Abstract
Electrochemical advanced oxidative processes (EAOP) are a promising route to destroy recalcitrant organic contaminants such as per- and polyfluoroalkyl substances (PFAS) in drinking water. Central to EAOP are catalysis-induced reactive free radicals for breaking the carbon fluorine bonds in PFAS. Generating these reactive species electrochemically at electrodes provides an advantage over other oxidation processes that rely on chemicals or other harsh conditions. Herein, we report on the performance of niobium (Nb) doped rutile titanium oxide (TiO2) as a novel EAOP catalytic material, combining theoretical modeling with experimental synthesis and characterization. Calculations based on density functional theory are used to predict the overpotential for oxygen evolution at these candidate electrodes, which must be high in order to oxidize PFAS. The results indicate a non-monotonic trend in which Nb doping below 6.25 at.% is expected to reduce performance relative to TiO2, while higher concentrations up to 12.5 at.% lead to increased performance, approaching that of state-of-the-art Magnéli Ti4O7. TiO2 samples were synthesized with Nb doping concentration at 10 at.%, heat treated at temperatures from 800 to 1100 °C, and found to exhibit high oxidative stability and high generation of reactive oxygen free radical species. The capability of Nb-doped TiO2 to destroy two common species of PFAS in challenge water was tested, and moderate reduction by ~ 30% was observed, comparable to that of Ti4O7 using a simple three-electrode configuration. We conclude that Nb-doped TiO2 is a promising alternative EAOP catalytic material with increased activity towards generating reactive oxygen species and warrants further development for electrochemically destroying PFAS contaminants.
Highlights
Electrochemical advanced oxidative processes (EAOP) are a promising route to destroy recalcitrant organic contaminants such as per- and polyfluoroalkyl substances (PFAS) in drinking water
A high overpotential for oxygen evolution reaction (OER) is an important criterion for catalytic materials for high ROS production[10]. This activity can be predicted in terms of a theoretical overpotential for OER based on a combined descriptor, GO∗ − GHO∗, which is the difference between free energies of binding of the O* and HO* intermediates of the OER mechanism[7]
The results for rutile T iO2, Nb-doped TiO2 (NTO), and Magnéli phase T i4O7 are shown in Fig. 1, and the density functional theory (DFT) results used to calculate the quantity
Summary
Electrochemical advanced oxidative processes (EAOP) are a promising route to destroy recalcitrant organic contaminants such as per- and polyfluoroalkyl substances (PFAS) in drinking water. We conclude that Nb-doped TiO2 is a promising alternative EAOP catalytic material with increased activity towards generating reactive oxygen species and warrants further development for electrochemically destroying PFAS contaminants. DFT-based methods have frequently been leveraged for applications relating to hydrogen and oxygen evolution reactions (HER and OER, respectively) for water splitting[6] Wide ranges of both metals and oxides have been computationally screened and placed on “volcano plots” that exhibit a peak in catalytic activity at certain moderate binding energies of critical intermediate s pecies[7]. We adapt these methods for a different purpose, i.e., to screen materials for the breakdown of recalcitrant contaminants such as PFAS In this case, we apply DFT calculations to seek materials far from the peak in the volcano plot and exhibiting suppressed OER, which is a prerequisite of electrode materials for electrochemical advanced oxidative processes (EAOP)[10]. Alternatives such as doped-tin oxides, lead oxides, and titanium oxides have been the subject of interest for identifying costeffective, stable, novel EAOP catalytic m aterials[10]
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