Quantitative 2D electrooxidative carbon nanotube filter model: Insight into reactive sites

In this study, the electrooxidative carbon nanotube (CNT) filtration of sorptive methyl orange (MO) and non-sorptive ferrocyanide was investigated by both experiment and numerical simulation. The two-dimensional numerical model includes target molecule; mass transport, adsorption, and electron transfer and product desorption. For MO, the model was calibrated with experimental reaction rates from the mass- and electron-transfer limited regimes and accurately predicted effluent concentrations over a much larger range of conditions. For ferrocyanide, five CNT electrodes of various specific surface area and surface oxygen content were utilized and a similar single reaction site model to MO accurately predicted kinetics at low anode potentials while a two-site model was necessary at higher potentials yielding insight into the CNT reactive sites. For example, at low anode potentials (⩽0.2 V), the kinetics have a linear correlation with CNT surface area indicating the sp2 conjugated sidewall sites are dominant. In contrast, at higher anode potentials (⩾0.3 V), the kinetics were significantly greater than expected from CNT surface area and the differential kinetics have a linear correlation with the CNT surface oxygen content indicating the CNT tips were also electroactive. The spatial distribution of internal electrode mass transport and surface reactivity is discussed.