Toward the electrochemical valorization of glycerol: Fourier transform infrared spectroscopic and chromatographic studies

Article abstract of DOI:10.1021/cs400559d

Glycerol electrooxidation reaction has been investigated by electrochemical, spectroelectrochemical, and chromatographic methods on palladium-nickel and palladium-silver nanoparticules supported on carbon Vulcan XC 72R. These materials, prepared by the so-called "Bromide Anion Exchange" method, exhibited high activity toward the glycerol electrooxidation in alkaline medium showing furthermore an important shift of the onset potential toward low potential values. Electrolysis coupled with high-performance liquid chromatography (HPLC) and in situ Fourier transform infrared spectroscopy (FTIRS) measurements have been used to determine the various compounds generated in the oxidative conversion of this three hydroxyl groups carbon molecule. Some products with high added value such as glycerate and tartronate have been identified. In situ FTIRS results have furthermore shown the pH decrease in the thin layer near the electrode. These results will positively serve as guidelines for future works on the potential use of glycerol in fuel cell devices in a cogeneration of high value chemicals and energy process.

Full text of DOI:10.1021/cs400559d

Research Article
pubs.acs.org/acscatalysis
Toward the Electrochemical Valorization of Glycerol: Fourier
Transform Infrared Spectroscopic and Chromatographic Studies
Yaovi Holade, Claudia Morais,* Karine Servat, Teko W. Napporn, and K. Boniface Kokoh
́
́
Universite de Poitiers, IC2MP CNRS UMR 7285, 4 rue Michel Brunet−B27, BP 633, 86022 Poitiers cedex, France
S
* Supporting Information
ABSTRACT: Glycerol electrooxidation reaction has been investigated by
electrochemical, spectroelectrochemical, and chromatographic methods on
palladium−nickel and palladium−silver nanoparticules supported on carbon
Vulcan XC 72R. These materials, prepared by the so-called “Bromide Anion
Exchange” method, exhibited high activity toward the glycerol electrooxidation
in alkaline medium showing furthermore an important shift of the onset
potential toward low potential values. Electrolysis coupled with high-
performance liquid chromatography (HPLC) and in situ Fourier transform
infrared spectroscopy (FTIRS) measurements have been used to determine the
various compounds generated in the oxidative conversion of this three hydroxyl
groups carbon molecule. Some products with high added value such as
glycerate and tartronate have been identified. In situ FTIRS results have
furthermore shown the pH decrease in the thin layer near the electrode. These
results will positively serve as guidelines for future works on the potential use
of glycerol in fuel cell devices in a cogeneration of high value chemicals and energy process.
KEYWORDS: glycerol electrooxidation, palladium-based nanoparticles, FTIR spectroelectrochemistry, bromide anion exchange,
chromatographic analysis
electrooxidize alcohols such as ethanol or glycerol as compared
with platinum. Not only has Pd revealed notable anodic peak
current, but the alcohol oxidation remarkably increased on the
surface of the PdPt bimetallic catalyst. This enhancement of the
activity in PdPt alloys was attributed to an electronic effect in
which the d-band center of Pd was raised by the presence of
nearby Pt.3,31 Several authors also showed an important shift in
the onset potential and a significant increase in the peak current
for the alcohol oxidation on the PdAu anode materials.30,32−34
Indeed, although Au nanoparticles may not directly lead to
practically active electrocatalysts for the alcohol oxidation, they
can enhance the activity of precious metal catalysts. Besides, the
association of palladium and gold was shown to decrease the
hydrogen absorption in the Pd network and therefore does
contribute to the durability of Pd-based materials.35 Other
cocatalysts such as bismuth,32 ruthenium,3,4,36 and rhodium37
were also proposed. They were shown to suitably promote a
bifunctional mechanism through the formation hydroxyl ions at
low potential, improving the oxidative conversion of glycerol. In
particular, if the purpose is to perform thorough conversion of
glycerol as fuel in a PEM system, rhodium would be an
appropriate candidate as it promotes the cleavage of the C−C
bonds in the glycerol molecule.37 Nickel and silver which are very
stable in alkaline medium might also be of great interest since
INTRODUCTION
■
Because of the depletion of fossil resources, small organic
molecules became electrocatalytically attractive, making their
fundamental studies in electrocatalysis an important topic.1−5
The potential use of these molecules, principally alcohols, is of
particular interest because of their potential application in direct
fuel cells6−8 and their possible chemical/industrial valorization.9
Glycerol may be particularly interesting since its selective
oxidation leads to products with high added value owing to its
three hydroxyl groups.10,11 Furthermore, since its production
exceeds by far its present consumption an alternative utilization is
desirable, and the possible use of glycerol in direct oxidation fuel
cells appears as an interesting option. In spite of its quite low
specific energy (4.4 against 6.1 kWh·kg−1 for methanol), glycerol
is attractive compared with other fuels because it is non toxic, has
a low volatility, and its transport and storage are not
demanding.12,13
Despite these advantages, the electrocatalytic oxidation of
glycerol has been less studied than other organic molecules.
Nevertheless, during the past decade, some studies were devoted
to glycerol oxidation in the fields of organic chemistry (KMnO4,
K2Cr2O7),14,15 heterogeneous catalysis with oxygen as oxidizing
agent,16−18 and in electrocatalysis, where the electrocatalysts
showed the best activity in alkaline medium.1,3,19 Various
bimetallic electrocatalysts involving palladium and nickel, silver,
gold, or platinum have shown to be electrocatalytically active
toward alcohol oxidation in alkaline medium.3,10,20−30 Palladium
is of particular interest because it was shown to efficiently
Received: March 19, 2013
Revised: September 9, 2013
Published: September 12, 2013
© 2013 American Chemical Society
2403
dx.doi.org/10.1021/cs400559d | ACS Catal. 2013, 3, 2403−2411

ACS Catalysis
Research Article
they enable to decrease markedly the Pd amount without
significantly decreasing the activity. Indeed, these cocatalysts are
well-known to promote an enhancement of the performance the
Pd electrode materials.25,26,28,38−43
reference and counter electrodes, respectively. The reference
electrode was separated from the solution by a Luggin capillary
tip. The working electrode consisted of a catalytic powder
deposited on a glassy carbon disk used as conductive carrier with
a 3 mm diameter. The catalytic ink was prepared following a
procedure similar to that described by Wilson and co-workers65
for fuel cell catalytic ink preparation using the conductive
ionomer Nafion (5 wt %).66−68 Briefly, the metal/C powder (4
mg) was added in a homogeneous mixture of 50 μL of Nafion
solution (5 wt % from Sigma-Aldrich) and 375 μL of ultra pure
water. Three microliters of the homogeneous catalytic ink were
finally deposited with a syringe onto the freshly polished glassy
carbon disk, and the solvent was evaporated in a stream of
ultrapure nitrogen at room temperature. This amount of material
represents 116.40 μg cm−2 of metal loading. All the CV
experiments were performed at room temperature (21 1 °C)
with 0.1 mol L−1 NaOH (Sigma-Aldrich, 97%), 0.1 mol L−1
glycerol (ReagentPlus >99.0%, Sigma-Aldrich), and ultrapure
water (18.2 MΩ cm at 20 °C). The solution was first
deoxygenated by bubbling nitrogen for 30 min before any
electrochemical experiment.
Coupling electrochemical methods to analytical (chromatog-
raphy, online or differential electrochemical mass spectrometry)
and spectroscopic (in situ FTIR)11,44−49 measurements permit
to identify the intermediate species and the final reaction
products which might be very helpful to evaluate the activity of an
electrode catalyst toward an organic compound and to
understand the reaction mechanism. This knowledge can also
allow tailoring a catalyst composition for a selective reaction
process to added value products. The judicious combination of
electrochemical, spectroscopic, and chromatographic methods
allows therefore to gain clear information about the electro-
oxidation of an organic compound such as glycerol. Recently, the
combination of cyclic voltammetry measurements with HPLC
and/or in situ spectroscopic techniques was reported for the
study of glycerol electrooxidation.11,30,32,46−48,50 Nevertheless
there is still a lack of data about the glycerol electrooxidation
reaction.
In this context, the purpose of the present work is to
investigate the glycerol electrooxidation in alkaline medium on
nickel and silver modified Pd anode materials synthesized by a
soft and clean method. Indeed, it is well-known that the
microemulsion synthesis method currently used to obtain these
materials leads to catalysts with some sites inaccessible by the
reactive species. The method used herein is expected to provide
catalysts with high active surface areas and consequently lead to
an enhancement of the oxidation current densities at lower
potential values. As stated above glycerol is a renewable and
sustainable organic compound which can possibly be used in a
valuable chemicals and energy cogeneration approach.10,19,30,51
For this purpose it is of great interest to provide insights on the
mechanism of its electrooxidation on these PdM/C (M = Ag, Ni)
nanocatalysts in alkaline medium, and this is another objective of
the present study. To this aim chromatographic analysis (HPLC)
and in situ FTIR spectroelectrochemistry experiments have been
combined to identify both the intermediates and the final
products of the glycerol oxidation process and hopefully propose
a preferential pathway for this reaction.
Electrolysis and High-Performance Liquid Chromatog-
raphy (HPLC). Electrolysis was fulfilled in a Pyrex two-
compartment cell.69 The compartments were separated with
an anion-exchange membrane (35 μm thickness, from
Fumatech). The electrochemical measurements were conducted
using a Potentiostat (Voltalab PGZ 402 from Radiometer
Analytical) which was controlled by a computer with Volta-
master 4 software. The electrical connection of the carbon Toray
sheet was established with a gold wire thanks to a carbon paste.
The ink was prepared like for the CV experiments. Fifty
microliters of the catalytic ink were deposited onto each side of a
carbon Toray sheet (1.5 cm2 geometric surface area), and the
solvent was evaporated with a N2 stream at room temperature. A
glassy carbon slab (10.8 cm2 geometric surface area) and RHE
served as counter and reference electrodes, respectively. The
programmed potential electrolysis (PPE) setup consisted of two
potential levels: a first potential plateau was set at Eox = 0.8 V vs
RHE (20 s for electrooxidation of organic molecules) and a
second one fixed at Edes = 1.4 V vs RHE for 1 s allowed the
removal of the poison species or the regeneration of the catalytic
surface.
The final products of glycerol electrooxidation were
determined by analyzing collected samples with high-perform-
ance liquid chromatography (Dionex system P680 HPLC). It
works with an isocratic elution and mainly includes an
autosampler (ASI 100 Automated Sample Injector), a sample
loop (20 μL), and an ion-exclusion column (Aminex HPX-87H),
which operated at room temperature. The analytes were
separated with diluted sulfuric acid (3.3 mmol L−1 H2SO4,
MERCK 96%) used as eluent with a 0.6 mL min−1 flow rate. The
chromatograph was equipped with an UV−vis detector (λ = 210
nm) followed by a refractive index detector (IOTA2). The
assignments of the different peaks and the quantitative
determination of the products were done with external standards,
that is, by comparing the retention times with those of reference
samples prepared with the expected products of glycerol
electrooxidation.10,11
MATERIALS AND METHODS
■
Nanoscale Materials Synthesis by “Bromide Anion
Exchange”. PdNi/C and PdAg/C catalysts with different
atomic compositions have been synthesized using the “Bromide
Anion Exchange” (BAE) method; first proposed for gold−
platinum supported catalysts.52 The details about this method
modified for palladium−nickel and palladium−silver catalyst
synthesis are reported elsewhere.38 BAE consists of a soft and
clean method which does not involve any organic compound as
surfactant or stabilizer as a few other methods.53−61 The obtained
materials were characterized by different physicochemical
methods, namely, differential and thermogravimetric analysis
(DTA/TGA), X-ray diffraction (XRD), and Transmission
Electron Microscopy (TEM). Physicochemical characterizations
indicated a metal loading of about 30 wt % and particle sizes from
3 to 6 nm (Supporting Information, Table S1).
In Situ Fourier Transform Infrared (FTIR) Spectroscopy
Measurements Coupled with Electrochemical Experi-
ments. In situ FTIR measurements were carried out in a Bruker
IFS 66v spectrometer which was modified for beam reflection on
the electrode surface at a 65° incidence angle. A 10−6 bar vacuum
Cyclic Voltammetry (CV). CV experiments were performed
by using an analogical potentiostat EG&G PARC Model 362
(Princeton Applied Research) in a conventional three-electrode
cell.62−64 A Reversible Hydrogen Electrode (RHE) and a slab of
glassy carbon of 6.48 cm2 geometrical surface area were used as
2404
dx.doi.org/10.1021/cs400559d | ACS Catal. 2013, 3, 2403−2411

ACS Catalysis
Research Article
was used to remove all interferences from atmospheric water and
CO2. The detector was a MCT (HgCdTe) type, cooled
beforehand by liquid nitrogen. The spectral resolution was 4
cm−1, and the FTIR spectra were recorded in the 1000 cm−1 and
4000 cm−1 IR region. A special tailored cell, in a three-electrode
spectroelectrochemical cell, fitted with an MIR transparent
window (CaF2) was used for in situ FTIR experiments.70−72 The
potentiostat was the same used in CV experiments. A slab of
glassy carbon and RHE served as counter and reference
electrodes, respectively. The working electrode consisted of 3
μL of catalytic ink deposited on a glassy carbon disk (8 mm
diameter). To minimize the absorption of the infrared beam by
the solution, the working electrode was pressed against the
window and a thin layer of electrolytic solution was obtained.
Two methods were used. The first one was the SPAIRS (Single
Potential Alteration IR Spectroscopy), carried out in the
potential range of 0.3−1.45 V vs RHE. The electrode reflectivity
REi was recorded at different potentials Ei, each separated by 0.05
V during the first voltammogram at a sweep rate of 1 mV s−1. The
second spectroelectrochemical method was a Chronoamperom-
etry/FTIRS coupling which was performed at 0.8 V vs RHE for 1
h with a spectrum acquisition every 3 min.
RESULTS AND DISCUSSION
■
Electrochemical Characterization of the Catalysts. The
electrochemical characterization of the catalysts was performed
by CV experiments in a N2-satured 0.1 mol L−1 NaOH solution.
Figure 1a shows the cyclic voltammograms of PdNi/C catalysts.
The lower potential limit has been set to 0.30 V vs RHE to avoid
the irreversible insertion of hydrogen in the palladium crystal
lattice.73,74 During the positive-direction scan, the hydrogen
desorption occurs at about 0.59 V vs RHE (peak A1), and the
palladium surface oxidation takes place at about 0.70 V vs RHE
(peak A2). The reduction of PdO occurs during the backward
scan between 0.55 and 0.50 V vs RHE, depending on the nickel
content in the catalyst composition (peak A3). The upper
potential limit was set to 1.6 V vs RHE for Ni containing
materials to see the NiOOH formation peak at about 1.5 V vs
RHE (peak B1) and the corresponding reduction peak (B2)
occurring during the negative-direction sweep. Figure 1b shows
steady-state cyclic voltammograms of PdAg/C nanocatalysts.
Peak C1 is due to the Ag2O reduction, and peak C2 is attributed to
palladium oxide reduction. It should be noted that when the
silver atomic composition is lower than or equal to 50%, the
intensity of peak C1 (Ag2O reduction) decreases with cycling and
disappears after 20 cycles. This behavior was already reported for
PdAg/C catalysts and attributed to a surface rearrangement
which can be due to either its enrichment in Pd or to the
dissolution of the Ag species when setting the upper potential
limit to 1.6 V vs RHE.21,28,75 To exclude the latter hypothesis the
bulk electrolyte solution was analyzed, and no dissolved metal
species like Ag+ were found.38 Furthermore, a careful comparison
of the shape of cyclic voltammograms obtained for monometallic
Pd/C and bimetallic PdAg/C materials at high potential values
indicates the presence of silver in the catalyst, supporting
therefore the first hypothesis of the migration of Pd atoms to the
surface without Ag dissolution. As an additional proof, we
provide cyclic voltammogram of Pd30Ag70/C in 0.1 mol L−1 at 50
mV s−1 (Supporting Information, Figure S1). Indeed, when the
silver atomic composition is higher than 50% in the PdAg/C
materials the reduction peaks of Ag2O do not disappear,
remaining stable after 22 complete cyclic voltammograms. The
active surface area of Pd/C catalyst was evaluated by integrating
Figure 1. Cyclic voltammograms of (a) PdxNi(100−x)/C and (b)
PdxAg(100−x)/C catalysts recorded at 50 mV s−1 in 0.1 mol L−1 NaOH
electrolytic solution (20th cycle).
the reduction peak of PdO. A charge density of 424 μC cm−2 was
associated with the reduction of the formed PdO monolayer.76,77
−1
The obtained value of 69 m2 g was found to be twice larger than
Pd
that found by Simoes et al.77 for a Pd/C catalyst synthesized by
̃
the “water in oil” microemulsion method. Therefore, the
synthesis of the nanomaterials by the BAE method appears as
a promising process for obtaining electrocatalysts with high
active surface areas. For the PdAg/C catalysts, the active surface
areas were also calculated by integration of the reduction zones of
PdO and Ag2O. The complete method is described in section 3 of
Supporting Information, Figure S2. A charge density of 420 μC
cm−2 was associated to the reduction of the Ag2O monolayer.78
The obtained specific electrochemical surface areas (SECSA)
values calculated for PdAg/C electrode materials by dividing the
active surface area by the palladium weight were 47, 16, 34, 72, 72
m2 g for the Pd90Ag10/C, Pd80Ag20/C, Pd70Ag30/C, Pd60Ag40/
−1
Pd
C, and Pd50Ag50/C compositions, respectively. In particular, it
can be noticed that regarding the identical total metal loadings
and particle size distributions obtained for the bimetallic
catalysts, the low SECSA values reported herein for the
Pd80Ag20/C and Pd70Ag30/C catalysts are quite unexpected.
These values might nevertheless be explained by a lesser
dispersion of metallic nanoparticles onto the supporting carbon
for these two compositions. Because of the presence of Ni(OH)2
species in the PdNi/C catalysts, it is not possible to evaluate their
active surface areas.38
2405
dx.doi.org/10.1021/cs400559d | ACS Catal. 2013, 3, 2403−2411

ACS Catalysis
Research Article
Electrochemical Activity toward Glycerol Oxidation.
The glycerol electrooxidation reaction was considered during the
positive variation of the electrode potential between 0.30−1.45 V
vs RHE. The choice of the potential range was motivated by the
possible application of catalysts in fuel cells or in electrosynthesis.
The polarization curves are depicted in Figures 2a and b
content (Figure 2b), in agreement with results reported in the
literature.21,28 The promoting effect of the second metal has been
explained by a bifunctional mechanism and/or electronic
(ligand) effects.21,75 According to Y. Wang et al.,75 who have
noticed the facility of removing CO from the PdAg/C catalysts
surface, while Ag/C is not active in CO stripping, Ag promotes
the removal of adsorbed CO or similar poisoning intermediates
out of the surface of PdAg/C catalysts by a bifunctional
mechanism. According to the d-band theory, when Ag is added
into the structure of Pd, the d-band of Pd may be shifted up. As a
result, more hydroxyls may be adsorbed at the catalyst surface
and thus, more active sites are released.21
Electrolysis Experiments and HPLC Analysis. Electrolysis
experiments were performed in alkaline solution for the different
catalyst compositions. With the purpose of separating and
identifying the final products of the glycerol electrooxidation in
alkaline medium, the bulk solutions were analyzed by HPLC at
different electrolysis times. Thus, a periodic PPE has been
established to oxidize glycerol and chromatographically analyze
the reaction products. The profile of the electrode potential as a
function of the electrolysis time is reported in Supporting
Information, Figure S5. For each cycle of electrolysis, the first
potential plateau which was set to Eox = 0.8 V vs RHE for 20 s,
corresponds to the organic molecules’ electrooxidation, and the
second at Edes = 1.4 V vs RHE for 1 s, allows the regeneration of
the electrode surface. The choice of the potential value 0.8 V
RHE was done taking into consideration the prospective
application in fuel cell devices. Indeed, for a fuel cell application
the fundamental characteristics are the cell tension and the
output power. The more interesting anodic material will be the
one that provides the higher density current at low potential
values. The chosen potential value is therefore a compromise
between both aspects. The oxidation currents at Eox = 0.8 V vs
RHE (Iox) and the quantity of electricity (Qox) are shown in
Figures 3a and 3b, respectively, for selected catalysts. It can be
noticed that whatever the anode catalyst the current intensity
decreases fast during the first half hour before stabilization. As
expected from previous reported results and evidenced by the
obtained Qox values herein, the presence of Ag or Ni in the Pd-
based catalysts improves the ability of the electrode material to
oxidize glycerol. The Pd70Ag30/C composition is of particular
interest as it presents high oxidation current values and the
smoothest stabilization curve. It should be noticed that all
electrolysis experiments were performed during 4 h. However,
after 2 h a slow and progressive decrease of the current intensity
was observed which may be reasonably attributed to glycerol
consumption and/or to oxygenated species production which are
strongly adsorbed on the catalyst surface and cause its
deactivation even with the desorption potential plateau.
Figure 2. Polarization curves of glycerol oxidation on (a) PdxNi(100−x)/C
and (b) PdxAg(100−x)/C catalysts at 50 mV s−1 in 0.1 mol L−1 NaOH
electrolytic solution containing 0.1 mol L−1 of glycerol.
respectively for PdNi/C and PdAg/C catalysts. The catalytic
activity was evaluated as a mass specific current (MSC,
normalized with the mass of the most noble metal, Pd). The
PdNi/C bimetallic catalysts exhibit good reaction kinetics at low
potentials, as illustrated by the shift of the onset potential toward
low potential values when compared with the Pd/C polarization
curve. This improvement is attributed to a bifunctional
mechanism of the oxidative desorption at low potential involving
the transfer of OH− species from the Ni(OH)2 surface toward
adsorbed organic molecules close to the palladium surface where
the reaction takes place.3,25 The forward oxidation current peak is
attributed to the oxidation of freshly chemisorbed species
coming from glycerol adsorption. Afterward, the current
decreases because of the deactivation of the catalyst caused by
the palladium surface oxidation. The reverse oxidation peak
(Supporting Information, Figure S3) is associated with removal
of carbonaceous species not completely oxidized in the forward
scan.26 One can also clearly observe the improvement of the
catalytic activity of palladium with the increase of the silver
Qualitative HPLC analyses are depicted in Figure 4 for
different compositions of PdNi/C (Figure 4a) and PdAg/C
(Figure 4b) nanocatalysts. The identified product distributions
are given in Supporting Information, Figure S6 for three
representative catalysts. The first important result is that all the
catalysts are able to break the C−C bond of glycerol at ambient
temperature. The second point is that the identified products
were the same for the different studied compositions, the
distributions of those products being slightly different. Indeed for
all the compositions the major identified products were glycolic
and glyceric acids. Figures 4c and d display glycerol molar
conversions after 2 and 4 h of electrolysis for the different nickel
or silver atomic compositions, respectively, of the PdM/C
catalysts. It can be noticed that the Pd80Ni20/C material exhibits
2406
dx.doi.org/10.1021/cs400559d | ACS Catal. 2013, 3, 2403−2411

ACS Catalysis
Research Article
and glycerol. So, glycerate, tartronate, glycolate, oxalate, and
formate ions were the determined final products of glycerol
electrooxidation in alkaline medium on PdM/C (M = Ag; Ni)
catalysts. The presence of large amounts of glyceric acid indicates
that the pathway involving glyceraldehyde is the most probable.
However, in our operating conditions the hydroxypyruvic acid
retention time is very close to that of tartronic acid and so we
could not reach a conclusion about its presence in the analyzed
solution. Furthermore, glycolic acid being a product of both
proposed pathways, we cannot affirm as this stage that the
glyceraldehyde pathway for the glycerol oxidation is the only one.
So, complementary analyses by in situ FTIR experiments were
undertaken to try to identify all the intermediates species.
In Situ Infrared Reflectance Spectroscopy Measure-
ments. To get further insights on how glycerol is electrooxidized
in alkaline medium on these nanoscale PdM/C materials, in situ
spectroelectrochemical experiments have been undertaken.
Figure 5 displays the single potential alteration infrared spectra
(SPAIRS) obtained for Pd/C and Pd60Ni40/C catalysts during
glycerol oxidation in alkaline medium. The first important result
is that adsorbed CO is formed at low potential values (ca. 0.40 V
vs RHE for Pd/C and 0.50 V vs RHE for PdNi/C) indicating the
cleavage of C−C bond as reported in literature.10,81 Indeed, the
band at about 1887 cm−1 on Pd/C and 1895 cm−1 on Pd60Ni40/C
is attributed to bridged CO (μ2-(CO)) resulting from the rapid
dissociative adsorption of glycerol at low potential values. For the
PdAg/C catalysts (Supporting Information, Figure S7), the weak
vibration band at 1888 cm−1 indicates the presence of bridged
CO. Another important result is the evidence of CO2 production
on the different nanocatalysts (asymmetric stretching band at
2343 cm−1).47,48 This band appears at about 1.0 V vs RHE for
bimetallic compositions and at about 1.1 V vs RHE for the
monometallic composition and corresponds to the decrease of
the absorbed CO band. According to Demarconnay et al.,82 CO2
appears in alkaline medium as soon as carbonate (1390−1420
cm−1 band)48,82 could not be further formed because of the lack
of OH−. Jeffery and Camara48 have proved by simple calculation
in the thin layer (IR window-electrode interface) that the
depletion of OH− is enough to justify a sensible pH change
during glycerol electrooxidation in alkaline medium. One can
furthermore recognize the O−H stretching vibration band of
carboxylic acid toward 3000 cm−1 confirming the acidification of
the thin layer between the working electrode and the CaF2
window.83−85
So far, it is herein clearly shown that (i) adsorbed CO is
formed during the oxidation of glycerol for Pd-bases nano-
catalysts, which results in poisoning of the catalysts at low
potential values, and (ii) the main reaction products are glycolate
and glycerate as evidenced by HPLC results and CO2 as shown in
the FTIR spectra. Based on the obtained spectra, we will
undertake now to get further insights on the glycerol oxidation
mechanism on Pd-based materials, and in particular we will
attempt to identify the reaction intermediates. This will hopefully
help us to reach conclusions about the preferential pathway and
the crucial steps of the glycerol oxidation reaction in alkaline
medium on Pd-based nanomaterials. The characteristic FTIR
spectra of the different possible products or intermediates
recorded in alkaline medium are depicted in Supporting
Information, Figure S8 and will be used herein to reach
conclusions on their presence in the electrolyte solution. So, for
the Pd/C catalyst (Figure 5a) at low potential values (0.4−0.6 V
vs RHE) the only identified intermediate is adsorbed CO. At
higher potentials values (0.7−1.0 V vs RHE) evidence of the
Figure 3. Evolution of the oxidation current (Iox: black) and the quantity
of electricity (Qox: blue) during the long-term electrolyses of glycerol
(0.1 mol L−1) in NaOH (0.1 mol L−1) at 0.8 V vs RHE on (a) PdNi/C
and (b) PdAg/C catalysts.
the best conversion (ca. 28.4% after 2 h of electrolysis and 31.3%
after 4 h of electrolysis). These values are definitely low when
compared with the ones reported in heterogeneous catalysis
obtained in batch reactors, but they are, to our knowledge, the
highest values reported for electrochemical glycerol conversion.
In fact, Roquet and co-workers obtained the highest glycerol
conversion of about 50% but after 48 h, instead of 4 h in the
present work, of electrolysis at 0.79 V vs RHE on bulk platinum
electrode (0.1 M NaOH + 0.1 M Glycerol).1 More recently the
Koper group also reported quantitative work, but their results did
not show significant glycerol conversion.11,46,49 The results
reported herein are therefore very promising because they prove
for the first time the potential conversion of glycerol on cheaper
and more abundant metals than platinum. It should be pointed
out that no correlation was found between the best glycerol
conversion and the lower onset potential value.
It is commonly accepted that the oxidation of glycerol may
imply two pathways involving glyceraldehyde and/or DHA
(dihydroxyacetone). Nevertheless, both species, being unstable
in alkaline medium because of the aldolization reaction,79,80
could not be identified. Particularly the absence of glyceralde-
hyde can be explained by the Cannizzaro reaction which is a non
Faradaic process that leads in alkaline solution to both glycerate
2407
dx.doi.org/10.1021/cs400559d | ACS Catal. 2013, 3, 2403−2411

ACS Catalysis
Research Article
Figure 4. Chromatograms obtained from HPLC analysis after 4 h of electrolysis at 0.8 V vs RHE in 0.1 mol L−1 NaOH electrolytic solution containing
0.1 mol L−1 of glycerol for (a) PdNi/C and (b) PdAg/C catalysts. (Eluent: 3.3 mol L−1 H2SO4; injection: 20 μL; column Aminex HPX 87-H, 30 cm, at
room temperature). (c, d) Glycerol conversion as a function of the molar compositions of the PdNi/C and PdAg/C catalysts, respectively, after 2 h and 4
h of electrolysis.
presence of other compounds could be found, namely, glycerate
(1107 cm−1 and 1416 cm−1 bands), glycolate (1076 cm−1, 1326
cm−1 and 1410 cm−1 bands), oxalate (sharp band at 1308 cm−1),
and formate (1350, 1382 cm−1).10,48,82,83 We cannot exclude at
these potentials the formation of small amounts of tartronate
and/or mesoxalate ions (characteristic IR bands at 1100, 1338,
and 1438 cm−1) in the thin layer electrolyte solution. It should be
stated that the intense band at 1583 cm−1 is characteristic of
different carboxylate ions and there also might be a contribution
of the water bending vibration to this band.47 Afterward, for
potential values higher than 1.0 V vs RHE, CO2 formation takes
place while the amount of glycolate increases. A broad vibration
band is also clearly visible between 1650 and 1720 cm−1
indicating the presence of adsorbed glyceraldehyde and carbonyl
species.
low potential values for PdNi/C. Indeed, for potential values
ranging from 0.3 to 0.7 V vs RHE two vibrational bands at 1595
and 1700 cm−1 are observed. The band at 1595 cm−1 can be
assigned to different carboxylates such as glycerate and tartronate
whereas the one at 1700 cm−1 is characteristic of adsorbed
carbonyl species.
Based on these results we can therefore conclude that the
pathway for the electrooxidation of glycerol on Pd-based
materials in alkaline medium is the glyceraldehyde one.
An in situ IR study coupled with chronoamperometry
experiments has also been performed. Chronoamperometry
was carried out at 0.8 V vs RHE in the presence of 0.1 mol L−1 of
glycerol. The obtained spectra are reported in Figure 6 for Pd/C
and Pd70Ag30/C catalysts and in Supporting Information, Figure
S9 for the Pd60Ni40/C composition. The absence of the band at
2343 cm−1 confirms the hypothesis that carbon dioxide is
produced from CO(ads) at high potential values. In fact, the
applied potential (0.8 V vs RHE) is not high enough to
electrooxidize all adsorbed CO as shown by CO stripping
experiments combined with FTIR spectroscopy measure-
ments.38 This is consistent with first Demarconnay et al.82 and
second Jeffery and Camara48 observations. Another point is the
Finally, the positive direction bands at 1050 and 2700 cm−1 are
attributed to glycerol and OH− consumption,10,32,48,82 whereas
the band at about 3700 cm−1 corresponds to water stretching
vibrations.
The spectra obtained for the bimetallic catalysts (Figures 5b
and Supporting Information, Figure S7) were similar to those
obtained for the Pd/C material. The major difference occurs at
2408
dx.doi.org/10.1021/cs400559d | ACS Catal. 2013, 3, 2403−2411

ACS Catalysis
Research Article
Figure 5. SPAIR spectra recorded in 0.1 mol L−1 NaOH electrolyte
containing 0.1 mol L−1 of glycerol at 1 mV s−1 on (a) Pd/C and (b)
Pd60Ni40/C and catalysts at potentials ranging from 0.30 to 1.45 V vs
RHE.
Figure 6. FTIR spectra recorded during chronoamperometry in 0.1 mol
L−1 NaOH + 0.1 mol L−1 glycerol on (a) Pd/C and (b) Pd70Ag30/C
catalysts at 0.8 V vs RHE.
enhancement of some vibration bands that were hardly visible in
the previous SPAIR spectra and that might be assigned to
adsorbed species, confirming therefore the progressive poisoning
of the catalysts. Indeed, the bands at 1225 and 1680 cm−1 have
been assigned to adsorbed glyceraldehyde, whereas the band at
about1380 cm−1 has been shown to be characteristic of adsorbed
glycerate.10 Chronoamperometry experiments coupled with in
situ FTIR measurements confirm therefore the glyceraldehyde
pathway for the glycerol electrooxidation in alkaline medium on
Pd-based materials bringing to light adsorbed glyceraldehyde and
glycerate as intermediates.
is possible to have other sequences, the composition of the
catalyst and the potential of electrolysis being the keys of such
processes. It is too early to talk about the reaction mechanism of
glycerol electrooxidation. Indeed, the knowledge of the RDS
(rate determining step(s)) is a prerequisite.
CONCLUSION
■
Nanoscale palladium−nickel or silver bimetallic electrocatalysts
have been prepared by a synthetic route without the use of an
organic compound as surfactant. We have shown by CV
experiments the enhancement of the reaction kinetics in the
low potential region for the different obtained bimetallic
catalysts. The onset potential (ca. 0.4 V vs RHE) for the glycerol
oxidation proves that one can replace platinum in the
Glycerol Electrooxidation Scheme. According to HPLC
analyses and in situ infrared measurements, we propose herein a
scheme for glycerol electrooxidation in alkaline medium on the
Pd-based nanocatalysts (Figure 7). Depending on the catalyst, it
2409
dx.doi.org/10.1021/cs400559d | ACS Catal. 2013, 3, 2403−2411

ACS Catalysis
Research Article
Figure 7. Proposed reaction scheme for glycerol electrooxidation on PdNi/C and PdAg/C nanocatalysts in alkaline medium. The dotted arrows indicate
that the reaction product was not explicitly determined by HPLC.
(3) Bianchini, C.; Shen, P. K. Chem. Rev. 2009, 109, 4183.
(4) Bambagioni, V.; Bianchini, C.; Marchionni, A.; Filippi, J.; Vizza, F.;
Teddy, J.; Serp, P.; Zhiani, M. J. Power Sources 2009, 190, 241.
(5) Hong, P.; Luo, F.; Liao, S.; Zeng, J. Int. J. Hydrogen Energy 2011, 36,
8518.
(6) Steele, B. C. H.; Heinzel, A. Nature 2001, 414, 345.
(7) Matsuoka, K.; Iriyama, Y.; Abe, T.; Matsuoka, M.; Ogumi, Z. J.
Power Sources 2005, 150, 27.
electrocatalysts by palladium, which is at least 50 times more
abundant on earth than platinum, combined with less expensive
non noble metals (Ni, Ag) for obtaining better kinetics. By
combining HPLC and in situ FTIR measurements, we were able
to confirm that the glycerol electrooxidation on such catalysts
involves glyceraldehyde as a reaction intermediate and that the
major products are glycolate and glycerate ions. Furthermore
carbon dioxide (2343 cm−1) and carboxylic acid (3000 cm−1)
production during glycerol electrooxidation in alkaline medium
(pH = 13) was also demonstrated, confirming the important
change of pH nearby the electrode surface. The nature of the
catalysts and the applied potential to perform electrolysis are
therefore the important parameters that control the selectivity of
the glycerol electrooxidation reaction toward the cogeneration of
added value chemicals and the electric energy production in a
fuel cell configuration.
(8) Ilie, A.; Simoes, M.; Baranton, S.; Coutanceau, C.; Martemianov, S.
J. Power Sources 2011, 196, 4965.
(9) Schafer, H. J. C. R. Chim. 2011, 14, 745.
̈
(10) Simoes, M.; Baranton, S.; Coutanceau, C. Appl. Catal., B 2010, 93,
̃
354.
(11) Kwon, Y.; Schouten, K. J. P.; Koper, M. T. M. ChemCatChem
2011, 3, 1176.
(12) Lamy, C.; Lima, A.; LeRhun, V.; Delime, F.; Coutanceau, C.;
́
Leger, J. M. J. Power Sources 2002, 105, 283.
(13) Antolini, E. J. Power Sources 2007, 170, 1.
(14) Zhou, C.-H.; Beltramini, J. N.; Fan, Y.-X.; Lu, G. Q. Chem. Soc.
Rev. 2008, 37, 527.
ASSOCIATED CONTENT
* Supporting Information
■
S
(15) Katryniok, B.; Kimura, H.; Skrzynska, E.; Girardon, J.-S.;
Fongarland, P.; Capron, M.; Ducoulombier, R.; Mimura, N.; Paul, S.;
Dumeignil, F. Green Chem. 2011, 13, 1960.
(16) Carrettin, S.; McMorn, P.; Johnston, P.; Griffin, K.; Kiely, C. J.;
Hutchings, G. J. Phys. Chem. Chem. Phys. 2003, 5, 1329.
(17) Ketchie, W. C.; Murayama, M.; Davis, R. J. J. Catal. 2007, 250,
264.
Electrochemical surface area evaluation, cyclic voltammogram of
Pd30Ag70/C, effect of scan rate on glycerol electrooxidation,
electrolysis current curves, product distributions, SPAIR spectra
of the Pd80Ag20/C catalyst, reference IR spectra, chronoamper-
ommetry/IR spectra of Pd60Ni40/C. This material is available
free of charge via the Internet at http://pubs.acs.org.
(18) Zhang, Y.; Cui, X.; Shi, F.; Deng, Y. Chem. Rev. 2011, 112, 2467.
(19) Zhang, Z.; Xin, L.; Li, W. Appl. Catal., B 2012, 119−120, 40.
(20) Ksar, F. a.; Ramos, L.; Keita, B.; Nadjo, L.; Beaunier, P.; Remita,
H. Chem. Mater. 2009, 21, 3677.
(21) Nguyen, S. T.; Law, H. M.; Nguyen, H. T.; Kristian, N.; Wang, S.;
Chan, S. H.; Wang, X. Appl. Catal., B 2009, 91, 507.
(22) Singh, R. N.; Singh, A.; Anindita. Carbon 2009, 47, 271.
(23) Singh, R. N.; Singh, A.; Anindita. Int. J. Hydrogen Energy 2009, 34,
2052.
AUTHOR INFORMATION
Corresponding Author
*E-mail: claudia.gomes.de.morais@univ-poitiers.fr.
Notes
■
The authors declare no competing financial interest.
REFERENCES
■
(24) Xu, J. B.; Zhao, T. S.; Shen, S. Y.; Li, Y. S. Int. J. Hydrogen Energy
2010, 35, 6490.
(1) Roquet, L.; Belgsir, E. M.; Leg
1994, 39, 2387.
́
er, J. M.; Lamy, C. Electrochim. Acta
(25) Shen, S. Y.; Zhao, T. S.; Xu, J. B.; Li, Y. S. J. Power Sources 2010,
195, 1001.
(2) Arechederra, R. L.; Treu, B. L.; Minteer, S. D. J. Power Sources 2007,
173, 156.
2410
dx.doi.org/10.1021/cs400559d | ACS Catal. 2013, 3, 2403−2411

ACS Catalysis
Research Article
(26) Zhao, Y.; Yang, X.; Tian, J.; Wang, F.; Zhan, L. Int. J. Hydrogen
Energy 2010, 35, 3249.
(27) Lee, Y. W.; Kim, M.; Kim, Y.; Kang, S. W.; Lee, J.-H.; Han, S. W. J.
(64) Wang, J. Analytical Electrochemistry, 2nd ed.; John Wiley & Sons
Inc.: New York, 2004.
(65) Wilson, M. S.; Valerio, J. A.; Gottesfeld, S. Electrochim. Acta 1995,
40, 355.
Phys. Chem. C 2010, 114, 7689.
(66) Srinivasan, S.; Ticianelli, E. A.; Derouin, C. R.; Redondo, A. J.
Power Sources 1988, 22, 359.
(67) Srinivasan, S.; Manko, D. J.; Koch, H.; Enayetullah, M. A.;
Appleby, A. J. J. Power Sources 1990, 29, 367.
(68) Holze, R.; Ahn, J. J. Membr. Sci. 1992, 73, 87.
(69) Kokoh, K. B.; Leger, J. M.; Beden, B.; Huser, H.; Lamy, C.
́
Electrochim. Acta 1992, 37, 1909.
(70) Corrigan, D. S.; Leung, L. W. H.; Weaver, M. J. Anal. Chem. 1987,
59, 2252.
(71) Beden, B.; Lamy, C. In Spectroelectrochemistry; Gale, R., Ed.;
Springer: New York: 1988; p 189.
(72) Bae, I. T.; Xing, X.; Liu, C. C.; Yeager, E. J. Electroanal. Chem.
Interfacial Electrochem. 1990, 284, 335.
(73) Tateishi, N.; Yahikozawa, K.; Nishimura, K.; Takasu, Y.
Electrochim. Acta 1992, 37, 2427.
(74) Hu, C.-C.; Wen, T.-C. Electrochim. Acta 1995, 40, 495.
(75) Wang, Y.; Sheng, Z. M.; Yang, H.; Jiang, S. P.; Li, C. M. Int. J.
Hydrogen Energy 2010, 35, 10087.
(28) Li, G.; Jiang, L.; Jiang, Q.; Wang, S.; Sun, G. Electrochim. Acta
2011, 56, 7703.
(29) Zhang, Z.; Xin, L.; Sun, K.; Li, W. Int. J. Hydrogen Energy 2011, 36,
12686.
(30) Simoes, M.; Baranton, S.; Coutanceau, C. ChemSusChem 2012, 5,
̃
2106.
(31) Lin, S.-C.; Chen, J.-Y.; Hsieh, Y.-F.; Wu, P.-W. Mater. Lett. 2011,
65, 215.
(32) Simoes, M.; Baranton, S.; Coutanceau, C. Appl. Catal., B 2011,
̃
110, 40.
(33) Mougenot, M.; Caillard, A.; Simoes, M.; Baranton, S.;
Coutanceau, C.; Brault, P. Appl. Catal., B 2011, 107, 372.
(34) Nie, M.; Tang, H.; Wei, Z.; Jiang, S. P.; Shen, P. K. Electrochem.
Commun. 2007, 9, 2375.
(35) Al-Odail, F. A.; Anastasopoulos, A.; Hayden, B. E. Phys. Chem.
Chem. Phys. 2010, 12, 11398.
(36) Bianchini, C. In Interfacial Phenomena in Electrocatalysis; Vayenas,
C. G., Ed.; Springer: New York, 2011; Vol. 51, p 203.
(37) Ferreira, R. S., Jr.; Janete Giz, M.; Camara, G. A. J. Electroanal.
Chem. 2013, 697, 15.
(38) Holade, Y.; Morais, C.; Arrii-Clacens, S.; Servat, K.; Napporn, T.
W.; Kokoh, K. B. Electrocatalysis 2013, 4, 167.
(39) Pires, F. I.; Villullas, H. M. Int. J. Hydrogen Energy 2012, 37, 17052.
(40) Li, R.; Wei, Z.; Huang, T.; Yu, A. Electrochim. Acta 2011, 56, 6860.
(41) Liu, Z.; Zhang, X.; Hong, L. Electrochem. Commun. 2009, 11, 925.
́
(42) Ramos-Sanchez, G.; Yee-Madeira, H.; Solorza-Feria, O. Int. J.
Hydrogen Energy 2008, 33, 3596.
(43) Martinez-Casillas, D. C.; Vazquez-Huerta, G.; Perez-Robles, J. F.;
Solorza-Feria, O. J. New Mater. Electrochem. Syst. 2010, 13, 163.
(44) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals
and Applications, 2nd ed.; John Wiley & Son, Inc.: New York, 2001.
(45) Scholz, F. Electroanalytical Methods: Guide to Experiments and
Applications; Springer-Verlag: Berlin, Germany, 2010.
(46) Kwon, Y.; Koper, M. T. M. Anal. Chem. 2010, 82, 5420.
(47) Gomes, J.; Tremiliosi-Filho, G. Electrocatalysis 2011, 2, 96.
(48) Jeffery, D. Z.; Camara, G. A. Electrochem. Commun. 2010, 12,
1129.
(76) Grden, M.; Lukaszewski, M.; Jerkiewicz, G.; Czerwinski, A.
Electrochim. Acta 2008, 53, 7583.
(77) Simoes, M.; Baranton, S.; Coutanceau, C. J. Phys. Chem. C 2009,
̃
113, 13369.
(78) Maheswari, S.; Sridhar, P.; Pitchumani, S. Electrocatalysis 2012, 3,
13.
(79) Yaylayan, V. A.; Ismail, A. A. Carbohydr. Res. 1995, 276, 253.
(80) A. Yaylayan, V.; Harty-Majors, S.; A. Ismail, A. Carbohydr. Res.
1999, 318, 20.
(81) Sun, S.-G. In Electrocatalysis; Lipkowski, J., Ross, P. N., Eds.;
Wiley-VCH, Inc.: New York, 1998; p 243.
(82) Demarconnay, L.; Brimaud, S.; Coutanceau, C.; Leg
́
er, J. M. J.
Electroanal. Chem. 2007, 601, 169.
(83) Pouchert, C. J. The Aldrich Library of Infrared Spectra, 3rd ed.;
Aldrich Chemical Company, Inc.: Milwaukee, WI, 1981.
(84) E. Bourguet, C. A. Les techniques de laboratoire: Purification et
analyse des composes
Paris, France, 2008.
́
organiques; Edition Ellipses, THECHNOSUP:
(85) Orozco, G.; Per
Chem. 2000, 495, 71.
́
ez, M. C.; Rincon, A.; Gutierrez, C. J. Electroanal.
́
́
(49) Kwon, Y.; Birdja, Y.; Spanos, I.; Rodriguez, P.; Koper, M. T. M.
ACS Catal. 2012, 2, 759.
(50) Fernandez, P. S.; Martins, M. E.; Camara, G. A. Electrochim. Acta
́
2012, 66, 180.
(51) Alcaide, F.; Cabot, P.-L.; Brillas, E. J. Power Sources 2006, 153, 47.
(52) Tonda-Mikiela, P.; Napporn, T. W.; Morais, C.; Servat, K.; Chen,
A.; Kokoh, K. B. J. Electrochem. Soc. 2012, 159, H828.
(53) Boutonnet, M.; Kizling, J.; Stenius, P.; Maire, G. Colloids Surf.
1982, 5, 209.
(54) Tojo, C.; Blanco, M. C.; Rivadulla, F.; Lop
Langmuir 1997, 13, 1970.
(55) Solla-Gullon, J.; Rodes, A.; Montiel, V.; Aldaz, A.; Clavilier, J. J.
́
ez-Quintela, M. A.
́
Electroanal. Chem. 2003, 554−555, 273.
(56) Capek, I. Adv. Colloid Interface Sci. 2004, 110, 49.
(57) Guo, S.; Dong, S.; Wang, E. ACS Nano 2009, 4, 547.
(58) Carbone, L.; Cozzoli, P. D. Nano Today 2010, 5, 449.
(59) Wang, D.; Zhao, P.; Li, Y. Sci. Rep. 2011, 1, 37.
(60) Wang, Z.-L.; Yan, J.-M.; Wang, H.-L.; Ping, Y.; Jiang, Q. Sci. Rep.
2012, 2, 598.
́
(61) Ingert, D. In Les nanosciences: 2. Nanomateriaux et nanochimie, 2
́
ed.; Marcel Lahmani, C. B., Houdy, P., Eds.; Editions Belin: Paris,
France, 2012; p 463.
(62) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals
and Applications, 1st ed.; John Wiley & Son, Inc.: New York, 1980.
(63) Eklund, J. C.; Bond, A. M.; Alden, J. A.; Compton, R. G. In
Advances in Physical Organic Chemistry; Bethell, D., Ed.; Academic Press:
New York, 1999; Vol. 32, p 1.
2411
dx.doi.org/10.1021/cs400559d | ACS Catal. 2013, 3, 2403−2411