Electrochemical reduction of the carbonyl functional group: The importance of adsorption geometry, molecular structure, and electrode surface structure

Article abstract of DOI:10.1021/jacs.9b05397

This paper studies the electrochemical hydrogenation of the carbonyl functional group of acetophenone and 4-acetylpyridine at platinum single-crystal electrodes. Comparison with results obtained for the hydrogenation of acetone featuring an isolated carbo

Full text of DOI:10.1021/jacs.9b05397

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Article
Electrochemical Reduction of the Carbonyl Functional Group: The Importance
of Adsorption Geometry, Molecular Structure and Electrode Surface Structure
Christoph J. Bondue, and Marc T.M. Koper
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b05397 • Publication Date (Web): 05 Jul 2019
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Electrochemical Reduction of the Carbonyl Functional Group: The
Importance of Adsorption Geometry, Molecular Structure and
Electrode Surface Structure
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a
a
Christoph J. Bondue and Marc T. M. Koper *
a
Leiden Institute of Chemistry, Leiden University, P.O. Box 9502, 2300 RA Leiden, The Netherlands.
ABSTRACT: This paper studies the electrochemical hydrogenation of the carbonyl functional group of acetophenone and
4
-acetylpyridine at platinum single crystal electrodes. Comparison with results obtained for the hydrogenation of acetone
featuring an isolated carbonyl functional group reveals the influence of the phenyl ring and the pyridine ring, respectively.
Lack of acetone adsorption at Pt(111) and Pt(100) due to a weak interaction between surface and carbonyl functional group
renders these surfaces inactive for the hydrogenation of acetone. Adsorption through a strong interaction with the phenyl
ring of acetophenone activates the Pt(111) and Pt(100) surfaces for the hydrogenation of the acetyl substituent. In
agreement with previous results for acetone reduction, the Pt(100) surface is specifically active for the hydrogenolysis
reaction, breaking the C-O bond, whereas the other surfaces only hydrogenate the carbonyl functionality. In contrast to
the phenyl ring, the pyridine ring has a very different effect: due to the dominant interaction of the N-atom of the
pyridine ring with the platinum electrode, a vertical adsorption mode is realized. The resulting large physical distance
between carbonyl functional group and electrode surface inhibits the hydrogenation at all platinum surfaces. This also
holds for the Pt(110) electrode, which is otherwise active for the electrochemical hydrogenation of the isolated carbonyl
functional group of aliphatic ketones. Our results show how the combination of molecular structure of the reactant and
surface structure of the catalyst determine the selective electroreduction of functionalized ketones.
group in the substrate molecules interacts strongly with
the Pt(111) and Pt(100) surface, thus enabling the
INTRODUCTION
In recent years, there has been an increased interest in
the electrochemical hydrogenation of organic molecules
that feature two or more functional groups. Studies on
activation of the carbonyl functional group.
Since benzene binds to metal surfaces through an
interaction between its π-electron system and the
1
-2
the electrochemical reduction of acetophenone ,
13, 17-20
electrode surface
,
it can be expected that
2-4
5-7
benzaldehyde , 5-hydroxymethyllfurfural and ethyl
acetophenone adsorbs to platinum electrodes through the
phenyl ring. Pyridine, on the other hand, can adsorb to
metal electrodes in two ways: horizontally through an
8-9
pyruvate have been performed in the context of the
electrorefining of biomass derived compounds. To design
efficient electrochemical processes for these conversions,
it is important to utilize our knowledge on the
24
interaction via its π-electron system (similar to benzene)
18, 21-22
or vertically through the N-atom
. At platinum
10-16
electrochemistry of simple organic molecules
and on
conceptual
18,
electrodes, pyridine generally adsorbs in a vertical mode
13, 17-22
adsorption structures
to build
a
21-22
.
No changes to this adsorption mode would be
understanding on how (inert) functional groups can
influence the electrochemistry of complex organic
molecules.
expected when an acetyl substituent as in 4-
acetylpyridine is introduced.
Here, we compare the hydrogenation of the carbonyl
functional group of acetone, acetophenone and 4-
acetylpyridine to establish the impact of an inert
functional group (phenyl ring and pyridine ring). We
show that adsorption through the phenyl ring in a
horizontal mode facilitates the hydrogenation of the
carbonyl functional group, while adsorption through the
N-atom of the pyridine ring in a vertical mode inhibits the
hydrogenation reaction. Our results provide a new
conceptual understanding about how one functional
group in the substrate molecule can alter the reactivity of
another functional group through an influence on the
adsorption mode. In addition we show that combination
of molecular structure of the reactant and surface
In
a
previous paper, we have investigated the
electrochemical hydrogenation of acetone at platinum
single crystal electrodes . This study showed that acetone
23
reduction only proceeds at step sites, while the
crystallographic orientation of the steps determines
whether the reduction only yields 2-propanol or in
addition also propane. Computational studies indicate
that the reduction of the carbonyl functional group is
favorable at the Pt(111) and Pt(100) electrode. Nonetheless
no reduction proceeds in practice, since the adsorption of
acetone is unfavorable at these surfaces . This raises the
question whether the hydrogenation of the carbonyl
functional group is possible when an (inert) functional
23
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structure of the catalyst determines the selectivity of tht damage the laser beam was attenuated by a factor of 10
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electroreduction of functionalized ketones.
resulting in a measured beam energy at the location of
the Raman cell of 0.28 W.
EXPERIMENTAL
Chemicals and Materials: The blank electrolyte was
High
Pressure
Liquid
Chromatography
(HPLC)
Measurements: Samples for HPLC measurements were
generated through long-term electrolysis. Electrolysis was
conducted in an H-cell of which the working electrode
compartment was equipped with an reference electrode.
The compartments of working electrode and counter
electrode were separated with a Nafion membrane. The
electrolyte volume of the working electrode compartment
was 20 ml. From this volume a sample of 100 µl was taken
for HPLC-analysis.
prepared from MilliQ water and either H
2 4 4
SO or HClO
(
both suprapure, Merck). The ketones used were
acetophenone (Laboratory Reagent Grade, Fisher
Scientific), 4-acetylpyridine (alfa Aesar 98%) and acetone
(Sigma Aldrich, HPLC garde). All solutions were freed
from oxygen by purging with argon (6.0, Linde). All
potentials were measured versus a reversible hydrogen
electrode (RHE) in contact with the blank electrolyte. A
platinum wire was used as a counter electrode. Cyclic
voltammograms (CV) were recorded on an Iviumstat
potentiostat (Ivium Technologies).
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For compound separation after long-term electrolysis of
acetophenone an Aminex HPX 87-H (Bio-Rad) column
filled with Sugar SH1011 (Shodex) in combination with an
eluent of 0.5 mM H SO was used. The temperature of the
Single Crystal Electrodes: Bead type single crystal
electrodes (Icryst) where used to collect CVs and to
2
4
0
column was kept at 85 C and compounds were detected
by measuring the refractive index.
conduct
measurements.
electrochemical
Experiments
mass
involving
spectroscopy
long-term
For compound separation after long-term electrolysis of
acetylpyridine an Phenomenex (Kintex) column was used.
In this case the eluent consisted to 30% of acetonitrile
electrolysis were conducted with disk shaped single
crystal electrodes of 1 cm in diameter. Both types of single
crystal electrodes were prepared by the Clavilier
2 4
and to 70% of a solution of 10 mM Na HPO in water with
25
method : The crystals were flame annealed and cooled
down over MilliQ water in an atmosphere of 20%
hydrogen and 80% argon. Afterwards the electrode was
dipped into water saturated with the hydrogen-argon
mixture and transferred with a droplet attached to the
surface to the electrochemical cell. Glassware used in this
the pH adjusted to 8.2. The temperature of the column
0
was kept at 60 C and compounds were detected by
measuring the absorption of UV-light with a wavelength
of 230 nm. Prior to HPLC analysis the samples were
neutralized by adding appropriate amounts of Na HPO .
2 4
study was kept in
a cleaning solution of acidic
RESULTS
permanganate for at least 8 hours. Prior to use, the
glassware was rinsed with acidic peroxide solution and
boiled several times in MilliQ water.
Figure 1 compares the cyclic voltammograms (CV)
obtained at Pt(111) in an electrolyte of 0.1 M H SO with
2
4
those obtained in the presence of 0.01 M acetone
(Figure 1A), acetophenone (Figure 1B) or 4-acetylpyridine
(Figure 1C). In the CV obtained in the blank electrolyte a
sharp spike at approximately 0.5 V appears. This spike is
due to the transition of an unordered sulfate overlayer on
Online Electrochemical Mass Spectrometry (OLEMS): The
details of our OLEMS setup have been reported
26
elsewhere : A Teflon frit inserted into an KelF-tip is
brought in close contact with the surface of the electrode,
which resides in a hanging meniscus configuration. The
Teflon frit serves as an interface between the vacuum of
the mass spectrometer and the electrolyte. Volatile
products formed in the course of the electrochemical
reaction evaporate through the tip into the vacuum of the
mass spectrometer. A PrismaPlus mass spectrometer
3
× 7
)
푅19.1°
the Pt(111) electrode to a
(
superstructure
29-30
.
The sulfate spike can only be observed when large
(
111)-terraces are present that are not covered by any other
adsorbate than sulfate. In the presence of acetone neither
the sulfate spike nor hydrogen adsorption is suppressed,
indicating that acetone does not adsorbs to the Pt(111)
surfaces. Indeed, we have shown in a previous publication
that it is thermodynamically unfavorable for the carbonyl
functional group of acetone to interact with surface sites
(
Pfeiffer Vacuum) operating in the multiple-ion detection
mode was used in this study.
Surface Enhanced Raman Spectroscopy (SERS): Electrodes
used for SERS studies were prepared by a protocol
23
that feature highly coordinated platinum atoms .
27
established by Zou and Weaver : First a polycrystalline
gold electrode is roughened by 20 cycles of potential steps
between 1.2 V (5 seconds) and -0.6 V (20 seconds) vs.
Ag/AgCl in a solution of 0.5 M KCl. galvanostatic
platinum deposition is conducted with a current density
In contrast to acetone, the presence of carbonyl
compounds featuring an aromatic ring suppresses the
sulfate spike completely. Furthermore, hydrogen
adsorption is shifted to lower potentials in the presence of
the aromatic compounds. Both the suppression of the
hydrogen adsorption and the sulfate spike indicate that
acetophenone and 4-acetylpyridene adsorb strongly to
the Pt(111) surface. The absence of acetone adsorption at
Pt(111) suggests that the adsorption of the aromatic
carbonyl compounds does not proceed through an
interaction between the surface and the carbonyl
2
of 400 µA/cm (20 seconds) from a solution of 100 µM
H
2
PtCl
were collected at a LabRam HR800 (Horiba Jobin Yvon)
confocal microscope, which featured HeNe-Laser
632.81 nm). A detailed description of the setup and the
6 2 4
in 0.35 M Na HPO . SERS spectra at this electrode
a
(
28
methodology is given by Lai et al. . Each spectrum shown
is the average of 16 spectra taken. In order to avoid beam
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functional group. This is supported by the CVs in Figure 1
obtained in the presence of 10 mM benzene (blue curve in
Figure 1B) and pyridine (blue curve in Figure 1C). These
compounds affect the
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Figure 1: Comparison of the CV obtained at Pt(111) in the blank electrolyte of 0.1 M H
presence of 0.01 M ketone (red curve). A: CV in the absence (black) and presence of acetone (red); B: CV in the absence
black) and presence of acetophenone (red) or benzene (0.01 M, blue); Panel C: CV in the absence (black) and presence of
-acetylpyridine (red) or pyridne (0.01 M, blue). The insets show the enlarged potential region just positive of hydrogen
2 4
SO (black curve) and in the
(
4
evolution. Currents are normalized to the geometric electrode area, which should correspond to the real surface area for
an atomically smooth single crystal electrode.
hydrogen adsorption and the sulfate in a similar way as
the respective carbonyl compound. This suggests that the
adsorption of the carbonyl compounds is dominated by
an interaction between Pt(111) surface and the
phenyl/pyridine ring. In the discussion of Figure S1 and S2
in the Supporting Information, we arrive at the same
conclusion for the adsorption at Pt(100) and Pt(110)
surfaces. It is difficult to determine from the CVs in
Figure 1 whether a reduction proceeds at Pt(111) electrode.
In the potential region near the onset of hydrogen
evolution there is no additional shoulder in the CV that
would indicate the reduction of the carbonyl compound
compound might result from experimental inaccuracies
when bringing the electrode in the hanging meniscus
configuration. Figure S1 and S2 show that the situation is
similar for the Pt(100) and the Pt(110) electrode.
In the presence of acetophenone, a reversible process
around 0.03 V is observed in the CV of Pt(111). The linear
scan rate dependence of the peak current (as shown in
Figure S3 of the Supporting Information) indicates that
this pertains to a surface process that is not related to the
reduction of acetophenone. The same behavior is
observed in the presence of benzene and has been
ascribed previously to the adsorption of hydrogen
(
c.f. insets in Figure 1). The hydrogenation reaction might
20
underneath the benzene adlayer .
take place in parallel to hydrogen evolution. Small
differences in the current around 0 V between the CVs
obtained in the presence and absence of the carbonyl
In order to check whether acetophenone is hydrogenated
at Pt(111) and at other basal planes, we conducted OLEMS
measurements and HPLC analysis after long-term
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electrolysis. Figure 2 shows the OLEMS results obtained
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for the reduction of acetophenone at the three basal
planes (111), (110) and (100) of platinum. Upon cycling into
the potential region of hydrogen evolution, a signal
evolves for mass 91 at Pt(100), while no such signal
appears in the ionic current for the same mass at Pt(111)
and Pt(110). The ionic current for mass 91 does not follow
the
faradaic
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Figure 2: CVs obtained with a sweep rate of 0.5 mV/sec at Pt(111) (A), Pt(110) (B) and Pt(100) (C) in an electrolyte of 0.3 M
SO , containing 0.03 M acetophenone. Panels D, E and F show ionic current for mass 91. Arrows indicate the direction
of the potential scan. Currents are normalized on the geometric electrode area, which should correspond to the real
surface area for an atomically smooth single crystal electrode.
H
2
4
current in Figure 2C. This is due to a complex diffusion
patterns of products in the electrolyte from the electrode
surface to the inlet of the mass spectrometer resulting in a
smeared out and delayed signal for ethylbenzene.
Although it is not possible to quantify our OLEMS data,
the appearance of a signal for mass 91 gives qualitative
proof for the reduction of acetophenone to the
corresponding hydrocarbon (ethylbenzene) at the Pt(100)
electrode. Neither the Pt(111) nor the Pt(110) electrode are
able to deoxygenate the carbonyl functional group of
acetophenone.
masses 112 and 126, respectively. However, no signals were
observed for either mass, which shows that ethylbenzene
is the only volatile product formed in the course of
acetophenone reduction.
Other possible hydrogenation products of acetophenone,
such as 1-phenylethanol and 1-cyclohexylethanol, cannot
be detected by OLEMS, as alcohols do not pass readily
from the aqueous phase into the vacuum of the mass
spectrometer.
Therefore,
we
electrolyzed
an
acetophenone containing electrolyte for 2 h at each
platinum basal plane and conducted an HPLC analysis of
the electrolyte afterwards. Figure 3A shows the HPLC
results of samples obtained for each of the basal planes
and Figure 3B shows standards of 0.5 mM acetophenone,
1-phenylethanol and 1-cyclohexylethanol, respectively.
The electrochemical hydrogenation of acetophenone
could
also
yield
ethylcyclohexane
and
cyclohexylmethylketone, respectively. Their formation
should come along with a signal in the ionic current for
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After long-term electrolysis all samples contain
a
compound that eludes after 70.5 minutes, which indicates
the presence of 1-phenylethanol. The compound eluding
between 134 and 149 minutes is acetophenone. Despite
the broad signal due to acetophenone, the presence of 1-
cyclohexylethanol with a retention time of 132 minutes
should have been detected if it had formed in the course
of the electrochemical reduction of acetophenone. The
current-time transients corresponding to the data
presented in Figure 3 and for electrolysis conducted at
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.01 V and 0.05 V are shown in Figures S4-S6 in the SI.
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Figure S7 in the SI also shows that with decreasing
potential the current efficiency for 1-phenylethanol
production drops from 35% at 0.05 V to 15% at 0.01 V as
hydrogen evolution accelerates faster with potential than
ketone reduction.
2 4
Figure 3: A: HPLC analysis of a sample taken from an electrolyte of 0.1 M H SO containing 0.03 M acetophenone that has
been electrolyzed at 0.03 V for 2 hours at a platinum electrode (surface structure as indicated). B: HPLC-analysis of
.5 mM acetophenone (black), 0.5 mM 1-pheylethanol (red) and cyclohexylmethylketone (blue) in water.
0
Our HPLC data show that all three basal planes of
platinum can reduce the carbonyl functional group of
acetophenone to the corresponding alcohol. The OLEMS
data show that the Pt(100) electrode has the unique
ability among the platinum basal planes to catalyze in
addition the hydrogenolysis. Neither HPLC nor OLEMS
give any indication for the formation of either
cyclohexylmethylketone,
1-cyclohexylethanol
or
ethylcyclohexane which would form if the phenyl ring
was hydrogenated in addition to the carbonyl functional
group. This is in line with literature results that show that
platinum single crystals hydrogenate benzene only at
potentials lower than 0 V . Hence, selective reduction
of the carbonyl functional group in acetophenone is
1
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possible at platinum electrodes and the selective
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formation of 1-phenylethanol is possible by the
appropriate choice of the surface structure of the
electrocatalyst.
As for acetophenone, a solution of 0.1 M acid containing
0
.03 M of 4-acetylpyridine was electrolyzed for 2 h at a
potential of 0.03 V at each of the three platinum basal
planes. Because of their basic character, pyridine
derivatives and piperidine derivatives, which are potential
hy- drogenation products of acetylpyridine, interact very
strongly with water and do not pass from the aqueous
phase into the gas phase. Therefore, OLEMS is not a
suitable tool to detect hydrogenation products of 4-
acetylpyridine and we only conducted HPLC analysis after
long-term electrolysis. The HPLC results for each sample
taken after electrolysis of a 4-acetylpyridne containing
electrolyte with the indicated single crystal electrode are
shown in
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Figure 4A. Figure 4B shows the HPLC data for the 10 mM
standards of 4-acetylpyridine eluding after 7.25 minutes
(black curve), of 1-(pyridine-4-yl)ethan-1-ol eluding after
5.87 minutes (red curve) and of 4-ethylpyridne eluding
after 13.36 minutes (blue curve). Aside from a broad peak
around 7 minutes due to the elusion of 4-acetylpyridine,
no signal is observed due to a compound eluding after
5
.87 minutes (alcohol), 13.36 minutes (hydrocarbon), or
any other compound. The results of Figure 4 show that 4-
acetylpyridne cannot be reduced at any of the three
platinum basal planes under conditions that show the
hydrogenation of acetophenone at each of the three
platinum basal planes.
Surface-Enhanced Raman Spectroscopy (SERS) was
employed to obtain information on the adsorption of
aceto-
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Figure 4: A: HPLC analysis of a sample taken from an electrolyte of 0.1 M H SO containing 0.03 M 4-acetylpyridine that
has been electrolyzed at 0.03 V for 2 hours with a platinum electrode (surface structure as indicated). The corresponding
current time transients are shown in Figure S8 in the SI. B: HPLC-analysis of 0.01 M 4-acetylpyridne (black), 0.01 M 1-
(
pyridine-4-yl)ethan-1-ol (red) and 4-ethylpyridne (blue) in water.
phenone and 4-acetylpyridine on
platinum electrode. Figure 5 shows the SERS spectra of
acetophenone and 4-acetylpyridine adsorbed on
platinum electrode at 0.4 V. The full spectra (from
a
polycrystalline
respect to its location in the spectrum of the pure
3
1
substance . This suggests that the strength of the C=O
bond is not reduced significantly as would be expected if
the carbonyl func-tional group interacted strongly with
the platinum surface.
a
-1
-1
200 cm
to 3500 cm ) of acetophenone and 4-
acetylpyridine at Figures S9 and S10, respectively. We
The ring breathing mode of acetophenone appears at
-1
assign the intense band at 1022 cm in the red curve of
Figure 5 to the in-plane ring breathing mode of 4-
-1
9
99 cm in the black curve of Figure 5, corresponding to a
-1
red shift of 1 cm compared to the band position at
-1
acetylpyridine that occurs at 993 cm in the spectrum of
-1
1
000 cm in the spectrum of the bulk compound (c.f.
3
1
the free compound . Upon adsorption the ring breathing
mode of pyridine derivatives blue shifts due to the surface
Figure S10B in the Supporting Information). Benzene
derivatives experience a red shift of the ring breathing
32
bonding through the N-atom . The significant blue shift
-1
mode by about 1 – 14 cm upon adsorption through the
-1
of 29 cm of the ring breathing mode indicates therefore
33
aromatic π-electron system . The red shift of the ring
that 4-acetylpyridine binds through the N-atom to the
platinum surface, in a vertical adsorption mode. This is in
agreement with the general tendency of pyridine to bind
breathing mode of acetophenone indicates therefore that
the compound binds horizontally to the platinum surface.
This adsorption mode agrees with other benzene
18,
21-22
vertically to platinum surfaces
.
However,
derivatives, which generally realize flat adsorption
considering the molecular geometry of 4-acetylpyridine,
an interaction between the surface and the carbonyl
functional group cannot be realized when the molecule
adsorbs in a vertical adsorption mode. This is also
reflected by the band position of the symmetrical C=O
13, 17-22, 34
geometries on metal electrodes
. Different from the
spectrum of 4-acetylpyridine, for which the ring C-C
-1
-1
stretching vibration (at 1567 cm and 1599 cm ,
respectively) and the symmetrical C=O-stretching
vibration are well separated, bands corresponding to
-1
stretch vibration occurring at 1691 cm in the red curve of
these
modes
merge
into
a
-1
Figure 5. The band is redshifted by only 2 cm with
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Figure 5: A: SERS spectra obtained at a platinum covered gold electrode in 0.1 M H SO containing 10 mM 4-Acetylpyridin (red
curve) or 0.01 M acetophenone (black curve). B: Band position of the C=O stretch vibration as a function of the applied
potential. The slope of the linear regression (m) gives the Stark tuning experienced by the carbonyl functional group.
multiplet in the spectrum of acetophenone. However, in
band position of the ring breathing mode that
acetophenone realizes a horizontal adsorption geometry
at platinum, while 4-acetylpyriden binds vertically to the
same surface. In the discussion of Figure S9 and S10 in the
Supporting Information we derive these adsorption
geometries also from the relative band intensity of the
ring breathing mode.
-1
the multiplet, a peak at 1664 cm appears, which is close
to the band position of the symmetrical C=O stretch
-1
various potentials are shown in vibration at 1678 cm in
the spectrum of bulk acetophenone, and which is
therefore assigned to the same mode of adsorbed
-1
acetophenone. The red shift of 12 cm indicates a slight
weakening of the C=O bond. This might be due to the
adsorption of acetophenone at those surface sites that do
not interact strongly with carbonyl functional groups
Additional information on the adsorption mode can be
derived from Figure 5B, which shows how the applied
potential affects the band position of the symmetrical
carbonyl stretch vibration. The carbonyl functional group
of 4-acetylpyridine experiences a relatively weak Stark
23
(
such as Pt(111) and Pt(100) terraces ). Adsorption at step
and kink sites that interact more strongly with the
carbonyl functional group should result in a considerable
weakening of the C=O bond and a stronger red shift of
the corresponding band. However, these bands would be
hidden in the multiplet.
-1
tuning slope of 13.8 cm /V as compared to a Stark tuning
-1
slope of 33.1 cm /V for acetophenone. The relatively
small Stark shift indicates that the electric field caused by
the potential difference between electrode and electrolyte
has already declined considerably at the location of the
carbonyl functional group. Electrodes covered with
pyridine in the vertical adsorption mode have a small
double layer capacity, since the electric field is shielded
The SERS spectra of Figure 5 support the notion derived
from the CVs in Figure 1 as well as those in Figure S1 and
S2 of the supporting Information: adsorption of
acetophenone and 4-acetylpyridne is dominated by an
interaction between surface and the phenyl and pyridine
ring, respectively. Furthermore, we can derive from the
24, 35-38
effectively by the pyridine ring
. The pyridine ring of
adsorbed 4-acetylpyridine can shield the carbonyl
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away from the surfaces, which impairs the electron
functional group from the electric field only when the
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ring resides between electrode and carbonyl functional
group. Such a geometry is only realized in a more or less
vertical adsorption mode. Hence, the low Stark tuning
slope agrees with a vertical adsorption mode of 4-
acetylpyridine. The higher Stark tuning slope experienced
by the carbonyl functional group of acetophenone means
that it is located closer to the surface where a change in
the electrode potential causes a large change of the
electric field. This is in line with the flat adsorption mode
derived from the band position of the ring breathing
mode, though it is difficult to derive from the Stark effect
alone more information on the adsorption mode.
transfer. This is also true for the Pt(110) electrode, which
is otherwise active for the hydrogenation of the isolate
carbonyl functional group in acetone. That is, adsorption
through the N-atom of 4-acetylpyridne inhibits the
hydrogenation reaction.
CONCLUSION
In this paper, we have investigated the electrochemical
hydrogenation of functionalized ketones on platinum
single crystal electrodes. The insight that the a secondary
functional group affects the reducibility of the carbonyl
functional group through an influence on the adsorption
geometry is rather intuitive: the vertical adsorption mode
brought about by the adsorption through the N-atom of
the pyridine ring in 4-acetylpyridine creates a physical
distance between carbonyl functional group and electrode
surface. This renders the Pt(110) electrode, otherwise most
active for the reduction of acetone, inactive for the
reduction of the carbonyl functional group of 4-
acetylpyridine. The horizontal adsorption mode of the
phenyl ring in acetophenone activates the otherwise
inactive Pt(111) electrode. Here adsorption through the
phenyl ring reduces the physical distance between
electrode and carbonyl functional group. Despite its
simplicity, the finding that a secondary, inert functional
group (here phenyl and pyridine ring) affects the
reactivity of another (here carbonyl functional group) by
defining the adsorption mode on the electrode, poses a
new concept for electrocatalysis.
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DISCUSSION
We first compare the results obtained for the
electrochemical hydrogenation of acetophenone with our
previous results for the electrochemical hydrogenation of
23
acetone at platinum single crystal electrodes . Stepped
Pt(111) crystals reduce acetone nearly exclusively to 2-
propanol, while stepped Pt(100) electrodes reduce
acetone in addition to propane. Qualitatively the same
selectivity pattern is observed here for the hydrogenation
of acetophenone. HPLC and OLEMS data in Figure 2 and
3
show that acetophenone is reduced selectively to the
corresponding alcohol at Pt(111) and Pt(110) electrodes,
while ethylbenzene is formed (in addition to 1-
phenylethanol) in the course of acetophenone reduction
at the Pt(100) electrode. The finding that the Pt(100)
electrode catalyzes the deoxygenation/hydrogenolysis of
acetophenone is another example of the interesting
In addition, we showed that the electrocatalytic
hydrogenation of acetophenone displays the same
sensitivity towards the crystallographic orientation of the
39-40
ability of (100) terraces to make and break bonds
. An
important difference with acetone reduction is that all
three basal planes of platinum show at least some
moderate activity for the hydrogenation of the carbonyl
functional group of acetophenone, while pristine Pt(111)
and Pt(100) electrodes do not display any activity for
23
platinum electrode as observed earlier for acetone ,.
Selective formation of the alcohol is observed when
electrodes with an (111) or an (110) surface structure are
employed. Platinum electrodes with an (100) surface
structure perform in addition the hydrogenolysis to the
hydrocarbon. The high structural sensitivity of (100)
surfaces for hydrogenolysis is agreement with previous
23
acetone reduction . The inactivity of these surfaces for
acetone reduction has been assigned to their inability to
23
bind acetone . Figure 1 shows that acetophenone binds to
the Pt(111) surface, whereas acetone does not. CV and
SERS data suggest that adsorption of acetophenone to
Pt(111) and Pt(100) is brought about by an interaction
between surface and phenyl ring. The horizontal
adsorption mode of acetophenone forces the carbonyl
functional group in close contact with the Pt(111) surface.
In this situation, the Pt(111) surface has the ability to
reduce the carbonyl functional group.
23
results on acetone electroreduction , and more generally
with the ability of (100) surface sites to break or make
39-40
bonds between, C, N and O
.
ASSOCIATED CONTENT
Supporting Information. Contains CV data at Pt(100) and
Pt(110) in the presence of acetone, acetophenenone, 4-
acetylpyridine, benzene and pyridine, scan rate dependence
of CV at Pt(15,15,14) in the presence of acetophenone and full
SERS-spectra of acetophenone and 4-acetylpyridine. This
material is available free of charge via the Internet at
http://pubs.acs.org.
The important implication of this conclusion is that the
electrochemical conversion of one functional group can
be enabled by a strong interaction between the electrode
surface and another inert functional group in the same
substrate molecule. On the other hand, our results
obtained for 4-acteylpyridine show that this also requires
a certain adsorption geometry. Although the interaction
between electrode surface and pyridine ring brings about
adsorption as well, no activation of the carbonyl
functional group can be achieved. In the vertical
adsorption mode the acetyl functional group is directed
AUTHOR INFORMATION
Corresponding Author
*m.koper@lic.leidenuniv.nl
Author Contributions
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Page 10 of 12
The manuscript was written through contributions of all
authors. All authors have given approval to the final version
of the manuscript.
adsorbed ethyl pyruvate on Pt{h k l} and epitaxial Pd/Pt{h k l}
adlayers. Journal of Electroanalytical Chemistry 2010, 640 (1–2),
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A DEMS study of the electroreduction and oxidation of 3-buten-
-one and 2-butanone adsorbates on platinum in sulphuric
solutions. Journal of Electroanalytical Chemistry 1998, 454 (1–2),
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1
Zinola, C. F.; Rodrı ́g uez, J. L.; Arévalo, M. C.; Pastor, E.,
Funding Sources
2
This research received funding from the Netherlands
Organization for Scientific Research (NWO) in the
framework of the fund New Chemical Innovations (project:
1
61-172.
11.
Zinola, C. F.; Castro Luna, A. M., Electrochemical
7
31.015.204 ELECTROGAS) with financial support of Akzo
Behavior and Semiempirical Approach to the Electroadsorption
of 3-Buten-2-one and 2-Butanone on Platinum. Journal of Colloid
and Interface Science 1999, 219 (2), 260-274.
Nobel Chemicals, Shell Global Solutions, Magneto Special
Anodes (Evoqua Water Technologies) and Elson
Technologies.
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2.
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influence of the surface structure on the adsorption of ethene,
ethanol and cyclohexene as studied by DEMS. Electrochimica
Acta 1995, 40 (1), 99-107.
ACKNOWLEDGMENT
We gratefully acknowledge financial support from the
Netherlands Organization for Scientific Research (NWO),
Akzo Nobel Chemicals, Shell Global Solutions, Magneto
Special Anodes (Evoqua Water Technologies) and Elson
Technologies.
13.
Baltruschat, H.; Ernst, S., Molecular Adsorbates at
Single-Crystal Platinum-Group Metals and Bimetallic Surfaces.
ChemPhysChem 2011, 12 (1), 56-69.
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4.
Lai, S. C. S.; Koper, M. T. M., Electro-oxidation of
ethanol and acetaldehyde on platinum single-crystal electrodes.
Faraday Discussions 2009, 140 (0), 399-416.
ABBREVIATIONS
1
5.
Lai, S. C. S.; Koper, M. T. M., The Influence of Surface
CV, Cyclic Voltammogram; SERS, Surface Enhanced Raman
Spectroscopy; OLEMS, On-Line Electrochemical Mass
Spectroscopy; HPLC, High Pressure Liquid Chromatography.
Structure on Selectivity in the Ethanol Electro-oxidation
Reaction on Platinum. The Journal of Physical Chemistry Letters
2
010, 1 (7), 1122-1125.
6. Bondue, C. J.; Koper, M. T. M.,
1
A mechanistic
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ACS Paragon Plus Environment