Photocatalytic oxidation of benzene to phenol using dioxygen as an oxygen source and water as an electron source in the presence of a cobalt catalyst

Article abstract of DOI:10.1039/c7sc02495a

Photocatalytic hydroxylation of benzene to phenol by dioxygen (O₂) occurs under visible light irradiation of an O₂-saturated acetonitrile solution containing [Ru^II(Me₂phen)₃]²⁺ as a photocatalyst, [Co^III(Cp?)(bpy)(H₂O)]²⁺ as an efficient catalyst for both the water oxidation and benzene hydroxylation reactions, and water as an electron source in the presence of Sc(NO₃)₃. The present study reports the first example of photocatalytic hydroxylation of benzene with O₂ and H₂O, both of which are the most green reagents, under visible light irradiation to afford a high turnover number (e.g., >500). Mechanistic studies revealed that the photocatalytic reduction of O₂ to H₂O₂ is the rate-determining step, followed by efficient catalytic hydroxylation of benzene to phenol with H₂O₂, paving a new way for the photocatalytic oxygenation of substrates by O₂ and water.

Full text of DOI:10.1039/c7sc02495a

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Photocatalytic oxidation of benzene to phenol using dioxygen as
an oxygen source and water as an electron source in the presence
of a cobalt catalyst†
Received 00th July 20xx,
Accepted 00th July 20xx
Ji Won Han,‡a Jieun Jung,‡ab Yong-Min Lee,a Wonwoo Nam*a and Shunichi Fukuzumi*ac
DOI: 10.1039/x0xx00000x
Photocatalytic hydroxylation of benzene to phenol by dioxygen (O2) occurs under visible light irradiation of an O2-saturated
acetonitrile solution containing [RuII(Me2phen)3]2+ as a photocatalyst, [CoIII(Cp*)(bpy)(H2O)]2+ as an efficient catalyst for
both water oxidation and benzene hydroxylation reactions, and water as an electron source in the presence of Sc(NO3)3.
The present study reports the first example of photocatalytic hydroxylation of benzene with O2 and H2O, both of which are
the most green reagents, under visible light irradiation to afford a high turnover number (e.g., >500). Mechanistic studies
revealed that the photocatalytic reduction of O2 to H2O2 is the rate-determining step, followed by efficient catalytic
hydroxylation of benzene to phenol with H2O2, paving a new way of photocatalytic oxygenation of substrates by O2 and
water.
www.rsc.org/
(TON) of the catalytic hydroxylation of benzene to phenol are
still low (e.g., 13).20
Introduction
We report herein an efficient photocatalytic hydroxylation
Phenol, which is an important precursor for many chemicals
and industrial products (e.g., dyes, polymers, etc.), is currently
pro-duced from benzene by a three step cumene process.1,2
Since the efficiency of the cumene process is low (~5% yield of
phenol) under severe conditions (e.g., high temperature, high
pressure, and strong acidic conditions), it is highly desired to
develop a one-step synthesis of phenol from benzene using
homogeneous and heterogeneous inorganic catalysts.3-5
Among various oxidants, hydrogen peroxide (H2O2) is
frequently used as a green oxidant, which produces water or
dioxygen (O2) as products, in the catalytic benzene
hydroxylation.6-13 O2 is a more ideal oxidant than H2O2 because
of its abundance in nature, low cost, and environmental
benignity.4,14,15 However, catalytic aerobic oxidation of
benzene to phenol has required sacrificial reducing agents
such as H2, ascorbic acid, and NADH analogs.16 In addition, the
catalytic aerobic oxidation of benzene without sacrificial
reducing agents has so far required harsh conditions, such as
high temperatures or UV light photoirradiation.14,15,17,18
Moreover, although the photocatalytic hydroxylation of
benzene to phenol using organic photocatalysts without
overoxidation was demonstrated,19-22 the turnover numbers
of benzene to phenol (PhOH) by O2 as an oxidant in the
presence of Sc(NO3)3 and utilizing [RuII(Me2phen)3]2+ (Me2phen
= 4,7-dimethyl-1,10-phenanthroline) as a photocatalyst, water
as an electron source, and [CoIII(Cp*)(bpy)(H2O)]2+ (Cp* = η5-
‡ These authors contributed equally to this work.
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decreased with time due to the oxidative degradation of the
catalyst (see Experimental Secꢀon; Fig. S1, ESI†).
1
p-Benzoquinone was also produced during the
photocatalytic oxidation of benzene by O2, and the yield of p-
benzoquinone increased as the reaction solution was
irradiated for longer time but the yield of PhOH remained
constant (Fig. 1). The observation of the induction period for
the formation of p-benzoquinone in Fig. 1 suggests that p-
benzoquinone is produced by the further oxidation of PhOH.
The production of p-benzoquinone from PhOH was
independently confirmed (Fig. S2, ESI†). The TON for the
production of both phenol and benzoquinone based on
1 was
determined to be 500(20), where TON of p-benzoquinone is
counted three times because p-benzoquinone is the six-
electron oxidized product whereas phenol is the two-electron
Fig. 1 Time courses of products [PhOH (blue) and p-benzoquinone (red)]
obtained in the photocatalytic oxidation of benzene by O2 with
[RuII(Me2phen)3]2+ (1.0 mM) in the presence of a catalytic amount of 1 (1.0 oxidized product. When the reaction conditions were changed
µM) in an O2-saturated solvent mixture of MeCN and H2O (v/v = 23:2)
containing Sc(NO3)3 (100 mM) and benzene (1.0 M) under photoirradiation
(white light) at 298 K.
by increasing the concentration of the catalyst and decreasing
the concentration of benzene substrate, a much higher
product yield of phenol (~30%) based on benzene was
obtained (Fig. 2) than that in Fig. 1. The quantum yield (QY)
was estimated by eqn (2),
pentamethylcyclopentadienyl and bpy = 2,2-bipyridine) as an
efficient catalyst for water oxidation as well as benzene
hydroxylation to attain TON of greater than 500 (Scheme 1).
Mechanistic aspects of the benzene hydroxylation reaction
under the visible light irradiation conditions have been
discussed as well.
where R (mol s–1) is PhOH production rate and I (einstein s–1) is
the rate of the number of incident photons. The quantum yield
was determined from the amount of PhOH produced during
the photocatalytic reaction under photoirradiation (λ = 440
nm) for 1 h to be 1.7(2)% (Fig. S3, ESI†). The photocatalyꢀc
reactivity increased with increasing the concentration of
benzene as well as the concentration of [RuII(Me2phen)3]2+
(Fig. S4, ESI†). Negligible amount of PhOH was produced in the
Results and Discussion
[CoIII(Cp*)(bpy)(H2O)]2+
(
1
)
and [RuII(Me2phen)3]2+ were
synthesised according to the literature methods.23,24 Visible
light irradiation of an O2-saturated solution of CH3CN (MeCN)
absence of
1, indicating that 1 is an essential component in the
and H2O (v/v = 23:2) containing a catalytic amount of 1 (1.0
M), [RuII(Me2phen)3]2+ (1.0 mM), Sc(NO3)3 (100 mM), and
benzene hydroxylation reaction.
µ
benzene (1.0 M) at 298 K resulted in the formation of PhOH
The photocatalytic hydroxylation reaction proceeds as
shown in Scheme 1. Visible light irradiation of
[eqn (1)], as
shown in Fig. 1. PhOH is produced via the catalytic oxygenation
of hydrogen peroxide (H2O2) with 1, which acts as not only a
benzene oxygenation catalyst but also a water oxidation
catalyst with [RuIII(Me2phen)3]3+ produced by photoinduced
electron transfer from the excited state of [RuII(Me2phen)3]2+
to O2 in the presence of Sc3+, as shown in Scheme 1. It should
be noted that the catalyst
1 acts as an efficient catalyst for
water oxidation in the photocatalytic production of H2O2 in the
presence of Sc(NO3)3, as reported previously.23,25
[RuII(Me2phen)3]2+ acts as a more efficient photocatalyst as
compared to [RuII(bpy)3]2+ for the O2 reduction by the excited
state, because the one-electron oxidation potential of
[RuII(Me2phen)3]2+* (Eox* = –1.01 V vs. NHE) is more negative
than that of [RuII(bpy)3]2+* (Eox* = –0.84 V vs. NHE).23 UV-vis
spectrum of the photocatalyst, [RuII(Me2phen)3]2+, during the
photocatalytic oxidation remained almost identical to that of
initial stage at 445 nm, indicating that the overall reactivity
2 | J. Name., 2012, 00, 1-3
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Fig. 3 Time courses of concentrations of [RuIII(Me2phen)3]3+ (red) and H2O2
(blue) produced from H2O and O2 in the photocatalytic oxidation of
[RuII(Me2phen)3]2+ by O2 under photoirradiation (white light) of an air-
saturated solvent mixture of MeCN and H2O (v/v = 23:2) containing
[RuII(Me2phen)3]2+ (100 µM) in the presence of Sc(NO3)3 (100 mM) at 298 K.
Fig. 4 Time courses of the photocatalytic production of H2O2 from H2O and
O2 with [RuII(Me2phen)3]2+ (0.10 mM) in the presence of a catalytic amount
of 1 [0.50 mM (red), 1.0 mM (orange), 1.5 mM (green), and 2.0 mM (blue)]
in air-saturated solvent mixture of MeCN and H2O (v/v = 23:2) containing
Sc(NO3)3 (100 mM) under photoirradiation (white light) at 298 K.
[RuII(Me2phen)3]2+ resulted in the formation of H2O2 by (Scheme 1, reaction pathways a – c), and the production of
oxidative
quenching
of
the
photoexcited
state H2O2 is obtained from the photooxidation of [RuII(Me2phen)3]2+
[RuII(Me2phen)3]2+* (* denotes the excited state) with O2 in the by O2
presence of Sc(NO3)3 (Scheme 1, reaction pathways b and c), [eqn (3)] and the catalytic water oxidation by
as reported for the case of [RuII(bpy)3]2+.26 The time courses of [RuIII(Me2phen)3]3+ in the presence of
[RuIII(Me2phen)3]3+ generation and H2O2 production are overall reaction is the
obtained to give the stoichiometry of the photochemical
1
[eqn (4)].23 Thus, the
reaction, as shown in eqn (3) (Fig. 3).
photocatalytic oxidation of H2O by O2 to produce H2O2, as given by
eqn (5). Rates of photocatalytic oxidation of H2O by O2 to produce
The lifetime of [RuII(Me2phen)3]2+* and emission quenching
have been determined to obtain rate constants (ket) of
photoinduced electron transfer in the absence and presence of
Sc(NO3)3 (Fig. S5 and Table S1, ESI†). The ket values in both the
absence and presence of Sc(NO3)3 are close to the diffusion-
limited value, showing that Sc(NO3)3 does not affect the
oxidative quenching of [RuII(Me2phen)3]2+* by O2. The emission
spectra of [RuII(Me2phen)3]2+ in the absence and presence of
Sc(NO3)3 taken in a solvent mixture of MeCN and H2O (v/v =
23:2) containing different concentrations of O2 indicate that
the amount of O2 in air is large enough for efficient H2O2
photogeneraꢀon (Fig. S5d, ESI†).
H2O2 in the presence of
1 were also investigated with various
concentrations of 1 at 298 K (Fig. S6, ESI†). The zeroth-order
rate constants (kobs) were obtained from the initial slopes of
the plots of the amount of H2O2 produced vs. time, being
proportional to concentration of 1 (Fig. 4; Fig. S6, ESI†).
The catalytic water oxidation by [RuIII(Me2phen)3]3+ with
1
to evolve O2 [eqn (6); Scheme 1, reaction pathway a] was
Visible light irradiation of the reaction solution without
benzene results in the formation of H2O2, as shown in Fig. 4
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independently confirmed, where O2 was evolved,
accompanied by the formation of [RuII(Me2phen)3]2+. The
evolved O2 concentrations were investigated by a Clark oxygen
electrode, showing an increase in the amount of the O2
evolved after addition of [RuIII(Me2phen)3]3+ (1.0 mM) to a
solvent mixture of MeCN and H2O (1.0 mL; v/v = 9:1)
containing Sc(NO3)3 (100 mM) and various concentrations of
(Fig. 5), where the O2 yield increased with increasing
concentration to approach 100% (1/4 of the initial
concentration of [RuIII(Me2phen)3]3+). The reaction rate was
accelerated with increasing concentration of to account for
oxidation of water by [RuIII(Me2phen)3]3+ with
under acidic
1
1
1
1
conditions due to the presence of Sc(NO3)3. Formation of
[RuII(Me2phen)3]2+ accompanied by water oxidation was also
confirmed by monitoring UV-vis spectral changes (Fig. S7a,
Fig. 7 Time courses of the products from photoinduced reaction under
photoirradiation (white light) on an O2-saturated solvent mixture of MeCN
and H2O (v/v = 23:2) containing 1 (0.50 mM), [RuII(Me2phen)3]2+ (0.10 mM),
and Sc(NO3)3 (100 mM) in the absence (blue) and presence (red) of benzene
(1.0 M) at 298 K.
ESI†). The time courses of the oxidation of
1 by
[RuIII(Me2phen)3]3+ was obtained by spectral changes at λ = 445
nm due to [RuII(Me2phen)3]2+, showing that the rate of the
[RuII(Me2phen)3]2+
formation
from
[RuIII(Me2phen)3]3+
[RuII(Me2phen)3]2+ (0.10 mM), and Sc(NO3)3 (100 mM) under
photoirradiation (white light) were examined in the absence
and presence of benzene to compare their reactivities (Fig. 7).
The rate of the H2O2 production from H2O and O2 in the
absence of benzene was slightly higher than that of PhOH
generation in the photocatalytic hydroxylation of benzene,
indicating that H2O2 produced in the photocatalytic oxidation
of H2O by O2 reacted with benzene efficiently in the presence
increased with the increase of the concentration of
ESI†).
1 (Fig. S7b,
Catalytic oxidation of benzene to phenol with H2O2 [eqn
(7)] has also been confirmed in a solvent mixture of MeCN and
H2O (23:2) containing Sc(NO3)3 in order to stabilize H2O2 with
the same conditions for the overall photocatalytic reaction as
shown in Fig. 6 (Scheme 1, reaction pathway d). The rate of
of
1
to produce PhOH.27 In fact, the rate of PhOH production
M after 1 h) from H2O2 (1.0 mM) and benzene 1.0 M)
(0.50 mM, red dots in Fig. 6) is much faster than that of
(59
µ
with
1
benzene
hydroxylation
increased
with
increasing
PhOH production (estimated to be 7.5 μM after 1 h) in the
photocatalytic reaction (Fig. 7), because concentration of H2O2
produced in the photocatalytic oxidation of H2O by O2 is much
smaller than 1.0 mM. Since the rate of PhOH production from
H2O2 is proportional to the concentration of H2O2 (Fig. S8,
concentrations of
1
(Fig. 6) and H2O2 (Fig. S8, ESI†).
Combination of the photocatalytic oxidation of H2O by O2 to
produce H2O2 [eqn (5)] and the catalytic hydroxylation of
benzene to phenol by H2O2 [eqn (7)] affords the overall
photocatalytic oxidation of benzene to phenol by O2 [eqn (1)].
The photocatalytic reactions of an O2-saturated solution of
MeCN and H2O (v/v
= 23:2) containing 1 (0.50 mM),
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H2O to O2 with catalysis of
was also catalysed by
peaks assignable to bpy and Cp* of
the photocatalytic production of H2O2, indicating
1
. Benzene hydroxylation to PhOH
1
. As reported previously, the NMR
remained the same after
acted as a
1
1
homogeneous catalyst during the reaction.23 However, the
catalytically active intermediates such as the Co(IV)-oxo
species29 have yet to be detected in the catalytic benzene
hydroxylation with H2O2. There was no deuterium kinetic
isotope effect (KIE) for benzene (Fig. 2), indicating that the C-H
bond cleavage is not involved in the rate-determining step of
the photocatalytic hydroxylation reaction.
Fig. 9 Comparison of relative abundances of authentic PhOH (blue) and the
produced PhOH (red) in the photocatalytic hydroxylation of benzene by O2
with 1 (0.10 mM) in the presence of [RuII(Me2phen)3]2+ (1.0 mM), Sc(NO3)3
Conclusions
In conclusion, we have shown that benzene is oxidized to phenol by
(100 mM), and benzene (2.0 M) under photoirradiation (white light) of an dioxygen with a high TON (e.g., 500) under visible light irradiation of
a reaction solution containing [RuII(Me2phen)3]2+ as a photocatalyst,
[CoIII(Cp*)(bpy)(H2O)]2+ as an efficient catalyst for both water
oxidation and benzene hydroxylation, and water as an electron
source in the presence of Sc(NO3)3. The combination of the
photocatalytic H2O2 production by H2O oxidation with O2 and the
catalytic hydroxylation of benzene to phenol with H2O2
demonstrated in this study has paved a new road towards direct
oxygenation of substrates by O2 as the most environmentally
benign oxidant as well as oxygen source without any electron
source except H2O, which is also the most environmentally benign
hydrogen source.
16O2-saturated solvent mixture of MeCN and H218O (v/v = 23:2) at 298 K for
24 h. The peaks at m/z = 94 and 96 correspond to Ph16OH and Ph18OH,
respectively.
ESI†), the rate of PhOH production in the photocatalytic
hydroxylation of benzene by O2 corresponds to that of PhOH
production in the catalytic oxidation of benzene with 127 μM
H2O2, which may be the steady state concentration during the
photocatalytic hydroxylation of benzene.
Then, the 18O2-labeling experiments were performed to
confirm that oxygen atom in the phenol product in eqn (1)
derived from O2 (Fig. 8). Indeed, Ph18OH (73%) was formed as
the major product together with Ph16OH (27%) in the 18O2-
labeling experiments, indicating that the oxygen atom in the
PhOH product obtained in the photocatalytic oxidation of
benzene by H218O2 derived from 18O2 (98% 18O-enriched; Fig.
8). Formation of Ph16OH (27%) indicates that 16O2 was
produced by the oxidation of H216O by [RuIII(Me2phen)3]3+,
leading to production of H216O2 to oxidize benzene to produce
Ph16OH instead of Ph18OH (Scheme 1, reaction pathway a). The
18O-labeling experiments were also performed by replacing
H216O with H218O to support that oxygen atom in the phenol
product derived from O2 rather than H2O (Fig. 9). In this case,
Ph16OH (66%) was the major product together with Ph18OH
(34%), consistent with the 18O2-labeling experiments described
above.
Experimental section
Materials
All solvents and chemicals were of reagent-grade quality, obtained
commercially and used without further purification, unless
otherwise noted. Ruthenium(III) chloride hydrate, ammonium
hexafluorophosphate, n-butyllithium solution (2.7 M in heptane), n-
pentane, and tetrahydrofuran were purchased from Aldrich
Chemicals. The chemicals, such as 4,7-dimethyl-1,10-
phenanthroline (Me2phen), silver sulphate, lead dioxide, and
The photocatalytic mechanism of benzene hydroxylation to
phenol by O2 is summarized in Scheme 1. The photoexcitation
of [RuII(Me2phen)3]2+ in an O2-saturated solvent mixture of
MeCN and H2O (v/v = 23:2) containing Sc(NO3)3 resulted in
electron transfer from the triplet excited state of
[RuII(Me2phen)3]2+ to O2 to produce [RuIII(Me2phen)3]3+ and the
•–
,
O2•––Sc3+ complex.23,28 The strong binding of Sc3+ to O2
which was detected by EPR, inhibits the back electron transfer
from the O2•––Sc3+ complex to [RuIII(Me2phen)3]3+, followed by
disproportion with H2O to produce H2O2.23 H2O2 has attracted
considerable attention as an ideal solar fuel for a one-
compartment H2O2 fuel cell with the theoretical maximum
output potential of 1.09 V, which is comparable to that of a
hydrogen fuel cell (1.23 V).23 [RuIII(Me2phen)3]3+ can oxidize
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cobalt(II) chloride, were supplied from Alfa Aesar. 2,2’-Bipyridine, [RuIII(Me2phen)3]3+ (100 mM) to an O2-saturated solvent mixture
1,2,3,4,5-pentamethylcyclopentadiene, and TiIV(O)(tpyp) (tpyp = (1.0 mL) of MeCN and H2O (v/v = 9:1) containing 1 (10 – 200 μM)
5,10,15,20-tetra(4-pyridyl)porphyrinato anion) were obtained from and monitored the O2 evolved.
Tokyo Chemical Industry Co., Ltd. Sc(NO3)3•4H2O was supplied from
Mitsuwa Chemicals Co., Ltd. 18O2 gas (98% 18O-enriched) was
purchased from ICON Services Inc. (Summit, NJ. USA). Purification
Acknowledgements
of water (18.2 MΩ cm) was performed with a Milli-Q system
This work was supported by SENTAN projects from Japan
(Millipore, Direct-Q 3 UV). Acetonitrile was dried according to
published procedures and distilled prior to use.30 The cobalt(III)
Science and Technology Agency (JST), JSPS KAKENHI (No.
starting complex, [CoIII(Cp*)(bpy)(H2O)]2+ (1, Cp*
=
η5-
2,2-bipyridine), and
complex, Research Program (2017R1D1A1B03029982 to Y.M.L. and
16H02268) to S.F. and by the CRI (NRF-2012R1A3A2048842 to
W.N.), GRL (NRF-2010-00353 to W.N.), and Basic Science
pentamethylcyclopenta-dienyl and bpy
=
tris(4,7-dimethyl-1,10-phenanthroline)ruthenium(II)
[RuII(Me2phen)3]2+, were prepared according to the published
methods.25,31
2017R1D1A1B03032615 to S.F.) through the NRF of Korea.
Notes and references
Instrumentation
1
M. Weber, M. Weber and M. Kleine-Boymann, Phenol.
Ullmann’s Encyclopedia of Industrial Chemistry, Wiley-VCH
Verlag GmbH & Co. KGaA, Weinheim, Germany, 2004, vol.
26, p 503.
UV-vis spectra were recorded on a Hewlett Packard 8453 diode
array spectrophotometer equipped with a UNISOKU Scientific
Instruments Cryostat USP-203A. Product analysis for oxidation
reactions was performed with an Agilent Technologies 6890N gas
chromatograph (GC) and a Thermo Finnigan (Austin, Texas, U.S.A.)
FOCUS DSQ (dual stage quadrupole) mass spectrometer interfaced
with Finnigan FOCUS gas chromatograph (GC-MS). The amount of
evolved oxygen was recorded by Clark-type oxygen electrode made
by Hansatech Ltd.
2
3
(a) R. Molinari and T. Poerio, Asia-Pac. J. Chem. Eng., 2010,
191; (b) R. J. Schmidt, Appl. Catal. A, 2005, 280, 89.
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(a) L. Balducci, D. Bianchi, R. Bortolo, R. D'Aloisio, M. Ricci, R.
Tassinari and R. Ungarelli, Angew. Chem., Int. Ed., 2003, 42
4937; (b) J.-H. Yang, G. Sun, Y. Gao, H. Zhao, P. Tang, J. Tan,
A.-H. Lu and D. Ma, Energy Environ. Sci., 2013, , 793.
,
6
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S. Fukuzumi and K. Ohkubo, Asian J. Org. Chem., 2015, 4,
836.
Product analysis
(a) X. Cai, Q. Wang, Y. Liu, J. Xie, Z. Long, Y. Zhou and J.
Wang, ACS Sustainable Chem. Eng., 2016,
Wang, L. Shi, N. Li and Y. Ma, RSC Adv., 2017,
4
7
, 4986; (b) W.
, 12738.
Products formed in the oxidation of benzene by 1 and O2 in the
presence of [RuII(Me2phen)3]2+ and Sc(NO3)3 in a solvent mixture of
MeCN and H2O (v/v = 23:2) at 298 K were identified by GC and GC-
MS by comparison of the mass peaks and retention time of the
products with respect to authentic samples, and the product yields
were determined by comparing the responsive peak areas of
sample products against standard curves prepared with known
authentic compounds using internal standard decane. Quantum
yield (QY) of photocatalytic hydroxylation of benzene has been
determined under visible light irradiation of monochromatized light
using an Asahi spectra (MAX-302). The amount of hydrogen
peroxide produced was determined by spectroscopic titration with
an acidic solution of [TiIVO(tpypH4)]4+ complex.31 The
[TiIVO(tpypH4)]4+ (50 μM) was prepared by dissolving 3.4 mg of the
TiO(tpyp) complex into water (100 mL) containing hydrochloric acid
(50 mM). A small portion (100 μL) of the photocatalytic reaction
solution was taken and diluted up to 1.0 mL with water. To 0.25 mL
of the diluted sample, 0.25 mL of perchloric acid (4.8 M) and 0.25
mL of the [TiIVO(tpypH4)]4+ (50 μM) were added. The mixed solution
was then allowed to stand for 5 min at room temperature. This
sample solution was diluted up to 2.5 mL with water and used for
the spectroscopic measurement. The absorbance at λ = 434 nm (AS)
due to [TiIVO(tpypH4)]4+ was measured by using a Hewlett Packard
8453 diode array spectrophotometer. In a similar manner, a blank
solution was prepared by adding distilled water instead of the
sample solution in the same volume with its absorbance designated
as AB. The difference in absorbance at 434 nm was determined as
follows: ΔA434 = AB – AS. Based on ΔA434 and the volume of the
solution, the amount of hydrogen peroxide was determined
according to the literature.23 A Clark-type oxygen electrode was
used to obtain oxygen evolution data, and calibrated daily using Ar
deoxygenated and oxygen saturated atmospheric solutions. Clark
electrode experiments were performed by adding 10 μL of
6
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