Kinetics and mechanisms of decomposition reaction of hydrogen peroxide in presence of metal complexes

Hydrogen peroxide was discovered in 1818 and has been used in bleaching for over a century [1]. H2O2 on its own is a relatively weak oxidant under mild conditions: It can achieve some oxidations unaided, but for the majority of applications it requires activation in one way or another. Some activation methods, e.g., Fenton's reagent, are almost as old [2]. However, by far the bulk of useful chemistry has been discovered in the last 50 years, and many catalytic methods are much more recent. Although the decomposition of hydrogen peroxide is often employed as a standard reaction to determine the catalytic activity of metal complexes and metal oxides [3,4], it has recently been extensively used in intrinsically clean processes and in end-of-pipe treatment of effluent of chemical industries [5,6]. Furthermore, the adoption of H2O2 as an alternative of current industrial oxidation processes offer environmental advantages, some of which are (1) replacement of stoichiometric metal oxidants, (2) replacement of halogens, (3) replacement or reduction of solvent usage, and (4) avoidance of salt by-products. On the other hand, wasteful decomposition of hydrogen peroxide due to trace transition metals in wash water in the fabric bleach industry, was also recognized [7]. The low intrinsic reactivity of H2O2 is actually an advantage, in that a method can be chosen which selectively activates it to perform a given oxidation. There are three main active oxidants derived from hydrogen peroxide, depending on the nature of the activator; they are (1) inorganic oxidant systems, (2) active oxygen species, and (3) per oxygen intermediates. Two general types of mechanisms have been postulated for the decomposition of hydrogen peroxide in the presence of transition metal complexes. The first is the radical mechanism (outer sphere), which was proposed by Haber and Weiss for the Fe(III)-H2O2 system [8]. The key features of this mechanism were the discrete formation of hydroxyl and hydroperoxy radicals, which can form a redox cycle with the Fe(II)/Fe(III) couple. The second is the peroxide complex mechanism, which was proposed by Kremer and Stein [9]. The significant difference in the peroxide complex mechanism is the two-electron oxidation of Fe(III) to Fe(V) with the resulting breaking of the peroxide oxygen-oxygen bond. It is our intention in this article to briefly summarize the kinetics as well as the mechanisms of the decomposition of hydrogen peroxide, homogeneously and heterogeneously, in the presence of transition metal complexes. © 2000 John Wiley & Sons, Inc. Int J Chem Kinet 32: 643–666, 2000