Multi-electron oxygen reduction by a hybrid visible-light-photocatalyst consisting of metal-oxide semiconductor and self-assembled biomimetic complex

Article abstract of DOI:10.1002/anie.201408352

Adsorption experiments and density functional theory (DFT) simulations indicated that Cu(acac)₂ is chemi-sorbed on the monoclinic sheelite (ms)-BiVO₄ surface to form an O₂-bridged binuclear complex (OBBC/BiVO₄) like hemo-cyanin. Multi-electron reduction of O₂ is induced by the visible-light irradiation of the OBBC/BiVO₄ in the same manner as a blue Cu enzyme. The drastic enhancement of the O₂ reduction renders ms-BiVO₄ to work as a good visible-light photocatalyst without any sacrificial reagents. As a model reaction, we show that this biomimetic hybrid photocatalyst exhibits a high level of activity for the aerobic oxidation of amines to aldehydes in aqueous solution and imines in THF solution at 25°C giving selectivities above 99% under visible-light irradiation.

Full text of DOI:10.1002/anie.201408352

Angewandte
Chemie
DOI: 10.1002/anie.201408352
Multi-Electron Oxygen Reduction
Multi-Electron Oxygen Reduction by a Hybrid Visible-Light-
Photocatalyst Consisting of Metal-Oxide Semiconductor and Self-
Assembled Biomimetic Complex**
Shin-ichi Naya, Tadahiro Niwa, Ryo Negishi, Hisayoshi Kobayashi,* and Hiroaki Tada*
Abstract: Adsorption experiments and density functional
theory (DFT) simulations indicated that Cu(acac)2 is chemi-
sorbed on the monoclinic sheelite (ms)-BiVO4 surface to form
an O2-bridged binuclear complex (OBBC/BiVO4) like hemo-
cyanin. Multi-electron reduction of O2 is induced by the visible-
light irradiation of the OBBC/BiVO4 in the same manner as
a blue Cu enzyme. The drastic enhancement of the O2
reduction renders ms-BiVO4 to work as a good visible-light
photocatalyst without any sacrificial reagents. As a model
reaction, we show that this biomimetic hybrid photocatalyst
exhibits a high level of activity for the aerobic oxidation of
amines to aldehydes in aqueous solution and imines in THF
solution at 258C giving selectivities above 99% under visible-
light irradiation.
light absorptivity and the driving force for O2 reduction, i.e.,
the decrease in the band gap necessarily lowers the con-
duction band (CB) minimum or the reducing ability of the
excited electrons, because the valence band (VB) maximum,
mainly consisting of O2p orbitals, is almost constant (+ 2.94 V
vs. standard hydrogen electrode, SHE).[4] Thus, the CB
minimum for the metal oxide semiconductors with Eg <
2.5 eV is located below + 0.44 V vs. SHE, which means that
the photocatalysts cannot use O2 with the standard redox
potential of oxygen (E0(O2/O2 )) of ꢀ0.284 V[5] as a one-
ꢀ
electron acceptor. In fact, sacrificial electron acceptors such
as Ag+ ions (E0(Ag+/Ag) =+ 0.799 V)[5] are necessary for
BiVO4 to operate as a photocatalyst. Thermodynamically, the
increase in the number of electrons participating in the O2
reduction facilitates its progress (E0(O2/H2O2) =+ 0.695 V
and E0(O2/H2O) =+ 1.229 V).[5] If metal oxide semiconduc-
tors with Eg < 2.5 eV can be endowed with electrocatalytic
activity for the multi-electron O2 reduction, the method
would be accessible for efficient and selective oxidative
chemical transformation processes. A fascinating approach is
the hybridization of metal oxide semiconductors exhibiting
a strong absorption in the wide spectral range and molecular
metal complexes with highly efficient and selective electro-
catalytic activity.[6,7] Although this type of hybrid photo-
catalysts has recently attracted much interest for H2 gener-
ation,[8] CO2 reduction,[9,10] and environmental purification,[11]
there is no report on oxidative organic synthesis.
Here we show that BiVO4 with an O2-bridged (acetyl-
acetonato)copper(II) binuclear complexes on the surface
gives rise to the multi-electron O2 reduction leading to a high
level of visible-light activity for the oxidation of amines to
aldehydes and imines. Both of them are important versatile
intermediates for fine chemicals and pharmaceuticals.[12]
Several conventional synthetic methods use stoichiometric
amounts of harmful or explosive reagents accompanied by the
emission of the corresponding amount of pollutants.[13,14] The
development of “green” efficient and selective catalytic
processes to produce aldehydes and imines is highly desired.
BiVO4 can take the crystal forms of monoclinic sheelite
(ms-) and tetragonal zircon structures with Egꢀs of 2.4 and
2.9 eV, respectively.[15] ms-BiVO4 particles with a specific
surface area of 0.54 m2 gꢀ1 were synthesized by a liquid-phase
method.[16] Bis(acetylacetonato)copper(II) (Cu(acac)2) and
bis(hexafluoroacetylacetonato)copper(II) (Cu(hfacac)2) were
used as the copper complexes, and (ethylenediaminetetra-
acetato)copper(II) ([Cu(edta)]2ꢀ) for comparison. The
adsorption property of Cu(acac)2 on ms-BiVO4 was studied.
Figure 1a shows the Cu adsorption isotherm on ms-BiVO4
from the solution of Cu(acac)2 at 298 K and the Langmuir
I
n industry, approximately 30% of fine chemicals are
produced by oxidation processes.[1] The development of
“green” oxidative synthetic routes utilizing the sunlight and
O2 in the air as the energy source and oxidizing agent,
respectively, is crucial for reducing CO2 emissions and saving
energy. O2 reduction is the key process in photocatalytic
reactions as well as in fuel cells and biological energy
conversion. Among a variety of visible-light photocatalysts
developed so far,[2] metal oxide semiconductors represented
[3]
by BiVO4 are very attractive because of the moderate
oxidation ability and the good physicochemical stability. Since
the solar spectrum peaks at around 500 nm, the metal oxide
should possess an absorption edge longer than 500 nm or
a band gap (Eg) smaller than 2.5 eV for an efficient use of the
sunlight. However, there is a trade-off between the visible-
[*] Dr. S.-i. Naya, Prof. Dr. H. Tada
Environmental Research Laboratory, Kinki University
3-4-1, Kowakae, Higashi-Osaka, Osaka 577-8502 (Japan)
T. Niwa, R. Negishi, Prof. Dr. H. Tada
Department of Applied Chemistry
School of Science and Engineering
Kinki University
3-4-1, Kowakae, Higashi-Osaka, Osaka 577-8502 (Japan)
E-mail: h-tada@apch.kindai.ac.jp
Prof. Dr. H. Kobayashi
Department of Chemistry and Materials Technology
Kyoto Institute of Technology
Matsugasaki, Sakyo-ku, Kyoto, 606-8585 (Japan)
E-mail: hisa@kit.ac.jp
[**] This work was supported by a Grant-in-Aid for Scientific Research
(C) No. 24550239 and MEXT-Supported Program for the Strategic
Research Foundation at Private Universities.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201408352.
Angew. Chem. Int. Ed. 2014, 53, 1 – 5
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to be 1.417 for the Cu atom. The actual oxidation number of
the Cu ion in the complex is significantly reduced from the
formal oxidation number of + 2 as a result of coordination by
the acetylacetonato ligands. In the biosystem, the active
center of hemocyanin is known to involve an O2-bridged CuI
binuclear complex (OBBC).[18] The assumption for the
formation of an OBBC on the ms-BiVO4 surface (OBBC/
ms-BiVO4) rationalizes the abnormally large adsorption
amount of Cu(acac)2 with the uptake of O2 from the solution.
Figure 2b is the DFT-optimized structure showing an O2-
bridging structure with the bond of O2 shortened from
1.208 ꢂ for the free molecule to 1.204 ꢂ due to the complex-
ation. The enthalpy of the OBBC formation (DadH0) was
calculated to be ꢀ44.2 kJmolꢀ1 from the energy change with
the uptake of O2 into the (Cu-complex)dimer (E(Cu-com-
plex)dimer-O2–E((Cu-complex)dimer)). These results indi-
cate that an OBBC can be spontaneously formed on the ms-
BiVO4 surface against the entropy loss ((DadH0ꢀ DadG0)/T=
ꢀ54.0 Jmolꢀ1 Kꢀ1).
Figure 1. a) Cu adsorption isotherm on ms-BiVO4 from the Cu(acac)2
solution at 298 K and the Langmuir plot. b) Change in the amount of
dissolved O2 in an aqueous suspension of BiVO4 with the addition of
Cu(acac)2 in a closed system.
plot. The adsorption follows the Langmuir-type behavior
suggesting that Cu(acac)2 is chemisorbed on the ms-BiVO4
surface. From the Langmuir plot, the saturated adsorption
amount (Gs) and the equilibrium constant (K) were calculated
to be 1.52 moleculenmꢀ2 and 8.50 ꢁ 104 mꢀ1, respectively. The
K value yields the standard Gibbs energy of adsorption
(DadG0) of ꢀ28.12 kJmolꢀ1. Curiously, the cross-sectional area
of Cu(acac)2 in the saturated adsorption state (Gsꢀ1 = 0.66 nm2
moleculeꢀ1) is significantly smaller than the value for the free
molecule estimated by the density functional theory (DFT)
calculations (0.97 nm2 moleculeꢀ1). The concentration of dis-
solved O2 in an aqueous suspension of ms-BiVO4 was
measured by a dissolved O2 meter in a closed system.
Figure 1b shows the change in the concentration of dissolved
O2 with the stepwise addition of Cu(acac)2, where C expresses
the nominal Cu(acac)2 concentration at each step. The O2
concentration decreases with an increase in the concentration
of Cu(acac)2 added. Evidently, Cu(acac)2 is chemisorbed on
the BiVO4 surface, and its adsorption is accompanied by the
uptake of O2 from the solution.
The adsorption state of Cu(acac)2 on the ms-BiVO4
surface was studied by X-ray photoelectron spectroscopy
(XPS). Cu2p-XPS spectra were measured for Cu(acac)2-
adsorbed ms-BiVO4 with varying amount of Cu(acac)2 (Fig-
ure S1). A signal appears with the Cu(acac)2 adsorption at the
binding energy (EB) of 931.8 eV due to the emission from
Cu2p3/2, and the shake-up features unique to CuII are
absent.[17] Figure 2a shows the optimized structure of Cu-
(acac)2 by DFT calculations. The atomic charge was estimated
The effect of the surface Cu complex on the O2 reduction
property of ms-BiVO4 was studied by electrochemical meas-
urements for ms-BiVO4 film-coated fluorine-doped tin oxide
electrodes (ms-BiVO4/FTO) in the dark. Figure 3 shows
current (J)–potential (E) curves for ms-BiVO4/FTO in
Figure 3. Dark current (J)–potential (E/V vs. SHE) curves for the ms-
BiVO4/FTO electrodes in deaerated (a) and aerated (b) 0.1m NaClO4
solutions containing various amounts of Cu(acac)2.
deaerated (a) and aerated (b) 0.1m NaClO4 solutions
containing various amounts of Cu(acac)2. In the absence of
O2, only a small current flows at E <+ 0.2 V regardless of the
addition of Cu(acac)2. In the presence of O2, the addition of
Cu(acac)2 causes a drastic increase in the current scaling with
an increase in the Cu(acac)2 concentration. Interestingly,
there are two current shoulders around + 0.12 V (p1) and
ꢀ0.2 V (p2). The potentials for p1 and p2 are more positive
than the E0(O2/O2 ) value. The p1 and p2 can be attributed to
ꢀ
the four- and two-electron reductions of O2, respectively.
Clearly, the surface modification by Cu(acac)2 endows ms-
BiVO4 with the ability for multi-electron O2 reduction, and
thus, we can expect that OBBC/ms-BiVO4 works as a visible-
light photocatalyst in the presence of O2.
As a model reaction, the OBBC/ms-BiVO4-photocata-
lyzed oxidation of benzylamine was chosen. ms-BiVO4
particles (0.2 g) were added to 200 mL of aqueous (2%
acetonitrile) or THF (2% acetonitrile) solutions of Cu(acac)2
or Cu(hfacac)2. The small amount of acetonitrile was added
for the complexes to be dissolved completely. After the
Figure 2. Optimized structures of Cu(acac)2 (a) and Cu(acac)2-O2-
Cu(acac)2 dimer complex (b) with neutral triple and quintet states,
respectively, by the hybrid-type DFT calculations.
2
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The square-planar Cu(acac)2 complex can
further have basic solvent molecules like
THF as axial ligands,[20] whose coordina-
tion bond is rather weak due to the Jahn–
Teller effect. Thus, the axial ligand may be
partially replaced by O2 to form the
OBBC. In Cu(edta), edta strongly coor-
dinates to Cu2+ as a hexadentate ligand to
form a stable complex, which would be
inactive for the ligand exchange with O2.
Cu(acac)2 and Cu(hfacac)2 do not absorp
at l > 350 nm and therefore do not partic-
ipate in the photoprocess. Visible-light
irradiation of OBBC/ms-BiVO4 excites
the electrons from the VB to the CB of
ms-BiVO4. The VB holes oxidize benzyl-
Figure 4. a) Time course for the oxidation of benzylamine with and without copper(II)
complexes. b) Time course for the control experiments of the oxidation of benzylamine.
Reaction conditions: ms-BiVO4 0.2 g, benzylamine (500 mm) solution (200 mL,
H2O:acetonitrile=98:2 v/v) with and without copper(II) complex (4.0 mm), visible-light
(l>430 nm, 6 mWcmꢀ2) irradiation at 258C under aerated or deaerated conditions.
suspension had been stirred for 1 h under aerobic conditions
Table 1: OBBC/ms-BiVO4-photocatalyzed oxidation of benzylamines.[a]
in the dark, it was irradiated by visible light (l > 430 nm, I420–
485 nm = 6 mWcmꢀ2). Figure 4a shows the time course for the
reactions with and without addition of copper complexes. No
reaction occurred in the dark as well as under irradiation in
the absence of copper complexes or in the presence of
Cu(edta). Visible-light irradiation of ms-BiVO4 in the pres-
ence of Cu(acac)2 or Cu(hfacac)2 induces the efficient
oxidation of benzylamine to benzaldehyde. Chemical analysis
by inductively coupled plasma spectroscopy of the particles
recovered after the reaction confirmed that no Cu was
deposited as metal and oxides on the surface. The turnover
number (TON) for benzaldehyde generation after 2 h is 25. A
serious problem that is common for molecular complex/
semiconductor photocatalysts is the degradation of the
complex. In the present system, the amount of generated
benzaldehyde increases in proportion to the irradiation time.
The OBBC is self-repaired by the self-assembling nature even
if it partially degrades during the reaction. As shown in
Figure 4b, O2 is necessary for the reaction to be sustained.
Previously, loading Pt nanoparticles on WO3 was reported to
boost its visible-light activity for the decomposition of acetic
acid and acetaldehyde.[19] The enhanced activity was attrib-
uted to the multi-electron O2 reduction affected by the Pt
nanoparticles. The photocatalytic activity of Cu(hfacac)2/ms-
BiVO4 is far greater than that of Pt/ms-BiVO4 and Au/ms-
BiVO4. Evidently, the very high level of photocatalytic
activity stems from the cooperative effect of Cu(acac)2 or
Cu(hfacac)2 and ms-BiVO4, where the copper complex acts
not as a sacrificial agent but as an excellent electrocatalyst for
the O2 reduction. Table 1 summarizes the results on the
OBBC/ms-BVO4-photocatalyzed oxidation of benzylamine
derivatives. In every case, the corresponding aldehydes are
generated in high selectivity (> 99%). Further, the reactions
were carried out by changing the solvent from H2O to THF. In
this case, the corresponding imines are selectively formed
(> 99%; Table 2).
Substrate
Product
Yield [mm]
112
Selectivity [%]
>99
83
>99
68
>99
108
>99
[a] Reaction conditions: ms-BiVO4 0.2 g, amine (500 mm) solution
(200 mL, H2O:acetonitrile=98:2 v/v) with Cu(hfacac)2 (4.0 mm), visible-
light (l>430 nm, 6 mWcmꢀ2) irradiation for 2 h at 258C under aerated
conditions.
Table 2: OBBC/ms-BiVO4-photocatalyzed oxidation of benzylamines.[a]
Substrate
Product
Yield [mm]
166
Selectivity [%]
>99
228
>99
188
>99
154+20
>99
[a] Reaction conditions: ms-BiVO4 0.2 g, amine (500 mm) solution
(200 mL, THF : acetonitrile=98:2 v/v) with Cu(hfacac)2 (4.0 mm),
visible-light (l>430 nm, 6 mWcmꢀ2) irradiation for 2 h at 258C under
aerated conditions.
amines to yield the corresponding imines and H+. In the
presence of H2O, the resulting imines undergo hydrolysis to
afford aldehydes. In the absence of H2O as shown in Table 2,
the imines react with another amine to give the substituted
imines. The VB maximum of BiVO4 is raised up to + 2.53 V
from + 2.94 V for the usual O2p band by the mixing of O2p
and Bi6s orbitals.[21] The high selectivity of the reactions can
be partly attributed to the mild oxidation ability of the VB
holes. On the other hand, the direct copper complex–BiVO4
bonding and the concentration of O2 near the surface enable
On the basis of these results, we present the a basic
reaction mechanism for the oxidation of benzylamine deriv-
atives to the corresponding aldehydes and imines. In the dark,
Cu(acac)2 or Cu(hfacac)2 molecules are self-assembled on the
ms-BiVO4 surface with the uptake of O2 to form an OBBC.
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were determined by high-performance liquid chromatography (LC-6
AD, SPD-6 A, C-R8 A (Shimadzu)) [measurement conditions:
column = Shim-pack CLC-ODS (4.6 mm ꢁ 150 mm) (Shimadzu);
eluent = acetonitrile; flow rate = 1.0 mLminꢀ1; l = 280 nm].
the rapid electron transfer from the CB of BiVO4 to O2 in the
OBBC through the copper ions.[7,22] Consequently, the multi-
electron reduction of O2 efficiently proceeds through the pair
of Cu ions in the OBBC, being accompanied by the coupling
with H+. H2O2 was not detected from the solutions after the
reaction. The CB electrons have a potential (+ 0.13 Vat pH 0)
that is high enough for the four-electron reduction of O2 but
insufficient for its two-electron reduction (Figure 3b). The
Received: August 19, 2014
Published online: && &&, &&&&
Keywords: biomimetic complex · oxidation · photocatalysis ·
+
.
H -transfer-coupled electron transfer may proceed in a con-
oxygen reduction · visible light
ꢀ
certed fashion with the O O bond cleavage to produce
H2O.[23] This reminds us of laccase with a trinuclear Cu site
that catalyzes the four-electron reduction of O2.[18] The
regeneration of the OBBC by the ligand exchange between
the H2O and O2 completes the catalytic cycle.
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We have shown that a ms-BiVO4 light harvester self-
integrated with an O2-bridged binuclear copper complex on
the surface is an efficient electrocatalyst for the multi-electron
reduction of O2 under visible-light irradiation. This biomi-
metic hybrid photocatalyst exhibits high levels of visible-light
activity for the selective aerobic oxidation of benzylamines to
the corresponding aldehydes and imines without a sacrificial
reagent. We anticipate that the exploitation of each compo-
nent of the hybrid photocatalyst will pave the way to new
“green” routes for the synthesis of valuable organic com-
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Experimental Section
Catalyst preparation and characterization: According to the liter-
ature reported by Kohtani et al., ms-BiVO4 was prepared from
Bi(NO3)2 and NH4VO3.[16] X-ray photoelectron spectroscopic (XPS)
measurements were performed on a Kratos Axis Nova X-ray
photoelectron spectrometer with a monochromated Al Ka X-ray
source operated at 15 kVand 10 mAusing C1s as the energy reference
(284.6 eV).
Electrochemical measurements:
A paste of the ms-BiVO4
precursor was prepared from Bi(NO3)3 (1 mmol), NH4VO3
(1 mmol), poly(ethylene glycol) 20000 Da (4 g), Triton X-100 (1
drop), and conc. HNO3 (2 mL). The paste was coated on fluorine-
doped SnO2 (FTO)-film-coated glass substrates (sheet resistance =
10 W/square), and the samples were heated in air at 673 K for 4 h to
form mp-ms-BiVO4/FTO electrodes. The electrochemical cells com-
prising of the mp-ms-BiVO4/FTO j Cu(acac)2 + 0.1m NaClO4 (aque-
ous electrolyte solution) j glassy carbon were fabricated. The active
area of the cell was 3.0 ꢁ 2.5 cm2. The current density (mAcmꢀ2) was
measured at the dark rest potential by using a galvanostat/potentio-
stat (HZ-5000, Hokuto Denko).
Oxidation of benzylamine derivatives: A suspension of ms-
BiVO4 (200 mg) in a solution (H2O:acetonitrile = 98:2 v/v or tetrahy-
drofuran:acetonitrile = 98:2) of benzylamine (500 mm, 200 mL) with
Cu-complex (4 mm) was stirred at 298 K for 30 min in the dark.
Irradiation was started using a 300 W Xe lamp (HX-500, Wacom)
with a cut-off filter Y-45 (AGC TECHNO GLASS) in a double jacket
type reaction cell (81 mm in diameter and 77 mm in length). The light
intensity integrated from 420 to 485 nm (I420–485) through a Y-45
optical filter was adjusted to 6.0 mWcmꢀ2. The yield and selectivity
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Chemie
Communications
Multi-Electron Oxygen Reduction
S.-i. Naya, T. Niwa, R. Negishi,
Light-driven oxidation of benzylamines to
aldehydes and imines with air at 258C
with selectivities of more than 99% and
without sacrificial reagents is achieved by
a biomimetic hybrid photocatalyst. The
catalyst is obtained by chemisorption of
Cu(acac)2 onto BiVO4 to form an O2-
bridged binuclear complex. The multi-
electron reduction of O2 is induced by
visible-light irradiation. vb=valence
band, cb=conducting band.
H. Kobayashi,* H. Tada*
&&&& — &&&&
Multi-Electron Oxygen Reduction by
a Hybrid Visible-Light-Photocatalyst
Consisting of Metal-Oxide
Semiconductor and Self-Assembled
Biomimetic Complex
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