>I</em>. D and <em>E</em> are constants at a particular temperature at a fixed [H⁺], and [Ir]_T gives the total concentration of iridium added as catalyst. An intermediate involving the oxidant catalyst and substrate has been proposed as being formed in pre-eqilibriurn steps before the electron transfer step through the Ir(III)/Ir(IV) catalytic cycle. The electron transfer (inner-sphere mechanism) leads to the partially oxidised product (i.e. ethoxide radical) which is rapidly oxidised by cerium(IV) at a fast step. From the hydrogensulphate dependence, Ce(SO₄)²⁺ has been found as the kinetically active species.