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 mil d conditions: It can achieve some oxidations unaided, but for the majority of applications it requires activation in one way or another. Some activati on 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 stan dard reaction to determine the catalytic activity of metal complexes and me tal oxides [3,4], it has recently been extensively used in intrinsically cl ean processes and in end-of-pipe treatment of effluent of chemical industri es [5,6]. Furthermore, the adoption of H2O2 as an alternative of current in dustrial oxidation processes offer environmental advantages, some of which are (I) replacement of stoichiometric metal oxidants, (2) replacement of ha logens, (3) replacement or reduction of solvent usage, and (4) avoidance of salt by-products. On the other hand, wasteful decomposition of hydrogen pe roxide due to trace transition metals in wash water in the fabric bleach in dustry, was also recognized [7]. The low intrinsic reactivity of H2O2 is actually an advantage, in that a me thod can be chosen which selectively activates it to perform a given oxidat ion. There are three main active oxidants derived from hydrogen peroxide, d epending on the nature of the activator; they are (1) inorganic oxidant sys tems, (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 fir st is the radical mechanism (outer sphere), which was proposed by Haber and Weiss for the Fe(III)-H2O3 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 perox ide complex mechanism, which was proposed by Kremer and Stein [9]. The sign ificant difference in the peroxide complex mechanism is the two-electron ox idation of Fe(III) to Fe(V) with the resulting breaking of the peroxide oxy gen-oxygen bond. It is our intention in this article to briefly summarize the kinetics as we ll as the mechanisms of the decomposition of hydrogen peroxide, homogeneous ly and heterogeneously, in the presence of transition metal complexes. (C) 2000 John Wiley & Sons. Inc.