Enhanced photocatalytic activity of a B12-based catalyst co-photosensitized by TiO2 and Ru(II) towards dechlorination

Article abstract of DOI:10.1039/c7ra13037f

A novel hybrid photocatalyst denoted as B₁₂-TiO₂-Ru(ii) was prepared by co-immobilizing a B₁₂ derivative and trisbipyridine ruthenium (Ru(bpy)₃²⁺) on the surface of a mesoporous anatase TiO₂ microspheres and was characterized by DRS, XRD, SEM and BET et al. By using the hybrid photocatalyst, DDT was completely didechlorinated and a small part of tridechlorinated product was also detected in the presence of TEOA only after 30 min of visible light irradiation. Under simulated sunlight, the hybrid exhibited a significantly enhanced photocatalytic activity for dechlorination compared with B₁₂-TiO₂ under the same condition or itself under visible light irradiation due to the additivity in the contribution of UV and visible part of the sunlight to the electron transfer. In addition, this hybrid catalyst can be easily reused without loss of catalytic efficiency. This is the first report on a B₁₂-based photocatalyst co-sensitized by two photosensitizers with wide spectral response.

Full text of DOI:10.1039/c7ra13037f

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PAPER
Enhanced photocatalytic activity of a B12-based
catalyst co-photosensitized by TiO2 and Ru(II)
towards dechlorination†
Cite this: RSC Adv., 2018, 8, 662
a
a
c
Ying Sun, Wei Zhang, Tian-Yi Ma,b Yu Zhang,a Hisashi Shimakoshi,
*
c
a
*
Yoshio Hisaeda
and Xi-Ming Song
A novel hybrid photocatalyst denoted as B12–TiO2–Ru(II) was prepared by co-immobilizing a B12 derivative
and trisbipyridine ruthenium (Ru(bpy)32+) on the surface of a mesoporous anatase TiO2 microspheres and
was characterized by DRS, XRD, SEM and BET et al. By using the hybrid photocatalyst, DDT was completely
didechlorinated and a small part of tridechlorinated product was also detected in the presence of TEOA only
after 30 min of visible light irradiation. Under simulated sunlight, the hybrid exhibited a significantly
enhanced photocatalytic activity for dechlorination compared with B12–TiO2 under the same condition
or itself under visible light irradiation due to the additivity in the contribution of UV and visible part of the
sunlight to the electron transfer. In addition, this hybrid catalyst can be easily reused without loss of
Received 4th December 2017
Accepted 18th December 2017
DOI: 10.1039/c7ra13037f
catalytic efficiency. This is the first report on
a B12-based photocatalyst co-sensitized by two
rsc.li/rsc-advances
photosensitizers with wide spectral response.
catalysts based on B12 were obtained.13,14 In 2011, Hisaeda
group14 immobilized a B12 derivative and trisbipyridyl ruthe-
nium proportionally on the same polymer backbone through
radical polymerization reaction, and the obtained copolymer
catalyst showed high catalytic activity for dehalogenation reac-
tion. They proved that the electron transfer rate from the
photosensitizer to the Co center of the B12 catalyst was signi-
cantly accelerated attributing to the short distance between the
catalyst and the photosensitizer in the copolymer.
TiO2 as a semiconductor has been widely applied in various
elds such as dye-sensitized solar cell, organic pollution
degradation, organic synthesis and so on for its strong oxida-
tion and reduction abilities generated by UV light irradia-
tion.15–18 Hiseada group has successfully immobilized B12 on
P25 TiO2 or AMT600 TiO2 nanoparticles and the obtained
hybrid catalysts showed high catalytic activity for dehalogena-
tion reactions under UV light irradiation.19–22 In the hybrid
systems, TiO2 served not only as a catalyst support but also an
effective photosensitizer. In 2011, Reisner group23 reported
a self-assembled system in which a cobalt catalyst (Et3NH)
[CoIIICl(dmgH)2 (pyridyl-4-hydrophosphonate)] was attached on
a ruthenium dye [Ru(bpy)3]2+-sensitized P25 TiO2 nanoparticles.
It was proved that TiO2 played an important role as an electron
mediator in the hybrid and promoted the electron transfer from
the excited [Ru(bpy)3]2+ moieties to the cobalt catalyst under
visible light irradiation. In addition, in recent years, many
efforts have been focused on the photocatalytic applications of
mesoporous TiO2 microspheres for their high surface areas,
suitable pore structures, and uniform pore size distributions,
which could supply more active sites for adsorption and
Introduction
Photocatalysis has played a special role in both pollutant
degradation and organic synthesis from the viewpoint of eco-
friendliness in recent decades.1–5 As photocatalysts, vitamin
B12 and its derivatives have exhibited excellent photocatalytic
activity for decomposing halogenated organic compounds due
to the supernucleophilic reactivity of Co(I) species, which could
be produced from the Co(III) or Co(II) center of B12 and its
derivatives under ultraviolet light irradiation in the presence of
electron donor precursors.6–11 In 2004, Hisaeda et al.12 rstly
reported the photocatalytic activity of a heptamethyl cobyrinate
perchlorate, [Co(II)7C1ester]ClO4, in methanol for DDT dechlo-
rination under visible light irradiation in the presence of
2+
Ru(bpy)3 as photosensitizer and proved that the Co(I) species
could be formed through accepting electrons from the photo-
sensitizer under visible light irradiation. In order to establish
a more efficient, economical and environmental friendly visible-
light photocatalytic system, heterogenous catalysis has been
performed through immobilizing the B12 catalyst and a photo-
sensitizer onto catalyst supports and some recycled hybrid
aLiaoning Key Laboratory for Green Synthesis and Preparative Chemistry of Advanced
Materials, College of Chemistry, Liaoning University, Shenyang 110036, P. R. China.
E-mail: weizhanghx@lnu.edu.cn; songlab@lnu.edu.cn; Fax: +86-24-62207922; Tel:
+86-24-62207792
bSchool of Chemical Engineering, University of Adelaide, Adelaide, SA 5005, Australia
cDepartment of Chemistry and Biochemistry, Graduate School of Engineering, Kyushu
University, Motooka, Fukuoka 819-0395, Japan
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c7ra13037f
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photocatalytic reactions.24–27 Aiming to develop more efficient under vigorously stirring and then HCl (5 mol Lꢀ1) was added to
photocatalyst for dechlorination, in this paper, a novel B12
-
the mixture dropwise until much precipitate appeared. Then
based hybrid catalyst co-sensitized by TiO2 and Ru(bpy)32+, B12– the precipitate was ltered and washed with a minimal amount
TiO2–Ru(II), was prepared through immobilizing a vitamin B12 of water, CH2Cl2, Et2O respectively. Aer being dried in vacuum
1
derivative, cobyrinic acid, and a derivative of trisbipyridine for 6 h, a dark red crystal was obtained. H NMR (CD3CN), d:
ruthenium, Ru(dcb)(bpy)2(PF6)2, on the surface of mesoporous 7.39 (q, 4H), 7.68 (dd, 4H), 7.80 (d, 2H), 7.87 (d, 2H), 8.06 (q, 4H),
anatase TiO2 microspheres as shown in Fig. 1. The catalytic 8.50 (dd, 4H), 9.19 (s, 2H).
properties of this hybrid catalyst for 1,1-bis(4-chlorophenyl)-
2,2,2-trichloroethane (DDT) dechlorination under visible light
irradiation or simulated sunlight irradiation were reported.
This is the rst report on B12-based photocatalyst co-sensitized
by two photosensitizers with wide spectral response.
Preparation of B12–TiO2–Ru(II)
Mesoporous anatase TiO2 microspheres (30 mg) was added into
the methanol solution (6 mL) containing [(CN)(H2O)Cob(III)
7COOH]Cl (8 ꢁ 10ꢀ4 mol Lꢀ1) and Ru(dcb)(bpy)2(PF6)2 (1.6 ꢁ
10ꢀ3 mol Lꢀ1). Aer the mixture was stirred at room tempera-
ture for 4 h, the hybrid B12–TiO2–Ru(II) was obtained by
centrifugation and washed with methanol for three times.
Experimental
Materials
1,1-Bis(4-chlorophenyl)-2,2,2-trichloroethane (DDT), Ru(bpy)2-
Cl2, Tetrabutyl titanate, and Ru(bpy)3Cl2 was purchased from J &
K Scientic Ltd. Cyanoaqua cobyrinic acid [(CN)(H2O)Cob(III)
General catalytic procedure
A methanol solution (5 mL) containing B12–TiO2–Ru(II) (3 mg),
DDT (2.4 ꢁ 10ꢀ3 mol Lꢀ1) and triethanolamine (0.2 mol Lꢀ1
)
7COOH]Cl,28
[(CN)(H2O)Cob(III)7C1ester]Cl,19
mesoporous
anatase TiO2 microspheres,25 4,40-diethyl ester-2,20-bipyridine
(deep) and Ru(dcb)(bpy)2(PF6)2 (ref. 29) were synthesized
according to the literatures. Other reagents were purchased
from Sinopharm Chemical Reagent Beijing Co. (SCRC). All the
reagents were of analytical grade and were used as received
without further purication.
was degassed aer three of freeze–pump–thaw circles. Aer the
solution was irradiated for 0.5 h using a xenon lamp with a l $
420 nm optical lter and a heat cut-off lter or a AM 1.5 optical
lter, the catalyst was separated by centrifugation. Methanol
was evaporated under reduced pressure and the residue was
dissolved in CHCl3 (5 mL). The CHCl3 layer was extracted with
water (10 mL) to remove TEOA, and then the solvent was
evaporated under reduced pressure to dryness. The residues
were analyzed by 1H NMR using 1,4-dioxane as the internal
standard. The hybrid catalyst B12–TiO2–Ru(II) was washed with
methanol and then reused aer adding fresh triethanolamine
and the substrate.
Characterization
The SEM images and EDS analysis were acquired using a Hita-
chi SU-8010 equipped with an EDX analyzer operated at an
accelerating voltage of 5 kV. The TEM images were obtained on
a JEM 2100 operating at 200 kV. The UV-vis diffuse reectance
spectroscopy (DRS) measurements were obtained on a UV-vis
spectrometer (Shimadzu UV-2550) using BaSO4 as a reference
standard. The specic surface area of the samples was
measured by the Brunauer–Emmett–Teller (BET) method using
nitrogen adsorption and desorption isotherms on a Micro-
metrics ASAP 2020 system. The UV-vis spectra were obtained on
a Perkin Elmer Lamda 25 spectrophotometer. The NMR spectra
were recorded on a Mercury Vx-300 MHz NMR spectrometer.
XRD patterns were obtained on a Bruker D8-Advance. The
luminescence spectra were measured using a Hitachi F-4500
spectrophotometer in MeOH at room temperature. A 500 W
xenon lamp (CHFXQ 500 W, Global xenon lamp power) with a l
$ 420 nm optical lter, AM 1.5 optical lter and a heat cut-off
lter provided visible light or simulated sunlight illumination.
Results and discussion
Fig. 1 illustrates the preparation procedure of the hybrid B12
–
TiO2–Ru(II). The mesoporous anatase TiO2 microspheres were
rstly synthesized by hydrothermal reaction and then the
catalyst [(CN)(H2O)Cob(III)7COOH]Cl and the photosensitizer
Ru(dcb)(bpy)2(PF6)2 were graed on the surface of mesoporous
anatase TiO2 microspheres through their carboxylic groups only
at room temperature. Aer the decoration, the colour of TiO2
microspheres changed from white to saffron yellow as shown in
Fig. 1. The Co and Ru contents in the hybrid B12–TiO2–Ru(II)
were 6.83 ꢁ 10ꢀ5 mol gꢀ1 and 1.5 ꢁ 10ꢀ4 mol gꢀ1, which were
determined through detecting the absorbance change of the
characteristic peaks of [(CN)(H2O)Cob(III)7COOH]Cl and
Ru(dcb)(bpy)2(PF6)2 at 523 nm and 480 nm respectively in the
supernatant by UV-vis spectra. The Co content in this hybrid
Synthesis of Ru(dcb)(bpy)2(PF6)2
Ru(dcb)(bpy)2(PF6)2 was synthesized according to the litera- was obviously increased and was 5 times higher than that of the
ture29 with slight modication. Ru(bpy)2Cl2$2H2O (75 mg, 0.15 hybrid B12–TiO2 (AMT600) reported in the literature,22 which
mmol) and 4,40-diethyl ester-2,20-bipyridine (44 mg, 0.15 mmol) should be attributed to the large surface area and pores of the
were dissolved in ethanol (15 mL) and the mixture reuxed for mesoporous anatase TiO2 microspheres. In addition, it is worth
8 h. The solution was allowed to cool, ltered, and taken to to mention that the ratio of Co and Ru in the hybrid was about
dryness. The residue was dissolved in NaOH solution 1 : 2.2 which was similar to their ratio 1 : 2 in the raw solution
(0.3 mol Lꢀ1, 15 mL) and reuxed for 1 h and cooled to room before adding TiO2. This result indicated that the proportional
temperature. A large excess of NH4PF6 was added to the solution co-immobilization of these two different molecules, [(CN)(H2O)
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Fig. 1 Illustration of the preparation of the hybrid B12–TiO2–Ru(II).
Cob(III)7COOH]Cl and Ru(dcb)(bpy)2(PF6)2, on the surface of absorption from 400 nm to 600 nm shown by the hybrid B12
–
TiO2 microspheres could be easily realized by one pot reaction. TiO2–Ru(II) fully indicated that the B12 catalyst and the Ru(II)
In this hybrid, the Ru(II) complexes and the TiO2 microspheres photosensitizer had been co-immobilized on the surface of
as photosensitizer are expected to be electron donors for B12 mesoporous anatase TiO2 microspheres successfully. The
activation to form reactive Co(I) species.
hybrid could absorb not merely ultraviolet but also visible light.
Fig. 2 shows the UV-vis diffuse reectance spectra of the
Fig. 3 gives the XRD patterns of the hybrid B12–TiO2–Ru(II)
hybrid B12–TiO2–Ru(II) and mesoporous anatase TiO2 micro- and the raw material mesoporous anatase TiO2 microspheres.
sphere and the UV-vis spectra of [(CN)(H2O)Cob(III)7COOH]Cl The as-prepared TiO2 exhibited seven obviously diffraction
and Ru(dcb)(bpy)2(PF6)2 in methanol. The mesoporous anatase peaks at 2q ¼ 25.4ꢂ, 37.9ꢂ, 48.1ꢂ, 55.0ꢂ, 62.8ꢂ, 68.8ꢂ and 75.0ꢂ,
TiO2 microspheres strongly absorbed ultraviolet light and had which corresponding to the (101), (004), (200), (211), (204), (116)
very weak absorption aer 400 nm.25 The catalyst [(CN)(H2O) and (215) crystal faces of anatase TiO2 according to the JCPDS
Cob(III)7COOH]Cl exhibited
a
dominant absorption peak No. 21-1272.25 The sharp diffraction peaks indicated that the
located at lmax ¼ 490 nm accompanied by a prominent shoulder crystallinity of the anatase TiO2 was relatively high. Aer the
located at 523 nm and a small absorption peak at 404 nm.28 modication with the two functional molecules, the shape and
Ru(dcb)(bpy)2(PF6)2 exhibits a broad characteristic absorptions the intensity of the diffraction peaks did not change signi-
at lmax ¼ 480 nm accompanied a small shoulder at 423 nm, cantly, conrming that the crystallinity of the mesoporous
which coincided with the literature.29 Comparing with the UV- anatase TiO2 microspheres was well kept.
vis spectra of the above reference compounds, the broad
Fig. 2 UV-vis diffuse reflectance spectra of B12–TiO2–Ru(II) and TiO2,
and
UV-vis
spectra
of
[(CN)(H2O)Cob(III)7COOH]Cl
and Fig. 3 XRD patterns of the hybrid B12–TiO2–Ru(II) and the raw material
Ru(dcb)(bpy)2(PF6)2 in methanol.
mesoporous anatase TiO2 microspheres.
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The morphologies and structures of the mesoporous anatase catalytic activity in a heterogeneous catalytic system. Therefore,
TiO2 microspheres and the hybrid B12–TiO2–Ru(II) were char- the dispersity of the mesoporous TiO2 microspheres and the
acterized by SEM and TEM as shown in Fig. 4. Before or aer the hybrid B12–TiO2–Ru(II) in methanol was evaluated and the
modication, the support TiO2 exhibited good spherical pictures were taken aer standing for 1 h at room temperature.
morphology with an uniform size of about 500 nm in diameter As shown in Fig. 6, both the mesoporous anatase TiO2 micro-
and each TiO2 microsphere was stacked by innumerable inter- spheres and the hybrid B12–TiO2–Ru(II) dispersed well in
connected nanocrystals. Compared with TiO2 nanocrystals (P25 methanol though their diameter were larger than that of the P25
or AMT600), the larger particle diameter could make them more or AMT600.
easily separated from the dispersion and recycled.
Nitrogen adsorption–desorption isotherms and the Barrett–
The chemical composition of the hybrid B12–TiO2–Ru(II) was Joyner–Halenda (BJH) pore-size distribution analysis are usually
also determined by EDS as shown in Fig. 5. The element Ti was employed to detect structural information for porous materials.
derived from TiO2, while the Ru, P and F peaks arose from the The nitrogen adsorption–desorption isotherms at 77 K of the
photosensitizer Ru(dcb)(bpy)2(PF6)2, Co peak originated from mesoporous anatase TiO2 microspheres and the hybrid B12
–
[(CN)(H2O)Cob(III)7COOH]Cl and the C and N peaks originated TiO2–Ru(II) are shown in Fig. 7. The as-prepared TiO2 micro-
from both Ru(dcb)(bpy)2(PF6)2 and [(CN)(H2O)Cob(III)7COOH] spheres showed an apparent capillary condensation step at
Cl. The element O was originated from Ru(dcb)(bpy)2(PF6)2, a relative pressure (P/P0) of 0.5–0.85, which was corresponded to
TiO2 and [(CN)(H2O)Cob(III)7COOH]Cl.
a pore size of 8 nm obtained by the BJH method. Their BET
High dispersity of a heterocatalyst in the reaction medium specic surface area was 172 m2 gꢀ1, which is three times that of
could signicantly increase the effective collision probability P25 (ref. 19) and AMT600,21 and the total pore volume was
between the catalyst and the substrate and thus promote its 0.64 cm3 gꢀ1. From the isotherms of the hybrid B12–TiO2–Ru(II),
it can be conrmed that the pore size of the hybrid reduced to
7 nm and the BET specic surface area and the total pore
volume decreased to 159 m2 gꢀ1 and 0.55 cm3 gꢀ1 respectively,
which should be attributed to the modication of the B12
catalyst [(CN)(H2O)Cob(III)7COOH]Cl and the photosensitizer
Ru(dcb)(bpy)2(PF6)2. These data indicated that the two func-
tional molecules were immobilized not only onto the surface
but also inside the pore of the TiO2 microspheres.
Formation of the Co(I) species of the B12 complex in the
presence of TiO2 was monitored by the UV-vis spectral change in
methanol containing a sacricial reductant triethanolamine
(TEOA) under visible light irradiation (l $ 420 nm). In order to
avoid that the UV-vis spectral change of B12 in solution can't be
detected sensitively aer it was xed on the TiO2 microspheres,
Fig. 4 SEM images of TiO2 (A) and B12–TiO2–Ru(II) (B); TEM images of
TiO2 (C) and B12–TiO2–Ru(II) (D).
Fig. 6 Dispersions of TiO2 (a) and B12–TiO2–Ru(II) (b) in methanol after
Fig. 5 EDS spectrum of the hybrid B12–TiO2–Ru(II).
standing for 1 h at room temperature.
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Fig. 7 Nitrogen adsorption–desorption isotherms measured at 77 K and the pore-size distribution (inset) of the mesoporous anatase TiO2
microspheres (left) and the hybrid B12–TiO2–Ru(II) (right).
the compounds [(CN)(H2O)Cob(III)7C1ester]Cl and Ru(bpy)3Cl2 the distance among B12, TiO2 and Ru(II) would be sharply
without carboxylic groups (see Fig. S1, ESI†), were used instead shortened and the electron transfer from the photosensitizer to
of [(CN)(H2O)Cob(III)7COOH]Cl and Ru(dcb)(bpy)2(PF6)2 the Co center of the B12 catalyst via TiO2 is expected to be further
respectively. As shown in Fig. 8a, the mixture of [(CN)(H2O) accelerated under visible light irradiation.
Cob(III)7C1ester]Cl, Ru(bpy)3Cl2 and TiO2 produced three char-
The visible light photocatalytic activity of the hybrid B12–
acteristic peaks at 352 nm, 463 nm and 542 nm respectively. TiO2–Ru(II) for DDT dechlorination was investigated rstly
Comparing to the UV-vis absorption spectra of [(CN)(H2O) using triethanolamine as the electron sacricer in methanol.
Cob(III)7C1ester]Cl and Ru(bpy)3Cl2 (see Fig. S2, ESI†), the The results are summarized in Table 1. When using the hybrid
absorbance at 352 nm and 542 nm was assigned to the Co(III) B12–TiO2–Ru(II) as catalyst, DDT conversion was 65% only aer
species of the B12 complex and the peak at 463 nm was origi- 10 min of visible light irradiation and four kinds of dechlori-
nated from both Co(III) and Ru(II) before irradiation. Aer irra- nated products were obtained including one mono-
diation for 25 minutes, one new peak at 392 nm was clearly dechlorinated
product
1,1-bis(4-chlorophenyl)-2,2-
observed, which was typical for the Co(I) state of the B12 dichloroethane (DDD, 35% yield), three di-dechlorinated
complex,13 and simultaneously the peaks at 352 nm and 542 nm products 1,1-bis(4-chlorophenyl)-2-chloroethylene (DDMU, 4%
disappeared and the absorbance at 463 nm decreased. In yield)
and
1,1,4,4-tetrakis(4-chlorophenyl)-2,3-dichloro-2-
contrast, a slow Co(I) formation was observed in the mixture of butene (TTDB (E/Z), 9% yield), one tri-dechlorinated product
the B12 catalyst and the photosensitizer without TiO2 (Fig. 8b). methyl 2,2-bis(4-chlorophenyl) acetate (DDA methyl ester, 3%
Through the comparison, it was proved that the introduction of yield) (entry 1 in Table 1). When the irradiation time was
TiO2 can efficiently promote the formation of Co(I) species, increased to 30 min, not only DDT was totally converted, but
attributing to the semiconductor property of TiO2 as an electron also the mono-dechlorinated product DDD disappeared
mediator, not only accelerating the electron transfer from the completely. Accordingly, the yields of the di- and tri-
Ru(II) photosensitizer to the Co center of the catalyst, but also dechlorinated products rose and one more di-dechlorinated
effectively suppressing the back electron-transfer process. product 1,1-bis(4-chlorophenyl)-2-chloroethane (DDMS) was
When both the B12 catalyst and the Ru(II) photosensitizer were also found (entry 2 in Table 1). TTDB (E/Z) were the products of
graed on the TiO2 microspheres to give a new hybrid catalyst, coupling reaction between two didechlorinated carbene
Fig. 8 UV-vis spectral change of [(CN)(H2O)Cob(III)7C1ester]Cl (5 ꢁ 10ꢀ6 mol Lꢀ1) with TiO2 (a) and without TiO2 (b) in the presence of
Ru(bpy)3Cl2 (5 ꢁ 10ꢀ6 mol Lꢀ1) and TEOA (0.2 mol Lꢀ1) by visible light irradiation (l $ 420 nm) under N2 atmosphere in methanol.
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Table 1 Photocatalytic dechlorination of DDT catalyzed by B12–TiO2–Ru(II) under visible light irradiationa
Product yieldb (%)
Conversion
Entry
Catalytic system
Irradiation time(min)
(%)
DDD
DDMU
DDMS
TTDB (E/Z)
DDA
TONc
TOFc (hꢀ1
)
1
2
B
B
B
12–TiO2–Ru(II)
12–TiO2–Ru(II)
12–TiO2–Ru(II)
10
30
30
30
30
30
30
30
65
35
0
—
—
16
7
4
40
—
—
—
—
—
Trace
Trace
13
—
—
—
—
—
Trace
9
3
6
52
119
0
0
9
—
5
30
312
238
0
100
Trace
0
24
13
20
—
—
—
—
—
10
3d
4e
5f
—
—
—
—
—
Trace
TiO2
0
B12–TiO2
18
—
10
60
6g
7h
8e,h
Ru(II)–TiO2
B12, Ru(bpy)3
2+
11
42
10
21
B12, Ru(bpy)32+, TiO2
a
B
12–TiO2–Ru(II) ¼ 3 mg, [DDT] ¼ 2.4 ꢁ 10ꢀ3 mol Lꢀ1, [TEOA] ¼ 0.2 mol Lꢀ1, MeOH: 5 mL, irradiation: l $ 420 nm, 50 mW cmꢀ2, distance: 10 cm.
b
DDT conversion and the product yields were determined by 1H HMR. c Turnover numbers (TON) and the turnover frequency (TOF) were based on
d
e
f
g
h
the concentration of B12 and the product yields. In the dark. TiO2 ¼ 3 mg.
B
12–TiO2 ¼ 3 mg. Ru(II)–TiO2 ¼ 3 mg. [Ru(bpy)3Cl2] ¼ 1.1 ꢁ
10ꢀ4 mol Lꢀ1, [(CN)(H2O)Cob(III)7C1ester]Cl ¼ 4.9 ꢁ 10ꢀ5 mol Lꢀ1
.
intermediates,12 therefore di-dechlorinated products obtained here indicated that aer the introduction of TiO2 to the simple
with a high yield of 93% aer 30 min of irradiation, which was mixture, especially aer the co-immobilization of the B12 cata-
the highest among the B12-based photocatalytic systems re- lyst and the Ru(II) photosensitizer on TiO2 microspheres, reac-
ported in the literatures. In addition, from the data it can be tion efficiency was signicantly enhanced compared to that of
concluded that DDD could be further degraded as soon as it was the simple mixture system, which should be attributed to the
produced in such a hybrid catalytic system. The reaction did not very important role of the mesoporous anatase TiO2 micro-
proceed under dark conditions or using TiO2 instead of the spheres not only as an adsorption reagent for the B12 catalyst,
hybrid B12–TiO2–Ru(II) (entries 3 and 4 in Table 1). When we the photosensitizer and the substrate DDT or DDD etc. but also
used the hybrid B12–TiO2 or Ru(II)–TiO2 (see Fig. S3–S5, ESI†) as an electron mediator.
alone (entries 5 and 6 in Table 1), the DDT conversion decreased
TiO2 and the Ru(II) moieties in the hybrid could be excited at
to 24% and 13% respectively and DDD was detected as the the same time under sunlight irradiation because they absorb
single product aer 30 min of irradiation. In contrast to the light in UV and visible region respectively. Therefore, the hybrid
result in the hybrid system (entry 2 in Table 1), the DDT B12–TiO2–Ru(II) should exhibited enhanced photocatalytic
conversion was only 11% with DDD as the single product when activity for dechlorination under sunlight irradiation. To eval-
a 1 : 2.2 mixture of the B12 catalyst [(CN)(H2O)Cob(III)7C1ester]Cl uate the co-photosensitized effect of TiO2 and the Ru(II) complex
2+
and the photosensitizer Ru(bpy)3 was used under the same on the B12 catalyst, DDT dechlorination was subsequently
experimental conditions (entry 7 in Table 1). While adding carried out under simulated sunlight (in Table 2). Aer 5 min of
a certain amount of mesoporous TiO2 microspheres into the simulated sunlight irradiation, the DDT conversion was 49%
simple mixture system (entry 8 in Table 1), the DDT conversion and ve dechlorinated products including DDD, DDMU, TTDB
was increased to 42% and the di-dechlorinated products TTDB (E/Z) and DDA were found when using the hybrid B12–TiO2
(E/Z) were found with 10% yield except DDD. The results shown alone as catalyst (entry 2 in Table 2). While using the hybrid B12
–
Table 2 Photocatalytic dechlorination of DDT catalyzed by B12–TiO2–Ru(II) under simulated sunlight irradiationa
Product yieldb (%)
Irradiation time
Entry (%)
Catalyst
(min)
Conversion
DDD
DDMU
DDMS
TTDB (E/Z)
DDA
TONc
TOFc (hꢀ1
)
1
2
3
4
B
B
B
B
12–TiO2–Ru(II)
12–TiO2
12–TiO2–Ru(II)
12–TiO2
5
5
10
10
70
49
100
74
22
13
29
25
4
2
15
2
2
17
12
19
20
4
2
5
4
67
41
173
70
804
492
1038
42
Trace
5
Trace
a
B
12–TiO2–Ru(II) ¼ 3 mg, B12–TiO2 ¼ 3 mg, [DDT] ¼ 2.4 ꢁ 10ꢀ3 mol Lꢀ1, [TEOA] ¼ 0.2 mol Lꢀ1, MeOH: 5 mL, irradiation illuminant: simulated
sunlight, 50 mW cmꢀ2, distance: 10 cm. DDT conversion and the product yields were determined by 1H HMR. Turnover numbers (TON) and
b
c
the turnover frequency (TOF) were based on the concentration of B12 and the product yields.
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Table 3 Recycled catalysis of B12–TiO2–Ru(II)a
Product yield (%)b
Conversion
(%)
Entry
1
Irradiation illuminant (irradiation time)
Visible light (30 min)
Cycle
DDD
DDMU
DDMS
TTDB (E/Z)
DDA
1
2
3
1
2
3
100
100
100
70
69
68
0
0
0
22
23
23
40
38
39
4
3
3
13
14
12
2
1
1
20
19
19
17
17
16
6
7
6
4
3
3
2
Simulated sunlight (5 min)
a
B
12–TiO2–Ru(II) ¼ 3 mg, [DDT] ¼ 2.4 ꢁ 10ꢀ3 mol Lꢀ1, [TEOA] ¼ 0.2 mol Lꢀ1, MeOH: 5 mL, irradiation conditions: 50 W cmꢀ2, distance: 10 cm.
b
1
DDT conversion and the product yields were determined by H HMR.
TiO2–Ru(II) instead, the DDT conversion increased to 70% and catalytic activity aer 3 recyclings even at lower conversion. The
one more product DDMS were detected (entry 1 in Table 2). recovered B12–TiO2–Ru(II) aer 3 recyclings irradiated by visible
When the irradiation time was prolonged to 10 min, the DDT light was characterized by UV-vis spectrum and SEM respectively
conversion reached 100% in the hybrid B12–TiO2–Ru(II) system in Fig. 9, showing that the morphology and the optical property
(entry 3 in Table 2), which was still 1.4 times that of the hybrid of the hybrid had no distinct change compared with its original.
B12–TiO2 system (entry 4 in Table 2) and 1.5 times that of itself
Finally, we investigated the photosensitizing mechanism of
under visible light irradiation (entry 1 in Table 1). Comparing the Ru(II) complex in the hybrid. In general, reductive and
the data, the effect of co-photosensitization was clearly
observed. TiO2 was activated by the UV part of simulated
sunlight and then transferred the electrons directly to the Co(I)
2+
center of the B12 catalyst, while at the same time the Ru(bpy)3
moieties in the hybrid were excited by the visible part and
transferred its electrons to the Co(I) center through TiO2. In
other words, the hybrid B12–TiO2–Ru(II) realized the simulta-
neous utilization of both UV and visible part of the sunlight with
high efficiency and the contribution of UV and visible part of the
sunlight to the electron transfer has additivity.
Furthermore, the recycled catalysis of this hybrid catalyst B12–
TiO2–Ru(II) under visible light irradiation and simulated sunlight
irradiation was performed respectively since it can be easily
separated from the reaction system by centrifugation and the
relevant data were shown in Table 3. According to the data, this
hybrid catalyst still kept high catalytic activity aer 3 recyclings
under 30 min of visible-light irradiation (entry 1 in Table 3). The
recyclability of the hybrid B12–TiO2–Ru(II) at lower conversion
Fig. 10 The steady-state emission of Ru(bpy)3Cl2 before and after
has also been investigated under 5 min of simulated sunlight
irradiation (entry 2 in Table 3). These data indicated that this
hybrid catalyst had good stability and recyclability, and kept its
adding [(CN)(H2O)Cob(III)7C1ester]Cl and TiO2 in methanol.
([Ru(bpy)3Cl2] ¼ [[(CN)(H2O)Cob(III)7C1ester]Cl] ¼ 4.7 ꢁ 10ꢀ5 mol Lꢀ1
,
TiO2 ¼ 0.2 g Lꢀ1, lex ¼ 450 nm).
Fig. 9 UV-vis spectrum (left) and SEM (right) of B12–TiO2–Ru(II) after 3 recyclings irradiated by visible light.
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Fig. 11 Proposed mechanism for DDT dechlorination catalyzed by B12–TiO2–Ru(II) under simulated sunlight.
oxidative quenching mechanisms exist in the complex part was tridechlorinated only aer 30 min of visible light
2+
Ru(bpy)3 when it is activated by visible light irradiation.30 To irradiation. The catalytic activity was much higher than that of
elucidate the quenching mechanism, the steady-state emission the other reported B12-based photocatalytic systems. Especially,
from the excited state of the Ru(II) complex before and aer under simulated sunlight, the hybrid exhibited a signicantly
adding the B12 complex and TiO2 was measured, as shown in enhanced photocatalytic activity for dechlorination compared
Fig. 10. Aer exciting the metal to ligand charge transfer (MLCT) with B12–TiO2 at the same condition or itself under visible light
transition band of the Ru(II) complex at 450 nm, a strong irradiation attributing to the additivity in the contribution of UV
emission at 600 nm was observed, while the complex [(CN)(H2O) and visible part of the sunlight to the electron transfer. In
Cob(III)7C1ester]Cl showed no emission at this position. Aer addition, this hybrid catalyst can be easily reused without loss
adding [(CN)(H2O)Cob(III)7C1ester]Cl to the solution of of catalytic efficiency. Therefore, the presented design strategy
Ru(bpy)32+, the emission intensity of the Ru(II) complexes was for photocatalytic system is efficient in full utilization of solar
only slightly decreased, but if further adding a small amount of energy and it would be readily applicable to the design of an
TiO2 microspheres, the excited state of the Ru(II) complex was eco-friendly catalyst.
quenched obviously, which indicated that electron transfer
2+
should proceed efficiently between the excited state Ru(bpy)3
*
Conflicts of interest
and TiO2 microspheres, which is consistent with the litera-
ture.31 Therefore, the so-called oxidative quenching mechanism
could be expected for the visible-light-driven catalytic reaction
in this hybrid system.
There are no conicts to declare.
The photocatalytic mechanism of the hybrid system under Acknowledgements
sunlight was illustrated in Fig. 11. The photosensitizer
2+
We are grateful for nancial support from Science Foundation
Ru(bpy)3 was excited by visible light irradiation and then
injected one electron to the conduction band of the support
TiO2 microspheres, and simultaneously, some electrons in the
valence band of TiO2 moved up to the conduction band under
UV light irradiation. Subsequently, the Co(III) center of the B12
derivative accepted two electrons from the conduction band of
TiO2 and was reduced to the Co(I) species. The super-
nucleophilic Co(I) species had a high reactivity with organic
halides to induce the oxidative addition of the alkylating agents
to the metal center with dehalogenation.19,32 The obtained
of Educational Department of Liaoning Province (LYB201604)
and the National Natural Science Foundation of China
(51773085, 21203082).
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