Investigating the Kinetics and Mechanism of Organic Oxidation in Parallel with the Oxygen Evolution Reaction

Abstract

In this paper, the mechanism of organic oxidation in parallel with the oxygen evolution reaction at an electrode following the “active” anode mechanism is investigated. The active anode (IrO2-Sb2O5-SnO2/Ti) was prepared via standard thermal decomposition method and 4-nitrophenol (4-NP) chosen as the model organic compound. It is firstly confirmed that this anode does follow the “active” anode mechanism, with the rate of 4-NP oxidation being dependent on the coverage adsorbed oxygen on the surface of the anode. This surface coverage can be estimated by fitting steady-state polarisation curves with a micro-kinetic model describing the oxygen evolution behaviour of the anode. This surface coverage dependent oxidation rate can only be observed at relatively low overpotentials where mass transport limitations are avoided. At high overpotentials, the rate of oxidation is completely controlled by mass transfer of 4-NP to the anode surface, with the measured and calculated rate constants agreeing closely. It is also shown that the instantaneous current efficiency can be directly calculated from the measured pseudo first-order rate constant in both the kinetic and mass transport limited regimes. Using this analysis method, it was found that the instantaneous current efficiency for 4-NP oxidation is less than 100% in both regimes and only approached 100% at very low overpotentials. This finding is important as in prior literature, it is often believed that the instantaneous current efficiency of electrochemical wastewater oxidation will be 100% provided that mass transfer does not limit the process, due to an underlying assumption that the rate of organic oxidation is much larger than the OER.