Towards applications in catalysis: Investigation on photocatalytic activities of a derivative family of the Keplerate type molybdenum-oxide based polyoxometalates

Article abstract of DOI:10.1039/c4ra07700h

The photocatalytic potential of a derivative family of the Keplerate type nano-porous Mo-O based polyoxometalates with general formula [Mo₇₂^VIMo₆₀^VO₃₇₂(L)₃₀(H₂O)₇₂]^n- (L = CH₃COO^-, SO₄^2-) (denoted Mo₁₃₂) has been evaluated by a prototype photocatalytic decoloration reaction of aqueous rhodamine B. The Mo₁₃₂ anions are found to be photocatalytically active centers, however they are unstable and subjected to decomposition in the solution form. The introduction of organic counter cations such as tetrabutylammonium (n-Bu₄N⁺) and dioctadecyldimethylammonium (DODA⁺) can endow significant stability to the giant anions during the catalytic process. TOC changes and GC/MS measurements were done to identify the degradation products. Among the derivatives, compound 2 composed of n-Bu₄N⁺ and Mo₁₃₂ (L = CH₃COO^-) has been found to be the most active one towards the photocatalytic decoloration of RhB solution. The analytical mechanism indicates that both OH radicals and ¹O₂ participate in the photo degradation process. This journal is

Full text of DOI:10.1039/c4ra07700h

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Towards applications in catalysis: investigation on
photocatalytic activities of a derivative family of the
Keplerate type molybdenum–oxide based
polyoxometalates†
Cite this: RSC Adv., 2014, 4, 54928
*
*
Yunshan Zhou, Libo Qin, Chao Yu, Tian Xiong, Lijuan Zhang, Waqar Ahmad
and Hao Han
The photocatalytic potential of a derivative family of the Keplerate type nano-porous Mo–O based
V
nꢀ
2ꢀ
CH3COOꢀ, SO4
)
polyoxometalates with general formula [Mo72VIMo60 O372(L)30(H2O)72
]
(L
¼
(denoted Mo132) has been evaluated by a prototype photocatalytic decoloration reaction of aqueous
rhodamine B. The Mo132 anions are found to be photocatalytically active centers, however they are
unstable and subjected to decomposition in the solution form. The introduction of organic counter
cations such as tetrabutylammonium (n-Bu4N+) and dioctadecyldimethylammonium (DODA+) can
endow significant stability to the giant anions during the catalytic process. TOC changes and GC/MS
measurements were done to identify the degradation products. Among the derivatives, compound 2
composed of n-Bu4N+ and Mo132 (L ¼ CH3COOꢀ) has been found to be the most active one towards
the photocatalytic decoloration of RhB solution. The analytical mechanism indicates that both OH
radicals and 1O2 participate in the photo degradation process.
Received 28th July 2014
Accepted 7th October 2014
DOI: 10.1039/c4ra07700h
www.rsc.org/advances
magnetic,9 optical properties,10 water decontamination,11 envi-
ronment related carbon dioxide picking and so on.12
Introduction
On the other hand, it is well known that polyoxometalates
have been widely used as acids, reductant–oxidants, photo-
chemical or electrochemical catalysts for industrial organic
reactions13,14 because of their easily adjustable catalytic prop-
erties, high thermal stability, non-toxicity and environment
friendly character. Being new members of polyoxometalates, the
Keplerate type POMs have also been expected to function as
catalysts. Indeed, interesting catalytic activities like reversible
cleavage and methyl tert-butyl ether synthesis,15 regioselective
Huisgen reactions,16 and selective oxidation of suldes under
conned conditions17 have already been reported. However,
Polyoxometalates (POMs) constitute an enormous class of
inorganic clusters, which can be characterized by a fascinating
variety of architectures, topologies and excellent physicochem-
ical properties including strong Brønsted acidity, high proton
mobility, fast multi-electron transfer, high solubility in various
solvents and resistance to the hydrolytic and oxidative degra-
dations in solutions.1,2 Bearing in mind that increasing the size
and complexity of POMs can generate variable functionality of
interest, the unique nano-sized porous POMs of Keplerate type
V
nꢀ
with general formula [Mo72VIMo60 O372(L)30(H2O)72
]
(L ¼
CH3COOꢀ, SO42ꢀ, HCOOꢀ, et al.) (denoted Mo132, Scheme 1)
3,4
¨
have been designed and synthesized by the Muller's group.
The Keplerates are composed of 72 MoVI and 60 MoV, that
constitute a capsule of 2.9 nm, 20 {Mo9O9} pores and a cavity
connected outside via 20 channels, these inner shells can be
3ꢀ
modied by various ligands such as CH3COOꢀ, HPO2ꢀ, PO4
and SO42ꢀ, which correspond to different anionic and cationic
charges in the charge balance. So far, a variety of applications
for such unique POMs have been found in elds like
nano-separation chemistry,5 cell simulation,6 materials,7,8
State Key Laboratory of Chemical Resource Engineering, Institute of Science, Beijing
University of Chemical Technology, Beijing 100029, P. R. China. E-mail: zhouys@
mail.buct.edu.cn; Fax: +86-10-64414640; Tel: +86-10-64414640
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ra07700h
Scheme 1 Polyhedral representation of the structure of the anion
[Mo72VIMo60 O372(L)30(H2O)72
]
nꢀ (L ¼ CH3COOꢀ, SO42ꢀ, HCOOꢀ, .).
V
Mo(VI) atoms are shown in blue and green, and Mo(V) atoms in red.
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photocatalytic behaviour of the Keplerate type POMs, which as on the degradation effect. The light intensity was measured by a
expected is very interesting and important both from a theo- radiometer (FZ-A, Photoelectric instrument factory of Beijing
retical and practical view, still remains unexplored.
Normal University, China).
In this paper, the photocatalytic potential of the nano-porous
V
Keplerates type POMs, namely, ((NH4)42[Mo72VIMo60 O372
-
Synthesis of the compounds
(CH3COO)30(H2O)72]$ca. 300H2O$ca. 10CH3COONH4 1 (ref. 3),
V
(NH4)42[Mo72VIMo60 O372(CH3COO)30(H2O)72]$ca. 300H2O$ca.
V
(NH4)18(nBu4N)24[Mo72VIMo60 O372(H2O)72(CH3COO)30]$ca.
10CH3COONH4 (compound 1) was synthesized as described in
V
7CH3COONH4$ca. 173H2O 2 (ref. 18), (NH4)72[Mo72VIMo60 O372
-
literature.3 Elemental analysis (%) calcd: C 3.36, H 3.78, N 2.55,
(SO4)30(H2O)72]$ca. 200H2O
3 (ref. 4), (NH4)44(n-Bu4N)28
Mo 44.25; found: C 3.45, H 3.69, N 2.63, Mo 43.96. IR (n/cmꢀ1
,
V
[Mo72VIMo60 O372(SO4)30(H2O)72]$ca. 210H2O 4 and (DODA)40
KBr): 3424 (m), 3170 (m), 2958 (w, (nas(C–H)), 2862 (w, ns(C–H)),
1622 (m, n(H2O)), 1546 (m, n(COOꢀ)), 1441 (sh), 1403 (m,
ns(COOꢀ), nas(NH4+)), 969 (m), 934 (w–m, Mo]O), 855 (m), 792
(s), 723 (s), 629 (w), 568 (s) (Fig. S1†).
V
(NH4)2[Mo72VIMo60 O372(CH3COO)30(H2O)72]$ca. 50H2O 5 (ref. 8)),
were investigated via degradation of a rhodamine B(RhB)
solution (difficult to degrade completely by conventional
methods and also suspected to be carcinogenic), which is oen
used as a prototype to examine the catalytic behavior of a
photocatalyst.
(NH4)18(n-Bu4N)24[Mo72VIMo60 O372(CH3COO)30(H2O)72$ca.
V
7CH3COONH4$ca. 173H2O (compound 2) was prepared as
described in literature.18 Elemental analysis (%) calcd: C 17.61,
H 4.27, N 2.20, Mo 40.53; found: C 17.41, H 4.20, N 2.23, Mo
40.62. IR (n/cmꢀ1, KBr): 3430 (m), 3173 (m), 2968 (m, nas(C–H)),
2870 (m, ns(C–H)), 1623 (m, n(H2O)), 1557 (m, n(COOꢀ)),
Experimental
Materials and instruments
1440 (sh), 1404 (m, ns(COOꢀ), nas(NH4 )), 978 (m), 951 (w–m)
+
All chemicals and reagents were of analytical grade and used
without further purication. Commercially available photo-
catalyst TiO2 (Aladdin reagent, EP grade, average size 0.2–0.4
mm) was used as received. IR spectra were recorded on a
MAGNA-IR 750 (Nicolet) spectrophotometer with KBr pellets in
the 400–4000 cmꢀ1 region. Thermogravimetric analyses were
carried ꢀo1ut on a NETZSCH STA 449C unit at a heating rate of 5
ꢁC min under air atmosphere. Elemental analyses for C, H
and N were performed on a Perkin-Elmer 240C analytical
instrument. Analyses for Mo were performed by the SPECTRO
ARCOS type inductively coupled plasma spectrometer. UV-vis
absorption spectra were recorded on a SHIMADZU UV-2550
spectrometer. GC/MS analyses were carried out on a Shi-
madzu GC/MS-QP2010 instrument equipped with a DB-5MS
(Mo]O), 855 (m), 801 (s), 732 (s), 633 (w), 576 (s) (Fig. S1†).
V
(NH4)72[Mo72VIMo60 O372(SO4)30(H2O)72]$ca.
200H2O
(compound 3) was synthesized as described in literature.4
Elemental analysis (%) calcd: H 3.03, N 3.64, Mo 45.72; found: H
2.96, N 3.54, Mo 45.64. IR (n/cmꢀ1, KBr): 3418 (s), 3172 (s), 1629
(m, n(H2O)), 1440 (sh), 1401 (m, nas(NH4+)), 1185 (w), 1134 (w),
1041 (w, nas(SO42ꢀ), 966 (m), 849 (m), 795 (s), 720 (s), 630 (w), 567
(s) (Fig. S2†).
V
(NH4)44(n-Bu4N)28[Mo72VIMo60 O372(SO4)30(H2O)72]$ca. 210H2O
(compound 4) was prepared with the following procedure: a
15 mL aqueous solution of compound 3 (0.25 g) and a 15 mL
aqueous solution of n-Bu4NBr (0.23 g) (molar ratio compound 3:
n-Bu4NBr ¼ 1 : 72), both having a pH of ca. 3 adjusted by using
dilute CH3COOH (30%), respectively, were mixed together
under fast stirring. Aer 20 min. of stirring, the resulting brown
precipitates were ltered, washed with de-ionized water till the
negative AgNO3 test and dried in air. Elemental analysis (%)
calcd, C 15.75, H 5.15, N 2.95, Mo 37.07; found: C 15.78, H 4.28,
N 2.81, Mo 37.15; IR (n/cmꢀ1, KBr): 3418 (s), 3172 (s), 2968
(nas(C–H)), 2878 (ns(C–H)), 1641 (m, n(H2O)), 1461 (w), 1407
capillary column (30
m
ꢂ
0.25 mm). The temperature
program of the column was set as follows: at 60 ꢁC, hold time ¼
1 min; from 60 to 250 ꢁC, rate 20 ꢁC minꢀ1. The samples for GC/
MS analyses were prepared as follows: Aer the catalysts were
removed by centrifugation, the remaining aqueous solution was
collected and evaporated to near dryness under reduced pres-
sure; methanol (10 mL each time) was added and evaporated
three times to remove water completely; nally, the remaining
residue was dissolved in 0.5 mL of methanol. TOC determina-
tion of the samples (ltered through polycarbonate membranes
with an average pore diameter of 15 nm) was performed on a
Shimadzu TOC-5000A analyzer. Photoluminescence spectra
were recorded on a Hitachi F-7000 uorescence spectropho-
tometer with emission slit of 5 nm by using a 150 W xenon lamp
as the light source. ICP analyses for Mo were conducted on a
Spectro Arcos Eop Axial View Inductively Coupled Plasma
Spectrometer.
+
(m, n(NH4 )), 1166 (w), 1119 (w), 1056 (w, nas(SO42ꢀ)), 972 (m),
855 (m), 792 (s), 729 (s), 633 (w), 570 (s) (Fig. S2†).
V
(DODA)40(NH4)2[Mo72VIMo60 O372(CH3COO)30(H2O)72$ca.
50H2O (compound 5) was prepared according to the reference.8
Elemental analysis (%) calcd, C 42.49, H 7.99, N 1.32, Mo 28.35;
found: C 42.55, H 7.89, N 1.23, Mo 28.42; IR (n/cmꢀ1, KBr): 3436
(s), 2932 (nas(C–H)), 2860 (ns(C–H)), 1659 (m, n(H2O)), 1548 (m,
+
n(COOꢀ)), 1467 (w), 1410 (m, n(NH4 ), ns(COOꢀ)), 978 (m), 951
(sh), 852 (m), 801 (s), 732 (s), 630 (w), 573 (s) (Fig. S3†).
The TG-DTA curve (Fig. S4†) and thermogravimetric analysis
for compound 4 are given in the ESI† part.
During the catalytic reaction processes, all illumination
processes were performed using a BYLAB UV-III UV light
desktop equipped with a 12 W lamp (light intensity 13 mW
Catalytic reaction
cmꢀ2) of irradiation wavelength 365 nm, except one experiment RhB was dissolved in deionized water to get a 2.0–6.0 mg Lꢀ1
where a 250 W mercury lamp (light intensity 22 mW cmꢀ2) was solution and the desired pH was adjusted with dilute acetic acid
employed as a light source to test the inuence of light intensity or ammonia. Compounds 2, 4 and 5 were ground by using agate
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Table 2 Effect of pH on photocatalytic decolorizing efficiency of 50
mL of 2.0 mg Lꢀ1 RhB solution over 2.0 mg of compound 2
mortar. To 50 mL RhB solution with desired concentration in a
100 mL beaker, compounds 1–5 of desired amounts were added
under stirring, respectively. Dark treatments for 15 min were
performed to avoid the effect of adsorption of the dye before all
the measurement. Reaction samples were taken at regular
Batch
pH
Equilibrium timeb (h)
Decoloration ratio (%)
No. 1
No. 2
No. 3
No. 4
No. 5a
No. 6
6.0
5.0
4.0
3.5
3.5
3.0
3.5
4.0
3.0
6.0
<1.25
1.5
66.9
92.2
96.1
100.0
100.0
85.6
intervals to analyze the absorbance change of RhB (lmax
554 nm) using a UV-vis spectrophotometer.
¼
Absorbance and RhB concentration were governed by the
Lambert–Beer law: A ¼ log(I0/I) ¼ 3cl, where A: absorbance
(measured in 1 cm quartz cells, cmꢀ1); I0, I: intensity of light
entering and leaving the optical cell, respectively; 3: extinction
coefficient or absorptivity (L mgꢀ1 cmꢀ1); l: optical path length
(cm); c: concentration of dye (mg Lꢀ1). The absorbance change
of RhB (lmax ¼ 554 nm) corresponded to the concentration
changes of RhB, so the decoloration ratio (DC) was calculated by
the equation: DC ¼ [(A0 ꢀ At)/A0] ꢂ 100%, where A0 represents
the absorbance before the reaction, and At the absorbance at a
given time.
a
250 W mercury lamp was used as a light source instead of the BYLAB
UV-III UV light desktop with a power 12 W and an irradiation
b
wavelength of 365 nm. Time when the degradation ratio remains
unchanged.
Additionally, it was also found that both the speed and the
decoloration ratio of catalysis remarkably enhanced by
increasing the dosage of catalyst (0.5–3.0 mg) for compound 2
(Fig. S5,† Table 3) at optimal pH range (3.0–5.0).
Initial concentration of RhB also affects the decoloration
ratio; the apparent exponential decay curves were obtained for
0.5–6 mg Lꢀ1 of the RhB solution when the degradation rate was
plotted versus time (Fig. 3). It exhibits an optimum dye
concentration (2.0 mg Lꢀ1) at pH 3.5 in the presence of 2.0 mg of
compound 2. The decomposition rate slowed down with
increasing concentration of RhB solution, and the decomposi-
tion ratio reached up to ca. 80% with long reaction time of 23 h
by using 6.0 mg Lꢀ1 concentration of RhB solution. This
phenomenon suggests that when initial dye concentration is
beyond a threshold, effective light penetration is seriously
hindered and competitive absorption starts to inhibit the
photochemical formation.20 In other words, UV light cannot
penetrate into the RhB containing solution when the concen-
tration of RhB is too high. As a result, only a small percentage of
POMs can absorb UV light, which leads to the lower decolor-
ation ratio.
The experimental results showed that when compound 1
(quite soluble in water) was added to the RhB solution, the
concentration of RhB in solution was notably decreased with
the passage of time, which was demonstrated by the decrease of
the characteristic RhB peak at 554 nm. However, unlike
compound 2, compound 1 was unstable and decomposed
simultaneously during the catalysis process, which was
Results and discussion
Factors inuencing the photocatalytic effect
In the preliminary experiments, compound 2 was used as a
catalyst and it was found that RhB was remarkably degraded
under UV irradiation as compared to other conditions (see
Table 1). The results showed that UV irradiation and the catalyst
are compulsory for the degradation of RhB.
The effect of pH on photocatalytic decolorizing efficiency
using compound 2 has been done and the RhB solution was
completely decolored with a decoloration ratio of almost 100%
under optimized conditions (pH 3–5) (Table 2). The experi-
mental results strengthened the fact that acidic condition is
favorable for the degradation of RhB molecules as for other
polyoxometalates.19 However, at very low pH, brown colour was
observed in the system indicating the dissolution of compound
2, which leads to easy decomposition of the anion, while at very
high pH compound 2 is also apt to directly decompose.
The light intensity has signicant inuence on the decolor-
ation results, as almost a 100% decoloration ratio was observed
within 75 min (Table 2, Fig. 2) by using a 250 W mercury lamp,
while the same 100% decoloration ratio was achieved aer 6 h
when a BYLAB UV-III UV light desktop with power of 12 W was
used at irradiation wavelength of 365 nm under the same
reaction condition.
Table 1 Preliminary RhB degradation results under various conditionsa
Batch
Compound 2
UV light
Sunlight
Reaction time (h)
Decoloration ratio (%)
No. 1
No. 2
No. 3
No. 4
No. 5
7
7
3
7
6
6
6
6
6
1.5
3.1
52.7
92.2
1.3
7
3
7
3
3
3
3
7
3
7
7
a
Symbols 3 and 7 mean that the item is present and absent, respectively. Reaction condition: 2.0 mg of compound 2, 50 mL of 2 mg Lꢀ1 RhB
solution and pH ¼ ca. 5.1.
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Fig. 1 UV-vis absorption spectral changes of RhB during the catalytic
process (the arrow marks the increase of reaction time). Reaction
condition: pH 3.5, 2.0 mg of compound 2 and 50 mL of 2 mg Lꢀ1 RhB
solution.
Fig. 3 Effects of RhB concentration (50 mL) under pH 3.5 in the
presence of 2.0 mg of compound 2.
compared to compound 1, compound 3 decomposed more
easily in water during the catalytic process due to its higher
anionic charge, which was revealed by fast disappearance of the
characteristic absorption at 450 nm that was ascribed to the
Keplerate type POM anion (Fig. 4b). In contrast to compounds 1
and 3, the characteristic absorption of compounds 2 and 5 was
not found in the reaction solution, indicating that they are quite
insoluble in water. It can be deduced from the results that the
{Mo132} anions in the compounds are the active centers under
UV-irradiation to degrade RhB. However, the water-soluble
compounds 1 and 3 were naturally unstable in the solution
and could be decomposed completely within a certain period of
time, this means that the Keplerate type {Mo132} anions need to
be stabilized and protected in aqueous solution. In this context,
combination of {Mo132} anions and n-Bu4N+ or DODA+ results in
the formation of a new kind of heterogeneous catalysts, viz.,
compound 2, 4, and 5, where the organic cations endow
remarkable stability of the anions because of their water-
tolerant properties and hydrophobic interactions between the
alkyl chains of n-Bu4N+ or DODA+ cations in the hybrid mate-
rials in the solid state as a reported in a case for the giant wheel
system with DODA+ cations.21
Surprisingly, when the quite stable compound 5 was used as
photocatalyst, the decoloration ratio of RhB was calculated to
ca. 21% with quite a long reaction time of 23 h (Fig. S6†) under
the same conditions (50 mL of 2.0 mg Lꢀ1 RhB solution with pH
3.5, 2.0 mg of compound 5). Further, for comparison,
commercially available TiO2 was used instead of compound 2 to
degrade RhB, it was found that the decomposition ratio could
reach ca. 76%, which is inferior to compound 2, within the 4 h
reaction time under the same experimental conditions
Fig. 2 UV-vis absorption spectral changes of RhB during the catalytic
process using a 250 W mercury lamp as a light source. The arrow
marks the increase of reaction time. Reaction condition: pH 3.5,
2.0 mg of the compound 2.
recognized from the reduction of the characteristic absorption
at 450 nm that is ascribed to the Keplerate type POM anion
(Fig. 4a).3,4,7,8 Aer complete decomposition of compound 1,
absorbance intensity of the characteristic RhB peak at 554 nm
corresponding to the concentration of RhB did not change any
more. A similar situation was observed for compound 3. As
Table 3 Effect of catalyst dosage of compound 2 on photocatalytic
decolorizing efficiency with 50 mL of 2.0 mg Lꢀ1 RhB solution
Batch pH Dosage of catalyst Time (h) Decoloration ratio (%)
(Fig. S7†).
No. 1
No. 2
No. 3
No. 4
No. 5
No. 6
No. 7
3.5 0.5
3.5 1.0
3.5 1.5
3.5 2.0
3.5 3.0
4.0 1.0
4.0 2.0
6.0
6.0
6.0
6.0
3.5
6.0
6.0
22.0
50.0
88.0
100.0
99.8
62.3
V
On the other hand, considering that the [Mo72VIMo60
-
O
372(SO4)30(H2O)72
]
72ꢀ bears (ꢀ)72 charges, which is larger than
that of [Mo72VIMo60 O372(CH3COO)30(H2O)72]42ꢀ, it is inter-
esting to consider the charge inuence on the catalytic activities
of their corresponding derivatives. Therefore, compound 4
(a derivative of compound 3) was tested for RhB solution
V
100.0
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V
nꢀ
[Mo72VIMo60 O372(L)30(H2O)72
]
structures. Though the differ-
ence in the number of n-Bu4N+ cations in compounds 2 and 4 do
not look so large, the decoloration results demonstrate that
such small difference can inuence the photochemical effect
due to the large size of the n-Bu4N+. Therefore, it is speculated
that the proper amount of n-Bu4N+ not only endow remarkable
stability due to its hydrophobicity but also allow reactant
molecules to access the catalytic centers because of the short
alkyl chains and small steric hindrances, while excess amount
of n-Bu4N+ surrounding the anion dissipate the advantage. In
the case of compound 5, the anion is encapsulated tightly by
amphiphile DODA+ with a straight alkyl chain at the anion
surface8 which causes less approach to the catalytically active
centers, so less catalytic effect was found for compound 5.
In summary, based on the above experimental results, it can
be concluded that the compound 2 has the highest photo-
catalytic activities among the derivative's family, and the
optimal condition for RhB degradation was determined as, 50
mL of 2.0 mg Lꢀ1 RhB solution, pH 3.5–4.0, dosage of
compound 2 2.0–3.0 mg, and reaction time 3–6 h.
TOC measurement and identication of nal products in the
decoloration of RhB
The TOC changes in the RhB solution during the progress of
photodegradation reected the mineralization degree of the
dye. The initial RhB solution contained about 1.85 ppm of TOC.
In order to check the adsorption of RhB on the catalysts, the dye
solution was stirred in the dark for 0.5 h under the following
conditions: 50 mL of 2.0 mg Lꢀ1 RhB solution with pH 3.5,
2.0 mg of the compounds 2, 4 and 5, respectively. Aer
adsorption of RhB over the three compounds reached an equi-
librium, it was found that the TOC values of the bulk solution
dropped to ca. 1.79 ppm showing that ca. 3% of RhB was
adsorbed on the catalyst for each case.The TOC changes of RhB
under UV-light irradiation using compound 2 as catalyst are
shown in Fig. 5. The TOC removal of ca. 27.9% (from 1.79 to
1.29 ppm) was achieved aer 6 h stirring when a complete
decoloration of the dye solution (line a in Fig. 5) was nished.
The TOC values decreased from 1.79 to 1.40 ppm aer 17 h
stirring when compound 4 was used as a catalyst, correspond-
ing to a TOC removal of ca. 21.8%. While in the case of
compound 5 as catalyst, the TOC was removed only by 4.5%
(from 1.79 to 1.70 ppm) aer 23 h stirring. Notably, when the
intensity of UV light was enhanced by using a 250 W mercury
lamp (light intensity 22 mW cmꢀ2) instead of BYLAB UV-III UV
light desktop with a power of 12 W (light intensity 13 mW cmꢀ2),
the TOC value dropped from 1.79 to 1.26 ppm, corresponding to
29.6% removal of TOC, within a much shorter time of 75 min
(line b in Fig. 5) by using the compound 2 as catalyst. These
results agreed quite well with the photocatalytic decolorizing
efficiency of the catalysts obtained from the above UV-vis
absorption spectral results.
Fig. 4 UV-vis absorption spectral changes of RhB solution during the
catalytic process with (a) compound 1 and (b) compound 3 as catalyst.
The dash line represents UV-vis absorption spectra of compound 1
and compound 3, respectively. The arrow marks the increase of
reaction time. Reaction condition: 50 mL of 2.0 mg Lꢀ1 RhB solution
and pH 3.5.
decoloration under the same condition (50 mL of 2.0 mg Lꢀ1
RhB solution with pH 3.5, 2.0 mg of compound 4) as that for
compound 2. It was found that when compound 4 was used as a
catalyst the characteristic absorption of the RhB solution
decreased to ca. 73% aer 17 h stirring under UV light (Fig. S8†).
The adsorption site did not dri as in the case of compound 2.
As compared to compound 2, compound 4 showed weak cata-
lytic activities under the same condition.
Taken the above results into account, it is known that these
three heterogenous catalysts showed the following order:
compound 2 > compound 4 [ compound 5 in view of catalytic
effect. The reason for the phenomena can be tentatively
explained as follows: in the case of n-Bu4N+ as cations for
V
{Mo132} anions, each [Mo72VIMo60 O372(SO4)30(H2O)72
]
72ꢀ anion
has potential to combine more n-Bu4N+cations than each
[Mo72VIMo60 O372(CH3COO)30(H2O)72
]
anion because of
V
42
difference in the negative charge numbers, which causes the
number of accessible catalytic active centers of compound 4 less
than that of compound 2. This may be responsible for the
lower catalytic effect of compound 4 because the anions in
It should be noted that the color of the RhB solution faded
gradually upon its decomposition in the presence of the cata-
lysts, and concomitantly, the characteristic absorbance peak of
RhB at 554 nm gradually decreased without any peak shi
compounds
2
and
4
are nearly the same except their
anionic charges
due to different in the
L
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Table 4 Predominant products of the photodegradation of RhB and
their amounts detected by GC/MS using compound 2 as a catalyst
Retention time
(min)
Relative product
amount
Detected product
1.42
1.48
0.23
1.00
1.57
1.64
0.36
0.11
Fig. 5 Temporary changes of TOC during photodegradation of RhB
(pH 3.5) using compound 2 by (a) the BYLAB UV-III UV light desktop
with a power 12 W and an irradiation wavelength of 365 nm; (b) a 250
W high pressure mercury lamp.
(Fig. 1). This observation of no peak shi indicates that the
opening of the benzene ring was the main process during the
catalytic process.22 Further, the small peaks between 200 nm
and 450 nm gradually decreased without a peak shi and nally
disappeared, indicating that the fragments generated as a result
of ring-opening degraded easily during the catalytic process
with time.
GC/MS was used to analyze the nal products and the cor-
responding concentrations of RhB degradation. The results (see
Table 4) showed that the photodegradation of RhB in the
presence of different catalysts under the UV light produced the
same products with different concentrations. In this paper, four
predominant products were analysed and some other products
with smaller intensities were neglected. These products mainly
came from the partially cleaved conjugated xanthene structure
of RhB during the degradation reaction. It should be noted that
the photodegradation products of RhB produced under
different experimental systems may differ from each other, and
a direct and simple comparison is not allowed.23,24
applied for this photocatalytic processes needs to be studied. As
it is known that hydroxyl radicals involved in a photoreaction
can be detected by the photoluminescence (PL) technique using
terephthalic acid as a probe molecule because the hydroxyl
radicals react readily with terephthalic acid to produce the
highly uorescent product 2-hydroxyterephthalic acid.30 The
principle of this method is based on the PL signal of
2-hydroxyterephthalic acid at 425 nm. Moreover, the PL inten-
sity of 2-hydroxyterephthalic acid is proportional to the
concentration of cOH radicals which are produced from POMs
catalysis. So, a similar procedure for the measurement of pho-
tocatalytic activity was carried out where just the RhB aqueous
solution was replaced by aqueous solution of terephthalic acid
(5 ꢂ 10ꢀ4 M) with NaOH solution (2 ꢂ 10ꢀ3 M). A gradual
increase in PL intensity at about 425 nm (excited by 315 nm UV-
light) was observed with increasing irradiation time for
compounds 2, 4, and 5 (Fig. 6 and S9†), while no PL increase was
observed in the absence of UV light or in the absence of the
compounds. This observation indicates that the catalysts
indeed produce OH radicals under UV illumination. It was also
observed that the PL intensity change was not linear vs. the
irradiation time, and this can be ascribed to the fact that Mo132
catalyst is not stable in such a high basic condition and Mo132
catalyst may decompose gradually.
Importantly, under the same condition, the PL intensities
were different for the different compounds: the intensity for
compound 2 > the intensity for compound 4 [ the intensity for
compound 5 (Fig. 6 and S9†). This result is quite consistent with
their catalytic activities observed above.
In order to get more insight into the possible active species
involved during the degradation process in the present study,
isopropanol31 and KI32 (effective scavengers for OH radicals),
benzoquinone33 (an effective scavenger for O2cꢀ radicals) were
introduced into the reaction system under the same experi-
mental conditions in separate experiments (Fig. 7), respectively.
The understanding for the mechanism of catalytic reaction
Generally, it is accepted that under UV irradiation, excitation of
the POM molecule induces an electronic transition from the
highest occupied molecular orbital (HOMO) to the lowest
unoccupied molecular orbital (LUMO). The excited state of POM
(POM*) in a water solution can generate OH radicals25–27 which
are extremely powerful oxidizing agents for organic substrates.
The reduced POM (POMꢀ) produced from the POM* by
abstracting an electron from the substrates can deliver the
electron to electron acceptors28,29 such as O2, metal ions, and
then change into the oxidized POM form again. In other words,
the hydroxyl free radical mechanism was commonly involved in
POMs-based photocatalysis.
So far, no literature has reported about photocatalytic
activities and mechanisms for the Keplerate type Mo–O based
polyoxometalates. Whether the hydroxyl free radicals were
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Fig. 7 Plotted degradation kinetics of RhB under UV irradiation on
addition of NaN3 (a), isopropanol (b), KI (c), benzoquinone (d), and no
scavenger added (e). Reaction condition: 50 mL of 2 mg Lꢀ1 RhB
solution, 2 mg of compound 2, 2 mmol Lꢀ1 of scavenger (i.e., NaN3,
isopropanol, KI, benzoquinone).
Separation, stability and recovery of the catalyst
As compound 2 is found to be the most active one towards the
photocatalytic decoloration of RhB solution under the obtained
optimal condition, the recovery and reuse of compound 2 was
tested. At the completion of the photocatalytic reaction, the
suspensions were collected by ltration through a poly-
carbonate lm with an average pore diameter of 15 nm and
thoroughly washed with water to remove the RhB which was
attached on the surface of catalyst. Then the catalyst was reused
in the catalytic experiments. The catalytic activity of the recov-
ered compound 2 for the decoloration of RhB was still satis-
factory aer 5 repeated experiments (Fig. 8). The slight
reduction of decoloration ratio was owed to unavoidable loss of
the catalyst during the repeating regeneration processes. The IR
spectra of the catalyst did not show distinct changes aer 5
repeated experiments (Fig. S10†). Decomposition of compound
2 in solution under catalytic conditions was excluded by per-
forming ICP analysis for Mo in the ltration aer complete
removal of the suspension on the completion of the photo-
catalytic reaction. These indicate unambiguously that
compound 2 remained quite stable and its structure remained
intact under the catalytic condition. Otherwise, the situation as
illustrated in Fig. 4a would have taken place.
Fig. 6 Photoluminescence spectral changes of the compound 2
under UV light irradiation (a), and photoluminescence intensity
changes at lmax ¼ 425 nm vs. UV light irradiation time of the
compound 2, 4 and 5. (b) in a 5 ꢂ 10ꢀ4 M basic solution of terephthalic
acid (excitation light l ¼ 315 nm, voltage ¼ 500 V, EX slit ¼ 10 nm, EM
slit ¼ 10 nm).
A little effect on the degradation rate of RhB upon addition of
benzoquinone into the reaction system was found, suggesting
that the O2cꢀ radicals were not the dominant active oxygen
species in the present reaction system. Notably, the degradation
rate of RhB upon addition of either isopropanol or KI into the
reaction system decreased greatly, which further conrmed that
the OH radicals were the active oxygen species, a result quite
consistent with the PL measurement. In addition, it was found
that addition of NaN3 (ref. 34) (an effective scavenger for both
OH radicals and 1O2) led to a remarkable suppression in the
photodegradation rate of RhB, indicating that besides OH
radicals 1O2 also took part in the photo degradation process.
Taking into account that the Keplerates consist of 12
VI
pentagonal {(MoVI)Mo5
O (H2O)6} units and 30 linear
21
{Mo2VO4(L)} linkers, we believe that the 12 pentagonal
{(MoVI)Mo5 O21(H2O)6} units of Keplerates having MoVI centers
VI
rather than 30 linear {Mo2VO4(L)} linkers are the active centers
to generate OH radicals, because the oxidation state of the Mo
species in the {Mo2VO4 (L)}unit is already (+)5.
Fig. 8 The catalytic activity of recovered compound 2 versus cycling
runs.
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¨
¨
11 C. Schaffer, A. M. Todea, H. Bogge, O. A. Petina,
Conclusions
¨
E. T. K. Haupt and A. Muller, Chem.–Eur. J., 2011, 17,
9634–9639.
The photocatalytic activities of a derivative family of the Keplerate
¨
¨
12 S. Garai, E. T. K. Haupt, H. Bogge, A. Merca and A. Muller,
type nano-porous Mo–O based polyoxometalates with general
formula [Mo72VIMo60 O372(L)30(H2O)72]nꢀ (L ¼ CH3COOꢀ, SO4
)
V
2ꢀ
Angew. Chem., Int. Ed., 2012, 51, 10528–10531.
13 I. V. Kozhevnikov, Chem. Rev., 1998, 8, 171–198.
14 M. Sadakane and E. Steckhan, Chem. Rev., 1998, 98, 219–237.
were evaluated for the rst time by a prototype photocatalytic
decoloration of an RhB solution. The Mo132 anions were found to
be the active centers under UV irradiation to degrade RhB, while
they were unstable in solution and need to be protected by
organic cations endowing stability of the anions due to the
resultant water-tolerant property and the hydrophobic interac-
tions between the alkyl chains of cations in the hybrid materials
in a solid state. An analytical mechanism indicates that both OH
´
15 S. Kopilevich, A. Gil, M. Garcia-Rates, J. B. Avalos, C. Bo,
¨
A. Muller and I. A. Weinstock, J. Am. Chem. Soc., 2012, 34,
13082–13088.
16 C. Besson, S. Schmitz, K. M. Capella, S. Kopilevich,
¨
I. A. Weinstock and P. Kogerler, Dalton Trans., 2012, 41,
9852–9854.
1
17 N. V. Izarova, O. A. Kholdeeva, M. N. Sokolov and V. P. Fedin,
Russ. Chem. Bull., Int. Ed, 2009, 58, 134–137.
18 Y. Zhou, Z. Shi, L. Zhang, S. Hassan and N. Qu, Appl. Phys. A:
Mater. Sci. Process., 2013, 113, 563–568.
19 N. Lu, Y. H. Zhao, H. B. Liu, Y. H. Guo, X. Yuan, H. Xu,
H. F. Peng and H. W. Qin, J. Hazard. Mater., 2012, 199–
120, 1–8.
radicals and O2 participate in the photo degradation process.
Compound 2 showed the highest photocatalytic activity due to
the largest formation rate of hydroxyl radicals. It is expected
that this study will open a new perspective for the Keplerate
type nano-porous Mo–O based polyoxometalates with respect
to possible applications in photocatalysis which is yet
unexplored.
20 I. Arslan-Alaton, Dyes Pigm., 2004, 60, 167–176.
21 S.-i. Noro, R. Tsunashima, Y. Kamiya, K. Uemura, H. Kita,
L. Cronin, T. Akutagawa and T. Nakamura, Angew. Chem.,
Int. Ed., 2009, 48, 8703–8706.
22 T. Shiragami, S. Fukami, Y. J. Waka and S. Yanagida, J. Phys.
Chem., 1993, 97, 12882–12887.
Acknowledgements
The nancial support of the Natural Science Foundation of
China, PCSIRT (no. IRT1205) and Beijing Engineering Center
for Hierarchical Catalysts is greatly acknowledged. Prof. Xue
Duan of Beijing University of Chemical Technology is greatly
acknowledged for his kind support.
23 C. Chen, W. Zhao and P. Lei, Chem.–Eur. J., 2004, 10, 1956–
1965.
24 P. Lei, C. Chen and J. Yang, Environ. Sci. Technol., 2005, 39,
8466–8474.
25 E. Androulaki, A. Hiskia, D. Dimotikali, C. Minero, P. Calza,
E. Pelizzetti and E. Papaconstantinou, Environ. Sci. Technol.,
2000, 34, 2024–2028.
Notes and references
1 C. L. Hill, Chem. Rev., 1998, 98, 1–387.
2 D. Long, R. Tsunashima and L. Cronin, Angew. Chem., Int.
Ed., 2010, 49, 1736–1758.
26 A. Mylonas, A. Hiskia and E. Papaconstantinou, J. Mol. Catal.
A: Chem., 1996, 114, 191–200.
¨
¨
3 A. Muller, E. Krickemeyer, H. Bogge, M. Schmidtmann and
F. Peters, Angew. Chem., Int. Ed., 1998, 37, 3359–3363.
27 T. Yamase and T. Kurozumi, J. Chem. Soc., Dalton Trans.,
1983, 10, 2205–2209.
28 A. Troupis, A. Hiskia and E. Papaconstantinou, New J. Chem.,
2001, 25, 361–363.
¨
¨
4 A. Muller, E. Krickemeyer, H. Bogge, M. Schmidtmann,
B. Botar and M. O. Talismanova, Angew. Chem., Int. Ed.,
2003, 42, 2085–2090.
¨
5 A. Muller, S. K. Das, S. Talismanov, S. Roy, E. Beckmann,
29 A. Troupis, A. Hiskia and E. Papaconstantinou, Angew.
Chem., Int. Ed., 2002, 41, 1911–1914.
¨
H. Bogge, M. Schmidtmann, A. Merca, A. Berkle,
L. Allouche, Y. Zhou and L. Zhang, Angew. Chem., Int. Ed.,
2003, 42, 5039–5044.
30 J. Yu, G. Dai and B. Cheng, J. Phys. Chem. C, 2010, 114,
19378–19385.
¨
¨
6 A. Muller, D. Rehder, E. T. K. Haupt, A. Merca, H. Bogge,
31 G. T. Li, K. H. Wong, X. W. Zhang, C. Hu, J. C. Yu,
R. C. Y. Chan and P. K. Wong, Chemosphere, 2009, 76,
1185–1191.
¨
M. Schmidtmann and G. Heinze-Bruckner, Angew. Chem.,
Int. Ed., 2004, 43, 4466–4470.
7 D. G. Kurth, A. Chesne, D. Volkmer, M. Ruttorf, B. Richer and
32 C. Karunakaran and R. Dhanalakshmi, Sol. Energy Mater.
Sol. Cells, 2008, 92, 1315–1321.
¨
A. Muller, Chem. Mater., 2000, 12, 2829–2831.
8 D. Volkmer, D. G. Kurth, H. Schnablegger, P. Lehmann,
33 C. Schweitzer and R. Schmidt, Chem. Rev., 2003, 103, 1685–
1757.
¨
M. J. Koop and A. Muller, J. Am. Chem. Soc., 2000, 122,
1995–1998.
34 S. Zhao, X. H. Wang and M. X. Huo, Appl. Catal., B, 2010, 97,
127–134.
¨
¨
9 P. Kogerler, B. Tsukerblat and A. Muller, Dalton Trans., 2010,
39, 21–36.
10 L. Zhang, Y. Zhou and R. Han, Eur. J. Inorg. Chem., 2010, 17,
2471–2475.
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