In situ identification of intermediates of benzyl chloride reduction at a silver electrode by SERS coupled with DFT calculations

Article abstract of DOI:10.1021/ja1024639

Aiming to deeply understand the electrocatalytic mechanism of silver on reduction of benzyl chloride, we carried out an in situ electrochemical surface-enhanced Raman spectroscopic study to characterize various surface species in different electrode potential regions. A further analysis with DFT calculation reveals that the benzyl radical and its anionic derivate bonded on a silver electrode are the key intermediates, implying that the pathway could drastically differ from the outer sphere concerted electron reduction at inert electrodes.

Full text of DOI:10.1021/ja1024639

Published on Web 06/24/2010
In Situ Identification of Intermediates of Benzyl Chloride Reduction at a Silver
Electrode by SERS Coupled with DFT Calculations
†
†
†
†
†
‡
An Wang, Yi-Fan Huang, Ujjal Kumar Sur, De-Yin Wu, Bin Ren, Sandra Rondinini,
,
§
,†
Christian Amatore,* and Zhong-Qun Tian*
State Key Laboratory of Physical Chemistry of Solid Surfaces and Department of Chemistry, College of Chemistry
and Chemical Engineering, Xiamen UniVersity, 361005, Xiamen, China, Ecole Normale Superieure, D e´ partement de
Chimie, UMR 8640, 24 Rue Lhomond, F-75231 Paris Cedex 5, France, and Dipartimento di Chimica Fisica ed
Elettrochimica, UniVersit a` degli Studi di Milano, Via Golgi 19, 20133 Milan, Italy
Received March 24, 2010; E-mail: zqtian@xmu.edu.cn; christian.amatore@ens.fr
them at the molecular level, SERS spectra were recorded at different
Abstract: Aiming to deeply understand the electrocatalytic mech-
potentials spanning the voltammetric wave range from -0.6 to -2.2
anism of silver on reduction of benzyl chloride, we carried out an
V vs SCE (Figure 1b). At potentials positive to -1.2 V, the spectral
in situ electrochemical surface-enhanced Raman spectroscopic
2
features resembled that of free PhCH Cl though a few significant
study to characterize various surface species in different electrode
potential regions. A further analysis with DFT calculation reveals
that the benzyl radical and its anionic derivate bonded on a silver
electrode are the key intermediates, implying that the pathway
could drastically differ from the outer sphere concerted electron
reduction at inert electrodes.
changes indicated a weak interaction between benzyl chloride and
the silver surface (see Supporting Information). When the cathode
was held at ca. -1.2 V, the SERS spectrum changed dramatically.
A strong and broad peak appeared at ca. 800 cm- and reached its
maximum intensity at -1.4 V. This indicates that new species
formed at (or near) the surface as soon as the electrochemical
reduction occurred at a significant rate. A second drastic spectral
change happened when the potential was set beyond -1.6 V. A
peak grew gradually at ca. 1000 cm- and reached its maximum at
-1.8 V, i.e., when the voltammetric peak was displayed. This new
peak then is most likely due to the final reaction product(s) adsorbed
at the surface, its decay indicating a progressive desorption at more
negative potentials. This sequence of specific SERS features reflects
the presence in each potential range of possibly important reaction
intermediates or product(s).
1
1
Characterization of reaction mechanisms and intermediates of
complex chemical reactions is a central topic in organic chemistry,
catalysis, electrochemistry, polymer chemistry, toxicology, biology,
1
etc. In this context, the electrochemical reductive cleavage of
carbon-halogen bonds at inert electrodes has played an important
role in electroorganic synthesis, environmental applications, and
2
electron transfer mechanisms. Recent works evidenced that silver
(
Ag) cathodes possess surprisingly high electrocatalytic activity
Two distinct spectral features were observed at potentials around
3
-6
toward these reactions.
However, voltammetric data alone,
-
3
1.4 V (Figure 1b). The first one was a broad peak centered at
lacking in situ characterization of transient surface intermediates,
could not help unravel the exact origin of this strong catalysis by
Ag.
-1
50 cm that could be assigned to Ag-C stretching bands,
indicating that at least one reaction intermediate was strongly bound
to the surface via a C-Ag bond. Since a halide anion is one
On the other hand, surface-enhanced Raman spectroscopy
necessary product of any two-electron reduction sequence at a silver
(
SERS) is a powerful in situ technique for probing the existence
2-6
electrode,
previous works hypothesized that their known interac-
7
of transient surface species, especially when coupled with density
functional theory (DFT). So far, the combined SERS-DFT studies
on electrochemical systems are very limited and the majority
focused on electrosorption behavior in aqueous solution. Herein,
we report the first SERS-DFT study on electrochemical reduction
of benzyl chloride (PhCH
tion with silver surface cathodes may play a significant role in the
3,4,9
overall catalytic effect.
However, the characteristic potential
8
-1
dependent Ag-Cl vibration band at approximately 230-240 cm
could not be observed in this work, showing that under our
10
conditions the chloride ion did not bind appreciably. This presum-
ably reflects competition by other species (see below), probably
reinforced by the strong electrostatic repulsion from negatively
2
Cl) at a Ag electrode in acetonitrile
(
CH CN). The result reveals that the electrocatalytic reaction
3
proceeds from a weakly adsorbed PhCH
bound strongly to the Ag surface.
2
Cl to benzyl intermediates
9
charged surfaces.
A second remarkable feature consisted of an intense and
Figure 1a shows cyclic voltammograms (CVs) of 5 mM PhCH
at a Ag electrode, exhibiting a single irreversible reduction peak
with a peak potential of E
) -1.82 V vs SCE at 0.5 V/s (R ≈
As already reported, the voltammetric behavior was then
2
Cl
-1
unusually broad peak at ∼800 cm . To the best of our knowledge,
no one has acquired surface-enhanced Raman data relevant to the
p
2
PhCH Cl reduction. Owing to the many possible reaction pathways
4
-6
0
.3).
and intermediates which could be envisioned, we relied on DFT to
2
a
quite similar to that observed at inert glassy carbon electrodes
except for the remarkable positive shift by ca. 0.5 V of the CV
peak.
11
unambiguously identify the corresponding surface species.
Figure 2 presents DFT simulated spectra (see Supporting
Information for methods) for the free benzyl radical and benzyl
anion whose formation could be hypothesized based on the final
To observe the presence of key surface intermediates possibly
3b
generated during the Ag-electrocatalyzed process and characterize
5
electrolysis products (see below) and compares them to the spectra
predicted for the same species bound to the Ag surface as well as
to the experimental spectrum. The simulated spectra of the bound
benzyl moieties displayed strong and characteristic bands between
†
Xiamen University.
‡
Universit a` degli Studi di Milano.
§
Ecole Normale Superieure.
9
534 9 J. AM. CHEM. SOC. 2010, 132, 9534–9536
10.1021/ja1024639  2010 American Chemical Society

C O M M U N I C A T I O N S
Figure 1. (a) CV of 5 mM PhCH
2 3
Cl in 0.1 M TEAP + CH CN at a Ag electrode with different scan rates. (b) Potential dependent SERS spectra of
2
PhCH Cl on a Ag electrode.
1
8
50 and 900 cm- which were absent for the free species.
directly or ultimately a bound benzyl radical and then the bound
benzyl anion. Desorption of the benzyl anion should then lead to
the final products. Such a pathway drastically differs from the outer
Furthermore, a superposition of the simulated spectra of the bound
benzyl moieties, as illustrated in Figure 2f, shows that the overlap
of the bands affords a broad peak which is quite similar to the
experimental one (Figure 2c). Additionally, other bands such as
the ones at ca. 530 and 1200 cm- were also successfully predicted
for the bonded benzyl species (see Supporting Information for
detailed assignment of all bands). This indicates that both neutral
and anionic benzyl-silver adducts were formed, the anionic species
being the dominant one over the main voltammetric wave potential
range.
2
sphere concerted electron reduction at inert electrodes and, hence,
modifies the thermodynamics and kinetics of the first reduction
stage. Such important mechanistic changes must then be responsible
for the important anodic shift of the voltammetric wave at silver
electrodes. A thorough kinetic and thermodynamic investigation
of the mechanistic features outlined here requires quantitative
predictions of adsorption energies of all surface species and their
cross-comparison with electrochemical data. This work is currently
1
1
2
in progress using DFT.
-
-
(
PhCH Cl--Ag ) + e f (PhCH --Ag ) + Cl
(1)
(2)
(3)
(4)
2
n
2
n
-
-
(
PhCH --Ag ) + e f (PhCH --Ag )
2 n 2 n
-
-
2
(
PhCH --Ag ) f Ag +PhCH
2
n
n
-
PhCH f products
2
In summary, combining SERS and DFT has afforded crucial in
situ molecular entities, revealing the presence of unexpected key
surface intermediates underlying the strong electrocatalytic proper-
ties of silver electrodes. Although metals with intrinsically high
SERS activity are limited to gold, silver, and copper, mechanisms
involving various inactive or weakly active metals such as iron,
cobalt, nickel, platinum, palladium, etc. may also be investigated
using core-shell nanoparticles. This strategy allows the outer
metallic layer to “borrow” SERS activity from the SERS active
Figure 2. DFT calculated Raman spectra of the possible solvated reaction
intermediates: (a) free benzyl radical, (b) free benzyl anion, (d) benzyl
radical-Ag adduct, (e) benzyl anion-Ag adduct. These are compared with
4 4
the experimental SERS spectrum at -1.4 V vs SCE (c) and a 1:5
superposition of the predicted spectra in d and e (f).
4
5
core so as to yield enhancement factors up to 10 or 10 at the
1
3
surface of the shell. Hence, SERS may be used to gather in situ
mechanistic information in many areas where strong interaction
between reactants and catalytic metal surfaces are important, such
as organic chemistry, catalysis, polymer chemistry, toxicology,
astrochemistry, etc.
The same strategy was used to analyze the spectrum at -1.9 V.
5
According to reported electrolyses, reduction of benzyl chloride
at a silver electrode yields toluene and 3-phenylpropanenitrile as
the major products. Comparison of DFT-simulated and experimental
spectra established that 3-phenylpropanenitrile was formed in a
silver-bound state (see Supporting Information for details). Toluene
presence could not be characterized by SERS, though formation
of toluene is a necessary step on the route to 3-phenylpropane-
Acknowledgment. This work was supported by NSF of China
(
No. 20620130427) and MOST of China (No. 2007DFC40440),
CNRS, ENS, and UPMC (UMR 8640 and LIA XiamENS).
5
Supporting Information Available: Experimental procedures,
detailed molecular configuration and assignment of the peaks in the
spectra, discussion of the products, complete ref 11. This material is
available free of charge via the Internet at http://pubs.acs.org.
nitrile. This indicates that toluene was not prone to interact
sufficiently with the negative silver surface to provide a significant
SERS signature.
The above integrated SERS and DFT study provides important
mechanistic information, revealing that the catalytic property of
silver cathodes stems from the fact that they involve reduction of
benzyl chloride weakly adsorbed on the silver surface so as to yield
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