Dramatically enhanced cleavage of the C-C bond using an electrocatalytically coupled reaction

Article abstract of DOI:10.1021/ja301992h

This paper describes a generalized approach for the selective electrocatalytic C-C bond splitting in aliphatic alcohols at low temperature in aqueous state, with ethanol as an example. We show that selective C-C bond cleavage, leading to carbon dioxide, is possible in high pH aqueous media at low overpotentials. This improved selectivity and activity is achieved using a solution-born co-catalyst based on Pb(IV) acetate, which controls the mode of the ethanol adsorption so as to facilitate direct activation of the C-C bond. The simultaneously formed under-potentially deposited (UPD) Pb and surface lead hydroxide change the functionality of the catalyst surface for efficient promotion of CO oxidation. The resulting catalyst retains an unprecedented ability to sustain the full oxidation reaction pathway on an extended time scale of hours as opposed to minutes without addition of Pb(IV) acetate.

Full text of DOI:10.1021/ja301992h

Article
pubs.acs.org/JACS
Dramatically Enhanced Cleavage of the C−C Bond Using an
Electrocatalytically Coupled Reaction
Qinggang He,†,⊥ Badri Shyam,‡,∥ Katerina Macounova,§ Petr Krtil,§ David Ramaker,‡
̌
́
and Sanjeev Mukerjee*,†
†Department of Chemistry and Chemical Biology, Northeastern University, 360 Huntington Avenue, Boston, Massachusetts 02115,
United States
‡Department of Chemistry, The George Washington University, 725 21st Street NW, Washington D.C. 20052, United States
§J. Heyrovsky Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolejsk
̌
ova 3, CZ-182 23 Prague 8, Czech
Republic
S
* Supporting Information
ABSTRACT: This paper describes a generalized approach for
the selective electrocatalytic C−C bond splitting in aliphatic
alcohols at low temperature in aqueous state, with ethanol as
an example. We show that selective C−C bond cleavage,
leading to carbon dioxide, is possible in high pH aqueous
media at low overpotentials. This improved selectivity and
activity is achieved using a solution-born co-catalyst based on
Pb(IV) acetate, which controls the mode of the ethanol
adsorption so as to facilitate direct activation of the C−C bond. The simultaneously formed under-potentially deposited (UPD) Pb
and surface lead hydroxide change the functionality of the catalyst surface for efficient promotion of CO oxidation. The resulting
catalyst retains an unprecedented ability to sustain the full oxidation reaction pathway on an extended time scale of hours as
opposed to minutes without addition of Pb(IV) acetate.
to potentials in the oxygen evolution region in alkaline media.11
The oxidation process is, however, far from controlled at these
conditions.
1. INTRODUCTION
The controlled electrocatalytic cleavage of the C−C bond in
organic molecules, particularly in aliphatic alcohols, remains
Regardless of the media, Pt-based materials represent the
one of the most challenging problems in electrochemistry.
benchmark catalysts for ethanol oxidation. There have been
Aside from a fundamental interest in directing a multielectron
transfer to selectively cleave or form C−C bonds,1 the reaction
several strategies devised for enhancing the activity and
selectivity of electrocatalytic ethanol oxidation,12−15 some
is also of great practical significance in the development of fuel
cell-based electric energy generators.2,3 This implementation
based on incorporation of adatoms to the surface. The
enhancement has been attributed to steric hindrance of the
enabling higher alcohol oxidation can significantly improve the
energy density of these low-temperature fuel cells. The
surface, or electronic ligand and/or bifunctional effects when
the adatoms are directly involved in the catalytic process.16 The
inertness of the C−C bonds in electrocatalytic alcohol
activity of purely metal-based catalytic systems is reported to be
surpassed by multifunctional catalysts featuring metal oxide-
oxidation is counterintuitive, since the C−C bonds are
relatively weak and can be, in the case of hydrocarbons,
based oxidation promoters like, for example, MgO,17 CeO2,18
ZrO2,19 or SnOx.7,20 Despite the apparent improvement of the
activity in the ethanol oxidation, the resulting selectivity of the
oxidation process toward the desired 12 electron process still
significantly lags behind expectations, with a gradual
deactivation of the catalyst surface in the course of minutes.
The rationalization for the C−C bond inactivity in ethanol
oxidation arises because the ethanol molecule primarily adsorbs
on the metal surface via the oxygen on the C1 carbon. The first
electron removed from the ethanol molecule facilitates
formation of the energetically more stable aldehyde, which
cracked either thermally or catalytically.4 Taking ethanol as an
example, a cleavage of the C−C bond in the oxidation process
leading eventually to CO2 formation (12 electrons) is severely
disfavored kinetically, especially in acid media.5 The electro-
catalytic oxidation of aliphatic alcohols proceeds readily on Pt-
based metal surfaces but usually yields just 2−4 electrons per
alcohol molecule, and depending on both the electrode material
and the pH, rarely involves C−C bond splitting.6−8 The
resulting products are mostly the undesired aldehydes or
corresponding acids at low pH. In contrast, in alkaline media,
more complete oxidation occurs, mainly due to higher
sensitivity to structural and morphological moieties such as
the relative population of crystallite planes.9,10 Ethanol
oxidation, selective to CO2, has been reported for Pt polarized
Received: February 28, 2012
© XXXX American Chemical Society
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reference scan. Any edge shift corrections applied to the reference foils
were also applied to their respective sample scans. A post-edge
normalization procedure was then applied to the aligned scans as
described previously.30 Difference spectra were obtained using the
equation
then easily desorbs from the surface preventing further charge
transfer that could attack the ethanol C−C bond.21,22 It may be
envisaged that controlling the approach, and hence the
adsorption mode of ethanol, toward the substrate could be
instrumental in steering the selectivity of the oxidation process
toward the C−C bond splitting essential for the complete
oxidation route.
The present paper shows for the first time the successful
application of this concept, utilizing a solution-based co-catalyst
to direct the ethanol adsorption, while modifying the catalyst
surface chemistry to oxidize selectively the C−C bond in
ethanol. Differential electrochemical mass spectrometry
(DEMS) and in situ X-ray absorption near-edge spectroscopy
(XANES) data are used to show and rationalize the behavior of
the Pb(IV) acetate under reaction conditions, thus, modifying
the behavior of the Pt/C nanoparticulate catalyst to retain an
unprecedented activity and selectivity toward C−C bond
breaking on the time scale of hours.
Δμ = μ(V) − μ(0.54 V)
(1)
where μ(V) is the sample at various potentials and μ(0.54) is the
reference signal at 0.54 V, which is considered the cleanest region of
Pt, that is, relatively free of any adsorbed H, OH, or Ox species. The
Δμ spectra are then compared to theoretical curves (Δμt) constructed
using the FEFF8.0 code31 as described elsewhere.32 This was
accomplished using the relationship
Δμt = μ(Pt6X) − μ(Pt6)
(2)
where X is H or O in a specified binding site with respect to the
absorbing Pt atom and Pt6 is a 6-Pt cluster with a Pt−Pt bond distance
of 2.77 Å as described by Janin et al.33 It should be noted that
theoretical Δμ curves are generally shifted by 1−5 eV and scaled by a
multiplication factor for optimal comparison with experimental data.
2.3. DEMS. The analysis of the ethanol oxidation products
generated in potentiostatic oxidation of ethanol was done by means
of DEMS in a single compartment three electrode flow through cell
made of PTFE. The DEMS apparatus consisted of a Prisma QMS200
quadrupole mass spectrometer (Balzers) connected to a TSU071E
turbomolecular drag pumping station (Balzers). The DEMS data were
recorded simultaneously with the current corresponding to the
potentiostatic ethanol oxidation both in the presence and in absence
of the co-catalyst. The treatment of the raw data along with
information on construction of the calibration curves is given in the
Supporting Information. The DEMS and XAS measurements were not
carried out simultaneously owing to the vastly differing experimental
requirements and constraints of the two techniques.
2. EXPERIMENTAL METHODS
2.1. Electrochemical Characterization. The deposited Pb
electrode was prepared as follows: (1) A Pt/C (E-TEK, 30%)
electrode was cycled in 0.25 M KOH + 1 mM Pb(Ac)4 between
potential limits of 1.1 and 0.06 V ending at 0.06 V. (2) The electrode
was taken out and transferred into 0.25 M KOH + 1 M ethanol after
washing. (3) A cyclic voltammogram between potential limits of 0.2
and 1.1 V was obtained followed by a 1 h chronoamperometry (CA)
test at 0.55 V. The “component” Pb samples, Pt4Pb/C and PtRuPb0.3/
C, were synthesized from a Pt/C (30%, E-TEK) and PtRu/C (40%, E-
TEK) catalyst and the Pb added by Li’s method.23 Detailed
information about the chemicals and procedures for the electro-
chemical characterization can be found in the Supporting Information.
2.2. X-ray Absorption Spectroscopy. All experiments were
conducted at room temperature in an in situ electrochemical XAS cell
based on a previously reported design.24 The cells consisted of a 30 wt
% Pt/VXC72 (E-TEK) working electrode (WE), a Grafoil counter
electrode (CE), and a reversible hydrogen reference electrode (RHE).
Grafoil was chosen as a CE to eliminate any interference at the Pt L3,2
edges and decrease X-ray beam attenuation. In all cases, Au wire
(99.999%, Alfa-Aesar) was utilized as a current collector and
mechanically pressed against the back side of the electrode in a
fashion which did not expose the gold to the X-ray beam. The
platinum working electrodes were activated by potential cycling
(0.05−1.2 V vs RHE at 10 mV s−1) in clean 0.25 M KOH. Following
the activation step, the clean electrolyte was removed from the cell by
syringe and replaced with 0.25 M KOH + xM PbAc4. XAS data were
collected from −250 to ca. 1000 eV above the Pt L3 edge with the WE
fixed at various static potentials along the anodic sweep of the cyclic
voltammetric (CV) measurement. Between extended X-ray absorption
fine structure (EXAFS) scans, the potential was cycled around
completely to “clean” the electrode surface. A full set of EXAFS scans
was also obtained in clean 0.25 M KOH to provide clean reference
scans and as H2O activation standards. The measurements were made
at beamline X11-A (National Synchrotron Light Source, Brookhaven
National Laboratory, Upton, NY) with the Si(111) monochromator
detuned by 40% in order to reject the higher harmonics from the
beam. Data were collected in transmission mode using gas ionization
detectors (I0, I1, or I2) with a nominal nitrogen/argon gas mixture to
allow ∼10% photon absorption in I0 and 70% in I1. The sample was
placed between the I0 and I1 detectors, while a Pt reference foil (4 μm,
Alfa Aesar) was positioned between I1 and I2.
3. RESULTS AND DISCUSSION
The ethanol oxidation on Pt nanoparticulate (3−4 nm)
electrocatalysts is relatively sluggish and proceeds without
apparent C−C bond splitting in the potential window between
0.55 and 0.9 V (vs RHE, pH 14). The addition of Pb(IV)
acetate (mM conc., in electrolyte soln.) significantly facilitates
the oxidation process as indicated by a negative shift in the
onset potential of ca. 150 mV, and a 3-fold increase of the
current density, confirming the significant improvement of the
ethanol oxidation kinetics (Figure 1). Although variation of
The IFEFFIT suite25 (version 1.2.8, IFEFFIT Copyright 2005,
Matthew Newville, University of Chicago, http://cars9.uchicago.edu/
ifeffit/) was utilized for background subtraction (AUTOBK).26 The
Δμ analysis technique has been described in great detail else-
where.27−29 Briefly, XAS reference scans were carefully calibrated to
the edge energy (11564 eV, Pt L3) and aligned to one standard
Figure 1. Cyclic voltammograms of Pt/C (E-TEK, 30%), in 0.25 M
KOH + 1 M ethanol with 0, 1, and 3 mM Pb(IV); Pt loading, 15 μg/
cm2; scan rate, 10 mV/s. The CVs are normalized based on the
geometric area of the glassy carbon electrode.
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Pb(IV) acetate concentration does not necessarily alter the
overall ethanol oxidation rate, it alters the potential window
available for oxidation due to a change of the surface chemistry
(Figure 1). The effect of the Pb(IV) acetate on the surface
chemistry is also reflected in the cathodic response, which
indicates more efficient removal of the oxidation reaction
products from the electrode surface than when the Pb co-
catalyst is absent. The remarkable stability of the Pt catalyst in
the presence of Pb(IV) acetate (in soln.) is also evident from
the chronoamperometric experiments, wherein the presence of
the co-catalyst leads to a retention of ca. 56% of the initial
current after 1 h (Figure 2). Similar yet slightly inferior results
Figure 3. Chronoamperometry data, at the indicated potentials (vs
RHE) in 0.25 M KOH + 1 M ethanol; Pt loading, 15 μg/cm2; room
temperature. Inset: Plots of steady current from the same data.
Figure 2. Chronoamperometry data for Pt/C (E-TEK, 30%), Pt4Pb/
C (30%), PtRuPb0.3/C (40%), and Pt/C (E-TEK, 30%) with Pb
deposited in 0.25 M KOH + 1 M ethanol. Also shown is data for Pt/C
(E-TEK, 30%) in the same 0.25 M KOH + 1 M ethanol solution but
with 1 mM Pb(IV) acetate or 1 mM Pb(II) acetate added; Pt loading,
15 μg/cm2.
are found with Pb(II) acetate. The ethanol oxidation on Pt
modified by lead deposition (UPD) shows a much steeper drop
in activity compared with the case where Pb acetate is present
in the electrolyte solution, although the behavior is superior to
pure Pt, which retains just 3% of the original activity after 1 h.
The trends summarized in Figure 3 reflect the complex nature
of the Pb(IV) acetate functionality in the catalytic process.
The chronoamperometric response shown in Figure 3 shows
the clear advantage of Pb(IV) as an electrolyte additive in a
homogeneous−heterogeneous co-catalyst role for not only
enhancing the activity but also ensuring sustained current
densities as indicated in the steady-state current versus
potential plot (inset, Figure 3). The presence of Pb(IV) in
solution is expected to form a dynamic equilibrium with UPD
adatoms of Pb formed at the Pt surface; the simultaneous
presence of Pb in solution enables an apparent bifunctional
mechanism to be combined with an apparent solution-based
process responsible for the extraordinary activity retention.
These chronoamperometric responses also indicate interesting
differences in the capacitive currents, in the presence of co-
catalyst wherein PtRu shows much higher capacitive charging
currents as compared to response from Pt. This is particularly
manifest at lower potentials (Figure 3).
Figure 4. Proposed mechanism for EtOH oxidation on Pt.
often, particularly on the Pt(111) planes, only 4 electrons are
evolved to produce acetic acid (CH3CH2OH + H2O →
CH3COOH + 4e− + 4H+) (the OH path). Bimetallics such as
PtSn and PtMo normally give higher currents with ethanol than
pure Pt, but unfortunately, the selectivity is shifted predom-
inantly toward acetic acid product (path C2a), that is, less
efficient use of the ethanol fuel.5,37 To accomplish the complete
oxidation of ethanol, the initial adsorption of C atoms has to
follow a C1 or C2 pathway; both of which have the potential of
forming the crucial π bonded species on Pt enabling effective
cleavage of the C−C bond. These are denoted as the C1 and
C2π pathways. These steps are, however, highly energetically
disfavored as shown by recent DFT calculations.34 As we will
show below, addition of a Pb(IV) co-catalyst enables the
selective formation of such π-bonded ethylene species.
To rationalize the electrochemical enhancement and
mechanism (e.g., the C1 path or the C2π in Figure 4) of
complete ethanol oxidation with the aid of a co-catalyst,
unambiguous detection and quantification of CO2 is essential.
The lack of chemical sensitivity of many electrochemical
techniques and the interfering effect of residual carbonates were
Figure 4 gives a proposed ethanol oxidation reaction (EOR)
mechanism based on DFT calculations,34 and FTIR data.5,35,36
Lai et al.36 have shown that the orientation of the ethanol
adsorption dictates which path the oxidation follows. Most
avoided in this work by labeling the substrate ethanol with 13
C
in the C2 position. The 13CO2 (m/z = 45) detected in the
reaction mixture, at levels well exceeding the natural abundance
C
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of 13C, during chronoamperometric oxidation experiments
mass spectroscopically proves conclusively the C−C bond
splitting. The selectivity of the anodic process toward C−C
bond cleavage significantly increases in the presence of Pb co-
catalyst (Figure 5).
dramatically than that of methanol with the assistance of
Pb(IV) co-catalyst. It indicates that the C−C bond in ethanol
can be efficiently broken by using Pb(IV) salts.
As can be seen from Figure 7, lead(IV) acetate from the
electrolyte deposits at the Pt surface via underpotential
Figure 5. Charge fractions corresponding to CO2 formation in
catalytic oxidation of ethanol (0.01 M) in 0.25 M KOH on
nanocrystalline Pt in presence and absence of lead co-catalyst. Data
were extracted from potentiostatic oxidation experiments with
CH313CH2OH.
Figure 7. Cyclic voltammograms for Pt/C (E-TEK, 30%), in 0.25 M
KOH with x mM Pb(IV) acetate as indicated; Pt loading, 15 μg/cm2.
Inset shows Δμ magnitudes reflecting OH/Pt coverage with (red) and
wiyhout Pb (black) and also highlights the difference between the 0.0
and 0.25 mM CV curves (see also Supporting Information Figures S2
and S3).
The CO2 selectivity increases with time (Figure 5) and
reaches ca. 50% of its initial activity value after approximately 1
h (Figure 2). This trend confirms the anticipated competition
between the C2π and C1 kinetic oxidation mechanisms (Figure
4) at the surface. While the C2π mechanism deactivates rather
quickly, the co-catalyst assisted C1 mechanism apparently is
deactivation resistant. The selectivity toward CO2 production
follows the same potential dependence as the observed catalytic
current. Detailed accounts of the mass spectrometric experi-
ments are given in the Supporting Information (Figure S4 in
Supporting Information).
The promoting effect of Pb(IV) on electrooxidation of
methanol and ethanol can be distinguished from the steady-
state current density values obtained at different potentials
shown in Figure 6. It can be clearly seen that the
electrooxidation of ethanol can be enhanced much more
deposition at potentials negative of 0.4 V. The coverage of
Pb adatoms on the Pt surface grows as the concentration of
Pb(IV) ions increases and becomes saturated at the
concentration about 0.5 mM. The UPD lead layer suppresses
the hydrogen adsorption (Figure 7) and oxidizes to stable Pb
(OH)2 at 0.6 V, which is removed from the surface as PbO2
when potential moves further positively.38 The formation of the
UPD lead layer and its subsequent oxidation can be
correspondingly tracked in the increase of intensity of in situ
XANES derived Δμ data (which is referred to as ‘Δμ-XANES’
and tracks OH adsorption) shown in Figure S3 (Supporting
Information) and Figure 7, and also shows up as sudden jumps
in the current response following the potentiostatic oxidation of
ethanol in the presence of Pb(IV) acetate at potentials positive
to 0.6 V (Figure 3); these jumps reflect the sudden increase of
capacitance resulting from hydroxide growth. The bifunctional
effect of the UPD lead layer is confined to the region of the lead
hydroxide stability, which is broader on pure Pt than on the
more complex Pt−Ru catalysts (Figure 3).
The bifunctional mechanism is instrumental in explaining the
increased activity of anode catalysts based on the combination
of Pt with oxophilic metals such as Ru, Sn, or Mo or in this case
Pb in, for example, methanol or formic acid oxidation. The
oxophilic metal provides the OH species needed to oxidize
adsorbed CO. There is no indication of particular specific attack
to the C−C bond by the bifunctional catalysts; in fact, the data
shown in Figure 2 suggest rather inefficient C−C bond
splitting, if the effect of the lead in the catalytic process is
restricted to UPD deposited adatoms. The results reveal a
pronounced deactivation of Pt−Pb alloy or Pt−Pb UPD
catalysts with time (Figure 2). The key aspect of the reported
efficient C−C bond splitting is, therefore, the presence of
Pb(IV) acetate in solution and its interaction with the C−C
bond of ethanol preceding the electron transfer. This
Figure 6. Chronoamperometry test comparison of Pt/C (E-TEK,
30%), as a function of potential in 0.25 M KOH + 1 M ethanol/
methanol with 0 and 1 mM Pb(IV); Pt loading, 15 μg/cm2.
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interaction is not of a purely redox nature, as shown by the
almost identical response for divalent as well as tetravalent lead
acetate.
The preferred adsorption mode of the ethanol at the
electrode normally involves binding through the oxygen (i.e.,
the OH path); this is facilitated by the electrostatics of the
surface at anodic potentials.39 In contrast, the interaction
between ethanol and Pt is tracked in the Δμ-XANES data
plotted in Figure 8. The data acquired on Pt in the absence of
previously reported experimental Δμ-XANES data for ethylene
on Pt(111).42 Both proposed adsorption modes assume an
attachment of the ethanol to the surface via the C1 carbon,
either σ-bonded or ethylene-like π-bonded. Both modes of
adsorption allow for direct activation of the C−C bond, but the
C1 adsorption has to be directed somehow by interaction with
the co-catalyst to overcome the electrostatically preferred
adsorption via the oxygen end of the molecule.
Similar effects with cations directing the adsorption at the
electrodes were described recently.43 These interactions have
been reported to occur between hydrated cations in the outer
Helmholtz plane (OHP) and specifically adsorbed OH in the
inner Helmholtz plane (IHP) as illustrated in Figure 9. Note
Figure 9. Possible mechanism for how noncovalent interactions with
the hydrated Pb cations may provide a preferred orientation of
acetaldehyde anion with the CH3 end pointing toward the surface as it
approaches.
Figure 8. (i) Plot of Δμ = μ(V) − μ(0.5 V, cl) for Pt with (1 mM
PbAc4) and without PbAc4 in 0.2 M ethanol + 0.25 M KOH at the
indicated potentials, mV. (ii) Theoretically calculated Δμ = μ(Ad/Pt6)
− M(Pt6) using FEFF8 for the indicated adsorbates as illustrated; CO/
Pt in either an atop or bridged site, and for ethanol in a π-bonded
geometry (that for the σ-bonded is nearly identical so it is not shown).
Also shown is experimental Δμ data for π-bonded ethylene.
that the electrostatic interaction between the waters hydrating
the Pb cation and the approaching ethanol causes the carbon
end of the molecule to preferably approach the electrode
surface. An alternative explanation can be derived from the
known fact that Pb complexes like Pb(Ac)4 and Pb(benzoate)4
are able to facilitate C−C and CO bond dissociation in
nonaqueous media,44 forming chelates with alcohol molecules.
A similar effect may be operating also in this case. The effect
of the co-catalyst activity is summarized in the scheme shown in
Figure 10. In panel a, it shows that a possible Pb−acetate−
chelate complex and Pb adatoms can be formed simultaneously
due to the presence of Pb(n+) ions in the solution when the
potential is lower than 0.3 V. This is an important role of the
co-catalyst namely to isolate the ethanol from the Pt substrate
while modifying the Pt surface. This cannot be realized by the
bifunctional catalysts (e.g., PtRu and PtSn) or UPD adatom
modified Pt catalysts. In panel b, ethanol molecules near the
surface, through their energetically favorable binding with the
Pb(IV), are weakly bonded to the surface through the carbon as
opposed to the usual binding through the electron rich oxygen
atom (OH pathway). The adsorbed moiety then reacts with the
surface OH species on Pb adatoms to directly cleave the C−C
Pb(IV) acetate (red curves in Figure 8(i)) can be rationalized in
terms of the accumulation of bicarbonate and chemisorbed CO
in ‘bridged’ and ‘atop’ positions at the electrode surface. The
bicarbonate is present at the surface only at potentials close to
0.1 V and yields chemisorbed CO at more positive potentials.
The chemisorbed CO moves more to the atop position with
decreasing surface coverage at more positive potential.40,41 This
tendency for CO adsorption is essentially in accordance with
previous studies carried out by means of infrared spectroscopy
by Lai et al.36 When the lead acetate-based co-catalyst is present
(blue curves in Figure 8(i)), the observed Δμ-XANES data
indicates accumulation of atop CO on the surface at potentials
positive to 0.5 V. The Δμ features dominating at potentials
negative to 0.3 V, that is, at potentials negative to the ethanol
oxidation window, centered at −5 eV cannot be attributed to
species present on a pure Pt electrode surface and therefore
suggest an adsorbed ethanol moiety. The quantum chemistry-
based adsorption models best fitting the experimental Δμ-
XANES data are depicted in Figure 8(ii) and also agree with
E
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Figure 10. Working mechanism of electrooxidation of ethanol on the Pt electrode with the assistance of Pb acetate at different potentials in alkaline
media.
Present Addresses
bond, giving rise to CO2 in the potential range 0.4−0.8 V. In
panel c, at potentials above 0.6 V, more Pb adatoms are
expected to leave the surface so that more C2 products are
produced. On the other hand, it indicates that it is a self-
regeneration system because the Pb adatoms are oxidized and
released back into solution to regenerate the Pb(IV) complex.
⊥Environmental Energy Technologies Division, Lawrence
Berkeley National Laboratory, Berkeley, CA 94720
∥X-ray Science Division, Advanced Photon Source, Argonne
National Laboratory, Argonne, IL 60439-4858
Notes
The authors declare no competing financial interest.
4. CONCLUSIONS
A successful method was developed to significantly enhance the
efficiency of electro-oxidation of ethanol in a high pH
environment through the addition of Pb(IV) cations. The
combination of electrochemical and spectrometric data shows
for the first time an effective strategy for combining
homogeneous and heterogeneous processes directing both
the activity as well as the selectivity of electrocatalytic
oxidations of organic molecules, specifically involving cleavage
of the C−C bond.
A cathodic shift of 150 mV of the onset potential for ethanol
oxidation was observed on a Pt/C electrode in the presence of
1 mM Pb(IV). Chronoamperometric measurements reveal that
a steady-state current remains 1 h later (up to 56% of the
initial), and is higher than any reported to date. The lead ions
were found to play a dual role in the catalysis: Pb(OH)2 is
adsorbed on the Pt surface and produces favorably adsorbed
OH at lower potentials to oxidize the CO and other carbonyl
species, and Pb(II) and/or Pb(IV) ions form a complex with
ethanol before it reaches the electrode surface, which appears to
be beneficial for the C−C bond breakage either by influencing
the orientation of the ethanol as it approaches the surface or
even by assisting in the bond breaking directly. The excellent
sustained activity shown in the case of ethanol oxidation
accompanied with high selectivity to complete oxidation
outlines the potential of this strategy for use in electrocatalytic
power generating applications.
ACKNOWLEDGMENTS
■
The authors deeply appreciate the financial assistance of the
Army Research Office under the Single Investigator grant. Use
of the National Synchrotron Light Source, Brookhaven
National Laboratory, was supported by the U.S. Department
of Energy, Office of Science, Office of Basic Energy Sciences,
under Contract No. DE-AC02-98CH10886.
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S
* Supporting Information
Detailed electrochemical characterizations, in situ Δμ XANES
analysis, DEMS experiments, and oxygen reduction reaction
data in the presence of Pb(IV). This material is available free of
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AUTHOR INFORMATION
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