Mechanistic studies of electrode-assisted catalytic oxidation by flavinium and acridinium cations

Article abstract of DOI:10.1021/cs5005135

Electrochemical behavior of flavinium (Et-Fl⁺) and acridinium (Acr⁺) cations is presented, in order to investigate their activity toward catalytic water oxidation. Cyclic voltammograms of Acr⁺ and Et-Fl⁺ in acetonitrile are qualitatively similar, with oxidation peaks at highly positive potentials, and these oxidation peaks depend strongly on the type of the working electrode being used. However, the two model compounds exhibit different behaviors in the presence of water: while Et-Fl ⁺ facilitates electrocatalytic water oxidation through an electrode-assisted mechanism, water oxidation is not accelerated in the presence of Acr⁺. A comparative study of variable scan-rate cyclic voltammetry, concentration dependence, and spectroelectrochemical behavior of two model compounds suggest that Et-Fl⁺ and Acr⁺ exhibit different reaction pathways with the electrode surface. On the basis of the experimental results, a mechanism is proposed to account for the observed differences in electrocatalysis.

Full text of DOI:10.1021/cs5005135

Research Article
pubs.acs.org/acscatalysis
Mechanistic Studies of Electrode-Assisted Catalytic Oxidation by
Flavinium and Acridinium Cations
†
†
†
‡
‡
Xin Yang, Janitha Walpita, Ekaterina Mirzakulova, Shameema Oottikkal, Christopher M. Hadad,
,†
and Ksenija D. Glusac*
†Department of Chemistry, Center for Photochemical Sciences, Bowling Green State University, Bowling Green, Ohio 43403, United
States
‡
Department of Chemistry, The Ohio State University, Columbus, Ohio 43210, United States
*
S Supporting Information
+
ABSTRACT: Electrochemical behavior of flavinium (Et-Fl ) and
+
acridinium (Acr ) cations is presented, in order to investigate their
+
activity toward catalytic water oxidation. Cyclic voltammograms of Acr
+
and Et-Fl in acetonitrile are qualitatively similar, with oxidation peaks at
highly positive potentials, and these oxidation peaks depend strongly on
the type of the working electrode being used. However, the two model
compounds exhibit different behaviors in the presence of water: while Et-
+
Fl facilitates electrocatalytic water oxidation through an electrode-
assisted mechanism, water oxidation is not accelerated in the presence of
+
Acr . A comparative study of variable scan-rate cyclic voltammetry,
concentration dependence, and spectroelectrochemical behavior of two
+
+
model compounds suggest that Et-Fl and Acr exhibit different reaction
pathways with the electrode surface. On the basis of the experimental
results, a mechanism is proposed to account for the observed differences
in electrocatalysis.
KEYWORDS: electrocatalytic water oxidation, iminium ions, electrode-assisted, pseudobase, electron transfer
2
7−34
INTRODUCTION
studies of model Ru-based bimetallic catalysts,
notable
■
1
−6
progress in the field occurred since the discovery of the first
molecular catalyst in 1982. For example, the key O−O bond
formation of the Ru blue dimer is currently thought to occur via
Electrocatalytic oxidation of water to oxygen
key processes that needs to be improved for the development
of efficient solar fuel cells.
intermediates, such as hydrogen peroxide and hydroxyl radical,
water oxidation needs to occur via a simultaneous, proton-
is one of the
26
7
,8
To avoid high-energy
V
a nucleophilic attack of water to the Ru O species to form a
2
8,29
hydroperoxyl intermediate.
Furthermore, significant im-
+
coupled four-electron transfer process (2H O → O + 4H +
provement in the catalyst’s stability was achieved by the
discovery of new model Ru-based catalysts that exhibit a single
While some of these monometallic
catalysts oxidize water via the monomolecular mechanism
involving the above-mentioned nucleophilic attack of water to
2
2
−
9
4
e ). This requirement for synchronicity poses substantial
2
4,35−41
challenges in the development of catalysts that evolve oxygen
from water at sufficiently low overpotentials. The low catalytic
overpotential can only be achieved if all four one-electron
oxidation steps in the water oxidation pathway exhibit the same
metallic site.
V
the Ru O intermediate, the complexes discovered by Sun
10
change in free energy, which is a tall order to fill. Even if high-
energy intermediates are avoided, water oxidation is signifi-
cantly thermodynamically uphill and requires large potentials
appear to undergo a bimolecular O−O bonding mechanism, in
which a Ru−O−O−Ru peroxide is made by a radical coupling
24
mechanism. More recently, mononuclear iridium complexes
have been reported to efficiently catalyze water oxidation.
However, it is important to note that some of the Ir-based
catalysts were found to undergo ligand oxidation to form
iridium oxides that are highly active toward water oxida-
(
E° = +1.23 V vs NHE), causing most molecular catalysts to
42−44
undergo other unwanted chemical reactions (oxidative
damage). In an attempt to overcome these challenges, a great
scientific effort is aimed at studies of well-defined molecular
water-oxidation catalysts, whose mechanistic details can be
readily investigated using available spectroscopic techni-
45−47
tion.
1
1−25
ques.
Most of the currently known oxygen-evolving molecular
catalysts are complexes made of second and third row transition
Received: April 17, 2014
Revised: June 12, 2014
Published: June 30, 2014
2
4,26−47
metals, such as Ru and Ir.
Due to extensive mechanistic
©
2014 American Chemical Society
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Research Article
Inspired by the catalytic water oxidation in natural
similarities are found in the electrochemical behaviors of Et-Fl+
and Acr , the acridinium ion does not facilitate the electro-
catalytic oxidation of water to oxygen. A plausible mechanistic
2
+
photosynthetic centers, a large scientific effort was dedicated
1
6,19−21,48,49
to the studies of model manganese complexes.
However, these studies have shown that the manganese
complexes are unstable outside the protein environment and
that only a few complexes were found to be catalytically
explanation of these differences is presented.
EXPERIMENTAL SECTION
Methods. All chemicals were purchased from commercial
suppliers and used without further purification. H and
■
2
1,50,51
active.
Most of these catalysts, except for the recent
51
report by Åkermark2, required a sacrificial oxidant as an
1
13
C
1,50
oxygen-atom donor.
Even though Mn-based molecular
NMR spectra were recorded on a Bruker Avance 300 MHz
system. GC-MS spectra were measured on a Shimadzu GC-
MS-Q5050A spectrometer. UV/vis absorption spectra were
recorded on an Agilent 8453 UV Spectrophotometer in a 1 cm
catalysts are scarce, significant research progress was achieved
recently using other first-row transition metal complexes, such
11−16,52−56
as Fe, Co, and Cu-based catalysts.
Our group is interested in organocatalytic molecular
+
quartz cell. 10-Methyl-9-phenylacridinium perchlorate (Acr )
5
7,58
frameworks for oxygen evolution.
Fully organic water
was purchased from TCI America. Et-Fl+ was synthesized
oxidation catalysts have not been reported in the literature,
likely due to the chemical instability of organic compounds
under strongly oxidizing conditions required for water
oxidation. Despite this disadvantage, organic catalysts should
not be neglected, since they offer some advantages over the
currently known molecular systems: (i) There are fewer
concerns about limited resources, as the organic molecules
are made of earth-abundant elements (C, H, O, and N). (ii)
Synthetic organic chemistry is a mature field, offering easy
access to a wide variety of molecular motifs for catalysis. (iii)
Organic catalysts are likely to be less toxic than the metal-
containing analogs.
57
according to the previously published procedure.
Cyclic Voltammetry. Cyclic voltammetry was performed
using a BASi epsilon potentiostat in a VC-2 voltammetry cell
(
Bioanalytical Systems) using glassy carbon (3 mm diameter,
MF-2012, Bioanalytical Systems), fluorine-doped tin oxide
FTO (area 4.5 cm , Hartford Glass), boron-doped diamond
BDD (area 2 cm , Fraunhofer USA), and Pt (1.6 mm diameter,
2
2
MF-2013, Bioanalytical Systems) working electrodes; a
platinum wire auxiliary electrode (MW-4130, Bioanalytical
+
Systems); and a nonaqueous Ag/Ag reference electrode (MF-
2
062, Bioanalytical Systems). Acetonitrile was purchased from
Sigma-Aldrich (anhydrous, 99.8%) and purified by reflux over
We recently found that a simple flavinium ion (Et-Fl+,
Scheme 1) facilitates the electrocatalytic water oxidation at high
CaH for 8 h, followed by distillation. Tetrabutylammonium
2
perchlorate (TBAP) was purchased from Sigma-Aldrich,
recrystallized from methanol, and dried under a vacuum.
Scheme 1. Structures of N(5)-Ethylflavinium Perchlorate
Electrochemical potentials were referenced to NHE by adding
+
60
(
(
Et-Fl ) and N-Methyl-9-phenylacridinium Perchlorate
0
.548 V to the experimental potentials.
+
a
Acr )
Bulk Electrolysis. Bulk electrolysis was performed in a
custom-designed two-compartment gastight electrochemical
cell under an argon atmosphere. One arm of the cell contained
(
i) a Pt electrode (0.25″ × 4″, Home Science Tools), (ii) a Ag/
AgCl aqueous reference electrode (Bioanalytical Systems), (iii)
an oxygen sensor (FOXY-R, Ocean Optics), (iv) a Schlenk line
outlet connected to a round-bottom flask, (v) a gas inlet. The
second arm contained a Pt wire as an auxiliary electrode and a
gas outlet port. Electrolysis was carried out using an EC Epsilon
potentiostat (BioanalyticalSystem) at +2.1 V vs Ag/AgCl in a
0
.1 M phosphate buffer at different pH values. The water for
electrolysis was deionized using a water purification system
a
The red arrows show the sites where hydroxide attack occurs to
(Barnstead Nanopure System). Prior to each experiment, the
generate the corresponding pseudobase derivatives Et-FlOH and
AcrOH.
sensor was calibrated using a two-point reading (20.9% O in
2
the air and 0% O in the argon-purged cell). An empty cell was
2
degassed with argon for 2 h. In parallel, a prepared solution of
1.5 mM Acr in the phosphate buffer at pH = 2, 7, and 11 was
5
9
+
overpotentials. This preliminary study provided two key
mechanistic insights regarding the catalysis: (i) the hydroxy-
lated flavin FlOH+ was identified as a likely catalytic
intermediate using UV/vis spectroelectrochemical measure-
ments, and (ii) the surface of the working electrode plays an
important role in catalysis (the electrocatalytic current was
observed on glassy carbon (GC) and platinum (Pt) electrodes,
while no catalysis was observed in the case of the fluoride-
doped tin oxide (FTO) electrode).
purged by argon in a round-bottom flask connected to the cell
+
by a Schlenk connector. The Acr solution was then transferred
to the cell through a Schlenk connection. The oxygen sensor
probe was placed in the headspace, and data were collected at
10 s intervals. Before electrolysis was initiated, the O signal was
2
monitored for 10 min to ensure there was no leakage of O2
from the air outside the cell. After ensuring there were no leaks,
the electrolysis was initiated and continued for 2 h. Conversion
In an attempt to identify which functional groups of Et-Fl+
are responsible for the catalytic activity, we investigate here the
electrochemical behavior of a structurally similar, but
significantly simpler, derivative: N-methyl-9-phenylacridinium
perchlorate (Acr , Scheme 1). This manuscript describes the
electrochemical behavior of Acr and contrasts it to that of Et-
of %O into micromoles was obtained from the known volumes
2
of the solution (V = 100 mL) and the headspace (V = 70
S
H
mL) using Henry’s Law.
Spectroelectrochemistry. UV/vis spectroelectrochemistry
+
was done using a Pt mesh working electrode, nonaqueous Ag/
+
+
Ag reference, and Pt wire as an auxiliary electrode, and the
+
Fl . The results of our study suggest that, even though some
absorption spectra were recorded on a HP 8453 UV/vis
2
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spectrophotometer. The spectroelectrochemical cell was
purchased from Bioanalytical Systems (EF-1350). A solution
of 1 mM Acr in acetonitrile containing 0.1 M TBAP was
of the text addresses their similarities, and the second section
reports the important differences between them.
+
+
+
2. Et-Fl vs Acr : Similarities. 2.1. Cyclic Voltammetry.
+
degassed with argon prior to each experiment. The changes in
the absorption were monitored in 6 s intervals after applying a
potential of +2.7 V vs NHE. In addition, the chemical oxidation
of 1.0 mM AcrOH solution was performed with 1.5 mM
Cu(ClO ) ·6H O in acetonitrile, and the UV/vis absorption
The cyclic voltammogram of Acr in acetonitrile is qualitatively
very similar to that of Et-Fl (Figure 2). The reversible one-
+
4
2
2
spectra were recorded as a function of time.
Computational Methods. All geometries were optimized
using Becke’s three-parameter hybrid exchange functional with
the Lee−Yang−Parr correlation functional (B3LYP) meth-
6
1,62
od,
along with other DFT functionals (S2, Supporting
Information), with the 6-311++G** basis set in the Gaussian
63
0
9 suite of software. All stationary points were verified to be
minima by harmonic vibrational frequency calculations. In
order to estimate the effect of solvation, polarizable continuum
model (PCM) calculations for acetonitrile as a solvent and its
effect on the oxidation potential were calculated with respect to
6
4
NHE at +4.4 V.
RESULTS AND DISCUSSION
+
■
Figure 2. Cyclic voltammograms of 2 mM Acr (red) and 2 mM Et-
+
1
. Model Compounds. The aim of this study is to pinpoint
Fl (blue) in acetonitrile (electrolyte: TBAP). Scan rate: 100 mV/s.
Electrodes: platinum working electrode, platinum counter electrode,
+
the structural motifs of Et-Fl that are responsible for its
electrocatalytic activity. For this purpose, an investigation of a
+
nonaqueous Ag/Ag reference electrode.
+
significantly simpler model compound Acr (Scheme 1) is
+
presented. While Et-Fl exhibits a number of functional groups,
+
•
+
electron reduction of Acr to generate neutral Acr radical
appears at −0.9 V vs Ag/Ag , which is consistent with literature
reports. At positive potentials, three oxidation peaks were
observed in the presence of Acr and Et-Fl (red circles and
blue stars in Figure 2). Importantly, an increase in the current is
observed at potentials above +2 V (vs Ag/Ag ), which is in the
case of Acr shifted by ∼0.2 V to a more positive potential
relative to the corresponding peak of Et-Fl . These results
Acr contains only the aromatic iminium ion moiety as a
+
+
possible site for electrocatalysis. The iminium ion Acr forms
the pseudobase intermediate AcrOH in basic solution (Figure
). This is an important property, since our previous study of
65
+
+
1
+
+
+
+
+
suggest that Acr exhibits similar oxidation behavior to Et-Fl .
Since the oxidation behavior of Et-Fl was previously shown
+
to strongly depend on the type of the working electrode used in
59
the experiment, the effect of the working electrode was
+
investigated for Acr as well (Figure 3). The electrodes used in
this study are as follows: platinum (Pt), glassy carbon (GC),
+
Figure 1. UV/vis absorption spectra of Acr in acetonitrile/water =
1000:1 mixture at varying pH values: pH 7 (black), 10.4 (red), 11.1
(
green), 11.3 (blue), and 12.3 (purple).
Et-Fl electrochemistry identified the intermediate Et-FlOH+
+
59
as a plausible intermediate in catalysis. On the other hand,
+
+
pseudobase formations from Et-Fl and Acr differ in two
significant ways: (i) Et-FlOH formation occurs more readily
59
(
pseudobase pK = 3.5) than the formation of AcrOH
a
(
pseudobase pK = 11.1, Figure 1); (ii) the hydroxide ion
Figure 3. Cyclic voltammograms of 2 mM Acr+ in acetonitrile
a
+
attaches in the 4-position of Acr , while the 2-hydroxy product
is formed in the case of Et-Fl (Scheme 1). Thus, Acr+
resembles Et-Fl+ only by the presence of iminium ion/
pseudobase equilibrium. This manuscript compares the electro-
chemical behavior of the two model compounds: the first part
containing 0.1 M TBAP at the (a) Pt electrode, (b) GC electrode, (c)
FTO electrode, (d) BDD electrode (scan rate: 100 mV/s). Scan
direction: blue curve, +0.5 → −1.5 → +0.5 V; red curve, +0.5 → +2.5
→ −1.5 → +0.5 V. Black curve shows a baseline scan. I /I is the ratio
+
cat
d
of currents at +2 and −1.0 V.
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fluoride-doped tin oxide (FTO), and boron-doped diamond
simple outer-sphere one-electron oxidation at 1.32 V (Figure
S2, Supporting Information). The results show that the
(
BDD) electrode. These electrodes were selected because they
66
+
+
exhibit substantially different surface chemistry. As shown in
Figure 3, the reduction potential of Acr does not depend on
calculated potentials for Et-Fl (E
= 2.44 V) are very
Et‑Fl
+
close to the experimentally observed anodic potentials on the
+
the type of the electrode used (reduction occurs at −0.9 V vs
Pt working electrode (E
= 2.40 V). However, the calculated
Et‑Fl
+
potential for Acr+ (2.02 V) is much lower than the
experimentally determined value (2.60 V). We have repeated
calculations of oxidation potential using different DFT
functionals (S2, Supporting Information), but the calculated
data show a lower oxidation potential for Acr+ than the
experimentally determined potential. On the basis of the
calculated Acr+ potential (2.02 V), we conclude that the
Ag/Ag for all four electrodes), which is indicative of an outer-
67
sphere electron-transfer process. In contrast, the oxidation
+
potential and current density of Acr is strongly electrode-
dependent: the current above 1.9 V is observed on Pt, GC, and
BDD electrodes, while no oxidation was observed on the FTO
electrode. These electrochemical results are consistent with
+
59
those observed previously in cyclic voltammograms of Et-Fl .
The strong effect of the working electrode on the anodic
+
experimental one-electron oxidation of Acr occurs with slow
+
+
oxidation of Acr and Et-Fl suggest that these iminium cations
interact with the electrode surface prior to the charge transfer
kinetics, even when the Pt working electrode is used (2.60 V).
This large difference in the calculated and experimental
67
+
(
inner-sphere electron transfer mechanism).
The current observed at ∼2 V decreases in the following
order of working electrodes: Pt > GC > BDD > FTO, as can be
potentials suggests that the interaction of Acr with the surface
of the Pt electrode is not sufficiently strong to provide fast
charge transfer kinetics. On the other hand, a good match
between the calculated and experimental oxidation potentials of
+
deduced from the I /I ratios reported in Figure 3. Thus, Acr
cat
d
+
and Et-Fl interact most efficiently with the platinum electrode
and least efficiently with the FTO electrode. This electrode-
dependent behavior has prompted us to determine computa-
tionally the standard potentials for Acr+ and Et-Fl+ in
acetonitrile using the previously developed DFT (density
+
+
Et-Fl implies a stronger interaction of Et-Fl with the Pt
surface.
+
Even though cyclic voltammograms of Acr show a strong
dependence on the working electrode material, this interaction
between Acr+ and the electrode’s surface appears to be
68
functional theory, B3LYP) methodology (Table 1, more
transient: the baseline cyclic voltammogram collected using+
the working electrode that was previously kept in the Acr
solution under an applied potential of 2.1 V for 10 min showed
no presence of any electrochemically active species adsorbed on
the electrode’s surface (Figure S1, Supporting Information).
The question that remains unanswered is, what type of
Table 1. Calculated (B3LYP/6-311++G**, with PCM
(
Acetonitrile) As Solvent) and Experimental Oxidation
+
+
Potentials of Et-Fl and Acr in Acetonitrile
oxidation potential vs NHE (V)
+
+
interaction between Acr /Et-Fl and the electrode’s surface
exists to cause the observed trend? The adsorption of organic
molecules to surfaces of several types of electrodes was
compounds
calculation
experiment
Et-Fl+
Acr+
2.44
2.02
1.32
2.40
a
a
2.60
observed previously, and the adsorption characteristics were
Me N-Acr+
1.30
69−71
2
found to strongly depend on the applied potential.
For
a
Anodic peak potentials for irreversible signals on Pt working
example, pyridine-based derivatives form flat adsorbates at
negative potentials (due to π interactions), while the
perpendicular orientation is preferred at high potentials (via
electrode.
7
2
+
information is available in section S2, Supporting Information).
nitrogen lone pairs interactions). In the case of Acr and Et-
+
For comparison purposes, the experimental and calculated
Fl , the nitrogen centers are alkylated, so the lone pair
+
potentials were also obtained for NMe -Acr , which exhibits a
interactions between these molecules and the electrode’s
2
+
+
Figure 4. Cyclic voltammograms of (a) Acr and (b) Et-Fl at different concentrations: 1 mM (black), 2 mM (brown), 3 mM (light blue), 4 mM
green), 5 mM (dark blue), 6 mM (pink), 7 mM (purple), 8 mM (orange), 9 mM (red). Inset: Plot of current vs salt concentration at 2.1 V (purple
(
+
+
line), 2.5 V (red line), and the corresponding reduction potentials (−0.1 V for Et-Fl and −1.0 V for Acr (black line); scan rate: 100 mV/s, Pt
working electrode).
2
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surface are not possible. On the other hand, the π interactions
with the electrode surface are a plausible mode of adsorption in
the case of Acr+ and Et-Fl , since the strong interactions
between positively charged organic molecules and the surface
+
71
of the gold electrode were observed at positive potentials.
Previous studies suggest that all four electrodes generate
hydroxylated surface S−OH species, where S is the electrode’s
surface. The hydroxylated species on Pt and GC electrodes are
considered to be “active,” as they tend to undergo further
66
oxidation to generate S−O species. On the other hand, BDD
and FTO electrodes are considered to be “inactive,” as they do
not undergo additional oxidation steps. Thus, it appears that
+
+
the catalytic current observed for Acr and Et-Fl is stronger on
+
electrodes that are known to generate S−O species. On the
Figure 5. Cyclic voltammograms of 2 mM Acr in acetonitrile with
varying concentrations of pH 7 water (0, 50, 100, 150, 200, 250, 300,
350, 400, and 450 mM) at Pt (b) and GC (d) electrodes. The
corresponding baseline scans are shown in panels a and c for Pt and
GC electrodes, respectively. Sweep rate: 100 mV/s. Scan direction:
+
+
basis of these findings, we hypothesize that Acr and Et-Fl
selectively interact with S−O species, which causes the
observed trend of current decrease from Pt to the FTO
electrode. However, further spectroscopic studies are needed to
address the structure of adduct formed between these iminium
ions and the surface of the electrode.
+0.5 → +2.5 → −1.0 → +2.5 V.
increase in the current above +2 V, which is equivalent to the
increase in the background current (Figure 5a and c) due to
water oxidation by the working electrodes. Thus, even though
an increase in the current at ∼2 V is observed in the presence of
Acr and maintained in the presence of water, this process does
not facilitate the electrocatalytic water oxidation. The opposite
It is interesting to mention that the Ru-based blue dimer is
also catalytically inactive on an ITO (indium−tin oxide)
7
3
electrode, which exhibits similar electrochemical behavior as
the FTO electrode. The catalysis by the blue dimer was
+
observed on the ITO electrode in the presence of an electron+
mediator. Similarly, we find that the oxidation peak of Acr
+
behavior was observed for Et-Fl , where increasing concen-
tration of water caused a significant increase in the current at
appears on the FTO electrode when ferrocene is used as an
electron mediator (Figure S6, Supporting Information). These
results indicate that the anodic current increase is absent for
iminium ions on the FTO electrode due to slow electron
transfer kinetics, and that this charge transfer process can be
accelerated by the addition of an electron transfer mediator.
The fact that Pt and GC electrodes do not require an electron
transfer mediator suggests that iminium ions interact with the
surfaces of Pt and GC electrodes in a way that facilitates the
electron transfer process.
potentials above +2 V.
+
3
.2. Bulk Electrolysis. Additional evidence that Acr does not
facilitate electrocatalytic water oxidation is obtained from the
controlled potential electrolysis (Figure 6). The oxygen
2.2. Concentration Dependence. An almost linear depend-
ence of the current on the iminium ion concentration suggests
that the rate-determining step of the process at potentials above
+
+
+
1.9 V is monomolecular with respect to Acr and Et-Fl (red
and purple-colored lines in inset plots of Figure 4). However, a
closer inspection of the cyclic voltammograms (Figure 4)
+
+
shows that the anodic peaks of both Acr and Et-Fl undergo
shape and intensity changes at increasing concentrations. While
+
+
the one-electron reduction peaks of Acr at −0.9 V and Et-Fl
at +0.1 V are almost identical (once the currents are scaled to
account for different iminium ion concentrations by plotting I/c
on the y axis), the oxidation peaks show different behavior: (i)
the Acr peak at +1.9 V and Et-Fl peaks at +1.7 and +2.1 V
undergo a decrease in the peak current, and the peak potentials
shift to more positive values at higher concentrations. These
results indicate that the electrochemical reaction is more
efficient at lower concentrations of the iminium ion, possibly
due to the fact that the catalysis is limited by the availability of
+
+
Figure 6. Oxygen evolution during controlled potential electrolysis at
+2.1 V vs Ag/AgCl of 0.1 M aqueous phosphate solution in the
presence (red) and absence (black) of 1.5 mM iminium ions. The
purple line in each panel represents the O evolution calculated from
2
+
the faradaic efficiencies (FE) reported below. (a) Et-Fl , pH = 2, FE:
0%, Pt working electrode. (b) Acr , pH = 11, FE: 55%, Pt electrode.
(c) Et-Fl , pH = 2, FE: 35%, GC electrode. (d) Acr , pH = 11, FE:
45%, GC electrode.
+
7
+
+
the oxide sites on the electrode’s surface.
. Et-Fl+ vs Acr : Differences. 3.1. Water Addition.
+
3
+
+
Despite the similarities between Et-Fl and Acr presented in
the previous section, the two iminium ions exhibit different
behavior in the presence of water. While the addition of water
to the acetonitrile solution of Et-Fl gives rise to an increase in
the current at potentials above +2 V, the Acr solution is not
evolution was measured during controlled potential electrolysis
+
+
at 2.1 V vs Ag/AgCl in aqueous Acr and Et-Fl solutions. In
the absence of iminium ions, oxygen evolution was observed
due to electrocatalytic water oxidation by the working
electrodes (GC and Pt, black curves in Figure 6). In the
+
59
+
+
+
affected (Figure 5). The cyclic voltammograms of Acr on Pt
presence of Acr , a negligible increase in the oxygen evolution
can be observed relative to the blank in the case of the Pt
and GC electrodes (Figure 5b and d) exhibit only a modest
2
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electrode, while a significant decrease was observed for the GC
rates, and their growth coincides with a decrease in the catalytic
current intensity at +2 V. Thus, these two peaks are likely
signatures of the species formed during catalysis, which can be
observed at higher scan rates, when the rate of catalysis is
slower than the rate of potential sweep. Similar peaks are
+
electrode (red curves, Figure 6b and d). Compared to Acr ,
+
electrolysis of aqueous Et-Fl solution gave rise to a noticeable
increase in the oxygen relative to the blank scans (Figures 6a
5
9
and c), which is consistent with our previous report.
+
+
+
Thus, despite the fact that Acr and Et-Fl exhibit similar
electrochemical behavior in a nonaqueous medium, these two
compounds exhibit different behavior in the presence of water:
observed in the case of Et-Fl : in addition to the two reversible
one-electron reduction peaks at −0.15 and −0.75 V observed at
all scan rates, the high scan rate (e.g., 500 mV/s) voltammo-
grams exhibit two new peaks at +0.2 and −0.4 V.
+
+
while Et-Fl facilitates electrocatalytic water oxidation, Acr
does not (even suppresses the process at the GC electrode).
Since Acr and Et-Fl+ generate pseudobase derivatives at
different pH values, we performed the experiments at two
Importantly, the same two peaks at +0.2/+0.3 and −0.4/−
0.6 V are observed at higher scan rate voltammograms of the
baseline. These peaks are particularly large in the presence of
water at pH = 7 (Figure 7a), while they are barely noticeable in
the presence of water at pH = 2 (Figure 7c). Similar peaks were
observed during anodic oxidation of water on the platinum
working electrode and are assigned to the reduction of surface
+
+
+
different pH values (pH = 2 for Et-Fl and pH = 11 for Acr ),
in order to maintain the pH at values just below the
corresponding pseudobase pK values for the two systems.
a
+
To investigate whether Acr exhibits different behavior at other
74,75
pH values, we performed the controlled potential electrolysis at
other pH values (pH = 2 and 7). The results of this study
oxides.
On the basis of these reports, we assign the peaks at
+0.2 and −0.4 V to the reduction of surface oxides S−O and
S−OH that are formed as intermediates during the anodic
oxidation of water at the Pt working electrode. Since the loss of
catalysis in baseline scans occurs at the same scan rates and with
the same intermediates as in the presence of iminium ions, we
conclude that the rate-determining step in electrode-assisted
catalysis by iminium ions involves the reaction of surface oxides
at the electrode’s surface with iminium ions.
+
showed that Acr does not facilitate the oxygen evolution at
either of these pH values (Figure S3, Supporting Information).
3.3. Scan Rate Dependence. The investigation of dynamics
of the electrocatalytic process was attempted using scan-rate
+
+
dependent cyclic voltammetry of Et-Fl and Acr in the 0.01 to
V/s range (Figure 7). At low scan rates (e.g., 30 mV/s), the
5
The scan-rate dependent voltammograms were analyzed
using a model in which the reversible charge transfer is followed
by a catalytic reaction, as follows:
+
Im ⇌ Im2+ + e− ket
(1)
Im2 + R → I + + O kcat
+
m
(2)
where I + is one of the iminium ions (Et-Fl or Acr ) and R/O
+
+
m
is the system being oxidized during catalysis (e.g., S−OH to S−
O). The theory for the above model is well-known for cyclic
76
voltammetry, and the kcat values are usually estimated by
−1/2
obtaining the ratio of I /I as a function of v
(where Icat is
cat
d
the current during catalysis, I is the peak current for reversible
d
electron transfer in the absence of the substrate R, and v is the
scan rate). A particularly simple relationship is obtained when
the rate of catalysis is significantly higher than the scan rate: the
current is directly proportional to kcat and independent of the
scan rate. This limiting condition was used previously to
evaluate rates of catalysis for other water oxidation
Figure 7. Scan rate dependent cyclic voltammograms of 1.5 mM Acr+
+
(
b) and 1.5 mM Et-Fl (d) in acetonitrile with 450 mM water (pH = 7
+
+
for Acr and pH = 2 for Et-Fl ) at the Pt electrode. Corresponding
baseline scans in the absence of iminium ions are shown in a for
acetonitrile containing 450 mM pH = 7 water and c for acetonitrile
containing 450 mM pH = 2 water. Scan rate: 30 mV/s (black), 500
mV/s (red). Electrolyte: TBAP.
56,77,78
catalysts.
+
+
In the case of Et-Fl and Acr assisted catalysis, the limiting
conditions were not achieved experimentally, likely due to small
kcat values (even at very low scan rates, the current was still
dependent on scan rate). Due to this complication, the
current at potentials above +2 V is large, with baseline-
+
subtracted I /I ratios reaching values above 20 (for Acr ) and
8
estimates of kcat values were obtained by comparing our
cat
d
+
−1/2
(for Et-Fl ). These results suggest that the low-scan rate
experimental I /I vs v
plots with the plots simulated using
cat
d
provides sufficient time for the electrocatalysis to occur, as can
be concluded by the recovery of the iminium ion reduction
peaks in the reverse scan and absence of any additional
reduction peaks. In contrast, the higher scan rates (e.g., 500
mV/s) lead to a decrease in the current above +2 V and the
appearance of additional reduction peaks during the reduction
scan, both of which suggest a loss of catalytic activity. For
the model presented in eqs 1 and 2 (simulation details are
presented in section S4 of the Supporting Information). The
experimental values for Icat were obtained by subtracting the
baseline current obtained in the absence of iminium ions.
Both model compounds exhibit an increase in I /I values as
cat
d
−1/2
the scan rate is decreased (increasing v
values, Figure 8). At
−1/2
−1/2
high scan rates (v
∼ 0.45 (V/s) ), the I /I values for
cat
d
+
+
+
example, the return scan upon oxidation of Acr at potentials
Et-Fl and Acr are ∼2, suggesting that iminium ions undergo a
above +2 V generates three reduction peaks, at +0.3, −0.6, and
two-electron oxidation process prior to the rate-determining
electrocatalytic step. At low scan rates (v− = 22.3 (V/s) ),
1/2
−1/2
−
0.9 V. While the reduction peak at −0.9 V is present at all
+
+
scan rates and is assigned to the one-electron reduction of Acr ,
the I /I ratios at +2.5 V increase to values of ∼25 for Acr
cat
d
+
the peaks at +0.3 and −0.6 V are present only at higher scan
and ∼10 for Et-Fl , suggesting that the rate of the
2
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ACS Catalysis
Research Article
Ru-based catalyst, it is important to keep in mind that the two
values were obtained at substantially different potentials (2.1 V
+
+
+
vs Ag/Ag in the case of Et-Fl and 1.3 V vs Ag/Ag in the case
77
of Ru-based catalyst).
.4. Spectroelectrochemistry. Previous spectroelectrochem-
ical studies on Et-Fl provided valuable information regarding
the intermediates formed during the electrocatalytic process.
For example, oxidation of Et-Fl in nonaqueous acetonitrile was
monitored by the appearance of the Et-Fl absorption band at
75, 440, and 500 nm. In the case of Acr , no clear evidence for
the formation of Acr could be obtained (Figure 10a). Anodic
oxidation of Acr in the absence of water exhibited a slow
decrease of the Acr bands at 425 nm. Even after 2 min of
controlled potential electrolysis at 2.1 V vs Ag/Ag , more than
0% of the initial Acr absorption was still observed. This
behavior is in sharp contrast to Et-Fl , where most of the Et-
3
+
59
+
2+
+
2
2+
+
+
+
+
6
+
59
−
1/2
Figure 8. Baseline subtracted I /I ratios versus v
at 2.1 V (black
+
cat
d
Fl absorption decayed within 1 min of electrolysis under
+
+
line), 2.3 V (purple line), and 2.5 V (red line) vs Ag/Ag . (a) Acr in
acetonitrile and 450 mM pH = 7 water mixture. (b) Acr in
acetonitrile. (c) Et-Fl in acetonitrile and 450 mM pH = 2 water
mixture. (d) Et-Fl in acetonitrile (electrolyte: 0.1 M TBAP, Pt
identical experimental conditions. We attribute this behavior to
+
−1
+
+
the fact that k = 10 s for Acr is 10 times larger than k = 1
cat cat
−1
+
+
s
for Et-Fl . The higher rate of catalysis causes faster recovery
+
of Acr , giving the appearance of slower decay of its absorption
bands. For the same reason, no identifiable absorption bands
for oxidized Acr2+ were observed, even though a weak shoulder
can be observed in the 500−650 nm range (Figure 10a).
Previous studies on Et-Fl indicated that oxidized Et-Fl
reacts with water to form a hydroxylated Et-FlOH derivative.
working electrode, c = 1.5 mM).
+
electrocatalytic process is higher in the case of Acr . Despite
+
faster electrocatalysis by Acr , this process is not affected by the
+
2+
59
presence of water: the I /I values are almost identical in the
cat
d
+
absence (Figure 8b) and presence (Figure 8a) of water. On the
+
+
In this study, spectroelectrochemical analysis of Acr oxidation
in the presence of water did not reveal the formation of an
other hand, the I /I ratios of Et-Fl (Figures 8c and d) are
cat
d
increased in the presence of water, particularly at 2.1 V
+
+
+
analogous AcrOH derivative (Figure 10b). The oxidation leads
potential. These results clearly indicate that Acr and Et-Fl
+
to a decrease of Acr absorption bands at 370 and 425 nm
catalyze two different electrochemical processes.
To evaluate the rate constants k for electrocatalysis by Acr+
without the appearance of any new absorption bands in the
visible range. To identify the spectroscopic signature of
AcrOH , chemical oxidation of AcrOH was performed in the
cat
+
and Et-Fl , scan-rate dependent experiments were compared
+
with the simulated data (Figure 9). The simulations were
−1
presence of Cu(II) ions (Figure 10d). The oxidation generated
performed for kcat values in the 1−100 s range (Figure 9c),
and the details of simulation are presented in the Supporting
Information. The comparison of the experimental and the
simulated I /I ratios suggest that k for electrocatalysis at 2.1
+
AcrOH with the absorption band in the 600−800 nm range,
which is qualitatively similar to the previously reported
79
absorption of the triphenylamine radical cation. Since anodic
cat
d
cat
+
−1
−1
+
+
V by Acr is ∼10 s and ∼1 s in the case of Et-Fl . Similar
rate analysis was previously reported for the Ru-based catalyst
oxidation of Acr in the presence of water does not exhibit
detectable absorption in the 600−800 nm range, we conclude
7
7
+
by Meyer et al., where the kcat value was found to increase
that the AcrOH ion is not formed.
from 0.003 s− in aqueous solution to a value of 1 s in a
1
−1
Two possible scenarios could explain the observed lack of
+
water−propylene carbonate solvent mixture. Even though k
=
AcrOH formation. One explanation is that the pseudobase pKa
cat
−1
+
1
s
observed for Et-Fl is equivalent to that reported for the
value for the formation of AcrOH (pK = 11, Figure 1) is much
a
−
1/2
+
+
Figure 9. Experimental and simulated I /I values vs v
for 1.5 mM Acr (panel a) and Et-Fl (panel b) on Pt electrode in acetonitrile and 450
cat
d
+
+
mM water (pH = 2 for Et-Fl and pH = 7 for Acr ) solution at different potentials: 2.1 V (black), 2.3 V (purple), 2.5 V (red).
2
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ACS Catalysis
Research Article
+
+
structural differences between Acr and Et-Fl , both model
compounds exhibit a noticeable current increase at potentials
above +1.9 V, and the intensity of this current is strongly
dependent on the working electrode material. (ii) The
electrocatalytic water oxidation was observed only in the
+
+
presence of Et-Fl , while Acr electrochemistry was not affected
by the introduction of water. On the basis of these findings, we
+
conclude that electrocatalysis by Et-Fl involves two important
processes, one of which is the interaction of the iminium ion
with the electrode surface. This interaction is strongly surface-
dependent and gives rise to the current at potentials above +1.9
+
V. The structurally simpler Acr model preserves this behavior.
The second process involves the electrocatalytic oxidation o+f
water, and this important aspect is not preserved in the Acr
model.
Our experimental data do not provide sufficient information
on what is the cause of a current increase observed for both
iminium ions in acetonitrile. One possible explanation is that
+
+
Acr and Et-Fl catalyze a certain electrochemical process and
that this process does not involve water molecules. Since amine
radical cations tend to perform H atom abstractions
80,81
efficiently,
the electrochemical process in question could
be the H atom abstraction by oxidized iminium ions from
hydroxylated electrode surface (S−OH → S−O + H) or,
•
•
alternatively, from the solvent (CH CN → H + CH CN).
3
2
+
The process would be monomolecular with respect to Acr , as
observed using the concentration-dependent experiments
(
=
Figure 4), and the rate of H atom abstraction would be k
10 s , as obtained using the scan-rate dependent
cat
−
1
experiments (Figure 9). Furthermore, the process would not
be affected by the addition of water. Our attempts to
characterize the oxidized products of acetonitrile (such as
succinonitrile) after bulk electrolysis were not successful, as no
Figure 10. UV−vis spectra collected upon electrochemical oxidation
+
+
+
1
of (a) 2 mM Acr at 2.4 V vs Ag/Ag in acetonitrile; (b) 2 mM Acr in
product was detectable by H NMR.
+
+
acetonitrile with 20 M pH 7 water at 2.4 V vs Ag/Ag ; (c) 2 mM Acr
The second possible explanation for the observed current
increase at potentials above +1.9 V could be the adsorption/
desorption of iminium ions to/from the electrode surface. For
example, it is known that oxidative or reductive adsorption and
desorption occurs efficiently in some classes of organic
+
in acetonitrile with 20 M pH 11 water at 2.4 V vs Ag/Ag , 0 (red), 2
(
4
brown), 5 (lime green), 10 (green), 17 (light blue), 30 (dark blue),
5 (purple), 60 (pink), 75 (orange), and 90 s (black); (d) UV−vis
absorption changes upon chemical oxidation of 1 mM AcrOH using
.5 mM Cu(ClO ) ·6H O, 0 (red), 2 (brown), 5 (lime green), 10
1
(
4
2
2
8
2,83
+
compounds, such as thiols.
In the case of Acr and Et-
green), 17 (light blue), 30 (dark blue), 45 (purple), 60 (pink), and 75
+
s (black).
Fl , the increase in the current could be due to oxidation of the
adsorbed iminium ion on the electrode’s surface. The lack of
the return peak during the cathodic scan can be explained by
desorption of the iminium ion after oxidation. In the case of Et-
Fl , the adsorbed iminium ion film catalyzes the water
oxidation, while the adsorbed Acr film exhibits no catalytic
higher than the pseudobase pK for the formation of Et-FlOH
a
5
9
+
(
pK = 3.5). To investigate whether AcrOH formation occurs
a
+
+
at higher pH values, the anodic oxidation of Acr was
performed in the presence of pH = 11 water. Since the
formation of AcrOH+ was not observed (Figure 10c), we
+
behavior. Our future experiments will investigate the electro-
+
+
conclude that the difference in pseudobase pK values cannot
chemical adsorption/desorption of Et-Fl and Acr in more
detail.
a
+
be used to explain the lack of AcrOH formation.
+
The second explanation for the lack of AcrOH formation
It is generally considered that molecular catalysts offer
mechanistic insights into the water oxidation mechanism by
their heterogeneous analogs. While transition-metal homoge-
neous catalysts mimic the catalysis by heterogeneous metal
oxides, the organic iminium ions presented here are the model
systems for water oxidation by N-doped graphene electro-
+
involves the fact that kcat for Acr in the absence of water is 10
+
times higher than k for Et-Fl . This high rate of electro-
cat
2+
+
catalysis by Acr quickly regenerates Acr and does not provide
sufficient time for Acr to react with water and generate
detectable amounts of AcrOH . We postulate that the lack of
AcrOH formation and the absence of electrocatalytic water
2
+
+
+
84
catalysts. The N-doped carbon-based materials have been
+
oxidation by Acr are due to a high rate of electrochemical
extensively investigated for electrocatalytic oxygen reduc-
process catalyzed by Acr+ in the absence of water. The
following section proposes the chemical origin for this
85−87
tion,
and more recently for the reverse reaction, the
84,88,89
electrocatalytic water oxidation.
The studies of water
electrocatalytic process.
oxidation by N-doped graphene have identified pyridinic and
+
3
.5. Possible Reasons for Different Behaviors of Acr and
quarternary nitrogen centers as the crucial components of the
+
89−91
EtFl . Two important conclusions can be drawn from the
catalytic site,
but a detailed understanding of the
experimental data presented above: (i) Despite significant
mechanism is lacking due to the difficulties associated with
2
642
dx.doi.org/10.1021/cs5005135 | ACS Catal. 2014, 4, 2635−2644

ACS Catalysis
Research Article
the identification of the intermediates formed at the
heterogeneous catalyst. We anticipate that the studies of well-
defined molecular systems, such as iminium ions presented
here, will provide useful insights into electrocatalytic water
oxidation by N-doped graphitic materials.
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The electrochemical behavior of two iminium ions (Acr and
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(
The presented molecular systems will likely serve as models
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ASSOCIATED CONTENT
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AUTHOR INFORMATION
Corresponding Author
E-mail: kglusac@bgsu.edu.
■
1
(
*
Notes
(
The authors declare no competing financial interest.
(
ACKNOWLEDGMENTS
This work was supported by National Science Foundation
CHE-1055397 to K. D. G and DMR-1212842 to C. M. H.).
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dx.doi.org/10.1021/cs5005135 | ACS Catal. 2014, 4, 2635−2644