Kinetics and mechanism of ruthenium(III) catalysed oxidation of formic acid by cerium(IV) in aqueous sulfuric acid media

Abstract

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 °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[Ce^IV]_T[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 °C, [H₂SO₄]= 1.0 mol dm^–3 and I= 2.75 mol dm^–3 the values of 10²k_b and 10²k_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^IIIH (through hydride transfer from the C–H bond of metal-bound formate) and CO₂ followed by the rapid oxidation of Ru^IIIH to Ru^III by cerium(IV). The activation parameters are ΔH^‡= 46 ± 3 kJ mol^–1, ΔS^‡=–125 ± 5 J K^–1 mol^–1(for the k_b path) and ΔH^‡= 47 ± 3 kJ mol^–1, ΔS^‡=–120 ± 5 J K^–1 mol^–1(for the k_c path).