Boosting the photoreduction activity of Cr(vi) in metal-organic frameworks by photosensitiser incorporation and framework ionization

Article abstract of DOI:10.1039/c9ta13339a

It is mandatory to decontaminate Cr(vi) species from water streams due to their high toxicity. The photoreduction of hazardous Cr(vi) to nontoxic Cr(iii) is a very promising approach to control Cr(vi) pollution. Although some MOFs and their composites have been employed as photocatalysts to reduce toxic Cr(vi), their catalytic activities are always limited by inefficient visible light absorption and inferior Cr(vi) adsorption performances. Herein, cationic Ru-UiO-dmbpy(1) that is decorated with a high density of cationic charges and also embedded with catalytically competent and visible light harvesting moieties (i.e. Ru(bpy)3) was successfully synthesized by a sequential mix-and-match and ionization process. The presence of Ru(bpy)3 moieties makes Ru-UiO-dmbpy(1) an excellent visible-light harvester, extending the absorption edge from 420 nm for pristine UiO-bpy to about 780 nm for Ru-UiO-bpy/Ru-UiO-dmbpy(1). Driven by ion exchange, Ru-UiO-dmbpy(1) shows a large adsorption rate constant (k2) of 1.33 × 10-2 g mg-1 min-1 in the adsorption of Cr2O72-, which is 8.26 times that of pristine UiO-bpy and 6.27 times that of the non-ionization counterpart Ru-UiO-bpy(1) under the same adsorption conditions. The equilibrium Cr2O72- uptake capacity of Ru-UiO-dmbpy(1) reaches 101.8 mg g-1, which is about 1.98 and 1.81 times that of UiO-bpy and Ru-UiO-bpy(1), respectively. With an efficient visible light absorption ability and superior Cr(vi) adsorption performance, Ru-UiO-dmbpy(1) shows a reaction rate constant (k1) of 0.011 min-1 in the photoreduction of Cr(vi) without any sacrificial agents under visible light illumination, which is much larger than those of pristine UiO-bpy (0.003 min-1) and its non-ionization counterpart Ru-UiO-bpy(1) (0.007 min-1) under the same conditions. The result unambiguously demonstrates that the Cr(vi) photoreduction activity could be largely enhanced by photosensitiser introduction and framework ionization. To further evaluate the catalytic performance of Ru-UiO-dmbpy(1), Cr(vi) reduction was also investigated in the presence of various sacrificial agents. Remarkably, a high Cr(vi) photoreduction rate of 6.4 mgCr(VI) gcatalyst-1 min-1 was obtained when benzyl alcohol was used as a sacrificial agent, which is a record value among those reported for MOF-based photocatalysts. This journal is

Full text of DOI:10.1039/c9ta13339a

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DOI: 10.1039/C9TA13339A
ARTICLE
Boosting the photoreduction activity of Cr(VI) in metal-organic
frameworks by photosensitiser incorporation and framework
ionization
Received 00th January 20xx,
Accepted 00th January 20xx
He-Qi Zheng,a, 1 Xing-Hao He,a, 1 Yong-Nian Zeng,a Wei-Hua Qiu,a Jin Chen,a Gao-Juan Cao,a Rong-
Guang Lin,a Zu-Jin Lin,*a,b,c and Banglin Chen*b
DOI: 10.1039/x0xx00000x
It is mandatory to decontaminate Cr(VI) species from water streams due to their high toxicity. The photoreduction of
hazardous Cr(VI) to nontoxic Cr(III) is a very promising approach to harness Cr(VI) pollution. Althouth some MOFs and their
composites have been employed as photocatalysts to reduce toxic Cr(VI), their catalytic activities always limited by inefficent
visible light adsorption and inferior Cr(VI) adsorptive performances. Herein, a cationic Ru-UiO-dmbpy(1) that decorated with
high density of cationic charges and as well embeded catalytically competent and visible light havesting moiteis (i.e.
Ru(bpy)3) was successfully synthesized by a sequential mix-and-match and ionization process. The presence of Ru(bpy)3
moieties makes Ru-UiO-dmbpy(1) an excellent visible-light harvester, extending the adsortpion edge from 420 nm for
pristine UiO-bpy to about 780 nm for Ru-UiO-bpy/Ru-UiO-dmbpy(1). Driven by ion exchange, Ru-UiO-dmbpy(1) shows a
large adsorption rate constant (k2) of 1.33×10-2 g mg-1 min-1 in the adsorption of Cr2O72-, which is 8.26 times that of pristine
UiO-bpy and 6.27 times that of non-ionization counterpart Ru-UiO-bpy(1) under the same adsorption conditions. The
equlibrium Cr2O72- uptake capacity of Ru-UiO-dmbpy(1) reaches to 101.8 mg/g, which is about 1.98 and 1.81 times that of
UiO-bpy and Ru-UiO-bpy(1), respectively. With an efficient visible light absorption ability and superior Cr(VI) adsorption
performance, Ru-UiO-dmbpy(1) shows a reaction rate constant (k1) of 0.011 min-1 in the photorduction of Cr(VI) without any
sacrifice agents under visible light illumination, which is much larger than those of prinstine UiO-bpy (0.003 min-1) and its
non-ionization counterpart Ru-UiO-bpy(1) (0.007 min-1) under the same conditions. The result umambiguously demonstrats
the Cr(VI) photoreduction activity could be largely enhanced by photosensitiser introduction and framework ionization. To
further evaluate the catalytic performance of Ru-UiO-dmbpy(1), Cr(VI) reduction was also investigated in the presence of
various sacrifice agents. Remarkably, a high Cr(VI) photoreduction rate of 13.3 mgCr(VI)/gcatalyst/min was obtained when benzyl
alcohol was used as sacrifice agent, which is a recorded value among the reported MOF-based photocatalysts.
environmental benign Cr(III) is regarded as the most
prospective technique due to its efficiency, low cost, no
hazardous chemicals production, and energy conservation.7 An
1. Introduction
The massive use of hexavalent chromium Cr(VI) in modern
industries including chromium electroplating, leather tanning,
pigment production, metallurgy etc. has resulted in
unneglectable Cr(VI) pollution worldwide.1 Cr(VI) is
carcinogenic, which is classified into a priority pollutant by the
U.S. Environment Protection Agency (EPA).2 Several
technologies including ion exchange,3 adsorption,4 membrane
separation,5 and photocatalytic reduction6 have been proposed
to address Cr(VI) pollution. Among them, photocatalytic
reduction that transforms toxic Cr(VI) species into
array of semiconductors including TiO2, ZnO, CdS, C3N4 and their
derivatives/composites have been employed as photocatalysts
to reduce Cr(VI) in water media.8 However, their reduction
activities under visible light irradiation are usually limited by
poor Cr(VI) adsorptive ability, inferior visible light harvesting,
fast recombination of photoinduced carriers and etc.9
Therefore, it is still a daunting challenge to develop new
photocatalysts to tackle the above problems for efficient
reduction of toxic Cr(VI).
Metal-organic frameworks (MOFs), comprising of organic
ligands and metal ions, have gained immense attention in the
last three decades. Due to their large specific surface area,
designable structure, modulatable pore functionalization,
a. Department of Applied Chemistry, College of Life Science, Fujian Agriculture and
Forestry University, Fuzhou, Fujian 350002, People’s Republic of China. E-mail:
linzujin@fafu.edu.cn.
MOFs have been widely applied in adsorption,10, 11 separation,12
b. Department of Chemistry, University of Texas at San Antonio, One UTSA Circle,
San Antonio, TX 78249-0698, United States. E-mail: banglin.chen@utsa.edu.
c. State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the
Structure of Matter, Chinese Academy of Sciences, Fuzhou, 350002, People’s
Republic of China.
18
sensing,13-16 heterogeneous catalysis,17,
and so on. MOF-
based photocatalysts could be regarded as a new type of
semiconductor, whose HOMO-LUMO gaps could be considered
as their band gaps.19 Benefiting from the adjustable chemical
compositions, the band gap of MOFs can be finely tuned in the
1 These authors contributed equally
Electronic Supplementary Information (ESI) available: PXRD, IR, 1H NMR, ICP, GC-
MS, and other tables and figures. See DOI: 10.1039/x0xx00000x
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2.1.1 Materials [2,2'-bipyridine]-5,5'-dicarbVoiexwylAircticle Oanclinide
DOI: 10.1039/C9TA13339A
(H2bpydc) and cis-[Ru(bpy)2Cl2] (bpy=2,2'-bipyridine) were
molecular level. In this regard, MOF-based photocatalysts could
be superior to traditional inorganic semiconductor
photocatalysts whose band gaps are difficult to adjust. In this
context, several water-tolerant MOFs like ZIF-8, MIL-68(Fe),
MIL-53(Fe), and MIL-100(Fe) have been employed as
photocatalysts to reduce Cr(VI) in water medium under visible
light illumination.20-24 These MOFs, however, cannot effectively
absorb visible light, inhibiting their reduction performances. To
extend the absorption edge to visible light region, organic linker
decoration or photosensitizer incorporation were adopted to
reduce the bandgap of MOFs and improve the visible light
utilization. For example, by graft of amino groups into BDC2-
ligands, Wu et al. reported that MIL-68(In)-NH2 has a better
photoreduction activity than its unfunctional MIL-68(In)
counterpart due to the extension of the absorption edge to
visible region.25 Ye and Cao et al. successfully introduced
porphyrin units (a typical photosensitizer) into UiO-66 type
framework, whose photoreduction activity is much higher than
that of pristine UiO-66.26 Compared with traditional inorganic
semiconductors, another important merit of MOFs is their open
framework, which could effectively inhibit the recombination of
photogenerated electron-hole pairs due to their short spatial
transmission distance. The photogenerated electrons or holes
can facilely move to the substrates that interpenetrate into the
cavies of the frameworks and quickly proceed redox reactions.
Therefore, the mass transfer of substrates over MOF-based
catalysts is an another vital parameter for Cr(VI)
photoreduction. Albeit, most work only concerns the
modulation of visible light absorption performance, and the
effect of Cr(VI) adsorption over MOF catalysts is overlooked.
Therefore, it is highly desirable to investigate the relationship
between Cr(VI) adsorption performances and the
photoreduction activities for the further development of
efficient photocatalysts to reduce Cr(VI).
purchased from admas beta. Other reagents were brought from
Tansoole. All reagents and solvents are analytical grade and
used without further purification.
[Ru(bpy)2(bpydc)]Cl2 (denoted as Ru(bpy))29 and MOF-86733
(denoted as UiO-bpy hereafter for convenience) were
synthesized according to the reported literatures.
2.2 Synthesis of Ru-UiO-dmbpy
The synthetic process of Ru-UiO-bpy is similar to that of MOF-
867 or UiO-bpy.29 Taking Ru-UiO-bpy(1) as an example, DMF
(7.2 mL) and glacial acetic acid (0.4 mL) were added to a Teflon-
lined stainless steel vessel (21 mL) containing a mixture of ZrCl4
(46.7 mg, 0.2 mmol), 2,2’-bipyridine-5,5’-dicarboxylate
(H2bpydc) (48.8 mg, 0.2 mmol), and Ru(bpy) (4.6 mg, 0.02
mmol). The resulting mixture was sonicated for about 20 min.
Subsequently, the vessel was placed in an oven and heated at
120 °C for 24 hours. After naturally cooling, an orange solid was
isolated by centrifugation, sequentially washed with clean DMF
and methanol, and then dried at 60 °C overnight under vacuum
to get Ru-UiO-bpy(1) sample.
The N-methylation of open 2,2’-bpy units in Ru-UiO-bpy(1)
was
proceed
as
followings.33
Excessive
methyl
trifluoromethanesulfonate (TfOMe, 1 mL) was added to a vial
containing 50 mg of Ru-UiO-bpy(1) (0.023 mmol) to form a
suspension. The suspension was stirred at room temperature
for about 5 h. The solid was isolated by centrifugation, washed
with ethanol, and then dried at 60 °C under vacuum overnight
to get N-methylated sample Ru-UiO-dmbpy(1).
By tuning the feeding molar ratio of H2bpydc to Ru(bpy) from
10:2 and 10:0.5, two other materials denoted as Ru-UiO-bpy(2)
and Ru-UiO-bpy(0.5) respectively were also synthesized by the
same procedure. After N-methylation, the corresponding
materials were denoted as Ru-UiO-dmbpy(2) and Ru-UiO-
dmbpy(0.5), respectively.
2+
Ru(bpy)3 (bpy = 2,2'-bipyridine) and its derivatives, an
representative type of photosensitizers who can efficiently
harvest photons within visible light, are one of the most
attracting molecular photocatalysts.27 Various photocatalytic
reactions have been successfully established in terms of their
long lifetime of MLCT state and their easiness to undergo redox
2.3 Physical measurements
Powder X-ray diffraction (PXRD) was carried out on a Rigaku
MiniFlex2 diffractometer working with Cu Kα radiation. Fourier
transform infrared (FT-IR) spectra were measured on a
PerkinElmer Spectrum One. Inductively coupled plasma (ICP)
measurements were performed on an Ultima2 spectrometer;
To monitor Ru and Zr leaching by ICP, the reaction solution after
photocatalytic reaction was filtered by a syringe filter (0.24 μm)
prior to ICP measurements. Solid UV–vis diffuse reflectance
spectroscopy (UV–vis DRS) was tested on a Shimadzu UV-2600
with BaSO4 as the reference and transformed to the absorption
spectra based on the Kubelka-Munk relationship. Fluorescent
spectra were collected on a FS5 Spectrofluorometer (Edinburgh
Instruments Ltd.). Fluorescent lifetimes were measured on a
FLS980 Spectrofluorometer (Edinburgh Instruments Ltd.) with
the excitation wavelength of 475 nm and monitoring
wavelength of 650 nm. Scanning electron microscopy (SEM)
images were collected in JSM6700-F and SU-8010 field emission
scanning electron microscopes. X-ray photoelectron
2+
quenching.28 Lin et al pioneered to incorporate Ru(bpy)3
moieties into MOFs to study their application in
photocatalysis.29-32 Although many progresses have been made,
2+
the application of MOFs embedding Ru(bpy)3 units for
photoreduction of toxic Cr(VI) has not yet been reported.
2+
Herein, Ru-UiO-dmbpy(1) embedding Ru(bpy)3 moieties and
decorating with high density of cationic charges was
synthesized by photosensitizer introduction and framework
ionization. The physical properties, Cr(VI) adsorption
performances, and Cr(VI) photoreduction behaviors of Ru-UiO-
dmbpy and the pristine sample UiO-bpy and its non-ionization
counterpart Ru-UiO-bpy(1) were fully investigated. Besides, a
mechanism concerning the Cr(VI) reduction was also proposed.
2. Experimental
2.1 Materials and methods
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2−
spectroscopy (XPS) was measured on an ESCALAB 250Xi Xray Cr2O7 adsorption was carried out in a 200 mL quartz reactor.
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photoelectron spectrometer (Thermo Fisher) using an Al Kα Typically, 10 mg of adsorbent was added tDoOaIn: 1a0q.1u0e39o/uCs9TsAo1lu33t3io9nA
1
2-
source (15 kV, 10 mA). H NMR was performed on an AVANCE (40 mL) of K2Cr2O7 with an initial Cr2O7 concentration of 50
III Bruker Biospin spectrometer (400 MHz). N2 sorption ppm, and the resulting suspension was vigorously stirred in the
isotherms were performed at 77 K on a Micrometrics ASAP 2460 dark. During this process, 1.0 mL of the suspension was taken at
surface area and pore size analyser. Electron paramagnetic the preset time and the concentration of Cr2O72- in filtrate was
resonance (EPR) spectra were collected on a Bruker-BioSpin evaluated by UV–vis spectroscopy by diphenylcarbazide (DPC)
E500 spectrometer at room temperature. Perfectlight (PLS- method.10
SXE300) with a 300 W xenon lamp equipped with an ultraviolet
cut-off filter (> 420 nm) was used as the irradiation source.
2.6 Photocatalytic tests
Typically, the photocatalytic experiment was carried out as
2−
2.4 Electrochemistry measurements
following steps. 10.0 mg of photocatalyst and 40 mL of Cr2O7
The catalyst-modified electrode was prepared on a fluorine aqueous solution with an initial concentration of 50 ppm were
doped tin oxide (FTO) glass. The FTO slide was previously placed in a 200-mL quartz reactor. The mixture was vigorously
2−
protected using Scotch tape to make sure that the exposed area stirred in the dark for 1 hour to reach Cr2O7 adsorption-
of the working electrode was controlled at 1.0 cm2. 5 mg of desorption equilibrium over the catalyst. Subsequentially, the
photocatalyst, 0.7 mL of DMF, and 0.3 mL of alcohol was mixed reactor was illuminated by visible light. During this process, 1.0
and sonicated to get a homogeneous slurry. FTO slide was dip mL of the suspension was taken from the reactor at pre-set time
2−
coated with 60 μL of the above-prepared slurry. After air drying, interval, filtered by
a syringe filter, and the Cr2O7
o
the working electrode was further dried at 70 C for 24 h to concentration in the filtrate was measured by UV–vis
improve adhesion. All the electrochemistry measurements spectroscopy using the DPC method. The reduction percentage
were carried out on a ZM6ex electrochemical station (Zahner, of Cr(VI) was calculated by the following equation:
Germany) in a three-electrode quartz cell. The Mott-Schottky
Reduction percentage of Cr(VI) (%) = (C0 - Ct)/C0 × 100%
and photocurrent analyses were measured in 0.2 M Na2SO4 where C0 and Ct are the Cr2O72- concentration when the reaction
aqueous solutions, which were purged with N2 before system was illuminated at 0 and t min, respectively.
measurements. The light irradiation source was the same as
that used in the photocatalytic tests.
The cycled experiment was carried out as followings. After
reaction, Ru-UiO-dmbpy(1) was collected by centrifugation,
washed sequential by NaCl aqueous solution, 0.1 M HNO3
solution, and ethanol. The solid then dried under vacuum at 60
2.5 Cr2O72− adsorption experiments
Prior to the adsorption experiments, Cr2O72− aqueous solutions °C overnight for the next run.
2−
with preset concentrations were prepared. The pH of Cr2O7
aqueous solutions was adjusted by 0.1 M of HCl and NaOH.
Fig. 1 Synthetic procedure of (a) UiO-bpy, as well as (b) Ru-UiO-bpy and Ru-UiO-dmbpy.
During the Cr(VI) photoreduction process, the Cr(VI)
adsorption and desorption over the catalysts occur
simultaneously. It is difficult to calculate the exact amount of
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Cr(VI) reduced. For convenience, we assume that the adsorbed
Cr(VI) proportionately decreases with that in the reaction
solutions. It is reasonable because all Cr(VI) were completely
reduced under the optimal conditions.
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DOI: 10.1039/C9TA13339A
3. Results and discussion
3.1 Structure and Characterization
UiO-bpy (i.e., MOF-867, Fig. 1a), a thermal-stable and water-
tolerant MOF that is self-assembled by ZrCl4 and 5,5'-dicarboxy-
2,2'-bipyridine (H2bpydc) ligand, was selected as the platform to
introduce the photosensitiser and catalytically competent
Ru(bpy). As shown in Fig. 1b, Ru-UiO-bpy was synthesized by the
mix-and-match strategy, whose synthetic procedure is similar
to that of UiO-bpy except that H2bpydc is partially replaced with
Ru(bpy). PXRD result confirms that Ru-UiO-bpy retains UiO-bpy
type framework when the feeding molar ratio of Ru(bpy) to
Fig. 2 (a) PXRD patterns of UiO-bpy, Ru-UiO-bpy(1) and Ru-UiO-dmbpy(1). (b) FT-IR
H2bpydc is smaller than 2:10 (Fig. 2a and S1). The successful N- spectra and (c) N2 adsorption isotherms of UiO-bpy, Ru-UiO-bpy(1) and Ru-UiO-
dmbpy(1). (d) SEM image of the as-synthesized Ru-UiO-dmbpy(1) sample.
methylation of 2,2’-bpy moieties in Ru-UiO-bpy is verified by the
appearance of three new adsorption peaks at about 1263, 1174
and 1032 cm-1 in the IR spectra of Ru-UiO-dmbpy samples (Fig.
2b and S4), which could be assigned to TfO- anions that serve as
counterions in the cavities of cationic Ru-UiO-dmbpy
framework.33 Taking Ru-UiO-dmbpy(1) as an example, about
89% of 2,2’-bpy moieties have been N-methylated, as indicated
by the 1H NMR spectrum of the digested Ru-UiO-dmbpy(1)
sample where about 71% of dimethylated product and about
18% of monomethylated ligands are observed (Fig. S9-10).
PXRD patterns of Ru-UiO-dmbpy(1) and Ru-UiO-bpy(1) match
well with those of experimental and stimulated UiO-bpy,
demonstrating the good retention of UiO-bpy/MOF-867 type
structure after photosensitiser introduction and N-methylation
(Fig. 2a and S2). The Brunauer-Emmett-Teller (BET) surface
areas and pore volumes are 1367 m2 g-1 and 0.956 cm3 g-1 for
UiO-bpy, and 692 m2 g-1 and 0.829 cm3 g-1 for Ru-UiO-bpy(1),
which further decreased to 174 m2 g-1 and 0.205 cm3 g-1 for Ru-
UiO-dmbpy(1) (Fig. 2c and Fig. S5). Such a large decrement in
both the surface area and pore volume of Ru-UiO-dmbpy(1) is
attributed to cavity occupancy by the introduced Ru(bpy)3
moieties, pyridyl N+–CH3 groups, and also the TfO- counterions.
Scanning electron microscopy (SEM) images reveal that Ru-UiO-
dmbpy(1) shows distorted octahedral particles with sizes of
780 nm for Ru-UiO-bpy(1)/Ru-UiO-dmbpy(1), corresponding to
the colour change from white for UiO-bpy to dark orange for Ru-
UiO-bpy(1)/Ru-UiO-dmbpy(1) (Fig. 3b). The broad absorption
band in the visible light for Ru-UiO-bpy(1)/Ru-UiO-dmbpy(1) is
mainly attributed to the metal-to-ligand charge transfer (MLCT)
of the incorporating Ru(bpy)3 moieties.34
Fig. 3 (a) UV-visible diffuse reflectance spectra of H2bpydc, Ru(bpy), UiO-bpy, Ru-UiO-
bpy(1) and Ru-UiO-dmbpy(1), respectively. (b) Photographs of their powdery samples.
3.2 Cr(VI) adsorption behaviours on photocatalysts
The incorporation of Ru(bpy)3 photosensitiser and the high
porosity of Ru-UiO-bpy/Ru-UiO-dmbpy inspired us to
investigate their photoreduction of toxic Cr(VI) under visible
light irradiation. MOFs can be seen as nano-scale inorganic
semiconductor/photosensitiser composites When the metal
clusters and organic ligands are separately considered as
nanoscale semiconductor quantum dots and photosensitisers
respectively. One of the prerequisites to initiate the
photocatalytic redox reaction is close contact between the
nanocatalysts and substrates that facilitates the electron/hole
1
ranging from 50 to 100 nm (Fig. 2d). HNMR spectrum of HF-
digested Ru-UiO-dmbpy(1) sample reveals that the molar ratio
of H2bpydc to Ru(bpy) is about 18.2 (Fig. S9), indicating that
about one in nineteen of H2bpydc is replaced with Ru(bpy)
ligand. ICP measurement show that the molar ratio of Zr to Ru
in Ru-UiO-dmbpy(1) is 17.9 (Table S1), corresponding to one in
eighteen H2bpydc is replaced with Ru(bpy), matching well with
the 1H NMR result. The incorporation of Ru(bpy)3 moieties into
UiO-bpy type structure results in tremendous change of UV-vis
diffuse reflectance spectra. As shown in Fig. 3a, the absorption
wavelength edge is extended from 420 nm for UiO-bpy to ca.
2-
transfer. Therefore, the adsorption efficiency of Cr2O7 over
photocatalysts is closely related to the Cr(VI) photoreduction
2-
activity. Keeping this in mind, Cr2O7 adsorptive performance
over these MOF-based catalysts was evaluated in aqueous
solutions at pH 3. As shown in Fig. S11, with only 0.25 g L-1
adsorbent dosage and within 30 min, Ru-UiO-dmbpy(1) could
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2-
adsorb about 49.4% of Cr2O7 in an aqueous solution with an aqueous solution with an initial concentration ofV5ie0w pArpticmle,Oanlninde
initial Cr2O72- concentration of 50 ppm (40 mL), confirming the pH of 3.0. Prior to visible light illuminatDioOnI:,1t0h.1e03m9/iCx9tTuAr1e33w39aAs
2-
high adsorption efficiency of Cr2O7 over Ru-UiO-dmbpy(1). vigorously stirred in the dark for 1 h to reach adsorption-
Under this condition, the maximum Cr2O72- uptake over Ru-UiO- desorption equilibrium of Cr(VI). As shown in Fig. 5a, no
2-
dmbpy(1) reaches to 101.8 mg/g, which is 1.81 times that of its observable decrease of Cr2O7 content is observed in the
non-ionized counterpart Ru-UiO-bpy(1) (56.3 mg/g) and about absence of catalyst after illumination by visible light for 120 min.
1.98 times that of pristine UiO-bpy (51.4 mg/g) (Fig. 4b). Only about 29% of apparent Cr(VI) decrease (including
Considering that the order of BET surface areas and pore adsorption and photoreduction) is found for UiO-bpy, 30% for
volumes is UiO-bpy > Ru-UiO-bpy > Ru-UiO-dmbpy, the reverse mechanical mixture of Ru(bpy) and H2bpydc, and 54% for Ru-
order of Cr2O72- adsorption capacity under the same adsorption UiO-bpy. Impressively, Ru-UiO-dmbpy(1) shows an excellent
condition is mainly ascribed to the framework ionization. Ru- photocatalytic activity for Cr(VI) reduction and it could remove
UiO-dmbpy(1), which has more negative counterions (i.e. about 80% of Cr(VI) under the identical conditions. Control
-
F3CSO3 and Cl- counterions) in the cavities of framework than experiment shows that Cr2O72- concentration almost remained
non-ionic counterpart Ru-UiO-bpy(1) (i.e. Cl- ) and neutral UiO- unchanged after further preservation in the dark for 2 h.
2-
bpy (no counterions) for ion exchange, shows largest Cr2O7
Therefore, the Cr(VI) concentration decrease after adsorption-
uptake capacity. To investigate the Cr(VI) adsorption kinetics, desorption equilibrium is mainly ascribed to the visible light
2-
the Cr2O7 adsorption amount versus contact time are shown driven photodegradation. To further evaluate the
in Fig. 4b and these data are fitted by pseudo-second-order photocatalytic behaviours of these MOFs, the experimental
model (Fig S12). High correlation coefficients (> 0.989) are kinetic data were further fitted by the pseudo-first-order model
2-
obtained, indicating that the Cr2O7 adsorption over UiO-bpy, and the results were shown in Figure 5b and Table S3. The Cr(VI)
Ru-UiO-bpy(1), and Ru-UiO-dmbpy(1) follow the pseudo- reduction over the mechanical mixture of Ru(bpy) and H2bpydc,
second-order model (Figure S12 and Table S2). Remarkably, Ru- UiO-bpy, Ru-UiO-bpy(1), and Ru-UiO-dmbpy(1) obey the
UiO-dmbpy(1) also shows a highest adsorption rate than UiO- pseudo-first-order model, as demonstrated by the high
bpy and Ru-UiO-bpy(1), as demonstrated by the adsorption rate correlation coefficients (> 0.946). The calculated rate constant
constant (k2) at the same adsorption time under the same of Ru-UiO-dmbpy(1) is 0.011 min-1, which is about 1.6, 3.7, 5.5
adsorption conditions. The adsorption rate constant (k2) of Ru- times for mechanical mixture of Ru(bpy) and H2bpydc (0.002
UiO-dmbpy(1) reaches to 1.33×10-2 g mg-1 min-1 which is 8.26 min-1), UiO-bpy (0.003 min-1), and Ru-UiO-bpy(1) (0.007 min-1),
times that of pristine UiO-bpy (2.87×10-3 g mg-1 min-1) and 6.27 respectively. In view of the very similar Cr(VI) adsorption
times that of non-ionization counterpart Ru-UiO-bpy(1) performance between UiO-bpy and Ru-UiO-bpy(1), the larger
(2.12×10-3 g mg-1 min-1). The initial adsorption rate h of Ru-UiO- photoreduction rate constant of Ru-UiO-bpy(1) could be
dmbpy(1) is 17.7 and 19.9 times that of UiO-bpy and Ru-UiO- ascribed to its more efficient visible light response. While the
bpy, respectively. The largest Cr(VI) adsorption capacity coupled better Cr(VI) adsorption performance contributes to the larger
with the highest Cr(VI) adsorption rate of Ru-UiO-dmbpy(1) rate constant of Ru-UiO-dmbpy(1) than that of Ru-UiO-bpy(1).
demonstrated the importance of ionization process for the The largest rate constant of Ru-UiO-dmbpy(1) indicates its
improvement of the Cr(VI) adsorption performance, which highest activity in the Cr(VI) photoreduction, which can be well
largely enhanced the mass transfer of Cr(VI) and could boost the explained by the synergistic of efficient visible light harvesting
photocatalytic activity of Cr(VI) reduction.
and the excellent Cr(VI) adsorption performance. Besides, the
presence of largest electrostatic attractions between anionic
Cr2O7 and cationic Ru-UiO-dmbpy(1) as well as the largest
2-
electrostatic repulsions between Cr3+ and cationic Ru-UiO-
dmbpy(1) may also improve the mass transfer of Cr3+ and
Cr2O72-, which also benefits for the Cr(VI) photocatalytic
reduction activity. These results unambiguously demonstrate
the combination of photosensitizer introduction and
framework ionization is a very effect strategy to boost the Cr(VI)
photoreduction activity.
Based on the above results, Ru-UiO-dmbpy(1) was chosen as
representative photocatalyst to investigate its Cr(VI)
Fig. 4 (a) The pseudo-second-order adsorption rate constant (k2) over UiO-bpy,
Ru-UiO-bpy(1), and Ru-UiO-dmbpy(1), respectively. (b) Effect of the contact time
photoreduction behaviours. For
a typical photocatalytic
2-
on the adsorption amount of Cr2O7 over UiO-bpy, Ru-UiO-bpy(1), and Ru-UiO-
process, the electron-hole pairs will be generated upon light
irradiation. The separation efficiency of the photogenerated
electrons and holes could be enhanced when the holes (h+) are
rapidly consumed, leading to the acceleration of the total redox
reaction including Cr(VI) photoreduction half reaction. For this
reason, some electron donors including methanol, ethanol, and
ammonium oxalate have been employed as hole scavengers in
the photoreduction of Cr(VI) to improve the photocatalytic
dmbpy(1), respectively. Adsorption conditions: adsorbent, 10 mg; Cr2O72- solution,
40 mL, 50 ppm; temperature, 25 °C; and pH, 3.0.
3.3 Photocatalytic reduction of Cr(VI)
To evaluate the photoreduction performance, the
photocatalytic reaction was initially performed under the
2-
following conditions: 10 mg of photocatalyst, 40 mL of Cr2O7
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activities.25 Herein, triethanolamine (TEOA), methanol,
Journal Name
To further confirm the effectiveness of Ru-UiOV-idewmAbrtpicyle(O1n)linine
ammonium oxalate (AO), and also benzyl alcohol (BA) were the photoreduction of Cr(VI), the photoDcaOtIa: 1ly0t.1ic03e9x/Cp9eTrAim13e33n9tAs
selected as sacrificial agents to promote the total redox were also carried out in aqueous solutions with various initial
reaction. As shown in Fig. 5c, benzyl alcohol significantly boosts contents of Cr2O72-. As shown in Fig. S16, Ru-UiO-dmbpy(1)
2-
the Cr(VI) photoreduction in comparison with other sacrificial could completely reduce Cr(VI) in aqueous solutions of Cr2O7
agents (including no additional sacrificial or water as sacrificial (40 mL) with an initial Cr2O72- concentration lower than 100 ppm
agent). Therefore, benzyl alcohol was selected as the sacrificial within 30 min. Notably, the photoreduction rate reaches to 13.3
agent in this Cr(VI) photoreduction system.
mgCr(VI)/gcatalyst/min when the initial concentration of Cr2O72- was
pH value is another vital parameter to affect the 100 ppm. This value is equal to that of H2TCPP(I-)Meim-UiO-
photoreduction activity of Cr(VI). Previous studies reveals that 66,26 and much larger than those of the representative MOF-
Cr(VI) photoreduction activity could be enhanced at low pH.35 based catalysts like NH2-UiO-66 (0.2 mgCr(VI)/gcatalyst/min),23 NH2-
As expected, the photocatalytic activity of Ru-UiO-dmbpy(1) MIL-125 (1.6 mgCr(VI)/gcatalyst/min),43 Pt@MIL-100(Fe)/40 (1.3
also strongly relied on pH (Fig. 5d). At pH 3.0, Ru-UiO-dmbpy(1) mgCr(VI)/gcatalyst/min),42 and HPMo@MIL-100(Fe)/20 (5.0
could completely reduce Cr(VI) under visible light irradiation for mgCr(VI)/gcatalyst/min).22 As far as we know, this value represents
30 min. Under the same conditions, however, the apparent the largest reduction rate among MOF-based photocatalysts for
Cr(VI) reduction percentage significantly decreases to 65.7% for Cr(VI) reduction (Table S9). Such a high reduction rate is mainly
2-
pH 4 and 70% for pH 5, respectively. The catalytic efficiency ascribed to the excellent Cr2O7 adsorption performance and
lowing is further confirmed by the rate constant decreasing with excellent visible light utilization. The above result confirms that
the pH increasing, which is obtained from kinetic data fitted by it is an effective strategy to construct efficient photocatalysts
the pseudo-first-order kinetic model (Fig. S14). The decline of for Cr(VI) photo-degradation under visible light illumination via
photocatalytic activity under low acidic environments is not photosensitiser introduction and framework ionization.
2-
only ascribed to the decrement in oxidation potential of Cr2O7
,
but also ascribed to the potential formed Cr(OH)3 precipitates
that could block the cavities of the framework and mask the
photoactive sites.36 Therefore, the optimal pH was 3.0, and the
following experiments were conducted at this pH value.
To evaluate the effect of the incorporated Ru(bpy)3 content
to photoreduction activity, the as-synthesized three Ru-UiO-
dmbpy frameworks are screened under the optimal
photocatalytic conditions. Fig. 5e showed that the Cr(VI)
photoreduction activity first enhances and then declines with
the increment in the feeding ratio of H2bpydc to Ru(bpy) from
10:0.5 to 10:2 and the optimal ligand feeding ratio is found at
10:1 (corresponding to Ru-UiO-dmbpy(1) material). Ru-UiO-
dmbpy(1) has a rate constant of 0.128 min-1, which is much
larger than those of Ru-UiO-dmbpy(0.5) (0.025 min-1) and Ru-
UiO-dmbpy(2) (0.043 min-1), matching well with the above
order of photocatalytic efficiency (Fig. 5f, S15 and Table S4). The
preliminary increase of Ru(bpy) ligand would increase the
photocatalytic active sites and enhance the visible light
harvesting. However, further increase of the Ru(bpy) amount
will deteriorate the framework crystallinity due to the large
steric hindrance of Ru(bpy)3 moieties (Fig. S1-2). In addition,
Ru(bpy)3 moieties also inevitably blocked the cavities of Ru-UiO-
dmbpy, lowing the accessibility of photocatalytic sites and
deteriorating the mass transfer of the subtract. The rate
constant of Ru-UiO-dmbpy(1) is larger than those of the
Fig. 5 (a) Photocatalytic experiments under the initial reaction conditions and (b) Plots
of ln(C/C0) versus time for Cr(VI) reduction over various catalysts. The effects of (c) hole
scavengers, (d) pH values, (e) Ru-UiO-dmbpy composites on the Cr(VI) photoreduction
performance and (f) K value over various Ru-UiO-dmbpy composites.
recently
reported
representative
inorganic-based
photocatalysts including OH-TiO2 (0.079 min-1),37 g-C3N4/BiVO4
(0.063 min-1),38 and g-C3N4/SnS2/SnO2 (0.0517)39 but a little
smaller than BMO-S1 (0.164 min-1)40 and CdS-ZnIn2S4 (0.179
min-1).41 Specially, such a larger rate constant is comparable to
that of H2TCPP(I-)Meim-UiO-66 (0.1541 min-1)26 and only
much smaller than that of Pt@MIL-100(Fe) (0.5618 min-1)42
among the MOF-based photocatalysts, verifying the high
catalytic activity of Ru-UiO-dmbpy(1) in Cr(VI) photoreduction
under visible light irradiation.
XPS was employed to determine the photoreduction product.
As shown in Fig. S18, a couple of peaks with binding energies of
577.2 eV and 587.2 eV is observed in Ru-UiO-dmbpy(1) after
photocatalytic reaction, which can be assigned to Cr 2p3/2 and
Cr 2p1/2 of Cr3+, respectively. The absence of binding energies of
Cr(VI) at 580 eV for and 589 eV for hexavalent chromium species
6 | J. Name., 2012, 00, 1-3
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further indicates all the toxic Cr(VI) species have been reduced (LUMO) of UiO-bpy, Ru-UiO-bpy(1), and Ru-UiO-dmbpy(1) were
View Article Online
to Cr3+ during photoreduction process.44
-0.84, -0.88, and -0.92 V vs Ag/AgCl at pH = 6.8 respectively,
corresponding to -0.64, -0.68, and -0.72 V vs normal hydrogen
electrode (NHE). The flat-band potentials of these materials are
more negative than the electrode potential of Cr2O72-/Cr3+ (i.e.,
DOI: 10.1039/C9TA13339A
3.4 Recycling experiments
The recoverability and reusability of a heterogeneous catalyst is
pivotal for their practical applications.45 Ru-UiO-dmbpy(1) could
be recovered by centrifugation and washing with an aqueous
solution of HNO3 (0.1 M). The photocatalytic activity of the
recovered Ru-UiO-dmbpy(1) almost remains unchanged after
seven successive runs (Fig. 6). Benefiting from the high stability
of UiO-type framework, the recycled Ru-UiO-dmbpy(1) could
well retain its structure, as demonstrated by PXRD (Fig. S19).
The morphologies of the pristine and recycled samples are very
similar to each other (Fig. 2d, S20), and only a very low Ru
leaching of less than 0.185 ppm was detected even in the
seventh photocatalytic run (Table S6). Besides, the BET surface
area of the recycled Ru-UiO-dmbpy(1) is 293.1 m2 g-1, which is
larger than that of as-synthesized Ru-UiO-dmbpy(1) sample
(i.e., 174 m2 g-1) (Fig. S21). The larger BET surface area of Ru-
UiO-dmbpy(1) can be well explained by the partial replacement
of larger CF3SO3- by smaller Cl-, as verified by the EDS spectrum
where the molar ratio of Cl- increases from 14.5 (calculated
value) for the as-synthesized Ru-UiO-dmbpy(1) to 1.28
(experimental value) for the recycled sample (Fig. S22). These
results further confirm the good stability of Ru-UiO-dmbpy(1)
photocatalyst. The good reusability and high chemical/photo-
stability of Ru-UiO-dmbpy(1) makes it a very promising
photocatalyst for Cr(VI) decontamination in practical
applications.
0.51 V vs NHE at pH = 6.8), verifying their feasibilities as
2-
photocatalysts to reduce Cr2O7
theoretically could transfer from these materials to Cr2O7
since the electrons
2- 46
.
The slopes of the Mott-Schottky plots are all positive, indicating
that these materials could be regarded as n-type semiconductor
where the conductive band potential can be regarded as the
flat-band potential. According to the Tauc plots (Fig. 7d-f, Fig.
S23), the bandgap of UiO-bpy was 3.53 eV, which decreased to
2.16 V for Ru-UiO-bpy(1) and 2.13 V for Ru-UiO-dmbpy(1)
respectively, corresponding to the absorption wavelength edge
ranging from 420 nm for UiO-bpy to about 780 nm for Ru-UiO-
bpy(1)/Ru-UiO-dmbpy(1). The highest occupied molecular
orbital (HOMO) of UiO-bpy, Ru-UiO-bpy, and Ru-UiO-dmbpy(1)
are 2.89, 1.48, and 1.41 V vs normal hydrogen electrode (NHE),
respectively, which are obtained from the band gap and LUMO
(Fig. 7a-c). Compared with UiO-bpy, Ru-UiO-dmbpy(1)/Ru-UiO-
bpy(1) have a more suitable band structure and wider visible
light absorption, making them as better photocatalysts than
UiO-bpy to reduce Cr2O72- under the visible light illumination.
Fig. 6 Reusability of Ru-UiO-dmbpy(1) for the photoreduction of Cr2O72- under irradiation
by visible light for 30 and 60 min.
3.5 Plausible Mechanism
The above experimental results have shown that Ru-UiO-
dmbpy(1) is an better material than pristine UiO-bpy and its
non-ionization counterpart Ru-UiO-bpy(1) for the adsorption
and photoreduction of Cr2O72-. To fully understand the
photocatalytic mechanism, the band structure of the catalysts,
the separation efficiency and transfer processes of the
photogenerated charge carries over the these photocatalysts
are discussed detailly.
Mott-Schottky plots and UV–vis DRS were used to investigate
the band structure of the catalysts. Based on the result of Mott-
Schottky plots at different frequencies (i.e., 500, 1000, and 1500
Hz) (Fig. 7a-c, S23), the lowest unoccupied molecular orbitals
Fig. 7 Typical Mott–Schottky plots for (a) UiO-bpy, (b) Ru-UiO-bpy(1) and (c) Ru-UiO-
dmbpy(1) at frequencies of 500, 1000 and 1500 Hz. Tauc plots and bandgaps for (d) UiO-
bpy (e) Ru-UiO-bpy(1) and (f) Ru-UiO-dmbpy(1).
In principle, a low resistance and a small arc radius in an EIS
Nyquist plot means a fast charge transfer in a semiconductor,
which could enhance the separation efficiency of electron and
hole pairs and thus improves the photocatalytic activity. As
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shown in Fig. 8a, Ru-UiO-dmbpy(1) has a smaller radius and a
Journal Name
Electron paramagnetic resonance (EPR) analyses were used
View Article Online
lower transfer resistance than UiO-bpy and Ru-UiO-bpy(1), to study the electron transfer processes.DAOsI:s1h0o.1w03n9/iCn9FTiAg1.33S3294A,
implying the faster interfacial charge transfer in Ru-UiO- no obvious signal is observed when Ru-UiO-dmbpy(1) is
dmbpy(1). The result matches well with the experimental preserved in the dark. After visible light illumination for 10 min,
results where Ru-UiO-dmbpy(1) shows a higher catalytic activity however, an ESR signal at about g = 2.003 was clearly observed,
for Cr(VI) reduction than UiO-bpy and Ru-UiO-bpy(1). (Fig. 5a). implying that the photogenerated electrons were transferred
The transient photocurrent spectra of UiO-bpy, Ru-UiO-bpy(1), from Ru(bpy)3 moieties to Zr6 clusters (Fig. S24).26
and Ru-UiO-dmbpy(1) were collected under the intermittent
Based on the above experimental results and related
47, 48
visible light illumination (> 420 nm). An apparent photocurrent mechanisms previously reported,10,
a mechanism
response of MOFs coated electrodes in an off-on mode was concerning the adsorption and photoreduction of Cr2O72- over
detected (Fig. 8b), confirming the competence of these MOFs in Ru-UiO-dmbpy(1) is proposed and illustrated in Fig. 9. Prior to
the production and transfer of photogenerated electrons and reaction, the anionic Cr2O7 are quickly adsorbed into the
2-
holes under visible light illumination. Among them, Ru-UiO- cavities of cationic Ru-UiO-dmbpy(1) via ion exchange. Upon
dmbpy(1) shows a largest photocurrent upon visible light irradiation by visible light, Ru(bpy)3 moieties in Ru-UiO-
illumination, implying the best photocatalytic activity of Ru- dmbpy(1) strongly absorb visible light and spontaneously
UiO-dmbpy(1).
generate photogenerated electron and hole pairs via metal-to-
The separation of the photogenerated electron and hole pairs ligand charge transfer (MLCT).49 Therefore, the photogenerated
could be further analyse by steady-state photoluminescent electrons mainly located at bpy, while the holes locate at Ru.
spectrum. To some extent, the luminescent intensity reflects Meanwhile, the photogenerated electrons on bpy could also
the separation efficiency of electron-hole pairs, and a stronger transfer to Zr6O4(OH)4 SBUs via ligand-to-metal charge transfer
luminescence usually implies better recombination and low (LMCT) or dmbpydc2- linkers via ligand-to-ligand charge transfer
separation efficiency. As shown in Fig. 8c, the photoluminescent (LLCT). The combination of MLCT, LMCT and LLCT effectively
intensities of Ru-UiO-dmbpy(1) is much weaker than that of Ru- separate the photogenerated electron-hole pairs, thus largely
UiO-bpy(1), indicating the obstruction of electron-hole boosting the photocatalytic activities of Ru-UiO-dmbpy(1). The
recombination after framework ionization. Time-resolved photogenerated electrons then involves in the reduction of
luminescence was further employed to investigate the charge Cr(VI), while the photogenerated holes are scavenged by benzyl
2-
transfer process. Fig. 8d and Table S7 show the alcohol. During the photocatalytic process, anionic Cr2O7 is
photoluminescent lifetime of Ru-UiO- dmbpy(1)
preferred to be adsorbed into the cavities of cationic
frameworks while the reductive product Cr3+ is prone to being
precluded out due to the electrostatic interactions, which
effectively improve the Cr(VI) adsorption performance of Ru-
UiO-dmbpy(1). The excellent Cr(VI) adsorption performance of
Ru-UiO-dmbpy(1) also effectively boosts the photocatalytic
activities of Ru-UiO-dmbpy(1).
Fig. 9 The proposed mechanism of Cr(VI) photoreduction when Ru-UiO-dmbpy(1) acted
as the photocatalyst under visible light irradiation.
Fig. 8 (a) EIS Nyquist plots of UiO-bpy Ru-UiO-bpy(1) and Ru-UiO-dmbpy(1), respectively.
(b) Transient photocurrent response of UiO-bpy Ru-UiO-bpy(1) and Ru-UiO-dmbpy(1)
under the visible light irradiation (420 nm–780 nm). (c) Luminescent spectra of UiO-bpy
Ru-UiO-bpy(1) and Ru-UiO-dmbpy(1) in DMF solution. (d) Time-resolved luminescent
decay of Ru-UiO-bpy(1), Ru-UiO-dmbpy(1) suspensions before and after the in-situ
2-
addition of Cr2O7
.
Conclusions
suspension is 620 ns, which significantly shortens to 188 ns after
in-situ addition of Cr2O72-. Such an apparent lifetime shortening
In summary, Ru(bpy)3 functionalized and ionized MOFs, Ru-UiO-
dmbpy, was successfully prepared via the mix-and-match
synthetic strategy and ionization process. The introduction of
photosensitive Ru(bpy)3 moieties largely expanded the spectral
demonstrates that the long-lived photogenerated electrons are
2-
easily transferred from Ru(bpy)3 moieties to Cr2O7
.
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11. H. Li, L. Li, R.-B. Lin, W. Zhou, Z. Zhang, S. Xiang and B. Chen,
response from 420 nm for UiO-bpy to about 780 nm for Ru-UiO-
bpy(1)/Ru-UiO-dmbpy(1). The framework ionization endows
Ru-UiO-dmbpy(1) with highest Cr2O7 adsorption rate and
largest Cr2O72- uptake capacity via ion-exchange in comparation
View Article Online
EnergyChem, 2019, 1, 100006.
DOI: 10.1039/C9TA13339A
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to pristine UiO-bpy and its non-ionized Ru-UiO-bpy(1)
2-
counterparts. As a result of the best Cr2O7 adsorptive
performance and efficient visible-light utilization, Ru-UiO-
dmbpy(1) shows a more efficiency in Cr(VI) photoreduction
than UiO-bpy and Ru-UiO-bpy under visible light illumination.
Further investigations show that Ru-UiO-dmbpy(1) can
effectively reduce Cr(VI) to Cr(III) with a recorded reductive rate
of 13.3 mgCr(VI)/gcatalyst/min when benzyl alcohol acts as
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Acknowledgements
This work was financially supported by the National Natural
Science Foundation of China (NSFC) (21520102001), Fujian
Agriculture and Forestry University (118360020), Outstanding
Youth Research Training Program of Fujian Agriculture and
Forestry University (XJQ201616 and XJQ201815), and the State
Key Laboratory of Structural Chemistry, Fujian Institute of
Research on the Structure of Matter, Chinese Academy of
Sciences (20170028). Z.-J. Lin also acknowledges the support of
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Table of Contents
Ru-UiO-dmbpy was successfully synthesized by sequential PS introduction and
framework ionization, whose Cr(VI) photoreduction efficiency is largely boosted in
comparation with the pristine UiO-bpy and non-ionized Ru-UiO-bpy counterpart due
to the great improvement of the visible light harvesting ability and Cr(VI) adsorption
performance.