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

Kinetics and mechanism of iridium(III) (ca. 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 degrees C) under the conditions [C2H5OH](T) >> [Ce(IV)](T) >> [Ir](T). The rate law conforms to: -d ln[Ce(IV)]/dt= k(0) - A(1)[C2H5OH](T) [Ir](T)/(B-1 + C-1 [Ir](T)) D [C2H5OH](T) +E where A(1), B-1, C-1, D and E 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-eqilibrium 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.