X. Yang et al. / Journal of Catalysis 373 (2019) 116–125
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with sacrificial reagents, such as triethanolamine, Na2SO3 or
2.2. Typical experimental procedure for photocatalytic test
methanol [21–23]. Though both electrons and holes possess excel-
lent reducing and oxidizing capacities, respectively, the simultane-
ous exploitation of photogenerated holes and electrons to promote
both photocatalytic reactions of oxidation and reduction in a single
system has rarely been reported. Jiang’s group recently demon-
strated a strategy in which hydrogen was produced by the photo-
generated electrons while the benzylamine oxidation was
catalyzed by the holes over a Pt/PCN-777 composite [24]. The
recent development of such coupled systems that fully exploit
photoexcited electrons and holes has recently attracted great inter-
est for the greater utility of photocatalytic materials [25–27].
The selective oxidation of alcohols to benzaldehydes is a promis-
ing approach for the production of high-value-added chemicals and
products [28–31]. Meanwhile nitrobenzene is an electron-
accepting substrate that can be reduced by high-negative-value
conduction band electrons [32–35], but it still remains challenging
to improve its low conversion and selectivity. Here, experimental
characterizations in conjunction with DFT calculations reveal that
OVs in situ induced through surface complexation, which broaden
the light absorption range and act as a bridge to enable the transfer
of the photoinduced electrons. The benzyl alcohol oxidation half
reaction mediated by photogenerated holes is desired. Simultane-
ously, rather than recombine with their counterpart holes, efficient
electron transport may occur from the TiO2 conduction band to
nitrobenzene for the other half reaction. Moreover, the TiO2 with
OVs demonstrate high activity and long stability in this coupled
system. This work provides a new perspective on utilizing the com-
plexation between the reactants and catalyst to achieve a defect
engineering strategy for synergic photocatalysis.
In a typical procedure, 40 mg of TiO2, 31 mL (0.3 mmol) of benzyl
alcohol and 10 mL (0.1 mmol) of nitrobenzene were added into
10 mL of reagent grade benzotrifluoride (BTF), n-dodecane was
used as an internal standard. The mixture was evacuated and back-
filled with N2 (1.0 atm) five times to completely remove air and
allowed to stir for 30 min in the dark. The reaction mixture was
then magnetically stirred at 800 rpm and illuminated by the 300-
W Xenon Illuminator System with an CM 1 filter in the wavelength
region of 380–760 nm. At a certain time after the photocatalytic
reaction, an aliquot of the reaction solutions (60 mL) were pulled
out, filtered by a one-time plastic filter and diluted by 0.5 mL etha-
nol for gas chromatograph analysis (Agilent 7890A equipped with
an FID detector). The conversion of alcohol, nitrobenzene, and yield
of aldehyde and aniline were defined as follows:
Conversion ð%Þ ¼ ½ðC0 ꢀ CrÞ=C0ꢁ ꢂ 100
Yield ð%Þ ¼ Ct=C0 ꢂ 100
ð1Þ
ð2Þ
C0 is the initial concentration of reactant; Cr and Ct are the remain-
ing concentrations of reactant and target product in the reaction
system. In particular, the yield of the Schiff base is based on the ini-
tial concentration of nitrobenzene. The products were confirmed by
gas chromatography mass spectrometry (430 GC Varian, USA) and
comparing their retention times with those of standard samples.
When the photocatalytic experiments completed, the reaction mix-
tures were centrifuged to separate the photocatalyst from reaction
mixture and dried in an oven at 70 °C overnight for the following
cycling photoactivity test and further characterization. For clarity,
the catalysts after the photocatalytic reaction in the coupled sys-
tems were denoted as used TiO2, unless otherwise stated. To
explore the scope of our catalyst, the benzyl alcohol was replaced
by 0.3 mmol of different diverse aromatic alcohols while other
experimental parameters were remained. Controlled photoactivity
experiments using radical scavengers, triethanolamine (0.3 mmol)
as the scavenger for photogenerated holes, and CCl4 (0.3 mmol) as
the scavenger for electrons, were performed in a similar manner
to the above photocatalytic experiment with the radical scavengers
added to the reaction system.
2. Experimental
2.1. Materials and methods
All of the reagents were purchased from commercial suppliers
and used without further purification. Anatase TiO2 microspheres
were prepared by the microwave-assisted method [36]. Typically,
3.6 mL of TiCl4 solution (14% in dichloromethane) was slowly
added into a mixture of 24 mL of methanol and 12 mL of acetic acid
under magnetic stirring in an ice bath. The clear mixture solution
was then transferred into a 20 mL vial for microwave reaction.
The synthesis was performed in a 400-W microwave oven heated
at 130 °C for 1 h (Initiator 8 EXP, Biotage Corp). The white TiO2
powders could be obtained after washing several times with etha-
nol and drying at 70 °C under vacuum. The fluorinated TiO2 was
prepared by adding 10 mM NaF into a 0.5 g/L TiO2 aqueous solu-
tion, evaporating the water from the suspension and drying at
70 °C for 2 h.
2.3. Electrochemistry measurements
The catalyst-modified electrode was prepared on a fluorine-
doped tin oxide (FTO) glass. The FTO slide was previously protected
using Scotch tape to ensure the exposed area of the working elec-
trode was controlled at 1.0 cm2. The FTO slide was dip coated with
40 mL of slurry, which was obtained from the mixture of 10 mg of
photocatalyst, 0.7 mL of DMF and 0.3 mL of alcohol under sonica-
tion for a certain time. After air drying, the working electrode
was further dried at 150 °C for 2 h to improve adhesion. All the
electrochemistry measurements were carried out using a ZM6ex
electrochemical station (Zahner, Germany) in a three-electrode
quartz cell. The Mott-Schottky and photocurrent analyses were
measured in a 0.2 M Na2SO4 aqueous solution and purged with
N2 before the measurement. The light irradiation source was the
same as that used in the aforementioned photoactivity tests. Cyclic
voltammetry was carried out using 0.2 M solution of tetra-n-
butylammonium hexafluorophosphate (TBAPF6) in CH3CN as the
supporting electrolyte. A glassy carbon electrode was used as the
working electrode for the measurement of the oxidative potential
of benzyl alcohol.
Powder X-ray diffraction (XRD) was recorded by a Rigaku Mini-
Flex 600 diffractometer with Cu Ka irradiation. The morphologies
of the samples were examined by scanning electron microscopy
(SEM, JSM6700) and high-resolution transmission electron micro-
scopy (TEM, JEM2010). The Brunauer-Emmett-Teller (BET) specific
surface area of the samples was analyzed by nitrogen adsorption in
a Micromeritics ASAP 2020 apparatus. UV–visible diffuse reflec-
tance spectroscopy was measured by a Shimadzu UV-2600 with
BaSO4 as the reference and transformed to the absorption spectra
according to the Kubelka-Munk relationship. The EPR spectra were
obtained on a Bruker-BioSpin E500 spectrometer at room temper-
ature. The irradiation source was same light source used in our
photocatalytic experiments described below. A pump wavelength
of 400 nm was used for the time-resolved photoluminescence
(TRPL), which originates from the frequency-doubled 800 nm laser
pulses generated from a Coherent Libra Regenerative Amplifier
using a Beta Barium Borate (BBO) crystal.