KINETICS AND MECHANISM OF RUTHENIUM(III) CATALYZED OXIDATION OF FORMIC-ACID BY CERIUM(IV) IN AQUEOUS SULFURIC-ACID MEDIA

The kinetics of oxidation of formic acid by cerium(IV) in the presence of ruthenium(III) (ca. 10(-6)mol dm-3) in aqueous sulfuric acid media has been followed at different temperatures (30-50-degrees-C). The rate of disappearance of cerium(IV) in the reaction has been found to be zero order with respect to cerium(IV) concentration. At a fixed [H+], under the conditions, [HCO2H]T much greater than [Ce(IV)]T much greater than [Ru]T the observed zero-order rate constant (k0) conforms to: -d[Ce(IV)]t/dt = k0 = [Ru]T[HCO2H]T{k(b) + k(c)[HCO2H]T} where [Ru]T and [HCO2H]T represent the total concentration of ruthenium(III) and formic acid respectively. At 40-degrees-C, [H2SO4] = 1.0 mol dm-3 and I = 2.75 mol dm-3 the values of 10(2)k(b) and 10(2)k(c) are 6.0 +/- 0.1 dm3 mol-1 s-1 and 5.4 +/- 0.1 dm6 mol-2 s-1 respectively. Both k(b) and k(c) are found to have an inverse hydrogen-ion dependence. Out of the different possible mono- and bis-complexes, Ru(III)(HCO2-) and Ru(III)(HCO2-)(HCO2H) have been found to be kinetically active in the slow oxidative steps (through an inner-sphere mechanism) leading to Ru(III)H (through hydride transfer from the C-H bond of metal-bound formate) and CO2 followed by the rapid oxidation of Ru(III)H to Ru(III) by cerium(IV). The activation parameters are DELTAH(double dagger) = 46 +/- 3 kJ mol-1, DELTAS(double dagger) = -125 +/- 5 J K-1 mol-1 (for the k(b) path) and DELTAH(double dagger) = 47 +/- 3 kJ mol-1, DELTAS(double dagger) = -120 +/- 5 J K-1 mol-1 (for the k(c) path).