A function-switchable metal-free photocatalyst for the efficient and selective production of hydrogen and hydrogen peroxide

Article abstract of DOI:10.1039/d0ta03974h

Photocatalytic water splitting to produce hydrogen and hydrogen peroxide is the main goal of photocatalysis. However, the reactions of both are water based and thus, it is extremely challenging to conduct both processes on one catalyst. Herein, a function-switchable metal-free catalyst was designed and synthesized through the polycondensation of procyanidin and 4-methoxybenzaldehyde. Impressively, the catalyst developed here not only split water to produce hydrogen via a four-electron reaction process in the absence of air, but also generated hydrogen peroxide by preferentially reducing oxygen via a two-electron pathway in air. The production rates of hydrogen and hydrogen peroxide reached 252.02 and 1385.42 μmol h-1 g-1, respectively. Through in situ transient photovoltage tests and kinetic analysis, a clear and fundamental understanding of the highly efficient and switchable functions was acquired for this photocatalyst. This study highlights the unique behavior and provides an insight into the design of function-switchable catalysts and the regulation of catalytic reactions. This journal is

Full text of DOI:10.1039/d0ta03974h

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A function-switchable metal-free photocatalyst for efficient and
selective production of hydrogen and hydrogen peroxide
Received 00th January 20xx,
Accepted 00th January 20xx
Qingyao Wu, Yan Liu, Jingjing Cao, Yue Sun, Fan Liao, Yang Liu,* Hui Huang,* Mingwang Shao,*
and Zhenhui Kang*
DOI: 10.1039/x0xx00000x
Photocatalytic water splitting to produce hydrogen and hydrogen peroxide are the core issue of photocatalysis. Thought
they are both water-based reactions, it’s extremely troublesome to conduct the two processes on one catalyst. Herein, a
function-switchable metal-free catalyst was design and synthesized through the polycondensation of procyanidin and 4-
methoxybenzaldehyde. Impressively, the catalyst developed here not only was expected to split water to produce
hydrogen by four-electron reaction process in air free, but also to generate hydrogen peroxide by preferentially reducing
oxygen with two-electron pathway in air. The production rates of hydrogen and hydrogen peroxide reached 252.02 and
1385.42 μmol h-1g-1, respectively. Through in-situ transient photovoltage tests and kinetic analysis, a clear and
fundamental understanding on the highly efficient and switchable functions was acquired over this photocatalyst. This
study highlights a unique behavior, and provides an insight into the design of function-switchable catalyst and the
regulation of catalytic reactions.
www.rsc.org/
synthesize hydrogen peroxide13-16. It is significant to regulate
hydrogen and hydrogen peroxide production on one photocatalyst
for both high efficiency photocatalyst design as well as the wide
1. Introduction
Environmental pollution and the energy crisis spur the exploitation
of clean and renewable energy sources1-3. Hydrogen (H2), a green
and carbon-free energy source, has been shown to be an ideal
candidate for replacing fossil fuels4. Generating hydrogen directly
from water through photocatalysis is one of the most promising and
scalable ways5. On the other hand, hydrogen peroxide (H2O2) widely
used in field of medicine, chemical industry and other aspects is
also a powerful alternative to conventional fuels if it can be
prepared by simply, clean and cheap approaches, such as
photosynthesis method6,7. The most satisfactory situation is that, in
the photocatalytic system, solar energy is converted directly into
stored chemical energy (H2 and H2O2) through a water-based
reaction without releasing any by-products, and the H2 and H2O2
generation only needs light, water and oxygen8,9. However, the
mechanism and free energy of hydrogen produced by water
splitting are different from that of hydrogen peroxide by oxygen
reduction10-12. Generally, in virtually all photocatalytic researches,
the focus was placed on the single-functional catalytic reactions, in
which photocatalyst could only achieve a single reaction, such as
just splitting water to produce hydrogen or oxygen reduction to
application of photocatalytic technology.
Recent reports have demonstrated some bifunctional properties
of photocatalysts, such as hydrogen production and olefin
hydrogenation17, hydrogen production and organic degradation18,
hydrogen production and reduction of carbon dioxide19. However,
bifunctional catalysts that can both produce hydrogen and
hydrogen peroxide are few reported. Due to the low yields of
hydrogen peroxide product, an intermediate in water splitting, and
it is easily decomposed20. Notably, in photocatalysis, catalytic
material with low-cost, safety, high efficiency and function-
switchable, is considered as one of the most ideal photocatalysts.
Very recently, the resorcinol-formaldehyde resin was reported as a
low-bandgap metal-free photocatalyst for the synthesis of
hydrogen peroxide by a single function way21. In this field, the most
challenging obstacle is the design and synthesis of efficient
bifunctional metal-free photocatalysts, based on water splitting, for
both hydrogen evolution and hydrogen peroxide artificial synthesis.
We aim to fabricate
a
function-switchable metal-free
photocatalyst for efficient and selective production of hydrogen and
hydrogen peroxide. In this work, a metal-free polymer catalyst
procyanidin-methoxy-benzaldehyde (PC-MB) was prepared by
polymerization of light-sensitive dye procyanidin (PC) and 4-
methoxybenzaldehyde (MB). Impressively, these catalysts exhibit
excellent light absorption and function-switchable catalytic
characteristics, that is, hydrogen or hydrogen peroxide could be
selectively generated by controlling the reaction atmosphere. The
material PC-MB possesses the ability of overall water splitting only
in the absence of air, in which, the H2 evolution was coupled with
a
Institute of Functional Nano and Soft Materials Laboratory (FUNSOM), Jiangsu
Key Laboratory for Carbon-Based Functional Materials
University, Suzhou 215123, PR China.
E-mail: zhkang@suda.edu.cn; mwshao@suda.edu.cn; hhuang0618@suda.edu.cn;
yangl@suda.edu.cn
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
& Devices, Soochow
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the O2 formation via a one-step 4e- pathway. The efficiency of H2 monochromatic Mg Kα X-ray source (hν=1283.3 eV).VDieywnAarmticliecOlinglihnet
evolution is 252.02 μmol h-1g-1 and the conversion efficiency from scattering (DLS) was employed to deterDmOinI:e10t.1h0e39/pDa0rtTiAc0le39s7i4zHe
solar to hydrogen (STH) can be up to 0.85% at the illumination distribution of the samples. Organic element analyzer (Vario
intensity of 1.75 mW cm-2. After switching the function, only MACRO cube) was used for the combustion elemental analysis (EA).
hydrogen peroxide was detected when catalyst is in air, by which,
UV/VIS/NIR spectrophotometer (Lambda 750, Perkinelmer) was
this sole product was obtained via a two-channel pathway (water employed to acquire the UV-vis absorption spectra. Ultraviolet
oxidation reaction and oxidation reduction reaction). The yield of photoelectron spectroscopy (UPS) measurements were performed
hydrogen peroxide can reach 1385.42 μmol h-1g-1 in air (illumination with He I (21.22 eV) as the monochromatic light source and a total
intensity is 31.27 mW cm-2). With evidence from electrochemical instrumental energy resolution of 100 meV.
experiments, in-situ transient photovoltage (TPV) tests and kinetic
All the electrochemical measurements were conducted on a CHI
analysis, a clear and fundamental understanding was acquired on 920C workstation (CH Instruments, Shanghai, China), using a
the highly efficient and selective production of hydrogen and standard three-electrode system, of which a carbon electrode is
hydrogen peroxide over the function-switchable photocatalyst.
used as the reference electrode, a saturated calomel electrode
(SCE) as the counter electrode and a glass carbon (GC) electrode as
working electrode. The photoresponse-time curves were obtained
at open circuit potential (OCP) in 0.1 M sodium sulphate (Na2SO4)
with a 100 W light-emitting diode (LED) lamp as the light source.
Electrochemical impedance spectra (EIS) measurements were
carried out at open circuit potential as well, with a frequency range
from 1 MHz to 0.01 Hz and Ac voltage amplitude of 5 mV in
ultrapure water.
TPV was measured on a home-made system. The TPV was excited
with a nanosecond laser radiation pulse (wavelength of 355 nm and
the repetition rate is 5 Hz) from a third harmonic Nd:YAG
(Beamtech Optronics Co., Ltd). The signal of the TPV was amplified
by the amplifier and recorded by the oscilloscope. All the
measurements were performed at room temperature and under
ambient pressure.
2. Experimental section
2.1 Materials.
All chemicals were used without any further purification. Na2CO3
(AR, 98%) and HNO3 (AR, 98%) were purchased from Aladdin.
Procyanidins (AR, 98%) was purchased from Yuanye. 4-
methoxybenzaldehyde (AR, 98%) was purchased from Sinopharm
Group chemical Reagent Co., Ltd.
2.2 Preparation of PC-MB cross-linked organic semiconductor
photocatalyst.
The interconnected organic semiconductor PC-MB was synthesized
by using Na2CO3 as the catalyst, through a sol-gel process,
crosslinking polymerization and freeze-drying technique. Typically,
0.5 g of procyanidins (PC) and 0.1 g of Na2CO3 were dissolved in 10
mL deionized water, and then 0.9 mL of 4-methoxybenzaldehyde
(MB) was added. Next, different amounts of HNO3 (4 mol L-1) were
added to adjust the pH values of the mixture solution, which are 1,
3, 5 are 7, respectively. After stirring for 30 min at room
temperature, the reactors were sealed to prevent the volatilization
of the solution. The precursor was heated at 80 °C for 48 h to form
PC-MB gel. After that, the PC-MB gel was washed with ethanol and
water to remove the residual organic solvent. Finally, the PC-MB gel
was frozen in liquid nitrogen (-196 °C) for 15 minutes, and
transferred to the vacuum chamber of the freeze-drying device
(operating at -53 °C and 0.113 mbar) for at least 60 h to obtain PC-
MB cross-linked organic semiconductor photocatalyst. The samples
synthesized at different pH values were labeled as PC-MB-1, PC-MB-
3, PC-MB-5, and PC-MB-7, respectively.
2.4 Photocatalytic activity exploration.
The production and detection of hydrogen peroxide (H2O2). The
photocatalytic hydrogen peroxide production properties of the as-
obtained samples were evaluated using
a
multichannel
photochemical reaction system (PCX50A, Beijing Perfectlight Co.
Ltd, China) with a visible light source (λ ≥ 420 nm). No sacrificial or
cocatalysts were needed. In a typically process, 40 mg photocatalyst
was dispersed in 20 mL ultrapure water (in a 40 mL reaction
bottles), and then the photocatalytic reactions were exposed to
visible light for 12 h in air. A series of conditions were changed to
explore the optimal activity and mechanism of the reaction, such as
changing the amount of catalyst, lighting time, excitation
wavelength, adding sacrificial agent, pH value of the reaction
environment and reaction atmosphere (air, N2, O2 and air free), etc.
At the end of each reaction, the catalyst was centrifuged, washed,
dried, and then used for the cycle reactions. To detect the amount
of H2O2 generated during the photocatalytic process, potassium
permanganate titration was employed. 10 mL of the supernatant
with 5 mL H2SO4 (4 M) was titrated by KMnO4 (0.1 M) to calculate
the amount of the hydrogen peroxide produced.
The evolution and test of hydrogen (H2). The activity of
photocatalytic hydrogen was investigated in a vacuum adaptive
reactor with a xenon lamp as the light source (λ ≥ 420 nm). 40 mg
as-prepared catalyst was dispersed in 80 mL of ultrapure water,
then vacuumed for 30 min to reach an air-free environment, and
finally added visible light for 12 h. The collected gas was detected
by a gas chromatograph (GC-7890) set up with 5A molecular sieves
column and a thermal conductivity detector (TCD) was utilized.
2.3 Materials characterization.
Powder X-ray diffraction (XRD) was carried out to characterized the
crystal structure of the as-prepared products by using a PIXcel3D X-
ray diffractometer (Empyrean, Holland Panalytical) with Cu Kα
radiation (λ=0.154178 nm). The scanning electron microscope (SEM)
was applied to characterize the surface morphology of the samples.
Transmission electron microscopy (TEM) was measured by using a
FEI-Tecnai F20 transmission electron microscope with an
accelerating voltage of 200 kV. The Fourier transform infrared (FTIR)
spectra were collected by using a Hyperion spectrophotometer
(Bruker) at the scan range of 400-4000 cm-1. X-ray photoelectron
spectroscopy (XPS) measurements were conducted on a KRATOS
Axis ultra-DLD X-ray photo-electron spectroscope with
a
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absorption spectrum. The curve of (αhν)2 versus hν obtained by UV-
3. Results and Discussion
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Vis spectrum conversion is illustrated in Fig.D1ObI,: i1n0.w10h3i9c/hDα0T, Ah0,3a9n7d4Hν
4. Characterization and Band Structure of Photocatalysts.
are correspond to the absorption coefficient, Planck’s constant, and
frequency of the incident light, respectively. In the ordinate, r=2
denotes the directed band gap of a material, while r=1 represents
the indirect band gap of a material31,32. The curve in Fig. 1b
illustration fits well linearly at r=2, indicating that the photocatalyst
PC-BM-3 is a direct band gap material. A low Eg value of PC-BM-3,
which may be caused by π-stacking according to the previous
report21, could be calculated as 1.88 eV by measuring the x-axis
intercept of the extrapolated line from the linear state of the curve.
The UV-Vis absorption diagrams of other catalysts are accessed in
Fig. S7, ESI†.
As shown in Fig. 1a, the postulated schematic of the procyanidin-
methoxy-benzaldehyde (PC-MB) polymer, formed by the
polycondensation of PC and MB, was demonstrated briefly. Under
acidic condition, MB with aldehyde group was activated by
hydrogen proton (Fig. S1a, ESI†)22, and then the activated MB was
connected to PC containing the resorcinol group through catechol
group, forming a phenolic resin chain by electrophilic substitution
(Fig. S1b and S1c, ESI†)23-25. The phenolic resin chains are further
cross-linked to form macromolecular polymers PC-MB (Fig. S1d,
ESI†)22,26, which has a cross-networked structure. Here we mainly
introduce the PC-MB-3, which was formed by polymerization at a
pH value of 3 and exhibits the optimal photocatalytic performance.
The X-ray diffraction (XRD) patterns in Fig. S2, ESI† exhibit the
similar broad diffraction peaks for different PC-MB catalysts, at 19°
and 45° respectively, indicating the amorphous structure of PC-MB
polymer. It has been reported that the diffraction peak of 19°
represents the π-stacked aromatics21. Full X-ray photoelectron
spectroscopy (XPS) spectra of PC-BM are available in Fig. S3a, d, g,
and j ESI†, which indicate that the catalysts consist only of C and O.
High-resolution XPS spectra of C 1s (Fig. S3b, e, h, and k, ESI†) can
be divided into three peaks, in which the positions at 284.6, 286.3
and 288.8 eV are assigned to graphitic carbon, C-O and C=O bonds,
respectively19,27. The XPS monitored at O 1s level (Fig. S3c, f, i, and l,
ESI†) also reveals three components assigned to C-O, C=O and
adsorbed O28. The content analysis of PC-MB catalysts was further
confirmed by combustion elemental analysis (EA, Table S1, ESI†).
The surface functional groups of the reactants and photocatalysts
were further confirmed by Fourier transform infrared (FT-IR, shown
in Fig. S4, ESI†). The comparison between the reactants and the
polymerization reveals that the characteristic peaks of polymer PC-
MB-3 are all derived from the two reactants. The polymer resin
shows distinct bands from PC, linker, BM and residual groups,
including phenyls and their substituents, methyl and methylene
groups, C-O, C=C, etc29.
The morphology of the catalyst was observed by scanning
electron microscopy (SEM). As can be seen from Fig. S5a, ESI†, the
PC-BM-3 is mainly composed of microspheres with a particle
diameter of about 0.5-5 μm, which is confirmed by dynamic light
scattering (DLS, Fig. S5b, ESI†) analysis (80-1000 nm). The magnified
image of catalyst particles shows that the microspheres have rough
surface (inset in Fig. S5a, ESI†), exposing more specific surface area,
which is more conducive to the catalytic reaction. The transmission
electron microscopy (TEM, Fig. S6, ESI†) offers more surface details,
showing clearly that the microspheres are polymerized by irregular
fragments.
As indicated in Fig. 1b, the ultraviolet-visible (UV-Vis) diffuse
reflection spectrum reveals that the prepared polymer
photocatalyst PC-MB-3 has strong absorption in the both the
ultraviolet and visible light regions. Its absorption range can be
extended to the near infrared region (λ ≧ 700 nm), which assigns
to the charge transfer transition of the polymerized monomer
pair21,30, indicating that the catalyst could more effectively improve
the utilization of solar energy. The optical band gap of the catalyst
was calculated by the Tauc plot that derived from the UV-Vis
Fig. 1 Postulated schematic and the band structure of PC-MB-3. (a)
Reaction process of catalyst PC-MB-3 in an acidic environment. (b)
UV-Vis absorption spectrum of PC-MB-3 (Inset is the corresponding
Tauc plot). (c) UPS spectrum of PC-MB-3. The horizontal dashed red
line marks the baseline, and the other red dashed lines are tangent
to the curve. (d) CV curve of PC-MB-3 modified glassy carbon (GC)
electron in N2-saturated anhydrous acetonitrile (0.1 M BMIMPF6)
with ferrocene as the internal standard. A carbon electrode and an
Ag/AgCl (3 M KCl) electrode were used as the counter electrode and
the reference electrode with a scan rate of 50 mV s-1. (e) The band
structure diagram of PC-MB-3 with respect to the redox potential of
H2O and H2O2.
In addition to the appropriate band gap, for photocatalyst, it’s
also important to properly match the conduction band and valence
band level with the redox potential of photocatalytic reaction.
Therefore, other characterizations have been carried out to further
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explore the band structure of the sample. As shown in Fig. 1c, the
ultraviolet photoelectron spectroscopy (UPS) was conducted to
determine the ionization potential (equivalent to the valence band
energy Ev) of catalyst PC-MB-3. By subtracting the width of the He I
UPS spectrum from the exciting energy (21.22 eV), the valence band
(Ev) of -5.98 eV will be obtained. Thus, the conduction band (Ec) is
determined to be -4.1 eV by calculating from (Ev-Eg).
View Article Online
DOI: 10.1039/D0TA03974H
In order to study the band structure of the catalyst more
accurately, furthermore, the cyclic voltammetry (CV) experiment
(shown in Fig. 1d) was employed to identify the highest occupied
molecular orbital (HOMO) and lowest unoccupied molecular orbital
(LUMO) positions of the sample PC-MB-3, in which the ferrocene
redox system was used as the external standard. It is clearly seen
from the CV curve that there are two pairs of redox peaks, where
one is from the commercial ferrocene and the other is from the
catalyst itself. According to the formulas (mentioned in the
method33,34), the HOMO and LUMO levels of PC-MB-3 were
Fig. 2 Photocatalytic overall water splitting of the photocatalysts. (a)
Gases productions of different PC-MB photocatalysts. (b) Typical
time course of H2 production for the stability of PC-MB-3. (c)
Hydrogen evolution and the conversation efficiency from solar
energy to hydrogen energy (STH) of PC-MB-3 at different incident
light intensity. (d) Wavelength course of H2 evolution and
corresponding UV-vis spectrum of PC-MB-3.
ox
red
E
E
onset
onset
calculated from the onset of oxidation (
) and reduction (
)
potential with the energy level of ferrocene (4.8 eV below the
vacuum level) as a reference, which are -6.07 and -4.17 V,
respectively. To better demonstrate the relationship between CB
and VB of the catalyst and the redox potentials of H2O, electron
volts are converted to electrochemical energy potentials in volts
according to the reference standard, where 0 V versus RHE is equal
to -4.44 eV versus evac. As shown in Fig. 1e, the CB of PC-MB-3 is
above the reduction level for H2O to H2, while the VB is lower than
the oxidation level from water to oxygen, but it cannot reach the
oxidation level from water to hydrogen peroxide. Therefore, the
catalyst could realize the direct decomposition of water to
hydrogen and oxygen. On the other hand, the CB is far lower than
the reduction level from oxygen to hydrogen peroxide, so the
oxygen reduction will be achieved theoretically.
In the following experiments, the photocatalytic performance of
the optimum sample PC-MB-3 for water decomposition was
studied. Fig. 2c shows the amount of hydrogen produced and the
conversation efficiency from solar energy to hydrogen energy (STH)
under different incident light intensity (12 h visible light irradiation),
indicating that hydrogen production gradually increased with the
increased light intensity through the experiment range. Note that
the STH value shows an inverse trend to hydrogen yield when light
intensity increases, and the STH could reach as high as 0.85% at the
light intensity of 1.75 mW cm-2, much higher than the previously
reported values1,35. This is most likely because low light intensity
reduced the loss of solar energy, which in turn increases the
utilization of solar energy. Next, we researched the dependence of
the hydrogen evolution rate on incident wavelength of the catalyst
PC-MB-3 (Fig. 2d). As the incident wavelength increases, the
produced hydrogen will gradually decrease, indicating that the
catalytic activity is positively correlated with the photo-absorption
by the catalyst.
Photocatalytic activity of producing hydrogen peroxide. The
photocatalytic activity of as-prepared PC-MB for photocatalytic
reduction of oxygen to hydrogen peroxide was carried out at room
temperature and pressure by visible light irradiation (λ ≥ 420 nm,
illumination intensity = 31.27 mW cm-2), and the generated
hydrogen peroxide was detected by potassium permanganate
titration (KMnO4, 0.1M). Fig. 3a displays the hydrogen peroxide
yield of PC-MB after 12 h of photocatalytic reaction, revealing that
PC-MB-3 shows the best catalytic activity, with the evolution of
hydrogen peroxide reaching 665 μmol (1385.42 μmol h-1g-1), while
PC-MB-7 shows the worst catalytic activity. This result is likely to
that of photocatalytic overall water splitting, which further proves
that catalyst PC-MB-3 has higher catalytic activity than other PC-MB
catalysts under the excitation of visible light. Similarly, PC-MB-3 was
3.2 Photocatalytic Properties of Catalysts.
Photocatalytic activity in hydrogen evolution. The photocatalytic
activity of as-prepared catalysts for overall water splitting was
evaluated under visible-light irradiation (λ ≥ 420 nm, light intensity
= 3.85 mW cm-2) and vacuum condition without any sacrificial agent
and cocatalyst. Fig. 2a presents the hydrogen and oxygen
productions by different catalysts after 12 h under light irradiation.
Obviously, the polymer PC-MB-3 produces the largest amount of
gases in these catalysts, where H2 and O2 production rates are
120.97 and 56.82 μmol, respectively. It’s roughly twice as much as
PC-MB-7, which produced the least amount of hydrogen and
oxygen, 56.43 and 24.30 μmol, respectively. The further analysis of
PC-MB-3 shows that the molar ratio of hydrogen to oxygen
production is 2.1, which is effectively consistent with the
approximately stoichiometric vale of 2 for overall water splitting.
And the hydrogen evolution rate of PC-MB-3 could be up to 10.08
μmol h-1 (252.02 μmol h-1g-1). The cyclic stability experiments (Fig.
2b) were carried out to investigate the stability of PC-MB-3, which
exhibits favorable stability with hardly reduced hydrogen
production after reusable irradiated under visible light (λ ≥ 420 nm)
for 5 times (each time is 12 h).
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tested the stability of photocatalytic reduction of oxygen to wavelengths (365, 420, 535 and 630 nm) are illustraVteiedw AinrticFleigO.n3linde,
produce hydrogen peroxide. As seen from Fig. 3b, the catalyst PC- indicating a desirable consistent with UV-VDisOsI:p1e0c.1t0ru3m9/D. 0ATQAY03v9a7l4uHe
MB-3 could be used for drying and recycling at least 4 times to decreases with the increase of incident wavelength, and it could still
produce hydrogen peroxide via photocatalysis under the same reach to 1.44 % with the longest wavelength at 630 nm in
condition (exposure in visible light for 12 h). Certainly, the amount photocatalytic oxide reduction.
After the photocatalytic reactions, the collected catalyst was
performed by FT-IR and XPS measurements to detect the stability of
the sample. As shown in Fig. S8, ESI†, the FT-IR spectra of the
sample before and after the catalytic reactions were similar, and
the characteristic peaks show no obvious change, indicating that
the structure of the catalyst was stable before and after reaction. In
the XPS patterns of PC-MB-3 (Fig. S9, ESI†), although the contents
of C-O and Oads were relatively increased after the catalytic
reactions, the total composition did no significantly changes.
Therefore, the PC-MB-3 catalyst expresses a good photostability.
of recovered catalyst and hydrogen peroxide produced was
decreasing in each cycle, notwithstanding, but the evolution rate
remained essentially the same while only decreased for the fourth
time. During the process of centrifugation, filtration and drying,
part of the catalyst will be absorbed on the surface of the container
and difficult to recover due to the small particle size. In addition,
repeated drying may destroy the structure of the catalyst and then
reduce its activity.
3.3 Study on the Photocatalytic Mechanism and Process.
Investigated the photocatalytic process by photocatalytic
experiments. For the sake of studying the mechanism and the
favorable conditions for the photocatalytic reaction of PC-MB-3, a
series of experiments were carried out. Fig. 4a compares the
reactivity of PC-MB-3 in different gas atmospheres, and it was
found that, with respect to the reaction in air, the produced
hydrogen peroxide increased slightly under oxygen saturation
condition, while the evaluated hydrogen peroxide significantly
decreased under nitrogen saturation. When the photocatalytic
reaction took place in vacuum, only hydrogen but no hydrogen
peroxide was detected, indicating that photocatalytic water
splitting is a four-electron reaction process, which can directly
decompose water into hydrogen and oxygen. These results further
certify that the production of hydrogen peroxide is a photocatalytic
oxygen reduction process. Above all, the PC-MB-3 catalyst has the
characteristics of functional and selective regulation. That is to say,
photocatalytic reactions produce hydrogen peroxide in the
presence of oxygen and hydrogen in the absence of oxygen. Then
the variation trend of hydrogen peroxide evolution over time was
investigated (Fig. 4b), suggesting that the production of hydrogen
peroxide increased with the duration of light, while no hydrogen
peroxide was detected without light exciting. However, due to the
gradual inactivation of the catalysis and the reaction of oxygen in
the water, the total production of hydrogen peroxide began to
decline after 72 h of photocatalytic reaction. Comparing the curves
(continuous light and light off after 72 h) in Fig. 4b, the thermal
effect (from the continuous irradiate) and the degradation of
hydrogen peroxide by the catalyst itself are also non-negligible
reasons for the decrease in hydrogen peroxide evolution. The
degradation curve of hydrogen peroxide (Fig. S10, ESI†) further
proves the ability of the catalyst to degrade hydrogen peroxide.
To determine the mechanism of photocatalytic hydrogen
peroxide generation of as-prepared PC-MB-3, active species
capturing experiments were conducted. In this exploration, AgNO3,
EDTA-2Na, and benzoquinone (BQ) were used as the electron (e-),
Fig. 3 Photocatalytic oxygen reduction of the photocatalysts. (a)
Comparison of H2O2 evolutions among PC-MB-1, PC-MB-3, PC-MB-5
and PC-MB-7. (b) Stability of photocatalytic production of hydrogen
peroxide by PC-MB-3. (c) Typical photocatalyst weight course of
H2O2 production from O2 under visible light irradiation. (d)
Wavelength-dependent apparent quantum yield (AQY) of oxygen
reduction by PC-MB-3.
An addition experiment was performed likewise to demonstrate
the relevance between the concentration and photocatalytic
activity property of PC-MB-3. The controls were experimented
under the same condition except for the concentration of the
catalyst. The result in Fig. 3c suggests that the initial increase of the
catalyst concentration promote the formation of hydrogen peroxide
until reaching a maximum, but the production of hydrogen peroxide
remains relatively stable when the weight of the catalyst PC-MB-3 is
further increasing. On the contrary, the catalytic efficiency is
proportional to catalyst content when it rises to 40 mg PC-MB-3 in
20 mL ultrapure water, yet when the concentration of PC-MB-3 is
higher than 40 mg, the activity turns to a gradual decline process,
which is probably due to the fast recombination of the electron-
hole pairs and the low light absorption at high concentrations. In
fact, there was no significant difference in catalytic activity at the
weight below 60 mg, and the difference in activity (Fig. 3c) is mainly
due to the error caused by the low weight of catalytic. Apparent
quantum yield (AQY) values as a function of the incident light
-
hole (h+), and superoxide radical (·O2 ) scavengers, respectively. Fig.
4c shows the data characterizing the produced amount of hydrogen
peroxide in the presence of different scavenger agents (1 mmol L-1),
exemplifying that the addition of AgNO3 reduces the photocatalytic
activity while the addition of EDTA-2Na increases it. The reason is
that the trapping of electrons impedes the reaction, while the
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capture of holes promotes the use of electrons. Surprisingly, the acidity of the reaction solution, the higher production efficiency of
View Article Online
DOI: 10.1039/D0TA03974H
produced hydrogen peroxide hardly changes with the addition of hydrogen peroxide, which is consistent with the experimental
BQ. Thus, as previously reported in the literature36, hydrogen results in Fig. 4d.
peroxide was formed by a one-step two-electron direct reduction Research
(O2 + 2H+ + 2e- → H2O2) pathway. In this mechanism, the existence electrochemistry tests. To further study the photocatalystic
of oxygen enhanced the production of hydrogen peroxide (as mechanism of the polymer catalyst PC-MB, series of
the
photocatalytic
mechanism
by
photo-
a
analyzed in Fig. 4a) and the protons are favorable for the coupled electrochemical tests were characterized. The time-current (i-t)
electron transfer. Therefore, we investigated the relationship curves from PC-MB-1 to PC-MB-7 were collected to estimate the
between catalyst activity and concentration of hydrogen protons in photo-response properties through an intermittent light on-off way
the reaction solution (Fig. 4d). Comparing the photocatalytic activity under open circuit potential. As shown in Fig. 5a, all the PC-MB
of PC-MB-3 from pH 3 to 5, it is noted that PC-MB-3 was sensitively samples show an elevated mutation of photocurrent once exposed
affected by the concentration of protons. This is probably because it to light, which reveals very quickly separated electron-hole pairs.
is much facilitated for PC-MB-3 framework to conduct proton, and
higher proton conductivity should favor the production of hydrogen
peroxide. However, when the proton concentration reaches a
certain level (pH ≤ 3), the overall production of hydrogen peroxide
would be much less affected by the pH value. The following
theoretical calculations have also proved this claim.
Fig. 4 Experimental investigation of photocatalytic reaction process.
(a) Comparison of H2 and H2O2 by photocatalytic reaction in
different gas environment. (b) The production of hydrogen peroxide
Fig. 5 Photo-electrochemical exploration of photocatalytic reaction
mechanism. (a) Photo-response curves and (b) EIS Nyquist plots of
PC-MB-1, PC-MB-3, PC-MB-5, PC-MB-7. (c) TPV patterns of PC-MB-
1, PC-MB-3, PC-MB-5, and PC-MB-7. (d) Comparison of the in-situ
TPV with PC-MB-3 in different reaction conditions (wavelength of
355 nm and pulse width of 5 ns). (e) The RRDE collection
experiment for PC-MB-3 with a rotating speed of 1600 rpm in N2-
saturation water. (f) The RRDE collection experiment for PC-MB-3
with a rotating speed of 1600 rpm in O2-saturation water.
changes with time (blue curve: continuous irradiated by visible
light; red curve: remove the light after irradiated 72 h). (c) The
amount of photocatalytic hydrogen peroxide production by adding
different sacrificial agents. (d) The relationship between the
concentration of H+ in the reaction solution and the photocatalytic
H2O2 evolution rate.
In the air environment, the reaction process of two-electron
oxygen reduction is as mentioned above. Thus, we can get the
relationship between hydrogen proton concentration and hydrogen
peroxide production (detailed derivation in supporting
information).
And then the photocurrent decays smoothly to a stable value,
which is mainly caused by the redistribution of charges between the
interfaces during illumination. What’s more, the photocurrent
generated by light excited of PC-MB-3 is higher than others, which
also explains why PC-MB-3 emerges higher photocatalytic activity.
Next, the ability of the transfer generated charge carries was
investigated by electrochemical impedance spectra (EIS) in
ultrapure water. Fig. 5b depicts the EIS Nyquist plot of the samples,
3
0.5
2
2
+
―
2
[
]
[
]
ℎ
푟퐻 푂 ,푚푖푛
=
2푘1푘02.5
(
푝푂0
)
푒
퐾푤푎푡푒푟
(1)
2
2
2
2
As shown in formula 1, the high or low solubility of [H+] is
beneficial to the formation of H2O2, however, the hydrogen
peroxide is unstable under alkaline condition. Thus, the higher the
6 | J. Name., 2018, 00, 1-3
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Journal Name ARTICLE
where a smaller semicircle is considered to a faster interfacial functional selectivity of photocatalyst for producing VhieywdrAortgicelenOnalninde
DOI: 10.1039/D0TA03974H
electron transfer speed. It can be easily observed that PC-MB-3 hydrogen peroxide can be realized.
displays the smallest semicircle, demonstrating that the charge
recombination is more steadily inhibited. The TPV tests of the
samples (Fig. 5c) were carried out to demonstrate the action of
different catalysts (from PC-MB-1 to PC-MB-7) under light
excitation. It’s clearly to point out that more charges were excited
on PC-MB-3 than others, which is more favorable for the
photocatalytic reactions. A more detailed observation in the
illustration shows that the attenuation of the TPV curves is not the
prime-type attenuation, indicating a redistribution of surface charge
after photo-excitation, which is consistent with the i-t curves in Fig.
5a. According to the above data and analyses, PC-MB-3 has both
great light absorption and surface photovoltage due to the
appropriate degree of polymerization crosslinking, which eventually
improve the separation efficiency and charge transfer speed to
enhance catalytic performance.
Fig. 6 The mechanism diagram of hydrogen or hydrogen peroxide
produced over the PC-MB-3 through photocatalytic reaction in
To better understand the catalytic functional selection of PC-MB-
3 with H2 and H2O2 production via two-channel pathway, the in-situ
TPV tests were measured under different conditions (Fig. 5d).
different
environments.
Obviously, PC-MB-3 shows
a high photovoltage intensity in
5. Conclusions
nitrogen-saturated ethyl acetate, indicating that no reaction occurs
in this case and no photogenic charge is consumed. With the
addition of water, the photovoltage decreased as the holes is
consumed in the process of water oxidation. Compared with the
photovoltage in nitrogen-saturated ethyl acetate and water
condition, the optical-voltage has a large attenuation under the
oxygen-saturated ethyl acetate, meaning that, in the present
reaction system, the oxygen reduction rate is higher than that of
water oxidation process. To determine the electron changes in the
catalytic reactions in the two pathway (with and with oxygen), the
electron transfer number (n) of PC-MB-3 was characterized via
rotating ring-disk electrode (RRDE) curves. As depicted in Fig. 5e,
the ring current (red line) does not change while the disk current
saliently improves when the light is applied to the ultrapure water
without oxygen. Therefore, according to the formula mentioned in
supporting information, the electron transfer number of PC-MB-3
was calculated to be 4 for the water splitting. Fig. 5f shows the
electron transfer number in photocatalytic oxygen reduction
process. The test was performed at the potential of 0.3 V (vs RHE),
which avoids electrochemical oxygen reduction. After the
irradiation of visible light, there are relatively strong saltation in
both disk and ring disk, and the electron transfer number was
calculated to be 1.98, indicating the photocatalytic oxygen
reduction reaction is a two-electron process.
In this paper, we successfully designed and synthesized the
bifunctional metal-free catalyst PC-MB-3 by phenolic condensation,
and realized precise regulation for H2 and H2O2 production on one
catalyst. The catalytic reaction pathway of the catalyst PC-MB-3 can
be controlled by adjusting the reaction atmosphere, which exhibits
the characteristics of switchable function and excellent catalytic
activity. The in-situ TPV and photoelectrochemical experiments
show that the case of redox reactions of water, a four-electron
process, occurs to produce H2 and O2 in the absence of air. In case
of the air, oxygen preferentially participates in the reaction, and
H2O2 is generated through two-electron reduction. The apparent
reaction kinetics analysis of hydrogen proton concentration ([H+])
and reactivity further demonstrated that the influence of acidic
condition is conducive to hydrogen peroxide production. The
conversion efficiency from solar to hydrogen (STH) can be up to
0.85% at the illumination intensity of 1.78 mW cm-2, and the yield
of H2O2 can reach 1385.42 μmol h-1g-1 at the illumination intensity
of 31.27 mW cm-2. Our study on the switch function of water-based
catalytic reactions is of great significance in catalytic design,
catalytic reaction regulation and other aspects, and it may lay a
foundation for industrialization process of photocatalytic hydrogen
and hydrogen peroxide production. This field of function-switchable
metal-free photocatalyst is one that continues putting much
emphasis by both experimentalists and theoreticians.
Based on the above exploration and analysis, a reasonable
mechanism of PC-MB-3 was explained, as illustrated in Fig. 6. Under
the illumination of visible light (λ ≥ 420 nm), PC-MB-3 can easily
absorb visible light to produce photogenic electron-hole pairs. In
the absence of oxygen, the electrons generated by conduction band
and the holes generated by valence band work together to
decompose water directly to produce hydrogen and oxygen (shown
in the red part). In the presence of oxygen, however, the oxidation
of water is completely inhibited, and the photogenic electrons will
combine with the oxygen in water, reducing the oxygen to
hydrogen peroxide (shown in the black part). Therefore, by
controlling the change of oxygen in the reaction process, the
Conflicts of interest
There are no conflicts to declare
Acknowledgements
This work is supported by National MCF Energy R&D Program
(2018YFE0306105), Innovative Research Group Project of the
National Natural Science Foundation of China (51821002), National
Natural Science Foundation of China (51725204, 21771132,
51972216, 52041202), Natural Science Foundation of Jiangsu
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28. V. R. Battula, S. Kumar, D. K. Chauhan, S. Samanta, K. Kailasam,
Province (BK20190041, BK20190828), Key-Area Research and
Development Program of GuangDong Province (2019B010933001),
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Appl. Catal. B 2019, 244, 313-319.
DOI: 10.1039/D0TA03974H
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Journal of Materials Chemistry A
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DOI: 10.1039/D0TA03974H
Function-switchable metal-free catalyst splits water to produce hydrogen in air free,
while hydrogen peroxide is generated preferentially in air.