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Electrocatalytic reduction of chlorophenoxy acids at palladium-modified glassy carbon electrodes
Electrochimica Acta 52 (2007) 7028–7034
Electrocatalytic reduction of chlorophenoxy acids at palladium-modified glassy carbon electrodes Innocenzo G. Casella ∗ , Michela Contursi Dipartimento di Chimica dell’Universit`a, Via N. Sauro 85, Potenza 85100, Italy Received 2 February 2007; received in revised form 10 May 2007; accepted 12 May 2007 Available online 18 May 2007
Abstract Palladium species can be immobilized on a glassy carbon electrode by voltage cycling between 0.0 and −0.4 V versus SCE in solutions containing 0.5 mM Na2 PdCl6 in order to facilitate the electrocatalytic reduction of chlorophenoxycarboxylic acids in solutions buffered at pH 7. Cyclic voltammetry, measurements at the rotating disc electrode (RDE) and chronoamperometric techniques were used in order to investigate the electrochemical behaviour of the modified electrodes (GC/Pd) towards the catalytic reduction of chlorophenoxycarboxylic acids. A reaction mechanism is proposed and discussed. A probable scheme for the electroreduction of chlorophenoxycarboxylic species in neutral medium involves a simultaneous and competitive adsorption of the organic molecules and hydrogen atoms on the catalytic sites, followed by an irreversible hydrodechlorination reaction. 2,4-Chlorophenoxyacetic acid can be dehalogenated to a chlorine-free product in neutral aqueous solutions at relatively low cathodic polarizations and at ambient temperature using a GC/Pd electrode. © 2007 Elsevier Ltd. All rights reserved. Keywords: Chlorophenoxy acids; Reduction; Palladium; Electrodes
1. Introduction Chlorophenoxy acids are widely employed as agricultural pesticides to control the growth of broad leaf weeds and other vegetable products. Because of the free acid character and the salt form, these substances are sufficiently soluble in water and consequently may leach down into ground water and later contaminate drinking water. Due to their high toxicity and low biodegradation properties, the presence and accumulation of these substances in aquatic systems and biological organisms represents a severe toxicological risk [1,2]. The evidence of the toxicity of chlorophenoxy acids demands the development of selective and sensitive analytical methods for their trace determination in the aquatic environment. Analytical procedures for the determination of chlorophenoxy acids are generally based on gas chromatography in combination with mass spectrometry [3,4] or selective detectors based on electron-capture methods [5]. Nevertheless, the determination of these compounds by gas chromatography is
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Corresponding author. E-mail address: [email protected] (I.G. Casella).
0013-4686/$ – see front matter © 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2007.05.025
complicated due to their low volatility caused by hydrogen bonding and their polar nature. Thus, masking of these polar groups by derivatization to their corresponding esters in needed to yield products, which are volatile. In contrast to gas chromatography, high-performance liquid chromatography with UV–vis or mass spectrometry detectors have the advantage of being suitable for thermally labile and polar phenoxy acids as well, and has been employed for their quantitative determinations [6–8]. In this context, liquid chromatography in conjunction with electrochemical detection was successfully adopted for the analytical determination of traces of chlorophenoxy acid herbicides [9]. Nevertheless, the analytical performance of the electrochemical detection methods is greatly influenced by the particular electrode material and its morphological properties. Thus, one of the dominant themes in electrochemical research has been the attempt to regulate the chemical composition and morphological structure of the electrode surface in order to obtain a desired degree of electrocatalytic activity, selectivity and physical stability in various technological applications, such as electroanalysis and electrosynthesis fields. On the other hand, the low biodegradability of chlorophenoxy acid herbicides led to the development of environmentally clean redox methods capable of destroying such compounds
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in aqueous medium to avoid their dangerous accumulation in aquatic and biological environments. In this respect, the electrocatalytic reduction or oxidation can be advantageously applied to the extensive treatment of industrial and/or biological matrices to eliminate these pollutants. Thus, the search for a suitable catalyst material is of high importance in technological applications. In this respect, it has been discovered that metal porphyrins can be used for electrocatalytic dehalogenation of organohalides with a significant increase of the reduction currents and a shift of potentials towards less negative values with respect to untreated traditional electrodes [10–12]. In addition, electrochemical reductive dehalogenation of chlorinated organic compounds on palladium-loaded cathode materials in organic and aqueous matrices has been extensively studied [13–19]. However, the specific voltammetric and chronoamperometric behaviour of the palladium-coated cathode towards chlorophenoxy acids has not yet been investigated. Recently, in our laboratory we have electrodeposited palladium films on traditional electrode substrates from acid solutions and characterized these modified electrodes as sensing probes toward the oxidation or reduction of several electroactive molecules [20,21]. As a consequence, in this work, we aim to investigate the electrochemical properties of a glassy carbon electrode modified with an electrodeposited palladium film for the electroreduction of chlorophenoxy acids in water solutions buffered by 40 mM NaH2 PO4 /Na2 HPO4 to pH 7. In particular, considering the good solubility in aqueous solutions, the electrocatalytic properties of the GC/Pd electrode were investigated using 2,4-dichlorophenpoxyacetic acid as a model compound.
meter with a combined glass pH electrode model 91-02 was used for all pH measurements. Current densitiy values (mA cm−2 ) are quoted with respect to the apparent geometric area of the electrode substrate. Unless otherwise specified, voltammetric experiments were performed in unstirred solutions and were carried out at ambient temperature (21 ± 2 ◦ C). When necessary, the solutions were deoxygenated by bubbling with nitrogen prior to the electrochemical experiments. For the bulk electrolysis experiments, the anolyte solution and reference electrode (SCE) were separated from the catholyte solution by means of a Luggin capillary containing 40 mM NaH2 PO4 /Na2 HPO4 . During electrolysis, aliquots of catholyte were periodically withdrawn from the cell and analyzed to measure the amounts of chloride ions formed at different electrolysis times by an ion chromatographic technique (IC). All chromatographic analyses were performed using a metalfree pump Mod. PU-1580i (Jasco, Tokio, Japan) equipped with a rotary injection valve Mod. 7125i (Rheodyne, Cotati, CA, USA), a 20 L sample loop and an ED40 conductance detector (Dionex, Sunnyvale, CA, USA). The separations were carried out with a Dionex AS11 column (250 mm × 4 mm) using a 5 mM Na2 CO3 solution as mobile phase. Background conductivity was suppressed with an ASRS Ultra II 4 mm suppressor (Dionex) operating under external water recycle (2 mL min−1 ) and a current of 100 mA. Typical ion chromatographic separations were carried out at a flow rate of 0.8 mL min−1 and the chloride ions were eluted after about 7.8 min.
2. Experimental
The surface modification of the glassy carbon electrodes was performed by voltage cycling (50 mV s−1 ) between 0.0 and −0.4 V versus SCE for 35 cycles in deaerated solution containing 0.5 mM Na2 PdCl6 and 25 mM HCl. The resulting modified GC/Pd electrodes were then rinsed with pure water and transferred to an electrochemical cell containing the supporting electrolyte. Traces of palladium oxides were removed from the glassy carbon substrate by polishing the electrode surface with 0.05 m ␣-alumina powder on a polishing micro cloth and by washing with twice-distilled water. The thickness of the deposited film on the glassy carbon electrode was indirectly evaluated by electrochemical analysis. Assuming that all the surface palladium redox sites are electroactive on the voltammetric time scale, the surface concentration of the deposited palladium species was determined by oxidizing the palladium species at 1.3 V for 30 s in 50 mM H2 SO4 , and then determined the charge under the reduction peak when the potential was switched in the cathodic direction at 10 mV s−1 [13]. Thus, electrodeposited films of about 0.3–0.6 g cm−2 of palladium surface concentration were obtained and assuming that the density of the films is about 12 g cm−3 [22], a film thickness of about 25–50 nm was estimated.
2.1. Chemicals All selected chlorophenoxy acids were of analytical-reagent grade and were purchased from Sigma–Aldrich (Steinheim, Germany). Solutions were prepared daily from analytical-reagent grade chemicals (Aldrich-Chemie) without further purification and by using ultrapure water supplied by a Milli-Q RG unit from Millipore (Bedford, MA, USA). Unless otherwise specified, experiments were performed in buffered solutions (pH 7) containing 40 mM NaH2 PO4 /Na2 HPO4 . 2.2. Apparatus The voltammetric experiments were performed with an Autolab PGSTAT 30 Potentiostat/Galvanostat (Eco Chemie, Utrecht, The Netherlands) and the data were acquired using an Autolab GPES software package version 4.8. Cyclic voltammetry (CV) was done in a three-electrode cell using a glassy carbon electrode modified with a deposited Pd film as a working electrode, a SCE reference electrode and a platinum foil counter electrode. The electrode substrate (3 mm diameter) used in CV was purchased from Amel (Milan, Italy). Rotating disk electrode (RDE) measurements were performed using a model EDI101 rotator from Radiometer (Copenhagen) connected with a speed control unit (CTV101, Radiometer). A Thermo Orion model 420 pH
2.3. Electrode modification
3. Results and discussion Fig. 1 shows steady-state cyclic voltammograms obtained with a GC/Pd electrode in buffered solution at pH 7 with differ-
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2,4-DA 2,4-DP 2,4-DB 2,4,5-TA
Chargea (mC mM−1 )
Ep Ia (mV mM−1 )
Sensitivityb (A mM−1 )
2.1 2.1 2.7 2.6
55 62 104 91
25 28 30 39
a The charges were evaluated by integration of the reduction wave during the cathodic sweep between −0.3 and 1.0 V vs. SCE (50 mV s−1 ) in buffered phosphate solution containing the analyte in the range of concentration comprised between 0.0 and 1.0 mM. b The sensitivities were evaluated by measuring the relevant currents at −0.8 V vs. SCE during the cathodic sweep.
Fig. 1. Cyclic voltammograms (5th cycle) at GC/Pd electrode in buffered solutions at pH 7 (20 mM NaH2 PO4 plus 20 mM Na2 HPO4 ). Curves: dotted, buffered solutions; dashed, buffered solutions containing 2.8 mM 2,4-DA; solid, buffered solutions containing 6.0 mM 2,4-DA. Scan rate, 50 mV s−1 .
ent amounts of 2,4-dichlorophenoxyacetic acid (2,4-DA). As can be seen, a complex reduction wave between −0.5 and −1.1 V during the cathodic sweep is observed, and the relevant currents increase with increasing 2,4-DA concentration. This reduction wave is generally composed of two ill-defined peaks, IIc1 and IIc2 and the relevant peak potentials shift negatively in the presence of increasing concentration of 2,4-DA. The relevant reduction currents, measured at −1.0 V versus SCE, increase linearly with increasing analyte concentration up to about 6 mM. During the positive potential scan, the major voltammetric signal includes a well-resolved peak (Ia ) at about −0.25 V associated with the electrodesorption/desorbtion processes of adsorbed hydrogen atoms formed during the previous negative potential scan [23]. Under cathodic conditions, hydrogen atoms are strongly adsorbed on the Pd surface and/or can be absorbed to a large extent into the Pd lattice [24] with formation of either ␣-PdH and -PdH phase [25]. In the presence of 2,4-DA, the intensity of the anodic peak Ia decreases and the peak potential shifts markedly to lower potentials with increasing analyte concentration. The presence of 2,4-DA has no effect on either palladium oxide formation or its relevant reformation of Pd0 species. In fact, the wave associated with the oxygen evolution process and the cathodic peak Ic , attributed to the reduction of palladium oxide species, appear to be rather independent of the 2,4-DA concentration. Similar current–potential profiles were observed in the hydrogen region for other compounds, such as 2-(2,4-dichlorophenoxy)propionic acid (2,4-DP), 4-(2,4-dichlorophenoxy)butyric acid (2,4-DB) and 2,4,5-trichlorophenoxy acetic acid (2,4,5-TA). Nevertheless, there are some differences in terms of the magnitude of the electroreduction currents and variations of the peak potential Ia with increasing analyte concentration. Table 1 summarizes the relevant results. As can be seen, the molar sensitivity (A mM−1 ) decreases in the order 2,4,5-TA > 2,4-DB > 2,4DP > 2,4-DA. Similarly, the oxidation wave Ia is shifted toward
negative potentials (Ep Ia ) and decreased in peak height (Ip Ia ) in the same order of the investigated molecules. In particular, in the presence of 2,4,5-TA, a marked inhibition of the palladium oxide formation, oxygen evolution and reformation of Pd0 species (Ic ) was observed. These facts indicate that the intensity of surface adsorption of chlorophenoxylic acids on the palladium catalytic sites is directly related to the number of chloride atoms per molecule and molecular size of analytes. If log(IIc1 ) and log(IIc2 ) are plotted versus log(C) for the electroreduction of 2,4-DA, two straight lines are found in the concentration range between 1.5 and 12.0 mM. The best-fit lines for graphs relevant to peaks IIc1 and IIc2 show slopes of 0.55 and 0.52, respectively, with correlation coefficients >0.978 (six experimental points in the graphs). The fractional reaction order obtained for the electroreduction of 2,4-DA supports the hypothesis that the reduction reaction involves a kinetic control on the overall electrochemical process. Fig. 2 shows a series of linear sweep voltammograms (I–E curves) obtained at various pH values between 2.5 and 10.3. As can be seen, while the peak potentials for IIc1 and IIc2 , related to the reduction process of 2,4-DA, are shifted to more cathodic values with increasing pH values, the electroreduction currents are nearly independent of pH. The slopes of plots of Ep versus pH for waves IIc1 and IIc2 were 76 ± 5 and 58 ± 7 mV pH−1 unit, respectively (correlation coefficients >0.982). This result indicates that the reduction of chlorophenoxycarboxylic acids involves approximately a 1:1 ratio of protons to electrons, as predicted for the hydrodechlorination processes over palladiumloaded cathode materials [14,15,18]. Table 2 summarizes the influence of pH on the charge and peak potentials associated with the reduction of 2,4-DA. As observed, in contrast with the electrochemical behaviour for reaction processes involving proton exchange, the charges evaluated under the reduction waves were found to be rather independent of pH. This behaviour suggests that the electroreduction of chlorophenoxycarboxylic acids on GC/Pd electrodes involves a preliminary slow competitive electroadsorption/absorption of hydrogen atoms and analyte with a subsequent concerted reaction between the relevant adsorbed species. In addition, the fact that hydrogen diffuses into bulk palladium during the electroadsorption process and re-diffuses out of the Pd lattice at a finite rate [25] may be considered a reason for
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Table 2 Effect of pH on the reduction charge and peak potentials of 2,4-DA evaluated in cyclic voltammetry on the GC/Pd electrode pH
Reduction charge (mC)
IIc1 (V)
IIc2 (V)
2.8 3.2 3.6 5.3 6.0 6.6 7.0 7.5 8.0 9.0 10.0
0.51 0.46 0.41 0.50 0.44 0.52 0.46 0.42 0.47 0.39 0.40
−0.58 −0.77 −0.82 −0.86 −0.91 −0.93 −1.0 −1.0 – – –
– – – −0.57 −0.56 −0.63 −0.67 −0.72 −0.83 −0.86 −0.87
The charge was evaluated during the negative sweep of the potential (5th cycle, 50 mV s−1 ) in phosphate solution containing 1.5 mM 2,4-DA by integration of the reduction wave between the lower potential limit (−1.2 V vs. SCE) and the potential just after the Ic1 wave. The net charge was obtained by difference between the charges evaluated in the presence and absence of 2,4-DA. Experimental conditions as in Fig. 2.
palladium catalyst with the relatively low negative polarizations and background currents related to hydrogen evolution, buffered phosphate solutions at pH 7 were chosen for this electrochemical investigation. To ascertain the electrode kinetics of the reduction of chlorophenoxycarboxylic acids on GC/Pd, measurements at a rotating disc electrode were performed with different rotation speeds (ω) at various concentrations of 2,4-DA. Fig. 3 shows the relevant voltammograms. As can be seen, the complex reduction wave composed of peaks IIc1 and IIc2 increases with increasing rotation speeds, whereas the opposite behaviour for the electrodesorption processes of hydrogen atoms (peak Ia ) was observed. In addition, the peak potential IIc2 is shifted towards more negative values with increasing rotation speed, whereas peak Ic remains essentially unchanged. The marked lowering of peak Ia with rotation speed can be related to two concomitant factors: (i) if the electrochemical reduction of
Fig. 2. I–E curves (5th sweep) obtained in phosphate solutions containing 2.5 mM 2,4-DA at various pH. The pH was adjusted to the desired value by addition of H3 PO4 or NaOH. Other conditions as in Fig. 1.
the unexpected low effects of pH on the reduction charge during the cathodic sweeps. Thus, using a GC/Pd electrode as a cathode, the catalytic reduction of chlorophenoxycarboxylic acids can be nicely conducted over a wide range of pH values. Nevertheless, to combine the good electrochemical activity of the
Fig. 3. Cyclic voltammograms (5th cycle) at a GC/Pd rotating disk electrode in deoxygenated and buffered solutions at pH 7 containing 1.5 mM 2,4-DA at different rotation rates. Other condition as in Fig. 1.
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Fig. 4. Koutecky–Levich plots for the electrocatalytic reduction of 2,4-DA. Experimental conditions as in Fig. 3.
chlorophenoxycarboxylic acids is assisted by adsorbed hydrogen atoms previously generated onto the electrode surface, the increase in the reduction reaction of the organic molecule causes a decrease of the surface concentration of adsorbed hydrogen; (ii) a rapid physical desorption of hydrogen atoms from the Pd0 sites before their electrodesorption reaction causes the decrease. At increased rotation speeds, plots of the peak IIc2 current versus ω1/2 diverged significantly from straight lines intersecting the origin (not shown here). This suggests that some surface kinetic steps are involved in the overall reduction process. Fig. 4 shows the RDE results in terms of a Koutecky–Levich plot. As can be seen, the intercepts are positive and the slopes are inversely proportional to the concentration of 2,4-DA. This result indicates a surface kinetic limitation of the overall reduction process, excluding charge diffusion within the electroactive film. Thus, in the frame work of the Sav´ean–Andrieux model [26], the reduction currents can be limited by diffusion of reactants within the palladium film (i.e., diffusion of hydrogen atoms into/out of the Pd lattice) and/or by cross-exchange reactions involving, in particular, a preliminary adsorption of the organic molecules on the palladium catalytic sites and a subsequent sequence of hydrogenation/dechlorination reactions. It is interesting to observe the effect of changing the upper potential limit on the kinetics of the reduction of chlorophenoxycarboxylic acids. The influence of the upper applied potential on the reduction currents for 6.0 mM 2,4-DA is shown in Fig. 5. As can be seen, when the higher potential limit was decreased, the reduction wave was noticeably smaller. In particular, peak IIc1 disappeared completely for anodic potential limits lower than 0.2 V. On the contrary, the charge under peak Ia remains rather unchanged when the higher potential limits were shifted negatively, indicating that the kinetics of the electrodesorption/adsorption–absorption processes of hydrogen atoms are not influenced by the formation of palladium oxide species. To define the causes regarding the direct dependence of the electroreduction efficiencies with higher potential limits, various factors can be considered: (i) the reactant species (i.e.,
Fig. 5. Influence of the upper potential limit on the reduction currents at the GC/Pd electrode in buffered solution containing 6.0 mM 2,4-DA.
chlorophenoxy acids) were preliminarily adsorbed on the palladium oxide sites and reacted with activated hydrogen atoms on the catalyst surface; (ii) there is a change in spatial distribution of the supported palladium clusters with a subsequent increase in interfacial area where the reduction reaction proceeds; (iii) there is a favorable desorption process of reaction products and/or reaction intermediate during the anodic sweep. Although a competitive adsorption process of reactants on the palladium oxide species and/or a change in roughness factor of the electrode surface layer induced during the anodic excursion of the potentials cannot be excluded, a probable explanation could be associated with an efficient desorption process of fouling products from the electrode surface. We imply that reduction products strongly adsorbed on the catalytic sites gradually poison the catalyst because the new chlorophenoxycarboxyl molecules are unable to displace the adsorbed reaction products and/or reaction intermediate in the cathodic range of potentials. Taking into account our experimental results and in agreement with literature [12,14,15,17,27], a scheme for the electroreduction of the chlorophenoxy acid species on the GC/Pd electrode can be hypothesized:
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where Pd(H)ads , Pd(H)abs , Pd(2,4-DA)ads , Pd(2-CA)ads and Pd(PA)ads refer to adsorbed species, such as hydrogen atoms, 2,4-DA, 2-chlorophenoxy acid (2-PA) and phenoxyacid (PA) species, respectively. The Pd(OH)n species represent the oxidized palladium species formed during the anodic sweep of the potentials. As can be seen, the mechanism of the electrocatalytic hydrogenation of organic molecules involves, in the first instance, the direct interaction between adsorbed/chemisorbed hydrogen atoms and adsorbed organic molecules. The organic molecule may occupy more than one site of the catalyst and its adsorption energy depends strongly on the nature of the metal and state of the surface (i.e., roughness, porous surface, degree of surface dispersion, etc.) and on the nature of organic reactants [28]. It is important to point out that the efficiency of the dechlorination process of chlorophenoxy species depends on the relative rates of the hydrogenation step and of hydrogen desorption process. If the hydrogenation step is too slow with respect to hydrogen desorption, only hydrogen evolution occurs. In addition, the proposed reaction mechanism suggests that the formation of palladium oxide species (Pd(OH)n ) induces an efficient desorption of reaction products (and/or intermediates), thus avoiding irreversible fouling of the catalytic sites. Considering the presence of a functional acid group attached to an aromatic ring, it may inhibit 2-chlorine elimination due to the steric hindrance; in agreement with literature [12,14], the dechlorination process very probably proceeds through a 4-chlorine cleavage followed by 2-chlorine elimination. Thus, the reduction reaction proceeds via a probable multi-step process involving a competitive adsorption step of the organic molecules and hydrogen atoms, followed by an irreversible hydrodechlorination reaction. However, to define a more detailed mechanism and an accurate and unambiguous definition of the kinetic character of the reduction currents, additional experimental proofs are necessary. To verify the electrocatalytic capacity of the GC/Pd electrode and to validate the proposed reaction scheme, chronoamperometric measurements in deoxygenated and buffered solutions at pH 7 containing increasing concentrations of 2,4-DA were carried out. Fig. 6 shows a comparison of the relevant I–t curves obtained with a GC/Pd electrode under a constant applied potential of −0.8 V versus SCE and under pulsed chronoamperometric detection (PCD). The PCD experiments were carried out with a selected waveform of potentials of −0.8 V (E1 ) for 150 ms (t1 ) and +1.0 V (E2 ) for 100 ms (t2 ). Under PCD, the currents were sampled at the applied potential of −0.8 V (E1 ). As can be seen, the success of PCD is probably due to the multistep potential waveform which manages sequentially the detection step (E1 ) followed by a surface cleaning process at sufficiently positive potential (E2 ). The chronoamperometric signal obtained under constant applied potential (Fig. 6, curve A) appears almost unchanged as a function of 2,4-DA concentration. On the contrary, the formation of palladium oxides on the electrode surface induces a more efficient desorption process of reaction products and/or reaction intermediates, avoiding fouling effects of the catalyst. From Fig. 6 (curve C), it can be observed that, after the first addition of 1.0 mM 2,4-DA, the chronoamperometric response remains rather insensitive to subsequent addition of analyte. To
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Fig. 6. Chronoamperometric measurements obtained at a GC/Pd electrode in degassed solutions at pH 7: (A) constant applied potential of −0.8 V versus SCE; a–e represent the consecutive addition of 2,4-DA in 1.0 mM steps; (B) pulsed chronoamperometric detection (PCD) using a selected waveform of potentials of −0.8 V for 150 ms and +1.0 V for 100 ms; a–e represent the consecutive addition of 2,4-DA in 0.25 mM steps; (C) PCD measurement using the same waveform adopted in (B); a–e represent the consecutive addition of 2,4-DA in 1.0 mM steps. The solution was stirred with a magnetic bar at about 300 rpm.
a first approximation, this behaviour can be due to the strong adsorption phenomenon of reactants on the surface electrode. On the other hand, chronoamperometric measurements carried out using addition steps of 0.25 mM 2,4-DA show an I–t profile virtually identical to those observed using addition steps of 1.0 mM 2,4-DA (see curve B). This behaviour appears not completely justified by a simple adsorption process of reactant on the electrode surface, and the reason for the observed saturation phenomenon is not clear at the present time. Although a calibration plot for 2,4-DA analysis appears nonlinear under the present experimental conditions, the GC/Pd electrode responds rapidly to initial millimolar addition of 2,4-DA, and the high reduction current remains largely unchanged over long electrolysis times (i.e., 1200 s) without apparent undesired electrode poisoning effects (see Fig. 6, curves B and C). Therefore, the GC/Pd electrode used under PCD conditions shows a good catalytic efficiency in neutral aqueous solutions at relatively low negative polarizations without any apparent fouling effects towards the reduction of chlorophenoxycarboxylic acids. The conversion of 2,4-DA was evaluated as the total amount of chloride produced during the bulk electrolysis process and the current efficiency of the dechlorination reaction was calculated from the amount of chloride and the charge passed assuming that two electrons and two protons are required for the removal of one chloride ion. Fig. 7 shows the relevant results in terms of amount of chloride produced and current efficiency on the charge passed during the electrolysis process. As can be seen, during the initial 0.8 C of charge passed, corresponding to about 1.5 h of electrolysis time, the amount of chloride increases linearly with charge, while the current efficiency shows an opposite behaviour. For higher electrolysis times (i.e., charge >0.8 C), no appreciable additional chloride formation was observed and, consequently, the current efficiency decreased markedly. These observations
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and/or reaction intermediates avoiding irreversible fouling of the catalytic sites. Chronoamperometric experiments carried out under pulsed chronoamperometric conditions, using a selected waveform of potentials, show high and reproducible reduction currents indicating a good dechlorination efficiency of organochlorine pollutants in neutral aqueous solutions. Acknowledgements This work was supported by Ministero dell’Universita’ e della Ricerca Scientifica e Tecnologica (MURST, COFIN 2006). The anonymous reviewers are thanked for constructive criticism and appreciated comments. References Fig. 7. Chloride formation and the corresponding current efficiency of the 2,4-DA dechlorination at GC/Pd cathode on the total charge passed during the electrolysis process. Electrolysis conditions: −0.8 V (E1 ) for 150 ms (t1 ) and +1.0 V (E2 ) for 100 ms (t2 ); Solution: degassed (N2 ) 40 mM NaH2 PO4 /Na2 HPO4 solution plus 2.0 mM 2,4-DA. The solution was stirred with a magnetic bar at about 500 rpm. The chloride measurements were carried out by IC technique, under experimental conditions as reported in the Section 2.
indicate that reaction products, such as chloride species can be strongly adsorbed on the palladium catalytic sites and gradually poison the catalyst because new reactant molecules are unable to displace the adsorbed reaction products. Thus, as a consequence, for prolonged electrolysis times (i.e., >1.5 h), the current efficiency decreased sensibly. In order to increase the efficiency reduction for long times of electrolysis, the design of an electrolysis cell with a “flow-through” arrangement should be likely chosen for cell construction. Such a cell design should facilitate the effective transport of reagents to and reaction products from the surface cathode avoiding stagnant zones. 4. Conclusions We prepared a GC/Pd electrode for the electrocatalytic reduction of chlorophenoxycarboxyl acids in water solutions buffered at pH 7. The electrochemical behaviour of the palladium catalyst was investigated by the use of cyclic voltammetry and chronoamperometric techniques. A reaction mechanism of the chorophenxyacetic acids at the GC/Pd electrode is proposed and critically discussed. The reduction reaction proceeds via a probable multi-step process involving a competitive adsorption step of chlorophenoxycarboxyl species and hydrogen atoms on the catalytic sites, followed by irreversible hydrodechlorination reaction steps. The formation of palladium oxide species (Pd(OH)n ) during the positive potential sweep induces an efficient desorption process of surface reaction products
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Electrocatalytic reduction of chlorophenoxy acids at palladium-modified glassy carbon electrodes Innocenzo G. Casella ∗ , Michela Contursi Dipartimento di Chimica dell’Universit`a, Via N. Sauro 85, Potenza 85100, Italy Received 2 February 2007; received in revised form 10 May 2007; accepted 12 May 2007 Available online 18 May 2007
Abstract Palladium species can be immobilized on a glassy carbon electrode by voltage cycling between 0.0 and −0.4 V versus SCE in solutions containing 0.5 mM Na2 PdCl6 in order to facilitate the electrocatalytic reduction of chlorophenoxycarboxylic acids in solutions buffered at pH 7. Cyclic voltammetry, measurements at the rotating disc electrode (RDE) and chronoamperometric techniques were used in order to investigate the electrochemical behaviour of the modified electrodes (GC/Pd) towards the catalytic reduction of chlorophenoxycarboxylic acids. A reaction mechanism is proposed and discussed. A probable scheme for the electroreduction of chlorophenoxycarboxylic species in neutral medium involves a simultaneous and competitive adsorption of the organic molecules and hydrogen atoms on the catalytic sites, followed by an irreversible hydrodechlorination reaction. 2,4-Chlorophenoxyacetic acid can be dehalogenated to a chlorine-free product in neutral aqueous solutions at relatively low cathodic polarizations and at ambient temperature using a GC/Pd electrode. © 2007 Elsevier Ltd. All rights reserved. Keywords: Chlorophenoxy acids; Reduction; Palladium; Electrodes
1. Introduction Chlorophenoxy acids are widely employed as agricultural pesticides to control the growth of broad leaf weeds and other vegetable products. Because of the free acid character and the salt form, these substances are sufficiently soluble in water and consequently may leach down into ground water and later contaminate drinking water. Due to their high toxicity and low biodegradation properties, the presence and accumulation of these substances in aquatic systems and biological organisms represents a severe toxicological risk [1,2]. The evidence of the toxicity of chlorophenoxy acids demands the development of selective and sensitive analytical methods for their trace determination in the aquatic environment. Analytical procedures for the determination of chlorophenoxy acids are generally based on gas chromatography in combination with mass spectrometry [3,4] or selective detectors based on electron-capture methods [5]. Nevertheless, the determination of these compounds by gas chromatography is
∗
Corresponding author. E-mail address: [email protected] (I.G. Casella).
0013-4686/$ – see front matter © 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2007.05.025
complicated due to their low volatility caused by hydrogen bonding and their polar nature. Thus, masking of these polar groups by derivatization to their corresponding esters in needed to yield products, which are volatile. In contrast to gas chromatography, high-performance liquid chromatography with UV–vis or mass spectrometry detectors have the advantage of being suitable for thermally labile and polar phenoxy acids as well, and has been employed for their quantitative determinations [6–8]. In this context, liquid chromatography in conjunction with electrochemical detection was successfully adopted for the analytical determination of traces of chlorophenoxy acid herbicides [9]. Nevertheless, the analytical performance of the electrochemical detection methods is greatly influenced by the particular electrode material and its morphological properties. Thus, one of the dominant themes in electrochemical research has been the attempt to regulate the chemical composition and morphological structure of the electrode surface in order to obtain a desired degree of electrocatalytic activity, selectivity and physical stability in various technological applications, such as electroanalysis and electrosynthesis fields. On the other hand, the low biodegradability of chlorophenoxy acid herbicides led to the development of environmentally clean redox methods capable of destroying such compounds
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in aqueous medium to avoid their dangerous accumulation in aquatic and biological environments. In this respect, the electrocatalytic reduction or oxidation can be advantageously applied to the extensive treatment of industrial and/or biological matrices to eliminate these pollutants. Thus, the search for a suitable catalyst material is of high importance in technological applications. In this respect, it has been discovered that metal porphyrins can be used for electrocatalytic dehalogenation of organohalides with a significant increase of the reduction currents and a shift of potentials towards less negative values with respect to untreated traditional electrodes [10–12]. In addition, electrochemical reductive dehalogenation of chlorinated organic compounds on palladium-loaded cathode materials in organic and aqueous matrices has been extensively studied [13–19]. However, the specific voltammetric and chronoamperometric behaviour of the palladium-coated cathode towards chlorophenoxy acids has not yet been investigated. Recently, in our laboratory we have electrodeposited palladium films on traditional electrode substrates from acid solutions and characterized these modified electrodes as sensing probes toward the oxidation or reduction of several electroactive molecules [20,21]. As a consequence, in this work, we aim to investigate the electrochemical properties of a glassy carbon electrode modified with an electrodeposited palladium film for the electroreduction of chlorophenoxy acids in water solutions buffered by 40 mM NaH2 PO4 /Na2 HPO4 to pH 7. In particular, considering the good solubility in aqueous solutions, the electrocatalytic properties of the GC/Pd electrode were investigated using 2,4-dichlorophenpoxyacetic acid as a model compound.
meter with a combined glass pH electrode model 91-02 was used for all pH measurements. Current densitiy values (mA cm−2 ) are quoted with respect to the apparent geometric area of the electrode substrate. Unless otherwise specified, voltammetric experiments were performed in unstirred solutions and were carried out at ambient temperature (21 ± 2 ◦ C). When necessary, the solutions were deoxygenated by bubbling with nitrogen prior to the electrochemical experiments. For the bulk electrolysis experiments, the anolyte solution and reference electrode (SCE) were separated from the catholyte solution by means of a Luggin capillary containing 40 mM NaH2 PO4 /Na2 HPO4 . During electrolysis, aliquots of catholyte were periodically withdrawn from the cell and analyzed to measure the amounts of chloride ions formed at different electrolysis times by an ion chromatographic technique (IC). All chromatographic analyses were performed using a metalfree pump Mod. PU-1580i (Jasco, Tokio, Japan) equipped with a rotary injection valve Mod. 7125i (Rheodyne, Cotati, CA, USA), a 20 L sample loop and an ED40 conductance detector (Dionex, Sunnyvale, CA, USA). The separations were carried out with a Dionex AS11 column (250 mm × 4 mm) using a 5 mM Na2 CO3 solution as mobile phase. Background conductivity was suppressed with an ASRS Ultra II 4 mm suppressor (Dionex) operating under external water recycle (2 mL min−1 ) and a current of 100 mA. Typical ion chromatographic separations were carried out at a flow rate of 0.8 mL min−1 and the chloride ions were eluted after about 7.8 min.
2. Experimental
The surface modification of the glassy carbon electrodes was performed by voltage cycling (50 mV s−1 ) between 0.0 and −0.4 V versus SCE for 35 cycles in deaerated solution containing 0.5 mM Na2 PdCl6 and 25 mM HCl. The resulting modified GC/Pd electrodes were then rinsed with pure water and transferred to an electrochemical cell containing the supporting electrolyte. Traces of palladium oxides were removed from the glassy carbon substrate by polishing the electrode surface with 0.05 m ␣-alumina powder on a polishing micro cloth and by washing with twice-distilled water. The thickness of the deposited film on the glassy carbon electrode was indirectly evaluated by electrochemical analysis. Assuming that all the surface palladium redox sites are electroactive on the voltammetric time scale, the surface concentration of the deposited palladium species was determined by oxidizing the palladium species at 1.3 V for 30 s in 50 mM H2 SO4 , and then determined the charge under the reduction peak when the potential was switched in the cathodic direction at 10 mV s−1 [13]. Thus, electrodeposited films of about 0.3–0.6 g cm−2 of palladium surface concentration were obtained and assuming that the density of the films is about 12 g cm−3 [22], a film thickness of about 25–50 nm was estimated.
2.1. Chemicals All selected chlorophenoxy acids were of analytical-reagent grade and were purchased from Sigma–Aldrich (Steinheim, Germany). Solutions were prepared daily from analytical-reagent grade chemicals (Aldrich-Chemie) without further purification and by using ultrapure water supplied by a Milli-Q RG unit from Millipore (Bedford, MA, USA). Unless otherwise specified, experiments were performed in buffered solutions (pH 7) containing 40 mM NaH2 PO4 /Na2 HPO4 . 2.2. Apparatus The voltammetric experiments were performed with an Autolab PGSTAT 30 Potentiostat/Galvanostat (Eco Chemie, Utrecht, The Netherlands) and the data were acquired using an Autolab GPES software package version 4.8. Cyclic voltammetry (CV) was done in a three-electrode cell using a glassy carbon electrode modified with a deposited Pd film as a working electrode, a SCE reference electrode and a platinum foil counter electrode. The electrode substrate (3 mm diameter) used in CV was purchased from Amel (Milan, Italy). Rotating disk electrode (RDE) measurements were performed using a model EDI101 rotator from Radiometer (Copenhagen) connected with a speed control unit (CTV101, Radiometer). A Thermo Orion model 420 pH
2.3. Electrode modification
3. Results and discussion Fig. 1 shows steady-state cyclic voltammograms obtained with a GC/Pd electrode in buffered solution at pH 7 with differ-
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2,4-DA 2,4-DP 2,4-DB 2,4,5-TA
Chargea (mC mM−1 )
Ep Ia (mV mM−1 )
Sensitivityb (A mM−1 )
2.1 2.1 2.7 2.6
55 62 104 91
25 28 30 39
a The charges were evaluated by integration of the reduction wave during the cathodic sweep between −0.3 and 1.0 V vs. SCE (50 mV s−1 ) in buffered phosphate solution containing the analyte in the range of concentration comprised between 0.0 and 1.0 mM. b The sensitivities were evaluated by measuring the relevant currents at −0.8 V vs. SCE during the cathodic sweep.
Fig. 1. Cyclic voltammograms (5th cycle) at GC/Pd electrode in buffered solutions at pH 7 (20 mM NaH2 PO4 plus 20 mM Na2 HPO4 ). Curves: dotted, buffered solutions; dashed, buffered solutions containing 2.8 mM 2,4-DA; solid, buffered solutions containing 6.0 mM 2,4-DA. Scan rate, 50 mV s−1 .
ent amounts of 2,4-dichlorophenoxyacetic acid (2,4-DA). As can be seen, a complex reduction wave between −0.5 and −1.1 V during the cathodic sweep is observed, and the relevant currents increase with increasing 2,4-DA concentration. This reduction wave is generally composed of two ill-defined peaks, IIc1 and IIc2 and the relevant peak potentials shift negatively in the presence of increasing concentration of 2,4-DA. The relevant reduction currents, measured at −1.0 V versus SCE, increase linearly with increasing analyte concentration up to about 6 mM. During the positive potential scan, the major voltammetric signal includes a well-resolved peak (Ia ) at about −0.25 V associated with the electrodesorption/desorbtion processes of adsorbed hydrogen atoms formed during the previous negative potential scan [23]. Under cathodic conditions, hydrogen atoms are strongly adsorbed on the Pd surface and/or can be absorbed to a large extent into the Pd lattice [24] with formation of either ␣-PdH and -PdH phase [25]. In the presence of 2,4-DA, the intensity of the anodic peak Ia decreases and the peak potential shifts markedly to lower potentials with increasing analyte concentration. The presence of 2,4-DA has no effect on either palladium oxide formation or its relevant reformation of Pd0 species. In fact, the wave associated with the oxygen evolution process and the cathodic peak Ic , attributed to the reduction of palladium oxide species, appear to be rather independent of the 2,4-DA concentration. Similar current–potential profiles were observed in the hydrogen region for other compounds, such as 2-(2,4-dichlorophenoxy)propionic acid (2,4-DP), 4-(2,4-dichlorophenoxy)butyric acid (2,4-DB) and 2,4,5-trichlorophenoxy acetic acid (2,4,5-TA). Nevertheless, there are some differences in terms of the magnitude of the electroreduction currents and variations of the peak potential Ia with increasing analyte concentration. Table 1 summarizes the relevant results. As can be seen, the molar sensitivity (A mM−1 ) decreases in the order 2,4,5-TA > 2,4-DB > 2,4DP > 2,4-DA. Similarly, the oxidation wave Ia is shifted toward
negative potentials (Ep Ia ) and decreased in peak height (Ip Ia ) in the same order of the investigated molecules. In particular, in the presence of 2,4,5-TA, a marked inhibition of the palladium oxide formation, oxygen evolution and reformation of Pd0 species (Ic ) was observed. These facts indicate that the intensity of surface adsorption of chlorophenoxylic acids on the palladium catalytic sites is directly related to the number of chloride atoms per molecule and molecular size of analytes. If log(IIc1 ) and log(IIc2 ) are plotted versus log(C) for the electroreduction of 2,4-DA, two straight lines are found in the concentration range between 1.5 and 12.0 mM. The best-fit lines for graphs relevant to peaks IIc1 and IIc2 show slopes of 0.55 and 0.52, respectively, with correlation coefficients >0.978 (six experimental points in the graphs). The fractional reaction order obtained for the electroreduction of 2,4-DA supports the hypothesis that the reduction reaction involves a kinetic control on the overall electrochemical process. Fig. 2 shows a series of linear sweep voltammograms (I–E curves) obtained at various pH values between 2.5 and 10.3. As can be seen, while the peak potentials for IIc1 and IIc2 , related to the reduction process of 2,4-DA, are shifted to more cathodic values with increasing pH values, the electroreduction currents are nearly independent of pH. The slopes of plots of Ep versus pH for waves IIc1 and IIc2 were 76 ± 5 and 58 ± 7 mV pH−1 unit, respectively (correlation coefficients >0.982). This result indicates that the reduction of chlorophenoxycarboxylic acids involves approximately a 1:1 ratio of protons to electrons, as predicted for the hydrodechlorination processes over palladiumloaded cathode materials [14,15,18]. Table 2 summarizes the influence of pH on the charge and peak potentials associated with the reduction of 2,4-DA. As observed, in contrast with the electrochemical behaviour for reaction processes involving proton exchange, the charges evaluated under the reduction waves were found to be rather independent of pH. This behaviour suggests that the electroreduction of chlorophenoxycarboxylic acids on GC/Pd electrodes involves a preliminary slow competitive electroadsorption/absorption of hydrogen atoms and analyte with a subsequent concerted reaction between the relevant adsorbed species. In addition, the fact that hydrogen diffuses into bulk palladium during the electroadsorption process and re-diffuses out of the Pd lattice at a finite rate [25] may be considered a reason for
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Table 2 Effect of pH on the reduction charge and peak potentials of 2,4-DA evaluated in cyclic voltammetry on the GC/Pd electrode pH
Reduction charge (mC)
IIc1 (V)
IIc2 (V)
2.8 3.2 3.6 5.3 6.0 6.6 7.0 7.5 8.0 9.0 10.0
0.51 0.46 0.41 0.50 0.44 0.52 0.46 0.42 0.47 0.39 0.40
−0.58 −0.77 −0.82 −0.86 −0.91 −0.93 −1.0 −1.0 – – –
– – – −0.57 −0.56 −0.63 −0.67 −0.72 −0.83 −0.86 −0.87
The charge was evaluated during the negative sweep of the potential (5th cycle, 50 mV s−1 ) in phosphate solution containing 1.5 mM 2,4-DA by integration of the reduction wave between the lower potential limit (−1.2 V vs. SCE) and the potential just after the Ic1 wave. The net charge was obtained by difference between the charges evaluated in the presence and absence of 2,4-DA. Experimental conditions as in Fig. 2.
palladium catalyst with the relatively low negative polarizations and background currents related to hydrogen evolution, buffered phosphate solutions at pH 7 were chosen for this electrochemical investigation. To ascertain the electrode kinetics of the reduction of chlorophenoxycarboxylic acids on GC/Pd, measurements at a rotating disc electrode were performed with different rotation speeds (ω) at various concentrations of 2,4-DA. Fig. 3 shows the relevant voltammograms. As can be seen, the complex reduction wave composed of peaks IIc1 and IIc2 increases with increasing rotation speeds, whereas the opposite behaviour for the electrodesorption processes of hydrogen atoms (peak Ia ) was observed. In addition, the peak potential IIc2 is shifted towards more negative values with increasing rotation speed, whereas peak Ic remains essentially unchanged. The marked lowering of peak Ia with rotation speed can be related to two concomitant factors: (i) if the electrochemical reduction of
Fig. 2. I–E curves (5th sweep) obtained in phosphate solutions containing 2.5 mM 2,4-DA at various pH. The pH was adjusted to the desired value by addition of H3 PO4 or NaOH. Other conditions as in Fig. 1.
the unexpected low effects of pH on the reduction charge during the cathodic sweeps. Thus, using a GC/Pd electrode as a cathode, the catalytic reduction of chlorophenoxycarboxylic acids can be nicely conducted over a wide range of pH values. Nevertheless, to combine the good electrochemical activity of the
Fig. 3. Cyclic voltammograms (5th cycle) at a GC/Pd rotating disk electrode in deoxygenated and buffered solutions at pH 7 containing 1.5 mM 2,4-DA at different rotation rates. Other condition as in Fig. 1.
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Fig. 4. Koutecky–Levich plots for the electrocatalytic reduction of 2,4-DA. Experimental conditions as in Fig. 3.
chlorophenoxycarboxylic acids is assisted by adsorbed hydrogen atoms previously generated onto the electrode surface, the increase in the reduction reaction of the organic molecule causes a decrease of the surface concentration of adsorbed hydrogen; (ii) a rapid physical desorption of hydrogen atoms from the Pd0 sites before their electrodesorption reaction causes the decrease. At increased rotation speeds, plots of the peak IIc2 current versus ω1/2 diverged significantly from straight lines intersecting the origin (not shown here). This suggests that some surface kinetic steps are involved in the overall reduction process. Fig. 4 shows the RDE results in terms of a Koutecky–Levich plot. As can be seen, the intercepts are positive and the slopes are inversely proportional to the concentration of 2,4-DA. This result indicates a surface kinetic limitation of the overall reduction process, excluding charge diffusion within the electroactive film. Thus, in the frame work of the Sav´ean–Andrieux model [26], the reduction currents can be limited by diffusion of reactants within the palladium film (i.e., diffusion of hydrogen atoms into/out of the Pd lattice) and/or by cross-exchange reactions involving, in particular, a preliminary adsorption of the organic molecules on the palladium catalytic sites and a subsequent sequence of hydrogenation/dechlorination reactions. It is interesting to observe the effect of changing the upper potential limit on the kinetics of the reduction of chlorophenoxycarboxylic acids. The influence of the upper applied potential on the reduction currents for 6.0 mM 2,4-DA is shown in Fig. 5. As can be seen, when the higher potential limit was decreased, the reduction wave was noticeably smaller. In particular, peak IIc1 disappeared completely for anodic potential limits lower than 0.2 V. On the contrary, the charge under peak Ia remains rather unchanged when the higher potential limits were shifted negatively, indicating that the kinetics of the electrodesorption/adsorption–absorption processes of hydrogen atoms are not influenced by the formation of palladium oxide species. To define the causes regarding the direct dependence of the electroreduction efficiencies with higher potential limits, various factors can be considered: (i) the reactant species (i.e.,
Fig. 5. Influence of the upper potential limit on the reduction currents at the GC/Pd electrode in buffered solution containing 6.0 mM 2,4-DA.
chlorophenoxy acids) were preliminarily adsorbed on the palladium oxide sites and reacted with activated hydrogen atoms on the catalyst surface; (ii) there is a change in spatial distribution of the supported palladium clusters with a subsequent increase in interfacial area where the reduction reaction proceeds; (iii) there is a favorable desorption process of reaction products and/or reaction intermediate during the anodic sweep. Although a competitive adsorption process of reactants on the palladium oxide species and/or a change in roughness factor of the electrode surface layer induced during the anodic excursion of the potentials cannot be excluded, a probable explanation could be associated with an efficient desorption process of fouling products from the electrode surface. We imply that reduction products strongly adsorbed on the catalytic sites gradually poison the catalyst because the new chlorophenoxycarboxyl molecules are unable to displace the adsorbed reaction products and/or reaction intermediate in the cathodic range of potentials. Taking into account our experimental results and in agreement with literature [12,14,15,17,27], a scheme for the electroreduction of the chlorophenoxy acid species on the GC/Pd electrode can be hypothesized:
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where Pd(H)ads , Pd(H)abs , Pd(2,4-DA)ads , Pd(2-CA)ads and Pd(PA)ads refer to adsorbed species, such as hydrogen atoms, 2,4-DA, 2-chlorophenoxy acid (2-PA) and phenoxyacid (PA) species, respectively. The Pd(OH)n species represent the oxidized palladium species formed during the anodic sweep of the potentials. As can be seen, the mechanism of the electrocatalytic hydrogenation of organic molecules involves, in the first instance, the direct interaction between adsorbed/chemisorbed hydrogen atoms and adsorbed organic molecules. The organic molecule may occupy more than one site of the catalyst and its adsorption energy depends strongly on the nature of the metal and state of the surface (i.e., roughness, porous surface, degree of surface dispersion, etc.) and on the nature of organic reactants [28]. It is important to point out that the efficiency of the dechlorination process of chlorophenoxy species depends on the relative rates of the hydrogenation step and of hydrogen desorption process. If the hydrogenation step is too slow with respect to hydrogen desorption, only hydrogen evolution occurs. In addition, the proposed reaction mechanism suggests that the formation of palladium oxide species (Pd(OH)n ) induces an efficient desorption of reaction products (and/or intermediates), thus avoiding irreversible fouling of the catalytic sites. Considering the presence of a functional acid group attached to an aromatic ring, it may inhibit 2-chlorine elimination due to the steric hindrance; in agreement with literature [12,14], the dechlorination process very probably proceeds through a 4-chlorine cleavage followed by 2-chlorine elimination. Thus, the reduction reaction proceeds via a probable multi-step process involving a competitive adsorption step of the organic molecules and hydrogen atoms, followed by an irreversible hydrodechlorination reaction. However, to define a more detailed mechanism and an accurate and unambiguous definition of the kinetic character of the reduction currents, additional experimental proofs are necessary. To verify the electrocatalytic capacity of the GC/Pd electrode and to validate the proposed reaction scheme, chronoamperometric measurements in deoxygenated and buffered solutions at pH 7 containing increasing concentrations of 2,4-DA were carried out. Fig. 6 shows a comparison of the relevant I–t curves obtained with a GC/Pd electrode under a constant applied potential of −0.8 V versus SCE and under pulsed chronoamperometric detection (PCD). The PCD experiments were carried out with a selected waveform of potentials of −0.8 V (E1 ) for 150 ms (t1 ) and +1.0 V (E2 ) for 100 ms (t2 ). Under PCD, the currents were sampled at the applied potential of −0.8 V (E1 ). As can be seen, the success of PCD is probably due to the multistep potential waveform which manages sequentially the detection step (E1 ) followed by a surface cleaning process at sufficiently positive potential (E2 ). The chronoamperometric signal obtained under constant applied potential (Fig. 6, curve A) appears almost unchanged as a function of 2,4-DA concentration. On the contrary, the formation of palladium oxides on the electrode surface induces a more efficient desorption process of reaction products and/or reaction intermediates, avoiding fouling effects of the catalyst. From Fig. 6 (curve C), it can be observed that, after the first addition of 1.0 mM 2,4-DA, the chronoamperometric response remains rather insensitive to subsequent addition of analyte. To
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Fig. 6. Chronoamperometric measurements obtained at a GC/Pd electrode in degassed solutions at pH 7: (A) constant applied potential of −0.8 V versus SCE; a–e represent the consecutive addition of 2,4-DA in 1.0 mM steps; (B) pulsed chronoamperometric detection (PCD) using a selected waveform of potentials of −0.8 V for 150 ms and +1.0 V for 100 ms; a–e represent the consecutive addition of 2,4-DA in 0.25 mM steps; (C) PCD measurement using the same waveform adopted in (B); a–e represent the consecutive addition of 2,4-DA in 1.0 mM steps. The solution was stirred with a magnetic bar at about 300 rpm.
a first approximation, this behaviour can be due to the strong adsorption phenomenon of reactants on the surface electrode. On the other hand, chronoamperometric measurements carried out using addition steps of 0.25 mM 2,4-DA show an I–t profile virtually identical to those observed using addition steps of 1.0 mM 2,4-DA (see curve B). This behaviour appears not completely justified by a simple adsorption process of reactant on the electrode surface, and the reason for the observed saturation phenomenon is not clear at the present time. Although a calibration plot for 2,4-DA analysis appears nonlinear under the present experimental conditions, the GC/Pd electrode responds rapidly to initial millimolar addition of 2,4-DA, and the high reduction current remains largely unchanged over long electrolysis times (i.e., 1200 s) without apparent undesired electrode poisoning effects (see Fig. 6, curves B and C). Therefore, the GC/Pd electrode used under PCD conditions shows a good catalytic efficiency in neutral aqueous solutions at relatively low negative polarizations without any apparent fouling effects towards the reduction of chlorophenoxycarboxylic acids. The conversion of 2,4-DA was evaluated as the total amount of chloride produced during the bulk electrolysis process and the current efficiency of the dechlorination reaction was calculated from the amount of chloride and the charge passed assuming that two electrons and two protons are required for the removal of one chloride ion. Fig. 7 shows the relevant results in terms of amount of chloride produced and current efficiency on the charge passed during the electrolysis process. As can be seen, during the initial 0.8 C of charge passed, corresponding to about 1.5 h of electrolysis time, the amount of chloride increases linearly with charge, while the current efficiency shows an opposite behaviour. For higher electrolysis times (i.e., charge >0.8 C), no appreciable additional chloride formation was observed and, consequently, the current efficiency decreased markedly. These observations
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and/or reaction intermediates avoiding irreversible fouling of the catalytic sites. Chronoamperometric experiments carried out under pulsed chronoamperometric conditions, using a selected waveform of potentials, show high and reproducible reduction currents indicating a good dechlorination efficiency of organochlorine pollutants in neutral aqueous solutions. Acknowledgements This work was supported by Ministero dell’Universita’ e della Ricerca Scientifica e Tecnologica (MURST, COFIN 2006). The anonymous reviewers are thanked for constructive criticism and appreciated comments. References Fig. 7. Chloride formation and the corresponding current efficiency of the 2,4-DA dechlorination at GC/Pd cathode on the total charge passed during the electrolysis process. Electrolysis conditions: −0.8 V (E1 ) for 150 ms (t1 ) and +1.0 V (E2 ) for 100 ms (t2 ); Solution: degassed (N2 ) 40 mM NaH2 PO4 /Na2 HPO4 solution plus 2.0 mM 2,4-DA. The solution was stirred with a magnetic bar at about 500 rpm. The chloride measurements were carried out by IC technique, under experimental conditions as reported in the Section 2.
indicate that reaction products, such as chloride species can be strongly adsorbed on the palladium catalytic sites and gradually poison the catalyst because new reactant molecules are unable to displace the adsorbed reaction products. Thus, as a consequence, for prolonged electrolysis times (i.e., >1.5 h), the current efficiency decreased sensibly. In order to increase the efficiency reduction for long times of electrolysis, the design of an electrolysis cell with a “flow-through” arrangement should be likely chosen for cell construction. Such a cell design should facilitate the effective transport of reagents to and reaction products from the surface cathode avoiding stagnant zones. 4. Conclusions We prepared a GC/Pd electrode for the electrocatalytic reduction of chlorophenoxycarboxyl acids in water solutions buffered at pH 7. The electrochemical behaviour of the palladium catalyst was investigated by the use of cyclic voltammetry and chronoamperometric techniques. A reaction mechanism of the chorophenxyacetic acids at the GC/Pd electrode is proposed and critically discussed. The reduction reaction proceeds via a probable multi-step process involving a competitive adsorption step of chlorophenoxycarboxyl species and hydrogen atoms on the catalytic sites, followed by irreversible hydrodechlorination reaction steps. The formation of palladium oxide species (Pd(OH)n ) during the positive potential sweep induces an efficient desorption process of surface reaction products
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