DOI: 10.1595/147106708X292517 A Disordered Copper ...organic chlorides, bromides and iodides were car-ried out using three-electrode cells allowing a total catholyte volume of about

84

For achieving novel reactions in organic elec-trochemistry the design and construction of anideal, multi-purpose electrode remains a perennialgoal (1). Within the cathodic range, the use of mer-cury is now banned for environmental reasons (2).Platinum, owing to both its cost and its weakhydrogen over-voltage, is difficult to use over awide cathodic domain. Various carbon interfaces(such as graphite, glassy carbon and carbon felts)may provide useful working electrodes but they arenot always inactive. Their neutrality towards elec-trochemical insertion of ions as well as theirtendency to graft free radicals have been noted (3).Electrodes modified with functionalised conduc-tive polymers have been claimed to be useful ininterfacial synthesis since they can mimic some ofthe mechanisms of organic chemistry on solid sup-ports (4). Consequently, some new insights inelectrochemical synthesis may be linked to the

development of solid electrodes with very specificproperties. Following this approach, pure silver (5)as well as other solid metal electrodes modified byadatoms (6) could offer interesting prospects whentailored to specific reactions.

Two-electron cathodic cleavage reactionsinvolving halides, sulfones, sulfonamides or tosy-lates are of importance in organic synthesis, sincethey can be applied to deprotection processes, seefor example (7). However, most of these reactionshave been reported as slow electrochemicalprocesses, depending on the electrochemicalpotential necessary to cleave the carbon–hetero-atom bonds. Such cleavage reactions have beenshown to occur at quite negative potentials – basi-cally lower than –2 V vs. SCE. Thus the use ofaqueous or water-wet organic solvents is inappro-priate when considering most solid metals for useas cathodes.

Platinum Metals Rev., 2008, 52, (2), 84–95

A Disordered Copper-Palladium AlloyUsed as a Cathode MaterialTHE ONE-ELECTRON CLEAVAGE OF CARBON–HALOGEN BONDS

By Philippe Poizot and Lydia Laffont-DantrasLRCS, UMR 6007, Université de Picardie Jules Verne, 33 rue Saint-Leu, 80039 Amiens Cedex, France

and Jacques SimonetLaboratoire MaSCE, UMR 6226, Université de Rennes 1, Campus de Beaulieu, 35042 Rennes Cedex, France;

E-mail: [email protected]

A novel method of forming a palladised copper (Cu/Pd) interface of well defined structure isdescribed. The CuPd alloy is straightforwardly obtained by immersing a copper substratein acidic solutions of palladium salts. Depending on the composition of the salt/acid solution,the copper surface is virtually instantly covered with a CuPd deposit. With nitric and sulfuricacid solutions and the corresponding Pd(II)-based salt, the deposit is composed of nanoparticlesof disordered CuPd alloy dispersed at the copper interface. The alloy-modified surface wassuccessfully used as an efficient promoter of bond cleavage reactions, especially those ofcarbon–iodide and carbon–bromide bonds in alkyl halides. The catalytic activity is specificallycharacterised by a very large shift in potential as between the use of a regular glassy carbonsurface and the palladised copper interface. With alkyl halides (RBr and RI), the shift towardless cathodic potentials is so large that it enables the one-electron cleavage of C–I andC–Br bonds. This method should enable the heterogeneous generation of free alkyl radicalsas transients in electrochemical reactions. These novel cathodic materials could also be ofconsiderable interest for the disposal of halogenated waste.

DOI: 10.1595/147106708X292517

The tailoring of surfaces by means of a specif-ic deposit of catalyst is an alternative approach tonovel electrode design. Here, the activationpotential may become so large as to transformthe nature of the cathodic reactions. Largepotential shifts can be observed, which in princi-ple enable the overall reaction process to befundamentally changed. Following thisapproach, we have developed a very convenientmethod to produce a modified electrode basedon a copper-palladium alloy (8). Preliminaryresults have shown that the as-obtained Cu/Pdinterface appears particularly efficient in acceler-ating the cleavage of carbon–halogen bonds.These cleavage reactions have often been notedas possessing a high activation energy to achievethe first electron transfer (9–14).

In the present paper, we intend to fully definethe characteristics of the copper-palladium layerproduced onto copper substrates by displace-ment reactions from several Pd(II)-basedprecursors, and to investigate the efficiency ofthe as-produced surface towards the electro-chemical cleavage of several alkyl halides RX(with X = Cl, Br and I). At regular solid metallicelectrodes, these cleavages are commonly report-ed to occur with very large activation energies. Aseries of organic halides were therefore used asprobes to compare their cleavage modes as con-ventionally observed at glassy carbon electrodes(GCEs) with those at palladised surfaces. Thepalladised surfaces were prepared by electro-chemical deposition (onto platinum or glassycarbon substrates under the same experimentalconditions). The advantages of this Cu-Pd sur-face, such as its great catalytic activity, itsstability, and its simplicity of synthesis will bepresented. The use and specificity of this newinterface are discussed in terms of potential shiftvalues related to its catalytic efficiency. It mustbe borne in mind that these interfaces specifical-ly promote unexpected one-electron processes,which involve the transient formation of freealkyl radicals. With aryl halides, however, reduc-tion processes retain a two-electron mechanism.This is in agreement with the observed high reac-tivity of aryl radicals (15, 16).

ExperimentalFormation of the Palladised CopperInterface

The copper/palladium interface was simplyprepared by dipping into a fresh acidic solution ofPd(II) for 15 s a copper substrate (grid or sheet)previously cleaned with acetone. Three differentsolutions were prepared by dissolving a palladiumsalt Pd(Yn–)2/n (with Y = SO4

2–, NO3– and Cl–) into

the corresponding acid. For example, 1 g of palla-dium(II) sulfate dihydrate (Pd(SO4)·2H2O) (AlfaAesar) was dissolved in 100 cm3 of 0.1 N H2SO4

solution. The dipping procedure produced a virtu-ally instant deposit onto the copper surface, due tothe displacement of copper by palladium cations,together with the unexpected formation of a pal-ladised copper interface. The shiny layer appearedto be quite stable, and sonication had no visibleeffect on its adhesion to the copper substrate. Toprevent any residue of acidic impurities more orless strongly adsorbed onto the surface, a prelimi-nary cleaning step is recommended; the modifiedelectrode is dipped into a dilute aqueous solutionof tetramethylammonium hydroxide, followed byrinsing with water, alcohol and finally acetonebefore drying with a hot air flow (at about 60ºC).Such electrodes were easily reused, giving coherentdata, since they were rinsed regularly following theabove procedure.

Texture and Structure AnalysisModified surface samples were first examined

by X-ray diffraction (XRD) measurements atroom temperature. However, the nanometric scaleof the deposits made it difficult to identify the as-produced material correctly. To preciselydetermine the structure/texture of thecopper/palladium interface, investigations weremade using high-resolution transmission electronmicroscopy (HRTEM). Commercially availablecopper grids for electron microscopy were used asthe substrate to study the Cu/Pd interface. Threesamples were prepared for TEM investigation bydipping a copper grid into a fresh acidic solutionof Pd(Yn–)2/n. Electron-transparent specimenswere obtained. The TEM and HRTEM imagingwere performed using a FEI Tecnai F20 S-TWIN

Platinum Metals Rev., 2008, 52, (2) 85

microscope. The elemental composition was alsodetermined by energy dispersive spectroscopy(EDS) at nanometre resolution. The diffractionpatterns were obtained using the selected area elec-tron diffraction (SAED) mode or by Fouriertransform of the HRTEM imaging.

Electrochemical Procedures: Salts andSolvents

In all the electrochemical experiments, tetra-n-butylammonium tetrafluoroborate (TBABF4) wasused as the supporting salt at a fixed concentrationof 0.1 M. Its purity (at least 98%, Aldrich) wasconsidered suitable for the experiments; there wasno further purification. The dimethylformamide(DMF) solvent (SDS, France) was typicallyemployed without drying. However, if ultra-drysolutions were required, DMF stored over activat-ed alumina was used. Alumina activation was byheating at 340ºC under vacuum overnight.Alumina could be added into the electrode cell ifnecessary, and this in situ drying technique gave amoisture level well below 100 ppm. It is worthmentioning that the procedures given below donot require extremely dry solutions. If one wishesto reach potentials as low as –2 V vs. SCE, thesolution could be dried more efficiently to avoidhydrogen evolution via the reduction of residualwater, thereby increasing the electrical yield of theoverall organic cleavage. The organic halides (RX)used in the present work were purchased fromAldrich (minimum purity 95%) and used as sup-plied.

All electrochemical experiments were per-formed under an inert atmosphere (dry argon)using a three-electrode cell with a glass separator,as described elsewhere (8). Potential values givenin this study are quoted versus SCE. The electrodesused here had an apparent surface area ofS = 0.8 mm2, except for those using copper as asubstrate (S = 1.6 or 3.2 mm2). Glassy carbon, purepalladium disc and copper electrodes were alwayscarefully polished with silicon carbide paper(Struer) or with Norton polishing paper (grades 02and 03). Before use, the conventional workingelectrodes were rinsed twice with water, then alco-hol and finally acetone before drying with a hot air

flow. Palladised electrodes (including those usedfor comparison purposes) were prepared by a gal-vanostatic deposit of Pd from a palladium chloridesolution onto several types of metallic substrates(platinum, gold or palladium). The plating bathcontained 10 g l–1 of PdCl2 (Alfa Aesar) in aqueous0.1 N HCl. In the present experiments, the chargedensity for galvanostatic deposition was4 mC mm–2 throughout, with current densities ofthe order of a few hundreds of μA cm–2.

Coulometry and ElectrolysesCoulometric experiments and electrolyses of

organic chlorides, bromides and iodides were car-ried out using three-electrode cells allowing a totalcatholyte volume of about 5 to 10 cm3. The anod-ic compartment was separated by a fritted glass ofweak porosity. Substrate volume was about0.1 mM. In order to avoid disturbance resultingfrom the possible presence of copper oxide,which could depend on the history of the coppersubstrate, the solution was always pre-electrolysedbefore adding the RX compound to the cell.Owing to the high reactivity of the Cu/Pd inter-face towards impurities (in particular dioxygen),there was efficient argon bubbling in all cases,ensuring a good reproducibility of results,especially in voltammetry.

ResultsCharacterisation of the Palladised CopperInterface

A complete TEM study was performed on allthe samples in order to determine the texture, thestructure and the precise composition of the palla-dium-based layer. The bright field image (Figure1(a)) shows a dendritic-like growth of the layer(particle size < 50 nm) obtained with the palladiumsulfate solution. The HRTEM image of one part ofthe bright field image represents one of the 12 nmnanoparticles of which the dendrite was com-posed. The morphology and dimensions (around10–15 nm) of these particles are homogeneous,and each is well crystallised. For the two othersamples, prepared with palladium chloride and pal-ladium nitrate solutions, the growth of thePd-based layer is not dendritic (Figures 1(b) and


1(c), respectively). However, the layer is alwayscomposed by the juxtaposition of nanoparticles,the size of which is closely dependent on the pre-cursors and varies from 3 to 5 nm (Figure 1(b))and from 5 to 8 nm (Figure 1(c)). All thesenanoparticles are well crystallised. The SAED pat-terns obtained from these three samples areidentical, as shown in Figure 1(d). They are com-posed of diffraction circles due to randomlyoriented nanoparticles. Thanks to the‘ProcessDiffraction’ software, a line profile of theelectron diffraction pattern may be plotted, simi-larly to the X-ray diffraction pattern (Figure 1(d)).Hence it was determined that the layer can berelated to the CuPd disordered alloy (ICDD cardNo. 48-1551, space group: Fm3m). The nature andcrystallographic properties of this layer are pre-sented and developed in the Discussion section ofthis article. The EDS analysis of these layers (notgiven here) systematically indicates a Cu:Pd ratio

near to unity, corroborating the CuPd alloy forma-tion. It was concluded that, for all samples, thePd-based modified electrodes consisted of a thinlayer of the stoichiometric CuPd alloy.Additionally, as mentioned in a previous paper(17), it is worth noting that, when using palladiumchloride as the salt in solution, the sparingly solu-ble compound copper(I) chloride (CuCl) may alsobe incorporated into the surface electrode layer, inwhich case Cu(I) is stabilised by the excess chlo-ride as a transient in the redox process.

Primary Alkyl IodidesWhen using Cu-Pd electrodes, and in particular

those obtained with palladium sulfate and nitratesolutions, the results regarding the electrochemicalreduction of primary alkyl halides differ from thosealready described for conventional solid electrodes.Several alkyl iodides such as 1-iodobutane,1-iodohexane, 1-iodooctane and 1-iodohexadecane


20 40 60 80 100 120 140 1602θ (Cu Kα)

160140120100

80604020

Inte

nsity

, a.u

.CuPd

(ICDD No. 48–1551)*

*

***

*

***

(d)(c)

(a) (b)

50 nm

5 nm

50 nm100 nm

5 nm

5 nm

0

Fig. 1 Bright field images of the palladium-based layer obtained with the solution of: (a) palladium sulfate; (b) palladium chloride; and (c) palladium nitrate, (insets: HRTEM images of CuPd nanoparticles composing theselayers); (d) common SAED pattern of these three layers combined with a graph similar to an X-ray diffraction patternwhich enables characterisation of the disordered CuPd alloy

were tested (see Figure 2). 1-Iodobutane wasfound to exhibit a strong activation phenomenonat any Cu-Pd electrode. This activation is quantifi-able as a positive shift in potential with respect toresults obtained at a GCE, which is supposed to bethe ideal type of electrode with zero activitytowards alkyl halide molecules. Thus under stan-dard conditions, the half-peak potential ofiodobutane (Ep/2 = –1.93 V vs. SCE) has beenunambiguously assigned to the classical two-elec-tron reaction step, which is regarded as unchangedby the nature of the interface. By contrast, using aCu-Pd electrode prepared from a PdSO4 solutionyields a cathodic step that is strongly shifted inpotential, and of much smaller limiting current(Ep/2 = –1.44 V). This electrochemical process isdiffusion-controlled, but strongly irreversible. Thetransfer coefficient estimated from half-peak widthmeasurements was found to be smaller than 0.2.The catalytic efficiency of the Cu/Pd interface wasalso compared with those of palladised interfacessuch as smooth platinum and polished palladium.

As already reported (18) palladised surfaces such asthose of Pt/Pd electrodes led to a higher potentialshift (Ep/2 = –1.36 V) as compared with that of aGCE. As a general trend, all types of palladisedsurfaces exhibit highly significant potential shiftswith alkyl iodides. Peak currents are halved whenusing Cu-Pd electrodes, suggesting that the overallelectrochemical process has become a one-elec-tron reaction. Coulometric measurements verifiedthis proposal satisfactorily with all the primaryalkyl iodides tested. It is worth noting that smoothcopper also exhibits a one-electron step with butyliodide, but located at more negative potentials, typ-ically –1.82 V vs. SCE. Therefore the soleparticipation of the copper substrate in the overallactivation process is vanishingly unlikely.

Whatever the formation mode, the one-elec-tron reduction process at Cu-Pd surfaces could beverified by microcoulometric measurements onmillimolar amounts of reactant. At low reductionpotentials (Er < –1.2 V vs. SCE) the measuredcharge values are consistently very close to 1F mol–1. Since nitrones (classically used as spinmarkers) do not disturb electrochemical reductionreactions of alkyl iodides, complementary investigations were performed with N-tert-butyl-α-phenylnitrone (PBN). In all cases, a distinctone-electron process was observed, whereas theformation of paramagnetic nitroxides was demon-strated. Under these conditions, 1-iodobutaneproduces a six-line ESR signal with the couplingconstants aH = 3.16 G and aN = 14.9 G. Thisremains in good agreement with previous resultsobtained at regular palladised surfaces (18).

Alkyl BromidesA large suite of long-chain primary alkyl bro-

mides were reduced at different solid electrodesand their voltammetric data were compared.Almost all of this range exhibited a two-electronirreversible step at a GCE at quite strongly reduc-ing potentials (i.e. Ep/2 < –2.5 V vs. SCE). Smoothpalladium electrodes also yielded a main reductionstep that occurs at very negative potentials (withina comparable potential range < –2.4 V) since theelectrolyte was thoroughly dried by addingactivated alumina in situ. The reduction of


i, μA

E, V/SCE

–3.0 –2.0 –1.0

10

2

1

A

B

C

Fig. 2 Voltammetry of 1-iodohexane (concentration:9 mM) in 0.1 M TBABF4 using DMF as solvent, recordedat different microelectrodes. Scan rate: 100 mV s–1

(A) Response at a GCE (S = 0.8 mm2)(B) Response at a palladised platinum electrode (S = 0.8mm2)(C) Response at a Cu-Pd modified electrode preparedfrom a palladium sulfate solution (S = 1.6 mm2)

short-chain alkyl bromides at palladised electrodesmay be effective at much less negative potentialsthan –2 V, but currents are generally small.However, these voltammetric steps exhibited akinetically controlled character, and appeared tovanish completely upon repeated scans withincreasing alkyl chain length. At a smooth copperinterface, a reduction step was generally observedbeyond –1.8 V, together with the possible occur-rence of an adsorption-like step attributable to thereduction of copper oxide at moderate potentials.By contrast, Cu-Pd modified electrodes yieldedsurprising results: much larger reduction steps foralkyl bromides were consistently observable atmuch higher potentials than –2.0 V (see Figure 3for the case of 1-bromodecane). The stepobtained from the second scan is generally S-shaped, with the overall current indicating aprocess close to a one-electron transfer. Thenature of the step strongly suggests a kind of self-inhibition, probably due to the adsorption at theelectrode surface of the free radical produced.

Voltammetric experiments have shown that theshape of the reduction step is strongly sensitive toimpurities in the solution. Thus, with traces ofdioxygen, there is no pre-peak and the main step isclearly shifted. The second scan of an ‘impurity-free’ solution also exhibits such a potential shift,

probably underlining that the catalysis is sloweddown by the decay of the free active surface. Theactivated surface can only be regenerated by rins-ing the electrode according to the proceduredescribed above. However, a pre-peak of variableheight is obtained with a freshly produced micro-electrode, depending on the nature and theconcentration of the alkyl bromides (see Figure 4in the case of 1-bromohexadecane). The totalheight of the overall cathodic step is a linear func-tion of alkyl bromide concentration. With R =n-propyl, n-butyl, n-pentyl, n-hexyl, n-octyl and n-decyl, the total current of the reduction step(found to be diffusion-controlled throughout) isroughly half of the current observed at a GCE.This observation argues in favour of a one-elec-tron reduction. Within the scanned potential range(i.e. –2.5 ≤ E ≤ –0.5 V vs. SCE), there is no appear-ance of a second step attributable to the reductionof the free radical release by the alkyl halide reduc-tion. Moreover, it has been verified that there is noevidence of a partial reduction of the alkyl halideonto a pure copper cathode at potentials above–2 V.

In order to produce cheap and strongly acti-vated electrodes, we attempted to build aCu/Pd interface onto a GCE. Copper was


10

–3.0 –2.0 –1.0

1021

A

B

E, V/SCE

i, μA

Fig. 3 Voltammetry of 1-bromodecane (concentration:9 mM) in 0.1 M TBABF4 using DMF as solvent, recordedat different microelectrodes. Scan rate: 100 mV s–1

(A) Response at a GCE (S = 0.8 mm2)(B) Response at a Cu-Pd modified electrode preparedfrom a palladium chloride solution (S = 1.6 mm2). Firsttwo sweeps

25

–3.0 –2.0 –1.0

2 1A

B

E, V/SCE

i, μA

Fig. 4 Voltammetry of 1-bromohexadecane(concentration: 9 mM) in 0.1 M TBABF4 using DMF assolvent, recorded at different microelectrodes. Scan rate:100 mV s–1

(A) Response at a GCE (S = 0.8 mm2)(B) Response at a Cu-Pd modified electrode preparedfrom a palladium sulfate solution (S = 1.6 mm2). Firsttwo sweeps

galvanostatically deposited from a copper nitratesolution prepared by dissolving 0.1 g of the salt in100 cm3 of 0.1 N HNO3. The charge density waslimited to 5 × 10–3 C mm–2 and the current wasfixed at 0.5 mA. After obtaining the copperdeposit, (estimated average thickness ≈ 0.2 μm),the electrode was briefly dipped into a palladiumsulfate solution. The glassy carbon surface emergedshiny. Results from use of the interface as avoltammetric electrode were interesting, since thedegree of activation appeared extremelyfavourable. The ‘pre-peak’ turned out to be themain peak (see Figure 5, curve (C)). If the mainreduction step decays during repetitive sweeps, abrief pause at 0 V may regenerate most of the orig-inal current. It is likely that a finely divided depositof the alloy Cu-Pd can be superimposed on thethin copper deposit, producing quite a large activat-ed surface. This procedure has so far only beenachieved with a glassy carbon support.

Our observations suggest that using Cu-Pd elec-trodes at much less negative potentials than those

already reported with conventional electrodematerials leads, at least with alkyl bromides, toone-electron processes. To verify this hypothesis,an extended series of coulometric experimentswas carried out on a large suite of primary alkylhalides. Cu-Pd electrodes (visible as a brightmetallic deposit) formed from palladium sulfate ornitrate solutions could be reused for a large num-ber of experiments without any apparentdeterioration in efficiency. This was not the casewith electrodes produced from palladium chlo-ride, which turned blue over time, probably due tothe oxidation of residual cuprous ions inside thelayer. It was found for all alkyl bromides in theseries that the Cu-Pd electrode then consistentlyproduced a one-electron process. Finally, theanalysis by gas chromatography/mass spectrome-try (GC/MS) of the R–Br electrolysis productsshowed that R–R dimers and/or mixtures ofR(H)/R(–H) in equal amounts were obtained withR = C8, C10 and C12.

The formation of free alkyl radicals in the cleav-age of primary alkyl bromides at Cu-Pd cathodes isstrongly corroborated by the spin marker tech-nique. 10–20 mg of the alkyl halide, dissolved in5 cm3 of DMF, was reduced in the presence of athreefold excess of N-tert-butyl-α-phenylnitrone(PBN) (electrolysis current = 10–15 mA). By wayof example, the reduction current for 1-bromohep-tane at –1.5 V on a Cu-Pd electrode vanishedcompletely at 1 F mol–1. ESR analysis of the elec-trolyte in the absence of dioxygen disclosed astrong paramagnetic signal, fully attributable to thetrapping of the n-heptyl radical. The nitroxide rad-ical obtained (see Structure 1) displayed a six-rayspectrum with coupling constants aN =14.379 Gand aH = 2.614 G.

6-Bromo-1-hexene, which is known to afford acyclisable free radical, usable as a ‘radical clock’,gives very similar results. Thus the reduction at aGCE shows Ep/2 = –2.34 V, whereas the use of a


25

–3.0 –2.0 –1.0

21

A

C

E, V/SCE

i, μA

B

2

1

5

Fig. 5 Voltammetry of 1-bromodecane (concentration:9 mM) in 0.1 M TBABF4 using DMF as solvent, recordedat different microelectrodes. Scan rate: 100 mV s–1

(A) Response at a GCE (S = 0.8 mm2)(B) Response at a freshly made CuPd cathode preparedfrom a palladium chloride solution (S = 3.2 mm2). Firsttwo sweeps(C) Response at a GCE first covered by a galvanostaticdeposit of copper (S = 0.8 mm2) and then treated bypalladium sulfate solution. First two steps

N

H

Ph

CH3(CH2)6

tBu

O

C

1

Cu-Pd working electrode (still prepared withPdSO4) produces a spectacular shift toEp/2 = –1.40 V. As shown in Figure 6(a), the pres-ence of nitrone at the reduction led to twoparamagnetic transients, with the formation of twoparent nitroxides. It is presently premature toassign these two nitroxides to the trapping of theuncyclised and cyclised n-hexenyl radical.

Finally, fixed potential electrolyses on alkyl bro-mides (all exhibiting one-electron processes) led tomixtures of R–R, RH and R(–H). The ratioRH:R(–H) was equal to 1, as shown by GC/MSexperiments with C8, C10 and C12 bromides.

n-Alkyl ChloridesIt was found possible to reduce 1-chloroalkanes

at Cu-Pd cathodes. An appreciable potential shiftwas also observed. However, in all cases, half-peakpotentials were still located at very negative

potentials (E < –2.5 V vs. SCE). This precludesobtaining one-electron reduction processes similarto those observed with alkyl bromides and iodides.

DiscussionIn order to characterise the electrochemical effi-

ciency of our as-prepared electrodes, whatever thepalladium precursor used, the primary objectivewas to unambiguously identify the structure of thedeposited layer. Electron diffraction was an appro-priate technique here, since the nanometric scale ofthe metallic particles made valid identification diffi-cult when using a conventional XRD analysis. Forall deposits, the electron diffraction line profileenabled identification of the deposited layer as adisordered CuPd alloy, thanks to a perfect matchwith the diffraction data given by Nekrasov (19)and more specifically by Zhu et al. (20). The latterperformed a thorough study of clusters of


Inte

nsity

, a.u

.

3450 3480 3510Magnetic field, Gauss

40,000

0

–40,000 g = 2.00641aN = 14.161 GaH = 2.397 G

Inte

nsity

, a.u

.

100,000

0

–100,000

g = 2.01aN = 14.692 GaH = 2.564 G

3420 3440 3460 3480 3500 3520 3540Magnetic field, Gauss

(a)

(b)

Fig. 6 ESR signals obtained from:(a) 6-bromo-1-hexene and (b) phenyliodide when reduced in 0.1 MTBABF4 using DMF and withdissolved TBPN (threefold excess). Inboth cases, reductions werecompleted after a total consumptionof 1 F mol–1 based on the halideamount

disordered CuPd (i.e. nanoparticles) via a theoret-ical approach using the bond order simulation(BOS) model for metals and the corrected effectmedium (CEM) theory. The simulation model ofZhu et al. predicts diffraction patterns and rela-tive peak intensities, which are in goodagreement with the reported experimental data.

Having demonstrated the formation of thedisordered CuPd phase via TEM investigations,data regarding the Cu-Pd system must be consid-ered, since the disordered structure is notexpected at room temperature. Thermo-dynamically speaking, below the solidus, the Cu-Pd system is first characterised by a continuoussolid solution showing a face-centred cubic(f.c.c.) structure (21) with a lattice spacing rang-ing from 3.615 Å (pure copper) to 3.892 Å (purepalladium) (22). At the 50:50 atomic composi-tion, the disordered CuPd A1-type alloy (solidsolution) has a cell constant close to 3.77 Å (22),and is formed of copper and palladium that ran-domly occupy, with 50% probability, each site ofthe f.c.c. structure (Table I). As the temperaturedecreases (T < 600ºC), ordering of Cu and Pdatoms is energetically favoured (23–25) and thecubic CuPd alloy adopts the b.c.c.-based struc-ture (CsCl-type). This phase, which is alsoreferred to as B2 or CuPd (β), shows an alterna-tion of (001) planes of Cu and Pd (Table I). It isworth noting that the f.c.c.-based L10 orderedsuperstructure (CuAu-type) with alternating

(001) planes of Cu atoms and (001) planes of Pdatoms does not exist (Table I). The competitionbetween the B2 and L10 ordered phases of CuPdresolves in favour of the former, thanks to a sub-stantially lower energy of formation (for moredetails see (25)). Moreover, the high stability ofthe B2-type structure is substantiated empiricallyby the recent discovery of the correspondingmineral (skaergaardite) (26). Consequently, underour experimental conditions, the layer growthmust be kinetically controlled, since it leads tothe A1-type alloy, a metastable phase at roomtemperature.

A noteworthy result of this study is that wesucceeded in synthesising very straightforwardlynanoparticles of the disordered CuPd alloy byimmersing a copper substrate (grid, sheet, Cuelectrodeposit) into a fresh acidic solution of apalladium salt. Other methods known to date arevery much more complicated, usually involvingpolyvinylpyrrolidone (PVP) as stabiliser to obtainnanoparticles. Esumi’s method (27) (or adapta-tions) yields this alloy at nanometric scale bythermal decomposition of mixtures of copperand palladium precursors in high-boiling organicsolvents (20, 27–29) or by condensing Pd and Cuatoms at 350ºC under ultra-high vacuum (30, 31).Interestingly, CuPd alloys also show potential asgas-phase catalysts in enhancing the selectivity ofhydrogenation of dienes (32) and the reductionof NO by CO (30, 33, 34).


Phase formula CuPd “CuPd(α)”* CuPd(β)

Pearson symbol cF4 tP4 cP2

Space group Fm3m P4/mmm Pm3m

Strukturbericht A1 L10 B2designation

Crystal structure

Pd

Cu

Cu or Pd

Table I

Crystal Structure Information for the f.c.c. and b.c.c. Copper-Palladium Alloys

*Note the L10 structure is given only for comparison since “CuPd(α)” phase does not exist for energetic reasons

ConclusionIt may be concluded that a simple redox dis-

placement reaction between Cu0 and Pd2+,operating in acidic solution at room temperatureoffers a route to a thin and very stable layer, charac-terised as a well crystallised nanometric CuPd alloy.The displacement is simply achieved in the presenceof Pd(II)-based salts such as sulfate, nitrate andchloride. However, a pure CuPd alloy is formedonly with palladium sulfate and nitrate. Electro-chemical data obtained from the reduction of alarge series of organic halides (mainly iodides andbromides) showed that the use of such alloys ascathode materials very strongly activates the cleav-age of the carbon–halide bond, sometimesdisplaying a +1 V shift in potential. There were nostrong passivating phenomena during the electroly-ses, even though a moderate decay of the cathodiccurrent could be observed after a few minutes. Thedeposit was shown to act as a porous material, andits structure may change dramatically with time; thiscorroborates the assumption that palladium reactswith alkyl halides (Figure 7 depicts the modificationin morphology of the Cu-Pd layer during the reduc-tion of alkyl bromides).

We have already mentioned (18) the use of pal-ladium deposits as modifier of the cathode surface(for example deposits onto platinum or glassy car-bon). The mode of action of palladium probablystems from the finely divided nature of the deposits(nanosized particles). Hitherto it was believed thatelectrolytic deposition (from Pd2+ in acidic solu-tions) was a prerequisite for electrocatalytic activity(here quantified mainly in terms of a shift of themain voltammetric step toward much less negativepotentials).

The mode of catalysis is not yet fully determined,but it is conceivable as the insertion of palladiuminto the carbon–halide bond, giving a stronglyadsorbed chain species such as C–Pd–X. Such aninsertion may corroborate the catalytic hypothesis,given the constant regeneration of the copper-palla-dium alloy (see Scheme I), promoted by the strongelectronic interaction between Pd and Cu uponalloying with a specific feature (33, 34).

In the process proposed here, the rates ofadsorption and insertion of palladium into the

C–halogen bond would be rapid compared with dif-fusion of the electroactive species. As stressedabove, catalysis by the Cu-Pd surface is of very greatinterest for C–Br bond cleavage reactions. The C–Icleavage reaction is also facilitated, but results arequite similar to those already observed with pal-ladised surfaces. Very often (but not invariably), thepotential shift is so large that the cathodic reactionis fundamentally changed, and turns out to bemono-electronic. The method may therefore beseen as an efficient source of free radicals (with apossible coupling reaction outside the cathodiclayer) more or less strongly adsorbed at the inter-face. These results are in full agreement withprevious estimates by Lund et al. (35–37) concern-ing the standard potentials corresponding to thereduction of a large number of free alkyl radicals inDMF between –1.39 and –1.72 V vs. SCE, undervery similar experimental conditions.


200 nm

200 nm

(a)

(b)

Fig. 7 SEM images of the Cu-Pd layer before and afterreduction of 1-bromodecane (concentration 2 × 10–2 M).The metal layer (shown in (a)) has been obtained after adipping of a copper sheet into PdSO4 (see Experimentalsection) for 2 minutes. The structural change (b) of thelayer (consecutive to the catalytic reduction of the RBrcompound) was obtained by electrolysis at –1.9 V vs. SCEafter 2 C cm–2 have passed through the cell. Averagecurrent density: 0.5 mA cm–2


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Scheme Iads = adsorbed; sol = solution; ≡ represents that an interaction existsbetween Cu and Pd atoms in the solidstate

RBrCu-Pdinterface

[RBr]ads

Pd

Cu[R ≡ Br]ads

e–

– Br– [R•]ads + Cu-Pd R•sol

R•sol

R–R (coupling)

R(H) + R(–H) (disproportionation)

References

AcknowledgementsThe authors are grateful to Professor Viatcheslav Jouikov (Laboratoire MaSCE) for the ESR measure-ments and to Michèle Nelson (LRCS) for helpful assistance.


Jacques Simonet is Directeur deRecherche Emérite in theElectrochemistry Group, Université deRennes 1 (UMR 6226), France. Hisprincipal interests are organicelectrochemistry, the activation oforganic reactions by electron transfer,electro-polymerisation and the formation

of redox polymers. He also researches on the reversible cathodiccharging of precious metals (platinum and palladium) in super-dry conditions, in contact with polar organic solvents containingelectrolytes, mimicking Zintl phases for transition metals.

Philippe Poizot is presently AssistantProfessor at the Department ofChemistry (LRCS, UMR 6007) of theUniversité de Picardie Jules Verne(Amiens, France) where he studiedChemistry, and completed his Ph.D. inMaterials Science in 2001. His researchtopics are mainly focused on the

lithium-ion battery and the synthesis of nanostructuredelectrode materials using soft chemistry routes such aselectrodeposition.

Lydia Laffont-Dantras is Assistant Professorat the Department of Chemistry (LRCS,UMR 6007) of the Université de PicardieJules Verne (Amiens, France). Her principalinterest is the study of organic and inor-ganic compounds by transmission electronmicroscopy (TEM) and electron energy lossspectroscopy (EELS). Her research work is

currently focused on the characterisation (morphology andnanostructure) of electrochemical devices such as electrochromicthin films or lithium-ion batteries by TEM and EELS.

The Authors

and references therein34 A. Rochefort, M. Abon, P. Delichère and J. C.

Bertolini, Surf. Sci., 1993, 294, (1–2), 4335 D. Occhialini, S. U. Pedersen and H. Lund, Acta

Chem. Scand., 1990, 44, (7), 715

36 D. Occhialini, J. S. Kristensen, K. Daasbjerg and H.Lund, Acta Chem. Scand., 1992, 46, (5), 474

37 D. Occhialini, K. Daasbjerg and H. Lund, ActaChem. Scand., 1993, 47, (11), 1100

Documents

DOI: 10.1595/147106708X292517 A Disordered Copper ...organic chlorides, bromides and iodides were car-ried out using three-electrode cells allowing a total catholyte volume of about