Mechanism for catalytic ozonation of p-nitrophenol in water with titanate nanotube supported manganese oxide

Article abstract of DOI:10.1039/c5ra19808a

Manganese oxide supported on titanate nanotubes (TNT) was prepared by an impregnation method and used as a catalyst for ozonation of p-nitrophenol (PNP) in an aqueous solution. Characterization results indicated that the manganese oxide was highly dispersed on the surface of TNT. The synthesized catalyst exhibited high activity for the mineralization of PNP with ozone, and about 95% of the total organic carbon was removed at 45 min. The degradation of PNP was mainly due to the oxidative process in solution, and the hydroxyl radical reaction played an important role for the degradation of its ozonation products (formic acid and oxalic acid). The negatively charged surface and surface acid sites of the support favored the adsorption of ozone, while the highly dispersed MnO_x accelerated the decomposition of adsorbed ozone into hydroxyl radicals. A possible mechanism for the catalytic ozonation of PNP was proposed.

Full text of DOI:10.1039/c5ra19808a

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PAPER
Mechanism for catalytic ozonation of p-
nitrophenol in water with titanate nanotube
supported manganese oxide
Cite this: RSC Adv., 2015, 5, 101975
Shengtao Xing,* Xiaoyang Lu, Xinjian Zhang, Yiyao Zhang, Zichuan Ma and Yinsu Wu*
Manganese oxide supported on titanate nanotubes (TNT) was prepared by an impregnation method and
used as a catalyst for ozonation of p-nitrophenol (PNP) in an aqueous solution. Characterization results
indicated that the manganese oxide was highly dispersed on the surface of TNT. The synthesized catalyst
exhibited high activity for the mineralization of PNP with ozone, and about 95% of the total organic
carbon was removed at 45 min. The degradation of PNP was mainly due to the oxidative process in
solution, and the hydroxyl radical reaction played an important role for the degradation of its ozonation
products (formic acid and oxalic acid). The negatively charged surface and surface acid sites of the
Received 25th September 2015
Accepted 19th November 2015
support favored the adsorption of ozone, while the highly dispersed MnO
x
accelerated the
DOI: 10.1039/c5ra19808a
decomposition of adsorbed ozone into hydroxyl radicals. A possible mechanism for the catalytic
ozonation of PNP was proposed.
www.rsc.org/advances
the solution pH. At pHs lower than the pH of the point of zero
charge (PZC), the particle surface is positively charged, while it
Introduction
14
Water pollution with organic pollutants has become a serious has negative charges above it. Zhang et al. suggested that not
environmental problem in the past few decades. Heterogeneous all surface hydroxy groups of FeOOH possessed the same high
catalytic ozonation has recently attracted considerable attention catalytic activity, but the weak surface MeO–H bonds were
as a promising technology for the degradation and minerali- favorable sites for promoting cOH generation from aqueous
1,2
15
zation of refractory organic pollutants in water. Signicant ozone. Li et al. reported that the positively charged surface of
efforts have been made on the preparation of highly active PdO/CeO played an important role for the catalytic ozonation
2
16
heterogeneous ozonation catalysts. These catalysts can promote of pyruvic acid. On the contrary, some researchers found that
the generation of hydroxyl radicals from ozone decomposition negatively charged surface groups had a major effect on ozone
17,18
and lead to higher efficiency of ozone consumption. However, decomposition.
On the other hand, the surface hydroxy
their application is limited because the mechanisms of these groups of metal oxide can behave as Br ¨o nsted acid sites. Some
3
processes are still unclear.
metal oxides have Lewis acid sites and/or Lewis bases. Since
The efficiency of catalytic ozonation depends on the struc- ozone can be regarded as a Lewis base, it can react with surface
19
ture and surface properties of the catalysts. Transition metal acid groups of metal oxide. For instance, Bing et al. reported
oxides with variable electronic structures have exhibited excel- that the combination of both Lewis acid sites of iron and
4
–8
lent catalytic activity for ozonation of organic pollutants. In aluminium onto Fe
2
O
3
/Al
2
20
O
3
@SBA-15 enhanced the formation
À
order to increase the active surface, mesoporous materials with of cOH and O
large surface areas were used as the supports of metal oxides.
The supported oxides with multivalence oxidation states and reactants (ozone and/or organic molecules) on the surface
2
c
radicals. However, some researchers found
9–13
that alumina did not decompose ozone, and the adsorption of
21–24
high dispersion can enhance the interfacial electron transfer, might be necessary for its catalytic effect.
Therefore, the fate
causing the high catalytic activity. In aqueous systems, metal of the catalytic activity of metal oxide should be further
oxide particles can be hydrated and form surface hydroxy investigated.
groups, which are hypothesized to be active sites in catalytic
In the present work, manganese oxide was highly dispersed
ozonation. Nevertheless, there are full of contradictory reports on the surface of titanate nanotube (TNT) by a simple impreg-
on the catalytic activity of metal oxides. These surface hydroxy nation method. TNT was selected as the support because it
groups tend to dissociate or protonate, which is dependent on contains acid sites and has unusual morphology. The structure,
morphology and surface properties of the catalyst were analyzed
by different characterization methods. p-Nitrophenol (PNP),
a common compound in waste waters, was selected to evaluate
the catalytic ozonation activity of the catalyst. The results
College of Chemistry and Material Sciences, Hebei Normal University, Shijiazhuang
0
8
50024, PR China. E-mail: stxing07@sina.com; wuyinsu02@163.com; Fax: +86 311
0787402; Tel: +86 311 80787406
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À1
indicated that the catalyst signicantly enhanced the mineral- 1 mL min and wavelength of 210 nm. The mobile phase
ization of PNP. The correlation between the surface properties consisted of 60% acetonitrile and 40% water. Organic inter-
and catalytic activity of the catalyst was discussed. A possible mediates were determined using a Poroshell 120 SB-Aq
À1
reaction mechanism was also proposed.
column (4.6–100 mm, 5 mm) at a ow rate of 0.5 mL min
The mobile phase consisted of the solution of 0.1% H
.
3
PO
4
/
H
2
O. The concentration of 7-hydroxycoumarin was measured
with a Hitachi F-4600 uorescence spectrometer at 455 nm
excited at 332 nm). In order to avoid the degradation of 7-
Experimental
Catalyst preparation
(
All chemicals were of analytical grade and used without further hydroxycoumarin by excessive cOH, the ozone output was
À1
purication. Deionized water was used in this study. TNT was decreased to 0.2 mg min . Each experiment was run in
2
5
prepared by a hydrothermal method. 1 g of anatase phase TiO
triplicate. Data were the arithmetic mean of three measured
2
À1
powders were added into 50 mL of 10 mol L NaOH aqueous values.
solution. The suspension was transferred into an autoclave and
heated at 150 C for 24 h. The product was washed with dilute
ꢀ
HCl and distilled water. Supported manganese oxide was
Results and discussion
Characterization of catalysts
prepared by an impregnation method with Mn(NO
3 2
) as the
metal precursor. 0.67 g of Mn(NO ) was dissolved in 100 mL of
3
2
distilled water. Then, 1 g of TNT was added to this solution. The XRD patterns of the products are shown in Fig. 1. The
ꢀ
ꢀ
ꢀ
Aer impregnation, the sample was washed with deionized diffraction peaks of TNT located at 11 , 25 and 49 can be
water to remove the excess manganese that was not connected assigned to the (001), (011) and (020) faces of Na $nH
x
H
2ÀxTi
3
O
7
2
O
ꢀ
tightly to the surface of titanate, and dried at 110 C for several (JCPDS 31-1329), respectively. According to previous report,
hours and calcined in air at 400 C for 2 h. As a reference, TNT layered hydrogen titanates nanotubes would be transformed to
ꢀ
ꢀ
was also calcined in air at 400 C for 2 h.
anatase with the loss of the tubular morphology aer calcination
at 400–600 C. Therefore, the TNT sample calcined at 400 C
should be Na H Ti O $nH O. The XRD pattern of MnO /TNT is
ꢀ
26
ꢀ
Catalyst characterization
x
2Àx
3
7
2
x
similar to that of pure TNT. No XRD diffraction peaks of
manganese oxide were observed due to its low content and good
dispersion. EDX analysis reveals that the Mn content loaded on
TNT is about 4 wt%.
TEM images in Fig. 2 show that both TNT and MnO /TNT are
x
nanotubes with diameters of about 10 nm and lengths of several
hundred nanometers. Comparing to TNT, the nanotubular
X-ray diffraction (XRD) analysis was performed on a Bruker D8-
Advance using Cu Ka radiation. The morphologies were exam-
ined on a Hitachi H-7500 transmission electron microscopy
(TEM) at 80 kV and a JEM-2100 high-resolution TEM at 200 kV.
The content of Mn was determined by induced coupled plasma
spectrometer (ICP)-MS (Thermo Fisher). X-ray photoelectron
spectroscopy (XPS) data were obtained by an AXIS-Ultra
instrument using monochromatic Al Ka radiation (225 W, 15
mA, 15 kV). Nitrogen adsorption–desorption measurements
were carried out with a Quantachrome NOVA 4000e surface area
analyzer at 77 K. Infrared spectra were recorded on a Nicolet iS
structure of MnO /TNT became uniform and the tube length
x
became long. The reason might be that some nanotubes
collapsed during calcination and the loaded manganese
increased the thermal stability of TNT. Manganese oxide was
not detected by HRTEM imaging, which could be due to its low
content and amorphous nature. Manganese oxide might be
5
0 Fourier transformation infrared (FTIR) spectrometer with
À1
KBr disks in the range of 4000–400 cm at room temperature.
The zeta potential of the catalyst suspensions was measured
with a Malvern 3000 Zetasizer with three consistent readings.
Catalytic activity measurements
À1
In a typical experiment, 0.5 L of 40 mg L PNP solution and
0
.1 g catalyst were added into a 1 L cylindrical reactor. Ozone
À1
gas (4 mg min output) generated from a 3S-A3 laboratory
ozonizer (Tonglin Technology, China) was continuously fed
to the suspension under magnetical stirring. Samples were
withdrawn and ltered through a millipore lter (pore size
0
.22 mm) for analysis. The residual ozone was removed by
À1
adding 0.1 mol L Na
2
S
2
O
3
solution. The concentration of
aqueous ozone was measured with the indigo method (Ozone
Standards Committee method). The total organic carbon
(
TOC) of the solution was determined with a Phoenix 8000
TOC analyzer. PNP was measured by a high performance
liquid chromatography (HPLC, Agilent 1200) using a ZORBAX
Eclipse XDB-C18 column (4.6–250 mm, 5 mm) at a ow rate of Fig. 1 XRD patterns of TNT, MnO
x
/TNT and Na
x
3 7 2
H2ÀxTi O $nH O.
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x
Fig. 4 Nitrogen adsorption–desorption isotherms of TNT and MnO /
TNT.
x
Fig. 2 TEM images of (a) TNT, (b) MnO /TNT, and HRTEM image of (c)
MnO /TNT.
x
Surface properties of a catalyst, such as surface charge and
acidity, are very important for the heterogeneous catalytic
ozonation of organic pollutants. Fig. 5 shows the zeta potential
as a function of pH for different catalyst suspensions. The PZC
dispersed into the pore channels of TNT, which is similar with
27,28
the literatures.
MnO /TNT was characterized by XPS to conrm the chemical
state of Mn and Ti (Fig. 3). The peaks located at 458.1 and 463.9
x
of TNT and MnO /TNT were measured to be about 1.9 and 1.7,
x
4
+
4+
29,30
eV were attribute to Ti 2p3/2 and Ti 2p1/2, respectively.
The broad and asymmetric Mn 2p peaks imply multiple oxida-
tion states of Mn in MnO /TNT. The peaks around 653.2 and
41.3 eV were assigned to the Mn 2p1/2 and Mn 2p3/2 transitions
x
respectively. The introduction of MnO did not change the
surface charge property of TNT. The surface of the catalysts was
negatively charged at pH above 2, and the surface charge density
increased signicantly with increasing pH.
x
6
2
+/3+ 31
of Mn
x
. The surface concentration of Na on MnO /TNT was
The adsorption of base molecules (e.g. pyridine or
ammonia) combined with vibrational spectroscopic tech-
niques, is well known for characterizing the surface acid
much lower than that on TNT, suggesting that Mn ion was
loaded on TNT via an ion exchange with Na ion. Then, highly
dispersed MnO was formed by calcination.
33
x
sites. In order to study the interaction between base mole-
The surface area of TNT was calculated to be about 143.6
cules and catalyst surface in aqueous system, the catalyst was
added to an aqueous pyridine or ammonia solution, and the
suspension was vigorously stirred for 2 h to achieve
adsorption/desorption equilibrium. Then the catalyst was
separated from solution by centrifugation and dried in an
2
À1
2
À1
m g , smaller than that of MnO /TNT (171.2 m g ). The
x
reason is that some nanotubes of TNT collapsed during calci-
nation. As shown in Fig. 4, both of the isotherms are type IV.
The hysteresis loop is type H3, indicating the existence of
32
narrow slit-shaped pores.
oven for analysis. The FTIR spectra of the catalysts are shown
À1
in Fig. 6. The band at 910 cm
corresponded to the
3
4
stretching vibration of Ti–O bonds. The weak band around
Fig. 3 (a) Ti 2p and (b) Mn 2p spectra of MnO
spectra of TNT and MnO /TNT.
x
/TNT, and (c) Na 1s Fig. 5 Plot of the zeta potential as a function of pH for different
À3
À1
x
3
catalyst suspensions in the presence of KNO (10 mol L ).
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Fig. 6 FTIR spectra of different catalysts: (a) TNT, (b) TNT after
adsorption of ammonia, (c) TNT after adsorption of pyridine, (d) MnO
x
/
TNT, (e) MnO /TNT after adsorption of ammonia, (f) MnO /TNT after
x
x
adsorption of pyridine.
À1
1
630 cm was assigned to the H–O–H bending vibration of
water. For TNT aer adsorption of pyridine, the intensity of
À1
this band around 1630 cm became stronger, which might
be due to the protonation of pyridine molecule by the
3
3
Br ¨o nsted acid sites. According to the literature, the coor-
dination interaction of water molecules with oxide surface is
accompanied with protonization, and the strong proton
centers on the TNT surface can interact with pyridine mole-
+
35
Fig. 7 (a) Degradation of PNP and (b) TOC removal during ozonation
and catalytic ozonation of PNP at pH 6.5.
cules with a creation of pyridinium ion (PyH ). The broad
À1
band at 400–600 cm further conrms the adsorption of
3
5
pyridine on TNT. The TNT sample aer adsorption of
ammonia exhibited similar FTIR spectra of the sample aer the oxidative process in solution. The effect of catalytic ozona-
À1
adsorption of pyridine except for a new band at 1402 cm
assigned to the asymmetric deformation mode of NH
,
tion in the presence of MnO /TNT is much higher than the
combined effect of adsorption on the catalyst surface and
x
+
4
3
6,37
cations adsorbed on Br ¨o nsted acid sites.
The strong band ozonation without catalyst, demonstrating that MnO /TNT is an
around 1630 cm could be due to ammonia adsorbed on effective ozonation catalyst. Catalytic ozonation usually exhibits
/TNT aer adsorption of pyridine high effectiveness in the mineralization of organic pollutants.
and ammonia, the bands at 400–600 and 1630 cm were The TOC removal efficiencies during the degradation of PNP are
x
À1
3
8
Lewis acid sites. For MnO
x
À1
À1
much weaker and the band at 1402 cm was not observed. shown in Fig. 7b. About 58% of TOC was removed at 45 min in
The result indicates that TNT exhibited higher adsorption the presence of ozone alone. The addition of TNT did not
capacities for base molecules and might have more acid sites enhance the mineralization of PNP, and the TOC removal effi-
than MnO
x
/TNT.
ciency was only 52%. However, the TOC removal increased
dramatically with the addition of MnO /TNT, and about 95% of
x
TOC was removed at a reaction time of 30 min. The maximum
Catalytic ozonation of PNP
À1
concentration of leached Mn in the solution was 0.85 mg L
.
Fig. 7a displays the degradation of PNP in single ozonation and For comparison, the catalytic activity of the dissolved Mn was
catalytic ozonation using various catalysts at pH 6.5. Only 81% tested. However, only about 60% of TOC was removed. The
of PNP was removed at 45 min in the absence of catalyst. The contribution of homogeneous catalysis could be ignored. The
addition of catalyst enhanced the degradation of PNP. In the results indicate that the TNT support is inactive and the sup-
presence of TNT, 91% of PNP was removed at 45 min, while with ported Mn is the active component for this reaction. In our
MnO
x
/TNT PNP was completely removed at 30 min. The previous work, various mesoporous composite metal oxides
18
adsorption efficiencies of PNP on TNT and MnO /TNT were were prepared and used as catalyst for ozonation of PNP. The
x
about 10% and 3%, respectively. The increased removal of PNP highest TOC removal efficiency (95%) was achieved by Mn–Co–
in TNT suspension was due to its adsorption on TNT, while the Fe at a reaction time of 60 min. MnO
x
/TNT exhibited much
/TNT was
degradation of PNP in MnO
/TNT suspension was mainly due to higher activity than Mn–Co–Fe. The stability of MnO
x
x
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also examined. As shown in Fig. 8, the TOC removal efficiencies
were all above 85% in each cycle, and no apparent reduction
was observed aer six cycles. The result indicates that MnOx/
TNT is a highly effective and stable ozonation catalyst.
Previous studies conrmed that the main ozonation products
of nitrophenol are carboxylic acids that could not be oxidized in
39,40
ozonation.
The solution pH decreased from 6.5 to 3.3 during
the single ozonation process, indicating the accumulation of
organic acids in solution. While the solution pH in the presence
of MnO /TNT changed less (from 6.5 to 4.0) than that in ozona-
x
tion, which might be due to the removal of the produced organic
acids. The organic intermediates generated during the degrada-
tion process were analyzed by HPLC and identied by compar-
ison with standards. Formic acid and oxalic acid were detected
during the degradation of PNP, and their retention times were 2.8
and 2.9 min, respectively. The concentrations of the acids in
catalytic ozonation process were much lower than that in ozon-
ation process. It has been demonstrated that formic acid and
oxalic acid are hardly oxidized by ozone molecules and their
Fig. 9 Catalytic ozonation of formic acid and oxalic acid with MnO
TNT at different pH.
x
/
À2
À1 À1
ozonation rate constants are only 5.0 and <4.0 Â 10
M
s
,
with increasing pH, which inhibits the adsorption of anions.
41
respectively. Because hydroxyl radicals are highly potent Surface reaction involving adsorbed pollutants is suggested to
chemical species with a little selectivity, formic acid and oxalic play an important role in heterogeneous catalytic ozonation
10,18
acid can be readily oxidized by hydroxyl radicals with the reaction process.
When the initial pH decreased from 6.5 to 3.5, the
8
6
À1
41
rate constant of 1.3 Â 10 and 1.4 Â 10 M s , respectively.
adsorption efficiencies for formic acid and oxalic acid increased
Fig. 9 presents the catalytic ozonation of formic acid and oxalic from 10.7% to 23.5% and 5.4% to 9.1%, respectively. The result
acid with MnO /TNT at different pH. Both formic acid and oxalic indicates that the contribution of surface reactions to the
x
acid were signicantly degraded in the catalytic ozonation removal of formic acid and oxalic acid would be strengthened
process, suggesting that the catalyst can enhance the generation with decreasing pH, which is similar with the phenomena in
4
2
of active radicals from ozone decomposition. When the initial pH literature.
decreased from 6.5 to 3.5, the TOC removal efficiencies for formic
acid and oxalic acid increased from 54% to 92% and 36% to 65%,
Mechanism discussion
respectively. This may be associated with the adsorption capacity
of the catalyst toward the acids at different pH values. On one It is well known that coumarin can react with hydroxyl radicals
hand, the increasing pH promotes the dissociation of the acids to form uorescent 7-hydroxycoumarin. Fig. 10a shows the
into anion species that are prone to be adsorbed on the catalyst formation of 7-hydroxycoumarin in different ozonation
surface. On the other hand, the catalyst surface is negatively systems. Only a small amount of 7-hydroxycoumarin was
charged in the tested pH range, and the charge density increases formed in ozonation and TNT catalytic ozonation, while MnOx/
TNT resulted in signicant 7-hydroxycoumarin generation. The
result suggests that the supported MnO indeed promoted the
x
decomposition of ozone into cOH. In addition, sodium sulte
(
Na
its inuence on TOC removal in ozonation and catalytic ozon-
ation processes. Although Na SO can also react with ozone
2 3
SO , 4 mM) was selected as a cOH scavenger to investigate
2
3
with a signicant rate, this effect can be neglected due to the
relatively high concentration of ozone in the reaction
43,44
medium.
As shown in Fig. 10b, the addition of Na SO had
2 3
slight effect on the single ozonation of PNP, which indicates
that the organic pollutants were mainly degraded by ozone
molecules. In contrast, the TOC removal in the presence of
x 2 3
MnO /TNT was signicantly inhibited by adding Na SO , indi-
cating that cOH was the main active species in this catalytic
ozonation reaction. Meanwhile, a series of ozone decomposi-
tion experiments were carried out. As shown in Fig. 11, about
90% of O3 was decomposed within 30 min without catalyst,
while the ozone decomposition rate was markedly enhanced in
the presence of catalysts. The concentration of ozone decreased
more rapidly in TNT suspension than MnO /TNT suspension
x
Fig. 8 TOC removal for six successive catalytic ozonation experi-
ments with MnO /TNT.
x
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within 25 min. Previous experiments have shown that TNT did
not enhance the mineralization of PNP, and the corresponding
TOC removal efficiency was even less than that without catalyst
(Fig. 7). The reason might be that ozone molecules were only
adsorbed on the surface of TNT and were not decomposed into
cOH. Organic molecules in TNT suspension were mainly
degraded by ozone molecules. Characterization results have
x
demonstrated that TNT has more acid sites than MnO /TNT
and its surface is negatively charged at pH above 2. Since ozone
can be regarded as a Lewis base and an electrophilic agent, it
can be readily adsorbed on the surface of TNT. On the contrary,
the organic intermediates of PNP were hardly adsorbed on TNT.
The reactions between ozone and organic molecules were
inhibited to some extent, leading to lower reactivity. In
comparison with TNT, MnO
mineralization of PNP and the decomposition of ozone, indi-
cating that the supported MnO can catalytically decompose
x
/TNT markedly enhanced both the
x
ozone into cOH. It can be deduced that the surface acid sites
and negatively charged surface groups of the TNT support are
active sites for the adsorption of ozone, while the highly
dispersed MnO is the active component for the decomposition
x
of ozone into cOH.
Based on the results obtained under different experimental
conditions, we proposed a possible mechanism for the miner-
alization of PNP in this catalytic ozonation reaction. Ozone
molecules were rstly adsorbed onto the support, and some of
them were transformed into cOH by MnO highly dispersed on
x
TNT. The TNT support with negatively charged surface and acid
sites favored the adsorption of ozone, while the highly
x
dispersed MnO with multiple oxidation states enhanced the
Fig. 10 (a) Formation of 7-hydroxycoumarin during ozonation and
catalytic ozonation of coumarin (20 mg L ), and (b) effect of sodium
sulfite (4 mM) on the ozonation and catalytic ozonation of PNP at pH
electron transfer involved in the catalytic decomposition reac-
tion of ozone. The transformation of ozone into cOH was
signicantly accelerated due to the synergic effect between TNT
À1
6
.5.
and MnO . On the other hand, PNP was oxidized into carboxylic
x
acids by ozone and cOH in solution. Subsequently, the produced
carboxylic acids were further oxidized into CO by active radi-
2
before 15 min, and then reached a plateau. In contrast, the
concentration of ozone in MnO /TNT suspension continuously
decreased aer 15 min, and ozone was completely decomposed
cals. Additionally, the solution pH decreased due to the accu-
mulation of organic acids in solution, and some of the
produced carboxylic acids were adsorbed on the catalyst, which
facilitate their surface reactions with radicals.
x
Conclusions
x
MnO was highly dispersed on the surface of TNT by a simple
impregnation method. The synthesized sample was highly
effective for the mineralization of PNP in aqueous solution by
ozone. PNP was oxidized by ozone molecules and active radicals
in aqueous solution, and its ozonation intermediates were
identied to be small molecular carboxylic acids such as formic
acid and oxalic acid. The produced acids were further oxidized
by hydroxyl radicals. The TNT support with negatively charged
surface and acid sites favored the adsorption of ozone, while the
highly dispersed MnO accelerated the decomposition of ozone
x
into hydroxyl radicals. This study offers a new approach for the
optimization of supported catalyst with potential applications
in heterogeneous catalytic ozonation.
Fig. 11 Decomposition of ozone in aqueous dispersions of various
catalysts at pH 6.5.
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2
2 B. Kasprzyk-Hordern and J. Nawrocki, Ozone: Sci. Eng., 2003,
Acknowledgements
25, 185.
´
This work was supported by the National Natural Science Foun- 23 B. Kasprzyk-Hordern, U. Raczyk-Stanisławiak, J. Swietlik and
dation of China (no. 21207032), the Natural Science Foundation of
J. Nawrocki, Appl. Catal., B, 2006, 62, 345.
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´
´
Hebei Education Department (no. ZD20131005).
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