Building up an electrocatalytic activity scale of cathode materials for organic halide reductions

Article abstract of DOI:10.1016/j.electacta.2004.10.047

A wide investigation on the electrochemical activity of four model organic bromides has been carried out in acetonitrile on nine cathodes of widely different affinity for halide anions (Pt, Zn, Hg, Sn, Bi, Pb, Au, Cu, Ag), and the electrocatalytic activities of the latter have been evaluated with respect to three possible inert reference cathode materials, i.e. glassy carbon, boron-doped diamond, and fluorinated boron-doped diamond. A general electrocatalytic activity scale for the process is proposed, with a discussion on its modulation by the configuration of the reacting molecule, and its connection with thermodynamic parameters accounting for halide adsorption.

Full text of DOI:10.1016/j.electacta.2004.10.047

Electrochimica Acta 50 (2005) 2331–2341
Building up an electrocatalytic activity scale of cathode
materials for organic halide reductions
C. Bellomunnoa, D. Bonanomia, L. Falciolaa,1, M. Longhia,1, P.R. Mussinia,∗,1
,
L.M. Doubovab,1, G. Di Silvestroc
a
Department of Physical Chemistry and Electrochemistry, University of Milano, Via Golgi 19, 20133 Milano, Italy
b
IENI-CNR, Corso Stati Uniti 4, 35127 Padova, Italy
c
Department of Organic and Industrial Chemistry, University of Milano, Via Venezian 21, 20133 Milano, Italy
Received 2 March 2004; received in revised form 18 June 2004; accepted 8 October 2004
Available online 21 November 2004
Abstract
A wide investigation on the electrochemical activity of four model organic bromides has been carried out in acetonitrile on nine cathodes
of widely different affinity for halide anions (Pt, Zn, Hg, Sn, Bi, Pb, Au, Cu, Ag), and the electrocatalytic activities of the latter have been
evaluated with respect to three possible inert reference cathode materials, i.e. glassy carbon, boron-doped diamond, and fluorinated boron-
doped diamond. A general electrocatalytic activity scale for the process is proposed, with a discussion on its modulation by the configuration
of the reacting molecule, and its connection with thermodynamic parameters accounting for halide adsorption.
© 2004 Elsevier Ltd. All rights reserved.
Keywords: Organic electrocatalysis; Organic halide reduction; Halide adsorption; Specifically interacting cathode materials; Non-interacting cathode materials
(GC, BDD, FBDD)
1. Introduction
molecule, and therefore must be included in the reaction
intermediate. In other words, electrocatalytic processes
involve inner-sphere electron transfers [2] requiring to
take into account not only the substrate intrinsic reactiv-
ity, but also specific adsorption/desorption effects, con-
cerning not only the substrate but also the solvent and
the supporting electrolyte [3];
Surprisingly enough, careful investigations of organic
electrocatalytic processes are still very rare, in spite of their
intrinsic great potentialities, and unlike the many extensive
investigations having been carried out so far on inorganic
electrocatalytic processes. This lack can be explained by the
fact that many major research groups are now concentrating
on urgent topics connected with fuel cell development [1], but
also by the intrinsic complexity of organic electrocatalysis:
(b) with respect to inorganic electrocatalysis, because of the
structural complexityof the reactingmolecule (the chem-
ical environment surrounding a given active group not
only modulates its intrinsic reactivity, but also the elec-
trocatalytic effect exerted on it by the electrode material
[4]).
(a) with respect to non-electrocatalytic organic electrochem-
ical processes, because the electrode must be regarded
not as an inert electron source (appropriate e.g. for in-
vestigations on the intrinsic reactivity of the organic
substrate), but as an interacting partner for the reacting
Therefore, a correct investigation on a given organic elec-
trocatalytic process should necessarily be an extensive, sys-
tematic, “multivariate” search, requiring to change one by
onemanyparameters(theelectrodematerial, thesurfacemor-
phology, thestructureoftheorganicmolecule, thesolvent, the
supporting electrolyte), while keeping the others constant. A
∗
Corresponding author. Tel.: +39 02 50314211; fax: +39 02 50314300.
E-mail address: patrizia.mussini@unimi.it (P.R. Mussini).
ISE member.
1
0013-4686/$ – see front matter © 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2004.10.047

2332
C. Bellomunno et al. / Electrochimica Acta 50 (2005) 2331–2341
partial example is provided by the recent work of some of us
concerning the electrocatalytic activity of silver for the reduc-
tion of organic halides, and including investigations on the
effects of morphology and state of the active surface [5,6], of
the molecular structure [4] and of the supporting electrolyte
in constant acetonitrile solvent [3].
Actually the electrocatalytic reduction of organic halides
RX, assumed [4] to proceed via an attenuated radical interme-
diate involving three “actors”, R · · · X · · · Me (quite similar
to the one featuring as activated complex in atom-transfer
radical polymerizations [7]) appears an ideal model case for
a detailed study of organic electrocatalysis. In fact
rial assumed free of electrocatalytic effects, such as the
glassy carbon one [4], while the authors considered only
electrocatalytic materials in their investigations.
In this context, the present work is aimed to achieve a
general treatment of the effects of the cathode material in
the electrocatalytic reduction of organic halides, along the
following methodological guidelines:
• three model organic bromides have been chosen among
those tested in our cited previous screening [4] (acetobro-
moglucose, a glycosyl bromide; benzyl bromide; and 8-
bromooctanol, an alkyl bromide for which silver displays
a marked catalytic effect probably on account of the pres-
ence of the hydroxy group acting as “adsorption auxiliary”
[4]), plus halothane for comparison with Langmaier and
Samec’s work;
• nine cathode materials have been selected (Pt, Zn, Hg, Sn,
Bi, Pb, Au, Cu, Ag), having widely different electronic
properties and therefore showing very low to very high
affinity for halides in specific adsorption experiments [8],
and each surface has been prepared and/or activated along
specific procedures;
• all experiments have been carried out in the same medium,
i.e. acetonitrile (ACN) + 0.1 M tetraethylammonium per-
chlorate TEAP or tetraethylammonium tetrafluoborate
TEATFB, having proved to be the least specifically inter-
acting media for organic halide reduction in our previous
screenings on silver [3,4,6];
• the reduction of organic halides is a key process in organic
chemistry;
• many authoritative research groups have thoroughly inves-
tigated both specific adsorption of halide anions onto metal
electrodes (e.g. [8,9])and non-electrocatalytic reduction of
organic halides [10];
• other authoritative research groups have recently applied
computational chemistry methods to the problem of spe-
cific halide adsorption [11,12];
• organic halide reductions have already been carried out,
several times and with interesting results [10,13–18], on
electrocatalytic cathode materials, but such works were
predominantly application-oriented, rather than aimed to
a systematic investigation of the underlying mechanism.
An exception to the above limitations is the recent,
brilliant work by Langmaier and Samec [19], which, al-
though prompted by a specific applicative problem, i.e. the
electroanalytical determination of anaesthetic halothane (2-
bromo-2-chloro-1,1,1-trifluoroethane) in human and animal
metabolism, actually features an extensive research on the
electrochemical reduction of the above substrate on many
cathodic materials and several media, providing convincing
mechanistic schemes and interpretations of the trends of both
reduction potentials (in connection with Me · · · X bonding
energies) and reduction currents (in connection with solvent
viscosities). However, many aspects of the above work should
be modified in the perspective of a truly general investigation
on RX electrocatalytic reductions, especially the following
ones:
• among the tested cathodes three possible non-catalytic ref-
erence materials have been included, i.e. glassy carbon
(GC), a popular electrode for mechanistic investigations
of outer-sphere electron transfers to organic halides (e.g.
[20]) and the even more inert boron doped diamond (BDD)
and fluorinated boron doped diamond (FBDD).
2. Experimental
The model organic bromides (Fig. 1) ␣-acetobromo-
glucose ABG 1 (FLUKA), benzyl bromide BzBr 2 (Aldrich),
8-bromooctanol 3 (Aldrich), and halothane 4 (Aldrich), were
used as received with the exception of ABG which was
purified from CaCO3 (added in traces as a stabilizer by
the producer) by filtration in diethyl ether. Voltammograms
were recorded for each substrate and cathode material in
ACN (Merck HPLC grade, water ≤0.05%) + 0.1 M TEAP
(or TEATFB (Fluka), because of recent limitations on per-
chlorate availability in our country; in any case, in a par-
allel work on the medium effect on the electrocatalytic re-
duction of organic halides [21] we verified that the two
supporting electrolytes result in no significant differences
in the CV patterns for a given system), at three concentra-
tions (0.001, 0.002 and 0.003 M) and at scan rates ranging
20–500 mV s−1. The cyclovoltammetric investigations were
carried out on carefully deaerated solutions in a cell ther-
mostated at 298 K, by an Autolab PGSTAT 12 or an Auto-
(1) the target molecule is a very specific one (a bromide of
high intrinsic reactivity, and featuring no less than four
other different halide atoms on its two-carbon backbone);
(2) all the cathode surfaces are prepared along a single, sim-
ple standard procedure of mechanical + electrochemical
polishing, irrespective of specific requirements of the dif-
ferent metals tested;
(3) the experiments are predominantly carried out in 1 M
NaOH in water or methanol, i.e. in media powerfully
interacting with many of the tested cathodes;
(4) above all, the electrocatalytic effect of a given cathode
material is conveniently evaluated in terms of positive
shift of peak potential with respect to a cathode mate-

C. Bellomunno et al. / Electrochimica Acta 50 (2005) 2331–2341
2333
Bismuth cathodes were spindle-shaped pieces
(length = 1–1.5 cm; maximum diameter ∼0.2 cm; Aldrich
99.999%) suspended by small alligator clamps, with the
working surface delimited by TeflonTM ribbon; they were
activated by polarization in a saturated KI solution with 1%
HCl (5 min, 0.7 A cm−2) [24].
Mercury cathodes were either a Metrohm E410 HDME
(drop surface [determined by weight] = 0.022 cm2), or the
SDME 6.1246.020 included in the above Metrohm stand
(drop surface [determined by weight] = 0.0046 cm2).
Tin cathodes were wires (length = 1 cm, diame-
ter = 0.1 cm, Aldrich 99.999%) polarized in a 20 cm3 30%
HClO4 + 80 cm3 CH3COOH solution (40 s, 0.6 A cm−2),
then kept at −1.7 V (SCE), in a 0.0015 M Na2SO4 solution
for 30 min [25].
Fig. 1. The model organic halides selected for this investigation.
Zinc cathodes were wires (Aldrich 99.995%,
length = 1 cm, diameter = 0.05 cm) submitted to gal-
vanostatic steps (0/0.2 A cm−2, 5 s each, total time 2 min)
in a 50% H3PO4 solution, then kept for 20 min at −1.4 V
(SCE) in the working solution [26].
Platinum cathodes were ultrapure platinum wires
(length = 1 cm, diameter = 0.05 cm) polished both mechani-
cally, with wet alumina on filter paper, and electrochemically
by 30 s steps (−0.25/0/1.5 V), followed by cycling in 0.5 M
H2SO4.
Glassy carbon cathodes were either an AMEL GC disk
electrode (diameter = 0.25 cm) or a Metrohm 6.1204.110 GC
disk electrode (diameter = 0.2 cm). They were polished by
wiping with acetone and then with the working solvent (a pro-
cedure affording reproducible results for our systems; soni-
cation proved useless).
Boron-doped diamond cathodes and fluorinated boron-
doped diamond cathodes were 0.225 and 0.36 cm2 plates,
respectively (courtesy of A. De Battisti and S. Ferro, Univer-
sity of Ferrara; the characterization of such electrode materi-
als is reported in [27,28]) glued by colloidal graphite (Leit-C)
to a Pt wire and embedded into a concrete matrix (Saratoga
Unistop, selected for its satisfactory inertness to acetonitrile)
leaving it to dry for several days before use. The electrodes
were polished by wiping with acetone and then with the work-
ing solvent.
lab PGSTAT 30 potentiostat/galvanostat (EcoChemie, The
Netherlands) run by PCs with GPES software and equipped
with a Metrohm 663 VA Stand. The reference electrode
was the aqueous saturated calomel one (SCE), while the
counter electrode was either a carbon or a platinum one.
To allow intersolvental comparison along the IUPAC fer-
rocene criterion [22] the half-wave potential of ferrocene
E
1/2 = (Ep,c + Ep,a) was measured on a GC electrode in our
ACN + 0.1 M TEATFB medium against our aqueous SCE,
yielding 0.394 V.
Controlled-surface polycrystalline silver electrodes were
prepared on Pt wire supports (diameter: 0.05 cm, length:
1 cm) by silver electrodeposition against a silver foil anode
in six-electrode batches from a 10 g dm−3 KAg(CN)2 bath at
a current density i = 2 mA cm−2 for 15 h, followed by 5 min
sonication in the operating solvent in a Bransonic 221 ultra-
sound bath. This procedure affords reproducible, controlled
surfaces [5].
Polycrystalline copper cathodes were freshly prepared on
Cu wire supports (diameter = 0.05 cm, length = 1 cm) by cop-
per electrodeposition from a CuSO4 bath at i = 0.5 mA cm−2
for 15 min, followed by 5 min sonication in the operating sol-
vent.
Gold cathodes were ultrapure gold wires (length = 1.5 cm,
diameter = 0.1 cm), or a gold disk (Metrohm 6.1204.140, di-
ameter = 0.2 cm), polished both mechanically, with wet alu-
mina on filter paper, and electrochemically, by cycling in
0.5 M H2SO4.
3. Results and discussion
Lead cathodes were thin lead plates (1 cm × 0.5 cm ×
0.05 cm; Aldrich 99.95%) suspended by small platinum alli-
gatorclamps, withtheworkingsurfacedelimitedbyTeflonTM
ribbon; they were mechanically polished, and then activated
by immersion in 0.5 M H2SO4 at −0.55 V for 20 min, fol-
lowed by repeated immersions in boiling CH3COONH4 so-
lution, and by verification of open circuit potential in 0.5 M
H2SO4 [23]. The background CV usually featured a typi-
cal oxide reduction peak at about −1.5 V (SCE) in the first
cycles, which however disappeared upon prolonged cycling.
Substrate additions were made only then, and with concurrent
nitrogen flowing.
3.1. The problem of the reference, non-catalytic cathode
material
As it has been shown in a former work on silver cath-
odes [4], the molecular structure surrounding a given active
group powerfully modulates not only its intrinsic reactivity,
but also the electrocatalytic activity of the cathode in the pro-
cess, since even groups not involved in the faradaic process
can influence both the extent and mode of specific adsorp-
tion. For example, the electrocatalytic activity of silver for
the reduction of bromides featuring one or more “adsorp-

2334
C. Bellomunno et al. / Electrochimica Acta 50 (2005) 2331–2341
Table 1
tion auxiliaries” such as hydroxyl, phenyl, and (additional)
halide groups is even two or three times greater than with
the analogous, purely aliphatic bromides [4]. To estimate the
extent of the electrocatalytic effect of a given cathode ma-
terial Me in the reduction of a given substrate, normalized
with respect to the substrate intrinsic reactivity, it is conve-
nient to consider the first reduction peak potential difference
∆ = (Ep,Me − Ep,Ref), where the “Ref” subscript denotes an
inert reference material that can be regarded purely as an elec-
tron source. It should be underlined that the mechanism of the
reduction reaction can be different on the different electrodes,
according to the possibly different reaction intermediates, as
it is, inter alia, implicit in the concept of catalysis; never-
theless the above difference is significant, since it accounts
for the difference in activation energy for the uptake of the
first electron via any reaction mechanism. Now glassy carbon
(GC), a popular electrode for mechanistic investigations of
outer-sphere electron transfers to organic halides [20], could
beaconvenientcandidatefortheinertreferenceelectrode, but
its inertness has been questioned on account of the significant
amount of oxygenated functions usually present on its sur-
face [29]. The more recently appeared boron doped diamond
(BDD) electrode looks a more appropriate inert reference
candidate, being predominantly hydrogen-terminated, which
makes it inter alia rather inert to adsorption while retaining
CV features (peak potentials Ep, peak current densities ip normalized with
respect to concentration c = 0.003 mol dm−3) obtained in solvent ACN at
0.2 V s−1 for the reduction of the four model organic bromides on the three
cathode materials tested as non-specifically interacting reference materials
GC
BDD
FBDD
Acetobromoglucose 1
Supporting electrolyte
Ep (V)
TEAP
TEATFB TEATFB
−2.491 −2.672
−2.97
−0.48
Ep − Ep,GC (V)
0
−0.181
0.283
(−ip/c) (A cm−2 mol−1 dm3) 0.398
Benzyl bromide 2
Supporting electrolyte
Ep (V)
TEATFB TEAP
TEATFB
−2.584
−0.674
0.555
−1.910 −2.326
Ep − Ep,GC (V)
0
−0.416
0·623
(−ip/c) (A cm−2 mol−1 dm3) 0·748
8-Bromooctanol 3
Supporting electrolyte
Ep (V)
TEAP
−2.79
TEAP
No signal before background
Halothane 4
Supporting electrolyte
Ep (V)
TEATFB TEATFB TEATFB
−1.827 −2.195
−2.37 (0.5 V s−1
)
Ep − Ep,GC (V)
0
−0.336
0.68
(−ip/c) (A cm−2 mol−1 dm3) 0.73
0.287
ophilic substitution. In particular, the intrinsic reactivity
of the latter compound is so low that no wave can be ob-
served before the background on the less active diamond
electrodes;
reversible to quasi-reversible electron transfer for inorganic
model redox couples such as Fe(CN)6−3/−4, Ru(NH3)6
and IrCl6
,
+2/+3
−2/−3
[30]. Moreover, the fluorinated boron-doped
• the reduction potential sequence on the three carbon-based
electrodes follows (excepting compound 3, of such a low
intrinsic activity that its reduction can be observed only
on the GC electrode) the sequence GC > BDD > FBDD
(possibly a sequence of decreasing Lewis acidity of the
reacting surfaces). The last features can be convincingly
justified in terms of BDD (and even more FBDD) being
less specifically interacting than GC with the reacting sub-
strate on account of the increasing H (and F) versus O(H)
ratio. FBDD appears by far the less active surface, but
probably the hydrogen-terminated BDD is the more ap-
propriate inert reference, considering that the fluorinated
FBDD surface can even exert a repulsive effect on the ap-
proaching halide leaving group of the reacting molecule.
Anyway, in the development of an electrocatalytic activity
scale as described below, we will use GC as the reference
electrode since its behaviour approaches that of BDD, and
it is the only carbon-based electrode on which all the four
molecules tested exhibit a reduction peak in the observable
window.
diamond (FBDD) has very recently been introduced, fea-
turing a consistent number of fluoride atoms on its surface,
which results in a more hydrophobic character and therefore
in significant deactivation of the hydrogen evolution reac-
tion in water [31]; for similar reasons we foresaw that FBDD
should be even less favourable to halide reduction, since the
F adatoms could have a repulsive effect on the approaching
halide leaving group of the reacting molecule.
Therefore as a preliminary step to the comparison of the
electrocatalytic materials, we have studied the electrochemi-
cal activity of our model substrates on the above three carbon-
based electrodes, GC, BDD, and FBDD. The results are sum-
marized in Table 1 and visualized in Figs. 2–5. It appears that
• the potential window available in ACN + 0.1 M TEAP (or
TEATFB) on the diamond electrodes (BDD, FBDD) is
slightly wider on the negative side, 100–200 mV, than on
GC; such background (normalized with respect to the elec-
trode surface) features on diamond-based electrodes a gen-
erally lower capacitance than on GC, as it happens in water
[30];
The electrocatalytic activity scale: To enlighten the exist-
ing relationship between the cathode material and its elec-
trocatalytic effect on the target process, we have studied the
reactivity of the above four model molecules on nine cathode
materials, showing very low to very high affinity for halides
in specific adsorption experiments [8]: Zn, Pt, Hg, Sn, Bi,
Pb, Au, Cu, Ag, each one being prepared along the specific
procedure described in Section 2.
• the CV characteristics are, whenever observable, single,
irreversible peaks;
• the intrinsic reactivity of the four tested bromides (ac-
counted for by the peak potentials on the non-catalytic
electrodes) follows the order halothane 4 ≥ benzyl bro-
mide 2 acetobromoglucose 1 ≫ 8-Br-octanol 3, a se-
quence following the substrates’ SN1 character in nucle-

C. Bellomunno et al. / Electrochimica Acta 50 (2005) 2331–2341
2335
Fig. 2. CV features for the reduction of substrate acetobromoglucose 1 on different cathode materials, recorded at 0.2 V s−1 scan rate, in ACN + 0.1 M TEAP
(or TEATFB) medium.
Table 2
CV features (peak potentials Ep, peak current densities ip normalized with respect to concentration c = 0.003 mol dm−3) obtained in solvent ACN for the
electrocatalytic reduction of model substrate acetobromoglucose at 0.2 V s−1 on the cathode materials tested as electrocatalytic materials for the target reaction
plus the glassy carbon reference electrode, for comparison
Cathode
D2◦98(M
(kJ mol−1
)
Geometric surface (cm2)
0.1 M supporting electrolyte
Ep (V)
−ip/c (A cm−2 mol−1 dm3)
Ep − Ep,GC (V)
Br)
Ag
Pb
Au
Cu
Hg
Sn
Bi
Pt
Zn
GC
293
247
220.3
338
72.8
>565
267
0.161
1
TEATFB
TEAP
TEAP
TEATFB
TEAP
TEAP
TEATFB
TEAP
TEAP
−1.284
−1.835
−1.844
−1.880
−1.933
−1.938
−2.082
−2.23
0.60
0.20
0.40
0.32
0.41
0.22
0.42
0.57
1.163
0.656
0.647
0.611
0.558
0.553
0.409
0.26
0.636
0.159
0.022
0.385
0.942
0.19
142
309
0.143
0.049
No signal
−2.491
TEAP
0.40
0
Bonding energies D2◦98(M
(kJ mol−1) [19,33], are also quoted.
Br)

2336
C. Bellomunno et al. / Electrochimica Acta 50 (2005) 2331–2341
Table 3
CV features (peak potentials Ep, peak current densities ip normalized with respect to concentration c = 0.003 mol dm−3) obtained in solvent ACN for the
electrocatalytic reduction of model substrate benzyl bromide at 0.2 V s−1 on the cathode materials tested as electrocatalytic materials for the target reaction plus
the glassy carbon reference electrode, for comparison
Cathode
D2◦98(M
(kJ mol−1
)
Geometric surface (cm2)
0.1 M supporting
electrolyte
Ep (V)
−ip/c (A cm−2 mol−1 dm3)
Ep − Ep,GC (V)
Br)
Ag
Hg
Pb
Bi
Au
Cu
Sn
Zn
Pt
293
72.8
247
267
220.3
338
>565
142
309
0.161
0.022
1
0.942
0.636
0.206
0.385
0.143
0.157
0.049
TEATFB
TEAP
TEAP
TEATFB
TEAP
TEATFB
TEAP
TEAP
TEAP
TEATFB
−1.047
−1.465
−1.531
−1.594
−1.635
−1.686
−1.867
−1.902
0.71
0.30
0.863
0.437
0.379
0.316
0.275
0.224
0.043
0.008
0.54
1.08
0.23
Background anticipation, ∼−2.2 V
−1.910 0.75
GC
0
Bonding energies D2◦98(M
(kJ mol−1) [19,33], are also quoted.
Br)
Table 4
CV features (peak potentials Ep, peak current densities ip normalized with respect to concentration c = 0.003 mol dm−3) obtained in solvent ACN for the
electrocatalytic reduction of model substrate 8-bromooctanol at 0.2 V s−1 on the cathode materials tested as electrocatalytic materials for the target reaction
plus the glassy carbon reference electrode, for comparison
Cathode
D2◦98(M
(kJ mol−1
)
Geometric surface (cm2)
0.1 M supporting electrolyte
Ep (V)
−ip/c (A cm−2 mol−1 dm3)
Ep − Ep,GC (V)
Br)
Ag
Pb
Au
Hg
Pt
293
247
220.3
72.8
142
0.161
1
0.636
0.022
0.19
0.049
TEAP
TEAP
TEAP
TEAP
TEAP
TEAP
−1.572
−2.201
−2.31
0.55
0.13
1.215
0.586
0.473
No signal before background
−2.38
0.57
0.30
0.387
0
GC
−2.787
Bonding energies D2◦98(M
(kJ mol−1) [19,33], are also quoted.
Br)
Table 5
CV features (peak potentials Ep, peak current densities ip normalized with respect to concentration c = 0.003 mol dm−3) obtained in solvent ACN for the
electrocatalytic reduction of model substrate halothane at 0.2 V s−1 on the cathode materials tested s electrocatalytic materials for the target reaction plus the
glassy carbon reference electrode, for comparison
Cathode D2◦98(M
(kJ mol−1
)
Geometric surface (cm2) 0.1 M supporting electrolyte Ep (V)
−ip/c (A cm−2 mol−1 dm3) Ep − Ep,GC (V)
Br)
Hg
Ag
Pb
Cu
Au
GC
72.8
293
247
338
220.3
0.015
0.161
1
0.2
0.314
0.049
TEATFB
TEATFB
TEATFB
TEATFB
TEATFB
TEATFB
−0.851, −0.998 1.05
0.976
0.707
0.598
0.560
0.396
0
−1.120
−1.229
−1.274
−1.431
−1.827
0.96
1.09
0.91
0.67
0.73
Bonding energies D2◦98(M
(kJ mol−1) [19,33], are also quoted.
Br)
The results are summarized in Tables 2 (substrate aceto-
bromoglucose), 3 (substrate benzyl bromide), 4 (substrate 8-
bromooctanol) and 5 (substrate halothane) and synopsized in
the corresponding Figs. 2–5.
generally very similar (especially in the ABG and halothane
cases), a good agreement when considering the wide range
of forms and dimensions of the electrodes tested and the fact
that current densities have been calculated by normalization
of the currents against the geometric area, since reliable cri-
teria for active surface determination were unavailable for
many cathode materials. However, some significant excep-
tions do appear, and can be ascribed to partial surface deac-
tivation, or to the active surface differing significantly from
the geometric one, or to specific effects such as the forma-
tion of an organometallic product (especially some Hg and Pb
cases resulting in more complex signals). On the average the
current densities show an increase in the sequence ABG < Br-
Octanol ≈ BzBr < halothane, possibly connected with the in-
creasing diffusion coefficients of the tested substrates (con-
The reduction processes are both chemically and electro-
chemically irreversible; increasing the substrate concentra-
tion from 0.001 to 0.003 M usually (but not always) results
in a negative potential shift of a few to many tenths of mV
per decade (according to the cathode/substrate system), con-
sistently with the observations by Langmaier and Samec for
the halothane case in other reaction media [19], which they
ascribed to significant specific adsorption of the substrate on
the cathode surface. The current densities obtained on the
different cathode materials (always featuring a linear depen-
dencyonv0.5, typicalofdiffusional, notadsorptive, peaks)are

C. Bellomunno et al. / Electrochimica Acta 50 (2005) 2331–2341
2337
Fig. 3. CV features for the reduction of substrate benzyl bromide 2 on different cathode materials, recorded at 0.2 V s−1 scan rate, in ACN + 0.1 M TEAP (or
TEATFB) medium.
sistently with their decreasing bulkiness and hydrophilicity).
Moreover, dealing with irreversible peaks, the reaction mech-
anism should be also taken into account; however, in our case
all the model bromidesshouldundergo a first concerted disso-
ciative electron transfer, which should be the rate determining
step according to many authoritative works in the literature
[32–37].
From our “electrocatalytic activity parameter”
∆ = (Ep,Me − Ep,GC) a scale of electrocatalytic activity
can be deduced for each model substrate, the four scales
being plotted together in Fig. 6. It is evident that
• both Cu and Au, preceding and following Ag in the same
group of the periodic table, feature average catalytic activ-
ities (∆ = 0.3–0.7 V);
• a similar, average catalytic activity range (∆ = 0.3–0.7 V)
is provided by a series of metals belonging to the following
twogroupsintheperiodictableandhavingstrongtendency
to organometallic compound formation (Hg, Pb, and to a
lesser degree, Bi, Sn);
• low catalytic effects, or no effects at all, are obtained on Pt
and Zn, which never gave well defined peaks;
• of course, having taken GC as the reference material, BDD
and FBDD result in increasingly negative ∆ values.
• silver is by far the most catalytic cathode material for the
target process, resulting in a dramatic anticipation of the
reduction potential (i.e. lowering of the activation energy
for the uptake of the first electron) with respect to the non
catalytic reference material, the relevant ∆ values ranging
0.6–1.3 V according to the bromide molecular structure;
It is worthwhile noticing that the best electrocatalytic ma-
terials in the scale in Fig. 6 include the materials resulting
the most efficient catalysts in the preparative works available
in the literature, even concerning very different halides (e.g.
geminal polychlorides in environmentally oriented papers,

2338
C. Bellomunno et al. / Electrochimica Acta 50 (2005) 2331–2341
Fig. 4. CV features for the reduction of substrate 8-bromooctanol 3 on different cathode materials, recorded at 0.2 V s−1 scan rate, in ACN + 0.1 M TEAP (or
TEATFB) medium.
such as [13,14,18]). This confirms the general validity of our
scale for the process under study.
• itseffectsonthestabilityofthethree-actor(Me · · · X · · · R)
reaction intermediate typical of the electrocatalytic pro-
cess;
Besides the above general features common to all model
substrates, the electrocatalytic effects significantly depend,
in both their extents and sequence, on the molecular struc-
ture surrounding the active bromide group, as it is evident
considering the last columns in Tables 2–5 (where for each
substance the cathode materials are listed in decreasing order
of our “electrocatalytic activity parameter” ∆) and also look-
ing at Fig. 6. For instance, catalytic effects for the reduction
of benzyl bromide are significantly and constantly lower (by
from one-third to one-half) with respect to catalytic effects for
the reduction of glycosyl bromide ABG and of bromooctanol;
lower electrocatalytic effects are obtained for halothane, too,
featuring a remarkable smoothing of the silver activity and
an equally remarkable enhancement of that of mercury: actu-
ally, it is the first halide case so far encountered (including our
former parallel Hg/Ag screening of about 40 organic halides
[4]) where silver is not the most electrocatalytic cathode, and
it must be connected with the formation of an organometallic
compound, as it is evident from the relevant peak shape.
It is worthwhile underlining that, as expected, intrinsic
reactivity and electrocatalytic effects are not necessarily pro-
portional: halothane and benzyl bromide, although resulting
muchmoreintrinsicallyreactivethanABGandbromooctanol
(considering the scale of reduction potentials on the inert ref-
erence cathode), result in significantly lower electrocatalytic
effects. In fact the molecular structure is likely to modulate
the electrocatalytic capability of the metal according with one
or more of the following features:
• its possible promoting formation of organometallic prod-
ucts (several evident examples of which appear in
Figs. 2–5);
• its possible effects on concurrent coadsorption or desorp-
tion of solvent and/or supporting electrolyte.
The higher electrocatalytic effects on ABG and bromooc-
tanol could therefore be connected with them both featuring
oxygen-containing groups acting as adsorption auxiliaries
[4], although also the phenyl group of benzyl bromide and the
chloride atom of halothane can specifically interact with the
electrode surfaces. Of course the adsorption of other groups
should be such to support and not to hinder, or compete with,
the adsorption of the reacting group. Hindering or compe-
tition by neighbour halide atoms could explain the compar-
atively much lower catalytic effect of silver on halothane
(actually a sort of hydrophobic halide bundle). A program
of SERS investigations is currently being planned to acquire
experimental information on the adsorption geometry of our
model molecules on such SERS-active electrocatalytic sur-
faces as Ag, Au, and Cu.
Considering the above many factors affecting the reaction
mechanism it is evident that finding out a quantitative rela-
tionship justifying the experimental electrocatalytic activity
scale shown in Fig. 6 should indeed not be an easy task.
Of course a key ad hoc parameter must be the X · · · Me or
X− · · · Me adsorption energy, as it is in well known, inor-
ganic electrocatalytic processes leading to the so popular and
useful “volcano-like” plots.
• its higher or lower cooperation in the specific adsorption
of the reacting molecule [4], and its effects on the distance
and accessibility of the reaction site with respect to the
cathode surface;
Computational electrochemistry groups are certainly
aware of this important problem. Significantly, authoritative

C. Bellomunno et al. / Electrochimica Acta 50 (2005) 2331–2341
2339
Fig. 5. CV features for the reduction of substrate halothane 4 on different cathode materials, recorded at 0.2 V s−1 scan rate, in ACN + 0.1 M TEATFB medium.
theoretical studies have recently appeared, dealing with the
problem of computing adsorption energies, charge distribu-
tions, halogen bonding energies, equilibrium bonding dis-
tances, and adsorbate charge for Men X or Men X− sys-
tems (Ignaczak and Gomes [11], Koper and Van Santen [12]).
Unfortunately, both works refer to vacuum; this surely ac-
counts for the calculated metal−halide affinity sequences
being mostly in contraddiction with experimental results ob-
tained in adsorption, reactivity (including our works) and
SERS investigations. Actually, in his cited work [12], Koper
underlines that, since water stabilizes a negative charge on
the halide, the electrostatic interaction of a solvated halide
with a metal surface will be substantially different from the
halogen/halide interaction at the metal/vacuum surface.
Moreover, themetalworkfunction ΦM shouldbemodified
ꢀ
into ΦM as suggested by Trasatti [38].
Therefore, at present, thermodynamic parameters on
halide ion adsorption in solution can be derived only by
experimental data, particularly spectroscopic X Me bond-
ing energies. In their pioneer work [19] Langmaier and
Samec, starting from the assumption of a reaction mech-
anism hinging on an adsorptive rate determining step fol-
lowed by uptake of the first electron, come to predict a
volcano-like relationship between the reduction potentials
obtained on the tested metals and the relevant X Me bond-
ing energies, derived from a tabulation of spectroscopic re-
sults [39]. In their particular, non-standard case (see the
discussion in Section 1), referring to halothane in water

2340
C. Bellomunno et al. / Electrochimica Acta 50 (2005) 2331–2341
Fig. 6. The experimental electrocatalytic activity scale (referred to the non-catalytic GC assumption) of the tested cathode materials Me for the reduction of
four model organic bromides in ACN with TEAP or TEATFB supporting electrolytes.
or methanol with NaOH supporting electrolyte, such re-
lationship actually appears approximately verified, so that
they even manage to discuss it in terms of coverage de-
grees, being optimal on silver on account of its intermediate
halide bonding energies among the tested metals (in analogy
with the case of the hydrogen evolution reaction on plat-
inum).
(but only neglecting the anomalous case of mercury); how-
ever, the same does not seem to hold for ABG (triangles),
the substrate corresponding to the highest catalytic effects
(even if bismuth and platinum, the most deviating points,
come from rather critical CV experiments);
• mercury represents a strong anomaly, particularly concern-
ing the halothane case (which however is very likely justi-
fied by the formation of an organometallic compound). A
similar anomaly, although less remarkable, appears also in
Fig. 7 in Ref. [19], although only in the case of water and
not in the methanol one.
To verify if the Langmaier and Samec relationship could
be generalized, we have plotted against the same spectro-
scopic bonding energies (referred to the silver one, as the
authors did) the four ∆ = Ep,Me − Ep,GC series obtained for
our model substrates in ACN solvent with the nearly inert
supporting electrolytes TEAP or TEATFB. For the sake of
discussion, it is useful to underline that the so obtained dia-
gram, appearing in Fig. 7, is directly comparable with Fig. 7
in Ref. [19], the only difference being that the latter’s zero
level was assigned to the reduction potential of the most cat-
alytic electrode (Ag) since these authors performed no stan-
dardization against a non-catalytic electrode. In our figure
we followed our criterion instead than the authors’ one since,
apart from the importance of knowing the total extents of
the electrocatalytic effects (rather than their level with re-
spect to the maximum one), normalizing against the most cat-
alytic electrode is unappropriate when more than one halide
structure has to be compared, since the electrocatalytic ef-
fects of silver are different for each structure. Fig. 7 shows
that
• each substrate corresponds to a characteristic significantly
different both in shape and level on the electrocatalytic
activity scale;
• the characteristics pertaining to substrates BzBr (circles),
halothane (crosses), and Br-octanol (squares) can be con-
sidered roughly volcano-shaped with a maximum on silver
Fig. 7. The experimental electrocatalytic effects of a series of cathode ma-
terials Me for the reduction of the four organic bromide substrates ABG
(triangles), BzBr (circles), Br-octanol (squares) and halothane (crosses) in
ACN medium, plotted as a function of the relevant MeBr bonding energies,
referred to the AgBr one.

C. Bellomunno et al. / Electrochimica Acta 50 (2005) 2331–2341
2341
Therefore our experimental results only vaguely comply
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The financial support of Italy’s MIUR (FIRB and COFIN)
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