Hydrogenated Cu2OAu@CeO2 Z-scheme catalyst for photocatalytic oxidation of amines to imines

Article abstract of DOI:10.1039/c8cy01689e

The design and fabrication of highly active visible light photocatalysts for organic synthesis reactions are particularly challenging for solar energy utilization and conversion. Herein, hydrogenated Z-scheme yolk-shell Cu₂OAu@CeO₂ (H-Cu₂OAu@CeO₂) photocatalysts were synthesized using cubic Cu₂O as the starting core material via surface Au deposition and oxidation etching process, followed by hydrogenation treatment. When compared with CeO₂, Cu₂O@CeO₂, and Cu₂O@CeO₂Au nanocomposites, optimized H-Cu₂OAu@CeO₂ showed remarkably higher visible light oxidation activity for the synthesis of imines from amines at ambient pressure and room temperature. The remarkably enhanced photoactivity of the H-Cu₂OAu@CeO₂ composite mainly derives from the enhanced photoinduced charge separation efficiency, porous yolk-shell structure, proper surface defects, and well-maintained strong oxidation/reduction capabilities. The Z-scheme charge transfer process and photocatalytic reaction mechanism of the H-Cu₂OAu@CeO₂ composites were also provided through spectral and photoelectrochemical analyses together with the investigation of structure and photocatalytic oxidation reactions. This study provides a probable approach for designing unique Z-scheme catalysts.

Full text of DOI:10.1039/c8cy01689e

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Hydrogenated Cu O\Au@CeO Z-scheme catalyst
2
2
for photocatalytic oxidation of amines to imines†
Cite this: Catal. Sci. Technol., 2018,
8, 5535
Yingying Liu, Yajie Chen,* Wei Zhou,
Xin Zhang and Guohui Tian
Baojiang Jiang,
*
The design and fabrication of highly active visible light photocatalysts for organic synthesis reactions are
particularly challenging for solar energy utilization and conversion. Herein, hydrogenated Z-scheme yolk–
shell Cu
starting core material via surface Au deposition and oxidation etching process, followed by hydrogenation
treatment. When compared with CeO , Cu O@CeO , and Cu O@CeO \Au nanocomposites, optimized H–
Cu O\Au@CeO showed remarkably higher visible light oxidation activity for the synthesis of imines from
amines at ambient pressure and room temperature. The remarkably enhanced photoactivity of the H–
Cu O\Au@CeO composite mainly derives from the enhanced photoinduced charge separation efficiency,
2 2 2 2 2
O\Au@CeO (H–Cu O\Au@CeO ) photocatalysts were synthesized using cubic Cu O as the
2
2
2
2
2
2
2
2
2
Received 14th August 2018,
Accepted 13th September 2018
porous yolk–shell structure, proper surface defects, and well-maintained strong oxidation/reduction capa-
bilities. The Z-scheme charge transfer process and photocatalytic reaction mechanism of the H–
2 2
Cu O\Au@CeO composites were also provided through spectral and photoelectrochemical analyses to-
DOI: 10.1039/c8cy01689e
gether with the investigation of structure and photocatalytic oxidation reactions. This study provides a
probable approach for designing unique Z-scheme catalysts.
rsc.li/catalysis
conducted involving the reactions of amines to imines. For
Introduction
example, oxide semiconductors (e.g., TiO , CeO , Nb O ),
2
2
2 5
During recent years, considerable attention has been paid to
the utilization of sunlight energy, which is regarded as an
abundant and clean energy source for different applications
metal sulfides (e.g., CdS, ZnIn S ), and mesoporous graphite
2 4
3 4
C N have showed photocatalytic oxidation ability of amines
2
1–25
to imines.
Unfortunately, fast photoinduced charge re-
1
–3
(e.g., driving chemical reactions). In the field of catalytic re-
combination rate of a single-component catalyst is difficult
to meet the stringent requirements of practical applications,
including selectivity and productivity. As compared to single-
component catalysts, heterostructured catalysts constructed
by coupling two semiconductors have been confirmed to
show considerably enhanced photoactivity; therefore, the
construction of a heterostructure material was considered to
actions, exploiting low-cost and efficient catalytic systems
with high stability is a major goal. Among these, photocata-
lytic technology is a promising strategy because it can com-
4–6
bine catalysis with the potential utilization of solar energy.
Among the organic reactions, the synthesis of imines derived
7–12
from amines is a kind of critical organic reaction.
The re-
sultant imines are widely used intermediates, pharmaceuti-
be an effective approach to resolve the disadvantages men-
1
3,14
26,27
cals, and fine chemicals.
In earlier studies, the transformation of amines to imines
tioned above.
For instance, for type-II band alignment,
heterostructured Cu O/CeO photocatalysts have exhibited
2
2
1
5,16
were performed mainly on noble metals (Pd, Au, Pt).
higher visible light photocatalytic degradation performance
than single Cu O and CeO because of their accelerated sepa-
When compared with noble metals, low-cost and abundant
inorganic semiconductors photocatalysts have attracted in-
creasing attention because of their intrinsic catalytic
2
2
2
8–30
ration rate of photogenerated carriers.
creased oxidation/reduction ability of the Cu
structure is detrimental to any improvements in the
However, the de-
2
O/CeO hetero-
2
1
7–20
properties.
Some significant studies have been
3
1
photocatalytic activity. Therefore, simultaneously improving
photogenerated charge carriers separation and maintaining
individual oxidation/reduction ability is considerably desir-
able. The construction of a Z-scheme system photocatalyst
can effectively realize this objective. Recently, the Z-scheme
system photocatalysts have garnered increased attention be-
cause the strong oxidation and reduction capabilities of the
Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education
of the People's Republic of China, School of Chemistry and Materials Science,
Heilongjiang University, 150080 Harbin, P. R. China.
E-mail: chenyajie1970@163.com, tiangh@hlju.edu.cn; Fax: +86 451 86661259;
Tel: +86 451 86608781
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
c8cy01689e
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3
2–37
components in the composite can be maintained.
In ad-
parison, the experiments of Cu O loaded with different Au
2
dition, the morphology and structure considerably influence
the photocatalytic activity. Some special structures, such as
hollow and core-shell structures, have demonstrated in-
contents were also done. Here, 0.35 mL, 1.05 mL, and 1.40
4 2
mL HAuCl solutions were used to prepare the Cu O\Au
samples.
3
8
creased photocatalytic activity.
In particular, yolk–shell
nanostructures possess the advantages of both core–shell
and hollow structures, which can significantly enhance light
utilization and photocatalytic activity. For example, Pt@CeO2
composites with tunable yolk–shell and core–shell structures
have been prepared, which have exhibited remarkable visible
Synthesis of hydrogenated Cu
2 2 2 2
O\Au@CeO (H–Cu O\Au@CeO )
Cu O\Au@CeO was firstly synthesized via a template etching
2
2
method. Under magnetic stirring, 2.5 mL NaCl aqueous solu-
tion was added in a beaker containing 200 mL absolute etha-
nol. Further, 50 mg Cu O\Au was dispersed in the above NaCl
2
solution to obtain well-distributed suspension under ultra-
sonic dispersion for 15 min. Then, 50 mL ethanol solution
4 2 3 6
containing 0.011 g (NH ) CeIJNO ) was dropped into the
above suspension solution at 45 °C. The obtained final pow-
der was washed and dried (60 °C). Similarly, Cu O@CeO was
2
also obtained under a similar synthesis route using Cu O to
3
9
light photocatalytic activity. However, no Z-scheme system
involving Cu O/CeO yolk–shell nanostructure composites
2
2
has been designed and applied in photocatalytic organic re-
actions up to now.
In this work, in order to implement the synthesis of imines
from amines under visible light irradiation, hydrogenated
Z-scheme yolk–shell Cu O\Au@CeO photocatalyst was pre-
2
2
2
2
replace Cu
ing Cu O\Au@CeO to 300 °C and maintaining it under Ar/H
2
2 2 2
O\Au. H–Cu O\Au@CeO was obtained by calcin-
pared, and its fabrication route is illustrated in Scheme S1.†
2
2
Cubic Cu O was first prepared. Then, the surfaces of cubic
2
(
7%) atmosphere for 10 min, and the resultant product was
termed as H–Cu O\Au@CeO . As a comparison, Au nano-
particles were also loaded on the outer side of Cu O@CeO ,
2
Cu O were loaded with Au nanoparticles via chemical reduc-
tion. The following oxidation etching process produced the
2
2
2
2
Z-scheme yolk–shell Cu O\Au@CeO composite. The photo-
2
2
2 2
and the sample was labeled as Cu O@CeO \Au.
catalytic activity of the optimal Z-scheme yolk–shell
Cu O\Au@CeO composite was remarkably higher than that of
the control samples, such as individual CeO2 and common
yolk–shell Cu O@CeO composite. Moreover, the subsequent
2
2
Characterization
2
2
The crystal structure of the powder samples was determined
using an X-ray diffractometer (D8 Advance), and Cu Kα was
the radiation source (40 kV, 44 mA, λ = 1.5406 Å). Surface
morphology and elemental distributions of the samples were
analyzed by scanning electron microscopy-energy-dispersive
X-ray spectrometer (SEM-EDS, S-4800, EDAX). Transmission
electron microscopy (TEM) was performed using a JEOL 2100
system operated at 200 kV. In order to perform TEM charac-
terization, after ultrasonic dispersion, ethanol suspensions
containing catalysts were dropped onto copper grids (400
mesh) coated with carbon. The diffuse reflectance spectra
hydrogenation treatment can further improve the photocata-
lytic performance by reforming the surface/interface defects of
Cu O\Au@CeO . The origin of significantly enhanced photo-
2 2
catalytic performance and reaction mechanism for the optimal
Z-scheme yolk–shell composite catalyst were explored.
Experimental section
Synthesis of cubic Cu
2
O nanocrystals
Firstly, 0.0372 g EDTA–2Na and 0.0250 g CuSO ·5H O were
4
2
separately added into 100 mL distilled water to form an aque-
ous solution (0.01 M). An appropriate amount of D-glucose
and NaOH were also dissolved in 10 mL distilled water to
form 0.1 M and 1 M aqueous solutions, respectively. Then,
(
DRS) were obtained from a UV-2550 UV-vis absorption
spectrophotometer equipped with an ISR-240A integrating
sphere using BaSO as the reference sample. Raman spectra
4
were obtained from a laser Raman spectrometer (HR800,
Horiba) using 632.8 nm excitation wavelength and 1800 L
1
00 mL EDTA–2Na solution and 10 mL D-glucose solution
were put in a 250 mL breaker containing 100 mL CuSO4
aqueous solution. After NaOH solution (10 mL, 1 M) was
mixed with the above solution, the beaker containing the
mixed solution was put into a water bath (80 °C) and kept for
−
1
mm grating. The valence states of the elements were deter-
mined on an X-ray photoelectron spectrometer (XPS, VG
ESCALABMK II); Al Kα (hv = 1486.8 eV) was used as the X-ray
source. The C 1s peak at 284.6 eV was used as the internal
standard. Fluorescence spectra were obtained from the
FLS920 fluorescence spectrophotometer (Edinburgh) using a
He–Cd laser source, and the excitation wavelength was 320
nm. The metal composition of the optimal catalyst was
detected via inductively coupled plasma emission spectrome-
ter (ICP, Optimal 7000).
3
0 min. After filtration, the obtained precipitate (cubic Cu
2
O)
was washed, followed by drying at 60 °C.
2
Synthesis of Cu O\Au
In a typical synthesis process, 0.05 g Cu O was added in dis-
2
tilled water (28 mL); then, 10 mL 1% PVP (polyvinyl pyrrolid-
one; mol. wt: 10 000) solution was added in the above solu-
tion under stirring (30 min). Finally, the optimal 0.7 mL
−
3
−1
Electrochemical test
4
HAuCl solution (2.5 × 10 g mL ) was mixed with the above
solution. After stirring for 3 h, the obtained precipitate was
filtrated and washed, followed by drying at 60 °C. For com-
The electrochemical test was performed on an electro-
chemical analyzer (BAS100B), and a three-electrode system
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was applied. Saturated Ag/AgCl was used as the reference
electrode, and Pt foil (1 cm × 1.5 cm) was used as the counter
electrode. A Xe lamp (300 W, CEL-HXF-300) was used as the
light source; it was equipped with a 400 nm cutoff filter.
Here, 0.1 M Na SO solution bubbled with nitrogen gas was
not be identified in the XRD pattern of Cu O\Au@CeO be-
2
2
cause of its low content. The XRD diffraction peak intensity
of H–Cu O\Au@CeO is slightly lower than that of
Cu O\Au@CeO due to the decrease in the crystallinity after
2
2
2
2
hydrogenation treatment. Moreover, there exists a weak dif-
2
4
used as the electrolyte. Electrochemical impedance spectro-
scopy (EIS) data was obtained using an electrochemical ana-
lyzer (Zahner Elektrik, Germany) over a 100–0.05 Hz fre-
fraction peak (43.3°) of Cu derived from the partial reduction
4
2
of Cu O. The Raman spectra in Fig. 1B further confirmed
2
the formation of Cu O and CeO (Fig. 1B). In Fig. 1B, the
2
2
−
1
43
quency range under bias of −0.8
V
with sinusoidal
peak at 459 cm corresponds to the F2g skeletal vibration.
However, in the Raman spectrum of H–Cu O\Au@CeO , the
peak (459 cm ) corresponding to CeO changes to 465 cm ,
perturbations of 10 mV.
2
2
−
1
−1
2
Photocatalytic performance test
which can be attributed to the production of oxygen vacan-
cies (Vo's) originating from the hydrogenation treatment. In
4
4
Photocatalytic tests were carried out in a 3 mL quartz vessel.
A mixed solution of 1.5 mL deionized water (H O), 15 μL or-
optimal H–Cu O\Au@CeO , the molar ratio of Cu : Ce : Au is
2
2
2
about 39 : 18 : 1 according to the ICP test.
SEM and TEM characterizations were performed to dem-
onstrate the morphology and structure of the catalysts. As
ganic reactants, and 15 mg photocatalyst was added into the
quartz vessel. Further, oxygen was bubbled through the reac-
tor for 15 min to enhance the oxygen adsorption of the
photocatalyst. Then, the vessel was plugged using a rubber
stopper. The vessel was maintained at room temperature (20–
2
shown in Fig. 2A, the outer surface of cubic Cu O is smooth.
However, the outer surface of cubic Cu O\Au is rough and
2
uniformly covered by Au nanoparticles (8–10 nm) (Fig. 2B).
The TEM image further confirmed the cubic structure of
2
3 °C) by circulating water through the outer jacket. The sam-
ples were collected before and at regular intervals during irra-
diation, and the catalyst was removed by filtration. The light
source was a Xe lamp (300 W, CEL-HXF-300); a 400 nm cutoff
filter was employed. The light density was 5.8 mW cm . After
light irradiation, the photocatalyst particles were filtered, and
the product was extracted using acetonitrile. It was then ana-
lyzed by gas chromatography-mass spectrometry (6890GC-
Cu O (Fig. 2C), and uniformly distributed Au nanoparticles
2
2
on the outer surface of Cu O can also be seen from the inset
−
2
of Fig. 2D. The diameter of Au nanoparticles is about 8–10
nm. The EDS element mapping results also proved the uni-
form distribution of Au nanoparticles on the outer surface of
2
Cu O (Fig. S1†). From Fig. 3A, it can be seen that rough po-
rous CeO shells were formed on the outside of the Cu O\Au
2
2
5
2 4
973MSD, Agilent) after drying with Na SO powder. The
because of the partial sacrifice of Cu
2
O during the etching re-
electron spin resonance (ESR) spectra were obtained using an
A300 ESR spectrometer at room temperature, where DMPO
was used as the spin-trap reagent in a methanol solution. In-
strument settings conditions: receiver gain, 1 × 10 ; time con-
stant, 10.24 ms; modulation amplitude, 3 G; center field,
action, and uniform yolk–shell structure nanocubes were ob-
served (Fig. 3B), demonstrating the successful preparation of
3
2 2
yolk–shell structure Cu O\Au@CeO . Fig. 3C shows the
HRTEM image (the box in Fig. 3B); interplanar spacings of
0
CeO
.31 and 0.19 nm correspond to the (111) and (220) facets of
3
507 G; microwave power, 6.35 mW; sweep width, 80 G;
4
1
2
,
respectively.
After hydrogenation treatment, the
sweep time, 42 s.
Results and discussion
Material characterization
X-ray diffraction (XRD) analysis was performed to determine
the structures of the catalysts. In Fig. 1A, all the diffraction
peaks located at 36.5°, 42.4°, 60.9°, and 73.1° matched with
4
0
the cuprite structure Cu O (JCPDS: 05-0667). The diffraction
2
peaks at 56.3°, 47.5°, 33.1°, and 28.6° correspond to CeO
with a fluorite structure (JCPDS: 81-0792). However, Au can-
2
4
1
Fig. 2 SEM (A) and TEM (C) images of Cu
images of Cu O\Au.
2
O; SEM (B) and TEM (D)
Fig. 1 (A) XRD patterns and (B) Raman spectra of different samples.
2
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Fig. 4 XPS spectra of (A) Ce 3d, (B) Cu 2p, (C) O 1s, and (D) Au 4f of
different samples. Core-level XPS spectra are shown using color maps,
and the corresponding deconvoluted peaks are shown using solid
lines. The spectra decomposition was executed using the XPS PEAK 41
program with Gaussian functions after the subtraction of a linear
background.
tence of satellite peaks at 941.2 and 961.9 eV in the spectra
indicates that there is marginal content of the chemical state
2
+
48
of Cu oxidation state (CuO). The O 1s spectra are shown
in Fig. 4C. The two deconvoluted peaks at around 529.3 and
531.4 eV are related to lattice oxygen and hydroxyl-type oxy-
Fig. 3 SEM images of Cu
2
O\Au@CeO
(C) and H–Cu
(E) and H–Cu O\Au@CeO
2
2
(A) and H–Cu
O\Au@CeO
(F).
2
O\Au@CeO
2
(B).
4
9
TEM images of Cu
2
O\Au@CeO
2
2
2
(D). HRTEM
gen species/molecular water, respectively. In the Au 4f XPS
spectrum (Fig. 4D), the peaks at 84.0 and 87.7 eV can be at-
images of Cu O\Au@CeO
2
2
2
5
0
tributed to metallic Au atoms in the composite catalysts.
UV-vis absorption spectra are employed to investigate the
optical properties of the photocatalysts. In Fig. 5A, steep ab-
sorption edges can be found at 450 and 635 nm in the ab-
entire cubic morphology and yolk–shell structure of
Cu O\Au@CeO were effectively maintained (Fig. 3D and E).
The corresponding HRTEM image is shown in Fig. 3F. Apart
from the lattice fringe (0.19 nm) of the CeO (220) plane,
there exists a thin amorphous CeO surface layer, which is
2
2
sorption spectra of CeO
pared with CeO , the increased visible light absorption of
Cu O\Au@CeO mainly comes from the introduction of Cu O.
2 2
and Cu O, respectively. When com-
2
2
2
2
2
2
4
4
produced from the hydrogenation treatment.
Meanwhile, the existence of Au and Cu nanoparticles can ex-
hibit localized surface plasmon resonance (SPR) effect. More-
over, H–Cu O\Au@CeO exhibited the strongest visible light
2
2
Optical and electrochemical properties
absorption among these samples because of the existence of
surface defects generated from hydrogenation.
Surface defects such as the existence of Vo's were con-
2 2
firmed by ESR test (Fig. S3†). For Cu O@CeO \Au, weak ab-
sorption at about 550 nm can be found from its UV-vis ab-
sorption spectrum, which originated from the SPR effect of
The survey XPS spectrum (Fig. S2†) confirms the existence of
Ce, O, Cu, and Au in H–Cu O\Au@CeO . In Fig. 4A, the peaks
assigned to Ce 3d3 are marked as u‴, u″, and u. The peaks
2
2
4
+
/2
4
+
corresponding to Ce 3d5/2 are marked as v″, v‴, and v. Simi-
larly, the peaks attributed to Ce 3d3/2 and Ce 3d5/2 are
marked as u′ and v′, respectively.
sults reveal the coexistence of Ce and Ce in the samples.
3
+
3+
4
5,46
The deconvolution re-
Au nanoparticles. The direct optical bandgaps of CeO
2
and
3
+
4+
2
Cu O were measured by plotting the graph of (ahv) versus
2
3
+
Moreover, the relative concentration of Ce
in H–
photon energy (hv) (Fig. 5B). Here, a is the optical absorption
coefficient, and hv can be calculated as hv = 1240/wavelength.
Cu O\Au@CeO is larger than that in Cu O\Au@CeO esti-
2
2
2
2
mated via the integrated peak area in the spectra through the
equation (Fig. S2†), implying that the hydrogenation treat-
2 2
The direct optical bandgaps of CeO and Cu O were 2.87 and
1.95 eV, respectively, measured by extrapolating the linear re-
gion of the square of the absorption curve to the x-axis. These
3+
4+
ment induced the formation of Vo's of Ce /Ce redox pairs
4
6
by causing lattice distortion and charge imbalance in CeO2.
The peaks of Cu 2p1/2 (952.2 eV) and Cu 2p3/2 (932.4 eV) are
2
values are in good agreement with the reports of CeO (2.8–
4
6,51
3.0 eV) and Cu O (2.0–2.2 eV).
2
0
1+ 47
associated with Cu or Cu . The peaks at 934.1 (Cu 2p3/2)
Photoluminescence is an important characterization tech-
2
+ 45
52
and 953.9 eV (Cu 2p1/2) can be ascribed to Cu . The exis-
nique to analyze the behavior of photogenerated carriers.
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Fig. 5 (A) UV-vis absorption spectra and (B) calculated bandgaps of
different catalysts; steady state (C) and transient state (D) photo-
luminescence spectra of the different catalysts.
In Fig. 5C, the steady state photoluminescence spectra show
emission bands at around 427 and 465 nm, originating from
Fig.
6
(A) Photocurrent response of different samples with and
5
3
without visible light irradiation under bias of 0.4 V versus NHE. (B)
Nyquist plots of different samples. Supporting electrolyte is 0.1 M
the defects within the Ce 4f and O 2p levels, respectively.
Obviously, these composite catalysts including Cu2-
O@CeO \Au showed relatively lower photoluminescence
intensity than pure CeO . In particular, H–Cu O\Au@CeO
2
2 4
Na SO .
2
2
2
showed the lowest photoluminescence intensity. It indicates
that the inhibited photoinduced charge recombination in the
composites was because of the existence of proper surface de-
fects (Vo's) and the synergy between Cu O, Au, and CeO .
lysts in deionized water. The N-benzylidene benzylamine de-
rived from benzylamine under aerobic oxidation was first
studied. A high yield (88.0%) and good selectivity (96.5%) to
N-benzylidene benzylamine product were obtained in the
2
2
Moreover, the transient state photoluminescence spectro-
scopy tests (Fig. 5D) also prove that the average fluorescence
lifetime of H–Cu O\Au@CeO is the longest in these catalysts
presence of H–Cu
tion. However, when using Cu
Cu O@CeO , CeO \Au, and CeO
2
2
O\Au@CeO
2
after 5 h visible light irradia-
O\Au@CeO , Cu O@CeO \Au,
as the photocatalysts in an
2
2
2
2
2
2
2
2
2
(
Table S1†). Hence, this H–Cu
2
O\Au@CeO
2
Z-scheme yolk–
oxygen atmosphere, despite having similar selectivity, rela-
tively low yields of 72%, 56.2%, 41.3%, 40.3%, and 21.6% are
obtained, respectively. The significantly enhanced photocata-
shell system is better than an ordinary heterojunction
structure.
Transient photocurrent tests were implemented to demon-
strate the accelerated photoinduced charge transfer and sepa-
ration of different samples. As evident from Fig. 6A, the H–
lytic activity of H–Cu
Firstly, the enhanced photoinduced charge separation and
transfer rate in Z-scheme Cu O@CeO composites can accel-
2 2
O\Au@CeO arise from several factors.
2
2
Cu
sity than Cu
indicate that the Z-scheme structure and proper surface de-
fects in H–Cu O\Au@CeO could generate additional charge
carriers, thereby promoting their effective separation. Simi-
larly, EIS tests were also performed. In Fig. 6B, the smallest
2
O\Au@CeO
2
electrode showed higher photocurrent den-
erate the photocatalytic organic reaction rate. Secondly, sig-
nificantly enhanced visible light absorption of the composites
because of hydrogenation can indeed contribute toward the
generation of additional photogenerated charge carriers.
Meanwhile, hot electrons can be produced from Au nano-
particles during visible light irradiation, and some hot
2
O, Cu
2
O@CeO , and Cu O\Au@CeO . The results
2
2
2
2
2
arc size can be observed for the H–Cu
suggesting the lowest interfacial charge transfer resistance
Rct), resulting in the highest charge transfer and separation
efficiency. This agrees with the photoluminescence spectra
2
O\Au@CeO
2
electrode,
electrons can leap into the conduction band of Cu
2
O to par-
ticipate in the catalytic reactions. Thirdly, the Vo's (generated
3
+
(
from the formed Ce after hydrogenation) can also provide a
higher carrier concentration and promote enhanced carrier
5
4
43
and transient photocurrent results.
separation, which can be proven by photochemical and
2
photoelectrochemical tests. Moreover, the porous CeO shell
and hollow interlayer can also promote the reactants transfer.
In addition, the relatively high adsorption capacity (Table
Photocatalytic activities
A series of organic synthesis reactions of imines from amines
were performed to evaluate the photocatalytic activity of cata-
S2†) of H–Cu
ment of photocatalytic activity.
2 2
O\Au@CeO contributes toward the enhance-
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In photocatalytic tests, some control experiments were
also carried out. The photocatalytic tests of H–
Cu O\Au@CeO with different Au contents confirmed that
the moderate content of Au can make H–Cu O\Au@CeO ex-
of the catalyst and participate in the reaction, so the conver-
sion rate and yield decrease. However, the selectivity exhibits
no obvious change (Fig. S5C†). The aerobic oxidation reac-
tions of some other amines were also carried out. The experi-
mental results are listed in Table 1. Obviously, the aerobic ox-
idation of amines was realized. Moreover, higher yields can
be obtained from benzylamines containing electron-donating
groups such as OCH3 and CH3 in comparison to
benzylamines with electron-withdrawing groups (e.g., Cl),
which is in agreement with a previous report. In addition,
for the para-substituted benzylamines, a higher reaction rate
can be provided in comparison to the ortho and meta isomers
due to steric effects.
2
2
2
2
hibit optimal photocatalytic activity (Fig. S4†). The same or-
ganic reaction occurring in air and under de-aerated condi-
tions showed relatively low yields of 69% and 15% (Fig. 7B),
respectively. Meanwhile, the control experiments also indi-
cated that there was a very low conversion rate under dark or
no catalyst conditions (Fig. 7B). Meanwhile, when compared
with the reported catalysts, the catalyst in this work still
achieves satisfying conversion and selectivity (Table S3†). The
results verified that the oxidative reactions could be driven
under the existence of a catalyst, molecular oxygen, and light
irradiation. The effect of substrate concentration on the
photocatalytic reactions (Fig. S5†) was also studied. It can be
seen that the conversion rate and yield increased with the
benzylamine concentration up to 90 mM; it then decreased at
higher concentrations. This can be attributed to the fact that
when the concentration is less than or equal to 90 mM, more
benzylamine molecules can reach the surface active sites of
the catalysts and participate in the oxidation reaction, lead-
ing to a higher conversion rate (Fig. S5A†) and yield (Fig.
S5B†). When the benzylamine concentration is too high,
some of the benzylamine molecules cannot reach the surface
5
5
To reveal the underlying cause of the dramatic enhance-
ment in activity and corresponding intrinsic mechanism of
photocatalytic aerobic oxidation of amines, radical scavenger
tests were performed. Isopropanol (IPA), benzoquinone (BQ),
and ammonium oxalate (AO) were selected as the trapping
−
+
56
agents of ˙OH, ˙O , and h , respectively. As shown in
2
Fig. 7C, the introduction of BQ and AO led to a significant
decrease in the benzylamine conversion, demonstrating that
−
+
˙O2 and h act as the main active species. The significantly
−
important role of ˙O
2
in the photocatalytic reaction was fur-
ther confirmed by ESR measurements. As presented in
−
Fig. 7D, four obvious signals assigned to DMPO–˙O2 can be
observed when H–Cu
visible light illumination, indicating that ˙O
2
O\Au@CeO
2
acts as the catalyst under
−
2
can be gener-
5
7
ated from H–Cu O\Au@CeO .
The control experiments
2
2
−
show that ˙O
Cu O\Au@CeO
2
signal cannot be found in the absence of H–
or under dark conditions. Meanwhile, the
2
2
weak ESR signal of DMPO–˙OH adduct can be found (Fig.
S6†). This indicated that only a small quantity of ˙OH radicals
were produced in this photocatalytic system. The above ESR
and radical trapping tests confirmed the synergistic effects of
+
−
h and ˙O
2
in the photocatalytic reaction.
The improved photocatalytic activity of H–Cu O\Au@CeO
2 2
can be elucidated from the formation of the Z-scheme band
structure system (Fig. 8). According to the XPS valence spec-
tra (Fig. S7†), the valence band (VB) positions of CeO
2
and
Cu O were located at 2.44 and 0.63 V, respectively. As evident
2
from Fig. 5B, the bandgaps of CeO
.95 eV, respectively. Therefore, the conduction band (CB) po-
sitions of CeO2 and Cu O were located at about −0.30 and
2 2
and Cu O were 2.74 and
1
2
Fig.
photocatalysts: a) Cu
Cu O@CeO \Au, f) Cu
conditions: 15 mg photocatalysts, 1.5 mL H
7
(A) Time course of benzylamines yield over various
O, b) CeO , c) CeO \Au, d) Cu O@CeO , e)
O\Au@CeO , g) H–Cu O\Au@CeO . Reaction
O, 15 μL substrates, visible
−
1.32 V versus NHE, respectively. The designed schematic dia-
gram of the energy band structure of H–Cu O\Au@CeO is
shown in Fig. 8. In the Z-scheme H–Cu O\Au@CeO system,
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
light (λ > 400 nm), illumination time is 5 h, oxygen atmosphere, room
temperature (20–23 °C); (B) time course of benzylamines yield over H–
2 2
Cu O and CeO can be simultaneously excited under visible
light irradiation. Photoinduced electrons from the CB of
Cu
photocatalysts, (b) dark, (c) N
oxidation of benzylamines over H–Cu
2
O\Au@CeO
2
under different reaction conditions: (a) no
atmosphere, (d) air; (C) photocatalytic
O\Au@CeO using different
CeO could transfer to the VB of Cu O via metallic Au to re-
2
2
2
combine with the holes. This combination process was very
fast. Hence, the charge carriers' separation efficiency was
2
2
trapping agents: isopropanol (IPA) for ˙OH, benzoquinone (BQ) for
−
+
˙
O
2
,
and ammonium oxalate (AO) for h ; (D) ESR spectra of
considerably enhanced. Electrons in the CB of Cu O were
2
2−
2
2
DMPO–˙O
in H–Cu
O\Au@CeO
dispersion under different
−
+
captured by O
2
, so ˙O
2
was produced. The h in the VB of
O to form ˙OH radicals. Therefore,
the produced predominant active species ˙O2 and h could
effectively oxidize various amines to their corresponding
conditions: a) reaction conditions: 15 mg photocatalysts, 1.5 mL
methanol dispersion, 15 μL of substrates under visible light irradiation
CeO directly oxidized H
2
2
−
+
(λ > 400 nm), illumination time is 2 h, oxygen atmosphere, room
temperature (20–23 °C), b) no photocatalysts, c) dark.
5540 | Catal. Sci. Technol., 2018, 8, 5535–5543
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2 2 2 2 2
Table 1 Photocatalytic oxidation of a series of benzylic amines over H–Cu O\Au@CeO . Reaction conditions: 15 mg H–Cu O\Au@CeO , 1.5 mL H O, 15
μL substrate, visible light (λ > 400 nm), illumination time is 5 h, oxygen atmosphere, reaction temperature is 20–23 °C
Selectivity (%)
Conversion
Entry
1
Substrate
Product
By-product
(%)
N-Benzylidene benzylamines
Benzaldehydes
13.5
91.2
96.5
2
3
91.9
71.6
94.8
69.5
15.2
30.5
4
5
90.4
75.4
61.0
56.8
39
43.2
2−
imines. Meanwhile,
electrons could jump to the CB of Cu
by the molecular oxygen to yield superoxide radicals (˙O ).
2 2
Moreover, the Vo's on H–Cu O\Au@CeO can accelerate the
photoinduced charge separation and improve the photocata-
lytic performance. Conversely, in a traditional type-II hetero-
2
structure model (Fig. S8†), the electrons in the CB of Cu O
and Au jump to that of CeO
would leap to that of Cu O. However, ˙O could not be pro-
a
small number of generated hot
can be extracted by ˙O from the amine radial cations to gen-
erate imine intermediates, and the hydrolysis of imine inter-
mediates leads to the formation of intermediate benzalde-
hyde and ammonia gas, which were proven by the GC-MS
(Fig. S9†) and ion chromatography test results (Fig. S10†).
Then, benzaldehyde molecules are transferred to the primary
amines to produce the corresponding imines (Table 1).
2
O and then be captured
−
2
+
2
, and the h in the VB of CeO
2
2 2
The photocatalytic cycling stability of H–Cu O\Au@CeO
−
was also tested. After six reaction cycles, only a slow decrease
in the photocatalytic activity can be found (Fig. S11†). Mean-
while, conversion and selectivity have no obvious decrease af-
ter the reusability test (Fig. S12†), indicating the excellent ac-
2
2
2
duced because CeO has a more positive CB-edge potential
(
−0.30 V) than O
2
/˙O (−0.33 V). Similarly, ˙OH radicals can-
2−
not be produced because Cu O has a more negative VB (+0.63
2
−
V). However, the production and significant role of ˙O
2
and
2 2
tive stability of the H–Cu O/Au@CeO catalyst. Furthermore,
˙
OH were verified in the active species trapping experiments
the different characterizations (XRD, Raman, XPS, SEM, and
(Fig. 7C). Therefore, a Z-scheme structure mechanism is rea-
TEM) of the H–Cu O\Au@CeO catalyst after the tests were
2
2
sonable. The above experiment results showed that the pro-
done (Fig. S13–S16†). The results show that the catalyst has
relatively high morphology and structural stability. In addi-
tion, the H–Cu O\Au@CeO catalyst can be applied to other
organic synthesis reactions. For example, the photocatalytic
synthesis of aromatic aldehyde from the corresponding aro-
matic alcohol derivatives were also achieved (Table S4†). This
+
−
2
duced h and ˙O species should exert a synergistic catalytic
role on the photocatalytic reaction. The photocatalytic reac-
2
2
−
tion mechanism was also proposed (Fig. 8). The electrons (e )
−
+
in the CB of Cu
2 2 2
O reacted with O to produce ˙O . The h in
the VB of CeO oxidized the anchored amine to produce the
2
5
8
cation radical complex. Meanwhile, the hydrogen atoms
2 2
suggests that the H–Cu O\Au@CeO catalyst may have wide
applications in photocatalytic organic syntheses.
Conclusions
In summary, we have developed
Z-scheme yolk–shell structure photocatalyst. The prepared H–
Cu O\Au@CeO catalysts can significantly enhance the charge
a H–Cu O\Au@CeO
2 2
2
2
separation efficiency and maintain strong oxidation/reduc-
tion capabilities. The transformation of amines into imines
was realized with high productivity and excellent selectivity
under aerobic oxidation conditions. Besides, the composite
catalysts showed potential performance in other organic reac-
tions and possessed excellent stability without compromising
on the photocatalytic efficiency. More detailed studies were
also performed to clarify the factors that determine the
Fig. 8 Photocatalytic reaction mechanism of imines derived from
amines under aerobic conditions over H–Cu O\Au@CeO .
2 2
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Catalysis Science & Technology
catalytic activity and applications to some other oxidation re-
actions. This synthesis strategy can contribute toward fabri-
cating Z-scheme catalysts yielding both high oxidation and
excellent reduction ability.
20 J. Chen, J. Cen, X. L. Xu and X. N. Li, Catal. Sci. Technol.,
2016, 6, 349–362.
21 H. J. Chen, C. Liu, M. Wang, C. F. Zhang, N. C. Luo, Y. H. Wang,
H. Abroshan, G. Li and F. Wang, ACS Catal., 2017, 7, 3632–3638.
2
2
2
2
2 X. J. Lang, W. H. Ma, Y. B. Zhao, C. C. Chen, H. W. Ji and
J. C. Zhao, Chem. – Eur. J., 2012, 18, 2624–2631.
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Tanaka, ACS Catal., 2011, 1, 1150–1153.
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Conflicts of interest
The authors declare no competing financial interest.
Acknowledgements
We thank the financial support from the National Natural Sci-
ence Foundation of China (51772079, 51672073 21771061),
Natural Science Foundation of Heilongjiang Province of
China (B2017009).
3
506–3512.
2
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