Mechanism of p-Substituted Phenol Oxidation at a Ti4O7 Reactive

Article pubs.acs.org/est

Mechanism of p‑Substituted Phenol Oxidation at a Ti4O7 Reactive Electrochemical Membrane Amr M. Zaky‡ and Brian P. Chaplin*,† †

Department of Chemical Engineering, University of Illinois at Chicago, 810 South Clinton Avenue, Chicago, Illinois 60607, United States ‡ Department of Civil and Environmental Engineering and Villanova Center for the Advancement of Sustainable Engineering, Villanova University, Villanova, Pennsylvania 19085, United States S Supporting Information *

ABSTRACT: This research investigated the removal mechanisms of pnitrophenol, p-methoxyphenol, and p-benzoquinone at a porous Ti4O7 reactive electrochemical membrane (REM) under anodic polarization. Cross-flow filtration experiments and density functional theory (DFT) calculations indicated that p-benzoquinone removal was primarily due to reaction with electrochemically formed OH•, while the dominant removal mechanism of p-nitrophenol and p-methoxyphenol was a function of the anodic potential. At low anodic potentials (1.7−1.8 V/SHE), p-nitrophenol and p-methoxyphenol were removed primarily by an electrochemical adsorption/polymerization mechanism on the REM. Increasing anodic potentials (1.9−3.2 V/SHE) resulted in the electroassisted adsorption mechanism contributing far less to p-methoxyphenol removal compared to p-nitrophenol. DFT calculations indicated that an increase in anodic potential resulted in a shift in p-methoxyphenol removal from a 1e− direct electron transfer (DET) reaction that resulted in radical formation and significant adsorption/polymerization, to a 2e− DET reaction that formed nonadsorbing products (i.e., p-benzoquinone). However, the anodic potentials were too low for the 2e− DET reaction to be thermodynamically favorable for p-nitrophenol. The decreased COD adsorption for p-nitrophenol at higher anodic potentials was attributed to reaction of soluble/adsorbed organics with OH•. These results provide the first mechanistic explanation for p-substituted phenolic compound removal during advanced electrochemical oxidation processes.

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INTRODUCTION

Recent work has shown that the use of porous substoichimetric TiO2 (Ti4O7) anodes in flow-through filtration mode creates a reactive electrochemical membrane (REM), which combines microfiltration with electrochemical oxidation.16 The micrometer-sized pores of the REM produced a high electroactive surface area and advection-enhanced mass transfer rates approximately 10-fold higher than those obtained in traditional flow-by mode.16 The water treatment potential of the REM is promising, as a high removal (i.e., 99.9%) of pmethoxyphenol, a toxic phenolic compound present in industrial effluents,19 was achieved by electrochemical adsorption and oxidation by OH•.16 However, a detailed understanding of these removal processes is still lacking, and further research is needed to thoroughly understand the proposed removal mechanisms. Numerous studies have focused on electrochemical oxidation of substituted phenols at EAOP electrodes, including chlorinated phenols,8,9,20−31 p-substituted phenols,16,32−35 and multisubstituted phenols.29,36 These compounds are present in

Electrochemical advanced oxidation processes (EAOPs) are methods that generate hydroxyl radicals (OH•) via water oxidation at an anode surface. Facilitated by the development of stable anode materials, EAOPs are being increasingly used for water treatment applications. Numerous studies have shown the ability of EAOPs to degrade various recalcitrant organic compounds by a combination of OH•-mediated oxidation and direct electron transfer (DET) reactions at the anode surface.1−17 EAOPs primarily use flat plate electrodes operated in flow-by mode. This flow configuration results in a large hydrodynamic diffuse boundary layer (∼100 μm), which prevents rapid reaction rates of contaminants due to diffusional limitations. Therefore, EAOPs operate at relatively low applied current densities (e.g., 1.75 V relative to the blank electrolyte solution, and a distinct current peak is observed at 1.88 V (Figure 1a). During the second LSV scan, the peak is attenuated, and during the third scan, a peak is no longer observed (Figure 1a). This behavior is attributed to electrochemical adsorption of p-NP, which prevents access of aqueous p-NP to the electrode.29,46,47 The LSV scans with 25 mM p-MP showed increased current relative to the blank electrolyte at potentials >0.8 V and a current peak at ∼2.0 V (Figure 1b). Successive scans were also performed with 25 mM p-MP (results not shown), but decreased current was not observed, indicating that adsorption was minimal. LSV experiments were also conducted with p-BQ, but current peaks were not observed (results not shown). Oxidation Experiments. Various studies have investigated the oxidation of phenolic compounds at EAOP anodes (e.g., boron-doped diamond (BDD), doped-SnO2, PbO2, and Ti4O7).1,8,9,16,20−31,36 These studies suggest three oxidation pathways, as shown in Scheme 1. The pathways consist of the following: (A) 1e− DET reaction resulting in polymer 5859

dx.doi.org/10.1021/es5010472 | Environ. Sci. Technol. 2014, 48, 5857−5867

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Scheme 1. Proposed Reaction Pathways for p-Substituted Phenol Electrochemical Oxidation

was discussed in our prior study.16 The validity of this mechanism will be tested in the DFT Modeling section. The trends for COD removal during p-BQ oxidation differ from the other organic compounds. The oxidation of p-BQ has been reported to occur via oxidation with OH•,48 and is highly resistant to DET reactions.15 The COD profiles in Figure 2c support this mechanism, and show that the p-BQ oxidation kinetics are zeroth order and not significantly different from the kinetic limited oxidation rate (dashed line). Zeroth-order kinetics for COD removal continues up to current densities of 1.0 mA cm−2. The potentials of these experiments were 1.8− 2.4 V/SHE, which are in the range associated with the onset of OH• production.49 Due to the rapid reaction between OH• and p-BQ, OH• production is likely shifted toward lower potentials and therefore enhances substrate oxidation.50 The rN,COD values determined for p-BQ were normalized to the theoretical current controlled oxidation rate, assuming 100% current efficiency (rC,COD). The corresponding rN,COD/rC,COD values are plotted in Figure 2d, and are not significantly different from 1.0 for current densities ≤1.0 mA cm−2. Of the organic compounds tested, only p-BQ showed an increasing removal rate upon increasing current densities, which is consistent with a reaction that is dependent on OH• concentrations. The COD removal at longer reaction times during the p-BQ experiment conducted at 1.0 mA cm−2 deviates from zeroth-order behavior, as removal is likely affected by oxidation products or mass transfer (SI Figure S-4). At a current density of 3.5 mA cm−2, COD profiles are first-order, suggesting that COD oxidation may be limited by mass-transport control, the blocking of active sites by kinetically slow reacting intermediates, establishment of a steady-state OH• surface concentration,51 or the blocking of electrode sites by O2 bubble formation (SI Figures S-4). The kN,COD/km value for p-BQ at a 3.5 mA cm−2 current density was 0.47 ± 0.01 and was not plotted in Figure 2d due to data overlap. The percentage of organic compounds adsorbed to the REM after each experiment (%CODad,f) showed different trends with respect to the applied current density for each compound (Table 1). During the oxidation of p-MP, it was observed that COD adsorption steadily decreased upon increasing current density, from 47.4 and 44.7% (duplicate experiments) of the initial COD concentration at a 0.2 mA cm−2 current density, to 4.11 and 3.17% (duplicate experiments) of the initial COD concentration at a 3.5 mA cm−2 current density. The COD

Profiles of the COD concentration versus time at a current density of 0.2 mA cm−2 are shown in Figure 2 for the three compounds tested and a summary of all experiments is provided in Table 1. Representative concentrations in both the feed (CF,COD) and permeate (CP,COD) are shown, and the difference between the two concentrations at a given time represents the COD removed upon a single pass through the REM pores. Individual RP,COD values are provided in Table 1. The dashed lines in Figure 2a−c represent the theoretical maximum COD removal at 100% anodic current efficiency. The solid red lines in Figure 2a−c represent the maximum COD removal under mass-transport control, which was determined by the average km value in each experiment. Each compound tested showed a different trend with regard to COD removal. The removal of both p-NP and p-MP was mass-transport limited, and therefore COD removal could only reach the mass-transfer limit (solid red line) if complete adsorption and/or complete oxidation to CO2 were occurring. At lower current densities, COD removal was often greater than the current-limited rate (dashed line), indicating that compound adsorption was a dominant removal mechanism (Pathway A, Scheme 1). This scenario was the case for p-NP at current densities between 0.2 and 1.0 mA cm−2, where adsorption was a significant removal mechanism (Figure 2a and SI Figure S-2). Figure 2d contains a plot of kN,COD values normalized by km (kN,COD/km). At the lowest current density (0.2 mA cm−2), the kN,COD/km value for p-NP was only slightly lower than 1.0 at the 95% confidence interval, indicating that pNP was removed almost entirely by an electrochemical adsorption mechanism (Pathway A, Scheme 1). As current density increased, kN,COD/km values for p-NP were much less than unity, indicating that the higher anodic potential facilitated other pathways shown in Scheme 1, which will be discussed in the DFT Modeling section. Similar trends were observed for p-MP, where the measured COD removal was greater than the current-limited rates at low current densities (≤0.5 mA cm−2) (Figure 2b and SI Figure S3). However, unlike p-NP, kN,COD/km values for p-MP were between 0.31 ± 0.02 and 0.44 ± 0.08 (Figure 2d), indicating that both Pathways A and B were likely active at the lower current densities tested. At higher current densities, the generation of OH• would allow Pathway C to contribute to p-MP oxidation and also the oxidation of adsorbed organics, as 5860


p-BQ

0.73b 1.67 1.88 2.13 3.12 0.51b 1.83 1.93 2.27 3.25 0.77b 1.81 1.91 2.37 2.92

measured anode potential (V vs SHE) 51.1 45.8 37.2 43.8 39.4 36.0 39.8 49.6 48.7 36.1 40.1 52.6 38.7 38.2 38.6

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± 1.9 3.8 2.7 3.7 6.4 1.2 2.5 5.6 5.2 9.8 3.0 16 3.7 4.1 4.6

mass transfer rate constat (km)d (L m−2 h−1) 0 42.6 39.5 47.7 44.7 0 37.3 50.4 46.9 40.6 ± ± ± ±

± ± ± ± 2.8 4.9 2.0 3.0

5.7 1.0 3.3 3.0

normalized rate constant (kN,p‑X) (L m−2 h−1)

b

0 92.9 99.9 99.9 59.1 0 96.9 97.9 98.2 98.5 ± ± ± ±

± ± ± ± 2.6 1.1 0.73 1.7

6.8 0.12 0.17 6.4

removal in permeate (RP) (%) 100, 3.71, 6.98, 5.04, 16.0, 100, 2.81, 2.76, 4.87, 12.1,

100 10.8 0.00 3.05 9.06 100 9.94 5.70 2.86 17.3

p-X remaininge (%)

c

± ± ± ±

± ± ± ± 5.4 4.0 5.1 2.4

0.91 1.5 0.71 3.3

18.0 ± 0.57

0 14.4 14.4 14.7 17.2 0 33.8 38.4 29.6 25.1 0

normalized rate constant (kN,COD) (L m−2 h−1)

0 0.21 ± 0.040 0.40 ± 0.038 0.78 ± 0.068

initial rate (rN,COD) (mg m−2 h−1) 0 30.1 24.5 25.6 24.7 0 84.3 48.3 46.3 76.5 0 14.7 20.4 25.2 17.1 ± ± ± ±

± ± ± ±

± ± ± ±

4.0 3.7 3.8 3.0

4.4 14 12 3.9

3.1 9.8 3.5 3.8

removal in permeate (RP) (%) 100, 39.8, 41.3, 39.9, 54.7, 100, 15.0, 9.55, 17.9, 39.4, 100, 72.4, 51.9, 44.6, 39.9,

100 41.2 33.5 34.7 31.6 100 16.2 5.14 8.64 24.4 100 69.5 54.2 45.1 39.1

COD remaininge (%)

COD

0, 47.4, 22.2, 6.05, 4.11, 0, 81.5, 73.6, 65.0, 42.0, 0, 3.72, 6.05, 3.81, 4.75,

0 44.7 26.4 5.67 3.17 0 71.3 70.1 69.1 45.4 0 6.98 7.20 5.12 6.48

adsorbed CODe,f (%) 0, 12.8, 36.5, 54.1, 41.2, 0, 3.5, 16.8, 17.1, 18.6, 0, 23.9, 42.1, 51.6, 55.4,

0 14.1 40.1 59.6 65.2 0 12.5 24.7 22.3 30.2 0 23.5 38.6 49.8 54.4

COD oxidatione,g (%)

109, 76.8, 47.9, 25.7,

18.6, 33.7, 18.3, 8.92,

86.8, 99.0, 73.3, 25.9,

113 78.2 51.7 25.2

66.5 42.3 27.3 11.8

88.8 109 80.8 39.3

current efficiency (CECOD)e,h(%)

d

Current density calculated using nominal surface area of inner REM wall. Open circuit potential. Data for current densities between 0.0 and 1.0 reprinted (adapted) with permission from ref 16. Determined by measured membrane flux (J). eFinal concentration at end of the experiment (values for duplicate experiments). fFinal adsorbed mass remaining at end of experiment (values for duplicate experiments). gCalculated by (COD oxidation = 100% − % COD remaining − % adsorbed COD). hCumulative current efficiency for experiment (values for duplicate experiments).

a

0.00 0.20 0.50 1.00 3.50 0.00 0.20 0.50 1.00 3.50 0.00 0.20 0.50 1.00 3.50

p-MPc

p-NP

current density (mA/cm2)

compound (p-X)

a

p-X

Table 1. Summary of the Experimental Results Obtained during the Oxidation of p-Substituted Phenols

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Figure 2. Profiles of COD concentration versus time where solid squares represent the feed concentration and hollow squares represent the permeate concentration for (a) p-NP, (b) p-MP, and (c) p-BQ. The dashed lines represent current controlled removal at 100% Faradaic current efficiency determined by SI eq S-1. The solid red lines represent the mass-transfer controlled rate determined by SI eq S-2. Solid lines through feed data represent a regression line. Panel (d) shows the ratio of kN,COD/km on the left axis for p-NP and p-MP, and rN,COD/rC,COD on the right axis for pBQ.

adsorption followed a similar trend for p-NP, but values were much higher than those for p-MP (Table 1). During the oxidation of p-BQ, it was observed that COD adsorption was minimal and approximately the same for all oxidation experiments (3.72−7.20% of initial COD concentration). The adsorption results suggest that the oxidation mechanisms of the three organic compounds differ, which will be further investigated using DFT modeling. The percent COD oxidation values reported in Table 1 are plotted versus the measured anodic potential in order to more easily observe trends in the data (SI Figure S-5). As shown in SI Figure S-5, plateaus in the percent COD oxidation upon increasing anodic potentials were observed for all three compounds. This trend is indicative of a process that is limited by mass transport. The plateaus for COD oxidation occur at similar values for p-NP and p-BQ, as both of these compounds showed negligible adsorption at anodic potentials greater than 2.0 V (Table 1). However, the much lower plateau value for COD oxidation observed for p-NP is likely due to the presence of adsorbed polymer on the REM surface, which is supported by the adsorbed COD values reported in Table 1. The adsorbed polymer likely blocked reactive sites for water oxidation to OH• and therefore inhibited COD removal. Data from duplicate experiments showed the largest variability for experiments whose measured anodic potentials were greater than 3.0 V (i.e., p-MP and p-NP at 3.5 mA cm−2), which may be attributed to O2 bubble formation within the REM pores, which blocked reaction sites to different extents in the duplicate experiments. DFT Modeling. DFT modeling was used to interpret the experimental results and determine a mechanism for the oxidation of the p-substituted phenolic compounds. Specifically, DFT modeling was used to understand why p-MP and p-NP

apparently reacted by different mechanisms during oxidation experiments. Simulations were conducted to determine an Ea profile as a function of anodic potential for the 1e− direct oxidation of p-MP (Pathway A, Scheme 1), as shown in reaction 4a. The subsequent deprotonation step is shown in

reaction 4b.The energy profiles of the reactants and products at an anodic potential of 0.85 V are shown in Figure 3a, and the Ea profile as a function of anodic potential is shown in Figure 3b. The Ea at 0.85 V was 13.0 kJ mol−1, and corresponded to the potential at which current began to flow in response to the addition of p-MP in LSV experiments (Figure 1b). The Ea profile shown in Figure 3b indicates that reaction 4a was activationless at potentials ≥1.16 V. This potential is lower than anodic potentials measured during oxidation experiments, indicating that Pathway A is feasible during all experiments. The shaded region in Figure 3b indicates the anodic potential range during oxidation experiments. The phenoxy radicals formed in reaction 4b are known to polymerize.17 Therefore, it was of interest to calculate the potential at which the 2e− DET reaction shown in Pathway B was feasible (Scheme 1). Model simulations were performed to 5862



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Figure 3. (a) Energy profiles as a function of COH bond length at an electrode potential of 0.85 V/SHE for the reactant (p-MP) and product (pMP•+ + e−) shown in rxn. 4a. (b) Activation energy calculations for p-MP as a function of electrode potential for reactions shown in rxn. 4a and rxn. 5a. (c) Energy profiles as a function of COH bond length at an electrode potential of 2.02 V/SHE for the reactant (p-NP) and product (p-NP•+ + e−) shown in rxn. 4a. (d) Activation energy calculations for p-NP as a function of electrode potential for reactions shown in rxn. 4a and rxn. 5a. Shaded boxes show the range of anodic potentials measured in the experiments.

increasing current densities during p-MP oxidation experiments (Table 1). The phenoxonium ion product formed in reaction 5b (p-MP+) is rapidly converted to p-BQ, as shown in reaction 6. The increasing formation of p-BQ upon increasing current

determine the Ea profile as a function of anodic potential for the direct oxidation of p-MP•+, as shown in reaction 5a. The

density has been previously confirmed experimentally for the oxidation of p-MP in the REM system,16 providing further evidence to support DFT modeling results. Reactions 4a and 5a were also simulated for p-NP oxidation. Energy profiles for reaction 4a are displayed in Figure 3c at an electrode potential of 2.02 V and Ea values as a function of anodic potential are shown in Figure 3d. Modeling results determined Ea = 67.0 kJ mol−1 at 1.76 V for p-NP undergoing reaction 4a, and the reaction was activationless at potentials ≥2.35 V. These results compared well with the anodic peak current observed at 1.88 V in LSV experiments (Figure 1a). Anodic potentials during p-NP oxidation experiments were between 1.83 and 3.25 V, indicating that reaction 4a was feasible. The Ea profile was also calculated for p-NP•+ undergoing reaction 5a, and it was found that Ea = 43.8 kJ mol−1 at 3.32 V and was activationless at potentials ≥4.18 V. As shown in Figure 3d and Table 1, the highest anodic potential during p-NP oxidation experiments was 3.25 V, indicating that reaction 5a was likely not a significant oxidation pathway. These results support the experimental findings that suggested p-NP underwent electrochemical adsorption (Pathway A) to a greater extent than p-MP. Reaction 5a was thermodynamically favorable for p-MP, but not for p-NP. The p-NP• formed via

subsequent deprotonation step is shown in reaction 5b. As shown in Figure 3b, reaction 5a has an Ea = 32.0 kJ mol−1 at an anodic potential of 2.11 V, and the reaction was activationless at potentials ≥2.72 V. The Ea calculated at 2.11 V is low enough that oxidation could occur at room temperature, and the potential is close to that of the oxidation peak that was observed at 2.0 V in LSV experiments containing p-MP (Figure 1b). Two separate current peaks for reaction 4a and reaction 5a were not observed in LSV experiments containing p-MP, likely because either current generated from the two reactions occurred at overlapping potentials and/or dispersion in the anodic current was caused by ohmic drop with depth into the porous REM. Anodic potentials in p-MP oxidation experiments increased from 1.7 to 3.12 V at a current density between 0.2 and 3.5 mA cm−2. Therefore, DFT results suggest that Pathway B was an increasingly favorable oxidation mechanism at increasing current densities. This hypothesis is supported by the observed trend of decreasing compound adsorption at 5863



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their electrons more tightly and are not expected to be sites for OH• attack. Upon removing an electron from p-MP to form pMP• (reaction 4a,b), the reactive sites change. Fukui function calculations indicate that the C2, C6, and C4 atoms are the most likely sites for electrophilic attack by OH•. Upon removing an additional electron to form p-MP+ (reaction 5a,b), the reactive sites change again. Fukui function calculations indicate that the C4 atom is the most likely site for nucleophilic attack, which supports reaction Pathway B shown in Scheme 1, and indicates that nucleophilic attack by water at the C4 atom of p-MP+ is the likely mechanism for pBQ formation. The C3 and C5 atoms of p-MP+ were shown to be the most likely sites for electrophilic attack by OH•. Fukui function calculations for p-NP indicate that the C4 and C6 atoms are the most likely sites for electrophilic attack by OH•. Similarly, a previous study concluded that the C2, C4, and C6 atoms were the likely sites for electrophilic attack based on calculated C atom charges.54 Upon removing an electron from p-NP to form p-NP• (reaction 4a,b), calculated Fukui function values indicate that the C2, C4, and C6 atoms are the most likely sites for electrophilic attack. Upon removing an additional electron to form p-NP+ (reaction 5a,b), Fukui function calculations indicate that the C4 atom is the most likely site for nucleophilic attack, and electrophilic attack was not observed to be favorable at the phenol ring for this compound. Results shown in Figure 3 suggest that high anodic potentials are necessary to form p-NP+, suggesting it was not present at significant concentrations in our experiments. However, other studies that oxidized p-NP at BDD electrodes detected p-BQ as an intermediate, and therefore these results indicate that Pathway B (Scheme 1) is feasible for p-NP if the anodic potential is sufficient to allow the 2e− DET reactions. The results presented in this study provide conclusive experimental and DFT modeling evidence that the dominant mechanism for p-substituted phenolic compound removal at a Ti4O7 anode is a function of both the electrode potential and the substituent type. Electron donating substituents (e.g.,  OCH3 groups) increase the electron density of the phenolic ring, and allow DET reactions to proceed at lower anodic potentials relative to p-substituted phenolic compounds with electron withdrawing substituents (e.g., NO2). Therefore, the anodic potential at which the mechanism for p-substituted phenolic compound removal switches from the 1e− polymerization mechanism to the 2e− oxidation mechanism is determined by the electronegativity of the substituent. The results of this study can be used to develop a technology for the electrochemical adsorption of phenolic compounds, which would mimic packed bed electrode columns that were developed for heavy-metal removal.55 Such a strategy would utilize a downstream cathode and, due to ohmic drop with depth into the electrode, would create an increasing anodic potential in the direction of flow.55 Therefore, mixtures of phenolic compounds would be polymerized at different depths within the anode pores. Either offline electrochemical treatment or chemical/electrochemical desorption methods could be explored as potential regeneration methods. For this strategy to be a viable remediation technique, the adsorption capacity and energy consumption of the anode must be determined and compared to existing adsorption technologies (e.g., activated carbon).

reactions 4a,b underwent adsorption/polymerization at the REM surface and was retained by the micrometer-sized pores. Theoretical calculations support experimental results that did not detect the formation of p-BQ during p-NP oxidation. The lower %CODad,f value observed for p-NP oxidation experiments at higher current densities suggests that OH• formation was responsible for the oxidation of the adsorbed organic compounds and not a shift to Pathway B. The oxidation of p-NP at other EAOP anodes (i.e., Sb-doped SnO2 and Bi-doped PbO2) is consistent with the mechanism found above, where p-BQ was not detected during p-NP oxidation,8,52 but was observed during p-MP oxidation.52 However, studies have shown that the oxidation of p-NP at BDD anodes formed p-BQ, and two separate oxidation peaks were detected in cyclic voltammetry scans,32 suggesting that Pathway B was active during p-NP oxidation at BDD anodes. The second oxidation peak was observed at ∼2.2−2.6 V/ SHE,32,33 which is significantly less than potentials calculated in our study for p-NP oxidation by Pathway B (Scheme 1). These results indicate that the BDD surface may catalyze p-NP oxidation. Although prior studies have not investigated this hypothesis, the surficial C atoms of (111) diamond are phenolic in shape and internuclear distances (1.544 Å) are similar to CC bonds in phenolic compounds,53 suggesting catalytic effects are possible. The direct oxidation of p-BQ was also investigated for reaction 7. Results determined that p-BQ underwent an

activationless DET reaction at potentials ≥3.11 V, which supports LSV experiments that did not observe direct oxidation in the presence of p-BQ, and previous studies that report p-BQ is highly resistant to direct oxidation.15 At anodic potentials of ∼3.0 V, significant water oxidation occurs, which makes it difficult to observe oxidation peaks during LSV scans. Although DFT simulations suggest that direct oxidation of p-BQ was possible at the highest current density used experimentally (3.5 mA cm−2 and 2.9 V), at this potential a significant quantity of OH• is formed, and therefore p-BQ likely reacted with OH• instead of through the DET pathway. The measured secondorder rate constant for the reaction between p-BQ and OH• is 1.2 × 109 M−1 s−1,48 which is in the diffusion-limited range. DFT modeling results confirm that Pathway C is the dominant reaction pathway for p-BQ, as previously suggested.15 The geometry optimized structures for p-MP, p-NP, and their oxidation products are shown in Figure 4. Also included in Figure 4 are atomic charges of the C atoms in the phenol ring. Upon the removal of successive electrons from these compounds, the charges on the C1 and C4 atoms increase for both p-MP and p-NP. The largest changes in electron density for p-MP occur on the C1 and C4 atoms, and the largest change in electron density for p-NP occurs on the C4 atom. Fukui functions were calculated for the C atoms in the phenol ring (SI Table S-1), and it was determined for p-MP that electrophilic attack by OH• was most likely at the C1 and C4 atoms. Although the charges of the other C atoms are more electronegative than the C1 and C4 atoms, these atoms hold 5864



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Figure 4. Geometry optimized structures for (a) p-MP, (b) p-MP•, (c) p-MP+, (d) p-NP, (e) p-NP•, and (f) p-NP+. Hirshfeld atomic charges of C atoms in the phenol ring are shown, and arrows indicate preferred sites for electrophilic attack by OH• and nucleophilic attack by water. Atom key: C = gray; O = red; and H = white.

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(3) Comninellis, C.; Pulgarin, C. Electrochemical oxidation of phenol for waste-water treatment using SnO2 anodes. J. Appl. Electrochem. 1993, 23 (2), 108−112. (4) Borras, C.; Berzoy, C.; Mostany, J.; Scharifker, B. Oxidation of pmethoxyphenol on SnO2Sb2O5 electrodes: Effects of electrode potential and concentration on the mineralization efficiency. J. Appl. Electrochem. 2006, 36 (4), 433−439. (5) Adams, B.; Tian, M.; Chen, A. Design and electrochemical study of SnO2-based mixed oxide electrodes. Electrochim. Acta 2009, 54 (5), 1491−1498. (6) Zhuo, Q.; Deng, S.; Yang, B.; Huang, J.; Yu, G. Efficient electrochemical oxidation of perfluorooctanoate using a Ti/SnO2 SbBi anode. Environ. Sci. Technol. 2011, 45 (7), 2973−2979. (7) Sharifian, H.; Kirk, D. W. Electrochemical oxidation of phenol. J. Electrochem. Soc. 1986, 133 (5), 921−924. (8) Borras, C.; Laredo, T.; Mostany, J.; Scharifker, B. R. Study of the oxidation of solutions of p-chlorophenol and p-nitrophenol on Bidoped PbO2 electrodes by UV−vis and FTIR in situ spectroscopy. Electrochim. Acta 2004, 49 (4), 641−648. (9) Borras, C.; Laredo, T.; Scharifker, B. R. Competitive electrochemical oxidation of p-chlorophenol and p-nitrophenol on Bi-doped PbO2. Electrochim. Acta 2003, 48 (19), 2775−2780. (10) Borras, C.; Rodriguez, P.; Laredo, T.; Mostany, J.; Scharifker, B. R. Electrooxidation of aqueous p-methoxyphenol on lead oxide electrodes. J. Appl. Electrochem. 2004, 34 (6), 583−589. (11) Liu, Y.; Liu, H.; Li, Y. Comparative study of the electrocatalytic oxidation and mechanism of nitrophenols at Bi-doped lead dioxide anodes. Appl. Catal., B 2008, 84 (1−2), 297−302. (12) Zhao, G. H.; Zhang, Y. G.; Lei, Y. Z.; Lv, B. Y.; Gao, J. X.; Zhang, Y. A.; Li, D. M. Fabrication and Electrochemical treatment

ASSOCIATED CONTENT

S Supporting Information *

Experimental setup, REM filtration experimental results, and Fukui Function values. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: 312-996-0288; fax: 312-996-0808; e-mail: chaplin@ uic.edu. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Funding for this work was provided by the National Science Foundation (CBET-1159764). We thank Mr. Thomas Hoffman and Mr. Kai Ding for assistance in filtration experiments.

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REFERENCES

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Mechanism of p-Substituted Phenol Oxidation at a Ti4O7 Reactive

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