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

Abstract

The kinetics of oxidation of formic acid by cerium(IV) in the presence of iridium(lll) (<i>ca.</i> 10^-6 mol dm^{- 3}) in aqueous sulphuric acid media have been followed at different temperatures (30-50°C). At fixed [H⁺], under the conditions [HCO₂H]_T.»[Ce^IV]_T»[lr]_T, the rate of disapperance of cerium(IV) in the reaction has been found to be first order with respect to cerium(IV) and the observed first order rate constant conforms to: <img src='http://www.niscair.res.in/jinfo/equation18.gif' border=0> where [HCO₂H]_T and [Ir]_T represent the total concentrations of formic acid and iridium(III) respectively; A, B, C, D and E are constants at a particular temperature and at a fixed [H ⁺] The values of the constants (at 30°C, 1.0 mol dm^-3 sulphuric acid, <i>l=</i> 3.0 mol dm^-3) are: A/EE=6X10^-3 s^-1; B/D= 1.0 mol dm^{- 3}; C= 6.0, and D/E= 2.3 x 10^{- 6} mol dm^-3. Formation of a dinuclear complex involving the oxidant, catalyst and the substrate before the electron transfer step has been proposed in the reaction mechanism. The electron transfer (innersphere mechanism) occurs within the dinuclear complex to give the partially oxidised product (i.e. formate radical) which is rapidly oxidised by Ce(IV) at a fast step.