Kinetics and mechanism of ruthenium(III) catalysed 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 °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[CeIV]T[Ru]T the observed zero-order rate constant (k0) conforms to: –d[CeIV]t/dt=k0=[Ru]T[HCO2H]T{kb+kc[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 102kb and 102kc are 6.0 ± 0.1 dm3 mol–1 s–1 and 5.4 ± 0.1 dm6 mol–2 s–1 respectively. Both kb and kc are found to have an inverse hydrogen-ion dependence. Out of the different possible mono- and bis-complexes, RuIII(HCO2–) and RuIII(HCO2–)(HCO2H) have been found to be kinetically active in the slow oxidative steps (through an inner-sphere mechanism) leading to RuIIIH (through hydride transfer from the C–H bond of metal-bound formate) and CO2 followed by the rapid oxidation of RuIIIH to RuIII by cerium(IV). The activation parameters are ΔH‡= 46 ± 3 kJ mol–1, ΔS‡=–125 ± 5 J K–1 mol–1(for the kb path) and ΔH‡= 47 ± 3 kJ mol–1, ΔS‡=–120 ± 5 J K–1 mol–1(for the kc path).