Impedance Spectroscopy and Differential Electrochemical Mass Spectrometry As Tools for Characterization of Mediated Oxidation of Organic Pollutants

Abstract

Persistent Organic Pollutants (POP) are hazardous environmental contaminants of mainly synthetic origin, highly toxic and chemically inert. They accumulate in the environment and in living organisms. Most POPs do not ignite themselves and they cannot be combusted. Therefore it is very important to find an alternative processes of POP neutralization. Mediated Electrochemical Oxidation has been proven to be an efficient approach, in which those dangerous substances are electrochemically combusted under ambient conditions. Mediated electrochemical oxidation is an indirect electrolysis, where pollutants are oxidized through the mediation of some electrochemically generated redox reagents, which act as an intermediary for electrons shuttling between the electrode and the target molecule. This technique enables the possibility to oxidize an (electro)inactive compounds.1 Recently, we have derived the exact analytical solution for impedance of catalytic processes involving redox mediators. The results obtained for general case in the rigorous treatment provide a powerful research tool that can be used in studies of many important catalytic processes.2 Unusual impedance characteristics have been found for systems having unequal diffusivities of the oxidized and reduced forms of the redox mediator. Depending on the ratio of diffusion coefficients the spectra are either markedly deformed at low frequency range or an inductive loop appears at the complex plane. It is impossible to fit these spectra in the whole frequency range, neither with the Gerischer impedance nor with any other known transfer function.2 We show that the key parameters of the studied systems, such as both heterogeneous and homogeneous rate constants, which are limiting factors for the electrochemical combustion efficiency, can be readily obtained even from a single impedance spectrum.2 In this research, silver(II) compounds were used as redox mediators for electrochemical combustion of organic molecules.3 As reported by us previously, the redox potential of the system Ag(II)/Ag(I) can reach up to 2.9V in sulfuric acid media.4 Moreover the average lifetime of the produced radical oxidant was estimated to be in order of seconds.5 Initially, investigations were carried out for a model organic molecule - acetic acid, which is known to be difficult to oxidize, and next for 1,2-dichlorobenzene an organic compound from POP group. The catalytic activity of Ag(II) was characterized mainly by using electrochemical methods with a particular emphasis placed on Electrochemical Impedance Spectroscopy (EIS). We have also performed analysis of volatile products by using mass spectrometry coupled with electrochemical measurements DEMS (Differential Electrochemical Mass Spectroscopy). This technique allows to detect in situ volatile electrooxidation products and/or intermediates. The latter were monitored for catalytic electrooxidation of acetic acid and 1,2-dichlorobenzene in function of electrode potential, for both compounds a large amount of CO2 was detected. 1. M. Panizza and G. Cerisola, Chemical Reviews, 2009, 109, 6541-6569. 2. R. Jurczakowski and P. Polczynski, J Phys Chem C, 2014, 118, 7980-7988. 3. M. Fleischmann and D. Pletcher, Tetrahedron Letters, 1968, 9, 6255-6258. 4. P. Polczynski, R. Jurczakowski and W. Grochala, Chem Commun, 2013, 49, 7480-7482. 5. P. Polczynski, R. Jurczakowski and W. Grochala, J Phys Chem C, 2013, 117, 20689-20696.