NN bond cleavage of azobenzene: Via photocatalytic hydrogenation with Dy-doped Zn(O,S): The progress from hydrogen evolution to green chemical conversion

Article abstract of DOI:10.1039/c9cy00502a

Hydrogen-evolved Dy-doped Zn(O,S) photocatalysts with different amounts of Dy precursor have been synthesized, characterized and further utilized for hydrogenation reactions. The Dy catalysts are first examined using electrochemical impedance spectroscopy, photo responsivity, and the hydrogen evolution reaction. Dy-doped Zn(O,S) with 10% Dy precursor shows the best photocatalyst performance with the highest H₂ production rate of 8.160 mmol g^-1 h^-1. This catalyst is chosen for conducting the hydrogenation reaction. It is our intention to confirm the correlation between the photocatalytic H₂ production rate and the hydrogenation reactions of 4-nitrophenol to 4-aminophenol and azobenzene to aniline, which is a step further in the hydrogenation reaction to challenge the NN bond cleavage of azobenzene. A 100% photo conversion of 60 ppm azobenzene to aniline in 6 h was confirmed under a low-power UV light illumination in a 10% ethanol solution. Kinetic steps and kinetic mechanisms are proposed, which involve the important reaction steps of solvation, adsorption, pinning, and surface hydrogenation reaction operating together.

Full text of DOI:10.1039/c9cy00502a

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ISSN 2044-4753
PAPER
Qingzhu Zhang et al.
Catalytic mechanism of C–F bond cleavage: insights from QM/MM
analysis of fluoroacetate dehalogenase
rsc.li/catalysis

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N=N bond cleavage of azobenzene via photocatalytic hydrogenation with Dy-
doped Zn(O,S): The progress from hydrogen evolution to green chemical
conversion
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Hairus Abdullah, Dong-Hau Kuo*, Noto Susanto Gultom
Department of Materials Science and Engineering, National Taiwan University of Science and
Technology, No.43, Sec. 4, Keelung Road, Taipei 10607, Taiwan
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*Fax: +011-886-2-27303291. E-mail: dhkuo@mail.ntust.edu.tw
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Graphical Abstract:
Hydrogen
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ABSTRACT:
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Hydrogen-evolved Dy-doped Zn(O,S) photocatalyst with different amounts of Dy precursor has
been synthesized, characterized and further utilized for hydrogenation reactions. The Dy-
catalysts are firstly examined with electrochemical impedance spectroscopy, photo responsivity,
and hydrogen evolution reaction. Dy-Zn(O,S) with 10% Dy precduror shows the best
photocatalyst performance with a highest H production rate of 8.160 mmol/g·h. This catalyst is
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chosen for conducting the hydrogenation reaction. It is our intention to confirm the
correlationship between photocatalytic H production rate and hydrogenation reactions of 4-
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nitrophenol to 4-aminophenol and azobenzene to aniline, which is a step further in the
hydrogenation reaction to challenge the N=N bond cleave of azobenzene. A 100% photo
conversion of 60 ppm azobenzene to aniline in 6 h was confirmed under a low power UV light
illumination in a 10% ethanol solution. Kinetic steps and kinetic mechanism are proposed, which
involve the important reaction steps of solvation, adsorption, pinning, and surface hydrogenation
reaction to operate together.
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Introduction
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Nowadays, energy and environmental issues have emerged as important topics at global level.
As fossil fuel has been the main source of energy, our society depends on fossil fuel for most of
their activities. However, the process to obtain the energy from fossil fuel releases CO to
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atmosphere and generates other environmental problems. The amount of CO in air has
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significantly increased and caused the elevation of global temperature. Therefore, naturally
obtained energy carriers are urgently required to lower the negative effects of energy harnessing
to environment. One of the most fascinating works to alleviate the global warming effects is the
research on hydrogen evolution. Hydrogen as one of the most efficient and clean energy carriers
has a great potential to replace fossil fuel in the future for its harmless combustion product of
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water. There are many reports on metal oxide, sulphide, phosphide, and oxysulfide solid
solution photocatalysts for efficient photocatalytic water splitting using sacrificial reagents (such
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as Na SO , ethanol, and ethanolamine) in the past several decades. Photocatalysis with
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different kinds of semiconductor materials have been identified as one of the great methods to
produce hydrogen due to its simplicity in applications, by which hydrogen can be generated with
simply mixing semiconductor powders in water solution under the illumination with light.
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Chemical conversion with photocatalytic hydrogenation reaction (PHR) is also an on-going
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issue for the green technology. It is an important fundamental of chemistry to promote
environment-friendly chemical conversion for synthesizing useful substances. The application
using TiO semiconductor as a photocatalytic oxidant for detoxifying harmful chemicals has
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been avidly studied, because the photo-generated OH• radical can oxidize many organics. For
photocatalytic reduction reaction, the efficient conversion of the toxic 4-nitrophenol (4-NP) to a
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useful 4-aminophenol (4-AP) chemical with the Zn(O,S)-based nanoparticles without using
reducing agents had been successfully developed in our group, following the success in HER.7, 9,
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For most of works on the 4-NP-to-4-AP conversion, ammonium formate and sodium
borohydrate as proton sources were applied to accelerate the reduction reaction.1,19 The
advantage of using our photocatalyst for liquid phase hydrogenation is the reaction cleanness and
safeness due to the in-situ proton generation for HER and chemical conversion. Therefore, PHR
of Zn(O,S)-based catalyst is promising and encouraging for further exploration to widen its
environment-friendly application.1, 9, 11-13
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Aniline is one of the most important intermediates for polyurethane, dyestuffs and
pharmaceuticals in industries.14-18 Currently, aniline production involves two steps, where
benzene is firstly nitrated with a concentrated mixture of nitric acid and sulfuric acid at 50-60 °C
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to yield nitrobenzene (NB), followed by the catalytic hydrogenation at 200–300 °C. The NB-
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to-aniline reaction also occurs at 90-125 C under high H pressure,
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of 3 MPa in ionic liquid, and at 25 C with electrocatalysis. For PHR, NB also can be
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with N H ·H O in water.
Similar to the 4-NP-to-4-AP reaction, the NB-to-aniline one at a
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mild condition also needs the strong reducing agent to provide protons.
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Azobenzene (AB) with the N=N bond is a model chemical for the carcinogenic azo dyes
widely used in textile industries. While aniline is mainly synthesized from nitrobenzene, its
formation from AB by photocatalysis is less reported. Tada et al. in 2000 used Pt/TiO catalyst to
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complete the first study for the photocatalytic reduction of azobenzene. Their rate of reduction
of AB to hydrazobenzene (HAB) increased with the loading of Pt to reach ~100% conversion,
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91% selectivity for HAB, and 9% for aniline after 1h reaction. The amount of aniline increased
to 19.2% after 3h reaction. Shiraishi et al. in 2018 reported PHR of AB had 95% selectivity for
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HAB and 5% for aniline. At this stage, HAB remains the predominant product for the PHR of
AB, although Density Functional Theory (DFT) calculation indicates that, in the gas phase
reaction, the formation of aniline on catalyst by the reaction with AB in the presence of a source
of hydrogen is energetically favored.
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The present work intends to demonstrate that Dy-doped Zn(O,S) nanoparticles (NPs) with
the photocatalytic HER capability can be used for the green chemical conversion through PHR.
Two kinds of photocatalytic chemical conversions are selected: one is the conversion of 4-NP to
4-AP with sodium sulfate as a hole scavenger and the other is the conversion of AB to aniline
with 10% ethanol in aqueous solution. The total AB-to-aniline reaction can be expressed as:
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Zn(O,S) with different Dy contents were characterized and tested at the first stage to find the best
composition for the photocatalytic chemical conversions at the second stage. A complete
conversion of AB to aniline had been achieved. The photocatalytic activities of Dy-doped
Zn(O,S) for the hydrogenation of nitro and azo groups were evaluated and elucidated with
proposed mechanism.
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Experimental Methods
Materials.
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Zinc acetate dihydrate and dysprosium acetate hydrate were purchased from Alfa Aesar and
Sigma Aldrich, respectively. Thioacetamide (TAA) as a sulfur source was obtained from
Shanghai Aladdin Bio-Chem Technology Co., LTD. Hydrazine monohydrate (N H ·H O) was
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provided by Fisher Scientific U.K. limited. 99% 4-nitrophenol and 97%+ azobenzene were
purchased from Alfa Aesar and Acros Organics, respectively. Sodium sulfite anhydrous
(Na SO ) was purchased from Alfa Aesar. All the chemicals were analitical grade without
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further purification.
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Synthesis of Dy-doped Zn(O,S) NPs.
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Dy-doped Zn(O,S) powders were prepared based on the previous work with different amounts
of Dy(CH COO) ·xH O precursor at 0%, 5%, 10%, and 20% for samples of Dy-Zn(O,S)-0, Dy-
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Zn(O,S)-5, Dy-Zn(O,S)-10, and Dy-Zn(O,S)-20, respectively. In a typical preparation for Dy-
Zn(O,S)-10 NPs, 20 mmol Zn(Ac) dihydrate and 2 mmol Dy(CH COO) ·xH O were first
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dissolved in 500 mL distilled water, then followed by adding 10 mmol TAA. The solution was
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stirred to totally dissolve all the precursors, followed by heating to 95 C with the ramp of 1.5
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C/min. After the solution temperature reached 95 C, 0.5 mL N H ·H O was added into the
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solution and continuously stirred for 4 h. Finally, the obtained white precipitate was collected by
centrifugation after washing for three times with alcohol. To remove all the volatile species, the
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white powder was dried at 70 C for 12 h in vacuum oven. The other catalyst powders for 0%,
5%, and 20% were prepared by the same experimental procedure with an appropriate amount of
Dy(CH COO) ·xH O.
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Characterization techniques.
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Morphology and microstructure of catalyst powder were both examined with field emission
scanning electron microscopy (FE-SEM, JSM 6500F, JEOL, Tokyo, Japan). Powder X-ray
diffraction (XRD) patterns of Dy-Zn(O,S) NPs with different contents of Dy precursor were
recorded by Bruker D2-phaser diffractometer using Cu Kα radiation with a wavelength of 1.5418
Å. Optical properties of Dy-Zn(O,S) NPs were examined with diffuse reflectance spectra (DRS)
measurement using a Jasco V-670 UV-visible-near IR spectrophotometer. Energy bonding of
each element in Dy-Zn(O,S) NPs was examined with X-ray photoelectron spectroscopy (XPS)
measurement which was carried out on a VG ESCA Scientific Theta Probe spectrometer system
with Al Kα (1486.6 eV) source. Electrochemical impedance spectroscopy (EIS) measurement
was conducted by using a Bio-Logic Science Instruments with EC-Lab® software. Glassy
carbon electrode (GCE), Ag/AgCl electrode, and platinum plate were respectively used as the
working, reference, and counter electrodes in an electrochemical cell. High performance liquid
chromatography (HPLC, Shimadzu LC-2010AHT) measurement was used to ensure the
conversion of 4-NP to 4-AP after photo reaction with each time interval of one hour. The
reaction solution was detected with UV detector at the monitoring wavelength of 323 nm and
with fluorescence detector at the emission and excitation wavelengths of 300 nm and 323 nm,
respectively. The 4-NP and 4-AP in aliquots were examined with ODS Hypersil (C18) 250*4.6
column. The mobile phase was 20% methanol solution containing 5 mM tetrabutylammonium
phosphate with the flowing rate of 1 mL/min at room temperature. To differentiate the
azobenzene and aniline in aliquots, the reaction solution was collected and analyzed with Jasco
V-670 UV−visible−near IR spectrophotometer and GC-MS with flame ionization detector (FID).
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Photocatalytic HER experiment.
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The capability of Dy-Zn(O,S) to evolve hydrogen was tested in 500 mL borosilicate reactor
containing 450 mL solution with 10% ethanol as a hole scavenger reagent. To evaluate the HER,
100 mg Dy-Zn(O,S) powders with different amounts of Dy were applied and the experiments
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were run for 5 h with 4 W × 4 blacklight UV tube lamps (0.088 mW/cm ) illumination. The
experiments were done with vigorous stiring to prevent the precipitation of catalyst powder
during the photocatalytic session. Prior to starting the HER experiment, the reactor which
containing catalyst-dispersed ethanol solution was purged by Ar with flowing rate of 100
mL/min for 1 h to remove all the atmospheric air in reactor. The existences of atmospheric air in
reactor were then evaluated with GC. If the signals from oxygen and nitrogen appeared in GC
analysis, the purging process had to continue. After the air in reactor had been sterilized, the
illumination system would be turned on and an aliquot was taken every 30 min. The sampling
process needed several minutes to flow the evolved hydrogen from reactor to GC system set with
the retention time of 0.6 min. Finally, the hydrogen amount was measured based on the peak
area.
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Photocatalytic hydrogenation of 4-NP to 4-AP.
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The hydrogenation of 4-NP experiment was conducted in a 500 mL reactor which was inserted
with 4 W × 4 UV blacklight lamps during the photocatalytic session. The reactor was
continuously flushed with Ar gas during reaction. In the experiment, 200 mg Dy-Zn(O,S)
powder was well dispersed in 450 mL 4-NP solution (30 ppm) containing 100 mg Na SO . Prior
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to starting the light illumination, the solution was stirred in dark condition for 30 min. An aliquot
was taken before the light illumination. After starting the light illumination, aliquots were taken
at the time interval of 15 min to easily observe the 4-NP conversion with UV-vis spectroscopy.
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To ensure the total conversion of 4-NP to 4-AP, another experiment with the same condition was
carried out for 2 h with the aliquots were hourly taken. The aliquots were examined with HPLC
to ensure the conversion product of 4-AP. UV detector with the monitoring wavelength of 323
nm for detecting 4-NP and fluorescent detector with the excitation and emission wavelengths at
300 nm and 323 nm, respectively, for 4-AP were used during the HPLC measurement.
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Photocatalytic hydrogenation of azobenzene to aniline.
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The hydrogenation reaction for 15 ppm AB was also carried out in the same reactor as used for
the 4-NP reduction reaction. During the experiment, Ar gas was continuously flowed through the
reactor. To convert azobenzene to aniline, 200 mg catalyst powder and and 15 ppm AB were
dispersed in 400 mL aqueous solution with 10% ethanol. Ethanol was necessary not only for the
dissolution of azobenzene but also for being a hole scavenger reagent during PHR. To reach the
adsorption and desorption equilibrium between catalyst and azobenzene, the catalyst-dispersed
azobenzene solution was stirred for 30 min before starting the light illumination. Aliquots were
taken with a certain time interval before and during photocatalytic session and examined with
UV-vis spectroscopy. To further ensure the conversion product of aniline, GC-MS with flame
ionization detector (FID) was used to analyzed the product from the reaction with 60 ppm AB of
much high concentration.
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Results and discussion
X-ray diffraction (XRD) structure analysis.
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Fig. 1 shows the XRD crystal structure of Dy-Zn(O,S) powders prepared with different Dy
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precursor contents. XRD pattern of Dy-free Zn(O,S)-0 agreed with our previous work , with the
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standard JCPDS files of ZnO and ZnS evaluated in Fig. S1 in supplementary data. The main
peaks of Zn(O,S)-0 solid solution in the planes of (111), (220), and (311) were located between
those of cubic ZnO (JCPDS #65-2880) and ZnS (JCPDS #05-0566). With increasing the Dy
contents, XRD patterns of Dy-Zn(O,S) did not show the peak shifts, which indicated Dy-Zn(O,S)
still remained solid solution with Dy embedded at the cation site. The average crystalline size of
all Dy-Zn(O,S), calculated with the (111) peak, was about 2.5 nm.
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111)
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20% Dy-doped Zn(O,S)
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Fig. 1. XRD patterns of Dy-doped Zn(O,S) with different contents of dysprosium.
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Scanning electron microscopy (SEM) analysis.
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The morphology and microstructure of Dy-Zn(O,S) with different Dy contents are examined and
shown in Figure S2. It showed all the particles with different contents of Dy did not exhibit
different morphologies. The particles less than 100 nm were aggregated to form larger
agglomerates. To show all the elements in Dy-Zn(O,S)-10 NPs, FE-SEM elemental mapping was
carried out on catalyst surface with a depicted-red rectangle area on the image for the mapping
scan, as shown in Fig. 2. The signals related with Zn, O, S, and Dy elements in Dy-Zn(O,S)-10
NPs were collected and shown as the blue, green, red, and gray dot patterns in Fig. 2. The
elemental mapping notified Dy had a lower signal intensity for its lower content. The amount of
Dy in catalyst was confirmed with electron dispersed spectroscopy (EDS) analysis. Due to the
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of Dy precursor. The atomic ratio values of Dy/(Zn+Dy) were 0%, 0.25%, 2.70%, and 4.20% for
Dy-Zn(O,S)-0, Dy-Zn(O,S)-5, Dy-Zn(O,S)-10, and Dy-Zn(O,S)-20, respectively. The overall
EDS analysis results were presented in Table S1. The excessive Dy did not precipitate and was
washed away during the powder preparation stage.
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Fig. 2. SEM image of Dy-Zn(O,S)-10 nanoparticles and its elemental mapping at the depicted-
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Transmission electron microscopy (TEM) analysis.
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Fig. 3a shows the TEM image of Dy-Zn(O,S)-10 NPs. The particle size of Dy-Zn(O,S) was very
tiny and aggregated to form larger sphere-like particles with the size of 100 nm. The tiny size of
Dy-Zn(O,S)-10 NPs was consistent with the crystalline size calculated with XRD data by
Scherer equation. Figs. 3b and 3c show the lattice fringes and selected area electron diffraction
(SAED) ring patterns of Dy-Zn(O,S) NPs, respectively. The lattice fringes in Fig. 3b show the
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lattice parameters of 2.67, 2.92, and 2.94 Å are between those of ZnO (111) and ZnS (111)
planes and consistent with our previous works 1, 11 The varying lattice fringe values in Fig. 3b
were related to broad ring patterns, as shown in Fig. 3c. As a broad (111) ring pattern was found,
the depicted-outer and -inner ring patterns corresponding with ZnO (111) and ZnS (111) planes,
respectively, were provided to clearly approve the formation of Dy-Zn(O,S) solid solution. The
broad ring patterns were also observed for (220) and (311) planes. A random distribution of the S
content in Dy-Zn(O,S) is the major reason for the broad diffraction ring.
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a
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Fig. 3. (a) Low and (b) high magnification of HRTEM images of Dy-Zn(O,S)-10 with (c) the
broad ring pattern obtained from SAED. The broad ring pattern for (111) was located between
the depicted outer and inner ring patterns of ZnO and ZnS, indicating the formation of solid
solution of oxysulfide.
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Diffuse reflectance spectroscopy (DRS) analysis.
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2
2
2
2
2
2
2
81
82
83
84
85
86
87
88
89
90
Fig. S3a show diffuse reflectance spectra of Dy-Zn(O,S) with different Dy amounts. Compared
with the Dy-free Zn(O,S), all the Dy-Zn(O,S) showed an improved absorbance in the UV range.
Based on the DRS measurements, the converted Tauc plots are shown in Fig. S3b. Tauc plot was
m
obtained with the formula of αhν = A (hν - Eg) for Eg > hν, where α is the material optical
absorbance, hν is the photon energy; Eg is the bandgap energy; and m = 0.5 and 2 are for the
materials with direct and indirect allowed transition band gap, respectively. The direct bandgap
values of Dy-Zn(O,S) were about 3.54–3.57 eV, which were between those of ZnO (3.2 eV) and
ZnS (4.1 eV) due to the formation of solid solution. Based on the optical property data, the
blacklight UV tube lamps with the wavelength of 352 nm were selected as a light source to meet
the requirement for our photocatalysts.
2
91
X-ray photoelectron spectroscopy (XPS) analysis.
2
2
2
2
2
2
92
93
94
95
96
97
To identify the chemical state of each element in catalysts, XPS measurement as one of the
surface-sensitive quantitative spectroscopic techniques was carried out on Dy-Zn(O,S)-10. Fig. 4
shows the XPS spectra of (a) Zn 2p, (b) Dy 4d, (c) O 1s, and (d) S 2p orbitals for Dy-Zn(O,S)-
10. Fig. 4a shows the binding energy values of Zn (2p ) and Zn (2p ) at 1046.9 eV and 1023.8
1
/2
3/2
2
8
eV, respectively, which are consistent with literature data. Fig. 4b shows the binding energy
2
8
value of Dy (4d) at 152.5 eV with low energy intensity for its low content. Fig. 4c shows the
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6

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2
3
3
3
98
99
00
01
02
binding energy values of O (1s) at 529.6 eV, 530.3 eV, and 531.4 eV, which are contributed
from the oxygen in lattice (O ), oxygen vacancy (O ), and oxygen adsorbed (O ) as hydroxide,
L
v
ads
1
respectively, on catalyst surfaces. 69.89% O , 15.59% O , and 14.52% O were calculated
L
v
ads
from the peak area of oxygen in Fig. 4c. Fig. 4d shows the binding energy values of S (2p ) and
1
/2
1
, 28
S (2p ) at 164.7 eV and 163.2 eV, respectively, and were consistent with literature data.
3
/2
3
3
3
3
3
03
04
05
06
07
3
3
3
3
3
3
3
08
09
10
11
12
13
14
Fig. 4. XPS spectra of (a) Zn 2p, (b) Dy 4d, (c) O 1s, and (d) S 2p orbitals for Dy-Zn(O,S)-10.
3
15
Electrochemical impedance spectroscopy (EIS) analysis.
3
3
3
16
17
18
To examine the electron transfer properties of Dy-Zn(O,S) NPs with different Dy contents during
photocatalytic reaction, EIS measurement was carried out in a 0.1 M KCl solution at the
frequency range from 50 KHz to 0.1 Hz, window potential from -1 – 1 V, scan rate of 10 mV,
1
7

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3
3
3
3
3
3
3
3
19
20
21
22
23
24
25
26
27
and signal amplitude of 10 mA. The catalyst-dispersed solution was prepared with 5 mg powder
in 1 mL Nafion solution (DI water : isopropanol = 3 : 1) and then coated on GCE. Fig. 5 shows
the electrochemical impedance spectra of Dy-Zn(O,S) with different Dy contents on a Nafion-
fixed GCE electrode. The spectra were fitted with the equivalent Randles circuit to obtain the
resistance values of 34.2 KΩ, 14.9 KΩ, 6.56 KΩ, and 21.4 KΩ for Dy-Zn(O,S)-0, Dy-Zn(O,S)-5,
Dy-Zn(O,S)-10, and Dy-Zn(O,S)-20, respectively. Dy-Zn(O,S)-10 with 6.56 KΩ showed the
lowest resistance, indicating the most efficient electron transfer property at the interface between
electrode and electrolyte during the EIS measurement. From the experimental data, Dy-Zn(O,S)-
10 is expected to have the excellent photocatalytic activity.
3
3
3
3
3
3
28
29
30
31
32
33
1
1
1
1
6000
4000
2000
0000
8
6
4
2
000
000
000
000
0
0% Dy-doped Zn(O,S)
% Dy-doped Zn(O,S)
10% Dy-doped Zn(O,S)
0% Dy-doped Zn(O,S)
5
2
0
5000
10000
15000
20000
25000
30000
35000
Z re (ohms)
3
3
3
3
3
34
35
36
37
38
Fig. 5. Electrochemical impedance spectra of Dy-Zn(O,S) with different Dy contents on a
Nafion-fixed GCE electrode in 0.1 M KCl solution. The inset shows the equivalent Randles
circuit for curve fitting.
Photoresponse analysis.
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8

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3
3
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3
3
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3
3
3
39
40
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44
45
46
47
48
One of the indications for photocatalytic activity is the photocurrent induced by photon during
light illumination. If a higher current is induced by photon, a higher catalytic activity can occur
to support HER or hydrogenation reaction. Fig. 6 shows the photoresponses of Dy-Zn(O,S) at
different Dy contents on modified GCE as working electrodes in 0.1 M KCl solution under UV
LED light illumination with the chopping frequency of 25 mHz. The photo current values were
measured to be 102, 117, 192, and 136 pA for Dy-Zn(O,S)-0, Dy-Zn(O,S)-5, Dy-Zn(O,S)-10,
and Dy-Zn(O,S)-20, respectively. Dy-Zn(O,S)-10 achieved the highest photo current of 192 pA.
The other feature in Fig. 6 is the longer tail for the Dy-Zn(O,S)-10, which indicates a slower
recombination rate between hole and electron after the light illumination is turned off. The photo
current data was also consistent with EIS data in Fig. 5.
3
3
3
3
3
3
3
3
3
49
50
51
52
53
54
55
56
57
0
25
50
75
100
125
150
175
200
2
-
4
.0x10
.5x10
.0x10
20%
-4
1
1
-4
-5
5.0x10
0
.0
-
4
2
1
1
5
.0x10
.5x10
.0x10
.0x10
10%
-
4
-4
-5
0
.0
-4
2
1
1
5
0
.0x10
.5x10
.0x10
.0x10
.0
5%
-4
-4
-5
0%
-
4
2
1
6
.0x10
.3x10
.7x10
-
4
-5
0
.0
0
25
50
75
100
125
150
175
200
Time (s)
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9

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58
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60
61
Fig. 6. Photoresponses of Dy-Zn(O,S) with different Dy contents on a Nafion-fixed GCE
working electrode in 0.1 M KCl solution under UV LED light illumination with a chopping
frequency of 25 mHz.
3
62
Assessment of photocatalytic HER.
3
3
3
3
3
3
3
3
3
63
64
65
66
67
68
69
70
71
Fig. 7 shows the amounts of hydrogen evolved from 10% ethanol solution with different Dy-
Zn(O,S) catalysts. The experimental data indicated Dy-Zn(O,S) prepared with different Dy
precursor contents of 0, 5, 10, and 20% could achieve 25.8, 37.6, 40.8, and 31.1 mmol/g,
respectively, for 5 h photocatalytic reaction. The maximum amount of evolved hydrogen reached
40.8 mmol/g or 8.160 mmol/g·h, which was significantly enhanced by almost 60%, as compared
1
to that with Dy-free Zn(O,S). The color of Dy-Zn(O,S) powders were changed to gray color
during the HER reaction but reversibly changed back. This phenomenon supports the reversible
process with the lattice oxygen removal for oxygen vacancy at the light-on condition and the
annihilation of oxygen vacancy at the light-off condition.
45000
40000
35000
30000
25000
20000
15000
10000
0
5
1
% Dy
% Dy
0% Dy
20% Dy
5000
0
0
1
2
3
4
5
Time (h)
3
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0

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3
3
73
74
75
76
Fig. 7. Hydrogen evolution amounts from a 10% ethanol solution in the presence of Dy-Zn(O,S)
with different Dy contents under blacklight UV tube lamps illumination.
Evaluation of photocatalytic hydrogenation of 4-NP to 4-AP.
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
After the evaluations with EIS, photoresponse, and HER, Dy-Zn(O,S)-10 was selected for green
chemical conversion. Fig. 8 shows the variation of UV-vis spectra with reaction time for 30 ppm
4-NP catalyzed with (a) Na SO +Dy-Zn(O,S)-10, (b) Na SO +Zn(O,S), and (c) Dy-Zn(O,S)-10
2
3
2
3
after illuminated with 4 W × 4 UV blacklight tube lamps. Fig. 8a shows the total conversion of
NP to nitrophenolate with a characteristic peak at 400 nm for Dy-Zn(O,S) in the presence of
sulfite ions. The nitrophenolate ions gradually converted to 4-AP in 45 min, accompanied with
the disappearance of peak at 400 nm and the appearance of peaks at 302 and 230 nm. Fig. 8b
shows the chemical conversion with Dy-free Zn(O,S). It displayed a similar conversion behavior
to that with Dy-doped one but took more than 60 min to complete the conversion. The data
indicated Dy doping also improved the photocatalytic activity of Zn(O,S) for chemical
conversion of 4-NP. The absorbance at 400 nm in UV-vis spectra is due to the formation of
nitrophenolate ions after adding Na SO . As the solution pH becomes higher, nitrophenolate ions
2
3
will be formed and adsorbed on catalyst surfaces, subsequently, the absorbance peak at 400 nm
becomes lower for Dy-doped Zn(O,S). The data suggests more positively oxygen-vacancy-sites
of Dy-doped Zn(O,S) have more adsorption of negatively nitrophenolate ions as compared to
Dy-free Zn(O,S), therefore the reduction of nitrophenol on Dy-doped Zn(O,S) is faster. Fig. 8c
shows the chemical conversion of 4-NP without the addition of Na SO . Here, there was no
2
3
nitrophenolate peak at 400 nm before the light illumination. The conversion from 4-NP with
peak at 317 nm occurred slowly not only to 4-AP with peaks at 302 and 230 nm but also to
nitrophenolate with peak at 400 nm. Therefore, 4-NP could be well reduced to 4-AP with Dy-
2
1

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98
99
00
01
02
Zn(O,S)-10 in Na SO solution at a relatively short time. After the conversion, the powder was
2 3
evaluated with XRD to study its structural stability. As shown in Fig. S4, no observable
structural change can be detected. Other M-Zn(O,S) photocatalysts with M= Sn, Mn, and La had
been evaluated, however a twice conversion time was needed.9, 29, 30 Therefore, it is one of the
purposes to use the excellent Dy-Zn(O,S) for conversion of azobenzene to aniline, which is
commonly known as a sluggish conversion reaction.
4
4
4
4
4
03
04
05
06
07
The 4-NP-to-4-AP conversion was further analyzed with HPLC. 4-NP and 4-AP were
respectively monitored with UV and fluorescent detectors at the retention times of 31.8 min and
18.3 min. Fig. S5 shows HPLC plots of (a) 4-NP and (b) 4-AP solutions after photocatalytic
reduction in the presence of Dy-Zn(O,S)-10. The conversion was complete in 2 h with the
gradual increased peak intensity of 4-AP and the gradually decreased intensity of 4-NP.
4
4
4
4
4
4
4
08
09
10
11
12
13
14
To understand the kinetic mechanism of 4-NP reduction, the experiments to observe the
evolved hydrogen with and without 30 ppm 4-NP solution in the presence of Dy-Zn(O,S)-10 and
Na SO were executed with GC-TCD. Fig. S6 shows the amounts of evolved hydrogen during
2
3
the photocatalytic session in the presence of Dy-Zn(O,S)-10 with and without 4-NP in solution.
The data showed the amount of the evolved hydrogen in 4-NP solution was lower than that in 4-
NP-free solution, implying the evolved hydrogen ions on catalyst were not reduced by electron
to form hydrogen gas, but used for the reduction of 4-NP to 4-AP.
4
15
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2

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4
4
4
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4
4
4
4
4
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4
4
4
4
4
4
4
4
4
4
4
4
17
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28
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31
32
33
34
35
36
37
38
39
40
41
42
43
a) 3.5
4-NP
(
Sulfite ions
at 210 nm
400 nm
3
2
2
.0
.5
.0
0
1
3
4
min
5 min
0 min
5 min
1.5
4-AP
1
0
0
.0
.5
.0
2
30 nm 302 nm
250 300
2
00
350
400
450
500
(
b) 7
Sulfite ions
at 210 nm
4
-NP
400 nm
6
0
min
15 min
0 min
45 min
0 min
5
4
3
2
1
0
3
6
4
-AP
2
30 nm
302 nm
2
00 250 300 350 400 450 500
4-NP
0
1
3
4
6
7
min
(
c)2.00
317 nm
5 min
0 min
5 min
0 min
5 min
90 min
20 min
180 min
1.75
1.50
1.25
1.00
0.75
0.50
0.25
0.00
1
4-NP
400 nm
4
4
4
4
44
45
46
47
302 nm
4-AP
2
00
250
300
350
400
450
500
Wavelength (nm)
2
3

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4
4
4
48
49
50
51
Fig. 8. The variation of UV-vis spectra with reaction time for 30 ppm 4-NP catalyzed with (a)
Na SO +Dy-Zn(O,S)-10, (b) Na SO +Zn(O,S), and (c) Dy-Zn(O,S)-10 after illuminated with 4
2
3
2
3
W × 4 UV blacklight tube lamps.
4
52
Evaluation of photocatalytic hydrogenation of azobenzene to aniline.
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
Fig. 9a shows the UV absorbance spectra of 15 ppm azobenzene solution after 0-min, 15-min,
30-min, and 60-min photo reaction with Dy-Zn(O,S)-10. As the trans-azobenzene easily changed
to cis-azobenzene under light (UV or vis) illumination, the specific absorbance spectrum of cis-
azobenzene was observed before light illumination for PHR. After 15-min photo reaction, the
UV absorbance peaks of AB were shifted to the characteristic peaks of aniline at 237 nm. 48%
aniline was formed in 15 min from AB, after we compared it with the peak intensity of 15 ppm
standard aniline at the same condition. An appropriate hydrogenation reaction step is proposed
and shown as an inset in Fig. 9a. To ensure the formation of aniline as a hydrogenation product,
the other experiment with higher concentration of 60 ppm azobenzene was carried out and the
aliquots after 0-h, 2-h, 4-h, and 6-h reactions were taken for GC-MS measurement. The result of
GC-MS measurement after 6 h reaction was shown in Fig. 9b and it revealed that 60 ppm
azobenzene was totally converted to aniline after 6-h reaction. Although the UV-vis spectra data
indicated about 48% of the azobenzene had been hydrogenated to aniline in 15 min, however to
achieve 100 % azobenzene-hydrogenated aniline, a relatively lengthy photocatalytic session was
required. It might be due to the much higher AB concentration or the converted aniline on
catalyst to cover the active sites of catalyst. The existences of azobenzene and aniline during 2 h,
4 h, and 6 h reactions were shown in the inset in Fig. 9b. During the photocatalytic conversion
reaction at 2 h and 4 h, azobenzene and aniline were still existed in solution. However, after 6-h
reaction, only aniline was detected as a product of azobenzene hydrogenation. Commonly,
2
4

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4
4
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84
85
hydrogenation reaction for chemicals with azo bond is quite hard, however the experimental data
indicate that the double bond of nitrogen in azobenzene could be broken and hydrogenated with
Dy-Zn(O,S) photocatalyst. To show the effect of Dy doping, pure Zn(O,S) (without Dy doping)
was used to hydrogenate azobenzene with the same experimental condition. The result indicated
pure Zn(O,S) could not totally convert azobenzene to aniline as one of the characteristic cis-
azobenzene peaks at 430 nm in UV-vis measurement (Fig. S7) was still shown up, which
revealed intermediates might remain for Dy-free Zn(O,S) catalyst. To show a hydrogenation
reaction to use the hydrogen generated from HER on Dy-Zn(O,S), an experiment to measure the
evolved hydrogen with and without azobenzene was conducted for the solution. Fig. S8 shows
the hydrogen evolution obtained from HER on Dy-Zn(O,S)-10 nanoparticles with and without 60
ppm azobenzene in 10% ethanol solution. It was obviously shown that the evolved hydrogen
amount from azobenzene-contained solution was lower than that from azobenzene-free solution.
After the conversion, the powder was evaluated with XRD to study its structural stability. As
shown in Fig. S9, no observable structural change can be detected.
4
4
86
87
4
4
4
4
4
4
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5

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5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
95
96
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00
01
02
03
04
05
06
07
08
09
10
11
12
13
14
15
16
17
N
N
1
1
1
.8
.6
.4
N
N
(
a)
UV
Vis
trans-azobenzene
cis-azobenzene
Aniline
-
+
2
e
H
0
1
3
min
5 min
0 min
1.2
1.0
0.8
H
N
N
H
60 min
hydrazobenzene
-
+
2e
H
cis-azobenzene
NH2
0.6
0.4
0.2
0.0
Aniline
200
250
300
350
400
450
500
Wavelength (nm)
N
2
(
b)
0 h
Azobenzene exist
Aniline not exist
2 h
exist
exist
4 h
exist
exist
6 h
not exist
exist
CO
2
Mass spectra for AB solution after 6 h reaction
Standard mass spectra for aniline
5
5
5
5
18
19
20
21
Fig. 9. (a) UV-vis absorbance spectra of 15 ppm azobenzene solution with different photo
reaction times and (b) GC-MS spectra of 60 ppm azobenzene solution after 6 h reaction with a
inset table to show the existences of azobenzene and aniline in different reaction times.
2
6

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23
Photocatalytic hydrogenation mechanism
5
5
5
5
5
5
5
5
5
5
24
25
26
27
28
29
30
31
32
33
The basic reaction steps for PHR needs to be consistent HER. Based on the experimental data
which showed the solution color reversibly changed, PHR is expected to be initiated with the
water oxidation to form oxygen vacancy which further reduces water to form H ions. As the H+
ions are available on catalyst surface, there are two possibilities for it, either to form hydrogen
gas or to undergo hydrogenation reaction. For Dy-Zn(O,S), the photocatalytic reactions are
+
3
1
proposed and presented with the Kröger–Vink notation for defects. As Dy-Zn(O,S) catalyst is
illuminated with UV light, electron and hole are generated on conduction and valence bands,
respectively, as shown in Eq. (1). The generated hole will oxidize water with the assistance of
active surface oxygen to form hydroxide and positively-charged oxygen vacancy, as indicated in
Eq (2).1, 7, 11-13, 31
Photon
5
5
34
35
(Zn,Dy)(O,S)
h+ + e-
(1)
(2)
2
-
+ 2h+
-
2+
H O + O
2
2OH + V
O
surf
5
5
5
36
37
38
The formation of active surface oxygen is evaluated for the next paragraphs. Dy-doped Zn(O,S)
can have the defect equation expressed below:
3
ZnO
Dy O  2Dy +VZn2- + 3OO0+
+
(3)
2
3
Zn
5
5
5
39
40
41
However, Dy-Zn(O,S) had lower electrical resistance than the Dy-free Zn(O,S), as shown in Fig.
5. To support the evidence in electrical resistance, the defect equation in Zn(O,S) for oxygen
vacancy together with electrons is listed in Eq. (4) below.
ZnO
OO0+  1/2 O +V + 2e-
2+
(4)
5
42
2
O
2
7

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44
Together with the intrinsic Schottky defect equation of Eq. (5): null  VZn2- + VO2+, the Dy-
Zn(O,S) has the overall defect reaction in Eq. (6) below:
2
3
ZnO
+
0+
5
5
5
5
5
5
5
5
5
5
5
45
46
47
48
49
50
51
52
53
54
55
Dy O  2Dy + 1/2 O + 2O +2e-
(6)
2
Zn
2
O
The Dy-on-Zn defect is a donor one to increase electron concentration and low electrical
resistance, as shown in Fig. 5. Therefore, the Dy doping is to lower electrical resistance with the
electron contribution in Eq. (6). Based upon the equilibrium constant of Eq. (4), the higher e-
concentration leads to the lower [VO2+]. For the Dy-free Zn(O,S), on the other hand, the defect
-
2-
+ OO0+, is achieved by subtracting Eq. (4) from Eq. (5). This
equation (7): 1/2 O + 2e = V
2
Zn
mechanism involves the electron elimination to lead Zn(O,S) with high electrical resistance, as
-
supported with Fig. 5. Based upon the equilibrium constant of Eq. (7), the lower e concentration
for Dy-free Zn(O,S) leads to the lower [VZn2-] or higher [VO2+]. With the interaction between
point defect and surface/interface, Eq. (8) and Eq. (9) below have to be kept.
ZnZn0+  VZn2- + Znsurf2+
OO0+  VO2+ + Osurf2-
(8)
(9)
5
5
5
5
5
5
5
5
5
56
57
58
59
60
61
62
63
64
As observed in Table S2 in supplementary data, the Dy-free Zn(O,S) has a higher VO2+ content,
but [VO2+] is lower with the addition of Dy. That is [VO2+]Dy-free > [VO2+]Dy, so [Osurf2-]Dy-free
<
[Osurf2-]Dy based upon Eq. (9) can be obtained. The meaning for [Osurf2-]Dy-free < [Osurf2-]Dy is that
Dy-Zn(O,S) has more negatively charged surface oxygen ions than Dy-free Zn(O,S). The more
negatively charged catalyst is expected to have stronger interaction with other chemicals through
Eq. (2). The interaction between defect and interface becomes severe, when the particle size
becomes small with a small radius of curvature. Therefore, more active surface oxygen ions
occur on Dy-Zn(O,S), as compared with Dy-free Zn(O,S).
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Page 29 of 36
Catalysis Science & Technology
View Article Online
DOI: 10.1039/C9CY00502A
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After the active surface oxygen-involved water oxidation in Eq. (2), the oxygen vacancy
sites then can trap water molecule to form the adsorbed H O and to weaken its O-H bond. This
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interaction between adsorbed water and oxygen vacancy site on catalyst surface finally generates
hydrogen ions, as shown in Eq. (10). Without the 4-NP and AB, the proton in Eq. (10) is used for
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the HER mechanism. With 4-NP, Na SO has been added to act not only as an electron transfer
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3
3
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mediated agent for forming nitrophenolate ion, but also as the hole scavenger to be oxidized by
photo generated hole to SO3*- radical. The schematic diagram for the kinetic mechanism for PHR
of 4-NP is shown in Fig. S10, where the reaction steps demonstrate the continuous
hydrogenation reactions with the aids of protons and electrons.
+
2
+
0+
H O + V
2
2H + O
O surf
(10)
O
To be consistent with our proposed mechanism for HER and the 4-NP conversion, the
initiation of photocatalytic AB-to-aniline conversion must involve Eqs. (1) – (3) for producing
proton, electron, and active surface oxygen. For the AB-to-aniline conversion, the addition of
ethanol is not only important for the dissolution of AB in water but also useful to act as a hole
scavenger to initiate Eq. (11) below for generating important species of oxygen vacancy and
lowering the hole concentration. The oxygen vacancy needs to interact with H O for protons and
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with AB for the pinning of its N in the N=N bond. Although Dy-Zn(O,S) is expected to have
more active surface oxygen ions due to [Osurf2-]Dy-free < [Osurf2-]Dy, too much Dy-doping content to
form too much oxygen vacancy is not beneficial because of the lattice relaxation to weaken the
the interaction with H O or azobenzene. An optimal Dy content in Zn(O,S) is required to achieve
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the best reaction condition. The schematic diagram for the kinetic mechanism for the PHR of
azobenzene is shown in Fig. 10. The pinned c-AB goes through continuous PHR with the
available protons and electrons to form aniline.
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