Water oxidation intermediates applied to catalysis: Benzyl alcohol oxidation

Article abstract of DOI:10.1021/ja210718u

Four distinct intermediates, Ru^IV=O²⁺, Ru ^IV(OH)³⁺, Ru^V=O³⁺, and Ru ^V(OO)³⁺, formed by oxidation of the catalyst [Ru(Mebimpy)(4,4′-((HO)₂OPCH₂)

Full text of DOI:10.1021/ja210718u

Communication
pubs.acs.org/JACS
Water Oxidation Intermediates Applied to Catalysis: Benzyl Alcohol
Oxidation
Aaron K. Vannucci, Jonathan F. Hull, Zuofeng Chen, Robert A. Binstead, Javier J. Concepcion,
and Thomas J. Meyer*
Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599, United States
S
* Supporting Information
Scheme 1. Water Oxidation Mechanism for nanoITO|1-
ABSTRACT: Four distinct intermediates, RuIVO2+,
PO3H2
RuIV(OH)3+, RuVO3+, and RuV(OO)3+, formed by oxida-
tion of the catalyst [Ru(Mebimpy)(4,4′-((HO)2OPCH2)2-
bpy)(OH2)]2+ [Mebimpy = 2,6-bis(1-methylbenzimidazol-
2-yl) and 4,4′-((HO)2OPCH2)2bpy = 4,4′-bismethylene-
phosphonato-2,2′-bipyridine] on nanoITO (1-PO3H2) have
been identified and utilized for electrocatalytic benzyl
alcohol oxidation. Significant catalytic rate enhancements
are observed for RuV(OO)3+ (∼3000) and RuIV(OH)3+
(∼2000) compared to RuIVO2+. The appearance of an
intermediate for RuIVO2+ as the oxidant supports an
O-atom insertion mechanism, and H/D kinetic isotope
effects support net hydride-transfer oxidations for
O-atom transfer from H2O gives RuIII-OOH3+, which can be
further oxidized to the peroxide RuIV(OO)2+ followed by slow
O2 evolution. Additional oxidation of the RuIV peroxide to
RuV(OO)3+ increases the lability of O2, leading to re-entry into
the catalytic cycle through O2 loss and regeneration of RuIII-
OH2+. Evidence has been obtained for intermediate
RuIV(OO)2+/RuIII-OOH2+ and RuIII-OOH2+/RuII(HOOH)2+
couples both in solution and on electrode surfaces.2,7,9
Any mechanism for water oxidation near the thermodynamic
potential for the H2O/O2 couple is necessarily complex due to
the requirement for 4e−/4H+ loss and O−O bond formation.
Mechanistic complexity, however, creates an opportunity
to examine intermediates that appear in the catalytic cycle,
RuIVO2+, RuIV(OH)3+, RuVO3+, and RuV(OO)3+, for
possible catalytic activity. We show here that all four have
catalytic reactivity toward benzyl alcohol (BnOH), which is
chosen as a model substrate. The advantages of surface-bound
catalysis are exploited to identify and utilize conditions that
allow for the reactivity of each to be evaluated.
Figure 1 shows cyclic voltammograms (CVs) of nanoITO|1-
PO3H2 at pH 5 before and after entry into a water oxidation
cycle. Limiting the potential scan to <1.35 V avoids the water
oxidation cycle, which occurs following RuIVO2+→RuVO3+
oxidation at Ep,a ≈ 1.5 V. Oxidative peak currents for the
RuII-OH22+→RuIII-OH2+ oxidation wave at E1/2 = 0.75 V vary
linearly with scan rate, ν, as expected for a surface-bound couple.8
At scan rates <5 mV/s, integrated peak currents for the RuIII-
OH2+/RuIVO2+ wave are comparable to those for the RuII-
OH22+/RuIII-OH2+ couple. At scan rates >10 mV/s, the anodic
peak current, ip,a, for the PCET process RuIII-OH2+→RuIVO2+
RuIV(OH)3+ and RuV(OO)3+. These results illustrate the
importance of multiple reactive intermediates under
catalytic water oxidation conditions and possible control
of electrocatalytic reactivity on modified electrode surfaces.
n impressive scope of oxidative reactivity has been
A
identified for high oxidation state Ru oxo complexes,
including a family of single-site water oxidation catalysts based
on RuVO3+ polypyridyl complexes, for which mechanisms of
water oxidation have been elucidated.1 Related single-site
catalysts with phosphonate (−PO3H2)-modified ligands have
been shown to function as water oxidation electrocatalysts
when bound to planar ITO (Sn(IV)-doped In2O3) and FTO
(fluorine-doped SnO2) electrodes2 and, more recently, to films
of nanostructured ITO (nanoITO).3,4 Surface attachment can be
useful in avoiding over oxidation5 and mechanistically insightful
due to the absence of diffusion of the catalyst.5 Surface attach-
ment also provides an interfacial configuration appropriate
for catalytic, electrocatalytic, fuel cell, and photoelectrocatalytic
applications.6
The mechanism of interfacial water oxidation by [Ru-
(Mebimpy)(4,4′-((HO)2OPCH2)2bpy)(OH2)]2+ (1-PO3H2)
[Mebimpy = 2,6-bis(1-methylbenzimidazol-2yl) and 4,4′-
((HO)2OPCH2)2bpy = 4,4′-bismethylenephosphonato-2,2′-bi-
pyridine] bound to ITO and nanoITO electrodes2,6,7 is shown
in Scheme 1. Key elements in the mechanism include stepwise
proton-coupled electron transfer (PCET) oxidation from
nanoITO|Ru-OH2 to higher oxidation states RuIVO2+ and
2+
RuVO3+. This process occurs through RuIII-OH2+ or RuIII-
OH23+, with pKa = 2.3 for the latter. In acidic solutions, oxidative
activation from RuIII-OH2+/RuIII-OH2 occurs through
3+
Received: November 22, 2011
Published: February 6, 2012
RuIV(OH)3+.7,8 Further oxidation to RuVO3+ followed by
© 2012 American Chemical Society
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Communication
(40 mM in pH 5, I = 0.064 M OAc−, 23 2 °C) to a cuvette
containing nanoITO|RuIVO2+, generated by oxidation of RuII-
2+
OH2 in a pH 9 solution of NaOCl, with the cell aligned
perpendicularly to the spectrometer light-path. Characteristic
2+
spectral changes for reduction to nanoITO|RuII-OH2 with
λmax = 498 nm and isosbestic points at 614 and 423 nm
occurred with addition of BnOH. Importantly, there was no
spectroscopic evidence for RuIII-OH2+ (λmax = 380 nm at pH 5)
or RuIII-OH22+ (λmax = 650 nm at pH 1) as intermediates on the
surface, suggesting a concerted 2e− mechanism.
Figure 1. CVs of nanoITO|1-PO3H2 at 30 mV s−1 at pH 5 (I = 0.064 M
acetate anion (OAc−)). (a) nanoITO|1-PO3H2 before water oxidation.
The wave for the thermodynamic formation of RuIVO2+ at Ep,a
≈
Initial oxidation of RuIVO2+ to the intermediate species with
1.0 V is only slightly discernible, followed by the wave for the
RuIV(OH)3+/RuIII-OH2+ couple at Ep,a = 1.24 V. (b) nanoITO|1-PO3H2
after scans to 1.5 V through the water oxidation cycle. The new waves at
E1/2 = 0.34 and 0.25 V arise from a peroxide intermediate (see text).
λmax = 493 nm occurs with kox = (1.3 0.02) × 10−2 s−1. λmax
then shifts with time to 498 nm, characteristic of RuII-OH22+
,
indicating initial formation of a RuII intermediate followed by
solvolysis to give nanoITO|RuII-OH22+. These observations are
consistent with a mechanism in which oxidation of BnOH occurs
by C−H insertion to give a coordinated aldehyde hydrate, which
undergoes solvolysis to give nanoITO|RuII-OH22+ (eqs 1a,b).
The same reactivity was observed for BnOH oxidation by the
RuIVO2+ oxidant [RuIV(tpy)(4,4′-((HO)2OPCH2)2bpy)(O)]2+
(tpy = 2,2′:6′,2″-terpyridine) on TiO2.6
decreases as a new wave appears at E1/2 = 1.24 V. The new wave
arises from the direct oxidation of RuIII-OH2+ in a chemically
reversible but kinetically inhibited RuIV(OH)3+/RuIII-OH2+
couple, which is the sole Ru(IV/III) couple observed at pH ≤ 2.
As noted below, the pKa for RuIV(OH)3+ is ∼3. The skewed,
narrow reduction wave observed in Figure 1a is scan rate and
pH dependent and arises from the autocatalytic reduction of
RuIVO2+ to RuIII-O+ followed by rapid proton transfer to give
RuIII-OH2+.8
IV
2+
nanoITO|Ru O + PhCH OH
2
II
2+
→ nanoITO|Ru ‐O(H)CH(OH)Ph
,
k
IV
ox,Ru
O
The CV waveform in Figure 1a is maintained through multiple
scans until the potential window is extended to 1.5 V, resulting in
electrocatalytic water oxidation. After water oxidation at the
interface, the waveform in Figure 1a is transformed into the
waveform shown in Figure 1b. The noticeable difference
between the two CVs in Figure 1 is the appearance of two
new waves at E1/2 = 0.34 and 0.25 V that arise at the expense of
the RuIII-OH2+/RuII-OH22+ peak at 0.75 V. As observed in an
earlier study,8 the new waves are due to RuIV(OO)2+ formation
on the electrode surface and result from the pH-dependent
peroxide couples RuIII-OOH2+/RuII(HOOH)2+ and RuIV(OO)2+/
RuIII-OOH2+, which are known stable complexes.2,7,9 The
RuIV(OO)2+→RuV(OO)3+ oxidation wave occurs at ∼1.3 V.
These observations provide conditions for generating each of
the intermediates RuIVO2+, RuIV(OH)3+, and RuV(OO)3+
and investigating their oxidation chemistry.
(1a)
II
2+
nanoITO|Ru ‐O(H)CH(OH)Ph + H O
2
II
2+
→ nanoITO|Ru ‐OH
+ PhCHO + H O,
k
2
2
solv
(1b)
In order to verify benzaldehyde as a major product, a
controlled potential electrolysis of nanoITO|1-PO3H2 in the
presence of 20 mM BnOH was performed at 1.05 V with
stirring for 13 h. A steady-state current density of ∼8000 μA/cm3
was maintained during the electrolysis (SI). GC analysis showed
that benzaldehyde was formed with a faradaic efficiency of 64%.
Oxidation by nanoITO|RuIV(OH)3+. Electrocatalytic oxida-
tion of BnOH by nanoITO|RuIV(OH)3+ was investigated by
cyclic voltammetry at pH 5. As noted above, RuIII-OH2+→
RuIV(OH)3+ oxidation occurs at E1/2 = 1.24 V. Further evidence
for surface-bound RuIV(OH)3+ is shown in CVs of nanoITO|1-
PO3H2 at pH 1 (Figure S2), where the magnitude of ip,a and
the chemical reversibility of the RuIV(OH)3+/RuIII-OH2+ couple
are enhanced relative to those at pH 5.
Oxidation by nanoITO|RuIVO2+. Figure 2 shows
spectral changes that accompany the addition of BnOH
Figure 3 shows CVs of nanoITO|1-PO3H2 with increasing
concentrations of BnOH. There is no evidence for the peroxide
Figure 2. UV−vis monitoring of the reaction between BnOH
(40 mM) and nanoITO|RuIVO2+ in pH 5, I = 0.064 M OAc− at
23 2 °C, showing the appearance of an initial intermediate at λmax
=
493 nm followed by nanoITO|RuII-OH22+ at λmax = 498 nm. The inset
shows an analysis of the data by using a biexponential A→B→C model
obtained via the software package SPECFIT/32 using normalized
concentrations and an experimental time offset t0 ≈ 30 s, where A is
Figure 3. CVs of nanoITO|1-PO3H2 at 20 mV/s (pH 5, I = 0.064 M
OAc−, 23
2 °C) with added increments of BnOH (2, 4, 6, and
8 mM) showing the electrocatalytic activity of RuIV(OH)3+.
nanoITO|RuIVO2+, B is an intermediate (see eq 1b), and C is
couples on the surface of the electrode under these conditions.
No sign of current enhancement was observed at the potential
2+
nanoITO|RuII-OH2
.
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Journal of the American Chemical Society
Communication
for the RuIVO2+/RuIII-OH2+ couple (Ep,a = 1.0 V) on the CV
time scale, consistent with the slow oxidation observed in
Figure 1. This indicates that catalytic oxidation of BnOH on the
CV time scale occurs from RuIV(OH)3+.
RuIII-OOH2+/RuII(HOOH)2+ couples on the electrode surface,
as described above. A previous DFT study showed two possible
forms for both RuIV(OO)2+ and the oxidized RuV(OO)3+: an
O2-chelating, 7-coordinate complex for which there is literature
9
2−
As shown in Figure S8, BnOH oxidation by nanoITO|
RuIV(OH)3+ is acid dependent. The electrochemical kinetic
measurements were repeated at 1.24 V, and kobs values were
obtained as a function of various acid concentrations (HNO3)
from pH 4 to 1. The dependence of kobs with [H+] provides
evidence for saturation at high acid concentrations, consistent
with the rate law in eq 2 and the mechanism in eqs 3 and 4.
In eq 2, [RuIVtot] is the total concentration of RuIV (RuIV(OH)3+,
RuIVO2+), Ka is the acid dissociation constant for nanoITO|
precedence, or an “open”, 6-coordinate Ru complex with O2
terminally bound.9 Slow interconversion between the two
forms has been predicted.9 DFT calculations favor the chelate
form in RuIV and the “open” form in RuV, with the latter
undergoing kinetically facile O2 release. We presume that the
relatively stable peroxide intermediate observed on the
electrode surface is the chelate form, RuIV(OO)2+. This form
is also accessible by O2 substitution for water in air-dried
electrodes (eq 6).
RuIV(OH)3+, and kRu OH is the rate constant for nanoITO|
IV
II
2+
RuIV(OH)3+ oxidation of BnOH. Based on the inverse−inverse
nanoITO|Ru ‐OH
+ O
2
2
plot in the inset in Figure S8, pKa = 3.2 0.3 for nanoITO|
IV
2+
→ nanoITO|Ru (OO) + H O
(6)
RuIV(OH)3+ and kRu OH = 11.1
0.4 M−1 s−1, which was
2
IV
confirmed by steady-state current measurements at 150 s at an
applied potential of 1.24 V (SI). The number of moles of active
RuIV(OH)3+ catalyst on the surface was determined by the
integrated current under the RuIV(OH)3+/RuIII-OH2+ wave in
the absence of substrate and eq 5, where Q is the integrated
charge, n is the number of electrons transferred (=1), F is the
Faraday constant, A is the area of the electrode, and Γ is the
surface coverage of active catalyst in mol/cm2. The remaining
sites on the electrode surface (RuIII and RuIVO2+) were
assumed to be inactive.
The peroxide waves are persistent on the electrode surface
and can be studied separately from water oxidation by limiting
the potential window to <1.34 V during CV analysis. Figure 4
IV
rate=k [Ru
obs
]
(2a)
(2b)
(3)
tot
+
[H ]
Figure 4. CVs of nanoITO|1-PO3H2 at 20 mV/s (pH 5, I = 0.064 M
OAc−, 23 2 °C), with added BnOH (2, 4, 6, and 8 mM), showing
the catalytic activity of RuV(OO)3+.
k
= k
K [BnOH]
IV
obs
a
+
Ru OH
1 + K [H ]
a
IV
2+
IV
+
3+
nanoITO|Ru O + H
shows CVs for the catalytic response of nanoITO|RuIV(OO)2+
in the presence of increasing amounts of BnOH. Catalytic rate
constants were evaluated by steady-state current measurements
after 150 s with stirring at 1.27 V by application of eq 7 to the
−1
→ nanoITO|Ru (OH)
,
K
a
IV
3+
nanoITO|Ru (OH) + PhCH OH
2
data (SI). From the measurements, kRu (OO) = 28.1 0.5 M−1 s−1
(pH 5, I = 0.064 M OAc−, 23 2 °C).
II
2+
+
V
3+
→ nanoITO|Ru ‐OH
+ PhCHO + H ,
k
IV
2
Ru OH
(4)
i
= n FAΓk
cat.
[BnOH]
ox,BnOH
cat.
(7)
Q = nFAΓ
(5)
Similar to RuIV(OH)3+ as the oxidant, kH O/kD O = 1.1 0.1
2
2
To gain insight into the catalytic mechanism, kinetic isotope
effects (KIEs) were evaluated by measurements in D2O and
and kC H CH OH/kC D CD OH = 2.7 0.1, and there is a lack of
6
5
2
6
5
2
dependence on O2 for RuV(OO)3+ as the oxidant, suggesting a
net hydride transfer mechanism, possibly by H-rebound. This
mechanism would generate benzaldehyde and RuIII(OOH)2+
that could be oxidized back into the catalytic cycle as shown in
eqs 8 and 9. Controlled potential electrolysis of nanoITO|
RuV(OO)3+ in the presence of 20 mM BnOH at 1.29 V for 16 h
produced a steady-state current of ∼13 mA/cm3, corresponding
to ∼2400 2e− turnovers. Benzaldehyde was the major product
determined by GC analysis, with a 66% faradaic efficiency.
with the deuterated benzyl-d7 alcohol, which gave (k /k ) =
H2O D2O
1.1
0.1 and (kC H CH OH/kC D CD OH) = 3.0
0.2. The
6
5
2
6
5
2
magnitudes of these KIE values indicate that water plays a small
role in the rate-determining step, and the C−H/C−D KIE of
3 suggests a possible hydride transfer mechanism, similar to
results obtained in a previous study on a related oxidant.10
Deaeration with N2 had no effect, ruling out a free radical
mechanism involving radical capture by oxygen. Controlled
potential electrolysis of nanoITO|RuIV(OH)3+ in the presence of
20 mM BnOH at 1.24 V for 16 h resulted in a steady-state
current density of ∼11000 μA/cm3. Benzaldehyde was the
major product (57% faradaic efficiency) as determined by GC
analysis.
V
3+
nanoITO|Ru (OO) + PhCHOH
III 2+
+
→ nanoITO|Ru (OOH) + PhCHO + H ,
k
V
3+
ox,Ru (OO)
(8)
(9)
Oxidation by nanoITO|RuV(OO)3+. Catalytic rates for
BnOH oxidation were further increased following an oxidative
scan through the catalytic water oxidation wave, which led to
the appearance of waves for the RuIV(OO)2+/RuIII-OOH2+ and
III
2+
−
nanoITO|Ru (OOH) − 2e
→ nanoITO|Ru (OO) + H
V
3+
+
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Journal of the American Chemical Society
Communication
The RuVO3+ form of the catalyst is also active toward
Notes
electrocatalytic BnOH oxidation, but nanoITO|RuVO3+ is
The authors declare no competing financial interest.
only accessible by RuIVO2+→RuVO3+ oxidation at Ep,a
≈
ACKNOWLEDGMENTS
1.5 V. At this potential, BnOH oxidation overlaps with H2O
oxidation. The overlapping catalytic processes make a detailed
analysis of BnOH oxidation by RuVO3+ difficult in aqueous
solutions. Qualitatively, oxidation currents at 1.5 V increased with
increasing amounts of added BnOH, suggesting that oxidation
of BzOH by nanoITO|RuVO3+ is comparable in rate to the
oxidation of BnOH by nanoITO|RuV(OO)3+. Currently, measure-
ments are underway in propylene carbonate/H2O mixtures in
order to study the catalytic activity of RuVO3+ without
complication from extensive background water oxidation.
■
Funding by the Center for Catalytic Hydrocarbon Function-
alization, an Energy Frontier Research Center (EFRC) funded
by the U.S. Department of Energy (DOE), Office of Science,
Office of Basic Energy Sciences, under Award DE-SC0001298
supporting J.F.H. and the electrochemical experiments of
A.K.V. and the UNC EFRC Solar Fuels, an EFRC funded by
the U.S. DOE, Office of Science, Office of Basic Energy
Sciences, under Award DE-SC0001011 for supporting R.A.B.
and J.J.C. and the material prepatation performed by .A.K.V. is
gratefully acknowledged. Support for the purchase of the
instrumentation from UNC EFRC (Solar Fuels, An EFRC
funded by the U.S. DOE, Office of Science, Office of Basic
Energy Sciences under Award Number DE-SC0001011) and
from UNC Solar Energy Research Center Instrumentation
Facility, funded by the U.S. DOE, Office of Energy Efficiency &
Renewable Energy, under Award Number DE-EE0003188 are
gratefully acknowledged.
Table 1 summarizes kinetic parameters, KIE values, and
turnover numbers from the controlled potential electrolysis
Table 1. Rate Constants, KIE Values, and Turnover
Numbers for RuIVO2+, RuIV(OH)3+, and RuV(OO)3+ on
a
nanoITO
KIE
b
k
cat, M−1 s−1
BnOH-d7
D2O
TON
nanoITO|RuV(OO)3+
nanoITO|RuIV(OH)3+
nanoITO|RuIVO2+
V
28.1
11.1
0.01
2.7
3.0
c
1.1
1.1
c
2440
400
70
REFERENCES
■
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a
3+/RuIII‑OOH2+
E°(Ru (OO)
)
IV(OH)3+/RuII‑OH2
)
2+
= 0.89 V, E°(Ru
= 0.99 V,
IV
2+
O2+/RuII‑OH2
)
E°(Ru
= 0.90 V at pH 5, I = 0.064 OAc−, 23 0.2 °C.
b
Mols of benzaldehyde produced from mols of catalyst on the
c
electrode surface 10%. Not available.
experiments. By using the advantages of the surface-bound catalyst
on nanoITO, we have been able to identify and exploit a series of
high oxidation state intermediates found in water oxidation cycles.
These results are remarkable in illustrating a cascading increase in
reactivity by over a factor of ∼3000 for BnOH oxidation by a
single catalytic system with E°′ values for 2e−/1H+ couples at pH 5
that vary by only ∼0.1 V. The appearance of contributions to
BnOH oxidation by four different forms of the same catalyst,
RuVO3+, RuV(OO)3+, RuIV(OH)3+, and RuIVO2+, also raises a
warning flag about interpreting data on oxo-catalyzed oxidations
with excess oxidants, notably ceric ammonium sulfate. Under these
conditions, with added water, there could be contributions from
several forms of the catalyst.
Detailed mechanistic analyses are currently under investigation,
but there are indications of what may be exploitable mechanistic
diversity. RuIVO2+ oxidizes BnOH through a discrete
intermediate, presumably by C−H insertion. The appearance of
moderate C−H/C−D KIE values and the absence of an O2 effect
point to hydride transfer or H-atom rebound mechanisms for
nanoITO|RuIV(OH)3+ and nanoITO|RuV(OO)3+. Such pathways
[e.g., RuIV(OH)3++ PhCH2OH → RuII-OH22+ + PhCHOH+;
RuV(OO)3+ + PhCH2OH → RuIII-OOH2+ + PhCHOH+] are
appealing because they avoid high-energy radical intermediates.
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ASSOCIATED CONTENT
■
S
* Supporting Information
Additional information as noted in the text. This material is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
tjmeyer@email.unc.edu
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dx.doi.org/10.1021/ja210718u | J. Am. Chem. Soc. 2012, 134, 3972−3975