Electrocatalytic reduction of dioxygen by Mn(iii): Meso -tetra(N -methylpyridinium-4-yl)porphyrin in universal buffer

Article abstract of DOI:10.1039/c9dt01436e

The electrochemical characterization of manganese(iii) meso-tetra(N-methylpyridinium-4-yl)porphyrin pentachloride ([Mn(TMPyP)Cl][Cl]₄) via cyclic voltammetry (CV) and UV-vis spectroelectrochemistry (UV-vis SEC) was performed across the entire p

Full text of DOI:10.1039/c9dt01436e

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Electrocatalytic Reduction of Oxygen by Mn(III)
meso-Tetra(N-methylpyridinium-4-yl)porphyrin in
Universal Buffer
Lauren E. Lieske, Shelby L. Hooe, Asa W. Nichols, and Charles W. Machan*
Department of Chemistry, University of Virginia, McCormick Road, P.O. Box 400319,
Charlottesville, VA, 22904-4319, United States
The electrochemical characterization of manganese (III) meso-tetra(N-methylpyridinium-4-
yl)porphyrin pentachloride ([Mn(TMPyP)Cl][Cl]4) via cyclic voltammetry (CV) and UV-vis
spectroelectrochemistry (UV-vis SEC) was performed across the entire pH domain in aqueous
buffered conditions. Assessment of the homogeneous electrocatalytic efficiency for the oxygen
reduction reaction (ORR) from pH 3 to 6 using rotating-ring disk electrode experiments (RRDE)
found it to be selective for water (82 to 93%). The observed efficiency for water is in contrast to
previous reports on electrocatalytic ORR activity by Mn porphyrins in aqueous systems, which
identified H2O2 as the primary product using indirect RDE methods only. The results described
here are consistent with recent reports on the electrocatalytic behavior of Mn porphyrins under
nonaqueous conditions, where the similar selectivity for water was also determined by RRDE
methods. At pH 1, UV-vis SEC experiments also revealed that decomposition was occurring; free-
base porphyrin was observed after the application of reducing potentials.
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Introduction
The reduction of dioxygen (O2) to water is of fundamental interest to the study of renewable
energy utilization and green chemistry.1 Biological enzymes can use O2 as a thermodynamic sink
for reducing equivalents or as a chemical oxidant2 and selective oxygen reduction has been
continuously studied as the cathodic half-reaction in fuel cells. The use of O2 as an oxidant also
continues to attract interest in commodity chemical synthesis.3 The variability and geographic
dependence of renewable energy sources like wind and solar has generated specific interest in the
storage of energy from renewable resources in chemical bonds via electrochemical ‘water
splitting’. Water splitting converts two equivalents of H2O to one of O2 (through an oxidation half-
reaction) and two equivalents of H2 (through a reduction half-reaction). Reversing this conversion
electrochemically is the basis for the simplest possible fuel cell, enabling the use of the stored
renewable energy on demand.4 The O2 reduction reaction (ORR) is a kinetically difficult reaction
and fundamental mechanistic studies on model compounds5 have been used to inform new catalyst
development and shift away from Pt metal, the best cathodic electrode material known.6
In aqueous solutions, the electrochemical reduction of O2 can proceed either by a two-
electron/two-proton pathway to yield hydrogen peroxide (H2O2), a four-proton/four-electron
pathway to yield water, or by a stepwise (2+2) mechanism which yields H2O via two-electron/two-
proton reduction of the intermediate H2O2.7 In fuel cells, the generation of peroxide and reactive
oxygen radical species is known to degrade proton exchange membranes, reducing the stability of
the cell and making selective reduction to H2O a priority for both efficiency and device lifetime.8
Developing catalysts with earth-abundant metals with high selectivity for the ORR would address
a fundamental need for cost-effective materials.
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Due to the highly selective nature of O2 reduction to H2O by the iron porphyrin active site in
cytochrome c oxidase, porphyrin-based ligand scaffolds have been developed in order to study the
reactivity of synthetic metal complexes with O2.9 To date, iron- and cobalt-based porphyrin
systems have received significant attention for homogenous ORR catalytic studies, with
comparatively less known about their manganese analogues.7 Herein we report the electrochemical
characterization of the water-soluble manganese (III) meso-tetra(N-methylpyridinium-4-yl)
porphyrin pentachloride ([Mn(TMPyP)Cl]4+) from pH 1-14 in aqueous buffered conditions via
cyclic voltammetry (CV), UV-vis spectroelectrochemistry (SEC), and rotating-ring disk electrode
(RRDE) studies. Under acidic conditions (above pH 1) and in the presence of O2,
[Mn(TMPyP)Cl]4+ functions as a competent electrocatalyst for the 4e- pathway to water, with
efficiencies from 82 to 93% established by rotating ring-disc electrode RRDE experiments.
Previous studies on this catalyst reported that [Mn(TMPyP)Cl]4+ was active for the
electrocatalytic reduction of O2 with H2O2 as the major product.10, 11 However, studies on the
related [Fe(TMPyP)]5+ system reported that selectivity for the four-electron reduction to water was
dependent on low catalyst concentrations and the involvement of a heterogeneous response from
catalyst adsorption on the electrode.7, 12-14 A recent report on the activity of [Mn(porphyrin)]+
derivatives for the electrocatalytic reduction of O2 to H2O under non-aqueous conditions motivated
us to re-examine [Mn(TMPyP)Cl]4+ under aqueous conditions.15 With [Mn(TMPyP)Cl]4+ in
Britton-Robinson buffer as the electrolyte solution,16 from pH 3 to 6, our results indicate catalytic
selectivity for H2O instead of H2O2. Unlike [Fe(TMPyP)]5+, control testing showed electrode
adsorption did not occur, meaning that the observed catalytic response for [Mn(TMPyP)Cl]4+ is
homogeneous in nature. The disparity between our results and those previously published appear
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to arise from experimental methodology: prior reports used indirect RDE methods,10-11 while we
have directly quantified H2O2 production using RRDE methods.
Results and Discussion
In initial CV studies of [Mn(TMPyP)Cl]4+ (Figure 1) under Ar saturation, the E1/2 values for
the Mn(III)/(II) reduction were determined from pH 1-14. Britton-Robinson buffer,16-17 adjusted
with a NaOH solution, (Table S1) was used as a universal buffer system from pH 2-13 in order to
maintain a similar reaction environment for each set of experimental conditions.18 This
necessitated a systematic comparison to previous reports, in order to establish that the behavior in
the universal buffer system did not differ substantively. Nota bene, the equilibrium displacement
reaction of Cl– by H2O is known to occur in aqueous solution, resulting in the formation of the
diaquo species [Mn(III)(TMPyP)(OH2)2]5+.19 Based on the reported pKa1 and pKa2 values of 8.0
and 10.6 for [Mn(III)(TMPyP)(OH2)2]5+,19-20 by pH 11 the aquo ligands should both be partially
deprotonated to generate the Mn-dihydroxo species [Mn(III)(TMPyP)(OH)2]3+.19, 21
Figure 1. (A) Structure of the solid state water-soluble manganese (III) meso-tetra(N-
methylpyridinium-4-yl) porphyrin pentachloride ([Mn(TMPyP)Cl]4+) (B) Diaquo species
expected to predominate in aqueous solution, [Mn(TMPyP)(OH2)2]5+.
At all pH values greater than 2, the ratio of ipc/ipa values were around 1 for the Mn(III)/(II)
couple, however the peak-to-peak separation (ΔE) was consistent with a quasi-reversible redox
event with ΔE > 60 mV (Tables S2-S14; Figures S1-S13).22 We note that the large observed ΔE
can be attributed in part to the anticipated structural reorganization of the Mn center undergoing
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reduction from d4 to d5: previous reports with Mn porphyrin complexes have established that the
coordination environment of the parent species goes from six-coordinate to five-coordinate upon
reduction.19-21 At pH 2 and below, the Mn(III)/(II) redox feature becomes irreversible, restricting
the analysis of the pH-dependent standard potentials, E1/2, under Ar saturation to pH 3-14 (Figure
S14). Notably, the quasi-reversible shape of Mn(III/II) redox couple from pH 3-13 (Figure S14)
is characteristic of waveforms which fall under the KO regime of the CE kinetic zone diagram.22-
23
We attribute this to pre-equilibrium processes involving multiple metal-ligand (buffer
components, Cl– and H2O) exchange reactions that are dependent on the underlying Mn aquo acid-
base equilibria. The increased asymmetry of ΔE and decreasing ipc/ipa values of the Mn(III)/(II)
reduction at higher scan rates are consistent with the interception of a Keq process for a CE reaction
(Tables S2-S14; Figures S1-S13).21-23
The potential-pH plot shown in Figure 2, revealed two distinct regimes for the Mn(III)/(II)
reduction. The first regime (Figure 2, black) was located from pH 3-10 where the E1/2 was
independent of pH, which is consistent with previous results.10, 21, 24-25 In this region the reduction
of Mn(III)/(II) is not dependent on proton transfer and can be described by eq (1) for 3 ≤ pH < 8.0
and eq (2) for 8.0 < pH < 10.6
Figure 2. Potential-pH plot constructed from CV data obtained on the Mn(III)/(II) (black and blue)
and Mn(IV)/(III) (green) reduction features of [Mn(TMPyP)Cl]4+ in Britton-Robinson buffer
conditions under Ar saturation.
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5 +
4 +
―
(
)
(
[
(
)
]
[
[
]
(1)
(2)
푀푛(퐼퐼퐼)퐿 푂퐻2
+ 푒 ⇌ 푀푛(퐼퐼)퐿 푂퐻2
+ 퐻2푂
2
4 +
푀푛(퐼퐼퐼)퐿(푂퐻) 푂퐻2
+ 푒 ― ⇌[푀푛(퐼퐼)퐿(푂퐻)]3 + + 퐻2푂
)
]
where L refers to the porphyrin ligand TMPyP. The second regime extends between pH 11 and 14,
where the E1/2 shifts towards more reducing potentials with a slope of 39 mV/pH (Figure 2, blue).
The Nernstian dependence deviates slightly from 59 mV/pH for Mn(III)/(II) across this pH range,
which suggests a 1H+/1e– reaction with slow proton transfer kinetic parameters or a competing
equilibrium process. The proposed electrochemical reaction is described by eq (3) for 10.6 < pH
< 14.19 Another contributing factor in this region may be an additional acid-base equilibrium of
the product Mn(II) species formed at more basic conditions, [Mn(II)(TMPyP)(OH)]3+; the pKa1 of
this species has previously been characterized as 12.4.19
+ 퐻 + /푒 ― ⇌[푀푛(퐼퐼)퐿(푂퐻)]3 + + 퐻2푂
3 +
[
푀푛(퐼퐼퐼)퐿(푂퐻)2
]
(3)
CVs obtained from pH 10-14 also revealed a second feature at more positive potentials, which
previous reports assign to a Mn(IV)/(III) reduction (Figure S15).24, 26-27 This redox wave shifts
from quasi-reversible at pH 10, where the two species ([Mn(III)(TMPyP)(OH2)(OH)]4+ and
[Mn(III)(TMPyP)(OH)2]3+) should exist, to reversible from pH 11-14 (Tables S15).19, 21 The
known pKa values of the Mn(IV) porphyrin should establish two regions for potential-pH
dependence involving Mn(IV)/(III) (7.5 < pH < 10.7 and 10.7 < pH ≤ 14), which have good overlap
with those of the Mn(III) species. The Mn(IV)/(III) reduction feature also shifts towards negative
potentials at higher pH values from pH 11 to 14, with a slope of 97 mV/pH suggesting a 2H+/e–
process as described by eq (4) and (5) (Figure 2, green). Limited electrochemical reversibility
restricts our analysis to the second region only, however.28-30 The Nernstian dependence of E1/2 on
pH in this region appears to describe a 2H+/e– reaction more than a H+/e– one, for which both eq
(4) and (5) have been proposed previously,21 but again deviation from the expected 118 mV/pH
occurs. It should be noted that based on known pKa values, the putative product in eq (4) should
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not be stable, the dihydroxo species [Mn(III)(TMPyP)(OH)2]3+ is expected under these conditions.
Conversely, eq (5) proposes a Mn(IV) dioxo species where only one double bond is possible based
on d electron count.31 One possible explanation for the observed deviation from 118 mV/pH is a
dimerization reaction involving the Mn(IV) porphyrin to generate [[Mn(IV)(TMPyP)]2(µ2-O)]10+,
evidence of which is observed spectroscopically in the broadening of the Soret band, vide infra
(Figure 3B).21, 32-33 Clearly, the competing acid-base and dimerization equilibria correspond to a
complex reaction environment where it is difficult to assign speciation unequivocally. The general
consensus in published reports19-21, 28, 30, 32, 34-36 is to formulate the monomeric Mn(IV) species at
pH > 11 as [Mn(IV)(TMPyP)(O)(OH)]3+, which suggests that the potential-pH slope for the
Mn(IV)/(III) reduction from pH 11 to 14 should describe a H+/e- reaction, eq (6); the observed
deviation from 59 mV/pH is then likely to be a consequence of the competing dimerization
equilibrium eq (7).
3 +
+
―
[푀푛(퐼푉)퐿(푂)(푂퐻)]3 + +2퐻 /푒 ⇌ 푀푛(퐼퐼퐼)퐿(푂퐻)(푂퐻2)
[
]
(4)
(5)
(6)
(7)
2 +
3 +
+
―
[
]
[
]
푀푛(퐼푉)(푂)2
+ 2퐻 /푒 ⇌ 푀푛(퐼퐼퐼)퐿(푂퐻)2
3 +
+
―
[푀푛(퐼푉)퐿(푂)(푂퐻)]3 + + 퐻 /푒 ⇌ 푀푛(퐼퐼퐼)(푂퐻)2
[
]
10 +
2[푀푛(퐼푉)퐿(푂)(푂퐻)]3 + ⇌ [푀푛(퐼푉)퐿]2 휇2푂
+2푂퐻 ―
[
(
)
]
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Figure 3. UV-vis SEC of 0.1 mM [Mn(III)(TMPyP)Cl]4+ in a pH 10 Britton-Robinson Buffer
solution (A) under reducing conditions and (B) under oxidizing conditions. Conditions:
Honeycomb platinum electrode and a Ag/AgCl sat’d KCl aqueous reference electrode corrected
to NHE.
UV-vis spectroscopic studies were subsequently used to evaluate the stability of
[Mn(TMPyP)Cl]4+. UV-vis absorbance data were collected with equimolar amounts of the free-
base porphyrin and [Mn(TMPyP)Cl]4+ at pH 1 and pH 14, using 1 M HCl/0.1 M NaCl and 1 M
KOH/0.1 M NaCl, respectively: no demetallation was observed under these conditions (Figure
S16). Analysis of [Mn(TMPyP)Cl]4+ from pH 1 to 14 by UV-vis also revealed a decrease in the
absorbance intensity of the Soret band (462 nm), with the concomitant appearance of a band at
453 nm at pH values greater than 10.6 consistent with the diaquo species undergoing two pKa
transitions to form [Mn(III)TMPyP(OH)2]3+ (Figure S17-S18).19, 24, 26
With this information in hand, UV-vis spectroelectrochemical (SEC) studies were conducted
under an Ar atmosphere to assess the speciation in solution as a function of potential and pH. At
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pH 1, as the potential is moved to more reducing potentials from +0.40 to +0.10 V vs NHE to
examine the Mn(III)/(II) reduction, a decrease in the intensity of the Soret band at 462 nm with a
corresponding blue shift from 462 to 440 nm occurs along with a blue shift in the Q band from
560 nm to 502 nm (Figure S19). Upon returning the potential to 0.40 V vs NHE, UV-vis spectra
consistent with free base TMPyP are observed (Figure S16A, S19). These results indicate that
upon reduction at +0.10 V vs NHE, [Mn(II)(TMPyP)(OH2)]4+ is unstable under reducing
conditions and de-metalation occurs. This is consistent with previous reports that suggest the
decomposition of analogous Mn porphyrins at a pH of 2 and below.34-35 We note that with the
Universal Buffer, the same Mn(III)/(II) reduction at a pH of 3 shows some degradation: moving
from +0.20 V to -0.10 V vs NHE caused a shift in the Soret band from 463 nm to 450 nm that was
only partially reversible after 4 minutes at reducing potentials (Figure S20A). Returning the cell
potential to +0.20 V vs NHE resulted in the appearance of a new band at 422 nm and restored only
70% of the original absorbance intensity at 463 nm, consistent with partial demetallation (Figure
S20A). For comparison, when the Soret band is monitored during the same Mn(III)/(II) reduction
at pH 7, a blue shift is still observed as reducing potentials are applied stepwise from +0.20 to -
0.10 V vs NHE (Figure S20B). Under these pH conditions, however, this process is reversible:
returning the potential to +0.20 V vs NHE restores the features of the original absorbance spectrum
and no new bands are observed.
Following spectral changes in the absorbance of the Soret band during the Mn(III)/(II)
reduction at pH 10 revealed a new band at 448 nm with the disappearance of the parent species at
462 nm, suggesting the quantitative formation of [Mn(II)(TMPyP)(OH)]3+ (Figure 3A; eq (2)).19
Conversely, as the potential is moved stepwise to more oxidizing potentials from +0.35 to +0.80
V vs NHE to probe the changes corresponding to the Mn(IV/III) reduction, a blue shift of the Soret
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band is observed with the appearance of a new band at 427 nm (Figure 3B), which is indicative
of the formation of [Mn(IV)(O)(TMPyP)(OH)]3+, Figure 1, eq (4)-(6). It is worth noting here
that Mn(IV) porphyrins are known to form µ2-oxo-dimers [(Mn(IV)(TMPyP)(OH))2(µ2-O)]8+ in
alkaline solutions eq (7) (Figure 3B).32-33 The large half-width observed for the Soret band at 427
nm in this experiment is characteristic of this type of Mn-µ2-oxo-dimer and is observed at pH
values at or greater than 10 with the formation of a Mn(IV) oxidation state.32-33
At pH 14 as the potential is moved stepwise from +0.10 V to -0.20 V vs NHE to probe
Mn(III)/(II) reduction, a shoulder appears on the Soret band at 416 nm along with a concomitant
red shift in the Q band, which suggests the generation of [Mn(II)TMPyP(OH)]3+ (eq (3); Figure
S21A).19 N.B. the pKa2 of the parent Mn(III) diaquo is 10.6.19 As the potential is moved stepwise
to more oxidizing potentials from +0.15 to +0.50 V vs NHE to observe changes corresponding to
the Mn(IV/III) reduction,
a
shoulder at 425 nm appears, representative of
[Mn(IV)(TMPyP)(O)(OH)]3+ eq (5) and the corresponding formation of the
[(Mn(IV)(TMPyP)(OH))2(µ2-O)]8+ dimer based on the broadening of this feature (Figure S21B).19
These speciation results are generally consistent with prior reports, suggesting that the use of the
universal buffer system does not have significant consequences on the electrochemical behavior
of the parent [Mn(TMPyP)Cl]4+ species.19-21
Based on the observed speciation of [Mn(TMPyP)Cl]4+, we were interested in observing if the
distinct potential-pH regimes would have a significant effect on its product selectivity, efficiency
or activity for the electrocatalytic ORR. [Mn(TMPyP)Cl]4+ was initially examined via CV studies
under variable O2 saturation conditions (max [O2] = 1.3 mM7, 37) across all pH values (Figure S22-
35). At pH 1, minimal increases in catalytic current were observed with changes in [O2] (Figure
S22). Between pH 2-14, however, current increased proportionally to the concentration of O2,
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indicative of a catalytic response (Figure S23-35). A series of control CVs obtained at all pH
values demonstrated that a heterogenous reduction response from the electrode occurred at more
negative potentials than the molecular regime in all cases (Figure S36-S49). A plot of the catalytic
response for ORR at each pH showed two regimes based on pH (Figure 4). From pH 3 to pH 10,
the catalytic response shifted to positive potentials with increasing pH. However, from pH 11 to
14, a shift back towards negative potentials occurred with increasing pH values. Interestingly, the
appearance of two distinct waves was also observed in the catalytic feature for pH 7-11 (Figure
4). These pH-dependent differences in the shape of the CV wave may be suggestive of a different
mechanism occurring at more basic potentials in comparison to acidic conditions (Figure 5); it is
also noteworthy that this pH regime spans the pKa1 and pKa2 values of 8.0 and 10.6 for
[Mn(III)(TMPyP)(OH2)2]5+.19-20 It is possible that at more basic pH values concerted reduction and
protonation steps involving superoxide/peroxide-type intermediates could require more reducing
potentials than acidic conditions.
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Figure 4. Linear-sweep voltammetry of [Mn(TMPyP)Cl]4+ (partial plots of CV data),
demonstrating reactivity under O2 saturation conditions with corresponding shifts (A) from
pH 3-10 and (B) from pH 11-14. Conditions: Glassy carbon working electrode, glassy
carbon counter electrode, 3.0 M NaCl Ag/AgCl reference electrode; Scan rate 100 mV/s.
Previous studies on the [Fe(TMPyP)]5+ system in acidic conditions found that a heterogenous
pathway arising from adsorption on the electrode contributed significantly to the observed
activity.17 Likewise, non-specific adsorption was also reported in a prior study of
[Mn(TMPyP)Cl]4+.10 To ensure a similar mechanism was not occurring for [Mn(TMPyP)Cl]4+, a
rinse test was performed at pH values 1, 3, 6, 7, and 12 (Figure S50-S54). No adsorption was
observed at any of the tested pH values, suggesting a purely homogenous response.
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Figure 5. CVs of [Mn(TMPyP)Cl]4+ under Ar and O2 saturation conditions in (A) pH 6 buffer (B)
pH 12 buffer solution. Conditions: glassy carbon working electrode, glassy carbon counter
electrode, 3.0 M NaCl Ag/AgCl reference electrode; scan rate 100 mV/s.
Catalytic efficiency was subsequently determined using RRDE experiments (See Supporting
Information). These results suggested [Mn(TMPyP)(Cl)]4+ was primarily selective for the four-
electron pathway to H2O (82 to 93%) from pH 3 to 6 (Figure S55-S70, Table 1). Reliable product
analysis could not be performed at pH values outside of this range because the background disk
and ring current were larger than the observed catalytic current (Figure S71). The quasi-
reversibility of the Mn(III)/(II) reduction under non-catalytic conditions as a result of the
competing chemical equilibria described previously precludes accurate use of foot-of-the-wave
analysis (FOWA)38 and variable scan-rate studies did not establish a scan rate-independent
catalytic response because of the background electrode response at higher scan rates. In order to
estimate an upper limit for the TOFmax value of this system, we used the icat/ip ratio method as
described in the Experimental Methods.39-40 For completeness, overpotential (η) values
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calculated using the E1/2 of Mn(III)/(II) under Ar saturation conditions are included in Table 1 for
reference.
Table 1. Catalytic efficiencies, overpotentials and TOFmax determined for [Mn(TMPyP)Cl]4+ from
pH 3 to 6.
Current
electrons
pH
Efficiency
for H2O
Overpotential (η)
TOFmax (s-1)b
transferred (n)a
3
4
5
6
82 (±4) %
89 (±2) %
93 (±1) %
85 (±5) %
3.64
3.77
3.86
3.69
1.05 V
0.99 V
0.91 V
0.85 V
4.3
2.1
2.2
3.9
a – determined from current efficiencies based on RRDE data; b – determined from icat/ip ratios at
0.1 Vs-1.
To investigate the possibility of a 2+2 pathway, where H2O2 is generated and subsequently
reduced to H2O, CVs in pH 3 and 6 buffers were obtained in the presence of 1 mM and 4 mM
H2O2. Prior reports indicate that catalase-type reactivity is expected at pH 7 and higher, with the
fastest rates at the highest pH values; this background reactivity precludes accurate CV
measurements for the 2+2 pathway at pH values > 7.41-42 Minimal electrocatalytic activity is
observed for H2O2 reduction at pH 3, a region where significant catalase-type reactivity is not
expected (Figure S72).42 Only slight activity for H2O2 reduction occurs at 1 mM concentrations
at pH 6, suggesting that the contribution of the 2+2 pathway is minimal (Figure S73). A stability
test of [Mn(TMPyP)Cl]4+ in the presence of H2O2 was also conducted at pH 3 and 6, which showed
that the rate of reaction, as monitored by the Soret band, was much slower than the timescale of
our electrochemical experiments, with no observed decrease in absorbance over 30 minutes
(Figure S74-75).
At the pH values between 3 and 6, where electrocatalytic activity could be measured, the
resting species should be primarily [Mn(III)(TMPyP)(OH2)2]5+, according to our potential-pH and
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UV-vis absorbance data. Upon a single-electron reduction, the resultant [Mn(II)(TMPyP)(OH2)]4+
•–
species can bind O2 to generate a superoxide species, [Mn(III)(O2 )(TMPyP)]4+. We presume the
subsequent reduction of the superoxide to a Mn-bound hydroperoxo species is facile at the same
potential under these conditions. Although a sufficient pH to protonate the hydroperoxide anion is
present (<11.67), the Faradaic efficiencies obtained from RRDE experiments suggest that at the
operating potential, reduction and cleavage of the O–O bond to complete the overall 4e– pathway
and ultimately generate two equivalents of H2O is the dominant pathway. The less-than-
quantitative efficiencies suggest that the O–O bond cleavage step could be rate-limiting. As noted
above, contributions from catalase-type reactivity are expected to be minimal at these pH values.41-
42
Conclusion
In summary, we have electrochemically characterized [Mn(III)(TMPyP)Cl]4+ and assessed its
electrocatalytic behavior with respect to O2 reduction through the entire pH domain. Mechanistic
studies have established that the use of the universal buffer system does not appear to have any
consequences on the observed electrochemical behavior, suggesting a comparison between this
and previous studies is reasonable. Speciation studies conducted by UV-vis absorbance and SEC
methods suggest that at pH values greater than 10, dimer formation is possible upon oxidation.
These methods also demonstrated that decomposition occurred at pH 1 after the application of
reducing potentials, which had not been previously reported with a different buffer system at this
pH.11 Where catalytic efficiency could be assessed from pH 3 to 6, efficiencies for H2O ranged
from 82 to 93% (the pH range for quantitative electrocatalytic efficiency assessment was limited
by the heterogeneous electrode response). Further studies assessing whether the O–O bond
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cleavage step is rate-determining and the kinetic parameters of the overall mechanism are the
subject of future study. The notable difference in efficiency with prior publications is attributed to
the method of reaction efficiency analysis: here direct RRDE methods were used, whereas in
previous studies reaction efficiency was assessed via indirect RDE methods that could not
accurately assess reaction efficiency.10-11
Materials and Methods
General
All chemicals and solvents (ACS or HPLC grade) were commercially available and used as
received unless otherwise indicated. Mn(III) meso-tetra(N-methylpyridinium-4-yl)porphyrin
pentachloride was obtained from Frontier Scientific. Gas cylinders were obtained from Praxair (Ar
as 5.0; O2 as 4.0). Gas mixing for variable concentration experiments was accomplished using a
gas proportioning rotameter from Omega Engineering (O2 saturation in aqueous solutions is 1.3
mM7, 37). UV-vis absorbance spectra were recorded on a Cary 60 from Agilent. A Mettler Toledo
(LE438) pH probe using a glass electrode calibrated with standard buffers (pH 4.00, 7.01 and
10.01) was used to determine the aqueous pH of solutions used in all experiments in this study.
Aqueous Buffer Solutions16-17
The buffer solution utilized to conduct the majority of the electrochemical studies from pH 2-13
was the “universal” Britton-Robinson buffer.17 The Britton-Robinson Buffer was prepared on a 1
L scale and was checked with a pH probe prior to use to ensure a pH of 2 was maintained. The
buffer solution contained 2.47 g of H3BO3 (0.04 M), 1.55 mL H3PO4 (0.04 M), and 2.29 mL
CH3COOH (0.04 M) and 1 L of DI water. Experimental solutions at other pH values were prepared
by titrating 0.2 M NaOH into Britton-Robinson stock buffer solution (described above) to reach
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the desired pH. Experiments at pH 1 used a buffer of 1.0 M HCl with 0.1 M NaCl and at pH 14 a
buffer of 1.0 M KOH and 0.1 M NaCl was used.
Electrochemistry
All electroanalytical experiments were performed using a Metrohm Autolab PGSTAT302N
potentiostat. All CV experiments used a glassy carbon working electrode (⌀ = 3 mm, CH
instruments), glassy carbon rod counter electrode (Type 2, ⌀ = 3 mm, Alfa Aesar), and 3.0 M NaCl
aqueous silver/silver chloride reference electrode (BASi MF-2052). Experiments were performed
in a modified scintillation vial (20 mL volume) as a single-chamber cell with a cap modified with
ports for all electrodes and a sparging needle. All data referenced to 3.0 M NaCl aqueous Ag/AgCl
which were then converted to NHE (+209 mV).43 Manual iR compensation was performed to
minimize ohmic drop using systematic testing to determine the optimal resistance correction for
each experiment. Rotating ring-disk electrode (RRDE) experiments were conducted using a
Metrohm rotator with a Metrohm electrode consisting of a glassy carbon disk (⌀ = 5 mm) and a Pt
ring. The collection efficiency of the RRDE electrode was experimentally determined to be
26.6%.44 The counter electrode in the RRDE experiments was a glassy carbon rod in a gas diffusion
tube (Type 2, ⌀ = 3 mm; Alfa Aesar) and the aqueous reference was a Ag/AgCl/3 M KCI electrode
(+210 mV vs NHE45) inserted in a 50 mM KCl/pure buffer salt bridge (Metrohm) system.
RRDE Experiments (adapted)44, 46
RRDE experiments were performed under both Ar and O2 saturation conditions, using 50 mL of
the appropriate Britton-Robinson buffer solution (adjusted to the appropriate pH using a 0.2 M
NaOH solution), glassy carbon disk electrode (⌀ = 5 mm), Pt ring electrode, glassy carbon counter
electrode, Ag/AgCl 3 M KCl reference electrode; scan rate 0.02 V/s.
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The working solution was sparged with argon to saturation was achieved and confirmed by CV.
The analyte [Mn(TMPyP)Cl]4+ (0.25 mM) was then dissolved in solution. A standard CV was
taken of the solution to confirm the potential window to be used for the experiment (+0.15 to -0.75
V vs. Ag/AgCl). At the beginning of the experiment, the platinum ring was set to +1.2 V vs.
Ag/AgCl and the glassy carbon disk was set to 0 V vs. Ag/AgCl. LSVs were obtained for rotation
rates between 400 and 1000 rpm under the described conditions. Between each LSV, the electrode
was sonicated in ethanol. The cleaning procedure for the electrode was standardized by taking
repeated LSVs at the same rotation rates to confirm repeated scans at the same rotation rate were
exact overlays of one another. This same procedure was repeated for O2 saturation conditions,
which were achieved by sparging the solution with O2 for approximately 15 minutes. Between
experiments with different pH values, the RRDE electrode was polished on alumina and
electrochemically pretreated in a 0.5 M sulfuric acid solution where the ring was held at +1.6 V
vs. Ag/AgCl for three minutes and then -0.3 V vs. Ag/AgCl for three minutes. This was then
repeated, and the ring was then held at +0.2 V vs. Ag/AgCl for 2 minutes prior to use.
For all RRDE data analysis, the background current generated at the experimental pH under
argon and O2 were obtained and subtracted from the data obtained with the analyte solution at the
corresponding rotation rate under otherwise identical conditions. The RRDE data plotted in the SI
reflect this background current-corrected data. The difference between the corrected values for the
amount of current produced at the disk under O2 saturation conditions and the amount of current
produced at the disk under argon saturation conditions was taken as the corrected disk current for
O2 saturation conditions (idisk corrected) for each rotation rate. The difference between the corrected
values for the amount of current produced at the ring under O2 saturation conditions and the amount
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of current produced at the ring under argon saturation conditions was taken as the corrected ring
current for O2 saturation conditions (iring corrected) for each rotation rate.
To calculate the percent of H2O2 generated catalytically, the idisk corrected was multiplied by the
corresponding Nempirical value for the specific rotation rate to determine the maximum amount of
ring current for H2O2 production (iring max). The ratio of iring corrected to iring max was multiplied by a
factor of 100 to determine the H2O2% generated at the ring across all measured rotation rates.
푖푟푖푛푔 푐표푟푟푒푐푡푒푑
%퐻2푂2 = (100)
푖푑푖푠푘 푐표푟푟푒푐푡푒푑(푐표푙푙푒푐푡푖표푛 푒푓푓푖푐푒푛푐푦)
Levich and Koutecky-Levich theory was applied to these experiments to calculate the diffusion
coefficients for each of the conditions under which the experiment was run (argon and O2
saturation). The slope of the Levich plot was obtained by plotting the square root of the angular
rotation rate versus the limiting current for both argon and O2. The slope of these Levich plots
were then used to calculate the diffusion coefficients under each of the experimental conditions,
argon and O2 saturation, using the following equation
3
푠푙표푝푒
퐷 =
(
)
0.620푛퐹퐴ν ―1/6퐶0∗
where D is the diffusion coefficient, slope is the slope from the Levich plot, n is the number of
electrons, F is Faraday’s constant, A is the area of the electrode, 휈 is the kinematic viscosity of the
solvent (0.010038 cm2/s for H2O47) and C*0 is the concentration of O2 at saturation in H2O (1.3
mM7, 37).
Description of Number of Electrons Transferred (ncat) and Overpotential (η) Calculations:
The production of H2O2 is a 2-electron process and the production of O2 is a 4-electron process.
To determine the total number of electrons transferred, the Faradaic efficiency for H2O2
determined from RRDE is multiplied by two then added to the product of the remaining current
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efficiency multiplied by four (which represents the water selectivity). The sum of those two
numbers represent the total number of electrons transferred in the RRDE experiment.
An adjustment for the standard reduction potential, 1.229 V vs NHE48 of dioxygen reduction to
water, was made for each pH where the number of electrons was determined following:
퐸0푂푅푅 = 퐸푂 /퐻 푂 ― (0.0592푝퐻)
2
2
This allowed for the overpotential to be determined from the equation below using the E1/2 of the
Mn(III)/(II) reduction for [Mn(TMPyP)Cl]4+ at each pH.
휂 = 퐸1/2(푐푎푡) ― 퐸푂0푅푅
TOFmax Calculation from CV data39-40
Utilizing the number of electrons calculated for the conversion of O2 to H2O by
[Mn(TMPyP)Cl]4+ from RRDE, the maximum turnover frequency (TOFmax) was calculated. Under
the assumption that this system follows pseudo-first order kinetics for the catalytic process, the
observed catalytic current plateau under O2 results in the following description, if H2O is assumed
to be the proton donor
푦
푖푐푎푡 = 푛푐푎푡퐹퐴퐶0푐푎푡
[
]푦[
]
퐷푘푐푎푡 푂2 퐻2푂
푦
[
]푦[
]
푇푂퐹푚푎푥 = 푘푐푎푡 푂2 퐻2푂
where ncat is the number of electrons determined by RRDE, F is Faraday’s constant, A is the surface
퐶0
area of the working electrode, 푐푎푡is the catalyst concentration in the bulk solution, D is the
diffusion coefficient for the electrocatalyst, kcat is the rate constant for the overall process, [O2] is
the substrate concentration and y is the reaction order of each substrate. Normalization of this
catalytic current observed under O2 by the non-catalytic process leads to a relationship between
the TOFmax and the ratio icat/ip described as
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3
퐹
푅푇
푖푝 = 0.4463푛푝 2퐹퐴퐶푐0푎푡
퐷푣
푖푐푎푡 푛푐푎푡
1
푅푇
=
푇푂퐹푚푎푥
3
푖푝
20.4463 퐹푣
푛푝
2
푛푝3
푖푐푎푡
퐹푣
푇푂퐹푚푎푥 = 0.1992
(
)
2
푅푇 푖푝
푛푐푎푡
where np is the number of electrons transferred in the non-catalytic process (np=1), R is the
universal gas constant, T is the temperature, and v is the scan rate. Each TOFmax value was
determined at 0.1 Vs-1 and as reported in Table 1; the calculation of these values under the non-
ideal conditions presented here places an upper limit on the likely TOFmax for this catalyst.
UV-vis Spectroelectrochemistry
UV-vis SEC experiments were performed utilizing a platinum Honeycomb
Spectroelectrochemical Cell Kit from Pine Research Instrumentation coupled with an Agilent Cary
60 UV-vis, Metrohm PGSTAT302N, and a 4.0 M KCl gel Ag/AgCl low profile reference electrode
from Pine Research Instrumentation. Data was referenced to NHE by adding 0.199 V to the
potential at which it was collected.49 The desired experimental pH was obtained by utilizing the
Britton-Robinson buffer described above with pH adjustments by 0.2 M NaOH (pH = 7 and 10),
or utilizing the solutions described below for pH 1 and 14. A background spectrum of the pH
adjusted solutions was obtained prior to adding analyte, and this spectrum was utilized to reduce
background noise through subtraction. A stock solution of [Mn(TMPyP)Cl]4+ was generated by
dissolving 9.4 mg of [Mn(TMPyP)Cl]4+ in DI water. An aliquot of this stock solution (1 mL) was
then diluted with 9 mL of pH adjusted buffer to generate ~0.1 mM solution of [Mn(TMPyP)Cl]4+.
The solution was then loaded in the UV-vis SEC cell, with special care taken to prevent air bubbles
within the honeycomb electrode. The cell and solution were degassed with Ar sparging for 10
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minutes prior to measurement. The resting potential was set in a potential region where only a
single species should be present (according to the reduction waves of interest seen in CV
experiments) and was held for 30 s prior to collection of a spectrum. The potential was then stepped
in 50 mV increments with a subsequent 30 s electrolysis prior to collection of a UV-vis spectrum.
The potential was not changed until two consecutive spectra were obtained as overlays under these
conditions. The honeycomb electrode was cleaned after electrolysis by first rinsing with DI water,
and then taking 20 CVs with a start potential of –0.3 V vs Ag/AgCl, upper vertex, in 0.5 M H2SO4
as described in the Honeycomb Spectroelectrochemical Cell Kit Manual.
Rinse Test Procedure
A stock solution of the Britton-Robinson buffer was made for pH values of 3, 6, 7 and 12. At
each pH, a CV was taken of the blank buffer solution under Ar saturation. After this initial CV, 1
mM of [Mn(TMPyP)(Cl)]4+ was added to the solution and sonicated to ensure dissolution. The
electrode was then poised at the potential of the Mn(III/II) reduction under an Ar atmosphere. The
working electrode was then removed and gently dried with a Kimwipe® before being placed into
a blank electrolyte solution saturated with Ar. A CV was taken through the same potential window
as the solution containing the 1 mM of catalyst to confirm that no adsorption had taken place
(Figure S50-S54).
ASSOCIATED CONTENT
Electrochemical, UV- vis spectrochemical and UV-vis spectroelectrochemical characterization of
[Mn(TMPyP)Cl]4+; control CVs, adsorption tests, and RRDE results with calculation of number
of electrons transferred. (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail: machan@virginia.edu
ORCID
Charles W. Machan: 00000-0002-5182-1138
Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval
to the final version of the manuscript.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
The authors thank the University of Virginia for generous funding.
ABBREVIATIONS
[Mn(TMPyP)]5+)], Manganese (III) meso-Tetra (N-methyl-4-pyridyl) porphine pentachloride
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