Chemical and Electrochemical Studies of Interactions between Iron(III

Food Chem. ... Org. Process Res. .... The Fe(III)/Fe(II) couple in acidic (0.01−0.1 M HNO3) aqueous solutions was ... Industrial & Engineering Chemi...
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Langmuir 1998, 14, 3082-3089

Chemical and Electrochemical Studies of Interactions between Iron(III) Ions and an Activated Carbon Surface M. Pakuła,† S. Biniak,*,‡ and A. SÄ wia¸ tkowski§ Naval Academy, 81-918 Gdynia, Poland, Faculty of Chemistry, Nicholas Copernicus University, 87-100 Torun˜ , Poland, and Military Technical Academy, 00-908 Warszawa, Poland Received May 29, 1997. In Final Form: March 20, 1998 Activated carbon (CWZ) produced from hardwood (HPSDD, Hajno´wka, Poland) was powdered (to 0.0600.075-mm grain size) and then deashed (concentrated HF and HCl). A part of the deashed carbon was oxidized with concentrated HNO3. The surface chemical properties of both the nonoxidized and oxidized samples were studied by various methods: neutralization of surface acidic groups with bases of different strength, estimation of total oxygen contents, and determination of water vapor adsorption isotherms and sorption capacity toward Fe3+ ions. The porous structures of carbon samples were estimated from a benzene adsorption-desorption isotherm. Spectroscopic studies (XPS and FTIR) of carbon samples without and with adsorbed iron ions were also performed. A correlation of different determinations was discussed. Using cyclic voltametry, the electrochemical behavior of the powdered activated electrodes (PACEs) was compared with that of solid graphite and glassy carbon electrodes. The Fe(III)/Fe(II) couple in acidic (0.01-0.1 M HNO3) aqueous solutions was investigated as the reference system. The influence of the surface chemistry of the powdered carbon material in terms of its electrochemical properties was discussed, also.

Introduction Owing to their widespread use in electroanalysis, electrosynthesis, electrosorption, and electrocatalysis, electrodes prepared with various forms of carbon materials have been extensively investigated, with particular attention being paid to factors affecting electrode kinetics and background currents.1-11 Several different types of carbon have been employed in these studies, including pyrographite,3-5 glassy carbon,4-6 carbon fibers,7,8 and carbon powder.9-11 Metal and metal complexes are immobilized at the surface of carbon electrodes as well as other carbon supports in order to preconcentrate them from dilute solution,12 investigate the reaction of surface redox system,3-6,9 or manufacture sensors13 for catalytic14 and electrocatalytic purposes.11,15 Significant efforts have been directed toward an understanding of the relationship between surface structure and electron transfer (ET) reactivity for carbon electrodes.1,4 The most important phenomena affecting ET * To whom correspondence should be addressed. † Naval Academy. ‡ Nicholas Copernicus University. § Military Technical Academy. (1) Kinoshita, K. Carbon, Electrochemical and Physicochemical Properties; Wiley: New York, 1988. (2) McCreery, R. L. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1991; Vol. 17, p 221. (3) Hollax, E.; Cheng, D. S. Carbon 1985, 23, 655. (4) McCreery, R. L.; Cline, K. K.; McDermott, C. A.; McDermott, M. T. Colloids Surf. A 1994, 93, 211. (5) Chen, P.-H.; Fryling, M. A.; McCreery, R. L. Anal. Chem. 1995, 67, 3115. (6) Taylor, R. J.; Humffray, A. A. Electroanal. Chem. Interfacial Electrochem. 1973, 42, 347. (7) Biniak, S.; Dzielen˜dziak, B.; Siedlewski, J. Carbon 1995, 33, 1255. (8) Kozłowski, C.; Sherwood, P. M. A. J. Chem. Soc., Faraday Trans. 1 1985, 81, 2745. (9) Pakuła, M.; SÄ wia¸ tkowski, A.; Biniak, S. J. Appl. Electrochem. 1995, 25, 1038. (10) SÄ wia¸ tkowski, A.; Pakuła, M.; Biniak, S. Electrochim. Acta 1997, 42, 1441. (11) Antonucci, P. L.; Alderucci, V.; Giordano, N.; Cocke, D. L.; Kim, H. J. Appl. Electrochem. 1994, 24, 58. (12) Kasem, K. K.; Abruna, H. D. J. Electroanal. Chem. 1988, 242, 87.

reactivity mentioned in the literature are (i) surface purity (dependent on the electrode’s history), (ii) the microstructure (and porosity) of carbon materials, and (iii) the presence of surface functional groups (mainly oxides). Unfortunately, many common electrode preparation procedures affect more than one of these variables. If we are to understand the phenomena controlling the reactivity of a carbon surface on a molecular level, knowledge of the chemical structure of the surface is essential. In this report we relate the surface chemistry of activated carbon (AC) to its electrochemical properties. By combining investigations of surface chemistry based on FTIR spectroscopy, XPS, and adsorption capacity toward water vapor and Fe3+ ions, as well as on neutralization techniques,16 with observation of electrochemical behavior of carbon material during cyclovoltammetric experiments, we tried to elucidate the relationship between results obtained by these different methods. Experimental Section Commercially available activated carbon (CWZ) manufactured from hardwood (HPSDD, Hajno´wka, Poland) was used as the electrode material. Prior to measurements, it was powdered to a grain size of 0.067-0.075 mm and then deashed by treatment with concentrated HF and HCl acids.17 Part of this ashless carbon was oxidized with concentrated HNO3 at 353 K for 4 h and then washed and dried under vacuum (10-2 hPa) at 423 K. The procedure used for carbon purification ensured the removal of adsorbed nitric acid and nitrogen oxides from the oxidized carbon surface.18,19 The nonmodified and oxidized carbon samples will be denoted CWZ-NM and CWZ-Ox, respectively. To minimize (13) Oungpipat, W.; Southwell-Keely, P.; Alexander, P. W. Analyst 1995, 120, 1559. (14) Grzybek, T.; Papp, H. Appl. Catal. B 1992, 1, 271. (15) Yamanaka, I.; Otsuka, K. J. Chem. Soc., Faraday Trans. 1 1994, 90, 451. (16) Bo¨hm, H. P.; Diehl, E.; Heck, W.; Sappok, R. Angew. Chem. 1964, 76, 742. (17) Biniak, S.; Pakuła, M.; SÄ wia¸ tkowski, A.; Zie¸ tek, S. Przem. Chem. 1989, 68, 468. (18) Biniak, S.; Szyman´ski, G.; Siedlewski, J.; SÄ wia¸ tkowski, A. Carbon 1997, 35, 1799.

S0743-7463(97)00562-3 CCC: $15.00 © 1998 American Chemical Society Published on Web 05/05/1998

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Table 1. Selected Chemical Properties of the Activated Carbon Electrode Materials Studied carbon sample

COOH

CWZ-NM CWZ-Ox

0.09 0.84

a

surface functional groups (mequiv/g) COO >CsOH >CdO 0.01 0.45

0.23 0.71

0.18 0.10

basic

total oxygen amount (wt %)

primary adsorption center a0 (mM/g)

Fe (III) ion adsorptiona (mM/g)

0.24 0.02

2.9 9.7

1.39 3.85

0.48 1.60

From 0.05 M Fe(NO3)3 solution (pH ) 2).

weathering effects, all further measurements were carried out on freshly dried samples. The surface chemical properties of the carbon materials were studied by neutralization with bases of different strength16 and diluted HCl, measurement of pH of carbon slurries (1 g of AC in 100 cm3 of 0.1 M NaCl solution),18 determination of total oxygen content (with accuracy of 0.2%),20 and determination of the number of primary adsorption centers from water vapor adsorption isotherms according to the DubininSierpinski method.21 The adsorption isotherms were determined using the volumetric method (microburets) at 25 °C. To characterize the porous structure of the carbons, the benzene adsorption-desorption isotherms were determined.17 Using the Dubinin-Stoeckli equation,22 some parameters characterizing the microporous structure were calculated. The mesopore surface area was also estimated.22 The adsorption capacities toward iron(III) ions for both carbon samples were determined using ferric nitrate solution (0.05 M, pH ) 2). The pH was adjusted with nitric acid. Analytically pure chemicals and doubly distilled water were used. Carbon samples with and without adsorbed iron ions were also studied spectroscopically (FTIR and XPS). Transmission IR spectra of the carbon samples were obtained using a PerkinElmer FTIR Spectrum 2000 spectrometer. The carbon-KBr mixtures in a ratio of 1:300 were ground in an agate mortar and then desorbed under vacuum (10-2 Pa) and finally pressed in a hydraulic press. Before the spectrum of a sample was recorded, the background line was obtained arbitrarily and subtracted. The spectra were recorded from 4000 to 450 cm-1 at a scan rate of 0.2 cm s-1, and the number of interferograms at a nominal resolution of 4 cm-1 was fixed at 25. XPS spectra were obtained with an EscaLab 210 (V. G. Scientific Ltd.) photoelectron spectrometer using nonmonochromatized Al KR radiation (1486.6 eV), the source being operated at 15 kV and 34 mA. Prior to XPS measurement all the carbon samples were dried for 2 h at 373 K. The vacuum in the analysis chamber was always better than 5 × 10-10 Pa. Survey scans were collected from 0 to 1200 eV with a pass energy of 50 eV. High-resolution scans were performed over the 280-295, 524542, and 705-735 eV ranges with the pass energy adjusted to 20 eV. Each spectral region was scanned between 10 and 200 times, depending on the intensity of the signal, to obtain an accetable signal-to-noise ratio at reasonable acquisition times. For calibration purposes, the carbon 1s electron bond energy corresponding to graphitic carbon was referenced at 284.5 eV. After subtraction of the baseline (Shirley-type), curve-fitting was performed using a general multidimensional nonlinear optimalization method (Simplex)23 and assuming a Gaussian/ Lorentzian mix of variable proportion (usually near 0.3) peak shape. This peak-fitting was repeated until an acceptable fit was obtained (error ) 5%). Cyclic voltammetry (CV) was performed using the typical three-electrode system and electrochemical cell described elsewhere.9 The working powdered activated carbon electrode (PACE) design9 was modified in that a change in electrical contact with carbon material was made: a Pt plate (surface area 0.78 cm2) was used instead of wire. After prior vacuum desorption (10-2 Pa), the powdered carbon (mass 50 mg) was placed in an electrode container and drenched with a deareated solution to obtain a ∼3 mm sedimentation layer. Although the electrode design ensures that the shape of the CV curves recorded is (19) Zawadzki, J. In Chemistry and Physics of Carbon; Thrower, P. A., Ed.; Marcel Dekker: New York, 1988; Vol. 21, p 147. (20) Kopycki, W.; Fraise, D.; Binkowski, J. Chem. Anal. 1980, 25, 829 (in Polish). (21) Dubinin, M. M.; Sierpinski, V. V. Carbon 1981, 19, 402. (22) Dubinin, M. M. Carbon 1985, 23, 373. (23) ECLIPSE, version 2.0; VG Scientific 1994

Table 2. Surface Characteristics of the Carbon carbon sample

W0° (cm3 g-1)

x0 (nm)

δ (nm)

CWZ-NM CWZ-Ox

0.224 0.139

0.459 0.415

0.122 0.131

reproducible, estimation of the specific capacitance is now not possible because the potential and current distribution in the electrode bed are unknown.10 The electrochemical behavior of the prepared PACEs was compared with those of solid graphite (RG) (Radelkis, OP-C-711-C, 6 mm diameter) and glassy carbon (GC) (3 mm diameter) electrodes. The respective surface areas exposed to electrolyte were 0.780, 0.238, and 0.071 cm2 for the powdered carbon, graphite and glassy carbon electrodes. Prior to the experiments, the solid electrodes were polished with 1000grit silicon carbide paper (SICA) and washed in double-distilled water and blank electrolyte solution. A Pt gauze was used as the counter electrode. The Fe(III)/Fe(II) couple in aqueous acidic (0.01-0.1 M HNO3) solutions was selected to act as the model redox system. Prior to measurements, the solutions were deareated by bubbling N2 though them. The CV curves were recorded over a wide range of sweep potential (1-250 mV s-1), each time following equilibration of the system (usually 24 h after immersion). The equilibration time was estimated by measurements of the Fe3+ reduction peak current: this rose during the first 20 h but was stable during the next 100 h. All potentials were measured and reported against a potassiumchloride-saturated calomel electrode (SCE).

Results and Discussion The results of the physicochemical investigation are set out in Tables 1 and 2. The surface chemical structures of the activated carbon materials tested were highly diverse and depended on the chemical treatment they had been subjected to (Table 1). The oxidation of activated carbon led to a 3.5-fold increase in the total amount of bound oxygen, especially in the form of acidic surface groups such as carboxylic, lactonic, and hydroxylic.16,19 At the same time, the number of basic groups as well as the pH of the carbon slurry in NaCl solution decreased markedly. The hydrophilic and hydrophobic properties were determined from an analysis of water vapor adsorption isotherms over a range of low and medium relative adsorbate pressures. Hence, with the aid of the DubininSierpinski equation21 the concentration of primary centers for water adsorption (a0) could be calculated. The values of ao for the carbon samples examined (Table 1) were closely dependent on the chemical structure of their surface. An almost 3-fold increase in a0 was observed following carbon oxidation. Similar changes in the value of a0 due to the oxidation of carbon materials have been reported in the literature (e.g. refs 24 and 25). The surface oxidative modification procedure used in our work caused striking changes in the microporosity of the carbon sample tested (Table 2). Following oxidation, their total micropore volume (W0°) decreased nearly 40%, although the micropore half-width (x0) at the maximum of the distribution curve and the variance parameter (δ) deminished only slightly. By contrast, their mesoporosity (24) Kaz´mierczak, J.; Biniak, S.; SÄ wia¸ tkowski, A.; Radeke, K.-H. J. Chem. Soc., Faraday Trans. 1991, 87, 3557. (25) Barton, S. S.; Evans M. J. B.; MacDonald, J. A. F. Adsorpt. Sci. Technol. 1993, 10, 75.

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Pakuła et al.

Scheme 1

did not change after oxidation (Sme ) 112 m2 g-1), which could indicate that oxygen-containing functional groups can make large parts of micropores inaccessible to adsorbate molecules (benzene). Surface chemical differences influenced the sorption capacity toward iron ions. Oxidative modification of the carbon surface increased the equilibrium quantity of Fe3+ adsorbed from aqueous solution (0.05 M Fe(NO3)3, pH ) 2) by a factor of more than three. Various surface groups can participate in iron ion sorption (Table 1). The higher number of adsorbed ions recorded following oxidative modification yielded the best correlation with the larger number of surface carboxyl- and hydroxyl-like functional groups. The sorption of metal ions influenced not only the numbers of such groups but also the adjacent ones. We believe that the formation of surface iron complexes with oxygen functional groups is dominant in sorption processes. Examples of possible surface structures formed during the adsorption of ionic iron species on oxidized carbon are given in Scheme 1. The IR measurements applied here (KBr-pelt technique) make it impossible to compare quantitatively the FTIR spectra obtained for different carbons, but they can indicate which individual chemical structures may or may not be present in the carbon.19,26 In the FTIR spectra of the carbon materials (Figure 1) the band of stretching OH vibrations (3600-3100 cm-1) was due to surface hydroxylic groups and chemisorbed water. Below 2000 cm-1, spectrum a for CWZ-NM carbon shows absorption typical of surface and structural oxygen species.19 The presence of bands at 1740 cm-1 (i), 1630 cm-1 (ii), and 1560 cm-1 (iii) can be respectively attributed to stretching vibrations of CdO moieties in (i) carboxylic, ester, lactonic, or anhydride groups; (ii) chinone and/or iono-radical structures; and (iii) conjugated systems like diketone, keto ester, and keto-enol structures.19 Since H2O is sorbed on the surface of activated carbons with the participation of both specific (hydrogen bonds, chemisorption due to surface oxide hydratation) and nonspecific interactions (physical adsorption), the bands in the 15001600-cm-1 region can also be described by OH binding vibrations. The complicated nature of the adsorption bands in the 1650-1500-cm-1 region suggests that aromatic ring bands and double-bond (CdC) vibrations (26) Vinke, P.; van der Eijk, M.; Verbree, M.; Voskamp, A. F.; van Bekkum, H. Carbon 1994, 32, 675.

Figure 1. FTIR spectra of carbons: (a) CWZ-NM; (b) CWZOx; (c) CWZ-NM after Fe(III) adsorption; (d) CWZ-Ox after Fe(III) adsorption.

overlap the aforesaid CdO streching vibration bands and OH binding vibration bands. Another broad band in the 1470-1380-cm-1 range consists of a series of overlapping absorption bands ascribable to the deformation vibration of surface hydroxyl groups and in-plane vibrations of CsH in various CdCsH structures. The partially resolved peaks forming the absorption band in the 1260-1000cm-1 region can be assigned to etheric (symmetrical stretching vibrations), epoxide and phenolic (vibrations at 1180 cm-1) structures existing in different structural environments. After oxidation (spectrum b) the band characteristic of carbonyl moieties in a carboxylic acid (1750-1700 cm-1) increases and there is a simultaneous, considerable increase in the band intensities of these carbonyl functional groups in different surroundings. Oxidation also changes the shape of the overlapping bands in the “fingerprint” region (1400-1000 cm-1), mainly enhancing absorption in the 1250-1000-cm-1 range (maximum near 1180 cm-1). This suggests an increase in the number of ether and hydroxylic structures on the carbon investigated. The relative decrease in intensity of C-H moiety bands (near 2900 cm-1) after oxidation may indicate that oxygen surface species are also formed at the expense of the aliphatic residues in the carbon material. A relative increase in the OH band (near 3400 cm-1) and a decrease in the carboxylic band (1740 cm-1) following Fe(III) ion adsorption were commonly recorded for both carbon materials (spectra c and d). For nonmodified carbon a new band at the wavelength 1380 cm-1 was observed and could be ascribed to NO3- ions adsorbed at basic centers of carbon or/and as an ion compensating for adsorbed iron-containing species (Scheme 1). The relative increase in overlapping bands in the 1250-1000-cm-1 range (maximum at 1120-cm-1) may be confirmation of additional hydratation of the adsorbent surface. These observations suggest that the adsorption of iron(III) hydroxide species (FeOH2+, [Fe(OH)2]+) and iron(III) aqua complexes may take place with the participation of surface oxygen functionalities. This adsorption may be partially due to ion exchange. Figure 2 shows the XPS spectral changes in the C 1s (a-d), O 1s (e-h), and Fe 2p3/2 (i-j) peaks of CWZ-NM

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Figure 2. High-resolution XPS spectra of C 1s (a-d), O 1s (e-h) and Fe 2p3/2 (i-j) regions of CWZ-NM (a, c) and CWZ-Ox (b, d) before (a, b) and after (c, d) Fe3+ ion adsorption.

and CWZ-Ox samples with and without adsorbed iron ions. There were marked differences between experimental (dots) and synthesized (continuous) lines in all the spectra. The positions of fitted peaks (dashed lines) were determined according to literature data27-39 as well as to empirically derived values. The relative areas (%) of the fitted peaks were also calculated. The C 1s spectrum of a nonmodified carbon sample (a) shows a shoulder on the high-energy side and comprises the main peaks for binding energy (BE) at 284.5 (54%), 285.9 (33%), 288.8 (7.3%), and 290.7 (2.5%) eV corresponding to CdC (graphitic), CsO (CsOH, CsOsC), CdO (carbonyl and/ or quinone-like), and COO (carboxylic acid, lactone, anhydride) surface moieties, respectively. Reference data can be found in the literature.8,11,14,28-30 The O 1s spectrum for the CWZ-NM carbon sample displays one main peak corresponding to the CsO (532.8 eV, 81.4%) moiety in different surface oxygen-containing functional groups. The two other peaks at 530.7 eV (7.8%) and 534.7 eV (10.8%) can be assigned to CdO species and adsorbed water respectively.28-30 Oxidative modification enhanced the quantity of oxygen (and therefore of surface oxygen-containing groups) more than 3-fold (Table 1). The magnitude of the total C 1s peak (in arbitrary units) changed only slightly after (27) Kulkarni, G. U.; Rao, C. N. R.; Roberts, M. W. Langmuir 1995, 11, 2572. (28) Grzybek, T. Pol. J. Chem. 1994, 68, 1649. (29) Gai, P. L.; Billinge, B. H. M.; Brown, A. M. Carbon 1989, 27, 41. (30) Ohwaki, T.; Ishida, H. J. Adhesion 1995, 52, 167. (31) Fournier, J.; Lalande, G.; Coˆte´, R.; Guay, D.; Dodelet, J.-P. Electrochim. Acta 1996, 41, 1689.

oxidationsonly a shoulder in Figure 2b clearly suggests the presence of larger quantities of oxygen-containing species. The fitted spectrum exhibits a relatively smaller graphitic carbon peak (43.6%). Likewise, the relative peak intensity of the CsO (36.1%), CdO (7.7%), and COO (3.9%) moieties increased. Furthermore, a new peak at 287.3 eV (7.8%) appears, which has been assigned to carbon in keto-enolic and/or iono-radical equilibria (CsO in the free-radical semiquinone group, carboxylo-carbonate ionic species). The presence of such stable structures has been observed on the surface of carbon materials by a number of authors.19,34 These assignments agree very well with the extensive XPS studies made on commercially available carbon used as supports by Alderucci et al.32 and Alberts et al.33 A highly oxidized carbon signal (287-288 eV) was also observed by Desimoni et al.35 and Goodenough et al.,36 but the species could not be identified precisely. The O 1s peak of CWZ-Ox carbon (f) consists of three peaks (as for CWZ-NM carbon)sthe main one (at 532.8 eV) was relatively smaller (71.7%), and the next two were larger (14.6% and 13.7% for >CdO groups and adsorbed H2O, respectively). The considerable peak intensity, attributed (32) Alderuci, V.; Pino, L.; Antonucci, P. L.; Roh, W.; Kim, H.; Cocke, D. L.; Antonucci, V. Mater. Chem. Phys. 1995, 41, 9. (33) Albers, P.; Deller, K.; Despeyroux, B. M.; Prescher, G.; Scha¨fer, A.; Seibold, K. J. Catal. 1994, 150, 368. (34) Leon y Leon, C. A.; Radovic, L. R. In Chemistry and Physics of Carbon; Thrower, P. A., Ed.; Marcel Dekker: New York, 1994; Vol. 24, p 213. (35) Desimoni, E.; Casella, G. I.; Cataldi, T. R. I.; Salvi, A. M.; Rotunno, T.; Di Croce, E. Surf. Interface Anal. 1992, 18, 623. (36) Goodenough, J. B.; Hamnett, A.; Kennedy, B. J.; Manoharan, R.; Weeks, S. A. Electrochim. Acta 1990, 35, 199.

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to adsorbed water, in the O 1s region for both the nonmodified and the oxidized carbon indicates that their surfaces are highly hydrated. The iron ions adsorbed on the carbon surface studied by XPS give Fe 2p lines at binding energies of ca. 725 eV (2 p1/2) and 711 eV (2 p2/3), and they influence the intensity and binding energies of the C 1s and O 1s spectra. The C 1s spectrum in a nonmodified carbon sample following Fe3+ ion adsorption (c) also consists of a number of peaks, some of which have shifted toward the high-energy side. The CdC peak (284.5 eV, 27.6%) has shrunk slightly. Moreover, the presence of two peaks at 285.9 eV (26.1%) and 287.3 eV (27.1%), in the chemical shift range of C-O (C-O-C) and C-O-(Fe) moieties, respectively, confirms the suggestion that adsorbed, highly hydrated (aqua complex) Fe3+ and FeOH2+ ions interact with oxygencontaining surface functional groups, as is shown in Scheme 1. Changes in peak shape in the chemical shift range corresponding to CdO and COO groups (288.8 eV, 8.2%, and 290.7 eV, 3.9%) can be ascribed to iron carboxylate formation, resulting in isolated Fe3+ cations. These results are confirmed by changes in the O 1s spectrum (f): only the peaks at 534.7 eV (61.4%) and 532.8 eV (36.0%) in the chemical shift range corresponding to surface CsO moieties in different surface groups and adsorbed water are seen. The Fe 2p3/2 spectrum (i) comprises the peaks at 710.3 eV (36.5%), 712.1 eV (36.9%), and 714.5 eV (26.6%). The spectrum has no shoulder at lower binding energies, which suggests that a reduced chemical form of ironlike Fe2+ (708.7-707.1 eV) or even metallic Fe (707.2-706.7 eV) or iron carbide (706.9 eV), is absent.31-33 The BE values of the first two peaks (710.3 and 712.1 eV) are near to the literature data for Fe3+ (710.8-711.8 eV); the third one (714.5 eV) may be ascribed to iron ions complexed with electronegative surface ligands.37 The results obtained for oxidized carbon following the adsorption of Fe3+ ions are similar. The C 1s spectrum (d) consists of several peaks, some of which have shifted toward the high-energy side. The CdC peak (284.5 eV) has become the smallest (27.3%) in relation to those of other carbon samples; furthermore, there is an increase in peak size, ascribed to C-O-(Fe) moieties (287.3 eV, 40.2%) as well as peaks in the chemical shift range of carboxylate species (288.8-290.7 eV, 10.7%). CsO moieties (285.9 eV, 17.0%) ascribed to structural CsOsC groups are also present. The O 1s spectrum (h) consists of three peaks: the two main peaks at 532.8 eV (33.6%) and 534.7 eV (53.7%), ascribed as above, and an additional small one at 530.5 eV (1.9%), ascribed to isolated CdO moieties. The Fe 2p3/2 spectrum (j) comprises the peaks at 710.3 eV (43.6%), 712.1 eV (36.9%), and 714.5 eV (19.4%). A relative increase in the peaks typical of Fe3+ is correlated with enhanced adsorption and ion-exchange following oxidation. This suggests that iron ions could have been adsorbed at different active centers (surface oxides) and at a variety of sites surrounding the adsorbed iron. It is clear that the spin-orbit splitting, multiple oxidation states, satellite structure, and adsorbent surface heterogeneity complicate the analysis of the Fe 2p spectra.38 However, the general tendency observed in XPS signals for the studied samples confirms the results obtained with other methods. Recently, results similar to those of XPS studies were reported for some other metalsupported activated carbon materials.39 (37) McIntyre, N. S.; Chan, T. C. In Practical Surface Analysis. Vol. 1. Auger and X-ray Photoelectron Spectroscopy; Briggs, D., Seah, M. P., Eds.; Wiley: New York, 1994. (38) Lin, T.-C.; Seshardi, G.; Kelber, J. A. Appl. Surf. Sci. 1997, 119, 83.

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Figure 3. Cyclic voltammograms of studied carbon electrode materials: (a, a′) CWZ-NM; (b, b′) CWZ-Ox; (c, c′) Radelkis graphite electrode; (d, d′) glassy carbon electrode. Curves recorded in 0.01 M HNO3 without (a-d) and in the presence of (a′-d′) 0.05 M Fe(NO3)3. Sweep rate v ) 0.005 V s-1.

The CWZ-NM and CWZ-Ox activated carbons described and characterized above were used as powdered electrode materials in cyclic voltammetry experiments. An aqueous solution of 0.1 or 0.01 M HNO3 as background electrolyte and iron(III) nitrate as depolarizer was employed. Additionally, for comparison of electrochemical behavior, some measurements with the use of commercial carbon electrodes (made from pyrographite and glassy carbon) were carried out. By way of example, CV curves for the electrode materials studied, recorded in the absence (Figure 3a-d) and in the presence (Figure 3a′-d′) of Fe3+ ions (0.05 M) in background electrolyte (0.01 M HNO3), are illustrated. These background currents depend on the kind of carbon samples used as electrode materials as well as on the surface area of electrodes in contact with the solution. The estimated capacitive surface (geometric) current densities in the absence of electroactive species obtained from a-d cyclic voltammograms (Figure 3) at the potential E ) 0.75 V are 256.4, 134.6, 66.1, and 13.9 µA cm-2 for CWZ-NM, CWZ-Ox, pirographite (RG), and a glassy carbon electrode (GC), respectively. The capacitance of powdered carbon electrodes calculated from these data (v ) 5 mV s-1) is nearly 20 times higher than that of GC, which implies that a much larger area participates in the electrochemical process. On the other hand, if the mesopore area of powdered material (112 m2 g-1) is taken into consideration, capacitances considerably lower than those quoted in the literature for carbon electrodes are obtained.38-40 Furthermore, estimated capacitances related to the mass of powdered electrode materials (0.80 and 0.39 F g-1 for D-NM and D-Ox carbon respectively) (39) Park, S. H.; McClain, S.; Tian, Z. R.; Suib, S. L.; Karwacki, C. Chem. Mater. 1997, 9, 176.

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Figure 4. Dependence of the anodic (a) and cathodic (b) peak currents on the Fe(III) concentration in 0.01 M HNO3: (1) CWZ-NM; (2) CWZ-Ox; (3) Redelkis graphite electrode; (4) glassy carbon electrode; v ) 0.0075 V s-1.

are significantly lower than the double-layer capacity of activated carbon materials.40 The overall conclusion should be that probably only part of the elctrode material or an unknown part of the external carbon particles’ surface (and macroporous surface area) participated in the electrochemical process. The potential and current distribution in packed bed electrodes composed of powdered activated carbon have been studied: the specific capacitance varied between 17 and 58 µF cm-2 for different types of AC.41 Our previous study of the influences of surface chemical structure on the electrochemical behavior (background currents) of PACEs9,10 showed a decrease in current density as well as the creation of quinone-like electroactive surface oxides following oxidative modification of carbon material. Activated carbon materials can be used for preconcentration (by adsorption) of diluted electroactive species (e.g. metal ions). Characterizing the electrochemical behavior of these systems is therefore important for both electroanalytical and catalytic purposes.42 On the other hand, how the adsorbed electroactive species interacts with the electrode material depends on its surface chemistry.42-46 Whereas much attention has been accorded to the effects of surface treatment of graphite, glassy carbon, carbon fibers, carbon black, and pyrolitic carbon electrodes on electron transfer,4-5,40,43-48 the electrochemical properties of activated carbon have been investigated only occasionally, despite their numerous applications (e.g. fuel cell).34,42 (40) Momma, T.; Liu, X.-J.; Osaka, T.; Ushio, Y.; Sawada, Y. J. Power Sources 1996, 60, 249. (41) Card, J. C.; Valentin, G.; Storck, A. J. Electroanal. Soc. 1990, 137, 2736. (42) Solar, J. M.; Leon y Leon, C. A.; Osseo-Asare, K.; Radovic, L. R. Carbon 1990, 28, 369. (43) McDermot, C. A.; Kneten, K. R.; McCreery, R. L. J. Electrochem. Soc. 1993, 140, 2593. (44) Frysz, C. A.; Shui, X.-P.; Chung, D. D. L. Carbon 1994, 32, 1499. (45) Rice, M. E.; Galus, Z.; Adams, R. N. J. Electroanal. Chem. 1983, 143, 89. (46) Wightman, M. R.; Deakin, M. R.; Kovach, P. M.; Kuhr, W. G.; Stutts, K. J. J. Electrochem. Soc. 1984, 131, 1578.

The CV curves recorded for such carbons in the presence of iron(III) ions in solution (a′-d′, Figure 3) display a pair of peaks ascribed to the Fe3+/Fe2+ redox couple. The peak potentials depend on the kind of electrode used. In commercial carbon electrodes (RG and GC) they are similar to those quoted in the literature.3,5 The peak currents also depend on the type of electrode materials used. The relationships between anodic (a) and cathodic (b) peak current densities and the depolarizer concentrations shown in Figure 4 are linear over the concentration range studied. The linearity of these dependences for PACEs is not as good as that of commercial carbon electrodes. The linearity of the anodic peak current densities was inferior in all cases, which indicates that some interaction could have occurred between the electrogenerated Fe2+ ions and the electrode material surface. Thus, it can be stated that both the porous structure of an electrode material and its surface chemical properties influence the charge-transfer kinetics of this process. The formal redox potentials (Ef) of the Fe3+/Fe2+ couple were estimated as average anodic and cathodic peak potentials at all the depolarizer concentrations and scan rates used. The respective charge-transfer coefficients (nR and nβ) and rate constants (ks(a) and ks(c)) for the anodic and cathodic processes were calculated49 from experimentally estimated peak and half-peak potentials and from diffusion coefficients of Fe3+ and Fe2+ ions gleaned from the literature (0.35 × 10-5 and 0.51 × 10-5 cm2 s-1, respectively).45 The average values of these parameters at a selected depolarizer concentration ([Fe3+] ) 0.01 M) and several potential scan rates are given in Table 3. Additionally, the proportions between cathodic and anodic peak currents in all the systems investigated have been recored. As the differences between anodic and cathodic peak potentials for powdered electrodes are much higher than those for (47) Adams, R. N. Electrochemistry of Solid Electrodes, Marcel Dekker: New York, 1969; p 221. (48) Motta, N.; Guadelupe, A. R. Anal. Chem. 1994, 66, 566. (49) Nicholson, R. S. Anal. Chem. 1965, 37, 1351.

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Pakuła et al.

Table 3. Redox Potentials and Kinetic Data of Systems Examined sample

Ep,a,V

Ep,c, V

Ef*, V

nR

nβ

ks (a) (×105 cm s-1)

ks (c) (×105 cm s-1)

ip,a mA

ip,c/ip,a

CWZ-NM CWZ-Ox RG GC

0.700 0.640 0.540 0.530

0.165 0.170 0.450 0.450

0.455 0.445 0.480 0.480

0.25 0.30 0.72 0.72

0.25 0.26 0.72 0.78

2.03 2.48 7.06 9.36

5.14 5.40 65.5 63.5

0.30 0.32 0.124 0.0242

1.73 1.47 1.15 1.20

a

Calculated as average values of anodic and cathodic peak potentials for all depolarizer concentrations and scan rates used.

Figure 5. (a) Cyclic voltammograms of the carbon electrodes (CWZ-NMsdashed line; CWZ-Oxssolid line) with preadsorbed Fe3+ ions recorded in 0.01 M HNO3 solution (v ) 0.005 V s-1). (b) Ep,c vs log(v) plots for Fe3+ ions reduction in bulk solution on CWZ-NM (1) and CWZ-Ox (2), and preadsorbed on CWZ-Ox (3) carbon electrodes.

commercial solid electrodes, charge-transfer processes can be expected to be irreversible when the electrode surface becomes increasingly heterogeneous. The Ef values obtained for the commercial electrodes used are close to the literature data.3-5,43 The kinetic parameters (R, ks) quoted in Table 3 show that charge transfer is much slower at powdered electrodes and that oxidative modification accelerates this process only slightly (aproximately 20%) in relation to that for nonmodified carbon. Other autors5,6,43 report very large increases (10- to 100fold) in ks for the Fe3+/Fe2+ couple following carbon oxidaton. The differences observed for powdered carbons can be explained by the fact that nonmodified carbon is partially covered by surface oxides (see IR and XPS spectra) and that the surfaces are additionally covered by adsorbed iron-containing ionic species. The adsorbed layer can play a significant role in charge-transfer processes a fact that seems to be confirmed by the long (24 h) equlibration (increase) of peak currents of the Fe3+/Fe2+ couple. A similar period is required for equilibrium of cation adsorption from aqueous solution to be reached.50 These data require more evidence, and some studies on electrochemical systems will be performed in our further work. The CV curves for powdered active carbon electrodes with previously adsorbed ionic iron species are shown in (50) SÄ wia¸ tkowski, A.; Szyman´ski, G.; Biniak, S. In Fundamentals of Adsorption; LeVan, M. D., Ed.; Kluwer Academic Publishers: Boston, 1996; p 913.

Figure 5a. The presence of a pair of peaks, ascribed to the Fe(III)/Fe(II) redox couple (Ep,c ) 0.31 V, Ep,a ) 0.63 V), is very clear in the case of the oxidized carbon sample, which exhibits a higher adsorption capacity toward iron ions (Table 1). By contrast, the nonmodified carbon sample produced only a scarcely visible cathodic peak. In view of the preadsorption conditions (acidic solution), these peaks should be ascribed to the iron ions bound to the carbon surface. The experimental variations in peak potential of Fe3+ ion reduction in solution (for CWN-NM and CWN-Ox) and preadsorbed on CWZ-Ox carbon vs the logarithm of the sweep rate are given in Figure 5b (lines 1-3, respectively). There is a more positive shift in reduction peak potentials for oxidized carbons, which may indicate a greater irreversibility of the electrochemical process.46 The relationship Ep,c vs log(v) for oxidized carbon is similar, both for the reduction of adsorbed iron (line 3) and for iron in bulk solution (line 2). On the other hand, the differences between nonmodified (line 1) and oxidized samples are clear. All this is indicative of the significance of surface functional groups and the adsorption process in the electrochemical reactions of powdered activated carbons. Conclusion Oxidative modification of an activated carbon surface raises considerably the amount of oxygen attached, particularly in the form of strongly acidic functional groups, and significantly increases its capacity to adsorb

Iron(III) Ions and an Activated Carbon Surface

water molecules and iron(III) ions. The spectral studies suggest that not only ion-exchange adsorption but also the formation of surface complexes with the participation of iron ionic species present in aqueous solution are possible. Oxidation of the carbon surface influences the charge-transfer reaction in the Fe(III)/Fe(II) redox couple and, simultaneously, increases the irreversibility of this process.

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Acknowledgment. This work was partially supported by Polish Committee for Scientific Research, project 3 TO9B 026 12. The authors extend their gratitude to Dr. J. Sobczak, Institute of Physical Chemistry of the Polish Academy of Science, ul. Kasprzaka 44/52, PL-01-224 Warszawa, for the XPS measurements. LA9705625