Novel System Based on Divalent Silver for Mediated Electrochemical Oxidation of Persistent Organic Pollutants
In this research, for mediated electrochemical oxidation, we have chosen silver(II) compounds as redox mediators.4 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.5 Moreover the average lifetime of the produced radical oxidant was estimated to be in order of seconds.6 The catalytic activity of Ag(II) was characterized mainly by using electrochemical methods with a particular emphasis placed on Electrochemical Impedance Spectroscopy (EIS). Recently, we have derived the exact solution for the analysis of catalytic systems. Kinetics of the pseudo-first order chemical reaction with depolarizer reproducing was included in the model using the proposed element.7
Here 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.7 Exemplary results for electrocatalytic oxidation of 1,2-dichlorobenzene are shown in Figure 1. Before the addition of the pesticide, two semicircles are observed on the Niquist plot of impedance registered in sulfuric acid containing silver(I) (-○-). The first semicircle, related to the coupling of the charge transfer resistance with the double layer capacitance, is followed by the second skewed semicircle characteristic for the Gerischer Impedance and related to catalytic oxidation of the acid.6 The determined homogenous reaction rate constant for the latter is equal to k=0.0100±0.0003 dm3∙mol-1∙s-1. The nonzero value is related to a finite half-life time of the electrogenerated oxidizer. After addition of the pesticide (-●-), the second semicircle decreases markedly. According to our theoretical model, this is related to a higher rate constant of the chemical homogeneous step.7 First semicircle is also smaller because the resistance of the charge transfer is inversely proportional to the square root of the homogeneous reaction rate constant.7 The homogenous reaction rate constant of pesticide oxidation is equal to 120±5 dm3∙mol-1∙s-1, which is four order of magnitude higher than that observed in pure sulfuric acid. This result indicates that electrogenerated Ag(II) effectively oxidize 1,2-dichlorobenzene. Products and possible mechanism of this reaction will be discussed.
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