Electrochemical Production of Hydroxyl Radical at Polycrystalline Nb-Doped TiO₂ Electrodes and Estimation of the Partitioning between Hydroxyl Radical and Direct Hole Oxidation Pathways

Abstract

The use of TiO2 as a photocatalyst for the destruction of organic chemical pollutants in aqueous systems has been extensively studied. One obstacle to the effective utilization of these systems is the relatively inefficient use of the solar spectrum by the photocatalyst. In addition, light delivery to the photocatalyst can be impeded by UV-absorbing components in mixed effluent streams. We present a novel use of TiO2 as a catalyst for the oxidative degradation of organic compounds in water that uses a potential source instead of light to generate reactive oxidants. Application of an anodic bias of >+2 V vs NHE to titanium electrodes coated with niobium-doped, polycrystalline TiO2 particles electrochemically generates hydroxyl radicals at the TiO2 surface. This process has been demonstrated to efficiently degrade a variety of environmentally important pollutants. In addition, these electrodes offer a unique opportunity to probe mechanistic questions in TiO2 catalysis. By comparing substrate degradation rates with increases in current density upon substrate addition, the extent of degradation due to direct oxidation and •OH oxidation can be quantified. The branching ratio for these two pathways depends on the nature of the organic substrate. Formate is shown to degrade primarily via a hydroxyl radical mechanism at these electrodes, whereas the current increase data for compounds such as 4-chlorocatechol indicate that a higher percentage of their degradation may occur through direct oxidation. In addition, the direct oxidation pathway is shown to be more important for 4-chlorocatechol, a strongly adsorbing substrate, than for 4-chlorophenol, which does not adsorb strongly to TiO2.

Photocatalytical Properties of TiO₂ Nanotubes

“…The surface hydroxyl (OH) groups of TiO 2 have been recognized to play an important role in the photocatalytic behavior, because they not only undertake the adsorption of reactant molecules but also directly participate in the reaction mechanism by trapping of photogenerated holes to form hydroxyl radicals [57][58][59] from Ti-OH bonds was ascribed to the strong interaction between Ti ions and OH groups. Actually, the defect structure of TNTs formed by oxygen vacancies favors the adsorption of water on the surface and then dissociates water into hydroxyl groups and protons [61][62][63].…”

Modeling the photocatalytic degradation of formic acid in a reactor with immobilized catalyst

“…The degradation of formic acid is reported to proceed both via indirect (the hydroxyl route) (Kesselman, Weres, Lewis, & Ho mann, 1997;Peterson, Turner, & Nozik, 1991) and via direct attack (uptake of h + ) (Bideau, Claudel, & Otterbein, 1980;Pelizzetti & Minero, 1993;Prairie, Evans, Stange, & Martinez, 1993;Upadhya & Ollis, 1997), presented in Table 1 as reaction (R8) and (R9), respectively. Following the measurements of Kesselman et al (1997), the indirect route is taken as the dominant reaction route.…”

Electrochemical Incineration of Organic Pollutants for Wastewater Treatment: Past, Present and Prospect

“…Klamklang (Klamklang, 2006, Klamklang, 2007and Klamklang et al, 2010 has found that increasing current density from 5 to 10 mA/cm 2 , leads to less degradation rate of oxalic acid by electrochemical oxidation. This behavior is characteristic of mass transfer-controlled processes (Kesselman, 1997). In such systems, the increase of current density cannot increase the organic removal efficiency at the electrode, but only favors oxygen evolution as the anodic side reaction which hides the electrode and prevents contact between hydroxyl radicals and organic pollutants.…”