Efficient acceptorless dehydrogenation of hydrogen-rich N-heterocycles photocatalyzed by Ni(OH)2@CdSe/CdS quantum dots

Article abstract of DOI:10.1039/d1cy00366f

Hydrogen storage using liquid organic hydrogen carriers (LOHCs) is a promising hydrogen storage technology; however, the hydrogen release process typically requires a high temperature. Developing dehydrogenation technology under mild conditions is highly desirable. Herein, a new approach for photocatalytic acceptorless dehydrogenation of hydrogen-rich LOHCs using Ni(OH)2@CdSe/CdS QDs as the photocatalyst was demonstrated. 1,2,3,4-Tetrahydroquinoline (THQ), iso-THQ, indoline, and their derivatives were selected as hydrogen-rich substrates, which exhibit excellent dehydrogenation efficiency with the release of hydrogen photocatalyzed by Ni(OH)2@CdSe/CdS QDs. Up to 100% yields of hydrogen and over 90% yields of complete dehydrogenation products were obtained at ambient temperature. Isotope tracer studies indicate a stepwise pathway, beginning with the photocatalytic oxidation of the substrate to release a proton and followed by proton exchange with heavy water. This work provides a promising alternative strategy to develop highly efficient, low cost and earth-abundant photocatalysts for acceptorless dehydrogenation of hydrogen-rich LOHCs.

Full text of DOI:10.1039/d1cy00366f

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Efficient acceptorless dehydrogenation of
hydrogen-rich N-heterocycles photocatalyzed by
Cite this: DOI: 10.1039/d1cy00366f
Ni(OH)2@CdSe/CdS quantum dots†
a
Yanpeng Liu,ab Tianjun Yu,
a Yi Zeng, ab Jinping Chen,
*
*
bc
ab
Guoqiang Yang
and Yi Li
Hydrogen storage using liquid organic hydrogen carriers (LOHCs) is a promising hydrogen storage
technology; however, the hydrogen release process typically requires a high temperature. Developing
dehydrogenation technology under mild conditions is highly desirable. Herein, a new approach for
photocatalytic acceptorless dehydrogenation of hydrogen-rich LOHCs using Ni(OH)2@CdSe/CdS QDs as
the photocatalyst was demonstrated. 1,2,3,4-Tetrahydroquinoline (THQ), iso-THQ, indoline, and their
derivatives were selected as hydrogen-rich substrates, which exhibit excellent dehydrogenation efficiency
with the release of hydrogen photocatalyzed by Ni(OH)2@CdSe/CdS QDs. Up to 100% yields of hydrogen
and over 90% yields of complete dehydrogenation products were obtained at ambient temperature.
Isotope tracer studies indicate a stepwise pathway, beginning with the photocatalytic oxidation of the
substrate to release a proton and followed by proton exchange with heavy water. This work provides a
promising alternative strategy to develop highly efficient, low cost and earth-abundant photocatalysts for
acceptorless dehydrogenation of hydrogen-rich LOHCs.
Received 1st March 2021,
Accepted 15th April 2021
DOI: 10.1039/d1cy00366f
rsc.li/catalysis
evidence that the liquid organic hydrogen carrier (LOHC)
technology is a very attractive alternative, because it makes
Introduction
As an environmentally friendly, abundant and high energy
density gas fuel, hydrogen is an ideal candidate energy carrier
to address environmental pollution and energy shortage in
the world.1–5 Although the concept of “hydrogen economy”
has been put forward by many governments as a strategy to
complete the nation's transformation toward the hydrogen
energy society, a great challenge facing the implementation is
how to store and transport hydrogen safely and efficiently.6
Liquid hydrogen storage and high-pressure hydrogen storage
are the most common methods in use at present, but both
methods run the risk of hydrogen leaks and explosions.7
Recent developments in hydrogen storage have provided
high density hydrogen storage possible at ambient
temperature and pressure.8–12 The LOHC technology involves
reversible hydrogenation and acceptorless dehydrogenation
between hydrogen-poor and hydrogen-rich organic liquids,
achieving storage and release of hydrogen. Most studies on
LOHC systems focused on the hydrogen capacity and the
efficiency of acceptorless dehydrogenation reactions between
hydrogen-poor and hydrogen-rich pairs. Benzene/cyclo-hexane,
toluene/methylcyclohexane and naphthalene/decalin are the
most studied hydrogen-poor/hydrogen-rich pairs because of
their high theoretical hydrogen capacity, but high reaction
temperatures are required for high conversion efficiency.13–19
Several groups have made efforts to decrease the reaction
enthalpy by introducing nitrogen atoms,20–24 but the
temperature of thermocatalytic dehydrogenation reactions is
still higher than 110 °C. In order to avoid the inconvenience
caused by the temperature of thermally promoted reactions,
light-induced acceptorless dehydrogenation reaction may
become an alternative way.
a Key Laboratory of Photochemical Conversion and Optoelectronic Materials,
Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing,
100190, China. E-mail: tianjun_yu@mail.ipc.ac.cn, yili@mail.ipc.ac.cn
b University of Chinese Academy of Sciences, Beijing, 100049, P. R. China
c Key Laboratory of Photochemistry, Institute of Chemistry, Chinese Academy of
Sciences, Beijing, 100190, P. R. China
Tanaka, Crabtree, Field, and Saito are the pioneers in the
field of photocatalytic acceptorless dehydrogenation from
alkanes using noble metals iridium and rhodium or cheap
metal iron and cobalt complexes.25–28 Because of the great
energy barrier for the C(sp3)-H band activation of alkanes,
reactions previously reported in the literature are inefficient and
† Electronic supplementary information (ESI) available: GC-FID analyses of THQ
and QL, GC-TCD analyses of H2 and D2, XPS survey and high-resolution XPS
spectra of CdSe/CdS QDs and Ni(OH)2@CdSe/CdS QDs, control experiments of
the photocatalytic hydrogen release over Ni(OH)2@CdSe/CdS QDs, time
dependent photocatalytic H2 and D2 release over Ni(OH)2@CdSe/CdS QDs, and
ESI-MS spectra of dehydrogenation products. See DOI: 10.1039/d1cy00366f
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require ultraviolet light to initiate.29–31 Recently, several
examples of photocatalytic dehydrogenation of hydrogen-rich
organic liquids have demonstrated that the reaction efficiency
has increased dramatically by replacing alkanes with N-
heterocycles.32–35 However, noble metals are often
indispensable in these systems, such as ruthenium, platinum,
palladium, and so on. Further, a homogenous system is greatly
limited in application by the tedious synthesis and recyclability
process of these costly noble metal complexes. To date, h-BCN,
MoS2/ZnIn2S4, and Pd/TiO2 have been reported as
heterogeneous catalysts for the photocatalytic acceptorless
dehydrogenation of N-heterocycles.36–38 However, h-BCN and
MoS2/ZnIn2S4 are effective only for the dehydrogenation of
tetrahydroquinolines and tetrahydroisoquinolines, and Pd/TiO2
contains precious metals. Therefore, developing an economical,
refluxed under N2-saturated conditions to generate CdSe QDs
of desired sizes controlled by the refluxing time of 3.0 h.
Synthesis of CdSe/CdS QDs
CdSe QDs are used as the core materials to prepare CdSe/CdS
core–shell QDs in situ. Firstly, the pre-prepared MPA–CdSe
QDs were purified to remove the extra ligands and impurities
in water by precipitation method. A stock solution of 200 mL
MPA–CdSe QDs was concentrated to 20 mL and then was
precipitated by adding an excessive amount of isopropanol.
The obtained precipitate was dispersed by using 200 mL
redistilled water and then was heated to 50 °C under an N2
atmosphere with vigorous stirring. Na2S (60 mL, 5 mM) and
CdCl2·2.5H2O (40 mL, 5 mM) aqueous solutions were in turn
pumped into the CdSe QD solution with a speed of 20 mL
min−1 and 15 mL min−1, respectively. After the addition of
these precursors, the solution was reacted for another 2 h to
ensure the growth of CdSe/CdS QDs.
efficient
heterogeneous
photocatalytic
acceptorless
dehydrogenation system is still highly desirable.
CdSe/CdS core–shell quantum dots (QDs) have received
significant attention in the past few decades because of their
fascinating lower cost, earth-abundant optical and electronic
properties.39–41 Herein, we for the first time report an efficient
photocatalytic acceptorless dehydrogenation of hydrogen-rich
N-heterocycles (tetrahydroquinolines, tetrahydroisoquinolines
and indolines) catalyzed by surface Ni(OH)2-loaded CdSe/CdS
core–shell QDs (Ni(OH)2@CdSe/CdS). Tetrahydroquinoline
and indoline derivatives are dehydrogenated to their
corresponding N-heteroaromatics with excellent yields,
accompanied by the release of hydrogen upon blue light
Characterization and measurements
Absorption and photoluminescence spectra were measured with
a Shimadzu UV-2550PC spectrometer and a Hitachi F-4600
spectrometer, respectively. X-ray diffraction patterns were
obtained by using a Bruker D8 Focus under Cu-Kα radiation.
High-resolution transmission electron microscopy (HRTEM)
images were obtained by using a JEM-2100F microscope with
an accelerating voltage of 200 kV. X-ray photoelectron
spectroscopy (XPS) was carried out on a Thermo ESCALAB 250
XPS spectrophotometer with Al-Kα radiation. The binding
energy scale was calibrated using the C 1s peak at 284.8 eV.
Photoelectrochemical and cyclic voltammetry measurements
were performed on a CHI 760E in a standard three electrode
cell. Working electrodes were designed by adding ethanol and
irradiation. This work provides
a
new strategy for
photocatalytic acceptorless dehydrogenation of hydrogen-rich
organic liquids, as well as clear experimental evidence on the
dehydrogenation mechanism, demonstrating potential for
application to LOHC technology.
Nafion
suspension
containing
CdSe/CdS
QDs
or
Experimental
Materials
Ni(OH)2@CdSe/CdS QDs onto glassy carbon electrodes and 2
cm2 area of clean FTO for electrochemical impedance
spectroscopy (EIS) and transient photocurrent response
measurements, respectively. ESI-MS results were obtained using
a Waters Q-TOF Premier apparatus.
If not specially indicated, reagents and solvents were used as
commercially available without further purification.
Ultrapure water with an 18.2 MΩ cm resistivity (Millipore)
was used for all experiments.
Photocatalytic dehydrogenation experiments
To a glass reactor containing a magnetic stirring bar was added
CdSe/CdS QDs, NiCl2·6H2O, H2O/CH3CN (V/V = 2/3, 5 mL) and
hydrogen-rich N-heterocycles. The solution was degassed in
vacuo and refilled with N2 3 times in an ice-water bath, and then
2 mL ultrapure CH4 was injected into the system to work as the
internal standard for quantitative GC analysis. Then the
reaction mixture was stirred for 24 h upon 420 nm LED
irradiation (I = 50 mW cm−2) at ambient temperature. The gas
phase was analysed for the H2 yield by GC-TCD using a 5 Å
molecular sieve column (2 m × 2 mm) and N2 as the carrier gas.
The gas phase was analysed for the D2 yield by GC-TCD using a
15% MnCl2@γ-Al2O3 column (2 m × 2 mm) and He as the
carrier gas. Then the generated dehydrogenation products were
analysed by GC-FID using an SH-Rtx-5 column (30 m × 0.25
Synthesis of CdSe QDs
CdSe QDs stabilized by 3-mercaptopropionic acid were made
according to the reported methods.42 Firstly, 40 mg Se
powder (0.5 mmol) was transferred to a Na2SO3 (100 mL, 15
mM) aqueous solution. Then, the resulting mixture was
degassed by evacuating and then refluxed until the Se powder
was dissolved completely to obtain a transparent Na2SeSO3
solution. Colloidal MPA–CdSe QDs were prepared by
introducing a freshly prepared Na2SeSO3 (10 mL, 5 mM)
solution into a 190 mL N2-saturated solution containing
CdCl2·2.5H2O and MPA at pH 11.0. The concentration of Cd2+
was set to be 1 mM, and the feed ratios of Cd : MPA : Se were
1 : 1.5 : 0.25. The resulting reaction mixture was immediately
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mm) and N2 as the carrier gas. Later on, Ni(OH)2@CdSe/CdS
QDs were separated from the reaction mixture by centrifugation
at 8000 r min−1 for 5 min, and the products were confirmed by
ESI-MS.
In order to evaluate the performance of CdSe and CdSe/CdS
QDs, photocatalytic acceptorless dehydrogenation of 1,2,3,4-
tetrahydroquinoline (THQ) to quinoline (QL) was conducted. A
reaction system consisting of the CdSe or CdSe/CdS core–shell
QDs and the substrate THQ was dispersed in acetonitrile–water
binary solvent via a 30 min ultrasound treatment before the
photocatalytic reaction. The photocatalytic dehydrogenation
reaction was started by injecting a certain amount of NiCl2 into
the reaction systems upon 420 nm light irradiation with a 50
mW cm−2 intensity. Both the CdSe and CdSe/CdS QDs catalytic
systems show hydrogen release upon irradiation, and the
amount of hydrogen released is closely related to the irradiation
time, as shown in Fig. 2a. The control experiments indicated
that no hydrogen release was observed in the absence of
irradiation, or CdSe or CdSe/CdS QDs, indicating that they are
essential, and NiCl2 itself cannot act as a photocatalyst alone for
the dehydrogenation reaction. The photocatalytic activity of the
CdSe QD system can only last for 12 h, and that of the CdSe/
CdS QD system remains almost unchanged for 24 h under the
experimental conditions, demonstrating that the CdSe/CdS
core–shell QDs are more stable during the photocatalytic
dehydrogenation of THQ. The effects of CdSe/CdS QD and NiCl2
concentrations on the yield of dehydrogenation were also
investigated. The yields of 39% for the formation of QL and
44% for the hydrogen release were obtained by gas
chromatography using 20 mM THQ, 0.6 mg mL−1 CdSe/CdS
QDs and 0.4 mM NiCl2 in H2O/CH3CN (V/V = 2/3) mixed
solvent. Further optimization experiments for the amount of
CdSe/CdS QDs and NiCl2 were conducted to improve the yield
of QL by varying one condition while keeping all the other
conditions identical. The yield of QL increases with the
Results and discussion
The formation of the CdSe/CdS core–shell structure was
confirmed by a series of characterization techniques. Typical
excitonic peaks of CdSe and CdSe/CdS QDs at 433 nm and 452
nm can be observed in their UV–vis absorption spectra (Fig. 1a),
accompanied with a bathochromic shift of emission caused by
the CdS shell formation (Fig. 1b).43 The band-gap energies (Eg)
are estimated to be 2.66 and 2.60 eV for CdSe and CdSe/CdS
QDs from their Tauc plots, respectively (Fig. 1c), indicating that
the CdS shell enhances the visible-light harvesting ability. The
X-ray diffraction (XRD) patterns demonstrate that the
characteristic peaks corresponding to the (111), (220) and (311)
planes located at 25.4°, 42.0°, and 49.7° for CdSe shift to 26.0°,
43.3°, and 50.8° for CdSe/CdS, moving consistently toward the
values of CdS QDs (Fig. 1d). The transmission electron
microscopy (TEM) and high-resolution TEM (Fig. 1e and f)
images show that the CdSe QDs are nearly spherical
nanoparticles with an average diameter of 3.0 nm, while the
obtained CdSe/CdS QDs have an ellipsoidal shape with an
average width and length of 3.5 and 5.0 nm. The interplanar
spacings of the (111) crystal plane are 0.353 nm and 0.338 nm
for CdSe and CdSe/CdS QDs, respectively. Moreover, the (220)
crystal plane of cubic CdS with a lattice spacing of 0.207 nm
was also found in the CdSe/CdS QDs, giving another strong
evidence of the formation of CdSe/CdS core/shell QDs.
Fig. 1 (a and b) Absorption and normalized emission spectra of CdSe and CdSe/CdS QDs. The emission spectra were obtained under 400 nm
excitation. (c) Tauc plots of CdSe and CdSe/CdS QDs. (d) XRD patterns of CdSe and CdSe/CdS QDs. (e and f) TEM and HRTEM (inset) images of
CdSe and CdSe/CdS QDs.
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Fig. 2 (a) Time dependent photocatalytic hydrogen release over CdSe and CdSe/CdS QDs, respectively ([CdSe] = [CdSe/CdS] = 0.5 mg mL−1
[NiCl2] = 0.2 mM, [THQ] = 20 mM). (b) Influence of the concentration of CdSe/CdS QDs on the photocatalytic dehydrogenation of THQ ([NiCl2] =
0.4 mM, [THQ] = 20 mM). (c) Influence of the concentration of NiCl2 on the photocatalytic dehydrogenation of THQ ([CdSe/CdS] = 2.8 mg mL−1
,
,
[THQ] = 20 mM). The photocatalytic reactions were performed in H2O/CH3CN (V/V = 2/3) for 24 h under blue LED irradiation (λ = 420 nm, I = 50
mW cm−2).
concentration of CdSe/CdS QDs when the concentration is
below 2.8 mg mL−1 (Fig. 2b), which can be ascribed to more
photons absorbed by the reaction system. Further increasing
the concentration of CdSe/CdS QDs results in no improvement
of the QL yield, which may be caused by two possible reasons:
1) the redundant increase of CdSe/CdS QDs for the limited
incident photons and 2) the self-quenching caused by high
concentration of CdSe/CdS QDs. Fig. 2c and S1 and S2†
demonstrate that the presence of NiCl2 can significantly
increase the dehydrogenation efficiency. In the absence of
NiCl2, the yield of QL is only 28% after 24 hours of irradiation.
The yield of QL increases with the increase in the concentration
of NiCl2, and reaches 80% at a NiCl2 concentration of 1.6 mM,
indicating that the presence of NiCl2 enhances the
photocatalytic activity of dehydrogenation. The ESI-MS of the
reaction mixture (Fig. S3†) after removing the photocatalyst by
centrifugation further proved that THQ was completely
dehydrogenated to QL instead of partially dehydrogenating to
1,2-dihydroquinoline or 3,4-dihydroquinoine.
The high photocatalytic
activity of acceptorless
dehydrogenation of THQ encouraged us to explore the
application of this photocatalytic system to other substrates.
THQ derivatives and several other N-heterocycles were examined
under the optimal conditions ([CdSe/CdS] = 2.8 mg mL−1,
[NiCl2] = 1.6 mM, H2O/CH3CN (V/V) = 2/3), and the results are
summarized in Table 1. 2-MeTHQ and 4-MeTHQ (THQ bearing
Table 1 Acceptorless dehydrogenation of hydrogen-rich N-heterocycles
Entrya
1
Substrate
Product
Time (h)
24
Conversionb (%)
95
GC yieldb (%)
80
H2 yieldc (%)
95
2
3
24
24
97
82
88
94
100
100
4
24
71
47
61
5
6
24
24
64
85
38
78
59
72
7
24
100
86
100
8
9
12
12
100
98
93
92
100
98
a
Reaction conditions: hydrogen-rich N-heterocycles (20 mM), CdSe/CdS QDs (2.8 mg mL−1), NiCl2 (1.6 mM), and H2O/CH3CN (V/V = 2/3, 5 mL),
under irradiation with 420 nm LEDs at room temperature (I = 50 mW cm−2). Conversions and yields were determined by GC-FID analysis
b
using a peak area normalization method. c Hydrogen yields were determined by GC-TCD analysis using CH4 as the internal standard.
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2-methyl and 4-methyl groups) were converted to their
corresponding 2-MeQL and 4-MeQL in 82% and 88% yields,
respectively (entries 2 and 3). Meanwhile, the hydrogen yields
were up to 94% and 100%, which are very close to the
conversions of the substrate. The adsorption of the
dehydrogenation products on the surface of CdSe/CdS QDs may
be responsible for their lower yields than conversion. The
higher conversions of 2-MeTHQ and 4-MeTHQ than that of
THQ are attributed to stabilization by the methyl group, which
is similar to previous results in the literature.44 When the
aromatic ring was substituted with a methoxyl group at the 7
position, 7-MeOQL was obtained only in 47% yield (entry 4).
The substrates were also extended to tetrahydroisoquinoline
(iso-THQ)
and
its
derivatives.
The
photocatalytic
dehydrogenation of iso-THQ produces isoquinoline (iso-QL) in
only 38% yield (entry 5). The low yield of iso-QL can be ascribed
to the difficult oxidation process of iso-THQ caused by
nonconjugation of the secondary amine with the phenyl
group.45 When the electron donating substituents methyl and
Fig. 3 (a–f) STEM image and the corresponding elemental mapping
images of the Ni(OH)2@CdSe/CdS QDs obtained from the photocatalytic
system after 24 hours of irradiation. (g) High-resolution XPS spectrum of
Ni 2p distributed on the surface of Ni(OH)2@CdSe/CdS QDs.
phenyl were attached to the
1 position, 1-methyl and
1-phenylisoquinlines were obtained in 78 and 86% yields,
respectively (entries 6 and 7), which also benefits from the
stabilization by the substituents. Notably, the dehydrogenation
reactions of tetrahydroisoquinolines are complete rather than
partial, which is concluded from the ESI-MS analysis of the
reaction mixture (Fig. S4–S9†). More exciting results were
obtained by using indoline and 2-methyl indoline substrates.
The GC yields of indole and 2-methylindole were as high as
93% and 92%, accompanied with the generation of hydrogen in
100% and 98% yield, respectively (entries 8 and 9), which can
be ascribed to their single hydrogen removal process.
The optimization experiments demonstrate that NiCl2 plays
a crucial role in the improvement of the photocatalytic activity.
To investigate the behavior of NiCl2 during the photocatalytic
reaction, we have analyzed the catalyst composition after
photocatalytic dehydrogenation reaction by means of scanning
TEM (STEM) and elemental mapping analysis (Fig. 3a–f). The
elemental mapping images show that the photocatalyst is
composed of Cd, Se, S and Ni elements. The high consistency of
Ni distribution with other elements indicates that Ni is
deposited on the surface of the CdSe/CdS QDs in the
photocatalytic reaction system, which is further validated by
X-ray photoelectron spectroscopy (XPS) measurements. The
specific bonding and chemical states of Ni were also analyzed
by XPS (Fig. 3g and S10 and S11†). As shown in Fig. 3g, the
high-resolution XPS spectrum of Ni 2p reveals two typical peaks
located at 856.7 and 874.2 eV, which are consistent with the Ni
2p3/2 and Ni 2p1/2 spin–orbit peaks of Ni(OH)2, respectively.46,47
Two peaks located at 862.1 and 880.3 eV are assigned to the
corresponding satellite peaks of Ni(OH)2. These indicate that
NiCl2 converts to Ni(OH)2 on the surface of the CdSe/CdS QDs,
forming Ni(OH)2@CdSe/CdS QDs. The in situ formed Ni(OH)2
captures electrons from the CdSe/CdS QDs and becomes a new
active site for proton reduction because of the less negative
potential of Ni2+/Ni than that of CdS,48 thus greatly improving
the catalytic activity of the photocatalyst.
To gain an insight into the electronic level structure, we
carried out cyclic voltammetry (CV) measurements to analyse
the valence band (EVB) of CdSe and CdSe/CdS QDs, which is
approximately equal to that of the oxidation potential (Epa).49
Both of them exhibit an Epa approximate to 1.10 V (vs. NHE),
corresponding to the EVB of the CdSe QDs (Fig. S12†).
Additionally, the CdSe/CdS QDs present another Epa of
approximately 1.49 V, corresponding to the EVB of the CdS
shell. Considering that the conduction band (ECB) is equal to
the difference between EVB and Eg, the ECB of the CdSe QDs
is calculated to be −1.56 V. The ECB of the CdS shell is
estimated to be −1.50 V by the difference between the EVB of
the CdSe core and the Eg of the CdSe/CdS QDs, which is
attributed to the spatially indirect transition from the valence
band of the CdSe core to the lowest conduction band of the
CdS shell.43,50 Obviously, the CdSe/CdS QDs formed a quasi-
type II carrier distribution (Fig. 4), which is beneficial for
Fig. 4 Energy level diagram of the proposed mechanism for the
photocatalytic
acceptorless
dehydrogenation
of
THQ
by
Ni(OH)2@CdSe/CdS QDs.
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improving the charge separation efficiency.43,51,52 After
absorbing photons, electrons and holes are generated and
transferred to the conduction band of the CdS shell and the
valence band of CdSe, respectively. Since the reduction
potential (ERE) of Ni(OH)2 (−0.72 V)53 is more positive than
the ECB of the CdS shell, the electron transfer from the CdS
shell to the catalyst Ni(OH)2 is thermodynamically feasible.
The emission of CdSe/CdS QDs is dramatically quenched by
Ni(OH)2 (Fig. S13†), indicating that the electron transfer
occurs.54,55 THQ is directly oxidized by the holes randomly
distributed on the surface of the CdSe/CdS QDs, because the
EVB of the CdSe/CdS QDs is more positive than the Epa of
THQ (Fig. S14†). Transient photocurrent response and
electrochemical impedance spectroscopy (EIS) measurements
of the CdSe/CdS QDs and Ni(OH)2@CdSe/CdS QDs were
performed to investigate the interfacial charge transport
behavior. As shown in Fig. 5a, the transient photocurrent
response of the Ni(OH)2@CdSe/CdS QDs are remarkably
higher than that of the CdSe/CdS QDs, indicating the more
effective separation and transition of the photoinduced
electron/hole pairs. The Nyquist plot of the Ni(OH)2@CdSe/
CdS QDs in the dark exhibits a smaller semicircular arc
diameter compared to that of the CdSe/CdS QDs (Fig. 5b),
suggesting a lower charge-transfer resistance.
dehydrogenation of THQ over Ni(OH)2@CdSe/CdS QDs. After
24 hours of irradiation, only deuterium gas (D2) was detected
in the gas phase of the reaction system, accompanied with
the production of QL and the consumption of THQ in the
liquid phase (Fig. S15 and S16†). The reaction carried out in
pure acetonitrile solvent hardly detected the formation of H2
and QL. Obviously, the removal of hydrogen from THQ is not
in the form of H2. A reasonable inference is that THQ is
firstly oxidized and then releases a proton. The released
protons exchange rapidly with D2O, and the produced
deuterons are reduced to D2. No H2 product detected in the
gas phase can be ascribed to the large amount of D2O in the
system and the quick exchange reaction, indicating that
almost all released protons are converted to deuterons. Even
though a trace amount of protons exists in the system, the
generated H2 is lower than the detection limit of gas
chromatography. On the basis of the isotope tracer studies,
the photocatalytic dehydrogenation and the control
experiments, a possible mechanism for the photocatalytic
acceptorless dehydrogenation of THQ is proposed and
illustrated in Scheme 1. Upon photoexcitation of
Ni(OH)2@CdSe/CdS QDs,
a valence band hole and a
conduction band electron are produced. The substrate is
then oxidized by the hole to generate the corresponding
cation radical, which was indirectly proved through the
formation of an adduct with 5,5-dimethy-1-pyrroline N-oxide
(DMPO) by ESI-MS (Fig. S17†). Following the deprotonation
of the cation radical, another electroneutral intermediate is
formed. The electroneutral intermediate undergoes the
second photosensitized oxidation and deprotonation process
in turn to form a double bond in a nitrogenous six-
membered ring, which was confirmed by the partial
dehydrogenation product signal in the ESI-MS spectrum (Fig.
S8†). Subsequent isomerization allows the partially
dehydrogenated intermediate to be continually oxidized by
the valence band holes and to remove protons. After four
oxidation and deprotonation processes, the initial hydrogen-
rich substrate eventually becomes an unsaturated organic
hydrogen carrier. The protons detached from the substrate
Understanding the mechanism of the hydrogen release
process will lead to designing active photocatalysts for the
acceptorless dehydrogenation of hydrogen-rich LOHCs.
Hence, an isotope tracer experiment was conducted by
replacing H2O–CH3CN with a D2O–CH3CN mixed solvent to
identify the elementary steps in the acceptorless
Fig. 5 (a) The transient photocurrent response of CdSe/CdS QDs and
Ni(OH)2@CdSe/CdS QDs upon visible-light illumination (λ ≥ 420 nm).
(b) Nyquist plot of CdSe/CdS QDs and Ni(OH)2@CdSe/CdS QDs.
Scheme 1 Proposed mechanism for the photocatalytic acceptorless
dehydrogenation of THQ catalyzed by Ni(OH)2@CdSe/CdS QDs.
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exchange with heavy water, and then, with the help of the
cocatalyst Ni(OH)2 deposited on the surface of the CdSe/CdS
QDs, D+ is reduced to form D2.
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Conclusion
In summary, an efficient photocatalyst based on
Ni(OH)2@CdSe/CdS QDs has been reported for its activity
toward acceptorless dehydrogenation of hydrogen-rich
N-heterocycles. Ni(OH)2@CdSe/CdS QDs are proved to be highly
active for the photocatalytic dehydrogenation of THQ, iso-THQ,
indoline, and their derivatives, with as high as 93% yield of
dehydrogenation products and 100% yield of released hydrogen.
The admirable activity of the photocatalytic system can be
attributed to the heterojunction structure of the CdSe/CdS QDs
and the suitable redox potential of the Ni(OH)2 co-catalysts. As a
consequence, the combination of these photocatalysts raises
the possibility that this system might be applied to hydrogen
release at ambient temperature. The present research provides a
powerful complement or potential competitor to the thermally
promoted dehydrogenation systems, especially suitable for the
application in a sunny environment.
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Author contributions
19 G. Lee, Y. Jeong, B.-G. Kim, J. S. Han, H. Jeong, H. B. Na and
J. C. Jung, Catal. Commun., 2015, 67, 40–44.
Yanpeng Liu: investigation, formal analysis, writing – original
draft. Tianjun Yu: conceptualization, supervision, writing –
review & editing, funding acquisition. Yi Zeng: supervision,
funding acquisition. Jinping Chen: supervision. Guoqiang
Yang: supervision. Yi Li: conceptualization, writing – review &
editing, supervision, funding acquisition.
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Conflicts of interest
24 Y. H. Han, Z. Y. Wang, R. R. Xu, W. Zhang, W. X. Chen, L. R.
Zheng, J. Zhang, J. Luo, K. L. Wu, Y. Q. Zhu, C. Chen, Q.
Peng, Q. Liu, P. Hu, D. S. Wang and Y. D. Li, Angew. Chem.,
Int. Ed., 2018, 57, 11262–11266.
There are no conflicts to declare.
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This work is supported by the National Natural Science
Foundation of China (No. 21672226, 21673264, 21573266,
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