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The cathodic insertion of tetraalkylammonium iodides into palladium
Electrochemistry Communications 3 (2001) 209±214
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The cathodic insertion of tetraalkylammonium iodides into palladium Charles Cougnon, Jacques Simonet
*
Laboratoire d'Electrochimie Mol eculaire et Macromol eculaire, UMR CNRS 6510, Campus de Beaulieu, Universit e de Rennes 1, 35042 Rennes Cedex, France Received 3 January 2001; received in revised form 25 January 2001; accepted 9 February 2001
Abstract The cathodic behavior of tetraalkylammonium iodides was studied at a polished palladium cathode in aprotic dimethylformamide (DMF). Coulometric and electrochemical quartz crystal microbalance analyses allowed to show the formation of phases, the stoichiometry of which was fully determined. The reversible insertion of tetraalkylammonium cations as well as the electrolyte into palladium was demonstrated. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Palladium cathodes; Tetraalkylammonium iodides; Insertion phases; Chemically modi®ed cathodes
1. Introduction The cathodic reduction of onium salts ± principally at mercury cathodes ± has appeared to be an ecient way of carbon±heteroatom bond cleavage. Thus, some trialkylsulfonium and tetraalkylammonium cations were reported to cleave under heterogeneous electron transfer at metallic cathodes [1±5]. Thus, benzyltrimethylammonium, trimethyl sulfonium and tertiarybutyldimethylsulfonium cations appeared to be a convenient source of benzyl, methyl and tertiarybutyl radicals, respectively. As an example, alkylation via alkylradicals produced homogeneously (reduction of ammonium salts, used as electrolyte by radical anions) have been described [6]. Thus, alkylation of acenaphthylene and anthracene was achieved by this way. On the other hand, redox catalysis of Me4 N by phenanthrene anion radical was demonstrated to occur. But what happens with tetraalkylammonium cations (like Me4 N and n-Bu4 N , the mostly used for analytical purposes) at the cathodic interface? For the ®rst time, Horner used electrogenerated tetraalkylammonium amalgams as a reducing reagent [7,8]. Rather similarly, ammonium cations could cathodically form at graphite (especially highly oriented pyrolitic graphites) a * Corresponding author. Tel.: +33-2992-86292; fax: +33-299286292. E-mail address: [email protected] (J. Simonet).
lamellar compounds where stages such as C24 NBu4 and C12 NMe4 were demonstrated to be cathodically produced [9±11]. The behavior of tetraalkylammonium salts at various electrodes (mercury, lead, bismuth, tin or antimony) were extensively described by Kariv-Miller [12,13]. It was concluded that direct onium reduction did not lead to electrolyte degradation but mainly to the insertion of these cations. So, the ``reduction'' of tetramethylammonium ions at lead and mercury cathodes principally aord Pb5 N Me4 and Hg5 N Me4 phases [14,15]. Actually, phases possessing such a stoichiometry resemble Zintl phases [16,17] currently obtained with alkali metals but under fundamentally dierent experimental conditions. Very recently, it has been pointed out that cathodically polarized platinum could insert not only tetraalkylammonium cations but also associated anions when the used solvent (mainly dimethylformamide (DMF)) was carefully dried [18]. The formation of organometallic salts involving the electrolyte was clearly established and the stoichiometry of insertion de®ned [19]. The authors wish to report here the behavior of palladium when used as a cathode material and negatively polarized in the presence of tetraalkylammonium iodides, currently used as electrolyte. The aim of this preliminary paper is to stress that cathodic reduction may be an ecient tool to modify precious metals according to a new procedure and possibly build new materials for catalysis.
1388-2481/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 8 - 2 4 8 1 ( 0 1 ) 0 0 1 3 5 - 7
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C. Cougnon, J. Simonet / Electrochemistry Communications 3 (2001) 209±214
2. Experimental 2.1. Tetraalkylammonium salts and solvent All salts studied in the present paper were tetraalkylammonium iodides. In most experiments, electrolyte concentration was 0.1 M in DMF. Salts were purchased from Fluka (puriss. grade) and were used without any further puri®cation after being drastically dried under vacuum at 100°C during 48 h. DMF (purum quality), purchased from SDS, was checked to contain less than 50 ppm of water. Moreover, solvent was carefully stocked in situ over neutral alumina (Merck), previously activated for several hours under vacuum at 300°C. Finally, all experiments were performed under a dry argon atmosphere. Lastly, electrolyte solutions were maintained in contact of alumina into the cell during coulometric and EQCM experiments. 2.2. Electrochemical procedures Cyclic voltammetry investigations were carried out by using a classical three-electrode cell. Experiments were performed with an lAUTOLAB potentiostat from Eco Chemie BV, connected to a computer equipped with a general purpose electrochemical system (GPES) software (version 4.5 for Windows). For analytical purposes, the working electrode was a palladium disk (8 10 3 cm2 ) and the counter electrode was a glassy carbon rod. All potentials were referred to the system Ag/AgI/I n-Bu4 N 0.1 M in DMF. Beforehand, the working electrode was carefully polished with silicon carbide (Struers) using successively smaller particle sizes (from 18 to 5 lm). After that the electrode was extensively polished using diamond powder (6 and 3 lm). Finally, the working electrode was rinsed with ethanol and acetone and then dried. Between each scan, the electrode surface was re-polished with diamond powder (3 lm). On the other hand, for macroelectrolyses investigations, Pd sheets, purchased from Strem (puriss grade: 99.95%, area: 1 cm2 , 0.1 mm thickness) were employed ± only one time each ± without any further treatment. 2.3. Chronocoulometric investigations on thin Pd layer Coulometric experiments were carried out on palladium ®lm electrodes which were electrochemically prepared by depositing palladium from a solution of 10 g/l PdCl2 in 0.1 M HCl onto a polished gold disk (2 10 3 cm2 ). The plating was done at a constant current (10 2 A/cm2 ). All experiments were performed using the same substrate as the one described above but at dierent thicknesses of palladium deposit.
2.4. Electrochemical quartz (EQCM) instrumentation
crystal
microbalance
Simultaneous cyclic voltammetric and mass balance experiments were carried out with an oscillator module (Seiko EG&G Quartz Cristal Analyzer 917) connected to an EG&G PAR 273 potentiostat. This device was used under computer control using a research electrochemistry software (ECHEM 270, version 4.3 for Windows). In the experiments, 9 MHz AT-cut quartz crystals (EG&G) coated with gold and then electrochemically plated with a thin ®lm of Pd were employed. Plating of Pd was achieved still following the procedure which is described in Section 2.3. EQCM measurements were achieved in a Te¯on three-electrode cell. The working electrode was the palladium-plated quartz crystal itself. In all these experiments the apparent area of the quartz crystal was about 0.2 cm2 . Microgravimetric data, reported in terms of mass change Dm, were calculated using the Sauerbrey equation that links resonant frequency and mass modi®cation [20]. 2.5. Scanning electron microscopy (SEM) experiments Surfaces treated electrochemically (palladium samples were rinsed using a mixture of alcohol and acetone in an ultrasound vat for 2 h) were analyzed with a JEOL scanning microscope (model JSM-6301-F). This device was equipped with a ®eld emission gun in which an ultra vacuum was held using an ionic pump (10 9 Torr). The accelerating tension was 7 kV, and resolu tion was 15 A.
3. Results and discussion 3.1. Voltammetry at a stationary palladium electrode Tetraalkylammonium iodides studied display, at slow scan rates (0.2 V/s), an irreversible cathodic peak (or wave) always associated with a corresponding anodic peak (Epa) showing a re-oxidation process (Table 1). For example n-Hex4 NI (0.1 M) alone in dry DMF (maintained carefully on neutral alumina) exhibits a wave that indicates a slow electron transfer (Fig. 1). Cathodic currents in the presence of Me4 NI and Bu4 NI were found to vary linearly with the square root of the scan rate up to 5 V/s and show a global diusional process. They also vary strongly with the salt concentration, the nature and the size of the tetraalkylammonium cation. Thus, the higher is the bulkiness of the cation, the smaller is the current limit of the cathodic wave.
C. Cougnon, J. Simonet / Electrochemistry Communications 3 (2001) 209±214 Table 1 Potential values and current peaks (cathodic and anodic) obtained during cyclic voltammetry at a palladium cathode (8 10 relative to a 0.1 M tetraalkylammonium salt solution in DMFa Electrolyte b
Me4 NI Et4 NI n-Bu4 NI n-Hex4 NI a b
Epc (V) 1:93 2:14 2:32 2:36
Ipc (mA/cm2 ) 2:11 1:87 1:62 1:37
Epa (V) 1:25 1:21 0:81 0:7
211 3
cm2 ) freshly polished
Ipa (mA/cm2 )
Epa
0.72 0.54 0.29 0.17
0.68 0.93 1.51 1.66
Epc (V)
Sweep potential: 0.2 V/s. Potentials are all referred to Ag/AgI/I 0.1 M. a, transfer coecient, was estimated from the width of cathodic peaks. Me4 NI was analyzed at 10 mM in presence of a 0.1 M tetrabutylammonium iodide, due to this weak solubility.
Fig. 1. Voltammetric behavior of a stationary palladium electrode (8 10 3 cm2 ) in contact with 0.1 M n-Hex4 NI in dry DMF. Scan rate: 0.2 V/s. Potential was referred to the Ag/AgI/I 0.1 M system.
3.2. Microelectrolyses and SEM analysis of treated palladium surfaces Preparative microelectrolyses were achieved at quite negative potentials (between 2:2 and 2:4 V). The applied amount of electricity (Qred ) were relatively small (from 1 to 7.5 C/cm2 ). When a Pd sample cathodically exposed to Et4 NI 0.1 M in DMF (Fig. 2(b)) is compared to a blank surface (Fig. 2(a)), dramatic structure changes appears. In any case, speci®c surface alterations are noticed. SEM probes on crystals and dark deposits on the surface of treated palladium sheets (Fig. 3(a)), showed that elements such as C, Pd and I are present (Fig. 3(b)). Crystals and black areas can be probably assigned to a palladium phase in a progressed oxidation by air. Lastly, a prolonged contact with air allows the total emergence of the tetraalkylammonium salts on the bulk of palladium, strongly suggesting its insertion in the course of the cathodic process. 3.3. Microcoulometries on thin ®lms of palladium deposited electrochemically Thin ®lms of palladium were deposited onto glassy carbon and gold by galvanostatic reduction in a 0.1 M HCl solution of PdCl2 . These two latter materials did
Fig. 2. SEM analysis of a palladium sheet treated with a 0.1 M Et4 NI solution. (a) exhibits the surface of a commercial palladium sheet. (b) shows the surface modi®cation of the same sample cathodically treated (electrolyzed at E 2:2 V in 0.1 M Et4 NI in dry DMF). The amount of electricity consumed was 1 C/cm2 .
not show insertion compounds (blank experiments). Mostly, it was veri®ed that Pd2 gave a well-de®ned step at 0:2 V (SCE) and the deposit was achieved almost in quantitative yield. According to simple calculations taking into account the radius of a palladium atom [21] the average coverage of gold by palladium (1.37 A), should yield a layer of about 0.1±5 lm. Under these
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C. Cougnon, J. Simonet / Electrochemistry Communications 3 (2001) 209±214
Fig. 3. Surface changes of the same palladium sheet seen in Fig. 2(b). (a) aggregation of matter which takes shape during oxidation process. (b) electron density spectroscopy (EDS) analysis results of the matter observed above. (c) EDS analysis of a commercial palladium sheet (blank experiment).
conditions, thin ®lms of palladium deposited at a microgold electrode (2 10 3 cm2 ) were electrolyzed in a 0.1 M tetraalkylammonium solution in dry DMF at the level of the peak potential reported in Table 1. More than 30 s were necessary to fully saturate the Pd deposit (Q < 1 C/cm2 ). The charge recording shows (Fig. 4) that a continuous quantity of cathodic electricity amount (Qc ) was consumed, probably corresponding both to the reduction of the solvent and to the cathodic treatment of the metal. In return, the oxidation charge (Qa ) depends
only on cluster collapse and can be considered to be speci®c to palladium phase formation. Thus, according that the decomposition of the bulk material require the transfer of one electron, Qa allows to determine ratio of R4 N /Pdx . In consequence, tetraalkylammonium solutions were electrolyzed at the peak potential (during 60 s) and, immediatly reoxidized at 0 V until completion (zero current). Table 2 exhibits coulometric results deduced by integration of the oxidation current (Qa ).
C. Cougnon, J. Simonet / Electrochemistry Communications 3 (2001) 209±214
213
Fig. 4. Chronocoulometric investigation of 0.1 M n-Hex4 NI in dry DMF. Reduction at E 2:35 V of a thin ®lm of palladium (deposit charge from PdCl2 0:1 C/cm2 deposited onto a microgold electrode (2 10 3 cm2 ) during 60 s and immediately after oxidation at E 0 V until completion. Table 2 Microcoulometric analysis resultsa Electrolyte (0.1 M) Me4 NI Et4 NI n-Bu4 NI n-Hex4 NI
E (V) reduction 1:90 2:20 2:30 2:35
E (V) oxidation
x values for (R4 N )Pdx insertion
0 0 0 0
2:13 0:09 2:01 0:08 2:10 0:15 2:07 0:13
a
Ratio x (see Section 3.3) of palladium atoms per electron found for several tetraalkylammonium salts. The palladium layer was electrodeposited onto a gold microelectrode (2 10 3 cm2 ) with a charge between 2:5 10 2 and 25 10 2 C/cm2 . The layer was saturated during 60 s and recovered electricity was integrated at 0 V (oxidation at the level of the discharge anodic peak).
3.4. Mass increase analysis by EQCM Coulometric measurements have previously shown that each electron transfer involves the participation of two palladium atoms (Table 2); it now remained to check whether the transfer of each electron was associated with the insertion of one cation. All experiments were carried out with known weights of deposited palladium which permit to quantify the mass increase. In Fig. 5(a), the curves (corresponding to dierent amounts of Pd deposited) show a regular mass increase upon potential during the ®rst times and reach a plateau assigned to the saturation of palladium ®lms. It is noteworthy to check that the mass increases are proportional to the quantity of palladium (Fig. 5(b)), attesting a mass phenomenon and not a surface modi®cation. Thus, it must be pointed out that the electron transfer relative to the metal was accompanied not only by the insertion of one ammonium cation but also by the insertion of electrolyte in a concomitant manner. In the case of iodides Pd2 ; N R4 ; y NR4 I;
Fig. 5. EQCM results obtained during electrolysis at E 2:35 V vs Ag/AgI/I 0.1 M of dierent amounts of deposit palladium onto a gold quartz blade, in 0.1 M n-Hex4 NI solution. (a) exhibits experimental mass increase until completion (fully saturation of palladium layers). (b) shows the linear relationship between these mass increases upon the amounts of palladium deposited.
Table 3 Mass investigation results in DMF containing tetraalkyliodides at a palladium cathodea Entry
Electrolyte (0.1 M)
1 2 3 4
Me4 NI Et4 NI n-Bu4 NI n-Hex4 NI
Applied potential (V) 1.90 2.20 2.30 2.35
y values 0:92 0:02 0:95 0:05 0:92 0:04 0:97 0:06
a
Use of EQCM technique in the measurement of insertion of electrolyte (Pt2 , R4 N , yR4 NI) in palladium during the cathodic reduction. With each salt at least four experiments were achieved.
Table 3 gathers EQCM results. As a matter of fact, all these measurements ®t quite well in all cases with the value y 1. This value was constantly found, whatever the experimental conditions (concentration of the salt, thickness of the Pd layer). These values allowed to exclude practically concomitant insertion of solvent. Moreover, no weight loss was noticed in the course of experiments.
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C. Cougnon, J. Simonet / Electrochemistry Communications 3 (2001) 209±214
4. Conclusion The cathodic polarization of palladium in aprotic DMF containing tetraalkylammonium iodides led to complex phases corresponding to the insertion of the electrolyte into the metallic bulk. These phases were de®ned thanks to microcoulometric analysis and EQCM technique. The use of palladium deposits of dierent thicknesses has demonstrated the electrolyte insertion into the metallic bulk. Therefore the described process (insertion into palladium) cannot be assigned only to a surface phenomenon. Analysis revealed that the stoichiometry of phases (clusters?) does not vary at all with the size of tetraalkylammonium cation: as a matter of fact, tetramethylammonium aords the same stoichiometry as tetra n-hexylammonium. At this stage, questions arise about the electronic conductivity of the growing layer in the course of electrolyses. Lastly, instability of cathodic phases of palladium in the contact of dioxygen leads to a more or less fast re-oxidation process in which both Pd0 is regenerated and inserted electrolyte totally emerges from the metallic bulk, as fully demonstrated by means of the SEM and EQCM techniques. Quite frequently, the oxidation by air resembles a corrosion phenomenon especially with formation of pin holes and grain boundaries. Acknowledgements The authors are grateful to the University of Rennes 1 and CNRS (UMR 6510) for partial ®nancial and
technical support. They also thank Prof. Le Lannic for his precious contribution in SEM experiments. References [1] S.D. Ross, M. Finkelstein, R.C. Petersen, J. Am. Chem. Soc. 82 (1960) 1582. [2] J.S. Mayell, A.J. Bard, J. Am. Chem. Soc. 85 (1963) 421. [3] C.E. Dahm, D.G. Peters, J. Electroanal. Chem. 402 (1996) 91. [4] E.A.H. Hall, J. Simonet, H. Lund, J. Electroanal. Chem. 100 (1979) 197. [5] A. Ghanimi, J. Simonet, N. J. Chem. 21 (1997) 257. [6] P. Martigny, J. Simonet, J. Electroanal. Chem. 101 (1979) 275. [7] L. Horner, in: M.M. Baizer, H. Lund (Eds.), Organic Electrochemistry, second ed., New York, 1991 (Chapter 12). [8] L. Horner, H. Neumann, Chem. Ber. 98 (1965) 1715. [9] J. Simonet, H. Lund, J. Electroanal. Chem. 75 (1977) 719. [10] G. Bernard, J. Simonet, J. Electroanal. Chem. 96 (1979) 249. [11] J. Berthelot, M. Jubault, J. Simonet, J. Chem. Soc. Chem. Commun. 284 (1982) 759. [12] V. Svetlicic, P.B. Lawin, E. Kariv-Miller, J. Electroanal. Chem. 284 (1990) 185. [13] E. Kariv-Miller, P.D. Christian, V. Svetlicic, Langmuir 11 (1995) 1817. [14] E. Kariv-Miller, P.B. Lawin, J. Electroanal. Chem. 247 (1988) 345. [15] V. Svetlicic, E. Kariv-Miller, J. Electroanal. Chem. 209 (1986) 91. [16] R. Nesper, Prog. Solid State Chem. 20 (1990) 1. [17] J.D. Corbett, Chem. Rev. 85 (1985) 383. [18] J. Simonet, Y. Astier, C. Dano, J. Electroanal. Chem. 451 (1998) 5. [19] J. Simonet, E. Labaume, J. Rault-Berthelot, Electrochem. Commun. 1 (1999) 252. [20] G. Sauerbrey, Z. Phys. 155 (1959) 206. [21] B.K. Vainshtein, V.M. Fridkin, V.L. Indenbom, Structure of Crystals, second ed., Springer, Berlin, 1995 (Chapter 1).
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The cathodic insertion of tetraalkylammonium iodides into palladium Charles Cougnon, Jacques Simonet
*
Laboratoire d'Electrochimie Mol eculaire et Macromol eculaire, UMR CNRS 6510, Campus de Beaulieu, Universit e de Rennes 1, 35042 Rennes Cedex, France Received 3 January 2001; received in revised form 25 January 2001; accepted 9 February 2001
Abstract The cathodic behavior of tetraalkylammonium iodides was studied at a polished palladium cathode in aprotic dimethylformamide (DMF). Coulometric and electrochemical quartz crystal microbalance analyses allowed to show the formation of phases, the stoichiometry of which was fully determined. The reversible insertion of tetraalkylammonium cations as well as the electrolyte into palladium was demonstrated. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Palladium cathodes; Tetraalkylammonium iodides; Insertion phases; Chemically modi®ed cathodes
1. Introduction The cathodic reduction of onium salts ± principally at mercury cathodes ± has appeared to be an ecient way of carbon±heteroatom bond cleavage. Thus, some trialkylsulfonium and tetraalkylammonium cations were reported to cleave under heterogeneous electron transfer at metallic cathodes [1±5]. Thus, benzyltrimethylammonium, trimethyl sulfonium and tertiarybutyldimethylsulfonium cations appeared to be a convenient source of benzyl, methyl and tertiarybutyl radicals, respectively. As an example, alkylation via alkylradicals produced homogeneously (reduction of ammonium salts, used as electrolyte by radical anions) have been described [6]. Thus, alkylation of acenaphthylene and anthracene was achieved by this way. On the other hand, redox catalysis of Me4 N by phenanthrene anion radical was demonstrated to occur. But what happens with tetraalkylammonium cations (like Me4 N and n-Bu4 N , the mostly used for analytical purposes) at the cathodic interface? For the ®rst time, Horner used electrogenerated tetraalkylammonium amalgams as a reducing reagent [7,8]. Rather similarly, ammonium cations could cathodically form at graphite (especially highly oriented pyrolitic graphites) a * Corresponding author. Tel.: +33-2992-86292; fax: +33-299286292. E-mail address: [email protected] (J. Simonet).
lamellar compounds where stages such as C24 NBu4 and C12 NMe4 were demonstrated to be cathodically produced [9±11]. The behavior of tetraalkylammonium salts at various electrodes (mercury, lead, bismuth, tin or antimony) were extensively described by Kariv-Miller [12,13]. It was concluded that direct onium reduction did not lead to electrolyte degradation but mainly to the insertion of these cations. So, the ``reduction'' of tetramethylammonium ions at lead and mercury cathodes principally aord Pb5 N Me4 and Hg5 N Me4 phases [14,15]. Actually, phases possessing such a stoichiometry resemble Zintl phases [16,17] currently obtained with alkali metals but under fundamentally dierent experimental conditions. Very recently, it has been pointed out that cathodically polarized platinum could insert not only tetraalkylammonium cations but also associated anions when the used solvent (mainly dimethylformamide (DMF)) was carefully dried [18]. The formation of organometallic salts involving the electrolyte was clearly established and the stoichiometry of insertion de®ned [19]. The authors wish to report here the behavior of palladium when used as a cathode material and negatively polarized in the presence of tetraalkylammonium iodides, currently used as electrolyte. The aim of this preliminary paper is to stress that cathodic reduction may be an ecient tool to modify precious metals according to a new procedure and possibly build new materials for catalysis.
1388-2481/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 8 - 2 4 8 1 ( 0 1 ) 0 0 1 3 5 - 7
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C. Cougnon, J. Simonet / Electrochemistry Communications 3 (2001) 209±214
2. Experimental 2.1. Tetraalkylammonium salts and solvent All salts studied in the present paper were tetraalkylammonium iodides. In most experiments, electrolyte concentration was 0.1 M in DMF. Salts were purchased from Fluka (puriss. grade) and were used without any further puri®cation after being drastically dried under vacuum at 100°C during 48 h. DMF (purum quality), purchased from SDS, was checked to contain less than 50 ppm of water. Moreover, solvent was carefully stocked in situ over neutral alumina (Merck), previously activated for several hours under vacuum at 300°C. Finally, all experiments were performed under a dry argon atmosphere. Lastly, electrolyte solutions were maintained in contact of alumina into the cell during coulometric and EQCM experiments. 2.2. Electrochemical procedures Cyclic voltammetry investigations were carried out by using a classical three-electrode cell. Experiments were performed with an lAUTOLAB potentiostat from Eco Chemie BV, connected to a computer equipped with a general purpose electrochemical system (GPES) software (version 4.5 for Windows). For analytical purposes, the working electrode was a palladium disk (8 10 3 cm2 ) and the counter electrode was a glassy carbon rod. All potentials were referred to the system Ag/AgI/I n-Bu4 N 0.1 M in DMF. Beforehand, the working electrode was carefully polished with silicon carbide (Struers) using successively smaller particle sizes (from 18 to 5 lm). After that the electrode was extensively polished using diamond powder (6 and 3 lm). Finally, the working electrode was rinsed with ethanol and acetone and then dried. Between each scan, the electrode surface was re-polished with diamond powder (3 lm). On the other hand, for macroelectrolyses investigations, Pd sheets, purchased from Strem (puriss grade: 99.95%, area: 1 cm2 , 0.1 mm thickness) were employed ± only one time each ± without any further treatment. 2.3. Chronocoulometric investigations on thin Pd layer Coulometric experiments were carried out on palladium ®lm electrodes which were electrochemically prepared by depositing palladium from a solution of 10 g/l PdCl2 in 0.1 M HCl onto a polished gold disk (2 10 3 cm2 ). The plating was done at a constant current (10 2 A/cm2 ). All experiments were performed using the same substrate as the one described above but at dierent thicknesses of palladium deposit.
2.4. Electrochemical quartz (EQCM) instrumentation
crystal
microbalance
Simultaneous cyclic voltammetric and mass balance experiments were carried out with an oscillator module (Seiko EG&G Quartz Cristal Analyzer 917) connected to an EG&G PAR 273 potentiostat. This device was used under computer control using a research electrochemistry software (ECHEM 270, version 4.3 for Windows). In the experiments, 9 MHz AT-cut quartz crystals (EG&G) coated with gold and then electrochemically plated with a thin ®lm of Pd were employed. Plating of Pd was achieved still following the procedure which is described in Section 2.3. EQCM measurements were achieved in a Te¯on three-electrode cell. The working electrode was the palladium-plated quartz crystal itself. In all these experiments the apparent area of the quartz crystal was about 0.2 cm2 . Microgravimetric data, reported in terms of mass change Dm, were calculated using the Sauerbrey equation that links resonant frequency and mass modi®cation [20]. 2.5. Scanning electron microscopy (SEM) experiments Surfaces treated electrochemically (palladium samples were rinsed using a mixture of alcohol and acetone in an ultrasound vat for 2 h) were analyzed with a JEOL scanning microscope (model JSM-6301-F). This device was equipped with a ®eld emission gun in which an ultra vacuum was held using an ionic pump (10 9 Torr). The accelerating tension was 7 kV, and resolu tion was 15 A.
3. Results and discussion 3.1. Voltammetry at a stationary palladium electrode Tetraalkylammonium iodides studied display, at slow scan rates (0.2 V/s), an irreversible cathodic peak (or wave) always associated with a corresponding anodic peak (Epa) showing a re-oxidation process (Table 1). For example n-Hex4 NI (0.1 M) alone in dry DMF (maintained carefully on neutral alumina) exhibits a wave that indicates a slow electron transfer (Fig. 1). Cathodic currents in the presence of Me4 NI and Bu4 NI were found to vary linearly with the square root of the scan rate up to 5 V/s and show a global diusional process. They also vary strongly with the salt concentration, the nature and the size of the tetraalkylammonium cation. Thus, the higher is the bulkiness of the cation, the smaller is the current limit of the cathodic wave.
C. Cougnon, J. Simonet / Electrochemistry Communications 3 (2001) 209±214 Table 1 Potential values and current peaks (cathodic and anodic) obtained during cyclic voltammetry at a palladium cathode (8 10 relative to a 0.1 M tetraalkylammonium salt solution in DMFa Electrolyte b
Me4 NI Et4 NI n-Bu4 NI n-Hex4 NI a b
Epc (V) 1:93 2:14 2:32 2:36
Ipc (mA/cm2 ) 2:11 1:87 1:62 1:37
Epa (V) 1:25 1:21 0:81 0:7
211 3
cm2 ) freshly polished
Ipa (mA/cm2 )
Epa
0.72 0.54 0.29 0.17
0.68 0.93 1.51 1.66
Epc (V)
Sweep potential: 0.2 V/s. Potentials are all referred to Ag/AgI/I 0.1 M. a, transfer coecient, was estimated from the width of cathodic peaks. Me4 NI was analyzed at 10 mM in presence of a 0.1 M tetrabutylammonium iodide, due to this weak solubility.
Fig. 1. Voltammetric behavior of a stationary palladium electrode (8 10 3 cm2 ) in contact with 0.1 M n-Hex4 NI in dry DMF. Scan rate: 0.2 V/s. Potential was referred to the Ag/AgI/I 0.1 M system.
3.2. Microelectrolyses and SEM analysis of treated palladium surfaces Preparative microelectrolyses were achieved at quite negative potentials (between 2:2 and 2:4 V). The applied amount of electricity (Qred ) were relatively small (from 1 to 7.5 C/cm2 ). When a Pd sample cathodically exposed to Et4 NI 0.1 M in DMF (Fig. 2(b)) is compared to a blank surface (Fig. 2(a)), dramatic structure changes appears. In any case, speci®c surface alterations are noticed. SEM probes on crystals and dark deposits on the surface of treated palladium sheets (Fig. 3(a)), showed that elements such as C, Pd and I are present (Fig. 3(b)). Crystals and black areas can be probably assigned to a palladium phase in a progressed oxidation by air. Lastly, a prolonged contact with air allows the total emergence of the tetraalkylammonium salts on the bulk of palladium, strongly suggesting its insertion in the course of the cathodic process. 3.3. Microcoulometries on thin ®lms of palladium deposited electrochemically Thin ®lms of palladium were deposited onto glassy carbon and gold by galvanostatic reduction in a 0.1 M HCl solution of PdCl2 . These two latter materials did
Fig. 2. SEM analysis of a palladium sheet treated with a 0.1 M Et4 NI solution. (a) exhibits the surface of a commercial palladium sheet. (b) shows the surface modi®cation of the same sample cathodically treated (electrolyzed at E 2:2 V in 0.1 M Et4 NI in dry DMF). The amount of electricity consumed was 1 C/cm2 .
not show insertion compounds (blank experiments). Mostly, it was veri®ed that Pd2 gave a well-de®ned step at 0:2 V (SCE) and the deposit was achieved almost in quantitative yield. According to simple calculations taking into account the radius of a palladium atom [21] the average coverage of gold by palladium (1.37 A), should yield a layer of about 0.1±5 lm. Under these
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C. Cougnon, J. Simonet / Electrochemistry Communications 3 (2001) 209±214
Fig. 3. Surface changes of the same palladium sheet seen in Fig. 2(b). (a) aggregation of matter which takes shape during oxidation process. (b) electron density spectroscopy (EDS) analysis results of the matter observed above. (c) EDS analysis of a commercial palladium sheet (blank experiment).
conditions, thin ®lms of palladium deposited at a microgold electrode (2 10 3 cm2 ) were electrolyzed in a 0.1 M tetraalkylammonium solution in dry DMF at the level of the peak potential reported in Table 1. More than 30 s were necessary to fully saturate the Pd deposit (Q < 1 C/cm2 ). The charge recording shows (Fig. 4) that a continuous quantity of cathodic electricity amount (Qc ) was consumed, probably corresponding both to the reduction of the solvent and to the cathodic treatment of the metal. In return, the oxidation charge (Qa ) depends
only on cluster collapse and can be considered to be speci®c to palladium phase formation. Thus, according that the decomposition of the bulk material require the transfer of one electron, Qa allows to determine ratio of R4 N /Pdx . In consequence, tetraalkylammonium solutions were electrolyzed at the peak potential (during 60 s) and, immediatly reoxidized at 0 V until completion (zero current). Table 2 exhibits coulometric results deduced by integration of the oxidation current (Qa ).
C. Cougnon, J. Simonet / Electrochemistry Communications 3 (2001) 209±214
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Fig. 4. Chronocoulometric investigation of 0.1 M n-Hex4 NI in dry DMF. Reduction at E 2:35 V of a thin ®lm of palladium (deposit charge from PdCl2 0:1 C/cm2 deposited onto a microgold electrode (2 10 3 cm2 ) during 60 s and immediately after oxidation at E 0 V until completion. Table 2 Microcoulometric analysis resultsa Electrolyte (0.1 M) Me4 NI Et4 NI n-Bu4 NI n-Hex4 NI
E (V) reduction 1:90 2:20 2:30 2:35
E (V) oxidation
x values for (R4 N )Pdx insertion
0 0 0 0
2:13 0:09 2:01 0:08 2:10 0:15 2:07 0:13
a
Ratio x (see Section 3.3) of palladium atoms per electron found for several tetraalkylammonium salts. The palladium layer was electrodeposited onto a gold microelectrode (2 10 3 cm2 ) with a charge between 2:5 10 2 and 25 10 2 C/cm2 . The layer was saturated during 60 s and recovered electricity was integrated at 0 V (oxidation at the level of the discharge anodic peak).
3.4. Mass increase analysis by EQCM Coulometric measurements have previously shown that each electron transfer involves the participation of two palladium atoms (Table 2); it now remained to check whether the transfer of each electron was associated with the insertion of one cation. All experiments were carried out with known weights of deposited palladium which permit to quantify the mass increase. In Fig. 5(a), the curves (corresponding to dierent amounts of Pd deposited) show a regular mass increase upon potential during the ®rst times and reach a plateau assigned to the saturation of palladium ®lms. It is noteworthy to check that the mass increases are proportional to the quantity of palladium (Fig. 5(b)), attesting a mass phenomenon and not a surface modi®cation. Thus, it must be pointed out that the electron transfer relative to the metal was accompanied not only by the insertion of one ammonium cation but also by the insertion of electrolyte in a concomitant manner. In the case of iodides Pd2 ; N R4 ; y NR4 I;
Fig. 5. EQCM results obtained during electrolysis at E 2:35 V vs Ag/AgI/I 0.1 M of dierent amounts of deposit palladium onto a gold quartz blade, in 0.1 M n-Hex4 NI solution. (a) exhibits experimental mass increase until completion (fully saturation of palladium layers). (b) shows the linear relationship between these mass increases upon the amounts of palladium deposited.
Table 3 Mass investigation results in DMF containing tetraalkyliodides at a palladium cathodea Entry
Electrolyte (0.1 M)
1 2 3 4
Me4 NI Et4 NI n-Bu4 NI n-Hex4 NI
Applied potential (V) 1.90 2.20 2.30 2.35
y values 0:92 0:02 0:95 0:05 0:92 0:04 0:97 0:06
a
Use of EQCM technique in the measurement of insertion of electrolyte (Pt2 , R4 N , yR4 NI) in palladium during the cathodic reduction. With each salt at least four experiments were achieved.
Table 3 gathers EQCM results. As a matter of fact, all these measurements ®t quite well in all cases with the value y 1. This value was constantly found, whatever the experimental conditions (concentration of the salt, thickness of the Pd layer). These values allowed to exclude practically concomitant insertion of solvent. Moreover, no weight loss was noticed in the course of experiments.
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4. Conclusion The cathodic polarization of palladium in aprotic DMF containing tetraalkylammonium iodides led to complex phases corresponding to the insertion of the electrolyte into the metallic bulk. These phases were de®ned thanks to microcoulometric analysis and EQCM technique. The use of palladium deposits of dierent thicknesses has demonstrated the electrolyte insertion into the metallic bulk. Therefore the described process (insertion into palladium) cannot be assigned only to a surface phenomenon. Analysis revealed that the stoichiometry of phases (clusters?) does not vary at all with the size of tetraalkylammonium cation: as a matter of fact, tetramethylammonium aords the same stoichiometry as tetra n-hexylammonium. At this stage, questions arise about the electronic conductivity of the growing layer in the course of electrolyses. Lastly, instability of cathodic phases of palladium in the contact of dioxygen leads to a more or less fast re-oxidation process in which both Pd0 is regenerated and inserted electrolyte totally emerges from the metallic bulk, as fully demonstrated by means of the SEM and EQCM techniques. Quite frequently, the oxidation by air resembles a corrosion phenomenon especially with formation of pin holes and grain boundaries. Acknowledgements The authors are grateful to the University of Rennes 1 and CNRS (UMR 6510) for partial ®nancial and
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