Understanding Reductive Dechlorination of Trichloroethene on Boron

Nov 5, 2008 - rotating disk electrode reactor. ... Reduction of TCE resulted in production of acetate and ... Rotating Disc Electrode (RDE) Experiment...
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Environ. Sci. Technol. 2008, 42, 9344–9349

Understanding Reductive Dechlorination of Trichloroethene on Boron-Doped Diamond Film Electrodes DHANANJAY MISHRA, ZHAOHUI LIAO, AND JAMES FARRELL* Department of Chemical and Environmental Engineering, University of Arizona, Tucson, Arizona 85721

Received June 30, 2008. Revised manuscript received August 30, 2008. Accepted October 8, 2008.

This research investigated reduction of trichloroethylene (TCE) at boron-doped diamond (BDD) film cathodes using a rotating disk electrode reactor. Rates of TCE reduction were determined as functions of the electrode potential and TCE concentration over a temperature range between 2 and 32 °C. Reduction of TCE resulted in production of acetate and chloride ions with no detectable intermediate products. At a current density of 15 mA/cm2 and concentrations below 0.75 mM, reaction rates were first order with respect to TCE concentration, with surface area normalized rate constants 2 orders of magnitude greater than those for iron electrodes. Density functional theory (DFT) simulations were used to evaluate activation barriers for reduction by direct electron transfer, and for reaction with four functional groups commonly found on BDD surfaces. The DFT calculated activation barrier for direct electron transfer was more than 4 times greater than the experimentally measured value of 22 kJ/mol. In contrast, the DFT activation barrier for reaction at a deprotonated hydroxyl site on a tertiary carbon atom (tCsO-) of 24 kJ/mol was in close agreement with the experimental value. Both experiments and quantum mechanical simulations support a TCE reduction mechanism that involves chemically adsorbed intermediates.

Introduction The main obstacle to developing commercially viable electrochemical water treatment technologies is the need for an electrode material that is stable under both anodic and cathodic polarization. Both anodic and cathodic stability are needed because polarization reversals are periodically required to remove carbonate scale and biofilms that may build-up on the cathode surface. Noble metals, such as platinum, are anodically and cathodically stable, but they have high catalytic activity for water electrolysis and are prone to fouling by chemically adsorbed compounds (1, 2). Dimensionally stable electrodes, such as titanium metal coated with various catalysts, are also prone to fouling; they suffer from catalyst leaching when anodically polarized (3) and accelerated caustic wear when cathodically polarized (4). Boron-doped diamond (BDD) films coated on metal and semiconducting substrates overcome the deficiencies of other electrode materials because of their high anodic and cathodic * Corresponding author e-mail: [email protected]; phone: (520) 621-2465; fax: (520) 621-6048. 9344

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stabilities, resistance to fouling, low catalytic activity for water electrolysis, and effectiveness without the need for catalysts (1, 5). Freshly prepared BDD surfaces are terminated with hydrogen atoms (6-9). Anodic polarization introduces oxygenated functional groups onto the surface and has also been found to remove sp2-hybridized carbon impurities via oxidation to CO2 (7, 10, 11). After anodic polarization, up to 20% of the surface C atoms have been found to contain oxygenated functional groups (12), such as, carboxyl, carbonyl, and hydroxyl groups (6, 8, 15, 16). These oxygenated groups are suspected to be involved in mediating electron transfer at BDD electrodes (8, 13) and remain on the surface even after cathodic polarization (12, 17). Cathodic polarization is suspected to reduce carbonyl groups to carboxyl groups (17) and to produce carbon radicals on the diamond surface via the electrochemical desorption step involved hydrogen evolution (18). BDD electrodes have been extensively studied for use as anodes in water treatment applications (1, 19-21). However, there have been very few investigations of BDD electrodes employed as cathodes. Bouamrane et al. (24) studied the effectiveness of BDD for reducing nitrate to ammonia, and several studies (25-27) have investigated metal deposition, including mercury, silver, copper, and cadmium. We are not aware of any published reports on the use of BDD cathodes for reductive destruction of organic contaminants in water. The goals of this research were to investigate the effectiveness of a BDD cathode for reductive destruction of a typical organic compound that is often found in industrial wastewaters and contaminated groundwaters. Trichloroethylene (TCE) was selected for study because its reductive dechlorination by a wide range of corroding metals (28-32) and electrode materials (33-35) has been extensively investigated. Experiments were performed to measure rates of TCE reduction as functions of concentration and electrode potential over temperatures ranging from 2 to 32 °C. The experiments were complemented with density functional theory (DFT) simulations to investigate possible rate-limiting mechanisms for TCE reduction.

Materials and Methods Rotating Disc Electrode (RDE) Experiments. Experiments measuring TCE dechlorination rates were performed in a previously described (33) 30 mL temperature-controlled glass-reaction cell. The working electrode consisted of a boron-doped diamond (BDD) film coated on a 1.13 cm diameter p-silicon disk (Adamant Technologies, Neuchatel, Switzerland). As reported by the manufacturer, the polycrystalline diamond film had a thickness of 1 µm ((5%), a resistivity of 15 mΩ cm ((30%), and a crystal size of ∼0.5 µm. Prior to use, the electrode was conditioned for 12 h via anodic polarization at a current density of 20 mA/cm2. The electrode was mounted in a Princeton Applied Research (PAR) (Oak Ridge, TN) model 616 rotating disk electrode (RDE) holder. To eliminate mass transfer limitations, the disk electrode was rotated at a speed of 1000 rpm. A 0.3 mm diameter by 4 cm long platinum wire (Aesar, Ward Hill, MA) was used as the counter electrode, and a PAR Hg/Hg2SO4 (saturated K2SO4) electrode was used as the reference. The counter electrode was encased within a Nafion (Dupont) proton permeable membrane in order to prevent oxidation of any product or reactant species. Electrode potentials and currents were controlled using a PAR model 273A potentiostat and were corrected for uncompensated solution resistance. All po10.1021/es801815z CCC: $40.75

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tentials are reported with respect to the standard hydrogen electrode (SHE). To minimize the effect of reaction products on the measurement of TCE reaction rates, the experimental conditions attempted to mimic a differential reactor with constant reactant concentrations and near zero product concentrations. Experiments measuring TCE dechlorination rates were performed at constant aqueous TCE concentrations ranging from 0.067 to 7.5 mM in 10 mM CaSO4 background electrolyte solutions with an initial pH value of 7. Constant TCE concentrations were achieved by continuously purging the reaction cell with 25 mL/min of humidified nitrogen gas containing TCE at a fixed concentration. The experiments were performed for the minimum amount of time required to reach product concentrations that could be accurately measured (∼0.5-8 h), and only the final composition of the solution was analyzed. Temperatures were controlled to within (0.2 °C using a circulating water bath. Flow Cell Experiments. The possibility for loss of volatile reaction products through the electrode shaft opening in the RDE reactor necessitated the use of a gastight, flow-through cell for determining TCE reaction products and carbon mass balances. Two previously described BDD disks were mounted inside a hollow Teflon cylinder in a parallel plate arrangement with an interelectrode gap of 1 cm. The anode and cathode chambers, each with a volume of 1 cm3, were separated by a Nafion membrane. TCE containing solutions were fed through the cell using a liquid chromatography pump via 0.16 cm diameter Teflon tubing. Product Analyses. Rates of TCE dechlorination were determined from rates of product species generation in the RDE reactor. Chloride and acetate concentrations were determined by triplicate analyses of 5 mL aqueous samples using ion chromatography. Liquid phase volatile reaction products were determined from 25 µL aqueous samples extracted into 1 g of pentane and analyzed using a gas chromatograph (GC) equipped with electron capture and flame ionization detectors. In the flow cell experiments, headspace samples were also taken and analyzed by GC. Quantum Mechanical Simulations. Density functional theory (DFT) simulations were performed to calculate the activation barriers for different possible TCE reaction mechanisms. DFT calculations were performed using the DMol3 (36, 37) package in the Accelrys Materials Studio (38) modeling suite using a personal computer. All simulations used doublenumeric with polarization (DNP) basis sets (39) and the gradient corrected Becke-Lee-Yang-Parr (BLYP) (40, 41) functional for exchange and correlation. The nuclei and core electrons were described by DFT optimized semilocal pseudopotentials (42). Implicit solvation was incorporated into all simulations using the COSMO-ibs (43) model. Implicit solvation incorporates the electrostatic effects of solvation and the energy associated with solute induced cavity formation in the solvent. Activation energies for reduction of TCE via an outersphere electron transfer mechanism were calculated using the methods described in Anderson and Kang (18). Activation barriers for TCE reaction at different functional groups on a diamond cluster were calculated to investigate surface catalyzed TCE reduction. A diamond cluster consisting of 10 carbon atoms was used to simulate a reactive site on the BDD surface. Functional groups were added to the cluster by replacing surface terminating hydrogen atoms with oxygen atoms, or by removing hydrogen atoms without replacement. The four functional groups investigated were: (i) a hydrogenterminated tertiary carbon atom (tCsH); (ii) a carbonyl group on a secondary carbon atom (dCdO); (iii) a tertiary carbon radical (tC•); and (iv) a deprotonated hydroxyl group on a tertiary carbon atom (tCsO-). In addition to providing the reaction site, the functional groups in the simulated

FIGURE 1. TCE reaction rates (•) on a 1 cm2 BDD disk cathode operated at a current density of 15 mA/cm2 and an electrode potential of -2.20 V/SHE at 22 °C. Also shown are the Faradaic current efficiencies (0) for TCE conversion to acetate. cluster also affect the potential at which the reaction occurs. The effective electrode potential for each cluster on the SHE scale is equal to the ionization potential (IP) of the cluster minus 4.6 eV in order to convert the vacuum potential scale to the SHE scale (18). Simulation of reactions at the four functional groups were performed by placing a TCE molecule approximately 10 Å away from the diamond cluster and performing an energy minimization. These energy minimums correspond to different physically adsorbed states on the diamond cluster. The second simulation for each site began by decreasing the distance between the TCE molecule and the diamond cluster by ∼50%, followed by an energy minimization. These simulations produced the final reaction products at each site. The transition states were determined by connecting the physically adsorbed stationary points with the final products using a quadratic synchronous transit (QST) method (44) and refined using an eigenvector following method (45). The energy minimized structures and transition states were verified by frequency calculations. Imaginary frequencies with wave numbers smaller than 20 cm-1 were considered numerical artifacts of the integration grid and convergence criteria (46, 47).

Results and Discussion RDE Experiments. Figure 1 shows TCE reaction rates and Faradaic current efficiencies as a function of concentration for a current density of 15 mA/cm2 and a potential of -2.20 V/SHE. At TCE concentrations below 0.75 mM, the near linear relationship between the reaction rate and concentration indicates that TCE reduction was first-order with respect to TCE concentration. At TCE concentrations greater than 1 mM, reaction rates were nearly independent of concentration, thereby exhibiting kinetics that approached zeroth-order with respect to TCE concentration. This type of behavior is typical for surface reactions whose rates become increasingly limited by the availability of reaction sites with increasing reactant concentrations (48). In the concentration range where the reaction rates were first-order in TCE concentration, rates of TCE reduction can be expressed in terms of pseudo-first-order rate constants normalized for the surface area and solution volume used in the experiments. These rate constants are a lumped parameter since they may depend on the rate of more than one step along the reaction pathway. The normalized rate constants can be compared to normalized pseudo-first-order rate constants measured for TCE reduction by other materials. For example, the reaction rate of 0.56 µmol/h measured at a TCE concentration of 0.75 mM translates into a surface area normalized pseudo-first-order rate constant of 6.9 × 10-6 cm/s. This value is more than 2 orders of magnitude greater than those measured for TCE reduction by an iron cathode (49), and 8-4000 times greater than those for TCE reduction by zerovalent iron filings (31). VOL. 42, NO. 24, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 2. Rate of TCE reduction (•) in µmol h-1 and current density (0) in mA cm-2 as a function of electrode potential (V/ SHE) for a TCE concentration of 7.5 mM at 22 °C. Also shown are the electron transfer coefficients (r b) and their 95% confidence intervals. As determined from the flow-cell experiments, reduction of TCE resulted in the stoichiometric production of acetate with no detectable intermediate products. The absence of detectable intermediates indicates that the reaction involves surface bound intermediates, or that the intermediate products are fast reacting. For example, TCE reduction at iron surfaces has been found to produce chloroacetylene as a rapidly reacting intermediate that may reduced to ethylene (50, 51) or hydrolyzed to acetate (52-54). Acetate production accounted for 94-102% of the TCE mass balance and 3 Cl- ions were released per TCE molecule reacted. TCE reduction was accompanied by a decline in solution pH values, with an average of 4 moles of H+ produced per mole of TCE reduction. The overall stoichiometry for this reaction can be written as cathode reaction: CHClCCl2 + 2H2O + 2e- f CH3COO- + 2 H+ + 3 Cl- (1) anode reaction: H2O f 1/2O2+2e-+2H+

(2)

The Faradaic current efficiency for TCE reduction can be calculated using the stoichiometry of 2 moles of electrons per mole of TCE. The current efficiencies in Figure 1 are much less than 1% and indicate that hydrogen evolution was the predominant cathode reaction. Although hydrogen evolution predominated, hydrogen bubbles did not form on the electrode surface because of the high rotation speed of the RDE. Figure 2 shows TCE reaction rates as a function of electrode potential at a concentration of 7.5 mM where the reaction rates were zeroth order in TCE concentration. The data in Figure 2 show that there are two apparent electron transfer coefficients (55) (R b) for TCE reduction. At potentials between -0.45 and -0.95 V/SHE, b R ) 0.017 ( 0.003. One possible explanation for this behavior may be a change in the rate-limiting step at potentials more negative than -0.95 V/SHE. Another possible explanation is that the number of active sites for TCE reduction increases with decreasing electrode potentials between -0.45 and -0.95 V/SHE. This would result in an artificially high b R in this potential range, because decreasing potentials not only increase the reaction rate at each site but also increase the number of reactive sites. This explanation is consistent with results from previous investigators reporting that oxygenated functional groups may be reduced (e.g., reduction of carbonyl to carboxyl groups) by cathodic polarization (7, 15, 17). The apparent activation energy for TCE reduction was determined at a potential of -1.25 V/SHE by measuring TCE reaction rates at 2, 12, 22, and 32 °C at a concentration of 3 mM. Under these conditions, TCE reaction rates were found 9346

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FIGURE 3. (a) Energy profiles as a function of the C-Cl bond length at an electrode potential of -1.25 V/SHE for the reactants (Cl2CCHCl + e-) and products (Cl2CCH-Cl-) for vertical electron transfer. Zero of the energy scale represents the energy of the reactants at an electrode potential of 0 V/ SHE. (b) Activation energies as a function of electrode potential for direct oxidation based on the calculations in part (a). to be zeroth order in TCE concentration. This ensured that the reactive sites for TCE reduction were all saturated, and thus the experiments at different temperatures were all performed at the same surface concentration of TCE. Constant TCE concentrations at the reactive sites are required to make the measured activation energy as close to the intrinsic activation energy as possible. An Eyring plot of the reaction rates in µmol/h versus T-1 (K-1) yields an apparent activation energy of 22.1 ( 1.8 kJ/mol. This measured activation energy can be used to assess possible reaction mechanisms by comparing it to results from DFT simulations. DFT Simulations. TCE reduction by BDD electrodes may occur via an outer-sphere or inner-sphere reaction mechanism (55). Outer-sphere electron transfer reactions involve direct electron transfer, and no bond breaking or atomic rearrangements accompany the electron transfer step. In contrast, inner-sphere electron transfer involves bond breaking and/or atomic rearrangements. The activation energies for direct reduction of TCE by an outer-sphere mechanism were calculated as a function of the electrode potential using the methods described in Anderson and Kang (18). DFT simulations indicate that the addition of one electron to TCE leads to scission of the C-Cl bond on the carbon atom containing only one Cl atom. Therefore, stretching of this C-Cl bond can be taken as the reaction coordinate for direct TCE reduction. Figure 3a shows the energy of the reactant and products for the reaction Cl2CCHCl + e- f Cl2CCH•+Cl-

(3)

as a function of the C-Cl bond length at a potential of -1.25 V/SHE. The reactant energies were calculated by varying the length of the C-Cl bond from its minimum energy length of 1.734 Å, followed by geometry optimization of the structure. The product energies were calculated using the atomic

FIGURE 4. (a) Initial structure, (b) transition state, and (c) final product for TCE reaction at the tCsO- site at an effective electrode potential of -0.55 V/SHE. Atom key: C, gray; Cl, green; H, white; and O, red.

FIGURE 5. (a) Initial structure, (b) transition state, and (c) final product for TCE reaction at the dCdO site at an effective electrode potential of -0.55 V/SHE. C, gray; Cl, green; H, white; and O, red.

FIGURE 6. (a) Initial structure, (b) transition state, and (c) final product for TCE reaction at the tC• site at an effective electrode potential of -1.07 V/SHE. C, gray; Cl, green; H, white; and O, red. positions determined from the optimized reactant structures, followed by self-consistent field optimization of the electronic configurations. The product energies, therefore, represent those for vertical electron transfer, which is an outer-sphere mechanism based on the Born-Oppenheimer approximation that changes in electronic configuration happen much faster than changes in atomic configuration (56). Reactant energies as a function of electrode potential were determined by shifting the energy profile of the reactant species upward by 96.5 kJ (i.e., 1.0 eV) to decrease the electrode potential by 1.0 V, and downward by 96.5 kJ to increase the electrode potential by 1.0 V (18). Intersection of the product and reactant energy profiles yields the bond length of transition state and the activation energy for the reaction, as illustrated in Figure 3a. The more negative the electrode potential, the shorter the C-Cl bond stretching required for the reactant and product energy profiles to intersect. By shifting the reactants energy profile up and down, activation energies as a function of electrode potential were calculated, as shown in Figure 3b.

Figure 3b shows that the activation energy for direct electron transfer at a potential of -1.25 V/SHE is 93.4 kJ/ mol. This value is much greater than the experimentally measured value of 22.1 kJ/mol at -1.25 V/SHE. The fact that the activation barrier calculated for direct electron transfer is more than four times greater than the experimentally measured value indicates that TCE reduction at a potential of -1.25 V/SHE does not involve direct electron transfer. The absence of the diamond surface in the calculations should not affect this conclusion, because physical adsorption of TCE on a noncatalytic site on the diamond surface would most likely increase the activation barrier for direct electron transfer. This arises from the effect of solvation on the anion radical produced by the vertical electron transfer step. A previous DFT study on direct reduction of carbon tetrachloride showed that solvation lowers the energy of the product anion much more than it lowers the energy of the uncharged reactant molecule (57). Because adsorption VOL. 42, NO. 24, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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decreases interactions with the solvent, physical adsorption should lead to increased activation barriers for direct electron transfer. The data in Figure 3b also support the conclusion that outer-sphere electron transfer is not the rate-limiting step for TCE reduction at other potentials. According to the Butler-Volmer equation, activation energies for outer-sphere electron transfer reactions should decline with decreasing electrode potential (E), according to (55) f Ea ) Eeq a + RF(E - Eeq)

(4)

where Eeq a is the activation energy at the equilibrium potential

and F is the Faraday constant. The b R values calculated from the slope of the data in Figure 3b range from 0.8 to 0.39 over the potential range investigated in Figure 2. Values in this range are much larger than the experimentally measured b R values of 0.10 and 0.017, which are more characteristic of inner-sphere electron transfer reactions, whose rates are independent of (or only weakly dependent on) the electrode potential (55). DFT calculations were also performed for reaction mechanisms involving catalysis by different functional groups on the diamond cluster. TCE did not react or form a chemical adsorption complex at the ≡C-H site, but did form chemically adsorbed structures at the deprotonated hydroxyl (tCsO-), carbonyl (dCdO), and carbon radical (tC•) sites. Figure 4 illustrates the initial structure, the transition state, and the final product for TCE reaction at the tCsO- site. The IP of the isolated, solvated diamond cluster of 4.05 eV yields an effective electrode potential of -0.55 V/SHE. Complex formation of TCE with the diamond cluster resulted in the loss of one chloride ion and the formation of a bond between a carbon atom in TCE and an oxygen atom on the cluster surface. The overall reaction energy was -42 kJ/mol with an activation barrier of 24 kJ/mol, which is close to the experimentally measured value of 22.1 kJ/mol. Figure 5 illustrates the initial structure, transition state, and final product for TCE reaction at the dCdO site at a potential of -0.55 V/SHE. The overall reaction energy at this site was +54 kJ/mol with an activation barrier of 151 kJ/mol. The positive overall reaction energy and the high activation barrier indicate that complex formation at the dCdO should not measurably contribute to TCE reduction. The initial structure, transition state, and final product for TCE reaction at the tC• site at a potential of -1.07 V/SHE is shown in Figure 6. The overall reaction energy at this site was -222 kJ/mol with an activation barrier of 113 kJ/mol. Although the overall reaction energy is thermodynamically favorable, the activation barrier is much higher than at the tCsOsite, and therefore, reactions at the tC• site should not measurably contribute to TCE reduction at room temperature at a potential of -1.07 V/SHE. Comparison with Other Systems. The main pathway for TCE reduction at iron surfaces produces primarily ethene and ethane with only trace quantities of chlorinated intermediates (28-31). In contrast, the primary pathway for TCE reduction by graphite electrodes produces dichloroethylene isomers and vinyl chloride as intermediates between TCE and ethene, ethane or acetylene (34). It is not surprising that the products of TCE reduction at BDD electrodes are different from those at iron electrodes. However, given that diamond and graphite surfaces are both composed of carbon atoms, terminated with oxygen or hydrogen atoms, the difference in TCE reaction products at BDD and graphite electrodes is unexpected. One possible reason for this difference is that BDD surfaces are composed of sp3 hybridized C atoms bound to either two or three other C atoms, whereas graphite surfaces contain sp2-hybridized C atoms, bound to either one or two other C atoms. As illustrated in Figure 5, only the sp3 hybridized C atoms bound to three other C atoms give 9348

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rise to the tCsO- sites. As indicated by the DFT modeling, only these sites had a sufficiently low activation barrier to produce appreciable reaction rates at room temperature. Therefore, since graphite does not contain tertiary carbon atoms, TCE reduction on graphite surfaces does not occur at tCsO- sites and produce acetate. Differences in O2 reduction mechanisms at sp2- and sp3-hybridized carbon atoms on BDD electrodes with graphitic impurities have been previously reported (58). This suggests that specific functional groups on BDD electrodes play an important role in mediating reduction of organic and inorganic compounds.

Acknowledgments Thanks to the National Science Foundation Chemical and Transport Systems Directorate (CTS-0522790) and the Donors of the American Chemical Society Petroleum Research Fund (PRF 43535-AC5) for support of this work.

Supporting Information Available Eyring plot of the TCE reaction rates at an electrode potential of -1.25 V/SHE (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.

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