Photocatalytic synthesis of N-benzyleneamine from benzylamine on ultrathin BiOCl nanosheets under visible light

Article abstract of DOI:10.1016/j.jcat.2019.10.018

Ultrathin BiOCl nanosheets (NST) with the thickness about 1.5 nm was prepared as a photocatalyst for the oxidation of benzylamine (BA) to N-benzyleneamine under visible light. The photocatalytic activity of NST is over 2.5 times higher than that of its bu

Full text of DOI:10.1016/j.jcat.2019.10.018

Journal of Catalysis xxx (xxxx) xxx
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Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Photocatalytic synthesis of N-benzyleneamine from benzylamine on
ultrathin BiOCl nanosheets under visible light
Yahang Ren a, Junhua Zou a, Kaiqiang Jing a,b, Yanyang Liu a, Binbin Guo a,b, Yujie Song a, Yan Yu c,
Ling Wu a,b,
⇑
a State Key Laboratory of Photocatalysis on Energy and Environment, Fuzhou University, Fuzhou 350116, PR China
b State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, PR China
c Key Laboratory of Eco-materials Advanced Technology (Fuzhou University), Fujian Province University, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Ultrathin BiOCl nanosheets (NST) with the thickness about 1.5 nm was prepared as a photocatalyst for
the oxidation of benzylamine (BA) to N-benzyleneamine under visible light. The photocatalytic activity
of NST is over 2.5 times higher than that of its bulk counterpart. More oxygen vacancies (OVs) in NST
were revealed by EPR and XPS. UV-DRS result indicates that light adsorption of NST can be extended
to visible light due to the presence of OVs. In addition, rich OVs result in Bi atoms in sublayer to be
exposed as Lewis acid sites. In situ FTIR spectrum shows that BA is chemosorbed on the Lewis acid sites
forming surface -N∙∙∙Bi- coordination species, resulting in the activation of BA. The surface coordination
species would also result in the red shift of the photo-absorption spectrum. Finally, a synergistic mech-
anism based on surface coordination activation and photocatalysis was proposed to explain the enhanced
photocatalytic activity over NST.
Received 7 August 2019
Revised 27 September 2019
Accepted 14 October 2019
Available online xxxx
Keywords:
Ultrathin BiOCl nanosheets
Oxygen vacancy
Surface coordination
Photocatalytic
Ó 2019 Elsevier Inc. All rights reserved.
Benzylamine
N-benzyleneamine
1. Introduction
catalyst for photocatalysis oxidation coupling of BA to N-
benzyleneamine [5,19]. In addition, based on the semiconductor
Catalytic oxidation of amine to imine has always been a
research hotspot since imines are indispensable intermediates for
fine chemicals and pharmaceuticals [1–3]. Traditionally, imines
are produced though the dehydration condensation of amines
and carbonyl compounds. However, the carbonyl compounds are
extremely reactive and make the condensation difficult to control
[4]. In recent years, it has been reported that imines can be direct
oxidation from two moles of the primary amines in the presence of
a catalyst. Nevertheless, the reaction requires high temperatures,
typically >100 °C, as well as the use of precious metals [5]. There-
fore, an efficient and economical method is desired for the synthe-
sis of imines from amines under mild conditions.
As a green process, photocatalysis has been applied in many
fields, such as water splitting, CO2 reduction and pollutants degra-
dation [6–9]. Photocatalytic organic synthesis has also attracted
wide attention [10–15]. Photocatalytic conversion of benzylamine
to the corresponding imine under visible light appears to be an
ideal green approach [3,16–18]. It was reported that the conver-
sion of BA would be improved by loading precious metals on a
valence band theory, a photocatalytic process is generally
explained by the action of electron-hole pairs [20]. However, the
coordination and activation of reactant molecules on the catalyst
surface were less concentrated on [21]. Therefore, to explore the
interaction between reactant molecules and the surface of a cata-
lyst is necessary to understand the intrinsic mechanism for photo-
catalytic organic transformation at a molecular level.
Ultrathin two-dimensional nanosheets has attracted attention
due to their larger surface area, more active sites, and faster
electron-hole separation capabilities, such as monolayer Bi2MoO6
nanosheets [22,23], monolayer HxTiyO4ꢀH2O nanosheets [24–26]
and monolayer HNb3O8 nanosheets [21], which could show excel-
lent photocatalytic activity in organic reactions [27]. BiOCl, as one
of the typical layered compounds, has aroused extensive attention
in recent years [28–32]. It consists of [Bi2O2]2+ layers sandwiched
between two halogen ions slabs. The thickness of monolayer BiOCl
nanosheets is only about 0.7 nm [33]. More unsaturated sites, such
as oxygen defects, could be produced on the surface of NST as
reducing its thickness into molecular level. The OVs induced local
state could extend the photoresponse to visible light and effec-
tively capture charge carriers, resulting in enhancing photoreactiv-
ity [34,35]. Moreover, native OVs possess abundant localized
⇑
Corresponding author.
E-mail address: wuling@fzu.edu.cn (L. Wu).
https://doi.org/10.1016/j.jcat.2019.10.018
0021-9517/Ó 2019 Elsevier Inc. All rights reserved.
Please cite this article as: Y. Ren, J. Zou, K. Jing et al., Photocatalytic synthesis of N-benzyleneamine from benzylamine on ultrathin BiOCl nanosheets under
visible light, Journal of Catalysis, https://doi.org/10.1016/j.jcat.2019.10.018

2
Y. Ren et al. / Journal of Catalysis xxx (xxxx) xxx
electrons which can easily activate O2 to further producing reactive
oxygen species [36,37]. In addition, more exposed unsaturation
metal sites could act as Lewis acid sites. The unsaturated metal
centers can coordinate with organic molecules and activate them
[38]. This process is critical to the overall photocatalytic reaction.
Therefore, NST could as an ideal catalyst for photocatalytic synthe-
sis of N-benzyleneamine from BA. Finally, this structure provides a
reference for researching the role of defects in photocatalytic
organic synthesis due to its unique electronic construction.
Herein, we prepared ultrathin BiOCl nanosheets with a thick-
ness of about 1.5 nm for photocatalytic oxidation coupling of BA
to N-benzyleneamine, which exhibit superior photocatalytic per-
formance compared to its bulk counterpart. The unique ultrathin
structure, large specific surface area and strong light absorption
were revealed by TEM, AFM, BET and UV–vis DRS. OVs on the sur-
face of nanosheets were investigated by EPR and XPS. In addition,
the interactions between BA molecules and surface acidic sites of
nanosheets were presented by in situ FTIR. Based on the detailed
characterization, a possible mechanism was proposed at a mole-
cule level for the photocatalytic oxidation coupling BA to N-
benzyleneamine over NST.
adsorption-desorption isotherms were obtained on Micrometric
3020 M. The samples were degassed in vacuum at 160 °C for 8 h
and then analyzed at 77 K. X-ray photoelectron spectra (XPS) were
carried on a PHI Quantum 2000 XPS system with Al Ka as the exci-
tation source and the reference C1s binding energies is 284.8 eV.
Electron paramagnetic resonance (EPR) signals were recorded on
a Bruker A300 spectrometer at room temperature using a 300 W
Xe lamp with a 400 nm cutoff filters as light source (Beijing Trus-
tech, PLS-SXE300c). NH3 temperature-programmed desorption
(NH3– TPD) was carried out on a Micromeritics AutoChem II
2920 instrument. 100 mg of a sample was pretreated at 260 °C
for 2 h under helium stream and then cooled to 30 °C. Then, 3%
NH3/He gas was introduced and maintained for 30 min at flow rate
30 mL/min. Desorption was conducted with increasing tempera-
ture up to 270 °C (10 °C/min).
2.4. In-situ FTIR measurement
The in-situ FTIR spectra of BA and pyridine adsorbed on BiOCl
samples were carried out on a NICOLET IS50 Fourier transform
infrared (FT-IR) spectrometer. A total of 64 scans at a resolution
of 4 cmꢁ1 were performed to obtain each spectrum. Firstly, a pow-
der sample (16 mg BiOCl + 8 mg KBr) was pressed into a self-
supporting IR disk. The disk was then put into the sample holder
which could be moved vertically along the cell’s tube. Before mea-
surements, the disk was treated under dynamic vacuum
(4 ꢂ 10ꢁ2 hPa) at 200 °C for 3 h to remove surface contaminants.
2. Experimental section
2.1. Reagents and chemicals
All reagents with analytical grade were used as received with-
out further purification. NaCl, mannitol, polyvinylpyrrolidone K30
(PVPK30) and bismuth nitrate pentahydrate (Bi(NO3)3ꢀ5H2O) are
purchased from Sinopharm Chemical Reagent Co. (SCRC). Benzy-
lamine (99%) and N-benzyleneamine (99%) are obtained from J&K
Scientific Ltd. and Alfa Aesar China Co., Ltd., respectively.
After the disk cooling to room temperature, 20 lL of BA or pyridine
was spiked into the cell with a syringe via a septum. After adsorp-
tion equilibrium was reached for 30 min later, FTIR spectrum of a
sample was collected. The physisorbed BA or pyridine were
removed by a further evacuation at 150 °C for 3 min under
4 ꢂ 10ꢁ2 hPa, and then, another FTIR spectrum was taken for
revealing the information of organic substrates chemisorbed on a
sample.
2.2. Catalyst preparation
Ultrathin BiOCl nanosheets (denoted as NST) was prepared by
using a modified method reported by Xie et al [33]. Firstly,
0.800 g of PVPk30 and 0.002 mol of Bi(NO3)3ꢀ5H2O were succes-
sively dissolved in 50 mL of mannitol solution (0.1 M) under vigor-
ous stirring. Then, 10 mL of saturated sodium chloride solution was
slowly added into the mixed solution under continuous stirring.
Next, the mixture was continually stirred for 10 min and then
transferred into a 100 mL Teflon-lined stainless-steel autoclave,
which was heated at 160 °C for 5 h. The precipitates were collected
by centrifugation and washed with deionized water and ethanol to
remove residual ions. Finally, the products were dried at 60 °C for
6 h. In order to compare, bulk BiOCl (denoted as Bulk) was also pre-
pared in the same procedure without PVPK30.
2.5. Electrochemistry measurement
The working electrode was prepared on fluorine-doped tin
oxide (FTO) glass. Firstly, FTO glasses were successively cleaned
by sonicating in chloroform, acetone and ethanol for 30 min. Then,
an FTO slide was coated with 10 lL of slurry. The slurry was
obtained by sonicating the mixture of 5 mg of a sample and
0.5 mL of DMF for 1 h. After the slide was dried at 60 °C for 1 h,
the uncoated part of the electrode was isolated with an epoxy resin
and the exposed area of the electrode was 0.25 cm2. The electro-
chemical measurements were performed in a conventional three
electrode cell, using a Pt plate and a saturated Ag/AgCl electrode
as counter electrode and reference electrode, respectively. The
working electrode was immersed in a 0.2 M of Na2SO4 aqueous
solution without any additive for 30 s before measurement. The
Mott-Schottky plots were obtained at three different frequencies
(0.5 k, 1 k and 1.5 kHz) on a Zahner electrochemical workstation.
Then, we make a line tangent to its straight line, which is indicated
by the black symbol. Three tangent lines intersect on the X-axis to
obtain the flat-band potential value [39–41].
2.3. Materials characterization
The X-ray diffraction (XRD) of prepared samples were charac-
terized on the Bruker D8 Advance X-ray diffractometer with Ni-
filtered Cu Kirradiation (k = 1.5406 Å) at 40 kV and 40 mA. The field
emission scanning electron microscope (SEM) was used to reveal
the morphologies of the samples on a FEI Quanta 200F electron
microscope. Transmission electron microscopy (TEM) and higher-
resolution transmission electron microscopy (HRTEM) images
were acquired on a JEOL model JEM2010 EX microscope at an
accelerating voltage of 200 kV to further investigate the structure
and morphology of samples. Atomic force microscopy (AFM) to
2.6. Photocatalytic activity test
The photocatalytic oxidation coupling of benzylamine (BA) was
carried out as follows. A mixture of BA (0.2 mmol) and 8 mg of a
sample was added with the solvent of acetonitrile (1 mL) in a
10 mL of Pyrex glass reaction tube. Then, the mixture was stirred
for 30 min at O2 atmosphere to evenly mix the sample in the solu-
tion. Next, the suspension was irradiated by a 300 W Xe arc lamp
with a 400 nm-Cut filter. After irradiated for 10 h, the mixture
determine the thickness of nanosheets were recorded on
a
Nanoscope E multimode. UV–vis diffuse reflectance spectra (UV–
vis DRS) were got on an ultraviolet–visible spectrophotometer
(Cary 500) with Barium sulfate as reference. Nitrogen
Please cite this article as: Y. Ren, J. Zou, K. Jing et al., Photocatalytic synthesis of N-benzyleneamine from benzylamine on ultrathin BiOCl nanosheets under
visible light, Journal of Catalysis, https://doi.org/10.1016/j.jcat.2019.10.018

Y. Ren et al. / Journal of Catalysis xxx (xxxx) xxx
3
was centrifuged to completely remove the catalyst particles. The
supernatant solution was determined with a Shimadzu Gas Chro-
matograph (GC-2014C). Gas chromatography-mass spectrometry
(GC-MS) was carried out to confirm the identity of the products
on an Agilent gas chromatograph 7890B-mass spectrometry
5977B. The conversion of BA and the selectivity of N-
benzyleneamine were defined as follows:
between adjacent points is 45°, which is corresponding to the the-
oretical value of the angle between the (1 1 0) and (2 0 0) planes of
BiOCl. This means a group of diffraction points is indexed to the
[0 0 1] region axis, which is in good agreement with the XRD result.
Moreover, in order to further characterize the thickness of NST, we
performed the AFM measurement. The AFM image (Fig. 3d–f) sug-
gests the thickness of NST is about 1.5 nm. Considering that the c
parameter of BiOCl is 7.37 Å [33], it means NST with a 1.5 nm
thickness consists of nearly 2 [ClABiAOABiACl] units.
Conv
ersionð%Þ ¼ ½ðC0 ꢁ CBAÞ=C0ꢃ ꢂ 100
The changes of morphological for a sample may have an impor-
tant influence on its surface area. Therefore, N2 adsorption-
desorption was carried out to determine the surface area of pre-
pared samples. As shown in Fig. S3, the surface areas for NST and
Bulk are 9.3 m2/g and 4.3 m2/g, respectively. The surface area of
NST is 2 times higher than that of Bulk because NST has overcome
the spatial inhibition of adjacent layers. The more active sites
would be exposed on the surface of the sample NST with the large
surface area and ultrathin character. Therefore, more reagent
molecules could be adsorbed on the active sites to enhance the
photocatalytic activity.
Selecti
v
ityð%Þ ¼ ½CNꢁbenzyleneamine ꢂ 2=ðC0 ꢁ CBAÞꢃ ꢂ 100
Yieldð%Þ ¼ ð2 ꢂ CNꢁbenzyleneamine=C0Þ ꢂ 100
Where C0 is the initial concentration of BA.
3. Results and discussion
The crystal phases of samples were confirmed by XRD analysis.
As shown in Fig. 1, the single-phase diffraction patterns of pre-
pared samples are matched well with the published data (JCPDS
No. 6-0249) [42]. No characteristic peak of any other phase is
observed. This indicates that the BiOCl samples with pure phase
were successfully prepared. Furthermore, the half width of the
peaks for NST are significantly broadened than those for Bulk,
revealing the existence of different morphology in NST.
FTIR was carried out to verify the purity of the prepared sam-
ples. It can be seen from the Fig. S2 that the spectra of NST and Bulk
are similar. The peaks at 3455 cmꢁ1 and 1637 cmꢁ1 can be
assigned to the stretching vibration and the bending vibration of
O-H for the adsorbed H2O, respectively. The stretching vibrations
for C-H at about 2900 cmꢁ1 and N-H at about 3300 cmꢁ1 are not
observed, indicating the absence of residual PVP in the prepared
samples. Therefore, this result shows the prepared samples with
high purity.
The morphological features of the prepared samples were char-
acterized by scanning electron microscopy (SEM) and transmission
electron microscopy (TEM). It is apparent that the bulk sample
(Fig. 2a and b) is typical squarelike plates. NST (Fig. 2c and d) exhi-
bits thinner and typical nanosheet morphology. Moreover, the
transparent feature image of TEM (Fig. 3a) also indicates NST is
ultrathin. The HRTEM image (Fig. 3b) shows that the spacing
between the lattice stripes is 0.275 nm, that indicates the angle
of 90° matches well with the (1 1 0) atomic planes of BiOCl. As
shown in Fig. 3c, the corresponding selected-area electron diffrac-
tion (SAED) pattern displays a regular point distribution, indicating
the nanosheets exist in the form of the single-crystalline. The angle
X-ray photoelectron spectroscopy (XPS) was conducted to
determine the chemical composition of the prepared samples
and the chemical state of each element. Fig. S4 displays the survey
of the prepared samples, which shows that Bi, O and Cl elements
are presented in both of samples. The peak at about 400 eV for N
is not observed, which further reveal the prepared samples with
high purity. As shown in Fig. 4, O1s spectra of NST and Bulk can
be de-convoluted into three peaks, which represent the lattice oxy-
gen (O1), coordination of oxygen in OH (O2) and adsorbed oxygen
species (O3), respectively [43]. The binding energy of each peak is
shown in Table 1. Simultaneously, the estimated atom percentages
(at. %) of adsorbed oxygen species for NST is 17.5%. It is higher than
that for Bulk (6.7%), suggesting that NST has more oxygen vacan-
cies. The estimated atom percentages (at. %) of oxygen in OH
(38.2%) for NST is also higher than that for Bulk (18.8%), which
can be attributed to the large surface area of NST. Therefore, the
estimated atom percentages (at. %) of lattice oxygen for NST
(44.3%) is lower than that for Bulk (74.5%) due to the more Bi-O
bonds for Bulk. It can be clearly seen that the binding energy of
oxygen species in NST shows lower values compared with Bulk.
These shifts demonstrate that more electrons are transferred from
Bi to O atoms in NST, implying the electrons in Bi-O bonds are
redistributed due to more OVs in NST [22,44]. Meanwhile, Bi 4f
spectra of NST and Bulk (Fig. 4(2)) have further confirmed these
results. Both of them can be de-convoluted into two peaks, Bi
4f7/2 and Bi 4f5/2, that indexed to Bi3+. Bi 4f peaks of NST shift to
higher binding energy compared with Bulk, which may result from
the adjacent oxygen vacancies with high attracting electron effect.
Subsequently, the existence of OVs in BiOCl samples was fur-
ther confirmed by the characterization of electron paramagnetic
resonance (EPR). The EPR signal (Fig. 5) at g = 2.003 is observed
over NST, attributed to OVs [45–47]. Under the same conditions,
the signal at g = 2.003 is difficult to observe for Bulk. This result
further reveals much more OVs could be produced on the surface
of the nanosheets as reducing its thickness into molecular level.
Moreover, the large surface area of the nanosheets is also the rea-
son for the high exposure of oxygen vacancies. Thence, more Bi
atoms would be exposed as Lewis acid sites due to the existence
of more OVs. Finally, the Lewis acid sites in NST may be attributed
to its ultrathin structure and the larger specific surface area.
More importantly, it is widely known that surface defects can
strongly affect the optical properties of semiconductors. Thence,
the existence of OVs in BiOCl samples may further change their
photocatalytic activities by strongly effect on the band structure
[43,48]. UV/vis absorption spectra (DRS) (Fig. 6b) reveal that the
band gap of NST and Bulk is about 2.9 eV and 3.3 eV, respectively.
Fig. 1. XRD patterns of the prepared BiOCl samples.
Please cite this article as: Y. Ren, J. Zou, K. Jing et al., Photocatalytic synthesis of N-benzyleneamine from benzylamine on ultrathin BiOCl nanosheets under
visible light, Journal of Catalysis, https://doi.org/10.1016/j.jcat.2019.10.018

4
Y. Ren et al. / Journal of Catalysis xxx (xxxx) xxx
Fig. 2. SEM images of the prepared samples, Bulk (a and b), NST (c and d).
Fig. 3. TEM (a), HRTEM (b), SAED (c) and AFM images (d–f) for BiOCl nanosheet.
That is to say, the presence of surface OVs extends the absorption
range of NST by generating defect levels [22]. The Mott–Schottky
plots display the flat-band potential of NST (Fig. 6c) and Bulk
(Fig. S5) are ca. ꢁ1.1 V and ꢁ0.8 V vs. Ag/AgCl at pH 6.8, respec-
tively, corresponding to ca. ꢁ0.9 V and ꢁ0.6 V vs. NHE, respec-
tively. Compared with O2/O2.ꢁ potential (ꢁ0.28 V vs. NHE), the
positions of the conduction band (CB) over two samples are more
negative. This result means that the photogenerated electrons
can reduce oxygen molecules to superoxide radicals, which are
the important reactive intermediates in oxidation reactions. There-
fore, according to the optical absorption spectra, the valence band
(VB) maximum of both NST and Bulk would occur at about 2.0 V
and 2.7 V.
The prepared samples were used in the oxidation of BA to N-
benzyleneamine at 25 °C with oxygen atmosphere under visible
light irradiation. The conversion of BA over NST is increased with
Please cite this article as: Y. Ren, J. Zou, K. Jing et al., Photocatalytic synthesis of N-benzyleneamine from benzylamine on ultrathin BiOCl nanosheets under
visible light, Journal of Catalysis, https://doi.org/10.1016/j.jcat.2019.10.018

Y. Ren et al. / Journal of Catalysis xxx (xxxx) xxx
5
Fig. 4. XPS spectra for NST (a) and Bulk (b), O 1s (1) and Bi 4f (2).
Table 1
the extension of illumination time. After irradiation for 10 h,
the conversion of BA has reached 87.6% and the selectivity
of N-benzyleneamine reached 96.9% (Table 2, entry 1).
N-benzyleneamine as the main product is further identified by
GC-MS (Fig. S6). However, the conversion of BA over Bulk is only
36.9% in the identical conditions (Table 2, entry 2). Furthermore,
N-benzyleneamine is not detected without a catalysis and in the
dark condition (Table 2, entry 5). Very low conversion of BA is
obtained in N2 atmosphere (Table 2, entry 4). In addition, no pro-
duct is detected at 80 °C in the present of a catalyst under the dark
(Table 2, entry 6). These results indicate that the oxidative coupling
BA is carried out under visible light with O2 atmosphere. Then,
recycling experiments for the photocatalytic process were con-
ducted to confirm the stability of NST. After three successive recy-
cles, NST keeps high activities (Fig. S7). What’s more, no obvious
change is observed in the XRD patterns for fresh and used NST
(Fig. S8), indicating that the prepared NST possesses high stability.
Our results undoubtedly demonstrate that NST shows superior
photocatalytic performance compared with Bulk.
Atom percentages of O 1 s peaks determined by the XPS analysis.
Sample
NST
Oxygen species
Position (eV)
at. %
O1
O2
O3
O1
O2
O3
529.5
531.0
532.2
530.0
531.5
532.7
44.3
38.2
17.5
74.5
18.8
6.7
Bulk
In order to explore the best reaction time, the experiments for
the conversion of BA with relation to time have been done. As is
shown in Fig. S9, the conversion of BA and the selectivity of N-
benzyleneamine were obtained by changing reaction time from
4 h to 12 h. It is obviously that the conversion of BA gradually
increased as the reaction time prolongation. The conversion of BA
is up to 87.6% when irradiation for 10 h. Simultaneously, the selec-
tivity of N-benzyleneamine is keeping at 96.9%. However, when the
reaction time reached 12 h, the selectivity of N-benzyleneamine
began to decrease with the conversion of BA increased. This result
Fig. 5. EPR spectra of the prepared samples under visible light, Bulk (A) and NST (B).
1/2
Fig. 6. DRS spectra (a) and the bandgap (insert b) of the prepared samples, Bulk (1) and NST (2). The bandgap of BiOCl samples are estimated by a related curve of (
versus photon energy plotted. Mott-Schottky plots (c) of NST.
ahm)
Please cite this article as: Y. Ren, J. Zou, K. Jing et al., Photocatalytic synthesis of N-benzyleneamine from benzylamine on ultrathin BiOCl nanosheets under
visible light, Journal of Catalysis, https://doi.org/10.1016/j.jcat.2019.10.018

6
Y. Ren et al. / Journal of Catalysis xxx (xxxx) xxx
Table 2
Photocatalytic performances for the oxidation of BA over NST/Bulk under visible light irradiationa.
Entry
Catalyst
hv
Atm.
Conv. (%)
Sel. (%)
Yield (%)
TONc
TOFd (hꢁ1
)
1
2
3
4
5
6
7
NST
Bulk
NST
NST
NST
NST
–
+
+
+
+
–
O2
O2
Air
N2
O2
O2
O2
87.6
36.9
62.0
2.6
Null
Null
Null
96.9
99.0
94.3
98.0
Null
Null
Null
84.9
36.5
58.5
2.5
Null
Null
Null
2.8
1.2
2.0
0.08
Null
Null
Null
0.28
0.12
0.2
0.008
Null
Null
Null
b
–
+
a
b
c
Reaction condition: benzylamine (0.2 mmol), catalyst (8 mg, 0.03 mmol), acetonitrile (1 mL), O2 (1 atm), irradiation time (10 h).
T = 80 . NST ultrathin BiOCl nanosheets, Bulk: Bulk BiOCl.
Moles of N-benzyleneamine produced per mole of a catalyst.
d
Moles of N-benzyleneamine produced per mole of a catalyst per hour [49,50].
may be due to the excessive oxidation of BA. Therefore, the best
reaction time is 10 h for the oxidative coupling benzylamine to
N-benzyleneamine.
of pyridine on NST that there are two strong peaks at 1433 and
1485 cmꢁ1, which can be considered as pyridine adsorbed on the
Lewis acid sites [52]. These peaks also remained even after heating
under vacuum condition. It is suggested that the Lewis acid sites
are derived from the unsaturated surface Bi atoms. The intensity
of the corresponding peaks on Bulk is reduced compared to NST.
Therefore, it is concluded from the above results that there are
more Lewis acid sites on NST than Bulk, which may be attributed
to the larger specific surface area and ultrathin structure for the
nanosheets. The acidic sites on the surface of NST play a crucial role
in photocatalytic organic conversion. In the progress of photocat-
alytic oxidation coupling of BA, the Lewis acid sites can serve as
The structure and surface properties of a catalyst have impor-
tant influences on reaction system. The mainly exposed surface
of the prepared BiOCl samples is (0 0 1) surface. The O terminated
(0 0 1) surface is more thermodynamically stable [35]. Therefore, O
atoms are the outermost exposed atoms and Bi atoms are the sub-
layer atoms for (0 0 1) surface. If OVs are generated, Bi atoms in
secondary layer will be exposed as Lewis acid sites [51]. Therefore,
in situ FTIR was used to characterize the Lewis acid sites on NST
and Bulk. It can be known from the in situ FTIR spectra (Fig. 7a)
Fig. 7. In situ FTIR spectra of NST (a and c) and Bulk (b and d) before and after adsorbed pyridine (a and b) and BA (c and d). Conditions (1) After degassing at 200 for 3 h. (2)
Adsorption for 30 min at room temperature (physisorption + chemisorption). (3) Further evacuation of excess probe molecules at 150 for 3 min under 4 ꢂ 10ꢁ2 hPa
(chemisorption). (4) FTIR spectra of BA.
Please cite this article as: Y. Ren, J. Zou, K. Jing et al., Photocatalytic synthesis of N-benzyleneamine from benzylamine on ultrathin BiOCl nanosheets under
visible light, Journal of Catalysis, https://doi.org/10.1016/j.jcat.2019.10.018

Y. Ren et al. / Journal of Catalysis xxx (xxxx) xxx
7
the active sites of the reaction. Therefore, the higher photocatalytic
activity for NST may assign to the more acidic sites on the surface
of nanosheets.
BA molecules are adsorbed on the surface of NST, which is one of
the reasons why the absorption band edge of NST redshifted more
than that of Bulk. These are consistent with the in-suit FTIR results.
Furthermore, EPR measurements were carried out to confirm
the important role of O2 molecules in photocatalysis. As shown
in Fig. 9, neither Bulk nor NST shows the ESR signal at air atmo-
sphere in the dark condition. Under visible light irradiation, super-
oxide radical species trapped by 5,5-dimethyl-1-pyrroline N-oxide
(DMPO) are observed. The signal intensity of NST significantly
stronger than that of Bulk. This result not only confirms the gener-
ation of ÅOꢁ2 [55,56], but shows that NST can produce more ÅOꢁ2 than
Bulk. This is due to the large number of Lewis acid sites on the sur-
face of NST. The Lewis acid sites can serve as active sites for trap-
ping and activating oxygen molecules by forming the ABiꢀ ꢀ ꢀO—
Oꢀ ꢀ ꢀBiA complexes. Therefore, NST with rich Lewis acid sites can
be introduced as an efficient photocatalyst to activate O2. In order
to further illustrate the role of ÅOꢁ2 in the reaction, we added p-
benzoquinone (an effective superoxide radical quencher [57]) to
the reaction system, the conversion of BA over NST decreased sig-
nificantly (Fig. S11). Therefore, superoxide radicals play an impor-
tant role in this photocatalytic reaction. In addition, when
methanol (an effective hole scavenger [58]) is added to the reaction
system (Fig. S11), the photocatalytic activity of NST is remarkably
suppressed. This result indicates that holes are also important for
the reaction.
Previous work on photocatalysis oxidative coupling BA reported
that ammonia gas was generated during the reaction [56]. There-
fore, ammonium ions may exist in the reaction system. In order
to verify the conclusion, we used Nessler reagent to measure the
solution before and after the irradiation [59]. As shown in
Fig. S12, the reacted solution turns to yellow by adding Nessler
reagent. However, the color for the only N-benzyleneamine solu-
tion or initiative solution before irradiation does not change. This
infers the presence of ammonium ions in the reaction, that is,
ammonia gas is produced during the photocatalytic process. Fur-
thermore, benzaldehyde was detected in the solution after the
reaction by gas chromatography.
NH3-TPD was conducted to further characterize the properties
of the surface for prepared samples [53,54]. As shown in Fig. S10,
the TPD signal peak of NH3 at about 100 °C may be attributed to
the interaction between NH3 and the weak acid sites on NST. The
desorption peak occurring at about 254 °C was due to the presence
of moderate acid sites. In contrast, the TPD result for the bulk sam-
ple shows only one weak desorption peaks at about 263 °C. The
concentration of acid sites could be estimated by the total area
under the curve for all the desorption peaks. It is clear that the total
acidity of NST is much larger than that of Bulk. Since Bi atoms
could as Lewis acid sites in the prepared BiOCl samples, there are
more Lewis acid sites in NST. This result is consistent with the
results of in-situ FTIR. Moreover, moderate acid sites in NST facil-
itate the adsorption of the reactants and the desorption of the pro-
duct to increase the photocatalytic activity.
The interactions between the samples and BA molecules have
been revealed by in-situ FTIR of BA absorption. As shown in
Fig. 7c, BA molecules are chemosorbed and physisorbed on the sur-
face of NST. The peak at 1647 cmꢁ1 can be assigned to the bending
vibration of N-H bond. There two obvious peaks are observed at
1490 and 1453 cmꢁ1, which can be assigned to the stretching
vibration of C@C in a aromatic ring [21]. In addition, there is a peak
appearing at 1319 cmꢁ1, which represents the stretching vibration
of C-N bond in BA, the surface -Nꢀ ꢀ ꢀBi- coordination species would
be formed. After heating at 150 °C under vacuum condition for
3 min, the peaks at the corresponding positions still remained. It
is further indicated the BA molecules are chemosorbed on the sur-
face of NST. However, compared to the strong chemisorption of BA
on NST, the adsorption signal intensity of BA on Bulk is much
weaker. This result illustrates that NST have more reactive sites.
The interaction between the unsaturated active sites on the sur-
face of samples and BA molecules was further characterized by UV-
DRS. As is shown in Fig. 8, Bulk can only absorb UV light. NST has
light absorption in the visible range due to in the present of OVs
[43]. When BA molecules were absorbed, the absorption spectra
for both Bulk and NST have a red shift and extend their absorption
ranges. Considering that BA molecules could be adsorbed on Lewis
acid sites, we propose that the surface complexes are formed in the
samples and BA. The formed complexes may further extend the
light response and be excited under visible light irradiation by sur-
face charge transfer from ‘‘ligand to metal” [21]. Moreover, there
are more unsaturated acid sites on the surface of NST, so that more
Based on the above analysis, a feasible reaction mechanism is
proposed to explain the photocatalytic oxidation of BA to N-
benzyleneamine under visible light irradiation. Firstly, BA mole-
cules are adsorbed and activated on Lewis acid sites on the surface
of NST via the surface species -Nꢀ ꢀ ꢀBi-. The remaining Lewis acid
Fig. 9. EPR spectra of the prepared samples with the presence of DMPO in the
solvent of acetonitrile, Bulk in dark (1), NST in dark (2), Bulk under visible light (c),
NST under visible light (d).
Fig. 8. UV/Vis DRS spectra of Bulk (1), Bulk absorbed BA (2), NST (3) and NST
adsorbed BA (4).
Please cite this article as: Y. Ren, J. Zou, K. Jing et al., Photocatalytic synthesis of N-benzyleneamine from benzylamine on ultrathin BiOCl nanosheets under
visible light, Journal of Catalysis, https://doi.org/10.1016/j.jcat.2019.10.018

8
Y. Ren et al. / Journal of Catalysis xxx (xxxx) xxx
the design of efficient photocatalysts for green and universal pho-
tocatalysis applications.
Declaration of Competing Interest
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared
to influence the work reported in this paper.
Acknowledgements
This work was supported by the National Natural Science Foun-
dation of China (21872032 and 51672048).
Scheme 1. Schematic illustration proposed feasible reaction mechanism for
selective oxidation of BA to N-benzyleneamine over the samples.
Appendix A. Supplementary material
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.jcat.2019.10.018.
sites allow more O2 molecules to be adsorbed and activated by
forming the ABiꢀ ꢀ ꢀO—Oꢀ ꢀ ꢀBiA complexes. As shown in Scheme 1,
two reaction pathways may exist in NST system. In pathway A,
under visible light irradiation, direct surface charge transfer takes
place from coordinated BA molecules to NST (ligand-to-metal)
(Scheme 1A1). Simultaneously, excited electrons reduce oxygen
molecules to ÅOꢁ2 . What’s more, since NST would generate photo-
generated electrons and holes under visible light (pathway B).
The adsorbed O2 molecules are also reduced to ÅOꢁ2 by photogener-
ated electrons. At the same time, holes can abstract electrons from
adsorbed BA molecules to yield BA cations (Scheme 1B1). Subse-
quently, the activated BA on NST would be deprotonated under
the assistance of ÅOꢁ2 (Scheme 1C), forming active intermediate
species (PhACH@NH). PhACH@NH would produce benzaldehyde
with water and release ammonia (Scheme 1D). The generated
benzaldehyde further condenses with the free BA to produce
N-benzyleneamine (Scheme 1E). Finally, releasing of N-
benzyleneamine from the surface of NST. The regeneration of Lewis
acid sites on the surface of BiOCl sample would be reused to absorb
BA and O2 molecules achieving the reaction cycles. However, Bulk
is only responsive to UV, its photocatalytic process is just per-
formed as pathway A. Therefore, more Lewis acid sites and two
pathways coexist in NST system, which contributes to the remark-
able photocatalytic performance for the oxidative coupling of BA.
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4. Conclusion
In summary, ultrathin BiOCl nanosheets (NST) with the thick-
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Please cite this article as: Y. Ren, J. Zou, K. Jing et al., Photocatalytic synthesis of N-benzyleneamine from benzylamine on ultrathin BiOCl nanosheets under
visible light, Journal of Catalysis, https://doi.org/10.1016/j.jcat.2019.10.018

Y. Ren et al. / Journal of Catalysis xxx (xxxx) xxx
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Please cite this article as: Y. Ren, J. Zou, K. Jing et al., Photocatalytic synthesis of N-benzyleneamine from benzylamine on ultrathin BiOCl nanosheets under
visible light, Journal of Catalysis, https://doi.org/10.1016/j.jcat.2019.10.018