New kinetic and mechanistic findings in the oxidation of hydroxylamine by Cerium(IV) in perchloric acid media

Abstract

AbstractThe oxidation of hydroxylamine (HONH2) by Ce(IV) in perchloric acid media took place fast and was followed by a stopped‐flow spectrometer. A rate law of −d[Ce(IV)]tot/dt = k′[HONH2]tot[Ce(IV)]tot was proved, where k′ stands for the observed second‐order rate constant and [Ce(IV)]tot and [HONH2]tot represent the total concentrations of Ce(IV) and HONH2, respectively. The dependencies of k′ on [H+] and on ionic strength were investigated. Rapid scan spectra recorded for a typical reaction course and a stoichiometric measurement unraveled several additional characters for the redox reaction. Based on experimental results, a reaction mechanism is proposed, which involves Ce4+, Ce(OH)3+, and Ce(OH)22+ attacking on HONH3+ in parallel. Moreover, these parallel attacking reactions are the rate‐determining steps, generating free radicals •ONH2, which are rapidly oxidized to nitrate by Ce(IV). Rate constants of these rate‐determining steps have been evaluated; a reactivity trend of Ce4+ > Ce(OH)3+ > Ce(OH)22+ is disclosed, but the reactivity differences among them are very small. The rate‐determining steps are rationalized in terms of an outer‐sphere electron transfer, and possible transition states are proposed in the cases of Ce(OH)3+ and Ce(OH)22+ to account for the small reactivity differences. The obtained kinetic and mechanistic results in the present work are compared with those acquired previously for the same reaction but in sulfuric acid and nitric media. It is concluded that the reaction media can play a dominant role in determining the reaction kinetics, mechanisms, and the product(s) in the oxidation of hydroxylamine by Ce(IV).