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It has been traditionally assumed that the reduction of hydrogen peroxide proceeds by a simple, first-order, 2-electron reduction, to yield water as the product. Under this model, a plot of the reciprocal of the current vs the reciprocal square root of the rotation rate, known as a Koutecky-Levich plot, should be linear, from which kinetic information can be obtained based on the magnitude of the intercepts as a function of the overpotential. However, close inspection of many such plots reported in the literature do exhibit some curvature especially at high rotation rates and low overpotential, a behavior that has been largely ignored but which may affect the interpretation of such plots1. One reaction seldom considered in this context is the well-documented heterogeneous disproportionation of hydrogen peroxide, a process known to occur on many surfaces that yields dioxygen2, a species that could subsequently reduce and thus contribute to the overall current observed (see Scheme 1). According to this Scheme, the current derived from the mass balances associated with peroxide and dioxygen is expressed in Equation 1. Shown in Panel A, Figure 1, are a series of Koutecky-Levich plots based upon this formalism for three reasonable values of the heterogeneous electron transfer rates for the processes involved for disproportionation rates spanning the entire domain. As clearly indicated, and for certain values of these parameters, the plots display the same type of curvature as those reported in the literature. Shown in Panel B of the Figure are curves for a fictitious set of parameters which were deliberately chosen to obtain a curvature equal in magnitude but opposite in direction. Several methods are currently being considered in order to measure independently the rates of peroxide disproportionation for the same type of rotating disk electrodes used for collecting data of the type discussed herein3. Acknowledgments This work was supported by a grant from NSF, CHE-1412060 References 1. S. J. Amirfakhri, J.-L. Meunier and D. Berk, Electrochimica Acta, 114, 551 (2013). 2. E. R. Vago and E. J. Calvo, Journal of Electroanalytical Chemistry, 339, 41 (1992). 3. A. Muthukrishnan, Y. Nabae, T. Okajima and T. Ohsaka, ACS Catalysis, 5, 5194 (2015). Figure 1 |
