Photocatalytic oxidation of arylalcohols to aromatic aldehydes promoted by hydroxyl radicals over a CoP/CdS photocatalyst in water with hydrogen evolution

Article abstract of DOI:10.1039/c8cy00138c

Cobalt phosphide (CoP) combined with CdS was employed as a photocatalyst to oxidize arylalcohols into aromatic aldehydes or ketones in water. This was accompanied by the reduction of water, and the quantitative yield of hydrogen evolution was much higher than the chemical equivalent of hydrogen gas. Electron spin resonance spectroscopy and quenching experiments demonstrated that hydroxyl free radicals, originating from water splitting, promoted the oxidation of arylalcohols, while the holes in the valence band of the photocatalyst were reduced by the ^-OH and organic substrates. The overall reaction generates high-value-added organics. This photocatalytic reaction is atom-economical, in accordance with the concept of sustainable development.

Full text of DOI:10.1039/c8cy00138c

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PAPER
Qingzhu Zhang et al.
Catalytic mechanism of C–F bond cleavage: insights from QM/MM
analysis of fluoroacetate dehalogenase
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Photocatalytic oxidation of arylalcohols to aromatic aldehydes
promoted by hydroxyl radicals over CoP/CdS photocatalyst in
water with hydrogen evoluꢀon†
Received 00th January 20xx,
Accepted 00th January 20xx
a,b‡
c‡
a
c
c
a
a,c
DOI: 10.1039/x0xx00000x
Yong Xu, Ling-Zhen Zeng, Zhi-Cheng Fu, Cong Li, Zhi Yang, Yong Chen and Wen-Fu Fu*
www.rsc.org/
The cobalt phosphide (CoP) combined with CdS was employed as a photocatalyst to oxidize arylalcohols into aromatic
aldehydes or ketones in water. This was accompanied by the reduction of water, and hydrogen evolution was much more
than the chemical equivalent of hydrogen gas. Electron spin resonance spectroscopy and quenching experiments
demonstrated that hydroxyl free radicals, originating from water splitting, promoted the oxidation of arylalcohols, while
–
holes in the valence band of the photocatalyst were reduced by the OH and organic substrates. The overall reaction
generates high-value-added organics. This photocatalytic reaction is atom-economical, in accordance with the concept of
sustainable development.
However, if a system were developed in which the electrons of
+
the conduction band were captured by H to produce H , this
1
. Introduction
2
would be an ideal replacement for fossil fuels.
Utilization of solar energy to drive chemical reactions has
received increasing attention, being an energy-saving, effective
CdS, as a representative type II-VI semiconductor, is deemed
a promising candidate for transformation of solar energy into
1
and green approach. In 1972, Fujishima and Honda reported a
-3
1
,28
4
solar-driven alternative to traditional chemical synthesis. The
chemical energy on account of its suitable band gap.
However, the application of pure CdS is circumscribed by the
fast recombination of photogenerated electron-hole pairs as
well as the propensity to suffer photocorrosion. Therefore, a
great many measures have been taken to boost the activity
and stability of CdS, among which the combination of CdS with
conversion of alcohols into aldehydes is an important reaction
process in both chemical research and industry, being widely
5
used in the production of food, perfumes and pharmaceuticals.
The traditional method of transforming alcohols to aldehydes
not only uses toxic or caustic oxidants but also requires relatively
high pressure and temperature. As an alternative, oxidation of
2
9,30
a cocatalyst is particularly simple but effective.
By
promoting rapid electron transfer from CdS into the cocatalyst,
alcohols to aldehydes or ketones by using O
in both homogeneous and heterogeneous catalytic systems.
Palmisano et al. studied the oxidation of benzyl alcohols by O
2
has been reported
6
,7 the combined system gives rise to efficient and longer-lasting
photoinduced charge separation through lessening the
2
8
,9 recombination of electron-hole pairs. Cobalt phosphide (CoP)
using TiO
2
as catalyst under ultraviolet light irradiation.
is an example of non-noble-metal cocatalysts which exhibits
high photocatalytic hydrogen-generating activity when it
combined with CdS. The generation rate of hydrogen can
However, because about 50% of total solar energy is in the
visible region of the spectrum, it is necessary to develop visible-
light-responsive catalysts. Typical visible-light-responsive
-
1
-1
31
reach up to 254,000 μmol h
g under sunlight irradiation.
semiconductors include dye-sensitized or rare-earth-doped TiO
1
transition metal oxides and sulfides, and carbon nitride.
2
,
0-19 Transition metal phosphides have also been intensively
studied as catalysts for hydrodesulfurization, as they possess
Recently, numerous efforts have been made to use O
electron acceptor and Au/CeO , Au/TiO2, CdS-UiO-66(NH ), or
Pt/BiWO as photocatalysts under visible-light irradiation.
2
as
high electrical conductivity and metalloid characteristics,
3
2
2
2
2
0-27 similarly to zero-valent metals.
6
In view of the excellent photocatalytic activity of this class of
promising materials, we attempted to further extend their
applicability to photocatalytic oxidation of organics. Herein we
report the use of a composite CoP/CdS material as visible-light
catalyst for the oxidation of arylalcohols to aromatic aldehydes
or ketones in water. The reaction was accompanied by
hydrogen generation, and the quantitative yields of hydrogen
evolution were much more than the chemical equivalent of
hydrogen gas. It was found that hydroxyl free radicals,
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produced by photoinduced splitting of water, drove the with Ar as a carrier gas) with a 5 Å molecular sieve column (3
efficient catalytic oxidation of arylalcohols. Electron spin m × 2 m) and a thermal-conductivity detector.
resonance (ESR) spectral and isotope labeling experiments and
2
.3 Photoelectrochemical (PEC) measurements
product analysis provided direct evidence on the mechanistic
interpretation and origin of hydrogen evolution.
Photocurrent measurements were conducted on a CHI 660e
electrochemical workstation (Chenhua Instrument, China)
using a conventional three-electrode configuration. A catalyst
electrode served as the working electrode, with a platinum foil
as the counter electrode and an Ag/AgCl (saturated KCl)
2
. Experimental
2.1 Preparation
electrode as the reference electrode. Then, 0.2 M of Na
aqueous solution (pH = 6.8) was used as the electrolyte,
through which was bubbled for 20 min prior to
measurement. A white light-emitting diode (LED) (30 × 3 W, λ
420 nm) light source was used for irradiation. The working
2 4
SO
2
.1.1Preparation of CoP nanoparticles. CoP nanoparticles were
prepared through a solid-state thermal phosphorization
reaction. Typically, 2.0 g NaOH was added to a 500 mL of
aqueous solution containing 594 mg sodium citrate and 2,379
N
2
≥
2 2
mg CoCl •6H O in a round-bottom flask. After stirring for 15
electrodes were prepared as follows: 3 mg of sample was
dispersed in 40 μL isopropanol with 5 μL Nafion by sonication
for 20 min. The slurry was then evenly spread onto a 3.0 × 1.0
min, the generated suspension was separated by
centrifugation and washed with a large amount of water. The
obtained Co(OH)
temperature of 100
Co(OH) and 1,000 mg NaH
form a uniform mixture, and transferred into a vessel. Then,
the vessel was directly heated to 300 C for 4 h under Ar
2
was dried in a vacuum oven at the
C. After this, 200 mg of oven-dried
PO were ground in a mortar to
2
cm conducting indium tin oxide glass substrate with an active
2
area of about 1.0 cm . The film was dried in air. The
°
2
2
2
photoresponses of the samples with the light on and off were
measured at 0.0 V. Electrochemical impedance spectroscopy
°
(EIS) measurementswere carried out using an AC amplitude of
atmosphere in a tube furnace. Subsequently, the sample was
cooled naturally to room temperature in continuous Ar flow.
Finally, the obtained black solid was washed with water as well
1
0 mV in the frequency range of 0.01 Hz–1000 kHz at 0.05 V.
2
.4 Photocatalytic test
as hydrochloric acid several times and dried in a vacuum oven. Typically, 5 mL of water containing 0.1 mmol substrate and 2.5
2
4
5
.1.2 Preparation of CdS nanoparticles. In a typical procedure, mg CoP/CdS photocatalyst was added to a 15 mL cuvette
00 mL (0.14 M) of aqueous Na S solution was added slowly to sealed with a rubber septum placed on top of a magnetic
00 mL (0.14 M) of Cd(OAc) solution with vigorous stirring. stirrer. A stream of N2 was then passed into the reaction
2
2
The resulting yellow precipitate was stirred for 24 h and then system for 10 min. White LEDs (30 × 3 W, λ ≥ 420 nm) were
allowed to settle for 24 h. The yellow slurry was filtered and used as the irradiation light source. The LEDs were positioned
washed with 200 mL of water, then transferred to a dried 3 cm away from the sample, which was kept under continuous
Teflon-lined autoclave with a capacity of 100 mL. The stirring at room temperature. The mixture was stirred for 30
autoclave was sealed and heated at 200 °C for 24 h, after min before irradiation.
which the system was allowed to cool to room temperature
naturally. The obtained yellow solid was filtered and then
washed with 500 mL of water and 100 mL of ethanol 3. Results and discussion
consecutively. After drying at 80
obtained yellow solid was ground into fine particles.
.2 Characterization
The samples were characterized by powder X-ray diffraction
XRD) on a Bruker AXS D8 X-ray diffractometer with Cu Kα
°
C for 5 h in the oven, the 3.1 Characterization of photocatalysts
CoP was prepared by a solid-state thermal phosphorization
3
1
2
reaction. Its TEM and high-resolution TEM (HRTEM) images
are depicted in Fig. S1 in the electronic supplementary
informaꢀon (ESI†). Fig. 1a shows the XRD paꢁern of the
prepared orthorhombic CoP nanoparticles (JCPDS 29-0497),
with no impurity peaks. Five distinct diffraction peaks at about
(
radiation (λ =1.54056 Å) to identify their phases. The particle
size and lattice fringes of the samples were analyzed on a
transmission electron microscope (TEM; JEM 2100F) with an
accelerating voltage of 200 kV. Ultraviolet-visible diffuse
3
1.7°, 36.5°, 46.3°, 48.4° and 56.6° can be seen, corresponding
to the (011), (111), (112), (211) and (301) reflections of the
CoP phase, respectively. The hexagonal CdS nanospheres
reflectance spectra (DRS) were measured on
a
(
JCPDS 65-3414) were synthesized according to a conventional
spectrophotometer (Hitachi UV-3010) using BaSO4 as a
reference. ESR signals were recorded at room temperature
28
method (Fig. S2 and S3†). The CoP/CdS composite material
was obtained via grinding CoP nanoparticles and CdS
nanospheres together in a mortar to obtain uniformity. The
XRD pattern (Fig. 1b) shows no obvious peaks of CoP after
loading 5 wt% CoP on the as-prepared CdS. This may be
with a Bruker ESR E500 spectrometer. The chemical structure
1
and the quantity of the products were determined by
nuclear magnetic resonance (NMR)
H
spectroscopy
(BrukerAvance 400 spectrometer). The produced hydrogen because the peak intensity of CoP is much weaker than that of
was measured by gas chromatography (GC-2014C, Shimadzu, CdS, and CoP is highly dispersed in the CdS nanospheres. Fig.
2
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The resulting mixture was extracted thoroughly using
dichloromethane as extraction agent, and the extracted liquid
2 4
was dried with anhydrous Na SO . The chemical structures and
1
the quantity of products were confirmed via H NMR
spectroscopy. Control experiments showed that photocatalyst,
light and substrate were all indispensable for the
photocatalytic oxidation reaction (Fig. 2a). The conversion was
only 1.7% in the absence of CoP. However, when the
photocatalyst CoP/CdS (5 wt%) was added into the aqueous
solution, the conversion reached 88.0% with 109.1 μmol
hydrogen evolution. In anhydrous acetonitrile, no oxidation
product or hydrogen gas was detected (Fig. S5†). These results
showed that water plays a crucial role in this catalytic system.
To further understand the reaction process, D O was used in
2
place of H O. Under these conditions, D was detected by gas
2
2
chromatography with He as carrier gas (Fig. S6†), indicaꢀng
that the H detected in the main experiment originated from
2
water. The molar ratio between produced H2 and the
theoretical H2 generation, including the main product
aldehyde and the byproducts, was calculated to be 109.1:98.1.
This clearly showed that although the amount of hydrogen
production is directly related to the extent of arylalcohol
oxidation, the partial contribution of all the water splitting to
total amounts of hydrogen can not be excluded. The content
of CoP has a great influence on the efficiency of photocatalytic
oxidation. Fig. 2b compares the photocatalytic activity of
hydrogen generation at different ratios of CoP/CdS (1 wt%, 2.5
wt%, 5 wt% and 10 wt%). With increasing CoP concentration in
the range from 1 wt% to 5 wt%, the production of hydrogen
gradually rose. The reason for this was the exposure of more
active sites, resulting in higher catalytic activity. However, the
amount of produced hydrogen fell when the content of CoP
was further increased to 10 wt%. This can be tentatively
1
c shows TEM images of the 5 wt% CoP/CdS photocatalyst,
which confirms that the CoP nanoparticles are polydispersed.
As shown in the HRTEM image (Fig. 1d), the CoP nanoparticles
are less than 10 nm in diameter, and the lattice fringes with
spacings of 0.19 and 0.33 nm correspond to the (211) and
(002) planes of CoP and CdS, respectively. The transient
photocurrent responses of CdS and CoP/CdS showed that
charge transfer was more efficient for the latter, thereby
suppressing charge recombination as well as lengthening the
lifetime of charge carriers (Fig. 1e). It can be seen from the EIS
Nyquist plots that the impedance radius is smaller for CoP/CdS
than for pure CdS, which further confirms the decrease of
charge transfer resistance on the surface of the former (Fig.
1
f). Ultraviolet-visible DRS of the as-synthesized samples were
shown in Fig. S4. In comparison with the band gap of the CdS
2.18 eV), that of CoP/CdS is approximately 2.23 eV. Thus, the
(
formation of a CoP@CdS may efficiently promote conversion
of solar energy.
3.2 Photocatalytic oxidation of arylalcohols
Photodriven oxidation was carried out in aqueous solution
under anaerobic conditions at room temperature. The chosen
1
substrate was 4-methylbenzyl alcohol, which has a distinct H
NMR spectrum, and can be accurately weighed because it is a
solid. In a typical process, 5 mL of aqueous solution containing
4
-methylbenzyl alcohol (0.1 mmol) and photocatalyst
CoP/CdS) in a 15 mL cuvette was irradiated by a white light
LED (λ > 420 nm) under N atmosphere with magnetic stirring.
(
2
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attributed to the greater coverage of CoP nanoparticles on the prominent, whereas the peak at 5.10 ppm, corresponding to
surface of CdS, partly hindering the absorption of visible light. group –OH, weakened as the reaction time was prolonged.
1
Therefore, the optimum loading amount of CoP is 5 wt% in the After 15 h of irradiation, the H NMR spectrum (top in Fig. 3)
present study. The dosage of photocatalyst also has a displayed a weak peak at 12.8 ppm, corresponding to –COOH.
significant impact on conversion (Fig. 2c). The production of 4-Methyl benzoic acid (m/z 135.0452) was detected by mass
hydrogen was maximized when 2.5 mg photocatalyst was spectral analysis at this point in time (Fig. S8†), and the
added to the catalytic system. By contrast, with larger subsequent reaction was very slow. To study the selectivity of
amounts of photocatalyst, the amount of hydrogen in fact products, 4-methoxybenzyl alcohol was chosen as reaction
slightly decreased. In addition, hydrogen production activity is substrate because of the distinct peaks of its products in the
well known to be associated with pH. Therefore, a series of aromatic region. As shown in Fig. S9 and S10, the main product
experiments were performed with the pH ranging from 0 to
14. The resulting amounts of hydrogen evolution are shown in
Fig. S7. In acid medium, the conversion and selectivity were
affected only slightly in comparison with neutral pH. However,
under alkaline conditions, the conversion rate decreased
heavily while the selectivity was slightly increased (Fig. 2d).
The possible reason for the reduced hydrogen production in
alkaline environments is the low concentration of hydrogen
ions, which suppresses hydrogen generation.
1
H NMR spectra of reactant and product at various times are
shown in Fig. S5 and Fig. 3. The characteristic peak at 9.95 ppm
for –CHO of 4-methyl benzaldehyde became increasingly
is 4-methoxybenzaldehyde, and the byproducts include
benzhydrol, benzoyl and benzoin. However, benzoic acid was
not found after 5 h irradiation. In the above-mentioned
isotope labeling experiment, the holes generated in the
photocatalyst under light irradiation were able to oxidize OH−
2
of H O to produce hydroxyl radicals. Subsequently, benzyl
alcohol was attacked by the hydroxyl radicals to yield
hydroxybenzyl radicals, which may undergo coupling to
hydrobenzoin. However, the hydrobenzoin is easily oxidized to
1
1
form benzoyl and benzoin.
To gain mechanistic insight into the photocatalytic oxidation
process, an ESR spin-trapping experiment was performed,
using the trapping agent DMPO (dimethyl pyridine N-oxide). In
the dark, no free radical signal peak was observed (Fig. S11†).
However, significant peaks appeared under white LED
irradiation (Fig. 4), the changes of signal intensity with
irradiation time were depicted Fig. S11. By computer
simulation, the peak was resolved into two sets of peaks with a
calculated intensity ratio of approximately 1:4, indicating that
they arose from both carbon free radicals and hydroxyl free
radicals. In a separate experiment, 1 equivalent of TEMPO
(
2,2,6,6-Tetramethyl-1-piperidinylox, 0.1 mmol) was added
4
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into the photocatalytic system as a free radical quencher. After conduction band (Eqn (4)). Based on the results attained, it
h visible-light irradiation, no measurable product was found, was tentatively suggested that pathway (1) dominated the
5
except for the original substrate and TEMPO, by NMR photocatalytic oxidative reaction. In addition, byproducts are
spectroscopy and mass spectrometry (Fig. S12†). This further generated through free radical coupling reactions.
verifies that the photocatalytic oxidation of arylalcohols was
driven by a free radical reaction. Therefore, formation of
benzhydrol may occur via the decarboxylation of the
byproduct benzoic acid to form aryl radicals, which then
reacted with hydroxybenzyl radicals to produce benzhydrol via
C-C coupling. In addition, a set of free radical quenching
experiments was also conducted by employing EDTA-2Na and
▪
▪
ArCH
▪
－
OH + OH → ArCHO + H
2
O
(1)
(2)
(3)
(4)
+
+
ArCH
▪
－
OH + h → ArCHO + H
OH → ArCHO + ArCH
OH
2
2
ArCH
▪
－
-
－
OH → ArCHO + e + H
+
ArCH
Other benzyl alcohols with different substituents were
tert-butyl alcohol as quenching agents for holes and hydroxyl introduced into this catalytic system, and the rates of
radicals, respectively (Table S1†). The conversion was reduced substrate transformation and hydrogen evolution were
1
from 88.0% to 3.2% under conditions of hydroxyl radical determined by H NMR and GC analysis (Table 1, Fig. S13†).
quenching. This result is consistent with the outcome of the The substrates differed widely in their conversion rates, which
ESR experiment, indicating that hydroxyl radicals are the main can be partially ascribed to the solubility of the reactants in
active species. Based on the above analysis, a likely mechanism water. However, the substituent groups also had a large direct
for the complete reaction process is as follows. First, electrons influence on the conversion. The reaction degree increased
are excited from the valence band to the conduction band of with electron-donating substituents, which increase the
CdS under visible-light irradiation. The excited electrons are electron density of the substrate, making it more easily
delivered to the cocatalyst CoP, where they are combined with oxidized. Comparing entries 5 and 6, the oxidation rate of
hydrogen ions to generate hydrogen gas. Free hydroxyl ions diphenyl methanol was faster than that of methyl benzyl
supply electrons into the holes in the valence band to produce alcohol, which can be attributed to the conjugation effect. The
the main active species, hydroxyl radicals. Through the action stability of the CoP/CdS hybrid catalyst was also investigated.
of hydroxyl radicals and holes with substrates, α-H atom is The conversion dropped to 66.2% from 88.0% after 3
abstracted from benzyl alcohol to create hydroxybenzyl consecutive cycles (Fig. S14†), but the catalyꢀc acꢀvity was
radicals. The hydroxybenzyl radicals are then transformed to recovered by adding 2 mg CdS into the system. This reveals
aromatic aldehydes by four possible pathways: hydrogen that CdS is susceptible to photocorrosion during long-term
extraction reaction (Eqn (1)), hole oxidation (Eqn (2)), irradiation (Fig. S15†).
disproportionation (Eqn (3)) and electron transport to the
4
. Conclusions
In summary, we have constructed a noble-metal-free system
for visible-light-catalyzed oxidation of arylalcohols to aromatic
aldehydes. The hybrid photocatalytic system comprises CoP as
cocatalyst combined with CdS. The reaction is accomplished in
water, and accompanied by hydrogen evolution. The observed
2
amounts of H are much more thanthe chemical equivalent of
hydrogen gas calculated by producted aldehydes or ketones
and byproducts. Mechanistic study revealed that hydroxyl free
radicals, generated by photocatalytic water splitting, are the
driving species for the reaction, and water acts as a hydrogen
source confirmed by isotope labeling experiment. The system
developed here meets the requirements of green chemistry
and is more atom-economical than existing methods.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work was financially supported by the Natural Science
Foundation of China (21267025, 21273257, 21367026 and
2
1371175) and the Strategic Priority Research Program of the
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Chinese Academy of Sciences (XDB17030300) for funding
support.
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C Pa l tea al ys se i sd So c ni eo nt c ae d &j u Ts et cmh an rog l ion gs y
View Article Online
DOI: 10.1039/C8CY00138C
Journal Name
ARTICLE
Graphical abstract
Hydroxyl free radicals generated by visible-light-catalyzed water
splitting over CoP/CdS drive the oxidation of arylalcohols with
hydrogen evolution
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