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Study on the electrodissolution and roughening of a palladium electrode in chloride containing solutions
Journal of Electroanalytical Chemistry 660 (2011) 80–84
Contents lists available at ScienceDirect
Journal of Electroanalytical Chemistry journal homepage: www.elsevier.com/locate/jelechem
Study on the electrodissolution and roughening of a palladium electrode in chloride containing solutions Shu Chen a,b, Wei Huang b, Jufang Zheng c, Zelin Li b,⇑ a Key Laboratory of Theoretical Chemistry and Molecular Simulation of Ministry of Education of China, School of Chemistry and Chemical Engineering, Hunan University of Science and Technology, Xiangtan 411201, China b Key Laboratory of Chemical Biology and Traditional Chinese Medicine Research, Ministry of Education of China, College of Chemistry and Chemical Engineering, Hunan Normal University, Changsha 410081, China c Key Laboratory of the Ministry of Education for Advanced Catalysis Materials, Institute of Physical Chemistry, Zhejiang Normal University, Jinhua 321004, China
a r t i c l e
i n f o
Article history: Received 29 January 2011 Received in revised form 21 May 2011 Accepted 7 June 2011 Available online 15 June 2011 Keywords: Palladium In situ Raman spectroscopy Electrodissolution Micro-nanostructure Electrocatalysis
a b s t r a c t Potential-dependent surface electrooxidation processes of a Pd electrode in chloride containing solutions have been investigated in detail by means of in situ confocal Raman spectroscopy for the first time. In a solution of HCl, characteristic Raman bands for the oxidative coordination of Pd with Cl, the transformation of soluble Pd(II) to Pd(IV) complexes, the electrooxidation of Cl into Cl2, and the redox between Cl2 and Pd were all detected unambiguously during the potential ascending. While in a solution of KCl, insoluble salt films of K2PdCl4 and K2PdCl6 were found on the electrode surface due to their poor solubility. Moreover, electrochemical roughing of Pd electrode by electrodissolution–electrodeposition cycling in the HCl solution with a surfactant have been utilized to construct micro-nanostructures of Pd, which display high activity toward the electrooxidation of ethanol in an alkaline medium and obvious surface enhanced Raman spectroscopy (SERS) signals for pyridine probe molecules. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction Electrodissolution of metals and alloys involves complicated surface processes like adsorption/desorption, film formation of oxides or insoluble salts, pitting corrosion, coordination, and gas evolution. Raman spectroelectrochemistry is a promising tool to understand the physicochemical processes that occur at these metal/solution interfaces on the molecular level, and has been employed to investigate the electrodissolution course of many metals like Cu [1,2], Au [3–5], Co [6], Hg [7], Fe [8–10], Fe–P [11], carbon steel [12–15], Au–Sn [16], Pt–Ni alloy [17] and so on. Electrooxidation of Pd has been intensively investigated in noncomplex acids (e.g. perchloric or sulfuric acid) and in alkaline solutions [18–21], emphasizing the formation and growth of insoluble oxide/hydroxide layers in mild electrolyte. Their oxidation state and chemical composition at different potential limit are determined by diversiform surface analytical and materials characterization techniques [18]. Comparatively, in strong acid/basic medium and in the presence of anions (e.g. Cl or I), the electrooxidation of Pd facilitate the formation of kinds of soluble intermediates and products [22–30], which pronounces the electrodissolution of Pd by hydration or complexation of Pd cations. The electrooxidation
⇑ Corresponding author. Tel.: +86 731 88871533; fax: +86 731 88872531. E-mail address: [email protected] (Z. Li). 1572-6657/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jelechem.2011.06.008
of Pd in chloride containing electrolyte has received less attention up to the present and the understanding of electrochemically formed species only derived from electrochemical measurements [31–33]. Therefore, further investigation especially using in situ spectroscopy is required to confirm the possible species and intermediates involved in the electrodissolution. In this paper, in situ potential-dependent Raman spectroscopy has been performed to clarify the species in the electrooxidation of Pd in chloride containing solutions. With the knowledge of species at different potentials, we construct micro-nanostructures of Pd for electrocatalysis on the electrode surface by electrodissolution–electrodeposition cycling. 2. Experimental Raman measurements were performed with a Renishaw RM1000 confocal Raman spectrometer (Gloucestershire, UK) in a self-designed spectroscopic cell made by Teflon with a quartz window. The objective equipped on a DMLM Leica microscope was of long working distance (ca. 8 mm) with 50 magnifications. The exciting wavelength was 785 nm from a diode laser with a power of ca. 18.5 mW on the sample. More detailed descriptions on the instrument can be found elsewhere [5]. Normal electrochemical measurements were carried out with three electrodes using CHI 660C electrochemical station (Chenhua, Shanghai, China). The working electrode was a Pd disk made from a 1 mm diameter wire
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(purity P99.99%) procured from Tianjin Aida Co., Ltd. (China). The current density is related the geometric area of the electrode unless indicated otherwise. Prior to use the electrode was polished with 1000# metallographic paper and cleaned with ultrasonic waves in ultra-pure water (from a water purification system of Millipore Corp., USA). Then, the electrode subjected potential cycling in the range of 0.6 to 0.9 V in 0.5 M H2SO4 until the curves repeated. The roughness factor of the polished electrode was 20 with the conversion factor of 424 lC cm2 for the reduction of monolayer Pd oxides. The potential quoted here is versus the saturated mercurous sulfate electrode (SMSE). Cyclic voltammetry was applied to roughen the Pd electrode in 1 M HCl solution (M = mol dm3) containing a certain amount of cetyltrimethyl ammonium bromide (CTAB). Surface morphology of the Pd electrode was obtained with a JSM-6380LV scanning electron microscopy (SEM). SERS spectra were measured under the excitation laser of 633 nm with ca. 8.2 mW power and the collection time was 60 s. Pyridine was chosen as the probe molecule, which was detected by immersing the roughened Pd electrode in the solution of 0.01 M pyridine + 0.1 M KClO4. All experiments were performed at room temperature (20 °C) and solutions were freshly prepared with analytical grade chemicals and ultra-pure water. 3. Results and discussion 3.1. Electrochemical behaviors of Pd in HCl
The effect of Cl ions on the electrodissolution of Pd is presented in Fig. 1 with different HCl concentrations. The cyclic voltammograms in the blank solution of aqueous perchloric acid in Fig. 1a (dotted and dashed lines) resembled those reported in numerous investigations for polycrystalline Pd [18–21], where adsorption/ absorption–desorption of hydrogen and the formation–reduction
Fig. 1. (a) Cyclic voltammograms of the Pd electrode in 1 M HClO4 (dotted and dashed line) and in 1 M HClO4 + 0.01 M HCl (solid line) within different potential ranges at a scan rate of 100 mV s1. (b) Linear sweep voltammograms at 100 mV s1 for the Pd electrode by changing the HCl concentration from 0.1 to 1 M as indicated. (c) Series of anodic sweep voltammograms in 0.1 M HCl at different scan rate and the relationship between first anodic peak current and square root of scan rate.
of surface Pd (hydro)oxides proceeded in the lower and higher potential ranges, respectively. In order to exclude the interference from the oxidation of absorbed hydrogen into bulk Pd [30,34,35], the lower potential limit was set such that it was not less than 0.3 V in the electrochemical experiments for anodic dissolution. In 1 M HClO4, the oxidation of Pd started at around 0.2 V in the positive-going potential scan and the reduction of the oxides began at 0.1 V in the negative-going scan. In the presence of 0.01 M Cl ions (the solid line in Fig. 1a), two pairs of new redox peaks appeared, which are attributed to the formation and reduction of two possible complex ions of Pd with chloride [31–33]: 2
Pd þ 4Cl ¼ PdCl4 þ 2e 2 PdCl4
þ 2Cl ¼
2 PdCl6
ðiÞ
þ 2e
ðiiÞ
Evidence for the involvement of such soluble complexes will be resolved latter by Raman spectra (Section 3.2). Notably, the two anodic peak currents increased remarkably (Fig. 1b) by increasing the concentration of HCl. Moreover, the first anodic peak current (Ip,a) varied linearly with the square root of the scan rates (m1/2) and the numbers of electron transferred were estimated closed to 2, as shown in Fig. 1c [36]. 3.2. In situ Raman spectroscopy for the electrooxidation of Pd in 1 M HCl Fig. 2 shows the Raman spectra of Pd in 1 M HCl for a sequence of anodic potentials applied. There were no Raman peaks below 0 V before the electrodissolution of Pd. The onset of surface oxidation was signaled near 0.1 V with a discernible band at 304 cm1, locating at the lower end of the first anodic climbing branch in Fig. 1b. This band 2 is assigned to the Pd–Cl stretching (v1) in PdCl4 [37] from Reaction (i) which gained in intensity with the increase of potential. Note that the stretching of Pd—Clads for the monolayer of adsorbed Cl is too weak to be detected. At 0.7 V, which situates at the lower end of the second ascending branch in Fig. 1b, the peak at 304 cm1 was substituted by a double band of Pd–Cl stretching (v1, v2) at 2 293/315 cm1 for the species of PdCl6 [38] from Reaction (ii). Immediately following this doublet, three more new peaks appeared at 535 (shoulder), 540 and 547 cm1 while E P 0.8 V and their inten-
Fig. 2. Potential-dependent in situ Raman spectra for the Pd electrode in 1 M HCl during increasing the potential. The collection time for each spectrum was 10 s with a 785 nm laser.
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sities got stronger till 1.1 V. The bands appeared as three peaks can be ascribed to the fundamentals and the hot bands of the isotopic Cl2 molecules [39,40]. The produced Cl2 (g) at 0.8 V and above agitated the solution near the electrode surface and dispersed some 2 PdCl6 into the bulk solution, and that is the reason why the spec2 trum intensity of PdCl6 was the strongest at 0.7 V. When E P 1.2 V where oxygen released, the triplet and the doublet remarkably weakened and strengthened, respectively. We suppose that these three peaks are related with the formation of Cl2:
2Cl ! Cl2 þ 2e
ðiiiÞ
To confirm the supposition, a Cl2 containing nonaqueous solution was prepared by bubbling chlorine gas through the cyclohexane (C6H12). The triplet at 535, 540, 547 cm1 was observed indeed (Fig. 3a) for the solution. More interestingly, we also confirmed that the produced Cl2 can react with Pd chemically forming 2 PdCl6 very likely by the following reaction:
2
Pd þ 2Cl2 þ 2Cl ¼ PdCl6
ðivÞ 2
And it might undergo two steps like 2
Pd þ Cl2 þ 2Cl ! PdCl4 2 PdCl4
þ Cl2 !
2 PdCl6
ðvÞ ðviÞ
We confirmed Reaction (iv) by setting the circuit open after polarizing the electrode at 1 V (Fig. 3b). The intensities of the dou2 blet for PdCl6 and of the triplet for Cl2 went oppositely upon open2 ing the circuit because the moieties of Cl2 and PdCl6 were consumed and produced, respectively, as expressed in Reaction 2 (iv). The exhaustion of Cl2 and the diffusion of PdCl6 account for the vanishing and diminishing of the triplet and doublet, respectively, in 30 s (Fig. 3b). Moreover, the opposite tendency in inten2 sity for Cl2 (triplet) and for PdCl6 (doublet) at 1.2 V in Fig. 2 can also be explained by Reaction (iv) because the convection induced by oxygen evolution (vii) can speed Reaction (iv).
H2 O ! ð1=2ÞO2 þ 2Hþ þ 2e
Fig. 4. In situ Raman spectra at selected potentials for the Pd electrode in 1 M KCl solution. Its anodic sweep voltammogram at 100 mV s1 was shown in the inset.
305 cm1 for PdCl4 was always there. These facts indicate that salt films of K2PdCl4, K2PdCl6 precipitated on the surface: 2
PdCl4 þ 2Kþ ¼ K2 PdCl4 ðsÞ 2 PdCl6
þ
þ 2K ¼ K2 PdCl6 ðsÞ
ðviiiÞ ðixÞ
Although the triplet for Cl2 did not show up for some reasons, Reactions (iv)–(vi), incorporating (viii) and (ix), should be still the reason for the increase in intensity after opening the circuit. We observed that the anodic peak current related K2PdCl4 (the first peak) was larger in 1 M HCl (Fig. 1b) than in 1 M KCl (the inset of Fig. 4). However, opposite situation appeared for the anodic peak current related K2PdCl6 (the second peak). These facts suggest that the film of K2PdCl4 rather than K2PdCl6 blocked the electrochemical reaction on the surface in some extent.
ðviiÞ 3.4. A potential-dependent species sketch for the electrooxidation of Pd
3.3. In situ Raman spectroscopy for the electrooxidation of Pd in 1M KCl Raman bands for Pd(II) and Pd(IV) species also occurred with 1 M KCl (Fig. 4) at 0.3 V and 0.9 V, respectively, corresponding to the oxidative current peaks (the inset of Fig. 4). Again, the doublet 2 at 293/324 cm1 for the PdCl6 got stronger in intensity upon opening the circuit then. However, the intensity of the doublet did not decrease for a long period of time, and the small peak at
Fig. 3. (a) Raman spectra for the sample of liquid cyclohexane before and after absorbing chlorine gas. (b) In situ Raman spectra for the Pd electrode when the cell was set at 1 V and then let the circuit open. The exposure time for CCD was 10 s.
Based on the spectroelectrochemical results and discussion, the electrooxidation processes of Pd are summarized in Fig. 5. In the HCl solution, the Pd electrode is electrooxidized into two soluble 2 2 complexes PdCl4 (aq) and PdCl6 (aq) successively with the potential increase; Cl can be then oxidized into gaseous Cl2 (g) parallel 2 to the formation of PdCl6 (aq), so the oxidation of Pd can also proceed chemically by reacting with the produced Cl2 (g); and the convection induced by the O2 (g) evolution at still higher potentials 2 can accelerate the chemical reactions of Pd with Cl2 into PdCl6 (aq). Nevertheless, insoluble salts K2PdCl4 (s) and K2PdCl6 (s) precipitate on the electrode in the solution of KCl. Thus, we have gotten an entire sketch for the species involved in the electrooxidation process of Pd in chloride containing solutions.
Fig. 5. A mechanistic scheme for the anodic processes of Pd in chloride containing solutions with the potential increase.
S. Chen et al. / Journal of Electroanalytical Chemistry 660 (2011) 80–84
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3.5. Electrochemical roughening of Pd and the application in electrocatalysis and SERS Pd electrodes have been roughened previously in 1 M H2SO4, a noncomplex medium, involving formation and reduction of Pd oxides [41,42]. We performed roughening of Pd electrodes here in 1 M HCl involving electrodissolution and electrodeposition of Pd under the guidance of above species sketch (Fig. 5). In 1 M HCl, the upper potential limit in the cyclic voltammetry played a key role in the resultant morphology. As shown in Fig. 6a and b, microspheres consisting of small aggregated Pd nanoparticles were obtained with an upper potential limit of 0.6 V. The Pd nanoparticles were formed by the repeated electrodissolution of bulk Pd and electrodeposition of the dissolved Pd(II) during the potential cycling. The produced nanoparticles can stick on the rough surface aggregating into microparticles under the action of strong electrolyte (1 M HCl). Since these particles were protected by the surfactant CTAB, they were not as active as the bulk electrode which kept electrodissolution. Therefore, assisted with the CTAB surfactant (the solid line), much higher surface area was obtained than roughening the Pd electrode in the blank HCl solution (the dash line in Fig. 7a). However, seriously etched macroporous structure occurred without the aggregated nanoparticles while the upper potential was changed to 1.0 V (Fig. 6c and d), and the solution turned into orange. The surface area of these etched pores is much lower than that of the microspheres. These phenomena can be ascribed to massive anodic dissolution of Pd into Pd(IV) complex and strong chemical etching by Cl2 evolution in this region as discussed in Section 3.2. Compared with the polished Pd electrode, the roughened Pd electrode in Fig. 6a possess very high surface area, as seen in Fig. 7a in 1 M NaOH background solution, and displayed superior catalytic activity for the electrooxidation of ethanol, as shown in Fig. 7b. Apparently, the onset potential for the oxidation of ethanol was about 100 mV lower and the anodic current density was about 5 times higher. Fig. 8 shows the in situ potential-dependent Raman spectra of pyridine adsorbed on the surface of roughed Pd. The Pd
Fig. 7. Cyclic voltammograms at 100 mV s1 on the electrodes of the roughed (real line) and the smooth Pd electrode (dotted line) in a solution of (a) 1 M NaOH and (b) 1 M NaOH + 1 M ethanol. The dash line in (a) indicated the CVs of Pd electrode treated in a solution of 1 M HCl without CTAB for comparison. The current density in (b) was normalized by the real surface areas.
Fig. 8. In situ SERS spectra of pyridine on the roughed Pd electrode in 0.01 M pyridine + 0.1 M KClO4. The peak at 937 cm1 is attributed to ClO4 .
Fig. 6. Typical SEM images with different magnifications for the Pd electrode treated in different potential range (a and b) 0.6 to 0.6 V for 300 cycles, (c and d) 0.6 to 1.0 V for 100 cycles in a solution of 1 M HCl + 10 mM CTAB.
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micro/nanostructure exhibits an obvious SERS activity, as evidenced by the typical band at 1004 cm1 for the typical ring breath vibration (v1), other peaks located at 629, 1561, 1591 cm1 are all the characteristic bands (v6a, v12, v8a) related to absorbed pyridine [42]. 4. Conclusions Electrodissolution processes of Pd in chloride containing solutions have been elucidated by in situ Raman spectroscopy. The species involved in a wide anodic potential window have been identified, including different soluble complexes and gas evolution. By potential cycling treatments of the Pd electrode in a given potential range, we can get micro-nanostructured surface with high area through the electrodissolution–electrodeposition of Pd. It affords us an efficient way to prepare high performance electrode materials of Pd for applications in electrocatalysis and SERS. Acknowledgments We are grateful for the financial supports of this research from Natural Science Foundation of Zhejiang Province of China (Grant No. Y4090658), the Open Foundation of Key Laboratory of the Ministry of Education for Advanced Catalysis Materials & Zhejiang Key Laboratory for Reactive Chemistry on Solid Surfaces (Grant No. DH201001), Ph.D. Programs Foundation of the Education Ministry of China (Grant No. 20104306110003), and National Natural Science Foundation of China (Grant Nos. 20673103 and 21003045). References [1] S.T. Mayer, R.H. Muller, J. Electrochem. Soc. 139 (1992) 426–434. [2] G.L. Chen, H. Lin, J.H. Lu, L. Wen, J.Z. Zhou, Z.H. Lin, J. Appl. Electrochem. 38 (2008) 1501–1508. [3] B.H. Loo, J. Phys. Chem. 86 (1982) 433–437. [4] B. Bozzini, A. Fanigliulo, J. Appl. Electrochem. 32 (2002) 1043–1048. [5] Z.L. Li, T.H. Wu, Z.J. Niu, W. Huang, H.D. Nie, Electrochem. Commun. 6 (2004) 44–48. [6] J.A. Calderón, O.R. Mattos, O.E. Barcia, S.I. Córdoba de Torresi, J.E. Pereira da Silva, Electrochim. Acta 47 (2002) 4531–4551. [7] A.G. Brolo, M. Odziemkowski, J. Porter, D.E. Irish, J. Raman Spectrosc. 33 (2002) 136–141.
[8] D. Larroumet, D. Greenfield, R. Akid, J. Yarwood, J. Raman Spectrosc. 38 (2007) 1577–1585. [9] D. Larroumet, D. Greenfield, R. Akid, J. Yarwood, J. Raman Spectrosc. 39 (2008) 1740–1748. [10] E.-S.M. Sherif, R.M. Erasmus, J.D. Comins, Electrochim. Acta 55 (2010) 3657–3663. [11] H. Tanabe, T. Shibuya, N. Kobayashi, T. Misawa, ISIJ Int. 37 (1997) 278–282. [12] M. Odziemkowski, J. Flis, D.E. Irish, Electrochim. Acta 39 (1994) 2225–2236. [13] S. Simard, M. Odziemkowski, D.E. Irish, L. Brossard, H. Ménarte, J. Appl. Electrochem. 31 (2001) 913–920. [14] C.T. Lee, M.S. Odziemkowski, D.W. Shoesmith, J. Electrochem. Soc. 153 (2006) B33–B41. [15] M. Reffass, R. Sabot, M. Jeannin, C. Berziou, Ph. Refait, Electrochim. Acta 52 (2007) 7599–7606. [16] S. Chen, Y.P. Chu, J.F. Zheng, Z.L. Li, Electrochim. Acta 54 (2009) 1102–1108. [17] S. Chen, S. Wu, J.F. Zheng, Z.L. Li, J. Electroanal. Chem. 628 (2009) 55–59. [18] M. Grden´, M. Łukaszewski, G. Jerkiewicz, A. Czerwin´, Electrochim. Acta 53 (2008) 7583–7598. [19] K. Gossner, E. Mizera, J. Electroanal. Chem. 125 (1981) 347–358. [20] L.D. Burke, D.T. Buckley, J. Electrochem. Soc. 143 (1996) 845–854. [21] C.C. Hu, T.C. Wen, Electrochim. Acta 41 (1996) 1505–1514. [22] K. Juodkazis, J. Juodkazyte, B. Šebeka, G. Stalnionis, A. Lukinskas, Russ. J. Electrochem. 39 (2003) 954–959. [23] R. Schumacher, W. Helbig, I. Hass, M. Wünsche, H. Meyer, J. Electroanal. Chem. 354 (1993) 59–70. [24] A.E. Bolzán, M.E. Martins, A.J. Arvía, J. Electroanal. Chem. 172 (1984) 221–233. [25] T. Solomun, J. Electroanal. Chem. 302 (1991) 31–46. [26] S.H. Cadle, J. Electrochem. Soc. 121 (1974) 645–648. [27] M. Grden´, J. Kotowski, A. Czerwin´ski, J. Solid State Electrochem. 3 (1999) 348–351. [28] M. Lukaszewski, A. Czerwinski, J. Electroanal. Chem. 589 (2006) 38–45. [29] M. Lukaszewski, A. Czerwinski, J. Alloys Compd. 473 (2009) 220–226. [30] M. Lukaszewski, T. Kedra, A. Czerwinski, Electrochem. Commun. 11 (2009) 978–982. [31] J.A. Harrison, T.A. Whitfield, Electrochim. Acta 28 (1983) 1229–1236. [32] J. Genescá, L. Victori, Platinum Met. Rev. 30 (1986) 80–83. [33] J. Genescá, R. Durán, Electrochim. Acta 32 (1987) 541–544. [34] M.W. Breiter, J. Electroanal. Chem. 81 (1977) 275–284. [35] C.C. Hu, T.C. Wen, J. Electrochem. Soc. 141 (1994) 2996–3001. [36] A.J. Bard, L.R. Faulkner, Electrochemical Methods, second ed., J. Wiley & Sons, New York, 2001. [37] P.L. Goggin, J. Mink, J. Chem. Soc., Dalton Trans. (1974) 1479–1483. [38] M. Debeau, H. Poulet, Spectrochim. Acta. 25A (1969) 1553–1562. [39] G. Hochenbleicher, H.W. Schrötter, Appl. Spectrosc. 25 (1971) 360–362. [40] H. Chang, D.M. Hwang, J. Raman Spectrosc. 7 (1978) 254–260. [41] T. Kessler, A. Visintin, A.E. Bolzan, G. Andreasen, R.C. Salvarezza, W.E. Triaca, A.J. Arvia, Langmuir 12 (1996) 6587–6596. [42] Z. Liu, Z.L. Yang, L. Cui, B. Ren, Z.Q. Tian, J. Phys. Chem. C 111 (2007) 1770–1775.
Contents lists available at ScienceDirect
Journal of Electroanalytical Chemistry journal homepage: www.elsevier.com/locate/jelechem
Study on the electrodissolution and roughening of a palladium electrode in chloride containing solutions Shu Chen a,b, Wei Huang b, Jufang Zheng c, Zelin Li b,⇑ a Key Laboratory of Theoretical Chemistry and Molecular Simulation of Ministry of Education of China, School of Chemistry and Chemical Engineering, Hunan University of Science and Technology, Xiangtan 411201, China b Key Laboratory of Chemical Biology and Traditional Chinese Medicine Research, Ministry of Education of China, College of Chemistry and Chemical Engineering, Hunan Normal University, Changsha 410081, China c Key Laboratory of the Ministry of Education for Advanced Catalysis Materials, Institute of Physical Chemistry, Zhejiang Normal University, Jinhua 321004, China
a r t i c l e
i n f o
Article history: Received 29 January 2011 Received in revised form 21 May 2011 Accepted 7 June 2011 Available online 15 June 2011 Keywords: Palladium In situ Raman spectroscopy Electrodissolution Micro-nanostructure Electrocatalysis
a b s t r a c t Potential-dependent surface electrooxidation processes of a Pd electrode in chloride containing solutions have been investigated in detail by means of in situ confocal Raman spectroscopy for the first time. In a solution of HCl, characteristic Raman bands for the oxidative coordination of Pd with Cl, the transformation of soluble Pd(II) to Pd(IV) complexes, the electrooxidation of Cl into Cl2, and the redox between Cl2 and Pd were all detected unambiguously during the potential ascending. While in a solution of KCl, insoluble salt films of K2PdCl4 and K2PdCl6 were found on the electrode surface due to their poor solubility. Moreover, electrochemical roughing of Pd electrode by electrodissolution–electrodeposition cycling in the HCl solution with a surfactant have been utilized to construct micro-nanostructures of Pd, which display high activity toward the electrooxidation of ethanol in an alkaline medium and obvious surface enhanced Raman spectroscopy (SERS) signals for pyridine probe molecules. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction Electrodissolution of metals and alloys involves complicated surface processes like adsorption/desorption, film formation of oxides or insoluble salts, pitting corrosion, coordination, and gas evolution. Raman spectroelectrochemistry is a promising tool to understand the physicochemical processes that occur at these metal/solution interfaces on the molecular level, and has been employed to investigate the electrodissolution course of many metals like Cu [1,2], Au [3–5], Co [6], Hg [7], Fe [8–10], Fe–P [11], carbon steel [12–15], Au–Sn [16], Pt–Ni alloy [17] and so on. Electrooxidation of Pd has been intensively investigated in noncomplex acids (e.g. perchloric or sulfuric acid) and in alkaline solutions [18–21], emphasizing the formation and growth of insoluble oxide/hydroxide layers in mild electrolyte. Their oxidation state and chemical composition at different potential limit are determined by diversiform surface analytical and materials characterization techniques [18]. Comparatively, in strong acid/basic medium and in the presence of anions (e.g. Cl or I), the electrooxidation of Pd facilitate the formation of kinds of soluble intermediates and products [22–30], which pronounces the electrodissolution of Pd by hydration or complexation of Pd cations. The electrooxidation
⇑ Corresponding author. Tel.: +86 731 88871533; fax: +86 731 88872531. E-mail address: [email protected] (Z. Li). 1572-6657/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jelechem.2011.06.008
of Pd in chloride containing electrolyte has received less attention up to the present and the understanding of electrochemically formed species only derived from electrochemical measurements [31–33]. Therefore, further investigation especially using in situ spectroscopy is required to confirm the possible species and intermediates involved in the electrodissolution. In this paper, in situ potential-dependent Raman spectroscopy has been performed to clarify the species in the electrooxidation of Pd in chloride containing solutions. With the knowledge of species at different potentials, we construct micro-nanostructures of Pd for electrocatalysis on the electrode surface by electrodissolution–electrodeposition cycling. 2. Experimental Raman measurements were performed with a Renishaw RM1000 confocal Raman spectrometer (Gloucestershire, UK) in a self-designed spectroscopic cell made by Teflon with a quartz window. The objective equipped on a DMLM Leica microscope was of long working distance (ca. 8 mm) with 50 magnifications. The exciting wavelength was 785 nm from a diode laser with a power of ca. 18.5 mW on the sample. More detailed descriptions on the instrument can be found elsewhere [5]. Normal electrochemical measurements were carried out with three electrodes using CHI 660C electrochemical station (Chenhua, Shanghai, China). The working electrode was a Pd disk made from a 1 mm diameter wire
81
S. Chen et al. / Journal of Electroanalytical Chemistry 660 (2011) 80–84
(purity P99.99%) procured from Tianjin Aida Co., Ltd. (China). The current density is related the geometric area of the electrode unless indicated otherwise. Prior to use the electrode was polished with 1000# metallographic paper and cleaned with ultrasonic waves in ultra-pure water (from a water purification system of Millipore Corp., USA). Then, the electrode subjected potential cycling in the range of 0.6 to 0.9 V in 0.5 M H2SO4 until the curves repeated. The roughness factor of the polished electrode was 20 with the conversion factor of 424 lC cm2 for the reduction of monolayer Pd oxides. The potential quoted here is versus the saturated mercurous sulfate electrode (SMSE). Cyclic voltammetry was applied to roughen the Pd electrode in 1 M HCl solution (M = mol dm3) containing a certain amount of cetyltrimethyl ammonium bromide (CTAB). Surface morphology of the Pd electrode was obtained with a JSM-6380LV scanning electron microscopy (SEM). SERS spectra were measured under the excitation laser of 633 nm with ca. 8.2 mW power and the collection time was 60 s. Pyridine was chosen as the probe molecule, which was detected by immersing the roughened Pd electrode in the solution of 0.01 M pyridine + 0.1 M KClO4. All experiments were performed at room temperature (20 °C) and solutions were freshly prepared with analytical grade chemicals and ultra-pure water. 3. Results and discussion 3.1. Electrochemical behaviors of Pd in HCl
The effect of Cl ions on the electrodissolution of Pd is presented in Fig. 1 with different HCl concentrations. The cyclic voltammograms in the blank solution of aqueous perchloric acid in Fig. 1a (dotted and dashed lines) resembled those reported in numerous investigations for polycrystalline Pd [18–21], where adsorption/ absorption–desorption of hydrogen and the formation–reduction
Fig. 1. (a) Cyclic voltammograms of the Pd electrode in 1 M HClO4 (dotted and dashed line) and in 1 M HClO4 + 0.01 M HCl (solid line) within different potential ranges at a scan rate of 100 mV s1. (b) Linear sweep voltammograms at 100 mV s1 for the Pd electrode by changing the HCl concentration from 0.1 to 1 M as indicated. (c) Series of anodic sweep voltammograms in 0.1 M HCl at different scan rate and the relationship between first anodic peak current and square root of scan rate.
of surface Pd (hydro)oxides proceeded in the lower and higher potential ranges, respectively. In order to exclude the interference from the oxidation of absorbed hydrogen into bulk Pd [30,34,35], the lower potential limit was set such that it was not less than 0.3 V in the electrochemical experiments for anodic dissolution. In 1 M HClO4, the oxidation of Pd started at around 0.2 V in the positive-going potential scan and the reduction of the oxides began at 0.1 V in the negative-going scan. In the presence of 0.01 M Cl ions (the solid line in Fig. 1a), two pairs of new redox peaks appeared, which are attributed to the formation and reduction of two possible complex ions of Pd with chloride [31–33]: 2
Pd þ 4Cl ¼ PdCl4 þ 2e 2 PdCl4
þ 2Cl ¼
2 PdCl6
ðiÞ
þ 2e
ðiiÞ
Evidence for the involvement of such soluble complexes will be resolved latter by Raman spectra (Section 3.2). Notably, the two anodic peak currents increased remarkably (Fig. 1b) by increasing the concentration of HCl. Moreover, the first anodic peak current (Ip,a) varied linearly with the square root of the scan rates (m1/2) and the numbers of electron transferred were estimated closed to 2, as shown in Fig. 1c [36]. 3.2. In situ Raman spectroscopy for the electrooxidation of Pd in 1 M HCl Fig. 2 shows the Raman spectra of Pd in 1 M HCl for a sequence of anodic potentials applied. There were no Raman peaks below 0 V before the electrodissolution of Pd. The onset of surface oxidation was signaled near 0.1 V with a discernible band at 304 cm1, locating at the lower end of the first anodic climbing branch in Fig. 1b. This band 2 is assigned to the Pd–Cl stretching (v1) in PdCl4 [37] from Reaction (i) which gained in intensity with the increase of potential. Note that the stretching of Pd—Clads for the monolayer of adsorbed Cl is too weak to be detected. At 0.7 V, which situates at the lower end of the second ascending branch in Fig. 1b, the peak at 304 cm1 was substituted by a double band of Pd–Cl stretching (v1, v2) at 2 293/315 cm1 for the species of PdCl6 [38] from Reaction (ii). Immediately following this doublet, three more new peaks appeared at 535 (shoulder), 540 and 547 cm1 while E P 0.8 V and their inten-
Fig. 2. Potential-dependent in situ Raman spectra for the Pd electrode in 1 M HCl during increasing the potential. The collection time for each spectrum was 10 s with a 785 nm laser.
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sities got stronger till 1.1 V. The bands appeared as three peaks can be ascribed to the fundamentals and the hot bands of the isotopic Cl2 molecules [39,40]. The produced Cl2 (g) at 0.8 V and above agitated the solution near the electrode surface and dispersed some 2 PdCl6 into the bulk solution, and that is the reason why the spec2 trum intensity of PdCl6 was the strongest at 0.7 V. When E P 1.2 V where oxygen released, the triplet and the doublet remarkably weakened and strengthened, respectively. We suppose that these three peaks are related with the formation of Cl2:
2Cl ! Cl2 þ 2e
ðiiiÞ
To confirm the supposition, a Cl2 containing nonaqueous solution was prepared by bubbling chlorine gas through the cyclohexane (C6H12). The triplet at 535, 540, 547 cm1 was observed indeed (Fig. 3a) for the solution. More interestingly, we also confirmed that the produced Cl2 can react with Pd chemically forming 2 PdCl6 very likely by the following reaction:
2
Pd þ 2Cl2 þ 2Cl ¼ PdCl6
ðivÞ 2
And it might undergo two steps like 2
Pd þ Cl2 þ 2Cl ! PdCl4 2 PdCl4
þ Cl2 !
2 PdCl6
ðvÞ ðviÞ
We confirmed Reaction (iv) by setting the circuit open after polarizing the electrode at 1 V (Fig. 3b). The intensities of the dou2 blet for PdCl6 and of the triplet for Cl2 went oppositely upon open2 ing the circuit because the moieties of Cl2 and PdCl6 were consumed and produced, respectively, as expressed in Reaction 2 (iv). The exhaustion of Cl2 and the diffusion of PdCl6 account for the vanishing and diminishing of the triplet and doublet, respectively, in 30 s (Fig. 3b). Moreover, the opposite tendency in inten2 sity for Cl2 (triplet) and for PdCl6 (doublet) at 1.2 V in Fig. 2 can also be explained by Reaction (iv) because the convection induced by oxygen evolution (vii) can speed Reaction (iv).
H2 O ! ð1=2ÞO2 þ 2Hþ þ 2e
Fig. 4. In situ Raman spectra at selected potentials for the Pd electrode in 1 M KCl solution. Its anodic sweep voltammogram at 100 mV s1 was shown in the inset.
305 cm1 for PdCl4 was always there. These facts indicate that salt films of K2PdCl4, K2PdCl6 precipitated on the surface: 2
PdCl4 þ 2Kþ ¼ K2 PdCl4 ðsÞ 2 PdCl6
þ
þ 2K ¼ K2 PdCl6 ðsÞ
ðviiiÞ ðixÞ
Although the triplet for Cl2 did not show up for some reasons, Reactions (iv)–(vi), incorporating (viii) and (ix), should be still the reason for the increase in intensity after opening the circuit. We observed that the anodic peak current related K2PdCl4 (the first peak) was larger in 1 M HCl (Fig. 1b) than in 1 M KCl (the inset of Fig. 4). However, opposite situation appeared for the anodic peak current related K2PdCl6 (the second peak). These facts suggest that the film of K2PdCl4 rather than K2PdCl6 blocked the electrochemical reaction on the surface in some extent.
ðviiÞ 3.4. A potential-dependent species sketch for the electrooxidation of Pd
3.3. In situ Raman spectroscopy for the electrooxidation of Pd in 1M KCl Raman bands for Pd(II) and Pd(IV) species also occurred with 1 M KCl (Fig. 4) at 0.3 V and 0.9 V, respectively, corresponding to the oxidative current peaks (the inset of Fig. 4). Again, the doublet 2 at 293/324 cm1 for the PdCl6 got stronger in intensity upon opening the circuit then. However, the intensity of the doublet did not decrease for a long period of time, and the small peak at
Fig. 3. (a) Raman spectra for the sample of liquid cyclohexane before and after absorbing chlorine gas. (b) In situ Raman spectra for the Pd electrode when the cell was set at 1 V and then let the circuit open. The exposure time for CCD was 10 s.
Based on the spectroelectrochemical results and discussion, the electrooxidation processes of Pd are summarized in Fig. 5. In the HCl solution, the Pd electrode is electrooxidized into two soluble 2 2 complexes PdCl4 (aq) and PdCl6 (aq) successively with the potential increase; Cl can be then oxidized into gaseous Cl2 (g) parallel 2 to the formation of PdCl6 (aq), so the oxidation of Pd can also proceed chemically by reacting with the produced Cl2 (g); and the convection induced by the O2 (g) evolution at still higher potentials 2 can accelerate the chemical reactions of Pd with Cl2 into PdCl6 (aq). Nevertheless, insoluble salts K2PdCl4 (s) and K2PdCl6 (s) precipitate on the electrode in the solution of KCl. Thus, we have gotten an entire sketch for the species involved in the electrooxidation process of Pd in chloride containing solutions.
Fig. 5. A mechanistic scheme for the anodic processes of Pd in chloride containing solutions with the potential increase.
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3.5. Electrochemical roughening of Pd and the application in electrocatalysis and SERS Pd electrodes have been roughened previously in 1 M H2SO4, a noncomplex medium, involving formation and reduction of Pd oxides [41,42]. We performed roughening of Pd electrodes here in 1 M HCl involving electrodissolution and electrodeposition of Pd under the guidance of above species sketch (Fig. 5). In 1 M HCl, the upper potential limit in the cyclic voltammetry played a key role in the resultant morphology. As shown in Fig. 6a and b, microspheres consisting of small aggregated Pd nanoparticles were obtained with an upper potential limit of 0.6 V. The Pd nanoparticles were formed by the repeated electrodissolution of bulk Pd and electrodeposition of the dissolved Pd(II) during the potential cycling. The produced nanoparticles can stick on the rough surface aggregating into microparticles under the action of strong electrolyte (1 M HCl). Since these particles were protected by the surfactant CTAB, they were not as active as the bulk electrode which kept electrodissolution. Therefore, assisted with the CTAB surfactant (the solid line), much higher surface area was obtained than roughening the Pd electrode in the blank HCl solution (the dash line in Fig. 7a). However, seriously etched macroporous structure occurred without the aggregated nanoparticles while the upper potential was changed to 1.0 V (Fig. 6c and d), and the solution turned into orange. The surface area of these etched pores is much lower than that of the microspheres. These phenomena can be ascribed to massive anodic dissolution of Pd into Pd(IV) complex and strong chemical etching by Cl2 evolution in this region as discussed in Section 3.2. Compared with the polished Pd electrode, the roughened Pd electrode in Fig. 6a possess very high surface area, as seen in Fig. 7a in 1 M NaOH background solution, and displayed superior catalytic activity for the electrooxidation of ethanol, as shown in Fig. 7b. Apparently, the onset potential for the oxidation of ethanol was about 100 mV lower and the anodic current density was about 5 times higher. Fig. 8 shows the in situ potential-dependent Raman spectra of pyridine adsorbed on the surface of roughed Pd. The Pd
Fig. 7. Cyclic voltammograms at 100 mV s1 on the electrodes of the roughed (real line) and the smooth Pd electrode (dotted line) in a solution of (a) 1 M NaOH and (b) 1 M NaOH + 1 M ethanol. The dash line in (a) indicated the CVs of Pd electrode treated in a solution of 1 M HCl without CTAB for comparison. The current density in (b) was normalized by the real surface areas.
Fig. 8. In situ SERS spectra of pyridine on the roughed Pd electrode in 0.01 M pyridine + 0.1 M KClO4. The peak at 937 cm1 is attributed to ClO4 .
Fig. 6. Typical SEM images with different magnifications for the Pd electrode treated in different potential range (a and b) 0.6 to 0.6 V for 300 cycles, (c and d) 0.6 to 1.0 V for 100 cycles in a solution of 1 M HCl + 10 mM CTAB.
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micro/nanostructure exhibits an obvious SERS activity, as evidenced by the typical band at 1004 cm1 for the typical ring breath vibration (v1), other peaks located at 629, 1561, 1591 cm1 are all the characteristic bands (v6a, v12, v8a) related to absorbed pyridine [42]. 4. Conclusions Electrodissolution processes of Pd in chloride containing solutions have been elucidated by in situ Raman spectroscopy. The species involved in a wide anodic potential window have been identified, including different soluble complexes and gas evolution. By potential cycling treatments of the Pd electrode in a given potential range, we can get micro-nanostructured surface with high area through the electrodissolution–electrodeposition of Pd. It affords us an efficient way to prepare high performance electrode materials of Pd for applications in electrocatalysis and SERS. Acknowledgments We are grateful for the financial supports of this research from Natural Science Foundation of Zhejiang Province of China (Grant No. Y4090658), the Open Foundation of Key Laboratory of the Ministry of Education for Advanced Catalysis Materials & Zhejiang Key Laboratory for Reactive Chemistry on Solid Surfaces (Grant No. DH201001), Ph.D. Programs Foundation of the Education Ministry of China (Grant No. 20104306110003), and National Natural Science Foundation of China (Grant Nos. 20673103 and 21003045). References [1] S.T. Mayer, R.H. Muller, J. Electrochem. Soc. 139 (1992) 426–434. [2] G.L. Chen, H. Lin, J.H. Lu, L. Wen, J.Z. Zhou, Z.H. Lin, J. Appl. Electrochem. 38 (2008) 1501–1508. [3] B.H. Loo, J. Phys. Chem. 86 (1982) 433–437. [4] B. Bozzini, A. Fanigliulo, J. Appl. Electrochem. 32 (2002) 1043–1048. [5] Z.L. Li, T.H. Wu, Z.J. Niu, W. Huang, H.D. Nie, Electrochem. Commun. 6 (2004) 44–48. [6] J.A. Calderón, O.R. Mattos, O.E. Barcia, S.I. Córdoba de Torresi, J.E. Pereira da Silva, Electrochim. Acta 47 (2002) 4531–4551. [7] A.G. Brolo, M. Odziemkowski, J. Porter, D.E. Irish, J. Raman Spectrosc. 33 (2002) 136–141.
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