Manganese-Catalyzed Autoxidation of Aqueous Hydrazine: Kinetics, Mechanism, and Catalyst-Inhybitor Conversion

Article abstract of DOI:10.1021/j150650a022

Spectrophotometric evidence and kinetic data suggest that the autoxidation of aqueous hydrazine is a chain reaction propagated by hydrogen peroxide, diimine(N2H2), and hydrazyl (N2H3^.), perhydroxy (O2^-.), and hydroxy (OH^.) radicals.At 25 deg C, the rate constants for the reactions between hydrazyl radical and oxygen and between hydrazyl radical and hydrogen peroxide have been estimated to be 13 and 95 M^-1s^-1, respectively.The manganese-catalyzed reaction shows variable reaction orders over a narrow catalyst concentration range.Below 5.0 x 1E-5 M catalyst concentration, the reaction is first order with respect to manganese, oxygen, and hydrazine, respectively, and second order with respect to basicity.Above 2.0 x 1E-4 M catalyst concentration, the reaction orders switch to zeroth order for manganese and second order for oxygen.The switch in the reaction orders is caused by a change in the relative contributions of two competing termination reactions in different manganese(II) concentration ranges.The dominant termination reaction switches from the unimolecular adsorption of the perhydroxy radical at low manganese concentration to the unimolecular adsorption of a manganese-hydrazyl radical complex at high manganese concentration.The ability of manganese(II) to tie up hydrazyl radical causes manganese to undergo a catalyst-inhibitor conversion at a high metal concentration.A simple and self-consistent reaction mechanism has been developed which correctly predicts the switch in the oxygen order and satisfactorily accounts for all experimental findings.The result of this study support the postulate that the underlying cause of catalyst-inhibitor conversion in any liquid-phase autoxidation reaction is the ability of the metal ion to tie up and deactivate a radical chain carrier through the reversible formation of a metal-radical complex which is unreactive in the propagation chain but active in the termination reaction sequence.