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

The kinetics of oxidation of ethanol by cerium(IV) in presence of ruthenium(III) (in the order of 10 −7 mol dm −3 ) in aqueous sulfuric acid media have been followed at different temperatures (25–40°C). The rate of disappearance of cerium(IV) in the title reaction increases sharply with increasing [C 2 H 5 OH] to a value independent of [C 2 H 5 OH] over a large range (0.2–1.0 mol dm −3 ) in which the rate law conforms to:\documentclass{article}\pagestyle{empty}\begin{document}$$ -2.303\frac{{d\log [{\rm Ce}^{{\rm IV}}]}}{{dt}} = k_{obsd} = (k_c + k_d [{\rm H}^{\rm + }]^{ - 1})[{\rm Ru}]_T $$\end{document}where [Ru] T gives the total ruthenium (III) concentration. The values of 10 −3 k c and 10 −3 k d are 3.6 ± 0.1 dm 3 mol −1 s −1 and 3.9 ± 0.2 s −1 , respectively, at 40°C, I = 3.0 mol dm −3 . The proposed mechanism involves the formation of ruthenium(III) − substrate complex which undergoes oxidation at the rate determining step by cerium(IV) to form ruthenium(IV) − substrate complex followed by the rapid red‐ox decomposition giving rise to the catalyst and ethoxide radical which is oxidized by cerium(IV) rapidly. The mechanism is consistent with the existence of the complexes Ru III · (C 2 H 5 OH) and Ru III · (C 2 H 5 O − ) and both are kinetically active. The overall bisulphate dependence conforms to: k obsd = A [Ru] T /{1 + C[HSO 4 − ]} where A = 2.2 × 10 4 dm 3 mol −1 s −1 , C = 1.3 at 40°C, [H + ] = 0.5 mol dm −3 , and I = 3.0 mol dm −3 . The observations are consistent with the Ce(SO 4 ) 2 as the kinetically active species. © 1995 John Wiley & Sons, Inc.